Mechanisms and dynamics of AKAP79/150-orchestrated multi-protein signalling complexes in brain and peripheral nerve

Abstract

A-kinase anchoring proteins (AKAPs) have emerged as a converging point of diverse signals to achieve spatiotemporal resolution of directed cellular regulation. With the extensive studies of AKAP79/150 in regulation of ion channel activity, the major questions to be posed centre on the mechanism and functional role of synergistic regulation of ion channels by such signalling proteins. In this review, we summarize recent discoveries of AKAP79/150-mediated modulation of voltage-gated neuronal M-type (KCNQ, Kv7) K(+) channels and L-type CaV 1 Ca(2+) channels, on both short- and longer-term time scales, highlighting the dynamics of the macromolecular signalling complexes in brain and peripheral nerve We also discuss several models for the possible mechanisms of these multi-protein assemblies and how they serve the agenda of the neurons in which they occur.

Figure 1. Model of 3‐step rearrangement of AKAP–KCNQ channel complex

See Kosenko et al. (2012) and Kosenko & Hoshi (2013). AKAP79/150 organizes a signalling complex at the carboxyl terminus of KCNQ channels, including PKC and CaM. Stable KCNQ–CaM association is required for maintaining tonic PIP₂ affinity. Stimulation of G_q/11‐coupled muscarinic receptors depletes PIP₂ and activates AKAP79/150‐anchored PKC, which phosphorylates KCNQ subunits near the CaM binding sites of KCNQ channels. PKC phosphorylation, or ${\mathrm{Ca}}_{\mathrm{i}}^{2+}$ signals, triggers CaM conformational changes and/or dissociation from the cytoplasmic tail of KCNQ channels, resulting in lowered PIP₂ affinity and suppressed channel activity. AKAP79/150 may act as an acceptor to bind to dissociated CaM. Thus, together with PIP₂ depletion, receptor stimulation suppresses KCNQ channel activity by inducing CaM rearrangement and dissociation from the channel complex. Note that the depression of M‐current by reducing channel opening is schematically indicated here by a squeezing together of the two halves of the channel, and a thinning of the directional arrow of potassium flux. See the text for issues concerning this model.

Figure 2. Modified concurrent dual mode of KCNQ channel inhibition

In this model, phosphorylation of KCNQ channels functions to reduce their PIP₂ affinity, priming them to small changes in PIP₂ abundance induced by physiological stimulation of receptors. A, activation of receptors that are part of AKAP79150–KCNQ complexes rapidly depletes PIP₂ and activates AKAP79/150‐anchored PKC, which phosphorylates KCNQ subunits near the CaM binding site on the B helix, altering the configuration of CaM and KCNQ channels. KCNQ channels thus sense both PIP₂ depletion and phosphorylation‐mediated PIP₂‐affinity reduction. B, activation of receptors located close to IP₃ receptors, but outside of the AKAP–KCNQ complexes, induces intracellular Ca²⁺ rises and Ca²⁺ binding to CaM, which undergoes a conformational change on the overlapping binding sites with AKAP150 on the carboxy‐terminus of KCNQ channels. This induces a lowered interaction between KCNQ and AKAP79/150, thus preventing PKC phosphorylation.

Figure 3. Model of putative AKAP79/150‐orchestrated multi‐channel protein complexes

AKAP79/150 expresses as a homodimer, and each protomer of AKAP79/150 binds to one PKA molecule, and could physically couple two distinct ion channel complexes together to form one large macro‐molecular super‐complex, such as containing the M‐channel and L‐channel we illustrate here. In this large multi‐channel protein complex, channels are also likely to be functionally linked by AKAP79/150 (Dixon et al. 2012), via direct coupling of their gating, indirect coupling via Ca²⁺ or changes in [PIP₂], or more slowly via transcriptional regulation.