Synthetic matrix enhances transplanted satellite cell engraftment in dystrophic and aged skeletal muscle with comorbid trauma

Optimization of Interleukin-10 incorporation for dendritic cells embedded in Poly(ethylene glycol) hydrogels

Translational research in biomaterials and immunoengineering is leading to the development of novel advanced therapeutics to treat diseases such as cancer, autoimmunity, and viral infections. Dendritic cells (DCs) are at the center of these therapeutics given that they bridge innate and adaptive immunity. The biomaterial system developed herein uses a hydrogel carrier to deliver immunomodulatory DCs for amelioration of autoimmunity. This biomaterial vehicle is comprised of a poly (ethylene glycol)-4 arm maleimide (PEG-4MAL) hydrogels, conjugated with the immunosuppressive cytokine, interleukin-10, IL-10, and cross-linked with a collagenase-degradable peptide sequence for the injectable delivery of immunosuppressive DCs to an anatomical disease-relevant site of the cervical lymph nodes, for intended application to treat multiple sclerosis. The amount of IL-10 incorporated in the hydrogel was optimized to be 500 ng in vitro, based on immunological endpoints. At this concentration, DCs exhibited the best viability, most immunosuppressive phenotype, and protection against proinflammatory insult as compared with hydrogel-incorporated DCs with lower IL-10 loading amounts. Additionally, the effect of the degradability of the PEG-4MAL hydrogel on the release rate of incorporated IL-10 was assessed by varying the ratio of degradable peptides: VPM (degradable) and DTT (nondegradable) and measuring the IL-10 release rates. This IL-10-conjugated hydrogel delivery system for immunosuppressive DCs is set to be assessed for in vivo functionality as the immunosuppressive cytokine provides a tolerogenic environment that keeps DCs in their immature phenotype, which consequently enhances cell viability and optimizes the system's immunomodulatory functionality.

Enhancing volumetric muscle loss (VML) recovery in a rat model using super durable hydrogels derived from bacteria

Bacteria can be programmed to deliver natural materials with defined biological and mechanical properties for controlling cell growth and differentiation. Here, we present an elastic, resilient and bioactive polysaccharide derived from the extracellular matrix of Pantoea sp. BCCS 001. Specifically, it was methacrylated to generate a new photo crosslinkable hydrogel that we coined Pantoan Methacrylate or put simply PAMA. We have used it for the first time as a tissue engineering hydrogel to treat VML injuries in rats. The crosslinked PAMA hydrogel was super elastic with a recovery nearing 100 %, while mimicking the mechanical stiffness of native muscle. After inclusion of thiolated gelatin via a Michaelis reaction with acrylate groups on PAMA we could also guide muscle progenitor cells into fused and aligned tubes - something reminiscent of mature muscle cells. These results were complemented by sarcomeric alpha-actinin immunostaining studies. Importantly, the implanted hydrogels exhibited almost 2-fold more muscle formation and 50 % less fibrous tissue formation compared to untreated rat groups. In vivo inflammation and toxicity assays likewise gave rise to positive results confirming the biocompatibility of this new biomaterial system. Overall, our results demonstrate that programmable polysaccharides derived from bacteria can be used to further advance the field of tissue engineering. In greater detail, they could in the foreseeable future be used in practical therapies against VML.

Recent trends in 3D bioprinting technology for skeletal muscle regeneration

Skeletal muscle is a pro-regenerative tissue, that utilizes a tissue-resident stem cell system to effect repair upon injury. Despite the demonstrated efficiency of this system in restoring muscle mass after many acute injuries, in conditions of severe trauma such as those evident in volumetric muscle loss (VML) (>20 % by mass), this self-repair capability is unable to restore tissue architecture, requiring interventions which currently are largely surgical. As a possible alternative, the generation of artificial muscle using tissue engineering approaches may also be of importance in the treatment of VML and muscle diseases such as dystrophies. Three-dimensional (3D) bioprinting has been identified as a promising technique for regeneration of the complex architecture of skeletal muscle. This review discusses existing treatment strategies following muscle damage, recent progress in bioprinting techniques, the bioinks used for muscle regeneration, the immunogenicity of scaffold materials, and in vitro and in vivo maturation techniques for 3D bio-printed muscle constructs. The pros and cons of these bioink formulations are also highlighted. Finally, we present the current limitations and challenges in the field and critical factors to consider for bioprinting approaches to become more translationa and to produce clinically relevant engineered muscle. STATEMENT OF SIGNIFICANCE: This review discusses the physiopathology of muscle injuries and existing clinical treatment strategies for muscle damage, the types of bioprinting techniques that have been applied to bioprinting of muscle, and the bioinks commonly used for muscle regeneration. The pros and cons of these bioinks are highlighted. We present a discussion of existing gaps in the literature and critical factors to consider for the translation of bioprinting approaches and to produce clinically relevant engineered muscle. Finally, we provide insights into what we believe will be the next steps required before the realization of the application of tissue-engineered muscle in humans. We believe this manuscript is an insightful, timely, and instructive review that will guide future muscle bioprinting research from a fundamental construct creation approach, down a translational pathway to achieve the desired impact in the clinic.

