The Rcs signal transduction system is a phosphorelay responsible for sensing enterobacterial cell envelope stresses. In Escherichia coli, the Rcs system is required to survive treatment with A22 and mecillinam, antibiotics that perturb cell size. To test whether size changes are correlated with envelope damage and thereby sensed by the Rcs system, we tuned E. coli cell size via A22 treatment, mutations in the cell-shape determinant MreB, and mechanically confined growth. In all conditions, cell width was strongly correlated with Rcs activation, and RcsF, the outer-membrane-localized upstream component, was essential for responding to cell width changes. Several gene deletions that induce Rcs resulted in cells that were wider than wild-type. Cryo-electron microscopy revealed that the periplasm of a wide MreB mutant is ~3 nm thinner than in wild-type cells, bringing RcsF closer to the downstream, inner-membrane-localized components of the signaling cascade. Conversely, extending the RcsF linker region in wild-type cells by ~3 nm increased Rcs activity. Thus, we propose that the Rcs system responds to changes in cell width due to altered periplasmic thickness.

The Asgard phylum has emerged as a model to study eukaryogenesis because of their close relatedness with the eukaryotes. In this study, we use FtsZ proteins from a member of the class Odinarchaeia as representatives to investigate the probable origin, evolution, and assembly of the FtsZ/tubulin protein superfamily in Asgard archaea. We performed a comparative analysis of the biochemical properties and cytoskeletal assembly of FtsZ1 and FtsZ2, the two FtsZ isoforms in the Odinarchaeota metagenome. Our electron microscopy analysis reveals that OdinFtsZ1 assembles into curved single protofilaments, while OdinFtsZ2 forms stacked spiral ring-like structures. Upon sequence analysis, we identified an N-terminal amphipathic helix in OdinFtsZ1, which mediates direct membrane tethering. In contrast, OdinFtsZ2 is recruited to the membrane by the anchor OdinSepF via OdinFtsZ2’s C-terminal tail. Overall, we report the presence of two distant evolutionary paralogs of FtsZ in Odinarchaeota, with distinct filament assemblies and differing modes of membrane targeting. Our findings highlight the diversity of FtsZ proteins in the archaeal phylum Asgardarchaeota, providing valuable insights into the evolution and differentiation of tubulin-family proteins.

The meiotic segregation pattern to generate haploid gametes is mediated by step-wise cohesion removal by separase, first from chromosome arms in meiosis I, and then from the pericentromere in meiosis II. In mammalian oocytes, separase is tightly controlled during the hours-long prometaphase and until chromosome segregation in meiosis I, activated for a short time window, and again inhibited until metaphase II arrest is lifted by fertilization. Centromeric cohesin is protected from cleavage by Sgo2-PP2A in meiosis I. It remained enigmatic how tight control of alternating separase activation and inactivation is achieved during the two divisions in oocytes, and when cohesin protection is put in place and removed. Using complementation assays in knock-out mouse models, we established the contributions of cyclin B1 and securin for separase inhibition during both divisions. When eliminating separase inhibition, we found that cohesin is not robustly protected at meiosis I resumption and during metaphase II arrest. Importantly, in meiosis II, the sole event required for cleavage of pericentromeric cohesin besides separase activation is prior kinetochore individualization in meiosis I.

Ribosome-associated protein quality control (RQC) is a surveillance system that identifies and processes aberrant mRNAs with collided ribosomes. ZNF598 plays a key role by ubiquitinating the 40S subunit of collided ribosomes. However, how ZNF598 distinguishes stalled from transient ribosome collisions remains unclear. To address this, we developed a method to visualize the binding of a single protein to a specific mRNA while simultaneously determining its translation status. By endogenously tagging ZNF598 with HaloTag, we observed its strong interaction with RQC reporter mRNAs. We discovered that multiple ZNF598s engage with a single RQC mRNA, suggesting that ZNF598 recognizes more than just the leading collided ribosome in a queue. Overexpressing ZNF598 increased the ribosomal clearance rate, indicating that it is a rate-limiting factor for RQC. Interestingly, a subset of supposedly “normal” mRNAs may be damaged and targeted by ZNF598, underscoring the importance of RQC to maintain the proteome quality even in unstressed conditions. Under global UV-induced RNA damage, ZNF598 recruitment to the reporter RQC mRNA diminished, highlighting its role as a limiting factor in managing widespread ribosome collisions.

Homeostatic control of cellular oxygen availability is a crucial feature of all eukaryotic life, and central to this process is the ability to sense oxygen across a broad range of concentrations and time scales. Much of our understanding of the molecular mechanisms underpinning oxygen sensing has been obtained using cell culture models, yet the biophysical properties of oxygen combined with the complex nature of cellular O2 consumption can make the interpretation of such data difficult. In this commentary, we have outlined some of the main problems encountered in measuring and manipulating cell monolayer oxygenation in vitro, and contextualised them using both historical and contemporary examples.

