UCSF

Lissencephaly (smooth brain) is a brain malformation disorder resulted from defective neuronal migration during early development. Deletion or mutation of many genes involved in the regulation of the microtubule (MT) and actin cytoskeletons have been identified as a direct causes of lissencephaly, including the human LIS1 (lissencephaly-1) gene. Loss-of-function phenotypes of lissencephaly-causing protein are linked with MT-actin dysfunction in post-mitotic neurons, which disrupts nucleokinesis and leading process dynamics. Haploinsufficiency of human LIS1 is responsible for reduced gyri/sulci in the brains from isolated lissencephaly sequence patients. We are particularly interested in mouse LIS1 functions in early development and used mouse embryonic fibroblasts (MEFs) and neural progenitors (NPs) to investigate molecular mechanisms regulated by LIS1. Previously, we demonstrated that loss of LIS1 in these cells led to cell death of these progenitors. However, the understanding of the detailed cellular processes regulated by LIS1 has been limited thus far. By performing timelapse live-imaging from Lis1 mutant MEFs, we demonstrate here that LIS1 is required for the spatiotemporal regulation of the mitotic spindle during mitotic cell division to dictate proper spindle orientation, anchored to the cell cortex by astral MT plus-end tips. Furthermore, LIS1 is also essential for normal centrosome number maintenance in MEFs. Surprisingly, we found that Lis1 mutant MEFs displayed severe defects in cytokinesis, a final critical step of cell separation. RhoA-actomyosin-contractile ring components and signaling pathways important for cleavage furrow specification were misregulated in Lis1 mutant MEFs, suggesting that mouse LIS1 mainly suppress RhoA GTPase activity to precisely control actomyosin-mediated cortical contractility at the cell cortex. Taken together, we concluded that LIS1 is a key molecular switch to balance MTs and actomyosin cytoskeletons to coordinate the interaction between mitotic spindle-associated cellular machinery and actin-based cell cortex membranes throughout the mitotic cell division. These results further suggest that LIS1 particulates in critical protein networks that determine cell survival or death of NPs through the regulation of mitosis and cytokinesis.