β2 Adrenergic Receptor Complexes with the L-Type Ca2+ Channel CaV1.2 and AMPA-Type Glutamate Receptors: Paradigms for Pharmacological Targeting of Protein Interactions

Abstract

Formation of signaling complexes is crucial for the orchestration of fast, efficient, and specific signal transduction. Pharmacological disruption of defined signaling complexes has the potential for specific intervention in selected regulatory pathways without affecting organism-wide disruption of parallel pathways. Signaling by epinephrine and norepinephrine through α and β adrenergic receptors acts on many signaling pathways in many cell types. Here, we initially provide an overview of the signaling complexes formed between the paradigmatic β₂ adrenergic receptor and two of its most important targets, the L-type Ca²⁺ channel Ca_V1.2 and the AMPA-type glutamate receptor. Importantly, both complexes contain the trimeric G_s protein, adenylyl cyclase, and the cAMP-dependent protein kinase, PKA. We then discuss the functional implications of the formation of these complexes, how those complexes can be specifically disrupted, and how such disruption could be utilized in the pharmacological treatment of disease.

Keywords: AKAP; Gs; PSD-95; adenylyl cyclase; cAMP; norepinephrine.

Figure 1

The β₂ AR–AC-PKA-Ca_V1.2 complex. Green arrows indicate binding of the N terminus of AKAP5 to the N terminus of AC, the C terminus of AKAP5 to the distal C terminus of α₁ 1.2, and the so far undefined regions of AKAP5 to the N terminus and the loop between domains I and II of α₁ 1.2. AKAP5 links in this way AC, PKA, and PP2B to Ca_V1.2. The β₂ AR binds with its C terminus to the region around S1928 in the distal C terminus of α₁ 1.2 (red arrow). PP2A and PP2B also bind directly to α₁ 1.2 about 40 and 50 residues downstream of S1928 (purple and blue arrows). Activation of β₂ AR–G_s-AC-cAMP-PKA signaling leads to S1928 phosphorylation by PKA (dashed green line) and upregulation of Ca_V1.2 activity, both of which are reversed by the β₁1.2-associated PP2A (dashed purple line). Abbreviations: AC, adenylyl cyclase; AKAP5, A-kinase anchor protein 5; AR, adrenergic receptor; PKA, cAMP-dependent protein kinase; PP, protein phosphatase.

Figure 2

Proposed model for the regulation of vascular smooth muscle cell (VSMC) excitability by macromolecular complexes. The magnitude of Ca²⁺ influx via Ca_V1.2 is critical for the control of excitation-contraction and excitation-transcription coupling in these cells. Under physiological conditions, K⁺ channels oppose pressure-induced depolarization to limit Ca_V1.2 activity and VSMC contractility. The activity of K⁺ and Ca_V1.2 channels can be regulated by cAMP-dependent protein kinase (PKA), protein kinase C (PKC), and the protein phosphatase PP2B, which are targeted to the specific channels and G protein-coupled receptors (GPCRs) by AKAP5 and/or PSD-95, and their function may be altered during pathological conditions. Both PKA and PKC can phosphorylate Ca_V1.2 on S1928 (dashed blue lines) but are regulated by different GPCRs. PKA can also regulate K⁺ channels of the K_V1 family. In turn, K_V1 channels negatively control Ca_V1.2 activity. Whether the hypothesized interactions (solid red lines), including those involving the GPCRs that mediate angiotensin II (angII) and high glucose (HG) signaling, PSD-95, and AKAP5, occur in native VSMCs is unclear.

Figure 3

The β₂ AR–G_s-AC-PKA-GluA1 complex. PSD-95 is a highly prevalent and central structural protein of excitatory glutamatergic synapses. AMPARs are linked to PSD-95 via the binding of their auxiliary TARP (γ) subunits to the first two PDZ domains of PSD-95, whereas the β₂ AR binds to the third PDZ domain of PSD-95. Both PSD-95 and its homolog, SAP97, bind to AKAP5 via their SH3-GK modules. SAP97 binds to the C terminus of the GluA1 subunit of AMPAR, recruiting PKA and PP2B to the vicinity of AMPAR. Through the SAP97-AKAP5 interaction, AC is also localized close to GluA1. Stimulation of β₂ AR induces cAMP increase and PKA activation, increasing S845 phosphorylation on GluA1 and, consequently, AMPAR activity. Abbreviations: AC, adenylyl cyclase; AKAP5, A-kinase anchor protein 5; AMPAR, AMPA-type glutamate receptor; AR, adrenergic receptor; PKA, cAMP-dependent protein kinase; PP, protein phosphatase; PSD-95, postsynaptic density 95; SAP97, synapse-associated protein 97; TARP, transmembrane AMPAR regulatory protein.

Figure 4

The β₂ adrenergic receptor (β₂ AR)–Ca_V1.2 and β₂ AR–GluA1 signaling complexes participate in prolonged theta tetanus long-term potentiation (PTT-LTP). β₂ AR activation is required in the induction of PTT-LTP. Stimulation of β₂ AR augments Ca_V1.2 and AMPA-type glutamate receptor (AMPAR) channel activity via phosphorylation of Ca_V1.2 on S1928 and GluA1 on S845, respectively, by A-kinase anchor protein (AKAP)-anchored cAMP-dependent protein kinase (PKA). The upregulation in AMPAR activity increases depolarization upon synaptic transmission and thereby increases Ca_V1.2 activation, in addition to the increased open probability of Ca_V1.2 due to S1928 phosphorylation. Phosphorylation of both S845 and S1928 is required for the induction of PTT-LTP. Adapted from Reference with permission from AAAS.