Investigating the synergistic effect of combined neurotrophic factor concentration gradients to guide axonal growth

Three-Dimensional Engineered Peripheral Nerve: Toward a New Era of Patient-Specific Nerve Repair Solutions

Reconstruction of peripheral nerve injuries (PNIs) with substance loss remains challenging because of limited treatment solutions and unsatisfactory patient outcomes. Currently, nerve autografting is the first-line management choice for bridging critical-sized nerve defects. The procedure, however, is often complicated by donor site morbidity and paucity of nerve tissue, raising a quest for better alternatives. The application of other treatment surrogates, such as nerve guides, remains questionable, and it is inefficient in irreducible nerve gaps. More importantly, these strategies lack customization for personalized patient therapy, which is a significant drawback of these nerve repair options. This negatively impacts the fascicle-to-fascicle regeneration process, critical to restoring the physiological axonal pathway of the disrupted nerve. Recently, the use of additive manufacturing (AM) technologies has offered major advancements to the bioengineering solutions for PNI therapy. These techniques aim at reinstating the native nerve fascicle pathway using biomimetic approaches, thereby augmenting end-organ innervation. AM-based approaches, such as three-dimensional (3D) bioprinting, are capable of biofabricating 3D-engineered nerve graft scaffolds in a patient-specific manner with high precision. Moreover, realistic in vitro models of peripheral nerve tissues that represent the physiologically and functionally relevant environment of human organs could also be developed. However, the technology is still nascent and faces major translational hurdles. In this review, we spotlighted the clinical burden of PNIs and most up-to-date treatment to address nerve gaps. Next, a summarized illustration of the nerve ultrastructure that guides research solutions is discussed. This is followed by a contrast of the existing bioengineering strategies used to repair peripheral nerve discontinuities. In addition, we elaborated on the most recent advances in 3D printing and biofabrication applications in peripheral nerve modeling and engineering. Finally, the major challenges that limit the evolution of the field along with their possible solutions are also critically analyzed.

Nanofibrous bicomponent scaffolds for the dual delivery of NGF and GDNF: controlled release of growth factors and their biological effects

Electrospun fibrous scaffolds capable of providing dual growth factor delivery in a controlled manner have distinctive advantages for tissue engineering. In this study, we have investigated the formation, structure, and characteristics/properties of fibrous bicomponent scaffolds for the dual delivery of glial cell line-derived neurotrophic factor (GDNF) and nerve growth factor (NGF) for peripheral nerve tissue regeneration. GDNF and NGF were incorporated into core-shell structured poly(lactic-co-glycolic acid) (PLGA) and poly (D,L-lactic acid) (PDLLA) nanofibers, respectively, through emulsion electrospinning. Using dual-source dual-power electrospinning, bicomponent scaffolds composed of GDNF/PLGA fibers and NGF/PDLLA fibers with different fiber component ratios were produced. The structure, properties, and in vitro release behavior of mono- and bicomponent scaffolds were systematically investigated. Concurrent and sustained release of GDNF and NGF from bicomponent scaffolds was achieved and their release profiles could be tuned. In vitro biological investigations were conducted. Rat pheochromocytoma cells were found to attach, spread, and proliferate on all scaffolds. The release of growth factors from scaffolds could induce much improved neurite outgrowth and neural differentiation. GDNF and NGF released from GDNF/PLGA scaffolds and NGF/PDLLA scaffolds, respectively, could induce dose-dependent neural differentiation separately. GDNF and NGF released from bicomponent scaffolds exerted a synergistic effect on promoting neural differentiation.

Machine intelligence for nerve conduit design and production

Nerve guidance conduits (NGCs) have emerged from recent advances within tissue engineering as a promising alternative to autografts for peripheral nerve repair. NGCs are tubular structures with engineered biomaterials, which guide axonal regeneration from the injured proximal nerve to the distal stump. NGC design can synergistically combine multiple properties to enhance proliferation of stem and neuronal cells, improve nerve migration, attenuate inflammation and reduce scar tissue formation. The aim of most laboratories fabricating NGCs is the development of an automated process that incorporates patient-specific features and complex tissue blueprints (e.g. neurovascular conduit) that serve as the basis for more complicated muscular and skin grafts. One of the major limitations for tissue engineering is lack of guidance for generating tissue blueprints and the absence of streamlined manufacturing processes. With the rapid expansion of machine intelligence, high dimensional image analysis, and computational scaffold design, optimized tissue templates for 3D bioprinting (3DBP) are feasible. In this review, we examine the translational challenges to peripheral nerve regeneration and where machine intelligence can innovate bottlenecks in neural tissue engineering.

Differentiation of neural-type cells on multi-scale ordered collagen-silica bionanocomposites

Cells respond to biophysical and biochemical signals. We developed a composite filament from collagen and silica particles modified to interact with collagen and/or present a laminin epitope (IKVAV) crucial for cell-matrix adhesion and signal transduction. This combines scaffolding and signaling and shows that local tuning of collagen organization enhances cell differentiation.

