Peripheral Motor and Sensory Nerve Conduction following Transplantation of Undifferentiated Autologous Adipose Tissue–Derived Stem Cells in a Biodegradable U.S. Food and Drug Administration–Approved Nerve Conduit:

Exosomes and tissue engineering: A novel therapeutic strategy for nerve regenerative

Damage to nerves negatively impacts quality of life and causes considerable morbidity. Self-regeneration is a special characteristic of the nervous system, yet how successful regeneration is accomplished remains unclear. Research on nerve regeneration is advancing and accelerating successful nerve recovery with potential new approaches. Eukaryote cells release extracellular vesicles (EVs), which control intercellular communication in both health and disease. More and more, EVs such as microvesicles and exosomes (EXOs) are being recognized as viable options for cell-free therapies that address complex tissue regeneration. The present study highlights the functional relevance of EVs in regenerative medicine for nerve-related regeneration. A subclass of EVs, EXOs were first identified as a way for cells to expel undesirable cell products. These nanovesicles have a diameter of 30-150 nm and are secreted by a variety of cells in conditions of both health and illness. Their benefits include the ability to promote endothelial cell growth, inhibit inflammation, encourage cell proliferation, and regulate cell differentiation. They are also known to transport functional proteins, metabolites, and nucleic acids to recipient cells, thus playing a significant role in cellular communication. EXOs impact an extensive array of physiological functions, including immunological responses, tissue regeneration, stem cell conservation, communication within the central nervous system, and pathological processes involving cardiovascular disorders, neurodegeneration, cancer, and inflammation. Their biocompatibility and bi-layered lipid structure (which shields the genetic consignment from deterioration and reduces immunogenicity) make them appealing as therapeutic vectors. They can pass through the blood brain barrier and other major biological membranes because of their small size and membrane composition. The creation of modified EXOs is a dynamic area of research that supports the evaluation of diverse therapeutic freights, improvement of target selectivity, and manufacturing optimization.

The limelight of adipose-derived stem cells in the landscape of neural tissue engineering for peripheral nerve injury

BACKGROUND AND AIMS

Challenges in treating peripheral nerve injury include prolonged repair time and insufficient functional recovery. Stem cell therapy coupled with neural tissue engineering has been shown to induce nerve regeneration following peripheral nerve injury. Among these stem cells, adipose-derived stem cells (ADSCs) are preferred due to their accessibility, expansion, multidirectional differentiation, and production of essential nutrient factors for nerve growth. In recent years, ADSC-laden nerve guide conduit has been utilized to enhance the therapeutic effects of tissue-engineered nerve grafts. This review explores existing research that recognizes the roles played by ADSCs in inducing peripheral nerve regeneration following injury and summarizes the different methods of application of ADSC-laden nerve conduit in neural tissue engineering.

A Novel Superparamagnetic Multifunctional Nerve Scaffold: A Remote Actuation Strategy to Boost In Situ Extracellular Vesicles Production for Enhanced Peripheral Nerve Repair

Extracellular vesicles (EVs) have inherent advantages over cell-based therapies in regenerative medicine because of their cargos of abundant bioactive cues. Several strategies are proposed to tune EVs production in vitro. However, it remains a challenge for manipulation of EVs production in vivo, which poses significant difficulties for EVs-based therapies that aim to promote tissue regeneration, particularly for long-term treatment of diseases like peripheral neuropathy. Herein, a superparamagnetic nanocomposite scaffold capable of controlling EVs production on-demand is constructed by incorporating polyethyleneglycol/polyethyleneimine modified superparamagnetic nanoparticles into a polyacrylamide/hyaluronic acid double-network hydrogel (Mag-gel). The Mag-gel is highly sensitive to a rotating magnetic field (RMF), and can act as mechano-stimulative platform to exert micro/nanoscale forces on encapsulated Schwann cells (SCs), an essential glial cell in supporting nerve regeneration. By switching the ON/OFF state of the RMF, the Mag-gel can scale up local production of SCs-derived EVs (SCs-EVs) both in vitro and in vivo. Further transcriptome sequencing indicates an enrichment of transcripts favorable in axon growth, angiogenesis, and inflammatory regulation of SCs-EVs in the Mag-gel with RMF, which ultimately results in optimized nerve repair in vivo. Overall, this research provides a noninvasive and remotely time-scheduled method for fine-tuning EVs-based therapies to accelerate tissue regeneration, including that of peripheral nerves.

Review on electrically conductive smart nerve guide conduit for peripheral nerve regeneration

At present, peripheral nerve injuries (PNIs) are one of the leading causes of substantial impairment around the globe. Complete recovery of nerve function after an injury is challenging. Currently, autologous nerve grafts are being used as a treatment; however, this has several downsides, for example, donor site morbidity, shortage of donor sites, loss of sensation, inflammation, and neuroma development. The most promising alternative is the development of a nerve guide conduit (NGC) to direct the restoration and renewal of neuronal axons from the proximal to the distal end to facilitate nerve regeneration and maximize sensory and functional recovery. Alternatively, the response of nerve cells to electrical stimulation (ES) has a substantial regenerative effect. The incorporation of electrically conductive biomaterials in the fabrication of smart NGCs facilitates the function of ES throughout the active proliferation state. This article overviews the potency of the various categories of electroactive smart biomaterials, including conductive and piezoelectric nanomaterials, piezoelectric polymers, and organic conductive polymers that researchers have employed latterly to fabricate smart NGCs and their potentiality in future clinical application. It also summarizes a comprehensive analysis of the recent research and advancements in the application of ES in the field of NGC.

