Calpain inhibitors confer biochemical, but not electrophysiological, protection against anoxia in rat optic nerves

The endogenous calpain inhibitor calpastatin attenuates axon degeneration in murine Guillain-Barré syndrome

Axon degeneration accounts for the poor clinical outcome in Guillain-Barré syndrome (GBS), yet no treatments target this key pathogenic stage. Animal models demonstrate anti-ganglioside antibodies (AGAb) induce axolemmal complement pore formation through which calcium flux activates the intra-axonal calcium-dependent proteases, calpains. We previously showed protection of axonal components using soluble calpain inhibitors in ex vivo GBS mouse models, and herein, we assess the potential of axonally-restricted calpain inhibition as a neuroprotective therapy operating in vivo. Using transgenic mice that over-express the endogenous human calpain inhibitor calpastatin (hCAST) neuronally, we assessed distal motor nerve integrity in our established GBS models. We induced immune-mediated injury with monoclonal AGAb plus a source of human complement. The calpain substrates neurofilament and AnkyrinG, nerve structural proteins, were assessed by immunolabelling and in the case of neurofilament, by single-molecule arrays (Simoa). As the distal intramuscular portion of the phrenic nerve is prominently targeted in our in vivo model, respiratory function was assessed by whole-body plethysmography as the functional output in the acute and extended models. hCAST expression protects distal nerve structural integrity both ex and in vivo, as shown by attenuation of neurofilament breakdown by immunolabelling and Simoa. In an extended in vivo model, while mice still initially undergo respiratory distress owing to acute conduction failure, the recovery phase was accelerated by hCAST expression. Axonal calpain inhibition can protect the axonal integrity of the nerve in an in vivo GBS paradigm and hasten recovery. These studies reinforce the strong justification for developing further animal and human clinical studies using exogenous calpain inhibitors.

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.

Calpains mediate axonal cytoskeleton disintegration during Wallerian degeneration

In both the central nervous system (CNS) and peripheral nervous system (PNS), transected axons undergo Wallerian degeneration. Even though Augustus Waller first described this process after transection of axons in 1850, the molecular mechanisms may be shared, at least in part, by many human diseases. Early pathology includes failure of synaptic transmission, target denervation, and granular disintegration of the axonal cytoskeleton (GDC). The Ca(2+)-dependent protease calpains have been implicated in GDC but causality has not been established. To test the hypothesis that calpains play a causal role in axonal and synaptic degeneration in vivo, we studied transgenic mice that express human calpastatin (hCAST), the endogenous calpain inhibitor, in optic and sciatic nerve axons. Five days after optic nerve transection and 48 h after sciatic nerve transection, robust neurofilament proteolysis observed in wild-type controls was reduced in hCAST transgenic mice. Protection of the axonal cytoskeleton in sciatic nerves of hCAST mice was nearly complete 48 h post-transection. In addition, hCAST expression preserved the morphological integrity of neuromuscular junctions. However, compound muscle action potential amplitudes after nerve transection were similar in wild-type and hCAST mice. These results, in total, provide direct evidence that calpains are responsible for the morphological degeneration of the axon and synapse during Wallerian degeneration.

Axonal degeneration as a therapeutic target in the CNS

Degeneration of the axon is an important step in the pathomechanism of traumatic, inflammatory and degenerative neurological diseases. Increasing evidence suggests that axonal degeneration occurs early in the course of these diseases and therefore represents a promising target for future therapeutic strategies. We review the evidence for axonal destruction from pathological findings and animal models with particular emphasis on neurodegenerative and neurotraumatic disorders. We discuss the basic morphological and temporal modalities of axonal degeneration (acute, chronic and focal axonal degeneration and Wallerian degeneration). Based on the mechanistic concepts, we then delineate in detail the major molecular mechanisms that underlie the degenerative cascade, such as calcium influx, axonal transport, protein aggregation and autophagy. We finally concentrate on putative therapeutic targets based on the mechanistic prerequisites.

