Photocrosslinkable, biodegradable hydrogels with controlled cell adhesivity for prolonged siRNA delivery to hMSCs to enhance their osteogenic differentiation

Developing hydrogels for gene therapy and tissue engineering

Hydrogels are a class of highly absorbent and easily modified polymer materials suitable for use as slow-release carriers for drugs. Gene therapy is highly specific and can overcome the limitations of traditional tissue engineering techniques and has significant advantages in tissue repair. However, therapeutic genes are often affected by cellular barriers and enzyme sensitivity, and carrier loading of therapeutic genes is essential. Therapeutic gene hydrogels can well overcome these difficulties. Moreover, gene-therapeutic hydrogels have made considerable progress. This review summarizes the recent research on carrier gene hydrogels for the treatment of tissue damage through a summary of the most current research frontiers. We initially introduce the classification of hydrogels and their cross-linking methods, followed by a detailed overview of the types and modifications of therapeutic genes, a detailed discussion on the loading of therapeutic genes in hydrogels and their characterization features, a summary of the design of hydrogels for therapeutic gene release, and an overview of their applications in tissue engineering. Finally, we provide comments and look forward to the shortcomings and future directions of hydrogels for gene therapy. We hope that this article will provide researchers in related fields with more comprehensive and systematic strategies for tissue engineering repair and further promote the development of the field of hydrogels for gene therapy.

Bioactive Phenylboronic Acid-Functionalized Hyaluronic Acid Hydrogels Induce Chondro-Aggregates and Promote Chondrocyte Phenotype

Hydrogels are extensively investigated as biomimetic extracellular matrix (ECM) scaffolds in tissue engineering. The physiological properties of ECM affect cellular behaviors, which is an inspiration for cell-based therapies. Photocurable hyaluronic acid (HA) hydrogel (AHAMA-PBA) modified with 3-aminophenylboronic acid, sodium periodate, and methacrylic anhydride simultaneously is constructed in this study. Chondrocytes are then cultured on the surface of the hydrogels to evaluate the effect of the physicochemical properties of the hydrogels on modulating cellular behaviors. Cell viability assays demonstrate that the hydrogel is non-toxic to chondrocytes. The existence of phenylboronic acid (PBA) moieties enhances the interaction of chondrocytes and hydrogel, promoting cell adhesion and aggregation through filopodia. RT-PCR indicates that the gene expression levels of type II collagen, Aggrecan, and Sox9 are significantly up-regulated in chondrocytes cultured on hydrogels. Moreover, the mechanical properties of the hydrogels have a significant effect on the cell phenotype, with soft gels (≈2 kPa) promoting chondrocytes to exhibit a hyaline phenotype. Overall, PBA-functionalized HA hydrogel with low stiffness exhibits the best effect on promoting the chondrocyte phenotype, which is a promising biomaterial for cartilage regeneration.

3D Bioprinting in Otolaryngology: A Review

The evolution of tissue engineering and 3D bioprinting has allowed for increased opportunities to generate musculoskeletal tissue grafts that can enhance functional and aesthetic outcomes in otolaryngology-head and neck surgery. Despite literature reporting successes in the fabrication of cartilage and bone scaffolds for applications in the head and neck, the full potential of this technology has yet to be realized. Otolaryngology as a field has always been at the forefront of new advancements and technology and is well poised to spearhead clinical application of these engineered tissues. In this review, current 3D bioprinting methods are described and an overview of potential cell types, bioinks, and bioactive factors available for musculoskeletal engineering using this technology is presented. The otologic, nasal, tracheal, and craniofacial bone applications of 3D bioprinting with a focus on engineered graft implantation in animal models to highlight the status of functional outcomes in vivo; a necessary step to future clinical translation are reviewed. Continued multidisciplinary efforts between material chemistry, biological sciences, and otolaryngologists will play a key role in the translation of engineered, 3D bioprinted constructs for head and neck surgery.

