Doxycycline-regulated GDNF expression promotes axonal regeneration and functional recovery in transected peripheral nerve

Phosphorus-carrying cascade molecules: inner architecture to biomedical applications

Cascade molecules are nearly uniform-sized macromolecules of small molecules or linear polymer cores built around symmetric branching units. A wide range of biological properties can be achieved with phosphorus-containing dendrimers, depending on their terminal functions, ranging from biomaterials to imaging, drug delivery, and acting as a drug by themselves. This feature article presents significant examples of phosphorus-containing dendrimers used to develop biochips, support cell cultures, carry or deliver biomacromolecules and drugs, bioimaging, and combinational benefits. Because of the thermal stability, ferrocene function, and physical and chemical properties of phosphorus, dendrimers show greater rigidity, mobility, and strength. These dendrimers will be discussed as having a favorable effect on cell growths, especially on neuronal cells, as well as human immune cells like natural killer cells and monocytes, which have a crucial part in preventing cancerous and viral infections. Several phosphorus dendrimers are effective as drugs by themselves (drug per se) and show their activity against neurodegenerative diseases, cancer, inflammation, ocular hypertension, and transmissible spongiform encephalopathies (TSEs) in both in vivo and in vitro. The present review discusses the synthetic route, fabrications, and biomedical applications of phosphorus-containing dendrimers. The toxicity of these dendrimers was also reported.

Gene Therapy for Regenerative Medicine

The development of biological methods over the past decade has stimulated great interest in the possibility to regenerate human tissues. Advances in stem cell research, gene therapy, and tissue engineering have accelerated the technology in tissue and organ regeneration. However, despite significant progress in this area, there are still several technical issues that must be addressed, especially in the clinical use of gene therapy. The aims of gene therapy include utilising cells to produce a suitable protein, silencing over-producing proteins, and genetically modifying and repairing cell functions that may affect disease conditions. While most current gene therapy clinical trials are based on cell- and viral-mediated approaches, non-viral gene transfection agents are emerging as potentially safe and effective in the treatment of a wide variety of genetic and acquired diseases. Gene therapy based on viral vectors may induce pathogenicity and immunogenicity. Therefore, significant efforts are being invested in non-viral vectors to enhance their efficiency to a level comparable to the viral vector. Non-viral technologies consist of plasmid-based expression systems containing a gene encoding, a therapeutic protein, and synthetic gene delivery systems. One possible approach to enhance non-viral vector ability or to be an alternative to viral vectors would be to use tissue engineering technology for regenerative medicine therapy. This review provides a critical view of gene therapy with a major focus on the development of regenerative medicine technologies to control the in vivo location and function of administered genes.

GDNF-Loaded Polydopamine Nanoparticles-Based Anisotropic Scaffolds Promote Spinal Cord Repair by Modulating Inhibitory Microenvironment

Spinal cord injury (SCI) is a devastating injury that causes permanent loss of sensation and motor function. SCI repair is a significant challenge due to the limited regenerating ability of adult neurons and the complex inflammatory microenvironment. After SCI, the oxidative stress induced by excessive reactive oxygen species (ROS) often leads to prolonged neuroinflammation that results in sustained damage to the spinal cord tissue. Polydopamine (PDA) shows remarkable capability in scavenging ROS to treat numerous inflammatory diseases. In this study, glial cell-derived neurotrophic factor (GDNF)-loaded PDA nanoparticle-based anisotropic scaffolds for spinal cord repair are developed. It is found that mesoporous PDA nanoparticles (mPDA NPs) in the scaffolds efficiently scavenge ROS and promote microglia M2 polarization, thereby inhibiting inflammatory response at the injury site and providing a favorable microenvironment for nerve cell survival. Furthermore, the GDNF encapsulated in mPDA NPs promotes corticospinal tract motor axon regeneration and its locomotor functional recovery. Together, findings from this study reveal that the GDNF-loaded PDA/Gelatin scaffolds hold potential as an effective artificial transplantation material for SCI treatment.

GDNF Gene Therapy to Repair the Injured Peripheral Nerve

A spinal root avulsion is the most severe proximal peripheral nerve lesion possible. Avulsion of ventral root filaments disconnects spinal motoneurons from their target muscles, resulting in complete paralysis. In patients that undergo brachial plexus nerve repair, axonal regeneration is a slow process. It takes months or even years to bridge the distance from the lesion site to the distal targets located in the forearm. Following ventral root avulsion, without additional pharmacological or surgical treatments, progressive death of motoneurons occurs within 2 weeks (Koliatsos et al., 1994). Reimplantation of the avulsed ventral root or peripheral nerve graft can act as a conduit for regenerating axons and increases motoneuron survival (Chai et al., 2000). However, this beneficial effect is transient. Combined with protracted and poor long-distance axonal regeneration, this results in permanent function loss. To overcome motoneuron death and improve functional recovery, several promising intervention strategies are being developed. Here, we focus on GDNF gene-therapy. We first introduce the experimental ventral root avulsion model and discuss its value as a proxy to study clinical neurotmetic nerve lesions. Second, we discuss our recent studies showing that GDNF gene-therapy is a powerful strategy to promote long-term motoneuron survival and improve function when target muscle reinnervation occurs within a critical post-lesion period. Based upon these observations, we discuss the influence of timing of the intervention, and of the duration, concentration and location of GDNF delivery on functional outcome. Finally, we provide a perspective on future research directions to realize functional recovery using gene therapy.

Phosphorus dendrimers as powerful nanoplatforms for drug delivery, as fluorescent probes and for liposome interaction studies: A concise overview

Gene therapy is a new and promising tool to treat many severe diseases and the silencing of proteins is the safest and the most efficient tool to treat diseases because it does not induce changes in human genome and avoids a huge problem encompassing insertional mutagenesis. Using small RNAs to switch on/off target proteins is limited due to existence of some barriers for them in the human body (blood RNAses, serum albumins, cell walls, etc). For therapeutic applications they need the efficient and non-toxic carrier which will deliver them into cell cytoplasm. Within the huge range of carriers available, dendrimers can be underlined as new promising efficient carriers. This review summarizes several findings in phosphorus dendrimers based on in vitro and in vivo studies. As a result, we can conclude that advantages of phosphorus dendrimers are strong interaction with siRNA/DNA and formation of small and compact positively charged complexes of high and fast penetration into cells; efficient release of siRNA/pDNA in endosomes due to "proton sponge" effect; possibility of their modification including addition of fluorescent probes - in this case fluorescent dendrimer can be used both as a gene carrier and a tracer of delivery into cells. Additional benefit of using fluorescent phosphorus dendrimers is their ability to monitor the macrophage physiological status in vitro and in vivo.

