The upregulation of GLAST-1 is an indirect antiapoptotic mechanism of GDNF and neurturin in the adult CNS.

Glial cell-line-derived neurotrophic factor (GDNF) and neurturin (NTN) protect retinal ganglion cells (RGCs) from axotomy-induced apoptosis. It is likely that neuroprotection by GDNF or NTN in the adult central nervous system (CNS) involves indirect mechanisms and independent signal transduction events. Extracellular glutamate is a trigger of apoptosis in injured RGCs, and glutamate transporter levels can be upregulated by GDNF. Therefore, GDNF may indirectly protect RGCs by enhancing glutamate uptake in the retina. We studied the upregulation of the glutamate transporters GLAST-1 and GLT-1 by GDNF and NTN, and the intracellular pathways required for GDNF/NTN neuroprotection. GDNF required phosphoinositide-3 kinase (PI3K) and Src activity to upregulate GLAST-1 and GLT-1. NTN required PI3K activity to upregulate GLAST-1 and did not affect GLT-1 levels. PI3K activity was also important for GDNF and NTN neuroprotection following optic nerve transection. However, GDNF also required Src and mitogen-activated protein kinase activity to prevent RGC apoptosis. RNA interference demonstrated that the upregulation of GLAST-1 by GDNF and NTN is required to rescue RGCs. Thus, additional independent signal transduction events, together with the upregulation of GLT-1 by GDNF, differentiate the biological activity of GDNF from NTN. Furthermore, the upregulation of the glial glutamate transporter GLAST-1 by both factors is an indirect neuroprotective mechanism in the CNS.