A calpain-2 selective inhibitor enhances learning & memory by prolonging ERK activation

Identification and neuroprotective properties of NA-184, a calpain-2 inhibitor

Our laboratory has shown that calpain-2 activation in the brain following acute injury is directly related to neuronal damage and the long-term functional consequences of the injury, while calpain-1 activation is generally neuroprotective and calpain-1 deletion exacerbates neuronal injury. We have also shown that a relatively selective calpain-2 inhibitor, referred to as C2I, enhanced long-term potentiation and learning and memory, and provided neuroprotection in the controlled cortical impact (CCI) model of traumatic brain injury (TBI) in mice. Using molecular dynamic simulation and Site Identification by Ligand Competitive Saturation (SILCS) software, we generated about 130 analogs of C2I and tested them in a number of in vitro and in vivo assays. These led to the identification of two interesting compounds, NA-112 and NA-184. Further analyses indicated that NA-184, (S)-2-(3-benzylureido)-N-((R,S)-1-((3-chloro-2-methoxybenzyl)amino)-1,2-dioxopentan-3-yl)-4-methylpentanamide, selectively and dose-dependent inhibited calpain-2 activity without evident inhibition of calpain-1 at the tested concentrations in mouse brain tissues and human cell lines. Like NA-112, NA-184 inhibited TBI-induced calpain-2 activation and cell death in mice and rats, both male and females. Pharmacokinetic and pharmacodynamic analyses indicated that NA-184 exhibited properties, including stability in plasma and liver and blood-brain barrier permeability, that make it a good clinical candidate for the treatment of TBI.

Revisiting the calpain hypothesis of learning and memory 40 years later

In 1984, Gary Lynch and Michel Baudry published in Science a novel biochemical hypothesis for learning and memory, in which they postulated that the calcium-dependent protease, calpain, played a critical role in regulating synaptic properties and the distribution of glutamate receptors, thereby participating in memory formation in hippocampus. Over the following 40 years, much work has been done to refine this hypothesis and to provide convincing arguments supporting what was viewed at the time as a simplistic view of synaptic biochemistry. We have now demonstrated that the two major calpain isoforms in the brain, calpain-1 and calpain-2, execute opposite functions in both synaptic plasticity/learning and memory and in neuroprotection/neurodegeneration. Thus, calpain-1 activation is required for triggering long-term potentiation (LTP) of synaptic transmission and learning of episodic memory, while calpain-2 activation limits the magnitude of LTP and the extent of learning. On the other hand, calpain-1 is neuroprotective while calpain-2 is neurodegenerative, and its prolonged activation following various types of brain insults leads to neurodegeneration. The signaling pathways responsible for these functions have been identified and involve local protein synthesis, cytoskeletal regulation, and regulation of glutamate receptors. Human families with mutations in calpain-1 have been reported to have impairment in motor and cognitive functions. Selective calpain-2 inhibitors have been synthesized and clinical studies to test their potential use to treat disorders associated with acute neuronal damage, such as traumatic brain injury, are being planned. This review will illustrate the long and difficult journey to validate a bold hypothesis.

Inhibition of ERK1/2 regulates cognitive function by decreasing expression levels of PSD-95 in the Hippocampus of CIH Rats

Obstructive sleep apnea syndrome (OSAS) is a potentially severe sleep disorder characterized by intermittent hypoxia, and there is growing evidence that OSAS can lead to cognitive decline. Extracellular signal-regulated protein kinase 1/2 (ERK1/2) plays a key role in synaptic plasticity. We established CIH model in male SD rats and examined their expression of p-ERK1/2 and PSD-95, as well as in CIH group, the effect of SL327 on the expression of p-ERK1/2 and PSD-95 in hippocampus of CIH model rats was observed by pretreating the experimental rats with SL327 during peak time of p-ERK1/2 expression. Mean oxygen saturation in the tail artery was lower in the CIH group. In the CIH groups exhibited increased escape latencies in the navigation test and decreased numbers of platform crossings in the space exploration test. Reduced volume, irregular structure, deepened cytoplasmic eosinophilic staining in the cytoplasm, and decreased nuclear size were found in hippocampal neurons in the 28-d CIH 、28-d CIH + SL327 group. The hippocampus of CIH rats^, p-ERK expressions gradually increased with prolonged CIH exposure but decreased after SL327 treatment. Moreover, PSD-95 expressions gradually reduced in the 14-d CIH, 21-d CIH, and 28-d CIH groups but increased in the SL327-treated group. The SL327 intervention decreased p-ERK1/2 expression, increased PSD-95 expression, and improved cognitive function in CIH rats. The present findings provide some insights into the mechanisms underlying OSAS-associated cognitive impairment.

