GDNF and glioma: Glioma cells secrete several factors that serve as a chemoattractants for microglial cells, such as the hepatocyte growth factor (HGF), colony-stimulating factor 1 (CSF-1), monocyte chemoattractant protein-1 (MCP-1/CCL2), stromal cell-derived factor-1 (SDF-1), and glial-derived neurotrophic factor (GDNF) [128–131].