The effects of cold preservation and subimmunosuppressive doses of FK506 on axonal regeneration in murine peripheral nerve isografts

Interpretation of Data from Translational Rodent Nerve Injury and Repair Models

This article highlights the use of rodents as preclinical models to evaluate the management of nerve injuries, describing the pitfalls and value from rodent nerve injury and regeneration outcomes, as well as treatments derived from these rodent models. The anatomic structure, size, and cellular and molecular differences and similarities between rodent and human nerves are summarized. Specific examples of success and failure when assessing outcome metrics are presented for context. Evidence for translation to clinical practice includes the topics of electrical stimulation, Tacrolimus (FK506), and acellular nerve allografts.

Local delivery of FK506 to a nerve allograft is comparable to systemic delivery at suppressing allogeneic graft rejection

The objective of this study was to determine if locally delivered FK506 could prevent allogeneic nerve graft rejection long enough to allow axon regeneration to pass through the nerve graft. An 8mm mouse sciatic nerve gap injury repaired with a nerve allograft was used to assess the effectiveness of local FK506 immunosuppressive therapy. FK506-loaded poly(lactide-co-caprolactone) nerve conduits were used to provide sustained local FK506 delivery to nerve allografts. Continuous and temporary systemic FK506 therapy to nerve allografts, and autograft repair were used as control groups. Serial assessment of inflammatory cell and CD4+ cell infiltration into the nerve graft tissue was performed to characterize the immune response over time. Nerve regeneration and functional recovery was serially assessed by nerve histomorphometry, gastrocnemius muscle mass recovery, and the ladder rung skilled locomotion assay. At the end of the study, week 16, all the groups had similar levels of inflammatory cell infiltration. The local FK506 and continuous systemic FK506 groups had similar levels of CD4+ cell infiltration, however, it was significantly greater than the autograft control. In terms of nerve histmorphometry, the local FK506 and continunous systemic FK506 groups had similar amounts of myelinated axons, although they were significantly lower than the autograft and temporary systemic FK506 group. The autograft had significantly greater muscle mass recovery than all the other groups. In the ladder rung assay, the autograft, local FK506, and continuous systemic FK506 had similar levels of skilled locomotion performance, whereas the temporary systemic FK506 group had significanty better performance than all the other groups. The results of this study suggest that local delivery of FK506 can provide comparable immunosuppression and nerve regeneration outcomes as systemically delivered FK506.

Local FK506 drug delivery enhances nerve regeneration through fresh, unprocessed peripheral nerve allografts

OBJECTIVE

Nerve allografts offer many advantages in the reconstruction of peripheral nerve gaps: they retain their native microstructure, contain pro-regenerative Schwann cells, are widely available, and avoid donor site morbidity. Unfortunately, clinical use of nerve allografts is limited by the need for systemic immunosuppression and its adverse effects. To eliminate the toxicity of the systemic immunosuppressant FK506, we developed a local FK506 drug delivery system (DDS) to provide drug release over 28 days. The study objective was to investigate if the local FK506 DDS enhances nerve regeneration in a rodent model of nerve gap defect reconstruction with immunologically-disparate nerve allografts.

METHODS

In male Lewis rats, a common peroneal nerve gap defect was reconstructed with either a 20 mm nerve isograft from a donor Lewis rat or a 20 mm fresh, unprocessed nerve allograft from an immunologically incompatible donor ACI rat. After 4 weeks of survival, nerve regeneration was evaluated using retrograde neuronal labelling, quantitative histomorphometry, and serum cytokine profile.

RESULTS

Treatment with both systemic FK506 and the local FK506 DDS significantly improved motor and sensory neuronal regeneration, as well as histomorphometric indices including myelinated axon number. Rats with nerve allografts treated with either systemic or local FK506 had significantly reduced serum concentrations of the pro-inflammatory cytokine IL-12 compared to untreated vehicle control rats with nerve allografts. Serum FK506 levels were undetectable in rats with local FK506 DDS.

INTERPRETATION

The local FK506 DDS improved motor and sensory nerve regeneration through fresh nerve allografts to a level equal to that of either systemic FK506 or nerve isografting. This treatment may be clinically translatable in peripheral nerve reconstruction or vascularized composite allotransplantation.