Erythropoietin-Induced Autophagy Protects Against Spinal Cord Injury and Improves Neurological Function via the Extracellular-Regulated Protein Kinase Signaling Pathway

The objective of this study was to explore the neuroprotective molecular mechanisms of erythropoietin (EPO) in rats following spinal cord injury (SCI). First, a standard SCI model was established. After drug or saline treatment was administered, locomotor function was evaluated in rats using the Basso, Beattie, and Bresnahan (BBB) locomotor rating scale. H&E, Nissl, and TUNEL staining were performed to assess the ratio of cavities, number of motor neurons, and apoptotic cells in the damaged area. The relative protein and mRNA expressions were examined using western blot and qRT-PCR analyses, and the inflammatory markers, axon special protein, and neuromuscular junctions (NMJs) were detected by immunofluorescence. Both doses of EPO notably improved locomotor function, but high-dose EPO was more effective than low-dose EPO. Moreover, EPO reduced the cavity ratio, cell apoptosis, and motor neuron loss in the damaged area, but enhanced the autophagy level and extracellular-regulated protein kinase (ERK) activity. Treatment with an ERK inhibitor significantly prevented the effect of EPO on SCI, and an activator mimicked the benefits of EPO. Further investigation revealed that EPO promoted SCI-induced autophagy via the ERK signaling pathway. EPO activates autophagy to promote locomotor function recovery in rats with SCI via the ERK signaling pathway.

Combining timed GDNF and ChABC gene therapy to promote long-distance regeneration following ventral root avulsion and repair

Ventral root avulsion leads to severe motoneuron degeneration and prolonged distal nerve denervation. After a critical period, a state of chronic denervation develops as repair Schwann cells lose their pro-regenerative properties and inhibitory factors such as CSPGs accumulate in the denervated nerve. In rats with ventral root avulsion injuries, we combined timed GDNF gene therapy delivered to the proximal nerve roots with the digestion of inhibitory CSPGs in the distal denervated nerve using sustained lentiviral-mediated chondroitinase ABC (ChABC) enzyme expression. Following reimplantation of lumbar ventral roots, timed GDNF-gene therapy enhanced motoneuron survival up to 45 weeks and improved axonal outgrowth, electrophysiological recovery, and muscle reinnervation. Despite a timed GDNF expression period, a subset of animals displayed axonal coils. Lentiviral delivery of ChABC enabled digestion of inhibitory CSPGs for up to 45 weeks in the chronically denervated nerve. ChABC gene therapy alone did not enhance motoneuron survival, but led to improved muscle reinnervation and modest electrophysiological recovery during later stages of the regeneration process. Combining GDNF treatment with digestion of inhibitory CSPGs did not have a significant synergistic effect. This study suggests a delicate balance exists between treatment duration and concentration in order to achieve therapeutic effects.

Regeneration of adult rat sensory and motor neuron axons through chimeric peroneal nerve grafts containing donor Schwann cells engineered to express different neurotrophic factors

Large peripheral nerve (PN) defects require bridging substrates to restore tissue continuity and permit the regrowth of sensory and motor axons. We previously showed that cell-free PN segments repopulated ex vivo with Schwann cells (SCs) transduced with lentiviral vectors (LV) to express different growth factors (BDNF, CNTF or NT-3) supported the regeneration of axons across a 1 cm peroneal nerve defect (Godinho et al., 2013). Graft morphology, the number of regrown axons, the ratio of myelinated to unmyelinated axons, and hindlimb locomotor function differed depending on the growth factor engineered into SCs. Here we extend these observations, adding more LVs (expressing GDNF or NGF) and characterising regenerating sensory and motor neurons after injection of the retrograde tracer Fluorogold (FG) into peroneal nerve distal to grafts, 10 weeks after surgery. Counts were also made in rats with intact nerves and in animals receiving autografts, acellular grafts, or grafts containing LV-GFP transduced SCs. Counts and analysis of FG positive (⁺) DRG neurons were made from lumbar (L5) ganglia. Graft groups contained fewer labeled sensory neurons than non-operated controls, but this decrease was only significant in the LV-GDNF group. These grafts had a complex fascicular morphology that may have resulted in axon trapping. The proportion of FG⁺ sensory neurons immunopositive for calcitonin-gene related peptide (CGRP) varied between groups, there being a significantly higher percentage in autografts and most neurotrophic factor groups compared to the LV-CNTF, LV-GFP and acellular groups. Furthermore, the proportion of regenerating isolectin B₄⁺ neurons was significantly greater in the LV-NT-3 group compared to other groups, including autografts and non-lesion controls. Immunohistochemical analysis of longitudinal graft sections revealed that all grafts contained a reduced number of choline acetyltransferase (ChAT) positive axons, but this decrease was significant only in the GDNF and NT-3 graft groups. We also assessed the number and phenotype of regrowing lumbar FG⁺ motor neurons in non-lesioned animals, and in rats with autografts, acellular grafts, or in grafts containing SCs expressing GFP, CNTF, NGF or NT-3. The overall number of FG⁺ motor neurons per section was similar in all groups; however in tissue immunostained for NeuN (expressed in α- but not γ-motor neurons) the proportion of NeuN negative FG⁺ neurons ranged from about 40-50% in all groups except the NT-3 group, where the percentage was 82%, significantly more than the SC-GFP group. Immunostaining for the vesicular glutamate transporter VGLUT-1 revealed occasional proprioceptive terminals in 'contact' with regenerating FG⁺ α-motor neurons in PN grafted animals, the acellular group having the lowest counts. In sum, while all graft types supported sensory and motor axon regrowth, there appeared to be axon trapping in SC-GDNF grafts, and data from the SC-NT-3 group revealed greater regeneration of sensory CGRP⁺ and IB₄⁺ neurons, preferential regeneration of γ-motor neurons and perhaps partial restoration of monosynaptic sensorimotor relays.

The Effects of Leukocyte-Platelet Rich Fibrin (L-PRF) on Suppression of the Expressions of the Pro-Inflammatory Cytokines, and Proliferation of Schwann Cell, and Neurotrophic Factors

This study evaluates the use of L-PRF as an autologous scaffold in nerve regeneration, and Schwann cells (SCs) proliferation and secretion of neurotrophic factors and its anti-inflammatory effect on SC Porphyromonas Gingivalis-Lipopolysaccharide (PG-LPS)-induced inflammatory responses in vitro. SEM was done to investigate various features of L-PRF. L-PRF-extracts was used to investigate the release of growth factors and treatment of SCs line. ELISA was applied to examine the release of IGF-1. The proliferative effect of L-PRF on SCs was assessed with CCK-8 assay. The effect of L-PRF on the mRNA and protein expression of SC neurotrophic factors were analyzed by RT-qPCR and ELISA. CCK-8 assay and RT-qPCR were used to determine the required concentration and the action time of PG-LPS before the anti-inflammatory effect of L-PRF was determined by measuring the changes in IL-1β, IL-6, and TNF-a with RT-qPCR and ELISA. There are different features in L-PRF. Fourteen days was sufficient to release adequate GF. The mRNA expressions of the pro-inflammatory cytokines were notably raised by PG-LPS in 3-hours treatment. L-PRF can increase SC proliferation, neurotrophic factors secretion, and suppress SC PG-LPS-induced inflammatory responses in vitro. L-PRF has the potential as an autologous biological additive for peripheral nerve regeneration in the event of nerve inflammation and injuries.

Long-gap peripheral nerve repair through sustained release of a neurotrophic factor in nonhuman primates

Severe injuries to peripheral nerves are challenging to repair. Standard-of-care treatment for nerve gaps >2 to 3 centimeters is autografting; however, autografting can result in neuroma formation, loss of sensory function at the donor site, and increased operative time. To address the need for a synthetic nerve conduit to treat large nerve gaps, we investigated a biodegradable poly(caprolactone) (PCL) conduit with embedded double-walled polymeric microspheres encapsulating glial cell line–derived neurotrophic factor (GDNF) capable of providing a sustained release of GDNF for >50 days in a 5-centimeter nerve defect in a rhesus macaque model. The GDNF-eluting conduit (PCL/GDNF) was compared to a median nerve autograft and a PCL conduit containing empty microspheres (PCL/Empty). Functional testing demonstrated similar functional recovery between the PCL/GDNF-treated group (75.64 ± 10.28%) and the autograft-treated group (77.49 ± 19.28%); both groups were statistically improved compared to PCL/Empty-treated group (44.95 ± 26.94%). Nerve conduction velocity 1 year after surgery was increased in the PCL/GDNF-treated macaques (31.41 ± 15.34 meters/second) compared to autograft (25.45 ± 3.96 meters/second) and PCL/Empty (12.60 ± 3.89 meters/second) treatment. Histological analyses included assessment of Schwann cell presence, myelination of axons, nerve fiber density, and g-ratio. PCL/GDNF group exhibited a statistically greater average area occupied by individual Schwann cells at the distal nerve (11.60 ± 33.01 μm2) compared to autograft (4.62 ± 3.99 μm2) and PCL/Empty (4.52 ± 5.16 μm2) treatment groups. This study demonstrates the efficacious bridging of a long peripheral nerve gap in a nonhuman primate model using an acellular, biodegradable nerve conduit.

Modern Trends for Peripheral Nerve Repair and Regeneration: Beyond the Hollow Nerve Guidance Conduit

Peripheral nerve repair and regeneration remains among the greatest challenges in tissue engineering and regenerative medicine. Even though peripheral nerve injuries (PNIs) are capable of some degree of regeneration, frail recovery is seen even when the best microsurgical technique is applied. PNIs are known to be very incapacitating for the patient, due to the deprivation of motor and sensory abilities. Since there is no optimal solution for tackling this problem up to this day, the evolution in the field is constant, with innovative designs of advanced nerve guidance conduits (NGCs) being reported every day. As a basic concept, a NGC should act as a physical barrier from the external environment, concomitantly acting as physical guidance for the regenerative axons across the gap lesion. NGCs should also be able to retain the naturally released nerve growth factors secreted by the damaged nerve stumps, as well as reducing the invasion of scar tissue-forming fibroblasts to the injury site. Based on the neurobiological knowledge related to the events that succeed after a nerve injury, neuronal subsistence is subjected to the existence of an ideal environment of growth factors, hormones, cytokines, and extracellular matrix (ECM) factors. Therefore, it is known that multifunctional NGCs fabricated through combinatorial approaches are needed to improve the functional and clinical outcomes after PNIs. The present work overviews the current reports dealing with the several features that can be used to improve peripheral nerve regeneration (PNR), ranging from the simple use of hollow NGCs to tissue engineered intraluminal fillers, or to even more advanced strategies, comprising the molecular and gene therapies as well as cell-based therapies.

Enhanced regeneration and reinnervation following timed GDNF gene therapy in a cervical ventral root avulsion

Avulsion of spinal nerve roots is a severe proximal peripheral nerve lesion. Despite neurosurgical repair, recovery of function in human patients is disappointing, because spinal motor neurons degenerate progressively, axons grow slowly and the distal Schwann cells which are instrumental to supporting axon extension lose their pro-regenerative properties. We have recently shown that timed GDNF gene therapy (dox-i-GDNF) in a lumbar plexus injury model promotes axon regeneration and improves electrophysiological recovery but fails to stimulate voluntary hind paw function. Here we report that dox-i-GDNF treatment following avulsion and re-implantation of cervical ventral roots leads to sustained motoneuron survival and recovery of voluntary function. These improvements were associated with a twofold increase in motor axon regeneration and enhanced reinnervation of the hand musculature. In this cervical model the distal hand muscles are located 6,5 cm from the reimplantation site, whereas following a lumber lesion this distance is twice as long. Since the first signs of muscle reinnervation are observed 6 weeks after the lesion, this suggests that regenerating axons reached the hand musculature before a critical state of chronic denervation has developed. These results demonstrate that the beneficial effects of timed GDNF-gene therapy are more robust following spinal nerve avulsion lesions that allow reinnervation of target muscles within a relatively short time window after the lesion. This study is an important step in demonstrating the potential of timed GDNF-gene therapy to enhance axon regeneration after neurosurgical repair of a severe proximal nerve lesion.

