Neuronal fodrin proteolysis occurs independently of excitatory amino acid-induced neurotoxicity

Reactive protoplasmic and fibrous astrocytes contain high levels of calpain-cleaved alpha 2 spectrin

Calpain, a family of calcium-dependent neutral proteases, plays important roles in neurophysiology and pathology through the proteolytic modification of cytoskeletal proteins, receptors and kinases. Alpha 2 spectrin (αII spectrin) is a major substrate for this protease family, and the presence of the αII spectrin breakdown product (αΙΙ spectrin BDP) in a cell is evidence of calpain activity triggered by enhanced intracytoplasmic Ca(2+) concentrations. Astrocytes, the most dynamic CNS cells, respond to micro-environmental changes or noxious stimuli by elevating intracytoplasmic Ca(2+) concentration to become activated. As one measure of whether calpains are involved with reactive glial transformation, we examined paraffin sections of the human cerebral cortex and white matter by immunohistochemistry with an antibody specific for the calpain-mediated αΙΙ spectrin BDP. We also performed conventional double immunohistochemistry as well as immunofluorescent studies utilizing antibodies against αΙΙ spectrin BDP as well as glial fibrillary acidic protein (GFAP). We found strong immunopositivity in selected protoplasmic and fibrous astrocytes, and in transitional forms that raise the possibility of some of fibrous astrocytes emerging from protoplasmic astrocytes. Immunoreactive astrocytes were numerous in brain sections from cases with severe cardiac and/or respiratory diseases in the current study as opposed to our previous study of cases without significant clinical conditions that failed to reveal such remarkable immunohistochemical alterations. Our study suggests that astrocytes become αΙΙ spectrin BDP immunopositive in various stages of activation, and that spectrin cleavage product persists even in fully reactive astrocytes. Immunohistochemistry for αΙΙ spectrin BDP thus marks reactive astrocytes, and highlights the likelihood that calpains and their proteolytic processing of spectrin participate in the morphologic and physiologic transition from resting protoplasmic astrocytes to reactive fibrous astrocytes.

Degradation of βII-Spectrin Protein by Calpain-2 and Caspase-3 Under Neurotoxic and Traumatic Brain Injury Conditions

A major consequence of traumatic brain injury (TBI) is the rapid proteolytic degradation of structural cytoskeletal proteins. This process is largely reflected by the interruption of axonal transport as a result of extensive axonal injury leading to neuronal cell injury. Previous work from our group has described the extensive degradation of the axonally enriched cytoskeletal αII-spectrin protein which results in molecular signature breakdown products (BDPs) indicative of injury mechanisms and to specific protease activation both in vitro and in vivo. In the current study, we investigated the integrity of βII-spectrin protein and its proteolytic profile both in primary rat cerebrocortical cell culture under apoptotic, necrotic, and excitotoxic challenge and extended to in vivo rat model of experimental TBI (controlled cortical impact model). Interestingly, our results revealed that the intact 260-kDa βII-spectrin is degraded into major fragments (βII-spectrin breakdown products (βsBDPs)) of 110, 108, 85, and 80 kDa in rat brain (hippocampus and cortex) 48 h post-injury. These βsBDP profiles were further characterized and compared to an in vitro βII-spectrin fragmentation pattern of naive rat cortex lysate digested by calpain-2 and caspase-3. Results revealed that βII-spectrin was degraded into major fragments of 110/85 kDa by calpain-2 activation and 108/80 kDa by caspase-3 activation. These data strongly support the hypothesis that in vivo activation of multiple protease system induces structural protein proteolysis involving βII-spectrin proteolysis via a specific calpain and/or caspase-mediated pathway resulting in a signature, protease-specific βsBDPs that are dependent upon the type of neural injury mechanism. This work extends on previous published work that discusses the interplay spectrin family (αII-spectrin and βII-spectrin) and their susceptibility to protease proteolysis and their implication to neuronal cell death mechanisms.

Initial biological qualification of SBDP-145 as a biomarker of compound-induced neurodegeneration in the rat

Detection of compound-related neurodegeneration is currently limited to brain histopathology in veterinary species and functional measurements such as electroencephalography and observation of clinical signs in patients. The objective of these studies was to investigate whether concentrations of spectrin breakdown product 145 (SBDP-145) in cerebrospinal fluid (CSF) correlate with the severity of neurodegeneration in rats administered neurotoxic agents, as part of a longer term objective of developing in vivo biomarkers of neurotoxicity for use in non-clinical and clinical safety studies. Non-erythroid alpha-II spectrin is a cytoskeletal protein cleaved by the protease calpain when this enzyme is activated by dysregulation of calcium in injured cells. Calcium dysregulation is also associated with some toxicological responses in animals, and may be sufficient to activate neuronal calpain and produce SBDPs that can be released into CSF. Neurotoxicants (kainic acid, 2-chloropropionic acid, bromethalin, and pentylenetetrazole) known to affect different portions of the brain were administered to rats in dose-response and time-course studies in which neurodegeneration was measured by histopathology and SBDP-145 concentrations in CSF were measured by ELISA. We consistently observed >3-fold increases in SBDP-145 concentration in rats with minimal to slight neurodegenerative lesions, and 20 to 150-fold increases in animals with more severe lesions. In contrast, compounds that caused non-degenerative changes in central nervous system (CNS) did not increase SBDP-145 in CSF. These data support expanded use of SBDP-145 as a biomarker for monitoring compound-induced neurodegeneration in pre-clinical studies, and support the investigation of clinical applications of this biomarker to promote safe dosing of patients with compounds that have potential to cause neurodegeneration.

Composite fatty acid ether amides suppress growth of liver cancer cells in vitro and in an in vivo allograft mouse model

BACKGROUND

The heterogeneity of liver cancer, in particular hepatocellular carcinoma (HCC), portrays the requirement of multiple targets for both its treatment and prevention. Multifaceted agents, minimally or non-toxic for normal hepatocytes, are required to address the molecular diversity of HCC, including the resistance of putative liver cancer stem cells to chemotherapy.

