Proteolysis of the protein inhibitor of calcium-dependent proteases produces lower molecular weight fragments that retain inhibitory activity

Calpain Inhibitors as Potential Therapeutic Modulators in Neurodegenerative Diseases

It is considered a significant challenge to understand the neuronal cell death mechanisms with a suitable cure for neurodegenerative disorders in the coming years. Calpains are one of the best-considered "cysteine proteases activated" in brain disorders. Calpain is an important marker and mediator in the pathophysiology of neurodegeneration. Calpain activation being the essential neurodegenerative factor causing apoptotic machinery activation, it is crucial to develop reliable and effective approaches to prevent calpain-mediated apoptosis in degenerating neurons. It has been recently seen that the "inhibition of calpain activation" has appeared as a possible therapeutic target for managing neurodegenerative diseases. A systematic literature review of PubMed, Medline, Bentham, Scopus, and EMBASE (Elsevier) databases was conducted. The present article reviews the basic pathobiology and role of selective calpain inhibitors used in various neurodegenerative diseases as a therapeutic target.

Single-Step Purification of Calpain-1, Calpain-2, and Calpastatin Using Anion-Exchange Chromatography

Purification and separation of calpains and calpastatin are used to determine the individual activities of calpain-1 and calpain-2 and their inhibitor calpastatin. We discuss here a method to purify these enzymes using dialysis followed by separation using anion-exchange chromatography coupled with gradient elution. Swollen DEAE Sephacel is used as the column matrix in this method. Calpastatin and both domains of calpain are weakly basic molecules that effectively bind with the DEAE Sephacel and separate well using a stepwise, increasing gradient of NaCl to elute the proteins. Calpastatin binds most weakly with the column matrix, so it elutes first, followed by calpain-1 and, finally, calpain-2.

Calpains and neuronal damage in the ischemic brain: The swiss knife in synaptic injury

The excessive extracellular accumulation of glutamate in the ischemic brain leads to an overactivation of glutamate receptors with consequent excitotoxic neuronal death. Neuronal demise is largely due to a sustained activation of NMDA receptors for glutamate, with a consequent increase in the intracellular Ca(2+) concentration and activation of calcium- dependent mechanisms. Calpains are a group of Ca(2+)-dependent proteases that truncate specific proteins, and some of the cleavage products remain in the cell, although with a distinct function. Numerous studies have shown pre- and post-synaptic effects of calpains on glutamatergic and GABAergic synapses, targeting membrane- associated proteins as well as intracellular proteins. The resulting changes in the presynaptic proteome alter neurotransmitter release, while the cleavage of postsynaptic proteins affects directly or indirectly the activity of neurotransmitter receptors and downstream mechanisms. These alterations also disturb the balance between excitatory and inhibitory neurotransmission in the brain, with an impact in neuronal demise. In this review we discuss the evidence pointing to a role for calpains in the dysregulation of excitatory and inhibitory synapses in brain ischemia, at the pre- and post-synaptic levels, as well as the functional consequences. Although targeting calpain-dependent mechanisms may constitute a good therapeutic approach for stroke, specific strategies should be developed to avoid non-specific effects given the important regulatory role played by these proteases under normal physiological conditions.

[Calpain system dysregulation in rat brain at beta-amyloid-induced neurodegeneration]

Experimental evidences of calcium-dependent proteolysis dysregulation in brain of murine model of Alzheimer disease were obtained. Experimental treatment consisted in intra-hippocampal injection of amyloid beta-peptide (AP1-40) promoted activation of main calpain forms in murine brain along with decrease incontent of natural calpain inhibitor, calpastatin. As a result of prognostic experiment on the correction of neurodegeneration induced in murine the neuroprotective properties of steroid hormone estradiol were confirmed and one of the possible protective action mechanisms was suggested. Obtained results allow considering both biochemical modifications in protein facilities of pathology-affected brain and the mechanisms of neurodegeneration and neuroprotection.

