Involvement of TRPM7 in cell growth as a spontaneously activated Ca2+ entry pathway in human retinoblastoma cells

Essential role for TRPM6 in epithelial magnesium transport and body magnesium homeostasis

Magnesium is an important cofactor for many biological processes such as protein synthesis, nucleic acid stability and neuromuscular excitability. The extracellular magnesium concentration is regulated tightly by the extent of intestinal absorption and renal excretion. Despite their critical role in magnesium handling, the molecular mechanisms mediating transepithelial transport are still not understood completely. Recently, genetic studies in patients with primary hypomagnesaemia and secondary hypocalcaemia (HSH), a combined defect of intestinal magnesium absorption and renal magnesium conservation, have identified "transient receptor potential (melastatin) 6" (TRPM6) as the first component involved directly in epithelial magnesium reabsorption. TRPM7, the closest homologue of TRPM6, has a central role in Mg(2+) uptake in vertebrate cells since TRPM7-deficient cells become Mg(2+) deficient and are not viable. TRPM7 has been characterized functionally as a constitutively active ion channel permeable for a variety of cations including calcium and magnesium and regulated by intracellular concentrations of magnesium and/or magnesium-nucleotide complexes. Both proteins share the unique feature of cation channels fused to serine/threonine kinase domains. This review summarizes recent data that has emerged from molecular genetic, biochemical and electrophysiological studies on these fascinating two new proteins and their involvement in epithelial magnesium transport.

Transient Receptor Potential Melastatin 7 Ion Channels Regulate Magnesium Homeostasis in Vascular Smooth Muscle Cells

Magnesium modulates vascular smooth muscle cell (VSMC) function. However, molecular mechanisms regulating VSMC Mg 2+ remain unknown. Using biochemical, pharmacological, and genetic tools, the role of transient receptor potential membrane melastatin 7 (TRPM7) cation channel in VSMC Mg 2+ homeostasis was evaluated. Rat, mouse, and human VSMCs were studied. Reverse transcriptase polymerase chain reaction and immunoblotting demonstrated TRPM7 presence in VSMCs (membrane and cytosol). Angiotensin II (Ang II) and aldosterone increased TRPM7 expression. Gene silencing using small interfering RNA (siRNA) against TRPM7, downregulated TRPM7 (mRNA and protein). Basal [Mg 2+ ] i , measured by mag fura-2AM, was reduced in siRNA-transfected cells (0.39±0.01 mmol/L) versus controls (0.54±0.01 mmol/L; P <0.01). Extracellular Mg 2+ dose-dependently increased [Mg 2+ ] i in control cells (E max 0.70±0.02 mmol/L) and nonsilencing siRNA-transfected cells (E max 0.71±0.04 mmol/L), but not in siRNA-transfected cells (E max 0.5±0.01 mmol/L). The functional significance of TRPM7 was evaluated by assessing [Mg 2+ ] i and growth responses to Ang II in TRPM7 knockdown cells. Acute Ang II stimulation decreased [Mg 2+ ] i in control and TRPM7-deficient cells in a Na + -dependent manner. Chronic stimulation increased [Mg 2+ ] i in control, but not in siRNA-transfected VSMCs. Ang II–induced DNA and protein synthesis, measured by 3 [H]-thymidine and 3 [H]-leucine incorporation, respectively, were increased in control and nonsilencing cells, but not in TRPM7 knockdown VSMCs. Our data indicate that VSMCs possess membrane-associated, Ang II–, and aldosterone-regulated TRPM7 channels, which play a role in regulating basal [Mg 2+ ] i , transmembrane Mg 2+ transport and DNA and protein synthesis. These novel findings identify TRPM7 as a functionally important regulator of Mg 2+ homeostasis and growth in VSMCs.

Phosphorylation of Annexin I by TRPM7 Channel-Kinase

TRPM7 is an unusual bifunctional molecule consisting of a TRP ion channel fused to a protein kinase domain. It has been shown that TRPM7 plays a key role in the regulation of intracellular magnesium homeostasis as well as in anoxic neuronal death. TRPM7 channel has been characterized using electrophysiological techniques; however, the function of the kinase domain is not known and endogenous substrates for the kinase have not been reported previously. Here we have identified annexin 1 as a substrate for TRPM7 kinase. Phosphorylation of annexin 1 by TRPM7 kinase is stimulated by Ca2+ and is dramatically increased in extracts from cells overexpressing TRPM7. Phosphorylation of annexin 1 by TRPM7 kinase occurs at a conserved serine residue (Ser5) located within the N-terminal amphipathic alpha-helix of annexin 1. The N-terminal region plays a crucial role in interaction of annexin 1 with other proteins and membranes, and therefore, phosphorylation of annexin 1 at Ser5 by TRPM7 kinase may modulate function of annexin 1.

Activation of the melastatin-related cation channel TRPM3 by D-erythro-sphingosine [corrected]

TRPM3, a member of the melastatin-like transient receptor potential channel subfamily (TRPM), is predominantly expressed in human kidney and brain. TRPM3 mediates spontaneous Ca2+ entry and nonselective cation currents in transiently transfected human embryonic kidney 293 cells. Using measurements with the Ca2+-sensitive fluorescent dye fura-2 and the whole-cell patch-clamp technique, we found that D-erythro-sphingosine, a metabolite arising during the de novo synthesis of cellular sphingolipids, activated TRPM3. Other transient receptor potential (TRP) channels tested [classic or canonical TRP (TRPC3, TRPC4, TRPC5), vanilloid-like TRP (TRPV4, TRPV5, TRPV6), and melastatin-like TRP (TRPM2)] did not significantly respond to application of sphingosine. Sphingosine-induced TRPM3 activation was not mediated by inhibition of protein kinase C, depletion of intracellular Ca2+ stores, and intracellular conversion of sphingosine to sphingosine-1-phosphate. Although sphingosine-1-phosphate and ceramides had no effect, two structural analogs of sphingosine, dihydro-D-erythro-sphingosine and N,N-dimethyl-D-erythro-sphingosine, also activated TRPM3. Sphingolipids, including sphingosine, are known to have inhibitory effects on a variety of ion channels. Thus, TRPM3 is the first ion channel activated by sphingolipids.