Neuro-taxis: Neuronal movement in gradients of chemical and physical environments

Environmental chemical and physical cues dynamically interact with migrating neurons and sprouting axons, and in particular, the gradients of environmental cues are regarded as one of the factors intimately involved in the neuronal movement. Since a growth cone was first described by Cajal, more than one century ago, chemical gradients have been suggested as one of the mechanisms by which the neurons determine proper paths and destinations. However, the gradients of physical cues, such as stiffness and topography, which also interact constantly with the neurons and their axons as a component of the extracellular environments, have rarely been noted regarding the guidance of neurons, despite their gradually increasingly reported influences in the case of nonneuronal-cell migration. In this review, we discuss chemical (i.e., chemo- and hapto-) and physical (i.e., duro-) taxis phenomena on the movement of neurons including axonal elongation. In addition, we suggest topotaxis, the most recently proposed physical-taxis phenomenon, as another potential mechanism in the neuronal movement, based on the reports of neuronal recognition of and responses to nanotopography.

Determining Neurotrophin Gradients in Vitro To Direct Axonal Outgrowth Following Spinal Cord Injury

A spinal cord injury can damage neuronal connections required for both motor and sensory function. Barriers to regeneration within the central nervous system, including an absence of neurotrophic stimulation, impair the ability of injured neurons to reestablish their original circuitry. Exogenous neurotrophin administration has been shown to promote axonal regeneration and outgrowth following injury. The neurotrophins possess chemotrophic properties that guide axons toward the region of highest concentration. These growth factors have demonstrated potential to be used as a therapeutic intervention for orienting axonal growth beyond the injury lesion, toward denervated targets. However, the success of this approach is dependent on the appropriate spatiotemporal distribution of these molecules to ensure detection and navigation by the axonal growth cone. A number of in vitro gradient-based assays have been employed to investigate axonal response to neurotrophic gradients. Such platforms have helped elucidate the potential of applying a concentration gradient of neurotrophins to promote directed axonal regeneration toward a functionally significant target. Here, we review these techniques and the principles of gradient detection in axonal guidance, with particular focus on the use of neurotrophins to orient the trajectory of regenerating axons.

Biomimetic Photocurable Three-Dimensional Printed Nerve Guidance Channels with Aligned Cryomatrix Lumen for Peripheral Nerve Regeneration

Repair and regeneration of critically injured peripheral nerves is one of the most challenging reconstructive surgeries. Currently available and FDA approved nerve guidance channels (NGCs) are suitable for small gap injuries, and their biological performance is inferior to that of autografts. Development of biomimetic NGCs with clinically relevant geometrical and biological characteristics such as topographical, biochemical, and haptotactic cues could offer better regeneration of the long-gap complex nerve injuries. Here, in this study, we present the development and preclinical analysis of three-dimensional (3D) printed aligned cryomatrix-filled NGCs along with nerve growth factor (NGF) (aCG + NGF) for peripheral nerve regeneration. We demonstrated the application of these aCG + NGF NGCs in the enhanced and successful regeneration of a critically injured rat sciatic nerve in comparison to random cryogel-filled NGCs, multichannel and clinically preferred hollow conduits, and the gold standard autografts. Our results indicated similar effect of the aCG + NGF NGCs viz-a-viz that of the autografts, and they not only enhanced the overall regenerated nerve physiology but could also mimic the cellular aspects of regeneration. This study emphasizes the paradigm that these biomimetic 3D printed NGCs will lead to a better functional regenerative outcome under clinical settings.

All roads lead to glaucoma: Induced retinal injury cascades contribute to a common neurodegenerative outcome

Glaucoma describes a distinct optic neuropathy with complex etiology and a variety of associated risk factors, but with similar pathological endpoints. Risk factors such as age, increased intraocular pressure (IOP), low mean arterial pressure, and autoimmune disease, can all be associated with death of retinal ganglion cells (RGCs) and optic nerve head remodeling. Today, IOP management remains the standard of care, even though IOP elevation is not pathognomonic of glaucoma, and patients can continue to lose vision despite effective IOP control. A contemporary view of glaucoma as a complex, neurodegenerative disease has developed, along with the recognition of a need for new disease modifying retinal treatment strategies and improved outcomes. However, the distinction between risk factors triggering the disease process and retinal injury responses is not always clear. In this review, we attempt to distinguish between the various triggers, and their association with subsequent key RGC injury mechanisms. We propose that distinct glaucomatous risk factors result in similar retinal and optic nerve injury cascades, including oxidative and metabolic stress, glial reactivity, and altered inflammatory responses, which induce common molecular signals to induce RGC apoptosis. This organization forms a coherent disease framework and presents conserved targets for therapeutic intervention that are not limited to specific risk factors.

Umbilical cord milking for neonates who are depressed at birth: a randomized trial of feasibility

OBJECTIVE

To evaluate the feasibility and safety of umbilical cord milking (UCM) in neonates who are depressed at birth.

STUDY DESIGN

This is a quasi-randomized, non-blinded, controlled trial on infants (≥35 weeks) who were depressed at birth. UCM (cord milked three times) was performed during the even months and the neonates born during the odd months were in the control group. Primary outcome was feasibility and safety.

