Calpain involvement in the remodeling of cytoskeletal anchorage complexes

Origin and flow-mediated remodeling of the murine and human extraembryonic circulation systems

The transduction of mechanical stimuli produced by blood flow is an important regulator of vascular development. The vitelline and umbilico-placental circulations are extraembryonic vascular systems that are required for proper embryonic development in mammalian embryos. The morphogenesis of the extraembryonic vasculature and the cardiovascular system of the embryo are hemodynamically and molecularly connected. Here we provide an overview of the establishment of the murine and human vitelline and umbilico-placental vascular systems and how blood flow influences various steps in their development. A deeper comprehension of extraembryonic vessel development may aid the establishment of stem-cell based embryo models and provide novel insights to understanding pregnancy complications related to the umbilical cord and placenta.

Podocyte Injury in Diabetic Kidney Disease in Mouse Models Involves TRPC6-mediated Calpain Activation Impairing Autophagy

SIGNIFICANCE STATEMENT

Autophagy protects podocytes from injury in diabetic kidney disease (DKD). Restoring glomerular autophagy is a promising approach to limit DKD. This study demonstrates a novel regulatory mechanism of autophagy that blocks this critical protection of the glomerular filtration barrier. We demonstrated that TRPC6 induced in podocytes in mouse models of diabetes mediates calpain activation, thereby impairing podocyte autophagy, causing injury and accelerating DKD. Furthermore, this study provides proof of principle for druggable targets for DKD because restoration of podocyte autophagy by calpain inhibitors effectively limits glomerular destruction.

BACKGROUND

Diabetic kidney disease is associated with impaired podocyte autophagy and subsequent podocyte injury. The regulation of podocyte autophagy is unique because it minimally uses the mTOR and AMPK pathways. Thus, the molecular mechanisms underlying the impaired autophagy in podocytes in diabetic kidney disease remain largely elusive.

METHODS

This study investigated how the calcium channel TRPC6 and the cysteine protease calpains deleteriously affect podocyte autophagy in diabetic kidney disease in mice. We demonstrated that TRPC6 knockdown in podocytes increased the autophagic flux because of decreased cysteine protease calpain activity. Diabetic kidney disease was induced in vivo using streptozotocin with unilateral nephrectomy and the BTBR ob/ob mouse models.

RESULTS

Diabetes increased TRPC6 expression in podocytes in vivo with decreased podocyte autophagic flux. Transgenic overexpression of the endogenous calpain inhibitor calpastatin, as well as pharmacologic inhibition of calpain activity, normalized podocyte autophagic flux, reduced nephrin loss, and prevented the development of albuminuria in diabetic mice. In kidney biopsies from patients with diabetes, we further confirmed that TRPC6 overexpression in podocytes correlates with decreased calpastatin expression, autophagy blockade, and podocyte injury.

CONCLUSIONS

Overall, we discovered a new mechanism that connects TRPC6 and calpain activity to impaired podocyte autophagy, increased podocyte injury, and development of proteinuria in the context of diabetic kidney disease. Therefore, targeting TRPC6 and/or calpain to restore podocyte autophagy might be a promising therapeutic strategy for diabetic kidney disease.

Joining forces: crosstalk between mechanosensitive PIEZO1 ion channels and integrin-mediated focal adhesions

Both integrin-mediated focal adhesions (FAs) and mechanosensitive ion channels such as PIEZO1 are critical in mechanotransduction processes that influence cell differentiation, development, and cancer. Ample evidence now exists for regulatory crosstalk between FAs and PIEZO1 channels with the molecular mechanisms underlying this process remaining unclear. However, an emerging picture is developing based on spatial crosstalk between FAs and PIEZO1 revealing a synergistic model involving the cytoskeleton, extracellular matrix (ECM) and calcium-dependent signaling. Already cell type, cell contractility, integrin subtypes and ECM composition have been shown to regulate this crosstalk, implying a highly fine-tuned relationship between these two major mechanosensing systems. In this review, we summarize the latest advances in this area, highlight the physiological implications of this crosstalk and identify gaps in our knowledge that will improve our understanding of cellular mechanosensing.

PIEZO1 and PECAM1 interact at cell-cell junctions and partner in endothelial force sensing

Two prominent concepts for the sensing of shear stress by endothelium are the PIEZO1 channel as a mediator of mechanically activated calcium ion entry and the PECAM1 cell adhesion molecule as the apex of a triad with CDH5 and VGFR2. Here, we investigated if there is a relationship. By inserting a non-disruptive tag in native PIEZO1 of mice, we reveal in situ overlap of PIEZO1 with PECAM1. Through reconstitution and high resolution microscopy studies we show that PECAM1 interacts with PIEZO1 and directs it to cell-cell junctions. PECAM1 extracellular N-terminus is critical in this, but a C-terminal intracellular domain linked to shear stress also contributes. CDH5 similarly drives PIEZO1 to junctions but unlike PECAM1 its interaction with PIEZO1 is dynamic, increasing with shear stress. PIEZO1 does not interact with VGFR2. PIEZO1 is required in Ca2+-dependent formation of adherens junctions and associated cytoskeleton, consistent with it conferring force-dependent Ca2+ entry for junctional remodelling. The data suggest a pool of PIEZO1 at cell junctions, the coming together of PIEZO1 and PECAM1 mechanisms and intimate cooperation of PIEZO1 and adhesion molecules in tailoring junctional structure to mechanical requirement.

Minimal expression of dysferlin prevents development of dysferlinopathy in dysferlin exon 40a knockout mice

Dysferlin is a Ca²⁺-activated lipid binding protein implicated in muscle membrane repair. Recessive variants in DYSF result in dysferlinopathy, a progressive muscular dystrophy. We showed previously that calpain cleavage within a motif encoded by alternatively spliced exon 40a releases a 72 kDa C-terminal minidysferlin recruited to injured sarcolemma. Herein we use CRISPR/Cas9 gene editing to knock out murine Dysf exon 40a, to specifically assess its role in membrane repair and development of dysferlinopathy. We created three Dysf exon 40a knockout (40aKO) mouse lines that each express different levels of dysferlin protein ranging from ~ 90%, ~ 50% and ~ 10-20% levels of wild-type. Histopathological analysis of skeletal muscles from all 12-month-old 40aKO lines showed virtual absence of dystrophic features and normal membrane repair capacity for all three 40aKO lines, as compared with dysferlin-null BLAJ mice. Further, lipidomic and proteomic analyses on 18wk old quadriceps show all three 40aKO lines are spared the profound lipidomic/proteomic imbalance that characterises dysferlin-deficient BLAJ muscles. Collective results indicate that membrane repair does not depend upon calpain cleavage within exon 40a and that ~ 10-20% of WT dysferlin protein expression is sufficient to maintain the muscle lipidome, proteome and membrane repair capacity to crucially prevent development of dysferlinopathy.

Reg-1α, a New Substrate of Calpain-2 Depending on Its Glycosylation Status

Reg-1α/lithostathine, a protein mainly associated with the digestive system, was previously shown to be overexpressed in the pre-clinical stages of Alzheimer’s disease. In vitro, the glycosylated protein was reported to form fibrils at physiological pH following the proteolytic action of trypsin. However, the nature of the protease able to act in the central nervous system is unknown. In the present study, we showed that Reg-1α can be cleaved in vitro by calpain-2, the calcium activated neutral protease, overexpressed in neurodegenerative diseases. Using chemical crosslinking experiments, we found that the two proteins can interact with each other. Identification of the cleavage site using mass spectrometry, between Gln⁴ and Thr⁵, was found in agreement with the in silico prediction of the calpain cleavage site, in a position different from the one reported for trypsin, i.e., Arg¹¹-Ile¹² peptide bond. We showed that the cleavage was impeded by the presence of the neighboring glycosylation of Thr⁵. Moreover, in vitro studies using electron microscopy showed that calpain-cleaved protein does not form fibrils as observed after trypsin cleavage. Collectively, our results show that calpain-2 cleaves Reg-1α in vitro, and that this action is not associated with fibril formation.

