Miniaturized system of neurotrophin patterning for guided regeneration

Promotion of Adrenal Pheochromocytoma (PC-12) Cell Proliferation and Outgrowth Using Schwann Cell-Laden Gelatin Methacrylate Substrate

Peripheral nerve injuries cause different degrees of nerve palsy and function loss. Due to the limitations of autografts, nerve tissue engineering (TE) scaffolds incorporated with various neurotrophic factors and cells have been investigated to promote nerve regeneration. However, the molecular mechanism is still poorly understood. In this study, we co-cultured Schwann cells (SCs) and rat adrenal pheochromocytoma (PC-12) cells on 50% degrees of methacryloyl substitution gelatin methacrylate (GelMA) scaffold. The SCs were encapsulated within the GelMA, and PC-12 cells were on the surface. A 5% GelMA was used as the co-culture scaffold since it better supports SCs proliferation, viability, and myelination and promotes higher neurotrophic factors secretion than 10% GelMA. In the co-culture, PC-12 cells demonstrated a higher cell proliferation rate and axonal extension than culturing without SCs, indicating that the secretion of neurotrophic factors from SCs can stimulate PC-12 growth and axonal outgrowth. The mRNA level for neurotrophic factors of SCs in 5% GelMA was further evaluated. We found significant upregulation when compared with a 2D culture, which suggested that this co-culture system could be a potential scaffold to investigate the mechanism of how SCs affect neuronal behaviors.

Micropatterned nanolayers immobilized with nerve growth factor for neurite formation of PC12 cells

BACKGROUND

Nerve regeneration is important for the treatment of degenerative diseases and neurons injured by accidents. Nerve growth factor (NGF) has been previously conjugated to materials for promotion of neurogenesis.

MATERIALS AND METHODS

Photoreactive gelatin was prepared by chemical coupling of gelatin with azidobenzoic acid (P-gel), and then NGF was immobilized on substrates in the presence or absence of micropatterned photomasks. UV irradiation induced crosslinking reactions of P-gel with itself, NGF, and the plate for immobilization.

RESULTS

By adjustment of the P-gel concentration, the nanometer-order height of micropatterns was controlled. NGF was quantitatively immobilized with increasing amounts of P-gel. Immobilized NGF induced neurite outgrowth of PC12 cells, a cell line derived from a pheochromocytoma of the rat adrenal medulla, at the same level as soluble NGF. The immobilized NGF showed higher thermal stability than the soluble NGF and was repeatedly used without loss of biological activity. The 3D structure (height of the formed micropattern) regulated the behavior of neurite guidance. As a result, the orientation of neurites was regulated by the stripe pattern width.

CONCLUSION

The micropattern-immobilized NGF nanolayer biochemically and topologically regulated neurite formation.

β3 Integrin Promotes Long-Lasting Activation and Polarization of Vascular Endothelial Growth Factor Receptor 2 by Immobilized Ligand

Supplemental Digital Content is available in the text.

Objective—

During neovessel formation, angiogenic growth factors associate with the extracellular matrix. These immobilized factors represent a persistent stimulus for the otherwise quiescent endothelial cells (ECs), driving directional EC migration and proliferation and leading to new blood vessel growth. Vascular endothelial growth factor receptor 2 (VEGFR2) is the main mediator of angiogenesis. Although VEGFR2 signaling has been deeply characterized, little is known about its subcellular localization during neovessel formation. Aim of this study was the characterization and molecular determinants of activated VEGFR2 localization in ECs during neovessel formation in response to matrix-immobilized ligand.

Approach and Results—

Here we demonstrate that ECs stimulated by extracellular matrix–associated gremlin, a noncanonical VEGFR2 ligand, are polarized and relocate the receptor in close contact with the angiogenic factor–enriched matrix both in vitro and in vivo. GM1 (monosialotetrahexosylganglioside)-positive planar lipid rafts, β₃ integrin receptors, and the intracellular signaling transducers focal adhesion kinase and RhoA (Ras homolog gene family, member A) cooperate to promote VEGFR2 long-term polarization and activation.