Calcium dependence of damage to mouse motor nerve terminals following oxygen/glucose deprivation

Motor nerve terminals are especially sensitive to an ischemia/reperfusion stress. We applied an in vitro model of this stress, oxygen/glucose deprivation (OGD), to mouse neuromuscular preparations to investigate how Ca(2+) contributes to stress-induced motor terminal damage. Measurements using an ionophoretically-injected fluorescent [Ca(2+)] indicator demonstrated an increase in intra-terminal [Ca(2+)] following OGD onset. When OGD was terminated within 20-30min of the increase in resting [Ca(2+)], these changes were sometimes reversible; in other cases [Ca(2+)] remained high and the terminal degenerated. Endplate innervation was assessed morphometrically following 22min OGD and 120min reoxygenation (32.5°C). Stress-induced motor terminal degeneration was Ca(2+)-dependent. Median post-stress endplate occupancy was only 26% when the bath contained the normal 1.8mM Ca(2+), but increased to 81% when Ca(2+) was absent. Removal of Ca(2+) only during OGD was more protective than removal of Ca(2+) only during reoxygenation. Post-stress endplate occupancy was partially preserved by pharmacological inhibition of various routes of Ca(2+) entry into motor terminals, including voltage-dependent Ca(2+) channels (ω-agatoxin-IVA, nimodipine) and the plasma membrane Na(+)/Ca(2+) exchanger (KB-R7943). Inhibition of a Ca(2+)-dependent protease with calpain inhibitor VI was also protective. These results suggest that most of the OGD-induced motor terminal damage is Ca(2+)-dependent, and that inhibition of Ca(2+) entry or Ca(2+)-dependent proteolysis can reduce this damage. There was no significant difference between the response of wild-type and presymptomatic superoxide dismutase 1 G93A mutant terminals to OGD, or in their response to the protective effect of the tested drugs.

Attenuation of microtubule associated protein-2 degradation after mild head injury by mexiletine and calpain-2 inhibitor

The objective of the study was to address the early effects of mild, closed, head injuries on neuronal stability and the prevention of microtubule-associated protein-2 (MAP-2) degradation by mexiletine and calpain-2 inhibitor. Twenty-four rats were divided into four groups: control group (1); trauma group without treatment (2); mexiletine-pretreated and subjected to trauma group (3); trauma subjected and then calpain-2 inhibitor received group (4). All animals were subjected to mild, closed, head trauma. Frontal lobes were removed and processed for staining and immunofluorescent labelling of MAP-2 cytoskeletal proteins, which were evaluated by confocal microscopy in serial optical sections showing the three dimensional cytoarchitecture of affected areas. MAP-2 decoration in almost all neurons obtained from traumatized brain regions drastically diminished, while minute filamentous and granular profiles in axons and/or dendrites were retained together implying a massive degradation/depolymerization of microtubules. In contrast, in mexiletine-pretreated animals, MAP-2 positivity in axonal and perikaryonal profiles was fairly retained, which clearly depicts the protective role of mexiletine after trauma. Compared with mexiletine-pretreated group, calpain-2 inhibitor treated group displayed a less well-preserved MAP-2 expression. Mexiletine can prevent cytoskeletal structure and protein degradation after mild head trauma. Calpain-2 inhibitor prevents protein degradation, but cytoskeletal organization is better preserved with mexiletine.

Calpain Inhibitor MDL-28170 Reduces the Functional and Structural Deterioration of Corpus Callosum following Fluid Percussion Injury

It is known that calpain activation is involved in human traumatic brain injury (TBI) and that calpain inhibition can have neuroprotective effects on both gray matter and white matter injury of TBI models. However, the role of calpain activation in the corpus callosum remains unclear and requires elucidation given its potential clinical relevance. We evaluated the neuroprotective effects of calpain inhibitor MDL-28170 on corpus callosum function and structural destruction using a fluid percussion injury (FPI) model. The therapeutic time window for a single administration of MDL-28170 was up to 4 h post injury in protecting the corpus callosum structural integrity, and up to 30 min in protecting the axonal function evaluated 1 day following injury. When given 30 min prior injury, MDL-28170 showed neuroprotective effects that lasted up to 7 days. However, 30 min post injury administration of the drug afforded neuroprotection only up to 3 days. In contrast, two additional reinforcement injections at 24 and 48 h in addition to 30 min post FPI significantly protected both axonal function and structural integrity that lasted 14 days following FPI. Our data indicated that calpain inhibitor MDL-28170 is an effective neuroprotectant for axonal injury in corpus callosum following FPI with a therapeutic time window up to 4 hours. Although delayed treatment (2 or 4 h post FPI) was effective in protecting the axonal structure, the axons saved may not be as functional as normal fibers. Multiple drug administrations may be necessary for achieving a persisting effectiveness of this compound.

