Calpain-mediated truncation of rat brain AMPA receptors increases their Triton X-100 solubility

Interleukin-10 and PD150606 modulate expression of AMPA receptor GluA1 and GluA2 subunits under hypoxic conditions

The goal of this study was to evaluate the effects of anti-inflammatory cytokine, interleukin-10 (IL-10), and calpain inhibitor, PD150606, on the expression of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits in rat hippocampal slices exposed to repeated brief hypoxic episodes. We studied both individual and combinatory effects of PD150606 and IL-10 on the expression of AMPA receptor subunits under hypoxic conditions for GluA1 and GluA2 as well as their phosphorylated forms - pSer831-GluA1 and pSer880-GluA2. Additionally, we studied whether brief hypoxic episodes and IL-10 may affect mRNA expression of transcriptional factors such as hypoxia-inducible factor-1α and nuclear factor κB (NF-κB). Western blotting analysis of hippocampal slice homogenates revealed that IL-10 and PD150606, both individually and in combination, ameliorate hypoxia-induced decrease in the expression of GluA1 and pSer831-GluA1, with different level of efficiency measured at 10, 50, and 90 min after hypoxia induction. Interestingly, brief hypoxic episodes did not induce any changes in the expression of GluA2 and pSer880-GluA2 subunits, whereas PD150606 showed biphasic effect, decreasing the expression of GluA2 and pSer880-GluA2 at 10 min and potentiating it at 90 min after hypoxia induction. IL-10 alone did not show any effect but was able to reverse PD150606 action on the expression of pSer880-GluA2 at 10 min and further potentiated it for GluA2 at 90 min after hypoxia. Finally, PCR analysis revealed that modulation of GluA1 and GluA2 expressions by hypoxia, and IL-10 was not associated with changes in the expression of hypoxia-inducible factor-1α and nuclear factor-κB (NF-κB) transcriptional factors.

Agonist-induced down-regulation of AMPA receptors in oligodendrocyte progenitors

Prolonged exposure of oligodendrocyte progenitor cultures to non-toxic concentrations of glutamate receptor agonists for 24 h decreased cellular proliferation mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. Since prolonged agonist stimulation can regulate the expression of various families of receptors, we examined this possibility. Pretreatment of progenitor cultures with 100 μM kainic acid (KA) for 1-24 h caused a time-dependent decrease in AMPA receptor activity, determined by agonist-induced (45)Ca(2+) uptake. The maximum effect (70-80% decrease), observed in the 24 h-pretreated cells, was accompanied by a significant reduction in AMPA receptor subunits, as determined by Western blotting. GluR2/3 and GluR4 subunits were the most affected. Receptor down-regulation and (45)Ca(2+) uptake were only partially reversible upon KA removal. Furthermore, 24 h co-treatment of cultures with CNQX blocked the KA-induced decreases in calcium uptake. To address whether calpain, a calcium-activated protease, was implicated in the regulation of the AMPA receptor subunits, cultures were treated with the specific inhibitor PD150606 alone or in combination with KA for 24 h. Calpain inhibition significantly increased GluR1 in both conditions and partly reversed downregulation of GluR4 by KA. Collectively, these results indicate that calpain is not involved in the agonist-induced down-regulation of AMPA receptors subunits 2/3 in oligodendrocyte progenitors, while it downregulates GluR1 and GluR4.

Role of the ubiquitin-proteasome system in brain ischemia: friend or foe?

