MIF does only marginally enhance the pro-regenerative capacities of DFO in a mouse-osteotomy-model of compromised bone healing conditions

A Dose-Dependent Spatiotemporal Response of Angiogenesis Elicited by Zn Biodegradation during the Initial Stage of Bone Regeneration

Zinc (Zn) plays a crucial role in bone metabolism and imbues biodegradable Zn-based materials with the ability to promote bone regeneration in bone trauma. However, the impact of Zn biodegradation on bone repair, particularly its influence on angiogenesis, remains unexplored. This study reveals that Zn biodegradation induces a consistent dose-dependent spatiotemporal response in angiogenesis,both in vivo and in vitro. In a critical bone defect model, an increase in Zn release intensity from day 3 to 10 post-surgery is observed. By day 10, the CD31-positive area around the Zn implant significantly surpasses that of the Ti implant, indicating enhanced angiogenesis. Furthermore,angiogenesis exhibits a distance-dependent pattern closely mirroring the distribution of Zn signals from the implant. In vitro experiments demonstrate that Zn extraction fosters the proliferation and migration of human umbilical vein endothelial cells and upregulates the key genes associated with tube formation, such as HIF-1α and VEGF-A, peaking at a concentration of 22.5 µM. Additionally, Zn concentrations within the range of 11.25-45 µM promote the polarization of M0-type macrophages toward the M2-type, while inhibiting polarization toward the M1-type. These findings provide essential insights into the biological effects of Zn on bone repair, shedding light on its potential applications.

Complex Spatio-Temporal Interplay of Distinct Immune and Bone Cell Subsets during Bone Fracture Healing

BACKGROUND

The healing of a bone injury is a highly complex process involving a multitude of different tissue and cell types, including immune cells, which play a major role in the initiation and progression of bone regeneration.

METHODS

We histologically analyzed the spatio-temporal occurrence of cells of the innate immune system (macrophages), the adaptive immune system (B and T lymphocytes), and bone cells (osteoblasts and osteoclasts) in the fracture area of a femoral osteotomy over the healing time. This study was performed in a bone osteotomy gap mouse model. We also investigated two key challenges of successful bone regeneration: hypoxia and revascularization.

RESULTS

Macrophages were present in and around the fracture gap throughout the entire healing period. The switch from initially pro-inflammatory M1 macrophages to the anti-inflammatory M2 phenotype coincided with the revascularization as well as the appearance of osteoblasts in the fracture area. This indicates that M2 macrophages are necessary for the restoration of vessels and that they also play an orchestrating role in osteoblastogenesis during bone healing. The presence of adaptive immune cells throughout the healing process emphasizes their essential role for regenerative processes that exceeds a mere pathogen defense. B and T cells co-localize consistently with bone cells throughout the healing process, consolidating their crucial role in guiding bone formation. These histological data provide, for the first time, comprehensive information about the complex interrelationships of the cellular network during the entire bone healing process in one standardized set up. With this, an overall picture of the spatio-temporal interplay of cellular key players in a bone healing scenario has been created.

CONCLUSIONS

A spatio-temporal distribution of immune cells, bone cells, and factors driving bone healing at time points that are decisive for this process-especially during the initial steps of inflammation and revascularization, as well as the soft and hard callus phases-has been visualized. The results show that the bone healing cascade does not consist of five distinct, consecutive phases but is a rather complex interrelated and continuous process of events, especially at the onset of healing.

Oxidative stress as a key modulator of cell fate decision in osteoarthritis and osteoporosis: a narrative review

During aging and after traumatic injuries, cartilage and bone cells are exposed to various pathophysiologic mediators, including reactive oxygen species (ROS), damage-associated molecular patterns, and proinflammatory cytokines. This detrimental environment triggers cellular stress and subsequent dysfunction, which not only contributes to the development of associated diseases, that is, osteoporosis and osteoarthritis, but also impairs regenerative processes. To counter ROS-mediated stress and reduce the overall tissue damage, cells possess diverse defense mechanisms. However, cellular antioxidative capacities are limited and thus ROS accumulation can lead to aberrant cell fate decisions, which have adverse effects on cartilage and bone homeostasis. In this narrative review, we address oxidative stress as a major driver of pathophysiologic processes in cartilage and bone, including senescence, misdirected differentiation, cell death, mitochondrial dysfunction, and impaired mitophagy by illustrating the consequences on tissue homeostasis and regeneration. Moreover, we elaborate cellular defense mechanisms, with a particular focus on oxidative stress response and mitophagy, and briefly discuss respective therapeutic strategies to improve cell and tissue protection.

A buprenorphine depot formulation provides effective sustained post-surgical analgesia for 72 h in mouse femoral fracture models

Adequate pain management is essential for ethical and scientific reasons in animal experiments and should completely cover the period of expected pain without the need for frequent re-application. However, current depot formulations of Buprenorphine are only available in the USA and have limited duration of action. Recently, a new microparticulate Buprenorphine formulation (BUP-Depot) for sustained release has been developed as a potential future alternative to standard formulations available in Europe. Pharmacokinetics indicate a possible effectiveness for about 72 h. Here, we investigated whether the administration of the BUP-Depot ensures continuous and sufficient analgesia in two mouse fracture models (femoral osteotomy) and could, therefore, serve as a potent alternative to the application of Tramadol via the drinking water. Both protocols were examined for analgesic effectiveness, side effects on experimental readout, and effects on fracture healing outcomes in male and female C57BL/6N mice. The BUP-Depot provided effective analgesia for 72 h, comparable to the effectiveness of Tramadol in the drinking water. Fracture healing outcome was not different between analgesic regimes. The availability of a Buprenorphine depot formulation for rodents in Europe would be a beneficial addition for extended pain relief in mice, thereby increasing animal welfare.

