Mitochondrial regulation of stem cell osteogenic differentiation: A key driver for bone regeneration

Abstract

Mesenchymal stem cells (MSCs) are multipotent stromal cells that serve as progenitors for connective tissue and have emerged as a crucial resource in the field of tissue engineering owing to their capacity to differentiate into multiple cell lineages. MSCs-based bone regeneration strategies hold immense therapeutic potential, yet their efficacy is critically limited by inefficient osteogenic differentiation. Mounting evidence positions mitochondria as central regulators of this process, extending beyond their traditional role as cellular powerhouses. Mitochondrial regulation not only influences the induction rate of MSCs differentiation, but also determines the differentiation pathway and the ultimate fate of the resulting cells. To date, research in bone regeneration engineering has predominantly focused on the application of stem cell-based biomaterials, with limited attention given to mitochondrial development. We aim to provide a novel research perspective for targeted mitochondrial interventions in bone regeneration engineering by elucidating the mechanisms through which mitochondria regulate osteogenic differentiation of MSCs.

Key Words: Mesenchymal stem cells; Mitochondrial; Bone regeneration; Oxidative stress; Mitophagy

Core Tip: Mesenchymal stem cells (MSCs) hold promise for bone regeneration, but their potential is limited by poor osteogenic differentiation. Mitochondria play a pivotal role in bone defect repair, where their functions in energy metabolism, oxidative stress, dynamics, and mitophagy significantly regulate the osteogenic differentiation of MSCs. This work elucidated the mechanisms and therapeutic strategies of mitochondrial-targeted interventions to enhance MSC osteogenic differentiation, providing novel perspectives for developing mitochondria-focused bone regenerative medicine.

INTRODUCTION

The skeletal system serves as the structural foundation for the human body, supporting posture and facilitating movement. It also plays a protective role for vital organs and is actively involved in essential physiological processes, including hematopoiesis, endocrine regulation, and immune responses[1,2]. However, severe trauma, infections, tumors, and metabolic disorders frequently result in refractory bone defects, presenting a significant challenge to achieving optimal bone regeneration[3,4]. Mesenchymal stem cells (MSCs) are a type of multipotent adult stem cells that exhibit extensive self-renewal capacity and the ability to differentiate into multiple cell lineages[5,6]. In recent years, MSCs have been demonstrated to exhibit significant capabilities, including osteogenic differentiation, modulation of the immune microenvironment, and facilitation of bone tissue repair following injury. Consequently, MSCs have shown promising therapeutic potential in the field of bone regenerative medicine[7,8]. At present, there are still some intractable problems in the targeted regulation of MSCs, such as low specific differentiation efficiency, the lack of delivery systems and side effects[9].

To develop effective stem cell-based therapies for bone regeneration, it is crucial to gain a comprehensive understanding of the physiological mechanisms underlying bone metabolism and remodeling. The regenerative capacity of bone tissue is dependent on the dynamic balance between osteoblast-mediated bone formation and osteoclast-mediated bone resorption[10,11]. The maintenance of this bone balance depends on the coordinated regulation of multiple cellular functions and molecular mechanisms. Accumulating evidence suggests that mitochondria play a critical role in modulating the osteogenic differentiation of MSCs[12,13]. Mitochondria are double-membrane-bound organelles that not only produce adenosine triphosphate (ATP) through oxidative phosphorylation (OxPhos) to supply energy for various cellular physiological activities, but also play a regulatory role in the osteogenic differentiation of MSCs by modulating oxidative stress, inflammatory responses and autophagy[14,15]. Some preliminary animal experiments and clinical findings have indicated that targeted mitochondrial interventions can be broadly applied in the treatment of bone defects and metabolic bone diseases[16,17]. Innovative pharmaceuticals, biomaterials, and delivery systems have all exhibited substantial developmental potential[13,17].

This study aims to discuss the potential molecular mechanisms of mitochondrial involvement in MSCs-mediated bone repair, and to elaborate on the latest research progress in targeting and intervening in mitochondria to achieve bone regeneration, thereby providing valuable ideas for the development of innovative bone regeneration therapies.

