APP-dependent regulation of GDNF expression and its involvement in neuromuscular junction

OBJECTIVES: Amyloid precursor protein (APP) plays a crucial role in Alzheimer's disease (AD) pathogenesis since its proteolytical cleavage generates β-amyloid peptide, the major constituent of senile plaques. However, APP precise physiological role has not been clarified yet. Interestingly, we observed that APP regulates the transcription of GDNF (Glial cell-line Derived Neutrophic Factor) in cell lines and in transgenic mice. The aim of our work is to investigate this novel APP function. METHODS: Following microarray analysis, quantitative RT-PCR, ELISA and reporter gene assays were performed on mouse embryonic fibroblast cell line (MEF) knock-out for APP. APP expression was monitored during myotube differentiation in C2C12 mouse myoblast has packed or not with a GDNF expression plasmid. Studies on APP knock-out (APP KO) mice included grip strength tests, mechanic measurements on isolated extensor digitorum longus (EDL) and immunohistochemistry (IHC) to investigate muscular atrophy and neuromuscular junctions (NMJs) formation. RESULTS: We found that GDNF mRNA and protein levels together with GDNF promoter transcriptional activity are downregulated in APP KO MEFs compared to control cells. In C2C12, we observed that consequent to myotube differentiation GDNF and APP protein levels increase. Moreover transfection of the GDNF-encoding plasmid favors myotube formation and correlates with APP increase. We showed that APP KO mice muscular phenotype and defective neuromuscular synapses are associated with GDNF downregulation. CONCLUSIONS: Our findings highlight for the first time a role for APP in the regulation of GDNF expression with implications in muscular differentiation and NMJs formation and function