The decisive early phase of bone regeneration

Bone has a remarkable endogenous regenerative capacity that enables scarless healing and restoration of its prior mechanical function, even under challenging conditions such as advanced age and metabolic or immunological degenerative diseases. However - despite much progress - a high number of bone injuries still heal with unsatisfactory outcomes. The mechanisms leading to impaired healing are heterogeneous, and involve exuberant and non-resolving immune reactions or overstrained mechanical conditions that affect the delicate regulation of the early initiation of scar-free healing. Every healing process begins phylogenetically with an inflammatory reaction, but its spatial and temporal intensity must be tightly controlled. Dysregulation of this inflammatory cascade directly affects the subsequent healing phases and hinders the healing progression. This Review discusses the complex processes underlying bone regeneration, focusing on the early healing phase and its highly dynamic environment, where vibrant changes in cellular and tissue composition alter the mechanical environment and thus affect the signalling pathways that orchestrate the healing process. Essential to scar-free healing is the interplay of various dynamic cascades that control timely resolution of local inflammation and tissue self-organization, while also providing sufficient local stability to initiate endogenous restoration. Various immunotherapy and mechanobiology-based therapy options are under investigation for promoting bone regeneration.

Scaffold Guided Bone Regeneration for the Treatment of Large Segmental Defects in Long Bones

Bone generally displays a high intrinsic capacity to regenerate. Nonetheless, large osseous defects sometimes fail to heal. The treatment of such large segmental defects still represents a considerable clinical challenge. The regeneration of large bone defects often proves difficult, since it relies on the formation of large amounts of bone within an environment impedimental to osteogenesis, characterized by soft tissue damage and hampered vascularization. Consequently, research efforts have concentrated on tissue engineering and regenerative medical strategies to resolve this multifaceted challenge. In this review, we summarize, critically evaluate, and discuss present approaches in light of their clinical relevance; we also present future advanced techniques for bone tissue engineering, outlining the steps to realize for their translation from bench to bedside. The discussion includes the physiology of bone healing, requirements and properties of natural and synthetic biomaterials for bone reconstruction, their use in conjunction with cellular components and suitable growth factors, and strategies to improve vascularization and the translation of these regenerative concepts to in vivo applications. We conclude that the ideal all-purpose material for scaffold-guided bone regeneration is currently not available. It seems that a variety of different solutions will be employed, according to the clinical treatment necessary.

HIF-1α Regulates Bone Homeostasis and Angiogenesis, Participating in the Occurrence of Bone Metabolic Diseases

In the physiological condition, the skeletal system's bone resorption and formation are in dynamic balance, called bone homeostasis. However, bone homeostasis is destroyed under pathological conditions, leading to the occurrence of bone metabolism diseases. The expression of hypoxia-inducible factor-1α (HIF-1α) is regulated by oxygen concentration. It affects energy metabolism, which plays a vital role in preventing bone metabolic diseases. This review focuses on the HIF-1α pathway and describes in detail the possible mechanism of its involvement in the regulation of bone homeostasis and angiogenesis, as well as the current experimental studies on the use of HIF-1α in the prevention of bone metabolic diseases. HIF-1α/RANKL/Notch1 pathway bidirectionally regulates the differentiation of macrophages into osteoclasts under different conditions. In addition, HIF-1α is also regulated by many factors, including hypoxia, cofactor activity, non-coding RNA, trace elements, etc. As a pivotal pathway for coupling angiogenesis and osteogenesis, HIF-1α has been widely studied in bone metabolic diseases such as bone defect, osteoporosis, osteonecrosis of the femoral head, fracture, and nonunion. The wide application of biomaterials in bone metabolism also provides a reasonable basis for the experimental study of HIF-1α in preventing bone metabolic diseases.

Coaxially Fabricated Dual-Drug Loading Electrospinning Fibrous Mat with Programmed Releasing Behavior to Boost Vascularized Bone Regeneration

In clinical treatment, the bone regeneration of critical-size defects is desiderated to be solved, and the regeneration of large bone segment defects depends on early vascularization. Therefore, overcoming insufficient vascularization in artificial bone grafts may be a promising strategy for critical-size bone regeneration. Herein, a novel dual-drug programmed releasing electrospinning fibrous mat (EFM) with a deferoxamine (DFO)-loaded shell layer and a dexamethasone (DEX)-loaded core layer is fabricated using coaxial electrospinning technology, considering the temporal sequence of vascularization and bone repair. DFO acts as an angiogenesis promoter and DEX is used as an osteogenesis inducer. The results demonstrate that the early and rapid release of DFO promotes angiogenesis in human umbilical vascular endothelial cells and the sustained release of DEX enhances the osteogenic differentiation of rat bone mesenchymal stem cells. DFO and DEX exert synergetic effects on osteogenic differentiation via the Wnt/β-catenin signaling pathway, and the dual-drug programmed releasing EFM acquired perfect vascularized bone regeneration ability in a rat calvarial defect model. Overall, the study suggests a low-cost strategy to enhance vascularized bone regeneration by adjusting the behavior of angiogenesis and osteogenesis in time dimension.