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Anchoring's the way

SBKB [doi:10.1038/sbkb.2011.62]
Featured Article - February 2012
Short description: New insights into the regulation of AQP0 reveal mechanisms involved in cataract development.

The C-terminal domain of AQP0 has an AKAP2 binding site (green) and a helical segment (yellow) where PKA-mediated phosphorylation occurs. (PDB: 2B6P) figure courtesy of Tamir Gonen.

While most tissues receive nutrients via blood vessels, the ocular lens, which must be transparent to properly focus light, lacks a conventional vascular system. Instead, the lens has an internal circulation system based on membrane channel and transporter proteins. These proteins maintain lens transparency by enabling a constant flow of water, ions, second messengers, and metabolites. Cataract, a clouding of the ocular lens, is the leading cause of blindness worldwide.

Aquaporins are transmembrane proteins that regulate the flow of water across biological membranes. One of the primary proteins involved in maintaining the flow of water throughout the lens is the water channel aquaporin-0 (AQP0). The opening and closing of AQP0 are regulated by several mechanisms. For example, binding of Ca2+/calmodulin closes the pore, whereas phosphorylation of AQP0 in the calmodulin-binding region opens the channel. However, the details involved in this process had not been previously identified.

Now, Gold and colleagues (PSI TEMIMPS), have identified the players involved in opening AQP0 by phosphorylation. The authors noted that the sequence surrounding the phosphorylation site in AQP0 corresponds to a protein kinase A (PKA) recognition motif. Because PKA is often brought into contact with its substrates by A-kinase anchoring proteins (AKAPs), the authors proceeded to identify potential AKAPs in the lens. Immunoblotting of lens homogenates and fixed lenses revealed that AKAP2 interacts with PKA and localizes to the lens cortex. AQP0 was also found to associate with AKAP2 via the channel's cytoplasmic tail. The authors verified by mass spectrometry that PKA could phosphorylate Ser235 in AQP0. Ser235 is located in the calmodulin-binding domain of AQP0, and phosphorylation at this site prevents binding between AQP0 and calmodulin. Therefore, the authors developed a model by which AKAP2 brings PKA and AQP0 into close proximity, enabling phosphorylation and subsequent activation of AQP0 by PKA and regulating water permeability in the lens.

To verify the relevance of their findings, the authors disrupted the interaction between PKA and AQP0 in the lens in vivo. Displacement of PKA led to cataract development in the lens and increased lens opacity. This work represents the first direct demonstration of the function of AKAP2 and elucidates one of the mechanisms that lead to cataract development.

Jennifer Cable

References

  1. M.G. Gold et al. AKAP2 anchors PKA with aquaporin-0 to support ocular lens transparency.
    EMBO Mol Med. 4, 1-12 (2011). doi:10.1002/emmm.201100184

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