Differential RET Signaling Pathways Drive Development of the Enteric Lymphoid and Nervous Systems

Cis and trans signaling mechanisms direct different developmental responses to ligands for the receptor tyrosine kinase RET. RET Signaling in Cis and Trans Development of the enteric (gastrointestinal) organs requires coordinated growth of tissues from various embryonic layers. Evidence suggests that ligands of the receptor tyrosine kinase RET are used in different tissues to control distinct developmental end points. Lymphoid tissue initiator (LTin) cells are thought to function in the early development of Peyer’s patches (PPs), which are secondary lymphoid organs of the gut important for mucosal immunity. The formation of the enteric nervous system, which enervates the lymphoid tissue, depends on interactions between neural crest cells and stroma cells of the gut wall. RET signaling requires the presence of co-receptors, which bind to ligands, in the same cell (in cis), or RET co-receptors can be cleaved from cells, leading to the possibility of RET signaling in trans; however, the physiological relevance of such signaling is uncertain. Patel et al. investigated lymphoid tissue morphogenesis in mice and found that whereas development of the enteric nervous tissue depended on RET signaling in cis, aggregation of LTin cells and development of lymphoid tissue were driven by RET signaling in trans and depended on the local availability of RET co-receptors and ligands. During the early development of the gastrointestinal tract, signaling through the receptor tyrosine kinase RET is required for initiation of lymphoid organ (Peyer’s patch) formation and for intestinal innervation by enteric neurons. RET signaling occurs through glial cell line–derived neurotrophic factor (GDNF) family receptor α co-receptors present in the same cell (signaling in cis). It is unclear whether RET signaling in trans, which occurs in vitro through co-receptors from other cells, has a biological role. We showed that the initial aggregation of hematopoietic cells to form lymphoid clusters occurred in a RET-dependent, chemokine-independent manner through adhesion-mediated arrest of lymphoid tissue initiator (LTin) cells. Lymphoid tissue inducer cells were not necessary for this initiation phase. LTin cells responded to all RET ligands in trans, requiring factors from other cells, whereas RET was activated in enteric neurons exclusively by GDNF in cis. Furthermore, genetic and molecular approaches revealed that the versatile RET responses in LTin cells were determined by distinct patterns of expression of the genes encoding RET and its co-receptors. Our study shows that a trans RET response in LTin cells determines the initial phase of enteric lymphoid organ morphogenesis, and suggests that differential co-expression of Ret and Gfra can control the specificity of RET signaling.