Cloning and expression of the human transient receptor potential 4 (TRP4) gene: localization and functional expression of human TRP4 and TRP3

Mammalian homologues of the Drosophila transient receptor potential (TRP) p rotein have been proposed to function as ion channels, and in some cases as store-operated or capacitative calcium entry channels. However, for each o f the mammalian TRP proteins, different laboratories have reported distinct modes of cellular regulation. In the present study we describe the cloning and functional expression of the human form of TRP4 (hTRP4), and compare i ts activity with another well studied protein, hTRP3. When hTRP3 was transi ently expressed in human embryonic kidney (HEK)-293 cells, basal bivalent c ation permeability (barium) was increased. Whole-cell patch-clamp studies o f hTRP4 expressed in Chinese hamster ovary cells revealed a constitutively active non-selective cation current which probably underlies the increased bivalent cation entry. Barium entry into hTRP4-transfected HEK-293 cells wa s not further increased by phospholipase: C (PLC)-linked receptor activatio n, by intracellular calcium store depletion with thapsigargin, or by a synt hetic diacylglycerol, 1-oleoyl-2-acetyl-sn-glycerol (OAG). In contrast, tra nsient expression of hTRP3 resulted in a bivalent cation influx that was ma rkedly increased by PLC-linked receptor activation and by OAG, but not by t hapsigargin. Despite the apparent differences in regulation of these two pu tative channel proteins, green fluorescent protein Fusions of both molecule s localized similarly to the plasma-membrane, notably in discrete punctate regions suggestive of specialized signalling complexes. Our findings indica te that while both hTRP4 and hTRP3 can apparently function as cation channe ls, their putative roles as components of capacitative calcium entry channe ls are not readily demonstrable by examining their behaviour when exogenous ly expressed in cells.