Cox Robert M, Munoz-Garcia Agusti, Jurkowitz Marianne S, Williams Joseph B
Department of Evolution, Ecology, and Organismal Biology, Ohio State University, Columbus, Ohio 43210, USA.
Physiol Biochem Zool. 2008 Jan-Feb;81(1):97-105. doi: 10.1086/522652. Epub 2007 Nov 20.
Skin is an important avenue of water loss in terrestrial birds, so environmental conditions that necessitate water conservation should favor physiological mechanisms that reduce cutaneous water loss (CWL). Skin resistance to CWL is conferred by a barrier of lipid molecules located in the stratum corneum (SC), the outer layer of the epidermis. In mammals, SC barrier function depends on the conversion of cerebrosides to ceramides by the enzyme beta -glucocerebrosidase ( beta -GlcCer'ase). Avian SC contains both cerebrosides and ceramides, suggesting that observed plasticity in CWL may be mediated by changes in beta -GlcCer'ase activity and resultant SC lipid composition. We tested the hypothesis that changes in ambient humidity would alter beta -GlcCer'ase activity by acclimating house sparrows (Passer domesticus) to either dry (6.5 g H(2)O m(-3) absolute humidity) or humid (31 g H(2)O m(-3)) conditions for 5 and 21 d at 30 degrees C and then measuring beta -GlcCer'ase activity from SC homogenates. Our results provide the first characterization of beta -GlcCer'ase activity in any nonmammalian vertebrate. Relative to nonacclimated controls, both dry- and humid-acclimated sparrows had significantly elevated beta -GlcCer'ase activity at 21 d postacclimation. Across individuals, we observed negative correlations between beta -GlcCer'ase activity and both CWL and SC ceramide content. Although dry- and humid-acclimated sparrows did not differ in beta -GlcCer'ase activity, these results are consistent with our findings that both humidity treatments caused a reduction in CWL and similar changes in SC lipid composition. Our results demonstrate physiological plasticity in CWL and provide tentative support for a role of beta -GlcCer'ase in mediating this response.
皮肤是陆生鸟类水分流失的重要途径，因此需要节约用水的环境条件应有利于减少皮肤水分流失（CWL）的生理机制。皮肤对CWL的抵抗力由位于角质层（SC）（表皮的外层）中的脂质分子屏障赋予。在哺乳动物中，SC屏障功能取决于β-葡萄糖脑苷脂酶（β-GlcCer'ase）将脑苷脂转化为神经酰胺的过程。鸟类的SC同时含有脑苷脂和神经酰胺，这表明观察到的CWL可塑性可能由β-GlcCer'ase活性的变化以及由此产生的SC脂质组成的变化介导。我们通过将家雀（Passer domesticus）在30摄氏度下分别适应干燥（绝对湿度6.5 g H₂O m⁻³）或潮湿（31 g H₂O m⁻³）条件5天和21天，然后测量SC匀浆中的β-GlcCer'ase活性，来检验环境湿度变化会改变β-GlcCer'ase活性这一假设。我们的结果首次对任何非哺乳动物脊椎动物的β-GlcCer'ase活性进行了表征。相对于未适应的对照组，干燥和潮湿适应的麻雀在适应后21天时β-GlcCer'ase活性均显著升高。在个体之间，我们观察到β-GlcCer'ase活性与CWL和SC神经酰胺含量之间呈负相关。尽管干燥和潮湿适应的麻雀在β-GlcCer'ase活性上没有差异，但这些结果与我们的发现一致，即两种湿度处理均导致CWL降低以及SC脂质组成发生类似变化。我们的结果证明了CWL的生理可塑性，并为β-GlcCer'ase在介导这种反应中的作用提供了初步支持。
