Scaffolds for central nervous system tissue engineering

Traumatic injuries to the brain and spinal cord of the central nervous system (CNS) lead to severe and permanent neurological deficits and to date there is no universally accepted treatment. Owing to the profound impact, extensive studies have been carried out aiming at reducing inflammatory responses and overcoming the inhibitory environment in the CNS after injury so as to enhance regeneration. Artificial scaffolds may provide a suitable environment for axonal regeneration and functional recovery, and are of particular importance in cases in which the injury has resulted in a cavitary defect. In this review we discuss development of scaffolds for CNS tissue engineering, focusing on mechanism of CNS injuries, various biomaterials that have been used in studies, and current strategies for designing and fabricating scaffolds.     

Using a 3-D multicellular simulation of spinal cord injury with live cell imaging to study the neural immune barrier to nanoparticle uptake

Development of nanoparticle (NP) based therapies to promote regeneration in sites of central nervous system (CNS;i.e.brain and spinal cord) pathology relies critically on the availability of experimental models that offer biologically valid predictions of NP fate in vivo.However,there is a major lack of biological models that mimic the pathological complexity of target neural sites in vivo,particularly the responses of resident neural immune cells to NPs.Here,we have utilised a previously developed in vitro model of traumatic spinal cord injury (based on 3-D organotypic slice arrays) with dynamic time lapse imaging to reveal in real-time the acute cellular fate of NPs within injury foci.We demonstrate the utility of our model in revealing the well documented phenomenon of avid NP sequestration by the intrinsic immune cells of the CNS (the microglia).Such immune sequestration is a known translational barrier to the use of NP-based therapeutics for neurological injury.Accordingly,we suggest that the utility of our model in mimicking microglial sequestration behaviours offers a valuable investigative tool to evaluate strategies to overcome this cellular response within a simple and biologically relevant experimental system,whilst reducing the use of live animal neurological injury models for such studies.     

中枢神经系统损伤修复生物材料研究进展

中枢神经系统损伤导致神经细胞死亡、组织破坏,造成神经功能永久性缺失,是长期困扰生物医学界的一大难题,目前尚无有效疗法。组织工程技术不仅能通过纳米生物材料为神经细胞和神经纤维生长提供结构支持,还能同时递送各种有利于神经再生修复的活性信号分子,有望在促进中枢神经损伤组织修复的同时,实现神经功能的重建,为中枢神经损伤再生修复带来希望。结合国内外有关中枢神经系统组织工程研究的最新进展,对中枢神经修复生物材料设计的主要策略、以及包括天然生物大分子和合成高分子在内的多种中枢神经修复生物材料的应用进行了详细综述。     

Macroporous hydrogels based on 2-hydroxyethyl methacrylate. Part 6: 3D hydrogels with positive and negative surface charges and polyelectrolyte complexes in spinal cord injury repair

Macroporous hydrogels are artificial biomaterials commonly used in tissue engineering, including central nervous system (CNS) repair. Their physical properties may be modified to improve their adhesion properties and promote tissue regeneration. We implanted four types of hydrogels based on 2-hydroxyethyl methacrylate (HEMA) with different surface charges inside a spinal cord hemisection cavity at the Th8 level in rats. The spinal cords were processed 1 and 6 months after implantation and histologically evaluated. Connective tissue deposition was most abundant in the hydrogels with positively-charged functional groups. Axonal regeneration was promoted in hydrogels carrying charged functional groups; hydrogels with positively charged functional groups showed increased axonal ingrowth into the central parts of the implant. Few astrocytes grew into the hydrogels. Our study shows that HEMA-based hydrogels carrying charged functional groups improve axonal ingrowth inside the implants compared to implants without any charge. Further, positively charged functional groups promote connective tissue infiltration and extended axonal regeneration inside a hydrogel bridge.     

A Robust Intrinsically Green Fluorescent Poly(Amidoamine) Dendrimer for Imaging and Traceable Central Nervous System Delivery in Zebrafish

Intrinsically fluorescent poly(amidoamine) dendrimers (IF‐PAMAM) are an emerging class of versatile nanoplatforms for in vitro tracking and bio‐imaging. However, limited tissue penetration of their fluorescence and interference due to auto‐fluorescence arising from biological tissues limit its application in vivo. Herein, a green IF‐PAMAM (FGP) dendrimer is reported and its biocompatibility, circulation, biodistribution and potential role for traceable central nervous system (CNS)‐targeted delivery in zebrafish is evaluated, exploring various routes of administration. Key features of FGP include visible light excitation (488 nm), high fluorescence signal intensity, superior photostability and low interference from tissue auto‐fluorescence. After intravenous injection, FGP shows excellent imaging and tracking performance in zebrafish. Further conjugating FGP with transferrin (FGP‐Tf) significantly increases its penetration through the blood–brain barrier (BBB) and prolongs its circulation in the blood stream. When administering through local intratissue microinjection, including intracranial and intrathecal injection in zebrafish, both FGP and FGP‐Tf exhibit excellent tissue diffusion and effective cellular uptake in the brain and spinal cord, respectively. This makes FGP/FGP‐Tf attractive for in vivo tracing when transporting to the CNS is desired. The work addresses some of the major shortcomings in IF‐PAMAM and provides a promising application of these probes in the development of drug delivery in the CNS.     

