Calpain-1 and calpain-2 play opposite roles in retinal ganglion cell degeneration induced by retinal ischemia/reperfusion injury

Reln-Dab1 pathway mitigates retinal ganglion cell apoptosis in retinal ischemia-reperfusion injury

Ischemia-reperfusion (I/R) injury is associated with a variety of retinal diseases, resulting in loss of the number of ganglion cells (RGCs), retinal structural disorders, and retinal dysfunction. The Reelin protein is an important regulator of neuronal migration and synaptogenesis, and the Reln signaling pathway plays an essential role in regulating the targeted projection of RGC dendrites and neuronal survival, which has not been reported in retinal I/R injury. The aim of this study was to investigate the expression, role and mechanism of Reln in retinal I/R injury. By establishing Reln-CreERT2 mTmG transgenic mice, it was observed that the expression of Reln initially decreased and then increased after retinal I/R injury. After supplementing exogenous Reelin protein and adeno-associated virus (AAV)-targeted regulation of Reln in vivo, morphological and functional experiments demonstrated its effectiveness in protecting RGCs survival, maintaining morphological integrity of the retina, and inhibiting post-injury retinal dysfunction. Furthermore, integrin β1 (Itgb1) was identified as the main receptor through which Reelin exerts neuroprotective effects while regulating retinal I/R injury repair through the Dab1-PI3K-Akt pathway. These findings provide evidence supporting Reln pathway's role in maintaining retinal homeostasis and facilitating injury repair. Moreover, these findings have significant implications for identifying new targets for preventing and treating various retinal diseases.

Optic nerve injury impairs intrinsic mechanisms underlying electrical activity in a resilient retinal ganglion cell

Retinal ganglion cells (RGCs) are the sole output neurons of the retina and convey visual information to the brain via their axons in the optic nerve. Following injury to the optic nerve, RGC axons degenerate and many cells die. For example, a model of axon injury, the optic nerve crush (ONC), kills ∼80% of RGCs after 2 weeks. Surviving cells are biased towards 'resilient' types, including several with sustained firing to light stimulation. RGC survival may depend on activity, and there is limited understanding of how or why activity changes following optic nerve injury. Here we quantified the electrophysiological properties of a highly resilient RGC type, the sustained ON-Alpha (AlphaONS) RGC, 7 days after ONC with extracellular and whole-cell patch clamp recording. Both light- and current-driven firing were reduced after ONC, but synaptic inputs were largely intact. Resting membrane potential and input resistance were relatively unchanged, while voltage-gated currents were impaired, including a reduction in voltage-gated sodium channel current and channel density in the axon initial segment. Hyperpolarization or chelation of intracellular calcium partially rescued firing rates. Extracellular recordings at 3 days following ONC showed normal light-evoked firing from AlphaONS RGCs and other Alpha RGCs, including susceptible types. These data suggest that an injured resilient RGC reduces its activity by 1 week after injury as a consequence of reduced voltage-gated current and downregulation of intrinsic excitability via a Ca2+-dependent mechanism. Reduced excitability may be due to degradation of the axon but could also be energetically beneficial, preserving energy for survival and regeneration. KEY POINTS: Retinal ganglion cell (RGC) types show diverse rates of survival after axon injury. A resilient RGC type (sustained ON-Alpha RGC) maintains its synaptic inputs 1 week after injury. The resilient RGC type shows diminished firing and reduced expression of axon initial segment genes 1 week after injury Activity deficits reflect dysfunction of intrinsic properties (Na+ channels, intracellular Ca2+), not changes to synaptic input. Both resilient and susceptible Alpha RGC types show intact firing at 3 days after injury, suggesting that activity at this time point does not predict resilience.

Loss of Stim2 in zebrafish induces glaucoma-like phenotype

Calcium is involved in vision processes in the retina and implicated in various pathologies, including glaucoma. Rod cells rely on store-operated calcium entry (SOCE) to safeguard against the prolonged lowering of intracellular calcium ion concentrations. Zebrafish that lacked the endoplasmic reticulum Ca2+ sensor Stim2 (stim2 knockout [KO]) exhibited impaired vision and lower light perception-related gene expression. We sought to understand mechanisms that are responsible for vision impairment in stim2 KO zebrafish. The single-cell RNA (scRNA) sequencing of neuronal cells from brains of 5 days postfertilization larvae distinguished 27 cell clusters, 10 of which exhibited distinct gene expression patterns, including amacrine and γ-aminobutyric acid (GABA)ergic retinal interneurons and GABAergic optic tectum cells. Five clusters exhibited significant changes in cell proportions between stim2 KO and controls, including GABAergic diencephalon and optic tectum cells. Transmission electron microscopy of stim2 KO zebrafish revealed decreases in width of the inner plexiform layer, ganglion cells, and their dendrites numbers (a hallmark of glaucoma). GABAergic neuron densities in the inner nuclear layer, including amacrine cells, as well as photoreceptors significantly decreased in stim2 KO zebrafish. Our study suggests a novel role for Stim2 in the regulation of neuronal insulin expression and GABAergic-dependent vision causing glaucoma-like retinal pathology.

