Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (10 results)

  • Article
  • Peer Reviewed
UCLA Previously Published Works (2019)
Mitochondria are essential organelles that have recently emerged as hubs for several metabolic and signaling pathways in the cell. Mitochondrial morphology is regulated by constant fusion and fission events to maintain a functional mitochondrial network and to remodel the mitochondrial network in response to external stimuli. Although the role of mitochondria in later stages of spermatogenesis has been investigated in depth, the role of mitochondrial dynamics in regulating early germ cell behavior is relatively less-well understood. We previously demonstrated that mitochondrial fusion is required for germline stem cell (GSC) maintenance in the Drosophila testis. Here, we show that mitochondrial fission is also important for regulating the maintenance of early germ cells in larval testes. Inhibition of Drp1 in early germ cells resulted in the loss of GSCs and spermatogonia due to the accumulation of reactive oxygen species (ROS) and activation of the EGFR pathway in adjacent somatic cyst cells. EGFR activation contributed to premature germ cell differentiation. Our data provide insights into how mitochondrial dynamics can impact germ cell maintenance and differentiation via distinct mechanisms throughout development.
    Cover page: Mitochondrial fission regulates germ cell differentiation by suppressing ROS-mediated activation of Epidermal Growth Factor Signaling in the Drosophila larval testis
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2020)
    Although typically upregulated upon cellular stress, autophagy can also be utilized under homeostatic conditions as a quality control mechanism or in response to developmental cues. Here, we report that autophagy is required for the maintenance of somatic cyst stem cells (CySCs) in the Drosophila testis. Disruption of autophagy in CySCs and early cyst cells (CCs) by the depletion of autophagy-related (Atg) genes reduced early CC numbers and affected CC function, resembling decreased epidermal growth factor receptor (EGFR) signaling. Indeed, our data indicate that EGFR acts to stimulate autophagy to preserve early CC function, whereas target of rapamycin (TOR) negatively regulates autophagy in the differentiating CCs. Finally, we show that the EGFR-mediated stimulation of autophagy regulates lipid levels in CySCs and CCs. These results demonstrate a key role for autophagy in regulating somatic stem cell behavior and tissue homeostasis by integrating cues from both the EGFR and TOR signaling pathways to control lipid metabolism.
      Cover page: EGFR Signaling Stimulates Autophagy to Regulate Stem Cell Maintenance and Lipid Homeostasis in the Drosophila Testis
      • Thesis
      • Peer Reviewed
      UCLA Electronic Theses and Dissertations (2024)

      Adult stem cells are crucial for tissue homeostasis and repair. Their fate is tightly regulated by intrinsic genetic factors and extrinsic cues within the stem cell niche. These niches are adaptable, responding to stimuli to maintain homeostasis, but the molecular details of this regulation are not fully understood. This study explores a mechanism that regulates the fate of somatic support cells in the Drosophila testis to maintain tissue homeostasis.The hub in the Drosophila testis niche secretes factors that regulate germline stem cells (GSCs) and cyst stem cells (CySCs). Our lab previously reported that the Snail family transcriptional regulator Escargot (Esg) is necessary for maintaining the identity of both CySCs and hub cells. Depleting esg from hub cells leads to their conversion into CySCs, resulting in the loss of both GSC and CySC populations. Similarly, esg depletion from early somatic cells causes CySCs to differentiate prematurely into daughter cyst cells. These findings indicate that Esg is central to determining somatic cell fates, although the mechanisms are not fully understood. In the adult testis, Egfr activity is low in hub cells under homeostatic conditions but active in CySCs and early cyst cells, regulating their differentiation. Interestingly, Esg protein levels are significantly higher in hub cells than in cyst cells. We hypothesized that Esg influences somatic cell fate by repressing the Egfr signaling cascade. Our results show that repressing Egfr signaling suppresses hub-to-CySC conversion upon esg depletion. Conversely, overexpressing esg in CySCs increases the number of early cyst cells and induces CySC-to-hub cell conversion, which depends on Egfr activity. Constitutive Egfr activation in CySCs prevents this conversion upon esg overexpression. These findings support the hypothesis that Escargot acts upstream of Egfr to regulate somatic cell identity and maintain tissue homeostasis in the testis.