RESULTS

A total of 101 infants were enrolled (50 UCM group and 51 control group) between January 2015 and October 2016. UCM was performed in 95% of infants (59/62) who qualified to receive UCM. There were no significant differences in resuscitation delay, resuscitation efforts, and short-term outcomes between the two groups.

CONCLUSIONS

UCM is feasible for term and late preterm infants who are depressed at birth. A larger clinical trial is needed to evaluate long-term benefits of UCM in neonates with HIE.

Incorporation and release of dual growth factors for nerve tissue engineering using nanofibrous bicomponent scaffolds

Electrospun fibrous scaffolds have been extensively used as cell-supporting matrices or delivery vehicles for various biomolecules in tissue engineering. Biodegradable scaffolds with tunable degradation behaviors are favorable for various resorbable tissue replacements. In nerve tissue engineering, delivery of growth factors (GFs) such as nerve growth factor (NGF) and glial cell line-derived neurotrophic factor (GDNF) from scaffolds can be used to promote peripheral nerve repair. In this study, using the established dual-source dual-power electrospinning technique, bicomponent scaffolds incorporated with NGF and GDNF were designed and demonstrated as a strategy to develop scaffolds providing dual GF delivery. NGF and GDNF were encapsulated in poly(D, L-lactic acid) (PDLLA) and poly(lactic-co-glycolic acid) (PLGA) nanofibers, respectively, via emulsion electrospinning. Bicomponent scaffolds with various mass ratios of GDNF/PLGA fibers to NGF/PDLLA fibers were fabricated. Their morphology, structure, properties, and the in vitro degradation were examined. Both types of core-shell structured fibers were evenly distributed in bicomponent scaffolds. Robust scaffolds with varying component ratios were fabricated with average fiber diameter ranging from 307 ± 100 nm to 688 ± 129 nm. The ultimate tensile stress and elastic modulus could be tuned ranging from 0.23 ± 0.07 MPa to 1.41 ± 0.23 MPa, 11.1 ± 3.0 MPa to 75.9 ± 3.3 MPa, respectively. Adjustable degradation was achieved and the weight loss of scaffolds ranged from 9.2% to 44.0% after 42 day degradation test. GDNF and NGF were incorporated with satisfactory encapsulation efficiency and their bioactivity were well preserved. Sustained release of both types of GFs was also achieved.

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

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

Advances in the neurological and neurosurgical management of peripheral nerve trauma

Peripheral nerve trauma frequently affects younger people and may result in significant and long-lasting functional disability. Currently, diagnosis and monitoring of peripheral nerve injury relies on clinical and electrodiagnostic information, supplemented by intraoperative electrophysiological studies. However, in a significant proportion of nerve injuries, the likelihood of spontaneous regeneration resulting in good functional outcome remains uncertain and unnecessary delays to treatment may be faced while monitoring for recovery. Advances in non-invasive imaging techniques to diagnose and monitor nerve injury and regeneration are being developed, and have the potential to streamline the decision-making process. In addition, advances in operative and non-operative treatment strategies may provide more effective ways to maximise functional outcomes following severe peripheral nerve trauma. This review discusses these advances in light of the current state of the art of management of peripheral nerve trauma.

Microfluidic gradients reveal enhanced neurite outgrowth but impaired guidance within 3D matrices with high integrin ligand densities

The density of integrin-binding ligands in an extracellular matrix (ECM) is known to regulate cell migration speed by imposing a balance of traction forces between the leading and trailing edges of the cell, but the effect of cell-adhesive ligands on neurite chemoattraction is not well understood. A platform is presented here that combines gradient-generating microfluidic devices with 3D protein-engineered hydrogels to study the effect of RGD ligand density on neurite pathfinding from chick dorsal root ganglia-derived spheroids. Spheroids are encapsulated in elastin-like polypeptide (ELP) hydrogels presenting either 3.2 or 1.6 mM RGD ligands and exposed to a microfluidic gradient of nerve growth factor (NGF). While the higher ligand density matrix enhanced neurite initiation and persistence of neurite outgrowth, the lower ligand density matrix significantly improved neurite pathfinding and increased the frequency of growth cone turning up the NGF gradient. The apparent trade-off between neurite extension and neurite guidance is reminiscent of the well-known trade-off between adhesive forces at the leading and trailing edges of a migrating cell, implying that a similar matrix-mediated balance of forces regulates neurite elongation and growth cone turning. These results have implications in the design of engineered materials for in vitro models of neural tissue and in vivo nerve guidance channels.

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.

Recent advances in nerve tissue engineering

Nerve injury secondary to trauma, neurological disease or tumor excision presents a challenge for surgical reconstruction. Current practice for nerve repair involves autologous nerve transplantation, which is associated with significant donor-site morbidity and other complications. Previously artificial nerve conduits made from polycaprolactone, polyglycolic acid and collagen were approved by the FDA (USA) for nerve repair. More recently, there have been significant advances in nerve conduit design that better address the requirements of nerve regrowth. Innovations in materials science, nanotechnology, and biology open the way for the synthesis of new generation nerve repair conduits that address issues currently faced in nerve repair and regeneration. This review discusses recent innovations in this area, including the use of nanotechnology to improve the design of nerve conduits and to enhance nerve regeneration.

