Defining the relative impact of muscle versus Schwann cell denervation on functional recovery after delayed nerve repair

Muscle preservation in proximal nerve injuries: a current update

Optimal recovery of muscle function after proximal nerve injuries remains a complex and challenging problem. After a nerve injury, alterations in the affected muscles lead to atrophy, and later degeneration and replacement by fat-fibrous tissues. At present, several different strategies for the preservation of skeletal muscle have been reported, including various sets of physical exercises, muscle massage, physical methods (e.g. electrical stimulation, magnetic field and laser stimulation, low-intensity pulsed ultrasound), medicines (e.g. nutrients, natural and chemical agents, anti-inflammatory and antioxidants, hormones, enzymes and enzyme inhibitors), regenerative medicine (e.g. growth factors, stem cells and microbiota) and surgical procedures (e.g. supercharge end-to-side neurotization). The present review will focus on methods that aimed to minimize the damage to muscles after denervation based on our present knowledge.

Early in vivo detection of denervation-induced atrophy by luminescence transient nanothermometry

Denervation induces skeletal muscle atrophy due to the loss of control and feedback with the nervous system. Unfortunately, muscle atrophy only becomes evident days after the denervation event when it could be irreversible. Alternative diagnosis tools for early detection of denervation-induced muscle atrophy are, thus, required. In this work, we demonstrate how the combination of transient thermometry, a technique already used for early diagnosis of tumors, and infrared-emitting nanothermometers makes possible the in vivo detection of the onset of muscle atrophy at short (<1 day) times after a denervation event. The physiological reasons behind these experimental results have been explored by performing three dimensional numerical simulations based on the Pennes' bioheat equation. It is concluded that the alterations in muscle thermal dynamics at the onset of muscle atrophy are consequence of the skin perfusion increment caused by the alteration of peripheral nervous autonomous system. This work demonstrates the potential of infrared luminescence thermometry for early detection of diseases of the nervous system opening the venue toward the development of new diagnosis tools.

Nerve function restoration following targeted muscle reinnervation after varying delayed periods

Targeted muscle reinnervation has been proposed for reconstruction of neuromuscular function in amputees. However, it is unknown whether performing delayed targeted muscle reinnervation after nerve injury will affect restoration of function. In this rat nerve injury study, the median and musculocutaneous nerves of the forelimb were transected. The proximal median nerve stump was sutured to the distal musculocutaneous nerve stump immediately and 2 and 4 weeks after surgery to reinnervate the biceps brachii. After targeted muscle reinnervation, intramuscular myoelectric signals from the biceps brachii were recorded. Signal amplitude gradually increased with time. Biceps brachii myoelectric signals and muscle fiber morphology and grooming behavior did not significantly differ among rats subjected to delayed target muscle innervation for different periods. Targeted muscle reinnervation delayed for 4 weeks can acquire the same nerve function restoration effect as that of immediate reinnervation.

The Dynamics of Nerve Degeneration and Regeneration in a Healthy Milieu and in Diabetes

Appropriate animal models, mimicking conditions of both health and disease, are needed to understand not only the biology and the physiology of neurons and other cells under normal conditions but also under stress conditions, like nerve injuries and neuropathy. In such conditions, understanding how genes and different factors are activated through the well-orchestrated programs in neurons and other related cells is crucial. Knowledge about key players associated with nerve regeneration intended for axonal outgrowth, migration of Schwann cells with respect to suitable substrates, invasion of macrophages, appropriate conditioning of extracellular matrix, activation of fibroblasts, formation of endothelial cells and blood vessels, and activation of other players in healthy and diabetic conditions is relevant. Appropriate physical and chemical attractions and repulsions are needed for an optimal and directed regeneration and are investigated in various nerve injury and repair/reconstruction models using healthy and diabetic rat models with relevant blood glucose levels. Understanding dynamic processes constantly occurring in neuropathies, like diabetic neuropathy, with concomitant degeneration and regeneration, requires advanced technology and bioinformatics for an integrated view of the behavior of different cell types based on genomics, transcriptomics, proteomics, and imaging at different visualization levels. Single-cell-transcriptional profile analysis of different cells may reveal any heterogeneity among key players in peripheral nerves in health and disease.

Traumatic peripheral nerve injuries: diagnosis and management

PURPOSE OF REVIEW

To review advances in the diagnostic evaluation and management of traumatic peripheral nerve injuries.

RECENT FINDINGS

Serial multimodal assessment of peripheral nerve injuries facilitates assessment of spontaneous axonal regeneration and selection of appropriate patients for early surgical intervention. Novel surgical and rehabilitative approaches have been developed to complement established strategies, particularly in the area of nerve grafting, targeted rehabilitation strategies and interventions to promote nerve regeneration. However, several management challenges remain, including incomplete reinnervation, traumatic neuroma development, maladaptive central remodeling and management of fatigue, which compromise functional recovery.

SUMMARY

Innovative approaches to the assessment and treatment of peripheral nerve injuries hold promise in improving the degree of functional recovery; however, this remains a complex and evolving area.

