Molecular Biology of the Cell最新文献

Cancer cell invasion relies on dynamic cell shape changes, which originate from protrusive and contractile intracellular forces. Previous studies revealed that contractile forces are controlled by positive-feedback amplification of the contraction regulator Rho by Lbc GEFs. These GEFs were previously linked to tumor progression; however, the underlying mechanisms are poorly understood. Here, we generated a mouse melanoma model in which cytosolic levels of the Lbc GEF GEF-H1 are controlled by light. Using this model, we found that increased GEF-H1 levels strongly stimulate cell contraction dynamics. Interestingly, increased contraction dynamics rapidly induced expansion of tumor spheroids via a focal adhesion kinase-dependent mechanism. Furthermore, long-term stimulation led to the escape of individual cells from spheroids. These findings reveal new insights into the oncogenic roles of Lbc GEFs and how they might promote tumor cell invasion. We propose a mechanism in which increased cell contraction dynamics result in asymmetric pulling forces at the tumor border, promoting the detachment and escape of individual cells.

Lipid droplets are increasingly recognized as necessary organelles. However, the cellular pathways that regulate lipid droplets have only been defined in select fungi, algae, plants, and animals. Our experiments expand the study of lipid droplets to an evolutionarily distinct model organism, the ciliate Tetrahymena thermophila. We identify conserved pathways that promote lipid droplet homeostasis while also uncovering features that suggest adaptation. We show that Tetrahymena accumulate lipid droplets in response to nutrient deprivation, including starvation and the stationary phase. Pulse-chase experiments with a fluorescent fatty acid analogue demonstrate lipid trafficking to lipid droplets in starved cultures. Unlike other cell types, starved Tetrahymena appear to use both peroxisomes and mitochondria (not vacuoles) for further fatty acid catabolism. We observe cooccurence of the fluorescent fatty acid analogue with markers of peroxisomes and a subpopulation of mitochondria, suggesting specialized catabolic roles for both organelles. We demonstrate a decrease in survival following starvation in the presence of inhibitors of mitochondrial fatty acid import or peroxisomal fatty acid metabolism. Together, our experiments add Tetrahymena to the expanding list of eukaryotes that increase lipid droplets in response to nutrient depletion while also uncovering important and distinct roles for mitochondrial and peroxisomal catabolism in survival pathways.

Centromeres are essential chromosomal components that ensure proper cell division by serving as assembly sites for kinetochores, which connect chromosomes to spindle microtubules. Centromeres are marked by the evolutionarily conserved centromere-specific histone H3 variant, CENP-A, which is deposited into centromere nucleosomes during G₁ in human cells. Centromeres retain cohesin, a ring-like protein complex, during mitosis, protecting sister chromatid cohesion and centromere transcription to prevent chromosome missegregation. Previous work in Drosophila has suggested that centromere transcription and centromeric RNAs are important for CENP-A deposition in chromatin. During mitosis, centromeric cohesin is critical for centromere transcription. However, it is not clear how or whether centromeric transcription and cohesin contribute to CENP-A deposition in G₁ in human cells. To address these questions, we combined a cell synchronization strategy with the Auxin Inducible Degron technology and transcription inhibition in human cells. In contrast with Drosophila cells, our results demonstrated that neither centromeric transcription nor cohesin is required for CENP-A deposition in human cells. Our data demonstrate clear differences in the CENP-A deposition mechanism between human and Drosophila cells. These findings provide deeper insights into the plasticity underlying centromere maintenance and highlight evolutionary divergence in centromere maintenance systems across species.

Phenotypic heterogeneity-distinct molecular and behavioral variations within a population-significantly influences collective invasion and tumor progression. Here, we use a molecular approach to explore how the underlying metabolic heterogeneity in non-small cell lung cancer (NSCLC) influences invasion and pack patterning. Assessment of  three-dimensional (3D) pack patterning revealed invasive heterogeneity across NSCLC cell lines and patient-derived samples. Flow cytometry identified IL13RA2 as a biomarker for invasive potential, enabling isolation of subpopulations with distinct invasive characteristics. By integrating a cell surface biomarker (IL13RA2±) with mitochondrial membrane potential (TMRM), we identified and isolated three distinct subpopulations. Two-dimensional (2D) analyses revealed differences in mitochondrial polarity and transcriptional programs associated with migration and oxygensensitivity. In 3D, these subpopulations invaded with distinct patterns, from contiguous circular packs to structured chains. Assessments under varied oxygen tension demonstrated that oxygen availability and subpopulation metabolism together influence collective invasion patterning. When recombined at ratios recapitulating the original population, both stochastic and opportunistic cooperative dynamics emerged, dependent on subpopulation composition and oxygen levels. Our molecular approach, integrating cell surface and metabolic characteristics, enables the isolation of unique subpopulations and demonstrates that phenotypic and metabolic heterogeneity, population composition, and oxygen availability collectively pattern invasion packs and drive collective invasion.