Modified gellan gum-based hydrogel with enhanced mechanical properties for application as a cell carrier for cornea endothelial cells

Recently, the number of people suffering from visual loss due to eye diseases is increasing rapidly around the world. However, due to the severe donor shortage and the immune response, corneal replacement is needed. Gellan gum (GG) is biocompatible and widely used for cell delivery or drug delivery, but its strength is not suitable for the corneal substitute. In this study, a GM hydrogel was prepared by blending methacrylated gellan gum with GG (GM) to give suitable mechanical properties to the corneal tissue. In addition, lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP), a crosslinking initiator, was added to the GM hydrogel. After the photo-crosslinking treatment, it was named GM/LAP hydrogel. GM and GM/LAP hydrogels were analyzed for physicochemical properties, mechanical characterization, and transparency tests to confirm their applicability as carriers for corneal endothelial cells (CEnCs). Also, in vitro studies were performed with cell viability tests, cell proliferation tests, cell morphology, cell-matrix remodeling analysis, and gene expression evaluation. The compressive strength of the GM/LAP hydrogel was improved compared to the GM hydrogel. The GM/LAP hydrogel showed excellent cell viability, proliferation, and cornea-specific gene expression than the GM hydrogel. Crosslinking-improved GM/LAP hydrogel can be applied as a promising cell carrier in corneal tissue engineering.

Recent Progress on Polysaccharide-Based Hydrogels for Controlled Delivery of Therapeutic Biomolecules

A plethora of applications using polysaccharides have been developed in recent years due to their availability as well as their frequent nontoxicity and biodegradability. These polymers are usually obtained from renewable sources or are byproducts of industrial processes, thus, their use is collaborative in waste management and shows promise for an enhanced sustainable circular economy. Regarding the development of novel delivery systems for biotherapeutics, the potential of polysaccharides is attractive for the previously mentioned properties and also for the possibility of chemical modification of their structures, their ability to form matrixes of diverse architectures and mechanical properties, as well as for their ability to maintain bioactivity following incorporation of the biomolecules into the matrix. Biotherapeutics, such as proteins, growth factors, gene vectors, enzymes, hormones, DNA/RNA, and antibodies are currently in use as major therapeutics in a wide range of pathologies. In the present review, we summarize recent progress in the development of polysaccharide-based hydrogels of diverse nature, alone or in combination with other polymers or drug delivery systems, which have been implemented in the delivery of biotherapeutics in the pharmaceutical and biomedical fields.

Hydrogel microspheres for spatiotemporally controlled delivery of RNA and silencing gene expression within scaffold-free tissue engineered constructs

Delivery systems for controlled release of RNA interference (RNAi) molecules, including small interfering (siRNA) and microRNA (miRNA), have the potential to direct stem cell differentiation for regenerative musculoskeletal applications. To date, localized RNA delivery platforms in this area have focused predominantly on bulk scaffold-based approaches, which can interfere with cell-cell interactions important for recapitulating some native musculoskeletal developmental and healing processes in tissue regeneration strategies. In contrast, scaffold-free, high density human mesenchymal stem cell (hMSC) aggregates may provide an avenue for creating a more biomimetic microenvironment. Here, photocrosslinkable dextran microspheres (MS) encapsulating siRNA-micelles were prepared via an aqueous emulsion method and incorporated within hMSC aggregates for localized and sustained delivery of bioactive siRNA. siRNA-micelles released from MS in a sustained fashion over the course of 28 days, and the released siRNA retained its ability to transfect cells for gene silencing. Incorporation of fluorescently labeled siRNA (siGLO)-laden MS within hMSC aggregates exhibited tunable siGLO delivery and uptake by stem cells. Incorporation of MS loaded with siRNA targeting green fluorescent protein (siGFP) within GFP-hMSC aggregates provided sustained presentation of siGFP within the constructs and prolonged GFP silencing for up to 15 days. This platform system enables sustained gene silencing within stem cell aggregates and thus shows great potential in tissue regeneration applications. STATEMENT OF SIGNIFICANCE: This work presents a new strategy to deliver RNA-nanocomplexes from photocrosslinked dextran microspheres for tunable presentation of bioactive RNA. These microspheres were embedded within scaffold-free, human mesenchymal stem cell (hMSC) aggregates for sustained gene silencing within three-dimensional cell constructs while maintaining cell viability. Unlike exogenous delivery of RNA within culture medium that suffers from diffusion limitations and potential need for repeated transfections, this strategy provides local and sustained RNA presentation from the microspheres to cells in the constructs. This system has the potential to inhibit translation of hMSC differentiation antagonists and drive hMSC differentiation toward desired specific lineages, and is an important step in the engineering of high-density stem cell systems with incorporated instructive genetic cues for application in tissue regeneration.

