Self-Assembling Peptide Hydrogels as Functional Tools to Tackle Intervertebral Disc Degeneration

Using Chemistry To Recreate the Complexity of the Extracellular Matrix: Guidelines for Supramolecular Hydrogel-Cell Interactions

Hydrogels have emerged as a promising class of extracellular matrix (ECM)-mimicking materials in regenerative medicine. Here, we briefly describe current state-of-the-art of ECM-mimicking hydrogels, ranging from natural to hybrid to completely synthetic versions, giving the prelude to the importance of supramolecular interactions to make true ECM mimics. The potential of supramolecular interactions to create ECM mimics for cell culture is illustrated through a focus on two different supramolecular hydrogel systems, both developed in our laboratories. We use some recent, significant findings to present important design principles underlying the cell-material interaction. To achieve cell spreading, we propose that slow molecular dynamics (monomer exchange within fibers) is crucial to ensure the robust incorporation of cell adhesion ligands within supramolecular fibers. Slow bulk dynamics (stress-relaxation─fiber rearrangements, τ1/2 ≈ 1000 s) is required to achieve cell spreading in soft gels (<1 kPa), while gel stiffness overrules dynamics in stiffer gels. Importantly, this resonates with the findings of others which specialize in different material types: cell spreading is impaired in case substrate relaxation occurs faster than clutch binding and focal adhesion lifetime. We conclude with discussing considerations and limitations of the supramolecular approach as well as provide a forward thinking perspective to further understand supramolecular hydrogel-cell interactions. Future work may utilize the presented guidelines underlying cell-material interactions to not only arrive at the next generation of ECM-mimicking hydrogels but also advance other fields, such as bioelectronics, opening up new opportunities for innovative applications.

Stimuli-Responsive Delivery Systems for Intervertebral Disc Degeneration

As a major cause of low back pain, intervertebral disc degeneration is an increasingly prevalent chronic disease worldwide that leads to huge annual financial losses. The intervertebral disc consists of the inner nucleus pulposus, outer annulus fibrosus, and sandwiched cartilage endplates. All these factors collectively participate in maintaining the structure and physiological functions of the disc. During the unavoidable degeneration stage, the degenerated discs are surrounded by a harsh microenvironment characterized by acidic, oxidative, inflammatory, and chaotic cytokine expression. Loss of stem cell markers, imbalance of the extracellular matrix, increase in inflammation, sensory hyperinnervation, and vascularization have been considered as the reasons for the progression of intervertebral disc degeneration. The current treatment approaches include conservative therapy and surgery, both of which have drawbacks. Novel stimuli-responsive delivery systems are more promising future therapeutic options than traditional treatments. By combining bioactive agents with specially designed hydrogels, scaffolds, microspheres, and nanoparticles, novel stimuli-responsive delivery systems can realize the targeted and sustained release of drugs, which can both reduce systematic adverse effects and maximize therapeutic efficacy. Trigger factors are categorized into internal (pH, reactive oxygen species, enzymes, etc.) and external stimuli (photo, ultrasound, magnetic, etc.) based on their intrinsic properties. This review systematically summarizes novel stimuli-responsive delivery systems for intervertebral disc degeneration, shedding new light on intervertebral disc therapy.

Chemical Modifications in Hyaluronic Acid-Based Electrospun Scaffolds

Hyaluronic acid (HA) is a natural, non-sulfated glycosaminoglycan (GAG) present in ECM. It is involved in different biological functions with appealing properties in cosmetics and pharmaceutical preparations as well as in tissue engineering. Generally, HA has been electrospun in blends with natural or synthetic polymers to produce fibers having diameters in the order of nano and micro-scale whose pores can host cells able to regenerate damaged tissues. In the last decade, a rich literature on electrospun HA-based materials arose. Chemical modifications were generally introduced in HA scaffolds to favour crosslinking or conjugation with bioactive molecules. Considering the high solubility of HA in water, HA-based electrospun scaffolds are cross-linked to increase the stability in biological fluids. Crosslinking is necessary also to avoid the release of HA from the hybrid scaffold when implanted in-vivo. Furthermore, to endow the HA based scaffolds with new chemical or biological properties, conjugation of bioactive molecules to HA was widely reported. Herein, we review the existing research classifying chemical modifications on HA and HA-based electrospun fibers into three categories: i) in-situ crosslinking of electrospun HA-based scaffolds ii) off-site crosslinking of electrospun HA-based scaffolds; iii) conjugation of biofunctional molecules to HA with focus on peptides.

