Black M M, Slaughter T, Fischer I
Department of Anatomy and Cell Biology, Temple University School of Medicine, Philadelphia, Pennsylvania 19140.
J Neurosci. 1994 Feb;14(2):857-70. doi: 10.1523/JNEUROSCI.14-02-00857.1994.
Microtubule-associated protein 1b (MAP1b) 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 MAP1b in growing axons would help define how MAP1b participates in axon growth. Therefore, we have stained cultured sympathetic neurons with various antibodies against MAP1b, and then used digital image processing and analysis procedures to quantify MAP1b distribution, phosphorylation and association with microtubules (MTs) in actively elongating axons. MAP1b is present on MTs all along the axon. Quantitative analyses showed that MAP1b has a nonuniform distribution along growing axons. It is present at relatively low and constant levels along the axon shaft until approximately 130 microns from the axon tip, where the amount of MAP1b begins to increase sharply and reaches a peak close to the growth cone. The peak amount of MAP1b in the distal axon is an order of magnitude greater than the average amount in the axon shaft. The enrichment of MAP1b in the distal axon was observed for total MAP1b and assembled MAP1b, and was even more pronounced for phosphorylated MAP1b. This distribution pattern remains after correcting the relative amount of MAP1b along the axon for variations in axonal volume. Thus, the concentration of MAP1b in the distal axon exceeds by severalfold that in the rest of the axon. The amount of assembled MAP1b relative to the amount of MT polymer also varies along the axon, and is greatest distally near the growth cone. This pattern of MAP1b localization in axons focuses attention on the distal axon and growth cone as the principal sites of MAP1b function in axon growth. We discuss the possibility that MAP1b regulates MT dynamics in the distal axon so that it is properly coordinated with growth cone events involved in axon extension.
微管相关蛋白1b（MAP1b）在积极延伸轴突的神经元中表达水平特别高，尽管它似乎是轴突生长所必需的，但其作用性质尚不清楚。我们推断，详细描述MAP1b在生长轴突中的定位将有助于确定MAP1b如何参与轴突生长。因此，我们用各种抗MAP1b抗体对培养的交感神经元进行染色，然后使用数字图像处理和分析程序来量化MAP1b在积极伸长的轴突中的分布、磷酸化以及与微管（MTs）的结合情况。MAP1b存在于整个轴突的微管上。定量分析表明，MAP1b在生长轴突上的分布不均匀。沿着轴突干，它以相对较低且恒定的水平存在，直到距离轴突尖端约130微米处，此时MAP1b的量开始急剧增加，并在靠近生长锥的地方达到峰值。轴突远端MAP1b的峰值量比轴突干中的平均量高一个数量级。总MAP1b和组装好的MAP1b在轴突远端都有富集现象，而磷酸化的MAP1b更为明显。在校正轴突中MAP1b的相对量以消除轴突体积变化的影响后，这种分布模式仍然存在。因此，轴突远端MAP1b的浓度比轴突其余部分高出几倍。组装好的MAP1b相对于微管聚合物的量也沿轴突变化，在靠近生长锥的远端最大。MAP1b在轴突中的这种定位模式将注意力集中在轴突远端和生长锥上, 它们是MAP1b在轴突生长中发挥功能的主要部位。我们讨论了MAP1b调节轴突远端微管动力学，从而使其与轴突延伸过程中涉及生长锥的事件适当协调的可能性。
