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WDR62 is a spindle pole-associated scaffold protein with pleiotropic functions during corticogenesis. Recessive mutations in WDR62 are associated with structural brain abnormalities and account for the second most common cause of autosomal recessive primary microcephaly (MCPH), indicating WDR62 as a critical hub for human brain development. Here, we investigated a C-terminal truncating mutation (D955AfsX112) in WDR62 using induced pluripotent stem cells (iPSCs) obtained from a patient with MCPH2. We generated neuroepithelial stem (NES) cells and cerebro-cortical progenitors and neurons from patient-derived and isogenic retro-mutated iPSC lines. We found that WDR62 dysfunction resulted in impaired cell cycle progression and alterations of the neurogenic trajectories of iPSC neuroderivatives. Moreover, we report WDR62 localization at the Golgi apparatus during interphase, both in human neural progenitors in vitro and in human fetal brain tissue. WDR62 shuttling from the Golgi apparatus to spindle poles is dynamic and microtubule-dependent. Impairment of WDR62 function and localization results in severe neurodevelopmental abnormalities, thus delineating new mechanisms in MCPH etiology.

Recessive mutations in WD repeat domain 62 (WDR62) cause microcephaly and a wide spectrum of severe brain malformations. Disruption of the mouse ortholog results in microcephaly underlain by reduced proliferation of neocortical progenitors during late neurogenesis, abnormalities in asymmetric centrosome inheritance leading to neuronal migration delays, and altered neuronal differentiation. Spindle pole localization of WDR62 and mitotic progression are defective in patient-derived fibroblasts, which, similar to mouse neocortical progenitors, transiently arrest at prometaphase. Expression of WDR62 is closely correlated with components of the chromosome passenger complex (CPC), a key regulator of mitosis. Wild type WDR62, but not disease-associated mutant forms, interacts with the CPC core enzyme Aurora kinase B and staining of CPC components at centromeres is altered in patient-derived fibroblasts. Our findings demonstrate critical and diverse functions of WDR62 in neocortical development and provide insight into the mechanisms by which its disruption leads to a plethora of structural abnormalities.

Coordinated Hair Growth Wnt/β-catenin signaling is a key pathway that plays a conserved role in regulating stem cell function during adult tissue regeneration. Using time-lapse imaging of live mice, Deschene et al. (p. 1353 ) show that genetic activation of β-catenin within hair follicle stem cells generates axes of hair growth by coordinated cell divisions and cell movements, even when the normal niches—the dermal papillae—are laser-ablated. Activated β-catenin enhances Wnt ligand secretion, and these ligands can then activate Wnt signaling in adjacent cells that do not have activated β-catenin, indicating how activated stem cells could influence neighboring cells during normal growth and in cancer.

Ezrin is a member of the ezrin–radixin–moesin family of membrane-actin cytoskeleton cross-linkers that participate in a variety of cellular processes. In B cells, phosphorylation of ezrin at different sites regulates multiple processes, such as lipid raft coalescence, BCR diffusion, microclustering, and endosomal JNK activation. In this study, we generated mice with conditional deletion of ezrin in the B cell lineage to investigate the physiological significance of ezrin’s function in Ag receptor–mediated B cell activation and humoral immunity. B cell development, as well as the proportion and numbers of major B cell subsets in peripheral lymphoid organs, was unaffected by the loss of ezrin. Using superresolution imaging methods, we show that, in the absence of ezrin, BCRs respond to Ag binding by accumulating into larger and more stable signaling microclusters. Loss of ezrin led to delayed BCR capping and accelerated lipid raft coalescence. Although proximal signaling proteins showed stronger activation in the absence of ezrin, components of the distal BCR signaling pathways displayed distinct effects. Ezrin deficiency resulted in increased B cell proliferation and differentiation into Ab-secreting cells ex vivo and stronger T cell–independent and -dependent responses to Ag in vivo. Overall, our data demonstrate that ezrin regulates amplification of BCR signals and tunes the strength of B cell activation and humoral immunity.