Stem cell therapy for degenerative disc disease: Bridging the gap between preclinical promise and clinical potential

Stem cell therapy has gained attention in the field of regenerative medicine due to its potential to restore damaged tissue. This article focuses on the application of stem cell therapy for treating spinal pathologies, particularly intervertebral disc degeneration. Disc degeneration is a major cause of low back pain and is characterized by changes in the matrix and inflammation. Animal studies have demonstrated that the implantation of mesenchymal stem cells (MSCs) yields promising results, including increased disc height, improved hydration, and reduced inflammation. However, the number of clinical trials remains limited, necessitating further research to optimize MSCs therapy. Although preclinical studies offer valuable insights, caution is needed when extrapolating these findings to clinical practice. Stem cell therapy still faces multiple challenges, such as the durability and survival of MSCs upon implantation, uncertain pathways to discogenic differentiation, and the adverse impact of a harsh microenvironment on cell survival. The avascular nature of the intervertebral disc and dynamic loading conditions also affect the adaptation of transplanted cells. Despite these obstacles, stem cell therapy holds promise as a potential treatment for disc degeneration, and ongoing research aims to fill the current gap in conclusive data.

Hydrogel-Based Strategies for Intervertebral Disc Regeneration: Advances, Challenges and Clinical Prospects

Millions of people worldwide suffer from low back pain and disability associated with intervertebral disc (IVD) degeneration. IVD degeneration is highly correlated with aging, as the nucleus pulposus (NP) dehydrates and the annulus fibrosus (AF) fissures form, which often results in intervertebral disc herniation or disc space collapse and related clinical symptoms. Currently available options for treating intervertebral disc degeneration are symptoms control with therapy modalities, and/or medication, and/or surgical resection of the IVD with or without spinal fusion. As such, there is an urgent clinical demand for more effective disease-modifying treatments for this ubiquitous disorder, rather than the current paradigms focused only on symptom control. Hydrogels are unique biomaterials that have a variety of distinctive qualities, including (but not limited to) biocompatibility, highly adjustable mechanical characteristics, and most importantly, the capacity to absorb and retain water in a manner like that of native human nucleus pulposus tissue. In recent years, various hydrogels have been investigated in vitro and in vivo for the repair of intervertebral discs, some of which are ready for clinical testing. In this review, we summarize the latest findings and developments in the application of hydrogel technology for the repair and regeneration of intervertebral discs.

Injectable smart stimuli-responsive hydrogels: pioneering advancements in biomedical applications

Hydrogels have established their significance as prominent biomaterials within the realm of biomedical research. However, injectable hydrogels have garnered greater attention compared with their conventional counterparts due to their excellent minimally invasive nature and adaptive behavior post-injection. With the rapid advancement of emerging chemistry and deepened understanding of biological processes, contemporary injectable hydrogels have been endowed with an "intelligent" capacity to respond to various endogenous/exogenous stimuli (such as temperature, pH, light and magnetic field). This innovation has spearheaded revolutionary transformations across fields such as tissue engineering repair, controlled drug delivery, disease-responsive therapies, and beyond. In this review, we comprehensively expound upon the raw materials (including natural and synthetic materials) and injectable principles of these advanced hydrogels, concurrently providing a detailed discussion of the prevalent strategies for conferring stimulus responsiveness. Finally, we elucidate the latest applications of these injectable "smart" stimuli-responsive hydrogels in the biomedical domain, offering insights into their prospects.

4D Printing Shape-Morphing Hybrid Biomaterials for Advanced Bioengineering Applications

Four-dimensional (4D) printing is an innovative additive manufacturing technology used to fabricate structures that can evolve over time when exposed to a predefined environmental stimulus. 4D printed objects are no longer static objects but programmable active structures that accomplish their functions thanks to a change over time in their physical/chemical properties that usually displays macroscopically as a shapeshifting in response to an external stimulus. 4D printing is characterized by several entangled features (e.g., involved material(s), structure geometry, and applied stimulus entities) that need to be carefully coupled to obtain a favorable fabrication and a functioning structure. Overall, the integration of micro-/nanofabrication methods of biomaterials with nanomaterials represents a promising approach for the development of advanced materials. The ability to construct complex and multifunctional triggerable structures capable of being activated allows for the control of biomedical device activity, reducing the need for invasive interventions. Such advancements provide new tools to biomedical engineers and clinicians to design dynamically actuated implantable devices. In this context, the aim of this review is to demonstrate the potential of 4D printing as an enabling manufacturing technology to code the environmentally triggered physical evolution of structures and devices of biomedical interest.

