Molecular mechanisms controlling cell proliferation, differentiation and fate determination during C. elegans development
Co-workers: Peter Appleford, Charles Brabin, Toby Braun, Samantha Hughes, Sara Maxwell
Research in my group focuses on developmental genetics in the nematode Caenorhabditis elegans, Our overall aim is to understand how gene networks encode developmental programmes, and in doing this we seek to integrate molecular mechanisms into a whole organism level of understanding. One major interest at present is the regulation of cell number during development. The transition from cell proliferation to differentiation is a key regulatory step in the development and subsequent maintenance of an organism's tissues and organs, and is of course at the heart of disease processes such as cancer. At present, the mechanisms controlling the balance between proliferation and differentiation of cells are poorly understood and we aim to use C. elegans as a powerful model organism in which to gain a molecular understanding of how this balance is achieved during normal development. Stem cells have the ability to both self-renew as well as give rise to differentiating daughters that can sometimes generate (or even re-generate) a particular tissue over the lifetime of an organism. Stem cells therefore need to constantly juggle the conflicting demands of proliferation and differentiation in order for a multicellular organism to develop and operate properly.
Our main focus at present involves trying to understand the molecular genetics of cell proliferation and differentiation in a particular stem cell lineage, the seam cell lineage, involved in the generation of male specific sense organs. Seam cells in C. elegans are neuroectodermal cells that provide a useful paradigm for the stem cell mode of division, as they divide throughout larval development to produce one daughter that retains the seam stem cell fate of further proliferation, and one daughter that differentiates, as a result of asymmetric division. They also undergo symmetrical self-renewing divisions at the beginning of each larval stage (and additionally in males) in order to expand the number of progenitors.
Hence, mutations that cause defects in seam cell division patterns tend to result in defects in the morphogenesis of the sensory rays of the male tail. The great strength of using C. elegans as a model system for studying cell division patterns, apart from its excellent genetic and genomic resources, is the fact that the developmental cell lineage is invariant and has been completely described. This means that lineage aberrations can be analysed at cellular resolution.
We have isolated and analysed two genes, rnt-1 and bro-1, which act as rate-limiting regulators of seam cell proliferation, promoting self-renewal. These genes are homologous to mammalian Runx and CBFβ factors, which are known to be de-regulated in various forms of cancer, particularly those, like leukaemias, that involve stem cell lineage aberrations. Thus, our work on these genes in C. elegans is likely to shed light on the function and regulation of these important cancer-associated genes. We are currently using various genetic and biochemical approaches to seek other, novel factors that function to regulate cell number in this stem cell lineage with the aim of furthering our knowledge of the genetic framework in which Runx/CBFβ factors, and other genes controlling stem cell biology, operate. For example, we have recently undertaken a genome-wide RNAi screen designed to identify novel C. elegans stem cell regulators. Some of the genes we have identified act in the established RNT-1/BRO-1 pathway while others act in parallel pathways, opening up new areas for further analysis.
- Peter J Appleford and Alison Woollard. RUNX genes find a niche in stem cell biology. J Cell Biochem. 108, 14-21, 2009
- Toby Braun and Alison Woollard. RUNX factors in development: lessons from invertebrate model systems. Blood Cells Mol. Dis. 43, 43-48, 2009
- Peter J Appleford, Maria Gravato-Nobre, Toby Braun and AlisonWoollard. Identification of cis-regulatory elements from the C.elegans T-box gene mab-9 reveals a role for mab-9 in hypodermalfunction. Dev. Biol. 317, 695-704, 2008
- Roger Pocock, Marina Mione, Sagair Hussein, Sara Maxwell, MarcoPontecorvi, Sobia Aslam, Dianne Gerrelli, Jane C. sowden and AlisonWoollard. Neuronal function of Tbx20 conserved from nematodes tovertebrates. Dev. Biol. 317, 671-85, 2008
- Kagoshima, H, Nimmo, R, Saad, N, Tanaka, J, Miwa, Y, Mitani, S,Kohara, Y & Woollard, A. The C. elegans CBFb homologue BRO-1interacts with the Runx factor, RNT-1, to promote stem cellproliferation and self-renewal. Development, 134, 3905-15, 2007
Figure 1: Cartoon of seam stem cell division patterns during post-embryonic hermaphrodite development. In males, extra symmetrical divisions in the posterior seam lineages (V5, V6 and T) expand the pool of seam progenitor cells that eventually give rise to male specific neuroblasts, the ray precursor cells. Male specific ray sub-lineages then give rise to nine sensory rays found on each side of the tail
Figure 2: Over-expression of rnt-1 and bro-1 in C. elegans causes hyperplasia of the stem-like seam cells (white nuclei visible). In humans, mis-expression of the rnt-1 and bro-1 homologues Runx and CBFβ are associated with various cancers including Leukaemias. Therefore, worms provide an important model system in which to analyse these genes
Figure 3: Stem-like seam cell expressing rnt-1::gfp (green) in the nucleus and the ajm-1::cherry (red) adherens junction marker outlining the cells