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Tin Tin Su

Publications and source records attributed to Tin Tin Su.

At least 19 recordsLinked to original sources
PubMed2026

Matrix Metalloprotease 1 (Mmp1) promotes cell fate change for epithelial-to-epithelial transition during regeneration after radiation damage in Drosophila.

Ionizing radiation (IR) is used to treat cancer, but therapeutic failure occurs when surviving cancer cells change fate and regenerate tumors through acquired stem cell-like properties. While transcriptional mechanisms underlying cell fate plasticity have been characterized, the cellular processes enabling cell movement during tissue regeneration remain unclear. We reported previously that hinge cells of the Drosophila larval wing disc convert to pouch fate and translocate to help regenerate the pouch that suffers from more IR-induced apoptosis. We report here that IR increases the expression of extracellular proteins in the hinge, including secreted proteases and cell adhesion modulators. Functional validation using RNA interference revealed that secreted Matrix Metalloprotease 1 (Mmp1) and the related secreted protease homolog Scarface (Scaf) are required in hinge cells for IR-induced cell fate conversion and translocation. IR, we found, induces Mmp1 and scaf transcripts in hinge cells via cell-autonomous JNK signaling. Overexpression of Mmp1 specifically in the hinge was sufficient to elevate Mmp1 protein levels throughout the disc and induced cell fate change of both hinge and non-hinge cells but only in the context of irradiation. Confocal imaging in a time course demonstrated that cells undergoing fate conversion remain within the epithelial layer with little evidence for delamination or epithelial-mesenchymal transition (EMT). We propose that remodeling of the extracellular environment is a critical mechanism that enables cellular reorganization during tissue regeneration. Mmp enzymes are important for cancer biology because of their role in ECM remodeling, extracellular signaling, and EMT. Our findings demonstrate for the first time that Mmp1 is necessary and sufficient for one epithelial cell type to switch to another epithelial cell type after radiation damage. These results provide a mechanistic basis for radiation therapy-induced cell fate plasticity.

Animals↗
PubMed2007

Analysis of the cell division cycle in Drosophila.

Drosophila melanogaster presents in an unparalleled opportunity to study the regulation of the cell division cycle in the context of cellular differentiation, growth regulation and the development of a multicellular organism. The complexity of Drosophila cell cycles and the large number of techniques available can, however, be overwhelming. We aim to provide here (1) an overview of cell cycle regulation and techniques in Drosophila and (2) a detailed description of techniques we recently used to study embryonic mitoses.

Animals↗
PubMed2006

Drosophila ATR in double-strand break repair.

The ability of a cell to sense and respond to DNA damage is essential for genome stability. An important aspect of the response is arrest of the cell cycle, presumably to allow time for repair. Ataxia telangiectasia mutated (ATM) and ATR are essential for such cell-cycle control, but some observations suggest that they also play a direct role in DNA repair. The Drosophila ortholog of ATR, MEI-41, mediates the DNA damage-dependent G2-M checkpoint. We examined the role of MEI-41 in repair of double-strand breaks (DSBs) induced by P-element excision. We found that mei-41 mutants are defective in completing the later steps of homologous recombination repair, but have no defects in end-joining repair. We hypothesized that these repair defects are the result of loss of checkpoint control. To test this, we genetically reduced mitotic cyclin levels and also examined repair in grp (DmChk1) and lok (DmChk2) mutants. Our results suggest that a significant component of the repair defects is due to loss of MEI-41-dependent cell cycle regulation. However, this does not account for all of the defects we observed. We propose a novel role for MEI-41 in DSB repair, independent of the Chk1/Chk2-mediated checkpoint response.

Animals↗
PubMed2006

Contribution of growth and cell cycle checkpoints to radiation survival in Drosophila.

