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First published online July 31, 2009
Journal of Experimental Biology 212, 2595-2603 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.030627

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Activation of extracellular signal-regulated kinases during dehydration in the African clawed frog, Xenopus laevis

Amal Idris Malik and Kenneth B. Storey^*

Institute of Biochemistry and Department of Biology, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6

^* Author for correspondence (e-mail: kenneth_storey{at}carleton.ca)

Accepted 14 May 2009

In its native environment the African clawed frog, Xenopus laevis, can experience seasonally arid conditions that impose dehydration stress. Activation of intracellular signal transduction cascades can mediate and coordinate biochemical responses to ameliorate dehydration stress. This study examines the extracellular signal-regulated kinase (ERK) signaling cascade, analyzing responses of both upstream and downstream components in six tissues of X. laevis experiencing medium and high levels of dehydration, 16.6±1.59 and 28.0±1.6% of total body water lost, respectively. Immunoblotting was used to assess the three tiers in this mitogen-activated protein kinase (MAPK) cascade: the initiating MAPK kinase kinases (c-Raf, MEKK), the MAPK kinase (MEK1/2), and finally the MAPK (ERK1/2). The amount of active phosphorylated c-Raf^Ser338 rose by 2- to 2.5-fold under high dehydration in muscle, lung and skin whereas MEKK protein levels rose in these organs and also increased 4-fold in liver. As a result, phosphorylated active MEK1/2^Ser217/221 increased significantly by 2- to 6-fold during dehydration which, in turn, led to 2- to 6-fold increases in phospho-ERK^{Thr202/Tyr204} content in all tissues except skin. Given this clear demonstration of ERK cascade activation, two downstream targets of ERK2 were then evaluated. The amount of phosphorylated active transcription factor, STAT3^Ser727 and p90 ribosomal S6 kinase (RSK^Ser380) increased particularly in muscle, lung and kidney. Furthermore, RSK activation was correlated with a 5- to 8-fold increase in phosphorylation of its target, S6 ribosomal protein. Overall, the results show a strong conserved activation of the ERK cascade in X. laevis tissues in response to dehydration.

Key words: signal transduction cascade, mitogen-activated protein kinase, signal transducers and activators of transcription, p90 ribosomal S6 kinase, S6 ribosomal protein, dehydration tolerance

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