Effects of BMP-2 dose and delivery of microvascular fragments on healing of bone defects with concomitant volumetric muscle loss

Bioprinted dermis with human adipose tissue-derived microvascular fragments promotes wound healing

Tissue-engineered skin is an effective material for treating large skin defects in a clinical setting. However, its use is limited owing to vascular complications. Human adipose tissue-derived microvascular fragments (HaMVFs) are vascularized units that form vascular networks by rapid reassembly. In this study, we designed a vascularized bionic skin tissue using a three-dimensional (3D) bioprinter of HaMVFs and human fibroblasts encapsulated in a hybrid hydrogel composed of GelMA, HAMA, and fibrinogen. Tissues incorporating HaMVFs showed good in vitro vascularization and mechanical properties after UV crosslinking and thrombin exposure. Thus, the tissue could be sutured appropriately to the wound. In vivo, the vascularized 3D bioprinted skin promoted epidermal regeneration, collagen maturation in the dermal tissue, and vascularization of the skin tissue to accelerate wound healing. Overall, vascularized 3D bioprinted skin with HaMVFs is an effective material for treating skin defects and may be clinically applicable to reduce the necrosis rate of skin grafts.

Tibial bone strength is negatively affected by volumetric muscle loss injury to the adjacent muscle in male mice

This study's objective was to investigate how contractile strength loss associated with a volumetric muscle loss (VML) injury affects the adjacent tibial bone structural and functional properties in male C57BL/6J mice. Mice were randomized into one of two experimental groups: VML-injured mice that were injured at age 12 weeks and aged to 20 weeks (8 weeks postinjury, VML) and 20-week-old age-matched uninjured mice (Uninjured-20). Tibial bone strength, mid-diaphysis cortical geometry, intrinsic material properties, and metaphyseal trabecular bone structure were assessed by three-point bending and microcomputed tomography (µCT). The plantar flexor muscle group (gastrocnemius, soleus, plantaris) was analyzed for its functional capacities, that is, peak-isometric torque and peak-isokinetic power. VML-injured limbs had 25% less peak-isometric torque and 31% less peak-isokinetic power compared to those of Uninjured-20 mice (p < 0.001). Ultimate load, but not stiffness, was significantly less (10%) in tibias of VML-injured limbs compared to those from Uninjured-20 (p = 0.014). µCT analyses showed cortical bone thickness was 6% less in tibias of VML-injured limbs compared to Uninjured-20 (p = 0.001). Importantly, tibial bone cross-section moment of inertia, the primary determinant of bone ultimate load, was 16% smaller in bones of VML-injured limbs compared to bones from Uninjured-20 (p = 0.046). Metaphyseal trabecular bone structure was also altered up to 23% in tibias of VML-injured limbs (p < 0.010). These changes in tibial bone structure and function after a VML injury occur during a natural maturation phase between the age of 12 and 20 weeks, as evidenced by Uninjured-20 mice having greater tibial bone size and strength compared to uninjured-aged 12-week mice.

A New Biomaterial Derived from Aloe vera-Acemannan from Basic Studies to Clinical Application

Aloe vera is a kind of herb rich in polysaccharides. Acemannan (AC) is considered to be a natural polysaccharide with good biodegradability and biocompatibility extracted from Aloe vera and has a wide range of applications in the biomedical field due to excellent immunomodulatory, antiviral, antitumor, and tissue regeneration effects. In recent years, clinical case reports on the application of AC as a novel biomedical material in tissue regenerative medicine have emerged; it is mainly used in bone tissue engineering, pulp-dentin complex regeneration engineering, and soft tissue repair, among other operations. In addition, multiple studies have proved that the new composite products formed by the combination of AC and other compounds have excellent biological and physical properties and have broader research prospects. This paper introduces the preparation process, surface structure, and application forms of AC; summarizes the influence of acetyl functional group content in AC on its functions; and provides a detailed review of the functional properties, laboratory studies, clinical cutting-edge applications, and combined applications of AC. Finally, the current application status of AC from basic research to clinical treatment is analyzed and its prospects are discussed.

