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 (Ca 2+ ) 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 Ca 2+ 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 Ca 2+ concentration and an altered dose response to pregnenolone sulfate (PS) activation. Trpm3 -mutant lenses shared the downregulation of genes involved in insulin/peptide secretion and the upregulation of genes involved in Ca 2+ 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.