The role of voltage-gated Ca2+ channels in anoxic injury of spinal cord white matter

Blockade of L-Type Ca2+ Channel Activity Alleviates Oligodendrocyte Pathology following Brain Injury in Male Rats

A growing body of studies suggests that Ca²⁺ signaling controls a variety of biological processes in brain elements. Activation of L-type voltage-operated Ca²⁺ channels (VOCCs) plays a role in the development of oligodendrocyte (OL) lineage loss, and indicates that the blocking of these channels may be an effective way to inhibit OL lineage cell loss. For this study, 10.5-day-old male Sprague-Dawley rats were used to generate cerebellar tissue slices. The slice tissues were cultured and randomly allocated to one of four groups (six each) and treated as follows: Group I, (sham control); Group II, 0.1% dimethyl sulfoxide (DMSO) only (vehicle control); Group III, injury (INJ); Group IV, (INJ and treatment with NIF). The injury was simulated by exposing the slice tissues to 20 min of oxygen-glucose deprivation (OGD). At 3 days post-treatment, the survival, apoptosis, and proliferation of the OL lineages were measured and compared. Results: In the INJ group, there was a decrease in mature myelin basic protein+ OLs (MBP+ OLs) and their precursors, NG2⁺ OPCs (Nerve-glia antigen 2+ oligodendrocyte precursor cell), compared with controls. A significant elevation was observed in the NG2⁺ OPCs and apoptotic MBP⁺ OLs as confirmed by a TUNEL assay. However, the cell proliferation rate was decreased in NG2⁺ OPCs. NIF increased OL survival as measured by apoptosis rate in both OL lineages and preserved the rate of proliferation in the NG2⁺ OPCs. Conclusions: Activation of L-type VOCCs may contribute to OL pathology in association with reduced mitosis of OPCs following brain injury as a strategy to treat demyelinating diseases.

Animal toxins: As an alternative therapeutic target following ischemic stroke condition

Globally, Ischemic stroke (IS) has become the second leading cause of mortality and chronic disability. The process of IS has triggered by the blockages of blood vessels to form clots in the brain which initiates multiple interactions with the key signaling pathways, counting excitotoxicity, acidosis, ionic imbalance, inflammation, oxidative stress, and neuronal dysfunction of cells, and ultimately cells going under apoptosis. Currently, FDA has approved only tissue plasminogen activator therapy, which is effective against IS with few limitations. However, the mechanism of excitotoxicity and acidosis has spurred the investigation of a potential candidate for IS therapy. Acid-sensing ion channels (ASICs) and Voltage-gated Ca²⁺ channels (VDCCs) get activated and disturb the brain's normal physiology. Animal toxins are novel inhibitors of ASICs and VDCCs channels and have provided neuroprotective insights into the pathophysiology of IS. This review will discuss the potential directions of translational ASICs and VDCCs inhibitors research for clinical therapies.

Direct Ryanodine Receptor-2 Knockout in Primary Afferent Fibers Modestly Affects Neurological Recovery after Contusive Spinal Cord Injury

Neuronal ryanodine receptors (RyR) release calcium from internal stores and play a key role in synaptic plasticity, learning, and memory. Dysregulation of RyR function contributes to neurodegeneration and negatively impacts neurological recovery after spinal cord injury (SCI). However, the individual role of RyR isoforms and the underlying mechanisms remain poorly understood. To determine whether RyR2 plays a direct role in axonal fate and functional recovery after SCI, we bred Advillin-Cre: tdTomato (Ai9) reporter mice with "floxed" RyR2 mice to directly knock out (KO) RyR2 function in dorsal root ganglion neurons and their spinal projections. Adult 6- to 8-week-old RyR2KO and littermate controls were subjected to a contusive SCI and their dorsal column axons were imaged in vivo using two-photon excitation microscopy. We found that direct RyR2KO in dorsal column primary afferents did not significantly alter secondary axonal degeneration after SCI. We next assessed behavioral recovery after SCI and found that direct RyR2KO in primary afferents worsened open-field locomotor scores (Basso Mouse Scale subscore) compared to littermate controls. However, both TreadScan™ gait analysis and overground kinematic gait analysis tests revealed subtle, but no fundamental, differences in gait patterns between the two groups after SCI. Subsequent removal of spared afferent fibers using a dorsal column crush revealed similar outcomes in both groups. Analysis of primary afferents at the lumbar (L3-L5) level similarly revealed no noticeable differences between groups. Together, our results support a modest contribution of dorsal column primary afferent RyR2 in neurological recovery after SCI.

Altered Expression of Ion Channels in White Matter Lesions of Progressive Multiple Sclerosis: What Do We Know About Their Function?

Despite significant advances in our understanding of the pathophysiology of multiple sclerosis (MS), knowledge about contribution of individual ion channels to axonal impairment and remyelination failure in progressive MS remains incomplete. Ion channel families play a fundamental role in maintaining white matter (WM) integrity and in regulating WM activities in axons, interstitial neurons, glia, and vascular cells. Recently, transcriptomic studies have considerably increased insight into the gene expression changes that occur in diverse WM lesions and the gene expression fingerprint of specific WM cells associated with secondary progressive MS. Here, we review the ion channel genes encoding K⁺, Ca²⁺, Na⁺, and Cl^- channels; ryanodine receptors; TRP channels; and others that are significantly and uniquely dysregulated in active, chronic active, inactive, remyelinating WM lesions, and normal-appearing WM of secondary progressive MS brain, based on recently published bulk and single-nuclei RNA-sequencing datasets. We discuss the current state of knowledge about the corresponding ion channels and their implication in the MS brain or in experimental models of MS. This comprehensive review suggests that the intense upregulation of voltage-gated Na⁺ channel genes in WM lesions with ongoing tissue damage may reflect the imbalance of Na⁺ homeostasis that is observed in progressive MS brain, while the upregulation of a large number of voltage-gated K⁺ channel genes may be linked to a protective response to limit neuronal excitability. In addition, the altered chloride homeostasis, revealed by the significant downregulation of voltage-gated Cl^- channels in MS lesions, may contribute to an altered inhibitory neurotransmission and increased excitability.

