Investigators: Ronald Roepman (group leader), Jeroen van Reeuwijk (post-doc), Dorus A. Mans (post-doc), Heleen H. Arts (graduate student), Karlien L.M. Coene (graduate student), Stef J.F. Letteboer (senior research technician), Sylvia E.C. van Beersum (research technician);
Usher syndrome: Hannie Kremer (co-group leader), Erwin van Wijk (graduate student), and Ferry F.J. Kersten (graduate student).

Summary:

Our challenging research goal is to functionally characterize proteins implicated in inherited retinal disorders, such as retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA). In addition, we study proteins that are disrupted in syndromes that include retinal degeneration as one of the characteristics, such as Usher syndrome (retinal degeneration and deafness) and Joubert syndrome (brain malformations, retinal degeneration, and kidney dysfunction). A common characteristic of these disorders is that the disrupted proteins are often localized in the basal body and microtubule-based axoneme of the photoreceptor “connecting” cilium, as well as in (primary) cilia of the overlapping affected tissues. As such, these disorders are often classified as “ciliopathies”.

The photoreceptor outer segment is basically a specialized primary cilium, where the photoreceptive opsin pigments are packed in stacks of about a 1000 membrane discs. The connecting cilium is the transition zone of this cilium that acts both as a gatekeeper and conveyor belt for transport from the inner segment, where all proteins and membranes are synthesized, to the outer segment. As 10% of the outer segments are shed each day to renew the outer segments, the logistic protein activity in this tiny connecting cilium region is immense. Important and well-studied basic players in this process are the intra flagellar transport proteins (IFTs) and motor proteins of the dynein and kinesin family. However, the total number of different proteins that are present in this small organelle, in the retina as well as other tissues, may be well over a thousand. A correct function therefore requires a well-balanced transition machinery, and disruption in any of the required components may lead to retinal degeneration due to a triggered apoptotic programme of the cells.

It is our initial aim to dissect the associated protein networks that are localized in the primary cilia of the cells, using yeast two-hybrid methods and proteomics approaches, and analyze their functional features using cell-based assays. Analysis of the complex interplay of these ciliary protein networks will provide essential knowledge of the molecular basis of blinding ciliopathies, which will pave the way for development of therapies.

Recent key publications:

Arts, H.H., Doherty, D., van Beersum, S.E., Parisi, M.A., Letteboer, S.J.F., Gorden, N.T., Peters, T.A., Märker, T., Voesenek, K., Kartono, A., Ozyurek, H., Farin, F.M., Kroes, H.Y., Wolfrum, U., Brunner, H.G., Cremers, F.P.M., Glass, I.A., Knoers, N.V.A.M., and Roepman, R. (2007). Mutations in the gene encoding the basal body protein RPGRIP1L, a nephrocystin-4 interactor, cause Joubert syndrome. Nat. Genet., 39, 882-888.

den Hollander, A.I., Koenekoop, R.K., Mohamed, M.D., Arts, H.H., et al., Cremers, F.P.M., Inglehearn, C.F., and Roepman, R. (2007). Mutations in LCA5, encoding the novel ciliary protein lebercilin, cause Leber congenital amaurosis. Nat. Genet., 39, 889-895.

Gosens, I, van Wijk, E., Kersten, F.F., Krieger, E., van der Zwaag, B., Märker, T., Letteboer, S.J.F., Dusseljee, S., Peters, T., Spierenburg, H.A., Punte, I.M., Wolfrum, U., Cremers, F.P.M., Kremer, H., and Roepman, R. (2007). MPP1 links the Usher protein network and the Crumbs protein complex in the retina. Hum. Mol. Genet., 16, 1993-2003.

van Wijk, E., van der Zwaag, B., Peters, T.A., Zimmerman, U., Märker, T., te Brinke, H., Kersten, F., Aller, E., Cremers, C.W., Cremers, F.P.M., Wolfrum, U., Knipper, M., Roepman, R.*, and Kremer, H.* (2006). The DFNB31 gene product whirlin connects to the Usher protein network in cochlea and retina by direct association with USH2A and VLGR1. Hum. Mol. Genet., 15, 751-765.*equal senior authors.

Roepman, R., Letteboer, S.J.F., Arts, H.H., van Beersum, S.E.C., Lu, X., Krieger, E., Ferreira, P.A., and Cremers, F.P.M. (2005). Interaction of nephrocystin-4 and RPGRIP1 is disrupted by nephronophthisis or Leber congenital amaurosis-associated mutations. Proc. Natl. Acad. Sci. USA, 102, 18520-18525.

Funding:

We are grateful to the following organisations for their financial support:

(NWO, Vidi-91786396)	(European Community FP6; LSHG-CT-2005 512036: Functional Genomics of the retina in health and disease)	(Dutch Kidney Foundation; C04.2112)
(Foundation for Retinal Research, USA)	(Foundation Fighting Blindness, USA)	(British Retinitis Pigmentosa Society, UK, #GR552)

Other relevant links: