Senescent Schwann cells induced by aging and chronic denervation impair axonal regeneration following peripheral nerve injury

Following peripheral nerve injury, successful axonal growth and functional recovery require Schwann cell (SC) reprogramming into a reparative phenotype, a process dependent upon c-Jun transcription factor activation. Unfortunately, axonal regeneration is greatly impaired in aged organisms and following chronic denervation, which can lead to poor clinical outcomes. While diminished c-Jun expression in SCs has been associated with regenerative failure, it is unclear whether the inability to maintain a repair state is associated with the transition into an axonal growth inhibition phenotype. We here find that reparative SCs transition into a senescent phenotype, characterized by diminished c-Jun expression and secretion of inhibitory factors for axonal regeneration in aging and chronic denervation. In both conditions, the elimination of senescent SCs by systemic senolytic drug treatment or genetic targeting improved nerve regeneration and functional recovery, increased c-Jun expression and decreased nerve inflammation. This work provides the first characterization of senescent SCs and their influence on axonal regeneration in aging and chronic denervation, opening new avenues for enhancing regeneration and functional recovery after peripheral nerve injuries.

Necroptosis inhibition counteracts neurodegeneration, memory decline, and key hallmarks of aging, promoting brain rejuvenation

Age is the main risk factor for the development of neurodegenerative diseases. In the aged brain, axonal degeneration is an early pathological event, preceding neuronal dysfunction, and cognitive disabilities in humans, primates, rodents, and invertebrates. Necroptosis mediates degeneration of injured axons, but whether necroptosis triggers neurodegeneration and cognitive impairment along aging is unknown. Here, we show that the loss of the necroptotic effector Mlkl was sufficient to delay age-associated axonal degeneration and neuroinflammation, protecting against decreased synaptic transmission and memory decline in aged mice. Moreover, short-term pharmacologic inhibition of necroptosis targeting RIPK3 in aged mice, reverted structural and functional hippocampal impairment, both at the electrophysiological and behavioral level. Finally, a quantitative proteomic analysis revealed that necroptosis inhibition leads to an overall improvement of the aged hippocampal proteome, including a subclass of molecular biofunctions associated with brain rejuvenation, such as long-term potentiation and synaptic plasticity. Our results demonstrate that necroptosis contributes to age-dependent brain degeneration, disturbing hippocampal neuronal connectivity, and cognitive function. Therefore, necroptosis inhibition constitutes a potential geroprotective strategy to treat age-related disabilities associated with memory impairment and cognitive decline.

Carotid Body-Mediated Chemoreflex Function in Aging and the Role of Receptor-Interacting Protein Kinase

Ventilatory impairment during aging has been linked to carotid body (CB) dysfunction. Anatomical/morphological studies evidenced CB degeneration and reductions in the number of CB chemoreceptor cells during aging. The mechanism(s) related to CB degeneration in aging remains elusive. Programmed cell death encompasses both apoptosis and necroptosis. Interestingly, necroptosis can be driven by molecular pathways related to low-grade inflammation, one hallmark of the aging process. Accordingly, we hypothesized that necrotic cell death dependent on receptor-interacting protein kinase-3 (RIPK3) may contribute, at least in part, to impair CB function during aging. Adult (3 months) and aged (24 months) wild type (WT) and RIPK3^-/- mice were used to study chemoreflex function. Aging results in significant reductions in both the hypoxic (HVR) and hypercapnic ventilatory responses (HCVR). Adult RIPK3^-/- mice showed normal HVR and HCVR compared to adult WT mice. Remarkable, aged RIPK3^-/- mice displayed no reductions in HVR nor in HCVR. Indeed, chemoreflex responses obtained in aged RIPK3^-/- KO mice were undistinguishable from the ones obtained in adult WT mice. Lastly, we found high prevalence of breathing disorders during aging and this was absent in aged RIPK3^-/- mice. Together our results support a role for RIPK3-mediated necroptosis in CB dysfunction during aging.

Neuronal activity-dependent ATP enhances the pro-growth effect of repair Schwann cell extracellular vesicles by increasing their miRNA-21 loading

Functional recovery after peripheral nerve injuries is critically dependent on axonal regeneration. Several autonomous and non-cell autonomous processes regulate axonal regeneration, including the activation of a growth-associated transcriptional program in neurons and the reprogramming of differentiated Schwann cells (dSCs) into repair SCs (rSCs), triggering the secretion of neurotrophic factors and the activation of an inflammatory response. Repair Schwann cells also release pro-regenerative extracellular vesicles (EVs), but is still unknown whether EV secretion is regulated non-cell autonomously by the regenerating neuron. Interestingly, it has been described that nerve activity enhances axonal regeneration by increasing the secretion of neurotrophic factors by rSC, but whether this activity modulates pro-regenerative EV secretion by rSC has not yet been explored. Here, we demonstrate that neuronal activity enhances the release of rSC-derived EVs and their transfer to neurons. This effect is mediated by activation of P2Y receptors in SCs after activity-dependent ATP release from sensory neurons. Importantly, activation of P2Y in rSCs also increases the amount of miRNA-21 present in rSC-EVs. Taken together, our results demonstrate that neuron to glia communication by ATP-P2Y signaling regulates the content of SC-derived EVs and their transfer to axons, modulating axonal elongation in a non-cell autonomous manner.

Multiple events case-control study in a prospective cohort to identify systemic, cellular, and molecular biomarkers of obesity-induced accelerated aging in 30-years-olds: the ObAGE study protocol

Background

Aging is characterized by a progressive loss of capacities linked to fundamental alterations/damage in multiple cellular and molecular pathways. It is the most significant risk factor for all non-communicable diseases (NCDs). Another contributing factor to the rise in NCDs is obesity. It has been suggested that obesity not only accelerates the onset of metabolic imbalances but also decreases lifespan and impacts cellular and molecular processes in a manner similar to aging. Obesity might accelerate the pace of aging. Guided by a lifecourse approach, we will explore how exposure to obesity in critical developmental stages disrupt homeostatic resilience mechanisms that preserve physiological integrity, inducing an early expression of aging phenotypes. Also, we will determine whether exposure to early psychosocial adversity influences vulnerability to obesity as a risk factor for accelerated aging.

Methods

Multiple events case–control study embedded in a prospective cohort of Chileans at 30-31y, 50% females, of low- to-middle socioeconomic status, who participated in nutrition research since birth. At 23y, 25% had obesity and cardiometabolic risk was high. We will use a multi-layer approach including: anthropometric assessment; DXA scan for body composition; abdominal ultrasound of the liver; stool samples collection and sequencing of the ribosomal RNA 16S gene to characterize the gut microbiome; determination of age-related pro-inflammatory cytokynes and anti-inflammatory miokynes. For the first time in Chile, we will address age-related epigenetic changes using the Horvath´s epigenetic clock. In a subset we will conduct a controlled physical challenge to characterize physical resilience (autophagy).

