Calcium-activated neutral proteinase (calpain) system in aging and Alzheimer's disease

Alterations in the ryanodine receptor calcium release channel correlate with Alzheimer's disease neurofibrillary and beta-amyloid pathologies

Investigation of the integrity of the ryanodine receptor in Alzheimer's disease is important because it plays a critical role in the regulation of calcium release from the endoplasmic reticulum in brain, impairment of which is believed to contribute to the pathogenesis of Alzheimer's disease. The present study compared ryanodine receptor levels and their functional modulation in particulate fractions from control and Alzheimer's disease temporal cortex, occipital cortex and putamen. Relationships between ryanodine receptor changes and the progression of Alzheimer's disease pathology were determined by examining autoradiographic [3H]ryanodine binding in entorhinal cortex/anterior hippocampus sections from 22 cases that had been staged for neurofibrillary changes and beta-amyloid deposition. A significant (P < 0.02) 40% decrease in the Bmax for [3H]ryanodine binding and significantly higher IC50 values for both magnesium and Ruthenium Red inhibition of [3H]ryanodine binding were detected in Alzheimer's disease temporal cortex particulate fractions compared to controls. Immunoblot analyses showed Type 2 ryanodine receptor holoprotein levels to be decreased by 20% (P < 0.05) in these Alzheimer's disease cases compared to controls. No significant differences were detected in [3H]ryanodine binding comparing control and Alzheimer's disease occipital cortex or putamen samples. The autoradiography study detected increased [3H]ryanodine binding in the subiculum, CA2 and CA1 regions in cases with early (stage I-II) neurofibrillary pathology when compared to Stage 0 cases. Analysis of variance of data with respect to the different stages of neurofibrillary pathology revealed significant stage-related declines of [3H]ryanodine binding in the subiculum (P < 0.02) with trends towards significant decreases in CA1, CA2 and CA4. Post-hoc testing with Fisher's PLSD showed significant reductions (74-94%) of [3H]ryanodine binding in the subiculum, and CA1-CA4 regions of the late isocortical stage (V-VI) cases compared to the early entorhinal stage I-II cases. [3H]Ryanodine binding also showed significant declines with staging for beta-amyloid deposition in the entorhinal cortex (P < 0.01) and CA4 (P < 0.05) with trends towards a significant decrease in the dentate gyrus. We conclude that alterations in ryanodine receptor binding and function are very early events in the pathogenesis of Alzheimer's disease, and may be fundamental to the progression of both neurofibrillary and beta-amyloid pathologies.

Endogenous bax translocation in SH-SY5Y human neuroblastoma cells and cerebellar granule neurons undergoing apoptosis

Changes at the mitochondria are an early, required step in apoptosis in various cell types. We used western blot analysis to demonstrate that the proapoptotic protein Bax translocated from the cytosolic to the mitochondrial fraction in SH-SY5Y human neuroblastoma cells undergoing staurosporine- or EGTA-mediated apoptosis. Levels of mitochondrial Bax increased 15 min after staurosporine treatment. In EGTA-treated cells, increased levels of mitochondrial Bax were seen at 4 h, consistent with a slower onset of apoptosis in EGTA versus staurosporine treatments. We also demonstrate the concomitant translocation of cytochrome c from the mitochondrial to the cytosolic fractions. We correlated these translocations with changes in caspase-3-like activity. An increase in caspase-3-like activity was evident 2 h after staurosporine treatment. Inhibition of the mitochondrial permeability transition had no effect on Bax translocation or caspase-3-like activity in staurosporine-treated SH-SY5Y cells. In primary cultures of cerebellar granule neurons undergoing low K(+)-mediated apoptosis, Bax translocation to the mitochondrial fraction was evident at 3 h. Cytochrome c release into the cytosol was not significant until 8 h after treatment. These data support a model of apoptosis in which Bax acts directly at the mitochondria to allow the release of cytochrome c.

Altered calcium homeostasis and mitochondrial dysfunction in cortical synaptic compartments of presenilin-1 mutant mice

Alzheimer's disease is characterized by amyloid beta-peptide deposition, synapse loss, and neuronal death, which are correlated with cognitive impairments. Mutations in the presenilin-1 gene on chromosome 14 are causally linked to many cases of early-onset inherited Alzheimer's disease. We report that synaptosomes prepared from transgenic mice harboring presenilin-1 mutations exhibit enhanced elevations of cytoplasmic calcium levels following exposure to depolarizing agents, amyloid beta-peptide, and a mitochondrial toxin compared with synaptosomes from nontransgenic mice and mice overexpressing wild-type presenilin-1. Mitochondrial dysfunction and caspase activation following exposures to amyloid beta-peptide and metabolic insults were exacerbated in synaptosomes from presenilin-1 mutant mice. Agents that buffer cytoplasmic calcium or that prevent calcium release from the endoplasmic reticulum protected synaptosomes against the adverse effect of presenilin-1 mutations on mitochondrial function. Abnormal synaptic calcium homeostasis and mitochondrial dysfunction may contribute to the pathogenic mechanism of presenilin-1 mutations.

Altered calpastatin protein levels following traumatic brain injury in rat

Pathological activation of the intracellular Ca2+-dependent proteases calpains may be responsible for the neuronal pathology associated with neurodegenerative diseases and acute traumas to the central nervous system. Though calpain activation has been shown definitively in traumatic brain injury (TBI), no studies have investigated calpastatin (CAST), the calpains' endogenous and specific inhibitor, after TBI. The present study examined temporal changes in CAST protein following controlled cortical impact injury in the rat. Western blot analyses of CAST in cortex and hippocampus detected two bands corresponding to molecular weights of 130 kDa [high-molecular-weight (HMW)] and 80 kDa [low-molecular-weight (LMW)]. A modest decrease in the HMW band in conjunction with a significant increase in the LMW band was observed in cortex ipsilateral to the site of impact following TBI. Examination of ipsilateral hippocampus revealed an increasing trend in the LMW band after injury, while no changes were observed in the HMW band. Thus, observable changes in CAST levels appear to occur several hours after reported calpain activation and cleavage of other substrates. In addition, a new analysis was performed on previously published data examining calpain activity in the same tissue samples used in the present study. These data suggest an association between decreases in calpain activity and accumulation of LMW CAST in the ipsilateral cortex following TBI. The present study cannot exclude proteolytic processing of CAST to LMW forms. However, the absence of reciprocity between changes in LMW and HMW bands in consistent with other data suggesting that rat brain could contain different CAST isoforms.