Endogenous slow and fast myosin dynamics in myofibers isolated from mice expressing GFP-Myh7 and Kusabira Orange-Myh1

Skeletal muscle consists of slow and fast myofibers in which different myosin isoforms are expressed. Approximately 300 myosins form a single thick filament in the myofibrils, where myosin is continuously exchanged. However, endogenous slow and fast myosin dynamics have not been fully understood. To elucidate those dynamics, here we generated mice expressing green fluorescence protein-tagged slow myosin heavy chain (GFP-Myh7) and Kusabira Orange fluorescence protein-tagged fast myosin heavy chain (KuO-Myh1). First, these mice enabled us to distinguish between GFP- and KuO-myofibers under fluorescence microscopy: GFP-Myh7 and KuO-Myh1 were exclusively expressed in slow myofibers and fast myofibers, respectively. Next, to monitor endogenous myosin dynamics, fluorescence recovery after photobleaching (FRAP) was conducted. The mobile fraction (Mf) of GFP-Myh7 and that of KuO-Myh1 were almost constant values independent of the regions of the myofibers and the muscle portions where the myofibers were isolated. Intriguingly, proteasome inhibitor treatment significantly decreased the Mf in GFP-Myh7 but not in KuO-Myh1 myofibers, indicating that the response to a disturbance in protein turnover depended on muscle fiber type. Taken together, the present results indicated that the mice we generated are promising tools not only for distinguishing between GFP- and KuO-myofibers but also for studying the dynamics of endogenous myosin isoforms by live-cell fluorescence imaging.

Mechanosensitive Piezo1 channels mediate renal fibrosis

Kidney fibrosis is the final common pathway of progressive kidney diseases, the underlying mechanisms of which is not fully understood. The purpose of the current study is to investigate a role of Piezo1, a mechanosensitive nonselective cation channel, in kidney fibrosis. In human fibrotic kidneys, Piezo1 protein expression was markedly upregulated. The abundance of Piezo1 protein in kidneys of mice with UUO or with folic-acid treatment was significantly increased. Inhibition of Piezo1 with GsMTx4 markedly ameliorated UUO or folic acid-induced kidney fibrosis. Mechanical stretch, compression or stiffness induced Piezo1 activation and pro-fibrotic responses in human HK2 cells and primary cultured mouse proximal tubular cells (mPTCs), which were greatly prevented by inhibition or silence of Piezo1. TGFβ-1 induced increased Piezo1 expression and pro-fibrotic phenotypic alterations in HK2 cells and mPTCs, which was again markedly prevented by inhibition of Piezo1. Activation of Piezo1 by Yoda1, a Piezo1 agonist, caused calcium influx and profibrotic responses in HK2 cells and induced calpain2 activation, followed by talin1 cleavage and upregulation of integrinβ1. Also, Yoda1 promoted the link between ECM and integrinβ1. In conclusion, Piezo1 is involved in the progression of kidney fibrosis and pro-fibrotic alterations in renal proximal tubular cells, likely through activating calcium-calpain2-integrinβ1 pathway.

Calcium-Mediated Calpain Activation and Microtubule Dissociation in Cell Model of Hereditary Sensory Neuropathy Type-1 Expressing V144D SPTLC1 Mutation

Hereditary sensory neuropathy type 1A (HSN1A) is an autosomal, dominantly inherited peripheral neuropathy caused by mutations in serine palmitoyl transferase long chain 1 (SPTLC1), involved in the de novo synthesis of sphingolipids. We have previously reported calcium imbalance, as well as mitochondrial and ER stress in both HSN1 patient lymphoblasts and a transiently transfected cell model. In this study, we investigated the role of the Ca2+-activated protease calpain in destabilizing the cell cytoskeleton, by examining calpain activity in SH-SY5Y cells overexpressing the V144D mutant and changes in microtubule-associated proteins (MAP). Intramitochondrial Ca2+ was found to be significantly depleted and cytoplasmic Ca2+ increased in the V144D mutant. Subsequently, calpain and proteasome activity were increased and calpain substrates, microtubule associated proteins MAP2, and tau were significantly reduced in the microtubule fraction of the mutant. Significant changes were also found in motor proteins dynein and KIF2A detected in the microtubule fraction of cells overexpressing the V144D mutation. There was also a reduction in anterograde and retrograde mitochondrial transport velocities in the V144D mutant. These findings strongly implicate cytoskeletal aberration caused by Ca2+ dysregulation and subsequent loss of microtubule transport functions as the cause of axonal dying back that is characteristic of HSN1.

Orai3 Calcium Channel Regulates Breast Cancer Cell Migration through Calcium-Dependent and -Independent Mechanisms

Orai3 calcium (Ca²⁺) channels are implicated in multiple breast cancer processes, such as proliferation and survival as well as resistance to chemotherapy. However, their involvement in the breast cancer cell migration processes remains vague. In the present study, we exploited MDA-MB-231 and MDA-MB-231 BrM2 basal-like estrogen receptor-negative (ER^-) cell lines to assess the direct role of Orai3 in cell migration. We showed that Orai3 regulates MDA-MB-231 and MDA-MB-231 BrM2 cell migration in two distinct ways. First, we showed that Orai3 remodels cell adhesive capacities by modulating the intracellular Ca²⁺ concentration. Orai3 silencing (siOrai3) decreased calpain activity, cell adhesion and migration in a Ca²⁺-dependent manner. In addition, Orai3 interacts with focal adhesion kinase (FAK) and regulates the actin cytoskeleton, in a Ca²⁺-independent way. Thus, siOrai3 modulates cell morphology by altering F-actin polymerization via a loss of interaction between Orai3 and FAK. To summarize, we demonstrated that Orai3 regulates cell migration through a Ca²⁺-dependent modulation of calpain activity and, in a Ca²⁺-independent manner, the actin cytoskeleton architecture via FAK.

Tributyltin Alters Calcium Levels, Mitochondrial Dynamics, and Activates Calpains Within Dorsal Root Ganglion Neurons

Tributyltin (TBT) remains a global health concern. The primary route of human exposure to TBT is either through ingestion or skin absorption, but TBT's effects on the peripheral nervous system have still not been investigated. Therefore, we exposed in vitro sensory dorsal root ganglion (DRG) neurons to TBT at a concentration of 50-200 nM, which is similar to the observed concentrations of TBT in human blood samples. We observed that TBT causes extensive axon degeneration and neuronal death in the DRG neurons. Furthermore, we discovered that TBT causes an increase in both cytosolic and mitochondrial calcium levels, disrupts mitochondrial dynamics, decreases neuronal ATP levels, and leads to the activation of calpains. Additional experiments demonstrated that inhibition of calpain activation prevented TBT-induced fragmentation of neuronal cytoskeletal proteins and neuronal cell death. Thus, we conclude that calpain activation is the key executioner of TBT-induced peripheral neurodegeneration.

Deeper and Deeper on the Role of BK and Kir4.1 Channels in Glioblastoma Invasiveness: A Novel Summative Mechanism?

