Calpain-1 cleaves and activates caspase-7

The intersection between cysteine proteases, Ca2+ signalling and cancer cell apoptosis

Apoptosis is a highly complex and regulated cell death pathway that safeguards the physiological balance between life and death. Over the past decade, the role of Ca2+ signalling in apoptosis and the mechanisms involved have become clearer. The initiation and execution of apoptosis is coordinated by three distinct groups of cysteines proteases: the caspase, calpain and cathepsin families. Beyond its physiological importance, the ability to evade apoptosis is a prominent hallmark of cancer cells. In this review, we will explore the involvement of Ca2+ in the regulation of caspase, calpain and cathepsin activity, and how the actions of these cysteine proteases alter intracellular Ca2+ handling during apoptosis. We will also explore how apoptosis resistance can be achieved in cancer cells through deregulation of cysteine proteases and remodelling of the Ca2+ signalling toolkit.

Regulated Cell Death of Retinal Ganglion Cells in Glaucoma: Molecular Insights and Therapeutic Potentials

Glaucoma is a group of diseases characterized by the degeneration of retinal ganglion cells (RGCs) and progressive, irreversible vision loss. High intraocular pressure (IOP) heightens the likelihood of glaucoma and correlates with RGC loss. While the current glaucoma therapy prioritizes lower the IOP; however, RGC, and visual loss may persist even when the IOP is well-controlled. As such, discovering and creating IOP-independent neuroprotective strategies for safeguard RGCs is crucial for glaucoma management. Investigating and clarifying the mechanism behind RGC death to counteract its effects is a promising direction for glaucoma control. Empirical studies of glaucoma reveal the role of multiple regulated cell death (RCD) pathways in RGC death. This review delineates the RCD of RGCs following IOP elevation and optic nerve damage and discusses the substantial benefits of mitigating RCD in RGCs in preserving visual function.

Calpain: the regulatory point of myocardial ischemia-reperfusion injury

Calpain is a conserved cysteine protease readily expressed in several mammalian tissues, which is usually activated by Ca²⁺ and with maximum activity at neutral pH. The activity of calpain is tightly regulated because its aberrant activation will nonspecifically cleave various proteins in cells. Abnormally elevation of Ca²⁺ promotes the abnormal activation of calpain during myocardial ischemia-reperfusion, resulting in myocardial injury and cardiac dysfunction. In this paper, we mainly reviewed the effects of calpain in various programmed cell death (such as apoptosis, mitochondrial-mediated necrosis, autophagy-dependent cell death, and parthanatos) in myocardial ischemia-reperfusion. In addition, we also discussed the abnormal activation of calpain during myocardial ischemia-reperfusion, the effect of calpain on myocardial repair, and the possible future research directions of calpain.

Proteomic Analysis of Huntington's Disease Medium Spiny Neurons Identifies Alterations in Lipid Droplets

Huntington's disease (HD) is a neurodegenerative disease caused by a CAG repeat expansion in the Huntingtin (HTT) gene. The resulting polyglutamine (polyQ) tract alters the function of the HTT protein. Although HTT is expressed in different tissues, the medium spiny projection neurons (MSNs) in the striatum are particularly vulnerable in HD. Thus, we sought to define the proteome of human HD patient-derived MSNs. We differentiated HD72 induced pluripotent stem cells and isogenic controls into MSNs and carried out quantitative proteomic analysis. Using data-dependent acquisitions with FAIMS for label-free quantification on the Orbitrap Lumos mass spectrometer, we identified 6,323 proteins with at least two unique peptides. Of these, 901 proteins were altered significantly more in the HD72-MSNs than in isogenic controls. Functional enrichment analysis of upregulated proteins demonstrated extracellular matrix and DNA signaling (DNA replication pathway, double-strand break repair, G1/S transition) with the highest significance. Conversely, processes associated with the downregulated proteins included neurogenesis-axogenesis, the brain-derived neurotrophic factor-signaling pathway, Ephrin-A: EphA pathway, regulation of synaptic plasticity, triglyceride homeostasis cholesterol, plasmid lipoprotein particle immune response, interferon-γ signaling, immune system major histocompatibility complex, lipid metabolism and cellular response to stimulus. Moreover, proteins involved in the formation and maintenance of axons, dendrites, and synapses (e.g., Septin protein members) were dysregulated in HD72-MSNs. Importantly, lipid metabolism pathways were altered, and using quantitative image, we found analysis that lipid droplets accumulated in the HD72-MSN, suggesting a deficit in the turnover of lipids possibly through lipophagy. Our proteomics analysis of HD72-MSNs identified relevant pathways that are altered in MSNs and confirm current and new therapeutic targets for HD.

Changes in calpain-2 expression during glioblastoma progression predisposes tumor cells to temozolomide resistance by minimizing DNA damage and p53-dependent apoptosis

BACKGROUND

Glioblastoma multiforme (GBM) is characterized by an unfavorable prognosis for patients affected. During standard-of-care chemotherapy using temozolomide (TMZ), tumors acquire resistance thereby causing tumor recurrence. Thus, deciphering essential molecular pathways causing TMZ resistance are of high therapeutic relevance.

METHODS

Mass spectrometry based proteomics were used to study the GBM proteome. Immunohistochemistry staining of human GBM tissue for either calpain-1 or -2 was performed to locate expression of proteases. In vitro cell based assays were used to measure cell viability and survival of primary patient-derived GBM cells and established GBM cell lines after TMZ ± calpain inhibitor administration. shRNA expression knockdowns of either calpain-1 or calpain-2 were generated to study TMZ sensitivity of the specific subunits. The Comet assay and ɣH2AX signal measurements were performed in order to assess the DNA damage amount and recognition. Finally, quantitative real-time PCR of target proteins was applied to differentiate between transcriptional and post-translational regulation.

RESULTS

Calcium-dependent calpain proteases, in particular calpain-2, are more abundant in glioblastoma compared to normal brain and increased in patient-matched initial and recurrent glioblastomas. On the cellular level, pharmacological calpain inhibition increased the sensitivities of primary glioblastoma cells towards TMZ. A genetic knockdown of calpain-2 in U251 cells led to increased caspase-3 cleavage and sensitivity to neocarzinostatin, which rapidly induces DNA strand breakage. We hypothesize that calpain-2 causes desensitization of tumor cells against TMZ by preventing strong DNA damage and subsequent apoptosis via post-translational TP53 inhibition. Indeed, proteomic comparison of U251 control vs. U251 calpain-2 knockdown cells highlights perturbed levels of numerous proteins involved in DNA damage response and downstream pathways affecting TP53 and NF-κB signaling. TP53 showed increased protein abundance, but no transcriptional regulation.

CONCLUSION

TMZ-induced cell death in the presence of calpain-2 expression appears to favor DNA repair and promote cell survival. We conclude from our experiments that calpain-2 expression represents a proteomic mode that is associated with higher resistance via "priming" GBM cells to TMZ chemotherapy. Thus, calpain-2 could serve as a prognostic factor for GBM outcome.