Stress: metaplastic effects in the hippocampus

Memory impairments, which occur regularly across species as a result of aging, disease and psychological insults (for example, stress), constitute a useful area for investigation into the neurobiological basis of learning and memory. Memory researchers have identified the hippocampus as a crucial brain structure involved in key aspects of memory formation. The most widely accepted putative mechanisms of memory storage in this structure are LTP and LTD. The hippocampus is enriched with receptors for corticosterone (a glucocorticoid hormone released in response to stress) and it plays a role in glucocorticoid negative feedback and, therefore, some hippocampal functioning might be particularly susceptible to stress. In support of this view, stress-induced modifications in learning, synaptic plasticity and endangerment of neurons have been seen in the hippocampus. Stress and glucocorticoids appear to exert a metaplastic effect through the modulation of Ca2+ levels. We propose a synaptic model that provides a conceptual scaffold to structure our understanding of the manifold actions of stress on the hippocampus. Accordingly, we suggest that stress-induced metaplasticity could disrupt Ca2+ homeostasis and thus endanger hippocampal neurons.

Calcium ion transients in neutrophils from patients with sporadic Alzheimer's disease

Abnormalities involving intracellular calcium homeostasis have been detected in Alzheimer's disease brain and fibroblasts as well as presenilin-1 mutation-bearing cells. In the present study we investigated inositol(1,4,5)trisphosphate-mediated calcium transients as well as calcium responses via mechanisms not related to surface receptors in Alzheimer's disease polymorphonuclear (PMN) granulocytes, using the tripeptide formyl-methionyl-leucyl-phenyl alanine (fMLP) and calcium ionophore ionomycin, respectively. fMLP elicited a biphasic response with an initial, fast increase in intracellular free calcium concentrations followed by a second, lower phase with no significant differences in either maximal response or time course between Alzheimer's disease granulocytes and controls. Similarly, the calcium signal elicited after ionomycin exposure was unchanged in Alzheimer's disease PMN. In conclusion, these results indicate that calcium mobilization from intracellular stores and via cross-membrane mechanisms is intact in Alzheimer's disease granulocytes.

Immunoblot analyses of the relative contributions of cysteine and aspartic proteases to neurofilament breakdown products following experimental brain injury in rats

Analyses using either one or two-dimensional gel electrophoresis were performed to identify the contribution of several proteases to lower molecular weight (MW) neurofilament 68 (NF68) break down products (BDPs) detected in cortical homogenates following unilateral cortical impact injury in rats. One dimensional immunoblot of BDPs obtained from in vitro cleavage of enriched neurofilaments (NF) by purified micro-calpain, m-calpain, cathepsin, B, cathepsin D, and CPP32 (caspase-3) were compared to in vivo samples from rats following traumatic brain injury (TBI). Comparison of these blots provided information on the relative contribution of different cysteine or aspartic proteases to NF loss following brain injury. As early as 3 hrs post-injury, cortical impact resulted in the presence of several lower MW NF68 immunopositive bands having patterns similar to those previously reported to be produced by calpain mediated proteolysis of neurofilaments. Only micro-calpain and m-calpain in vitro digestion of enriched neurofilaments contributed to the presence of the low MW 57 kD NF68 break down product (BDP) detected in post-TBI samples. Cathepsin B, cathepsin D, and caspase-3 failed to produce either the 53 kD or 57 kD NF BDPs. Further, 1 and 2 dimensional peptide maps containing a 1:1 ratio of in vivo and in vitro tissue samples showed complete comigration of lower MW immunopositive spots produced by TBI or in vitro incubation with m-calpain, thus providing additional evidence for the potential role of calpain activation to the production of NF68 BDPs following TBI. More importantly, 2-dimensional gel electrophoresis detected that immunopositive NF68 spots shifted to the basic pole (+) suggesting that dephosphorylation of the NF68 subunit pool may be associated with NF protein loss following TBI, an observation not previously noted in any model of experimental brain injury.

Catecholamines potentiate amyloid beta-peptide neurotoxicity: involvement of oxidative stress, mitochondrial dysfunction, and perturbed calcium homeostasis

Oxidative stress and mitochondrial dysfunction are implicated in the neuronal cell death that occurs in physiological settings and in neurodegenerative disorders. In Alzheimer's disease (AD) degenerating neurons are associated with deposits of amyloid beta-peptide (A beta), and there is evidence for increased membrane lipid peroxidation and protein oxidation in the degenerating neurons. Cell culture studies have shown that A beta can disrupt calcium homeostasis and induce apoptosis in neurons by a mechanism involving oxidative stress. We now report that catecholamines (norepinephrine, epinephrine, and dopamine) increase the vulnerability of cultured hippocampal neurons to A beta toxicity. The catecholamines were effective in potentiating A beta toxicity at concentrations of 10-200 microM, with the higher concentrations (100-200 microM) themselves inducing cell death. Serotonin and acetylcholine were not neurotoxic and did not modify A beta toxicity. Levels of membrane lipid peroxidation, and cytoplasmic and mitochondrial reactive oxygen species, were increased following exposure to neurons to A beta, and catecholamines exacerbated the oxidative stress. Subtoxic concentrations of catecholamines exacerbated decreases in mitochondrial energy charge and transmembrane potential caused by A beta, and higher concentrations of catecholamines alone induced mitochondrial dysfunction. Antioxidants (vitamin E, glutathione, and propyl gallate) protected neurons against the damaging effects of A beta and catecholamines, whereas the beta-adrenergic receptor antagonist propanolol and the dopamine (D1) receptor antagonist SCH23390 were ineffective. Measurements of intracellular free Ca2+ ([Ca2+]i) showed that A beta induced a slow elevation of [Ca2+]i which was greatly enhanced in cultures cotreated with catecholamines. Collectively, these data indicate a role for catecholamines in exacerbating A beta-mediated neuronal degeneration in AD and, when taken together with previous findings, suggest roles for oxidative stress induced by catecholamines in several different neurodegenerative conditions.

