Circadian disruption by shifting the light-dark cycle negatively affects bone health in mice

The specificities, influencing factors, and medical implications of bone circadian rhythms

Physiological processes within the human body are regulated in approximately 24-h cycles known as circadian rhythms, serving to adapt to environmental changes. Bone rhythms play pivotal roles in bone development, metabolism, mineralization, and remodeling processes. Bone rhythms exhibit cell specificity, and different cells in bone display various expressions of clock genes. Multiple environmental factors, including light, feeding, exercise, and temperature, affect bone diurnal rhythms through the sympathetic nervous system and various hormones. Disruptions in bone diurnal rhythms contribute to the onset of skeletal disorders such as osteoporosis, osteoarthritis and skeletal hypoplasia. Conversely, these bone diseases can be effectively treated when aimed at the circadian clock in bone cells, including the rhythmic expressions of clock genes and drug targets. In this review, we describe the unique circadian rhythms in physiological activities of various bone cells. Then we summarize the factors synchronizing the diurnal rhythms of bone with the underlying mechanisms. Based on the review, we aim to build an overall understanding of the diurnal rhythms in bone and summarize the new preventive and therapeutic strategies for bone disorders.

Human nonunion tissues display differential gene expression in comparison to physiological fracture callus

The healing of bone fractures can become aberrant and lead to nonunions which in turn have a negative impact on patient health. Understanding why a bone fails to normally heal will enable us to make a positive impact in a patient's life. While we have a wealth of molecular data on rodent models of fracture repair, it is not the same with humans. As such, there is still a lack of information regarding the molecular differences between normal physiological repair and nonunions. This study was designed to address this gap in our molecular knowledge of the human repair process by comparing differentially expressed genes (DEGs) between physiological fracture callus and two different nonunion types, hypertrophic (HNU) and oligotrophic (ONU). RNA sequencing data revealed over ∼18,000 genes in each sample. Using the physiological callus as the control and the nonunion samples as the experimental groups, bioinformatic analyses identified 67 and 81 statistically significant DEGs for HNU and ONU, respectively. Out of the 67 DEGs for the HNU, 34 and 33 were up and down-regulated, respectively. Similarly, out of the 81 DEGs for the ONU, 48 and 33 were up and down-regulated, respectively. Additionally, we also identified common genes between the two nonunion samples; 8 (10.8 %) upregulated and 12 (22.2 %) downregulated. We further identified many biological processes, with several statistically significant ones. Some of these were related to muscle and were common between the two nonunion samples. This study represents the first comprehensive attempt to understand the global molecular events occurring in human nonunion biology. With further research, we can perhaps decipher new molecular pathways involved in aberrant healing of human bone fractures that can be therapeutically targeted.

Circadian disturbances by altering the light-dark cycle negatively affects hematopoietic function of bone marrow in mice

Circadian rhythms in metabolically active tissues are crucial for maintaining physical health. Circadian disturbance (CD) can cause various health issues, such as metabolic abnormalities and immune and cognitive dysfunctions. However, studies on the role of CD in immune cell development and differentiation, as well as the rhythmic expression of the core clock genes and their altered expression under CD, remain unclear. Therefore, we exposed C57bl/6j mice to repeated reversed light-dark cycles for 90 days to research the effects of CD on bone marrow (BM) hematopoietic function. We also researched the effects of CD on endogenous circadian rhythms, temporally dependent expression in peripheral blood and myeloid leukocytes, environmental homeostasis within BM, and circadian oscillations of hematopoietic-extrinsic cues. Our results confirmed that when the light and dark cycles around mice were frequently reversed, the circadian rhythmic expression of the two main circadian rhythm markers, the hypothalamic clock gene, and serum melatonin, was disturbed, indicating that the body was in a state of endogenous CD. Furthermore, CD altered the temporally dependent expression of peripheral blood and BM leukocytes and destroyed environmental homeostasis within the BM as well as circadian oscillations of hematopoietic-extrinsic cues, which may negatively affect BM hematopoiesis in mice. Collectively, these results demonstrate that circadian rhythms are vital for maintaining health and suggest that the association between CD and hematopoietic dysfunction warrants further investigation.