The ubiquitin-proteasome system (UPS) is a catalytic machinery that targets numerous cellular proteins for degradation, thus being essential to control a wide range of basic cellular processes and cell survival. Degradation of intracellular proteins via the UPS is a tightly regulated process initiated by tagging a target protein with a specific ubiquitin chain. Neurons are particularly vulnerable to any change in protein composition, and therefore the UPS is a key regulator of neuronal physiology. Alterations in UPS activity may induce pathological responses, ultimately leading to neuronal cell death. Brain ischemia triggers a complex series of biochemical and molecular mechanisms, such as an inflammatory response, an exacerbated production of misfolded and oxidized proteins, due to oxidative stress, and the breakdown of cellular integrity mainly mediated by excitotoxic glutamatergic signaling. Brain ischemia also damages protein degradation pathways which, together with the overproduction of damaged proteins and consequent upregulation of ubiquitin-conjugated proteins, contribute to the accumulation of ubiquitin-containing proteinaceous deposits. Despite recent advances, the factors leading to deposition of such aggregates after cerebral ischemic injury remain poorly understood. This review discusses the current knowledge on the role of the UPS in brain function and the molecular mechanisms contributing to UPS dysfunction in brain ischemia with consequent accumulation of ubiquitin-containing proteins. Chemical inhibitors of the proteasome and small molecule inhibitors of deubiquitinating enzymes, which promote the degradation of proteins by the proteasome, were both shown to provide neuroprotection in brain ischemia, and this apparent contradiction is also discussed in this review.

Targeting calpain in synaptic plasticity

INTRODUCTION

Calpains represent a family of neutral, calcium-dependent proteases, which modify the function of their target proteins by partial truncation. These proteases have been implicated in numerous cell functions, including cell division, proliferation, migration, and death. In the CNS, where µ-calpain and m-calpain are the main calpain isoforms, their activation has been linked to synaptic plasticity as well as to neurodegeneration. This review will focus on the role of calpains in synaptic plasticity and discuss the possibility of developing methods to manipulate calpain activity for therapeutic purposes.

AREAS COVERED

This review covers the literature showing how calpains are implicated in synaptic plasticity and in a number of conditions associated with learning impairment. The possibility of developing new drugs targeting these enzymes for treating these conditions is discussed.

EXPERT OPINION

As evidence accumulates that calpain activation participates in neurodegeneration and cancer, there is interest in developing therapeutic approaches using direct or indirect calpain inhibition. In particular, a peptide derived from the calpain truncation site of mGluR1α was shown to decrease neurodegeneration following neonatal hypoxia/ischemia. More selective approaches need to be developed to target calpain or some of its substrates for therapeutic indications associated with deregulation of synaptic plasticity.

Calpain 2 activated through N-methyl-D-aspartic acid receptor signaling cleaves CPEB3 and abrogates CPEB3-repressed translation in neurons

Long-term memory requires the activity-dependent reorganization of the synaptic proteome to modulate synaptic efficacy and consequently consolidate memory. Activity-regulated RNA translation can change the protein composition at the stimulated synapse. Cytoplasmic polyadenylation element-binding protein 3 (CPEB3) is a sequence-specific RNA-binding protein that represses translation of its target mRNAs in neurons, while activation of N-methyl-d-aspartic acid (NMDA) receptors alleviates this repression. Although recent research has revealed the mechanism of CPEB3-inhibited translation, how NMDA receptor signaling modulates the translational activity of CPEB3 remains unclear. This study shows that the repressor CPEB3 is degraded in NMDA-stimulated neurons and that the degradation of CPEB3 is accompanied by the elevated expression of CPEB3's target, epidermal growth factor receptor (EGFR), mostly at the translational level. Using pharmacological and knockdown approaches, we have identified that calpain 2, activated by the influx of calcium through NMDA receptors, proteolyzes the N-terminal repression motif but not the C-terminal RNA-binding domain of CPEB3. As a result, the calpain 2-cleaved CPEB3 fragment binds to RNA but fails to repress translation. Therefore, the cleavage of CPEB3 by NMDA-activated calpain 2 accounts for the activity-related translation of CPEB3-targeted RNAs.

Calpain-mediated regulation of stargazin in adult rat brain

Changes in AMPA receptors have been proposed to underlie changes in synaptic efficacy in hippocampus and other brain structures. Calpain activation has also been discussed as a potential mechanism to produce lasting modifications of synaptic structure and function. Stargazin is a member of the family of transmembrane AMPA receptor associated proteins (TARPs), which participates in trafficking of AMPA receptors and regulates their kinetic properties. We report here that preincubation of thin (20 μm) frozen rat brain sections with calcium changes the immunological properties of stargazin, an effect totally blocked by a calpain inhibitor. Immunocytochemistry indicates that in situ calpain activation produces a decreased immunoreactivity for stargazin in the neuropil throughout the brain, and Western blots confirmed that a similar treatment decreased stargazin levels. Interestingly, the same treatment did not modify the immunoreactivity for another TARP member, γ-8, although it increased immunoreactivity in cell bodies in hippocampus, an effect that was not blocked by calpain inhibition. These results strongly suggest the involvement of calpain in the regulation of AMPA receptor targeting and function through truncation of stargazin.

