Intrinsic and extrinsic spindle orientation mechanisms in mammalian epidermis

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ABSTRACT Proper control of stem cell division is critical for tissue morphogenesis and homeostasis. When dysregulated, it can lead to hypoplasia and stem cell exhaustion on the one hand, or tissue overgrowth and cancer on the other. But mitosis is more than simple proliferation, as cell division can be controlled not only in time but also in space. Oriented cell divisions (OCDs) are an example of the latter, and for stem and progenitor cells, choices between division axes can dictate cell fate outcomes and impact tissue architecture. In stratified epithelia such as the epidermis, basal progenitors divide either within the plane of the epithelium, or perpendicular to it. Evidence suggests that planar divisions are generally self-renewing symmetric cell divisions (SCDs) while perpendicular divisions are differentiative asymmetric cell divisions (ACDs). Previous work from our lab has shown that ACDs are directed by a complex of polarity and spindle orientation proteins—converging on the critical scaffolding protein LGN (Gpsm2)—which localize asymmetrically at the apical cell cortex. More recently, we have found that the paralog AGS3 (Gpsm1) seems to oppose LGN, and functions in promoting SCDs through an unknown mechanism. In addition, we recently made the surprising discovery that division orientation is not fixed during metaphase, as previously thought, but can be further refined during late stages of mitosis. In this process, which we term “telophase correction,” roughly one-third of basal cells enter anaphase at oblique angles, but then reorient to either planar or perpendicular. We have learned that cell-cell adhesions—specifically, the mechanosensing components of the adherens junction—are important for telophase correction to occur, and can operate independently of LGN. This demonstrates that in addition to intrinsic cues such as the LGN complex, extrinsic factors such as the local tissue microenvironment influence the final division axis. Despite what we and others have learned about the molecular control of ACDs, major knowledge gaps exist in understanding how oriented divisions shape tissue architecture both during normal development and in congenital skin diseases such as epidermolysis bullosa and ectodermal dysplasia. Specifically, the objectives of this proposal are to develop a better understanding of 1) what regulates SCDs and how the choice between SCD/ACD is made (SA1), 2) how cell-cell adhesion, cell-matrix, and local cell density impact division orientation and fate decisions (SA2). To achieve these goals, we will leverage a combination of innovative approaches, centered on our rapid, high-throughput technique—lentiviral ultrasound-guided gene inactivation and gene expression (LUGGIGE)— which we will utilize to generate mouse models of both gene loss and of specific mutations found in human diseases. Combined with ex vivo imaging of skin explants and in vivo proteomic approaches to characterize the LGN and AGS3 interactomes using TurboID, these comprehensive studies will provide insights into the cell- intrinsic and extrinsic cues that determine division orientation, and how they operate during normal epidermal growth and in blistering and dysplastic skin diseases.