In the last decades, increasing evidence has revealed that a large number of channel protein and ion pumps exhibit impaired expression in cancers. This dysregulation is responsible for high proliferative rates as well as migration and invasiveness, reflected in the recently coined term oncochannelopathies. In glioblastoma (GBM), the most invasive and aggressive primary brain tumor, GBM cells modify their ionic equilibrium in order to change their volume as a necessary step prior to migration. This mechanism involves increased expression of BK channels and downregulation of the normally widespread Kir4.1 channels, as noted in GBM biopsies from patients. Despite a large body of work implicating BK channels in migration in response to an artificial intracellular calcium rise, little is known about how this channel acts in GBM cells at resting membrane potential (RMP), as compared to other channels that are constitutively open, such as Kir4.1. In this review we propose that a residual fraction of functionally active Kir4.1 channels mediates a small, but continuous, efflux of potassium at the more depolarized RMP of GBM cells. In addition, coinciding with transient membrane deformation and the intracellular rise in calcium concentration, brief activity of BK channels can induce massive and rapid cytosolic water loss that reduces cell volume (cell shrinkage), a necessary step for migration within the brain parenchyma.

The actin binding protein α-actinin-2 expression is associated with dendritic spine plasticity and migrating granule cells in the rat dentate gyrus following pilocarpine-induced seizures

α-actinin-2 (α-actn-2) is an F-actin-crosslinking protein, localized in dendritic spines. In vitro studies suggested that it is involved in spinogenesis, morphogenesis, actin organization, cell migration and anchoring of the NR1 subunit of the N-methyl-D-aspartate (NMDA) receptors in dendritic spines. However, little is known regarding its function in vivo. We examined the levels of α-actn-2 expression within the dentate gyrus (DG) during the development of chronic limbic seizures (epileptogenesis) induced by pilocarpine in rats. In this model, plasticity of the DG glutamatergic granule cells including spine loss, spinogenesis, morphogenesis, neo-synaptogenesis, aberrant migration, and alterations of NMDA receptors have been well characterized. We showed that α-actn-2 immunolabeling was reduced in the inner molecular layer at 1-2 weeks post-status epilepticus (SE), when granule cell spinogenesis and morphogenesis occur. This low level persisted at the chronic stage when new functional synapses are established. This decreased of α-actn-2 protein is concomitant with the recovery of drebrin A (DA), another actin-binding protein, at the chronic stage. Indeed, we demonstrated in cultured cells that in contrast to DA, α-actn-2 did not protect F-actin destabilization and DA inhibited α-actn-2 binding to F-actin. Such alteration could affect the anchoring of NR1 in dendritic spines. Furthermore, we showed that the expression of α-actn-2 and NR1 are co-down-regulated in membrane fractions of pilocarpine animals at chronic stage. Last, we showed that α-actn-2 is expressed in migrating newly born granule cells observed within the hilus of pilocarpine-treated rats. Altogether, our results suggest that α-actn-2 is not critical for the structural integrity and stabilization of granule cell dendritic spines. Instead, its expression is regulated when spinogenesis and morphogenesis occur and within migrating granule cells. Our data also suggest that the balance between α-actn-2 and DA expression levels may modulate NR1 anchoring within dendritic spines.

CAPN6 in disease: An emerging therapeutic target (Review)

As a member of the calpain protein family, calpain6 (CAPN6) is highly expressed mainly in the placenta and embryos. It plays a number of important roles in cellular processes, such as the stabilization of microtubules, the main-tenance of cell stability, the control of cell movement and the inhibition of apoptosis. In recent years, various studies have found that CAPN6 is one of the contributing factors associated with the tumorigenesis of uterine tumors and osteosarcoma, and that CAPN6 participates in the development of tumors by promoting cell proliferation and angiogenesis, and by inhibiting apoptosis, which is mainly regulated by the phosphatidylinositol 3 kinase (PI3K)/protein kinase B (Akt) pathway. Due to its abnormal cellular expression, CAPN6 has also been found to be associated with a number of diseases, such as white matter damage and muscular dystrophy. Therefore, CAPN6 may be a novel therapeutic target for these diseases. In the present review, the role of CAPN6 in disease and its possible use as a target in various therapies are discussed.

In Full Force. Mechanotransduction and Morphogenesis during Homeostasis and Tissue Regeneration

The interactions of form and function have been the focus of numerous studies in the context of development and more recently regeneration. Our understanding on how cells, tissues and organs sense and interpret external cues, such as mechanical forces, is becoming deeper as novel techniques in imaging are applied and the relevant signaling pathways emerge. These cellular responses can be found from bacteria to all multicellular organisms such as plants and animals. In this review, we focus on hemodynamic flow and endothelial shear stress during cardiovascular development and regeneration, where the interactions of morphogenesis and proper function are more prominent. In addition, we address the recent literature on the role of extracellular matrix and fibrotic response during tissue repair and regeneration. Finally, we refer to examples where the integration of multi-disciplinary approaches to understand the biomechanics of cellular responses could be utilized in novel medical applications.

ACYP2 contributes to malignant progression of glioma through promoting Ca2+ efflux and subsequently activating c-Myc and STAT3 signals

Background

Acylphosphatase 2 (ACYP2) is involved in cell differentiation, energy metabolism and hydrolysis of intracellular ion pump. It has been reported as a negative regulator in leukemia and a positive regulator in colon cancer, respectively. However, its biological role in glioma remains totally unclear.

Methods

We performed quantitative RT-PCR (qRT-PCR), immunohistochemistry (IHC) and western blot assays to evaluate ACYP2 expression. The functions of ACYP2 in glioma cells were determined by a series of in vitro and in vivo experiments, including cell proliferation, colony formation, cell cycle, apoptosis, migration, invasion and nude mouse tumorigenicity assays. In addition, western blot and co-immunoprecipitation (Co-IP) assays were used to identify its downstream targets.

Results

Knocking down ACYP2 in glioma cells significantly inhibited cell proliferation, colony formation, migration, invasion and tumorigenic potential in nude mice, and induced cell cycle arrest and apoptosis. Conversely, ectopic expression of ACYP2 in glioma cells dramatically promoted malignant phenotypes of glioma cells. Mechanistically, ACYP2 promoted malignant progression of glioma cells through regulating intracellular Ca²⁺ homeostasis via its interaction with PMCA4, thereby activating c-Myc and PTP1B/STAT3 signals. This could be effectively reversed by Ca²⁺ chelator BAPTA-AM or calpain inhibitor calpeptin.

Conclusions

Our data demonstrate that ACYP2 functions as an oncogene in glioma through activating c-Myc and STAT3 signals via the regulation of intracellular Ca²⁺ homeostasis, and indicate that ACYP2 may be a potential therapeutic target and prognostic biomarker in gliomas.