Designing biomaterials for the delivery of RNA therapeutics to stimulate bone healing

Ribonucleic acids (small interfering RNA, microRNA, and messenger RNA) have been emerging as a promising new class of therapeutics for bone regeneration. So far, however, research has mostly focused on stability and complexation of these oligonucleotides for systemic delivery. By comparison, delivery of RNA nanocomplexes from biomaterial carriers can facilitate a spatiotemporally controlled local delivery of osteogenic oligonucleotides. This review provides an overview of the state-of-the-art in the design of biomaterials which allow for temporal and spatial control over RNA delivery. We correlate this concept of spatiotemporally controlled RNA delivery to the most relevant events that govern bone regeneration to evaluate to which extent tuning of release kinetics is required. In addition, inspired by the physiological principles of bone regeneration, potential new RNA targets are presented. Finally, considerations for clinical translation and upscaled production are summarized to stimulate the design of clinically relevant RNA-releasing biomaterials.

Self-Assemblable Polymer Smart-Blocks for Temperature-Induced Injectable Hydrogel in Biomedical Applications

Self-assembled temperature-induced injectable hydrogels fabricated via self-assembly of polymer smart-blocks have been widely investigated as drug delivery systems and platforms for tissue regeneration. Polymer smart-blocks that can be self-assembly play an important role in fabrication of hydrogels because they can self-assemble to induce the gelation of their copolymer in aqueous solution. The self-assembly occurs in response to an external stimulus change, such as temperature, pH, glucose, ionic strength, light, magnetic field, electric field, or their combination, which results in property transformations like hydrophobicity, ionization, and conformational change. The self-assembly smart-block based copolymers exist as a solution in aqueous media at certain conditions that are suitable for mixing with bioactive molecules and/or cells. However, this solution turns into a hydrogel due to the self-assembly of the smart-blocks under exposure to an external stimulus change in vitro or injection into the living body for a controllable release of loaded bioactive molecules or serving as a biomaterial scaffold for tissue regeneration. This work reports current scenery in the development of these self-assembly smart-blocks for fabrication of temperature-induced injectable physically cross-linked hydrogels and their potential application as drug delivery systems and platforms for tissue engineering.

Covalently tethering siRNA to hydrogels for localized, controlled release and gene silencing

Small interfering RNA (siRNA) has found many applications in tissue regeneration and disease therapeutics. Effective and localized siRNA delivery remains challenging, reducing its therapeutic potential. Here, we report a strategy to control and prolong siRNA release by directly tethering transfection-capable siRNA to photocrosslinked dextran hydrogels. siRNA release is governed via the hydrolytic degradation of ester and/or disulfide linkages between the siRNA and hydrogels, which is independent of hydrogel degradation rate. The released siRNA is shown to be bioactive by inhibiting protein expression in green fluorescent protein-expressing HeLa cells without the need of a transfection agent. This strategy provides an excellent platform for controlling nucleic acid delivery through covalent bonds with a biomaterial and regulating cellular gene expression, which has promising potential in many biomedical applications.

Messenger RNA Delivery for Tissue Engineering and Regenerative Medicine Applications

The ability to control cellular processes and precisely direct cellular reprogramming has revolutionized regenerative medicine. Recent advances in in vitro transcribed (IVT) mRNA technology with chemical modifications have led to development of methods that control spatiotemporal gene expression. Additionally, there is a current thrust toward the development of safe, integration-free approaches to gene therapy for translational purposes. In this review, we describe strategies of synthetic IVT mRNA modifications and nonviral technologies for intracellular delivery. We provide insights into the current tissue engineering approaches that use a hydrogel scaffold with genetic material. Furthermore, we discuss the transformative potential of novel mRNA formulations that when embedded in hydrogels can trigger controlled genetic manipulation to regenerate tissues and organs in vitro and in vivo. The role of mRNA delivery in vascularization, cytoprotection, and Cas9-mediated xenotransplantation is additionally highlighted. Harmonizing mRNA delivery vehicle interactions with polymeric scaffolds can be used to present genetic cues that lead to precise command over cellular reprogramming, differentiation, and secretome activity of stem cells-an ultimate goal for tissue engineering.