Light-reactive dextran gels with immobilized guidance cues for directed neurite growth in 3D models

Immobilized NT-3 enhanced DRG neurite growth while Sema3A strongly repelled it, versus neutravidin controls, in a hydrogel choice point model.

Directed migration in neural tissue engineering

Directed cell migration is particularly important in neural tissue engineering, where the goal is to direct neurons and support cells across injured nerve gaps. Investigation of the gradients present in the body during development provides an approach to guiding cells in peripheral and central nervous system tissue, but many different types of gradients and patterns can accomplish directed migration. The focus of this review is to describe current research paradigms in neural tissue gradients and review their effectiveness for directed migration. The review explores directed migration achieved through the use of chemical, adhesive, mechanical, topographical, and electrical types of gradients. Few studies investigate combined gradients, though it is known that a combination of therapies is necessary for reconnection of neuronal circuitry. To date, there has been no systematic review of gradient approaches to neural tissue engineering. By looking at effectiveness of various scaffold cue presentation and methods to combine these strategies, the potential for nerve repair is increased.

Thick collagen-based 3D matrices including growth factors to induce neurite outgrowth

Designing synthetic microenvironments for cellular investigations is a very active area of research at the crossroads of cell biology and materials science. The present work describes the design and functionalization of a three-dimensional (3D) culture support dedicated to the study of neurite outgrowth from neural cells. It is based on a dense self-assembled collagen matrix stabilized by 100-nm-wide interconnected native fibrils without chemical crosslinking. The matrices were made suitable for cell manipulation and direct observation in confocal microscopy by anchoring them to traditional glass supports with a calibrated thickness of ∼50μm. The matrix composition can be readily adapted to specific neural cell types, notably by incorporating appropriate neurotrophic growth factors. Both PC-12 and SH-SY5Y lines respond to growth factors (nerve growth factor and brain-derived neurotrophic factor, respectively) impregnated and slowly released from the support. Significant neurite outgrowth is reported for a large proportion of cells, up to 66% for PC12 and 49% for SH-SY5Y. It is also shown that both growth factors can be chemically conjugated (EDC/NHS) throughout the matrix and yield similar proportions of cells with longer neurites (61% and 52%, respectively). Finally, neurite outgrowth was observed over several tens of microns within the 3D matrix, with both diffusing and immobilized growth factors.

Microtissue engineered constructs with living axons for targeted nervous system reconstruction

As a common feature of many neurological diseases and injury, the loss of axon pathways can have devastating effects on function. Here, we demonstrate a new strategy to restore damaged axon pathways using transplantable miniature constructs consisting of living neurons and axonal tracts internalized within hydrogel tubes. These hydrogel microconduits were developed through an iterative process to support neuronal survival and directed axon growth. The design included hollow agarose tubes providing a relatively stiff outer casing to direct constrained unidirectional outgrowth of axons through a central soft collagen matrix, with overall dimensions of 250 μm inner diameter ×500 μm outer diameter and extending up to several centimeters. The outer casing was also designed to provide structural support of neuronal/axonal cultures during transplantation of the construct. Using neuron culture conditions optimized for the microconduits, dissociated dorsal root ganglia neurons were seeded in the collagen at one end of the conduits. Over the following week, high-resolution confocal microscopy demonstrated that the neurons survived and the somata remained in a tight cluster at the original seeding site. In addition, robust outgrowth of axons from the neurons was found, with axon fascicles constrained in a longitudinal projection along the internal collagen canal and extending over 5 mm in length. Notably, this general geometry recapitulates the anatomy of axon tracts. As such, these constructs may be useful to repair damaged axon projections by providing a transplantable bridge of living axons. Moreover, the small size of the construct permits follow-on studies of minimally invasive transplantation into potentially sensitive regions of the nervous system.

Standardized criterion to analyze and directly compare various materials and models for peripheral nerve regeneration

Progress in understanding conditions for optimal peripheral nerve regeneration has been stunted due to lack of standardization of experimental conditions and assays. In this paper we review the large database that has been generated using the Lundborg nerve chamber model and compare various theories for their ability to explain the experimental data. Data were normalized based on systematic use of the critical axon elongation, the gap length at which the probability of axon reconnection between the stumps is just 50%. Use of this criterion has led to a rank-ordering of devices or treatments and has led, in turn, to conclusions about the conditions that facilitate regeneration. Experimental configurations that have maximized facilitation of peripheral nerve regeneration are those in which the tube wall comprised degradable polymers, including collagen and certain synthetic biodegradable polymers, and was cell-permeable rather than protein-permeable. Tube fillings that showed very high regenerative activity were suspensions of Schwann cells, a solution either of acidic or basic fibroblast growth factor, insoluble ECM substrates rather than solutions or gels, polyamide filaments oriented along the tube axis and highly porous, insoluble analogs of the ECM with specific structure and controlled degradation rate. It is suggested that the data are best explained by postulating that the quality of regeneration depends on two critical processes. The first is compression of stumps and regenerating nerve by a thick myofibroblast layer that surrounds these tissues and blocks synthesis of a nerve of large diameter (pressure cuff theory). The second is synthesis of linear columns of Schwann cells that serve as tracks for axon elongation (basement membrane microtube theory). It is concluded that experimental configurations that show high regenerative activity suppress the first process while facilitating the second.