GDNF-ADSCs-APG embedding enhances sciatic nerve regeneration after electrical injury in a rat model

The pluripotency of adipose-derived stem cells (ADSCs) makes them appropriate for tissue repair and wound healing. Owing to the repair properties of autologous platelet-rich gel (APG), which is based on easily accessible blood platelets, its clinical use has been increasingly recognized by physicians. The aim of this study was to investigate the effect of combined treatment with ADSCs and APG on sciatic nerve regeneration after electrical injury. To facilitate the differentiation of ADSCs, glial cell line-derived neurotrophic factor (GDNF) was overexpressed in ADSCs by lentivirus transfection. GDNF-ADSCs were mingled with APG gradient concentrations, and in vitro, cell proliferation and differentiation were examined with 5-ethynyl-2'-deoxyuridine staining and immunofluorescence. A rat model was established by exposing the sciatic nerve to an electrical current of 220 V for 3 seconds. Rat hind-limb motor function and sciatic nerve regeneration were subsequently evaluated. Rat ADSCs were characterized by high expression of CD90 and CD105, with scant expression of CD34 and CD45. We found that GDNF protein expression in ADSCs was elevated after Lenti-GDNF transfection. In GDNF-ADSCs-APG cultures, GDNF was increasingly produced while tissue growth factor-β was reduced as incubation time was increased. ADSC proliferation was augmented and neuronal nuclei (NeuN) and glial fibrillary acidic protein (GFAP) expression were upregulated in GDNF-ADSCs-APG. In addition, limb motor function and nerve axon growth were improved after GDNF-ADSCs-APG treatment. In conclusion, our study demonstrates the combined effect of ADSCs and APG in peripheral nerve regeneration and may lead to treatments that benefit patients with electrical injuries.

Time-restricted release of multiple neurotrophic factors promotes axonal regeneration and functional recovery after peripheral nerve injury

Delivery of multiple neurotrophic factors (NTFs), especially with time-restricted release kinetics, holds great potential for nerve repair. In this study, we utilized the tetracycline-regulatable Tet-On 3G system to control the expression of c-Jun, which is a common regulator of multiple NTFs in Schwann cells (SCs). In vitro, Tet-On/c-Jun-modified SCs showed a tightly controllable secretion of multiple NTFs, including glial cell line-derived NTF, nerve growth factor, brain-derived NTF, and artemin, by the addition or removal of doxycycline (Dox). When Tet-On/c-Jun-transduced SCs were grafted in vivo, the expression of NTFs could also be regulated by oral administration or removal of Dox. Fluoro-Gold retrograde tracing results indicated that a biphasic NTF expression scheme (Dox+3/-9, NTFs were up-regulated for 3 wk and declined to physiologic levels for another 9 wk) achieved more axonal regeneration than continuous up-regulation of NTFs (Dox+12) or no NTF induction (Dox-12). More importantly, the Dox+3/-9-group animals showed much better functional recovery than the animals in the Dox+12 and Dox-12 groups. Our findings, for the first time, demonstrated drug-controllable expression of multiple NTFs in nerve repair cells both in vitro and in vivo. These findings provide new hope for developing an optimal therapeutic alternative for nerve repair through the time-restricted release of multiple NTFs using Tet-On/c-Jun-modified SCs.-Huang, L., Xia, B., Shi, X., Gao, J., Yang, Y., Xu, F., Qi, F., Liang, C., Huang, J., Luo, Z. Time-restricted release of multiple neurotrophic factors promotes axonal regeneration and functional recovery after peripheral nerve injury.

Qian-Zheng-San promotes regeneration after sciatic nerve crush injury in rats

Qian-Zheng-San, a traditional Chinese prescription consisting of Typhonii Rhizoma, Bombyx Batryticatus, Scorpio, has been found to play an active therapeutic role in central nervous system diseases. However, it is unclear whether Qian-Zheng-San has therapeutic value for peripheral nerve injury. Therefore, we used Sprague-Dawley rats to investigate this. A sciatic nerve crush injury model was induced by clamping the right sciatic nerve. Subsequently, rats in the treatment group were administered 2 mL Qian-Zheng-San (1.75 g/mL) daily as systemic therapy for 1, 2, 4, or 8 weeks. Rats in the control group were not administered Qian-Zheng-San. Rats in sham group did not undergo surgery and systemic therapy. Footprint analysis was used to assess nerve motor function. Electrophysiological experiments were used to detect nerve conduction function. Immunofluorescence staining was used to assess axon counts and morphological analysis. Immunohistochemical staining was used to observe myelin regeneration of the sciatic nerve and the number of motoneurons in the anterior horn of the spinal cord. At 2 and 4 weeks postoperatively, the sciatic nerve function index, nerve conduction velocity, the number of distant regenerated axons and the axon diameter of the sciatic nerve increased in the Qian-Zheng-San treatment group compared with the control group. At 2 weeks postoperatively, nerve fiber diameter, myelin thickness, and the number of motor neurons in the lumbar spinal cord anterior horn increased in the Qian-Zheng-San treatment group compared with the control group. These results indicate that Qian-Zheng-San has a positive effect on peripheral nerve regeneration.

A microfluidic platform to study the effects of GDNF on neuronal axon entrapment

BACKGROUND

One potential treatment strategy to enhance axon regeneration is transplanting Schwann Cells (SCs) that overexpress glial cell line-derived neurotrophic factor (GDNF). Unfortunately, constitutive GDNF overexpression in vivo can result in failure of regenerating axons to extend beyond the GDNF source, a phenomenon termed the "candy-store" effect. Little is known about the mechanism of this axon entrapment in vivo.

