CNS & Neurological Disorders - Drug Targets (Formerly Current Drug Targets - CNS & Neurological Disorders) - Volume 7, Issue 3, 2008

Proteases play important roles in the pathogenesis of injuries and diseases of the central nervous system (CNS). Different classes of proteases such as calpains, caspases, and cathepsins may work independently or co-operatively to carry out the proteolysis of key proteins in the CNS cells leading to cell death and neurological problems [1-5]. Increased proteolytic activities contribute to neurodegeneration in CNS injuries such as ischemic brain injury (IBI), traumatic brain injury (TBI), spinal cord injury (SCI) and also in CNS diseases such as Alzheimer's disease (AD), glaucoma, Pakinson's disease (PD), and multiple sclerosis (MS). Moreover, activation of phospholipases can contribute to disruption of the blood-brain-barrier (BBB) causing inflammation in the CNS disorders [6]. Contemporary investigations in different laboratories have confirmed the unequivocal roles of proteases and phospholipases in the pathogenesis of these and other CNS disorders and suggested therapeutic strategies for prevention of expression and activity of proteases and phospholipases [7, 8]. I have the pleasure and privilege to put forward to the readers this volume of the CNS & Neurological Disorders - Drug Targets that contains nine review articles from prominent research groups delineating the roles of proteases and phospholipases in the pathogenesis of IBI, TBI, SCI, AD, PD, and MS and also indicating the prospective therapeutic strategies. Liu et al. have described the enormous potential of erythropoietin (EPO), a glycoprotein hormone and cytokine, for the treatment of IBI, TBI, and PD. The mechanism of EPO mediated amelioration of neurological disorders includes prevention of neurodegeneration and promotion of angiogenesis and neurogenesis. The therapeutic action of EPO is mediated through the EPO receptor, which is expressed in the CNS cells. Therapeutic efficacy of EPO includes decreases in ischemic infarct and hemorrhage volume, and neuronal apoptosis, and increases in survival rates in animal models. It is encouraging that some clinical trials with EPO in neurological diseases have shown desirable outcomes. Administration of EPO has proven to be safe in animals and adult human patients, although safety features of EPO in neonates and infants still need to be evaluated. So far, available data suggest that EPO is poised to be a promising therapeutic agent for the treatment of neurological disorders. Wang et al. have explained importance of targeting extracellular matrix proteolysis for prevention of hemorrhagic complications due to ischemic stroke therapy with the serine protease tissue plasminogen activator (tPA), the only stoke treatment approved by the US Food and Drug Administration (FDA). Although the thrombolytic activity of tPA helps achieving vascular reperfusion and clinical benefit, in reality tPA is administered in only about 2-5% of stroke patients in the US because of high risks of symptomatic intracranial hemorrhage and low therapeutic time window to minimize hemorrhagic complications. Currently, combination strategies are being explored to increase thrombolytic efficacy of tPA for beneficial reperfusion with simultaneous decrease in neurotoxicity and hemorrhagic complications. Because dysregulated extracellular proteases initiate the breakdown of neurovascular matrix to disrupt the BBB causing edema and/or hemorrhage, targeting the extracellular matrix proteolysis within the neurovascular unit may provide a new strategy for improving the safety and efficacy of the thrombolytic reperfusion therapy of stroke. Adibhatla and Hatcher have cautioned that combination of the thrombolytic activity of the serine protease tPA and the inhibition of the matrix metalloproteases (MMPs) may not be a viable therapeutic strategy for treatment of ischemic stroke. Use of tPA as a thrombolytic therapy for stroke is also associated with high risks of hemorrhage and inflammation due to the factual possibility of disruption of the BBB with activation of MMPs. Inhibition of MMPs may result in either beneficial or detrimental effects depending on timing of treatment of IBI. Although MMPs cause disruption of the BBB and neuronal damage during early injury phase of stroke, MMPs also contribute to vascular remodeling, angiogenesis, neurogenesis, and axonal regeneration during the later repair phase of stroke. Any treatment regimen targeted to MMPs must consider the conflicting effects of MMPs during the early and later phases of IBI. Titsworth et al. have presented the role of secretory phospholipase A2 (sPLA2) in inflammation in CNS disorders, especially in SCI. sPLA2 is a lipolytic enzyme and thus hydrolyzes the glycerophospholipids to produce free fatty acids and lysophospholipids, which are precursors of bioactive eicosanoids and platelet-activating factor (PAF).

