Nuclear calpain regulates Ca2+-dependent signaling via proteolysis of nuclear Ca2+/calmodulin-dependent protein kinase type IV in cultured neurons

Membrane-anchored calpains - hidden regulators of growth and development beyond plants?

Calpains are modulatory proteases that modify diverse cellular substrates and play essential roles in eukaryots. The best studied are animal cytosolic calpains. Here, we focus on enigmatic membrane-anchored calpains, their structural and functional features as well as phylogenetic distribution. Based on domain composition, we identified four types of membrane-anchored calpains. Type 1 and 2 show broad phylogenetic distribution among unicellular protists and streptophytes suggesting their ancient evolutionary origin. Type 3 and 4 diversified early and are present in brown algae and oomycetes. The plant DEK1 protein is the only representative of membrane-anchored calpains that has been functionally studied. Here, we present up to date knowledge about its structural features, putative regulation, posttranslational modifications, and biological role. Finally, we discuss potential model organisms and available tools for functional studies of membrane-anchored calpains with yet unknown biological role. Mechanistic understanding of membrane-anchored calpains may provide important insights into fundamental principles of cell polarization, cell fate control, and morphogenesis beyond plants.

New Perspectives on the Role of Nuclear Proteases in Cell Death Pathways

Multiple factors can trigger cell death via various pathways, and nuclear proteases have emerged as essential regulators of these processes. While certain nuclear proteases have been extensively studied and their mechanisms of action are well understood, others remain poorly characterized. Regulation of nuclear protease activity is a promising therapeutic strategy that could selectively induce favorable cell death pathways in specific tissues or organs. Thus, by understanding the roles of newly discovered or predicted nuclear proteases in cell death processes, we can identify new pharmacological targets for improving therapeutic outcomes. In this article, we delved into the role of nuclear proteases in several types of cell death and explore potential avenues for future research and therapeutic development.

Calpain Inhibitors as Potential Therapeutic Modulators in Neurodegenerative Diseases

It is considered a significant challenge to understand the neuronal cell death mechanisms with a suitable cure for neurodegenerative disorders in the coming years. Calpains are one of the best-considered "cysteine proteases activated" in brain disorders. Calpain is an important marker and mediator in the pathophysiology of neurodegeneration. Calpain activation being the essential neurodegenerative factor causing apoptotic machinery activation, it is crucial to develop reliable and effective approaches to prevent calpain-mediated apoptosis in degenerating neurons. It has been recently seen that the "inhibition of calpain activation" has appeared as a possible therapeutic target for managing neurodegenerative diseases. A systematic literature review of PubMed, Medline, Bentham, Scopus, and EMBASE (Elsevier) databases was conducted. The present article reviews the basic pathobiology and role of selective calpain inhibitors used in various neurodegenerative diseases as a therapeutic target.

Effects of nuclear factor I phosphorylation on calpastatin (CAST) gene variant expression and subcellular distribution in malignant glioma cells

Malignant glioma (MG) is the most lethal primary brain tumor. In addition to having inherent resistance to radiation treatment and chemotherapy, MG cells are highly infiltrative, rendering focal therapies ineffective. Genes involved in MG cell migration and glial cell differentiation are up-regulated by hypophosphorylated nuclear factor I (NFI), which is dephosphorylated by the phosphatase calcineurin in MG cells. Calcineurin is cleaved and thereby activated by calpain proteases, which are, in turn, inhibited by calpastatin (CAST). Here, we show that the CAST gene is a target of NFI and has NFI-binding sites in its intron 3 region. We also found that NFI-mediated regulation of CAST depends on NFI's phosphorylation state. We noted that occupation of CAST intron 3 by hypophosphorylated NFI results in increased activation of an alternative promoter. This activation resulted in higher levels of CAST transcript variants, leading to increased levels of CAST protein that lacks the N-terminal XL domain. CAST was primarily present in the cytoplasm of NFI-hypophosphorylated MG cells, with a predominantly perinuclear immunostaining pattern. NFI knockdown in NFI-hypophosphorylated MG cells increased CAST levels at the plasma membrane. These results suggest that NFI plays an integral role in the regulation of CAST variants and CAST subcellular distribution. Along with the previous findings indicating that NFI activity is regulated by calcineurin, these results provide a foundation for further investigations into the possibility of regulatory cross-talk between NFI and the CAST/calpain/calcineurin signaling pathway in MG cells.

A Calpain-Like Protein Is Involved in the Execution Phase of Programmed Cell Death of Entamoeba histolytica

Oxygen or nitrogen oxidative species and chemical stress induce the programmed cell death (PCD) of Entamoeba histolytica trophozoites. PCD caused by the aminoglycoside G418 is reduced by incubation with the cysteine protease inhibitor E-64; however, no typical caspases or metacaspases have been detected in this parasite. Calpain, a cysteine protease activated by calcium, has been suggested to be part of a specific PCD pathway in this parasite because the specific calpain inhibitor Z-Leu-Leu-Leu-al diminishes the PCD of trophozoites. Here, we predicted the hypothetical 3D structure of a calpain-like protein of E. histolytica and produced specific antibodies against it. We detected the protein in the cytoplasm and near the nucleus. Its expression gradually increased during incubation with G418, with the highest level after 9 h of treatment. In addition, a specific calpain-like siRNA sequence reduced the cell death rate by 65%. All these results support the hypothesis that the calpain-like protein is one of the proteases involved in the execution phase of PCD in E. histolytica. The hypothetical interactome of the calpain-like protein suggests that it may activate or regulate other proteins that probably participate in PCD, including those with EF-hand domains or other calcium-binding sites.

Involvement of calpain in the neuropathogenesis of Alzheimer's disease

Alzheimer’s disease (AD) is the most common (60% to 80%) age‐related disease associated with dementia and is characterized by a deterioration of behavioral and cognitive capacities leading to death in few years after diagnosis, mainly due to complications from chronic illness. The characteristic hallmarks of the disease are extracellular senile plaques (SPs) and intracellular neurofibrillary tangles (NFTs) with neuropil threads, which are a direct result of amyloid precursor protein (APP) processing to Aβ, and τ hyperphosphorylation. However, many indirect underlying processes play a role in this event. One of these underlying mechanisms leading to these histological hallmarks is the uncontrolled hyperactivation of a family of cysteine proteases called calpains. Under normal physiological condition calpains participate in many processes of cells’ life and their activation is tightly controlled. However, with an increase in age, increased oxidative stress and other excitotoxicity assaults, this regulatory system becomes impaired and result in increased activation of these proteases involving them in the pathogenesis of various diseases including neurodegeneration like AD. Reviewed here is a pool of data on the implication of calpains in the pathogenesis of AD, the underlying molecular mechanism, and the potential of targeting these enzymes for AD therapeutics.