METHODS

We designed and synthesized two fatty acid ethers of isopropylamino propanol, C16:0-AIP-1 and C18:1-AIP-2 (jointly named AIPs), and evaluated their anti-proliferative effects on the human HCC cell line Huh7 and the murine hepatoma cell line BNL 1MEA.7R.1, both in vitro and in an in vivo allograft mouse model.

RESULTS

We found that AIP-1 and AIP-2 inhibited proliferation and caused cell death in both Huh7 and BNL 1MEA.7R.1 cells. Importantly, AIP-1 and AIP-2 were found to block the activation of putative liver cancer stem cells as manifested by suppression of clonal 'carcinosphere' development in growth factor-free and anchorage-free medium. The AIPs exhibited a relatively low toxicity against normal human or rat hepatocytes in primary cultures. In addition, we found that the AIPs utilized multifaceted pathways that mediate both autophagy and apoptosis in HCC, including the inhibition of AKTs and CAMK-1. In immune-competent mice, the AIPs significantly reduced BNL 1MEA.7R.1 cell-driven tumor allograft development, with a higher efficiency than sorafenib. A combination of AIP-1 + AIP-2 was most effective in reducing the tumor allograft incidence.

CONCLUSIONS

AIPs represent a novel class of simple fatty acid derivatives that are effective against liver tumors via diverse pathways. They show a low toxicity towards normal hepatocytes. The addition of AIPs may represent a new avenue towards the management of chronic liver injury and, ultimately, the prevention and treatment of HCC.

A transient asymmetric distribution of XNOA 36 mRNA and the associated spectrin network bisects Xenopus laevis stage I oocytes along the future A/V axis

In Xenopus oogenesis, the mechanisms governing the localisation of molecules crucial for primary axis determination have been uncovered in recent years. In stage I oocytes, the mitochondrial cloud (MC) entraps RNAs implicated in germ line specification and other RNAs, such as Xwnt-11 and Xlsirts, that are later delivered to the vegetal pole. Microfilaments and microtubules gradually develop in the cytoplasm, sustaining organelles as well as the MC. At stage III, other mRNAs migrate to the vegetal hemisphere through a microtubule-dependent mechanism. We report here the isolation of a cDNA encoding XNOA 36, a highly conserved protein, whose function is to date not fully understood. The XNOA 36 transcript is abundantly accumulated in stage I oocytes where it decorates a filamentous network. At the end of stage I the transcript gradually segregates in a sector of the oocyte surrounding the MC and opposite the ovarian hylum. Here, XNOA 36 mRNA distributes in a gradient-like pattern extending from a peripheral network towards the interior of the oocyte. This distribution is similar to that of alpha-spectrin mRNA. Both mRNAs are segregated in one half of the 250 microm oocytes, with the MC located between the XNOA 36/alpha-spectrin mRNA-labelled and unlabelled regions. XNOA 36 mRNA localisation was uncoupled from that of alpha-spectrin mRNA by cytochalasin B or ice-nocodazole treatments, suggesting that the two transcripts rely on different mechanisms for their localisation. However, immunolocalisation experiments coupled with in situ hybridisation revealed that the XNOA 36 transcript co-localises with the protein spectrin. This observation, together with the finding that XNOA 36 mRNA co-precipitates with spectrin, indicates that these two molecules interact physically. In conclusion, our data suggest that XNOA 36 mRNA is localized and/or anchored in the oocyte through a cytoskeletal network containing spectrin. The putative implications of this finding are discussed.

Sequential degradation of alphaII and betaII spectrin by calpain in glutamate or maitotoxin-stimulated cells

Calpain-catalyzed proteolysis of II-spectrin is a regulated event associated with neuronal long-term potentiation, platelet and leukocyte activation, and other processes. Calpain proteolysis is also linked to apoptotic and nonapoptotic cell death following excessive glutamate exposure, hypoxia, HIV-gp120/160 exposure, or toxic injury. The molecular basis for these divergent consequences of calpain action, and their relationship to spectrin proteolysis, is unclear. Calpain preferentially cleaves II spectrin in vitro in repeat 11 between residues Y1176 and G1177. Unless stimulated by Ca++ and calmodulin (CaM), betaII spectrin proteolysis in vitro is much slower. We identify additional unrecognized sites in spectrin targeted by calpain in vitro and in vivo. Bound CaM induces a second II spectrin cleavage at G1230*S1231. BetaII spectrin is cleaved at four sites. One cleavage only occurs in the absence of CaM at high enzyme-to-substrate ratios near the betaII spectrin COOH-terminus. CaM promotes II spectrin cleavages at Q1440*S1441, S1447*Q1448, and L1482*A1483. These sites are also cleaved in the absence of CaM in recombinant II spectrin fusion peptides, indicating that they are probably shielded in the spectrin heterotetramer and become exposed only after CaM binds alphaII spectrin. Using epitope-specific antibodies prepared to the calpain cleavage sites in both alphaII and betaII spectrin, we find in cultured rat cortical neurons that brief glutamate exposure (a physiologic ligand) rapidly stimulates alphaII spectrin cleavage only at Y1176*G1177, while II spectrin remains intact. In cultured SH-SY5Y cells that lack an NMDA receptor, glutamate is without effect. Conversely, when stimulated by calcium influx (via maitotoxin), there is rapid and sequential cleavage of alphaII and then betaII spectrin, coinciding with the onset of nonapoptotic cell death. These results identify (i) novel calpain target sites in both alphaII and betaII spectrin; (ii) trans-regulation of proteolytic susceptibility between the spectrin subunits in vivo; and (iii) the preferential cleavage of alphaII spectrin vs betaII spectrin when responsive cells are stimulated by engagement of the NMDA receptor. We postulate that calpain proteolysis of spectrin can activate two physiologically distinct responses: one that enhances skeletal plasticity without destroying the spectrin-actin skeleton, characterized by preservation of betaII spectrin; or an alternative response closely correlated with nonapoptotic cell death and characterized by proteolysis of betaII spectrin and complete dissolution of the spectrin skeleton.

