The Transcription Factor PRDM8 Is Required for Rod and Cone Bipolar Cell Maintenance

Abstract

Purpose:: We identified PRDM8 (PR-domain containing 8) as an abundant cDNA in a screen for retinal transcription factors. Prdm8 is a putative zinc finger transcription factor, conserved among vertebrates. At least 5 other PR-domain family members are critical regulators of cell fate and maintenance in other model systems, suggesting that Prdm8 may play a comparable role in the developing retina. To define the roles of Prdm8 in the developing and mature retina, we generated a Prdm8 null mouse.

Methods:: Prdm8 expression in WT mice was characterized by Northern blot and RNA in situ hybridization. The Prdm8 ORF was disrupted with a nuclear-localized eGFP reporter (Prdm8^GFP/GFP) to generate a loss of function allele. Cell-specific immunohistochemical markers were used to determine the effect of Prdm8 loss-of-function on the retina.

Results:: During retinal development, Prdm8 expression is low in the neuroblast layer, but expression is pronounced in differentiating cells. In adult mouse, the Prdm8 transcript is expressed predominantly in the nervous system, most notably in the retina, hippocampus and cerebellum. Heterozygous and homozygous mutants are viable and express eGFP in retina and brain. eGFP is expressed by all major cell populations of the retina, with highest expression in rod bipolar cells and other INL cells. Early in life (P6), following bipolar cell genesis, Prdm8^GFP/GFP retinas appear morphologically normal. Prdm8 is not required for bipolar cell specification because cells of the INL of mutant retinas uniformly express the pan-bipolar marker Chx10, and the number of Chx10 (+) cells is comparable to WT. In contrast, adult Prdm8^GFP/GFP mutant retinas exhibit abnormal retinal morphology; the INL is hypocellular and the inner and outer plexiform layers are disorganized. The mutant retinas have a complete loss of rod (PKCα +) bipolar cells and apparently, of Type 2 OFF (recoverin, bHLHb5+) cone bipolar cells, while other cone bipolar cell types are being examined. Finally, Prdm8 mutants exhibit significant b-wave deficits on ERGs (see abstract by M. Klein et al). In addition to retinal defects, homozygous mutants display skin lesions with high penetrance, which we believe develop from excessive grooming.

Conclusions:: We conclude that Prdm8 is required for the maintenance of rod bipolar cells and probably of Type2 OFF cone bipolar cells. Defects in the human PRDM8 gene may be associated with congenital stationary night blindness phenotypes.

Keywords: development • bipolar cells • electroretinography: non-clinical