Calpain-mediated proteolysis as driver and modulator of polyglutamine toxicity

Among posttranslational modifications, directed proteolytic processes have the strongest impact on protein integrity. They are executed by a variety of cellular machineries and lead to a wide range of molecular consequences. Compared to other forms of proteolytic enzymes, the class of calcium-activated calpains is considered as modulator proteases due to their limited proteolytic activity, which changes the structure and function of their target substrates. In the context of neurodegeneration and - in particular - polyglutamine disorders, proteolytic events have been linked to modulatory effects on the molecular pathogenesis by generating harmful breakdown products of disease proteins. These findings led to the formulation of the toxic fragment hypothesis, and calpains appeared to be one of the key players and auspicious therapeutic targets in Huntington disease and Machado Joseph disease. This review provides a current survey of the role of calpains in proteolytic processes found in polyglutamine disorders. Together with insights into general concepts behind toxic fragments and findings in polyglutamine disorders, this work aims to inspire researchers to broaden and deepen the knowledge in this field, which will help to evaluate calpain-mediated proteolysis as a unifying and therapeutically targetable posttranslational mechanism in neurodegeneration.

The role of calcium-calpain pathway in hyperthermia

Hyperthermia is a promising anticancer treatment modality. Heat stress stimulates proteolytic machineries to regulate cellular homeostasis. Calpain, an intracellular calcium (Ca2+)-dependent cysteine protease, is a modulator that governs various cellular functions. Hyperthermia induces an increase in cytosolic Ca2+ levels and triggers calpain activation. Contrastingly, pre-exposure of cells to mild hyperthermia induces thermotolerance due to the presence of cellular homeostatic processes such as heat shock response and autophagy. Recent studies suggest that calpain is a potential key molecule that links autophagy and apoptosis. In this review, we briefly introduce the regulation of intracellular Ca2+ homeostasis, basic features of calpains with their implications in cancer, immune responses, and the roles and cross-talk of calpains in cellular protection and cell death in hyperthermia.

Calpain as a therapeutic target in cancer

INTRODUCTION

Calpain-1 and calpain-2 are prototypical classical isoforms of the calpain family of calcium-activated cysteine proteases. Their substrate proteins participate in a wide range of cellular processes, including transcription, survival, proliferation, apoptosis, migration, and invasion. Dysregulated calpain activity has been implicated in tumorigenesis, suggesting that calpains may be promising therapeutic targets.

AREAS COVERED

This review covers clinical and basic research studies implicating calpain-1 and calpain-2 expression and activity in tumorigenesis and metastasis. We highlight isoform specific functions and provide an overview of substrates and cancer-related signalling pathways affected by calpain-mediated proteolytic cleavage. We also discuss efforts to develop clinically relevant calpain specific inhibitors and spotlight the challenges facing inhibitor development.

EXPERT OPINION

Rationale for targeting calpain-1 and calpain-2 in cancer is supported by pre-clinical and clinical studies demonstrating that calpain inhibition has the potential to attenuate carcinogenesis and block metastasis of aggressive tumors. The wide range of substrates and cleavage products, paired with inconsistencies in model systems, underscores the need for more complete understanding of physiological substrates and how calpain cleavage alters their function in cellular processes. The development of isoform specific calpain inhibitors remains an important goal with therapeutic potential in cancer and other diseases.

Effects of intracellular calcium accumulation on proteins encoded by the major genes underlying amyotrophic lateral sclerosis

The aetiology of Amyotrophic Lateral Sclerosis (ALS) is still poorly understood. The discovery of genetic forms of ALS pointed out the mechanisms underlying this pathology, but also showed how complex these mechanisms are. Excitotoxicity is strongly suspected to play a role in ALS pathogenesis. Excitotoxicity is defined as neuron damage due to excessive intake of calcium ions (Ca²⁺) by the cell. This study aims to find a relationship between the proteins coded by the most relevant genes associated with ALS and intracellular Ca²⁺ accumulation. In detail, the profile of eight proteins (TDP-43, C9orf72, p62/sequestosome-1, matrin-3, VCP, FUS, SOD1 and profilin-1), was analysed in three different cell types induced to raise their cytoplasmic amount of Ca²⁺. Intracellular Ca²⁺ accumulation causes a decrease in the levels of TDP-43, C9orf72, matrin3, VCP, FUS, SOD1 and profilin-1 and an increase in those of p62/sequestosome-1. These events are associated with the proteolytic action of two proteases, calpains and caspases, as well as with the activation of autophagy. Interestingly, Ca²⁺ appears to both favour and hinder autophagy. Understanding how and why calpain-mediated proteolysis and autophagy, which are physiological processes, become pathological may elucidate the mechanisms responsible for ALS and help discover new therapeutic targets.

The role of ATF6 in Cr(VI)-induced apoptosis in DF-1 cells

Hexavalent chromium (Cr(VI)) is a common heavy metal pollutant in environment and has been proved possessing the cytotoxicity. In this study, we aimed to investigate the role of activating transcription factor 6 (ATF-6) in apoptosis of chicken embryo fibroblasts cell line (DF-1) induced by Cr(VI). Firstly, DF-1 cells were exposed to Cr(VI) to establish the cytotoxicity model, then the cell apoptosis and ATF-6 protein level were analyzed. By silencing ATF-6 gene, changes of the apoptosis rate and apoptotic proteins were examined. To further explore the regulatory mechanism of ATF-6, endoplasmic reticulum (ER) stress, mitochondrial function, reactive oxygen species (ROS) level, as well as the related pathway were evaluated. Results showed that Cr(VI) can result in DF-1 cell apoptosis, along with mitochondrial membrane potential (MMP) reducing and ER stress. Meanwhile, ATF-6 silencing lowered the apoptosis rate and ER stress level, showing with the decrease of XBP-1, PERK, GRP78, Caspase-12, Cleaved Caspase-3 and the increase of Bcl-2. Further analysis found that ATF-6 silencing down-regulated ROS and caused MMP loss, suggesting that ATF-6 silencing inhibited Cr(VI)-induced mitochondrial damage. In conclusion, this study indicate that ATF-6 plays an important regulatory role in Cr(VI)-induced DF-1 cell apoptosis through the ER stress and mitochondrial pathway.

Calpain-calpastatin system and cancer progression

The calpain system is required by many important physiological processes, including the cell cycle, cytoskeleton remodelling, cellular proliferation, migration, cancer cell invasion, metastasis, survival, autophagy, apoptosis and signalling, as well as the pathogenesis of a wide range of disorders, in which it may function to promote tumorigenesis. Calpains are intracellular conserved calcium-activated neutral cysteine proteinases that are involved in mediating cancer progression via catalysing and regulating the proteolysis of their specific substrates, which are important signalling molecules during cancer progression. μ-calpain, m-calpain, and their specific inhibitor calpastatin are the three molecules originally identified as comprising the calpain system and they contain several crucial domains, specific motifs, and functional sites. A large amount of data supports the roles of the calpain-calpastatin system in cancer progression via regulation of cellular adhesion, proliferation, invasion, metastasis, and cellular survival and death, as well as inflammation and angiogenesis during tumorigenesis, implying that the inhibition of calpain activity may be a potential anti-cancer intervention strategy targeting cancer cell survival, invasion and chemotherapy resistance.

