Abstract
Purpose: :
The Gpnmb gene encodes a type I transmembrane protein that has been reported to be expressed in melanocytes, dendritic cells, and a small list of other cell types. In mice, Gpnmb mutation contributes to a pigment dispersing iris disease that ultimately results in a form of pigmentary glaucoma. The goal of this experiment was to further characterize the biological functions of Gpnmb by characterizing the ocular distribution of GPNMB protein, testing the involvement of Gpnmb in candidate genetic pathways via epistasis, and assessing gene expression profiles associated with Gpnmb mutation..
Methods: :
In order to perform experiments that were stringently controlled for genetic background, we utilized congenic strains of mice in which the DBA/2J–derived GpnmbR150X mutation has been transferred to the C57BL/6J genetic background. These B6 congenic strains deficient for Gpnmb were compared to wild type C57BL/6J mice utilizing a variety of anti–GPNMB antibodies, genetic crosses, and gene expression assays.
Results: :
Antibody labeling demonstrates GPNMB is expressed in multiple tissues, strikingly including the iris and RPE. Whereas Rag1 deficiency had no influence on Gpnmb phenotypes, Tyr mutation completely rescued the Gpnmb mutant phenotype. Western blots of purified melanosome preparations demonstrate the presence of GPNMB within iris melanosomes. Quantitative RT–PCR assays of the 67 genes whose products are also known to be present in melanosomes demonstrate that Gpnmb mutation does not influence the expression of key genes such as Tyrp1, Tyr, or Si, but does result in a signature suggestive of oxidative stress.
Conclusions: :
Previous investigations of the pigment dispersing iris disease of DBA/2J mice have suggested that Gpnmb contributes to this disease via both melanosomal and immune mediated mechanisms. The current experiments strongly support a melanosomal role for Gpnmb and suggest that its influence on ocular immune parameters likely does not depend on adaptive immunity. Further experiments are warranted to investigate the suggestion that Gpnmb may play a role in reactions involving oxidative stress, and to address whether these pathways contribute to human pigmentary glaucoma.
Keywords: melanocytes • genetics • gene/expression