Local Calcium Signaling in Airway Smooth Muscle Cells

Abstract

Potentially fatal asthma attacks may result from airway hyperresponsiveness (AHR), which is the exaggerated contractile response of airway smooth muscle cells (ASMCs) to nonspecific stimuli. A better understanding of Ca²⁺ signaling in ASMC contraction can help develop advanced therapeutics for asthma. A common elementary form of Ca²⁺ signaling is the Ca²⁺ spark (i.e., a local transient Ca²⁺ release event). Ca²⁺ sparks occur as a result of the coordinated opening of a cluster of ryanodine receptors (RyRs) and play a fundamental role in skeletal, cardiac, and smooth muscle cells. This chapter summarizes the recent advances from our work and that of others in studies of Ca²⁺ sparks in ASMCs. Ca²⁺ sparks have been observed in equine, porcine, guinea-pig, and mouse ASMCs. Classical parasympathetic stimulation or membrane depolarization will activate native Gq protein-coupled muscarinic M3 receptors (M₃Rs) and phospholipase C (PLC), generating inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG) from phosphatidylinositol 4,5-bisphosphate (PIP₂) in ASMCs. IP₃ will activate IP₃ receptors (IP₃Rs), inducing Ca²⁺ release, which may locally induce further Ca²⁺ release from RyR2, increasing Ca²⁺ sparks and associated contraction. Meanwhile, DAG activates protein kinase C-ε (PKCε), which inhibits Ca²⁺ sparks and contraction through RyR1. Calcineurin (CaN) promotes Ca²⁺ sparks and contraction through RyR1, in contrast to the role of PKCε. In ASMCs, basal Ca²⁺ sparks directly mediate a contractile force, as seen during RyR activation. These local Ca²⁺ events are also capable of regulating membrane potential through spontaneous transient inward currents (STICs) and spontaneous transient outward currents (STOCs). At rest with the membrane potential closer to K⁺ equilibrium potential (E _K), Ca²⁺ sparks will preferentially activate TMEM16A-encoded Ca²⁺-activated Cl⁻ channels, depolarizing the cell membrane and promoting contraction. As the membrane potential rises closer to Cl⁻ equilibrium potential (E _Cl), Ca²⁺ sparks will begin to activate big-conductance Ca²⁺-activated K⁺ (BK) channels, leading to cell hyperpolarization and preventing contraction. A disruption in this balancing of cell excitability could play a role in asthmatic AHR. During asthma, Ca²⁺ sparks, TMEM16A expression, and STICs are increased, providing a mechanistic setting for AHR, whereas the STOC pathway cannot maintain balance and a lower level of cell excitability, resulting in excessive contraction. Therefore, Ca²⁺ sparks and the associated signaling axis in ASMCs may become new and effective targets for asthma therapeutics.