Brain regulates weight bearing bone through PGE2 skeletal interoception: implication of ankle osteoarthritis and pain

Bone mechanical loading reduces heart rate and increases heart rate variability in mice

Cardiovascular disease and osteoporosis are clinically associated. Bone adapts to mechanical forces by altering its overall structure and mass. In response to mechanical strain bone cells release signaling molecules and activate the nervous system. Bone also exhibits endocrine functions that modulate a number of tissues including the heart. We hypothesized that bone mechanical loading acutely alters cardiac function via neural and/or endocrine mechanisms. To test this hypothesis, we performed in vivo tibia mechanical loading in anesthetized mice while monitoring heart parameters using electrocardiogram (ECG). An immediate, transient reduction in resting heart rate was observed during tibial loading in both adult male and female mice (p < 0.01) with concurrent increases in heart rate variability (HRV) (p < 0.01). ECG intervals, PR, QRS and QTc were unaffected with loading. In further studies, we found that at least 3 N of load was necessary to elicit this heart response in adult mice. With aging to 11-12 months the responsiveness of the heart to loading was blunted, suggesting this bone-heart connection may weaken with age. Administration of lidocaine around the tibia significantly diminished the heart rate response to bone loading (p < 0.05). Moreover, pre-treatment with sympathetic antagonist propranolol inhibited this heart rate response to loading (p < 0.05), while parasympathetic antagonist atropine did not (p > 0.05). This suggests that a neuronal afferent pathway in the hindlimb and reduction in efferent sympathetic tone mediate this bone-neuro-heart reflex. In conclusion, the findings that tibia bone loading age-dependently modulates heart function support the concept of physiological coupling of the skeletal and cardiovascular systems.

Interoception and aging

Interoception refers to the body's perception and regulation of internal physiological states and involves complex neural mechanisms and sensory systems. The current definition of interoception falls short of capturing the breadth of related research; here, we propose an updated definition. Homeostasis, a foundational principle of integrated physiology, is the process by which organisms dynamically maintain optimal balance across all conditions through neural, endocrine, and behavioral functions. This review examines the role of interoception in body homeostasis. Aging is a complex process influenced by multiple factors and involving multiple levels, including physical, psychological, and cognitive. However, interoceptive and aging interoceptive interactions are lacking. A new perspective on interoception and aging holds significant implications for understanding how aging regulates interoception and how interoception affects the aging process. Finally, we summarize that arachidonic acid metabolites show promise as biomarkers of interoception-aging. The aim of this study is to comprehensively analyze interoceptive-aging interactions, understand the aging mechanism from a novel perspective, and provide a theoretical basis for exploring anti-aging strategies.

Cross-regulation of inflammation and metabolic mechanisms in osteoarthritis: recent advances bridging the gap to novel treatments

Osteoarthritis (OA) is a debilitating degenerative disease of the joints and one of the most prevalent joint disorders affecting millions of individuals worldwide. This disease is highlighted by significant morbidity owing to encumbering joint pain and functional impairment. OA ensues following disruption of normal homeostasis in the joint resulting from aging, metabolic changes, or as a consequence of joint injury (referred to as post-traumatic OA). These processes are largely driven by low-grade inflammation that gradually compromises the anabolic and protective activities of joint resident cells including chondrocytes, synovial fibroblasts (SFs) and immune cells. Ample research suggests that the process of cartilage deterioration is the endpoint of complex pathologic processes culminating with synovitis, subchondral bone sclerosis, osteophyte formation, aberrant remodeling, and ultimately articular cartilage degradation. There remains a great need for identifying early markers and a "window of opportunity" to enable timely interventions in OA. However, this effort is hampered by the complex nature of the disease and its comorbidities. Joint holistic approaches using recent unbiased multi-omic tools are currently at the forefront promising better understanding of OA development. Currently, there are no meaningful disease-modifying drugs to treat OA, with surgical procedures as the ultimate effective intervention for end stage OA patients. The disability, pain, and surgical costs associated with OA management position this disease among the costliest and onerous for our society. This mini review will highlight advances in the last two decades and major obstacles limiting progress in OA research with particular emphasis on metabolic and inflammatory comorbidities.

Bone-derived factors mediate crosstalk between skeletal and extra-skeletal organs

Bone has long been acknowledged as a fundamental structural entity that provides support and protection to the body's organs. However, emerging research indicates that bone plays a crucial role in the regulation of systemic metabolism. This is achieved through the secretion of a variety of hormones, cytokines, metal ions, extracellular vesicles, and other proteins/peptides, collectively referred to as bone-derived factors (BDFs). BDFs act as a medium through which bones can exert targeted regulatory functions upon various organs, thereby underscoring the profound and concrete implications of bone in human physiology. Nevertheless, there remains a pressing need for further investigations to elucidate the underlying mechanisms that inform the effects of bone on other body systems. This review aims to summarize the current findings related to the roles of these significant modulators across different organs and metabolic contexts by regulating critical genes and signaling pathways in vivo. It also addresses their involvement in the pathogenesis of various diseases affecting the musculoskeletal system, circulatory system, glucose and lipid metabolism, central nervous system, urinary system, and reproductive system. The insights gained from this review may contribute to the development of innovative therapeutic strategies through a focused approach to bone secretomes. Continued research into BDFs is expected to enhance our understanding of bone as a multifunctional organ with diverse regulatory roles in human health.

