A novel method for establishing daily in vivo concentration gradients of soluble nerve growth factor (NGF)

Growth factors and molecular-driven plasticity in neurological systems

It has been almost 70 years since the discovery of nerve growth factor (NGF), a period of a dramatic evolution in our understanding of dynamic growth, regeneration, and rewiring of the nervous system. In 1953, the extraordinary finding that a protein found in mouse submandibular glands generated a halo of outgrowing axons has now redefined our concept of the nervous system connectome. Central and peripheral neurons and their axons or dendrites are no longer considered fixed or static "wiring." Exploiting this molecular-driven plasticity as a therapeutic approach has arrived in the clinic with a slate of new trials and ideas. Neural growth factors (GFs), soluble proteins that alter the behavior of neurons, have expanded in numbers and our understanding of the complexity of their signaling and interactions with other proteins has intensified. However, beyond these "extrinsic" determinants of neuron growth and function are the downstream pathways that impact neurons, ripe for translational development and potentially more important than individual growth factors that may trigger them. Persistent and ongoing nuances in clinical trial design in some of the most intractable and irreversible neurological conditions give hope for connecting new biological ideas with clinical benefits. This review is a targeted update on neural GFs, their signals, and new therapeutic ideas, selected from an expansive literature.

Unleashing Intrinsic Growth Pathways in Regenerating Peripheral Neurons

Common mechanisms of peripheral axon regeneration are recruited following diverse forms of damage to peripheral nerve axons. Whether the injury is traumatic or disease related neuropathy, reconnection of axons to their targets is required to restore function. Supporting peripheral axon regrowth, while not yet available in clinics, might be accomplished from several directions focusing on one or more of the complex stages of regrowth. Direct axon support, with follow on participation of supporting Schwann cells is one approach, emphasized in this review. However alternative approaches might include direct support of Schwann cells that instruct axons to regrow, manipulation of the inflammatory milieu to prevent ongoing bystander axon damage, or use of inflammatory cytokines as growth factors. Axons may be supported by a growing list of growth factors, extending well beyond the classical neurotrophin family. The understanding of growth factor roles continues to expand but their impact experimentally and in humans has faced serious limitations. The downstream signaling pathways that impact neuron growth have been exploited less frequently in regeneration models and rarely in human work, despite their promise and potency. Here we review the major regenerative signaling cascades that are known to influence adult peripheral axon regeneration. Within these pathways there are major checkpoints or roadblocks that normally check unwanted growth, but are an impediment to robust growth after injury. Several molecular roadblocks, overlapping with tumour suppressor systems in oncology, operate at the level of the perikarya. They have impacts on overall neuron plasticity and growth. A second approach targets proteins that largely operate at growth cones. Addressing both sites might offer synergistic benefits to regrowing neurons. This review emphasizes intrinsic aspects of adult peripheral axon regeneration, emphasizing several molecular barriers to regrowth that have been studied in our laboratory.

Nerve growth factor activates autophagy in Schwann cells to enhance myelin debris clearance and to expedite nerve regeneration

Rationale: Autophagy in Schwann cells (SCs) is crucial for myelin debris degradation and clearance following peripheral nerve injury (PNI). Nerve growth factor (NGF) plays an important role in reconstructing peripheral nerve fibers and promoting axonal regeneration. However, it remains unclear if NGF effect in enhancing nerve regeneration is mediated through autophagic clearance of myelin debris in SCs.

Methods: In vivo, free NGF solution plus with/without pharmacological inhibitors were administered to a rat sciatic nerve crush injury model. In vitro, the primary Schwann cells (SCs) and its cell line were cultured in normal medium containing NGF, their capable of swallowing or clearing degenerated myelin was evaluated through supplement of homogenized myelin fractions.

Results: Administration of exogenous NGF could activate autophagy in dedifferentiated SCs, accelerate myelin debris clearance and phagocytosis, as well as promote axon and myelin regeneration at early stage of PNI. These NGF effects were effectively blocked by autophagy inhibitors. In addition, inhibition of the p75 kD neurotrophin receptor (p75^NTR) signal or inactivation of the AMP-activated protein kinase (AMPK) also inhibited the NGF effect as well.

