Calcium Influx through Plasma-Membrane Nanoruptures Drives Axon Degeneration in a Model of Multiple Sclerosis

Mitochondrial translocator-protein ligand etifoxine reduces pain symptoms and protects against motor dysfunction development following peripheral nerve injury in rats

Peripheral nerve injury enhances mitochondrial translocator protein (TSPO) expression in the spinal cord and dorsal root ganglia (DRG), which is associated with neuroinflammation and mitochondrial dysfunction contributing to chronic pain development. Here, we investigate the effect of TSPO ligand Etifoxine, on the development of chronic pain and motor dysfunction following sciatic nerve injury. Mechanical and thermal sensitivity, as well as motor function, were measured in rats before and after sciatic nerve crush (SNC). Rats were treated with the Etifoxine (50 mg/kg, twice daily) for one week. At the end of the experiment, RT-PCR and immunohistochemistry (IHC) were performed to assess mitochondrial stress and neuroinflammation. Additionally, high-resolution respirometry (O2k) was used to evaluate mitochondrial function in the spinal cord following mitochondrial permeability transition pore (mPTP) induction by Ca2+. Etifoxine treatment post-SNC alleviated mechanical and thermal hypersensitivity, as well as motor dysfunction in rats. In addition, Etifoxine treatment modulates neuroinflammation and mitochondrial stress. Specifically, we found a significant reduction in microglia presence and the transcription of pro-inflammatory cytokines (TNFα, IL-6, IL-1β) in the DRG and spinal cord of the SNC/etifoxine-treated group. Furthermore, Etifoxine treatment prevent the decline in mitochondrial respiration, including non-phosphorylation, ATP-linked respiration, and maximal respiration, after mPTP induction by Ca2+. Our findings suggest that TSPO-ligand Etifoxine protects against motor dysfunction and the development of chronic pain by reducing neuroinflammation and apoptosis in the DRG and spinal cord. Importantly, the beneficial effects of TSPO-ligands are reflected in the restoration of the mitochondrial function under challenging conditions.

Multiple sclerosis: what have we learned and can we still learn from electron microscopy

Multiple sclerosis (MS) is an inflammatory neurodegenerative disease marked by the formation of demyelinated lesions in the central nervous system. MS lesions can undergo remyelination, temporarily alleviating symptoms, but as the disease advances, remyelination becomes less effective. Beyond lesions, normal-appearing brain tissue exhibits subtle alterations, potentially indicating a broader, diffuse pathology and/or increased susceptibility to lesion formation. The pathology of MS varies between grey and white matter lesions and their normal-appearing regions, which most likely relates to their distinct cellular composition. Despite insights gained from MRI studies, serum and blood analyses, and post-mortem tissue examination, the molecular mechanisms driving MS lesion formation and persistent demyelination remain poorly understood. Exploring less conventional methods, such as electron microscopy (EM), may provide valuable new insights. EM offers detailed, nanometre-scale structural analysis that may enhance findings from immunohistochemistry and 'omics' approaches on MS brain tissue. Although earlier EM studies from before the 1990's provided some foundational data, advancements in EM technology now enable more comprehensive and detailed structural analysis. In this review we outline the pathogenesis of MS, summarize current knowledge of its ultrastructural features, and highlight how cutting-edge EM techniques could uncover new insights into pathological processes, including lesion formation, remyelination failure and diffuse pathology, which may aid therapeutic development.

Molecular Motors in Myelination and Their Misregulation in Disease

Molecular motors are cellular components involved in the intracellular transport of organelles and materials to ensure cell homeostasis. This is particularly relevant in neurons, where the synaptic components synthesized in the soma need to travel over long distances to their destination. They can walk on microtubules (kinesins and dyneins) or actin filaments (myosins), the major components of cell cytoskeleton. While kinesins mostly perform the anterograde transport of intracellular components toward the plus ends of microtubules located distally in cell processes, cytoplasmic dyneins allow the retrograde flux of intracellular cargo toward the minus ends of microtubules located at the cell soma. Axon myelination represents a major aspect of neuronal maturation and is essential for neuronal function, as it speeds up the transmission of electrical signals. Increasing evidence supports a role for molecular motors in the homeostatic control of myelination. This role includes the trafficking of myelin components along the processes of myelinating cells and local regulation of pathways that ensure axon wrapping. Dysfunctional control of the intracellular transport machinery has therefore been linked to several brain pathologies, including demyelinating diseases. These disorders include a broad spectrum of conditions characterized by pathological demyelination of axons within the nervous system, ultimately leading to axonal degeneration and neuronal death, with multiple sclerosis representing the most prevalent and studied condition. This review highlights the involvement of molecular motors in the homeostatic control of myelination. It also discusses studies that have yielded insights into the dysfunctional activity of molecular motors in the pathophysiology of multiple sclerosis.

Del Río Hortega's insights into oligodendrocytes: recent advances in subtype characterization and functional roles in axonal support and disease

Pío Del Río Hortega (1882-1945) was a giant of modern neuroscience and perhaps the most impactful member of Cajal's School. His contributions to clarifying the structure of the nervous system were key to understanding the brain beyond neurons. He uncovered microglia and oligodendrocytes, the latter until then named mesoglia. Most importantly, the characterization of oligodendroglia subtypes he made has stood the omics revolution that added molecular details relevant to comprehend their biological properties. Astounding as it may seem on today's eyes, he postulated a century ago that oligodendrocytes provide trophic support to axons, an idea that is now beyond doubt and under scrutiny as dysfunction at the axon-myelin unit is key to neurodegeneration. Here, we revised recent key advancements in oligodendrocyte biology that shed light on Hortega's ideas a century ago.

Identification and development of TRPM4 antagonists to counteract neuronal excitotoxicity

Neurodegeneration in central nervous system disorders is linked to dysregulated neuronal calcium. Direct inhibition of glutamate-induced neuronal calcium influx, particularly via N-methyl-D-aspartate receptors (NMDAR), has led to adverse effects and clinical trial failures. A more feasible approach is to modulate NMDAR activity or calcium signaling indirectly. In this respect, the calcium-activated non-selective cation channel transient receptor potential melastatin 4 (TRPM4) has been identified as a promising target. However, high affinity and specific antagonists are lacking. Here, we conducted high-throughput screening of a compound library to identify high affinity TRPM4 antagonists. This yielded five lead compound series with nanomolar half-maximal inhibitory concentration values. Through medicinal chemistry optimization of two series, we established detailed structure-activity relationships and inhibition of excitotoxicity in neurons. Moreover, we identified their potential binding site supported by electrophysiological measurements. These potent TRPM4 antagonists are promising drugs for treating neurodegenerative disorders and TRPM4-related pathologies, potentially overcoming previous therapeutic challenges.

