Dan Halm (Committee Member), David Ladle (Committee Member), James Olson (Advisor)
Master of Science (MS)
Cells rely on a constant volume not only for structural stability but also for normal homeostatic processes to occur. In the brain and CNS, cells can regain their normal volume through a process termed regulatory volume decrease (RVD). A key component of a cells' response to cell swelling is the activation of channel(s) responsible for the efflux of chloride current, ICl,swell. Volume regulated anion channels (VRAC) which mediate ICl,swell have been implicated in controlling cell volume during RVD, but the mechanisms which activate this channel are not completely understood. In this study, I examined the role of G protein-coupled signaling via P2Y2 purinergic receptors for activation of VRAC in osmotically swollen human astrocytoma cells. Whole cell patch clamp recordings were performed on 1321N1 cells stably transfected to express the human P2Y2 receptor. This tumor cell does not show VRAC activation in the native cell type; however, VRAC activation is displayed in the transfected 1321N1 cells. Cells were sequentially perfused with isoosmotic (290 mOsm) and hypoosmotic (200 mOsm and 250 mOsm) solutions containing 100 mM CsCl as the major electrolyte. The same CsCl concentration was used in the isoosmotic patch electrode solution. Voltage clamp recordings lasting 100 msec were made in 20 mV steps between -100 mV and +120 mV every 30 sec. For some experiments, inhibitors and activators of GPCRs, purinergic signaling, and ATP release pathways were added to the perfusate solutions. In others, pertussis toxin was added to the patch electrode solution. Hypoosmotic exposure evoked an outwardly rectifying current which inactivated over time at the higher membrane voltages and was inhibited by DCPIB; characteristics ascribed to VRAC. A smaller VRAC current was observed when cells were perfused with a 250 mOsm hypoosmotic CsCl solution. VRAC was not activated in isoosmotic solutions but the VRAC response in hypoosmotic solutions was enhanced in the presence of thrombin. VRAC activation was completely blocked by adding suramin to the hypoosmotic solution or pertussis toxin to the patch electrode solution, both of which are inhibitors of G-protein signaling pathways. Carbenoxolone, which can block ATP release from swollen cells and can directly inhibit VRAC also blocked VRAC activation. Adding ATP did not rescue cells from this inhibition; however, adding thrombin to activate a separate G-protein coupled signaling pathway did result in a partial recovery. In contrast, meclofenamate, another ATP efflux inhibitor had no effect on the VRAC response to hypoosmotic exposure. I conclude GPCRs are necessary for VRAC activation in these human astrocytoma cells. Exogenous thrombin increased an osmotically-sensitive current but, like exogenous ATP, had no effect on VRAC activation during isoosmotic exposure. If endogenously released ATP is responsible for VRAC activation via the P2Y2 receptor, it is not effluxed via gap junction hemi-channels. Future studies are needed to measure ATP release during hypoosmotic exposure and identify the specific GPCR that mediates VRAC activation.
Department or Program
Department of Neuroscience, Cell Biology, and Physiology
Year Degree Awarded
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