Micromolar Ca2+ requiring protease from human platelets: purification, partial characterization and effect on the cytoskeletal proteins

Role of calpains in the injury-induced dysfunction and degeneration of the mammalian axon

Axonal injury and degeneration, whether primary or secondary, contribute to the morbidity and mortality seen in many acquired and inherited central nervous system (CNS) and peripheral nervous system (PNS) disorders, such as traumatic brain injury, spinal cord injury, cerebral ischemia, neurodegenerative diseases, and peripheral neuropathies. The calpain family of proteases has been mechanistically linked to the dysfunction and degeneration of axons. While the direct mechanisms by which transection, mechanical strain, ischemia, or complement activation trigger intra-axonal calpain activity are likely different, the downstream effects of unregulated calpain activity may be similar in seemingly disparate diseases. In this review, a brief examination of axonal structure is followed by a focused overview of the calpain family. Finally, the mechanisms by which calpains may disrupt the axonal cytoskeleton, transport, and specialized domains (axon initial segment, nodes, and terminals) are discussed.

Characterization of the bovine neurofilament NF-M protein and cDNA sequence, and identification of in vitro and in vivo calpain cleavage sites

Neurofilaments are the major components of the neuronal cytoskeleton, and accumulations of these proteins are associated with several important human diseases. Here we report the cloning and sequencing of bovine NF-M, the first NF-M cloned from a large domestic mammal. The bovine sequence proves to be generally more similar to that of human NF-M than the previously described mammalian sequences, suggesting that bovine neurofilaments are a useful model for biochemical studies of application to humans. However, we noted some unusual features within the 16 lys-ser-pro (KSP2) type phosphopeptide repeats and also note that the number of these repeats correlates well with the size of animal. We also characterized two in vitro calpain cleavage sites by direct peptide sequencing, finding that both are located in the glutamic acid rich E segment. Finally, we show biochemically that the more abundant and stable of these calpain fragments can also be detected in vivo.

Autolysis parallels activation of mu-calpain

The kinetics of autolysis and activation of mu-calpain were measured with microtubule-associated protein 2 (MAP2) as a very sensitive substrate. The initial rate of MAP2 hydrolysis was found to be a linear function of the autolysed 76 kDa form of mu-calpain large subunit at both 10 and 300 microM Ca2+, and both straight lines intersected the origin. This finding supports the view that native mu-calpain is an inactive proenzyme and that activation is accompanied by autolysis. The first-order rate constant of autolysis, K1(aut), was determined at different Ca2+ concentrations: the half-maximal value was at pCa2+ = 3.7 (197 microM Ca2+), whereas the maximal value was 1.52 s-1, at 30 degrees C. The Ca(2+)-induced activation process was then monitored by using our novel, continuous fluorimetric assay with labelled MAP2 as substrate. The first-order rate constant of activation, k1(act), was derived as the reciprocal of the lag phase ('transit time') at the initial part of the progress curve: half-maximum was at pCa2+ = 3.8 (158 microM Ca2+) and the maximum value was 2.15 s-1. The good agreement between the kinetic parameters of mu-calpain autolysis and activation is remarkable. We claim that this is the first kinetically correct determination of the rate constant of autolysis of mu-calpain. Pre-activated mu-calpain has a Ca2+ requirement that is almost three orders of magnitude smaller [half-maximal activation at pCa2+ = 6.22 (0.6 microM Ca2+)]. We cannot exclude the possibility that the activation process involves other mechanistic steps, e.g. the rapid dissociation of the mu-calpain heterodimer, but we state that in our conditions in vitro autolysis and activation run in close parallel.

Histopathological changes at central nodes of Ranvier after stretch-injury

While the brain readily deforms when exposed to rotational loads as experienced in violent movements of the head, axons are able only to sustain tensile loads. Two discrete classes of axonal injury have been posited: disruptive axonal injury, where axons are physically torn or fragmented at the time of the insult, and nondisruptive axonal injury, where there is a hypothesised "perturbation" of the axolemma which leads to a cascade of pathobiological changes which result in axotomy over a period between 2 and 24 h after the initial insult. In the latter, it is posited that the node of Ranvier is that part of the axon which is the initial locus of axonal damage/ histopathological change. This paper describes the ultrastructure of nodal blebs, axolemma limited protrusions of the nodal axoplasm into the perinodal space, in which the nodal dense undercoating has been lost and aggregates of membranous profiles occur within the axoplasm. In addition, this paper provides novel data for disruption of the axonal cytoskeleton in nodes where blebs occur within 15 min of stretch-injury. The cytoskeletal disruption is visualised in thin sections as an almost total loss of microtubules together with a reduced density of neurofilaments within the nodal axoplasm. The loss of microtubules is posited to result in a disruption of fast axonal transport which results in the focal accumulation of membranous organelles in adjacent paranodal regions of the axon to form so-called "axonal swellings." Cytochemical and freeze-fracture studies provide evidence for structural reorganisation of the nodal axolemma after stretch-injury, and it is posited that these changes provide a route for uncontrolled influx of calcium which leads to loss of axonal integrity which potentiates axotomy. It is suggested that increased understanding of regulatory mechanisms that control ion channel activity will greatly increase our understanding of responses of neurones to trauma.

Calpain processing of brain microtubules from the Atlantic cod, Gadus morhua

Microtubules isolated from Atlantic cod (Gadus morhua) brains retained assembly competence and ultraculture, although treatment with rabbit calpain resulted in loss of MAPs. In addition, spirals and aberrant structures formed when calpain I was activated post assembly. No such effect was seen with calpain II. Soluble fractions from cod brain were found to contain proteolytic activity that could be blocked by exogenously added calpastatin. Calpain was also isolated from cod muscle tissue with 10 times less yield, compared to rabbit lung. On the basis of Ca(2+)-requirements for activation in the mM range, electrophoretic mobility, antigenicity and hydrophobicity, we conclude that the proteolytic activity was attributable to calpain II. There was no difference in effects of rabbit and cod calpain II on cod microtubule proteins, indicating that calpain is a conserved protein. Our results suggest that calpains might be involved in the Ca(2+)-dependent irreversible regulation of cod brain microtubules.

Calpains (intracellular calcium-activated cysteine proteinases): structure-activity relationships and involvement in normal and abnormal cellular metabolism

1. Calpains (calcium-activated cysteine proteinases) have evolved by gene fusion events involving calmodulin-like genes, cysteine proteinase genes and other sequences of unknown origin. 2. The enzymes are composed of two non-identical subunits, each of which contains functional calcium-binding sequences. 3. Calpains are inhibited by the endogenous protein inhibitor, calpastatin and some calmodulin antagonists are also inhibitors of calpain. A number of synthetic proteinase inhibitors also inhibit calpains. 4. Calpains can be activated by phospholipids, an endogenous protein activator and some amino acid derivatives. 5. Various protein substrates for calpains have been recognized in vitro, but the identity of in situ substrates remains unclear. 6. Proposals have been made for calpain function, including involvement in signal transduction, platelet activation, cell fusion, mitosis and cytoskeleton and contractile protein turnover. 7. Calpain and calpastatin expression is altered in a number of abnormal states including muscular dystrophy, muscle denervation and tenotomy, hypertension and platelet abnormalities.

Thrombin-induced platelet aggregation involves an indirect proteolytic cleavage of aggregin by calpain

