Abstract
Sulfated glycoprotein-1 (SGP-1/prosaposin) exists as a sulfated secreted protein or as a lysosomal precursor of four smaller saposin molecules. The protein exhibits ubiquitous expression, evolutionary conservation, and diverse tissue inducibility. The lysosomal form of SGP-1 plays a role in the hydrolysis of glycolipids and sphingomyelin. The function of the secreted form of SGP-1 is still unclear. However, it could act as a glycolipid transfer protein, since several gangliosides (a series) 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 immunostained the same tissues with an anti-SGP-1 antibody. The SGP-1 cDNA construct was generated by amplifying a rat testicular Zap cDNA library by PCR (polymerase chain reaction) with two synthetic oligonucleotide primers. A positive signal of 1.7 KB was isolated, subcloned into the pGEM-7Zf (+). Sequence analysis showed a near-identical nucleotide and amino acid similarity to a previous rat SGP-1 cDNA. The majority of the heterogeneites were conservative substitutions. Northern blot analysis demonstrated that all examined rat tissue and organs have SGP-1 mRNA. Immunocytochemistry identified two staining patterns in the cytoplasm of positive cells: (a) a granular reaction characteristic of lysosomes in the supranuclear and basal regions of epithelial cells and in the perinuclear region of neurons; and (b) a homogeneous reaction in the cytoplasm of Sertoli cells, Type II pneumocytes, macrophages, and epithelial cells lining the choroid plexus. The latter staining pattern could be characteristic of cells that exhibit a secretory routing of SGP- 1. The production of SGP-1 by a variety of specialized cells lining fluid compartments suggests that its secreted form has a role in the transport of lipids in biological fluids, possibly by the formation of soluble complexes with glycolipids. Similarly, the lysosomal form of SGP-1/prosaposin and their derived saposins also solubilizes certain glycolipids to promote their degradation by specific hydrolases.