TGFbeta trophic factors differentially modulate motor axon outgrowth and protection from excitotoxicity

Transforming growth factor (TGF) beta-like trophic factors have been shown to be protective in acute neuronal injury paradigms. In the current study, we analyzed and compared members of this growing family, including glial cell line-derived neurotrophic factor (GDNF), neurturin, nodal, persephin, and TGFbeta1, for protection against chronic glutamate toxicity. In parallel, we developed a organotypic spinal cord culture system to study the ability of these factors to promote motor axon outgrowth across white matter. Using these systems, we were able to differentiate the neuroprotective effect of the TGFbeta-like factors from their motor axon outgrowth-promoting activity. GDNF, neurturin, persephin, nodal, and TGFbeta1 all protected against excitotoxic motor neuron degeneration. Low amounts of GDNF (1 ng/ml) and high concentrations of neurturin induced vigorous motor axon outgrowth. In contrast, nodal, persephin, and TGFbeta1 did not induce motor axon outgrowth. Both GDNF and neurturin bind to Ret receptor complexes and were capable of activating the MAP kinase pathway. A specific inhibitor of MAP kinase kinase, PD98059, inhibited the motor axon outgrowth-promoting activity of the GDNF but not the neuroprotective activity. Similarly, the specific PI3K inhibitors, LY294002 and wortmannin, were able to inhibit the promotion of motor axon outgrowth by GDNF, but did not affect neuroprotective activity. Our results suggest that the neurite outgrowth-promoting effect of GDNF is mediated through the PI3K and MAP kinase pathways. The neuroprotective effect of GDNF appears to be through a separate pathway.