Abstract
Neurodegenerative diseases, such as Alzheimer’s disease (AD), Parkinson’s disease (PD), Multiple Sclerosis (MS), and Huntington’s disease (HD), although distinct in their clinical manifestations, share a common hallmark: a disrupted neuroinflammatory environment orchestrated by dysregulation of neuroglial intercellular communication. Neuroglial crosstalk is physiologically ensured by extracellular mediators and by the activity of connexins (Cxs), the forming proteins of gap junctions (Gjs) and hemichannels (HCs), which maintain intracellular and extracellular homeostasis. However, accumulating evidence suggests that Cxs can also act as pathological pore in neuroinflammatory conditions, thereby contributing to neurodegenerative phenomena such as synaptic dysfunction, oxidative stress, and ultimately cell death. This review explores mechanistic insights of Cxs-mediated intercellular communication in the progression of neurodegenerative diseases and discusses the therapeutic potential of targeting Cxs to restore cellular homeostasis.
1 Introduction
Neurodegenerative diseases are characterized by progressive neuronal loss in the central nervous system (CNS) and/or peripheral nervous system (PNS). To date, standard therapies for neurodegenerative diseases are not curative, and patient improvements are limited, highlighting the need for more effective treatments (). Neurodegenerative diseases, such as Alzheimer’s disease (AD), Parkinson’s disease (PD), multiple sclerosis (MS) and Huntington’s disease (HD), represent a diverse group of disorders with distinct clinical and pathological features (). The heterogeneity of these disorders is based on clinical symptoms such as cognitive decline and motor impairments and areas affected by neurodegeneration, ranging from brain regions critical for memory and movement to the myelin sheath in MS (; ).
A shared hallmark between these diseases is the accumulation of proteins aggregates, such as the amyloid-beta (Aβ) plaques or misfolded protein and neuroinflammatory mediators, which drive oxidative stress, neurodegeneration and cell death (; ). Neuroinflammation is strongly supported by resident glial cells activation, including microglia and astrocytes, which modulate neuronal activity leading to alterations in their function and morphology. The CNS is an immune privileged tissue, and microglia represent the resident immune cells acting as phagocyting cells and protecting CNS and contributing to homeostasis (). Solid evidence suggest that their functions and phenotype can be associated with neuroinflammatory and neurotoxic stimulation and it has been linked to CNS degenerative diseases (). Reactive astrocytes may also contribute to neurodegeneration and show synaptotoxic effects, contributing to synapse alterations and neuronal suffering in preclinical models (; ; ; Teleanu et al., 2022). In line with this evidence, it has been observed that A1-like astrocytes are abundant in key CNS areas of AD, PD, MS and HD patients, actively driving neuronal death and synaptic loss processes ().
Glial cells communicate with each other and with neurons through gap junctions (GJs), which allow to exchange ions and small molecules, such as glucose, glutathione, glutamate, between their intracellular compartments cyclic adenosine monophosphate (cAMP) (). The structure of GJs is composed by connexins (Cxs), a protein family with four transmembrane domains, two extracellular loops, one intracellular loop and one intracellular carboxy-tail (Vicario and Parenti, 2022). Six Cx units form a hemichannel (HC) and two docked HCs of adjacent cells constitute a GJ. HCs are also involved in the interaction between intracellular and extracellular spaces and can be classified as homomeric or heteromeric, based on their Cxs composition (Vicario and Parenti, 2022; Figure 1). Up to now, 21 human Cx isoforms have been identified and the protein names have been assigned on their molecular weight in kilodaltons (). In addition to their canonical channel function, Cxs are involved in additional biological processes, including cell migration and metabolism, interaction with cytoskeleton, and mRNA transcription regulation, which collectively may have an indirect impact on intercellular crosstalk (; Su et al., 2024; Figure 1).
FIGURE 1
Connexins exhibit distinct subtype-specific expression patterns in CNS-resident cells. Neurons primarily express Cx36 and Cx45, mainly involved in electrical synapses formation and synaptic plasticity phenomena. In astrocytes, Cx43 and Cx30 are the predominant isoforms, contributing to cell adhesion, energy metabolism and processes related to neurodegeneration. Microglia express Cx26, Cx32 and Cx43, which are involved in regulating inflammation and neurotoxic signaling. Cx29, Cx32 and Cx47 are predominantly expressed by oligodendrocytes, providing metabolic support and contributing to myelin formation (Vicario et al., 2017; Figure 1).
In the last years, many studies have focused on GJ- and HC-forming Cxs dysregulation that, affecting homeostatic balance and cell membrane permeability, sustains the onset and progression of neurodegenerative diseases (; Vicario et al., 2017; ). In particular, Cxs expression levels have been associated with several physiological and pathological functions in the CNS, such as memory and behavior, neuroglia homeostasis and reactive gliosis, implying Cxs as new potential targets for the treatment of neurodegenerative diseases (). Cxs phosphorylation also represents a critical aspect of Cx function, triggered by different kinases in response to intracellular signals, including changes in calcium levels, oxidative stress and activation of signaling pathway. These include the gating (opening and closing) of GJs, the trafficking and assembly of Cxs into the plasma membrane and their degradation. In the context of neurodegenerative processes, altered phosphorylation can disrupt intercellular communication, contributing to pathological remodeling of GJs. However, several aspects remain controversial and need to be further explored (Schulz et al., 2015; ).
This review focuses on the involvement of neuroglia in neuroinflammation, exploring how glial Cxs contribute to the pathophysiology and progression of AD, PD, MS, and HD and exploring possibilities to target Cxs as a potential effective strategy for the treatment of neurodegenerative diseases.
2 Connexins’ role in Alzheimer’s disease (AD)
Alzheimer’s disease is the most common cause of dementia, characterized by cognitive decline, with pathological features that remain untreatable. Alterations in glial Cxs are implicated in AD neurotoxicity and increased HCs activity exacerbates neurodegeneration and gliotransmitter release (Table 1). Key pathological hallmarks, such as extracellular Aβ plaques and hyperphosphorylated tau aggregates, drive neurodegeneration and synaptic loss (Scheltens et al., 2016; Scheltens et al., 2021). Despite extensive research and numerous therapeutic strategies, no effective treatments are currently available, with prevention remaining the primary approach (Soria Lopez et al., 2019; ). Recent research findings revealed that, beyond neurons, also microglia and astrocytes are involved in AD pathogenesis, showing their role as potential therapeutic targets. Indeed, glial cells sustain neuroinflammatory responses, whose long-term effects are neuronal suffering and cell death (). While microglia activation may result in synapses loss through a complement-dependent process and the secretion of inflammatory mediators (Swanson et al., 2020; ), astrocytes exhibit neurotoxic reactivity triggered by Aβ aggregates and cytokines release (Shao et al., 1997; Stevenson et al., 2020). However, glial-induced inflammation can have both beneficial and detrimental effects in AD (Uddin and Lim, 2022).
TABLE 1
| Disease | Model | Cxs modulation | Effects | References |
| Alzheimer’s disease | Mouse primary astrocytes treated with Aβ25–35 | Increased Cx43 intracellular pool and activity | Neurotoxicity and neuronal damage | |
| APPswe/PS1dE9 mouse model | Increased Cx43 HCs activity | High Ca2+ levels in astrocytes; neuronal suffering | Yi et al., 2016 | |
| Old 5XFAD mouse model | Increased Cx43 and Cx30, reduced Cx47 expression | Loss of interaction between astrocytes and oligodendrocytes | ||
| APP/PS1 mouse model of early AD | Increased Cx43 phosphorylation levels | Unknown | ||
| Mouse primary microglia treated with Aβ25–35 | Increased Cx43 and Panx1 activities | Neurotoxic alterations | ||
| Parkinson’s disease | Rotenone-induced rat PD model | Increased Cx43 expression and phosphorylation levels | Unknown | |
| Rotenone-stimulated primary cultured astrocytes | Reduced Cx43 expression | Dysfunction of astrocyte GJs | Zhang et al., 2011 | |
| Mouse primary astrocytes treated with α-synuclein | Increased opening of Cx43 and Panx1 HCs | Increased release of ATP and oxidative stress | ||
| Acute PD animal model induced by MPTP treatment | Increased astroglial Cx43 and Cx30 expressions in striatum | Astrogliosis; astrocyte neuroprotection | ||
| Multiple sclerosis | EAE mouse model | Increased Cx43 expression in the lumbar spinal cord | Disruption of glial syncytium; inflammation | Takase et al., 2024 |
| Human post-mortem cervical spinal cord tissues from MS patients | Increased Cx43 expression in demyelinated lesions | Dysfunction of astrocyte network | ||
| Acute phase in EAE mouse model | Reduced Cx47 and Cx32 GJs around MS lesions | Demyelination and axonal loss | ||
| Human post-mortem brain samples from MS patients | Reduced Cx32 and Cx47, and increased Cx43 GJs levels around MS lesion | Alterations in myelin structure and function | ||
| Huntington’s disease | Human post-mortem HD brain samples | Increased Cx43 expression in caudate nucleus | Astrogliosis; alterations in the neuronal environment | Vis et al., 1998 |
Connexins (Cxs) modulation in neurodegenerative diseases.
