Expression of the Opa1 mitochondrial protein in retinal ganglion cells:: Its downregulation causes aggregation of the mitochondrial network

PURPOSE. Mutations in the mitochondrial dynamin-related GTPase OPA1 cause autosomal dominant optic atrophy (ADOA), but the pathophysiology of this disease is unknown. As a first step in functional studies, this study was conducted to evaluate the expression of Opa1 in whole retina and in isolated retinal ganglion cells (RGCs) and to test the effects of Opa1 downregulation in cultured RGCs. METHODS. Opa1 mRNA isoforms from total retina and from RGCs freshly isolated by immunopanning were determined by RT-PCR. Protein expression was examined by immunohistochemistry and Western blot with antibodies against Opa1 and cytochrome c, and the mitochondrial network was visualized with a mitochondrial marker. Short interfering ( si) RNA targeting OPA1 mRNAs were transfected to cultured RGCs and mitochondrial network phenotypes were followed for 15 days, in comparison with those of cerebellar granule cells (CGCs). RESULTS. Opa1 expression did not predominate in rat postnatal RGCs as found by immunohistochemistry and Western blot analysis. The pattern of mRNA isoforms was similar in whole retina and RGCs. After a few days in culture, isolated RGCs showed fine mitochondrial punctiform structures in the soma and neurites that colocalized with cytochrome c and Opa1. Opa1 knockdown in RGCs induced mitochondrial network aggregation at a higher rate than in CGCs. CONCLUSIONS. Results suggest that the level of expression and the mRNA isoforms do not underlie the vulnerability of RGCs to OPA1 mutations. However, aggregation of the mitochondrial network induced by the downregulation of Opa1 appears more frequent in RGCs than in control CGCs.