MINI REVIEW article

Front. Mol. Neurosci., 15 July 2022

Sec. Brain Disease Mechanisms

Volume 15 - 2022 | https://doi.org/10.3389/fnmol.2022.932497

Neurotrophin Crosstalk in the Etiology and Treatment of Neuropsychiatric and Neurodegenerative Disease

  • 1. Nash Family Department of Neuroscience, Icahn School of Medicine at Mount Sinai, New York, NY, United States

  • 2. Graduate School of Biomedical Sciences, Icahn School of Medicine at Mount Sinai, New York, NY, United States

  • 3. Icahn School of Medicine at Mount Sinai, Friedman Brain Institute, New York, NY, United States

  • 4. Brookdale Department of Geriatrics and Palliative Medicine, Icahn School of Medicine at Mount Sinai, New York, NY, United States

Abstract

This article reviews the current progress in our understanding of the mechanisms by which growth factors, including brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF), and select neurotrophin-regulated gene products, such as VGF (non-acronymic) and VGF-derived neuropeptides, function in the central nervous system (CNS) to modulate neuropsychiatric and neurodegenerative disorders, with a discussion of the possible therapeutic applications of these growth factors to major depressive disorder (MDD) and Alzheimer’s disease (AD). BDNF and VEGF levels are generally decreased regionally in the brains of MDD subjects and in preclinical animal models of depression, changes that are associated with neuronal atrophy and reduced neurogenesis, and are reversed by conventional monoaminergic and novel ketamine-like antidepressants. Downstream of neurotrophins and their receptors, VGF was identified as a nerve growth factor (NGF)- and BDNF-inducible secreted protein and neuropeptide precursor that is produced and trafficked throughout the CNS, where its expression is greatly influenced by neuronal activity and exercise, and where several VGF-derived peptides modulate neuronal activity, function, proliferation, differentiation, and survival. Moreover, levels of VGF are reduced in the CSF of AD subjects, where it has been repetitively identified as a disease biomarker, and in the hippocampi of subjects with MDD, suggesting possible shared mechanisms by which reduced levels of VGF and other proteins that are similarly regulated by neurotrophin signaling pathways contribute to and potentially drive the pathogenesis and progression of co-morbid neuropsychiatric and neurodegenerative disorders, particularly MDD and AD, opening possible therapeutic windows.

Introduction

Major depressive disorder (MDD) is among the most prevalent psychiatric disorders across the world with a high economic and psychological burden. According to the World Mental Health (WMH) survey (), depression has an average annual prevalence of 6%, with 20% of the world’s population fulfilling the criteria for MDD once in their lifetime, and the economic burden of depression in the United States estimated to be $83.1 billion in 2000 () and increased from $236.6 billion to $326.2 billion between 2010 and 2018 (). Females are at a two-fold higher risk than males, after puberty, of developing MDD, although it can affect either sex at any age, with a median age of onset of 25 years, peaking from mid-late adolescence until the early 40 s ().

The oldest and still currently employed treatment option for MDD is monoamine therapy, based on the hypothesized interaction of monoamines norepinephrine and serotonin that modulates this condition (). Widely used MDD therapeutics include selective serotonin and norepinephrine uptake inhibitors (SSRIs and SNRIs), which increase synaptic serotonin or norepinephrine levels, respectively, resembling treatment with monoamine oxidase inhibitors (MAOIs), although SSRIs and SNRIs are more widely used and have significantly lower rates of relapse than MAOI therapy (). Although the treatment rates for MDD have increased, and different treatments have been introduced, patient response rates have remained relatively low (), and treatment-resistant depression is common, encouraging the search for novel antidepressant treatment strategies that result from a better understanding of disease pathophysiology.