Identification and functional characterization of three caspases in Takifugu obscurus in response to bacterial infection

Caspases are evolutionarily conserved proteases, which are inextricably linked with the apoptosis and immune system in mammals. However, the expression pattern and function of some caspases remain largely unknown in pufferfish. In this study, three different pufferfish caspases (caspase-2 (Pfcasp-2), caspase-3 (Pfcasp-3), and caspase-8 (Pfcasp-8)) were characterized, and their expression patterns and functions were determined following Aeromonas hydrophila infection. The open reading frames of Pfcasp-2, -3, and -8 are 1,320, 846, and 1455 bp, respectively. Analyses of sequence alignment and phylogenetic tree showed that casp-2, -3, and -8 share 52%-65%, 33%-40%, 63%-78% overall sequence identities with those of other vertebrates, respectively. 3D structures of Pfcasp-2, -3, and -8 enjoy conservation in core area together, while each owns a distinctive profile. Comparisons of deduced amino acid sequences indicated that Pfcaspases possessed the caspase domain and conserved active sites like 'HG' and 'QACXG' (X for R or G). qRT-PCR results revealed that Pfcasp-2, -3, and -8 were expressed constitutively in a wide range of organs, especially in immune-related organs including whole blood and kidney. In vitro, the expressions of the three caspases (Pfcasp-2, 3, and -8) and immune-related genes (IgM and IL-8) were significantly up-regulated in kidney leukocytes after A. Hydrophila challenge and inhibitors treatment. The expressions of Pfcasp-2 and Pfcasp-3 were successfully inhibited in the kidney leukocytes by Ac-DEVD-CHO (an inhibitor to caspase-3), but the expression of Pfcasp-8 was not affected. Cellular localization analysis showed that the distribution of Pfcasp-2, -3, and -8 was in cytoplasm. Further, overexpression of Pfcasp-2, -3, or -8 was found to cause DNA damage and apoptosis, suggesting that three caspases may be related to apoptosis and mediate different apoptosis pathways in pufferfish. Moreover, the expressions of these caspases were also up-regulated in whole blood and kidney after A. hydrophila challenge, indicating their possible involvement in the immune response against A. hydrophia stimulation. Taken together, the results of this study suggest that the caspase-2,-3, and -8 may play an important role in the apoptosis and immune response in pufferfish.

A Novel Probe for Spliceosomal Proteins that Induces Autophagy and Death of Melanoma Cells Reveals New Targets for Melanoma Drug Discovery

Background/Aims:

Despite recent advances in melanoma drug discovery, the average overall survival of patients with late stage metastatic melanoma is approximately 3 years, suggesting a need for approaches that identify new melanoma targets. We have previously reported a discovery of novel anti-melanoma compound 2155–14 (Onwuha-Ekpete et al., J Med Chem. 2014 Feb 27; 57(4):1599–608). In the report presented herein we aim to identify its target(s) and mechanism of action.

Methods:

We utilized biotinylated analog of 2155–14 to pull down its targets from melanoma cells. Proteomics in combination with western blot were used to identify the targets. Mechanism of action of 2155–14 was determined using flow cytometry, RT-PCR, microscopy, western blot, and enzymatic activity assays. Where applicable, one-way analysis of variance (ANOVA) was used followed by Dunnett post hoc test.

Results:

In the present study, we identified ATP-dependent RNA helicase DDX1 and heterogeneous nuclear ribonucleoproteins (hnRNPs) H1, H2 and A2/B1 as targets of anti-melanoma compound 2155–14. To the best of our knowledge, this is a first report suggesting that these proteins could be targeted for melanoma therapy. Mechanistic investigations showed that 2155–14 induces ER stress leading to potentiation of basal autophagy resulting in melanoma cell death in BRAF and NRAS mutated melanoma cells.

Conclusion:

Identification of mode of action of 2155–14 may provide insight into novel therapies against a broad range of melanoma subtypes. These studies were enabled by the novel probe derived from a mixture-based library, an important class of chemical biology tools for discovering novel targets.

Calpain Activation Is the Major Cause of Cell Death in Photoreceptors Expressing a Rhodopsin Misfolding Mutation

The majority of mutations in rhodopsin (RHO) cause misfolding of the protein and has been linked to degeneration of photoreceptor cells in the retina. A lot of attention has been set on targeting ER stress for the development of new therapies for inherited retinal degeneration caused by mutations in the RHO gene. Nevertheless, the cell death pathway activated by RHO misfolded protein is still debated. In this study, we analyzed the retina of the knock-in mouse expressing the P23H misfolded mutant RHO. We found persistent unfolded protein response (UPR) during degeneration. Interestingly, long-term stimulation of the PERK branch of ER stress had a protective effect by phosphorylating nuclear factor erythroid 2-related factor 2 (NRF2) transcription factor, associated with antioxidant responses. Otherwise, we provide evidence that increased intracellular calcium and activation of calpains strongly correlated with rod photoreceptor cell death. By blocking calpain activity, we significantly decreased the activation of caspase-7 and apoptosis-inducing factor (AIF), two cell death effectors, and cell demise, and effectively protected the retina from degeneration caused by the P23H dominant mutation in RHO.

Expression and effect of Calpain9 gene genetic polymorphism on slaughter indicators and intramuscular fat content in chickens

Calpain 9 (CAPN9) is expressed in the stomach and small intestine. CAPN9 has regulatory roles in hypertension, heart disease, gastric mucosal defense, and kidney disease. The involvement of CAPN9 has not been reported in the development of chickens. CAPN9 mRNA was found in adipose and muscle tissue in this study. Two linkage single nucleotide polymorphisms (SNP; G7518A and C7542G) in intron 4 were screened from 160 birds of the D2 chicken line. The 2 mutation sites were associated with carcass weight, evisceration weight, abdominal fat weight (AFW), abdominal fat percentage (AFP), and breast muscle percentage (all P < 0.05). Intramuscular fat (IMF) content was not significantly different in the 3 genotypes. But, the AA(7518)/GG(7542) genotype had the highest IMF content, highest breast muscle weight, and lower AFW and AFP. Moreover, the mRNA level of CAPN9 in abdominal fat tissue was significantly different (P < 0.05 or P < 0.01) between any 2 genotypes, consistent with AFW and AFP. In summary, the expression of CAPN9 in adipose and breast muscle tissue is reported for the first time. CAPN9 affected production performance of chickens. As a marker, the linkage G7518A and C7542G polymorphisms in intron 4 of CAPN9 could affect the production traits by regulating mRNA expression. The findings concerning the marker enrich the theoretical foundation for molecular breeding of high-quality broilers.

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.

Hypercapnia increases airway smooth muscle contractility via caspase-7-mediated miR-133a-RhoA signaling

The elevation of carbon dioxide (CO2) in tissues and the bloodstream (hypercapnia) occurs in patients with severe lung diseases, including chronic obstructive pulmonary disease (COPD). Whereas hypercapnia has been recognized as a marker of COPD severity, a role for hypercapnia in disease pathogenesis remains unclear. We provide evidence that CO2 acts as a signaling molecule in mouse and human airway smooth muscle cells. High CO2 activated calcium-calpain signaling and consequent smooth muscle cell contraction in mouse airway smooth muscle cells. The signaling was mediated by caspase-7-induced down-regulation of the microRNA-133a (miR-133a) and consequent up-regulation of Ras homolog family member A and myosin light-chain phosphorylation. Exposure of wild-type, but not caspase-7-null, mice to hypercapnia increased airway contraction and resistance. Deletion of the Caspase-7 gene prevented hypercapnia-induced airway contractility, which was restored by lentiviral transfection of a miR-133a antagonist. In a cohort of patients with severe COPD, hypercapnic patients had higher airway resistance, which improved after correction of hypercapnia. Our data suggest a specific molecular mechanism by which the development of hypercapnia may drive COPD pathogenesis and progression.

