The ultrastructural localization of calcium-activated protease "calpain" in rat brain

Calpains and neuronal damage in the ischemic brain: The swiss knife in synaptic injury

The excessive extracellular accumulation of glutamate in the ischemic brain leads to an overactivation of glutamate receptors with consequent excitotoxic neuronal death. Neuronal demise is largely due to a sustained activation of NMDA receptors for glutamate, with a consequent increase in the intracellular Ca(2+) concentration and activation of calcium- dependent mechanisms. Calpains are a group of Ca(2+)-dependent proteases that truncate specific proteins, and some of the cleavage products remain in the cell, although with a distinct function. Numerous studies have shown pre- and post-synaptic effects of calpains on glutamatergic and GABAergic synapses, targeting membrane- associated proteins as well as intracellular proteins. The resulting changes in the presynaptic proteome alter neurotransmitter release, while the cleavage of postsynaptic proteins affects directly or indirectly the activity of neurotransmitter receptors and downstream mechanisms. These alterations also disturb the balance between excitatory and inhibitory neurotransmission in the brain, with an impact in neuronal demise. In this review we discuss the evidence pointing to a role for calpains in the dysregulation of excitatory and inhibitory synapses in brain ischemia, at the pre- and post-synaptic levels, as well as the functional consequences. Although targeting calpain-dependent mechanisms may constitute a good therapeutic approach for stroke, specific strategies should be developed to avoid non-specific effects given the important regulatory role played by these proteases under normal physiological conditions.

Calpain-1 and Calpain-2: The Yin and Yang of Synaptic Plasticity and Neurodegeneration

Many signaling pathways participate in both synaptic plasticity and neuronal degeneration. While calpains participate in these phenomena, very few studies have evaluated the respective roles of the two major calpain isoforms in the brain, calpain-1 and calpain-2. We review recent studies indicating that calpain-1 and calpain-2 exhibit opposite functions in both synaptic plasticity and neurodegeneration. Calpain-1 activation is required for the induction of long-term potentiation (LTP) and is generally neuroprotective, while calpain-2 activation limits the extent of potentiation and is neurodegenerative. This duality of functions is related to their associations with different PDZ-binding proteins, resulting in differential subcellular localization, and offers new therapeutic opportunities for a number of indications in which these proteases have previously been implicated.

Protein synthesis and consolidation of memory-related synaptic changes

Although sometimes disputed, it has been assumed for several decades that new proteins synthesized following a learning event are required for consolidation of subsequent memory. Published findings and new results described here challenge this idea. Protein synthesis inhibitors did not prevent Theta Bust Stimulation (TBS) from producing extremely stable long-term potentiation (LTP) in experiments using standard hippocampal slice protocols. However, the inhibitors were effective under conditions that likely depleted protein levels prior to attempts to induce the potentiation effect. Experiments showed that induction of LTP at one input, and thus a prior episode of protein synthesis, eliminated the effects of inhibitors on potentiation of a second input even in depleted slices. These observations suggest that a primary role of translation and transcription processes initiated by learning events is to prepare neurons to support future learning. Other work has provided support for an alternative theory of consolidation. Specifically, if the synaptic changes that support memory are to endure, learning events/TBS must engage a complex set of signaling processes that reorganize and re-stabilize the spine actin cytoskeleton. This is accomplished in fast (10 min) and slow (50 min) stages with the first requiring integrin activation and the second a recovery of integrin functioning. These results align with, and provide mechanisms for, the long-held view that memories are established and consolidated over a set of temporally distinct phases. This article is part of a Special Issue entitled SI: Brain and Memory.

Pharmacological enhancement of memory or cognition in normal subjects

The possibility of expanding memory or cognitive capabilities above the levels in high functioning individuals is a topic of intense discussion among scientists and in society at large. The majority of animal studies use behavioral endpoint measures; this has produced valuable information but limited predictability for human outcomes. Accordingly, several groups are pursuing a complementary strategy with treatments targeting synaptic events associated with memory encoding or forebrain network operations. Transcription and translation figure prominently in substrate work directed at enhancement. Notably, the question of why new proteins would be needed for a now-forming memory given that learning-driven synthesis presumably occurred throughout the immediate past has been largely ignored. Despite this conceptual problem, and some controversy, recent studies have reinvigorated the idea that selective gene manipulation is a plausible route to enhancement. Efforts to improve memory by facilitating synaptic encoding of information have also progressed, in part due of breakthroughs on mechanisms that stabilize learning-related, long-term potentiation (LTP). These advances point to a reductionistic hypothesis for a diversity of experimental results on enhancement, and identify under-explored possibilities. Cognitive enhancement remains an elusive goal, in part due to the difficulty of defining the target. The popular view of cognition as a collection of definable computations seems to miss the fluid, integrative process experienced by high functioning individuals. The neurobiological approach obviates these psychological issues to directly test the consequences of improving throughput in networks underlying higher order behaviors. The few relevant studies testing drugs that selectively promote excitatory transmission indicate that it is possible to expand cortical networks engaged by complex tasks and that this is accompanied by capabilities not found in normal animals.

Distinct roles for μ-calpain and m-calpain in synaptic NMDAR-mediated neuroprotection and extrasynaptic NMDAR-mediated neurodegeneration

Prolonged calpain activation is widely recognized as a key component of neurodegeneration in a variety of pathological conditions. Numerous reports have also indicated that synaptic activation of NMDA receptors (NMDARs) provides neuroprotection against a variety of insults. Here, we report the paradoxical finding that such neuroprotection involves calpain activation. NMDAR activation in cultured rat cortical neurons was neuroprotective against starvation and oxidative stress-induced damage. It also resulted in the degradation of two splice variants of PH domain and Leucine-rich repeat Protein Phosphatase 1 (PHLPP1), PHLPP1α and PHLPP1β, which inhibit the Akt and ERK1/2 pathways. Synaptic NMDAR-induced neuroprotection and PHLPP1 degradation were blocked by calpain inhibition. Lentiviral knockdown of PHLPP1 mimicked the neuroprotective effects of synaptic NMDAR activation and occluded the effects of calpain inhibition on neuroprotection. In contrast to synaptic NMDAR activation, extrasynaptic NMDAR activation had no effect on PHLPP1 and the Akt and ERK1/2 pathways, but resulted in calpain-mediated degradation of striatal-enriched protein tyrosine phosphatase (STEP) and neuronal death. Using μ-calpain- and m-calpain-selective inhibitors and μ-calpain and m-calpain siRNAs, we found that μ-calpain-dependent PHLPP1 cleavage was involved in synaptic NMDAR-mediated neuroprotection, while m-calpain-mediated STEP degradation was associated with extrasynaptic NMDAR-induced neurotoxicity. Furthermore, m-calpain inhibition reduced while μ-calpain knockout exacerbated NMDA-induced neurotoxicity in acute mouse hippocampal slices. Thus, synaptic NMDAR-coupled μ-calpain activation is neuroprotective, while extrasynaptic NMDAR-coupled m-calpain activation is neurodegenerative. These results help to reconcile a number of contradictory results in the literature and have critical implications for the understanding and potential treatment of neurodegenerative diseases.

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.