Dysregulation and loss of neuronal density in the hippocampus during chronic stress are associated with MDD and may be responsible for symptoms, such as depressed mood, anhedonia, and low energy or fatigue, which are thought to result from induced changes in intracellular signaling, gene modification, neuronal function, and connectivity, in the limbic brain regions that regulate mood and drive (; ). Chronic stress, aging, and depressive mood reduce neurogenesis in the hippocampus and increase neuronal apoptosis in the rodent cerebral cortex (). Currently used antidepressants generally positively affect neurotransmitter signaling in the limbic system, rescuing neuronal loss and the deficits in neurogenesis and intracellular signaling that are associated with anhedonia and low mood (; ; ). This suggests a potentially shared mechanism for antidepressant actions that neurotrophic growth factors and downstream proteins they regulate could therapeutically target. Although not explained by common genetic variants (), major depression is frequently comorbid with Alzheimer’s disease (AD), and both conditions have significant heritable components and shared dysfunction in neuroinflammation, oxidative stress, cellular signaling, and neurotransmission (). Given the evidence of common pathways and associated changes in MDD and AD, genome-wide association and multiscale causal network studies have sought to uncover molecular signatures and genetic variations shared between AD and MDD, indicative of potential comorbidity (; ; ; ).

Neurotrophins and Depression

Neurotrophic factors and their signaling pathways have been proposed to be major targets responsible for the efficacy of antidepressant therapy (, ; ). These growth factors regulate peripheral nervous system (PNS) and CNS neuronal generation, proliferation, differentiation, and survival, and include the classical neurotrophin family, specifically nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4 (NT-4), and cytokine family members, such as vascular endothelial growth factor (VEGF) () (Figure 1).

FIGURE 1

), the BDNF/TrkB signaling cascade (), and calcium channels which activate GluR1 (). Other BDNF mimetic/agonists, including LM22A-4 () and 7,8-DHF (), have shown antidepressant efficacy via TrkB activation. Additionally, vascular endothelial growth factor (VEGF) is shown to induce antidepressant effects via PI3K/PKB/mTORC1 pathways that activate CREB. Activated CREB increases BDNF and VGF transcription, leading to increased BDNF and VGF translation and secretion, and stimulation of BDNF/TrkB signaling – an autoregulatory loop.

Brain-Derived Neurotrophic Factor Function in Major Depressive Disorder

In addition to their major role in neuronal survival and function during development and in adulthood, growth factors, such as BDNF exert important pro-survival and functional effects in models of neurological, neuropsychiatric, and neurodegenerative disorders (), mediated in many cases through changes in gene expression (). Studies of postmortem human brain tissues have revealed reduced BDNF mRNA and protein levels in the limbic regions of the brain, including the hippocampus, prefrontal cortex (PFC), and amygdala of depressed patients and suicide subjects (; ). In animal models, a significant reduction of BDNF expression in the dentate gyrus of the hippocampus in rats exposed to chronic restraint stress has been reported (; ). However, in the ventral tegmental area (VTA)-nucleus acumbens (NAc) limbic projections, increased BDNF release is associated with increased susceptibility to pro-depressive behavior (; ), while in models of chronic and sub-chronic variable stress (SCVS), reduced BDNF expression in hippocampal dentate gyrus and NAc was reported (; ; ).

The human BDNF Val66Met gene polymorphism is the most widely studied variation with a single nucleotide polymorphism (SNP) at nucleotide 196 leading to a substitution of methionine (Met) for valine (Val) at codon 66, Val66Met, which interferes with BDNF trafficking, sorting, and secretion, and is associated with cognitive deficits (; ; ; ). Meta-analysis pooling of gene association studies of human subjects with the Val66Met BDNF SNP showed an association with depression in late life only. However, the Val66Met SNP (1) failed to predict MDD risk, (2) was correlated with MDD in males only, and () had increased chronicity of MDD with no association with disease recurrence (; ; ; ; ). Meta-analysis further indicated that the BDNF Met allele carriers had an increased antidepressant response compared with Val/Val homozygotes in the Asian population (; ), whereas, another study revealed varied responses to a different conventional antidepressant in Val/Val homozygotes and Met carriers in depressed Caucasian subjects (). Perturbed BDNF expression has therefore been linked to depressive disorders (; ; ; ; ; ; ).

