Endogenous annexin A1 (AnxA1) modulates early‐phase gestation and offspring sex‐ratio skewing

Annexin A1 (AnxA1) is a glucocorticoid‐regulated anti‐inflammatory protein secreted by phagocytes and other specialised cells. In the endocrine system, AnxA1 controls secretion of steroid hormones and it is abundantly expressed in the testis, ovaries, placenta and seminal fluid, yet its potential modulation of fertility has not been described. Here, we observed that AnxA1 knockout (KO) mice delivered a higher number of pups, with a higher percentage of female offsprings. This profile was not dependent on the male features, as sperm from KO male mice did not present functional alterations, and had an equal proportion of Y and X chromosomes, comparable to wild type (WT) male mice. Furthermore, mismatched matings of male WT mice with female KO yielded a higher percentage of female pups per litter, a phenomenon which was not observed when male KO mice mated with female WT animals. Indeed, AnxA1 KO female mice displayed several differences in parameters related to gestation including (i) an arrested estrous cycle at proestrus phase; (ii) increased sites of implantation; (iii) reduced pre‐ and post‐implantation losses; (iv) exacerbated features of the inflammatory reaction in the uterine fluid during implantation phase; and (v) enhanced plasma progesterone in the beginning of pregnancy. In summary, herein we highlight that AnxA1 pathway as a novel determinant of fundamental non‐redundant regulatory functions during early pregnancy.