Inhibition of calpain blocks platelet secretion, aggregation, and spreading

Calpain activity is negatively regulated by a KCTD7-Cullin-3 complex via non-degradative ubiquitination

Calpains are a class of non-lysosomal cysteine proteases that exert their regulatory functions via limited proteolysis of their substrates. Similar to the lysosomal and proteasomal systems, calpain dysregulation is implicated in the pathogenesis of neurodegenerative disease and cancer. Despite intensive efforts placed on the identification of mechanisms that regulate calpains, however, calpain protein modifications that regulate calpain activity are incompletely understood. Here we show that calpains are regulated by KCTD7, a cytosolic protein of previously uncharacterized function whose pathogenic mutations result in epilepsy, progressive ataxia, and severe neurocognitive deterioration. We show that KCTD7 works in complex with Cullin-3 and Rbx1 to execute atypical, non-degradative ubiquitination of calpains at specific sites (K398 of calpain 1, and K280 and K674 of calpain 2). Experiments based on single-lysine mutants of ubiquitin determined that KCTD7 mediates ubiquitination of calpain 1 via K6-, K27-, K29-, and K63-linked chains, whereas it uses K6-mediated ubiquitination to modify calpain 2. Loss of KCTD7-mediated ubiquitination of calpains led to calpain hyperactivation, aberrant cleavage of downstream targets, and caspase-3 activation. CRISPR/Cas9-mediated knockout of Kctd7 in mice phenotypically recapitulated human KCTD7 deficiency and resulted in calpain hyperactivation, behavioral impairments, and neurodegeneration. These phenotypes were largely prevented by pharmacological inhibition of calpains, thus demonstrating a major role of calpain dysregulation in KCTD7-associated disease. Finally, we determined that Cullin-3-KCTD7 mediates ubiquitination of all ubiquitous calpains. These results unveil a novel mechanism and potential target to restrain calpain activity in human disease and shed light on the molecular pathogenesis of KCTD7-associated disease.

Multidrug Resistance in Cancer: Understanding Molecular Mechanisms, Immunoprevention and Therapeutic Approaches

Cancer is one of the leading causes of death worldwide. Several treatments are available for cancer treatment, but many treatment methods are ineffective against multidrug-resistant cancer. Multidrug resistance (MDR) represents a major obstacle to effective therapeutic interventions against cancer. This review describes the known MDR mechanisms in cancer cells and discusses ongoing laboratory approaches and novel therapeutic strategies that aim to inhibit, circumvent, or reverse MDR development in various cancer types. In this review, we discuss both intrinsic and acquired drug resistance, in addition to highlighting hypoxia- and autophagy-mediated drug resistance mechanisms. Several factors, including individual genetic differences, such as mutations, altered epigenetics, enhanced drug efflux, cell death inhibition, and various other molecular and cellular mechanisms, are responsible for the development of resistance against anticancer agents. Drug resistance can also depend on cellular autophagic and hypoxic status. The expression of drug-resistant genes and the regulatory mechanisms that determine drug resistance are also discussed. Methods to circumvent MDR, including immunoprevention, the use of microparticles and nanomedicine might result in better strategies for fighting cancer.

The key role of Calpain in COVID-19 as a therapeutic strategy

COVID-19 is one of the viral diseases that has caused many deaths and financial losses to humans. Using the available information, this virus appears to activate the host cell-death mechanism through Calpain activation. Calpain inhibition can stop its downstream cascade reactions that cause cell death. Given the main roles of Calpain in the entry and pathogenicity of the SARS-CoV-2, its inhibition can be effective in controlling the COVID-19. This review describes how the virus activates Calpain by altering calcium flow. When Calpain was activated, the virus can enter the target cell. Subsequently, many complications of the disease, such as inflammation, cytokine storm and pulmonary fibrosis, are caused by virus-activated Calpain function. Calpain inhibitors appear to be a potential drug to control the disease and prevent death from COVID-19.

Cleavage of talin by calpain promotes platelet-mediated fibrin clot contraction

Calpain-catalyzed talin cleavage in platelets promotes fibrin clot contraction.

Calpain cleaves talin in proximity to vinculin binding sites, likely initiating vinculin binding to talin to promote clot contraction.

Blood clot contraction is driven by traction forces generated by the platelet cytoskeleton that are transmitted to fibrin fibers via the integrin αIIbβ3. Here we show that clot contraction is impaired by inhibitors of the platelet cytosolic protease calpain. We used subtiligase-mediated labeling of amino termini and mass spectrometry to identify proteolytically cleaved platelet proteins involved in clot contraction. Of 32 calpain-cleaved proteins after TRAP stimulation, 14 were cytoskeletal, most prominently talin and vinculin. A complex of talin and vinculin constitutes a mechanosensitive clutch connecting integrins bound to the extracellular matrix with the actin cytoskeleton. Accordingly, we focused on talin and vinculin. Talin is composed of an N-terminal head domain and a C-terminal rod domain organized into a series of 4- and 5-helix bundles. The bundles contain 11 vinculin binding sites (VBSs), each of which is an α-helix packed into a bundle interior and requiring structural rearrangement to initiate vinculin binding. We detected 8 calpain-mediated cleavages in talin, 2 previously identified in unstructured regions and 6 in α-helical regions in proximity to a VBS. There is evidence in vitro that applying mechanical force across talin enables vinculin binding to the talin rod. However, we found that inhibiting platelet cytoskeletal contraction had no effect on talin cleavage, indicating that talin cleavage by calpain in platelets does not require cytoskeleton-generated tensile force. Therefore, it is likely that calpain acts in the later stages of clot retraction through focal adhesion disassembly.

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.

