Adrenomedullin, calcitonin gene-related peptide and their receptors: evidence for a decreased placental mRNA content in preeclampsia and HELLP syndrome

Abstract

Objective: The human placenta expresses a variety of vasoactive substances and neuropeptides, which play an important role in the regulation of placental blood flow in both the maternal and foetal compartment and are therefore of critical importance for foetal growth and development. Our study was planned to examine placental mRNA amounts of vasodilatory adrenomedullin (AM), calcitonin gene-related peptide (CGRP) and their receptors (AM-R and CGRP-R) in preeclampsia and HELLP syndrome (hemolysis, elevated liver enzymes, low platelets). These are severe maternal conditions leading to an altered uteroplacental and fetoplacental perfusion and a higher risk for foetal growth retardation, premature delivery, infant mortality, and even maternal death. Study Design: We included 17 patients with preeclampsia, four women with HELLP syndrome and 34 controls. After delivery, the mRNA levels of AM, AM-R, CGRP, CGRP-R, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and β-actin were measured in placental villi and chorionic plates using quantitative real-time PCR. Results: AM/β-actin and AM/GAPDH mRNA ratios were significantly lower in placental villi in preeclampsia than in controls (P<0.05) as were CGRP/β-actin and CGRP/GAPDH mRNA ratios in chorionic plates (P<0.05). Placental AM-R and CGRP-R mRNA amounts were unaffected. Conclusion: Our data show a reduction of AM and CGRP mRNAs in contrast to unchanged mRNA levels of their receptors in placenta specimens of women with preeclampsia or HELLP syndrome.

Introduction

The human placenta expresses various vasoactive substances and neuropeptides, which may play an important role in the local regulation of placental blood flow in both the maternal and foetal compartment and are therefore of critical importance for foetal growth and development [1]. One of these is adrenomedullin (AM), which is a 52-amino acid peptide with vascular smooth muscle relaxing and therefore hypotensive properties [2]. It belongs to a peptide superfamiliy together with α-calcitonin gene-related peptide (CGRP) and shows cross-reactions with CGRP at common receptor sites. The human analogous to the rat adrenomedullin receptor has been cloned by our group [3]. In recent times, receptor-activity-modifying proteins, so-called RAMPs were identified [4], regulating the ligand specificity of a calcitonin-receptor like receptor which can therefore act either as a CGRP receptor or an AM receptor. There seems to be a physiological role for this peptidergic system in controlling blood flow in various organs including the uterus. AM mRNA has been detected in the uterus of nonpregnant rats, with an almost two-fold increase in pregnancy [5]. AM-binding sites show a higher density in the pregnant rat uterus, indicating a physiological role in the control of uterine vascular tone [5]. CGRP and its binding sites are widely distributed within the brain and the cardiovascular system, where it elicits vasorelaxation via both, endothelium-dependent and independent mechanisms [2].

Preeclampsia with hypertension, proteinuria and oedema is a severe maternal condition typically diagnosed in the latter half of gestation which contributes to foetal growth retardation, infant mortality due to reduced placental perfusion, premature delivery and maternal death. Underlying mechanisms include vascular endothelial dysfunction, increased vascular reactivity and platelet activation. HELLP syndrome is the most severe complication with accompanying hemolysis, elevated liver enzymes, low platelet counts. Despite an extensive effort to elucidate the causes of pregnancy-induced hypertension, its pathogenesis and pathophysiology still remain obscure [6]. Obviously, preeclampsia is not a distinct disease but a description of symptoms with different pathophysiological conditions. In an animal model for preeclampsia due to chronic inhibition of nitric oxide production CGRP has been shown to reverse hypertension and pup mortality [7].

Preeclampsia leads to disturbed uteroplacental and therefore fetoplacental perfusion. We thus tested, whether mRNA levels of AM and CGRP as well as of AM-R and CGRP-R may be altered in placental tissue specimens from patients with preeclampsia or HELLP syndrome to further investigate the local mechanisms of reduced fetoplacental perfusion. We focussed on mRNA levels due to rational and methodical reasons (e.g. application of real-time PCR with a high degree of sensitivity, specificity and a quantitative approach, analysis of primary gene activities instead of secondary effects, avoidance of cross-reactivities compared with immunological post-translational methods).