A Proteomics Approach to Identify Candidate Proteins Secreted by Muller Glia that Protect Ganglion Cells in the Retina

The retinal Muller glial cells, can enhance the survival and activity of neurons, especially of retinal ganglion cells (RGCs), which are the neurons affected in diseases such as glaucoma, diabetes, and retinal ischemia. It has been demonstrated that Muller glia release neurotrophic factors that support RGC survival, yet many of these factors remain to be elucidated. To define these neurotrophic factors, a quantitative proteomic approach was adopted aiming at identifying neuroprotective proteins. First, the conditioned medium from porcine Muller cells cultured in vitro under three different conditions were isolated and these conditioned media were tested for their capacity to promote survival of primary adult RGCs in culture. Mass spectrometry was used to identify and quantify proteins in the conditioned medium, and osteopontin (SPP1), clusterin (CLU), and basigin (BSG) were selected as candidate neuroprotective factors. SPP1 and BSG significantly enhance RGC survival in vitro, indicating that the survival-promoting activity of the Muller cell secretome is multifactorial, and that SPP1 and BSG contribute to this activity. Thus, the quantitative proteomics strategy identify proteins secreted by Muller glia that are potentially novel neuroprotectants, and it may also serve to identify other bioactive proteins or molecular markers.