Calpain and synaptic function

Pregnenolone sulfate modulation of N-methyl-D-aspartate receptors is phosphorylation dependent

Pregnenolone sulfate (PS), an endogenously occurring neurosteroid, has been shown to modulate the activity of several neurotransmitter-gated channels, including the N-methyl-D-aspartate receptor (NMDAR). NMDARs are glutamate-gated ion channels involved in excitatory synaptic transmission, synaptic plasticity, and excitotoxicity. To determine the mechanism that controls PS sensitivity of NMDARs, we measured NMDAR responses induced by exogenous agonist application in voltage-clamped HEK293 cells expressing NR1/NR2B NMDARs and cultured rat hippocampal neurons. We report that PS potentiates the amplitude of whole-cell recorded NMDAR responses in cultured hippocampal neurons and HEK293 cells; however, the potentiating effect of PS on NMDAR in outside-out patches isolated from cultured hippocampal neurons and HEK293 cells was lost within 2 min after patch isolation in a neurosteroid-specific manner. The rate of diminution of the PS potentiating effect was slowed by protein phosphatase inhibitors. Treatment of cultured hippocampal neurons with a nonspecific protein kinase inhibitor and a specific protein kinase A (PKA) inhibitor diminished PS-induced potentiation, which was recovered by adding a PKA, but not a protein kinase C (PKC), activator. These results suggest that the effect of PS on NMDARs is controlled by cellular mechanisms that are mediated by dephosphorylation/phosphorylation pathways.

Ca2+-dependent, stimulus-specific modulation of the plasma membrane Ca2+ pump in hippocampal neurons

The plasma membrane Ca(2+) ATPase (PMCA) plays a major role in restoring Ca(2+) to basal levels following transient elevation by neuronal activity. Here we examined the effects of various stimuli that increase [Ca(2+)](i) on PMCA-mediated Ca(2+) clearance from hippocampal neurons. We used indo-1-based microfluorimetry in the presence of cyclopiazonic acid to study the rate of PMCA-mediated recovery of Ca(2+) elevated by a brief train of action potentials. [Ca(2+)](i) recovery was described by an exponential decay and the time constant provided an index of PMCA-mediated Ca(2+) clearance. PMCA function was assessed before and for >or=60 min following a 10-min priming stimulus of either 100 microM N-methyl-d-aspartate (NMDA), 0.1 mM Mg(2+) (reduced extracellular Mg(2+) induces intense excitatory synaptic activity), 30 mM K(+), or control buffer. Recovery kinetics slowed progressively following priming with NMDA or 0.1 mM Mg(2+); in contrast, Ca(2+) clearance initially accelerated and then slowly returned to initial rates following priming with 30 mM K(+)-induced depolarization. Treatment with 10 muM calpeptin, an inhibitor of the Ca(2+) activated protease calpain, prevented the slowing of kinetics observed following treatment with NMDA but had no affect on the recovery kinetics of control cells. Calpeptin also blocked the rapid acceleration of Ca(2+) clearance following depolarization. In calpeptin-treated cells, 0.1 mM Mg(2+) induced a graded acceleration of Ca(2+) clearance. Thus in spite of producing comparable increases in [Ca(2+)](i), activation of NMDA receptors, depolarization-induced activation of voltage-gated Ca(2+) channels and excitatory synaptic activity each uniquely affected Ca(2+) clearance kinetics mediated by the PMCA.

Glutamate and neurotrophic factors in neuronal plasticity and disease

Glutamate's role as a neurotransmitter at synapses has been known for 40 years, but glutamate has since been shown to regulate neurogenesis, neurite outgrowth, synaptogenesis, and neuron survival in the developing and adult mammalian nervous system. Cell-surface glutamate receptors are coupled to Ca(2+) influx and release from endoplasmic reticulum stores, which causes rapid (kinase- and protease-mediated) and delayed (transcription-dependent) responses that change the structure and function of neurons. Neurotrophic factors and glutamate interact to regulate developmental and adult neuroplasticity. For example, glutamate stimulates the production of brain-derived neurotrophic factor (BDNF), which, in turn, modifies neuronal glutamate sensitivity, Ca(2+) homeostasis, and plasticity. Neurotrophic factors may modify glutamate signaling directly, by changing the expression of glutamate receptor subunits and Ca(2+)-regulating proteins, and also indirectly by inducing the production of antioxidant enzymes, energy-regulating proteins, and antiapoptotic Bcl-2 family members. Excessive activation of glutamate receptors, under conditions of oxidative and metabolic stress, may contribute to neuronal dysfunction and degeneration in diseases ranging from stroke and Alzheimer's disease to psychiatric disorders. By enhancing neurotrophic factor signaling, environmental factors such as exercise and dietary energy restriction, and chemicals such as antidepressants may optimize glutamatergic signaling and protect against neurological disorders.