Stimulation-induced changes in diffusion and structure of calmodulin and calmodulin-dependent protein kinase II proteins in neurons

Calcium/calmodulin-dependent protein kinase II (CaMKII) and calmodulin (CaM) play essential roles in synaptic plasticity, which is an elementary process of learning and memory. In this study, fluorescence correlation spectroscopy (FCS) revealed diffusion properties of CaM, CaMKIIα and CaMKIIβ proteins in human embryonic kidney 293 (HEK293) cells and hippocampal neurons. A simultaneous multiple-point FCS recording system was developed on a random-access two-photon microscope, which facilitated efficient analysis of molecular dynamics in neuronal compartments. The diffusion of CaM in neurons was slower than that in HEK293 cells at rest, while the diffusion in stimulated neurons was accelerated and indistinguishable from that in HEK293 cells. This implied that activity-dependent binding partners of CaM exist in neurons, which slow down the diffusion at rest. Diffusion properties of CaMKIIα and β proteins implied that major populations of these proteins exist as holoenzymatic forms. Upon stimulation of neurons, the diffusion of CaMKIIα and β proteins became faster with reduced particle brightness, indicating drastic structural changes of the proteins such as dismissal from holoenzyme structure and further fragmentation.

Calcium influx-mediated translocation of m-calpain induces Ku80 cleavage and enhances the Ku80-related DNA repair pathway

Proteomic analysis of ionomycin-treated and untreated mammary epithelial MCF10A cells elucidated differences in Ku80 cleavage. Ku80, a subunit of the Ku protein complex, is an initiator of the non-homologous, end-joining (NHEJ), double-strand breaks (DSBs) repair pathway. The nuclear Ku80 was cleaved in a calcium concentration-dependent manner by m-calpain but not by m-calpain. The cleavage of nuclear Ku80 at its α/β domain was validated by Western blotting analysis using flag-tagged expression vectors of truncated versions of Ku80 and a flag antibody and was confirmed in m-calpain knock-down cells and in vitro cell-free evaluation with recombinant proteins of calpains, Ku70, and Ku80. In addition, the cleaved Ku80 still formed a Ku heterodimer and promoted DNA DSB repair activity. Taken together, these findings indicate that translocated m-calpain enhances the NHEJ pathway through the cleavage of Ku80. Based on the present study, m-calpain in DNA repair pathways might be a novel anticancer drug target, or its mechanism might be a possible route for resistance acquisition of DNA damage-inducing chemotherapeutics.

ER Stress-Mediated Signaling: Action Potential and Ca(2+) as Key Players

The proper functioning of the endoplasmic reticulum (ER) is crucial for multiple cellular activities and survival. Disturbances in the normal ER functions lead to the accumulation and aggregation of unfolded proteins, which initiates an adaptive response, the unfolded protein response (UPR), in order to regain normal ER functions. Failure to activate the adaptive response initiates the process of programmed cell death or apoptosis. Apoptosis plays an important role in cell elimination, which is essential for embryogenesis, development, and tissue homeostasis. Impaired apoptosis can lead to the development of various pathological conditions, such as neurodegenerative and autoimmune diseases, cancer, or acquired immune deficiency syndrome (AIDS). Calcium (Ca(2+)) is one of the key regulators of cell survival and it can induce ER stress-mediated apoptosis in response to various conditions. Ca(2+) regulates cell death both at the early and late stages of apoptosis. Severe Ca(2+) dysregulation can promote cell death through apoptosis. Action potential, an electrical signal transmitted along the neurons and muscle fibers, is important for conveying information to, from, and within the brain. Upon the initiation of the action potential, increased levels of cytosolic Ca(2+) (depolarization) lead to the activation of the ER stress response involved in the initiation of apoptosis. In this review, we discuss the involvement of Ca(2+) and action potential in ER stress-mediated apoptosis.

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.

ROS-Induced Nuclear Translocation of Calpain-2 Facilitates Cardiomyocyte Apoptosis in Tail-Suspended Rats

Isoproterenol (ISO) induced nuclear translocation of calpain-2 which further increased susceptibility of cardiomyocyte apoptosis in tail-suspended rats. The underlying mechanisms remain elusive. In the present study, the results showed that ISO (10 nM) significantly elevated NADPH oxidases (NOXs) activity and NOXs-derived ROS productions which induced nuclear translocation of calpain-2 in cardiomyocytes of tail-suspended rats. In contrast, the inhibition of NADPH oxidase or cleavage of ROS not only reduced ROS productions, but also resisted nuclear translocation of calpain-2 and decreased ISO-induced apoptosis of cardiomyocyte in tail-suspended rats. ISO also increased the constitutive binding between calpain-2 and Ca(2+)/calmodulin-dependent protein kinase II δB (CaMK II δB) in nuclei, concomitant with the promotion of CaMK II δB degradation and subsequent down-regulation of Bcl-2 mRNA expression and the ratio of Bcl-2 to Bax protein in tail-suspended rat cardiomyocytes. These effects of ISO on cardiomyocytes were abolished by a calpain inhibitor PD150606. Inhibition of calpain significantly reduced ISO-induced loss of the mitochondrial membrane potential, cytochrome c release into the cytoplasm, as well as the activation of caspase-3 and caspase-9 in mitochondrial apoptotic pathway. In summary, the above results suggest that ISO increased NOXs-derived ROS which activated nuclear translocation of calpain-2, subsequently nuclear calpain-2 degraded CaMK II δB which reduced the ratio of Bcl-2 to Bax, and finally the mitochondria apoptosis pathway was triggered in tail-suspended rat cardiomyocytes. Therefore, calpain-2 may represent a potentially therapeutic target for prevention of oxidative stress-associated cardiomyocyte apoptosis.

A beacon of hope in stroke therapy-Blockade of pathologically activated cellular events in excitotoxic neuronal death as potential neuroprotective strategies

Excitotoxicity, a pathological process caused by over-stimulation of ionotropic glutamate receptors, is a major cause of neuronal loss in acute and chronic neurological conditions such as ischaemic stroke, Alzheimer's and Huntington's diseases. Effective neuroprotective drugs to reduce excitotoxic neuronal loss in patients suffering from these neurological conditions are urgently needed. One avenue to achieve this goal is to clearly define the intracellular events mediating the neurotoxic signals originating from the over-stimulated glutamate receptors in neurons. In this review, we first focus on the key cellular events directing neuronal death but not involved in normal physiological processes in the neurotoxic signalling pathways. These events, referred to as pathologically activated events, are potential targets for the development of neuroprotectant therapeutics. Inhibitors blocking some of the known pathologically activated cellular events have been proven to be effective in reducing stroke-induced brain damage in animal models. Notable examples are inhibitors suppressing the ion channel activity of neurotoxic glutamate receptors and those disrupting interactions of specific cellular proteins occurring only in neurons undergoing excitotoxic cell death. Among them, Tat-NR2B9c and memantine are clinically effective in reducing brain damage caused by some acute and chronic neurological conditions. Our second focus is evaluation of the suitability of the other inhibitors for use as neuroprotective therapeutics. We also discuss the experimental approaches suitable for bridging our knowledge gap in our current understanding of the excitotoxic signalling mechanism in neurons and discovery of new pathologically activated cellular events as potential targets for neuroprotection.

