Journal of Cell Biology最新文献

Despite the well-established role of condensin II in mitotic chromosome assembly, its function in interphase chromosome organization remains poorly understood. Here, we applied multiscale FISH techniques to human cell lines engineered for single or double depletion of condensin II and cohesin and examined their functional collaboration at two distinct stages of the cell cycle. Our results demonstrate that a functional interplay between condensin II and cohesin during the mitosis-to-G1 transition is critical for establishing chromosome territories (CTs) in the newly assembling nucleus. During the G2 phase, condensin II and cohesin cooperate to maintain global CT morphology, although they act at different genomic scales. Strikingly, double depletion of both complexes causes CTs to collapse and accumulate abnormally at the nucleolar periphery. Based on these findings, we will discuss how the condensin and cohesin complexes act in an orderly and cooperative manner to orchestrate chromatin dynamics across genomic scales, thereby supporting higher-order chromosome organization throughout the cell cycle.

Vertebrate genes function in specific tissues and stages, so their functional studies require conditional knockout or editing. In zebrafish, spatiotemporally inducible genome editing, particularly during early embryogenesis, remains challenging. Here, we establish inducible Cas9-based editing in defined cell types and stages. The nCas9ERT2 fusion protein, consisting of Cas9 and an estrogen receptor flanked by two nuclear localization signals, is usually located in the cytoplasm and efficiently translocated into nuclei upon 4-hydroxytamoxifen (4-OHT) treatment in cultured cells or embryos. As a proof of concept, we demonstrate that genes in primordial germ cells in embryos and germ cells in adult ovaries from a transgenic line with stable expression of nCas9ERT2 and gRNAs can be mutated by 4-OHT induction. The system also works in early mouse embryos. Thus, this inducible nCas9ERT2 approach enables temporospatial gene editing at the organismal level, expanding the tissue- and stage-specific gene-editing toolkit.

Loss-of-function studies are a central approach to understanding gene/protein function. In mice, this often relies upon heritable recombination at the DNA level. This approach is slow and nonreversible, which limits both spatial and temporal resolution of analysis. Recently, degron techniques that directly target proteins for degradation have been successfully used to quickly and reversibly knock down proteins. Currently, these systems have been limited by lack of tissue/cell type specificity. Here, we generated mice that allow spatial and temporal control of GFP-tagged protein degradation. This DegronGFP line leads to degradation of GFP-tagged proteins in different cellular compartments and in distinct cell types. Further, it is rapid and reversible. We used DegronGFP to probe the function of the glucocorticoid receptor in the epidermis and demonstrate that it has distinct functions in proliferative and differentiated cells-an analysis that would not have been possible with traditional recombination approaches. We propose that the ability to use GFP knock-in lines for loss-of-function analysis will provide additional motivation for generation of these useful tools.

Hundreds of mitochondrial proteins rely on N-terminal presequences for organellar targeting and import. While generally described as positively charged amphiphilic helices, presequences lack a consensus motif and thus likely promote protein import into mitochondria with variable efficiencies. Indeed, the concept of presequence strength underlies biological models such as stress sensing, yet a quantitative analysis of what dictates strong versus weak presequences is lacking. Furthermore, the extent to which presequence strength affects mitochondrial function and cellular fitness remains unclear. Here, we capitalize on the MitoLuc protein import assay to define multiple aspects of presequence strength. We find that select presequences, including those that regulate the mitochondrial unfolded protein response (UPRmt), impart differential import efficiencies during mitochondrial uncoupling. Surprisingly, we find that presequences beyond those associated with stress signaling promote highly variable import efficiency in vitro, suggesting presequence strength may influence a broader array of processes than currently appreciated. We exploit this variability to demonstrate that only presequences that promote robust in vitro import can fully rescue defects in respiratory growth in complex IV-deficient yeast, suggesting that presequence strength dictates metabolic potential. Collectively, our findings demonstrate that presequence strength can describe numerous metrics, such as total imported protein, maximal import velocity, or sensitivity to uncoupling, suggesting that the annotation of presequences as weak or strong requires more nuanced characterization than typically performed. Importantly, we find that such variability in presequence strength meaningfully affects cellular fitness beyond stress signaling, suggesting that organisms may broadly exploit presequence strength to fine-tune mitochondrial import and thus organellar homeostasis.

Epithelial tissues are populated with accessory cells including pigment-producing melanocytes, which must migrate between tightly adherent epithelial cells, but how cells migrate through confined epithelial spaces without impairing barrier function is poorly understood. Using live imaging of the mouse epidermis, we captured the migration of embryonic melanocytes (melanoblasts) while simultaneously visualizing the basement membrane or epithelial surfaces. We show that melanoblasts migrate through basal and suprabasal layers of the epidermis where they use keratinocyte surfaces, as well as the basement membrane, as substrates for migration. Melanoblasts form atypical and dynamic E-cadherin attachments to keratinocytes that largely lack cytoplasmic catenins known to anchor E-cadherin to F-actin. We show E-cadherin is needed in both melanoblasts and keratinocytes to stabilize migratory protrusions, and that depleting E-cadherin results in reduced melanoblast motility and ventral depigmentation in adult mice. These findings illustrate how migratory cells modify the cell adhesion machinery to invade between connected epithelial cells without interrupting the skin barrier.

