Translational regulation mechanisms of aquaporin‐4 supramolecular organization in astrocytes

Abstract

The two predominant isoforms of Aquaporin-4 (AQP4), AQP4-M23 and AQP4-M1, assemble in the plasma membrane to form supramolecular structures called Orthogonal Array of Particles (OAPs) whose dimension is tightly associated to the M1/M23 ratio. Here, we explore translational regulation contribution to M1/M23 expression in primary cultures of rat astrocytes, and analyze the role of M1 mRNA 5'untranslated region (5'UTR) in this mechanism. Using isoform-specific RNAi we found that in rat astrocytes primary cultures a large proportion of M23 protein derives from M1 mRNA translation. Furthermore, 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 reinitiation mechanism are able to modulate the M1/M23 ratio and consequently, OAPs formation. These mechanisms are likely to be shared by different species, including human, and they can also be assumed to play a role in those pathophysiological situations where the organization of AQP4 in supramolecular structures (OAPs) is involved. Finally, we report that, when transfected in Hela cells, the longer rat AQP4 isoform, called Mz, which is not present in human impairs OAPs formation.

AQP4-Dependent Water Transport Plays a Functional Role in Exercise-Induced Skeletal Muscle Adaptations

“…This finding suggests that post-trascriptional regulation mechanisms may be involved. In particular, as recently reported [30], translational regulation of AQP4-M1 mRNA via Leaky Scanning and Reinitiation mechanisms, associated to an out-of-frame uORF, is able to modulate the M1/M23 ratio and AQP4 protein abundance. Many different types of cell stress are able to modify the re-initiation efficiency [31].…”

Different Aquaporin-4 Expression in Glioblastoma Multiforme Patients with and without Seizures

“…The lack of correlation between membrane expression and mRNA levels may be due to a leaky scanning mechanism during translation of AQP-4 mRNA (25). Consistent with this hypothesis, different isoforms of AQP-4 have been identified, but only the M23 isoform is capable of forming OAPs, leading to polarized expression of the protein on astrocytic end feet.…”

“…The M1 isoform, instead, is unable to form OAPs. In AQP-4-M1 mRNA, two possible translation initiation signals were identified and may lead to the possible formation of M1 as well as M23 isoforms from the same mRNA (25). It is therefore possible that a shift in balance between the two isoforms underlies the difference in cell membrane expression, despite unchanged levels of mRNA.…”

Factors determining the density of AQP4 water channel molecules at the brain–blood interface

“…This observation does not necessarily imply that the level of endfoot AQP4 is controlled at the transcriptional level. Thus, the transcriptional regulation of Aqp4 is complex (Pisani et al 2011; Rossi et al 2010), requiring further studies.…”