Current concepts of the cellular and molecular pathophysiology of multiple sclerosis

Multiple sclerosis (MS) is the most common demyelinating disease. It poses many challenges both clinically and scientifically. Progress made in understanding the genetics, immunology, and neurobiology of MS to date has positioned the field for further breakthroughs both in understanding the etiology and pathogenesis as well as the development of rationally based therapeutics. This review will cover fundamental aspects of the clinical and pathologic features of MS. Identified genetic markers will be considered as well as the evolving understanding of immunologic and neurobiological aspects of the disease. The development of immune therapy based on this knowledge is already apparent and it is likely that neuroprotective therapies will evolve to complement immune modulation in treating the disease.

Hypoxia-Induced Cytoskeleton Disruption in Alveolar Epithelial Cells

Alveolar hypoxia, a common feature of many respiratory disorders, has been previously reported to induce functional changes, particularly a decrease of transepithelial Na and fluid transport. In polarized epithelia, cytoskeleton plays a regulatory role in transcellular and paracellular transport of ions and fluid. We hypothesized that exposure to hypoxia could damage cytoskeleton organization, which in turn, may adversely affect ion and fluid transport. Primary rat alveolar epithelial cells (AEC) were exposed to either mild (3% O(2)) or severe (0.5% O(2)) hypoxia for 18 h or to normoxia (21% O(2)). First, mild and severe hypoxia induced a disorganization of actin, a major protein of the cytoskeleton, reflected by disruption of F-actin filaments. Second, alpha-spectrin, an apical cytoskeleton protein, which binds to actin cytoskeleton and Na transport proteins, was cleaved by hypoxia. Pretreatment of AEC by a caspase inhibitor (z-VAD-fmk; 90 microM) blunted hypoxia-induced spectrin cleavage as well as hypoxia-induced decrease in surface membrane alpha-ENaC and concomitantly induced a partial recovery of hypoxia-induced decrease of amiloride-sensitive Na transport at 3% O(2). Finally, tight junctions (TJs) proteins, which are linked to actin and are a determinant of paracellular permeability, were altered by mild and severe hypoxia: hypoxia induced a mislocalization of occludin from the TJ to cytoplasm and a decrease in zonula occludens-1 protein level. These modifications were associated with modest changes in paracellular permeability at 0.5% O(2,) as assessed by small 4-kD dextran flux and transepithelial resistance measurements. Together, these findings indicate that hypoxia disrupted cytoskeleton and TJ organization in AEC and may participate, at least in part, to hypoxia-induced decrease in Na transport.

Hypoxic injury of isolated axons is independent of ionotropic glutamate receptors

Axonal injury in white matter is an important consequence of many acute neurological diseases including ischemia. A role for glutamate-mediated excitotoxicity is suggested by observations from in vitro and in situ models that AMPA/kainate blockers can reduce axonal injury. We assessed axonal vulnerability in primary murine neuronal cultures, with axons isolated from their cell bodies using a compartmented chamber design. Transient removal of oxygen and glucose in the axon compartment resulted in irreversible loss of axon length and neurofilament labeling. This injury was not prevented by addition of ionotropic glutamate receptor blockers and could not be reproduced by glutamate receptor agonists. However, hypoxic injury was prevented by blockade of voltage-gated sodium channels, inhibition of calpain and removal of extracellular calcium. These results suggest that isolated, unmyelinated axons are vulnerable to hypoxic injury which is mediated by influx of sodium and calcium but is independent of glutamate receptor activation.

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.

Spatiotemporal distribution of spectrin breakdown products induced by anoxia in adult rat optic nerve in vitro

Hypoxic/ischemic and traumatic injury to central nervous system myelinated axons is heavily dependent on accumulation of Ca ions in the axoplasm, itself promoted by Na influx from the extracellular space. Given the high density of nodal Na channels, we hypothesized that nodes of Ranvier might be particularly vulnerable to Ca overload and subsequent damage, as this is the expected locus of maximal Na influx. Adult rat optic nerves were exposed to in vitro anoxia and analyzed immunohistochemically for the presence of spectrin breakdown. Cleavage of spectrin became detectable between 15 and 30 mins of anoxia, and increased homogeneously along the lengths of fibers; localized breakdown was not observed at nodes of Ranvier at any time point analyzed. Spectrin breakdown was also found in glial processes surrounding axons. Confocal imaging of axoplasmic Ca also revealed a gradual and nonlocalized increase as anoxia progressed, without evidence of Ca 'hot-spots' anywhere along the axons at any time between 0 and 30 mins of anoxic exposure in vitro. Calculations of Ca diffusion rates indicated that even if Ca entered or was released focally in axons, this ion would diffuse rapidly into the internodes and likely produce diffuse injury by activating Ca-dependent proteases. Western blot analysis for voltage-gated Na channel protein revealed that key functional proteins such as these are also degraded by anoxia/ischemia. Thus, proteolysis of structural and functional proteins will conspire to irreversibly injure central axons and render them nonfunctional, eventually leading to transection, degradation, and Wallerian degeneration.

