Translational Science
Science Made Simple Cellular basis of detrusor smooth muscle contraction Martin C. Michel Department of Pharmacology, Johannes Gutenberg University, Mainz, Germany
The cellular mechanisms, and particularly the signal transduction pathways, controlling contraction and relaxation of detrusor smooth muscle are insufficiently understood [1]. A better understanding could lead to novel therapeutics for patients with detrusor over- or underactivity, making this a question of potential clinical relevance. What determines smooth muscle tone? At the cellular level, smooth muscle contraction in the detrusor and other tissues is primarily driven by an increase in the free intracellular Ca2+ concentration; however, the extent of smooth muscle contraction in response to an intracellular Ca2+ concentration is determined by the phosphorylation state of several enzymes
[2]. If these enzymes are in the right state, a little Ca2+ can go a long way; if they are not, more Ca2+ is required to reach the same level of contraction. What are those enzymes? Ultimately, smooth muscle tone depends on the phosphorylation state of the myosin light chain (Fig. 1). This is increased by one enzyme, myosin light chain kinase (MLCK), and decreased by another, myosin light chain phosphatase (MLCP). Ca2+ in conjunction with calmodulin increases the activity of MLCK leading to myosin light chain phosphorylation and smooth muscle contraction; however, any phosphorylation of myosin light chain can be reversed by MLCP. The activity of MLCP is mainly regulated
Fig. 1 Simplified scheme of how muscarinic receptors and b-adrenoceptors interact at the cellular level to regulate detrusor smooth muscle tone. Muscarinic receptors promote elevation of intracellular Ca2+ via release from intracellular stores (sarcoplasmic reticulum [SR]) and via influx from the extracllular space through Ca2+ channels; this activates myosin light chain kinase (MLCK), which in turns phosphorylates myosin light chain (MLC). Muscarinic receptors also activate protein kinase C (PKC) and Rho kinase (RhoK), which phosphorylate myosin light chain phosphatase (MLCP), so that MLCP can no longer dephosphorylate MLC. This yields stronger contraction for a given Ca2+ concentration, i.e. Ca2+ sensitization. b-adrenoceptors inhibit Ca2+ influx and Ca2+ sensitization, but the precise molecular steps mediating this remain under discussion.
β-Adrenoceptor
Ca channel
Muscarinic receptor
SR
RhoK
cAMP
PKC
Ca
MLC
Ca
MLC MLCK
P MLCP MLC
P
MLC P
© 2015 The Authors BJU International © 2015 BJU International | doi:10.1111/bju.13216 Published by John Wiley & Sons Ltd. www.bjui.org
BJU Int 2016; 117: 177–178 wileyonlinelibrary.com
Michel
by phosphorylation of an associated protein, which in turn is promoted by protein kinases including Rho kinase and protein kinase C. Moreover, these protein kinases can also phosphorylate a protein called CPI-17, which when phosphorylated in turn causes additional inhibition of MLCP. What is Ca2+ sensitivity? The relationship between contractile force and Ca2+ concentration is called ‘Ca2+ sensitivity’ [2]. Some contractile agents, including agonists at muscarinic acetylcholine receptors, increase Ca2+ sensitivity and hence cause a contractile response greater than expected based on the elevation of intracellular Ca2+ they cause; such agonists are said to have Ca2+-sensitizing properties. Conversely, some relaxing agents, including agonists at b-adrenoceptors, decrease Ca2+ sensitivity and hence cause a greater relaxation response than expected based on their effects on intracellular Ca2+; such agonists are said to cause Ca2+ desensitization. Two recent studies shed additional light on the signal transduction processes involved in Ca2+ sensitization and Ca2+ desensitization. In this issue of the journal, Hayashi et al. [3] have used isolated human detrusor strips to study the interaction between cyclic AMP (cAMP), a second messenger generated in response to e.g. b-adrenoceptor agonists, and carbachol, an agonist at muscarinic receptors. They show in functional experiments that cAMP can inhibit carbacholinduced Ca2+ sensitization. The relaxation induced by cAMP was further enhanced when protein kinase C was inhibited, whereas concomitant inhibition of Rho kinase had little effect. In another study, Cernecka et al. [4] have not directly studied Ca2+ sensitization but in confirmation of the data by Hayashi et al. [3] found that inhibition of Rho kinase only inconsistently enhanced relaxation by two b-adrenoceptor agonists in human detrusor strips; however, consistent enhancement was observed in rat detrusor strips. In the absence of a contractile agent, the b-adrenoceptor agonists markedly reduced myosin light chain phosphorylation, whether or not Rho kinase was inhibited. In contrast, carbachol transiently increased myosin light chain phosphorylation; the additional presence of the badrenoceptor agonists did not cause inhibition of this response and, if anything, caused minor additional increases. What does all of this mean? The contraction of human detrusor smooth muscle cells is primarily stimulated by the
178
© 2015 The Authors BJU International © 2015 BJU International
endogenous muscarinic agonist acetylcholine. This can be inhibited by muscarinic receptor antagonists, but they have limited efficacy as, particularly in pathophysiological settings, additional contractile agents including ATP and bradykinin come into play. b-adrenoceptor agonists can cause relaxation against all types of contractile stimuli, albeit somewhat less against contraction induced by a muscarinic agonist than that by another stimulus [5]. This differential relaxation may reflect the Ca2+ sensitization caused by the muscarinic agonist. A better understanding of the mechanisms causing Ca2+ sensitization and desensitization may lead to novel therapeutics for overactive and/or underactive detrusor.
