AKAP2 anchors PKA with aquaporin‐0 to support ocular lens transparency

Matthew G Gold,Tamir Gonen,Lorene K Langeberg,James E Bruce,Susan E O'Neill,Steve L Reichow,Chad R Weisbrod,John D Scott

doi:10.1002/emmm.201100184

Matthew G Gold, Tamir Gonen + Show 6 more

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https://doi.org/10.1002/emmm.201100184

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Abstract

A decline in ocular lens transparency known as cataract afflicts 90% of individuals by the age 70. Chronic deterioration of lens tissue occurs as a pathophysiological consequence of defective water and nutrient circulation through channel and transporter proteins. A key component is the aquaporin-0 (AQP0) water channel whose permeability is tightly regulated in healthy lenses. Using a variety of cellular and biochemical approaches we have discovered that products of the A-kinase anchoring protein 2 gene (AKAP2/AKAP-KL) form a stable complex with AQP0 to sequester protein kinase A (PKA) with the channel. This permits PKA phosphorylation of serine 235 within a calmodulin (CaM)-binding domain of AQP0. The additional negative charge introduced by phosphoserine 235 perturbs electrostatic interactions between AQP0 and CaM to favour water influx through the channel. In isolated mouse lenses, displacement of PKA from the AKAP2–AQP0 channel complex promotes cortical cataracts as characterized by severe opacities and cellular damage. Thus, anchored PKA modulation of AQP0 is a homeostatic mechanism that must be physically intact to preserve lens transparency.

Highlights

20 million people worldwide suffer from severely reduced vision caused by clouding of the ocular lens known as cataract (Brian & Taylor, 2001)
A-kinase anchoring protein 2 (AKAP2) is the principal A-kinase anchoring protein in the lens To identify potential A-kinase anchoring proteins (AKAPs) in the mammalian lens, sheep lens homogenates were probed with 32P-radiolabelled RII subunits of protein kinase A (PKA)
Kinase anchoring was demonstrated by the detection of immune complexes formed between AKAP2 and the regulatory (RII) and catalytic (C) subunits of PKA that were absent in IgG controls (Fig 1E, lanes 2 and 3)

Summary

Introduction

20 million people worldwide suffer from severely reduced vision caused by clouding of the ocular lens known as cataract (Brian & Taylor, 2001). Vascular system, and its mature fibre cells contain no organelles, which enables minimal light scattering To accommodate this unusual physiology, the lens has developed an internal circulation system based upon an array of membrane channel and transporter proteins (Mathias et al, 2007). Foremost amongst these is the water channel aquaporin-0 (AQP0), known as major intrinsic polypeptide (MIP), which constitutes more than 60% of the total membrane protein content of fibre cells (Bloemendal et al, 1972). Consistent with its critical role in the lens, mutation or malfunction of AQP0 causes a loss of lens transparency known as cataract (Berry et al, 2000; Francis et al, 2000; Gu et al, 2007)

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