Characterization of human placental glycosaminoglycans and regional binding to VAR2CSA in malaria infected erythrocytes

Abstract

Placental malaria is a serious problem in sub-Saharan Africa. Young women are particular susceptible to contracting this form of malaria during their first or second pregnancy despite previously acquired immunity from past infections. Placental malaria is caused by Plasmodium falciparum parasites expressing VAR2CSA on the erythrocyte surface. This protein adheres to a low-sulfated chondroitin sulfate-A found in placental tissue causing great harm to both mother and developing fetus. In rare cases, the localization of infected erythrocytes to the placenta can even result in the vertical transmission of malaria. In an effort to better understand this infection, chondroitin sulfate was isolated from the cotyledon part of the placenta, which should be accessible for parasite adhesion, as well as two non-accessible parts of the placenta to serve as controls. The placental chondroitin sulfate structures and their VAR2CSA binding were characterized. All portions of human placenta contained sufficient amounts of the appropriate low-sulfated chondroitin sulfate-A to display high-affinity binding to a recombinant truncated VAR2CSA construct, as determined using surface plasmon resonance. The cotyledon is the only placental tissue accessible to parasites in the bloodstream, suggesting it is the primary receptor for parasite infected red blood cells.

Figure 1

Drawing of human placenta with sections analyzed labeled as umbilical cord, chorionic plate and cotyledon.

Figure 2

Common chondroitin disaccharides formed through chondroitin lyase treatment (top) and chondroitin sulfate undersulfated dodecasaccharide sequence (bottom) proposed to bind to infected erythrocytes.

Figure 3

Normalized average chondroitin sulfate disaccharide composition based on data in Table 2. Standard deviation represents variation between placenta samples.

Figure 4

SPR sensorgrams of isolated chondroitin sulfate (from different regions of placenta) and MP907 interaction. A: Cotyledon; B: Chorionic Plate; C: Umbilical Cord. Concentrations of MP907 (from top to bottom): 125, 63, 32, and 16 nM, respectively. The black curves are the fitting curves. SPR experiments relied on duplicate injections performed after the regeneration of the chip surface. Standard errors were calculated from the global fitting of different protein concentrations and are presented in Table 2.