Combining Constitutively Active Rheb Expression and Chondroitinase Promotes Functional Axonal Regeneration after Cervical Spinal Cord Injury

Abstract

After spinal cord injury (SCI), severed axons in the adult mammalian CNS are unable to mount a robust regenerative response. In addition, the glial scar at the lesion site further restricts the regenerative potential of axons. We hypothesized that a combinatorial approach coincidentally targeting these obstacles would promote axonal regeneration. We combined (1) transplantation of a growth-permissive peripheral nerve graft (PNG) into an incomplete, cervical lesion cavity; (2) transduction of neurons rostral to the SCI site to express constitutively active Rheb (caRheb; a Ras homolog enriched in brain), a GTPase that directly activates the growth-promoting pathway mammalian target of rapamycin (mTOR) via AAV-caRheb injection; and (3) digestion of growth-inhibitory chondroitin sulfate proteoglycans within the glial scar at the distal PNG interface using the bacterial enzyme chondroitinase ABC (ChABC). We found that expressing caRheb in neurons post-SCI results in modestly yet significantly more axons regenerating across a ChABC-treated distal graft interface into caudal spinal cord than either treatment alone. Excitingly, we found that caRheb+ChABC treatment significantly potentiates the formation of synapses in the host spinal cord and improves the animals' ability to use the affected forelimb. Thus, this combination strategy enhances functional axonal regeneration following a cervical SCI.

Keywords: Rheb; axon regeneration; chondroitinase; mTOR; peripheral nerve graft; spinal cord injury.

Figure 1

Quantification of Transduced Neurons within Spinal Cord and Brainstem following Intraspinal Injection of AAV-GFP or AAV-caRheb/GFP Rostral to a C2Hx GFP⁺ neurons in serially collected spinal cord and brain tissue sections from animals injected with either AAV5-GFP or AAV5-caRheb were manually counted. Many GFP⁺ neurons were found in spinal cord and different brainstem regions. The total numbers of GFP⁺ neurons in spinal cord (A), reticular formation (B), vestibular nuclei (C), and red nucleus (D) were compared for statistical significance between groups using Student’s t test. There was no significant different between groups in any of the regions (p > 0.05). Representative images from spinal cord sections from rats receiving injections of AAV-GFP (E–G) or AAV-caRheb (H–J) that were stained for GFP (green) and pS6 (red). Many AAV-GFP transduced neurons (E and G, arrows) do not show immunoreactivity for pS6 (F and G, arrows). The merged image confirms that neurons expressing GFP control do not express pS6 (G). On the other hand, many AAV-caRheb transduced neurons (H and J, arrowheads) have high levels of pS6 (I and J, arrowheads). Quantification of the percentage of transduced, GFP⁺ neurons in spinal cord that are also pS6⁺ (K). Data are shown as mean ± SEM, n = 4, ***p < 0.001. Scale bar represents 100 μm.

Figure 2

Retrograde Labeling of Neurons Projecting Axons into a Peripheral Nerve Graft In animals injected with AAV5-GFP or AAV5-caRheb, the retrograde tracer True Blue was applied on the exposed distal end of PNGs to retrogradely label neurons whose axons reached the end of the PNG. In both groups, TB⁺ neurons were identified in spinal cord gray matter (A), reticular formation (B), and vestibular nuclei (C). There was no significant difference in the number of TB⁺ neurons between groups in spinal cord and both brain regions. An image of a representative section shows that the majority of TB⁺ neurons in spinal cord rostral to the injury are located within the intermediate gray (D). Data are shown as mean ± SEM, n = 4. Scale bar represents 100 μm.

Figure 3

Quantification of Myelinated Axons in a PNG Segments of PN that were grafted into the C2Hx sites from animals transduced with AAV-GFP (A, n = 4) or AAV-caRheb (B, n = 3) were processed for semithin sectioning to assess the number of myelinated axons that grew into the transplant. Many myelinated axons were found to have extended into the PNG in both groups (indicated by arrowheads). There was no statistical difference in the number of myelinated axons in PNG between groups (C). Mean ± SEM. Scale bar represents 20 μm.