Single Loci and Haplotypes in CAPN1 and CAST Genes are Associated with Growth, Biometrics, and in Vivo Carcass Traits in Santa Inês Sheep

µ-calpain (CAPN1) and calpastatin (CAST) genes play key roles in protein turnover. The present study aimed to identify the variants in these genes associated with growth and ultrasound carcass traits in Santa Inês sheep. A sample of 192 no full sibling Santa Inês lambs was used. Fragments of the CAST and CAPN1 genes were amplified and next-generation sequencing was performed in the MiSeq platform. Variants in the CAPN1 and CAST sequences were then detected using bioinformatic tools. Withers and croup heights, body length, thoracic and croup widths, thoracic and leg girths, body depth, carcass fat score, rib eye area, fat thickness, body weights were recorded at weaning and at 140 days post-weaning, and average daily gain post-weaning was calculated. Both single-locus and haplotype association analyses were performed with the model as follows: farm (2 levels), year (4 levels), the month of birth (12 levels), and the covariate age of the animal. The fragments amplified included 4,514 bp between the 20th and 23rd exons of CAST as well as 3,927 bp between the 12th and 21st exons of CAPN1. In these regions, 58 (CAST) and 45 (CAPN1) variants were identified. In the CAST gene, the single-locus analysis revealed 22 suggestive additive effects (P<0.05) on several growth and carcass traits. Moreover, haplotype substitutions were associated with rib eye area (–0.689±0.290), average daily gain (–23.6±10.4), thoracic girth (–2.72±1.27), body length (–3.38±1.49), and leg girth (–2.84±1.37). Regarding the CAPN1 gene, the single-locus analysis identified seven suggestive additive effects, while only one haplotype replacement effect on fat thickness (–0.0143±0.0053) was detected. The results of the present study suggest that variants in the CAPN1 and CAST genes are associated with growth and ultrasound carcass traits in Santa Inês sheep, which may be a source of information to improve knowledge regarding the genetic control of these traits.

LOXL2 promotes oncogenic progression in alveolar rhabdomyosarcoma independently of its catalytic activity

Rhabdomyosarcoma (RMS) is the most common soft tissue malignancy in childhood and adolescence. Patients with the most aggressive histological variant have an unfavorable prognosis due to a high metastasis incidence. Lysyl oxidase-like 2 (LOXL2) is a lysyl oxidase, member of a family of extracellular matrix (ECM) crosslinking enzymes that recently have emerged as important regulators of tumor progression and metastasis. We report that LOXL2 is overexpressed in RMS, suggesting a potential role for LOXL2 in RMS oncogenic progression. Consistently, transient and stable LOXL2 knockdown decreased cell migratory and invasive capabilities in two ARMS cell lines. Furthermore, introduction of LOXL2 in RMS non-expressing cells using wild type or mutated (catalytically inactive) constructs resulted in increased cell migration, cell invasion and number and incidence of spontaneous lung metastasis in vivo, independently of its catalytic activity. To further study the molecular mechanism associated with LOXL2 expression, a pull-down assay on LOXL2-transfected cells was performed and analyzed by mass spectrometry. The intermediated filament protein vimentin was validated as a LOXL2-interactor. Thus, our results suggest an oncogenic role of LOXL2 in RMS by regulating cytoskeleton dynamics and cell motility capabilities.

The Beneficial Effects of Taurine to Counteract Sarcopenia

Aging is a multifactorial process characterized by several features including low-grade inflammation, increased oxidative stress and reduced regenerative capacity, which ultimately lead to alteration in morpho-functional properties of skeletal muscle, thus promoting sarcopenia. This condition is characterized by a gradual loss of muscle mass due to an unbalance between protein synthesis and degradation, finally conveying in functional decline and disability. The development of specific therapeutic approaches able to block or reverse this condition may represent an invaluable tool for the promotion of a healthy aging among elderly people. It is well established that changes in the quantity and the quality of dietary proteins, as well as the intake of specific amino acids, are able to counteract some of the physiopathological processes related to the progression of the loss of muscle mass and may have beneficial effects in improving the anabolic response of muscle in the elderly. Taurine is a non-essential amino acid expressed in high concentration in several mammalian tissues and particularly in skeletal muscle where it is involved in the modulation of intracellular calcium concentration and ion channel regulation and where it also acts as an antioxidant and anti-inflammatory factor. The aim of this review is to summarize the pleiotropic effects of taurine on specific muscle targets and to discuss its role in regulating signaling pathways involved in the maintenance of muscle homeostasis. We also highlight the potential use of taurine as a therapeutic molecule for the amelioration of skeletal muscle function and performance severely compromised during aging.

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.

In vivo induction of membrane damage by β-amyloid peptide oligomers

Exposure to the β-amyloid peptide (Aβ) is toxic to neurons and other cell types, but the mechanism(s) involved are still unresolved. Synthetic Aβ oligomers can induce ion-permeable pores in synthetic membranes, but whether this ability to damage membranes plays a role in the ability of Aβ oligomers to induce tau hyperphosphorylation, or other disease-relevant pathological changes, is unclear. To examine the cellular responses to Aβ exposure independent of possible receptor interactions, we have developed an in vivo C. elegans model that allows us to visualize these cellular responses in living animals. We find that feeding C. elegans E. coli expressing human Aβ induces a membrane repair response similar to that induced by exposure to the CRY5B, a known pore-forming toxin produced by B. thuringensis. This repair response does not occur when C. elegans is exposed to an Aβ Gly37Leu variant, which we have previously shown to be incapable of inducing tau phosphorylation in hippocampal neurons. The repair response is also blocked by loss of calpain function, and is altered by loss-of-function mutations in the C. elegans orthologs of BIN1 and PICALM, well-established risk genes for late onset Alzheimer's disease. To investigate the role of membrane repair on tau phosphorylation directly, we exposed hippocampal neurons to streptolysin O (SLO), a pore-forming toxin that induces a well-characterized membrane repair response. We find that SLO induces tau hyperphosphorylation, which is blocked by calpain inhibition. Finally, we use a novel biarsenical dye-tagging approach to show that the Gly37Leu substitution interferes with Aβ multimerization and thus the formation of potentially pore-forming oligomers. We propose that Aβ-induced tau hyperphosphorylation may be a downstream consequence of induction of a membrane repair process.

Molecular Insights into Muscle Homeostasis, Atrophy and Wasting

Muscle homeostasis is guaranteed by a delicate balance between synthesis and degradation of cell proteins and its alteration leads to muscle wasting and diseases. In this review, we describe the major anabolic pathways that are involved in muscle growth and homeostasis and the proteolytic systems that are over-activated in muscle pathologies. Modulation of these pathways comprises an attractive target for drug intervention.

How cells channel their stress: Interplay between Piezo1 and the cytoskeleton

Cells constantly encounter mechanical stimuli in their environment, such as dynamic forces and mechanical features of the extracellular matrix. These mechanical cues are transduced into biochemical signals, and integrated with genetic and chemical signals to modulate diverse physiological processes. Cells also actively generate forces to internally transport cargo, to explore the physical properties of their environment and to spatially position themselves and other cells during development. Mechanical forces are therefore central to development, homeostasis, and repair. Several molecular and biophysical strategies are utilized by cells for detecting and generating mechanical forces. Here we discuss an important class of molecules involved in sensing and transducing mechanical forces - mechanically-activated ion channels. We focus primarily on the Piezo1 ion channel, and examine its relationship with the cellular cytoskeleton.

Fate of tenogenic differentiation potential of human bone marrow stromal cells by uniaxial stretching affected by stretch-activated calcium channel agonist gadolinium

The role for mechanical stimulation in the control of cell fate has been previously proposed, suggesting that there may be a role of mechanical conditioning in directing mesenchymal stromal cells (MSCs) towards specific lineage for tissue engineering applications. Although previous studies have reported that calcium signalling is involved in regulating many cellular processes in many cell types, its role in managing cellular responses to tensile loading (mechanotransduction) of MSCs has not been fully elucidated. In order to establish this, we disrupted calcium signalling by blocking stretch-activated calcium channel (SACC) in human MSCs (hMSCs) in vitro. Passaged-2 hMSCs were exposed to cyclic tensile loading (1 Hz + 8% for 6, 24, 48, and 72 hours) in the presence of the SACC blocker, gadolinium. Analyses include image observations of immunochemistry and immunofluorescence staining from extracellular matrix (ECM) production, and measuring related tenogenic and apoptosis gene marker expression. Uniaxial tensile loading increased the expression of tenogenic markers and ECM production. However, exposure to strain in the presence of 20 μM gadolinium reduced the induction of almost all tenogenic markers and ECM staining, suggesting that SACC acts as a mechanosensor in strain-induced hMSC tenogenic differentiation process. Although cell death was observed in prolonged stretching, it did not appear to be apoptosis mediated. In conclusion, the knowledge gained in this study by elucidating the role of calcium in MSC mechanotransduction processes, and that in prolonged stretching results in non-apoptosis mediated cell death may be potential useful for regenerative medicine applications.