Lipid bilayers form the basis of organellar architecture, structure, and compartmentalization in the cell. Decades of biophysical, biochemical, and imaging studies on purified or in vitro-reconstituted liposomes have shown that variations in lipid composition influence the physical properties of membranes, such as thickness and curvature. However, similar studies characterizing these membrane properties within the native cellular context have remained technically challenging. Recent advancements in cellular cryo-electron tomography (cryo-ET) imaging enable high-resolution, three-dimensional views of native organellar membrane architecture preserved in near-native conditions. We previously developed a "Surface Morphometrics" pipeline that generates surface mesh reconstructions to model and quantify cellular membrane ultrastructure from cryo-ET data. Here, we expand this pipeline to measure the distance between the phospholipid head groups of the membrane bilayer as a readout of membrane thickness. Using this approach, we demonstrate thickness variations both within and between distinct organellar membranes. We show that organellar membrane thickness positively correlates with other features, such as membrane curvedness, in cells. Further, we show that subcompartments of the mitochondrial inner membrane exhibit varying membrane thicknesses that are independent of whether the mitochondria are in fragmented or elongated networks. We also demonstrate that our technique, when applied to three-dimensional data, yields results that match existing measurements obtained from two-dimensional data of in vitro samples. Finally, we demonstrate that large membrane-associated macromolecular complexes exhibit distinct density profiles that correlate with local variations in membrane thickness. Overall, our updated Surface Morphometrics pipeline provides a framework for investigating how changes in membrane composition in various cellular and disease contexts affect organelle ultrastructure and function.

Abl-interactor (Abi) proteins induce actin polymerization by activating Wiskott-Aldrich syndrome protein (WASp) or SCAR/WASP-family verprolin-homologous protein. Loss of mammalian Abi1 causes myeloproliferative neoplasm; however, little is known about how the Abi family of actin-regulatory proteins regulates blood cell homeostasis. Here, we demonstrate that Drosophila Abi promotes plasmatocyte-to-crystal cell transdifferentiation but represses plasmatocyte-to-lamellocyte transdifferentiation through Notch signaling. Consistent with a previously demonstrated role of clathrin-mediated endocytosis (CME) in Notch signaling activation, we find that Abi promotes Notch-CME by recruiting WASp and the Notch receptor to nascent sites of CME. Finally, we demonstrate that CME and crystal cell formation are inhibited by Abelson (Abl)-mediated phosphorylation of Abi but require PTP61F, a phosphatase that reverses this phosphorylation. Our findings identify Abi as a critical integrator of actin remodeling and Notch-CME and reveal opposing roles of Abl and PTP61F in regulating Abi activity to maintain blood cell homeostasis.

Interactions between actin filaments and microtubules (MTs) are essential, but how those mechanisms are orchestrated in complex developing systems is poorly understood. Here we show that actin-MT cross talk regulates actin cable assembly and the assembly and organization of MTs in Drosophila nurse cells during oogenesis. We found that a stable, acetylated MT meshwork develops concurrently with actin cable initiation and requires acetylation for its maintenance. These γ-tubulin-nucleated MTs appear to be cortically tethered via Patronin and Shortstop, extend into the cytoplasm, and coalign with the elongating actin cables. We showed that this MT network is required for actin cable initiation and elongation. We further demonstrated that actin filament assembly via Diaphanous and Enabled promotes cortical tethering of MTs and that loss of the actin filament bundlers Quail/Villin, Singed/Fascin, and Fimbrin resulted in fewer, shorter, and more highly coaligned MTs. Together, our data reveal multiple modes of coordinated actin-MT cross talk that are instrumental for oogenesis.

Exosomes are multivesicular body-derived extracellular vesicles that are secreted by metazoan cells. Exosomes have utility as disease biomarkers, and exosome-mediated miRNA secretion has been proposed to facilitate tumor growth and metastasis. Previously, we demonstrated that the Lupus La protein (La) mediates the selective incorporation of miR-122 into metastatic breast cancer-derived exosomes; however, the mechanism by which La itself is sorted into exosomes remains unknown. Using unbiased proximity labeling proteomics, biochemical fractionation, superresolution microscopy, and genetic tools, we establish that the selective autophagy receptor p62 sorts La and miR-122 into exosomes. We then performed small RNA sequencing and found that p62 depletion reduces the exosomal secretion of tumor suppressor miRNAs and results in their accumulation within cells. Our data indicate that p62 is a quality control factor that modulates the miRNA composition of exosomes. Cancer cells may exploit p62-dependent exosome cargo sorting to eliminate tumor suppressor miRNAs and thus to promote cell proliferation.

The type I intermediate filament proteins keratin 14 (K14) and keratin 15 (K15) are common to all complex epithelia. K14 is highly expressed by progenitor keratinocytes, in which it provides essential mechanical integrity and gates keratinocyte entry into differentiation by sequestering YAP1, a transcriptional effector of Hippo signaling, to the cytoplasm. K15 has long been used as a marker of hair bulge stem cells, though its specific role in skin epithelia is unknown. Here, we show that the lack of two biochemical determinants, a cysteine residue within the stutter motif of the central rod domain and a 14-3-3 binding site in the N-terminal head domain, renders K15 unable to effectively sequester YAP1 in the cytoplasm like K14 does. We combine insight obtained from cell culture and transgenic mouse models with computational analyses of transcriptomics data and propose a model in which a higher K15:K14 ratio promotes a progenitor state and antagonizes differentiation in keratinocytes of the epidermis.