Neurorepair and Regeneration of the Brain: A Decade of Bioscaffolds and Engineered Microtissue

Advances in 3D tissue models for neural engineering: self-assembled versus engineered tissue models

Neural tissue engineering has emerged as a promising field that aims to create functional neural tissue for therapeutic applications, drug screening, and disease modelling. It is becoming evident in the literature that this goal requires development of three-dimensional (3D) constructs that can mimic the complex microenvironment of native neural tissue, including its biochemical, mechanical, physical, and electrical properties. These 3D models can be broadly classified as self-assembled models, which include spheroids, organoids, and assembloids, and engineered models, such as those based on decellularized or polymeric scaffolds. Self-assembled models offer advantages such as the ability to recapitulate neural development and disease processes in vitro, and the capacity to study the behaviour and interactions of different cell types in a more realistic environment. However, self-assembled constructs have limitations such as lack of standardised protocols, inability to control the cellular microenvironment, difficulty in controlling structural characteristics, reproducibility, scalability, and lengthy developmental timeframes. Integrating biomimetic materials and advanced manufacturing approaches to present cells with relevant biochemical, mechanical, physical, and electrical cues in a controlled tissue architecture requires alternate engineering approaches. Engineered scaffolds, and specifically 3D hydrogel-based constructs, have desirable properties, lower cost, higher reproducibility, long-term stability, and they can be rapidly tailored to mimic the native microenvironment and structure. This review explores 3D models in neural tissue engineering, with a particular focus on analysing the benefits and limitations of self-assembled organoids compared with hydrogel-based engineered 3D models. Moreover, this paper will focus on hydrogel based engineered models and probe their biomaterial components, tuneable properties, and fabrication techniques that allow them to mimic native neural tissue structures and environment. Finally, the current challenges and future research prospects of 3D neural models for both self-assembled and engineered models in neural tissue engineering will be discussed.

Cell competition promotes metastatic intestinal cancer through a multistage process

Cell competition plays an instrumental role in quality control during tissue development and homeostasis. Nevertheless, cancer cells can exploit this process for their own proliferative advantage. In our study, we generated mixed murine organoids and microtissues to explore the impact of cell competition on liver metastasis. Unlike competition at the primary site, the initial effect on liver progenitor cells does not involve the induction of apoptosis. Instead, metastatic competition manifests as a multistage process. Initially, liver progenitors undergo compaction, which is followed by cell-cycle arrest, ultimately forcing differentiation. Subsequently, the newly differentiated liver cells exhibit reduced cellular fitness, rendering them more susceptible to outcompetition by intestinal cancer cells. Notably, cancer cells leverage different interactions with different epithelial populations in the liver, using them as scaffolds to facilitate their growth. Consequently, tissue-specific mechanisms of cell competition are fundamental in driving metastatic intestinal cancer.

Cell adhesion molecule immobilized gold surfaces for enhanced neuron-electrode interfaces

To provide a long-term solution for increasing the biocompatibility of neuroprosthetics, approaches to reduce the side effects of invasive neuro-implantable devices are still in need of improvement. Physical, chemical, and bioactive design aspects of the biomaterials are proven to be important for providing proper cell-to-cell, cell-to-material interactions. Particularly, modification of implant surfaces with bioactive cues, especially cell adhesion molecules (CAMs) that capitalize on native neural adhesion mechanisms, are promising candidates in favor of providing efficient interfaces. Within this concept, this study utilized specific CAMs, namely N-Cadherin (Neural cadherin, N-Cad) and neural cell adhesion molecule (NCAM), to enhance neuron-electrode contact by mimicking the cell-to-ECM interactions for improving the survival of cells and promoting neurite outgrowth. For this purpose, representative gold electrode surfaces were modified with N-Cadherin, NCAM, and the mixture (1:1) of these molecules. Modifications were characterized, and the effect of surface modification on both differentiated and undifferentiated neuroblastoma SH-SY5Y cell lines were compared. The findings demonstrated the successful modification of these molecules which subsequently exhibited biocompatible properties as evidenced by the cell viability results. In cell culture experiments, the CAMs displayed promising results in promoting neurite outgrowth compared to conventional poly-l-lysine coated surfaces, especially NCAM and N-Cad/NCAM modified surfaces clearly showed significant improvement. Overall, this optimized approach is expected to provide an insight into the action mechanisms of cells against the local environment and advance processes for the fabrication of alternative neural interfaces.

