Ethidium bromide staining reveals rapid cell dispersion in the rat dentate gyrus following ouabain-induced injury

Cognitive deficits caused by a disease-mutation in the α3 Na(+)/K(+)-ATPase isoform

The Na⁺/K⁺-ATPases maintain Na⁺ and K⁺ electrochemical gradients across the plasma membrane, a prerequisite for electrical excitability and secondary transport in neurons. Autosomal dominant mutations in the human ATP1A3 gene encoding the neuron-specific Na⁺/K⁺-ATPase α₃ isoform cause different neurological diseases, including rapid-onset dystonia-parkinsonism (RDP) and alternating hemiplegia of childhood (AHC) with overlapping symptoms, including hemiplegia, dystonia, ataxia, hyperactivity, epileptic seizures, and cognitive deficits. Position D801 in the α₃ isoform is a mutational hotspot, with the D801N, D801E and D801V mutations causing AHC and the D801Y mutation causing RDP or mild AHC. Despite intensive research, mechanisms underlying these disorders remain largely unknown. To study the genotype-to-phenotype relationship, a heterozygous knock-in mouse harboring the D801Y mutation (α₃^+/D801Y) was generated. The α₃^+/D801Y mice displayed hyperactivity, increased sensitivity to chemically induced epileptic seizures and cognitive deficits. Interestingly, no change in the excitability of CA1 pyramidal neurons in the α₃^+/D801Y mice was observed. The cognitive deficits were rescued by administration of the benzodiazepine, clonazepam, a GABA positive allosteric modulator. Our findings reveal the functional significance of the Na⁺/K⁺-ATPase α₃ isoform in the control of spatial learning and memory and suggest a link to GABA transmission.

Potential Roles of Electrogenic Ion Transport and Plasma Membrane Depolarization in Apoptosis

Apoptosis is characterized by the programmed activation of specific biochemical pathways leading to the organized demise of cells. To date, aspects of the intracellular signaling machinery involved in this phenomenon have been extensively dissected and characterized. However, recent studies have elucidated a novel role for changes in the intracellular milieu of the cells as important modulators of the cell death program. Specially, intracellular ionic homeostasis has been reported to be a determinant in both the activation and progression of the apoptotic cascade. Several apoptotic insults trigger specific changes in ionic gradients across the plasma membrane leading to depolarization of the plasma membrane potential (PMP). These changes lead to ionic imbalance early during apoptosis. Several studies have also suggested the activation and/or modulation of specific ionic transport mechanisms including ion channels, transporters and ATPases, as mediators of altered intracellular ionic homeostasis leading to PMP depolarization during apoptosis. However, the role of PMP depolarization and of the changes in ionic homeostasis during the progression of apoptosis are still unclear. This review summarizes the current knowledge regarding the causes and consequences of PMP depolarization during apoptosis. We also review the potential electrogenic ion transport mechanisms associated with this event, including the net influx/efflux of cations and anions. An understanding of these mechamisms could lead to the generation of new therapeutic approaches for a variety of diseases involving apoptosis.

Cell proliferation and granule cell dispersion in human hippocampal sclerosis

Granule cell dispersion (GCD) is observed in approximately 40% of cases of hippocampal sclerosis (HS) in patients with epilepsy. Studies in animal models suggest that GCD may be a consequence of enhanced proliferation of granule cell precursors as a result of seizures. We quantified the number of cells in cycle in subfields of the hippocampus with immunohistochemistry for Mcm2 in 14 HS cases with or without severe GCD compared to 6 epilepsy patients without classical HS or GCD as well as 5 postmortem controls. Higher numbers of Mcm2-positive cells were seen in the region of the granule cell layer in patients with severe GCD, and immunolabeling with Geminin and Ki-67 confirmed a proportion were progressing through cycle. Double labeling with Mcm2 and GFAP confirmed the majority of these cycling cells were GFAP-negative and occasional cells stained colocalized with stem cell marker nestin. These findings support the view that GCD may be a phenomenon related to increased progenitor cell proliferation in patients with hippocampal damage and chronic epilepsy.

Na+, K+-ATPase: the new face of an old player in pathogenesis and apoptotic/hybrid cell death

The Na(+), K(+)-ATPase is a ubiquitous membrane transport protein in mammalian cells, responsible for establishing and maintaining high K(+) and low Na(+) in the cytoplasm required for normal resting membrane potentials and various cellular activities. The ionic homeostasis maintained by the Na(+), K(+)-ATPase is also critical for cell growth, differentiation, and cell survival. Although the toxic effects of blocking the Na(+), K(+)-ATPase by ouabain and other selective inhibitors have been known for years, the mechanism of action remained unclear. Recent progress in two areas has significantly advanced our understanding of the role and mechanism of Na(+), K(+)-ATPase in cell death. Along with increased recognition of apoptosis in a wide range of disease states, Na(+), K(+)-ATPase deficiency has been identified as a contributor to apoptosis and pathogenesis. More importantly, accumulating evidence now endorses a close relationship between ionic homeostasis and apoptosis, namely the regulation of apoptosis by K(+) homeostasis. Since Na(+), K(+)-ATPase is the primary system for K(+) uptake, dysfunction of the transport enzyme and resultant disruption of ionic homeostasis have been re-evaluated for their critical roles in apoptosis and apoptosis-related diseases. In this review, instead of giving a detailed description of the structure and regulation of Na(+), K(+)-ATPase, the author will focus on the most recent evidence indicating the unique role of Na(+), K(+)-ATPase in cell death, including apoptosis and the newly recognized "hybrid death" of concurrent apoptosis and necrosis in the same cells. It is also hoped that discussion of some seemingly conflicting reports will inspire further debate and benefit future investigation in this important research field.

Detection of early tissue injury in vivo using fluorescent cell death markers in rat dentate gyrus

Fluorescent markers of cell death offer superior selectivity and sensitivity, although their applicability in detecting tissue injury under in vivo conditions remains uncertain. Here we examined whether ethidium bromide and Hoechst 33342, two widely used markers for cell necrosis and apoptosis, can be used in vivo to detect different types of cell death induced by Na,K-ATPase inhibition. Microinfusion of fluorescent markers and ouabain was made unilaterally into adult rat dentate gyrus. It was found that, at different time points post-injury, dentate cells that were exposed to ouabain but not to vehicle control showed marked loss of membrane integrity and exhibited nuclear condensation, as revealed by ethidium bromide and Hoechst 33342 staining, respectively. However, this pattern of cell death was not associated with DNA fragmentation and formation of apoptotic bodies, suggesting involvement of atypical cell apoptosis.

Overexpression and nuclear accumulation of glyceraldehyde-3-phosphate dehydrogenase in a transgenic mouse model of Huntington's disease

Huntington's disease is due to an expansion of CAG repeats in the huntingtin gene. Huntingtin interacts with several proteins including glyceraldehyde-3-phosphate dehydrogenase (GAPDH). We performed immunohistochemical analysis of GAPDH expression in the brains of transgenic mice carrying the huntingtin gene with 89 CAG repeats. In all wild-type animals examined, GAPDH was evenly distributed among the different cell types throughout the brain. In contrast, the majority of transgenic mice showed GAPDH overexpression, with the most prominent GAPDH changes observed in the caudate putamen, globus pallidus, neocortex, and hippocampal formation. Double staining for NeuN and GFAP revealed that GAPDH overexpression occurred exclusively in neurons. Nissl staining analysis of the neocortex and caudate putamen indicated 24 and 27% of cell loss in transgenic mice, respectively. Subcellular fluorescence analysis revealed a predominant increase in GAPDH immunostaining in the nucleus. Thus, we conclude that mutation of huntingtin is associated with GAPDH overexpression and nuclear translocation in discrete populations of brain neurons.

Porcine reproductive and respiratory syndrome virus-induced cell death exhibits features consistent with a nontypical form of apoptosis

Porcine reproductive and respiratory syndrome virus (PRRSV), an arterivirus, belongs to a group of RNA viruses that are cytopathic for macrophages and establish persistent infections. Apoptosis is the presumed mechanism of cell death in monkey kidney cell lines and porcine alveolar macrophages after infection with European PRRSV isolates. However, evidence from in vivo and in vitro studies using North American strains have failed to identify apoptosis in cells supporting virus replication and suggest that apoptosis is present in only uninfected bystander cells. The purpose of this study was to evaluate the mechanism of cell death following the infection of MARC-145 cells with wild-type (P6) and a cell culture-adapted (P136) strains derived from the North American isolate SDSU-23983. At 2 days after infection with P136, cytoplasmic blebbing and nuclear condensation were absent in monolayers containing almost 90% infected cells. By day 3, these infected cells detached and showed evidence of apoptosis, including nuclear condensation and inter-nucleosomal DNA fragmentation. Apoptosis in single infected floating cells was confirmed by the co-localization of FITC-anti-digoxigenin antibody, used to detect uridine-digoxigenin-labled nuclear DNA in a terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end-labeling assay, and Texas red-labeled PRRSV antibody. A majority of infected floating cells were also positive for the uptake of trypan blue, an indicator of necrotic cell death. These results demonstrate that apoptosis does occur in PRRSV infected cells, but is a late event during PRRSV replication and rapidly culminates in a necrotic-like death.

