The receptor tyrosine kinase Ret plays a critical role in the development of the mammalian excretory and enteric nervous systems. Differential splicing of the primary Ret transcript results in the generation of two main isoforms, Ret9 and Ret51, whose C-terminal amino acid tails diverge after tyrosine (Y) 1062. Monoisoformic mice expressing only Ret9 develop normally and are healthy and fertile. In contrast, animals expressing only Ret51 have aganglionosis of the distal gut and hypoplastic kidneys. By generating monoisoformic mice in which Y1062 of Ret9 has been mutated to phenylalanine, we demonstrate that this amino acid has a critical role in Ret9 signaling that is necessary for the development of the kidneys and the enteric nervous system. These findings argue that the distinct activities of Ret9 and Ret51 result from the differential regulation of Y1062 by C-terminal flanking sequences. However, a mutation which places Y1062 of Ret51 in a Ret9 context improves only marginally the ability of Ret51 to support renal and enteric nervous system development. Finally, monoisoformic mice expressing a variant of Ret9 in which a C-terminal PDZ-binding motif was mutated develop normally and are healthy. Our studies identify Y1062 as a critical regulator of Ret9 signaling and suggest that Ret51-specific motifs are likely to inhibit the activity of this isoform.

Mesenchymal-epithelial interactions are an important source of information for pattern formation during organogenesis. In the developing excretory system, one of the secreted mesenchymal factors thought to play a critical role in patterning the growth and branching of the epithelial ureteric bud is GDNF. We have tested the requirement for GDNF as a paracrine chemoattractive factor by altering its site of expression during excretory system development. Normally, GDNF is secreted by the metanephric mesenchyme and acts via receptors on the Wolffian duct and ureteric bud epithelium. Misexpression of GDNF in the Wolffian duct and ureteric buds resulted in formation of multiple, ectopic buds, which branched independently of the metanephric mesenchyme. This confirmed the ability of GDNF to induce ureter outgrowth and epithelial branching in vivo. However, in mutant mice lacking endogenous GDNF, kidney development was rescued to a substantial degree by GDNF supplied only by the Wolffian duct and ureteric bud. These results indicate that mesenchymal GDNF is not required as a chemoattractive factor to pattern the growth of the ureteric bud within the developing kidney, and that any positional information provided by the mesenchymal expression of GDNF may provide for renal branching morphogenesis is redundant with other signals.

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