Focal increases of axoplasmic Ca2+, aggregation of sodium-calcium exchanger, N-type Ca2+ channel, and actin define the sites of spheroids in axons undergoing oxidative stress

Therapeutic potential of human microglia transplantation in a chimeric model of CSF1R-related leukoencephalopathy

Microglia replacement strategies are increasingly being considered for the treatment of primary microgliopathies like adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP). However, available mouse models fail to recapitulate the diverse neuropathologies and reduced microglia numbers observed in patients. In this study, we generated a xenotolerant mouse model lacking the fms-intronic regulatory element (FIRE) enhancer within Csf1r, which develops nearly all the hallmark pathologies associated with ALSP. Remarkably, transplantation of human induced pluripotent stem cell (iPSC)-derived microglial (iMG) progenitors restores a homeostatic microglial signature and prevents the development of axonal spheroids, white matter abnormalities, reactive astrocytosis, and brain calcifications. Furthermore, transplantation of CRISPR-corrected ALSP-patient-derived iMG reverses pre-existing spheroids, astrogliosis, and calcification pathologies. Together with the accompanying study by Munro and colleagues, our results demonstrate the utility of FIRE mice to model ALSP and provide compelling evidence that iMG transplantation could offer a promising new therapeutic strategy for ALSP and perhaps other microglia-associated neurological disorders.

Identifying the Phenotypes of Diffuse Axonal Injury Following Traumatic Brain Injury

Diffuse axonal injury (DAI) is a significant feature of traumatic brain injury (TBI) across all injury severities and is driven by the primary mechanical insult and secondary biochemical injury phases. Axons comprise an outer cell membrane, the axolemma which is anchored to the cytoskeletal network with spectrin tetramers and actin rings. Neurofilaments act as space-filling structural polymers that surround the central core of microtubules, which facilitate axonal transport. TBI has differential effects on these cytoskeletal components, with axons in the same white matter tract showing a range of different cytoskeletal and axolemma alterations with different patterns of temporal evolution. These require different antibodies for detection in post-mortem tissue. Here, a comprehensive discussion of the evolution of axonal injury within different cytoskeletal elements is provided, alongside the most appropriate methods of detection and their temporal profiles. Accumulation of amyloid precursor protein (APP) as a result of disruption of axonal transport due to microtubule failure remains the most sensitive marker of axonal injury, both acutely and chronically. However, a subset of injured axons demonstrate different pathology, which cannot be detected via APP immunoreactivity, including degradation of spectrin and alterations in neurofilaments. Furthermore, recent work has highlighted the node of Ranvier and the axon initial segment as particularly vulnerable sites to axonal injury, with loss of sodium channels persisting beyond the acute phase post-injury in axons without APP pathology. Given the heterogenous response of axons to TBI, further characterization is required in the chronic phase to understand how axonal injury evolves temporally, which may help inform pharmacological interventions.

Non-prophylactic resveratrol-mediated protection of neurite integrity under chronic hypoxia is associated with reduction of Cav1.2 channel expression and calcium overloading

Cellular hypoxia is a major cause of oxidative stress, culminating in neuronal damage in neurodegenerative diseases. Numerous ex vivo studies have implicated that hypoxia episodes leading to disruption of Ca²⁺ homeostasis and redox status contribute to the progression of various neuropathologies and cell death. Isolation and maintenance of primary cell culture being cost-intensive, the details of the time course relationship between Ca²⁺ overload, L-type Ca²⁺ channel function, and neurite retraction under chronic and long-term hypoxia remain undefined. In order to explore the effect of oxidative stress and Ca²⁺ overload on neurite length, first, we developed a 5-day-long neurite outgrowth model using N2a cell line. Second, we propose a chronic hypoxia model to investigate the modulation of the L-type Ca²⁺ channel (Cav1.2) and oxidative resistance gene (OXR1) expression level during the process of neurite retraction and neuronal damage over 32 h. Thirdly, we developed a framework for quantitative analysis of cytosolic Ca²⁺, superoxide formation, neurite length, and constriction formation in individual cells using live imaging that provides an understanding of molecular targets. Our findings suggest that an increase in cytosolic Ca²⁺ is a feature of an early phase of hypoxic stress. Further, we demonstrate that augmentation in the L-type channel leads to amplification in Ca²⁺ overload, ROS accumulation, and a reduction in neurite length during the late phase of hypoxic stress. Next, we demonstrated that non-prophylactic treatment of resveratrol leads to the reduction of calcium overloading under chronic hypoxia via lowering of L-type channel expression. Finally, we demonstrate that resveratrol-mediated reduction of Cav1.2 channel and STAT3 expression are associated with retention of neurite integrity. The proposed in vitro model assumes significance in the context of drug designing and testing that demands monitoring of neurite length and constriction formations by imaging before animal testing.

