The brown adipocyte differentiation pathway in birds: An evolutionary road not taken

Background Thermogenic brown adipose tissue has never been described in birds or 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 (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.Nadejda V Mezentseva, Jaliya S Kumaratilake and Stuart A Newma

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Mesenchymal cells; blue line, stained mesenchymal cells; red line, limb bud preadipocytes (LBPAs); pink line, limb bud mesenchyme (LBM) adipocytes grown in chicken serum; green line, avian brown adipocyte-like cells (ABALCs).<p><b>Copyright information:</b></p><p>Taken from "The brown adipocyte differentiation pathway in birds: An evolutionary road not taken"</p><p>http://www.biomedcentral.com/1741-7007/6/17</p><p>BMC Biology 2008;6():17-17.</p><p>Published online 21 Apr 2008</p><p>PMCID:PMC2375860.</p><p></p

The brown adipocyte differentiation pathway in birds: An evolutionary road not taken-5

California Santa Cruz [43] genome browsers.<p><b>Copyright information:</b></p><p>Taken from "The brown adipocyte differentiation pathway in birds: An evolutionary road not taken"</p><p>http://www.biomedcentral.com/1741-7007/6/17</p><p>BMC Biology 2008;6():17-17.</p><p>Published online 21 Apr 2008</p><p>PMCID:PMC2375860.</p><p></p

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Lls grown in adipocyte differentiation medium (ADM) for 22 days with 10% horse serum. Transmission electron micrograph of a single cell from 22-day culture as in (a); L, lipid droplet. Oil-red and hematoxylin staining of light micrographs of 8-day cell cultures.Adipocytes from LBM cells grown in ADM with 10% horse serum. Adipocytes from vascular stromal cells (VSCs) grown in ADM with 10% horse serum. Adipocytes from LBM grown in ADM with 10% chicken serum. LBM cells grown in ADM with 10% FBS. Scale bars: (a) 10 μm, (b) 1 μm; (c)-(f) 50 μm<p><b>Copyright information:</b></p><p>Taken from "The brown adipocyte differentiation pathway in birds: An evolutionary road not taken"</p><p>http://www.biomedcentral.com/1741-7007/6/17</p><p>BMC Biology 2008;6():17-17.</p><p>Published online 21 Apr 2008</p><p>PMCID:PMC2375860.</p><p></p

The brown adipocyte differentiation pathway in birds: An evolutionary road not taken-9

DM) with 10% horse serum for 22 days. Adipocyte from the abdominal fat of a 20-day chicken embryo. Cy, cytoplasm; C, capillary; N, nucleus; M, mitochondrion; L, lipid droplet. Scale bars: 1 μm.<p><b>Copyright information:</b></p><p>Taken from "The brown adipocyte differentiation pathway in birds: An evolutionary road not taken"</p><p>http://www.biomedcentral.com/1741-7007/6/17</p><p>BMC Biology 2008;6():17-17.</p><p>Published online 21 Apr 2008</p><p>PMCID:PMC2375860.</p><p></p

The brown adipocyte differentiation pathway in birds: An evolutionary road not taken-4

Alyzed using one-way analysis of variance comparing means by the Tukey test. Results are shown as means ± standard error (= 3)<p><b>Copyright information:</b></p><p>Taken from "The brown adipocyte differentiation pathway in birds: An evolutionary road not taken"</p><p>http://www.biomedcentral.com/1741-7007/6/17</p><p>BMC Biology 2008;6():17-17.</p><p>Published online 21 Apr 2008</p><p>PMCID:PMC2375860.</p><p></p

The brown adipocyte differentiation pathway in birds: An evolutionary road not taken-7

UCP1 is known to be absent are indicated by a black dot. The question mark indicates the span within the evolutionary history of the amniotes during which UCP1 and the capacity to produce functional brown fat was lost.<p><b>Copyright information:</b></p><p>Taken from "The brown adipocyte differentiation pathway in birds: An evolutionary road not taken"</p><p>http://www.biomedcentral.com/1741-7007/6/17</p><p>BMC Biology 2008;6():17-17.</p><p>Published online 21 Apr 2008</p><p>PMCID:PMC2375860.</p><p></p

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Y luciferase activities were compared using one-way analysis of variance using the Tukey test. Results are shown as means ± standard error (= 8).<p><b>Copyright information:</b></p><p>Taken from "The brown adipocyte differentiation pathway in birds: An evolutionary road not taken"</p><p>http://www.biomedcentral.com/1741-7007/6/17</p><p>BMC Biology 2008;6():17-17.</p><p>Published online 21 Apr 2008</p><p>PMCID:PMC2375860.</p><p></p

The brown adipocyte differentiation pathway in birds: An evolutionary road not taken-0

Lls grown in adipocyte differentiation medium (ADM) for 22 days with 10% horse serum. Transmission electron micrograph of a single cell from 22-day culture as in (a); L, lipid droplet. Oil-red and hematoxylin staining of light micrographs of 8-day cell cultures.Adipocytes from LBM cells grown in ADM with 10% horse serum. Adipocytes from vascular stromal cells (VSCs) grown in ADM with 10% horse serum. Adipocytes from LBM grown in ADM with 10% chicken serum. LBM cells grown in ADM with 10% FBS. Scale bars: (a) 10 μm, (b) 1 μm; (c)-(f) 50 μm<p><b>Copyright information:</b></p><p>Taken from "The brown adipocyte differentiation pathway in birds: An evolutionary road not taken"</p><p>http://www.biomedcentral.com/1741-7007/6/17</p><p>BMC Biology 2008;6():17-17.</p><p>Published online 21 Apr 2008</p><p>PMCID:PMC2375860.</p><p></p

The brown adipocyte differentiation pathway in birds: An evolutionary road not taken-1

DM) with 10% horse serum for 22 days. Adipocyte from the abdominal fat of a 20-day chicken embryo. Cy, cytoplasm; C, capillary; N, nucleus; M, mitochondrion; L, lipid droplet. Scale bars: 1 μm.<p><b>Copyright information:</b></p><p>Taken from "The brown adipocyte differentiation pathway in birds: An evolutionary road not taken"</p><p>http://www.biomedcentral.com/1741-7007/6/17</p><p>BMC Biology 2008;6():17-17.</p><p>Published online 21 Apr 2008</p><p>PMCID:PMC2375860.</p><p></p