Design, synthesis, and optimization of novel epoxide incorporating peptidomimetics as selective calpain inhibitors

Hyperactivation of the calcium-dependent cysteine protease calpain 1 (Cal1) is implicated as a primary or secondary pathological event in a wide range of illnesses and in neurodegenerative states, including Alzheimer's disease (AD). E-64 is an epoxide-containing natural product identified as a potent nonselective, calpain inhibitor, with demonstrated efficacy in animal models of AD. By use of E-64 as a lead, three successive generations of calpain inhibitors were developed using computationally assisted design to increase selectivity for Cal1. First generation analogues were potent inhibitors, effecting covalent modification of recombinant Cal1 catalytic domain (Cal1cat), demonstrated using LC-MS/MS. Refinement yielded second generation inhibitors with improved selectivity. Further library expansion and ligand refinement gave three Cal1 inhibitors, one of which was designed as an activity-based protein profiling probe. These were determined to be irreversible and selective inhibitors by kinetics studies comparing full length Cal1 with the general cysteine protease papain.

Progesterone-estrogen interactions in synaptic plasticity and neuroprotection

17ß-Estradiol and progesterone exert a number of physiological effects throughout the brain due to interactions with several types of receptors belonging to the traditional family of intracellular hormonal receptors as well as to membrane-bound receptors. In particular, both hormones elicit rapid modifications of neuronal excitability that have been postulated to underlie their effects on synaptic plasticity and learning and memory. Likewise, both hormones have been shown to be neuroprotective under certain conditions, possibly due to the activation of pro-survival pathways and the inhibition of pro-apoptotic cascades. Because of the similarities in their cellular effects, there have been a number of questions raised by numerous observations that progesterone inhibits the effects of estrogen. In this manuscript, we first review the interactions between 17ß-estradiol (E2) and progesterone (P4) in synaptic plasticity, and conclude that, while E2 exerts a clear and important role in long-term potentiation of synaptic transmission in hippocampal neurons, the role of P4 is much less clear, and could be accounted by the direct or indirect regulation of GABAA receptors. We then discuss the neuroprotective roles of both hormones, in particular against excitotoxicity. In this case, the neuroprotective effects of these hormones are very similar to those of the neurotrophic factor BDNF. Interestingly, P4 antagonizes the effects of E2, possibly through the regulation of estrogen receptors or of proteins associated with the receptors or interactions with signaling pathways activated by E2. Overall, this review emphasizes the existence of common molecules and pathways that participate in the regulation of both synaptic plasticity and neurodegeneration.

Axonopathy in an α-synuclein transgenic model of Lewy body disease is associated with extensive accumulation of C-terminal-truncated α-synuclein

Progressive accumulation of α-synuclein (α-syn) in limbic and striatonigral systems is associated with the neurodegenerative processes in dementia with Lewy bodies (DLB) and Parkinson's disease (PD). The murine Thy-1 (mThy1)-α-syn transgenic (tg) model recapitulates aspects of degenerative processes associated with α-syn accumulation in these disorders. Given that axonal and synaptic pathologies are important features of DLB and PD, we sought to investigate the extent and characteristics of these alterations in mThy1-α-syn tg mice and to determine the contribution of α-syn c-terminally cleaved at amino acid 122 (CT α-syn) to these abnormalities. We generated a novel polyclonal antibody (SYN105) against the c-terminally truncated sequence (amino acids 121 to 123) of α-syn (CT α-syn) and performed immunocytochemical and ultrastructural analyses in mThy1-α-syn tg mice. We found abundant clusters of dystrophic neurites in layers 2 to 3 of the neocortex, the stratum lacunosum, the dentate gyrus, and cornu ammonis 3 of the hippocampus, striatum, thalamus, midbrain, and pons. Dystrophic neurites displayed intense immunoreactivity detected with the SYN105 antibody. Double-labeling studies with antibodies to phosphorylated neurofilaments confirmed the axonal location of full-length and CT α-syn. α-Syn immunoreactive dystrophic neurites contained numerous electrodense laminated structures. These results show that neuritic dystrophy is a prominent pathologic feature of the mThy1-α-syn tg model and suggest that CT α-syn might play an important role in the process of axonal damage in these mice as well as in DLB and PD.

Calpain dysregulation in Alzheimer's disease

Alzheimer's disease (AD) is characterized by the presence of senile plaques and neurofibrillary tangles in the neocortex and hippocampus of AD patients. In addition, a marked decrease in synaptic contacts has been detected in these affected brain areas. Due to its prevalence in the aging population, this disease has been the focus of numerous studies. The data obtained from those studies suggest that the mechanisms leading to the formation of the hallmark lesions of AD might be linked. One of such mechanisms seems to be the dysregulation of calcium homeostasis that results in the abnormal activation of calpains. Calpains are a family of Ca(2+)-dependent cysteine proteases that play a key role in multiple cell functions including cell development, differentiation and proliferation, axonal guidance, growth cone motility, and cell death, among others. In this paper, we briefly reviewed data on the structure of these proteases and their regulation under normal conditions. We also summarized data underscoring the participation of calpains in the neurodegenerative mechanisms associated with AD.

N-methyl-D-aspartate receptor- and calpain-mediated proteolytic cleavage of K+-Cl- cotransporter-2 impairs spinal chloride homeostasis in neuropathic pain

Loss of synaptic inhibition by γ-aminobutyric acid and glycine due to potassium chloride cotransporter-2 (KCC2) down-regulation in the spinal cord is a critical mechanism of synaptic plasticity in neuropathic pain. Here we present novel evidence that peripheral nerve injury diminishes glycine-mediated inhibition and induces a depolarizing shift in the reversal potential of glycine-mediated currents (E(glycine)) in spinal dorsal horn neurons. Blocking glutamate N-methyl-D-aspartate (NMDA) receptors normalizes synaptic inhibition, E(glycine), and KCC2 by nerve injury. Strikingly, nerve injury increases calcium-dependent calpain activity in the spinal cord that in turn causes KCC2 cleavage at the C terminus. Inhibiting calpain blocks KCC2 cleavage induced by nerve injury and NMDA, thereby normalizing E(glycine). Furthermore, calpain inhibition or silencing of μ-calpain at the spinal level reduces neuropathic pain. Thus, nerve injury promotes proteolytic cleavage of KCC2 through NMDA receptor-calpain activation, resulting in disruption of chloride homeostasis and diminished synaptic inhibition in the spinal cord. Targeting calpain may represent a new strategy for restoring KCC2 levels and tonic synaptic inhibition and for treating chronic neuropathic pain.

Is the amyloid hypothesis of Alzheimer's disease therapeutically relevant?

The conventional view of AD (Alzheimer's disease) is that much of the pathology is driven by an increased load of β-amyloid in the brain of AD patients (the 'Amyloid Hypothesis'). Yet, many therapeutic strategies based on lowering β-amyloid have so far failed in clinical trials. This failure of β-amyloid-lowering agents has caused many to question the Amyloid Hypothesis itself. However, AD is likely to be a complex disease driven by multiple factors. In addition, it is increasingly clear that β-amyloid processing involves many enzymes and signalling pathways that play a role in a diverse array of cellular processes. Thus the clinical failure of β-amyloid-lowering agents does not mean that the hypothesis itself is incorrect; it may simply mean that manipulating β-amyloid directly is an unrealistic strategy for therapeutic intervention, given the complex role of β-amyloid in neuronal physiology. Another possible problem may be that toxic β-amyloid levels have already caused irreversible damage to downstream cellular pathways by the time dementia sets in. We argue in the present review that a more direct (and possibly simpler) approach to AD therapeutics is to rescue synaptic dysfunction directly, by focusing on the mechanisms by which elevated levels of β-amyloid disrupt synaptic physiology.

