E. Salisbury, F. Gannon, N. Liles, E. Rodenberg, Z. Lazard, M. Merched-Sauvage, E. Olmsted-Davis, A. Davis

We have previously reported the rapid appearance and expansion of brown adipocytes in a model of heterotopic ossification (HO).  This model relies on induction of HO by delivery of adenovirus transduced cells expressing BMP2 to the quadriceps muscle of a mouse.  Microarray and immunohistochemical analysis of this model, as compared to a control that received similar cells transduced with an empty cassette adenovirus, shows the immediate elevation, within 48 hours, of key genes involved in energy metabolism, oxygen regulation, and adipogenesis.  We postulated that these cells function to regulate localized oxygen tension through their characteristic robust aerobic respiration, which plays a functional role in patterning of the newly forming bone and cartilage.  The study presented here extends this study by attempting to characterize the origin of the brown adipocytes, the regulation that may surround their differentiation and function, and the crosstalk that these cells have with other inflammatory molecules, including production of lymphatics.  The brown adipocytes are defined by positive expression of uncoupling protein 1 (UCP1) which is thought to be restricted to this cell type.  We further demonstrate the co-expression of two other markers, preadipocyte factor-1 (Pref-1), and peroxisome proliferator- activated receptor gamma coactivator 1 alpha (PGC-1α) in these cells, confirming the adipocyte phenotype.  To determine the origin of the cells we have employed a tracking mouse which expresses beta-galactosidase in cells which express or have expressed fatty acid binding protein-4 (FABP4) at one point of their lifespan.  Interestingly, we observe the expression of this marker on the sheath of adjacent peripheral nerves.  These nerve associated cells, which also express Pref-1 and UCP1, appear to be migrating from the nerve sheath towards the site of new bone formation.  Interestingly, at a slightly later time frame we observe the expression of osterix within these migrating cells.  From this preliminary data, we hypothesize that the nerve may house tentative progenitors essential for production of HO.  Finally, we are investigating the role of inflammatory mediators in this process.  Determination of the functional role of adipose, inflammation, and innervation in bone formation, may provide novel targets for the design of drugs that can inhibit HO, as well as targets for molecular imaging agents that can detect HO prior to the deposition of matrix.

Disclosures: None