A Cataract-Causing Mutation in the TRPM3 Cation Channel Disrupts Calcium Dynamics in the Lens

Area of Science:

• Molecular Biology
• Genetics
• Ophthalmology

Background:

• TRPM3 (transient receptor potential melastatin 3) is a calcium ion (Ca²⁺) channel involved in heat sensitivity and steroid activation.
• A specific missense mutation (p.I65M) in the TRPM3 gene is linked to inherited, early-onset progressive cataracts in humans and mice.

Purpose of the Study:

• To investigate the pathogenetic mechanisms of the cataract-causing TRPM3 mutation using a 'knock-in' mouse model and human cell lines.
• To determine if the mutation leads to a gain-of-function or loss-of-function of the TRPM3 channel in the context of cataract formation.

Main Methods:

• Generation and analysis of 'knock-in' mutant mice and human lens epithelial cell lines (HLE-B3) carrying the TRPM3 p.I65M mutation.
• Measurement of intracellular ion concentrations (Ca²⁺, Na⁺, K⁺), water content, and gene expression profiles in mutant and wild-type lenses.
• Biochemical analysis of TRPM3 channel activity, including sensitivity to Ca²⁺ and activation by pregnenolone sulfate (PS).

Main Results:

• Homozygous mutant lenses showed elevated cytosolic Ca²⁺, altered Na⁺/K⁺ balance, and increased water content.
• Mutant lenses and cell lines exhibited increased phosphorylation of MAPK1/ERK2 and MAPK3/ERK1.
• Mutant TRPM3 channels displayed heightened Ca²⁺ sensitivity and altered PS dose-response, while TRPM3-deficient lenses did not show cataract phenotypes.

Conclusions:

• The cataract-causing TRPM3 mutation results in a gain-of-function, characterized by altered channel activity and cellular ion imbalance.
• These findings implicate TRPM3 dysfunction as a key factor in the pathogenesis of this inherited form of cataract.