Opticin binds retinal growth hormone in the embryonic vitreous.

Abstract

Opticin is a small leucine-rich repeat proteoglycan that is abundant in several ocular tissues, including the vitreous. Like other proteoglycans, opticin may have the ability to bind and regulate the release of growth factors. Previous work has shown that isoforms of growth hormone (GH) are present in the embryonic retina, where they may act as a growth factor. The current study was conducted to investigate the possibility that opticin binds retinal GH in the vitreous of the chick embryo. The vitreous and retina of embryonic day-8 chicks were examined for the presence of opticin and GH, by Western immunoblot analysis, coimmunoprecipitation, and immunocytochemistry. Opticin associated with GH in the embryonic vitreous to produce a 60- to 62-kDa complex. Opticin and GH were also colocalized in the retina in retinal ganglion cells. The binding of retinal GH by opticin in the vitreous suggests that GH, secreted by the retinal ganglion cells, may be sequestered and concentrated in the vitreous and could act there as a paracrine differentiation factor in ocular development. During development, opticin could therefore regulate growth factor-like actions of retinal GH, both in the vitreous and the retinal ganglion cells. The physiological roles of GH in this location remain to be determined, but may include the regulation of cell proliferation and cell death.