Creating and Transferring an Innervated, Vascularized Muscle Flap Made from an Elastic, Cellularized Tissue Construct Developed In Situ

Reanimating facial structures following paralysis and muscle loss is a surgical objective that would benefit from improved options for harvesting appropriately sized muscle flaps. The objective of this study is to apply electrohydrodynamic processing to generate a cellularized, elastic, biocomposite scaffold that could develop and mature as muscle in a prepared donor site in vivo, and then be transferred as a thin muscle flap with a vascular and neural pedicle. First, an effective extracellular matrix (ECM) gel type is selected for the biocomposite scaffold from three types of ECM combined with poly(ester urethane)urea microfibers and evaluated in rat abdominal wall defects. Next, two types of precursor cells (muscle-derived and adipose-derived) are compared in constructs placed in rat hind limb defects for muscle regeneration capacity. Finally, with a construct made from dermal ECM and muscle-derived stem cells, protoflaps are implanted in one hindlimb for development and then microsurgically transferred as a free flap to the contralateral limb where stimulated muscle function is confirmed. This construct generation and in vivo incubation procedure may allow the generation of small-scale muscle flaps appropriate for transfer to the face, offering a new strategy for facial reanimation.

Precise Monitoring of Returning Sensation in Digital-Nerve Lesions by Three-Dimensional Imaging: A Proof-of-Concept Study

SUMMARY

Digital-nerve lesions result in a loss of tactile sensation reflected by an anesthetic area (AA) at the radial or ulnar aspect of the respective digit. Available tools to monitor the recovery of tactile sense have been criticized for their lack of validity. Precise quantification of AA dynamics by three-dimensional (3D) imaging could serve as an accurate surrogate to monitor recovery after digital-nerve repair. For validation, AAs were marked on digits of healthy volunteers to simulate the AA of an impaired cutaneous innervation. The 3D models were composed from raw images that had been acquired with a 3D camera to precisely quantify relative AA for each digit (3D models, n = 80). Operator properties varied with regard to individual experience in 3D imaging and image processing. In addition, the concept was applied in a clinical case study. Results showed that images taken by experienced photographers were rated as better quality ( P < 0.001) and needed less processing time ( P = 0.020). Quantification of the relative AA was not altered significantly, regardless of experience level of the photographer ( P = 0.425) or image assembler ( P = 0.749). The proposed concept allows precise and reliable surface quantification of digits and can be performed consistently without relevant distortion by lack of examiner experience. Routine 3D imaging of the AA has the great potential to provide visual evidence of various returning states of sensation and to convert sensory nerve recovery into a metric variable with high responsiveness to temporal progress.

Engineered hydrogels for peripheral nerve repair

Peripheral nerve injury (PNI) is a complex disease that often appears in young adults. It is characterized by a high incidence, limited treatment options, and poor clinical outcomes. This disease not only causes dysfunction and psychological disorders in patients but also brings a heavy burden to the society. Currently, autologous nerve grafting is the gold standard in clinical treatment, but complications, such as the limited source of donor tissue and scar tissue formation, often further limit the therapeutic effect. Recently, a growing number of studies have used tissue-engineered materials to create a natural microenvironment similar to the nervous system and thus promote the regeneration of neural tissue and the recovery of impaired neural function with promising results. Hydrogels are often used as materials for the culture and differentiation of neurogenic cells due to their unique physical and chemical properties. Hydrogels can provide three-dimensional hydration networks that can be integrated into a variety of sizes and shapes to suit the morphology of neural tissues. In this review, we discuss the recent advances of engineered hydrogels for peripheral nerve repair and analyze the role of several different therapeutic strategies of hydrogels in PNI through the application characteristics of hydrogels in nerve tissue engineering (NTE). Furthermore, the prospects and challenges of the application of hydrogels in the treatment of PNI are also discussed.

An enzyme-free technique enables the isolation of a large number of adipose-derived stem cells at the bedside

Adipose tissue derived stromal cells (ADSCs) play a crucial role in research and applications of regenerative medicine because they can be rapidly isolated in high quantities. Nonetheless, their purity, pluripotency, differentiation capacity, and stem cell marker expression might vary greatly depending on technique and tools used for extraction and harvesting. There are two methods described in the literature for isolating regenerative cells from adipose tissue. The first technique is enzymatic digestion, which utilizes many enzymes to remove stem cells from the tissue they reside in. The second method involves separating the concentrated adipose tissue using non-enzymatic, mechanical separation methods. ADSCs are isolated from the stromal-vascular fraction (SVF) of processed lipoaspirate, which is the lipoaspirate's aqueous portion. The purpose of this work was to evaluate a unique device 'microlyzer' for generating SVF from adipose tissue using a mechanical technique that required minimal intervention. The Microlyzer was examined using tissue samples from ten different patients. The cells that were retrieved were characterized in terms of their cell survival, phenotype, proliferation capacity, and differentiation potential. The number of progenitor cells extracted only from the microlyzed tissue was in comparable amount to the number of progenitor cells acquired by the gold standard enzymatic approach. The cells that were collected from each group exhibit similar levels of viability as well as proliferation rates. In addition, the differentiation potentials of the cells derived from the microlyzed tissue were investigated, and it was discovered that cells isolated through microlyzer entered the differentiation pathways more quickly and displayed a greater level of marker gene expression than cells isolated by enzymatic methods. These findings suggest that microlyzer, particularly in regeneration investigations, will allow quick and high rate cell separation at the bedside.

