Identification of novel regulators of mesenchymal stem cell commitment to the brown adipocyte lineage
McDonald, Meghan Elizabeth
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Obesity is associated with an imbalance resulting from increased caloric intake and decreased energy expenditure. Humans have two types of adipose tissue: white adipose tissue (WAT), specialized for the storage of excess energy as lipid; and brown adipose tissue (BAT), which catabolizes lipid, releasing heat. Enhancing the development and/or activity of brown adipose tissue (BAT) or brown-like (beige) adipocytes within WAT is proposed as a means to enhance energy expenditure in obese individuals. Brown adipocyte progenitors share common origins with vascular cells; however, the mechanisms regulating commitment to these lineages are not understood. Bone Morphogenic Protein 7 (BMP7), a member of the Transforming Growth Factor β (TGFβ) superfamily, promotes the development of brown adipocytes. The goal of this study was to identify novel mechanisms regulating the commitment of mesenchymal stem cells (MSCs) to the brown adipocyte lineage, and to characterize the contrasting effects of BMP7 and TGFβ1 on cell fate. To address these questions, we used MSC culture models and identified several genes that are selectively regulated by BMP7 during BAT lineage commitment. These include the transcription factor Zinc Finger Protein of the Cerebellum 1 (Zinc1), Gremlin1, a secreted BMP antagonist, and regulators of cell shape, the Rho-associated protein kinases, Rock1/2. Repression of Gremlin1 and Zic1 expression are necessary for MSCs to undergo brown adipogenesis. It is well-established that TGFβ1 activates ROCK, and induces elongated, myofibroblast morphology in MSCs. We demonstrate that BMP7, in contrast, represses ROCK activity, altering actin dynamics and promoting a broadened morphology. Consistent with these findings, inhibition of ROCK activity or the downstream transcription factor Serum Response Factor (SRF), promotes brown adipocyte development. We conclude from these results that changes in cell shape and gene expression programs that are responsive to the status of the actin cytoskeleton are critical mediators of BAT lineage commitment. Our study has identified multiple genes involved in a brown fat/myofibroblast phenotypic switch. Modulation of ROCK or SRF activity may provide a novel means of promoting the development of brown/beige adipocytes in obese individuals. Further delineation of mechanisms regulating BAT development will lead to the identification of novel targets for anti-obesity therapeutics.
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