Tracking of Nascent Matrix Deposition during Muscle Stem Cell Activation across Lifespan Using Engineered Hydrogels

Adult stem cells occupy a niche that contributes to their function, but how stem cells rebuild their microenvironment after injury remains an open-ended question. Herein, biomaterial-based systems and metabolic labeling are utilized to evaluate how skeletal muscle stem cells deposit extracellular matrix. Muscle stem cells and committed myoblasts are observed to generate less nascent matrix than muscle resident fibro-adipogenic progenitors. When cultured on substrates that matched the stiffness of physiological uninjured and injured muscles, muscle stem cells increased nascent matrix deposition with activation kinetics. Reducing the ability to deposit nascent matrix by an inhibitor of vesicle trafficking (Exo-1) attenuated muscle stem cell function and mimicked impairments observed from muscle stem cells isolated from old muscles. Old muscle stem cells are observed to deposit less nascent matrix than young muscle stem cells, which is rescued with therapeutic supplementation of insulin-like growth factors. These results highlight the role of nascent matrix production with muscle stem cell activation.

Biomaterials-Based Technologies in Skeletal Muscle Tissue Engineering

For many clinically prevalent severe injuries, the inherent regenerative capacity of skeletal muscle remains inadequate. Skeletal muscle tissue engineering (SMTE) seeks to meet this clinical demand. With continuous progress in biomedicine and related technologies including micro/nanotechnology and 3D printing, numerous studies have uncovered various intrinsic mechanisms regulating skeletal muscle regeneration and developed tailored biomaterial systems based on these understandings. Here, the skeletal muscle structure and regeneration process are discussed and the diverse biomaterial systems derived from various technologies are explored in detail. Biomaterials serve not merely as local niches for cell growth, but also as scaffolds endowed with structural or physicochemical properties that provide tissue regenerative cues such as topographical, electrical, and mechanical signals. They can also act as delivery systems for stem cells and bioactive molecules that have been shown as key participants in endogenous repair cascades. To achieve bench-to-bedside translation, the typical effect enabled by biomaterial systems and the potential underlying molecular mechanisms are also summarized. Insights into the roles of biomaterials in SMTE from cellular and molecular perspectives are provided. Finally, perspectives on the advancement of SMTE are provided, for which gene therapy, exosomes, and hybrid biomaterials may hold promise to make important contributions.

Engineered Synthetic Matrices for Human Intestinal Organoid Culture and Therapeutic Delivery

Human intestinal organoids (HIOs) derived from pluripotent stem cells or adult stem cell biopsies represent a powerful platform to study human development, drug testing, and disease modeling in vitro, and serve as a cell source for tissue regeneration and therapeutic advances in vivo. Synthetic hydrogels can be engineered to serve as analogs of the extracellular matrix to support HIO growth and differentiation. These hydrogels allow for tuning the mechanical and biochemical properties of the matrix, offering an advantage over biologically derived hydrogels such as Matrigel. Human intestinal organoids have been used for repopulating transplantable intestinal grafts and for in vivo delivery to an injured intestinal site. The use of synthetic hydrogels for in vitro culture and for in vivo delivery is expected to significantly increase the relevance of human intestinal organoids for drug screening, disease modeling, and therapeutic applications.

Quantification of local matrix deposition during muscle stem cell activation using engineered hydrogels

Adult stem cells occupy a niche that contributes to their function, but how stem cells remodel their microenvironment remains an open-ended question. Herein, biomaterials-based systems and metabolic labeling were utilized to evaluate how skeletal muscle stem cells deposit extracellular matrix. Muscle stem cells and committed myoblasts were observed to generate less nascent matrix than muscle resident fibro-adipogenic progenitors. When cultured on substrates that matched the stiffness of physiological uninjured and injured muscles, the increased nascent matrix deposition was associated with stem cell activation. Reducing the ability to deposit nascent matrix in muscle stem cells attenuated function and mimicked impairments observed from muscle stem cells isolated from old aged muscles, which could be rescued with therapeutic supplementation of insulin-like growth factors. These results highlight how nascent matrix production is critical for maintaining healthy stem cell function.

Optimization of Xenografting Methods for Generating Human Skeletal Muscle in Mice

Xenografts of human skeletal muscle generated in mice can be used to study muscle pathology and to test drugs designed to treat myopathies and muscular dystrophies for their efficacy and specificity in human tissue. We previously developed methods to generate mature human skeletal muscles in immunocompromised mice starting with human myogenic precursor cells (hMPCs) from healthy individuals and individuals with facioscapulohumeral muscular dystrophy (FSHD). Here, we examine a series of alternative treatments at each stage in order to optimize engraftment. We show that (i) X-irradiation at 25Gy is optimal in preventing regeneration of murine muscle while supporting robust engraftment and the formation of human fibers without significant murine contamination; (ii) hMPC lines differ in their capacity to engraft; (iii) some hMPC lines yield grafts that respond better to intermittent neuromuscular electrical stimulation (iNMES) than others; (iv) some lines engraft better in male than in female mice; (v) coinjection of hMPCs with laminin, gelatin, Matrigel, or Growdex does not improve engraftment; (vi) BaCl2 is an acceptable replacement for cardiotoxin, but other snake venom preparations and toxins, including the major component of cardiotoxin, cytotoxin 5, are not; and (vii) generating grafts in both hindlimbs followed by iNMES of each limb yields more robust grafts than housing mice in cages with running wheels. Our results suggest that replacing cardiotoxin with BaCl2 and engrafting both tibialis anterior muscles generates robust grafts of adult human muscle tissue in mice.

