Molecular characterization of paan-ag, an hla-g-like gene expressed in baboon (papio anubis) placenta

Abstract

The human class Ib major histocompatibility complex (MHC) molecule, HLA-G, is unique in its limited polymorphism, high expression in the placenta and generation of multiple transcripts by alternative splicing. The proteins encoded by these transcripts are believed to modulate maternal-foetal immunological relationships during pregnancy. The placentas of both monkeys and baboons contain messages encoded by a novel MHC molecule, which is evolutionarily related to the HLA-A locus but shares unique characteristics with HLA -G. In this study, it was demonstrated that in the placenta of the olive baboon (Papio anubis), the HLA-G-like gene, Paan-AG, is alternatively spliced to generate at least seven transcripts. Four of these messages were predicted to encode membrane-bound class Ib MHC glycoprotein isoforms (Paan-AG I, Paan-AG2, Paan- AG3, Paan-AG4) and one to encode a soluble isoform (sPaan-AGl), and all these transcripts have HLA-G counterparts. Two other transcripts, predicted to encode a truncated soluble protein, were detected in the placenta and all the non-placental tissues tested in this study, including cycling uterine endometrium, heart, liver, lung, kidney, skeletal muscle, thymus and spleen. The full-length transcript, Paan-AG 1, was also detected in these samples, but in the non-placental tissues, the transcripts contained multiple stop codons. The alternatively spliced transcripts, with the exception of Paan- AG 1, were also detected in unfertilised baboon oocytes, zygotes and preimplantation embryos. Immunohistochemical studies using Paan-AG isoform-specific antibodies showed that Paan-AG 1 and soluble Paan-AG proteins were expressed III syncytiotrophoblast in both first trimester and term placenta, and in extravillous cells in decidua and basal plate in term placenta, and also in the prostate gland. Paan-AG2 protein was not expressed by these tissues. Because of the structural similarities and common features of organ-specific expression and splicing of the message, studies on Paan-AG may be of value in investigating the functions of the class Ib proteins in human pregnancy. The nucleotide sequence data described in this thesis are available in the DDBJ/EMBJ/GenBank databases under the Accession numbers AY055027-A Y055058.