Umbilical cord blood mononuclear cell transplantation for neonatal hypoxic-ischemic encephalopathy

Despite recent advances in the treatment of neonatal hypoxic-ischemic encephalopathy (HIE) using therapeutic hypothermia, at least 30% of the cooled infants will die or have moderate/severe neurological disability. Umbilical cord blood cells (UCBCs), which are readily available at birth, have been shown to reduce sensorimotor and/or cognitive impairments in several models of brain damage, representing a promising option for the treatment of neurological diseases. In this review, we discuss recent preclinical studies that assessed the effects of UCBC transplantation in the Rice-Vannucci animal model of HIE. We also review the possible cell types and mechanisms involved in the therapeutic effect of UCBC transplantation, including neuroprotection, immunomodulation, and stimulation of neural plasticity and regeneration. In addition, we discuss how neuroimaging methods, such as bioluminescence imaging, nuclear-medicine imaging, or magnetic resonance imaging, could be used to evaluate the biodistribution of UCBCs in both preclinical and clinical studies.

Sensory axon regeneration: rebuilding functional connections in the spinal cord

Functional regeneration within the adult spinal cord remains a formidable task. A major barrier to regeneration of sensory axons into the spinal cord is the dorsal root entry zone. This region displays many of the inhibitory features characteristic of other central nervous system injuries. Several experimental treatments, including inactivation of inhibitory molecules (such as Nogo and chondroitin sulfate proteoglycans) or administration of neurotrophic factors (such as nerve growth factor, neurotrophin3, glial-derived neurotrophic factor and artemin), have been found to promote anatomical and functional regeneration across this barrier. However, there have been relatively few experiments to determine whether regenerating axons project back to their appropriate target areas within the spinal cord. This review focuses on recent advances in sensory axon regeneration, including studies assessing the ability of sensory axons to reconnect with their original synaptic targets.

Facile micropatterning of dual hydrogel systems for 3D models of neurite outgrowth

Understanding how microenvironmental factors influence neurite growth is important to inform studies in nerve regeneration, plasticity, development, and neurophysiology. In vitro models attempting to more accurately mimic the physiological environment by provision of a 3D growth matrix may provide useful foundations. Some limitations of thick 3D culture models include hampered solute transport, less-robust neurite growth than on 2D substrates, and difficulty in achieving spatial control of growth. To this end, we describe a 3D dual hydrogel model for embryonic rat day 15 dorsal root ganglion tissue explant growth using a digital micromirror device for dynamic mask projection photolithography. The photolithography method developed allowed simple, reproducible, one-step fabrication of thick hydrogel constructs on a variety of substrates, including permeable cell culture inserts. The relationships between projected mask size, crosslinked hydrogel resolution, and gel thickness were characterized, and resolution was found generally to decrease with increasing gel thickness. Cell viability in thick (481 μm) hydrogel constructs was significantly greater on permeable supports than glass, suggesting transport limitations were somewhat alleviated. The observed neurite growth was abundant and occurred in a spatially controlled manner throughout the 3D environment, a crucial step in the quest for a more effective biomimetic model of neurite outgrowth.

Wallerian degeneration and peripheral nerve conditions for both axonal regeneration and neuropathic pain induction

Wallerian degeneration is a cascade of stereotypical events in reaction to injury of nerve fibres. These events consist of cellular and molecular alterations, including macrophage invasion, activation of Schwann cells, as well as neurotrophin and cytokine upregulation. This review focuses on cellular and molecular changes distal to various types of peripheral nerve injury which simultaneously contribute to axonal regeneration and neuropathic pain induction. In addition to the stereotypical events of Wallerian degeneration, various types of nerve damage provide different conditions for both axonal regeneration and neuropathic pain induction. Wallerian degeneration of injured peripheral nerve is associated with an inflammatory response including rapid upregulation of the immune signal molecules like cytokines, chemokines and transcription factors with both beneficial and detrimental effects on nerve regeneration or neuropathic pain induction. A better understanding of the molecular interactions between the immune system and peripheral nerve injury would open the possibility for targeting these inflammatory mediators in therapeutic interventions. Understanding the pleiotropic effects of cytokines/chemokines, however, requires investigating their highly specific pathways and precise points of action.

Effect of controlled co-delivery of synergistic neurotrophic factors on early nerve regeneration in rats

Present interventions to repair severed peripheral nerves provide slow and poor early axonal regeneration, which may cause unsatisfactory functional reinnervation. To improve early axonal regeneration in a 10 mm rat sciatic nerve gap model, we developed collagen nerve conduits loaded with the synergistically acting glial cell line-derived neurotrophic factor (GDNF) and nerve growth factor (NGF). For controlling the concomitant GDNF and NGF release, the collagen tubes were cross-linked by a dehydro-thermal treatment (110 degrees C; 20 mbar; 5 days) prior to impregnating the tubes with GDNF and NGF and by coating drug-loaded tubes with layers of poly(lactide-co-glycolide). The conduits made of cross-linked collagen released low initial amounts of GDNF and NGF (2% of both during first 3 days) and enhanced significantly the early (2 weeks) nerve regeneration in terms of axonal outgrowth and Schwann cell migration in a 10 mm rat sciatic nerve gap model, as compared to the conduits made of non-cross-linked collagen releasing higher initial amounts of GDNF and NGF (12-16% within 3 days), or those releasing GDNF alone. The enhancement of early axonal regeneration using controlled co-delivery of multiple synergistic neurotrophic factors is an important requisite for eventually establishing functional connections with the target organ.