Alterations in glial expression and function of GJs are implicated in AD neurotoxicity (). In vitro studies have shown that Aβ25–35 decreased GJs assembly but increased Cx43 intracellular pool and activity, indeed Aβ can alter Cx43 function in astrocytes through an increased internalization (; Figure 2). Similarly, in APPswe/PS1dE9 mice, a mouse model of familial AD, it has been reported an increased Cxs expression in astrocytes nearby amyloid plaques in murine brain (Yi et al., 2016).
FIGURE 2
Reactive astrocytes exposed to amyloid plaques showed an increased activity of HCs as compared to the overall astrocyte population (). Cx43-based HCs were the main factors involved in such a phenomenon, but also pannexin1 (Panx1)-formed channels played a role in reactive astrocytes surrounding amyloid plaques. In particular, Cx43 HCs activity was triggered by enhanced calcium levels into astrocytes, instead Panx1 activation was prompted by inflammation ().
Such an increased HC and pannexon activity in astrocytes can be associated with toxic effects mediated by small molecules, such as ATP and glutamate, acting on either neurons and/or microglia.
Before the development of Aβ deposits, young APP/PS1 mice at 2 months showed Cx43 and Cx30 distributions superimposable to that of control mice. Instead, in older animals (4 months), Cxs expression was altered nearby dense core of Aβ plaques, while 18 months-old mice presented a high immunoreactivity for both Cxs in most of the plaques ().
In contrast, a model of early AD exhibited an altered HCs activity in hippocampal astrocytes. Mice with intracerebroventricular (icv) injection of Aβ1–42 showed increased Cx43 phosphorylation levels, demonstrating that HCs activity is also regulated by AD progression ().
Indeed, in the early phases of AD, murine models showed apparent discrepancies between 2 months-old APP/PS1 mice versus Aβ1–42 icv-injected mice. Such observations can be attributed to the direct increase in Aβ1–42 levels in the icv model, as opposed to the gradual accumulation processes characterizing APP/PS1model, where potential compensatory phenomena might take place during the early stages of the disease (). As a matter of fact, older 5XFAD mice, a widely used AD mouse model, showed higher expression of Cx43 and Cx30, while Cx47 was reduced (). Interestingly, the reduction of Cx47 has been linked to impaired metabolic support provided by oligodendrocytes to neurons, leading to a subsequent decrease in myelination (). Furthermore, reduced colocalization of Cx47 GJs plaques with Cx43 plaques has been observed, suggesting a shift toward the formation of astrocyte-astrocyte GJ and/or HCs rather than astrocyte-oligodendrocyte connections. This shift could further compromise oligodendrocytes homeostasis, contributing to the dysregulation of myelination processes and exacerbating axonal dysfunction ().
In astroglial targeted-Cx43 knockout (KO) in APP/PS1 mice it has been observed a reduction in neuronal damage and a decreased oxidative stress in neurons localized in hippocampus and related to amyloid plaques, coupled with improved neuronal functionality (Yi et al., 2016). In addition, in APP/PS1 mice, deletion of astroglial Cx43 reduced astrogliosis and increased synapses functionality but did not influence amyloid plaques arrangement. Cx43 KO mice showed decreased levels of factors involved in astroglial activation, such as an increase in GFAP positive cells and GLAST mRNA expression levels. Moreover, astroglial Cx43 KO in APP/PS1 mice enhanced cognitive functions and, re-expression of Cx43 reverted these beneficial effects ().
Besides mice models, post-mortem AD brains also showed an increased Cx43 immunoreactivity in astrocytes in contact with amyloid plaques (). Furthermore, Aβ stimulates IL-1β and TNF production in astrocytes and increases Cx43-based HCs activity. Of note, inhibition of IL-1β and TNF signaling axis reverted this phenomenon (; ).
The gene encoding Cx43 (Gja1) is the top key driver of an astrocyte enriched subnetwork correlated to AD, regulating the expression of multiple AD risk factor genes (; Zhao et al., 2021). Indeed, Gja1 is upregulated throughout AD progression and is involved in the modulation of inflammatory and immune processes in astrocytes (). Moreover, an RNA-seq analysis on Gja1–/– astrocytes revealed that Gja1 controls the expression of numerous genes implicated in Aβ metabolism. Indeed, Gja1–/– astrocytes exhibited decreased Apolipoprotein E levels and altered Aβ phagocytosis, revealing a substantial neuroprotective function (; ).
Increased expression and hyperactivity of Cx43 in astrocytes induces the release of gliotransmitters, such as ATP and glutamate. ATP stimulates astroglial purinergic P2Y1 receptors, causing astrogliosis through autocrine signaling, while the release of glutamate induces excitotoxicity and neuronal death (). Moreover, many studies confirmed that pro-inflammatory cytokines, such as TNF, could reinforce glutamate release from astrocytes and microglia, leading to neurotoxicity (Takeuchi et al., 2006; ). In addition, another mechanism involved in the pathophysiology of AD could be the enhanced intracellular calcium levels, implicating the activation of endoplasmic reticulum stress, following Cx43 overexpression ().
An increase in Cx43 and Panx1 activities was also observed in microglia after Aβ25–35 treatment. ATP and glutamate released in extracellular medium from microglia treated with Aβ25–35 can induce HCs opening in neurons, leading to neurotoxic alterations and harmful effects ().
Pharmacological blockade of Cxs functionality has shown promising effects in modulating neuroinflammation and cognitive function. INI-0602 is a GJ inhibitor that, by binding to the outer loop of Cxs, leads to the irreversible internalization of Cxs, effectively blocking GJ/HC formations. Indeed, in APP/PS1 mice, it has been demonstrated that INI-0602 was able to inhibit microglia derived glutamate, with a consequent improvement in murine cognitive abilities (Figure 2; Takeuchi et al., 2011). Interestingly, the use of boldine, an alkaloid from the boldo tree that acts by blocking the activity of HCs has shown promising effects, although the mechanism of action is not fully understood (Yi et al., 2017). A long-term boldine administration in APP/PS1 mice reduced glial HCs activity and the consequent ATP and glutamate release, improving neuronal fitness.
Recently, evidence showed that adenosine A2A receptors (A2AR) are involved in the regulation of Aβ effects on Cx43 HCs activity in astrocytes, indeed A2AR pharmacological and genetic blockade prevented HCs alteration related to Aβ. Moreover, throughout primary cultures of astrocytes, it has been demonstrated that protein kinase C (PKC) pathway was involved in these A2AR-induced effects. Thus, A2AR can modulate HCs activity of hippocampal astrocytes in an early AD phase (). Moreover, A2AR antagonists blocked ATP release mediated by Cx43 HCs in Aβ1–42-exposed astrocytes (). Taken together, this evidence suggests that targeting glial Cxs could be a potential effective strategy to reduce neurotoxicity and ameliorate AD patients’ cognitive functions.
3 Connexins’ role in Parkinson’s disease (PD)
Parkinson’s disease is a progressive neurodegenerative disease, with a global prevalence of more than six million individuals (Tolosa et al., 2021). PD is characterized by a degeneration of dopaminergic neurons in the substantia nigra pars compacta and the abnormal accumulation of α-synuclein, forming Lewy bodies and Lewy neurites. The disruption of dopaminergic transmission leads to the onset of extrapyramidal motor symptoms, including tremor at rest, bradykinesia and akinesia (Figure 3; ). Moreover, PD is also associated with a variety of non-motor symptoms, such as memory loss, depression, pain and sleep disorders (Tolosa et al., 2021). Although the exact molecular mechanism of PD remains poorly understood, neuroinflammation, oxidative stress and alteration of CNS architecture are considered as key factors in its pathogenesis.
FIGURE 3
In PD, disruptions in the intricate neuroglia crosstalk have emerged as pivotal contributors to the disease’s pathogenesis and progression. Accumulating evidence suggest an active role for non-neuronal cells, where the possible alteration of astrocytes and microglial functions and the disruption of neuron-to-glia crosstalk may contribute to PD pathogenesis and progression (; ). Aberrant functioning of Cxs-based channels and related signaling pathways in glial networks has been implicated in dopaminergic neuronal degeneration, neuroinflammatory responses, and impaired neuroprotection (; ; Table 1). Thus, the molecular signaling mediated by glial GJs appears to be critical in this context. The maintenance of the physiological concentration of several molecules at cytoplasmic and extracellular levels, such as glutamate and ions like potassium, as well as the regulation of pH in astrocytes, largely depends by Cx43 functionality (Vicario et al., 2020). These mechanisms are of critical importance in the regulation of dopaminergic neurons firing and homeostasis (Wang et al., 2020). This suggests that changes in the levels of Cx43 HCs or alterations in the Cx43 gating mechanism are key processes in the development of the disease.
Over the last decades, numerous studies have focused their attention on alteration of Cx43 dynamics and its role in the progression of PD. Kawasaki and collaborators showed an increase of Cx43 expression in rotenone-induced PD model, both in vitro and in vivo (). Rotenone is a mitochondrial complex I inhibitor, widely used to induce oxidative stress and subsequent cell death, mimicking parkinsonism in rodents when injected systemically (). Increased protein levels of Cx43 were detected in the substantia nigra pars compacta and pars reticulata, and striatum, suggesting that aberrant coupling between glial cells could be crucial in the neurodegenerative process that drive PD development (). In addition, rotenone treatment promoted the phosphorylation of Cx43, which is critical for intercellular communication properties occurring at several stages of the Cx “life cycle,” including trafficking and degradation, assembly/disassembly and permeability (; Zhang et al., 2023). However, other evidence reports a marked reduction in Cx43 expression and permeability on rotenone-stimulated primary cultured astrocytes, which is associated with impaired GJs communication and increased apoptosis (Zhang et al., 2011). Such a discrepancy could be related to the different concentration of rotenone used in these studies. While the former used a concentration of 8 nM rotenone, the latter used a concentration in the range of 20–30 nM. This could suggest that at lower doses, rotenone may enhance the phosphorylation of Cx43, potentially increasing gap junctional intercellular communication, while higher concentrations may induce a different effect by disrupting Cx43 functionality.
