SummaryTissue regeneration requires dynamic cellular adaptation to the wound environment. It is currently unclear how this is orchestrated at the cellular level and how cell fate is affected by severe tissue damage. Here we dissect cell fate transitions during colonic regeneration in a mouse dextran sulfate sodium (DSS) colitis model, and we demonstrate that the epithelium is transiently reprogrammed into a primitive state. This is characterized by de novo expression of fetal markers as well as suppression of markers for adult stem and differentiated cells. The fate change is orchestrated by remodeling the extracellular matrix (ECM), increased FAK/Src signaling, and ultimately YAP/TAZ activation. In a defined cell culture system recapitulating the extracellular matrix remodeling observed in vivo, we show that a collagen 3D matrix supplemented with Wnt ligands is sufficient to sustain endogenous YAP/TAZ and induce conversion of cell fate. This provides a simple model for tissue regeneration, implicating cellular reprogramming as an essential element.

SummaryRegeneration and homeostasis in the adult intestinal epithelium is driven by proliferative resident stem cells, whose functional properties during organismal development are largely unknown. Here, we show that human and mouse fetal intestine contains proliferative, immature progenitors, which can be expanded in vitro as Fetal Enterospheres (FEnS). A highly similar progenitor population can be established during intestinal differentiation of human induced pluripotent stem cells. Established cultures of mouse fetal intestinal progenitors express lower levels of Lgr5 than mature progenitors and propagate in the presence of the Wnt antagonist Dkk1, and new cultures can be induced to form mature intestinal organoids by exposure to Wnt3a. Following transplantation in a colonic injury model, FEnS contribute to regeneration of colonic epithelium by forming epithelial crypt-like structures expressing region-specific differentiation markers. This work provides insight into mechanisms underlying development of the mammalian intestine and points to future opportunities for patient-specific regeneration of the digestive tract.

SummaryHuman pluripotent stem cells (hPSCs) could provide an infinite source of clinically relevant cells with potential applications in regenerative medicine. However, hPSC lines vary in their capacity to generate specialized cells, and the development of universal protocols for the production of tissue-specific cells remains a major challenge. Here, we have addressed this limitation for the endodermal lineage by developing a defined culture system to expand and differentiate human foregut stem cells (hFSCs) derived from hPSCs. hFSCs can self-renew while maintaining their capacity to differentiate into pancreatic and hepatic cells. Furthermore, near-homogenous populations of hFSCs can be obtained from hPSC lines which are normally refractory to endodermal differentiation. Therefore, hFSCs provide a unique approach to bypass variability between pluripotent lines in order to obtain a sustainable source of multipotent endoderm stem cells for basic studies and to produce a diversity of endodermal derivatives with a clinical value.

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RAC1 activity is critical for intestinal homeostasis, and is required for hyperproliferation driven by loss of the tumour suppressor gene Apc in the murine intestine. To avoid the impact of direct targeting upon homeostasis, we reasoned that indirect targeting of RAC1 via RAC-GEFs might be effective. Transcriptional profiling of Apc deficient intestinal tissue identified Vav3 and Tiam1 as key targets. Deletion of these indicated that while TIAM1 deficiency could suppress Apc-driven hyperproliferation, it had no impact upon tumourigenesis, while VAV3 deficiency had no effect. Intriguingly, deletion of either gene resulted in upregulation of Vav2, with subsequent targeting of all three (Vav2−/−Vav3−/−Tiam1−/−), profoundly suppressing hyperproliferation, tumourigenesis and RAC1 activity, without impacting normal homeostasis. Critically, the observed RAC-GEF dependency was negated by oncogenic KRAS mutation. Together, these data demonstrate that while targeting RAC-GEF molecules may have therapeutic impact at early stages, this benefit may be lost in late stage disease.

An overactive Toll-like receptor (TLR) pathway is a hallmark of several autoimmune diseases and some types of B cell lymphomas. TLR signaling is a main mediator of inflammatory signals in B cells and causes NF-κB activation, which has numerous stimulatory effects, including promotion of B cell survival and proliferation. A key factor in TLR signaling is the adaptor protein MyD88. Downstream of MyD88, the interleukin-1 receptor-associated kinase 4 (IRAK4) links TLR signaling to the NF-κB pathway. Because of the overactive TLR signaling in several autoimmune diseases, and as some B cell lymphomas carry activating mutations in MyD88, there is strong interest in developing inhibitors of this pathway. MyD88 itself is not a promising target, because it is difficult to develop inhibitors for adaptor proteins. Fortunately, IRAK4 is essential for signaling through MyD88, and numerous examples of efficient kinase inhibitors exist. In this issue, Kelly et al. now succeed in identifying two novel and very promising IRAK4 inhibitors and present for the first time exciting preclinical studies with these molecules.The two inhibitors, called ND-2158 and ND-2110, bind to the ATP pocket of IRAK4, thereby inhibiting its kinase activity. These two molecules have high affinities for IRAK4 and possess good pharmacological properties. Importantly, in a screen of 334 kinases, ND-2158 and ND-2110 proved to be highly specific for IRAK4. ND-2158 and ND-2110 were efficient in reducing disease severity in mouse models of pathological inflammatory responses. In studies with cell lines from diffuse large B cell lymphoma (DLBCL), specifically those lines of the activated B cell-like (ABC) subtype of DLBCL with a specific activating mutation in MyD88 (L265P), they showed inhibition of IRAK4 and NF-κB activity upon incubation with these inhibitors. Xenograft studies with DLBCL cell lines revealed the in vivo efficiency of ND-2158 in reducing tumor growth. As it is likely that in a potential future clinical application IRAK4 inhibition alone may not be curative, the authors studied combined application of the IRAK4 inhibitors with inhibitors of other pathogenic factors in ABC DLBCL. For example, coapplication of ibrutinib, an inhibitor of B cell receptor signaling, which is chronically active in ABC DLBCL, had synergistic effects on tumor growth in mice. The present study lays the ground for the development of clinical studies with the two IRAK4 inhibitors. It is impressive that this stage has already been reached, considering that MyD88 mutations were identified in ABC DLBCL only four years ago. As this type of DLBCL has the worst prognosis, and 30% of ABC DLBCL carries the L265P mutation, this new treatment approach may be valuable for a substantial fraction of DLBCL patients. Furthermore, patients affected by several other types of B cell lymphomas carrying this mutation, or patients with autoimmune diseases might also benefit from treatment with these inhibitors. Considering the recent clinical success with ibrutinib in the treatment of ABC D...

Hair follicle regeneration is controlled by an intricate relationship between epidermal stem cells and their microenvironment. A recent report in Nature by Rompolas et al. (2012) uses two-photon live imaging to interrogate the spatial organization and cellular requirements for hair follicle regeneration by epidermal stem cells and their immediate progeny.

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