Tejero Rocío, Lopez-Manzaneda Mario, Arumugam Saravanan, Tabares Lucía
Department of Medical Physiology and Biophysics, School of Medicine, University of Seville, Avda. Sánchez Pizjuán, 4. 41009 Seville, Spain.
Hum Mol Genet. 2016 Nov 1;25(21):4703-4716. doi: 10.1093/hmg/ddw297.
Spinal muscular atrophy (SMA) is the most frequent genetic cause of infant mortality. The disease is characterized by progressive muscle weakness and paralysis of axial and proximal limb muscles. It is caused by homozygous loss or mutation of the SMN1 gene, which codes for the Survival Motor Neuron (SMN) protein. In mouse models of the disease, neurotransmitter release is greatly impaired, but the molecular mechanisms of the synaptic dysfunction and the basis of the selective muscle vulnerability are unknown. In the present study, we investigated these open questions by comparing the molecular and functional properties of nerve terminals in severely and mildly affected muscles in the SMNΔ7 mouse model. We discovered that synaptotagmin-1 (Syt1) was developmentally downregulated in nerve terminals of highly affected muscles but not in low vulnerable muscles. Additionally, the expression levels of synaptotagmin-2 (Syt2), and its interacting protein, synaptic vesicle protein 2 (SV2) B, were reduced in proportion to the degree of muscle vulnerability while other synaptic proteins, such as syntaxin-1B (Stx1B) and synaptotagmin-7 (Syt7), were not affected. Consistently with the extremely low levels of both Syt-isoforms, and SV2B, in most affected neuromuscular synapses, the functional analysis of neurotransmission revealed highly reduced evoked release, altered short-term plasticity, low release probability, and inability to modulate normally the number of functional release sites. Together, we propose that the strong reduction of Syt2 and SV2B are key factors of the functional synaptic alteration and that the physiological downregulation of Syt1 plays a determinant role in muscle vulnerability in SMA.
脊髓性肌萎缩症（SMA）是婴儿死亡最常见的遗传原因。该疾病的特征是进行性肌肉无力以及躯干和近端肢体肌肉麻痹。它由SMN1基因的纯合缺失或突变引起，该基因编码生存运动神经元（SMN）蛋白。在该疾病的小鼠模型中，神经递质释放受到严重损害，但突触功能障碍的分子机制以及选择性肌肉易损性的基础尚不清楚。在本研究中，我们通过比较SMNΔ7小鼠模型中严重和轻度受影响肌肉神经末梢的分子和功能特性，研究了这些悬而未决的问题。我们发现，在受影响严重的肌肉神经末梢中，突触结合蛋白-1（Syt1）在发育过程中表达下调，但在低易损性肌肉中未下调。此外，突触结合蛋白-（Syt2）及其相互作用蛋白突触小泡蛋白2（SV2）B的表达水平与肌肉易损程度成比例降低，而其他突触蛋白，如 syntaxin-1B（Stx1B）和突触结合蛋白-7（Syt7），则不受影响。与大多数受影响的神经肌肉突触中两种Syt亚型和SV2B的极低水平一致，神经传递的功能分析显示诱发释放高度降低、短期可塑性改变、释放概率低以及无法正常调节功能性释放位点的数量。我们共同提出，Syt2和SV2B的强烈减少是功能性突触改变的关键因素，而Syt1的生理性下调在SMA的肌肉易损性中起决定性作用。