Suppression of calpain expression by NSAIDs is associated with inhibition of cell migration in rat duodenum

Non-steroidal anti-inflammatory drugs (NSAIDs) are widely used for the alleviation of pain and inflammation, but these drugs are also associated with a suite of negative side effects. Gastrointestinal (GI) toxicity is particularly concerning since it affects an estimated 70% of individuals taking NSAIDs routinely, and evidence suggests the majority of toxicity is occurring in the small intestine. Traditionally, NSAID-induced GI toxicity has been associated with indiscriminate inhibition of cyclooxygenase isoforms, but other mechanisms, including inhibition of cell migration, intestinal restitution, and wound healing, are likely to contribute to toxicity. Previous efforts demonstrated that treatment of cultured intestinal epithelial cells (IEC) with NSAIDs inhibits expression and activity of calpain proteases, but the effects of specific inhibition of calpain expression in vitro or the effects of NSAIDs on intestinal cell migration in vivo remain to be determined. Accordingly, we examined the effect of suppression of calpain protease expression with siRNA on cell migration in cultured IECs and evaluated the effects of NSAID treatment on epithelial cell migration and calpain protease expression in rat duodenum. Our results show that calpain siRNA inhibits protease expression and slows migration in cultured IECs. Additionally, NSAID treatment of rats slowed migration up the villus axis and suppressed calpain expression in duodenal epithelial cells. Our results are supportive of the hypothesis that suppression of calpain expression leading to slowing of cell migration is a potential mechanism through which NSAIDs cause GI toxicity.

A possible therapeutic potential of quercetin through inhibition of μ-calpain in hypoxia induced neuronal injury: a molecular dynamics simulation study

The neuroprotective property of quercetin is well reported against hypoxia and ischemia in past studies. This property of quercetin lies in its antioxidant property with blood-brain barrier permeability and anti-inflammatory capabilities. µ-Calpain, a calcium ion activated intracellular cysteine protease causes serious cellular insult, leading to cell death in various pathological conditions including hypoxia and ischemic stroke. Hence, it may be considered as a potential drug target for the treatment of hypoxia induced neuronal injury. As the inhibitory property of µ-calpain is yet to be explored in details, hence, in the present study, we investigated the interaction of quercetin with µ-calpain through a molecular dynamics simulation study as a tool through clarifying the molecular mechanism of such inhibition and determining the probable sites and modes of quercetin interaction with the µ-calpain catalytic domain. In addition, we also investigated the structure-activity relationship of quercetin with μ-calpain. Affinity binding of quercetin with µ-calpain had a value of -28.73 kJ/mol and a Ki value of 35.87 µM that may be a probable reason to lead to altered functioning of µ-calpain. Hence, quercetin was found to be an inhibitor of µ-calpain which might have a possible therapeutic role in hypoxic injury.

Inhibition of Kv channel expression by NSAIDs depolarizes membrane potential and inhibits cell migration by disrupting calpain signaling

Clinical use of non-steroidal anti-inflammatory drugs (NSAIDs) is well known to cause gastrointestinal ulcer formation via several mechanisms that include inhibiting epithelial cell migration and mucosal restitution. The drug-affected signaling pathways that contribute to inhibition of migration by NSAIDs are poorly understood, though previous studies have shown that NSAIDs depolarize membrane potential and suppress expression of calpain proteases and voltage-gated potassium (Kv) channel subunits. Kv channels play significant roles in cell migration and are targets of NSAID activity in white blood cells, but the specific functional effects of NSAID-induced changes in Kv channel expression, particularly on cell migration, are unknown in intestinal epithelial cells. Accordingly, we investigated the effects of NSAIDs on expression of Kv1.3, 1.4, and 1.6 in vitro and/or in vivo and evaluated the functional significance of loss of Kv subunit expression. Indomethacin or NS-398 reduced total and plasma membrane protein expression of Kv1.3 in cultured intestinal epithelial cells (IEC-6). Additionally, depolarization of membrane potential with margatoxin (MgTx), 40mM K(+), or silencing of Kv channel expression with siRNA significantly reduced IEC-6 cell migration and disrupted calpain activity. Furthermore, in rat small intestinal epithelia, indomethacin and NS-398 had significant, yet distinct, effects on gene and protein expression of Kv1.3, 1.4, or 1.6, suggesting that these may be clinically relevant targets. Our results show that inhibition of epithelial cell migration by NSAIDs is associated with decreased expression of Kv channel subunits, and provide a mechanism through which NSAIDs inhibit cell migration and may contribute to NSAID-induced gastrointestinal (GI) toxicity.

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.

Modulation of intracellular calcium levels by calcium lactate affects colon cancer cell motility through calcium-dependent calpain

Cancer cell motility is a key phenomenon regulating invasion and metastasis. Focal adhesion kinase (FAK) plays a major role in cellular adhesion and metastasis of various cancers. The relationship between dietary supplementation of calcium and colon cancer has been extensively investigated. However, the effect of calcium (Ca2+) supplementation on calpain-FAK-motility is not clearly understood. We sought to identify the mechanism of FAK cleavage through Ca2+ bound lactate (CaLa), its downstream signaling and role in the motility of human colon cancer cells. We found that treating HCT116 and HT-29 cells with CaLa immediately increased the intracellular Ca2+ (iCa2+) levels for a prolonged period of time. Ca2+ influx induced cleavage of FAK into an N-terminal FAK (FERM domain) in a dose-dependent manner. Phosphorylated FAK (p-FAK) was also cleaved in to its p-N-terminal FAK. CaLa increased colon cancer cells motility. Calpeptin, a calpain inhibitor, reversed the effects of CaLa on FAK and pFAK cleavage in both cancer cell lines. The cleaved FAK translocates into the nucleus and modulates p53 stability through MDM2-associated ubiquitination. CaLa-induced Ca2+ influx increased the motility of colon cancer cells was mediated by calpain activity through FAK and pFAK protein destabilization. In conclusion, these results suggest that careful consideration may be given in deciding dietary Ca2+ supplementation to patient undergoing treatment for metastatic cancer.

Modulation of intracellular calcium levels by calcium lactate affects colon cancer cell motility through calcium-dependent calpain

Cancer cell motility is a key phenomenon regulating invasion and metastasis. Focal adhesion kinase (FAK) plays a major role in cellular adhesion and metastasis of various cancers. The relationship between dietary supplementation of calcium and colon cancer has been extensively investigated. However, the effect of calcium (Ca²⁺) supplementation on calpain-FAK-motility is not clearly understood. We sought to identify the mechanism of FAK cleavage through Ca²⁺ bound lactate (CaLa), its downstream signaling and role in the motility of human colon cancer cells. We found that treating HCT116 and HT-29 cells with CaLa immediately increased the intracellular Ca²⁺ (iCa²⁺) levels for a prolonged period of time. Ca²⁺ influx induced cleavage of FAK into an N-terminal FAK (FERM domain) in a dose-dependent manner. Phosphorylated FAK (p-FAK) was also cleaved in to its p-N-terminal FAK. CaLa increased colon cancer cells motility. Calpeptin, a calpain inhibitor, reversed the effects of CaLa on FAK and pFAK cleavage in both cancer cell lines. The cleaved FAK translocates into the nucleus and modulates p53 stability through MDM2-associated ubiquitination. CaLa-induced Ca²⁺ influx increased the motility of colon cancer cells was mediated by calpain activity through FAK and pFAK protein destabilization. In conclusion, these results suggest that careful consideration may be given in deciding dietary Ca²⁺ supplementation to patient undergoing treatment for metastatic cancer.