The role of myosin V in exocytosis and synaptic plasticity

In neuroscience, myosin V motor proteins have attracted attention since they are highly expressed in brain, and absence of myosin Va in man leads to a severe neurological disease called Griscelli syndrome. While in some cells myosin V is described to act as a vesicle transport motor, an additional role in exocytosis has emerged recently. In neurons, myosin V has been linked to exocytosis of secretory vesicles and recycling endosomes. Through these functions, it is implied in regulating important brain functions including the release of neuropeptides by exocytosis of large dense-core vesicles and the insertion of neurotransmitter receptors into post-synaptic membranes. This review focuses on the role of myosin V in (i) axonal transport and stimulated exocytosis of large dense-core vesicles to regulate the secretion of neuroactive substances, (ii) tethering of the endoplasmic reticulum at cerebellar synapses to permit long-term depression, (iii) recycling of α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors at hippocampal synapses during long-term potentiation, and (iv) recycling of nicotinic acetylcholine receptors at the neuromuscular junction. Myosin V is thus discussed as an important modulator of synaptic plasticity.

Regulation of calpain-2 in neurons: implications for synaptic plasticity

The family of calcium-dependent neutral proteases, calpains, was discovered more than 30 years ago, but their functional roles in the nervous system under physiological or pathological conditions still remain unclear. Although calpain was proposed to participate in synaptic plasticity and in learning and memory in the early 1980s, the precise mechanism regarding its activation, its target(s) and the functional consequences of its activation have remained controversial. A major issue has been the identification of roles of the two major calpain isoforms present in the brain, calpain-1 and calpain-2, and the calcium requirement for their activation, which exceeds levels that could be reached intracellularly under conditions leading to changes in synaptic efficacy. In this review, we discussed the features of calpains that make them ideally suited to link certain patterns of presynaptic activity to the structural modifications of dendritic spines that could underlie synaptic plasticity and learning and memory. We then summarize recent findings that provide critical answers to the various questions raised by the initial hypothesis, and that further support the idea that, in brain, calpain-2 plays critical roles in developmental and adult synaptic plasticity.

Brain-derived neurotrophic factor and epidermal growth factor activate neuronal m-calpain via mitogen-activated protein kinase-dependent phosphorylation

Calpain is a calcium-dependent protease that plays a significant role in synaptic plasticity, cell motility, and neurodegeneration. Two major calpain isoforms are present in brain, with mu-calpain (calpain1) requiring micromolar calcium concentrations for activation and m-calpain (calpain2) needing millimolar concentrations. Recent studies in fibroblasts indicate that epidermal growth factor (EGF) can activate m-calpain independently of calcium via mitogen-activated protein kinase (MAPK)-mediated phosphorylation. In neurons, MAPK is activated by both brain-derived neurotrophic factor (BDNF) and EGF. We therefore examined whether these growth factors could activate m-calpain by MAPK-dependent phosphorylation using cultured primary neurons and HEK-TrkB cells, both of which express BDNF and EGF receptors. Calpain activation was monitored by quantitative analysis of spectrin degradation and by a fluorescence resonance energy transfer (FRET)-based assay, which assessed the truncation of a calpain-specific peptide flanked by the FRET fluorophore pair DABCYL and EDANS. In both cell types, BDNF and EGF rapidly elicited calpain activation, which was completely blocked by MAPK and calpain inhibitors. BDNF stimulated m-calpain but not mu-calpain serine phosphorylation, an effect also blocked by MAPK inhibitors. Remarkably, BDNF- and EGF-induced calpain activation was preferentially localized in dendrites and dendritic spines of hippocampal neurons and was associated with actin polymerization, which was prevented by calpain inhibition. Our results indicate that, in cultured neurons, both BDNF and EGF activate m-calpain by MAPK-mediated phosphorylation. These results strongly support a role for calpain in synaptic plasticity and may explain why m-calpain, although widely expressed in CNS, requires nonphysiological calcium levels for activation.

17-Beta-estradiol increases neuronal excitability through MAP kinase-induced calpain activation

17-Beta-estradiol (E2) is a steroid hormone involved in numerous brain functions. E2 regulates synaptic plasticity in part by enhancing NMDA receptor function and spine density in the hippocampus, resulting in increased long-term potentiation and facilitation of learning and memory. As the calcium-dependent neutral protease, calpain, is also involved in these processes, we tested whether E2 could activate calpain and examined the functional consequences of E2-mediated calpain activation in hippocampus. Calpain activity was analyzed by a fluorescence resonance energy transfer (FRET)-based assay that allows both quantitative determination and spatial resolution. E2 rapidly activated calpain in cultured cortical and hippocampal neurons, prominently in dendrites and dendritic spines. E2-induced calpain activation was mediated through mitogen-activated protein kinase (MAPK), as it was completely blocked by MEK inhibitors. It was also calcium-independent, as it was still evident in presence of the calcium chelator, BAPTA-AM. Activation of ERalpha and ERbeta receptors by specific agonists stimulated calpain activity. Finally, the rapid E2-mediated increase in excitability in acute hippocampal slices was prevented by a membrane-permeable calpain inhibitor. Furthermore, E2 treatment of acute hippocampal slices resulted in increased actin polymerization and membrane levels of GluR1 but not GluR2/3 subunits of AMPA receptors; both effects were also blocked by a calpain inhibitor. Our results indicate that E2 rapidly stimulates calpain activity through MAP kinase-mediated phosphorylation, resulting in increased membrane levels of AMPA receptors. These effects could be responsible for E2-mediated increase in neuronal excitability and facilitation of cognitive processes.

Calcium in the initiation, progression and as an effector of Alzheimer's disease pathology

The cause(s) of sporadic Alzheimer’s disease (sAD) are complex and currently poorly understood. They likely result from a combination of genetic, environmental, proteomic and lipidomic factors that crucially occur only in the aged brain. Age-related changes in calcium levels and dynamics have the potential to increase the production and accumulation of both amyloid-β peptide (Aβ) and τ pathologies in the AD brain, although these two pathologies themselves can induce calcium dyshomeostasis, particularly at synaptic membranes. This review discuses the evidence for a role for calcium dyshomeostasis in the initiation of pathology, as well as the evidence for these pathologies themselves disrupting normal calcium homeostasis, which lead to synaptic and neuronal dysfunction, synaptotoxicity and neuronal loss, underlying the dementia associated with the disease.

Inhibition of calpains improves memory and synaptic transmission in a mouse model of Alzheimer disease

Calpains are calcium-dependent enzymes that determine the fate of proteins through regulated proteolytic activity. Calpains have been linked to the modulation of memory and are key to the pathogenesis of Alzheimer disease (AD). When abnormally activated, calpains can also initiate degradation of proteins essential for neuronal survival. Here we show that calpain inhibition through E64, a cysteine protease inhibitor, and the highly specific calpain inhibitor BDA-410 restored normal synaptic function both in hippocampal cultures and in hippocampal slices from the APP/PS1 mouse, an animal model of AD. Calpain inhibition also improved spatial-working memory and associative fear memory in APP/PS1 mice. These beneficial effects of the calpain inhibitors were associated with restoration of normal phosphorylation levels of the transcription factor CREB and involved redistribution of the synaptic protein synapsin I. Thus, calpain inhibition may prove useful in the alleviation of memory loss in AD.

