Ciliary neurotrophic factor immunoreactivity in rat intramuscular nerve during reinnervation through a silicone tube after severing of the rat sciatic nerve

Surgical repair in humans after traumatic nerve injury provides limited functional neural regeneration in adults

Traumatic nerve injuries result in devastating loss of neurologic function with unpredictable functional recovery despite optimal medical management. After traumatic nerve injury and denervation, regenerating axons must traverse a complex environment in which they encounter numerous barriers on the way to reinnervation of their target muscle. Outcomes of surgical intervention alone have unfortunately reached a plateau, resulting in often unsatisfactory functional recovery. Over the past few decades, many improvements were developed to supplement and boost the results of surgical repair. Biological optimization of Schwann cells, macrophages, and degradation enzymes have been studied due to the key roles of these components in axonal development, maintenance and response to injury. Moreover, surgical techniques such as nerve grafting, conduits, and growth factor supplementation are also employed to enhance the microenvironment and nerve regeneration. Yet, most of the roadblocks to recovery after nerve injury remain unsolved. These roadblocks include, but are not limited to: slow regeneration rates and specificity of target innervation, the presence of a segmental nerve defect, and degeneration of the target end-organ after prolonged periods of denervation. A recognition of these limitations is necessary so as to develop new strategies to improve functional regeneration for these life changing injuries.

Complementary effects of two growth factors in multifunctionalized silk nanofibers for nerve reconstruction

With the aim of forming bioactive guides for peripheral nerve regeneration, silk fibroin was electrospun to obtain aligned nanofibers. These fibers were functionalized by incorporating Nerve Growth Factor (NGF) and Ciliary NeuroTrophic Factor (CNTF) during electrospinning. PC12 cells grown on the fibers confirmed the bioavailability and bioactivity of the NGF, which was not significantly released from the fibers. Primary neurons from rat dorsal root ganglia (DRGs) were grown on the nanofibers and anchored to the fibers and grew in a directional fashion based on the fiber orientation, and as confirmed by growth cone morphology. These biofunctionalized nanofibers led to a 3-fold increase in neurite length at their contact, which was likely due to the NGF. Glial cell growth, alignment and migration were stimulated by the CNTF in the functionalized nanofibers. Organotypic culture of rat fetal DRGs confirmed the complementary effect of both growth factors in multifunctionalized nanofibers, which allowed glial cell migration, alignment and parallel axonal growth in structures resembling the ‘bands of Bungner’ found in situ. Graftable multi-channel conduits based on biofunctionalized aligned silk nanofibers were developed as an organized 3D scaffold. Our bioactive silk tubes thus represent new options for a biological and biocompatible nerve guidance conduit.

Peripheral nerve regeneration and neurotrophic factors

The role of neurotrophic factors in the maintenance and survival of peripheral neuronal cells has been the subject of numerous studies. Administration of exogenous neurotrophic factors after nerve injury has been shown to mimic the effect of target organ-derived trophic factors on neuronal cells. After axotomy and during peripheral nerve regeneration, the neurotrophins NGF, NT-3 and BDNF show a well defined and selective beneficial effect on the survival and phenotypic expression of primary sensory neurons in dorsal root ganglia and of motoneurons in spinal cord. Other neurotrophic factors such as CNTF, GDNF and LIF also exert a variety of actions on neuronal cells, which appear to overlap and complement those of the neurotrophins. In addition, there is an indirect contribution of GGF to nerve regeneration. GGF is produced by neurons and stimulates proliferation of Schwann cells, underlining the close interaction between neuronal and glial cells during peripheral nerve regeneration. Different possibilities have been investigated for the delivery of growth factors to the injured neurons, in search of a suitable system for clinical applications. The studies reviewed in this article show the therapeutic potential of neurotrophic factors for the treatment of peripheral nerve injury and for neuropathies.