Biomaterials in Traumatic Brain Injury: Perspectives and Challenges

Traumatic brain injury (TBI) is a leading cause of mortality and long-term impairment globally. TBI has a dynamic pathology, encompassing a variety of metabolic and molecular events that occur in two phases: primary and secondary. A forceful external blow to the brain initiates the primary phase, followed by a secondary phase that involves the release of calcium ions (Ca2+) and the initiation of a cascade of inflammatory processes, including mitochondrial dysfunction, a rise in oxidative stress, activation of glial cells, and damage to the blood-brain barrier (BBB), resulting in paracellular leakage. Currently, there are no FDA-approved drugs for TBI, but existing approaches rely on delivering micro- and macromolecular treatments, which are constrained by the BBB, poor retention, off-target toxicity, and the complex pathology of TBI. Therefore, there is a demand for innovative and alternative therapeutics with effective delivery tactics for the diagnosis and treatment of TBI. Tissue engineering, which includes the use of biomaterials, is one such alternative approach. Biomaterials, such as hydrogels, including self-assembling peptides and electrospun nanofibers, can be used alone or in combination with neuronal stem cells to induce neurite outgrowth, the differentiation of human neural stem cells, and nerve gap bridging in TBI. This review examines the inclusion of biomaterials as potential treatments for TBI, including their types, synthesis, and mechanisms of action. This review also discusses the challenges faced by the use of biomaterials in TBI, including the development of biodegradable, biocompatible, and mechanically flexible biomaterials and, if combined with stem cells, the survival rate of the transplanted stem cells. A better understanding of the mechanisms and drawbacks of these novel therapeutic approaches will help to guide the design of future TBI therapies.

Advances in electrode interface materials and modification technologies for brain-computer interfaces

Recent advances in neuroelectrode interface materials and modification technologies are reviewed. Brain-computer interface is the new method of human-computer interaction, which not only can realise the exchange of information between the human brain and external devices, but also provides a brand-new means for the diagnosis and treatment of brain-related diseases. The neural electrode interface part of brain-computer interface is an important area for electrical, optical and chemical signal transmission between brain tissue system and external electronic devices, which determines the performance of brain-computer interface. In order to solve the problems of insufficient flexibility, insufficient signal recognition ability and insufficient biocompatibility of traditional rigid electrodes, researchers have carried out extensive studies on the neuroelectrode interface in terms of materials and modification techniques. This paper introduces the biological reactions that occur in neuroelectrodes after implantation into brain tissue and the decisive role of the electrode interface for electrode function. Following this, the latest research progress on neuroelectrode materials and interface materials is reviewed from the aspects of neuroelectrode materials and modification technologies, firstly taking materials as a clue, and then focusing on the preparation process of neuroelectrode coatings and the design scheme of functionalised structures.

Poly(N-acryloylglycine-acrylamide) Hydrogel Mimics the Cellular Microenvironment and Promotes Neurite Growth with Protection from Oxidative Stress

