GFRA1 and glioblastoma: In addition, GNDF (glial cell line-derived neurotrophic factor), secreted by astrocytes, induces the invasion of GBM cells by activating RET (rearranged during transfection)/GFRα1 (GDNF family receptor alpha-1) receptors and pro-tumoral signaling pathways, such as MAPK (mitogen-activated protein kinases) and PI3K (phosphatidylinositol 3-kinase)/Akt [44].