BDNF and neurodegenerative disease: In one particular example, we presented a polyion complex (PIC) composed of poly(ethylene glycol)113-b-poly(glutamic acid)50 (PEG-PLE) and brain-derived neurotrophic factor (BDNF), termed Nano-BDNF PEG-PLE, as a therapeutic candidate for the treatment of neurodegenerative diseases [10,11].