Pancreatic Ductal Morphogenesis and the Pdx1 Homeodomain Transcription Factor

Abstract

Embryonic development of the pancreas is marked by an early phase of dramatic morphogenesis, in which pluripotent progenitor cells of the developing pancreatic epithelium give rise to the full array of mature exocrine and endocrine cell types. The genetic determinants of acinar and islet cell lineages are somewhat well defined; however, the molecular mechanisms directing ductal formation and differentiation remain to be elucidated. The complex ductal architecture of the pancreas is established by a reiterative program of progenitor cell expansion and migration known as branching morphogenesis, or tubulogenesis, which proceeds in mouse development concomitantly with peak Pdx1 transcription factor expression. We therefore evaluated Pdx1 expression with respect to lineage-specific markers in embryonic sections of the pancreas spanning this critical period of duct formation and discovered an unexpected population of nonislet Pdx1-positive cells displaying physical traits of branching. We then established a 3D cell culture model of branching morphogenesis using primary pancreatic duct cells and identified a transient surge of Pdx1 expression exclusive to branching cells. From these observations we propose that Pdx1 might be involved temporally in a program of gene expression sufficient to facilitate the biochemical and morphological changes necessary for branching morphogenesis. INTRODUCTIONThe primary function of the exocrine pancreas is to produce and secrete digestive enzymes for export to the small intestine. The collection and transport of these enzymes are facilitated by an intricate ductal network of branched epithelial tubules, such that enzymes secreted into smaller peripheral ducts ultimately feed into the larger main pancreatic duct, which in turn flows into the duodenum. This complex structure is established during embryonic development by a coordinated mechanism of progenitor cell proliferation and migration known as branching morphogenesis (Jorgensen et al., 2007). Some of the genetic and biochemical events directing this process are shared among the many organs served by ductal networks-such as the lung, breast, and kidney-and are also somewhat conserved across species (Lu and Werb, 2008). In general, branching morphogenesis is initiated by mesenchymal signaling to epithelial cell growth factor receptors, inducing multiple responses within those epithelial cells to 1) undertake temporary cytoskeletal reorganization that will facilitate cell motility, 2) inhibit proliferation, and 3) suppress the original mesenchymal growth factor signal (Metzger and Krasnow, 1999;Affolter et al., 2003). The specific mesenchymal growth factors and epithelial growth factor receptors, their cognate signaling pathways, and transcription factors involved in such processes contributing to branching morphogenesis in the pancreas are not well understood and remain to be elucidated.Pancreatic organogenesis is dependent on the homeodomain transcription factor Pdx1, as demonstrated by pancreatic agenesis observed in Pdx1-null mic...

Pancreatic ductal cells in development, regeneration, and neoplasia

“…Gu et al showed that pancreatic ducts emerge from Pdx1-expressing cells that separate from the endocrine and exocrine lineages between E9.5 and E11.5 (41). It has been demonstrated that non-islet Pdx1-positive cells display physical traits of ductal branching starting at E14.5, and there is even some evidence of these cells - which are insulin negative and K19 positive - as early as E12.5 (42). In addition, a 3D cell culture model of branching morphogenesis, using primary pancreatic duct cells, identified a transient surge of Pdx1 expression exclusive to branching cells (42).…”

“…It has been demonstrated that non-islet Pdx1-positive cells display physical traits of ductal branching starting at E14.5, and there is even some evidence of these cells - which are insulin negative and K19 positive - as early as E12.5 (42). In addition, a 3D cell culture model of branching morphogenesis, using primary pancreatic duct cells, identified a transient surge of Pdx1 expression exclusive to branching cells (42). This suggests that Pdx1 might be involved temporally in a program of gene expression sufficient to facilitate the biochemical and morphological changes necessary for branching ductal morphogenesis, both during development and in the adult pancreas.…”

Three-dimensional culture model to distinguish normal from malignant human bronchial epithelial cells

“…Therefore, we have used a three-dimensional culture model of normal and cancer human bronchial epithelial cells (HBECs). Different studies have used a reconstituted Matrigel (BD Biosciences, Franklin Lakes, NJ, USA) derived from the Englebreth-Holm-Swarm tumour, composed of laminin, collagen IV and entactin, to study the morphology of human epithelial cell in three-dimensional culture from breast [11], intestine [12,13], pancreas [14,15] and metastatic tumour cells [16]. Lung epithelial cells from distal regions of rodent airways have also been shown to grow in Matrigel [17,18] and more recently in normal HBECs [19].…”

Pdx1 regulates pancreas tubulogenesis and E-cadherin expression

“…Pdx1 is expressed in the foregut endoderm at E8.5 (Villasenor et al, 2008) and in both dorsal and ventral pancreatic buds by E9.5. By late gestation, Pdx1 expression becomes restricted to endocrine cells and later exclusively to β-cells (Guo et al, 2013;Wescott et al, 2009). Although Pdx1 is required for the expression of insulin, developmental targets are only now being identified (Khoo et al, 2012;Oliver-Krasinski et al, 2009;Raum et al, 2015).…”