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

Fear conditioning downregulates miR-138 expression in the hippocampus to facilitate the formation of fear memory

Fear memory is important for the survival of animals and is associated with certain anxiety disorders, such as posttraumatic stress disorder. A thorough understanding of the molecular mechanisms of fear memory, especially associative fear memory, is imperative. MicroRNA-138 (miR-138) is a widely distributed microRNA in the brain and is locally enriched at synaptic sites. The role of miR-138 in the formation of fear memory is still largely unknown. In this study, a contextual fear conditioning (CFC) paradigm, bioinformatic methods, a luciferase assay, real-time PCR and western blot were used to evaluate the detailed effects of miR-138 on fear memory. We found that miR-138 transiently decreased in the dorsal hippocampus (DH) after CFC training. Upregulation or downregulation of miR-138 in the DH with miR-138 agomir or antagomir treatment significantly impaired or enhanced the formation of CFC memory, respectively. Moreover, the effects of miR-138 in the DH on the formation of CFC memory were achieved by changing the expression of the downstream target gene calpain 1 (Capn1). Taken together, both the in-vitro evidence and the in-vivo evidence presented in this study support the involvement of miR-138 in CFC memory formation, at least partly via the regulation of Capn1-mediated synaptic plasticity changes. Therapeutic use of miR-138/Capn1 is promising as an alternative option in the treatment of fear memory-related anxiety disorders.

Identification of Calpain-Activated Protein Functions

As opposed to proteasome-mediated proteolysis that leads to protein degradation, calpain proteases carry out limited proteolytic cleavages of their substrates. The cleavage of some substrates can produce active fragments that perform functions that are different from those performed by the full-length proteins. Therefore, cleavage by calpains can operate as a posttranslational modification and increase the functional diversity of target proteins. Nevertheless, activation of protein function by calpain cleavage is still an understudied area in molecular biology. Identifying and functionally characterizing by products generated by calpain cleavage could lead to the discovery of biomarkers and the identification of novel drug targets for the treatment of human diseases. This chapter contains a workflow designed to experimentally characterize novel calpain substrates, including identification of potential calpain targets via Western blotting, characterization of calpain cleavage sites, and the study of cellular functions played by such cleaved products. We will employ MYC as an example for these experiments.

Basolateral amygdala calpain is required for extinction of contextual fear-memory

Extinction of fear-memory is essential for emotional and mental changes. However, the mechanisms underlying extinction of fear-memory are largely unknown. Calpain is a type of calcium-dependent protease that plays a critical role in memory consolidation and reconsolidation. Whether calpain functions in extinction of fear-memory is unknown, as are the molecular mechanisms. In this study, we investigated the pivotal role of calpain in extinction of fear-memory in mice, and assessed its mechanism. Conditioned stimulation/unconditioned stimulation-conditioned stimulation paradigms combined with pharmacological methods were employed to evaluate the action of calpain in memory extinction. Our data demonstrated that intraperitoneal or intra-basolateral amygdala (BLA) injection of calpain inhibitors could eliminate extinction of fear-memory in mice. Moreover, extinction of fear-memory paradigm-activated BLA calpain activity, which degraded suprachiasmatic nucleus circadian oscillatory protein (SCOP) and phosphatase and tensin homolog (PTEN), subsequently contributing to activation of a protein kinase B (AKT)-mammalian target of the rapamycin (mTor) signaling pathway. Additionally, cAMP-response element binding protein (CREB) phosphorylation was also augmented following extinction of fear-memory. Calpain inhibitor blocked the signaling pathway activation induced by extinction of fear-memory. Additionally, intra-BLA injection of rapamycin or cycloheximide also blocked the extinction of fear-memory. Conversely, intra-BLA injection of PTEN inhibitor, bpV, reversed the effect of calpeptin on extinction of fear-memory. Together, our data confirmed the function of BLA calpain in extinction of fear-memory, likely via degrading PTEN and activating AKT-mTor-dependent protein synthesis.