MRI of Capn15 Knockout Mice and Analysis of Capn 15 Distribution Reveal Possible Roles in Brain Development and Plasticity

The Small Optic Lobe (SOL) family of calpains are intracellular cysteine proteases that are expressed in the nervous system and play an important role in neuronal development in both Drosophila, where loss of this calpain leads to the eponymous small optic lobes, and in mouse and human, where loss of this calpain leads to eye anomalies. Some human individuals with biallelic variants in CAPN15 also have developmental delay and autism. However, neither the specific effect of the loss of the Capn15 protein on brain development nor the brain regions where this calpain is expressed in the adult is known. Here we show using small animal MRI that mice with the complete loss of Capn15 have smaller brains overall with larger decreases in the thalamus and subregions of the hippocampus. These losses are not seen in Capn15 conditional knockout (KO) mice where Capn15 is knocked out only in excitatory neurons in the adult. Based on β-galactosidase expression in an insert strain where lacZ is expressed under the control of the Capn15 promoter, we show that Capn15 is expressed in adult mice, particularly in neurons involved in plasticity such as the hippocampus, lateral amygdala and Purkinje neurons, and partially in other non-characterized cell types. The regions of the brain in the adult where Capn15 is expressed do not correspond well to the regions of the brain most affected by the complete knockout suggesting distinct roles of Capn15 in brain development and adult brain function.

The Alpha Keto Amide Moiety as a Privileged Motif in Medicinal Chemistry: Current Insights and Emerging Opportunities

Over the years, researchers in drug discovery have taken advantage of the use of privileged structures to design innovative hit/lead molecules. The α-ketoamide motif is found in many natural products, and it has been widely exploited by medicinal chemists to develop compounds tailored to a vast range of biological targets, thus presenting clinical potential for a plethora of pathological conditions. The purpose of this perspective is to provide insights into the versatility of this chemical moiety as a privileged structure in drug discovery. After a brief analysis of its physical-chemical features and synthetic procedures to obtain it, α-ketoamide-based classes of compounds are reported according to the application of this motif as either a nonreactive or reactive moiety. The goal is to highlight those aspects that may be useful to understanding the perspectives of employing the α-ketoamide moiety in the rational design of compounds able to interact with a specific target.

Changes in neurodegeneration-related miRNAs in brains from CAPN1-/- mice

Calpain-1 knock-out (KO) mice exhibit enhanced susceptibility to neurodegeneration due to the lack of the neuroprotective function of calpain-1. Dicer has been shown to play a fundamental role in the biogenesis of most miRNAs. Here, we identified 45 differentially expressed miRNAs (DE miRNAs) in the brain of calpain-1 KO mice, as compared to wild-type mice. In particular, among all the DE miRNAs, 7 neurodegeneration-related miRNAs were found to be down-regulated in calpain-1 KO mice. We also found that Dicer is cleaved by calpain-1 in mouse brain, which generates an active fragment of Dicer with RNAse III activity and increases miRNA formation. Levels of active Dicer were reduced in brain homogenates from calpain-1 KO mice and incubation with calpain-1 and calcium restored Dicer activity and miRNA expression. Our results indicate that calpain-1 deletion results in decreased levels of active Dicer and changes in neurodegenerative-related miRNAs. These findings could account for some of the pathological changes found in brain of various mammals, including humans, with calpain-1 mutations or down-regulation.

Cognitive Decline of Rats with Chronic Fluorosis Is Associated with Alterations in Hippocampal Calpain Signaling