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

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

The use of immobilized neurotrophins to support neuron survival and guide nerve fiber growth in compartmentalized chambers

We answered two major questions: (1) does retrograde signaling involve retrograde transport of nerve growth factor (NGF); and (2) is a gradient of immobilized NGF sufficient to promote and guide local axonal growth? To answer these questions, we developed a technique that resulted in stably immobilized NGF and combined this with compartmented chambers. NGF was photochemically-immobilized on a chitosan surface either in the cell body (CB) compartment, distal axon (DA) compartment, or both. Neuron survival and axon outgrowth were found to be insignificantly different from positive controls where soluble NGF was present. When NGF was immobilized on chitosan surfaces in the DA compartment, and in the absence of soluble NGF, neuron survival was observed, likely due to the retrograde signal of the activated TrkA receptor and NGF-induced signals, but not the retrograde signal of NGF itself. Axons were guided towards the higher end of the step concentration gradient of NGF that was photoimmobilized on the chitosan surface in the DA compartment by laser confocal patterning, demonstrating axonal guidance. These studies provide better insight into NGF signaling mechanisms which are important to both understanding developmental disorders and degenerative diseases of the nervous system, as well as improving design strategies to promote nerve regeneration after injury.

Osteochondral interface tissue engineering using macroscopic gradients of bioactive signals

Continuous gradients exist at osteochondral interfaces, which may be engineered by applying spatially patterned gradients of biological cues. In the present study, a protein-loaded microsphere-based scaffold fabrication strategy was applied to achieve spatially and temporally controlled delivery of bioactive signals in three-dimensional (3D) tissue engineering scaffolds. Bone morphogenetic protein-2 and transforming growth factor-beta(1)-loaded poly(D,L-lactic-co-glycolic acid) microspheres were utilized with a gradient scaffold fabrication technology to produce microsphere-based scaffolds containing opposing gradients of these signals. Constructs were then seeded with human bone marrow stromal cells (hBMSCs) or human umbilical cord mesenchymal stromal cells (hUCMSCs), and osteochondral tissue regeneration was assessed in gradient scaffolds and compared to multiple control groups. Following a 6-week cell culture, the gradient scaffolds produced regionalized extracellular matrix, and outperformed the blank control scaffolds in cell number, glycosaminoglycan production, collagen content, alkaline phosphatase activity, and in some instances, gene expression of major osteogenic and chondrogenic markers. These results suggest that engineered signal gradients may be beneficial for osteochondral tissue engineering.

Controlled growth factor delivery for tissue engineering

Growth factors play a crucial role in information transfer between cells and their microenvironment in tissue engineering and regeneration. They initiate their action by binding to specific receptors on the surface of target cells and the chemical identity, concentration, duration, and context of these growth factors contain information that dictates cell fate. Hence, the importance of exogenous delivery of these molecules in tissue engineering is unsurprising, considering their importance for tissue regeneration. However, the short half-lives of growth factors, their relatively large size, slow tissue penetration, and their potential toxicity at high systemic levels, suggest that conventional routes of administration are unlikely to be effective. In this review, we provide an overview of the design criteria for growth factor delivery vehicles with respect to the growth factor itself and the microenvironment for delivery. We discuss various methodologies that could be adopted to achieve this localized delivery, and strategies using polymers as delivery vehicles in particular.

Bayesian model predicts the response of axons to molecular gradients

Axon guidance by molecular gradients plays a crucial role in wiring up the nervous system. However, the mechanisms axons use to detect gradients are largely unknown. We first develop a Bayesian "ideal observer" analysis of gradient detection by axons, based on the hypothesis that a principal constraint on gradient detection is intrinsic receptor binding noise. Second, from this model, we derive an equation predicting how the degree of response of an axon to a gradient should vary with gradient steepness and absolute concentration. Third, we confirm this prediction quantitatively by performing the first systematic experimental analysis of how axonal response varies with both these quantities. These experiments demonstrate a degree of sensitivity much higher than previously reported for any chemotacting system. Together, these results reveal both the quantitative constraints that must be satisfied for effective axonal guidance and the computational principles that may be used by the underlying signal transduction pathways, and allow predictions for the degree of response of axons to gradients in a wide variety of in vivo and in vitro settings.

