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Douglas A Melton

Publications and source records attributed to Douglas A Melton.

At least 19 recordsLinked to original sources
PubMed2006

Notch signaling reveals developmental plasticity of Pax4(+) pancreatic endocrine progenitors and shunts them to a duct fate.

Relatively little is known about the developmental signals that specify the types and numbers of pancreatic cells. Previous studies suggested that Notch signaling in the pancreas inhibits differentiation and promotes the maintenance of progenitor cells, but it remains unclear whether Notch also controls cell fate choices as it does in other tissues. To study the impact of Notch in progenitors of the beta cell lineage, we generated mice that express Cre-recombinase under control of the Pax4 promoter. Lineage analysis of Pax4(+) cells demonstrates they are specified endocrine progenitors that contribute equally to four islet cell fates, contrary to expectations raised by the dispensable role of Pax4 in the specification of the alpha and PP subtypes. In addition, we show that activation of Notch in Pax4(+) progenitors inhibits their differentiation into alpha and beta endocrine cells and shunts them instead toward a duct fate. These observations reveal an unappreciated degree of developmental plasticity among early endocrine progenitors and raise the possibility that a bipotent duct-endocrine progenitor exists during development. Furthermore, the redirection of Pax4(+) cells from alpha and beta endocrine fates toward a duct cell type suggests a positive role for Notch signaling in duct specification and is consistent with the more widely defined role for Notch in cell fate determination.

Journal Article↗
PubMed2006

Control of developmental regulators by Polycomb in human embryonic stem cells.

Polycomb group proteins are essential for early development in metazoans, but their contributions to human development are not well understood. We have mapped the Polycomb Repressive Complex 2 (PRC2) subunit SUZ12 across the entire nonrepeat portion of the genome in human embryonic stem (ES) cells. We found that SUZ12 is distributed across large portions of over two hundred genes encoding key developmental regulators. These genes are occupied by nucleosomes trimethylated at histone H3K27, are transcriptionally repressed, and contain some of the most highly conserved noncoding elements in the genome. We found that PRC2 target genes are preferentially activated during ES cell differentiation and that the ES cell regulators OCT4, SOX2, and NANOG cooccupy a significant subset of these genes. These results indicate that PRC2 occupies a special set of developmental genes in ES cells that must be repressed to maintain pluripotency and that are poised for activation during ES cell differentiation.

Animals↗
PubMed2006

The vascular basement membrane: a niche for insulin gene expression and Beta cell proliferation.

Endocrine pancreatic beta cells require endothelial signals for their differentiation and function. However, the molecular basis for such signals remains unknown. Here, we show that beta cells, in contrast to the exocrine pancreatic cells, do not form a basement membrane. Instead, by using VEGF-A, they attract endothelial cells, which form capillaries with a vascular basement membrane next to the beta cells. We have identified laminins, among other vascular basement membrane proteins, as endothelial signals, which promote insulin gene expression and proliferation in beta cells. We further demonstrate that beta1-integrin is required for the beta cell response to the laminins. The proposed mechanism explains why beta cells must interact with endothelial cells, and it may apply to other cellular processes in which endothelial signals are required.

Animals↗
PubMed2005

Beta-catenin is essential for pancreatic acinar but not islet development.

Despite our increasingly sophisticated understanding of transcriptional regulation in pancreas development, we know relatively little about the extrinsic signaling pathways involved in this process. We show here that the early pancreatic epithelium exhibits a specific enrichment in unphosphorylated beta-catenin protein, a hallmark of activation of the canonical Wnt signaling pathway. To determine if this pathway is functionally required for normal pancreas development, we have specifically deleted the beta-catenin gene in these cells. Pancreata developing without beta-catenin are hypoplastic, although their early progenitors appear normal and exhibit no premature differentiation or death. Surprisingly, and in marked contrast to its role in the intestine, loss of beta-catenin does not significantly perturb islet endocrine cell mass or function. The major defect of the beta-catenin-deficient pancreas is an almost complete lack of acinar cells, which normally comprise the majority of the organ. beta-Catenin appears to be cell-autonomously required for the specification of acinar cells, rather than for their survival or maintenance, as deletion of beta-catenin specifically in differentiated acinar cells has no effect. Thus, our data are consistent with a crucial role for canonical Wnt signals in acinar lineage specification and differentiation.

Animals↗
PubMed2005

Core transcriptional regulatory circuitry in human embryonic stem cells.