Regulation of CaMKII signaling in cardiovascular disease

Heart failure (HF) is a major cause of death in the developed countries (Murray and Lopez, 1996; Koitabashi and Kass, 2012). Adverse cardiac remodeling that precedes heart muscle dysfunction is characterized by a myriad of molecular changes affecting the cardiomyocyte. Among these, alterations in protein kinase pathways play often an important mediator role since they link upstream pathologic stress signaling with downstream regulatory programs and thus affect both the structural and functional integrity of the heart muscle. In the context of cardiac disease, a profound understanding for the overriding mechanisms that regulate protein kinase activity (protein-protein interactions, post-translational modifications, or targeting via anchoring proteins) is crucial for the development of specific and effective pharmacological treatment strategies targeting the failing myocardium. In this review, we focus on several mechanisms of upstream regulation of Ca²⁺-calmodulin-dependent protein kinase II that play a relevant pathophysiological role in the development and progression of cardiovascular disease; precise targeting of these mechanisms might therefore represent novel and promising tools for prevention and treatment of HF.

Cytosolic HMGB1 controls the cellular autophagy/apoptosis checkpoint during inflammation

The intracellular protein HMGB1 is released from cells and acts as a damage-associated molecular pattern molecule during many diseases, including inflammatory bowel disease (IBD); however, the intracellular function of HMGB1 during inflammation is poorly understood. Here, we demonstrated that cytosolic HMGB1 regulates apoptosis by protecting the autophagy proteins beclin 1 and ATG5 from calpain-mediated cleavage during inflammation. Colitis in mice with an intestinal epithelial cell-specific Hmgb1 deletion and patients with IBD were both characterized by increased calpain activation, beclin 1 and ATG5 cleavage, and intestinal epithelial cell (IEC) death compared with controls. In vitro cleavage assays and studies of enteroids verified that HMGB1 protects beclin 1 and ATG5 from calpain-mediated cleavage events that generate proapoptotic protein fragments. Together, our results indicate that HMGB1 is essential for mitigating the extent and severity of inflammation-associated cellular injury by controlling the switch between the proautophagic and proapoptotic functions of beclin 1 and ATG5 during inflammation. Moreover, these studies demonstrate that HMGB1 is pivotal for reducing tissue injury in IBD and other complex inflammatory disorders.

Role of calcium and calpain in the downregulation of voltage-gated sodium channel expression by the pyrethroid pesticide deltamethrin

Voltage-gated sodium channels (Na(v)) are essential for initiation and propagation of action potentials. Previous in vitro studies reported that exposure to the Na(v) toxins veratridine and α scorpion toxin cause persistent downregulation of Na(v) mRNA in vitro. However the mechanism of this downregulation is not well characterized. Here, we report that the type-II pyrethroid deltamethrin, which has a similar mechanism as these toxins, elicited an approximate 25% reduction in Na(v) 1.2 and Na(v) 1.3 mRNA in SK-N-AS cells. Deltamethrin-induced decreases of Na(v) mRNA were blocked with the Na(v) antagonist tetrodotoxin, demonstrating a primary role for interaction with Na(v). Pre-treatment with the intracellular calcium chelator BAPTA-AM and the calpain inhibitor PD-150606 also prevented these decreases, identifying a role for intracellular calcium and calpain activation. Because alterations in Na(v) expression and function can result in neurotoxicity, additional studies are warranted to determine whether or not such effects occur in vivo.

Calpain activation and CaMKIV reduction in spinal cords from hSOD1G93A mouse model

Amyotrophic Lateral Sclerosis (ALS), a severe neurodegenerative disease, affects the upper and lower motor neurons in the brain and spinal cord. In some studies, ALS disease progression has been associated with an increase in calcium-dependent degeneration processes. Motoneurons are specifically vulnerable to sustained membrane depolarization and excessive elevation of intracellular calcium concentration. The present study analyzed intracellular events in embryonic motoneurons and adult spinal cords of the hSOD1G93A ALS mouse model. We observed activation of calpain, a calcium-dependent cysteine protease that degrades a variety of substrates, and a reduction in calcium-calmodulin dependent protein kinase type IV (CaMKIV) levels in protein extracts from spinal cords obtained at several time-points of hSOD1G93A mice disease progression. However, in cultured embryonic motoneurons these differences between controls and hSOD1G93A mutants are not evident. Our results support the hypothesis that age-dependent changes in calcium homeostasis and resulting events, e.g., calpain activation and CaMKIV processing, are involved in ALS pathogenesis.

Differential functions of calpain 1 during epithelial cell death and adipocyte differentiation in mammary gland involution

Calpains become activated in the mammary gland early during weaning, cleaving several proteins located mainly in the cell membrane, but also in other organelles such as lysosomes, mitochondria and nuclei. By immunofluorescence and Western blot analysis, we have demonstrated the nuclear translocation of calpain-1 and calpain-2, together with the cleavage of several cytoplasmic nucleoporins in epithelial cells of the lobulo-alveolar compartment. In vivo and in vitro calpain inhibition prevented this nucleoporin degradation. In addition, calpain-1 was also present in the nucleus of non-epithelial mammary tissue cells, concomitant with adipocyte re-differentiation. Calpain-1 was internalized within nuclei and found to be present in the nuclear chromatin-enriched fraction, associated with histone H3. Furthermore, we have demonstrated, both in vivo and in vitro, the cleavage of the N-terminal residue of histone H3 by calpain-1. Calpain-1 co-localized with both H3K4me3 (histone H3 trimethylated at Lys4) and H3K27me3 (histone H3 trimethylated at Lys27) at the nuclear periphery, a bivalent epigenetic signal essential for cell differentiation. Using ChIP assays we could confirm the presence of calpain-1 in the promoters of key genes expressed in adipose tissue, such as Cebpa (CCAAT/enhancer-binding protein α) and Lep (leptin). The results of the present study highlight a dual role for calpain-1 in the weaned gland after the pregnancy/lactation cycle, controlling programmed cell death and participating in the epigenetic programme during adipocyte differentiation.

Role of calmodulin-calmodulin kinase II, cAMP/protein kinase A and ERK 1/2 on Aeromonas hydrophila-induced apoptosis of head kidney macrophages

The role of calcium (Ca²⁺) and its dependent protease calpain in Aeromonas hydrophila-induced head kidney macrophage (HKM) apoptosis has been reported. Here, we report the pro-apoptotic involvement of calmodulin (CaM) and calmodulin kinase II gamma (CaMKIIg) in the process. We observed significant increase in CaM levels in A. hydrophila-infected HKM and the inhibitory role of BAPTA/AM, EGTA, nifedipine and verapamil suggested CaM elevation to be Ca²⁺-dependent. Our studies with CaM-specific siRNA and the CaM inhibitor calmidazolium chloride demonstrated CaM to be pro-apoptotic that initiated the downstream expression of CaMKIIg. Using the CaMKIIg-targeted siRNA, specific inhibitor KN-93 and its inactive structural analogue KN-92 we report CaM-CaMKIIg signalling to be critical for apoptosis of A. hydrophila-infected HKM. Inhibitor studies further suggested the role of calpain-2 in CaMKIIg expression. CaMK Kinase (CaMKK), the other CaM dependent kinase exhibited no role in A. hydrophila-induced HKM apoptosis. We report increased production of intracellular cAMP in infected HKM and our results with KN-93 or KN-92 implicate the role of CaMKIIg in cAMP production. Using siRNA to PKACA, the catalytic subunit of PKA, anti-PKACA antibody and H-89, the specific inhibitor for PKA we prove the pro-apoptotic involvement of cAMP/PKA pathway in the pathogenicity of A. hydrophila. Our inhibitor studies coupled with siRNA approach further implicated the role of cAMP/PKA in activation of extracellular signal-regulated kinase 1 and 2 (ERK 1/2). We conclude that the alteration in intracellular Ca²⁺ levels initiated by A. hydrophila activates CaM and calpain-2; both pathways converge on CaMKIIg which in turn induces cAMP/PKA mediated ERK 1/2 phosphorylation leading to caspase-3 mediated apoptosis of infected HKM.