Detecting the active conformation of calpain with calpastatin-based reagents

The specific, calcium-dependent, high affinity interaction between calpain and its endogenous inhibitor calpastatin was exploited to selectively detect the calcium-bound, catalytically competent, conformation of calpain in vitro. Modification of calpastatin domain-1 (Val(114)-Ser(270)) or its N-terminal fragment (Val(114)-Pro(202)), at selected unique cysteine residues with maleimide-AlexaFluor546 did not compromise calpastatin function (inhibition of calpain) or its binding with calpain. Ca(2+)-dependent binding between catalytically dead calpain-2 (Cys(105)Ala) fused with eGFP and these fluorigenic calpastatin peptides generates fluorescent resonance energy transfer (FRET). The FRET signal documents proximity of calpain-2, C-terminally linked fluorophore to specific sites within calpastatin when the proteins form a complex. These results provide important insights into the calcium-dependent interaction between calpain and calpastatin and for holo-calpain-2 in solution experimentally validate some key features of their predicted interactions. These data also provide proof of concept that the calpastatin-based reagents may be useful to selectively detect the active conformation of calpain.

Calpain-mediated signaling mechanisms in neuronal injury and neurodegeneration

Calpain is a ubiquitous calcium-sensitive protease that is essential for normal physiologic neuronal function. However, alterations in calcium homeostasis lead to persistent, pathologic activation of calpain in a number of neurodegenerative diseases. Pathologic activation of calpain results in the cleavage of a number of neuronal substrates that negatively affect neuronal structure and function, leading to inhibition of essential neuronal survival mechanisms. In this review, we examine the mechanistic underpinnings of calcium dysregulation resulting in calpain activation in the acute neurodegenerative diseases such as cerebral ischemia and in the chronic neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, prion-related encephalopathy, and amylotrophic lateral sclerosis. The premise of this paper is that analysis of the signaling and transcriptional consequences of calpain-mediated cleavage of its various substrates for any neurodegenerative disease can be extrapolated to all of the neurodegenerative diseases vulnerable to calcium dysregulation.

Mechanistic role of calpains in postischemic neurodegeneration

The calpain family of proteases is causally linked to postischemic neurodegeneration. However, the precise mechanisms by which calpains contribute to postischemic neuronal death have not been fully elucidated. This review outlines the key features of the calpain system, and the evidence for its causal role in postischemic neuronal pathology. Furthermore, the consequences of specific calpain substrate cleavage at various subcellular locations are explored. Calpain substrates within synapses, plasma membrane, endoplasmic reticulum, lysosomes, mitochondria, and the nucleus, as well as the overall effect of postischemic calpain activity on calcium regulation and cell death signaling are considered. Finally, potential pathways for calpain-mediated neurodegeneration are outlined in an effort to guide future studies aimed at understanding the downstream pathology of postischemic calpain activity and identifying optimal therapeutic strategies.

Role of calpain in skeletal-muscle protein degradation

Although protein degradation is enhanced in muscle-wasting conditions and limits the rate of muscle growth in domestic animals, the proteolytic system responsible for degrading myofibrillar proteins in skeletal muscle is not well defined. The goals of this study were to evaluate the roles of the calpains (calcium-activated cysteine proteases) in mediating muscle protein degradation and the extent to which these proteases participate in protein turnover in muscle. Two strategies to regulate intracellular calpain activities were developed: overexpression of dominant-negative m-calpain and overexpression of calpastatin inhibitory domain. To express these constructs, L8 myoblast cell lines were transfected with LacSwitch plasmids, which allowed for isopropyl beta-D-thiogalactoside-dependent expression of the gene of interest. Inhibition of calpain stabilized fodrin, a well characterized calpain substrate. Under conditions of accelerated degradation (serum withdrawal), inhibition of m-calpain reduced protein degradation by 30%, whereas calpastatin inhibitory domain expression reduced degradation by 63%. Inhibition of calpain also stabilized nebulin. These observations indicate that calpains play key roles in the disassembly of sarcomeric proteins. Inhibition of calpain activity may have therapeutic value in treatment of muscle-wasting conditions and may enhance muscle growth in domestic animals.