THE MECHANISMS AND STRATEGIES FOR TARGETING MITOCHONDRIA IN BONE REGENERATION

Mitochondria serve as critical regulatory hubs that modulate cell proliferation and differentiation. Through OxPhos, mitochondria produce substantial amounts of ATP, which not only supplies the essential energy required for the osteogenic differentiation of MSCs, but also actively participates in the regulation of complex osteogenesis-related signaling pathways. The mechanisms through which mitochondria regulate the osteogenic differentiation of MSCs and modulate the immune microenvironment are primarily associated with mitochondrial energy metabolism, oxidative stress, mitochondrial dynamics, and mitophagy (Figure 1).

Figure 1 Mitochondria play a crucial role in regulating the osteogenic differentiation of mesenchymal stem cells through various mechanisms, including energy metabolism, oxidative stress regulation, mitochondrial dynamics, and autophagy. ATP: Adenosine triphosphate; OxPhos: Oxidative phosphorylation; TCA: Tricarboxylic acid; NF-κB: Nuclear factor kappa B; ROS: Reactive oxygen species; PI3K: Phosphoinositide 3-kinase; Akt: Protein kinase B; Wnt: Wingless/integrase; MSC: Mesenchymal stem cell; ER: Endoplasmic reticulum; PINK1: PTEN-induced kinase 1.

Mitochondrial energy metabolism

Cellular energy metabolism consists of glycolysis, which occurs in the cytoplasm, and OxPhos, which takes place in the mitochondria. Previous study have indicated that during osteogenic differentiation of MSCs, a metabolic shift occurs[18]. Specifically, osteoprogenitor cells predominantly rely on glycolysis for energy production. However, upon initiation of osteogenic differentiation, OxPhos becomes activated. Therefore, mitochondrial OxPhos plays a key role in maintaining osteogenic function. Shum et al[19] found that the expression level of hypoxia-inducible factor 1 is down-regulated during osteogenic induction of MSCs, and this reduction in hypoxia-inducible factor 1 activity directly promotes the activation of OxPhos. Subsequently, Smith and Eliseev[20] reported that bone morphogenetic protein 2- and wingless/integrase 3a-induced osteogenic differentiation relies on the activation of protein kinase B-mediated mitochondrial OxPhos.

The regulatory mechanisms underlying energy metabolism are highly complex, making effective modulation of mitochondrial energy metabolism a significant challenge. Nevertheless, several promising advancements have been made in this field to date. Liu et al[21] developed a wireless piezoelectric hydrogel capable of generating effective electrical signals in response to mechanical stress. This system can induce the osteogenic differentiation of inflamed periodontal ligament stem cells by modulating cellular energy metabolism and enhancing ATP synthesis[21]. Wang et al[22] demonstrated that amorphous calcium zinc phosphate effectively enhances macrophage mitochondrial OxPhos by inhibiting the coupling between the endoplasmic reticulum and mitochondria, thereby promoting optimal bone tissue repair. Another study revealed that a sponge-like scaffold incorporating melatonin could markedly enhance mitochondrial energy metabolism in MSCs, promote ATP production, and thereby significantly improve vascularized bone regeneration in animal models of bone defects[23].

Mitochondrial oxidative stress

Oxidative stress arises when the equilibrium between oxidation and antioxidation within the body is disturbed, resulting in the excessive accumulation of reactive oxygen species (ROS). These ROS promote osteoclast expression while simultaneously inhibiting the runt-related transcription factor 2 and wingless/integrase-1 signaling pathways, thereby impairing osteogenic activity[24]. Mitochondria are the primary organelles responsible for maintaining redox homeostasis in the body. Preserving stable mitochondrial function is crucial for alleviating oxidative stress and supporting effective osteogenesis. Li et al[25] identified that the mitochondrial calcium uniporter is important in regulating mitochondrial calcium transport. Overexpression of mitochondrial calcium uniporter promotes the generation of ROS, which subsequently suppresses the activation of the bone morphogenetic protein/Smad signaling pathway, thereby contributing to osteogenic dysfunction[25].