Cell cycle checkpoints contribute to survival after exposure to ionizing radiation (IR) by arresting the cell cycle and permitting repair. As such, yeast and mammalian cells lacking checkpoints are more sensitive to killing by IR. We reported previously that Drosophila larvae mutant for grp (encoding a homolog of Chk1) survive IR as well as wild type despite being deficient in cell cycle checkpoints. This discrepancy could be due to differences either among species or between unicellular and multicellular systems. Here, we provide evidence that Grapes is needed for survival of Drosophila S2 cells after exposure to similar doses of IR, suggesting that multicellular organisms may utilize checkpoint-independent mechanisms to survive irradiation. The dispensability of checkpoints in multicellular organisms could be due to replacement of damaged cells by regeneration through increased nutritional uptake and compensatory proliferation. In support of this idea, we find that inhibition of nutritional uptake (by starvation or onset of pupariation) or inhibition of growth factor signaling and downstream targets (by mutations in cdk4, chico, or dmyc) reduced the radiation survival of larvae. Further, some of these treatments are more detrimental for grp mutants, suggesting that the need for compensatory proliferation is greater for checkpoint mutants. The difference in survival of grp and wild-type larvae allowed us to screen for small molecules that act as genotype-specific radiation sensitizers in a multicellular context. A pilot screen of a small molecule library from the National Cancer Institute yielded known and approved radio-sensitizing anticancer drugs. Since radiation is a common treatment option for human cancers, we propose that Drosophila may be used as an in vivo screening tool for genotype-specific drugs that enhance the effect of radiation therapy.

Animals↗
PubMed2006

Ionizing radiation induces caspase-dependent but Chk2- and p53-independent cell death in Drosophila melanogaster.

Ionizing radiation (IR) can induce apoptosis via p53, which is the most commonly mutated gene in human cancers. Loss of p53, however, can render cancer cells refractory to therapeutic effects of IR. Alternate p53-independent pathways exist but are not as well understood as p53-dependent apoptosis. Studies of how IR induces p53-independent cell death could benefit from the existence of a genetically tractable model. In Drosophila melanogaster, IR induces apoptosis in the imaginal discs of larvae, typically assayed at 4-6 hr after exposure to a LD(50) dose. In mutants of Drosophila Chk2 or p53 homologs, apoptosis is severely diminished in these assays, leading to the widely held belief that IR-induced apoptosis depends on these genes in Drosophila. In this article, we show that IR-induced apoptosis still occurs in the imaginal discs of chk2 and p53 mutant larvae, albeit with a delay. We demonstrate that this phenomenon is a true apoptotic response because it requires caspase activity and the chromosomal locus that encodes the pro-apoptotic genes reaper, hid, and grim. We also show that Chk2- and p53-independent apoptosis is IR dose-dependent and is therefore probably triggered by a DNA damage signal. We conclude that Drosophila has Chk2- and p53-independent pathways to activate caspases and induce apoptosis in response to IR. This work establishes Drosophila as a model for p53-independent apoptosis, which is of potential therapeutic importance for inducing cell death in p53-deficient cancer cells.

Animals↗
PubMed2006

Catastrophic household expenditure for health care in a low-income society: a study from Nouna District, Burkina Faso.

OBJECTIVE: To quantify the extent of catastrophic household health care expenditure and determine the factors responsible for it in Nouna District, Burkina Faso. METHODS: We used the Nouna Health District Household Survey to collect data on 800 households during 2000-01 for our analysis. The determinants of household catastrophic expenditure were identified by multivariate logistic regression method. FINDINGS: Even at very low levels of health care utilization and modest amount of health expenditure, 6-15% of total households in Nouna District incurred catastrophic health expenditure. The key determinants of catastrophic health expenditure were economic status, household health care utilization especially for modern medical care, illness episodes in an adult household member and presence of a member with chronic illness. CONCLUSION: We conclude that the poorest members of the community incurred catastrophic health expenses. Setting only one threshold/cut-off value to determine catastrophic health expenses may result in inaccurate estimation leading to misinterpretation of important factors. Our findings have important policy implications and can be used to ensure better access to health services and a higher degree of financial protection for low-income groups against the economic impact of illness.