Conclusions—

A ligand anchored to the extracellular matrix induces VEGFR2 polarization in ECs. Long-lasting VEGFR2 relocation is closely dependent on lipid raft integrity and activation of β₃ integrin pathway. The study of the endothelial responses to immobilized growth factors may offer insights into the angiogenic process in physiological and pathological conditions, including cancer, and for a better engineering of synthetic tissue scaffolds to blend with the host vasculature.

Non-Faradaic electrical impedimetric investigation of the interfacial effects of neuronal cell growth and differentiation on silicon nanowire transistors

Silicon nanowire field-effect transistor (SiNW FET) devices have been interfaced with cells; however, their application for noninvasive, real-time monitoring of interfacial effects during cell growth and differentiation on SiNW has not been fully explored. Here, we cultured rat adrenal pheochromocytoma (PC12) cells, a type of neural progenitor cell, directly on SiNW FET devices to monitor cell adhesion during growth and morphological changes during neuronal differentiation for a period of 5-7 d. Monitoring was performed by measuring the non-Faradaic electrical impedance of the cell-SiNW FET system using a precision LCR meter. Our SiNW FET devices exhibited changes in impedance parameters during cell growth and differentiation because of the negatively charged cell membrane, seal resistance, and membrane capacitance at the cell/SiNW interface. It was observed that during both PC12 cell growth and neuronal differentiation, the impedance magnitude increased and the phase shifted to more negative values. However, impedance changes during cell growth already plateaued 3 d after seeding, while impedance changes continued until the last observation day during differentiation. Our results also indicate that the frequency shift to above 40 kHz after growth factor induction resulted from a larger coverage of cell membrane on the SiNWs due to distinctive morphological changes according to vinculin staining. Encapsulation of PC12 cells in a hydrogel scaffold resulted in a lack of trend in impedance parameters and confirmed that impedance changes were due to the cells. Moreover, cytolysis of the differentiated PC12 cells led to significant changes in impedance parameters. Equivalent electrical circuits were used to analyze the changes in impedance values during cell growth and differentiation. The technique employed in this study can provide a platform for performing investigations of growth-factor-induced progenitor cell differentiation.

Capture of endothelial cells under flow using immobilized vascular endothelial growth factor

We demonstrate the ability of immobilized vascular endothelial growth factor (VEGF) to capture endothelial cells (EC) with high specificity under fluid flow. To this end, we engineered a surface consisting of heparin bound to poly-L-lysine to permit immobilization of VEGF through the C-terminal heparin-binding domain. The immobilized growth factor retained its biological activity as shown by proliferation of EC and prolonged activation of KDR signaling. Using a microfluidic device we assessed the ability to capture EC under a range of shear stresses from low (0.5 dyne/cm²) to physiological (15 dyne/cm²). Capture was significant for all shear stresses tested. Immobilized VEGF was highly selective for EC as evidenced by significant capture of human umbilical vein and ovine pulmonary artery EC but no capture of human dermal fibroblasts, human hair follicle derived mesenchymal stem cells, or mouse fibroblasts. Further, VEGF could capture EC from mixtures with non-EC under low and high shear conditions as well as from complex fluids like whole human blood under high shear. Our findings may have far reaching implications, as they suggest that VEGF could be used to promote endothelialization of vascular grafts or neovascularization of implanted tissues by rare but continuously circulating EC.