The C2 domain of calpain 5 contributes to enzyme activation and membrane localization

The enzymatic characteristics of the ubiquitous calpain 5 (CAPN5) remain undescribed despite its high expression in the central nervous system and links to eye development and disease. CAPN5 contains the typical protease core domains but lacks the C terminal penta-EF hand domain of classical calpains, and instead contains a putative C2 domain. This study used the SH-SY5Y neuroblastoma cell line stably transfected with CAPN5-3xFLAG variants to assess the potential roles of the CAPN5 C2 domain in Ca²⁺ regulated enzyme activity and intracellular localization. Calcium dependent autoproteolysis of CAPN5 was documented and characterized. Mutation of the catalytic Cys81 to Ala or addition of EGTA prevented autolysis. Eighty μM Ca²⁺ was sufficient to stimulate half-maximal CAPN5 autolysis in cellular lysates. CAPN5 autolysis was inhibited by tri-leucine peptidyl aldehydes, but less effectively by di-Leu aldehydes, consistent with a more open conformation of the protease core relative to classical calpains. In silico modeling revealed a type II topology C2 domain including loops with the potential to bind calcium. Mutation of the acidic amino acid residues predicted to participate in Ca²⁺ binding, particularly Asp531 and Asp589, resulted in a decrease of CAPN5 membrane association. These residues were also found to be invariant in several genomes. The autolytic fragment of CAPN5 was prevalent in membrane-enriched fractions, but not in cytosolic fractions, suggesting that membrane association facilitates the autoproteolytic activity of CAPN5. Together, these results demonstrate that CAPN5 undergoes Ca²⁺-activated autoproteolytic processing and suggest that CAPN5 association with membranes enhances CAPN5 autolysis.

Loss of lenalidomide-induced megakaryocytic differentiation leads to therapy resistance in del(5q) myelodysplastic syndrome

Interstitial deletion of the long arm of chromosome 5 (del(5q)) is the most common structural genomic variant in myelodysplastic syndromes (MDS)¹. Lenalidomide (LEN) is the treatment of choice for patients with del(5q) MDS, but half of the responding patients become resistant² within 2 years. TP53 mutations are detected in ~20% of LEN-resistant patients³. Here we show that patients who become resistant to LEN harbour recurrent variants of TP53 or RUNX1. LEN upregulated RUNX1 protein and function in a CRBN- and TP53-dependent manner in del(5q) cells, and mutation or downregulation of RUNX1 rendered cells resistant to LEN. LEN induced megakaryocytic differentiation of del(5q) cells followed by cell death that was dependent on calpain activation and CSNK1A1 degradation^4,5. We also identified GATA2 as a LEN-responsive gene that is required for LEN-induced megakaryocyte differentiation. Megakaryocytic gene-promoter analyses suggested that LEN-induced degradation of IKZF1 enables a RUNX1-GATA2 complex to drive megakaryocytic differentiation. Overexpression of GATA2 restored LEN sensitivity in the context of RUNX1 or TP53 mutations by enhancing LEN-induced megakaryocytic differentiation. Screening for mutations that block LEN-induced megakaryocytic differentiation should identify patients who are resistant to LEN.

Procoagulant Phosphatidylserine-Exposing Platelets in vitro and in vivo

The physiological heterogeneity of platelets leads to diverse responses and the formation of discrete subpopulations upon platelet stimulation. Procoagulant platelets are an example of such subpopulations, a key characteristic of which is exposure either of the anionic aminophospholipid phosphatidylserine (PS) or of tissue factor on the activated platelet surface. This review focuses on the former, in which PS exposure on a subpopulation of platelets facilitates assembly of the intrinsic tenase and prothrombinase complexes, thereby accelerating thrombin generation on the activated platelet surface, contributing importantly to the hemostatic process. Mechanisms involved in platelet PS exposure, and accompanying events, induced by physiologically relevant agonists are considered then contrasted with PS exposure resulting from intrinsic pathway-mediated apoptosis in platelets. Pathologies of PS exposure, both inherited and acquired, are described. A consideration of platelet PS exposure as an antithrombotic target concludes the review.

A positive feedback loop involving nuclear factor IB and calpain 1 suppresses glioblastoma cell migration

Glioblastoma (GBM) is a brain tumor that remains largely incurable because of its highly-infiltrative properties. Nuclear factor I (NFI)-type transcription factors regulate genes associated with GBM cell migration and infiltration. We have previously shown that NFI activity depends on the NFI phosphorylation state and that calcineurin phosphatase dephosphorylates and activates NFI. Calcineurin is cleaved and activated by calpain proteases whose activity is, in turn, regulated by an endogenous inhibitor, calpastatin (CAST). The CAST gene is a target of NFI in GBM cells, with differentially phosphorylated NFIs regulating the levels of CAST transcript variants. Here, we uncovered an NFIB-calpain 1-positive feedback loop mediated through CAST and calcineurin. In NFI-hyperphosphorylated GBM cells, NFIB expression decreased the CAST-to-calpain 1 ratio in the cytoplasm. This reduced ratio increased autolysis and activity of cytoplasmic calpain 1. Conversely, in NFI-hypophosphorylated cells, NFIB expression induced differential subcellular compartmentalization of CAST and calpain 1, with CAST localizing primarily to the cytoplasm and calpain 1 to the nucleus. Overall, this altered compartmentalization increased nuclear calpain 1 activity. We also show that nuclear calpain 1, by cleaving and activating calcineurin, induces NFIB dephosphorylation. Of note, knockdown of calpain 1, NFIB, or both increased GBM cell migration and up-regulated the pro-migratory factors fatty acid-binding protein 7 (FABP7) and Ras homolog family member A (RHOA). In summary, our findings reveal bidirectional cross-talk between NFIB and calpain 1 in GBM cells. A physiological consequence of this positive feedback loop appears to be decreased GBM cell migration.

Intercellular Vesicular Transfer by Exosomes, Microparticles and Oncosomes - Implications for Cancer Biology and Treatments

Intercellular communication is a normal feature of most physiological interactions between cells in healthy organisms. While cells communicate directly through intimate physiology contact, other mechanisms of communication exist, such as through the influence of soluble mediators such as growth factors, cytokines and chemokines. There is, however, yet another mechanism of intercellular communication that permits the exchange of information between cells through extracellular vesicles (EVs). EVs are microscopic (50 nm−10 μM) phospholipid bilayer enclosed entities produced by virtually all eukaryotic cells. EVs are abundant in the intracellular space and are present at a cells' normal microenvironment. Irrespective of the EV “donor” cell type, or the mechanism of EV biogenesis and production, or the size and EV composition, cancer cells have the potential to utilize EVs in a manner that enhances their survival. For example, cancer cell EV overproduction confers benefits to tumor growth, and tumor metastasis, compared with neighboring healthy cells. Herein, we summarize the current status of knowledge on different populations of EVs. We review the situations that regulate EV release, and the factors that instruct differential packaging or sorting of EV content. We then highlight the functions of cancer-cell derived EVs as they impact on cancer outcomes, promoting tumor progression, metastases, and the mechanisms by which they facilitate the creation of a pre-metastatic niche. The review finishes by focusing on the beneficial (and challenging) features of tumor-derived EVs that can be adapted and utilized for cancer treatments, including those already being investigated in human clinical trials.

