In C. elegans, Wnt signaling regulates a number of asymmetric cell divisions. During telophase, WRM-1/beta-catenin localizes asymmetrically to the anterior cortex and the posterior daughter's nucleus. However, cortical WRM-1's functions are not known. Here, we use a membrane-targeted form of WRM-1 to show that cortical WRM-1 inhibits Wnt signaling and the nuclear localization of WRM-1. These functions are mediated by APR-1/APC, which regulates WRM-1 nuclear export. We also show that APR-1 as well as PRY-1/Axin and Dishevelled homologs localize asymmetrically to the cortex. Our results suggest a model in which cortical WRM-1 recruits APR-1 to the anterior cortex before and during division, and the cortical APR-1 stimulates WRM-1 export from the anterior nucleus at telophase. Because beta-catenin and APC are localized to the cortex in many cell types in different species, our results suggest that these cortical proteins may regulate asymmetric divisions or Wnt signaling in other organisms as well.

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Wnt signaling plays important roles in cell polarization in diverse organisms, and loss of cell polarity is an early event in tumorigenesis caused by mutations in Wnt pathway genes. Despite this, the precise roles of Wnt proteins in cell polarization have remained elusive. In no organism has it been shown that the asymmetric position of a Wnt signal is essential to establishing a cell's polarity. Attempts to test this by ubiquitous expression of Wnt genes have suggested that Wnt signals might act only as permissive factors in cell polarization. Here we find, by using cell manipulations and ectopic gene expression in C. elegans, that the position from which Wnt signals are presented can determine the polarity of both embryonic and postembryonic cells. Furthermore, the position from which a Wnt signal is presented can determine the polarity of Frizzled receptor localization, suggesting that the polarizing effect of Wnt is likely to be direct. These results demonstrate that Wnt proteins can function as positional cues in establishing cell polarity.

Extrinsic signals received by a cell can induce remodeling of the cytoskeleton, but the downstream effects of cytoskeletal changes on gene expression have not been well studied. Here, we show that during telophase of an asymmetric division in C. elegans, extrinsic Wnt signaling modulates spindle structures through APR-1/APC, which in turn promotes asymmetrical nuclear localization of WRM-1/β-catenin and POP-1/TCF. APR-1 that localized asymmetrically along the cortex established asymmetric distribution of astral microtubules, with more microtubules found on the anterior side. Perturbation of the Wnt signaling pathway altered this microtubule asymmetry and led to changes in nuclear WRM-1 asymmetry, gene expression, and cell-fate determination. Direct manipulation of spindle asymmetry by laser irradiation altered the asymmetric distribution of nuclear WRM-1. Moreover, laser manipulation of the spindles rescued defects in nuclear POP-1 asymmetry in wnt mutants. Our results reveal a mechanism in which the nuclear localization of proteins is regulated through the modulation of microtubules.

Summary Cellular interactions between neighboring axons are essential for global topographic map formation. Here we show that axonal interactions also precisely instruct the location of synapses. Motoneurons form en passant synapses in Caenorhabditis elegans. While axons from the same neuron class significantly overlap, each neuron innervates a unique and tiled segment of the muscle field by restricting its synapses to a distinct subaxonal domain—a phenomenon we term “synaptic tiling”. Using DA8 and DA9 motoneurons as a model, we found that the synaptic tiling requires Plexin(PLX-1) and two transmembrane Semaphorins. In the Plexin or Semaphorin mutants, synaptic domains from both neurons expand and overlap with each other without guidance defects. In a Semaphorin-dependent manner, PLX-1 is concentrated at the synapse-free axonal segment, delineating the tiling border. Furthermore, Plexin inhibits presynapse formation by suppressing synaptic F-actin through its cytoplasmic GAP domain. Hence, contact-dependent, intra-axonal Plexin signaling specifies synaptic circuits by inhibiting synapse formation at the subcellular loci.