Hydrolytic hydrogels tune mesenchymal stem cell persistence and immunomodulation for enhanced diabetic cutaneous wound healing

Diabetes is associated with an altered global inflammatory state with impaired wound healing. Mesenchymal stem/stromal cells (MSC) are being explored for treatment of diabetic cutaneous wounds due to their regenerative properties. These cells are commonly delivered by injection, but the need to prolong the retention of MSC at sites of injury has spurred the development of biomaterial-based MSC delivery vehicles. However, controlling biomaterial degradation rates in vivo remains a therapeutic-limiting challenge. Here, we utilize hydrolytically degradable ester linkages to engineer synthetic hydrogels with tunable in vivo degradation kinetics for temporally controlled delivery of MSC. In vivo hydrogel degradation rate can be controlled by altering the ratio of ester to amide linkages in the hydrogel macromers. These hydrolytic hydrogels degrade at rates that enable unencumbered cutaneous wound healing, while enhancing the local persistence MSC compared to widely used protease-degradable hydrogels. Furthermore, hydrogel-based delivery of MSC modulates local immune responses and enhances cutaneous wound repair in diabetic mice. This study introduces a simple strategy for engineering tunable degradation modalities into synthetic biomaterials, overcoming a key barrier to their use as cell delivery vehicles.

Bioprocessing Considerations towards the Manufacturing of Therapeutic Skeletal and Smooth Muscle Cells

Tissue engineering approaches within the muscle context represent a promising emerging field to address the current therapeutic challenges related with multiple pathological conditions affecting the muscle compartments, either skeletal muscle or smooth muscle, responsible for involuntary and voluntary contraction, respectively. In this review, several features and parameters involved in the bioprocessing of muscle cells are addressed. The cell isolation process is depicted, depending on the type of tissue (smooth or skeletal muscle), followed by the description of the challenges involving the use of adult donor tissue and the strategies to overcome the hurdles of reaching relevant cell numbers towards a clinical application. Specifically, the use of stem/progenitor cells is highlighted as a source for smooth and skeletal muscle cells towards the development of a cellular product able to maintain the target cell's identity and functionality. Moreover, taking into account the need for a robust and cost-effective bioprocess for cell manufacturing, the combination of muscle cells with biomaterials and the need for scale-up envisioning clinical applications are also approached.

Cell-extracellular matrix mechanotransduction in 3D

Mechanical properties of extracellular matrices (ECMs) regulate essential cell behaviours, including differentiation, migration and proliferation, through mechanotransduction. Studies of cell-ECM mechanotransduction have largely focused on cells cultured in 2D, on top of elastic substrates with a range of stiffnesses. However, cells often interact with ECMs in vivo in a 3D context, and cell-ECM interactions and mechanisms of mechanotransduction in 3D can differ from those in 2D. The ECM exhibits various structural features as well as complex mechanical properties. In 3D, mechanical confinement by the surrounding ECM restricts changes in cell volume and cell shape but allows cells to generate force on the matrix by extending protrusions and regulating cell volume as well as through actomyosin-based contractility. Furthermore, cell-matrix interactions are dynamic owing to matrix remodelling. Accordingly, ECM stiffness, viscoelasticity and degradability often play a critical role in regulating cell behaviours in 3D. Mechanisms of 3D mechanotransduction include traditional integrin-mediated pathways that sense mechanical properties and more recently described mechanosensitive ion channel-mediated pathways that sense 3D confinement, with both converging on the nucleus for downstream control of transcription and phenotype. Mechanotransduction is involved in tissues from development to cancer and is being increasingly harnessed towards mechanotherapy. Here we discuss recent progress in our understanding of cell-ECM mechanotransduction in 3D.

Hydrogel delivery of purinergic enzymes improves cardiac ischemia/reperfusion injury

RATIONALE

The innate immune response contributes to cardiac injury in myocardial ischemia/reperfusion (MI/R). Neutrophils are an important early part of the innate immune response to MI/R. Adenosine, an endogenous purine, is a known innate immune modulator and inhibitor of neutrophil activation. However, its delivery to the heart is limited by its short half-life (<30 s) and off-target side effects. CD39 and CD73 are anti-inflammatory homeostatic enzymes that can generate adenosine from phosphorylated adenosine substrate such as ATP released from injured tissue.

OBJECTIVE

We hypothesize that hydrogel-delivered CD39 and CD73 target the local early innate immune response, reduce neutrophil activation, and preserve cardiac function in MI/R injury.

METHODS AND RESULTS

We engineered a poly(ethylene) glycol (PEG) hydrogel loaded with the adenosine-generating enzymes CD39 and CD73. We incubated the hydrogels with neutrophils in vitro and showed a reduction in hydrogen peroxide production using Amplex Red. We demonstrated availability of substrate for the enzymes in the myocardium in MI/R by LC/MS, and tested release kinetics from the hydrogel. On echocardiography, global longitudinal strain (GLS) was preserved in MI/R hearts treated with the loaded hydrogel. Delivery of purinergic enzymes via this synthetic hydrogel resulted in lower innate immune infiltration into the myocardium post-MI/R, decreased markers of macrophage and neutrophil activation (NETosis), and decreased leukocyte-platelet complexes in circulation.

CONCLUSIONS

In a rat model of MI/R injury, CD39 and CD73 delivered via a hydrogel preserve cardiac function by modulating the innate immune response.

Bead-jet printing enabled sparse mesenchymal stem cell patterning augments skeletal muscle and hair follicle regeneration

Transplantation of mesenchymal stem cells (MSCs) holds promise to repair severe traumatic injuries. However, current transplantation practices limit the potential of this technique, either by losing the viable MSCs or reducing the performance of resident MSCs. Herein, we design a "bead-jet" printer, specialized for high-throughput intra-operative formulation and printing of MSCs-laden Matrigel beads. We show that high-density encapsulation of MSCs in Matrigel beads is able to augment MSC function, increasing MSC proliferation, migration, and extracellular vesicle production, compared with low-density bead or high-density bulk encapsulation of the equivalent number of MSCs. We find that the high-density MSCs-laden beads in sparse patterns demonstrate significantly improved therapeutic performance, by regenerating skeletal muscles approaching native-like cell density with reduced fibrosis, and regenerating skin with hair follicle growth and increased dermis thickness. MSC proliferation within 1-week post-transplantation and differentiation at 3 - 4 weeks post-transplantation are suggested to contribute therapy augmentation. We expect this "bead-jet" printing system to strengthen the potential of MSC transplantation.