Thiol-Epoxy "Click" Chemistry to Engineer Cytocompatible PEG-Based Hydrogel for siRNA-Mediated Osteogenesis of hMSCs

Thiol-epoxy "click" chemistry is employed for the first time to engineer a new cytocompatible PEG-based hydrogel system in aqueous media with the ability to encapsulate human mesenchymal stem cells (hMSCs) and control their fate for tissue regeneration. Cells were easily encapsulated into the hydrogels and exhibited high cell viability over 4 weeks of culture regardless of the presence of siRNA, complexed with polyethylenimine (PEI) in the form of siRNA/PEI nanocomplexes, indicating the biocompatibility of the developed hydrogel. Loading pro-osteogenic siNoggin in the hydrogel significantly enhanced the osteogenesis of encapsulated hMSCs, demonstrating the potential application of this system in tissue engineering.

RNA interfering molecule delivery from in situ forming biodegradable hydrogels for enhancement of bone formation in rat calvarial bone defects

RNA interference (RNAi) may be an effective and valuable tool for promoting the growth of functional tissue, as short interfering RNA (siRNA) and microRNA (miRNA) can block the expression of genes that have negative effects on tissue regeneration. Our group has recently reported that the localized and sustained presentation of siRNA against noggin (siNoggin) and miRNA-20a from in situ forming poly(ethylene glycol) (PEG) hydrogels enhanced osteogenic differentiation of encapsulated human bone marrow-derived mesenchymal stem cells (hMSCs). Here, the capacity of the hydrogel system to accelerate bone formation in a rat calvarial bone defect model is presented. After 12 weeks post-implantation, the hydrogels containing encapsulated hMSCs and miRNA-20a resulted in more bone formation in the defects than the hydrogels containing hMSCs without siRNA or with negative control siRNA. This localized and sustained RNA interfering molecule delivery system may provide an excellent platform for healing bony defects and other tissues.

STATEMENT OF SIGNIFICANCE

Delivery of RNAi molecules may be a valuable strategy to guide cell behavior for tissue engineering applications, but to date there have been no reports of a biomaterial system capable of both encapsulation of cells and controlled delivery of incorporated RNA. Here, we present PEG hydrogels that form in situ via Michael type reaction, and that permit encapsulation of hMSCs and the concomitant controlled delivery of siNoggin and/or miRNA-20a. These RNAs were chosen to suppress noggin, a BMP-2 antagonist, and/or PPAR-γ, a negative regulator of BMP-2-mediated osteogenesis, and therefore promote osteogenic differentiation of hMSCs and subsequent bone repair in critical-sized rat calvarial defects. Simultaneous delivery of hMSCs and miRNA-20a enhanced repair of these defects compared to hydrogels containing hMSCs without siRNA or with negative control siRNA. This in situ forming PEG hydrogel system offers an exciting platform for healing critical-sized bone defects by localized, controlled delivery of RNAi molecules to encapsulated hMSCs and surrounding cells.

Toward Customized Extracellular Niche Engineering: Progress in Cell-Entrapment Technologies

The primary aim in tissue engineering is to repair, replace, and regenerate dysfunctional tissues to restore homeostasis. Cell delivery for repair and regeneration is gaining impetus with our understanding of constructing tissue-like environments. However, the perpetual challenge is to identify innovative materials or re-engineer natural materials to model cell-specific tissue-like 3D modules, which can seamlessly integrate and restore functions of the target organ. To devise an optimal functional microenvironment, it is essential to define how simple is complex enough to trigger tissue regeneration or restore cellular function. Here, the purposeful transition of cell immobilization from a cytoprotection point of view to that of a cell-instructive approach is examined, with advances in the understanding of cell-material interactions in a 3D context, and with a view to further application of the knowledge for the development of newer and complex hierarchical tissue assemblies for better examination of cell behavior and offering customized cell-based therapies for tissue engineering.

Photolabile Hydrogels Responsive to Broad Spectrum Visible Light for Selective Cell Release

We introduce an efficient method for the preparation of photolabile polymer linkers to be used in the fabrication of bioorthogonal and photodegradable hydrogels. The versatility of this synthesis strategy allows for incorporation of a series of chromophores responsive to addressable wavelengths of UV and broad spectrum visible light. Consequently, selective release of different cell types from composite hydrogels by user-defined timing can be achieved by irradiating the materials with different wavelengths of light.