The overall objective of this project is to investigate the molecular basis of signal transduction in human melanopsin (MO), the recently discovered light-receptor in photosensitive retinal ganglion cells, which underlies the control of circadian rhythms and pupillary response. Because melanopsin is involved in a variety of physiological functions including sleep, mental alertness, eating habits, and hormonal levels, as well potentially involved in a variety of disorders including sleep disorders, seasonal affected disorders and glaucoma, the NIH has highlighted melanopsin as a priority for future research. Remarkably, the properties of melanopsin strongly resemble invertebrate rhodopsin instead of the extensively studied vertebrate """"""""visual"""""""" rhodopsins. Similarities include a close primary sequence homology and signaling through the Gq-protein (phospholipaseC/inositol triphosphate) pathway instead of the Gt-protein cyclic nucleotide pathway. Importantly, melanopsin and its analog invertebrate rhodopsins such as squid rhodopsin (sRh) serve as models for investigating the signal transduction mechanism in the hundreds of GPCRs in human cells. Such GPCRs signal through the Gq-protein pathway and are inhibited by 2-arrestin2 instead of the more specialized visual 2-arrestin. Prominent examples include serotonin, histamine, adrenergic, muscarinic and calcitonin receptors which are targets of current and potentially new drugs. A key feature of melanopsin and invertebrate rhodopsins but not vertebrate rhodopsins is their optical bistability. This property allows them to be """"""""cycled"""""""" between two different stable states using two different colors of light. In this project, we will exploit this two-photon property in order to investigate the detailed structural changes occurring upon light activation in melanopsin, sRh and the complexes they formed with 2- arrestin2 and Gq-protein. This research will be facilitated by the application of several advanced FTIR difference techniques, many developed in our laboratory, in conjunction with site-directed mutagenesis and isotope labeling. Application of this approach has led previously to several milestones including the first detailed characterization of the conformational changes which occur during vertebrate rhodopsin photoactivation and the proton pumping mechanism of bacteriorhodopsin. We have recently demonstrated the ability of this approach to also detect and characterize structural changes in key residues and internal water molecules that lie in the interfacial contact region between membrane protein signaling receptors such as sensory rhodopsin II and its cognate transducer. HtrII In preliminary studies, we have measured static and time resolve FTIR difference spectra of squid rhodopsin and its 2-arrestin2 complex. By using isotope editing, we can characterize conformational changes separately in the receptor and 2-arrestin2 components. The proposed studies will also benefit from our recent development of methods to: i) measure sub-picosecond protein changes;ii) probe minute quantities of membrane proteins including single crystals using time-resolved FTIR microscopy and iii) rapidly in vitro express membrane proteins in nanolipoparticles (NLPs). This work will be facilitated by close collaborations with the laboratories of Dr. J. Navarro at the University of Texas Medical Branch, Galveston who will prepare sRho/2-arrestin2 crystals and perform parallel x-ray crystallographic studies;Dr. W. DeGrip at the University of Nijmegen whose laboratory has expressed and characterized functional recombinant melanopsin and Dr. M. Coleman at the LLNL and UC Davis whose laboratory has developed cell-free techniques to express GPCRs in NLPs.
The overall objective of this project is to investigate the mechanism by which human melanopsin, the recently discovered light-receptor in the retina, controls the body's internal clock as well as pupillary response. Understanding melanopsin is important because it is involved in key physiological processes including sleep, mental alertness, eating habits, and hormonal levels as well as disorders involving these processes. The application of advanced infrared spectroscopic methods developed in our laboratory will allow us to determine the detailed molecular response of melanopsin and the complexes it forms with other proteins to light on time scales as short as one trillionth of a second.