Encapsulated vaterite-calcite CaCO3 particles loaded with Mg2+ and Cu2+ ions with sustained release promoting osteogenesis and angiogenesis

Bioactive cations, including calcium, copper and magnesium, have shown the potential to become the alternative to protein growth factor-based therapeutics for bone healing. Ion substitutions are less costly, more stable, and more effective at low concentrations. Although they have been shown to be effective in providing bone grafts with more biological functions, the precise control of ion release kinetics is still a challenge. Moreover, the synergistic effect of three or more metal ions on bone regeneration has rarely been studied. In this study, vaterite-calcite CaCO3 particles were loaded with copper (Cu2+) and magnesium (Mg2+). The polyelectrolyte multilayer (PEM) was deposited on CaCuMg-CO3 particles via layer-by-layer technique to further improve the stability and biocompatibility of the particles and to enable controlled release of multiple metal ions. The PEM coated microcapsules were successfully combined with collagen at the outmost layer, providing a further stimulating microenvironment for bone regeneration. The in vitro release studies showed remarkably stable release of Cu2+ in 2 months without initial burst release. Mg2+ was released in relatively low concentration in the first 7 days. Cell culture studies showed that CaCuMg-PEM-Col microcapsules stimulated cell proliferation, extracellular maturation and mineralization more effectively than blank control and other microcapsules without collagen adsorption (Ca-PEM, CaCu-PEM, CaMg-PEM, CaCuMg-PEM). In addition, the CaCuMg-PEM-Col microcapsules showed positive effects on osteogenesis and angiogenesis in gene expression studies. The results indicate that such a functional and controllable delivery system of multiple bioactive ions might be a safer, simpler and more efficient alternative of protein growth factor-based therapeutics for bone regeneration. It also provides an effective method for functionalizing bone grafts for bone tissue engineering.

Biomimetic sponges improve functional muscle recovery following composite trauma

There is a dearth of therapies that are safe and effective for the treatment of volumetric muscle loss (VML), defined as the surgical or traumatic loss of muscle tissue, resulting in functional impairment. To address this gap in orthopedic care, we developed a porous sponge-like scaffold composed of extracellular matrix (ECM) proteins (e.g., gelatin, collagen, and laminin-111) and an immunosuppressant drug, FK-506. While the majority of VML injuries occur in orthopedic trauma cases, preclinical models typically study muscle injuries in isolation without a concomitant bone fracture. The goal of this study was to investigate the extent to which FK506 loaded biomimetic sponges support functional muscle regeneration and fracture healing in a composite trauma model involving VML injury to the tibialis anterior muscle and osteotomy (OST) to the tibia. In this model, implantation of the FK-506 loaded biomimetic sponges limited the extent of inflammation while increasing the total number of myofibers, mean myofiber cross-sectional area, myosin-to-collagen ratio, and peak isometric torque compared to untreated VML+OST muscles on Day 28. Although all tibia fractures were bridged by Day 28 post-injury, fracture healing was impaired in response to an adjacent VML injury. Sponge treatment increased bone callus volume, yet the bridged mineralized bone volume was not significantly different. Taken together, these results suggest that biomimetic sponges primarily benefitted muscle repair and may provide a promising therapy for traumatized muscle.

The vascularization paradox of non-union formation

Despite major research efforts to elucidate mechanisms of non-union formation, failed fracture healing remains a common complication in orthopedic surgery. Adequate vascularization has been recognized as a crucial factor for successful bone regeneration, as newly formed microvessels guarantee the supply of the callus tissue with vital oxygen, nutrients, and growth factors. Accordingly, a vast number of preclinical studies have focused on the development of vascularization strategies to stimulate fracture repair. However, recent evidence suggests that stimulation of blood vessel formation is an oversimplified approach to support bone regeneration. This review discusses the role of vascularization during bone regeneration and delineates a phenomenon, for which we coin the term “the vascularization paradox of non-union-formation”. This view is based on the results of a variety of experimental studies that suggest that the callus tissue of non-unions is indeed densely vascularized and that pro-angiogenic mediators, such as vascular endothelial growth factor, are sufficiently expressed at the facture site. By gaining further insights into the molecular and cellular basis of non-union vascularization, it may be possible to develop more optimized treatment approaches or even prevent the non-union formation in the future.

Microvascular fragments in microcirculation research and regenerative medicine

Adipose tissue-derived microvascular fragments (MVF) are functional vessel segments, which rapidly reassemble into new microvasculatures under experimental in vitro and in vivo conditions. Accordingly, they have been used for many years in microcirculation research to study basic mechanisms of endothelial cell function, angiogenesis and microvascular network formation in two- and three-dimensional environments. Moreover, they serve as vascularization units for musculoskeletal regeneration and implanted biomaterials as well as for the treatment of myocardial infarction and the generation of prevascularized tissue organoids. Besides, multiple factors determining the vascularization capacity of MVF have been identified, including their tissue origin and cellular composition, the conditions for their short- and long-term storage as well as their implantation site and the general health status and medication of the recipient. The next challenging step is now the successful translation of all these promising experimental findings into clinical practice. If this succeeds, a multitude of future therapeutic applications may significantly benefit from the remarkable properties of MVF.