Translational bioengineering strategies for peripheral nerve regeneration: opportunities, challenges, and novel concepts

Peripheral nerve injuries remain a challenging problem in need of better treatment strategies. Despite best efforts at surgical reconstruction and postoperative rehabilitation, patients are often left with persistent, debilitating motor and sensory deficits. There are currently no therapeutic strategies proven to enhance the regenerative process in humans. A clinical need exists for the development of technologies to promote nerve regeneration and improve functional outcomes. Recent advances in the fields of tissue engineering and nanotechnology have enabled biomaterial scaffolds to modulate the host response to tissue repair through tailored mechanical, chemical, and conductive cues. New bioengineered approaches have enabled targeted, sustained delivery of protein therapeutics with the capacity to unlock the clinical potential of a myriad of neurotrophic growth factors that have demonstrated promise in enhancing regenerative outcomes. As such, further exploration of combinatory strategies leveraging these technological advances may offer a pathway towards clinically translatable solutions to advance the care of patients with peripheral nerve injuries. This review first presents the various emerging bioengineering strategies that can be applied for the management of nerve gap injuries. We cover the rationale and limitations for their use as an alternative to autografts, focusing on the approaches to increase the number of regenerating axons crossing the repair site, and facilitating their growth towards the distal stump. We also discuss the emerging growth factor-based therapeutic strategies designed to improve functional outcomes in a multimodal fashion, by accelerating axonal growth, improving the distal regenerative environment, and preventing end-organs atrophy.

Functional Reconstruction of Denervated Muscle by Xenotransplantation of Neural Cells from Porcine to Rat

Neural cell transplantation targeting peripheral nerves is a potential treatment regime for denervated muscle atrophy. This study aimed to develop a new therapeutic technique for intractable muscle atrophy by the xenotransplantation of neural stem cells derived from pig fetuses into peripheral nerves. In this study, we created a denervation model using neurotomy in nude rats and transplanted pig-fetus-derived neural stem cells into the cut nerve stump. Three months after transplantation, the survival of neural cells, the number and area of regenerated axons, and the degree of functional recovery by electrical stimulation of peripheral nerves were compared among the gestational ages (E 22, E 27, E 45) of the pigs. Transplanted neural cells were engrafted at all ages. Functional recovery by electric stimulation was observed at age E 22 and E 27. This study shows that the xenotransplantation of fetal porcine neural stem cells can restore denervated muscle function. When combined with medical engineering, this technology can help in developing a new therapy for paralysis.

Spinal motor neuron transplantation to enhance nerve reconstruction strategies: Towards a cell therapy

Nerve transfers have become a powerful intervention to restore function following devastating paralyzing injuries. A major limitation to peripheral nerve repair and reconstructive strategies is the progressive, fibrotic degeneration of the distal nerve and denervated muscle, eventually precluding recovery of these targets and thus defining a time window within which reinnervation must occur. One proven strategy in the clinic has been the sacrifice and transfer of an adjacent distal motor nerve to provide axons to occupy, and thus preserve (or "babysit"), the target muscle. However, available nearby nerves are limited in severe brachial plexus or spinal cord injury. An alternative and novel proposition is the transplantation of spinal motor neurons (SMNs) derived from human induced pluripotent stem cells (iPSCs) into the target nerve to extend their axons to occupy and preserve the targets. These cells could potentially be delivered through minimally invasive or percutaneous techniques. Several reports have demonstrated survival, functional innervation, and muscular preservation following transplantation of SMNs into rodent nerves. Advances in the generation, culture, and differentiation of human iPSCs now offer the possibility for an unlimited supply of clinical grade SMNs. This review will discuss the previous reports of peripheral SMN transplantation, outline key considerations, and propose next steps towards advancing this approach to clinic. Stem cells have garnered great enthusiasm for their potential to revolutionize medicine. However, this excitement has often led to premature clinical studies with ill-defined cell products and mechanisms of action, particularly in spinal cord injury. We believe the peripheral transplantation of a defined SMN population to address neuromuscular degeneration will be transformative in augmenting current reconstructive strategies. By thus removing the current barriers of time and distance, this strategy would dramatically enhance the potential for reconstruction and functional recovery in otherwise hopeless paralyzing injuries. Furthermore, this strategy may be used as a permanent axon replacement following destruction of lower motor neurons and would enable exogenous stimulation options, such as pacing of transplanted SMN axons in the phrenic nerve to avoid mechanical ventilation in high cervical cord injury or amyotrophic lateral sclerosis.

Functional Recovery following Repair of Long Nerve Gaps in Senior Patient 2.6 Years Posttrauma

Sensory nerve grafts are the clinical “gold standard” for repairing peripheral nerve gaps. However, reliable good-to-excellent recovery develops only for gaps less than 3–5 cm, repairs performed less than 3–5 months posttrauma, and patients aged less than 20–25 years. As the value of any variable increases, the extent of recovery decreases precipitously, and if the values of any two or all increase, there is little to no recovery. One 9-cm-long and two 11-cm-long nerve gaps in a 56-year-old patient were repaired 2.6 years posttrauma. They were bridged with two sensory nerve grafts within an autologous platelet-rich plasma-filled collagen tube. Both were connected to the proximal ulnar nerve stump, with one graft end to the distal motor and the other to the sensory nerve branches. Although presurgery the patient suffered chronic level 10 excruciating neuropathic pain, it was reduced to 6 within 2 months, and did not increase for more than 2 years. Motor axons regenerated across the 9-cm gap and innervated the appropriate two measured muscles, with limited muscle fiber recruitment. Sensory axons regenerated across both 11-cm gaps and restored normal topographically correct sensitivity to stimuli of all sensory modalities, including static two-point discrimination of 5 mm, and pressure of 2.83 g to all regions innervated by both sensory nerves. This novel technique induced a significant long-term reduction in chronic excruciating neuropathic pain while promoting muscle reinnervation and complete sensory recovery, despite the values of all three variables that reduce or prevent axon regeneration and recovery being simultaneously large.