Ethological and histopathological Characteristics of SCI model rats
We selected different critical time points to remove and collect scar tissue from SCI model rats (Fig. 1). We first checked the pathology characteristics after SCI by HE, Nissl, Tunel and immunofluorescence methods until Day 180 (Fig. 2). Tissue staining showed that a large number of cell apoptosis occurred within 24 hours after SCI, the cell bodies of neurons shrunk, and the number of Nissl bodies decreased rapidly. Nestin (GFAP confined the border of lesion) cells began to appear in the surrounding tissue of the injured area on the 3rd day after injury. On the 5th day after injury, a transparent scar tissue visible to the naked eye appeared in the tissue absent area. At this time, Tunel staining showed that the number of apoptotic cells reached a peak, and the number of Nestin+ cells increased significantly. On the 7th day after injury, some Nestin positive stained cells began to increase at the center of the injured area, and the maximum number was on the 10th day after injury. On the 15th day after injury, transparent scar formation with high toughness was visible to the naked eye. At this time, cell apoptosis stopped, and no Nestin and Tuj1 positive stained cells were found in the injured area. After 30 days, there was no significant changes in histopathological staining until the 180th day after the injury. During this period, the scar tissue began to shrink gradually, and the spinal cord tissue at the end of the injured area also began to shrink. HE staining showed that at the early stage of injury, the tissue was wrapped and entangled by fiber strips and isolated from the external environment. The fiber strips gradually present a grid-like structure, and later forms a cavity.
Resected the scar tissue of the SCI model rats can promotes functional recovery
According to the pathology characteristics of different stages after SCI. We selected the 7 time points – the 5th, 7th, 10th, 15th, 30th, 45th, 60th days – as the research time points for the scar resection. We grouped the SCI rats according to time points: Day 5 group, Day 7 group, Day 10 group, Day 15 group, Day 30 group, Day 45 group, Day 60 group, and the model rats without scar resection were the control group.
After the scar tissue was resected, the motor function of the rats began to show rapid recovery. The BBB results showed that significantly-obvious motor improvements in rats, whose scar tissue was resected within 30 days after SCI, especially those in Day 7, Day 10, and Day 15 groups (Fig. 3a).
The CMEP waveforms were detected in rats with SCI after excised scar tissue (Fig. 3b). The CSEP test results of rats in each group showed that N33 waves and P40 waves were tested in rats with scar tissue resected after 30 days, and the latent period of the main wave was delayed and smaller than normal (Fig. 3c).
The CMEP test results of the rats whose scar tissue was resected in each group showed that the peak-to-peak amplitude of P-N waves of the single excitation decreased by more than 80% of the normal value (Fig. 3d), but there were differences among the groups. On the 120th day after modeling, the latent period of the main wave of the left limbs of the rats in Day 7 group and Day 10 group was significantly shortened compared with the control group, and the latent period of the main wave of right limbs of the rats in Day 10 and Day 15 was significantly shortened (Fig. 3e). The peak- to-peak amplitude of P-N waves of the single excitation of the bilateral limbs showed the Day 10 group was higher than that of other groups (Fig. 3f).
Neuronal and axonal regeneration in the re-formed scar
In order to further verify and find out the reasons for the recovery of motor function of rats, we detected the neural regeneration after injury. The results showed that after the first scar tissue resection, Tuj1-positive neurons and axonal regeneration were found in the reformed scar tissue of each group. In contrast, no Tuj1-positive cells and no axons survival were found in the injured areas of the rats without scar tissue resection (Fig. 4, 5). In the re-formed scar tissues after scar resection, the number of Tuj1-positive cells (Fig. 3g) and regenerated axons also increased significantly, especially the total amount of axons in Day 7, Day 10 and Day 15 groups increased (Fig. 3h).
High-throughput sequencing analysis suggest that the recovery mechanism is related to changes in the scar tissue environment
To investigate the mechanisms involved in functional recovery, we selected scar tissues at 7th, 10th and 15th when motor function recovery was most significant and performed RNA-seq analysis. S27-Day 7 group, S210-Day 10 group and S215-Day 15 group is that after the scar tissue was resected according to the corresponding resection time point, and starting from the resection time point, the reformed scar tissue was collected after 15 days of continuous feeding; S230-Day 7 group, S220-Day 10 group and S210-Day 15 group is that the scar tissue of the model rats was resected on the 10th day after the establishment of the rat model, and starting from the resection operation time, the re-formed scar tissue was respectively collected after 7,10,15 days of continuous feeding (Fig. 1c).
Comparison is S17-Day 7 group, S110-Day 10 group, S115-Day 15 group, starting from the 1st day after modeling, the scar tissue at the corresponding time was collected on the 7th day, 10th day and 15th day respectively and then used as a comparison (Fig. 1c).
We compared the differential genes in the following groups: S27-S17; S210-S110; S215-S115; S230-S17; S220-S110; S210-S115 (Fig. 6a, b). The differential genes and significant degree in scar tissues before and after scars resection were the most obvious on the 7th day after SCI, and the number of the significant difference genes were the greatest and the difference was the most obvious. As time went by, the significant differential genes showed a rapidly reduction. GO enrichment analysis showed that the significantly differential genes were distributed in the pathways related to promoting recovery of nerve function such as synaptic function, axonal myelin, reducing irregular scars, reducing tissue damage, promoting angiogenesis, and promoting neuron regeneration, etc. (Fig. 7a). KEGG enrichment analysis showed that significantly differential genes were related to nervous system, immune system, synaptic function and PI3k-Akt signaling pathway (Fig. 7b, c). After family distribution of differential genes, functional genes were most significantly distributed in zf-C2H2, Homeobox, and bHLH families, and were significantly correlated with the expression of βⅢ-tubulin (Fig. 7d). These key genes jointly promoted the expression of βⅢ-tubulin by playing different roles. At the same time, it was found that Tubb3 and Tubb6 genes were significantly different in tissues before and after early scar resection, and maintaining the stability of microtubules played a key role in promoting axons regeneration. We used Venn diagram to analyze the shared differential genes having entered the stable phase and found that the immune system was involved throughout the process of promoting regeneration. The genes related to immunity and neutrophil regulation were significantly different in the group of promoting βⅢ-tubulin expression and these differential genes were significantly associated with the release of neutrophil extracellular traps.