Short Peptide Nanofiber Biomaterials Ameliorate Local Hemostatic Capacity of Surgical Materials and Intraoperative Hemostatic Applications in Clinics

Short designer self-assembling peptide (dSAP) biomaterials are a new addition to the hemostat group. It may provide a diverse and robust toolbox for surgeons to integrate wound microenvironment with much safer and stronger hemostatic capacity than conventional materials and hemostatic agents. Especially in noncompressible torso hemorrhage (NCTH), diffuse mucosal surface bleeding, and internal medical bleeding (IMB), with respect to the optimal hemostatic formulation, dSAP biomaterials are the ingenious nanofiber alternatives to make bioactive neural scaffold, nasal packing, large mucosal surface coverage in gastrointestinal surgery (esophagus, gastric lesion, duodenum, and lower digestive tract), epicardiac cell-delivery carrier, transparent matrix barrier, and so on. Herein, in multiple surgical specialties, dSAP-biomaterial-based nano-hemostats achieve safe, effective, and immediate hemostasis, facile wound healing, and potentially reduce the risks in delayed bleeding, rebleeding, post-operative bleeding, or related complications. The biosafety in vivo, bleeding indications, tissue-sealing quality, surgical feasibility, and local usability are addressed comprehensively and sequentially and pursued to develop useful surgical techniques with better hemostatic performance. Here, the state of the art and all-round advancements of nano-hemostatic approaches in surgery are provided. Relevant critical insights will inspire exciting investigations on peptide nanotechnology, next-generation biomaterials, and better promising prospects in clinics.

The Role of Extracellular Matrix (ECM) Adhesion Motifs in Functionalised Hydrogels

To create functional tissue engineering scaffolds, biomaterials should mimic the native extracellular matrix of the tissue to be regenerated. Simultaneously, the survival and functionality of stem cells should also be enhanced to promote tissue organisation and repair. Hydrogels, but in particular, peptide hydrogels, are an emerging class of biocompatible scaffolds which act as promising self-assembling biomaterials for tissue engineering and regenerative therapies, ranging from articular cartilage regeneration at joint defects, to regenerative spinal cord injury following trauma. To enhance hydrogel biocompatibility, it has become imperative to consider the native microenvironment of the site for regeneration, where the use of functionalised hydrogels with extracellular matrix adhesion motifs has become a novel, emerging theme. In this review, we will introduce hydrogels in the context of tissue engineering, provide insight into the complexity of the extracellular matrix, investigate specific adhesion motifs that have been used to generate functionalised hydrogels and outline their potential applications in a regenerative medicine setting. It is anticipated that by conducting this review, we will provide greater insight into functionalised hydrogels, which may help translate their use towards therapeutic roles.

Controlling the Self-Assembly and Material Properties of β-Sheet Peptide Hydrogels by Modulating Intermolecular Interactions

Self-assembling peptides are a promising biomaterial with potential applications in medical devices and drug delivery. In the right combination of conditions, self-assembling peptides can form self-supporting hydrogels. Here, we describe how balancing attractive and repulsive intermolecular forces is critical for successful hydrogel formation. Electrostatic repulsion is tuned by altering the peptide's net charge, and intermolecular attractions are controlled through the degree of hydrogen bonding between specific amino acid residues. We find that an overall net peptide charge of +/-2 is optimal to facilitate the assembly of self-supporting hydrogels. If the net peptide charge is too low then dense aggregates form, while a high molecular charge inhibits the formation of larger structures. At a constant charge, altering the terminal amino acids from glutamine to serine decreases the degree of hydrogen bonding within the assembling network. This tunes the viscoelastic properties of the gel, reducing the elastic modulus by two to three orders of magnitude. Finally, hydrogels could be formed from glutamine-rich, highly charged peptides by mixing the peptides in combinations with a resultant net charge of +/-2. These results illustrate how understanding and controlling self-assembly mechanisms through modulating intermolecular interactions can be exploited to derive a range of structures with tuneable properties.

Synthetic extracellular matrices with function-encoding peptides

The communication of cells with their surroundings is mostly encoded in the epitopes of structural and signalling proteins present in the extracellular matrix (ECM). These peptide epitopes can be incorporated in biomaterials to serve as function-encoding molecules to modulate cell-cell and cell-ECM interactions. In this Review, we discuss natural and synthetic peptide epitopes as molecular tools to bioengineer bioactive hydrogel materials. We present a library of functional peptide sequences that selectively communicate with cells and the ECM to coordinate biological processes, including epitopes that directly signal to cells, that bind ECM components that subsequently signal to cells, and that regulate ECM turnover. We highlight how these epitopes can be incorporated in different biomaterials as individual or multiple signals, working synergistically or additively. This molecular toolbox can be applied in the design of biomaterials aimed at regulating or controlling cellular and tissue function, repair and regeneration.