Calpain and synaptic function

Proteolysis by calpain is a unique posttranslational modification that can change integrity, localization, and activity of endogenous proteins. Two ubiquitous calpains, mu-calpain and m-calpain, are highly expressed in the central nervous system, and calpain substrates such as membrane receptors, postsynaptic density proteins, kinases, and phosphatases are localized to the synaptic compartments of neurons. By selective cleavage of synaptically localized molecules, calpains may play pivotal roles in the regulation of synaptic processes not only in physiological states but also during various pathological conditions. Activation of calpains during sustained synaptic activity is crucial for Ca2+-dependent neuronal functions, such as neurotransmitter release, synaptic plasticity, vesicular trafficking, and structural stabilization. Overactivation of calpain following dysregulation of Ca2+ homeostasis can lead to neuronal damage in response to events such as epilepsy, stroke, and brain trauma. Calpain may also provide a neuroprotective effect from axotomy and some forms of glutamate receptor overactivation. This article focuses on recent findings on the role of calpain-mediated proteolytic processes in potentially regulating synaptic substrates in physiological and pathophysiological events in the nervous system.

Glutamate-induced apoptosis in neuronal cells is mediated via caspase-dependent and independent mechanisms involving calpain and caspase-3 proteases as well as apoptosis inducing factor (AIF) and this process is inhibited by equine estrogens

BACKGROUND

Glutamate, a major excitatory amino acid neurotransmitter, causes apoptotic neuronal cell death at high concentrations. Our previous studies have shown that depending on the neuronal cell type, glutamate-induced apoptotic cell death was associated with regulation of genes such as Bcl-2, Bax, and/or caspase-3 and mitochondrial cytochrome c. To further delineate the intracellular mechanisms, we have investigated the role of calpain, an important calcium-dependent protease thought to be involved in apoptosis along with mitochondrial apoptosis inducing factor (AIF) and caspase-3 in primary cortical cells and a mouse hippocampal cell line HT22.

RESULTS

Glutamate-induced apoptotic cell death in neuronal cells was associated with characteristic DNA fragmentation, morphological changes, activation of calpain and caspase-3 as well as the upregulation and/or translocation of AIF from mitochondria into cytosol and nuclei. Our results reveal that primary cortical cells and HT22 cells display different patterns of regulation of these genes/proteins. In primary cortical cells, glutamate induces activation of calpain, caspase-3 and translocation of AIF from mitochondria to cytosol and nuclei. In contrast, in HT22 cells, only the activation of calpain and upregulation and translocation of AIF occurred. In both cell types, these processes were inhibited/reversed by 17beta-estradiol and Delta8,17beta-estradiol with the latter being more potent.

CONCLUSION

Depending upon the neuronal cell type, at least two mechanisms are involved in glutamate-induced apoptosis: a caspase-3-dependent pathway and a caspase-independent pathway involving calpain and AIF. Since HT22 cells lack caspase-3, glutamate-induced apoptosis is mediated via the caspase-independent pathway in this cell line. Kinetics of this apoptotic pathway further indicate that calpain rather than caspase-3, plays a critical role in the glutamate-induced apoptosis. Our studies further indicate that glutamate- induced changes of these proteins can be inhibited by estrogens, with Delta8,17beta-estradiol, a novel equine estrogen being more potent than 17beta-estradiol. To our knowledge, this is the first demonstration that glutamate-induced apoptosis involves regulation of multiple apoptotic effectors that can be inhibited by estrogens. Whether these observations can help in the development of novel therapeutic approaches for the prevention of neurodegenerative diseases with estrogens and calpain inhibitors remains to be investigated.

? and ? spectrin distribution during the differentiation of pyriform cells in follicles of lizardPodarcis sicula

Using alpha and beta spectrin mammalian antibodies on Western blotting, we demonstrated that lizard ovarian follicles contain two isoforms of alpha spectrin, Mr 94 and 134 kDa, and a 230 kDa beta spectrin, and that their pattern modifies in relation to pyriform cell differentiation. In fact, a positive immunoreaction is firstly evident within follicular epithelium of previtellogenic follicles when small cells differentiate into pyriform cells via intermediate cells. Later on, immunostain is present in pyriform cells and in the oocyte cortex that previously appears unstained. It is noteworthy that immunostain is also present on small cells located in contact with the oocyte membrane, but not on those located under the basal lamina and among pyriform cells, not engaged in pyriform cell differentiation. During the subsequent stages of previtellogenic phase, spectrin immunostain over the follicular epithelium and in the oocyte cortex does not change. By contrast, in vitellogenic follicles, when the follicular epithelium is constituted only by small cells, immunostain is evident at the level of the oocyte cortex and the cytoplasm of regressing pyriform cells. The present data strongly suggest that the alpha and beta spectrin pattern put in evidence during the different phases of lizard oocyte growth is related to the differentiation of small into pyriform cells, where such protein may guarantee a relationship between surface glycoproteins (Andreuccetti et al., 2001: Anat Rec 263:1-9), and the cytoskeleton distribution (Maurizii et al., 2000: Raf Mol Reprod Dev 57:159-166). Furthermore, the distribution of spectrin mRNA, similar to that observed for the protein, demonstrates that spectrin, once synthesized within pyriform cells, is transferred through intercellular bridges in the oocyte cortex, thus confirming that pyriform cells are nurse that significantly are involved in the oocyte growth. Finally, the present data demonstrate that alpha spectrin of lizard ovarian follicles has Mr quite different from those so far reported and may constitute a new group of isoforms. This important result will be the focus of future experiments. Mol. Reprod. Dev. 67: 101-107, 2004.

