Regulation of Myosin Light-Chain Phosphatase Activity to Generate Airway Smooth Muscle Hypercontractility

Abstract

Smooth muscle is a central structure involved in the regulation of airway tone. In addition, it plays an important role in the development of some pathologies generated by alterations in contraction, such as hypercontractility and the airway hyperresponsiveness observed in asthma. The molecular processes associated with smooth muscle contraction are centered around myosin light chain (MLC) phosphorylation, which is controlled by a balance in the activity of myosin light-chain kinase (MLCK) and myosin light-chain phosphatase (MLCP). MLCK activation depends on increasing concentrations of intracellular Ca²⁺, while MLCP activation is independent of Ca²⁺. MLCP contains a phosphatase subunit (PP1c) that is regulated through myosin phosphatase target subunit 1 (MYPT1) and other subunits, such as glycogen-associated regulatory subunit and myosin-binding subunit 85 kDa. Interestingly, MLCP inhibition may contribute to exacerbation of smooth muscle contraction by increasing MLC phosphorylation to induce hypercontractility. Many pathways inhibiting MLCP activity in airway smooth muscle have been proposed and are focused on inhibition of PP1c, inhibitory phosphorylation of MYPT1 and dissociation of the PP1c-MYPT1 complex.

Keywords: contraction; kinase network in contraction; myosin phosphatase; myosin phosphatase target subunit 1; phosphatase subunit 1c.

FIGURE 1

Regulation of myosin light-chain phosphatase (MLCP) through regulation of myosin phosphatase target subunit 1 (MYPT1). (A) Active MLCP dephosphorylates the light chain of myosin (MLC20) at the Ser¹⁹ residue and induces smooth muscle relaxation. (B) Signaling pathway mediated by RhoA kinase (ROCK). Several agonists activate the RhoA–ROCK pathway. RhoA activation is dependent on GTP and can induce ROCK activity. ROCK phosphorylates the MYPT1 subunit of MLCP to inhibit its activity and maintain smooth muscle contraction. (C) Signaling mediated by integrin-linked kinase (ILK). ILK phosphorylates MYPT1 at Thr⁶⁹⁶, inhibiting its phosphatase activity or activating the endogenous PP1c inhibitors CPI-17 and phosphatase holoenzyme inhibitor (PHI-1) by phosphorylation. (D) Signaling mediated by Zipper-interacting protein kinase (ZIPK). ZIPK phosphorylates MYPT1 at Thr⁶⁹⁶ to inactivate MLCP. (E) Signaling mediated by p21-activated kinase (PAK-1). (F–H) Activation pathway of MYPT1 mediated by protein kinases. (F) Protein kinase C (PKC) is involved in the activation of CPI-17, PHI-1 and arachidonic acid and dissociation of the PP1c–MYPT1 complex, which directly inactivates MLCP and results in sustained contraction. (G,H) Activation of cAMP-dependent protein kinase A (PKA) and cGMP-dependent protein kinase G (PKG) is responsible for phosphorylation of MYPT1 at various serine residues and one threonine residue. (I) Phosphorylation of the regulatory subunit MYPT1 at Thr⁸⁵⁰ and PKC/arachidonic acid induce dissociation of the PP1c-MYPT1 complex. (J) After dissociation of the complex, PP1c binds to another of its regulatory subunits, such as glycogen-targeting subunit of protein phosphatase 1 (GM) or myosin-binding subunit 85 kDa (MBS85). GM also competes with MYPT1 for binding with the PP1c catalytic subunit. PKA is involved in dissociation of the PP1c–GM complex. (K) Transgelin-2 (TAGLN2) dephosphorylates MYPT1 at Thr⁸⁵³. The black and blue arrows show the pathways in airway smooth muscle and in tissues other than airway smooth muscle, respectively. The blue dotted lines indicate the known pathways leading to dissociation of the PP1c–MYPT1 and PP1c–GM complexes.