Active Magnesium Boride/Alginate Hydrogels Rejuvenate Senescent Cells

The clearance of senescent cells may be detrimental to low cell density diseases, such as intervertebral disc degeneration (IVDD), and rejuvenating these cells presents a formidable obstacle. In this study, we investigate a mild-alkalization strategy employing magnesium boride-alginate (MB-ALG) hydrogels to rejuvenate senescent cells associated with age-related diseases. MB-ALG hydrogels proficiently ensnare senescent cells owing to their surface roughness. The hydrolysis of MB-ALG hydrogels liberates hydroxide ions (OH-), effecting a transition from an acidic microenvironment (pH ∼ 6.2) to a mildly alkaline state (pH ∼ 8.0), thereby fostering senescent cell proliferation via activation of the PI3K/Akt/mTOR pathway. Additionally, H2 aids in ROS clearance, which reduces cellular oxidative stress. And, Mg2+ rejuvenates senescent cells by inhibiting Ca2+ influx and fine-tuning the sirt1-p53 signaling pathways. Both in vitro and in vivo experiments conducted on rat intervertebral discs corroborate the sustained antisenescence and rejuvenation properties of MB-ALG hydrogels, with effects persisting for up to 12 weeks postoperation. These discoveries elucidate the role of mild-alkalization in dictating cellular destiny and provide key insights for addressing age-related diseases.

Actomyosin-II protects axons from degeneration induced by mild mechanical stress

Whether, to what extent, and how the axons in the central nervous system (CNS) can withstand sudden mechanical impacts remain unclear. By using a microfluidic device to apply controlled transverse mechanical stress to axons, we determined the stress levels that most axons can withstand and explored their instant responses at nanoscale resolution. We found mild stress triggers a highly reversible, rapid axon beading response, driven by actomyosin-II-dependent dynamic diameter modulations. This mechanism contributes to hindering the long-range spread of stress-induced Ca2+ elevations into non-stressed neuronal regions. Through pharmacological and molecular manipulations in vitro, we found that actomyosin-II inactivation diminishes the reversible beading process, fostering progressive Ca2+ spreading and thereby increasing acute axonal degeneration in stressed axons. Conversely, upregulating actomyosin-II activity prevents the progression of initial injury, protecting stressed axons from acute degeneration both in vitro and in vivo. Our study unveils the periodic actomyosin-II in axon shafts cortex as a novel protective mechanism, shielding neurons from detrimental effects caused by mechanical stress.

How does Nogo-A signalling influence mitochondrial function during multiple sclerosis pathogenesis?

Multiple sclerosis (MS) is a severe neurological disorder that involves inflammation in the brain, spinal cord and optic nerve with key disabling neuropathological outcomes being axonal damage and demyelination. When degeneration of the axo-glial union occurs, a consequence of inflammatory damage to central nervous system (CNS) myelin, dystrophy and death can lead to large membranous structures from dead oligodendrocytes and degenerative myelin deposited in the extracellular milieu. For the first time, this review covers mitochondrial mechanisms that may be operative during MS-related neurodegenerative changes directly activated during accumulating extracellular deposits of myelin associated inhibitory factors (MAIFs), that include the potent inhibitor of neurite outgrowth, Nogo-A. Axonal damage may occur when Nogo-A binds to and signals through its cognate receptor, NgR1, a multimeric complex, to initially stall axonal transport and limit the delivery of important growth-dependent cargo and subcellular organelles such as mitochondria for metabolic efficiency at sites of axo-glial disintegration as a consequence of inflammation. Metabolic efficiency in axons fails during active demyelination and progressive neurodegeneration, preceded by stalled transport of functional mitochondria to fuel axo-glial integrity.

STING orchestrates the neuronal inflammatory stress response in multiple sclerosis

Inflammation-induced neurodegeneration is a defining feature of multiple sclerosis (MS), yet the underlying mechanisms remain unclear. By dissecting the neuronal inflammatory stress response, we discovered that neurons in MS and its mouse model induce the stimulator of interferon genes (STING). However, activation of neuronal STING requires its detachment from the stromal interaction molecule 1 (STIM1), a process triggered by glutamate excitotoxicity. This detachment initiates non-canonical STING signaling, which leads to autophagic degradation of glutathione peroxidase 4 (GPX4), essential for neuronal redox homeostasis and thereby inducing ferroptosis. Both genetic and pharmacological interventions that target STING in neurons protect against inflammation-induced neurodegeneration. Our findings position STING as a central regulator of the detrimental neuronal inflammatory stress response, integrating inflammation with glutamate signaling to cause neuronal cell death, and present it as a tractable target for treating neurodegeneration in MS.

Modeling and simulation for prediction of multiple sclerosis progression

In light of extensive work that has created a wide range of techniques for predicting the course of multiple sclerosis (MS) disease, this paper attempts to provide an overview of these approaches and put forth an alternative way to predict the disease progression. For this purpose, the existing methods for estimating and predicting the course of the disease have been categorized into clinical, radiological, biological, and computational or artificial intelligence-based markers. Weighing the weaknesses and strengths of these prognostic groups is a profound method that is yet in need and works directly at the level of diseased connectivity. Therefore, we propose using the computational models in combination with established connectomes as a predictive tool for MS disease trajectories. The fundamental conduction-based Hodgkin-Huxley model emerged as promising from examining these studies. The advantage of the Hodgkin-Huxley model is that certain properties of connectomes, such as neuronal connection weights, spatial distances, and adjustments of signal transmission rates, can be taken into account. It is precisely these properties that are particularly altered in MS and that have strong implications for processing, transmission, and interactions of neuronal signaling patterns. The Hodgkin-Huxley (HH) equations as a point-neuron model are used for signal propagation inside a small network. The objective is to change the conduction parameter of the neuron model, replicate the changes in myelin properties in MS and observe the dynamics of the signal propagation across the network. The model is initially validated for different lengths, conduction values, and connection weights through three nodal connections. Later, these individual factors are incorporated into a small network and simulated to mimic the condition of MS. The signal propagation pattern is observed after inducing changes in conduction parameters at certain nodes in the network and compared against a control model pattern obtained before the changes are applied to the network. The signal propagation pattern varies as expected by adapting to the input conditions. Similarly, when the model is applied to a connectome, the pattern changes could give an insight into disease progression. This approach has opened up a new path to explore the progression of the disease in MS. The work is in its preliminary state, but with a future vision to apply this method in a connectome, providing a better clinical tool.

From progression to progress: The future of multiple sclerosis

Significant advances have been made in the diagnosis and treatment of multiple sclerosis in recent years yet challenges remain. The current classification of MS phenotypes according to disease activity and progression, for example, does not adequately reflect the underlying pathophysiological mechanisms that may be acting in an individual with MS at different time points. Thus, there is a need for clinicians to transition to a management approach based on the underlying pathophysiological mechanisms that drive disability in MS. A Canadian expert panel convened in January 2023 to discuss priorities for clinical discovery and scientific exploration that would help advance the field. Five key areas of focus included: identifying a mechanism-based disease classification system; developing biomarkers (imaging, fluid, digital) to identify pathologic processes; implementing a data-driven approach to integrate genetic/environmental risk factors, clinical findings, imaging and biomarker data, and patient-reported outcomes to better characterize the many factors associated with disability progression; utilizing precision-based treatment strategies to target different disease processes; and potentially preventing disease through Epstein-Barr virus (EBV) vaccination, counselling about environmental risk factors (e.g. obesity, exercise, vitamin D/sun exposure, smoking) and other measures. Many of the tools needed to meet these needs are currently available. Further work is required to validate emerging biomarkers and tailor treatment strategies to the needs of individual patients. The hope is that a more complete view of the individual's pathobiology will enable clinicians to usher in an era of truly personalized medicine, in which more informed treatment decisions throughout the disease course achieve better long-term outcomes.

