Membrane progesterone receptor e (paqr9) is necessary for chorion elevation in zebrafish

Paqr9 is a gene encoding membrane progestin receptor e (mPRe), the fifth subtype of the five mPR subtypes, and is currently classified a member of the progestin and adipoQ receptor (PAQR) family, which consists of 11 genes. To elucidate the physiological functions of the mPR subtypes, we established gene knockout (KO) fish via genome editing of seven paqr genes in zebrafish and analyzed their phenotypes. The null-mutant strain of paqr9 (paqr9^-/-) that we established in this study presented reduced chorion elevation and a high percentage of abnormal embryos. Embryos exhibit various kinds of abnormal morphology, which are thought to be caused by insufficient elevation of the chorion. Immunohistochemical staining of ovaries with an anti-Paqr9 antibody revealed that Paqr9 was expressed in the periplasm of oocytes and the surface of chorion in the wild type, whereas signals were absent in paqr9^-/- zebrafish. In histological sections, the periplasmic connection between the oocyte plasma membrane and chorion was absent in paqr9^-/- oocytes. The number of cortical alveoli (CA) that are responsible for chorion elevation was significantly reduced in paqr9^-/- zebrafish. SEM revealed that fiber-supported knob-like structures (KSs) on the chorion were absent in paqr9^-/- zebrafish. These results indicate that Paqr9 is required for the preparation of CA during oogenesis. Insufficient formation of the chorion resulted in the abnormal development of embryos.