NEW METHOD

We present a reproducible in vitro culture platform using a microfluidic device to model axon entrapment and investigate mechanisms by which GDNF causes axon entrapment. The device is comprised of three culture chambers connected by two sets of microchannels, which prevent cell soma from moving between chambers but allow neurites to grow between chambers. Neurons from dorsal root ganglia were seeded in one end chamber while the effect of different conditions in the other two chambers was used to study neurite entrapment.

RESULTS

The results showed that GDNF-overexpressing SCs (G-SCs) can induce axon entrapment in vitro. We also found that while physiological levels of GDNF (100 ng/mL) promoted neurite extension, supra-physiological levels of GDNF (700 ng/mL) induced axon entrapment.

COMPARISON WITH EXISTING METHOD

All previous work related to the "candy-store" effect were done in vivo. Here, we report the first in vitro platform that can recapitulate the axonal entrapment and investigate the mechanism of the phenomenon.

CONCLUSIONS

This platform facilitates investigation of the "candy-store" effect and shows the effects of high GDNF concentrations on neurite outgrowth.

Phosphorus dendrimers for nanomedicine

From biomaterials to imaging, and from drug delivery to drugs by themselves, phosphorus-containing dendrimers offer a large palette of biological properties, depending essentially on their types of terminal functions. The most salient examples of phosphorus dendrimers used for the elaboration of bio-chips and of supports for cell cultures, for imaging biological events, and for carrying and delivering drugs or biomacromolecules are presented in this feature article. Several phosphorus dendrimers can be considered also as drugs per se (by themselves) in particular to fight against cancers, neurodegenerative diseases, and inflammation, both in vitro and in vivo. Toxicity assays are also reported.

Sustained delivery of glial cell-derived neurotrophic factors in collagen conduits for facial nerve regeneration

Facial nerve injury caused by traffic accidents or operations may reduce the quality of life in patients, and recovery following the injury presents unique clinical challenges. Glial cell-derived neurotrophic factor (GDNF) is important in nerve regeneration; however, soluble GDNF rapidly diffuses into body fluids, making it difficult to achieve therapeutic efficacy. In this work, we developed a rat tail derived collagen conduit to connect nerve defects in a simple and safe manner. GDNF was immobilized in the collagen conduits via chemical conjugation to enable controlled release of GDNF. The GDNF delivery system prevented rapid diffusion from the site without impacting bioactivity of GDNF; degradation of the collagen conduit was inhibited owing to the chemical conjugation. The artificial nerve conduit was then used to examine facial nerve regeneration across a facial nerve defect. Following transplantation, the artificial nerve conduits degraded gradually without causing dislocations and serious inflammation, with good integration into the host tissue. Functional and histological tests indicated that the artificial nerve conduits were able to guide the axons to grow through the defect, reaching the distal stumps. The degree of nerve regeneration in the group that was treated with the artificial nerve conduit approached that of the autograft group, and exceeded that of the other conduit grafted groups.

STATEMENT OF SIGNIFICANCE

In this study, we developed artificial nerve conduits consisting of GDNF immobilized on collagen, with the aim of providing an environment for nerve regeneration. Our results show that the artificial nerve conduits guided the regeneration of axons to the distal nerve segment. GDNF was immobilized stably in the artificial nerve conduits, and therefore retained a sufficient concentration at the target site to effectively promote the regeneration process. The artificial nerve conduits exhibited good biocompatibility and facilitated nerve regeneration and functional recovery with an efficacy that was close to that of an autograft, and better than that of the other conduit grafted groups. Our approach provides an effective delivery system that overcomes the rapid diffusion of GDNF in body fluids, promoting peripheral nerve regeneration. The artificial nerve conduit therefore qualifies as a putative candidate material for the fabrication of peripheral nerve reconstruction devices.

Which Dendrimer to Attain the Desired Properties? Focus on Phosphorhydrazone Dendrimers

Among the six Critical Nanoscale Design Parameters (CNDPs) proposed by Prof. Donald A. Tomalia, this review illustrates the influence of the sixth one, which concerns the elemental composition, on the properties of dendrimers. After a large introduction that summarizes different types of dendrimers that have been compared with PolyAMidoAMine (PAMAM) dendrimers, this review will focus on the properties of positively and negatively charged phosphorhydrazone (PPH) dendrimers, especially in the field of biology, compared with other types of dendrimers, in particular PAMAM dendrimers, as well as polypropyleneimine (PPI), carbosilane, and p-Lysine dendrimers.

An update-tissue engineered nerve grafts for the repair of peripheral nerve injuries

Peripheral nerve injuries (PNI) are caused by a range of etiologies and result in a broad spectrum of disability. While nerve autografts are the current gold standard for the reconstruction of extensive nerve damage, the limited supply of autologous nerve and complications associated with harvesting nerve from a second surgical site has driven groups from multiple disciplines, including biomedical engineering, neurosurgery, plastic surgery, and orthopedic surgery, to develop a suitable or superior alternative to autografting. Over the last couple of decades, various types of scaffolds, such as acellular nerve grafts (ANGs), nerve guidance conduits, and non-nervous tissues, have been filled with Schwann cells, stem cells, and/or neurotrophic factors to develop tissue engineered nerve grafts (TENGs). Although these have shown promising effects on peripheral nerve regeneration in experimental models, the autograft has remained the gold standard for large nerve gaps. This review provides a discussion of recent advances in the development of TENGs and their efficacy in experimental models. Specifically, TENGs have been enhanced via incorporation of genetically engineered cells, methods to improve stem cell survival and differentiation, optimized delivery of neurotrophic factors via drug delivery systems (DDS), co-administration of platelet-rich plasma (PRP), and pretreatment with chondroitinase ABC (Ch-ABC). Other notable advancements include conduits that have been bioengineered to mimic native nerve structure via cell-derived extracellular matrix (ECM) deposition, and the development of transplantable living nervous tissue constructs from rat and human dorsal root ganglia (DRG) neurons. Grafts composed of non-nervous tissues, such as vein, artery, and muscle, will be briefly discussed.

