mRNA expression of the murine glycoprotein (transmembrane) nmb (Gpnmb) gene is linked to the developing retinal pigment epithelium and iris.

The murine homologue of the human glycoprotein (transmembrane) NMB (GPNMB) gene was identified by subtractive cloning from in vitro cultured murine primary osteoblast cells and subsequent RACE-PCR. GPNMB is a highly glycosylated type I transmembrane protein that shares significant sequence homology to several melanosomal proteins. Increasing expression of Gpnmb mRNA was observed during differentiation of murine primary osteoblast cell cultures. To address the potential functions of GPNMB we analysed its mRNA-expression during murine embryonic development. In early development Gpnmb mRNA is detected at high levels in the outer layer of the retina. Later in development expression gets restricted to the retinal pigment epithelium and iris. At the cytoplasmic domain of GPNMB, a conserved di-leucin-based endosomal/melanosomal-sorting signal (ExxPLL) was located, present as well in several known melanosomal proteins. To analyse the subcellular localization we used EGFP-tagged GPNMB transfected in COS7 and HEK293 cells. In both non-pigmented cell lines, the EGFP-GPNMB fusion protein was localized to vesicular, endosomal like structures. Sequence homology to known melanosomal proteins, mRNA expression and subcellular localization are suggestive for GPNMB as an intracellular, endosomal/melanosomal compartment specific protein important for melanin biosynthesis and the development of the retinal pigment epithelium and iris. As the gene coding for human GPNMB was localized to chromosome 7p15, a locus involved in the human inherited disease cystoid macular edema, also known as dominant cystoid macular dystrophy (OMIM 153880) we highly suggest that GPNMB is a candidate gene for this human inherited disease.