Besides its direct effects on astrocytic functions and survival, Cx43-mediated signaling has been shown to promote neuroinflammatory response following CNS insults (). Crosstalk between activated microglia and astrocytes is thought to be a key regulator of neuroinflammation in PD (). Via immunofluorescence staining, an increase in Cx43 intensity was reported in the substantia nigra of animals that received brain injections of lipopolysaccharide (LPS) (Voronkov et al., 2023). LPS has the capability to induce inflammatory-driven dopaminergic neurons degeneration and has been used as the common toxicant for PD modeling (). Along with this finding, the study showed severe astrogliosis, that was coupled by long-term (up to 3 months) alteration of the glial network with a redistribution of Cx43 in the astrocytes end-feet (Voronkov et al., 2023). This was accompanied by an increase of Iba1+ cells (i.e., microglia) with hypertrophic soma around the injection area (Voronkov et al., 2023). It can be speculated that the observed changes in Cx43 localization contributes to the prolonged maintenance of the neuroinflammatory state. One possible hypothesis is that the Cx43 redistribution enhances coupling with microglia, promoting the exchange of pro-inflammatory cytokines within the glial network, thereby sustaining the reactive gliosis. Indeed, an increase of mRNA expression levels of typical pro-inflammatory markers (TNF, Il-1β and iNOS) has been observed, with an increase in the levels of the anti-inflammatory cytokines (Il-10, TGFβ and Arg1) in a model of LPS-induced PD, suggesting a chronic activation of microglia (Zhao et al., 2022). Interestingly, the injection of Gap27, a mimetic peptide able to selectively block Cx43-based GJs and HCs, mostly attenuated microgliosis, leading to a decrease of the inflammatory response triggered by LPS. Moreover, Cx43 inhibition was able to attenuate the damage to dopaminergic neurons as well as the reduction of dopamine and its metabolites in vivo caused by LPS (Zhao et al., 2022; Figure 3).
Furthermore, it has been demonstrated that α-synuclein might affect astrocytic HC function, triggering the opening of Cx43- and Panx1-based channels, leading to abnormal calcium signaling, excessive release of gliotransmitters such as ATP and glutamate and increased oxidative stress. Consistent with this evidence, knockdown of Cx43 with siRNA or blocking with Gap19, completely abolished ATP release induced by α-synuclein (). These changes contribute to a vicious cycle of astrocytic dysfunction and neuronal degeneration, which fuel neuroinflammation (Figure 3).
In addition, studies have shown that α-synuclein oligomers can bind to Cx32, facilitating their uptake by neurons and oligodendrocytes (). Evidence from post-mortem brain tissue of PD patients further supports a direct interaction between α-synuclein and Cx32. This interaction promotes the intercellular transfer of α-synuclein, thereby contributing to the propagation of PD ().
Another important piece of evidence highlighting the contribution of GJs coupling in PD comes from a study conducted by Schwab and colleagues, showing the critical role of the neuronal Cx36 in enhancing neuronal synchrony in the basal ganglia. This study, conducted on post-mortem human tissues, revealed a significant overexpression of Cx36 in putamen and globus pallidus (GP) of PD patients, highlighting its role in pathological network activity associated with the motor deficits observed in PD (Schwab et al., 2014).
Modulating Cxs expression and gating properties could be a key strategy in understanding the pathogenesis of PD, by interfering with the inflammatory cascade (). To this end, the effects of conditional inactivation of the glucocorticoid receptor (GR) on astrocytes were investigated in GRCx30CreERT mice, using a model of PD induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) injection (). This study found that the deletion of GR on astrocytes led to a significant loss of dopaminergic neurons after MPTP treatment in the substantia nigra of mutant mice. Furthermore, in the absence of GR, it has been observed an increased activity of Cx43 HCs that, not only enhanced calcium levels in astrocytes, but also trigger microglial reactivity following MPTP induction, causing a robust inflammatory response. This was coupled with increased levels of specific inflammatory markers, including intercellular adhesion molecule 1 (ICAM-1), TNF and Il-1β. Analyses of post-mortem brain tissue from PD patients revealed a significant decrease in the number of astrocytes expressing GR in the substantia nigra, suggesting the existence of a link between the neuroinflammatory cascade and the activity of Cx43 ().
Astrocytes, through GJs, sculpt an environment that ensures proper neuronal functions. In addition to Cx43, astrocytes also abundantly express Cx30, which is mainly located in astrocytic processes around neurons in the gray matter, constituting an important component of the astrocytic metabolic network (). It has been observed that both Cx43 and Cx30 are upregulated in astrocytes in the striatum of an acute PD animal model induced by MPTP treatment (). However, Cx30 KO mice showed an increased susceptibility to MPTP, exacerbating the loss of dopaminergic neurons. This may suggest that other aspects of Cx functions are required for neuronal survival. Cx30 KO mice, treated with MPTP, showed a reduced expression of genes associated with the neuroprotective response of astrocytes in the striatum as compared to wild-type (WT) mice (). This may suggest that Cx30 is involved in the regulation of the neuroprotective properties of astrocytes within the tissue; thus, enhancing Cx30 function could be a potential therapeutic approach for PD patients.
4 Connexins’ role in multiple sclerosis (MS)
Multiple sclerosis (MS) is a chronic, autoimmune, inflammatory disorder of the CNS characterized by progressive demyelination, neuroinflammation, and neurodegeneration (). A number of factors associated with the onset of MS have been reported to be related to the incidence of the disease. For example, an increase in MS prevalence is observed with the distance from the equator and in certain geographical areas (). Relapsing remitting MS (RRMS) is the most common clinical form of MS affecting 85–90% of people aged 20–40 years old and it is characterized by symptomatic attacks (relapses) followed by periods of recovery (remissions). RRMS frequently develops into secondary progressive MS (SPMS), characterized by the progressive and irreversible accumulation of neurological impairment (). The involvement of Cxs-based intercellular communication in MS has garnered significant attention due to their role in glial cell, inflammation, and myelination. Redistribution of Cxs plaques and modulation of intercellular exchanges has been linked to either sustained inflammation and defects in myelination, highlighting the need of further research into Cxs influence on immune response and remyelination (Table 1).
Multiple sclerosis is the result of a primary defect in the immune system, which mediates direct damage to the oligodendrocytes, the CNS myelinating cells, resulting in secondary damage to neurons. This aberrant environment leads to the activation of the CNS-resident cells such as microglia and astrocytes. To summarize, from a neuropathological point of view, MS hallmarks include lesion in the CNS white and gray matter areas with variable degree of demyelination, immune cells infiltration, reactive gliosis and neurodegeneration (Figure 4; ; ).
FIGURE 4
In this scenario, Cxs-mediated neuroimmune crosstalk undoubtedly plays an active role in driving the neurodegeneration process and it is increasingly recognized as a critical factor in disease progression (). However, the exact mechanism by which Cxs activity influences these processes remains controversial. Evidence of altered Cxs protein has been observed in MS patient biopsies as well as in murine preclinical model. Notably, increased expression of Cx43 has been reported in the experimental autoimmune encephalomyelitis (EAE) model of MS. EAE is a well-established and widely used preclinical model, where animals are immunized with myelin-derived peptides, resulting in a progressive disease resembling the relapsing-remitting or progressive forms of MS (). Takase et al. (2024) reported changes in Cx43, observing an overexpression in the lumbar spinal cord of EAE mice throughout the time course of the disease. Evidence from human post-mortem cervical spinal cord tissues confirmed an upregulation of Cx43 in peri-plaque demyelinated lesions, coupled with an accumulation of macrophages and microglia ().
In accordance with its role in sustaining MS progression, recent reports indicate that astroglia-specific Cx43 ablation attenuates EAE clinical signs, with reduced F4/80+ macrophages and CD45+ cells infiltration (Une et al., 2021).
Additionally, administration of INI-0602, a pan-Cx blocker that inhibits GJ/HC opening (Umebayashi et al., 2014) demonstrated a therapeutic effect on EAE mice, by preventing excessive demyelination in the spinal cord, as evidenced by an increase of the myelin basic protein-immunopositive (MBP+) area in INI-0602-treated mice. The effects of Cx43 deletion/blockade are also evident in the regulation of the neuroinflammatory environment during the course of EAE. Astroglia-Cx43 conditional (c-)KO mice showed reduced activation of astrocytes toward a pro-inflammatory (A1) phenotype during acute EAE, and enhanced anti-inflammatory responses in spinal microglia before the onset of the EAE. However, during the peak phase of the disease, both wild-type (WT) and Cx43cKO mice displayed similar pro-inflammatory profiles (Une et al., 2021).
