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Joseph Brzezinski, Ko Uoon Park, Thomas Reh; Mechanisms of retinal bipolar cell genesis in Blimp1 (Prdm1) mutant mice. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5136.
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© ARVO (1962-2015); The Authors (2016-present)
During development, Otx2 is expressed by cells that become photoreceptors and bipolar cells. The transcription factor Blimp1 (Prdm1) is transiently expressed in Otx2+ cells. Mice that lack Blimp1 generate excess bipolar cells at the expense of photoreceptors through unknown mechanisms. We tested how Blimp1 regulates photoreceptor versus bipolar cell fate choice.
We conducted Blimp1 lineage tracing experiments by crossing Blimp1-Cre mice to multiple Lox-stop-Lox reporter strains to test whether Blimp1 is made solely by Otx2 cells that become photoreceptors or whether it is made by all Otx2 cells and is subsequently repressed by cells that adopt bipolar identity. Next, we used two methods to test whether photoreceptors in Blimp1 conditional knockout (CKO) mice transdifferentiate into bipolar cells. First, we crossed Nrl-GFP mice, which specifically labels rods, to Blimp1 CKO mice to see if bipolars can be generated from existing rods. Second, we conducted EdU (5-ethynyl-2’-deoxyuridine) birthdating to determine whether bipolar cells in Blimp1 CKO mice can be generated embryonically; a time when photoreceptors, but not bipolar cells, are normally formed.
A substantial fraction of bipolar cells were seen in our Blimp1 lineage tracing experiments. Thus, Blimp1 downregulation may be required for bipolar cell formation. Premature Blimp1 downregulation, seen in Blimp1 CKO mice, may also cause existing photoreceptors to convert into bipolar cells. We tested this by crossing Nrl-GFP mice to Blimp1 CKO mice. Control mice lacked Nrl-GFP+ bipolar cells whereas Blimp1 CKO mice had many Nrl-GFP+ bipolars. This suggests that rods can directly convert into bipolar cells. If true, progenitors that become postmitotic embryonically should adopt bipolar fate. We conducted EdU birthdating experiments from embryonic day (E) 13.5 to E17.5 to test this. Birthdated bipolar cells were seen in all Blimp1 CKO retinas examined, but were absent from control mice as predicted.
Our data suggest that bipolar cells are generated from a subset of Otx2+ cells that rapidly downregulate Blimp1. In the absence of Blimp1, nascent photoreceptors cannot maintain their identity and transdifferentiate into bipolar cells. Together, these data suggest that cross-repressive interactions are required to stabilize photoreceptor and bipolar cell fates.
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