Aeromonas hydrophila is a natural fish pathogen and is known to induce apoptosis of HKM. Head kidney is an important immune-organ in fish and HKM are critical for immunity against the invading pathogen. The mechanisms of cell death induced by A. hydrophila are incompletely characterized. We have studied the role of Ca²⁺-dependent signalling pathways in the induction of A. hydrophila-induced HKM apoptosis. We observed that A. hydrophila infection led to increased CaM expression in infected HKM which was Ca²⁺-dependent. The inhibitor and siRNA studies suggested CaM to be pro-apoptotic and triggered CaMKIIg expression in the infected HKM. Calpain-2 appeared to influence CaMKIIg expression. However, further studies are needed to understand the process. We report that the CaM-CaMKIIg pathway is important for initiating cAMP production within the infected HKM. The pro-apoptotic activation of cAMP dependent PKA was quite evident. The activation of ERK 1/2 was observed in the HKM and results clearly suggested the pro-active role of cAMP/PKA in the process. Thus we conclude that CaM-CaMKIIg initiates the cAMP/PKA pathway that induces ERK 1/2 phosphorylation to promote caspase-3 mediated apoptosis of the A. hydrophila-infected HKM.

Inhibition of the AIF/CypA complex protects against intrinsic death pathways induced by oxidative stress

Delayed neuronal cell death largely contributes to the progressive infarct development and associated functional impairments after cerebral ischemia or brain trauma. Previous studies exposed a key role for the interaction of the mitochondrial protein apoptosis-inducing factor (AIF) and cytosolic cyclophilin A (CypA) in pathways of programmed cell death in neurons in vitro and in vivo. These studies suggested that pro-apoptotic activities of AIF, such as its translocation to the nucleus and subsequent DNA degradation, depend on the physical interaction of AIF with CypA. Hence, this protein complex may represent a new pharmacological target for inhibiting the lethal action of AIF on the brain tissue. In this study, we show that the AIF amino-acid residues 370–394 mediate the protein complex formation of AIF with CypA. The synthetic AIF(370–394) peptide inhibited AIF/CypA complex formation in vitro by binding CypA with a K_D of 12 μM. Further, the peptide exerted pronounced neuroprotective effects in a model of glutamate-induced oxidative stress in cultured HT-22 cells. In this model system of AIF-dependent cell death, the AIF(370–394) peptide preserved mitochondrial integrity, as detected by measurements of the mitochondrial membrane potential and quantification of mitochondrial fragmentation. Further, the AIF(370–394) peptide inhibited perinuclear accumulation of fragmented mitochondria, mitochondrial release of AIF to the nucleus and glutamate-induced cell death to a similar extent as CypA-siRNA. These data indicate that the targeting of the AIF-CypA axis is an effective strategy of neuroprotection.

Alterations of Ca²⁺-responsive proteins within cholinergic neurons in aging and Alzheimer's disease

The molecular basis of selective neuronal vulnerability in Alzheimer's disease (AD) remains poorly understood. Using basal forebrain cholinergic neurons (BFCNs) as a model and immunohistochemistry, we have demonstrated significant age-related loss of the calcium-binding protein calbindin-D(28K) (CB) from BFCN, which was associated with tangle formation and degeneration in AD. Here, we determined alterations in RNA and protein for CB and the Ca(2+)-responsive proteins Ca(2+)/calmodulin-dependent protein kinase I (CaMKI), growth-associated protein-43 (GAP43), and calpain in the BF. We observed progressive downregulation of CB and CaMKI RNA in laser-captured BFCN in the normal-aged-AD continuum. We also detected progressive loss of CB, CaMKIδ, and GAP43 proteins in BF homogenates in aging and AD. Activated μ-calpain, a calcium-sensitive protease that degrades CaMKI and GAP43, was significantly increased in the normal aged BF and was 10 times higher in AD BF. Overactivation of μ-calpain was confirmed using proteolytic fragments of its substrate spectrin. Substantial age- and AD-related alterations in Ca(2+)-sensing proteins most likely contribute to selective vulnerability of BFCN to degeneration in AD.

Temporal analysis of the magnaporthe oryzae proteome during conidial germination and cyclic AMP (cAMP)-mediated appressorium formation

Rice blast disease caused by Magnaporthe oryzae is one of the most serious threats to global rice production. During the earliest stages of rice infection, M. oryzae conidia germinate on the leaf surface and form a specialized infection structure termed the appressorium. The development of the appressorium represents the first critical stage of infectious development. A total of 3200 unique proteins were identified by nanoLC-MS/MS in a temporal study of conidial germination and cAMP-induced appressorium formation in M. oryzae. Using spectral counting based label free quantification, observed changes in relative protein abundance during the developmental process revealed changes in the cell wall biosynthetic machinery, transport functions, and production of extracellular proteins in developing appressoria. One hundred and sixty-six up-regulated and 208 down-regulated proteins were identified in response to cAMP treatment. Proteomic analysis of a cAMP-dependent protein kinase A mutant that is compromised in the ability to form appressoria identified proteins whose developmental regulation is dependent on cAMP signaling. Selected reaction monitoring was used for absolute quantification of four regulated proteins to validate the global proteomics data and confirmed the germination or appressorium specific regulation of these proteins. Finally, a comparison of the proteome and transcriptome was performed and revealed little correlation between transcript and protein regulation. A subset of regulated proteins were identified whose transcripts show similar regulation patterns and include many of the most strongly regulated proteins indicating a central role in appressorium formation. A temporal quantitative RT-PCR analysis confirmed a strong correlation between transcript and protein abundance for some but not all genes. Collectively, the data presented here provide the first comprehensive view of the M. oryzae proteome during early infection-related development and highlight biological processes important for pathogenicity.

Neuroprotection by GDNF in the ischemic brain

The glial cell line-derived neurotrophic factor (GDNF) was first identified as a survival factor for midbrain dopaminergic neurons, but additional studies provided evidences for a role as a trophic factor for other neurons of the central and peripheral nervous systems. GDNF regulates cellular activity through interaction with glycosyl-phosphatidylinositol-anchored cell surface receptors, GDNF family receptor-α1, which might signal through the transmembrane Ret tyrosine receptors or the neural cell adhesion molecule, to promote cell survival, neurite outgrowth, and synaptogenesis. The neuroprotective effect of exogenous GDNF has been shown in different experimental models of focal and global brain ischemia, by local administration of the trophic factor, using viral vectors carrying the GDNF gene and by transplantation of GDNF-expressing cells. These different strategies and the mechanisms contributing to neuroprotection by GDNF are discussed in this review. Importantly, neuroprotection by GDNF was observed even when administered after the ischemic injury.

