Ttk69 acts as a master repressor of enteroendocrine cell specification inDrosophilaintestinal stem cell lineages

Abstract

In adult Drosophila midgut, intestinal stem cells (ISCs) periodically produce progenitor cells that undergo a binary fate choice determined primarily by the levels of Notch activity that they receive, before terminally differentiating into enterocytes (ECs) or enteroendocrine (EE) cells. Here we identified Ttk69, a BTB domain-containing transcriptional repressor, as a master repressor of EE cell specification in the ISC lineages. Depletion of ttk69 in progenitor cells induced ISC proliferation and caused all committed progenitor cells to adopt EE fate, leading to the production of supernumerary EE cells in the intestinal epithelium. Conversely, forced expression of Ttk69 in progenitor cells was sufficient to prevent EE cell specification. The expression of Ttk69 was not regulated by Notch signaling, and forced activation of Notch, which is sufficient to induce EC specification of normal progenitor cells, failed to prevent EE cell specification of Ttk69-depleted progenitors. Loss of Ttk69 led to derepression of the acheate-scute complex (AS-C) genes scute and asense, which then induced prospero expression to promote EE cell specification. These studies suggest that Ttk69 functions in parallel with Notch signaling and acts as a master repressor of EE cell specification in Drosophila ISC lineages primarily by suppressing AS-C genes.

Keeping intestinal stem cell differentiation on the Tramtrack

“…On the other hand, knockdown of ttk69 allowed Su(H) C EBs to differentiate toward EE cells, indicating that Ttk69 functions to prevent EC-committed EB from adopting EE cell fate. Moreover, 15 Taken together, these observations are consistent with a role for Ttk69 as a master EE cell fate repressor, but not an EC fate promoter.…”

“…1C). 15 Therefore we concluded that Ttk69 but not Ttk88 regulates EE cell specification from progenitor cells. We further studied the genetic relationship between Ttk69 and Notch signaling in EC/ EE cell specification.…”

“…In addition, overactivation of Notch signaling by expressing an intracellular domain of Notch in ttk69 clones failed to rescue EC specification. 15 These observations suggest that Ttk69 acts in parallel with Notch signaling in regulating the binary fate choice of intestinal progenitor cells.…”

“…From this screen, we identified a BTB-domain containing transcriptional repressor Ttk69, which plays a vital role in repressing EE cell specification. 15 We first observed excessive EE cell production in the intestinal epithelium when ttk was conditionally knocked down in ISCs and EBs for 7 d by expressing ttk-RNAi with esg-Gal4. As the ttk locus encodes both Ttk69 as well as Ttk88 by alternative splicing, 16 we subsequently determined which isoform was involved in EE cell specification.…”

“…1C). 15 We favor the former one for a number of reasons. On one hand, forced expression of Ttk69 is sufficient to repress EE cell specification but not to promote EC specification when Notch is disrupted.…”