Brain fodrin: substrate for calpain I, an endogenous calcium-activated protease

Selective release of calpain produced alphalI-spectrin (alpha-fodrin) breakdown products by acute neuronal cell death

Activation of calpain results in the breakdown of alpha II spectrin (alpha-fodrin), a neuronal cytoskeleton protein, which has previously been detected in various in vitro and in vivo neuronal injury models. In this study, a 150 kDa spectrin breakdown product (SBDP150) was found to be released into the cell-conditioned media from SH-SY5Y cells treated with the calcium channel opener maitotoxin (MTX). SBDP150 release can be readily quantified on immunoblot using an SBDP150-specific polyclonal antibody. Increase of SBDP150 also correlated with cell death in a time-dependent manner. MDL28170, a selective calpain inhibitor, was the only protease inhibitor tested that significantly reduced MTX-induced SBDP150 release. The cell-conditioned media of cerebellar granule neurons challenged with excitotoxins (NMDA and kainate) also exhibited a significant increase of SBDP150 that was attenuated by pretreatment with an NMDA receptor antagonist, R(-)-3-(2-carbopiperazine-4-yl)-propyl-1-phosphonic acid (CPP), and MDL28170. In addition, hypoxic/hypoglycemic challenge of cerebrocortical cultures also resulted in SBDP150 liberation into the media. These results support the theory that an antibody-based detection of SBDP150 in the cell-conditioned media can be utilized to quantify injury to neural cells. Furthermore, SBDP150 may potentially be used as a surrogate biomarker for acute neuronal injury in clinical settings.

Extracellular matrix molecules, long-term potentiation, memory consolidation and the brain angiotensin system

Considerable evidence now suggests an interrelationship among long-term potentiation (LTP), extracellular matrix (ECM) reconfiguration, synaptogenesis, and memory consolidation within the mammalian central nervous system. Extracellular matrix molecules provide the scaffolding necessary to permit synaptic remodeling and contribute to the regulation of ionic and nutritional homeostasis of surrounding cells. These molecules also facilitate cellular proliferation, movement, differentiation, and apoptosis. The present review initially focuses on characterizing the ECM and the roles of cell adhesion molecules (CAMs), matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs), in the maintenance and degradation of the ECM. The induction and maintenance of LTP is described. Debate continues over whether LTP results in some form of synaptic strengthening and in turn promotes memory consolidation. Next, the contribution of CAMs and TIMPs to the facilitation of LTP and memory consolidation is discussed. Finally, possible roles for angiotensins, MMPs, and tissue plasminogen activators in the facilitation of LTP and memory consolidation are described. These enzymatic pathways appear to be very important to an understanding of dysfunctional memory diseases such as Alzheimer's disease, multiple sclerosis, brain tumors, and infections.

Dentate hilar mossy cells and somatostatin-containing neurons are immunoreactive for the alpha8 integrin subunit: characterization in normal and kainic acid-treated rats

Integrins are heterodimeric cell surface receptors composed of different alpha and beta subunits that mediate cell-cell and cell-extracellular matrix interactions. They have been implicated in the regulation of neuronal migration, differentiation, process outgrowth, and plasticity. The alpha8 integrin subunit associates exclusively with the beta1 subunit to form a receptor (alpha8beta1) for fibronectin, vitronectin, tenascin, and osteopontin. In a previous study, we demonstrated that hippocampal dentate hilar neurons are immunoreactive for alpha8. The present study identifies the major types of alpha8-immunoreactive hilar neurons and characterizes the effects of kainic acid-induced seizures on alpha8-immunoreactivity in these cells. Examination of the hilus in normal rats revealed alpha8-immunoreactivity in the somatodendritic compartments of large hilar neurons identified as mossy cells, including a subset of dendritic thorny excrescences that were contacted by large mossy fiber terminals. alpha8-immunoreactivity also was found in approximately 71% of somatostatin-containing hilar cells. Kainic acid-induced seizures dramatically and rapidly altered the levels and distribution of alpha8-immunoreactivity in hilar neurons. After 1.5 h of seizures, alpha8-immunoreactivity in their dendrites was reduced greatly. One day after kainic acid treatment, labeling was diminished throughout the somatodendritic compartments of most hilar cells. This decrease appeared to be transient, since alpha8 labeling returned to normal levels in surviving hilar neurons within 2 weeks of treatment. In addition, many alpha8-immunoreactive hilar neurons, particularly in caudal dentate regions, were lost 3-5 weeks after kainic acid treatment. Our findings suggest that alpha8beta1 may mediate adhesive interactions of the dendritic processes of mossy cells and somatostatin-containing hilar neurons with other cellular elements or with extracellular matrix components. They also suggest that alpha8 may be susceptible to activity-dependent proteolysis that could modulate its function in the somatodendritic compartment of these cells.

The pathogenic activation of calpain: a marker and mediator of cellular toxicity and disease states

Over-activation of calpain, a ubiquitous calcium-sensitive protease, has been linked to a variety of degenerative conditions in the brain and several other tissues. Dozens of substrates for calpain have been identified and several of these have been used to measure activation of the protease in the context of experimentally induced and naturally occurring pathologies. Calpain-mediated cleavage of the cytoskeletal protein spectrin, in particular, results in a set of large breakdown products (BDPs) that are unique in that they are unusually stable. Over the last 15 years, measurements of BDPs in experimental models of stroke-type excitotoxicity, hypoxia/ischemia, vasospasm, epilepsy, toxin exposure, brain injury, kidney malfunction, and genetic defects, have established that calpain activation is an early and causal event in the degeneration that ensues from acute, definable insults. The BDPs also have been found to increase with normal ageing and in patients with Alzheimer's disease, and the calpain activity may be involved in related apoptotic processes in conjunction with the caspase family of proteases. Thus, it has become increasingly clear that regardless of the mode of disturbance in calcium homeostasis or the cell type involved, calpain is critical to the development of pathology and therefore a distinct and powerful therapeutic target. The recent development of antibodies that recognize the site at which spectrin is cleaved has greatly facilitated the temporal and spatial resolution of calpain activation in situ. Accordingly, sensitive spectrin breakdown assays now are utilized to identify potential toxic side-effects of compounds and to develop calpain inhibitors for a wide range of indications including stroke, cerebral vasospasm, and kidney failure.

Calpain-dependent proteolysis of NF2 protein: involvement in schwannomas and meningiomas

The neurofibromatosis type 2 (NF2) protein, known as merlin or schwannomin, is a tumor suppressor, and the NF2 gene has been found to be mutated in the majority of schwannomas and meningiomas, including both sporadically occurring and familial NF2 cases. Although the development of these tumors depends on the loss of merlin, the presence of tumors lacking detectable NF2 mutations suggests different mechanisms for inactivating merlin. Recent studies have demonstrated cleavage of merlin by calpain, a calcium-dependent neutral cysteine protease, and marked activation of the calpain system resulting in the degradation of merlin in these tumors. Increased turnover of merlin by calpain in some schwannomas and meningiomas exemplifies tumorigenesis linked to the calpain-mediated proteolytic pathway.

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

Calpains are ubiquitous Ca(2+)-activated neutral proteases that have been implicated in ischemic and traumatic CNS injury. Ischemia and trauma of central white matter are dependent on Ca2+ accumulation, and calpain overactivation likely plays a significant role in the pathogenesis. Adult rat optic nerves, representative central white matter tracts, were studied in an in vitro anoxic model. Functional recovery following 60 min of anoxia and reoxygenation was measured electrophysiologically. Calpain activation was assessed using western blots with antibodies against calpain-cleaved spectrin breakdown products. Sixty minutes of in vitro anoxia increased the amount of spectrin breakdown approximately 20-fold over control, with a further increase after reoxygenation to >70 times control, almost as much as 2 h of continuous anoxia. Blocking voltage-gated Na+ channels with tetrodotoxin or removing bath Ca2+ was highly neuroprotective electrophysiologically and resulted in a marked reduction of spectrin degradation. The membrane-permeable calpain inhibitors MDL 28,170 and calpain inhibitor-I (10-100 microM) were effective at reducing spectrin breakdown in anoxic and reoxygenated optic nerves, but no electrophysiological improvement was observed. We conclude that calpain activation is an important step in anoxic white matter injury, but inhibition of this Ca(2+)-dependent process in isolation does not improve functional outcome, probably because other deleterious Ca(2+)-activated pathways proceed unchecked.

