In order to replicate, most pathogens need to enter their target cells. Many viruses enter the host cell through an endocytic pathway and hijack endosomes for their journey towards sites of ...replication. For delivery of their genome to the host cell cytoplasm and to avoid degradation, viruses have to escape this endosomal compartment without host detection. Viruses have developed complex mechanisms to penetrate the endosomal membrane and have evolved to co-opt several host factors to facilitate endosomal escape. Conversely, there is an extensive variety of cellular mechanisms to counteract or impede viral replication. At the level of cell entry, there are cellular defense mechanisms that recognize endosomal membrane damage caused by virus-induced membrane fusion and pore formation, as well as restriction factors that block these processes. In this Cell Science at a Glance article and accompanying poster, we describe the different mechanisms that viruses have evolved to escape the endosomal compartment, as well as the counteracting cellular protection mechanisms. We provide examples for enveloped and non-enveloped viruses, for which we discuss some unique and unexpected cellular responses to virus-entry-induced membrane damage.
During development and regeneration, proliferation of tissue-specific stem cells is tightly controlled to produce organs of a predetermined size. The molecular determinants of this process remain ...poorly understood. Here, we investigate the function of Yap1, the transcriptional effector of the Hippo signaling pathway, in skin biology. Using gain- and loss-of-function studies, we show that Yap1 is a critical modulator of epidermal stem cell proliferation and tissue expansion. Yap1 mediates this effect through interaction with TEAD transcription factors. Additionally, our studies reveal that α-catenin, a molecule previously implicated in tumor suppression and cell density sensing in the skin, is an upstream negative regulator of Yap1. α-catenin controls Yap1 activity and phosphorylation by modulating its interaction with 14-3-3 and the PP2A phosphatase. Together, these data identify Yap1 as a determinant of the proliferative capacity of epidermal stem cells and as an important effector of a “crowd control” molecular circuitry in mammalian skin.
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► Yap is a critical modulator of epidermal stem cell proliferation ► In epidermis, Yap's function is mediated by TEAD transcription factors ► α-catenin is a cell density sensor regulating Yap activity in the epidermis ► A 14-3-3/α-catenin complex limits PP2A-mediated Yap dephosphorylation
Microtubules are subjected to a variety of post-translational modifications (PTMs). The combination of different α- and β-tubulin isoforms and PTMs are referred to as the tubulin code. PTMs are ...generated by a suite of enzymes thought to affect tubulin-interacting proteins. One PTM is the cyclic removal and ligation of the C-terminal tyrosine of α-tubulin. This has been implicated in cellular processes such as mitosis, cardiomyocyte contraction, and neuronal function. Recently, vasohibins (VASHs) were identified as the first tubulin-detyrosinating enzymes, A cell-autonomous role for VASHs in regulating the cytoskeleton was unexpected due to their previous association with angiogenesis. This review discusses the functionality of the tubulin detyrosination cycle, the biology of VASHs, and highlights the emerging questions accompanying this link.
The tubulin code is the result of different tubulin isoforms in combination with PTMs found on the microtubules. Together, this generates heterogeneous microtubules.
Tubulin detyrosination affects microtubule-associated proteins that function in a wide-range of biological processes.
After microtubule depolymerization, TTL re-ligates the tyrosine to α-tubulin, making tubulin detyrosination a cyclic event.
VASHs can proteolytically remove the C-terminal tyrosine from α-tubulin. Yet-to-be-identified additional enzymes can also perform this reaction.
VASHs are studied as secreted factors regulating angiogenesis. They are dependent on their interaction with the small protein SVBP that is required for VASH stability but might have additional functions.
Tubulin is subjected to a number of posttranslational modifications to generate heterogeneous microtubules. The modifications include removal and ligation of the C-terminal tyrosine of α-tubulin. The ...enzymes responsible for detyrosination, an activity first observed 40 years ago, have remained elusive. We applied a genetic screen in haploid human cells to find regulators of tubulin detyrosination. We identified SVBP, a peptide that regulates the abundance of vasohibins (VASH1 and VASH2). Vasohibins, but not SVBP alone, increased detyrosination of α-tubulin, and purified vasohibins removed the C-terminal tyrosine of α-tubulin. We found that vasohibins play a cell type–dependent role in detyrosination, although cells also contain an additional detyrosinating activity. Thus, vasohibins, hitherto studied as secreted angiogenesis regulators, constitute a long-sought missing link in the tubulin tyrosination cycle.
Phospholipids are asymmetrically distributed in the plasma membrane. This asymmetrical distribution is disrupted during apoptosis, exposing phosphatidylserine (PtdSer) on the cell surface. Using a ...haploid genetic screen in human cells, we found that ATP11C (adenosine triphosphatase type 11C) and CDC50A (cell division cycle protein 50A) are required for aminophospholipid translocation from the outer to the inner plasma membrane leaflet; that is, they display flippase activity. ATP11C contained caspase recognition sites, and mutations at these sites generated caspase-resistant ATP11C without affecting its flippase activity. Cells expressing caspase-resistant ATP11C did not expose PtdSer during apoptosis and were not engulfed by macrophages, which suggests that inactivation of the flippase activity is required for apoptotic PtdSer exposure. CDC50A-deficient cells displayed PtdSer on their surface and were engulfed by macrophages, indicating that PtdSer is sufficient as an "eat me" signal.
