GDNF and cancer: The crosstalk between cancer cells and nerves (including neurons, axons, SCs, immunocytes, and other stroma cells)—through neurotrophins, glial cell-derived neurotrophic factor (GDNF), chemokines/cytokines, midkine family molecules, neural adhesion molecules, axon guidance molecules, neurotransmitters/neuropeptides, and so forth —mimics the interactions by activated SCs, macrophages, injured axons, or target (innervated) tissues and nerves [14,39,40,41,42,43,44,45,46,47].