Protective effects of calpain inhibitors against neuronal damage caused by cytotoxic hypoxia in vitro and ischemia in vivo

The Cytoskeletal Elements MAP2 and NF-L Show Substantial Alterations in Different Stroke Models While Elevated Serum Levels Highlight Especially MAP2 as a Sensitive Biomarker in Stroke Patients

In the setting of ischemic stroke, the neurofilament subunit NF-L and the microtubule-associated protein MAP2 have proven to be exceptionally ischemia-sensitive elements of the neuronal cytoskeleton. Since alterations of the cytoskeleton have been linked to the transition from reversible to irreversible tissue damage, the present study investigates underlying time- and region-specific alterations of NF-L and MAP2 in different animal models of focal cerebral ischemia. Although NF-L is increasingly established as a clinical stroke biomarker, MAP2 serum measurements after stroke are still lacking. Therefore, the present study further compares serum levels of MAP2 with NF-L in stroke patients. In the applied animal models, MAP2-related immunofluorescence intensities were decreased in ischemic areas, whereas the abundance of NF-L degradation products accounted for an increase of NF-L-related immunofluorescence intensity. Accordingly, Western blot analyses of ischemic areas revealed decreased protein levels of both MAP2 and NF-L. The cytoskeletal alterations are further reflected at an ultrastructural level as indicated by a significant reduction of detectable neurofilaments in cortical axons of ischemia-affected areas. Moreover, atomic force microscopy measurements confirmed altered mechanical properties as indicated by a decreased elastic strength in ischemia-affected tissue. In addition to the results from the animal models, stroke patients exhibited significantly elevated serum levels of MAP2, which increased with infarct size, whereas serum levels of NF-L did not differ significantly. Thus, MAP2 appears to be a more sensitive stroke biomarker than NF-L, especially for early neuronal damage. This perspective is strengthened by the results from the animal models, showing MAP2-related alterations at earlier time points compared to NF-L. The profound ischemia-induced alterations further qualify both cytoskeletal elements as promising targets for neuroprotective therapies.

Neuronal Cell Death

Neuronal cell death occurs extensively during development and pathology, where it is especially important because of the limited capacity of adult neurons to proliferate or be replaced. The concept of cell death used to be simple as there were just two or three types, so we just had to work out which type was involved in our particular pathology and then block it. However, we now know that there are at least a dozen ways for neurons to die, that blocking a particular mechanism of cell death may not prevent the cell from dying, and that non-neuronal cells also contribute to neuronal death. We review here the mechanisms of neuronal death by intrinsic and extrinsic apoptosis, oncosis, necroptosis, parthanatos, ferroptosis, sarmoptosis, autophagic cell death, autosis, autolysis, paraptosis, pyroptosis, phagoptosis, and mitochondrial permeability transition. We next explore the mechanisms of neuronal death during development, and those induced by axotomy, aberrant cell-cycle reentry, glutamate (excitoxicity and oxytosis), loss of connected neurons, aggregated proteins and the unfolded protein response, oxidants, inflammation, and microglia. We then reassess which forms of cell death occur in stroke and Alzheimer's disease, two of the most important pathologies involving neuronal cell death. We also discuss why it has been so difficult to pinpoint the type of neuronal death involved, if and why the mechanism of neuronal death matters, the molecular overlap and interplay between death subroutines, and the therapeutic implications of these multiple overlapping forms of neuronal death.

Hippocampal calpain is required for the consolidation and reconsolidation but not extinction of contextual fear memory

Memory consolidation, reconsolidation, and extinction have been shown to share similar molecular signatures, including new gene expression. Calpain is a Ca2+-dependent protease that exerts its effects through the proteolytic cleavage of target proteins. Neuron-specific conditional deletions of calpain 1 and 2 impair long-term potentiation in the hippocampus and spatial learning. Moreover, recent studies have suggested distinct roles of calpain 1 and 2 in synaptic plasticity. However, the role of hippocampal calpain in memory processes, especially memory consolidation, reconsolidation, and extinction, is still unclear. In the current study, we demonstrated the critical roles of hippocampal calpain in the consolidation, reconsolidation, and extinction of contextual fear memory in mice. We examined the effects of pharmacological inhibition of calpain in the hippocampus on these memory processes, using the N-Acetyl-Leu-Leu-norleucinal (ALLN; calpain 1 and 2 inhibitor). Microinfusion of ALLN into the dorsal hippocampus impaired long-term memory (24 h memory) without affecting short-term memory (2 h memory). Similarly, this pharmacological blockade of calpain in the dorsal hippocampus also disrupted reactivated memory but did not affect memory extinction. Importantly, the systemic administration of ALLN inhibited the induction of c-fos in the hippocampus, which is observed when memory is consolidated. Our observations showed that hippocampal calpain is required for the consolidation and reconsolidation of contextual fear memory. Further, the results suggested that calpain contributes to the regulation of new gene expression that is necessary for these memory processes as a regulator of Ca2+-signal transduction pathway.

Genome-independent hypoxic repression of estrogen receptor alpha in breast cancer cells

BACKGROUND

About 75-80% of breast tumors express the estrogen receptor alpha (ER-α) and are treated with endocrine-target therapeutics, making this the premier therapeutic modality in the breast cancer clinic. However, acquired resistance is common and about 20% of resistant tumors loose ER-α expression via unknown mechanisms. Inhibition of ER-α loss could improve endocrine therapeutic efficacy, benefiting a significant number of patients. Here we test whether tumor hypoxia might commonly produce ER-α loss.

METHODS

Using standard molecular and cellular biological assays and a work station/incubator with controllable oxygen levels, we analyze the effects of hypoxia on ER-α protein, mRNA, and transcriptional activity in a panel of independently-derived ER-α positive cell lines. These lines were chosen to represent the diverse genetic backgrounds and mutations commonly present in ER-α positive tumors. Using shRNA-mediated knockdown and overexpression studies we also elucidate the role of hypoxia-inducible factor 1-alpha (HIF-1α) in the hypoxia-induced decrease in ER-α abundance.

RESULTS

We present the first comprehensive overview of the effects of bona fide low environmental oxygen (hypoxia) and HIF-1α activity on ER-α abundance and transcriptional activity. We find that stabilized HIF-1α induces rapid loss of ER-α protein in all members of our diverse panel of breast cancer cell lines, which involves proteolysis rather than transcriptional repression. Reduced ER-α severely attenuates ER-α directed transcription, and inhibits cell proliferation without overt signs of cell death in the cell lines tested, despite their varying genomic backgrounds.

CONCLUSIONS

These studies reveal a common hypoxia response that produces reduced ER-α expression and cell cycle stalling, and demonstrate a common role for HIF-1α in ER-α loss. We hypothesize that inhibitors of HIF-1α or the proteasome might stabilize ER-α expression in breast tumors in vivo, and work in combination with endocrine therapies to reduce resistance. Our data also suggests that disease re-occurrence in patients with ER-α positive tumors may arise from tumor cells chronically resident in hypoxic environments. We hypothesize that these non-proliferating cells may survive undetected until conditions change to oxygenate the environment, or cells eventually switch to proliferation via other signaling pathways.

Can 'calpain-cathepsin hypothesis' explain Alzheimer neuronal death?