Animal Venom Peptides as a Treasure Trove for New Therapeutics Against Neurodegenerative Disorders

BACKGROUND

Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and cerebral ischemic stroke, impose enormous socio-economic burdens on both patients and health-care systems. However, drugs targeting these diseases remain unsatisfactory, and hence there is an urgent need for the development of novel and potent drug candidates.

METHODS

Animal toxins exhibit rich diversity in both proteins and peptides, which play vital roles in biomedical drug development. As a molecular tool, animal toxin peptides have not only helped clarify many critical physiological processes but also led to the discovery of novel drugs and clinical therapeutics.

RESULTS

Recently, toxin peptides identified from venomous animals, e.g. exenatide, ziconotide, Hi1a, and PcTx1 from spider venom, have been shown to block specific ion channels, alleviate inflammation, decrease protein aggregates, regulate glutamate and neurotransmitter levels, and increase neuroprotective factors.

CONCLUSION

Thus, components of venom hold considerable capacity as drug candidates for the alleviation or reduction of neurodegeneration. This review highlights studies evaluating different animal toxins, especially peptides, as promising therapeutic tools for the treatment of different neurodegenerative diseases and disorders.

Animal Toxins as Therapeutic Tools to Treat Neurodegenerative Diseases

Neurodegenerative diseases affect millions of individuals worldwide. So far, no disease-modifying drug is available to treat patients, making the search for effective drugs an urgent need. Neurodegeneration is triggered by the activation of several cellular processes, including oxidative stress, mitochondrial impairment, neuroinflammation, aging, aggregate formation, glutamatergic excitotoxicity, and apoptosis. Therefore, many research groups aim to identify drugs that may inhibit one or more of these events leading to neuronal cell death. Venoms are fruitful natural sources of new molecules, which have been relentlessly enhanced by evolution through natural selection. Several studies indicate that venom components can exhibit selectivity and affinity for a wide variety of targets in mammalian systems. For instance, an expressive number of natural peptides identified in venoms from animals, such as snakes, scorpions, bees, and spiders, were shown to lessen inflammation, regulate glutamate release, modify neurotransmitter levels, block ion channel activation, decrease the number of protein aggregates, and increase the levels of neuroprotective factors. Thus, these venom components hold potential as therapeutic tools to slow or even halt neurodegeneration. However, there are many technological issues to overcome, as venom peptides are hard to obtain and characterize and the amount obtained from natural sources is insufficient to perform all the necessary experiments and tests. Fortunately, technological improvements regarding heterologous protein expression, as well as peptide chemical synthesis will help to provide enough quantities and allow chemical and pharmacological enhancements of these natural occurring compounds. Thus, the main focus of this review is to highlight the most promising studies evaluating animal toxins as therapeutic tools to treat a wide variety of neurodegenerative conditions, including Alzheimer's disease, Parkinson's disease, brain ischemia, glaucoma, amyotrophic lateral sclerosis, and multiple sclerosis.

Nimodipine confers clinical improvement in two models of experimental autoimmune encephalomyelitis

Multiple sclerosis is characterised by inflammatory neurodegeneration, with axonal injury and neuronal cell death occurring in parallel to demyelination. Regarding the molecular mechanisms responsible for demyelination and axonopathy, energy failure, aberrant expression of ion channels and excitotoxicity have been suggested to lead to Ca2+ overload and subsequent activation of calcium-dependent damage pathways. Thus, the inhibition of Ca2+ influx by pharmacological modulation of Ca2+ channels may represent a novel neuroprotective strategy in the treatment of secondary axonopathy. We therefore investigated the effects of the L-type voltage-gated calcium channel blocker nimodipine in two different models of mouse experimental autoimmune encephalomyelitis (EAE), an established experimental paradigm for multiple sclerosis. We show that preventive application of nimodipine (10 mg/kg per day) starting on the day of induction had ameliorating effects on EAE in SJL/J mice immunised with encephalitic myelin peptide PLP139-151 , specifically in late-stage disease. Furthermore, supporting these data, administration of nimodipine to MOG35-55 -immunised C57BL/6 mice starting at the peak of pre-established disease, also led to a significant decrease in disease score, indicating a protective effect on secondary CNS damage. Histological analysis confirmed that nimodipine attenuated demyelination, axonal loss and pathological axonal β-amyloid precursor protein accumulation in the cerebellum and spinal cord in the chronic phase of disease. Of note, we observed no effects of nimodipine on the peripheral immune response in EAE mice with regard to distribution, antigen-specific proliferation or activation patterns of lymphocytes. Taken together, our data suggest a CNS-specific effect of L-type voltage-gated calcium channel blockade to inflammation-induced neurodegeneration.

Differential expression of ryanodine receptor isoforms after spinal cord injury

Ryanodine receptors (RyRs) are highly conductive intracellular Ca²⁺ release channels and are widely expressed in many tissues, including the central nervous system. RyRs have been implicated in intracellular Ca²⁺ overload which can drive secondary damage following traumatic injury to the spinal cord (SCI), but the spatiotemporal expression of the three isoforms of RyRs (RyR1-3) after SCI remains unknown. Here, we analyzed the gene and protein expression of RyR isoforms in the murine lumbar dorsal root ganglion (DRG) and the spinal cord lesion site at 1, 2 and 7 d after a mild contusion SCI. Quantitative RT PCR analysis revealed that RyR3 was significantly increased in lumbar DRGs and at the lesion site at 1 and 2 d post contusion compared to sham (laminectomy only) controls. Additionally, RyR2 expression was increased at 1 d post injury within the lesion site. RyR2 and -3 protein expression was localized to lumbar DRG neurons and their spinal projections within the lesion site acutely after SCI. In contrast, RyR1 expression within the DRG and lesion site remained unaltered following trauma. Our study shows that SCI initiates acute differential expression of RyR isoforms in DRG and spinal cord.

