Cardiac high molecular weight calmodulin binding protein contains calpastatin activity

Calmodulin-binding proteins: A journey of 40 years

The proteins which bind to calmodulin in a Ca²⁺-dependent and reversible manner are known as calmodulin-binding proteins. These proteins are involved in a multitude of processes in which Ca²⁺ and calmodulin play crucial roles. Our group elucidated the mechanism and importance of these proteins in normal and diseased conditions. Various calmodulin-binding proteins were discovered and purified from bovine tissue including a heat stable calmodulin-binding protein 70, calmodulin-dependent protein kinase VI and a high molecular weight calmodulin-binding protein (HMWCaMBP). We observed a complex interplay occurs between these and other Ca²⁺ and calmodulin-binding proteins during cardiac ischemia and reperfusion. Purified cardiac HMWCaMBP is a homolog form of calpastatin and an inhibitor of the Ca²⁺-activated cysteine proteases, calpains and therefore can have cardioprotective role in ischemic conditions. Calcineurin is a Ca²⁺ and calmodulin-dependent serine/threonine protein phosphatase showed increased phosphatase activity in ischemic heart through its direct interaction with Hsp70 and expression of calcineurin following ischemia suggests self-repair and favorable survival outcomes. Calcineurin was also found to be present in other tissues including the eye; where its expression and calcineurin phosphatase activity varied. In neurons, calcineurin may play a key role in initiating apoptosis-related pathways especially in epilepsy. In colorectal cancer we demonstrated high calcineurin phosphatase activity and simultaneous overexpression of calcineurin. The impact of calcineurin signaling on neuronal apoptosis in epilepsy and its use as a diagnostic marker for colorectal cancer requires in-depth study.

Expression of calcineurin, calpastatin and heat shock proteins during ischemia and reperfusion

Objective

Calcineurin (CaN) interacts with calpains (Calpn) and causes cellular damage eventually leading to cell death. Calpastatin (Calp) is a specific Calpn inhibitor, along with CaN stimulation has been implicated in reduced cell death and self-repair. Molecular chaperones, heat shock proteins (Hsp70 and Hsp90) acts as regulators in Calpn signaling. This study aims to elucidate the role of CaN, Calp and Hsps during induced ischemia and reperfusion in primary cardiomyocyte cultures (murine).

Methods and results

Protein expression was analyzed concurrently with viability using flow cytometry (FACS) in ischemia- and reperfusion-induced murine cardiomyocyte cultures. The expression of Hsp70 and Hsp90, both being molecular chaperones, increased during ischemia with a concurrent increase in death of cells expressing these proteins. The relative expression of Hsp70 and Hsp90 during ischemia with respect to CaN was enhanced in comparison to Calp. Reperfusion slightly decreased the number of cells expressing these chaperones. There was no increase in death of cells co-expressing Hsp70 and Hsp90 along with CaN and Calp. CaN expression peaked during ischemia and subsequent reperfusion reduced its expression and cell death. Calp expression increased both during ischemia and subsequent reperfusion but cell death decreased during reperfusion.

Conclusion

The present study adds to the existing knowledge that Hsp70, Hsp90, CaN and Calp interact with each other and play significant role in cardio protection.

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Differential expression of calcineurin and calpastatin during ischemia and reperfusion.

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Enhanced ischemia induced cell death in cells expressing Hsp70 and Hsp90.

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Cardio protective role of calcineurin, calpastatin, Hsp70 and Hsp90.

Ischemia and reperfusion induce differential expression of calpastatin and its homologue high molecular weight calmodulin-binding protein in murine cardiomyocytes

In the heart, calpastatin (Calp) and its homologue high molecular weight calmodulin-binding protein (HMWCaMBP) regulate calpains (Calpn) by inhibition. A rise in intracellular myocardial Ca²⁺ during cardiac ischemia activates Calpn thereby causing damage to myocardial proteins, which leads to myocyte death and consequently to loss of myocardial structure and function. The present study aims to elucidate expression of Calp and HMWCaMBP with respect to Calpn during induced ischemia and reperfusion in primary murine cardiomyocyte cultures. Ischemia and subsequently reperfusion was induced in ∼80% confluent cultures of neonatal murine cardiomyocytes (NMCC). Flow cytometric analysis (FACS) has been used for analyzing protein expression concurrently with viability. Confocal fluorescent microscopy was used to observe protein localization. We observed that ischemia induces increased expression of Calp, HMWCaMBP and Calpn. Calpn expressing NMCC on co-expressing Calp survived ischemic induction compared to NMCC co-expressing HMWCaMBP. Similarly, living cells expressed Calp in contrast to dead cells which expressed HMWCaMBP following reperfusion. A significant difference in the expression of Calp and its homologue HMWCaMBP was observed in localization studies during ischemia. The current study adds to the existing knowledge that HMWCaMBP could be a putative isoform of Calp. NMCC on co-expressing Calp and Calpn-1 survived ischemic and reperfusion inductions compared to NMCC co-expressing HMWCaMBP and Calpn-1. A significant difference in expression of Calp and HMWCaMBP was observed in localization studies during ischemia.

