Why is placentation abnormal in preeclampsia?

Abstract

The causes of preeclampsia remain one of the great medical mysteries of our time. This syndrome is thought to occur in two stages with abnormal placentation leading to a maternal inflammatory response. Specific regions of the placenta have distinct pathological features. During normal pregnancy, cytotrophoblasts emigrate from the chorionic villi and invade the uterus, reaching the inner third of the myometrium. This unusual process is made even more exceptional by the fact that the placental cells are hemi-allogeneic, co-expressing maternal and paternal genomes. Within the uterine wall, cytotrophoblasts deeply invade the spiral arteries. Cytotrophoblasts migrate up these vessels and replace, in a retrograde fashion, the maternal endothelial lining. They also insert themselves amongst the smooth muscle cells that form the tunica media. As a result, the spiral arteries attain the physiological properties that are required to adequately perfuse the placenta. In comparison, invasion of the venous side of the uterine circulation is minimal, sufficient to enable venous return. In preeclampsia, cytotrophoblast invasion of the interstitial uterine compartment is frequently shallow, although not consistently so. In many locations, spiral artery invasion is incomplete. There are many fewer endovascular cytotrophoblasts and some vessels retain portions of their endothelial lining with relatively intact muscular coats while others are not modified. Work from our group showed that these defects mirror deficits in the differentiation program that enables cytotrophoblast invasion of the uterine wall. During normal pregnancy, invasion is accompanied by downregulation of epithelial-like molecules that are indicative of their ectodermal origin and upregulation of numerous receptors and ligands that are typically expressed by endothelial or vascular smooth muscle cells. For example, the expression of epithelial-cadherin, the cell-cell adhesion molecule that many ectodermal derivatives use to adhere to one another, becomes nearly undetectable, replaced by vascular-endothelial cadherin, which serves the same purpose in blood vessels. Invading cytotrophoblasts also modulate vascular endothelial growth factor ligands and receptors, at some point in the differentiation process expressing every (mammalian) family member. Molecules in this family play crucial roles in vascular and trophoblast biology, including prevention of apoptosis. In preeclampsia, this process of vascular mimicry is incomplete, which we theorize hinders the cells interactions with spiral arterioles. What causes these aberrations? Given what is known from animal models and human risk factors, reduced placental perfusion and/or certain disease states (metabolic, immune and cardiovascular) lie upstream. Recent evidence suggests the surprising conclusion that isolation and culture of cytotrophoblasts from the placentas of pregnancies complicated by preeclampsia enables normalization of their gene expression. The affected molecules include SEMA3B, which downregulates vascular endothelial growth factor signaling through the PI3K/AKT and GSK3 pathways. Thus, some aspects of the aberrant differentiation of cytotrophoblasts within the uterine wall that is observed in situ may be reversible. The next challenge is asking what the instigating causes are. There is added urgency to finding the answers, as these pathways could be valuable therapeutic targets for reversing abnormal placental function in patients.