Neuroprotection with delayed calpain inhibition after transient forebrain ischemia

Therapeutic time windows of compounds against NMDA receptors signaling pathways for ischemic stroke

Much evidence has proved that excitotoxicity induced by excessive release of glutamate contributes largely to damage caused by ischemia. In view of the key role played by NMDA receptors in mediating excitotoxicity, compounds against NMDA receptors signaling pathways have become the most promising type of anti-stroke candidate compounds. However, the limited therapeutic time window for neuroprotection is a key factor preventing NMDA receptor-related compounds from showing efficacy in all clinical trials for ischemic stroke. In this perspective, the determination of therapeutic time windows of these kinds of compounds is useful in ensuring a therapeutic effect and accelerating clinical application. This mini-review discussed the therapeutic time windows of compounds against NMDA receptors signaling pathways, described related influence factors and the status of clinical studies. The purpose of this review is to look for compounds with wide therapeutic time windows and better clinical application prospect.

Extending the Calpain-Cathepsin Hypothesis to the Neurovasculature: Protection of Brain Endothelial Cells and Mice from Neurotrauma

The calpain-cathepsin hypothesis posits a key role for elevated calpain-1 and cathepsin-B activity in the neurodegeneration underlying neurotrauma and multiple disorders including Alzheimer's disease (AD). AD clinical trials were recently halted on alicapistat, a selective calpain-1 inhibitor, because of insufficient exposure of neurons to the drug. In contrast to neuroprotection, the ability of calpain-1 and cathepsin-B inhibitors to protect the blood-brain barrier (BBB), is understudied. Since cerebrovascular dysfunction underlies vascular dementia, is caused by ischemic stroke, and is emerging as an early feature in the progression of AD, we studied protection of brain endothelial cells (BECs) by selective and nonselective calpain-1 and cathepsin-B inhibitors. We show these inhibitors protect both neurons and murine BECs from ischemia-reperfusion injury. Cultures of primary BECs from ALDH2 -/- mice that manifest enhanced oxidative stress were sensitive to ischemia, leading to reduced cell viability and loss of tight junction proteins; this damage was rescued by calpain-1 and cathepsin-B inhibitors. In ALDH2 -/- mice 24 h after mild traumatic brain injury (mTBI), BBB damage was reflected by significantly increased fluorescein extravasation and perturbation of tight junction proteins, eNOS, MMP-9, and GFAP. Both calpain and cathepsin-B inhibitors alleviated BBB dysfunction caused by mTBI. No clear advantage was shown by selective versus nonselective calpain inhibitors in these studies. The lack of recognition of the ability of calpain inhibitors to protect the BBB may have led to the premature abandonment of this therapeutic approach in AD clinical trials and requires further mechanistic studies of cerebrovascular protection by calpain-1 inhibitors.

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.

Calpains and neuronal damage in the ischemic brain: The swiss knife in synaptic injury

The excessive extracellular accumulation of glutamate in the ischemic brain leads to an overactivation of glutamate receptors with consequent excitotoxic neuronal death. Neuronal demise is largely due to a sustained activation of NMDA receptors for glutamate, with a consequent increase in the intracellular Ca(2+) concentration and activation of calcium- dependent mechanisms. Calpains are a group of Ca(2+)-dependent proteases that truncate specific proteins, and some of the cleavage products remain in the cell, although with a distinct function. Numerous studies have shown pre- and post-synaptic effects of calpains on glutamatergic and GABAergic synapses, targeting membrane- associated proteins as well as intracellular proteins. The resulting changes in the presynaptic proteome alter neurotransmitter release, while the cleavage of postsynaptic proteins affects directly or indirectly the activity of neurotransmitter receptors and downstream mechanisms. These alterations also disturb the balance between excitatory and inhibitory neurotransmission in the brain, with an impact in neuronal demise. In this review we discuss the evidence pointing to a role for calpains in the dysregulation of excitatory and inhibitory synapses in brain ischemia, at the pre- and post-synaptic levels, as well as the functional consequences. Although targeting calpain-dependent mechanisms may constitute a good therapeutic approach for stroke, specific strategies should be developed to avoid non-specific effects given the important regulatory role played by these proteases under normal physiological conditions.

[Modulation of Ca(2+)-Dependent Proteiolysis under the Action of Weak Low-Frequency Magnetic Fields]

The study aimed to determine the molecular targets of magnetic fields in living objects. Time-dependent effects of weak low-frequency magnetic field tuned to the parametric resonance for calcium ions were studied on model organisms (fish, whelk). The dynamics of Ca(2+)-dependent proteinase activity under the exposure to magnetic fields with given parameters was determined and minimal time of exposure in order to achieve inactivation of these proteinases was find out as well. As hyperactivation of Ca(2+)-dependent proteinases is a basis of degenerative pathology development the therapeutic potential of weak low-frequency magnetic fields enabling to modulate Ca(2+)-dependent proteinase activity is supported.

