There may be clues however from studies of Dll1 where Neratinib in situ hybridisation indicates that high (and maybe stable) Delta expression occurs in supra-Paneth cell positions in cells that also express high levels of Atoh1 ( Figure 4) [ 13•]. Low-level oscillations may occur at the lower cell positions containing the intercalated, Lgr5+ population. Additionally, lower levels of Delta are seen in individual cells higher in the crypt and even on the villus (though the bHLH and Hes proteins are not), commensurate with Notch signalling playing roles later in the specification/differentiation programme (see below) [ 13•]. Notch also regulates

Ngn3, a bHLH that is absolutely required for secretory cells to adopt enteroendocrine fate [32]. The molecular mechanism of regulation of Ngn3 by Notch signalling is analogous to the regulation of Atoh1 as well as Ngn2 in the nervous system; where Notch activation inhibits Ngn3 expression, suppressing enteroendocrine cell formation and promoting alternate enterocyte or goblet fates [7, 33•• and 34]. It is striking that enteroendocrine numbers are limited but not eliminated by Notch activation in Ngn3 positive cells while Notch

activation driven by the villin promoter, that acts earlier in crypt specification results in complete enteroendocrine cell loss showing context-dependence of Notch sensitivity [33•• and 35]. In terms of plasticity the iterative role of Notch signalling means that find more the pathway is accessible to cells throughout the crypt to villus axis. After epithelial cell depletion, surviving cells have

a number of options to be restored to a stem cell state. At the level of an individual cell this may require regaining Cell press low-level oscillatory Notch signals associated with the poised state perhaps by altering the stability or post-translational regulation of the bHLH proteins that promote fate decisions [36]. Alternatively, in maturing enterocytes [37 and 38], upregulation of Hes family proteins could actively promote Ascl2 while suppressing Atoh1 expression and function. Notably the Ascl2 axis with potentially competing roles for elements of the Notch pathway also allows input and crosstalk from the Wnt pathway. Cell interactions favouring acquisition of stemness might include occupying a vacant cell position adjacent to a DeltaHi expressing cell to promote active Notch signalling in neighbours. The outline circuitry defined by the bHLH/Hes axis regulation can be fleshed out by a variety of post-transcriptional interactions and modification to limit or potentiate available Notch signalling in a context dependent manner. For example Notch transcript itself can be sequestered by regulatory microRNAs such as miR-34a. Downregulation of miR-34a following damage could promote not only acquisition of stemness but allow for rapid expansion of stem cells by symmetric divisions [39•].