Nucleic Acid Sequence of Feline Preprorelaxin and Its Localization within the Feline Placenta1

The cat placenta is known to secrete large amounts of relaxin. We employed uteroplacental tissue at approximately Day 35 of gestation to determine the nucleic acid sequence of feline preprorelaxin using reverse transcription- and rapid amplification of cDNA ends-polymerase chain reaction. Feline preprorelaxin cDNA was found to consist of 540 base pairs encoding a protein of 180 amino acids (aa). We identified a signal peptide of 25 aa, a B domain of 33 aa, a C domain of 98 aa, and an A domain of 24 aa. The putative receptor binding region in the N'-terminal part of the B domain contained one substitution from the classical GRELVR motif (L→F). Feline preprorelaxin shared highest homology with porcine and equine preprorelaxin. Northern analysis revealed a specific 1-kilobase transcript present in total RNA of feline uteroplacental tissue but not of liver tissue. Nonradioactive in situ hybridization was used to localize relaxin mRNA, and immunohistochemistry was used to localize the relaxin hormone and cytokeratin, in tissues of the feto-maternal interface recovered from two queens at Day 35 of gestation. Specific hybridization signals for relaxin mRNA were exclusively detected in cells located in the lamellar placental labyrinth but were absent from other placental and non-placental uterine parts. The cells expressing relaxin mRNA also displayed immunoreactivity for cytokeratin and were, therefore, identified as trophoblast cells. Immunoreactive relaxin colocalized in those placental areas expressing relaxin mRNA. Trophoblast cells located at the villous chorioallantoic tips invading the endometrium and extravillous trophoblast cells in the junctional placental zone were devoid of relaxin.