In vivo assessment further showed that the blockade of Cx43 HCs by INI-0602, suppressed microglial activation, as evidenced by a more ramified morphology, and decreased astrocytes activation in the spinal cord. Additionally, INI-0602 administration decreased levels of pro-inflammatory cytokines (Il-6, INF-γ) and increased anti-inflammatory Il-10 in the cerebrospinal fluid (CSF) of EAE mice. These findings suggest that activated microglia drive astrocytes toward a pro-inflammatory phenotype, exacerbating neuroinflammation (Takase et al., 2024).
One proposed mechanism by which Cx43 exacerbates and sustains the neuroinflammation in MS is linked to the aberrant release of ATP, which serves as activation signal of the NOD-like receptor protein 3 (NLRP3) inflammasome (; ). The blockade of Cx43 HCs, mediated by tonebersat in EAE model, effectively reduces astrocytes and microglia activation, thus slowing disease progression. This, in turn, limits the formation of the NLRP3 complex that correlates with lower levels of Caspase-1 cleavage, which are crucial steps in inflammasome-driven neuroinflammation (Figure 4; ).
In addition to direct ATP release, Cx43 HCs may facilitate the propagation of the neuroinflammatory response acting downstream to the P2X7 receptor (P2X7R)-Cx43-β3 integrin axis. This receptor is activated by ATP released from astrocytes through different mechanisms, including Cx43 HCs. Analyzing spinal cord of EAE rats, found a close interaction between these two proteins, particularly near infiltrated CD4+ T cells. Microglial cells also express the P2X7R, thus it is plausible that a similar mechanism of interaction occurs in microglia, thereby contributing to neuroinflammation. Furthermore, this study highlighted the role of mitochondria in regulating astrocytic coupling. Blocking specific mitochondrial transporters significantly augmented the ATP release from astrocytes, enhancing the immune cell-induced astrocytic calcium response (). Given that Cx43 is not only localized at the plasma membrane, but also within mitochondria, another layer of complexity must be considered (). This means that the mitochondrial localization of Cx43 may have a direct influence on astrocytic mitochondrial functions, including the regulation of calcium dynamics in response to cellular stress.
Thus far, if from the one hand there is the increasing assumption that high levels of Cx43 are detected in the CNS during EAE progression, on the other hand, different lines of evidence reveal a more nuanced and controversial role of Cx43. It has been observed that mutant mice lacking both Cx43 and Cx30 exhibited an altered neurochemical microenvironment and hypomyelinating phenotype (). However, these changes did not translate into more severe EAE clinical symptoms or increased inflammatory cells infiltration ().
The idea that GJs, particularly Cx43 in astrocytes, may have distinct roles at different stages of EAE is increasingly convincing. Indeed, the study of Lutz and colleagues takes into account a narrow window of time, between the onset and the peak of the disease (i.e., 25 days post immunization, dpi). It is possible that early microenvironmental changes caused by the deletion of Cx43 could alter myelination without directly influencing the severity of clinical symptoms, which may depend more on immune cells infiltration and neurodegeneration, taking place in the chronic phase (). This is further supported by the observation that pharmacological blockade of Cx43 resulted in a significant improvement in EAE clinical score, particularly evident around 22 dpi and became more consistent at 50 dpi, during the relapsing phase (Une et al., 2021). Another important consideration is the possibility that, although Cx43 is a key player in forming GJs, other Cxs may partially compensate for its absence, ensuring some level of communication between glial cells.
Oligodendrocytes express both Cx32 and Cx47, which form GJs with their main astrocytic partner Cx43 (). The homomeric combination of Cx43-Cx47 is critical for maintaining potassium buffering, nutrient homeostasis and myelin formation properties by oligodendrocytes ().
A redistribution of Cx47-formed GJs plaque to the intracellular compartment has been observed following the loss of Cx43, at 14 dpi subsequent EAE induction (). In line with this evidence, a similar distribution pattern was found on human post-mortem brain samples from MS patients, that revealed reduced Cx32 and Cx47, and increased Cx43 GJ plaques around demyelinating lesions and on the normal appearing white matter (NAWM) (Figure 4). Moreover, there was an increase of Cx47 in oligodendrocytes precursors cells (OPCs), which are recruited to areas of demyelination. However, these OPCs exhibited limited coupling with astrocytes as indicated by the reduced formation of heterotypic Cx43–Cx47 astrocyte-to-oligodendrocyte GJs (; ). Cx47 has been studied in the context of myelinogenesis and demyelination and has been associated with developing oligodendrocytes and in specific CNS areas in the adulthood (). It is interesting that Cx47 expression in oligodendrocytes is an early effect anticipating Cx32 and Cx29 expression, all typical Cxs expressed by mature oligodendrocytes (Vicario et al., 2017). It has been demonstrated that Cx47 is highly expressed by astrocytes during demyelination and that a shift in Cx47 expression from astrocytes to oligodendrocytes is taking place during spontaneous remyelination. Thus, sustaining Cx47-based intercellular crosstalk between astrocytes and oligodendrocytes might be of critical importance during remyelination ().
To summarize, the common idea that comes up from the above-mentioned mechanism is that an imbalance in Cxs expression during MS may result in the formation of unpaired HCs, disrupting glial syncytium and promoting inflammation and demyelination.
5 Connexins’ role in Huntington’s disease (HD)
Huntington’s disease is a neurodegenerative disorder, characterized by abnormal long CAG repeat expansions in the Huntingtin gene, that causes the expansion of a polyglutamine (polyQ) stretch in the huntingtin protein (HTT) (Shin et al., 2005; ). The mutant huntingtin (mHTT) shows altered functions and protein-protein interactions, leading to neurological symptoms and brain pathology (). Indeed, HTT is expressed in different tissues and, intracellularly, colocalizes with many organelles: nucleus, Golgi apparatus and endoplasmic reticulum (Strong et al., 1993; ). HTT plays important roles in different pathways due to its scaffolding role in protein complex formations, indeed more than 200 interaction partners are known and studied ().
Intercellular communication is vital for maintaining neuronal homeostasis and supporting physiological neuronal function. The HD early phase is characterized by mutant HTT self-aggregation, involved in the appearance of pathological symptoms (; ). Moreover, mHTT leads to a progressive neuronal dysfunction and neuronal death in the striatum and cortex, resulting in motor dysfunction and cognitive impairment (). Neuroinflammation is correlated with HD pathophysiology, indeed reactive astrocytes and microglia are detectable in brain regions involved in HD (Sapp et al., 2001; ). Reactive astrocytes and microglia, expressing mutant HTT, play central roles in neuroinflammation, synaptic dysfunction, and neuronal death. Altered Cxs-mediated signaling, impaired glial homeostasis, and changes in myelinogenic potential further exacerbate the pathophysiological cascade in HD (; ; Table 1).
At the beginning, inflammatory mechanisms are effective to compensate the alterations related to protein aggregations and deposits; however, inflammation can cause neuronal impairment, supporting HD degeneration. Several post-mortem studies revealed a correlation between HD severity and the levels of astroglial and microglial activation (). Furthermore, mHTT can induce the activation of microglia, that undergo morphological changes, hyperproliferation and production of inflammatory cytokines. These cytokines could alter microglia phagocytic capacity, leading to mHTT accumulation and its consequent detrimental effect (; ). Moreover, microglia could also play a significant role in synaptic impairment during HD progression (Savage et al., 2020).
Recently, it has been demonstrated that microglia from WT mice in co-culture with mHTT striatal neurons had an amoeboid shape, an augmented proliferation and cytokine IL-6 levels, and, despite their activation, they increased mHTT neuronal viability (Savage et al., 2020). Conversely, mHTT microglia seems to induces neuronal suffering and cell death, in vivo (). Reactive microglia and altered cytokine levels were also detected in the brains of HD patients ().
Similarly, from the analysis of post-mortem HD brains, it has been observed that also astrocytes express mHTT. In co-cultures, mHTT astrocytes lead to striatal neurons death, while in a double transgenic model, mHTT astrocytic expression significantly altered neuronal functions (; ; ). Using a snRNAseq, it has been demonstrated that astrocytes in post-mortem human HD cingulate cortex showed multiple expression profiles, that could be grouped in three states with different levels of GFAP, metallothionein and quiescent protoplasmic genes expression levels. Moreover, as compared to control, HD astrocytes exhibited a downregulation of genes involved in protoplasmic astrocyte function and lipid synthesis ().
Pathological changes were also observed in microglia and astrocytes localized in the midcingulate cortex, a brain area involved in motor and cognitive functions. In particular, these alterations were observed in HD cases with prevalent mood symptoms ().
Recently, some studies revealed that HD patients presented demyelination and white matter loss, that could originate before symptoms (Tabrizi et al., 2012). Thus, an alteration in myelinogenic oligodendrocytes may be related to HD pathophysiology (Teo et al., 2016). demonstrated that glial progenitor cells derived from human embryonic stem cells (hESCs) expressing mHTT show an altered maturation, leading to dysfunctional astrocytes and demyelination in vivo. In addition, mHTT human glial chimeras show an impaired astrocyte differentiation, confirming that defects in glial differentiation could be related to HD pathogenesis and symptoms ().
engrafting mice with mHTT-expressing human glial progenitor cells, revealed that mHTT glia can induce HD phenotype in healthy mice, also showing a decline in motor performance. However, normal glia can improve behavior and survival of transgenic HD mice, reestablishing interstitial potassium homeostasis and reducing HD progression (). These results confirm the important role of glia in HD progression, revealing cell-based treatment as a potential strategy against HD.