Discussion

ObAGE is in an excellent position to: approach aging as a process whose expression involves multiple factors from the early stages of a person's life; understand how longitudinal changes in health trajectories impact the biological mechanisms of aging; identify potential resilience mechanisms that help prevent unhealthy aging. Because SLS participants are still young, our research setting combined with advanced scientific techniques may identify individuals or groups at risk of early onset health issues. Results from ObAGE may pave the way to address the contribution of obesity to aging through lifespan from cells to systems and might be instrumental to developing interventions to improve health span in the Chilean population.

Trial registration

The proposed study does not consider any health care intervention on human participants.

Aβ oligomers trigger necroptosis-mediated neurodegeneration via microglia activation in Alzheimer's disease

Alzheimer’s disease (AD) is a major adult-onset neurodegenerative condition with no available treatment. Compelling reports point amyloid-β (Aβ) as the main etiologic agent that triggers AD. Although there is extensive evidence of detrimental crosstalk between Aβ and microglia that contributes to neuroinflammation in AD, the exact mechanism leading to neuron death remains unknown. Using postmortem human AD brain tissue, we show that Aβ pathology is associated with the necroptosis effector pMLKL. Moreover, we found that the burden of Aβ oligomers (Aβo) correlates with the expression of key markers of necroptosis activation. Additionally, inhibition of necroptosis by pharmacological or genetic means, reduce neurodegeneration and memory impairment triggered by Aβo in mice. Since microglial activation is emerging as a central driver for AD pathogenesis, we then tested the contribution of microglia to the mechanism of Aβo-mediated necroptosis activation in neurons. Using an in vitro model, we show that conditioned medium from Aβo-stimulated microglia elicited necroptosis in neurons through activation of TNF-α signaling, triggering extensive neurodegeneration. Notably, necroptosis inhibition provided significant neuronal protection. Together, these findings suggest that Aβo-mediated microglia stimulation in AD contributes to necroptosis activation in neurons and neurodegeneration. As necroptosis is a druggable degenerative mechanism, our findings might have important therapeutic implications to prevent the progression of AD.

Enhanced Activity of Exportin-1/CRM1 in Neurons Contributes to Autophagy Dysfunction and Senescent Features in Old Mouse Brain

Brain aging is characterized by dysfunctional autophagy and cellular senescence, among other features. While autophagy can either promote or suppress cellular senescence in proliferating cells, in postmitotic cells, such as neurons, autophagy impairment promotes cellular senescence. CRM1 (exportin-1/XPO1) exports hundreds of nuclear proteins into the cytoplasm, including the transcription factors TFEB (the main inducer of autophagy and lysosomal biogenesis genes) and STAT3, another autophagy modulator. It appears that CRM1 is a modulator of aging-associated senescence and autophagy, because pharmacological inhibition of CRM1 improved autophagic degradation in flies, by increasing nuclear TFEB levels, and because enhanced CRM1 activity is mechanistically linked to senescence in fibroblasts from Hutchinson-Gilford progeria syndrome patients and old healthy individuals; furthermore, the exogenous overexpression of CRM1 induced senescence in normal fibroblasts. In this work, we tested the hypothesis that impaired autophagic flux during brain aging occurs due to CRM1 accumulation in the brain. We found that CRM1 levels and activity increased in the hippocampus and cortex during physiological aging, which resulted in a decrease of nuclear TFEB and STAT3. Consistent with an autophagic flux impairment, we observed accumulation of the autophagic receptor p62/SQSTM1 in neurons of old mice, which correlated with increased neuronal senescence. Using an in vitro model of neuronal senescence, we demonstrate that CRM1 inhibition improved autophagy flux and reduced SA-β-gal activity by restoring TFEB nuclear localization. Collectively, our data suggest that enhanced CRM1-mediated export of proteins during brain aging perturbs neuronal homeostasis, contributing to autophagy impairment, and neuronal senescence.

Dementia in Latin America: Paving the way toward a regional action plan

Across Latin American and Caribbean countries (LACs), the fight against dementia faces pressing challenges, such as heterogeneity, diversity, political instability, and socioeconomic disparities. These can be addressed more effectively in a collaborative setting that fosters open exchange of knowledge. In this work, the Latin American and Caribbean Consortium on Dementia (LAC-CD) proposes an agenda for integration to deliver a Knowledge to Action Framework (KtAF). First, we summarize evidence-based strategies (epidemiology, genetics, biomarkers, clinical trials, nonpharmacological interventions, networking, and translational research) and align them to current global strategies to translate regional knowledge into transformative actions. Then we characterize key sources of complexity (genetic isolates, admixture in populations, environmental factors, and barriers to effective interventions), map them to the above challenges, and provide the basic mosaics of knowledge toward a KtAF. Finally, we describe strategies supporting the knowledge creation stage that underpins the translational impact of KtAF.

GERO Cohort Protocol, Chile, 2017-2022: Community-based Cohort of Functional Decline in Subjective Cognitive Complaint elderly

BACKGROUND

With the global population aging and life expectancy increasing, dementia has turned a priority in the health care system. In Chile, dementia is one of the most important causes of disability in the elderly and the most rapidly growing cause of death in the last 20 years. Cognitive complaint is considered a predictor for cognitive and functional decline, incident mild cognitive impairment, and incident dementia. The GERO cohort is the Chilean core clinical project of the Geroscience Center for Brain Health and Metabolism (GERO). The objective of the GERO cohort is to analyze the rate of functional decline and progression to clinical dementia and their associated risk factors in a community-dwelling elderly with subjective cognitive complaint, through a population-based study. We also aim to undertake clinical research on brain ageing and dementia disorders, to create data and biobanks with the appropriate infrastructure to conduct other studies and facilitate to the national and international scientific community access to the data and samples for research.

METHODS

The GERO cohort aims the recruitment of 300 elderly subjects (> 70 years) from Santiago (Chile), following them up for at least 3 years. Eligible people are adults not diagnosed with dementia with subjective cognitive complaint, which are reported either by the participant, a proxy or both. Participants are identified through a household census. The protocol for evaluation is based on a multidimensional approach including socio-demographic, biomedical, psychosocial, neuropsychological, neuropsychiatric and motor assessments. Neuroimaging, blood and stool samples are also obtained. This multidimensional evaluation is carried out in a baseline and 2 follow-ups assessments, at 18 and 36 months. In addition, in months 6, 12, 24, and 30, a telephone interview is performed in order to keep contact with the participants and to assess general well-being.