Several classes of antidepressant drugs, including MAOIs, SSRIs, tricyclic antidepressants (TCAs), and SNRIs, increase BDNF expression in the brain in healthy rodents. Rapid-acting antidepressants, such as ketamine increase TrkB phosphorylation in the rat hippocampus (). Utilizing selective transcription and translation inhibitors, the antidepressant efficacy of ketamine and other NMDA receptor antagonists was found to require a robust increase in BDNF translation but not transcription (; ). BDNF protein levels are also differentially expressed in the brain following acute and chronic antidepressant treatment. Repeated application of electroconvulsive shock (ECS), but not a single application, produced 40–100% increases in BDNF levels in the hippocampus, cortex, amygdala, and brainstem; however, chronic but not acute treatment with antidepressants, such as desipramine (TCA), fluoxetine (SSRI), and phenelzine (MAOI), increased BDNF protein levels in the frontal cortex (10–30%) but not in the hippocampus, olfactory bulb, amygdala and brainstem ().

Brain-derived neurotrophic factor (BDNF), however, lacks suitable pharmacokinetics for systemic administration due to its short plasma half-life and poor blood brain barrier (BBB) penetration. Consequently, BDNF mimetics with better pharmacokinetics have been administered that target the tropomyosin-like receptor kinase B (TrkB) receptor for BDNF. The TrkB agonist LM22A has been intranasally administered to rodents (), showing promising results by activating TrkB, PI3K-Akt, and Ras-MAPK pathways, which play a critical role in MDD (; ; ; ). Interestingly, the levels of BDNF and its receptor TrkB are inversely related; increased TrkB expression accompanies decreased BDNF expression in a repeated immobilization stress model and the forced swim test (; ). Expression and activation of brain BDNF/TrkB pathways are also highly region- and circuit-specific. For example, a single bilateral infusion of TrkB agonist 7,8-Dihydroxyflavone (7,8-DHF), but not the TrkB antagonist ANA-12, into the infralimbic part of the medial prefrontal cortex (mPFC), dentate gyrus () and CA3 region exerted antidepressant effects in learned helplessness (LH) rat model, while the effect was absent following infusion of 7,8-DHF into the prelimbic region of the PFC (; ). In contrast, bilateral infusion of ANA-12 but not 7,8-DHF, into NAc exerted an antidepressant-like effect in LH rats (). Furthermore, intraperitoneal (IP) injection of 7,8-DHF or ANA-12 also led to antidepressant and pro-depressant responses, respectively (). While BDNF/TrkB activation has antidepressant efficacy, proBDNF binding to its receptor p75NTR has the opposite effect; proBDNF/p75NTR signaling is upregulated, and BDNF/TrkB signaling is downregulated by chronic mild unpredictable stress in rats ().

The Role of Vascular Endothelial Growth Factor in Major Depressive Disorder

The antidepressant efficacies of ketamine and desipramine are reduced by gene ablation of BDNF and vascular endothelial growth factor (VEGF), respectively (; ). VEGF is a potent mitogen and survival factor for endothelial cells and neurons, and additionally modulates synaptic transmission in the adult hippocampus and subventricular zone (; ), neuroprotection (), and hippocampal neurogenesis (), consistent with a potential role in MDD. The VEGF family consists of six members: VEGF (A-E) and placental-derived growth factor (P1DF). VEGF-A is spliced into four variants, with VEGF120 and VEGF164 known to be the predominant brain variants (). Furthermore, in addition to its regulation by antidepressants, VEGF is also modulated by exercise, a potent stimulator of neurogenesis and neurotrophic growth factor expression (; ; ). Nevertheless, some studies have found no correlation between BDNF and VEGF levels with aerobic exercise in depressed patients (). Currently, exercise is thought to affect executive functioning by (1) increasing oxygen saturation and angiogenesis in brain areas crucial for task performance, (2) increasing neurotransmitter levels, and () upregulating neurotrophic growth factors, such as BDNF (; ). Given the evidence that exercise stimulates brain angiogenesis and increases oxygen saturation, implicating a role for VEGF, it is interesting that reported that blockade of peripheral VEGF signaling in a rat exercise model reduced CNS neurogenesis, and that removal of VEGF-antagonist increased neurogenesis, suggesting that exercise-induced neurogenesis requires peripheral VEGF signaling. The biological effects of VEGF are transduced by two receptor tyrosine kinases, fetal liver kinase (Flk-1 or VEGFR2) and fms-like tyrosine kinase (Flt-1), and by a family of putative coreceptors called neuropilins (NRPs) (). Therefore, pharmacological modulation of VEGF receptors could have efficacy in MDD, consistent with the recent finding that neuronal VEGF-VEGFR2 (Flk-1) pathways play a key role in the rapid antidepressant actions of ketamine (). Recent studies further demonstrate that antidepressant actions of VEGF in mPFC are blocked by neutralizing anti-BDNF antiserum, while antidepressant actions of BDNF in mPFC are blocked by VEGF ablation or neutralizing anti-VEGF antiserum (; ). VEGF, therefore, functions in complex interplay with BDNF in the mechanism of action of ketamine. Nevertheless, the neuropathological effects of VEGF in MDD and other neuropsychiatric conditions, including schizophrenia require additional investigation (; ).