Calcium release from intra-axonal endoplasmic reticulum leads to axon degeneration through mitochondrial dysfunction

Axonal degeneration represents an early pathological event in neurodegeneration, constituting an important target for neuroprotection. Regardless of the initial injury, which could be toxic, mechanical, metabolic, or genetic, degeneration of axons shares a common mechanism involving mitochondrial dysfunction and production of reactive oxygen species. Critical steps in this degenerative process are still unknown. Here we show that calcium release from the axonal endoplasmic reticulum (ER) through ryanodine and IP3 channels activates the mitochondrial permeability transition pore and contributes to axonal degeneration triggered by both mechanical and toxic insults in ex vivo and in vitro mouse and rat model systems. These data reveal a critical and early ER-dependent step during axonal degeneration, providing novel targets for axonal protection in neurodegenerative conditions.

Systemic effects induced by intralesional injection of ω-conotoxin MVIIC after spinal cord injury in rats

Background

Calcium channel blockers such as conotoxins have shown a great potential to reduce brain and spinal cord injury. MVIIC neuroprotective effects analyzed in in vitro models of brain and spinal cord ischemia suggest a potential role of this toxin in preventing injury after spinal cord trauma. However, previous clinical studies with MVIIC demonstrated that clinical side effects might limit the usefulness of this drug and there is no research on its systemic effects. Therefore, the present study aimed to investigate the potential toxic effects of MVIIC on organs and to evaluate clinical and blood profiles of rats submitted to spinal cord injury and treated with this marine toxin. Rats were treated with placebo or MVIIC (at doses of 15, 30, 60 or 120 pmol) intralesionally following spinal cord injury. Seven days after the toxin administration, kidney, brain, lung, heart, liver, adrenal, muscles, pancreas, spleen, stomach, and intestine were histopathologically investigated. In addition, blood samples collected from the rats were tested for any hematologic or biochemical changes.

Results

The clinical, hematologic and biochemical evaluation revealed no significant abnormalities in all groups, even in high doses. There was no significant alteration in organs, except for degenerative changes in kidneys at a dose of 120 pmol.

Conclusions

These findings suggest that MVIIC at 15, 30 and 60 pmol are safe for intralesional administration after spinal cord injury and could be further investigated in relation to its neuroprotective effects. However, 120 pmol doses of MVIIC may provoke adverse effects on kidney tissue.

Role of calpains in the injury-induced dysfunction and degeneration of the mammalian axon

Axonal injury and degeneration, whether primary or secondary, contribute to the morbidity and mortality seen in many acquired and inherited central nervous system (CNS) and peripheral nervous system (PNS) disorders, such as traumatic brain injury, spinal cord injury, cerebral ischemia, neurodegenerative diseases, and peripheral neuropathies. The calpain family of proteases has been mechanistically linked to the dysfunction and degeneration of axons. While the direct mechanisms by which transection, mechanical strain, ischemia, or complement activation trigger intra-axonal calpain activity are likely different, the downstream effects of unregulated calpain activity may be similar in seemingly disparate diseases. In this review, a brief examination of axonal structure is followed by a focused overview of the calpain family. Finally, the mechanisms by which calpains may disrupt the axonal cytoskeleton, transport, and specialized domains (axon initial segment, nodes, and terminals) are discussed.

Neuroprotection in multiple sclerosis: a therapeutic approach

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system that is pathologically characterized by inflammatory demyelination and neurodegeneration. Axonal damage, along with neuronal loss, occurs from disease onset and may lead to progressive and permanent disability. In contrast with the inflammatory pathways, the molecular mechanisms leading to MS neurodegeneration remain largely elusive. With improved understanding of these mechanisms, new potential therapeutic targets for neuroprotection have emerged. We review the current understanding of neurodegenerative processes at play in MS and discuss potential outcome measures and targets for neuroprotection trials.

Three Ca2+ channel inhibitors in combination limit chronic secondary degeneration following neurotrauma

Following neurotrauma, cells beyond the initial trauma site undergo secondary degeneration, with excess Ca2+ a likely trigger for loss of neurons, compact myelin and function. Treatment using inhibitors of specific Ca2+ channels has shown promise in preclinical studies, but clinical trials have been disappointing and combinatorial approaches are needed. We assessed efficacy of multiple combinations of three Ca2+ channel inhibitors at reducing secondary degeneration following partial optic nerve transection in rat. We used lomerizine to inhibit voltage gated Ca2+ channels; oxidised adenosine-triphosphate (oxATP) to inhibit purinergic P2X7 receptors and/or 2-[7-(1H-imidazol-1-yl)-6-nitro-2,3-dioxo-1,2,3,4-tetrahydro quinoxalin-1-yl]acetic acid (INQ) to inhibit Ca2+ permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. Only the three Ca2+ channel inhibitors delivered in combination significantly preserved visual function, as assessed using the optokinetic nystagmus visual reflex, at 3 months after injury. Preservation of retinal ganglion cells was partial and is unlikely to have accounted for differential effects on function. A range of the Ca2+ channel inhibitor combinations prevented swelling of optic nerve vulnerable to secondary degeneration. Each of the treatments involving lomerizine significantly increased the proportion of axons with normal compact myelin. Nevertheless, limiting decompaction of myelin was not sufficient for preservation of function in our model. Multiple combinations of Ca2+ channel inhibitors reduced formation of atypical node/paranode complexes; outcomes were not associated with preservation of visual function. However, prevention of lengthening of the paranodal gap that was only achieved by treatment with the three Ca2+ channel inhibitors in combination was an important additional effect that likely contributed to the associated preservation of the optokinetic reflex using this combinatorial treatment strategy.