The study was designed to evaluate an influence of excessive fluoride (F^-) intake on cognitive capacities of adult rats and on proteins of memory-related calpain signaling in hippocampus. Control animals were given water with natural F^- content of 0.4 ppm; rats from other groups consumed the same water supplemented with 5, 20, and 50 ppm F^- (as NaF) for 12 months. The efficiency of learning and memory formation was evaluated by novel object recognition (NOR) and Morris water maze tests. The expression of enzymes of calpain-1 and calpain-2 signaling in hippocampus was detected by Western blotting. Excessive F^- consumption had moderate impact on short-term memory, but impaired spatial learning and long-term memory of animals. Intoxication of rats with 5-50 ppm F^- led to stimulation of calpain-1 in hippocampal cells and its translocation from cytosol to membranes, accompanied by activation of GTPase RhoA. Exposure to 20-50 ppm F^- resulted in proteolytic cleavage of phosphatase PHLPP1 and increased expression of phospho-ERK1/2 kinase with insignificant decline of total ERK1/2 activity. In contrast, F^- did not change the expression of calpain-2 and its substrates-phosphatase PTEN and kinase mTOR. However, F^- intake led to downregulation of cAMP-response element binding protein (CREB) and brain-derived neurotrophic factor (BDNF). Thus, altered expression of calpain-1 and its downstream effectors at a background of stable activity of calpain-2 indicates overstimulation of signaling pathways of early LTP phase and disrupted link between early and late LTP phases, most probably due to altered activity of transcriptional and neurotrophic factors.

Calpain-1 and Calpain-2 in the Brain: Dr. Jekill and Mr Hyde?

While the calpain system has now been discovered for over 50 years, there is still a paucity of information regard-ing the organization and functions of the signaling pathways regulated by these proteases, although calpains play critical roles in many cell functions. Moreover, calpain overactivation has been shown to be involved in numerous diseases. Among the 15 calpain isoforms identified, calpain-1 (aka µ-calpain) and calpain-2 (aka m-calpain) are ubiquitously distributed in most tissues and organs, including the brain. We have recently proposed that calpain-1 and calpain-2 play opposite functions in the brain, with calpain-1 activation being required for triggering synaptic plasticity and neuroprotection (Dr. Jekill), and calpain-2 limiting the extent of plasticity and being neurodegenerative (Mr. Hyde). Calpain-mediated cleavage has been ob-served in cytoskeleton proteins, membrane-associated proteins, receptors/channels, scaffolding/anchoring proteins, and pro-tein kinases and phosphatases. This review will focus on the signaling pathways related to local protein synthesis, cytoskele-ton regulation and neuronal survival/death regulated by calpain-1 and calpain-2, in an attempt to explain the origin of the op-posite functions of these 2 calpain isoforms. This will be followed by a discussion of the potential therapeutic applications of selective regulators of these 2 calpain isoforms.

Improvement in inflammation is associated with the protective effect of Gly-Pro-Glu and cycloprolylglycine against Aβ-induced depletion of the hippocampal somatostatinergic system

Glycine-proline-glutamate (GPE) is a cleaved tripeptide of IGF-I that can be processed to cycloprolylglycine (cPG) in the brain. IGF-I protects the hippocampal somatostatinergic system from β-amyloid (Aβ) insult and although neither IGF-I-derived peptides bind to IGF-I receptors, they exert protective actions in several neurological disorders. As their effects on the hippocampal somatostatinergic system remain unknown, the objective of this study was to evaluate if cPG and/or GPE prevent the deleterious effects of Aβ_25-35 infusion on this system and whether changes in intracellular-related signaling and interleukin (IL) content are involved in their protective effect. We also determined the effect of cPG or GPE co-administration with Aβ_25-35 on IL secretion in glial cultures and the influence of these ILs on signaling activation and somatostatin synthesis in neuronal cultures. cPG or GPE co-administration reduced Aβ-induced cell death and pro-inflammatory ILs, increased IL-4 and partially avoided the reduction of components of the somatostatinergic system affected by Aβ_25-35. GPE increased activation of Akt and CREB and reduced GSK3β activation and astrogliosis, whereas cPG increased phosphorylation of extracellular signal-regulated kinases. Both peptides converged in the activation of mTOR and S6 kinase. Co-administration of these peptides with Aβ_25-35 to glial cultures increased IL-4 and reduced IL-1β; this release of IL-4 could be responsible for activation of Akt and increased somatostatin in neuronal cultures. Our findings suggest that cPG and GPE exert protective effects against Aβ on the somatostatinergic system by a reduction of the inflammatory environment that may activate different pro-survival pathways in these neurons.