Contribution of calpains to myocardial ischaemia/reperfusion injury

Loss of calcium (Ca(2+)) homeostasis contributes through different mechanisms to cell death occurring during the first minutes of reperfusion. One of them is an unregulated activation of a variety of Ca(2+)-dependent enzymes, including the non-lysosomal cysteine proteases known as calpains. This review analyses the involvement of the calpain family in reperfusion-induced cardiomyocyte death. Calpains remain inactive before reperfusion due to the acidic pHi and increased ionic strength in the ischaemic myocardium. However, inappropriate calpain activation occurs during myocardial reperfusion, and subsequent proteolysis of a wide variety of proteins contributes to the development of contractile dysfunction and necrotic cell death by different mechanisms, including increased membrane fragility, further impairment of Na(+) and Ca(2+) handling, and mitochondrial dysfunction. Recent studies demonstrating that calpain inhibition contributes to the cardioprotective effects of preconditioning and postconditioning, and the beneficial effects obtained with new and more selective calpain inhibitors added at the onset of reperfusion, point to the potential cardioprotective value of therapeutic strategies designed to prevent calpain activation.

Potential contribution of oxidative stress and inflammation to anxiety and hypertension

Previously, we have published that pharmacological induction of oxidative stress causes anxiety-like behavior in rats and also is associated with hypertension in these animals. Here, we report that sub-chronic induction of oxidative stress via pharmacological induction leads to i) reduction in glyoxalase (GLO)-1 and glutathione reductase (GSR)-1 expression; ii) calpain mediated reduction of brain derived neurotrophic factor (BDNF) levels; iii) NFκB mediated upregulation of proinflammatory factors interleukin (IL)-6 and tumor necrosis factor (TNF)-α and elevated angiotensin (AT)-1 receptor levels in hippocampus, amygdala and locus coeruleus regions of the brain. Acute oxidative stress has opposite effects. We speculate that regulation of GLO1, GSR1, BDNF, NFκB and AT-1 receptor may contribute to anxiety-like behavior and hypertension in rats.

Calcium-induced cleavage of DNA topoisomerase I involves the cytoplasmic-nuclear shuttling of calpain 2

Important to the function of calpains is temporal and spatial regulation of their proteolytic activity. Here, we demonstrate that cytoplasm-resident calpain 2 cleaves human nuclear topoisomerase I (hTOP1) via Ca(2+)-activated proteolysis and nucleoplasmic shuttling of proteases. This proteolysis of hTOP1 was induced by either ionomycin-caused Ca(2+) influx or addition of Ca(2+) in cellular extracts. Ca(2+) failed to induce hTOP1 proteolysis in calpain 2-knockdown cells. Moreover, calpain 2 cleaved hTOP1 in vitro. Furthermore, calpain 2 entered the nucleus upon Ca(2+) influx, and calpastatin interfered with this process. Calpain 2 cleavage sites were mapped at K(158) and K(183) of hTOP1. Calpain 2-truncated hTOP1 exhibited greater relaxation activity but remained able to interact with nucleolin and to form cleavable complexes. Interestingly, calpain 2 appears to be involved in ionomycin-induced protection from camptothecin-induced cytotoxicity. Thus, our data suggest that nucleocytoplasmic shuttling may serve as a novel type of regulation for calpain 2-mediated nuclear proteolysis.

Calcium/calmodulin-dependent protein kinase IV involvement in the pathophysiology of glucotoxicity in rat pancreatic β-cells

Glucotoxicity is a critical component of the pathophysiology of type 2 diabetes mellitus; however, the molecular mechanisms of glucotoxicity are still not fully understood. We have attempted to determine the protein kinases involved in glucotoxicity in pancreatic β-cells by the use of a new technique. Using Multi-PK antibodies, which are capable of detecting a wide variety of protein kinases, we analyzed the protein kinase that correlated with insulin synthesis in INS-1 cells under glucotoxic conditions. When expression patterns of protein kinases in INS-1 cells were analyzed by Western blotting with Multi-PK antibodies, a kinase of 63 kd was significantly reduced concomitant with the decrease of insulin secretion under glucotoxic conditions. To identify the 63-kd kinase, we used a unique 2-dimensional gel electrophoretic technique and MicroRotofor (Bio-Rad Laboratories, Tokyo, Japan) electrophoresis. From the molecular size of a native kinase/cyanogen bromide fragment and pI value, the 63-kd protein kinase was deduced to be CaMKIV. This was confirmed by Western blotting analysis using anti-CaMKIV antibodies. The decreased CaMKIV levels under glucotoxic conditions recovered to original levels after changing the medium to a normal glucose concentration. Recombinant CaMKIV was degraded in a Ca²+-dependent manner by incubation with cell lysates from INS-1 cells under glucotoxic conditions, and degradation was protected by calpain inhibitor. Furthermore, CaMKIV was reduced in the pancreatic islets of diabetic Otsuka Long-Evans Tokushima fatty rats, whereas that of nondiabetic Long-Evans Tokushima Otsuka rats was not. This study suggests that the abnormal regulation of CaMKIV is a component of β-cell dysfunction caused by high glucose.

Molecular determinants of survival motor neuron (SMN) protein cleavage by the calcium-activated protease, calpain

Spinal muscular atrophy (SMA) is a leading genetic cause of childhood mortality, caused by reduced levels of survival motor neuron (SMN) protein. SMN functions as part of a large complex in the biogenesis of small nuclear ribonucleoproteins (snRNPs). It is not clear if defects in snRNP biogenesis cause SMA or if loss of some tissue-specific function causes disease. We recently demonstrated that the SMN complex localizes to the Z-discs of skeletal and cardiac muscle sarcomeres, and that SMN is a proteolytic target of calpain. Calpains are implicated in muscle and neurodegenerative disorders, although their relationship to SMA is unclear. Using mass spectrometry, we identified two adjacent calpain cleavage sites in SMN, S192 and F193. Deletion of small motifs in the region surrounding these sites inhibited cleavage. Patient-derived SMA mutations within SMN reduced calpain cleavage. SMN(D44V), reported to impair Gemin2 binding and amino-terminal SMN association, drastically inhibited cleavage, suggesting a role for these interactions in regulating calpain cleavage. Deletion of A188, a residue mutated in SMA type I (A188S), abrogated calpain cleavage, highlighting the importance of this region. Conversely, SMA mutations that interfere with self-oligomerization of SMN, Y272C and SMNΔ7, had no effect on cleavage. Removal of the recently-identified SMN degron (Δ268-294) resulted in increased calpain sensitivity, suggesting that the C-terminus of SMN is important in dictating availability of the cleavage site. Investigation into the spatial determinants of SMN cleavage revealed that endogenous calpains can cleave cytosolic, but not nuclear, SMN. Collectively, the results provide insight into a novel aspect of the post-translation regulation of SMN.