Conclusions: NGF effect on promoting early nerve regeneration is closely associated with its accelerating autophagic clearance of myelin debris in SCs, which probably regulated by the p75^NTR/AMPK/mTOR axis. Our studies thus provide strong support that NGF may serve as a powerful pharmacological therapy for peripheral nerve injuries.

Beyond Trophic Factors: Exploiting the Intrinsic Regenerative Properties of Adult Neurons

Injuries and diseases of the peripheral nervous system (PNS) are common but frequently irreversible. It is often but mistakenly assumed that peripheral neuron regeneration is robust without a need to be improved or supported. However, axonal lesions, especially those involving proximal nerves rarely recover fully and injuries generally are complicated by slow and incomplete regeneration. Strategies to enhance the intrinsic growth properties of reluctant adult neurons offer an alternative approach to consider during regeneration. Since axons rarely regrow without an intimately partnered Schwann cell (SC), approaches to enhance SC plasticity carry along benefits to their axon partners. Direct targeting of molecules that inhibit growth cone plasticity can inform important regenerative strategies. A newer approach, a focus of our laboratory, exploits tumor suppressor molecules that normally dampen unconstrained growth. However several are also prominently expressed in stable adult neurons. During regeneration their ongoing expression “brakes” growth, whereas their inhibition and knockdown may enhance regrowth. Examples have included phosphatase and tensin homolog deleted on chromosome ten (PTEN), a tumor suppressor that inhibits PI3K/pAkt signaling, Rb1, the protein involved in retinoblastoma development, and adenomatous polyposis coli (APC), a tumor suppressor that inhibits β-Catenin transcriptional signaling and its translocation to the nucleus. The identification of several new targets to manipulate the plasticity of regenerating adult peripheral neurons is exciting. How they fit with canonical regeneration strategies and their feasibility require additional work. Newer forms of nonviral siRNA delivery may be approaches for molecular manipulation to improve regeneration.

Establishment of nerve growth factor gradients on aligned chitosan-polylactide /alginate fibers for neural tissue engineering applications

Nerve conduits containing aligned fibrous fillers with gradiently distributed signal molecules are essential for long-gap nerve repair. This study was to develop an approach for establishing nerve growth factor (NGF) gradients onto the aligned chitosan-polylactide (CH-PLA) fibers. CH-PLA containing 37wt% of PLA was spun into fibers using a wet-spinning technique. CH-PLA fibers showed much higher wet-state tensile strength, enhanced degradation tolerance and significantly lower swelling degree in comparison to chitosan fibers. The CH-PLA fibers with diameters from 40 to 60μm were selected and segmentally coated in bundles using NGF-contained alginate solutions to establish NGF gradients lengthwise along fibers. The diameter of resulting NGF-loaded CH-PLA/alginate fibers was well controlled within a range between 60 and 120μm. Calcium ion crosslinked alginate coating layers on fibers showed abilities to administer the sustainable NGF release in a gradient distribution manner for at least 5 weeks. NGF-induced neurite outgrowth of PC12 cells confirmed that bioactivity of NGF released from fibers was well retained.

Conserved dopamine neurotrophic factor-transduced mesenchymal stem cells promote axon regeneration and functional recovery of injured sciatic nerve

Peripheral nerve injury (PNI) is a common disease that often results in axonal degeneration and the loss of neurons, ultimately leading to limited nerve regeneration and severe functional impairment. Currently, there are no effective treatments for PNI. In the present study, we transduced conserved dopamine neurotrophic factor (CDNF) into mesenchymal stem cells (MSCs) in collagen tubes to investigate their regenerative effects on rat peripheral nerves in an in vivo transection model. Scanning electron microscopy of the collagen tubes demonstrated their ability to be resorbed in vivo. We observed notable overexpression of the CDNF protein in the distal sciatic nerve after application of CDNF-MSCs. Quantitative analysis of neurofilament 200 (NF200) and S100 immunohistochemistry showed significant enhancement of axonal and Schwann cell regeneration in the group receiving CDNF-MSCs (CDNF-MSCs group) compared with the control groups. Myelination thickness, axon diameter and the axon-to fiber diameter ratio (G-ratio) were significantly higher in the CDNF-MSCs group at 8 and 12 weeks after nerve transection surgery. After surgery, the sciatic functional index, target muscle weight, wet weight ratio of gastrocnemius muscle and horseradish peroxidase (HRP) tracing demonstrated functional recovery. Light and electron microscopy confirmed successful regeneration of the sciatic nerve. The greater numbers of HRP-labeled neuron cell bodies and increased sciatic nerve index values (SFI) in the CDNF-MSCs group suggest that CDNF exerts neuroprotective effects in vivo. We also observed higher target muscle weights and a significant improvement in muscle atrophism in the CDNF-MSCs group. Collectively, these findings indicate that CDNF gene therapy delivered by MSCs is capable of promoting nerve regeneration and functional recovery, likely because of the significant neuroprotective and neurotrophic effects of CDNF and the superior environment offered by MSCs and collagen tubes.