Limiting spinal cord injury by pharmacological intervention

The direct primary mechanical trauma to neurons, glia and blood vessels that occurs with spinal cord injury (SCI) is followed by a complex cascade of biochemical and cellular changes which serve to increase the size of the injury site and the extent of cellular and axonal loss. The aim of neuroprotective strategies in SCI is to limit the extent of this secondary cell loss by inhibiting key components of the evolving injury cascade. In this review we will briefly outline the pathophysiological events that occur in SCI, and then review the wide range of neuroprotective agents that have been evaluated in preclinical SCI models. Agents will be considered under the following categories: antioxidants, erythropoietin and derivatives, lipids, riluzole, opioid antagonists, hormones, anti-inflammatory agents, statins, calpain inhibitors, hypothermia, and emerging strategies. Several clinical trials of neuroprotective agents have already taken place and have generally had disappointing results. In attempting to identify promising new treatments, we will therefore highlight agents with (1) low known risks or established clinical use, (2) behavioral data gained in clinically relevant animal models, (3) efficacy when administered after the injury, and (4) robust effects seen in more than one laboratory and/or more than one model of SCI.

Neuroprotection in multiple sclerosis: a therapeutic challenge for the next decade

Multiple sclerosis (MS) is the commonest cause of progressive neurological disability amongst young, Caucasian adults. MS is considered to be an auto-immune disease that results from an attack against myelin, the layer which surrounds axons. The pathophysiology of MS is complex, with both demyelination and axonal degeneration contributing to what is essentially an inflammatory neurodegenerative disease. Axonal loss is increasingly being accepted as the histopathological correlate of neurological disability. Currently, the underpinnings of neurodegeneration in MS, and how to promote neuroprotection are only partly understood. No established treatments that directly reduce nervous system damage or enhance its repair are currently available. Moreover, the ability of currently available immunomodulatory therapies used to treat MS, such as interferon-beta, to prevent long-term disability is uncertain. Results from short-term randomized-controlled trials suggest a partial benefit with regards to disability outcomes, but this is yet to be established in long-term studies. Novel neuroprotective agents have been identified in preclinical studies but their development is being hampered by the absence of appropriate clinical platforms to test them. In this article, we will discuss some of the principal therapeutic candidates that could provide neuroprotection in MS and emerging methodologies by which to test them.

Regulation of store-operated and voltage-operated Ca2+ channels in the proliferation and death of oligodendrocyte precursor cells by golli proteins

OPCs (oligodendrocyte precursor cells) express golli proteins which, through regulation of Ca²⁺ influx, appear to be important in OPC process extension/retraction and migration. The aim of the present study was to examine further the role of golli in regulating OPC development. The effects of golli ablation and overexpression were examined in primary cultures of OPCs prepared from golli-KO (knockout) and JOE (golli J37-overexpressing) mice. In OPCs lacking golli, or overexpressing golli, differentiation induced by growth factor withdrawal was impaired. Proliferation analysis in the presence of PDGF (platelet-derived growth factor), revealed that golli enhanced the mitogen-stimulated proliferation of OPCs through activation of SOCCs (store-operated Ca²⁺ channels). PDGF treatment induced a biphasic increase in OPC intracellular Ca²⁺, and golli specifically increased Ca²⁺ influx during the second SOCC-dependent phase that followed the initial release of Ca²⁺ from intracellular stores. This store-operated Ca²⁺ uptake appeared to be essential for cell division, since specific SOCC antagonists completely blocked the effects of PDGF and golli on OPC proliferation. Additionally, in OPCs overexpressing golli, increased cell death was observed after mitogen withdrawal. This phenomenon could be prevented by exposure to VOCC (voltage-operated Ca²⁺ channel) blockers, indicating that the effect of golli on cell death involved increased Ca²⁺ influx through VOCCs. The results showed a clear effect of golli on OPC development and support a role for golli in modulating multiple Ca²⁺-regulatory events through VOCCs and SOCCs. Our results also suggest that PDGF engagement of its receptor resulting in OPC proliferation proceeds through activation of SOCCs.

The critical role of voltage-dependent calcium channel in axonal repair following mechanical trauma

Membrane disruption following mechanical injury likely plays a critical role in the pathology of spinal cord trauma. It is known that intracellular calcium is a key factor that is essential to membrane resealing. However, the differential role of calcium influx through the injury site and through voltage dependent calcium channels (VDCC) has not been examined in detail. Using a well-established ex vivo guinea-pig spinal cord white matter preparation, we have found that axonal membrane resealing was significantly inhibited following transection or compression in the presence of cadmium, a non-specific calcium channel blocker, or nimodipine, a specific L-type calcium channel blocker. Membrane resealing was assessed by the changes of membrane potential and compound action potential (CAP), and exclusion of horseradish peroxidase 60 min following trauma. Furthermore, 1 microM BayK 8644, a VDCC agonist, significantly enhanced membrane resealing. Interestingly, this effect was completely abolished when the concentration of BayK 8644 was increased to 30 microM. These data suggest that VDCC play a critical role in membrane resealing. Further, there is likely an appropriate range of calcium influx through VDCC which ensures effective axonal membrane resealing. Since elevated intracellular calcium has also been linked to axonal deterioration, blockage of VDCC is proposed to be a clinical treatment for various injuries. The knowledge gained in this study will likely help us better understand the role of calcium in various CNS trauma, which is critical for designing new approaches or perhaps optimizing the effectiveness of existing methods in the treatment of CNS trauma.