3D presentation of a neurotrophic factor for the regulation of neural progenitor cells

Background: Adequate cell–scaffold interactions and neurotrophin support are essential factors for neural regeneration. Aim: To provide insight into the biofunctionalization of complex 3D scaffolds with nanoscale precision, as well as the effect of spatial distribution of brain-derived neurotrophic factor (BDNF) and its prolonged stimulation in combination with enhanced cell affinity of nanofibrous scaffolds on the survival/proliferation and neurite outgrowth. Methods & materials: We developed a versatile approach using layer-by-layer self-assembly to incorporate cell adhesion and spatial representation of neurotrophic factors into complex nanofibrous scaffolds. Results: Heparin/poly-L-lysine (PLL) polyelectrolyte multilayers (PEMs) were deposited on electrospun poly-ε-caprolatone nanofibers. Well-controlled amounts of BDNF were immobilized on the PEM-modified nanofibers. In addition, longer neurite outgrowth was observed in neural progenitor cells cultured on PLL-terminating PEM scaffolds. The immobilized BDNF on PLL-terminated PEM scaffolds resulted in significantly longer neurites and higher cell numbers (p < 0.01) compared with BDNF-free and BDNF-adsorbed PLL-terminating scaffolds. Interestingly, there was no upregulation of TrkB-FL, TrkB-T1 or GAP-43 mRNAs with immobilized BDNF in day 5 cultures. Discussion & conclusion: This work reinforces the importance of the combinatorial effects of biomaterial scaffold nanostructure and spatial presentation of neurotrophins in directing neural progenitor cell fates.

Original submitted 18 January 2013; Revised submitted 3 May 2013

Misdirection and guidance of regenerating axons after experimental nerve injury and repair

Misdirection of regenerating axons is one of the factors that can explain the limited results often found after nerve injury and repair. In the repair of mixed nerves innervating different distal targets (skin and muscle), misdirection may, for example, lead to motor axons projecting toward skin, and vice versa—that is, sensory axons projecting toward muscle. In the repair of motor nerves innervating different distal targets, misdirection may result in reinnervation of the wrong target muscle, which might function antagonistically. In sensory nerve repair, misdirection might give an increased perceptual territory. After median nerve repair, for example, this might lead to a dysfunctional hand.

Different factors may be involved in the misdirection of regenerating axons, and there may be various mechanisms that can later correct for misdirection. In this review the authors discuss these different factors and mechanisms that act along the pathway of the regenerating axon. The authors review recently developed evaluation methods that can be used to investigate the accuracy of regeneration after nerve injury and repair (including the use of transgenic fluorescent mice, retrograde tracing techniques, and motion analysis). In addition, the authors discuss new strategies that can improve in vivo guidance of regenerating axons (including physical guidance with multichannel nerve tubes and biological guidance accomplished using gene therapy).

Specific immobilization of biotinylated fusion proteins NGF and Sema3A utilizing a photo-cross-linkable diazirine compound for controlling neurite extension

In this study we report the successful synthesis of N-(2-mercaptoethyl)-3-(3-methyl-3H-diazirine-3-yl) propanamide (N-MCEP-diazirine), with sulfhydryl and amine photoreactive ends to allow recombinant protein tethering to chitosan films. This regimen allows mimicry of the physiological endeavor of axon pathfinding in the nervous system where neurons rely on cues for guidance during development and regeneration. Our strategy incorporates strong covalent and noncovalent interactions, utilizing N-MCEP-diazirine, maleimide-streptavidin complex, and two custom biotinylated-fusion proteins, nerve growth factor (bNGF), and semaphorin3A (bSema3A). Synthetic yield of N-MCEP-diazirine was 87.3 ± 1.9%. Characteristic absorbance decrease at 348 nm after N-MCEP-diazirine exposure to UV validated the photochemical properties of the diazirine moiety, and the attachment of cross-linker to chitosan films was verified with Fourier transform infrared spectroscopy (FTIR). Fluorescence techniques showed no significant difference in the detection of immobilized proteins compared to absorbing the proteins to films (p < 0.05); however, in vitro outgrowth of dorsal root ganglia (DRG) was more responsive to immobilized bNGF and bSema3A compared to adsorbed bNGF and bSema3A over a 5 day period. Immobilized bNGF significantly increased DRG length over time (p < 0.0001), but adsorbed bNGF did not increase in axon extension from day 1 to day 5 (p = 0.4476). Immobilized bSema3A showed a significant decrease in neurite length (524.42 ± 57.31 μm) at day 5 compared to adsorbed bSema3A (969.13 ± 57.31 μm). These results demonstrate the superiority of our immobilization approach to protein adsorption because biotinylated-fusion proteins maintain their active confirmation and their tethering can be spatially controlled via a UV activated N-MCEP-diazirine cross-linker.