Maintaining mitochondrial antioxidant capacity and effectively eliminating excessive ROS are critical processes in promoting bone repair. Lao et al[26] integrated metformin-loaded zeolitic imidazolate framework-8 nanoparticles with methacrylated gelatin hydrogels to develop a multifunctional composite system. This system demonstrated the ability to repair mitochondrial dysfunction, scavenge excessive ROS, and consequently promote the regeneration of diabetic-induced bone defects[26]. Another study has found that the delivery of berberine via nanocarriers can effectively ameliorate mitochondrial dysfunction, suppress excessive ROS production, and consequently enhance alveolar bone regeneration in diabetic conditions[27]. Interestingly, Xu et al[28] reported that copper-containing alloys can significantly enhance vascularized bone formation by inducing macrophage polarization toward the M2 phenotype. This immunomodulatory effect was found to be dependent on the elevated levels of mitochondrial ROS generated in response to copper-containing alloy stimulation. The aforementioned findings suggest that precise regulation of mitochondrial ROS levels represents a promising and important direction for future research in the field of bone regeneration medicine.

Mitochondrial dynamics

Mitochondrial dynamics refer to the continuous process by which mitochondria undergo morphological, quantitative, and positional changes during physiological activities. Dysregulation of mitochondrial dynamics can lead to various pathological conditions, and maintaining the balance of these dynamic processes is crucial for determining cell fate. Previous study has demonstrated that the dynamic behaviors of mitochondria, including fusion and fission, play a key role in the proliferation and differentiation of MSCs[29,30]. For example, mitochondrial mitofusin-1/2 has been identified as an important regulator in the osteogenic differentiation of MSCs[29]. Another study has demonstrated that mitochondrial dynamin-related protein 1 (DRP1) is crucial in the osteoclast differentiation of MSCs via the RANKL signaling pathway[30].

Investigating the regulation of mitochondrial dynamics holds significant potential for enhancing the osteogenic differentiation of MSCs. An et al[31] developed a type of human periodontal ligament stem cell sheets activated by graphene oxide quantum dots and confirmed that these sheets could enhance osteogenic differentiation through the regulation of mitochondrial dynamics, specifically by promoting mitochondrial fusion and inhibiting fission. Another study revealed that a bone implant complex incorporating a dual-nutrient-element coating composed of ZnO and Sr(OH)₂, combined with polyetheretherketone, effectively downregulated DRP1 gene expression, restored mitochondrial dynamic balance, and significantly enhanced osteogenic activity[32]. Similarly, Ma et al[33] demonstrated that silicon can enhance mitochondrial functional transfer through its interaction with DRP1 and Fis1, thereby facilitating angiogenesis and bone regeneration (Figure 2A). These studies suggest that modulating signaling pathways associated with mitochondrial dynamics provides novel therapeutic strategies for addressing bone defects, and metal ions demonstrate significant potential for therapeutic application. Greater emphasis should be placed on metal ions in the future.

Figure 2 The strategies for targeting mitochondria in bone regeneration. A: Targeting mitochondrial dynamics, particularly by enhancing functional mitochondrial transfer, can significantly promote bone regeneration potential. For instance, silicified collagen scaffolds significantly improve bone repair outcomes in this way; B: Modulating the level of mitophagy can effectively enhance the osteogenic differentiation capacity of mesenchymal stem cells. For example, WW domain-containing coiled-coil adaptor protein promotes mesenchymal stem cell osteogenesis by activating mitophagy through protecting the key mitophagy initiator. Citation for Figure 2A: Ma YX, Lei C, Ye T, Wan QQ, Wang KY, Zhu YN, Li L, Liu XF, Niu LZ, Tay FR, Mu Z, Jiao K, Niu LN. Silicon Enhances Functional Mitochondrial Transfer to Improve Neurovascularization in Diabetic Bone Regeneration. Adv Sci (Weinh) 2025; 12: E2415459. Copyright© The Authors 2025. Published by Wiley-VCH GmbH. The article is open access (Supplementary material). Citation for Figure 2B: Fan S, Li J, Zheng G, Ma Z, Peng X, Xie Z, Liu W, Yu W, Lin J, Su Z, Xu P, Wang P, Wu Y, Shen H, Ye G. WAC Facilitates Mitophagy-mediated MSC Osteogenesis and New Bone Formation via Protecting PINK1 from Ubiquitination-Dependent Degradation. Adv Sci (Weinh) 2025; 12: E2404107. Copyright© The Authors 2024. Published by Wiley-VCH GmbH. The article is open access (Supplementary material).