The integrin-adhesome is required to maintain muscle structure, mitochondrial ATP production, and movement forces in Caenorhabditis elegans

The integrin-adhesome network, which contains >150 proteins, is mechano-transducing and located at discreet positions along the cell-cell and cell-extracellular matrix interface. A small subset of the integrin-adhesome is known to maintain normal muscle morphology. However, the importance of the entire adhesome for muscle structure and function is unknown. We used RNA interference to knock down 113 putative Caenorhabditis elegans homologs constituting most of the mammalian adhesome and 48 proteins known to localize to attachment sites in C. elegans muscle. In both cases, we found >90% of components were required for normal muscle mitochondrial structure and/or proteostasis vs. empty vector controls. Approximately half of these, mainly proteins that physically interact with each other, were also required for normal sarcomere and/or adhesome structure. Next we confirmed that the dystrophy observed in adhesome mutants associates with impaired maximal mitochondrial ATP production (P < 0.01), as well as reduced probability distribution of muscle movement forces compared with wild-type animals. Our results show that the integrin-adhesome network as a whole is required for maintaining both muscle structure and function and extend the current understanding of the full complexities of the functional adhesome in vivo.—Etheridge, T., Rahman, M., Gaffney, C. J., Shaw, D., Shephard, F., Magudia, J., Solomon, D. E., Milne, T., Blawzdziewicz, J., Constantin-Teodosiu, D., Greenhaff, P. L., Vanapalli, S. A., Szewczyk, N. J. The integrin-adhesome is required to maintain muscle structure, mitochondrial ATP production, and movement forces in Caenorhabditis elegans.

Piezo1 integration of vascular architecture with physiological force

The mechanisms by which physical forces regulate endothelial cells to determine the complexities of vascular structure and function are enigmatic. Studies of sensory neurons have suggested Piezo proteins as subunits of Ca(2+)-permeable non-selective cationic channels for detection of noxious mechanical impact. Here we show Piezo1 (Fam38a) channels as sensors of frictional force (shear stress) and determinants of vascular structure in both development and adult physiology. Global or endothelial-specific disruption of mouse Piezo1 profoundly disturbed the developing vasculature and was embryonic lethal within days of the heart beating. Haploinsufficiency was not lethal but endothelial abnormality was detected in mature vessels. The importance of Piezo1 channels as sensors of blood flow was shown by Piezo1 dependence of shear-stress-evoked ionic current and calcium influx in endothelial cells and the ability of exogenous Piezo1 to confer sensitivity to shear stress on otherwise resistant cells. Downstream of this calcium influx there was protease activation and spatial reorganization of endothelial cells to the polarity of the applied force. The data suggest that Piezo1 channels function as pivotal integrators in vascular biology.

Disruption and molecular characterization of calpains-related (MoCAPN1, MoCAPN3 and MoCAPN4) genes in Magnaporthe oryzae

Calpains are intracellular, cysteine proteases found in plants, animals and fungi functioning as signal transduction components in different cellular pathways including sporulation and alkaline adaptation in fungi. Calpains-related MoCAPN1 (MGG_14872), MoCAPN3 (MGG_15810) and MoCAPN4 (MGG_04818) genes from Magnaporthe oryzae genome which are 2604, 3513 and 771-bp in length and encoding identical proteins of 867, 1170 and 256 amino acids were functionally characterized for different phenotypes through gene disruption method. All the mutants except those for MoCAPN1 showed normal phenotypes. In pathogenicity test, the mutants did not lead to any visible changes in phenotypes causing similar blast lesions on blast susceptible rice and barley leaves as those of the Guy-11 strain suggesting no major role in pathogenicity. Germ tubes formation, appressorium formation, mycelium radial growth and mating with 2539 strain were indistinguishable among the mutants and Guy-11 strains. Cell wall integrity (congo red) test, stress response under chemical pressure (ZnSO4, CuSO4 and CdCl2), osmotic and oxidative (NaCl and H2O2) stress response, growth response on glucose and nitrogen deficient media resulted in similar results in the mutants and Guy-11 strains. However, mutants for ΔMoCAPN1 gene produced reduced (0.57±0.15B and 0.54±0.05B) conidia compared to that (1.69±0.13A) of the Guy-11 strain showing its involvement in conidiation.

Cryptosporidiuminfections: molecular advances

Cryptosporidiumhost cell interaction remains fairly obscure compared with other apicomplexans such asPlasmodiumorToxoplasma. The reason for this is probably the inability of this parasite to complete its life cyclein vitroand the lack of a system to genetically modifyCryptosporidium. However, there is a substantial set of data about the molecules involved in attachment and invasion and about the host cell pathways involved in actin arrangement that are altered by the parasite. Here we summarize the recent advances in research on host cell infection regarding the excystation process, attachment and invasion, survival in the cell, egress and the available data on omics.

Caspase-8 as a regulator of tumor cell motility

The caspases are a family of ubiquitously expressed cysteine proteases best known for their roles in programmed cell death. However, caspases play a number of other roles in vertebrates. In the case of caspase-8, loss of expression is an embryonic lethal phenotype, and caspase-8 plays roles in suppressing cellular necrosis, promoting differentiation and immune signaling, regulating autophagy, and promoting cellular migration. Apoptosis and migration require localization of caspase-8 in the periphery of the cells, where caspase-8 acts as part of distinct biosensory complexes that either promote migration in appropriate cellular microenvironments, or cell death in inappropriate settings. In the cellular periphery, caspase-8 interacts with components of the focal adhesion complex in a tyrosine-kinase dependent manner, promoting both cell migration in vitro and metastasis in vivo. Mechanistically, caspase-8 interacts with components of both focal adhesions and early endosomes, enhancing focal adhesion turnover and promoting rapid integrin recycling to the cell surface. Clinically, this suggests that the expression of caspase-8 may not always be a positive prognostic sign, and that the role of caspase-8 in cancer progression is likely context-dependent.

Calpain-dependent cytoskeletal rearrangement exploited for anthrax toxin endocytosis

The protective antigen component of Bacillus anthracis toxins can interact with at least three distinct proteins on the host cell surface, capillary morphogenesis gene 2 (CMG2), tumor endothelial marker 8, and β1-integrin, and, with the assistance of other host proteins, enters targeted cells by receptor-mediated endocytosis. Using an antisense-based phenotypic screen, we discovered the role of calpains in this process. We show that functions of a ubiquitous Ca(2+)-dependent cysteine protease, calpain-2, and of the calpain substrate talin-1 are exploited for association of anthrax toxin and its principal receptor, CMG2, with higher-order actin filaments and consequently for toxin entry into host cells. Down-regulated expression of calpain-2 or talin-1, or pharmacological interference with calpain action, did not affect toxin binding but reduced endocytosis and increased the survival of cells exposed to anthrax lethal toxin. Adventitious expression of wild-type talin-1 promoted toxin endocytosis and lethality, whereas expression of a talin-1 mutant (L432G) that is insensitive to calpain cleavage did not. Disruption of talin-1, which links integrin-containing focal adhesion complexes to the actin cytoskeleton, facilitated association of toxin bound to its principal cell-surface receptor, CMG2, with higher-order actin filaments undergoing dynamic disassembly and reassembly during endocytosis. Our results reveal a mechanism by which a bacterial toxin uses constitutively occurring calpain-mediated cytoskeletal rearrangement for internalization.