Erythropoietin (EPO) was first identified as a hematopoietic cytokine that stimulates proliferation and differentiation of erythroid progenitor cells and was approved by the Food and Drug Administration as a treatment for chronic renal disease patients with anemia. In neural tissues, EPO is working via EPO receptors and induces non-hematopoietic effects. Recent studies have demonstrated that EPO exerts therapeutic potentials on neurological disorders such as ischemic stroke, intracerebral hemorrhage, subarachnoid hemorrhage, traumatic brain injury, and Parkinson's disease. EPO treatment has been shown to reduce the ischemic infarct and hemorrhage volume, decrease neuronal death including apoptosis, and improve survival rates in animal models. The mechanism of EPO action in neurological disorders involves neuroprotection and promotion of neurogenesis and angiogenesis. Clinical trials of EPO treatments in neurological diseases have accumulated positive results. In stroke patients, EPO treatment may reduce infarct volume and improve functional outcomes. EPO administration has proven safe in animal studies and adult human patients, although safety and efficacy data in neonates and infants are incomplete and long-term multi-center patient evaluations are necessary. Available information suggests that EPO is a promising therapeutic drug for the treatment of neurological diseases.

To date, tPA-based thrombolytic therapy is the only FDA-approved treatment for achieving vascular reperfusion and clinical benefit, but this agent is given to only about 2-5% of stroke patients in the United States of America. This may be related, in part, to the elevated risks of symptomatic intracranial hemorrhage, and the consequently reduced therapeutic time window. Recent efforts have aimed at identifying new combination strategies that might increase thrombolytic efficacy of tPA to benefit reperfusion, while reducing its associated neurotoxicity and hemorrhagic complications. Emerging experimental studies demonstrate that the breakdown of neurovascular matrix initiates blood-brain barrier disruption with edema and/or hemorrhage. Perturbation of extracellular homeostasis triggered by dysregulated extracellular proteases may underlie processes responsible for the hemorrhagic complications of thrombolytic stroke therapy. This short review summarizes experimental investigations of this field in pre-clinical stroke models. The data strongly suggest that targeting the extracellular matrix proteolytic imbalance within the neurovascular unit may provide new approaches for improving the safety and efficacy of thrombolytic reperfusion therapy of stroke.

Today there exists only one FDA-approved treatment for ischemic stroke; i.e., the serine protease tissue-type plasminogen activator (tPA). In the aftermath of the failed stroke clinical trials with the nitrone spin trap/radical scavenger, NXY-059, a number of articles raised the question: are we doing the right thing? Is the animal research truly translational in identifying new agents for stroke treatment? This review summarizes the current state of affairs with plasminogen activators in thrombolytic therapy. In addition to therapeutic value, potential side effects of tPA also exist that aggravate stroke injury and offset the benefits provided by reperfusion of the occluded artery. Thus, combinational options (ultrasound alone or with microspheres/nanobubbles, mechanical dissociation of clot, activated protein C (APC), plasminogen activator inhibitor-1 (PAI-1), neuroserpin and CDP-choline) that could offset tPA toxic side effects and improve efficacy are also discussed here. Desmoteplase, a plasminogen activator derived from the saliva of Desmodus rotundus vampire bat, antagonizes vascular tPA-induced neurotoxicity by competitively binding to low-density lipoprotein relatedreceptors (LPR) at the blood-brain barrier (BBB) interface, minimizing the tPA uptake into brain parenchyma. tPA can also activate matrix metalloproteinases (MMPs), a family of endopeptidases comprised of 24 mammalian enzymes that primarily catalyze the turnover and degradation of the extracellular matrix (ECM). MMPs have been implicated in BBB breakdown and neuronal injury in the early times after stroke, but also contribute to vascular remodeling, angiogenesis, neurogenesis and axonal regeneration during the later repair phase after stroke. tPA, directly or by activation of MMP-9, could have beneficial effects on recovery after stroke by promoting neurovascular repair through vascular endothelial growth factor (VEGF). However, any treatment regimen directed at MMPs must consider their pleiotropic nature and the likelihood of either beneficial or detrimental effects that might depend on the timing of the treatment in relation to the stage of brain injury.