Biological nerve conduit model with de-epithelialized human amniotic membrane and adipose-derived mesenchymal stem cell sheet for repair of peripheral nerve defects

In this study, a biological conduit, consisting of an adipocyte-derived mesenchymal stem cell (AdMSCs) sheet and amniotic membrane (AM), was designed for the reconstruction of peripheral nerve defects. To evaluate the effect of the produced conduit on neural regeneration, a 10-mm sciatic nerve defect was created in rats, and experiments were carried out on six groups, i.e., sham control group (SC), negative control group (NC), nerve autograft group (NG), the biological conduit (AdMSCs + AM) group, the commercial PGA tube conduit (PGA) group, and the conduit only consisting of AM (AM) group. The effects of different nerve repair methods on the peripheral nerve and gastrocnemius muscle were evaluated by functional, histological, and immunohistochemical tests. When the number of myelinated axons was compared between the groups of AdMSCs + AM and PGA, it was higher in the AdMSCs + AM group (p < 0.05). The percentage of gastrocnemius collagen bundle area of AdMSCs + AM group was found to be statistically lower than the PGA group (p < 0.05). The muscle fiber diameter of AdMSCs + AM group was lower than that of the NG group, but significantly higher than that of the PGA group and the AM group (p < 0.001). Muscle weight index was significantly higher in the AdMSCs + AM group compared to the PGA group (p < 0.05). It was observed that nerve regeneration was faster in the AdMSCs + AM group, and there was an earlier improvement in pin-prick score and sciatic functional index compared to the PGA group and the AM group. In conclusion, the biological conduit prepared from the AdMSCs sheet and AM is regarded as a new biological conduit that can be used as an alternative treatment method to nerve autograft in clinical applications.

Peripheral Nerve Regeneration-Adipose-Tissue-Derived Stem Cells Differentiated by a Three-Step Protocol Promote Neurite Elongation via NGF Secretion

The lack of supportive Schwann cells in segmental nerve lesions seems to be one cornerstone for the problem of insufficient nerve regeneration. Lately, adipose-tissue-derived stem cells (ASCs) differentiated towards SC (Schwann cell)-like cells seem to fulfill some of the needs for ameliorated nerve recovery. In this study, three differentiation protocols were investigated for their ability to differentiate ASCs from rats into specialized SC phenotypes. The differentiated ASCs (dASCs) were compared for their expressions of neurotrophins (NGF, GDNF, BDNF), myelin markers (MBP, P0), as well as glial-marker proteins (S100, GFAP) by RT-PCR, ELISA, and Western blot. Additionally, the influence of the medium conditioned by dASCs on a neuron-like cell line was evaluated. The dASCs were highly diverse in their expression profiles. One protocol yielded relatively high expression rates of neurotrophins, whereas another protocol induced myelin-marker expression. These results were reproducible when the ASCs were differentiated on surfaces potentially used for nerve guidance conduits. The NGF secretion affected the neurite outgrowth significantly. It remains uncertain what features of these SC-like cells contribute the most to adequate functional recovery during the different phases of nerve recovery. Nevertheless, therapeutic applications should consider these diverse phenotypes as a potential approach for stem-cell-based nerve-injury treatment.

Augmenting Peripheral Nerve Regeneration with Adipose-Derived Stem Cells

Peripheral nerve injuries (PNIs) are common and debilitating, cause significant health care costs for society, and rely predominately on autografts, which necessitate grafting a nerve section non-locally to repair the nerve injury. One possible approach to improving treatment is bolstering endogenous regenerative mechanisms or bioengineering new nervous tissue in the peripheral nervous system. In this review, we discuss critical-sized nerve gaps and nerve regeneration in rats, and summarize the roles of adipose-derived stem cells (ADSCs) in the treatment of PNIs. Several regenerative treatment modalities for PNI are described: ADSCs differentiating into Schwann cells (SCs), ADSCs secreting growth factors to promote peripheral nerve growth, ADSCs promoting myelination growth, and ADSCs treatments with scaffolds. ADSCs' roles in regenerative treatment and features are compared to mesenchymal stem cells, and the administration routes, cell dosages, and cell fates are discussed. ADSCs secrete neurotrophic factors and exosomes and can differentiate into Schwann cell-like cells (SCLCs) that share features with naturally occurring SCs, including the ability to promote nerve regeneration in the PNS. Future clinical applications are also discussed.

Comparison of Collagen and Human Amniotic Membrane Nerve Wraps and Conduits for Peripheral Nerve Repair in Preclinical Models: A Systematic Review of the Literature

INTRODUCTION

 Collagen and human amniotic membrane (hAM) are Food and Drug Administration (FDA)-approved biomaterials that can be used as nerve wraps or conduits for repair of peripheral nerve injuries. Both biomaterials have been shown to reduce scarring and fibrosis of injured peripheral nerves. However, comparative advantages and disadvantages have not been definitively shown in the literature. The purpose of this systematic review is to comprehensively evaluate the literature regarding the roles of hAM and collagen nerve wraps and conduits on peripheral nerve regeneration in preclinical models.

METHODS

 The MEDLINE database was queried using the PubMed search engine on July 7, 2019, with the following search strategy: ("amniotic membrane" OR "amnion") OR ("collagen conduit" OR "nerve wrap")] AND "nerve." All resulting articles were screened by two independent reviewers. Nerve type, lesion type/injury model, repair type, treatment, and outcomes were assessed.