Nerve conduits for peripheral nerve surgery

Autologous nerve grafts are the current criterion standard for repair of peripheral nerve injuries when the transected nerve ends are not amenable to primary end-to-end tensionless neurorrhaphy. However, donor-site morbidities such as neuroma formation and permanent loss of function have led to tremendous interest in developing an alternative to this technique. Artificial nerve conduits have therefore emerged as an alternative to autologous nerve grafting for the repair of short peripheral nerve defects of less than 30 mm; however, they do not yet surpass autologous nerve grafts clinically. A thorough understanding of the complex biological reactions that take place during peripheral nerve regeneration will allow researchers to develop a nerve conduit with physical and biological properties similar to those of an autologous nerve graft that supports regeneration over long nerve gaps and in large-diameter nerves. In this article, the authors assess the currently available nerve conduits, summarize research in the field of developing these conduits, and establish areas within this field in which further research would prove most beneficial.

PTEN inhibition enhances neurite outgrowth in human embryonic stem cell-derived neuronal progenitor cells

We investigated the role of PTEN (phosphatase and tensin homolog deleted on chromosome 10) during neurite outgrowth of human embryonic stem cell (hESC)-derived neuronal progenitors. PTEN inhibits phosphoinositide 3-kinase (PI3K)/Akt signaling, a common and central outgrowth and survival pathway downstream of neuronal growth factors. It is known that PTEN inhibition, by either polymorphic mutation or gene deletion, can lead to the development of tumorigenesis (Stambolic et al., ; Tamura et al., ). However, temporary inhibition of PTEN, through pharmacological manipulation, could regulate signaling events such as the PI3K/Akt signaling pathway, leading to enhanced recovery of central nervous system (CNS) injury and disease. We demonstrate that pharmacological inhibition of PTEN in hESC-derived neuronal progenitors significantly increased neurite outgrowth in vitro in a dose- and time-dependent manner. Our results indicate that inhibition of PTEN augments neurite outgrowth beyond that of traditional methods such as elevation of intracellular cyclic adenosine monophosphate (cAMP) levels, and depends on upregulation of the PI3K/Akt signaling pathway and its downstream effectors, such as mammalian target of rapamycin (mTOR). PTEN inhibition also rescued neurite outgrowth over an inhibitory substrate in vitro. These findings indicate a remarkable impact on hESC-derived neuronal progenitor plasticity through PTEN inhibition. Overall, these findings identify a novel therapeutic strategy for neurite outgrowth in CNS injury and disease.

The challenges and beauty of peripheral nerve regrowth

This review provides an overview of selected aspects of peripheral nerve regeneration and potential avenues to explore therapeutically. The overall coordinated and orchestrated pattern of recovery from peripheral nerve injury has a beauty of execution and progress that rivals all other forms of neurobiology. It involves changes at the level of the perikaryon, coordination with important peripheral glial partners, the Schwann cells, a controlled inflammatory response, and growth that overcomes surprising intrinsic roadblocks. Both regenerative axon growth and collateral sprouting encompass fascinating aspects of this story. Better understanding of peripheral nerve regeneration may also lead to enhanced central nervous system recovery.

Nerve-muscle-endplate band grafting: a new technique for muscle reinnervation

BACKGROUND

Because currently existing reinnervation methods result in poor functional recovery, there is a great need to develop new treatment strategies.

OBJECTIVE

To investigate the efficacy of our recently developed nerve-muscle-endplate band grafting (NMEG) technique for muscle reinnervation.

