Airway Smooth Muscle Dysfunction in Asthma: Releasing the Anchor

Airway smooth muscle (ASM) dysfunction is of cardinal importance in asthma, and encompasses hypercontractility, hypertrophy, excessive migration, and secretion of cytokines (1, 2). Reflecting this, β-agonists, which directly induce ASM relaxation, are a mainstay therapy for short-term asthma symptom relief (3). Inhaled corticosteroids, which are central to long-term asthma control, also exert critical effects on ASM function (4). Whereas substantial progress has been made in treating severe asthma with type II inflammatory features using biologic therapy (5), the role of these agents in directly modifying ASM function is poorly understood (6). Moreover, there is significant prevalence of asthma endotypes with minimal evidence of type II inflammation and endotypes in which ASM pathophysiology plays a dominant role (5, 6). Thus, strategies to more effectively target ASM in asthma continue to have considerable therapeutic potential.

In addition to the importance of ASM in asthma, features of asthma-related signaling in ASM make this tissue tractable as a therapeutic target. Specifically, G protein–coupled receptors (GPCRs), which are frequently druggable, signal to protein kinase A (PKA), which in turn regulates ASM contraction, proliferation, hypertrophy, and migration (7). Thus, a more detailed understanding of the pathways through which transduction of such signals occurs, such as β-agonist–mediated induction of PKA activity, could lead to adjunct therapies to enhance the therapeutic effects of this well-established treatment approach. Intriguingly, in cultured models, β-agonists mitigate many of the critical features of ASM-related asthma pathophysiology, with their effects extending beyond acute relaxation to repression of cellular behavior implicated in long-term ASM remodeling (8, 9). Similarly, induction of the prostanoid receptors EP2R and EP4R, which also activate PKA, reduces hallmarks of ASM remodeling in cultured ASM (10). Despite this broad activity in preclinical models of ASM dysfunction, there is little evidence that β-agonists reduce ASM remodeling in clinical asthma, and pharmacologic targeting of EP2R and EP4R for asthma remains in the developmental stages. Molecular control of PKA activity, which includes restriction of PKA activity to specific cellular compartments, has thus emerged as critical to understanding both the therapeutic activity of β-agonists and for defining new targets to further block or reverse elements of ASM dysfunction that are not effectively targeted with current asthma therapies.

In this issue of the Journal, Javed and colleagues (pp. 133–144) provide novel insight in this area (11). A central mechanism of PKA regulation that potentially controls desensitization to β-agonists and cellular compartmentalization of signaling responses is the activity of the A-kinase–anchoring proteins (AKAPs) (12). Members of the AKAP family serve as scaffolds that control cAMP-PKA signaling through binding directly to the regulatory subunits of PKA. This serves to confine the enzyme to discrete compartments within the cell through a tethering mechanism, which in turn limits the availability of specific targets of PKA to areas of AKAP-driven colocalization. AKAPs are ubiquitous, and there are more than 50 family members, but specific functions of PKA that are controlled through AKAPs vary by cell type. In ASM, global inhibition of AKAPs using a pan inhibitor, Ht-31, did not broadly change the phosphorylation of PKA substrates, but appeared to lengthen β-agonist–stimulated, plasma membrane–delineated cAMP transients and was also reported to enhance contractility (12, 13). This left open the question whether individual AKAPs, specifically AKAP78 (Ezrin) and AKAP12 (Gravin), which are among the most abundant AKAPs in ASM (13), may exert differential effects on PKA signaling in ASM.

Javed and colleagues addressed this important question by combining pharmacologic manipulation of PKA signaling and ASM contraction with targeted knockdown of Ezrin and Gravin in ASM. Using this system, they assayed a suite of cellular markers and physiologic properties of direct relevance to ASM pathobiology in asthma. Individual knockdown reduced PKA-mediated phosphorylation of pVASP (phospho–vasodilator-stimulated phosphoprotein), and combined knockdown had a more pronounced effect, supporting the role of both AKAPs in directing PKA-mediated phosphorylation. This work also revealed an intriguing role for both Gravin and Ezrin in controlling MLC20 (myosin light chain 20) activity in which knockdown of these proteins individually inhibited the increased expression of phosphorylated MLC20 in response to procontractile stimulation, with an associated inhibitory effect on ASM contraction. This complicated analysis of the impacts of Gravin and Ezrin on β-agonist–mediated reductions of phosphorylated MLC20, but nevertheless implicates the AKAPs in controlling ASM contractility.

In contrast to these potentially beneficial effects of reducing expression or activity of these two AKAPs, knockdown of Ezrin and Gravin caused β-agonists to lose their inhibitory effect on PDGF-induced ASM migration in vitro. These aggregate data refine the results from prior studies reporting on pan-AKAP inhibition. Although the effects of AKAPs on therapeutically important pathways in ASM are complex, targeting of Ezrin or Gravin individually emerges as a potential strategy to broaden the clinical effects of β-agonists or other small molecules that activate PKA via GPCRs.

Despite the physiologic interest and importance of these findings, a number of critical gaps will need to be addressed before AKAP targeting emerges as a viable strategy to manipulate ASM function in asthma. In addition to the standard challenges of identifying druglike compounds that target AKAPs selectively, translational models of AKAP function in ASM must be developed. The case for AKAP targeting would also be further strengthened by exploring the evidence for genetic variation linked to Ezrin or Gravin function or regulation and asthma. In that regard, it is intriguing that rs6924755, which is an expression quantitative trait locus for Ezrin, achieved borderline genome-wide significance with a P value of approximately 8.7 × 10⁻⁶ for FEV₁/FVC ratio (14), a trait that would be expected to be highly relevant to some asthma endotypes. Additional interrogation of these and other AKAP loci for genetic associations with asthma or lung function may further bolster the emerging biochemical evidence implicating these scaffold proteins as critical mediators of asthma pathobiology and ASM therapeutic responses. The weight of such cross-disciplinary evidence would provide a strong anchor for building programmatic efforts to target AKAPs as a novel treatment for the vexing issue of airway remodeling in asthma.