Role of Ptf1a and Modulation of Its Activity During Early Differentiation of the Chick Retina

Abstract

Purpose: : The pancreas transcription factor 1a (Ptf1a), is necessary for the specification of horizontal cells and the vast majority of amacrine cell subtypes. Here we show that, in contrast to rodents, misexpression of Ptf1a by itself promotes horizontal and amacrine cell fates in chick development. We therefore used this avian model to decipher the role of Ptf1a during retinal development and the modulation of its activity by its cofactors.

Methods: : Ptf1a expression was analyzed by qPCR and immunochemistry. Chick optic vesicles were injected at embryonic day 2 (E2) with an empty avian virus (RCAS) or a Ptf1a coding virus (RCAS-Ptf1a) and the fate of Ptf1a-infected RPC was analyzed by immunohistochemistry and flow cytometry. Similarly, various mutant forms of Ptf1a that are unable to interact with mammalian Suppressor of Hairless (RBP-J) or/and its paralogue RBP-L were misexpressed and the resulting phenotypes were analyzed.

Results: : Ptf1a is expressed transiently from E6 to E12 in post-mitotic neurons of the chick retina but, unlike rodents, is also detected in some mitotic PCNA-positive and BrdU-labeled progenitors during early neurogenesis. Misexpression of Ptf1a in chick eye did not induce precocious cell cycle exit as assayed by scoring the mitotic index but resulted in a modification of the representation of the different retinal cell types. Ptf1a misexpression induced at E7 a strong decrease in the number of brn3a-positive ganglion cells and visinin-positive photoreceptors while it increased the number of AP2a-amacrine and prox1-horizontal cells. Both lim1- and islet1-horizontal cell subtypes were found in the lineage of Ptf1a-infected RPC. By E12 Ptf1a misexpression resulted in a marked disorganization of the retina. Misexpression of various mutant forms of Ptf1a that are unable to interact with its partners had graded effects on cell specification.

Conclusions: : Together, our results indicate that in the chick retina Ptf1a misexpression by itself mirrors the effects of its invalidation in the mouse. Ptf1a misexpression is sufficient to promote horizontal/amacrine cell genesis and to inhibit ganglion cells differentiation. Moreover, progenitors that misexpress Ptf1a retain their ability to progress through cell cycle. Our study demonstrates that Ptf1a acts as part of a transcription complex containing endogenous RBP cofactors.

Keywords: retinal development • transcription factors • gene/expression