Human retinal pigment epithelial SPARC expression and age: an immunohistochemical study

The aim of this study was to investigate the temporal and spatial expression of the matricellular protein SPARC (Secreted Protein, Acidic and Rich in Cysteine; also known as osteonectin) in human retinal pigment epithelial (RPE) cells, and compare the results with Bruch's membrane thickness, employing immuno-histochemistry. Eyes from 36 human donors, 16 being 65 years old, were included in the study. Intensity of SPARC immunoreactivity was evaluated using Aequitas Image Analysis software with two-way analysis of variance (ANOVA). Bruch's membrane thickness was assessed by profile analysis and the association between RPE SPARC immunoreactivity and Bruch's membrane thickness was investigated by fitting a linear mixed effects model to the data set. Intensity of SPARC immunostaining in RPE cells was significantly lower in older donors (p<0.05 and p<0.001 for posterior and peripheral RPE cells, respectively). The anatomical localisation of the RPE cells also affected the intensity of SPARC staining, which was lower in posterior compared to peripheral cells (p<0.01). No correlation was observed between SPARC immunoreactivity in RPE cells and the thickness of the underlying Bruch's membrane, in either posterior or peripheral regions. Our results suggest that RPE cell SPARC levels decline with age, a change that may play a role in the pathogenesis of age-related diseases such as age-related macular degeneration.