Collinge et al. (1994) established the role of PrP on neuronal excitability by electrophysiological studies in hippocampal pyramidal neurons and Purkinje cells, from non-transgenic (N-Tg) mice and conditional PrP^C-null mice (Prnp0/0). Interestingly, the histopathological evaluation of Prnp0/0 did not exhibit significant variations with N-Tg mice, but it showed alterations on feedback mechanisms controlling frequency and patterning of neuronal firing, such as input resistance (Rinp), Ca²⁺-activated K⁺ current (IAHP), and afterhyperpolarization (AHP) current (Collinge et al., 1994; Colling et al., 1996; Herms et al., 2001; Mallucci et al., 2002).

One of the main modulators in the generation and shaping of APs is voltage-dependent calcium channels (VGCCs or CaV) (Llinas et al., 1976; Campiglio and Flucher, 2015). Electrophysiological and immunohistochemical studies have shown that PrP^C is able to maintain neuronal excitability at the presynaptic level. This is achieved by stabilization and interaction with α2δ-1 auxiliary subunit of VGCC channels in a GPI anchor-dependent manner (Table 1; Rutishauser et al., 2009; Senatore et al., 2012; Alvarez-Laviada et al., 2014). Furthermore, co-expression of PrP with different Ca²⁺ channel subunits in Xenopus oocytes and mammalian tsA-201 cells has shown that PrP is able to modulate the amplitude peak of Ca²⁺ currents of the CaV2.1/β4/α2δ-2 and CaV2.1/β1b/α2δ-1 channels (Alvarez-Laviada et al., 2014). In contrast, in cerebellar granule neurons (CGN) of the transgenic mouse of PrP Tg (PG14), which synthesizes a misfolded mutant variant of PrP (PrPmut) that is partially retained in the ER, it was observed that PrPmut can impair α2δ-1 auxiliary subunit anterograde trafficking, reducing intracellular Ca²⁺ influx and glutamate transmission into the synaptic cleft (Senatore et al., 2012). Furthermore, PrP^C modulates neuronal membrane excitability, synaptic integration of voltage threshold, and the repolarization process of the APs, mediated by their functional interaction with the Kv4.2 (voltage-gated K channels)/DPP6 (dipeptidyl aminopeptidase-like protein 6) complex at the neuronal cell surface (Schmitt-Ulms et al., 2004; Kim et al., 2008; Mercer et al., 2013). Electrophysiological studies in HEK293T cells transiently transfected with the Kv4.2/DPP6 channel complex have shown that PrP^C is able to increase the amplitude peak and depolarizing potential of A-type K⁺ currents, as well as it shifts the activation curve of the Kv4.2 channels to more depolarized potentials in a DPP6-dependent form (Mercer et al., 2013). Further studies are needed to understand the link between PrP^C, its misfolding, and the neuronal activity-dependent signaling pathways during the APs.

PrP^C also participates in the regulation of excitatory postsynaptic responses through its functional interaction with ionotropic receptors, namely, N-methyl-D-aspartate receptor (NMDARs) (Khosravani et al., 2008a; You et al., 2012), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPARs) (Watt et al., 2012; Chater and Goda, 2014), kainate receptor (KARs) (Carulla et al., 2011), and α7 nicotinic acetylcholine receptors (α7nAChRs) (Zanata et al., 2002; Beraldo et al., 2010; Roffe et al., 2010; Table 1).

Increasing studies indicate that PrP^C would be a key mediator in the maintenance of glutamatergic synapses, mediated by their interaction with NR1 and NR2 subunits of NMDAR (Khosravani et al., 2008a; You et al., 2012; Gasperini et al., 2015). It was observed that PrP^C ablation induced an overexpression and S-nitrosylation of the NR2A and NR2B subunits of NMDAR, altering its kinetic properties. PrP^C ablation induced a slow inactivation of the channel triggering an abnormal increase in neuronal excitability (Gasperini et al., 2015). Meanwhile, overexpression of mouse PrP^C showed decreased activity of NMDAR (Maglio et al., 2004; Khosravani et al., 2008a; Gasperini et al., 2015; Huang et al., 2018). Additionally, recent studies have shown that the neuroprotective effects of PrP^C associated with downregulation of NMDAR would occur in a Cu²⁺−dependent manner (Gasperini et al., 2015; Huang et al., 2018). More studies are needed to establish the interaction sites of PrP^C in the modulation of NMDAR activity.

