Biochemical and immunological analyses of cytoskeletal domains of neurons

We have used cultured sympathetic neurons to identify microtubule proteins (tubulin and microtubule-associated proteins [MAPs]) and neurofilament (NF) proteins in pure preparations of axons and also to examine the distribution of these proteins between axons and cell bodies + dendrites. Pieces of sympathetic ganglia containing thousands of neurons were plated onto culture dishes and allowed to extend neurites. Dendrites remained confined to the ganglionic explant or cell body mass (CBM), while axons extended away from the CBM for several millimeters. Axons were separated from cell bodies and dendrites by dissecting the CBM away from cultures, and the resulting axonal and CBM preparations were analyzed using biochemical, immunoblotting, and immunoprecipitation methods. Cultures were used after 17 d in vitro, when 40-60% of total protein was in the axons. The 68,000-mol-wt NF subunit is present in both axons and CBM in roughly equal amounts. The 145,000- and 200,000-mol-wt NF subunits each consist of several variants which differ in phosphorylation state; poorly and nonphosphorylated species are present only in the CBM, whereas more heavily phosphorylated forms are present in axons and, to a lesser extent, the CBM. One 145,000-mol-wt NF variant was axon specific. Tubulin is roughly equally distributed between CBM and axon-like neurites of explant cultures. MAP-1a, MAP-1b, MAP-3, and the 60,000-mol- wt MAP are also present in the CBM and axon-like neurites and show distribution patterns similar to that of tubulin. In contrast, MAP-2 was detected only in the CBM, while tau and the 210,000-mol-wt MAP were greatly enriched in axons compared to the CBM. In immunostaining analyses, MAP-2 localized to cell bodies and dendrite-like neurites, but not to axon-like neurites, whereas antibodies to tubulin and MAP-1b localized to all regions of the neurons. The regional differences in composition of the neuronal cytoskeleton presumably generate corresponding differences in its structure, which may, in turn, contribute to the morphological differences between axons and dendrites.