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.

Calpain-2 Inhibitors as Therapy for Traumatic Brain Injury

While calpains have long been implicated in neurodegeneration, no calpain inhibitor has been developed for the treatment of neurodegeneration. This is partly due to the lack of understanding of the specific functions of most of the 15 members of the calpain family. Work from our laboratory over the last 5-10 years has revealed that calpain-1 and calpain-2, two of the major calpain isoforms in the brain, play opposite roles in both synaptic plasticity/learning and memory and neuroprotection/neurodegeneration. Thus, calpain-1 activation is required for triggering certain forms of synaptic plasticity and for learning some types of information and is neuroprotective. In contrast, calpain-2 activation limits the extent of synaptic plasticity and of learning and is neurodegenerative. These results have been validated with the use of calpain-1 knock-out mice and mice with a selective calpain-2 deletion in excitatory neurons of the forebrain. Through a medicinal chemistry campaign, we have identified a number of selective calpain-2 inhibitors and shown that these inhibitors do facilitate learning of certain tasks and are neuroprotective in a number of animal models of acute neurodegeneration. One of these inhibitors, NA-184, is currently being developed for the treatment of traumatic brain injury, and clinical trials are being planned.

Genome-wide analyses identify novel risk loci for cluster headache in Han Chinese residing in Taiwan

BACKGROUND

Cluster headache is a highly debilitating neurological disorder with considerable inter-ethnic differences. Genome-wide association studies (GWAS) recently identified replicable genomic loci for cluster headache in Europeans, but the genetic underpinnings for cluster headache in Asians remain unclear. The objective of this study is to investigate the genetic architecture and susceptibility loci of cluster headache in Han Chinese resided in Taiwan.

METHODS

We conducted a two-stage genome-wide association study in a Taiwanese cohort enrolled from 2007 through 2022 to identify the genetic variants associated with cluster headache. Diagnosis of cluster headache was retrospectively ascertained with the criteria of International Classification of Headache Disorders, third edition. Control subjects were enrolled from the Taiwan Biobank. Genotyping was conducted with the Axiom Genome-Wide Array TWB chip, followed by whole genome imputation. A polygenic risk score was developed to differentiate patients from controls. Downstream analyses including gene-set and tissue enrichment, linkage disequilibrium score regression, and pathway analyses were performed.

RESULTS

We enrolled 734 patients with cluster headache and 9,846 population-based controls. We identified three replicable loci, with the lead SNPs being rs1556780 in CAPN2 (odds ratio = 1.59, 95% CI 1.42‒1.78, p = 7.61 × 10^-16), rs10188640 in MERTK (odds ratio = 1.52, 95% CI 1.33‒1.73, p = 8.58 × 10^-13), and rs13028839 in STAB2 (odds ratio = 0.63, 95% CI 0.52‒0.78, p = 2.81 × 10^-8), with the latter two replicating the findings in European populations. Several previously reported genes also showed significant associations with cluster headache in our samples. Polygenic risk score differentiated patients from controls with an area under the receiver operating characteristic curve of 0.77. Downstream analyses implicated circadian regulation and immunological processes in the pathogenesis of cluster headache.

CONCLUSIONS

This study revealed the genetic architecture and novel susceptible loci of cluster headache in Han Chinese residing in Taiwan. Our findings support the common genetic contributions of cluster headache across ethnicities and provide novel mechanistic insights into the pathogenesis of cluster headache.

Inhibition of Calpain Attenuates Degeneration of Substantia Nigra Neurons in the Rotenone Rat Model of Parkinson's Disease

In the central nervous system (CNS), calcium homeostasis is a critical determinant of neuronal survival. Calpain, a calcium-dependent neutral protease, is widely expressed in the brain, including substantia nigra (SN) dopaminergic (DA) neurons. Though calpain is implicated in human Parkinson's disease (PD) and corresponding animal models, the roles of specific ubiquitous calpain isoforms in PD, calpain-1 and calpain-2, remain poorly understood. In this study, we found that both isoforms are activated in a nigrostriatal pathway with increased phosphorylated synuclein following the administration of rotenone in Lewis rats, but calpain isoforms played different roles in neuronal survival. Although increased expression of calpain-1 and calpain-2 were detected in the SN of rotenone-administered rats, calpain-1 expression was not altered significantly after treatment with calpain inhibitor (calpeptin); this correlated with neuronal survival. By contrast, increased calpain-2 expression in the SN of rotenone rats correlated with neuronal death, and calpeptin treatment significantly attenuated calpain-2 and neuronal death. Calpain inhibition by calpeptin prevented glial (astroglia/microglia) activation in rotenone-treated rats in vivo, promoted M2-type microglia, and protected neurons. These data suggest that enhanced expression of calpain-1 and calpain-2 in PD models differentially affects glial activation and neuronal survival; thus, the attenuation of calpain-2 may be important in reducing SN neuronal loss in PD.

