Molecular and cellular basis of synovial joint formation

Abstract

Synovial joints are the most common joints facilitating skeletal movement. Healthy articular cartilage in synovial joints provides a smooth, wear-resistant structure that reduces friction and absorbs impacts. These joints are encased in a joint capsule, with an outer layer of fibrous connective tissues and a synovial inner layer, stabilized by ligaments and tendons. Degeneration of articular cartilage is a hallmark of joint diseases affecting millions of people worldwide. Damaged articular cartilage does not heal well and the reason is not well understood. To develop an effective biological treatment would require a clear understanding of how synovial joints are formed in development, and the progenitor cells that contribute to different structures of a joint.

Here, I identified for the first time that Lgr5, a well-known stem cell marker, is highly expressed in the interzone region of a developing joint. Concurrent with the initiation of interzone formation, Lgr5 is expressed and maintained at the early stages of joint formation, but is down-regulated with joint maturation. Relative to an early interzone marker (Gdf5), Lgr5 is expressed later in development and is more restricted to the central region of the interzone. Using the Lgr5-eGFP-IRES-CreERT2 mice as a novel genetic tool to track the fate of Lgr5-expressinginterzone cells in vivo, I showed that Lgr5 marks a pool of progenitor cells that contribute to the major tissues of a joint, including the articular cartilage, ligaments and tendons, meniscus, and the synovial membrane. I further identified an extracellular matrix gene, Collagen XXII (Col22a1), as a marker for early-differentiated articular chondrocytes, with Lgr5and Col22a1co-expressing cells as potential intermediate progenitor cells committed to become articular chondrocytes. I propose this sequential gene expression pattern represents the progressive differentiation of interzone cells towards an articular chondrocyte cell fate.

Using these new gene markers and mouse reagents, I further studied the molecular basis of abnormal joint formation in a mouse model for Brachydactyly type A1 (BDA1), with missing middle phalangeal bone in digit V. A previous hypothesis proposed that this is due to the failure of interzone initiation. Here, I showed that the formation of all phalangeal joints is initiated, and the missing phalangeal joint in digit V results from a failure of interzone maintenance leading to joint fusion. The BDA1 mouse carries anE95Kmutation in Ihh that results in an excessive IHH signaling into the interzone. I provided additional information showing this resulted in an expanded interzone region expressing Gdf5but delayed onset of Lgr5expression that lead to the failure of joint cavitation in digit V development at the stage when Col22a1is expressed, and thus a joint is not formed.

Together, I provided new molecular insights into the maintenance of the interzone cells in synovial joint formation and the cellular basis leading to joint fusion in the mouse model for BDA1.Importantly, I have identified a pool of Lgr5-expressing progenitor cells in joint formation that could be explored for the development of potential therapeutic treatments of damaged joints.