Cells differentiate when transcription factors (TFs) bind accessible cis-regulatory elements to establish specific gene expression programs. In differentiating embryonic stem (ES) cells, chromatin at lineage-restricted genes becomes sequentially accessible1-4, probably by virtue of “pioneer” TF activity5, but tissues may utilize other strategies in vivo. Lateral inhibition is a pervasive process in which one cell forces a different identity on its neighbors6, and it is unclear how chromatin in equipotent progenitors undergoing lateral inhibition quickly enables distinct, transiently reversible cell fates. Here we report the chromatin and transcriptional underpinnings of differentiation in mouse small intestine crypts, where Notch signaling mediates lateral inhibition to assign progenitor cells into absorptive or secretory lineages7-9. Transcript profiles in isolated LGR5+ intestinal stem cells (ISC)10 and secretory and absorptive progenitors indicated that each cell population was distinct and the progenitors specified. Nevertheless, secretory and absorptive progenitors showed comparable levels of H3K4me2 and H3K27ac histone marks and DNaseI hypersensitivity - signifying accessible, permissive chromatin - at most of the same cis-elements. Enhancers acting uniquely in progenitors were well-demarcated in LGR5+ ISC, revealing early priming of chromatin for divergent transcriptional programs, and retained active marks well after lineages were specified. On this chromatin background, ATOH1, a secretory-specific TF, controls lateral inhibition through Delta-like Notch ligand genes and also drives numerous secretory lineage genes. Depletion of ATOH1 from specified secretory cells converted them into functional enterocytes, indicating prolonged responsiveness of marked enhancers to presence or absence of a key TF. Thus, lateral inhibition and intestinal crypt lineage plasticity involve interaction of a lineage-restricted TF with broadly permissive chromatin established in multipotent stem cells.

Mutations in PKD1, which encodes polycystin-1 (PC1), contribute to Ͼ85% of cases of autosomal dominant polycystic kidney disease (ADPKD). The planar cell polarity (PCP) pathway is necessary for the oriented cell division and convergent extension that establishes and maintains the structure of kidney tubules, but the role of this pathway in the pathophysiology of ADPKD is incompletely understood. Here, we show that inactivation of Pkd1 in postnatal developing mouse kidneys leads to a defect in oriented cell division in precystic kidney tubules. We also observed this defect in precystic Pkd1-inactivated mature kidneys subjected to ischemia-reperfusion injury as a "third hit." Cystic kidneys exhibited striking upregulation and activation of Frizzled 3 (Fz3), a regulator of PCP, and its downstream effector, CDC42. Precystic kidneys demonstrated upregulation of CDC42, but the localization of the polarity proteins Par3 and Par6 was similar to control. Fz3 was expressed on the cilia of cystic kidneys but barely detected on the cilia of normal kidneys. In vitro, PC1 and Fz3 antagonized each other to control CDC42 expression and the rate of cell migration in HEK293T cells. Taken together, our data suggest that PC1 controls oriented cell division and that aberrant PCP signaling contributes to cystogenesis.

Wnt signaling pathways are essential for embryonic patterning, and they are disturbed in a wide spectrum of diseases, including cancer. An unresolved question is how the different Wnt pathways are supported and regulated. We previously established that the postsynaptic density 95/disc-large/zona occludens (PDZ) protein syntenin binds to syndecans, Wnt coreceptors, and known stimulators of protein kinase C (PKC)␣ and CDC42 activity. Here, we show that syntenin also interacts with the C-terminal PDZ binding motif of several Frizzled Wnt receptors, without compromising the recruitment of Dishevelled, a key downstream Wnt-signaling component. Syntenin is coexpressed with cognate Frizzled during early development in Xenopus. Overexpression and down-regulation of syntenin disrupt convergent extension movements, supporting a role for syntenin in noncanonical Wnt signaling. Syntenin stimulates c-jun phosphorylation and modulates Frizzled 7 signaling, in particular the PKC␣/CDC42 noncanonical Wnt signaling cascade. The syntenin-Frizzled 7 binding mode indicates syntenin can accommodate Frizzled 7-syndecan complexes. We propose that syntenin is a novel component of the Wnt signal transduction cascade and that it might function as a direct intracellular link between Frizzled and syndecans. INTRODUCTIONWnt proteins are involved in cell proliferation, differentiation, polarity, migration, and apoptosis, controlling a variety of processes during embryonic development and adult homeostasis (Logan and Nusse, 2004). Various inborn and acquired diseases are based on aberrant Wnt signaling (Johnson and Rajamannan, 2006). Wnt signaling requires the interplay of multiple proteins (http://www.stanford.edu/ ϳrnusse/wntwindow.html), but the reception and transduction of Wnt signals are predominantly based on the binding of Wnt proteins to Frizzled (Fz) cell surface receptors (Yang-Snyder et al., 1996). The various Fz receptors differ in their spatial and temporal expression patterns and in their relative affinities for ligand. Receptor activation by Wnt somehow activates Dishevelled (Dsh), the most upstream component of the cytosolic signal transduction cascade. The role of Dsh is intricate because downstream of Dsh, the signaling branches into either the canonical Wnt/ ␤-catenin or the noncanonical (␤-catenin-independent) pathway (Boutros and Mlodzik, 1999). Activation of the canonical pathway drives ␤-catenin-dependent transcription of target genes, controls tissue-specific cell fate decisions during embryogenesis, and regulates cell proliferation in adult tissues (Logan and Nusse, 2004). The noncanonical pathway controls reorganization of the actin cytoskeleton, tissue polarity, and cell movement (Strutt, 2003). Several molecular cascades, which may overlap, seem to function in noncanonical Wnt signaling (Veeman et al., 2003;Kohn and Moon, 2005). One is similar to the planar cell polarity or PCP pathway in Drosophila, and it activates small GTPases of the Rho family and the c-Jun-NH 2 -terminal kinase (JNK). Another, triggered by Wn...