Identification of compounds that cause axonal dieback without cytotoxicity in dorsal root ganglia explants and intervertebral disc cells with potential to treat pain via denervation

Low back pain, knee osteoarthritis, and cancer patients suffer from chronic pain. Aberrant nerve growth into intervertebral disc, knee, and tumors, are common pathologies that lead to these chronic pain conditions. Axonal dieback induced by capsaicin (Caps) denervation has been FDA-approved to treat painful neuropathies and knee osteoarthritis but with short-term efficacy and discomfort. Herein, we propose to evaluate pyridoxine (Pyr), vincristine sulfate (Vcr) and ionomycin (Imy) as axonal dieback compounds for denervation with potential to alleviate pain. Previous literature suggests Pyr, Vcr, and Imy can cause undesired axonal degeneration, but no previous work has evaluated axonal dieback and cytotoxicity on adult rat dorsal root ganglia (DRG) explants. Thus, we performed axonal dieback screening using adult rat DRG explants in vitro with Caps as a positive control and assessed cytotoxicity. Imy inhibited axonal outgrowth and slowed axonal dieback, while Pyr and Vcr at high concentrations produced significant reduction in axon length and robust axonal dieback within three days. DRGs treated with Caps, Vcr, or Imy had increased DRG cytotoxicity compared to matched controls, but overall cytotoxicity was minimal and at least 88% lower compared to lysed DRGs. Pyr did not lead to any DRG cytotoxicity. Further, neither Pyr nor Vcr triggered intervertebral disc cell death or affected cellular metabolic activity after three days of incubation in vitro. Overall, our findings suggest Pyr and Vcr are not toxic to DRGs and intervertebral disc cells, and there is potential for repurposing these compounds for axonal dieback compounds to cause local denervation and alleviate pain.

Fluorogenic Chemical Probes for Wash-free Imaging of Cell Membrane Damage in Ferroptosis, Necrosis, and Axon Injury

Selectively labeling cells with damaged membranes is needed not only for identifying dead cells in culture, but also for imaging membrane barrier dysfunction in pathologies in vivo. Most membrane permeability stains are permanently colored or fluorescent dyes that need washing to remove their non-uptaken extracellular background and reach good image contrast. Others are DNA-binding environment-dependent fluorophores, which lack design modularity, have potential toxicity, and can only detect permeabilization of cell volumes containing a nucleus (i.e., cannot delineate damaged volumes in vivo nor image non-nucleated cell types or compartments). Here, we develop modular fluorogenic probes that reveal the whole cytosolic volume of damaged cells, with near-zero background fluorescence so that no washing is needed. We identify a specific disulfonated fluorogenic probe type that only enters cells with damaged membranes, then is enzymatically activated and marks them. The esterase probe MDG1 is a reliable tool to reveal live cells that have been permeabilized by biological, biochemical, or physical membrane damage, and it can be used in multicolor microscopy. We confirm the modularity of this approach by also adapting it for improved hydrolytic stability, as the redox probe MDG2. We conclude by showing the unique performance of MDG probes in revealing axonal membrane damage (which DNA fluorogens cannot achieve) and in discriminating damage on a cell-by-cell basis in embryos in vivo. The MDG design thus provides powerful modular tools for wash-free in vivo imaging of membrane damage, and indicates how designs may be adapted for selective delivery of drug cargoes to these damaged cells: offering an outlook from selective diagnosis toward therapy of membrane-compromised cells in disease.

Diversity in homeostatic calcium set points predicts retinal ganglion cell survival following optic nerve injury in vivo

Retinal ganglion cell (RGC) degeneration drives vision loss in blinding conditions. RGC death is often triggered by axon degeneration in the optic nerve. Here, we study the contributions of dynamic and homeostatic Ca2+ levels to RGC death from axon injury. We find that axonal Ca2+ elevations from optic nerve injury do not propagate over distance or reach RGC somas, and acute and chronic Ca2+ dynamics do not affect RGC survival. Instead, we discover that baseline Ca2+ levels vary widely between RGCs and predict their survival after axon injury, and that lowering these levels reduces RGC survival. Further, we find that well-surviving RGC types have higher baseline Ca2+ levels than poorly surviving types. Finally, we observe considerable variation in the baseline Ca2+ levels of different RGCs of the same type, which are predictive of within-type differences in survival.

Targeting the TCA cycle can ameliorate widespread axonal energy deficiency in neuroinflammatory lesions

Inflammation in the central nervous system can impair the function of neuronal mitochondria and contributes to axon degeneration in the common neuroinflammatory disease multiple sclerosis (MS). Here we combine cell-type-specific mitochondrial proteomics with in vivo biosensor imaging to dissect how inflammation alters the molecular composition and functional capacity of neuronal mitochondria. We show that neuroinflammatory lesions in the mouse spinal cord cause widespread and persisting axonal ATP deficiency, which precedes mitochondrial oxidation and calcium overload. This axonal energy deficiency is associated with impaired electron transport chain function, but also an upstream imbalance of tricarboxylic acid (TCA) cycle enzymes, with several, including key rate-limiting, enzymes being depleted in neuronal mitochondria in experimental models and in MS lesions. Notably, viral overexpression of individual TCA enzymes can ameliorate the axonal energy deficits in neuroinflammatory lesions, suggesting that TCA cycle dysfunction in MS may be amendable to therapy.

Myelin insulation as a risk factor for axonal degeneration in autoimmune demyelinating disease

Axonal degeneration determines the clinical outcome of multiple sclerosis and is thought to result from exposure of denuded axons to immune-mediated damage. Therefore, myelin is widely considered to be a protective structure for axons in multiple sclerosis. Myelinated axons also depend on oligodendrocytes, which provide metabolic and structural support to the axonal compartment. Given that axonal pathology in multiple sclerosis is already visible at early disease stages, before overt demyelination, we reasoned that autoimmune inflammation may disrupt oligodendroglial support mechanisms and hence primarily affect axons insulated by myelin. Here, we studied axonal pathology as a function of myelination in human multiple sclerosis and mouse models of autoimmune encephalomyelitis with genetically altered myelination. We demonstrate that myelin ensheathment itself becomes detrimental for axonal survival and increases the risk of axons degenerating in an autoimmune environment. This challenges the view of myelin as a solely protective structure and suggests that axonal dependence on oligodendroglial support can become fatal when myelin is under inflammatory attack.

Cytotoxic CNS-associated T cells drive axon degeneration by targeting perturbed oligodendrocytes in PLP1 mutant mice

Myelin defects lead to neurological dysfunction in various diseases and in normal aging. Chronic neuroinflammation often contributes to axon-myelin damage in these conditions and can be initiated and/or sustained by perturbed myelinating glia. We have previously shown that distinct PLP1 mutations result in neurodegeneration that is largely driven by adaptive immune cells. Here we characterize CD8+ CNS-associated T cells in myelin mutants using single-cell transcriptomics and identify population heterogeneity and disease-associated changes. We demonstrate that early sphingosine-1-phosphate receptor modulation attenuates T cell recruitment and neural damage, while later targeting of CNS-associated T cell populations is inefficient. Applying bone marrow chimerism and utilizing random X chromosome inactivation, we provide evidence that axonal damage is driven by cytotoxic, antigen specific CD8+ T cells that target mutant myelinating oligodendrocytes. These findings offer insights into neural-immune interactions and are of translational relevance for neurological conditions associated with myelin defects and neuroinflammation.

