Differential Activation of Calpain-1 and Calpain-2 following Kainate-Induced Seizure Activity in Rats and Mice

Plasma proteomics in epilepsy: Network-based identification of proteins associated with seizures

PURPOSE

Identification of potential biomarkers of seizures.

METHODS

In this exploratory study, we quantified plasma protein intensities in 15 patients with recent seizures compared to 15 patients with long-standing seizure freedom. Using TMT-based proteomics we found fifty-one differentially expressed proteins.

RESULTS

Network analyses including co-expression networks and protein-protein interaction networks, using the STRING database, followed by network centrality and modularity analyses revealed 22 protein modules, with one module showing a significant association with seizures. The protein-protein interaction network centered around this module identified a subnetwork of 125 proteins, grouped into four clusters. Notably, one cluster (mainly enriching inflammatory pathways and Gene Ontology terms) demonstrated the highest enrichment of known epilepsy-related genes.

CONCLUSION

Overall, our network-based approach identified a protein module linked with seizures. The module contained known markers of epilepsy and inflammation. The results also demonstrate the potential of network analysis in discovering new biomarkers for improved epilepsy management.

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.

Changes in activity of µ- and m-calpains and signs of neuroinflammation in the hippocampus and striatum of rats after single intraperitoneal injection of subseptic dose of endotoxin

Some mechanisms of neuronal degeneration in endotoxinemia are already well described, but need to be detailed. In this study, we tested the effect of a single intraperitoneal injection of a LPS sub-septic dose (1 mg/kg of animal weight) on calpain activity in the striatum and hippocampus. We showed, that in the hippocampus the day after LPS administration an increase in production of IL-1β and TNF-α mRNA, followed by elevated mRNA expression and activity of µ- and m-calpains without signs of microglia activation is observed. In striatal cells, the day after LPS injection an increase in expression of IL-1β, TNF-α, IBA-1, m-calpain and calpastatin mRNA is revealed, which only intensifies over time. The elicited changes are accompanied by a decrease in motor behavior, which can be considered as a sign of sickness behavior. In the hippocampus, 180 days after LPS administration expression of TNF-α, content and activity of µ-calpain are increased. In the striatum, elevation in expression of TNF-α, IBA-1, µ- and m-calpain mRNA, with hyperactivation of only m-calpain, is observed. Significantly reduced motor activity can be a consequence of LPS-induced neuronal death. A long-lasting endotoxin activates microglia that damage neurons via proinflammation cytokines and calpain hyperactivation. The endotoxin hypothesis of neurodegeneration is unproven, but if correct, then neurodegeneration may be reduced by decreasing endotoxin-induced neuroinflammation and m-calpain hyperactivation. Therefore, the drugs, that decrease endotoxin-induced neuroinflammation and differently inhibit µ- or m-calpain, can be used to prevent or reduce the severity of neurodegeneration.

The actin binding protein drebrin helps to protect against the development of seizure-like events in the entorhinal cortex

The actin binding protein drebrin plays a key role in dendritic spine formation and synaptic plasticity. Decreased drebrin protein levels have been observed in temporal lobe epilepsy, suggesting the involvement of drebrin in the disease. Here we investigated the effect of drebrin knockout on physiological and pathophysiological neuronal network activities in mice by inducing gamma oscillations, involved in higher cognitive functions, and by analyzing pathophysiological epileptiform activity. We found that loss of drebrin increased the emergence of spontaneous gamma oscillations suggesting an increase in neuronal excitability when drebrin is absent. Further analysis showed that although the kainate-induced hippocampal gamma oscillations were unchanged in drebrin deficient mice, seizure like events measured in the entorhinal cortex appeared earlier and more frequently. The results suggest that while drebrin is not essential for normal physiological network activity, it helps to protect against the formation of seizure like activities during pathological conditions. The data indicate that targeting drebrin function could potentially be a preventive or therapeutic strategy for epilepsy treatment.

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.

