Addition of fibronectin to alginate matrix improves peripheral nerve regeneration in tissue-engineered conduits

Alginate Hydrogels as Biomaterials

[Image: see text] Alginate hydrogels are proving to have a wide applicability as biomaterials. They have been used as scaffolds for tissue engineering, as delivery vehicles for drugs, and as model extracellular matrices for basic biological studies. These applications require tight control of a number of material properties including mechanical stiffness, swelling, degradation, cell attachment, and binding or release of bioactive molecules. Control over these properties can be achieved by chemical or physical modifications of the polysaccharide itself or the gels formed from alginate. The utility of these modified alginate gels as biomaterials has been demonstrated in a number of in vitro and in vivo studies.Micro-CT images of bone-like constructs that result from transplantation of osteoblasts on gels that degrade over a time frame of several months leading to improved bone formation.

Repair of Experimental Alveolar Bone Defects by Tissue-Engineered Bone

Alveolar bone resorption caused by periodontal diseases remains a difficult clinical problem to treat. Our purpose here was to develop protocols for repairing experimental horizontal alveolar bone defects. The procedure entailed isolating bone marrow stromal cells (BMSC). They were expanded and induced in vitro into osteogenic cells in a defined medium. Induced BMSCs were mixed with calcium alginate to form a gel form of cell-scaffold construct for developing engineered bone. A horizontal alveolar bone defect was created in 15 mongrel dogs, which was 5 mm high on each of two buccal sides at the location of mandibular premolar 3, 4, and molar 1. Without bias, the animals were separated into the following groups: (1) cell-scaffold construct as the experimental group; (2) calcium alginate alone as the control group A; (3) untreated as the control group B. Block sections of the defects were collected at 4, 12, and 24 weeks postsurgery, respectively, and processed for gross and histological observation as well as x-ray examination. The results showed that in vitro induced BMSCs exhibited an osteogenic phenotype. Histologically, bone nodule structure was observed in the tissue of the experimental group at 4 weeks postsurgery and the engineered bone became more mature after 12 weeks, which was similar to normal bone. At 12 weeks postsurgery, the height of repaired alveolar bone reached 2.43 +/- 0.93 mm, 0.98 +/- 0.87 mm, 0.78 +/- 0.75 mm for the experimental group, control groups A and B, respectively, with a significant difference between the experimental and control groups (p < 0.01). The average level of buccal alveolar ridge in experimental group, control groups A and B reached 48.59%, 19.74%, and 15.76% of the height of normal alveolus, respectively, with a significant difference between the experimental group and two control groups (p < 0.01). We thus conclude that BMSCs can be induced to become osteogenic and can be used as seed cells to engineer bone tissue and repair experimental alveolar bone defects.

Tissue-engineered peripheral nerve grafting by differentiated bone marrow stromal cells

Bone marrow stromal cells are multipotential stem cells that contribute to the differentiation of tissues such as bone, cartilage, fat and muscle. In the experiment, we found that bone marrow stromal cells can be induced to differentiate into cells expressing characteristic markers of Schwann cells, such as S-100 and glial fibrillary acidic protein, promoting peripheral nerve regeneration. Tissue-engineered bioartificial nerve grafting of rats by differentiated bone marrow stromal cells was applied for bridging a 10 mm-long sciatic nerve defect. Twenty-eight inbred strains of female F344 rats weighing 160 approximately 200 g were randomly divided into four nerve grafting groups, with seven rats in each group. Differentiated bone marrow stromal cell-laden group: poly(lactic-co-glycolic) acid tubes with an intrinsic framework were seeded with syngeneic bone marrow stromal cells which were induced for 5 days; Schwann cell-laden group: poly(lactic-co-glycolic) acid tubes with an intrinsic framework were seeded with syngeneic Schwann cells; acellular group: poly(lactic-co-glycolic) acid tubes were only filled with an intrinsic framework; autografts group. Three months later, a series of examinations was performed, including electrophysiological methods, walking track analysis, immunohistological staining of nerves, immunostaining of S-100 and neurofilament, and axon counts. The outcome indicated that bone marrow stromal cells are able to differentiate into Schwann-like cells and Schwann-like cells could promote nerve regeneration. Bone marrow stromal cells may be potentially optional seed cells for peripheral nerve tissue engineering because of abilities of promoting axonal regeneration.

