Distinct roles of neurofilament and tubulin gene expression in axonal growth.

Tubulin and the neurofilament (NF) proteins, which are major constituents of the axonal cytoskeleton, play distinct roles in longitudinal and radial growth of axons. The predominantly longitudinal growth of axons in developing neurons correlates with relatively high levels of expression for a particular tubulin gene, encoding the class II beta tubulin isotype, whereas levels of gene expression for two other beta tubulin isotypes (classes I and IV) are comparable in developing and maturing neurons. Gene expression for the 68 kDa NF protein (NF68) is low during longitudinal growth. Conversely, relatively high levels of gene expression for NF68 and low levels for class II beta tubulin in maturing neurons correlate with the radial growth of axons. The developmental pattern of gene expression is recapitulated during axonal regeneration. Expression of the class II beta tubulin isotype correlates with the outgrowth (elongation) of regenerating sprouts.