Bone-brain interaction: mechanisms and potential intervention strategies of biomaterials

Following the discovery of bone as an endocrine organ with systemic influence, bone-brain interaction has emerged as a research hotspot, unveiling complex bidirectional communication between bone and brain. Studies indicate that bone and brain can influence each other's homeostasis via multiple pathways, yet there is a dearth of systematic reviews in this area. This review comprehensively examines interactions across three key areas: the influence of bone-derived factors on brain function, the effects of brain-related diseases or injuries (BRDI) on bone health, and the concept of skeletal interoception. Additionally, the review discusses innovative approaches in biomaterial design inspired by bone-brain interaction mechanisms, aiming to facilitate bone-brain interactions through materiobiological effects to aid in the treatment of neurodegenerative and bone-related diseases. Notably, the integration of artificial intelligence (AI) in biomaterial design is highlighted, showcasing AI's role in expediting the formulation of effective and targeted treatment strategies. In conclusion, this review offers vital insights into the mechanisms of bone-brain interaction and suggests advanced approaches to harness these interactions in clinical practice. These insights offer promising avenues for preventing and treating complex diseases impacting the skeleton and brain, underscoring the potential of interdisciplinary approaches in enhancing human health.

Targeting skeletal interoception: a novel mechanistic insight into intervertebral disc degeneration and pain management

Despite being a leading cause of chronic pain and disability, the underlying mechanisms of intervertebral disc (IVD) degeneration (IVDD) remain unclear. Emerging evidence suggests that mechanosensation (the ability of the skeletal system to perceive mechanical and biochemical signals) mediated by abnormal mechanical loading plays a critical role in the regulation of IVD health. This review examines the complex interactions amongIVDs, intraosseous sensory mechanisms, and the central nervous system (CNS), with a particular focus on the roles of pathways such as PGE2/EP4, Wnt/β-catenin, and NF-κB. This review elucidates the manner in which mechanical stress and aberrant signaling disrupt the homeostasis of the nucleus pulposus (NP), cartilaginous endplate (CEP) and annulus fibrosus (AF), thereby driving degeneration and exacerbating pain. Furthermore, targeted therapeutic strategies, including the modulation of skeletal interoception and dynamic mechanical loading, present novel avenues for reversing IVDD progression. By integrating skeletal biology with spinal pathology, this work offers a novel perspective on the pathogenesis of IVDD and identifies promising strategies for clinical intervention. These findings highlight the potential of targeting skeletal interoception to mitigate IVDD and associated pain, paving the way for innovative, mechanism-driven therapies.

Lipoteichoic Acid Rescued Age-Related Bone Loss by Enhancing Neuroendocrine and Growth Hormone Secretion Through TLR2/COX2/PGE2 Signalling Pathway

The phenomenon of brain-bone crosstalk pertains to the intricate interaction and communication pathways between the central nervous system and the skeletal system. Disruption in brain-bone crosstalk, particularly in disorders such as osteoporosis, can result in skeletal irregularities. Consequently, investigating and comprehending this communication network holds paramount importance in the realm of bone disease prevention and management. In this study, we found that Staphylococcus aureus lipoteichoic acid promoted the conversion of arachidonic acid to PGE2 by interacting with TLR2 receptors acting on the surface of microglial cells in the pituitary gland, leading to the upregulation of COX-2 expression. Subsequently, PGE2 bound to the EP4 receptor of growth hormone-secreting cells and activated the intracellular CREB signalling pathway, promoting GH secretion and ameliorating age-related bone loss.

Angelicae dahuricae radix alleviates simulated microgravity induced bone loss by promoting osteoblast differentiation

Bone loss caused by long-duration spaceflight seriously affects the skeletal health of astronauts. There are many shortcomings in currently available treatments for weightlessness-induced bone loss. The aim of this study was to evaluate the preventive effect of Angelica dahuricae Radix (AR) on simulated microgravity-induced bone loss. Here, we established a hind limb unloading (HLU) mouse model and treated HLU mice with AR (2 g/kg) for 4 weeks. Results indicated that AR significantly inhibited simulated microgravity-induced bone loss. In addition, the components in AR were analyzed using UPLC-MS/MS; results showed that a total of 224 compounds were detected in AR, which mainly contained 7 classes of components. Moreover, the network pharmacological predictions suggested that active ingredients of AR might act on PTGS2 to prevent bone loss. These results elucidate the efficacy of AR in preventing microgravity-induced bone loss and its potential for use in protecting the bone health of astronauts.

Hormonal Control of Bone Architecture Throughout the Lifespan: Implications for Fracture Prediction and Prevention

BACKGROUND

Skeletal modeling in childhood and adolescence and continuous remodeling throughout the lifespan are designed to adapt to a changing environment and resist external forces and fractures. The flux of sex steroids in men and women, beginning from fetal development and evolving through infancy, childhood, puberty, young adulthood, peri/menopause transition, and postmenopause, is critical for bone size, peak bone mass, and fracture resistance.

OBJECTIVE

This review will highlight how changes in sex steroids throughout the lifespan affect bone cells and the consequence of these changes on bone architecture and strength.

METHODS

Literature review and discussion.

RESULTS

The contributions of estrogen and testosterone on skeletal development have been difficult to study due to the reciprocal and intertwining contributions of one on the other. Although orchiectomy in men renders circulating testosterone absent, circulating estrogen also declines due to testosterone being the substrate for estradiol. The discovery of men with absent estradiol or resistance to estrogen and the study of mouse models led to the understanding that estrogen has a larger direct role in skeletal development and maintenance in men and women. The mechanistic reason for larger bone size in men is incompletely understood but related to indirect effects of testosterone on the skeleton, such as higher muscle mass leading to larger mechanical loading. Declines in sex steroids during menopause in women and androgen deprivation therapies in men have profound and negative effects on the skeleton. Therapies to prevent such bone loss are available, but how such therapies can be tailored based on bone size and architecture remains an area of investigation.

CONCLUSION

In this review, the elegant interplay and contribution of sex steroids on bone architecture in men and women throughout the lifespan is described.