The multi-faceted nature of age-associated osteoporosis

Age-associated osteoporosis (AAOP) poses a significant health burden, characterized by increased fracture risk due to declining bone mass and strength. Effective prevention and early treatment strategies are crucial to mitigate the disease burden and the associated healthcare costs. Current therapeutic approaches effectively target the individual contributing factors to AAOP. Nonetheless, the management of AAOP is complicated by the multitude of variables that affect its development. Main intrinsic and extrinsic factors contributing to AAOP risk are reviewed here, including mechanical unloading, nutrient deficiency, hormonal disbalance, disrupted metabolism, cognitive decline, inflammation and circadian disruption. Furthermore, it is discussed how these can be targeted for prevention and treatment. Although valuable as individual targets for intervention, the interconnectedness of these risk factors result in a unique etiology for every patient. Acknowledgement of the multifaceted nature of AAOP will enable the development of more effective and sustainable management strategies, based on a holistic, patient-centered approach.

Melatonin as a chronobiotic/cytoprotective agent in bone. Doses involved

Because the chronobiotic and cytoprotective molecule melatonin diminishes with age, its involvement in postmenopausal and senescence pathology has been considered since long. One relevant melatonin target site in aging individuals is bone where melatonin chronobiotic effects mediated by MT1 and MT2 receptors are demonstrable. Precursors of bone cells located in bone marrow are exposed to high quantities of melatonin and the possibility arises that melatonin acts a cytoprotective compound via an autacoid effect. Proteins that are incorporated into the bone matrix, like procollagen type I c-peptide, augment after melatonin exposure. Melatonin augments osteoprotegerin, an osteoblastic protein that inhibits the differentiation of osteoclasts. Osteoclasts are target cells for melatonin as they degrade bone partly by generating free radicals. Osteoclast activity and bone resorption are impaired via the free radical scavenger properties of melatonin. The administration of melatonin in chronobiotic doses (less than 10 mg daily) is commonly used in clinical studies on melatonin effect on bone. However, human equivalent doses allometrically derived from animal studies are in the 1-1.5 mg/kg/day range for a 75 kg human adult, a dose rarely used clinically. In view of the absence of toxicity of melatonin in phase 1 pharmacological studies with doses up to 100 mg in normal volunteers, further investigation is needed to determine whether high melatonin doses have higher therapeutic efficacy in preventing bone loss.

Abrupt Photoperiod Changes Differentially Modulate Hepatic Antioxidant Response in Healthy and Obese Rats: Effects of Grape Seed Proanthocyanidin Extract (GSPE)

Disruptions of the light/dark cycle and unhealthy diets can promote misalignment of biological rhythms and metabolic alterations, ultimately leading to an oxidative stress condition. Grape seed proanthocyanidin extract (GSPE), which possesses antioxidant properties, has demonstrated its beneficial effects in metabolic-associated diseases and its potential role in modulating circadian disruptions. Therefore, this study aimed to assess the impact of GSPE administration on the liver oxidant system of healthy and diet-induced obese rats undergoing a sudden photoperiod shift. To this end, forty-eight photoperiod-sensitive Fischer 344/IcoCrl rats were fed either a standard (STD) or a cafeteria diet (CAF) for 6 weeks. A week before euthanizing, rats were abruptly transferred from a standard photoperiod of 12 h of light/day (L12) to either a short (6 h light/day, L6) or a long photoperiod (18 h light/day, L18) while receiving a daily oral dose of vehicle (VH) or GSPE (25 mg/kg). Alterations in body weight gain, serum and liver biochemical parameters, antioxidant gene and protein expression, and antioxidant metabolites were observed. Interestingly, GSPE partially ameliorated these effects by reducing the oxidative stress status in L6 through an increase in GPx1 expression and in hepatic antioxidant metabolites and in L18 by increasing the NRF2/KEAP1/ARE pathway, thereby showing potential in the treatment of circadian-related disorders by increasing the hepatic antioxidant response in a photoperiod-dependent manner.

