Centrosomal Enrichment and Proteasomal Degradation of SYS-1/β-catenin Requires the Microtubule Motor Dynein

SYS-1/beta-catenin inheritance and regulation by Wnt-signaling during asymmetric cell division

Asymmetric cell division (ACD) allows daughter cells of a polarized mother to acquire different developmental fates. In C. elegans , the Wnt/β-catenin Asymmetry (WβA) pathway oversees many embryonic and larval ACDs; here, a Wnt gradient induces an asymmetric distribution of Wnt signaling components within the dividing mother cell. One terminal nuclear effector of the WβA pathway is the transcriptional activator SYS-1/β-catenin. SYS-1 is sequentially negatively regulated during ACD; first by centrosomal regulation and subsequent proteasomal degradation and second by asymmetric activity of the β-catenin "destruction complex" in one of the two daughter cells, which decreases SYS-1 levels in the absence of WβA signaling. However, the extent to which mother cell SYS-1 influences cell fate decisions of the daughters is unknown. Here, we quantify inherited SYS-1 in the differentiating daughter cells and the role of SYS-1 inheritance in Wnt-directed ACD. Photobleaching experiments demonstrate the GFP::SYS-1 present in daughter cell nuclei is comprised of inherited and de novo translated SYS-1 pools. We used a photoconvertible DENDRA2::SYS-1, to directly observe the dynamics of inherited SYS-1. Photoconversion during mitosis reveals that SYS-1 clearance at the centrosome preferentially degrades older SYS-1, and this accumulation is regulated via dynein trafficking. Photoconversion of the EMS cell during Wnt-driven ACD shows daughter cell inheritance of mother cell SYS-1. Additionally, loss of centrosomal SYS-1 increased inherited SYS-1 and, surprisingly, loss of centrosomal SYS-1 also resulted in increased levels of de novo SYS-1 in both EMS daughter cells. Lastly, we show that daughter cell negative regulation of SYS-1 via the destruction complex member APR-1/APC is key to limit both the de novo and the inherited SYS-1 pools in both the E and the MS cells. We conclude that regulation of both inherited and newly translated SYS-1 via centrosomal processing in the mother cell and daughter cell regulation via Wnt signaling are critical to maintain sister SYS-1 asymmetry during ACD.

Wnt signalling in cell division: from mechanisms to tissue engineering

Wnt signalling is an essential player in tissue formation, notably in the regulation of stem cell function. Wnt signalling is best known for its roles in G1/S progression. However, a complex Wnt programme that also mediates mitotic progression and asymmetric cell division (ACD) is emerging. Recent developments in this area have provided mechanistic insights as well as tools to engineer or target Wnt signalling for translational and therapeutic purposes. Here, we discuss the bidirectional relationship between Wnt activity and mitosis. We emphasise how various Wnt-dependent mechanisms control spindle dynamics, chromosome segregation, and ACD. Finally, we illustrate how knowledge about these mechanisms has been successfully employed in tissue engineering for regenerative medicine applications.