Elevation of cerebral proteases after systemic administration of aluminum

The levels of three proteases in the cerebral cortex of rats following a three week exposure to aluminum, were measured. The activity of apopain (CPP32), an interleukin 1beta converting enzyme (ICE)-like cysteine protease specifically associated with apoptosis, was increased following dosing with aluminum. The activity of calcium-activated neutral protease, calpain, was also increased. However, the enzyme activity of trypsin-like serine protease, known to be elevated by oxidative events, was unchanged. Since aluminum is suspected as a possible factor in the pathogenesis of Alzheimer's disease and other neurological diseases, it is speculated that changed levels in proteolytic enzymes may relate to the neurotoxicity of aluminum.

Mechanism of action of a new component of the Ca(2+)-dependent proteolytic system in rat brain: the calpain activator

Rat brain contains a calpain activator specific for the mu-form of the proteinase. We now report that this protein factor binds to the catalytic 80 kDa calpain subunit, promoting the dissociation of the heterodimer structure of the proteinase. The successive steps of the activation process, namely the two autoproteolytic steps producing the 78 kDa and the 75 kDa calpain forms, result in a 100 times faster rate. The activator competes with calpastatin and associates with the inner surface of plasma membranes. Based on its properties, the calpain activator can be visualised as the molecule indicating the sites for calpain activation at which the proteinase can also elude the negative control exerted by calpastatin.

The 5-HT1A receptor agonist BAY x 3702 prevents staurosporine-induced apoptosis

The 5-HT1A receptor agonist (-)-(R)-2-[4-[[(3,4-dihydro-2H-1-benzopyran-2-yl)methyl]amino]butyl]-1,2 -benzisothiazol-3(2H)-one1,1-dioxide monohydrochloride (BAY x 3702) was recently shown to have pronounced neuroprotective effects in rat models of cerebral ischemia and traumatic brain injury. In the present study we investigated the neuroprotective effects of BAY x 3702 in primary cultures of hippocampal and cortical neurons. Cell death was induced by 25 nM of the apoptosis inducing agent staurosporine and analyzed 24 h later by release of lactate dehydrogenase, formation of apoptotic bodies and DNA fragmentation. A significant neuroprotection was seen after pretreatment of the affected neurons with 50 pM to 1 microM BAY x 3702. The effects of BAY x 3702 were completely blocked by the selective 5-HT1A receptor antagonist N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride) (WAY-100635). These results indicate that low concentrations of BAY x 3702 protect cortical as well as hippocampal neurons from apoptotic cell death via a 5-HT1A receptor mediated pathway.

Properties of calpastatin forms in rat brain

Four recombinant calpastatin forms, deduced from rat brain mRNAs and differing in the number of inhibitory repetitive domains from zero to four, were expressed and characterized for their inhibitory efficiency on mu- and m-calpain. Although the most effective one is a truncated calpastatin form composed of the N-terminal region (domain L) and a single inhibitory domain, all inhibitors are more active against mu-calpain, but are preferentially degraded and inactivated by m-calpain. The protein form composed exclusively of a domain L is deprived of any inhibitory activity but prevents inhibition of calpain by the other calpastatin forms, indicating that this calpastatin region could be relevant in the recognition of the proteinase. A calpastatin form having molecular properties similar to those of the recombinant truncated calpastatin, has also been found in rat brain. It does not derive from proteolysis of a higher molecular mass precursor. The expression of multiple calpastatin forms may be relevant for the specific modulation of the different calpain isozymes normally present in a single cell type.

Loss of inositol 1,4,5-trisphosphate receptor sites and decreased PKC levels correlate with staging of Alzheimer's disease neurofibrillary pathology

Inositol 1,4,5-trisphosphate (IP3), inositol 1,3,4,5-tetrakisphosphate (IP4) and protein kinase C (PKC) play important roles in the phosphoinositide hydrolysis signal transducing pathway. Several studies have shown severe deficits in both IP3 receptor levels and PKC levels and activity in Alzheimer's disease brain, although the relationship of these changes to disease pathology is poorly understood. In the present study, we determined the autoradiographic localization of [3H]IP3 and [3H]IP4 binding to their calcium mobilizing receptor sites and [3H]phorbol 12,13-dibutyrate ([3H]PDBu) binding to PKC in sections of entorhinal cortex/hippocampal formation and cerebellum from 24 cases that had been staged for Alzheimer's disease-related neurofibrillary changes and amyloid deposition according to Braak and Braak [Acta Neuropathol. Berl., 82 (1991) 239-259]. Results indicated that [3H]IP3 binding showed a trend towards a decline with staging for neurofibrillary changes in the entorhinal region (0.05 < P < 0.10, ANOVA) and subiculum (0.05 < P < 0.10). In the former region, [3H]IP3 binding showed a significant decline with staging for amyloid deposition (P < 0.05). [3H]IP3 binding in the CA1 region showed statistically significant declines with respect to both neurofibrillary changes and amyloid staging (P < 0.05). [3H]IP3 binding levels in the other hippocampal subregions were too low to quantify accurately. The binding of [3H]IP4 showed no significant changes with either neurofibrillary changes or amyloid staging in any of the regions investigated. In contrast, [3H]PDBu binding showed significant declines with neurofibrillary staging in the entorhinal region (P < 0.01), subiculum (P < 0.001), CA1 (P < 0.001), CA2 (P < 0.001), CA3 (P < 0.001) and CA4 (P < 0.0001) regions and the dentate gyrus (P < 0.0001). Of these regions, only the subiculum showed a significant decline of [3H]PDBu binding with amyloid staging. There were no significant neurofibrillary or amyloid stage-related changes in either [3H]IP3, [3H]IP4 or [3H]PDBu binding in the molecular layer of the cerebellum. These findings suggest that reduced IP3 receptor and PKC levels in the entorhinal cortex/hippocampal formation reflect and may be important for the progression of Alzheimer's disease neurofibrillary pathology. The data also suggests that hippocampal IP3 receptor loss is related to the extent of amyloid deposition.

m-Calpain (calcium-activated neutral proteinase) in Alzheimer's disease brains

An antibody specific for the calpain isoform m-calpain was used to resolve conflicting results from several studies on the possible role of m-calpain in the pathogenesis of Alzheimer's disease (AD). Levels of the enzyme in both cytosolic and membranous fractions of brain tissue were determined by Western blot analysis. We also demonstrated changes in m-calpain molecules in AD brains using high-resolution 2D gel electrophoresis (2DE). The levels of the m-calpain isoform detected in the cytosolic fraction were significantly increased in AD brains when compared with the levels in controls. On 2DE, m-calpain molecules resolved into eight main spots. These spots were detected in AD brains as well as in control brains, suggesting that the calpain molecule was not qualitatively changed in AD. Quantitative analysis of the m-calpain spots on 2DE, on the other hand, indicated that the ratio of the intensity of four protein spots in the acidic region to that of the total spots was increased in AD brains.

