Signals for the initiation of parturition at term have long remained a mystery. Our work suggests that the fetus signals the mother when it is ready to be born through production of surfactant components by the maturing lung. We observed that the major surfactant protein, SP-A, secreted in increasing amounts by the mouse fetal lung into amniotic fluid (AF) after 17.5 days post-coitum (dpc) (term = 19.5 dpc), serves as a hormonal signal for parturition. Our prior studies revealed that the temporal induction of SP-A secretion was associated with increased proinflammatory cytokine production by amniotic fluid (AF) MΦ, myometrial NF-κB activation and increased contractile gene expression. Accordingly, mice carrying a targeted deletion of the SP-A (Sftpa) gene or its putative receptor, toll-like receptor 2 (TLR2), manifested a delay (~12 h) in parturition associated with decreased induction of contractile gene expression in myometrium.
In light of our findings that steroid receptor coactivators (SRC) are important for transcriptional upregulation of SFTPA in fetal lung type II cells (Fig. 1) and those of O’Malley and colleagues that Src-1/Src-2 double knockout (dKO) mice die at birth of respiratory distress, we crossed mice that were doubly heterozygous for Src-1 and Src-2 (Src-1/-2 dhet). Intriguingly, these mice manifested a striking delay in parturition (~38 h), which was far more pronounced than that of Sftpa-deficient mice (~12 h). The parturition delay in Src-1/-2 double-deficient (Src-1/-2d/d) mice was associated with a marked decrease in SP-A levels in fetal lungs and AF and reduced levels of NF-κB activation, CAP gene expression and PGF2α levels in maternal myometrium. Circulating maternal P4 also was elevated at term, due to a failure of luteolysis. Remarkably, wild-type (WT) females bred to Src-1/-2d/d males manifested similarly delayed labor with decreased activation of NF-κB and CAP genes and elevated P4, strongly suggesting that the defect responsible for the delay in labor is fetal in origin.
The pronounced delay in labor (~38 h) in mice carrying Src-1/-2d/d fetuses, compared to that of Sftpa-deficient mice (~12 h), suggests that signaling molecules in addition to SP-A were altered by Src-1/-2d/d. Importantly, levels of the inflammatory phospholipid, platelet-activating factor (PAF), previously implicated in the initiation of parturition, were significantly decreased in the fetal lungs and AF of Src-1/-2d/d fetuses compared to WT. As noted, Src-1/-2 dKO mice die at birth of alveolar collapse/atelectasis, suggesting that surfactant glycerophospholipids also may be altered. In this regard, we found that AF levels of the major surfactant glycerophospholipid, dipalmitoylphosphatidylcholine (DPPC), were significantly reduced in Src-1/-2 dKO fetuses compared to WT. Furthermore, the glucocorticoid-regulated enzyme, lysophosphatidylcholine acyltransferase 1 (LPCAT1), which catalyzes synthesis of both DPPC and PAF, also was significantly decreased in lungs of Src-1/-2d/d fetuses vs. WT.
Based on our collective findings, we propose that towards term, there is increased activation and binding of key transcription factors (e.g. TTF-1 and NF-κB) to response elements upstream of the SFTPA gene and of glucocorticoid receptor (GR) to response elements upstream of the LPCAT1 gene (Fig. 6). These transcription factors recruit SRC-1 and SRC-2, resulting in increased SP-A and LPCAT1 expression and secretion of SP-A and PAF into AF. These immune modulators activate immune cells that migrate to the maternal uterus leading to an activation of inflammatory transcription factors (e.g. NF-κB and AP-1). This contributes to the decline in PR function, with increased CAP gene expression and synthesis of PGF2α, which circulates to the ovary to inhibit steroidogenic acute regulatory protein (StAR), promote luteolysis and cause a decline in circulating P4. The decrease in P4/PR function causes further upregulation of CAP gene expression culminating in labor (Fig. 6).
Currently, we are testing the hypothesis that upon secretion by fetal lung, SP-A and PAF bind to and alter the polarization state of MΦ, which also arise from the fetal lung. We further propose that these MΦ traffic to the maternal decidua, where they alter the decidual leukocyte landscape and trigger a cascade of inflammatory signals leading to a break in maternal-fetal tolerance culminating in parturition.