Co-regulation of Sox9 and TGFβ1 transcription factors in mesenchymal stem cells regenerated the intervertebral disc degeneration

Background

Intervertebral disc (IVD) shows aging and degenerative changes earlier than any other body connective tissue. Its repair and regeneration provide a considerable challenge in regenerative medicine due to its high degree of infrastructure and mechanical complexity. Mesenchymal stem cells, due to their tissue resurfacing potential, represent many explanatory pathways to regenerate a tissue breakdown.

Methods

This study was undertaken to evaluate the co-regulation of Sox9 and TGFβ1 in differentiating human umbilical cord mesenchymal stem cells (hUC-MSC) into chondrocytes. The combinatorial impact of Sox9 and TGFβ1 on hUC-MSCs was examined in vitro by gene expression and immunocytochemical staining. In in vivo, an animal model of IVD degeneration was established under a fluoroscopic guided system through needle puncture of the caudal disc. Normal and transfected MSCs were transplanted. Oxidative stress, pain, and inflammatory markers were evaluated by qPCR. Disc height index (DHI), water content, and gag content were analyzed. Histological examinations were performed to evaluate the degree of regeneration.

Results

hUC-MSC transfected with Sox9+TGFβ1 showed a noticeable morphological appearance of a chondrocyte, and highly expressed chondrogenic markers (aggrecan, Sox9, TGFβ1, TGFβ2, and type II collagens) after transfection. Histological observation demonstrated that cartilage regeneration, extracellular matrix synthesis, and collagen remodeling were significant upon staining with H&amp;E, Alcian blue, and Masson's trichrome stain on day 14. Additionally, oxidative stress, pain, and inflammatory markers were positively downregulated in the animals transplanted with Sox9 and TGFβ1 transfected MSCs.

Conclusion

These findings indicate that the combinatorial effect of Sox9 and TGFβ1 substantially accelerates the chondrogenesis in hUC-MSCs. Cartilage regeneration and matrix synthesis were significantly enhanced. Therefore, a synergistic effect of Sox9 and TGFβ1 could be an immense therapeutic combination in the tissue engineering of cartilaginous joint bio-prostheses and a novel candidate for cartilage stabilization.

Protein-engineered biomaterials for cartilage therapeutics and repair

Cartilage degeneration and injury are major causes of pain and disability that effect millions, and yet treatment options for conditions like osteoarthritis (OA) continue to be mainly palliative or involve complete replacement of injured joints. Several biomaterial strategies have been explored to address cartilage repair either by the delivery of therapeutics or as support for tissue repair, however the complex structure of cartilage tissue, its mechanical needs, and lack of regenerative capacity have hindered this goal. Recent advances in synthetic biology have opened new possibilities for engineered proteins to address these unique needs. Engineered protein and peptide-based materials benefit from inherent biocompatibility and nearly unlimited tunability as they utilize the body's natural building blocks to fabricate a variety of supramolecular structures. The pathophysiology and needs of OA cartilage are presented here, along with an overview of the current state of the art and next steps for protein-engineered repair strategies for cartilage.

Intervertebral Disc Tissue Engineering Using Additive Manufacturing

Intervertebral disc (IVD) degeneration is one of the major causes of lower back pain, a common health condition that greatly affects the quality of life. With an increasing elderly population and changes in lifestyle, there exists a high demand for novel treatment strategies for damaged IVDs. Researchers have investigated IVD tissue engineering (TE) as a way to restore biological and mechanical functions by regenerating or replacing damaged discs using scaffolds with suitable cells. These scaffolds can be constructed using material extrusion additive manufacturing (AM), a technique used to build three-dimensional (3D), custom discs utilising computer-aided design (CAD). Structural geometry can be controlled via the manipulation of printing parameters, material selection, temperature, and various other processing parameters. To date, there are no clinically relevant TE-IVDs available. In this review, advances in AM-based approaches for IVD TE are briefly discussed in order to achieve a better understanding of the requirements needed to obtain more effective, and ultimately clinically relevant, IVD TE constructs.