The ankyrin repeat-rich membrane spanning (ARMS)/Kidins220 scaffold protein is regulated by activity-dependent calpain proteolysis and modulates synaptic plasticity

The expression of forms of synaptic plasticity, such as the phenomenon of long-term potentiation, requires the activity-dependent regulation of synaptic proteins and synapse composition. Here we show that ARMS (ankyrin repeat-rich membrane spanning protein)/Kidins220, a transmembrane scaffold molecule and BDNF TrkB substrate, is significantly reduced in hippocampal neurons after potassium chloride depolarization. The activity-dependent proteolysis of ARMS/Kidins220 was found to occur through calpain, a calcium-activated protease. Moreover, hippocampal long-term potentiation in ARMS/Kidins220(+/-) mice was enhanced, and inhibition of calpain in these mice reversed these effects. These results provide an explanation for a role for the ARMS/Kidins220 protein in synaptic plasticity events and suggest that the levels of ARMS/Kidins220 can be regulated by neuronal activity and calpain action to influence synaptic function.

Synthetic calpain activator boosts neuronal excitability without extra Ca2+

Earlier we have shown that an equimolar mixture of calpastatin subdomains A and C (19 amino acids each) strongly activates m-calpain in vitro. In the present work we developed a membrane-permeable activator system, by conjugating an oligo-arginine tail to both peptides. We tested calpain activation as well as synaptic excitability on rat brain slices ex vivo. In hippocampal slices both basic excitability and long-term synaptic efficacy were significantly increased upon treatment with the activator. We propose that the activator peptide conjugates can be used with any mammalian cell, to specifically challenge the calpain system apparently without raising cytoplasmic Ca2+. Such an effector may be a useful tool in dissecting intracellular mechanisms involving the calpain system.

Mechanistic role of calpains in postischemic neurodegeneration

The calpain family of proteases is causally linked to postischemic neurodegeneration. However, the precise mechanisms by which calpains contribute to postischemic neuronal death have not been fully elucidated. This review outlines the key features of the calpain system, and the evidence for its causal role in postischemic neuronal pathology. Furthermore, the consequences of specific calpain substrate cleavage at various subcellular locations are explored. Calpain substrates within synapses, plasma membrane, endoplasmic reticulum, lysosomes, mitochondria, and the nucleus, as well as the overall effect of postischemic calpain activity on calcium regulation and cell death signaling are considered. Finally, potential pathways for calpain-mediated neurodegeneration are outlined in an effort to guide future studies aimed at understanding the downstream pathology of postischemic calpain activity and identifying optimal therapeutic strategies.

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.

Calpain regulation of AMPA receptor channels in cortical pyramidal neurons

AMPA receptors (AMPARs) are the principal glutamate receptors mediating fast excitatory synaptic transmission in neurons. Aberrant extracellular glutamate has long been recognized as a hallmark phenomenon during neuronal excitotoxicity. Excessive glutamate triggers massive Ca(2+) influx through NMDA receptors (NMDARs), which in turn can activate Ca(2+)-dependent protease, calpain. In the present study, we found that prolonged NMDA treatment (100 microM, 10 min) caused a sustained and irreversible suppression of AMPAR-mediated currents in cortical pyramidal neurons, which was largely blocked by selective calpain inhibitors. Biochemical and immunocytochemical studies demonstrated that in cortical cultures, prolonged glutamate or NMDA treatment reduced the level of surface and total GluR1, but not GluR2, subunits in a calpain-dependent manner. Consistent with the in vitro data, in animals exposed to transient ischaemic insults, calpain was strongly activated, and the AMPAR current density and GluR1 expression level were substantially reduced. Moreover, calpain inhibitors blocked the ischaemia-induced depression of AMPAR currents, and the NMDAR-induced, calpain-mediated depression of AMPA responses was occluded in ischaemic animals. Taken together, our studies show that overstimulation of NMDARs reduces AMPAR functions in cortical pyramidal neurons through activation of endogenous calpain, and calpain mediates the ischaemia-induced synaptic depression. The down-regulation of AMPARs by calpain provides a negative feedback to dampen neuronal excitability in excitotoxic conditions like ischaemia and epilepsy.

