G12/13 signaling in asthma

Abstract

Shortening of airway smooth muscle and bronchoconstriction are pathognomonic for asthma. Airway shortening occurs through calcium-dependent activation of myosin light chain kinase, and RhoA-dependent calcium sensitization, which inhibits myosin light chain phosphatase. The mechanism through which pro-contractile stimuli activate calcium sensitization is poorly understood. Our review of the literature suggests that pro-contractile G protein coupled receptors likely signal through G_12/13 to activate RhoA and mediate calcium sensitization. This hypothesis is consistent with the effects of pro-contractile agonists on RhoA and Rho kinase activation, actin polymerization and myosin light chain phosphorylation. Recognizing the likely role of G_12/13 signaling in the pathophysiology of asthma rationalizes the effects of pro-contractile stimuli on airway hyperresponsiveness, immune activation and airway remodeling, and suggests new approaches for asthma treatment.

Keywords: Airway hyperresponsiveness; Airway remodeling; Anticholinergic agents; Asthma; Bronchoconstriction; Calcium sensitization; G12/13; Inflammation; Muscarinic 3 acetylcholine receptor; RhoA.

Fig. 1

G protein signaling in smooth muscle contraction. Two G protein signaling pathways contribute to airway smooth muscle contraction. Activation of G_q/11 upon receptor-dependent guanine nucleotide exchange stimulates the calcium-dependent contractile pathway, whereby GTP-bound G_q/11 allosterically activates PLCβ-dependent hydrolysis of phosphoinositide bisphosphate (PIP2) into IP3 and DAG, which promote intracellular calcium flux, thereby activating MLCK-dependent phosphorylation of myosin light chain and actomyosin cross-bridge cycling (blue nodes on right side of figure). Calcium sensitization (fuchsia and violet nodes in the center and left of the figure) is mediated by GTP-bound RhoA, which can be generated downstream of activation of either G_q/11 or G_12/13, although G_12/13 are more potent activators [29]. PKC phosphorylation of CPI-17 promotes inhibition of myosin light chain phosphatase, which increases net MLC phosphorylation. GTP-bound RhoA stimulates actin polymerization through activation of formins (fuchsia nodes) and ROCK activity (violet nodes), which inhibit both filament severing and MLCP. ROCK also phosphorylates CPI-17, which further inhibits MLCP. (Created with BioRender.com)

Fig. 2

G_12/13 signaling in inflammation. G_12/13-dependent activation of RhoA and ROCK promotes inflammation in both Th2-high (a) and Th2-low asthma (c). G_12/13 can also stimulate NFκB-dependent production of inflammatory cytokines (b). (Created with BioRender.com)

Fig. 3

G_12/13 signaling in proliferation. G_12/13 activate RhoGEFs that generate GTP-bound RhoA, activating ROCK and stimulating proliferative gene transcription via AP-1 (a) and SRE (b) promoters. GTP-bound RhoA inhibits phosphorylation of Lats 1/2, thereby blocking negative regulation of the Hippo pathway (c). G_12/13 bind to c-Abl, affecting ERK-dependent proliferation (d). (Created with BioRender.com)

Fig. 4

Targets for regulating G_12/13 signaling. Hyperactive G_12/13 signaling in airway smooth muscle can be attenuated by antagonizing upstream receptors (red), by developing direct inhibitors of G₁₂, G₁₃ (green) or complexes between G_12/13 and RhoGEFs (blue), or by inhibiting RhoA or ROCK (purple). (Created with BioRender.com)