Tissue development and regeneration depend on cell-cell interactions and signals that target stem cells and their immediate progeny. However, the cellular behaviours that lead to a properly regenerated tissue are not well understood. Using a new, non-invasive, intravital two-photon imaging approach we study physiological hair-follicle regeneration over time in live mice. By these means we have monitored the behaviour of epithelial stem cells and their progeny during physiological hair regeneration and addressed how the mesenchyme influences their behaviour. Consistent with earlier studies, stem cells are quiescent during the initial stages of hair regeneration, whereas the progeny are more actively dividing. Moreover, stem cell progeny divisions are spatially organized within follicles. In addition to cell divisions, coordinated cell movements of the progeny allow the rapid expansion of the hair follicle. Finally, we show the requirement of the mesenchyme for hair regeneration through targeted cell ablation and long-term tracking of live hair follicles. Thus, we have established an in vivo approach that has led to the direct observation of cellular mechanisms of growth regulation within the hair follicle and that has enabled us to precisely investigate functional requirements of hair-follicle components during the process of physiological regeneration. © 2012 Macmillan Publishers Limited. All rights reserved.

Individual cell types are defined by architecturally and functionally specialized cortical domains. The Ezrin, Radixin, and Moesin (ERM) proteins play a major role in organizing cortical domains by assembling membrane protein complexes and linking them to the cortical actin cytoskeleton. Many studies have focused on the individual roles of the ERM proteins in stabilizing the membrane–cytoskeleton interface, controlling the distribution and function of apical membrane complexes, regulating the small GTPase Rho, or establishing cell–cell junctions. We previously found that deletion of the mouse Ezrin gene yields severe defects in apical integrity throughout the developing intestinal epithelium, resulting in incomplete villus morphogenesis and neonatal death. However, the molecular function of Ezrin in building the apical surface of the intestinal epithelium was not clear. By deleting Ezrin in the adult mouse intestinal epithelium, we provide evidence that Ezrin performs multiple molecular functions that collaborate to build the functional apical surface of the intestinal epithelium in vivo. The loss of Ezrin-mediated apical integrity in the adult intestine yields severe morphological consequences during intestinal homeostasis, including defects in cell geometry, extrusion, junctional remodeling, and spindle orientation. Surprisingly, deletion of Ezrin either before or after villus morphogenesis yields villus fusion, revealing a previously unrecognized step in intestinal homeostasis. Our studies indicate that the function of Ezrin in building and maintaining the apical domain is essential not only for intestinal morphogenesis but also for homeostasis in the mature intestine.

The oncogenic tyrosine kinase Src has a role in cancer development, especially by promoting invasive and metastatic behavior. It is, however, unclear which of the Src-regulated signaling cascades induce malignant phenotype in three-dimensional environment. One of Src substrates is ezrin, a cytoskeletal organiser and regulator of signal transduction. Ezrin expression correlates with poor outcome of diverse cancers and is essential in experimental metastatic osteosarcoma. We reconstituted genetically ezrin-deficient cells with wild-type (WT) or phosphorylation-deficient Y477F ezrin together with constitutively active Src. In two-dimensional cultures, Src induced malignant features regardless of the presence or absence of ezrin. In contrast, only WT ezrin-expressing cells grew efficiently in soft agar or in suspension. In Matrigel, only WT ezrin significantly promoted growth and invasion, and was targeted to specific regions on the plasma membrane. WT and Y477F ezrin-expressing cells showed marked differences only when growing or scattering in three-dimensional matrix. Additional experiments showed that Y477-phosphorylated ezrin is also needed for the growth of Src-transformed epithelial cells in three-dimensional matrix. Cells lacking functional ezrin had reduced cyclin D levels and fewer cells in G2+S phase, possibly as a consequence of abnormal mTOR signaling, as ezrin Y477F cells showed lower expression of phosphorylated intermediates downstream of mTOR than WT cells. We conclude that the pathways activated by Src depend on the type of environment and that ezrin is a crucial element of Src-induced malignant features in cells growing inside three-dimensional environment.

The molecular signals that control the maintenance and activation of liver stem/progenitor cells are poorly understood, and the role of liver progenitor cells in hepatic tumorigenesis is unclear. We report here that liver-specific deletion of the neurofibromatosis type 2 (Nf2) tumor suppressor gene in the developing or adult mouse specifically yields a dramatic, progressive expansion of progenitor cells throughout the liver without affecting differentiated hepatocytes. All surviving mice eventually developed both cholangiocellular and hepatocellular carcinoma, suggesting that Nf2−/− progenitors can be a cell of origin for these tumors. Despite the suggested link between Nf2 and the Hpo/Wts/Yki signaling pathway in Drosophila, and recent studies linking the corresponding Mst/Lats/Yap pathway to mammalian liver tumorigenesis, our molecular studies suggest that Merlin is not a major regulator of YAP in liver progenitors, and that the overproliferation of Nf2−/− liver progenitors is instead driven by aberrant epidermal growth factor receptor (EGFR) activity. Indeed, pharmacologic inhibition of EGFR blocks the proliferation of Nf2−/− liver progenitors in vitro and in vivo, consistent with recent studies indicating that the Nf2-encoded protein Merlin can control the abundance and signaling of membrane receptors such as EGFR. Together, our findings uncover a critical role for Nf2/Merlin in controlling homeostasis of the liver stem cell niche.