Altered expression of calcineurin, calpain, calpastatin and HMWCaMBP in cardiac cells following ischemia and reperfusion

A rise in intracellular myocardial Ca(2+) during cardiac ischemia activates calpain (Calpn) thereby causing damage to myocardial proteins, which leads to myocyte death and consequently to loss of myocardial structure and function. Calcineurin (CaN) interacts with Calpn and causes cellular damage eventually leading to cell death. Calpastatin (Calp) and high molecular weight calmodulin-binding protein (HMWCaMBP) (homolog of Calp), inhibit Calpn activity and thus prevent cell death. CaN stimulation can also result in self-repair of damaged cardiomyocytes. The present study attempts to elucidate the expression of these proteins in cells under pre-ischemic condition (control), following ischemia induction and also reperfusion subsequent to ischemia. For the first time, flow cytometric analysis (FACS) has been used for analyzing protein expression concurrently with viability. We induced ischemia and subsequently reperfusion in 80% confluent cultures of neonatal murine cardiomyocytes (NMCC). Viability following induction was assessed with 7-AAD staining and the cells were simultaneously checked for protein expression by FACS. We observed that ischemia induction results in increased expression of CaN, Calp and Calpn. HMWCaMBP expression was reduced in live cells following ischemia which suggests that there is a poor survival outcome of cells expressing HMWCaMBP thereby making it a potential biomarker for such cells. Most live cells following ischemia expressed CaN pointing towards self-repair and favorable survival outcomes.

Potential role of high molecular weight calmodulin-binding protein in cardiac injury

Ca(2+) is a major determinant of many biochemical processes in various cell types and is a critical second messenger in cell signalling. High molecular weight calmodulin-binding protein (HMWCaMBP) was originally discovered and purified in the authors' laboratory. It was identified as a homologue of calpastatin - an inhibitor of Ca(2+)-activated cysteine proteases (calpains). Decreased expression of HMWCaMBP in ischemia suggests that it is proteolyzed by calpains during ischemia and reperfusion. In normal myocardial muscle, HMWCaMBP may protect its substrate from calpains, but during an early stage of ischemia/reperfusion with increased Ca(2+) influx, calpain activity exceeds HMWCaMBP activity, leading to proteolysis of HMWCaMBP and other protein substrates, resulting in cellular damage. The role of HMWCaMBP in ischemia/reperfusion is yet to be explored. The present review summarizes developments from the authors' laboratory in the area of HMWCaMBP.

Overexpression of m-Calpain in Human Colorectal Adenocarcinomas

Background: Calpains represent a well-conserved family of Ca2+-dependent proteolytic enzymes. Recently, the importance of calpain in the metastatic process has received great attention. To investigate whether m-calpain contributes to the pathogenesis of colorectal cancer, we investigated the expression of m-calpain and its inhibitors, calpastatin and high-molecular-weight calmodulin-binding protein (HMWCaMBP), in human colorectal surgical specimens. Methods: Fifty cases of colon carcinoma were evaluated for this study. Of 50 cases evaluated, we presented in this report six cases for m-calpain, calpastatin and HMWCaMBP protein expression by Western blot analyses was done in both normal and invasive tumor components of human samples. In addition, immunohistochemistry analysis was also carried out in all patients. Results: The activity and protein expression of m-calpain was significantly higher in colorectal adenocarcinoma than in normal colonic mucosa. This finding was corroborated by immunohistochemical studies that showed strong cytoplasmic staining in the colon tumors with m-calpain antibody. The decreased expression of these calpain inhibitors (calpastatin and HMWCaMBP) paralleled increased activity and expression of calpain in colorectal adenocarcinoma and the well-documented involvement of this Ca2+-dependent protease in colon tumor. Conclusion: Increased activity and moderate staining of m-calpain in polyps show the usage of this enzyme as a marker for the early detection of colorectal adenocarcinoma using immunologic approaches. These findings represent the first description of calpain overexpression in colorectal cancer. This has implications with regard to the design of chemotherapeutic drugs as well as in monitoring colorectal cancer in early stages of the metastatic process.