Self-assembling peptide nanofiber hydrogels for central nervous system regeneration

Central nervous system (CNS) presents a complex regeneration problem due to the inability of central neurons to regenerate correct axonal and dendritic connections. However, recent advances in developmental neurobiology, cell signaling, cell--matrix interaction, and biomaterials technologies have forced a reconsideration of CNS regeneration potentials from the viewpoint of tissue engineering and regenerative medicine. The applications of a novel tissue regeneration-inducing biomaterial and stem cells are thought to be critical for the mission. The use of peptide nanofiber hydrogels in cell therapy and tissue engineering offers promising perspectives for CNS regeneration. Self-assembling peptide undergo a rapid transformation from liquid to gel upon addition of counterions or pH adjustment, directly integrating with the host tissue. The peptide nanofiber hydrogels have mechanical properties that closely match the native central nervous extracellular matrix, which could enhance axonal growth. Such materials can provide an optimal three dimensional microenvironment for encapsulated cells. These materials can also be tailored with bioactive motifs to modulate the wound environment and enhance regeneration. This review intends to detail the recent status of self-assembling peptide nanofiber hydrogels for CNS regeneration.     

Systemic Delivery of Monoclonal Antibodies to the Central Nervous System for Brain Tumor Therapy

As an essential component of immunotherapy, monoclonal antibodies (mAbs) have emerged as a class of powerful therapeutics for treatment of a broad range of diseases. For central nervous system (CNS) diseases, however, the efficacy remains limited due to their inability to enter the CNS. A platform technology is reported here that enables effective delivery of mAbs to the CNS for brain tumor therapy. This is achieved by encapsulating the mAbs within nanocapsules that contain choline and acetylcholine analogues; such analogues facilitate the penetration of the nanocapsules through the brain–blood barrier and the delivery of mAbs to tumor sites. This platform technology uncages the therapeutic power of mAbs for various CNS diseases that remain poorly treated.     

The effects of functional magnetic nanotubes with incorporated nerve growth factor in neuronal differentiation of PC12 cells

In this in vitro study the efficiency of magnetic nanotubes to bind with nerve growth factor (NGF) and the ability of NGF-incorporated magnetic nanotubes to release the bound NGF are investigated using rat pheochromocytoma cells (PC12 cells). It is found that functional magnetic nanotubes with NGF incorporation enabled the differentiation of PC12 cells into neurons exhibiting growth cones and neurite outgrowth. Microscope observations show that filopodia extending from neuron growth cones were in close proximity to the NGF-incorporated magnetic nanotubes, at times appearing to extend towards or into them. These results show that magnetic nanotubes can be used as a delivery vehicle for NGF and thus may be exploited in attempts to treat neurodegenerative disorders such as Parkinson's disease with neurotrophins. Further neurite outgrowth can be controlled by manipulating magnetic nanotubes with external magnetic fields, thus helping in directed regeneration.     

Biomaterials for the central nervous system

Biomaterials are widely used to help treat neurological disorders and/or improve functional recovery in the central nervous system (CNS). This article reviews the application of biomaterials in (i) shunting systems for hydrocephalus, (ii) cortical neural prosthetics, (iii) drug delivery in the CNS, (iv) hydrogel scaffolds for CNS repair, and (v) neural stem cell encapsulation for neurotrauma. The biological and material requirements for the biomaterials in these applications are discussed. The difficulties that the biomaterials might face in each application and the possible solutions are also reviewed in this article.     