Neurons are highly specialized post-mitotic cells, so their homeostasis and survival depend on the tightly-regulated, continuous protein degradation, synthesis, and turnover. In neurons, autophagy is indispensable to facilitate recycling of long-lived, damaged proteins and organelles in a lysosome-dependent manner. Since lysosomal proteolysis under basal conditions performs an essential housekeeping function, inhibition of the proteolysis exacerbates level of neurodegeneration. The latter is characterized by an accumulation of abnormal proteins or organelles within autophagic vacuoles which reveal as 'granulo-vacuolar degenerations' on microscopy. Heat-shock protein70.1 (Hsp70.1), as a means of molecular chaperone and lysosomal stabilizer, is a potent survival protein that confers neuroprotection against diverse stimuli, but its depletion induces neurodegeneration via autophagy failure. In response to hydroxynonenal generated from linoleic or arachidonic acids by the reactive oxygen species, a specific oxidative injury 'carbonylation' occurs at the key site Arg469 of Hsp70.1. Oxidative stress-induced carbonylation of Hsp70.1, in coordination with the calpain-mediated cleavage, leads to lysosomal destabilization/rupture and release of cathepsins with the resultant neuronal death. Hsp70.1 carbonylation which occurs anywhere in the brain is indispensable for neuronal death, but extent of calpain activation should be more crucial for determining the cell death fate. Importantly, not only acute ischemia during stroke but also chronic ischemia due to ageing may cause calpain activation. Here, role of Hsp70.1-mediated lysosomal rupture is discussed by comparing ischemic and Alzheimer neuronal death. A common neuronal death cascade may exist between cerebral ischemia and Alzheimer's disease.

The indirect NMDAR inhibitor flupirtine induces sustained post-ischemic recovery, neuroprotection and angioneurogenesis

N-methyl-D-aspartate receptor (NMDAR) activation induces excitotoxicity, contributing to post-stroke brain injury. Hitherto, NMDAR deactivation failed in clinical trials due to insufficient pre-clinical study designs and drug toxicity. Flupirtine is an indirect NMDAR antagonist being used as analgesic in patients. Taking into account its tolerability profile, we evaluated effects of flupirtine on post-stroke tissue survival, neurological recovery and brain remodeling. Mice were exposed to stroke and intraperitoneally treated with saline (control) or flupirtine at various doses (1-10 mg/kg) and time-points (0-12 hours). Tissue survival and cell signaling were studied on day 2, whereas neurological recovery and tissue remodeling were analyzed until day 84. Flupirtine induced sustained neuroprotection, when delivered up to 9 hours. The latter yielded enhanced neurological recovery that persisted over three months and which was accompanied by enhanced angioneurogenesis. On the molecular level, inhibition of calpain activation was noted, which was associated with increased signal-transducer-and-activator-of-transcription-6 (STAT6) abundance, reduced N-terminal-Jun-kinase and NF-κB activation, as well as reduced proteasomal activity. Consequently, blood-brain-barrier integrity was stabilized, oxidative stress was reduced and brain leukocyte infiltration was diminished. In view of its excellent tolerability, considering its sustained effects on neurological recovery, brain tissue survival and remodeling, flupirtine is an attractive candidate for stroke therapy.

The indirect NMDAR antagonist acamprosate induces postischemic neurologic recovery associated with sustained neuroprotection and neuroregeneration

Cerebral ischemia stimulates N-methyl-d-aspartate receptors (NMDARs) resulting in increased calcium concentration and excitotoxicity. Yet, deactivation of NMDAR failed in clinical studies due to poor preclinical study designs or toxicity of NMDAR antagonists. Acamprosate is an indirect NMDAR antagonist used for patients with chronic alcohol dependence. We herein analyzed the therapeutic potential of acamprosate on brain injury, neurologic recovery and their underlying mechanisms. Mice were exposed to cerebral ischemia, treated with intraperitoneal injections of acamprosate or saline (controls), and allowed to survive until 3 months. Acamprosate yielded sustained neuroprotection and increased neurologic recovery when given no later than 12 hours after stroke. The latter was associated with increased postischemic angioneurogenesis, albeit acamprosate did not stimulate angioneurogenesis itself. Rather, increased angioneurogenesis was due to inhibition of calpain-mediated pro-injurious signaling cascades. As such, acamprosate-mediated reduction of calpain activity resulted in decreased degradation of p35, increased abundance of the pro-survival factor STAT6, and reduced N-terminal-Jun-kinase activation. Inhibition of calpain was associated with enhanced stability of the blood-brain barrier, reduction of oxidative stress and cerebral leukocyte infiltration. Taken into account its excellent tolerability, its sustained effects on neurologic recovery, brain tissue survival, and neural remodeling, acamprosate is an intriguing candidate for adjuvant future stroke treatment.

Excitotoxicity and stroke: identifying novel targets for neuroprotection

Excitotoxicity, the specific type of neurotoxicity mediated by glutamate, may be the missing link between ischemia and neuronal death, and intervening the mechanistic steps that lead to excitotoxicity can prevent stroke damage. Interest in excitotoxicity began fifty years ago when monosodium glutamate was found to be neurotoxic. Evidence soon demonstrated that glutamate is not only the primary excitatory neurotransmitter in the adult brain, but also a critical transmitter for signaling neurons to degenerate following stroke. The finding led to a number of clinical trials that tested inhibitors of excitotoxicity in stroke patients. Glutamate exerts its function in large by activating the calcium-permeable ionotropic NMDA receptor (NMDAR), and different subpopulations of the NMDAR may generate different functional outputs, depending on the signaling proteins directly bound or indirectly coupled to its large cytoplasmic tail. Synaptic activity activates the GluN2A subunit-containing NMDAR, leading to activation of the pro-survival signaling proteins Akt, ERK, and CREB. During a brief episode of ischemia, the extracellular glutamate concentration rises abruptly, and stimulation of the GluN2B-containing NMDAR in the extrasynaptic sites triggers excitotoxic neuronal death via PTEN, cdk5, and DAPK1, which are directly bound to the NMDAR, nNOS, which is indirectly coupled to the NMDAR via PSD95, and calpain, p25, STEP, p38, JNK, and SREBP1, which are further downstream. This review aims to provide a comprehensive summary of the literature on excitotoxicity and our perspectives on how the new generation of excitotoxicity inhibitors may succeed despite the failure of the previous generation of drugs.

Furoxans (1,2,5-oxadiazole-N-oxides) as novel NO mimetic neuroprotective and procognitive agents

Furoxans (1,2,5-oxadiazole-N-oxides) are thiol-bioactivated NO-mimetics that have not hitherto been studied in the CNS. Incorporation of varied substituents adjacent to the furoxan ring system led to modulation of reactivity toward bioactivation, studied by HPLC-MS/MS analysis of reaction products. Attenuated reactivity unmasked the cytoprotective actions of NO in contrast to the cytotoxic actions of higher NO fluxes reported previously for furoxans. Neuroprotection was observed in primary neuronal cell cultures following oxygen glucose deprivation (OGD). Neuroprotective activity was observed to correlate with thiol-dependent bioactivation to produce NO(2)(-), but not with depletion of free thiol itself. Neuroprotection was abrogated upon cotreatment with a sGC inhibitor, ODQ, thus supporting activation of the NO/sGC/CREB signaling cascade by furoxans. Long-term potentiation (LTP), essential for learning and memory, has been shown to be potentiated by NO signaling, therefore, a peptidomimetic furoxan was tested in hippocampal slices treated with oligomeric amyloid-β peptide (Aβ) and was shown to restore synaptic function. The novel observation of furoxan activity of potential therapeutic use in the CNS warrants further studies.

