HuR-positive stress granules: Potential targets for age-related osteoporosis

Aging impairs osteoblast function and bone turnover, resulting in age-related bone degeneration. Stress granules (SGs) are membrane-less organelles that assemble in response to stress via the recruitment of RNA-binding proteins (RBPs), and have emerged as a novel mechanism in age-related diseases. Here, we identified HuR as a bone-related RBP that aggregated into SGs and facilitated osteogenesis during aging. HuR-positive SG formation increased during osteoblast differentiation, and HuR overexpression mitigated the reduction in SG formation observed in senescent osteoblasts. Moreover, HuR positively regulated the mRNA stability and expression of its target β-catenin by binding and recruiting β-catenin into SGs. As a potential therapeutic target, HuR activator apigenin (API) enhanced its expression and thus aided osteoblasts differentiation. API treatment increased HuR nuclear export, enhanced the recruitment of β-catenin into HuR-positive SGs, facilitated β-catenin nuclear translocation, and contributed osteogenesis. Our findings highlight the roles of HuR and its SGs in promoting osteogenesis during skeletal aging and lay the groundwork for novel therapeutic strategies against age-related skeletal disorders.

Mechanical induction of osteoanabolic Wnt1 promotes osteoblast differentiation via Plat

Physical activity-induced mechanical stimuli play a crucial role in preserving bone mass and structure by promoting bone formation. While the Wnt pathway is pivotal for mediating the osteoblast response to loading, the exact mechanisms are not fully understood. Here, we found that mechanical stimulation induces osteoblastic Wnt1 expression, resulting in an upregulation of key osteogenic marker genes, including Runx2 and Sp7, while Wnt1 knockdown using siRNA prevented these effects. RNAseq analysis identified Plat as a major target through which Wnt1 exerts its osteogenic influence. This was corroborated by Plat depletion using siRNA, confirming its positive role in osteogenic differentiation. Moreover, we demonstrated that mechanical stimulation enhances Plat expression, which, in turn leads to increased expression of osteogenic markers like Runx2 and Sp7. Notably, Plat depletion by siRNA prevented this effect. We have established that Wnt1 regulates Plat expression by activating β-Catenin. Silencing Wnt1 impairs mechanically induced β-Catenin activation, subsequently reducing Plat expression. Furthermore, our findings showed that Wnt1 is essential for osteoblasts to respond to mechanical stimulation and induce Runx2 and Sp7 expression, in part through the Wnt1/β-Catenin/Plat signaling pathway. Additionally, we observed significantly reduced Wnt1 and Plat expression in bones from ovariectomy (OVX)-induced and age-related osteoporotic mouse models compared with non-OVX and young mice, respectively. Overall, our data suggested that Wnt1 and Plat play significant roles in mechanically induced osteogenesis. Their decreased expression in bones from OVX and aged mice highlights their potential involvement in post-menopausal and age-related osteoporosis, respectively.

Down-expression of miR-494-3p in senescent osteocyte-derived exosomes inhibits osteogenesis and accelerates age-related bone loss via PTEN/PI3K/AKT pathway

Aims

To investigate the effects of senescent osteocytes on bone homeostasis in the progress of age-related osteoporosis and explore the underlying mechanism.

Methods

In a series of in vitro experiments, we used tert-Butyl hydroperoxide (TBHP) to induce senescence of MLO-Y4 cells successfully, and collected conditioned medium (CM) and senescent MLO-Y4 cell-derived exosomes, which were then applied to MC3T3-E1 cells, separately, to evaluate their effects on osteogenic differentiation. Furthermore, we identified differentially expressed microRNAs (miRNAs) between exosomes from senescent and normal MLO-Y4 cells by high-throughput RNA sequencing. Based on the key miRNAs that were discovered, the underlying mechanism by which senescent osteocytes regulate osteogenic differentiation was explored. Lastly, in the in vivo experiments, the effects of senescent MLO-Y4 cell-derived exosomes on age-related bone loss were evaluated in male SAMP6 mice, which excluded the effects of oestrogen, and the underlying mechanism was confirmed.

Results

The CM and exosomes collected from senescent MLO-Y4 cells inhibited osteogenic differentiation of MC3T3-E1 cells. RNA sequencing detected significantly lower expression of miR-494-3p in senescent MLO-Y4 cell-derived exosomes compared with normal exosomes. The upregulation of exosomal miR-494-3p by miRNA mimics attenuated the effects of senescent MLO-Y4 cell-derived exosomes on osteogenic differentiation. Luciferase reporter assay demonstrated that miR-494-3p targeted phosphatase and tensin homolog (PTEN), which is a negative regulator of the phosphoinositide 3-kinase (PI3K)/AKT pathway. Overexpression of PTEN or inhibition of the PI3K/AKT pathway blocked the functions of exosomal miR-494-3p. In SAMP6 mice, senescent MLO-Y4 cell-derived exosomes accelerated bone loss, which was rescued by upregulation of exosomal miR-494-3p.