A BODIPY fluorescent microplate assay for measuring activity of calpains and other proteases

The use of 4,4-difluoro-5,7-dimethyl-4-bora-3a, 4a-diaza-s-indacene-3-propionic acid (BODIPY-FL) labeled casein in autoquenching assays of proteolytic activity has been recently described, and we have adapted this assay to measurement of calpain activity. BODIPY-FL coupled to casein at a ratio of 8 mol of BODIPY-FL/mol of casein or higher produces a BODIPY-FL-casein substrate that can be used in an autoquenching assay of calpain proteolytic activity. This assay has a number of advantages for measuring calpain activity. (1) The procedure does not require precipitation and removal of undegraded protein, so it is much faster than other procedures that require a precipitation step, and it can be used directly in kinetic assays of proteolytic activity. (2) The BODIPY-FL-casein assay is easily adapted to a microtiter plate format, so it can be used to screen large numbers of samples. (3) Casein is an inexpensive and readily available protein substrate that more closely mimics the natural substrates of endoproteinases, such as the calpains, than synthetic peptide substrates do. Casein has K(m) values for micro- and m-calpain that are similar to those of other substrates such as fodrin or MAP2 that may be "natural" substrates for the calpains, and there is no reason to believe that calpain hydrolysis of casein is inherently different from hydrolysis of fodrin or MAP2, which are much less accessible as substrates for protease assays. (4) The BODIPY-FL-casein assay is capable of detecting 10 ng ( approximately 5 nM) of calpain and is nearly as sensitive as the most sensitive calpain assay reported thus far. (5) The BODIPY-FL-casein assay is as reproducible as the FITC-casein assay, whose reproducibility is comparable to or better than the reproducibility of other methods used to assay calpain activity. The BODIPY-FL-casein assay is a general assay for proteolytic activity and can be used with any protease that cleaves casein.

Synthesis, assembly, and turnover of alpha and beta-erythroid and nonerythroid spectrins in rat hippocampal neurons

The synthesis and turnover of alpha-erythroid, beta-erythroid, alpha-nonerythroid and beta-nonerythroid spectrins was investigated in cultured rat hippocampal neurons. [35S]methionine and subunit specific antibodies were used to label and immunoprecipitate newly synthesized spectrins in 12- to 14-day-old cultures. Synthesis experiments, performed under normal resting conditions, showed that the ratio of newly synthesized alpha-erythroid/beta-erythroid and alpha-nonerythroid/beta-nonerythroid spectrins is 1/1 (mol/mol) both in the soluble and insoluble fractions. Soluble and insoluble alpha and beta erythroid spectrin turn over rapidly (half-life=16-24 min). Soluble nonerythroid alpha-spectrin (half-life=80 min) and beta spectrin (half-life=53 min) turn over more slowly than their insoluble counterparts (30-34 min). The nonerythroid alpha spectrin turnover was significantly different (p<0.05) from the other measurements except for nonerythroid beta spectrin, indicating that these subunits are protected from rapid proteolytic degradation until they are assembled in the membrane skeleton.

Moderate posttraumatic hypothermia decreases early calpain-mediated proteolysis and concomitant cytoskeletal compromise in traumatic axonal injury

Traumatic brain injury (TBI) in animals and man generates widespread axonal injury characterized by focal axolemmal permeability changes, induction of calpain-mediated proteolysis, and neurofilament side-arm modification associated with neurofilament compaction (NFC) evolving to axonal disconnection. Recent observations have suggested that moderate hypothermia is neuroprotective in several models of TBI. Nevertheless, the pathway by which hypothermia prevents traumatic axonal injury (TAI) is still a matter of debate. The present study was conducted to evaluate the effects of moderate, early posttraumatic hypothermia on calpain-mediated spectrin proteolysis (CMSP), implicated in the pathogenesis of TAI. Using moderate (32 degrees C) hypothermia of 90 min duration without rewarming, the density of CMSP immunoreactive/damaged axons was quantified via LM analysis in vulnerable brain stem fiber tracts of hypothermic and normothermic rats subjected to impact acceleration TBI (90 min postinjury survival). To assess the influence of posthypothermic rewarming, a second group of animals was subjected to 90 min of hypothermia followed by 90 min of rewarming to normothermic levels when CMSP was analyzed to detect if any purported CMSP prevention persisted (180 min postinjury survival). Additionally, to determine if this protection translated into comparable cytoskeletal protection in the same foci showing decreased CMSP, antibodies targeting altered/compacted NF subunits were also employed. Moderate hypothermia applied in the acute postinjury period drastically reduced the number of damaged axons displaying CMSP at both time points and significantly reduced NFC immunoreactivity at 180 min postinjury. These results suggest that the neuroprotective effects of hypothermia in TBI are associated with the inhibition of axonal/cytoskeletal damage.

Postinjury cyclosporin A administration limits axonal damage and disconnection in traumatic brain injury

Recent observations concerning presumed calcium-induced mitochondrial damage and focal intraaxonal proteolysis in the pathogenesis of traumatic axonal injury (TAI) have opened new perspectives for therapeutic intervention. Studies from our laboratory demonstrated that cyclosporin A (CsA), a potent inhibitor of Ca2+-induced mitochondrial damage, administered 30 min prior to traumatic brain injury preserved mitochondrial integrity in those axonal foci destined to undergo delayed disconnection. We attributed this neuroprotection to the inhibition by CsA of mitochondrial permeability transition (MPT). Additional experiments proved that CsA pretreatment also significantly reduced calcium-induced, calpain-mediated spectrin proteolysis (CMSP) and neurofilament compaction (NFC), pivotal events in the pathogenesis of axonal failure and disconnection. Given these provocative findings the goal of the current study was to evaluate the potential of CsA to inhibit calcium-induced axonal damage in a more clinically relevant postinjury treatment paradigm. To this end, cyclosporin A was administered intrathecally to Sprague Dawley rats 30 min following impact acceleration traumatic brain injury. The first group of animals were sacrificed 120 min postinjury and the density of CMSP and NFC immunoreactive damaged axonal segments of CsA-treated and vehicle-treated injured animals were quantitatively analyzed. A second group of CsA- versus vehicle-treated rats was sacrificed at 24 h postinjury to compare the density of damaged axons displaying beta amyloid precursor protein (APP) immunoreactivity, a signature protein of axonal perturbation and disconnection. Postinjury CsA administration resulted in a significant decrease (>60%) in CMSP/NFC immunoreactivity in corticospinal tracts and medial longitudinal fasciculi. A similar decrease was detected in the density of APP immunoreactive damaged axons, indicating an attenuation of axonal disconnection at 24 h postinjury in CsA-treated animals. These results once again suggest that the maintenance of the functional integrity of the mitochondria can prevent TAI, presumably via the preservation of the local energy homeostasis of the axon. Moreover and perhaps more importantly, these studies also demonstrate the efficacy of CsA administration when given in the early posttraumatic period. Collectively, our findings suggest that a therapeutic window exists for the use of drugs targeting mitochondria and energy regulation in traumatic brain injury.

Calpastatin is up-regulated in response to hypoxia and is a suicide substrate to calpain after neonatal cerebral hypoxia-ischemia

In a model of cerebral hypoxia-ischemia in the immature rat, widespread brain injury is produced in the ipsilateral hemisphere, whereas the contralateral hemisphere is left undamaged. Previously, we found that calpains were equally translocated to cellular membranes (a prerequisite for protease activation) in the ipsilateral and contralateral hemispheres. However, activation, as judged by degradation of fodrin, occurred only in the ipsilateral hemisphere. In this study we demonstrate that calpastatin, the specific, endogenous inhibitor protein to calpain, is up-regulated in response to hypoxia and may be responsible for the halted calpain activation in the contralateral hemisphere. Concomitantly, extensive degradation of calpastatin occurred in the ipsilateral hemisphere, as demonstrated by the appearance of a membrane-bound 50-kDa calpastatin breakdown product. The calpastatin breakdown product accumulated in the synaptosomal fraction, displaying a peak 24 h post-insult, but was not detectable in the cytosolic fraction. The degradation of calpastatin was blocked by administration of CX295, a calpain inhibitor, indicating that calpastatin acts as a suicide substrate to calpain during hypoxia-ischemia. In summary, calpastatin was up-regulated in areas that remain undamaged and degraded in areas where excessive activation of calpains and infarction occurs.

DY-9760e, a novel calmodulin antagonist, reduces brain damage induced by transient focal cerebral ischemia

Perturbations in Ca2+ homeostasis have been proposed to lead to neuronal damage after cerebral ischemia. DY-9760e (3-[2-[4-(3-chloro-2-methylphenyl)-1-piperazinyl]ethyl]-5,6-dimethoxy-1- (4-imidazolylethyl)-1H-indazole dihydrochloride 3.5 hydrate) is a novel calmodulin antagonist. In this study, we examined the effects of DY-9760e on brain damage in rats subjected to transient (1 h) focal cerebral ischemia. DY-9760e (0.25-1.00 mg kg(-1) h(-1)) was intravenously infused for 6 h, starting 1 h after middle cerebral artery occlusion. Treatment with DY-9760e 0.25, 0.50 and 1.00 mg kg(-1) h(-1) reduced infarct volume by 30, 42 (P < 0.05), and 60% (P < 0.05), respectively. Furthermore, the effect of DY-9760e on ischemia-induced fodrin breakdown was examined in the same model. Pronounced fodrin breakdown was observed in the cerebral cortex and striatum at 24 h after ischemia. DY-9760e caused potent suppression of fodrin breakdown in the cerebral cortex at the same doses as those that had a protective action against cerebral infarction. From these results DY-9760e may have a therapeutic effect against cerebral ischemic damage in the acute stage.