Combination of engineered neural cell adhesion molecules and GDF-5 for improved neurite extension in nerve guide concepts

Current therapeutical approaches for the treatment of severe lesions in the peripheral nervous system rely on the use of autologous tissue or the body's own Schwann cells. However, these approaches are limited and alternative strategies for peripheral nerve regeneration are required. Here we evaluate combinations of a variety of neuronal regeneration factors including engineered cell adhesion molecules and growth factors in embryonic model neurons to test the possible improvement of artificial nerve guides by cooperative mechanisms. Cell adhesion molecules L1 and neurofascin synergistically promote neurite elongation. The outgrowth promoting properties of both proteins can be combined and further increased within one chimeric protein. Addition of growth and differentiation factor 5 (GDF-5) further enhances neurite outgrowth in a substrate-independent manner. This effect is not due to a protective mode of action of GDF-5 against pro-apoptotic stimuli. Consequently, the study supports the idea that different modes of action of pro-regenerative factors may contribute synergistically to neurite outgrowth and emphasizes the applicability of combinations of proteins specifically involved in development of the nervous system for therapeutical approaches.

Roles of nitric oxide, malondialdehyde, and fibronectin in an experimental peripheral nerve ischemia-reperfusion model

Although there are many studies of the neuropathology of the ischemic degeneration of peripheral nerves, the pathogenesis is not well-understood. The roles of several biomolecules on this process were previously reported. An adhesion molecule, fibronectin, which is applied locally (as a conduit material), is very effective in nerve recovery. This study was carried out to evaluate the roles of fibronectin, lipid peroxidation, and nitric oxide (NO) in an experimental model of peripheral nerves. Ischemia and reperfusion injury of sciatic nerves was rendered by clamping the femoral artery and vein. Rats were divided into nine groups. Ischemia and reperfusion were not applied to group 1. In group 2, only ischemia was performed, but reperfusion was not accomplished. For groups 3-9, 1, 2, and 24 h and 1, 2, 3, and 4 weeks of reperfusion were applied following 3 h of ischemia. Then NO, malondialdehyde (MDA), and fibronectin levels were observed in serum samples of rats. Colorimetric and nephelometric assays were used for determination of the levels of these parameters. In this study, all biochemical parameters were found to be increased in the ischemia groups when compared with the control group 1 (P < 0.05). A significant difference was observed between study groups with respect to MDA, NO, and fibronectin levels (P < 0.05). Also, some correlations were established between biochemical parameters in the same group, depending on the varying reperfusion time (r > 0.50). Ischemia causes some important changes in biochemical parameters, and depending on the reperfusion time, nerve injury continues for a while. In our study, we observed that serum levels of MDA decreased in the periods when NO and fibronectin simultaneously increased. Such increases may contribute to neural recovery, and there may be interactions among them.

Alginate hydrogel and matrigel as potential cell carriers for neurotransplantation

Development of biosynthetic conduits carrying extracellular matrix molecules and cell lines expressing neurotrophic growth factors represents a novel and promising strategy for spinal cord and peripheral nerve repair. In the present in vitro study, the compatibility and growth-promoting effects of (i) alginate hydrogel, (ii) alginate hydrogel complemented with fibronectin, and (iii) matrigel were compared between olfactory ensheathing cells (OECs), Schwann cells (SCs), and bone marrow stromal cells (BMSCs). Neurite outgrowth from embryonic dorsal root ganglia (DRG) neurons was used to assess the efficacy of the hydrogels alone or in combination with cultured cells to promote axonal regeneration. The result showed that alginate hydrogel transformed OECs, SCs, and BMSCs into atypical cells with spherical shape and inhibited their metabolic activity. Combination of alginate hydrogel with fibronectin promoted only OECs proliferation. Alginate hydrogel also inhibited outgrowth of DRG neurites, although this effect was attenuated by addition of fibronectin, SCs, or BMSCs. In contrast, matrigel stimulated cell proliferation, preserved the typical morphological features of the cultured cells and induced massive sprouting of DRG neurites. Addition of cultured cells to matrigel did not further improve DRG neurite outgrowth. The present findings suggest that addition of extracellular matrix should be considered when engineering biosynthetic scaffolds on the basis of alginate hydrogels.