IL-10-Functionalized Hydrogels Support Immunosuppressive Dendritic Cell Phenotype and Function

Biomaterial systems such as hydrogels enable localized delivery and postinjection modulation of cellular therapies in a wide array of contexts. Biomaterials as adjuvants have been an active area of investigation, but the study of functionalized biomaterials supporting immunosuppressive cell therapies for tolerogenic applications is still nascent. Here, we developed a 4-arm poly(ethylene-glycol)-maleimide (PEG-4MAL) hydrogel functionalized with interleukin-10 (IL-10) to improve the local delivery and efficacy of a cell therapy against autoimmune disease. The biophysical and biochemical properties of PEG-4MAL hydrogels were optimized to support dendritic cell (DC) viability and an immature phenotype. IL-10-functionalized PEG-4MAL (PEG-IL10) hydrogels exhibited controlled IL-10 release, extended the duration of DC support, and protected DCs from inflammatory assault. After incorporation in PEG-IL10 hydrogels, these DCs induced CD25+FoxP3+ regulatory T cells (Tregs) during in vitro coculture. These studies serve as a proof-of-concept for improving the efficacy of immunosuppressive cell therapies through biomaterial delivery. The flexible nature of this system enables its widespread application across a breadth of other tolerogenic applications for future investigation.

Transplantation to study satellite cell heterogeneity in skeletal muscle

Skeletal muscle has a remarkable capacity to regenerate throughout life, which is mediated by its resident muscle stem cells, also called satellite cells. Satellite cells, located periphery to the muscle fibers and underneath the basal lamina, are an indispensable cellular source for muscle regeneration. Satellite cell transplantation into regenerating muscle contributes robustly to muscle repair, thereby indicating that satellite cells indeed function as adult muscle stem cells. Moreover, satellite cells are a heterogenous population in adult tissue, with subpopulations that can be distinguished based on gene expression, cell-cycle progression, ability to self-renew, and bi-potential ability. Transplantation assays provide a powerful tool to better understand satellite cell function in vivo enabling the separation of functionally distinct satellite cell subpopulations. In this review, we focus on transplantation strategies to explore satellite cells’ functional heterogeneity, approaches targeting the recipient tissue to improve transplantation efficiency, and common strategies to monitor the behaviour of the transplanted cells. Lastly, we discuss some recent approaches to overcome challenges to enhance the transplantation potential of muscle stem cells.

Nanomaterial for Skeletal Muscle Regeneration

Skeletal muscle has an innate regenerative capacity to restore their structure and function following acute damages and injuries. However, in congenital muscular dystrophies, large volumetric muscle loss, cachexia, or aging, the declined regenerative capacity of skeletal muscle results in muscle wasting and functional impairment. Recent studies indicate that muscle mass and function are closely correlated with morbidity and mortality due to the large volume and location of skeletal muscle. However, the options for treating neuromuscular disorders are limited. Biomedical engineering strategies such as nanotechnologies have been implemented to address this issue.In this review, we focus on recent studies leveraging nano-sized materials for regeneration of skeletal muscle. We look at skeletal muscle pathologies and describe various proof-of-concept and pre-clinical studies that have used nanomaterials, with a focus on how nano-sized materials can be used for skeletal muscle regeneration depending on material dimensionality.Depending on the dimensionality of nano-sized materials, their application have been changed because of their different physical and biochemical properties.Nanomaterials have been spotlighted as a great candidate for addressing the unmet needs of regenerative medicine. Nanomaterials could be applied to several types of tissues and diseases along with the unique characteristics of nanomaterials. However, when confined to muscle tissue, the targets of nanomaterial applications are limited and can be extended in future research.

Synthetic Matrix Scaffolds Engineer the In Vivo Tumor Immune Microenvironment for Immunotherapy Screening

Immunotherapy has emerged as one of the most powerful anti-cancer therapies but is stymied by the limits of existing preclinical models with respect to disease latency and reproducibility. Additionally, the influence of differing immune microenvironments within tumors observed clinically and associated with immunotherapeutic resistance cannot be tuned to facilitate drug testing workflows without changing model system or laborious genetic approaches. To address this testing platform gap in the immune oncology drug development pipeline, the authors deploy engineered biomaterials as scaffolds to increase tumor formation rate, decrease disease latency, and diminish variability of immune infiltrates into tumors formed from murine mammary carcinoma cell lines implanted into syngeneic mice. By altering synthetic gel formulations that reshape infiltrating immune cells within the tumor, responsiveness of the same tumor model to varying classes of cancer immunotherapies, including in situ vaccination with a molecular adjuvant and immune checkpoint blockade, diverge. These results demonstrate the significant role the local immune microenvironment plays in immunotherapeutic response. These engineered tumor immune microenvironments therefore improve upon the limitations of current breast tumor models used for immune oncology drug screening to enable immunotherapeutic testing relevant to the variability in tumor immune microenvironments underlying immunotherapeutic resistance seen in human patients.

Development of a High-Yield Isolation Protocol Optimized for the Retrieval of Active Muscle Satellite Cells from Mouse Skeletal Muscle Tissue

Background and Objectives

Difficulties often encountered in separating and purifying active muscle satellite cells (MSCs) from skeletal muscle tissues have limited the supply of cells for muscle therapy and artificial meat production. Here, we report an effective isolation protocol to economically and conveniently retrieve active MSCs from skeletal muscle tissues in mice.