The highly homologous proteins ezrin, radixin, and moesin link proteins to the actin cytoskeleton. The two family members expressed in T cells, ezrin and moesin, are implicated in promoting T cell activation and polarity. To elucidate the contributions of ezrin and moesin, we conducted a systematic analysis of their function during T cell activation. In response to TCR engagement, ezrin and moesin were phosphorylated in parallel at the regulatory threonine, and both proteins ultimately localized to the distal pole complex (DPC). However, ezrin exhibited unique behaviors, including tyrosine phosphorylation and transient localization to the immunological synapse before movement to the DPC. To ask whether these differences reflect unique requirements for ezrin vs moesin in T cell signaling, we generated mice with conditional deletion of ezrin in mature T cells. Ezrin−/− T cells exhibited normal immunological synapse organization based upon localization of protein kinase C-θ, talin, and phospho-ZAP70. DPC localization of CD43 and RhoGDP dissociation inhibitor, as well as the novel DPC protein Src homology region 2 domain-containing phosphatase-1, was also unaffected. However, recruitment of three novel DPC proteins, ezrin binding protein of 50 kDa, Csk binding protein, and the p85 subunit of PI3K was partially perturbed. Biochemical analysis of ezrin−/− T cells or T cells suppressed for moesin using small interfering RNA showed intact early TCR signaling, but diminished levels of IL-2. The defects in IL-2 production were more pronounced in T cells deficient for both ezrin and moesin. These cells also exhibited diminished phospholipase C-γ1 phosphorylation and calcium flux. We conclude that despite their unique movement and phosphorylation patterns, ezrin and moesin function together to promote T cell activation.

Objective We tested the ability of Notch pathway receptors Notch1 and Notch2 to regulate stem and epithelial cell homoeostasis in mouse and human gastric antral tissue. Design Mice were treated with the pan-Notch inhibitor dibenzazepine (DBZ) or inhibitory antibodies targeting Notch1 and/or Notch2. Epithelial proliferation, apoptosis and cellular differentiation were measured by histological and molecular approaches. Organoids were established from mouse and human antral glands; growth and differentiation were measured after treatment with Notch inhibitors. Results Notch1 and Notch2 are the predominant Notch receptors expressed in mouse and human antral tissue and organoid cultures. Combined inhibition of Notch1 and Notch2 in adult mice led to decreased epithelial cell proliferation, including reduced proliferation of LGR5 stem cells, and increased apoptosis, similar to the response to global Notch inhibition with DBZ. Less pronounced effects were observed after inhibition of individual receptors. Notch pathway inhibition with DBZ or combined inhibition of Notch1 and Notch2 led to increased differentiation of all gastric antral lineages, with remodelling of cells to express secretory products normally associated with other regions of the GI tract, including intestine. Analysis of mouse and human organoids showed that Notch signalling through Notch1 and Notch2 is intrinsic to the epithelium and required for organoid growth. Conclusions Notch signalling is required to maintain gastric antral stem cells. Notch1 and Notch2 are the primary Notch receptors regulating epithelial cell homoeostasis in mouse and human stomach.

Ezrin, Radixin, and Moesin (the ERM proteins) supply regulated linkage between membrane proteins and the actin cytoskeleton. The study of mammalian ERM proteins has been hampered by presumed functional overlap. We have found that Ezrin, the only ERM detected in epithelial cells of the developing intestine, provides an essential role in configuring the mouse intestinal epithelium. Surprisingly, Ezrin is not absolutely required for the formation of brush border microvilli or for the establishment or maintenance of epithelial polarity. Instead, Ezrin organizes the apical terminal web region, which is critical for the poorly understood process of de novo lumen formation and expansion during villus morphogenesis. Our data also suggest that Ezrin controls the localization and/or function of certain apical membrane proteins that support normal intestinal function. These in vivo studies highlight the critical function of Ezrin in the formation of a multicellular epithelium rather than an individual epithelial cell.