NLRP3-Mediated Piezo1 Upregulation in ACC Inhibitory Parvalbumin-Expressing Interneurons Is Involved in Pain Processing after Peripheral Nerve Injury

The anterior cingulate cortex (ACC) is particularly critical for pain information processing. Peripheral nerve injury triggers neuronal hyper-excitability in the ACC and mediates descending facilitation to the spinal dorsal horn. The mechanically gated ion channel Piezo1 is involved in the transmission of pain information in the peripheral nervous system. However, the pain-processing role of Piezo1 in the brain is unknown. In this work, we found that spared (sciatic) nerve injury (SNI) increased Piezo1 protein levels in inhibitory parvalbumin (PV)-expressing interneurons (PV-INs) but not in glutaminergic CaMKⅡ+ neurons, in the bilateral ACC. A reduction in the number of PV-INs but not in the number of CaMKⅡ+ neurons and a significant reduction in inhibitory synaptic terminals was observed in the SNI chronic pain model. Further, observation of morphological changes in the microglia in the ACC showed their activated amoeba-like transformation, with a reduction in process length and an increase in cell body area. Combined with the encapsulation of Piezo1-positive neurons by Iba1+ microglia, the loss of PV-INs after SNI might result from phagocytosis by the microglia. In cellular experiments, administration of recombinant rat TNF-α (rrTNF) to the BV2 cell culture or ACC neuron primary culture elevated the protein levels of Piezo1 and NOD-like receptor (NLR) family pyrin domain containing 3 (NLRP3). The administration of the NLRP3 inhibitor MCC950 in these cells blocked the rrTNF-induced expression of caspase-1 and interleukin-1β (key downstream factors of the activated NLRP3 inflammasome) in vitro and reversed the SNI-induced Piezo1 overexpression in the ACC and alleviated SNI-induced allodynia in vivo. These results suggest that NLRP3 may be the key factor in causing Piezo1 upregulation in SNI, promoting an imbalance between ACC excitation and inhibition by inducing the microglial phagocytosis of PV-INs and, thereby, facilitating spinal pain transmission.

Unraveling axonal mechanisms of traumatic brain injury

Axonal swellings (AS) are one of the neuropathological hallmark of axonal injury in several disorders from trauma to neurodegeneration. Current evidence proposes a role of perturbed Ca²⁺ homeostasis in AS formation, involving impaired axonal transport and focal distension of the axons. Mechanisms of AS formation, in particular moments following injury, however, remain unknown. Here we show that AS form independently from intra-axonal Ca²⁺ changes, which are required primarily for the persistence of AS in time. We further show that the majority of axonal proteins undergoing de/phosphorylation immediately following injury belong to the cytoskeleton. This correlates with an increase in the distance of the actin/spectrin periodic rings and with microtubule tracks remodeling within AS. Observed cytoskeletal rearrangements support axonal transport without major interruptions. Our results demonstrate that the earliest axonal response to injury consists in physiological adaptations of axonal structure to preserve function rather than in immediate pathological events signaling axonal destruction.

Modeling CSF-1 receptor deficiency diseases - how close are we?

The role of colony-stimulating factor-1 receptor (CSF-1R) in macrophage and organismal development has been extensively studied in mouse. Within the last decade, mutations in the CSF1R have been shown to cause rare diseases of both pediatric (Brain Abnormalities, Neurodegeneration, and Dysosteosclerosis, OMIM #618476) and adult (CSF1R-related leukoencephalopathy, OMIM #221820) onset. Here we review the genetics, penetrance, and histopathological features of these diseases and discuss to what extent the animal models of Csf1r deficiency currently available provide systems in which to study the underlying mechanisms involved.

Paranodal Axoglial Junctions, an Essential Component in Axonal Homeostasis

In vertebrates, a high density of voltage-gated Na⁺ channel at nodes of Ranvier and of voltage-gated K⁺ channel at juxtaparanodes is necessary for rapid propagation of action potential, that is, for saltatory conduction in myelinated axons. Myelin loops attach to the axonal membrane and form paranodal axoglial junctions (PNJs) at paranodes adjacent to nodes of Ranvier. There is growing evidence that the PNJs contribute to axonal homeostasis in addition to their roles as lateral fences that restrict the location of nodal axolemmal proteins for effective saltatory conduction. Perturbations of PNJs, as in specific PNJ protein knockouts as well as in myelin lipid deficient mice, result in internodal axonal alterations, even if their internodal myelin is preserved. Here we review studies showing that PNJs play crucial roles in the myelinated axonal homeostasis. The present evidence points to two functions in particular: 1) PNJs facilitate axonal transport of membranous organelles as well as cytoskeletal proteins; and 2) they regulate the axonal distribution of type 1 inositol 1,4,5-trisphosphate receptor (IP₃R1) in cerebellar Purkinje axons. Myelinated axonal homeostasis depends among others on the state of PNJs, and consequently, a better understanding of this dependency may contribute to the clarification of CNS disease mechanisms and the development of novel therapies.

