Extreme cell growth in support of stem cell proliferation and niche exit

K. Gordon, UNC R35 Abstract The specification and robust maintenance of the germ line stem cells and their differentiated descendants that form gametes are essential to the success of an organismal lineage. Stem cell niches are primary regulators of stem cell fate, which is characterized by continuous self-renewal with the capability of differentiation. Therefore

the structure of stem cell niches—which often have extensive cellular contacts with their supported stem cell populations that must be broken by daughter cells that go on to differentiate—appear crucial for the regulation of the cell fate decision. Such contacts are shared in a variety of stem cell niches, but studies relating niche

structure to function lag behind genetic analysis of stem cell regulation. Stem cell niches lie deep in tissues, making them difficult to observe, and stem cell divisions are stochastic. This proposal avoids these challenges by studying the germ line stem cell niche of C. elegans, an animal that is suited to multiplex in vivo imaging.

The overall goal for the research program is to discover how undifferentiated germ stem cells receive cues from their micro-environment to stimulate proliferation and the cell fate switch to gamete differentiation, and how the dynamic cell structures supporting germ stem cells form. Conserved proliferation and cell growth

pathways are relevant to human health. Pursuit of these questions will lead to insights that, like other findings made in this canonical stem cell niche model, may be broadly applicable to other stem cell systems. Overview of research and goals for the next five years: The Gordon Lab opened at UNC just before the

pandemic began. It pursues key questions like: How do the stem cell niche and gonad sheath cells grow and internally partition themselves to interact appropriately with germ cells at different steps of the differentiation program? Both the niche cell and sheath undergo substantial growth and transformation during development.

Both somatic cells will be genetically manipulated to determine if effects on germ cell proliferation are mediated by growth and partitioning of these regulatory cells. Cell-specific RNAi strains will be used to screen for genes resulting in diminished Sh1 growth and germ cell proliferation. What molecular mechanisms underlie oriented

cell divisions at the niche boundary and asymmetrical cell fate acquisition? Spindle orientation has been studied in great detail in early C. elegans embryos, but it was not known that divisions in proliferating germ cells were oriented. Adhesion between the niche and germ cells was a neglected area of inquiry until recent

findings implicated cadherin complexes—which orient spindles in the embryo—in the dramatic wrapping behavior of the niche around the stem cells. How does the stem cell niche mature from larvae to adults? RNA- seq of niche cells across the larva/adult transition will identify candidate genes that are correlated with the

transition from a migratory niche during gonad growth to a stationary, mature adult niche. These and known genes will be investigated by tagging and loss of function analysis in live-imaging studies of the transition. Together this work will address arising and long-standing questions about a classical stem cell model system.