Utility of MRI in Quantifying Tissue Injury in Cervical Spondylotic Myelopathy

Cervical spondylotic myelopathy (CSM) is a progressive disease that worsens over time if untreated. However, the rate of progression can vary among individuals and may be influenced by various factors, such as the age of the patients, underlying conditions, and the severity and location of the spinal cord compression. Early diagnosis and prompt treatment can help slow the progression of CSM and improve symptoms. There has been an increased use of magnetic resonance imaging (MRI) methods in diagnosing and managing CSM. MRI methods provide detailed images and quantitative structural and functional data of the cervical spinal cord and brain, allowing for an accurate evaluation of the extent and location of tissue injury. This review aims to provide an understanding of the use of MRI methods in interrogating functional and structural changes in the central nervous system in CSM. Further, we identified several challenges hindering the clinical utility of these neuroimaging methods.

Axolemmal nanoruptures arising from paranodal membrane injury induce secondary axon degeneration in murine Guillain-Barré syndrome

The major determinant of poor outcome in Guillain-Barré syndrome (GBS) is axonal degeneration. Pathways leading to primary axonal injury in the motor axonal variant are well established, whereas mechanisms of secondary axonal injury in acute inflammatory demyelinating polyneuropathy (AIDP) are unknown. We recently developed an autoantibody-and complement-mediated model of murine AIDP, in which prominent injury to glial membranes at the node of Ranvier results in severe disruption to paranodal components. Acutely, axonal integrity was maintained, but over time secondary axonal degeneration occurred. Herein, we describe the differential mechanisms underlying acute glial membrane injury and secondary axonal injury in this model. Ex vivo nerve-muscle explants were injured for either acute or extended periods with an autoantibody-and complement-mediated injury to glial paranodal membranes. This model was used to test several possible mechanisms of axon degeneration including calpain activation, and to monitor live axonal calcium signalling. Glial calpains induced acute disruption of paranodal membrane proteins in the absence of discernible axonal injury. Over time, we observed progressive axonal degeneration which was markedly attenuated by axon-specific calpain inhibition. Injury was unaffected by all other tested methods of protection. Trans-axolemmal diffusion of fluorescent proteins  and live calcium imaging studies indirectly demonstrated the presence of nanoruptures in the axon membrane. This study outlines one mechanism by which secondary axonal degeneration arises in the AIDP variant of GBS where acute paranodal loop injury is prominent. The data also support the development of calpain inhibitors to attenuate both primary and secondary axonal degeneration in GBS.

Neurons on tape: Automated Tape Collecting Ultramicrotomy-mediated volume EM for targeting neuropathology

In this chapter, we review Automated Tape Collecting Ultramicrotomy (ATUM), which, among other array tomography methods, substantially simplified large-scale volume electron microscopy (vEM) projects. vEM reveals biological structures at nanometer resolution in three dimensions and resolves ambiguities of two-dimensional representations. However, as the structures of interest-like disease hallmarks emerging from neuropathology-are often rare but the field of view is small, this can easily turn a vEM project into a needle in a haystack problem. One solution for this is correlated light and electron microscopy (CLEM), providing tissue context, dynamic and molecular features before switching to targeted vEM to hone in on the object's ultrastructure. This requires precise coordinate transfer between the two imaging modalities (e.g., by micro computed tomography), especially for block face vEM which relies on physical destruction of sections. With array tomography methods, serial ultrathin sections are collected into a tissue library, thus allowing storage of precious samples like human biopsies and enabling repetitive imaging at different resolution levels for an SEM-based search strategy. For this, ATUM has been developed to reliably collect serial ultrathin sections via a conveyor belt onto a plastic tape that is later mounted onto silicon wafers for serial scanning EM (SEM). The ATUM-SEM procedure is highly modular and can be divided into sample preparation, serial ultramicrotomy onto tape, mounting, serial image acquisition-after which the acquired image stacks can be used for analysis. Here, we describe the steps of this workflow and how ATUM-SEM enables targeting and high resolution imaging of specific structures. ATUM-SEM is widely applicable. To illustrate this, we exemplify the approach by reconstructions of focal pathology in an Alzheimer mouse model and CLEM of a specific cortical synapse.

Multiple sclerosis progression: time for a new mechanism-driven framework

Traditionally, multiple sclerosis has been categorised by distinct clinical descriptors-relapsing-remitting, secondary progressive, and primary progressive-for patient care, research, and regulatory approval of medications. Accumulating evidence suggests that the clinical course of multiple sclerosis is better considered as a continuum, with contributions from concurrent pathophysiological processes that vary across individuals and over time. The apparent evolution to a progressive course reflects a partial shift from predominantly localised acute injury to widespread inflammation and neurodegeneration, coupled with failure of compensatory mechanisms, such as neuroplasticity and remyelination. Ageing increases neural susceptibility to injury and decreases resilience. These observations encourage a new consideration of the course of multiple sclerosis as a spectrum defined by the relative contributions of overlapping pathological and reparative or compensatory processes. New understanding of key mechanisms underlying progression and measures to quantify progressive pathology will potentially have important and beneficial implications for clinical care, treatment targets, and regulatory decision-making.

Axons-on-a-chip for mimicking non-disruptive diffuse axonal injury underlying traumatic brain injury

Diffuse axonal injury (DAI) is the most severe pathological feature of traumatic brain injury (TBI). However, how primary axonal injury is induced by transient mechanical impacts remains unknown, mainly due to the low temporal and spatial resolution of medical imaging approaches. Here we established an axon-on-a-chip (AoC) model for mimicking DAI and monitoring instant cellular responses. Integrating computational fluid dynamics and microfluidic techniques, DAI was induced by injecting a precisely controlled micro-flux in the transverse direction. The clear correlation between the flow speed of injecting flux and the severity of DAI was elucidated. We next used the AoC to investigate the instant intracellular responses underlying DAI and found that the dynamic formation of focal axonal swellings (FAS) accompanied by Ca²⁺ surge occurs during the flux. Surprisingly, periodic axonal cytoskeleton disruption also occurs rapidly after the flux. These instant injury responses are spatially restricted to the fluxed axon, not affecting the overall viability of the neuron in the acute stage. Compatible with high-resolution live microscopy, the AoC provides a versatile system to identify early mechanisms underlying DAI, offering a platform for screening effective treatments to alleviate TBI.

