Nodal Promotes Functional Luteolysis via Down-Regulation of Progesterone and Prostaglandins E2 and Promotion of PGF2α Synthetic Pathways in Mare Corpus Luteum

In the present work, we investigated the role of Nodal, an embryonic morphogen from the TGF beta superfamily in corpus luteum (CL) secretory activity using cells isolated from equine CL as a model. Expression pattern of Nodal and its receptors activin receptor A type IIB (ACVR2B), activin receptor like kinase (Alk)-7, and Alk4, as well as the Nodal physiological role, demonstrate the involvement of this pathway in functional luteolysis. Nodal and its receptors were immune localized in small and large luteal cells and endothelial cells, except ACVR2B, which was not detected in the endothelium. Nodal mRNA in situ hybridization confirmed its transcription in steroidogenic and endothelial cells. Expression analysis of the aforementioned factors evidenced that Nodal and Alk7 proteins peaked at the mid-CL (P < .01), the time of luteolysis initiation, whereas Alk4 and ACVR2B proteins increased from mid-to late CL (P < .05). The Nodal treatment of luteal cells decreased progesterone and prostaglandin (PG) E-2 concentrations in culture media (P < .05) as well as mRNA and protein of secretory enzymes steroidogenic acute regulatory protein, cholesterol side-chain cleavage enzyme, cytosolic PGE(2) synthase, and microsomal PGE(2) synthase-1 (P < .05). Conversely, PGF(2 alpha) secretion and gene expression of PG-endoperoxidase synthase 2 and PGF(2 alpha) synthase were increased after Nodal treatment (P < .05). Mid-CL cells cultured with PGF(2 alpha) had increased Nodal protein expression (P < .05) and phosphorylated mothers against decapentaplegic-3 phosphorylation (P < .05). Finally, the supportive interaction between Nodal and PGF(2 alpha) on luteolysis was shown to its greatest extent because bothfactors together more significantly inhibited progesterone (P < .05) and promoted PGF(2 alpha) (P < .05) synthesis than Nodal or PGF(2 alpha) alone. Our results neatly pinpoint the sites of action of the Nodal signaling pathway toward functional luteolysis in the mare.