Domain structure of calpain: mapping the binding site for calpastatin

The peptide EKLGERDDTIPPEYRELLEKKTGV was synthesized to mimic the central consensus sequence of calpastatin, the specific, endogenous inhibitor of the calpains (EC 3.4.22.17). The peptide competitively inhibits hydrolysis of casein by either micro- or milli-calpain but does not affect the activity of other proteases. This inhibitory peptide was preferentially cross-linked to milli-calpain in the presence of calcium using the heterobifunctional cross-linking reagent m-maleimidobenzoyl-N-hydroxysuccinimide ester. Cross-linking of the peptide was blocked by calpastatin. The site of cross-linking for the peptide within milli-calpain was localized using random chemical cleavage of the enzyme-peptide complex at cysteine residues. Calpain fragments were identified as amino-terminal fragments through reactivity with a peptide-specific antiserum or as non-amino-terminal fragments through incorporation of 14C from 14CN. Analysis of the control and cross-linked fragments, from experiments using both milli-calpain and micro-calpain, maps the chemical cross-linking site to cysteine-497 and localizes the binding site for the calpastatin-like peptide to this highly conserved region of domain III of calpains catalytic subunit.

Identification of two calpastatin forms in rat skeletal muscle and their susceptibility to digestion by homologous calpains

Two forms of calpastatin, differing in their specificity for the homologous calpain isozymes I and II, have been separated from rat skeletal muscle extracts and purified to homogeneity. Calpastatin I, the first form to elute in chromatography on DE32, is more effective against calpain I, while calpastatin II is more effective as an inhibitor of calpain II. Based on their molecular mass (approximately 105 kDa) both calpastatin forms belong to the high molecular mass class found in muscles of other animal species (Murachi, T., 1989, Biochem. Int. 18, 263-294). For calpain I, which is active with low (mu-M) concentrations of Ca2+, maximum inhibition with either calpastatin form was observed over a wide range of Ca2+ concentrations. With calpain II, which requires high (mM) concentrations of Ca2+ for activity, maximum inhibition required Ca2+ concentrations above 1 mM. Both calpastatin forms were found to be highly sensitive to degradation by calpain II, but almost completely resistant to degradation by calpain I. Degradation of calpastatin by calpain II is competitively inhibited by the addition of a calpain substrate. Isovaleryl carnitine (IVC), an intermediate product of L-leucine catabolism, previously demonstrated to be a potent and specific activator of rat skeletal muscle calpain II (Pontremoli, S., Melloni, E., Viotti, P. L., Michetti, M., Di Lisa, F., and Siliprandi, N., 1990. Biochem. Biophys. Res. Commun. 167, 373-380) greatly enhances the rate of degradation of calpastatins by calpain II. IVC, which decreases the Ca2+ requirement for maximal calpain II activity, also decreases the concentration of Ca2+ required for digestion of the inhibitor. For calpain II, regulation by either calpastatins may occur only in the presence of high [Ca2+].

An endogenous inhibitor of calcium-activated neutral protease in UMX 7.1 hamster dystrophy

An endogenous inhibitor for calcium-activated neutral protease (CANP) from skeletal and cardiac muscles of muscular dystrophic hamsters (UMX 7.1) was compared with that from normal control animals at 4 and 10 weeks of age by Western blotting using antibody raised against CANP inhibitor. Fragmented CANP inhibitor was found in dystrophic skeletal muscles in all cases at both ages, while only intact inhibitor was detected in the skeletal muscle of the normal hamsters. A total absence of intact inhibitor was shown in one 10-week-old dystrophic hamster. In contrast, there was little difference in CANP inhibitor from heart between dystrophic and control hamsters at 4 weeks. However, fragmentation similar to that in skeletal muscle was seen in the heart inhibitor in a few of the 10-week-old dystrophic hamsters.

The calpains

In recent years interest has increased concerning the characterization of the structural-functional properties and the identification of the physiological role of non-lysosomal intracellular proteinases. Among these, calpain, a calcium-dependent cysteine proteinase ubiquitously present in a variety of tissues and cells, has been most extensively investigated in terms of activation, regulatory mechanisms, specificity and biological function. This review discusses each of these points on the basis of the most recent results concerning the general characteristics of calpain activity, and its preferential site of action within the cell as related to the specific functions of the proteinase in different cell types. As with other proteinases, calpain has to be under a continuous spatial and temporal control, and the structural and functional properties of the natural calpain inhibitor, calpastatin, must also be considered. The calpain-calpastatin system is the functional proteolytic unit that governs the activity of this intracellular proteolytic system, which is tightly correlated to the control of calcium homeostasis and thereby to the biological process of transmembrane signalling.