In HD human brain, caudate nucleus (CN) and GP of the basal ganglia are areas strongly affected. Examining Cxs expression and distribution in these regions, it was demonstrated that Cx50 was not expressed, while Cx40 was identified in endothelial cells of blood vessels. Instead, Cx26 and Cx32 showed a superimposable distribution both in normal and HD brain, with a higher expression in GP as compared to CN. Cx43 was the only Cx that revealed a significantly different expression between healthy and HD brains. Indeed, Cx43 was increased in CN of HD human brains, and this was correlated with a higher expression of GFAP, that demonstrated a robust astrogliosis (Vis et al., 1998; ). However, Cxs role in HD has not been fully elucidated and new studies about this topic are needed.
6 Conclusion
In recent years, several efforts have been made to broaden the knowledge of the processes that drive the progression of neurodegenerative diseases. This review highlighted the role of Cxs in glial communication, neuroinflammation and the disruption of homeostasis in neuroinflammatory-driven diseases, which are summarized in Table 1.
All explored neurodegenerative disorders, AD, PD, MS and HD, appear to be coupled with a general overexpression of Cx43 and increased intercellular communication between astrocyte and microglia.
This set of evidence supports the idea that Cxs upregulation, particularly Cx43, is a critical and a common factor in neurodegenerative diseases, where its act as “pathological” pore, supporting the notion that Cxs are not merely passive markers of disease, but active contributors to its progression. This makes Cxs ideal targets for therapeutic strategies aimed at reducing inflammation and restoring cellular homeostasis.
However, the complexity in their regulation must be considered. For instance, phosphorylation state of Cx43, its interaction with other cellular proteins, and its localization within cells and mitochondria can all impact its role in disease. Last but not least, the non-channel functions of Cxs are of particular importance, such as the ability to bind to DNA and RNA, exerting a modulatory function of genes with consequent functional impact (). This means that the ideal therapeutic strategy should target specific aspect of Cxs function, without disrupting their beneficial roles in maintaining homeostasis.
Statements
Author contributions
SD: Conceptualization, Visualization, Writing – original draft, Writing – review and editing. SD’A: Visualization, Writing – original draft, Writing – review and editing. NV: Supervision, Writing – review and editing. RP: Conceptualization, Supervision, Writing – review and editing.
Funding
The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. This study was partially funded by the grant Piano di Incentivi per la Ricerca di Ateneo 2020–2022 - PIA.CE.RI. 2020–2022-Linea di intervento 3-Starting Grant, CHRONOS and by the grant Piano di Incentivi per la Ricerca di Ateneo 2024–2026 - PIA.CE.RI. 2024–2026-Linea di intervento 1, ACHIEVE from the University of Catania to NV and by the grant National Biodiversity Future Center (NBFC) by European Union NextGenerationEU, PNRR, project no. CN00000033; CUP: E63C22004340006; META-CONNECT—Spoke 6 to NV and RP.
Acknowledgments
We thank Servier Medical Art (smart.servier.com, accessed in October 2024).
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Generative AI statement
The author(s) verify and take full responsibility for the use of generative AI in the preparation of this manuscript. Generative AI was used. The version 4.0 of ChatGPT developed by OpenAI (https://www.openai.com), was used for language refinement of this manuscript.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
References
1
Al-DalahmahO.SosunovA. A.ShaikA.OforiK.LiuY.VonsattelJ. P.et al (2020). Single-nucleus RNA-seq identifies Huntington disease astrocyte states.Acta Neuropathol. Commun.8:19. 10.1186/s40478-020-0880-6
2
AlteenM. G.DemeJ. C.AlvarezC. P.LoppnauP.HutchinsonA.SeitovaA.et al (2023). Delineation of functional subdomains of Huntingtin protein and their interaction with HAP40.Structure311121–1131.e6. 10.1016/j.str.2023.06.002
3
AngeliS.KousiappaI.StavrouM.SargiannidouI.GeorgiouE.PapacostasS. S.et al (2020). Altered expression of glial gap junction proteins Cx43, Cx30, and Cx47 in the 5XFAD model of Alzheimer’s disease.Front. Neurosci.14:582934.
4
AsghariK.NiknamZ.Mohammadpour-AslS.ChodariL. (2024). Cellular junction dynamics and Alzheimer’s disease: a comprehensive review.Mol. Biol. Rep.51:273. 10.1007/s11033-024-09242-w
5
BasuR.SarmaJ. D. (2018). Connexin 43/47 channels are important for astrocyte/oligodendrocyte cross-talk in myelination and demyelination.J. Biosci.431055–1068. 10.1007/s12038-018-9811-0
6
BennettM. V.GarreJ. M.OrellanaJ. A.BukauskasF. F.NedergaardM.SaezJ. C. (2012). Connexin and pannexin hemichannels in inflammatory responses of glia and neurons.Brain Res14873–15. 10.1016/j.brainres.2012.08.042
7
BenraissA.WangS.HerrlingerS.LiX.Chandler-MilitelloD.MauceriJ.et al (2016). Human glia can both induce and rescue aspects of disease phenotype in Huntington disease.Nat. Commun.7:11758. 10.1038/ncomms11758
8
BetarbetR.ShererT. B.MacKenzieG.Garcia-OsunaM.PanovA. V.GreenamyreJ. T. (2000). Chronic systemic pesticide exposure reproduces features of Parkinson’s disease.Nat. Neurosci.31301–1306. 10.1038/81834
9
BloemB. R.OkunM. S.KleinC. (2021). Parkinson’s disease.Lancet3972284–2303. 10.1016/S0140-6736(21)00218-X
10
BoenglerK.LeybaertL.Ruiz-MeanaM.SchulzR. (2022). Connexin 43 in mitochondria: what do we really know about its function?Front. Physiol.13:928934. 10.3389/fphys.2022.928934
11
BoschM.KielianT. (2014). Hemichannels in neurodegenerative diseases: is there a link to pathology?Front. Cell Neurosci.8:242. 10.3389/fncel.2014.00242
12
BoulayA. C.GilbertA.Oliveira MoreiraV.BlugeonC.PerrinS.PouchJ.et al (2018). Connexin 43 controls the astrocyte immunoregulatory phenotype.Brain Sci.8:50. 10.3390/brainsci8040050
13
BradfordJ.ShinJ. Y.RobertsM.WangC. E.ShengG.LiS.et al (2010). Mutant huntingtin in glial cells exacerbates neurological symptoms of Huntington disease mice.J. Biol. Chem.28510653–10661. 10.1074/jbc.M109.083287
14
CharabatiM.WheelerM. A.WeinerH. L.QuintanaF. J. (2023). Multiple sclerosis: neuroimmune crosstalk and therapeutic targeting.Cell1861309–1327. 10.1016/j.cell.2023.03.008
15
CompstonA.ColesA. (2008). Multiple sclerosis.Lancet3721502–1517. 10.1016/S0140-6736(08)61620-7
16
ConstantinescuC. S.FarooqiN.O’BrienK.GranB. (2011). Experimental autoimmune encephalomyelitis (EAE) as a model for multiple sclerosis (MS).Br. J. Pharmacol.1641079–1106. 10.1111/j.1476-5381.2011.01302.x
17
CragnoliniA. B.LampitellaG.VirtuosoA.ViscovoI.PanetsosF.PapaM.et al (2020). Regional brain susceptibility to neurodegeneration: what is the role of glial cells?Neural Regen. Res.15838–842. 10.4103/1673-5374.268897
18
CrottiA.BennerC.KermanB. E.GosselinD.Lagier-TourenneC.ZuccatoC.et al (2014). Mutant Huntingtin promotes autonomous microglia activation via myeloid lineage-determining factors.Nat. Neurosci.17513–521. 10.1038/nn.3668
19
DejanovicB.ShengM.HansonJ. E. (2024). Targeting synapse function and loss for treatment of neurodegenerative diseases.Nat. Rev. Drug Discov.2323–42. 10.1038/s41573-023-00823-1
20
DenaroS.D’AprileS.TorrisiF.ZappalaA.MarrazzoA.Al-KhrasaniM.et al (2024). Sigma-1 receptor targeting inhibits connexin 43 based intercellular communication in chronic neuropathic pain.Inflamm. Res.731711–1726. 10.1007/s00011-024-01926-0