Calpain proteolysis of alpha II-spectrin in the normal adult human brain

The proteolysis of alphaII-spectrin by calpain may be physiologically involved with synaptic remodeling, long-term potentiation, and memory formation. Calpain activation may also mediate neuronal apoptosis, responses to hypoxic insult, and excitotoxic injury. Surprisingly little is known of the activity of these calpain-mediated processes in the adult human brain. Using an antibody that specifically recognizes calpain-cleaved alphaII-spectrin, we have mapped the topographic distribution of the major alphaII-spectrin break-down product (alphaII-bdp1) in six adult brains examined post-mortem. All brains were from patients without evident neurological disease. Focally positive alphaII-bdp1 was consistently detected in the neuropil of the cortical gray matter, in occasional pyramidal neurons, and in rare reactive astrocytes in the cerebral cortex and hippocampus. Cerebellar Purkinje cells were more frequently, and more intensely, immunopositive. In all fields, staining was most intense in the soma and dendrites of neurons. There was no correlation of the frequency of positive cells with the postmortem interval or clinical condition. While these findings do not rigorously exclude contributions from postmortem calpain activation, they do suggest that a low-level of calpain processing of alphaII-spectrin is likely to be a constitutive process in the adult human brain.

Caspase remodeling of the spectrin membrane skeleton during lens development and aging

Terminal differentiation of lens fiber cells resembles the apoptotic process in that organelles are lost, DNA is fragmented, and changes in membrane morphology occur. However, unlike classically apoptotic cells, which are disintegrated by membrane blebbing and vesiculation, aging lens fiber cells are compressed into the center of the lens, where they undergo cell-cell fusion and the formation of specialized membrane interdigitations. In classically apoptotic cells, caspase cleavage of the cytoskeletal protein alpha-spectrin to approximately 150-kDa fragments is believed to be important for membrane blebbing. We report that caspase(s) cleave alpha-spectrin to approximately 150-kDa fragments and beta-spectrin to approximately 120- and approximately 80-kDa fragments during late embryonic chick lens development. These fragments continue to accumulate with age so that in the oldest fiber cells of the adult lens, most, if not all, of the spectrin is cleaved to discrete fragments. Thus, unlike classical apoptosis, where caspase-cleaved spectrin is short lived, lens fiber cells contain spectrin fragments that appear to be stable for the lifetime of the organism. Moreover, fragmentation of spectrin results in reduced membrane association and thus may lead to permanent remodeling of the membrane skeleton. Partial and specific proteolysis of membrane skeleton components by caspases may be important for age-related membrane changes in the lens.

alpha-Spectrin has a stage-specific asymmetrical localization during Xenopus oogenesis

Xenopus oocyte organization largely depends upon the cytoskeleton distribution, which is dynamically regulated during oogenesis. An actin-based cytoskeleton is present in the cortex starting from stage 1. At stages 4-6, a complex and polarized cytoskeleton network forms in the cytoplasm. In this paper, we studied the distribution of spectrin, a molecule that has binding sites for several cytoskeletal proteins and is responsible for the determination of regionalized membrane territories. The localization of alpha-spectrin mRNA was analyzed during Xenopus oogenesis by in situ hybridization on both whole mount and sections, utilizing a cDNA probe encoding a portion of Xenopus alpha-spectrin. Furthermore, an antibody against mammalian alpha-spectrin was used to localize the protein. Our results showed a stage-dependent mRNA localization and suggested that spectrin may participate in the formation of specific domains in oocytes at stages 1 and 2 and 4-6. Mol. Reprod. Dev. 55:229-239, 2000.

Synthesis, assembly, and turnover of alpha and beta-erythroid and nonerythroid spectrins in rat hippocampal neurons

The synthesis and turnover of alpha-erythroid, beta-erythroid, alpha-nonerythroid and beta-nonerythroid spectrins was investigated in cultured rat hippocampal neurons. [35S]methionine and subunit specific antibodies were used to label and immunoprecipitate newly synthesized spectrins in 12- to 14-day-old cultures. Synthesis experiments, performed under normal resting conditions, showed that the ratio of newly synthesized alpha-erythroid/beta-erythroid and alpha-nonerythroid/beta-nonerythroid spectrins is 1/1 (mol/mol) both in the soluble and insoluble fractions. Soluble and insoluble alpha and beta erythroid spectrin turn over rapidly (half-life=16-24 min). Soluble nonerythroid alpha-spectrin (half-life=80 min) and beta spectrin (half-life=53 min) turn over more slowly than their insoluble counterparts (30-34 min). The nonerythroid alpha spectrin turnover was significantly different (p<0.05) from the other measurements except for nonerythroid beta spectrin, indicating that these subunits are protected from rapid proteolytic degradation until they are assembled in the membrane skeleton.

Characterization of NF-L and betaIISigma1-spectrin interaction in live cells

Neurofilaments (NFs) are neuron-specific intermediate filaments (IFs) composed of three different subunits, NF-L, NF-M, and NF-H. NFs move down the axon with the slow component of axonal transport, together with microtubules, microfilaments, and alphaII/betaII-spectrin (nonerythroid spectrin or fodrin). It has been shown that alphaII/betaII-spectrin is closely associated with NFs in vivo and that betaII-spectrin subunit binds to NF-L filaments in vitro. In the present study we seek to elucidate the relationship between NF-L and betaII-spectrin in vivo. We transiently transfected full-length NF-L and carboxyl-terminal deleted NF-L mutants in SW13 Cl.2 Vim- cells, which lack an endogenous IF network and express alphaII/betaIISigma1-spectrin. Double-immunofluorescence and electron microscopy studies showed that a large portion of betaIISigma1-spectrin colocalizes with the structures formed by NF-L proteins. We found a similar association between NF-L proteins and actin. However, coimmunoprecipitation experiments in transfected cells and the yeast two-hybrid system results failed to demonstrate a direct interaction of NF-L with betaIISigma1-spectrin in vivo. The presence of another protein that acts as a bridge between the membrane skeleton and neurofilaments or modulating their association may therefore be required.