The use of chitosan-based scaffolds to enhance regeneration in the nervous system

Various biomaterials have been proposed to build up scaffolds for promoting neural repair. Among them, chitosan, a derivative of chitin, has been raising more and more interest among basic and clinical scientists. A number of studies with neuronal and glial cell cultures have shown that this biomaterial has biomimetic properties, which make it a good candidate for developing innovative devices for neural repair. Yet, in vivo experimental studies have shown that chitosan can be successfully used to create scaffolds that promote regeneration both in the central and in the peripheral nervous system. In this review, the relevant literature on the use of chitosan in the nervous tissue, either alone or in combination with other components, is overviewed. Altogether, the promising in vitro and in vivo experimental results make it possible to foresee that time for clinical trials with chitosan-based nerve regeneration-promoting devices is approaching quickly.

Hyaluronic acid-based scaffold for central neural tissue engineering

Central nervous system (CNS) regeneration with central neuronal connections and restoration of synaptic connections has been a long-standing worldwide problem and, to date, no effective clinical therapies are widely accepted for CNS injuries. The limited regenerative capacity of the CNS results from the growth-inhibitory environment that impedes the regrowth of axons. Central neural tissue engineering has attracted extensive attention from multi-disciplinary scientists in recent years, and many studies have been carried out to develop cell- and regeneration-activating biomaterial scaffolds that create an artificial micro-environment suitable for axonal regeneration. Among all the biomaterials, hyaluronic acid (HA) is a promising candidate for central neural tissue engineering because of its unique physico-chemical and biological properties. This review attempts to outline current biomaterials-based strategies for CNS regeneration from a tissue engineering point of view and discusses the main progresses in research of HA-based scaffolds for central neural tissue engineering in detail.

The Effect of Growth Factors and Soluble Nogo-66 Receptor Protein on Transplanted Neural Stem/Progenitor Survival and Axonal Regeneration after Complete Transection of Rat Spinal Cord

Adult central mammalian axons show minimal regeneration after spinal cord injury due to loss of oligodendrocytes, demyelination of surviving axons, absence of growth-promoting molecules, and inhibitors of axonal outgrowth. In the present study, we attempted to address these impediments to regeneration by using a combinatory strategy to enhance cell survival and regeneration after complete spinal cord transection (SCT) in adult rats. The strategy comprised: 1) adult rat brain-derived neural stem/progenitor cells (NSPCs) preseeded on laminin-coated chitosan channels; 2) extramedullary chitosan channels to promote axonal regrowth and reduce the barrier caused by scarring; 3) local delivery of a novel rat soluble Nogo-66 receptor protein [NgR(310)ecto-Fc, referred to as NgR] to block the inhibitory effect of myelin-based inhibitors; and 4) local delivery of basic fibroblast growth factor, epidermal growth factor, and platelet-derived growth factor to enhance survival and promote differentiation of transplanted cells. Compared with our previous studies where brain-derived NSPCs preseeded in extramedullary chitosan channels were implanted in the same SCT model but without growth factors and NgR, the present channel–growth factor combination produced greater numbers of surviving NSPCs after SCT. Also, the growth factors promoted preferential differentiation of NSPCs toward oligodendrocytes, while NgR significantly decreased astrocytic differentiation of NSPCs. NgR alone or in combination with NSPCs significantly enhanced the total number of myelinated fibers in the bridge and increased the area of the bridging tissue between the cord stumps. The combination of NgR, growth factors, and NSPCs had synergistic effect on bridge formation. However, only a small number of descending corticospinal tract axons grew into the central portions of the bridges as shown by anterograde tracing of the corticospinal tract with BDA. The majority of the regenerated axons in the channels originated from local host neurons adjacent to the tissue bridges. In conclusion, we showed that growth factors increased survival of transplanted NSPCs whereas NgR enhanced axonal regeneration, but the combination did not have additive effects on functional recovery or regeneration.

