Department of Biological Sciences, University of Missouri, Columbia, Columbia, MO 65211, USA.
J Exp Biol. 2010 Jan 1;213(1):128-36. doi: 10.1242/jeb.033787.
The evolution of larger mammals resulted in a corresponding increase in peripheral nerve length. To ensure optimal nervous system functionality and survival, nerve conduction velocities were likely to have increased to maintain the rate of signal propagation. Increases of conduction velocities may have required alterations in one of the two predominant properties that affect the speed of neuronal transmission: myelination or axonal diameter. A plausible mechanism to explain faster conduction velocities was a concomitant increase in axonal diameter with evolving axonal length. The carboxy terminal tail domain of the neurofilament medium subunit is a determinant of axonal diameter in large caliber myelinated axons. Sequence analysis of mammalian orthologs indicates that the neurofilament medium carboxy terminal tail contains a variable lysine-serine-proline (KSP) repeat sub-domain flanked by two highly conserved sub-domains. The number of KSP repeats within this region of neurofilament medium varies among species. Interestingly, the number of repeats does not change within a species, suggesting that selective pressure conserved the number of repeats within a species. Mapping KSP repeat numbers onto consensus phylogenetic trees reveals independent KSP expansion events across several mammalian clades. Linear regression analyses identified three subsets of mammals, one of which shows a positive correlation in the number of repeats with head-body length. For this subset of mammals, we hypothesize that variations in the number of KSP repeats within neurofilament medium carboxy terminal tail may have contributed to an increase in axonal caliber, increasing nerve conduction velocity as larger mammals evolved.
大型哺乳动物的进化导致外周神经长度相应增加。为了确保神经系统的最佳功能和生存，神经传导速度可能会增加，以维持信号传播的速度。传导速度的增加可能需要改变影响神经元传输速度的两个主要特性之一：髓鞘形成或轴突直径。一个合理的解释更快的传导速度的机制是与进化的轴突长度同时增加轴突直径。神经丝中间亚单位的羧基末端尾部是大直径有髓轴突轴突直径的决定因素。哺乳动物同源物的序列分析表明，神经丝中间羧基末端尾部含有一个可变的赖氨酸-丝氨酸-脯氨酸（KSP）重复亚域，两侧是两个高度保守的亚域。该神经丝中间区域的 KSP 重复数在物种间有所不同。有趣的是，在一个物种内重复数不会改变，这表明选择性压力在一个物种内保持了重复数的不变。将 KSP 重复数映射到共识系统发育树上揭示了几个哺乳动物进化枝中的独立 KSP 扩展事件。线性回归分析确定了哺乳动物的三个亚组，其中一组显示出重复数与头身长度之间的正相关。对于这组哺乳动物，我们假设神经丝中间羧基末端尾部 KSP 重复数的变化可能导致轴突口径增加，从而随着大型哺乳动物的进化增加神经传导速度。