DISCUSSION

Our work will allow us to determine multidimensional risks factors associated with functional decline and conversion to dementia in elderly with subjective cognitive complain. The aim of our GERO group is to establish the capacity to foster cutting edge and multidisciplinary research on aging in Chile including basic and clinical research.

TRIAL REGISTRATION

NCT04265482 in ClinicalTrials.gov. Registration Date: February 11, 2020. Retrospectively Registered.

Insulin-like growth factor 2 (IGF2) protects against Huntington's disease through the extracellular disposal of protein aggregates

Impaired neuronal proteostasis is a salient feature of many neurodegenerative diseases, highlighting alterations in the function of the endoplasmic reticulum (ER). We previously reported that targeting the transcription factor XBP1, a key mediator of the ER stress response, delays disease progression and reduces protein aggregation in various models of neurodegeneration. To identify disease modifier genes that may explain the neuroprotective effects of XBP1 deficiency, we performed gene expression profiling of brain cortex and striatum of these animals and uncovered insulin-like growth factor 2 (Igf2) as the major upregulated gene. Here, we studied the impact of IGF2 signaling on protein aggregation in models of Huntington's disease (HD) as proof of concept. Cell culture studies revealed that IGF2 treatment decreases the load of intracellular aggregates of mutant huntingtin and a polyglutamine peptide. These results were validated using induced pluripotent stem cells (iPSC)-derived medium spiny neurons from HD patients and spinocerebellar ataxia cases. The reduction in the levels of mutant huntingtin was associated with a decrease in the half-life of the intracellular protein. The decrease in the levels of abnormal protein aggregation triggered by IGF2 was independent of the activity of autophagy and the proteasome pathways, the two main routes for mutant huntingtin clearance. Conversely, IGF2 signaling enhanced the secretion of soluble mutant huntingtin species through exosomes and microvesicles involving changes in actin dynamics. Administration of IGF2 into the brain of HD mice using gene therapy led to a significant decrease in the levels of mutant huntingtin in three different animal models. Moreover, analysis of human postmortem brain tissue and blood samples from HD patients showed a reduction in IGF2 level. This study identifies IGF2 as a relevant factor deregulated in HD, operating as a disease modifier that buffers the accumulation of abnormal protein species.

Axonal Degeneration in AD: The Contribution of Aβ and Tau

Alzheimer's disease (AD) represents the most common age-related neurodegenerative disorder, affecting around 35 million people worldwide. Despite enormous efforts dedicated to AD research over decades, there is still no cure for the disease. Misfolding and accumulation of Aβ and tau proteins in the brain constitute a defining signature of AD neuropathology, and mounting evidence has documented a link between aggregation of these proteins and neuronal dysfunction. In this context, progressive axonal degeneration has been associated with early stages of AD and linked to Aβ and tau accumulation. As the axonal degeneration mechanism has been starting to be unveiled, it constitutes a promising target for neuroprotection in AD. A comprehensive understanding of the mechanism of axonal destruction in neurodegenerative conditions is therefore critical for the development of new therapies aimed to prevent axonal loss before irreversible neuronal death occurs in AD. Here, we review current evidence of the involvement of Aβ and tau pathologies in the activation of signaling cascades that can promote axonal demise.

Collateral Sprouting of Peripheral Sensory Neurons Exhibits a Unique Transcriptomic Profile

Peripheral nerve injuries result in motor and sensory dysfunction which can be recovered by compensatory or regenerative processes. In situations where axonal regeneration of injured neurons is hampered, compensation by collateral sprouting from uninjured neurons contributes to target reinnervation and functional recovery. Interestingly, this process of collateral sprouting from uninjured neurons has been associated with the activation of growth-associated programs triggered by Wallerian degeneration. Nevertheless, the molecular alterations at the transcriptomic level associated with these compensatory growth mechanisms remain to be fully elucidated. We generated a surgical model of partial sciatic nerve injury in mice to mechanistically study degeneration-induced collateral sprouting from spared fibers in the peripheral nervous system. Using next-generation sequencing and Ingenuity Pathway Analysis, we described the sprouting-associated transcriptome of uninjured sensory neurons and compare it with the activated by regenerating neurons. In vitro approaches were used to functionally assess sprouting gene candidates in the mechanisms of axonal growth. Using a novel animal model, we provide the first description of the sprouting transcriptome observed in uninjured sensory neurons after nerve injury. This collateral sprouting-associated transcriptome differs from that seen in regenerating neurons, suggesting a molecular program distinct from axonal growth. We further demonstrate that genetic upregulation of novel sprouting-associated genes activates a specific growth program in vitro, leading to increased neuronal branching. These results contribute to our understanding of the molecular mechanisms associated with collateral sprouting in vivo. The data provided here will therefore be instrumental in developing therapeutic strategies aimed at promoting functional recovery after injury to the nervous system.

Schwann cell reprogramming into repair cells increases miRNA-21 expression in exosomes promoting axonal growth

Functional recovery after peripheral nerve damage is dependent on the reprogramming of differentiated Schwann cells (dSCs) into repair Schwann cells (rSCs), which promotes axonal regeneration and tissue homeostasis. Transition into a repair phenotype requires expression of c-Jun and Sox2, which transcriptionally mediates inhibition of the dSC program of myelination and activates a non-cell-autonomous repair program, characterized by the secretion of neuronal survival and regenerative molecules, formation of a cellular scaffold to guide regenerating axons and activation of an innate immune response. Moreover, rSCs release exosomes that are internalized by peripheral neurons, promoting axonal regeneration. Here, we demonstrate that reprogramming of Schwann cells (SCs) is accompanied by a shift in the capacity of their secreted exosomes to promote neurite growth, which is dependent on the expression of c-Jun (also known as Jun) and Sox2 by rSCs. Furthermore, increased expression of miRNA-21 is responsible for the pro-regenerative capacity of rSC exosomes, which is associated with PTEN downregulation and PI3-kinase activation in neurons. We propose that modification of exosomal cargo constitutes another important feature of the repair program of SCs, contributing to axonal regeneration and functional recovery after nerve injury.

Metformin protects from oxaliplatin induced peripheral neuropathy in rats

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After oxaliplatin treatment rats developed mechanical and cold hyperalgesia.

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We observed intraepidermal nerve fiber degeneration, and mild spinal cord gliosis.

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Co treatment with Metformin could prevent all these pathological outcomes.

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This suggests metformin as a candidate drug to prevent oxaliplatin-induced neuropathy.

