Auto and cross regulatory elements control Onecut expression in the ascidian nervous system

Abstract

The expression pattern of Onecut genes in the central and peripheral nervous systems is highly conserved in invertebrates and vertebrates but the regulatory networks in which they are involved are still largely unknown. The presence of three gene copies in vertebrates has revealed the functional roles of the Onecut genes in liver, pancreas and some populations of motor neurons. Urochordates have only one Onecut gene and are the closest living relatives of vertebrates and thus represent a good model system to understand its regulatory network and involvement in nervous system formation. In order to define the Onecut genetic cascade, we extensively characterized the Onecut upstream cis-regulatory DNA in the ascidian Ciona intestinalis. Electroporation experiments using a 2.5kb genomic fragment and of a series of deletion constructs identified a small region of 262bp able to reproduce most of the Onecut expression profile during embryonic development. Further analyses, both bioinformatic and in vivo using transient transgenes, permitted the identification of transcription factors responsible for Onecut endogenous expression. We provide evidence that Neurogenin is a direct activator of Onecut and that an autoregulatory loop is responsible for the maintenance of its expression. Furthermore, for the first time we propose the existence of a direct connection among Neurogenin, Onecut and Rx transcription factors in photoreceptor cell formation.

The central nervous system of ascidian larvae

“…The cell lineage of the photoreceptors in Ciona thus requires clarification. Specification of the ocellus‐associated photoreceptor lineages involves a Neurogenin ‐to‐ Onecut ‐to‐ Rx genetic cascade with Rx then activating Arrestin and Opsin1 gene expression . Nodal expression on the left side of the larvae represses Rx gene expression (and thus Arrestin and Opsin1 ), restricting photoreceptor cell formation to only the right side (Figure (b)).…”

Regulatory elements retained during chordate evolution: Coming across tunicates

“…Experiments were performed in triplicate, comparing at least 100 embryos for each single electroporated construct as described in Pezzotti et al . ().…”

“…LacZ staining of transgenic embryos was observed by histochemical detection of bgalactosidase activity. Experiments were performed in triplicate, comparing at least 100 embryos for each single electroporated construct as described in Pezzotti et al (2014).…”

The ascidian pigmented sensory organs: structures and developmental programs

“…As further step in this regulatory cascade, we recently demonstrated that the Neurogenin transcription factor is involved in the direct HNF6/OC1 activation in photoreceptor cells (Pezzotti et al, 2014). An autoregulatory loop has also been evidenced, as responsible for the maintenance of HNF6/OC1 expression in these territories (Pezzotti et al, 2014). It would be interesting to investigate if these regulatory mechanisms, that control the expression of the C. intestinalis OC gene in photoreceptor territories, are also conserved in vertebrates.…”