Traction injuries to the proximal brachial plexus frequently involve nerve root avulsion, and regenerating nerves must grow across considerable distances to achieve reinnervation of the target tissues. Restoration of biceps function in patients with superior trunk and upper spinal nerve root lesions at C5 and C6 is a priority in the presence of brachial plexus injury. This functional restoration improves morbidity considerably. In the Oberlin transfer, the ulnar nerve is used to reinnervate the biceps muscle.22 The nerve transfer technique takes advantage of neuroplasticity, and a functionally redundant nerve is used as a donor for another function.23 The free end of the donor nerve is connected near the terminal motor branches of the injured nerve. This effectively converts a proximal nerve lesion into a distal nerve lesion, leading to notable reduction in the time and distance required for regrowth. When nerve transfer is performed correctly, donor site deficits are minimal, and invaluable functions are regained in the affected limb.
Existing treatments of peripheral nerve injuries offer limited capacity to effect true functional neural regeneration. Challenges with direct nerve repair include limited autograft material, poor or absent scaffolding, dense scar formation, misrouting of the growth cone, and end-organ atrophy. The use of autograft to repair segmental nerve defects induces donor site morbidity, and insufficient donor material may be available (Figure 5). Use of axonal guidance channels is an alternative to autograft. Currently, use of these channels is reserved for short segmental defects in sensory nerves.24 Studies on segmental nerve defects have revealed the existence of critical nerve gap length where the efficacy of tube conduits declines.25 Studies are under way to explore the use of various biomaterials within axonal guidance channels to improve peripheral nerve repair.26
Denervation atrophy of target tissues is one of the most significant issues associated with peripheral nerve repair. In adults, neural regeneration occurs at the rate of 1 to 3 mm per day.27 For proximal peripheral nerve lesions, the nerve must cover substantial distances, and adipose and fibrous changes in the denervated muscle are required for the nerve to reach its target.28 Prolonged target deprivation drastically reduces the ability of motor neurons to regenerate and causes Schwann cells associated with the target tissues to lose their growth-supportive phenotypes. 29–31 End-organ atrophy is addressed somewhat with nerve transfer. However, even the most successful nerve transfer leads to only partial restoration of muscle function, and preinjury strength is never fully recovered.32 Alternative treatment methods at the molecular and cellular levels have been proposed to improve peripheral nerve repair.
Increasing the rate of neural regeneration is the goal with the most widely studied therapeutic strategies. Numerous studies have established the role of neurotrophic factors, such as nerve growth factor and glial cell line-derived neurotrophic factor, in the development and regrowth of peripheral nerves; failure to sustain their levels has been implicated in poor neural regeneration.33–35 Thus, neurotrophic factor supplementation of axonal guidance channels could promote and improve the rate of regeneration after peripheral nerve injury. One proposed method involves Schwann cells.36 Schwann cells are critical to regenerative processes, such as wallerian degeneration, and these cells produce and secrete neurotrophic factors. Adipose-derived, skin-derived, and mesenchymal stem cells have been successfully isolated, differentiated, and used as Schwann cell precursors in peripheral nerve repair.37–39 Stem cells provide prompt and abundant sources of Schwann cells without the complications of graft rejection and allograft immunosensitivity. Bone marrow stromal cells can also produce neurotrophic factors, but harvest of these cells involves a more invasive process.40–42
These attempts at improving on peripheral nerve regeneration have largely been experimental. Furthermore, increasing the rate of neural regeneration does not sufficiently address the crucial issue of end-organ atrophy.
The neuromuscular junction (NMJ) is the interface between the peripheral nerves and muscle at which critical actions occur. The NMJ is composed of three cellular constituents: the terminal branch of the motor axon, terminal Schwann cells (TSCs), and muscle fibers containing acetylcholine receptors (AChRs) at the motor end plates43 (Figure 6). Through this interface, nerves transmit signals for muscle contraction and provide trophic support to target tissues. Loss of this support during peripheral nerve injury induces end-organ changes through an underlying mechanism that is not fully understood. Stabilization of the NMJ may reduce the rate of end-organ atrophy after major nerve injury, resulting in better functional outcomes with peripheral nerve repair.