RESULTS

 Two hundred and fifty-eight articles were identified, and 44 studies remained after application of inclusion and exclusion criteria. Seventeen studies utilized hAM, whereas 27 studies utilized collagen wraps or conduits. Twenty-three (85%) of the collagen studies utilized conduits, and four (15%) utilized wraps. Six (35%) of the hAM studies utilized conduits and 11 (65%) utilized wraps. Two (9%) collagen studies involving a conduit and one (25%) involving a wrap demonstrated at least one significant improvement in outcomes compared with a control. While none of the hAM conduit studies showed significant improvements, eight (73%) of the studies investigating hAM wraps showed at least one significant improvement in outcomes.

CONCLUSION

 The majority of studies reported positive outcomes, indicating that collagen and hAM nerve wraps and conduits both have the potential to enhance peripheral nerve regeneration. However, relatively few studies reported significant findings, except for studies evaluating hAM wraps. Preclinical models may help guide clinical practice regarding applications of these biomaterials in peripheral nerve repair.

Adipose Tissue Uses in Peripheral Nerve Surgery

Currently, many different techniques exist for the surgical repair of peripheral nerves. The degree of injury dictates the repair and, depending on the defect or injury of the peripheral nerve, plastic surgeons can perform nerve repairs, grafts, and transfers. All the previously listed techniques are routinely performed in human patients, but a novel addition to these peripheral nerve surgeries involves concomitant fat grafting to the repair site at the time of surgery. Fat grafting provides adipose-derived stem cells to the injury site. Though fat grafting is performed as an adjunct to some peripheral nerve surgeries, there is no clear evidence as to which procedures have improved outcomes resultant from concomitant fat grafting. This review explores the evidence presented in various animal studies regarding outcomes of fat grafting at the time of various types of peripheral nerve surgery.

Perspective: Why and How Ubiquitously Distributed, Vascular-Associated, Pluripotent Stem Cells in the Adult Body (vaPS Cells) Are the Next Generation of Medicine

A certain cell type can be isolated from different organs in the adult body that can differentiate into ectoderm, mesoderm, and endoderm, providing significant support for the existence of a certain type of small, vascular-associated, pluripotent stem cell ubiquitously distributed in all organs in the adult body (vaPS cells). These vaPS cells fundamentally differ from embryonic stem cells and induced pluripotent stem cells in that the latter possess the necessary genetic guidance that makes them intrinsically pluripotent. In contrast, vaPS cells do not have this intrinsic genetic guidance, but are able to differentiate into somatic cells of all three lineages under guidance of the microenvironment they are located in, independent from the original tissue or organ where they had resided. These vaPS cells are of high relevance for clinical application because they are contained in unmodified, autologous, adipose-derived regenerative cells (UA-ADRCs). The latter can be obtained from and re-applied to the same patient at the point of care, without the need for further processing, manipulation, and culturing. These findings as well as various clinical examples presented in this paper demonstrate the potential of UA-ADRCs for enabling an entirely new generation of medicine for the benefit of patients and healthcare systems.

Bioactive Nanofiber-Based Conduits in a Peripheral Nerve Gap Management-An Animal Model Study

The aim was to examine the efficiency of a scaffold made of poly (L-lactic acid)-co-poly(ϵ-caprolactone), collagen (COL), polyaniline (PANI), and enriched with adipose-derived stem cells (ASCs) as a nerve conduit in a rat model. P(LLA-CL)-COL-PANI scaffold was optimized and electrospun into a tubular-shaped structure. Adipose tissue from 10 Lewis rats was harvested for ASCs culture. A total of 28 inbred male Lewis rats underwent sciatic nerve transection and excision of a 10 mm nerve trunk fragment. In Group A, the nerve gap remained untouched; in Group B, an excised trunk was used as an autograft; in Group C, nerve stumps were secured with P(LLA-CL)-COL-PANI conduit; in Group D, P(LLA-CL)-COL-PANI conduit was enriched with ASCs. After 6 months of observation, rats were sacrificed. Gastrocnemius muscles and sciatic nerves were harvested for weight, histology analysis, and nerve fiber count analyses. Group A showed advanced atrophy of the muscle, and each intervention (B, C, D) prevented muscle mass decrease (p < 0.0001); however, ASCs addition decreased efficiency vs. autograft (p < 0.05). Nerve fiber count revealed a superior effect in the nerve fiber density observed in the groups with the use of conduit (D vs. B p < 0.0001, C vs. B p < 0.001). P(LLA-CL)-COL-PANI conduits with ASCs showed promising results in managing nerve gap by decreasing muscle atrophy.

Dexpanthenol reduces fibrosis and aids repair following nerve laceration and neurorrhaphy

The aim of the present study was to investigate the effect of dexpanthenol on nerve healing following neurorrhaphy in lacerated peripheral nerves. A total of 30 mature Sprague Dawley rats were used. Surgical sciatic nerve dissection and repair was performed on an experimental group of 20 rats. The remaining 10 rats were designated as the control group. The experimental group was divided into 2 subgroups. The surgery + saline group (SSLE; n=10) was given 1 ml/kg 0.9% sodium chloride saline intraperitoneally. The surgery + dexpanthenol group (SDPL; n=10) rats were given 500 mg/kg/day dexpanthenol intraperitoneally. Histological evaluation of the sciatic nerve tissue revealed that the fibrosis score was significantly lower in the SDPL group than in the SSLE group (P<0.001). Electrophysiological evaluation of compound muscle action potential (CMAP) indicated that the CMAP level in the SDPL group was significantly higher than that of the SSLE group (P<0.001), and the CMAP latency period was lower in the SDPL group compared with the SSLE group (P<0.001). In addition, the SDPL group malondialdehyde level was significantly lower than that of the SSLE group (P<0.001). Functional evaluation with an inclined plane test revealed a significant difference between the SSLE (39.6±5.5^˚) and SDPL (79.1±6.93^˚) groups (P<0.001). Dexpanthenol was observed to have a positive effect on nerve tissue repaired with neurorrhaphy in a rat sciatic model of laceration-type injuries similar to those frequently encountered in the clinic.