The cAMP/PKA Pathway Inhibits Beta-amyloid Peptide Release from Human Platelets

The main component of Alzheimer's disease (AD) is the amyloid-beta peptide (Aβ), the brain of these patients is characterized by deposits in the parenchyma and cerebral blood vessels known as cerebral amyloid angiopathy (CAA). On the other hand, the platelets are the major source of the Aβ peptide in circulation and once secreted can activate the platelets and endothelial cells producing the secretion of several inflammatory mediators that finally end up unchaining the CAA and later AD. In the present study we demonstrate that cAMP/PKA pathway plays key roles in the regulation of calpain activation and secretion of Aβ in human platelets. We confirmed that inhibition of platelet functionality occurred when platelets were incubated with forskolin (molecule that rapidly increased cAMP levels). In this sense we found that platelets pre-incubated with forskolin (20 μM) present a complete inhibition of calpain activity and this effect is reversed using an inhibitor of protein kinase A. Consequentially, when platelets were inhibited by forskolin a reduction in the processing of the APP with the consequent decrease in the Aβ peptide secretion was observed. Therefore our study provides novel insight in relation to the mechanism of processing and release of the Aβ peptide from human platelets.

Mouse Platelet Ral GTPases Control P-Selectin Surface Expression, Regulating Platelet-Leukocyte Interaction

OBJECTIVE

RalA and RalB GTPases are important regulators of cell growth, cancer metastasis, and granule secretion. The purpose of this study was to determine the role of Ral GTPases in platelets with the use of platelet-specific gene-knockout mouse models.

APPROACH AND RESULTS

This study shows that platelets from double knockout mice, in which both GTPases have been deleted, show markedly diminished (≈85% reduction) P-selectin translocation to the surface membrane, suggesting a critical role in α-granule secretion. Surprisingly, however, there were only minor effects on stimulated release of soluble α- and δ-granule content, with no alteration in granule count, morphology, or content. In addition, their expression was not essential for platelet aggregation or thrombus formation. However, absence of surface P-selectin caused a marked reduction (≈70%) in platelet-leukocyte interactions in blood from RalAB double knockout mice, suggesting a role for platelet Rals in platelet-mediated inflammation.

CONCLUSIONS

Platelet Ral GTPases primarily control P-selectin surface expression, in turn regulating platelet-leukocyte interaction. Ral GTPases could therefore be important novel targets for the selective control of platelet-mediated immune cell recruitment and inflammatory disease.

Twinfilin 2a regulates platelet reactivity and turnover in mice

Regulated reorganization of the actin cytoskeleton is a prerequisite for proper platelet production and function. Consequently, defects in proteins controlling actin dynamics have been associated with platelet disorders in humans and mice. Twinfilin 2a (Twf2a) is a small actin-binding protein that inhibits actin filament assembly by sequestering actin monomers and capping filament barbed ends. Moreover, Twf2a binds heterodimeric capping proteins, but the role of this interaction in cytoskeletal dynamics has remained elusive. Even though Twf2a has pronounced effects on actin dynamics in vitro, only little is known about its function in vivo. Here, we report that constitutive Twf2a-deficient mice (Twf2a^-/-) display mild macrothrombocytopenia due to a markedly accelerated platelet clearance in the spleen. Twf2a^-/- platelets showed enhanced integrin activation and α-granule release in response to stimulation of (hem) immunoreceptor tyrosine-based activation motif (ITAM) and G-protein-coupled receptors, increased adhesion and aggregate formation on collagen I under flow, and accelerated clot retraction and spreading on fibrinogen. In vivo, Twf2a deficiency resulted in shortened tail bleeding times and faster occlusive arterial thrombus formation. The hyperreactivity of Twf2a^-/- platelets was attributed to enhanced actin dynamics, characterized by an increased activity of n-cofilin and profilin 1, leading to a thickened cortical cytoskeleton and hence sustained integrin activation by limiting calpain-mediated integrin inactivation. In summary, our results reveal the first in vivo functions of mammalian Twf2a and demonstrate that Twf2a-controlled actin rearrangements dampen platelet activation responses in a n-cofilin- and profilin 1-dependent manner, thereby indirectly regulating platelet reactivity and half-life in mice.

Comparison of the protein expression of calpain-1, calpain-2, calpastatin and calmodulin between gastric cancer and normal gastric mucosa

The understanding of molecular mechanisms that are involved in the development and the progression of gastric cancer (GC) are of importance for the diagnosis and treatment. The calpain system, which contains the calpains and the endogenous inhibitor, has been suggested as an important factor in the tumorigenesis and migration of colorectal adenocarcinoma, breast and ovarian cancer, and as a prognostic marker for GC. However, the expression level of calpain system proteins in GC and normal-appearing peritumoral gastric mucosa remain unknown. The present study investigated the expression of calpain-1 (CAPN1), calpain-2 (CAPN2), calpastatin and calmodulin (CaM) in GC and uninvolved gastric mucosa tissues with immunohistochemistry. Results demonstrated that CAPN2 protein level increased in GCs compared with normal tissues, while calpastatin and CaM protein level decreased. No evident alterations were observed for CAPN1. Although the protein expression of all these four proteins was not in association with the clinical variables of GC in the present study, higher calpain enzyme activity could be a negative prognostic marker, since calpains are responsible for the generation of active forms of certain proteins that facilitate the progression of cancer. The ratio of (CAPN1 × CAPN2)/(calpastatin × CaM) may serve as a potential index for diagnosis of GC.

Association between type 2 diabetes mellitus, biochemical factors and UCSNP-43 polymorphisms of CALPIN-10 gene in patients with atherosclerosis of coronary artery disease in Southern Iran population

Introduction: Genetic variations in the calpain 10 gene (CALPIN-10), single nucleotide polymorphisms-43 (SNP-43), have increased the risk of type 2 diabete mellitus (T2DM) and coronary artery disease (CAD).