Burkina Faso↗
PubMed2006

Determinants of household health expenditure on western institutional health care.

We try to identify determinants of illness reporting, provider choice and resulting expenditure with different econometric models using data from a representative household panel survey of 800 households in Nouna health district, Burkina Faso, during 2000-2001. The factors "being an adult", "married", "illness occurred in rainy season" and "severe illness" significantly increased the magnitude of health expenditure. Compared to malaria, individuals spent more on other infectious diseases, injury and the other disease category. In contrast, people were less likely to spend on chronic illness. An individual who belonged to a household headed by a female, a literate household head and with a higher household expenditure had a significantly positive association with the magnitude of expenditure. Findings from this study can be used for policy implication to improve health system performance in Burkina Faso through enhancing health care utilization.

Adult↗
PubMed2006

Cellular responses to DNA damage: one signal, multiple choices.

DNA double-strand breaks (DSBs) produce a number of cellular responses, some mutually exclusive. Depending on where on the chromosome it occurs, a DSB may become preserved inside a telomere or eliminated by repair. A cell may arrest division via checkpoint activation to fix DSBs or commit suicide by apoptosis. What determines the outcome: to bury, fix, or succumb to DNA DSBs? With this question in mind, we review recent data on cellular responses to DSBs.

Animals↗
PubMed2005

Drosophila Wee1 interacts with members of the gammaTURC and is required for proper mitotic-spindle morphogenesis and positioning.

BACKGROUND: Wee1 kinases delay entry into mitosis by phosphorylating and inactivating cyclin-dependent kinase 1 (Cdk1). Loss of this activity in many systems, including Drosophila, leads to premature mitotic entry. RESULTS: We report here that Drosophila Wee1 (dwee1) mutant embryos show mitotic-spindle defects that include ectopic foci of microtubule organization, formation of multipolar spindles from adjacent centrosome pairs, and promiscuous interactions between neighboring spindles. Furthermore, centrosomes are displaced from the embryo cortex in dwee1 mutants. These defects are not observed to the same extent in embryos in which nuclei also enter mitosis prematurely as a result of a lack of checkpoint control or in embryos with elevated Cdk1 activity. dWee1 physically interacts with members of the gamma-tubulin ring complex (gammaTuRC), and gamma-tubulin is phosphorylated in a dwee1-dependent manner in embryo extracts. CONCLUSIONS: Some of the abnormalities in dwee1 mutant embryos cannot be explained by premature entry into mitosis or bulk elevation of Cdk1 activity. Instead, dWee1 is also required for phosphorylation of gamma-tubulin, centrosome positioning, and mitotic-spindle integrity. We propose a model to account for these requirements.

Animals↗
PubMed2005

Regulation of mitosis in response to damaged or incompletely replicated DNA require different levels of Grapes (Drosophila Chk1).

Checkpoints monitor the state of DNA and can delay or arrest the cell cycle at multiple points including G1-S transition, progress through S phase and G2-M transition. Regulation of progress through mitosis, specifically at the metaphase-anaphase transition, occurs after exposure to ionizing radiation (IR) in Drosophila and budding yeast, but has not been conclusively demonstrated in mammals. Here we report that regulation of metaphase-anaphase transition in Drosophila depends on the magnitude of radiation dose and time in the cell cycle at which radiation is applied, which may explain the apparent differences among experimental systems and offer an explanation as to why this regulation has not been seen in mammalian cells. We further document that mutants in Drosophila Chk1 (Grapes) that are capable of delaying the progress through mitosis in response to IR are incapable of delaying progress through mitosis when DNA synthesis is blocked by mutations in an essential replication factor encoded by double park (Drosophila Cdt1). We conclude that DNA damage and replication checkpoints operating in the same cell cycle at the same developmental stage in Drosophila can exhibit differential requirements for the Chk1 homolog. The converse situation exists in fission yeast where loss of Chk1 is more detrimental to the DNA damage checkpoint than to the DNA replication checkpoint. It remains to be seen which of these two different uses of Chk1 homologs are conserved in mammals. Finally, our results demonstrate that Drosophila provides a unique opportunity to study the regulation of the entry into, and progress through, mitosis by DNA structure checkpoints in metazoa.