Pregabalin Mediates Retinal Ganglion Cell Survival From Retinal Ischemia/Reperfusion Injury Via the Akt/GSK3β/β-Catenin Signaling Pathway

Purpose

Progressive retinal ganglion cell (RGC) loss induced by retinal ischemia/reperfusion (RIR) injury leads to irreversible visual impairment. Pregabalin (PGB) is a promising drug for neurodegenerative diseases. However, with regard to RGC survival, its specific role and exact mechanism after RIR injury remain unclear. In this study, we sought to investigate whether PGB could protect RGCs from mitochondria-related apoptosis induced by RIR and explore the possible mechanisms.

Methods

C57BL/6J mice and primary RGCs were pretreated with PGB prior to ischemia/reperfusion modeling. The retinal structure and cell morphology were assessed by immunochemical assays and optical coherence tomography. CCK8 was used to assay cell viability, and an electroretinogram was performed to detect RGC function. Mitochondrial damage was assessed by a reactive oxygen species (ROS) assay kit and transmission electron microscopy. Western blot and immunofluorescence assays quantified the expression of proteins associated with the Akt/GSK3β/β-catenin pathway.

Results

Treatment with PGB increased the viability of RGCs in vitro. Consistently, PGB preserved the normal thickness of the retina, upregulated Bcl-2, reduced the ratio of cleaved caspase-3/caspase-3 and the expression of Bax in vivo. Meanwhile, PGB improved mitochondrial structure and prevented excessive ROS production. Moreover, PGB restored the amplitudes of oscillatory potentials and photopic negative responses following RIR. The mechanisms underlying its neuroprotective effects were attributed to upregulation of the Akt/GSK3β/β-catenin pathway. However, PGB-mediated neuroprotection was suppressed when using MK2206 (an Akt inhibitor), whereas it was preserved when treated with TWS119 (a GSK3β inhibitor).

Conclusions

PGB exerts a protective effect against RGC apoptosis induced by RIR injury, mediated by the Akt/GSK3β/β-catenin pathway.

Calpains as mechanistic drivers and therapeutic targets for ocular disease

Ophthalmic neurodegenerative diseases encompass a wide array of molecular pathologies unified by calpain dysregulation. Calpains are calcium-dependent proteases that perpetuate cellular death and inflammation when hyperactivated. Calpain inhibition trials in other organs have faced pharmacological challenges, but the eye offers many advantages for the development and testing of targeted molecular therapeutics, including small molecules, peptides, engineered proteins, drug implants, and gene-based therapies. This review highlights structural mechanisms underlying calpain activation, distinct cellular expression patterns, and in vivo models that link calpain hyperactivity to human retinal and developmental disease. Optimizing therapeutic approaches for calpain-mediated eye diseases can help accelerate clinically feasible strategies for treating calpain dysregulation in other diseased tissues.

Retinal Glutamate Neurotransmission: From Physiology to Pathophysiological Mechanisms of Retinal Ganglion Cell Degeneration

Glutamate neurotransmission and metabolism are finely modulated by the retinal network, where the efficient processing of visual information is shaped by the differential distribution and composition of glutamate receptors and transporters. However, disturbances in glutamate homeostasis can result in glutamate excitotoxicity, a major initiating factor of common neurodegenerative diseases. Within the retina, glutamate excitotoxicity can impair visual transmission by initiating degeneration of neuronal populations, including retinal ganglion cells (RGCs). The vulnerability of RGCs is observed not just as a result of retinal diseases but has also been ascribed to other common neurodegenerative and peripheral diseases. In this review, we describe the vulnerability of RGCs to glutamate excitotoxicity and the contribution of different glutamate receptors and transporters to this. In particular, we focus on the N-methyl-d-aspartate (NMDA) receptor as the major effector of glutamate-induced mechanisms of neurodegeneration, including impairment of calcium homeostasis, changes in gene expression and signalling, and mitochondrial dysfunction, as well as the role of endoplasmic reticular stress. Due to recent developments in the search for modulators of NMDA receptor signalling, novel neuroprotective strategies may be on the horizon.

