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Lori Sussel

Publications and source records attributed to Lori Sussel.

8 recordsLinked to original sources
PubMed2007

Nkx2.2-repressor activity is sufficient to specify alpha-cells and a small number of beta-cells in the pancreatic islet.

The homeodomain protein Nkx2.2 (Nkx2-2) is a key regulator of pancreatic islet cell specification in mice; Nkx2.2 is essential for the differentiation of all insulin-producing beta-cells and of the majority of glucagon-producing alpha-cells, and, in its absence, these cell types are converted to a ghrelin cell fate. To understand the molecular functions of Nkx2.2 that regulate these early cell-fate decisions during pancreatic islet development, we created Nkx2.2-dominant-derivative transgenic mice. In the absence of endogenous Nkx2.2, the Nkx2.2-Engrailed-repressor derivative is sufficient to fully rescue glucagon-producing alpha-cells and to partially rescue insulin-producing beta-cells. Interestingly, the insulin-positive cells that do form in the rescued mice do not express the mature beta-cell markers MafA or Glut2 (Slc2a2), suggesting that additional activator functions of Nkx2.2 are required for beta-cell maturation. To explore the mechanism by which Nkx2.2 functions as a repressor in the islet, we assessed the pancreatic expression of the Groucho co-repressors, Grg1, Grg2, Grg3 and Grg4 (Tle1-Tle4), which have been shown to interact with and modulate Nkx2.2 function. We determined that Grg3 is highly expressed in the embryonic pancreas in a pattern similar to Nkx2.2. Furthermore, we show that Grg3 physically interacts with Nkx2.2 through its TN domain. These studies suggest that Nkx2.2 functions predominantly as a transcriptional repressor during specification of endocrine cell types in the pancreas.

Animals↗
PubMed2006

Gata6 is an important regulator of mouse pancreas development.

Gata4, Gata5, and Gata6 represent a subfamily of zinc-finger transcriptional regulators that are important in the development and differentiation of numerous tissues, including many endodermally-derived organs. We demonstrate that Gata4 and Gata6 have overlapping expression patterns in the early pancreatic epithelium. Later, Gata4 becomes restricted to exocrine tissue and Gata6 becomes restricted to a subset of endocrine cells. In addition, we show Gata6, but not Gata4, physically interacts with Nkx2.2, an essential islet transcription factor. To begin determining the roles that Gata4 and Gata6 play during pancreatic development, we expressed Gata4-Engrailed and Gata6-Engrailed dominant repressor fusion proteins in the pancreatic epithelium and in the islet. At e17.5, transgenic Gata6-Engrailed embryos exhibit two distinct phenotypes: a complete absence of pancreas or a reduction in pancreatic tissue. In the embryos that do form pancreas, there is a significant reduction of all pancreatic cell types, with the few differentiated endocrine cells clustered within, or in close proximity to, enlarged ductal structures. Conversely, the majority of transgenic Gata4-Engrailed embryos do not have a pancreatic phenotype. This study suggests that Gata6 is an important regulator of pancreas specification.

Animals↗
PubMed2006

FoxA2, Nkx2.2, and PDX-1 regulate islet beta-cell-specific mafA expression through conserved sequences located between base pairs -8118 and -7750 upstream from the transcription start site.

The MafA transcription factor is both critical to islet beta-cell function and has a unique pancreatic cell-type-specific expression pattern. To localize the potential transcriptional regulatory region(s) involved in directing expression to the beta cell, areas of identity within the 5' flanking region of the mouse, human, and rat mafA genes were found between nucleotides -9389 and -9194, -8426 and -8293, -8118 and -7750, -6622 and -6441, -6217 and -6031, and -250 and +56 relative to the transcription start site. The identity between species was greater than 75%, with the highest found between bp -8118 and -7750 ( approximately 94%, termed region 3). Region 3 was the only upstream mammalian conserved region found in chicken mafA (88% identity). In addition, region 3 uniquely displayed beta-cell-specific activity in cell-line-based reporter assays. Important regulators of beta-cell formation and function, PDX-1, FoxA2, and Nkx2.2, were shown to specifically bind to region 3 in vivo using the chromatin immunoprecipitation assay. Mutational and functional analyses demonstrated that FoxA2 (bp -7943 to -7910), Nkx2.2 (bp -7771 to -7746), and PDX-1 (bp -8087 to -8063) mediated region 3 activation. Consistent with a role in transcription, small interfering RNA-mediated knockdown of PDX-1 led to decreased mafA mRNA production in INS-1-derived beta-cell lines (832/13 and 832/3), while MafA expression was undetected in the pancreatic epithelium of Nkx2.2 null animals. These results suggest that beta-cell-type-specific mafA transcription is principally controlled by region 3-acting transcription factors that are essential in the formation of functional beta cells.

Animals↗
PubMed2006

A Wnt1-regulated genetic network controls the identity and fate of midbrain-dopaminergic progenitors in vivo.