Neurite consolidation is an active process requiring constant repression of protrusive activity

During development, neurons extend projections that pathfind to reach their appropriate targets. These projections are composed of two distinct domains: a highly dynamic growth cone and a stable neurite shaft, which is considered to be consolidated. Although the regulation of these domains is critical to the appropriate formation of neural networks, the molecular mechanisms that regulate neurite shape remain poorly understood. Here, we show that calpain protease activity localizes to the neurite shaft, where it is essential for the repression of protrusive activity by limiting cortactin levels and inhibiting actin polymerization. Correspondingly, inhibition of calpain by branching factors induces the formation of new growth cones along the neurite shaft through cAMP elevation. These findings demonstrate that neurite consolidation is an active process requiring constant repression of protrusive activity. We also show that sprouting is, at least in part, accomplished by turning off the mechanism of consolidation.

SMN complex localizes to the sarcomeric Z-disc and is a proteolytic target of calpain

Spinal muscular atrophy (SMA) is a recessive neuromuscular disease caused by mutations in the human survival motor neuron 1 (SMN1) gene. The human SMN protein is part of a large macromolecular complex involved in the biogenesis of small ribonucleoproteins. Previously, we showed that SMN is a sarcomeric protein in flies and mice. In this report, we show that the entire mouse Smn complex localizes to the sarcomeric Z-disc. Smn colocalizes with alpha-actinin, a Z-disc marker protein, in both skeletal and cardiac myofibrils. Furthermore, this localization is both calcium- and calpain-dependent. Calpains are known to release proteins from various regions of the sarcomere as a part of the normal functioning of the muscle; however, this removal can be either direct or indirect. Using mammalian cell lysates, purified native SMN complexes, as well as recombinant SMN protein, we show that SMN is a direct target of calpain cleavage. Finally, myofibers from a mouse model of severe SMA, but not controls, display morphological defects that are consistent with a Z-disc deficiency. These results support the view that the SMN complex performs a muscle-specific function at the Z-discs.

Calpain inhibitors alter the excitable membrane properties of cultured aplysia neurons

The calpain superfamily of calcium-dependent papain-like cysteine proteases constitutes highly conserved proteases that function to posttranslationally modify substrates by partial proteolysis. Calpains are known to proteolyze >100 substrates that lack strong sequence homology. Consequently, the calpain superfamily has been implicated in playing a central role in diverse physiological and pathological processes. Investigation of the physiological functions of calpains, on the one hand, and the need to develop pharmacological reagents to inhibit calpain-mediated pathological processes, on the other hand, led to the development of numerous calpain inhibitors. Using cultured Aplysia neurons and voltage-clamp analysis, we report here that the calpain inhibitors calpeptin, MG132, and the calpain inhibitor XII inhibit voltage-gated potassium conductance and moderately reduce the sodium conductance. These consequently lead to spike broadening and increased calcium influx. Such alterations of the excitable membrane properties may alter the normal patterns of neuronal and muscle electrical activities and thus should be taken into account when evaluating the effects of calpain inhibitors as protective/therapeutic drugs and as research tools.

Calpain activity contributes to the control of SNAP-25 levels in neurons

Calpains are a family of calcium-dependent proteases with abundant expression in the CNS, and potent in cleaving some synaptic components. Assessment of calpain activity by its fluorescent substrate, Boc-Leu-Met-CMAC, revealed that cultured neurons display a significant level of constitutive enzyme activity. Notably, calpain activity differs in distinct neuronal populations, with a significantly higher level of activity in GABAergic cells. Using selectively-enriched cultures of fast-spiking GABAergic interneurons, we show that calpain activity partially contributes to the post-translational down regulation of SNAP-25, a calpain substrate, in differentiated GABA cells. In addition, we demonstrate that SNAP-25 is cleaved by calpain in response to acute seizures induced by intraperitoneal kainate injection in vivo. These data indicate that calpains in neurons are active even at physiological calcium concentrations and that different levels of calpain activation in selected neuron subtypes may contribute to the pattern of synaptic protein expression.

