EXPRESSION AND TISSUE DISTRIBUTION OF RAT SULFATED GLYCOPROTEIN-1 (PROSAPOSIN)

Sulfated glycoprotein-1 (SGP-1/prosaposin) exists as a sulfated secret ed protein or as a lysosomal precursor of four smaller saposin molecul es. The protein exhibits ubiquitous expression, evolutionary conservat ion, and diverse tissue inducibility. The lysosomal form of SGP-1 play s a role in the hydrolysis of glycolipids and sphingomyelin. The funct ion of the secreted form of SGP-1 is still unclear. However, it could act as a glycolipid transfer protein, since several gangliosides (a se ries) were found to bind with high affinity to prosaposin. To identify cell types that produce SGP-1 mRNA, we constructed an SGP-1 cDNA and used for screening of different rat tissues by Northern blot analysis. To localize the translation product of SGP-1 transcripts, we immunost ained the same tissues with an anti-SGP-1 antibody. The SGP-1 cDNA con struct was generated by amplifying a rat testicular Zap cDNA library b y PCR (polymerase chain reaction) with two synthetic oligonucleotide p rimers. A positive signal of 1.7 KB was isolated, subcloned into the p GEM-7Zf (+). Sequence analysis showed a near-identical nucleotide and amino acid similarity to a previous rat SGP-1 cDNA. The majority of th e heterogeneites were conservative substitutions. Northern blot analys is demonstrated that all examined rat tissue and organs have SGP-1 mRN A. Immunocytochemistry identified two staining patterns in the cytopla sm of positive cells: (a) a granular reaction characteristic of lysoso mes in the supranuclear and basal regions of epithelial cells and in t he perinuclear region of neurons; and (b) a homgeneous reaction in the cytoplasm of Sertoli cells, Type II pneumocytes, macrophages, and epi thelial ails lining the choroid plexus. The latter staining pattern co uld be characteristic of cells that exhibit a secretory routing of SGP -1. The production of SGP-1 by a variety of specialized cells lining f luid compartments suggests that its secretd form has a role in the tra nsport of lipids in biological fluids, possibly by the formation of so luble complexes with glycolipids. Similarly, the lysosomal form of SGP -1/prosaposin and their derived saposins also solubilizes certain glyc olipids to promote their degradation by specific hydrolases.