The importance of the circadian trough in glucocorticoid signaling: a variation on B-flat

Glucocorticoid hormones are essential for health, but overexposure may lead to many detrimental effects, including metabolic, psychiatric, and bone disease. These effects may not only be due to increased overall exposure to glucocorticoids, but also to elevated hormone levels at the time of the physiological circadian trough of glucocorticoid levels. The late Mary Dallman developed a model that allows the differentiation between the effects of overall 24-hour glucocorticoid overexposure and the effects of a lack of circadian rhythmicity. For this, she continuously treated rats with a low dose of corticosterone (or "B"), which leads to a constant hormone level, without 24-hour overexposure using subcutaneously implanted pellets. The data from this "B-flat" model suggest that even modest elevations of glucocorticoid signaling during the time of the normal circadian trough of hormone secretion are a substantial contributor to the negative effects of glucocorticoids on health.

Night shift work and serum markers of bone turnover in male shift workers

Night shift work is related to sleep disorders, disruption of circadian rhythm and low serum levels of vitamin D. It is known that all these conditions can adversely affect bone mass. The rate of bone turnover can be assessed through the measurement of molecules called bone turnover markers, including C-terminal telopeptide fragment of type I collagen (CTX) and procollagen type I N-terminal propeptide (P1NP). In this study, we evaluated the serum levels of CTX, P1NP and 25-Hydroxy Vitamin D in 82 male subjects (42 daytime workers and 40 night shift workers) to assess the possible risk of osteoporosis in male shift workers. Serum levels of CTX and P1NP were found to be higher in night shift workers than in daytime workers. No significant difference was found in vitamin D levels between night shift and daytime workers. The increased CTX and P1NP levels reveal a higher rate of bone turnover in night shift workers and thus a possible increased risk of osteoporosis in this category of workers compared with daytime workers. In view of this, our results highlight the importance of further studies investigating the bone health in male night shift workers.

Developmental growth plate cartilage formation suppressed by artificial light at night via inhibiting BMAL1-driven collagen hydroxylation

Exposure to artificial light at night (LAN) can induce obesity, depressive disorder and osteoporosis, but the pernicious effects of excessive LAN exposure on tissue structure are poorly understood. Here, we demonstrated that artificial LAN can impair developmental growth plate cartilage extracellular matrix (ECM) formation and cause endoplasmic reticulum (ER) dilation, which in turn compromises bone formation. Excessive LAN exposure induces downregulation of the core circadian clock protein BMAL1, which leads to collagen accumulation in the ER. Further investigations suggest that BMAL1 is the direct transcriptional activator of prolyl 4-hydroxylase subunit alpha 1 (P4ha1) in chondrocytes, which orchestrates collagen prolyl hydroxylation and secretion. BMAL1 downregulation induced by LAN markedly inhibits proline hydroxylation and transport of collagen from ER to golgi, thereby inducing ER stress in chondrocytes. Restoration of BMAL1/P4HA1 signaling can effectively rescue the dysregulation of cartilage formation within the developmental growth plate induced by artificial LAN exposure. In summary, our investigations suggested that LAN is a significant risk factor in bone growth and development, and a proposed novel strategy targeting enhancement of BMAL1-mediated collagen hydroxylation could be a potential therapeutic approach to facilitate bone growth.

