A DEVELOPMENTAL SWITCH IN EXPRESSION FROM BLASTOCYST TO ENDOMETRIAL PLACENTAL-TYPE CYTOCHROME-P450 AROMATASE GENES IN THE PIG AND HORSE/

Pig blastocysts exhibit a transient period of estrogen production at p eriimplantation, with a second, more sustained period of estrogen synt hesis occurring in endometrium and placenta at later pregnancy. Previo usly we reported the isolation of cDNA clones encoding a novel isoform of cytochrome P450 aromatase (the terminal enzyme in the estrogen bio synthetic pathway) from porcine periimplantation blastocysts. The pres ent study investigated pregnancy-associated expression, in blastocysts and maternal reproductive tract tissues, of this and an additional mR NA transcript encoding a distinct P450 aromatase isoform. Restriction endonuclease and nucleotide sequence analyses of 44 cDNA clones demons trated that the major aromatase mRNA in periimplantation blastocysts a nd early-pregnancy endometrium and placenta (blastocyst-type) differed in sequence from the major aromatase mRNA expressed in endometrium an d placenta at midpregnancy (endometrial-type). The deduced blastocyst and endometrial aromatase isoform protein sequences had 93% similarity . A third type of aromatase mRNA, deleted in exons 4-6 sequences, also was cloned from blastocysts. This cDNA was identical in nucleotide se quence to the blastocyst full-length aromatase cDNA and specified an o pen reading frame of 354 amino acids for a putative aromatase-related protein containing the heme-binding domain. Expression of this shorter mRNA in blastocysts was confirmed by reverse transcription polymerase chain reaction. The 5'-untranslated exon sequences in the transcripts encoding the blastocyst-type aromatase isoform were distinct from tha t of the endometrial type, consistent with differential expression of multiple chromosomal genes. In periimplantation equine embryos, howeve r, embryonic and placental 5'-untranslated exon-containing transcripts were coexpressed. Results identify an aromatase isoform expressed in the endometrium and placenta at midpregnancy, demonstrate a transition in synthesis of aromatase isoform-specific mRNAs during placental dev elopment, and suggest the preferential involvement of the blastocyst a romatase isoform in synthesis of estrogenic molecules that may functio n in embryo-maternal signaling at periimplantation.