Melastatin family Transient Receptor Potential channels support spermatogenesis in planarian flatworms

The Transient Receptor Potential superfamily of proteins (TRPs) form cation channels that are abundant in animal sensory systems. Amongst TRPs, the Melastatin-related family (TRPMs) is composed of members that respond to temperature, pH, sex hormones, and various other stimuli. Some TRPMs exhibit enriched expression in the gonads of vertebrate and invertebrate species, but their contributions to germline development remain to be determined. We identified twenty-one potential TRPMs in the planarian flatworm Schmidtea mediterranea and analyzed their anatomical distribution of expression by whole-mount in situ hybridization. Enriched expression of two TRPMs (Smed-TRPM-c and Smed-TRPM-l) was detected in testis, whereas eight TRPM genes had detectable expression in patterns representative of neuronal and/or sensory cell types. Functional analysis of TRPM homologs by RNA-interference (RNAi) revealed that disruption of normal levels of Smed-TRPM-c expression impaired sperm development, indicating a role for this receptor in supporting spermatogenesis. Smed-TRPM-l RNAi alone did not result in a detectable phenotype, but it did increase sperm development deficiencies when combined with Smed-TRPM-c RNAi. Fluorescence in situ hybridization revealed expression of Smed-TRPM-c in early spermatogenic cells within testes, suggesting cell-autonomous regulatory functions in germ cells for this gene. In addition, Smed-TRPM-c RNAi resulted in reduced numbers of presumptive germline stem cell clusters in asexual planarians, suggesting that Smed-TRPM-c supports the establishment, maintenance, and/or expansion of spermatogonial germline stem cells. While further research is needed to identify the factors that trigger Smed-TRPM-c activity, these findings reveal one of the few known examples for TRPM function in the direct regulation of sperm development.