The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) 
(GDNF, neurturin, artemin, and persephin) are critical regulators of 
neurodevelopment and support the survival of midbrain dopaminergic and spinal 
motor neurons in vitro and in animal disease models making them attractive 
therapeutic candidates for treatment of neurodegenerative diseases. The GFLs 
signal through a multicomponent receptor complex comprised of a high affinity 
binding component (GDNF-family receptor alpha-component (GFRalpha1-GFRalpha4)) 
and the receptor tyrosine kinase RET. To begin characterization of GFL receptor 
specificity at the molecular level, we performed comprehensive 
homologue-scanning mutagenesis of GDNF, the prototypical member of the GFLs. 
Replacing short segments of GDNF with the homologous segments from persephin 
(PSPN) (which cannot bind or activate GFRalpha1.RET or GFRalpha2.RET) identified 
sites along the second finger of GDNF critical for activating the GFRalpha1.RET 
and GFRalpha2.RET receptor complexes. Furthermore, introduction of these regions 
from GDNF, neurturin, or artemin into PSPN demonstrated that they are sufficient 
for activating GFRalpha1. RET, but additional determinants are required for 
interaction with the other GFRalphas. This difference in the molecular basis of 
GFL-GFRalpha specificity allowed the production of GFRalpha1. RET-specific 
agonists and provides a foundation for understanding of GFL-GFRalpha.RET 
signaling at the molecular level.