Neural Precursor Death Is Central to the Pathogenesis of Intestinal Aganglionosis inRetHypomorphic Mice

Abstract

The RET tyrosine kinase is required for the migration, proliferation, and survival of the enteric neural crest-derived cells (ENCCs) that form the enteric nervous system (ENS). Hypomorphic RET alleles cause intestinal aganglionosis [Hirschsprung disease (HSCR)], in which delayed migration and successive nonapoptotic ENCC death are considered to be major contributory factors. The significance of ENCC death in intestinal aganglionosis, however, has remained unclear. We show that elevated expression of Bcl-xL inhibits ENCC death in both Ret-null and hypomorphic states. However, the rescued Ret-null mice showed ENS malfunction with reduced nitric oxide synthase expression in colonic neurons, revealing the requirement of RET for neuronal differentiation. In contrast, the inhibition of cell death allows morphologically and functionally normal ENS formation in Ret hypomorphic mice. These results indicate that ENCC death is a principal cause of intestinal aganglionosis in a Ret hypomorphic state, and suggest that the inhibition of cell death is a route to the prevention of HSCR.

Neural crest requires Impdh 2 for development of the enteric nervous system, great vessels, and craniofacial skeleton

“…Since we did not detect increased ENCDC apoptosis in mutant embryos at E13.5, this apparent regression might be explained by the dramatic lengthening of the midgut (post-ampullary small bowel) relative to the foregut (pre-ampullary small bowel) between E13.5 and E18.5. It is also possible that either apoptotic or non-apoptotic, non-necrotic cell death (Uesaka and Enomoto, 2010) occurs in ENCDCs at any time between E13.5 and E18.5 and was not detected due to event rarity or the time chosen for sampling. Alternatively, the nascent ENCDCs in the most distal region of colonized bowel could preferentially differentiate into glia.…”

Enteric neural progenitors are more efficient than brain-derived progenitors at generating neurons in the colon

“…It has been previously shown that both cell death and proliferation play important roles in the normal development of the ENS (20,56,68,69). Proliferation is essential for the migration of ENS progenitors (63), and some of the genes associated with Hirschsprung disease play a role in proliferation (46).…”

Building a second brain in the bowel

“…17 and Figure 2A). This is important because RET supports ENCDC survival, proliferation, and migration (2,(27)(28)(29)(30)(31)(32)(33), and homozygous Ret inactivation prevents ENCDCs from colonizing bowel distal to the stomach (34,35). Exogenous RA can substitute for an otherwise essential paraxial mesoderm interaction in quail to induce RET (17).…”

“…For example, removal of vagal neural tube segments where ENCDCs originate causes distal intestinal aganglionosis (50,51). RET mutations that reduce ENCDC proliferation or enhance cell death also cause bowel aganglionosis (27,28), as does treatment with mycophenolic acid, an inhibitor of guanine nucleotide synthesis that reduces ENCDC proliferation but does not prevent ENCDCs from migrating (20). Finally, mutations in endothelin receptor B (EDNRB) or its ligand endothelin-3 (EDN3) cause premature differentiation and cell cycle exit (52)(53)(54), leading to HSCR in humans and distal bowel aganglionosis in mice (55,56).…”

“…Birth-dating studies show that progenitors for neuronal subtypes exit the cell cycle at different times throughout development (89)(90)(91). The observation that factors influencing ENCDC proliferation (e.g., GDNF) or differentiation (BMPs) may alter the ratio of enteric neuron subtypes within the bowel in a way that depends on neuronal birth date suggests a genetically programmed lineage commitment tied to the timing of cell cycle exit (28,32,92,93 …”