Calpain modulates fear memory consolidation, retrieval and reconsolidation in the hippocampus

It has been proposed that long-lasting changes in dendritic spines provide a physical correlate for memory formation and maintenance. Spine size and shape are highly plastic, controlled by actin polymerization/depolymerization cycles. This actin dynamics are regulated by proteins such as calpain, a calcium-dependent cysteine protease that cleaves the structural cytoskeleton proteins and other targets involved in synaptic plasticity. Here, we tested whether the pharmacological inhibition of calpain in the dorsal hippocampus affects memory consolidation, retrieval and reconsolidation in rats trained in contextual fear conditioning. We first found that post-training infusion of the calpain inhibitor PD150606 impaired long-term memory consolidation, but not short-term memory. Next, we showed that pre-test infusion of the calpain inhibitor hindered memory retrieval. Finally, blocking calpain activity after memory reactivation disrupted reconsolidation. Taken together, our results show that calpain play an essential role in the hippocampus by enabling memory formation, expression and reconsolidation.

Regulatory role of calpain in neuronal death

Calpains are a group of calcium-dependent proteases that are over activated by increased intracellular calcium levels under pathological conditions. A wide range of substrates that regulate necrotic, apoptotic and autophagic pathways are affected by calpain. Calpain plays a very important role in neuronal death and various neurological disorders. This review introduces recent research progress related to the regulatory mechanisms of calpain in neuronal death. Various neuronal programmed death pathways including apoptosis, autophagy and regulated necrosis can be divided into receptor interacting protein-dependent necroptosis, mitochondrial permeability transition-dependent necrosis, pyroptosis and poly (ADP-ribose) polymerase 1-mediated parthanatos. Calpains cleave series of key substrates that may lead to cell death or participate in cell death. Regarding the investigation of calpain-mediated programed cell death, it is necessary to identify specific inhibitors that inhibit calpain mediated neuronal death and nervous system diseases.

Calpain-Mediated Degradation of Drebrin by Excitotoxicity In vitro and In vivo

The level of drebrin, an evolutionarily conserved f-actin-binding protein that regulates synaptic structure and function, is reduced in the brains of patients with chronic neurodegenerative diseases such as Alzheimer’s disease (AD) and Down’s syndrome (DS). It was suggested that excitotoxic neuronal death caused by overactivation of NMDA-type glutamate receptors (NMDARs) occurs in AD and DS; however, the relationship between excitotoxicity and drebrin loss is unknown. Here, we show that drebrin is a novel target of calpain-mediated proteolysis under excitotoxic conditions induced by the overactivation of NMDARs. In cultured rodent neurons, degradation of drebrin was confirmed by the detection of proteolytic fragments, as well as a reduction in the amount of full-length drebrin. Notably, the NMDA-induced degradation of drebrin in mature neurons occurred concomitantly with a loss of f-actin. Furthermore, pharmacological inhibition of f-actin loss facilitated the drebrin degradation, suggesting a functional linkage between f-actin and drebrin degradation. Biochemical analyses using purified drebrin and calpain revealed that calpain degraded drebrin directly in vitro. Furthermore, cerebral ischemia also induced the degradation of drebrin in vivo. These findings suggest that calpain-mediated degradation of drebrin is a fundamental pathology of neurodegenerative diseases mediated by excitotoxicity, regardless of whether they are acute or chronic. Drebrin regulates the synaptic clustering of NMDARs; therefore, degradation of drebrin under excitotoxic conditions may modulate NMDAR-mediated signal transductions, including pro-survival signaling. Overall, the results presented here provide novel insights into the molecular basis of cellular responses to excitotoxicity in vitro and in vivo.

E2F1 in neurons is cleaved by calpain in an NMDA receptor-dependent manner in a model of HIV-induced neurotoxicity