Impaired cerebellar plasticity and eye-blink conditioning in calpain-1 knock-out mice

Calpain-1 and calpain-2 are involved in the regulation of several signaling pathways and neuronal functions in the brain. Our recent studies indicate that calpain-1 is required for hippocampal synaptic plasticity, including long-term depression (LTD) and long-term potentiation (LTP) in field CA1. However, little is known regarding the contributions of calpain-1 to cerebellar synaptic plasticity. Low frequency stimulation (LFS, 5 Hz, 5 min)-induced LTP at parallel fibers to Purkinje cell synapses was markedly impaired in cerebellar slices from calpain-1 knock-out (KO) mice. Application of a selective calpain-2 inhibitor enhanced LFS-induced LTP in both wild-type (WT) and calpain-1 KO mice. Three protocols were used to induce LTD at these synapses: LFS (1 Hz, 15 min), perfusion with high potassium and glutamate (K-Glu) or dihydroxyphenylglycine (DHPG), a mGluR1 agonist. All three forms of LTD were impaired in calpain-1 KO mice. DHPG application stimulated calpain-1 but not calpain-2 in cerebellar slices, and DHPG-induced LTD impairment was reversed by application of a protein phosphatase 2A (PP2A) inhibitor, okadaic acid. As in hippocampus, BDNF induced calpain-1 activation and PH domain and Leucine-rich repeat Protein Phosphatase 1/suprachiasmatic nucleus oscillatory protein (PHLPP1/SCOP) degradation followed by extracellular signal-regulated kinase (ERK) activation, as well as calpain-2 activation leading to degradation of phosphatase and tensin homolog deleted on chromosome ten (PTEN) in cerebellar slices. The role of calpain-1 in associative learning was evaluated in the delay eyeblink conditioning (EBC). Calpain-1 KO mice exhibited significant learning impairment in EBC during the first 2 days of acquisition training. However, after 5 days of training, the percentage of conditioned responses (CRs) between calpain-1 KO and WT mice was identical. Both calpain-1 KO and WT mice exhibited typical extinction patterns. Our results indicate that calpain-1 plays critical roles in multiple forms of synaptic plasticity and associative learning in both hippocampus and cerebellum.

Involvement of calpain in the neuropathogenesis of Alzheimer's disease

Alzheimer’s disease (AD) is the most common (60% to 80%) age‐related disease associated with dementia and is characterized by a deterioration of behavioral and cognitive capacities leading to death in few years after diagnosis, mainly due to complications from chronic illness. The characteristic hallmarks of the disease are extracellular senile plaques (SPs) and intracellular neurofibrillary tangles (NFTs) with neuropil threads, which are a direct result of amyloid precursor protein (APP) processing to Aβ, and τ hyperphosphorylation. However, many indirect underlying processes play a role in this event. One of these underlying mechanisms leading to these histological hallmarks is the uncontrolled hyperactivation of a family of cysteine proteases called calpains. Under normal physiological condition calpains participate in many processes of cells’ life and their activation is tightly controlled. However, with an increase in age, increased oxidative stress and other excitotoxicity assaults, this regulatory system becomes impaired and result in increased activation of these proteases involving them in the pathogenesis of various diseases including neurodegeneration like AD. Reviewed here is a pool of data on the implication of calpains in the pathogenesis of AD, the underlying molecular mechanism, and the potential of targeting these enzymes for AD therapeutics.

M-Calpain Activation Facilitates Seizure Induced KCC2 Down Regulation

Potassium chloride co-transporter 2 (KCC2), a major chloride transporter that maintains GABA_A receptor inhibition in mature mammalian neurons, is down-regulated in the hippocampus during epileptogenesis. Impaired KCC2 function accelerates or facilitates seizure onset. Calpain, with two main subtypes of m- and μ-calpain, is a Ca²⁺-dependent cysteine protease that mediates the nonlysosomal degradation of KCC2. Although recent studies have demonstrated that calpain inhibitors exert antiepileptic and neuroprotective effects in animal models of acute and chronic epilepsy, whether calpain activation affects seizure induction through KCC2 degradation remains unknown. Our results showed that: (1) Blockade of calpain by non-selective calpain inhibitor MDL-28170 prevented convulsant stimulation induced KCC2 downregulation, and reduced the incidence and the severity of pentylenetetrazole (PTZ) induced seizures. (2) m-calpain, but not μ-calpain, inhibitor mimicked MDL-28170 effect on preventing KCC2 downregulation. (3) Phosphorylation of m-calpain has been significantly enhanced during seizure onset, which was partly mediated by the calcium independent MAPK/ERK signaling pathway activation. (4) MAPK/ERK signaling blockade also had similar effect as total calpain blockade on both KCC2 downregulation and animal seizure induction. The results indicate that upregulated m-calpain activation by MAPK/ERK during convulsant stimulation down regulates both cytoplasm- and membrane KCC2, and in turn facilitates seizure induction. This finding may provide a foundation for the development of highly effective antiepileptic drugs targeting of m-calpain.