The transcription factors OCT4, SOX2, and NANOG have essential roles in early development and are required for the propagation of undifferentiated embryonic stem (ES) cells in culture. To gain insights into transcriptional regulation of human ES cells, we have identified OCT4, SOX2, and NANOG target genes using genome-scale location analysis. We found, surprisingly, that OCT4, SOX2, and NANOG co-occupy a substantial portion of their target genes. These target genes frequently encode transcription factors, many of which are developmentally important homeodomain proteins. Our data also indicate that OCT4, SOX2, and NANOG collaborate to form regulatory circuitry consisting of autoregulatory and feedforward loops. These results provide new insights into the transcriptional regulation of stem cells and reveal how OCT4, SOX2, and NANOG contribute to pluripotency and self-renewal.

Animals↗
PubMed2005

Nuclear reprogramming of somatic cells after fusion with human embryonic stem cells.

We have explored the use of embryonic stem cells as an alternative to oocytes for reprogramming human somatic nuclei. Human embryonic stem (hES) cells were fused with human fibroblasts, resulting in hybrid cells that maintain a stable tetraploid DNA content and have morphology, growth rate, and antigen expression patterns characteristic of hES cells. Differentiation of hybrid cells in vitro and in vivo yielded cell types from each embryonic germ layer. Analysis of genome-wide transcriptional activity, reporter gene activation, allele-specific gene expression, and DNA methylation showed that the somatic genome was reprogrammed to an embryonic state. These results establish that hES cells can reprogram the transcriptional state of somatic nuclei and provide a system for investigating the underlying mechanisms.

Adult↗
PubMed2005

Direct regulation of intestinal fate by Notch.

The signals that maintain the proper balance between adult intestinal cell types are poorly understood. Loss-of-function studies have implicated the Notch pathway in the regulation of intestinal fate during development. However, it is unknown whether Notch has a role in maintaining the balance of different cell types in the adult intestine and whether it acts reversibly. To determine whether Notch has a direct effect on intestinal development and adult intestinal cell turnover, we have used a gain-of-function approach to activate Notch. Ectopic Notch signaling in adult intestinal progenitor cells leads to a bias against secretory fates, whereas ectopic Notch activation in the embryonic foregut results in reversible defects in villus morphogenesis and loss of the proliferative progenitor compartment. We conclude that Notch regulates adult intestinal development by controlling the balance between secretory and absorptive cell types. In the embryo, Notch activation perturbs morphogenesis, possibly through effects on stem or progenitor cells.

Animals↗
PubMed2005

Pten constrains centroacinar cell expansion and malignant transformation in the pancreas.

To determine the role of the phosphatidylinositol 3-kinase (PI3-K) pathway in pancreas development, we generated a pancreas-specific knockout of Pten, a negative regulator of PI3-K signaling. Knockout mice display progressive replacement of the acinar pancreas with highly proliferative ductal structures that contain abundant mucins and express Pdx1 and Hes1, two markers of pancreatic progenitor cells. Moreover, a fraction of these mice develop ductal malignancy. We provide evidence that ductal metaplasia results from the expansion of centroacinar cells rather than transdifferentiation of acinar cells. These results indicate that Pten actively maintains the balance between different cell types in the adult pancreas and that misregulation of the PI3-K pathway in centroacinar cells may contribute to the initiation of pancreatic carcinoma in vivo.

Animals↗
PubMed2004

How important are adult stem cells for tissue maintenance?

Most tissues undergo significant expansion during postnatal life, as well as a lifelong cellular turnover to compensate for cell loss. What is the identity of cells that give rise to newly differentiated cells? This fundamental yet understudied question is now attracting the attention of biologists and clinicians, triggered by reports that essentially every tissue can be replenished and repaired by multipotent adult stem cells. The identification of the "cell of origin" for a given tissue is essential for understanding its dynamics during adult life, and may have important therapeutic implications for both degenerative and neoplastic diseases. In this commentary, we briefly outline classic and current views on the question of the cell of origin. We also describe a general method that we have recently developed for addressing this issue, and its first application for the study of pancreatic beta cells.

Animals↗
PubMed2004

The Src family of tyrosine kinases is important for embryonic stem cell self-renewal.

cYes, a member of the Src family of non-receptor tyrosine kinases, is highly expressed in mouse and human embryonic stem (ES) cells. We demonstrate that cYes kinase activity is regulated by leukemia inhibitory factor (LIF) and serum and is down-regulated when cells differentiate. Moreover, selective chemical inhibition of Src family kinases decreases growth and expression of stem cell genes that mark the undifferentiated state, including Oct3/4, alkaline phosphatase, fibroblast growth factor 4, and Nanog. A synergistic effect on differentiation is observed when ES cells are cultured with an Src family inhibitor and low levels of retinoic acid. Src family kinase inhibition does not interfere with LIF-induced JAK/STAT3 (Janus-associated tyrosine kinases/signal transducer and activator of transcription 3) or p42/p44 MAPK (mitogen-activated protein kinase) phosphorylation. Together the results suggest that the activation of the Src family is important for maintaining mouse and human ES in an undifferentiated state and may represent a third, independent pathway, downstream of LIF in mouse ES cells.