Methods: We studied the control and CAD groups for association of association of SNP-43 in the CALPIN-10 gene with T2DM and other risk factors of its complications. Overall, we examined 452 individuals, 224 patients with CAD and 228 healthy subjects for CAD in Iranian population. All the subjects were genotyped for the CALPIN-10, SNP-43 by polymorphism chain reaction (PCR) and restriction fragment length polymorphism (RFLP) methods, using biochemical methods to detect fasting glucose and other biochemical factors in the blood sample. We assessed frequencies of SNP-43 alleles between CAD and normal population groups.

Results: In CAD patients, the GG allele was significantly associated with T2DM and GG allele was causing high level of glucose. But in control group, there was no relationship between them. Between clinical and biochemical risk factors with different genotypes there was no significant difference in the compared group.

Conclusion: The results of our study suggest no significant association between SNP-43 and the risk of T2DM. In other words, CALPIN-10 did not show a major diabetes gene pool capacity in normal southern Iranian population.

Involvement of calpains in adult neurogenesis: implications for stroke

Calpains are ubiquitous proteases involved in cell proliferation, adhesion and motility. In the brain, calpains have been associated with neuronal damage in both acute and neurodegenerative disorders, but their physiological function in the nervous system remains elusive. During brain ischemia, there is a large increase in the levels of intracellular calcium, leading to the activation of calpains. Inhibition of these proteases has been shown to reduce neuronal death in a variety of stroke models. On the other hand, after stroke, neural stem cells (NSC) increase their proliferation and newly formed neuroblasts migrate towards the site of injury. However, the process of forming new neurons after injury is not efficient and finding ways to improve it may help with recovery after lesion. Understanding the role of calpains in the process of neurogenesis may therefore open a new window for the treatment of stroke. We investigated the involvement of calpains in NSC proliferation and neuroblast migration in two highly neurogenic regions in the mouse brain, the dentate gyrus (DG) and the subventricular zone (SVZ). We used mice that lack calpastatin, the endogenous calpain inhibitor, and calpains were also modulated directly, using calpeptin, a pharmacological calpain inhibitor. Calpastatin deletion impaired both NSC proliferation and neuroblast migration. Calpain inhibition increased NSC proliferation, migration speed and migration distance in cells from the SVZ. Overall, our work suggests that calpains are important for neurogenesis and encourages further research on their neurogenic role. Prospective therapies targeting calpain activity may improve the formation of new neurons following stroke, in addition to affording neuroprotection.

Contribution of calpain to myoglobin efflux from cardiomyocytes during ischaemia and after reperfusion in anaesthetized rats

AIM

Calpain activation has a putative role in ischaemia-reperfusion injury of cardiomyocytes. This study clarified the in vivo contribution of calpain to disruption of cardiomyocyte sarcolemma during ischaemia and after reperfusion in anaesthetized rats.

METHODS

Using a microdialysis technique in the hearts of anaesthetized rats, we investigated the effects of the calpain inhibitors on myocardial interstitial myoglobin level in the ischaemic region during coronary occlusion and after reperfusion. The calpain inhibitors were administered locally via a dialysis probe. Two durations of coronary occlusion were tested.

RESULTS

Thirty-minute coronary occlusion: dialysate myoglobin concentration increased markedly from 385 ± 46 ng mL(-1) at baseline to 3701 ± 527 ng mL(-1) at 20-30 min of occlusion. After reperfusion, dialysate myoglobin concentration further increased, reaching a peak (12 296 ± 1564 ng mL(-1) ) at 10-20 min post-reperfusion and then declined gradually. The calpain inhibitors, MDL-28170 and SNJ-1945 did not change dialysate myoglobin concentration during occlusion but attenuated the increase after reperfusion to 6826 ± 1227 and 8130 ± 938 ng mL(-1) at 10-20 min post-reperfusion (P < 0.05), respectively. Ninety-minute coronary occlusion: dialysate myoglobin concentration increased from 516 ± 33 ng mL(-1) at baseline to 5463 ± 387 ng mL(-1) at 80-90 min after occlusion. After reperfusion, there was no significant increase in dialysate myoglobin concentration. MDL-28170 did not affect dialysate myoglobin concentration during occlusion or after reperfusion.

CONCLUSION

Calpain contributes to sarcolemmal disruption immediately after reperfusion following 30-min coronary occlusion, but has little effects during ischaemia and after reperfusion in 90-min coronary occlusion.

Altered expression of platelet proteins and calpain activity mediate hypoxia-induced prothrombotic phenotype

Oxygen-compromised environments, such as high altitude, air travel, and sports, and pathological conditions, such as solid tumors, have been suggested to be prothrombotic. Despite the indispensable role of platelets in thrombus formation, the studies linking hypoxia, platelet reactivity, and thrombus formation are limited. In the present study, platelet proteome/reactivity was analyzed to elucidate the acute hypoxia-induced prothrombotic phenotype. Rats exposed to acute simulated hypoxia (282 torr/8% oxygen) demonstrated a decreased bleeding propensity and increased platelet reactivity. Proteomic analysis of hypoxic platelets revealed 27 differentially expressed proteins, including those involved in coagulation. Among these proteins, calpain small subunit 1, a 28-kDa regulatory component for calpain function, was significantly upregulated under hypoxic conditions. Moreover, intraplatelet Ca(2+) level and platelet calpain activity were also found to be in accordance with calpain small subunit 1 expression. The inhibition of calpain activity demonstrated reversal of hypoxia-induced platelet hyperreactivity. The prothrombotic role for calpain was further confirmed by an in vivo model of hypoxia-induced thrombosis. Interestingly, patients who developed thrombosis while at extreme altitude had elevated plasma calpain activities and increased soluble P-selectin level. In summary, this study suggests that augmented calpain activity is associated with increased incidence of thrombosis under hypoxic environments.

Calpain-1 inhibitors for selective treatment of rheumatoid arthritis: what is the future?

Effective small-molecule treatment of inflammatory diseases remains an unmet need in medicine. Current treatments are either limited in effectiveness or invasive. The latest biologics prevent influx of inflammatory cells to damaged tissue. Calpain-1 is a calcium-activated cysteine protease that plays an important role in neutrophil motility. It is, therefore, a potential target for intervention in inflammatory disease. Many inhibitors of calpains have been developed but most are unselective and so unsuitable for drug use. However, recent series of α-mercaptoacrylate inhibitors target regulatory domains of calpain-1 and are much more specific. These compounds are effective in impairing the cell spreading mechanism of neutrophils in vitro and raise the possibility of treating rheumatoid arthritis with a pill; however, challenges still remain. Improved bioavailability is needed and solution of their precise mode of action should prompt the development of specific calpain-1 screens for novel classes of inhibitors.