Lack of phenotype for LTP and fear conditioning learning in calpain 1 knock-out mice

We previously proposed the hypothesis that calpain activation played an important role in long-term potentiation (LTP) of synaptic transmission in hippocampus. Two forms of calpain are predominant in brain tissues, calpain 1 (mu-calpain), activated by micromolar calcium concentration and calpain 2 (m-calpain), activated by millimolar calcium concentration in vitro. In the present study, we tested the role of calpain 1 in LTP and in learning and memory using calpain 1 knock-out mice. Changes in learning and memory were assessed using both context and tone fear conditioning. No differences in freezing responses were observed between the knock-out and the wild-type animals during the acquisition phase of the training, eliminating the possibility that the knock-out animals could be differentially affected by the foot shock. Likewise, no differences in freezing responses elicited by either the context or the tone were observed during the retention phase. No differences in short-term potentiation (STP) or LTP were observed in hippocampal slices from the knock-out and matched wild-type mice. Several interpretations might explain these negative results. First, it is conceivable that calpain 2 plays a more dominant role in neurons, and that calpain 1 makes a minor contribution as opposed to its suspected predominant role in the hematopoietic system. Alternatively, it is conceivable that some as yet unknown compensatory mechanisms take effect, and that calpain 2 or another calpain isoform substitutes for the missing calpain 1.

Role of galanin receptor 1 and galanin receptor 2 activation in synaptic plasticity associated with 3',5'-cyclic AMP response element-binding protein phosphorylation in the dentate gyrus: studies with a galanin receptor 2 agonist and galanin receptor 1 knockout mice

The neuropeptide galanin was shown to impair cognitive performance and reduce hippocampal CA1 long-term potentiation (LTP) in rodents. However, the contribution of the two main galanin receptors; GalR1 and GalR2, present in the hippocampus to these effects is not known. In the present study, we determined the protein expression levels of GalR1 and GalR2 in the mouse dentate gyrus (DG) and used galanin (2-11), a recently introduced GalR2 agonist, and GalR1 knockout mice to examine the contribution of GalR1 and GalR2 to the modulation of LTP and 3',5'-cyclic AMP response element-binding protein (CREB)-dependent signaling cascades. In the DG, 57+/-5% of the galanin binding sites were GalR2, and the remaining population corresponded to GalR1. In hippocampal slices, galanin (2-11) fully blocked the induction of DG LTP, whereas galanin (1-29), a high affinity agonist for both GalR1 and GalR2, strongly but not fully attenuated the late phase of LTP by 80+/-1.5%. Application of galanin (1-29) or galanin (2-11) after LTP induction caused a transient reduction in the maintenance phase of LTP, with the larger effect displayed by superfusion of galanin (2-11). The induction and maintenance of DG LTP was not altered in the GalR1 knockout mice. Superfusion of galanin (1-29) or galanin (2-11) blocked the LTP induction to the same degree indicating a role for GalR2 in the induction phase of DG LTP. Furthermore, we analyzed the effects of GalR1 and/or GalR2 activation on DG LTP-induced CREB phosphorylation, associated with the late transcriptional effects of LTP. In the lateral part of the granule cell layer, high-frequency trains stimulation caused a significant increase in the level of CREB phosphorylation, which was significantly reduced by application of either galanin (1-29) or galanin (2-11), indicating that both GalR1 and/or GalR2 can mediate some of their effects on LTP through inhibition of CREB-related signaling cascades.