Engineering airway epithelium

Airway epithelium is constantly presented with injurious signals, yet under healthy circumstances, the epithelium maintains its innate immune barrier and mucociliary elevator function. This suggests that airway epithelium has regenerative potential (I. R. Telford and C. F. Bridgman, 1990). In practice, however, airway regeneration is problematic because of slow turnover and dedifferentiation of epithelium thereby hindering regeneration and increasing time necessary for full maturation and function. Based on the anatomy and biology of the airway epithelium, a variety of tissue engineering tools available could be utilized to overcome the barriers currently seen in airway epithelial generation. This paper describes the structure, function, and repair mechanisms in native epithelium and highlights specific and manipulatable tissue engineering signals that could be of great use in the creation of artificial airway epithelium.

Neural regenerative strategies incorporating biomolecular axon guidance signals

There are currently no acceptable cures for central nervous system injuries, and damage induced large gaps in the peripheral nervous system have been challenging to bridge to restore neural functionality. Innervation by neurons is made possible by the growth cone. This dynamic structure is unique to neurons, and can directly sense physical and chemical activity in its environment, utilizing these cues to propel axons to precisely reach their targets. Guidance can occur through chemoattractive factors such as neurotrophins and netrins, chemorepulsive agents like semaphorins and slits, or contact-mediated molecules such as ephrins and those located in the extracellular matrix. The understanding of biomolecular activity during nervous system development and injury has generated new techniques and tactics for improving and restoring function to the nervous system after injury. This review will focus on the major neuronal guidance molecules and their utility in current tissue engineering and neural regenerative strategies.

Pattern analysis and spatial distribution of neurons in culture

The nervous system is a complex, highly-ordered, integrated network of cells. Dispersed cultures of neurons enable investigations into intrinsic cellular functions without the complexities inherent in the intact nervous system. This culture process generates a homogeneously dispersed population that is assumed to be spatially random. Despite the vast number of studies utilizing dispersed neurons, few studies address the spatial distribution of large populations of neurons, in vitro. We used ink-jet printing and surface chemistry to define patterned areas of poly-lysine adhesion (∼50 μm spots) juxtaposed against a fluorinated-silane background. We quantitatively analysed populations of patterned neurons on printed protein spots, and unpatterned neurons. Using a microarray scanner, we acquired large images (72 mm × 22 mm) of patterns, and neurons with and without patterns. Fast Fourier transformation (FFT) image analysis was used to determine global alignment of neurons to patterns. Through point pattern analysis, we described the spatial organization of dispersed neurons with, or without, patterned substrates. Patterned neurons show spatial organization characteristics reminiscent of printed patterns, with spatial distributions representative of unpatterned neurons. Most notably, both patterned and unpatterned neurons show departure from null models of complete spatial randomness (CSR; a homogeneous Poisson process) at shorter distances with conformity to CSR occurring at longer distances. Cellular morphometrics show that when compared to their unpatterned counterparts, spot-patterned neurons exhibit a significant increase (p < 0.0001) in the mean dendritic circularity and an increase in the number of more circular neurons. Through neurite tracing, we show that dendritic processes are also highly confined to patterned areas, and that they are on average 58% shorter than dendrites of neurons without patterns. Our findings show that patterned areas change the spatial organization of the somata and dendrites of cultured neurons, and that traditional neuronal cultures deviate from CSR.