The novel calpain inhibitor A-705253 potently inhibits oligomeric beta-amyloid-induced dynamin 1 and tau cleavage in hippocampal neurons

We have previously shown that beta-amyloid (Abeta) oligomers induced dynamin 1 and tau cleavage in cultured hippocampal neurons. As a result of this cleavage, dynamin 1 levels decreased and a toxic tau fragment was generated. Abeta-induced cleavage of these proteins was calpain-mediated and impacted both synaptic vesicle recycling and the integrity of neuronal processes [Kelly, B.L., Vassar, R., Ferreira, A., 2005. Beta-amyloid-induced dynamin 1 depletion in hippocampal neurons. A potential mechanism for early cognitive decline in Alzheimer disease. J. Biol. Chem. 280, 31746-31753; Park, S.Y., Ferreira, A., 2005. The generation of a 17kDa neurotoxic fragment: an alternative mechanism by which tau mediates beta-amyloid-induced neurodegeneration. J. Neurosci. 25, 5365-5375; Kelly, B.L., Ferreira, A., 2006. Beta-amyloid-induced dynamin 1 degradation is mediated by N-methyl-d-aspartate receptors in hippocampal neurons. J. Biol. Chem. 281, 28079-28089, Kelly, B.L., Ferreira, A., 2007. Beta-amyloid disrupted synaptic vesicle endocytosis in cultured hippocampal neurons. Neuroscience 147, 60-70]. Building on previous reports, these results identified calpain as a potential target for therapeutic intervention in Alzheimer's disease. In the present study, we tested the ability of A-705253, a novel water-soluble calpain inhibitor with oral availability and enhanced metabolic stability, to prevent Abeta-induced dynamin 1 and tau cleavage in cultured hippocampal neurons. Quantitative Western blot analysis indicated that the incubation of these cells with A-705253 prior to the addition of oligomeric Abeta reduced both dynamin 1 and tau cleavage in a dose-dependent manner. In addition, our results showed that this calpain inhibitor significantly ameliorated the cleavage of these proteins when added simultaneously with oligomeric Abeta. Furthermore, our data indicated that the use of this calpain inhibitor could have some beneficial effects even when added after the cleavage of these proteins have been triggered by Abeta. Collectively, these results suggest that, indeed, specific calpain inhibitors could play an important role in the treatment of Alzheimer's disease.

Mechanistic role of calpains in postischemic neurodegeneration

The calpain family of proteases is causally linked to postischemic neurodegeneration. However, the precise mechanisms by which calpains contribute to postischemic neuronal death have not been fully elucidated. This review outlines the key features of the calpain system, and the evidence for its causal role in postischemic neuronal pathology. Furthermore, the consequences of specific calpain substrate cleavage at various subcellular locations are explored. Calpain substrates within synapses, plasma membrane, endoplasmic reticulum, lysosomes, mitochondria, and the nucleus, as well as the overall effect of postischemic calpain activity on calcium regulation and cell death signaling are considered. Finally, potential pathways for calpain-mediated neurodegeneration are outlined in an effort to guide future studies aimed at understanding the downstream pathology of postischemic calpain activity and identifying optimal therapeutic strategies.

LTP consolidation: Substrates, explanatory power, and functional significance

Long-term potentiation (LTP) resembles memory in that it is initially unstable and then, over about 30 min, becomes increasingly resistant to disruption. Here we present an hypothesis to account for this initial consolidation effect and consider implications that follow from it. Anatomical studies indicate that LTP is accompanied by changes in spine morphology and therefore likely involves cytoskeletal changes. Accordingly, theta bursts initiate calpain-mediated proteolysis of the actin cross-linking protein spectrin and trigger actin polymerization in spine heads, two effects indicative of cytoskeletal reorganization. Polymerization occurs within 2 min, has the same threshold as LTP, is dependent on integrins, and becomes resistant to disruption over 30 min. We propose that the stabilization of the new cytoskeletal organization, and thus of a new spine morphology, underlies the initial phase of LTP consolidation. This hypothesis helps explain the diverse array of proteins and signaling cascades implicated in LTP, as well as the often-contradictory results about contributions of particular molecules. It also provides a novel explanation for why LTP is potently modulated by factors likely to be released during theta trains (e.g., BDNF). Finally, building on evidence that normal patterns of activity reverse LTP, we suggest that consolidation provides a delay that allows brain networks to sculpt newly formed memories.

Lack of phenotype for LTP and fear conditioning learning in calpain 1 knock-out mice

We previously proposed the hypothesis that calpain activation played an important role in long-term potentiation (LTP) of synaptic transmission in hippocampus. Two forms of calpain are predominant in brain tissues, calpain 1 (mu-calpain), activated by micromolar calcium concentration and calpain 2 (m-calpain), activated by millimolar calcium concentration in vitro. In the present study, we tested the role of calpain 1 in LTP and in learning and memory using calpain 1 knock-out mice. Changes in learning and memory were assessed using both context and tone fear conditioning. No differences in freezing responses were observed between the knock-out and the wild-type animals during the acquisition phase of the training, eliminating the possibility that the knock-out animals could be differentially affected by the foot shock. Likewise, no differences in freezing responses elicited by either the context or the tone were observed during the retention phase. No differences in short-term potentiation (STP) or LTP were observed in hippocampal slices from the knock-out and matched wild-type mice. Several interpretations might explain these negative results. First, it is conceivable that calpain 2 plays a more dominant role in neurons, and that calpain 1 makes a minor contribution as opposed to its suspected predominant role in the hematopoietic system. Alternatively, it is conceivable that some as yet unknown compensatory mechanisms take effect, and that calpain 2 or another calpain isoform substitutes for the missing calpain 1.

Tat-calpastatin fusion proteins transduce primary rat cortical neurons but do not inhibit cellular calpain activity

Excessive activation of calpains (calcium-activated neutral proteases) is observed following spinal cord contusion injury, traumatic brain injury, stroke, and in neurodegenerative disorders including Alzheimer's disease. Calpain inhibition represents an attractive therapeutic target, but current calpain inhibitors possess relatively weak potency, poor specificity, and in many cases, limited cellular and blood-brain barrier permeability. We developed novel calpain inhibitors consisting of the endogenous inhibitor, calpastatin or its inhibitory domain I, fused to the protein transduction domain of the HIV trans-activator (Tat) protein (Tat(47-57)). The Tat-calpastatin fusion proteins were potent calpain inhibitors in a cell-free activity assay, but did not inhibit cellular calpain activity in primary rat cortical neurons when applied exogenously at concentrations up to 5 microM. The fusion proteins were able to transduce neurons, but were localized within endosome-like structures. A similar endosomal uptake was observed for Tat-GFP. Together, the results suggest that endosomal uptake of the Tat-calpastatin prevents its interaction with calpain in other cellular compartments. Endosomal uptake of proteins fused to the Tat protein transduction domain severely limits the applications of this methodology.

