Abstract
The failure of glial cell derived neurotropic factor to be efficacious in blinded clinical trials for Parkinson's disease may be due to alterations in signaling receptors and downstream signaling molecules. To test this hypothesis, brain sections were obtained from older adults with no motor deficit (n = 6), minimal motor deficits (n = 10), and clinical diagnosis of Parkinson's disease (n = 10) who underwent motor examination proximate to death. Quantitative unbiased stereology and densitometry were performed to analyze RET and phosphorylated ribosomal protein S6 expression in nigral neurons. Individuals with no motor deficit had extensive and intense RET and phosphorylated ribosomal protein S6 immunoreactive neurons in substantia nigra. The number and staining intensity of RET-immunoreactive neurons were reduced moderately in subjects with minimal motor deficits and severely reduced in Parkinson's disease relative to no motor deficit group. The number and staining intensity of phosphorylated ribosomal protein S6 was more markedly reduced in both subjects with minimal motor deficits and Parkinson's disease. Reductions in levels of RET and phosphorylated ribosomal protein S6 were recapitulated in a non-human primate genetic Parkinson's disease model based on over-expression of human mutant α-synuclein (A53T). These data indicate that for neurotrophic factors to be effective in patients with minimal motor deficits or PD, these factors would likely have to upregulate RET and phosphorylated ribosomal protein S6 immunoreactive neurons in substantia nigra .