Journal of Cell Biology最新文献

Microtubule and actin are the two major cytoskeletal polymers that form organized functional structures in the interior of eukaryotic cells. Although the structural mechanics of the cytoskeleton has been extensively studied by direct manipulations in in vitro reconstitution systems, such unambiguous characterizations inside the living cell are sparse. Here, we report a comprehensive analysis of how the microtubule and actin cytoskeletons structurally respond to direct intracellular load. Ferrofluid-based intracellular magnetic tweezers reveal rheological properties of the microtubule complex primarily determined by filamentous actin. The strain fields of the microtubule complex and actin meshwork follow the same scaling, suggesting that the two cytoskeletal systems behave as an integrated elastic body. The structural responses of single microtubules to contact and remote forces further evidence that the individual microtubules are enclosed by the elastic medium of actin. These results, directly characterizing the microtubule and actin cytoskeletons as an interacting continuum throughout the cytoplasm, serve as a cornerstone for the physical understanding of intracellular organization.

Macroautophagy (often-named autophagy), a catabolic process involving autophagy-related (Atg) genes, prevents the accumulation of harmful cytoplasmic components and mobilizes energy reserves in long-lived and self-renewing cells. Autophagy deficiency affects antigen presentation in conventional dendritic cells (DCs) without impacting their survival. However, previous studies did not address epidermal Langerhans cells (LCs). Here, we demonstrate that deletion of either Atg5 or Atg7 in LCs leads to their gradual depletion. ATG5-deficient LCs showed metabolic dysregulation and accumulated neutral lipids. Despite increased mitochondrial respiratory capacity, they were unable to process lipids, eventually leading them to ferroptosis. Finally, metabolically impaired LCs upregulated proinflammatory transcripts and showed decreased expression of neuronal interaction receptors. Altogether, autophagy represents a critical regulator of lipid storage and metabolism in LCs, allowing their maintenance in the epidermis.

Separase regulates multiple aspects of the metaphase-to-anaphase transition. Separase cleaves cohesin to allow chromosome segregation and localizes to vesicles to promote exocytosis. The anaphase-promoting complex/cyclosome (APC/C) activates separase by ubiquitinating its inhibitory chaperone, securin, triggering its degradation. How this pathway controls the exocytic function of separase is unknown. During meiosis I, securin is degraded over several minutes, while separase rapidly relocalizes from kinetochore structures at the spindle and cortex to sites of action on chromosomes and vesicles at anaphase onset. The loss of cohesin coincides with the relocalization of separase to the chromosome midbivalent at anaphase onset. APC/C depletion prevents separase relocalization, while securin depletion causes precocious separase relocalization. Expression of non-degradable securin inhibits chromosome segregation, exocytosis, and separase localization to vesicles but not to the anaphase spindle. We conclude that APC/C-mediated securin degradation controls separase localization. This spatiotemporal regulation will impact the effective local concentration of separase for more precise targeting of substrates in anaphase.

Protein secretion is an essential process that drives cell growth and communication. Enrichment of soluble secretory proteins into ER-derived transport carriers occurs via transmembrane cargo receptors that connect lumenal cargo to the cytosolic COPII coat. Here, we find that the cargo receptor, SURF4, recruits different SEC24 cargo adaptor paralogs of the COPII coat to export different cargoes. The secreted protease, PCSK9, requires both SURF4 and a co-receptor, TMED10, for export via SEC24A. In contrast, secretion of Cab45 and NUCB1 requires SEC24C/D. We further show that ER export signals of Cab45 and NUCB1 bind co-translationally to SURF4 via a lumenal pocket, contrasting prevailing models of receptor engagement only upon protein folding/maturation. Bioinformatics analyses suggest that strong SURF4-binding motifs are features of proteases, receptor-binding ligands, and Ca2+-binding proteins. We propose that certain classes of proteins are fast-tracked for rapid export to protect the health of the ER lumen.