Schwann cell nodal membrane disruption triggers bystander axonal degeneration in a Guillain-Barré syndrome mouse model

In Guillain-Barré syndrome (GBS), both axonal and demyelinating variants can be mediated by complement-fixing anti-GM1 ganglioside autoantibodies that target peripheral nerve axonal and Schwann cell (SC) membranes, respectively. Critically, the extent of axonal degeneration in both variants dictates long-term outcome. The differing pathomechanisms underlying direct axonal injury and the secondary bystander axonal degeneration following SC injury are unresolved. To investigate this, we generated glycosyltransferase-disrupted transgenic mice that express GM1 ganglioside either exclusively in neurons [GalNAcT-/--Tg(neuronal)] or glia [GalNAcT-/--Tg(glial)], thereby allowing anti-GM1 antibodies to solely target GM1 in either axonal or SC membranes, respectively. Myelinated-axon integrity in distal motor nerves was studied in transgenic mice exposed to anti-GM1 antibody and complement in ex vivo and in vivo injury paradigms. Axonal targeting induced catastrophic acute axonal disruption, as expected. When mice with GM1 in SC membranes were targeted, acute disruption of perisynaptic glia and SC membranes at nodes of Ranvier (NoRs) occurred. Following glial injury, axonal disruption at NoRs also developed subacutely, progressing to secondary axonal degeneration. These models differentiate the distinctly different axonopathic pathways under axonal and glial membrane targeting conditions, and provide insights into primary and secondary axonal injury, currently a major unsolved area in GBS research.

A new form of axonal pathology in a spinal model of neuromyelitis optica

Neuromyelitis optica is a chronic neuroinflammatory disease, which primarily targets astrocytes and often results in severe axon injury of unknown mechanism. Neuromyelitis optica patients harbour autoantibodies against the astrocytic water channel protein, aquaporin-4 (AQP4-IgG), which induce complement-mediated astrocyte lysis and subsequent axon damage. Using spinal in vivo imaging in a mouse model of such astrocytopathic lesions, we explored the mechanism underlying neuromyelitis optica-related axon injury. Many axons showed a swift and morphologically distinct 'pearls-on-string' transformation also readily detectable in human neuromyelitis optica lesions, which especially affected small calibre axons independently of myelination. Functional imaging revealed that calcium homeostasis was initially preserved in this 'acute axonal beading' state, ruling out disruption of the axonal membrane, which sets this form of axon injury apart from previously described forms of traumatic and inflammatory axon damage. Morphological, pharmacological and genetic analyses showed that AQP4-IgG-induced axon injury involved osmotic stress and ionic overload, but does not appear to use canonical pathways of Wallerian-like degeneration. Subcellular analysis demonstrated remodelling of the axonal cytoskeleton in beaded axons, especially local loss of microtubules. Treatment with the microtubule stabilizer epothilone, a putative therapy approach for traumatic and degenerative axonopathies, prevented axonal beading, while destabilizing microtubules sensitized axons for beading. Our results reveal a distinct form of immune-mediated axon pathology in neuromyelitis optica that mechanistically differs from known cascades of post-traumatic and inflammatory axon loss, and suggest a new strategy for neuroprotection in neuromyelitis optica and related diseases.

Acute axon damage and demyelination are mitigated by 4-aminopyridine (4-AP) therapy after experimental traumatic brain injury

Damage to long axons in white matter tracts is a major pathology in closed head traumatic brain injury (TBI). Acute TBI treatments are needed that protect against axon damage and promote recovery of axon function to prevent long term symptoms and neurodegeneration. Our prior characterization of axon damage and demyelination after TBI led us to examine repurposing of 4-aminopyridine (4-AP), an FDA-approved inhibitor of voltage-gated potassium (Kv) channels. 4-AP is currently indicated to provide symptomatic relief for patients with chronic stage multiple sclerosis, which involves axon damage and demyelination. We tested clinically relevant dosage of 4-AP as an acute treatment for experimental TBI and found multiple benefits in corpus callosum axons. This randomized, controlled pre-clinical study focused on the first week after TBI, when axons are particularly vulnerable. 4-AP treatment initiated one day post-injury dramatically reduced axon damage detected by intra-axonal fluorescence accumulations in Thy1-YFP mice of both sexes. Detailed electron microscopy in C57BL/6 mice showed that 4-AP reduced pathological features of mitochondrial swelling, cytoskeletal disruption, and demyelination at 7 days post-injury. Furthermore, 4-AP improved the molecular organization of axon nodal regions by restoring disrupted paranode domains and reducing Kv1.2 channel dispersion. 4-AP treatment did not resolve deficits in action potential conduction across the corpus callosum, based on ex vivo electrophysiological recordings at 7 days post-TBI. Thus, this first study of 4-AP effects on axon damage in the acute period demonstrates a significant decrease in multiple pathological hallmarks of axon damage after experimental TBI.

Hydrogen Ion Dynamics as the Fundamental Link between Neurodegenerative Diseases and Cancer: Its Application to the Therapeutics of Neurodegenerative Diseases with Special Emphasis on Multiple Sclerosis

The pH-related metabolic paradigm has rapidly grown in cancer research and treatment. In this contribution, this recent oncological perspective has been laterally assessed for the first time in order to integrate neurodegeneration within the energetics of the cancer acid-base conceptual frame. At all levels of study (molecular, biochemical, metabolic, and clinical), the intimate nature of both processes appears to consist of opposite mechanisms occurring at the far ends of a physiopathological intracellular pH/extracellular pH (pHi/pHe) spectrum. This wide-ranging original approach now permits an increase in our understanding of these opposite processes, cancer and neurodegeneration, and, as a consequence, allows us to propose new avenues of treatment based upon the intracellular and microenvironmental hydrogen ion dynamics regulating and deregulating the biochemistry and metabolism of both cancer and neural cells. Under the same perspective, the etiopathogenesis and special characteristics of multiple sclerosis (MS) is an excellent model for the study of neurodegenerative diseases and, utilizing this pioneering approach, we find that MS appears to be a metabolic disease even before an autoimmune one. Furthermore, within this paradigm, several important aspects of MS, from mitochondrial failure to microbiota functional abnormalities, are analyzed in depth. Finally, and for the first time, a new and integrated model of treatment for MS can now be advanced.

Mild Traumatic Brain Injury-Induced Disruption of the Blood-Brain Barrier Triggers an Atypical Neuronal Response

Mild TBI (mTBI), which affects 75% of TBI survivors or more than 50 million people worldwide each year, can lead to consequences including sleep disturbances, cognitive impairment, mood swings, and post-traumatic epilepsy in a subset of patients. To interrupt the progression of these comorbidities, identifying early pathological events is key. Recent studies have shown that microbleeds, caused by mechanical impact, persist for months after mTBI and are correlated to worse mTBI outcomes. However, the impact of mTBI-induced blood-brain barrier damage on neurons is yet to be revealed. We used a well-characterized mouse model of mTBI that presents with frequent and widespread but size-restricted damage to the blood-brain barrier to assess how neurons respond to exposure of blood-borne factors in this pathological context. We used immunohistochemistry and histology to assess the expression of neuronal proteins in excitatory and inhibitory neurons after mTBI. We observed that the expression of NeuN, Parvalbumin, and CamKII was lost within minutes in areas with blood-brain barrier disruption. Yet, the neurons remained alive and could be detected using a fluorescent Nissl staining even 6 months later. A similar phenotype was observed after exposure of neurons to blood-borne factors due to endothelial cell ablation in the absence of a mechanical impact, suggesting that entrance of blood-borne factors into the brain is sufficient to induce the neuronal atypical response. Changes in postsynaptic spines were observed indicative of functional changes. Thus, this study demonstrates That exposure of neurons to blood-borne factors causes a rapid and sustained loss of neuronal proteins and changes in spine morphology in the absence of neurodegeneration, a finding that is likely relevant to many neuropathologies.