Reduced blockade by extracellular Mg(2+) is permissive to NMDA receptor activation in cerebellar granule neurons that model a migratory phenotype

NMDA receptors (NMDAR) contribute to neuronal development throughout the CNS. However, their mode(s) of activation preceding synaptic maturation is unclear, as they are not co-localized with alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPARs) which normally provide sufficient depolarization to relieve voltage-dependent blockade by Mg(2+). We used cerebellar granule neurons (CGNs) cultured at a near-physiological KCl concentration to examine maturation-dependent changes in NMDAR responses. In contrast, most studies use KCl-supplemented medium to promote survival. At 2-4 days in vitro CGNs: (i) express developmental markers resembling the in vivo migratory phenotype; (ii) maintain a basal amount of calcium responsive element-binding protein phosphorylation that requires NMDARs and calcium/calmodulin-dependent kinases, but not AMPARs; (iii) exhibit NMDA-mediated Ca(2+) influx not effectively blocked by ambient Mg(2+) (0.75 mM) or AMPARs; (iv) maintain a more depolarized resting membrane potential and increased resistance compared to synaptically-connected CGNs. Moreover, migrating CGNs in explant cultures demonstrate NMDA-mediated Ca(2+) influx not effectively blocked by 0.75 mM Mg(2+), and NMDAR but not AMPAR antagonists slow migration. These data suggest the biophysical properties of immature CGNs render NMDARs less sensitive to Mg(2+) blockade, enhancing the likelihood of activation in the absence of AMPAR depolarization.

Calpain as a therapeutic target in traumatic brain injury

The family of calcium-activated neutral proteases, calpains, appears to play a key role in neuropathologic events following traumatic brain injury (TBI). Neuronal calpain activation has been observed within minutes to hours after either contusive or diffuse brain trauma in animals, suggesting that calpains are an early mediator of neuronal damage. Whereas transient calpain activation triggers numerous cell signaling and remodeling events involved in normal physiological processes, the sustained calpain activation produced by trauma is associated with neuron death and axonal degeneration in multiple models of TBI. Nonetheless, the causal relationship between calpain activation and neuronal death is not fully understood. Much remains to be learned regarding the endogenous regulatory mechanisms for controlling calpain activity, the roles of different calpain isoforms, and the in vivo substrates affected by calpain. Detection of stable proteolytic fragments of the submembrane cytoskeletal protein alphaII-spectrin specific for cleavage by calpains has been the most widely used marker of calpain activation in models of TBI. More recently, these protein fragments have been detected in the cerebrospinal fluid after TBI, driving interest in their potential utility as TBI-associated biomarkers. Post-traumatic inhibition of calpains, either direct or indirect through targets related to intracellular calcium regulation, is associated with attenuation of functional and behavioral deficits, axonal pathology, and cell death in animal models of TBI. This review focuses on the current state of knowledge of the role of calpains in TBI-induced neuropathology and effectiveness of calpain as a therapeutic target in the acute post-traumatic period.

Ca2+/calmodulin-dependent protein kinase IV links group I metabotropic glutamate receptors to fragile X mental retardation protein in cingulate cortex

Fragile X syndrome is caused by a lack of fragile X mental retardation protein (FMRP) due to silencing of the FMR1 gene. The metabotropic glutamate receptors (mGluRs) in the central nervous system contribute to higher brain functions including learning/memory, persistent pain, and mental disorders. Our recent study has shown that activation of Group I mGluR up-regulated FMRP in anterior cingulate cortex (ACC), a key region for brain cognitive and executive functions; Ca(2+) signaling pathways could be involved in the regulation of FMRP by Group I mGluRs. In this study we demonstrate that stimulating Group I mGluRs activates Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) in ACC neurons. In ACC neurons of adult mice overexpressing CaMKIV, the up-regulation of FMRP by stimulating Group I mGluR is enhanced. The enhancement occurs at the transcriptional level as the Fmr1 mRNA level was further elevated compared with wild-type mice. Using pharmacological approaches, we found that inhibition of CaMKIV could attenuate the up-regulation of FMRP by Group I mGluRs. CaMKIV contribute to the regulation of FMRP by Group I mGluRs probably through cyclic AMP-responsive element binding protein (CREB) activation, as manipulation of CaMKIV could simultaneously cause the change of CREB phosphorylation induced by Group I mGluR activation. Our study has provided strong evidence for CaMKIV as a molecular link between Group I mGluRs and FMRP in ACC neurons and may help us to elucidate the pathogenesis of fragile X syndrome.

Fatty acid acylation regulates trafficking of the unusual Plasmodium falciparum calpain to the nucleolus

The Plasmodium falciparum genome encodes a single calpain. By generating P. falciparum clones expressing C-terminally tagged calpain, we localized this protein to the nucleolus. Pf_calpain possesses an unusual and long N-terminal domain in which we identified three subregions that are highly conserved among Plasmodium species. Two have putative targeting signals: a myristoylation motif and a nuclear localization sequence. We assessed their functionality. Our data show that the nuclear localization sequence is an active nuclear import motif that contains an embedded signal conferring nucleolar localization on various chimeras. The N-terminus is myristoylated at Gly2 and palmitoylated at Cys3 and Cys22. Palmitoylation status has an important role in dictating P. falciparum calpain localization. The targeting signals function in mammalian cells as well as in the parasite. P. falciparum calpain is a unique nucleolar protein with an interesting mechanism of targeting.

Calpain-mediated signaling mechanisms in neuronal injury and neurodegeneration

Calpain is a ubiquitous calcium-sensitive protease that is essential for normal physiologic neuronal function. However, alterations in calcium homeostasis lead to persistent, pathologic activation of calpain in a number of neurodegenerative diseases. Pathologic activation of calpain results in the cleavage of a number of neuronal substrates that negatively affect neuronal structure and function, leading to inhibition of essential neuronal survival mechanisms. In this review, we examine the mechanistic underpinnings of calcium dysregulation resulting in calpain activation in the acute neurodegenerative diseases such as cerebral ischemia and in the chronic neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, prion-related encephalopathy, and amylotrophic lateral sclerosis. The premise of this paper is that analysis of the signaling and transcriptional consequences of calpain-mediated cleavage of its various substrates for any neurodegenerative disease can be extrapolated to all of the neurodegenerative diseases vulnerable to calcium dysregulation.

Apoptosis-suppressing and autophagy-promoting effects of calpain on oridonin-induced L929 cell death

Calpain, calcium-dependent cysteine protease, is reported here to impose the crucial influence on oridonin-induced L929 cell apoptosis and autophagy. We found that inhibition of calpain increased oridonin-induced Bax activation, cytochrome c release and PARP cleavage, indicating that calpain plays an anti-apoptotic role in oridonin-induced L929 cell apoptosis. To explore this potential anti-apoptotic mechanism, we inhibited calpain and proteasome activity in oridonin-induced L929 cell apoptosis, and discovered that the inducible IkappaBalpha proteolysis was partially blocked by the inhibition of either calpain or proteasome, but completely blocked by the inhibition of both. It demonstrated that calpain and proteasome were two distinct pathways participating in IkappaBalpha degradation. To further study the role of calpain in oridonin-induced L929 cell autophagy, we discovered that calpain inhibitor decreased oridonin-induced autophagy, as well as Beclin 1 activation and the conversion from LC3-I to LC3-II. Moreover, Inhibition of autophagy by 3-MA increased oridonin-induced apoptosis. In conclusion, besides suppressing apoptosis, calpain promotes autophagy in oridonin-induced L929 cell death, and inhibition of autophagy might contribute to up-regulation of apoptosis.

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.

The calpains: modular designs and functional diversity

The eukaryotic calpains are a family of calcium-dependent papain-like proteases and their non-enzymatic relatives whose varied physiological functions are beginning to be fully explored.

