Extracellular high mobility group box 1 (HMGB1) protein can operate in a synergistic fashion with different signal molecules promoting an increase of cell Ca²⁺ influx. However, the mechanisms responsible for this effect of HMGB1 are still unknown. Here we demonstrate that, at concentrations of agonist per se ineffective, HMGB1 potentiates the activation of the ionotropic glutamate N-methyl-D-aspartate receptor (NMDAR) in isolated hippocampal nerve terminals and in a neuroblastoma cell line. This effect was abolished by the NMDA channel blocker MK-801. The HMGB1-facilitated NMDAR opening was followed by activation of the Ca²⁺-dependent enzymes calpain and nitric oxide synthase in neuroblastoma cells, resulting in an increased production of NO, a consequent enhanced cell motility, and onset of morphological differentiation. We have also identified NMDAR as the mediator of HMGB1-stimulated murine erythroleukemia cell differentiation, induced by hexamethylenebisacetamide. The potentiation of NMDAR activation involved a peptide of HMGB1 located in the B box at the amino acids 130–139. This HMGB1 fragment did not overlap with binding sites for other cell surface receptors of HMGB1, such as the advanced glycation end products or the Toll-like receptor 4. Moreover, in a competition assay, the HMGB1_(130–139) peptide displaced the NMDAR/HMGB1 interaction, suggesting that it comprised the molecular and functional site of HMGB1 regulating the NMDA receptor complex. We propose that the multifunctional cytokine-like molecule HMGB1 released by activated, stressed, and damaged or necrotic cells can facilitate NMDAR-mediated cell responses, both in the central nervous system and in peripheral tissues, independently of other known cell surface receptors for HMGB1.

The 80 kDa human erythrocyte calpain, when exposed to Ca²⁺, undergoes autoproteolysis that generates a 75 kDa species, with an increase in Ca²⁺ affinity. It is demonstrated here that this proteolytic modification proceeds through an initial step producing a 78 kDa form which is rapidly converted to the 75 kDa one. In the presence of the calpain inhibitor E‐64, the 78 kDa form accumulates and only small amounts of the 75 kDa polypeptide are formed. Following loading of erythrocytes with micromolar concentration of Ca²⁺, in the presence of the ionophore A23187, the native 80 kDa calpain subunit is extensively translocated and retained at the plasma membrane, this process is accompanied by the appearance of only a small amount of the 75 kDa subunit which is released into the soluble fraction of the cells. Following exposure to μM Ca²⁺, membrane‐bound 80 kDa calpain is converted to the 78 kDa form, this conversion being linearly correlated with the expression of the proteinase activity. Taken together, these results demonstrate that the initial step in calpain activation involves Ca²⁺‐induced translocation to the inner surface of plasma membranes. In the membrane‐bound form the native inactive 80 kDa subunit is converted through intramolecular autoproteolysis to a locally active 78 kDa form. Further autoproteolytic intermolecular digestion converts the 78 kDa to the 75 kDa form, no longer being retained by the membrane. This process generates two active forms of calpain, with different intracellular localisations.

Acyl-CoA-binding protein, a 20-kDa homodimer that exerts many physiological functions, promotes activation of the classic calpain forms, most markedly that of the m-isozyme. This protein factor was purified from rat skeletal muscle and was also expressed inEscherichia coli. Both native and recombinant acyl-CoA-binding proteins show the same molecular properties and an identical capacity to decrease the [Ca²⁺] required for m-calpain activity. The binding of long-chain acyl-CoAs to acyl-CoA-binding protein does not modify the activating effect on calpains. Acyl-CoA-binding protein seems to be involved in the m-calpain regulation process, whereas the previously identified UK114 activator is a specific modulator of μ-calpain. Acyl-CoA-binding protein is proposed as a new component of the Ca²⁺-dependent proteolytic system. A comparative analysis among levels of classic calpains and their activator proteins is also reported.

It is generally accepted that the Ca(2+)-dependent interaction of calpain with calpastatin is the most relevant mechanism involved in the regulation of Ca(2+)-induced proteolysis. We now report that a calpain-calpastatin association can occur also in the absence of Ca(2+) or at very low Ca(2+) concentrations, reflecting the physiological conditions under which calpain retains its inactive conformational state. The calpastatin binding region is localized in the non-inhibitory L-domain containing the amino acid sequences encoded by exons 4-7. This calpastatin region recognizes a calpain sequence located near the end of the DII-domain. Interaction of calpain with calpastatins lacking these sequences becomes strictly Ca(2+)-dependent because, under these conditions, the transition to an active state of the protease is an obligatory requirement. The occurrence of the molecular association between Ca(2+)-free calpain and various recombinant calpastatin forms has been demonstrated by the following experimental results. Addition of calpastatin protected calpain from trypsin digestion. Calpain was coprecipitated when calpastatin was immunoprecipitated. The calpastatin molecular size increased following exposure to calpain. The two proteins comigrated in zymogram analysis. Furthermore, calpain-calpastatin interaction was perturbed by protein kinase C phosphorylation occurring at sites located at the exons involved in the association. At a functional level, calpain-calpastatin interaction at a physiological concentration of Ca(2+) represents a novel mechanism for the control of the amount of the active form of the protease potentially generated in response to an intracellular Ca(2+) influx.

Calpains, the thiol proteinases of the calcium‐dependent proteolytic system, are regulated by a natural inhibitor, calpastatin, which is present in brain tissue in two forms. Although both calpastatins are highly active on human erythrocyte calpain, only one form shows a high inhibitory efficiency with both rat brain calpain isozymes. The second calpastatin form is almost completely inactive against homologous proteinases and can be converted into an active one by exposure to a phosphoprotein phosphatase, also isolated from rat brain. Phosphorylation of the active calpastatin by protein kinase C and protein kinase A promotes a decrease in its inhibitory efficiency. The interconversion between the two inhibitor forms seems involved in the adjustment of the level of intracellular calpastatin activity on specific cell requirements.

Although several reports have indicated that eNOS is a highly sensitive calpain substrate, the occurrence of a concomitant Ca²⁺-dependent activation of the synthase and of the protease has never been analyzed in specific direct experiments. In this study, we have explored in vivo how eNOS can undergo Ca²⁺-dependent translocation and activation, protected against degradation by activated calpain. Here we demonstrate that following a brief exposure to Ca²⁺-loading, the cytosolic eNOS-HSP90 complex recruits calpain in a form in which the chaperone and the synthase are almost completely resistant to digestion by the protease. Furthermore, in the presence of the HSP90 inhibitor geldanamycin, a significant decrease in NO production and an extensive degradation of eNOS protein occurs, indicating that dissociation from membranes and association with the chaperone is correlated to the protection of the synthase. Experiments with isolated membrane preparations confirm the primary role of HSP90 in dissociation of eNOS from caveolae. Prolonged exposure of cells to Ca²⁺-loading resulted in an extensive degradation of both eNOS and HSP90, accompanied by a large suppression of NO production. We propose that the protective effect exerted by HSP90 on eNOS degradation mediated by calpain represents a novel and critical mechanism that assures the reversibility of the intracellular trafficking and activation of the synthase.