Incremental Growth Lines in Mouse Molar Dentin Represent 8-hr Ultradian Rhythm

Beyond circadian rhythms: emerging roles of ultradian rhythms in control of liver functions

The mammalian liver must cope with various metabolic and physiological changes that normally recur every day and primarily stem from daily cycles of rest-activity and fasting-feeding. Although a large body of evidence supports the reciprocal regulation of circadian rhythms and liver function, the research on the hepatic ultradian rhythms have largely been lagging behind. However, with the advent of more cost-effective high-throughput omics technologies, high-resolution time-lapse imaging, and more robust and powerful mathematical tools, several recent studies have shed new light on the presence and functions of hepatic ultradian rhythms. In this review, we will first very briefly discuss the basic principles of circadian rhythms, and then cover in greater details the recent literature related to ultradian rhythms. Specifically, we will highlight the prevalence and mechanisms of hepatic 12-h rhythms, and 8-h rhythms, which cycle at the second and third harmonics of circadian frequency. Finally, we also refer to ultradian rhythms with other frequencies and examine the limitations of the current approaches as well as the challenges related to identifying ultradian rhythm and addressing their molecular underpinnings.

Quantitative morphometric analysis of molar teeth and alveolar bone using micro-computed tomography in aged mice

OBJECTIVES

Irreversible morphological regressions of the teeth or related structures in older people can diminish their overall health. However, research on human aging in dentistry is complicated by several confounding factors. In this study, we conducted a morphometric analysis of the mandibular second molars and surrounding alveolar bone in C57BL/6 mice to evaluate age-related changes in the oral cavity.

METHODS

The animals were divided into five groups based on their age: 4 weeks (juvenile mice; n=5); 20 weeks (n=7), 50 weeks (n=5), 77 weeks (n=7), and 100 weeks (n=5); changes were evaluated using micro-computed tomography.

RESULTS

The molars of juvenile mice had sharp and pointed cusps and presented maximum heights. With age and occlusal wear, the cusp heights demonstrated a significant decrease (up to 75%) until the last stage of life. Conversely, apparent lesions were not observed on the basal portion of the crown, even in the most heavily worn teeth. The roots of the molars continued to grow in length at 4 weeks of age. Alveolar bone resorption begins to occur in middle age and continues throughout life. The proportion of vertical bone loss reached approximately 40% of the entire root length, demonstrating a remarkable increase between weeks 77 and 100.

CONCLUSIONS

Overall, these morphological changes were similar to those observed in humans. Therefore, it might be appropriate to use aged mice as an experimental model for basic and clinical research in geriatric dentistry.

Orthodontic Force-Induced BMAL1 in PDLCs Is a Vital Osteoclastic Activator

Orthodontic tooth movement (OTM) depends on periodontal ligament cells (PDLCs) sensing biomechanical stimuli and subsequently releasing signals to initiate alveolar bone remodeling. However, the mechanisms by which PDLCs sense biomechanical stimuli and affect osteoclastic activities are still unclear. This study demonstrates that the core circadian protein aryl hydrocarbon receptor nuclear translocator-like protein 1 (BMAL1) in PDLCs is highly involved in sensing and delivering biomechanical signals. Orthodontic force upregulates BMAL1 expression in periodontal tissues and cultured PDLCs in manners dependent on ERK (extracellular signal-regulated kinase) and AP1 (activator protein 1). Increased BMAL1 expression can enhance secretion of CCL2 (C-C motif chemokine 2) and RANKL (receptor activator of nuclear factor-κB ligand) in PDLCs, which subsequently promotes the recruitment of monocytes that differentiate into osteoclasts. The mechanistic delineation clarifies that AP1 induced by orthodontic force can directly interact with the BMAL1 promoter and activate gene transcription in PDLCs. Localized administration of the ERK phosphorylation inhibitor U0126 or the BMAL1 inhibitor GSK4112 suppressed ERK/AP1/BMAL1 signaling. These treatments dramatically reduced osteoclastic activity in the compression side of a rat orthodontic model, and the OTM rate was almost nonexistent. In summary, our results suggest that force-induced expression of BMAL1 in PDLCs is closely involved in controlling osteoclastic activities during OTM and plays a vital role in alveolar bone remodeling. It could be a useful therapeutic target for accelerating the OTM rate and controlling pathologic bone-remodeling activities.