Profiling the Role of SAPCD2 in the Control of Mammalian Mitotic Spindle Orientation

Cell division is fundamental to multicellular organisms, and understanding the molecular basis remains important for developing treatment strategies for various human diseases, including developmental disorders and cancers. During mitosis, proper segregation of cellular components in each daughter cell is required for epithelial morphogenesis, and asymmetric cell division is achieved through controlled orientation of the mitotic spindle. Molecular signalling through a ternary protein complex consisting of Gai, LGN, and NuMA is conserved in both vertebrate and invertebrate mitoses to orient the mitotic spindle, however its role in cell fate and tissue development signalling is context-dependent, and remains incompletely understood in mammalian systems. My work identifies Suppressor APC domain containing 2 (SAPCD2) as a regulator of the Gai-LGN-NuMA complex in mitotic spindle orientation signalling. SAPCD2 is essential for planar mitotic spindle orientation in both epithelial cell cultures and mouse retinal progenitor cells in vivo. Loss of SAPCD2 randomizes spindle orientation, which disrupts cyst morphogenesis in three-dimensional cultures, and increases terminal asymmetric cell divisions at the expense of symmetric cell divisions in the developing retina. SAPCD2 competes with NuMA for LGN binding, and prevents LGN accumulation at the cell cortex. Mass spectrometry-based proteomic approach revealed that SAPCD2 interacts with polarity proteins required for establishing epithelial cell polarity, an essential step upstream of mitotic spindle orientation. SAPCD2 is dispensable for tight junction biogenesis, an instructive process for formation of the the apical-basolateral domains, however interactions with polarity proteins mediates its localization to the apical domain through its C-terminal sequence that binds to PDZ (PSD-95/Dlg1/ZO-1) domains. In summary, this thesis reports the characterization of SAPCD2 as a regulator of the mitotic spindle orientation pathway, and provides mechanisms underlying mammalian-specific control of this pathway.Ph.D