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NMO Research

Determinants of aquaporin-4 assembly in orthogonal arrays revealed by live-cell single-molecule fluorescence imaging

Summary

We investigated the molecular determinants of aquaporin-4 (AQP4) assembly in orthogonal arrays of particles (OAPs) by visualizing fluorescently labeled AQP4 mutants in cell membranes using quantum-dot single-particle tracking and total internal reflection fluorescence microscopy. The fulllength ‘long’ (M1) form of AQP4 diffused freely in membranes and did not form OAPs, whereas the ‘short’ (M23) form of AQP4 formed OAPs and was nearly immobile. Analysis of AQP4 deletion mutants revealed progressive disruption of OAPs by the addition of three to seven residues at the AQP4- M23 N-terminus, with polyalanines as effective as native AQP4 fragments. OAPs disappeared upon downstream deletions of AQP4-M23, which, from analysis of point mutants, involves Nterminus interactions of residues Val24, Ala25 and Phe26. OAP formation was also prevented by introducing proline residues at sites just downstream from the hydrophobic N-terminus of AQP4-M23. AQP1, an AQP4 homolog that does not form OAPs, was induced to form OAPs upon replacement of its N-terminal domain with that of AQP4-M23. Our results indicate that OAP formation by AQP4-M23 is stabilized by hydrophobic intermolecular interactions involving N-terminus residues, and that absence of OAPs in AQP4-M1 results from non-selective blocking of this interaction by seven residues just upstream from Met23.

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Accepted 17 November 2008
Journal of Cell Science 122, 813-821 Published by The Company of Biologists 2009
doi:10.1242/jcs.042341

NMO Pathogenesis & Aquaporin 4

Abstract

Neuromyelitis optica (NMO) is a severe, debilitating human disease that predominantly features immunopathology in the optic nerves and the spinal cord. An lgGI autoantibody (NMO-lgG) that binds aquaporin 4 (AQP4) has been identified in the sera of a significant number of NMO patients, as well as in patients with two related neurologic conditions, bilateral optic neuritis (ON), and longitudinal extensive transverse myelitis (LETM), that are generally considered to lie within the NMO spectrum of diseases. NMO-lgG is not the only autoantibody found in NMO patient sera, but the correlation of pathology in central nervous system (CNS) with tissues that normally express high levels of AQP4 suggests NMO-lgG might be pathogenic. If this is the case, it is important to identify and understand the mechanism(s) whereby an immune response is induced against AQP4.

Introduction

There are many excellent reviews on the clinical and laboratory aspects of NMO, reviews that describe criteria for diagnosis, and paraclinical features of NMO and the NMO spectrum of disorders. We do not intend this to be a review of these issues. There is a complex, diverse array of “preceding environmental events” and perhaps unconnected immune-related events which are often associated with the period before patients are diagnosed with NMO. In this review we discuss in detail how the different isoform structures of AQP4 in different membrane locales and in different cell types might be related to pathology. Changes in AQP4 expression in CNS and non-CNS tissue can be regulated by inflammatory mediators induced during and following infection or by underlying autoimmunity and can result in the induction of AQP4-specific lymphocytes and ensuing pathogenesis.

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Published: 29 May 2008
Journal of Neuroinflammation 2008, 5:22 doi:10.1186/1742-2094-5-22
Received: 27 March 2008
Accepted: 29 May 2008
This article is available from: http://www.jneuroinflammation.com/content/5/1/22
© 2008 Grahn et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License
(http://creativecommons.org/licenses/by/2.0 ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited