Ion Channels, Cell Volume, Cell Proliferation and Apoptotic Cell Death

Ion channels in pediatric CNS Atypical Teratoid/Rhabdoid Tumor (AT/RT) cells: potential targets for novel therapeutic agents

The central nervous system Atypical Teratoid/Rhabdoid Tumor (CNS AT/RT) is a highly malignant neoplasm that commonly affects infants and young children, and has an extremely poor prognosis. Recently, a small subset of ion channels have been found to be over-expressed in a variety of malignant cells, thus emerging as potential therapeutic targets for difficult to treat tumors. We have studied the electrophysiological properties of AT/RT cell lines with particular attention to cell volume sensitive ion channels (VSC). This class of membrane proteins can play a fundamental role in cellular processes relevant to tumor development. We have found that chloride selective VSCs are particularly active in AT/RT cell lines, compared to non-tumor cells. We evaluated specific inhibitors for activity against chloride selective VSCs and consequently for their ability to inhibit the growth and survival of AT/RT cells in vitro. The results demonstrated that the extent of volume sensitive membrane current inhibition by these agents was correlated with their potency in AT/RT cell growth inhibition in vitro. In addition, we showed that ion channel inhibition enhanced the activity of certain anti-neoplastic agents, suggesting its value in effective drug combination protocols. Results presented provide preliminary in vitro data for possible evaluation of distinct ion channels as plausible therapeutic targets in the treatment of AT/RT.

Balance of unidirectional monovalent ion fluxes in cells undergoing apoptosis: why does Na+/K+ pump suppression not cause cell swelling?

Cells dying according to the apoptotic program, unlike cells dying via an unprogrammed mode, are able to avoid swelling and osmotic bursting with membrane disruption.There are indications that apoptosis is accompanied by suppression of the Na+/K+ pump and changes in the K+ and Cl− channels. It remains unclear how ion fluxes through individual ion pathways are integrated so as to induce loss of intracellular ions and concomitant apoptotic volume decrease. A decrease in activity of the sodium pump during apoptosis should cause cell swelling rather than shrinkage. We have made the first systemic analysis of the monovalent ion flux balance in apoptotic cells. Experimental data were obtained for human U937 cells treated with staurosporine for 4–5 h, which is known to induce apoptosis. The data include cellular Cl− content and fluxes, K+, Na+, water content and ouabain-sensitive and -resistant Rb+ fluxes.Unidirectional monovalent ion fluxeswere calculated using these data and a cell model comprising the double Donnan system with the Na+/K+ pump, Cl−, K+, Na+ channels, the Na+–K+–2Cl−cotransporter (NKCC), the Na+–Cl− cotransporter (NC), and the equivalent Cl−/Cl− exchange.Apoptotic cell shrinkage was found to be caused, depending on conditions, either by an increase in the integral channel permeability of membrane for K+ or by suppression of the pump coupledwith a decrease in the integral channel permeability of membrane for Na+. The decrease in the channel permeability of membrane for Na+ plays a crucial role in cell dehydration in apoptosis accompanied by suppression of the pump. Supplemental Table S1 is given for easy calculating flux balance under specified conditions.

Physiological roles of ion channels in adult neural stem cells and their progeny

Elucidation of the machinery of adult neurogenesis is indispensable for the treatment of neurodegenerative diseases by therapeutic drugs and/or by neural stem cell (NSC) transplantation. It is well known that membrane ion channels play a critical role in cell function, including proliferation, apoptosis and migration in a wide range of cells. In NSC research, interdisciplinary collaboration between cell biologists and membrane physiologists has been pursued principally to monitor ion channel and synaptic currents as a hallmark of neuronal differentiation and maturation of NSC progeny. Nevertheless, less attention had been paid to a functional role of ion channels in NSCs or their immature progeny. Recently, however, evidence regarding their functional relevance has started to accumulate. In focusing on the early stages of the neurogenic process during which NSCs give rise to neuroblasts, this review highlights the latent ability of ion channels to act as functional regulators of adult neurogenesis.