Ubiquitin-Dependent Control of Myogenic Development: Mechanistic Insights into Getting Huge, and Staying Huge

Metazoan development is dependent on the robust spatiotemporal execution of stem cell cell-fate determination programs. Although changes in transcriptional and translational landscapes have been well characterized throughout many differentiation paradigms, their regulatory mechanisms remain poorly understood. Ubiquitin has recently been found to be a key modulator of developmental programs. Ubiquitylation of target proteins occurs through a cascade of enzymatic reactions beginning with a ubiquitin activating enzyme (E1) which transfer the ubiquitin moiety to a ubiquitin conjugating enzyme (E2). The reaction is finalized by the transfer of ubiquitin to its target protein by a ubiquitin ligase (E3). Post-translational modification of proteins can lead to several different outcomes, depending on the context of the modification, known as the ubiquitin code. The precise spatiotemporal execution of ubiquitylation is critical for organismal development and homeostasis. Due to the modular and reversible nature of ubiquitylation, it is an ideal moiety in the control of a plethora of cellular processes. Cell-cell fusion is a frequent and essential event during development, whose dysregulation causes diseases ranging from infertility to muscle weakness. Critical to this process, cells repeatedly need to remodel their plasma membrane through orchestrated formation and disassembly of cortical actin filaments. In Chapter 2, I describe the identification of a ubiquitin-dependent toggle switch that establishes reversible actin bundling during mammalian cell fusion. My work identified KCTD10 as a modulator of the EPS8-IRSp53 complex, which stabilizes cortical actin bundles at sites of cell contact to push fusing cells towards each other. This work highlights how cytoskeletal rearrangements during development are precisely controlled, raising the possibility of modulating the efficiency of cell fusion for therapeutic benefit.Organismal development must rely on the timely and robust execution of quality control responses. However, how these responses modulate metazoan development is poorly understood. Showcasing the versatility of ubiquitin signaling, Chapters 3 and 4 provide insight into the role of ubiquitin in controlling stress and quality control responses. Chapter 3 describes the reductive stress response, in which FEM1B senses and reacts to persistent depletion of reactive oxygen species. Loss of ROS is detrimental for development, as it inhibits myogenesis. Concomitant to this stress response is the identification of multimerization quality control, regulated by BTBD9. MQC surveys multimeric BTB complex composition, ensuring that multimeric complexes contain the correct stoichiometries and compositions. MQC is critical for development, as loss of MQC als prevents myogenesis. These two chapters showcase the integration of ubiquitin signaling, stress/quality control pathways, and development. These writings provide a more holistic understanding into the robust regulatory underpinnings of organismal formatio