Calpain-induced proteolysis of beta-spectrins

The calcium-activated neutral protease calpain is activated in several pathological conditions. Calpain usually hydrolyses one or only a few peptide bonds in its substrate. One prominent substrate for calpain is spectrin and it has been shown that alpha-spectrin is the preferred substrate. We now show that the beta-chain of spectrin is also a substrate for calpain proteolysis, and that the cleavage site in each beta-subunit is located at the very C-terminal part of the molecule. Surprisingly, beta1sigma-spectrin is cleaved at a different site than betaIsigma2- and betaIIsigma1-spectrins despite their high degree of sequence identity.

Simultaneous degradation of alphaII- and betaII-spectrin by caspase 3 (CPP32) in apoptotic cells

The degradation of alphaII- and betaII-spectrin during apoptosis in cultured human neuroblastoma SH-SY5Y cells was investigated. Immunofluorescent staining showed that the collapse of the cortical spectrin cytoskeleton is an early event following staurosporine challenge. This collapse correlated with the generation of a series of prominent spectrin breakdown products (BDPs) derived from both alphaII- and betaII-subunits. Major C-terminal alphaII-spectrin BDPs were detected at approximately 150, 145, and 120 kDa (alphaII-BDP150, alphaII-BDP145, and alphaII-BDP120, respectively); major C-terminal betaII-spectrin BDPs were at approximately 110 and 85 kDa (betaII-BDP110 and betaII-BDP85, respectively). N-terminal sequencing of the major fragments produced in vitro by caspase 3 revealed that alphaII-BDP150 and alphaII-BDP120 were generated by cleavages at DETD1185*S1186 and DSLD1478*S1479, respectively. For betaII-spectrin, a major caspase site was detected at DEVD1457*S1458, and both betaII-BDP110 and betaII-BDP85 shared a common N-terminal sequence starting with Ser1458. An additional cleavage site near the C terminus, at ETVD2146*S2147, was found to account for betaII-BDP85. Studies using specific caspase or calpain inhibitors indicate that the pattern of spectrin breakdown during apoptosis differs from that during non-apoptotic cell death. We postulate that in concert with calpain, caspase rapidly targets critical sites in both alphaII- and betaII-spectrin and thereby initiates a rapid dissolution of the spectrin-actin cortical cytoskeleton with apoptosis.

Brain spectrin binding to the NMDA receptor is regulated by phosphorylation, calcium and calmodulin

The N-methyl-D-aspartate receptor (NMDA-R) and brain spectrin, a protein that links membrane proteins to the actin cytoskeleton, are major components of post-synaptic densities (PSDs). Since the activity of the NMDA-R channel is dependent on the integrity of actin and leads to calpain-mediated spectrin breakdown, we have investigated whether the actin-binding spectrin may interact directly with NMDA-Rs. Spectrin is reported here to interact selectively in vitro with the C-terminal cytoplasmic domains of the NR1a, NR2A and NR2B subunits of the NMDA-R but not with that of the AMPA receptor GluR1. Spectrin binds at NR2B sites distinct from those of alpha-actinin-2 and members of the PSD95/SAP90 family. The spectrin-NR2B interactions are antagonized by Ca2+ and fyn-mediated NR2B phosphorylation, but not by Ca2+/calmodulin (CaM) or by Ca2+/CaM-dependent protein kinase II-mediated NR2B phosphorylation. The spectrin-NR1 interactions are unaffected by Ca2+ but inhibited by CaM and by protein kinase A- and C-mediated phosphorylations of NR1. Finally, in rat synaptosomes, both spectrin and NR2B are loosened from membranes upon addition of physiological concentrations of calcium ions. The highly regulated linkage of the NMDA-R to spectrin may underlie the morphological changes that occur in neuronal dendrites concurrently with synaptic activity and plasticity.

Commitment and effector phases of the physiological cell death pathway elucidated with respect to Bcl-2 caspase, and cyclin-dependent kinase activities

Physiological cell deaths occur ubiquitously throughout biology and have common attributes, including apoptotic morphology with mitosis-like chromatin condensation and prelytic genome digestion. The fundamental question is whether a common mechanism of dying underlies these common hallmarks of death. Here we describe evidence of such a conserved mechanism in different cells induced by distinct stimuli to undergo physiological cell death. Our genetic and quantitative biochemical analyses of T- and B-cell deaths reveal a conserved pattern of requisite components. We have dissected the role of cysteine proteases (caspases) in cell death to reflect two obligate classes of cytoplasmic activities functioning in an amplifying cascade, with upstream interleukin-1beta-converting enzyme-like proteases activating downstream caspase 3-like caspases. Bcl-2 spares cells from death by punctuating this cascade, preventing the activation of downstream caspases while leaving upstream activity undisturbed. This observation permits an operational definition of the stages of the cell death process. Upstream steps, which are necessary but not themselves lethal, are modulators of the death process. Downstream steps are effectors of, and not dissociable from, actual death; the irreversible commitment to cell death reflects the initiation of this downstream phase. In addition to caspase 3-like proteases, the effector phase of death involves the activation in the nucleus of cell cycle kinases of the cyclin-dependent kinase (Cdk) family. Nuclear recruitment and activation of Cdk components is dependent on the caspase cascade, suggesting that catastrophic Cdk activity may be the actual effector of cell death. The conservation of the cell death mechanism is not reflected in the molecular identity of its individual components, however. For example, we have detected different cyclin-Cdk pairs in different instances of cell death. The ordered course of events that we have observed in distinct cases reflects essential thematic elements of a conserved sequence of modulatory and effector activities comprising a common pathway of physiological cell death.

