Chapter 22: Transplantation of olfactory ensheathing cells for peripheral nerve regeneration

The Effect of Schwann Cells/Schwann Cell-Like Cells on Cell Therapy for Peripheral Neuropathy

Peripheral neuropathy is a common neurological issue that leads to sensory and motor disorders. Over time, the treatment for peripheral neuropathy has primarily focused on medications for specific symptoms and surgical techniques. Despite the different advantages of these treatments, functional recovery remains less than ideal. Schwann cells, as the primary glial cells in the peripheral nervous system, play crucial roles in physiological and pathological conditions by maintaining nerve structure and functions and secreting various signaling molecules and neurotrophic factors to support both axonal growth and myelination. In addition, stem cells, including mesenchymal stromal cells, skin precursor cells and neural stem cells, have the potential to differentiate into Schwann-like cells to perform similar functions as Schwann cells. Therefore, accumulating evidence indicates that Schwann cell transplantation plays a crucial role in the resolution of peripheral neuropathy. In this review, we summarize the literature regarding the use of Schwann cell/Schwann cell-like cell transplantation for different peripheral neuropathies and the potential role of promoting nerve repair and functional recovery. Finally, we discuss the limitations and challenges of Schwann cell/Schwann cell-like cell transplantation in future clinical applications. Together, these studies provide insights into the effect of Schwann cells/Schwann cell-like cells on cell therapy and uncover prospective therapeutic strategies for peripheral neuropathy.

The Present and Future for Peripheral Nerve Regeneration

Peripheral nerve injury can have a potentially devastating impact on a patient's quality of life, resulting in severe disability with substantial social and personal cost. Refined microsurgical techniques, advances in peripheral nerve topography, and a better understanding of the pathophysiology and molecular basis of nerve injury have all led to a decisive leap forward in the field of translational neurophysiology. Nerve repair, nerve grafting, and nerve transfers have improved significantly with consistently better functional outcomes. Direct nerve repair with epineural microsutures is still the surgical treatment of choice when a tension-free coaptation in a well-vascularized bed can be achieved. In the presence of a significant gap (>2-3 cm) between the proximal and distal nerve stumps, primary end-to-end nerve repair often is not possible; in these cases, nerve grafting is the treatment of choice. Indications for nerve transfer include brachial plexus injuries, especially avulsion type, with long distance from target motor end plates, delayed presentation, segmental loss of nerve function, and broad zone of injury with dense scarring. Current experimental research in peripheral nerve regeneration aims to accelerate the process of regeneration using pharmacologic agents, bioengineering of sophisticated nerve conduits, pluripotent stem cells, and gene therapy. Several small molecules, peptides, hormones, neurotoxins, and growth factors have been studied to improve and accelerate nerve repair and regeneration by reducing neuronal death and promoting axonal outgrowth. Targeting specific steps in molecular pathways also allows for purposeful pharmacologic intervention, potentially leading to a better functional recovery after nerve injury. This article summarizes the principles of nerve repair and the current concepts of peripheral nerve regeneration research, as well as future perspectives. [Orthopedics. 2017; 40(1):e141-e156.].

Cell migration in the developing rodent olfactory system

The components of the nervous system are assembled in development by the process of cell migration. Although the principles of cell migration are conserved throughout the brain, different subsystems may predominantly utilize specific migratory mechanisms, or may display unusual features during migration. Examining these subsystems offers not only the potential for insights into the development of the system, but may also help in understanding disorders arising from aberrant cell migration. The olfactory system is an ancient sensory circuit that is essential for the survival and reproduction of a species. The organization of this circuit displays many evolutionarily conserved features in vertebrates, including molecular mechanisms and complex migratory pathways. In this review, we describe the elaborate migrations that populate each component of the olfactory system in rodents and compare them with those described in the well-studied neocortex. Understanding how the components of the olfactory system are assembled will not only shed light on the etiology of olfactory and sexual disorders, but will also offer insights into how conserved migratory mechanisms may have shaped the evolution of the brain.

Development and characterization of novel agar and gelatin injectable hydrogel as filler for peripheral nerve guidance channels

Injectable hydrogels are becoming of increasing interest in the field of tissue engineering thanks to their versatile properties and to the possibility of being injected into tissues or devices during surgery. In peripheral nerve tissue engineering, injectable hydrogels having shear-thinning properties are advantageous as filler of nerve guidance channels (NGCs) to improve the regeneration process. In the present work, gelatin-based hydrogels were developed and specifically designed for the insertion into the lumen of hollow NGCs through a syringe during surgery. Injectable hydrogels were obtained using an agar-gelatin 20:80 weight ratio, (wt/wt) blend crosslinked by the addition of genipin (A/GL_GP). The physicochemical properties of the A/GL_GP hydrogels were analysed, including their injectability, rheological, swelling and dissolution behaviour, and their mechanical properties under compression. The hydrogel developed showed shear-thinning properties and was applied as filler of NGCs. The A/GL_GP hydrogel was tested in vitro using different cell lines, among them Schwann cells which have been used because they have an important role in peripheral nerve regeneration. Viability assays demonstrated the lack of cytotoxicity. In vitro experiments showed that the hydrogel is able to promote cell adhesion and proliferation. Two- and three-dimensional migration assays confirmed the capability of the cells to migrate both on the surface and within the internal framework of the hydrogel. These data show that A/GL_GP hydrogel has characteristics that make it a promising scaffold material for tissue engineering and nerve regeneration. Copyright © 2014 John Wiley & Sons, Ltd.