Animals↗
PubMed2004

Adult pancreatic beta-cells are formed by self-duplication rather than stem-cell differentiation.

How tissues generate and maintain the correct number of cells is a fundamental problem in biology. In principle, tissue turnover can occur by the differentiation of stem cells, as is well documented for blood, skin and intestine, or by the duplication of existing differentiated cells. Recent work on adult stem cells has highlighted their potential contribution to organ maintenance and repair. However, the extent to which stem cells actually participate in these processes in vivo is not clear. Here we introduce a method for genetic lineage tracing to determine the contribution of stem cells to a tissue of interest. We focus on pancreatic beta-cells, whose postnatal origins remain controversial. Our analysis shows that pre-existing beta-cells, rather than pluripotent stem cells, are the major source of new beta-cells during adult life and after pancreatectomy in mice. These results suggest that terminally differentiated beta-cells retain a significant proliferative capacity in vivo and cast doubt on the idea that adult stem cells have a significant role in beta-cell replenishment.

Aging↗
PubMed2003

Global expression analysis of gene regulatory pathways during endocrine pancreatic development.

To define genetic pathways that regulate development of the endocrine pancreas, we generated transcriptional profiles of enriched cells isolated from four biologically significant stages of endocrine pancreas development: endoderm before pancreas specification, early pancreatic progenitor cells, endocrine progenitor cells and adult islets of Langerhans. These analyses implicate new signaling pathways in endocrine pancreas development, and identified sets of known and novel genes that are temporally regulated, as well as genes that spatially define developing endocrine cells from their neighbors. The differential expression of several genes from each time point was verified by RT-PCR and in situ hybridization. Moreover, we present preliminary functional evidence suggesting that one transcription factor encoding gene (Myt1), which was identified in our screen, is expressed in endocrine progenitors and may regulate alpha, beta and delta cell development. In addition to identifying new genes that regulate endocrine cell fate, this global gene expression analysis has uncovered informative biological trends that occur during endocrine differentiation.

Animals↗
PubMed2003

Notch signaling controls multiple steps of pancreatic differentiation.

Multiple cell types of the pancreas appear asynchronously during embryogenesis, which requires that pancreatic progenitor cell potential changes over time. Loss-of-function studies have shown that Notch signaling modulates the differentiation of these progenitors, but it remains unclear how and when the Notch pathway acts. We established a modular transgenic system to heritably activate mouse Notch1 in multiple types of progenitors and differentiated cells. We find that misexpression of activated Notch in Pdx1-expressing progenitor cells prevents differentiation of both exocrine and endocrine lineages. Progenitors remain trapped in an undifferentiated state even if Notch activation occurs long after the pancreas has been specified. Furthermore, endocrine differentiation is associated with escape from this activity, because Ngn3-expressing endocrine precursors are susceptible to Notch inhibition, whereas fully differentiated endocrine cells are resistant.

Animals↗
PubMed2003

Role of VEGF-A in vascularization of pancreatic islets.

Blood vessel endothelium has been recently shown to induce endocrine pancreatic development. Because pancreatic endocrine cells or islets express high levels of vascular endothelial growth factors, VEGFs, we investigated the role of a particular VEGF, VEGF-A, on islet vascularization and islet function. By deleting VEGF-A in the mouse pancreas, we show that endocrine cells signal back to the adjacent endothelial cells to induce the formation of a dense network of fenestrated capillaries in islets. Interestingly, VEGF-A is not required for the development of all islet capillaries. However, the few remaining capillaries found in the VEGF-A-deficient islets are not fenestrated and contain an unusual number of caveolae. In addition, glucose tolerance tests reveal that the VEGF-A-induced capillary network is not strictly required for blood glucose control but is essential for fine-tuning blood glucose regulation. In conclusion, we speculate that islet formation takes place in two sequential steps: in the first step, signals from blood vessel endothelium induce islet formation next to the vessels, and in the second step, the islets signal to the endothelium. The second step involves paracrine VEGF-A signaling to elaborate the interaction of islets with the circulatory system.

Animals↗