Differential distribution of calpain small subunit 1 and 2 in rat brain

Calpains, the Ca(2+)-dependent thiol proteases, are abundant in the nervous tissue. The ubiquitous enzyme forms in mammals are heterodimers consisting of a specific, micro or m, large (catalytic) subunit and, apparently, a common small (regulatory) subunit (CSS1). Recently, however, we described a second form of small subunit (CSS2), which is of restricted occurrence [Schád, E., Farkas, A., Jékely, G., Tompa, P. & Friedrich, P. (2002) Biochem. J., 362, 383-388]. Here we analysed the distribution of immunoreactivity in various parts of rat brain against two anti-CSS1 and two anti-CSS2 antibodies by correlated light and electron microscopy. Remarkably, the antibodies showed differential distribution in various parts of rat cortex: anti-CSS1 reacted mainly with perikarya and dendrites, whereas anti-CSS2 was more prominent in axons. In serial sections CSS2 and synaptophysin gave very similar patterns, i.e. these epitopes seem to colocalize. Electron microscopy confirmed that CSS1 was mainly localized postsynaptically in dendrites and somata, whereas CSS2 was found presynaptically. The hypothesis is advanced that these distinct distributions of calpain subunits may be related to the transport of these enzymes in nerve cells.

Integrins, synaptic plasticity and epileptogenesis

A number of processes are thought to contribute to the development of epilepsy including enduring increases in excitatory synaptic transmission, changes in GABAergic inhibition, neuronal cell death and the development of aberrant innervation patterns in part arising from reactive axonal growth. Recent findings indicate that adhesion chemistries and, most particularly, activities of integrin class adhesion receptors play roles in each of these processes and thereby are likely to contribute significantly to the cell biology underlying epileptogenesis. As reviewed in this chapter, studies of long-term potentiation have shown that integrins are important for stabilizing activity-induced increases in synaptic strength and excitability. Other work has demonstrated that seizures, and in some instances subseizure neuronal activity, modulate the expression of integrins and their matrix ligands and the activities of proteases which regulate them both. These same adhesion proteins and proteases play critical roles in axonal growth and synaptogenesis including processes induced by seizure in adult brain. Together, these findings indicate that seizures activate integrin signaling and induce a turnover in adhesive contacts and that both processes contribute to lasting changes in circuit and synaptic function underlying epileptogenesis.

Alteration of gene expression by chromosome loss in the postnatal mouse brain

Frequent chromosomal aneuploidy has recently been discovered in normal neurons of the developing and mature murine CNS. Toward a more detailed understanding of aneuploidy and its effects on normal CNS cells, we examined the genomes of cells in the postnatal subventricular zone (SVZ), an area that harbors a large number of neural stem and progenitor cells (NPCs), which give rise to neurons and glia. Here we show that NPCs, neurons, and glia from the SVZ are frequently aneuploid. Karyotyping revealed that approximately 33% of mitotic SVZ cells lost or gained chromosomes in vivo, whereas interphase fluorescence in situ hybridization demonstrated aneuploidy in postnatal-born cells in the olfactory bulb (OB) in vivo, along with neurons, glia, and NPCs in vitro. One possible consequence of aneuploidy is altered gene expression through loss of heterozygosity (LOH). This was examined in a model of LOH: loss of transgene expression in mice hemizygous for a ubiquitously expressed enhanced green fluorescent protein (eGFP) transgene on chromosome 15. Concurrent examination of eGFP expression, transgene abundance, and chromosome 15 copy number demonstrated that a preponderance of living SVZ and OB cells not expressing eGFP lost one copy of chromosome 15; the eGFP transgene was lost in these cells as well. Although gene expression profiling revealed changes in expression levels of several genes relative to GFP-expressing controls, cells with LOH at chromosome 15 were morphologically normal and proliferated or underwent apoptosis at rates similar to those of euploid cells in vitro. These findings support the view that NPCs and postnatal-born neurons and glia can be aneuploid in vivo and functional gene expression can be permanently altered in living neural cells by chromosomal aneuploidy.

Effect of chronic inhibition of calpains in the hippocampus on spatial discrimination learning and protein kinase C

Several behavioral and electrophysiological studies have suggested that a sustained activation of protein kinase C would be required to underlie persistent changes associated with memory formation. Limited proteolysis of PKCs by calpains, calcium-activated proteases, cleaves the catalytic and the regulatory domains, generating a free catalytic fragment termed PKM, constitutively active. In order to investigate the potential physiological importance of this limited proteolysis as a mechanism of PKC activation, we have studied the effect of the calpastatin peptide, a specific calpain inhibitor, on the learning of a spatial discrimination task in a radial maze. Thus, using osmotic micro-pumps, the calpastatin peptide was infused bilaterally into the dorsal hippocampus during the six sessions of training and the probe test. The treatment was shown to facilitate the performance of the mice on the two last training sessions and on the probe test. This behavioral effect was shown to correspond to the reduced calpain activity observed in the hippocampus at the very end of the 7-day infusion of the calpastatin peptide, suggesting a relation between both events. In addition, PKC activity measured immediately after the probe test was notably decreased in the membrane fraction of the hippocampus. Although protein levels of PKCs and calpains quantified by western blot were not affected by calpastatin infusion, we found a noticeable correlation between mu-calpain and PKCgamma levels confirming the particular relationship between both proteins. These results suggest that calpains influence on PKCs activity may affect cellular mechanisms during memory processes.

Calpain inhibitors: a treatment for Alzheimer's disease

Activation of the calpain system might contribute to the impairment of synaptic transmission in Alzheimer's disease (AD) (Liu et al., 1999; Rapoport, 1999; Selkoe, 1994). Calpains regulate the function of many proteins by limited proteolysis and initiate the complete degradation of other proteins. In particular, they modulate processes that govern the function and metabolism of proteins key to the pathogenesis of AD, including tau and amyloid precursor protein (APP). (Xie and Johnson, 1998; Wang, 2000). We have found that overexpression of APP(K670M:N671L) and PS1(M146L) proteins in hippocampal cultures derived from transgenic mice causes an increase in the frequency of spontaneous release of neurotransmitter. We have also found that calpain immunoreactive clusters are co-localized with immunoreactivity for the vesicle-associated presynaptic marker, synaptophysin. Moreover, application of calpain inhibitor reduces the frequency of spontaneous release of neurotransmitter. Therefore, we have hypothesized that calpains might contribute to the increase in transmitter release. Based on this hypothesis, we propose to test whether it is possible to restore normal synaptic transmission between cells derived from the transgenic model of AD by using calpain inhibitors. The transgenic mouse model also shows spatial learning impairment, a phenomenon that is thought to be associated with plastic changes at synaptic level. Therefore, we will also test whether we can rescue the learning impairment through a treatment with calpain inhibitors.

The purification and characterisation of m-calpain from ostrich brain

Calpains are intracellular cysteine proteases activated in a Ca(2+)-dependent manner. The purpose of the present study was to investigate the physico-chemical and kinetic properties of ostrich brain m-calpain. m-Calpain was purified by successive chromatographic steps on Toyopearl-Super Q 650s and Pharmacia Mono Q HR 5/5 columns. A Ca(2+) concentration of 5mM and a casein concentration of 5mg/ml were found to be necessary for optimum calpain activity. Ostrich m-calpain exhibited a M(r) of 84K using SDS-PAGE and a M(min) of 79.3K from amino acid analysis. The pH and temperature optima were found to be 7.5 and 37 degrees C, respectively. The amino acid composition of m-calpain revealed 700 residues. The N-terminal sequence of m-calpain showed sequence identity with chicken (27%), human (23%) and rabbit (18%) and Schistoma mansoni (9%).