Secondary release of the peripheral nerve with autologous fat derivates benefits for functional and sensory recovery

The reconstruction of nerve continuity after traumatic nerve injury is the gold standard in hand surgery. Immediate, tension-free, end-to-end nerve suture ensures the best prognosis. The recovery is mostly promising; however, in a few cases, insufficient outcomes in motor or sensory function are observed. Intra- and extra-fascicular scarring accompanies the nerve regeneration process and limits final outcomes. Secondary nerve release in those cases is recommended. Unfortunately, scarring recurrence cannot be eliminated after secondary revision and neurolysis. The supportive influences of mesenchymal stem cells in the process of nerve regeneration were observed in many preclinical studies. However, a limited number of studies in humans have analyzed the clinical usage of mesenchymal stem cells in peripheral nerve reconstruction and revisions. The objective of this study was to evaluate the effects of undifferentiated adipose-derived stromal/stem cell injection during a last-chance surgery (neurolysis, nerve release) on a previously reconstructed nerve. Three patients (one female, two males; mean age 59 ± 4.5 years at the time of injury), who experienced failure of reconstructions of median and ulnar nerves, were included in this study. During the revision surgery, nerve fascicles were released, and adipose-derived stromal/stem cells were administered through microinjections along the fascicles and around the adjacent tissues after external neurolysis. During 36 months of follow-up, patients noticed gradual signs of sensory and in consequence functional recovery. No adverse effects were observed. Simultaneous nerve release with adipose-derived stromal/stem cells support is a promising method in patients who need secondary nerve release after nerve reconstruction. This method can constitute an alternative procedure in patients experiencing recovery failure and allow improvement in cases of limited nerve regeneration. The study protocol was approved by the Institutional Review Board (IRB) at the Centre of Postgraduate Medical Education (No. 62/PB/2016) on September 14, 2016.

Advances and clinical challenges for translating nerve conduit technology from bench to bed side for peripheral nerve repair

Injuries to the peripheral nervous system remain a large-scale clinical problem. These injuries often lead to loss of motor and/or sensory function that significantly affects patients' quality of life. The current neurosurgical approach for peripheral nerve repair involves autologous nerve transplantation, which often leads to clinical complications. The most pressing need is to increase the regenerative capacity of existing tubular constructs in the repair of large nerve gaps through development of tissue-engineered approaches that can surpass the performance of autografts. To fully realize the clinical potential of nerve conduit technology, there is a need to reconsider design strategies, biomaterial selection, fabrication techniques and the various potential modifications to optimize a conduit microenvironment that can best mimic the natural process of regeneration. In recent years, a significant progress has been made in the designing and functionality of bioengineered nerve conduits to bridge long peripheral nerve gaps in various animal models. However, translation of this work from lab to commercial scale has not been achieve. The current review summarizes recent advances in the development of tissue engineered nerve guidance conduits (NGCs) with regard to choice of material, novel fabrication methods, surface modifications and regenerative cues such as stem cells and growth factors to improve regeneration performance. Also, the current clinical potential and future perspectives to achieve therapeutic benefits of NGCs will be discussed in context of peripheral nerve regeneration.

Adipose tissue stem cells in peripheral nerve regeneration-In vitro and in vivo

After peripheral nerve injury, Schwann cells (SCs) are crucially involved in several steps of the subsequent regenerative processes, such as the Wallerian degeneration. They promote lysis and phagocytosis of myelin, secrete numbers of neurotrophic factors and cytokines, and recruit macrophages for a biological debridement. However, nerve injuries with a defect size of >1 cm do not show proper tissue regeneration and require a surgical nerve gap reconstruction. To find a sufficient alternative to the current gold standard-the autologous nerve transplant-several cell-based therapies have been developed and were experimentally investigated. One approach aims on the use of adipose tissue stem cells (ASCs). These are multipotent mesenchymal stromal cells that can differentiate into multiple phenotypes along the mesodermal lineage, such as osteoblasts, chondrocytes, and myocytes. Furthermore, ASCs also possess neurotrophic features, that is, they secrete neurotrophic factors like the nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, ciliary neurotrophic factor, glial cell-derived neurotrophic factor, and artemin. They can also differentiate into the so-called Schwann cell-like cells (SCLCs). These cells share features with naturally occurring SCs, as they also promote nerve regeneration in the periphery. This review gives a comprehensive overview of the use of ASCs in peripheral nerve regeneration and peripheral nerve tissue engineering both in vitro and in vivo. While the sustainability of differentiation of ASCs to SCLCs in vivo is still questionable, ASCs used with different nerve conduits, such as hydrogels or silk fibers, have been shown to promote nerve regeneration.