The cellular response to lysosomal damage involves fine-tuned mechanisms of membrane repair, lysosome regeneration and lysophagy, but how these different processes are coordinated is unclear. Here we show in human cells that the deubiquitinating enzyme ATXN3 helps restore integrity of the lysosomal system after damage by targeting K48-K63-branched ubiquitin chains on regenerating lysosomes. We find that ATXN3 is required for lysophagic flux after lysosomal damage but is not involved in the initial phagophore formation on terminally damaged lysosomes. Instead, ATXN3 is recruited to a distinct subset of lysosomes that are decorated with phosphatidylinositol-(4,5)-bisphosphate and that are not yet fully reacidified. There, ATXN3, along with its partner VCP/p97, targets and turns over K48-K63-branched ubiquitin conjugates. ATXN3 thus facilitates degradation of a fraction of LAMP2 via microautophagy to regenerate the lysosomal membrane and to thereby reestablish degradative capacity needed also for completion of lysophagy. Our findings identify a key role of ATXN3 in restoring lysosomal function after lysosomal membrane damage and uncover K48-K63-branched ubiquitin chain-regulated regeneration as a critical element of the lysosomal damage stress response.

Macrophages are primary immune cells involved in obesity-triggered chronic low-grade inflammation in adipose tissues. Prostaglandin E2 (PGE2), mainly generated from macrophages, can regulate adipose tissue remodeling, yet the underlying mechanisms are not fully understood. Here, we observed that PGE2 receptor subtype 3 (EP3) was remarkably downregulated in adipose tissue macrophages from high-fat diet (HFD)-fed mice and patients with obesity. Notably, macrophage-specific deletion of EP3 exacerbated HFD-induced fat expansion, whereas EP3α isoform overexpression in macrophages alleviated obesity phenotypes. Further, EP3 deficiency suppressed secretion of anti-adipogenic matricellular protein SPARC from macrophages. SPARC deletion in macrophages abrogated the protection of EP3-overexpression against diet-induced obesity. Mechanistically, EP3 activation promoted SPARC expression by suppressing DNA methylation in macrophages through a PKA-Sp1-Dnmt1/3a signaling cascade. Finally, EP3 agonist treatment ameliorated HFD-induced obesity in mice. Thus, EP3 inhibits adipogenesis through promoting release of SPARC from macrophages, suggesting a novel therapeutic target for diet-induced obesity.

Caspase-4 and caspase-11 (CASP4/11) sense bacterial lipopolysaccharide (LPS). Currently available inhibitors of CASP4/11 also block the activity of caspase-1 (CASP1), which restricts their usefulness in the study of CASP4/11 functions, as well as their clinical potential for the treatment of LPS-linked diseases through CASP4/11 inhibition. Here, we identify mitochondrial cardiolipin as a selective inhibitor of CASP4/11-dependent cell death and inflammatory cytokine secretion, without affecting CASP1 function. Cardiolipin targets the CARD domain of CASP4/11, impeding its interaction with LPS to restrain CASP4/11 activation, thereby suppressing LPS-induced systemic inflammation in vivo. By identifying cardiolipin as a selective inhibitor of CASP4/11, we provide an urgently needed tool for studying caspase-4/11 and noncanonical inflammasome functions in inflammatory pathways and LPS-induced pathogenesis.

[Image: see text]

CX3CR1+ cells generate tissue macrophages in the developing heart and play cardioprotective roles in response to ischemic injuries in the adult heart. However, the origin and fate of CX3CR1+ cells during cardiogenesis remain unclear. Here, we performed genetic lineage tracing of CX3CR1+ cells and their progeny (termed Cx3cr1 lineage cells) in the mouse and demonstrated that they emerge from a subset of epiblast cells at embryonic day E6.5 and contribute to the parietal endoderm cells at E7.0. At E8.0–9.5 of development, Cx3cr1 lineage cells produced cardiomyocytes and endothelial cells via both de novo differentiation and fusion with pre-existing cardiomyocytes or endothelial cells, respectively. Cx3cr1 lineage cells persisted in the adult heart, comprising ~13% of cardiomyocytes and ~31% of endothelial cells. Additionally, CX3CR1+ cells differentiated from mouse embryonic stem cells generated cardiomyocytes, endothelial cells, and macrophages in vitro, ex vivo, and in vivo. Single-cell RNA sequencing revealed that Cx3cr1+ cells represent an intermediate cell population transitioning from embryonic stem cells to mesoderm. Taken together, embryonic CX3CR1+ cells constitute a multipotent epiblast-derived progenitor population that contributes not only to the formation of macrophages, but also of cardiomyocytes and endothelial cells.