Mutation of theNeurofibromatosis 2(NF2) tumor suppressor gene leads to cancer development in humans and mice. Recent studies suggest thatNf2loss also contributes to tumor metastasis. TheNf2-encoded protein, merlin, is related to the ERM (ezrin,radixin, andmoesin) family of membrane:cytoskeleton-associated proteins. However, the cellular mechanism whereby merlin controls cell proliferation from this location is not known. Here we show that the major cellular consequence ofNf2deficiency in primary cells is an inability to undergo contact-dependent growth arrest and to form stable cadherin-containing cell:cell junctions. Merlin colocalizes and interacts with adherens junction (AJ) components in confluent wild-type cells, suggesting that the lack of AJs and contact-dependent growth arrest inNf2−/−cells directly results from the absence of merlin at sites of cell:cell contact. Our studies indicate that merlin functions as a tumor and metastasis suppressor by controlling cadherin-mediated cell:cell contact.

A fundamental goal in cancer biology is to identify the cells and signalling pathways that are keys to induce tumour regression. Here we use a spontaneously self-regressing tumour, cutaneous keratoacanthoma (KAs), to identify physiological mechanisms that drive tumour regression. By using a mouse model system that recapitulates the behaviour of human KAs, we show that self-regressing tumours shift their balance to a differentiation programme during regression. Furthermore, we demonstrate that developmental programs utilized for skin hair follicle regeneration, such as Wnt, are hijacked to sustain tumour growth and that the retinoic acid (RA) signalling pathway promotes tumour regression by inhibiting Wnt signalling. Finally, we find that RA signalling can induce regression of malignant tumours that do not normally spontaneously regress, such as squamous cell carcinomas. These findings provide new insights into the physiological mechanisms of tumour regression and suggest therapeutic strategies to induce tumour regression., Keratoacanthomas are skin tumours that spontaneously regress but the mechanisms leading to regression are unknown. Here, using a mouse chemical carcinogenesis model, the authors show that tumour regression is driven by activation of retinoic acid signalling that induces Wnt inhibition and tumour differentiation.

Ciliary neurotrophic factor (CNTF) promotes the survival of motor neurons, in vitro and in vivo. Moreover, CNTF can block the degeneration of injured or diseased motor neurons in young rodents. Motor neuron degeneration (mnd) mutant mice display adult onset symptoms reflecting progressive motor debilitation and provide a model in which to test the hypothesis that CNTF can prevent the loss of these motor functions. We generated mnd mice that harbor a genomically integrated transgene, resulting in overexpression of the encoded CNTF protein in these mice. In contrast to the beneficial effects of CNTF in preventing motor neuron degeneration in other experimental paradigms, we report that overproduction of CNTF increased the rate of onset of motor disease symptoms in mnd mice and the presence of the transgene correlated with low adult body weight in mnd and wild-type genetic backgrounds. © 1996 John Wiley & Sons, Inc.

OBJECTIVE: Our purpose was to determine whether intrauterine bacterial inoculation leads to preterm delivery in mice. STUDY DESIGN: Fifty-four female CD-1 mice at 75% of the length of the gestational period (14.5 days) received either an intrauterine bacterial inoculum of 2 to 10 × 103Escherichia coli (n = 33), an intraperitoneal bacterial inoculum (n = 7), or an intrauterine injection of a sterile solution (n = 14). RESULTS: Delivery within 48 hours of surgery occurred in 91% of mice after intrauterine bacteria, in 0% after intraperitoneal bacteria, and in 7% after sterile intrauterine injection (p < 0.001). Intrauterine bacterial inoculation produced systemic infection (i.e., recovery of organisms from culture of the heart) in 50% of animals post partum. Intraperitoneal bacteria and intrauterine saline solution injections resulted in systemic infection rates of 20% and 0%, respectively, 48 hours after surgery. Five of seven animals injected with bacteria into the uterus had histologic evidence of metritis, mild in all cases. Intrauterine bacterial inoculation resulted in induction of ribonucleic acid transcripts for tumor necrosis factor-α, interleukin-1α, interleukin-1β, and cyclooxygenase-2. CONCLUSIONS: Intrauterine inoculation with Escherichia coli in mice leads to preterm delivery and the local induction of factors known to be involved in human preterm labor with infection. The observation that intraperitoneal bacterial inoculation does not result in preterm delivery suggests that in this model labor is the product of a local (uterine) stimulus.

