Observations on the blood and perineurial permeability barriers of surviving nerve allografts in immunodeficient and immunosuppressed rats

Structured wound angiogenesis instructs mesenchymal barrier compartments in the regenerating nerve

Organ injury stimulates the formation of new capillaries to restore blood supply raising questions about the potential contribution of neoangiogenic vessel architecture to the healing process. Using single-cell mapping, we resolved the properties of endothelial cells that organize a polarized scaffold at the repair site of lesioned peripheral nerves. Transient reactivation of an embryonic guidance program is required to orient neovessels across the wound. Manipulation of this structured angiogenic response through genetic and pharmacological targeting of Plexin-D1/VEGF pathways within an early window of repair has long-term impact on configuration of the nerve stroma. Neovessels direct nerve-resident mesenchymal cells to mold a provisionary fibrotic scar by assembling an orderly system of stable barrier compartments that channel regenerating nerve fibers and shield them from the persistently leaky vasculature. Thus, guided and balanced repair angiogenesis enables the construction of a "bridge" microenvironment conducive for axon regrowth and homeostasis of the regenerated tissue.

Perspectives on optimizing local delivery of drugs to peripheral nerves using mathematical models

Drug therapies for treating peripheral nerve injury repair have shown significant promise in preclinical studies. Despite this, drug treatments are not used routinely clinically to treat patients with peripheral nerve injuries. Drugs delivered systemically are often associated with adverse effects to other tissues and organs; it remains challenging to predict the effective concentration needed at an injured nerve and the appropriate delivery strategy. Local drug delivery approaches are being developed to mitigate this, for example via injections or biomaterial-mediated release. We propose the integration of mathematical modeling into the development of local drug delivery protocols for peripheral nerve injury repair. Mathematical models have the potential to inform understanding of the different transport mechanisms at play, as well as quantitative predictions around the efficacy of individual local delivery protocols. We discuss existing approaches in the literature, including drawing from other research fields, and present a process for taking forward an integrated mathematical-experimental approach to accelerate local drug delivery approaches for peripheral nerve injury repair. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology Neurological Diseases > Computational Models Neurological Diseases > Biomedical Engineering.

Efficacy of Short-Term FK506 Administration on Accelerating Nerve Regeneration

Background. The slow rate of nerve regeneration following injury can cause extended muscle denervation, leading to irreversible muscle atrophy, fibrosis, and destruction of motor endplates. The immunosuppressant FK506 (tacrolimus) has been shown to accelerate the rate of nerve regeneration and functional recovery. However, the toxic and immunosuppressive properties of FK506 make it undesirable for long-term use. Objective. To take advantage of the regeneration-enhancing effects of FK506 but avoid the potential adverse effects of long-term administration, the current study evaluates and quantifies the efficacy of short-term FK506 treatment in rat models. Methods. Clinically relevant transection and graft models were evaluated, and walking track analysis (WTA) was used to evaluate functional recovery. FK506 was administered for 5 and 10 days post transection injury and 10 and 20 days post graft injury. Both groups involving a short course were compared with the continuous administration group. Results. In the transection model, FK506 was administered for 5 and 10 days postoperatively. WTA demonstrated that 10 days of FK506 administration was sufficient to reduce functional recovery time by 29% compared with negative controls. In the graft model, FK506 was administered for 10 and 20 days postoperatively. Short treatment courses of 10 and 20 days reduced recovery time by 15% and 21%, respectively, compared with negative controls. Analysis of blood–nerve barrier (BNB) integrity demonstrated that FK506 facilitated early reconstitution of the BNB. Conclusions. The results of this study indicate that short-term FK506 delivery following nerve injury imparts a significant therapeutic effect.

FK506 promotes functional recovery in crushed rat sciatic nerve

In this study we examine whether the systemic administration of FK506 or Cyclosporin A (CsA) expedited functional recovery following an axonotmetic nerve injury, and compared their effects in a rat model. Seventy-five adult Buffalo rats received a crush injury to the right posterior tibial nerve and subsequently underwent either no treatment (group I), daily injections of FK506 (group II), or daily injections of CsA (group III). Walking track analysis demonstrated return of hindlimb function by 20 days postoperatively in group I, 14 days in group II, and 18 days in group III. The blood-nerve barrier (BNB) was reconstituted by postoperative day (POD) 7 in both FK506- and CsA-treated animals and by POD 13 in control animals. These results suggest that recovery of function is more rapid with daily administration of FK506 than with CsA or no treatment, perhaps because of earlier restoration of the blood-nerve barrier. Agents that facilitate nerve regeneration have the potential to limit the extent of motor endplate loss and muscle atrophy seen with prolonged denervation, thereby limiting permanent functional loss.

Immunosuppressive effect of 15-deoxyspergualin applied to peripheral nerve allotransplantation in the rat

15-Deoxyspergualin (15-DSG), which has a unique immunosuppressive action, was applied to peripheral nerve allotransplantation. Its effects on graft survival were experimentally assessed using inbred rats. The sciatic nerve (20 mm) allotransplantation model was created in two different strains. An attempt was made to answer the following two questions: (1) can short-term immunosuppression alone produce sufficient immunological tolerance to maintain graft survival indefinitely? (2) can graft rejection be prevented by chronic intermittent low-dose 15-DSG administration (2.5 mg/kg/day), and to what extent does nerve regeneration occur? To evaluate the efficacy of 15-DSG, a comparison was made with autografts, allografts with no immunosuppression, and allografts immunosuppressed with cyclosporine (CsA), a strong immunosuppressant. The results indicate that short-term 15-DSG therapy is incapable of inducing immunotolerance of peripheral nerve allografts. Because nerve conduction in the rejected allografts was better preserved than in the CsA group, short-course 15-DSG therapy appeared to provide better results than CsA therapy for peripheral nerve allotransplantation.

Nerve regenerating effect of short-course administration of cyclosporine after fresh peripheral nerve allotransplantation in the rat: comparison of nerve regeneration using different forms of donor nerve allografts

There is almost universal agreement that if cyclosporine (CsA), which is a potent immunosuppressant, is temporarily administered after surgery, regenerated axons will be maintained even after withdrawal of CsA following peripheral nerve allotransplantation. Thus, this experimental study was conducted to investigate whether a difference in donor nerve form, including thickness and length, influences nerve regeneration after withdrawal of immunosuppression with CsA. The findings suggest that as a result of immunosuppression with CsA, large-diameter nerve grafts are better able to induce nerve regeneration than small-diameter grafts, and after withdrawal of the immunosuppressant, thick nerve grafts are also better able to preserve regenerated axons against the rejection reaction than thin grafts. With regard to the length of the grafted nerve, short nerve allografts yield higher axon counts than long ones, the same as with autografts. The best way to induce nerve regeneration appears to be to transplant a short, thick nerve allograft, which is definitely capable of inducing many regenerated axons.