Na(+)-K(+)-ATPase inhibition and depolarization induce glutamate release via reverse Na(+)-dependent transport in spinal cord white matter

Excitotoxic mechanisms involving alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA)/kainate receptors play an important role in mediating cellular damage in spinal cord injury. However, the precise cellular mechanisms of glutamate release from non-synaptic white matter are not well understood. We examined how the collapse of transmembrane Na(+) and K(+) gradients induces reverse operation of Na(+)-dependent glutamate transporters, leading to glutamate efflux and injury to rat spinal dorsal columns in vitro. Compound action potentials were irreversibly reduced to 43% of control after ouabain/high K(+)/low Na(+) exposure (500 microM ouabain for 30 min to increase [Na(+)](i), followed by 1 h ouabain+high K(+) (129 mM)/low Na(+) (27 mM), to further reverse transmembrane ion gradients) followed by a 2 h wash. Ca(2+)-free perfusate was very protective (compound action potential amplitude recovered to 87% vs. 43%). The broad spectrum glutamate antagonist kynurenic acid (1 mM) or the selective AMPA antagonist GYKI52466 (30 microM) were partially protective (68% recovery). Inhibition of Na(+)-dependent glutamate transport with L-trans-pyrrolidine-2,4-dicarboxylic acid (1 mM) also provided significant protection (71% recovery), similar to that seen with glutamate receptor antagonists. Blocking reverse Na(+)-Ca(2+) exchange with KB-R7943 (10 microM) however, was ineffective in this paradigm (49% recovery). Semiquantitative glutamate immunohistochemistry revealed that levels of this amino acid were significantly depleted in axon cylinders and, to a lesser degree, in oligodendrocytes (but not in astrocytes) by ouabain/high K(+)/low Na(+), which was largely prevented by glutamate transport inhibition. Our data show that dorsal column white matter contains the necessary glutamate pools and release mechanisms to induce significant injury. When Na(+) and K(+) gradients are disrupted, even in the absence of reduced cellular energy reserves, reverse operation of Na(+)-dependent glutamate transport will release enough endogenous glutamate to activate AMPA receptors and cause substantial Ca(2+)-dependent injury. This mechanism likely plays an important role during ischemic and traumatic white matter injury, where collapse of transmembrane Na(+) and K(+) gradients occurs.

Sodium-potassium adenosine triphosphatase inhibition enhances membrane accumulation of DiI in rat hippocampus in vivo

Transient brain ischemia induces significant alterations in lipid structures of neuronal membranes, which are believed to result from lipid peroxidation and free radical attack. Such a membrane structural change may serve as an important histological marker of cell injury. In the present study, we examined how the dynamics of DiI/membrane incorporation may reflect early membrane metabolism and dynamic changes following sodium-potassium pump inhibition. Ouabain (1mM) was stereotactically co-administered with either 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate DiI (50 microg/ml) or ethidium homodimer (4 microM) into the granule cell layer of the adult rat hippocampus. Tissue was cryosectioned and examined with epifluorescence microscopy at 1, 2, 3, 4, 6, 8 and 72h post-injection. Alternate sections were stained with thionine or haematoxylin and eosin to evaluate morphological changes. Ouabain-induced pump inhibition resulted in a dramatic increase in DiI fluorescence in granule cell layer neurons as early as 4h post-injection. This increase in DiI incorporation coincided both spatially and temporally with the appearance of reactive changes characterizing early neuronal injury. However, the fluorescence increase was not a result of membrane breakdown because ethidium homodimer, a membrane-impermeable nucleic acid probe used for labeling cells with compromised membranes, when applied in a similar fashion, failed to show any fluorescence changes. The results of this study suggest that pump inhibition results in a specific increase in membrane lipophilicity possibly due to altered lipid structure.