Complex interplay between glutamate receptors and intracellular Ca2+ stores during ischaemia in rat spinal cord white matter

Electrophysiological recordings of propagated compound action potentials (CAPs) and axonal Ca(2+) measurements using confocal microscopy were used to study the interplay between AMPA receptors and intracellullar Ca(2+) stores in rat spinal dorsal columns subjected to in vitro combined oxygen and glucose deprivation (OGD). Removal of Ca(2+) or Na(+) from the perfusate was protective after 30 but not 60 min of OGD. TTX was ineffective with either exposure, consistent with its modest effect on ischaemic depolarization. In contrast, AMPA antagonists were very protective, even after 60 min of OGD where 0Ca(2+) + EGTA perfusate was ineffective. Similarly, blocking ryanodine receptor-mediated Ca(2+) mobilization from internal stores (0Ca(2+) + nimodipine or 0Ca(2+) + ryanodine), or inositol 1,4,5-trisphosphate (IP(3))-dependent Ca(2+) release (block of group 1 metabotropic glutamate receptors with 1-aminoindan-1,5-dicarboxylic acid, inhibition of phospholipase C with U73122 or IP(3) receptor block with 2APB; each in 0Ca(2+)) were each very protective, with the combination resulting in virtually complete functional recovery after 1 h OGD (97 +/- 32% CAP recovery versus 4 +/- 6% in artificial cerebrospinal fluid). AMPA induced a rise in Ca(2+) concentration in normoxic axons, which was greatly reduced by blocking ryanodine receptors. Our data therefore suggest a novel and surprisingly complex interplay between AMPA receptors and Ca(2+) mobilization from intracellular Ca(2+) stores. We propose that AMPA receptors may not only allow Ca(2+) influx from the extracellular space, but may also significantly influence Ca(2+) release from intra-axonal Ca(2+) stores. In dorsal column axons, AMPA receptor-dependent mechanisms appear to exert a greater influence than voltage-gated Na(+) channels on functional outcome following OGD.

Calcium channel blockers ameliorate disease in a mouse model of multiple sclerosis

Multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE), an animal model of MS, are inflammatory demyelinating diseases of the central nervous system. The inflammatory attacks lead to glial dysfunction and death, axonal damage, and neurological deficits. Numerous studies in rat suggest that extracellular calcium influx, via voltage-gated calcium channels (VGCC), contributes to white matter damage in acute spinal cord injury and stroke. Our immunohistochemical finding that mouse spinal cord axons display subunits of L-type VGCC also supports this hypothesis. Furthermore, we hypothesized that VGCC also play a role in EAE, and possibly, MS. In our study, administration of the calcium channel blockers (CCB) bepridil and nitrendipine significantly ameliorated EAE in mice, compared with vehicle-treated controls. Spinal cord samples showed reduced inflammation and axonal pathology in bepridil-treated animals. Our data support the hypothesis that calcium influx via VGCC plays a significant role in the development of neurological disability and white matter damage in EAE and MS.

Traumatic axonal injury induces proteolytic cleavage of the voltage-gated sodium channels modulated by tetrodotoxin and protease inhibitors

We demonstrated previously that dynamic stretch injury of cultured axons induces structural changes and Ca2+ influx modulated by tetrodotoxin (TTX)-sensitive voltage-gated sodium channels (NaChs). In the present study, we evaluated potential damage to the NaCh alpha-subunit, which can cause noninactivation of NaChs. In addition, we explored the effects of pre-injury and post-injury treatment with TTX and protease inhibition on proteolysis of the NaCh alpha-subunit and intra-axonal calcium levels ([Ca2+]i) over 60 min after trauma. After stretch injury, we found that [Ca2+]i continued to increase in untreated axons for at least 60 min. We also observed that the III-IV intra-axonal loop of the NaCh alpha-subunit was proteolyzed between 5 and 20 min after trauma. Pre-injury treatment of the axons with TTX completely abolished the posttraumatic increase in [Ca2+]i and proteolysis of the NaCh alpha-subunit. In addition, both pre-injury and post-injury inhibition of protease activity attenuated long-term increases in [Ca2+]i as well as mitigating degradation of the NaCh alpha-subunit. These results suggest a unique "feed-forward" deleterious process initiated by mechanical trauma of axons. Na+ influx through NaChs resulting from axonal deformation triggers initial increases in [Ca2+]i and subsequent proteolysis of the NaCh-subunit. In turn, degradation of the alpha-subunit promotes persistent elevations in [Ca2+]i, fueling additional pathologic changes. These observations may have important implications for developing therapeutic strategies for axonal trauma.

The Role of Excitotoxicity in Secondary Mechanisms of Spinal Cord Injury: A Review with an Emphasis on the Implications for White Matter Degeneration

Following an initial impact after spinal cord injury (SCI), there is a cascade of downstream events termed 'secondary injury', which culminate in progressive degenerative events in the spinal cord. These secondary injury mechanisms include, but are not limited to, ischemia, inflammation, free radical-induced cell death, glutamate excitotoxicity, cytoskeletal degradation and induction of extrinsic and intrinsic apoptotic pathways. There is emerging evidence that glutamate excitotoxicity plays a key role not only in neuronal cell death but also in delayed posttraumatic spinal cord white matter degeneration. Importantly however, the differences in cellular composition and expression of specific types of glutamate receptors in grey versus white matter require a compartmentalized approach to understand the mechanisms of secondary injury after SCI. This review examines mechanisms of secondary white matter injury with particular emphasis on glutamate excitotoxicity and the potential link of this mechanism to apoptosis. Recent studies have provided new insights into the mechanisms of glutamate release and its potential targets, as well as the downstream pathways associated with glutamate receptor activation in specific types of cells. Evidence from molecular and functional expression of glutamatergic AMPA receptors in white matter glia (and possibly axons), the protective effects of AMPA/kainate antagonists in posttraumatic white matter axonal function, and the vulnerability of oligodendrocytes to excitotoxic cell death suggest that glutamate excitotoxicity is associated with oligodendrocyte apoptosis. The latter mechanism appears key to glutamatergic white matter degeneration after SCI and may represent an attractive therapeutic target.

