Establishment of fibrillin-deficient osteoprogenitor cell lines identifies molecular abnormalities associated with extracellular matrix perturbation of osteogenic differentiation.

Fibrillin-1 and fibrillin-2 are structural components of the extracellular matrix which are also involved in modulating local TGFβ and BMP bioavailability. Loss of fibrillin-1 or fibrillin-2 is associated with perturbed osteoblast maturation principally as the result of unbalanced TGFβ and BMP signaling. Here, we demonstrated that stable expression of small hairpin RNAs against fibrillin-1(Fbn1) or fibrillin-2 (Fbn2) transcripts in the clonal osteoprogenitor cell line Kusa-A1 led to the same phenotypic and molecular manifestations as germline Fbn1- or Fbn2-null mutations in primary calvarial osteoblast cultures. Proof-of-concept experiments are also presented showing that Fbn1- or Fbn2-silenced Kusa-A1 cell lines are suitable models to identify candidate determinants of osteogenesis which are under the control of extracellular microfibrils. Specific findings included: the inference of a potential role for fibrillin-1-mediated cell-matrix interactions in regulating Kusa-A1 proliferation; the possibility of fibrillin-2 involvement in modulating the activity of transcription factor Runx2 by restricting microRNA expression and/or processing; and the suggestion that fibrillin-1 and fibrillin-2 influence Notch signaling indirectly by differentially regulating BMP signaling. Collectively, the data reiterated the notion that fibrillin-1 and fibrillin-2 exert opposite effects on osteoblast differentiation through the discrete modulation of a broad network of interacting signaling molecules.