METHODS

Twenty-five adult rats were used. Sternohyoid (SH) and sternomastoid (SM) muscles served as donor and recipient muscle, respectively. Neural organization of the SH and SM muscles and surgical feasibility of the NMEG technique were determined. An NMEG contained a muscle block, a nerve branch with nerve terminals, and a motor endplate band with numerous neuromuscular junctions. After a 3-month recovery period, the degree of functional recovery was evaluated with a maximal tetanic force measurement. Retrograde horseradish peroxidase tracing was used to track the origin of the motor innervation of the reinnervated muscles. The reinnervated muscles were examined morphohistologically and immunohistochemically to assess the extent of axonal regeneration.

RESULTS

Nerve supply patterns and locations of the motor endplate bands in the SH and SM muscles were documented. The results demonstrated that the reinnervated SM muscles gained motor control from the SH motoneurons. The NMEG technique yielded extensive axonal regeneration and significant recovery of SM muscle force-generating capacity (67% of control). The mean wet weight of the NMEG-reinnervated muscles (87% of control) was greater than that of the denervated SM muscles (36% of control).

CONCLUSION

The NMEG technique resulted in successful muscle reinnervation and functional recovery. This technique holds promise in the treatment of muscle paralysis.

Nerve surgery and gene therapy: a neurobiological and clinical perspective

Despite major microsurgical improvements the clinical outcome of peripheral nerve surgery is still regarded as suboptimal. Over the past decade several innovative techniques have been developed to extend the armamentarium of the nerve surgeon. This review evaluates the potential of gene therapy in the context of peripheral nerve repair. First the main challenges impeding peripheral nerve regeneration are presented. This is followed by a short introduction to gene therapy and an overview of its most important advantages over the classical delivery of therapeutic proteins. Next, this review focuses on the most promising viral vectors capable of targeting the peripheral nervous system and their first application in animal models. In addition, the challenges of translating these experimental results to the clinic, the limitations of current vectors and the further developments needed, are discussed. Finally, four strategies are presented on how gene therapy could help patients that have to undergo reconstructive nerve surgery in the future.

Dose and duration of nerve growth factor (NGF) administration determine the extent of behavioral recovery following peripheral nerve injury in the rat

Nerve growth factor (NGF) has been previously shown to support neuron survival and direct neurite outgrowth in vitro, and to enhance axonal regeneration in vivo. However, a systematic analysis of NGF dose and dose duration on behavioral recovery following peripheral nerve injury in rodents has not been previously investigated. Here, we show that NGF promotes a bell shaped dose-response, with an optimal threshold effect occurring at 800 pg/μl. High dose NGF inhibited regeneration. However, this effect could be reversed through functional blockade of p75 receptors, thus implicating these receptors as mediators of the inhibitory response. Longer term evaluation showed that animals administered NGF at 80 ng/day for 3 weeks had greater sensorimotor recovery compared to all other treatment groups. These animals made significantly fewer errors during skilled locomotion, and displayed both increased vertical and fore-aft ground reaction forces during flat surface locomotion. Furthermore, terminal electrophysiological and myological assessments (EMG, wet gastrocnemius muscle weights) corroborated the behavioral data. Overall, these data support the hypothesis that both appropriate dose and duration of NGF are important determinants of behavioral recovery following nerve injury in the rat.

Inducible nerve growth factor delivery for peripheral nerve regeneration in vivo

BACKGROUND

HEK-293 cells can be genetically modified to release and regulate nerve growth factor (NGF) in vitro. The aim of this study was to evaluate the impact of this NGF delivery system on peripheral nerve regeneration in vivo.

METHODS

HEK-293 cells were transfected with an ecdysone receptor, NGF cDNA, and herpes simplex virus-thymidine kinase suicide vector. NGF production is induced by ponasterone A and stopped by ganciclovir. A 13-mm sciatic nerve gap was bridged with Silastic conduits in 120 nude rats, and transfected HEK-293 cells were added, induced, and boostered to secrete bioactive NGF.

RESULTS

The induction of the cell line and additional booster with ponasterone A demonstrated significantly higher levels of bioactive NGF, enhanced macroscopic nerve growth, improved functional recovery, and histologic regeneration when compared with control groups after 7, 14, and 21 days, and 2 and 4 months. The treatment with ganciclovir resulted in suppression of the NGF production and decreased functional and histologic outcomes.