Methods and Results

We optimized an enzyme-based tissue digestion protocol for isolating skeletal muscle-derived primary cell population having a large number of active MSCs and described a method of differential plating (DP) for improving purity of active MSCs from skeletal muscle-derived primary cell population. Then, the age of the mouse appropriate to the isolation of a large number of active MSCs was elucidated. The best isolation yield of active MSCs from mouse skeletal muscle tissues was induced by the application of DP method to the primary cell population harvested from skeletal muscle tissues of 2-week-old mice digested in 0.2% (w/v) collagenase type II for 30 min at 37℃ and then in 0.1% (w/v) pronase for 5 min at 37℃.

Conclusions

The protocol we developed not only facilitates the isolation of MSCs but also maximizes the retrieval of active MSCs. Our expectation is that this protocol will contribute to the development of original technologies essential for muscle therapy and artificial meat industrialization in the future.

Advancements in 3D Cell Culture Systems for Personalizing Anti-Cancer Therapies

Over 90% of potential anti-cancer drug candidates results in translational failures in clinical trials. The main reason for this failure can be attributed to the non-accurate pre-clinical models that are being currently used for drug development and in personalised therapies. To ensure that the assessment of drug efficacy and their mechanism of action have clinical translatability, the complexity of the tumor microenvironment needs to be properly modelled. 3D culture models are emerging as a powerful research tool that recapitulates in vivo characteristics. Technological advancements in this field show promising application in improving drug discovery, pre-clinical validation, and precision medicine. In this review, we discuss the significance of the tumor microenvironment and its impact on therapy success, the current developments of 3D culture, and the opportunities that advancements that in vitro technologies can provide to improve cancer therapeutics.

Cellular and molecular modulation of rotator cuff muscle pathophysiology

Rotator cuff (RC) tendon tears are common shoulder injuries that result in irreversible and persistent degeneration of the associated muscles, which is characterized by severe inflammation, atrophy, fibrosis, and fatty infiltration. Although RC muscle degeneration strongly dictates the overall clinical outcomes, strategies to stimulate RC muscle regeneration have largely been overlooked to date. In this review, we highlight the current understanding of the cellular processes that coordinate muscle regeneration, and the roles of muscle resident cells, including immune cells, fibroadipogenic progenitors, and muscle satellite cells in the pathophysiologic regulation of RC muscles following injury. This review also provides perspectives for potential therapies to alleviate the hallmarks of RC muscle degeneration to address current limitations in postsurgical recovery.

Immunomodulation and Biomaterials: Key Players to Repair Volumetric Muscle Loss

Volumetric muscle loss (VML) is defined as a condition in which a large volume of skeletal muscle is lost due to physical insult. VML often results in a heightened immune response, resulting in significant long-term functional impairment. Estimates indicate that ~250,000 fractures occur in the US alone that involve VML. Currently, there is no active treatment to fully recover or repair muscle loss in VML patients. The health economics burden due to VML is rapidly increasing around the world. Immunologists, developmental biologists, and muscle pathophysiologists are exploring both immune responses and biomaterials to meet this challenging situation. The inflammatory response in muscle injury involves a non-specific inflammatory response at the injured site that is coordination between the immune system, especially macrophages and muscle. The potential role of biomaterials in the regenerative process of skeletal muscle injury is currently an important topic. To this end, cell therapy holds great promise for the regeneration of damaged muscle following VML. However, the delivery of cells into the injured muscle site poses a major challenge as it might cause an adverse immune response or inflammation. To overcome this obstacle, in recent years various biomaterials with diverse physical and chemical nature have been developed and verified for the treatment of various muscle injuries. These biomaterials, with desired tunable physicochemical properties, can be used in combination with stem cells and growth factors to repair VML. In the current review, we focus on how various immune cells, in conjunction with biomaterials, can be used to promote muscle regeneration and, most importantly, suppress VML pathology.

Milestones and current achievements in development of multifunctional bioscaffolds for medical application

Tissue engineering (TE) is a rapidly growing interdisciplinary field, which aims to restore or improve lost tissue function. Despite that TE was introduced more than 20 years ago, innovative and more sophisticated trends and technologies point to new challenges and development. Current challenges involve the demand for multifunctional bioscaffolds which can stimulate tissue regrowth by biochemical curves, biomimetic patterns, active agents and proper cell types. For those purposes especially promising are carefully chosen primary cells or stem cells due to its high proliferative and differentiation potential. This review summarized a variety of recently reported advanced bioscaffolds which present new functions by combining polymers, nanomaterials, bioactive agents and cells depending on its desired application. In particular necessity of study biomaterial-cell interactions with in vitro cell culture models, and studies using animals with in vivo systems were discuss to permit the analysis of full material biocompatibility. Although these bioscaffolds have shown a significant therapeutic effect in nervous, cardiovascular and muscle, tissue engineering, there are still many remaining unsolved challenges for scaffolds improvement.

A Hydrogel Strategy to Augment Tissue Adenosine to Improve Hindlimb Perfusion

[Figure: see text].

A 3D, Magnetically Actuated, Aligned Collagen Fiber Hydrogel Platform Recapitulates Physical Microenvironment of Myoblasts for Enhancing Myogenesis

Many cell responses that underlie the development, maturation, and function of tissues are guided by the architecture and mechanical loading of the extracellular matrix (ECM). Because mechanical stimulation must be transmitted through the ECM architecture, the synergy between these two factors is important. However, recapitulating the synergy of these physical microenvironmental cues in vitro remains challenging. To address this, a 3D magnetically actuated collagen hydrogel platform is developed that enables combined control of ECM architecture and mechanical stimulation. With this platform, it is demonstrated how these factors synergistically promote cell alignment of C2C12 myoblasts and enhance myogenesis. This promotion is driven in part by the dynamics of Yes-associated protein and structure of cellular microtubule networks. This facile platform holds great promises for regulating cell behavior and fate, generating a broad range of engineered physiologically representative microtissues in vitro, and quantifying the mechanobiology underlying their functions.