In this work, the glycine-based acryloyl monomer is polymerized to obtain a neurogenic polymeric hydrogel for regenerative applications. The synthesized poly(N-acryloylglycine-acrylamide) [poly(NAG-b-A)] nanohydrogel exhibits high swelling (∼1500%) and is mechanically very stable, biocompatible, and proliferative in nature. The poly(NAG-b-A) nanohydrogel provides a stable 3D extracellular mimetic environment and promotes healthy neurite growth for primary cortical neurons by facilitating cellular adhesion, proliferation, actin filament stabilization, and neuronal differentiation. Furthermore, the protective role of the poly(NAG-b-A) hydrogel for the neurons in oxidative stress conditions is revealed and it is found that it is a clinically relevant material for neuronal regenerative applications, such as for promoting nerve regeneration via GSK3β inhibition. This hydrogel additionally plays an important role in modulating the biological microenvironment, either as an agonist and antagonist or as an antioxidant. Furthermore, it favors the physiological responses and eases the neurite growth efficiency. Additionally, we found out that the conversion of glycine-based acryloyl monomers into their corresponding polymer modulates the mechanical performance, mimics the cellular microenvironment, and accelerates the self-healing capability due to the responsive behavior towards reactive oxygen species (ROS). Thus, the p(NAG-b-A) hydrogel could be a potential candidate to induce neuronal regeneration since it provides a physical cue and significantly boosts neurite outgrowth and also maintains the microtubule integrity in neuronal cells.

Protocol for designing and bioprinting multi-layered constructs to reconstruct an endothelial-epithelial 3D model

3D bioprinting has opened new possibilities and elevated tissue engineering complexity. Here, we present a protocol to design a 3D model with two cell lineage layers (A549 and HUVEC) to recreate multi-cell constructs. We describe the steps for slicing the constructs, handling hydrogels, and detailing the bioprinting setup. These 3D-bioprinted constructs can be adapted to various cell models-from primary cell cultures to commercial cell lines and induced pluripotent stem cells (IPCs)-and applications, including drug screening and disease modeling. For complete details on the use and execution of this protocol, please refer to Cruz et al.1.

Skin wound healing as a mirror to cardiac wound healing

NEW FINDINGS

What is the topic of this review? Wound healing is a general response of the body to injury and can be divided into three phases: inflammation, inflammation resolution and repair. In this review, we compare the wound-healing response of the skin after an injury and the wound-healing response of the heart after a myocardial infarction. What advances does it highlight? We highlight differences and similarities between skin and cardiac wound healing and summarize how skin can be used to provide information about the heart.

ABSTRACT

Wound healing is a general response of the body to injury. All organs share in common three response elements to wound healing: inflammation to prevent infection and stimulate the removal of dead cells, active anti-inflammatory signalling to turn off the inflammatory response, and a repair phase characterized by extracellular matrix scar formation. The extent of scar formed depends on the ability of endogenous cells that populate each organ to regenerate. The skin has keratinocytes that have regenerative capacity, and in general, wounds are fully re-epithelialized. Heart, in contrast, has cardiac myocytes that have little to no regenerative capacity, and necrotic myocytes are entirely replaced by scars. Despite differences in tissue regeneration, the skin and heart share many wound-healing properties that can be exploited to predict the cardiac response to pathology. We summarize in this review article our current understanding of how the response of the skin to a wounding event can inform us about the ability of the myocardium to respond to a myocardial infarction.

Toward a New Generation of Bio-Scaffolds for Neural Tissue Engineering: Challenges and Perspectives

Neural tissue engineering presents a compelling technological breakthrough in restoring brain function, holding immense promise. However, the quest to develop implantable scaffolds for neural culture that fulfill all necessary criteria poses a remarkable challenge for material science. These materials must possess a host of desirable characteristics, including support for cellular survival, proliferation, and neuronal migration and the minimization of inflammatory responses. Moreover, they should facilitate electrochemical cell communication, display mechanical properties akin to the brain, emulate the intricate architecture of the extracellular matrix, and ideally allow the controlled release of substances. This comprehensive review delves into the primary requisites, limitations, and prospective avenues for scaffold design in brain tissue engineering. By offering a panoramic overview, our work aims to serve as an essential resource, guiding the creation of materials endowed with bio-mimetic properties, ultimately revolutionizing the treatment of neurological disorders by developing brain-implantable scaffolds.

Dental Pulp Stem Cells for Salivary Gland Regeneration-Where Are We Today?

