Wong J, Oblinger M M
Department of Biological Chemistry and Structure, Chicago Medical School, Illinois 60064.
Metab Brain Dis. 1987 Dec;2(4):291-303. doi: 10.1007/BF00999699.
Neurofilaments (NFs) are predominant elements in large myelinated axons, where they are thought to serve the important function of maintaining axonal caliber. Previous studies have shown that changes in NF synthesis and axonal transport occur after axonal injury in rat dorsal root ganglion (DRG) cells. The resulting reduction in the NF supply to DRG axons is thought to be largely responsible for the observed decrease in axonal diameter in the proximal axonal stump after an injury. In the present study, we test the hypothesis that a change in NF gene expression precedes the changes in synthesis and transport of NF proteins. To address this hypothesis, the levels of mRNA encoding the 68-kilodalton (kd) neurofilament protein (NF68) in adult rat DRG neurons were assessed at different times after peripheral axotomy using in situ hybridization. For these studies we used a 35S-labeled cDNA probe to NF68. The levels of NF68 mRNA in sensory neurons located in ipsilateral fourth and fifth lumbar DRG at 1, 7, and 14 days after sciatic nerve crush were compared to those in normal DRG neurons using quantitative autoradiography. In large DRG neurons (greater than 1000 micron 2), the levels of NF68 mRNA were significantly reduced relative to normal at 1, 7, and 14 days after axotomy. Medium-sized cells (601-1000 micron 2) exhibited a reduction only at 14 days postinjury, and small-sized cells were not significantly affected. These findings indicate that larger DRG neurons which give rise to large myelinated sensory axons exhibit a change in NF gene expression after axonal injury. The observed changes in NF68 mRNA levels temporally precede changes in NF synthesis and transport in injured DRG cells. Thus, a change in NF gene expression may be an important component of an effective regenerative response and a critical step at which axonal caliber is regulated in injured neurons.
神经丝（NFs）是有髓大轴突中的主要成分，人们认为它们在维持轴突管径方面发挥着重要作用。先前的研究表明，大鼠背根神经节（DRG）细胞轴突损伤后，神经丝的合成和轴突运输会发生变化。DRG轴突中神经丝供应的减少被认为是损伤后近端轴突残端轴突直径减小的主要原因。在本研究中，我们检验了神经丝基因表达的变化先于神经丝蛋白合成和运输变化这一假设。为验证这一假设，我们使用原位杂交技术，在成年大鼠DRG神经元外周轴突切断后的不同时间，评估编码68千道尔顿（kd）神经丝蛋白（NF68）的mRNA水平。在这些研究中，我们使用了针对NF68的35S标记cDNA探针。通过定量放射自显影，将坐骨神经挤压后1天、7天和14天位于同侧第四和第五腰段DRG的感觉神经元中NF68 mRNA的水平与正常DRG神经元中的水平进行比较。在大型DRG神经元（大于1000微米2）中，轴突切断后1天、7天和14天，NF68 mRNA水平相对于正常水平显著降低。中等大小的细胞（601 - 1000微米2）仅在损伤后14天出现降低，而小细胞则未受到显著影响。这些发现表明，产生有髓大感觉轴突的较大DRG神经元在轴突损伤后神经丝基因表达发生了变化。在受损DRG细胞中，观察到的NF68 mRNA水平变化在时间上先于神经丝合成和运输的变化。因此，神经丝基因表达的变化可能是有效再生反应的重要组成部分，也是受损神经元调节轴突管径的关键步骤。
