life Review Characterization of Contractile Machinery of Vascular Smooth Muscles in Hypertension Qunhui Yang * and Masatoshi Hori Department of Veterinary Pharmacology, Graduate School of Agriculture and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-3-5841-7940; Fax: +81-3-5841-8183 Abstract: Hypertension is a key risk factor for cardiovascular disease and it is a growing public health problem worldwide. The pathophysiological mechanisms of vascular smooth muscle (VSM) contrac- tion contribute to the development of hypertension. Calcium (Ca2+)-dependent and -independent signaling mechanisms regulate the balance of the myosin light chain kinase and myosin light chain phosphatase to induce myosin phosphorylation, which activates VSM contraction to control blood pressure (BP). Here, we discuss the mechanism of the contractile machinery in VSM, especially RhoA/Rho kinase and PKC/CPI-17 of Ca2+ sensitization pathway in hypertension. The two signal- ing pathways affect BP in physiological and pathophysiological conditions and are highlighted in pulmonary, pregnancy, and salt-sensitive hypertension. Keywords: vascular smooth muscle contraction; hypertension; CPI-17 Citation: Yang, Q.; Hori, M. 1. Introduction Characterization of Contractile Three types of muscle tissues are found in vertebrates: skeletal muscle, cardiac muscle, Machinery of Vascular Smooth and smooth muscle [1]. Muscle contraction depends on the ATP-driven sliding of highly Muscles in Hypertension. Life 2021, organized arrays of actin filaments against arrays of myosin II filaments [2]. In smooth 11, 702. https://doi.org/10.3390/ muscle, phosphorylation at Thr18/Ser19 of the myosin regulatory light chain results in life11070702 myosin ATPase enzymatic activity that induces actin and myosin attachment to regulate smooth muscle contraction [3,4]. This process can be regulated by Ca2+-dependent and Academic Editors: Satoshi Komatsu Ca2+-independent mechanisms [1,5–7]. The Ca2+-dependent signaling pathway acts mainly and Xia Guo in combination with calmodulin (CaM) to form a complex that activates myosin light chain 2+ Received: 24 June 2021 kinase (MLCK). In contrast, the Ca -independent signaling pathway acts mainly through 2+ Accepted: 13 July 2021 the attenuation of myosin light chain phosphatase (MLCP) activity, i.e., Ca sensitization. Published: 16 July 2021 This review focuses on the two signaling pathways in vascular smooth muscle (VSM) contraction and the role of the Ca2+ sensitization pathway in hypertension. Publisher’s Note: MDPI stays neutral 2. Vascular Smooth Muscle Contractile Machinery with regard to jurisdictional claims in 2+ published maps and institutional affil- 2.1. Ca /Calmodulin/Myosin Light Chain Kinase iations. 2+ 2+ The intracellular free Ca concentration ([Ca ]i) is necessary to maintain basal 2+ 2+ vascular tone [8]. Increases in [Ca ]i initiates VSM contraction [9]. The [Ca ]i can be increased by triggering the Ca2+ influx. Depolarization of the cell membrane through voltage-dependent L-type Ca2+ channels (LTCCs), such as the opening of Cav1.2 chan- Copyright: © 2021 by the authors. nels in murine arterial myocytes or Cav1.3 channels in the canine basilar artery, is the Licensee MDPI, Basel, Switzerland. primary means of Ca2+ entry into arterial smooth muscle [10–12]. Ca2+ leaks through This article is an open access article non-specific Ca2+ movement across the plasma membrane; increases in intravascular pres- distributed under the terms and sure through stretch-activated Ca2+ channels could also activate voltage-dependent Ca2+ conditions of the Creative Commons channels [13]. The opening of the Ca2+-activated chloride channels (CaCCs) can also lead to Attribution (CC BY) license (https:// the depolarization of the membrane, followed by the opening of voltage-gated Ca2+ chan- creativecommons.org/licenses/by/ nels. Additionally, vasoconstrictor agonists enhance Ca2+ influx through transient receptor 4.0/). Life 2021, 11, 702. https://doi.org/10.3390/life11070702 https://www.mdpi.com/journal/life Life 2021, 11, 702 2 of 17 Life 2021, 11, 702 vasoconstrictor agonists enhance Ca2+ influx through transient receptor potential channels2 of 16 (TRPs), store-operated Ca2+ channels (SOCs) and receptor-operated nonselective cation channels (ROCs) [10,14–18]. potential channels (TRPs), store-operated Ca2+ channels (SOCs) and receptor-operated The [Ca2+]i can also be increased from intracellular stores through ryanodine recep- nonselective cation channels (ROCs) [10,14–18]. tors (RyRs)2+ and inositol-1, 4, 5-trisphosphate receptors (IP3Rs) in the sarcoplasmic reticu- The [Ca ]i can also be increased from intracellular stores through ryanodine receptors lum (SR) [19]. The precise