Synergistic NGF/B27 gradients position synapses heterogeneously in 3D micropatterned neural cultures

Native functional brain circuits show different numbers of synapses (synaptic densities) in the cerebral cortex. Until now, different synaptic densities could not be studied in vitro using current cell culture methods for primary neurons. Herein, we present a novel microfluidic based cell culture method that combines 3D micropatterning of hydrogel layers with linear chemical gradient formation. Micropatterned hydrogels were used to encapsulate dissociated cortical neurons in laminar cell layers and neurotrophic factors NGF and B27 were added to influence the formation of synapses. Neurotrophic gradients allowed for the positioning of distinguishable synaptic densities throughout a 3D micropatterned neural culture. NGF and B27 gradients were maintained in the microfluidic device for over two weeks without perfusion pumps by utilizing a refilling procedure. Spatial distribution of synapses was examined with a pre-synaptic marker to determine synaptic densities. From our experiments, we observed that (1) cortical neurons responded only to synergistic NGF/B27 gradients, (2) synaptic density increased proportionally to synergistic NGF/B27 gradients; (3) homogeneous distribution of B27 disturbed cortical neurons in sensing NGF gradients and (4) the cell layer position significantly impacted spatial distribution of synapses.

Large-scale analysis of neurite growth dynamics on micropatterned substrates

During both development and regeneration of the nervous system, neurons display complex growth dynamics, and several neurites compete to become the neuron's single axon. Numerous mathematical and biophysical models have been proposed to explain this competition, which remain experimentally unverified. Large-scale, precise, and repeatable measurements of neurite dynamics have been difficult to perform, since neurons have varying numbers of neurites, which themselves have complex morphologies. To overcome these challenges using a minimal number of primary neurons, we generated repeatable neuronal morphologies on a large scale using laser-patterned micron-wide stripes of adhesive proteins on an otherwise highly non-adherent substrate. By analyzing thousands of quantitative time-lapse measurements of highly reproducible neurite growth dynamics, we show that total neurite growth accelerates until neurons polarize, that immature neurites compete even at very short lengths, and that neuronal polarity exhibits a distinct transition as neurites grow. Proposed neurite growth models agree only partially with our experimental observations. We further show that simple yet specific modifications can significantly improve these models, but still do not fully predict the complex neurite growth behavior. Our high-content analysis puts significant and nontrivial constraints on possible mechanistic models of neurite growth and specification. The methodology presented here could also be employed in large-scale chemical and target-based screens on a variety of complex and subtle phenotypes for therapeutic discoveries using minimal numbers of primary neurons.

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.

Principles of design and biological approaches for improving the selectivity of cochlear implant electrodes

The perceptual performance of cochlear implant recipients seems to have reached a plateau in recent years. This may be attributable to inadequate neural selectivity of available intracochlear electrodes, caused by current spread and electrode interactions. Attempts to improve electrode selectivity have included manipulating the number and configuration of electrodes that are stimulated at any one time, displacing perilymph from the cochlea to restrict current flow along the cochlea, and reducing the distance between electrodes and neurons. One experimental approach by which the distance between neurons and electrodes may be reduced is to use neurotrophic factors to promote the regeneration of the peripheral dendrites of auditory neurons and guide them towards intracochlear electrodes. The likely requirements of a system for regenerating auditory neurons towards the cochlear electrode include either a stable release of neurotrophin, or transient neurotrophin followed by electrical stimulation; a close proximity of electrode to osseous spiral lamina or a polymer to bridge the gap between the two; guidance signals to attract neurons towards the electrode; patterning of the electrode surface to direct dendrites to electrode contacts and a 'stop' signal to arrest regeneration once the electrode has been reached.