Mitochondrial dysfunction as a cause of axonal degeneration in multiple sclerosis patients

OBJECTIVE

Degeneration of chronically demyelinated axons is a major cause of irreversible neurological disability in multiple sclerosis (MS) patients. Development of neuroprotective therapies will require elucidation of the molecular mechanisms by which neurons and axons degenerate.

METHODS

We report ultrastructural changes that support Ca2+-mediated destruction of chronically demyelinated axons in MS patients. We compared expression levels of 33,000 characterized genes in postmortem motor cortex from six control and six MS brains matched for age, sex, and postmortem interval. As reduced energy production is a major contributor to Ca2+-mediated axonal degeneration, we focused on changes in oxidative phosphorylation and inhibitory neurotransmission.

RESULTS

Compared with controls, 488 transcripts were decreased and 67 were increased (p < 0.05, 1.5-fold) in the MS cortex. Twenty-six nuclear-encoded mitochondrial genes and the functional activities of mitochondrial respiratory chain complexes I and III were decreased in the MS motor cortex. Reduced mitochondrial gene expression was specific for neurons. In addition, pre-synaptic and postsynaptic components of GABAergic neurotransmission and the density of inhibitory interneuron processes also were decreased in the MS cortex.

INTERPRETATION

Our data supports a mechanism whereby reduced ATP production in demyelinated segments of upper motor neuron axons impacts ion homeostasis, induces Ca2+-mediated axonal degeneration, and contributes to progressive neurological disability in MS patients.

Activation of ATP-sensitive potassium channels prevents the cleavage of cytosolic mu-calpain and abrogates the elevation of nuclear c-Fos and c-Jun expressions after hypoxic-ischemia in neonatal rat brain

The purpose of this study was to determine whether activation of ATP-sensitive K+ (KATP) channels with diazoxide (DIZ) is able to prevent the cleavage of cytosolic mu-calpain and abrogate the elevation of nuclear c-Fos and c-Jun protein (c-Fos, c-Jun) expressions after hypoxic-ischemia (HI) in brain. The model of hypoxic-ischemic brain injury (HIBI) was made in the 7-day-old Sprague-Dawley (SD) rats by left carotid arterial ligation and hypoxia (8% oxygen). DIZ was injected into the left lateral ventricle (5 microl, 1 mg/ml) before or post-hypoxic-ischemia (HI) insults. Western blot and computer image processing were used to detect the integrated density of nuclear c-Fos and c-Jun at 4 h and cleavage of cytosolic mu-calpain at 24 h after HI insults from cerebral cortical and hippocampal samples. Compared with HI controls (c-Fos=30.37+/-7.39 from cortical samples, 58.61+/-3.64 from hippocampal samples; c-Jun=52.48+/-14.23 from cortical samples, 35.55+/-4.73 from hippocampal samples), there was a significant down-regulation of c-Fos and c-Jun expressions from cortical and hippocampal samples in rats treated with DIZ before (c-Fos=11.10+/-4.64 from cortical samples, 4.82+/-3.38 from hippocampal samples; c-Jun=19.01+/-5.29 from cortical samples, 35.55+/-4.73 from hippocampal samples) or post- (c-Fos=18.81+/-7.93 from cortical samples, 11.33+/-7.05 from hippocampal samples; c-Jun=24.64+/-10.01 from cortical samples, 19.75+/-3.47 from hippocampal samples) HI insults. Furthermore, the ratio of 76 kD/80 kD of mu-calpain was down-regulated from cortical and hippocampal samples in rats treated with DIZ before or post-HI insults, demonstrating a significant difference compared with that observed in HI controls. Finally, the increase in DNA fragments caused by the HI injury was decreased or eliminated by the treatment with DIZ. These data suggests that activation of KATP channels by DIZ reduces the degree of mu-calpain proteolysis, and c-Fos and c-Jun expressions in immature brain may contribute to the neuroprotection of K(ATP) channel openers against HIBI.