Here, we report that the RTN3L-SEC24C endoplasmic reticulum autophagy (ER-phagy) receptor complex, the CUL3KLHL12 E3 ligase that ubiquitinates RTN3L, and the FIP200 autophagy initiating protein, target mutant proinsulin (Akita) condensates for lysosomal delivery at ER tubule junctions. When delivery was blocked, Akita condensates accumulated in the ER. In exploring the role of tubulation in these events, we unexpectedly found that loss of the Parkinson's disease protein, PINK1, reduced peripheral tubule junctions and blocked ER-phagy. Overexpression of the PINK1 kinase substrate, DRP1, increased junctions, reduced Akita condensate accumulation, and restored lysosomal delivery in PINK1-depleted cells. DRP1 is a dual-functioning protein that promotes ER tubulation and severs mitochondria at ER-mitochondria contact sites. DRP1-dependent ER tubulating activity was sufficient for suppression. Supporting these findings, we observed PINK1 associating with ER tubules. Our findings show that PINK1 shapes the ER to target misfolded proinsulin for RTN3L-SEC24C-mediated macro-ER-phagy at defined ER sites called peripheral junctions. These observations may have important implications for understanding Parkinson's disease.

VPS13 proteins mediate lipid transfer across membranes. Among them, VPS13B is associated with Golgi membranes, and VPS13B mutations cause Cohen syndrome. In this issue, Ugur et al. (https://doi.org/10.1083/jcb.202311189) and Du et al. (https://doi.org/10.1083/jcb.202402083) reveal new VPS13B interactors and their functions in Golgi organization and trafficking.

ER-phagy is an evolutionarily conserved mechanism crucial for maintaining cellular homeostasis. However, significant gaps persist in our understanding of how ER-phagy and the ER network vary across cell subtypes, tissues, and organs. Furthermore, the pathophysiological relevance of ER-phagy remains poorly elucidated. Addressing these questions requires developing quantifiable methods to visualize ER-phagy and ER architecture in vivo. We generated two transgenic mouse lines expressing an ER lumen-targeting tandem RFP-GFP (ER-TRG) tag, either constitutively or conditionally. This approach enables precise spatiotemporal measurements of ER-phagy and ER structure at single-cell resolution in vivo. Systemic analysis across diverse organs, tissues, and primary cultures derived from these ER-phagy reporter mice unveiled significant variations in basal ER-phagy, both in vivo and ex vivo. Furthermore, our investigation uncovered substantial remodeling of ER-phagy and the ER network in different tissues under stressed conditions such as starvation, oncogenic transformation, and tissue injury. In summary, both reporter models represent valuable resources with broad applications in fundamental research and translational studies.

Mitochondrial reactive oxygen species (ROS) function intrinsically within cells to induce cell damage, regulate transcription, and cause genome instability. However, we know little about how mitochondrial ROS production non-cell autonomously impacts cell-cell signaling. Here, we show that mitochondrial dysfunction inhibits the plasma membrane localization of cell surface receptors that drive cell-cell communication during oogenesis. Within minutes, we found that mitochondrial ROS impairs exocyst membrane binding and leads to defective endosomal recycling. This endosomal defect impairs the trafficking of receptors, such as the Notch ligand Delta, during oogenesis. Remarkably, we found that overexpressing RAB11 restores ligand trafficking and rescues the developmental defects caused by ROS production. ROS production from adjacent cells acutely initiates a transcriptional response associated with growth and migration by suppressing Notch signaling and inducing extra cellualr matrix (ECM) remodeling. Our work reveals a conserved rapid response to ROS production that links mitochondrial dysfunction to the non-cell autonomous regulation of cell-cell signaling.

Two recent papers by Szentgyörgyi et al. (http://doi.org/10.1083/jcb.202401167) and Pankiv et al. (http://doi.org/10.1083/jcb.202408173) provide new insights into the roles of BEACH domain proteins in membrane trafficking and cellular homeostasis. They explore which membranes they are recruited to, how they are recruited, and the potential coat-like functions of these proteins.

Joseph Grafton Gall (1928-2024), a founder of modern cell biology, made foundational discoveries on eukaryotic chromosomes and RNA biogenesis. His major contributions include the development of in situ hybridization (later called FISH), demonstration of one DNA double helix/chromosome, isolation of the first eukaryote gene, localization of satellite DNA to centromeric heterochromatin, determination of the first telomeric DNA sequence, and elucidating the structure and functions of Cajal bodies. He was an expert microscopist, a scholar of science history, and an avid naturalist. These attributes, together with his ready embrace of new technologies, contributed to his remarkable success. He was also an early and strong supporter of women in science. His contributions to science and mentoring were recognized by numerous awards including the American Society for Cell Biology's E.B. Wilson Medal, the Society for Developmental Biology's Lifetime Achievement Award, the Albert Lasker Special Achievement Award in Medical Research, and the AAAS Mentor Award for Lifetime Achievement.