A p53-Dependent Response Limits Epidermal Stem Cell Functionality and Organismal Size in Mice with Short Telomeres

Telomere maintenance is essential to ensure proper size and function of organs with a high turnover. In particular, a dwarf phenotype as well as phenotypes associated to premature loss of tissue regeneration, including the skin (hair loss, hair graying, decreased wound healing), are found in mice deficient for telomerase, the enzyme responsible for maintaining telomere length. Coincidental with the appearance of these phenotypes, p53 is found activated in several tissues from these mice, where is thought to trigger cellular senescence and/or apoptotic responses. Here, we show that p53 abrogation rescues both the small size phenotype and restitutes the functionality of epidermal stem cells (ESC) of telomerase-deficient mice with dysfunctional telomeres. In particular, p53 ablation restores hair growth, skin renewal and wound healing responses upon mitogenic induction, as well as rescues ESCmobilization defects in vivo and defective ESC clonogenic activity in vitro. This recovery of ESC functions is accompanied by a downregulation of senescence markers and an increased proliferation in the skin and kidney of telomerase-deficient mice with critically short telomeres without changes in apoptosis rates. Together, these findings indicate the existence of a p53-dependent senescence response acting on stem/progenitor cells with dysfunctional telomeres that is actively limiting their contribution to tissue regeneration, thereby impinging on tissue fitness

In vivo epigenetic reprogramming of primary human colon cancer cells enhances metastases

How metastases develop is not well understood and no genetic mutations have been reported as specific metastatic drivers. Here we have addressed the idea that epigenetic reprogramming by GLI-regulated pluripotent stemness factors promotes metastases. Using primary human colon cancer cells engrafted in mice, we find that transient expression of OCT4, SOX2, KLF4 +/− cMYC establishes an enhanced pro-metastatic state in the primary tumor that is stable through sequential engraftments and is transmitted through clonogenic cancer stem cells. Metastatic reprogramming alters NANOG methylation and stably boosts NANOG and NANOGP8 expression. Metastases and reprogrammed EMT-like phenotypes require endogenous NANOG, but enhanced NANOG is not sufficient to induce these phenotypes. Finally, reprogrammed tumors enhance GLI2, and we show that GLI2high and AXIN2low, which are markers of the metastatic transition of colon cancers, are prognostic of poor disease outcome in patients. We propose that metastases arise through epigenetic reprogramming of cancer stem cells within primary tumors

On flux-limited morphogenesis. Reply to the comments on >Morphogenetic action through flux-limited spreading>.

Spanish MINECO; Marie Curie FP6 (RTN035528-2); FP7(ITN238186); Fundación Areces; Juntad e Andalucía (Project P08-FQM-4267); Swiss National Science Foundation; the University of Geneva; European Research Council; Leenaards foundation; European Molecular Biology Organization (younginvestigatorprogram); Cantonet Republique de GenèvetoPeer Reviewe

Morphogenetic action through flux-limited spreading

A central question in biology is how secreted morphogens act to induce different cellular responses within a group of cells in a concentration-dependent manner. Modeling morphogenetic output in multicellular systems has so far employed linear diffusion, which is the normal type of diffusion associated with Brownian processes. However, there is evidence that at least some morphogens, such as Hedgehog (Hh) molecules, may not freely diffuse. Moreover, the mathematical analysis of such models necessarily implies unrealistic instantaneous spreading of morphogen molecules, which are derived from the assumptions of Brownian motion in its continuous formulation. A strict mathematical model considering Fick's diffusion law predicts morphogen exposure of the whole tissue at the same time. Such a strict model thus does not describe true biological patterns, even if similar and attractive patterns appear as results of applying such simple model. To eliminate non-biological behaviors from diffusion models we introduce flux-limited spreading (FLS), which implies a restricted velocity for morphogen propagation and a nonlinear mechanism of transport. Using FLS and focusing on intercellular Hh-Gli signaling, we model a morphogen gradient and highlight the propagation velocity of morphogen particles as a new key biological parameter. This model is then applied to the formation and action of the Sonic Hh (Shh) gradient in the vertebrate embryonic neural tube using our experimental data on Hh spreading in heterologous systems together with published data. Unlike linear diffusion models, FLS modeling predicts concentration fronts and the evolution of gradient dynamics and responses over time. In addition to spreading restrictions by extracellular binding partners, we suggest that the constraints imposed by direct bridges of information transfer such as nanotubes or cytonemes underlie FLS. Indeed, we detect and measure morphogen particle velocity in such cell extensions in different systems