Oxaliplatin is a commonly used drug to treat cancer, extending the rate of disease-free survival by 20% in colorectal cancer. However, oxaliplatin induces a disabling form of neuropathy resulting in more than 60% of patients having to reduce or discontinue oxaliplatin, negatively impacting their chance of survival. Oxaliplatin-induced neuropathies are accompanied by degeneration of sensory fibers in the epidermis and hyperexcitability of sensory neurons. These morphological and functional changes have been associated with sensory symptoms such as dysesthesia, paresthesia and mechanical and cold allodynia. Various strategies have been proposed to prevent or treat oxaliplatin-induced neuropathies without success. The anti-diabetic drug metformin has been recently shown to exert neuroprotection in other chemotherapy-induced neuropathies, so here we aimed to test if metformin can prevent the development of oxaliplatin-induced neuropathy in a rat model of this condition. Animals treated with oxaliplatin developed significant intraepidermal fiber degeneration, a mild gliosis in the spinal cord, and mechanical and cold hyperalgesia. The concomitant use of metformin prevented degeneration of intraepidermal fibers, gliosis, and the altered sensitivity. Our evidence further supports metformin as a new approach to prevent oxaliplatin-induced neuropathy with a potential important clinical impact.

The necroptosis pathway and its role in age-related neurodegenerative diseases: will it open up new therapeutic avenues in the next decade?

INTRODUCTION

Necroptosis is a programmed form of necrotic cell death. Growing evidence demonstrates that necroptosis contributes to cell demise in different pathological conditions including age-dependent neurodegenerative diseases (NDs). These findings open new avenues for understanding the mechanisms of neuronal loss in NDs, which might eventually translate into novel therapeutic interventions.

AREAS COVERED

We reviewed key aspects of necroptosis, in health and disease, focusing on evidence demonstrating its involvement in the pathogenesis of age-related NDs. We then highlight the activation of this pathway in the mechanism of axonal degeneration. We searched on PubMed the literature regarding necroptosis published between 2008 and 2020 and reviewed all publications were necroptosis was studied in the context of age-related NDs.

EXPERT OPINION

Axonal loss and neuronal death are the ultimate consequences of NDs that translate into disease phenotypes. Targeting degenerative mechanisms of the neuron appears as a strategy that might cover a wide range of diseases. Thus, the participation of necroptosis as a common mediator of neuronal demise emerges as a promising target for therapeutic intervention. Considering evidence demonstrating that necroptosis mediates axonal degeneration, we propose and discuss the potential of targeting necroptosis-mediated axonal destruction as a strategy to tackle NDs before neuronal loss occurs.

The p75NTR neurotrophin receptor is required to organize the mature neuromuscular synapse by regulating synaptic vesicle availability

The coordinated movement of organisms relies on efficient nerve-muscle communication at the neuromuscular junction. After peripheral nerve injury or neurodegeneration, motor neurons and Schwann cells increase the expression of the p75NTR pan-neurotrophin receptor. Even though p75NTR targeting has emerged as a promising therapeutic strategy to delay peripheral neuronal damage progression, the effects of long-term p75NTR inhibition at the mature neuromuscular junction have not been elucidated. We performed quantitative neuroanathomical analyses of the neuromuscular junction in p75NTR null mice by laser confocal and electron microscopy, which were complemented with electromyography, locomotor tests, and pharmacological intervention studies. Mature neuromuscular synapses of p75NTR null mice show impaired postsynaptic organization and ultrastructural complexity, which correlate with altered synaptic function at the levels of nerve activity-induced muscle responses, muscle fiber structure, force production, and locomotor performance. Our results on primary myotubes and denervated muscles indicate that muscle-derived p75NTR does not play a major role on postsynaptic organization. In turn, motor axon terminals of p75NTR null mice display a strong reduction in the number of synaptic vesicles and active zones. According to the observed pre and postsynaptic defects, pharmacological acetylcholinesterase inhibition rescued nerve-dependent muscle response and force production in p75NTR null mice. Our findings revealing that p75NTR is required to organize mature neuromuscular junctions contribute to a comprehensive view of the possible effects caused by therapeutic attempts to target p75NTR.

Publisher Correction: Non-canonical function of IRE1α determines mitochondria-associated endoplasmic reticulum composition to control calcium transfer and bioenergetics

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

c-Jun N-terminal kinase (JNK)-dependent internalization and Rab5-dependent endocytic sorting mediate long-distance retrograde neuronal death induced by axonal BDNF-p75 signaling

During the development of the sympathetic nervous system, signals from tropomyosin-related kinase receptors (Trks) and p75 neurotrophin receptors (p75) compete to regulate survival and connectivity. During this process, nerve growth factor (NGF)- TrkA signaling in axons communicates NGF-mediated trophic responses in signaling endosomes. Whether axonal p75 signaling contributes to neuronal death and how signaling endosomes contribute to p75 signaling has not been established. Using compartmentalized sympathetic neuronal cultures (CSCGs) as a model, we observed that the addition of BDNF to axons increased the transport of p75 and induced death of sympathetic neurons in a dynein-dependent manner. In cell bodies, internalization of p75 required the activity of JNK, a downstream kinase mediating p75 death signaling in neurons. Additionally, the activity of Rab5, the key GTPase regulating early endosomes, was required for p75 death signaling. In axons, JNK and Rab5 were required for retrograde transport and death signaling mediated by axonal BDNF-p75 in CSCGs. JNK was also required for the proper axonal transport of p75-positive endosomes. Thus, our findings provide evidence that the activation of JNK by p75 in cell bodies and axons is required for internalization to a Rab5-positive signaling endosome and the further propagation of p75-dependent neuronal death signals.

Mitochondria and Calcium Regulation as Basis of Neurodegeneration Associated With Aging

Age is the main risk factor for the onset of neurodegenerative diseases. A decline of mitochondrial function has been observed in several age-dependent neurodegenerative diseases and may be a major contributing factor in their progression. Recent findings have shown that mitochondrial fitness is tightly regulated by Ca²⁺ signals, which are altered long before the onset of measurable histopathology hallmarks or cognitive deficits in several neurodegenerative diseases including Alzheimer’s disease (AD), the most frequent cause of dementia. The transfer of Ca²⁺ from the endoplasmic reticulum (ER) to the mitochondria, facilitated by the presence of mitochondria-associated membranes (MAMs), is essential for several physiological mitochondrial functions such as respiration. Ca²⁺ transfer to mitochondria must be finely regulated because excess Ca²⁺ will disturb oxidative phosphorylation (OXPHOS), thereby increasing the generation of reactive oxygen species (ROS) that leads to cellular damage observed in both aging and neurodegenerative diseases. In addition, excess Ca²⁺ and ROS trigger the opening of the mitochondrial transition pore mPTP, leading to loss of mitochondrial function and cell death. mPTP opening probably increases with age and its activity has been associated with several neurodegenerative diseases. As Ca²⁺ seems to be the initiator of the mitochondrial failure that contributes to the synaptic deficit observed during aging and neurodegeneration, in this review, we aim to look at current evidence for mitochondrial dysfunction caused by Ca²⁺ miscommunication in neuronal models of neurodegenerative disorders related to aging, with special emphasis on AD.