Transgenic SCs expressing GDNF-IRES-DsRed impair nerve regeneration within acellular nerve allografts

Providing temporally regulated glial cell line-derived neurotrophic factor (GDNF) to injured nerve can promote robust axon regeneration. However, it is poorly understood why providing highly elevated levels of GDNF to nerve can lead to axon entrapment in the zone containing elevated GDNF. This limited understanding represents an obstacle to the translation of GDNF therapies to treat nerve injuries clinically. Here, we investigated how transgenic Schwann cells (SCs) overexpressing GDNF-IRES-DsRed impact nerve regeneration. Cultured primary SCs were transduced with lentiviruses (GDNF-overexpressing transgenic SCs), one of which provides the capability to express high levels of GDNF and regulate temporal GDNF expression. These SC groups were transplanted into acellular nerve allografts (ANAs) bridging a 14 mm rat sciatic nerve defect. GDNF-overexpressing transgenic SCs expressing GDNF for as little as 1 week decreased axon regeneration across ANAs and caused extensive extracellular matrix (ECM) remodeling. To determine whether additional gene expression changes beyond GDNF transgene expression occurred in GDNF-overexpressing transgenic SCs, microarray analysis of GDNF-overexpressing transgenic SCs compared to untreated SCs was performed. Microarray analysis revealed a set of common genes regulated in transgenic SC groups expressing high levels of GDNF compared to untreated SCs. A co-culture model of GDNF-overexpressing transgenic SCs with fibroblasts (FBs) revealed differential FB ECM-related gene expression compared to untreated SCs. These data suggest a component of axon entrapment is independent of GDNF's impact on axons. Biotechnol. Bioeng. 2017;114: 2121-2130. © 2017 Wiley Periodicals, Inc.

The role of growth factors in nerve regeneration

Nerve injuries result in functional loss in the innervated organ or body parts, and recovery is difficult unless surgical treatment has been done. Different surgical treatments have been suggested for nerve repair. Tissue engineering related to growth factors has arisen as an alternative approach for triggering and improving nerve regeneration. Therefore, the aim of this review is to provide a comprehensive analysis related to growth factors as tools for optimizing the regeneration process. Studies and reviews on the use of growth factors for nerve regeneration were compiled over the course of the review. According to literature review, it may be concluded that growth factors from different sources present promising treatment related to nerve regeneration involved in neuronal differentiation, greater myelination and axonal growth and proliferation of specific cells for nerve repair.

Finely Tuned Temporal and Spatial Delivery of GDNF Promotes Enhanced Nerve Regeneration in a Long Nerve Defect Model

The use of growth factors, such as glial cell line-derived neurotrophic factor (GDNF), for the treatment of peripheral nerve injury has been useful in promoting axon survival and regeneration. Unfortunately, finding a method that delivers the appropriate spatial and temporal release profile to promote functional recovery has proven difficult. Some release methods result in burst release profiles too short to remain effective over the regeneration period; however, prolonged exposure to GDNF can result in axonal entrapment at the site of release. Thus, GDNF was delivered in both a spatially and temporally controlled manner using a two-phase system comprised of an affinity-based release system and conditional lentiviral GDNF overexpression from Schwann cells (SCs). Briefly, SCs were transduced with a tetracycline-inducible (Tet-On) GDNF overexpressing lentivirus before transplantation. Three-centimeter acellular nerve allografts (ANAs) were modified by injection of a GDNF-releasing fibrin scaffold under the epineurium and then used to bridge a 3 cm sciatic nerve defect. To encourage growth past the ANA, GDNF-SCs were transplanted into the distal nerve and doxycycline was administered for 4, 6, or 8 weeks to determine the optimal duration of GDNF expression in the distal nerve. Live imaging and histomorphometric analysis determined that 6 weeks of doxycycline treatment resulted in enhanced regeneration compared to 4 or 8 weeks. This enhanced regeneration resulted in increased gastrocnemius and tibialis anterior muscle mass for animals receiving doxycycline for 6 weeks. The results of this study demonstrate that strategies providing spatial and temporal control of delivery can improve axonal regeneration and functional muscle reinnervation.

Regulable Transgene Expression in Dorsal Root Ganglia of a Replication-Defective Herpes Simplex Virus Type 1 Vector by Means of Sciatic Nerve Injection

BACKGROUND

Targeted and controllable gene delivery to neurons is essential to efforts to facilitate peripheral nerve regeneration. The authors investigated both the in vitro and in vivo expression profiles of a tetracycline-controlled, replication-defective, herpes simplex virus type 1-based vector.

METHODS

Mouse primary dorsal root ganglia cells were infected with QR9TO-LacZ in the absence or presence of tetracycline. LacZ gene expression was examined. It was also injected into sciatic nerves in CD-1 mice fed with and without tetracycline. LacZ expression in the upstream dorsal root ganglia was examined.

RESULTS

Following inoculation with QR9TO-LacZ, approximately 40 percent of the cultured primary dorsal root ganglia cells exhibited strong LacZ activity in the presence of tetracycline at 48 and 72 hours, whereas little was detected in those in the absence of tetracycline. Quantitative analysis revealed that the β-galactosidase activity within cells exposed to tetracycline increased 181-fold at 48 hours (p < 0.001) and 47-fold at 72 hours after infection (p < 0.05) compared with those without tetracycline. However, this LacZ transgene activity in the presence of tetracycline tapered off to less than sevenfold over baseline 168 hours after infection (p < 0.05). Furthermore, successful uptake of this replication-defective viral vector was evident in upstream dorsal root ganglia after sciatic nerve injection in mice. In addition, its expression profile was similar to that in vitro, as strong β-galactosidase activity was evident only in mice fed with a doxycycline-containing diet, and it tapered off by 168 hours.

CONCLUSION

The replication-defective herpes simplex virus type 1-based vector, which provides tightly regulated transgene expression in dorsal root ganglia by means of peripheral nerve injection, represents an appealing approach to improve peripheral nerve regeneration.

Nanoscale effects in dendrimer-mediated targeting of neuroinflammation

Neuroinflammation, mediated by activated microglia and astrocytes, plays a key role in the pathogenesis of many neurological disorders. Systemically-administered dendrimers target neuroinflammation and deliver drugs with significant efficacy, without the need for ligands. Elucidating the nanoscale aspects of targeting neuroinflammation will enable superior nanodevices for eventual translation. Using a rabbit model of cerebral palsy, we studied the in vivo contributions of dendrimer physicochemical properties and disease pathophysiology on dendrimer brain uptake, diffusion, and cell specific localization. Neutral dendrimers move efficiently within the brain parenchyma and rapidly localize in glial cells in regions of injury. Dendrimer uptake is also dependent on the extent of blood-brain-barrier breakdown, glial activation, and disease severity (mild, moderate, or severe), which can lend the dendrimer to be used as an imaging biomarker for disease phenotype. This new understanding of the in vivo mechanism of dendrimer-mediated delivery in a clinically-relevant rabbit model provides greater opportunity for clinical translation of targeted brain injury therapies.