Grafts of human adipose-derived stem cells into a biodegradable poly (acid lactic) conduit enhances sciatic nerve regeneration

Despite the regenerative potential of the Peripheral Nervous System (PNS), injuries with loss of a nerve segment make the functional recovery a challenge. This work aimed to investigate the effects of the association of biodegradable conduits of poly (lactic acid) (PLA) with human adipose-derived stem cells (hADSCs) on the regeneration of the sciatic nerve. C57BL / 6 male mice were submitted to sciatic nerve transection followed by tubulization with PLA conduit. Animals were allocated in two groups: the first received an injection of DMEM inside the conduit (DMEM) and the second received hADSCs inside it (hADSC). Sensory and motor functions were assessed by the pinprick test and electroneuromiography, respectively. To assess neuronal survival the retrograde tracer fluorogold was injected into the sciatic nerve distally to the lesion site. One week after that, animals were sacrificed, tissues harvested and processed for morphological evaluation. After eight weeks, all animals showed sensory recovery in the pinprick test and there was no significant difference between the two groups. The amplitude of the compound muscle action potential was higher in the hADSCs group. The number of myelinated nerve fibers, muscle cells and motor plates was higher in the hADSC group. There was also greater survival of sensory and motor neurons in the hADSC animals. These results suggest that the association of PLA conduit and cell therapy with hADSCs leads to a better functional and morphological recovery after sciatic nerve transection.

Ex-Vivo Stimulation of Adipose Stem Cells by Growth Factors and Fibrin-Hydrogel Assisted Delivery Strategies for Treating Nerve Gap-Injuries

Peripheral nerve injuries often result in lifelong disabilities despite advanced surgical interventions, indicating the urgent clinical need for effective therapies. In order to improve the potency of adipose-derived stem cells (ASC) for nerve regeneration, the present study focused primarily on ex-vivo stimulation of ASC by using growth factors, i.e., nerve growth factor (NGF) or vascular endothelial growth factor (VEGF) and secondly on fibrin-hydrogel nerve conduits (FNC) assisted ASC delivery strategies, i.e., intramural vs. intraluminal loading. ASC were stimulated by NGF or VEGF for 3 days and the resulting secretome was subsequently evaluated in an in vitro axonal outgrowth assay. For the animal study, a 10 mm sciatic nerve gap-injury was created in rats and reconstructed using FNC loaded with ASC. Secretome derived from NGF-stimulated ASC promoted significant axonal outgrowth from the DRG-explants in comparison to all other conditions. Thus, NGF-stimulated ASC were further investigated in animals and found to enhance early nerve regeneration as evidenced by the increased number of β-Tubulin III+ axons. Notably, FNC assisted intramural delivery enabled the improvement of ASC’s therapeutic efficacy in comparison to the intraluminal delivery system. Thus, ex-vivo stimulation of ASC by NGF and FNC assisted intramural delivery may offer new options for developing effective therapies.

Human platelet lysate stimulated adipose stem cells exhibit strong neurotrophic potency for nerve tissue engineering applications

Aim: We investigated a potential strategy involving human platelet lysate (HPL) as a media additive for enhancing the neurotrophic potency of human adipose stem cells (ASC). Materials & methods: Dorsal root ganglion explants, ASC and Schwann cells were used for in vitro axonal outgrowth experiments. Results: Remarkably, HPL-supplemented ASC promoted robust axonal outgrowth, in other words, four-times higher than fetal bovine serum-supplemented ASC and even matched to the level of Schwann cells. Further, analysis of regime of growth medium additive supplementation revealed the critical play of HPL in dorsal root ganglion and stem cells co-culture system for mounting effective axonal growth response. Conclusion: HPL supplementation significantly improved the neurotrophic potency of ASC as evidenced by the robust axonal outgrowth; these findings hold significance for nerve tissue engineering applications.

Differentiated adipose-derived stem cells promote peripheral nerve regeneration

INTRODUCTION

Many reports have indicated that adipose-derived stem cells (ADSCs) are effective for nerve regeneration. We investigated nerve regeneration by combining a polyglycolic acid collagen (PGA-c) tube, which is approved for clinical use, and Schwann cell-like differentiated ADSCs (dADSCs).

METHODS

Fifteen-millimeter-long gaps in the sciatic nerve of rats were bridged in each group using tubes (group I), with tubes injected with dADSCs (group II), or by resected nerve (group III).

RESULTS

Axonal outgrowth was greater in group II than in group I. Tibialis anterior muscle weight revealed recovery only in group III. Latency in nerve conduction studies was equivalent in group II and III, but action potential was lower in group II. Transplanted dADSCs maintained Schwann cell marker expression. ATF3 expression level in the dorsal root ganglia was equivalent in groups II and III.

DISCUSSION

dADSCs maintained their differentiated state in the tubes and are believed to have contributed to nerve regeneration.

Gene expression and growth factor analysis in early nerve regeneration following segmental nerve defect reconstruction with a mesenchymal stromal cell-enhanced decellularized nerve allograft

The purpose of this study was to evaluate the molecular mechanisms underlying nerve repair by a decellularized nerve allograft seeded with adipose-derived mesenchymal stromal cells (MSCs) and compare it to the unseeded allograft and autograft nerve.

METHODS

Undifferentiated MSCs were seeded onto decellularized nerve allografts and used to reconstruct a 10 mm gap in a rat sciatic nerve model. Gene expression profiles of genes essential for nerve regeneration and immunohistochemical staining (IHC) for PGP9.5, NGF, RECA-1, and S100 were obtained 2 weeks postoperatively.

RESULTS

Semi-quantitative RT-PCR analysis showed that the angiogenic molecule VEGFA was significantly increased in seeded allografts, and transcription factor SOX2 was downregulated in seeded allografts. Seeded grafts showed a significant increase in immunohistochemical markers NGF and RECA-1, when compared with unseeded allografts.

CONCLUSIONS

MSCs contributed to the secretion of trophic factors. A beneficial effect of the MSCs on angiogenesis was found when compared with the unseeded nerve allograft, but implanted MSCs did not show evidence of differentiation into Schwann cell-like cells.