Circadian rhythm disruption exacerbates the progression of macrophage dysfunction and alveolar bone loss in periodontitis

Macrophages are highly implicated in the progression of periodontitis, while circadian rhythm disruption (CRD) promotes the inflammatory response of macrophages in many diseases. However, the effects of CRD on periodontitis and the role of macrophages in this process remain unclear. Histone lysinedemethylase6a (Kdm6a), a histone demethylase, has recently been identified as a key regulator of macrophage-induced inflammation. Here, we established an experimental periodontitis model by injecting lipopolysaccharide (LPS) derived from Porphyromonas gingivalis with or without periodontal ligation in mice exposed to an 8-h time shift jet-lag schedule every 3 days. By histomorphometry, tartrate acid phosphatase (TRAP) staining, RT-qPCR, ELISA, immunohistochemistry and immunofluorescence analysis, we found that CRD promoted the inflammatory response, alveolar bone resorption, macrophage infiltration and Kdm6a expression in macrophages. Macrophage-specific Kdm6a knockout mice were further used to elucidate the effects of Kdm6a deficiency on periodontitis. Kdm6a deletion in macrophages rescued periodontal tissue inflammation, osteoclastogenesis, and alveolar bone loss in a mouse model of periodontitis. These findings suggest that CRD may intensify periodontitis by increasing the infiltration and activation of macrophages. Kdm6a promotes the inflammatory response in macrophages, which may participate in aggravated periodontitis via CRD.

Effect of Circadian Rhythm Disturbance on the Human Musculoskeletal System and the Importance of Nutritional Strategies

The circadian system in the human body responds to daily environmental changes to optimise behaviour according to the biological clock and also influences various physiological processes. The suprachiasmatic nuclei are located in the anterior hypothalamus of the brain, and they synchronise to the 24 h light/dark cycle. Human physiological functions are highly dependent on the regulation of the internal circadian clock. Skeletal muscles comprise the largest collection of peripheral clocks in the human body. Both central and peripheral clocks regulate the interaction between the musculoskeletal system and energy metabolism. The skeletal muscle circadian clock plays a vital role in lipid and glucose metabolism. The pathogenesis of osteoporosis is related to an alteration in the circadian rhythm. In the present review, we discuss the disturbance of the circadian rhythm and its resultant effect on the musculoskeletal system. We also discuss the nutritional strategies that are potentially effective in maintaining the system's homeostasis. Active collaborations between nutritionists and physiologists in the field of chronobiological and chrononutrition will further clarify these interactions. This review may be necessary for successful interventions in reducing morbidity and mortality resulting from musculoskeletal disturbances.

Using bioluminescence to image gene expression and spontaneous behavior in freely moving mice

Bioluminescence imaging (BLI) of gene expression in live animals is a powerful method for monitoring development, tumor growth, infections, healing, and other progressive, long-term biological processes. BLI remains an effective approach for reducing the number of animals needed to monitor dynamic changes in gene activity because images can be captured repeatedly from the same animals. When examining these ongoing changes, it is sometimes necessary to remove rhythmic effects on the bioluminescence signal caused by the circadian clock's daily modulation of gene expression. Furthermore, BLI using freely moving animals remains limited because the standard procedures can alter normal behaviors. Another obstacle with conventional BLI of animals is that luciferin, the firefly luciferase substrate, is usually injected into mice that are then imaged while anesthetized. Unfortunately, the luciferase signal declines rapidly during imaging as luciferin is cleared from the body. Alternatively, mice are imaged after they are surgically implanted with a pump or connected to a tether to deliver luciferin, but stressors such as this surgery and anesthesia can alter physiology, behavior, and the actual gene expression being imaged. Consequently, we developed a strategy that minimizes animal exposure to stressors before and during sustained BLI of freely moving unanesthetized mice. This technique was effective when monitoring expression of the Per1 gene that serves in the circadian clock timing mechanism and was previously shown to produce circadian bioluminescence rhythms in live mice. We used hairless albino mice expressing luciferase that were allowed to drink luciferin and engage in normal behaviors during imaging with cooled electron-multiplying-CCD cameras. Computer-aided image selection was developed to measure signal intensity of individual mice each time they were in the same posture, thereby providing comparable measurements over long intervals. This imaging procedure, performed primarily during the animal's night, is compatible with entrainment of the mouse circadian timing system to the light cycle while allowing sampling at multi-day intervals to monitor long-term changes. When the circadian expression of a gene is known, this approach provides an effective alternative to imaging immobile anesthetized animals and can removing noise caused by circadian oscillations and body movements that can degrade data collected during long-term imaging studies.