Conclusion

Reduced expression of miR-494-3p in senescent osteocyte-derived exosomes inhibits osteogenic differentiation and accelerates age-related bone loss via PTEN/PI3K/AKT pathway.

miR-34a-5p facilitates osteogenic differentiation of bone marrow mesenchymal stem cells and modulates bone metabolism by targeting HDAC1 and promoting ER-α transcription

OBJECTIVE

Metabolism is essential for bone development. The expressions of catabolic markers in chondrocytes show association with miR-34a-5p. This study discussed the mechanism by which miR-34a-5p regulates osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) as well as bone metabolism.

METHODS

Expressions of BMSC surface markers were determined via flow cytometry. Osteogenic differentiation of BMSCs was subsequently induced. miR-34a-5p mimic, oe-HDAC1, or ER-α activator Ferutinin was introduced in BMSCs. Alkaline phosphatase activity and calcification were detected. Expressions of miR-34a-5p, HDAC1, ER-α, and osteogenic markers were determined via RT-qPCR and Western blot. The binding relationship between miR-34a-5p and HDAC1 was verified by a dual-luciferase assay. Mice at the age of 6 months and 18 months were assigned to the young group and age group for in vivo experiments, and aged mice were treated with agomiR miR-34a-5p. Expressions of serum miR-34a-5p, HDAC1, ER-α, and bone metabolism markers in mice were determined.

RESULTS

Osteogenic medium-induced BMSCs manifested increased expressions of miR-34a-5p and ER-α and decreased HDAC1 expression. miR-34a-5p overexpression promoted osteogenic differentiation of BMSCs. miR-34a-5p targeted HDAC1. HDAC1 overexpression partially counteracted the promotional action of miR-34a-5p overexpression on osteogenic differentiation of BMSCs. miR-34a-5p overexpression activated ER-α. ER-α activator Ferutinin partially nullified the regulatory function of miR-34a-5p/HDAC1 on osteogenic differentiation of BMSCs. In vivo experiments showed that miR-34a-5p overexpression enhanced the potential of bone metabolism in aged mice.

CONCLUSION

miR-34a-5p overexpression promoted osteogenic differentiation of BMSCs and enhanced bone metabolism by promoting ER-α activation via targeting HDAC1.

Age Related Osteoporosis: Targeting Cellular Senescence

Age-related chronic diseases are an enormous burden to modern societies worldwide. Among these, osteoporosis, a condition that predisposes individuals to an increased risk of fractures, substantially contributes to increased mortality and health-care costs in elderly. It is now well accepted that advanced chronical age is one of the main risk factors for chronical diseases. Hence, targeting fundamental aging mechanisms such as senescence has become a promising option in the treatment of these diseases. Moreover, for osteoporosis, the main pathophysiological concepts arise from menopause causing estrogen deficiency, and from aging. Here, we focus on recent advances in the understanding of senescence-related mechanisms contributing to age-related bone loss. Furthermore, treatment options for senile osteoporosis targeting senescent cells are reviewed.

The Role of Novel Bone Forming Agents in the Treatment of Osteoporosis

Osteoporosis is a growing epidemic that leads to significant morbidity and mortality among the elderly population due to associated fractures that lead to disabilities and reduced quality of life. Bisphosphonates are well-established as a first-line and cost-effective treatment for osteoporosis. Unfortunately, clinicians are often uncertain as to how to select treatments when bisphosphonates are ineffective as initial treatment or contraindicated. Romosozumab and abaloparatide are 2 alternative agents that have been recently FDA approved for the treatment of osteoporosis in postmenopausal women at high risk for fracture or patients who have failed or are intolerant to other osteoporosis therapies. Currently, the National Osteoporosis Foundation (NOF) has no formal recommendations in regard to these 2 novel agents. The purpose of this review is to help guide pharmacists on how to ensure appropriate utilization of these 2 novel bone-forming agents as potential alternatives to bisphosphonate therapy by providing evidence-based recommendations according to the current literature and key counseling points.

Skeletal effects of whole-body vibration in adult and aged mice

Low-amplitude, whole-body vibration (WBV) may be anabolic for bone. Animal studies of WBV have not evaluated skeletal effects in aged animals. We exposed 75 male BALB/c mice (7 month/young-adult; 22 month/aged) to 5 weeks of daily WBV (15 min/day, 5 day/wk; 90 Hz sine wave) at acceleration amplitudes of 0 (sham), 0.3, or 1.0 g. Whole-body bone mineral content (BMC) increased with time in 7 month (p < 0.001) but not 22 month (p = 0.34) mice, independent of WBV (p = 0.60). In 7 month mice, lower-leg BMC increased with time in 0.3 and 1.0 g groups (p < 0.005) but not in the sham group (p = 0.09), indicating a positive WBV effect. In 22 month mice, there were no changes with time in lower-leg BMC (p = 0.11). WBV did not affect tibial trabecular or cortical bone structure (by microCT), dynamic indices of trabecular or cortical bone formation, trabecular osteoclast surface, or the mass of the reproductive fat pad (p > 0.05). Each of these outcomes was diminished in 7 month versus 22 month animals (p < 0.05). In summary, 5 weeks of daily exposure to low-amplitude WBV had no skeletal effects in aged male mice. The potential of WBV to enhance bone mass in age-related osteoporosis is not supported in this preclinical study.