Review
Growth factor delivery systems and repair strategies for damaged peripheral nerves

https://doi.org/10.1016/j.jconrel.2011.11.036Get rights and content

Abstract

Artificial nerve conduits (NCs) are, in certain cases, instrumental for repairing damaged peripheral nerves, although therapeutic efficacy remains often suboptimal. Considerable efforts have been made to improve the therapeutic performance of NCs. This article reviews recent developments in NC-technology for peripheral nerve regeneration with a main focus on growth factors delivery systems and repair strategies. Commonly used materials for NC fabrication include collagen, silk fibroin, and biodegradable aliphatic polyesters. The basic NC structure, i.e., a hollow tube, can be manufactured by diverse methods: spinning mandrel technology, sheet rolling, injection-molding, freeze-drying, and electro-spinning. Polymeric and cellular delivery systems for growth factors can be integrated in the NC wall or within luminal structures such as gels, fibers, or biological materials providing binding affinity for the bioactive compounds. NCs with sustained growth factor delivery generally improve significantly the axonal outgrowth in nerve defect models, although restoration of sensory and motor functions remains inferior to that achieved with autologous nerve grafts. To improve therapeutic outcomes, further biofunctionalization of NCs will be needed, i.e., adjusting degradation kinetics of NC scaffolding to be compatible with axonal regeneration; delivering multiple growth factors at individually optimized kinetics; incorporating modality specific glial cells and furnishing NCs with guiding nanostructures.

Introduction

Peripheral nerve injuries have become a major financial burden for the public; the frequency of such injuries has steadily increased over the past few decades representing several hundred thousand cases across the world [1]. In many cases, such injuries cause life-long disability due to lack of efficient therapeutic repair measures, particularly in case of severe nerve damages. Most commonly used treatments include end-to-end suturing and autologous nerve grafting [2] with functional clinical outcomes often remaining unsatisfactory. For example, reduced stretching capacity of up to 24% with end-to-end sutured nerves has been reported, which is due to changes during Wallerian degeneration, intra- and perineural fibrosis, and tissue adhesion [3]. Autologous nerve grafting is the current gold standard for the bridging of nerve gaps that are not amenable to direct suturing, although it is associated with morbidity, loss of sensation, painful neuroma formation and scarring at the donor site [4]. Furthermore, nerve grafts are needed rapidly and with an appropriate size for efficient reconstruction. Nerve allografts have also been used to overcome the limitations of autografts, but their use is impaired by host immune rejection [5]. The problems caused by autografts and allografts have led to the use of non-nervous tissue grafts originating from blood vessels, small intestinal submucosa, and skeletal muscle tissue [6], [7], [8]. Despite of preliminary success of the substitute tissues for bridging small-sized nerve gaps, the foreign tissue grafts are probably unsuitable to support nerve regeneration over critical size nerve injuries as they lack the appropriate biochemical and topographical elements. To solve some of the problems with biological grafts, nerve conduits (NCs) were proposed and are currently used as flexible and well tolerated alternative.

Biomaterials play a central role in the development of NCs as they significantly influence attachment, proliferation and migration of endogenously regenerating cells [9]. Thus, selection and processing of the biomaterial are critical. A suitable biomaterial should possess good biocompatibility, appropriate degradation properties, and be amenable for controlling secondary NC properties such as pore size, porosity, mechanical strength, and biological functionalization. A wide range of natural and synthetic polymers have been explored for the fabrication of NCs and some of them have been approved by regulatory authorities for use in human [10]; they include, e.g., NeuraGen® (Integra) and NeuroMatrix®/Neuroflex® (Stryker) both made of the cross-linked collagen, Neurotube® (Synovis) made of poly(glycolic acid), SaluBridge® and SaluTunnel® (SaluMedica) made of Salubria® hydrogel, and Neurolac® (Ascension) made of poly(d,l-lactide-co-caprolactone); incidentally, Salubria® is a hydrogel made of cross-linked poly(vinyl alcohol), which is non-biodegradable, treated by freeze–thawing cycles to obtain PVA with shear moduli in the range of 0.1–0.4 MPa [11]. However, presently marketed artificial nerve conduits have limited functional capacity to repair even small-sized nerve gaps [12]. This frequently results in complete failure of nerve regeneration or unsatisfactory clinical outcome. As an example, neuroma formation occurred in a human median nerve (2 cm gap) repaired by Neuragen® NC [13]. Thus, repair of critical nerve injuries requires not only rough guidance and protection from the surrounding, but also trophical (e.g., growth factors) and topographical (guiding nano- or microstructures) information from NCs.