Molecular fingerprint of high fat diet induced urinary bladder metabolic dysfunction in a rat model

AIMS/HYPOTHESIS

Diabetic voiding dysfunction has been reported in epidemiological dimension of individuals with diabetes mellitus. Animal models might provide new insights into the molecular mechanisms of this dysfunction to facilitate early diagnosis and to identify new drug targets for therapeutic interventions.

METHODS

Thirty male Sprague-Dawley rats received either chow or high-fat diet for eleven weeks. Proteomic alterations were comparatively monitored in both groups to discover a molecular fingerprinting of the urinary bladder remodelling/dysfunction. Results were validated by ELISA, Western blotting and immunohistology.

RESULTS

In the proteome analysis 383 proteins were identified and canonical pathway analysis revealed a significant up-regulation of acute phase reaction, hypoxia, glycolysis, β-oxidation, and proteins related to mitochondrial dysfunction in high-fat diet rats. In contrast, calcium signalling, cytoskeletal proteins, calpain, 14-3-3η and eNOS signalling were down-regulated in this group. Interestingly, we found increased ubiquitin proteasome activity in the high-fat diet group that might explain the significant down-regulation of eNOS, 14-3-3η and calpain.

CONCLUSIONS/INTERPRETATION

Thus, high-fat diet is sufficient to induce significant remodelling of the urinary bladder and alterations of the molecular fingerprint. Our findings give new insights into obesity related bladder dysfunction and identified proteins that may indicate novel pathophysiological mechanisms and therefore constitute new drug targets.

Targeting calpain in synaptic plasticity

INTRODUCTION

Calpains represent a family of neutral, calcium-dependent proteases, which modify the function of their target proteins by partial truncation. These proteases have been implicated in numerous cell functions, including cell division, proliferation, migration, and death. In the CNS, where µ-calpain and m-calpain are the main calpain isoforms, their activation has been linked to synaptic plasticity as well as to neurodegeneration. This review will focus on the role of calpains in synaptic plasticity and discuss the possibility of developing methods to manipulate calpain activity for therapeutic purposes.

AREAS COVERED

This review covers the literature showing how calpains are implicated in synaptic plasticity and in a number of conditions associated with learning impairment. The possibility of developing new drugs targeting these enzymes for treating these conditions is discussed.

EXPERT OPINION

As evidence accumulates that calpain activation participates in neurodegeneration and cancer, there is interest in developing therapeutic approaches using direct or indirect calpain inhibition. In particular, a peptide derived from the calpain truncation site of mGluR1α was shown to decrease neurodegeneration following neonatal hypoxia/ischemia. More selective approaches need to be developed to target calpain or some of its substrates for therapeutic indications associated with deregulation of synaptic plasticity.

Structure-function relationships in calpains

Calpains are a family of complex multi-domain intracellular enzymes that share a calcium-dependent cysteine protease core. These are not degradative enzymes, but instead carry out limited cleavage of target proteins in response to calcium signalling. Selective cutting of cytoskeletal proteins to facilitate cell migration is one such function. The two most abundant and extensively studied members of this family in mammals, calpains 1 and 2, are heterodimers of an isoform-specific 80 kDa large subunit and a common 28 kDa small subunit. Structures of calpain-2, both Ca2+-free and bound to calpastatin in the activated Ca2+-bound state, have provided a wealth of information about the enzyme's structure-function relationships and activation. The main association between the subunits is the pairing of their C-terminal penta-EF-hand domains through extensive intimate hydrophobic contacts. A lesser contact is made between the N-terminal anchor helix of the large subunit and the penta-EF-hand domain of the small subunit. Up to ten Ca2+ ions are co-operatively bound during activation. The anchor helix is released and individual domains change their positions relative to each other to properly align the active site. Because calpains 1 and 2 require ~30 and ~350 μM Ca2+ ions for half-maximal activation respectively, it has long been argued that autoproteolysis, subunit dissociation, post-translational modifications or auxiliary proteins are needed to activate the enzymes in the cell, where Ca2+ levels are in the nanomolar range. In the absence of robust support for these mechanisms, it is possible that under normal conditions calpains are transiently activated by high Ca2+ concentrations in the microenvironment of a Ca2+ influx, and then return to an inactive state ready for reactivation.

Calpain activity is essential in skin wound healing and contributes to scar formation

Wound healing is a multistep phenomenon that relies on complex interactions between various cell types. Calpains are ubiquitously expressed proteases regulating several processes including cellular adhesion and motility as well as inflammation and angiogenesis. Calpains can be targeted by inhibitors, and their inhibition was shown to reduce organ damage in various disease models. We aimed to assess the role of calpains in skin healing and the potential benefit of calpain inhibition on scar formation. We used a pertinent model where calpain activity is inhibited only in lesional organs, namely transgenic mice overexpressing calpastatin (CPST), a specific natural calpain inhibitor. CPST mice showed a striking delay in wound healing particularly in the initial steps compared to wild types (WT). CPST wounds displayed reduced proliferation in the epidermis and delayed re-epithelization. Granulation tissue formation was impaired in CPST mice, with a reduction in CD45+ leukocyte infiltrate and in CD31+ blood vessel density. Interestingly, wounds on WT skin grafted on CPST mice (WT/CPST) showed a similar delayed healing with reduced angiogenesis and inflammation compared to wounds on WT/WT mice demonstrating the implication of calpain activity in distant extra-cutaneous cells during wound healing. CPST wounds showed a reduction in alpha-smooth muscle actin (αSMA) expressing myofibroblasts as well as αSMA RNA expression suggesting a defect in granulation tissue contraction. At later stages of skin healing, calpain inhibition proved beneficial by reducing collagen production and wound fibrosis. In vitro, human fibroblasts exposed to calpeptin, a pan-calpain inhibitor, showed reduced collagen synthesis, impaired TGFβ-induced differentiation into αSMA-expressing myofibroblasts, and were less efficient in a collagen gel contraction assay. In conclusion, calpains are major players in granulation tissue formation. In view of their specific effects on fibroblasts a late inhibition of calpains should be considered for scar reduction.

Talin contains a C-terminal calpain2 cleavage site important in focal adhesion dynamics

Talin is a large (∼2540 residues) dimeric adaptor protein that associates with the integrin family of cell adhesion molecules in cell-extracellular matrix junctions (focal adhesions; FAs), where it both activates integrins and couples them to the actin cytoskeleton. Calpain2-mediated cleavage of talin between the head and rod domains has previously been shown to be important in FA turnover. Here we identify an additional calpain2-cleavage site that removes the dimerisation domain from the C-terminus of the talin rod, and show that an E2492G mutation inhibits calpain cleavage at this site in vitro, and increases the steady state levels of talin1 in vivo. Expression of a GFP-tagged talin1 E2492G mutant in CHO.K1 cells inhibited FA turnover and the persistence of cell protrusion just as effectively as a L432G mutation that inhibits calpain cleavage between the talin head and rod domains. Moreover, incorporation of both mutations into a single talin molecule had an additive effect clearly demonstrating that calpain cleavage at both the N- and C-terminal regions of talin contribute to the regulation of FA dynamics. However, the N-terminal site was more sensitive to calpain cleavage suggesting that lower levels of calpain are required to liberate the talin head and rod fragments than are needed to clip off the C-terminal dimerisation domain. The talin head and rod liberated by calpain2 cleavage have recently been shown to play roles in an integrin activation cycle important in FA turnover and in FAK-dependent cell cycle progression respectively. The half-life of the talin head is tightly regulated by ubiquitination and we suggest that removal of the C-terminal dimerisation domain from the talin rod may provide a mechanism both for terminating the signalling function of the talin rod and indeed for inactivating full-length talin thereby promoting FA turnover at the rear of the cell.

