Taylor M D, Vancura R, Patterson C L, Williams J M, Riekhof J T, Wright D E
Department of Anatomy and Cell Biology, University of Kansas Medical Center, Kansas City, KS 66160, USA.
J Comp Neurol. 2001 Apr 2;432(2):244-58. doi: 10.1002/cne.1100.
To investigate the effects of neurotrophin-3 (NT-3) on postnatal proprioceptive neurons and their targets, transgenic mice were generated that use the myosin light chain 1 (mlc) promoter to overexpress NT-3 in skeletal muscle. Ribonuclease protection assays revealed that NT-3 overexpression in hindlimb skeletal muscle began at embryonic day 14 (E14) and continued throughout adulthood. Overexpression of NT-3 during late embryogenesis resulted in increased numbers of large sensory and small fusimotor axons. Within a week of birth, mlc/NT-3 mice retract their limbs to the torso when lifted by the tail. Footprint analysis revealed that mlc/NT-3 mice had significant abnormalities in their gait compared with wild-types. Beam walking and rotorod analysis confirmed the poor limb control by mlc/NT-3 mice. These locomotive deficits progressively worsened with age and were likely related to the formation of morphologically abnormal muscle spindles. The most common spindle anomaly was the presence of excessive intrafusal bag fibers within individual muscle spindles. To assess the role of NT-3 in recovery from nerve injury, sciatic nerve crushes were performed in young adult mice. Two days after injury, mlc/NT-3 mice displayed significantly improved sciatic functional indexes and a significant increase in muscle spindles that remained associated with axons. The latter finding suggests that excess NT-3 in muscle may retard the degeneration of proprioceptive axons after nerve crush. Long-term survival after nerve injury in mlc/NT-3 mice did not induce further changes in spindle number or morphology. These findings demonstrate that, in addition to promoting embryonic proprioceptive neuron survival, postnatal overexpression of NT-3 in muscle leads to abnormal spindle formation and deficits in locomotive control. However, our results also show that NT-3 may be therapeutic for proprioceptive axons immediately after nerve injury by delaying axon degeneration.
为了研究神经营养因子-3（NT-3）对出生后本体感觉神经元及其靶标的影响，构建了转基因小鼠，其利用肌球蛋白轻链1（mlc）启动子在骨骼肌中过表达NT-3。核糖核酸酶保护试验显示，后肢骨骼肌中NT-3的过表达始于胚胎第14天（E14），并持续至成年期。胚胎后期NT-3的过表达导致大型感觉和小型肌梭运动轴突数量增加。出生后一周内，mlc/NT-3小鼠被尾巴提起时会将四肢缩回躯干。足迹分析显示，与野生型相比，mlc/NT-3小鼠的步态存在明显异常。光束行走和转棒分析证实mlc/NT-3小鼠的肢体控制能力较差。这些运动缺陷随着年龄的增长而逐渐恶化，可能与形态异常的肌梭形成有关。最常见的梭异常是单个肌梭内存在过多的梭内袋状纤维。为了评估NT-3在神经损伤恢复中的作用，对年轻成年小鼠进行了坐骨神经挤压。损伤后两天，mlc/NT-3小鼠的坐骨神经功能指数显著改善，与轴突相关的肌梭数量显著增加。后一发现表明，肌肉中过量的NT-3可能会延缓神经挤压后本体感觉轴突的退化。mlc/NT-3小鼠神经损伤后的长期存活并未导致梭数量或形态的进一步变化。这些发现表明，除了促进胚胎本体感觉神经元存活外，出生后肌肉中NT-3的过表达还会导致异常的梭形成和运动控制缺陷。然而，我们的结果还表明，NT-3可能通过延迟轴突退化对神经损伤后立即的本体感觉轴突具有治疗作用。