Actomyosin rings are specializations of the nonmuscle actomyosin cytoskeleton that drive cell shape changes during division, wound healing, and other events. Contractile rings throughout phylogeny and in a range of cellular contexts are built from conserved components, including nonmuscle myosin II, actin filaments, and cross-linking proteins. To explore whether diverse actomyosin rings generate contractile force and close via a common mechanism, we studied three instances of ring closure within the continuous cytoplasm of the Caenorhabditis elegans syncytial oogenic germline: mitotic cytokinesis of germline stem cells, apoptosis of meiotic compartments, and cellularization of oocytes. The three ring types exhibited distinct closure kinetics and component protein abundance dynamics. We formulated a physical model to relate measured closure speed and molecular composition dynamics to ring active stress and viscosity. We conclude that these ring intrinsic factors vary among the ring types. Our model suggests that motor and nonmotor cross-linkers' abundance and distribution along filaments are important to recapitulate observed closure dynamics. Thus, our findings suggest that across ring closure contexts, fundamental contractile mechanics are conserved, and the magnitude of contractile force is tuned via regulation of ring component abundance and distribution. These results motivate testable hypotheses about cytoskeletal regulation, architecture, and remodeling.

Cell size is strongly correlated with several biological processes, including the cell cycle and growth. Here, we investigated the regulation of stem cell size during Drosophila central nervous system (CNS) development and its association with cell fate. We note that neural stem cells (NSC) in different regions of the ventral nerve cord increase their size at different rates. Thoracic NSCs grow at a faster rate compared with those in the abdominal region during larval development. We show that in addition to the known role in apoptosis and nervous system remodeling, larval expression of abdA is crucial in regulating the rate of postembryonic NSCs size increase, their timely exit from G₂ phase and mitotic rate. We demonstrate that when abdA expression is lost in abdominal NSCs, their size increases, they exhibit a shorter G₂ phase, enter mitosis earlier, and divide more rapidly. Conversely, the introduction of abdA in thoracic NSCs slows their growth and delays their entry into mitosis. We demonstrate that abdA-mediated NSC size regulation acts downstream of their nutrition-induced activation, thereby fine-tuning the stem cell potential spatiotemporally. This study highlights the instructive role of abdA in regulating various fates of larval NSCs during CNS patterning.

At the end of the 19th century, Rayleigh and Plateau explained the physical principle behind the fragmentation of a liquid jet into regular droplets. The classical Rayleigh-Plateau instability concerns liquid jets governed by inertia and surface tension, whereas biological tubes are membrane-bounded and inertia-free. We therefore refer to the process observed here as a pearling instability, formally analogous to Rayleigh-Plateau but dominated by membrane mechanics. Although pearling-type instabilities have long been recognized in lipid tubes and some biological systems, a clear physiological example remained elusive. Here, we present results showing that pearling instability occurs during the physiological process of platelet formation. Platelets are formed from megakaryocytes by the extension of long protrusions, called proplatelets. As they extend in the bloodstream, proplatelets become pearled and detach, circulating in the peripheral blood before their fragmentation into calibrated platelets. We propose that this pearling, by creating regular constrictions along proplatelets, is key to the process of proplatelet fragmentation into individual platelets of calibrated size. Pearling instability thus acts as a mechanobiological regulator allowing local delivery of the right size platelets to the right place at the right time. Our observations quantitatively match parameter-free theoretical predictions for membrane pearling, supporting a unified physical picture.

Mitochondrial networks exhibit remarkable dynamics that are driven in part by fission and fusion events. However, there are other reorganizations of the network that do not involve fission and fusion. One such exception is the elusive "beads-on-a-string" morphological transition of mitochondria. During such transitions, the cylindrical tubes of the mitochondrial membrane transiently undergo shape changes to a string of "pearls" connected along thin tubes. These dynamics have been observed in many contexts and given disparate explanations. Here, we unify these observations by proposing a common underlying mechanism based on the biophysical properties of tubular fluid membranes, for which it is known that, under particular regimes of tension and pressure, membranes reach an instability and undergo a shape transition to a string of connected pearls. First, we use high-speed light-sheet microscopy to show that transient, short-lived pearling events occur spontaneously in the mitochondrial network in every cell type we have examined, including during T-cell activation, neuronal firing, and replicative senescence. This high-temporal data reveals two distinct classes of spontaneous pearling, triggered either by ionic flux or cytoskeleton tension. We then induce pearling with chemical, genetic, and mechanical perturbations and establish three main physical causes of mitochondrial pearling: 1) ionic flux producing internal osmotic pressure, 2) membrane packing lowering bending elasticity, and 3) external mechanical force increasing membrane tension. Pearling dynamics thereby reveal a fundamental biophysical facet of mitochondrial biology. We suggest that pearling should take its place beside fission and fusion as a key process of mitochondrial dynamics, with implications for physiology, disease, and aging.

Alternative splicing expands proteomic diversity and is tightly regulated by splicing factors, including the serine/arginine-rich (SR) protein family. Here, we analyze the poorly characterized protein SRSF12. Although SRSF12 is conserved across vertebrates, it is lowly expressed in most mammals, and we find that SRSF12 knockout mice do not display overt physiological or transcriptomic alterations. In contrast, SRSF12 is more highly expressed in primates, where it is predominantly transcribed in the testes, oocytes, and brain. SRSF12 colocalizes with other splicing components to nuclear speckles and interacts with splicing regulatory factors in cultured human cells. Strikingly, ectopic expression of SRSF12 in human cells induces widespread transcriptional changes, activating meiosis-, testis-, and brain-specific genes. SRSF12 overexpression also leads to mitotic arrest and cell death, phenotypes that require both its structured RNA recognition motif and intrinsically disordered arginine/serine-rich C-terminal domain. Together, our results suggest that SRSF12 has evolved primate-specific expression to regulate testis- and brain-specific genes.