**Persistent Human KIT Receptor Signaling Disposes Murine Placenta to Premature Di**ff**erentiation Resulting in Severely Disrupted Placental Structure and Functionality**

**Franziska Kaiser 1,**† **, Julia Hartweg 1,2,**† **, Selina Jansky 1,3,4, Natalie Pelusi 1,5 , Caroline Kubaczka 1,6, Neha Sharma 1,7, Dominik Nitsche 1,8 , Jan Langkabel <sup>1</sup> and Hubert Schorle 1,\***


Received: 8 July 2020; Accepted: 30 July 2020; Published: 31 July 2020

**Abstract:** Activating mutations in the human KIT receptor is known to drive severe hematopoietic disorders and tumor formation spanning various entities. The most common mutation is the substitution of aspartic acid at position 816 to valine (D816V), rendering the receptor constitutively active independent of ligand binding. As the role of the KIT receptor in placental signaling cascades is poorly understood, we analyzed the impact of KITD816V expression on placental development using a humanized mouse model. Placentas from KITD816V animals present with a grossly changed morphology, displaying a reduction in labyrinth and spongiotrophoblast layer and an increase in the Parietal Trophoblast Giant Cell (P-TGC) layer. Elevated differentiation to P-TGCs was accompanied with reduced differentiation to other Trophoblast Giant Cell (TGC) subtypes and by severe decrease in proliferation. The embryos display growth retardation and die in utero. KITD816V-trophoblast stem cells (TSC) differentiate much faster compared to wild type (WT) controls. In undifferentiated KITD816V-TSCs, levels of Phosphorylated Extracellular-signal Regulated Kinase (P-ERK) and Phosphorylated Protein Kinase B (P-AKT) are comparable to wildtype cultures differentiating for 3–6 days. Accordingly, P-TGC markers Placental Lactogen 1 (PL1) and Proliferin (PLF) are upregulated as well. The results reveal that KIT signaling orchestrates the fine-tuned differentiation of the placenta, with special emphasis on P-TGC differentiation. Appropriate control of KIT receptor action is therefore essential for placental development and nourishment of the embryo.

**Keywords:** KIT receptor; KITD816V; placental development; premature differentiation; trophoblast stem cell; trophoblast giant cell; spongiotrophoblast; invasion; embryonic growth retardation