A dual role for Integrin α6β4 in modulating hereditary neuropathy with liability to pressure palsies

Peripheral myelin protein 22 (PMP22) is a component of compact myelin in the peripheral nervous system. The amount of PMP22 in myelin is tightly regulated, and PMP22 over or under-expression cause Charcot-Marie-Tooth 1A (CMT1A) and Hereditary Neuropathy with Pressure Palsies (HNPP). Despite the importance of PMP22, its function remains largely unknown. It was reported that PMP22 interacts with the β4 subunit of the laminin receptor α6β4 integrin, suggesting that α6β4 integrin and laminins may contribute to the pathogenesis of CMT1A or HNPP. Here we asked if the lack of α6β4 integrin in Schwann cells influences myelin stability in the HNPP mouse model. Our data indicate that PMP22 and β4 integrin may not interact directly in myelinating Schwann cells, however, ablating β4 integrin delays the formation of tomacula, a characteristic feature of HNPP. In contrast, ablation of integrin β4 worsens nerve conduction velocities and non-compact myelin organization in HNPP animals. This study demonstrates that indirect interactions between an extracellular matrix receptor and a myelin protein influence the stability and function of myelinated fibers.

Axonal degeneration induced by glutamate-excitotoxicity is mediated by necroptosis

Neuronal excitotoxicity induced by glutamate leads to cell death and functional impairment in a variety of central nervous system pathologies. Glutamate-mediated excitotoxicity triggers neuronal apoptosis in the cell soma as well as degeneration of axons and dendrites by a process associated to calcium increase and mitochondrial dysfunction. Importantly, degeneration of axons initiated by diverse stimuli, including excitotoxicity, has been proposed as an important pathological event leading to functional impairment in neurodegenerative conditions. Here we demonstrate that excitotoxicity-induced axonal degeneration proceeds by a mechanism dependent on the necroptotic kinases RIPK1, RIPK3 and the necroptotic mediator MLKL. Inhibition of RIPK1, RIPK3 or MLKL prevent key steps in the axonal degeneration cascade including mitochondrial depolarization, the opening of the permeability transition pore and calcium dysregulation in the axon. Interestingly, the same excitotoxic stimuli lead to apoptosis in the cell soma, demonstrating the co-activation of two independent degenerative mechanisms in different compartments of the same cell. The identification of necroptosis as a key mechanism of axonal degeneration after excitotoxicity is an important initial step to develop novel therapeutic strategies for nervous system disorders.

Purification of Exosomes from Primary Schwann Cells, RNA Extraction, and Next-Generation Sequencing of Exosomal RNAs

Exosomes are small (30-150 nm) vesicles of endosomal origin secreted by most cell types. Exosomes contain proteins, lipids, and RNA species including microRNA, mRNA, rRNA, and long noncoding RNAs. The mechanisms associated with exosome synthesis and cargo loading are still poorly understood. A role for exosomes in intercellular communication has been reported in physiological and pathological conditions both in vitro and in vivo. Previous studies have suggested that Schwann cell-derived exosomes regulate neuronal functions, but the mechanisms are still unclear. Here, we describe protocols to establish rat neonatal Schwann cell cultures and to isolate exosomes from the conditioned medium of these cultures by differential ultracentrifugation. To analyze the RNA content of Schwann cell-derived exosomes, we detail protocols for RNA extraction and next-generation sequencing using miRNA and mRNA libraries. The protocol also includes RNA sequencing of Schwann cells, which allows the comparison between RNA content from cells and the secreted exosomes. Identification of RNAs present in Schwann cell-derived exosomes is a valuable tool to understand novel roles of Schwann cells in neuronal function in health and disease.

In Vitro Analysis of the Role of Schwann Cells on Axonal Degeneration and Regeneration Using Sensory Neurons from Dorsal Root Ganglia

Sensory neurons from dorsal root ganglion efficiently regenerate after peripheral nerve injuries. These neurons are widely used as a model system to study degenerative mechanisms of the soma and axons, as well as regenerative axonal growth in the peripheral nervous system. This chapter describes techniques associated to the study of axonal degeneration and regeneration using explant cultures of dorsal root ganglion sensory neurons in vitro in the presence or absence of Schwann cells. Schwann cells are extremely important due to their involvement in tissue clearance during axonal degeneration as well as their known pro-regenerative effect during regeneration in the peripheral nervous system. We describe methods to induce and study axonal degeneration triggered by axotomy (mechanical separation of the axon from its soma) and treatment with vinblastine (which blocks axonal transport), which constitute clinically relevant mechanical and toxic models of axonal degeneration. In addition, we describe three different methods to evaluate axonal regeneration using quantitative methods. These protocols constitute a valuable tool to analyze in vitro mechanisms associated to axonal degeneration and regeneration of sensory neurons and the role of Schwann cells in these processes.

Molecular analysis of axonal-intrinsic and glial-associated co-regulation of axon degeneration

Wallerian degeneration is an active program tightly associated with axonal degeneration, required for axonal regeneration and functional recovery after nerve damage. Here we provide a functional molecular foundation for our undertstanding of the complex non-cell autonomous role of glial cells in the regulation of axonal degeneration. To shed light on the complexity of the molecular machinery governing axonal degeneration we employ a multi-model, unbiased, in vivo approach combining morphological assesment and quantitative proteomics with in silico-based higher order functional clustering to genetically uncouple the intrinsic and extrinsic processes governing Wallerian degeneration. Highlighting a pivotal role for glial cells in the early stages fragmenting the axon by a cytokinesis-like process and a cell autonomous stage of axonal disintegration associated to mitochondrial dysfunction.

Disulfide cross-linked multimers of TDP-43 and spinal motoneuron loss in a TDP-43A315T ALS/FTD mouse model

Tar DNA binding protein 43 (TDP-43) is the principal component of ubiquitinated protein inclusions present in nervous tissue of most cases of both amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Previous studies described a TDP-43A315T transgenic mouse model that develops progressive motor dysfunction in the absence of protein aggregation or significant motoneuron loss, questioning its validity to study ALS. Here we have further characterized the course of the disease in TDP-43A315T mice using a battery of tests and biochemical approaches. We confirmed that TDP-43 mutant mice develop impaired motor performance, accompanied by progressive body weight loss. Significant differences were observed in life span between genders, where females survived longer than males. Histopathological analysis of the spinal cord demonstrated a significant motoneurons loss, accompanied by axonal degeneration, astrogliosis and microglial activation. Importantly, histopathological alterations observed in TDP-43 mutant mice were similar to some characteristic changes observed in mutant SOD1 mice. Unexpectedly, we identified the presence of different species of disulfide-dependent TDP-43 aggregates in cortex and spinal cord tissue. Overall, this study indicates that TDP-43A315T transgenic mice develop key features resembling key aspects of ALS, highlighting its relevance to study disease pathogenesis.

