The phototransduction cascade is the fastest known biological process and among the most intensively studied of the G-protein regulated signaling pathways. Light activation of the rhodopsin receptor molecule triggers a conformational change that allows rhodopsin to bind and activate the heterotrimeric transducin complex. Activated transducin decreases intracellular levels of cGMP by activating the effector molecule, phosphodiesterase, thus regulating gating of the plasma membrane channel. These events occur within a 200-millisecond time frame.

Mutations in rhodopsin, transducin and phosphodiesterase have been associated with the human visual diseases, retinitis pigmentosa (RP). Currently, no treatment exists to arrest the progressively blinding disease, RP. One of the major goals of research in the Lem lab is to elucidate the molecular mechanisms triggering the photoreceptor cell death that occurs in the course of retinal degenerative diseases. Rhodopsin mutants account for the largest proportion of retinal degenerations of known genetic etiology. For that reason, we have focused on rhodopsin-mediated degenerations using a rhodopsin knockout mouse generated in our lab. In addition to studying rhodopsin mutations associated with human RP, we are investigating functional elements common to many members of the family of G-protein coupled receptors, such as the C-terminal phosphorylation sites involved in receptor inactivation, and the C-terminal palmitoylation site.

A second objective of studies in the Lem lab are to investigate the role of transducin in retinal degenerative disease and phototransduction signaling mechanisms. Using a transducin knockout mouse produced in our lab, we are studying transducin mutants with defects in their ability to interact with a member of the family of RGS (regulator of G-protein signaling) proteins and the inhibitory PDE subunit. Effects of these mutations on the photoresponse will be investigated using biochemical and electrophysiological methods. The transducin knockout mouse is also being used to test the ‘equivalent light hypothesis’, which proposes that some cases of RP cause retinal degeneration by mimicking continuous light exposure in the absence of light. It is well documented that rodents exposed continuously to light suffer retinal degeneration. Knockout mice are being crossed to existing retinally degenerate transgenic mice to assess whether degeneration occurs in a signal-dependent or signal-independent fashion. Should ‘equivalent light’ be a mechanism of degeneration, one would predict that such a cross would demonstrate signal-dependent degeneration.