Calpains are downstream effectors of bax-dependent excitotoxic apoptosis

Excitotoxicity resulting from excessive Ca(2+) influx through glutamate receptors contributes to neuronal injury after stroke, trauma, and seizures. Increased cytosolic Ca(2+) levels activate a family of calcium-dependent proteases with papain-like activity, the calpains. Here we investigated the role of calpain activation during NMDA-induced excitotoxic injury in embryonic (E16-E18) murine cortical neurons that (1) underwent excitotoxic necrosis, characterized by immediate deregulation of Ca(2+) homeostasis, a persistent depolarization of mitochondrial membrane potential (Δψ(m)), and insensitivity to bax-gene deletion, (2) underwent excitotoxic apoptosis, characterized by recovery of NMDA-induced cytosolic Ca(2+) increases, sensitivity to bax gene deletion, and delayed Δψ(m) depolarization and Ca(2+) deregulation, or (3) that were tolerant to excitotoxic injury. Interestingly, treatment with the calpain inhibitor calpeptin, overexpression of the endogenous calpain inhibitor calpastatin, or gene silencing of calpain protected neurons against excitotoxic apoptosis but did not influence excitotoxic necrosis. Calpeptin failed to exert a protective effect in bax-deficient neurons but protected bid-deficient neurons similarly to wild-type cells. To identify when calpains became activated during excitotoxic apoptosis, we monitored calpain activation dynamics by time-lapse fluorescence microscopy using a calpain-sensitive Förster resonance energy transfer probe. We observed a delayed calpain activation that occurred downstream of mitochondrial engagement and directly preceded neuronal death. In contrast, we could not detect significant calpain activity during excitotoxic necrosis or in neurons that were tolerant to excitotoxic injury. Oxygen/glucose deprivation-induced injury in organotypic hippocampal slice cultures confirmed that calpains were specifically activated during bax-dependent apoptosis and in this setting function as downstream cell-death executioners.

Leupeptin enhances cell surface localization of fibroblast growth factor receptor 1 in adult sensory neurons by increased recycling

Fibroblast growth factors (FGFs) act as trophic factors during development and regeneration of the nervous system. FGFs mediate their responses by activation of four types of FGF receptors (FGFR1-4). FGFR1 is expressed in adult sensory neurons of dorsal root ganglia (DRG), and overexpression of FGFR1 enhances FGF-2-induced elongative axon growth in vitro. Ligand-induced activation of FGFR1 is followed by endocytosis and rapid lysosomal degradation. We previously reported that the lysosomal inhibitor leupeptin prevents degradation of FGFR1 and promotes FGF-2-induced elongative axon growth of DRG neurons overexpressing FGFR1. Therefore, we analyzed the effects of leupeptin on intracellular sorting of FGFR1 in PC12 pheochromocytoma cells and DRG neurons. Leupeptin increased colocalization of FGFR1 with lysosomes. Furthermore, leupeptin enhanced the cell surface localization of FGFR1 by increased receptor recycling and this effect was abolished by the recycling inhibitor monensin. In addition, a lysine mutant of FGFR1, which is preferentially recycled back to the cell surface, promoted elongative axon growth of DRG neurons similar to leupeptin. In contrast, the lysosomal inhibitor bafilomycin had no effect on surface localization of FGFR1, inhibited axon growth of DRG neurons and abolished the effects of leupeptin on receptor recycling. Together, our results strongly imply that increased recycling of FGFR1 promotes axon elongation, but not axonal branching, of adult DRG neurons in vitro.

Calpains and delayed calcium deregulation in excitotoxicity

Overactivation of glutamate receptors results in neurodegeneration in a variety of brain pathologies, including ischemia, epilepsy, traumatic brain injury and slow-progressing neurodegenerative disorders. In all these pathologies, it is well accepted that the calcium-dependent cysteine proteases calpains are key players in the mechanisms of neuronal cell death. Many research groups have been actively pursuing to establish a link between the deregulation of intracellular Ca(2+) homeostasis associated with excitotoxicity and calpain activity. It is well established that these two events are connected and interact synergistically to promote neurodegeneration, but whether calpain activity depends on or contributes to Ca(2+) deregulation is still under debate.

Moderation of calpain activity promotes neovascular integration and lumen formation during VEGF-induced pathological angiogenesis

Background

Successful neovascularization requires that sprouting endothelial cells (ECs) integrate to form new vascular networks. However, architecturally defective, poorly integrated vessels with blind ends are typical of pathological angiogenesis induced by vascular endothelial growth factor-A (VEGF), thereby limiting the utility of VEGF for therapeutic angiogenesis and aggravating ischemia-related pathologies. Here we investigated the possibility that over-exuberant calpain activity is responsible for aberrant VEGF neovessel architecture and integration. Calpains are a family of intracellular calcium-dependent, non-lysosomal cysteine proteases that regulate cellular functions through proteolysis of numerous substrates.

Methodology/Principal Findings

In a mouse skin model of VEGF-driven angiogenesis, retroviral transduction with dominant-negative (DN) calpain-I promoted neovessel integration and lumen formation, reduced blind ends, and improved vascular perfusion. Moderate doses of calpain inhibitor-I improved VEGF-driven angiogenesis similarly to DN calpain-I. Conversely, retroviral transduction with wild-type (WT) calpain-I abolished neovessel integration and lumen formation. In vitro, moderate suppression of calpain activity with DN calpain-I or calpain inhibitor-I increased the microtubule-stabilizing protein tau in endothelial cells (ECs), increased the average length of microtubules, increased actin cable length, and increased the interconnectivity of vascular cords. Conversely, WT calpain-I diminished tau, collapsed microtubules, disrupted actin cables, and inhibited integration of cord networks. Consistent with the critical importance of microtubules for vascular network integration, the microtubule-stabilizing agent taxol supported vascular cord integration whereas microtubule dissolution with nocodazole collapsed cord networks.

Conclusions/Significance

These findings implicate VEGF-induction of calpain activity and impairment of cytoskeletal dynamics in the failure of VEGF-induced neovessels to form and integrate properly. Accordingly, calpain represents an important target for rectifying key vascular defects associated with pathological angiogenesis and for improving therapeutic angiogenesis with VEGF.

Neuroprotection with delayed calpain inhibition after transient forebrain ischemia

OBJECTIVE

Delayed neurodegeneration after transient global brain ischemia offers a therapeutic window for inhibiting molecular injury mechanisms. One such mechanism is calpain-mediated proteolysis, which peaks 24 to 48 hrs after transient forebrain ischemia in rats. This study tests the hypothesis that delayed calpain inhibitor therapy can reduce brain calpain activity and neurodegeneration after transient forebrain ischemia.

DESIGN

Prospective randomized placebo-controlled animal trial.

SETTING

University research laboratory.

SUBJECTS

Adult male Long-Evans rats.

INTERVENTIONS

Rats subjected to 10-min transient forebrain ischemia were randomized to intravenous infusion of calpain inhibitor CEP-3453 or vehicle beginning 22 hrs after injury.

MEASUREMENTS AND MAIN RESULTS

In a dose-response study, a 60 mg/kg bolus followed by 30 mg/kg infusion was required to reduce postischemic brain calpain activity measured by Western blot of hippocampal homogenates at 48 hrs after injury. The same dosing protocol decreased degeneration of CA1 pyramidal neurons measured at 72 hrs after injury.

CONCLUSIONS

These results suggest a causal role for calpains in delayed postischemic neurodegeneration, and demonstrate a broad therapeutic window for calpain inhibition in this model.