Axonal Degeneration during Aging and Its Functional Role in Neurodegenerative Disorders

Aging constitutes the main risk factor for the development of neurodegenerative diseases. This represents a major health issue worldwide that is only expected to escalate due to the ever-increasing life expectancy of the population. Interestingly, axonal degeneration, which occurs at early stages of neurodegenerative disorders (ND) such as Alzheimer's disease, Amyotrophic lateral sclerosis, and Parkinson's disease, also takes place as a consequence of normal aging. Moreover, the alteration of several cellular processes such as proteostasis, response to cellular stress and mitochondrial homeostasis, which have been described to occur in the aging brain, can also contribute to axonal pathology. Compelling evidence indicate that the degeneration of axons precedes clinical symptoms in NDs and occurs before cell body loss, constituting an early event in the pathological process and providing a potential therapeutic target to treat neurodegeneration before neuronal cell death. Although, normal aging and the development of neurodegeneration are two processes that are closely linked, the molecular basis of the switch that triggers the transition from healthy aging to neurodegeneration remains unrevealed. In this review we discuss the potential role of axonal degeneration in this transition and provide a detailed overview of the literature and current advances in the molecular understanding of the cellular changes that occur during aging that promote axonal degeneration and then discuss this in the context of ND.

Herpes Simplex Virus Type 1 Neuronal Infection Perturbs Golgi Apparatus Integrity through Activation of Src Tyrosine Kinase and Dyn-2 GTPase

Herpes simplex virus type 1 (HSV-1) is a ubiquitous pathogen that establishes a latent persistent neuronal infection in humans. The pathogenic effects of repeated viral reactivation in infected neurons are still unknown. Several studies have reported that during HSV-1 epithelial infection, the virus could modulate diverse cell signaling pathways remodeling the Golgi apparatus (GA) membranes, but the molecular mechanisms implicated, and the functional consequences to neurons is currently unknown. Here we report that infection of primary neuronal cultures with HSV-1 triggers Src tyrosine kinase activation and subsequent phosphorylation of Dynamin 2 GTPase, two players with a role in GA integrity maintenance. Immunofluorescence analyses showed that HSV-1 productive neuronal infection caused a scattered and fragmented distribution of the GA through the cytoplasm, contrasting with the uniform perinuclear distribution pattern observed in control cells. In addition, transmission electron microscopy revealed swollen cisternae and disorganized stacks in HSV-1 infected neurons compared to control cells. Interestingly, PP2, a selective inhibitor for Src-family kinases markedly reduced these morphological alterations of the GA induced by HSV-1 infection strongly supporting the possible involvement of Src tyrosine kinase. Finally, we showed that HSV-1 tegument protein VP11/12 is necessary but not sufficient to induce Dyn2 phosphorylation. Altogether, these results show that HSV-1 neuronal infection triggers activation of Src tyrosine kinase, phosphorylation of Dynamin 2 GTPase, and perturbation of GA integrity. These findings suggest a possible neuropathogenic mechanism triggered by HSV-1 infection, which could involve dysfunction of the secretory system in neurons and central nervous system.

Altered potassium channel distribution and composition in myelinated axons suppresses hyperexcitability following injury

Neuropathic pain following peripheral nerve injury is associated with hyperexcitability in damaged myelinated sensory axons, which begins to normalise over time. We investigated the composition and distribution of shaker-type-potassium channels (Kv1 channels) within the nodal complex of myelinated axons following injury. At the neuroma that forms after damage, expression of Kv1.1 and 1.2 (normally localised to the juxtaparanode) was markedly decreased. In contrast Kv1.4 and 1.6, which were hardly detectable in the naïve state, showed increased expression within juxtaparanodes and paranodes following injury, both in rats and humans. Within the dorsal root (a site remote from injury) we noted a redistribution of Kv1-channels towards the paranode. Blockade of Kv1 channels with α-DTX after injury reinstated hyperexcitability of A-fibre axons and enhanced mechanosensitivity. Changes in the molecular composition and distribution of axonal Kv1 channels, therefore represents a protective mechanism to suppress the hyperexcitability of myelinated sensory axons that follows nerve injury.

Applying extracellular vesicles based therapeutics in clinical trials - an ISEV position paper

Extracellular vesicles (EVs), such as exosomes and microvesicles, are released by different cell types and participate in physiological and pathophysiological processes. EVs mediate intercellular communication as cell-derived extracellular signalling organelles that transmit specific information from their cell of origin to their target cells. As a result of these properties, EVs of defined cell types may serve as novel tools for various therapeutic approaches, including (a) anti-tumour therapy, (b) pathogen vaccination, (c) immune-modulatory and regenerative therapies and (d) drug delivery. The translation of EVs into clinical therapies requires the categorization of EV-based therapeutics in compliance with existing regulatory frameworks. As the classification defines subsequent requirements for manufacturing, quality control and clinical investigation, it is of major importance to define whether EVs are considered the active drug components or primarily serve as drug delivery vehicles. For an effective and particularly safe translation of EV-based therapies into clinical practice, a high level of cooperation between researchers, clinicians and competent authorities is essential. In this position statement, basic and clinical scientists, as members of the International Society for Extracellular Vesicles (ISEV) and of the European Cooperation in Science and Technology (COST) program of the European Union, namely European Network on Microvesicles and Exosomes in Health and Disease (ME-HaD), summarize recent developments and the current knowledge of EV-based therapies. Aspects of safety and regulatory requirements that must be considered for pharmaceutical manufacturing and clinical application are highlighted. Production and quality control processes are discussed. Strategies to promote the therapeutic application of EVs in future clinical studies are addressed.