Neuroregenerative effects of polyethylene glycol and FK-506 in a rat model of sciatic nerve injury

The recently introduced polyethylene glycol (PEG) treatment restores axonal continuity after nerve injury, leading to rapid recovery of nerve function. The impact of PEG therapy on neuroregeneration has not yet been compared with any intervention with an established proneuroregenerative potential. FK-506 is an immunosuppressive agent with documented proneuroregenerative potential in nerve injury models. The aim of this study was to compare the effects of PEG therapy and preinjury FK-506 administration in rats with sciatic nerve transection injury. Four groups of male Sprague Dawley rats (seven per group) underwent sciatic nerve transection with primary repair. Group A received placebo injections, group B placebo injections and PEG treatment, group C FK-506 injections, and group D both FK-506 injections and PEG treatment. Clinical outcomes were assessed by the skin prick test and Sciatic Functional Index (SFI). Regenerated nerves underwent histomorphometric analysis. The histomorphometric analysis demonstrated that compared with the controls, nerve specimens from all treated groups showed signs of enhanced neuroregeneration (higher mean axonal area) (p < 0.001). The histomorphometric parameters for group D (PEG + FK-506), mean axonal area (p < 0.001) and axonal count (p > 0.05), were significantly better than those in the other study groups. The Form factor was closest to its optimal values in group B (p < 0.0001). At the end of the study, mean skin prick test scores in all treated groups were significantly higher than those in controls (p > 0.05). During the first postoperative week, PEG-treated rats (groups B and D) presented with higher values of the SFI than animals from groups A and C, but the difference was not statistically significant. Combined therapy with PEG and FK-506 seems to produce better neuroregeneration outcomes than a simple suture-based repair complemented with either PEG or FK-506 treatment.

Evaluation of peripheral nerve regeneration via in vivo serial transcutaneous imaging using transgenic Thy1-YFP mice

This study uses the saphenous nerve crush model in Thy1-YFP mice and serial transcutaneous imaging to evaluate the rate of nerve regeneration under various FK-506 (tacrolimus) dosing regimens and in the presence of transgenic overexpression of glial cell line-derived neurotrophic factor (GDNF). Thy1-YFP transgenic mice received saphenous nerve crush and were monitored for axonal regeneration via transcutaneous imaging for 7 days. Group A received no FK-506. Groups B and C received FK-506 at 2 or 0.5 mg/kg/day, starting three days before injury (preload). Groups D and E received FK-506 at 2 or 0.5 mg/kg/day, starting on the day of injury. Group F consisted of double transgenic mice with central overexpression of GDNF by CNS astrocytes (GFAP-GDNF/Thy1-YFP). Length and rate of axonal regeneration were measured and calculated over time. Regardless of concentration, FK-506 preload (Groups B and C) improved length and rate of axonal outgrowth compared with controls (Group A) and no preload (Groups D and E). Surprisingly, central overexpression of GDNF (GFAP-GDNF) delayed and stunted axonal outgrowth. Saphenous nerve crush in Thy1-YFP mice represents a viable model for timely evaluation of therapeutic strategies affecting the rate of nerve regeneration. FK-506 administered three days prior to injury accelerates axonal regeneration beyond injury conditioned regeneration alone and may serve as a reliable positive control for the model. GDNF overexpression in the CNS impedes early axonal outgrowth.

Treatment modality affects allograft-derived Schwann cell phenotype and myelinating capacity

We used peripheral nerve allografts, already employed clinically to reconstruct devastating peripheral nerve injuries, to study Schwann cell (SC) plasticity in adult mice. By modulating the allograft treatment modality we were able to study migratory, denervated, rejecting, and reinnervated phenotypes in transgenic mice whose SCs expressed GFP under regulatory elements of either the S100b (S100-GFP) or nestin (Nestin-GFP) promoters. Well-differentiated SCs strongly expressed S100-GFP, while Nestin-GFP expression was stimulated by denervation, and in some cases, axons were constitutively labeled with CFP to enable in vivo imaging. Serial imaging of these mice demonstrated that untreated allografts were rejected within 20 days. Cold preserved (CP) allografts required an initial phase of SC migration that preceded axonal regeneration thus delaying myelination and maturation of the SC phenotype. Mice immunosuppressed with FK506 demonstrated mild subacute rejection, but the most robust regeneration of myelinated and unmyelinated axons and motor endplate reinnervation. While characterized by fewer regenerating axons, mice treated with the co-stimulatory blockade (CSB) agents anti-CD40L mAb and CTLAIg-4 demonstrated virtually no graft rejection during the 28 day experiment, and had significant increases in myelination, connexin-32 expression, and Akt phosphorylation compared with any other group. These results indicate that even with SC rejection, nerve regeneration can occur to some degree, particularly with FK506 treatment. However, we found that co-stimulatory blockade facilitate optimal myelin formation and maturation of SCs as indicated by protein expression of myelin basic protein (MBP), connexin-32 and phospho-Akt.