Triple growth factor delivery promotes functional bone regeneration following composite musculoskeletal trauma

Successful bone healing in severe trauma depends on early revascularization to restore oxygen, nutrient, growth factor, and progenitor cell supply to the injury. Therapeutic angiogenesis strategies have therefore been investigated to promote revascularization following severe bone injuries; however, results have been inconsistent. This is the first study investigating the effects of dual angiogenic growth factors (VEGF and PDGF) with low-dose bone morphogenetic protein-2 (BMP-2; 2.5 µg) on bone healing in a clinically challenging composite bone-muscle injury model. Our hydrogel-based delivery systems demonstrated a more than 90% protein entrapment efficiency and a controlled simultaneous release of three growth factors over 28 days. Co-stimulation of microvascular fragment constructs with VEGF and PDGF promoted vascular network formation in vitro compared to VEGF or PDGF alone. In an in vivo model of segmental bone and volumetric muscle loss injury, combined VEGF (5 µg) and PDGF (7.5 µg or 15 µg) delivery with a low dose of BMP-2 significantly enhanced regeneration of vascularized bone compared to BMP-2 treatment alone. Notably, the regenerated bone mechanics reached ~60% of intact bone, a value that was previously only achieved by delivery of high-dose BMP-2 (10 µg) in this injury model. Overall, sustained delivery of VEGF, PDFG, and BMP-2 is a promising strategy to promote functional vascularized bone tissue regeneration following severe composite musculoskeletal injury. Although this study is conducted in a clinically relevant composite injury model in rats using a simultaneous release strategy, future studies are necessary to test the regenerative potential of spatiotemporally controlled delivery of triple growth factors on bone healing using large animal models. STATEMENT OF SIGNIFICANCE: Volumetric muscle loss combined with delayed union or non-union bone defect causes deleterious effects on bone regeneration even with the supplementation of bone morphogenetic protein-2 (BMP-2). In this study, the controlled delivery of dual angiogenic growth factors (vascular endothelial growth factor [VEGF] + Platelet-derived growth factor [PDGF]) increases vascular growth in vitro. Co-delivering VEGF+PDGF significantly increase the bone formation efficacy of low-dose BMP-2 and improves the mechanics of regenerated bone in a challenging composite bone-muscle injury model.

Gait biomechanics: A clinically relevant outcome measure for preclinical research of musculoskeletal trauma

Traumatic injuries to the musculoskeletal system are the most prevalent of those suffered by United States Military Service members and accounts for two-thirds of initial hospital costs to the Department of Defense. These combat-related wounds often leave survivors with life-long disability and represent a significant impediment to the readiness of the fighting force. There are immense opportunities for the field of tissue engineering and regenerative medicine (TE/RM) to address these musculoskeletal injuries through regeneration of damaged tissues as a means to restore limb functionality and improve quality of life for affected individuals. Indeed, investigators have made promising advancements in the treatment for these injuries by utilizing small and large preclinical animal models to validate therapeutic efficacy of next-generation TE/RM-based technologies. Importantly, utilization of a comprehensive suite of functional outcome measures, particularly those designed to mimic data collected within the clinical setting, is critical for successful translation and implementation of these therapeutics. To that end, the objective of this review is to emphasize the clinical relevance and application of gait biomechanics as a functional outcome measure for preclinical research studies evaluating the efficacy of TE/RM therapies to treat traumatic musculoskeletal injuries. Specifically, common musculoskeletal injuries sustained by service members-including volumetric muscle loss, post-traumatic osteoarthritis, and composite tissue injuries-are examined as case examples to highlight the use of gait biomechanics as an outcome measure using small and large preclinical animal models.