Molecular and functional properties of a calpain activator protein specific for mu-isoforms

A natural calpain activator protein has been isolated from bovine brain and characterized in its properties and molecular structure. The protein is a homodimer with a molecular mass of about 30 kDa and results in being almost identical to UK114 goat liver protein. Significant similarities with mouse HR12 protein were also observed, whereas a lower degree of similarity was found with a family of heat-responsive proteins named YJGF and YABJ from Haemophilus influenzae and Bacillus subtilis, respectively. The brain activator expresses a strict specificity for the mu-calpain isoform, being completely ineffective on the m-calpain form. As expected, also UK114 was found to possess calpain-activating properties, indistinguishable from those of bovine brain activator. A protein showing the same calpain-activating activity has been also isolated from human red cells, indicating that this factor is widely expressed. All these activators are efficient on mu-calpain independently from the source of the proteinase. The high degree of specificity of the calpain activator for a single calpain isoform may be relevant for the understanding of sophisticated intracellular mechanisms underlying intracellular proteolysis. These data are indicating the existence of a new component of the Ca2+-dependent proteolytic system, constituted of members of a chaperonin-like protein family and capable of promoting intracellular calpain activation.

Oxidative stress inhibits calpain activity in situ

In this study, the effects of oxidative stress on calpain-mediated proteolysis and calpain I autolysis in situ were examined. Calpain activity was stimulated in SH-SY5Y human neuroblastoma cells with the calcium ionophore, ionomycin. Calpain-mediated proteolysis of the membrane-permeable fluorescent substrate N-succinyl-L-leucyl-L-leucyl-L-valyl-L-tyrosine-7-amido-4-methylcouma rin, as well as the endogenous protein substrates microtubule-associated protein 2, tau and spectrin, was measured. Oxidative stress, induced by addition of either doxorubicin or 2-mercaptopyridine N-oxide, resulted in a significant decrease in the extent of ionophore-stimulated calpain activity of both the fluorescent compound and the endogenous substrates compared with control, normoxic conditions. Addition of glutathione ethyl ester, as well as other antioxidants, resulted in the retention/recovery of calpain activity, indicating that oxidation-induced calpain inactivation was preventable/reversible. The rate of autolytic conversion of the large subunit of calpain I from 80 to 78 to 76 kDa was decreased during oxidative stress; however, the extent of calpain autolysis was not altered. These data indicate that oxidative stress may reversibly inactivate calpain I in vivo.

Secreted beta-amyloid precursor protein counteracts the proapoptotic action of mutant presenilin-1 by activation of NF-kappaB and stabilization of calcium homeostasis

Mutations in the presenilin-1 (PS-1) gene account for approximately 50% of the cases of autosomal dominant, early onset, inherited forms of Alzheimer's disease (AD). PS-1 is an integral membrane protein expressed in neurons and is localized primarily in the endoplasmic reticulum (ER). PS-1 mutations may promote neuronal degeneration by altering the processing of the beta-amyloid precursor protein (APP) and/or by engaging apoptotic pathways. Alternative processing of APP in AD may increase production of neurotoxic amyloid beta-peptide (Abeta) and reduce production of the neuroprotective alpha-secretase-derived form of APP (sAPPalpha). In differentiated PC12 cells expressing an AD-linked PS-1 mutation (L286V), sAPPalpha activated the transcription factor NF-kappaB and prevented apoptosis induced by Abeta. Treatment of cells with kappaB decoy DNA blocked the antiapoptotic action of sAPPalpha, demonstrating the requirement for NF-kappaB activation in the cytoprotective action of sAPPalpha. Cells expressing mutant PS-1 exhibited an aberrant pattern of NF-kappaB activity following exposure to Abeta, which was characterized by enhanced early activation of NF-kappaB followed by a prolonged depression of activity. Blockade of NF-kappaB activity in cells expressing mutant PS-1 by kappaB decoy DNA was associated with enhanced Abeta-induced increases of [Ca2+]i and mitochondrial dysfunction. Treatment of cells with sAPPalpha stabilized [Ca2+]i and mitochondrial function and suppressed oxidative stress by a mechanism involving activation of NF-kappaB. Blockade of ER calcium release prevented (and stimulation of ER calcium release by thapsigargin induced) apoptosis in cells expressing mutant PS-1, suggesting a pivotal role for ER calcium release in the proapoptotic action of mutant PS-1. Finally, a role for NF-kappaB in preventing apoptosis induced by ER calcium release was demonstrated by data showing that sAPPalpha prevents thapsigargin-induced apoptosis, an effect blocked by kappaB decoy DNA. We conclude that sAPPalpha stabilizes cellular calcium homeostasis and protects neural cells against the proapoptotic action of mutant PS-1 by a mechanism involving activation of NF-kappaB. The data further suggest that PS-1 mutations result in aberrant NF-kappaB regulation that may render neurons vulnerable to apoptosis.

