SUMMARY Cells in intestinal epithelia turn over rapidly due to damage from digestion and toxins produced by the enteric microbiota. Gut homeostasis is maintained by intestinal stem cells (ISCs) that divide to replenish the intestinal epithelium, but little is known about how ISC division and differentiation are coordinated with epithelial cell loss. We show here that when enterocytes (ECs) in the Drosophila midgut are subjected to apoptosis, enteric infection, or JNK-mediated stress signaling, they produce cytokines (Upd, Upd2, Upd3) that activate Jak/Stat signaling in ISCs, promoting their rapid division. Upd/Jak/Stat activity also promotes progenitor cell differentiation, in part by stimulating Delta/Notch signaling, and is required for differentiation in both normal and regenerating midguts. Hence, cytokine-mediated feedback enables stem cells to replace spent progeny as they are lost, thereby establishing gut homeostasis.

A central enzyme in the pathway of de novo lipogenesis, fatty acid synthase (FAS) 1 catalyzes all of the steps in the conversion of malonyl-CoA to palmitate. Expression of the FAS gene is controlled primarily at the level of transcription and is responsive to both hormonal and nutritional signals (1, 2). Previous work has shown that sterol regulatory element-binding proteins (SREBPs) play a critical role in the transcriptional regulation of a number of genes in the lipogenic pathway, including FAS, steroyl-CoA desaturase (SCD-1), and acetyl-CoA carboxylase (ACC) (3-8). Three SREBP isoforms have been described: SREBP-1a and Ϫ1c (also called ADD1), which are derived from the same gene through alternative splicing, and SREBP-2, which is encoded by a separate gene (9, 10). Although their transcriptional targets overlap significantly, studies suggest that SREBP-1 preferentially activates genes involved in lipogenesis, whereas SREBP-2 preferentially activates genes in the cholesterol biosynthetic pathway (11-14). SREBPs have been shown to regulate FAS expression through direct interaction with the FAS promoter at multiple sites (7, 15). Overexpression of nuclear SREBP-1 is sufficient to induce expression of the FAS gene in cultured cells as well as transgenic mice (5,8). Recent work has also implicated the nuclear receptors LXR␣ and LXR␤ in the control of lipogenesis. Both LXRs bind to DNA and regulate transcription of target genes in a heterodimeric complex with RXR (16). Although early studies on LXRs focused on their role in cholesterol metabolism, mice carrying a targeted disruption in the LXR␣ gene were noted to be deficient in expression of FAS, SCD-1, ACC, and SREBP-1, consistent with a role in lipogenesis as well (17). Further support for this idea came with the observation that the administration of the synthetic LXR ligand T1317 to mice triggers induction of the lipogenic pathway and raises plasma triglyceride levels (18). The demonstration that the SREBP-1c promoter is a direct target for regulation by LXR/RXR heterodimers provided a straightforward explanation for the ability of LXR ligands to induce hepatic lipogenesis (19,20). Until now, the effects of LXR activation on the expression of lipogenic genes, including FAS, have been presumed to be entirely indirect.We demonstrate here that the FAS promoter is a direct target for regulation by the LXR/RXR heterodimer as well as SREBPs. This novel mechanism for the regulation of FAS expression and lipogenesis by LXRs has implications for the development of LXR agonists as modulators of human lipid metabolism. EXPERIMENTAL PROCEDURESReagents and Plasmids-Expression plasmids for RXR␣ and LXR␣, and nuclear SREBP-1a, -1c, and -2 have been described (21,22). GW3965 (23) and T0901317 (18) were provided by Timothy M. Willson (GlaxoSmithKline). Ligands were dissolved in Me 2 SO prior to use in cell culture. The Ϫ1594, Ϫ700, Ϫ150, and Ϫ135 rat FAS promoter luciferase reporter constructs were described previously (3). Mutations were

Deciphering contributions of specific cell types to organ function is experimentally challenging. The Drosophila midgut is a dynamic organ with five morphologically and functionally distinct regions (R1-R5), each composed of multipotent intestinal stem cells (ISCs), progenitor enteroblasts (EBs), enteroendocrine cells (EEs), enterocytes (ECs), and visceral muscle (VM). To characterize cellular specialization and regional function in this organ, we generated RNA-sequencing transcriptomes of all five cell types isolated by FACS from each of the five regions, R1-R5. In doing so, we identify transcriptional diversities among cell types and document regional differences within each cell type that define further specialization. We validate cell-specific and regional Gal4 drivers; demonstrate roles for transporter Smvt and transcription factors GATAe, Sna, and Ptx1 in global and regional ISC regulation, and study the transcriptional response of midgut cells upon infection. The resulting transcriptome database (http://flygutseq.buchonlab.com) will foster studies of regionalization, homeostasis, immunity, and cell-cell interactions.