Circadian clock genes as promising therapeutic targets for bone loss

The circadian clock regulates many key physiological processes such as the sleep-wake cycle, hormone release, cardiovascular health, glucose metabolism and body temperature. Recent evidence has suggested a critical role of the circadian system in controlling bone metabolism. Here we review the connection between bone metabolism and the biological clock, and the roles of these mechanisms in bone loss. We also analyze the regulatory effects of clock-related genes on signaling pathways and transcription factors in osteoblasts and osteoclasts. Additionally, osteocytes and endothelial cells (ECs) regulated by the circadian clock are also discussed in our review. Furthermore, we also summarize the regulation of circadian clock genes by some novel modulators, which provides us with a new insight into a potential strategy to prevent and treat bone diseases such as osteoporosis by targeting circadian genes.

Co-regulation of circadian clock genes and microRNAs in bone metabolism

Mammalian bone is constantly metabolized from the embryonic stage, and the maintenance of bone health depends on the dynamic balance between bone resorption and bone formation, mediated by osteoclasts and osteoblasts. It is widely recognized that circadian clock genes can regulate bone metabolism. In recent years, the regulation of bone metabolism by non-coding RNAs has become a hotspot of research. MicroRNAs can participate in bone catabolism and anabolism by targeting key factors related to bone metabolism, including circadian clock genes. However, research in this field has been conducted only in recent years and the mechanisms involved are not yet well established. Recent studies have focused on how to target circadian clock genes to treat some diseases, such as autoimmune diseases, but few have focused on the co-regulation of circadian clock genes and microRNAs in bone metabolic diseases. Therefore, in this paper we review the progress of research on the co-regulation of bone metabolism by circadian clock genes and microRNAs, aiming to provide new ideas for the prevention and treatment of bone metabolic diseases such as osteoporosis.

Dark-light cycle disrupts bone metabolism and suppresses joint deterioration in osteoarthritic rats

Background

Light alteration affects the internal environment and metabolic homeostasis of the body through circadian rhythm disorders (CRD). CRD is one of the factors that induce and accelerate osteoarthritis (OA). Therefore, the aim of this study was to evaluate the effects of continuous dark-light (DL) cycle on joint inflammation, bone structure, and metabolism in normal and OA Sprague-Dawley (SD) rats.

Methods

Interleukin (IL)-1β, IL-6, inducible nitric oxide synthase (iNOS), and tumor necrosis factor (TNF)-α were used to evaluate the systemic inflammation in rats. The pathological changes and inflammatory reactions of the cartilage and synovium of the knee joint in rats were evaluated by Safranin O-fast green and immunological staining. Bone turnover was assessed by histomorphometry and μCT scanning, as well as bone metabolism markers and proteins. The expression changes of clock proteins BMAL1, NR1D1, PER3, and CRY1 in representative tissues were detected by western blotting.

Results

DL cycle significantly inhibited body weight gain in normal and OA rats. The levels of proinflammatory factors in the peripheral blood circulation and degradation enzymes in the cartilage were significantly decreased in OA+DL rats. DL cycle significantly destroyed the structure of subchondral bone in hindlimbs of OA rats and reduced trabecular bone numbers. The decrease of bone mineral density (BMD), percent bone volume with respect to total bone volume (BV/TV), trabecular number (TB.N), osteoclast number, and mineralization could also be found. The ratio of the receptor activator of nuclear factor-kappa B ligand/osteoprotegerin (RANKL/OPG) in the bone marrow of OA rats was markedly increased under DL, along with the activation of the mononuclear/phagocyte system. The expression of representative clock proteins and genes BMAL1, PER3, and CRY1 were markedly changed in the tissues of OA+DL rats.

Conclusions

These results suggested that DL cycle dampened the arthritis and promoted bone resorption and bone mass loss.

Graphical abstract

DL cycle affects bone turnover by regulating osteoclast production in osteoarthritic rats.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13075-022-02832-8.