Asymmetrical changes in the fodrin alpha subunit in the superior temporal cortices in schizophrenia

BACKGROUND

We examined possible abnormalities in neural structural proteins that may underlie morphometric changes reported in the left superior temporal cortices (Brodmann's area 22) of schizophrenics.

METHODS

Particulate proteins of the superior temporal cortices taken at autopsy from 11 schizophrenic and 9 control brains were fractionated by gel electrophoresis. Target proteins, identified by reading their amino acid sequences, were immunoquantified using the specific antibody.

RESULTS

Amino acid sequences of the 150-kDa proteins on sodium dodecyl sulfate/polyacrylamide gel electrophoresis, which were significantly increased on the left side of schizophrenic superior temporal cortices, revealed that they were proteolytic fragments of the alpha subunit of fodrin, a major cytoskeletal protein underlying the plasma membrane. Immunoquantification using the specific antibodies against alpha and beta subunits of fodrin indicated that there exist concomitant decreases in the full-length 240-kDa form and increases in the 150-kDa form of alpha-fodrin with no changes of the 235-kDa form of beta-fodrin in the left superior temporal cortices of the schizophrenic brains.

CONCLUSIONS

The findings may be a possible molecular basis for linking morphometric changes to neurochemical pathophysiology in schizophrenia.

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.

Calpain activation contributes to dendritic remodeling after brief excitotoxic injury in vitro

The calcium-dependent protease calpain may contribute to neuronal death in acute neurological insults and may be activated very early in the neuronal injury cascade. We assessed the role of calpain in a model of rapid, reversible dendritic injury in murine cortical cultures. Brief sublethal NMDA exposure (10-30 microM for 10 min) resulted in focal swellings, or varicosities, along the length of neuronal dendrites as visualized with the lipophilic membrane tracer Dil or with immunostaining using antibodies to the somatodendritic protein MAP2. These varicosities appeared within minutes of NMDA exposure and recovered spontaneously within 2 hr after NMDA removal. Addition of the calpain inhibitors MDL28,170, calpain inhibitors I and II, and leupeptin (all 1-100 microM) had little effect on the development of NMDA-induced dendrite injury. However, the resolution of varicosities was substantially delayed by addition of calpain inhibitors after sublethal excitotoxic exposure. Using Western blots and immunocytochemistry, we observed reactivity for a calpain-specific spectrin proteolytic fragment during the period of recovery from dendritic swelling, but not during its formation. Spectrin breakdown product immunoreactivity could be blocked by the calpain inhibitor MDL28,170 and appeared in neuronal cell bodies and neurites in a time course that paralleled dendritic recovery. These observations suggest that calcium-dependent proteolysis contributes to recovery of dendritic structure after NMDA exposure. Calpain activation is not necessarily detrimental and may play a role in dendritic remodeling after neuronal injury.

Ruthenium red protects against glutamate-induced neuronal death in cerebellar culture

Intracytosplasmic concentration of calcium dramatically increases after glutamate exposure in cultured neurons. Here, we investigated the effects of glutamate (100 microM, 15 min) on the intramitochondrial calcium homeostasis using 45Ca as a radioactive tracer. Calcium content increased in both cytoplasmic and mitochondrial fractions during the first 15 min following glutamate exposure. Whereas calcium content declined in the cytosolic fraction during the next 45 min, it remained elevated in the mitochondrial fraction. We next investigated the effects of ruthenium red, a mitochondrial calcium uptake blocker, on calcium homeostasis and neuronal viability after glutamate. Ruthenium red (100 microM) fully protected granule cells from glutamate exposure when performed in low sodium-chloride solution. Furthermore, ruthenium red substantially lessened calcium accumulation in mitochondrial fraction after glutamate exposure.

Time-related neuronal changes following middle cerebral artery occlusion: implications for therapeutic intervention and the role of calpain

Changes in neocortex and striatum were characterized over time following focal ischemia to the brain. Rats were subjected to permanent middle cerebral artery occlusion (MCA-O) and sacrificed 1, 3, 6, 12, or 24 h later. The affected tissue was processed for tetrazolium chloride (TTC) and cresyl violet staining, as well as for Western blots to detect calpain-induced spectrin proteolysis. Significant changes in cell size and spectrin breakdown occurred within the first hour of occlusion, with further, dramatic changes in these two early markers continuing over time. Initial evidence of cell loss was noted at 1 h postocclusion in the striatum and at 3 h in the neocortex. However, even in the center of the most affected portion of the neocortex, the majority of cells appeared to be intact through 6 h. By this time, a significant TTC-defined infarct also emerged. These quantitative data indicate that calpain-induced proteolysis occurs very soon after the ischemic insult, is correlated with earliest changes in cell hypotrophy, and precedes or occurs in tandem with evidence of significant cell loss. They also demonstrate that, while some cell loss occurs earlier than previously believed, the majority of cells remains morphologically intact well beyond what is typically thought to be the window of opportunity for intervention. The results thus raise the question of how long after the ischemic event pharmaceutic intervention might be employed to salvage substantial numbers of neurons.

Ultrastructural damage and neuritic beading in cold-stressed spinal neurons with comparisons to NMDA and A23187 toxicity

While exposure of cultured spinal neurons to mild hypothermia provides some protection from physical trauma (dendrotomy), profound cooling (< 17 degrees C) causes unrelated neuronal injury and death, which can be prevented by treatment with NMDA receptor antagonists. To investigate the mechanism of hypothermic neuronal injury we examined the ultrastructure of cultured spinal neurons after 2 h of cooling to 17 degrees C or 10 degrees C, with or without the presence of the NMDA receptor antagonist D-2-amino-5-phosphonovalerate, and with or without rewarming to 37 degrees C. These groups were compared to cultures exposed to NMDA or to the calcium ionophore A23187. Patterns of ultrastructural change, involving cytoskeletal disruption, mitochondrial abnormalities and vacuolization of the cytoplasm, suggest a common mechanism of injury in all treatment groups, involving an elevation of intracellular calcium. Some neurons exposed to hypothermia, NMDA or ionophore developed beaded dendrites. Microtubules were fragmented in varicosities but not in the intervening constrictions; other organelles were largely excluded from the constrictions. Varicosities may form when organelles and cytoplasm accumulate as the result of disruption of transport and membrane stabilizing proteins by proteases activated by calcium influx via NMDA mediated channels. The periodic nature of the swellings may reflect inherently discontinuous distribution of molecular subunits of the cytoskeleton.

