Cataracts and microphthalmia caused by a Gja8 mutation in extracellular loop 2

Area of Science:

• Ophthalmology
• Genetics
• Cell Biology

Background:

• The Gja8(R205G) mutation in mice leads to variable cataracts and smaller lenses, with homozygous mice exhibiting severe phenotypes.
• This mutation affects the connexin 50 (Cx50, also known as α8 connexin) protein, specifically altering its second extracellular loop.

Purpose of the Study:

• To investigate the molecular mechanisms by which the Gja8(R205G) mutation causes cataracts and microphthalmia.
• To elucidate the differential effects of the Cx50-R205G mutant protein on Cx50 and Cx46 (α3 connexin) gap junction function.

Main Methods:

• Immunohistology to examine protein localization and phosphorylation levels.
• Genetic analysis to determine the necessity of wild-type Cx46 for the observed phenotype.
• Electrophysiology in Xenopus oocytes to assess gap junction channel function.

Main Results:

• Cx50-R205G mutant proteins and wild-type Cx46 proteins formed abnormal aggregates instead of functional gap junctions.
• Phosphorylation of Cx46 was reduced in mutant lenses.
• The Cx50-R205G mutation required wild-type Cx46 to disrupt lens fiber and epithelial cells.
• Cx50-R205G mutant proteins blocked wild-type Cx50 channels but only affected the gating of wild-type Cx46 channels, indicating they cannot form functional channels alone.

Conclusions:

• The Gja8(R205G) mutation impairs Cx50 and Cx46 function differentially, leading to cataracts, small lenses, and microphthalmia.
• The findings provide molecular insights into Gja8-related eye abnormalities in both mice and humans, as the mutation site is conserved.