Regarding AMPA receptors, in vitro co-immunoprecipitation studies also revealed interactions with PrP^C (Kleene et al., 2007; Watt et al., 2012; Huang et al., 2018). Interestingly, it has been observed that the increase in the formation of PrP^C/AMPAR complex could exert neuroprotection in a Cu²⁺− and Zn²⁺−dependent manner, as well as AMPA-ergic activity (Watt et al., 2012; Huang et al., 2018). However, PrP^C modulation does not induce significant changes in the amplitude or channel kinetics nor the long-term depression (LTD) maintenance (Khosravani et al., 2008a; Huang et al., 2018). Remarkably, the mutant variant of PrP could exert excitotoxicity mediated by intracellularly retained GluA2 AMPAR subunit (Ghirardini et al., 2020).

It has been postulated that PrP^C has a neuroprotective function in association with KARs against neurotoxicity induced by kainite (KA), which induces neurodegeneration in presynaptic terminals (Carulla et al., 2011). Additionally, in vivo and in vitro evidence in Prnp0/0 mice indicated that PrP^C can also regulate synaptic transmission and exert neuroprotection against KA toxicity, in a GPI anchoring-dependent manner (Rangel et al., 2007; Carulla et al., 2015). More studies are needed to determine the direct action of PrP^C in channel kinetics and KARs activity, mediated by postsynaptic density protein 95 (PSD95) modulation.

Another postulated mechanism by which PrP^C would exert neuroprotection and promote neuritogenesis is related to its association with α7nAChRs/stress-inducible protein 1 (STI1) complex at the cell membrane (Beraldo et al., 2010). Effectively, in several neuron cell lines, it has been observed that PrP^C can upregulate several neuroprotective pathways, such as autophagic flux cAMP-dependent protein kinase 1 (PKA), and extracellular signal-regulated kinase 1 and 2 (ERK1/2) pathways, in a α7nAChRs-dependent manner (Beraldo et al., 2010; Jeong and Park, 2015).

The overall data suggest that PrP^C would be acting as a new player in the regulation of glutamatergic and cholinergic neurotransmission. However, further research is needed to identify regions involved in the association of ionotropic receptors and PrP^C, as well as the consequences of its disruption in the synaptic neurotransmission in a pathological context.

Aβ oligomers (AβOs)-induced neuronal toxicity is thought, at least partly, to be mediated by putative Aβ receptors. Among them, PrP^C has emerged as an important potential receptor, due to its high affinity to the oligomeric form of the peptide (Laurén et al., 2009; Smith et al., 2019; Legname and Scialo, 2020). A cloning cDNA screening from a mouse brain library in order to find a protein that binds to AβOs (Aβ1-42) found that the only high-affinity binding protein was PrP^C, an observation that has been further supported by other studies (Laurén et al., 2009; Corbett et al., 2020). In fact, in a systematic comparison of reported Aβ receptors, only PrP^C, Nogo receptor 1 (NgR1), and leukocyte immunoglobulin-like receptor subfamily member 2 (LilrB2) showed direct binding to synthetic Aβ assemblies. Interestingly, binding with human AD brains-derived soluble AβOs revealed strong affinity only for PrP^C, with a weak affinity for NgR1 and no detectable affinity for LilrB2 (Smith et al., 2019). Therefore, PrP^C is most likely an Aβ-binding receptor.

In contrast to what was observed between PrP^C and AβOs, experiments performed in vitro showed low-affinity interactions with Aβ monomers (Chen et al., 2010; Fluharty et al., 2013; Corbett et al., 2020). Solid-phase assays showed that there is neither interaction of monomeric Aβ1-42 with PrP23–231 nor full-length PrP^C (Corbett et al., 2020). However, immunoassay studies have revealed that PrP^C 23–39 and 93–119 can interact with monomeric Aβ1-42 (Kang et al., 2013). Reported sites of interaction between PrP^C and different Aβ species are summarized in Table 2.

