Distal Region of Growing Axons

Microtubule-associated protein 1 b (MAP1 b) is expressed at especially high levels in neurons actively extending axons, and although it appears to be required for axon growth, the nature of its role is unknown. We reasoned that a detailed description of the localization of MAP1 b in growing axons would help define how MAP1 b participates in axon growth. Therefore, we have stained cultured sympathetic neurons with various antibodies against MAP1 b, and then used digital image processing and analysis procedures to quantify MAP1 b distribution, phosphorylation and association with microtubules (MTs) in actively elongating axons. MAP1 b is present on MTs all along the axon. Quantitative analyses showed that MAP1 b has a nonuniform distribution along growing axons. It is present at relatively low and constant levels along the axon shaft until = 130 pm from the axon tip, where the amount of MAP1 b begins to increase sharply and reaches a peak close to the growth cone. The peak amount of MAP1 b in the distal axon is an order of magnitude greater than the average amount in the axon shaft. The enrichment of MAP1 b in the distal axon was observed for total MAP1 b and assembled MAP1 b, and was even more pronounced for phosphorylated MAP1 b. This distribution pattern remains after correcting the relative amount of MAP1 b along the axon for variations in axonal volume. Thus, the concentration of MAP1 b in the distal axon exceeds by severalfold that in the rest of the axon. The amount of assembled MAP1 b relative to the amount of MT polymer also varies along the axon, and is greatest distally near the growth cone. This pattern of MAP1 b localization in axons focuses attention on the distal axon and growth cone as the principal sites of MAP1 b function in axon growth. We discuss the possibility that MAP1 b regulates MT dynamics in the distal axon so that it is properly coordinated with growth cone events involved in axon extension. [