Teriflunomide Preserves Neuronal Activity and Protects Mitochondria in Brain Slices Exposed to Oxidative Stress

Teriflunomide (TFN) limits relapses in relapsing–remitting multiple sclerosis (RRMS) by reducing lymphocytic proliferation through the inhibition of the mitochondrial enzyme dihydroorotate dehydrogenase (DHODH) and the subsequent modulation of de novo pyrimidine synthesis. Alterations of mitochondrial function as a consequence of oxidative stress have been reported during neuroinflammation. Previously, we showed that TFN prevents alterations of mitochondrial motility caused by oxidative stress in peripheral axons. Here, we aimed to validate TFN effects on mitochondria and neuronal activity in hippocampal brain slices, in which cellular distribution and synaptic circuits are largely preserved. TFN effects on metabolism and neuronal activity were investigated by assessing oxygen partial pressure and local field potential in acute slices. Additionally, we imaged mitochondria in brain slices from the transgenic Thy1-CFP/COX8A)S2Lich/J (mitoCFP) mice using two-photon microscopy. Although TFN could not prevent oxidative stress-related depletion of ATP, it preserved oxygen consumption and neuronal activity in CNS tissue during oxidative stress. Furthermore, TFN prevented mitochondrial shortening and fragmentation of puncta-shaped and network mitochondria during oxidative stress. Regarding motility, TFN accentuated the decrease in mitochondrial displacement and increase in speed observed during oxidative stress. Importantly, these effects were not associated with neuronal viability and did not lead to axonal damage. In conclusion, during conditions of oxidative stress, TFN preserves the functionality of neurons and prevents morphological and motility alterations of mitochondria.

Rebound effects of NCX3 pharmacological inhibition: A novel strategy to accelerate myelin formation in oligodendrocytes

The Na+/Ca2+ exchanger NCX3 is an important regulator of sodium and calcium homeostasis in oligodendrocyte lineage. To date, no information is available on the effects resulting from prolonged exposure to NCX3 blockers and subsequent drug washout in oligodendroglia. Here, we investigated, by means of biochemical, morphological and functional analyses, the pharmacological effects of the NCX3 inhibitor, the 5-amino-N-butyl-2-(4-ethoxyphenoxy)-benzamide hydrochloride (BED), on NCXs expression and activity, as well as intracellular [Na+]i and [Ca2+]i levels, during treatment and following drug washout both in human MO3.13 oligodendrocytes and rat primary oligodendrocyte precursor cells (OPCs). BED exposure antagonized NCX activity, induced OPCs proliferation and [Na+]i accumulation. By contrast, 2 days of BED washout after 4 days of treatment significantly upregulated low molecular weight NCX3 proteins, reversed NCX activity, and increased intracellular [Ca2+]i. This BED-free effect was accompanied by an upregulation of NCX3 expression in oligodendrocyte processes and accelerated expression of myelin markers in rat primary oligodendrocytes. Collectively, our findings show that the pharmacological inhibition of the NCX3 exchanger with BED blocker maybe followed by a rebound increase in NCX3 expression and reversal activity that accelerate myelin sheet formation in oligodendrocytes. In addition, they indicate that a particular attention should be paid to the use of NCX inhibitors for possible rebound effects, and suggest that further studies will be necessary to investigate whether selective pharmacological modulation of NCX3 exchanger may be exploited to benefit demyelination and remyelination in demyelinating diseases.

Axonal spheroids in neurodegeneration

Axonal spheroids are bubble-like biological features that form on most degenerating axons, yet little is known about their influence on degenerative processes. Their formation and growth has been observed in response to various degenerative triggers such as injury, oxidative stress, inflammatory factors, and neurotoxic molecules. They often contain cytoskeletal elements and organelles, and, depending on the pathological insult, can colocalize with disease-related proteins such as amyloid precursor protein (APP), ubiquitin, and motor proteins. Initial formation of axonal spheroids depends on the disruption of axonal and membrane tension governed by cytoskeleton structure and calcium levels. Shortly after spheroid formation, the engulfment signal phosphatidylserine (PS) is exposed on the outer leaflet of spheroid plasma membrane, suggesting an important role for axonal spheroids in phagocytosis and debris clearance during degeneration. Spheroids can grow until they rupture, allowing pro-degenerative factors to exit the axon into extracellular space and accelerating neurodegeneration. Though much remains to be discovered in this area, axonal spheroid research promises to lend insight into the etiologies of neurodegenerative disease, and may be an important target for therapeutic intervention. This review summarizes over 100 years of work, describing what is known about axonal spheroid structure, regulation and function.