In vivo imaging and biodistribution of multimodal polymeric nanoparticles delivered to the optic nerve

The use of nanoparticles for targeted delivery of therapeutic agents to sites of injury or disease in the central nervous system (CNS) holds great promise. However, the biodistribution of nanoparticles following in vivo administration is often unknown, and concerns have been raised regarding potential toxicity. Using poly(glycidyl methacrylate) (PGMA) nanoparticles coated with polyethylenimine (PEI) and containing superparamagnetic iron oxide nanoparticles as a magnetic resonance imaging (MRI) contrast agent and rhodamine B as a fluorophore, whole animal MRI and fluorescence analyses are used to demonstrate that these nanoparticles (NP) remain close to the site of injection into a partial injury of the optic nerve, a CNS white matter tract. In addition, some of these NP enter axons and are transported to parent neuronal somata. NP also remain in the eye following intravitreal injection, a non-injury model. Considerable infiltration of activated microglia/macrophages occurs in both models. Using magnetic concentration and fluorescence visualization of tissue homogenates, no dissemination of the NP into peripheral tissues is observed. Histopathological analysis reveals no toxicity in organs other than at the injection sites. Multifunctional nanoparticles may be a useful mechanism to deliver therapeutic agents to the injury site and somata of injured CNS neurons and thus may be of therapeutic value following brain or spinal cord trauma.     

A Bioinspired Platform for Effective Delivery of Protein Therapeutics to the Central Nervous System

Central nervous system (CNS) diseases are the leading cause of morbidity and mortality; their treatment, however, remains constrained by the blood–brain barrier (BBB) that impedes the access of most therapeutics to the brain. A CNS delivery platform for protein therapeutics, which is achieved by encapsulating the proteins within nanocapsules that contain choline and acetylcholine analogues, is reported herein. Mediated by nicotinic acetylcholine receptors and choline transporters, such nanocapsules can effectively penetrate the BBB and deliver the therapeutics to the CNS, as demonstrated in mice and non‐human primates. This universal platform, in general, enables the delivery of any protein therapeutics of interest to the brain, opening a new avenue for the treatment of CNS diseases.     

Application potential of bone marrow mesenchymal stem cell (BMSCs) based tissue-engineering for spinal cord defect repair in rat fetuses with spina bifida aperta

Spina bifida aperta are complex congenital malformations resulting from failure of fusion in the spinal neural tube during embryogenesis. Despite surgical repair of the defect, most patients who survive with spina bifida aperta have a multiple system handicap due to neuron deficiency of the defective spinal cord. Tissue engineering has emerged as a novel treatment for replacement of lost tissue. This study evaluated the prenatal surgical approach of transplanting a chitosan–gelatin scaffold seeded with bone marrow mesenchymal stem cells (BMSCs) in the healing the defective spinal cord of rat fetuses with retinoic acid induced spina bifida aperta. Scaffold characterisation revealed the porous structure, organic and amorphous content. This biomaterial promoted the adhesion, spreading and in vitro viability of the BMSCs. After transplantation of the scaffold combined with BMSCs, the defective region of spinal cord in rat fetuses with spina bifida aperta at E20 decreased obviously under stereomicroscopy, and the skin defect almost closed in many fetuses. The transplanted BMSCs in chitosan–gelatin scaffold survived, grew and expressed markers of neural stem cells and neurons in the defective spinal cord. In addition, the biomaterial presented high biocompatibility and slow biodegradation in vivo. In conclusion, prenatal transplantation of the scaffold combined with BMSCs could treat spinal cord defect in fetuses with spina bifida aperta by the regeneration of neurons and repairmen of defective region.     

Materials considerations of implantable neuroengineering devices for clinical use

Implantable devices for recording and stimulation of the human nervous system offer promise for the treatment of disorders including spinal cord injury, stroke, traumatic brain injury, sensory and motor deficits, chronic pain, epilepsy, Parkinson’s disease and amputation. While advances in neuroengineering devices have been impressive, often the expectations and desires for a chronically implantable device remain unrealized. In the face of engineering approaches which perform well on the bench or in acute implantations is an immune response which is well-tuned to recognize foreign bodies, including the materials chosen for our innovations. Recent years have demonstrated a co-evolution of engineering solutions for neural disorders and knowledge of underlying biological hurdles. This review describes the state-of-the-art for implantable neuroengineering devices used for electrical recording and stimulation, the tissue response to these devices, and emerging technologies and materials to mitigate the tissue response. The test methods for candidate materials and paths to the commercial market are briefly described.     