Major nerve injury induces distinct morphologic changes at the NMJ. Dispersion of AChR clusters is a key feature of acute peripheral nerve injury44 (Figure 7). Agrin has been identified as a critical regulator of AChR cluster maintenance and formation45 (Figure 6, A). Agrin is a glycoprotein that is synthesized and released by the distal nerve terminal.46 During synaptogenesis, agrin is responsible for the conversion of growth cones into presynaptic terminals by inducing tyrosine phosphorylation of its postsynaptic receptor, muscle-specific kinase.47 Decreased levels of agrin have been associated with poor AChR cluster formation, and the loss of agrin after nerve injury may significantly contribute to the dispersion of AChR clusters48 (Figure 7, B and C). Thus, increasing local levels of agrin can potentially preserve the integrity of AChR clusters and reduce end-organ changes following peripheral nerve injury.
In addition to activating muscle-specific kinase, agrin maintains NMJ stability through critical interactions with components of motor end plate extracellular matrix, such as laminin and α-dystroglycan.49,50 Matrix metalloproteinase-3 (MMP-3) is a serine protease responsible for the degradation of agrin in the NMJ extracellular matrix. Levels of MMP-3 increase dramatically following peripheral nerve injury, leading to significantly reduced levels of agrin.51 Transgenic mice that lack MMP-3 have morphologically larger and more efficient end plates. Thus, pharmacologic inhibition of MMP-3 and increased levels of agrin could feasibly stabilize the NMJ and diminish end-organ atrophy subsequent to major nerve injury.
Change in AChR remodeling is also observed at the NMJ following acute peripheral nerve injury. The half-life of AChRs decreases from 10 days prior to denervation to 1 day after denervation; the mechanisms controlling AChR turnover are more directly dependent on the presence of the nerve than on the clustering of AChRs.52 The presence of the distal nerve stump can provide a therapeutic advantage by increasing AChR half-life and conserving AChR end plates.53 During acute nerve injury, the distal nerve stump immediately undergoes wallerian degeneration, but in slow wallerian degeneration mice, the process is delayed up to 2 weeks.54 This phenotype is attributed to a fusion protein product of ubiquitination factor E4B and nicotinamide mononucleotide adenylyltransferase-1; however, the mechanism of deferred wallerian degeneration is uncertain. Further study of these unique transgenic mice will provide insight into maintaining trophic support provided by the distal nerve stump and reducing changes in the denervated muscle.
Another potential therapeutic intervention makes use of TSCs and their role in the regenerative processes following peripheral nerve injury. TSCs are nonmyelinating glial cells associated with the distal nerve terminal and, as with their counterparts found outside the NMJ, TSCs provide trophic support to their environment. 55 TSCs modulate nerve growth throughout development and are actively involved in NMJ regeneration. 56 During nerve repair, TSCs extend processes and facilitate regrowth of motor axons to the muscles. 57 TSCs organize the NMJ for accepting and guiding the regenerating growth cone to sites of denervation. Much like Schwann cell supplementation of axonal guidance channels, Schwann cell precursors could also be differentiated into proregenerative TSCs and supplemented at the NMJ to promote successful reinnervation and peripheral nerve repair.
Major nerve injuries are severely debilitating, and no effective treatment option exists for functional neural regeneration. Surgical advancements in connective tissue reconstruction and nerve transfer have reached a plateau, and new treatments are required to improve outcomes. Attempts to increase the rate of nerve regrowth have been unsuccessful in resolving the limitations of peripheral nerve repair. A combination of multiple therapeutic approaches will be required to support successful neural regeneration.
Stabilization of the NMJ could potentially reduce end-organ atrophy following peripheral nerve injury. The NMJ is a dynamic structure, and the interactions between the terminal axon, TSCs, and motor end plate provide avenues for innovative therapeutic interventions. Other novel treatment strategies exist that involve targeting the NMJ, and further research is required into preservation of the NMJ as a method of treating patients with major nerve injury.
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