Phagocytosis and self-destruction break down dendrites of Drosophila sensory neurons at distinct steps of Wallerian degeneration

After injury, severed dendrites and axons expose the "eat-me" signal phosphatidylserine (PS) on their surface while they break down. The degeneration of injured axons is controlled by a conserved Wallerian degeneration (WD) pathway, which is thought to activate neurite self-destruction through Sarm-mediated nicotinamide adenine dinucleotide (NAD+) depletion. While neurite PS exposure is known to be affected by genetic manipulations of NAD+, how the WD pathway coordinates both neurite PS exposure and self-destruction and whether PS-induced phagocytosis contributes to neurite breakdown in vivo remain unknown. Here, we show that in Drosophila sensory dendrites, PS exposure and self-destruction are two sequential steps of WD resulting from Sarm activation. Surprisingly, phagocytosis is the main driver of dendrite degeneration induced by both genetic NAD+ disruptions and injury. However, unlike neuronal Nmnat loss, which triggers PS exposure only and results in phagocytosis-dependent dendrite degeneration, injury activates both PS exposure and self-destruction as two redundant means of dendrite degeneration. Furthermore, the axon-death factor Axed is only partially required for self-destruction of injured dendrites, acting in parallel with PS-induced phagocytosis. Lastly, injured dendrites exhibit a unique rhythmic calcium-flashing that correlates with WD. Therefore, both NAD+-related general mechanisms and dendrite-specific programs govern PS exposure and self-destruction in injury-induced dendrite degeneration in vivo.

Live imaging reveals the cellular events downstream of SARM1 activation

SARM1 is an inducible NAD+ hydrolase that triggers axon loss and neuronal cell death in the injured and diseased nervous system. While SARM1 activation and enzyme function are well defined, the cellular events downstream of SARM1 activity but prior to axonal demise are much less well understood. Defects in calcium, mitochondria, ATP, and membrane homeostasis occur in injured axons, but the relationships among these events have been difficult to disentangle because prior studies analyzed large collections of axons in which cellular events occur asynchronously. Here, we used live imaging of mouse sensory neurons with single axon resolution to investigate the cellular events downstream of SARM1 activity. Our studies support a model in which SARM1 NADase activity leads to an ordered sequence of events from loss of cellular ATP, to defects in mitochondrial movement and depolarization, followed by calcium influx, externalization of phosphatidylserine, and loss of membrane permeability prior to catastrophic axonal self-destruction.

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.

The basic science of optic nerve regeneration

Diverse insults to the optic nerve result in partial to total vision loss as the axons of retinal ganglion cells are destroyed. In glaucoma, axons are injured at the optic nerve head; in other optic neuropathies, axons can be damaged along the entire visual pathway. In all cases, as mammals cannot regenerate injured central nervous system cells, once the axons are lost, vision loss is irreversible. However, much has been learned about how retinal ganglion cells respond to axon injuries, and many of these crucial discoveries offer hope for future regenerative therapies. Here we review the current understanding regarding the temporal progression of axonal degeneration. We summarize known survival and regenerative mechanisms in mammals, including specific signaling pathways, key transcription factors, and reprogramming genes. We cover mechanisms intrinsic to retinal ganglion cells as well as their interactions with myeloid and glial cell populations in the retina and optic nerve that affect survival and regeneration. Finally, we highlight some non-mammalian species that are able to regenerate their retinal ganglion cell axons after injury, as understanding these successful regenerative responses may be essential to the rational design of future clinical interventions to regrow the optic nerve. In the end, a combination of many different molecular and cellular interventions will likely be the only way to achieve functional recovery of vision and restore quality of life to millions of patients around the world.

Ultrastructural Characterization of Human Oligodendrocytes and Their Progenitor Cells by Pre-embedding Immunogold

Oligodendrocytes are the myelinating cells of the central nervous system. They provide trophic, metabolic, and structural support to neurons. In several pathologies such as multiple sclerosis (MS), these cells are severely affected and fail to remyelinate, thereby leading to neuronal death. The gold standard for studying remyelination is the g-ratio, which is measured by means of transmission electron microscopy (TEM). Therefore, studying the fine structure of the oligodendrocyte population in the human brain at different stages through TEM is a key feature in this field of study. Here we study the ultrastructure of oligodendrocytes, its progenitors, and myelin in 10 samples of human white matter using nine different markers of the oligodendrocyte lineage (NG2, PDGFRα, A2B5, Sox10, Olig2, BCAS1, APC-(CC1), MAG, and MBP). Our findings show that human oligodendrocytes constitute a very heterogeneous population within the human white matter and that its stages of differentiation present characteristic features that can be used to identify them by TEM. This study sheds light on how these cells interact with other cells within the human brain and clarify their fine characteristics from other glial cell types.

Plasma membrane integrity: implications for health and disease

Plasma membrane integrity is essential for cellular homeostasis. In vivo, cells experience plasma membrane damage from a multitude of stressors in the extra- and intra-cellular environment. To avoid lethal consequences, cells are equipped with repair pathways to restore membrane integrity. Here, we assess plasma membrane damage and repair from a whole-body perspective. We highlight the role of tissue-specific stressors in health and disease and examine membrane repair pathways across diverse cell types. Furthermore, we outline the impact of genetic and environmental factors on plasma membrane integrity and how these contribute to disease pathogenesis in different tissues.

Chronic Demyelination and Axonal Degeneration in Multiple Sclerosis: Pathogenesis and Therapeutic Implications

PURPOSE OF REVIEW

Multiple sclerosis (MS) is the most common demyelinating disease of the central nervous system (CNS). Inflammatory attacks in MS lead to both demyelination and axonal damage. However, due to incomplete remyelination most MS lesions remain chronically demyelinated. In parallel, there is axonal degeneration in the CNS of MS patients, contributing to progressive disability. There are currently no approved therapies that adequately restore myelin or protect axons from degeneration. In this review, we will discuss the pathophysiology of axonal loss and chronic demyelination in MS and how understanding this pathophysiology is leading to the development of new MS therapeutics.

RECENT FINDINGS

Ongoing research into the function of oligodendrocytes and myelin has revealed the importance of their relationship with neuronal health. Demyelination in MS leads to a number of pathophysiologic changes contributing to axonal generation. Among these are mitochondrial dysfunction, persistent neuroinflammation, and the effects of reactive oxygen and nitrogen species. With this information, we review currently approved and investigational therapies designed to restore lost or damaged myelin and protect against neuronal degeneration. The development of therapies to restore lost myelin and protect neurons is a promising avenue of investigation for the benefit of patients with MS.

