Pph13 and Orthodenticle define a dual regulatory pathway for photoreceptor cell morphogenesis and function

Abstract

The function and integrity of photoreceptor cells are dependent upon the creation and maintenance of specialized apical structures: membrane discs/outer segments in vertebrates and rhabdomeres in insects. We performed a molecular and morphological comparison of Drosophila Pph13 and orthodenticle (otd) mutants to investigate the transcriptional network controlling the late stages of rhabdomeric photoreceptor cell development and function. Although Otd and Pph13 have been implicated in rhabdomere morphogenesis, we demonstrate that it is necessary to remove both factors to completely eliminate rhabdomere formation. Rhabdomere absence is not the result of degeneration or a failure of initiation, but rather the inability of the apical membrane to transform and elaborate into a rhabdomere. Transcriptional profiling revealed that Pph13 plays an integral role in promoting rhabdomeric photoreceptor cell function. Pph13 regulates Rh2 and Rh6, and other phototransduction genes, demonstrating that Pph13 and Otd control a distinct subset of Rhodopsin-encoding genes in adult visual systems. Bioinformatic, DNA binding and transcriptional reporter assays showed that Pph13 can bind and activate transcription via a perfect Pax6 homeodomain palindromic binding site and the Rhodopsin core sequence I (RCSI) found upstream of Drosophila Rhodopsin genes. In vivo studies indicate that Pph13 is necessary and sufficient to mediate the expression of a multimerized RCSI reporter, a marker of photoreceptor cell specificity previously suggested to be regulated by Pax6. Our studies define a key transcriptional regulatory pathway that is necessary for late Drosophila photoreceptor development and will serve as a basis for better understanding rhabdomeric photoreceptor cell development and function.

The transcription factor glass links eye field specification with photoreceptor differentiation in Drosophila

“…Both genes are required for rhabdomere formation and for the expression of several phototransduction genes (Vandendries et al, 1996;Tahayato et al, 2003;Zelhof et al, 2003;Mishra et al, 2010). Expression of Rh3 and Rh5 is directly regulated by Otd binding to their enhancers (Tahayato et al, 2003).…”

“…However, their mutant phenotypes are milder than that of glass (Vandendries et al, 1996;Tahayato et al, 2003;Zelhof et al, 2003;Mishra et al, 2010). Therefore, we tested whether hazy and otd act downstream of Glass.…”

“…It has been shown that rhabdomere formation, together with the expression of some of the proteins involved in phototransduction, is transcriptionally controlled by the redundant function of two homeodomain proteins: Orthodenticle (Otd) and Hazy. Both genes are expressed in all PRs and seem to act through separate pathways (Vandendries et al, 1996;Tahayato et al, 2003;Zelhof et al, 2003;Mishra et al, 2010). How the expression of Otd and Hazy is induced in PRs, and which transcription factors mediate between initial PR specification by the RDN and their final differentiation into functional PRs has not yet been resolved.…”

Analysis of the Drosophila Compound Eye with Light and Electron Microscopy

“…The rougheye phenotype of some calphotin alleles is the result of misoriented rhabdomeres and PRC death ( 18 ) . Mutations associated with genes like orthodenticle ( otd ) and PpH13 alter the shape of the rhabdomeres by changing the length/size of the microvilli, but keep photoreceptor number unaltered ( 19,20 ) . In hypomorphic alleles of the gene encoding Myosin Va or upon early expression of the membrane-bound cytoplasmic domain of Crumbs ectopic rhabdomeres form ( 21,22 ) .…”

Establishing and maintaining gene expression patterns: insights from sensory receptor patterning

“…Schematics shown in B and C were adapted with permission (Cagan and Ready, 1989). PRIMER Development 140 (3) homeodomain TF Pph13 activates Rh6 expression (Mishra et al, 2010). Repressive mechanisms then play a key role in restricting Rh expression to specific PR subtypes (Fig.…”