Secretory phospholipases A2 (sPLA2s) are a subfamily of lipolytic enzymes which hydrolyze the acyl bond at the sn-2 position of glycerophospholipids to produce free fatty acids and lysophospholipids. These products are precursors of bioactive eicosanoids and platelet-activating factor (PAF). The hydrolysis of membrane phospholipids by PLA2 is a rate-limiting step for generation of eicosanoids and PAF. To date, more than 10 isozymes of sPLA2 have been found in the mammalian central nervous system (CNS). Under physiological conditions, sPLA2s are involved in diverse cellular responses, including host defense, phospholipid digestion and metabolism. However, under pathological situations, increased sPLA2 activity and excessive production of free fatty acids and their metabolites may lead to inflammation, loss of membrane integrity, oxidative stress, and subsequent tissue injury. Emerging evidence suggests that sPLA2 plays a role in the secondary injury process after traumatic or ischemic injuries in the brain and spinal cord. Importantly, sPLA2 may act as a convergence molecule that mediates multiple key mechanisms involved in the secondary injury since it can be induced by multiple toxic factors such as inflammatory cytokines, free radicals, and excitatory amino acids, and its activation and metabolites can exacerbate the secondary injury. Blocking sPLA2 action may represent a novel and efficient strategy to block multiple injury pathways associated with the CNS secondary injury. This review outlines the current knowledge of sPLA2 in the CNS with emphasis placed on the possible roles of sPLA2 in mediating CNS injuries, particularly the traumatic and ischemic injuries in the brain and spinal cord.

Endolysosomal proteases such as cysteinyl and aspartyl cathepsins play diverse roles in inflammatory autoimmune diseases, cancers, and neurodegenerative diseases. Cysteinyl cathepsin B and aspartyl cathepsin D levels are markedly elevated in a variety of neurological disorders including Alzheimer's disease (AD), a leading cause of dementia in the elderly. Studies have also shown an increased cathepsin activity in AD patients where senile plaques and neuronal loss are marked features of the disease. Senile plaques contain amyloid-beta (Aβ) peptide, which is produced by proteolytic cleavage of the amyloid precursor protein (APP) by the proteases. In this article, we present the current knowledge of cysteinyl and aspartyl cathepsins in cellular and molecular events that lead to formation of senile plaques in AD. This article also focused on the role of cathepsin inhibitors as disease-modifying treatment strategies that could halt, or even prevent, this devastating neurological disorder.

The most popular current hypothesis is that Alzheimer's disease (AD) is caused by aggregates of the amyloid peptide (Aβ), which is generated by cleavage of the Aβ protein precursor (APP) by β-secretase (BACE-1) followed by γ- secretase. BACE-1 cleavage is limiting for the production of Aβ, making it a particularly good drug target for the generation of inhibitors that lower Aβ. A landmark discovery in AD was the identification of BACE-1 (a.k.a. Memapsin-2) as a novel class of type I transmembrane aspartic protease. Although BACE-2, a homologue of BACE-1, was quickly identified, follow up studies using knockout mice demonstrated that BACE-1 was necessary and sufficient for most neuronal Aβ generation. Despite the importance of BACE-1 as a drug target, development has been slow due to the incomplete understanding of its function and regulation and the difficulties in developing a brain penetrant drug that can specifically block its large catalytic pocket. This review summarizes the biological properties of BACE-1 and attempts to use phylogenetic perspectives to understand its function. The article also addresses the challenges in discovering a selective druglike molecule targeting novel mechanisms of BACE-1 regulation.