Diffuse Axonal Injury: Clinical Prognostic Factors, Molecular Experimental Models and the Impact of the Trauma Related Oxidative Stress. An Extensive Review Concerning Milestones and Advances

Traumatic brain injury (TBI) is a condition burdened by an extremely high rate of morbidity and mortality and can result in an overall disability rate as high as 50% in affected individuals. Therefore, the importance of identifying clinical prognostic factors for diffuse axonal injury (DAI) in (TBI) is commonly recognized as critical. The aim of the present review paper is to evaluate the most recent contributions from the relevant literature in order to understand how each single prognostic factor determinates the severity of the clinical syndrome associated with DAI. The main clinical factors with an important impact on prognosis in case of DAI are glycemia, early GCS, the peripheral oxygen saturation, blood pressure, and time to recover consciousness. In addition, the severity of the lesion, classified on the ground of the cerebral anatomical structures involved after the trauma, has a strong correlation with survival after DAI. In conclusion, modern findings concerning the role of reactive oxygen species (ROS) and oxidative stress in DAI suggest that biomarkers such as GFAP, pNF-H, NF-L, microtubule associated protein tau, Aβ42, S-100β, NSE, AQP4, Drp-1, and NCX represent a possible critical target for future pharmaceutical treatments to prevent the damages caused by DAI.

Generators of Pressure-Evoked Currents in Vertebrate Outer Retinal Neurons

(1) Background: High-tension glaucoma damages the peripheral vision dominated by rods. How mechanosensitive channels (MSCs) in the outer retina mediate pressure responses is unclear. (2) Methods: Immunocytochemistry, patch clamp, and channel fluorescence were used to study MSCs in salamander photoreceptors. (3) Results: Immunoreactivity of transient receptor potential channel vanilloid 4 (TRPV4) was revealed in the outer plexiform layer, K⁺ channel TRAAK in the photoreceptor outer segment (OS), and TRPV2 in some rod OS disks. Pressure on the rod inner segment evoked sustained currents of three components: (A) the inward current at <-50 mV (I_pi), sensitive to Co²⁺; (B) leak outward current at ≥-80 mV (I_po), sensitive to intracellular Cs⁺ and ruthenium red; and (C) cation current reversed at ~10 mV (I_pc). Hypotonicity induced slow currents like I_pc. Environmental pressure and light increased the FM 1-43-identified open MSCs in the OS membrane, while pressure on the OS with internal Cs⁺ closed a Ca²⁺-dependent current reversed at ~0 mV. Rod photocurrents were thermosensitive and affected by MSC blockers. (4) Conclusions: Rods possess depolarizing (TRPV) and hyperpolarizing (K⁺) MSCs, which mediate mutually compensating currents between -50 mV and 10 mV, serve as an electrical cushion to minimize the impact of ocular mechanical stress.

N-acetylcysteine reduces brain injury after delayed hypoxemia following traumatic brain injury

Preclinical investigations into neuroprotective agents for traumatic brain injury (TBI) have shown promise when administered before or very early after experimental TBI. However clinical trials of therapeutics demonstrating preclinical efficacy for TBI have failed to replicate these results in humans, a lost in translation phenomenon. N-acetylcysteine (NAC) is a potent anti-oxidant with demonstrated efficacy in pre-clinical TBI when administered early after primary injury. Utilizing our clinically relevant mouse model, we hypothesized that NAC administration in a clinically relevant timeframe could improve the brain's resilience to the secondary insult of hypoxemia. NAC or vehicle administered daily starting 2 h prior to hypoxemia (24 h after controlled cortical impact) for 3 doses in male mice reduced short-term axonal injury and hippocampal neuronal loss. Six month behavioral assessments including novel object recognition, socialization, Barnes maze, and fear conditioning did not reveal performance differences between sham controls and injured mice receiving NAC or saline vehicle. At 7 months after injury, NAC administered mice had reduced hippocampal neuronal loss but no reduction in lesion volume. In summary, our preclinical trial to test the neuroprotective efficacy of NAC against a secondary hypoxic insult after TBI demonstrated short and long-term neuropathological evidence of neuroprotection but a lack of detectable differences in long-term behavioral assessments between sham controls and injured mice limits conclusions on its impact on long-term neurobehavioral outcomes.

