γ-Synuclein as a marker of retinal ganglion cells

PURPOSE Previous studies have described gamma-synuclein as a protein highly expressed in retinal ganglion cells (RGCs), and a loss of RGCs correlates with a downregulation of gamma-synuclein gene expression in glaucoma. Here we asked whether gamma-synuclein expression in the retina can be considered a specific marker of RGCs. METHODS gamma-Synuclein expression was examined with immunohistochemistry in retinal sections from normal and glaucomatous human eyes. Primary cultures of RGCs from Sprague-Dawley rats purified by sequential immunopanning using a monoclonal antibody to Thy1-1, cultures of A7 immortalized optic nerve astrocytes from newborn rats, and the immortalized RGC-5 cell line were studied using immunofluorescence and quantitative RT-PCR. RESULTS gamma-Synuclein was highly expressed in RGCs in the human retina and was localized in cytoplasm adjacent to the RGC nuclear marker, Brn-3a. Axons of RGCs were immunopositive for gamma-synuclein in the nerve fiber layer (NFL), the lamina cribrosa and the retrobulbar optic nerve. In the optic nerve of glaucoma patients, axon swellings were likewise immunopositive, whereas in the retina of patients with retinoblastoma, NFL staining appeared reduced. In primary rat RGCs and in immortalized RGC-5 cultures, gamma-synuclein was localized predominantly in the perinuclear area and in cell processes. Among rat retinal cells in culture, all Brn-3a positive cells were stained with a gamma-synuclein antibody; rare gamma-synuclein-positive cells were not stained by the Brn-3a antibody. CONCLUSIONS gamma-Synuclein is selectively and abundantly expressed in human RGCs in vivo, primary rat RGCs in vitro, and immortalized RGC-5 cells. In pathology, gamma-synuclein abundance may vary between RGC somas and axons. Coincident Brn-3a and gamma-synuclein expression suggests that strong gamma-synuclein expression can be considered a marker of RGCs. Future translational approaches might include using a gamma-synuclein promoter for the specific delivery of siRNA or therapeutic proteins to RGCs.