BmCalpains are involved in autophagy and apoptosis during metamorphosis and after starvation in Bombyx mori

Apoptosis and autophagy play crucial roles during Bombyx mori metamorphosis and in response to various adverse conditions, including starvation. Recently, calpain, one of the major intracellular proteases, has been reported to be involved in apoptosis and autophagy in mammals. BmATG5 and BmATG6 have been identified to mediate apoptosis following autophagy induced by 20-hydroxyecdysone and starvation in B. mori. However, B. mori calpains and their functions remain unclear. In this study, phylogenetic analysis of calpains from B. mori, Drosophila melanogaster and Homo sapiens were performed and the results showed distinct close relationships of BmCalpain-A/B with DmCalpain-A/B, BmCalpain-C with DmCalpain-C, and BmCalpain-7 with HsCalpain-7. Then, the expression profiles of BmCalpains were analyzed by quantitative real-time polymerase chain reaction, and results showed that expression of BmCalpain-A/B, BmCalpain-C and BmCalpain-7 was significantly increased during B. mori metamorphosis and induced in the fat body and midgut of starved larvae, which is consistent with the expression profiles of BmAtg5, BmAtg6 and BmCaspase-1. Moreover, the apoptosis-associated cleavage of BmATG6 in Bm-12 cells was significantly enhanced when BmCalpain-A/B and BmCalpain-7 were induced by starvation, and was partially inhibited by the inhibitor of either calpain or caspase, but completely inhibited when both types of inhibitors were applied together. Our results indicated that BmCalpains, including BmCalpain-A/B, -C and -7, may be involved in autophagy and apoptosis during B. mori metamorphosis and after starvation, and may also contribute to the apoptosis-associated cleavage of BmATG6.

Furin inhibitor protects against neuronal cell death induced by activated NMDA receptors

The proprotein convertases (PCs) act as serine proteases and are known to convert diverse precursor proteins into their active forms. Among the PCs, furin has been considered to play a crucial role not only in embryogenesis, but also in the initiation and progression of certain pathologic conditions. However, the roles played by furin with respect to neuronal cell injuries remain to be determined. An excessive influx of Ca²⁺ through the N-methyl-d-aspartate (NMDA) receptor has been associated with diverse neurological and neurodegenerative disorders. The aim of this study was to achieve further insight into the pathophysiologic roles of furin in cultured cortical neurons. We demonstrated that furin inhibitors dose-dependently prevented neuronal injury induced by NMDA treatment. Neuronal injury induced by NMDA treatment was attenuated by the calpain inhibitor calpeptin. And the increase observed in the activity of calpain after NMDA treatment was significantly inhibited by these furin inhibitors. Furthermore, calpain-2 activity, which was evaluated by means of the immunoblotting assay, was increased by NMDA treatment. It was noteworthy that this increased activity was almost completely inhibited by a furin inhibitor. Our findings suggested that furin is involved in NMDA-induced neuronal injury by acting upstream of calpain.

Caspase-3 and caspase-8 expression in breast cancer: caspase-3 is associated with survival

Impaired apoptosis is one of the hallmarks of cancer. Caspase-3 and -8 are key regulators of the apoptotic response and have been shown to interact with the calpain family, a group of cysteine proteases, during tumorigenesis. The current study sought to investigate the prognostic potential of caspase-3 and -8 in breast cancer, as well as the prognostic value of combinatorial caspase and calpain expression. A large cohort (n = 1902) of early stage invasive breast cancer patients was used to explore the expression of caspase-3 and -8. Protein expression was examined using standard immunohistochemistry on tissue microarrays. High caspase-3 expression, but not caspase-8, is significantly associated with adverse breast cancer-specific survival (P = 0.008 and P = 0.056, respectively). Multivariate analysis showed that caspase-3 remained an independent factor when confounding factors were included (hazard ratio (HR) 1.347, 95% confidence interval (CI) 1.086–1.670; P = 0.007). The analyses in individual subgroups demonstrated the significance of caspase-3 expression in clinical outcomes in receptor positive (ER, PR or HER2) subgroups (P = 0.001) and in non-basal like subgroup (P = 0.029). Calpain expression had been previously assessed. Significant association was also found between high caspase-3/high calpain-1 and breast cancer-specific survival in the total patient cohort (P = 0.005) and basal-like subgroup (P = 0.034), as indicated by Kaplan–Meier analysis. Caspase-3 expression is associated with adverse breast cancer-specific survival in breast cancer patients, and provides additional prognostic values in distinct phenotypes. Combinatorial caspase and calpain expression can predict worse prognosis, especially in basal-like phenotypes. The findings warrant further validation studies in independent multi-centre patient cohorts.

TNFα induces Ca2+ influx to accelerate extrinsic apoptosis in hepatocellular carcinoma cells

BACKGROUND

Tumor necrosis factor-α has been proven an effective anticancer agent in preclinical studies. However, the translation of TNFα from research to clinic has been blocked by significant systemic toxicity and limited efficacy at maximal tolerated dose, which need urgently to be solved.

METHODS

The level of cytosolic Ca²⁺ was assessed by Fura-2 in HCC cells. After changing cytosolic Ca²⁺ level by using agonists or inhibitors, cell apoptosis was detected by flow cytometry. We also detected the effect of ionomycin or parvalbumin on the anti-tumor activity of TNFα in a mice model. Lastly, we studied the roles of cytosolic Ca²⁺ in the mitochondrial-dependent intrinsic apoptosis pathway.

RESULTS

Here, we demonstrated that TNFα induced extracellular Ca²⁺ influx into cytoplasm through transient receptor potential channel in HCC cells. Both cytosolic Ca²⁺ scavenger and Ca²⁺-binding protein PV effectively desensitized hepatocellular carcinoma cells to TNFα, whereas combination ionomycin or 1,4,5-inositol triphosphate significantly sensitized HCC cells to TNFα, indicating that the increased level of cytosolic Ca²⁺ was positively correlated with the TNFα-induced cell apoptosis in vitro. In a nude mice xenograft model, our data revealed that TNFα combined with ionomycin remarkably synergized the anti-tumor effect of TNFα. Furthermore, we found that TNFα-mediated extracellular Ca²⁺ influx accelerated TNFα-induced extrinsic apoptosis through activating calpain/IAP/caspase3 pathway.

CONCLUSIONS

Our study provides the evidence supporting a novel mechanism by which TNFα induces extracellular Ca²⁺ influx to enhance cell apoptosis and suggests that increasing the level of cytosolic Ca²⁺ might be an alternative strategy to improve the pro-apoptotic activity of TNFα in HCC cells, although suitable chemical or biological reagents need to be further tested.

