Current Neurovascular Research - Volume 2, Issue 1, 2005

The Na+ / Ca2+ exchangers, RNCX and SNCX, were cloned from mesangial cells of salt sensitive and salt resistant Dahl / Rapp rats, respectively, and differ at amino acid 218 (RNCXi / SNCXf) and in the exons expressed at the alternative splice site (RNCXB,D / SNCXB, D,F). These isoforms are also expressed in myocytes, neurons, and astrocytes where they maintain cytosolic calcium homeostasis. We demonstrated that cells expressing SNCX were more susceptible to oxidative stress than cells expressing RNCX. Others demonstrated that amyloid β peptide (Aβ) augments the adverse effects of oxidative stress on calcium homeostasis. Therefore, we sought to assess the effect of Aβ 1-40 on the abilities of OK-PTH cells stably expressing RNCX and SNCX and human glioma cells, SKMG1, to regulate cytosolic calcium homeostasis. Our studies showed that Aβ 1-40 (1 μM) did not affect RNCX activity, as assessed by changes in [Ca2+]i (D[Ca2+]i, 260 ± 10 nM to 267 ± 8 nM), while stimulating exchange activity 2.4 and 3 fold in cells expressing SNCX (100 ± 8 to 244 ± 12 nM) and in SKMG1 cells (90 ± 11 nM to 270 ± 18 nM), respectively. Our results also showed that Aβ 1- 40, while not affecting the rate of Mn2+ influx in cells expressing RNCX, stimulated the rate of Mn2+ influx 2.8 and 2.9 fold in cells expressing SNCX and in SKMG1 cells. Thus, our studies demonstrate that Aβ-induced cytosolic calcium increase is mediated through certain isoforms of the Na+ / Ca2+ exchanger and reveals a possible mechanism by which Aβ 1-40 can alter cytosolic calcium homeostasis.

Neuropathologies have been associated with neuronal de-differentiation and oxidative susceptibility. To address whether cellular states determines their oxidative vulnerability, we have challenged naive (undifferentiated) and nerve growth factor-induced differentiated pheochromocytoma (PC12) with methylglyoxal (MG), a model of carbonyl stress. MG dose-dependently induced greater apoptosis (24h) in naive (nPC12) than differentiated (dPC12) cells. This enhanced nPC12 susceptibility was correlated with a high basal oxidized cellular glutathione-to-glutathione disulfide (GSH / GSSG) redox and an MG-induced GSH-to-Disulfide (GSSG plus protein-bound SSG) imbalance. The loss of redox balance occurred at 30 min post-MG exposure, and was prevented by N-acetylcysteine (NAC) that was unrelated to de novo GSH synthesis. NAC was ineffective when added at 1h post-MG, consistent with an early window of redox signaling. This redox shift was kinetically linked to decreased BcL-2, increased Bax, and release of mitochondrial cytochrome c which preceded caspase-9 and -3 activation and poly ADP-ribose polymerase (PARP) cleavage (1-2h), consistent with mitochondrial apoptotic signaling. The blockade of apoptosis by cyclosporine A supported an involvement of the mitochondrial permeability transition pore. The enhanced vulnerability of nPC12 cells to MG and its relationship to cellular redox shifts will have important implications for understanding differential oxidative vulnerability in various cell types and their transition states.

Heme oxygenase-2 (HO-2) has been suggested to be a cytoprotective enzyme in a variety of in vivo experimental models. HO-2, the constitutive isozyme, is enriched in neurons and, under normal conditions, accounts for nearly all of brain HO activity. HO-2 deletion (HO-2- / -) leads to increased neurotoxicity in cultured brain cells and increased damage following transient cerebral ischemia in mice. Moreover, pharmacologic inhibition of HO activity significantly augments focal ischemic damage in wildtype (WT) mice, but does not further exacerbate it in HO-2- / - mice. The HO system shares some similarities with nitric oxide synthase (NOS), notably their syntheses of carbon monoxide (CO) and nitric oxide (NO), respectively, which are diffusible gases with numerous biological actions, including neurotransmission and vasodilation. While deletion of HO-2 results in greater stroke damage, the pharmacologic inhibition of neuronal nitric oxide synthase (nNOS), or its gene deletion, confers neuroprotection in animal models of transient cerebral ischemia. To investigate the interactions, the outcome of focal cerebral ischemia-reperfusion in double knockout (HO-2- / - X nNOS- / -) mice lacking both genes was compared to control WT mice. Wildtype and double knockout male mice underwent intraluminal middle cerebral occlusion for 2 hours, followed by reperfusion for 22 hours. Outcomes in neurologic deficits and infarct size were determined. No difference was observed between WT and double knockout mice in the volume of infarction, neurologic signs, decrease in relative cerebral blood flow during ischemia, or core body temperature. The results suggest that the deleterious action of nNOS would counteract the role of HO-2 in neuroprotection.