Regulation of degenerative spheroids after injury

Neuronal injury leads to rapid, programmed disintegration of axons distal to the site of lesion. Much like other forms of axon degeneration (e.g. developmental pruning, toxic insult from neurodegenerative disorder), Wallerian degeneration associated with injury is preceded by spheroid formation along axons. The mechanisms by which injury leads to formation of spheroids and whether these spheroids have a functional role in degeneration remain elusive. Here, using neonatal mouse primary sympathetic neurons, we investigate the roles of players previously implicated in the progression of Wallerian degeneration in injury-induced spheroid formation. We find that intra-axonal calcium flux is accompanied by actin-Rho dependent growth of calcium rich axonal spheroids that eventually rupture, releasing material to the extracellular space prior to catastrophic axon degeneration. Importantly, after injury, Sarm1-/- and DR6-/-, but not Wlds (excess NAD+) neurons, are capable of forming spheroids that eventually rupture, releasing their contents to the extracellular space to promote degeneration. Supplementation of exogenous NAD+ or expressing WLDs suppresses Rho-dependent spheroid formation and degeneration in response to injury. Moreover, injured or trophically deprived Sarm1-/- and DR6-/-, but not Wlds neurons, are resistant to degeneration induced by conditioned media collected from wild-type axons after spheroid rupture. Taken together, these findings place Rho-actin and NAD+ upstream of spheroid formation and may suggest that other mediators of degeneration, such as DR6 and SARM1, mediate post-spheroid rupture events that lead to catastrophic axon disassembly.

Teriflunomide preserves peripheral nerve mitochondria from oxidative stress-mediated alterations

Mitochondrial dysfunction is a common pathological hallmark in various inflammatory and degenerative diseases of the central nervous system, including multiple sclerosis (MS). We previously showed that oxidative stress alters axonal mitochondria, limiting their transport and inducing conformational changes that lead to axonal damage. Teriflunomide (TFN), an oral immunomodulatory drug approved for the treatment of relapsing forms of MS, reversibly inhibits dihydroorotate dehydrogenase (DHODH). DHODH is crucial for de novo pyrimidine biosynthesis and is the only mitochondrial enzyme in this pathway, thus conferring a link between inflammation, mitochondrial activity and axonal integrity. Here, we investigated how DHODH inhibition may affect mitochondrial behavior in the context of oxidative stress. We employed a model of transected murine spinal roots, previously developed in our laboratory. Using confocal live imaging of axonal mitochondria, we showed that in unmanipulated axons, TFN increased significantly the mitochondria length without altering their transport features. In mitochondria challenged with 50 µM hydrogen peroxide (H₂O₂) to induce oxidative stress, the presence of TFN at 1 µM concentration was able to restore mitochondrial shape, motility, as well as mitochondrial oxidation potential to control levels. No effects were observed at 5 µM TFN, while some shape and motility parameters were restored to control levels at 50 µM TFN. Thus, our data demonstrate an undescribed link between DHODH and mitochondrial dynamics and point to a potential neuroprotective effect of DHODH inhibition in the context of oxidative stress-induced damage of axonal mitochondria.

Cleavage of human tau at Asp421 inhibits hyperphosphorylated tau induced pathology in a Drosophila model

Hyperphosphorylated and truncated tau variants are enriched in neuropathological aggregates in diseases known as tauopathies. However, whether the interaction of these posttranslational modifications affects tau toxicity as a whole remains unresolved. By expressing human tau with disease-related Ser/Thr residues to simulate hyperphosphorylation, we show that despite severe neurodegeneration in full-length tau, with the truncation at Asp421, the toxicity is ameliorated. Cytological and biochemical analyses reveal that hyperphosphorylated full-length tau distributes in the soma, the axon, and the axonal terminal without evident distinction, whereas the Asp421-truncated version is mostly restricted from the axonal terminal. This discrepancy is correlated with the fact that fly expressing hyperphosphorylated full-length tau, but not Asp421-cleaved one, develops axonopathy lesions, including axonal spheroids and aberrant actin accumulations. The reduced presence of hyperphosphorylated tau in the axonal terminal is corroborated with the observation that flies expressing Asp421-truncated variants showed less motor deficit, suggesting synaptic function is preserved. The Asp421 cleavage of tau is a proteolytic product commonly found in the neurofibrillary tangles. Our finding suggests the coordination of different posttranslational modifications on tau may have an unexpected impact on the protein subcellular localization and cytotoxicity, which may be valuable when considering tau for therapeutic purposes.

p75NTR and DR6 Regulate Distinct Phases of Axon Degeneration Demarcated by Spheroid Rupture

