GDNF and Parkinson disease: A model neurotrophin, glial-derived neurotrophic factor (GDNF), which is neuroprotective in PD, was fused to the carboxyl terminus of the HC of the HIRMAb (Boado et al., 2008a), and this HIRMAb-GDNF fusion protein traversed the BBB via RMT on the IR, followed by binding of the GDNF domain of the fusion protein to the cognate neurotrophin receptor (NTR) on the neuronal cell membrane (Figure 4).