TSC1/2 regulates intestinal stem cell maintenance and lineage differentiation through Rheb-TORC1-S6K but independently of nutritional status or Notch regulation.

Tubular sclerosis complex gene products TSC1 and TSC2 have evolutionarily conserved roles in cell growth from Drosophila to mammals. Here we reveal important roles for TSC1/2 in regulating intestinal stem cell (ISC) maintenance and differentiation of the enteroendocrine cell lineage in the Drosophila midgut. Loss of either the Tsc1 or Tsc2 gene in ISCs causes rapid ISC loss through TORC1 hyperactivation, because ISCs can be efficiently rescued by mutation of S6k or by rapamycin treatment. In addition, overexpression of Rheb, which triggers TORC1 activation, recapitulates the phenotype caused by TSC1/2 disruption. Genetic studies suggest that TSC1/2 maintains ISCs independently of nutritional status or Notch regulation, probably by inhibiting cell delamination. We show that Tsc1/Tsc2 mutant ISCs can efficiently produce enterocytes but not enteroendocrine cells, and this altered differentiation potential is also caused by hyperactivation of TORC1. Reduced TORC1-S6K signaling by mutation of S6k, however, has no effect on ISC maintenance or cell lineage differentiation. Our studies demonstrate that hyperactivation of TORC1 following the loss of TSC1/2 is detrimental to stem cell maintenance and multiple lineage differentiation in the Drosophila ISC lineage, a mechanism that could be conserved in other stem cell lineages, including that in humans.

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