After peripheral axotomy, there is a selective retraction of synaptic terminals in contact with injured motoneurons. This process, which actively involves glial cells, is influenced by the expression of immune-related molecules. Since toll-like receptors (TLRs) are upregulated by astrocytes and microglia following lesions, they might be involved in synaptic plasticity processes. Therefore, we administered lipopolysaccharide (LPS) to enhance TLR4 expression in mice and studied retrograde changes in the spinal cord ventral horn following sciatic nerve crush. To this end, adult C57BL/6J male mice were subjected to unilateral sciatic nerve crush at the mid-thigh level and, after a survival time of seven and forty days (acute and chronic phases, respectively), the spinal cords were paraformaldehyde-fixed and dissected out for immunolabeling for synaptophysin, glial fibrillary acidic protein (GFAP) and ionized calcium-binding adapter molecule 1 (Iba1). The results show that TLR4 upregulation leads to synaptophysin downregulation close to spinal motoneuron cell bodies, indicating increased synaptic elimination. LPS exposure also further increases astrogliosis and microglial reactions in the both ventral and dorsal horns, especially ipsilateral to nerve axotomy, compared to those in untreated mice. Notably, LPS administration to TLR4-/- mice produces results similar to those observed in untreated wild-type counterparts, reinforcing the role of this receptor in the glial response to injury. Therefore, our results suggest that the overexpression of the TLR4 receptor results in augmented astrogliosis/microglial reactions and the excessive loss of synapses postinjury, which may, in turn, affect the motoneuronal regenerative response and functionality. Additionally, treatment with LPS increases the expression of β2-microglobulin, a subcomponent of MHC I. Importantly, the absence of TLR4 results in imbalanced axonal regeneration, inducing subsequent improvements and setbacks. In conclusion, our results show the involvement of TLR4 in the process of synaptic remodeling, indicating a new target for future research aimed at developing therapies for CNS and PNS repair.

中文翻译：

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Toll样受体4（TLR4）影响脊髓中的神经胶质反应和周围神经挤压后对损伤的神经反应。

周围轴突切开后，与受伤的运动神经元接触的突触末端选择性退缩。该过程积极涉及神经胶质细胞，受到免疫相关分子表达的影响。由于损伤后星形胶质细胞和小胶质细胞上调了Toll样受体（TLR），因此它们可能参与突触可塑性过程。因此，我们给予脂多糖（LPS）增强小鼠中TLR4的表达，并研究了坐骨神经挤压后脊髓腹角的逆行变化。为此，在大腿中部水平对成年C57BL / 6J雄性小鼠进行单侧坐骨神经挤压，并在​​其存活时间分别为7天和40天（分别为急性和慢性期）之后，将脊髓固定在低聚甲醛中，解剖后进行免疫标记，以进行突触素，胶质原纤维酸性蛋白（GFAP）和离子化钙结合衔接子分子1（Iba1）的免疫标记。结果表明，TLR4上调导致接近脊髓运动神经元细胞体的突触素下调，表明突触消除增加。与未治疗的小鼠相比，LPS暴露还进一步增加了腹角和背角的星形胶质化和小胶质细胞反应，尤其是在神经切断术的同侧。值得注意的是，向TLR4-/-小鼠施用LPS产生的结果类似于在未处理的野生型对应物中观察到的结果，从而增强了该受体在神经胶质对损伤的反应中的作用。所以，我们的研究结果表明，TLR4受体的过表达导致星形胶质细胞增生/小胶质细胞反应增强，损伤后突触过度丧失，这可能反过来影响动子神经元的再生反应和功能。另外，用LPS治疗可增加β2-微球蛋白（MHC I的一个亚成分）的表达。重要的是，不存在TLR4会导致轴突再生失衡，从而导致随后的改善和挫折。总之，我们的结果表明TLR4参与突触重塑过程，为旨在开发CNS和PNS修复疗法的未来研究提供了新的目标。LPS治疗可增加MHC I的一个亚成分β2-微球蛋白的表达。重要的是，不存在TLR4会导致轴突再生失衡，从而导致随后的改善和挫折。总之，我们的结果表明TLR4参与突触重塑过程，为旨在开发CNS和PNS修复疗法的未来研究提供了新的目标。LPS治疗可增加MHC I的一个亚成分β2-微球蛋白的表达。重要的是，不存在TLR4会导致轴突再生失衡，从而导致随后的改善和挫折。总之，我们的结果表明TLR4参与突触重塑过程，为旨在开发CNS和PNS修复疗法的未来研究提供了新的目标。