Synergistic effect of GDNF and NGF on axonal branching and elongation in vitro

There is a clinical need to enhance functional recovery of injured peripheral nerves. Local administration of neurotrophic factors (NTFs) after surgical repair has been proposed for this purpose. Little is known, however, on the optimal local dose and dosing frequency of NTFs in a peripheral nerve defect. For increasing our knowledge on biologically relevant local NTFs concentrations and for making available an in vitro assay for assessing the bioactivity of NTFs in connection with implantable localized delivery systems, we developed in this study a bioassay for NTFs, which is based on dorsal root ganglion (DRG) explants from E9 (9 days old) chicken embryos. Axonal elongation and extent of axonal branching was analyzed microscopically after addition of glial cell line-derived neurotrophic factor (GDNF) and nerve growth factor (NGF), each alone and in combination. GDNF significantly promoted axonal elongation, but only little axonal branching, whereas NGF induced extensive axonal branching with modest axonal elongation. The combination of GDNF and NGF exerted a synergistic effect on the axonal elongation, axonal branching and growth kinetics. GDNF and NGF also enhanced the expression of their respective functional receptors Ret and TrkA on the DRG neurons. This information should be relevant for the development of implants containing NTFs and on drug therapy of damaged peripheral nerves.

Microsphere-based seamless scaffolds containing macroscopic gradients of encapsulated factors for tissue engineering

Spatial and temporal control of bioactive signals in three-dimensional (3D) tissue engineering scaffolds is greatly desired. Coupled together, these attributes may mimic and maintain complex signal patterns, such as those observed during axonal regeneration or neovascularization. Seamless polymer constructs may provide a route to achieve spatial control of signal distribution. In this study, a novel microparticle-based scaffold fabrication technique is introduced as a method to create 3D scaffolds with spatial control over model dyes using uniform poly(D,L-lactide-co-glycolide) microspheres. Uniform microspheres were produced using the Precision Particle Fabrication technique. Scaffolds were assembled by flowing microsphere suspensions into a cylindrical glass mold, and then microspheres were physically attached to form a continuous scaffold using ethanol treatment. An ethanol soak of 1 h was found to be optimum for improved mechanical characteristics. Morphological and physical characterization of the scaffolds revealed that microsphere matrices were porous (41.1 +/- 2.1%) and well connected, and their compressive stiffness ranged from 142 to 306 kPa. Culturing chondrocytes on the scaffolds revealed the compatibility of these substrates with cell attachment and viability. In addition, bilayered, multilayered, and gradient scaffolds were fabricated, exhibiting excellent spatial control and resolution. Such novel scaffolds can serve as sustained delivery devices of heterogeneous signals in a continuous and seamless manner, and may be particularly useful in future interfacial tissue engineering investigations.

Spatially patterned gene expression for guided neurite extension

Axon pathfinding by localized expression of guidance molecules is critical for the proper development of the nervous system. In this report, we present a well-defined spatially patterned gene expression system to investigate neurite guidance in vitro. Nonviral gene delivery was patterned by combining substrate-mediated gene delivery with soft lithography techniques, and the amount of protein produced at the region of localized expression was varied by altering the vector concentration and the width of the pattern, highlighting the flexibility of the system. A neuronal coculture model was used to investigate responses to spatial patterns of nerve growth factor (NGF) expression. The soluble NGF gradient elicited a guidance cue, and the degree of guidance was governed by the distance a neuron was cultured from the pattern and the time between accessory cell and neuron seedings. A portion of the diffusible NGF bound to the culture surface in the extracellular space, and the surface-associated NGF supported neuron survival and neurite outgrowth. However, the surface-bound NGF gradient alone did not elicit a guidance signal, and in fact masked the guidance cue by soluble NGF gradients. Mathematical modeling of NGF diffusion was used to predict the concentration gradients, and both the absolute and fractional gradients capable of guiding neurites produced by patterned gene expression differed substantially from the values obtained with existing engineered protein gradients. Spatially patterned gene expression provides a versatile tool to investigate the factors that may promote neurite guidance.

Adenosine induces growth-cone turning of sensory neurons

The formation of appropriate connections between neurons and their specific targets is an essential step during development and repair of the nervous system. Growth cones are located at the leading edges of the growing neurites and respond to environmental cues in order to be guided to their final targets. Directional information can be coded by concentration gradients of substrate-bound or diffusible-guidance molecules. Here we show that concentration gradients of adenosine stimulate growth cones of sensory neurons (dorsal root ganglia) from chicken embryos to turn towards the adenosine source. This response is mediated by adenosine receptors. The subsequent signal transduction process involves cAMP. It may be speculated that the in vivo function of this response is concerned with the formation or the repair and regeneration of the peripheral nervous system.

Strategies and applications for incorporating physical and chemical signal gradients in tissue engineering

From embryonic development to wound repair, concentration gradients of bioactive signaling molecules guide tissue formation and regeneration. Moreover, gradients in cellular and extracellular architecture as well as in mechanical properties are readily apparent in native tissues. Perhaps tissue engineers can take a cue from nature in attempting to regenerate tissues by incorporating gradients into engineering design strategies. Indeed, gradient-based approaches are an emerging trend in tissue engineering, standing in contrast to traditional approaches of homogeneous delivery of cells and/or growth factors using isotropic scaffolds. Gradients in tissue engineering lie at the intersection of three major paradigms in the field-biomimetic, interfacial, and functional tissue engineering-by combining physical (via biomaterial design) and chemical (with growth/differentiation factors and cell adhesion molecules) signal delivery to achieve a continuous transition in both structure and function. This review consolidates several key methodologies to generate gradients, some of which have never been employed in a tissue engineering application, and discusses strategies for incorporating these methods into tissue engineering and implant design. A key finding of this review was that two-dimensional physicochemical gradient substrates, which serve as excellent high-throughput screening tools for optimizing desired biomaterial properties, can be enhanced in the future by transitioning from two dimensions to three dimensions, which would enable studies of cell-protein-biomaterial interactions in a more native tissue-like environment. In addition, biomimetic tissue regeneration via combined delivery of graded physical and chemical signals appears to be a promising strategy for the regeneration of heterogeneous tissues and tissue interfaces. In the future, in vivo applications will shed more light on the performance of gradient-based mechanical integrity and signal delivery strategies compared to traditional tissue engineering approaches.

