Rajala V.S. Raju

Calmodulin-dependent cyclic nucleotide phosphodiesterase (PDE1).

Abstract. Ca2+/calmodulin-dependent cyclic nucleotide phosphodiesterase (PDE1) is one of the key enzymes involved in the complex interactions between the cyclic nucleotide and Ca2+ second messenger systems. Currently, three genes encode PDE1, and alternate splicing of these genes gives rise to functionally different isozymes which exhibit distinct catalytic and regulatory properties. Some isozymes have similar kinetic and immunological properties but are differentially regulated by Ca2+ and calmodulin. These isozymes also differ in their mechanism of regulation by phosphorylation. Analysis of various regulatory reactions involving Ca2+ and cyclic adenosine monophosphate (cAMP) has revealed the importance of the time dependence of these reactions during cell activation; however, no measurement is available for the time of occurrence of specific regulatory reactions. cAMP-signalling systems provide a pivotal centre for achieving crosstalk regulation by various signalling pathways. It has been proposed that polypeptide sequences enriched in proline (P), glutamate (E), serine (S) and threonine (T), known as PEST motifs, serve as putative intramolecular signals for rapid proteolytic degradation by calpains. Calpains are Ca2+-dependent cysteine proteases that regulate various enzymes, transcription factors and structural proteins through limited proteolysis. Isozyme PDE1A2 has a PEST motif and acts as a substrate for m-calpain. In this paper, we have described PDE1A2 regulation by calpains and its physiological implications. cAMP is an important component of the signal transduction pathway and plays an integral role in various physiological processes such as gene transcription, various neuronal functions, cardiac muscle contraction, vascular relaxation, cell proliferation and a host of other functions. It is important to identify the cellular processes where PDE isoform(s) and cAMP response are altered. This will lead to better understanding of the pathology of disease states and development of novel therapeutics. The different PDE1 isozymes, although similar in kinetic properties, can be distinguished by various pharmacological agents. Our recent understanding of the role of PDE1 inhibitors such as ginseng, dihydropyridine antagonists and antiparkinsonian agents are described in this review. The exact function of PDE1 isozymes in various pathophysiological processes is not clear because most of the studies have been carried out in vitro; therefore, it is essential that further research be directed to in vivo studies.

Amantadine: an antiparkinsonian agent inhibits bovine brain 60 kDa calmodulin-dependent cyclic nucleotide phosphodiesterase isozyme

The effect of amantadine (an antiparkinsonian agent) on calmodulin-dependent cyclic nucleotide phosphodiesterase isozymes was investigated. Amantadine inhibited bovine brain 60 kDa calmodulin-dependent cyclic nucleotide phosphodiesterase but not the bovine brain 63 kDa, heart and lung calmodulin-dependent cyclic nucleotide phosphodiesterase isozymes. The inhibition of bovine brain 60 kDa calmodulin-dependent cyclic nucleotide phosphodiesterase was overcome by increasing the concentration of calmodulin. This suggests that amantadine may be an antagonist of calmodulin or act specifically and reversibly on the action of calmodulin. The bovine brain 60 kDa calmodulin-dependent cyclic nucleotide phosphodiesterase isozyme is predominantly expressed in the brain and its inhibition may result in increased intracellular levels of cyclic AMP (cAMP). The increased intracellular levels of cAMP have a protective role for dopaminergic neurons. The present findings suggest that amantadine may be a valuable tool to investigate the physiological role of 60 kDa calmodulin-dependent cyclic nucleotide phosphodiesterase isozyme in the progression of Parkinsons disease and gives a new insight into the action of this drug.