Midbrain neurons synthesizing the neurotransmitter dopamine play a central role in the modulation of different brain functions and are associated with major neurological and psychiatric disorders. Despite the importance of these cells, the molecular mechanisms controlling their development are still poorly understood. The secreted glycoprotein Wnt1 is expressed in close vicinity to developing midbrain dopaminergic neurons. Here, we show that Wnt1 regulates the genetic network, including Otx2 and Nkx2-2, that is required for the establishment of the midbrain dopaminergic progenitor domain during embryonic development. In addition, Wnt1 is required for the terminal differentiation of midbrain dopaminergic neurons at later stages of embryogenesis. These results identify Wnt1 as a key molecule in the development of midbrain dopaminergic neurons in vivo. They also suggest the Wnt1-controlled signaling pathway as a promising target for new therapeutic strategies in the treatment of Parkinson's disease.

Animals↗
PubMed2004

Ghrelin cells replace insulin-producing beta cells in two mouse models of pancreas development.

The pancreatic islet is necessary for maintaining glucose homeostasis. Within the pancreatic islet, the homeodomain protein Nkx2.2 is essential for the differentiation of all insulin-producing beta cells and a subset of glucagon-producing alpha cells (1). Mice lacking Nkx2.2 have relatively normal sized islets, but a large number of cells within the mutant islet fail to produce any of the four major islet hormones. In this study we demonstrate that Nkx2.2 mutant endocrine cells have been replaced by cells that produce ghrelin, an appetite-promoting peptide predominantly found in the stomach. Intriguingly, normal mouse pancreas also contains a small population of ghrelin-producing cells, defining a new islet "epsilon" cell population. The expansion of ghrelin-producing cells at the expense of beta cells may be a general phenomenon, because we demonstrate that Pax4 mutant mice display a similar phenotype. We propose that insulin and ghrelin cells share a common progenitor and that Nkx2.2 and Pax4 are required to specify or maintain differentiation of the beta cell fate. This finding also suggests that there is a genetic component underlying the balance between insulin and ghrelin in regulating glucose metabolism.

Amylases↗
PubMed2004

The concerted activities of Pax4 and Nkx2.2 are essential to initiate pancreatic beta-cell differentiation.

Pancreatic beta cells play a central role in maintaining glucose homeostasis because they secrete insulin in response to increased level of blood glucose; failure of this capacity constitutes a major component of the pathogenesis of diabetes. The identification of key regulators of pancreatic beta-cell differentiation is relevant for the overall understanding of this process and for future experiments aimed at regenerating insulin-producing beta cells from pancreatic or embryonic stem cells. Several studies using transgenic or knockout mice have established that the development and function of pancreatic beta cells are controlled by several genes encoding specific transcription factors. By inactivating the homeobox gene Pax4, we previously demonstrated that its function is required for the formation of mature insulin-producing cells. Here, we show that during pancreas ontogeny, Pax4 is expressed in differentiating endocrine cells, including beta cells. Pax4 activity appears essential for appropriate initiation of beta-cell differentiation because loss of Pax4 prevents the expression of Pdx1, HB9 and insulin in beta-cell precursors. This role of Pax4 appears to be accomplished via its genetic interaction with another homeobox gene, Nkx2.2.

Animals↗
PubMed2002

Transcription factor occupancy of the insulin gene in vivo. Evidence for direct regulation by Nkx2.2.

Consensus-binding sites for many transcription factors are relatively non-selective and found at high frequency within the genome. This raises the possibility that factors that are capable of binding to a cis-acting element in vitro and regulating transcription from a transiently transfected plasmid, which would not have higher order chromatin structure, may not occupy this site within the endogenous gene. Closed chromatin structure and competition from another DNA-binding protein with similar nucleotide specificity are two possible mechanisms by which a transcription factor may be excluded from a potential binding site in vivo. Multiple transcription factors, including Pdx-1, BETA-2, and Pax6, have been implicated in expression of the insulin gene in pancreatic beta cells. In this study, the chromatin immunoprecipitation assay has been used to show that these factors do, in fact, bind to insulin control region sequences in intact beta cells. In addition, another key islet-enriched transcription factor, Nkx2.2, was found to occupy this region using the chromatin immunoprecipitation assay. In vitro DNA-binding and transient transfection assays defined how Nkx2.2 affected insulin gene expression. Pdx-1 was also shown to bind within a region of the endogenous islet amyloid polypeptide, pax-4, and glucokinase genes that were associated with control in vitro. Because Pdx-1 does not regulate gene transcription in isolation, these sequences were examined for occupancy by the other insulin transcriptional regulators. BETA-2, Pax6, and Nkx2.2 were also found to bind to amyloid polypeptide, glucokinase, and pax-4 control sequences in vivo. These studies reveal the broad application of the Pdx-1, BETA-2, Pax6, and Nkx2.2 transcription factors in regulating expression of genes selectively expressed in islet beta cells.

Amyloid↗