Abstract
Purpose: :
The connecting cilium of the vertebrate photoreceptor cell is an important transport route of proteins from the biosynthetically active inner segment to the light-sensitive outer segment. Malfunction of the connecting cilium causes degeneration of outer segments and leads to photoreceptor loss in the human retina, such as that seen in retinitis pigmentosa (RP) or the Bardet Biedl Syndrome (BBS). Given the importance of the connecting cilium in photoreceptor outer segment formation, we search for genes that regulate cilia differentiation.
Methods: :
To identify such genes, we perform forward genetic chemical mutagenesis screens in the zebrafish model. The mutant genes are cloned and analyzed using a variety of cell biological, genetic, embryological, as well as biochemical approaches.
Results: :
We have recently found a new zebrafish mutant, jj203, characterized by defects in the photoreceptor connecting cilium. While in wild-type animals the connecting cilium is well differentiated at 3dpf, in jj203 mutants it fails to form at this stage. Consistent with this finding, electron microscopy reveals the absence of outer segments in jj203 retina at 3dpf. Surprisingly, this defect is transient, and the connecting cilium starts to recover by 4dpf. Although well differentiated, mutant connecting cilia do not reach the wild-type size even at 7dpf. The recovery of the connecting cilium is accompanied by a delayed formation of abnormal outer segments. In addition to the eye, mutations of the jj203 locus lead to cilia defects in other organs, including the olfactory placode, the lateral line, the pronephric system and the ear. Unexpectedly, the cilia of auditory cristae, but not maculae, remain normal at least until 7dpf. Staining with several markers specific to photoreceptor subtypes in the retina reveals that rod but not cone photoreceptors die in jj203 mutant embryos. We identified the jj203 gene via the positional candidate approach and show that it encodes a motor protein. The recovery of outer segments in jj203 mutants suggests that another motor protein substitutes for the jj203 function at later stages of photoreceptor differentiation. Our results also suggest that ciliary transport mechanisms may function differently in rod and cone photoreceptor cells.
Conclusions: :
The zebrafish jj203 locus encodes a motor protein of key importance for outer segment differentiation.
Keywords: photoreceptors • genetics • degenerations/dystrophies