5'-p-Fluorosulfonylbenzoyl adenosine (FSBA), a nucleotide analog of ADP, has been shown to inhibit ADP-induced shape change, aggregation and exposure of fibrinogen binding sites concomitant with covalent modification of a single surface membrane polypeptide of Mr 100,000 (aggregin). Since thrombin can aggregate platelets which have been modified by FSBA and are refractory to ADP, we tested the hypothesis that thrombin-induced platelet aggregation might involve cleavage of aggregin. At a low concentration of thrombin (0.05 U/ml), platelet aggregation, exposure of fibrinogen receptors and cleavage of aggregin in FSBA-modified platelets did not occur, indicating ADP dependence. In contrast, incubation of [3H]FSBA-labeled intact platelets with a higher concentration of thrombin (0.2 U/ml) resulted in cleavage of radiolabeled aggregin, aggregation, and exposure of fibrinogen binding sites. Under identical conditions, aggregin in membranes isolated from [3H]FSBA-labeled platelets was not cleaved by thrombin. Thrombin-induced platelet aggregation and cleavage of aggregin were concomitantly inhibited by a mixture of 2-deoxy-D-glucose, D-gluconic acid 1,5-lactone, and antimycin A. These results suggest that thrombin cleaves aggregin indirectly by activating an endogeneous protease. Thrombin is known to elevate intracellular Ca2+ concentration and thereby activates intracellular calcium dependent thiol proteases (calpains). In contrast to serine protease inhibitors, calpain inhibitors including leupeptin, antipain, and ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid (chelator of Ca2+) inhibited platelet aggregation and cleavage of aggregin in [3H]FSBA-labeled platelets. Leupeptin, at a concentration of 10-20 microM, used in these experiments, did not inhibit the amidolytic activity of thrombin, thrombin-induced platelet shape change, or the rise in intracellular Ca2+. Purified platelet calpain II caused aggregation of unmodified and FSBA-modified platelets and cleaved aggregin in [3H]FSBA-labeled platelets as well as in isolated membranes. The latter is in marked contrast to the action of thrombin on [3H]FSBA-labeled membranes. Thus, thrombin-induced platelet aggregation may involve intracellular activation of calpain which proteolytically cleaves aggregin thus unmasking latent fibrinogen receptors, a necessary prerequisite for platelet aggregation.

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

Calpain I and II (EC 3.4.22.17) are Ca2+-activated neutral thiol-proteases. Isolated brain tubulin and microtubule-associated proteins were found to be good substrates for proteolytic degradation by brain calpain I and II. The assembly of microtubules was totally inhibited when the calpains were allowed to act on microtubule proteins initially, and a complete disassembly was found after addition of calpain I to assembled microtubules. The high-molecular weight microtubule-associated proteins were degraded within a few minutes following incubation with calpain as shown by SDS-polyacrylamide gel electrophoresis and electron microscopy. When calpain was added to pre-formed microtubules, either in the presence or in the absence of microtubule-associated proteins, the proteolysis was significantly reduced. When tubulin was pre-assembled by taxol, the formation of proteolytic fragments was decreased indicating that assembly alters the availability of tubulin sites for proteolytic cleavage by calpain. Digested tubulin spontaneously formed aberrant polymers. No considerable change of apparent net charge was seen, thus indicating that calpain cleaves off fragments containing neutral amino acid residues and/or that the fragments of tubulin remain associated as an entity with the same charge as native tubulin. The results suggest that the calpains act as irreversible microtubule regulators.

Third form of calcium-activated neutral proteinase from calf brain: purification, partial characterization and comparison of properties with other forms

A third form (CANP3) of calcium-activated neutral proteinase (CANP) has been purified, 3900-fold, to near homogeneity from calf brain cortex. The purification procedure is based on the one recently developed for the purification of CANP1 and CANP2. The molecular weight of CANP3, as judged on SDS-polyacrylamide gel electrophoresis was Mr 78,000. A protein with an apparent Mr 17,000 co-purified with the proteinase. At neutral pH (7.2), it was maximally active at 260 microM CaCl2. In the presence of CaCl2, CANP1 and CANP3 were autolyzed very rapidly, whereas the autolysis of CANP2 was slow and gradual. The autolyzed CANP1 and CANP3 responded differently to CaCl2; CANP1 lost activity completely, whereas CANP3 was fully active at 0.5 microM CaCl2. Despite the opposite behavior of these proteinases in the presence of Ca2+, no significant differences in the peptide maps of the three proteinases were observed. Neurofilaments, neurotubules and myelin basic protein (MBP) were degraded by each of the proteinases. Monoclonal antibodies raised against CANP2 reacted almost equally with CANP1 and CANP3. As with CANP1 and CANP2, leupeptin and sulfhydryl-modifying compounds, NEM and iodoacetic acid, inhibited the activity of CANP3.