While Polycomb-based gene silencing relies upon the generation of large, condensed domains of histone H3K27 trimethylation, the mechanisms forming and maintaining these domains are incompletely understood. Several previously identified mechanisms concentrate PRC2 and its H3K27me3 activity, including allosteric activation, binding to H2Aub, and concentration into PcG bodies through weak multivalent interactions with other Polycomb group proteins. Here, using the Drosophila nurse cell system to study silencing initiation in developing tissue, we discover a new mechanism. The scaffolding protein Sex comb on midleg (Scm) first polymerizes into PcG bodies independently of PRC1 and PRC2. These Scm-containing bodies then recruit PRC2 through a direct, structured interaction between the Scm–Zf-FCS domain and the Pcl atypical Tudor domain. Finally, we show that the Scm–Zf-FCS/Pcl–Tudor interaction is required for viability and Hox gene regulation during somatic cell development. Our work discovers a new mechanism anchoring PRC2 to PcG bodies that explains how cells can rapidly create and maintain large domains of H3K27me3. It also raises a new possibility that cells could regulate Scm polymerization to determine the size and location of H3K27me3 domains.
The keratin intermediate filament (IF) cytoskeleton provides mechanical support to load-bearing epithelial cells, but its composition varies depending on cell type and tissue context. This variation, the large size of the keratin protein family, and emerging noncanonical functions of individual keratins suggest that the keratin cytoskeleton may have additional, more complicated roles. We report a surprising interaction between the keratin IF cytoskeleton in epithelial cells and soluble oligomers of vimentin, the principal IF component in mesenchymal cells. Rather than forming a cytoskeletal network, soluble vimentin functions as a recruitment factor, organizing regulatory kinases and nonmuscle myosin on keratin filaments to promote signaling. Soluble vimentin and keratin filaments interact through the intrinsically disordered head domain of each protein. Because vimentin preferentially interacts with keratin 6A-containing filaments, increasing keratin 6A expression, as occurs during skin wound healing and cancer development, triggers this myosin-activating signaling cascade. These results demonstrate how varying composition, soluble pools, and client-scaffold interactions through intrinsically disordered regions allow IFs to organize cellular signals in parallel to their canonical mechanical role, supporting context-dependent cell and tissue regulation.
How mammalian organ size is precisely regulated remains incompletely understood. This process depends on the coordinated balance between cell proliferation, differentiation, and apoptosis. In the heart, the epicardium plays a central role in embryonic cardiac growth as a source of multiple cardiac progenitor lineages. The transcription factor Wt1 is essential for epicardial development and maturation, and its loss disrupts epicardial maintenance and the generation of epicardial-derived cells (EPDCs). In addition, the epicardium produces mitogenic factors required for cardiomyocyte (CM) proliferation, including those of the insulin-like growth factor ( IGF ) signaling pathway. Consistently, functional inactivation of Igf1 or Igf2 in mice results in severe prenatal growth reduction, with mutant embryos ∼60% smaller than controls, underscoring the roles of these genes in organ and fetal growth. We recently identified the cardiac lymphatic vasculature as an unexpected regulator of heart size during embryonic development. Here, by combining single-cell RNA sequencing with loss-of-function mouse models, we uncover a critical regulatory network linking cardiac lymphatics, the epicardium, Wt1 , and Igf1 signaling that controls the final stages of embryonic cardiac growth. Our findings suggest that cardiac lymphatics act as a physiological quality control system for heart growth. Furthermore, using gain-of-function approaches in human epicardial organoids, we demonstrate that Reelin promotes epicardial fate, suggesting that the beneficial roles of lymphatics in adult cardiac repair are likely mediated through Reelin-dependent epicardial reactivation.
Lineage-defining transcription factors are key oncogenic drivers but remain difficult to target pharmacologically due to the absence of ligandable pockets. The molecular rules governing substrate recognition by large HECT ubiquitin ligases also remain incompletely understood, limiting efforts to exploit these enzymes for targeted protein degradation. Here we combine genome-wide CRISPR knockout screening with base editor tiling screens at amino acid resolution, both coupled to an endogenous knock-in reporter of the SCLC lineage oncogenic transcription factor ASCL1, to systematically interrogate the mechanisms governing its degradation. These complementary screens unbiasedly identify the HECT ubiquitin ligase HUWE1 as the dominant regulator of ASCL1 stability in small cell lung cancer (SCLC) and resolve a conserved C-terminal phospho-degron centered on Ser207 and terminal Trp/Phe residues that are required for HUWE1 docking and ubiquitin-mediated degradation. Unexpectedly, base editor screening further uncovers a previously unrecognized regulatory module within HUWE1: a short negatively charged helix that functions as an autoinhibitory gate controlling access of phospho-degron substrates to HUWE1. Charge-flipping mutations within this regulatory helix relieve autoinhibition and accelerate degradation of multiple HUWE1 phospho-degron substrates, including ASCL1 and the canonical HUWE1 substrate DDIT4. Stabilization of ASCL1 through degron disruption paradoxically impairs SCLC proliferation, revealing that dynamic proteasome-coupled turnover is required for transcription factor function. Together, these findings reveal molecular rules governing HUWE1 phospho-degron recognition and identify a regulatory gate controlling substrate engagement. They also illustrate a generalizable strategy for resolving degradation mechanisms of undruggable transcription factors in their endogenous cellular context.
Noninfectious granulomas organize immune cells into persistent tissue aggregates without an identifiable pathogen. Across diverse diseases, inflammation that should resolve instead persists, producing hallmark pathologic changes in tissue architecture that compromise organ function. Here, we synthesize recent studies to move beyond linear cytokine models and propose that granulomas persist through regulatory circuits linking immune cells, metabolism, and stroma. Maintenance reflects reinforcement among these local programs rather than a single inflammatory pathway. Comparisons across granulomatous diseases, including sarcoidosis, can distinguish disease-specific wiring from conserved maintenance programs. Defining these programs should identify strategies to dismantle established lesions.