Calbindin D28k blocks the proapoptotic actions of mutant presenilin 1: reduced oxidative stress and preserved mitochondrial function

Mutations in the presenilin 1 (PS-1) gene account for many cases of early-onset autosomal dominant inherited forms of Alzheimer's disease. Recent findings suggest that PS-1 mutations may sensitize neurons to apoptosis induced by trophic factor withdrawal and exposure to amyloid beta-peptide (Abeta). We now report that overexpression of the calcium-binding protein calbindin D28k prevents apoptosis in cultured neural cells expressing mutant PS-1 (L286V and M146V missense mutations). Elevations of the intracellular Ca2+ concentration and generation of reactive oxygen species induced by Abeta, and potentiated by mutant PS-1, were suppressed in calbindin-overexpressing cells. Impairment of mitochondrial function by Abeta (which preceded apoptosis) was exacerbated by PS-1 mutations and was largely prevented by calbindin. These findings suggest that PS-1 mutations render neurons vulnerable to apoptosis by a mechanism involving destabilization of cellular calcium homeostasis, which leads to oxidative stress and mitochondrial dysfunction.

Calpain I activation in rat hippocampal neurons in culture is NMDA receptor selective and not essential for excitotoxic cell death

Administration of glutamate (100 microM) to primary cultures of rat hippocampal neurons for 1 h led to calpain I activation as determined by monitoring the extent of spectrin breakdown with the antibodies designed to specifically recognize the calpain I-mediated spectrin breakdown products. Based on the studies with subtype selective antagonists of glutamate receptors, glutamate caused calpain I activation specifically through the activation of the NMDA receptor. In parallel experiments, the magnitude and the temporal profiles of Ca2+ rise were determined by Fura-2 microfluorimetry. Ca2+ influx through voltage-sensitive Ca2+ channels, even though leading to substantial Ca2+ rise, did not by itself activate calpain I. These results indicate that for calpain I activation, the source of Ca1+ influx is more important than the magnitude of Ca2+ rise. Glutamate-mediated calpain I activation was fully blocked by preincubation (30 min) of the cultures with calpain inhibitor I, calpain inhibitor II, or calpeptin (all 10 microM). The presence of calpain inhibitors did not, however, in any way ameliorate the massive excitotoxicity resulting from 16 h exposure to glutamate, indicating that calpain I activation and excitotoxicity are not causally related events. Similarly, preincubation with any of the tested calpain inhibitors was detrimental to the clearance of neuritic from a 10-min exposure to glutamate. Additionally, the presence of calpain inhibitors was detrimental to the clearance of neuritic varicosities resulting from a short-term sublethal exposure to glutamate, suggesting that a physiological level of calpain I activation might actually play an important homeostatic role in the restoration of normal cytoskeletal organization.

Rat brain contains multiple mRNAs for calpastatin

This work was undertaken to establish the forms of the calpain inhibitor, calpastatin, expressed in the brain tissue. Five cDNA clones were obtained and the corresponding amino acid sequences were deduced. Three of these proteins contain an N-terminal domain (domain L) and four inhibitory repeats typical of the calpastatin molecule. The other two are truncated forms, containing the domain L, free or associated with a single inhibitory repeat. Other differences, due to exon skipping, produce calpastatin forms with different susceptibility to posttranslational modifications. The more represented mRNA form corresponds to a calpastatin molecule containing the four inhibitory domains. These results may be useful to understand the involvement of calpain in the onset of acute and degenerative disorders of the central nervous system.

New inhibitors of calpain prevent degradation of cytoskeletal and myelin proteins in spinal cord in vitro

We have determined the effects of the calpain inhibitors AK275 and AK295 upon purified m-calpain and calcium-mediated degradation of neurofilament protein (NFP) in rat spinal cord in vitro. After incubation, the soluble radioactivity and/or extent of myelin basic protein (MBP) or NFP degradation was determined. Fifty percent of caseinolytic activity was inhibited by both inhibitors at 0.6 microM concentration, while more than 90% inhibition was seen at 1.6 microM. In contrast, 37% and 64% inhibition of MBP degradation was seen with AK295 and AK275, respectively, at 10 microM concentration. The extent of NFP degradation in spinal cord was quantified from immunoblot enhanced chemiluminescence. The calcium-mediated breakdown of NFP was inhibited by both AK275 and AK295, and the inhibition was dose-dependent. A 50% inhibition of NFP degradation was seen with AK295 at 10 microM and was almost completely inhibited at 25-50 microM. AK295 was slightly more potent than AK275. These studies suggest that these potent calpain inhibitors may be used therapeutically to provide neuroprotection in vivo in experimental central nervous system trauma and ischemia.

Altered gene expression for calpain/calpastatin system in motor neuron degeneration (Mnd) mutant mouse brain and spinal cord

The calcium-activated neutral proteases (CANP, calpains) have been implicated in both acute and chronic neurodegenerative processes. In the present study, we analyzed the in situ mRNA expression of calpain I and II and their endogenous inhibitor, calpastatin, in the motor neuron degeneration (Mnd) mutant mouse, which exhibits progressive dysfunction of the spinal cord and brain. As the disease progresses, the mutants show increasingly pronounced motor abnormalities which coincide with swelling of the spinal motor neurons, neocortex, hippocampal CA regions and cerebellar Purkinje cells. In situ hybridization studies show that the Mnd mice have a significantly higher level of calpain I, calpain II and calpastatin than the congenic controls in the following brain regions and cell types: hippocampal CA3 region, pyramidal cells, cerebellar Purkinje cells and spinal cord motor neurons. However, no differences in calpain or calpastatin mRNA levels are observed in glial and cerebellar granule cells of Mnd and control mice. Western blots and competitive RT-PCR analyses of brain and spinal cord homogenates are confirmative. Such altered gene expression in specific cell types of brain and spinal cord suggests the involvement of the calpain/calpastatin system.

Structure and physiological function of calpains

For a long time now, two ubiquitously expressed mammalian calpain isoenzymes have been used to explore the structure and function of calpain. Although these two calpains, mu- and m-calpains, still attract intensive interest because of their unique characteristics, various distinct homologues to the protease domain of mu- and m-calpains have been identified in a variety of organisms. Some of these 'novel' calpain homologues are involved in important biological functions. For example, p94 (also called calpain 3), a mammalian calpain homologue predominantly expressed in skeletal muscle, is genetically proved to be responsible for limb-girdle muscular dystrophy type 2A. Tra-3, a calpain homologue in nematodes, is involved in the sex determination cascade during early development. PalB, a key gene product involved in the alkaline adaptation of Aspergillus nidulans, is the first example of a calpain homologue present in fungi. These findings indicate various important functional roles for intracellular proteases belonging to the calpain superfamily.