Anaphase↗
PubMed2005

Grp/DChk1 is required for G2-M checkpoint activation in Drosophila S2 cells, whereas Dmnk/DChk2 is dispensable.

Cell-cycle checkpoints are signal-transduction pathways required to maintain genomic stability in dividing cells. Previously, it was reported that two kinases essential for checkpoint signalling, Chk1 and Chk2 are structurally conserved. In contrast to yeast, Xenopus and mammals, the Chk1- and Chk2-dependent pathways in Drosophila are not understood in detail. Here, we report the function of these checkpoint kinases, referred to as Grp/DChk1 and Dmnk/DChk2 in Drosophila Schneider's cells, and identify an upstream regulator as well as downstream targets of Grp/DChk1. First, we demonstrate that S2 cells are a suitable model for G(2)/M checkpoint studies. S2 cells display Grp/DChk1-dependent and Dmnk/DChk2-independent cell-cycle-checkpoint activation in response to hydroxyurea and ionizing radiation. S2 cells depleted for Grp/DChk1 using RNA interference enter mitosis in the presence of impaired DNA integrity, resulting in prolonged mitosis and mitotic catastrophe. Grp/DChk1 is phosphorylated in a Mei-41/DATR-dependent manner in response to hydroxyurea and ionizing radiation, indicating that Mei-41/ATR is an upstream component in the Grp/DChk1 DNA replication and DNA-damage-response pathways. The level of Cdc25(Stg) and phosphorylation status of Cdc2 are modulated in a Grp/DChk1-dependent manner in response to hydroxyurea and irradiation, indicating that these cell-cycle regulators are downstream targets of the Grp/DChk1-dependent DNA replication and DNA-damage responses. By contrast, depletion of Dmnk/DChk2 by RNA interference had little effect on checkpoint responses to hydroxyurea and irradiation. We conclude that Grp/DChk1, and not Dmnk/DChk2, is the main effector kinase involved in G(2)/M checkpoint control in Drosophila cells.

Animals↗
PubMed2004

Drosophila Wee1 kinase regulates Cdk1 and mitotic entry during embryogenesis.

Cyclin-dependent kinases (Cdks) are the central regulators of the cell division cycle. Inhibitors of Cdks ensure proper coordination of cell cycle events and help regulate cell proliferation in the context of tissues and organs. Wee1 homologs phosphorylate a conserved tyrosine to inhibit the mitotic cyclin-dependent kinase Cdk1. Loss of Wee1 function in fission or budding yeast causes premature entry into mitosis. The importance of metazoan Wee1 homologs for timing mitosis, however, has been demonstrated only in Xenopus egg extracts and via ectopic Cdk1 activation . Here, we report that Drosophila Wee1 (dWee1) regulates Cdk1 via phosphorylation of tyrosine 15 and times mitotic entry during the cortical nuclear cycles of syncytial blastoderm embryos, which lack gap phases. Loss of maternal dwee1 leads to premature entry into mitosis, mitotic spindle defects, chromosome condensation problems, and a Chk2-dependent block of subsequent development, and then embryonic lethality. These findings modify previous models about cell cycle regulation in syncytial embryos and demonstrate that Wee1 kinases can regulate mitotic entry in vivo during metazoan development even in cycles that lack a G2 phase.

Animals↗
PubMed2004

Telomeres: not all breaks are equal.

ATM, Rad50 and Mre11 have been shown to prevent telomere fusion in Drosophila, thereby extending the protective role of DNA damage checkpoint proteins to non-canonical telomeres formed without telomerase. How do these proteins help chromosomal termini escape fusion through 'repair' while promoting repair of induced DNA breaks?

Acid Anhydride Hydrolases↗
PubMed2004

Promiscuity rules? The dispensability of cyclin E and Cdk2.