Mammalian myristoyl CoA: protein N-myristoyltransferase

Myristoyl CoA:Protein N-myristoyltransferase (NMT) is the enzyme which catalyses the covalent transfer of myristate from myristoyl CoA to the amino-terminal glycine residue of protein substrates. Although NMT is ubiquitous in eukaryotic cells, the enzyme levels and cellular distribution vary among tissues. In this article, we describe the properties of mammalian NMT(s) with reference to subcellular distribution, molecular weights, substrate specificity and the possible involvement of NMT in pathological processes. The cytosolic fraction of bovine brain contains multiple forms of NMT activity whereas bovine spleen contains only a single form. In bovine brain and spleen, the cytosol contained majority of NMT activity. In contrast, rabbit colon and rat liver NMT activity was predominantly particulate. Regional differences in NMT activity have been observed in both rabbit intestine and bovine brain. Results from our laboratory along with the existing knowledge, provide evidence for the existence of tissue specific isozymes of NMT.

INHIBITION OF BOVINE BRAIN CALMODULIN-DEPENDENT CYCLIC NUCLEOTIDE PHOSPHODIESTERASE ISOZYMES BY DEPRENYL

Intracellular concentrations of cyclic nucleotides is regulated by cyclic nucleotide phosphodiesterases and calmodulin-dependent cyclic nucleotide phosphodiesterases (CaMPDE), one of the most intensively studied and best characterized phosphodiesterases. In the present study, the effect of an antiparkinsonian agent, deprenyl (selegeline hydrochloride) which is believed to be a selective inhibitor of monoamine oxidase-B, on bovine brain calmodulin-dependent cyclic nucleotide phosphodiesterase (CaMPDE) isozymes have been investigated. The findings indicated that deprenyl inhibited brain 60 kDa isozyme, however the inhibition for brain 63 kDa CaMPDE was observed to a lesser extent. The inhibition of brain 60 kDa CaMPDE was overcome by increasing the concentration of calmodulin suggesting that deprenyl may be calmodulin antagonist or act specifically and reversibly on the action of calmodulin. The 60 kDa CaMPDE isozyme is predominantly expressed in brain and its inhibition can result in increased intracellular levels of cAMP. The increased intracellular levels of cAMP have a protective role for dopaminergic neurons. Therefore, deprenyl may be a valuable tool to investigate the physiological roles of brain CaMPDE isozymes in progression of Parkinsons disease and gives a new insight into the action of this drug.

Expression of human N-myristoyltransferase inEscherichia coli. Comparison with N-myristoyltransferases expressed in different tissues

Myristoyl CoA:protein N-myristoyltransferase catalyzes the addition of myristate to the amino-terminal glycine residue of a number of eukaryotic proteins.Escherichia coli transformed with human NMT expression construct produced high levels of N-myristoyltransferase. Using the combination of ammonium sulfate precipitation, chromatography on SP-Sepharose fast flow and fast protein liquid chromatography on Mono-S, the enzyme was purified more than 100 fold with 40% yield. The hNMT fusion protein exhibited an apparent molecular weight of 53 kDa on SDS-polyacrylamide gel electrophoresis. Upon cleavage by the Enterokinase [(Asp)4-Lys], the hNMT exhibited an apparent molecular mass of 49 kDa without loss of catalytic activity. The hNMT activity could be greatly activated severalfold with the use of Tris, SDS, ethanol and acetonitrile. The catalytic activity of hNMT was potently inhibited in a concentration dependent manner by NIP711 a bovine brain NMT inhibitory protein with a half maximal inhibition of 31.0 nM. TheE. coli expressed hNMT was homogeneous and showed enzyme activity.

Coenzyme A dependent myristoylation and demyristoylation in the regulation of bovine spleen N-myristoyltransferase

