guanosine-5--o-(3-thiotriphosphate) and Night-Blindness

The Nougaret form of dominant stationary night blindness is linked to a G38D mutation in the rod transducin-alpha subunit (Talpha). In this study, we have examined the mechanism of Nougaret night blindness using transgenic mice expressing TalphaG38D. The biochemical, electrophysiological, and vision-dependent behavioral analyses of the mouse model revealed a unique phenotype of reduced rod sensitivity, impaired activation, and slowed recovery of the phototransduction cascade. Two key deficiencies in TalphaG38D function, its poor ability to activate PDE6 (cGMP phosphodiesterase) and decreased GTPase activity, are found to be the major mechanisms altering visual signaling in transgenic mice. Despite these defects, rod-mediated sensitivity in heterozygous mice is not decreased to the extent seen in heterozygous Nougaret patients.

Topics: Animals; Blotting, Western; Dark Adaptation; Disease Models, Animal; Electroretinography; Eye Proteins; Gene Expression; Guanosine 5'-O-(3-Thiotriphosphate); Immunohistochemistry; Membrane Potentials; Mice; Mice, Inbred C57BL; Mice, Transgenic; Night Blindness; Photic Stimulation; Retina; Retinal Rod Photoreceptor Cells; Sensory Thresholds; Transducin; Vision, Ocular

A missense mutation, G38D, was found in the rod transducin alpha subunit (Galpha(t)) in individuals with the Nougaret form of dominant stationary night blindness. To elucidate the mechanism of Nougaret night blindness, we have examined the key functional properties of the mutant transducin. Our data show that the G38D mutation does not alter the interaction between Galpha(t) and Gbetagamma(t) or activation of transducin by photoexcited rhodopsin (R*). The mutant Galpha(t) has only a modestly (approximately 2.5-fold) reduced k(cat) value for GTP hydrolysis. The GTPase activity of Galpha(t)G38D can be accelerated by photoreceptor regulator of G protein signaling, RGS9. Analysis of the Galpha(t)G38D interaction with cGMP phosphodiesterase revealed marked impairment of the mutant effector function. Galpha(t)G38D completely fails to bind the inhibitory PDE gamma subunit and activate the enzyme. Altogether, our results demonstrate a novel molecular mechanism in dominant stationary night blindness. In contrast to known forms of the disease caused by constitutive activation of the visual cascade, the Nougaret form has its origin in attenuated visual signaling due to loss of effector function by transducin G38D mutant.

Topics: Adenosine Diphosphate Ribose; GTP Phosphohydrolases; Guanosine 5'-O-(3-Thiotriphosphate); Humans; Mutagenesis, Site-Directed; Night Blindness; Transducin; Trypsin; Virulence Factors, Bordetella