Calpains as Potential Therapeutic Targets for Myocardial Hypertrophy

Despite advances in its treatment, heart failure remains a major cause of morbidity and mortality, evidencing an urgent need for novel mechanism-based targets and strategies. Myocardial hypertrophy, caused by a wide variety of chronic stress stimuli, represents an independent risk factor for the development of heart failure, and its prevention constitutes a clinical objective. Recent studies performed in preclinical animal models support the contribution of the Ca²⁺-dependent cysteine proteases calpains in regulating the hypertrophic process and highlight the feasibility of their long-term inhibition as a pharmacological strategy. In this review, we discuss the existing evidence implicating calpains in the development of cardiac hypertrophy, as well as the latest advances in unraveling the underlying mechanisms. Finally, we provide an updated overview of calpain inhibitors that have been explored in preclinical models of cardiac hypertrophy and the progress made in developing new compounds that may serve for testing the efficacy of calpain inhibition in the treatment of pathological cardiac hypertrophy.

Calpain-Mediated Alterations in Astrocytes Before and During Amyloid Chaos in Alzheimer's Disease

One of the changes found in the brain in Alzheimer's disease (AD) is increased calpain, derived from calcium dysregulation, oxidative stress, and/or neuroinflammation, which are all assumed to be basic pillars in neurodegenerative diseases. The role of calpain in synaptic plasticity, neuronal death, and AD has been discussed in some reviews. However, astrocytic calpain changes sometimes appear to be secondary and consequent to neuronal damage in AD. Herein, we explore the possibility of calpain-mediated astroglial reactivity in AD, both preceding and during the amyloid phase. We discuss the types of brain calpains but focus the review on calpains 1 and 2 and some important targets in astrocytes. We address the signaling involved in controlling calpain expression, mainly involving p38/mitogen-activated protein kinase and calcineurin, as well as how calpain regulates the expression of proteins involved in astroglial reactivity through calcineurin and cyclin-dependent kinase 5. Throughout the text, we have tried to provide evidence of the connection between the alterations caused by calpain and the metabolic changes associated with AD. In addition, we discuss the possibility that calpain mediates amyloid-β clearance in astrocytes, as opposed to amyloid-β accumulation in neurons.

The Regulatory NOD-Like Receptor NLRC5 Promotes Ganglion Cell Death in Ischemic Retinopathy by Inducing Microglial Pyroptosis

Retinal ischemia is a common pathological event that can result in retinal ganglion cell (RGC) death and irreversible vision loss. The pathogenic mechanisms linking retinal ischemia to RGC loss and visual deficits are uncertain, which has greatly hampered the development of effective treatments. It is increasingly recognized that pyroptosis of microglia contributes to the indirect inflammatory death of RGCs. In this study, we report a regulatory NOD-like receptor, NOD-, LRR- and CARD-containing 5 (NLRC5), as a key regulator on microglial pyroptosis and the retinal ischemia process. Through an in-depth analysis of our recently published transcriptome data, we found that NLRC5 was significantly up-regulated in retina during ischemia–reperfusion injury, which were further confirmed by subsequent detection of mRNA and protein level. We further found that NLRC5 was upregulated in retinal microglia during ischemia, while NLRC5 knockdown significantly ameliorated retinal ischemic damage and RGC death. Mechanistically, we revealed that knockdown of NLRC5 markedly suppressed gasdermin D (GSDMD) cleavage and activation of interleukin-1β (IL-1β) and caspase-3, indicating that NLRC5 promotes both microglial pyroptosis and apoptosis. Notably, we found that NLRC5 directly bound to NLRP3 and NLRC4 in inflammasomes to cooperatively drive microglial pyroptosis and apoptosis mediating retinal ischemic damage. Overall, these findings reveal a previously unidentified key contribution of NLRC5 signaling to microglial pyroptosis under ischemia or hypoxia conditions. This NLRC5-dependent pathway may be a novel therapeutic target for treatment of ischemic retinopathy.

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.

Calpain-1 and Calpain-2 in the Brain: New Evidence for a Critical Role of Calpain-2 in Neuronal Death

Calpains are a family of soluble calcium-dependent proteases that are involved in multiple regulatory pathways. Our laboratory has focused on the understanding of the functions of two ubiquitous calpain isoforms, calpain-1 and calpain-2, in the brain. Results obtained over the last 30 years led to the remarkable conclusion that these two calpain isoforms exhibit opposite functions in the brain. Calpain-1 activation is required for certain forms of synaptic plasticity and corresponding types of learning and memory, while calpain-2 activation limits the extent of plasticity and learning. Calpain-1 is neuroprotective both during postnatal development and in adulthood, while calpain-2 is neurodegenerative. Several key protein targets participating in these opposite functions have been identified and linked to known pathways involved in synaptic plasticity and neuroprotection/neurodegeneration. We have proposed the hypothesis that the existence of different PDZ (PSD-95, DLG and ZO-1) binding domains in the C-terminal of calpain-1 and calpain-2 is responsible for their association with different signaling pathways and thereby their different functions. Results with calpain-2 knock-out mice or with mice treated with a selective calpain-2 inhibitor indicate that calpain-2 is a potential therapeutic target in various forms of neurodegeneration, including traumatic brain injury and repeated concussions.