Xerostomia is the phenomenon of dry mouth and is mostly caused by hypofunction of the salivary glands. This hypofunction can be caused by tumors, head and neck irradiation, hormonal changes, inflammation or autoimmune disease such as Sjögren's syndrome. It is associated with a tremendous decrease in health-related quality of life due to impairment of articulation, ingestion and oral immune defenses. Current treatment concepts mainly consist of saliva substitutes and parasympathomimetic drugs, but the outcome of these therapies is deficient. Regenerative medicine is a promising approach for the treatment of compromised tissue. For this purpose, stem cells can be utilized due to their ability to differentiate into various cell types. Dental pulp stem cells are adult stem cells that can be easily harvested from extracted teeth. They can form tissues of all three germ layers and are therefore becoming more and more popular for tissue engineering. Another potential benefit of these cells is their immunomodulatory effect. They suppress proinflammatory pathways of lymphocytes and could therefore probably be used for the treatment of chronic inflammation and autoimmune disease. These attributes make dental pulp stem cells an interesting tool for the regeneration of salivary glands and the treatment of xerostomia. Nevertheless, clinical studies are still missing. This review will highlight the current strategies for using dental pulp stem cells in the regeneration of salivary gland tissue.

Stem Cell Therapies for Restorative Treatments of Central Nervous System Ischemia-Reperfusion Injury

Ischemic damage to the central nervous system (CNS) is a catastrophic postoperative complication of aortic occlusion subsequent to cardiovascular surgery that can cause brain impairment and sometimes even paraplegia. Over recent years, numerous studies have investigated techniques for protecting and revascularizing the nervous system during intraoperative ischemia; however, owing to a lack of knowledge of the physiological distinctions between the brain and spinal cord, as well as the limited availability of testing techniques and treatments for ischemia-reperfusion injury, the cause of brain and spinal cord ischemia-reperfusion injury remains poorly understood, and no adequate response steps are currently available in the clinic. Given the limited ability of the CNS to repair itself, it is of great clinical value to make full use of the proliferative and differentiation potential of stem cells to repair nerves in degenerated and necrotic regions by stem cell transplantation or mobilization, thereby introducing a novel concept for the treatment of severe CNS ischemia-reperfusion injury. This review summarizes the most recent advances in stem cell therapy for ischemia-reperfusion injury in the brain and spinal cord, aiming to advance basic research and the clinical use of stem cell therapy as a promising treatment for this condition.

A Gelatin Methacrylate-Based Hydrogel as a Potential Bioink for 3D Bioprinting and Neuronal Differentiation

Neuronal loss is the ultimate pathophysiologic event in central nervous system (CNS) diseases and replacing these neurons is one of the most significant challenges in regenerative medicine. Providing a suitable microenvironment for new neuron engraftment, proliferation, and synapse formation is a primary goal for 3D bioprinting. Among the various biomaterials, gelatin methacrylate (GelMA) stands out due to its Arg-Gly-Asp (RGD) domains, which assure its biocompatibility and degradation under physiological conditions. This work aimed to produce different GelMA-based bioink compositions, verify their mechanical and biological properties, and evaluate their ability to support neurogenesis. We evaluated four different GelMA-based bioink compositions; however, when it came to their biological properties, incorporating extracellular matrix components, such as Geltrex^TM, was essential to ensure human neuroprogenitor cell viability. Finally, Geltrex^TM: 8% GelMA (1:1) bioink efficiently maintained human neuroprogenitor cell stemness and supported neuronal differentiation. Interestingly, this bioink composition provides a suitable environment for murine astrocytes to de-differentiate into neural stem cells and give rise to MAP2-positive cells.