Species differences in fodrin proteolysis in the ischemic brain

There has been growing evidence that the breakdown of cytoskeletal proteins is an important biochemical change leading to ischemic neuronal death. In the present study, we investigated species differences in the susceptibility of fodrin to calpain activation induced by cerebral ischemia in gerbils, rats, and mice. In vivo fodrin proteolysis and degradation of microtubule-associated protein 2 after complete ischemia occurred more rapidly in the hippocampus and cerebral cortex of the gerbil brain than in the corresponding area of the rat and mouse brain. The N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 injected intraperitoneally before ischemia did not diminish fodrin degradation in the gerbil hippocampus. In vivo fodrin proteolysis was inhibited at 33 degrees C and enhanced at 41 degrees C compared with proteolysis at 37 degrees C during ischemia. However, in vitro fodrin proteolysis after addition of Ca2+ into the crude membrane fraction did not show any differences among three species. Although it is highly unlikely that the difference in the sensitivity of NMDA receptor or the sensitivity of calpain activation to calcium was the crucial determinant of susceptibility of fodrin degradation in the gerbil brain, the present study clearly demonstrated that fodrin in the gerbil brain was more susceptible to calpain activation induced by ischemia than that in the rat and mouse brains. Enhanced proteolysis may be one of the reasons neurons in the gerbil brain are highly vulnerable to ischemia.

Cortical ablation induces a spreading calcium-dependent, secondary pathogenesis which can be reduced by inhibiting calpain

Many forms of acute brain injury are associated with a secondary, glutamate- and calcium-dependent cascade which greatly exacerbates the final damage. The calcium-dependent protease, calpain, has been implicated as an important variable in this pathogenic process. The present studies tested (i) if similar secondary degeneration occurs following surgical ablation of a discrete area within rat visual cortex, (ii) if activation of calpain contributes to the secondary degeneration by spreading into areas adjacent to the ablation, and (iii) if blocking calpain's proteolytic effects reduces the secondary degeneration attendant to the lesion. Antibodies selective for a protein fragment specifically generated by calpain were used to map areas in which the protease had been activated. Labeling was present 5 min after surgery in a narrow zone surrounding the ablated region. The volume of the immunopositive staining increased twofold within 24 h and fivefold by 48 h, at which time it was equivalent in size to the original lesion. This staining pattern significantly decreased in size at 5 days postsurgery. Application of calpain inhibitors to the ablation site immediately after surgery reduced the spread of calpain activation by approximately 80%. Following cortical ablation, the volume of the lateral geniculate nucleus ipsilateral to the cortical ablation shrank by 46 +/- 3% in control rats but only by 31 +/- 5% in animals given the calpain inhibitors. These results establish that (i) a secondary degenerative cascade is unleashed following discrete cortical surgery which expands into brain areas clearly outside the initial perturbation site, (ii) the gradual expansion of calpain activation contributes to the underlying secondary pathology, and (iii) blocking calpain activity substantially reduces atrophy in areas anatomically connected, but physically distal to the damaged zone. The possible utility of topical applications of calpain inhibitors, or analogously acting drugs, in minimizing the secondary effects of brain surgery is discussed.

Application of fluorescence polarization to the steady-state enzyme kinetic analysis of calpain II

This paper presents the application of fluorescence polarization to the determination of dissociation constants for competitive inhibitors that bind to enzymes. This steady-state enzyme kinetic study measures the inhibition of the conversion of a fluorescently tagged substrate to a lower molecular weight fluorescent product by calpain II. It relies on the measurement of a parameter proportional to velocity, which is sufficient for this type of analysis. The strengths and limitations of the method are discussed. Inhibition constants for filamin and spectrin determined by this method are 125 nM and 13 nM respectively.

Selective coupling of mu-calpain activation with the NMDA receptor is independent of translocation and autolysis in primary cortical neurons

Excessive mu-calpain activation has been linked to several cellular pathologies including excitotoxicity and ischemia. In erythrocytes and other non-central nervous system (CNS) cells, calpain activation is thought to occur following a Ca2+-induced translocation of inactive cytosolic enzyme to membranes and subsequent autolysis. In the present report, we show that transiently exposing primary rat cortical neurons to lethal (50 microM) N-methyl-D-aspartic acid (NMDA) caused protracted calpain activation, measured as increased spectrin hydrolysis, but this was independent of translocation or autolysis of the protease. An anti-mu-calpain antibody showed that calpain was largely membrane associated in cortical neurons, and, consequently, neither translocation nor autolysis of the protease was observed following ionomycin or lethal NMDA treatment. By contrast, in rat erythrocytes, calpain was largely cytosolic and underwent rapid translocation and autolysis in response to ionomycin. Calpain-mediated spectrin hydrolysis was specifically coupled to Ca2+ entry through the NMDA receptor because nonspecific Ca2+ influx via ionomycin or KCl-mediated depolarization failed to activate the enzyme. Thus, calpain appears selectively linked to glutamate receptors in cortical neurons and regulated by mechanisms distinct from that occurring in many non-CNS cells. The data suggest that intracellular signals coupled to the NMDA receptor are responsible for activating calpain already associated with cellular membranes in cortical cells.

Novel pharmacologic strategies in the treatment of experimental traumatic brain injury: 1998

The mechanisms underlying secondary or delayed cell death following traumatic brain injury are poorly understood. Recent evidence from experimental models suggests that widespread neuronal loss is progressive and continues in selectively vulnerable brain regions for months to years after the initial insult. The mechanisms underlying delayed cell death are believed to result, in part, from the release or activation of endogenous "autodestructive" pathways induced by the traumatic injury. The development of sophisticated neurochemical, histopathological and molecular techniques to study animal models of TBI have enabled researchers to begin to explore the cellular and genomic pathways that mediate cell damage and death. This new knowledge has stimulated the development of novel therapeutic agents designed to modify gene expression, synthesis, release, receptor or functional activity of these pathological factors with subsequent attenuation of cellular damage and improvement in behavioral function. This article represents a compendium of recent studies suggesting that modification of post-traumatic neurochemical and cellular events with targeted pharmacotherapy can promote functional recovery following traumatic injury to the central nervous system.

Ca(2+)-dependent mechanisms of cell injury in cultured cortical neurons

The contributions of several Ca(2+)-dependent processes to neurotoxicity were examined in primary cultures of rat cortical neurons. The Ca2+ ionophore ionomycin induced a rapid loss of axonal morphology and concomitant release of inositol phosphates that preceded morphological alterations of neuronal cell bodies, choline and arachidonate release, and protein degradation. These events were followed by a degree of neuronal lysis proportional to the external Ca2+ concentration and exposure time. The phospholipase inhibitor neomycin decreased both arachidonate release and the phospholipid hydrolysis catalysed by phospholipases C and D. Proteolysis was abated by the protease inhibitor leupeptin, but not by lysosomal proteolysis inhibitors. Neuronal lysis was inhibited partially by either leupeptin or neomycin and almost completely by both in combination. However, neither agent, alone or in combination, affected the morphological derangements. The diacylglycerol lipase inhibitor RHC-80267 reduced arachidonate release, but not neuronal lysis. Phospholipase A2 inhibitors had no effect on either arachidonate release or lysis. Treatment of mixed cultures of neurons and glia with a Ca(2+)-dependent glutamate challenge caused similar morphological changes and a delayed neuronal lysis that was also diminished by leupeptin and neomycin, but not by inhibitors of lysosomal proteolysis. These data describe several distinct stages of Ca(2+)-dependent injury to cortical neurons, a key feature of which is the stimulation of protease, and phospholipase C and D activities. The initial stage is characterized by a rapid loss of axonal morphology and increased phosphatidylinositol hydrolysis. An intermediate stage involves changes in cell body morphology plus the degradation of neuronal protein and phosphatidylcholine. In a later stage, the loss of plasma membrane integrity denotes neuronal death.

