van den Heuvel, S. | bioRxiv Science

bioRxiv2018

Optogenetic dissection of mitotic spindle positioning in vivo

The position of the mitotic spindle determines the plane of cell cleavage, and thereby the location, size, and content of daughter cells. Spindle positioning is driven by dynein-mediated pulling forces exerted on astral microtubules. This process requires an evolutionarily conserved complex of G-GDP, GPR-1/2Pins/LGN, and LIN-5Mud/NuMA proteins. It remains unknown whether this complex merely forms a membrane anchor for dynein, or whether the individual components have additional functions, for instance through G-GTP or dynein activation. To functionally dissect this system, we developed a genetic strategy for light-controlled localization of endogenous proteins in C. elegans embryos. Controlled germline expression and membrane recruitment of the G regulators RIC-8Ric-8A and RGS-7Loco/RGS3, and replacement of G with a light-inducible membrane anchor demonstrated that G-GTP signaling is dispensable for pulling force generation. In the absence of G, cortical recruitment of GPR-1/2 or LIN-5, but not dynein itself, induced high pulling forces. Local recruitment of LIN-5 overruled normal cell-cycle and polarity regulation, and provided experimental control over the spindle and cell cleavage plane. Our results define G*GDP-GPR-1/2Pins/LGN as a regulatable membrane anchor, and LIN-5Mud/NuMA as a potent activator of dynein-dependent spindle positioning forces. This study also highlights the possibilities for optogenetic control of endogenous proteins within an animal system.

cell biology↗

bioRxiv2017

The tumor suppressor APC is an attenuator of spindle-pulling forces during C. elegans asymmetric cell division

The adenomatous polyposis coli (APC) tumor suppressor has dual functions in Wnt/{beta}-catenin signaling and accurate chromosome segregation, and is frequently mutated in colorectal cancers. Although APC contributes to proper cell division, the underlying mechanisms remain poorly understood. Here we show that C. elegans APR-1/APC is an attenuator of the pulling forces acting on the mitotic spindle. During asymmetric cell division of the C. elegans zygote, a LIN-5/NuMA protein complex localizes dynein to the cell cortex to generate pulling forces on astral microtubules that position the mitotic spindle. We found that APR-1 localizes to the anterior cell cortex in a Par-aPKC polarity-dependent manner and suppresses anterior centrosome movements. Our combined cell biological and mathematical analyses support the conclusion that cortical APR-1 reduces force generation by stabilizing microtubule plus ends at the cell cortex. Furthermore, APR-1 functions in coordination with LIN-5 phosphorylation to attenuate spindle pulling forces. Our results document a physical basis for spindle-pulling force attenuation, which may be generally used in asymmetric cell division, and when disrupted potentially contributes to division defects in cancer.\n\nSignificance StatementAPC (adenomatous polyposis coli) is a Wnt signaling component as well as a microtubule-associated protein, and its mutations are frequently associated with colorectal cancers in humans. Although APC stabilizes microtubules (MTs), its mechanical role during cell division is largely unknown. Here we show that APC is an attenuator of forces acting on the mitotic spindle during asymmetric cell division of the C. elegans zygote. We performed live-imaging, laser-microsurgery, and numerical simulation to show how APC suppresses spindle pulling force generation by stabilizing microtubule plus-ends and reducing microtubule catastrophe frequency at the cell cortex. Our study is the first to document a mechanical role for the APC protein, and provides a physical basis for spindle-pulling force attenuation.

cell biology↗