The cellular autophagy/apoptosis checkpoint during inflammation

Cell death is a major determinant of inflammatory disease severity. Whether cells live or die during inflammation largely depends on the relative success of the pro-survival process of autophagy versus the pro-death process of apoptosis. These processes interact and influence each other during inflammation and there is a checkpoint at which cells irrevocably commit to either one pathway or another. This review will discuss the concept of the autophagy/apoptosis checkpoint and its importance during inflammation, the mechanisms of inflammation leading up to the checkpoint, and how the checkpoint is regulated. Understanding these concepts is important since manipulation of the autophagy/apoptosis checkpoint represents a novel opportunity for treatment of inflammatory diseases caused by too much or too little cell death.

Association between myocardial cell apoptosis and calpain-1/caspase-3 expression in rats with hypoxic-ischemic brain damage

The present study aimed to investigate the association between myocardial cell apoptosis and calpain-1/caspase-3 expression in a rat model of hypoxic-ischemic brain damage (HIBD). A total of 64 newborn rats were divided into control (n=8; sacrificed on day 7) and HIBD groups (n=56). HIBD group rats were sacrificed 2, 12 or 24 h, or 2, 3, 5 or 7 days following HIBD (n=8/group). A terminal deoxynucleotidyl transferase dUTP nick-end labeling assay was performed to detect myocardial apoptotic cells and calculate the apoptosis index (AI), reverse transcription-polymerase chain reaction was performed to detect myocardial calpain-1/caspase-3 mRNA expression levels and a western blot analysis was conducted to detect calpain‑1 protein expression levels. The correlations between calpain‑1 and caspase‑3 expression levels and AI were analyzed. The results demonstrated that apoptotic myocardial cells in the HIBD groups were markedly increased compared with the control group, with AI peaking in the day 3 group. Caspase‑3 and calpain‑1 mRNA expression levels were increased from 2 and 12 h following HIBD, respectively, with the most elevated levels in the day 2 group. Compared with the control group, calpain‑1 protein expression levels were increased from 2 h, with the greatest expression levels in the day 3 group (P<0.05). Calpain‑1 mRNA and protein (76/80 kDa) expression levels demonstrated positive linear correlations with AI (r=0.786, P=0.001; and r=0.853, P=0.001, respectively) Caspase-3 mRNA expression levels were positively correlated with AI (r=0.894; P=0.001). In conclusion, the present study demonstrated that in rats with HIBD, there is a positive correlation between increased apoptosis of myocardial cells and expression levels of calpain-1 and caspase-3.

MiR-99a and MiR-491 Regulate Cisplatin Resistance in Human Gastric Cancer Cells by Targeting CAPNS1

Cisplatin is the first-line agent utilized for the clinical treatment of a wide variety of solid tumors including gastric cancer. However, the intrinsic or acquired cisplatin resistance is often occurred in patients with gastric cancer and resulted in failure of cisplatin therapy. In order to investigate if miRNA involves in cisplatin resistance of human gastric cancer, we first screened and compared the expression of miRNAs between cisplatin resistant gastric cancer cell lines SGC-7901/DDP and BGC-823/DDP and their sensitive parental cells by miRNAs microarray and followed by analysis of 2D-GE/MS to identify their target proteins. We found both miR-99a and miR-491 were upregulated while their target gene calpain small subunit 1 (CAPNS1) was downregulated in resistant gastric cancer cells. Dual-luciferase- reporter assays with wild-type and mutated CAPNS1 3'-UTR confirmed their specificity of targeting. Inhibition of miR-99a and miR-491, or overexpress CAPNS1 can enhance cisplatin sensitivity of the resistant cells while transfection of two miRNAs' mimics or si-CAPNS1 in the sensitive cells can induce their resistance. Moreover, our results demonstrated CAPNS1 positively regulated calpain1 and calpain2, the catalytic subunits of CAPNS1, and cleaved caspase3 which further cleaved PARP1 and directly induced apoptosis. Therefore, miR-99a and miR-491 might be work as novel molecules regulate cisplatin resistance by directly targeting CAPNS1 associated pathway in human gastric cancer cells.

Non-canonical programmed cell death mechanisms triggered by natural compounds

Natural compounds are the fundament of pharmacological treatments and more than 50% of all anticancer drugs are of natural origins or at least derived from scaffolds present in Nature. Over the last 25 years, molecular mechanisms triggered by natural anticancer compounds were investigated. Emerging research showed that molecules of natural origins are useful for both preventive and therapeutic purposes by targeting essential hallmarks and enabling characteristics described by Hanahan and Weinberg. Moreover, natural compounds were able to change the differentiation status of selected cell types. One of the earliest response of cells treated by pharmacologically active compounds is the change of its morphology leading to ultra-structural perturbations: changes in membrane composition, cytoskeleton integrity, alterations of the endoplasmic reticulum, mitochondria and of the nucleus lead to formation of morphological alterations that are a characteristic of both compound and cancer type preceding cell death. Apoptosis and autophagy were traditionally considered as the most prominent cell death or cell death-related mechanisms. By now multiple other cell death modalities were described and most likely involved in response to chemotherapeutic treatment. It can be hypothesized that especially necrosis-related phenotypes triggered by various treatments or evolving from apoptotic or autophagic mechanisms, provide a more efficient therapeutic outcome depending on cancer type and genetic phenotype of the patient. In fact, the recent discovery of multiple regulated forms of necrosis and the initial elucidation of the corresponding cell signaling pathways appear nowadays as important tools to clarify the immunogenic potential of non-canonical forms of cell death induction.

Tetra-O-Methyl Nordihydroguaiaretic Acid Broadly Suppresses Cancer Metabolism and Synergistically Induces Strong Anticancer Activity in Combination with Etoposide, Rapamycin and UCN-01

The ability of Tetra-O-methyl nordihydroguaiaretic acid (M₄N) to induce rapid cell death in combination with Etoposide, Rapamycin, or UCN-01 was examined in LNCaP cells, both in cell culture and animal experiments. Mice treated with M₄N drug combinations with either Etoposide or Rapamycin showed no evidence of tumor and had a 100% survival rate 100 days after tumor implantation. By comparison all other vehicles or single drug treated mice failed to survive longer than 30 days after implantation. This synergistic improvement of anticancer effect was also confirmed in more than 20 cancer cell lines. In LNCaP cells, M₄N was found to reduce cellular ATP content, and suppress NDUFS1 expression while inducing hyperpolarization of mitochondrial membrane potential. M₄N-treated cells lacked autophagy with reduced expression of BNIP3 and ATG5. To understand the mechanisms of this anticancer activity of M₄N, the effect of this drug on three cancer cell lines (LNCaP, AsPC-1, and L428 cells) was further examined via transcriptome and metabolomics analyses. Metabolomic results showed that there were reductions of 26 metabolites essential for energy generation and/or production of cellular components in common with these three cell lines following 8 hours of M₄N treatment. Deep RNA sequencing analysis demonstrated that there were sixteen genes whose expressions were found to be modulated following 6 hours of M₄N treatment similarly in these three cell lines. Six out of these 16 genes were functionally related to the 26 metabolites described above. One of these up-regulated genes encodes for CHAC1, a key enzyme affecting the stress pathways through its degradation of glutathione. In fact M₄N was found to suppress glutathione content and induce reactive oxygen species production. The data overall indicate that M₄N has profound specific negative impacts on a wide range of cancer metabolisms supporting the use of M₄N combination for cancer treatments.