Neuron-Oligodendrocyte Interactions in the Structure and Integrity of Axons

The myelination of axons by oligodendrocytes is a highly complex cell-to-cell interaction. Oligodendrocytes and axons have a reciprocal signaling relationship in which oligodendrocytes receive cues from axons that direct their myelination, and oligodendrocytes subsequently shape axonal structure and conduction. Oligodendrocytes are necessary for the maturation of excitatory domains on the axon including nodes of Ranvier, help buffer potassium, and support neuronal energy metabolism. Disruption of the oligodendrocyte-axon unit in traumatic injuries, Alzheimer's disease and demyelinating diseases such as multiple sclerosis results in axonal dysfunction and can culminate in neurodegeneration. In this review, we discuss the mechanisms by which demyelination and loss of oligodendrocytes compromise axons. We highlight the intra-axonal cascades initiated by demyelination that can result in irreversible axonal damage. Both the restoration of oligodendrocyte myelination or neuroprotective therapies targeting these intra-axonal cascades are likely to have therapeutic potential in disorders in which oligodendrocyte support of axons is disrupted.

Mechanisms and repair strategies for white matter degeneration in CNS injury and diseases

White matter degeneration is an important pathophysiological event of the central nervous system that is collectively characterized by demyelination, oligodendrocyte loss, axonal degeneration and parenchymal changes that can result in sensory, motor, autonomic and cognitive impairments. White matter degeneration can occur due to a variety of causes including trauma, neurotoxic exposure, insufficient blood flow, neuroinflammation, and developmental and inherited neuropathies. Regardless of the etiology, the degeneration processes share similar pathologic features. In recent years, a plethora of cellular and molecular mechanisms have been identified for axon and oligodendrocyte degeneration including oxidative damage, calcium overload, neuroinflammatory events, activation of proteases, depletion of adenosine triphosphate and energy supply. Extensive efforts have been also made to develop neuroprotective and neuroregenerative approaches for white matter repair. However, less progress has been achieved in this area mainly due to the complexity and multifactorial nature of the degeneration processes. Here, we will provide a timely review on the current understanding of the cellular and molecular mechanisms of white matter degeneration and will also discuss recent pharmacological and cellular therapeutic approaches for white matter protection as well as axonal regeneration, oligodendrogenesis and remyelination.

Phagocyte-mediated synapse removal in cortical neuroinflammation is promoted by local calcium accumulation

Cortical pathology contributes to chronic cognitive impairment of patients suffering from the neuroinflammatory disease multiple sclerosis (MS). How such gray matter inflammation affects neuronal structure and function is not well understood. In the present study, we use functional and structural in vivo imaging in a mouse model of cortical MS to demonstrate that bouts of cortical inflammation disrupt cortical circuit activity coincident with a widespread, but transient, loss of dendritic spines. Spines destined for removal show local calcium accumulations and are subsequently removed by invading macrophages or activated microglia. Targeting phagocyte activation with a new antagonist of the colony-stimulating factor 1 receptor prevents cortical synapse loss. Overall, our study identifies synapse loss as a key pathological feature of inflammatory gray matter lesions that is amenable to immunomodulatory therapy.

Enhancing mitochondrial activity in neurons protects against neurodegeneration in a mouse model of multiple sclerosis

While transcripts of neuronal mitochondrial genes are strongly suppressed in central nervous system inflammation, it is unknown whether this results in mitochondrial dysfunction and whether an increase of mitochondrial function can rescue neurodegeneration. Here, we show that predominantly genes of the electron transport chain are suppressed in inflamed mouse neurons, resulting in impaired mitochondrial complex IV activity. This was associated with post-translational inactivation of the transcriptional co-regulator proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α). In mice, neuronal overexpression of Ppargc1a, which encodes for PGC-1α, led to increased numbers of mitochondria, complex IV activity, and maximum respiratory capacity. Moreover, Ppargc1a-overexpressing neurons showed a higher mitochondrial membrane potential that related to an improved calcium buffering capacity. Accordingly, neuronal deletion of Ppargc1a aggravated neurodegeneration during experimental autoimmune encephalomyelitis, while neuronal overexpression of Ppargc1a ameliorated it. Our study provides systemic insights into mitochondrial dysfunction in neurons during inflammation and commends elevation of mitochondrial activity as a promising neuroprotective strategy.

Engine Failure in Axo-Myelinic Signaling: A Potential Key Player in the Pathogenesis of Multiple Sclerosis

Multiple Sclerosis (MS) is a complex and chronic disease of the central nervous system (CNS), characterized by both degenerative and inflammatory processes leading to axonal damage, demyelination, and neuronal loss. In the last decade, the traditional outside-in standpoint on MS pathogenesis, which identifies a primary autoimmune inflammatory etiology, has been challenged by a complementary inside-out theory. By focusing on the degenerative processes of MS, the axo-myelinic system may reveal new insights into the disease triggering mechanisms. Oxidative stress (OS) has been widely described as one of the means driving tissue injury in neurodegenerative disorders, including MS. Axonal mitochondria constitute the main energy source for electrically active axons and neurons and are largely vulnerable to oxidative injury. Consequently, axonal mitochondrial dysfunction might impair efficient axo-glial communication, which could, in turn, affect axonal integrity and the maintenance of axonal, neuronal, and synaptic signaling. In this review article, we argue that OS-derived mitochondrial impairment may underline the dysfunctional relationship between axons and their supportive glia cells, specifically oligodendrocytes and that this mechanism is implicated in the development of a primary cytodegeneration and a secondary pro-inflammatory response (inside-out), which in turn, together with a variably primed host's immune system, may lead to the onset of MS and its different subtypes.

Axon-Myelin Unit Blistering as Early Event in MS Normal Appearing White Matter

Objective

Multiple sclerosis (MS) is a chronic neuroinflammatory and neurodegenerative disease of unknown etiology. Although the prevalent view regards a CD4⁺‐lymphocyte autoimmune reaction against myelin at the root of the disease, recent studies propose autoimmunity as a secondary reaction to idiopathic brain damage. To gain knowledge about this possibility we investigated the presence of axonal and myelinic morphological alterations, which could implicate imbalance of axon‐myelin units as primary event in MS pathogenesis.

Methods

Using high resolution imaging histological brain specimens from patients with MS and non‐neurological/non‐MS controls, we explored molecular changes underpinning imbalanced interaction between axon and myelin in normal appearing white matter (NAWM), a region characterized by normal myelination and absent inflammatory activity.

Results

In MS brains, we detected blister‐like swellings formed by myelin detachment from axons, which were substantially less frequently retrieved in non‐neurological/non‐MS controls. Swellings in MS NAWM presented altered glutamate receptor expression, myelin associated glycoprotein (MAG) distribution, and lipid biochemical composition of myelin sheaths. Changes in tethering protein expression, widening of nodes of Ranvier and altered distribution of sodium channels in nodal regions of otherwise normally myelinated axons were also present in MS NAWM. Finally, we demonstrate a significant increase, compared with controls, in citrullinated proteins in myelin of MS cases, pointing toward biochemical modifications that may amplify the immunogenicity of MS myelin.