The regressive events associated with trophic deprivation are critical for sculpting a functional nervous system. After nerve growth factor withdrawal, sympathetic axons derived from male and female neonatal mice maintain their structural integrity for ∼18 h (latent phase) followed by a rapid and near unison disassembly of axons over the next 3 h (catastrophic phase). Here we examine the molecular basis by which axons transition from latent to catastrophic phases of degeneration following trophic withdrawal. Before catastrophic degeneration, we observed an increase in intra-axonal calcium. This calcium flux is accompanied by p75 neurotrophic factor receptor-Rho-actin-dependent expansion of calcium-rich axonal spheroids that eventually rupture, releasing their contents to the extracellular space. Conditioned media derived from degenerating axons are capable of hastening transition into the catastrophic phase of degeneration. We also found that death receptor 6, but not p75 neurotrophic factor receptor, is required for transition into the catastrophic phase in response to conditioned media but not for the intra-axonal calcium flux, spheroid formation, or rupture that occur toward the end of latency. Our results support the existence of an interaxonal degenerative signal that promotes catastrophic degeneration among trophically deprived axons.SIGNIFICANCE STATEMENT Developmental pruning shares several morphological similarities to both disease- and injury-induced degeneration, including spheroid formation. The function and underlying mechanisms governing axonal spheroid formation, however, remain unclear. In this study, we report that axons coordinate each other's degeneration during development via axonal spheroid rupture. Before irreversible breakdown of the axon in response to trophic withdrawal, p75 neurotrophic factor receptor-RhoA signaling governs the formation and growth of spheroids. These spheroids then rupture, allowing exchange of contents ≤10 kDa between the intracellular and extracellular space to drive death receptor 6 and calpain-dependent catastrophic degeneration. This finding informs not only our understanding of regressive events during development but may also provide a rationale for designing new treatments toward myriad neurodegenerative disorders.

Inhibition of Piezo1 attenuates demyelination in the central nervous system

Piezo1 is a mechanosensitive ion channel that facilitates the translation of extracellular mechanical cues to intracellular molecular signaling cascades through a process termed, mechanotransduction. In the central nervous system (CNS), mechanically gated ion channels are important regulators of neurodevelopmental processes such as axon guidance, neural stem cell differentiation, and myelination of axons by oligodendrocytes. Here, we present evidence that pharmacologically mediated overactivation of Piezo1 channels negatively regulates CNS myelination. Moreover, we found that the peptide GsMTx4, an antagonist of mechanosensitive cation channels such as Piezo1, is neuroprotective and prevents chemically induced demyelination. In contrast, the positive modulator of Piezo1 channel opening, Yoda-1, induces demyelination and neuronal damage. Using an ex vivo murine-derived organotypic cerebellar slice culture model, we demonstrate that GsMTx4 attenuates demyelination induced by the cytotoxic lipid, psychosine. Importantly, we confirmed the potential therapeutic effects of GsMTx4 peptide in vivo by co-administering it with lysophosphatidylcholine (LPC), via stereotactic injection, into the cerebral cortex of adult mice. GsMTx4 prevented both demyelination and neuronal damage usually caused by the intracortical injection of LPC in vivo; a well-characterized model of focal demyelination. GsMTx4 also attenuated both LPC-induced astrocyte toxicity and microglial reactivity within the lesion core. Overall, our data suggest that pharmacological activation of Piezo1 channels induces demyelination and that inhibition of mechanosensitive channels, using GsMTx4, may alleviate the secondary progressive neurodegeneration often present in the latter stages of demyelinating diseases.

Dynamin and reverse-mode sodium calcium exchanger blockade confers neuroprotection from diffuse axonal injury

Mild traumatic brain injury (mTBI) is a frequently overlooked public health concern that is difficult to diagnose and treat. Diffuse axonal injury (DAI) is a common mTBI neuropathology in which axonal shearing and stretching induces breakdown of the cytoskeleton, impaired axonal trafficking, axonal degeneration, and cognitive dysfunction. DAI is becoming recognized as a principal neuropathology of mTBI with supporting evidence from animal model, human pathology, and neuroimaging studies. As mitochondrial dysfunction and calcium overload are critical steps in secondary brain and axonal injury, we investigated changes in protein expression of potential targets following mTBI using an in vivo controlled cortical impact model. We show upregulated expression of sodium calcium exchanger1 (NCX1) in the hippocampus and cortex at distinct time points post-mTBI. Expression of dynamin-related protein1 (Drp1), a GTPase responsible for regulation of mitochondrial fission, also changes differently post-injury in the hippocampus and cortex. Using an in vitro model of DAI previously reported by our group, we tested whether pharmacological inhibition of NCX1 by SN-6 and of dynamin1, dynamin2, and Drp1 by dynasore mitigates secondary damage. Dynasore and SN-6 attenuate stretch injury-induced swelling of axonal varicosities and mitochondrial fragmentation. In addition, we show that dynasore, but not SN-6, protects against H2O2-induced damage in an organotypic oxidative stress model. As there is currently no standard treatment to mitigate cell damage induced by mTBI and DAI, this work highlights two potential therapeutic targets for treatment of DAI in multiple models of mTBI and DAI.