Inorganic dendrimers: recent advances for catalysis, nanomaterials, and nanomedicine

Dendrimers are hyperbranched polymers having a perfectly defined structure because they are synthesized step-by-step in an iterative fashion, and not by polymerization reactions. Some dendrimers are considered as inorganic, as they possess inorganic atoms at each branching point. Among numerous examples, two families of inorganic dendrimers have emerged as particularly promising: silicon-containing dendrimers, particularly carbosilanes, and phosphorus-containing dendrimers, particularly phosphorhydrazones. This tutorial review will display the main properties of both families of dendrimers in the fields of catalysis, materials and biology/nanomedicine. Emphasis will be put on the most recent and promising examples.

c-Jun gene-modified Schwann cells: upregulating multiple neurotrophic factors and promoting neurite outgrowth

Genetically modified Schwann cells (SCs) that overexpress neurotrophic factors (NFs), especially those that overexpress multiple NFs, hold great potential for promoting nerve regeneration. Currently, only one NF can be upregulated in most genetically modified SCs, and simultaneously upregulating multiple NFs in SCs remains challenging. In this study, we found that the overexpression of c-Jun, a component of the AP-1 transcription factor, effectively upregulated the expression and secretion of multiple NFs, including glial cell line-derived neurotrophic factor, brain-derived neurotrophic factor, artemin, leukemia inhibitory factor, and nerve growth factor. The c-Jun gene-modified SCs showed a normal morphology in scanning electron microscopy and fluorescent staining analysis. In addition, the c-Jun-modified SCs showed enhanced proliferation and migration abilities compared with vector control cells. We used transwell chambers to establish coculture systems imitating the in vivo conditions in which transplanted SCs might influence native SCs and neurons. We found that the c-Jun-modified SCs enhanced native SC migration and promoted the proliferation of native SCs in the presence of axons. Further analysis revealed that in the c-Jun group, the average length and the total area of neurites divided by the total area of the explant body were μm 1180±25 and 6.4±0.4, respectively, which were significantly greater compared with the other groups. These findings raise the possibility of constructing an optimal therapeutic alternative for nerve repair using c-Jun-modified SCs, which have the potential to promote axonal regeneration and functional recovery by upregulating multiple NFs. In addition, these cells exhibit enhanced migration and proliferation abilities, enhance the biological functions of native SCs, and promote neurite outgrowth.

Concentrated growth factor increases Schwann cell proliferation and neurotrophic factor secretion and promotes functional nerve recovery in vivo

Concentrated growth factor (CGF) is a newly generated complex that comprises a fibrin matrix incorporating growth factors and plasmatic and leukocyte cytokines. It has been widely used in bone regenerative medicine. However, the effect of CGF on peripheral nerve regeneration had not been previously investigated. The aim of the present study was to evaluate the possibility of using CGF for nerve regeneration by i) investigating the effect of CGF on the proliferation of Schwann cells (SCs) and secretion of neurotrophic factors nerve growth factor (NGF) and glial cell line‑derived neurotrophic factor (GDNF) in vitro; and ii) analyzing the effect of CGF on functional nerve recovery after nerve injury in vivo. CGF was prepared from venous blood taken from rats, and using scanning electron microscopy (SEM) we noted that it featured a fiber‑like appearance with pore size ranging from 0.1 to 1.0 µm. The soluble component of CGF was used to produce conditioned media with which to treat the Schwann cell line. A cell counting kit-8 assay and cell cycle analysis were both used to study the proliferative effect of CGF on SCs. Reverse transcription-quantitative PCR and western blot analysis demonstrated that there was an increase in the mRNA and protein expression of NGF and GDNF, both of which are markers of SC neurotrophic secretion. A model of sciatic nerve crush injury was established for the in vivo experiment, and CGF was found to increase the sciatic functional index (indicative of nerve function). We noted that CGF increased SC proliferation and secretion of neurotrophic factors in vitro, and promoted functional recovery after peripheral nerve injuries in vivo. These results suggest that CGF is a promising candidate biomaterial for peripheral nerve regeneration, and may potentially be utilized to repair nerve injuries.

Clinical and neurobiological advances in promoting regeneration of the ventral root avulsion lesion

Root avulsions due to traction to the brachial plexus causes complete and permanent loss of function. Until fairly recent, such lesions were considered impossible to repair. Here we review clinical repair strategies and current progress in experimental ventral root avulsion lesions. The current gold standard in patients with a root avulsion is nerve transfer, whereas reimplantation of the avulsed root into the spinal cord has been performed in a limited number of cases. These neurosurgical repair strategies have significant benefit for the patient but functional recovery remains incomplete. Developing new ways to improve the functional outcome of neurosurgical repair is therefore essential. In the laboratory, the molecular and cellular changes following ventral root avulsion and the efficacy of intervention strategies have been studied at the level of spinal motoneurons, the ventral spinal root and peripheral nerve, and the skeletal muscle. We present an overview of cell-based pharmacological and neurotrophic factor treatment approaches that have been applied in combination with surgical reimplantation. These interventions all demonstrate neuroprotective effects on avulsed motoneurons, often accompanied with various degrees of axonal regeneration. However, effects on survival are usually transient and robust axon regeneration over long distances has as yet not been achieved. Key future areas of research include finding ways to further extend the post-lesion survival period of motoneurons, the identification of neuron-intrinsic factors which can promote persistent and long-distance axon regeneration, and finally prolonging the pro-regenerative state of Schwann cells in the distal nerve.

