1. Regulation of Niche-Stem Cell Interaction by MT-nanotubes
Specialized environments called “niches” help to maintain stem cells by producing signals essential for stem cell maintenance. Stem cell-niche signaling has to be carefully regulated, since an excess of signal activation can lead to the tumorigenic overproliferation of stem cells, while its shortage can deplete stem cells, causing tissue degeneration. Thus, the niche signal has to meet two criteria 1) sufficient signal activation in stem cells, 2) no (or lower than threshold) signal activation in non-stem cells. Stem cells and their non-stem cell daughters are often juxtaposed to one another, and thus how niche signaling is spatially confined has been a mystery. It has been postulated that the secreted niche ligands “diffuse” only a short range, but how the range of diffusion could be tightly regulated remains unknown. We recently discovered previously unrecognized cellular protrusions, termed MT (microtubule-based)-nanotubes, that are specifically formed by germline stem cells (blue cells in the picture) and extend into the niche-forming hub cells (pink cells). Our preliminary studies indicate that MT-nanotubes promote BMP signaling (Dpp ligand-Tkv receptor), the ligand of which is required for stem cell maintenance. We hypothesize that MT-nanotubes mediate productive niche signaling such that only stem cells, but not their differentiating daughters experience enough niche-dependent signal transduction, providing a mechanistic basis for the short-range nature of niche signaling. We are now exploring further the molecular and cellular mechanisms underlying MT-nanotube-mediated niche-stem cell signaling.
2. Investigation of protein dynamics during asymmetric cell division
We have discovered that phosphorylated Mad (a downstream effector of niche signaling) exhibits clear asymmetric distribution during stem cell division even BEFORE cytokinesis is completed. Interestingly, this asymmetry is only observed in female germline stem cells (GSCs) but not in male's, indicating the existence of different regulatory mechanisms in male and female GSCs. We conducted RNAi candidate screening and identified the genes responsible for this asymmetry. In order to understand the dynamics of molecules during stem cell division, we employed mathematical modeling, specifically the Virtual Cell modeling platform (https://vcell.org/) developed in CCAM.
The physical nature of barriers and the manner whereby they impede the free diffusion of molecules during asymmetric division has rarely been studied in depth. Our approach will contribute to the discovery of novel asymmetries, and ultimately a comprehensive understanding of the molecular and cellular mechanisms of asymmetric stem cell division.
3. Investigation of microtubules’ function in peripheral ER stacking
We have discovered that a mutation of a microtubule modification enzyme affects the morphology of stacked endoplasmic reticulum (ER) sheets in female ovarian follicle cells. ER stacks have been hypothesized to permit maximal protein synthesis in professional secretory cells. Microtubules are reported to colocalize with ER tubules and studies have shown that microtubule-ER interactions are important in shaping the ER. However, the mechanism by which these interactions occur and their impact on ER structure remains unknown. We hypothesize that the post-translational modification of ⍺-tubulin is required for the organization of ER into stacked sheets. We are investigating the mechanism of formation and/or maintenance of peripheral ER morphology.