Cleavage of doublecortin-like kinase by calpain releases an active kinase fragment from a microtubule anchorage domain

Doublecortin-like kinase (DCLK) is widely expressed in postmitotic neurons throughout the embryonic nervous system. DCLK consists of an N-terminal doublecortin domain, responsible for its localization to microtubules, and a C-terminal serine-threonine kinase domain. Here we report that DCLK is a physiological substrate for the cysteine protease calpain. Cleavage of DCLK by calpain severs the kinase domain from its microtubule anchorage domain and releases it into the cytoplasm. The isolated kinase domain retains catalytic activity and is structurally similar to CPG16, a second product of the DCLK gene expressed in the adult brain that lacks the doublecortin domain. We propose that in neurons cleavage of DCLK by calpain represents a calcium responsive mechanism to regulate localization of the DCLK kinase domain.

Regulation of dendritic spine stability

Dendritic spines undergo several types of transformations, ranging from growth to collapse, and from elongation to shortening, and they experience dynamic morphological activity on a rapid time scale. Changes in spine number and morphology occur under pathological conditions like excitotoxicity, but also during normal central nervous system development, during hormonal fluctuations, and in response to neural activity under physiological circumstances. We briefly review evidence for various types of alterations in spines, and discuss the possible molecular basis for changes in spine stability. Filamentous actin appears to be the most important cytoskeletal component of spines, and a growing list of actin-associated and actin-regulatory proteins has been reported to reside within spines. We conclude that spines contain two distinct pools of actin filaments (one stable, the other unstable) that provide the spine with both a stable core structure and a dynamic, complex shape. Finally, we review the current state of knowledge of actin filament regulation, based on studies in nonneuronal cells.

Development and molecular organization of dendritic spines and their synapses

Nearly all excitatory input in the hippocampus impinges on dendritic spines which serve as multifunctional compartments that can, at the very least, selectively isolate and amplify incoming signals. Their importance to normal brain function is highlighted by the severe mental impairment observed in most individuals having poorly developed spines (Purpura, Science 1974;186:1126-1128). Distinct groups of membrane proteins, cytoskeletal elements, scaffolding proteins, and second messenger-related proteins are concentrated particularly in dendritic spines, but their ability to generate, maintain, and coordinately regulate spine structure or function is poorly understood. Here we review the unique molecular composition of dendritic spines along with the factors known to influence dendritic spine development in order to construct a model of dendritic spine development in relation to synaptogenesis.

Delayed and isoform-specific effect of NMDA exposure on neural cell adhesion molecules in hippocampus

Brief stimulation of N-methyl-D-aspartate (NMDA) receptors has been shown to generate proteolytic fragments from the extracellular domain of neural cell adhesion molecules (NCAMs). In the present study, hippocampal slice cultures were used to demonstrate that such brief stimulation is followed by a delayed increase in the 180-kDa isoform NCAM-180. The slices were exposed to NMDA for 30 s followed by rapid quenching with the antagonist AP5. Immunoassays of the experimental samples indicated that concentrations of NCAM-180 were elevated above matched controls 2-3 h after the NMDA exposure, but not at earlier or later time points. This effect was isoform-specific as concentrations of the 140-kDa NCAM species were not found to increase. Interestingly, similar selectivity was evident with prolonged infusions of NMDA where, in contrast to the effect of brief stimulation, NCAM-180 content was reduced to 50% while levels of NCAM-140 were unchanged. Together with previous findings, the data indicate that the synaptic chemistries activated by NMDA differentially regulate NCAM-180 at the translation level and by localized activation of proteases.

Ischemic cell death in brain neurons

This review is directed at understanding how neuronal death occurs in two distinct insults, global ischemia and focal ischemia. These are the two principal rodent models for human disease. Cell death occurs by a necrotic pathway characterized by either ischemic/homogenizing cell change or edematous cell change. Death also occurs via an apoptotic-like pathway that is characterized, minimally, by DNA laddering and a dependence on caspase activity and, optimally, by those properties, additional characteristic protein and phospholipid changes, and morphological attributes of apoptosis. Death may also occur by autophagocytosis. The cell death process has four major stages. The first, the induction stage, includes several changes initiated by ischemia and reperfusion that are very likely to play major roles in cell death. These include inhibition (and subsequent reactivation) of electron transport, decreased ATP, decreased pH, increased cell Ca(2+), release of glutamate, increased arachidonic acid, and also gene activation leading to cytokine synthesis, synthesis of enzymes involved in free radical production, and accumulation of leukocytes. These changes lead to the activation of five damaging events, termed perpetrators. These are the damaging actions of free radicals and their product peroxynitrite, the actions of the Ca(2+)-dependent protease calpain, the activity of phospholipases, the activity of poly-ADPribose polymerase (PARP), and the activation of the apoptotic pathway. The second stage of cell death involves the long-term changes in macromolecules or key metabolites that are caused by the perpetrators. The third stage of cell death involves long-term damaging effects of these macromolecular and metabolite changes, and of some of the induction processes, on critical cell functions and structures that lead to the defined end stages of cell damage. These targeted functions and structures include the plasmalemma, the mitochondria, the cytoskeleton, protein synthesis, and kinase activities. The fourth stage is the progression to the morphological and biochemical end stages of cell death. Of these four stages, the last two are the least well understood. Quite little is known of how the perpetrators affect the structures and functions and whether and how each of these changes contribute to cell death. According to this description, the key step in ischemic cell death is adequate activation of the perpetrators, and thus a major unifying thread of the review is a consideration of how the changes occurring during and after ischemia, including gene activation and synthesis of new proteins, conspire to produce damaging levels of free radicals and peroxynitrite, to activate calpain and other Ca(2+)-driven processes that are damaging, and to initiate the apoptotic process. Although it is not fully established for all cases, the major driving force for the necrotic cell death process, and very possibly the other processes, appears to be the generation of free radicals and peroxynitrite. Effects of a large number of damaging changes can be explained on the basis of their ability to generate free radicals in early or late stages of damage. Several important issues are defined for future study. These include determining the triggers for apoptosis and autophagocytosis and establishing greater confidence in most of the cellular changes that are hypothesized to be involved in cell death. A very important outstanding issue is identifying the critical functional and structural changes caused by the perpetrators of cell death. These changes are responsible for cell death, and their identity and mechanisms of action are almost completely unknown.

Increased calpain I-mediated proteolysis, and preferential loss of dephosphorylated NF200, following traumatic spinal cord injury

We investigated the hypothesis that the Ca2+-activated protease calpain is involved in the pathophysiology of spinal cord injury, and is linked to the proteolytic degradation of cytoskeletal proteins. We report here that levels of calpain I (mu-calpain)-mediated spectrin breakdown products are increased by 15 min post-injury, with peak levels reached by 2 h post-injury. The dephosphorylated form of the neurofilament protein NF200 is substantially lost over the same time-period. A 35-g compressive injury was applied to the midthoracic rat spinal cord for 1 min, and animals were killed at 15 min, 1, 2, 4, 8, 16, and 24 h post-injury. Calpain I-mediated spectrin breakdown products accumulated post-injury, with peak levels reached at 2 h. Secondly, we have demonstrated a progressive loss of the 200,000 mol. wt neurofilament protein NF200, a cytoskeletal calpain substrate, which began within 1-2 h post-injury. Densitometric analyses confirmed that loss of NF200 is a substrate-specific phenomenon, since (i) dephosphorylated NF200 was preferentially lost while phosphorylated NF200 was relatively spared, and (ii) actin, which is not a substrate for calpain, was relatively spared following spinal cord injury. Finally, we demonstrated calpain I-mediated spectrin breakdown within NF200-positive neuronal processes post-injury. We conclude that the accumulation of spectrin breakdown products is temporally and spatially correlated with loss of dephosphorylated NF200 after spinal cord injury.

