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Published ahead of print on December 12, 2003, doi:10.1165/rcmb.2003-0381OC
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American Journal of Respiratory Cell and Molecular Biology. Vol. 30, pp. 862-870, 2004
© 2004 American Thoracic Society
DOI: 10.1165/rcmb.2003-0381OC

Differential Translational Efficiency of ENaC Subunits During Lung Development

Gail Otulakowski, Bijan Rafii and Hugh O'Brodovich

CIHR Group in Lung Development, Research Institute of the Hospital for Sick Children; and Departments of Paediatrics and Physiology, University of Toronto, Toronto, Ontario, Canada

Address correspondence to: Gail Otulakowski, Ph.D., Programme in Lung Biology Research, Hospital for Sick Children Research Institute, 555 University Avenue, Toronto, ON, M5G 1X8 Canada. E-mail: gail.otulakowski{at}sickkids.ca

The amiloride-sensitive epithelial Na+ channel (ENaC), the rate-limiting step in epithelial Na+ transport, consists of three subunits: {alpha}, ß, and {gamma}. The abundance of mRNA encoding the {alpha}-subunit far surpasses the amount for other subunits, and considerably exceeds the predicted subunit protein stoichiometry. We evaluated 5'-untranslated region (UTR) expression and found that fetal rat lung uses alternative 5'UTRs for {alpha}-ENaC during development. Sucrose density gradient analysis of postnuclear supernatants from fetal rat lung homogenates demonstrated that all three ENaC subunits were associated with high molecular weight polysomes, indicating active translation of the mRNAs, but translational efficiency was much lower for the {alpha}-subunit. Sucrose density gradient distributions were comparable for the endogenously expressed {alpha}-ENaC 5'UTRs in rat lung at Fetal Day 20 or Postnatal Day 1 using Northern analysis. Although birth resulted in a global decrease in lung mRNA translation, the loading of ribosomes on ENaC subunit mRNAs was largely unaffected. Evaluation of cytokeratin 18 and vimentin mRNAs in these gradients suggested a cell-specific effect. We conclude that there are different translational efficiencies for ENaC subunits and that perinatal processes globally modulate lung mRNA translation.

Abbreviations: cytokeratin, CK • endoplasmic reticulum, ER • epithelial sodium channel, ENaC • eukaryotic initiation factor, eIF • polymerase chain reaction, PCR • protein S6 kinase, p70S6k • rapid amplification of cDNA ends, RACE • ribonuclease protection assay, RPA • RNA-ligation mediated, RLM • untranslated region, UTR




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