Effects of Akt/mTOR/p70S6K Signaling Pathway Regulation on Neuron Remodeling Caused by Translocation Repair

Peripheral nerve injury repair has been considered a difficult problem in the field of trauma for a long time. Conventional surgical methods are not applicable in some special types of nerve injury, prompting scholars to seek to develop more effective nerve translocation repair technologies. The purpose of this study was to explore the functional state of neurons in injured lower limbs after translocation repair, with a view to preliminarily clarify the molecular mechanisms underlying this process. Eighteen Sprague–Dawley rats were divided into the normal, tibial nerve in situ repair, and common peroneal nerve transposition repair tibial nerve groups. Nerve function assessment and immunohistochemical staining of neurofilament 200 (NF-200), protein kinase B (Akt), mammalian target of rapamycin (mTOR), and ribosomal protein S6 kinase (p70S6K) in the dorsal root ganglia were performed at 12 weeks after surgery. Tibial nerve function and neuroelectrophysiological analysis, osmic acid staining, muscle strength testing, and muscle fiber staining showed that the nerve translocation repair could restore the function of the recipient nerve to a certain extent; however, the repair was not as efficient as the in situ repair. Immunohistochemical staining showed that the translocation repair resulted in changes in the microstructure of neuronal cell bodies, and the expressions of Akt, mTOR, and p70S6K in the three dorsal root ganglia groups were significantly different (p < 0.05). This study demonstrates that the nerve translocation repair technology sets up a new reflex loop, with the corresponding neuroskeletal adjustments, in which, donor neurons dominate the recipient nerves. This indicates that nerve translocation repair technology can lead to neuronal remodeling and is important as a supplementary treatment for a peripheral nerve injury. Furthermore, the Akt/mTOR/p70S6K signaling pathway may be involved in the formation of the new neural reflex loop created as a result of the translocation repair.

[Experimental study of the effect of the sciatic nerve elongation on pain in rats]

OBJECTIVE

To investigate the effect of the sciatic nerve elongation on pain in rats.

METHODS

Thirty-six adult male Wistar rats of SPF grade, weighing 250-300 g. Eighteen of them were randomly divided into 3 groups, 6 rats in each group. They were sciatic nerve elongation group (group A), nerve no-elongation group (group B), and nerve ligation group (group C). The model of 10-mm sciatic nerve defect was established in all 3 groups. The sciatic nerve was extended at a speed of 1 mm/d for 14 days in group A. The group B was only installed with external fixation. The nerve stumps were ligated in the group C. At 3, 7, 10, and 14 days after operation, the foot injury was evaluated by the autotomy scoring scale. At 14 days after operation, the dorsal root ganglia (DRG) of L 4-S 1 spinal cord of rats in each group was observed by tumor necrosis factor α (TNF-α) immunohistochemical staining, and the primary antibodies were replaced by pure serum as negative control group. Another 18 rats were randomly divided into 3 groups, 6 rats in each group. They were sciatic nerve elongation group (group A1), nerve no-elongation group (group B1), positive control group (group C1). In groups A1 and B1, the 10-mm long sciatic nerve defect model was established by the same method as groups A and B, and then fixed with external fixation. Nerve elongation was done or not done without anesthesia at 3 days after operation. In group C1, no modeling was done and 20 μL 2.5% formaldehyde was injected into the toes. After 90 minutes, the dorsal horn of spinal cord of L 4-S 1 segment of rats was cutting for c-Fos immunohistochemical staining and the number of positive cells was counted. Primary antibodies were replaced with pure serum as negative control group.

RESULTS

The autotomy scores of rats in groups B and C gradually increased postoperatively, and group A remained stable at 0.25±0.50. The scores of group C were significantly higher than those of group A and group B at each time point postoperatively ( P<0.05). The scores of group A were significantly lower than those of group B at 10 and 14 days postoperatively ( P<0.05). TNF-α immunohistochemical staining showed that the TNF-α expression in group A was weak, slightly positive (+/-); in group B was positive (+); in group C was strongly positive (++); and the negative control group had no TNF-α expression (-). c-Fos immunohistochemical staining showed that the c-Fos expressions in groups A1 and B1 were weak positive, in group C1 was strong positive, and negative control group had no c-Fos positive expression. The number of c-Fos positive cells in groups A1, B1, C1, and negative control group were (21.5±6.6), (19.3±8.1), (95.6±7.4), and 0 cells/field, respectively, and group C1 was significantly higher than groups A1 and B1 ( P<0.05), there was no significant difference between group A1 and group B1 ( P>0.05).

CONCLUSION

Nerve elongation does not cause obvious pain neither during the operation of elongation nor throughout the whole elongation.

Reinnervation of spinal cord anterior horn cells after median nerve repair using transposition with other nerves

Our previous studies have confirmed that during nerve transposition repair to injured peripheral nerves, the regenerated nerve fibers of motor neurons in the anterior horn of the spinal cord can effectively repair distal nerve and target muscle tissue and restore muscle motor function. To observe the effect of nerve regeneration and motor function recovery after several types of nerve transposition for median nerve defect (2 mm), 30 Sprague-Dawley rats were randomly divided into sham operation group, epineurial neurorrhaphy group, musculocutaneous nerve transposition group, medial pectoral nerve transposition group, and radial nerve muscular branch transposition group. Three months after nerve repair, the wrist flexion test was used to evaluate the recovery of wrist flexion after regeneration of median nerve in the affected limbs of rats. The number of myelinated nerve fibers, the thickness of myelin sheath, the diameter of axons and the cross-sectional area of axons in the proximal and distal segments of the repaired nerves were measured by osmic acid staining. The ratio of newly produced distal myelinated nerve fibers to the number of proximal myelinated nerve fibers was calculated. Wet weights of the flexor digitorum superficialis muscles were measured. Muscle fiber morphology was detected using hematoxylin-eosin staining. The cross-sectional area of muscle fibers was calculated to assess the recovery of muscles. Results showed that wrist flexion function was restored, and the nerve grew into the distal effector in all three nerve transposition groups and the epineurial neurorrhaphy group. There were differences in the number of myelinated nerve fibers in each group. The magnification of proximal to distal nerves was 1.80, 3.00, 2.50, and 3.12 in epineurial neurorrhaphy group, musculocutaneous nerve transposition group, medial pectoral nerve transposition group, and radial nerve muscular branch transposition group, respectively. Nevertheless, axon diameters of new nerve fibers, cross-sectional areas of axons, thicknesses of myelin sheath, wet weights of flexor digitorum superficialis muscle and cross-sectional areas of muscle fibers of all three groups of donor nerves from different anterior horn motor neurons after nerve transposition were similar to those in the epineurial neurorrhaphy group. Our findings indicate that donor nerve translocation from different anterior horn motor neurons can effectively repair the target organs innervated by the median nerve. The corresponding spinal anterior horn motor neurons obtain functional reinnervation and achieve some degree of motor function in the affected limbs.