Carboxymethyl Chitosan and Gelatin Hydrogel Scaffolds Incorporated with Conductive PEDOT Nanoparticles for Improved Neural Stem Cell Proliferation and Neuronal Differentiation

Tissue engineering scaffolds provide biological and physiochemical cures to guide tissue recovery, and electrical signals through the electroactive materials possess tremendous potential to modulate the cell fate. In this study, a novel electroactive hydrogel scaffold was fabricated by assembling poly(3,4-ethylenedioxythiophene) (PEDOT) nanoparticles on a carboxymethyl chitosan/gelatin (CMCS/Gel) composite hydrogel surface via in situ chemical polymerization. The chemical structure, morphology, conductivity, porosity, swelling rate, in vitro biodegradation, and mechanical properties of the prepared hydrogel samples were characterized. The adhesion, proliferation, and differentiation of neural stem cells (NSCs) on conductive hydrogels were investigated. The CMCS/Gel-PEDOT hydrogels exhibited high porosity, excellent water absorption, improved thermal stability, and adequate biodegradability. Importantly, the mechanical properties of the prepared hydrogels were similar to those of brain tissue, with electrical conductivity up to (1.52 ± 0.15) × 10^-3 S/cm. Compared to the CMCS/Gel hydrogel, the incorporation of PEDOT nanoparticles significantly improved the adhesion of NSCs, and supported long-term cell growth and proliferation in a three-dimensional (3D) microenvironment. In addition, under the differentiation condition, the conductive hydrogel also significantly enhanced neuronal differentiation with the up-regulation of β-tubulin III expression. These results suggest that CMCS/Gel-PEDOT hydrogels may be an attractive conductive substrate for further studies on neural tissue repair and regeneration.

Biomaterial and tissue-engineering strategies for the treatment of brain neurodegeneration

The incidence of neurodegenerative diseases is increasing due to changing age demographics and the incidence of sports-related traumatic brain injury is tending to increase over time. Currently approved medicines for neurodegenerative diseases only temporarily reduce the symptoms but cannot cure or delay disease progression. Cell transplantation strategies offer an alternative approach to facilitating central nervous system repair, but efficacy is limited by low in vivo survival rates of cells that are injected in suspension. Transplanting cells that are attached to or encapsulated within a suitable biomaterial construct has the advantage of enhancing cell survival in vivo. A variety of biomaterials have been used to make constructs in different types that included nanoparticles, nanotubes, microspheres, microscale fibrous scaffolds, as well as scaffolds made of gels and in the form of micro-columns. Among these, Tween 80-methoxy poly(ethylene glycol)-poly(lactic-co-glycolic acid) nanoparticles loaded with rhynchophylline had higher transport across a blood-brain barrier model and decreased cell death in an in vitro model of Alzheimer’s disease than rhynchophylline or untreated nanoparticles with rhynchophylline. In an in vitro model of Parkinson’s disease, trans-activating transcriptor bioconjugated with zwitterionic polymer poly(2-methacryoyloxyethyl phosphorylcholine) and protein-based nanoparticles loaded with non-Fe hemin had a similar protective ability as free non-Fe hemin. A positive effect on neuron survival in several in vivo models of Parkinson’s disease was associated with the use of biomaterial constructs such as trans-activating transcriptor bioconjugated with zwitterionic polymer poly(2-methacryoyloxyethyl phosphorylcholine) and protein-based nanoparticles loaded with non-Fe hemin, carbon nanotubes with olfactory bulb stem cells, poly(lactic-co-glycolic acid) microspheres with attached DI-MIAMI cells, ventral midbrain neurons mixed with short fibers of poly-(L-lactic acid) scaffolds and reacted with xyloglucan with/without glial-derived neurotrophic factor, ventral midbrain neurons mixed with Fmoc-DIKVAV hydrogel with/without glial-derived neurotrophic factor. Further studies with in vivo models of Alzheimer’s disease and Parkinson’s disease are warranted especially using transplantation of cells in agarose micro-columns with an inner lumen filled with an appropriate extracellular matrix material.