Isolation of adipose tissue derived regenerative cells from human subcutaneous tissue with or without the use of an enzymatic reagent

Freshly isolated, uncultured, autologous adipose derived regenerative cells (ADRCs) have emerged as a promising tool for regenerative cell therapy. The Transpose RT system (InGeneron, Inc., Houston, TX, USA) is a system for isolating ADRCs from adipose tissue, commercially available in Europe as a CE-marked medical device and under clinical evaluation in the United States. This system makes use of the proprietary, enzymatic Matrase Reagent for isolating cells. The present study addressed the question whether the use of Matrase Reagent influences cell yield, cell viability, live cell yield, biological characteristics, physiological functions or structural properties of the ADRCs in final cell suspension. Identical samples of subcutaneous adipose tissue from 12 subjects undergoing elective lipoplasty were processed either with or without the use of Matrase Reagent. Then, characteristics of the ADRCs in the respective final cell suspensions were evaluated. Compared to non-enzymatic isolation, enzymatic isolation resulted in approximately twelve times higher mean cell yield (i.e., numbers of viable cells/ml lipoaspirate) and approximately 16 times more colony forming units. Despite these differences, cells isolated from lipoaspirate both with and without the use of Matrase Reagent were independently able to differentiate into cells of all three germ layers. This indicates that biological characteristics, physiological functions or structural properties relevant for the intended use were not altered or induced using Matrase Reagent. A comprehensive literature review demonstrated that isolation of ADRCs from lipoaspirate using the Transpose RT system and the Matrase Reagent results in the highest viable cell yield among published data regarding isolation of ADRCs from lipoaspirate.

The advances in nerve tissue engineering: From fabrication of nerve conduit to in vivo nerve regeneration assays

Peripheral nerve damage is a common clinical complication of traumatic injury occurring after accident, tumorous outgrowth, or surgical side effects. Although the new methods and biomaterials have been improved recently, regeneration of peripheral nerve gaps is still a challenge. These injuries affect the quality of life of the patients negatively. In the recent years, many efforts have been made to develop innovative nerve tissue engineering approaches aiming to improve peripheral nerve treatment following nerve injuries. Herein, we will not only outline what we know about the peripheral nerve regeneration but also offer our insight regarding the types of nerve conduits, their fabrication process, and factors associated with conduits as well as types of animal and nerve models for evaluating conduit function. Finally, nerve regeneration in a rat sciatic nerve injury model by nerve conduits has been considered, and the main aspects that may affect the preclinical outcome have been discussed.

Adipose Stem Cells Enhance Nerve Regeneration and Muscle Function in a Peroneal Nerve Ablation Model

Severe peripheral nerve injuries have devastating consequences on the quality of life in affected patients, and they represent a significant unmet medical need. Destruction of nerve fibers results in denervation of targeted muscles, which, subsequently, undergo progressive atrophy and loss of function. Timely restoration of neural innervation to muscle fibers is crucial to the preservation of muscle homeostasis and function. The goal of this study was to evaluate the impact of addition of adipose stem cells (ASCs) to polycaprolactone (PCL) nerve conduit guides on peripheral nerve repair and functional muscle recovery in the setting of a critical size nerve defect. To this end, peripheral nerve injury was created by surgically ablating 6 mm of the common peroneal nerve in a rat model. A PCL nerve guide, filled with ASCs and/or poloxamer hydrogel, was sutured to the nerve ends. Negative and positive controls included nerve ablation only (no repair), and reversed polarity autograft nerve implant, respectively. Tibialis anterior (TA) muscle function was assessed at 4, 8, and 12 weeks postinjury, and nerve and muscle tissue was retrieved at the 12-week terminal time point. Inclusion of ASCs in the PCL nerve guide elicited statistically significant time-dependent increases in functional recovery (contraction) after denervation; ∼25% higher than observed in acellular (poloxamer-filled) implants and indistinguishable from autograft implants, respectively, at 12 weeks postinjury (p < 0.05, n = 7-8 in each group). Analysis of single muscle fiber cross-sectional area (CSA) revealed that ASC-based treatment of nerve injury provided a better recapitulation of the overall distribution of muscle fiber CSAs observed in the contralateral TA muscle of uninjured limbs. In addition, the presence of ASCs was associated with improved features of re-innervation distal to the defect, with respect to neurofilament and S100 (Schwann cell marker) expression. In conclusion, these initial studies indicate significant benefits of inclusion of ASCs to the rate and magnitude of both peripheral nerve regeneration and functional recovery of muscle contraction, to levels equivalent to autograft implantation. These findings have important implications to improved nerve repair, and they provide input for future work directed to restoration of nerve and muscle function after polytraumatic injury. Impact Statement This works explores the application of adipose stem cells (ASCs) for peripheral nerve regeneration in a rat model. Herein, we demonstrate that the addition of ASCs in poloxamer-filled PCL nerve guide conduits impacts nerve regeneration and recovery of muscle function, to levels equivalent to autograft implantation, which is considered to be the current gold standard treatment. This study builds on the importance of a timely restoration of innervation to muscle fibers for preservation of muscle homeostasis, and it will provide input for future work aiming at restoring nerve and muscle function after polytraumatic injury.

Novel approaches using mesenchymal stem cells for curing peripheral nerve injuries

Peripheral nerve injury (PNI) is a common life-changing disability of peripheral nervous system with significant socioeconomic consequences. Conventional therapeutic approaches for PNI have several drawbacks such as need to autologous nerve scarifying, surplus surgery, and difficult accessibility to donor nerve; therefore, other therapeutic strategies such as mesenchymal stem cells (MSCs) therapy are getting more interesting. MSCs have been proved to be safe and efficient in numerous degenerative diseases of central and peripheral nervous systems. In this paper, we review novel biotechnological advancements in treating PNI using MSCs.