The calpain family is named for the calcium dependence of the papain-like, thiol protease activity of the well-studied ubiquitous vertebrate enzymes calpain-1 (μ-calpain) and calpain-2 (m-calpain). Proteins showing sequence relatedness to the catalytic core domains of these enzymes are included in this ancient and diverse eukaryotic protein family. Calpains are examples of highly modular organization, with several varieties of amino-terminal or carboxy-terminal modules flanking a conserved core. Acquisition of the penta-EF-hand module involved in calcium binding (and the formation of heterodimers for some calpains) seems to be a relatively late event in calpain evolution. Several alternative mechanisms for binding calcium and associating with membranes/phospholipids are found throughout the family. The gene family is expanded in mammals, trypanosomes and ciliates, with up to 26 members in Tetrahymena, for example; in striking contrast to this, only a single calpain gene is present in many other protozoa and in plants. The many isoforms of calpain and their multiple splice variants complicate the discussion and analysis of the family, and challenge researchers to ascertain the relationships between calpain gene sequences, protein isoforms and their distinct or overlapping functions. In mammals and plants it is clear that a calpain plays an essential role in development. There is increasing evidence that ubiquitous calpains participate in a variety of signal transduction pathways and function in important cellular processes of life and death. In contrast to relatively promiscuous degradative proteases, calpains cleave only a restricted set of protein substrates and use complex substrate-recognition mechanisms, involving primary and secondary structural features of target proteins. The detailed physiological significance of both proteolytically active calpains and those lacking key catalytic residues requires further study.

Physiological roles of the Ca2+/CaM-dependent protein kinase cascade in health and disease

Numerous hormones, growth factors and physiological processes cause a rise in cytosolic Ca2+, which is translated into meaningful cellular responses by interacting with a large number of Ca2(+)-binding proteins. The Ca2(+)-binding protein that is most pervasive in mediating these responses is calmodulin (CaM), which acts as a primary receptor for Ca2+ in all eukaryotic cells. In turn, Ca2+/CaM functions as an allosteric activator of a host of enzymatic proteins including a considerable number of protein kinases. The topic of this review is to discuss the physiological roles of a sub-set of these protein kinases which can function in cells as a Ca2+/CaM-dependent kinase signaling cascade. The cascade was originally believed to consist of a CaM kinase kinase that phosphorylates and activates one of two CaM kinases, CaMKI or CaMKIV. The unusual aspect of this cascade is that both the kinase kinase and the kinase require the binding of Ca2+/CaM for activation. More recently, one of the CaM kinase kinases has been found to activate another important enzyme, the AMP-dependent protein kinase so the concept of the CaM kinase cascade must be expanded. A CaM kinase cascade is important for many normal physiological processes that when misregulated can lead to a variety of disease states. These processes include: cell proliferation and apoptosis that may conspire in the genesis of cancer; neuronal growth and function related to brain development, synaptic plasticity as well as memory formation and maintenance; proper function of the immune system including the inflammatory response, activation of T lymphocytes and hematopoietic stem cell maintenance; and the central control of energy balance that, when altered, can lead to obesity and diabetes. Although the study of the CaM-dependent kinase cascades is still in its infancy continued analysis of the pathways regulated by these Ca2(+)-initiated signaling cascades holds considerable promise for the future of disease-related research.

Calmodulin-dependent kinase IV links Toll-like receptor 4 signaling with survival pathway of activated dendritic cells

Microbial products, including lipopolysaccharide (LPS), an agonist of Toll-like receptor 4 (TLR4), regulate the lifespan of dendritic cells (DCs) by largely undefined mechanisms. Here, we identify a role for calcium-calmodulin-dependent kinase IV (CaMKIV) in this survival program. The pharmacologic inhibition of CaMKs as well as ectopic expression of kinase-inactive CaMKIV decrease the viability of monocyte-derived DCs exposed to bacterial LPS. The defect in TLR4 signaling includes a failure to accumulate the phosphorylated form of the cAMP response element-binding protein (pCREB), Bcl-2, and Bcl-xL. CaMKIV null mice have a decreased number of DCs in lymphoid tissues and fail to accumulate mature DCs in spleen on in vivo exposure to LPS. Although isolated Camk4-/- DCs are able to acquire the phenotype typical of mature cells and release normal amounts of cytokines in response to LPS, they fail to accumulate pCREB, Bcl-2, and Bcl-xL and therefore do not survive. The transgenic expression of Bcl-2 in CaMKIV null mice results in full recovery of DC survival in response to LPS. These results reveal a novel link between TLR4 and a calcium-dependent signaling cascade comprising CaMKIV-CREB-Bcl-2 that is essential for DC survival.

Cerebral calpain in fatal falciparum malaria

Disruption of axonal transport may represent a final common pathway leading to neurological dysfunction in cerebral malaria (CM). Calpains are calcium (Ca2+)-activated cysteine proteases which have been implicated in axonal injury in neurological diseases of various aetiologies. In this study we examined the association between mu- and m-calpain, the specific inhibitor calpastatin, and axonal injury in post mortem brain tissue from patients who died from severe malaria. Calpains were associated with axons labelled for the beta-amyloid precursor protein that detects impaired axonal transport. Elevated levels of calpastatin were rarely observed in injured axons. There were increased numbers of neurones with mu-calpain in the nuclear compartment in severe malaria cases compared with non-neurological controls, and increased numbers of glia with nuclear mu-calpain in CM patients compared with non-CM malaria cases and non-neurological controls. There was marked redistribution of calpastatin in the sequestered Plasmodium falciparum-infected erythrocytes. Responses specific to malaria infection were ascertained following analysis of brain samples from fatal cases with acute axonal injury, HIV encephalitis, and progressive multifocal leucoencephalopathy. Our findings implicate a role for calpains in the modulation of disease progression in CM.

Calpain and synaptic function

Proteolysis by calpain is a unique posttranslational modification that can change integrity, localization, and activity of endogenous proteins. Two ubiquitous calpains, mu-calpain and m-calpain, are highly expressed in the central nervous system, and calpain substrates such as membrane receptors, postsynaptic density proteins, kinases, and phosphatases are localized to the synaptic compartments of neurons. By selective cleavage of synaptically localized molecules, calpains may play pivotal roles in the regulation of synaptic processes not only in physiological states but also during various pathological conditions. Activation of calpains during sustained synaptic activity is crucial for Ca2+-dependent neuronal functions, such as neurotransmitter release, synaptic plasticity, vesicular trafficking, and structural stabilization. Overactivation of calpain following dysregulation of Ca2+ homeostasis can lead to neuronal damage in response to events such as epilepsy, stroke, and brain trauma. Calpain may also provide a neuroprotective effect from axotomy and some forms of glutamate receptor overactivation. This article focuses on recent findings on the role of calpain-mediated proteolytic processes in potentially regulating synaptic substrates in physiological and pathophysiological events in the nervous system.