CONCLUSIONS

Transfected HEK-293 cells can be regulated to inducibly produce bioactive NGF in vivo over prolonged periods. This tissue-engineered nerve construct including the NGF delivery system is able to improve peripheral nerve regeneration and functional recovery and appears to be superior to nerve isografts.

PTEN inhibition to facilitate intrinsic regenerative outgrowth of adult peripheral axons

In vivo regeneration of peripheral neurons is constrained and rarely complete, and unfortunately patients with major nerve trunk transections experience only limited recovery. Intracellular inhibition of neuronal growth signals may be among these constraints. In this work, we investigated the role of PTEN (phosphatase and tensin homolog deleted on chromosome 10) during regeneration of peripheral neurons in adult Sprague Dawley rats. PTEN inhibits phosphoinositide 3-kinase (PI3-K)/Akt signaling, a common and central outgrowth and survival pathway downstream of neuronal growth factors. While PI3-K and Akt outgrowth signals were expressed and activated within adult peripheral neurons during regeneration, PTEN was similarly expressed and poised to inhibit their support. PTEN was expressed in neuron perikaryal cytoplasm, nuclei, regenerating axons, and Schwann cells. Adult sensory neurons in vitro responded to both graded pharmacological inhibition of PTEN and its mRNA knockdown using siRNA. Both approaches were associated with robust rises in the plasticity of neurite outgrowth that were independent of the mTOR (mammalian target of rapamycin) pathway. Importantly, this accelerated outgrowth was in addition to the increased outgrowth generated in neurons that had undergone a preconditioning lesion. Moreover, following severe nerve transection injuries, local pharmacological inhibition of PTEN or siRNA knockdown of PTEN at the injury site accelerated axon outgrowth in vivo. The findings indicated a remarkable impact on peripheral neuron plasticity through PTEN inhibition, even within a complex regenerative milieu. Overall, these findings identify a novel route to propagate intrinsic regeneration pathways within axons to benefit nerve repair.

Designing in vivo concentration gradients with discrete controlled release: a computational model

One promising neurorehabilitation therapy involves presenting neurotrophins directly into the brain to induce growth of new neural connections. The precise control of neurotrophin concentration gradients deep within neural tissue that would be necessary for such a therapy is not currently possible, however. Here we evaluate the theoretical potential of a novel method of drug delivery, discrete controlled release (DCR), to control effective neurotrophin concentration gradients in an isotropic region of neocortex. We do so by constructing computational models of neurotrophin concentration profiles resulting from discrete release locations into the cortex and then optimizing their design for uniform concentration gradients. The resulting model indicates that by rationally selecting initial neurotrophin concentrations for drug-releasing electrode coatings in a square 16-electrode array, nearly uniform concentration gradients (i.e. planar concentration profiles) from one edge of the electrode array to the other should be obtainable. DCR therefore represents a promising new method of precisely directing neuronal growth in vivo over a wider spatial profile than would be possible with single release points.

Practical considerations concerning the use of stem cells for peripheral nerve repair

In this review the authors intend to demonstrate the need for supplementing conventional repair of the injured nerve with alternative therapies, namely transplantation of stem or progenitor cells. Although peripheral nerves do exhibit the potential to regenerate axons and reinnervate the end organ, outcome following severe nerve injury, even after repair, remains relatively poor. This is likely because of the extensive injury zone that prevents axon outgrowth. Even if outgrowth does occur, a relatively slow growth rate of regeneration results in prolonged denervation of the distal nerve. Whereas denervated Schwann cells (SCs) are key players in the early regenerative success of peripheral nerves, protracted loss of axonal contact renders Schwann cells unreceptive for axonal regeneration. Given that denervated Schwann cells appear to become effete, one logical approach is to support the distal denervated nerve environment by replacing host cells with those derived exogenously. A number of different sources of stem/precursor cells are being explored for their potential application in the scenario of peripheral nerve injury. The most promising candidate, transplant cells are derived from easily accessible sources such as the skin, bone marrow, or adipose tissue, all of which have demonstrated the capacity to differentiate into Schwann cell-like cells. Although recent studies have shown that stem cells can act as promising and beneficial adjuncts to nerve repair, considerable optimization of these therapies will be required for their potential to be realized in a clinical setting. The authors investigate the relevance of the delivery method (both the number and differentiation state of cells) on experimental outcomes, and seek to clarify whether stem cells must survive and differentiate in the injured nerve to convey a therapeutic effect. As our laboratory uses skin-derived precursor cells (SKPCs) in various nerve injury paradigms, we relate our findings on cell fate to other published studies to demonstrate the need to quantify stem cell survival and differentiation for future studies.