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

Calpains (CANPs) are a family of calcium-dependent cysteine proteases under complex cellular regulation. By making selective limited proteolytic cleavages, they activate or alter the regulation of certain enzymes, including key protein kinases and phosphatases, and induce specific cytoskeletal rearrangements, accounting for their suspected involvement in intracellular signaling, vesicular trafficking, and structural stabilization. Calpain activity has been implicated in various aging phenomena, including cataract formation and erythrocyte senescence. Abnormal activation of the large stores of latent calpain in neurons induces cell injury and is believed to underlie neurodegeneration in excitotoxicity, Wallerian degeneration, and certain other neuropathologic states involving abnormal calcium influx. In Alzheimer's disease, we found the ratio of activated calpain I to its latent precursor isoform in neocortex to be threefold higher than that in normal individuals and those with Huntington's or Parkinson's disease. Immunoreactivity toward calpastatin, the endogenous inhibitor of calpain, was also markedly reduced in layers II-V of the neocortex in Alzheimer's disease. The excessive calpain system activation suggested by these findings represents a potential molecular basis for synaptic loss and neuronal cell death in the brain in Alzheimer's disease given the known destructive actions of calpain I and its preferential neuronal and synaptic localization. In surviving cells, persistent calpain activation may also contribute to neurofibrillary pathology and abnormal amyloid precursor protein trafficking/processing through its known actions on protein kinases and the membrane skeleton. The degree of abnormal calpain activation in the brain in Alzheimer's disease strongly correlated with the extent of decline in levels of secreted amyloid precursor protein in brain. Cytoskeletal proteins that are normally good calpain substrates become relatively calpain resistant when they are hyperphosphorylated, which may contribute to their accumulation in neurofibrillary tangles. As a major effector of calcium signals, calpain activity may mirror disturbances in calcium homeostasis and mediate important pathologic consequences of such disturbances.

Calpain inhibitors block Ca(2+)-induced suppression of neurite outgrowth in isolated hippocampal pyramidal neurons

Ca2+ is an important regulator of neurite elongation and growth cone movements but the mechanism(s) mediating these Ca(2+)-dependent effects is unclear. Since cytoskeletal proteins are rapidly degraded by Ca(2+)-dependent proteinases (calpains) in vitro and in vivo, we investigated whether Ca(2+)-induced pruning or regression of neuronal processes is mediated by calpains. Isolated hippocampal pyramidal-like neurons were cultured and the ability of the membrane-permeable calpain inhibitors ethyl(+)-(2S,3S)-3-[(S)-methyl-1-(3-methylbutylcarbamoyl)-butyl carbamoyl]-2 - oxiranecarboxylate (EST) and carbobenzoxyl-valyl-phenylalanyl-H (MDL 28170) to block the Ca2+ ionophore A23187-induced suppression in neurite outgrowth was investigated. Addition of 100 nM A23187 to the culture medium resulted in a retraction of dendrites without altering axonal elongation. The addition of 300 nM A23187 to the culture medium resulted in a significant decrease in the rate of axonal elongation as well as a retraction of dendritic processes. Administration of EST (5 or 20 microM) to the culture medium completely blocked the pruning effect of 100 nM A23187 on dendrites and of 300 nM A23187 on axons, while EST alone did not significantly affect neurite outgrowth rate. MDL 28170 (20 microM) showed the same effect as EST in preventing ionophore-induced pruning of dendrites and axons at 100 and 300 nM concentrations, respectively, of A23187. EST (20 microM) did not block the A23187-induced rise of [Ca2+]i as measured with fura-2. These results suggest that calpains play a role in Ca(2+)-induced pruning of neurites in isolated hippocampal pyramidal neurons.

Calpain inhibition: an overview of its therapeutic potential

Increasing evidence now suggests that excessive activation of the Ca(2+)-dependent protease calpain could play a key or contributory role in the pathology of a variety of disorders, including cerebral ischaemia, cataract, myocardial ischaemia, muscular dystrophy and platelet aggregation. In this review, Kevin Wang and Po-Wai Yuen discuss the evidence linking these disorders to calpain overactivation. At present, it is difficult to confirm the exact role of calpain in these disorders because of the lack of potent, selective and cell-permeable calpain inhibitors. However, given the multiple therapeutic indications for calpain, it appears that achievement of selective calpain inhibition is an important pharmacological goal.

Glutamate regulates intracellular calcium and gene expression in oligodendrocyte progenitors through the activation of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Oligodendrocytes and their progenitors (O-2A) express functional kainate- and DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-preferring glutamate receptors. The physiological consequences of activation of these receptors were studied in purified rat cortical O-2A progenitors and in the primary oligodendrocyte cell line CG-4. Changes in the mRNA levels of a set of immediate early genes were studied and were correlated to intracellular Ca2+ concentration, as measured by fura-2 Ca2+ imaging. Both in CG-4 and in cortical O-2A progenitors, basal mRNA levels of NGFI-A were much higher than c-fos, c-jun, or jun-b. Glutamate, kainate, and AMPA greatly increased NGFI-A mRNA and protein by activation of membrane receptors in a Ca(2+)-dependent fashion. Agonists at non-N-methyl-D-aspartate receptors promoted transmembrane Ca2+ influx through voltage-dependent channels as well as kainate and/or AMPA channels. The influx of Ca2+ ions occurring through glutamate-gated channels was sufficient by itself to increase the expression of NGFI-A mRNA. AMPA receptors were found to be directly involved in intracellular Ca2+ and NGFI-A mRNA regulation, because the effects of kainate were greatly enhanced by cyclothiazide, an allosteric modulator that selectively suppresses desensitization of AMPA but not kainate receptors. Our results indicate that glutamate acting at AMPA receptors regulates immediate early gene expression in cells of the oligodendrocyte lineage by increasing intracellular calcium. Consequently, modulation of these receptor channels may have immediate effects at the genomic level and regulate oligodendrocyte development at critical stages.