RNAi targeting micro-calpain increases neuron survival and preserves hippocampal function after global brain ischemia

The calpain family of cysteine proteases has a well-established causal role in neuronal cell death following acute brain injury. However, the relative contribution of calpain isoforms has not been determined in in vivo models. Identification of the calpain isoform responsible for neuronal injury is particularly important given the differential role of calpain isoforms in normal physiology. This study evaluates the role of m-calpain and micro-calpain in an in vivo model of global brain ischemia. Adeno-associated viral vectors expressing short hairpin RNAs targeting the catalytic subunits of micro- or m-calpain were used to knockdown expression of the targeted isoforms in adult rat hippocampal CA1 pyramidal neurons. Knockdown of micro-calpain, but not m-calpain, prevented calpain activity 72 h after 6-min transient forebrain ischemia, increased long-term survival and protected hippocampal electrophysiological function. These findings represent the first in vivo evidence that reducing expression of an individual calpain isoform can decrease post-ischemic neuronal death and preserve hippocampal function.

Knockdown of m-calpain increases survival of primary hippocampal neurons following NMDA excitotoxicity

The calpain family of cysteine proteases has a well-established causal role in neuronal cell death following acute brain injury. However, the relative contribution of calpain isoforms to the various forms of injury has not been determined as available calpain inhibitors are not isoform-specific. In this study, we evaluated the relative role of m-calpain and mu-calpain in a primary hippocampal neuron model of NMDA-mediated excitotoxicity. Baseline mRNA expression for the catalytic subunit of m-calpain (capn2 ) was found to be 50-fold higher than for the mu-calpain catalytic subunit (capn1) based on quantitative real-time PCR. Adeno-associated viral vectors designed to deliver short hairpin RNAs targeting capn1 or capn2 resulted in 60% and 90% knockdown of message respectively. Knockdown of capn2 but not capn1 increased neuronal survival after NMDA exposure at 21 days in vitro. Nuclear translocation of calpain substrates apoptosis inducing factor, p35/p25 and collapsin response mediator protein (CRMP) 2-4 was not detected after NMDA exposure in this model. However, nuclear translocation of CRMP-1 was observed and was prevented by capn2 knockdown. These findings provide insight into potential mechanisms of calpain-mediated neurodegeneration and have important implications for the development of isoform-specific calpain inhibitor therapy.

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.

Sequential degradation of alphaII and betaII spectrin by calpain in glutamate or maitotoxin-stimulated cells

Calpain-catalyzed proteolysis of II-spectrin is a regulated event associated with neuronal long-term potentiation, platelet and leukocyte activation, and other processes. Calpain proteolysis is also linked to apoptotic and nonapoptotic cell death following excessive glutamate exposure, hypoxia, HIV-gp120/160 exposure, or toxic injury. The molecular basis for these divergent consequences of calpain action, and their relationship to spectrin proteolysis, is unclear. Calpain preferentially cleaves II spectrin in vitro in repeat 11 between residues Y1176 and G1177. Unless stimulated by Ca++ and calmodulin (CaM), betaII spectrin proteolysis in vitro is much slower. We identify additional unrecognized sites in spectrin targeted by calpain in vitro and in vivo. Bound CaM induces a second II spectrin cleavage at G1230*S1231. BetaII spectrin is cleaved at four sites. One cleavage only occurs in the absence of CaM at high enzyme-to-substrate ratios near the betaII spectrin COOH-terminus. CaM promotes II spectrin cleavages at Q1440*S1441, S1447*Q1448, and L1482*A1483. These sites are also cleaved in the absence of CaM in recombinant II spectrin fusion peptides, indicating that they are probably shielded in the spectrin heterotetramer and become exposed only after CaM binds alphaII spectrin. Using epitope-specific antibodies prepared to the calpain cleavage sites in both alphaII and betaII spectrin, we find in cultured rat cortical neurons that brief glutamate exposure (a physiologic ligand) rapidly stimulates alphaII spectrin cleavage only at Y1176*G1177, while II spectrin remains intact. In cultured SH-SY5Y cells that lack an NMDA receptor, glutamate is without effect. Conversely, when stimulated by calcium influx (via maitotoxin), there is rapid and sequential cleavage of alphaII and then betaII spectrin, coinciding with the onset of nonapoptotic cell death. These results identify (i) novel calpain target sites in both alphaII and betaII spectrin; (ii) trans-regulation of proteolytic susceptibility between the spectrin subunits in vivo; and (iii) the preferential cleavage of alphaII spectrin vs betaII spectrin when responsive cells are stimulated by engagement of the NMDA receptor. We postulate that calpain proteolysis of spectrin can activate two physiologically distinct responses: one that enhances skeletal plasticity without destroying the spectrin-actin skeleton, characterized by preservation of betaII spectrin; or an alternative response closely correlated with nonapoptotic cell death and characterized by proteolysis of betaII spectrin and complete dissolution of the spectrin skeleton.

Critical role of PTEN in the coupling between PI3K/Akt and JNK1/2 signaling in ischemic brain injury

JNK pathway is an important pro-apoptotic kinase cascade mediating cell death in response to a variety of extracellular stimuli including excitotoxicity, which results in selective and delayed neuronal death in the hippocampal CA1. On the contrary, activation of the protein kinase Akt, which is controlled by the opposing actions of PI3K and PTEN, contributes to enhanced resistance to apoptosis through multiple mechanisms. We here demonstrate that the temporal pattern of Akt activation reversely correlates with JNK1/2 activation following various time points of ischemic reperfusion. However, the activation of JNK1/2 could be decreased by the elevation of Akt activation via increasing the tyrosine phosphorylation of PTEN by bpv(pic), a potent PTPases inhibitor for PTEN, or by intracerebroventricular infusion of PTEN antisense oligodeoxynucleotides (AS-ODNs). In contrast, JNK1/2 activation was significantly increased by preventing PTEN degradation after pretreatment with proteasome inhibitor. The neuroprotective effects of bpv(pic) and PTEN AS-ODNs were significant in the CA1 subfield after transient global ischemia. In conclusion, the present results clearly show that PTEN plays a key regulatory role in the cross-talk between cell survival PI3K/Akt pathway and pro-death JNK pathway, and raise a new possibility that agents targeting phosphatase PTEN may offer a great promise to expand the therapeutic options in protecting neurons form ischemic brain damage.

Retinal Penetration of Calpain Inhibitors in Rats After Oral Administration

Calpain-mediated proteolysis has been involved in neuronal cell death of retinal neurological degeneration. An aldehyde-based calpain inhibitor, SJA6017 (1), was effective following oral administration in a rat retinal ischemia model but had low oral bioavailability. The aim of this study was to identify calpain inhibitors with good retinal penetration after oral dosing. The orally bioavailable inhibitors, hemiacetal 3 (SNJ-1715), amphipathic ketoamide 5 (SNJ-1945), and pyridine ketoamide 6 (SNJ-2008), were evaluated for their retinal pharmacokinetic (PK) profiles. The retinal drug exposure of these inhibitors was more than tenfold higher than 1. Among these compounds, 5 exhibited the most favorable retinal PK properties, such as good penetration and long half-life. Comparisons of 5 and the structurally related ketoamide 6 suggested that the presence of a methoxy diethylene glycol moiety resulted in the inhibitor with high penetration into the retina and the sustained high retinal levels. Ketoamide 5 was selected as the development candidate for the treatment of retinal diseases.

Identification and characterization of PEBP as a calpain substrate

Calpains are calcium- and thiol-dependent proteases whose dysregulation has been implicated in a number of diseases and conditions such as cardiovascular dysfunction, ischemic stroke, and Alzheimer's disease (AD). While the effects of calpain activity are evident, the precise mechanism(s) by which dysregulated calpain activity results in cellular degeneration are less clear. In order to determine the impact of calpain activity, there is a need to identify the range of specific calpain substrates. Using an in vitro proteomics approach we confirmed that phosphatidylethanolamine-binding protein (PEBP) as a novel in vitro and in situ calpain substrate. We also observed PEBP proteolysis in a model of brain injury in which calpain is clearly activated. In addition, with evidence of calpain dysregulation in AD, we quantitated protein levels of PEBP in postmortem brain samples from the hippocampus of AD and age-matched controls and found that PEBP levels were approximately 20% greater in AD. Finally, with previous evidence that PEBP may act as a serine protease inhibitor, we tested PEBP as an inhibitor of the proteasome and found that PEBP inhibited the chymostrypsin-like activity of the proteasome by approximately 30%. Together these data identify PEBP as a potential in vivo calpain substrate and indicate that increased PEBP levels may contribute to impaired proteasome function.

