Molecular elucidation of receptor-operated Ca^<2+> permeable cation channels with TRP as candidate proteins

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Stimulation of G-protein coupled or tyrosine kinase receptors often leads to activation of Ca2+-permeable cation channels (ROCCs) which seem to participate in a wide variety of living cells by evoking Ca2+ entry from the extracellular space in direct and indirect ways. Although the molecular identification of ROCCs remains entirely elusive, recent progress in understanding the Drosophila's visual signal transduction has revealed that the vertebrate homologues of transient receptor potential protein (TRPs) are promising candidates for these channels. Based on this knowledge, we have launched on correlating native ROCCs with distinct TRP isoforms at molecular level using the following approach; (1) detailed comparison of cation currents due to native ROCCs and recombinantly expressed TRPs in HEK293 cells with electrophysiological and Ca2+ imaging techniques; (2) detection of TRP mRNAs by RT-PCR and in situ hybridization techniques with TRP isoform specific probes; (3) effects of TRP isof … More orm specific antisense oligonucleotides on native ROCCs. As the results of these studies, we have found that TRP6 is likely to be the essential component of alpha-1 adrenoceptor-activated ROCCs which seem to serve as a membrane depolarizer activating voltage-dependent Ca2+ entry as well as a direct Ca2+ entry pathway that is activated in a store depletion-independent fashion. Thus, TRP6 could be an interesting molecular target of development of anti-hypertensive drugs having entirely new mechanisms (Circ Res. 88, 325-332, 2001). We have also obtained the evidence supporting that TRP6, together with its relative protein, LTRPC2, may form a heretrooligomeric complex which reproduces a number of hallmarks of muscarinic ROCCs widely distributed in the whole gut, some part of brain and adrenal chromaffin cells. In this heterooligomeric configuration, it appears that the signal transduction pathway involved switches from pertussiss toxin-insensitive to sensitive pathways. Such heteromultimeric association of different isoforms of TRP and TRP-related proteins might be responsible for enormous biodiversity of Ca2+ entry associated with cell receptor stimulation. Less