The Dorothy Russell Memorial Lecture. The molecular and cellular sequelae of experimental traumatic brain injury: pathogenetic mechanisms

The mechanisms underlying secondary or delayed cell death following traumatic brain injury (TBI) are poorly understood. Recent evidence from experimental models of TBI suggest that diffuse and widespread neuronal damage and loss is progressive and prolonged for months to years after the initial insult in selectively vulnerable regions of the cortex, hippocampus, thalamus, striatum, and subcortical nuclei. The development of new neuropathological and molecular techniques has generated new insights into the cellular and molecular sequelae of brain trauma. This paper will review the literature suggesting that alterations in intracellular calcium with resulting changes in gene expression, activation of reactive oxygen species (ROS), activation of intracellular proteases (calpains), expression of neurotrophic factors, and activation of cell death genes (apoptosis) may play a role in mediating delayed cell death after trauma. Recent data suggesting that TBI should be considered as both an inflammatory and/or a neurodegenerative disease is also presented. Further research concerning the complex molecular and neuropathological cascades following brain trauma should be conducted, as novel therapeutic strategies continue to be developed.

The involvement of calpain-dependent proteolysis of the tumor suppressor NF2 (merlin) in schwannomas and meningiomas

Neurofibromatosis type 2 (NF2) protein, also known as merlin or schwannomin, is a tumor suppressor, and NF2 is mutated in most schwannomas and meningiomas. Although these tumors are dependent on NF2, some lack detectable NF2 mutations, which indicates that alternative mechanisms exist for inactivating merlin. Here, we demonstrate cleavage of merlin by the ubiquitous protease calpain and considerable activation of the calpain system resulting in the loss of merlin expression in these tumors. Increased proteolysis of merlin by calpain in some schwannomas and meningiomas exemplifies tumorigenesis linked to the calpain-mediated proteolytic pathway.

Memory and the brain: unexpected chemistries and a new pharmacology

Efforts to characterize long-term potentiation (LTP) and to identify its substrates have led to the discovery of novel synaptic chemistries, computational algorithms, and, most recently, pharmacologies. Progress has also been made in using LTP to develop a "standard model" of how unusual, but physiologically plausible, levels of afferent activity create lasting changes in the operating characteristics of synapses in the cortical telencephalon. Hypotheses of this type typically distinguish induction, expression, and consolidation stages in the formation of LTP. Induction involves a sequence consisting of theta-type rhythmic activity, suppression of inhibitory currents, intense synaptic depolarization, NMDA receptor activation, and calcium influx into dendritic spines. Calcium-dependent lipases, kinases, and proteases have been implicated in LTP induction. Regarding the last group, it has been recently reported that theta pattern stimulation activates calpain and that translational suppression of the protease blocks potentiation. It is thus likely that proteolysis is readily driven by synaptic activity and contributes to structural reorganization. LTP does not interact with treatments that affect transmitter release, has a markedly differential effect on the currents mediated by colocalized AMPA vs NMDA synaptic receptors, changes the waveform of the synaptic current, modifies the effects of drugs that modulate AMPA receptors, and is sensitive to the subunit composition of those receptors. These results indicate that LTP is expressed by changes in AMPA receptor operations. LTP is accompanied by modifications in the anatomy of synapses and spines, something which accounts for its extreme duration (weeks). As with various types of memory, LTP requires about 30 min to consolidate (become resistant to disruption). Consolidation involves adhesion chemistries and, in particular, activation of integrins, a class of transmembrane receptors that control morphology in numerous cell types. Platelet activating factor and adenosine may contribute to consolidation by regulating the engagement of latent integrins. How consolidation stabilizes LTP expression is a topic of intense investigation but probably involves modifications to one or more of the following: membrane environment of AMPA receptors; access of regulatory proteins (e.g., kinases, proteases) to the receptors; receptor clustering; and space available for receptor insertion. Attempts to enhance LTP have focused on the induction phase and resulted in a class of centrally active drugs ("ampakines") that positively modulate AMPA receptors. These compounds promote LTP in vivo and improve the encoding of variety of memory types in animals. Positive results have also been obtained in preliminary studies with humans.

Real time imaging of calcium-induced localized proteolytic activity after axotomy and its relation to growth cone formation

The emergence of a neuronal growth cone from a transected axon is a necessary step in the sequence of events that leads to successful regeneration. Yet, the molecular mechanisms underlying its formation after axotomy are unknown. In this study, we show by real time imaging of the free intracellular Ca2+ concentration, of proteolytic activity, and of growth cone formation that the activation of localized and transient Ca2+-dependent proteolysis is a necessary step in the cascade of events that leads to growth cone formation. Inhibition of this proteolytic activity by calpeptin, a calpain inhibitor, abolishes growth cone formation. We suggest that calpain plays a central role in the reorganization of the axon's cytoskeleton during its transition from a stable differentiated structure into a dynamically extending growth cone.

Temporal ordering of pathogenic events following transient global ischemia

Rats were subjected to transient global ischemia (four vessel occlusion) and time-related changes in the selectively vulnerable hippocampal field CA1 were characterized. The assessment included ex vivo field responses to afferent stimulation, silver staining, calpain-induced spectrin breakdown, chromatolysis, and cell death, beginning at 6 h post-ischemia and continuing until total disintegration of the pyramidal cells occurred several days later. The earliest change observed was a modest increase in the slope and amplitude of field CA1 potentials (at 6 h). The hyperresponsiveness was most apparent at higher stimulation currents and persisted unchanged at 16 h post-ischemia. Three effects became detectable within 24 h, post-ischemia: (a) an increase in concentrations of calpain-mediated, spectrin breakdown products; (b) enhanced silver staining in the deep pyramidal neurons of the field CA1 with lesser, though still apparent, staining of stratum radiatum, and (c) a decrease in amplitude and slope of field CA1 responses to afferent stimulation. Both the concentration of spectrin breakdown products and the intensity of silver staining progressively increased to a maximum at four days post ischemia, while the amplitude and slope of the field responses dropped to a very low level between 24 and 48 h. Disturbances of Nissl staining were finally evident at 48 h, with nearly complete disappearance of staining at five days post-ischemia. This study provides the first demonstration of a close and early temporal relationship between calpain proteolysis, subcellular damage to the pyramidal cells and their loss of function following global ischemia, prior to their eventual death.

Induction of growth cone formation by transient and localized increases of intracellular proteolytic activity

The formation of a growth cone at the tip of a transected axon is a crucial step in the subsequent regeneration of the amputated axon. During this process, the transected axon is transformed from a static segment into a motile growth cone. Despite the importance of this process for regeneration of the severed axon, little is known about the mechanisms underlying this transformation. Recent studies have suggested that Ca2+-activated proteinases underlay the morphological remodeling of neurons after injury. However, this hypothesis was never tested directly. Here we tested the ability of transient and localized increases in intracellular proteolytic activity to induce growth cone formation and neuritogenesis. Minute amounts of the proteinase trypsin were microinjected into intact axonal segments or somata of cultured Aplysia neurons, transiently elevating the intracellular protease concentration to 13-130 nM in the vicinity of the injection site. Such microinjections were followed by the formation of ectopic growth cones and irreversible neuritogenesis. Growth cones were not formed after external application of trypsin, microinjection of the carrier solution, or inactivated trypsin. Growth cone formation was not preceded by increases in free intracellular Ca2+ or changes in passive membrane properties, and was blocked by inhibitors of actin and tubulin polymerization. Trypsin-induced neuritogenesis was associated with ultrastructural alterations similar to those observed by us after axotomy. We conclude that local and transient elevations of cytoplasmic proteolytic activity can induce growth cone formation and neuritogenesis, and suggest that localized proteolytic activity plays a role in growth cone formation after axotomy.

The calpain proteolytic system in neonatal hypoxic-ischemia

Neonatal rats were subjected to transient cerebral hypoxic-ischemia (HI, unilateral occlusion of the common carotid artery +7.70% O2 for 100 min) and allowed to recover for up to 14 days. Calpain caseinolytic activity was found to increase in both hemispheres for at least 20 hr. Hypoxic exposure per se increased the activity of calpains, more pronounced in a membrane-associated fraction, probably through interaction with cellular components, whereas HI introduced a loss of activity, most likely through consumption and loss of proteases. Consecutive tissue sections were stained with antibodies against calpastatin, alpha-fodrin, the 150-kDa breakdown product of alpha-fodrin (FBDP, marker of calpain proteolysis) or microtubule-associated protein 2 (MAP-2, marker of dendrosomatic neuronal injury). Areas with brain injury displayed a distinct loss of MAP-2, which clearly delineated the infarct. FBDP accumulated in injured and borderline regions ipsilaterally, and a less conspicuous, transient increase in FBDP also occurred in the contralateral hemisphere, especially in the white matter. The cytosolic fraction (CF) and the membrane and microsomal fraction (MMF) of cortical tissue were subjected to Western blotting and stained with antibodies against calpain, calpastatin and the 150-kDa breakdown product of alpha-fodrin (FBDP). Calpain immunoreactivity decreased bilaterally in the CF during the insult (62-68% of controls) and remained significantly lower during early recovery, whereas the MMF showed no significant changes. This translocation of calpains coincided with the appearance of FBDP in the ipsilateral, HI hemisphere, displaying a significantly higher level of FBDP from immediately after the insult until at least 1 day of recovery (204-292% of controls). No significant changes in FBDP were found in the contralateral, undamaged hemisphere, despite translocation of calpains in both hemispheres, a prerequisite for calpain activation. This discrepancy may be related to changes in the endogenous inhibitor, calpastatin. Calpastatin protein was found to decrease during and shortly after HI in the ipsilateral, but not the contralateral, hemisphere. The inhibitory activity of calpastatin also tended to decrease after HI, indicating that a reduction of calpastatin may be necessary for extensive calpain activation to occur. The mRNA of m-calpain increased in the HI hemisphere 48 hr after the insult (167%, p < 0.001), a time point when the protein was also increased. In summary, our findings indicate that calpains are activated during HI and in the early phase of reperfusion after HI, preceding neuronal death.