Current Strategies for the Regeneration of Skeletal Muscle Tissue

Traumatic injuries, tumor resections, and degenerative diseases can damage skeletal muscle and lead to functional impairment and severe disability. Skeletal muscle regeneration is a complex process that depends on various cell types, signaling molecules, architectural cues, and physicochemical properties to be successful. To promote muscle repair and regeneration, various strategies for skeletal muscle tissue engineering have been developed in the last decades. However, there is still a high demand for the development of new methods and materials that promote skeletal muscle repair and functional regeneration to bring approaches closer to therapies in the clinic that structurally and functionally repair muscle. The combination of stem cells, biomaterials, and biomolecules is used to induce skeletal muscle regeneration. In this review, we provide an overview of different cell types used to treat skeletal muscle injury, highlight current strategies in biomaterial-based approaches, the importance of topography for the successful creation of functional striated muscle fibers, and discuss novel methods for muscle regeneration and challenges for their future clinical implementation.

Integrating Biomaterials and Genome Editing Approaches to Advance Biomedical Science

The recent discovery and subsequent development of the CRISPR-Cas9 (clustered regularly interspaced short palindromic repeat-CRISPR-associated protein 9) platform as a precise genome editing tool have transformed biomedicine. As these CRISPR-based tools have matured, multiple stages of the gene editing process and the bioengineering of human cells and tissues have advanced. Here, we highlight recent intersections in the development of biomaterials and genome editing technologies. These intersections include the delivery of macromolecules, where biomaterial platforms have been harnessed to enable nonviral delivery of genome engineering tools to cells and tissues in vivo. Further, engineering native-like biomaterial platforms for cell culture facilitates complex modeling of human development and disease when combined with genome engineering tools. Deeper integration of biomaterial platforms in these fields could play a significant role in enabling new breakthroughs in the application of gene editing for the treatment of human disease.

Engineered Heterochronic Parabiosis in 3D Microphysiological System for Identification of Muscle Rejuvenating Factors

Exposure of aged mice to a young systemic milieu revealed remarkable rejuvenation effects on aged tissues, including skeletal muscle. Although some candidate factors have been identified, the exact identity and the underlying mechanisms of putative rejuvenating factors remain elusive, mainly due to the complexity of in vivo parabiosis. Here, we present an in vitro muscle parabiosis system that integrates young- and old-muscle stem cell vascular niche on a three-dimensional microfluidic platform designed to recapitulate key features of native muscle stem cell microenvironment. This innovative system enables mechanistic studies of cellular dynamics and molecular interactions within the muscle stem cell niche, especially in response to conditional extrinsic stimuli of local and systemic factors. We demonstrate that vascular endothelial growth factor (VEGF) signaling from endothelial cells and myotubes synergistically contribute to the rejuvenation of the aged muscle stem cell function. Moreover, with the adjustable on-chip system, we can mimic both blood transfusion and parabiosis and detect the time-varying effects of anti-geronic and pro-geronic factors in a single organ or multi-organ systems. Our unique approach presents a complementary in vitro model to supplement in vivo parabiosis for identifying potential anti-geronic factors responsible for revitalizing aging organs.

From injectable to 3D printed hydrogels in maxillofacial tissue engineering: A review

Introduction

This review aims at describing different types of hydrogels in context to their composition, fabrication techniques and other specific features along with an insight into the latest advancements including smart hydrogels, 3D printed, programmable, shape memory and self-healing hydrogels for their applicability as scaffold in maxillofacial bone and cartilage tissue regeneration.

Methods

Electronic database searches were undertaken on PubMed, Ovid, Medline, Embase, ProQuest and science direct for English language literature, published for application of hydrogels in maxillofacial bone and cartilage tissue engineering. The search items used in this article were hydrogel, bone and cartilage tissue engineering, maxillofacial, clinical trials. Reviews and in vitro studies were excluded.

Results

Search for injectable hydrogel showed 4955 articles, when restricted to bone tissue engineering results were reduced to 463 and for cartilage engineering to 335; when we limited it to maxillofacial bone and cartilage tissue engineering, search results showed 49 articles to which 9 additional articles were included from references, after exclusion of in-vitro studies and duplicates 16 articles were obtained for our study. Similarly, for 3D printed hydrogels, result showed 1126 articles, which got restricted to 19 when searched for maxillofacial bone and cartilage engineering, then 2 additional articles were included directly from references, and finally after exclusion of the invitro studies and duplicates, a total of 5 articles were obtained.

Conclusion

Modifications in hydrogel can improve the mechanical properties, biocompatibility and unique chemistries for its use in bone and cartilage tissue engineering for future research.

Synthetic alternatives to Matrigel

Matrigel, a basement-membrane matrix extracted from Engelbreth-Holm-Swarm mouse sarcomas, has been used for more than four decades for a myriad of cell culture applications. However, Matrigel is limited in its applicability to cellular biology, therapeutic cell manufacturing and drug discovery owing to its complex, ill-defined and variable composition. Variations in the mechanical and biochemical properties within a single batch of Matrigel - and between batches - have led to uncertainty in cell culture experiments and a lack of reproducibility. Moreover, Matrigel is not conducive to physical or biochemical manipulation, making it difficult to fine-tune the matrix to promote intended cell behaviours and achieve specific biological outcomes. Recent advances in synthetic scaffolds have led to the development of xenogenic-free, chemically defined, highly tunable and reproducible alternatives. In this Review, we assess the applications of Matrigel in cell culture, regenerative medicine and organoid assembly, detailing the limitations of Matrigel and highlighting synthetic scaffold alternatives that have shown equivalent or superior results. Additionally, we discuss the hurdles that are limiting a full transition from Matrigel to synthetic scaffolds and provide a brief perspective on the future directions of synthetic scaffolds for cell culture applications.

