A novel function of cyclin-dependent kinase 5 (Cdk5) in bone integrity - a potential therapeutic target to treat osteoporosis

骨质疏松症是老年人群的主要疾病，经常用不能恢复骨形成的抗吸收剂治疗。因此，骨质量没有完全恢复。骨形成的增加目前是通过昂贵的生物制剂实现的。可以通过具有成本效益的小分子靶向的途径仍然没有充分探索。Cdk 5是一种脯氨酸导向的丝氨酸/苏氨酸激酶，可以用小分子如Roscovitine抑制。我们发现Cdk 5是成骨细胞（骨形成细胞）分化的负调节因子。在我们的初步工作中，我们证明，无论是siRNA敲低Cdk 5或抑制与Roscovitine在原代小鼠骨祖细胞有力地增强成骨细胞的分化和成熟。类似地，Cdk 5相互作用蛋白的敲低调节成骨细胞分化，表明以前在骨细胞中没有描述的Cdk 5信号网络。高剂量地塞米松暴露抑制成骨细胞分化，这是糖皮质激素诱导的骨质疏松症（GIO）中骨丢失的根本原因。有趣的是，通过siRNA敲低CDK 5或用Roscovitine抑制CDK 5消除了地塞米松的有害作用，表明这可能改善体内GIO。最后，我们有初步证据表明，在未激发的小鼠中应用Roscovitine可提高骨小梁质量。在这个建议中，我们的目标是解开Cdk 5在间充质祖细胞的功能，抑制成骨细胞在体内的谱系追踪条件Cdk 5基因敲除小鼠。我们将通过最先进的骨完整性分析（微型计算机断层扫描、动态骨组织形态计量学和生物力学测试）分析条件性Cdk 5敲除小鼠中间充质细胞和成骨细胞谱系中CDK 5缺失的影响。我们进一步的目标是生化表征Cdk 5在原代成骨细胞中导致分化抑制的信号网络。最后，我们将确定在何种程度上小分子干扰Cdk 5活性救援骨质疏松症模型小鼠。由于CDK 5抑制剂已经应用于临床试验，我们提出的研究将提供评估患者队列中骨代谢参数的基本原理，以验证CDK 5作为增加骨形成治疗骨质疏松症的靶点。

Osteoporosis, a major disease of the elderly population is frequently treated with anti-resorptive agents that fail to restore bone formation. Thus, bone quality is not fully restored. Increase in bone formation is currently achieved by costly biologicals. Pathways that can be targeted by cost-effective small molecules are still ill-explored. Cdk5, a proline-directed serine/threonine kinase, can be inhibited with small molecules, such as Roscovitine. We discovered that Cdk5 is a negative regulator of differentiation of osteoblasts, the bone forming cells. In our preliminary work we demonstrate that either siRNA knockdown of Cdk5 or inhibition with Roscovitine in primary murine osteoprogenitor cells potently enhance osteoblast differentiation and maturation. Similarly, knockdown of Cdk5 interacting proteins modulate osteoblast differentiation, suggesting a Cdk5 signaling network that had not been described in bone cells before. High dose exposure of dexamethasone inhibits osteoblast differentiation, an underlying cause of bone loss in glucocorticoid-induced osteoporosis (GIO). Intriguingly, knockdown of Cdk5 by siRNA or inhibition of CDK5 with Roscovitine abrogates the deleterious effects of dexamethasone, suggesting that this might ameliorate GIO in vivo. Finally, we have first preliminary evidence that application of Roscovitine in unchallenged mice elevates trabecular bone mass. In this proposal we aim to unravel the function of Cdk5 in mesenchymal progenitor cells to inhibit osteoblastogenesis in vivo by lineage tracing in conditional Cdk5 knockout mice. We will analyze the impact of loss of CDK5 in mesenchymal cells and osteoblast lineage in conditional Cdk5 knockout mice by state-of-the-art bone integrity analyses (micro computer tomography, dynamic bone histomorphometry and biomechanical tests). We further aim to biochemically characterize the signaling network of Cdk5 in primary osteoblasts that leads to inhibition of differentiation. Finally, we will determine to which extent small molecules interfering with Cdk5 activity rescue osteoporosis models in mice. Since CDK5 inhibitors are already applied in clinical trials, our proposed study will provide the rationale to assess bone metabolic parameters in patient cohorts to validate CDK5 as a target to increase bone formation for treatment of osteoporosis.