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.

Ca²⁺ and calpain control membrane expansion during the rapid cell spreading of neutrophils

Following adherence of neutrophils to the endothelium, neutrophils undergo a major morphological change that is a necessary prelude to their extravasation. We show here that this shape change is triggered by an elevation of cytosolic inositol (1,4,5)-trisphosphate (IP3), to provoke physiological Ca(2+) influx through a store-operated mechanism. This transition from a spherical to 'flattened' neutrophil morphology is rapid (∼100 seconds) and is accompanied by an apparent rapid expansion of the area of the plasma membrane. However, no new membrane is added into the plasma membrane. Pharmacological inhibition of calpain-activation, which is triggered by Ca(2+) influx during neutrophil spreading, prevents normal cell flattening. In calpain-suppressed cells, an aberrant form of cell spreading can occur where an uncoordinated and localised expansion of the plasma membrane is evident. These data show that rapid neutrophil spreading is triggered by Ca(2+) influx, which causes activation of calpain and release of furled plasma membrane to allow its apparent 'expansion'.

Sitagliptin: anti-platelet effect in diabetes and healthy volunteers

Sitagliptin, a selective dipeptidyl peptidase-4 inhibitor drug is used to treat type-2 diabetes (T2DM). We investigated the anti-platelet activity of sitagliptin in patients with T2DM and in in vitro samples obtained from healthy humans. Patients with T2DM (27 male + 23 female) were selected and followed up before (control) and after treatment with sitagliptin for up to 3 months. Platelets were isolated from the blood of sitagliptin treated patients and controls. Patients with T2DM treated with sitagliptin for 1and 3 months, showed 10 ± 2% and 30 ± 5% inhibition of platelet aggregation, respectively. For the in vitro study, platelets from 10 normal humans (n = 10) were isolated. Platelet aggregation, intracellular free calcium and tyrosine phosphorylation of multiple proteins were measured by aggregometer, spectrofluorometer and western blotting, respectively. Platelets pre-treated with 5 and 10 µg/ml of sitagliptin, showed 25 ± 4% and 40 ± 6% inhibition of thrombin-induced platelet aggregation, respectively. Sitagliptin decreased intracellular free calcium (2.5-fold) and tyrosine phosphorylation of multiple proteins in thrombin-induced platelet activation. Sitagliptin inhibited platelet aggregation in T2DM as well as in healthy humans. Sitagliptin has significant concentration-dependent anti-platelet activity. This activity was due to its inhibitory effect on intracellular free calcium and tyrosine phosphorylation.

Targeted gene inactivation reveals a functional role of calpain-1 in platelet spreading

BACKGROUND

Calpains are implicated in a wide range of cellular functions including the maintenance of hemostasis via the regulation of cytoskeletal modifications in platelets.

OBJECTIVES

Determine the functional role of calpain isoforms in platelet spreading.

METHODS AND RESULTS

Platelets from calpain-1(-/-) mice show enhanced spreading on collagen- and fibrinogen-coated surfaces as revealed by immunofluorescence, differential interference contrast (DIC) and scanning electron microscopy. The treatment of mouse platelets with MDL, a cell permeable inhibitor of calpains 1/2, resulted in increased spreading. The PTP1B-mediated enhanced tyrosine dephosphorylation in calpain-1(-/-) platelets did not fully account for the enhanced spreading as platelets from the double knockout mice lacking calpain-1 and PTP1B showed only a partial rescue of the spreading phenotype. In non-adherent platelets, proteolysis and GTPase activity of RhoA and Rac1 were indistinguishable between the wild-type (WT) and calpain-1(-/-) platelets. In contrast, the ECM-adherent calpain-1(-/-) platelets showed higher Rac1 activity at the beginning of spreading, whereas RhoA was more active at later time points. The ECM-adherent calpain-1(-/-) platelets showed an elevated level of RhoA protein but not Rac1 and Cdc42. Proteolysis of recombinant RhoA, but not Rac1 and Cdc42, indicates that RhoA is a calpain-1 substrate in vitro.

CONCLUSIONS

Potentiation of the platelet spreading phenotype in calpain-1(-/-) mice suggests a novel role of calpain-1 in hemostasis, and may explain the normal bleeding time observed in the calpain-1(-/-) mice.

Two-wavelength near-infrared fluorescence for the quantitation of drug antiplatelet effects in large animal model systems

OBJECTIVE

Intraoperative imaging of intravascular thrombi is limited by the inability of visible light to penetrate thick-walled vessels. Near-infrared (NIR) light has relatively high tissue penetration and low autofluorescence and scatter, offering significant advantages. We hypothesized that the development of 700-nm NIR fluorophores for platelet labeling, in conjunction with existing 800-nm NIR fluorophores, would permit simultaneous and separable quantitation of intravascular thrombi and measurement of the antiplatelet effect of drugs.

METHODS

We synthesized a series of lipophilic, cationic, polymethine indocyanine dyes (MHI-86, 94, 106, and 114) that emit at approximately 700 nm. Platelet uptake was optimized in vitro and the bioactivity and blood half-life of labeled platelets was characterized in vitro and in vivo. FeCl(3)-induced injury of the femoral arteries and intravascular thrombus formation was performed in 35-kg Yorkshire pigs. A combination of 700-nm and 800-nm NIR fluorophore-labeled platelets was used in conjunction with the fluorescence-assisted resection and exploration imaging system to image and quantify the antiplatelet effect of cilostazol and acetylsalicylic acid.

RESULTS

MHI-114 was incorporated at nearly 4.1 × 10(6) molecules per platelet without affecting platelet function. When infused into pigs, the signal-to-background ratio of MHI-114-labeled platelets exhibited a blood half-life of 16.4 ± 2.2 (mean ± SEM; n = 3) minute and generated a signal-to-background ratio of 2.5 ± 0.5 (mean ± SEM; n = 3) at the site of thrombi. Using dual-NIR-labeled platelet populations, cilostazol and acetylsalicylic acid were found to cause a reduction in platelet incorporation into thrombi of 51 ± 2% and 10 ± 1% (mean ± SEM; n = 3), respectively, relative to vehicle-only treated control thrombi.

