BDNF and amyotrophic lateral sclerosis: M2 microglia can secrete anti-inflammatory cytokines as well as neurotrophic factors such as brain-derived neurotrophic factor (BDNF), and insulin-like growth factor-1 (IGF1) which has the capacity to promote growth and differentiation of “neural stem cells” (55), while impairment of GH-IGF system has been shown in ALS (56).