Calpain in Breast Cancer: Role in Disease Progression and Treatment Response

The calpains are a family of intracellular cysteine proteases that function in a wide array of cellular activities, including cytoskeletal remodelling, survival and apoptosis. The ubiquitously expressed micro (µ)-calpain and milli (m)-calpain are archetypal family members that require calcium for function and can be inhibited by their endogenous inhibitor calpastatin. This review describes the role of the calpain system in the prognosis of breast cancer and disease progression, in addition to the role of the calpain system in the response to breast cancer treatments, including chemotherapeutic, endocrine and targeted therapies.

Caspase-7: a critical mediator of optic nerve injury-induced retinal ganglion cell death

Background

Axonal injury of the optic nerve (ON) is involved in various ocular diseases, such as glaucoma and traumatic optic neuropathy, which leads to apoptotic death of retinal ganglion cells (RGCs) and loss of vision. Caspases have been implicated in RGC pathogenesis. However, the role of caspase-7, a functionally unique caspase, in ON injury and RGC apoptosis has not been reported previously. The purpose of this study is to evaluate the role of caspase-7 in ON injury-induced RGC apoptosis.

Results

C57BL/6 (wildtype, WT) and caspase-7 knockout (Casp7^−/−) mice were used. We show that ON crush activated caspase-7 and calpain-1, an upstream activator of caspase-7, in mouse RGCs, as well as hydrolysis of kinectin and co-chaperone P23, specific substrates of caspase-7. ON crush caused a progressive loss of RGCs to 28 days after injury. Knockout of caspase-7 partially and significantly protected against the ON injury-induced RGC loss; RGC density at 28 days post ON crush in Casp7^−/− mice was approximately twice of that in WT ON injured retinas. Consistent with changes in RGC counts, spectral-domain optical coherence tomography analysis revealed that ON crush significantly reduced the in vivo thickness of the ganglion cell complex layer (including ganglion cell layer, nerve fiber layer, and inner plexiform layer) in the retina. The ON crush-induced thinning of retinal layer was significantly ameliorated in Casp7^−/− mice when compared to WT mice. Moreover, electroretinography analysis demonstrated a decline in the positive component of scotopic threshold response amplitude in ON crushed eyes of the WT mice, whereas this RGC functional response was significantly higher in Casp7^−/− mice at 28 days post injury.

Conclusion

Altogether, our findings indicate that caspase-7 plays a critical role in ON injury-induced RGC death, and inhibition of caspase-7 activity may be a novel therapeutic strategy for glaucoma and other neurodegenerative diseases of the retina.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-015-0039-2) contains supplementary material, which is available to authorized users.

Caspases: a molecular switch node in the crosstalk between autophagy and apoptosis

Autophagy and apoptosis are two important catabolic processes contributing to the maintenance of cellular and tissue homeostasis. Autophagy controls the turnover of protein aggregates and damaged organelles within cells, while apoptosis is the principal mechanism by which unwanted cells are dismantled and eliminated from organisms. Despite marked differences between these two pathways, they are highly interconnected in determining the fate of cells. Intriguingly, caspases, the primary drivers of apoptotic cell death, play a critical role in mediating the complex crosstalk between autophagy and apoptosis. Pro-apoptotic signals can converge to activate caspases to execute apoptotic cell death. In addition, activated caspases can degrade autophagy proteins (i.e., Beclin-1, Atg5, and Atg7) to shut down the autophagic response. Moreover, caspases can convert pro-autophagic proteins into pro-apoptotic proteints to trigger apoptotic cell death instead. It is clear that caspases are important in both apoptosis and autophagy, thus a detailed deciphering of the role of caspases in these two processes is still required to clarify the functional relationship between them. In this article, we provide a current overview of caspases in its interplay between autophagy and apoptosis. We emphasized that defining the role of caspases in autophagy-apoptosis crosstalk will provide a framework for more precise manipulation of these two processes during cell death.

Atrial Calpains: Mediators of Atrialmyopathies in Atrial Fibrillation

Atrial fibrillation (AF) is associated with substantial structural changes at cell and tissue level. Cellular hypertrophy, disintegration of sarcomeres, mitochondrial swelling and apoptosis have been described as typical histo-morphologic alterations in AF. Main initiators for cellular alterations in fibrillating atrial myocytes are cytosolic calcium overload and oxidative stress. Calpains are intracellular Ca2+- activated proteases and important mediators of calcium overload. Activation of calpains and down-regulation of the calpain inhibitor, calpastatin, contribute to myocardial damage in fibrillating atria. Thus, deregulations of the expression, activity, or subcellular localization of calpain within atrial myocytes have been established as important mediators of atrial myopathy during AF.

Calcium paradox induces apoptosis in the isolated perfused Rana ridibunda heart: involvement of p38-MAPK and calpain

“Calcium paradox” as a term describes the deleterious effects conferred to a heart perfused with a calcium-free solution followed by repletion, including loss of mechanical activity and sarcomere disruption. Given that the signaling mechanisms triggered by calcium paradox remain elusive, in the present study, we tried to investigate them in the isolated perfused heart from Rana ridibunda. Calcium paradox was found to markedly activate members of the MAPKs (p43-ERK, JNKs, p38-MAPK). In addition to lactate dehydrogenase (LDH) release in the perfusate (indicative of necrosis), we also confirmed the occurrence of apoptosis by using the TUNEL assay and identifying poly(ADP-ribose) polymerase (PARP) fragmentation and upregulated Bax expression. Furthermore, using MDL28170 (a selective calpain inhibitor), a role for this protease was revealed. In addition, various divalent cations were shown to exert a protective effect against the calcium paradox. Interestingly, SB203580, a p38-MAPK inhibitor, alleviated calcium-paradox-conferred apoptosis. This result indicates that p38-MAPK plays a pro-apoptotic role, contributing to the resulting myocardial dysfunction and cell death. To our knowledge, this is the first time that the calcium paradox has been shown to induce apoptosis in amphibians, with p38-MAPK and calpain playing significant roles.

Protein degradation systems in platelets

Protein synthesis and degradation are essential processes that allow cells to survive and adapt to their surrounding milieu. In nucleated cells, the degradation and/or cleavage of proteins is required to eliminate aberrant proteins. Cells also degrade proteins as a mechanism for cell signalling and complex cellular functions. Although the last decade has convincingly shown that platelets synthesise proteins, the roles of protein degradation in these anucleate cytoplasts are less clear. Here we review what is known about protein degradation in platelets placing particular emphasis on the proteasome and the cysteine protease calpain.

Targeting the XIAP/caspase-7 complex selectively kills caspase-3-deficient malignancies

Caspase-3 downregulation (CASP3/DR) in tumors frequently confers resistance to cancer therapy and is significantly correlated with a poor prognosis in cancer patients. Because CASP3/DR cancer cells rely heavily on the activity of caspase-7 (CASP7) to initiate apoptosis, inhibition of activated CASP7 (p19/p12-CASP7) by X-linked inhibitor of apoptosis protein (XIAP) is a potential mechanism by which apoptosis is prevented in those cancer cells. Here, we identify the pocket surrounding the Cys246 residue of p19/p12-CASP7 as a target for the development of a protein-protein interaction (PPI) inhibitor of the XIAP:p19/p12-CASP7 complex. Interrupting this PPI directly triggered CASP7-dependent apoptotic signaling that bypassed the activation of the apical caspases and selectively killed CASP3/DR malignancies in vitro and in vivo without adverse side effects in nontumor cells. Importantly, CASP3/DR combined with p19/p12-CASP7 accumulation correlated with the aggressive evolution of clinical malignancies and a poor prognosis in cancer patients. Moreover, targeting of this PPI effectively killed cancer cells with multidrug resistance due to microRNA let-7a-1-mediated CASP3/DR and resensitized cancer cells to chemotherapy-induced apoptosis. These findings not only provide an opportunity to treat CASP3/DR malignancies by targeting the XIAP:p19/p12-CASP7 complex, but also elucidate the molecular mechanism underlying CASP3/DR in cancers.