Exploration of orally available calpain inhibitors 2: peptidyl hemiacetal derivatives

We previously reported a potent calpain inhibitor 1 (SJA6017, N-(4-fluorophenyl)-l-valyl-l-leucinal), which displayed relatively low oral bioavailability (BA). Replacing the metabolically labile aldehyde moiety of 1with more chemically stable warheads, such as a cyclic hemiacetal, hydrazone, and alpha-ketoamide, provided the inhibitors with improved in vitro metabolic stability. Cyclic hemiacetal 2 was the most stable of these compounds. The optimization of 2 led to hemiacetal 8 (SNJ-1715) which exhibited high potency, good aqueous solubility, excellent oral BA, and prolonged plasma half-life in rats. Furthermore, 8 showed neuroprotective efficacy via oral administration in a rat retinal ischemia model.

Upregulation of calpastatin in regenerating and developing rat liver: role in resistance against hepatotoxicity

Acute liver failure induced by hepatotoxic drugs results from rapid progression of injury. Substantial research has shown that timely liver regeneration can prevent progression of injury leading to a favorable prognosis. However, the mechanism by which compensatory regeneration prevents progression of injury is not known. We have recently reported that calpain released from necrotic hepatocytes mediates progression of liver injury even after the hepatotoxic drug is cleared from the body. By examining expression of calpastatin (CAST), an endogenous inhibitor of calpain in three liver cell division models known to be resistant to hepatotoxicity, we tested the hypothesis that increased CAST in the dividing hepatocytes affords resistance against progression of injury. Liver regeneration that follows CCl(4)-induced liver injury, 70% partial hepatectomy, and postnatal liver development were used. In all three models, CAST was upregulated in the dividing/newly divided hepatocytes and declined to normal levels with the cessation of cell proliferation. To test whether CAST overexpression confers resistance against hepatotoxicity, CAST was overexpressed in the livers of normal SW mice using adenovirus before challenging them with acetaminophen (APAP) overdose. These mice exhibited markedly attenuated progression of liver injury and 57% survival. Whereas APAP-bioactivating enzymes and covalent binding of the APAP-derived reactive metabolites remained unaffected, degradation of calpain specific target substrates such as fodrin was significantly reduced in these mice. In conclusion, CAST overexpression could be used as a therapeutic strategy to prevent progression of liver injury where liver regeneration is severely hampered.

Homocysteine-mediated activation and mitochondrial translocation of calpain regulates MMP-9 in MVEC

Hyperhomocysteinemia (HHcy) is associated with atherosclerosis, stroke, and dementia. Hcy causes extracellular matrix remodeling by the activation of matrix metalloproteinase-9 (MMP-9), in part, by inducing redox signaling and modulating the intracellular calcium dynamics. Calpains are the calcium-dependent cysteine proteases that are implicated in mitochondrial damage via oxidative burst. Mitochondrial abnormalities have been identified in HHcy. The mechanism of Hcy-induced extracellular matrix remodeling by MMP-9 activation via mitochondrial pathway is largely unknown. We report a novel role of calpains in mitochondrial-mediated MMP-9 activation by Hcy in cultured rat heart microvascular endothelial cells. Our observations suggested that calpain regulates Hcy-induced MMP-9 expression and activity. We showed that Hcy activates calpain-1, but not calpain-2, in a calcium-dependent manner. Interestingly, the enhanced calpain activity was not mirrored by the decreased levels of its endogenous inhibitor calpastatin. We presented evidence that Hcy induces the translocation of active calpain from cytosol to mitochondria, leading to MMP-9 activation, in part, by causing intramitochondrial oxidative burst. Furthermore, studies with pharmacological inhibitors of calpain (calpeptin and calpain-1 inhibitor), ERK (PD-98059) and the mitochondrial uncoupler FCCP suggested that calpain and ERK-1/2 are the major events within the Hcy/MMP-9 signal axis and that intramitochondrial oxidative stress regulates MMP-9 via ERK-1/2 signal cascade. Taken together, these findings determine the novel role of mitochondrial translocation of calpain-1 in MMP-9 activation during HHcy, in part, by increasing mitochondrial oxidative stress.

Oestrogen receptor subtype-specific repression of calpain expression and calpain enzymatic activity in neuronal cells--implications for neuroprotection against Ca-mediated excitotoxicity

Calpains represent a superfamily of Ca2+-activated cysteine-proteases, which are important mediators of apoptosis and necrosis. In the brain, m-calpain and micro-calpain, the two ubiquitous calpain-isoforms, are strongly activated in neurones after an excitotoxic Ca2+ influx occurring, for example, during cerebral ischemia. Because oestrogen and its receptors (ERalpha/ERbeta) can exert neuroprotective activity, we investigated their influence on expression of calpains and their endogenous inhibitor, calpastatin. We found that ectopic expression of ERalpha in human neuroblastoma SK-N-MC cells led to a ligand-independent constitutive down-regulation of m-calpain accompanied by an up-regulation of micro-calpain expression. Up-regulation of micro-calpain was reversed in the presence of oestrogen, which, in turn, could be blocked by co-treatment with the oestrogen-receptor antagonist ICI 182,780. Expression of calpastatin was not altered, either in the absence or in the presence of oestrogen. Additional studies revealed that ERalpha-expressing cells exhibited decreased calpain enzymatic activity and increased survival when cells were exposed to the Ca2+ ionophore, ionomycin. Since all investigated effects could be observed exclusively in the presence of ERalpha, but not ERbeta, and since the effects are reduced when ERalpha and ERbeta are co-expressed, our data suggest a novel ER subtype-specific neuroprotective action by repressing calpain expression and calpain activity under conditions of a massive Ca2+ influx.

Cellular and molecular pathways of ischemic neuronal death

Three routes have been identified triggering neuronal death under physiological and pathological conditions. Excess activation of ionotropic glutamate receptors cause influx and accumulation of Ca2+ and Na+ that result in rapid swelling and subsequent neuronal death within a few hours. The second route is caused by oxidative stress due to accumulation of reactive oxygen and nitrogen species. Apoptosis or programmed cell death that often occurs during developmental process has been coined as additional route to pathological neuronal death in the mature nervous system. Evidence is being accumulated that excitotoxicity, oxidative stress, and apoptosis propagate through distinctive and mutually exclusive signal transduction pathway and contribute to neuronal loss following hypoxic-ischemic brain injury. Thus, the therapeutic intervention of hypoxic-ischemic neuronal injury should be aimed to prevent excitotoxicity, oxidative stress, and apoptosis in a concerted way.

