PgmNr W4168: Mechanisms of SYS-1/β-catenin centrosomal localization in early embryonic blastomeres.

Authors:
J. W. Thompson; Bryan Phillips


Institutes
University of Iowa, Iowa City, IA.


Keyword: Microtubules

Abstract:

Asymmetric cell division, the unequal distribution of cell fate determinants between daughter cells, is a critical system underlying the development and maintenance of the varied tissue types of a multicellular organism.  Despite widespread utilization of such divisions, the mechanisms responsible for induction of asymmetry are less understood due to the complex, pleiotropic, and often functionally redundant signaling systems involved.  Throughout C. elegans development, the Wnt/β-catenin asymmetry pathway induces the asymmetric distribution of Wnt signaling components to polarize a mother cell and therefore differentially regulate Wnt target genes in its daughter cells.  For instance, a proper Wnt signaling response results in accumulation of cytoplasmic SYS-1/b-catenin in the proximal ‘signaled’ cell, which, along with co-activator POP-1/TCF, activates genes to induce the corresponding cell fate.  Despite its asymmetric regulation immediately post-division, cytoplasmic SYS-1 localizes symmetrically to mitotic centrosomes.  Recent data from our lab indicate that centrosomal localization of SYS-1 serves as a clearance mechanism to increase the robustness of Wnt-mediated polarity.  However, the method of SYS-1 trafficking to and accumulation at the centrosome is unknown. In the early embryo, this centrosomal localization of the SYS-1 is dependent on the centrosomal scaffolding protein RSA-2.  When SYS-1 is prevented from loading to the centrosome by loss of RSA-2, it accumulates in the approximate region of kinetochore microtubules. This localization suggests that SYS-1 may be directly transported to centrosomes along microtubules via the action of molecular motors.  Specifically, we focused on dynein and dynactin components as the primary minus-end directed motor machinery. We conducted an RNAi and chemical screen specifically aimed at depleting these microtubules and microtubule transport proteins and quantified relative and proportionate SYS-1 centrosomal accumulation. This screen identified several dynein subunits that have a role in proper centrosomal localization of embryonic SYS-1/b-catenin and its regulators. Further, the preliminary data from microtubule-destabilizing and ATP-depleting chemical treatment indicate that both ATP and microtubules are important for SYS-1 centrosomal localization.  Together, these results suggest an active role for the multimeric dynein/dynactin cargo binding complex in SYS-1/b-catenin regulation and signaling status.