21
DiazE. F.LabraV. C.AlvearT. F.MelladoL. A.InostrozaC. A.OyarzunJ. E.et al (2019). Connexin 43 hemichannels and pannexin-1 channels contribute to the alpha-synuclein-induced dysfunction and death of astrocytes.Glia671598–1619. 10.1002/glia.23631
22
Diaz-CastroB.GangwaniM. R.YuX.CoppolaG.KhakhB. S. (2019). Astrocyte molecular signatures in Huntington’s disease.Sci. Transl. Med.11:eaaw8546. 10.1126/scitranslmed.aaw8546
23
DuggerB. N.DicksonD. W. (2017). Pathology of neurodegenerative diseases.Cold Spring Harb. Perspect. Biol.9:a028035. 10.1101/cshperspect.a028035
24
ErkkinenM. G.KimM. O.GeschwindM. D. (2018). Clinical neurology and epidemiology of the major neurodegenerative diseases.Cold Spring Harb. Perspect. Biol.10:a033118. 10.1101/cshperspect.a033118
25
FujitaA.YamaguchiH.YamasakiR.CuiY.MatsuokaY.YamadaK. I.et al (2018). Connexin 30 deficiency attenuates A2 astrocyte responses and induces severe neurodegeneration in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride Parkinson’s disease animal model.J. Neuroinflamm.15:227. 10.1186/s12974-018-1251-0
26
Gajardo-GomezR.LabraV. C.MaturanaC. J.ShojiK. F.SantibanezC. A.SaezJ. C.et al (2017). Cannabinoids prevent the amyloid beta-induced activation of astroglial hemichannels: a neuroprotective mechanism.Glia65122–137. 10.1002/glia.23080
27
GaoH. M.HongJ. S.ZhangW.LiuB. (2003). Synergistic dopaminergic neurotoxicity of the pesticide rotenone and inflammogen lipopolysaccharide: relevance to the etiology of Parkinson’s disease.J. Neurosci.231228–1236. 10.1523/JNEUROSCI.23-04-01228.2003
28
GiallongoS.LonghitanoL.DenaroS.D’AprileS.TorrisiF.La SpinaE.et al (2022). The role of epigenetics in neuroinflammatory-driven diseases.Int. J. Mol. Sci.23:15218. 10.3390/ijms232315218
29
GiaumeC.NausC. C.SaezJ. C.LeybaertL. (2021). Glial connexins and pannexins in the healthy and diseased brain.Physiol. Rev.10193–145. 10.1152/physrev.00043.2018
30
GomezG. I.AlvearT. F.RoaD. A.Farias-PastenA.VergaraS. A.MelladoL. A.et al (2024). Cx43 hemichannels and panx1 channels contribute to ethanol-induced astrocyte dysfunction and damage.Biol. Res.57:15. 10.1186/s40659-024-00493-2
31
GoshiN.MorganR. K.LeinP. J.SekerE. (2020). A primary neural cell culture model to study neuron, astrocyte, and microglia interactions in neuroinflammation.J. Neuroinflamm.17:155. 10.1186/s12974-020-01819-z
32
GreenC. R.NicholsonL. F. (2008). Interrupting the inflammatory cycle in chronic diseases–do gap junctions provide the answer?Cell Biol. Int.321578–1583. 10.1016/j.cellbi.2008.09.006
33
GrotemeyerA.McFlederR. L.WuJ.WischhusenJ.IpC. W. (2022). Neuroinflammation in Parkinson’s disease - Putative pathomechanisms and targets for disease-modification.Front. Immunol.13:878771. 10.3389/fimmu.2022.878771
34
GuoA.ZhangH.LiH.ChiuA.Garcia-RodriguezC.LagosC. F.et al (2022). Inhibition of connexin hemichannels alleviates neuroinflammation and hyperexcitability in temporal lobe epilepsy.Proc. Natl. Acad. Sci. U.S.A.119:e2213162119. 10.1073/pnas.2213162119
35
GuoM.WangJ.ZhaoY.FengY.HanS.DongQ.et al (2020). Microglial exosomes facilitate alpha-synuclein transmission in Parkinson’s disease.Brain1431476–1497. 10.1093/brain/awaa090
36
GutekunstC. A.LiS. H.YiH.MulroyJ. S.KuemmerleS.JonesR.et al (1999). Nuclear and neuropil aggregates in Huntington’s disease: relationship to neuropathology.J. Neurosci.192522–2534. 10.1523/JNEUROSCI.19-07-02522.1999
37
HauserS. L.CreeB. A. C. (2020). Treatment of multiple sclerosis: a review.Am. J. Med.1331380–1390.e2. 10.1016/j.amjmed.2020.05.049
38
HeJ. T.LiX. Y.YangL.ZhaoX. (2020). Astroglial connexins and cognition: memory formation or deterioration?Biosci. Rep.40:BSR20193510. 10.1042/BSR20193510
39
HuangX.SuY.WangN.LiH.LiZ.YinG.et al (2021). Astroglial connexins in neurodegenerative diseases.Front. Mol. Neurosci.14:657514. 10.3389/fnmol.2021.657514
40
ImarisioS.CarmichaelJ.KorolchukV.ChenC. W.SaikiS.RoseC.et al (2008). Huntington’s disease: from pathology and genetics to potential therapies.Biochem. J.412191–209. 10.1042/BJ20071619
41
JiangH.ZhangY.WangZ. Z.ChenN. H. (2023). Connexin 43: an interface connecting neuroinflammation to depression.Molecules28:1820. 10.3390/molecules28041820
42
KajiwaraY.WangE.WangM.SinW. C.BrennandK. J.SchadtE.et al (2018). GJA1 (connexin43) is a key regulator of Alzheimer’s disease pathogenesis.Acta Neuropathol. Commun.6:144. 10.1186/s40478-018-0642-x
43
KamT. I.HinkleJ. T.DawsonT. M.DawsonV. L. (2020). Microglia and astrocyte dysfunction in Parkinson’s disease.Neurobiol. Dis.144:105028. 10.1016/j.nbd.2020.105028
44
KamasawaN.SikA.MoritaM.YasumuraT.DavidsonK. G.NagyJ. I.et al (2005). Connexin-47 and connexin-32 in gap junctions of oligodendrocyte somata, myelin sheaths, paranodal loops and Schmidt-Lanterman incisures: implications for ionic homeostasis and potassium siphoning.Neuroscience13665–86. 10.1016/j.neuroscience.2005.08.027
45
KawasakiA.HayashiT.NakachiK.TroskoJ. E.SugiharaK.KotakeY.et al (2009). Modulation of connexin 43 in rotenone-induced model of Parkinson’s disease.Neuroscience16061–68. 10.1016/j.neuroscience.2009.01.080
46
KhakhB. S.GoldmanS. A. (2023). Astrocytic contributions to Huntington’s disease pathophysiology.Ann. N. Y. Acad. Sci.152242–59. 10.1111/nyas.14977
47
KotiniM.BarrigaE. H.LeslieJ.GentzelM.RauschenbergerV.SchambonyA.et al (2018). Gap junction protein Connexin-43 is a direct transcriptional regulator of N-cadherin in vivo.Nat. Commun.9:3846. 10.1038/s41467-018-06368-x
48
KutovaO. M.PospelovA. D.BalalaevaI. V. (2023). The multifaceted role of connexins in tumor microenvironment initiation and maintenance.Biology (Basel)12:204. 10.3390/biology12020204
49
KwakowskyA.ChawdharyB.de SouzaA.MeyerE.KayeA. H.GreenC. R.et al (2023). Tonabersat significantly reduces disease progression in an experimental mouse model of multiple sclerosis.Int. J. Mol. Sci.24:17454. 10.3390/ijms242417454
50
KwonH. S.KohS. H. (2020). Neuroinflammation in neurodegenerative disorders: the roles of microglia and astrocytes.Transl. Neurodegener.9:42. 10.1186/s40035-020-00221-2
51
LiddelowS. A.GuttenplanK. A.ClarkeL. E.BennettF. C.BohlenC. J.SchirmerL.et al (2017). Neurotoxic reactive astrocytes are induced by activated microglia.Nature541481–487. 10.1038/nature21029
52
LieuryA.ChanalM.AndrodiasG.ReynoldsR.CavagnaS.GiraudonP.et al (2014). Tissue remodeling in periplaque regions of multiple sclerosis spinal cord lesions.Glia621645–1658. 10.1002/glia.22705
53
Lopez-GonzalezI.SchluterA.AsoE.Garcia-EsparciaP.AnsoleagaB.LLorensF.et al (2015). Neuroinflammatory signals in Alzheimer disease and APP/PS1 transgenic mice: correlations with plaques, tangles, and oligomeric species.J. Neuropathol. Exp. Neurol.74319–344. 10.1097/NEN.0000000000000176
54
LucaciuS. A.LeightonS. E.HauserA.YeeR.LairdD. W. (2023). Diversity in connexin biology.J. Biol. Chem.299:105263. 10.1016/j.jbc.2023.105263
55
LutzS. E.RaineC. S.BrosnanC. F. (2012). Loss of astrocyte connexins 43 and 30 does not significantly alter susceptibility or severity of acute experimental autoimmune encephalomyelitis in mice.J. Neuroimmunol.2458–14. 10.1016/j.jneuroim.2012.01.007
56
LutzS. E.ZhaoY.GulinelloM.LeeS. C.RaineC. S.BrosnanC. F. (2009). Deletion of astrocyte connexins 43 and 30 leads to a dysmyelinating phenotype and hippocampal CA1 vacuolation.J. Neurosci.297743–7752. 10.1523/JNEUROSCI.0341-09.2009
57
MaatoukL.YiC.Carrillo-de SauvageM. A.CompagnionA. C.HunotS.EzanP.et al (2019). Glucocorticoid receptor in astrocytes regulates midbrain dopamine neurodegeneration through connexin hemichannel activity.Cell Death Differ.26580–596. 10.1038/s41418-018-0150-3
