Regulation of T-type calcium channels: signalling pathways and functional implications.

T-type calcium channels (T-channels) contribute to a wide variety of physiological functions, especially in the cardiovascular and nervous systems. Recent studies using knock-out mouse models have been instrumental in documenting further the role of T-channels in sleep, heartbeat, pain and epilepsy. Importantly, several novel aspects of the regulation of these channels have been identified over the last few years, providing new insights into their physiological and pathophysiological roles. Here, we review recent evidence supporting that the Cav3 subunits of T-channels are modulated by endogenous ligands such as anandamide, zinc, redox and oxidizing agents, as well as G-protein and protein kinases pathways. The study of T-channel mutations associated with childhood absence epilepsy has also revealed new aspects of Cav3 subunit trafficking. Collectively, these findings identify novel regulatory mechanisms involved in the fine tuning of T-channel expression and activity, and offer new directions for the design of novel therapeutic strategies targeting these channels.

Comparison of expressed human and mouse sodium/iodide symporters reveals differences in transport properties and subcellular localization.

The active transport of iodide from the bloodstream into thyroid follicular cells is mediated by the Na+/I- symporter (NIS). We studied mouse NIS (mNIS) and found that it catalyzes iodide transport into transfected cells more efficiently than human NIS (hNIS). To further characterize this difference, we compared (125)I uptake in the transiently transfected human embryonic kidney (HEK) 293 cells. We found that the V(max) for mNIS was four times higher than that for hNIS, and that the iodide transport constant (K(m)) was 2.5-fold lower for hNIS than mNIS. We also performed immunocytolocalization studies and observed that the subcellular distribution of the two orthologs differed. While the mouse protein was predominantly found at the plasma membrane, its human ortholog was intracellular in approximately 40% of the expressing cells. Using cell surface protein-labeling assays, we found that the plasma membrane localization frequency of the mouse protein was only 2.5-fold higher than that of the human protein, and therefore cannot alone account for the difference in the obtained V(max) values. We reasoned that the observed difference could also be caused by a higher turnover number for iodide transport in the mouse protein. We then expressed and analyzed chimeric proteins. The data obtained with these constructs suggest that the iodide recognition site could be located in the region extending from the N-terminus to transmembrane domain 8, and that the region between transmembrane domain 5 and the C-terminus could play a role in the subcellular localization of the protein.