The overarching goal of the Lassar lab is to understand how different cells types (i.e., articular or epiphyseal chondrocytes, ligaments and synoviocytes) emerge from a common precursor population during the formation of the synovial joint. Towards this goal, work in the Lassar lab focuses on the transcriptional regulatory pathways that regulate chondrocyte formation and maturation, elucidation of how mechanical loading regulates gene expression in the joint, and a molecular dissection of the signals and transcription factors that maintain articular cartilage stem cells. We are studying how the initial cartilage template is induced in the embryo and are trying to elucidate how chondrocytes “decide” whether to undergo maturation, which leads to endochondral ossification, or remain immature, as in the articular cartilage of our joints. The Lassar lab has recently identified a stem cell population for articular cartilage; and we are employing genome-wide ATAC-Seq, ChIP-Seq, and RNA-Seq methodologies to elucidate the transcriptional regulators that control both the induction and maintenance of these stem cells.

Identification of factors that are necessary for Sox9 to activate the chondrogenic differentiation program: The transcription factor Sox9 is critical for mesenchymal cells to commit to and execute the chondrogenic differentiation program; in its absence chondrogenesis is blocked. Sox9 both directly activates chondrocyte differentiation markers and induces the expression of Sox5 and Sox6, which work together with Sox9 to activate the chondrocyte differentiation program. However, in addition to its essential role in initiating the chondrogenic differentiation program, Sox9 is also expressed in a number of other cells types, including neural stem cells, oligodendrocyte precursors, intestinal stem cells, hair follicle stem cells, and the developing testis. Taken together, these findings indicate that the ability of Sox9 to induce the chondrocyte differentiation program is context dependent, suggesting that additional factors are necessary for Sox9 to activate chondrocyte-specific transcriptional targets. We have recently identified additional chondrogenic competence factors, and are studying how these factors establish the competence for Sox9 to induce chondrocyte formation.

Identification of the transcriptional network that controls the formation and maintenance of articular cartilage stem cells: Lineage tracing studies have suggested that growth plate and articular chondrocytes arise from distinct progenitor populations, such that articular chondrocytes share a common origin with synovial cells that line the joint cavity. The superficial-most layer of articular cartilage is distinguished from deeper layers by expression of lubricin. Lubricin is a secreted proteoglycan encoded by the Prg4 locus, and is highly expressed by both superficial zone articular chondrocytes and synoviocytes, and notably is expressed in the progenitor cells of the articular cartilage. We have recently mapped the DNA sequences that control the expression of Prg4/lubricin by performing ATAC-Seq analysis (which detects regions of the genome with increased accessibility for in vitro transposon insertion). This approach identified four regions surrounding the Prg4 locus that display increased accessibility for in vitro transposon insertion specifically in lubricin-expressing superficial zone chondrocytes, and which work in combination to drive reporter gene expression specifically in superficial zone chondrocytes. A bioinformatics analysis of both this ATAC-Seq data and complementary RNA-Seq data has identified transcription factor families that are specifically expressed in the Prg4/lubricin-expressing stem cells of articular cartilage. We are currently determining the role of these transcription factors in regulating either the formation or maintenance of articular cartilage stem cells; with the goal of employing this knowledge to restore articular cartilage stem cells in degenerating joint tissue.