Zebrafish Tsc1 Reveals Functional Interactions Between the Cilium and the TOR Pathway

The cell surface organelle called the cilium is essential for preventing kidney cyst formation and for establishing left-right asymmetry of the vertebrate body plan. Recent advances suggest that the cilium functions as a sensory organelle in vertebrate cells for multiple signaling pathways such as the hedgehog and the Wnt pathways. Prompted by kidney cyst formation in tuberous sclerosis complex (TSC) patients and rodent models, we investigated the role of the cilium in the TSC-TOR pathway using zebrafish. TSC1 and TSC2 genes are causal for TSC, and their protein products form a complex in the TOR pathway that integrates environmental signals to regulate cell growth, proliferation and survival. Two TSC1 homologs were identified in zebrafish, which we refer to as tsc1a and tsc1b. Morpholino knockdown of tsc1a led to a ciliary phenotype including kidney cyst formation and left-right asymmetry defects. Tsc1a was observed to localize to the Golgi, but morpholinos against it nonetheless acted synthetically with ciliary genes in producing kidney cysts. Consistent with a role of the cilium in the same pathway as Tsc genes, the TOR pathway is aberrantly activated in ciliary mutants, resembling the effect of tsc1a knockdown. Moreover, kidney cyst formation in ciliary mutants was blocked by the Tor inhibitor rapamycin. Surprisingly, we observed elongation of cilia in tsc1a knockdown animals. Together, these data suggest a signaling network between the cilium and the TOR pathway in that ciliary signals can feed into the TOR pathway and that Tsc1a regulates the length of the cilium itself.