Chlortetracycline and Demeclocycline Inhibit Calpains and Protect Mouse Neurons against Glutamate Toxicity and Cerebral Ischemia

Minocycline is a potent neuroprotective tetracycline in animal models of cerebral ischemia. We examined the protective properties of chlortetracycline (CTC) and demeclocycline (DMC) and showed that these two tetracyclines were also potent neuroprotective against glutamate-induced neuronal death in vitro and cerebral ischemia in vivo. However, CTC and DMC appeared to confer neuroprotection through a unique mechanism compared with minocycline. Rather than inhibiting microglial activation and caspase, CTC and DMC suppressed calpain activities. In addition, CTC and DMC only weakly antagonized N-methyl-D-aspartate (NMDA) receptor activities causing 16 and 14%, respectively, inhibition of NMDA-induced whole cell currents and partially blocked NMDA-induced Ca2+ influx, commonly regarded as the major trigger of neuronal death. In vitro and in vivo experiments demonstrated that the two compounds selectively inhibited the activities of calpain I and II activated following glutamate treatment and cerebral ischemia. In contrast, minocycline did not significantly inhibit calpain activity. Taken together, these results suggested that CTC and DMC provide neuroprotection through suppression of a rise in intracellular Ca2+ and inhibition of calpains.

Calpain and N-methyl-d-aspartate (NMDA)-induced excitotoxicity in rat retinas

Calpain-mediated proteolysis has been implicated as a major process in neuronal cell death in both acute insults and the chronic neurodegenerative disorders in the central nerves system. However, activation of calpain also plays a protective function in the early phase of excitotoxic neuronal death. The exact role of calpains in neuronal death and recovery after exposure to N-methyl-D-aspartate (NMDA) is not clearly known. The purpose of present study was to examine the involvement of mu- and m-calpain in NMDA-induced excitotoxicity in the adult rat retina. Increased immunoreactivity of mu-calpain was noted in RGC layer cells and in the inner nuclear layer with maximal expression at 12 h after NMDA injection. This was further confirmed with Western blotting. TdT-mediated biotin-dUTP nick end labeling (TUNEL) positive cells in the inner retina co-localized with moderate or intense mu-calpain immunoreactivity. In contrast, there was no remarkable change in m-calpain immunoreactivity at any time point after NMDA injection. Simultaneous injection of 2 nmol of a calpain inhibitor (calpain inhibitor II) significantly reduced the number of TUNEL-positive cells in the inner retina at 18 h after NMDA injection and preserved RGC-like cells counted at 7 days after injection. The results of this study showed that mu-calpain may be involved in mediating NMDA-induced excitotoxicity in the rat retina and calpain inhibitors may play a therapeutic role in NMDA related disease.

Identification of a novel calpain inhibitor using phage display

Calpains are calcium- and thiol-dependent proteases that cleave a variety of intracellular substrates. Overactivation of the calpains has been implicated in a number of diseases and conditions such as ischemic stroke indicating a need for the development of calpain inhibitors. A major problem with current calpain inhibitors has been specific targeting to calpain. To identify highly specific calpain interacting peptides, we developed a peptide-phage library screening method based on the calcium-dependent conformation change associated with calpain activation. A phage-peptide library representing greater than 2 billion expressed 12-mers was incubated with calpain I in the presence of calcium. The calcium-dependent bound phage was then eluted by addition of EGTA. After four rounds of selection we found a conserved 5-mer sequence represented by LSEAL. Synthetic LSEAL inhibited tau-calpain interaction and in vitro proteolysis of tau- and alpha-synuclein by calpains. Deletion of the portion of the tau protein containing a homologous sequence to LSEAL resulted in decreased calpain-mediated tau degradation. These data suggest that these peptides may represent novel calpastatin mimetics.

The calcium-dependent protease calpain causes endothelial dysfunction in type 2 diabetes

Cardiovascular complications are the leading cause of morbidity and mortality in diabetic patients. Endothelial dysfunction with impaired endothelial nitric oxide (NO) synthase (eNOS) activity is a widely accepted cause of diabetic vasculopathy. The mechanisms of endothelial dysfunction in diabetes remain elusive, thus limiting effective therapeutic interventions. We report novel evidence demonstrating that the calcium-dependent protease calpain causes endothelial dysfunction and vascular inflammation in the microcirculation of the ZDF (Zucker diabetic fatty) rat, a genetic rat model of type 2 diabetes. We found evidence of increased calpain activity and leukocyte trafficking in the microcirculation of ZDF rats. Inhibition of calpain activity significantly attenuated leukocyte-endothelium interactions in the vasculature of ZDF rats. Expression of cell adhesion molecules in the vascular endothelium of ZDF rats was consistently increased, and it was suppressed by calpain inhibition. In vivo measurement of endothelial NO availability demonstrated a 60% decrease in NO levels in the microcirculation of diabetic rats, which was also prevented by calpain inhibition. Immunoprecipitation studies revealed calpain-dependent loss of association between eNOS and the regulatory protein heat shock protein 90. Collectively, these data provide evidence for a novel mechanism of endothelial dysfunction and vascular inflammation in diabetes. Calpains may represent a new molecular target for the prevention and treatment of diabetic vascular complications.

Distinct mechanistic roles of calpain and caspase activation in neurodegeneration as revealed in mice overexpressing their specific inhibitors

Enzymatic proteolysis has been implicated in diverse neuropathological conditions, including acute/subacute ischemic brain injuries and chronic neurodegeneration such as Alzheimer disease and Parkinson disease. Calcium-dependent proteases, calpains, have been intensively analyzed in relation to these pathological conditions, but in vivo experiments have been hampered by the lack of appropriate experimental systems for a selective regulation of the calpain activity in animals. Here we have generated transgenic (Tg) mice that overexpress human calpastatin, a specific and the only natural inhibitor of calpains. In order to clarify the distinct roles of these cell death-associated cysteine proteases, we dissected neurodegenerative changes in these mice together with Tg mice overexpressing a viral inhibitor of caspases after intrahippocampal injection of kainic acid (KA), an inducer of neuronal excitotoxicity. Immunohistochemical analyses using endo-specific antibodies against calpain- and caspase-cleaved cytoskeletal components revealed that preclusion of KA-induced calpain activation can rescue the hippocampal neurons from disruption of the neuritic cytoskeletons, whereas caspase suppression has no overt effect on the neuritic pathologies. In addition, progressive neuronal loss between the acute and subacute phases of KA-induced injury was largely halted only in human calpastatin Tg mice. The animal models and experimental paradigm employed here unequivocally demonstrate their usefulness for clarifying the distinct contribution of calpain and caspase systems to molecular mechanisms governing neurodegeneration in adult brains, and our results indicate the potentials of specific calpain inhibitors in ameliorating excitotoxic neuronal damages.

Cystatin C prevents degeneration of rat nigral dopaminergic neurons: in vitro and in vivo studies

Destruction of nigrostriatal dopaminergic (DA) pathway triggers various persistent responses, such as inflammation and increased synthesis of neural growth factors, both in striatum and in substantia nigra. The pathological processes involved in such responses are poorly characterized and could contribute to secondary damage and/or regeneration in the central nervous system (CNS). Cystatin C was previously implicated in the process of neurodegeneration. However, its biological role during neurodegeneration is not understood and remains controversial. The present study identified an increased cystatin C mRNA level in the DA-depleted rat striatum, starting from the second week following a 6-OHDA-induced lesion. Immunohistochemical analysis confirmed the increase in cystatin C protein level in the striatum following DA depletion. Double-labeled fluorescence immunohistochemistry revealed that nigrostriatal neurons, astrocytes, and microglia contributed to the elevated level of cystatin C. Exposure to 6-hydroxydopamine, a DA-specific neurotoxin, resulted in DA neurons loss in the fetal mesencephalic cultures, an effect which could be partially reversed by treatment with cystatin C. Moreover, in vivo DA neurons survival study showed that administration of cystatin C in rats with 6-OHDA-induced lesion partially rescued the nigral DA neurons. The results indicate that the 6-OHDA lesioning induced a relatively slow but sustained up-regulation of cystatin C expression and suggest that the inhibitor may exert a neuroprotective action on DA neurons. The findings raise the possibility that cysteine proteinase inhibitors may be new candidates for neuroprotective treatment of Parkinson's disease. Cystatin C may be useful therapeutically in limiting neuropathy in Parkinson's disease.