Morphogenetic action through flux-limited spreading

A central question in biology is how secreted morphogens act to induce different cellular responses within a group of cells in a concentration-dependent manner. Modeling morphogenetic output in multicellular systems has so far employed linear diffusion, which is the normal type of diffusion associated with Brownian processes. However, there is evidence that at least some morphogens, such as Hedgehog (Hh) molecules, may not freely diffuse. Moreover, the mathematical analysis of such models necessarily implies unrealistic instantaneous spreading of morphogen molecules, which are derived from the assumptions of Brownian motion in its continuous formulation. A strict mathematical model considering Fick's diffusion law predicts morphogen exposure of the whole tissue at the same time. Such a strict model thus does not describe true biological patterns, even if similar and attractive patterns appear as results of applying such simple model. To eliminate non-biological behaviors from diffusion models we introduce flux-limited spreading (FLS), which implies a restricted velocity for morphogen propagation and a nonlinear mechanism of transport. Using FLS and focusing on intercellular Hh-Gli signaling, we model a morphogen gradient and highlight the propagation velocity of morphogen particles as a new key biological parameter. This model is then applied to the formation and action of the Sonic Hh (Shh) gradient in the vertebrate embryonic neural tube using our experimental data on Hh spreading in heterologous systems together with published data. Unlike linear diffusion models, FLS modeling predicts concentration fronts and the evolution of gradient dynamics and responses over time. In addition to spreading restrictions by extracellular binding partners, we suggest that the constraints imposed by direct bridges of information transfer such as nanotubes or cytonemes underlie FLS. Indeed, we detect and measure morphogen particle velocity in such cell extensions in different systems. © 2013 Elsevier B.V.Projects MTM2008-05271, MTM2011-23384; BFU2008-03320/BMC,BFU2011-25987; Consolider Program CSD2007-00008 MICINN; MarieCurie FP6 (RTN035528-2); FP7 (ITN238186) Fundación Areces; Junta de Andalucía ProjectP08-FQM-4267; Swiss National Science Foundation (SNFprofessorgrantnum-berPP0033-119169andSinergiagrantCRSII3-127456); University of Geneva; the European Research Council (contractnumberERC-2009-StG-243344-NEUROCHEMS); Leenaards foundationand; the European Molecular Biology Organization (younginvestigatorprogram)Peer Reviewe

Loss of WNT-TCF addiction and enhancement of HH-GLI1 signalling define the metastatic transition of human colon carcinomas

Previous studies demonstrate the initiation of colon cancers through deregulation of WNT-TCF signalling. An accepted but untested extension of this finding is that incurable metastatic colon carcinomas (CCs) universally remain WNT-TCF-dependent, prompting the search for WNT-TCF inhibitors. CCs and their stem cells also require Hedgehog (HH)-GLI1 activity, but how these pathways interact is unclear. Here we define coincident high-to-low WNT-TCF and low-to-high HH-GLI transitions in patient CCs, most strikingly in their CD133(+) stem cells, that mark the development of metastases. We find that enhanced HH-GLI mimics this transition, driving also an embryonic stem (ES)-like stemness signature and that GLI1 can be regulated by multiple CC oncogenes. The data support a model in which the metastatic transition involves the acquisition or enhancement of a more primitive ES-like phenotype, and the downregulation of the early WNT-TCF programme, driven by oncogene-regulated high GLI1 activity. Consistently, TCF blockade does not generally inhibit tumour growth; instead, it, like enhanced HH-GLI, promotes metastatic growth in vivo. Treatments for metastatic disease should therefore block HH-GLI1 but not WNT-TCF activities