58
MadeiraD.DiasL.SantosP.CunhaR. A.CanasP. M.AgostinhoP. (2021). Association between adenosine A(2A) receptors and connexin 43 regulates hemichannels activity and ATP release in astrocytes exposed to amyloid-beta peptides.Mol. Neurobiol.586232–6248. 10.1007/s12035-021-02538-z
59
MadeiraD.DominguesJ.LopesC. R.CanasP. M.CunhaR. A.AgostinhoP. (2023). Modification of astrocytic Cx43 hemichannel activity in animal models of AD: modulation by adenosine A(2A) receptors.Cell Mol. Life Sci.80:340. 10.1007/s00018-023-04983-6
60
MalhotraS.CostaC.EixarchH.KellerC. W.AmmanL.Martinez-BanaclochaH.et al (2020). NLRP3 inflammasome as prognostic factor and therapeutic target in primary progressive multiple sclerosis patients.Brain1431414–1430. 10.1093/brain/awaa084
61
MarkoullisK.SargiannidouI.GardnerC.HadjisavvasA.ReynoldsR.KleopaK. A. (2012a). Disruption of oligodendrocyte gap junctions in experimental autoimmune encephalomyelitis.Glia601053–1066. 10.1002/glia.22334
62
MarkoullisK.SargiannidouI.SchizaN.HadjisavvasA.RoncaroliF.ReynoldsR.et al (2012b). Gap junction pathology in multiple sclerosis lesions and normal-appearing white matter.Acta Neuropathol.123873–886. 10.1007/s00401-012-0978-4
63
MarquetteA.AisenbreyC.BechingerB. (2021). Membrane interactions accelerate the self-aggregation of huntingtin exon 1 fragments in a polyglutamine length-dependent manner.Int. J. Mol. Sci.22:6725. 10.3390/ijms22136725
64
MasakiK.SuzukiS. O.MatsushitaT.MatsuokaT.ImamuraS.YamasakiR.et al (2013). Connexin 43 astrocytopathy linked to rapidly progressive multiple sclerosis and neuromyelitis optica.PLoS One8:e72919. 10.1371/journal.pone.0072919
65
MaulikM.VasanL.BoseA.Dutta ChowdhuryS.SenguptaN.Das SarmaJ. (2020). Amyloid-beta regulates gap junction protein connexin 43 trafficking in cultured primary astrocytes.J. Biol. Chem.29515097–15111. 10.1074/jbc.RA120.013705
66
McFarlandK. N.ChakrabartyP. (2022). Microglia in Alzheimer’s disease: a key player in the transition between homeostasis and pathogenesis.Neurotherapeutics19186–208. 10.1007/s13311-021-01179-3
67
MeiX.EzanP.GiaumeC.KoulakoffA. (2010). Astroglial connexin immunoreactivity is specifically altered at beta-amyloid plaques in beta-amyloid precursor protein/presenilin1 mice.Neuroscience17192–105. 10.1016/j.neuroscience.2010.08.001
68
MichelucciA.HeurtauxT.GrandbarbeL.MorgaE.HeuschlingP. (2009). Characterization of the microglial phenotype under specific pro-inflammatory and anti-inflammatory conditions: effects of oligomeric and fibrillar amyloid-beta.J. Neuroimmunol.2103–12. 10.1016/j.jneuroim.2009.02.003
69
MielcarekM.InuabasiL.BondulichM. K.MullerT.OsborneG. F.FranklinS. A.et al (2014). Dysfunction of the CNS-heart axis in mouse models of Huntington’s disease.PLoS Genet.10:e1004550. 10.1371/journal.pgen.1004550
70
MilicevicK. D.BataveljicD. B.Bogdanovic PristovJ. J.AndjusP. R.NikolicL. M. (2023). Astroglial cell-to-cell interaction with autoreactive immune cells in experimental autoimmune encephalomyelitis involves P2X7 receptor, beta(3)-Integrin, and Connexin-43.Cells12:1786. 10.3390/cells12131786
71
MuzioL.ViottiA.MartinoG. (2021). Microglia in neuroinflammation and neurodegeneration: from understanding to therapy.Front. Neurosci.15:742065. 10.3389/fnins.2021.742065
72
NagyJ. I.LiW.HertzbergE. L.MarottaC. A. (1996). Elevated connexin43 immunoreactivity at sites of amyloid plaques in Alzheimer’s disease.Brain Res.717173–178. 10.1016/0006-8993(95)01526-4
73
NagyJ. I.PatelD.OchalskiP. A.StelmackG. L. (1999). Connexin30 in rodent, cat and human brain: selective expression in gray matter astrocytes, co-localization with connexin43 at gap junctions and late developmental appearance.Neuroscience88447–468. 10.1016/s0306-4522(98)00191-2
74
NanclaresC.BaraibarA. M.AraqueA.KofujiP. (2021). Dysregulation of astrocyte-neuronal communication in Alzheimer’s disease.Int. J. Mol. Sci.22:7887. 10.3390/ijms22157887
75
OrellanaJ. A.HernandezD. E.EzanP.VelardeV.BennettM. V.GiaumeC.et al (2010). Hypoxia in high glucose followed by reoxygenation in normal glucose reduces the viability of cortical astrocytes through increased permeability of connexin 43 hemichannels.Glia58329–343. 10.1002/glia.20926
76
OrellanaJ. A.ShojiK. F.AbudaraV.EzanP.AmigouE.SaezP. J.et al (2011). Amyloid beta-induced death in neurons involves glial and neuronal hemichannels.J. Neurosci.314962–4977. 10.1523/JNEUROSCI.6417-10.2011
77
OrellanaJ. A.von BernhardiR.GiaumeC.SaezJ. C. (2012). Glial hemichannels and their involvement in aging and neurodegenerative diseases.Rev. Neurosci.23163–177. 10.1515/revneuro-2011-0065
78
OrtonS. M.HerreraB. M.YeeI. M.ValdarW.RamagopalanS. V.SadovnickA. D.et al (2006). Sex ratio of multiple sclerosis in Canada: a longitudinal study.Lancet Neurol.5932–936. 10.1016/S1474-4422(06)70581-6
79
OsipovitchM.Asenjo MartinezA.MarianiJ. N.CornwellA.DhaliwalS.ZouL.et al (2019). Human ESC-derived chimeric mouse models of Huntington’s disease reveal cell-intrinsic defects in glial progenitor cell differentiation.Cell Stem Cell24107–122.e7. 10.1016/j.stem.2018.11.010
80
PalpagamaT. H.WaldvogelH. J.FaullR. L. M.KwakowskyA. (2019). The role of microglia and astrocytes in Huntington’s disease.Front. Mol. Neurosci.12:258. 10.3389/fnmol.2019.00258
81
PalpagamaT.MillsA. R.FergusonM. W.Vikas AnkealP.TurnerC.TippettL.et al (2023). Microglial and astrocytic responses in the human midcingulate cortex in Huntington’s disease.Ann. Neurol.94895–910. 10.1002/ana.26753
82
ParentiR.CicirataF.ZappalaA.CataniaA.La DeliaF.CicirataV.et al (2010). Dynamic expression of Cx47 in mouse brain development and in the cuprizone model of myelin plasticity.Glia581594–1609. 10.1002/glia.21032
83
PasseriE.ElkhouryK.MorsinkM.BroersenK.LinderM.TamayolA.et al (2022). Alzheimer’s disease: treatment strategies and their limitations.Int. J. Mol. Sci.23:13954. 10.3390/ijms232213954
84
RenR.ZhangL.WangM. (2018). Specific deletion connexin43 in astrocyte ameliorates cognitive dysfunction in APP/PS1 mice.Life Sci.208175–191. 10.1016/j.lfs.2018.07.033
85
ReyesJ. F.SackmannC.HoffmannA.SvenningssonP.WinklerJ.IngelssonM.et al (2019). Binding of alpha-synuclein oligomers to Cx32 facilitates protein uptake and transfer in neurons and oligodendrocytes.Acta Neuropathol.13823–47. 10.1007/s00401-019-02007-x
86
RochaN. P.CharronO.LathamL. B.ColpoG. D.Zanotti-FregonaraP.YuM.et al (2021). Microglia activation in basal ganglia is a late event in Huntington disease pathophysiology.Neurol. Neuroimmunol. Neuroinflamm.8:e984. 10.1212/NXI.0000000000000984
87
RossC. A.AylwardE. H.WildE. J.LangbehnD. R.LongJ. D.WarnerJ. H.et al (2014). Huntington disease: natural history, biomarkers and prospects for therapeutics.Nat. Rev. Neurol.10204–216. 10.1038/nrneurol.2014.24
88
SaezJ. C.SchalperK. A.RetamalM. A.OrellanaJ. A.ShojiK. F.BennettM. V. (2010). Cell membrane permeabilization via connexin hemichannels in living and dying cells.Exp. Cell Res.3162377–2389. 10.1016/j.yexcr.2010.05.026
89
SappE.KegelK. B.AroninN.HashikawaT.UchiyamaY.TohyamaK.et al (2001). Early and progressive accumulation of reactive microglia in the Huntington disease brain.J. Neuropathol. Exp. Neurol.60161–172. 10.1093/jnen/60.2.161
90
SavageJ. C.St-PierreM. K.CarrierM.El HajjH.NovakS. W.SanchezM. G.et al (2020). Microglial physiological properties and interactions with synapses are altered at presymptomatic stages in a mouse model of Huntington’s disease pathology.J. Neuroinflamm.17:98. 10.1186/s12974-020-01782-9
91