Calpain inhibitor 2 prevents axonal degeneration of opossum optic nerve fibers

The ultrastructural change that characterizes the onset of Wallerian degeneration is the disintegration of axoplasmic microtubules and neurofilaments, which are converted into an amorphous and granular material, followed by myelin breakdown. The mechanism underlying such processes is an increase in the amount of intracellular calcium, leading to activation of proteases called calpains. The aim of this study was to evaluate by quantitative ultrastructural analysis whether nerve fibers can be preserved by the use of an exogenous inhibitor of these proteases (calpain inhibitor-2, Mu-F-hF-FMK), after optic nerve crush. For that, the left optic nerves of opossums, Didelphis aurita, were crushed with the aid of a fine forceps, and half of them received a calpain inhibitor mixed with Elvax resin. Ninety-six hours after the lesion, the animals were reanesthetized and transcardially perfused, and the optic nerves were removed, the right ones being used as normal nerves. Afterward, the optic nerves were dissected and processed for routine transmission electron microscopy and quantitative and statistical analysis. The results of this analysis showed that the group that received the calpain inhibitor presented a reduction of astrogliosis, maintaining the optic nerve structure in an organized state; a significant decrease in the number of degenerating fibers; and a significant increase in the number of fibers with preserved cytoskeleton and preservation of axonal and myelin area and integrity, reducing the enlargement and edema of the axon. In conclusion, our findings suggest that calpain inhibitor is able to provide neuroprotection of the central nervous system fibers after a crush lesion.

Cathepsin B mRNA and protein expression following contusion spinal cord injury in rats

We provide the first data that cathepsin B (Cath B), a lysosomal cysteine protease, is up-regulated following contusion-spinal cord injury (SCI). Following T12 laminectomy and moderate contusion, Cath B mRNA and protein expression profiles were examined from 2 to 168 h post-injury in rats using real-time PCR and immunoblots, respectively. Contusion injury significantly increased [mRNA]Cath B in the injury site and adjacent segments over sham injury levels. While the largest [mRNA]Cath B induction (20-fold over naive) was seen in the injury site, the caudal segment routinely yielded [mRNA]Cath B levels greater than 10-fold over naive. Interestingly, sham injury animals also experienced mRNA induction at several time points at the injury site and in segments rostral and caudal to the injury site. Contusion injury also significantly elevated levels of Cath B proenzyme protein (37 kDa) over sham injury in the injury site (48, 72 and 168 h post-injury). Furthermore, significant protein increases of single and double chain Cath B (both active forms) occurred at the injury site at 72 and 168 h post-injury. Similar significant increases in Cath B protein levels were seen in areas adjacent to the injury site. The induction of Cath B mRNA and protein expression following contusion injury is previously undescribed and suggests that Cath B may potentially be involved in the secondary injury cascade, perhaps for as long as 1 week post-injury.

Inhibition of calpains, by treatment with leupeptin, improves motoneuron survival and muscle function in models of motoneuron degeneration

The effect of treatment with leupeptin, a calpain inhibitor, on motoneuron survival and muscle function was examined in in vitro and in vivo models of motoneuron degeneration. Exposure of primary rat motoneurons to alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) is an established in vitro model of excitotoxic motoneuron death. Here we show that leupeptin treatment improved motoneuron survival following exposure to AMPA (50 microM). Application of leupeptin (100 microM) to AMPA treated cultures rescued many motoneurons so that 74% (+/-3.4 S.E.M., n=5) survived compared with only 49% (+/-2.4 S.E.M., n=5) in untreated cultures. The effect of treatment with leupeptin on motoneuron survival and muscle function was also examined in vivo. In 3 day-old rats, the sciatic nerve was crushed and at the time of injury, a silicon implant containing leupeptin was inserted onto the lumbar spinal cord. The effect on long-term motoneuron survival and muscle function was assessed 12 weeks after injury. The results showed that there was long-term improvement in motoneuron survival in the leupeptin treated group. Thus, in untreated animals 12 weeks after nerve crush only 30% (+/-2.8. S.E.M., n=3) of sciatic motoneurons survived compared with 43% (+/-1.5 S.E.M., n=3) in the leupeptin-treated group. This improvement in motoneuron survival was reflected in a significant improvement in muscle function in the leupeptin-treated group. For example in the soleus muscle of treated rats 20.8 (+/-1.40 S.E.M., n=5) motor units survived compared with only 14.6 (+/-1.21 S.E.M., n=5) in untreated animals. Thus, treatment with leupeptin, a calpain inhibitor, rescues motoneurons from cell death and improves muscle function following nerve injury.

Calpain Inhibitor I Reduces Intestinal Ischemia-Reperfusion Injury in the Rat

In this study we evaluated the effect of calpain inhibitor I on splanchnic artery occlusion (SAO) shock-mediated injury. SAO shock was induced in rats by clamping both the superior mesenteric artery and the celiac trunk for 45 min. After 1 h of reperfusion, SAO-shocked rats developed a significant fall in mean arterial blood pressure. Western blot analysis of ileum revealed a marked decrease in of IkappaB-alpha expression, and immunohistochemical examination of necrotic ileum demonstrated a marked increase in the immunoreactivity to P-selectin, intracellular adhesion molecule (ICAM-1), nitrotyrosine formation, and nuclear enzyme poly[adenosine diphosphate (ADP)-ribose] synthase (PARS) activation. An increase in myeloperoxidase activity (143 +/- 22 4.5 U/100 mg wet tissue vs. 4.5 +/- 2.5 U/100 mg wet tissue of sham-operated rats) and in malondialdehyde levels (13.12 +/- 1.2 micromol/100 mg wet tissue vs. 3.9 +/- 1.1 micromol/100 mg wet tissue of sham-operated rats) was also observed in rats subjected to ischemia-reperfusion injury. Calpain inhibitor I, given intraperitoneally 30 min before ischemia at a dose of 15 mg/kg, significantly improved mean arterial blood pressure, markedly reduced IkappaB-alpha degradation and the intensity of P-selectin and ICAM-1 in the reperfused ileum. Calpain inhibitor I also significantly prevented neutrophil infiltration (32.95 +/- 9.82 U/100 mg wet tissue), reduced malondialdehyde levels (6.76 +/- 0.98 micromol/100 mg wet tissue) and markedly improved the histological status of the reperfused tissue. In conclusion, this study demonstrates that calpain inhibitor I exerts multiple protective effects in splanchnic artery occlusion-reperfusion shock and suggests that calpain inhibitor I may be a candidate for consideration as a therapeutic intervention for ischemia-reperfusion injury.

Tacrolimus and cyclosporine A are of no benefit to young rats with kaolin-induced hydrocephalus

Hydrocephalus causes damage to periventricular axons. Tacrolimus, cyclosporine A (CsA) and calpain inhibitors have been shown to protect axons in rat models of acute traumatic brain injury. We hypothesized that these agents would ameliorate the axon damage and behavioral effects in experimental hydrocephalus. Hydrocephalus was induced in 3-week-old rats by injection of kaolin into the cisterna magna. Tests of cognitive and motor function were performed on a weekly basis. In a blinded and randomized manner, tacrolimus (FK506; 3.6 mg/kg body weight) or CsA (10 mg/kg) was administered once daily by subcutaneous injection for 2 weeks, beginning 2 weeks after induction of hydrocephalus. In a separate experiment, calpain inhibitor I (10 mg/kg/day) was administered by continuous subcutaneous infusion. The brains were subjected to histopathological and biochemical analyses after 2 weeks of treatment. There was no statistically significant protection in regard to behavior, brain structure or brain composition in any of the experiments. However, there was biochemical and histological evidence of renal injury following chronic tacrolimus and CsA administration. Calcineurin inhibition does not offer significant protection in this rat model of hydrocephalus.

