AKAPs as Drug Targets

Due to the pleiotropic effects of cAMP signaling, it is no surprise that current therapies for some human diseases include the use of drugs that affect cAMP levels. For example, P-adrenergic agonists are a mainstay of asthma therapy due to cAMP-mediated relaxation of bronchial smooth muscle. In a complementary fashion, caffeine and theophylline have been used to inhibit phosphodiesterases. The problem with current cAMP-directed therapies is that many such drugs have unacceptable side effects. For example, P2-specific agonist therapy results in an increased risk of heart attacks (Salpeter et al. 2004). The discovery of AKAPs has brought the promise of more selective therapies that may target specific cAMP pathways. Because cAMP signaling and AKAP function in the cardiac myocyte have been especially well studied, AKAP-targeted therapy may be first directed towards heart disease.

During the induction of cardiac disease, increased catecholaminergic stimulation will induce cAMP signaling and the phosphorylation of PKA targets in the heart. Several lines of research suggest that this increased PKA activity will ultimately exacerbate the underlying cardiac condition. For example, overexpression of the P1-adren-ergic receptor, the Gas subunit, and the PKA catalytic subunit in the hearts of transgenic mice resulted in dilated cardiomyopathy (Iwase et al. 1997; Engelhardt et al. 1999; Bisognano et al. 2000; Antos et al. 2001). Moreover, the use of P-blockers to inhibit P-adrenergic receptor signaling is standard therapy in the management of heart failure (Lehnart et al. 2005; Adamson and Gilbert 2006; Shin et al. 2007). However, other research supports the notion that an increase in cAMP signaling is beneficial in cardiac disease. For example, overexpression of the type-VI adenylate cyclase in the hearts of G^ transgenic mice improved contractile function and increased survival of these mice (Roth et al. 1999, 2002). Further, the PDE3 inhibitor milrinone has been used for several years as a treatment for cardiac disease (Shin et al. 2007). This inhibitor increases cAMP concentration in the heart, resulting in an increase in contractility.

These seemingly contradictory results are hard to reconcile until one begins to look at the microdomains of cAMP signaling in the heart. When one compares the cAMP content in both the cytosolic and particulate fraction from the normal to the failing human myocardium, the reduction in cAMP content in the failing heart is much more pronounced in the particulate fraction than in the cyosolic (Movsesian 2004). Furthermore, phosphorylation of some PKA targets such as RyR2 is increased, while phosphorylation of other targets such as phospholamban is decreased in heart failure (Schwinger et al. 1999; Marx et al. 2000). Therefore, one goal for the treatment for heart disease might be to target individual PKA complexes, thereby affecting the phosphorylation of individual substrates. Given that AKAPs are distinguished by their binding partners and locations, it may be possible to invent therapeutic strategies that specifically manipulate AKAPs of interest.

To begin, perhaps the most elegant strategy for differential inhibition of PKA phosphorylation would be the expression of high-affinity Ht31-like molecules that would displace PKA from the AKAP complex. For example, Ht31 expression in myocytes was able to inhibit P-adrenergic-induced PKA phosphorylation of troponin I and myosin basic protein, but not P-adrenergic-induced phospholam-ban phosphorylation (Fink et al. 2001). One might imagine that Ht31 expression in the failing heart might reverse RyR2 hyper-phosphorylation, without further decrease of phospholamban phosphorylation. This concept was supported by the work of Pare et al. (2005). Disruption of PKA association with mAKAPp in myocytes inhibited adrenergic-induced cellular hypertrophy. Other AKAPs may also be potential targets for this approach. For example, AKAP 15/18-bound PKA is responsible for P-agonist-induced potentiation of the L-type calcium channel, such that direct PKA phosphorylation increases the open probability of the channel (Fraser et al. 1998; Gray et al. 1998). Calcium channel blockers are currently used for the treatment of left ventricular hypertrophy (Onose et al. 2001). Therefore, displacing PKA bound to AKAP15/18 would reduce P-AR stimulation of the L-type calcium channel, potentially having a similar effect in vivo as channel blockers.

AKAPs localize PKA to specific subcellular domains through their unique targeting domains, resulting in the efficient phosphorylation of local PKA substrates (see Fig. 1). Therefore, disruption of AKAP localization is a second potential therapeutic approach to limit the hyper-phosphorylation of PKA substrates in disease states. For example, mAKAPP is directed to the nuclear envelope through binding to the integral membrane protein nesprin-1a (Kapiloff et al. 1999; Pare et al. 2004). Overexpression of the mAKAPp spectrin-repeat targeting domain in cardiac myocytes displaces mAKAPp from nesprin-1a and the nuclear envelope. Importantly, displacement of mAKAPP from its normal cellular location is sufficient to block the induction of myocyte hypertrophy (Dodge-Kafka et al. 2005; Pare et al. 2005). Again, AKAP15/18 may be another useful example. AKAP15/18 associates with the L-type calcium channel through a leucine zipper motif (Hulme et al. 2003). A peptide mimicking this binding domain can compete with AKAP15/18 for channel binding, inhibiting P-adrenergic stimulation of the channel (Hulme et al. 2003).

Although AKAPs are defined by their ability to bind and localize PKA, most AKAPs are multivalent scaffolds that bind components of other signaling pathways (see Fig. 1). Just like Ht31 peptide will displace PKA from its anchoring proteins, other peptides may be derived that will displace other signaling enzymes from AKAP complexes. For example, the mAKAPP signalosome includes ERK5, which is involved in the induction of cardiac disease (Takahashi et al. 2005). Displacement of ERK5 from the mAKAPP signalosome by overexpression of a mAKAPP-derived binding peptide may prove useful to inhibit the cardiac disease.

Finally, the advent of RNA interference foretells an era in which genes may be selectively inhibited in patients at will. For example, one might imagine that future treatment of heart failure will include the introduction of mAKAPP-specific small interfering RNA that will attenuate the development of myocyte hypertrophy.

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