The calpain/calpastatin system has opposing roles in growth and metastatic dissemination of melanoma

Conventional calpains are ubiquitous cysteine proteases whose activity is promoted by calcium signaling and specifically limited by calpastatin. Calpain expression has been shown to be increased in human malignant cells, but the contribution of the calpain/calpastatin system in tumorigenesis remains unclear. It may play an important role in tumor cells themselves (cell growth, migration, and a contrario cell death) and/or in tumor niche (tissue infiltration by immune cells, neo-angiogenesis). In this study, we have used a mouse model of melanoma as a tool to gain further understanding of the role of calpains in tumor progression. To determine the respective importance of each target, we overexpressed calpastatin in tumor and/or host in isolation. Our data demonstrate that calpain inhibition in both tumor and host blunts tumor growth, while paradoxically increasing metastatic dissemination to regional lymph nodes. Specifically, calpain inhibition in melanoma cells limits tumor growth in vitro and in vivo but increases dissemination by amplifying cell resistance to apoptosis and accelerating migration process. Meanwhile, calpain inhibition restricted to host cells blunts tumor infiltration by immune cells and angiogenesis required for antitumor immunity, allowing tumor cells to escape tumor niche and disseminate. The development of highly specific calpain inhibitors with potential medical applications in cancer should take into account the opposing roles of the calpain/calpastatin system in initial tumor growth and subsequent metastatic dissemination.

Influence of DNA copy number and mRNA levels on the expression of breast cancer related proteins

For a panel of cancer related proteins, the aim was to shed light on which molecular level the expression of each protein was mainly regulated in breast tumors, and to investigate whether differences in regulation were reflected in different molecular subtypes. DNA, mRNA and protein lysates from 251 breast tumor specimens were analyzed using appropriate microarray technologies. Data from all three levels were available for 52 proteins selected for their known involvement in cancer, primarily through the PI3K/Akt pathway. For every protein, in cis Spearman rank correlations between the three molecular levels were calculated across all samples and within each intrinsic gene expression subtype, enabling 63 comparisons altogether due to multiple gene probes matching to single proteins. Subtype-specific relationships between the three molecular levels were studied by calculating the variance of subtype-specific correlation and differences between overall and average subtype-specific correlation. The findings were validated in an external dataset comprising 703 breast tumor specimens. The proteins were sorted into four groups based on the calculated rank correlation values between the three molecular levels. Group A consisted of eight proteins with significant correlation between DNA copy number levels and mRNA expression, and between mRNA expression and protein expression (Bonferroni adjusted p < 0.05). Group B consisted of 14 proteins with significant correlation between mRNA expression and protein expression. Group C consisted of 15 proteins with significant correlation between copy number levels and mRNA expression. For the remaining 25 proteins (group D), no significant correlations was observed. Stratification of tumors according to intrinsic subtype enabled identification of positive correlations between copy number levels, mRNA and protein expression that were undetectable when considering the entire sample set. Protein pairings that either demonstrated high variance in correlation values between subtypes, or between subtypes and the total dataset were studied in particular. The protein expression of cleaved caspase 7 was most highly expressed, and correlated highest to CASP7 gene expression within the basal-like subtype, accompanied by the lowest amounts of hsa-miR-29c. Luminal A-like subtype demonstrated highest amounts of hsa-miR-29c (a miRNA with a putative target sequence in CASP7 mRNA), low expression of cleaved caspase 7 and low correlation to CASP7 gene expression. Such pattern might be an indication of hsa-miR-29c miRNA functioning as a repressor of translation of CASP7 within the luminal-A subtype. Across the entire cohort no correlation was found between CCNB1 copy number and gene expression. However, within most gene intrinsic subtypes, mRNA and protein expression of cyclin B1 was found positively correlated to copy number data, suggesting that copy number can affect the overall expression of this protein. Aberrations of cyclin B1 copy number also identified patients with reduced overall survival within each subtype. Based on correlation between the three molecular levels, genes and their products could be sorted into four groups for which the expression was likely to be regulated at different molecular levels. Further stratification suggested subtype-specific regulation that was not evident across the entire sample set.

Temporal dependence of cysteine protease activation following excitotoxic hippocampal injury

Excitotoxic insults can lead to intracellular signaling cascades that contribute to cell death, in part by activation of proteases, phospholipases, and endonucleases. Cysteine proteases, such as calpains, are calcium (Ca(2+))-activated enzymes which degrade cytoskeletal proteins, including microtubule-associated proteins, tubulin, and spectrin, among others. The current study used the organotypic hippocampal slice culture model to examine whether pharmacologic inhibition of cysteine protease activity inhibits N-methyl-D-aspartate- (NMDA-) induced excitotoxic (20 μM NMDA) cell death and changes in synaptophysin immunoreactivity. Significant NMDA-induced cytotoxicity (as measured by propidium iodide [PI] uptake) was found in the CA1 region of the hippocampus at all timepoints examined (24, 72, 120 h), an effect significantly attenuated by co-exposure to the selective NMDA receptor antagonist DL-2-Amino-5-phosphonopentanoic acid (APV), but not MDL-28170, a potent cysteine protease inhibitor. Results indicated sparing of NMDA-induced loss of the synaptic vesicular protein synaptophysin in all regions of the hippocampus by MDL-28170, though only at early timepoints after injury. These results suggest Ca(2+)-dependent recruitment of cysteine proteases within 24h of excitotoxic insult, but activation of alternative cellular degrading mechanisms after 24h. Further, these data suggest that synaptophysin may be a substrate for calpains and related proteases.

Regulation and physiological roles of the calpain system in muscular disorders

Calpains, a family of Ca(2+)-dependent cytosolic cysteine proteases, can modulate their substrates' structure and function through limited proteolytic activity. In the human genome, there are 15 calpain genes. The most-studied calpains, referred to as conventional calpains, are ubiquitous. While genetic studies in mice have improved our understanding about the conventional calpains' physiological functions, especially those essential for mammalian life as in embryogenesis, many reports have pointed to overactivated conventional calpains as an exacerbating factor in pathophysiological conditions such as cardiovascular diseases and muscular dystrophies. For treatment of these diseases, calpain inhibitors have always been considered as drug targets. Recent studies have introduced another aspect of calpains that calpain activity is required to protect the heart and skeletal muscle against stress. This review summarizes the functions and regulation of calpains, focusing on the relevance of calpains to cardiovascular disease.