Cell and molecular neurobiology of presenilins: a role for the endoplasmic reticulum in the pathogenesis of Alzheimer's disease?

Mutations in genes encoding presenilin-1 (PS-1) and presenilin-2 (PS-2) cause many cases of autosomal dominant inherited forms of early-onset Alzheimer's disease (AD). PSs are expressed in neurons throughout the nervous system, with differences in abundance among cell populations. PS-1 and PS-2 each have six to eight transmembrane domains and are localized mainly in the endoplasmic reticulum (ER). PSs may interact with cytoskeletal proteins and beta-amyloid precursor protein (APP) in ways consistent with roles in membrane trafficking and APP processing. Expression of mutant PSs in cultured cells and transgenic mice results in increased production of an amyloidogenic-cytotoxic form of amyloid beta-peptide (Abeta). Neural cells expressing mutant PSs exhibit increased sensitivity to apoptosis induced by trophic factor withdrawal and Abeta. The proapoptotic action of mutant PSs involves perturbed calcium release from ER stores and increased levels of oxidative stress. PS mutations may also suppress neurotransmitter synthesis in cholinergic neurons, suggesting a role in regulation of neuronal phenotype. Homology of PSs with the C. elegans gene sel-12 and phenotypic similarities of PS-1 and Notch knockout mice suggest a developmental role for PSs in somitogenesis. Collectively, the emerging data suggest intriguing roles of PSs in neuronal plasticity and cell death and highlight the importance of the ER as a regulatory site involved in the pathogenesis of neuronal degeneration in AD.

A purine nucleoside phosphorylase (PNP) inhibitor induces apoptosis via caspase-3-like protease activity in MOLT-4 T cells

Children with congenital homozygous deficiency of purine nucleoside phosphorylase (PNP) have abnormalities in purine metabolism that result in T-cell selective immune deficiency. The mechanism of action for cell death has been attributed to intracellular accumulation of dGTP, a potent inhibitor of ribonucleotide reductase and subsequently DNA synthesis, in thymocytes and T-cells but not B-cells. However, the mode of cell death has not been determined to be either necrosis or apoptosis. To examine the involvement of apoptosis in T-cells following PNP inhibition, MOLT-4 cells, a human T cell leukemia cell line, were co-treated with the PNP inhibitor, CI-1000 (2-amino 3,5-dihydro-7-(3-thienylmethyl)-4H-pyrrolo[3,2-d]-pyrimidin-4-one HCl), and 2'-deoxyguanosine (dGuo) which resulted in a concentration-dependent loss of cell viability (trypan blue) and inhibition of tritiated thymidine ([3H]-TdR) uptake. Staining of cells with the DNA dye Hoechst 33,258 showed nuclear morphology characteristic of apoptosis. Western blots (24 h lysates) were probed with antibodies against several proteins implicated in apoptosis. Anti-PARP revealed the presence of an 85 kD PARP breakdown product while, anti-alpha-spectrin revealed the accumulation a 120 kD breakdown product, both suggestive of CPP32 cleavage (caspase-3; an ICE-like cysteine protease). Western blots also detected the loss of the intact 32 kD caspase-3 isoform, a biochemical event associated with caspase-3 activation. Corresponding fluorometric activity assays detected a marked increase in caspase-3-like activity using the substrate Ac-DEVD-MCA. Lastly, a pan caspase inhibitor (Z-D-DCB) and 2'-deoxycytidine (dCyd), which is known to prevent dGTP accumulation following PNP inhibition, were able to prevent cell death and all indicators of caspase-3-like activity in MOLT-4 cells co-treated with dGuo and CI-1000. In summary, we provided several lines of evidence for the role of apoptosis and the contribution of caspase-3-like proteases in T-cell death following PNP inhibition.

The Calpain Hypothesis of Neurodegeneration: Evidence for a Common Cytotoxic Pathway

Calpain's general function and pathogenic role in the CNS are reviewed. Collectively, the literature indicates that calpain proteolysis plays a common and important role in a variety of acute neurodegenerative conditions, including focal ischemia (stroke), global ischemia, traumatic brain injury, and spinal cord injury. This evidence indicates that 1) calpain is activated in an abnormally sustained fashion during cellular events commonly associated with neurodegeneration (e.g., excessive interstitial glutamate and cytosolic calcium); 2) many of calpain's preferred substrates are degraded as important components in these neurodegenerative conditions; 3) calpain activation occurs early in the pathogenic cascade of each, prior to onset of substantial cell death; and 4) calpain inhibitors can effectively reduce the severity of neuronal damage and loss of function normally associated with these acute neurodegenerative perturbations. Calpain proteolysis is also implicated in chronic neurodegenerative diseases, with the strength of current evidence varying among specific diseases. The evidence accumulated for a plausible role in Alzheimer's disease (AD) is currently the strongest. For example, empirical links have been established between abnormal calpain proteolysis and 1) the cellular formation of classic Alzheimer's pathology, such as β-amyloid plaques, neurofibrillary tangles, and Alz-50 immunoreactivity; 2) the brain regions with greatest concentrations of AD-related pathology; and 3) the degeneration of key brain pathways vulnerable in the early stages of the disease. Similar, though less extensive, evidence exists for a potential role of abnormal calpain proteolysis in Parkinson's disease. Finally, for several other chronic neurodegenerative conditions (e.g., Huntington's disease and amyotrophic lateral sclerosis), early evidence is emerging that calpain may also play some pathogenic role. Thus, these data support the possibility that uncontrolled calpain proteolysis may contribute to and/or accelerate the loss of neurons associated with a wide range of neurodegenerative conditions and may, therefore, represent an important, final common cytotoxic pathway for many diverse forms of neurodegeneration. NEUROSCIENTIST 3:314–327, 1997

Aluminum-induced structural alterations of the precursor of the non-A beta component of Alzheimer's disease amyloid

