Dusanka Deretic, PhD
Our research focuses on the understanding of the effects of the disease-causing mutations on membrane trafficking and photoreceptor health by studying the cellular function of the C-terminal domain of rhodopsin, mutations in which cause particularly severe forms of Autosomal Dominant Retinitis Pigmentosa (ADRP). Significantly, we found that the molecular machinery engaged by rhodopsin appears to be conserved in trafficking of other ciliary proteins, suggesting a common mechanism that underlies retinal and renal cystic diseases, and possibly other ciliopathies.
Recently, we uncovered one of the molecular mechanisms by which omega-3 docosahexaenoic acid (DHA) regulates rhodopsin trafficking and contributes to photoreceptor health. We found that the pairing of SNARE proteins, which are fusion regulators essential for rhodopsin trafficking, depends on free DHA. These studies are timely, given that the Age-Related Eye Disease Study 2 (AREDS2) conducted by the Clinical Trials Branch of the National Eye Institute includes a randomized trial of DHA in Age-Related Macular Degeneration (AMD).
The rhodopsin C-terminal amino acids VxPx comprise a sorting motif that regulates the budding of RTCs. The VxPx motif binds the small GTPase Arf4 and regulates its association with the trans-Golgi network (TGN), which is the site of assembly and function of a ciliary targeting complex. This complex is comprised of two small GTPases, Arf4 and Rab11, the Rab11/Arf effector FIP3, and the Arf GTPase-activating protein ASAP1. ASAP1 mediates GTP hydrolysis on Arf4 and acts as an Arf4 effector that regulates budding of post-TGN carriers, along with FIP3 and Rab11. As the VxPx motif is present in other ciliary membrane proteins, the Arf4-based targeting complex is likely a part of conserved machinery involved in the selection and packaging of cargo destined for the delivery to the cilium.Polarized membrane trafficking in retinal photoreceptors involves synthesis, sorting and transport, through the rod inner segments (RIS), of prodigious quantity of rhodopsin-laden membranes. They continuously renew specialized organelles filled with light sensitive disk membranes, the rod outer segments (ROS). Biosynthetic organelles are localized in the RIS, in the myoid region. Rhodopsin transport carriers (RTCs) traverse the ellipsoid region filled with mitochondria and fuse with the RIS plasma membrane only in the proximity of the cilium. Newly synthesized membranes are then delivered to the ROS.
Wang J, and Deretic D. (2015) The Arf and Rab11 effector FIP3 acts synergistically with the Arf GAP ASAP1 to direct Rabin8 in ciliary receptor targeting. J Cell Sci. J Cell Sci jcs.162925; Advance Online Article February 11, 2015, doi:10.1242/jcs.162925 (2015).
Wang J. and Deretic D (2014). Molecular complexes that direct rhodopsin transport to primary cilia. Prog Retin Eye Res. 2014 Jan;38:1-19. doi: 10.1016/j.preteyeres.2013.08.004. Epub 2013 Oct 14.
Deretic D. (2013) Crosstalk of Arf and Rab GTPases en route to cilia. Small GTPases. 2013 Apr-Jun;4(2):70-7. doi: 10.4161/sgtp.24396. Epub 2013 Apr 1.
Wang J, Morita Y, Mazelova J and Deretic D. (2012). Ciliary receptor targeting through the Arf GAP ASAP1, a scaffold linking Arf4 to the Rab11-Rab8 ciliogenesis cascade. EMBO J 31, 4057-4071.
Mazelova J, Ransom N, Astuto-Gribble L, Wilson MC and Deretic D. (2009) Syntaxin 3 and SNAP-25 pairing, regulated by omega-3 docosahexaenoic acid (DHA), controls the delivery of rhodopsin for the biogenesis of cilia-derived sensory organelles, the rod outer segments. J. Cell Sci. 122, 2003-2013.
Mazelova J, Astuto-Gribble L, Inoue H, Tam BM, Schonteich E, Prekeris R, Moritz OL, Randazzo PA and Deretic D. (2009) Ciliary targeting motif VxPx directs assembly of a trafficking module through Arf4. EMBO J. 28, 183-192.
Deretic D, Williams AH, Ransom N, Morel V, Hargrave PA and Arendt A. (2005) Rhodopsin C-terminus, the site of mutations causing retinal disease, regulates trafficking by binding to ARF4. Proc. Natl. Acad. Sci. USA. 102:3301-3306.