Background Ezrin/radixin/moesin (ERM) proteins are highly homologous proteins that function to link cargo molecules to the actin cytoskeleton. Ezrin and moesin are both expressed in mature lymphocytes, where they play overlapping roles in cell signaling and polarity, but their role in lymphoid development has not been explored. Methodology/Principal Findings We characterized ERM protein expression in lymphoid tissues and analyzed the requirement for ezrin expression in lymphoid development. In wildtype mice, we found that most cells in the spleen and thymus express both ezrin and moesin, but little radixin. ERM protein expression in the thymus was differentially regulated, such that ezrin expression was highest in immature thymocytes and diminished during T cell development. In contrast, moesin expression was low in early thymocytes and upregulated during T cell development. Mice bearing a germline deletion of ezrin exhibited profound defects in the size and cellularity of the spleen and thymus, abnormal thymic architecture, diminished hematopoiesis, and increased proportions of granulocytic precursors. Further analysis using fetal liver chimeras and thymic transplants showed that ezrin expression is dispensable in hematopoietic and stromal lineages, and that most of the defects in lymphoid development in ezrin−/− mice likely arise as a consequence of nutritional stress. Conclusions/Significance We conclude that despite high expression in lymphoid precursor cells, ezrin is dispensable for lymphoid development, most likely due to redundancy with moesin.

Ezrin, a member of the ezrin/moesin/radixin (ERM) family, localizes to microvilli of epithelia in vivo, where it functions as a bridge between actin filaments and plasma membrane proteins. In the eye, ezrin has been localized to both apical microvilli of Muller cells and retinal pigment epithelium (RPE) apical microvilli and basal infoldings. In the present study, we analyze these structures in the eyes of early postnatal ezrin knockout mice. This analysis indicates that the loss of ezrin leads to substantial reductions in the apical microvilli and basal infoldings in RPE cells and in the Muller cell apical microvilli. The absence of apical microvilli in the RPE is accompanied by the presence of microvilli-like inclusions (MIs) in the RPE cytoplasm. Finally, photoreceptors in the ezrin knockout animals show substantial retardation in development as compared to their wild type littermates.

Ezrin is a member of the Ezrin-Radixin-Moesin (ERM) family of membrane-actin cytoskeleton crosslinkers that participate in a variety of cellular processes. In B cells, phosphorylation of ezrin at different sites regulates multiple processes such as lipid raft coalescence, BCR diffusion, microclustering, and endosomal JNK activation. In this study, we generated mice with conditional deletion of ezrin in the B cell lineage to investigate the physiological significance of ezrin’s function in antigen receptor-mediated B cell activation and humoral immunity. B cell development, as well as the proportion and numbers of major B cell subsets in peripheral lymphoid organs were unaffected by the loss of ezrin. Using super resolution imaging methods we show that in the absence of ezrin, BCRs respond to antigen binding by accumulating into larger and more stable signaling microclusters. Loss of ezrin led to delayed BCR capping and accelerated lipid raft coalescence. Although proximal signaling proteins showed stronger activation in the absence of ezrin, components of the distal BCR signaling components displayed distinct effects. Ezrin deficiency resulted in increased B cell proliferation and differentiation into antibody-secreting cells ex vivo, and stronger T cell-independent and -dependent responses to antigen in vivo. Overall, our data demonstrate that ezrin regulates amplification of BCR signals and tunes the strength of B cell activation and humoral immunity.

Mutations in the neurofibromatosis type II ( NF2 ) tumor suppressor predispose humans and mice to tumor development. The study of Nf2 +/− mice has demonstrated an additional effect of Nf2 loss on tumor metastasis. The NF2 -encoded protein, merlin, belongs to the ERM (ezrin, radixin, and moesin) family of cytoskeleton:membrane linkers. However, the molecular basis for the tumor- and metastasis- suppressing activity of merlin is unknown. We have now placed merlin in a signaling pathway downstream of the small GTPase Rac. Expression of activated Rac induces phosphorylation and decreased association of merlin with the cytoskeleton. Furthermore, merlin overexpression inhibits Rac-induced signaling in a phosphorylation-dependent manner. Finally, Nf2 −/− cells exhibit characteristics of cells expressing activated alleles of Rac. These studies provide insight into the normal cellular function of merlin and how Nf2 mutation contributes to tumor initiation and progression.