Active zone proteins are dynamically associated with synaptic ribbons in rat pinealocytes

Synaptic ribbons (SRs) are prominent organelles that are abundant in the ribbon synapses of sensory neurons where they represent a specialization of the cytomatrix at the active zone (CAZ). SRs occur not only in neurons, but also in neuroendocrine pinealocytes where their function is still obscure. In this study, we report that pinealocyte SRs are associated with CAZ proteins such as Bassoon, Piccolo, CtBP1, Munc13–1, and the motorprotein KIF3A and, therefore, consist of a protein complex that resembles the ribbon complex of retinal and other sensory ribbon synapses. The pinealocyte ribbon complex is biochemically dynamic. Its protein composition changes in favor of Bassoon, Piccolo, and Munc13–1 at night and in favor of KIF3A during the day, whereas CtBP1 is equally present during the night and day. The diurnal dynamics of the ribbon complex persist under constant darkness and decrease after stimulus deprivation of the pineal gland by constant light. Our findings indicate that neuroendocrine pinealocytes possess a protein complex that resembles the CAZ of ribbon synapses in sensory organs and whose dynamics are under circadian regulation.

Concomitant degradation of beta-catenin and GSK-3 beta potently contributes to glutamate-induced neurotoxicity in rat hippocampal slice cultures

Increasingly, published evidence links glutamate with the pathogenesis of Alzheimer's disease. We investigated the molecular mechanism underlying glutamate-induced neurotoxicity in hippocampus, which is primarily linked to cognitive dysfunction in Alzheimer's disease. Acute exposure of rat hippocampal slices to glutamate significantly induced cell death, as determined by media lactate dehydrogenase levels and PI staining. Moreover, this was accompanied by Ca2+ influx and calpain-1 activation, as confirmed by the proteolytic pattern of spectrin. Notably, glutamate-induced calpain-1 activation decreased the level of beta-catenin, and this process appeared to be independent of glycogen synthase kinase 3beta (GSK-3beta), since glutamate also led to loss of GSK-3beta. Calpeptin, a calpain inhibitor, attenuated the glutamate-mediated degradations of spectrin, synaptophysin, and beta-catenin except GSK-3beta and modestly increased cell survival. In contrast, the NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (APV) effectively reduced all glutamate-evoked responses, i.e., the breakdowns of spectrin, synaptophysin, beta-catenin and GSK-3beta, and cell death. Pharmacological studies and in vitro calpain-1 proteolysis confirmed that in the glutamate-treated hippocampus, calpain-1-mediated decrease of beta-catenin could occur independently of GSK-3beta and of proteasome, and that GSK-3beta degradation is independent of calpain-1. These findings together provide the first direct evidence that glutamate promotes the down-regulations of beta-catenin and GSK-3beta, which potently contribute to neurotoxicity in hippocampus during excitotoxic cell death, and a molecular basis for the protection afforded by calpeptin and APV from the neurotoxic effect of glutamate.

Plasticity and repair in the post-ischemic brain

Stroke is the second commonest cause of death and the principal cause of adult disability in the world. In most cases ischemic injuries have been reported to induce mild to severe permanent deficits. Nevertheless, recovery is often dynamic, reflecting the ability of the injured neuronal networks to adapt. Plastic phenomena occurring in the cerebral cortex and in subcortical structures after ischemic injuries have been documented at the synaptic, cellular, and network level and several findings suggest that they may play a key role during neurorehabilitation in human stroke survivors. In particular, in vitro studies have demonstrated that oxygen and glucose deprivation (in vitro ischemia) exerts long-term effects on the efficacy of synaptic transmission via the induction of a post-ischemic long-term potentiation (i-LTP). i-LTP may deeply influence the plastic reorganization of cortical representational maps occurring after cerebral ischemia, inducing a functional connection of previously non-interacting neurons. On the other hand, there is evidence that i-LTP may exert a detrimental effect in the peri-infarct area, facilitating excitotoxic processes via the sustained, long-term enhancement of glutamate mediated neurotransmission. In the present work we will review the molecular and synaptic mechanisms underlying ischemia-induced synaptic plastic changes taking into account their potential adaptive and/or detrimental effects on the neuronal network in which they occur. Thereafter, we will consider the implications of brain plastic phenomena in the post-stroke recovery phase as well as during the rehabilitative and therapeutic intervention in human subjects.

Differential regulation of the NMDA receptor by acute and sub-chronic phencyclidine administration in the developing rat

Neurodegeneration induced by the NMDA receptor antagonist, phencyclidine (PCP), has been used to model the pathogenesis of schizophrenia in the developing rat. Acute and sub-chronic administration of PCP in perinatal rats results in different patterns of neurodegeneration. The potential role of an alteration in the membrane expression of NMDA receptors in PCP-induced degeneration is unknown. Acute PCP treatment on postnatal day 7 increased membrane levels of both NMDA receptor subunit 1 (NR1) and NMDA receptor subunit 2B (NR2B) proteins in the frontal cortex; conversely, NR1 and NR2B protein levels in the endoplasmic reticulum fraction were decreased. Acute PCP administration also resulted in increased membrane cortical protein levels of post-synaptic density-95, as well as the activation of calpain, which paralleled the observed increase in membrane expression of NR1 and NR2B. Further, administration of the calpain inhibitor, MDL28170, prevented PCP-induced up-regulation of NR1 and NR2B. On the other hand, sub-chronic PCP treatment on postnatal days 7, 9 and 11 caused an increase in NR1 and NR2A expression, which was accompanied by an increase in both NR1 and NR2A in the endoplasmic reticulum fraction. Sub-chronic PCP administration did not alter levels of post-synaptic density-95 and had no effect on activation of calpain. These data suggest that increased trafficking accounts for up-regulation of cortical NR1/NR2B subunits following acute PCP administration, while increased protein synthesis likely accounts for the increased expression of NR1/NR2A following sub-chronic PCP treatment of the developing rat. These results are discussed in the context of the differential neurodegeneration caused by acute and subchronic PCP administration in the developing rat brain.

