Localization of cystatin mRNA in chicken brain by in situ hybridization

The cystatins are a superfamily of proteins that inhibit the lysosomal cysteine proteinases, cathepsins B, H, and L. Members of this superfamily have been found in all tissues and biological fluids analyzed. Previous studies have shown that chicken cystatin mRNA is abundant in brain tissue. In this study, a definitive localization of chicken cystatin mRNA in chicken brain was determined by in situ hybridization. Chicken cystatin mRNA was heavily concentrated in the secretory epithelial cells of the choroid plexus. The rest of the brain failed to show a hybridization signal above that of the control sense strand probe, even after long exposures. We conclude that chicken cystatin is synthesized predominantly by the specialized secretory epithelial cells of the choroid plexus and secreted into the cerebrospinal fluid (CSF). We postulate that chicken cystatin functions to regulate proteinase activity in the CSF and therefore may function as a protective factor for the cellular elements of the central nervous system.