N-myristoyltransferase (NMT) is an essential eukaryotic enzyme that catalyzes the transfer of myristate to the NH2-terminal glycine residue of a number of important proteins of diverse function. Little is known about the control and regulation of NMT in higher eukaryotes. Bovine spleen N-myristoyltransferase has been purified and characterized [Raju, RVS, Kalra J & Sharma RK (1994) J Biol Chem 269:12080–12083]. The activation of bovine spleen NMT with thiol reducing compounds, and its inhibition by the oxidizing agent sodium iodate, suggest a role for oxidation/reduction in NMT regulation. Available knowledge concerning coenzyme A (CoA), the thiol in the cell, indicated that the agents tested on NMT could also reduce or oxidize CoA. The studies suggested that reduced CoA is the key regulator of NMT activity, while oxidized CoA did not allow NMT to promote myristoylation. Further, the process of myristoylation and demyristoylation may be governed by NMT, depending on the differential concentration of CoA. The process of demyristoylation could be blocked by excess CoA. We therefore hypothesize that the initial event in the regulation of NMT is an increase in cellular CoA concentration which could be coupled to an increase in protein myristoylation. Once the CoA concentration in the cell decreases due to oxidation, the demyristoylation process would be operative.

Biological significance of phosphorylation and myristoylation in the regulation of cardiac muscle proteins

Post-translational modification has long been recognized as a way in which the properties of proteins may be subtly altered after synthesis of the polypeptide chain is complete. Amongst the moieties most commonly encountered covalently attached to proteins are oligosaccharides, phosphate, acetyl, formyl and nucleosides. Protein phosphorylation and dephosphorylation is one of the most prevalent and best understood modifications employed in cellular regulation. The bovine heart calmodulin-dependent cyclic nucleotide phosphodiesterase (CaMPEDE) can be phosphorylated by cAMP-dependent protein kinase, resulting in a decrease in the enzymes affinity for Ca2+ and calmodulin (CaM). The phosphorylation of CaMPDE is blocked by Ca2+ and CaM and reversed by the CaM-dependent phosphatase (calcineurin). The dephosphorylation is accompanied by an increase in the affinity of the phosphodiesterase for CaM. Analysis of the complex regulatory properties of CaMPDE has led to the suggestion that fluxes of cAMP and Ca2+ during cell activations are closely coupled and that the CaMPDE play a key role in the signal coupling phenomenon. The high molecular weight calmodulin binding protein (HMWCaMBP) was phosphorylated by cAMP-dependent protein kinase. Phosphorylation of HMWCBP was higher in the absence of Ca2+/CaM then in the presence of Ca2+/CaM and reversed by the CaM-dependent phosphatase. Recently, it has become apparent that the binding of myristate to proteins is also widespread in eukaryotic cells and viruses and certainly is of great importance to the correct functioning of an organism. Myristoyl CoA:protein N-myristoyltransferase (NMT) catalyses the attachment of myristate to the amino-terminal glycine residue of various signal transduction proteins. Cardiac tissue express high levels of cAMP-dependent protein kinase whose catalytic subunit is myristoylated. The subcellular localization of bovine cardiac muscle NMT indicated a majority of the activity was localized in cytoplasm. Under native conditions the enzyme exhibited an apparent molecular mass of 50 kDa. Recovery of NMT activity, from both cytosol and particulate fractions, was found to be higher than the total activity in crude homogenates, suggesting that particulate fraction may contain an inhibitory activity towards NMT. Research in our laboratory has been focusing on the covalent modification of proteins and regulation of various signal transduction proteins. This special review is designed to summarize some aspects of the current work on co- and post-translational modification of proteins in cardiac muscle.

In vitro phosphorylation of bovine cardiac muscle high molecular weight calmodulin binding protein by cyclic AMP-dependent protein kinase and dephosphorylation by calmodulin-dependent phosphatase

High molecular weight calmodulin binding protein (HMWCaMBP) is one of the major proteins expressed in bovine cardiac muscle [9]. In this study, we report the phosphorylation and dephosphorylation of HMWCaMBP in vitro with a view to understand the function of this protein. The HMWCaMBP was phosphorylated by cAMP-dependent protein kinase with the incorporation of 2.30 mol of phosphate/mol of protein in the presence of EGTA. When phosphorylation was carried out in the presence of Ca2+/calmodulin (CaM), the incorporation of phosphate was reduced to 1.40 mol of phosphate/mol of protein. The decrease in the stoichometry of phosphorylation by Ca2+/CaM appears to be substrate directed i.e. due to the interaction of Ca2+/CaM with HMWCaMBP. The phosphorylated HMWCaMBP was unable to compete for free CaM in a CaM-dependent cyclic nucleotide phosphodiesterase (CaMPDE) assay. These results suggest that the phosphorylation sites may reside in or in proximity to the CaM-binding domain on HMWCaMBP since phosphorylated HMWCaMBP did not inhibit CaMPDE activity. HMWCaMBP was dephosphorylated by CaM-dependent phosphatase, calcineurin.