Calpastatin expression in porcine cardiac and skeletal muscle and partial gene structure

The expression in porcine skeletal and cardiac muscle of calpastatin, the specific endogenous inhibitor of the calpain proteolytic system, was examined 16 h after a single dose of a specific beta(2)-agonist. Immunoblotting of extracts indicated that treatment increased skeletal calpastatin 135-kDa band intensity (P < 0.01), while in cardiac combined 145- and 135-kDa band intensity decreased (P < 0.05). Treatment increased skeletal (P < 0.01) but not cardiac calpastatin mRNA steady-state levels. Three types of cardiac calpastatin mRNA transcripts were identified by 5'-RACE. Types I and II encoded a putative XL region that originated either from exon 1x(A) or exon 1x(B), arranged in tandem. Type III predominated in skeletal muscle and originated from exon 1u, which was located 40-50 kb 3' to exons 1x(A) and 1x(B). The region 5' to exon 1u may act as an independent promoter regulated by a cAMP-dependent mechanisms, thereby explaining the differential response of calpastatin to adrenergic stimulation in cardiac and skeletal muscle.

Decreased expression of high-molecular-weight calmodulin-binding protein and its correlation with apoptosis in ischemia-reperfused rat heart

A cardiac high-molecular-weight calmodulin-binding protein (HMWCaMBP) was previously identified as a homologue of the calpain inhibitor, calpastatin. In the present study, we investigated the expression of HMWCaMBP and calpains in rat heart after ischemia and reperfusion. Western blot analysis of normal rat heart extract with a polyclonal antibody raised against bovine HMWCaMBP indicated a prominent immunoreactive band of 140kDa. Both the expression and the activity of HMWCaMBP were decreased by ischemia reperfusion. Immunohistochemical studies showed strong-to-moderate HMWCaMBP immunoreactivity in normal heart and poor immunoreactivity in ischemia-reperfused heart muscle. However, the expression of micro-calpain and m-calpain in ischemia-reperfused heart was increased as compared to normal heart. The calpain inhibitory activity of ischemia-reperfused heart tissues was significantly lower as compared to normal heart tissues. The pre-ischemic and post-ischemic perfusion of hearts with a cell-permeable calpain inhibitor suppressed the increase in calpain expression but increased the HMWCaMBP expression. In-vitro HMWCaMBP was proteolyzed by micro-calpain and m-calpain. We also measured apoptosis in normal and ischemia-reperfused tissues. An increase in the number of apoptotic bodies was observed with increased duration of ischemia and reperfusion. Bcl-2 expression did not change in any of the groups, whereas Bax expression increased with ischemia-reperfusion and correlated well with the degree of apoptosis. Our findings suggest that HMWCaMBP may sequester calpains from its substrates in the normal myocardium, but it is susceptible to proteolysis by calpains during ischemia-reperfusion. Thus, decreased expression of HMWCaMBP may play an important role in myocardial injury.

Altered expression of high-molecular-weight calmodulin-binding protein in human ischaemic myocardium

A high-molecular-weight calmodulin-binding protein (HMWCaMBP) was previously identified and purified from the cytosolic fraction of bovine heart. Based on the sequence homology, amino acid analysis, antibody reactivity, and calpain inhibition, HMWCaMBP has been identified as a homologue of the calpain inhibitor calpastatin. In the present study the expression of HMWCaMBP was investigated in normal and ischaemic human myocardium. Western blot analysis of normal human cardiac muscle extract with the polyclonal antibody raised against bovine HMWCaMBP indicated a prominent immunoreactive band with a molecular mass of 140 kD. HMWCaMBP was localized in the cytoplasm and myofilaments of cardiac myocytes. Furthermore, Western blot analysis of normal and ischaemic cardiac tissues indicated a decrease in the expression of HMWCaMBP in ischaemic tissues. These studies were further substantiated by immunohistochemical studies, indicating strong to moderate HMWCaMBP immunoreactivity in normal cardiac muscle and poor to negative immunoreactivity in ischaemic muscle. The results obtained from the rat ischaemic model suggested that the expression of cardiac HMWCaMBP was significantly decreased during ischaemia/reperfusion. In addition, micro-calpain and m-calpain expression was higher in ischaemic cardiac tissue samples than in normal controls. The calpain inhibitory activity of ischaemic cardiac tissues was significantly lower than normal cardiac tissue samples. In some cases of cardiac ischaemia, HMWCaMBP highlighted the contraction band necrosis seen at the margins of a myocardial infarct. In vitro, HMWCaMBP was proteolysed by micro-calpain and m-calpain. These results indicate that HMWCaMBP could be susceptible to proteolysis by calpains during ischaemia or reperfusion and may play a contributory role in myocardial injury.

