Scaffolds That Regulate Cardiac Hypertrophy 51 mAkappkapde4D3 Complexes

Several AKAPs have been implicated in the coordination of cardiac hypertrophy. The muscle-specific AKAP (mAKAP, the 255-kDa muscle A-kinase anchoring protein) is a multivalent scaffold that binds PDE4D3 (and other signaling proteins, see below), is tethered to the nuclear envelope (through a protein-protein interaction with the integral nuclear membrane protein nesprin-1a), and plays a crucial role to synchronize PKA and PDE4D3 activity in cardiomyocytes (Fig. 3; Pare et al. 2005; Dodge-Kafka et al. 2005; Dodge et al. 2001). PDE4D3 accelerates local cAMP breakdown and tonically prevents a cAMP increase to a threshold level that would activate mAKAP-tethered PKA in resting cardiomyocytes. A PAR agonist-dependent rise in cAMP/PKA leads to PDE4D3 phosphorylation at S54 (which induces a two- to three-fold increase in PDE activity) and S13 (which increases PDE4D3 binding affinity for mAKAP), setting up a negative feedback loop that restores cAMP to low basal levels (Sette and Conti 1996; Carlisle et al. 2004). Experiments with fluorescent reporters of PKA activity provide compelling evidence that PDE4D3 plays a critical role to terminate signaling by AKAP-anchored PKA (Dodge-Kafka et al. 2005).

While the synergistic actions of PKA and PDE4D3 lead to localized pulses of cAMP at the nuclear membrane, their actions are counterbalanced by Extracellular signal-Regulated Kinase 5/big mitogen-activated protein kinase 1 (ERK5/BMK1), a cytokine-activated serine/threonine kinase that has been implicated in the induction of cardiac hypertrophy as well as in antiapoptotic mechanisms in vitro in cardiomyocytes cultures (during oxidative stress or during overexpression of the constitutively active form of MEK5a) and cardioprotection in vivo in the ischemic/ reperfused intact heart (Suzaki et al. 2002; Cameron et al. 2004). ERK5 indirectly anchors to mAKAP (through PDE4D3) and phosphorylates PDE4D3 at S579, leading to suppressed PDE activity (Hoffmann et al. 1999). cAMP acts reciprocally to

Nuclear membrane

Fig. 3 mAKAP binds PKA, anchors PDE4D3, and serves as an adaptor to recruit Epac1 and ERK5. (a) A modest rise in cAMP activates PKA (but not Epac1), leading to the phosphorylation of PDE4D3 at S13 (which increases PDE4D3 affinity for mAKAP) and S54 (which increase PDE4D3 activity). These PKA-dependent phosphorylations limit a local increase in cAMP and restrict mAKAP-anchored PKA activity. cAMP does not activate Epac1; there is no Epac1-dependent suppression of ERK5. The cytokine- (LIF-) dependent pathway leading to mobilization of the MEKK-MEK5-ERK5 pathway and cardiac hypertrophy is intact under these conditions. (b) Stimuli that activate ERK5 lead to PDE4D3-S579 phosphorylation and decreased PDE4D3 activity, leading to a local rise in cAMP and activation of Epac1. At high cAMP, activation of Epac1 and Rap1 suppresses ERK5 and abrogates ERK5-dependent inhibition of PDE4D3, permitting cAMP to return to low basal levels. The effect of the cAMP-Epac1-Rap1 pathway to inhibit ERK5 also suppresses LIF-dependent cardiac hypertrophy. mAKAP dephosphorylation by PP2A (which also is identified in mAKAP complexes) also contributes to the local control

Nuclear membrane

Nuclear membrane

inhibit mAKAP-anchored ERK5. Importantly, this inhibitory effect of cAMP is mediated through a PKA-independent mechanism involving Epac1 (a cAMP-dependent-GEF for the small Ras-related GTP binding proteins Rap1 and Rap2), which also localizes to the nuclear membrane and is recruited by PDE4D3 to the mAKAP complex (Dodge-Kafka et al. 2005). The ramifications of heart failure-dependent changes in ERK5 activation and its control by the cAMP-Epac1 signaling pathway have not been examined in any detail, but may be significant, given evidence that human heart failure is characterized by high levels of cAMP and decreased ERK5 expression, which would effectively prevent cardioprotection via this pathway (Takeishi et al. 2001).

Finally, it is worth noting that the presence of three cAMP effectors (with markedly different affinities for cAMP) on a single mAKAP scaffold might provide a mechanism to influence the timing of cAMP signals at the nuclear membrane. PKA (which is responsive to nM cAMP) is predicted to be activated more rapidly and persistently than PDE4D3 or Epac1 (which bind cAMP with much lower micromolar affinity (Dodge-Kafka et al. 2005).

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