Knight David, Tolley Lynn K, Kim David K, Lavidis Nick A, Noakes Peter G
School of Biomedical Sciences, and Special Research Center for Genomics and Bioinformatics, University of Queensland St Lucia, Queensland 4072, Australia.
J Physiol. 2003 Feb 1;546(Pt 3):789-800. doi: 10.1113/jphysiol.2002.030924.
beta2-Laminin is important for the formation of neuromuscular junctions in vertebrates. Previously, we have inactivated the gene that encodes for beta2-laminin in mice and observed predominantly prejunctional structural defects. In this study, we have used both intra- and extracellular recording methods to investigate evoked neurotransmission in beta2-laminin-deficient mice, from postnatal day 8 (P8) through to day 18 (P18). Our results confirmed that there was a decrease in the frequency of spontaneous release, but no change in the postjunctional response to such release. Analysis of evoked neurotransmission showed an increase in the frequency of stimuli that failed to elicit an evoked postjunctional response in the mutants compared to litter mate controls, resulting in a 50 % reduction in mean quantal content at mutant terminals. Compared to littermate controls, beta2-laminin-deficient terminals showed greater synaptic depression when subjected to high frequency stimulation. Furthermore, the paired pulse ratio of the first two stimuli was significantly lower in beta2-laminin mutant terminals. Statistical analysis of the binomial parameters of release showed that the decrease in quantal content was due to a decrease in the number of release sites without any significant change in the average probability of release. This suggestion was supported by the observation of fewer synaptic vesicle protein 2 (SV2)-positive varicosities in beta2-laminin-deficient terminals and by ultrastructural observations showing smaller terminal profiles and increased Schwann cell invasion in beta2-laminin mutants; the differences between beta2-laminin mutants and wild-type mice were the same at both P8 and P18. From these results we conclude that beta2-laminin plays a role in the early structural development of the neuromuscular junction. We also suggest that transmitter release activity may act as a deterrent to Schwann cell invasion in the absence of beta2-laminin.
β2-层粘连蛋白对脊椎动物神经肌肉接头的形成很重要。此前，我们已使小鼠中编码β2-层粘连蛋白的基因失活，并观察到主要是接头前结构缺陷。在本研究中，我们使用细胞内和细胞外记录方法，研究了出生后第8天（P8）至第18天（P18）的β2-层粘连蛋白缺陷小鼠的诱发神经传递。我们的结果证实，自发释放频率降低，但接头后对此类释放的反应没有变化。诱发神经传递分析表明，与同窝对照相比，突变体中未能引发诱发接头后反应的刺激频率增加，导致突变体终末的平均量子含量降低了50%。与同窝对照相比，β2-层粘连蛋白缺陷终末在高频刺激下表现出更大的突触抑制。此外，β2-层粘连蛋白突变体终末前两个刺激的配对脉冲比率显著更低。释放二项式参数的统计分析表明，量子含量的降低是由于释放位点数量减少，而释放的平均概率没有任何显著变化。β2-层粘连蛋白缺陷终末中突触小泡蛋白2（SV2）阳性曲张体较少，以及超微结构观察显示β2-层粘连蛋白突变体中终末轮廓较小且施万细胞侵入增加，支持了这一观点；β2-层粘连蛋白突变体与野生型小鼠之间的差异在P8和P18时相同。从这些结果我们得出结论，β2-层粘连蛋白在神经肌肉接头的早期结构发育中起作用。我们还提出，在没有β2-层粘连蛋白的情况下，递质释放活动可能对施万细胞的侵入起到抑制作用。