Bone morphogenetic protein 2: heterologous expression and potential in bone regeneration

Currently, bone morphogenetic protein 2 (BMP-2) is one of the two osteoinductive growth factors used in medical devices to promote bone formation. Typically, this protein is bought from commercial houses at high rates and in small quantities that are not enough to cover clinical needs. Because of this, it has been proposed that research centers use their own heterologous expression systems to have a constant supply of BMP-2. The aim of this study was to standardize the heterologous expression of BMP-2 and evaluate its osteoinductive activity in vitro. Our procedure for expression and purification was based on recombinant DNA technology using the plasmid pET-28 and IPTG as inductor. After extracting the protein from inclusion bodies, folding it and modifying it via a redox system, we observed via electrophoresis a 26 kDa dimer. We evaluated its osteoinductive activity in myoblastic C2C12 by quantifying enzymatically the activity of alkaline phosphate (ALP) and staining mineralization nodules. ALP activity is proportional to BMP-2 concentration, increasing 90% at 3 µg/mL. These cells form calcium nodules, mineralizing 50% of the area.

BMP-2 delivery strategy modulates local bone regeneration and systemic immune responses to complex extremity trauma

Bone nonunions arising from large bone defects and composite injuries remain compelling challenges for orthopedic surgeons. Biological changes associated with nonunions, such as systemic immune dysregulation, can contribute to an adverse healing environment. Bone morphogenetic protein 2 (BMP-2), an osteoinductive and potentially immunomodulatory growth factor, is a promising strategy; however, burst release from the clinical standard collagen sponge delivery vehicle can result in adverse side effects such as heterotopic ossification (HO) and irregular bone structure, especially when using supraphysiological BMP-2 doses for complex injuries at high risk for nonunion. To address this challenge, biomaterials that strongly bind BMP-2, such as heparin methacrylamide microparticles (HMPs), may be used to limit exposure and spatially constrain proteins within the injury site. Here, we investigate moderately high dose BMP-2 delivered in HMPs within an injectable hydrogel system in two challenging nonunion models exhibiting characteristics of systemic immune dysregulation. The HMP delivery system increased total bone volume and decreased peak HO compared to collagen sponge delivery of the same BMP-2 dose. Multivariate analyses of systemic immune markers showed the collagen sponge group correlated with markers that are hallmarks of systemic immune dysregulation, including immunosuppressive myeloid-derived suppressor cells, whereas the HMP groups were associated with immune effector cells, including T cells, and cytokines linked to robust bone regeneration. Overall, our results demonstrate that HMP delivery of moderately high doses of BMP-2 promotes repair of complex bone nonunion injuries and that local delivery strategies for potent growth factors like BMP-2 may positively affect the systemic immune response to traumatic injury.

Engineered Collagen Matrices

Collagen is the most abundant protein in mammals, accounting for approximately one-third of the total protein in the human body. Thus, it is a logical choice for the creation of biomimetic environments, and there is a long history of using collagen matrices for various tissue engineering applications. However, from a biomaterial perspective, the use of collagen-only scaffolds is associated with many challenges. Namely, the mechanical properties of collagen matrices can be difficult to tune across a wide range of values, and collagen itself is not highly amenable to direct chemical modification without affecting its architecture or bioactivity. Thus, many approaches have been pursued to design scaffold environments that display critical features of collagen but enable improved tunability of physical and biological characteristics. This paper provides a brief overview of approaches that have been employed to create such engineered collagen matrices. Specifically, these approaches include blending of collagen with other natural or synthetic polymers, chemical modifications of denatured collagen, de novo creation of collagen-mimetic chains, and reductionist methods to incorporate collagen moieties into other materials. These advancements in the creation of tunable, engineered collagen matrices will continue to enable the interrogation of novel and increasingly complex biological questions.

Effects of controlled dual growth factor delivery on bone regeneration following composite bone-muscle injury

