The expression and roles of inhibitor of DNA binding helix-loop-helix proteins in the developing and adult mouse retina.

Inhibitor of DNA binding (Id) proteins bind to and inhibit the function of basic helix-loop-helix (bHLH) transcription factors including those that regulate retinal development. However, little is known about the role of Id proteins in the growth and differentiation of the retina during development. The purpose of this study is to observe the expression of Id proteins in the developing and adult mouse retinas as the first step in investigating the functions of Id family members in the eye. The expression of Id1–4 was examined by real-time PCR, Western blot, and immunohistochemistry in wild-type and Id1/Id3 double-knockout mice. Id1–4 genes and proteins showed high expression levels in the retina at embryonic and early postnatal stages, whereas declined in the adult. Expression of Id proteins was observed in the inner neuroblastic layer (NBL) at embryonic (E) day 13.5 through 16.5. Id4 expression began at E18.5. By E18.5 and postnatal day 1, the expression of Id1–4 exhibited distinct yet overlapping patterns in the ganglion cell layer and inner part of NBL. In the adult, Ids were expressed in retinal ganglion cells, amacrine cells, bipolar cells, and horizontal cells. No Id expression was found in Müller cells. Id1 and Id3 double-knockout mice (Id1−/−/Id3−/−) showed smaller retinal size compared to wild-type or heterozygous littermates. However, histological analyses in Id1 and Id3 single-knockout retinas revealed no obvious defects in developmental phenotype. Our results indicate that the expression of the Id family may play an important role in regulating retinal progenitor cell proliferation and differentiation.