Sleep Disruption and Bone Health

PURPOSE OF REVIEW

Review recent literature investigating the relationship between bone health and sleep/circadian disruptions (e.g., abnormal sleep duration, night shift work).

RECENT FINDINGS

Short and long sleep are associated with low bone mineral density (BMD). Recent data from observational studies identified an increased risk of fracture in women with short sleep. Studies suggest that age, sex, weight change, and concurrent circadian misalignment may modify the effects of sleep restriction on bone metabolism. Interventional studies demonstrate alterations in bone metabolism and structure in response to circadian disruption that could underlie the increased fracture risk seen with night shift work. The effects of sleep and circadian disruption during adolescence may have lifelong skeletal consequences if they adversely impact bone modeling. Data suggest that short sleep and night shift work negatively impact bone metabolism and health. Rigorous studies of prevalent sleep and circadian disruptions are needed to determine mechanisms and develop prevention strategies to optimize lifelong skeletal health.

Circadian rhythm of markers of bone turnover in patients with chronic kidney disease

Patients with chronic kidney disease (CKD) have a high risk of bone fractures. A circadian rhythmicity in turnover and mineralization of bone appears to be of importance for bone health. In CKD disturbances in the circadian rhythm of various functions has been demonstrated and indeed the circadian rhythm in the mineral metabolism is disturbed. The aim of the present study was to compare the circadian rhythm of bone turnover markers in ten patients with CKD to ten healthy controls. Bone turnover markers (C-terminal telopeptide of type I collagen, tartrate-resistant acid phosphatase 5b, N-terminal propeptide of type I procollagen, bone alkaline phosphatase and osteocalcin) were measured every third hour for 24 h. All bone turnover markers displayed a significant circadian rhythm in both groups and there were no significant differences in the rhythmicity between the two groups (no group*time interaction). As expected, due to the reduced renal clearance, the overall level of C-terminal telopeptide of type I collagen and osteocalcin was higher in CKD compared to the healthy controls. The present study suggests that disturbances in the circadian rhythm of bone turnover do not explain the metabolic bone disease and increased risk of fractures in CKD.

REV-ERBs negatively regulate mineralization of the cementoblasts

OBJECTIVE

The role of circadian clock in cementogenesis is unclear. This study examines the role of REV-ERBs, one of circadian clock proteins, in proliferation, migration and mineralization of cementoblasts to fill the gap in knowledge.

METHODS

Expression pattern of REV-ERBα in cementoblasts was investigated in vivo and in vitro. CCK-8 assay, scratch wound healing assay, alkaline phosphatase (ALP) and alizarin red S (ARS) staining were performed to evaluate the effects of REV-ERBs activation by SR9009 on proliferation, migration and mineralization of OCCM-30, an immortalized cementoblast cell line. Furthermore, mineralization related markers including osterix (OSX), ALP, bone sialoprotein (BSP) and osteocalcin (OCN) were evaluated.

RESULTS

Strong expression of REV-ERBα was found in cellular cementum around tooth apex. Rev-erbα mRNA oscillated periodically in OCCM-30 and declined after mineralization induction. REV-ERBs activation by SR9009 inhibited proliferation but promoted migration of OCCM-30 in vitro. Results of ALP and ARS staining suggested that REV-ERBs activation negatively regulated mineralization of OCCM-30. Mechanically, REV-ERBs activation attenuated the expression of OSX and its downstream targets including ALP, BSP and OCN.

CONCLUSIONS

REV-ERBs are involved in cementogenesis and negatively regulate mineralization of cementoblasts via inhibiting OSX expression. Our study provides a potential target regarding periodontal and cementum regeneration.

