Characterization of Bazooka and aPKC in Asymmetric Drosophila Cyst Stem Cell Division

Asymmetric stem cell division is important for maintaining tissue homeostasis. Some stem cells undergo asymmetric stem cell divisions when they reside in local microenvironments (i.e. stem cell niches), which provide essential signals to maintain stem cell identity. The intrinsic cellular machinery of asymmetric stem cell division remains, however, poorly understood. In this study, the importance of polarity proteins (i.e. Bazooka and aPKC) in regulating the asymmetric Drosophila cyst stem cell (CySC) division was discussed. First of all, the localization of Bazooka and aPKC was visualized by confocal microscopy, and the expression was quantified by a pixel intensity analysis program. Results showed Bazooka was almost evenly distributed along CySC cortex from interphase to metaphase, but illustrated higher expression level in the basal cortex during anaphase. Additionally, aPKC also displayed different localizations at different cell cycles. These results demonstrated dynamic localization of Bazooka and aPKC during asymmetric CySCs division. Time-lapse live-cell imaging provides a means to directly investigate the dynamic cellular processes, demonstrating the function of Bazooka and aPKC in regulating the asymmetric CySCs divisions. Either Bazooka or aPKC knockdown severely disrupted the anaphase spindle repositioning, leading to symmetric stem cell division. Spindle pole dynamics and mitosis duration analysis illustrated that both spindle pole dynamics were disrupted in Bazooka and aPKC knockdown. Meanwhile, the E-cadherin, which is highly localized to the hub- CySC interface for wildtype, was disrupted and distributed throughout the cell cortex in Bazooka and aPKC knockdown. These findings demonstrated the role of Bazooka and aPKC in establishing polarization in the CySCs. Moreover, this study may also lead to further investigation of mechanisms of the stem cell niche and the cellular machinery ensuring asymmetric stem cell divisions.