The transcription factor E2F1 activates gene targets required for G1 -S phase progression and for apoptosis, and exhibits increased expression levels in neurons in several CNS diseases including HIV encephalitis, Alzheimer disease, and Parkinson's Disease. While E2F1 is known to regulate cell viability through activation of caspases, here we present evidence supporting the involvement of E2F1 in N-methyl-d-aspartate (NMDA) receptor-dependent, HIV-induced neuronal death mediated by calpains. Using an in vitro model of HIV-induced neurotoxicity that is dependent on NMDA receptor and calpain activation, we have shown that cortical neurons lacking functional E2F1 are less susceptible to neuronal death. In addition, we report that neuronal E2F1 is cleaved by calpain to a stable 55-kiloDalton fragment following NR2B-dependent NMDA receptor stimulation. This cleavage of E2F1 is protein conformation-dependent and involves at least two cleavage events, one at each terminus of the protein. Intriguingly, the stabilized E2F1 cleavage product is produced in post-mitotic neurons of all ages, but fails to be stabilized in cycling cells. Finally, we show that a matching E2F1 cleavage product is produced in human fetal neurons, suggesting that calpain cleavage of E2F1 may be produced in human cortical tissue. These results suggest neuronal E2F1 is processed in a novel manner in response to NMDA receptor-mediated toxicity, a mechanism implicated in HIV-associated neurocognitive disorders pathogenesis as well as several other diseases of the CNS. After crossing the blood-brain barrier, HIV-infected monocytes differentiate into macrophages and release excitotoxins and inflammatory factors including glutamate into the brain parenchyma (1). These factors stimulate neuronal N-Methyl-d-aspartate (NMDA) receptors (2), causing calcium influx (3) and subsequent activation of the cysteine protease calpain (4). Activated calpain cleaves multiple substrates including E2F1, producing a stabilized protein fragment with truncations at the N- and C-terminus (5). Calpain-cleaved E2F1 may contribute to calpain-mediated neuronal damage observed in NMDA receptor-mediated neurotoxicity (6).

Role of calcium and calpain in the downregulation of voltage-gated sodium channel expression by the pyrethroid pesticide deltamethrin

Voltage-gated sodium channels (Na(v)) are essential for initiation and propagation of action potentials. Previous in vitro studies reported that exposure to the Na(v) toxins veratridine and α scorpion toxin cause persistent downregulation of Na(v) mRNA in vitro. However the mechanism of this downregulation is not well characterized. Here, we report that the type-II pyrethroid deltamethrin, which has a similar mechanism as these toxins, elicited an approximate 25% reduction in Na(v) 1.2 and Na(v) 1.3 mRNA in SK-N-AS cells. Deltamethrin-induced decreases of Na(v) mRNA were blocked with the Na(v) antagonist tetrodotoxin, demonstrating a primary role for interaction with Na(v). Pre-treatment with the intracellular calcium chelator BAPTA-AM and the calpain inhibitor PD-150606 also prevented these decreases, identifying a role for intracellular calcium and calpain activation. Because alterations in Na(v) expression and function can result in neurotoxicity, additional studies are warranted to determine whether or not such effects occur in vivo.

Region-specific changes in activities of cell death-related proteases and nitric oxide metabolism in rat brain in a chronic unpredictable stress model

Effects of a chronic combined unpredictable stress on activities of two cell death-related proteases, calpain and cathepsin B, were studied along with indices of nitrergic system in rat brain structures. Male Wistar rats were subjected to a 2-week-long combined stress (combination of unpaired flash light and moderate footshock associated with a white noise session). Stress resulted in a significant loss in the body and thymus weight and increased defecation in the open field test, though neither motor and exploratory activity, nor plasma corticosterone differed from the respective control levels. Decreased calpain activity and increased cathepsin B activity were demonstrated in the hippocampus of stressed rats (previously we have shown that caspase-3 activity was significantly suppressed in the brain of rats subjected to same type of stress). A significant reduction in the number of NOS-containing neurons was accompanied by a chronic stressinduced decline in NOS activity in the neocortex. Similar changes were observed in the hippocampus. However, levels of NO metabolites were elevated in both structures. Thus, stress-induced structural modifications in the brain may be mediated by disturbances in the nitrergic system and increased lysosomal proteolysis.

