Two transcription factors, Pou4f2 and Isl1, are sufficient to specify the retinal ganglion cell fate

Abstract

As with other retinal cell types, retinal ganglion cells (RGCs) arise from multipotent retinal progenitor cells (RPCs), and their formation is regulated by a hierarchical gene-regulatory network (GRN). Within this GRN, three transcription factors-atonal homolog 7 (Atoh7), POU domain, class 4, transcription factor 2 (Pou4f2), and insulin gene enhancer protein 1 (Isl1)-occupy key node positions at two different stages of RGC development. Atoh7 is upstream and is required for RPCs to gain competence for an RGC fate, whereas Pou4f2 and Isl1 are downstream and regulate RGC differentiation. However, the genetic and molecular basis for the specification of the RGC fate, a key step in RGC development, remains unclear. Here we report that ectopic expression of Pou4f2 and Isl1 in the Atoh7-null retina using a binary knockin-transgenic system is sufficient for the specification of the RGC fate. The RGCs thus formed are largely normal in gene expression, survive to postnatal stages, and are physiologically functional. Our results indicate that Pou4f2 and Isl1 compose a minimally sufficient regulatory core for the RGC fate. We further conclude that during development a core group of limited transcription factors, including Pou4f2 and Isl1, function downstream of Atoh7 to determine the RGC fate and initiate RGC differentiation.retinal development | neural development | transcription factors | cell fate specification | gene regulation A central question in neural development is how the extreme cellular diversity in the central nervous system arises from multipotent neural progenitors. The neural retina is an excellent system to address this question because of its well-defined structure and stereotypical cellular composition. The six neuronal cell types and one glial cell type (Müller glia) form a welllaminated tissue with the various types of cells positioned at distinct layers (1). Many of these cell types are composed of multiple subtypes with distinct functions (2). All cell types in the retina originate from a common pool of retinal progenitor cells (RPCs) following a distinct temporal order (3-5). The ordered births of the retinal cell types are caused by changes of competence in RPCs for the various retinal cell types (6). Both intrinsic and extrinsic mechanisms are involved in regulating the production of the various retinal cell types, but the intrinsic factors, mostly transcription factors, appear to play more deterministic roles in directing progenitor cells toward specific cell fates (5). Many such transcription factors have been identified by loss-and gain-of-function analyses, but these studies often fail to reveal the specific roles these factors play in the development of the cell types with which they are involved (7-9). RPCs are heterogeneous, as has been demonstrated by the nonuniform expression of many RPC genes (10-13). RPCs expressing specific genes, particularly those encoding transcription factors, although still multipotent, tend to be biased for certain retinal cell types. In a few cases, specific fac...

Essential Roles of Tbr1 in the Formation and Maintenance of the Orientation-Selective J-RGCs and a Group of OFF-Sustained RGCs in Mouse

“…When RPCs exit the cell cycle, the post-mitotic precursor cells go through a series of changes of transcriptional regulatory programs. Atoh7 first appears in a subset of precursor cells to provide a competency state for RGC differentiation (Yang et al, 2003, Brown et al, 1998), and subsequently, Pou4f2 and Isl1 begin to express in some of these cells and direct them to differentiate into RGCs (Wu et al, 2015, Mu et al, 2008, Pan et al, 2008). However, the onset of Tbr1 expression in RGCs starts at a much later stage, around P0, suggesting that some of the early-born RGCs are likely to stay at an uncommitted state or in a protractedly long transcriptional program.…”

SoxC Transcription Factors Promote Contralateral Retinal Ganglion Cell Differentiation and Axon Guidance in the Mouse Visual System

“…The subsequent differentiation of postmitotic RGCs is mediated by various TFs in monocular and binocular species (Kanekar et al, 1997; Wu et al, 2015). However, TFs selectively mediating contralateral or ipsilateral RGC cell fate and differentiation were not identified.…”

Expression and function of the LIM-homeodomain transcription factor Islet-1 in the developing and mature vertebrate retina

“…These data suggest that Isl1 and Brn3b are not required for the specification of this cell type, but play a key role in their differentiation and survival (Mu et al, 2008;Pan et al, 2008;Sapkota et al, 2011;Li et al, 2014). However, it has recently been shown that ectopic expression of Isl1 and Brn3b in the Atoh7-null retina is sufficient for the specification of the retinal ganglion cell fate (Wu et al, 2015), suggesting that these transcription factors compose a minimally sufficient regulatory core for the retinal ganglion cell fate. These authors suggest that ectopic Isl1 and Brn3b can activate the native Isl1 and Brn3b genes, determining the retinal ganglion cell fate.…”

“…These transcription factors sustain their own expression and no longer rely on the activator Atoh7. Then, Isl1 and Brn3b activate the gene expression program required for RGC differentiation (Wu et al, 2015). Brn3b binds to Isl1 within the C-terminal region forming a complex to regulate target genes in developing retinal ganglion cells (Li et al, 2014).…”