Calmodulin-dependent cyclic nucleotide phosphodiesterase in human cerebral cortex and glioblastoma multiforme

BACKGROUND Calmodulin-dependent cyclic nucleotide phosphodiesterase (CaMPDE) has been extensively studied and characterized in normal mammalian tissues; however very little is known about this enzyme in human brain tumors. It has been established that high levels of this enzyme exist in non-central nervous system tumors, PDE inhibitors or cAMP analogues have been used to treat them. This study has examined the levels of CaMPDE in glioblastoma multiforme from six patients and has compared these to the levels of CaMPDE in four patients with normal cerebral tissue. In addition, an enzyme immune assay method (EIA) was developed in this study for the detection of CaMPDE in human cerebral tissue. This method is proposed to be used as an adjunct to the spectrophotometric method presently utilized. This would be beneficial in cases where small tissue samples, for example in stereotactic biopsy, are available. METHODS The CaMPDE activity and corresponding levels of expression in cerebral tissue from temporal lobectomies and both surgical extraction or stereotactic biopsy in patients with primary tumors were determined by spectrophotometric and EIA, respectively. The EIA was developed from the production of a polyclonal antibody against bovine brain 60 kDa CaMPDE isozyme. Cross reactivity of the antibody with human was confirmed using transblot and immunohistochemistry. RESULTS Utilising the EIA, there was found to be significant reduction in both catalytic activity (p < 0.001) and in quantitative protein expression (p < 0.001) in glioblastoma multiforme from patients when compared to normal cerebral cortex. Immunoblotting experiments and immunohistochemistry demonstrated that CaMPDE in glioblastoma multiforme failed to react with a polyclonal antibody raised against bovine brain 60 kDa CaMPDE isozyme, whereas the enzyme from normal tissue reacted with antibody. CONCLUSIONS Contrary to other studies on non-CNS tumors, the catalytic activity and the protein expression of CaMPDE is reduced in glioblastoma multiforme. The EIA method is a more sensitive in detecting CaMPDE than in the spectrophotometric method, especially when a small amount of tissue is available. Immunohistochemistry and the EIA may be useful in the future to use as markers for other types of brain tumors and not for glioblastoma multiforme as demonstrated.

Demonstration and purification of a myristoyl-CoA binding protein from bovine cardiac muscle

Protein myristoylation refers to the co-translational addition of myristoyl group to an amino-terminal glycine residue of a protein by the enzyme myristoyl-CoA:protein N-myristoyltransferase (NMT). The myristoylation reaction depends on the availability of the cellular pools of coenzyme A and myristate and their subsequent formation of myristoyl-CoA, the substrate of NMT. In the present study a myristoyl-CoA binding protein (MCBP) was purified using various column chromatographies: hydroxylapatite, DEAE Sepharose CL-6B and Sephacryl S-300 gel filtration. The purified protein exhibited an apparent molecular mass of 50 kDa on SDS-polyacrylamide gel electrophoresis. Incubation of protein with [1-(14)C]myristoyl-CoA followed by denaturing gel electrophoresis, fluorography and treatment with hydroxylamine yielded results that are highly suggestive of a covalent ester-linked acyl-protein complex. This complex formation was not observed in the crude cytosolic fractions. The addition of cytosolic fraction to a progressing acyl-protein complex, resulted in deacylation suggesting a role for thioesterase or/proteinases in the regulation of the acylation reaction in bovine cardiac muscle. The acyl-protein complex formation was not inhibited by NIP71, a 71 kDa NMT inhibitory protein from bovine brain.