Calpain-2 activation in mouse hippocampus plays a critical role in seizure-induced neuropathology

Calpain has been proposed to play a critical role in the development of epilepsy. Here we used conditional calpain-2 knock-out (C2CKO) mice in a C57/Bl6 background and a selective calpain-2 inhibitor to analyze the role of calpain-2 in epilepsy. Neurodegeneration was evident in various hippocampal subfields, in particular in mossy cells in the hilus of the dentate gyrus (DG) in C57/Bl6 mice 7 days after kainic acid (KA)-induced seizures. Calpain-2 activation was still observed in mossy cells 7 days after seizures. Calpain activation, astroglial and microglial activation, neurodegeneration, and cognitive impairment were absent in C2CKO mice and in C57/Bl6 mice treated with a selective calpain-2 inhibitor for 7 days after seizure initiation. Levels of the potassium chloride cotransporter 2 (KCC2) were decreased in mossy cells 7 days after seizures and this decrease was prevented by calpain-2 deletion or selective inhibition. Our results indicate that prolonged calpain-2 activation plays a critical role in neuropathology following seizures. A selective calpain-2 inhibitor could represent a therapeutic treatment for seizure-induced neuropathology.

The actin binding protein α-actinin-2 expression is associated with dendritic spine plasticity and migrating granule cells in the rat dentate gyrus following pilocarpine-induced seizures

α-actinin-2 (α-actn-2) is an F-actin-crosslinking protein, localized in dendritic spines. In vitro studies suggested that it is involved in spinogenesis, morphogenesis, actin organization, cell migration and anchoring of the NR1 subunit of the N-methyl-D-aspartate (NMDA) receptors in dendritic spines. However, little is known regarding its function in vivo. We examined the levels of α-actn-2 expression within the dentate gyrus (DG) during the development of chronic limbic seizures (epileptogenesis) induced by pilocarpine in rats. In this model, plasticity of the DG glutamatergic granule cells including spine loss, spinogenesis, morphogenesis, neo-synaptogenesis, aberrant migration, and alterations of NMDA receptors have been well characterized. We showed that α-actn-2 immunolabeling was reduced in the inner molecular layer at 1-2 weeks post-status epilepticus (SE), when granule cell spinogenesis and morphogenesis occur. This low level persisted at the chronic stage when new functional synapses are established. This decreased of α-actn-2 protein is concomitant with the recovery of drebrin A (DA), another actin-binding protein, at the chronic stage. Indeed, we demonstrated in cultured cells that in contrast to DA, α-actn-2 did not protect F-actin destabilization and DA inhibited α-actn-2 binding to F-actin. Such alteration could affect the anchoring of NR1 in dendritic spines. Furthermore, we showed that the expression of α-actn-2 and NR1 are co-down-regulated in membrane fractions of pilocarpine animals at chronic stage. Last, we showed that α-actn-2 is expressed in migrating newly born granule cells observed within the hilus of pilocarpine-treated rats. Altogether, our results suggest that α-actn-2 is not critical for the structural integrity and stabilization of granule cell dendritic spines. Instead, its expression is regulated when spinogenesis and morphogenesis occur and within migrating granule cells. Our data also suggest that the balance between α-actn-2 and DA expression levels may modulate NR1 anchoring within dendritic spines.

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.

GSK3-β promotes calpain-1-mediated desmin filament depolymerization and myofibril loss in atrophy

Phosphorylation by protein kinase GSK3-β is essential for desmin filament depolymerization by calpain-1 and the resulting myofibril destruction in muscle atrophy.

Myofibril breakdown is a fundamental cause of muscle wasting and inevitable sequel of aging and disease. We demonstrated that myofibril loss requires depolymerization of the desmin cytoskeleton, which is activated by phosphorylation. Here, we developed a mass spectrometry–based kinase-trap assay and identified glycogen synthase kinase 3-β (GSK3-β) as responsible for desmin phosphorylation. GSK3-β inhibition in mice prevented desmin phosphorylation and depolymerization and blocked atrophy upon fasting or denervation. Desmin was phosphorylated by GSK3-β 3 d after denervation, but depolymerized only 4 d later when cytosolic Ca²⁺ levels rose. Mass spectrometry analysis identified GSK3-β and the Ca²⁺-specific protease, calpain-1, bound to desmin and catalyzing its disassembly. Consistently, calpain-1 down-regulation prevented loss of phosphorylated desmin and blocked atrophy. Thus, phosphorylation of desmin filaments by GSK3-β is a key molecular event required for calpain-1–mediated depolymerization, and the subsequent myofibril destruction. Consequently, GSK3-β represents a novel drug target to prevent myofibril breakdown and atrophy.

