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Published ahead of print on December 12, 2003, doi:10.1165/rcmb.2003-0381OC

Am. J. Respir. Cell Mol. Biol., Volume 30, Number 6, June 2004, 862-870

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Submitted on October 28, 2003
Revised on December 9, 2003

Differential Translational Efficiency of ENaC Subunits During Lung Development

Gail Otulakowski1*, Bijan Rafii2, and Hugh O'Brodovich3

1 Lung Biology, Hospital for Sick Children Research Institute, Toronto, Ontario, Canada; Paediatrics, University of Toronto, Toronto, Ontario, Canada, 2 Lung Biology, Hospital for Sick Children Research Institute, Toronto, Ontario, Canada, 3 Lung Biology, Hospital for Sick Children Research Institute, Toronto, Ontario, Canada; Paediatrics, University of Toronto, Toronto, Ontario, Canada; Physiology, University of Toronto, Toronto, Ontario, Canada

* To whom correspondence should be addressed. 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}, {beta} 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 post-nuclear 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 1 day 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.




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