Specificity in the crosstalk of TGFβ/GDNF family members is determined by distinct GFR alpha receptors

Glial cell line-derived neurotrophic factor (GDNF) and neurturin (NRTN) are neurotrophic factors for parasympathetic neurons including ciliary ganglion (CG) neurons. Recently, we have shown that survival and signaling mediated by GDNF in CG neurons essentially requires transforming growth factor beta (TGF beta). We have provided evidence that TGF beta regulates the availability of the glycosyl phosphatidylinositol (GPI)-anchored GDNF receptor alpha 1 (GFR alpha 1) by promoting the recruitment of the receptor to the plasma membrane. We report now that in addition to GDNF, NRTN, but not persephin (PSPN) or artemin (ARTN), is able to promote survival of CG neurons. Interestingly, in contrast to GDNF, NRTN is not dependent on cooperation with TGF beta, but efficiently promotes neuronal survival and intracellular signaling in the absence of TGF beta. Additional treatment with TGF beta does not further increase the NRTN response. Both NRTN and GDNF exclusively bind to and activate their cognate receptors, GFR alpha 2 and GFR alpha 1, respectively, as shown by the use of receptor-specific neutralizing antibodies. Immunocytochemical staining for the two receptors on the surface of CG neurons reveals that, in contrast to the effect on GFR alpha 1, TGF beta is not required for recruitment of GFR alpha 2 to the plasma membrane. Moreover, binding of radioactively labeled GDNF but not NRTN is increased upon treatment of CG neurons with TGF beta. Disruption of TGF beta signaling does interfere with GDNF-, but not NRTN-mediated signaling and survival. We propose a model taking into account data from GFR alpha 1 crystallization and ontogenetic development of the CG that may explain the differences in TGF beta-dependence of GDNF and NRTN.