Cellular and molecular mechanisms regulating synovial joint development - Project Summary and Abstract Synovial joints are essential for body motion and quality of life. Their synovial cavity and lubricant-rich fluid permit unhindered joint motion and function and provide tissue protection and nourishment. While these aspects of synovial joint biology are well understood, little is known about how the cavity and its fluid actually develop during embryogenesis. At early fetal stages, the limb skeletal primordia are composed of continuous cartilaginous structures without joints. Joint development starts with appearance of an “interzone”, a tissue made of mesenchymal cells expressing the growth and differentiation factor 5 gene (Gdf5). We previously showed that Gdf5+ cell progenies produce most joint tissues over time and the synovial cavity forms in the middle of the interzone. Because the interzone cells are initially attached to each other, the cavitation process must involve their physical separation along the prospective articular line to facilitate the creation of a fluid-filled cavity. Previous studies indicated that interzone cells produce hyaluronan (HA) around the cavitation time, and this is accompanied by accumulation of a HA-rich matrix in local tissues. HA is a major component of extracellular matrix and synovial fluid and plays important roles in tissue homeostasis. In my preliminary studies, I found that just before cavitation onset, interzone cells in mouse embryo limbs express hyaluronan synthase 2 (HAS2, ‘the HA synthesizer’) and transmembrane protein 2 (TMEM2), a cell surface hyaluronidase that specifically cleaves high molecular weight HA into intermediate and biologically-active fragments. I also discovered that, morphologically, cavitation initiates with formation of microlumens along the prospective articular line and is completed soon afterwards when the pockets coalesce to generate a single one synovial cavity. This process is extremely rapid in the developing knee but is slower in digits. These and other novel data lead to my central hypothesis that joint cavitation is brought about by convergence of diverse but coordinated biological processes. Accordingly, Aim 1 is to determine the role of HAS2 and TMEM2 in joint cavitation using genetically modified mouse models. I will conditionally delete Has2 and/or Tmem2 in interzone cells (using Gdf5Cre mice) and subject resulting mutant embryos to detailed analysis. Aim 2 is to determine cellular and molecular mechanisms of cavitation. I will investigate downstream signaling pathways in response to changes in HA sizes and resulting interactions with cell surface CD44 receptor, regulating HA metabolism in synovial joint development and long-term maintenance. The project will provide novel insights into mechanisms underlying joint development and cavitation. In line with the K01 mechanism, the project will allow me to acquire new expertise in skeletal developmental and molecular biology and to integrate it with my previous training in bioengineering. This unique combination of expertise in two distinct but interrelated fields will allow me to establish an independent career distinct from my mentors and create novel therapeutic tools to repair and regenerate cartilage for the treatment of joint conditions such as osteoarthritis (OA).