Conflict of Interest Dr. Michel reports consultancy honoraria from AltheRX, grants and consultancy and speaker honorariy from Astellas, grants and consultancy and speaker honoraria from Pfizer, outside the submitted work, and is presently is an employee of Boehringer Ingelheim.
References 1 Frazier EP, Peters SLM, Braverman AS, Ruggieri MR Sr, Michel MC. Signal transduction underlying control of urinary bladder smooth muscle tone by muscarinic receptors and b-adrenoceptors. Naunyn-Schmiedeberg’s Arch Pharmacol 2008; 377: 449–62 2 Ratz PH, Berg KM, Urban NH, Miner AS. Regulation of smooth muscle calcium sensitivity: KCl as a calcium-sensitizing stimulus. Am J Physiol 2005; 288: C769–83 3 Hayashi M, Kajioka S, Itsumi M et al. Actions of cyclic 30 50 -adenosine monophosphate (cAMP) on calcium sensitisation in human detrusor smooth muscle contraction. BJU Int 2015; 117: 179–91 4 Cernecka H, Kersten K, Maarsingh H et al. b3-Adrenoceptor-mediated relaxation of rat and human urinary bladder: roles of BKCa channels and rho kinase. Naunyn-Schmiedeberg’s Arch Pharmacol 2015; 386: 749–59 5 Michel MC, Sand C. Effect of pre-contraction on b-adrenoceptormediated relaxation of rat urinary bladder. World J Urol 2009; 27: 711–5
Correspondence: Martin C. Michel, Department of Pharmacology, Johannes Gutenberg University, Obere Zahlbacher Str. 67, 51101 Mainz, Germany. e-mail: [email protected] Abbreviations: MLCK, myosin light chain kinase; MLCP, myosin light chain phosphatase.
Science Made Simple Cellular basis of detrusor smooth muscle contraction Martin C. Michel Department of Pharmacology, Johannes Gutenberg University, Mainz, Germany
The cellular mechanisms, and particularly the signal transduction pathways, controlling contraction and relaxation of detrusor smooth muscle are insufficiently understood [1]. A better understanding could lead to novel therapeutics for patients with detrusor over- or underactivity, making this a question of potential clinical relevance. What determines smooth muscle tone? At the cellular level, smooth muscle contraction in the detrusor and other tissues is primarily driven by an increase in the free intracellular Ca2+ concentration; however, the extent of smooth muscle contraction in response to an intracellular Ca2+ concentration is determined by the phosphorylation state of several enzymes
[2]. If these enzymes are in the right state, a little Ca2+ can go a long way; if they are not, more Ca2+ is required to reach the same level of contraction. What are those enzymes? Ultimately, smooth muscle tone depends on the phosphorylation state of the myosin light chain (Fig. 1). This is increased by one enzyme, myosin light chain kinase (MLCK), and decreased by another, myosin light chain phosphatase (MLCP). Ca2+ in conjunction with calmodulin increases the activity of MLCK leading to myosin light chain phosphorylation and smooth muscle contraction; however, any phosphorylation of myosin light chain can be reversed by MLCP. The activity of MLCP is mainly regulated
Fig. 1 Simplified scheme of how muscarinic receptors and b-adrenoceptors interact at the cellular level to regulate detrusor smooth muscle tone. Muscarinic receptors promote elevation of intracellular Ca2+ via release from intracellular stores (sarcoplasmic reticulum [SR]) and via influx from the extracllular space through Ca2+ channels; this activates myosin light chain kinase (MLCK), which in turns phosphorylates myosin light chain (MLC). Muscarinic receptors also activate protein kinase C (PKC) and Rho kinase (RhoK), which phosphorylate myosin light chain phosphatase (MLCP), so that MLCP can no longer dephosphorylate MLC. This yields stronger contraction for a given Ca2+ concentration, i.e. Ca2+ sensitization. b-adrenoceptors inhibit Ca2+ influx and Ca2+ sensitization, but the precise molecular steps mediating this remain under discussion.