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.

Postnatal calpain inhibition elicits cerebellar cell death and motor dysfunction

Calpain-1 deletion elicits neurodevelopmental disorders, such as ataxia. However, the function of calpain in postnatal neurodevelopment and its mechanisms remain unknown. In this study, we revealed that postnatal intraperitoneal injection of various calpain inhibitors attenuated cerebellar cytosolic calpain activity. Moreover, postnatal application of calpeptin (2 mg/kg) apparently reduced spectrin breakdown, promoted suprachiasmatic nucleus circadian oscillatory protein (SCOP) accumulation in cerebellar tissue. In addition, application of calpeptin decreased phosphorylated protein kinase B (p-AKT) level (p<0.05), as well as total AKT level (p<0.05). We also evidenced that administration of calpeptin obviously increased phosphorylation of mammalian target of rapamycin (p-mTor) (p<0.01). Apoptosis of granular cells and activation of caspase-3 (p<0.01) were facilitated after calpain inhibition. Importantly, cell numbers of granular cells were reduced and motor function was remarkably impaired in 4-month-old rats receiving postnatal calpain inhibition. Taken together, our data implicated that calpain activity in the postnatal period was critical for the cerebellar development. Postnatal calpain inhibition causes cerebellar granular cell apoptosis and motor dysfunction, likely through SCOP/AKT and p-mTor signaling pathways.

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.

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.

N-Butylphthalide Improves Cognitive Function in Rats after Carbon Monoxide Poisoning

Cognitive impairment is the most common neurologic sequelae after carbon monoxide (CO) poisoning, and the previous investigations have demonstrated that N-Butylphthalide (NBP) could exert a broad spectrum of neuroprotective properties. The current study aimed to investigate the effect of NBP on cognitive dysfunction in rats after acute severe CO poisoning. Rats were randomly divided into a normal control group, a CO poisoning group and a CO+NBP group. The animal model of CO poisoning was established by exposure to CO in a chamber, and then all rats received hyperbaric oxygen therapy once daily, while rats in CO+NBP group were administered orally NBP (6 mg/ 100g) by gavage twice a day additionally. The results indicated that CO poisoning could induce cognitive impairment. The ultrastructure of hippocampus was seriously damaged under transmission electron microscopy, and the expressions of calpain 1 and CaMK II proteins were significantly elevated after CO exposure according to the analysis of immunofluorescence staining and western blot. NBP treatment could evidently improve cognitive function, and maintain ultrastructure integrity of hippocampus. The expression levels of both calpain 1 and CaMK II proteins in CO+NBP group were considerably lower than that of CO poisoning group (P < 0.05). Taken together, this study highlights the molecular mechanism of cognitive dysfunction in rats after CO exposure via the upregulation of both calpain 1 and CaMK II proteins. The administration of NBP could balance the expressions of calpain 1 and CaMK II proteins and improve cognitive function through maintaining ultrastructural integrity of hippocampus, and thus may play a neuroprotective role in brain tissue in rats with CO poisoning.

Pathophysiology of status epilepticus

Status epilepticus (SE) is the maximal expression of epilepsy with a high morbidity and mortality. It occurs due to the failure of mechanisms that terminate seizures. Both human and animal data indicate that the longer a seizure lasts, the less likely it is to stop. Recent evidence suggests that there is a critical transition from an ictal to a post-ictal state, associated with a transition from a spatio-temporally desynchronized state to a highly synchronized state, respectively. As SE continues, it becomes progressively resistant to drugs, in particular benzodiazepines due partly to NMDA receptor-dependent internalization of GABA(A) receptors. Moreover, excessive calcium entry into neurons through excessive NMDA receptor activation results in activation of nitric oxide synthase, calpains, and NADPH oxidase. The latter enzyme plays a critical part in the generation of seizure-dependent reactive oxygen species. Calcium also accumulates in mitochondria resulting in mitochondrial failure (decreased ATP production), and opening of the mitochondrial permeability transition pore. Together these changes result in status epilepticus-dependent neuronal death via several pathways. Multiple downstream mechanisms including inflammation, break down of the blood-brain barrier, and changes in gene expression can contribute to later pathological processes including chronic epilepsy and cognitive decline.