Regarding the binding site in PrP^C for AβOs, it was shown that the unstructured N-terminal domain was relevant for this interaction (Laurén et al., 2009). In fact, when anti-PrP antibodies were used to interfere with the interaction, only 6D11 (which binds to amino acids 93–109 in mouse PrP) blocked the binding between Aβ assemblies and PrP^C with an IC₅₀ of 1 nM (Laurén et al., 2009). In addition, the deletion of a similar region (95–105) impaired Aβ binding to PrP^C (Laurén et al., 2009). Another site reported for this binding was the N-terminal basic amino acids 23–27 (KKRPK) in PrP (Legname and Scialo, 2020).

Protein misfolding and aggregation of Aβ peptide are key events in the onset of AD, especially AβOs, due its capacity to associate with the cell membrane and induce excitotoxicity (Puzzo et al., 2017; Cline et al., 2018). The main neurotoxic effects described for AβOs in AD are membrane disruption, synaptic failure, impaired LTP, and memory loss (Lambert et al., 1998; Cline et al., 2018). However, specific binding transducers of AβOs signals that mediate its neurotoxic effects are not yet clearly defined. Several works have postulated different interacting partners for Aβ assemblies in the cell membrane, namely, NMDAR (Rammes et al., 2018), APP (Puzzo et al., 2017), NgR1, nAChR, and PrP^C (Fabiani and Antollini, 2019; Smith et al., 2019; Zhang et al., 2019).

As mentioned earlier, PrP^C has been proposed as a high-affinity physiological receptor for soluble AβOs [see reviews Linden (2017) and Wiatrak et al. (2021)]. At present, in different animal AD models as well as in patients with AD, it has been established that PrP^C could be one of the best specific binding partners for AβOs-mediated inhibition of LTP and cognitive defects in the early stages of AD (Kostylev et al., 2015; Smith and Strittmatter, 2017; Smith et al., 2019; Corbett et al., 2020). With this knowledge, it has been proposed that PrP^C would play an important role in the onset of AD, occurring before clinical symptoms, such as movement and cognitive impairments associated with the late stages of the disease.

Laurén et al. (2009) have demonstrated that PrP^C is a high-affinity receptor for AβOs, being amino acids 95–110 of PrP^C involved in this interaction (Laurén et al., 2009). Solid-phase and ELISA-like assays showed further associations between AβOs and PrP^C (EC50 ∼30 nM) (Corbett et al., 2020). In cellular and animal models of Aβ toxicity, PrP^C was able to mediate impairment of synaptic plasticity, alteration in calcium transients, and reduction in the levels of synaptophysin (Riek et al., 1996; Laurén et al., 2009; Barry et al., 2011; Peters et al., 2015). Furthermore, these alterations can be rescued using antibodies that block the oligomer-binding site of Aβ in PrP^C (6D11) (Riek et al., 1996; Laurén et al., 2009; Barry et al., 2011; Peters et al., 2015). Regarding the mechanism by which PrP^C exerts its role as a receptor, it has been proposed that AβOs binding to PrP induces activation of Fyn, a Src kinase (SRK), through an undetermined TM partner (Figure 2; Malaga-Trillo and Ochs, 2016). After its activation, fyn phosphorylates NMDA receptor, which becomes transiently over-activated, producing excitotoxicity (Malaga-Trillo and Ochs, 2016). Fyn was already known to be relevant in the pathogenesis of AD, because it performs Tau phosphorylation. Tau is an axonal microtubule-associated protein, and phosphorylated Tau is the main constituent of neurofibrillary tangles in AD, which mediates Aβ toxicity at the post-synapse. The notion that AβO-induced Tau phosphorylation is mediated by PrP^C comes from assays in human and mice brain, as well as analyses in primary neuron cultures, which show that soluble Aβ binds to a PrP^C/Fyn complex and Prnp gene deletion uncouples AβOs and the Fyn/tau axis (Larson et al., 2012). Besides Tau phosphorylation, SRKs are able to regulate the stability at the neuronal plasma membrane of several synapse-relevant proteins as adhesion proteins and receptors (e.g., NMDAR, AMPAR, and GABAR) (Malaga-Trillo and Ochs, 2016). Therefore, PrP^C–Fyn interaction might be directly involved in the pathological characteristics of AD. The signal transduction pathway generated by the interaction between AβOs and PrP^C is depicted in Figure 2.

Tauopathies are a group of diseases that have in common the deposition of abnormal tau in the nervous system. They comprise AD, Pick’s disease, progressive supranuclear palsy, corticobasal degeneration, and primary age-related tauopathy, among others (Kovacs, 2015). Normal Tau, which is a microtubule-associated protein, plays a role in the stabilization of neuronal microtubules. In pathological conditions, tau undergoes phosphorylation and forms aggregates that are neurotoxic (Avila et al., 2004).

