Rat spongiotrophoblast-specific protein is predominantly a unique low sulfated chondroitin sulfate proteoglycan

We have previously demonstrated that the human placenta contains a uniquely low sulfated extracellular aggrecan family chondroitin sulfate proteoglycan (CSPG). This CSPG is a major receptor for the adherence of Plasmodium falciparum-infected red blood cells (IRBCs) in placentas, causing pregnancy-specific malaria. However, it is not known whether such low sulfated CSPGs occur in placentas of other animals and, if so, whether IRBCs bind to those CSPGs. In this study, we show that rat placenta contains a uniquely low sulfated extracellular CSPG bearing chondroitin sulfate (CS) chains, which comprise only similar to 2% 4-sulfated and the remainder nonsulfated disaccharides. Surprisingly, the core protein of the rat placental CSPG, unlike that of the human placental CSPG, is a spongiotrophoblast-specific protein (SSP), which is expressed in a pregnancy stage-dependent manner. The majority of rat placental SSP is present in the CSPG form, and only similar to 10% occurs without CS chain substitution. Of the total SSP-CSPG in rat placenta, similar to 57% is modified with a single CS chain, and similar to 43% carries two CS chains. These data together with the previous finding on human placental CSPG suggest that the expression of low sulfated CSPG is a common feature of animal placentas. Our data also show that the unique species-specific difference in the biology of the rat and human placentas is reflected in the occurrence of completely different CSPG core protein types. Furthermore, the rat SSP-CSPG binds P. falciparum IRBCs in aCS chain-dependent manner. Since IRBCs have been reported to accumulate in the placentas of malaria parasite-infected rodents, our results have important implications for exploiting pregnant rats as a model for studying chondroitin 4-sulfate-based therapeutics for human placental malaria.