The atypical calpains: evolutionary analyses and roles in Caenorhabditis elegans cellular degeneration

The calpains are physiologically important Ca²⁺-activated regulatory proteases, which are divided into typical or atypical sub-families based on constituent domains. Both sub-families are present in mammals, but our understanding of calpain function is based primarily on typical sub-family members. Here, we take advantage of the model organism Caenorhabditis elegans, which expresses only atypical calpains, to extend our knowledge of the phylogenetic evolution and function of calpains. We provide evidence that a typical human calpain protein with a penta EF hand, detected using custom profile hidden Markov models, is conserved in ancient metazoans and a divergent clade. These analyses also provide evidence for the lineage-specific loss of typical calpain genes in C. elegans and Ciona, and they reveal that many calpain-like genes lack an intact catalytic triad. Given the association between the dysregulation of typical calpains and human degenerative pathologies, we explored the phenotypes, expression profiles, and consequences of inappropriate reduction or activation of C. elegans atypical calpains. These studies show that the atypical calpain gene, clp-1, contributes to muscle degeneration and reveal that clp-1 activity is sensitive to genetic manipulation of [Ca²⁺]_i. We show that CLP-1 localizes to sarcomeric sub-structures, but is excluded from dense bodies (Z-disks). We find that the muscle degeneration observed in a C. elegans model of dystrophin-based muscular dystrophy can be suppressed by clp-1 inactivation and that nemadipine-A inhibition of the EGL-19 calcium channel reveals that Ca²⁺ dysfunction underlies the C. elegans MyoD model of myopathy. Taken together, our analyses highlight the roles of calcium dysregulation and CLP-1 in muscle myopathies and suggest that the atypical calpains could retain conserved roles in myofilament turnover.

Calpains are calcium activated non-lysosomal proteases that cleave proteins with exquisite selectivity. Proteins can be activated by calpain cleavage, because they are released from inhibitory constraints, or they can be targeted for further degradation to facilitate their normal physiological turnover or to promote cellular remodelling. Inappropriate calpain activity can lead to degenerative pathologies and cancers. Our understanding of calpain function is based primarily on typical calpains, which carry EF hand motifs that bind Ca²⁺ or mediate dimerization; however, typical and atypical calpains, which lack EF hand motifs, are both present in mammals. Hence, any therapeutic intervention designed to suppress degenerative conditions, particularly those caused by elevated Ca²⁺ levels, should also consider the potential involvement of atypical calpains. We have taken advantage of the model organism C. elegans, which only encodes atypical calpain proteins, to gain an understanding of the evolution and activities of these proteins. We show that the CLP-1 atypical calpain is normally expressed in muscle and localizes to sarcomeric sub-structures. We find that CLP-1 contributes to the muscle degeneration observed in a model of Duchenne muscular dystrophy. Our studies also highlight the importance of calcium dysregulation in promoting CLP-1 activity and muscle degeneration.

Proteomic study of calpeptin-induced differentiation on calpain-interacting proteins of C2C12 myoblast

Studies on skeletal muscle cell specification and development have demonstrated in the past that calpains interact with various transcriptional factors in regulating the cellular function. It has therefore, been assumed that transcriptional factors like myogenin, MyoD, Myf5, and MRF4 that are active during the myogenic differentiation might be affected and degraded by calpains. Therefore, to examine the biochemical adaptations of myoblasts during myocyte formation and muscle development comprehensively, the current study was designed to identify the effect of calpeptin (calpain inhibitors) on protein expression during differentiation of C2C12 mouse myoblast. Cells were proliferated to near 80% confluence under Dulbecco's modified eagle medium and differentiated further in 2% HS with 50 μM calpeptin. Incubated cells were collected at 0, 12, and 72 h and later the cell proteins were focused onto pH 4-7 IEF strip, followed by 12.5% SDS-PAGE. Obtained spots on the gels were compared and matched using commercial 2-DE analysis software and matched spots were identified by MALDI-ToF and/or Q-Tof systems. Conclusively, cell differentiation was observed to be active from 12 to 72 h however, calpeptin affected the differentiation process and cut down the rate of fusion by approximately 50%. Out of 41 proteins identified, 12 proteins were found to be upregulated where as 29 proteins were downregulated.

Calpains mediate integrin attachment complex maintenance of adult muscle in Caenorhabditis elegans

Two components of integrin containing attachment complexes, UNC-97/PINCH and UNC-112/MIG-2/Kindlin-2, were recently identified as negative regulators of muscle protein degradation and as having decreased mRNA levels in response to spaceflight. Integrin complexes transmit force between the inside and outside of muscle cells and signal changes in muscle size in response to force and, perhaps, disuse. We therefore investigated the effects of acute decreases in expression of the genes encoding these multi-protein complexes. We find that in fully developed adult Caenorhabditis elegans muscle, RNAi against genes encoding core, and peripheral, members of these complexes induces protein degradation, myofibrillar and mitochondrial dystrophies, and a movement defect. Genetic disruption of Z-line– or M-line–specific complex members is sufficient to induce these defects. We confirmed that defects occur in temperature-sensitive mutants for two of the genes: unc-52, which encodes the extra-cellular ligand Perlecan, and unc-112, which encodes the intracellular component Kindlin-2. These results demonstrate that integrin containing attachment complexes, as a whole, are required for proper maintenance of adult muscle. These defects, and collapse of arrayed attachment complexes into ball like structures, are blocked when DIM-1 levels are reduced. Degradation is also blocked by RNAi or drugs targeting calpains, implying that disruption of integrin containing complexes results in calpain activation. In wild-type animals, either during development or in adults, RNAi against calpain genes results in integrin muscle attachment disruptions and consequent sub-cellular defects. These results demonstrate that calpains are required for proper assembly and maintenance of integrin attachment complexes. Taken together our data provide in vivo evidence that a calpain-based molecular repair mechanism exists for dealing with attachment complex disruption in adult muscle. Since C. elegans lacks satellite cells, this mechanism is intrinsic to the muscles and raises the question if such a mechanism also exists in higher metazoans.

Muscle is a dynamic tissue that grows in response to use and nutrition and shrinks in response to lack of use, poor nutrition, or disease. Loss of muscle mass is an important public health problem, but we understand little of the genes that regulate muscle shrinkage. We have found that, in adult worm muscle, attachment to the basement membrane is continuously required to prevent catastrophic sub-cellular defects that result in impaired ability of muscle to function. We have also identified a group of proteases that are activated when the attachment fails to be properly maintained. Conversely, when these proteases are lacking in adult muscle, the muscles fail to maintain attachment to the basement membrane. Thus, we have discovered a group of proteases that appear to act to maintain attachment to the basement membrane and therefore to maintain muscle itself. Because these worms lack satellite cells, this maintenance system is intrinsic to muscle, thus raising the question whether a similar or identical system also works in humans.