Glutamate receptor antagonists inhibit calpain-mediated cytoskeletal proteolysis in focal cerebral ischemia

Excitatory amino acids may promote microtubular proteolysis observed in ischemic neuronal degeneration by calcium-mediated activation of calpain, a neutral protease. We tested this hypothesis in an animal model of focal cerebral ischemia without reperfusion. Spontaneously hypertensive rats were treated with 2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo-(F)quinoxaline (NBQX), a competitive antagonist of the neuronal receptor for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), or cis-4-[phosphono-methyl]-2-piperidine carboxylic acid (CGS 19755), a competitive antagonist of the N-methyl-d-aspartate (NMDA) receptor. After treatment, all animals were subjected to permanent occlusion of the middle cerebral artery for 6 or 24 h. Infarct volumes measured in animals pretreated with CGS 19755 after 24 h of ischemia were significantly smaller than those quantified in ischemic controls. Rats pretreated with NBQX showed partial amelioration of cytoskeletal injury with preserved immunolabeling of microtubule-associated protein 2 (MAP 2) at 6 and 24 h and reduced accumulation of calpain-cleaved spectrin byproducts only at 6 h. Prevention of cytoskeletal damage was more effective after pretreatment with CGS 19755, as shown by retention of MAP 2 immunolabeling and significant restriction of calpain activity at both 6 and 24 h. Preserved immunolabeling of tau protein was observed at 6 and 24 h only in animals pretreated with CGS 19755. Western analysis performed on ischemic cortex taken from controls or rats pretreated with either NBQX or CGS 19755 suggested that loss of tau protein immunoreactivity was caused by dephosphorylation, rather than proteolysis. These results demonstrate a crucial link between excitotoxic neurotransmission, microtubular proteolysis, and neuronal degeneration in focal cerebral ischemia.

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.

A calpain inhibitor attenuates cortical cytoskeletal protein loss after experimental traumatic brain injury in the rat

The capacity of a calpain inhibitor to reduce losses of neurofilament 200-, neurofilament 68- and calpain 1-mediated spectrin breakdown products was examined following traumatic brain injury in the rat. Twenty-four hours after unilateral cortical impact injury, western blot analyses detected neurofilament 200 losses of 65% (ipsilateral) and 36% (contralateral) of levels observed in naive, uninjured rat cortices. Neurofilament 68 protein levels decreased only in the ipsilateral cortex by 35% relative to naive protein levels. Calpain inhibitor 2, administered 10 min after injury via continuous arterial infusion into the right external carotid artery for 24 h, significantly reduced neurofilament 200 losses to 17% and 3% relative to naive neurofilament 200 protein levels in the ipsilateral and contralateral cortices, respectively. Calpain inhibitor administration abolished neurofilament 68 loss in the ipsilateral cortex and was accompanied by a reduction of putative calpain-mediated neurofilament 68 breakdown products. Spectrin breakdown products mediated by calpain 1 activation were detectable in both hemispheres 24 h after traumatic brain injury and were substantially reduced in animals treated with calpain inhibitor 2 both ipsilaterally and contralaterally to the site of injury. Qualitative immunofluorescence studies of neurofilament 200 and neurofilament 68 confirmed western blot data, demonstrating morphological protection of neuronal structure throughout cortical regions of the traumatically injured brain. Morphological protection included preservation of dendritic structure and reduction of axonal retraction balls. In addition, histopathological studies employing hematoxylin and eosin staining indicated reduced extent of contusion at the injury site. These data indicate that calpain inhibitors could represent a viable strategy for preserving the cytoskeletal structure of injured neurons after experimental traumatic brain injury in vivo.

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.

Microtubular proteolysis in focal cerebral ischemia

Calpain, a neutral protease activated by calcium, may promote microtubular proteolysis in ischemic brain. We tested this hypothesis in an animal model of focal cerebral ischemia without reperfusion. The earliest sign of tissue injury was observed after no more than 15 min of ischemia, with coiling of apical dendrites immunolabeled to show microtubule-associated protein 2 (MAP2). After 6 h of ischemia, MAP2 immunoreactivity was markedly diminished in the infarct zone. Quantitative Western analysis demonstrated that MAP2 was almost unmeasurable after 24 h of ischemia. An increase in calpain activity, shown by an antibody recognizing calpain-cleaved spectrin fragments, paralleled the loss of MAP2 immunostaining. Double-labeled immunofluorescent studies showed that intraneuronal calpain activity preceded evidence of MAP2 proteolysis. Perikaryal immunolabeling of tau protein became increasingly prominent between 1 and 6 h in neurons located within the transition zone between ischemic and unaffected tissue. Western blot experiments confirmed that dephosphorylation of tau protein occurred during 24 h of ischemia, but was not associated with significant loss of tau antigen. We conclude that focal cerebral ischemia is associated with early microtubular proteolysis caused by calpain.

Alterations in the neutral proteinase activities of central and peripheral nervous systems of acrylamide-, carbon disulfide-, or 2,5-hexanedione-treated rats

Proteinases are widespread in neuronal or nonneuronal eukaryotic cells. They are suggested to play an important role in the turnover of proteins in neuronal perikaryon and axon, and digestion of the transported cytoskeletal proteins in synaptic terminals. We examined the effect of acrylamide (50 mg/kg, ip), carbon disulfide (700 ppm, 9 h, 7 d a week), and 2,5-hexanedione (2,5-HD) (1% in drinking water) treatment of rats on mCANP (2 mM Ca2+), microCANP (0.1 mM Ca2+), and CINP (Ca(2+)-independent) activity in telencephalon + diencephalon (FB), rhombencephalon + mesencephalon (LB), spinal cord (SC), and sciatic nerve (SN). The proteinase activity was determined in the 30,000g supernatant fraction of tissues using 14C-methylated casein as the substrate. mCANP activity in FB, LB, and SC was inhibited only by acrylamide. Acrylamide or 2,5-HD treatment had no effect on microCANP and CINP activities of SN, whereas carbon disulfide enhanced microCANP after 15 d and CINP activity after 10 d. It is suggested that alteration in in vitro calpain activity shown by these chemicals may not be directly related to their neurotoxic effect. However, calpain may still be playing a role in this polyneuropathy by alteration in activity through inflow of Ca2+, release of Ca2+ from intracellular organelles, or other factors. Modification of cytoskeletal proteins making them more susceptible to proteases and the role of some other proteinase is also possible.

Increased M-calpain expression in the mesencephalon of patients with Parkinson's disease but not in other neurodegenerative disorders involving the mesencephalon: a role in nerve cell death?

