Summary Background Re-establishing epithelial integrity and biosynthetic capacity is critically important following tissue damage. The adult Drosophila abdominal epithelium provides an attractive new system to address how post-mitotic diploid cells contribute to repair. Results Puncture wounds to the adult Drosophila epidermis close initially by forming a melanized scab. We found that epithelial cells near the wound site fuse to form a giant syncytium, which sends lamellae under the scab to re-epithelialize the damaged site. Other large cells arise more peripherally by initiating endocycles and becoming polyploid, or by cell fusion. Rac GTPase activity is needed for syncytium formation, while the Hippo signaling effector Yorkie modulates both polyploidization and cell fusion. Large cell formation is functionally important because when both polyploidization and fusion are blocked, wounds do not re-epithelialize. Conclusions Our observations indicate that cell mass lost upon wounding can be replaced by polyploidization instead of mitotic proliferation. We propose that large cells generated by polyploidization or cell fusion are essential because they are better able than diploid cells to mechanically stabilize wounds, especially those containing permanent acellular structures, such as scar tissue.
Legionella pneumophila, the causative agent of Legionnaires' disease, grows within macrophages and manipulates target cell signaling. Formation of a Legionella-containing replication vacuole requires the function of the bacterial type IV secretion system (Dot/Icm), which transfers protein substrates into the host cell cytoplasm. A global microarray analysis was used to examine the response of human macrophage-like U937 cells to low-dose infections with L. pneumophila. The most striking change in expression was the Dot/Icm-dependent up-regulation of antiapoptotic genes positively controlled by the transcriptional regulator nuclear factor κB (NF-κB). Consistent with this finding, L. pneumophila triggered nuclear localization of NF-κB in human and mouse macrophages in a Dot/Icm-dependent manner. The mechanism of activation at low-dose infections involved a signaling pathway that occurred independently of the Toll-like receptor adaptor MyD88 and the cytoplasmic sensor Nod1. In contrast, high multiplicity of infection conditions caused a host cell response that masked the unique Dot/Icm-dependent activation of NF-κB. Inhibition of NF-κB translocation into the nucleus resulted in premature host cell death and termination of bacterial replication. In the absence of one antiapoptotic protein, plasminogen activator inhibitor–2, host cell death increased in response to L. pneumophila infection, indicating that induction of antiapoptotic genes is critical for host cell survival.
The past decade of research on Drosophila stem cells and niches has provided key insights. Fly stem cells do not occupy a special "state" based on novel "stem cell genes" but resemble transiently arrested tissue progenitors. Moreover, individual stem cells and downstream progenitors are highly dynamic and dispensable, not tissue bulwarks. Niches, rather than fixed cell lineages, ensure tissue health by holding stem cells and repressing cell differentiation inside, but not outside. We review the five best-understood adult Drosophila stem cells and argue that the fundamental biology of stem cells and niches is conserved between Drosophila and mice.
SummaryLegionella pneumophila possesses a large arsenal of type IV translocated substrates. Over 100 such proteins have been identified, but the functions of most are unknown. Previous studies have demonstrated that L. pneumophila activates NF-kB, a master transcriptional regulator of the mammalian innate immune response. Activation of NF-kB is dependent on the Legionella Icm/Dot type IV protein translocation system, consistent with the possibility that translocated bacterial proteins contribute to this response. To test this hypothesis, an expression library of 159 known and putative translocated substrates was created to evaluate whether ectopic production of a single L. pneumophila protein could activate NF-kB in mammalian cells. Expression of two of these proteins, LnaB (Legionella NF-kB activator B) and LegK1, resulted in~150-fold induction of NF-kB activity in HEK293T cells, levels similar to the strong induction that occurs with ectopic expression of the known activator Nod1. LnaB is a substrate of the Icm/Dot system, and in the absence of this protein, a partial reduction of NF-kB activation in host cells occurs after challenge by post-exponential phase bacteria. These data indicate that LnaB is an Icm/Dot substrate that contributes to NF-kB activation during L. pneumophila infection in host cells.
Tissue integrity and homeostasis often rely on the proliferation of stem cells or differentiated cells to replace lost, aged, or damaged cells. Recently, we described an alternative source of cell replacement- the expansion of resident, non-dividing diploid cells by wound-induced polyploidization (WIP). Here we show that the magnitude of WIP is proportional to the extent of cell loss using a new semi-automated assay with single cell resolution. Hippo and JNK signaling regulate WIP; unexpectedly however, JNK signaling through AP-1 limits rather than stimulates the level of Yki activation and polyploidization in the Drosophila epidermis. We found that polyploidization also quantitatively compensates for cell loss in a mammalian tissue, mouse corneal endothelium, where increased cell death occurs with age in a mouse model of Fuchs Endothelial Corneal Dystrophy (FECD). Our results suggest that WIP is an evolutionarily conserved homeostatic mechanism that maintains the size and synthetic capacity of adult tissues.
Polyploidy (the more than doubling of a cell’s genome) frequently arises during organogenesis, tissue repair, and age-associated diseases. Despite its prevalence, major gaps exist in how polyploid cells emerge and affect tissue function. Studies have begun to elucidate the signals required for polyploid cell growth as well as the advantages and disadvantages of polyploidy in health and disease. This review highlights the recent advances on the role and regulation of polyploidy in Drosophila and vertebrate models. The newly discovered versatility of polyploid cells has the potential to provide alternative strategies to promote tissue growth and repair, while limiting disease and dysfunction.
SUMMARY The dupA gene, encoding a putative tyrosine kinase/dual specificity phosphatase (Dusp), was identified in a screen for Dictyostelium discoideum mutants altered in supporting Legionella pneumophila intracellular replication. The absence of dupA resulted in hyperphosphorylation of ERK1, consistent with the loss of a phosphatase activity, as well as degradation of ERK2. ERK1 hyperphosphorylation mimicked the response of this protein after bacterial challenge of wild type amoebae. Similar to Dusps in higher eukaryotic cells, the amoebal dupA gene was induced after bacterial contact, indicating a response of Dusps that is conserved from amoebae to mammals. A large set of genes was misregulated in the dupA− mutant that largely overlaps with genes responding to L. pneumophila infection. Some of the amoebal genes appear to be involved in a response similar to innate immunity in higher eukaryotes, indicating there was misregulation of a conserved response to bacteria.
Tissue repair usually requires either polyploid cell growth or cell division, but the molecular mechanism promoting polyploidy and limiting cell division remains poorly understood. Here, we find that injury to the adult Drosophila epithelium causes cells to enter the endocycle through the activation of Yorkie-dependent genes ( Myc and E2f1 ). Myc is even sufficient to induce the endocycle in the uninjured post-mitotic epithelium. As result, epithelial cells enter S phase but mitosis is blocked by inhibition of mitotic gene expression. The mitotic cell cycle program can be activated by simultaneously expressing the Cdc25-like phosphatase String ( stg ), while genetically depleting APC/C E3 ligase fizzy-related ( fzr ) . However, forcing cells to undergo mitosis is detrimental to wound repair as the adult fly epithelium accumulates DNA damage, and mitotic errors ensue when cells are forced to proliferate. In conclusion, we find that wound-induced polyploidization enables tissue repair when cell division is not a viable option.
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