VGF Functions Downstream of Neurotrophin Signaling Pathways to Regulate Major Depressive Disorder

Brain-derived neurotrophic factor (BDNF) and TrkB play crucial roles in cognition, memory, and depression (; ; ), driven at least in part by downstream effectors, including VGF (non-acronymic) (; ). VGF is a neurosecretory protein that was first identified () as a nerve growth factor (NGF)-regulated transcript in rat pheochromocytoma cells (PC12), and was independently identified by the Wagner and Salton labs (; ), and belongs to the granin family (; ; ). VGF mRNA is widely expressed in the PNS and CNS, abundantly in the hypothalamus, consistent with known function in metabolic regulation and energy balance (; , ), and in the hippocampus, in line with important roles for VGF in memory and depression-like behavior (; , ; ; ). VGF undergoes proteolytic cleavage by prohormone convertases and proteases in the regulated secretory pathway to produce at least 12 VGF-derived peptides (; ). VGF-derived peptides have been reported in several CSF proteomic studies to be potential biomarkers of neuropsychiatric and neurodegenerative disorders, including MDD and AD (; ; ; ), consistent with possible functional roles for VGF in disease pathophysiology. Indeed, Vgf gene ablation impacts memory and hippocampal function (; ), and infusion of VGF-derived peptide TLQP-62 into the brain improves memory (; ). VGF expression in the hippocampus is decreased in mouse models of depression-like behavior and in postmortem samples of hippocampus and Brodmann area 25 from both medicated and unmedicated male and female patients with MDD compared to controls (, ), while expression is increased by exercise and chronic antidepressant treatment, including fluoxetine and ketamine (; ; , ). VGF overexpression in mouse vmPFC via infusion of AAV-VGF rescued behavioral deficits induced by chronic restraint stress (). Furthermore, ketamine and desipramine antidepressant efficacies were reduced by targeted VGF ablation in the hippocampus and/or vmPFC (, ), reminiscent of the reduction in antidepressant efficacy of voluntary exercise that was observed in germline VGF-deficient mice (). Taken together, these studies support a critical role of the neurotrophin-regulated gene Vgf in depressive behavior and antidepressant efficacy.

Procognitive and Antidepressant Efficacy of the VGF-Derived Peptide TLQP-62 – A Potentially Novel Therapeutic Agent for Major Depressive Disorder

Administration of the C-terminal VGF-derived peptide TLQP-62 (by convention named by its N-terminal 4 amino acids and length) to the hippocampus or vmPFC has antidepressant efficacy that was shown to be BDNF-dependent and increases pTrkB and pCREB levels (, ). TLQP-62 dependence on BDNF/TrkB was determined utilizing conditional BDNF knockout mice (, ), targeted shRNA approaches (), and BDNF/TrkB inhibitors (). Additional experiments implicated downstream activation of mTORC1, reduced bicaudal C homolog 1 (BICC1) signaling, and increased levels of synaptic GluA1 and pGluA1, following antidepressant TLQP-62 infusion into the hippocampus or PFC (, ,; ). TLQP-62 infused icv (intra-cerebroventricular) or into the hippocampus improved contextual fear memory or lipopolysaccharide (LPS)-induced memory dysfunction, and either activated BDNF/TrkB pathways or was shown to require BDNF expression (Figure 1) (; ), consistent with a general mechanism underlying VGF actions in the brain. A proposed autoregulatory feedback model suggests that increased BDNF/TrkB signaling results in Vgf gene induction and the production of VGF-derived peptides, including procognitive and antidepressant TLQP-62, which both stimulates and requires BDNF/TrkB signaling for efficacy (). Moreover, in neuronal cell cultures, pTrkB levels were significantly increased by 24 h of TLQP-62 treatment, which in addition increased neurogenesis and the proliferation of neural stem cells, and increased synaptic plasticity through actions on glutamatergic receptors, including mGluR5 and NMDA receptors (). Taken together, these studies suggest an important role for VGF and VGF-derived peptides as critical mediators of memory and depression-like behavior, and that novel VGF therapeutics may provide new approaches to MDD.