AMPA/kainate receptors mediate axonal morphological disruption in hypoxic white matter

We used acute brain slices to investigate the hypothesis that oxygen-glucose deprivation (OGD) induced loss of axon function and neurofilament labeling are correlated to axonal morphological disruption in the corpus callosum of adult brain. Coronal brain slices including corpus callosum were prepared from adult mice. White matter immunohistochemical properties and conduction along axons remained stable over 12 h after preparation. White matter injury was assessed by recording compound action potentials (CAPs) across corpus callosum, combined with immunofluorescence for axonal neurofilaments and by bright field microscopy of myelin profiles in semi-thin sections. OGD for 30 min resulted in irreversible loss of the CAPs, formation of axon heads and bulbs, and swelling of myelin profiles in slices examined 1h after OGD. In slices followed for 9 h after OGD, there was complete loss of neurofilament labeling and myelin profiles. Because overactivation of AMPA/kainate receptors mediates axon structural and functional disruption in hypoxic corpus callosum slices, we tested whether blockade of AMPA/kainate receptors reduced OGD-induced axonal morphological disruption. NBQX (30 microM), an AMPA/kainate receptor antagonist, prevented OGD-induced formation of axon heads and bulbs, swelling of myelin profiles, loss of neurofilament staining and preserved axonal morphology. These results expand our previous findings that the AMPA/kainate receptor activation contributes to axonal morphological disruption, as well as loss of electrical function.

Calpain inhibitor MDL 28170 protects hypoxic–ischemic brain injury in neonatal rats by inhibition of both apoptosis and necrosis

MDL 28170 is a CNS-penetrating calpain inhibitor, and we examined the effects of MDL 28170 on hypoxic-ischemic brain injury in immature brain using the Rice-Vannucci model. Immediately after hypoxic exposure, 24 mg/kg of MDL 28170 was injected intraperitoneally as an initial dose, followed by 12 mg/kg every 4 h for a total dose of 60 mg/kg over 12 h post-HI. A vehicle control group received peanut oil injection instead. Macroscopic evaluation of brain injury revealed the neuroprotective effect of MDL 28170 after 12 h post-HI. Neuropathological quantitative analysis of cell death showed that MDL 28170 significantly decreased the number of necrotic cells in all the examined regions except for cingular cortex, and the number of apoptotic cells in caudate putamen, parietal cortex, hippocampus CA1, and laterodorsal thalamus. Western blots showed that MDL 28170 suppressed 145/150 kDa subunits of alpha-spectrin breakdown products (SBDP) in cortex, hippocampus, thalamus, and striatum, and also 120-kDa subunit of SBDP in all regions except for striatum. This suggests that MDL 28170 inhibited activation of calpain and caspase-3, respectively. Our results indicate that post-hypoxic MDL 28170 injection is neuroprotective in HI newborn rat brain by decreasing both necrosis and apoptosis. SBDP expression also suggests that MDL 28170 injection inhibits both calpain and caspase-3 activation after HI insult.

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.

Aberrant chloride transport contributes to anoxic/ischemic white matter injury

Rundown of ionic gradients is a central feature of white matter anoxic injury; however, little is known about the contribution of anions such as Cl-. We used the in vitro rat optic nerve to study the role of aberrant Cl- transport in anoxia/ischemia. After 30 min of anoxia (NaN3, 2 mm), axonal membrane potential (V(m)) decreased to 42 +/- 11% of control and to 73 +/- 11% in the presence of tetrodotoxin (TTX) (1 microm). TTX + 4,4'-diisothiocyanatostilbene-2,2' disulfonic acid disodium salt (500 microm), a broad spectrum anion transport blocker, abolished anoxic depolarization (95 +/- 8%). Inhibition of the K-Cl cotransporter (KCC) (furosemide 100 microm) together with TTX was also more effective than TTX alone (84 +/- 14%). The compound action potential (CAP) area recovered to 26 +/- 6% of control after 1 hr anoxia. KCC blockade (10 microm furosemide) improved outcome (40 +/- 4%), and TTX (100 nm) was even more effective (74 +/- 12%). In contrast, the Cl- channel blocker niflumic acid (50 microm) worsened injury (6 +/- 1%). Coapplication of TTX (100 nm) + furosemide (10 microm) was more effective than either agent alone (91 +/- 9%). Furosemide was also very effective at normalizing the shape of the CAPs. The KCC3a isoform was localized to astrocytes. KCC3 and weaker KCC3a was detected in myelin of larger axons. KCC2 was seen in oligodendrocytes and within axon cylinders. Cl- gradients contribute to resting optic nerve membrane potential, and transporter and channel-mediated Cl- fluxes during anoxia contribute to injury, possibly because of cellular volume changes and disruption of axo-glial integrity, leading to propagation failure and distortion of fiber conduction velocities.

