The brown adipocyte differentiation pathway in birds: an evolutionary road not taken
Thermogenic brown adipose tissue has never been described in birds and other non-mammalian 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 (ABALC) with the morphological and many of the biochemical properties of terminally differentiated brown adipocytes. Although avian, and as we show here, lizard species lack the gene for UCP1, it is present in amphibian and fish species.
Therefore, while ABALC are 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-gamma (PPARgamma), and a coactivator of that factor specific to brown fat differentiation in mammals, PGC1alpha, are elevated in expression, as are mitochondrial volume and DNA. Furthermore, ABALC induction resulted in strong transcription from a transfected mouse UCP1 promoter.
Conclusions: 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.
Author: Nadejda V. Mezentseva, Jaliya S. Kumaratilake and Stuart A Newman
Credits/Source: BMC Biology 2008, 6:17
Published on: 2008-04-21
Copyright by the authors listed above - made available via BioMedCentral (Open Access). Please
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