The Malpighian tubules, which represent the Drosophila
functional equivalent of mammalian kidneys, have been studied extensively as a model for renal function and epithelial fluid transport 
. Several ion channels, including the IP3
-receptor and the TRPL channel have been shown to participate in fluid transport in Malpighian in response to capa family neuropeptides 
. However, the Drosophila
channels required for electrolyte homeostasis are not known.
Several observations in the current report indicate that TRPM functions in Mg2+ homeostasis. The TRPM protein is expressed in the principal cells of the Malpighian tubules, which are necessary for filtering electrolytes from the hemolymph and for excretion, and trpm mutations impair the morphology of the Malpighian tubules. Of primary importance in this study, the trpm animals were unable to handle high levels of Mg2+ in the diet. Supplementation with 30 mM Mg2+ led to a pronounced elevation of Mg2+ in the hemolymph, and a growth arrest during the larval period. The combination of these data indicated that loss of trpm resulted in altered Malpighian tubules, leading to impaired excretion of excessive Mg2+. Although we were unable to perform rescue experiments with a wild-type transgene, due to the large genomic region and the wide array of mRNA isoforms, the phenotypes described here were due to mutation of trpm, as we observed the same defects using either of two independent trpm alleles.
The exacerbation of the trpm
phenotype by high Mg2+
in the food, contrasts with the findings that Mg2+
supplementation suppresses the HSH symptoms due to mutations in TRPM6 
, the cell death resulting from loss of vertebrate TRPM7 
and the growth defect resulting from mutations in the worm gon-2
. These effects of high dietary Mg2+
are the consequence of the requirement for these other TRPM channels for intestinal and cellular absorption of Mg2+
. In contrast, loss of Drosophila trpm
did not appear to have a major impact on gut magnesium resorption, but predominatly impaired the renal organ and excretory function, thereby leading to a defect in the handling of elevated Mg2+
, resulting in hypermagnesemia. This role appears to be evolutionarily conserved, given the recent observation that the function of the excretory cell in worms in Mg2+
removal depends on the TRPM-related channel, GTL-2 
One of the salient defects exhibited by the trpm mutants was growth arrest. We suggest that this phenotype was a consequence of reduced anabolism, which was a secondary consequence of the perturbation in Mg2+ homeostasis. The problem in anabolism was evident by the smaller size of the cells in fat bodies and the reduced overall protein content.
While high Mg2+ largely enhanced the severity of the trpm larval growth phenotype, it appeared that supplementation of Mg2+ exclusively during the late-stage of larval development partially suppressed the biosynthetic defects in the fat body. In support of this conclusion, the protein content was much higher in larvae fed high Mg2+ on day 3 only, than in larvae maintained continuously on food with normal levels of Mg2+. These larvae, in contrast to larvae kept continuously on low Mg2+, displayed nearly normal hemolymph proteins. We propose that during early larval development when most larval growth is taking place, high Mg2+ in the diet is deleterious to trpm larvae because the hypermagnesemia suppresses feeding behavior and growth of tissues such as the fat bodies, which is a prerequisite for anabolic function. However, once morphogenesis and tissue growth is largely complete, as in late stage larvae, the high Mg2+ may contribute to Mg2+ resorption in tissues such as the fat bodies (), thereby promoting biosynthetic function.
Schematic showing the changes in Mg2+ homeostasis in trpm mutant larvae.
In summary, we found that the single fly TRPM homolog is an important regulator of Mg2+ homeostasis. Loss of fly TRPM impairs renal absorption and excretion of Mg2+, thereby resulting in defects in peripheral tissues. This represents the first requirement for a Drosophila TRP channel for Mg2+ homeostasis, growth and viability. The observation that the sole Drosophila TRPM channel functions in Mg2+ excretion underscores the broad evolutionarily conserved roles of TRPM channels in Mg2+ homeostasis.