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.

The zinc fingers of the small optic lobes calpain bind polyubiquitin

The small optic lobes (SOL) calpain is a highly conserved member of the calpain family expressed in the nervous system. A dominant negative form of the SOL calpain inhibited consolidation of one form of synaptic plasticity, non-associative facilitation, in sensory-motor neuronal cultures in Aplysia, presumably by inhibiting cleavage of protein kinase Cs (PKCs) into constitutively active protein kinase Ms (PKMs) (Hu et al. 2017a). SOL calpains have a conserved set of 5-6 N-terminal zinc fingers. Bioinformatic analysis suggests that these zinc fingers could bind to ubiquitin. In this study, we show that both the Aplysia and mouse SOL calpain (also known as Calpain 15) zinc fingers bind ubiquitinated proteins, and we confirm that Aplysia SOL binds poly- but not mono- or diubiquitin. No specific zinc finger is required for polyubiquitin binding. Neither polyubiquitin nor calcium was sufficient to induce purified Aplysia SOL calpain to autolyse or to cleave the atypical PKC to PKM in vitro. In Aplysia, over-expression of the atypical PKC in sensory neurons leads to an activity-dependent cleavage event and an increase in nuclear ubiquitin staining. Activity-dependent cleavage is partially blocked by a dominant negative SOL calpain, but not by a dominant negative classical calpain. The cleaved PKM was stabilized by the dominant negative classical calpain and destabilized by a dominant negative form of the PKM stabilizing protein KIdney/BRAin protein. These studies provide new insight into SOL calpain's function and regulation. Open Data: Materials are available on https://cos.io/our-services/open-science-badges/ https://osf.io/93n6m/.

EZH2 Methyltransferase Activity Controls Pten Expression and mTOR Signaling during Fear Memory Reconsolidation

Memory retrieval induces a transient period of increased transcriptional and translational regulation in neurons called reconsolidation, which is regulated by the protein kinase B (AKT)-mammalian target of rapamycin (mTOR) pathway. However, it is currently unknown how activation of the AKT-mTOR pathway is regulated during the reconsolidation process. Here, we found that in male rats retrieval of a contextual fear memory transiently increased Enhancer of Zeste Homolog 2 (EZH2) levels along with increased histone H3 lysine 27 trimethylation (H3K27me3) levels, which correlated with decreased levels of phosphatase and tensin homolog (PTEN), a potent inhibitor of AKT-mTOR-dependent signaling in the hippocampus. Further experiments found increased H3K27me3 levels and DNA methylation across the Pten promoter and coding regions, indicating transcriptional silencing of the Pten gene. Pten H3K27me3 levels did not change following training or after the retrieval of a remote (old) fear memory, suggesting that this mechanism of Pten repression was specific to the reconsolidation of a new memory. In vivo siRNA-mediated knockdown of Ezh2 in the hippocampus abolished retrieval-induced increases in H3K27me3 and prevented decreases in PTEN levels. Ezh2 knockdown attenuated increases in the phosphorylation of AKT and mTOR following retrieval, which could be restored by simultaneously reducing Pten, suggesting that H3K27me3 regulates AKT-mTOR phosphorylation via repression of Pten Consistent with these results, knockdown of Ezh2 in area CA1 before retrieval impaired memory on later tests. Collectively, these results suggest that EZH2-mediated H3K27me3 plays a critical role in the repression of Pten transcription necessary for AKT-mTOR activation and memory reconsolidation following retrieval.SIGNIFICANCE STATEMENT Understanding how critical translation pathways, like mTOR-mediated protein synthesis, are regulated during the memory storage process is necessary for improving memory impairments. This study tests whether mTOR activation is coupled to epigenetic mechanisms in the hippocampus following the retrieval of a contextual fear memory. Specifically, this study evaluates the role of epigenetic modifications in the form of histone methylation in downstream mTOR translational control during learning-dependent synaptic plasticity in neurons. Considering the broad implications of transcriptional and translational mechanisms in synaptic plasticity, psychiatric, and neurological and neurodegenerative disorders, these data are of interest to the neuroscience community due to the robust and specific regulation of mTOR signaling we found to be dependent on repressive histone methylation.

