Keratin and filament cell-cell junction cooperation in developmental and tumorigenic fate decisions - Carcinomas represent the majority of cancers diagnosed in the United States each year, including a yearly estimated 3-5 million cases of nonmelanoma skin cancer and 1 million cases of breast, lung, colon, and genital tract cancers. While these cancers originate in numerous organs, all carcinomas are tumors arising from epithelia. Epithelia are a tissue type characterized by sheets of polarized cells tightly attached to one another by cytoskeleton-linked cell-cell junctions, which form selectively permeable barriers. Epithelia are also characterized by their expression of cytoskeletal filament-forming proteins called keratins. Keratins are an expansive family of orthologous proteins exhibiting distinct biochemical properties in addition to their highly conserved intermediate filament-forming domains. Importantly, keratins are a determining feature of epithelial identity, and the alteration to keratin expression is a hallmark of carcinomas. While epithelia are specified during development, they undergo a continuous regenerative process throughout life known as homeostasis. Epithelial homeostasis is regulated by numerous signaling pathways which regulate cell proliferation, migration, and differentiation in developing organs and can lead to tumorigenesis when dysregulated. In the context of complex epithelial homeostasis, keratin proteins with distinct biochemical properties and transcriptional regulation were shown to cooperate to affect the localization of the pro-growth transcriptional co-activator YAP1 and regulate homeostatic cell fate decisions (cell division, differentiation, death). Here, we aim to further define the mechanism of keratin-dependent regulation of cell fate decisions by 1) investigating the keratin regulation of pro-growth fate decisions during embryogenesis and development, prior to the establishment of homeostasis, using a Xenopus laevis embryo model, and 2) testing this developmental mechanism in the context of cell fate decisions during cancer initiation using a Xenopus laevis tumor model. To define this mechanism, we will contextualize the keratin filament network within the broader epithelial cytoskeleton by investigating the cooperation between the keratin and actomyosin filament networks at cell-cell junctions, specifically (keratin-linked) desmosomes and (actin-linked) adherens junctions. This project conceptualizes the keratin filament network as a spring whose ability to store mechanical energy is proportional to its stiffness, which is determined by the post-translational modifications of its constituent keratin proteins. We seek to define how the stiffness of the keratin filament network regulates the transduction of mechanical information to transcriptional effectors of cell fate, including YAP1, through the cooperation with the mechanosensitive actomyosin network, desmosomes, and adherens junctions. Overall, this project seeks to determine a fundamental mechanism that regulates cell division and differentiation in epithelia that may be perturbed during cancer initiation. Through the elucidation of the mechanism, we aim to uncover a fundamental aspect of carcinogenesis, potentially informing the development of new and more effective prophylactics.