Cleavage of Tau by calpain in Alzheimer's disease: the quest for the toxic 17 kD fragment

The amyloid cascade hypothesis of Alzheimer's disease (AD) posits that the generation of β-amyloid (Aβ) triggers Tau neurofibrillary pathology. Recently a "17 kD" calpain-induced Tau fragment, comprising residues 45-230 (molecular weight [MW], 18.7 kD), was proposed to mediate Aβ-induced toxicity. Here, we demonstrate that the "17 kD" fragment is actually much smaller, containing residues 125-230 (molecular weight, 10.7 kD). Inducing Tau phosphorylation by okadaic acid or mimicking phosphorylation by Glu mutations at the epitopes of Alzheimer-diagnostic antibodies AT100/AT8/PHF1 could not prevent the generation of this fragment. The fragment can be induced not only by Aβ oligomers, but also by other cell stressors, e.g., thapsigargin (a Ca(2+)-ATPase inhibitor) or glutamate (an excitatory neurotransmitter). However, overexpression of neither Tau(45-230) nor Tau(125-230) fragment is toxic to Chinese hamster ovary (CHO) cells, neuroblastoma cells (N2a) or primary hippocampal neurons. Finally, the calpain-induced fragment can be observed both in Alzheimer's disease brains and in control normal human brains. We conclude that the 17 kD Tau fragment is not a mediator of Aβ-induced toxicity, leaving open the possibility that upstream calpain activation might cause both Tau fragmentation and toxicity.

Inhibition of calpain but not caspase activity by spectrin fragments

Calpains and caspases are ubiquitous cysteine proteases that are associated with a variety of cellular pathways. Calpains are involved in processes such as long term potentiation, cell motility and apoptosis, and have been shown to cleave non-erythroid (brain) α- and β-spectrin and erythroid β-spectrin. The cleavage of erythroid α-spectrin by calpain has not been reported. Caspases play an important role in the initiation and execution of apoptosis, and have been shown to cleave non-erythroid but not erythroid spectrin. We have studied the effect of spectrin fragments on calpain and caspase activities. The erythroid and non-erythroid spectrin fragments used were from the N-terminal region of α-spectrin, and C-terminal region of β-spectrin, both consisting of regions involved in spectrin tetramer formation. We observed that the all spectrin fragments exhibited a concentration-dependent inhibitory effect on calpain, but not caspase activity. It is clear that additional studies are warranted to determine the physiological significance of calpain inhibition by spectrin fragments. Our findings suggest that calpain activity is modulated by the presence of spectrin partial domains at the tetramerization site. It is not clear whether the inhibitory effect is substrate specific or is a general effect. Further studies of this inhibitory effect may lead to the identification and development of new therapeutic agents specifically for calpains, but not for caspases. Proteins/peptides with a coiled coil helical conformation should be studied for potential inhibitory effects on calpain activity.

Extrasynaptic NMDA receptors couple preferentially to excitotoxicity via calpain-mediated cleavage of STEP

NMDA receptor (NMDAR)-mediated excitotoxicity plays an important role in several CNS disorders, including epilepsy, stroke, and ischemia. Here we demonstrate the involvement of striatal-enriched protein tyrosine phosphatase (STEP) in this critical process. STEP(61) is an alternatively spliced member of the family that is present in postsynaptic terminals. In an apparent paradox, STEP(61) regulates extracellular signal-regulated kinase 1/2 (ERK1/2) and p38, two proteins with opposing functions; activated p38 promotes cell death, whereas activated ERK1/2 promotes cell survival. We found that synaptic stimulation of NMDARs promoted STEP(61) ubiquitination and degradation, concomitant with ERK1/2 activation. In contrast, extrasynaptic stimulation of NMDARs invoked calpain-mediated proteolysis of STEP(61), producing the truncated cleavage product STEP(33) and activation of p38. The calpain cleavage site on STEP was mapped to the kinase interacting motif, a domain required for substrate binding. As a result, STEP(33) neither interacts with nor dephosphorylates STEP substrates. A synthetic peptide spanning the calpain cleavage site efficiently reduced STEP(61) degradation and attenuated p38 activation and cell death in slice models. Furthermore, this peptide was neuroprotective when neurons were subjected to excitotoxicity or cortical slices were exposed to ischemic conditions. These findings suggest a novel mechanism by which differential NMDAR stimulation regulates STEP(61) to promote either ERK1/2 or p38 activation and identifies calpain cleavage of STEP(61) as a valid target for the development of neuroprotective therapy.