C-terminal truncation affects kinetic properties of GluR1 receptors

GluR1flop receptors in which the C-terminal 52 amino acids had been recombinantly removed were characterized with whole-cell recording and binding assays. Compared to wildtype GluR1, truncated receptors showed faster desensitization and deactivation and they recovered more slowly from desensitization. The EC50 for glutamate was increased 2-fold. In binding tests, K(D)s for [3H]fluorowillardiine were 1.5 times larger for truncated receptors. According to receptor simulations, most differences can be explained if the C-terminal domain is assumed to stabilize the ligand-bound closed and open states. The effects on response waveforms are different from those caused by phosphorylation, suggesting that the C-terminus influences receptor function in multiple ways. Truncated forms of GluR1 identical or similar to the one examined here may also be generated by calcium-activated proteases during intense synaptic activity. The lowered affinity and faster inactivation of these receptors suggests that their presence does not represent a risk for neuronal viability.

Interactions of postsynaptic density-95 and the NMDA receptor 2 subunit control calpain-mediated cleavage of the NMDA receptor

The calcium-dependent protease calpain cleaves the NMDA receptor 2 (NR2) subunit of the NMDA receptor both in vitro and in vivo and thus potentially modulates NMDA receptor function and turnover. We examined the ability of postsynaptic density-95 (PSD-95) protein to alter the calpain-mediated cleavage of NR2A and NR2B. Coexpression of PSD-95 with NMDA receptors in human embryonic kidney 293 cells blocked cleavage of NR2A and NR2B by NMDA receptor-activated calpain. NR2A cleavage by calpain occurred in the cell surface and intracellular fractions and required the presence of NR1 subunits. The blocking effect of PSD-95 did not result from decreased calpain activity, lowered intracellular calcium responses, or the blockade of internalization. Instead, this effect was eliminated by deletion of the C-terminal ESDV motif of NR2A or by overexpression of a palmitoylation-deficient PSD-95 mutant lacking the ability to cluster and to interact with NMDA receptors in situ, suggesting a role for association between the C terminus of NR2A and clustered PSD-95. Synapse-associated protein 102, a membrane-associated guanylate kinase interacting with NR2A but lacking palmitoylation motifs and the ability to cluster, did not protect NR2A from cleavage by calpain. Pharmacological inhibition of palmitoylation disrupted the interaction of PSD-95 with NMDA receptors in cortical neurons and allowed NR2A to be cleaved by calpain, whereas NR2A could not be cleaved in untreated neurons. These results indicate that PSD-95 clustering and direct association of NR2A and PSD-95 mediate the blocking effect of PSD-95 on calpain cleavage. PSD-95 could regulate the susceptibility of NMDA receptors to calpain-mediated cleavage during synaptic transmission and excitotoxicity.

Galanin type 2 receptors regulate neuronal survival, susceptibility to seizures and seizure-induced neurogenesis in the dentate gyrus

The neuropeptide galanin has been implicated in inhibiting seizures and protecting hippocampal neurons from excitotoxic injury. In the hippocampus galanin acts through two receptor subtypes, GalR1, expressed in CA1, and GalR2, abundant in dentate gyrus. We developed an approach to induce and to study selective semichronic knockdown of GalR2 in the rat hippocampus. A 50% reduction of GalR2 binding was achieved by continuous infusion of complementary peptide nucleic acid antisense oligonucleotide into the dentate gyrus. This resulted in an increase in the severity of self-sustaining status epilepticus induced by electrical stimulation of the perforant path, induced mild neuronal injury in the dentate hilus, augmented seizure-induced hilar injury and inhibited seizure-induced neurogenesis in the subgranular zone of the dentate gyrus. Our data suggest that in the dentate gyrus, galanin, acting through GalR2, inhibits seizures, promotes viability of hilar interneurons and stimulates seizure-induced neurogenesis. These results are important for understanding the role of galanin and galanin receptor subtypes in the hippocampus both under normal conditions and in excitotoxic injury.