Proteolytic Degradation of Hippocampal STEP61 in LTP and Learning

Striatal-enriched protein tyrosine phosphatase (STEP) modulates key signaling molecules involved in synaptic plasticity and neuronal function. It is postulated that STEP opposes the development of long-term potentiation (LTP) and that it exerts a restraint on long-term memory (LTM). Here, we examined whether STEP₆₁ levels are regulated during hippocampal LTP and after training in hippocampal-dependent tasks. We found that after inducing LTP by high frequency stimulation or theta-burst stimulation STEP₆₁ levels were significantly reduced, with a concomitant increase of STEP₃₃ levels, a product of calpain cleavage. Importantly, inhibition of STEP with TC-2153 improved LTP in hippocampal slices. Moreover, we observed that after training in the passive avoidance and the T-maze spontaneous alternation task, hippocampal STEP₆₁ levels were significantly reduced, but STEP₃₃ levels were unchanged. Yet, hippocampal BDNF content and TrkB levels were increased in trained mice, and it is known that BDNF promotes STEP degradation through the proteasome. Accordingly, hippocampal pTrkB^Tyr816, pPLCγ^Tyr783, and protein ubiquitination levels were increased in T-SAT trained mice. Remarkably, injection of the TrkB antagonist ANA-12 (2 mg/Kg, but not 0.5 mg/Kg) elicited LTM deficits and promoted STEP₆₁ accumulation in the hippocampus. Also, STEP knockout mice outperformed wild-type animals in an age- and test-dependent manner. Summarizing, STEP₆₁ undergoes proteolytic degradation in conditions leading to synaptic strengthening and memory formation, thus highlighting its role as a molecular constrain, which is removed to enable the activation of pathways important for plasticity processes.

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.

Calpain-2 as a therapeutic target for acute neuronal injury

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 calpain-1 and calpain-2 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 calpain-2 in acute neuronal injury and discuss the possibility of developing selective calpain-2 inhibitors for therapeutic purposes. Areas covered: This review covers the literature showing how calpain-2 is implicated in neuronal death in a number of pathological conditions. The possibility of developing new selective calpain-2 inhibitors for treating these conditions is discussed. Expert opinion: As evidence accumulates that calpain-2 activation participates in acute neuronal injury, there is interest in developing therapeutic approaches using selective calpain-2 inhibitors. Recent data indicate the potential use of such inhibitors in various pathologies associated with acute neuronal death. The possibility of extending the use of such inhibitors to more chronic forms of neurodegeneration is discussed.

Protection against TBI-Induced Neuronal Death with Post-Treatment with a Selective Calpain-2 Inhibitor in Mice

Traumatic Brain Injury (TBI) is a major cause of death and disability worldwide. The calcium-dependent protease, calpain, has been shown to be involved in TBI-induced neuronal death. However, whereas various calpain inhibitors have been tested in several animal models of TBI, there has not been any clinical trial testing the efficacy of calpain inhibitors in human TBI. One important reason for this could be the lack of knowledge regarding the differential functions of the two major calpain isoforms in the brain, calpain-1 and calpain-2. In this study, we used the controlled cortical impact (CCI) model in mice to test the roles of calpain-1 and calpain-2 in TBI-induced neuronal death. Immunohistochemistry (IHC) with calpain activity markers performed at different time-points after CCI in wild-type and calpain-1 knock-out (KO) mice showed that calpain-1 was activated early in cortical areas surrounding the impact, within 0-8 h after CCI, whereas calpain-2 activation was delayed and was predominant during 8-72 h after CCI. Calpain-1 KO enhanced cell death, whereas calpain-2 activity correlated with the extent of cell death, suggesting that calpain-1 activation suppresses and calpain-2 activation promotes cell death following TBI. Systemic injection(s) of a calpain-2 selective inhibitor, NA101, at 1 h or 4 h after CCI significantly reduced calpain-2 activity and cell death around the impact site, reduced the lesion volume, and promoted motor and learning function recovery after TBI. Our data indicate that calpain-1 activity is neuroprotective and calpain-2 activity is neurodegenerative after TBI, and that a selective calpain-2 inhibitor can reduce TBI-induced cell death.

