No abstract

Ca2+ loading of Jurkat and bovine aorta endothelium cells induces the degradation of the neuronal and endothelial nitric oxide synthases that are selectively expressed in these cell lines. For neuronal nitric oxide synthase, this process involves a conservative limited proteolysis without appreciable loss of catalytic activity. By contrast, endothelial nitic oxide synthase digestion proceeds through a parallel loss of protein and catalytic activity. The chaperone heat shock protein 90 (HSP90) is present in a large amount in Jurkat cells and at significantly lower levels in bovine aorta endothelium cells. The differing ratios of HSP90/nitric oxide synthase (NOS) occurring in the two cell types are responsible for the conservative or nonconservative digestion of NOS isozymes. Consistently, we demonstrate that, in the absence of Ca2+, HSP90 forms binary complexes with NOS isozymes or with calpain. When Ca2+ is present, a ternary complex containing the three proteins is produced. In this associated state, HSP90 and NOS forms are almost completely resistant to calpain digestion, probably due to a structural hindrance and a reduction in the catalytic efficiency of the protease. Thus, the recruitment of calpain in the HSP90–NOS complexes reduces the extent of the proteolysis of these two proteins. We have also observed that calpastatin competes with HSP90 for the binding of calpain in reconstructed systems. Digestion of the proteins present in the complexes can occur only when free active calpain is present in the system. This process can be visualized as a novel mechanism involving the association of NOS with HSP90 and the concomitant recruitment of active calpain in ternary complexes in which the proteolysis of both NOS isozymes and HSP90 is significantly reduced.

The regulatory mechanism of calpain activity is primarily based on the physiological control of intracellular Ca 2ϩ homeostasis, predominantly exerted by the plasma membrane Ca 2ϩ -ATPase and, more specifically, by the expression in all mammalian cells of a natural protein inhibitor named calpastatin (1-7).Calpastatin is a protein endowed with peculiar molecular properties characterized by the presence of four identical inhibitory domains, each one possessing, in its free form, an inhibitory capacity almost equivalent to that expressed by the native calpastatin molecule (2-4, 8 -11). Moreover, calpastatin also behaves as a substrate of calpain, which is degraded to single inhibitory domains as well as to inactive products as a result of more extensive digestion (8,(12)(13)(14)(15)(16). Conservative fragmentation has been attributed to -calpain activity, and degradation to inactive peptides attributed to digestion by m-calpain (8, 16).Despite this information and the numerous reports on the structural properties of calpain (17-22), including the presence of different isoforms in various tissues (23-27), the biological function of this protease and the efficiency of its "in vivo" intracellular regulation remain uncertain.It is often suggested (28 -34) that in brain, calpain is directly implicated in neurodegeneration and neuronal cell death as a consequence of a massive Ca 2ϩ influx occurring under pathological conditions, such as hypoxia and ischemia. This statement has been largely based on the protective effects exerted by synthetic protease inhibitors and by overexpression of calpastatin (35-40). However, in these reports, the fact that a large excess of calpastatin is normally present in brain is not considered, and no explanation has been given on the possible ineffectiveness of the inhibitor under the experimental conditions used (31). It must also be considered that the severe and irreversible cell damage observed under these pathological conditions occurs in the presence of a massive Ca 2ϩ influx, associated with the collapse of the membrane potential. On the basis of these considerations, it seems therefore reasonable to assume that in brain cells, as well as in other cell types, a moderate alteration in Ca 2ϩ homeostasis, even prolonged for a long period of time, could evoke adaptive compensatory defense mechanisms capable of preserving cell integrity.To explore in vivo the existence in the brain of such mechanisms and to characterize their molecular aspects, we have treated normal normotensive Milan strain rats (NMS) 2 with a high salt (sodium) diet (HSD) known to promote elevation in blood pressure in response to a mild increase in intracellular free [Ca 2ϩ ] in vascular smooth muscle, via an alteration in the Na ϩ /Ca 2ϩ exchanger (41)(42)(43). A role of the exchanger in the alteration of calcium homeostasis in brain has been previously suggested (44,45). To amplify the effects of this treatment we have exposed hypertensive rats (HMS) of the same Milan strain (46) to the same diet, because these an...

Human circulating PBMC (peripheral blood mononuclear cells) contain three calpain isoforms distinguishable on the basis of their chromatographic properties. Two of these proteases belong to the ubiquitous calpain subfamily, corresponding to the classical mu- and m-calpain forms. The third, which shows peculiar activating and regulatory properties, is an alternatively spliced calpain 3 (p94) form. This new calpain differs from calpain 3 in that it has lost IS1 insertion and exon 15, a lysine-rich sequence regarded as a nuclear translocation signal. PBMC p94-calpain undergoes activation and inactivation without the accumulation of a low-Ca2+-requiring form that is typical of the classical activation processes of mu- and m-calpain. Furthermore, it differs from the ubiquitous forms in that it displays a lower sensitivity to calpastatin. On the basis of these selective properties, it can be postulated that PBMC p94-calpain can be activated in response to specific stimuli that are not effective on the other calpain isoenzymes. The enzyme is preferentially expressed in B- and T-lymphocytes, whereas it is poorly expressed in natural killer cells and almost undetectable in polymorphonuclear cells. This distribution might reflect the specific function of this protease, which is preferentially present in cells devoted to the production of the humoral, rather than to the cellular, immune response.

Persistent dysregulation in Ca2؉ homeostasis is a pervasive pathogenic mechanism in most neurodegenerative diseases, and accordingly, calpain activation has been implicated in neuronal cells dysfunction and death. In this study we examined the intracellular functional state of the calpain-calpastatin system in ؊G93A (