Alteration of human neuroblastoma cell morphology and neurite extension with micropatterns

The spatial orientation of nerve cells plays a pivotal role in nerve regeneration. Here we report a new method for regulating neuronal cell morphology and guiding neurite extension on standard tissue culture dishes. Random copolymers of oligoethyleneglycol methacrylate and methacrylic acid [poly(OEGMA-co-MA)], microcontact printed on standard tissue culture dishes, resist cell attachment and remain intact in serum-containing medium for up to 2 weeks. Cell viability assay of SH-SY5Y cells demonstrated that poly(OEGMA-co-MA) on the substrate or in solution has no cytotoxic effect. When retinoic acid was added to SH-SY5Y cells, they extended neurites along the line patterns that are significantly longer than cells cultured on non-patterned culture dishes. The ability to guide neurite extension with micrometer precision is valuable for guiding directional growth of neurites and path finding of regenerating nerves.

Green fluorescent protein is a stable morphological marker for schwann cell transplants in bioengineered nerve conduits

Bioengineered systems incorporate cultured cells to mimic the substituted tissue. A labeling method is necessary to monitor the survival of transplanted cells within the host. This labeling method must be compatible with the histochemical methods used for morphological analysis. This study assessed (1) The in vitro characteristics of Schwann cells (SCs) labeled with green fluorescent protein (GFP), (2) the in vivo effect of transplanted GFP-SCs in a model of peripheral nerve injury, and (3) the compatibility of GFP-SCs with immunofluorescence histochemical techniques. SCs were retrovirally labeled with GFP and their growth characteristics were compared with those of nontransduced SCs (ntSCs). GFP-SCs were seeded in a resorbable nerve conduit for grafting into a 1-cm gap in rat sciatic nerve. Grafts were harvested after 2 weeks and immunofluorescent staining was performed to measure axonal and SC regeneration distances and to identify GFP-SCs. Results of GFP-SC vitality assays did not vary significantly from those of ntSC assays. GFP-SCs were readily located ex vivo and stimulated significantly better axonal and SC regeneration distances in comparison with empty conduits. These findings show that GFP labeling does not have a deleterious effect on SCs and that it is a useful labeling method for the study of bioengineered systems.

Building In Vitro Models of Organs

Tissue-engineering techniques are being used to build in vitro models of organs as substitutes for human donor organs for transplantation as well as in vitro toxicology testing (as alternatives to use of animals). Tissue engineering involves the fabrication of scaffolds from materials that are biologically compatible to serve as cellular supports and microhabitats in order to reconstitute a desired tissue or organ. Three organ systems that are currently the foci of tissue engineering efforts for both transplantation and in vitro toxicology testing purposes are discussed. These are models of the cornea, nerves (peripheral nerves specifically), and cardiovascular components. In each of these organ systems, a variety of techniques and materials are being used to achieve the same end results. In general, models that are designed with consideration of the developmental and cellular biology of the target tissues or organs have tended to result in morphologically and physiologically accurate models. Many of the models, with further development and refinement, have the potential to be useful as functional substitute tissues and organs for transplantation or for in vitro toxicology testing.

Physical and biological performance of a novel block copolymer nerve guide

Although the ability to regenerate is evident in the nervous system, lesioned neurites are unable to cross gaps in neuronal pathways. In order to bridge gaps, guiding cues are essential to direct neurite regrowth. To overcome many of the shortcomings of polymer-based nerve guides, we developed a bioresorbable nerve guide composed of a novel trimethylene carbonate-caprolacton block copolymer (TMC-CL). Pore formation was controlled by using special solvent/precipitation media compositions in combination with the pore forming agent poly ethylene glycol (PEG). NMR spectroscopy, shear force-, compression-, and permeation assays were used for conduit characterization. The polymer conduit has a semipermeable wall with submicron pores to allow free metabolite/drug exchange. In order to investigate the principle of temporally controlled expression of therapeutic proteins in nerve guides, Neuro-2a cells were genetically engineered to express the reporter gene product green fluorescent protein (GFP) under the control of the Tet-On system. When these transduced cells were encapsulated in nerve guides, GFP expression could be induced for days by adding the antibiotic tetracycline derivative doxycycline to the nerve guide environment. Furthermore, encapsulated dorsal root ganglia (DRG) produced long neurites in vitro. In subsequent in vivo experiments, nerve guides filled with Schwann cells (SC) were implanted into lesioned spinal cords of adult rats. Regeneration of spinal cord axons into nerve guides was promoted by co-implanted Schwann cells. The data suggest that the novel TMC-CL nerve guides provide a promising tool for neuroregeneration.

