TRANSLATIONAL CONTROL OF AQUAPORIN-4 GENE EXPRESSION

The Aquaporin-4 gene encodes the major water channel of the central nervous system. Two predominant mRNAs and protein isoforms called AQP4-M1 and AQP4-M23 have been described in all the AQP4 expressing tissues that assemble in the plasma membrane to form supramolecular structures called Orthogonal Array of Particles (OAPs). The dimension of OAPs is tightly associated to the M1/M23 ratio and appears to be tissue specific and functionally important. Interestingly, in all AQP4 expressing tissues the M1/M23 protein isoform ratio does not parallel with the M1/M23 mRNAs ratio suggesting a translational control. In the present study we analyzed in details the translational mechanisms that may be involved in the regulation of the two isoforms and in particular the role of M1 mRNA 5'untranslated region (5'UTR) was investigated. Using isoform-specific RNAi we found that in rat astrocytes primary cultures a large proportion of M23 protein derives from M1 mRNA translation. Site-specific mutagenesis of the 5'UTR sequence of AQP4-M1 mRNA indicates that a multiple-site leaky scanning mechanism, an out-of-frame upstream ORF (uORF), and a re-initiation mechanism are able to modulate the M1/M23 ratio and consequently, OAPs formation. These mechanisms are likely shared by different species, including human and we can speculate their role in pathophysiological situations in which the organization of AQP4 in supramolecular structures (OAPs) are involved.