Miniaturized system of neurotrophin patterning for guided regeneration

Understanding the fundamentals of cell behaviour is imperative for designing and improving engineering strategies for regenerative medicine. By combining the precision of confocal microscopy with photochemistry, nerve growth factor (NGF) was chemically immobilized on chitosan films either in distinct areas or as concentration gradients. Using rhodamine as a proxy for NGF, a series of immobilized concentration gradients were created, using the number of rastering scans within a defined area and the distance between each area as a way to control the resulting gradient. The same photochemistry was applied to create NGF patterns on chitosan films which were visualized by immunostaining, and the immobilized NGF remained bioactive as demonstrated with a neuron survival assay. Neuron survival was 73.2+/-1.3% after 3 days of culture on chitosan films with 30 ng/cm(2) of homogenously immobilized NGF, which was comparable to 74.8+/-3.4% neuron survival on chitosan with 50 ng/ml of soluble NGF present. Interestingly, when neurons were plated on a chitosan film that had distinct immobilized NGF-patterned areas surrounded by unmodified chitosan, the neurons remained predominantly as single cells in the NGF-patterned regions, but formed aggregates outside of these patterns on the plain chitosan film. Thus, the immobilized NGF pattern influenced neuron behaviour and can be used to further probe mechanisms of other neuron behaviour such as axon guidance. Importantly, the versatility of the confocal laser patterning technique reported here can be extended to other factors to elucidate fundamental cell functions, and hence design strategies in regenerative medicine.

Differences between the effect of anisotropic and isotropic laminin and nerve growth factor presenting scaffolds on nerve regeneration across long peripheral nerve gaps

Anisotropic scaffolds of agarose hydrogels containing gradients of laminin-1 (LN-1) and nerve growth factor (NGF) molecules were used to promote sciatic nerve regeneration across a challenging 20mm nerve gap in rats. Step and continuous gradient anisotropic scaffolds were fabricated and characterized, and regeneration was compared to that in isotropic scaffolds with uniform concentrations of LN-1 and NGF and sciatic nerve grafts harvested from syngenic rats. Polysulfone tubular guidance channels were used to present the agarose-based scaffolds to the nerve stumps. Four months after implantation, regenerating axons were observed in animals implanted with anisotropic scaffolds with gradients of both LN-1 and NGF molecules and nerve grafts, but not in animals with isotropic scaffold implants. Also, the scaffolds with gradients of either LN-1 or NGF, with the other component being uniformly distributed in the scaffold, did not elicit axonal regeneration. The total number of myelinated axons was similar for the anisotropic scaffold and the nerve graft conditions, with the anisotropic scaffolds having a higher density of axons than the nerve grafts. Axonal diameter distribution was similar for the anisotropic scaffolds and the nerve grafts. The nerve grafts and anisotropic scaffolds resulted in better functional outcome compared to isotropic scaffolds as measured by the relative gastrocnemius muscle weight (RGMW). Additionally the state of neuromuscular junctions as assessed by pre- and post-synaptic staining revealed that both the anisotropic scaffolds performed as well as nerve grafts.

Release rate controls biological activity of nerve growth factor released from fibrin matrices containing affinity‐based delivery systems

Previously, combinatorial techniques were used to identify peptide sequences exhibiting high, medium, and low affinity for heparin. Bidomain peptides were synthesized containing a transglutaminase sequence for one domain and one of the heparin-affinity sequences for the other domain. A delivery system was made consisting of bi-domain peptides, heparin, and nerve growth factor (NGF), which binds to heparin with moderate affinity. The goal of this research was to determine whether peptide affinity for heparin and the molar ratio of peptide to heparin affected the release rate of NGF from the delivery system and the biological activity of NGF released. This study also explored whether peptide affinity modulated biological activity independent of release rate. Mathematically modeling the delivery system confirmed that release could be controlled by both peptide affinity and molar ratio of peptide to heparin. Experimentally, the rate of NGF release from the delivery system was found to be affected by peptide affinity and molar ratio. The delivery system presented biologically active NGF as assayed by embryonic chick dorsal root ganglia (DRGs) neurite extension, where extension was similar to or increased for DRGs grown in fibrin matrices containing the delivery system compared to DRGs grown with NGF in the culture media. Furthermore, by modulating the molar ratio of peptide to heparin in the delivery system, similar release rates of NGF were obtained for different affinity peptides and these conditions promoted similar levels of neurite extension, demonstrating that release rate appears to be the main mechanism controlling the biological activity of released NGF.