        Cover page: Regulation of Somatic Cell Fate by Escargot in Drosophila Testis
        • Article
        • Peer Reviewed
        UC San Francisco Previously Published Works (2021)
        The Drosophila intestine is an excellent system for elucidating mechanisms regulating stem cell behavior. Here we show that the septate junction (SJ) protein Neuroglian (Nrg) is expressed in intestinal stem cells (ISCs) and enteroblasts (EBs) within the fly intestine. SJs are not present between ISCs and EBs, suggesting Nrg plays a different role in this tissue. We reveal that Nrg is required for ISC proliferation in young flies, and depletion of Nrg from ISCs and EBs suppresses increased ISC proliferation in aged flies. Conversely, overexpression of Nrg in ISC and EBs promotes ISC proliferation, leading to an increase in cells expressing ISC/EB markers; in addition, we observe an increase in epidermal growth factor receptor (Egfr) activation. Genetic epistasis experiments reveal that Nrg acts upstream of Egfr to regulate ISC proliferation. As Nrg function is highly conserved in mammalian systems, our work characterizing the role of Nrg in the intestine has implications for the treatment of intestinal disorders that arise due to altered ISC behavior.
          Cover page: Neuroglian regulates Drosophila intestinal stem cell proliferation through enhanced signaling via the epidermal growth factor receptor
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2017)
          Intestinal stem cells (ISCs) maintain the midgut epithelium in Drosophila melanogaster Proper cellular turnover and tissue function rely on tightly regulated rates of ISC division and appropriate differentiation of daughter cells. However, aging and epithelial injury cause elevated ISC proliferation and decreased capacity for terminal differentiation of daughter enteroblasts (EBs). The mechanisms causing functional decline of stem cells with age remain elusive; however, recent findings suggest that stem cell metabolism plays an important role in the regulation of stem cell activity. Here, we investigate how alterations in mitochondrial homeostasis modulate stem cell behavior in vivo via RNA interference-mediated knockdown of factors involved in mitochondrial dynamics. ISC/EB-specific knockdown of the mitophagy-related genes Pink1 or Parkin suppresses the age-related loss of tissue homeostasis, despite dramatic changes in mitochondrial ultrastructure and mitochondrial damage in ISCs/EBs. Maintenance of tissue homeostasis upon reduction of Pink1 or Parkin appears to result from reduction of age- and stress-induced ISC proliferation, in part, through induction of ISC senescence. Our results indicate an uncoupling of cellular, tissue, and organismal aging through inhibition of ISC proliferation and provide insight into strategies used by stem cells to maintain tissue homeostasis despite severe damage to organelles.
            Cover page: Pink1 and Parkin regulate <i>Drosophila</i> intestinal stem cell proliferation during stress and aging.
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2020)

            Background & aims

            The epithelia of the intestine and colon turn over rapidly and are maintained by adult stem cells at the base of crypts. Although the small intestine and colon have distinct, well-characterized physiological functions, it remains unclear if there are fundamental regional differences in stem cell behavior or region-dependent degenerative changes during aging. Mesenchyme-free organoids provide useful tools for investigating intestinal stem cell biology in vitro and have started to be used for investigating age-related changes in stem cell function. However, it is unknown whether organoids maintain hallmarks of age in the absence of an aging niche. We tested whether stem cell-enriched organoids preserved the DNA methylation-based aging profiles associated with the tissues and crypts from which they were derived.

            Methods

            To address this, we used standard human methylation arrays and the human epigenetic clock as a biomarker of age to analyze in vitro-derived, 3-dimensional, stem cell-enriched intestinal organoids.

            Results

            We found that human stem cell-enriched organoids maintained segmental differences in methylation patterns and that age, as measured by the epigenetic clock, also was maintained in vitro. Surprisingly, we found that stem cell-enriched organoids derived from the small intestine showed striking epigenetic age reduction relative to organoids derived from colon.

