The brown adipocyte differentiation pathway in birds: An evolutionary road not taken
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BioMed Central
Open Access
Research article
The brown adipocyte differentiation pathway in birds: An evolutionary road not taken Nadejda V Mezentseva1, Jaliya S Kumaratilake2 and Stuart A Newman*1 Address: 1Department of Cell Biology and Anatomy, New York Medical College, Valhalla, NY 10595, USA and 2Discipline of Anatomical Sciences, University of Adelaide, Adelaide, SA 5005, Australia Email: Nadejda V Mezentseva - [email protected]; Jaliya S Kumaratilake - [email protected]; Stuart A Newman* - [email protected] * Corresponding author
Published: 21 April 2008 BMC Biology 2008, 6:17
doi:10.1186/1741-7007-6-17
Received: 6 March 2008 Accepted: 21 April 2008
This article is available from: http://www.biomedcentral.com/1741-7007/6/17 © 2008 Mezentseva et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract Background: Thermogenic brown adipose tissue has never been described in birds or other nonmammalian vertebrates. Brown adipocytes in mammals are distinguished from the more common white fat adipocytes by having numerous small lipid droplets rather than a single large one, elevated numbers of mitochondria, and mitochondrial expression of the nuclear gene UCP1, the uncoupler of oxidative phosphorylation responsible for non-shivering thermogenesis. Results: We have identified in vitro inductive conditions in which mesenchymal cells isolated from the embryonic chicken limb bud differentiate into avian brown adipocyte-like cells (ABALCs) with the morphological and many of the biochemical properties of terminally differentiated brown adipocytes. Avian, and as we show here, lizard species lack the gene for UCP1, although it is present in amphibian and fish species. While ABALCs are therefore not functional brown adipocytes, they are generated by a developmental pathway virtually identical to brown fat differentiation in mammals: both the common adipogenic transcription factor peroxisome proliferator-activated receptor-γ (PPARγ), and a coactivator of that factor specific to brown fat differentiation in mammals, PGC1α, are elevated in expression, as are mitochondrial volume and DNA. Furthermore, ABALCs induction resulted in strong transcription from a transfected mouse UCP1 promoter. Conclusion: These findings strongly suggest that the brown fat differentiation pathway evolved in a common ancestor of birds and mammals and its thermogenicity was lost in the avian lineage, with the degradation of UCP1, after it separated from the mammalian lineage. Since this event occurred no later than the saurian ancestor of birds and lizards, an implication of this is that dinosaurs had neither UCP1 nor canonically thermogenic brown fat.
Background Most mammalian species have two types of adipose tissue, white and brown fat, both of which contain adi
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