The role of ALK4 signaling in skeletal homeostasis and pathogenesis

靶向信号通路的新的基于生物学的疗法具有巨大的潜力， 治疗慢性疾病和经常使人衰弱的疾病的新方法。一种这样的方法是产生配体陷阱， 其捕获表现出与受体胞外结构域的高亲和力结合的蛋白质， 阻止这些分子激活靶细胞。使用Acvr 2 A/B配体捕获剂的临床前研究 (2A/B-LT），TGF-β超家族中的2型受体，共同证明了减少典型Smad 2/3 信号传导减少了慢性疾病小鼠模型的疾病负担。根据这些调查结果， 计划中的临床试验正在评估2A/B-LT在肥胖、慢性阻塞性肺疾病、 肺病、癌症引起的恶病质、2型糖尿病、类风湿性关节炎和炎症性贫血。 这些慢性疾病中的大多数也有骨骼表现，以及骨量和 然而，为什么2A/B-LT治疗具有积极的效果， 在骨骼上，哪些细胞群直接受到2A/B-LT的影响，以及 降低Smad 2/3活化调节骨细胞反应在很大程度上未知。填补这些空白， 知识将是至关重要的，以确定使用2A/B-LT的任何骨骼副作用。 Alk 4（TGF-β超家族中的1型受体）在早期骨祖细胞中的遗传缺失导致 增加小鼠的骨量。此外，来自我们小组和其他人的数据表明，骨稳态可能 受由于Acvr 2A/B的共享使用而导致的Smad 2/3和Smad 1/5/8信号传导的相反变化的影响 BMP和Alk 4活化配体之间的关系。因此，我们假设循环配体信号传导 Alk 4通过与Smad 2/3竞争， Smad 1/5/8信号传导。完成我们提案中的目标将使我们能够确定骨细胞靶点， 这些TGF-β超家族配体陷阱（SA 1）决定了配体陷阱Alk 4-FC是否具有治疗潜力 在骨（SA 2）中，检查Alk 4-FC在2种慢性疾病模型（SA 3）中的疗效，并告知 包括骨骼对激活Smad 2/3信号传导的配体的应答的分子事件（SA 4）。的 本提案中提出的问题意义重大，因为它们填补了有关潜在关闭的关键知识空白， 2A/B-LT用于治疗慢性疾病时可能对骨骼产生的靶向作用。我们相信 我们通过完成研究获得的结果是有影响力的，因为它们突出了使用2A/B-LT的潜力 和/或Alk 4-FC以预防/调节随年龄发生的全身性骨丢失，以及增强骨 骨折修复此外，我们的研究将大大推进我们对Smad 2/3作用的理解， Smad 1/5/8信号在成人骨骼，并提供了一个框架，了解TGF-β超家族生物学 作为确保骨和其他靶组织内稳态的机制。

New biologically-based therapeutics that target signaling pathways have tremendous potential to provide novel treatments for chronic and often debilitating diseases. One such approach is the creation of ligand traps, which capture proteins that exhibit high-affinity binding to a receptor extracellular domain, and subsequently prevent these molecules from activating target cells. Preclinical studies employing ligand traps for Acvr2A/B (2A/B-LT), type 2 receptors in the TGF-β superfamily, collectively demonstrate that reducing canonical Smad2/3 signaling reduces the disease burden in mouse models of chronic illnesses. Based on these findings, ongoing and planned clinical trials are evaluating the efficacy of 2A/B-LT in patients with obesity, chronic obstructive pulmonary disease, cancer-induced cachexia, type 2 diabetes, rheumatoid arthritis, and anemia of inflammation. Most of these chronic illnesses also have skeletal manifestations, and beneficial changes in bone mass and strength have been reported in mice treated with 2A/B-LT. However, why 2A/B-LT treatment has positive effects on the skeleton, what cell populations are directly affected by 2A/B-LT, and the mechanism(s) by which decreasing Smad2/3 activation regulates bone cell responses are largely unknown. Filling these gaps in knowledge will be critical to identifying any skeletal side effects of using 2A/B-LT. Findings from our lab suggest that genetic deletion of Alk4, a type 1 receptor within the TGF-β superfamily, in early osteoprogenitors results in increased bone mass in mice. Furthermore, data from our group and others suggests bone homeostasis may be influenced by opposing changes in Smad2/3 and Smad1/5/8 signaling due to shared usage of Acvr2A/B between BMPs and Alk4-activating ligands. We therefore hypothesize that circulating ligands signaling through Alk4 regulate bone homeostasis and pathogenesis by competition between Smad2/3 and Smad1/5/8 signaling. Completion of the aims in our proposal will allow us to identify the bone cell targets of these TGF-β superfamily ligand traps (SA1), determine whether the ligand trap Alk4-FC has therapeutic potential in bone (SA2), examine the efficacy of Alk4-FC in 2 chronic disease models (SA3), and inform about the molecular events that comprise the skeletal response to ligands that activate Smad2/3 signaling (SA4). The questions addressed in this proposal are significant because they fill a critical knowledge gap about potential off- target effects on the skeleton that may occur when 2A/B-LT is used to treat chronic diseases. We believe the results we obtain by completing our studies are impactful because they highlight the potential of using 2A/B-LT and/or Alk4-FC to prevent/modulate the generalized bone loss that occurs with age, as well as to enhance bone fracture repair. Further, our studies will significantly advance our understanding of the role of Smad2/3 and Smad1/5/8 signaling in the adult skeleton, and provide a framework for understanding TGF-β superfamily biology as a mechanism for ensuring homeostasis in bone and other target tissues.