            Conclusions

            Our data validate the use of organoids as a model for studying human intestinal aging and introduce methods that can be used when modeling aging or age-onset diseases in vitro.
              Cover page: DNA Methylation Analysis Validates Organoids as a Viable Model for Studying Human Intestinal Aging
              • Article
              • Peer Reviewed
              UCLA Previously Published Works (2018)
              Intestinal barrier dysfunction is an evolutionarily conserved hallmark of aging, which has been linked to microbial dysbiosis, altered expression of occluding junction proteins, and impending mortality. However, the interplay between intestinal junction proteins, age-onset dysbiosis, and lifespan determination remains unclear. Here, we show that altered expression of Snakeskin (Ssk), a septate junction-specific protein, can modulate intestinal homeostasis, microbial dynamics, immune activity, and lifespan in Drosophila. Loss of Ssk leads to rapid and reversible intestinal barrier dysfunction, altered gut morphology, dysbiosis, and dramatically reduced lifespan. Remarkably, restoration of Ssk expression in flies showing intestinal barrier dysfunction rescues each of these phenotypes previously linked to aging. Intestinal up-regulation of Ssk protects against microbial translocation following oral infection with pathogenic bacteria. Furthermore, intestinal up-regulation of Ssk improves intestinal barrier function during aging, limits dysbiosis, and extends lifespan. Our findings indicate that intestinal occluding junctions may represent prolongevity targets in mammals.
                Cover page: Intestinal Snakeskin Limits Microbial Dysbiosis during Aging and Promotes Longevity
                • Article
                • Peer Reviewed
                UCLA Previously Published Works (2017)
                Ageing results in loss of tissue homeostasis across taxa. In the intestine of Drosophila melanogaster, ageing is correlated with an increase in intestinal stem cell (ISC) proliferation, a block in terminal differentiation of progenitor cells, activation of inflammatory pathways, and increased intestinal permeability. However, causal relationships between these phenotypes remain unclear. Here, we demonstrate that ageing results in altered localization and expression of septate junction proteins in the posterior midgut, which is quite pronounced in differentiated enterocytes (ECs) at tricellular junctions (TCJs). Acute loss of the TCJ protein Gliotactin (Gli) in ECs results in increased ISC proliferation and a block in differentiation in intestines from young flies, demonstrating that compromised TCJ function is sufficient to alter ISC behaviour in a non-autonomous manner. Blocking the Jun N-terminal kinase signalling pathway is sufficient to suppress changes in ISC behaviour, but has no effect on loss of intestinal barrier function, as a consequence of Gli depletion. Our work demonstrates a pivotal link between TCJs, stem cell behaviour, and intestinal homeostasis and provides insights into causes of age-onset and gastrointestinal diseases.
                  Cover page: Tricellular junctions regulate intestinal stem cell behaviour to maintain homeostasis
                  • Article
                  • Peer Reviewed
                  UCLA Previously Published Works (2014)
                  Snail family transcription factors are expressed in various stem cell types, but their function in maintaining stem cell identity is unclear. In the adult Drosophila midgut, the Snail homolog Esg is expressed in intestinal stem cells (ISCs) and their transient undifferentiated daughters, termed enteroblasts (EB). We demonstrate here that loss of esg in these progenitor cells causes their rapid differentiation into enterocytes (EC) or entero-endocrine cells (EE). Conversely, forced expression of Esg in intestinal progenitor cells blocks differentiation, locking ISCs in a stem cell state. Cell type-specific transcriptome analysis combined with Dam-ID binding studies identified Esg as a major repressor of differentiation genes in stem and progenitor cells. One critical target of Esg was found to be the POU-domain transcription factor, Pdm1, which is normally expressed specifically in differentiated ECs. Ectopic expression of Pdm1 in progenitor cells was sufficient to drive their differentiation into ECs. Hence, Esg is a critical stem cell determinant that maintains stemness by repressing differentiation-promoting factors, such as Pdm1.
                    Cover page: Escargot maintains stemness and suppresses differentiation in Drosophila intestinal stem cells
                    • Article
                    • Peer Reviewed
                    UCLA Previously Published Works (2016)
                    Human pluripotent stem cells (hPSCs) require precise control of post-transcriptional RNA networks to maintain proliferation and survival. Using enhanced UV crosslinking and immunoprecipitation (eCLIP), we identify RNA targets of the IMP/IGF2BP family of RNA-binding proteins in hPSCs. At the broad region and binding site levels, IMP1 and IMP2 show reproducible binding to a large and overlapping set of 3' UTR-enriched targets. RNA Bind-N-seq applied to recombinant full-length IMP1 and IMP2 reveals CA-rich motifs that are enriched in eCLIP-defined binding sites. We observe that IMP1 loss in hPSCs recapitulates IMP1 phenotypes, including a reduction in cell adhesion and increase in cell death. For cell adhesion, we find IMP1 maintains levels of integrin mRNA specifically regulating RNA stability of ITGB5 in hPSCs. Additionally, we show that IMP1 can be linked to hPSC survival via direct target BCL2. Thus, transcriptome-wide binding profiles identify hPSC targets modulating well-characterized IMP1 roles.
                      Cover page: Enhanced CLIP Uncovers IMP Protein-RNA Targets in Human Pluripotent Stem Cells Important for Cell Adhesion and Survival
                      Top