Growth factor and stem cell enhanced conduits in peripheral nerve regeneration and repair

OBJECTIVE

Despite the capacity for spontaneous axonal regeneration, recovery after severe peripheral nerve injury remains variable and often very poor. In addition, autologous nerve grafts, considered to be the 'gold standard' in nerve repair technique, are plagued by restricted donor tissue availability and donor site morbidity. Our primary objective is to highlight new and emerging methods of nerve repair, which have the potential to significantly improve both the functional and behavioral outcome after clinical nerve injury.

METHODS

A critical analysis of nerve injury and regeneration literature concentrating on outcome measures from both immediate and chronically denervated experimental works was conducted.

RESULTS

Results of numerous works employing both growth factor and stem cell enhanced nerve guidance conduits have shown encouraging results. However, further research is needed to optimize guidance conduit dynamics, bioavailability and delivery of both growth factors and stem cells to enhance peripheral nerve regeneration and functional recovery.

DISCUSSION

This review discusses current animal and clinical growth factor and stem cell studies, specifically focusing on future bio-engineering approaches in developing a nerve guidance conduit in the future.

Chapter 8: Current techniques and concepts in peripheral nerve repair

Despite the progress in understanding the pathophysiology of peripheral nervous system injury and regeneration, as well as advancements in microsurgical techniques, peripheral nerve injuries are still a major challenge for reconstructive surgeons. Thorough knowledge of anatomy, pathophysiology, and surgical reconstruction is a prerequisite of proper peripheral nerve injury management. This chapter reviews the currently available surgical treatment options for different types of nerve injuries in clinical conditions. In overview of direct nerve repair, various end-to-end coaptation techniques and the role of end-to-side repair for proximal nerve injuries is described. When primary repair cannot be performed without undue tension, nerve grafting or tubulization techniques are required. Current gold standard for bridging nerve gaps is nerve autografting. However, disadvantages of this approach, such as donor site morbidity and limited length of available graft material encouraged the search for alternative means of nerve gap reconstruction. Nerve allografting was introduced for repair of extensive nerve injuries. Tubulization techniques with natural or artificial conduits are applicable as an alternative for bridging short nerve defects without the morbidities associated with harvesting of autologous nerve grafts. Achieving better outcomes depends both on the advancements in microsurgical techniques and introduction of molecular biology discoveries into clinical practice. The field of peripheral nerve research is dynamically developing and concentrates on more sophisticated approaches tested at the basic science level. Future directions in peripheral nerve reconstruction including, tolerance induction and minimal immunosuppression for nerve allografting, cell based supportive therapies and bioengineering of nerve conduits are also reviewed in this chapter.

Artificial nerve tubes and their application for repair of peripheral nerve injury: an update of current concepts

Over the last 20 years, an increasing number of research articles have reported on the use of artificial nerve tubes to repair nerve defects. The development of an artificial nerve tube as an alternative to autogenous nerve grafting is currently a focus of interest for peripheral nerve repair. The clinical employment of tubes as an alternative to autogenous nerve grafts is mainly justified by the limited availability of donor tissue for nerve autografts and the related morbidity. Numerous studies indicate that short-distance defects in humans can be successfully treated by implantation of artificial nerve guides. This review provides a brief overview of various preclinical and clinical trials conducted to evaluate the utility of artificial nerve tubes for the regeneration of peripheral nerves. This review is also intended to help update hand surgeons on the rapid advances in tubulization techniques, and to provide them with indications of the various directions toward which future research can proceed. Future studies need to provide us with as much comparative information as possible on the effectiveness of different tubulization techniques, in order to guide the surgeon in choosing the best indications for their optimal clinical employment. Future progress in implant development can be expected from interdisciplinary approaches involving both materials and life sciences, leading to advances in neuro-tissue engineering that will be needed to effectively treat larger nerve defects.