Proteolytic processing of proNGF is necessary for mature NGF regulated secretion from neurons

Nerve growth factor mediates neuronal survival, synaptogenesis, and synaptic remodeling. We utilized primary hippocampal cultures to investigate the intrinsic motifs of proNGF that might contribute to its processing and subsequent allocation to a regulated versus constitutive secretory pathway. The addition of a carboxypeptidase E motif to proNGF did not alter the secretion of NGF. However, mutagenesis of proNGF proteolytic processing sites had significant effects on the final NGF product and its secretion. The furin recognition site (R118-S-K-R121) is essential for the proper processing of proNGF to its 13.5kDa mature product and mutating the furin site exposed an alternative processing site resulting in an intermediate NGF product of approximately 22kDa. Finally, inhibiting the processing of proNGF abolished regulated secretion of the resulting NGF product. These experiments demonstrate that hippocampal neurons harbor multiple pathways to process proNGF of which the furin consensus sequence is the preferred processing site.