The mAKAP complex participates in the induction of cardiac myocyte hypertrophy by adrenergic receptor signaling

Abstract

Maladaptive cardiac hypertrophy can progress to congestive heart failure, a leading cause of morbidity and mortality in the United States. A better understanding of the intracellular signal transduction network that controls myocyte cell growth may suggest new therapeutic directions. mAKAP is a scaffold protein that has recently been shown to coordinate signal transduction enzymes important for cytokine-induced cardiac hypertrophy. We now extend this observation and show mAKAP is important for adrenergic-mediated hypertrophy. One function of the mAKAP complex is to facilitate cAMPdependent protein kinase A-catalyzed phosphorylation of the ryanodine receptor Ca 2+ -release channel. Experiments utilizing inhibition of the ryanodine receptor, RNA interference of mAKAP expression and replacement of endogenous mAKAP with a mutant form that does not bind to protein kinase A demonstrate that the mAKAP complex contributes to pro-hypertrophic signaling. Further, we show that calcineurin A␤ ␤ associates with mAKAP and that the formation of the mAKAP complex is required for the full activation of the pro-hypertrophic transcription factor NFATc. These data reveal a novel function of the mAKAP complex involving the integration of cAMP and Ca 2+ signals that promote myocyte hypertrophy. Journal of Cell Science 5638 from the nuclear envelope, we showed that mAKAP was required for the induction of myocyte hypertrophy by this cytokine-regulated signaling pathway (Dodge-Kafka et al., 2005). We now report that mAKAP is important for the induction of myocyte hypertrophy by adrenergic receptor signaling. The ␤-adrenergic receptor stimulates adenylate cyclase and increases intracellular cAMP levels and, subsequently, PKA activity. mAKAP-bound PKA can phosphorylate associated RyR2 (Kapiloff, 2002;Marx et al., 2000;Ruehr et al., 2003), potentiating channel activity (Hain et al., 1995;. It is generally understood that RyR2 mediates Ca 2+ -induced Ca 2+ release from the sarcoplasmic reticulum during excitation-contraction coupling (Fill and Copello, 2002). We have found, however, a small pool of RyR2 associated with mAKAP at the nuclear envelope ). mAKAP-associated RyR2 might regulate the release of Ca 2+ from adjacent, peri-nuclear sarcoplasmic reticulum or the release of putative stores within the nuclear envelope (Abrenica and Gilchrist, 2000). We now present evidence that mAKAP is integral to the induction of ␤-adrenergic-stimulated hypertrophy by participating in the CaN-NFATc signaling pathway. We show further that mAKAP is also relevant to ␣-adrenergic signaling. In contrast to the ␤-adrenergic receptor, the ␣-adrenergic receptor activates phospholipase C and phosphatidyl inositol pathways, resulting in increased intracellular Ca 2+ levels and activation of MAP kinases (including MEK5 and ERK5) and CaN (Dorn and Force, 2005;Nicol et al., 2001). Given the ability of mAKAP to anchor ERK5, PKA, RyR2, and, as shown below, CaN, to the nuclear envelope, we propose that the mAKAP scaffold may serve as an integrator for hypertrophic signa...

The mAKAP signalosome and cardiac myocyte hypertrophy

“…The interlinked negative feedback loops are an example of how the signalosome processes upstream signals. As described below, calcineurin Ab may be an important effector for the mAKAP signalosome (13). We propose that calcineurin Ab is activated by Ca 2þ ions released locally by mAKAPb-associated RyR2.…”

“…NFATc translocates into the nucleus when the serine-rich domains are dephosphorylated by calcineurin. We have found that NFATc nuclear translocation in myocytes is reduced when mAKAPb expression is inhibited by RNA interference (13). In addition, NFATc transcriptional reporter activity depends on the presence of the scaffold protein (unpublished observations).…”

“…A target for these increased local Ca 2þ levels may be the Ca 2þ /calmodulin-dependent phosphatase calcineurin Ab. Calcineurin Ab-specific antibodies immunoprecipitate both mAKAPb and RyR2 from adult rat heart tissue extracts (13). The fact that calcineurin Ab can be detected in the complex is interesting since this isoform is uniquely required for the induction of cardiac hypertrophy in vivo (40).…”

“…mAKAPb is associated with the nuclear envelope in cardiac myocytes through the direct binding of nesprin-1a, a nuclear envelopelocalized integral membrane protein (9). mAKAPb is the core of a large signalosome containing several signal transduction molecules, including PKA (10,11), the phosphodiesterase PDE4D3 (12), the protein phosphatases PP2A and calcineurin Ab (11,13), ryanodine receptors (RyR2) (9,11,14,15), the small GTPase Rap1, the guanine exchange factor Epac1 (16), and the mitogen-activated proteins kinases (MAPKs) MEK5 Figure 1. mAKAP gene, mRNA, and protein structure.…”

“…Due to alternative splicing, translation of mAKAPb begins at mAKAPa residue 245, such that the only difference between mAKAPa (2314 residues, 255 kDa) and mAKAPb (2070 residues, 230 kDa) is the 244 residue N-terminal region unique to mAKAPa (8). Exons 5 -12 encode the spectrin repeat domains and exon 13 encodes a 'Signaling Domain' that binds PKA, RyR2, and PDE4D3 as indicated (10,13,33). Nesprin-1a binds the third spectrin repeat (9).…”