Proteolysis of tubulin and microtubule-associated proteins 1 and 2 by calpain I and II. Difference in sensitivity of assembled and disassembled microtubules

Calcium-activated neutral proteinases as regulators of cellular function. Implications for Alzheimer's disease pathogenesis

Evidence is emerging that calcium-activated neutral proteinases (CANPs) not only participate in intracellular protein turnover but help to regulate the functional reorganization of cytoskeletal proteins in response to calcium and second-messenger stimulation. The high concentration of CANPs in certain neurons has suggested prominent roles for this proteolytic system in neuronal and synaptic function. In addition to acting directly on specific constituents of the cytoplasmic and membrane-associated cytoskeletal networks, CANP may amplify its effects by modulating the activities of protein kinase C and possibly other kinases and phosphatases by limited proteolysis. Given its suspected involvement at the cytoskeleton-membrane interface, calcium-mediated proteolysis is an example of a metabolic process which, if impaired, could provide a unifying basis for the slow progressive development of diverse structural and functional abnormalities within neurons. The multiplicity of mechanisms regulating its activity makes the CANP system a vulnerable target for disruption from various sources. A working hypothesis is advanced that down-regulation (inhibition) of neuronal calcium-mediated proteolysis in Alzheimer's disease is one critical and early step in the development of neurofibrillary degeneration and altered membrane cytoskeleton dynamics, which leads to membrane injury, accumulation of abnormal proteins, and synaptic dysfunction.

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.

Calmodulin-binding proteins as calpain substrates

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Evidence that calcium may control neurite outgrowth by regulating the stability of actin filaments

We investigated the effects of calcium removal and calcium ionophores on the behavior and ultrastructure of cultured chick dorsal root ganglia (DRG) neurons to identify possible mechanisms by which calcium might regulate neurite outgrowth. Both calcium removal and the addition of calcium ionophores A23187 or ionomycin blocked outgrowth in previously elongating neurites, although in the case of calcium ionophores, changes in growth cone shape and retraction of neurites were also observed. Treatment with calcium ionophores significantly increased growth cone calcium. The ability of the microtubule stabilizing agent taxol to block A23187-induced neurite retraction and the ability of the actin stabilizing agent phalloidin to reverse both A23187-induced growth cone collapse and neurite retraction suggested that calcium acted on the cytoskeleton. Whole mount electron micrographs revealed an apparent disruption of actin filaments in the periphery (but not filopodia) of growth cones that were exposed to calcium ionophores in medium with normal calcium concentrations. This effect was not seen in cells treated with calcium ionophores in calcium-free medium or cells treated with the monovalent cation ionophore monensin, indicating that these effects were calcium specific. Ultrastructure of Triton X-100 extracted whole mounts further indicated that both microtubules and microfilaments may be more stable or extraction resistant after treatments which lower intracellular calcium. Taken together, the data suggest that calcium may control neurite elongation at least in part by regulating actin filament stability, and support a model for neurite outgrowth involving a balance between assembly and disassembly of the cytoskeleton.

Proteolytic cleavage of high-molecular-weight microtubule-associated proteins by the prostatic estramustine-binding protein

The rat prostatic estramustine-binding protein was found to inhibit assembly of microtubules in a concentration-dependent manner. The inhibition was caused by a proteolytic cleavage of the high-molecular-weight microtubule associated proteins (MAPs), as judged by sodium dodecyl sulfate-gel electrophoresis. A proteolytic fragment with a molecular weight of 199 kDa appeared, which remained bound to the assembled microtubules. Fragments of lower molecular weights (170, 149 kDa) were also found, but they did not bind to the assembled microtubules. Fragments with identical molecular weights were also found after incubation of purified MAP2 with the estramustine-binding protein, indicating that the fragments derive from MAP2. No proteolysis of tubulin, albumin, or casein was found. The estramustine-binding protein was found to be a Zn2+-dependent protease; it was inhibited by EDTA and reactivated by addition of 1 mM Zn2+. Its proteolytic activity was not affected by binding of the antimitotic drug estramustine.