A critical role of TRPM channel-kinase for human magnesium transport

Hereditary disorders of magnesium homeostasis comprise a heterogenous group of diseases mainly affecting the renal conservation of magnesium. In the past few years, genetic studies in affected individuals disclosed the first molecular components of epithelial magnesium transport: the tight junction protein paracellin-1 (claudin-16) was discovered as a key player in paracellular magnesium and calcium reabsorption in the thick ascending limb of Henle's loop and the gamma-subunit was identified as a component of renal Na+ -K+ -ATPase critical for transcellular magnesium reabsorption in the distal convoluted tubule. However, the molecular identity of proteins directly involved in cellular magnesium transport remained largely unknown until a series of recent studies highlighted the critical role of two members of the transient receptor potential (TRP) family, for body magnesium homeostasis. TRPM6 and TRPM7 belong to the melastatin-related TRPM subfamily of TRP channels whose eight members exhibit a significant diversity in domain structure as well as cation selectivity and activation mechanisms. Both proteins share the unique feature of an atypical kinase domain at their C-terminus for which they have been termed 'chanzymes' (channels plus enzymes). Whereas electrophysiological and biochemical analyses identified TRPM7 as an important player in cellular magnesium homeostasis, the critical role of TRPM6 for epithelial magnesium transport emerged from the discovery of loss-of-function mutations in patients with a severe form of hereditary hypomagnesaemia called primary hypomagnesaemia with secondary hypocalcaemia or HSH. The aim of this review is to summarize the data emerging from molecular genetic, biochemical and electrophysiological studies on these fascinating two new proteins combining ion channel and enzyme functions/properties.