Regarding its relation to PrP^C, in vitro and in vivo studies have found an association between PrP^C and hyperphosphorylated tau forms, particularly with tau N-terminal region (De Cecco et al., 2020; Legname and Scialo, 2020). Electrophysiological experiments showed that antibodies against PrP^C (6D11, MI-0131) could prevent LTP impairment induced by tau toxicity (Ondrejcak et al., 2018). At present, it has been reported that other antibodies against different epitopes of PrPC (POM 3, 4, 12) are able to impair the uptake of tau amyloid fibrils in mouse neuroblastoma cells (De Cecco et al., 2020). In contrast, it has been described that tau is a transcription regulator for PRPN gene in AD models (Lidon et al., 2020), linking both proteins in the progression of tauopathies.

The misfolding and accumulation of α-synuclein is involved in a group of pathologies known as synucleinopathies, such as PD, dementia with Lewy bodies (LBD), and multiple system atrophy (MSA). For instance, histopathological biomarker detected in patients with PD has been classically associated with abnormal deposits of α-syn, which mainly affects nigral dopaminergic system at the intracellular level, also called Lewy bodies (Kalia and Lang, 2015).

In these diseases, similar to other proteinopathies, fibrillar forms of α-syn spread from one cell to another. One of the mechanisms that this form of α-syn uses to enter cell is clathrin-dependent endocytosis, a process that requires the interaction with the TM protein lymphocyte-activation gene 3 (LAG3) (De Cecco and Legname, 2018). Other protein that was reported to be involved in the internalization of α-syn is PrP^C (Aulic et al., 2017). Cells that express PrP^C are able to internalize more amyloid α-syn fibrils compared to cells that do no express it; therefore, PrP^C favors cell-to-cell transmission (Aulic et al., 2017). In contrast, when these cells are infected with prions, α-syn reduces prion replication, especially due to PrP^C α cleavage, producing C1 and N1 that are neuroprotectors (Aulic et al., 2017).

Further analyses agreed on the connection between PrPC and α-syn: overexpression of PrPC in the striatum potentiates neurodegeneration, thereby altering α-syn propagation and toxicity. Electrophysiological and molecular approaches showed that antibodies against PrPC 6D11 could abolish LTP impairment, calcium dyshomeostasis, and cell degeneration induced by α-syn toxicity (Ferreira et al., 2017; Legname and Scialo, 2020).

Frontotemporal lobar degeneration (FTLD), a neurodegenerative syndrome in frontal and anterior temporal lobes (Rabinovici and Miller, 2010), and ALS, a motor neuron disorder characterized by degeneration in the upper and lower motor neurons (Prasad et al., 2019), are two distinct diseases that shared a histopathological hallmark: inclusion bodies composed of cytoplasmic deposits of the nuclear TDP-43 protein (Scialo et al., 2021). Under physiological conditions, TDP-43 is a transcriptional repressor that binds to chromosomally integrated TAR DNA. Nevertheless, a hyper-phosphorylated, ubiquitinated, and cleaved form of TDP-43 (pathological TDP-43) is the major disease protein in ubiquitin-positive, tau-, and α-synuclein-negative FTLD and in ALS (Mackenzie et al., 2011; Brauer et al., 2018).

It was observed in vitro that TDP-43 fibrils bind to recombinant PrP^C. Also, in vitro, it was shown that full-length mouse (Mu)PrP^C as well as human (Hu)PrP^C act as a membrane receptor of TDP-43 in its fibrillar conformation, inducing the formation of intracytoplasmic aggregates and cell death (Scialo et al., 2021). In addition, the overexpression of PrP^C in human and mouse cell lines was directly correlated with the internalization of TDP-43 fibrils. Increased internalization was associated with detrimental consequences in all PrP-overexpressing cell lines (Scialo et al., 2021).

As for other amyloids, treatment with TDP-43 fibrils induced a reduction in the accumulation of the misfolded form of PrP^C, PrP^Sc, in cells chronically infected with prions. Our results expand the list of misfolded proteins whose uptake and detrimental effects are mediated by PrP^C, which encompass almost all pathological amyloids involved in neurodegeneration (Scialo et al., 2021).