Intrinsic and extrinsic mechanisms regulating asymmetric cell division in C. elegans

不对称细胞分裂，即一个母细胞分裂产生两个子细胞的过程，这两个子细胞采用不同的细胞命运，是在发育过程中产生细胞多样性的关键机制。细胞内蛋白的不对称定位和细胞信号传导都有助于这一过程。尽管进行了大量的研究，但仍有许多悬而未决的问题。细胞信号是如何使不对称分裂的细胞极化的？生物体如何整合细胞信号来调节细胞内决定因子的不对称定位。秀丽隐杆线虫非常适合解决这些问题，因为大多数细胞是由不对称细胞分裂产生的。由于整个细胞谱系是已知的，细胞分裂的时间、位置和极性可以在单个细胞的分辨率上进行分析。

Asymmetric cell division, the process by which a mother cell divides to produce two daughter cells that adopt distinct cell fates, is a key mechanism that generates cellular diversity during development. Both the asymmetric localization of intracellular proteins and cell signaling contribute to this process. Despite numerous studies many outstanding questions remain. How does cell signaling polarize asymmetrically dividing cells? How does an organism integrate cell signaling to regulate the asymmetric localization of intracellular determinants. C. elegans is ideally suited address these questions since the majority of cells are generated by asymmetric cell division. Because the entire cell lineage is known, the timing, location and polarity of cell divisions can be analyzed at the resolution of single cells. My long-term goal is to elucidate the molecular and cellular mechanisms underlying asymmetric cell division in C. elegans. We have focused our analysis on the Wnt signalling pathway and HAM-1, a protein asymmetrically localized to the cell cortex in many dividing cells. In C. elegans, Wnt signaling regulates many asymmetric cell divisions. The Wnt pathway is a conserved cell-signaling pathway that is essential for development. In our previous NSERC funded research we showed that DSH-2, a key Wnt pathway component, regulates asymmetric neuroblast division during development and thus contributes to neuronal diversity. We undertook a highly successful genetic screen to identify genes that function with the Wnt/DSH-2 signaling pathway. One of the genes identified was hcf-1, a transcriptional co-activator. This was a novel finding since a role for hcf-1 in Wnt signalling has not been previously described in any experimental system. We proceeded to show that HCF-1 contributes to the asymmetric localization of the Wnt transcription factor POP-1/TCF. In this research proposal, we aim to determine the mechanism by which HCF-1 regulates POP-1 localization. Preliminary results indicates HCF-1 is likely affecting the acetylation of POP-1 and we will undertake experiments to test this hypothesis. We have cloned a second dsh-2 interacting gene and it encodes a novel protein. We will perform a combination of genetic, molecular and cell biological experiments to understand how this protein regulates Wnt dependent asymmetric cell division. We will also make use of whole genome sequencing to determine the molecular identity of additional mutations isolated in our genetic screen. Finally, we have shown that two other C. elegans Dishevelled homologs, DSH-1 and MIG-5, physically interact with HAM-1. This interaction provides a key link between cell signalling and the asymmetric localization of intracellular proteins. We propose experiments to investigate the role of Wnt signalling in the asymmetric localization of HAM-1.