The precursor of the non-A beta component of Alzheimer's disease amyloid (NACP) is a presynaptic protein whose function has been suspected to be tightly involved in neuronal biogenesis including synaptic regulations. NACP was suggested to seed the neuritic plaque formation in the presence of A beta during the development of Alzheimer's disease (AD). Recombinant NACP purified through heat treatment, DEAE-Sephacel anion-exchange, Sephacryl S-200 size-exclusion, and S-Sepharose cation-exchange chromatography steps appeared as a single band on SDS-PAGE with Mr of 19 kDa. Its N-terminal amino acid sequence clearly confirmed that the protein was NACP. Interestingly, however, the protein was split into a doublet on a nondenaturing (ND)-PAGE with equal intensities. The doublet was located slightly above a 45-kDa marker protein on a 12.5% ND-PAGE. In addition, the size of NACP was more carefully estimated as 53 kDa with high-performance gel-permeation chromatography using a TSK G3000sw size-exclusion column. Recently, Lansbury and his colleagues (Biochemistry 35, 13709-13715) have reported that NACP exists as an elongated "natively unfolded" structure which would make the protein more actively involved in protein-protein interactions and Kim (Mol. Cells 7, 78-83) has also shown that the natively unfolded protein is extremely sensitive to proteases. Here, we report that the structure of NACP could be altered by certain environmental factors. Aluminum, a suspected risk factor for AD, converged the doublet of NACP into a singlet with slightly lower mobility on ND-PAGE. Spectroscopic analysis employing uv absorption, intrinsic fluorescence, and circular dichroism indicated that NACP experienced the structural alterations in the presence of aluminum such as the secondary structure transition to generate about 33% alpha-helix. This altered structure of NACP became resistant to proteases such as trypsin, alpha-chymotrypsin, and calpain. Therefore, it is suggested that aluminum, which influences two pathologically critical processes in AD such as the protein turnover and the protein aggregation via the structural modifications, could participate in the disease.

Modulation of rat brain calpastatin efficiency by post-translational modifications

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.

Active site-directed antibodies identify calpain II as an early-appearing and pervasive component of neurofibrillary pathology in Alzheimer's disease

Calpain proteases influence intracellular signaling pathways and regulate cytoskeleton organization, but the neuronal and pathological roles of individual isoenzymes are unknown. In Alzheimer's disease (AD), the activated form of calpain I is significantly increased while the fate of calpain II has been more difficult to address. Here, calpain II antibodies raised to different sequences within a cryptic region around the active site, which becomes exposed during protease activation, were shown immunohistochemically to bind extensively to neurofibrillary tangles (NFT), neuritic plaques, and neuropil threads in brains from individuals with AD. Additional 'pre-tangle' granular structures in neurons were also intensely immunostained, indicating calpain II mobilization at very early stages of NFT formation. Total levels of calpain II remained constant in the prefrontal cortex of AD patients but were increased 8-fold in purified NFT relative to levels of calpain I. These results implicate activated calpain II in neurofibrillary degeneration, provide further evidence for the involvement of the calpain system in AD pathogenesis, and imply that neuronal calcium homeostasis is altered in AD.

Oxidation inhibits substrate proteolysis by calpain I but not autolysis

In this study, the effects of oxidation on calpain I autolysis and calpain-mediated proteolysis were examined. Calpain I was incubated with increasing concentrations of free calcium in the presence or absence of oxidant, and autolytic conversion of both the 80- and 30-kDa subunits was measured by immunoblotting utilizing monoclonal antibodies which recognize both autolyzed and non-autolyzed forms of each subunit, respectively. Autolytic conversion of the 80-kDa subunit of calpain I was not detected until free calcium concentration was greater than 40 microM, whereas autolysis of the 30-kDa subunit did not occur until the free calcium concentration was greater than 100 microM. In addition, autolytic conversion of either the 80- or 30-kDa subunit was not inhibited by the presence of oxidant. Calpain I activity was measured using the fluorescent peptide N-succinyl-L-leucyl-L-leucyl-L-valyl-L-tyrosine-7-amido-4- methylcoumarin or the microtubule-associated protein tau as substrate. Calpain I was found to have proteolytic activity at free calcium concentrations below that required for autolysis. Calpain I activity was strongly inhibited by oxidant at all calcium concentrations studied, suggesting that proteolytic activity of both the non-autolyzed 80-kDa and autolyzed 76-kDa forms was susceptible to oxidation. Interestingly, whereas oxidation did not inhibit autolytic conversion, the presence of high substrate concentrations did result in a significant reduction of autolysis without altering calpain proteolytic activity. Calpain I activity that had been inhibited by the presence of oxidant was recovered immediately by addition of the reducing agent dithiothreitol.

An immunocytochemical study of calpain II in the hippocampus of rats injected with kainate

The distributions of the kainate/DL-alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (KA/ AMPA) receptors GluR1 and calcium-activated neutral protease II (calpain II) in the hippocampus of normal and kainate-lesioned rats were studied by immunocytochemistry. There was a reduction in GluR1 immunoreactivity and a slight increase in calpain II immunoreactivity on the dendrites of pyramidal neurons in CA fields affected by the kainate at 18 h postinjection. Calpain II immunore-activity was associated with amyloid fibrils at electron microscopy. These fibrils were most often intracellular, in membrane-bound profiles, some of which were contacted by axon terminals and were identified as degenerating dendrites. There was extensive destruction of mitochondrial membranes in degenerating profiles, and accumulations of amyloid fibrils were often localised in mitochondria in a calpain-positive profile. This was unlike other, calpain-negative degenerating profiles, that contained tubulovesicular profiles or multilamellar bodies, where mitochondrial membranes were preserved. Many more calpain-positive profiles were observed at electron microscopy 6 days after kainate injection. The enzyme was present in macrophages and astrocytes in lesioned areas.

