During suboptimal growing conditions, C. elegans can enter a developmentally-arrested stage known as dauer that is specialized for dispersal and survival. Three independent signalling pathways (insulin-like, TGF-β, and cGMP) control entry into this stage and subsequently affect the changes that will occur in morphology, physiology, and behaviour. One of these changes is the establishment of cell cycle quiescence within every tissue of the animal.

To understand how this quiescence is established during dauer in the germ line stem cells, we performed a screen for dauer-specific germ line hyperplasia and isolated aak-2 and par-4 alleles that correspond to the mammalian genes AMPK and LKB1 (an AMPK activator). LKB1 mutations in humans are associated with Peutz-Jeghers syndrome, which predisposes afflicted individuals to various cancers.

We found that AMPK loss-of-function is not fully responsible for the LKB1 phenotype, implying that par-4/LKB1 has other unknown targets that may play roles in cancer development. A subsequent genetic screen confirmed that other AMPK subunits and LKB1-associated proteins are critical for germ line quiescence. We have also isolated several other mutations in the laboratory which phenocopy aak-2 and par-4. Cloning of these alleles should ultimately identify downstream targets of the AMPK and LKB1 complexes that may provide further insight concerning the mechanisms through which insulin signalling mediates cell growth and cancer onset.

Model

Sexually reproducing organisms must eliminate a pair of centrioles prior to the first zygotic division to avoid the formation of a multipolar spindle. During the development of the C. elegans germline, this process requires the activity of the p21/p27-like CDK inhibitor protein cki-2. cki-2 is presumably required to block CDK activity during the onset of oogenesis. In addition to oocyte-specific elimination of the centrosome, we are examining various developmental contexts exist where centrosome duplication must diverge from the intimate coupling of the centrosome cycle with the cell cycle.

By monitoring centrosome dynamics in contexts where cells undergo alternative cell cycles we hope to determine how the centrosome cycle adapts to these variations. In C. elegans, the intestinal and some hypodermal cells undergo endoreduplication during postembryonic development. We generated a centriolar GFP reporter to detect the presence of centrosomes in these tissues and are currently verifying the effects of various genes on this process.

Centrosomes

The precise spatiotemporal coordination between developmental pathways and the cell cycle machinery is essential to ensure timely and accurate progression through the cell division cycle together with controlled growth and differentiation of cells and organs.

The intestinal cells of C. elegans undergo three types of developmentally-regulated cell cycles: mitosis during embryogenesis, then karyokinesis (nuclear division without karyokinesis) and endocycles postembryonically. Thus, this organ provides a unique and powerful model system to study the regulation of these distinct cell cycles in a developmental context and to discover the molecular signals controlling the transitions between them.