A role for the membrane/cytoskeleton interface in the development and progression of cancer is established, yet poorly understood. The neurofibromatosis type II (NF2) tumor suppressor gene encodes a member of the ezrin/radixin/moesin (ERM) family of membrane/cytoskeleton linker proteins thought to be important for cell adhesion and motility. We report that in contrast to the narrow spectrum of benign tumors in human NF2 patients, Nf2 heterozygous mice develop a variety of malignant tumors. Using the fact that Nf2 is linked to the p53 tumor suppressor locus in the mouse we have also investigated the effects of genetic linkage of cancer-predisposing mutations on tumorigenesis and examined the genetic pathway to tumor formation involving Nf2 loss. Importantly, we observed a very high rate of metastasis associated with Nf2 deficiency, with or without loss of p53 function, and we provide experimental evidence supporting a role for Nf2 loss in metastatic potential. Together, our results suggest an important role for the NF2 tumor suppressor, and perhaps the ERM family in tumor formation and metastasis.

The neurofibromatosis type II (NF2) tumor suppressor encodes a putative cytoskeletal associated protein, the loss of which leads to the development of Schwann cell tumors associated with NF2 in humans. The NF2 protein merlin belongs to the band 4.1 family of proteins that link membrane proteins to the cytoskeleton and are thought to be involved in dynamic cytoskeletal reorganization. Beyond its membership in this family, however, the function of merlin remains poorly understood. In order to analyze the function of merlin during embryogenesis and to develop a system to study merlin function in detail, we have disrupted the mouse Nf2 gene by homologous recombination in embryonic stem cells. Most embryos homozygous for a mutation at the Nf2 locus fail between embryonic days 6.5 and 7.0, exhibiting a collapsed extraembryonic region and the absence of organized extraembryonic ectoderm. The embryo proper continues to develop, but fails to initiate gastrulation. These observations are supported by the expression patterns of markers of the extraembryonic lineage and the lack of expression of mesodermal markers in the mutant embryos. Mosaic studies demonstrate that merlin function is not required cell autonomously in mesoderm, and support the proposition that merlin function is essential for the development of extraembryonic structures during early mouse development.

Basic mechanisms of tissue development and regeneration are often hijacked during Cancer. A subset of skin squamous cell carcinoma is known to be able to spontaneously self-regress. These tumors are commonly referred as Keratoacanthoma and thought to arise from the skin hair follicle. Because skin hair follicles undergo continuous growth and regression, we hypothesized that the molecular mechanisms that regulate hair follicle regression drive Keratoacanthoma self-regression. In order to test this hypothesis we ask 1) what is the cellular origin of Keratoacanthoma, 2) and what signaling mechanisms are responsible for Keratoacanthoma self-regression. First, in order to understand the cellular origin of Keratoacanthoma, we utilize a genetic lineage tracing approach in combination with a DMBA-based chemical carcinogenesis protocol that recapitulates human Keratoacanthoma growth and self-regression in mice. Given that the location and identity of hair follicle stem cells has been identified, we utilize a spatially restricted Tamoxifen inducible CreER mouse line that specifically label hair follicle stem cells and their descendants (K19CreER;GFPr). By these means, we found that GFP labeled cells are observed within Keratoacanthoma tumors, thus suggesting that hair follicle stem cells contribute to the development of the self-regressing Keratoacanthoma tumors. Second, in order to identify the molecular mechanisms that drive Keratoacanthoma self-regression, we use a parallel approach of both human and mouse transcriptional analysis of Keratoacanthoma during either growth or regression. By focusing on signaling pathways that regulate hair follicle regression, we found that Wnt signaling is switched off during Keratoacanthoma self regression in both human and mouse Keratoacanthomas by immunofluorescence and real time PCR approaches. We are currently testing 1) whether Wnt is sufficient and/or required for Keratoacanthoma regression behavior and 2) what is upstream of Wnt down-regulation during the regression phase by genetic and sequencing approaches. The identification of the cells and signals that regulate Keratoacanthoma tumor growth and regression will provide key clues on the relevant cell populations and signaling that will likely be proper targets in other tumors beyond Keratoacanthomas, and allow for the design of treatment strategies that exploit stem cell regression pathways to switch off stem cell populations in cancerous tissues. Acknowledgements: New York Stem Cell Foundation (NYSCF) Postdoctoral Fellowship Grant (GZ) and ACS grant #121966-RSG-12-059-01-DDC (VG). Citation Format: Giovanni Zito, Ichiko Saotome, Valentina Greco. Stem cells and their signals during skin tumor regression. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5009. doi:10.1158/1538-7445.AM2013-5009