Promising avenues to improve the performance of NCs encompass integration of neurotrophic factors (NTFs), Schwann cells or stem cells, and luminal structures such as gels, multiple channels, or longitudinally aligned nanofibers. NTFs play an indispensable role for neuronal survival and axonal regeneration, which is a prerequisite for effective functional re-innervation of severed peripheral nerves [14], [15]. Widely studied neurotrophic factors for peripheral nerve repair include the neuotrophins [e.g., nerve growth factor (NGF), brain derived neurotrophic factor (BDNF), neurotrophins 3, 4 and 5 (NT-3, -4, -5)], the glial cell line-derived neurotrophic factor (GDNF) (a member of the glial cell line-derive neurotrophic factor family ligands), glial growth factor (GGF) (a member of the neuregulin family), neuropoietic cytokines [e.g., ciliary neuronotrophic factor (CNTF), leukemia inhibitory factor (LIF)], other cytokines [e.g., fibroblast growth factor-1 and -2 (FGF-1 and FGF-2), platelet-derived growth factor (PDGF)], and insulin-like growth factor-1 (IGF-1). For detailed information on the complex mechanism of action of growth factors in neural growth and repair, the reader is referred to pertinent textbooks such as the one from C. Bell [16]. For brief illustration, two very prominent growth factors used in peripheral nerve repair, i.e., NGF and GDNF, differ in their spectrum of action when applied as a single factor. While NGF promotes primarily survival and axonal outgrowth of sensory neurons, both in vitro and in vivo [17], GDNF was identified to be key factor for motor axonal regeneration [18]. Other studies showed, however, that GDNF also supports survival and regeneration of sensory neurons [19], [20]. Another striking difference between NGF and GDNF relates to their effects on axonal branching, as primarily induced by NGF, and axonal elongation, as mostly promoted by GDNF [21], [22]. Interestingly, recent studies demonstrated that the optimal combination of GDNF and NGF elicits synergistic effect on axonal regeneration in vitro and in vivo [22], [23]. DRG neurons co-expressing TrkA and RET receptors [22], [24], which are responsive for NGF and GDNF, may explain the observed synergistic effect on axonal growth, although complete understanding of underlying molecular mechanism still remains elusive. Altogether these findings reveal the importance of optimal combination of multiple synergistic growth factors for effective axonal regeneration, which has only been scarcely considered thus far.

Delivery of NTFs from NCs has been engineered by various means, e.g., by loading the NC lumen with simple NTF solutions, NTF-containing gel matrices, slow release microspheres or nano-fibers, embedding the NTF in the NC wall, or else by delivering the NTFs from mini-pumps or micro-injection ports [25]. Furthermore, NTFs may also be delivered naturally from specialized cells seeded into the NCs. Typical cells that deliver NTFs are Schwann cells and adipose-derived stem cells differentiated into Schwann-like cells. In this sense, the term “delivery system” is henceforth used for any technologies and materials, including native and genetically engineered cells, which can be applied in the fabrication of NCs for delivering biologically active substances at the site of interest, i.e. locally, for promoting axonal growth. The most promising of these approaches will be discussed in more detail later in this contribution. NCs delivering NTFs promote nerve regeneration, but cannot improve on aberrant axonal growth, which results in mismatched connections between the nerve cells and their peripheral targets [26]. Aberrant axonal growth may be attributed to several deficiencies, which include inadequate NTF dose and release kinetics, use of single factor rather than multiple growth factors — as acting naturally, and the lack of appropriate guidance at axon level. There has been a growing interest to develop new strategies for effective delivery of NTFs from polymer based drug delivery systems integrated into NCs. This article reviews widely studied materials and NTF delivery systems, summarizes the most pertinent evaluation systems for testing drug-loaded nerve conduits, and finally provides perspectives for advanced nerve repair strategies that may hold promise for enhancing therapeutic efficacy.

Section snippets

Materials

Biomaterials selection and design are critical for artificial NC development to serve as a potential alternative for autologous nerve grafts. A suitable material should possess good biocompatibility and support cellular growth, proliferation and migration. Appropriate mechanical strength is also required to resist compression or collapse and to allow easy surgical handling. Degradation kinetics of materials should be compatible with nerve regeneration rate, which is in the order of approx. 1–3 

Manufacturing methods of delivery systems

The basic scaffold of NCs is a hollow tube (Fig. 1B), which is commonly fabricated by spinning mandrel technology [50], [51] or gel spinning [52], film casting and subsequent rolling [53], [54], or molding and with subsequent freeze-drying [39]. More advanced fabrication techniques aim at integrating internal structures (Fig. 1B) such as multiple channels [55], longitudinally aligned fibers [56], micro-patterns or grooves [57], or hydrogels [58]. For the localized delivery of growth factors or

Methods for evaluation of nerve conduits performance

There is a huge diversity of methods for evaluating the performance of NCs, which vary depending on the study rationale or investigators' preferences. The diversity spans from different nerve defect models (animals, nerve, gap size) and NC implantation mode to the methods and time points of evaluating the biological outcome. A frequently used study model is the rat sciatic nerve with gap sizes varying from 10 to 20 mm and subsequent evaluation of anatomical, electrophysiological and behavioral

Impact of growth factors release kinetics on nerve regeneration

Artificial NCs have shown respectable biological performance in promoting axonal regeneration, although their usefulness seems to be limited to nerve gaps generally shorter than 20 mm [32], [33]. Regeneration of critical nerve injuries (> 20 mm) remains clinically challenging and will probably require the support of growth promoting factors [13], [34]. Among those, neurotrophic factors (NTFs) play an indispensable role to support neuronal survival and axonal regeneration [14], [15]. A variety of

Appraisal

Presently marketed NCs are limited to the repair of small-sized nerve gaps of generally max. 25 mm. Over such short distance, peripheral nerves have generally sufficient intrinsic regenerative capacity; as a response to the injury, a growth stimulating microenvironment is created, which is comprised of several neurotrophic factors, fibrin matrix, and Schwann cells actively proliferating in a distal-to-proximal gradient. In case of severe and more extended injuries, such organized

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