Nano-Neurogenesis for CNS Diseases and Disorders

Neurogenesis encompasses the formation and development of neurons in the mammalian brain, mainly occurring in hippocampus and the olfactory system. This process is rapid, accurate, and very sensitive to the external stressors including environment, diet, age, anxiety, stress, depression, diet, and hormones. The range of stressors is big and directly impacts the generation, maturation and migration, efficacy, and myelination of the neuronal cells. The field of regenerative medicine focuses on combating the direct or indirect effects of these stressors on the process of neurogenesis, and ensures increased general and neuronal communications and functioning. Understanding the deep secrets of brain signaling and devising ways to increase drug availability is tough, considering the complexity and intricate details of the neuronal networks and signaling in the CNS. It is imperative to understand this complexity and introduce potent and efficacious ways to combat diseases. This perspective offers an insight into how neurogenesis could be aided by nanotechnology and what plausible nanomaterials are available to culminate neurogenesis-related neurological disorders. The nanomaterials are promising as they are minute, robust, and effective and help in diagnostics and therapeutics such as drug delivery, maturation and neuroprotection, neurogenesis, imaging, and neurosurgery.

Genipin-Crosslinked, Proteosaccharide Scaffolds for Potential Neural Tissue Engineering Applications

Traumatic brain injuries (TBIs) are still a challenge for the field of modern medicine. Many treatment options such as autologous grafts and stem cells show limited promise for the treatment and the reversibility of damage caused by TBIs. Injury beyond the critical size necessitates the implementation of scaffolds that function as surrogate extracellular matrices. Two scaffolds were synthesised utilising polysaccharides, chitosan and hyaluronic acid in conjunction with gelatin. Both scaffolds were chemically crosslinked using a naturally derived crosslinker, Genipin. The polysaccharides increased the mechanical strength of each scaffold, while gelatin provided the bioactive sequence, which promoted cellular interactions. The effect of crosslinking was investigated, and the crosslinked hydrogels showed higher thermal decomposition temperatures, increased resistance to degradation, and pore sizes ranging from 72.789 ± 16.85 µm for the full interpenetrating polymer networks (IPNs) and 84.289 ± 7.658 μm for the semi-IPN. The scaffolds were loaded with Dexamethasone-21-phosphate to investigate their efficacy as a drug delivery vehicle, and the full IPN showed a 100% release in 10 days, while the semi-IPN showed a burst release in 6 h. Both scaffolds stimulated the proliferation of rat pheochromocytoma (PC12) and human glioblastoma multiforme (A172) cell cultures and also provided signals for A172 cell migration. Both scaffolds can be used as potential drug delivery vehicles and as artificial extracellular matrices for potential neural regeneration.

Newly Generated 3D Schwann-Like Cell Spheroids From Human Adipose-Derived Stem Cells Using a Modified Protocol

Peripheral nerve injury (PNI) is a relatively frequent type of trauma that results in the suffering of many patients worldwide every year. Schwann cells (SCs) are expected to be applied in cell therapy because of their ability to promote peripheral nerve regeneration. However, the lack of clinically renewable sources of SCs hinders the application of SC-based therapies. Adipose-derived stem cells (ADSCs) have generated great interest in recent years because of their multipotency and ease of harvest, and they have already been verified to differentiate into Schwann-like cells (SLCs) in vitro. However, the efficiency of differentiation and the functions of SLCs remain unsatisfactory. We newly generated three-dimensional (3D) SLC spheroids from ADSCs using a modified protocol with human recombinant peptide (RCP) petaloid μ-piece. Morphological analysis, gene expression analysis by qRT-PCR, ELISA measurement of the secretion capabilities of neurotrophic factors, and neurite formation assay were performed to evaluate the functions of these 3D SLCs in vitro. Motor function recovery was measured in a sciatic nerve injury mouse model to analyze the nerve regeneration-promoting effect of 3D SLCs in vivo. The differentiation efficiency and the secretion of neurotrophic factors were enhanced in 3D SLCs compared with conventional SLCs. 3D SLCs could more effectively promote neurite growth and longer neurite extension in a neuron-like SH-SY5Y model. Additionally, 3D SLCs had a better therapeutic effect on nerve regeneration after transplantation into the sciatic nerve injury mouse model. These findings demonstrated that the potential of ADSC-derived SLCs to promote nerve regeneration could be significantly increased using our modified differentiation protocol and by assembling cells into a 3D sphere conformation. Therefore, these cells have great potential and can be used in the clinical treatment of PNI.