Upregulation of calpain activity in neonatal rat brain after hypoxic-ischemia

Neonatal rats were subjected to transient cerebral hypoxic-ischemia (unilateral occlusion of the common carotid artery plus 7.7% O2 for 2 h) and allowed to recover for 0 min, 30 min or 20 h. The calpain and calpastatin activities were assayed in subcellular fractions of the ipsilateral, hypoxic-ischemic and the contralateral, hypoxic hemisphere. An upregulation of calpain activity occurred in the hypoxic hemisphere, both in the major, cytosolic fraction and in the hypotonic, membrane associated fraction (110% and 133% of controls, respectively). The hypoxic-ischemic hemisphere displayed a decrease in calpain activity in the cytosolic fraction but an increase in the hypotonic fraction (90% and 111% of controls, respectively). The changes in calpastatin activity were less pronounced. This indicates that an upregulation of calpain activity occurs in parallel with development of hypoxic-ischemic damage. However, this upregulation is not necessarily coupled to development of injury as lesions are not seen in the hypoxic hemisphere.

The 270 kDa splice variant of erythrocyte beta-spectrin (beta I sigma 2) segregates in vivo and in vitro to specific domains of cerebellar neurons

Spectrin isoforms arise from four distinct genes, three of which generate multiple alternative transcripts. With no biochemical restrictions on the assembly of alpha beta heterodimers, more than 25 distinct heterodimeric spectrin species may exist. Whether (and why) this subtle but substantial diversity is realized in any single cell is unknown. To address this question, sequence-specific antibodies to alternatively spliced regions of alpha- and beta-spectrin have been prepared. Reported here is the localization in rat cerebellar neurons at light and electron microscopic levels of an antibody against a unique sequence (beta I sigma 2-A = PGQHKDGQKSTGDERPT) from the 270 kDa transcript of the red cell beta-spectrin gene (spectrin beta I sigma 2). In this version, the 3′ sequence of erythroid beta-spectrin (beta I sigma 1) is replaced with an alternative sequence that shares substantial homology with the 3′ sequence of non-erythroid beta-spectrin (beta II sigma 1). The antibody to beta I sigma 2-A stains a single protein band at 270 kDa, determined by western blotting, in both rat cerebellum and in cultured cerebellar granule cells, and does not react with beta II sigma 1 spectrin (beta-fodrin). This antibody stains the dendritic spines of Purkinje cells in the molecular layer, and is concentrated at postsynaptic densities (PSDs) adjacent to synapsin I (which is confined to the presynaptic membrane). The soma of Purkinje cells do not stain. In the granular layer, cytoplasmic organelles and the postsynaptic densities of granular cells stain strongly. Astrocytes are also stained. In all cells, plasma membrane staining is confined to postsynaptic densities (PSD). The beta I sigma 2 isoform co-immunoprecipitates with non-erythroid alpha-spectrin (alpha II sigma), even though the distribution of alpha II sigma within neurons only partially overlaps that of beta I sigma 2. No hybrid beta I sigma 2 and beta II sigma 1 (beta-fodrin) spectrin complexes appear to exist. Spectrin beta I sigma 2 is also polarized in cultured rat cerebellar granule cells, where it is abundant in cell bodies but not neurites. The overall distribution of beta I sigma 2 is as a subset of the distribution of spectrins 240/235E previously detected with a generally reactive erythrocyte alpha beta-spectrin antibody. These findings establish the highly precise segregation of a beta-spectrin isoform to distinct cytoplasmic and membrane surface domains, indicate that it is complexed (partially) with non-erythroid alpha-spectrin, and demonstrate that cytoskeletal targeting mechanisms are preserved in cultured granular cells.(ABSTRACT TRUNCATED AT 400 WORDS)

N-Methyl-D-aspartate receptors are clustered and immobilized on dendrites of living cortical neurons

The response of nerve cells to synaptic inputs and the propagation of this activation is critically dependent on the cell-surface distribution of ion channels. In the hippocampus, Ca2+ influx through N-methyl-D-aspartate receptors (NMDAR) and/or voltage-dependent calcium channels on dendrites is thought to be critically involved in long-term potentiation, neurite outgrowth, epileptogenesis, synaptogenesis, and cell death. We report that conantokin-G (CntxG), a peptide from Conus geographus venom, competitively blocked with high affinity and specificity NMDAR-mediated currents in hippocampal neurons and is a reliable probe for exploring NMDAR distribution. Fluorescent derivatives of CntxG were prepared and used to directly determine NMDAR distribution on living hippocampal neurons by digital imaging and confocal fluorescence microscopy. In hippocampal slices, the CA1 dendritic subfield was strongly labeled by CntxG, whereas the CA3 mossy fiber region was not. On CA1 hippocampal neurons in culture, dendritic CntkG-sensitive NMDAR were clustered at sites of synaptic contacts, whereas somatic NMDAR were distributed diffusely and in patches. NMDAR distribution differed from the distribution of voltage-dependent calcium channels. A significant fraction of labeled NMDAR on somata and dendrites was found to be highly mobile: rates were consistent with the possible rapid recruitment of NMDAR to specific synaptic locations. The localization of NMDAR and modulation of this distribution demonstrated here may have important implications for the events that underlie neuronal processing and synaptic remodeling during associative synaptic modification.