The Role of the Carbohydrate Recognition Domain of Placental Protein 13 (PP13) in Pregnancy Evaluated with Recombinant PP13 and the DelT(221) PP13 Variant

INTRODUCTION: Placental protein 13 (PP13), a placenta specific protein, is reduced in the first trimester of pregnancy in women who subsequently develop preeclampsia. A naturally occurring PP13 deletion of thymidine at position 221 (DelT(221) or truncated variant) is associated with increased frequency of severe preeclampsia. In this study we compared the full length (wildtype) PP13 and the truncated variant. METHODS: Full length PP13 or its DelT(221) variant were cloned, expressed and purified from E-Coli. Both variants were administrated into pregnant rats at day 8 of pregnancy for slow release (>5 days) through osmotic pumps and rat blood pressure was measured. Animals were sacrificed at day 15 or day 21 and their utero-placental vasculature was examined. RESULTS: The DelT(221) variant (11 kDA) lacked exon 4 and a part of exon 3, and is short of 2 amino acids involved in the carbohydrate (CRD) binding of the wildtype (18 kDA). Unlike the wildtype PP13, purification of DelT(221) variant required special refolding. PP13 specific poly- clonal antibodies recognized both PP13 and DelT(221) but PP13 specific monoclonal antibodies recognized only the wildtype, indicating the loss of major epitopes. Wildtype PP13 mRNA and its respective proteins were both lower in PE patients compared to normal pregnancies. The DelT(221) mutant was not found in a large Caucasian cohort. Pregnant rats exposed to wildtype or DelT(221) PP13 variants had significantly lower blood pressure compared to control. The wildtype but not the DelT(221) mutant caused extensive vein expansion. CONCLUSION: This study revealed the importance of PP13 in regulating blood pressure and expanding the utero-placental vasculature in pregnant rats. PP13 mutant lacking amino acids of the PP13 CRD domain fails to cause vein expansion but did reduce blood pressure. The study provides a basis for replenishing patients at risk for preeclampsia by the full length but not the truncated PP13.