Identification of the skeletal progenitor cells forming osteophytes in osteoarthritis.

OBJECTIVES: Osteophytes are highly prevalent in osteoarthritis (OA) and are associated with pain and functional disability. These pathological outgrowths of cartilage and bone typically form at the junction of articular cartilage, periosteum and synovium. The aim of this study was to identify the cells forming osteophytes in OA. METHODS: Fluorescent genetic cell-labelling and tracing mouse models were induced with tamoxifen to switch on reporter expression, as appropriate, followed by surgery to induce destabilisation of the medial meniscus. Contributions of fluorescently labelled cells to osteophytes after 2 or 8 weeks, and their molecular identity, were analysed by histology, immunofluorescence staining and RNA in situ hybridisation. Pdgfrα-H2BGFP mice and Pdgfrα-CreER mice crossed with multicolour Confetti reporter mice were used for identification and clonal tracing of mesenchymal progenitors. Mice carrying Col2-CreER, Nes-CreER, LepR-Cre, Grem1-CreER, Gdf5-Cre, Sox9-CreER or Prg4-CreER were crossed with tdTomato reporter mice to lineage-trace chondrocytes and stem/progenitor cell subpopulations. RESULTS: Articular chondrocytes, or skeletal stem cells identified by Nes, LepR or Grem1 expression, did not give rise to osteophytes. Instead, osteophytes derived from Pdgfrα-expressing stem/progenitor cells in periosteum and synovium that are descendants from the Gdf5-expressing embryonic joint interzone. Further, we show that Sox9-expressing progenitors in periosteum supplied hybrid skeletal cells to the early osteophyte, while Prg4-expressing progenitors from synovial lining contributed to cartilage capping the osteophyte, but not to bone. CONCLUSION: Our findings reveal distinct periosteal and synovial skeletal progenitors that cooperate to form osteophytes in OA. These cell populations could be targeted in disease modification for treatment of OA

Defining the Role of Integrin-linked Kinase in Ureteric Bud Growth and Branching During Murine Kidney Development

The mammalian kidney develops through two major processes: nephrogenesis and renal branching morphogenesis. During branching morphogenesis, precise regulation of growth factor signaling is important for branching, elongation, and differentiation of the developing ureteric bud (UB). Defects in renal branching morphogenesis can lead to the development of kidneys incapable of supporting postnatal life. Despite their critical role in renal branching, the intracellular signaling pathways mediating the response to growth factor signaling in the developing kidney are poorly understood. Herein, I describe the crucial role of an intracellular scaffold protein, Integrin-linked Kinase (ILK), in kidney development. ILK is expressed within the developing UB and the conditional deletion of Ilk from this cell lineage during murine kidney development leads to postnatal renal hypoplasia or hypodysplasia. Characterization of the phenotype of Ilk-deficient kidneys demonstrated a role for Ilk in embryonic renal branching in vivo. At a mechanistic level, whole transcriptome analysis identified downstream gene targets of Ilk-dependent signaling involved in epithelial tube development that are mis-regulated in the absence of Ilk. The downregulated genes that were expressed in the developing UB included: i) known regulators of renal branching (Wnt11, Sox8, CXCR4), and ii) genes with unknown function in the developing kidney (Krt23, Slco4c1, Myb). Furthermore, activation of p38MAPK signaling was decreased in Ilk-deficient kidneys and a subset of Ilk-dependent genes requires p38MAPK activation for their expression in vitro. Loss of Ilk also results in misregulation of MAPK phosphatase activity in embryonic kidneys, with an upregulation of the Dual Specificity Phosphatase 8 (DUSP8). DUSP8 overexpression attenuates the activation of p38MAPK and decreases expression of Wnt11 and Krt23 in vitro. In sum, the combined approach of phenotypic, molecular, and genomic analysis of the role of ILK provided a comprehensive understanding of the role of this important protein in the developing kidney. This research improves our understanding of the regulation of intracellular signaling pathways during kidney development and provides knowledge that can be applied to the understanding and treatment of human renal disease.Ph