Involvement of the miR-137-3p/CAPN-2 Interaction in Ischemia-Reperfusion-Induced Neuronal Apoptosis through Modulation of p35 Cleavage and Subsequent Caspase-8 Overactivation

Background

Neuron survival after ischemia-reperfusion (IR) injury is the primary determinant of motor function prognosis. MicroRNA- (miR-) based gene therapy has gained attention recently. Our previous work explored the mechanisms by which miR-137-3p modulates neuronal apoptosis in both in vivo and in vitro IR models.

Methods

IR-induced motor dysfunction and spinal calpain (CAPN) subtype expression and subcellular localization were detected within 12 h post IR. Dysregulated miRs, including miR-137-3p, were identified by miR microarray analysis and confirmed by PCR. A luciferase assay confirmed CAPN-2 as a corresponding target of miR-137-3p, and their modulation of motor function was evaluated by intrathecal injection with synthetic miRs. CAPN-2 activity was measured by the intracellular Ca²⁺ concentration and mean fluorescence intensity in vitro. Neuronal apoptosis was detected by flow cytometry and TUNEL assay. The activities of p35, p25, Cdk5, and caspase-8 were evaluated by ELISA and Western blot after transfection with specific inhibitors and miRs.

Results

The IR-induced motor dysfunction time course was closely associated with upregulated expression of the CAPN-2 protein, which was mainly localized in neurons. The miR-137-3p/CAPN-2 interaction was confirmed by luciferase assay. The miR-137-3p mimic significantly improved IR-induced motor dysfunction and decreased CAPN-2 expression, even in combination with recombinant rat calpain-2 (rr-CALP2) injection, whereas the miR-137-3p inhibitor reversed these effects. Similar changes in the intracellular Ca²⁺ concentration, CAPN-2 expression, and CAPN-2 activity were observed when cells were exposed to oxygen-glucose deprivation and reperfusion (OGD/R) and transfected with synthetic miRs in vitro. Moreover, double fluorescence revealed identical neuronal localization of CAPN-2, p35, p25, and caspase-8. The decrease in CAPN-2 expression and activity was accompanied by the opposite changes in p35 activity and protein expression in cells transfected with the miR-137-3p mimic, roscovitine (a Cdk5 inhibitor), or Z-IETD-FMK (a caspase-8 inhibitor). Correspondingly, the abovementioned treatments resulted in a higher neuron survival rate than that of untreated neurons, as indicated by decreases in the apoptotic cell percentage and p25, Cdk5, caspase-8, and caspase-3 protein expression.

Conclusions

The miR-137-3p/CAPN-2 interaction modulates neuronal apoptosis during IR injury, possibly by inhibiting CAPN-2, which leads to p35 cleavage and inhibition of subsequent p25/Cdk5 and caspase-8 overactivation.

Downregulation of PHLPP1 ameliorates high glucose-evoked injury in retinal ganglion cells by attenuating apoptosis and oxidative stress through enhancement of Nrf2 activation

High glucose (HG)-induced oxidative stress contributes significantly to the pathogenesis of diabetic retinopathy. Pleckstrin homology domain and leucine rich repeat protein phosphatase 1 (PHLPP1) has emerged as a key regulator of oxidative stress implicated in various pathological processes. However, whether PHLPP1 participates in the regulation of HG-induced oxidative stress injury of retinal ganglion cells (RGCs) in diabetic retinopathy is undetermined. The purpose of this study was to explore the potential role and molecular mechanism of PHLPP1 in regulating HG-induced injury of RGCs. Our data showed that PHLPP1 expression was markedly elevated in RGCs from diabetic rats and HG-exposed RGCs. Our functional assay elucidated that knockdown of PHLPP1 improved cell viability and decreased cell apoptosis and reactive oxygen species (ROS) production in HG-exposed RGCs. Additionally, upregulation of PHLPP1 lowered cell viability and increased cell apoptosis and ROS production in HG-exposed RGCs. Mechanistically, knockdown of PHLPP1 resulted in an increase in nuclear factor erythroid-2 related factor 2 (Nrf2) nuclear expression and Nrf2/antioxidant response element (ARE)-mediated transcription associated with upregulation of glycogen synthase kinase-3β (GSK-3β) phosphorylation. Moreover, inhibition of GSK-3β significantly reversed the suppressive effect of PHLPP1 overexpression on Nrf2/ARE activation. Notably, the protective effect of PHLPP1 knockdown on HG-induced injury in RGCs was markedly abolished by Nrf2 inhibition. In conclusion, Our findings demonstrate that downregulation of PHLPP1 activates Nrf2/ARE signaling to protect RGCs from HG-induced apoptosis and oxidative stress. This study indicates a potential role of PHLPP1 in regulating HG-induced injury of RGCs during the development and progression of diabetic retinopathy.