Regulation of glucagon-like peptide-1 receptor and calcium-sensing receptor signaling by L-histidine

Receptor-specific agonists of the extracellular calcium-sensing receptor (CaSR) potentiate glucose-induced insulin secretion, an effect similar to that of glucagon-like peptide-1 (GLP-1). We have sequenced the full open reading frame of the CaSR from rat insulinoma (INS-1) cells and find that the predicted amino acid sequence of the receptor is identical with that of the receptor from the parathyroid gland. This receptor couples to both Gq/11 and Gi/o, and this dual coupling may partly explain the varying effects of nonspecific agonists on secretion reported previously. L-Histidine (L-His) increases the sensitivity of the CaSR to extracellular Ca2+ and potentiates glucose-dependent insulin secretion from INS-1 cells. This potentiation is partially inhibited at low extracellular [Ca2+] where the CaSR is ineffective. Coexpression of the CaSR and GLP-1 receptor (GLP-1R) produces a pertussis toxin-sensitive inhibition of GLP-1-induced cAMP production in response to elevated extracellular [Ca2+]. However, l-His potentiates cAMP response element reporter activity in INS-1 cells and in human embryonic kidney-293 cells expressing either the GLP-1R alone or the CaSR and GLP-1R. INS-1 cells express the RNA for the CaSR at a lower level than that for the GLP-1R. This difference in expression level of the receptors may explain the potentiation of insulin secretion by L-His despite coupling of the CaSR to Gi/o. In conclusion, L-His can potentiate both GLP-1R- and CaSR-activated signaling pathways, and these effects may play a role in the potentiation of glucose-induced insulin secretion in response to meals containing protein in addition to carbohydrates and fat.

Preservation of neurologic function during inflammatory demyelination correlates with axon sparing in a mouse model of multiple sclerosis

Axonal injury has been proposed as the basis of permanent deficits in the inflammatory, demyelinating disease, multiple sclerosis. However, reports on the degree of injury are highly variable, and the responsible mechanisms are poorly understood. We examined the relationships among long-term demyelination, inflammation, axonal injury, and motor function in a model of multiple sclerosis, in which mice develop chronic, immune-mediated demyelination of the spinal cord resulting from persistent infection with Theiler's virus. We studied two strains of mice, inbred SJL/J and C57BL/6x129 mice deficient in beta(2)-microglobulin and therefore CD8 lymphocytes. After 8 months of disease, SJL mice had considerably worse motor function than beta(2)-microglobulin-deficient mice. Motor dysfunction correlated linearly with the extent of demyelinated lesions in the spinal cord (lesion load) within each strain, but no difference in lesion load was present between strains. Also, the extent of remyelination did not differ between strains. Instead, the disparity in motor deficits reflected differences in the integrity of descending neurons. That is, retrograde labeling of reticulospinal, vestibulospinal, and rubrospinal neurons, although reduced in all chronically diseased mice, was two to seven times higher in beta(2)-microglobulin-deficient mice. The labeling was superior in beta(2)-microglobulin-deficient mice despite the fact that lesion expanse and therefore the number of axons traversing lesions were similar in both strains. Thus, by all criteria axons were equivalently demyelinated in SJL and beta(2)-microglobulin-deficient mice, but the extent of axonal injury differed significantly. These results indicate that mechanisms of demyelination and axonal injury are at least partly separable, and are consistent with the hypothesis that cytotoxic CD8 lymphocytes may selectively injure demyelinated axons. Additionally, the data suggest that axonal injury obligatorily results from chronic inflammatory demyelination and significantly contributes to neurological deficits.

Na(+)-K(+)-ATPase inhibition and depolarization induce glutamate release via reverse Na(+)-dependent transport in spinal cord white matter

Excitotoxic mechanisms involving alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA)/kainate receptors play an important role in mediating cellular damage in spinal cord injury. However, the precise cellular mechanisms of glutamate release from non-synaptic white matter are not well understood. We examined how the collapse of transmembrane Na(+) and K(+) gradients induces reverse operation of Na(+)-dependent glutamate transporters, leading to glutamate efflux and injury to rat spinal dorsal columns in vitro. Compound action potentials were irreversibly reduced to 43% of control after ouabain/high K(+)/low Na(+) exposure (500 microM ouabain for 30 min to increase [Na(+)](i), followed by 1 h ouabain+high K(+) (129 mM)/low Na(+) (27 mM), to further reverse transmembrane ion gradients) followed by a 2 h wash. Ca(2+)-free perfusate was very protective (compound action potential amplitude recovered to 87% vs. 43%). The broad spectrum glutamate antagonist kynurenic acid (1 mM) or the selective AMPA antagonist GYKI52466 (30 microM) were partially protective (68% recovery). Inhibition of Na(+)-dependent glutamate transport with L-trans-pyrrolidine-2,4-dicarboxylic acid (1 mM) also provided significant protection (71% recovery), similar to that seen with glutamate receptor antagonists. Blocking reverse Na(+)-Ca(2+) exchange with KB-R7943 (10 microM) however, was ineffective in this paradigm (49% recovery). Semiquantitative glutamate immunohistochemistry revealed that levels of this amino acid were significantly depleted in axon cylinders and, to a lesser degree, in oligodendrocytes (but not in astrocytes) by ouabain/high K(+)/low Na(+), which was largely prevented by glutamate transport inhibition. Our data show that dorsal column white matter contains the necessary glutamate pools and release mechanisms to induce significant injury. When Na(+) and K(+) gradients are disrupted, even in the absence of reduced cellular energy reserves, reverse operation of Na(+)-dependent glutamate transport will release enough endogenous glutamate to activate AMPA receptors and cause substantial Ca(2+)-dependent injury. This mechanism likely plays an important role during ischemic and traumatic white matter injury, where collapse of transmembrane Na(+) and K(+) gradients occurs.