Novel calpain families and novel mechanisms for calpain regulation in Aplysia

Calpains are a family of intracellular proteases defined by a conserved protease domain. In the marine mollusk Aplysia californica, calpains are important for the induction of long-term synaptic plasticity and memory, at least in part by cleaving protein kinase Cs (PKCs) into constitutively active kinases, termed protein kinase Ms (PKMs). We identify 14 genes encoding calpains in Aplysia using bioinformatics, including at least one member of each of the four major calpain families into which metazoan calpains are generally classified, as well as additional truncated and atypical calpains. Six classical calpains containing a penta-EF-hand (PEF) domain are present in Aplysia. Phylogenetic analysis determined that these six calpains come from three separate classical calpain families. One of the classical calpains in Aplysia, AplCCal1, has been implicated in plasticity. We identify three splice cassettes and an alternative transcriptional start site in AplCCal1. We characterize several of the possible isoforms of AplCCal1 in vitro, and demonstrate that AplCCal1 can cleave PKCs into PKMs in a calcium-dependent manner in vitro. We also find that AplCCal1 has a novel mechanism of auto-inactivation through N-terminal cleavage that is modulated through its alternative transcriptional start site.

The tyrosine phosphatase PTPN13/FAP-1 links calpain-2, TBI and tau tyrosine phosphorylation

Traumatic brain injury (TBI) increases the risk of Alzheimer's disease (AD). Calpain activation and tau hyperphosphorylation have been implicated in both TBI and AD. However, the link between calpain and tau phosphorylation has not been fully identified. We recently discovered that the two major calpain isoforms in the brain, calpain-1 and calpain-2, play opposite functions in synaptic plasticity and neuronal survival/death, which may be related to their different C-terminal PDZ binding motifs. Here, we identify the tyrosine phosphatase PTPN13 as a key PDZ binding partner of calpain-2. PTPN13 is cleaved by calpain-2, which inactivates its phosphatase activity and generates stable breakdown products (P13BPs). We also found that PTPN13 dephosphorylates and inhibits c-Abl. Following TBI, calpain-2 activation cleaved PTPN13, activated c-Abl and triggered tau tyrosine phosphorylation. The activation of this pathway was responsible for the accumulation of tau oligomers after TBI, as post-TBI injection of a calpain-2 selective inhibitor inhibited c-Abl activation and tau oligomer accumulation. Thus, the calpain-2-PTPN13-c-Abl pathway provides a direct link between calpain-2 activation and abnormal tau aggregation, which may promote tangle formation and accelerate the development of AD pathology after repeated concussions or TBI. This study suggests that P13BPs could be potential biomarkers to diagnose mTBI or AD.

Calpain-GRIP Signaling in Nucleus Accumbens Core Mediates the Reconsolidation of Drug Reward Memory

Exposure to drug-paired cues causes drug memories to be in a destabilized state and interfering with memory reconsolidation can inhibit relapse. Calpain, a calcium-dependent neutral cysteine protease, is involved in synaptic plasticity and the formation of long-term fear memory. However, the role of calpain in the reconsolidation of drug reward memory is still unknown. In the present study, using a conditioned place preference (CPP) model, we found that exposure to drug-paired contextual stimuli induced the activation of calpain and decreased the expression of glutamate receptor interacting protein 1 (GRIP1) in the nucleus accumbens (NAc) core, but not shell, of male rats. Infusions of calpain inhibitors in the NAc core immediately after retrieval disrupted the reconsolidation of cocaine/morphine cue memory and blocked retrieval-induced calpain activation and GRIP1 degradation. The suppressive effect of calpain inhibitors on the expression of drug-induced CPP lasted for at least 14 d. The inhibition of calpain without retrieval 6 h after retrieval or after exposure to an unpaired context had no effects on the expression of reward memory. Calpain inhibition after retrieval also decreased cocaine seeking in a self-administration model and this effect did not recover spontaneously after 28 d. Moreover, the knock-down of GRIP1 expression in the NAc core by lentivirus-mediated short-hairpin RNA blocked disruption of the reconsolidation of drug cue memories that was induced by calpain inhibitor treatment. These results suggest that calpain activity in the NAc core is crucial for the reconsolidation of drug reward memory via the regulation of GRIP1 expression.SIGNIFICANCE STATEMENT Calpain plays an important role in synaptic plasticity and long-term memory consolidation, however, its role in the reconsolidation of drug cue memory remains unknown. Using conditioned place preference and self-administration procedures, we found that exposure to drug-paired cues induced the activation of calpain and decreased glutamate receptor interacting protein 1 (GRIP1) expression in the nucleus accumbens (NAc) core. The inhibition of calpain activity in the NAc core immediately after retrieval disrupted the reconsolidation of cocaine/morphine cue memory that was blocked by prior GRIP1 knock-down. Our findings indicate that calpain-GRIP signaling is essential for the restabilization process that is associated with drug cue memory and the inhibition of calpain activity may be a novel strategy for the prevention of drug relapse.