Figure 4

Axonal Regeneration Promoted by Combining caRheb Expression and ChABC Treatment Axons within the graft were labeled with BDA. Representative images of transverse sections containing the PNG in the C4DQ were staining for BDA (red) and GFAP (blue). In animals treated with either GFP+PBS (A) or caRheb+PBS (B), few BDA-labeled axons grew beyond the graft-host interface (the boundary is denoted by the dashed lines surrounding the GFAP⁻ peripheral nerve; quantified in E). On the other hand, significantly more BDA-labeled axons were found growing out of the PNG following ChABC treatment in GFP+ChABC animals (C and E). caRheb expression enhanced this ChABC-facilitated regeneration (D and E). Quantitative analysis revealed that both GFP+ChABC and caRheb+ChABC animals had significantly more axons within 200 μm distal to the PNG than animals treated with GFP+PBS or caRheb+PBS. Additionally, caRheb+ChABC animals had more axons than GFP+ChABC animals within 100 μm from the PNG-spinal cord interface (E). Data are shown as mean ± SEM; n = 4; *p < 0.05 and **p < 0.01, comparing ChABC-treated animals with PBS-treated animals; #p < 0.05, comparing caRheb+ChABC animals with GFP+ChABC animals. Scale bar represents 35 μm.

Figure 5

Induction of c-Fos in Host Neurons after Electrical Stimulation of Peripheral Nerve Graft After electrical stimulation specifically of the PNG bridge, transverse sections containing the PN apposed to a C4DQ were processed for immunohistochemistry to visualize c-Fos expression. Animals that were treated with AAV-GFP+PBS (A) or AAV-caRheb+PBS (B) had few c-Fos⁺ nuclei within gray matter. The inset boxes (A′ and B′) show high-magnification images of the boxed areas in (A) and (B). Arrowheads point out the c-Fos⁺ nuclei in these sections. Significantly more c-Fos⁺ nuclei were observed in host spinal cord tissue in AAV-GFP+ChABC-treated animals (C). Combining caRheb expression and ChABC treatment of the CSPG further increased the number of c-Fos⁺ nuclei in spinal cord tissue ventral to the PNG (D). High-magnification image of boxed areas in (C) and (D) are shown in (C′) and (D′). Quantification and the result of one-way ANOVA of the number of c-Fos⁺ nuclei in each group are shown in (E) (*p < 0.05, **p < 0.01; error bars represent SEM). The numbers of c-Fos⁺ nuclei were binned to 100 μm intervals from the graft. (F) ChABC treatment increased the number of c-Fos⁺ nuclei close to the graft interface, and caRheb expression further augmented this increase. Data are shown as mean ± SEM; n = 4; *p < 0.05 and **p < 0.01, comparing ChABC-treated animals with PBS-treated animals; #p < 0.05 and ##p < 0.01, comparing caRheb+ChABC-treated animals with GFP+ChABC-treated animals. Scale bar represents 250 μm.

Figure 6

Forelimb Function Analyses after C2Hx Injuries (A–C) The ability of C2Hx/C4DQ-injured animals to use their affected, ipsilateral forepaws to groom after AAV5-GFP+PBS, AAV5-caRheb+PBS, AAV5-GFP+ChABC, or AAV5-caRheb+ChABC treatment was assessed. The grooming test has a score range from 0–5, with 5 indicating full grooming capability. The C2Hx/C4DQ injuries detrimentally affected the animals’ ability to use the ipsilateral forelimb to groom. The average scores were normalized and then compared between animal groups and across time points with a two-way ANOVA. At week 20, caRheb+ChABC animals had a significantly higher score than the other groups, including GFP+ChABC animals (A) (n = 8, *p < 0.05). Grooming scores for individual animals at week 20 (B) and week 24 (C) (the last time point) are shown. The only animals that reached a score ≥4 after injury received both AAV5-caRheb and ChABC treatments. (D) Locomotor recovery analyzed using the CatWalk system revealed that by week 24, animals that received caRheb+ChABC had higher maximum contact maximum intensity than animals in any of the other groups. This indicates that the ipsilateral forepaw of AAV5-caRheb+ChABC animals has more contact with the runway because the animals are putting more weight on their injured forepaws when the injured paws have maximum contact with the glass (C). Data shown as mean ± SEM, n = 8, *p < 0.05.