Cyp26 enzymes function in endoderm to regulate pancreatic field size

Abstract

The control of organ size and position relies, at least in part, upon appropriate regulation of the signals that specify organ progenitor fields. Pancreatic cell fates are specified by retinoic acid (RA), and proper size and localization of the pancreatic field are dependent on tight control of RA signaling. Here we show that the RAdegrading Cyp26 enzymes play a critical role in defining the normal anterior limit of the pancreatic field. Disruption of Cyp26 function causes a dramatic expansion of pancreatic cell types toward the anterior of the embryo. The cyp26a1 gene is expressed in the anterior trunk endoderm at developmental stages when RA is signaling to specify pancreas, and analysis of cyp26a1/giraffe (gir) mutant zebrafish embryos confirms that cyp26a1 plays the primary role in setting the anterior limit of the pancreas. Analysis of the gir mutants further reveals that cyp26b1 and cyp26c1 function redundantly to partially compensate for loss of Cyp26a1 function. We used cell transplantation to determine that Cyp26a1 functions directly in endoderm to modulate RA signaling and limit the pancreatic field. Taken together with our finding that endodermal expression of cyp26 genes is subject to positive regulation by RA, our data reveal a feedback loop within the endoderm. Such feedback can maintain consistent levels of RA signaling, despite environmental fluctuations in RA concentration, thus ensuring a consistent size and location of the pancreatic field.␤-cell ͉ insulin ͉ retinoic acid ͉ zebrafish

Vascular instruction of pancreas development

“…Conversely, blocking CXCR4 signaling with the inhibitor AMD3100 resulted in disrupted angioblast migration and reduced Pdx1 expression. Although it is suggested that angioblasts might induce PDX1 in a paracrine manner via retinoic acid, as studies in both zebrafish and chicks have proposed (Kumar et al, 2003;Kinkel et al, 2009), the exact nature of the vascular signal in question still remains elusive.…”

Retinoic Acid and the Development of the Endoderm

“…However, in chick explants, the effect of RA on promoting pancreas requires mesoderm suggesting that the RA is also acting on the mesoderm to drive pancreas differentiation [77]. Excess RA is able to drive an anterior expansion of the Pdx1 expression domain in zebrafish demonstrating that it is both necessary and sufficient for pancreas formation [124].…”

“…This provides a model where retinoic acid is necessary for formation of the pancreas and also creates a negative feedback loop that regulates pancreas size [124]. Size regulation by RA in the mouse pancreas, in particular the β-cell and α-cell mass, continues in the adult because vitamin A deficiency results in reduced mass that is subsequently restored when normal vitamin A levels are restored [127] …”

Retinoic acid signalling during development

“…RA may also act at later steps of specification of pancreatic endocrine cell lineages (Martin et al, 2005;Ostrom et al, 2008). Recent work indicates that RA may act more globally to coordinate the position of endoderm-derived organs along the foregut and midgut A-P axis (Bayha et al, 2009), and that CYP26 enzymes may restrict the extent of RA signalling and set up the limit of the pancreatic field (Kinkel et al, 2009). …”