Atypical Protein Kinase C isoforms maintain human placental syncytiotrophoblast polarity
Creators: Patel, Khushali
Publisher: name: University of Alberta Library
Year: 2023

<p>The human placental surface is lined by a giant multinucleate epithelial cell – the syncytiotrophoblast (ST). The ST acts a maternal-fetal interface and aids in the exchange of gas, nutrient exchange, and acts as a transient endocrine organ. This ST layer is formed and maintained by the underlying mononuclear proliferative villous cytotrophoblasts (vCTs). The ST is a highly polarized epithelium with dense microvilli emerging from its apical surface bathed in the maternal blood in the intervillous space. <br>
Microvilli (MV) are a hallmark of established apical-basal polarity in epithelial cells. These MV are membrane protrusions supported by an F-actin core. MV structure is dynamically maintained by constant actin treadmilling and cytoskeletal linker proteins belonging to the Ezrin/Radixin/Moesin (ERM) family aid in anchoring the F-actin core to the cell membrane. Previous electron micrographs reveal abundant expression of ezrin at the ST apical membrane. When phosphorylated, ezrin transforms from a dormant configuration to active configuration where it’s C-terminal end binds to the F-actin core and the N-terminus binds to the cell membrane. <br>
Polarity is the asymmetric distribution of biomolecules in a cell. Recent studies have revealed that mRNA transcribing for various transporters, vesicles and hormones are specifically expressed at the ST apical surface. Studies have also revealed that extracellular vesicles containing active biomolecules such as DNA, RNA and protein are actively released from the ST microvillar region. Thus, the ST and its microvilli seem to play a critical role in the normal functioning of the human placenta. However, active maintenance of polarity is critical for normal cellular homeostasis. Yet to date, no studies have investigated the maintenance of polarity in this highly polarized epithelium.<br><br>
Atypical Protein Kinase C (aPKC) isoforms are a part of the Par complex- an evolutionarily conserved regulator of apical-basal polarity. It is known that aPKC along with the scaffolding proteins Par-3 and Par-6 localize apically and determine the apical domain in epithelial cells. In humans, two full length isoforms – aPKCι and aPKCζ are expressed along with a constitutively active brain-specific N-terminal truncated aPKCζ isoform: PKMζ. APKCι/ζ isoforms have been shown to maintain polarity in various human epithelial cells. In the mice intestinal microvilli, aPKCι has been shown to regulate ezrin phosphorylation. While aPKCλ/ι knockout mice are known to be embryonically lethal at E9.5, aPKCζ knockout mice have no embryonic phenotype. However, later it was revealed that aPKCζ can partially compensate for the aPKCλ/ι knockout phenotype. Recent studies reveal the expression of aPKCι in human trophoblasts, and the critical role it plays in regulating trophoblast stem cell (TSC) to ST differentiation. However, there is a lack of literature that takes the compensatory nature of aPKCs into consideration and investigates the expression of multiple aPKC isoforms in the human trophoblasts.<br>
Data presented in this thesis reveals the expression of a novel aPKC isoform – aPKCζ III along with the full length aPKCι and aPKCζ isoforms in the human trophoblasts. APKCζ III is a N-terminal truncated isoform derived from an alternate promoter region of the PRKCZ gene. However, not much is known about the expression and function of this novel aPKC isoform in the human placenta. <br>
The data presented in this thesis also identify the apical localization of the aPKC isoforms along with ezrin and that aPKC kinase activity is partially responsible for ezrin phosphorylation at the ST apical surface. Inhibition of total aPKC kinase activity leads to a significant region-specific loss of ST apical F-actin and microvilli. Additionally, the loss of either one of the aPKC isoforms is enough to cause structural and functional alterations in the ST. <br>
In conclusion, the results presented in this thesis reveal a critical yet functionally redundant role of aPKC isoforms in maintaining ST apical polarity.  </p>