Deletion of the Capn1 Gene Results in Alterations in Signaling Pathways Related to Alzheimer's Disease, Protein Quality Control and Synaptic Plasticity in Mouse Brain

Calpains represent a family of calcium-dependent proteases participating in a multitude of functions under physiological or pathological conditions. Calpain-1 is one of the most studied members of the family, is ubiquitously distributed in organs and tissues, and has been shown to be involved in synaptic plasticity and neuroprotection in mammalian brain. Calpain-1 deletion results in a number of phenotypic alterations. While some of these alterations can be explained by the acute functions of calpain-1, the present study was directed at studying alterations in gene expression that could also account for these phenotypic modifications. RNA-seq analysis identified 354 differentially expressed genes (DEGs) in brain of calpain-1 knock-out mice, as compared to their wild-type strain. Most DEGs were classified in 10 KEGG pathways, with the highest representations in Protein Processing in Endoplasmic Reticulum, MAP kinase and Alzheimer's disease pathways. Most DEGs were down-regulated and validation of a number of these genes indicated a corresponding decreased expression of their encoded proteins. The results indicate that calpain-1 is involved in the regulation of a significant number of genes affecting multiple brain functions. They also indicate that mutations in calpain-1 are likely to be involved in a number of brain disorders.

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.

A high-salt diet aggravates retinal ischaemia/reperfusion injury

Ischaemia/reperfusion contributes to the pathophysiological process of many retinal diseases. Previous studies have shown that retinal ischaemia/reperfusion mainly results in neuronal degeneration, including thinning of the retina, retinal ganglion cell death and reductions in electroretinography. A high-salt diet contributes to the inflammatory response and tissue hypoperfusion and may be associated with ischaemia/reperfusion injury. In the present study, we investigated the influence of a high-salt diet on retinal ischaemia/reperfusion injury and explored the potential mechanism in a rat model. The results revealed that the high-salt diet aggravated ischaemia/reperfusion-induced thinning of the retina. A TUNEL assay and Brn-3a staining revealed substantially more severe cell death and loss of retinal ganglion cells, and electroretinography confirmed worse retinal function in the ischaemia/reperfusion eyes of rats fed the high-salt diet. These effects may be associated with upregulation of Caspase-3, Bax, Interleukin-1β and Interleukin-6 and decreased expression of nitric oxide. In summary, a high-salt diet aggravates ischaemia/reperfusion-induced retinal neuronal impairment by activating pro-apoptotic and pro-inflammatory signalling pathways and inhibiting vasodilation.

Calpain2 but not calpain1 mediated by calpastatin following glutamate-induced regulated necrosis in rat retinal neurons

The purpose of this study is to investigate whether calpastatin (CAST) plays an important role in the regulated necrosis (RN) in rat retinal neurons under an excessive glutamate condition and furthermore to investigate whether this process is regulated by calapin1 and calpain2. In the present study, glutamate triggered CAST inhibition, calpain2 activation and retinal neuronal RN after injury. The application of CAST active peptide could provide protective effects against activated calpain2 mediated RN. However, the calpain1 activity was not changed in these processes. Finally, in vivo studies further confirmed the role of the CAST-calpain2 pathway in cellular RN in the rat retinal ganglion cell layer and inner nuclear layer after glutamate excitation. In addition, flash electroretinogram results provided evidence that the impaired visual function induced by glutamate could recover after CAST peptide treatment. This research indicated that excessive glutamate may lead to CAST inhibition and activated calpain2, but not calpain1 activation, resulting in RN.

Calpain activation and neuronal death during early epileptogenesis

Epilepsy is a brain disorder characterized by a predisposition to suffer epileptic seizures. Acquired epilepsy might be the result of brain insults like head trauma, stroke, brain infection, or status epilepticus (SE) when one of these triggering injuries starts a transformative process known as epileptogenesis. There is some data to suggest that, during epileptogenesis, seizures themselves damage the brain but there is no conclusive evidence to demonstrate that spontaneous recurrent seizures themselves injure the brain. Our recent evidence indicates that calpain overactivation might be relevant for epileptogenesis. Here, we investigated if spontaneous recurrent seizures that occur during an early period of epileptogenesis show any correlation with the levels of calpain activation and/or expression. In addition, we also investigated a possible association between the occurrence of spontaneous seizures and increased levels of cell death, gliosis and inflammation (typical markers associated with epileptogenesis). We found that the number of spontaneous seizures detected prior to sample collection was correlated with altered calpain activity and expression. Moreover, the levels of hippocampal neurodegeneration were also correlated with seizure occurrence. Our findings suggest that, at least during early epileptogenesis, there is a correlation between seizure occurrence, calpain activity and neurodegeneration. Thus, this study opens the possibility that aberrant calpain reactivation by spontaneous seizures might contribute to the manifestation of future spontaneous seizures.

