Rachael Nimmo1, Hiroshi Kagoshima2 and Alison Woollard1. The seam cells of C. elegans are a good model system for understanding how cell fate and proliferation are regulated and coordinated during development. These hypodermal blast cells divide in a reiterated self renewing stem cell-like pattern at each larval stage producing an anterior cell that differentiates and fuses with
hyp7 and a posterior seam cell that divides again in the same way. Some of the seam cells also undergo extra proliferative symmetrical divisions in L2 larvae and in L3 males. We wish to understand how this pattern of cell division is programmed during development.. We identified the Runx homologue,
rnt-1(pka
mab-2) as a rate-limiting regulator of cell proliferation that specifically functions in seam cells. We also showed that the cyclin dependent kinase inhibitor orthologue, CKI-1 is negatively regulated by
rnt-1 revealing a possible mechanism by which this Runx factor regulates cell division. We have recently discovered that the timing of
rnt-1 overexpression is critically important for its hyperplastic effects. In addition it seems that RNT-1 may be inactivated by passage through S phase: a pulse of
rnt-1 overexpression in G2/M induces an extra round of division whereas overexpression prior to normal S phase entry fails to induce extra division. We have also found that the asymmetry of division may be overcome by
rnt-1 overexpression the anterior cell normally differentiates and fuses with the
hyp7 syncytium but in response to high levels of RNT-1 it is capable of remaining as a seam cell and dividing again. We propose that this failure to differentiate is caused by inappropriate cell cycle re-entry programmed by
rnt-1.. We have discovered that the CBFbeta homologue,
bro-1, is also required for seam cells to divide correctly. This implies that these two proteins interact in the same way as in other systems, namely that CBF beta binds to the Runt domain and increases Runx DNA binding affinity. Intriguingly, however, the proliferative effect of overexpression of
bro-1 appears not to be abrogated by
rnt-1 loss of function, indicating that
bro-1 may be capable of exerting its effects in a
rnt-1-independent manner. This result is unexpected as CBF beta proteins have not been previously shown to function separately from Runx factors. We are currently confirming this result and attempting to understand the additional mechanism by which
bro-1 acts.