CONCLUSIONS

New platelet-avid 700-nm NIR fluorophores permit simultaneous two-wavelength NIR fluorescence imaging and quantitation of intravascular thrombi in intact vessels approaching the size of humans and can be used to study the antiplatelet effect of drugs.

Calpain mediates epithelial cell microvillar effacement by enterohemorrhagic Escherichia coli

A member of the attaching and effacing (AE) family of pathogens, enterohemorrhagic Escherichia coli (EHEC) induces dramatic changes to the intestinal cell cytoskeleton, including effacement of microvilli. Effacement by the related pathogen enteropathogenic E. coli (EPEC) requires the activity of the Ca⁺²-dependent host protease, calpain, which participates in a variety of cellular processes, including cell adhesion and motility. We found that EHEC infection results in an increase in epithelial (CaCo-2a) cell calpain activity and that EHEC-induced microvillar effacement was blocked by ectopic expression of calpastatin, an endogenous calpain inhibitor, or by pretreatment of intestinal cells with a cell-penetrating version of calpastatin. In addition, ezrin, a known calpain substrate that links the plasma membrane to axial actin filaments in microvilli, was cleaved in a calpain-dependent manner during EHEC infection and lost from its normal locale within microvilli. Calpain may be a central conduit through which EHEC and other AE pathogens induce enterocyte cytoskeletal remodeling and exert their pathogenic effects.

Calpain activator dibucaine induces platelet apoptosis

Calcium-dependent calpains are a family of cysteine proteases that have been demonstrated to play key roles in both platelet glycoprotein Ibα shedding and platelet activation and altered calpain activity is associated with thrombotic thrombocytopenic purpura. Calpain activators induce apoptosis in several types of nucleated cells. However, it is not clear whether calpain activators induce platelet apoptosis. Here we show that the calpain activator dibucaine induced several platelet apoptotic events including depolarization of the mitochondrial inner transmembrane potential, up-regulation of Bax and Bak, down-regulation of Bcl-2 and Bcl-X_L, caspase-3 activation and phosphatidylserine exposure. Platelet apoptosis elicited by dibucaine was not affected by the broad spectrum metalloproteinase inhibitor GM6001. Furthermore, dibucaine did not induce platelet activation as detected by P-selectin expression and PAC-1 binding. However, platelet aggregation induced by ristocetin or α-thrombin, platelet adhesion and spreading on von Willebrand factor were significantly inhibited in platelets treated with dibucaine. Taken together, these data indicate that dibucaine induces platelet apoptosis and platelet dysfunction.

Calpain- and talin-dependent control of microvascular pericyte contractility and cellular stiffness

Pericytes surround capillary endothelial cells and exert contractile forces modulating microvascular tone and endothelial growth. We previously described pericyte contractile phenotype to be Rho GTPase- and α-smooth muscle actin (αSMA)-dependent. However, mechanisms mediating adhesion-dependent shape changes and contractile force transduction remain largely equivocal. We now report that the neutral cysteine protease, calpain, modulates pericyte contractility and cellular stiffness via talin, an integrin-binding and F-actin associating protein. Digital imaging and quantitative analyses of living cells reveal significant perturbations in contractile force transduction detected via deformation of silicone substrata, as well as perturbations of mechanical stiffness in cellular contractile subdomains quantified via atomic force microscope (AFM)-enabled nanoindentation. Pericytes overexpressing GFP-tagged talin show significantly enhanced contractility (~two-fold), which is mitigated when either the calpain-cleavage resistant mutant talin L432G or vinculin are expressed. Moreover, the cell-penetrating, calpain-specific inhibitor termed CALPASTAT reverses talin-enhanced, but not Rho GTP-dependent, contractility. Interestingly, our analysis revealed that CALPASTAT, but not its inactive mutant, alters contractile cell-driven substrata deformations while increasing mechanical stiffness of subcellular contractile regions of these pericytes. Altogether, our results reveal that calpain-dependent cleavage of talin modulates cell contractile dynamics, which in pericytes may prove instrumental in controlling normal capillary function or microvascular pathophysiology.

Global analysis of the rat and human platelet proteome - the molecular blueprint for illustrating multi-functional platelets and cross-species function evolution

Emerging evidences indicate that blood platelets function in multiple biological processes including immune response, bone metastasis and liver regeneration in addition to their known roles in hemostasis and thrombosis. Global elucidation of platelet proteome will provide the molecular base of these platelet functions. Here, we set up a high-throughput platform for maximum exploration of the rat/human platelet proteome using integrated proteomic technologies, and then applied to identify the largest number of the proteins expressed in both rat and human platelets. After stringent statistical filtration, a total of 837 unique proteins matched with at least two unique peptides were precisely identified, making it the first comprehensive protein database so far for rat platelets. Meanwhile, quantitative analyses of the thrombin-stimulated platelets offered great insights into the biological functions of platelet proteins and therefore confirmed our global profiling data. A comparative proteomic analysis between rat and human platelets was also conducted, which revealed not only a significant similarity, but also an across-species evolutionary link that the orthologous proteins representing "core proteome", and the "evolutionary proteome" is actually a relatively static proteome.

Integrin activation by Fam38A uses a novel mechanism of R-Ras targeting to the endoplasmic reticulum

The integrin family of heterodimeric cell-surface receptors are fundamental in cell-cell and cell-matrix adhesion. Changes to either integrin-ligand affinity or integrin gene expression are central to a variety of disease processes, including inflammation, cardiovascular disease and cancer. In screening for novel activators of integrin-ligand affinity we identified the previously uncharacterised multi-transmembrane domain protein Fam38A, located at the endoplasmic reticulum (ER). siRNA knockdown of Fam38A in epithelial cells inactivates endogenous beta1 integrin, reducing cell adhesion. Fam38A mediates integrin activation by recruiting the small GTPase R-Ras to the ER, which activates the calcium-activated protease calpain by increasing Ca(2+) release from cytoplasmic stores. Fam38A-induced integrin activation is blocked by inhibition of either R-Ras or calpain activity, or by siRNA knockdown of talin, a well-described calpain substrate. This highlights a novel mechanism for integrin activation by Fam38A, utilising calpain and R-Ras signalling from the ER. These data represent the first description of a novel spatial regulator of R-Ras, of an alternative integrin activation-suppression pathway based on direct relocalisation of R-Ras to the ER, and of a mechanism linking R-Ras and calpain signalling from the ER with modulation of integrin-ligand affinity.