Human umbilical cord mesenchymal stem cells promote peripheral nerve repair via paracrine mechanisms

Human umbilical cord-derived mesenchymal stem cells (hUCMSCs) represent a promising young-state stem cell source for cell-based therapy. hUCMSC transplantation into the transected sciatic nerve promotes axonal regeneration and functional recovery. To further clarify the paracrine effects of hUCMSCs on nerve regeneration, we performed human cytokine antibody array analysis, which revealed that hUCMSCs express 14 important neurotrophic factors. Enzyme-linked immunosorbent assay and immunohistochemistry showed that brain-derived neurotrophic factor, glial-derived neurotrophic factor, hepatocyte growth factor, neurotrophin-3, basic fibroblast growth factor, type I collagen, fibronectin and laminin were highly expressed. Treatment with hUCMSC-conditioned medium enhanced Schwann cell viability and proliferation, increased nerve growth factor and brain-derived neurotrophic factor expression in Schwann cells, and enhanced neurite growth from dorsal root ganglion explants. These findings suggest that paracrine action may be a key mechanism underlying the effects of hUCMSCs in peripheral nerve repair.

Recent advances in stem cell-mediated peripheral nerve repair

A major advance in the field of peripheral nerve repair has been the advent of stem and progenitor cell use to supplement the regenerative environment in animal models of nerve injury. As Schwann cell replacements, stem cells may be even better suited to promoting regeneration in these scenarios. We review the recent literature detailing the search for the definitive Schwann cell replacement cell, including a look at genetic modification of transplanted cells for nerve injury repair.

Gene therapy strategies for the treatment of spinal cord injury

Spinal cord injury is a complex pathology often resulting in functional impairment and paralysis. Gene therapy has emerged as a possible solution to the problems of limited neural tissue regeneration through the administration of factors promoting axonal growth, while also offering long-term local delivery of therapeutic molecules at the injury site. Of note, gene therapy is our response to the requirements of neural and glial cells following spinal cord injury, providing, in a time-dependent manner, growth substances for axonal regeneration and eliminating axonal growth inhibitors. Herein, we explore different gene therapy strategies, including targeting gene expression to modulate the presence of neurotrophic growth or survival factors and increase neural tissue plasticity. Special attention is given to describing advances in viral and non-viral gene delivery systems, as well as the available routes of gene delivery. Finally, we discuss the future of combinatorial gene therapies and give consideration to the implementation of gene therapy in humans.

A review of bioactive release from nerve conduits as a neurotherapeutic strategy for neuronal growth in peripheral nerve injury

Peripheral nerve regeneration strategies employ the use of polymeric engineered nerve conduits encompassed with components of a delivery system. This allows for the controlled and sustained release of neurotrophic growth factors for the enhancement of the innate regenerative capacity of the injured nerves. This review article focuses on the delivery of neurotrophic factors (NTFs) and the importance of the parameters that control release kinetics in the delivery of optimal quantities of NTFs for improved therapeutic effect and prevention of dose dumping. Studies utilizing various controlled-release strategies, in attempt to obtain ideal release kinetics, have been reviewed in this paper. Release strategies discussed include affinity-based models, crosslinking techniques, and layer-by-layer technologies. Currently available synthetic hollow nerve conduits, an alternative to the nerve autografts, have proven to be successful in the bridging and regeneration of primarily the short transected nerve gaps in several patient cases. However, current research emphasizes on the development of more advanced nerve conduits able to simulate the effectiveness of the autograft which includes, in particular, the ability to deliver growth factors.

Schwann cells transduced with a lentiviral vector encoding Fgf-2 promote motor neuron regeneration following sciatic nerve injury

Fibroblast growth factor 2 (FGF-2) is a trophic factor expressed by glial cells and different neuronal populations. Addition of FGF-2 to spinal cord and dorsal root ganglia (DRG) explants demonstrated that FGF-2 specifically increases motor neuron axonal growth. To further explore the potential capability of FGF-2 to promote axon regeneration, we produced a lentiviral vector (LV) to overexpress FGF-2 (LV-FGF2) in the injured rat peripheral nerve. Cultured Schwann cells transduced with FGF-2 and added to collagen matrix embedding spinal cord or DRG explants significantly increased motor but not sensory neurite outgrowth. LV-FGF2 was as effective as direct addition of the trophic factor to promote motor axon growth in vitro. Direct injection of LV-FGF2 into the rat sciatic nerve resulted in increased expression of FGF-2, which was localized in the basal lamina of Schwann cells. To investigate the in vivo effect of FGF-2 overexpression on axonal regeneration after nerve injury, Schwann cells transduced with LV-FGF2 were grafted in a silicone tube used to repair the resected rat sciatic nerve. Electrophysiological tests conducted for up to 2 months after injury revealed accelerated and more marked reinnervation of hindlimb muscles in the animals treated with LV-FGF2, with an increase in the number of motor and sensory neurons that reached the distal tibial nerve at the end of follow-up.

Viral transduction of primary Schwann cells using a Cre-lox system to regulate GDNF expression

Glial cell-line-derived neurotrophic factor (GDNF) is a potent neurotrophic factor known to enhance motor nerve regeneration following its delivery. However, recent studies have determined that extended GDNF delivery to regenerating axons can entrap motor axons at the site of GDNF delivery. This entrapment leads to reduced motor axons available to reinnervate muscle. To address this issue, we designed a cell-based GDNF expression system that can temporally regulate protein expression using an inducible gene excision mechanism to prevent entrapment at the site of expression. To design this system for regulation of GDNF expression, we transduced two lentiviral vectors, one containing a constitutively active GDNF transgene flanked by two loxP sites, and the other containing a tetracycline-inducible cre transgene along with its constitutively active transactivator, into Schwann cells (SCs). These SCs over-express GDNF, but expression can be suppressed through the administration of tetracycline family antibiotics, such as doxycycline. The engineered SCs produced significantly more GDNF as compared to untransduced controls, as measured by enzyme-linked immunosorbent assay (ELISA). Following doxycycline treatment, these SCs produced significantly lower levels of GDNF and induced less neurite extension as compared to untreated SCs. Engineered SCs treated with doxycycline showed a marked increase in Cre recombinase expression, as visualized by immunohistochemistry (IHC), providing evidence of a mechanism for the observed changes in GDNF expression levels and biological activity. This cell-based GDNF expression system could have potential for future in vivo studies to provide a temporally controlled GDNF source to promote axon growth.