N-myristoyltransferase

Myristoylation refers to the co-translational addition of a myristoyl group to an amino-terminal glycine residue of a protein by an ubiquitously distributed enzyme myristoyl-CoA:protein N-myristoyltransferase (NMT, EC 2.3.1.97). This review describes the basic enzymology, molecular cloning and regulation of NMT activity in various pathophysiological processes such as colon cancer and diabetes.

In vitro generation of an active calmodulin-independent phosphodiesterase from brain calmodulin-dependent phosphodiesterase (PDE1A2) by m-calpain

In the present study we have shown that bovine brain 60-kDa calmodulin-dependent cyclic nucleotide phosphodiesterase isozyme (CaMPDE - PDE1A2) is proteolyzed by a Ca2+-dependent cysteine protease, m-calpain. The proteolysis of PDE1A2 by m-calpain results in its conversion to a totally calmodulin (CaM)-independent form accompanied by degradation of PDE1A2 into a 45-kDa catalytic fragment and a 15-kDa fragment. The activity of PDE1A2 is unaffected by the presence or absence of CaM during cleavage, suggesting that the interaction between CaM and PDE1A2 does not alter substrate recognition by calpain. Furthermore, we provide evidence, based on the studies of CaM overlay and phosphorylation, that the cleavage site is not present either in the CaM-binding domain or phosphorylation site. N-terminal sequence analysis of the 45-kDa fragment indicated that cleavage occurs between residues 126Gln and 127Ala, and eliminates the CaM-dependent activity of carboxy termini PDE1A2. The present findings suggest that limited proteolysis in the brain through calpains could be an alternate mechanism for activating CaMPDE(s) and for regulating intracellular levels of cAMP.

Myristoyl-coA:protein N-myristoyltransferase from bovine cardiac muscle: molecular cloning, kinetic analysis, and in vitro proteolytic cleavage by m-calpain

Myristoyl-CoA:protein N-myristoyltransferase (NMT) catalyzes the attachment of myristate onto the amino terminal glycine residue of select polypeptides. Cardiac tissue expresses high levels of cAMP-dependent protein kinase whose catalytic subunit is myristoylated; however, cardiac muscle extracts were found to contain low NMT activities. Northern blot analysis of bovine heart poly(A)+ RNA probed with bovine spleen NMT cDNA revealed a 1.7-kb mRNA. Western blot analysis of cardiac muscle extracts with human NMT antibody indicated a prominent immunoreactive band with a molecular mass of 50 kDa. The expression of mRNA and protein levels in cardiac muscle is not correlated with NMT activities, suggesting the presence of regulators of the enzyme activity. We have isolated the cDNA encoding bovine cardiac muscle NMT (cNMT) by reverse transcription polymerase chain reaction. The single long open reading frame of 1248 bp of bovine cNMT specifies a protein of 416 amino acids with a predicted mass of 46,686 Da. The cDNA clone expressed in Escherichia coli resulted in the production of functionally active 50-kDa NMT. Ultrastructural and immunolocalization of NMT utilizing the immunogold labeling technique demonstrated cytoplasmic distribution with occasional mitochondrial and myofilaments localization of the NMT antibody. Cardiac muscle NMT has a higher affinity for myristoyl-CoA than toward palmitoyl-CoA. Substrate specificity indicated that cNMT has a higher affinity toward pp60src and M2 gene segment of reovirus type 3-derived peptide substrates than toward cAMP-dependent protein kinase-derived peptide. Primary translational product of cNMT sequence contained several regions rich in proline, glutamic acid, serine, and threonine, which are known as "PEST" regions. PEST-FIND analysis of the amino acid sequences indicated eight PEST regions were present in the cNMT. These PEST regions are suggested to be recognized by specific proteases, particularly Ca(2+)-dependent neutral proteases, calpains, which are responsible for the degradation of PEST-containing proteins. We have demonstrated the abolishment of NMT activity and NMT protein degradation in vitro by m-calpain. The proteolysis of cNMT by m-calpain and the abolishment of NMT activity was prevented by the calpain inhibitor, calpastatin. These observations indicate that calpains may regulate NMT activity.