The role of calpain in the regulation of ADAM17-dependent GPIbalpha ectodomain shedding

There are evidence that both a disintegrin and metalloproteinase 17 (ADAM17) and calpain are involved in platelet glycoprotein (GP)Ibalpha ectodomain cleavage. However, the relationship between the two enzymes in the shedding process remains unclear. Here we show that calcium ionophore A23187- and alpha-thrombin-induced GPIbalpha shedding is completely inhibited by the metalloproteinase inhibitor GM6001, whereas it is only partially inhibited by calpain inhibitors. Calpain activator dibucaine-induced GPIbalpha shedding was completely inhibited by both metalloproteinase and calpain inhibitors. On the other hand, calpain inhibitors did not inhibit GPIbalpha shedding induced by the reagents that specifically activate ADAM17. Furthermore, A23187-induced GPIbalpha shedding in Chinese hamster ovary cells expressing wild-type or mutant GPIb-IX was also partially inhibited by calpain inhibitors and almost completely inhibited by GM6001. Therefore, these data indicate that calpain plays an important role in the regulation of ADAM17-dependent GPIbalpha ectodomain shedding in both platelets and nucleated cells.

Redox control of platelet function

There has recently been a dramatic expansion in research in the area of redox biology with systems that utilize thiols to perform redox chemistry being central to redox control. Thiol-based reactions occur in proteins involved in platelet function, including extracellular platelet proteins. The alphaIIbbeta3 fibrinogen receptor contains free thiols that are required for the activation of this receptor to a fibrinogen-binding conformation. This process is under enzymatic control, with protein disulfide isomerase playing a central role in the activation of alphaIIbbeta3. Other integrins, such as the alpha2beta1 collagen receptor on platelets, are also regulated by protein disulfide isomerase and thiol metabolism. Low molecular weight thiols that are found in blood regulate these processes by converting redox sensitive disulfide bonds to thiols and by providing the appropriate redox potential for these reactions. Additional mechanisms of redox control of platelets involve nitric oxide that inhibits platelet responses, and reactive oxygen species that potentiate platelet thrombus formation. Specific nitrosative or oxidative modifications of thiol groups in platelets may modulate platelet function. Since many biologic processes are regulated by redox reactions that involve surface thiols, the extracellular redox state can have an important influence on health and disease status and may be a target for therapeutic intervention.

Replication of calpain-10 genetic association with carotid intima-media thickness

OBJECTIVE

Diabetes and atherosclerosis may share common genetic determinants. A prior study in Hispanics found association of haplotypes in the diabetes gene calpain-10 (CAPN10) with carotid artery intima-media thickness (CIMT). This study sought to replicate this association in an independent cohort.

METHODS

Four CAPN10 SNPs were genotyped and haplotypes determined in 487 Hispanic Americans from 143 families ascertained via an index case with hypertension. CIMT was measured from B-mode ultrasound, and glycemic traits quantified from euglycemic clamps. Association of SNPs and haplotypes with CIMT was determined.

RESULTS

The minor alleles of SNP-56 and SNP-63 were associated with increased CIMT in dominant and additive models. The association of haplotype 1112 with increased CIMT was replicated. No associations with fasting insulin, insulin secretion, or insulin sensitivity were observed.

CONCLUSIONS

CAPN10 association with CIMT was replicated, further supporting its role as a common genetic determinant of diabetes and atherosclerosis in Hispanics.

Phosphatidylserine surface expression and integrin alpha IIb beta 3 activity on thrombin/convulxin stimulated platelets/particles of different sizes

Platelets stimulated by a combination of thrombin/convulxin have been shown to develop two to three populations characterized by different phosphatidylserine (PS) surface expression and integrin alpha IIb beta 3 activity. To determine how these markers are distributed on the surface of platelets/particles, we studied Annexin V and PAC-1 binding to platelets/particles of different sizes by flow cytometry analysis and evaluated influences of calpain and caspase inhibitors on thrombin/convulxin-activated platelets. Analysed platelets/particles were divided by their sizes, according to the standard size beads, into seven populations from 0.37 to 4.8 microm. PAC-1 binding/microm(2) was almost equal in platelets/particles ranging from 1.2 to 4.8 microm and was significantly lower on smaller-sized particles sizes (0.37-0.7 microm). PS surface exposure/microm(2) was high in the particles of 0.37-1.2 microm and very low in platelets (2.6-4.8 microm). Upon thrombin/convulxin stimulation caspase inhibitors prevented microparticle (MP) formation, while a calpain inhibitor stimulated MP formation. It was also shown that stimulated platelets are heterogeneous not only in their ability to activate alpha IIb beta 3 integrin complex and expose PS on their surface, but also in the distribution of activation markers, which strongly depends on platelet/particle size and that platelets/particles of different sizes provide different responses to the same stimulus.

Thiosulfinates modulate platelet activation by reaction with surface free sulfhydryls and internal thiol-containing proteins

Thiosulfinates are characteristic flavors of Allium vegetables, with a highly reactive S-S=O group, that we previously showed to inhibit platelet aggregation through calpain-dependent mechanisms. With the aim to clarify the mode of action of these redox phytochemicals, we studied their effect on extracellular free sulfhydryls in relation to their effect on platelet responses (Ca2+ signals, release reaction, and aIIb3 integrin activation state). At the platelet surface, thiosulfinate dose-dependently increased the basal level of free sulfhydryls, independently of protein disulfide isomerase activity. This generation of new free sulfhydryls was associated with: (i) a three fold increase in labeling of resting platelets with an anti ligand-induced binding site antibody and (ii) marked inhibition of subsequent aIIb3 activation by agonists. Thiosulfinates increased the basal intracellular Ca2+ level of platelets. In activated platelets, they markedly inhibited the Ca2+ mobilization independently of the external Ca2+, the calpain-induced SNAP-23 cleavage and the granule release. In platelet free systems, thiosulfinates inhibited the activity of purified calpain and the free sulfhydryl of glutathione without any reducing properties on disulfides. The results demonstrate for the first time that thiosulfinates rapidly interact with sulfhydryls both at the platelet surface and inside the cell on intracellular cysteine-proteins, especially calpain. Inhibition of free cysteine and glutathione in whole blood may also contribute to their anti-aggregant properties. Such sulfur compounds are of interest for the development of a new class of antithrombotic agents.