Targeted gene inactivation of calpain-1 suppresses cortical degeneration due to traumatic brain injury and neuronal apoptosis induced by oxidative stress

Calpains are calcium-regulated cysteine proteases that have been implicated in the regulation of cell death pathways. Here, we used our calpain-1 null mouse model to evaluate the function of calpain-1 in neural degeneration following a rodent model of traumatic brain injury. In vivo, calpain-1 null mice show significantly less neural degeneration and apoptosis and a smaller contusion 3 days post-injury than wild type littermates. Protection from traumatic brain injury corroborated with the resistance of calpain-1 neurons to apoptosis induced by oxidative stress. Biochemical analysis revealed that caspase-3 activation, extracellular calcium entry, mitochondrial membrane permeability, and release of apoptosis-inducing factor from mitochondria are partially blocked in the calpain-1 null neurons. These findings suggest that the calpain-1 knock-out mice may serve as a useful model system for neuronal protection and apoptosis in traumatic brain injury and other neurodegenerative disorders in which oxidative stress plays a role.

The calpain system as a potential target for pelvic muscle reinforcement

The fascial and muscular components within the pelvic floor create a support mechanism that facilitates storage and voiding of urine. Their constituents are mainly fibrillar collagens I and III, which are responsible for maintaining tensile strength. Stretching and recoiling is enabled by the elastic fibers consisting of elastin on a scaffold of microfibrils, fibrillin-1 and -2. Calpains are intracellular Ca2+ -dependent cysteine proteases found in almost all eukaryotes and some bacteria. Calpains display limited proteolytic activity at neutral pH, proteolyzing substrates to transform and modulate their structures and activities, and are therefore called “modulator proteases”. By making selective limited proteolytic cleavages, they modulate the activity of enzymes, including key signaling molecules, and induce specific cytoskeletal rearrangements, accounting for their roles in signal transduction and structural stabilization. Understanding these mechanisms should provide avenues for novel therapeutic strategies to treat pathological processes such as urinary incontinence and pelvic prolapse.

The calpain system and cancer

The calpains are a conserved family of cysteine proteinases that catalyse the controlled proteolysis of many specific substrates. Calpain activity is implicated in several fundamental physiological processes, including cytoskeletal remodelling, cellular signalling, apoptosis and cell survival. Calpain expression is altered during tumorigenesis, and the proteolysis of numerous substrates, such as inhibitors of nuclear factor-κB (IκB), focal adhesion proteins (including, focal adhesion kinase and talin) and proto-oncogenes (for example, MYC), has been implicated in tumour pathogenesis. Recent evidence indicates that the increased expression of certain family members might influence the response to cancer therapies, providing justification for the development of novel calpain inhibitors.

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.

PARP-1 cleavage fragments: signatures of cell-death proteases in neurodegeneration

The normal function of poly (ADP-ribose) polymerase-1 (PARP-1) is the routine repair of DNA damage by adding poly (ADP ribose) polymers in response to a variety of cellular stresses. Recently, it has become widely appreciated that PARP-1 also participates in diverse physiological and pathological functions from cell survival to several forms of cell death and has been implicated in gene transcription, immune responses, inflammation, learning, memory, synaptic functions, angiogenesis and aging. In the CNS, PARP inhibition attenuates injury in pathologies like cerebral ischemia, trauma and excitotoxicity demonstrating a central role of PARP-1 in these pathologies. PARP-1 is also a preferred substrate for several 'suicidal' proteases and the proteolytic action of suicidal proteases (caspases, calpains, cathepsins, granzymes and matrix metalloproteinases (MMPs)) on PARP-1 produces several specific proteolytic cleavage fragments with different molecular weights. These PARP-1 signature fragments are recognized biomarkers for specific patterns of protease activity in unique cell death programs. This review focuses on specific suicidal proteases active towards PARP-1 to generate signature PARP-1 fragments that can identify key proteases and particular forms of cell death involved in pathophysiology. The roles played by some of the PARP-1 fragments and their associated binding partners in the control of different forms of cell death are also discussed.

Cordycepin induces apoptosis of CGTH W-2 thyroid carcinoma cells through the calcium-calpain-caspase 7-PARP pathway

Cordycepin, a nucleoside isolated from Cordyceps sinensis, is an inhibitor of polyadenylation and has an antitumor effect. We used CGTH W-2, a follicular thyroid carcinoma cell line, to study the mechanism of the anticancer effect of cordycepin. Cordycepin decreased cell viability and resulted in apoptosis but not necrosis. Cordycepin increased intracellular calcium levels triggering calpain activation, which led to apoptosis. BAPTA/AM and calpeptin inhibited the cordycepin-induced cleavage of caspase 7 and poly (ADP-ribose) polymerase (PARP), implying an upstream role of calcium and calpain. CGTH W-2 cells expressed four subtypes of adenosine receptors (AR), A1AR, A2AAR, A2BAR, and A3AR. Specific antagonists to AR subtypes all blocked cordycepin-induced apoptosis to different degrees. Small interfering RNA for A1AR and A3AR abrogated cordycepin-induced apoptosis. In conclusion, the cordycepin-induced apoptosis of CGTH W-2 cells is mediated by the calcium-calpain-caspase 7-PARP pathway, and ARs are involved in the apoptotic effect of cordycepin.

Caspase-3 activation is required for reovirus-induced encephalitis in vivo

Reovirus infection of neonatal mice provides a classic experimental system for understanding the molecular pathogenesis of central nervous system (CNS) viral infection. CNS tissue injury, caused by many human neurotropic viruses, including herpes viruses and West Nile virus, is associated with caspase-dependent apoptotic neuronal cell death. We have previously shown that reovirus-induced CNS tissue injury results from apoptosis and is associated with activation of both death-receptor and mitochondrial apoptotic pathways culminating in the activation of the downstream effector caspase, caspase-3. In order to directly investigate the role of caspase-3 in virus-induced neuronal death and CNS tissue injury during encephalitis, we have compared the pathogenesis of reovirus CNS infection in mice lacking the caspase-3 gene (caspase-3 (-/-)) to syngeneic wild-type mice. Prior studies of antiapoptotic treatments for reovirus-infected mice have indicated that protection from reovirus-induced neuronal injury can occur without altering the viral titer in the brains of infected mice. We now show that reovirus infection of caspase-3 (-/-) mice was associated with dramatic reduction in severity of CNS tissue injury, decreased viral antigen and titer in the brain, and enhanced survival of infected mice. Following intracerebral inoculation, the authors also show that virus spread from the brain to the eyes in reovirus-infected caspase-3 (-/-) mice, indicating that viral spread was intact in these mice. Examination of brains of long-term survivors of reovirus infection among caspase-3 (-/-) mice showed that these mice eventually clear their CNS viral infection, and do not manifest residual or delayed CNS tissue injury.

The dual role of calcium as messenger and stressor in cell damage, death, and survival

Ca(2+) is an important second messenger participating in many cellular activities; when physicochemical insults deregulate its delicate homeostasis, it acts as an intrinsic stressor, producing/increasing cell damage. Damage elicits both repair and death responses; intriguingly, in those responses Ca(2+) also participates as second messenger. This delineates a dual role for Ca(2+) in cell stress, making difficult to separate the different and multiple mechanisms required for Ca(2+)-mediated control of cell survival and apoptosis. Here we attempt to disentangle the two scenarios, examining on the one side, the events implicated in deregulated Ca(2+) toxicity and the mechanisms through which this elicits reparative or death pathways; on the other, reviewing the role of Ca(2+) as a messenger in the transduction of these same signaling events.