GDNF and amyotrophic lateral sclerosis: Astrocytes secrete many trophic factors, including nerve growth factor (NGF), glial cell line-derived neurotrophic factor (GDNF), insulin growing factor-1, and fibroblast growing factors (FGF) [189], especially during ALS [190], mature nerve growth factor (NGF) modulates neuronal differentiation and survival by binding to the Tyrosine receptor Kinase A (TrKA).