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.

Alpha 7 nicotinic receptors attenuate neurite development through calcium activation of calpain at the growth cone

The α7 nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel that plays an important role in cellular calcium signaling contributing to synaptic development and plasticity, and is a key drug target for the treatment of neurodegenerative conditions such as Alzheimer's disease. Here we show that α7 nAChR mediated calcium signals in differentiating PC12 cells activate the proteolytic enzyme calpain leading to spectrin breakdown, microtubule retraction, and attenuation in neurite growth. Imaging in growth cones confirms that α7 activation decreases EB3 comet motility in a calcium dependent manner as demonstrated by the ability of α7 nAChR, ryanodine, or IP3 receptor antagonists to block the effect of α7 nAChR on growth. α7 nAChR mediated EB3 comet motility, spectrin breakdown, and neurite growth was also inhibited by the addition of the selective calpain blocker calpeptin and attenuated by the expression of an α7 subunit unable to bind Gαq and activate calcium store release. The findings indicate that α7 nAChRs regulate cytoskeletal dynamics through local calcium signals for calpain protease activity.

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.

Pin1 Promotes Regulated Necrosis Induced by Glutamate in Rat Retinal Neurons via CAST/Calpain2 Pathway

The purpose of the current study was to investigate whether peptidyl-prolyl cis/trans isomerase NIMA-interacting 1 (Pin1) can interact with calpastatin (CAST) and regulate CAST/calpain2, under excessive glutamate conditions, and subsequently regulate necrosis in rat retinal neurons. Glutamate triggered CAST/calpain2-mediated necrosis regulation in primary cultured retinal neurons, as demonstrated by propidium iodide-staining and lactate dehydrogenase assay. Co-IP results and a computer simulation suggested that Pin1 could bind to CAST. Western blot, real-time quantitative polymerase chain reaction, immunofluorescence, and phosphorylation analysis results demonstrated that CAST was regulated by Pin1, as proven by the application of juglone (i.e., a Pin1 specific inhibitor). The retinal ganglion cell 5 cell line, combined with siRNA approach and flow cytometry, was then used to verify the regulatory pathway of Pin1 in CAST/calpain2-modulated neuronal necrosis that was induced by glutamate. Finally, in vivo studies further confirmed the role of Pin1 in CAST/calpain2-modulated necrosis following glutamate excitation, in the rat retinal ganglion cell and inner nuclear layers. In addition, a flash electroretinogram study provided evidence for the recovery of impaired visual function, which was induced by glutamate, with juglone treatment. Our work aims to investigate the involvement of the Pin1-CAST/calpain2 pathway in glutamate-mediated excitotoxicity.

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.

Inhibition of Starvation-Triggered Endoplasmic Reticulum Stress, Autophagy, and Apoptosis in ARPE-19 Cells by Taurine through Modulating the Expression of Calpain-1 and Calpain-2

Age-related macular degeneration (AMD) is a complex disease with multiple initiators and pathways that converge on death for retinal pigment epithelial (RPE) cells. In this study, effects of taurine on calpains, autophagy, endoplasmic reticulum (ER) stress, and apoptosis in ARPE-19 cells (a human RPE cell line) were investigated. We first confirmed that autophagy, ER stress and apoptosis in ARPE-19 cells were induced by Earle’s balanced salt solution (EBSS) through starvation to induce RPE metabolic stress. Secondly, inhibition of ER stress by 4-phenyl butyric acid (4-PBA) alleviated autophagy and apoptosis, and suppression of autophagy by 3-methyl adenine (3-MA) reduced the cell apoptosis, but the ER stress was minimally affected. Thirdly, the apoptosis, ER stress and autophagy were inhibited by gene silencing of calpain-2 and overexpression of calpain-1, respectively. Finally, taurine suppressed both the changes of the important upstream regulators (calpain-1 and calpain-2) and the activation of ER stress, autophagy and apoptosis, and taurine had protective effects on the survival of ARPE-19 cells. Collectively, this data indicate that taurine inhibits starvation-triggered endoplasmic reticulum stress, autophagy, and apoptosis in ARPE-19 cells by modulating the expression of calpain-1 and calpain-2.