ApoER2 and Reelin are expressed in regenerating peripheral nerve and regulate Schwann cell migration by activating the Rac1 GEF protein, Tiam1

ApoER2 and its ligand Reelin participate in neuronal migration during development. Upon receptor binding, Reelin induces the proteolytic processing of ApoER2 as well as the activation of signaling pathway, including small Rho GTPases. Besides its presence in the central nervous system (CNS), Reelin is also secreted by Schwann cells (SCs), the glial cells of the peripheral nervous system (PNS). Reelin deficient mice (reeler) show decreased axonal regeneration in the PNS; however neither the presence of ApoER2 nor the role of the Reelin signaling pathway in the PNS have been evaluated. Interestingly SC migration occurs during PNS development and during injury-induced regeneration and involves activation of small Rho GTPases. Thus, Reelin-ApoER2 might regulate SC migration during axon regeneration in the PNS. Here we demonstrate the presence of ApoER2 in PNS. After sciatic nerve injury Reelin was induced and its receptor ApoER2 was proteolytically processed. In vitro, SCs express both Reelin and ApoER2 and Reelin induces SC migration. To elucidate the molecular mechanism underlying Reelin-dependent SC migration, we examined the involvement of Rac1, a conspicuous small GTPase family member. FRET experiments revealed that Reelin activates Rac1 at the leading edge of SCs. In addition, Tiam1, a major Rac1-specific GEF was required for Reelin-induced SC migration. Moreover, Reelin-induced SC migration was decreased after suppression of the polarity protein PAR3, consistent with its association to Tiam1. Even more interesting, we demonstrated that PAR3 binds preferentially to the full-length cytoplasmic tail of ApoER2 corresponding to the splice-variant containing the exon 19 that encodes a proline-rich insert and that ApoER2 was required for SC migration. Our study reveals a novel function for Reelin/ApoER2 in PNS, inducing cell migration of SCs, a process relevant for PNS development and regeneration.

Exosomes: mediators of communication in eukaryotes

In addition to the established mechanisms of intercellular signaling, a new way of communication has gained much attention in the last decade: communication mediated by exosomes. Exosomes are nanovesicles (with a diameter of 40-120 nm) secreted into the extracellular space by the multivesicular endosome after its outer membrane fuses with the plasma membrane. Once released, exosomes modulate the response of the recipient cells that recognize them. This indicates that exosomes operate in a specific manner and participate in the regulation of the target cell. Remarkably, exosomes occur from unicellular organisms to mammals, suggesting an evolutionarily conserved mechanism of communication. In this review we describe the cascade of exosome formation, intracellular traffic, secretion, and internalization by recipient cells, and review their most relevant effects. We also highlight important steps that are still poorly understood.

Sprouting of axonal collaterals after spinal cord injury is prevented by delayed axonal degeneration

After an incomplete spinal cord injury (SCI), partial recovery of locomotion is accomplished with time. Previous studies have established a functional link between extension of axon collaterals from spared spinal tracts and locomotor recovery after SCI, but the tissular signals triggering collateral sprouting have not been identified. Here, we investigated whether axonal degeneration after SCI contributes to the sprouting of collaterals from axons spared after injury. To this end, we evaluated collateral sprouting from BDA-labeled uninjured corticospinal axons after spinal cord hemisection (SCI(H)) in wild type (WT) mouse and Wld(S) mouse strains, which shows a significant delay in Wallerian degeneration after injury. After SCI(H), spared fibers of WT mice extend collateral sprouts to both intact and denervated sides of the spinal cord distant from the injury site. On the contrary, in the Wld(S) mice collateral sprouting from spared fibers was greatly reduced after SCI(H). Consistent with a role for collateral sprouting in functional recovery after SCI, locomotor recovery after SCI(H) was impaired in Wld(S) mice compared to WT animals. In conclusion, our results identify axonal degeneration as one of the triggers for collateral sprouting from the contralesional uninjured fibers after an SCI(H). These results open the path for identifying molecular signals associated with tissular changes after SCI that promotes collateral sprouting and functional recovery.

Control of dopaminergic neuron survival by the unfolded protein response transcription factor XBP1

Parkinson disease (PD) is characterized by the selective loss of dopaminergic neurons of the substantia nigra pars compacta (SNpc). Although growing evidence indicates that endoplasmic reticulum (ER) stress is a hallmark of PD, its exact contribution to the disease process is not well understood. Here we report that developmental ablation of X-Box binding protein 1 (XBP1) in the nervous system, a key regulator of the unfolded protein response (UPR), protects dopaminergic neurons against a PD-inducing neurotoxin. This survival effect was associated with a preconditioning condition that resulted from induction of an adaptive ER stress response in dopaminergic neurons of the SNpc, but not in other brain regions. In contrast, silencing XBP1 in adult animals triggered chronic ER stress and dopaminergic neuron degeneration. Supporting this finding, gene therapy to deliver an active form of XBP1 provided neuroprotection and reduced striatal denervation in animals injected with 6-hydroxydopamine. Our results reveal a physiological role of the UPR in the maintenance of protein homeostasis in dopaminergic neurons that may help explain the differential neuronal vulnerability observed in PD.

The p75 neurotrophin receptor evades the endolysosomal route in neuronal cells, favouring multivesicular bodies specialised for exosomal release

The p75 neurotrophin receptor (p75, also known as NGFR) is a multifaceted signalling receptor that regulates neuronal physiology, including neurite outgrowth, and survival and death decisions. A key cellular aspect regulating neurotrophin signalling is the intracellular trafficking of their receptors; however, the post-endocytic trafficking of p75 is poorly defined. We used sympathetic neurons and rat PC12 cells to study the mechanism of internalisation and post-endocytic trafficking of p75. We found that p75 internalisation depended on the clathrin adaptor protein AP2 and on dynamin. More surprisingly, p75 evaded the lysosomal route at the level of the early endosome, instead accumulating in two different types of endosomes, Rab11-positive endosomes and multivesicular bodies (MVBs) positive for CD63, a marker of the exosomal pathway. Consistently, depolarisation by KCl induced the liberation of previously endocytosed full-length p75 into the extracellular medium in exosomes. Thus, p75 defines a subpopulation of MVBs that does not mature to lysosomes and is available for exosomal release by neuronal cells.

Schwann cell-derived exosomes enhance axonal regeneration in the peripheral nervous system

Axonal regeneration in the peripheral nervous system is greatly supported by Schwann cells (SCs). After nerve injury, SCs dedifferentiate to a progenitor-like state and efficiently guide axons to their original target tissues. Contact and soluble factors participate in the crosstalk between SCs and axons during axonal regeneration. Here we show that dedifferentiated SCs secrete nano-vesicles known as exosomes which are specifically internalized by axons. Surprisingly, SC-derived exosomes markedly increase axonal regeneration in vitro and enhance regeneration after sciatic nerve injury in vivo. Exosomes shift the growth cone morphology to a pro-regenerating phenotype and decrease the activity of the GTPase RhoA, involved in growth cone collapse and axon retraction. Altogether, our work identifies a novel mechanism by which SCs communicate with neighboring axons during regenerative processes. We propose that SC exosomes represent an important mechanism by which these cells locally support axonal maintenance and regeneration after nerve damage.