Towards stem cell therapies for skeletal muscle repair

Skeletal muscle is an ideal target for cell therapy. The use of its potent stem cell population in the form of autologous intramuscular transplantation represents a tantalizing strategy to slow the progression of congenital muscle diseases (such as Duchenne Muscular Dystrophy) or regenerate injured tissue following trauma. The syncytial nature of skeletal muscle uniquely permits the engraftment of stem/progenitor cells to contribute to new myonuclei and restore the expression of genes mutated in myopathies. Historically however, the implementation of this approach has been significantly limited by the inability to expand undifferentiated muscle stem cells (MuSCs) in culture whilst maintaining transplantation potential. This is crucial, as MuSC expansion and/or genetic manipulation is likely necessary for therapeutic applications. In this article, we review recent studies that have provided a number of important breakthroughs to tackle this problem. Progress towards this goal has been achieved by exploiting biochemical, biophysical and developmental paradigms to construct innovative in vitro strategies that are guiding stem cell therapies for muscle repair towards the clinic.

Engineering hydrogels with affinity-bound laminin as 3D neural stem cell culture systems

Laminin incorporation into biological or synthetic hydrogels has been explored to recapitulate the dynamic nature and biological complexity of neural stem cell (NSC) niches. However, the strategies currently explored for laminin immobilization within three-dimensional (3D) matrices do not address a critical aspect influencing cell-matrix interactions, which is the control over laminin conformation and orientation upon immobilization. This is a key feature for the preservation of the protein bioactivity. In this work, we explored an affinity-based approach to mediate the site-selective immobilization of laminin into a degradable synthetic hydrogel. Specifically, a four-arm maleimide terminated poly(ethylene glycol) (PEG-4MAL) macromer was functionalized with a mono-PEGylated recombinant human N-terminal agrin (NtA) domain, to promote high affinity binding of laminin. Different NtA concentrations (10, 50 and 100 μM) were used to investigate the impact of NtA density on laminin incorporation, hydrogel biophysical properties, and biological outcome. Laminin was efficiently incorporated for all the conditions tested (laminin incorporation >95%), and the developed hydrogels revealed mechanical properties (average storage modulus (G') ranging from 187 to 256 Pa) within the values preferred for NSC proliferation and neurite branching and extension. Affinity-bound laminin PEG-4MAL hydrogels better preserve laminin bioactivity, compared to unmodified hydrogels and hydrogels containing physically entrapped laminin, this effect being dependent on NtA concentration. This was evidenced by the 10 μM NtA-functionalized PEG-4MAL gels incorporating laminin that support enhanced human NSC proliferation and neurite extension, compared to the latter. Overall, this work highlights the potential of the proposed engineered matrices to be used as defined 3D platforms for the establishment of artificial NSC niches and as extracellular matrix-mimetic microenvironments to support human NSC transplantation.

Skeletal muscle as an experimental model of choice to study tissue aging and rejuvenation

Skeletal muscle is among the most age-sensitive tissues in mammal organisms. Significant changes in its resident stem cells (i.e., satellite cells, SCs), differentiated cells (i.e., myofibers), and extracellular matrix cause a decline in tissue homeostasis, function, and regenerative capacity. Based on the conservation of aging across tissues and taking advantage of the relatively well-characterization of the myofibers and associated SCs, skeletal muscle emerged as an experimental system to study the decline in function and maintenance of old tissues and to explore rejuvenation strategies. In this review, we summarize the approaches for understanding the aging process and for assaying the success of rejuvenation that use skeletal muscle as the experimental system of choice. We further discuss (and exemplify with studies of skeletal muscle) how conflicting results might be due to variations in the techniques of stem cell isolation, differences in the assays of functional rejuvenation, or deciding on the numbers of replicates and experimental cohorts.

Laminin-Inspired Cell-Instructive Microenvironments for Neural Stem Cells

Laminin is a heterotrimeric glycoprotein with a key role in the formation and maintenance of the basement membrane architecture and properties, as well as on the modulation of several biological functions, including cell adhesion, migration, differentiation and matrix-mediated signaling. In the central nervous system (CNS), laminin is differentially expressed during development and homeostasis, with an impact on the modulation of cell function and fate. Within neurogenic niches, laminin is one of the most important and well described extracellular matrix (ECM) proteins. Specifically, efforts have been made to understand laminin assembly, domain architecture, and interaction of its different bioactive domains with cell surface receptors, soluble signaling molecules, and ECM proteins, to gain insight into the role of this ECM protein and its receptors on the modulation of neurogenesis, both in homeostasis and during repair. This is also expected to provide a rational basis for the design of biomaterial-based matrices mirroring the biological properties of the basement membrane of neural stem cell niches, for application in neural tissue repair and cell transplantation. This review provides a general overview of laminin structure and domain architecture, as well as the main biological functions mediated by this heterotrimeric glycoprotein. The expression and distribution of laminin in the CNS and, more specifically, its role within adult neural stem cell niches is summarized. Additionally, a detailed overview on the use of full-length laminin and laminin derived peptide/recombinant laminin fragments for the development of hydrogels for mimicking the neurogenic niche microenvironment is given. Finally, the main challenges associated with the development of laminin-inspired hydrogels and the hurdles to overcome for these to progress from bench to bedside are discussed.