Mitophagy

Mitophagy is an important cellular pathway responsible for maintaining homeostasis, primarily involving the selective degradation and recycling of damaged mitochondria by autophagic mechanisms[34]. When mitophagy is excessively activated, resulting in a decrease in mitochondrial quantity, or when mitophagy is impaired, leading to the accumulation of dysfunctional mitochondria, mitochondrial function becomes compromised, which can trigger a range of pathological conditions, including disturbances in bone metabolism. Yang et al[35] showed that protein tyrosine phosphatase 1B was markedly upregulated in senescent MSCs, which contributed to reduced mitophagy and impaired osteogenic potential. Silencing protein tyrosine phosphatase 1B led to activation of the AMP-activated protein kinase signaling pathway, thereby enhancing mitophagy and alleviating MSC senescence[35]. Another study revealed that PTEN-induced kinase 1 (PINK1) is important in the regulation of mitophagy. Downregulation of PINK1 markedly compromises mitophagy function and inhibits the osteogenic differentiation of MSCs[36]. The investigation into the mechanisms of mitophagy has great clinical relevance for the development of novel therapeutic strategies aimed at enhancing bone repair.

Based on the aforementioned mechanism, Fan et al[37] indicated that the WW domain-containing coiled-coil adaptor protein is significantly associated with the regulation of mitophagy levels. WW domain-containing coiled-coil adaptor protein enhances the osteogenic differentiation of MSCs by activating mitophagy, a process through which it protects PINK1, a key initiator of mitophagy, from ubiquitin-mediated degradation[37] (Figure 2B). Li et al[38] developed a honeycomb bionic graphene oxide quantum dot/Layered double hydroxide composite nanocoating and demonstrated that it enhances the local bone regeneration environment by activating mitophagy through up-regulation of the BNIP3 signaling pathway. Similarly, another study found that graphene oxide quantum dots/methacrylated gelatin can activate mitophagy through the stimulation of the PINK1 pathway, thereby enhancing osteogenic differentiation[39]. In conclusion, the development of biomaterials capable of modulating mitophagy-related pathways, such as the PINK1-mediated regulation of mitophagy levels, represents a promising therapeutic strategy for the repair of bone defects.

LIMITATIONS AND FUTURE PERSPECTIVES

Targeted mitochondrial therapy represents a promising and strategic avenue of exploration in the field of bone regeneration engineering. However, challenges remain, including limited material stability and low drug loading efficiency when mitochondria are used as therapeutic targets. On the other hand, most studies on mitochondrial regulation to promote osteogenesis have been primarily confined to disease models, with a notable absence of clinical trials involving actual patients. Future research should focus on further elucidating the underlying mitochondrial-mediated osteogenic mechanisms in MSCs, thereby facilitating the development of more precise and effective mitochondrial-targeted therapeutic strategies.

CONCLUSION

Bone regeneration therapy based on MSCs holds significant clinical value. Specifically, mitochondria play a crucial role in the repair of bone defects, with their functions in energy metabolism, oxidative stress, mitochondrial dynamics, and mitophagy profoundly influencing the osteogenic differentiation of MSCs. Recent research has made remarkable progress in targeting mitochondrial interventions to enhance MSC-mediated osteogenesis. Molecular mechanism-based biomaterials and drug delivery systems have demonstrated considerable therapeutic potential. Therefore, the development of mitochondrial-targeted therapies for bone defect repair represents a significant area of frontier research.