Cell Guidance by 3D-Gradients in Hydrogel Matrices: Importance for Biomedical Applications

Concentration gradients of soluble and matrix-bound guidance cues in the extracellular matrix direct cell growth in native tissues and are of great interest for design of biomedical scaffolds and on implant surfaces. The focus of this review is to demonstrate the importance of gradient guidance for cells as it would be desirable to direct cell growth onto/into biomedical devices. Many studies have been described that illustrate the production and characterization of surface gradients, but three dimensional (3D)-gradients that direct cellular behavior are not well investigated. Hydrogels are considered as synthetic replacements for native extracellular matrices as they share key functions such as 2D- or 3D-solid support, fibrous structure, gas- and nutrition permeability and allow storage and release of biologically active molecules. Therefore this review focuses on current studies that try to implement soluble or covalently-attached gradients of growth factors, cytokines or adhesion sequences into 3D-hydrogel matrices in order to control cell growth, orientation and migration towards a target. Such gradient architectures are especially desirable for wound healing purposes, where defined cell populations need to be recruited from the blood stream and out of the adjacent tissue, in critical bone defects, for vascular implants or neuronal guidance structures where defined cell populations should be guided by appropriate signals to reach their proper positions or target tissues in order to accomplish functional repair.

The phosphorylation of vascular endothelial growth factor receptor-2 (VEGFR-2) by engineered surfaces with electrostatically or covalently immobilized VEGF

Growth factors are a class of signaling proteins that direct cell fate through interaction with cell-surface receptors. Although a myriad of possible cell fates stems from a growth factor binding to its receptor, the signaling cascades that result in one fate over another are still being elucidated. One possible mechanism by which nature modulates growth factor signaling is through the method of presentation of the growth factor--soluble or immobilized (matrix bound). Here we present the methodology to study signaling of soluble versus immobilized VEGF through VEGFR-2. We have designed a strategy to covalently immobilize VEGF using its heparin-binding domain to orient the molecule (bind) and a secondary functional group to mediate covalent binding (lock). This bind-and-lock approach aims to allow VEGF to assume a bioactive orientation before covalent immobilization. Surface plasmon resonance (SPR) demonstrated heparin and VEGF binding with surface densities of 60 ng/cm2 and 100 pg/cm2, respectively. ELISA experiments confirmed VEGF surface density and showed that electrostatically bound VEGF releases in cell medium and heparin solutions while covalently bound VEGF remains immobilized. Electrostatically bound VEGF and covalently bound VEGF phosphorylate VEGFR-2 in both VEGFR-2 transfected cells and VEGFR-2 endogenously producing cells. HUVECs plated on VEGF functionalized surfaces showed different morphologies between surface-bound VEGF and soluble VEGF. The surfaces synthesized in these studies allow for the study of VEGF/VEGFR-2 signaling induced by covalently bound, electrostatically bound, and soluble VEGF and may provide further insight into the design of materials for the generation of a mature and stable vasculature.

Chapter 9: Artificial scaffolds for peripheral nerve reconstruction

Posttraumatic peripheral nerve repair is one of the major challenges in restorative medicine and microsurgery. Despite the recent progresses in the field of tissue engineering, functional recovery after severe nerve lesions is generally partial and unsatisfactory. Autograft is still the best method to treat peripheral nerve lesions, although it has several drawbacks and does not allow complete functional recovery. Full recovery of nerve functionality could ideally be achieved by proper guiding axon regeneration toward the original target tissues, through the use of purposely engineered artificial nerve guidance channels (NGCs). In the last decade, artificial NGCs have been produced using a variety of both natural and synthetic, biodegradable and nonbiodegradable polymers. Several techniques have been developed to obtain porous and nonporous NGCs and to realize and incorporate bioactive fillers for NGCs. Some of the developed products have been approved for clinical applications. Many other NGC typologies have been object of interest and are currently under investigation. The current trend of nerve tissue engineering is the realization of biomimetic NGCs, providing chemotactic, topological, and haptotactic signalling to cells, respectively by surface functionalization with cell binding domains, the use of internal-oriented matrices/fibres and the sustained release of neurotrophic factors. The present contribution provides a balanced integration of the most recent achievements of tissue engineering in the field of peripheral nerve repair. By an accurate evaluation of the status of research, the review delineates the most promising directions to which research should address for consistent progress in the field of peripheral nerve repair.