PTEN: Local and Global Modulation of Neuronal Function in Health and Disease

Phosphatase and tensin homolog deleted on chromosome ten (PTEN) was recently revealed to be a synaptic player during plasticity events in addition to its well-established role as a general controlling factor in cell proliferation and neuronal growth during development. Alterations of these direct actions of PTEN at synapses may lead to synaptic dysfunction with behavioral and cognitive consequences. A recent paradigmatic example of this situation, Alzheimer's disease (AD), is associated with excessive recruitment of PTEN into synapses leading to pathological synaptic depression. By contrast, some forms of autism are characterized by failure to weaken synaptic connections, which may be related to insufficient PTEN signaling. Understanding the modulation of synaptic function by PTEN in these pathologies may contribute to the development of new therapies.

Differential role of calpain-dependent protein cleavage in intermediate and long-term operant memory in Aplysia

In addition to protein synthesis, protein degradation or protein cleavage may be necessary for intermediate (ITM) and long-term memory (LTM) to remove molecular constraints, facilitate persistent kinase activity and modulate synaptic plasticity. Calpains, a family of conserved calcium dependent cysteine proteases, modulate synaptic function through protein cleavage. We used the marine mollusk Aplysia californica to investigate the in vivo role of calpains during intermediate and long-term operant memory formation using the learning that food is inedible (LFI) paradigm. A single LFI training session, in which the animal associates a specific netted seaweed with the failure to swallow, generates short (30min), intermediate (4-6h) and long-term (24h) memory. Using the calpain inhibitors calpeptin and MDL-28170, we found that ITM requires calpain activity for induction and consolidation similar to the previously reported requirements for persistent protein kinase C activity in intermediate-term LFI memory. The induction of LTM also required calpain activity. In contrast to ITM, calpain activity was not necessary for the molecular consolidation of LTM. Surprisingly, six hours after LFI training we found that calpain activity was necessary for LTM, although this is a time at which neither persistent PKC activity nor protein synthesis is required for the maintenance of long-term LFI memory. These results demonstrate that calpains function in multiple roles in vivo during associative memory formation.

Deleting both PHLPP1 and CANP1 rescues impairments in long-term potentiation and learning in both single knockout mice

Calpain-1 (CANP1) has been shown to play a critical role in synaptic plasticity and learning and memory, as its deletion in mice results in impairment in theta-burst stimulation- (TBS) induced LTP and various forms of learning and memory. Likewise, PHLPP1 (aka SCOP) has also been found to participate in learning and memory, as PHLPP1 overexpression impairs hippocampus-dependent learning. We previously showed that TBS-induced LTP was associated with calpain-1 mediated truncation of PHLPP1.To better understand the roles of these 2 genes in synaptic plasticity and learning and memory, we generated a double knockout (DKO) mouse by crossing the parent strains. Surprisingly, DKO mice exhibit normal TBS-induced LTP, and the learning impairments in fear conditioning and novel object or novel location recognition were absent in the DKO mice. Moreover, TBS-induced ERK activation in field CA1 of hippocampal slices, which is impaired in both single deletion mice, was restored in the DKO mice. These results further strengthen the roles of both CANP1 and PHLPP1 in synaptic plasticity and learning and memory, and illustrate the complexities of the interactions between multiple pathways participating in synaptic plasticity.

Calpains: Master Regulators of Synaptic Plasticity

Although calpain was proposed to participate in synaptic plasticity and learning and memory more than 30 years ago, the mechanisms underlying its activation and the roles of different substrates have remained elusive. Recent findings have provided evidence that the two major calpain isoforms in the brain, calpain-1 and calpain-2, play opposite functions in synaptic plasticity. In particular, while calpain-1 activation is the initial trigger for certain forms of synaptic plasticity, that is, long-term potentiation, calpain-2 activation restricts the extent of plasticity. Moreover, while calpain-1 rapidly cleaves regulatory and cytoskeletal proteins, calpain-2-mediated stimulation of local protein synthesis reestablishes protein homeostasis. These findings have important implications for our understanding of learning and memory and disorders associated with impairment in these processes.