The canonical view of the mammalian cell cycle arose from studies of cultured cells rather than mutant organisms. It depicts the many complexes of cyclin and Cdk (cyclin/Cdk) as fulfilling unique and essential steps that dictate the sequential order of cell cycle events. Recent analyses of knockout mice challenge this view. Cdk2 and cyclin E, long thought to be essential, are largely dispensable. Here, we discuss the phenotypes of these and other cyclin/Cdk mutants in genetically tractable metazoa (mouse, fly, and nematode) and explore possible reasons behind similarities and differences among experimental systems and cell types.

Animals↗
PubMed2004

Embryonic cleavage cycles: how is a mouse like a fly?

The evolutionary advent of uterine support of embryonic growth in mammals is relatively recent. Nonetheless, striking differences in the earliest steps of embryogenesis make it difficult to draw parallels even with other chordates. We suggest that use of fertilization as a reference point misaligns the earliest stages and masks parallels that are evident when development is aligned at conserved stages surrounding gastrulation. In externally deposited eggs from representatives of all the major phyla, gastrulation is preceded by specialized extremely rapid cleavage cell cycles. Mammals also exhibit remarkably fast cell cycles in close association with gastrulation, but instead of beginning development with these rapid cycles, the mammalian egg first devotes itself to the production of extraembryonic structures. Previous attempts to identify common features of cleavage cycles focused on post-fertilization divisions of the mammalian egg. We propose that comparison to the rapid peri-gastrulation cycles is more appropriate and suggest that these cycles are related by evolutionary descent to the early cleavage stages of embryos such as those of frog and fly. The deferral of events in mammalian embryogenesis might be due to an evolutionary shift in the timing of fertilization.

Animals↗
PubMed2004

Relative contribution of DNA repair, cell cycle checkpoints, and cell death to survival after DNA damage in Drosophila larvae.

BACKGROUND: Components of the DNA damage checkpoint are essential for surviving exposure to DNA damaging agents. Checkpoint activation leads to cell cycle arrest, DNA repair, and apoptosis in eukaryotes. Cell cycle regulation and DNA repair appear essential for unicellular systems to survive DNA damage. The relative importance of these responses and apoptosis for surviving DNA damage in multicellular organisms remains unclear. RESULTS: After exposure to ionizing radiation, wild-type Drosophila larvae regulate the cell cycle and repair DNA; grp (DmChk1) mutants cannot regulate the cell cycle but repair DNA; okra (DmRAD54) mutants regulate the cell cycle but are deficient in repair of double strand breaks (DSB); mei-41 (DmATR) mutants cannot regulate the cell cycle and are deficient in DSB repair. All undergo radiation-induced apoptosis. p53 mutants regulate the cell cycle but fail to undergo apoptosis. Of these, mutants deficient in DNA repair, mei-41 and okra, show progressive degeneration of imaginal discs and die as pupae, while other genotypes survive to adulthood after irradiation. Survival is accompanied by compensatory growth of imaginal discs via increased nutritional uptake and cell proliferation, presumably to replace dead cells. CONCLUSIONS: DNA repair is essential for surviving radiation as expected; surprisingly, cell cycle regulation and p53-dependent cell death are not. We propose that processes resembling regeneration of discs act to maintain tissues and ultimately determine survival after irradiation, thus distinguishing requirements between muticellular and unicellular eukaryotes.

Animals↗
PubMed2004

Control of mitotic entry after DNA damage in Drosophila.

In the presence of DNA damage, cells delay the entry into mitosis, presumably to allow time for repair. Methods to detect the delay of mitosis in a multicellular model organism, Drosophila melanogaster, are described here. These include the collection of embryos and larvae, irradiation with x-rays to damage DNA, and fixing and staining of tissues with an antibody to phosphorylated histone H3 to measure the mitotic index. These methods should be useful in identifying potential mutants that are unable to regulate mitosis following DNA damage.

Animals↗