ScheltensP.BlennowK.BretelerM. M.de StrooperB.FrisoniG. B.SallowayS.et al (2016). Alzheimer’s disease.Lancet388505–517. 10.1016/S0140-6736(15)01124-1
92
ScheltensP.De StrooperB.KivipeltoM.HolstegeH.ChetelatG.TeunissenC. E.et al (2021). Alzheimer’s disease.Lancet3971577–1590. 10.1016/S0140-6736(20)32205-4
93
SchulzR.GorgeP. M.GorbeA.FerdinandyP.LampeP. D.LeybaertL. (2015). Connexin 43 is an emerging therapeutic target in ischemia/reperfusion injury, cardioprotection and neuroprotection.Pharmacol. Ther.15390–106. 10.1016/j.pharmthera.2015.06.005
94
SchwabB. C.HeidaT.ZhaoY.van GilsS. A.van WezelR. J. (2014). Pallidal gap junctions-triggers of synchrony in Parkinson’s disease?Mov. Disord.291486–1494. 10.1002/mds.25987
95
ShaoY.GearingM.MirraS. S. (1997). Astrocyte-apolipoprotein E associations in senile plaques in Alzheimer disease and vascular lesions: a regional immunohistochemical study.J. Neuropathol. Exp. Neurol.56376–381. 10.1097/00005072-199704000-00006
96
ShinJ. Y.FangZ. H.YuZ. X.WangC. E.LiS. H.LiX. J. (2005). Expression of mutant huntingtin in glial cells contributes to neuronal excitotoxicity.J. Cell Biol.1711001–1012. 10.1083/jcb.200508072
97
Soria LopezJ. A.GonzalezH. M.LegerG. C. (2019). Alzheimer’s disease.Handb. Clin. Neurol.167231–255. 10.1016/B978-0-12-804766-8.00013-3
98
StevensonR.SamokhinaE.RossettiI.MorleyJ. W.BuskilaY. (2020). Neuromodulation of glial function during neurodegeneration.Front. Cell Neurosci.14:278. 10.3389/fncel.2020.00278
99
StrongT. V.TagleD. A.ValdesJ. M.ElmerL. W.BoehmK.SwaroopM.et al (1993). Widespread expression of the human and rat Huntington’s disease gene in brain and nonneural tissues.Nat. Genet.5259–265. 10.1038/ng1193-259
100
SuY.VerkhratskyA.YiC. (2024). Targeting connexins: possible game changer in managing neuropathic pain?Trends Mol. Med.30642–659. 10.1016/j.molmed.2024.03.009
101
SwansonM. E. V.ScotterE. L.SmythL. C. D.MurrayH. C.RyanB.TurnerC.et al (2020). Identification of a dysfunctional microglial population in human Alzheimer’s disease cortex using novel single-cell histology image analysis.Acta Neuropathol. Commun.8:170. 10.1186/s40478-020-01047-9
102
TabriziS. J.ReilmannR.RoosR. A.DurrA.LeavittB.OwenG.et al (2012). Potential endpoints for clinical trials in premanifest and early Huntington’s disease in the TRACK-HD study: analysis of 24 month observational data.Lancet Neurol.1142–53. 10.1016/S1474-4422(11)70263-0
103
TakaseE. O.YamasakiR.NagataS.WatanabeM.MasakiK.YamaguchiH.et al (2024). Astroglial connexin 43 is a novel therapeutic target for chronic multiple sclerosis model.Sci. Rep.14:10877. 10.1038/s41598-024-61508-2
104
TakeuchiH.JinS.WangJ.ZhangG.KawanokuchiJ.KunoR.et al (2006). Tumor necrosis factor-alpha induces neurotoxicity via glutamate release from hemichannels of activated microglia in an autocrine manner.J. Biol. Chem.28121362–21368. 10.1074/jbc.M600504200
105
TakeuchiH.MizoguchiH.DoiY.JinS.NodaM.LiangJ.et al (2011). Blockade of gap junction hemichannel suppresses disease progression in mouse models of amyotrophic lateral sclerosis and Alzheimer’s disease.PLoS One6:e21108. 10.1371/journal.pone.0021108
106
TeleanuD. M.NiculescuA. G.LunguI. I.RaduC. I.VladacencoO.RozaE.et al (2022). An overview of oxidative stress, neuroinflammation, and neurodegenerative diseases.Int. J. Mol. Sci.23:5938. 10.3390/ijms23115938
107
TeoR. T.HongX.Yu-TaegerL.HuangY.TanL. J.XieY.et al (2016). Structural and molecular myelination deficits occur prior to neuronal loss in the YAC128 and BACHD models of Huntington disease.Hum. Mol. Genet.252621–2632. 10.1093/hmg/ddw122
108
TolosaE.GarridoA.ScholzS. W.PoeweW. (2021). Challenges in the diagnosis of Parkinson’s disease.Lancet Neurol.20385–397. 10.1016/S1474-4422(21)00030-2
109
UddinM. S.LimL. W. (2022). Glial cells in Alzheimer’s disease: from neuropathological changes to therapeutic implications.Ageing Res. Rev.78101622. 10.1016/j.arr.2022.101622
110
UmebayashiD.NatsumeA.TakeuchiH.HaraM.NishimuraY.FukuyamaR.et al (2014). Blockade of gap junction hemichannel protects secondary spinal cord injury from activated microglia-mediated glutamate exitoneurotoxicity.J. Neurotrauma311967–1974. 10.1089/neu.2013.3223
111
UneH.YamasakiR.NagataS.YamaguchiH.NakamutaY.IndiasariU. C.et al (2021). Brain gray matter astroglia-specific connexin 43 ablation attenuates spinal cord inflammatory demyelination.J. Neuroinflamm.18:126. 10.1186/s12974-021-02176-1
112
VicarioN.ParentiR. (2022). Connexins signatures of the neurovascular unit and their physio-pathological functions.Int. J. Mol. Sci.23:9510. 10.3390/ijms23179510
113
VicarioN.TurnaturiR.SpitaleF. M.TorrisiF.ZappalaA.GulinoR.et al (2020). Intercellular communication and ion channels in neuropathic pain chronicization.Inflamm. Res.69841–850. 10.1007/s00011-020-01363-9
114
VicarioN.ZappalaA.CalabreseG.GulinoR.ParentiC.GulisanoM.et al (2017). Connexins in the central nervous system: physiological traits and neuroprotective targets.Front. Physiol.8:1060. 10.3389/fphys.2017.01060
115
VisJ. C.NicholsonL. F.FaullR. L.EvansW. H.SeversN. J.GreenC. R. (1998). Connexin expression in Huntington’s diseased human brain.Cell Biol. Int.22837–847. 10.1006/cbir.1998.0388
116
VoronkovD. N.StavrovskayaA. V.PotapovI. A.GuschinaA. S.OlshanskiyA. S. (2023). Glial reaction in a neuroinflammatory model of Parkinson’s disease.Bull. Exp. Biol. Med.174693–698. 10.1007/s10517-023-05772-8
117
WangJ.WangF.MaiD.QuS. (2020). Molecular mechanisms of glutamate toxicity in Parkinson’s Disease.Front. Neurosci.14:585584. 10.3389/fnins.2020.585584
118
YiC.EzanP.FernandezP.SchmittJ.SaezJ. C.GiaumeC.et al (2017). Inhibition of glial hemichannels by boldine treatment reduces neuronal suffering in a murine model of Alzheimer’s disease.Glia651607–1625. 10.1002/glia.23182
119
YiC.MeiX.EzanP.MatoS.MatiasI.GiaumeC.et al (2016). Astroglial connexin43 contributes to neuronal suffering in a mouse model of Alzheimer’s disease.Cell Death Differ.231691–1701. 10.1038/cdd.2016.63
120
ZhangM.WangZ. Z.ChenN. H. (2023). Connexin 43 phosphorylation: implications in multiple diseases.Molecules28:4914. 10.3390/molecules28134914
121
ZhangS.LiangR.ZhouF.HuangX.DingJ. H.HuG. (2011). Reversal of rotenone-induced dysfunction of astrocytic connexin43 by opening mitochondrial ATP-sensitive potassium channels.Cell Mol. Neurobiol.31111–117. 10.1007/s10571-010-9560-6
122
ZhaoX.YaoH.LiX. (2021). Unearthing of key genes driving the pathogenesis of Alzheimer’s disease via bioinformatics.Front. Genet.12:641100. 10.3389/fgene.2021.641100
123
ZhaoY.QiY.LiQ.QuanH.LiuD.ZhouH. (2022). Connexin43 inhibition attenuated dopaminergic neuronal loss in the lipopolysaccharide-induced mice model of Parkinson’s disease.Neurosci. Lett.771:136471. 10.1016/j.neulet.2022.136471
Summary
Keywords
chronic neuroinflammation, neurodegeneration, connexin 43, Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, Huntington’s disease
Citation
Denaro S, D’Aprile S, Vicario N and Parenti R (2025) Mechanistic insights into connexin-mediated neuroglia crosstalk in neurodegenerative diseases. Front. Cell. Neurosci. 19:1532960. doi: 10.3389/fncel.2025.1532960
Received
22 November 2024
Accepted
20 January 2025
Published
11 February 2025
Volume
19 - 2025
Edited by
Yongjun Wang, Nantong University, China
Reviewed by
Diana Cunha-Reis, Biosystems and Integrative Sciences Institute (BioISI), Portugal
Rongchen Huang, University of Colorado, United States
Updates
Copyright
© 2025 Denaro, D’Aprile, Vicario and Parenti.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Rosalba Parenti, parenti@unict.it
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.