Diffuse Axonal Injury and Oxidative Stress: A Comprehensive Review

Traumatic brain injury (TBI) is one of the world’s leading causes of morbidity and mortality among young individuals. TBI applies powerful rotational and translational forces to the brain parenchyma, which results in a traumatic diffuse axonal injury (DAI) responsible for brain swelling and neuronal death. Following TBI, axonal degeneration has been identified as a progressive process that starts with disrupted axonal transport causing axonal swelling, followed by secondary axonal disconnection and Wallerian degeneration. These modifications in the axonal cytoskeleton interrupt the axoplasmic transport mechanisms, causing the gradual gathering of transport products so as to generate axonal swellings and modifications in neuronal homeostasis. Oxidative stress with consequent impairment of endogenous antioxidant defense mechanisms plays a significant role in the secondary events leading to neuronal death. Studies support the role of an altered axonal calcium homeostasis as a mechanism in the secondary damage of axon, and suggest that calcium channel blocker can alleviate the secondary damage, as well as other mechanisms implied in the secondary injury, and could be targeted as a candidate for therapeutic approaches. Reactive oxygen species (ROS)-mediated axonal degeneration is mainly caused by extracellular Ca²⁺. Increases in the defense mechanisms through the use of exogenous antioxidants may be neuroprotective, particularly if they are given within the neuroprotective time window. A promising potential therapeutic target for DAI is to directly address mitochondria-related injury or to modulate energetic axonal energy failure.

Reversible Axonal Dystrophy by Calcium Modulation in Frataxin-Deficient Sensory Neurons of YG8R Mice

Friedreich’s ataxia (FRDA) is a peripheral neuropathy involving a loss of proprioceptive sensory neurons. Studies of biopsies from patients suggest that axonal dysfunction precedes the death of proprioceptive neurons in a dying-back process. We observed that the deficiency of frataxin in sensory neurons of dorsal root ganglia (DRG) of the YG8R mouse model causes the formation of axonal spheroids which retain dysfunctional mitochondria, shows alterations in the cytoskeleton and it produces impairment of axonal transport and autophagic flux. The homogenous distribution of axonal spheroids along the neurites supports the existence of continues focal damages. This lead us to propose for FRDA a model of distal axonopathy based on axonal focal damages. In addition, we observed the involvement of oxidative stress and dyshomeostasis of calcium in axonal spheroid formation generating axonal injury as a primary cause of pathophysiology. Axonal spheroids may be a consequence of calcium imbalance, thus we propose the quenching or removal extracellular Ca²⁺ to prevent spheroids formation. In our neuronal model, treatments with BAPTA and o-phenanthroline reverted the axonal dystrophy and the mitochondrial dysmorphic parameters. These results support the hypothesis that axonal pathology is reversible in FRDA by pharmacological manipulation of intracellular Ca²⁺ with Ca²⁺ chelators or metalloprotease inhibitors, preventing Ca²⁺-mediated axonal injury. Thus, the modulation of Ca²⁺ levels may be a relevant therapeutic target to develop early axonal protection and prevent dying-back neurodegeneration.

Polarity of varicosity initiation in central neuron mechanosensation

Little is known about mechanical regulation of morphological and functional polarity of central neurons. In this study, we report that mechanical stress specifically induces varicosities in the axons but not the dendrites of central neurons by activating TRPV4, a Ca2+/Na+-permeable mechanosensitive channel. This process is unexpectedly rapid and reversible, consistent with the formation of axonal varicosities in vivo induced by mechanical impact in a mouse model of mild traumatic brain injury. In contrast, prolonged stimulation of glutamate receptors induces varicosities in dendrites but not in axons. We further show that axonal varicosities are induced by persistent Ca2+ increase, disassembled microtubules (MTs), and subsequently reversible disruption of axonal transport, and are regulated by stable tubulin-only polypeptide, an MT-associated protein. Finally, axonal varicosity initiation can trigger action potentials to antidromically propagate to the soma in retrograde signaling. Therefore, our study demonstrates a new feature of neuronal polarity: axons and dendrites preferentially respond to physical and chemical stresses, respectively.

