Abstract
Rods and cones are functionally and morphologically distinct. We previously identified N-myc downstream-regulated gene 1b (ndrg1b) in carp as a cone-specific gene. Here, we show that NDRG1b and its paralog, NDRG1a-1, contribute to photoreceptor outer segment (OS) formation in zebrafish. In adult zebrafish photoreceptors, NDRG1a-1 was localized in the entire cone plasma membranes, and also in rod plasma membranes except its OS. NDRG1b was expressed specifically in cones in the entire plasma membranes. In a developing retina, NDRG1a-1 was expressed in the photoreceptor layer, and NDRG1b in the photoreceptor layer plus inner nuclear layer. Based on our primary knockdown study suggesting that both proteins are involved in normal rod and cone OS development, NDRG1a-1 was overexpressed or NDRG1b was ectopically expressed in rods. These forced-expression studies in the transgenic fish confirmed the effect of these proteins on the OS morphology: rod OS morphology changed from cylindrical to tapered shape. These taper-shaped rod OSs were not stained with N,N’-didansyl cystine that effectively labels infolded membrane structure of cone OS. The result shows that rod OS membrane structure is preserved in these taper-shaped OSs and therefore, suggests that tapered OS morphology is not related to the infolded membrane structure in cone OS.
Highlights
Not known whether ndrg1a is ndrg1a-1 and/or ndrg1a-2, because two splice variants, ndrgia-1 and ndrg1a-2, were not distinguished in that study
Phylogenetic tree was constructed using determined carp NDRG1b amino acid sequence and those of other NDRG family proteins in the data base including NDRG1, NDRG2, NDRG3 and NDRG4 in other animals (Supplementary Fig. S1b). All of these results supported the notion that carp NDRG1L was the ortholog of zebrafish NDRG1b, which in turn suggests that zebrafish NDRG1b is expressed in zebrafish cones
We showed that NDRG1 family proteins are expressed in rod and cone photoreceptors in adult zebrafish, their subcellular localizations are different among the three
Summary
Not known whether ndrg1a is ndrg1a-1 and/or ndrg1a-2, because two splice variants, ndrgia-1 and ndrg1a-2, were not distinguished in that study. Zebrafish ndrg1b gene is preferentially expressed in the eye (http://zfin.org/action/ figure/all-figure-view/ZDB-PUB-040907–1?probeZdbID=ZDB-CDNA-040425–2588)[11]. We cloned mRNAs of zebrafish NDRG1 family proteins, ndrg1a-1, ndrg1a-2 and ndrg1b, and examined their protein localization in zebrafish rods and cones. Our study using morpholinos seemed to suggest involvement of ndrg1a-1 and ndrg1b in normal development of rods and cones. Ndrg1a-1, which is expressed in both zebrafish rods and cones, was overexpressed in rods, and ndrg1b, which is expressed in zebrafish cones as in carp, was ectopically expressed in zebrafish rods. Our results suggest that NDRG1 family proteins are involved in forming a normal OS shape in zebrafish photoreceptors
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