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-2/p35-p25/Cdk5 pathway is involved in the neuronal apoptosis induced by polybrominated diphenyl ether-153

Polybrominated diphenyl ethers (PBDEs) have been demonstrated to induce neurotoxicity in experimental rats and mice, with neuronal apoptosis as one of the major mechanisms, however, the mechanisms underlying PBDEs-induced neuronal apoptosis remain unclear. In this study, we aimed to investigate the role of calpain/p35-p25/Cdk5 pathway in BDE-153-induced neuronal apoptosis in the hippocampus and primary neurons in rats. Results showed that compared to the controls, neuronal apoptosis was significantly increased in vivo and ex vivo, as manifested by the increased hippocampus TUNEL-positive cell rates, apoptotic neurons in Hoechst and AO/EB staining, and the increased LDH activity and percentage of Annexin V-positive cells in rat hippocampus and primary neurons. Calpain activity was significantly increased in all the BDE-153-treated groups in vivo and ex vivo when compared to non-treatment controls. In addition, we showed that calpain-2 accounted for the calpain activation instead of calpain-1, as demonstrated by the up-regulated mRNA and protein expressions in calpain-2 but not calpain-1. Activated calpain truncated p35 into p25, which resulted in the p25/Cdk5 formation and activation. Calpain inhibitor PD150606 or p25/Cdk5 inhibitor Roscovitine relieved neuronal apoptosis mainly via inhibiting the p25/Cdk5 activation. Overall, the findings suggested that calpain-2/p35-p25/Cdk5 pathway was involved in BDE-153-induced neuronal apoptosis, which provides novel insight into the mechanisms of PBDE neurotoxicity.

A calpain inhibitor ameliorates seizure burden in an experimental model of temporal lobe epilepsy

In this study, we used the pilocarpine model of epilepsy to evaluate the involvement of calpain dysregulation on epileptogenesis. Detection of spectrin breakdown products (SBDPs, a hallmark of calpain activation) after induction of pilocarpine-induced status epilepticus (SE) and before appearance of spontaneous seizure suggested the existence of sustained calpain activation during epileptogenesis. Acute treatment with a cell permeable inhibitor of calpain, MDL-28170, resulted in a partial but significant reduction on seizure burden. The reduction on seizure burden was associated with a limited reduction on the generation of SBDPs but was correlated with a reduction in astrocytosis, microglia activation and cell sprouting. Together, these observations provide evidence for the role of calpain in epileptogenesis. In addition, provide proof-of-principle for the use of calpain inhibitors as a novel strategy to prevent epileptic seizures and its associated pathologies.

Degradomics in Neurotrauma: Profiling Traumatic Brain Injury

Degradomics has recently emerged as a subdiscipline in the omics era with a focus on characterizing signature breakdown products implicated in various disease processes. Driven by promising experimental findings in cancer, neuroscience, and metabolomic disorders, degradomics has significantly promoted the notion of disease-specific "degradome." A degradome arises from the activation of several proteases that target specific substrates and generate signature protein fragments. Several proteases such as calpains, caspases, cathepsins, and matrix metalloproteinases (MMPs) are involved in the pathogenesis of numerous diseases that disturb the physiologic balance between protein synthesis and protein degradation. While regulated proteolytic activities are needed for development, growth, and regeneration, uncontrolled proteolysis initiated under pathological conditions ultimately culminates into apoptotic and necrotic processes. In this chapter, we aim to review the protease-substrate repertoires in neural injury concentrating on traumatic brain injury. A striking diversity of protease substrates, essential for neuronal and brain structural and functional integrity, namely, encryptic biomarker neoproteins, have been characterized in brain injury. These include cytoskeletal proteins, transcription factors, cell cycle regulatory proteins, synaptic proteins, and cell junction proteins. As these substrates are subject to proteolytic fragmentation, they are ceaselessly exposed to activated proteases. Characterization of these molecules allows for a surge of "possible" therapeutic approaches of intervention at various levels of the proteolytic cascade.

Calpain research for drug discovery: challenges and potential

Calpains are a family of proteases that were scientifically recognized earlier than proteasomes and caspases, but remain enigmatic. However, they are known to participate in a multitude of physiological and pathological processes, performing 'limited proteolysis' whereby they do not destroy but rather modulate the functions of their substrates. Calpains are therefore referred to as 'modulator proteases'. Multidisciplinary research on calpains has begun to elucidate their involvement in pathophysiological mechanisms. Therapeutic strategies targeting malfunctions of calpains have been developed, driven primarily by improvements in the specificity and bioavailability of calpain inhibitors. Here, we review the calpain superfamily and calpain-related disorders, and discuss emerging calpain-targeted therapeutic strategies.