Ischemia-induced alterations of AMPA-type glutamate receptor subunit. Expression patterns in the rat retina--an immunocytochemical study

This study investigates whether retinal ischemia/reperfusion leads to alterations in the expression of AMPA-type glutamate receptor (AMPAR) subunits GluR1-4. In ischemia-vulnerable hippocampal neurons, a subunit-specific downregulation of GluR2 precedes the actual neurodegeneration. Our purpose was to study whether retinal ischemia induces a similar downregulation of GluR2 preceding the loss of ganglion and amacrine cells. A 60-min ischemic period was followed by reperfusion lasting between 2 h and 7 days. Changes in the expression patterns of GluR1-4 were assessed using immunocytochemistry. In the same sections, alterations in cell density, thickness of retinal layers, and density of apoptotic cells were investigated. Two-hour post-ischemia, GluR1 immunoreactivity was nearly absent from the inner plexiform layer (IPL). Thereafter, labeling intensity recovered slowly and was close to control levels at 7 days, albeit in a thinner IPL. The decrease in GluR2/3 labeling intensity was most profound at 4 h. The recovery of GluR2/3 staining intensity was slow, and staining was still decreased at 7 days. GluR2 immunoreactivity was not attenuated after ischemia. GluR4 labeling showed a similar time course as observed for GluR1, but the decrease in immunoreactivity was less profound and the recovery was nearly complete. The immunostaining of PKCalpha, a rod bipolar cell marker, was unaffected at all reperfusion times. The reduction of GluR staining preceded both the typical thinning of the IPL and the peak of cell loss, but coincided with a significant swelling of the IPL. In conclusion, retinal ischemia/reperfusion leads to differential changes in the expression of the different AMPA-type GluR subunits, which may affect excitatory synaptic transmission in the inner retina. However, no evidence was found for a preferential loss of GluR2 immunoreactivity that could account for selective neurodegeneration of amacrine and ganglion cells after retinal ischemia.

Different isoforms of synapse-associated protein, SAP97, are expressed in the heart and have distinct effects on the voltage-gated K+ channel Kv1.5

The SAP97 isoforms differ by alternatively spliced insertion domains that regulate protein localization and oligomerization. We used reverse transcription-PCR to identify SAP97 isoforms of human and rat myocardium. In Chinese hamster ovary cells, cloned protein expression was studied using Western blot, confocal imaging of green fluorescent protein-tagged proteins, and patch clamp technique. The two main cardiac SAP97 isoforms contained both I3 and I1B inserts and differed by the I1A insert. Both isoforms co-precipitated with hKv1.5 channels. Only the isoform lacking I1A increased the current (by 215 +/- 22%), whatever the level of channel expression. To examine the involvement of the proline-rich I1A insert in the effect of SAP97, a W623F mutation in the Src homology 3 domain was created, and that restored the effect of the SAP97 on current. SAP97 isoform containing an I1A and I2 domain instead of the I3 domain stimulated the current, whereas SAP97 after deletion of the Src homology 3 and guanylate kinase-like domains did not. In cells co-expressing I3(+I1A) or I3(-I1A), green fluorescent protein-tagged Kv1.5 channels were organized in plaque-like structures at the plasma membrane level, whereas intracellular aggregates of channels predominated with the I2 isoform. The two cardiac SAP97 isoforms have different effects on the hKv1.5 current, depending on their capacity to form channel clusters.

Inositol hexakisphosphate-mediated regulation of glutamate receptors in rat brain sections

D-myo-inositol 1,2,3,4,5,6-hexakisphosphate (InsP6), one of the most abundant inositol phosphates within cells, has been proposed to play a key role in vesicle trafficking and receptor compartmentalization. In the present study, we used in vitro receptor autoradiography, subcellular fractionation, and immunoblotting to investigate its effects on alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors. Qualitative and quantitative analysis of 3H-AMPA binding indicated that incubation of frozen-thawed brain sections with InsP6 at 35 degrees C enhanced AMPA receptor binding in several brain regions, with maximal increases in the hippocampus and cerebellum. Moreover, saturation kinetics demonstrated that InsP6-induced augmentation of AMPA binding was due to an increment in the maximal number of AMPA binding sites. At the immunological level, Western blots performed on crude mitochondrial/synaptic (P2) fractions revealed that InsP6 (but not InsP5 and InsP3) treatment increased glutamate receptor (GluR)1 and GluR2 subunits of AMPA receptors, an effect that was associated with concomitant reductions in microsomal (P3) fractions. Interestingly, the InsP6-induced modulation of AMPA receptor binding was blocked at room temperature, and pretreatment with heparin also dampered its action on both AMPA receptor binding and GluR subunits. These effects of InsP6 appear to be specific to AMPA receptors, as neither 3H-glutamate binding to NMDA receptors nor levels of NR1 and NR2A subunits in P2 and P3 fractions were affected. Taken together, our data strongly suggest that InsP6 specifically regulates AMPA receptor distribution, possibly through a clathrin-dependent process.