Traumatic Axonal Injury: Mechanisms and Translational Opportunities

Traumatic axonal injury (TAI) is an important pathoanatomical subgroup of traumatic brain injury (TBI) and a major driver of mortality and functional impairment. Experimental models have provided insights into the effects of mechanical deformation on the neuronal cytoskeleton and the subsequent processes that drive axonal injury. There is also increasing recognition that axonal or white matter loss may progress for years post-injury and represent one mechanistic framework for progressive neurodegeneration after TBI. Previous trials of novel therapies have failed to make an impact on clinical outcome, in both TBI in general and TAI in particular. Recent advances in understanding the cellular and molecular mechanisms of injury have the potential to translate into novel therapeutic targets.

Multiple therapeutic targets are emerging that offer the potential to reduce secondary brain injury at a cellular level. These include cytoskeletal and membrane stabilisation, control of calcium flux and calpain activation, optimisation of cellular energetics, and modulation of the inflammatory response.

Wallerian degeneration, as occurs following an axonal injury, is an active, cell-autonomous death pathway that involves failure of axonal transport to deliver key enzymes involved in NAD biosynthesis.

Chronic microglial activation occurs following traumatic brain injury (TBI) and may persist for decades afterwards. This ongoing response has been linked to long-term neurodegeneration, particularly of white matter tracts.

Phagoptosis is the process whereby physiologically stressed but otherwise viable neurons are phagocytosed by microglia in response to a range of eat-me signals induced by tissue injury.

Oxidative damage to mitochondria at the nodes of Ranvier precedes axon degeneration in ex vivo transected axons

Oxidative stress and mitochondrial dysfunction appear to contribute to axon degeneration in numerous neurological disorders. However, how these two processes interact to cause axonal damage-and how this damage is initiated-remains unclear. In this study we used transected motor axons from murine peripheral roots to investigate whether oxidative stress alters mitochondrial dynamics in myelinated axons. We show that the nodes of Ranvier are the initial sites of mitochondrial damage induced by oxidative stress. There, mitochondria became depolarized, followed by alterations of the external morphology and disruption of the cristae, along with reduced mitochondrial transport. These mitochondrial changes expanded from the nodes of Ranvier bidirectionally towards both internodes and eventually affected the entire mitochondrial population in the axon. Supplementing axonal bioenergetics by applying nicotinamide adenine dinucleotide and methyl pyruvate, rendered the mitochondria at the nodes of Ranvier resistant to these oxidative stress-induced changes. Importantly, this inhibition of mitochondrial damage protected the axons from degeneration. In conclusion, we present a novel ex vivo approach for monitoring mitochondrial dynamics within axons, which proved suitable for detecting mitochondrial changes upon exogenous application of oxidative stress. Our results indicate that the nodes of Ranvier are the site of initial mitochondrial damage in peripheral axons, and suggest that dysregulation of axonal bioenergetics plays a critical role in oxidative stress-triggered mitochondrial alterations and subsequent axonal injury. These novel insights into the mechanisms underlying axon degeneration may have implications for neurological disorders with a degenerative component.

Axonal transport plays a crucial role in mediating the axon-protective effects of NmNAT

Deficits in axonal transport are thought to contribute to the pathology of many neurodegenerative diseases. Expressing the slow Wallerian degeneration protein (Wld(S)) or related nicotinamide mononucleotide adenyltransferases (NmNATs) protects axons against damage from a broad range of insults, but the ability of these proteins to protect against inhibition of axonal transport has received little attention. We set out to determine whether these proteins can protect the axons of cultured hippocampal neurons from damage due to hydrogen peroxide or oxygen-glucose deprivation (OGD) and, in particular, whether they can reduce the damage that these agents cause to the axonal transport machinery. Exposure to these insults inhibited the axonal transport of both mitochondria and of the vesicles that carry axonal membrane proteins; this inhibition occurred hours before the first signs of axonal degeneration. Expressing a cytoplasmically targeted version of NmNAT1 (cytNmNAT1) protected the axons against both insults. It also reduced the inhibition of transport when cells were exposed to hydrogen peroxide and enhanced the recovery of transport following both insults. The protective effects of cytNmNAT1 depend on mitochondrial transport. When mitochondrial transport was inhibited, cytNmNAT1 was unable to protect axons against either insult. The protective effects of mitochondrially targeted NmNAT also were blocked by inhibiting mitochondrial transport. These results establish that NmNAT robustly protects the axonal transport system following exposure to OGD and reactive oxygen species and may offer similar protection in other disease models. Understanding how NmNAT protects the axonal transport system may lead to new strategies for neuroprotection in neurodegenerative diseases.