mAKAPβ signalosomes - A nodal regulator of gene transcription associated with pathological cardiac remodeling

Abstract

Striated myocytes compose about half of the cells of the heart, while contributing the majority of the heart's mass and volume. In response to increased demands for pumping power, including in diseases of pressure and volume overload, the contractile myocytes undergo non-mitotic growth, resulting in increased heart mass, i.e. cardiac hypertrophy. Myocyte hypertrophy is induced by a change in the gene expression program driven by the altered activity of transcription factors and co-repressor and co-activator chromatin-associated proteins. These gene regulatory proteins are subject to diverse post-translational modifications and serve as nuclear effectors for intracellular signal transduction pathways, including those controlled by cyclic nucleotides and calcium ion. Scaffold proteins contribute to the underlying architecture of intracellular signaling networks by targeting signaling enzymes to discrete intracellular compartments, providing specificity to the regulation of downstream effectors, including those regulating gene expression. Muscle A-kinase anchoring protein β (mAKAPβ) is a well-characterized scaffold protein that contributes to the regulation of pathological cardiac hypertrophy. In this review, we discuss the mechanisms how this prototypical scaffold protein organizes signalosomes responsible for the regulation of class IIa histone deacetylases and cardiac transcription factors such as NFAT, MEF2, and HIF-1α, as well as how this signalosome represents a novel therapeutic target for the prevention or treatment of heart failure.

Keywords: AKAP; Cardiac hypertrophy; Gene transcription; Kinase; cAMP.

Figure 1.. mAKAP is a perinuclear scaffold protein.

A. mAKAPβ is identical to residues 245–2314 of the mAKAPα isoform expressed in neurons, such that mAKAP residues are numbered according to mAKAPα (rat). Binding sites are shown for those mAKAP binding partners for which there is evidence of direct binding: PDK1, 3-phosphoinositide-dependent kinase-1 [15], AC5, adenylyl cyclase 5 [52], MEF2 and HDAC [31, 50], PLCε, phospholipase Cε [29], nesprin-1α [22], RyR, ryanodine receptor [84], calcineurin [64], PDE4D3, phosphodiesterase 4D3 [51], RSK3, p90 ribosomal S6 kinase 3 [85], PKA, protein kinase A [14], PP2A, protein phosphatase 2A [55]. B. mAKAPβ is localized to the myocyte nuclear envelope regardless of developmental stage or stress conditions [14, 18]. An adult rat ventricular myocyte is shown [18].

Figure 2.. Structure of a class IIa histone deacetylase.

HDAC5 is similar to other class IIa family members [86, 87]. CtBP, HP1 - binding sites for these co-repressors; MEF2 - binding site for the transcription factor; NLS and NES - nuclear localization and export signals. Phosphorylation sites and cysteine residues regulated by oxidation (Ox) are indicated.

Figure 3.. Regulation of perinuclear cAMP by mAKAPβ signalosomes.

AC5 presumably located on transverse tubules adjacent to the nuclear envelope can locally produce cAMP, followed by PKA phosphorylation that inhibits AC5 and activates PDE4D3 activity, resulting in decreased cAMP accumulation [54, 88]. Activation of ERK5 by MEK5 will lead to PDE4D3 inhibition and increased PKA activity [28]. When high cAMP levels activate the guanine nucleotide exchange factor Epac1, Rap1 will inhibit the ERK5 pathway, reversing the ERK5-mediated inhibition of PDE4D3 and limiting downstream signaling. In addition, there is an incoherent feedforward loop that will oppose PKA phosphorylation of PDE4D3 resulting from PKA phosphorylation and activation of PP2A in the complex [55].

Figure 4.. A model for bidirectional regulation of class IIa HDACc by mAKAPβ signalosomes.

Gβγ-activated PLCε at perinuclear mAKAPβ signalosomes will result in DAG and IP₂ production from PI4P on adjacent Golgi apparatus [89]. DAG activates PKCε and PKD [62], inducing HDAC nuclear export promoting MEF2-dependent gene expression. Intermittent, acute βAR stimulation inhibits PKD phosphorylation and HDAC nuclear export by inducing PKA HDAC phosphorylation at Ser-279 [50]. Chronic, persistent βAR stimulation results in greater cAMP elevation and further activation of PLCε by Epac1 and the small g-protein Rap1 that drives with PKD-dependent HDAC nuclear export [29]. Chronic, persistent βAR stimulation promotes myocyte hypertrophy by additional PKA-dependent pathways [21], potentially involving HDACs.

Figure 5.. Regulation of calcineurin-dependent transcription factors by mAKAPβ signalosomes.

A. Conserved Domains in NFATc Transcription Factors. AD, transactivation domain; NLS, nuclear localization signal; SRR, SP1, SP2 SP3, and KTS are serine-rich domains that are de-phosphoyrlated by calcineurin. The REL Homology Domain binds DNA. B. MEF2 structure and post-translational modification. C. RyR2 at mAKAPβ signalosomes is phosphorylated by bound PKA [19], presumably enhancing perinuclear Ca²⁺ levels that can activate local calcineurin Aβ [64]. We propose that MEF2D dynamically associates with chromatin, such that transient association with mAKAPβ facilitates its regulation by the phosphatase. Calcineurin-catalyzed de-phosphorylation of MEF2 results in MEF2 desumoylation and acetylation and an exchange of HDAC and p300 binding [90]. mAKAPβ-bound calcineurin also dephosphorylates NFATc transcription factors promoting their nuclear localization and hypertrophic gene expression [64].

Figure 6.. Regulation of HIF-1α by mAKAPβ signalosomes.

The association of E3-ubuiquitin ligase and HIF-1α with mAKAPβ confers bidirectional regulation of HIF-1α [78].