The spatial organization of chromosomes influences many nuclear processes including gene expression. The cohesin complex shapes the 3D genome by looping together CTCF sites along chromosomes. We show ...here that chromatin loop size can be increased and that the duration with which cohesin embraces DNA determines the degree to which loops are enlarged. Cohesin’s DNA release factor WAPL restricts this loop extension and also prevents looping between incorrectly oriented CTCF sites. We reveal that the SCC2/SCC4 complex promotes the extension of chromatin loops and the formation of topologically associated domains (TADs). Our data support the model that cohesin structures chromosomes through the processive enlargement of loops and that TADs reflect polyclonal collections of loops in the making. Finally, we find that whereas cohesin promotes chromosomal looping, it rather limits nuclear compartmentalization. We conclude that the balanced activity of SCC2/SCC4 and WAPL enables cohesin to correctly structure chromosomes.
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•Hi-C analysis demonstrates that chromatin loop size can be increased genome-wide•The duration with which cohesin embraces DNA determines the length of chromatin loops•Haploid genetics reveals that the SCC2/SCC4 complex promotes loop extension•Cohesin limits the compartmentalization of chromatin within the nucleus
Cohesin's dynamic association with DNA determines the length of chromatin loops and allows this complex to correctly structure chromosomes.
Loss of BRCA2 affects genome stability and is deleterious for cellular survival. Using a genome-wide genetic screen in near-haploid KBM-7 cells, we show that tumor necrosis factor-alpha (TNFα) ...signaling is a determinant of cell survival upon BRCA2 inactivation. Specifically, inactivation of the TNF receptor (TNFR1) or its downstream effector SAM68 rescues cell death induced by BRCA2 inactivation. BRCA2 inactivation leads to pro-inflammatory cytokine production, including TNFα, and increases sensitivity to TNFα. Enhanced TNFα sensitivity is not restricted to BRCA2 inactivation, as BRCA1 or FANCD2 inactivation, or hydroxyurea treatment also sensitizes cells to TNFα. Mechanistically, BRCA2 inactivation leads to cGAS-positive micronuclei and results in a cell-intrinsic interferon response, as assessed by quantitative mass-spectrometry and gene expression profiling, and requires ASK1 and JNK signaling. Combined, our data reveals that micronuclei induced by loss of BRCA2 instigate a cGAS/STING-mediated interferon response, which encompasses re-wired TNFα signaling and enhances TNFα sensitivity.
Microtubules are a major component of the cytoskeleton and can accumulate a plethora of modifications. The microtubule detyrosination cycle is one of these modifications; it involves the enzymatic ...removal of the C‐terminal tyrosine of α‐tubulin on assembled microtubules and the re‐ligation of tyrosine on detyrosinated tubulin dimers. This modification cycle has been implicated in cardiac disease, neuronal development, and mitotic defects. The vasohibin and microtubule‐associated tyrosine carboxypeptidase enzyme families are responsible for microtubule detyrosination. Their long‐sought discovery allows to review and summarise differences and similarities between the two enzymes families and discuss how they interplay with other modifications and functions of the tubulin code.
Microtubules undergo a plethora of post‐translational modifications, including removal and re‐ligation of the C‐terminal tyrosine of α‐tubulin. Microtubule detyrosination is mediated by two recently discovered evolutionary distinct enzyme families, the vasohibins and the microtubule‐associated tyrosine carboxypeptidases. Their long‐sought discovery allows to review and summarise the structure of the detyrosinases, how they bind microtubules, and their distinct enzymatic mechanism.
Although the genes essential for life have been identified in less complex model organisms, their elucidation in human cells has been hindered by technical barriers. We used extensive mutagenesis in ...haploid human cells to identify approximately 2000 genes required for optimal fitness under culture conditions. To study the principles of genetic interactions in human cells, we created a synthetic lethality network focused on the secretory pathway based exclusively on mutations. This revealed a genetic cross-talk governing Golgi homeostasis, an additional subunit of the human oligosaccharyltransferase complex, and a phosphatidylinositol 4-kinase β adaptor hijacked by viruses. The synthetic lethality map parallels observations made in yeast and projects a route forward to reveal genetic networks in diverse aspects of human cell biology.
The clinical benefit for patients with diverse types of metastatic cancers that has been observed upon blockade of the interaction between PD-1 and PD-L1 has highlighted the importance of this ...inhibitory axis in the suppression of tumour-specific T-cell responses. Notwithstanding the key role of PD-L1 expression by cells within the tumour micro-environment, our understanding of the regulation of the PD-L1 protein is limited. Here we identify, using a haploid genetic screen, CMTM6, a type-3 transmembrane protein of previously unknown function, as a regulator of the PD-L1 protein. Interference with CMTM6 expression results in impaired PD-L1 protein expression in all human tumour cell types tested and in primary human dendritic cells. Furthermore, through both a haploid genetic modifier screen in CMTM6-deficient cells and genetic complementation experiments, we demonstrate that this function is shared by its closest family member, CMTM4, but not by any of the other CMTM members tested. Notably, CMTM6 increases the PD-L1 protein pool without affecting PD-L1 (also known as CD274) transcription levels. Rather, we demonstrate that CMTM6 is present at the cell surface, associates with the PD-L1 protein, reduces its ubiquitination and increases PD-L1 protein half-life. Consistent with its role in PD-L1 protein regulation, CMTM6 enhances the ability of PD-L1-expressing tumour cells to inhibit T cells. Collectively, our data reveal that PD-L1 relies on CMTM6/4 to efficiently carry out its inhibitory function, and suggest potential new avenues to block this pathway.
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