Dorsal pancreas agenesis in retinoic acid-deficient Raldh2 mutant mice

Abstract

During embryogenesis, the pancreas arises from dorsal and ventral pancreatic protrusions from the primitive gut endoderm upon induction by different stimuli from neighboring mesodermal tissues. Recent studies have shown that Retinoic Acid (RA) signaling is essential for the development of the pancreas in non-mammalian vertebrates. To investigate whether RA regulates mouse pancreas development, we have studied the phenotype of mice with a targeted deletion in the retinaldehyde dehydrogenase 2 (Raldh2) gene, encoding the enzyme required to synthesize RA in the embryo. We show that Raldh2 is expressed in the dorsal pancreatic mesenchyme at the early stage of pancreas specification. RA-responding cells have been detected in pancreatic endodermal and mesenchymal cells. Raldh2-deficient mice do not develop a dorsal pancreatic bud. Mutant embryos lack Pdx 1 expression, an essential regulator of early pancreas development, in the dorsal but not the ventral endoderm. In contrast to Pdx 1-deficient mice, the early glucagon-expressing cells do not develop in Raldh2 knockout embryos. Shh expression is, as in the wild-type embryo, excluded from the dorsal endodermal region at the site where the dorsal bud is expected to form, indicating that the dorsal bud defect is not related to a mis-expression of Shh. Mesenchymal expression of the LIM homeodomain protein Isl 1, required for the formation of the dorsal mesenchyme, is altered in Raldh2--/-- embryos. The homeobox gene Hlxb9, which is essential for the initiation of the pancreatic program in the dorsal foregut endoderm, is still expressed in Raldh2--/-- dorsal epithelium but the number of HB9-expressing cells is severely reduced. Maternal supplementation of RA rescues early dorsal pancreas development and restores endodermal Pdx 1 and mesenchymal Isl 1 expression as well as endocrine cell differentiation. These findings suggest that RA signaling is important for the proper differentiation of the dorsal mesenchyme and development of the dorsal endoderm. We conclude that RA synthesized in the mesenchyme is specifically required for the normal development of the dorsal pancreatic endoderm at a stage preceding Pdx 1 function.

In vitro derivation of functional insulin-producing cells from human embryonic stem cells

“…It has been shown that mouse and human ES cells can be efficiently induced into Sox17-positive definitive endoderm by activin A treatment [18][19][20]. RA has been reported to be critical for early pancreas formation during zebrafish and mouse embryonic development [21][22][23][24]. In our study, as shown in Figure 3, when induced by activin A alone, human ES cells weakly express the pancreatic early transcription factors pdx1 and hnf4a.…”

“…The individual effect of activin A and RA on endoderm and pancreas development has also been demonstrated by several other groups [18][19][20][21][22][23][24]. It has been shown that mouse and human ES cells can be efficiently induced into Sox17-positive definitive endoderm by activin A treatment [18][19][20].…”

Isolation and characterization of centroacinar/terminal ductal progenitor cells in adult mouse pancreas

“…Our studies therefore do not address whether this enzymatic activity, especially as it relates to synthesis of retinoic acid, plays an important role in the function of these progenitors, or whether they serve as local sources of retinoic acid production in a manner that organizes surrounding cells. Retinoids have previously been shown to exert profound influences on vertebrate pancreas development (29)(30)(31), and retinoic acid is a critical component for directed differentiation of human ES cells into insulin-producing β-cells (32). In the hematopoietic system, in vitro inhibition of ALDH1 enzymatic activity has been reported to somewhat paradoxically lead to the expansion of undifferentiated hematopoietic progenitors (33), and studies using gene-targeted mice have suggested that Aldh1a1-specific enzymatic activity is dispensable for hematopoietic and neural progenitor cell activity (34).…”

Pancreas organogenesis: From bud to plexus to gland

“…The level of conservation in determining pancreatic boundary establishment by Cdx4 and Cyp26a1 across vertebrate species has not been determined. In RALDH2 null mice, which lack a critical RA-synthesizing enzyme, the dorsal pancreas fails to bud and the early glucagon þ cells do not develop (Molotkov et al, 2005;Martin et al, 2005). This dorsal pancreas agenesis phenotype in RALDH2 null mice is reminiscent of the pancreas phenotype observed in Islet1 (Isl1) or N-cadherin (Esni et al, 2001) knockout mice.…”