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TRPV4 and TRPM3 as Volume-Regulated Cation Channels Harteneck C; Schultz GTRP Ion Channel Function in Sensory Transduction and Cellular Signaling Cascades-/-Frontiers in Neuroscience 2007[]; ä (ä): äChanges of the extracellular osmolarity result in swelling or shrinkage of cells by increases or decreases in intracellular water along the ionic concentration gradient, thereby changing cell volume. For the viability of the cells, fast adaptation to the extracellular tonicity is essential and, therefore, a variety of cellular mechanisms protects cells from osmotic stress. By modulation of ion channels and transporters, cells modulate their intracellular ion concentrations in order to adapt to environmental conditions and facilitate cellular functions [1, 2]. The change in cell volume is accompanied by reorganization of the cytoskeleton, involving the structure-forming proteins as well as proteins involved in the regulation of cell architecture. Despite the variety of systems described to be involved in volume regulation like potassium and chloride channels, osmolytes, and others, the connections between the different systems are as unclear as the proteins sensing changes in tonicity and subsequent signaling cascades. We recently characterized two different TRP-homologous cation channels, TRPV4 and TRPM3, as proteins mediating calcium entry in cells upon extracellular application of hypotonic solutions [3, 4]. Proteins of the TRP family as integral membrane proteins form pores in the lipid bilayer of the plasma membrane, regulating ion fluxes across the membrane. Today the TRP superfamily is a superfamily of proteins subdivided into at least seven different subfamilies. High sequence similarity in the region of the pore-forming domains is the common feature of the classic (TRPC), melastatin-like (TRPM), and vanilloid-like (TRPV) subfamilies' functional properties, and proposed topology is the common theme of all TRP channels [5-7]. The cDNAs of two orphan proteins, named TRPV4 and TRPM3 according to sequence similarity and order of appearance, were cloned by their sequence similarity to Drosophila TRP. The proteins transiently expressed in HEK293 cells could be characterized as nonselective cation channels mediating calcium entry upon extracellular application of hypoosmolar solutions. This chapter summarizes the known data for TRPV4 and TRPM3.ä |
