Regulation of germline proliferation and differentiation by the environment: insulin, TOR and TGFß

We screened for genetic perturbations that interfere with the expansion of the germline progenitor pool during larval development, but that do not interfere with germ cell proliferation itself. The strategy took advantage of our previous work on mutants that exhibit a characteristic proximal germline tumor (Pepper et al., 2003(a,b); Killian and Hubbard, 2004, 2005; McGovern et al., 2009).

The strategy

In short, we determined that inadequate accumulation of germ cells in larval stages impairs DTC migration (since proliferating germ cells contribute to gonadal outgrowth together with intrinsic DTC migration programs). Inadequate DTC migration has the secondary consequence of sustaining Notch signaling in proximal germ cells delaying their differentiation. We further discovered that certain newly born proximal somatic cells thereby come into contact with undifferentiated germ cells when normally they would only contact differentiated germ cells.

We then discovered that these proximal somatic cells then act as a “latent niche”: they produce Notch ligands that induce the neighboring proximal germ cells to remain undifferentiated and proliferate. Thus, counter intuitively, interfering with timely and robust expansion of the germline progenitor pool in larval stages can cause tumor formation later in development.

This scenario provided a convenient genetic strategy to look for molecules that promote expansion of the germline progenitor pool: interfering with robust expansion of the larval progenitor pool enhances later tumor formation in a genetically sensitized background. Importantly, this strategy uses germ cell proliferation in the tumor as a read-out, and can therefore identify regulators of progenitor pool expansion as opposed to housekeeping genes required for cell proliferation per se (see Hubbard, 2011, for review).

Using this general strategy in both candidate and unbiased screening approaches, we found that components of the insulin, TOR and TGFß signaling pathways (Michaelson et al., 2010; Korta et al., 2012; Dalfó et al., 2012) are important for the expansion of the larval progenitor pool that occurs under favorable environmental conditions. We determined that the insulin receptor pathway and the S6K branch of TOR signaling pathway are required within the germ line to promote larval cell cycle progression. However, these pathways do not act in a linear fashion and their effects on the cell cycle are dissimilar. S6K additionally promotes the undifferentiated germ cell fate: loss of S6K potently enhances phenotypes associated with reduced Notch signaling and suppresses phenotypes associated with elevated Notch signaling. We also identified the TGFß pathway as a player in the expansion of the germline progenitor pool. We found that it acts within the DTC niche, not in the germ line, to promote expansion of the larval germline progenitor pool.

In the course of these studies, we also determined that the larval germline progenitors are particularly sensitive to environmental conditions. In particular, S6K mediates the effects of food abundance and protein intake on the larval cell cycle (Korta et al., 2012), and TGFß produced in two neurons mediates the effects of food and olfactory (pheromone) cues on the germ line (Dalfó et al., 2012). We are currently exploiting these findings to investigate the molecular basis for environmental influences on stem cells and to leverage facile phenotypic analysis in C. elegans for novel gene discovery and pathway interactions in vivo.

Relevant Publications:

Projects:

Regulation of germline proliferation and differentiation by the environment: insulin, TOR and TGFß

Germline proliferation and differentiation during aging

Stem cell dynamics: in vivo and in silico

Regulation of germline proliferation and differentiation by direct cell-cell communication: sheath cell-germline signaling