Adult tissue stem cells produce highly differentiated but short-lived cells throughout life, contributing to the tissue maintenance and repair. To balance between stem cell self-renewal and differentiation, stem cell often divide asymmetrically to produce two distinct daughters. Our lab challenges to reveal molecular and cellular mechanisms that regulate asymmetric stem cell division using male and female Drosophila germline stem cell systems. We are currently focusing on two specific questions, 1) How the niche signal is the spatially restricted with the emphasis on the novel stem cell specific structure, MT (microtubule based)-nanotubes? 2) How are the physical parameters of dividing stem cells regulated, and how does it contribute to precise fate asymmetry? Taking the advantage of small size and simple anatomy of Drosophila gonads, we conduct whole tissue live imaging to monitor in vivo stem cell behavior. We are generating tools to investigate various protein dynamics in asymmetrically dividing stem cells. Our effort will contribute to comprehensively understand how the extrinsic and intrinsic regulations are integrated to ensure the precise cell fate determination.
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 tumorigenic overproliferation of stem cells, while its shortage can deplete stem cells, causing tissue degeneration. Thus, 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 each other, and thus how specificity of spatially confined niche signaling is achieved has been a mystery. It has been postulated that the secreted niche ligands “diffuse” only in a short-range, but how the range of diffusion can be tightly regulated remained 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), a niche ligand required for stem cell maintenance. We hypothesize that MT-nanotubes function to mediate productive niche signaling such that only stem cells experience enough niche-dependent signal transduction, providing a mechanistic basis for the short-range nature of the niche signaling. We further explore molecular and cellular mechanisms of MT-nanotube-mediated niche-stem cell signaling.
2. Investigate the intracellular geometry during asymmetric cell division
Our preliminary studies have discovered that several proteins exhibits clear asymmetric distribution during GSC division even BEFORE the cytokinesis is completed. Interestingly, this asymmetry is only observed in female germline stem cells but not in male's, indicating the existence of distinct regulatory mechanisms between male and female GSCs. 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. We conducted RNAi candidate screening and identified the genes responsible for this asymmetry. Interestingly, many cellular structural components (nuclear envelope, fusome/spectrosome, ER) were found to be required for asymmetry formation, suggesting that the integrity of subcellular compartment is essential for successful asymmetric division. To understand the cellular geometry with emphasis on the physical requirements of the cell's compartmentalization to be effective in generation of stereotypical asymmetry, we will generate the intracellular "map" of dividing stem cell daughters. We investigate the behavior of proteins localize to the subcellular structures/organelle. We have developed a size model of female vs. male GSCs and are collecting local kinetic parameters (e.g. diffusion coefficients) using control mEOS (green to red photoconvertible protein upon UV light stimulation). Our approach will contribute to discovering novel asymmetries, thus comprehensive understanding of the molecular and cellular mechanisms of asymmetric stem cell division.