The pathogenesis of stroke-like episodes in mitochondrial encephalopathy, myopathy, lactic acidosis and stroke-like episodes (MELAS) is not fully understood although two main theories have been proposed; ischemic vascular hypothesis caused by “mitochondrial angiopathy” and generalized cytopathic hypothesis caused by “mitochondrial cytopathy”. Crucial molecular mechanism includes the lack of taurine modification at the wobble uridine of mutant transfer RNAsLeu(UUR) resulting in defective translation of cognate codons due to a defect in codon-anticodon interaction. Whereas recent clinical studies have shed light on the neuronal hyperexcitability, which may potentially initiate a cascade of stroke-like events. Stroke-like episodes are characterized by neuronal hyperexcitability, neuronal vulnerability, increased capillary permeability, and focal hyperaemia. It is recognized that stroke-like lesions not only evolve in the area incongruent to a vascular territory, but also potentially spread into the surrounding cortex with concomitant vasogenic edema presumably provoked by prolonged epileptic activities. Based on the clinical observations, we speculate that stroke-like episodes appear to be non-ischemic neurovascular events; once neuronal hyperexcitability developed in a localized brain region as a result from either mitochondrial dysfunction in the capillary endothelial cells, or in neurons or astrocytes, epileptic activities may depolarize the adjacent neurons leading to propagation of epileptic activities in the surrounding cortex. Increased capillary permeability provoked by epileptic activities in the presence of mitochondrial capillary angiopathy may cause unique edematous brain lesions predominantly involving the cortex. As a consequence, susceptible neuronal population in the cortex may result in neuronal loss with a laminar or pseudo-laminar distribution.

This article reviews recent studies on the alterations occurring in the brain vessel wall of the mdx mouse, an animal model with genetic defects in a region homologous with the human Duchenne muscular dystrophy (DMD) gene. These alterations affect both endothelial and astroglial cells and are associated with opened tight junctions, swollen perivascular astrocyte processes and a reduction in the expression of tight junctions associated proteins, ie. zonula occludens and of a specific water channel i.e. aquaporin-4, suggesting that some neurological dyspfunctions of mdx mice and DMD patients could be associated with changes in brain osmotic equilibrium.

Alzheimer's disease is a progressive disorder that results in the loss of cognitive function and memory. Although traditionally defined by the presence of extracellular plaques of amyloid-β peptide aggregates and intracellular neurofibrillary tangles in the brain, more recent work has begun to focus on elucidating the complexities of Alzheimer's disease that involve the generation of reactive oxygen species and oxidative stress. Apoptotic processes that are incurred as a function of oxidative stress affect neuronal, vascular, and monocyte derived cell populations. In particular, it is the early apoptotic induction of cellular membrane asymmetry loss that drives inflammatory microglial activation and subsequent neuronal and vascular injury. In this article, we discuss the role of novel cellular pathways that are invoked during oxidative stress and may potentially mediate apoptotic injury in Alzheimer's disease. Ultimately, targeting new avenues for the development of therapeutic strategies linked to mechanisms that involve inflammatory microglial activation, cellular metabolism, cell-cycle regulation, G-protein regulated receptors, and cytokine modulation may provide fruitful gains for both the prevention and treatment of Alzheimer's disease.

Constantly growing body of evidence suggests that hallmarks of oxidative stress are present in various central nervous system (CNS) disorders. Technological advantages in cell culturing made it possible to use neural cell / tissue cultures as experimental models for investigation of molecular mechanisms which underlie the development of oxidative stress condition, damage and adaptive responses to oxidative insults. This review is focused on the application of cell culture methodology for studies of oxidative stress condition in the brain. The review describes studies of biomarkers of oxidative stress-dependent cell damage and adaptive responses in various kinds of brain cell culture models. It discusses the use of cell / tissue culture models for elucidation of the role and pathogenesis of oxidative stress in neurodegenerative brain disorders, AIDS-associated brain pathology, drug abuse, and aging. The review underscores the importance of cell / tissue-based studies for testing of new antioxidants and development of therapeutic strategies for amelioration of oxidative damage in the CNS. The impact of new advances in gene and protein expression analysis on the cell / tissue culture-based research of oxidative stress in the CNS is also discussed.