Calpain cleavage of collapsin response mediator proteins in ischemic mouse brain

Collapsin response mediator proteins (CRMPs) are important brain-specific proteins with distinct functions in modulating growth cone collapse and axonal guidance during brain development. Our previous studies have shown that calpain cleaves CRMP3 in the adult mouse brain during cerebral ischemia [S.T. Hou et al. (2006) J. Neurosci., 26, 2241-2249]. Here, the expression of all CRMP family members (1-5) was examined in mouse brains that were subjected to middle cerebral artery occlusion. Among the five CRMPs, the expressions of CRMP1, CRMP3 and CRMP5 were the most abundant in the cerebral cortex and all CRMPs were targeted for cleavage by ischemia-activated calpain. Sub-cellular fractionation analysis showed that cleavage of CRMPs by calpain occurred not only in the cytoplasm but also in the synaptosomes isolated from ischemic brains. Moreover, synaptosomal CRMPs appeared to be at least one-fold more sensitive to cleavage compared with those isolated from the cytosolic fraction in an in-vitro experiment, suggesting that synaptosomal CRMPs are critical targets during cerebral ischemia-induced neuronal injury. Finally, the expression of all CRMPs was colocalized with TUNEL-positive neurons in the ischemic mouse brain, which further supports the notion that CRMPs may play an important role in neuronal death following cerebral ischemia. Collectively, these studies demonstrated that CRMPs are targets of calpains during cerebral ischemia and they also highlighted an important potential role that CRMPs may play in modulating ischemic neuronal death.

Emerging functions of the calpain superfamily of cysteine proteases in neuroendocrine secretory pathways

The first calpain protease was discovered over 40 years ago now, yet despite the vast amount of literature that has subsequently emerged detailing their involvement in the pathophysiology of a variety of human diseases, it is only in the last decade that calpain-mediated actions along the secretory pathway have begun to emerge. However, the number of secretory pathway substrates identified and their diversity of function continues to grow. This review summarizes our current knowledge of calpain-mediated mechanisms of action that are pertinent to synaptic vesicle assembly and budding, cytoskeletal organization, endosomal recycling, and exocytotic membrane fusion.

Picking up the pieces: the roles of functional remnants of calpain-mediated proteolysis

Calpain-mediated cleavage of neuronal targets has long been associated with excitotoxicity and synaptic plasticity. In this issue of Neuron, two papers by Xu et al. and Abe and Takeichi explore the respective roles of mGluR1alpha cleavage in excitotoxicity and beta-catenin cleavage in transcriptional control. Together, these papers show the functional importance of fragments of calpain-mediated cleavage.

Developmental and cell-selective variations in N-methyl-D-aspartate receptor degradation by calpain

NMDA receptors play critical roles in synaptic modulation and neurological disorders. In this study, we investigated the developmental changes in NR2 cleavage by NMDA receptor-activated calpain in cultured cortical and hippocampal neurons. Calpain activity increased with development, associated with increased expression of NMDA receptors but not of calpain I. The activation of calpain in immature and mature cortical cultures was inhibited by antagonists of NR1/2B and NR1/2A/2B receptors, whereas the inhibition of NR1/2B receptors did not alter calpain activation in mature hippocampal cultures. The degradation of NR2 subunits by calpain differed with developmental age. NR2A was not a substrate of calpain in mature hippocampal cultures, but was cleaved in immature cortical and hippocampal cultures. NR2B degradation by calpain in cortical cultures decreased with development, but the level of degradation of NR2B in hippocampal cultures did not change. The kinetics of NMDA receptor-gated whole cell currents were also modulated by calpain activation in a manner that varied with developmental stage in vitro. In early (but not later) developmental stages, calpain activation altered the NMDA-evoked current rise time and time constants for both desensitization and deactivation. Our data suggest that the susceptibility of the NMDA receptor to cleavage by calpain varies with neuronal maturity in a manner that may alter its electrophysiological properties.