Involvement of calpains in adult neurogenesis: implications for stroke

Calpains are ubiquitous proteases involved in cell proliferation, adhesion and motility. In the brain, calpains have been associated with neuronal damage in both acute and neurodegenerative disorders, but their physiological function in the nervous system remains elusive. During brain ischemia, there is a large increase in the levels of intracellular calcium, leading to the activation of calpains. Inhibition of these proteases has been shown to reduce neuronal death in a variety of stroke models. On the other hand, after stroke, neural stem cells (NSC) increase their proliferation and newly formed neuroblasts migrate towards the site of injury. However, the process of forming new neurons after injury is not efficient and finding ways to improve it may help with recovery after lesion. Understanding the role of calpains in the process of neurogenesis may therefore open a new window for the treatment of stroke. We investigated the involvement of calpains in NSC proliferation and neuroblast migration in two highly neurogenic regions in the mouse brain, the dentate gyrus (DG) and the subventricular zone (SVZ). We used mice that lack calpastatin, the endogenous calpain inhibitor, and calpains were also modulated directly, using calpeptin, a pharmacological calpain inhibitor. Calpastatin deletion impaired both NSC proliferation and neuroblast migration. Calpain inhibition increased NSC proliferation, migration speed and migration distance in cells from the SVZ. Overall, our work suggests that calpains are important for neurogenesis and encourages further research on their neurogenic role. Prospective therapies targeting calpain activity may improve the formation of new neurons following stroke, in addition to affording neuroprotection.

Introduction of calpain inhibitors in traumatic brain injury: a novel approach?

Traumatic brain injury (TBI) is a major cause of death and disability throughout the world. In recent years, researchers focused on the pathological significance of calcium accumulation in the brain after TBI. Neuronal calcium homeostasis disturbances may result in the activation of calpain a ubiquitous calcium-sensitive protease. The calpain family has a well-established causal role in neuronal cell death following acute brain injury: their activation has been observed to progressively increase after either contusive or diffuse brain trauma in animals, suggesting calpain to be a mediator of early neuronal damage. We hypothesize that pretreatment with the calpain inhibitors in population at objective risk (military soldiers' pre combat) in appropriate dose would open therapeutic time window expected to prevent and reduce extensive brain damage by providing optimal TBI neuroprotection. Additional therapeutic strategy for TBI, based on calpain modulating actions such as pretreatment with calpain inhibitors has been proposed. Since calpain overexpression has been well established in acute neuronal injury and further subsequent neurodegeneration, from a clinical viewpoint, we speculate that if this hypothesis proves correct pretreatment inhibitors introduction may become a therapeutic option for different brain pathologies to be approached and treated with.

A novel calpain inhibitor for treatment of transient retinal ischemia in the rat

After an acute ischemia/reperfusion of the rat retina, the activation of cytotoxic proteases, including calpain, results in necrosis and apoptosis of retinal ganglion cells resulting in their degeneration. Using a systemically administered calpain inhibitor that crosses the blood-retinal barrier would provide for novel systemic intervention that protects the retina from acute injury and loss of function. Herein, we study a novel calpain peptide inhibitor, cysteic-leucyl-argininal (CYLA), in an in-vivo rat model of retinal ischemia to determine functional protection using electroretinography. The CYLA prodrug was administered intraperitoneally before and/or after ischemia-reperfusion at concentrations of 20-40 mg/kg. We found that administering 20 mg/kg of CYLA only after ischemia provides significant preservation of retinal function.

Ischemia-induced calpain activation causes eukaryotic (translation) initiation factor 4G1 (eIF4GI) degradation, protein synthesis inhibition, and neuronal death

Persistent protein synthesis inhibition (PSI) is a robust predictor of eventual neuronal death following cerebral ischemia. We thus tested the hypothesis that persistent PSI inhibition and neuronal death are causally linked. Neuronal viability strongly correlated with both protein synthesis and levels of eukaryotic (translation) initiation factor 4G1 (eIF4G1). We determined that in vitro ischemia activated calpain, which degraded eIF4G1. Overexpression of the calpain inhibitor calpastatin or eIF4G1 resulted in increased protein synthesis and increased neuronal viability compared with controls. The neuroprotective effect of eIF4G1 overexpression was due to restoration of cap-dependent protein synthesis, as well as protein synthesis-independent mechanisms, as inhibition of protein synthesis with cycloheximide did not completely prevent the protective effect of eIF4G1 overexpression. In contrast, shRNA-mediated silencing of eIF4G1 exacerbated ischemia-induced neuronal injury, suggesting eIF4G1 is necessary for maintenance of neuronal viability. Finally, calpain inhibition following global ischemia in vivo blocked decreases in eIF4G1, facilitated protein synthesis, and increased neuronal viability in ischemia-vulnerable hippocampal CA1 neurons. Collectively, these data demonstrate that calpain-mediated degradation of a translation initiation factor, eIF4G1, is a cause of both persistent PSI and neuronal death.

RNAi targeting micro-calpain increases neuron survival and preserves hippocampal function after global brain ischemia

The calpain family of cysteine proteases has a well-established causal role in neuronal cell death following acute brain injury. However, the relative contribution of calpain isoforms has not been determined in in vivo models. Identification of the calpain isoform responsible for neuronal injury is particularly important given the differential role of calpain isoforms in normal physiology. This study evaluates the role of m-calpain and micro-calpain in an in vivo model of global brain ischemia. Adeno-associated viral vectors expressing short hairpin RNAs targeting the catalytic subunits of micro- or m-calpain were used to knockdown expression of the targeted isoforms in adult rat hippocampal CA1 pyramidal neurons. Knockdown of micro-calpain, but not m-calpain, prevented calpain activity 72 h after 6-min transient forebrain ischemia, increased long-term survival and protected hippocampal electrophysiological function. These findings represent the first in vivo evidence that reducing expression of an individual calpain isoform can decrease post-ischemic neuronal death and preserve hippocampal function.