Combinatorial regulation of a Blimp1 (Prdm1) enhancer in the mouse retina

Abstract

The mouse retina comprises seven major cell types that exist in differing proportions. They are generated from multipotent progenitors in a stochastic manner, such that the relative frequency of any given type generated changes over time. The mechanisms determining the proportions of each cell type are only partially understood. Photoreceptors and bipolar interneurons are derived from cells that express Otx2. Within this population, Blimp1 (Prdm1) helps set the balance between photoreceptors and bipolar cells by suppressing bipolar identity in most of the cells. How only a subset of these Otx2+ cells decides to upregulate Blimp1 and adopt photoreceptor fate is unknown. To understand this, we investigated how Blimp1 transcription is regulated. We identified several potential Blimp1 retinal enhancer elements using DNase hypersensitivity sequencing. Only one of the elements recapitulated Blimp1 spatial and temporal expression in cultured explant assays and within the retinas of transgenic mice. Mutagenesis of this retinal Blimp1 enhancer element revealed four discrete sequences that were each required for its activity. These included highly conserved Otx2 and ROR (retinoic acid receptor related orphan receptor) binding sites. The other required sequences do not appear to be controlled by Otx2 or ROR factors, increasing the complexity of the Blimp1 gene regulatory network. Our results show that the intersection of three or more transcription factors is required to correctly regulate the spatial and temporal features of Blimp1 enhancer expression. This explains how Blimp1 expression can diverge from Otx2 and set the balance between photoreceptor and bipolar fates.

Prdm13 is required for Ebf3+ amacrine cell formation in the retina

“…This was reinforced by our observation that some Ptf1a+ and Prdm13+ cells transiently co-expressed Otx2. This overlap is consistent with data suggesting that Otx2+ cells can adopt amacrine and horizontal cell identities (Baas et al, 2000; Brzezinski et al, 2013; Das et al, 2009; Mills et al, 2017). It has been shown that the Ptf1a+ lineage contains only horizontal and amacrine cells (Fujitani et al, 2006).…”

“…Since the Ptf1a lineage lacks photoreceptors (Fujitani et al, 2006), these double and triple labeled Otx2+ cells are already committed to becoming horizontals and/or amacrine cells. This plasticity in the Otx2+ lineage has been observed previously (Baas et al, 2000; Brzezinski et al, 2013; Das et al, 2009; Mills et al, 2017). We also co-labeled sections with antibodies against the ganglion cell marker Brn3 (Xiang et al, 1995).…”

Small molecule Photoregulin3 prevents retinal degeneration in the RhoP23H mouse model of retinitis pigmentosa

“…To further evaluate a role for PR3 as an RORb antagonist, we analyzed downstream RORb target gene expression in developing retina. One target that is specific to RORb, but not regulated by Nr2e3 is Prdm1 ( Liu et al, 2017 ; Wang et al, 2014 ; Mills et al, 2017 ). Prdm1 expression was unchanged in P0 retinal explants treated with PR3 ( Figure 1—figure supplement 1E ), supporting our hypothesis that the effects of PR3 are primarily mediated through modulation of Nr2e3.…”

How to lose your inheritance

“…What is the regulatory relationship between OTX2 and other transcription factors that act in developing PGCs? In the development of the mouse eye 11 , OTX2 drives the expression of BLIMP1 in a process linked to a switch between two types of cell fate, yet in the PGCs, BLIMP1 seems to function after OTX2 levels have declined.…”