The Calpain Hypothesis of Neurodegeneration: Evidence for a Common Cytotoxic Pathway

Calpain's general function and pathogenic role in the CNS are reviewed. Collectively, the literature indicates that calpain proteolysis plays a common and important role in a variety of acute neurodegenerative conditions, including focal ischemia (stroke), global ischemia, traumatic brain injury, and spinal cord injury. This evidence indicates that 1) calpain is activated in an abnormally sustained fashion during cellular events commonly associated with neurodegeneration (e.g., excessive interstitial glutamate and cytosolic calcium); 2) many of calpain's preferred substrates are degraded as important components in these neurodegenerative conditions; 3) calpain activation occurs early in the pathogenic cascade of each, prior to onset of substantial cell death; and 4) calpain inhibitors can effectively reduce the severity of neuronal damage and loss of function normally associated with these acute neurodegenerative perturbations. Calpain proteolysis is also implicated in chronic neurodegenerative diseases, with the strength of current evidence varying among specific diseases. The evidence accumulated for a plausible role in Alzheimer's disease (AD) is currently the strongest. For example, empirical links have been established between abnormal calpain proteolysis and 1) the cellular formation of classic Alzheimer's pathology, such as β-amyloid plaques, neurofibrillary tangles, and Alz-50 immunoreactivity; 2) the brain regions with greatest concentrations of AD-related pathology; and 3) the degeneration of key brain pathways vulnerable in the early stages of the disease. Similar, though less extensive, evidence exists for a potential role of abnormal calpain proteolysis in Parkinson's disease. Finally, for several other chronic neurodegenerative conditions (e.g., Huntington's disease and amyotrophic lateral sclerosis), early evidence is emerging that calpain may also play some pathogenic role. Thus, these data support the possibility that uncontrolled calpain proteolysis may contribute to and/or accelerate the loss of neurons associated with a wide range of neurodegenerative conditions and may, therefore, represent an important, final common cytotoxic pathway for many diverse forms of neurodegeneration. NEUROSCIENTIST 3:314–327, 1997

Central nervous system resuscitation

Traumatic injury to the central nervous system induces delayed neuronal death, which may be mediated by acute and chronic neurochemical changes. Experimental identification of these injury mechanisms and elucidation of the neurochemical cascade following trauma may provide enhanced opportunities for treatment with novel neuroprotective strategies.

Active site-directed antibodies identify calpain II as an early-appearing and pervasive component of neurofibrillary pathology in Alzheimer's disease

Calpain proteases influence intracellular signaling pathways and regulate cytoskeleton organization, but the neuronal and pathological roles of individual isoenzymes are unknown. In Alzheimer's disease (AD), the activated form of calpain I is significantly increased while the fate of calpain II has been more difficult to address. Here, calpain II antibodies raised to different sequences within a cryptic region around the active site, which becomes exposed during protease activation, were shown immunohistochemically to bind extensively to neurofibrillary tangles (NFT), neuritic plaques, and neuropil threads in brains from individuals with AD. Additional 'pre-tangle' granular structures in neurons were also intensely immunostained, indicating calpain II mobilization at very early stages of NFT formation. Total levels of calpain II remained constant in the prefrontal cortex of AD patients but were increased 8-fold in purified NFT relative to levels of calpain I. These results implicate activated calpain II in neurofibrillary degeneration, provide further evidence for the involvement of the calpain system in AD pathogenesis, and imply that neuronal calcium homeostasis is altered in AD.

Localized and transient elevations of intracellular Ca2+ induce the dedifferentiation of axonal segments into growth cones

The formation of a growth cone at the tip of a severed axon is a key step in its successful regeneration. This process involves major structural and functional alterations in the formerly differentiated axonal segment. Here we examined the hypothesis that the large, localized, and transient elevation in the free intracellular calcium concentration ([Ca2+]i) that follows axotomy provides a signal sufficient to trigger the dedifferentiation of the axonal segment into a growth cone. Ratiometric fluorescence microscopy and electron microscopy were used to study the relations among spatiotemporal changes in [Ca2+]i, growth cone formation, and ultrastructural alterations in axotomized and intact Aplysia californica neurons in culture. We report that, in neurons primed to grow, a growth cone forms within 10 min of axotomy near the tip of the transected axon. The nascent growth cone extends initially from a region in which peak intracellular Ca2+ concentrations of 300-500 microM are recorded after axotomy. Similar [Ca2+]i transients, produced in intact axons by focal applications of ionomycin, induce the formation of ectopic growth cones and subsequent neuritogenesis. Electron microscopy analysis reveals that the ultrastructural alterations associated with axotomy and ionomycin-induced growth cone formation are practically identical. In both cases, growth cones extend from regions in which sharp transitions are observed between axoplasm with major ultrastructural alterations and axoplasm in which the ultrastructure is unaltered. These findings suggest that transient elevations of [Ca2+]i to 300-500 microM, such as those caused by mechanical injury, may be sufficient to induce the transformation of differentiated axonal segments into growth cones.

Immunohistochemical study of calpain-mediated breakdown products to alpha-spectrin following controlled cortical impact injury in the rat

This study examined the effect of unilateral controlled cortical impact on the appearance of calpain-mediated alpha-spectrin breakdown products (BDPs) in the rat cortex and hippocampus at various times following injury. Coronal sections were taken from animals at 15 min, 1 h, 3 h, 6 h, and 24 h after injury and immunolabeled with an antibody that recognizes calpain-mediated BDPs to alpha-spectrin (Roberts-Lewis et al., 1994). Sections from a separate group of rats were also taken at the same times and stained with hematoxylin and eosin. Analyses of early time points (15 min, 1 h, 3 h, and 6 h following injury) revealed alpha-spectrin BDPs in structurally intact neuronal soma and dendrites in cortex ipsilateral to site of injury that was not present in tissue from sham-injured control rats. By 24 h after injury labeling was not restricted to clearly defined neuronal structures in ipsilateral cortex, although there was an increased extent of diffuse labeling. BDPs to alpha-spectrin in axons were not detected until 24 h after injury, in contrast to the more rapid accumulation of BDPs observed in neuronal soma and dendrites. The presence of BDPs to alpha-spectrin in the cortex at the site of impact, and in the rostral and contralateral cortex, coincided with morphopathology detected by hematoxylin and eosin. alpha-Spectrin BDPs were also observed in the hippocampus ipsilateral to the injury in the absence of overt cell death. This investigation provides further evidence that calpain is activated after controlled cortical impact and could contribute to necrosis at the site of injury. The appearance of calpain-mediated BDPs at sites distal to the contusion site and in the hippocampus also suggests that calpain activation may precede and/or occur in the absence of extensive morphopathological changes.

Localized and transient elevations of intracellular Ca2+ induce the dedifferentiation of axonal segments into growth cones

The formation of a growth cone at the tip of a severed axon is a key step in its successful regeneration. This process involves major structural and functional alterations in the formerly differentiated axonal segment. Here we examined the hypothesis that the large, localized, and transient elevation in the free intracellular calcium concentration ([Ca2+]i) that follows axotomy provides a signal sufficient to trigger the dedifferentiation of the axonal segment into a growth cone. Ratiometric fluorescence microscopy and electron microscopy were used to study the relations among spatiotemporal changes in [Ca2+]i, growth cone formation, and ultrastructural alterations in axotomized and intact Aplysia californica neurons in culture. We report that, in neurons primed to grow, a growth cone forms within 10 min of axotomy near the tip of the transected axon. The nascent growth cone extends initially from a region in which peak intracellular Ca2+ concentrations of 300-500 microM are recorded after axotomy. Similar [Ca2+]i transients, produced in intact axons by focal applications of ionomycin, induce the formation of ectopic growth cones and subsequent neuritogenesis. Electron microscopy analysis reveals that the ultrastructural alterations associated with axotomy and ionomycin-induced growth cone formation are practically identical. In both cases, growth cones extend from regions in which sharp transitions are observed between axoplasm with major ultrastructural alterations and axoplasm in which the ultrastructure is unaltered. These findings suggest that transient elevations of [Ca2+]i to 300-500 microM, such as those caused by mechanical injury, may be sufficient to induce the transformation of differentiated axonal segments into growth cones.

