A cataract-causing mutation in the TRPM3 cation channel disrupts calcium dynamics in the lens
TRPM3 belongs to the melastatin sub-family of transient receptor potential (TRPM) cation channels and has been shown to function as a steroid-activated, heat-sensitive, calcium ion (Ca2+) channel. A missense substitution (p.I65M) in the TRPM3 gene of humans (TRPM3) and mice (Trpm3) has been shown to underlie an inherited form of early-onset, progressive cataract. Here we model the pathogenetic effects of this cataract-causing mutation using 'knock-in' mutant mice and human cell-lines. Trpm3 and its intron-hosted micro-RNA gene (Mir204) were strongly co-expressed in the lens epithelium and other non-pigmented and pigmented ocular epithelia. Homozygous Trpm3-mutant lenses displayed elevated cytosolic Ca2+ levels and an imbalance of sodium (Na+) and potassium (K+) ions coupled with increased water content. Homozygous TRPM3-mutant human lens epithelial (HLE-B3) cell-lines and Trpm3-mutant lenses exhibited increased levels of phosphorylated mitogen-activated protein kinase 1/extracellular signal-regulated kinase 2 (MAPK1/ERK2/p42) and MAPK3/ERK1/p44. Mutant TRPM3-M65 channels displayed an increased sensitivity to external Ca2+ concentration and an altered dose response to pregnenolone sulfate (PS) activation. Trpm3-mutant lenses shared downregulation of genes involved in insulin/peptide secretion and upregulation of genes involved in Ca2+ dynamics. By contrast, Trpm3-deficient lenses did not replicate the pathophysiological changes observed in Trpm3-mutant lenses. Collectively, our data suggest that a cataract-causing substitution in the TRPM3 cation channel elicits a deleterious gain-of-function, rather than a loss-of-function, mechanism in the lens. Overall design: Mouse lens RNA samples (4 male and 4 female lenses per sample) were prepared in triplicate and cDNA libraries were synthesized, indexed, pooled, and sequenced.  Samples represented WT, Trpm3-KO, Trpm3-mutant (I/M), and Trpm3-mutant (M/M).