High Expression Of Syncytin-1 In Skeletal Muscles Retrogradely Damage Spinal Cord Anterior Horn Motor Neurons

The pathogeneses of motor neuron disease(MND) are unclear. RNA transcription and metabolic abnormalities, oxidative stress, glutamate toxicity, neurofilament accumulation and axonal transport disorder, protein misfolding, mitochondrial dysfunction, viral infections, poisoning, autoimmune mechanisms and environmental factors may be involved in the pathogeneses of MND. The glycoprotein syncytin-1, which is encoded by human endogenous retrovirus W envelope gene, was abnormally high expressed in the atrophic muscles of patients suffered from MND, but the correlation between the abnormal expression of syncytin-1 in the skeletal muscles and the impairment of anterior horn motor neurons of MND was uncertain. Syncytin-1 has many pathophysiological effects. It specifically high expressed in the placental syncytiotrophoblast, promoting cells fusion. It is involved in tumor formation. It mediates inflammatory cascade, thereby reduces the expression of myelin proteins and inpairs myelins and neurons. It is involved in the pathogeneses of multiple sclerosis(MS).Objective: To investigate the impairment of spinal cord anterior horn motor neurons retrogradely induced by abnormally high expression of endogenous retrovirus glycoprotein syncytin-1 in the mice skeletal muscles.Methods: Eight weeks old C57BL/6J male mice were implemented anesthesia with 10% chloral hydrate(each 300 mg with the amount of 1ml). The left anterior tibial muscle was exposed, and 200?l recombinant plasmid pCMV-tag2B-syncytin(1?g/?l) was injected with a 250?l microsyringe. Empty vector pCMV-tag2B(200?l, 1?g/?l) was used as control. The plasmids got from our previous studies. After the injection of plasmids, the weight of the mice was measured and recorded every four weeks. After the injection of plasmids, walking footprints and climbing barbed wire experiments were examined in all mice to evaluate the motor function on the 7, 28, 47, 80, 100, and 120 days. On the 120 days after the injection of plasmids, row concentric needle electrode EMG were measured in the gastrocnemius muscles for the resting potential, the light contraction potential, and the potential of strong contraction. Nerve conduction studies were measured for the motor nerve conduction velocity and F wave. The lumbar spinal cord and the anterior tibial muscle were collected on 120 days after the injection of plasmids. The number and morphology of motor neurons in the lumbar spinal cord anterior horn were observed with Nissl staining. The activation of astrocytes and microglia in the lumbar enlargement of spinal cord anterior horn were observed with immunohistochemistry staining and immunofluorescence staining. The protein levels of syncytin-1, sodium-dependent neutral amino acid transporter(ASCT1), inducible nitric oxide synthase(iNOS), tumornecrosis factors-?(TNF-?), and vascular endothelial growth factor(VEGF) in the skeletal muscles were detected by using of Western blot. Meantime, the mRNA levels of syncytin-1, ASCT1, iNOS, TNF-?, and VEGF were measured by RT-PCR.Results: Compared with the control group, the weight of mice in the experimental group had no obvious changes. Walking footprints and climbing barbed wire test in the experimental group did not tell obvious differences on the 7, 28, 47, 80, 100, and 120 days after the injection of plasmids. On the 120 days after the injection of plasmids, the electromyography of the gastrocnemius muscles of the mice in the experimental group showed neurogenic damage. Fibrillation potentials were shown at rest. Increased amplitude and extended time limit of MUAP were shown when the muscles light contracted. Denervation showing as simple phases was observed in the strong contracted muscles. Nerve conduction studies showed slowing down motor nerve conduction velocity. The detection rates of F wave declined. Nissl staining showed reduced motor neurons and atrophic morphology of motor neurons in the lumbar spinal anterior horn of the mice in experimental group. Immunohistochemistry and immunofluorescence staining showed the activation and increased numbers of astrocytes and microglia in the gray matter of the lumbar spinal cord anterior horn. The protein and mRNA levels of syncytin-1, iNOS, TNF-?, VEGF were increased in the skeletal muscles of the mice in experimental group, whereas the protein and mRNA levels of ASCT1 were decreased.Conclusion: 1. The motor functions of the mice, which were implanted with the recombinant plasmid expressing syncytin-1 in the skeletal muscles, had not been significantly impaired when the mice were observed as long as 120 days. 2. Excessive expression of syncytin-1 in the skeletal muscles of mice could induce the degeneration and deletions of motor neurons in enlargement of lumbar spinal anterior horn, at the meantime, activate astrocytes and microglia. Finally, neurogenic damage in the skeletal muscles appeared. 3. Abnormally high expression of syncytin-1 in the skeletal muscles of mice could lead to lower expression of amino acid transporter ASCT1, causing toxic effects of excitatory amino acids. Abnormally high expression of syncytin-1 in the skeletal muscles of mice could mediate the inflammatory cascade, causing increased release of the inflammatory factors iNOS and TNF-?, which may involve in the pathogenic mechanisms of the injury of spinal cord anterior horn motor neurons. Abnormally high expression of syncytin-1 in the skeletal muscles of mice could lead to increased production of the neuroprotective factor VEGF, which still was insufficient to prevent the injury of motor neurons.