Traumatic axonal injury induces calcium influx modulated by tetrodotoxin-sensitive sodium channels

Diffuse axonal injury (DAI) is one of the most common and important pathologies resulting from the mechanical deformation of the brain during trauma. It has been hypothesized that calcium influx into axons plays a major role in the pathophysiology of DAI. However, there is little direct evidence to support this hypothesis, and mechanisms of potential calcium entry have not been explored. In the present study, we used an in vitro model of axonal stretch injury to evaluate the extent and modulation of calcium entry after trauma. Using a calcium-sensitive dye, we observed a dramatic increase in intra-axonal calcium levels immediately after injury. Axonal injury in a calcium-free extracellular solution resulted in no change in calcium concentration, suggesting an extracellular source for the increased post-traumatic calcium levels. We also found that the post-traumatic change in intra-axonal calcium was completely abolished by the application of the sodium channel blocker tetrodotoxin or by replacement of sodium with N-methyl-d-glucamine. In addition, application of the voltage-gated calcium channel (VGCC) blocker omega-conotoxin MVIIC attenuated the post-traumatic increase in calcium. Furthermore, blockade of the Na(+)-Ca(2+) exchanger with bepridil modestly reduced the calcium influx after injury. In contrast to previously proposed mechanisms of calcium entry after axonal trauma, we found no evidence of calcium entry through mechanically produced pores (mechanoporation). Rather, our results suggest that traumatic deformation of axons induces abnormal sodium influx through mechanically sensitive Na(+) channels, which subsequently triggers an increase in intra-axonal calcium via the opening of VGCCs and reversal of the Na(+)-Ca(2+) exchanger.

Synchronized Ca2+signals mediated by Ca2+action potentials in the hippocampal neuron network in vitro

Periodic, synchronized Ca2+ signals appeared 30-120 min after the application of tetrodotoxin, 4-aminopyridine and Cs+, and became stable in interval (6-47s) for hours. The Ca2+ signals were accompanied by excitatory or inhibitory postsynaptic potentials (excitatory postsynaptic currents (EPSCs) for the former) and blocked by the simultaneous application of 6-cyano-7-nitroquinoxaline-2,3-dione and 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid or treatment with Ca2+ -free solution, nicardipine, or omega-conotoxin MVIIC (omegaCTX), but not with ryanodine, caffeine, thapsigargin or CPP alone. Nicardipine largely, but omegaCTX less, blocked Ca2+ action potentials or voltage pulse-induced Ca2+ currents at the cell soma, while omegaCTX completely blocked autaptic EPSCs. Ca2+ signals within a neuron occurred almost simultaneously in the cell soma and all the processes (> 200 microm), while the latency between Ca2+ signals of neighbouring neurons varied over hundreds of ms like that of Ca2 action potential induction from EPSPs. Ca2+ signals propagated in random directions throughout neural circuits. Thus, when Na+ and K+ channels are blocked, Ca2+ action potentials spontaneously occur somewhere in a neuron, eventually propagate via the cell soma to the presynaptic terminals and activate excitatory synaptic transmission, causing synchronized Ca2+ signals. The results further suggest that the axon of hippocampal neurones have the potential ability to convey coded information via Ca2+ action potentials.

Ionic mechanisms of aglycemic axon injury in mammalian central white matter

The authors investigated ionic mechanisms underlying aglycemic axon injury in adult rat optic nerve, a central white matter tract. Axon function was assessed using evoked compound action potentials (CAPs). Glucose withdrawal led to delayed CAP failure, an alkaline extracellular pH shift, and an increase in extracellular [K(+)]. Sixty minutes of glucose withdrawal led to irreversible axon injury. Aglycemic axon injury required extracellular calcium; the extent of injury progressively declined as bath [Ca(2+)] was decreased. To evaluate Ca(2+) movements during aglycemia, the authors recorded extracellular [Ca(2+)] ([Ca(2+)](o)) using Ca(2+)-sensitive microelectrodes. Under control conditions, [Ca(2+)](o) fell with a similar time course to CAP failure, indicating extracellular Ca(2+) moved to an intracellular position during aglycemia. The authors quantified the magnitude of [Ca(2+)]o decrease as the area below baseline [Ca(2+)]o during aglycemia and used this as a qualitative measure of Ca(2+) influx. The authors studied the mechanisms of Ca(2+) influx. Blockade of Na(+) influx reduced Ca(2+) influx and improved CAP recovery, suggesting Na(+)-Ca(2+) exchanger involvement. Consistent with this hypothesis, bepridil reduced axon injury. In addition, diltiazem or nifedipine decreased Ca(2+) influx and increased CAP recovery. The authors conclude aglycemic central white matter injury is caused by Ca(2+) influx into intracellular compartments through reverse Na(+)-Ca(2+) exchange and L-type Ca(2+) channels.

Voltage-dependent calcium channels in the rat retina: involvement in NMDA-stimulated influx of calcium

Rises in intracellular Ca2+ induced by activation of glutamate receptors are of ultimate importance for neuronal excitability and pathophysiological processes. In the present study, we aimed to elucidate the types of voltage-dependent Ca2+ channels involved in the NMDA-stimulated influx of Ca2+ into the isolated rat retina by using selective blockers. Additionally, the number of binding sites for radioligands labelling L- ([3H]nitrendipine), N- ([125I]omega-conotoxin MVIIA) and P/Q-type ([125I]omega-conotoxin MVIIC) Ca2+ channels was assessed in the rat retina and, for further comparison, in the rat cortex. Incubation of isolated rat retinas with 100 microM NMDA produced a three-fold increase in the influx of 45Ca2+ that was completely blunted by MK-801, a NMDA receptor antagonist, and partially attenuated (approximately 20%) by tetrodotoxin, a Na+ channel blocker. The L-type Ca2+ channel blocker nifedipine reduced NMDA-stimulated Ca2+ influx in a dose-related fashion, with a maximum reduction of approximately 50%. Similar effects were observed with verapamil and diltiazem. Blockers of N- and P/Q-type Ca2+ channels had no significant effect on the influx of Ca2+ evoked by NMDA. Co2+, a non-specific Ca2+ channel blocker, caused an inhibition of NMDA-stimulated Ca2+ influx similar to that of nifedipine. Therefore, of all voltage-dependent Ca2+ channels, L-type channels appear to make the greatest contribution (up to 50%) to the NMDA-stimulated influx of Ca2+ into the isolated rat retina. This finding contrasts with evidence obtained in brain neurones supporting a role for L-, N- and P/Q-type channels in NMDA-evoked Ca2+ signals. A comparison of the number of radioligand binding sites associated with L-, N- or P/Q-type Ca2+ channels in the rat cortex and retina revealed that such a difference cannot be ascribed to a distinct expression pattern of these channels in both tissues, although some variations were found. Interestingly, a different affinity of [3H]nitrendipine for L-type Ca2+ channels in the rat retina and cortex was observed which may reflect the expression of different classes of L-type channels in these tissues. The ability of L-type Ca2+ channel blockers to attenuate NMDA-stimulated Ca2+ influx may underlie their neuroprotective effects in the retina.