A Simple Dynamic Strategy to Deliver Stem Cells to Decellularized Nerve Allografts

BACKGROUND

The addition of adipose-derived mesenchymal stromal cells to decellularized nerve allografts may improve outcomes of nerve reconstruction. Prior techniques used for cell seeding are traumatic to both the mesenchymal stromal cells and nerve graft. An adequate, reliable, and validated cell seeding technique is an essential step for evaluating the translational utility of mesenchymal stromal cell-enhanced decellularized nerve grafts. The purpose of this study was to develop a simple seeding strategy with an optimal seeding duration.

METHODS

A dynamic bioreactor was used to seed rat and human mesenchymal stromal cells separately onto rat and human decellularized nerve allografts. Cell viability was evaluated by MTS assays and cellular topology after seeding was determined by scanning electron microscopy. Cell density and distribution were determined by Live/Dead assays and Hoechst staining at four different time points (6, 12, 24, and 72 hours). The validity and reliability of the seeding method were calculated.

RESULTS

Cells remained viable at all time points, and mesenchymal stromal cells exhibited exponential growth in the first 12 hours of seeding. Seeding efficiency increased significantly from 79.5 percent at 6 hours to 89.2 percent after 12 hours of seeding (p = 0.004). Both intrarater reliability (r = 0.97) and interrater reliability (r = 0.92) of the technique were high.

CONCLUSIONS

This study describes and validates a new method of effectively seeding decellularized nerve allografts with mesenchymal stromal cells. This method is reproducible, distributes cells homogenously over the graft, and does not traumatize the intraneural architecture of the allograft. Use of this validated seeding technique will permit critical comparison of graft outcomes.

Gellan Gum-based luminal fillers for peripheral nerve regeneration: an in vivo study in the rat sciatic nerve repair model

Peripheral nerve injuries (PNI) resulting in a gap to be bridged between the transected nerve ends are commonly reconstructed with autologous nerve tissue, but there is a need for valuable alternatives. This experimental work considers the innovative use of the biomaterial Gellan Gum (GG) as a luminal filler for nerve guidance channels made from chitosan with a 5% degree of acetylation. The engineered constructs should remodel the structural support given to regenerating axons by the so-called bands of Büngner. Four different GG formulations were produced by combining varying amounts of High-Acyl GG (HA-GG) and Methacrylated GG (MA-GG). The effective porosity of the freeze-dried networks was analysed by SEM and micro-CT 3D reconstructions, while the degradation and swelling abilities were characterized in vitro for up to 30 days. The metabolic activity and viability of immortalized Schwann cells seeded onto the freeze-dried networks were also evaluated. Finally, the developed hydrogel formulations were freeze-dried within the chitosan nerve guides and implanted in a 10 mm rat sciatic nerve defect. Functional and histomorphological analyses after 3, 6, and 12 weeks in vivo revealed that although it did not result in improved nerve regeneration, the NGC25:75 formulations could provide a basis for further development of GG scaffolds as luminal fillers for hollow nerve guidance channels.

The role of undifferentiated adipose-derived stem cells in peripheral nerve repair

Peripheral nerve injuries impose significant health and economic consequences, yet no surgical repair can deliver a complete recovery of sensory or motor function. Traditional methods of repair are less than ideal: direct coaptation can only be performed when tension-free repair is possible, and transplantation of nerve autograft can cause donor-site morbidity and neuroma formation. Cell-based therapy delivered via nerve conduits has thus been explored as an alternative method of nerve repair in recent years. Stem cells are promising sources of the regenerative core material in a nerve conduit because stem cells are multipotent in function, abundant in supply, and more accessible than the myelinating Schwann cells. Among different types of stem cells, undifferentiated adipose-derived stem cell (uASC), which can be processed from adipose tissue in less than two hours, is a promising yet underexplored cell type. Studies of uASC have emerged in the past decade and have shown that autologous uASCs are non-immunogenic, easy to access, abundant in supply, and efficacious at promoting nerve regeneration. Two theories have been proposed as the primary regenerative mechanisms of uASC: in situ trans-differentiation towards Schwann cells, and secretion of trophic and anti-inflammatory factors. Future studies need to fully elucidate the mechanisms, side effects, and efficacy of uASC-based nerve regeneration so that uASCs can be utilized in clinical settings.

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.

Stem Cell Transplantation for Peripheral Nerve Regeneration: Current Options and Opportunities

Peripheral nerve regeneration is a complicated process highlighted by Wallerian degeneration, axonal sprouting, and remyelination. Schwann cells play an integral role in multiple facets of nerve regeneration but obtaining Schwann cells for cell-based therapy is limited by the invasive nature of harvesting and donor site morbidity. Stem cell transplantation for peripheral nerve regeneration offers an alternative cell-based therapy with several regenerative benefits. Stem cells have the potential to differentiate into Schwann-like cells that recruit macrophages for removal of cellular debris. They also can secrete neurotrophic factors to promote axonal growth, and remyelination. Currently, various types of stem cell sources are being investigated for their application to peripheral nerve regeneration. This review highlights studies involving the stem cell types, the mechanisms of their action, methods of delivery to the injury site, and relevant pre-clinical or clinical data. The purpose of this article is to review the current point of view on the application of stem cell based strategy for peripheral nerve regeneration.