Featured characteristics and pivotal roles of satellite cells in skeletal muscle regeneration

Skeletal muscle, the essential organ for locomotion, as well as energy reservoir and expenditure, has robust regenerative capacity in response to mechanical stress and injury. As muscle-specific stem cells, satellite cells are responsible for providing new myoblasts during the process of muscle growth and regeneration. Self-renewal capacity and the fate of satellite cells are highly regulated and influenced by their surrounding factors, such as extracellular matrix and soluble proteins. The strong myogenic potential of satellite cells makes them a potential resource for stem cell therapy to cure genetic muscle disease and repair injured muscle. Here, we both review key features of satellite cells during skeletal muscle development and regeneration and summarize recent outcomes of satellite cell transplantation studies.

A strategy of tailoring polymorphs and nanostructures to construct self-reinforced nonswelling high-strength bacterial cellulose hydrogels

A serious decline in mechanical properties of polysaccharide hydrogels caused by swelling has always been a difficult problem which greatly limited their application especially in the medical field. Herein, nonswelling high-strength natural hydrogels based on self-reinforced double-crosslinked bacterial cellulose (SDBC) were prepared. Inspired by the concept of homogeneous composite materials, by regulating the ratio of LiOH/urea alkaline solvent, the aggregation structure and nanostructure of SDBC hydrogels can be controlled, thereby a unique nanofiber-network-self-reinforced (FNSR) structure was constructed and a new self-reinforcing mechanism is proposed. The prepared SDBC hydrogels have excellent mechanical properties at a high water content (>91%) for the combination of double-crosslinking and a unique FNSR structure, which can effectively prevent crack propagation and dissipate a large amount of energy. In particular, the compressive strength can reach 3.17 MPa which is 56 times that of native bacterial cellulose (BC). It is worth mentioning that no swelling occurs for the hydrogel, and the mechanical strength still remains in excess of 90% for 15 days in water, which is favorable for promising application in underwater equipment, implantable ionic devices, and tissue engineering scaffolds. This study also opens up a new horizon for the preparation of self-reinforced hydrogels.

Co-delivery of Wnt7a and muscle stem cells using synthetic bioadhesive hydrogel enhances murine muscle regeneration and cell migration during engraftment

Skeletal muscle possesses efficient ability to regenerate upon minor injuries, but its capacity to regenerate is severely compromised with traumatic injuries and muscle-associated diseases. Recent evidence suggests that skeletal muscle regeneration can be enhanced by transplantation of muscle satellite cells (MuSCs) or treatment with pro-myogenic factors, such as Wingless-type MMTV Integrated 7a (Wnt7a) protein. Although direct intramuscular injection is the simplest method to deliver MuSCs and Wnt7a for regenerative therapy, direct injections are not viable in many clinical cases where structural integrity is severely compromised. To address this challenge, we evaluated the feasibility of co-delivering pro-myogenic factors, such as Wnt7a, and MuSCs using a synthetic poly(ethylene glycol) (PEG)-based hydrogel to the affected skeletal muscles. The Wnt7a release rate can be controlled by modulating the polymer density of the hydrogel, and this release rate can be further accelerated through the proteolytic degradation of the hydrogel. Treating cryo-injured tibialis anterior (TA) muscles with Wnt7a-loaded hydrogels resulted in an improved regenerative response by day 14, measured by increased muscle fiber cross-sectional area, bulk TA mass, and the number of Pax7⁺ MuSCs at the injury site, compared to the TA muscles treated with Wnt7a-free hydrogels. Co-delivery of Wnt7a and primary MuSCs using the synthetic hydrogel to the cryo-injured TA muscles significantly increased cellular migration during the engraftment process. This work provides a synthetic biomaterial platform for advancing treatment strategies of skeletal muscle conditions where direct intramuscular injection may be challenging. Finally, the current outcomes establish an important foundation for future applications in treating severe muscle trauma and diseases, where the endogenous repair capacity is critically impaired. STATEMENT OF SIGNIFICANCE: Skeletal muscle injuries and diseases cause debilitating health consequences, including disability and diminished quality of life. Treatment using protein and stem cell-based therapeutics may help regenerate the affected muscles, but direct intramuscular injection may not be feasible in severe muscle injuries due to the gravely damaged tissue structure. In chronic muscle diseases, such as Duchenne muscular dystrophy, local treatment of the diaphragm, a muscle critical for respiration, may be necessary but direct injection is difficult due to its thin dimensions. To address this challenge, this work presents a synthetic and bioactive muscle "patch" that enables concurrent administration of proteins and muscle stem cells for accelerated muscle healing.

MBP-FGF2-Immobilized Matrix Maintains Self-Renewal and Myogenic Differentiation Potential of Skeletal Muscle Stem Cells

The robust capacity of skeletal muscle stem cells (SkMSCs, or satellite cells) to regenerate into new muscles in vivo has offered promising therapeutic options for the treatment of degenerative muscle diseases. However, the practical use of SkMSCs to treat muscle diseases is limited, owing to their inability to expand in vitro under defined cultivation conditions without loss of engraftment efficiency. To develop an optimal cultivation condition for SkMSCs, we investigated the behavior of SkMSCs on synthetic maltose-binding protein (MBP)-fibroblast growth factor 2 (FGF2)-immobilized matrix in vitro. We found that the chemically well-defined, xeno-free MBP-FGF2-immobilized matrix effectively supports SkMSC growth without reducing their differentiation potential in vitro. Our data highlights the possible application of the MBP-FGF2 matrix for SkMSC expansion in vitro.