Axonal L-type Ca2+ channels and anoxic injury in rat CNS white matter

We studied the magnitude and route(s) of Ca2+ flux from extra- to intracellular compartments during anoxia in adult rat optic nerve (RON), a central white matter tract, using Ca2+ sensitive microelectrodes to monitor extracellular [Ca2+] ([Ca2+]o). One hour of anoxia caused a rapid loss of the stimulus-evoked compound action potential (CAP), which partially recovered following re-oxygenation, indicating that irreversible injury had occurred. After an initial increase caused by extracellular space shrinkage, anoxia produced a sustained decrease of 0.42 mM (29%) in [Ca2+]o. We quantified the [Ca2+]o decrease as the area below baseline [Ca2+]o during anoxia and used this as a qualitative index of suspected Ca2+ influx. The degree of RON injury was predicted by the amount of Ca2+ leaving the extracellular space. Bepridil, 0 Na+ artificial cerebrospinal fluid or tetrodotoxin reduced suspected Ca2+ influx during anoxia implicating reversal of the Na+/Ca2+ exchanger as a route of Ca2+ influx. Diltiazem reduced suspected Ca2+ influx during anoxia, suggesting that Ca2+ influx via L-type Ca2+ channels is a route of toxic Ca2+ influx into axons during anoxia. Immunocytochemical staining was used to demonstrate and localize high-threshold Ca2+ channels. Only alpha1(C) and alpha1(D) subunits were detected, indicating that only L-type Ca2+ channels were present. Double labeling with anti-neurofilament antibodies or anti-glial fibrillary acidic protein antibodies localized L-type Ca2+ channels to axons and astrocytes.

Role of L- and N-type calcium channels in the pathophysiology of traumatic spinal cord white matter injury

Recent work has suggested a potential role for voltage-gated Ca(2+) channels in the pathophysiology of anoxic central nervous system white matter injury. To examine the relevance of these findings to neurotrauma, we conducted electrophysiological studies with inorganic Ca(2+) channels blockers and L- and N-subtype-specific calcium channel antagonists in an in vitro model of spinal cord injury. Confocal immunohistochemistry was used to examine for localization of L- and N-type calcium channels in spinal cord white matter tracts. A 30-mm length of dorsal column was isolated from the spinal cord of adult rats, pinned in an in vitro recording chamber and injured with a modified clip (2g closing force) for 15s. The functional integrity of the dorsal column was monitored electrophysiologically by quantitatively measuring the compound action potential at two points with glass microelectrodes. The compound action potential decreased to 71.4+/-2.0% of control (P<0. 05) after spinal cord injury. Removal of extracellular Ca(2+) promoted significantly greater recovery of compound action potential amplitude (86.3+/-7.6% of control; P< 0.05) after injury. Partial blockade of voltage-gated Ca(2+) channels with cobalt (20 microM) or cadmium (200 microM) conferred improvement in compound action potential amplitude. Application of the L-type Ca(2+) channel blockers diltiazem (50 microM) or verapamil (90 microM), and the N-type antagonist omega-conotoxin GVIA (1 microM), significantly enhanced the recovery of compound action potential amplitude postinjury. Co-application of the L-type antagonist diltiazem with the N-type blocker omega-conotoxin GVIA showed significantly greater (P<0.05) improvement in compound action potential amplitude than application of either drug alone. Confocal immunohistochemistry with double labelling for glial fibrillary acidic protein, GalC and NF200 demonstrated L- and N-type Ca(2+) channels on astrocytes and oligodendrocytes, but not axons, in spinal cord white matter. In conclusion, the injurious effects of Ca(2+) in traumatic central nervous system white matter injury appear to be partially mediated by voltage-gated Ca(2+) channels. The presence of L- and N-type Ca(2+) channels on periaxonal astrocytes and oligodendrocytes suggests a role for these cells in post-traumatic axonal conduction failure.

Important role of reverse Na(+)-Ca(2+) exchange in spinal cord white matter injury at physiological temperature

Spinal cord injury is a devastating condition in which most of the clinical disability results from dysfunction of white matter tracts. Excessive cellular Ca(2+) accumulation is a common phenomenon after anoxia/ischemia or mechanical trauma to white matter, leading to irreversible injury because of overactivation of multiple Ca(2+)-dependent biochemical pathways. In the present study, we examined the role of Na(+)-Ca(2+) exchange, a ubiquitous Ca(2+) transport mechanism, in anoxic and traumatic injury to rat spinal dorsal columns in vitro. Excised tissue was maintained in a recording chamber at 37 degrees C and injured by exposure to an anoxic atmosphere for 60 min or locally compressed with a force of 2 g for 15 s. Mean compound action potential amplitude recovered to approximately 25% of control after anoxia and to approximately 30% after trauma. Inhibitors of Na(+)-Ca(2+) exchange (50 microM bepridil or 10 microM KB-R7943) improved functional recovery to approximately 60% after anoxia and approximately 70% after traumatic compression. These inhibitors also prevented the increase in calpain-mediated spectrin breakdown products induced by anoxia. We conclude that, at physiological temperature, reverse Na(+)-Ca(2+) exchange plays an important role in cellular Ca(2+) overload and irreversible damage after anoxic and traumatic injury to dorsal column white matter tracts.