Both apoptosis and necrosis occur early after intracerebral grafting of ventral mesencephalic tissue: a role for protease activation

Neural transplantation is an experimental treatment for Parkinson's disease. Widespread clinical application of the grafting technique is hampered by a relatively poor survival (around 10%) of implanted embryonic dopamine neurones. Earlier animal studies have indicated that a large proportion of the grafted cells die during graft tissue preparation and within the first few days after intracerebral implantation. The present study was designed to reveal the prevalence of cell death in rat intrastriatal grafts at 90 min, 1, 3, 6 and 42 days after implantation. We examined apoptotic cell death using semi-thin and paraffin sections stained with methylene blue and an antibody against activated caspase 3, respectively. We identified abundant apoptotic cell death up to 3 days after transplantation. In addition, we studied calpain activation using an antibody specific for calpain-cleaved fodrin. We report a peak in calpain activity 90 min after grafting. Surprisingly, we did not observe any significant difference in the number of dopaminergic neurones over time. The present results imply that grafted cells may be victims of either an early necrotic or a later apoptotic cell death and that there is substantial cell death as early as 90 min after implantation.

A novel role for calpains in the endothelial dysfunction of hyperglycemia

Recent studies have reported that the activity of the calcium-dependent protease calpain is increased in acute inflammatory processes of the cardiovascular system. Because diabetes is associated with vascular inflammation, we hypothesized that increased calpain activity in response to hyperglycemia may play a role in diabetic cardiovascular disease. The effects of calpain inhibition on leukocyte-endothelium interactions induced by hyperglycemia were examined by intravital microscopy. Intraperitoneal administration of the selective calpain inhibitor benzyloxycarbonyl-leucyl-leucinal (5 micromol/L) prevented the up-regulation of leukocyte-endothelium interactions in response to 25 mmol/L D-glucose via a nitric oxide-dependent mechanism. Furthermore, treatment of rats with D-glucose significantly decreased basal endothelial NO release in mesenteric post-capillary venules, a phenomenon prevented by inhibition of calpain activity. Immunoprecipitation studies revealed that glucose induces loss of NO via a calpain-dependent decrease in the association of hsp90 with endothelial nitric oxide synthase. In addition, inhibition of calpain activity decreased endothelial cell surface expression of the pro-inflammatory adhesion molecules ICAM-1 and VCAM-1 during hyperglycemia. These data demonstrate that calpains contribute to important inflammatory events during hyperglycemia and that pharmacological inhibition of calpain activity attenuates leukocyte-endothelium interactions and preserves eNOS function.

Ischemic neuronal death in the rat hippocampus: the calpain-calpastatin-caspase hypothesis

Inappropriate imbalances between proteases and protease inhibitors are known to occur under cerebral ischemia and neurodegenerative processes, and could be contributors to various diseases that are characterized by excessive (ischemia, AIDS) or inadequate (cancer, autoimmunity) cell death. For instance, calpain is activated in various necrotic and apoptotic conditions, whereas caspase-3 is only activated in neuronal apoptosis. Caspases and calpains are cysteine proteases that require proteolytic cleavage for activation. The substrates cleaved by caspases include cytoskeletal and associated proteins, kinases, members of the Bcl-2 family of apoptosis-related proteins, presenilins, and DNA-modulating enzymes. Calpain substrates include cytoskeletal and associated proteins, kinases and phosphatases, membrane receptors and transporters, and steroid receptors. This article provides a review of the properties of caspases and calpains, their roles in cell death pathways following cerebral ischemia, and the substrates upon which they act. Because calpain inhibitors and caspase inhibitors appear to protect brain tissue by distinct mechanisms in cerebral ischemia, the possible therapeutic interactions between these drugs in a well-defined rodent model of global ischemia are briefly discussed and documented.

Cyclosporin A prevents calpain activation despite increased intracellular calcium concentrations, as well as translocation of apoptosis-inducing factor, cytochrome c and caspase-3 activation in neurons exposed to transient hypoglycemia

Blockade of mitochondrial permeability transition protects against hypoglycemic brain damage. To study the mechanisms downstream from mitochondria that may cause neuronal death, we investigated the effects of cyclosporin A on subcellular localization of apoptosis-inducing factor and cytochrome c, activation of the cysteine proteases calpain and caspase-3, as well as its effect on brain extracellular calcium concentrations. Redistribution of cytochrome c occurred at 30 min of iso-electricity, whereas translocation of apoptosis-inducing factor to nuclei occurred at 30 min of recovery following 30 min of iso-electricity. Active caspase-3 and calpain-induced fodrin breakdown products were barely detectable in the dentate gyrus and CA1 region of the hippocampus of rat brain exposed to 30 or 60 min of insulin-induced hypoglycemia. However, 30 min or 3 h after recovery of blood glucose levels, fodrin breakdown products and active caspase-3 markedly increased, concomitant with a twofold increase in caspase-3-like enzymatic activity. When rats were treated with neuroprotective doses of cyclosporin A, but not with FK 506, the redistribution of apoptosis-inducing factor and cytochrome c was reduced and fodrin breakdown products and active caspase-3 immuno-reactivity was diminished whereas the extracellular calcium concentration was unaffected. We conclude that hypoglycemia leads to mitochondrial permeability transition which, upon recovery of energy metabolism, mediates the activation of caspase-3 and calpains, promoting cell death.

Comparison of calpain and caspase activities in the adult rat brain after transient forebrain ischemia

The role of calpain and caspase family proteases in postischemic neuronal death remains controversial. This study compared the timing, location, and relative activity of calpains and caspases in the adult rat brain following 10 min of transient forebrain ischemia. Western blots of cortical, striatal, and hippocampal homogenates demonstrated a alpha-spectrin cleavage pattern indicative of predominant calpain activity, which peaked between 24 and 48 h after reperfusion. However, immunohistochemical evidence of both caspase 3 activation and caspase-mediated substrate cleavage was detected as early as 1 h and as late as 7 days after reperfusion in circumscribed neuronal populations. Simultaneous or sequential caspase and calpain activation was also observed suggesting the potential for interaction of these protease systems. The complex spatiotemporal pattern of calpain and caspase activity observed in this study provides important insights for the development and evaluation of therapeutic strategies to reduce protease-mediated injury following global brain ischemia.

Ritonavir inhibition of calcium-activated neutral proteases

Calpains (EC 3.4.22.17) are intracellular calcium-activated cysteine proteases that mediate tissue injury following post-ischemic and post-traumatic stress. Both human HIV protease and calpains share a similar secondary structure, where the active site is flanked by hydrophobic regions. The present study demonstrates that ritonavir, a hydrophobic HIV protease inhibitor, also inhibits calpain activity. In PC12 cell extracts assayed for calpain at maximal activity (2mM calcium), ritonavir exhibited competitive inhibition with a K(i) of 11+/-7.0 microM. Experiments with purified enzymes showed inhibition for both m- and mu-calpain isoforms (m-calpain, K(i)=9.2+/-1.2 microM; mu-calpain, K(i)=5.9+/-1.4 microM). Ritonavir also inhibited calcium-stimulated calpain activity in PC12 cells in situ. These results suggest that ritonavir or analogues of the drug should be investigated as cytoprotective agents in conditions where cell death or injury is mediated via calpain activation.