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Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Irvine Electronic Theses and Dissertations (2015)

Menopause is one of the earliest age related declines in humans. Uncovering factors that influence reproductive aging is of interest with regard to human reproductive health, and is also an important first step in understanding the overall aging process. The Caenorhabditis elegans hermaphrodite germline has been established as a highly-suitable model system, but many questions remain regarding the factors that modulate its aging.

Although IF is well known to extend lifespan in a variety of species including C. elegans, less is known about its effect on reproductive aging. I show that IF can delay worm reproductive cessation without reducing brood size. As a first step in establishing means to understand the ways in which IF exerts its effects on reproductive aging, I characterize germline changes that occur as reproductive activity is modulated; I chose this approach because reproductive aging is likely to be linked to reproductive activity and to germline stem cell (GSC) activity. Specifically, I find that GSCs of individuals with low reproduction rates randomly occupy a dormant state in which they do not cycle. I also show that increasing reproduction rate alters germline compartment makeup, which occurs independently of oocyte fertilization. A strong candidate for regulation of at least some of these responses is the stem cell niche provided by the distal tip cell (DTC). Indeed, niche architecture has been hypothesized to play an important role in stem cell regulation. The DTC extends processes whose functional significance is unknown. Using time-lapse imaging, I show this growth to be an active and highly-dynamic process. This suggests that process growth may be an important component of the regulation of GSC behavior.

Overall, I report striking, novel behaviors of the C. elegans reproductive system as a whole, and of GSCs in particular. I also report a new intervention by which reproductive cessation can be delayed without sacrificing brood size. This lays the foundation for future studies to identify the underlying causes of reproductive aging, and to identify ways in which this aging is intrinsically controlled and can be further manipulated by external intervention.

    Cover page: Reproductive senescence and control of germline stem cells in the Caenorhabditis elegans hermaphrodite germline
    • Article
    • Peer Reviewed
    UC Irvine Previously Published Works (2015)

    Background

    Stem cells are thought to play a critical role in minimizing the accumulation of mutations, but it is not clear which strategies they follow to fulfill that performance objective. Slow cycling of stem cells provides a simple strategy that can minimize cell pedigree depth and thereby minimize the accumulation of replication-dependent mutations. Although the power of this strategy was recognized early on, a quantitative assessment of whether and how it is employed by biological systems is missing.

    Results

    Here we address this problem using a simple self-renewing organ - the C. elegans gonad - whose overall organization is shared with many self-renewing organs. Computational simulations of mutation accumulation characterize a tradeoff between fast development and low mutation accumulation, and show that slow-cycling stem cells allow for an advantageous compromise to be reached. This compromise is such that worm germ-line stem cells should cycle more slowly than their differentiating counterparts, but only by a modest amount. Experimental measurements of cell cycle lengths derived using a new, quantitative technique are consistent with these predictions.

    Conclusions

    Our findings shed light both on design principles that underlie the role of stem cells in delaying aging and on evolutionary forces that shape stem-cell gene regulatory networks.
      Cover page: Control of Caenorhabditis elegans germ-line stem-cell cycling speed meets requirements of design to minimize mutation accumulation
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2016)
      Self-renewing organs often experience a decline in function in the course of aging. It is unclear whether chronological age or external factors control this decline, or whether it is driven by stem cell self-renewal-for example, because cycling cells exhaust their replicative capacity and become senescent. Here we assay the relationship between stem cell cycling and senescence in the Caenorhabditis elegans reproductive system, defining this senescence as the progressive decline in "reproductive capacity," i.e. in the number of progeny that can be produced until cessation of reproduction. We show that stem cell cycling diminishes remaining reproductive capacity, at least in part through the DNA damage response. Paradoxically, gonads kept under conditions that preclude reproduction keep cycling and producing cells that undergo apoptosis or are laid as unfertilized gametes, thus squandering reproductive capacity. We show that continued activity is in fact beneficial inasmuch as gonads that are active when reproduction is initiated have more sustained early progeny production. Intriguingly, continued cycling is intermittent-gonads switch between active and dormant states-and in all likelihood stochastic. Other organs face tradeoffs whereby stem cell cycling has the beneficial effect of providing freshly-differentiated cells and the detrimental effect of increasing the likelihood of cancer or senescence; stochastic stem cell cycling may allow for a subset of cells to preserve proliferative potential in old age, which may implement a strategy to deal with uncertainty as to the total amount of proliferation to be undergone over an organism's lifespan.
        Cover page: Intermittent Stem Cell Cycling Balances Self-Renewal and Senescence of the C. elegans Germ Line
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2015)
        Positional information derived from local morphogen concentration plays an important role in patterning. A key question is how morphogen diffusion and gene expression regulation shape positional information into an appropriate profile with suitably low noise. We address this question using a model system--the C. elegans germline--whose regulatory network has been well characterized genetically but whose spatiotemporal dynamics are poorly understood. We show that diffusion within the germline syncytium is a critical control of stem cell differentiation and that semi-permeable diffusion barriers present at key locations make it possible--in combination with a feedback loop in the germline regulatory network--for mitotic zone size to be robust against spatial noise in Notch signaling. Spatial averaging within compartments defined by diffusion barriers is an advantageous patterning strategy, which attenuates noise while still allowing for sharp transitions between compartments. This strategy could apply to other organs.
          Cover page: Semi-permeable Diffusion Barriers Enhance Patterning Robustness in the C. elegans Germline
          • Article
          • Peer Reviewed
          UC Irvine Previously Published Works (2015)

          Background

          Analysis of single cells in their native environment is a powerful method to address key questions in developmental systems biology. Confocal microscopy imaging of intact tissues, followed by automatic image segmentation, provides a means to conduct cytometric studies while at the same time preserving crucial information about the spatial organization of the tissue and morphological features of the cells. This technique is rapidly evolving but is still not in widespread use among research groups that do not specialize in technique development, perhaps in part for lack of tools that automate repetitive tasks while allowing experts to make the best use of their time in injecting their domain-specific knowledge.

          Results

          Here we focus on a well-established stem cell model system, the C. elegans gonad, as well as on two other model systems widely used to study cell fate specification and morphogenesis: the pre-implantation mouse embryo and the developing mouse olfactory epithelium. We report a pipeline that integrates machine-learning-based cell detection, fast human-in-the-loop curation of these detections, and running of active contours seeded from detections to segment cells. The procedure can be bootstrapped by a small number of manual detections, and outperforms alternative pieces of software we benchmarked on C. elegans gonad datasets. Using cell segmentations to quantify fluorescence contents, we report previously-uncharacterized cell behaviors in the model systems we used. We further show how cell morphological features can be used to identify cell cycle phase; this provides a basis for future tools that will streamline cell cycle experiments by minimizing the need for exogenous cell cycle phase labels.

          Conclusions

          High-throughput 3D segmentation makes it possible to extract rich information from images that are routinely acquired by biologists, and provides insights - in particular with respect to the cell cycle - that would be difficult to derive otherwise.
            Cover page: Analysis of in vivo single cell behavior by high throughput, human-in-the-loop segmentation of three-dimensional images
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