Ligand-independent control of extravillous trophoblast differentiation

During early pregnancy, the placenta adheres to the maternal decidua and gives rise to the extravillous trophoblast (EVT). This trophoblast subtype invades the deciuda and spiral arteries to enable an adequate nutrient support to the embryo and its immunological acceptance in an allogeneic environment. Although partly similar to cancer cell formation, EVT differentiation underlies strict control mechanisms of mostly unknown entities. On the one hand, various studies suggest extrinsic regulation of EVT differentiation by paracrine mechanisms, whereas on the other hand it remains a puzzling observation that EVT differentiation takes place in the absence of a pregnancy-related environment, as for instance noticed in differentiating in vitro trophoblast cultures. In this context, our preliminary data suggest that specific ERBB family members may partly control EVT function by inducing a constitutively active phenotype in these cells. Interestingly, the ligand-less oncogenic ERBB2 receptor and nuclear ERBB4, two inducers of ligand-independent signalling, are highly expressed in EVTs. Further in vitro analyses reveal a reciprocal expression pattern of two ERBB4 splice variants, the differentiation-promoting Cyt-1 and the pro- proliferative Cyt-2 variant, indicating a variant-specific role for nuclear ERBB4 during EVT differentiation. Moreover, we suggest that ERBB2 comprises the only membrane-associated ERBB member in invasive EVTs. This finding implies ligand-independent ERBB2 signalling, a so far exclusively described scenario in cancer cells. We firstly plan to refine our obtained expression data by performing flow cytometry-mediated cell sorting of specific EVT subtypes. In addition, by taking advantage of primary trophoblast model systems, we aim to study the role of ERBB2 and -4 during EVT-associated proliferation/cell cycle arrest, acquisition of an invasive phenotype and migration/invasion. Preliminary functional studies indicate a role for ERBB2 in EVTs, as its knockdown leads to changes in EVT-specific morphology and impairment of pro-invasive differentiation. In addition to loss-of- function studies, we aim to investigate downstream effectors of ERBB2 and -4 activities. Since overexpression of ERBB receptors is a common event in malignant transformation, we therefore aim to analyze a cohort of gestational trophoblast diseases by means of immunohistochemistry with respect to ERBB receptor expression, membranous dimer combinations and compartmental distribution. In summary, we here propose a novel concept, implying ligand-independent processes to operate during EVT differentiation.

Our research interests lie in the principles of mechanisms, which control early development of the human placenta. The placenta fulfills a variety of functions during pregnancy including nutrient supply of the embryo, exchange of gases and the production of pregnancy maintaining hormones. In addition, the placenta ensures maternal immunological acceptance of the genetically alien foetus. Another interesting aspect of human pregnancy includes invasion of the uterus by specific placenta-derived cells, the so-called extravillous trophoblast. In detail, these cells migrate from the placenta into the maternal uterus (during pregnancy referred to as decidua) in order to communicate with maternal immune cells and to dilate deciudal blood vessels. The latter phenomenon ensures the establishment of embryonic nutrient supply. However, failure in the function of the extravillous trophoblasts and consequently faulty remodeling of uterine vessels can provoke various pregnancy-related complications such as intrauterine growth retardation of the foetus or preeclampsia, the latter being characterized by high blood pressure in the mother. In this project we focused on studying specific pathways controlling critical functions in extravillous trophoblasts. Specifically we concentrated on the ErbB-signal pathway. Here, we were able to show that extravillous trophoblasts express two of the four ErbB receptor variants. Interestingly, we could demonstrate a specific ErbB receptor combination on the cellular surface of extravillous trophoblasts including ErbB2 and ErbB3. These receptors are believed to play an essential role in the development of various types of cancers in humans. Strikingly, in the course of this project we were able to confirm trophoblast-associated activation and cooperation of both receptors and could further show a pivotal function in extravillous trophoblasts. Interestingly, both receptors seem to be essential for the survival of invasive, decidual trophoblasts. In contrast, the ErbB1 receptor may play a role in cell division of non-invasive trophoblast. Along these lines, we could show an enhanced development of ErbB1-positive trophoblasts in complete mole placentas. This severe trophoblast disease is associated with lack of embryonic tissue leading to massive trophoblastic hyperplasia. Treatment of molar pregnancies requires surgical removal and in some case chemotherapeutic intervention. In summary, our data may contribute to a better understanding of physiological function of the placenta, and may further improve therapeutic options in the treatment of molar pregnancies.