2018 Theses Doctoral

Targeting primary cilia-mediated mechanotransduction to promote whole bone formation

Spasic, Milos

Osteoporosis is a devastating condition characterized by decreased bone mass, and affects over 50% of the population over 50 years old. Progression of osteoporosis results in significantly heightened risk of fracture leading to loss of mobility, prolonged rehabilitation, and even mortality due to extended hospitalization. Current therapeutic options exist to combat low bone mass, but these treatments are being met with increasing concern as reports emerge of atypical fractures and necrosis. Thus, new therapeutic strategies are required.
Bone is highly dynamic, and it has long been known that physical load is a potent stimulus of bone formation. Despite this, none of the current treatments for bone disease leverage the inherent mechanosensitivity of bone – the ability of bone cells to sense and respond to mechanical forces such as exercise. One potential therapeutic target is the primary cilium. Primary cilia are solitary antenna-like organelles, and over the last 20 years have been identified as a critical cellular mechanosensor. Primary cilia and cell mechanotransduction are critical to the function of numerous cells and tissues. Thus, understanding primary cilia-mediated mechanotransduction has potential applications in treating kidney and liver disease, atherosclerosis, osteoarthritis, and even certain cancers. Previous work from our group has demonstrated that disruption of the cilium impairs bone cell mechanosensitivity, resulting in abrogated whole bone adaptation in response to physical load.
In this thesis we examine the potential of targeting the primary cilium to enhance bone cell mechanosensitivity and promote whole bone formation. First, we demonstrate the pharmacologically increasing primary cilia length significantly enhances cell mechanotransduction. Next, we expand our list of candidate compounds to manipulate ciliogenesis through the use of high-throughput drug screening. We developed an automated platform for culturing, staining, imaging, and analyzing nearly 7000 small molecules with known biologic activity, and classify them based on mechanism of action. One of these compounds is then used in a co-culture model to study the effects of manipulating osteocyte primary cilia-mediated mechanosensing on pro-osteogenic paracrine signaling to promote the activity of bone-forming osteoblasts and osteogenic differentiation of mesenchymal stem cells. Finally, we translate our in vitro findings into an in vivo model of load-induced bone formation using the same compound to enhance cell mechanotransduction. We demonstrate that we can sensitize bones to mechanical stimulation to enhance load-induced bone formation in healthy and osteoporotic animals, with minimal adverse effects. Together, this work demonstrates the therapeutic potential and viability of targeting primary cilia-mediated mechanotransduction for treating bone diseases.