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的一种主要疾病，经常使用抗吸收药物治疗，但无法恢复骨形成。因此，骨骼质量并未完全恢复。目前，增加骨形成是通过昂贵的生物制品实现的。可以被成本效益高的小分子作为靶点的途径仍未得到充分探索。CDK5是一种脯氨酸导向的丝氨酸/苏氨酸激酶，可被小分子抑制，如罗可维汀。我们发现CDK5是成骨细胞分化的负调控因子，成骨细胞是骨形成细胞。在我们的初步工作中，我们证明了在原代小鼠成骨细胞中，CDK5的siRNA敲除或罗斯科维汀抑制都能有效地促进成骨细胞的分化和成熟。同样，CDK5相互作用蛋白的敲除调节了成骨细胞的分化，这表明了一个以前在骨细胞中未曾描述的CDK5信号网络。大剂量地塞米松可抑制成骨细胞分化，成骨细胞分化是糖皮质激素性骨质疏松症(GIO)骨丢失的根本原因。有趣的是，通过siRNA敲除CDK5或用罗索维汀抑制CDK5可以消除地塞米松的有害影响，这表明这可能会改善体内的GIO。最后，我们首次有初步证据表明，在未受挑战的小鼠中应用罗可维汀可提高骨小梁质量。在这项建议中，我们旨在通过对条件性CDK5基因敲除小鼠的谱系追踪，揭示CDK5在间充质祖细胞中抑制体内成骨细胞生成的功能。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)来分析CDK5在条件性CDK5基因敲除小鼠的间质细胞和成骨细胞谱系中丢失的影响。我们进一步的目标是从生物化学的角度描述CDK5在原代成骨细胞中导致分化抑制的信号网络。最后，我们将确定干扰CDK5活性的小分子在多大程度上拯救小鼠的骨质疏松症模型。由于CDK5抑制剂已经应用于临床试验，我们拟议的研究将为评估患者队列中的骨代谢参数提供理论基础，以验证CDK5作为促进骨形成的靶点来治疗骨质疏松症。

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.