Potential Mechanisms Underlying Comorbidity Between Alzheimer’s Disease and Major Depressive Disorder

The progressive neurodegenerative disorder Alzheimer’s disease (AD) is the most common form of dementia, affecting 50-75% of patients, and is reaching pandemic levels (), while Major Depressive Disorder (MDD) is a debilitating mental illness characterized by high prevalence and resistance to treatment. Both AD and MDD have devastating public health impacts, and comorbidity is frequent but not explained by common genetic variants () and could in part be driven by overlapping pathophysiologic mechanisms and signaling networks (Figure 2). BDNF protects against neurodegeneration through signaling pathways that activate the cyclic AMP response element-binding protein (CREB) (Figure 1), increasing hippocampal neurogenesis, controlling the amyloidogenic pathway and Aβ production, and regulating downstream gene expression in the hippocampus (; ; ). VEGF and BDNF pathways play critical, interdependent roles in the antidepressant efficacy of ketamine in MDD as noted above, while in early-onset and late-onset AD, VEGF-A expression in the brain and vasculature undergoes complex, stage-dependent dysregulation (). In the APP/PS1 AD mouse model, intraperitoneal administration of neutralizing anti-VEGF-A antibodies ameliorated capillary stalling, normalized BBB permeability, and increased cerebral blood flow (CBF) (capillary stalling and reduced CBF are associated with cognitive impairment in AD patients) (). On the other hand, delivery of VEGF-A to the brains of mouse AD models partially rescued vascular loss, reduced amyloid plaque load, and rescued impaired memory (; ) as did intraperitoneal VEGF administration (). VEGF-A signaling in the brain may, therefore, be impaired in AD (; ), much as it appears to be in MDD, suggesting a potential mechanism underlying disease comorbidity.

FIGURE 2

; ). Immune and glucocorticoid dysfunction are common to AD and MDD, leading to neuronal atrophy, as depicted. Expression of neurotrophic growth factors, including BDNF, VEGF, and VGF, and neuropeptides, such as TLQP-62, is also reduced in both AD and MDD. Treatment with antidepressants and increased exercise increase the levels of neurotrophins in MDD, helping to rescue neuronal function. In AD, exercise and future novel therapeutics may be similarly utilized to increase levels of these neurotrophins and select downstream gene products (VGF), halting the progression of neurodegenerative disease.

Among hundreds of genes involved in AD, the BDNF-regulated VGF gene was identified as a key network driver of AD pathogenesis and progression, and as a potential therapeutic target (; ). VGF levels are consistently decreased in brain tissue and CSF samples of patients with AD (; ; ; ; ; ; ; ; ; ) and postmortem brain tissue samples from depressed male and female subjects (, ) as compared with controls. VGF levels were also reduced in patients with MCI only in those who then progressed to AD, further indicating the potential role of VGF as a biomarker in dementia (; ). Importantly, overexpression of VGF in preclinical 5xFAD mouse AD models, via germline overexpression, AAV-mediated VGF delivery, or long-term icv TLQP-62 or TLQP-21 peptide administration, reduced amyloid load, phospho-tau levels, synaptic pathology, dystrophic neurite damage, astrogliosis, microgliosis, and memory impairment, and rescued neurogenesis and LTD deficits (; ). Thus, VGF is perhaps one of a limited number of proteins that are down-regulated in both AD and MDD.