Tyrosine phosphorylation of ionotropic glutamate receptors by Fyn or Src differentially modulates their susceptibility to calpain and enhances their binding to spectrin and PSD-95

Both tyrosine phosphorylation and calpain-mediated truncation of ionotropic glutamate receptors are important mechanisms for synaptic plasticity. Previous work from our laboratory has shown that calpain activation results in truncation of the C-terminal domains of several glutamate receptor subunits. To test whether and how tyrosine phosphorylation of glutamate ionotropic receptor subunits modulates calpain susceptibility, synaptic membranes were phosphorylated by Fyn or Src, two members of the Src family tyrosine kinases. Tyrosine phosphorylation of synaptic membranes by Src significantly reduced calpain-mediated truncation of both NR2A and NR2B subunits of NMDA receptors, but not of GluR1 subunits of AMPA receptors. In contrast, phosphorylation with Fyn significantly protected calpain-mediated truncation of GluR1 subunits of AMPA receptors, but enhanced calpain-mediated truncation of NR2A subunits of NMDA receptors. Similar results were observed with NR2A and NR2B C-terminal domain fusion proteins phosphorylated by Fyn or Src before incubation with calpain and calcium. In addition, phosphorylation of NR2A and NR2B C-terminal fusion proteins by Fyn or Src enhanced their binding to spectrin and PSD-95. Thus, tyrosine phosphorylation impairs or facilitates calpain-mediated truncation of glutamate receptor subunits, depending on which tyrosine kinase is activated. Such mechanisms could serve to regulate receptor integrity and location, in addition to modulating channel properties.

Proteolysis of glutamate receptor-interacting protein by calpain in rat brain: implications for synaptic plasticity

Activation of the calcium-dependent protease calpain has been proposed to be a key step in synaptic plasticity in the hippocampus. However, the exact pathway through which calpain mediates or modulates changes in synaptic function remains to be clarified. Here we report that glutamate receptor-interacting protein (GRIP) is a substrate of calpain, as calpain-mediated GRIP degradation was demonstrated using three different approaches: (i) purified calpain I digestion of synaptic membranes, (ii) calcium treatment of frozen-thawed brain sections, and (iii) NMDA-stimulated organotypic hippocampal slice cultures. More importantly, calpain activation resulted in the disruption of GRIP binding to the GluR2 subunit of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors. Because GRIP has been proposed to function as an AMPA receptor-targeting and synaptic-stabilizing protein, as well as a synaptic-organizing molecule, calpain-mediated degradation of GRIP and disruption of AMPA receptor anchoring are likely to play important roles in the structural and functional reorganization accompanying synaptic modifications in long-term potentiation and long-term depression.

Calpain-mediated degradation of PSD-95 in developing and adult rat brain

PSD-95 is a major postsynaptic density protein that is degraded as a result of synaptic activity. We used four different methods to test the hypothesis that calpain is involved in PSD-95 turnover. Treatment of synaptic membranes with purified calpain resulted in a decrease in immunoreactivity of the native 95 kDa protein and the appearance of two smaller molecular weight species, migrating at 50 and 36 kDa, respectively. Calcium treatment of frozen-thawed brain sections produced an identical digestion pattern, an effect blocked by calpain inhibitors. N-methyl-D-aspartate treatment of organotypic hippocampal cultures produced truncation of PSD-95 and accumulation of the 36 kDa species. Finally, calpain-generated degradation products of PSD95 were prominent in neonatal hippocampus, and disappeared with postnatal development. Our data suggest that PSD-95 is a substrate for calpain, and that calpain-mediated truncation contributes to PSD-95 turnover.