Calpain-dependent neurofilament breakdown in anoxic and ischemic rat central axons

Neurofilaments are key structural components of white matter axons. The effect of in vitro anoxia or oxygen-glucose deprivation (OGD) on the integrity of the 160 and 200 kDa neurofilament isoforms was studied by immunoblot, and correlated with physiological function. Adult rat optic nerves were exposed to 60 min of either anoxia or OGD. Compound action potential area recovered to 22+/-6% of control after 60 min of anoxia, and to 4+/-1% after 60 min of OGD. Ca(2+)-free (+EGTA) perfusate allowed complete recovery after OGD (108+/-42%). Tetrodotoxin (TTX, 1 microM) was less protective (45+/-6%). Both anoxia and OGD induced breakdown of neurofilament 160 (NF160) and NF200 revealed by the appearance of multiple lower molecular weight bands mainly in the 75-100 kDa range. Zero-Ca(2+)/EGTA completely prevented NF breakdown. TTX only partially reduced NF160 degradation. Non-phosphorylated NF200 appeared after reperfusion post-anoxia or OGD, and was also greatly reduced by zero-Ca(2+) or TTX. Calpain inhibitors (10 microM calpain inhibitor I or 50 microM MDL 28,170) significantly reduced NF160 and NF200 breakdown/dephosphorylation, but did not improve electrophysiological recovery. Significant calpain-mediated breakdown of NF160 and NF200 indicates structural damage to the axonal cytoskeleton, which was completely Ca(2+)-dependent. While pharmacological inhibition of calpain alone greatly reduced NF proteolysis, there was no concomitant improvement in function. These results imply that calpain inhibition is necessary but not sufficient for white matter protection, and emphasize the existence of multiple Ca(2+)-dependent degradative pathways activated in injured white matter.

Inhibition of rat PC12 cell calpain activity by glutathione, oxidized glutathione and nitric oxide

Calpain, a calcium activated neutral protease, is involved in mediating neurotoxicity resulting from conditions of oxidative stress and free radical formation, such as hypoxia and ischemia. Nitric oxide (NO) may also be involved in modulating the cytotoxic effects of oxidative stress. We investigated the roles of reduced glutathione (GSH), oxidized glutathione (GSSG), and NO in modulating calpain activity in PC12 cells. Cell extracts were treated with GSSG, GSH, or the NO-donor S-nitroso-N-acetylpenicillamine. Calpain activity was determined by means of a fluorescent assay. Non-linear regression analysis was used to determine the type of inhibition (competitive, uncompetitive, or non-competitive). GSH displayed uncompetitive inhibition, with K(i)=7.0+/-2.0 mM (Mean+/-SEM) while GSSG exhibited competitive inhibition with K(i)=2.5+/-0.3 mM. NO was an irreversible inhibitor of calpain activity. These results suggest that both GSH and GSSG may be important physiological modulators of calpain activity.

Calcium imaging in live rat optic nerve myelinated axons in vitro using confocal laser microscopy

Intracellular Ca(2+) plays a major role in the physiological responses of excitable cells, and excessive accumulation of internal Ca(2+) is a key determinant of cell injury and death. Many studies have been carried out on the internal Ca(2+) dynamics in neurons. In constrast, there is virtually no such information for mammalian central myelinated axons, due in large part to technical difficulty with dye loading and imaging such fine myelinated structures. We developed a technique to allow imaging of ionized Ca(2+) in live rat optic nerve axons with simultaneous electrophysiological recording in vitro at 37 degrees C using confocal microscopy. The K(+) salt of the Ca(2+)-sensitive indicator Oregon Green 488 BAPTA-2 and the Ca(2+)-insensitive reference dye Sulforhodamine 101 were loaded together into rat optic nerves using a low-Ca(2+)/low-Na(+) solution. Axonal profiles, confirmed immunohistochemically by double staining with neurofilament-160 antibodies, were clearly visualized by S101 fluorescence up to 800 microm from the cut ends. The Ca(2+) signal was very low at rest, just above the background fluorescence intensity, indicating healthy tissue, and increased significantly after caffeine (20 mM) exposure designed to release internal Ca(2+) stores. The health of imaged regions was further confirmed by a virtual absence of spectrin breakdown, which is induced by calpain activation in damaged CNS tissue. Red and green fluorescence decayed to no less than 70% of control after 60 min of recording at 37 degrees C, with the green:red fluorescence ratio increasing slightly by 21% after 60 min. Electrophysiological responses recorded simultaneously with confocal images remained largely stable as well.

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.