regulation of the [Ca2+]i is crucial for proper physiological VSM (RyRs) and inositol-1, 4, 5-trisphosphate receptors (IP Rs) in the sarcoplasmic reticulum function (Figure 1). In VSM, calcium performs most3 of its functions by interacting with (SR) [19]. The precise regulation of the [Ca2+] is crucial for proper physiological VSM specific Ca2+-binding proteins [20]. Calmodulini (CaM) is a critical Ca2+ sensor that acti- function (Figure1). In VSM, calcium performs most of its functions by interacting with vates the kinase of the myosin light chain (MLC), leading to MLC phosphorylation, actin- specific Ca2+-binding proteins [20]. Calmodulin (CaM) is a critical Ca2+ sensor that activates myosin interaction and VSM contraction [20,21]. These Ca2+-dependent signaling path- the kinase of the myosin light chain (MLC), leading to MLC phosphorylation, actin-myosin interactionways are central and VSM regulators contraction of differential [20,21]. These VSM Ca contractile2+-dependent functions signaling and pathwaysvascular disor- are centralders [22]. regulators of differential VSM contractile functions and vascular disorders [22]. Figure 1. The calcium/calmodulin/myosin light chain kinase signaling pathway-induced vascular smooth muscle con- traction. VSM contraction is increased by the intracellular levels of Ca2+. Activated G protein-coupled receptors (GPCRs) Figure 1. The calcium/calmodulin/myosin light chain kinase signaling pathway-induced vascular smooth muscle contrac- stimulate the plasma membrane-bound phospholipase C-b (PLC-b) via a G protein. Phosphatidylinositol 4, 5-bisphosphate tion. VSM contraction is increased by the intracellular levels of Ca2+. Activated G protein-coupled receptors (GPCRs) stim- (PI4,5)P2 is hydrolyzed by activated PLC-b to produce inositol 1, 4, 5-trisphsphate (IP3) and diacylglycerol (DAG). IP3 ulate the plasma membrane-bound phospholipase2+ C-β (PLC-β) via a G protein. Phosphatidylinositol 4, 5-bisphosphate diffuses(PI4,5)P2 through is hydrolyzed the cytosol by and activated releases PLC- Ca β tofrom produce the sarcoplasmic inositol 1, 4, reticulum 5-trisphspha (SR)te by (IP3) binding and todiacylglycerol and opening IP3-gated(DAG). IP3 2+ 2+ Cadiffusesrelease through channels the (IPcytosol3Rs). and The releases SR also Ca contains2+ fromregulated the sarcoplasmic Ca channels reticulum ryanodine (SR) by binding receptors to (RyRs) and opening to increase IP3-gated the 2+ 2+ 2+ 2+ CaCa2+level release in thechannels cytosol. (IP Ca3Rs). canThe alsoSR also be influxedcontains viaregulated voltage-gated Ca2+ channels Ca channel ryanodine (LTCC), receptors store-operated (RyRs) to increase Ca channels the Ca2+ (SOCs),level in receptor-operated the cytosol. Ca2+ Cacan2+ alsochannels be influxed (ROCs), via Ca voltage-gated2+-activated chloride Ca2+ channel channels (LTCC), (CaCCs), store-operated and transient Ca receptor2+ channels potential (SOCs), channelsreceptor-operated (TRPs). The Ca2+ increased channels Ca (ROCs),2+ binds Ca2+ calmodulin-activated chloride (CaM), thenchannels binds (CaCCs), myosin and light transient chain kinase receptor (MLCK), potential which chan- phosphorylatesnels (TRPs). The myosin increased light Ca chain,2+ binds stimulating calmodulin myosin (CaM), activity then tobinds combine myosin actin light to inducechain kinase contraction. (MLCK), which phosphor- ylates myosin light chain, stimulating myosin activity to combine actin to induce contraction. 2.2. Ca2+ Sensitization 2.2. Ca2+ Sensitization The regulation of MLCP activity is considered to be the most important mechanism un- derlyingThe the regulation regulation of of MLCP the Ca activity2+ sensitivity is considered of VSM to contractile be the most machinery important [23]. mechanism MLCP is aunderlying holoenzyme the composed regulation of threeof the subunits—a Ca2+ sensitivity 38-kDa of VSM catalytic contractile subunit machinery (type 1 phosphatase; [23]. MLCP PP1Cis a holoenzymeδ), a 20-kDa composed functionally of unknownthree subunits subunit,—a 38k-kDa and a large catalytic 110–130-kDa subunit (type regulatory 1 phos- proteinphatase; subunit PP1Cδ (myosin), a 20-kDa phosphatase functionally target unknown subunit; subunit, MYPT) and [ 24a large]. Various 110–130-kDa isoforms regu- of MYPTlatoryexist, protein and subunit the MYPT1 (myosin isoform phosphatase is present target in smoothsubunit; muscle MYPT) [ 25[24].]. TheVarious phosphory- isoforms lationof MYPT of MYPT1, exist, and conformational the MYPT1 isoform changes is pr inesent MLCP, in smooth and phosphorylation muscle [25]. The of phosphory- a smooth muscle-specificlation of MYPT1, inhibitor conformational protein, i.e., changes protein in kinase MLCP, C (PKC)-dependentand