Tailoring minimalist self-assembling peptides for localized viral vector gene delivery

Viral vector gene delivery is a promising technique for the therapeutic administration of proteins to damaged tissue for the improvement of regeneration outcomes in various disease settings including brain and spinal cord injury, as well as autoimmune diseases. Though promising results have been demonstrated, limitations of viral vectors, including spread of the virus to distant sites, neutralization by the host immune system, and low transduction efficiencies have stimulated the investigation of biomaterials as gene delivery vehicles for improved protein expression at an injury site. Here, we show how Nfluorenylmethyloxycarbonyl (Fmoc) self-assembling peptide (SAP) hydrogels, designed for tissue-specific central nervous system (CNS) applications via incorporation of the laminin peptide sequence isoleucine–lysine–valine–alanine–valine (IKVAV), are effective as biocompatible, localized viral vector gene delivery vehicles in vivo. Through the addition of a C-terminal lysine (K) residue, we show that increased electrostatic interactions, provided by the additional amine side chain, allow effective immobilization of lentiviral vector particles, thereby limiting their activity exclusively to the site of injection and enabling focal gene delivery in vivo in a tissue-specific manner. When the C-terminal lysine was absent, no difference was observed between the number of transfected cells, the volume of tissue transfected, or the transfection efficiency with and without the Fmoc-SAP. Importantly, immobilization of the virus only affected transfection cell number and volume, with no impact observed on transfection efficiency. This hydrogel allows the sustained and targeted delivery of growth factors post injury. We have established Fmoc-SAPs as a versatile platform for enhanced biomaterial design for a range of tissue engineering applications.

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Neuroprotective Effect of Nerve Growth Factor Loaded in Porous Silicon Nanostructures in an Alzheimer's Disease Model and Potential Delivery to the Brain

Nerve growth factor (NGF) plays a vital role in reducing the loss of cholinergic neurons in Alzheimer's disease (AD). However, its delivery to the brain remains a challenge. Herein, NGF is loaded into degradable oxidized porous silicon (PSiO₂) carriers, which are designed to carry and continuously release the protein over a 1 month period. The released NGF exhibits a substantial neuroprotective effect in differentiated rat pheochromocytoma PC12 cells against amyloid‐beta (Aβ)‐induced cytotoxicity, which is associated with Alzheimer's disease. Next, two potential localized administration routes of the porous carriers into murine brain are investigated: implantation of PSiO₂ chips above the dura mater, and biolistic bombardment of PSiO₂ microparticles through an opening in the skull using a pneumatic gene gun. The PSiO₂‐implanted mice are monitored for a period of 8 weeks and no inflammation or adverse effects are observed. Subsequently, a successful biolistic delivery of these highly porous microparticles into a live‐mouse brain is demonstrated for the first time. The bombarded microparticles are observed to penetrate the brain and reach a depth of 150 µm. These results pave the way for using degradable PSiO₂ carriers as potential localized delivery systems for NGF to the brain.     

Overcoming blood–brain barrier transport: Advances in nanoparticle-based drug delivery strategies

The blood–brain barrier (BBB), a unique structure in the central nervous system (CNS), protects the brain from bloodborne pathogens by its excellent barrier properties. Nevertheless, this barrier limits therapeutic efficacy and becomes one of the biggest challenges in new drug development for neurodegenerative disease and brain cancer. Recent breakthroughs in nanotechnology have resulted in various nanoparticles (NPs) as drug carriers to cross the BBB by different methods. This review presents the current understanding of advanced NP-mediated non-invasive drug delivery for the treatment of neurological disorders. Herein, the complex compositions and special characteristics of BBB are elucidated exhaustively. Moreover, versatile drug nanocarriers with their recent applications and their pathways on different drug delivery strategies to overcome the formidable BBB obstacle are briefly discussed. In terms of significance, this paper provides a general understanding of how various properties of nanoparticles aid in drug delivery through BBB and usher the development of novel nanotechnology-based nanomaterials for cerebral disease therapies.     

Nerve growth factor-immobilized electrically conducting fibrous scaffolds for potential use in neural engineering applications

Engineered scaffolds simultaneously exhibiting multiple cues are highly desirable for neural tissue regeneration. To this end, we developed a neural tissue engineering scaffold that displays submicrometer-scale features, electrical conductivity, and neurotrophic activity. Specifically, electrospun poly(lactic acid-co-glycolic acid) (PLGA) nanofibers were layered with a nanometer thick coating of electrically conducting polypyrrole (PPy) presenting carboxylic groups. Then, nerve growth factor (NGF) was chemically immobilized onto the surface of the fibers. These NGF-immobilized PPy-coated PLGA (NGF-PPyPLGA) fibers supported PC12 neurite formation ( 28.0±3.0% of the cells) and neurite outgrowth (14.2 μm median length), which were comparable to that observed with NGF (50 ng/mL) in culture medium ( 29.0±1.3%, 14.4 μm). Electrical stimulation of PC12 cells on NGF-immobilized PPyPLGA fiber scaffolds was found to further improve neurite development and neurite length by 18% and 17%, respectively, compared to unstimulated cells on the NGF-immobilized fibers. Hence, submicrometer-scale fibrous scaffolds that incorporate neurotrophic and electroconducting activities may serve as promising neural tissue engineering scaffolds such as nerve guidance conduits.