Loss of cyclin-dependent kinase 1 impairs bone formation, but does not affect the bone-anabolic effects of parathyroid hormone.

Bone mass is maintained by a balance between osteoblast-mediated bone formation and osteoclast-mediated bone resorption. Although recent genetic studies have uncovered various mechanisms that regulate osteoblast differentiation, the molecular basis of osteoblast proliferation remains unclear. Here, using an osteoblast-specific loss-of-function mouse model, we demonstrate that cyclin-dependent kinase 1 (Cdk1) regulates osteoblast proliferation and differentiation. Quantitative RT-PCR analyses revealed that Cdk1 is highly expressed in bone and is down-regulated upon osteoblast differentiation. We also noted that Cdk1 is dispensable for the bone-anabolic effects of parathyroid hormone (PTH). Cdk1 deletion in osteoblasts led to osteoporosis in adult mice due to low bone formation, but did not affect osteoclast formation in vivo Cdk1 overexpression in osteoblasts promoted proliferation, and conversely, Cdk1 knockdown inhibited osteoblast proliferation and promoted differentiation. Of note, we provide direct evidence that PTH's bone-anabolic effects occur without enhancing osteoblast proliferation in vivo Furthermore, we found that Cdk1 expression in osteoblasts is essential for bone fracture repair. These findings may help reduce the risk of nonunion after bone fracture and identify patients at higher risk for nonresponse to PTH treatment. Collectively, our results indicate that Cdk1 is essential for osteoblast proliferation and that it functions as a molecular switch that shifts osteoblast proliferation to maturation. We therefore conclude that Cdk1 plays an important role in bone formation.

The long noncoding RNA Crnde regulates osteoblast proliferation through the Wnt/ß-catenin signaling pathway in mice.

In the past decade, a growing importance has been placed on understanding the significance of long noncoding RNAs (lncRNAs) in regulating development, metabolism, and homeostasis. Osteoblast proliferation and differentiation are essential elements in skeletal development, bone metabolism, and homeostasis. However, the underlying mechanisms of lncRNAs in the process of osteoblast proliferation and differentiation remain largely unknown. Through comprehensive analysis of lncRNAs during bone formation, we show that colorectal neoplasia differentially expressed (Crnde), previously viewed as a cancer-related lncRNA, is an important regulator of osteoblast proliferation and differentiation. Crnde was found to be expressed in osteoblasts, and its expression was induced by parathyroid hormone. Furthermore, Crnde knockout mice developed a low bone mass phenotype due to impaired osteoblast proliferation and differentiation. Overexpression of Crnde in osteoblasts promoted their proliferation, and conversely, reduced Crnde expression inhibited osteoblast proliferation. Although ablation of Crnde inhibited osteoblast differentiation, overexpression of Crnde restored it. Finally, we provided evidence that Crnde modulates bone formation through Wnt/ß-catenin signaling. Therefore, our data suggest that Crnde is a novel regulator of bone metabolism.

Bone Turnover Markers as a New Predicting Factor for Nonunion After Spinal Fusion Surgery.

STUDY DESIGN: Retrospective observational study. OBJECTIVE: We investigated whether bone turnover markers could be a useful indicator for prediction of nonunion. SUMMARY OF BACKGROUND DATA: Nonunion is a major complication of lumbar spinal fusion surgery. The involvement of bone turnover in the process of bony union in spinal fusion surgery is, however, poorly understood. METHODS: Of the 74 patients analyzed, 13 were diagnosed with nonunion. We evaluated the significance of the following risk factors: age, sex, number of fused segments, serum levels of total alkaline phosphatase, procollagen type 1 amino-terminal propeptide (P1NP), tartrate-resistant acid phosphatase 5b (TRACP-5b), and albumin, and history of diabetes mellitus, cigarette smoking, or alcohol use. We also defined the bone turnover ratio (BTR) as a value that equals serum TRACP-5b concentration divided by serum P1NP concentration to evaluate patients' individual bone turnover balance and investigated the significance of BTR as a risk factor. RESULTS: Univariate analysis showed that older age, malnutrition, and lower P1NP are risk factors for nonunion. Stepwise logistic regression analysis revealed that in the presence of lower P1NP, higher TRACP-5b becomes a risk factor. Furthermore, we identified BTR as the most significant risk factor for nonunion. The optimum cut-off value of BTR by receiver-operating characteristic curve was 11.74. CONCLUSION: These findings show a relation between bone turnover and nonunion after spinal fusion surgery. The measurement of bone turnover markers could potentially be used to predict nonunion after spinal fusion surgery. LEVEL OF EVIDENCE: 4.

The Indispensable Role of Cyclin-Dependent Kinase 1 in Skeletal Development.

Skeletal development is tightly regulated through the processes of chondrocyte proliferation and differentiation. Although the involvement of transcription and growth factors on the regulation of skeletal development has been extensively studied, the role of cell cycle regulatory proteins in this process remains elusive. To date, through cell-specific loss-of-function experiments in vivo, no cell cycle regulatory proteins have yet been conclusively shown to regulate skeletal development. Here, we demonstrate that cyclin-dependent kinase 1 (Cdk1) regulates skeletal development based on chondrocyte-specific loss-of-function experiments performed in a mouse model. Cdk1 is highly expressed in columnar proliferative chondrocytes and is greatly downregulated upon differentiation into hypertrophic chondrocytes. Cdk1 is essential for proper chondrocyte proliferation and deletion of Cdk1 resulted in accelerated differentiation of chondrocytes. In vitro and ex vivo analyses revealed that Cdk1 is an essential cell cycle regulatory protein for parathyroid hormone-related peptide (PTHrP) signaling pathway, which is critical to chondrocyte proliferation and differentiation. These results demonstrate that Cdk1 functions as a molecular switch from proliferation to hypertrophic differentiation of chondrocytes and thus is indispensable for skeletal development. Given the availability of inhibitors of Cdk1 activity, our results could provide insight for the treatment of diseases involving abnormal chondrocyte proliferation, such as osteoarthritis.