Interpretation

Collectively, the impaired interaction of myelin and axons potentially leads to myelin disintegration. Conceptually, the ensuing release of (post‐translationally modified) myelin antigens may elicit a subsequent immune attack in MS. ANN NEUROL 2021;89:711–725

Blockade of metabotropic glutamate receptor 5 attenuates axonal degeneration in 6-hydroxydopamine-induced model of Parkinson's disease

Although there are numerous strategies to counteract the death of dopaminergic neurons in Parkinson's disease (PD), there are currently no treatments that delay or prevent the disease course, indicating that early protective treatments are needed. Targeting axonal degeneration, a key initiating event in PD, is required to develop novel therapies; however, its underlying molecular mechanisms are not fully understood. Here, we studied axonal degeneration induced by 6-hydroxydopamine (6-OHDA) in vitro and in vivo. We found that metabotropic glutamate receptor 5 (mGluR5) expression increased during 6-OHDA-induced axonal degeneration in primary neurons and that blockade of mGluR5 by its antagonists 2-methyl-6-(phenylethynyl)-pyridine (MPEP) and 3-[(2-methyl-1, 3-thiazol-4-yl) ethynyl]-pyridine (MTEP) almost completely attenuated the degenerative process in vitro. Furthermore, a rapid increase in intra-axonal calcium levels following 6-OHDA treatment was visualized using a calcium-sensitive fluorescence probe and a calcium chelator prevented the axonal degenerative process induced by 6-OHDA in vitro, whereas application of the mGluR5 antagonist MPEP partially attenuated the increase in intra-axonal calcium. The screening of calcium targets revealed that calpain activation and an increase in phosphorylated extracellular signal-regulated kinase (p-ERK) were calcium dependent during 6-OHDA-induced axonal degeneration in vitro. Consistent with these in vitro findings, blockade of mGluR5 with MPEP attenuated the degeneration of dopaminergic axons induced by 6-OHDA injection into the striatum prior to soma death in the early stage of PD in an in vivo animal model. In addition, MPEP inhibited the increase in mGluR5 expression levels, calpain activation and the elevation of p-ERK in the striatum triggered by 6-OHDA injection in vivo. Taken together, these data identify an mGluR5-calcium-dependent cascade that causes axonal degeneration, and suggest that mGluR5 antagonists could provide effective therapy to prevent the disease process of PD.

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.

Fluorescent Arylphosphonic Acids: Synergic Interactions between Bone and the Fluorescent Core

Herein, we report the third generation of fluorescent probes (arylphosphonic acids) to target calcifications, particularly hydroxyapatite (HAP). In this study, we use highly conjugated porphyrin-based arylphosphonic acids and their diesters, namely 5,10,15,20-tetrakis[m-(diethoxyphosphoryl)phenyl]porphyrin (m-H8 TPPA-OEt8 ) and 5,10,15,20-tetrakis [m-phenylphosphonic acid]porphyrin (m-H8 TPPA), in comparison with their positional isomers 5,10,15,20-tetrakis[p-(diisopropoxyphosphoryl)phenyl]porphyrin (p-H8 TPPA-iPr8 ) and 5,10,15,20-tetrakis [p-phenylphosphonic acid]porphyrin (p-H8 TPPA), which have phosphonic acid units bonded to sp2 carbon atoms of the fluorescent core. The conjugation of the fluorescent core is thus extended to the (HAP) through sp2 -bonded -PO3 H2 units, which generates increased fluorescence upon HAP binding. The resulting fluorescent probes are highly sensitive towards the HAP in rat bone sections. The designed probes are readily taken up by cells. Due to the lower reported toxicity of (p-H8 TPPA), these probes could find applications in monitoring bone resorption or adsorption, or imaging vascular or soft tissue calcifications for breast cancer diagnosis etc.

Motor learning promotes remyelination via new and surviving oligodendrocytes

Oligodendrocyte loss in neurological disease leaves axons vulnerable to damage and degeneration, and activity-dependent myelination may represent an endogenous mechanism to improve remyelination following injury. Here, we report that while learning a forelimb reach task transiently suppresses oligodendrogenesis, it subsequently increases OPC differentiation, oligodendrocyte generation, and myelin sheath remodeling in the forelimb motor cortex. Immediately followingdemyelination, neurons exhibit hyperexcitability, learning is impaired, and behavioral intervention provides no benefit to remyelination. However, partial remyelination restores neuronal and behavioral function allowing learning to enhance oligodendrogenesis, remyelination of denuded axons, and the ability of surviving oligodendrocytes to generate new myelinsheaths. Previously considered controversial, we show that sheath generation by mature oligodendrocytes is not only possible but also increases myelin pattern preservation following demyelination, presenting a new target for therapeutic interventions. Together, our findings demonstrate that precisely-timed motor learning improves recovery from demyelinating injury via enhanced remyelination from new and surviving oligodendrocytes.

Design of Calcium-Binding Proteins to Sense Calcium

Calcium controls numerous biological processes by interacting with different classes of calcium binding proteins (CaBP’s), with different affinities, metal selectivities, kinetics, and calcium dependent conformational changes. Due to the diverse coordination chemistry of calcium, and complexity associated with protein folding and binding cooperativity, the rational design of CaBP’s was anticipated to present multiple challenges. In this paper we will first discuss applications of statistical analysis of calcium binding sites in proteins and subsequent development of algorithms to predict and identify calcium binding proteins. Next, we report efforts to identify key determinants for calcium binding affinity, cooperativity and calcium dependent conformational changes using grafting and protein design. Finally, we report recent advances in designing protein calcium sensors to capture calcium dynamics in various cellular environments.

Repeat intravital imaging of the murine spinal cord reveals degenerative and reparative responses of spinal axons in real-time following a contusive SCI

Spinal cord injury (SCI) induces a secondary degenerative response that causes the loss of spared axons and worsens neurological outcome. The complex molecular mechanisms that mediate secondary axonal degeneration remain poorly understood. To further our understanding of secondary axonal degeneration following SCI, we assessed the spatiotemporal dynamics of axonal spheroid and terminal bulb formation following a contusive SCI in real-time in vivo. Adult 6-8 week old Thy1YFP transgenic mice underwent a T12 laminectomy for acute imaging sessions or were implanted with a custom spinal cord imaging chamber for chronic imaging of the spinal cord. Two-photon excitation time-lapse microscopy was performed prior to a mild contusion SCI (30 kilodyne, IH Impactor) and at 1-4 h and 1-14 days post-SCI. We quantified the number of axonal spheroids, their size and distribution, the number of endbulbs, and axonal survival from 1 h to 14 days post-SCI. Our data reveal that the majority of axons underwent swelling and axonal spheroid formation acutely after SCI resulting in the loss of ~70% of axons by 1 day after injury. In agreement, the number of axonal spheroids rapidly increased at 1 h after SCI and remained significantly elevated up to 14 days after SCI. Furthermore, the distribution of axonal spheroids spread mediolaterally over time indicative of delayed secondary degenerative processes. In contrast, axonal endbulbs were relatively sparse and their numbers peaked at 1 day after injury. Intriguingly, axonal survival significantly increased at 7 and 14 days compared to 3 days after SCI revealing a potential endogenous axonal repair process that mirrors the known spontaneous functional recovery after SCI. In support, ~43% of tracked axonal spheroids resolved over the course of observation revealing their dynamic nature. Furthermore, axonal spheroids and endbulbs accumulated mitochondria and excessive tubulin polyglutamylation suggestive of disrupted axonal transport as a shared mechanism. Collectively, this study provides important insight into both degenerative and recoverable responses of axons following contusive SCI in real-time. Understanding how axons spontaneously recover after SCI will be an important avenue for future SCI research and may help guide future clinical trials.