Bone adaptation to mechanical loading in mice is affected by circadian rhythms

Physical forces are critical for successful function of many organs including bone. Interestingly, the timing of exercise during the day alters physiology and gene expression in many organs due to circadian rhythms. Circadian clocks in tissues, such as bone, express circadian clock genes that target tissue-specific genes, resulting in tissue-specific rhythmic gene expression (clock-controlled genes). We hypothesized that the adaptive response of bone to mechanical loading is regulated by circadian rhythms. First, mice were sham loaded and sacrificed 8 h later, which amounted to tissues being collected at zeitgeber time (ZT)2, 6, 10, 14, 18, and 22. Cortical bone of the tibiae collected from these mice displayed diurnal expression of core clock genes and key osteocyte and osteoblast-related genes, such as the Wnt-signaling inhibitors Sost and Dkk1, indicating these are clock-controlled genes. Serum bone turnover markers did not display rhythmicity. Second, mice underwent a single bout of in vivo loading at either ZT2 or ZT14 and were sacrificed 1, 8, or 24 h after loading. Loading at ZT2 resulted in Sost upregulation, while loading at ZT14 led to Sost and Dkk1 downregulation. Third, mice underwent daily in vivo tibial loading over 2 weeks administered either in the morning, (ZT2, resting phase) or evening (ZT14, active phase). In vivo microCT was performed at days 0, 5, 10, and 15 and conventional histomorphometry was performed at day 15. All outcome measures indicated a robust response to loading, but only microCT-based time-lapse morphometry showed that loading at ZT14 resulted in a greater endocortical bone formation response compared to mice loaded at ZT2. The decreased Sost and Dkk1 expression coincident with the modest, but significant time-of-day specific increase in adaptive bone formation, suggests that circadian clocks influence bone mechanoresponse.

Circadian Rhythm Effects on the Molecular Regulation of Physiological Systems

Nearly every system within the body contains an intrinsic cellular circadian clock. The circadian clock contributes to the regulation of a variety of homeostatic processes in mammals through the regulation of gene expression. Circadian disruption of physiological systems is associated with pathophysiological disorders. Here, we review the current understanding of the molecular mechanisms contributing to the known circadian rhythms in physiological function. This article focuses on what is known in humans, along with discoveries made with cell and rodent models. In particular, the impact of circadian clock components in metabolic, cardiovascular, endocrine, musculoskeletal, immune, and central nervous systems are discussed. © 2021 American Physiological Society. Compr Physiol 11:1-30, 2021.

Chronobiology and Chronotherapy of Osteoporosis

Physiological circadian (ie, 24-hour) rhythms are critical for bone health. Animal studies have shown that genes involved in the intrinsic molecular clock demonstrate potent circadian expression patterns in bone and that genetic disruption of these clock genes results in a disturbed bone structure and quality. More importantly, circulating markers of bone remodeling show diurnal variation in mice as well as humans, and circadian disruption by, eg, working night shifts is associated with the bone remodeling disorder osteoporosis. In this review, we provide an overview of the current literature on rhythmic bone remodeling and its underlying mechanisms and identify critical knowledge gaps. In addition, we discuss novel (chrono)therapeutic strategies to reduce osteoporosis by utilizing our knowledge on circadian regulation of bone. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Loss of glucocorticoid rhythm induces an osteoporotic phenotype in female mice

Glucocorticoid (GC)-induced osteoporosis is a widespread health problem that is accompanied with increased fracture risk. Detrimental effects of anti-inflammatory GC therapy on bone have been ascribed to the excess in GC exposure, but it is unknown whether there is also a role for disruption of the endogenous GC rhythm that is inherent to GC therapy. To investigate this, we implanted female C57Bl/6J mice with slow-release corticosterone (CORT) pellets to blunt the rhythm in CORT levels without inducing hypercortisolism. Flattening of CORT rhythm reduced cortical and trabecular bone volume and thickness, whilst bone structure was maintained in mice injected with supraphysiologic CORT at the time of their endogenous GC peak. Mechanistically, mice with a flattened CORT rhythm showed disrupted circadian gene expression patterns in bone, along with changes in circulating bone turnover markers indicative of a negative balance in bone remodelling. Indeed, double calcein labelling of bone in vivo revealed a reduced bone formation in mice with a flattened CORT rhythm. Collectively, these perturbations in bone turnover and structure decreased bone strength and stiffness, as determined by mechanical testing. In conclusion, we demonstrate for the first time that flattening of the GC rhythm disrupts the circadian clock in bone and results in an osteoporotic phenotype in mice. Our findings indicate that at least part of the fracture risk associated with GC therapy may be the consequence of a disturbed GC rhythm, rather than excess GC exposure alone, and that a dampened GC rhythm may contribute to the age-related risk of osteoporosis.