Calpain activation contributes to dendritic remodeling after brief excitotoxic injury in vitro

The calcium-dependent protease calpain may contribute to neuronal death in acute neurological insults and may be activated very early in the neuronal injury cascade. We assessed the role of calpain in a model of rapid, reversible dendritic injury in murine cortical cultures. Brief sublethal NMDA exposure (10-30 microM for 10 min) resulted in focal swellings, or varicosities, along the length of neuronal dendrites as visualized with the lipophilic membrane tracer Dil or with immunostaining using antibodies to the somatodendritic protein MAP2. These varicosities appeared within minutes of NMDA exposure and recovered spontaneously within 2 hr after NMDA removal. Addition of the calpain inhibitors MDL28,170, calpain inhibitors I and II, and leupeptin (all 1-100 microM) had little effect on the development of NMDA-induced dendrite injury. However, the resolution of varicosities was substantially delayed by addition of calpain inhibitors after sublethal excitotoxic exposure. Using Western blots and immunocytochemistry, we observed reactivity for a calpain-specific spectrin proteolytic fragment during the period of recovery from dendritic swelling, but not during its formation. Spectrin breakdown product immunoreactivity could be blocked by the calpain inhibitor MDL28,170 and appeared in neuronal cell bodies and neurites in a time course that paralleled dendritic recovery. These observations suggest that calcium-dependent proteolysis contributes to recovery of dendritic structure after NMDA exposure. Calpain activation is not necessarily detrimental and may play a role in dendritic remodeling after neuronal injury.

Mechanisms of calpain proteolysis following traumatic brain injury: implications for pathology and therapy: implications for pathology and therapy: a review and update

Much recent research has focused on the pathological significance of calcium accumulation in the central nervous system (CNS) following cerebral ischemia, spinal cord injury (SCI), and traumatic brain injury (TBI). Disturbances in neuronal calcium homeostasis may result in the activation of several calcium-sensitive enzymes, including lipases, kinases, phosphatases, and proteases. One potential pathogenic event in a number of acute CNS insults, including TBI, is the activation of the calpains, calcium-activated intracellular proteases. This article reviews new evidence indicating that overactivation of calpains plays a major role in the neurodegenerative cascade following TBI in vivo. Further, this article presents an overview from in vivo and in vitro models of CNS injuries suggesting that administration of calpain inhibitors during the initial 24-h period following injury can attenuate injury-induced derangements of neuronal structure and function. Lastly, this review addresses the potential contribution of other proteases to neuronal damage following TBI.

Calpain activation contributes to dendritic remodeling after brief excitotoxic injury in vitro

The calcium-dependent protease calpain may contribute to neuronal death in acute neurological insults and may be activated very early in the neuronal injury cascade. We assessed the role of calpain in a model of rapid, reversible dendritic injury in murine cortical cultures. Brief sublethal NMDA exposure (10-30 microM for 10 min) resulted in focal swellings, or varicosities, along the length of neuronal dendrites as visualized with the lipophilic membrane tracer Dil or with immunostaining using antibodies to the somatodendritic protein MAP2. These varicosities appeared within minutes of NMDA exposure and recovered spontaneously within 2 hr after NMDA removal. Addition of the calpain inhibitors MDL28,170, calpain inhibitors I and II, and leupeptin (all 1-100 microM) had little effect on the development of NMDA-induced dendrite injury. However, the resolution of varicosities was substantially delayed by addition of calpain inhibitors after sublethal excitotoxic exposure. Using Western blots and immunocytochemistry, we observed reactivity for a calpain-specific spectrin proteolytic fragment during the period of recovery from dendritic swelling, but not during its formation. Spectrin breakdown product immunoreactivity could be blocked by the calpain inhibitor MDL28,170 and appeared in neuronal cell bodies and neurites in a time course that paralleled dendritic recovery. These observations suggest that calcium-dependent proteolysis contributes to recovery of dendritic structure after NMDA exposure. Calpain activation is not necessarily detrimental and may play a role in dendritic remodeling after neuronal injury.

Hyperthermia enhances spectrin breakdown in transient focal cerebral ischemia

Calpain-mediated spectrin degradation is triggered by cerebral ischemia and, when persistent, is thought to signal irreversible neuronal injury. Hyperthermia superimposed upon cerebral ischemia may exacerbate the injury process. In this study, we compared the extent of spectrin degradation in the brains of rats subjected to 1 h of transient proximal middle cerebral artery (MCA) clip-occlusion performed under conditions of cranial normothermia (37 degrees C) or mild cranial hyperthermia (39 degrees C). Immunocytochemical localization of spectrin breakdown products was achieved by the use of a rabbit polyclonal antibody which reacted selectively with calpain-generated fragments of brain spectrin. The perfusion times studied were 1, 4 or 24 h. Following normothermic MCA occlusion, spectrin immunoreactivity was present only occasionally and only in scattered cortical neurons immediately upon reperfusion and 1 h later; all normothermic brains showed space immunoreactivity at 4 h of reperfusion; and no immunoreactivity was detected at 24 h. By contrast, following hyperthermic MCA occlusion, moderate-to-intense immunostaining was present in cortical pyramidal neurons even immediately upon reperfusion and persisted at 1 h of reperfusion. At 4 and 24 h, most brains exhibited dense immunoreactivity associated with morphologically shrunken neurons. Following 24 h survival, semi-thick plastic sections revealed intact neuropil and only selective neuronal necrosis in normothermic rats. By contrast, pan-necrosis was evident 24 h after the hyperthermic ischemic insult. These results indicate that mild cranial hyperthermia superimposed upon transient focal ischemia markedly enhances calpain activation and spectrin degradation; this process appears to be an important mechanism by which hyperthermia exacerbates ischemic injury.

Oxidation inhibits substrate proteolysis by calpain I but not autolysis

In this study, the effects of oxidation on calpain I autolysis and calpain-mediated proteolysis were examined. Calpain I was incubated with increasing concentrations of free calcium in the presence or absence of oxidant, and autolytic conversion of both the 80- and 30-kDa subunits was measured by immunoblotting utilizing monoclonal antibodies which recognize both autolyzed and non-autolyzed forms of each subunit, respectively. Autolytic conversion of the 80-kDa subunit of calpain I was not detected until free calcium concentration was greater than 40 microM, whereas autolysis of the 30-kDa subunit did not occur until the free calcium concentration was greater than 100 microM. In addition, autolytic conversion of either the 80- or 30-kDa subunit was not inhibited by the presence of oxidant. Calpain I activity was measured using the fluorescent peptide N-succinyl-L-leucyl-L-leucyl-L-valyl-L-tyrosine-7-amido-4- methylcoumarin or the microtubule-associated protein tau as substrate. Calpain I was found to have proteolytic activity at free calcium concentrations below that required for autolysis. Calpain I activity was strongly inhibited by oxidant at all calcium concentrations studied, suggesting that proteolytic activity of both the non-autolyzed 80-kDa and autolyzed 76-kDa forms was susceptible to oxidation. Interestingly, whereas oxidation did not inhibit autolytic conversion, the presence of high substrate concentrations did result in a significant reduction of autolysis without altering calpain proteolytic activity. Calpain I activity that had been inhibited by the presence of oxidant was recovered immediately by addition of the reducing agent dithiothreitol.

