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Sasha Woods, Andrew Stewart, Cynthia Jung, Sarah Ross, Andrew Dick, Clea Warburton, Roderick McInnes, Denize Atan; Prdm8 and Bhlhb4 in the retina and CNS. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2478.
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© ARVO (1962-2015); The Authors (2016-present)
Understanding the genetic regulation of retinal development is key to advancing therapies for disease. Bhlhb4 is a transcription factor (TF) essential to the maturation of the major class of retinal interneurons, rod bipolar cells (RBs). Adult Bhlhb4-/- mice have b wave deficits on dark adapted ERGs, a defect of RB signalling that models the human disorder Congenital Stationary Night Blindness. RBs are born in normal numbers in Bhlhb4-/- retina but die from PN8, suggesting Bhlhb4 is required for RB survival. In the CNS, Bhlhb4 is expressed the developing forebrain and in adult cerebellum and hippocampus. We describe a second mouse model lacking the TF, Prdm8, that phenocopies the Bhlhb4-/- model. Both TFs regulate the final stages of RB development in the retina and have parallel functions in the regulation of interneuron development elsewhere in the CNS.
Prdm8 and Bhlhb4 expression in wildtype (wt) retina and CNS was analysed by immunofluorescence using cell specific markers. Retinal transcriptomes of Prdm8-/-, Bhlhb4-/- and wt mice at PN6 were compared using Affymetrix Murine Genome 430 2.0 arrays with a false discovery rate (FDR) of up to 10%. Differential gene expression between mutant and littermatched wt retina was quantified by rtPCR. Morphological differences between mutant and wt brains were analysed by MRI; and neurobehavioural phenotypes with SHIRPA screening tests and the Morris water maze.
Adult Prdm8-/- and Bhlhb4-/- retinas lack RBs and demonstrate b wave deficits on dark adapted ERGs, consistent with compromised RB circuitry. In both models, RBs are born in normal numbers but die apoptotically from PN8. Prdm8 and Bhlhb4 are co-expressed in RBs at PN5 to PN12 and from PN14 to adulthood. At PN6, Bhlhb4 is downregulated in Prdm8-/- retina (p=0.003) whilst Prdm8 expression is unchanged in Bhlhb4-/- retina (p=0.5). Non-overlapping genes are differentially expressed in Prdm8-/- and Bhlhb4-/- retina at PN6. In the latter, the most significantly downregulated genes, c-Rel and Neu3 (FDR=0%), are implicated in apoptosis. In the CNS, Prdm8 and Bhlhb4 are coexpressed in adult cerebellum and hippocampus. Prdm8-/- mice demonstrate neurobehavioural deficits and MRI changes consistent with impaired hippocampal and cerebellar development that overlap with deficits in Bhlhb4-/- mice.
We propose a model in which Prdm8 and Bhlhb4 coordinately regulate the development of specific interneuron subtypes in retina and brain.
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