Circadian rhythm disruption with high-fat diet impairs glycemic control and bone quality

Biological functions, including glycemic control and bone metabolism, are highly influenced by the body's internal clock. Circadian rhythms are biological rhythms that run with a period close to 24 hours and receive input from environmental stimuli, such as the light/dark cycle. We investigated the effects of circadian rhythm disruption (CRD), through alteration of the light/dark schedule, on glycemic control and bone quality of mice. Ten-week-old male mice (C57/BL6, n = 48) were given a low-fat diet (LFD) or a high-fat diet (HFD) and kept on a dayshift or altered schedule (RSS3) for 22 weeks. Mice were divided into four experimental groups (n = 12/group): Dayshift/LFD, Dayshift/HFD, RSS3/LFD, and RSS3/HFD. CRD in growing mice fed a HFD resulted in a diabetic state, with a 36.2% increase in fasting glucose levels compared to the Dayshift/LFD group. Micro-CT scans of femora revealed a reduction in inner and outer surface expansion for mice on a HFD and altered light schedule. Cancellous bone demonstrated deterioration of bone quality as trabecular number and thickness decreased while trabecular separation increased. While HFD increased cortical bone mineral density, its combination with CRD reduced this phenomenon. The growth of mineral crystals, determined by small angle X-ray scattering, showed HFD led to smaller crystals. Considering modifications of the organic matrix, regardless of diet, CRD exacerbated the accumulation of fluorescent advanced glycation end-products (fAGEs) in collagen. Strength testing of tibiae showed that CRD mitigated the higher strength in the HFD group and increased brittleness indicated by lower post-yield deflection and work-to-fracture. Consistent with accumulation of fAGEs, various measures of toughness were lowered with CRD, but combination of CRD with HFD protected against this decrease. Differences between strength and toughness results represent different contributions of structural and material properties of bone to energy dissipation. Collectively, these results demonstrate that combination of CRD with HFD impairs glycemic control and have complex effects on bone quality.

Distinct Glucocorticoid Receptor Actions in Bone Homeostasis and Bone Diseases

Glucocorticoids (GCs) are steroid hormones that respond to stress and the circadian rhythm. Pharmacological GCs are widely used to treat autoimmune and chronic inflammatory diseases despite their adverse effects on bone after long-term therapy. GCs regulate bone homeostasis in a cell-type specific manner, affecting osteoblasts, osteoclasts, and osteocytes. Endogenous physiological and exogenous/excessive GCs act via nuclear receptors, mainly via the GC receptor (GR). Endogenous GCs have anabolic effects on bone mass regulation, while excessive or exogenous GCs can cause detrimental effects on bone. GC-induced osteoporosis (GIO) is a common adverse effect after GC therapy, which increases the risk of fractures. Exogenous GC treatment impairs osteoblastogenesis, survival of the osteoblasts/osteocytes and prolongs the longevity of osteoclasts. Under normal physiological conditions, endogenous GCs are regulated by the circadian rhythm and circadian genes display oscillatory rhythmicity in bone cells. However, exogenous GCs treatment disturbs the circadian rhythm. Recent evidence suggests that the disturbed circadian rhythm by continuous exogenous GCs treatment can in itself hamper bone integrity. GC signaling is also important for fracture healing and rheumatoid arthritis, where crosstalk among several cell types including macrophages and stromal cells is indispensable. This review summarizes the complexity of GC actions via GR in bone cells at cellular and molecular levels, including the effect on circadian rhythmicity, and outlines new therapeutic possibilities for the treatment of their adverse effects.