The activity-dependent neuroprotective protein (ADNP), which plays a key role in neurogenesis and is essential for brain formation, is another protein that is decreased in postmortem AD brain samples. Accumulating mosaic somatic mutations in the ADNP and other genes in the brains of patients with AD have been proposed to contribute to disease progression, including impaired cognition, increased amyloid plaque burden, and tauopathy (; ; ). Low-level brain somatic mutations in glutamate and dopamine receptor signaling pathway genes have similarly been identified in neuropsychiatric diseases, specifically schizophrenia (; ). ADNP additionally functions in the pathophysiology of schizophrenia via a key regulatory role in autophagy, which when it is dysregulated also contributes to AD pathogenesis (, ; ). Recently identified ADNP interactions with SHANK3 () and SIRT1 () may provide critical new targets for understanding the role that ADNP plays in neuropsychiatric, neurodevelopmental, and neurodegenerative disease. Furthermore, the smallest active snippet of ADNP, an eight-amino-acid peptide NAP (NAPVSIPQ), protected against ADNP deficiencies in autism patients with Helsmoortel-Van Der Aa syndrome (; ; ; ). Therefore, therapeutic overexpression or CNS administration of these ADNP- or VGF-derived peptides have the potential to prevent the progression of comorbid neuropsychiatric and neurodegenerative diseases.

Neurotrophic growth factors and their regulated gene products, such as VGF, could regulate comorbid MDD and AD via shared actions on neurogenesis or synaptic plasticity. Neurogenesis is a dynamic process that is known to be regulated by various external stimuli, including physiological, pathological, and pharmacological changes, that also regulate hippocampal synaptogenesis (; ). These stimuli include exercise, enriched environment, antidepressants, and intrinsic factors that include neurotrophic growth factors, such as BDNF, VEGF, VGF, and VGF-derived neuropeptides, like TLQP-62 (; ; ; ). Several clinical studies have linked depression with decreased hippocampal volume associated with decreased pyramidal neuronal arborization (; ), and perhaps novel peptides like TLQP-62 could find therapeutic utility if they were able to restore neuronal loss and rescue hippocampal atrophy. However, clinical evidence is very limited, and therefore additional studies are required to understand the process of neuronal loss in patients suffering from depression (; ).

In pre-clinical studies, TLQP-62 infused intrahippocampally in rodents has antidepressant efficacy, and VGF overexpression or icv peptide administration reduces AD-related phenotypes in 5xFAD mice. However, an extension of these findings to clinical trials that assess novel therapeutics in human subjects, involving either CNS gene therapy (AAV-VGF) or intranasal peptide administration, still has significant technical hurdles to overcome. In this regard, it is somewhat sobering that rescue of reduced levels of BDNF in the hippocampus and entorhinal cortex by sustained BDNF gene delivery via viral vectors, in subjects with AD, did not have significant clinical efficacy (; ; ), although it does encourage the search for novel, alternative therapeutics that activate these pathways and may have better clinical results.

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.

Statements

Author contributions

RJ and SS wrote the manuscript. Both authors contributed to the article and approved the submitted version.

Funding

Research in the authors’ laboratory was supported by grants from the NIH and Cure Alzheimer’s Fund.

Acknowledgments

We acknowledge the contributions of Dr. Allen L. Pan and members of the Salton lab for helpful comments on this manuscript, and thank the Mount Sinai Instructional Technology Group for their assistance with the figure illustrations.

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.

References

Summary

Keywords

Alzheimer’s disease, TLQP-62, TrkB, VGF, MDD (major depressive disorder), BDNF (brain derived neurotrophic factor), VEGF – vascular endothelial growth factor, ADNP (activity dependent neuroprotective protein)

Citation

Joshi R and Salton SRJ (2022) Neurotrophin Crosstalk in the Etiology and Treatment of Neuropsychiatric and Neurodegenerative Disease. Front. Mol. Neurosci. 15:932497. doi: 10.3389/fnmol.2022.932497

Received

29 April 2022

Accepted

23 June 2022

Published

15 July 2022

Volume

15 - 2022

Edited by

Satoshi Deyama, Kanazawa University, Japan

Reviewed by

Illana Gozes, Tel Aviv University, Israel; Lucy Vulchanova, University of Minnesota Twin Cities, United States

Updates

Copyright

*Correspondence: Stephen R. J. Salton,

This article was submitted to Brain Disease Mechanisms, a section of the journal Frontiers in Molecular Neuroscience

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.

Outline

Figures

Cite article

Copy to clipboard


Export citation file


Share article

Article metrics