Parkinson's disease is characterized by the loss of dopaminergic neurons in the substantia nigra and, to a lesser extent, the ventral tegmental area and catecholaminergic cell group A8. However, among these dopaminergic neurons, those expressing the calcium buffering protein calbindin are selectively preserved, suggesting that a rise in intracellular calcium concentrations may be involved in the cascade of events leading to nerve cell death in Parkinson's disease. We therefore analysed immunohistochemically the expression of the calcium-dependent protease calpain II (m-calpain) in the mesencephalon of patients with Parkinson's disease, progressive supranuclear palsy or striatonigral degeneration, where nigral dopaminergic neurons degenerate, and matched controls without nigral involvement. Calpain immunoreactivity was found in fibers and neuronal perikarya in the substantia nigra, the ventral tegmental area, catecholaminergic cell group A8 and the locus coeruleus. In patients with Parkinson's disease but not with the other neurodegenerative disorders, m-calpain immunoreactivity was detected in fibers with an abnormal morphology and in Lewy bodies. Sequential double staining revealed that most of these m-calpain-positive fibers and neuronal perikarya co-expressed tyrosine hydroxylase, indicating that most m-calpain neurons are catecholaminergic. Quantitative analysis of m-calpain staining in the substantia nigra and locus coeruleus revealed an increased density of fibers and neuronal perikarya in parkinsonian patients in both structures. These data suggest that increased calcium concentrations may be associated with nerve cell death in Parkinson's disease.

Regional differences in gene expression for calcium activated neutral proteases (calpains) and their endogenous inhibitor calpastatin in mouse brain and spinal cord

The family of calpains (CANP or calcium activated neutral proteases) and their endogenous inhibitor calpastatin have been implicated in many neural functions; however, functional distinctions between the major calpain isoforms, calpain I and II, have not been clearly established. In the present study we analyzed the gene expression patterns for calpain I and II and calpastatin in mouse brain and spinal cord by measuring both their mRNA and protein levels. Our results show that the overall mRNA level measured by competitive reverse transcription polymerase chain reaction for calpain II is 15-fold higher and for calpastatin is three-fold higher than that for calpain I. Overall, both mRNA and protein expression levels for the calpains and calpastatin showed no significant difference between the spinal cord and the brain. The cellular distributions of mRNA for calpain I or calpastatin, measured by in situ hybridization, are relatively uniform throughout the brain. In contrast, calpain II gene expression is selectively higher in certain neuron populations including pyramidal neurons of the hippocampus and the deep neocortical layers, Purkinje cells of cerebellum, and motor neurons of the spinal cord. The motor neurons were the most enriched in calpain message. Motor neurons possessed 10-fold more calpain II mRNA than any other spinal cord cell type. The differential distribution of the two proteases in the brain and the spinal cord at the mRNA level indicates that the two calpain genes are differentially regulated, suggesting that they play different physiological roles in neuronal activities and that they may participate in the pathogenesis of certain regional neurological degenerative diseases.

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.

N-methyl-D-aspartate receptor-induced proteolytic conversion of postsynaptic class C L-type calcium channels in hippocampal neurons

Ca2+ influx controls multiple neuronal functions including neurotransmitter release, protein phosphorylation, gene expression, and synaptic plasticity. Brain L-type Ca2+ channels, which contain either alpha 1C or alpha 1D as their pore-forming subunits, are an important source of calcium entry into neurons. Alpha 1C exists in long and short forms, which are differentially phosphorylated, and C-terminal truncation of alpha 1C increases its activity approximately 4-fold in heterologous expression systems. Although most L-type calcium channels in brain are localized in the cell body and proximal dendrites, alpha 1C subunits in the hippocampus are also present in clusters along the dendrites of neurons. Examination by electron microscopy shows that these clusters of alpha 1C are localized in the postsynaptic membrane of excitatory synapses, which are known to contain glutamate receptors. Activation of N-methyl-D-aspartate (NMDA)-specific glutamate receptors induced the conversion of the long form of alpha 1C into the short form by proteolytic removal of the C terminus. Other classes of Ca2+ channel alpha1 subunits were unaffected. This proteolytic processing reaction required extracellular calcium and was blocked by inhibitors of the calcium-activated protease calpain, indicating that calcium entry through NMDA receptors activated proteolysis of alpha1C by calpain. Purified calpain catalyzed conversion of the long form of immunopurified alpha 1C to the short form in vitro, consistent with the hypothesis that calpain is responsible for processing of alpha 1C in hippocampal neurons. Our results suggest that NMDA receptor-induced processing of the postsynaptic class C L-type Ca2+ channel may persistently increase Ca2+ influx following intense synaptic activity and may influence Ca2+-dependent processes such as protein phosphorylation, synaptic plasticity, and gene expression.

Calpain inhibitors protect against depolarization-induced neurofilament protein loss of septo-hippocampal neurons in culture

We examined the effect of a 6 min depolarization with 60 mM KCl and 1.8, 2.8 or 5.8 mM extracellular CaCl2 on neurofilament proteins of high (NF-H), medium (NF-M) and low (NF-L) molecular weight in primary septohippocampal cultures. One day after depolarization, Western blot analyses revealed losses of all three neurofilament proteins. Increasing the extracellular calcium concentration from 1.8 to 5.8 mM CaCl2 in the presence of 60 mM KCl produced increased losses of all three neurofilament proteins to approximately 80% of control values in the absence of cell death. Calcium-dependent losses of the neurofilament proteins correlated with calcium-dependent increases in calpain 1-mediated breakdown products of alpha-spectrin. Calpain inhibitors 1 and 2, applied immediately after depolarization and made available to cultures for 24 h, reduced losses of all three neurofilament proteins to approximately 14% of control values. The protective effects of calpain inhibitors 1 and 2 were influenced by different levels of extracellular calcium. Qualitative immunohistochemical evaluations confirmed semiquantitative Western blot data on neurofilament loss and protection by calpain inhibitors 1 and 2. We propose that brief depolarization causes loss of neurofilament proteins, possibly due to calpain activation. Thus, calpain inhibitors could represent a viable strategy for preserving the cytoskeletal structure of injured neurons.

Distribution of mu-calpain proenzyme in the brain and other neural tissues in the rat

We raised antibodies against the acetyl N-terminal peptide of the human mu-calpain 80 kDa (80 K) subunit (N-acetyl SEETPVYCT-GVSAQVQKQRARELG) in the rabbit. A specific antibody was purified using N-acetyl SEEITPVYCTGVSAQVQKQ peptide-conjugated Sepharose 4B as an affinity gel support. Epitope analysis revealed that the purified antibody reacted only with mu-calpain N-terminal peptides containing N-acetyl SEETT structure but no reactions occurred with other analogous peptides. Western blot analysis showed that the antibody reacted with both human and rat mu-calpain proenzymes but not with the activated calpains lacking N-terminal peptide. Using this antibody we investigated immunohistochemically the distribution of mu-calpain proenzyme in central and peripheral nervous systems as well as other non-neural tissues in the rat. The proenzyme was detected mainly in neurons both in the central and peripheral nervous tissues, but not in non-neural tissues except for red blood cells. Immunoreaction was stronger in the perikarya and/or in the nuclei than in-the cytoplasm. Specificity of the antibody was verified by an absorption test. In summary, the mu-calpain proenzyme is mainly distributed in the perikarya and/or nuclei or neurons. Our present antibody specific to the N-terminus of the mu-calpain 80 K subunit could serve as a useful tool to detect various functions of mu-calpain as well as the damage in neurons caused by the enzyme.