Leishmania major: comparison of the cathepsin L- and B-like cysteine protease genes with those of other trypanosomatids

Cysteine proteases play important roles in the pathogenesis of several parasitic infections and have been proposed as targets for the structure-based strategy of drug design. As a first step toward applying this strategy to design inhibitors as antiparasitic agents for leishmaniasis, we have isolated and sequenced the full-length clones of two cysteine protease genes from Leishmania major. One of the genes is structurally similar to the cathepsin L-like family and the other is similar to the cathepsin B-like family of cysteine proteases. The L. major cathepsin L-like sequence has a proregion that shares high sequence similarity with other cathepsin L sequences but not cathepsin B sequences and has a proline/threonine-rich C-terminal extension. The cathepsin L-like gene occurs in multiple copies, whereas there may be only one copy of the cathepsin B-like gene. Northern blot analyses show that both genes are expressed in the promastigote and amastigote stages, and pulse field gel electrophoresis revealed that the cathepsin L- and B-like genes are each found on two nonhomologous chromosomes. The L. major L-like amino acid sequence is 75% identical to the L. mexicana sequence, 74% identical to the L. pifanoi sequence, 47% identical with the Trypanosoma cruzi sequence, 47% identical with the T. congolense sequence, and 45% identical with the T. brucei sequence. L. major is one of two trypanosomatid species for which a cathepsin B-like gene has been identified and sequenced; its amino acid sequence is 82% identical to the one from L. mexicana. Tree inference based on distance and parsimony methods of kinetoplastid cathepsin L proteins yielded independent support for phylogenetic hypotheses inferred from analyses of ribosomal RNA genes. Because the cathepsin L locus has a high level of phylogenetic signal with respect to trypanosomatid taxa, this locus has great potential utility for investigating the evolutionary history of trypanosomatids and related organisms.

Immunocytochemistry of tau phosphoserine 413 and tau protein kinase I in Alzheimer pathology

One unique phosphorylation site consistently found in paired helical filament tau, serine 413, is modified by tau protein kinase I/glycogen synthase kinase-3 beta but no other known tau kinase. Here we present immunocytochemistry from Alzheimer's disease brains showing that focal subpopulations of hippocampal CA1 pyramidal neurons and neuritic plaques are strongly reactive for tau protein kinase I/glycogen synthase kinase-3 beta and tau phosphoserine 413 in early stages of pathology. Colocalization of these epitopes suggests that tau protein kinase I/glycogen synthase kinase-3 beta abnormally phosphorylates tau and is in a position to disrupt neuronal metabolism in anatomical areas vulnerable to Alzheimer's disease.

Increased M-calpain expression in the mesencephalon of patients with Parkinson's disease but not in other neurodegenerative disorders involving the mesencephalon: a role in nerve cell death?

Parkinson's disease is characterized by the loss of dopaminergic neurons in the substantia nigra and, to a lesser extent, the ventral tegmental area and catecholaminergic cell group A8. However, among these dopaminergic neurons, those expressing the calcium buffering protein calbindin are selectively preserved, suggesting that a rise in intracellular calcium concentrations may be involved in the cascade of events leading to nerve cell death in Parkinson's disease. We therefore analysed immunohistochemically the expression of the calcium-dependent protease calpain II (m-calpain) in the mesencephalon of patients with Parkinson's disease, progressive supranuclear palsy or striatonigral degeneration, where nigral dopaminergic neurons degenerate, and matched controls without nigral involvement. Calpain immunoreactivity was found in fibers and neuronal perikarya in the substantia nigra, the ventral tegmental area, catecholaminergic cell group A8 and the locus coeruleus. In patients with Parkinson's disease but not with the other neurodegenerative disorders, m-calpain immunoreactivity was detected in fibers with an abnormal morphology and in Lewy bodies. Sequential double staining revealed that most of these m-calpain-positive fibers and neuronal perikarya co-expressed tyrosine hydroxylase, indicating that most m-calpain neurons are catecholaminergic. Quantitative analysis of m-calpain staining in the substantia nigra and locus coeruleus revealed an increased density of fibers and neuronal perikarya in parkinsonian patients in both structures. These data suggest that increased calcium concentrations may be associated with nerve cell death in Parkinson's disease.

The putatively protective Onchocerca volvulus neuronal protein E1 is a member of the death domain protein family

Here we show that E1, an ankyrin-related, potentially protective, neuronal protein of the human filarial nematode Onchocerca volvulus contains a death domain (DD), most similar to that of human Mort1/FADD (39% identity). In addition, sequence comparison of E1 to its homologue from Litomosoides sigmodontis and to Caenorhabditis elegans ankyrin defines two further putative functional domains. One represents the end of the spectrin-binding domain of ankyrins, the other an unique domain, most highly conserved between these nematodes, containing a calpain sequence motif. Thus, E1 may be involved in apoptosis, raising the possibility that protection against this parasitic helminth may be induced by apoptotic processes.

Deleterious network hypothesis of Alzheimer's disease

Numerous studies indicate that aberrant amyloid precursor protein metabolism, elevated peroxidative damage, depressed energy metabolism and altered calcium homeostasis are four pivotal deleterious factors in the pathogenesis of Alzheimer's disease. Cumulative evidence further suggests that these four factors are intimately interrelated, forming a deleterious network. Based on this new concept of 'deleterious network', a unifying hypothesis-the deleterious network hypothesis of Alzheimer's disease-is proposed. The main ideas of the hypothesis are delineated as follows: increases in free radical damage, alterations in amyloid precursor protein metabolism, impairment of energy metabolism and abnormalities of calcium homeostasis are four cornerstones of a deleterious network. Various risk factors of Alzheimer's disease can triger the network by promoting the occurrence of one of these key components, resulting in the biological abnormalities of Alzheimer's disease. Based on this new theory, a majority of the important observations about Alzheimer's disease can be explained consistently and succinctly.

Phospholipids inhibit proteolysis of protein kinase C alpha by mM calcium-requiring calpain

The alpha isoform of protein kinase C (PKC alpha) is rapidly hydrolyzed by mM Ca(2+)-requiring calpain (calcium-activated neutral proteinase) under cell-free conditions (Shea et al, 1994, FEBS Lett. 350:223). In the present study, we demonstrate that this hydrolysis is inhibited by phosphatidyl serine, diacylglycerol, phosphatidyl choline, phosphatidyl inositol, and phosphatidic acid. With the exception of phosphatidic acid, these phospholipids did not directly inhibit calpain activity as evidenced by degradation of [14C]azocasein, suggesting that the nature of inhibition of calpain-mediated PKC alpha degradation is due to an effect of phospholipids on PKC alpha conformation. These findings suggest that m calpain-mediated PKC alpha hydrolysis may be specifically minimized at the plasma membrane, and leave open the possibility that such a mechanism exists in situ. In addition, the unique inhibition of calpain activity by phosphatidic acid suggests the existence of a specific mechanism by which this phospholipid regulates PKC alpha activity.