The objective of this study was to investigate the controlled release of two growth factors (BMP-2 and VEGF) as a treatment strategy for bone healing in clinically challenging composite injuries, consisting of a femoral segmental bone defect and volumetric muscle loss. This is the first investigation of dual growth factor delivery in a composite injury model using an injectable delivery system consisting of heparin microparticles and alginate gel. The loading efficiency of growth factors into these biomaterials was found to be >90%, revealing a strong affinity of VEGF and BMP-2 to heparin and alginate. The system could achieve simultaneous or tunable release of VEGF and BMP-2 by varying the loading strategy. Single growth factor delivery (VEGF or BMP-2 alone) significantly enhanced vascular growth in vitro. However, no synergistic effect was observed for dual growth factor (BMP-2 + VEGF) delivery in vitro. Effective bone healing was achieved in all treatment groups (BMP-2, simultaneous or tunable delivery of BMP-2 and VEGF) in the composite injury model. The mechanics of the regenerated bone reached a maximum strength of ~52% of intact bone with tunable delivery of VEGF and BMP-2. Overall, simultaneous or tunable co-delivery of low-dose BMP-2 and VEGF failed to fully restore the mechanics of bone in this injury model. Given the severity of the composite injury, VEGF alone may not be sufficient to establish mature and stable blood vessels when compared with previous studies co-delivering BMP-2+VEGF enhanced bone tissue regeneration. Hence, future studies are warranted to develop an alternative treatment strategy focusing on better control over growth factor dose, spatiotemporal delivery, and additional growth factors to regenerate fully functional bone tissue. STATEMENT OF SIGNIFICANCE: We have developed an injectable delivery system consisting of heparin microparticles and an alginate hydrogel that is capable of delivering multiple growth factors in a tunable manner. We used this delivery system to deliver BMP-2 and VEGF in a rodent model of composite bone-muscle injury that mimics clinical type III open fractures. An advanced treatment strategy is necessary for these injuries in order to avoid the negative side effects of high doses of growth factors and because it has been shown that the addition of a muscle injury in this model attenuates the bone regenerative effect of BMP-2. This is the first study to test the effects of dual growth factor delivery (BMP-2/VEGF) on bone healing in a composite bone-muscle injury model and is expected to open up new directions in protein delivery for regenerative medicine.

Exosomal miRNA-128-3p from mesenchymal stem cells of aged rats regulates osteogenesis and bone fracture healing by targeting Smad5

Transplantation of mesenchymal stem cells (MSCs) has been considered an effective therapeutic treatment for a variety of diseases including bone fracture. However, there are associated complications along with MSCs transplantation. There is evidence to show that exosomes (Exos) derived from MSCs exert a similar paracrine function. In addition, repair capabilities of MSCs decline with age. Hence, this study aims to confirm whether the Exos protective function on osteogenic differentiation and fracture healing from aged MSCs was attenuated. This information was used in order to investigate the underlying mechanism. MSCs were successfully isolated and identified from young and aged rats, and Exos were then obtained. Aged-Exos exhibited significantly attenuated effects on MSCs osteogenic differentiation in vitro and facture healing in vivo. Using miRNA array analysis, it was shown that miR-128-3p was markedly upregulated in Aged-Exos. In vitro experiments confirmed that Smad5 is a direct downstream target of miR-128-3p, and was inhibited by overexpressed miR-128-3p. A series gain- and loss- function experiment indicated that miR-128-3P serves a suppressor role in the process of fracture healing. Furthermore, effects caused by miR-128-3P mimic/inhibitor were reversed by the application of Smad5/siSmad5. Taken together, these results suggest that the therapeutic effects of MSCs-derived Exos may vary according to differential expression of miRNAs. Exosomal miR-128-3P antagomir may act as a promising therapeutic strategy for bone fracture healing, especially for the elderly.

Involvement of Bone Marrow Multipotent Stromal Cells in the Processes Presumably Provoking Vascular Calcification

During serial transplantation of bone marrow derived from young and aged donor CBA mice to 5-month-old recipients, the counts of multipotent stromal cells (MSC) in transplants from young donors assessed at each passage surpassed those of aged donors by 3.2, 7.8, 3.0, and 2.2 times attesting to the age-related decrease of active pool of bone marrow MSC. The medullary curettage in mouse femur increased the total number of MSC and the number of osteogenic MSC both in the contralateral femur and in the bone marrow transplants attesting to spread of the effects of osteogenic factors after bone injury onto the bone tissue of the body even if this tissue if not topographically related to the skeleton. Combined and simultaneous administration of antigenic complex of S. typhimurium (or LPS) with BMP-2 markedly increased the count of osteogenic medullary MSC by 3.6 or 4.6 times in comparison with intact control or by 2.1 and 2.7 times in comparison with administration of BMP-2 alone, which probably resulted from enlargement of the pool of osteogenesis-inducible MSC due to inflammation. Addition of BMP-2 to the culture of splenic stromal cells where osteogenesis does not occur under normal conditions provoked appearance of MSC colonies with alkaline phosphatase activity attesting to involvement of inducible osteogenic MSC in vascular calcification. It can be hypothesized that the reaction to the age-related changes in the bone tissue and osteoporosis is similar to the reaction to bone marrow injury and includes initiation of systemic inflammation and elevation of blood BMP-2, both of which are prerequisite for vascular calcification.