RD3: a challenge and a promise

Abstract

Photoreceptor (PR) cells are highly specialized cells that convert light into electrical signals. Ten percent of their outer segment (OS) membranes (approximately 77 cm² of membrane) are renewed every day. Therefore, PR cells must possess an extraordinary trafficking system to provide all of the needed material to build up the OS discs through a 0.3 µm diameter connecting cilium. The mechanism of trafficking of membrane proteins in the retina and corresponding degenerative diseases is still elusive. The retinal degeneration(rd3) is the gene responsible for a murine autosomal recessive hereditary retinal degeneration, which is known as Leber Congenital Amaurosis 12 (LCA12). Degeneration starts at about two weeks of age and is completed between 2-4 months. We generated the first antibody against this protein and by a protein-protein interaction analysis discovered that RD3 protein directly interacts with guanylate cyclase 1 (GC1) and partially expresses in the OS. We also detected the major binding site between these two proteins and realized that RD3 is directly involved in trafficking of this crucial protein. In a separate study, we reported that RD3 negatively regulates GC1, which is crucial for efficient trafficking of GC1 during the trafficking path, and RD3 prevents unnecessary production of cGMP. It is possible that RD3 is still involved in regulating GC1 even after targeting. Several mutations that cause visual difficulties have been reported for the mouse and human ortholog of RD3. The symptoms these mutations cause are very similar to those reported for a more severe form of blindness referred to as LCA1. Therefore, RD3 might cause a broader range of retinal diseases. Gene replacement of RD3 has shown to restore the GC1 across the retina. This makes RD3 a novel therapeutic target for retinal targeting impaired degenerative diseases.

Keywords: Guanylate cyclase1; LCA12; Leber Congenital Amaurosis 1 (LCA1); Protein trafficking; Retina; Retina degeneration3.

Figure 1

RD3 regulates GC1 in either of the following possible mechanisms: (A) the RD3-GC1 second binding site (see the text) competes with one or a combination of the GC1-GCAP1 binding site [26], and consequently negatively regulates the GC1 by preventing binding of GCAP1 to GC1. The major RD3-GC1 binding sites (amino acid 1055–1068) [13] regulate the targeting of GC1 to photoreceptor outer segment; or (B) RD3 interacts with GCAP1 and causes conformational changes in this protein; therefore, GCAP1 cannot activate GC1. In this model, the RD3-GC1 main binding site (amino acid 1055–1068) regulates the targeting of GC1 to outer.