Mechanisms of Asymmetric Cell Division

Abstract

Asymmetric cell division is an important process by which a cell divides to give two different daughter cells. This process is important in stem cells whereby a single division produces two cells, one of which is the stem cell itself and the other, committed or differentiated daughter cell. This process is highly controlled, as failure of which would result in depletion of stem cells or uncontrolled cell proliferation, leading to diseases like Parkinson?s disease or cancer. Asymmetric cell division is well studied in the model organism Drosophila neuroblast and sensory organ precursor. It is known that proteins in the apical complex such as atypical Protein Kinase C (aPKC), Bazooka (Baz), Partitioning Defective 6 (Par6), Inscuteable, Partner of Inscuteable and G-protein alpha-I subunit direct cell fate determinants such as Numb, Partner of Numb, Miranda, Prospero, and Brain Tumor to localize as a basal crescent in the cell such that they would be inherited by only one of the daughter cells. However, detailed mechanism of this directing of cell fate determinants is largely unknown. Using a UAS-GAL4 system to drive expression of RNAi constructs in the central and peripheral nervous system, I identified clueless (clu) as one of the candidate genes that are involved in asymmetric cell division of the larval brain neuroblasts or sensory organ precursor cells. Clueless is expressed during the larval stages and enriched in the neuroblast. clu mutants display a weak but obvious phenotype of mislocalized Miranda and Numb crescent in a small percentage of dividing neuroblast. Deletion of clu in lethal giant larvae (lgl) mutants largely rescues the phenotype of mislocalized Miranda. The brain size of clu lgl double mutant is also considerably smaller than that of lgl mutants. Biochemistry data suggests that this rescue is due to down-regulation of aPKC. This down-regulation of aPKC is independent of transcription and translation and did not rescue the mislocalization of the apical aPKC crescent. Par6-aPKC can form a complex with either Baz or Lgl to exist in either an active or inactive complex. Co-immunoprecipitation of Clu showed that aPKC and Bazooka form a complex with Clu but not Lgl. This result suggests that Clu forms a complex with Par6-aPKC-Baz. Clu may be responsible for the stability of the active Par6-aPKC-Baz complex.