Adhesion and Growth of Bone Marrow Stromal Cells on Modified Alginate Hydrogels

Alginate is a biodegradable, immunocompatible biopolymer that is capable of immobilizing viable cells and bioactive factors. Few investigations have analyzed the efficacy of alginate gels as substrata for cell attachment and proliferation. Here we have compared the adhesion and subsequent growth of human and rat bone marrow stromal fibroblastic cells on unmodified alginate hydrogel surfaces. It was found that, in contrast to rat cells, human cells did not readily attach or proliferate on unmodified alginates. In attempts to enhance these features, or collagen type I was incorporated into the gels, with no significant improvements in prolonged human cell adherence. However, alginate gels containing both collagen type I and beta-tricalcium phosphate were found to enhance human cell adherence and proliferation. Furthermore, interactions between the collagen and beta-tricalcium phosphate prevented loss of the protein from the hydrogels. These results indicate that alginate gels containing collagen have potential uses as vehicles for delivery of adherent cells to a tissue site. In addition, gels containing beta-tricalcium phosphate, with or without collagen type I incorporation, have potential to support cell growth and differentiation in vitro before implantation. This study emphasizes the limitations of the uses of cells derived from experimental animals in certain model studies relating to human tissue engineering.

A composite poly-hydroxybutyrate-glial growth factor conduit for long nerve gap repairs

There is considerable evidence that peripheral nerves have the potential to regenerate in an appropriate microenvironment. We have developed a novel artificial nerve guide composed of poly 3-hydroxybutyrate (PHB) filled with glial growth factor (GGF) suspended in alginate hydrogel. Gaps of 2-4 cm in rabbit common peroneal nerve were bridged using a PHB conduit containing either GGF in alginate hydrogel (GGF) or alginate alone (Alginate), or with an empty PHB conduit (Empty). Tissues were harvested 21, 42 and 63 days post-operatively. Schwann cell and axonal regeneration were assessed using quantitative immunohistochemistry. At 21 days, addition of GGF increased significantly the distance of axonal and Schwann cells regeneration in comparison with that observed in Alginate and Empty conduits for both gap lengths. The axons bridged the 2-cm GGF conduits gap by 63 days, with a comparable rate of regeneration seen in 4-cm conduits. Schwann cells and axonal regeneration quantity was similar for both gap lengths in each group. However, at all time points the quantity of axonal and Schwann cells regeneration in GGF grafts was significantly greater than in both Alginate and Empty conduits, the latter showing better regeneration than Alginate conduits. The results indicate an inhibitory effect of alginate on regeneration, which is partially reversed by the addition of GGF to the conduits. In conclusion, GGF stimulates a progressive and sustainable regeneration increase in long nerve gap conduits.

Exogenous leukaemia inhibitory factor enhances nerve regeneration after late secondary repair using a bioartificial nerve conduit

The clinical outcome of peripheral nerve injuries remains disappointing, even in the ideal situation of a primary repair performed with optimal microsurgical techniques. Primary repair is appropriate for only about 85% of injuries, and outcome is worse following secondary nerve repair, partly owing to the reduced regenerative potential of chronically axotomised neurons. Leukaemia inhibitory factor (LIF) is a gp-130 neurocytokine that is thought to act as an 'injury factor', triggering the early-injury phenotype within neurons and potentially boosting their regenerative potential after secondary nerve repair. At 2-4 months after sciatic nerve axotomy in the rat, 1 cm gaps were repaired using either nerve isografts or poly-3-hydroxybutyrate conduits containing a calcium alginate and fibronectin hydrogel. Regeneration was determined by quantitative immunohistochemistry 6 weeks after repair, and the effect of incorporating recombinant LIF (100 ng/ml) into the conduits was assessed. LIF increased the regeneration distance in repairs performed after both 2 months (69%, P=0.019) and 4 months (123%, P=0.021), and was statistically comparable to nerve graft. The total area of axonal immunostaining increased by 21% (P>0.05) and 63% (P>0.05), respectively. Percentage immunostaining area was not increased in the 2 months group, but increased by 93% in the repairs performed 4 months after axotomy. Exogenous LIF, therefore, has a potential role in promoting peripheral nerve regeneration after secondary repair, and can be effectively delivered within poly-3-hydroxybutyrate bioartificial conduits used for nerve repair.