Magnesium is the second most-prevalent intracellular cation involved in a great variety of physiological and biochemical processes such as protein and nucleic acid synthesis, modulation of membrane transporters or signal transduction. Body magnesium is mainly distributed in bone and in the intracellular compartments of muscle and soft tissues. Serum magnesium levels are maintained at almost constant values. Homeostasis depends on the balance between intestinal absorption and renal excretion. Over the last years, a number of hereditary disorders of magnesium homeostasis have been clinically characterized. The genetic analysis of patients affected by these rare diseases represents a unique opportunity to gain insights into the physiology of cellular magnesium transport at the molecular level. Hypomagnesmia with Secondary Hypocalcemia (HSH) is characterized by extremely low serum magnesium levels and low serum calcium levels. Patients usually present within the first months of life with cerebral convulsions, muscle spasms or tetany. Untreated, this disorder may be fatal or result in permanent neurological damage. Relieve of clinical symptoms and normocalcemia are achieved by the administration of high oral doses of magnesium. Pathophysiological studies in affected patients pointed to a primary defect in intestinal magnesium transport. In some patients an additional renal magnesium leak was suspected. A gene locus for HSH was mapped to chromosome 9q22 and further refined to a critical interval of approximately 1cM. By using a positional candidate gene approach, we identified the TRPM6 gene within this region and detected mutations in TRPM6 in patients with HSH.The gene product TRPM6 is a new member of the long transient receptor potential channel or TRPM family and a close relative to TRPM7, a magnesium and calcium permeable ion channel. Unlike other TRP channels, TRPM6 and TRPM7 share the unique feature of a c-terminal alpha-kinase enzyme domain.By in-situ hybridization and RT-PCR, expression of TRPM6 was detected along the intestine as well as in the distal convoluted tubule, the main site of active transcellular magnesium reabsorption in the kidney. So far, we studied 12 families with typical clinical HSH. The mutational analysis of the TRPM6 gene in affected patients yielded mutations mostly truncating the TRPM6 protein, however one point mutation (S141L) was identified for which the pathomechanism remained unknown. Clinically, the observation of inappropriately high fractional excretion rates of magnesium in the presence of severe hypomagnesemia clearly indicates an additional role of renal magnesium wasting for the pathophysiology of HSH. Upon heterologous expression, TRPM6 alone failed to form functional channel complexes, as it was retained intracellularly. Considering the ability of oligomer formation demonstrated for other members of the TRP family and the overlapping expression pattern of TRPM6 with its closest relative TRPM7, we hypothesized that the two homologous TRPMs may be functionally linked and both together be involved in the process of epithelial magnesium absorption.In fact, coexpression of TRPM6 with TRPM7 in xenopus oocytes resulted in an amplification of TRPM7-evoked currents. Western blot analysis of oocyte lysates supported the functional data as the TRPM6 protein, when co-expressed with TRPM7, was detectable in the membrane fraction. In contrast, the naturally occurring (S141L) mutation resulted in a loss of TRPM7like current amplification and in an intracellular retention of the mutant TRPM6 protein.These results were confirmed by the observation of a TRPM7-dependant trafficking of TRPM6 to the plasma membrane in HEK293 cells. Consistently, the functional assessment of Mn2 entry into HEK cells by fura-2 fluorescence quenching showed an increase in TRPM7-evoked Mn2 influx upon co-expression with TRPM6. Finally, a direct and specific protein-protein interaction between TRPM6 and TRPM7 was shown by FRET (fluorescence resonance energy transfer) analysis. In line with the functional data obtained in xenopus oocytes, TRPM7 failed to promote a membrane insertion of the mutant (S141L)TRPM6 suggesting that the S141L mutation prevented the assembly of TRPM6/TRPM7 heteromultimers.In summary, these findings indicate a crucial role of TRPM6 for epithelial magnesium transport and suggest an important contribution of TRPM6/TRPM7 heteromultimerization for the formation of functional magnesium channels.