Specific cleavage of alpha-fodrin during Fas- and tumor necrosis factor-induced apoptosis is mediated by an interleukin-1beta-converting enzyme/Ced-3 protease distinct from the poly(ADP-ribose) polymerase protease

Interleukin-1beta-converting enzyme (ICE)/Ced-3 proteases play a critical role in apoptosis. One well characterized substrate of these proteases is the DNA repair enzyme poly(ADP-ribose) polymerase. We report here that alpha-fodrin, an abundant membrane-associated cytoskeletal protein, is cleaved rapidly and specifically during Fas- and tumor necrosis factor-induced apoptosis; this cleavage is mediated by an ICE/Ced-3 protease distinct from the poly(ADP-ribose) polymerase protease. Studies in cells treated with these apoptotic stimuli reveal that both fodrin and poly(ADP-ribose) polymerase proteolysis are inhibited by acetyl-Tyr-Val-Ala-Asp chloromethyl ketone and CrmA, specific inhibitors of ICE/Ced-3 proteases. However, fodrin proteolysis can be distinguished from poly(ADP-ribose) polymerase proteolysis by its relative insensitivity to acetyl-Asp-Glu-Val-Asp aldehyde (DEVD-CHO), a selective inhibitor of a subset of ICE/Ced-3 proteases that includes CPP32. DEVD-CHO protects cells from Fas-induced apoptosis but does not prevent fodrin proteolysis, indicating that cleavage of this protein can be uncoupled from apoptotic cell death. Moreover, purified fodrin is cleaved in vitro by CPP32 (but not by ICE) into fragments of the same size observed in vivo during apoptosis. These findings suggest that fodrin proteolysis in vivo may reflect the activity of multiple ICE/Ced-3 proteases whose partial sensitivity to DEVD-CHO reflects a limited contribution from CPP32, or an ICE/Ced-3 protease less sensitive than CPP32 to DEVD-CHO inhibition.

Inhomogeneous disappearance of myofilament-related cytoskeletal proteins in stunned myocardium of guinea pig

The decrease in Ca2+ responsiveness of myofilaments in stunned myocardium implies that there may be structural changes in proteins composing the contractile machinery. To elucidate the lesion in stunned myocardium, isolated guinea pig hearts were subjected to global ischemia at 37 degrees C and reperfused. SDS-PAGE revealed that the contents of desmin, alpha-actinin, and spectrin decreased in the myofibrillar fraction isolated from hearts reperfused after 60-minute ischemia compared with nonischemic control hearts. To examine the change of cytoskeletal proteins in stunned myocardium, immunohistochemical studies with antibodies against these proteins were performed after 15 minutes of ischemia. In stunned myocardium, the staining was largely intact, but there were some lesions where desmin was not stained and alpha-actinin and spectrin were only weakly identified. The percentage of normally stained areas in the myocardium (percent stained area), quantified by image processing, was significantly lower in stunned myocardium (79.6 +/- 3.6%, mean +/- SEM) than in nonischemic control myocardium (96.5 +/- 0.7%). Percent recovery of developed pressure significantly correlated with percent stained area (r = .82, P < .001). In hearts subjected to 15-minute ischemia but not reperfused, or in hearts reperfused with Ca(2+)-free solution after 15-minute ischemia, staining by the antibodies remained intact, suggesting that the change of the cytoskeletal proteins is mediated by Ca2+ overload during reperfusion. In hearts treated with the protease inhibitor leupeptin (50 mumol/L) or calpain inhibitor I (100 mumol/L), both developed pressure and staining were well preserved. These results indicate that contractile dysfunction in stunned myocardium has a strong correlation with the disappearance of cytoskeletal proteins that may be mediated by a Ca(2+)-dependent intracellular protease activated during reperfusion. The disruption of cytoskeletal proteins is a possible mechanism for stunning, although it may be a secondary effect of protease activation.

Metyrapone, an inhibitor of glucocorticoid production, reduces brain injury induced by focal and global ischemia and seizures

Increasing evidence indicates that glucocorticoids (GCs), produced in response to physical/emotional stressors, can exacerbate brain damage resulting from cerebral ischemia and severe seizure activity. However, much of the supporting evidence has come from studies employing nonphysiological paradigms in which adrenalectomized rats were compared with those exposed to constant GC concentrations in the upper physiological range. Cerebral ischemia and seizures can induce considerable GC secretion. We now present data from experiments using metyrapone (an 11-beta-hydroxylase inhibitor of GC production), which demonstrate that the GC stress-response worsens subsequent brain damage induced by ischemia and seizures in rats. Three different paradigms of brain injury were employed: middle cerebral artery occlusion (MCAO) model of focal cerebral ischemia; four-vessel occlusion (4VO) model of transient global forebrain ischemia; and kainic acid (KA)-induced (seizure-mediated) excitotoxic damage to hippocampal CA3 and CA1 neurons. Metyrapone (200 mg/kg body wt) was administered systemically in a single i.p. bolus 30 min prior to each insult. In the MCAO model, metyrapone treatment significantly reduced infarct volume and also preserved cells within the infarct. In the 4VO model, neuronal loss in region CA1 of the hippocampus was significantly reduced in rats administered metyrapone. Seizure-induced damage to hippocampal pyramidal neurons (assessed by cell counts and immunochemical analyses of cytoskeletal alterations) was significantly reduced in rats administered metyrapone. Measurement of plasma levels of corticosterone (the species-typical GC of rats) after each insult showed that metyrapone significantly suppressed the injury-induced rise in levels of circulating corticosterone. These findings indicate that endogenous corticosterone contributes to the basal level of brain injury resulting from cerebral ischemia and excitotoxic seizure activity and suggest that drugs that suppress glucocorticoid production may be effective in reducing brain damage in stroke and epilepsy patients.

An alpha-mercaptoacrylic acid derivative is a selective nonpeptide cell-permeable calpain inhibitor and is neuroprotective

Overactivation of calcium-activated neutral protease (calpain) has been implicated in the pathophysiology of several degenerative conditions, including stroke, myocardial ischemia, neuromuscular degeneration, and cataract formation. Alpha-mercaptoacrylate derivatives (exemplified by PD150606), with potent and selective inhibitory actions against calpain, have been identified. PD150606 exhibits the following characteristics: (i) Ki values for mu- and m-calpains of 0.21 microM and 0.37 microM, respectively, (ii) high specificity for calpains relative to other proteases, (iii) uncompetitive inhibition with respect to substrate, and (iv) it does not shield calpain against inactivation by the active-site inhibitor trans-(epoxysuccinyl)-L-leucyl-amido-3-methylbutane, suggesting a nonactive site action for PD150606. The recombinant calcium-binding domain from each of the large or small subunits of mu-calpain was found to interact with PD150606. In low micromolar range, PD15O6O6 inhibited calpain activity in two intact cell systems. The neuroprotective effects of this class of compound were also demonstrated by the ability of PD150606 to attenuate hypoxic/hypoglycemic injury to cerebrocortical neurons in culture and excitotoxic injury to Purkinje cells in cerebellar slices.

Calpain inhibitor AK295 attenuates motor and cognitive deficits following experimental brain injury in the rat

Marked increases in intracellular calcium may play a role in mediating cellular dysfunction and death following central nervous system trauma, in part through the activation of the calcium-dependent neutral protease calpain. In this study, we evaluated the effect of the calpain inhibitor AK295 [Z-Leu-aminobutyric acid-CONH(CH2)3-morpholine] on cognitive and motor deficits following lateral fluid percussion brain injury in rats. Before injury, male Sprague-Dawley rats (350-425 g) were trained to perform a beam-walking task and to learn a cognitive test using a Morris water maze paradigm. Animals were subjected to fluid percussion injury (2.2-2.4 atm; 1 atm = 101.3 kPa) and, beginning at 15 min postinjury, received a continuous intraarterial infusion of AK295 (120-140 mg/kg, n = 15) or vehicle (n= 16) for 48 hr. Sham (uninjured) animals received either drug (n = 5) or vehicle (n = 10). Animals were evaluated for neurobehavioral motor function at 48 hr and 7 days postinjury and were tested in the Morris water maze to evaluate memory retention at 7 days postinjury. At 48 hr, both vehicle- and AK295-treated injured animals showed significant neuromotor deficits (P< 0.005). At 7 days, injured animals that received vehicle continued to exhibit significant motor dysfunction (P< 0.01). However, brain-injured, AK295-treated animals showed markedly improved motor scores (P<0.02), which were not significantly different from sham (uninjured) animals. Vehicle-treated, injured animals demonstrated a profound cognitive deficit (P< 0.001), which was significantly attenuated by AK295 treatment (P< 0.05). To our knowledge, this study is the first to use a calpain inhibitor following brain trauma and suggests that calpain plays a role in the posttraumatic events underlying memory and neuromotor dysfunction.

Diminished microtubule-associated protein 2 (MAP2) immunoreactivity following cortical impact brain injury

This study employed Western blotting and qualitative immunohistochemistry to analyze the effects of cortical impact traumatic brain injury (TBI) on acute changes in MAP2 immunoreactivity in the rat cortex. We employed a lateral cortical impact injury device to induce severe TBI, which is associated with focal cortical contusion and neuronal death at the impact site. Three hours following TBI, Western blotting detected substantial MAP2 loss only in the cortex ipsilateral to the site of injury. Light microscopic studies of MAP2 revealed a prominent loss of MAP2 immunofluorescence in apical dendrites of pyramidal neurons within layers 3 and 5, as well as a loss of fine dendritic arborization within layer 1. These changes in MAP2 immunolabeling were associated with, but not exclusively restricted to, the presence of dark shrunken neurons labeled by hematoxylin and eosin staining, suggesting impending cell death. Alterations in MAP2 immunofluorescence were found both within and beyond areas of focal contusion and necrosis in the ipsilateral cortex. Thus, traumatic brain injury in rats can produce rapid and significant dendritic pathology within sites of contusion. However, immunohistochemical changes in MAP2 labeling outside of contused regions suggests that TBI-induced dendritic damage may not be exclusively associated with acute cell death.