Transcriptional profiling of depolarization-dependent phenotypic alterations in primary cultures of developing granule neurons

Rat cerebellar granule neurons cultured in medium supplemented with elevated KCl are extensively used as a model to examine the coupling between neural activity and Ca(2+)-dependent gene expression. Elevated (25 mM) KCl is believed to mimic endogenous neural activity because it promotes depolarization and Ca(+2)-dependent survival and some aspects of maturation. By comparison, at least half of the granule neurons grown in standard medium containing 5 mM KCl undergo apoptosis beginning approximately 4 days in vitro. However, accumulating evidence suggests that chronic depolarization induces phenotypic abnormalities whereas growth in chemically defined medium containing 5 mM KCl more closely resembles the constitutive phenotype. To examine this, oligonucleotide microarrays and RT-PCR of selected mRNAs were used to compare transcription profiles of cultures grown in 5 mM and 25 mM KCl. In some cases, N-methyl-D-aspartate (NMDA) which, like elevated KCl, promotes long-term survival was also tested. Robust changes in several gene groups were observed and indicated that growth in elevated KCl: induces expression of mRNAs that are not normally observed; represses expression of mRNAs that should be present; maintains expression of mRNAs that are markers of immature neurons. Supplementation of the growth medium with NMDA instead of elevated KCl produces similar abnormalities. Altogether, these data indicate that growth in 5 mM KCl more closely mimics survival and maturation of granule neurons in vivo and should therefore be adopted in future studies.

Implication of calpain in neuronal apoptosis

Apoptotic neuronal cell death is the cardinal feature of aging and neurodegenerative diseases, but its mechanisms remain obscure. Caspases, members of the cysteine protease family, are known to be critical effectors in central nervous system cellular apoptosis. More recently, the calcium-dependent proteases, calpains, have been implicated in cellular apoptotic processes. Indeed, several members of the Bcl-2 family of cell death regulators, nuclear transcription factors (p53) and caspases themselves are processed by calpains. Progressive regional loss of neurons underlies the irreversible pathogenesis of various neurodegenerative diseases such as Alzheimer's disease in adult brain. Alzheimer's disease is characterized by extracellular plaques of amyloid-beta peptide aggregates and intracellular neurofibrillary tangles composed of hyperphosphorylated tau leading to apoptotic cell death. In this review, we summarize the arguments showing that calpains modulate processes that govern the function and metabolism of these two key proteins in the pathogenesis of Alzheimer's disease. To conclude, this article reviews our understanding of calpain-dependent apoptotic neuronal cell death and the ability of these proteases to regulate intracellular signaling pathways leading to chronic neurodegenerative disorders such as Alzheimer's disease. Further research on these calpain-dependent mechanisms which promote or prevent cell apoptosis should help us to develop new approaches for preventing and treating neurodegenerative disorders.

Calpain 10: a mitochondrial calpain and its role in calcium-induced mitochondrial dysfunction

Calpains, Ca(2+)-activated cysteine proteases, are cytosolic enzymes implicated in numerous cellular functions and pathologies. We identified a mitochondrial Ca(2+)-inducible protease that hydrolyzed a calpain substrate (SLLVY-AMC) and was inhibited by active site-directed calpain inhibitors as calpain 10, an atypical calpain lacking domain IV. Immunoblot analysis and activity assays revealed calpain 10 in the mitochondrial outer membrane, intermembrane space, inner membrane, and matrix fractions. Mitochondrial staining was observed when COOH-terminal green fluorescent protein-tagged calpain 10 was overexpressed in NIH-3T3 cells and the mitochondrial targeting sequence was localized to the NH(2)-terminal 15 amino acids. Overexpression of mitochondrial calpain 10 resulted in mitochondrial swelling and autophagy that was blocked by the mitochondrial permeability transition (MPT) inhibitor cyclosporine A. With the use of isolated mitochondria, Ca(2+)-induced MPT was partially decreased by calpain inhibitors. More importantly, Ca(2+)-induced inhibition of Complex I of the electron transport chain was blocked by calpain inhibitors and two Complex I proteins were identified as targets of mitochondrial calpain 10, NDUFV2, and ND6. In conclusion, calpain 10 is the first reported mitochondrially targeted calpain and is a mediator of mitochondrial dysfunction through the cleavage of Complex I subunits and activation of MPT.

Depolarization and Ca(2+) down regulate CB1 receptors and CB1-mediated signaling in cerebellar granule neurons

Presynaptic terminals of cerebellar granule neurons are primary targets of cannabinoids, which act through type 1 G alpha(i/o)-coupled cannabinoid receptors (CB1) to modulate glutamate release. To study CB1 signaling investigators use primary cultures of granule neurons, typically grown in medium supplemented with elevated KCl to improve long-term survival. Herein, we demonstrate that CB1 expression and signaling are perturbed under these conditions. Specifically, immunochemical and RT-PCR assays indicate that depolarizing growth conditions decrease CB1 protein, mRNA and CB1-mediated inhibition of adenylyl cyclase compared to cultures grown in physiologic medium containing 5mM KCl. Depolarization-dependent downregulation of CB1 mRNA, like survival, is attenuated by L-type VDCC antagonists but not the Na(+)-channel antagonist, tetrodotoxin. Comparison of oligonucleotide microarrays from cultures grown in 5mM versus 25 mM KCl confirms that depolarization reduces CB1 mRNA, but not mRNAs encoding several G-protein subunits or adenylyl cyclases. However, significant alterations in synaptic signaling proteins that likely lie downstream of CB1 are observed, including K(+) channels, alpha-neurexins, cAMP-GEFII, Munc13-3, secretogranin and synaptotagmin. These findings make a compelling argument to adopt cultures grown in 5mM KCl for future study of CB1 signaling in granule neurons. Further, they suggest that a depolarization and Ca(2+)-dependent signaling pathway represses CB1 gene transcription.

Regulating cell migration: calpains make the cut

The calpain family of proteases has been implicated in cellular processes such as apoptosis, proliferation and cell migration. Calpains are involved in several key aspects of migration, including: adhesion and spreading; detachment of the rear; integrin- and growth-factor-mediated signaling; and membrane protrusion. Our understanding of how calpains are activated and regulated during cell migration has increased as studies have identified roles for calcium and phospholipid binding, autolysis, phosphorylation and inhibition by calpastatin in the modulation of calpain activity. Knockout and knockdown approaches have also contributed significantly to our knowledge of calpain biology, particularly with respect to the specific functions of different calpain isoforms. The mechanisms by which calpain-mediated proteolysis of individual substrates contributes to cell motility have begun to be addressed, and these efforts have revealed roles for proteolysis of specific substrates in integrin activation, adhesion complex turnover and membrane protrusion dynamics. Understanding these mechanisms should provide avenues for novel therapeutic strategies to treat pathological processes such as tumor metastasis and chronic inflammatory disease.

Calpain cleavage regulates the protein stability of p73

The function of p73, a transcription factor belonging to the p53 family, is finely regulated by its steady-state protein stability. p73 protein degradation/stabilization can be regulated by mechanisms in part dependent on the ubiquitin proteasome system (UPS): (i) Itch/NEDD4-like UPS degradation, (ii) NEDD8 UPS degradation, and (iii) NQO1 20S proteasome-dependent (but ubiquitin-independent) breakdown. Here, we show that, in vitro, Calpain I can cleave p73 at two distinct sites: the first proline-rich region and within the oligomerization domain. Consequently, different p73 isoforms can be degraded by calpains, i.e., both N-terminal isoforms (TAp73 and DeltaNp73) as well as the C-terminal isoforms (alpha, beta, gamma, delta). Moreover, overexpression of the specific endogenous calpain inhibitor, calpastatin, in cultured cells increased the steady-state p73 level. This suggests that calpains may play a physiological role in the regulation of p73 protein stability.