Regulation of plasma membrane Ca2+-ATPase in human platelets by calpain

The plasma membrane Ca(2+)-ATPase (PMCA) plays an essential role in maintaining low cytosolic Ca(2+) in resting human platelets by extruding Ca(2+) from the cytoplasm across the plasma membrane. Since PMCA is the main agent of Ca(2+) efflux in platelets, it is a key point for regulation of platelet Ca(2+) metabolism. PMCA has been shown to be an excellent substrate for the Ca(2+)-activated cysteine protease calpain, a major platelet protein that is turned on during platelet activation. The objectives of the present work were to determine if PMCA is degraded during thrombin- and collagen-mediated platelet activation, and if calpain is responsible. The kinetics of PMCA degradation during platelet activation were analysed using SDS polyacrylamide gel electrophoresis and immunoblotting. The role of calpain was tested using the calpain inhibitors calpeptin and ALLN. Platelet activation mediated by both collagen and thrombin resulted in degradation of 60% of platelet PMCA within 18 minutes. Calpeptin and ALLN significantly inhibited the rate and extent of PMCA degradation. We conclude that calpain-mediated degradation of PMCA during platelet activation likely contributes significantly to Ca(2+) regulation and, therefore, to platelet function.

Coordinated control of both insulin secretion and insulin action through calpain-10-mediated regulation of exocytosis?

Calpain-10 was first identified through a genome scan seeking to identify diabetes predisposition genes. Both genetic and functional data has since indicated that calpain-10 has an important role in insulin resistance and intermediate phenotypes, including those associated with adipocytes and skeletal muscle. Evidence presented in this issue by Brown, Yeaman, and Walker utilizes siRNA technology to specifically knock down calpain-10 expression, and suggests that calpain-10 facilitates GLUT4 translocation through effects on the distal secretory pathway. Calpain-10 is also an important molecule in the pancreatic beta-cell, where it has been shown to regulate exocytosis through partial proteolysis of a member of the secretory granule fusion machinery. In addition, calpain-10 has also been implicated in reorganization of the actin cytoskeleton that accompanies both GLUT4 vesicle translocation and insulin secretion. Taken together, these findings provide fresh hope for the development of novel diabetic treatments, utilizing either pharmacological activators that specifically target calpain-10, or through targeted calpain-10 gene therapy. Therapeutic intervention in this way could simultaneously enhance both insulin secretion and insulin action.

Emerging functions of the calpain superfamily of cysteine proteases in neuroendocrine secretory pathways

The first calpain protease was discovered over 40 years ago now, yet despite the vast amount of literature that has subsequently emerged detailing their involvement in the pathophysiology of a variety of human diseases, it is only in the last decade that calpain-mediated actions along the secretory pathway have begun to emerge. However, the number of secretory pathway substrates identified and their diversity of function continues to grow. This review summarizes our current knowledge of calpain-mediated mechanisms of action that are pertinent to synaptic vesicle assembly and budding, cytoskeletal organization, endosomal recycling, and exocytotic membrane fusion.

Hyperglycemia is a major determinant of albumin permeability in diabetic microcirculation: the role of mu-calpain

Increased permeability to albumin is a well-known feature of diabetic microvasculature and a negative prognostic factor of vascular complications. The mechanisms responsible for loss of the physiological albumin barrier in diabetic organs remain only partially understood. We have recently demonstrated that the protease mu-calpain is activated in hyperglycemia, which causes endothelial dysfunction and vascular inflammation. In the present study, we investigated whether mu-calpain is involved in the hyperpermeability of the diabetic vasculature. We also investigated the mechanistic roles of hyperglycemia and leukocyte adhesion in this process. Albumin permeability in the intact microcirculation of the Zucker diabetic fatty (ZDF) rat was quantified by intravital microscopy. Extravasation of albumin in the microcirculation of ZDF rats was significantly increased when compared with nondiabetic Zucker lean (ZL) rats. Microvascular albumin leakage was prevented by either antisense depletion of mu-calpain or pharmacological inhibition of calpain in vivo. Calpain inhibition also attenuated urinary albumin excretion in ZDF rats. Glucose concentrations in the range of those found in the blood of ZDF rats increased albumin permeability in nondiabetic ZL rats. Thus, this demonstrates a mechanistic role for hyperglycemia in the hypermeability of diabetes. Depletion of polymorphonuclear leukocytes in vivo failed to prevent glucose-induced hypermeability, which suggests that hyperglycemia can disrupt the physiological endothelial cell barrier of the microcirculation, even in the absence of increased overt leukocyte-endothelium interactions.

Double knockouts reveal that protein tyrosine phosphatase 1B is a physiological target of calpain-1 in platelets

Calpains are ubiquitous calcium-regulated cysteine proteases that have been implicated in cytoskeletal organization, cell proliferation, apoptosis, cell motility, and hemostasis. Gene targeting was used to evaluate the physiological function of mouse calpain-1 and establish that its inactivation results in reduced platelet aggregation and clot retraction potentially by causing dephosphorylation of platelet proteins. Here, we report that calpain-1 null (Capn1-/-) platelets accumulate protein tyrosine phosphatase 1B (PTP1B), which correlates with enhanced tyrosine phosphatase activity and dephosphorylation of multiple substrates. Treatment of Capn1-/- platelets with bis(N,N-dimethylhydroxamido)hydroxooxovanadate, an inhibitor of tyrosine phosphatases, corrected the aggregation defect and recovered impaired clot retraction. More importantly, platelet aggregation, clot retraction, and tyrosine dephosphorylation defects were rescued in the double knockout mice lacking both calpain-1 and PTP1B. Further evaluation of mutant mice by the ferric chloride-induced arterial injury model suggests that the Capn1-/- mice are relatively resistant to thrombosis in vivo. Together, our results demonstrate that PTP1B is a physiological target of calpain-1 and suggest that a similar mechanism may regulate calpain-1-mediated tyrosine dephosphorylation in other cells.