Time-related neuronal changes following middle cerebral artery occlusion: implications for therapeutic intervention and the role of calpain

Changes in neocortex and striatum were characterized over time following focal ischemia to the brain. Rats were subjected to permanent middle cerebral artery occlusion (MCA-O) and sacrificed 1, 3, 6, 12, or 24 h later. The affected tissue was processed for tetrazolium chloride (TTC) and cresyl violet staining, as well as for Western blots to detect calpain-induced spectrin proteolysis. Significant changes in cell size and spectrin breakdown occurred within the first hour of occlusion, with further, dramatic changes in these two early markers continuing over time. Initial evidence of cell loss was noted at 1 h postocclusion in the striatum and at 3 h in the neocortex. However, even in the center of the most affected portion of the neocortex, the majority of cells appeared to be intact through 6 h. By this time, a significant TTC-defined infarct also emerged. These quantitative data indicate that calpain-induced proteolysis occurs very soon after the ischemic insult, is correlated with earliest changes in cell hypotrophy, and precedes or occurs in tandem with evidence of significant cell loss. They also demonstrate that, while some cell loss occurs earlier than previously believed, the majority of cells remains morphologically intact well beyond what is typically thought to be the window of opportunity for intervention. The results thus raise the question of how long after the ischemic event pharmaceutic intervention might be employed to salvage substantial numbers of neurons.

Three distinct phases of fodrin proteolysis induced in postischemic hippocampus. Involvement of calpain and unidentified protease

BACKGROUND AND PURPOSE

Fodrin, a neuronal cytoskeleton protein, is proteolyzed by calpain after ischemic insult. We examined proteolysis of fodrin induced by global forebrain ischemia in gerbil hippocampus in spatial terms by using the antibody specific to the calpain-proteolyzed form of fodrin.

METHODS

In gerbils, a 10-minute forebrain ischemia was produced by occlusion of both carotid arteries. After recirculation, the hippocampus was processed for immunohistochemical and immunoblot study with the antibody against the calpain-proteolyzed form of fodrin. Additionally, short-term ischemia was studied to find the threshold of fodrin proteolysis.

RESULTS

Three phases of fodrin proteolysis distinct in chronology and distribution arose: (1) an early predegeneration phase in the molecular layer and stratum oriens of the CA1 and CA3 sectors within the first 15 minutes, which lasted up to 4 hours; (2) a late predegeneration phase in the whole CA1 sector, except for the pyramidal cells, between 12 hours and 2 days; and (3) a postdegeneration phase in the cytoplasm of the CA1 neurons, which arose in 3 to 7 days. A 4-minute (not a 3-minute) forebrain ischemia induced the late predegeneration phase of fodrin proteolysis and delayed neuronal death in CA1. Immunoblotting showed that the primary product of calpain action was further proteolyzed by an unidentified protease.

CONCLUSIONS

Calpain induced proteolysis of fodrin in ischemic hippocampus, and the late predegeneration phase of the proteolysis was closely associated with the delayed neuronal death in the CA1 sector. Calpain and another protease may play a role in the development of neuronal death after transient forebrain ischemia.

Cytoskeletal derangements following central nervous system injury: modulation by neurotrophic gene transfection

This paper reviews important new evidence indicating that traumatic brain injury can produce more widespread derangements to the neuronal cytoskeleton than previously recognized. Although cytoskeletal derangements in axons have long been documented, recent data suggest that traumatic brain injury can produce structural derangements to dendrites and cell bodies as well. Many of these investigations have employed in vivo models to provide important insights into mechanisms possibly mediating the acute loss of cytoskeletal proteins, including disturbances in calcium homeostasis and activation of calcium-dependent proteolytic enzymes. However, we have little understanding of processes mediating the recovery of cytoskeletal proteins following injury. This paper provides recent evidence from in vitro models of central nervous system injury that neurotrophic proteins can enhance the recovery of the neuronal cytoskeleton. Neurotrophin-based therapy could employ either administration of exogenous neurotrophic proteins and/or transfection of cDNA for appropriate neurotrophins.

Localization of immuno-analogues of erythrocyte protein 4.1 and spectrin in epidermis of psoriasis vulgaris

The presence and localization of immuno-analogues of human erythrocyte protein 4.1 and spectrin were examined in the epidermis of psoriasis vulgaris. Immunoblot analysis with antibodies against human erythrocyte protein 4.1 revealed that psoriatic epidermis contains a 4.1-like protein of 80 kDa, and also minor immunoreactive polypeptides, including a 45-kDa polypeptide. The 45-kDa band was not detected in non-lesional epidermis. Lesional epidermis of psoriasis contains spectrin-like proteins of 240 kDa. Analysis with immunofluorescence microscopy revealed that 4.1-like proteins were detected mainly in the cytoplasm of the suprabasal cells in lesional epidermis and in the peripheral cytoplasm of the basal cells in non-lesional epidermis. On the other hand, spectrin-like proteins were localized to the peripheral cytoplasm of basal keratinocytes in both lesional and non-lesional psoriatic epidermis. The present results indicate that proteins related to protein 4.1 and spectrin are consistently detected within epidermal cells of psoriasis, a chronic skin disease characterized by epidermal hyperplasia; the expression and distribution of protein 4.1 in lesional epidermis of psoriasis differs from that in non-lesional epidermis. These membrane skeletal proteins may be of significance in the hyperproliferative epidermis of psoriasis.

Local cerebral glucose utilization and cytoskeletal proteolysis as indices of evolving focal ischemic injury in core and penumbra

To ascertain the tempo of progression to irreversible injury in focal ischemia, we subjected halothane-anesthetized Sprague-Dawley rats to photochemically induced distal middle cerebral artery occlusion (dMCAO) combined with permanent ipsilateral and 1 h contralateral common carotid artery occlusions. Head temperature was maintained at 36 degrees C. At times centered at either 1.5 or 3 h post-dMCAO, the rate of local glucose metabolism (lCMRgl) was measured by 2-deoxyglucose autoradiography, and cytoskeletal proteolysis was assessed regionally by an immunoblotting procedure to detect spectrin breakdown products. At 1.5 h (n = 5), the cortical ischemic core was already severely hypometabolic (lCMRgl 15.5 +/- 10.8 mumol 100 g-1 min-1, mean +/- SD), whereas the cortical penumbral zone was hypermetabolic (69.0 +/- 9.7). (The lumped constant was verified to be unchanged by methylglucose studies). Neutral red pH studies at this time point showed that both the core and penumbral zones were equally acidotic. By 3 h post-dMCAO (n = 6), lCMRgl in the penumbral zone had fallen to low levels (15.4 +/- 2.2 mumol 100 g-1 min-1) equal to those of the ischemic core (16.7 +/- 4.5). Correspondingly, spectrin breakdown in the ischemic core was advanced at both 2 and 3.5 h post-dMCAO (36 +/- 18% and 33 +/- 18% of total spectrin, respectively), whereas in the penumbral zone spectrin breakdown was less extensive and more highly variable at both times (22 +/- 23% and 29 +/- 16%). We conclude that irreversible deterioration of the ischemic core, as evidenced by the onset of local cytoskeletal proteolysis, begins within 2 h of middle cerebral artery occlusion. In the ischemic penumbra, the transition from glucose hyper- to hypometabolism occurs by 3.5 h and is associated with a milder and more variable degree of spectrin breakdown.

AMPA-selective glutamate receptor subunits in the rat hippocampus during aging

The levels of mRNAs for the subunits of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-selective glutamate receptors (GluR-1, -2, -3, -4) in the rat hippocampus during aging were measured by Northern blotting. The distribution of these receptors was also examined at the protein level by immunoblotting using antibodies to GluR-1 and to an epitope common to GluR-2 and GluR-3 (denoted GluR-2/3). During aging a significant decrease of GluR-1 protein, but no change in the corresponding mRNA level, was observed. No differences in the level of GluR-2/3 protein and GluR-2, -3, and -4 mRNAs at the various ages examined (4, 12, and 24 months) were detected. Our results show that AMPA receptors are only slightly influenced by the aging process in the rat hippocampus. The slight decrease in GluR-1 protein content, not accompanied by a parallel decrease in the GluR-1 mRNA level, might be explained by a decreased translational efficiency or an increased protein degradation of the GluR-1 subunit.