Unbiased Proteomic Study of the Axons of Cultured Rat Cortical Neurons.

The axon is a long projection connecting a neuron to its targets. Here, the axons of cultured rat cortical neurons were isolated with micropatterned chips that enable the separation of axons from their cell bodies. Proteins extracted from isolated axons and whole neurons were subjected to analyses using two-dimensional liquid chromatography-tandem mass spectrometry (2D-LC-MS/MS) analyses without and with stable isotope dimethyl labeling, resulting in the identification of >2500 axonal proteins and 103 axon-enriched proteins. A strong correlation exists between the abundances of axonal proteins and their counterparts in whole neurons. The proteomic results confirm the axonal protein constituents of the subcellular structures documented in earlier electron microscopic studies. Cortical axons have proteins that are components of machineries for protein degradation and the synthesis of soluble, membrane, and secretory proteins, although axons lack conventional Golgi apparatus. Despite the fact that axons lack nucleus, nuclear proteins were identified, and 67 of them were found enriched in axons. Some of the results obtained by the MS-based studies were validated by quantitative Western blotting and immunofluorescence staining analyses. The results represent the first comprehensive description of the axonal protein landscape. The MS proteomics data are available via ProteomeXchange with identifier PXD005527.