Trehalose delays the progression of amyotrophic lateral sclerosis by enhancing autophagy in motoneurons

Amyotrophic lateral sclerosis (ALS) is a fatal motoneuron disease with no current effective treatment. Accumulation of abnormal protein inclusions containing SOD1, TARDBP, FUS, among other proteins, is a pathological hallmark of ALS. Autophagy is the major degradation pathway involved in the clearance of damaged organelles and protein aggregates. Although autophagy has been shown to efficiently degrade ALS-linked mutant protein in cell culture models, several studies suggest that autophagy impairment may also contribute to disease pathogenesis. In this report, we tested the potential use of trehalose, a disaccharide that induces MTOR-independent autophagy, in the development of experimental ALS. Administration of trehalose to mutant SOD1 transgenic mice significantly prolonged life span and attenuated the progression of disease signs. These effects were associated with decreased accumulation of SOD1 aggregates and enhanced motoneuron survival. The protective effects of trehalose were associated with increased autophagy levels in motoneurons. Cell culture experiments demonstrated that trehalose led to mutant SOD1 degradation by autophagy in NSC34 motoneuron cells and also protected primary motoneurons against the toxicity of conditioned media from mutant SOD1 transgenic astrocytes. At the mechanistic level, trehalose treatment led to a significant upregulation in the expression of key autophagy-related genes at the mRNA level including Lc3, Becn1, Sqstm1 and Atg5. Consistent with these changes, trehalose administration enhanced the nuclear translocation of FOXO1, an important transcription factor involved in the activation of autophagy in neurons. This study suggests a potential use of trehalose and enhancers of MTOR-independent autophagy for the treatment of ALS.

Diapause formation and downregulation of insulin-like signaling via DAF-16/FOXO delays axonal degeneration and neuronal loss

Axonal degeneration is a key event in the pathogenesis of neurodegenerative conditions. We show here that mec-4d triggered axonal degeneration of Caenorhabditis elegans neurons and mammalian axons share mechanistical similarities, as both are rescued by inhibition of calcium increase, mitochondrial dysfunction, and NMNAT overexpression. We then explore whether reactive oxygen species (ROS) participate in axonal degeneration and neuronal demise. C. elegans dauers have enhanced anti-ROS systems, and dauer mec-4d worms are completely protected from axonal degeneration and neuronal loss. Mechanistically, downregulation of the Insulin/IGF-1-like signaling (IIS) pathway protects neurons from degenerating in a DAF-16/FOXO–dependent manner and is related to superoxide dismutase and catalase-increased expression. Caloric restriction and systemic antioxidant treatment, which decrease oxidative damage, protect C. elegans axons from mec-4d-mediated degeneration and delay Wallerian degeneration in mice. In summary, we show that the IIS pathway is essential in maintaining neuronal homeostasis under pro-degenerative stimuli and identify ROS as a key intermediate of neuronal degeneration in vivo. Since axonal degeneration represents an early pathological event in neurodegeneration, our work identifies potential targets for therapeutic intervention in several conditions characterized by axonal loss and functional impairment.

Axonal degeneration and neuronal loss are currently considered crucial pathological factors in neurodegenerative diseases. Therefore, delaying or blocking these procesess is key for neuroprotection. In this work, we used an in vivo approach combining invertebrate (C. elegans) and vertebrate (mice) model systems to identify a novel and unexpected player in the mechanisms of axonal degeneration. Here, we demonstrate that both neuronal somas and axons degenerate through a step dependent on oxidative stress that can be efficiently delayed by genetic downregulation of a pathway controlling oxidative stress resistance. Impressively, we discovered that diapause formation, which is a state related to hibernating conditions, fully prevents neuronal degeneration. We uncovered new players in the degenerative mechanisms of neurons with relevance for several conditions associated to axonal degeneration, such as multiple sclerosis, motoneuron, and Parkinson diseases, offering novel potential targets for neuroprotection.

Transfer of vesicles from schwann cells to axons: a novel mechanism of communication in the peripheral nervous system

Schwann cells (SCs) are the glial component of the peripheral nervous system, with essential roles during development and maintenance of axons, as well as during regenerative processes after nerve injury. SCs increase conduction velocities by myelinating axons, regulate synaptic activity at presynaptic nerve terminals and are a source of trophic factors to neurons. Thus, development and maintenance of peripheral nerves are crucially dependent on local signaling between SCs and axons. In addition to the classic mechanisms of intercellular signaling, the possibility of communication through secreted vesicles has been poorly explored to date. Interesting recent findings suggest the occurrence of lateral transfer mediated by vesicles from glial cells to axons that could have important roles in axonal growth and axonal regeneration. Here, we review the role of vesicular transfer from SCs to axons and propose the advantages of this means in supporting neuronal and axonal maintenance and regeneration after nerve damage.

A BAX/BAK and cyclophilin D-independent intrinsic apoptosis pathway

Most intrinsic death signals converge into the activation of pro-apoptotic BCL-2 family members BAX and BAK at the mitochondria, resulting in the release of cytochrome c and apoptosome activation. Chronic endoplasmic reticulum (ER) stress leads to apoptosis through the upregulation of a subset of pro-apoptotic BH3-only proteins, activating BAX and BAK at the mitochondria. Here we provide evidence indicating that the full resistance of BAX and BAK double deficient (DKO) cells to ER stress is reverted by stimulation in combination with mild serum withdrawal. Cell death under these conditions was characterized by the appearance of classical apoptosis markers, caspase-9 activation, release of cytochrome c, and was inhibited by knocking down caspase-9, but insensitive to BCL-X(L) overexpression. Similarly, the resistance of BIM and PUMA double deficient cells to ER stress was reverted by mild serum withdrawal. Surprisingly, BAX/BAK-independent cell death did not require Cyclophilin D (CypD) expression, an important regulator of the mitochondrial permeability transition pore. Our results suggest the existence of an alternative intrinsic apoptosis pathway emerging from a cross talk between the ER and the mitochondria.

Mitochondria as a central sensor for axonal degenerative stimuli

Axonal degeneration is a major contributor to neuronal dysfunction in many neurological conditions and has additional roles in development. It can be triggered by divergent stimuli including mechanical, metabolic, infectious, toxic, hereditary and inflammatory stresses. Axonal mitochondria are an important convergence point as regulators of bioenergetic metabolism, reactive oxygen species (ROS), Ca²⁺ homeostasis and protease activation. The challenges likely to render axonal mitochondria more vulnerable than their cellular counterparts are reviewed, including axonal transport, replenishing nuclear-encoded proteins and maintenance of quality control, fusion and fission in locations remote from the cell body. The potential for mitochondria to act as a decision node in axon loss is considered, highlighting the need to understand the biology of axonal mitochondria and their contributions to degenerative mechanisms for novel therapeutic strategies.