Precision medicine strategies for spinal degenerative diseases: Injectable biomaterials with in situ repair and regeneration

As the population ages, spinal degeneration seriously affects quality of life in middle-aged and elderly patients, and prevention and treatment remain challenging for clinical surgeons. In recent years, biomaterials-based injectable therapeutics have attracted much attention for spinal degeneration treatment due to their minimally invasive features and ability to perform precise repair of irregular defects. However, the precise design and functional control of bioactive injectable biomaterials for efficient spinal degeneration treatment remains a challenge. Although many injectable biomaterials have been reported for the treatment of spinal degeneration, there are few reviews on the advances and effects of injectable biomaterials for spinal degeneration treatment. This work reviews the current status of the design and fabrication of injectable biomaterials, including hydrogels, bone cements and scaffolds, microspheres and nanomaterials, and the current progress in applications for treating spinal degeneration. Additionally, registered clinical trials were also summarized and key challenges and clinical translational prospects for injectable materials for the treatment of spinal degenerative diseases are discussed.

Development of Two-Layer Hybrid Scaffolds Based on Oxidized Polyvinyl Alcohol and Bioactivated Chitosan Sponges for Tissue Engineering Purposes

Oxidized polyvinyl alcohol (OxPVA) is a new polymer for the fabrication of nerve conduits (NCs). Looking for OxPVA device optimization and coupling it with a natural sheath may boost bioactivity. Thus, OxPVA/chitosan sponges (ChS) as hybrid scaffolds were investigated to predict in the vivo behaviour of two-layered NCs. To encourage interaction with cells, ChS were functionalized with the self-assembling-peptide (SAP) EAK, without/with the laminin-derived sequences -IKVAV/-YIGSR. Thus, ChS and the hybrid scaffolds were characterized for mechanical properties, ultrastructure (Scanning Electron Microscopy, SEM), bioactivity, and biocompatibility. Regarding mechanical analysis, the peptide-free ChS showed the highest values of compressive modulus and maximum stress. However, among +EAK groups, ChS+EAK showed a significantly higher maximum stress than that found for ChS+EAK-IKVAV and ChS+EAK-YIGSR. Considering ultrastructure, microporous interconnections were tighter in both the OxPVA/ChS and +EAK groups than in the others; all the scaffolds induced SH-SY5Y cells’ adhesion/proliferation, with significant differences from day 7 and a higher total cell number for OxPVA/ChS+EAK scaffolds, in accordance with SEM. The scaffolds elicited only a slight inflammation after 14 days of subcutaneous implantation in Balb/c mice, proving biocompatibility. ChS porosity, EAK 3D features and neuro-friendly attitude (shared with IKVAV/YIGSR motifs) may confer to OxPVA certain bioactivity, laying the basis for future appealing NCs.

Peptide-Based Low Molecular Weight Photosensitive Supramolecular Gelators

Over the last couple of decades, stimuli-responsive supramolecular gels comprising synthetic short peptides as building blocks have been explored for various biological and material applications. Though a wide range of stimuli has been tested depending on the structure of the peptides, light as a stimulus has attracted extensive attention due to its non-invasive, non-contaminant, and remotely controllable nature, precise spatial and temporal resolution, and wavelength tunability. The integration of molecular photo-switch and low-molecular-weight synthetic peptides may thus provide access to supramolecular self-assembled systems, notably supramolecular gels, which may be used to create dynamic, light-responsive “smart” materials with a variety of structures and functions. This short review summarizes the recent advancement in the area of light-sensitive peptide gelation. At first, a glimpse of commonly used molecular photo-switches is given, followed by a detailed description of their incorporation into peptide sequences to design light-responsive peptide gels and the mechanism of their action. Finally, the challenges and future perspectives for developing next-generation photo-responsive gels and materials are outlined.

Recent Advances in Managing Spinal Intervertebral Discs Degeneration

Low back pain (LBP) represents a frequent and debilitating condition affecting a large part of the global population and posing a worldwide health and economic burden. The major cause of LBP is intervertebral disc degeneration (IDD), a complex disease that can further aggravate and give rise to severe spine problems. As most of the current treatments for IDD either only alleviate the associated symptoms or expose patients to the risk of intraoperative and postoperative complications, there is a pressing need to develop better therapeutic strategies. In this respect, the present paper first describes the pathogenesis and etiology of IDD to set the framework for what has to be combated to restore the normal state of intervertebral discs (IVDs), then further elaborates on the recent advances in managing IDD. Specifically, there are reviewed bioactive compounds and growth factors that have shown promising potential against underlying factors of IDD, cell-based therapies for IVD regeneration, biomimetic artificial IVDs, and several other emerging IDD therapeutic options (e.g., exosomes, RNA approaches, and artificial intelligence).