Glaucoma is a group of irreversible blinding eye diseases affecting over 70 million people worldwide. Systemic delivery of calpain-1 inhibitors was proposed as a neuroprotection strategy for the prevention of progressive optic nerve damage in glaucoma. We present a general review of calpain-1 and an account of vast differences in processing of calpain- 1 in the trabecular meshwork (TM) and the optic nerve. Calpain-1 accumulates in the glaucomatous TM tissues in vivo. However, calpain-1 activity is substantially lower in the glaucomatous TM compared to controls, apparently owing to partial degradation, and modification by lipid oxidation products such as iso [4]levuglandin E2 (iso [4]LGE2). Treatment of calpain-1 with iso [4]LGE2 in vitro results in covalent modification, inactivation, and resistance to protease digestion. Iso [4]LGE2-modified calpain-1 appeared to undergo ubiquitination in the TM by cellular degradation machinery mediated by ubch1-2, ubch5,6 and E6-AP, E2 and E3 enzymes respectively. In the TM, iso [4]LGE2-modified calpain-1 loading impairs the cellular proteasome activity consistent with competitive inhibition and formation of suicidal high molecular weight aggregates. In contrast, higher calpain-1 activity, that appears to be under translational control, was observed in glaucomatous optic nerve compared to control. Therapeutic neuroprotection strategies using calpain-1 inhibitors will require consideration of such anatomic differences in its activity and biosynthesis.

Pathophysiology of idiopathic Parkinson's disease (PD) is associated with degeneration of dopaminergic neurons and inflammatory responses in the mid-brain substantia nigra (SN). However, central dopaminergic replenishment therapeutic strategy with L-3,4-dihydroxyphenylalanine (L-DOPA), the precursor for dopamine synthesis, does not fully rescue these cells in SN or improve motor function. Besides, prolonged use of L-DOPA worsens the clinical symptoms in PD patients. Thus, there is a possibility that other areas of central nervous system may also be affected in this disease. Spinal cord, the final coordinator of movement in the central nervous system, may be one such site that is critically affected during pathogenesis of this complex movement disorder. In this review, we summarize the evidence in support of involvement of calpain, a Ca2+-activated non-lysosomal protease, in spinal cord degeneration in two models of experimental parkinsonism induced by the neurotoxin 1-methyl-4-phenyl 1,2,3,6-tetrahydropyridine and also the environmental toxin rotenone. The key focus of this review is to discuss the role that calpain plays in disrupting the structural and functional integrity of the spinal cord in these experimental models of parkinsonism. A similar disruptive role of calpain has been reported earlier in SN of PD patients as well as in experimental PD animals. Studies in rodent and cell culture models of PD suggest that treatment with calpain inhibitors (e.g., calpeptin, MDL-28170) can prevent neuronal death and restore functions. Furthermore, the degradation of calpain substrates in both brain and spinal cord during pathogenesis of PD suggested a putative role of calpain, and calpain inhibition as a therapeutic strategy in PD.

Experimental autoimmune encephalomyelitis (EAE), a widely recognized animal model of multiple sclerosis (MS), is highly useful for studying inflammation, demyelination, and neurodegeneration in the central nervous system (CNS). EAE exhibits many similarities with MS, which is a chronic inflammatory disease affecting CNS white matter in humans. Various studies have indicated that EAE is a particularly useful animal model for understanding both the mechanisms of immune-mediated CNS pathology and also the progressive clinical course of MS. Demyelination and axonal dysfunction have previously been shown in MS and EAE but current evidences indicate that axonal damage and neuron death also occur, demonstrating that these diseases harbor a neurodegenerative component. Recent studies also have shown that the activation of calpain and caspase pathways contribute to the apoptotic death of oligodendrocytes and neurons, promoting the pathological events leading to neurological deficits. Apoptosis is involved in the disease-regulating as well as in the disease-promoting processes in EAE. This review discusses the major involvement of calpain and caspase pathways in causing demyelination and neurodegeneration in EAE animals.