Comparison of Schwann cells and olfactory ensheathing cells for peripheral nerve gap bridging

INTRODUCTION

Previously, we introduced the biogenic conduit (BC) as a novel autologous nerve conduit for bridging peripheral nerve defects and tested its regenerative capacity in a short- and long-term setting. The aim of the present study was to clarify whether intraluminal application of regeneration-promoting glial cells, including Schwann cells (SC) and olfactory ensheathing cells (OEC), displayed differential effects after sciatic nerve gap bridging.

MATERIAL AND METHODS

BCs were generated as previously described. The conduits filled with fibrin/SC (n = 8) and fibrin/OEC (n = 8) were compared to autologous nerve transplants (NT; n = 8) in the 15-mm sciatic nerve gap lesion model of the rat. The sciatic functional index was evaluated every 4 weeks. After 16 weeks, histological evaluation followed regarding nerve area, axon number, myelination index and N ratio.

RESULTS

Common to all groups was a continual improvement in motor function during the observation period. Recovery was significantly better after SC transplantation compared to OEC (p < 0.01). Both cell transplantation groups showed significantly worse function than the NT group (p < 0.01). Whereas nerve area and axon number were correlated to function, being significantly lowest in the OEC group (p < 0.001), both cell groups showed lowered myelination (p < 0.001) and lower N ratio compared to the NT group.

DISCUSSION

SC-filled BCs led to improved regeneration compared to OEC-filled BCs in a 15-mm-long nerve gap model of the rat.

Myelin-associated proteins block the migration of olfactory ensheathing cells: an in vitro study using single-cell tracking and traction force microscopy

Newly generated olfactory receptor axons grow from the peripheral to the central nervous system aided by olfactory ensheathing cells (OECs). Thus, OEC transplantation has emerged as a promising therapy for spinal cord injuries and for other neural diseases. However, these cells do not present a uniform population, but instead a functionally heterogeneous population that exhibits a variety of responses including adhesion, repulsion, and crossover during cell-cell and cell-matrix interactions. Some studies report that the migratory properties of OECs are compromised by inhibitory molecules and potentiated by chemical gradients. Here, we demonstrated that rodent OECs express all the components of the Nogo receptor complex and that their migration is blocked by myelin. Next, we used cell tracking and traction force microscopy to analyze OEC migration and its mechanical properties over myelin. Our data relate the decrease of traction force of OEC with lower migratory capacity over myelin, which correlates with changes in the F-actin cytoskeleton and focal adhesion distribution. Lastly, OEC traction force and migratory capacity is enhanced after cell incubation with the Nogo receptor inhibitor NEP1-40.

Role of the lesion scar in the response to damage and repair of the central nervous system

Traumatic damage to the central nervous system (CNS) destroys the blood–brain barrier (BBB) and provokes the invasion of hematogenous cells into the neural tissue. Invading leukocytes, macrophages and lymphocytes secrete various cytokines that induce an inflammatory reaction in the injured CNS and result in local neural degeneration, formation of a cystic cavity and activation of glial cells around the lesion site. As a consequence of these processes, two types of scarring tissue are formed in the lesion site. One is a glial scar that consists in reactive astrocytes, reactive microglia and glial precursor cells. The other is a fibrotic scar formed by fibroblasts, which have invaded the lesion site from adjacent meningeal and perivascular cells. At the interface, the reactive astrocytes and the fibroblasts interact to form an organized tissue, the glia limitans. The astrocytic reaction has a protective role by reconstituting the BBB, preventing neuronal degeneration and limiting the spread of damage. While much attention has been paid to the inhibitory effects of the astrocytic component of the scars on axon regeneration, this review will cover a number of recent studies in which manipulations of the fibroblastic component of the scar by reagents, such as blockers of collagen synthesis have been found to be beneficial for axon regeneration. To what extent these changes in the fibroblasts act via subsequent downstream actions on the astrocytes remains for future investigation.

Transplantation of Olfactory Ensheathing Cells as Adjunct Cell Therapy for Peripheral Nerve Injury

Traumatic events, such as work place trauma or motor vehicle accident violence, result in a significant number of severe peripheral nerve lesions, including nerve crush and nerve disruption defects. Transplantation of myelin-forming cells, such as Schwann cells (SCs) or olfactory ensheathing cells (OECs), may be beneficial to the regenerative process because the applied cells could mediate neurotrophic and neuroprotective effects by secretion of chemokines. Moreover, myelin-forming cells are capable of bridging the repair site by establishing an environment permissive to axonal regeneration. The cell types that are subject to intense investigation include SCs and OECs either derived from the olfactory bulb or the olfactory mucosa, stromal cells from bone marrow (mesenchymal stem cells, MSCs), and adipose tissue-derived cells. OECs reside in the peripheral and central nervous system and have been suggested to display unique regenerative properties. However, so far OECs were mainly used in experimental studies to foster central regeneration and it was not until recently that their regeneration-promoting activity for the peripheral nervous system was recognized. In the present review, we summarize recent experimental evidence regarding the regenerative effects of OECs applied to the peripheral nervous system that may be relevant to design novel autologous cell transplantation therapies.

Non-surgical therapies for peripheral nerve injury

BACKGROUND

Non-surgical approaches have been developed to enhance nerve recovery, which are complementary to surgery and are an adjunct to the reinnervation process.

SOURCES OF DATA

A search of PubMed, Medline, CINAHL, DH data and Embase databases was performed using the keywords 'peripheral nerve injury' and 'treatment'.

AREAS OF CONTROVERSY

Most of the conservative therapies are focused to control neuropathic pain after nerve tissue damage. Only physical therapy modalities have been studied in humans and their effectiveness is not proved.

GROWING POINTS

Many modalities have been experimented with to promote nerve healing and restore function in animal models and in vitro studies. Despite this, none have been actually translated into clinical practice.

AREAS TIMELY FOR DEVELOPING RESEARCH

The hypotheses proved in animals and in vitro should be translated to human clinical practice.