β-Adrenoceptor
Ca channel
Muscarinic receptor
SR
RhoK
cAMP
PKC
Ca
MLC
Ca
MLC MLCK
P MLCP MLC
P
MLC P
© 2015 The Authors BJU International © 2015 BJU International | doi:10.1111/bju.13216 Published by John Wiley & Sons Ltd. www.bjui.org
BJU Int 2016; 117: 177–178 wileyonlinelibrary.com
Michel
by phosphorylation of an associated protein, which in turn is promoted by protein kinases including Rho kinase and protein kinase C. Moreover, these protein kinases can also phosphorylate a protein called CPI-17, which when phosphorylated in turn causes additional inhibition of MLCP. What is Ca2+ sensitivity? The relationship between contractile force and Ca2+ concentration is called ‘Ca2+ sensitivity’ [2]. Some contractile agents, including agonists at muscarinic acetylcholine receptors, increase Ca2+ sensitivity and hence cause a contractile response greater than expected based on the elevation of intracellular Ca2+ they cause; such agonists are said to have Ca2+-sensitizing properties. Conversely, some relaxing agents, including agonists at b-adrenoceptors, decrease Ca2+ sensitivity and hence cause a greater relaxation response than expected based on their effects on intracellular Ca2+; such agonists are said to cause Ca2+ desensitization. Two recent studies shed additional light on the signal transduction processes involved in Ca2+ sensitization and Ca2+ desensitization. In this issue of the journal, Hayashi et al. [3] have used isolated human detrusor strips to study the interaction between cyclic AMP (cAMP), a second messenger generated in response to e.g. b-adrenoceptor agonists, and carbachol, an agonist at muscarinic receptors. They show in functional experiments that cAMP can inhibit carbacholinduced Ca2+ sensitization. The relaxation induced by cAMP was further enhanced when protein kinase C was inhibited, whereas concomitant inhibition of Rho kinase had little effect. In another study, Cernecka et al. [4] have not directly studied Ca2+ sensitization but in confirmation of the data by Hayashi et al. [3] found that inhibition of Rho kinase only inconsistently enhanced relaxation by two b-adrenoceptor agonists in human detrusor strips; however, consistent enhancement was observed in rat detrusor strips. In the absence of a contractile agent, the b-adrenoceptor agonists markedly reduced myosin light chain phosphorylation, whether or not Rho kinase was inhibited. In contrast, carbachol transiently increased myosin light chain phosphorylation; the additional presence of the badrenoceptor agonists did not cause inhibition of this response and, if anything, caused minor additional increases. What does all of this mean? The contraction of human detrusor smooth muscle cells is primarily stimulated by the
178
© 2015 The Authors BJU International © 2015 BJU International
endogenous muscarinic agonist acetylcholine. This can be inhibited by muscarinic receptor antagonists, but they have limited efficacy as, particularly in pathophysiological settings, additional contractile agents including ATP and bradykinin come into play. b-adrenoceptor agonists can cause relaxation against all types of contractile stimuli, albeit somewhat less against contraction induced by a muscarinic agonist than that by another stimulus [5]. This differential relaxation may reflect the Ca2+ sensitization caused by the muscarinic agonist. A better understanding of the mechanisms causing Ca2+ sensitization and desensitization may lead to novel therapeutics for overactive and/or underactive detrusor.
Conflict of Interest Dr. Michel reports consultancy honoraria from AltheRX, grants and consultancy and speaker honorariy from Astellas, grants and consultancy and speaker honoraria from Pfizer, outside the submitted work, and is presently is an employee of Boehringer Ingelheim.
References 1 Frazier EP, Peters SLM, Braverman AS, Ruggieri MR Sr, Michel MC. Signal transduction underlying control of urinary bladder smooth muscle tone by muscarinic receptors and b-adrenoceptors. Naunyn-Schmiedeberg’s Arch Pharmacol 2008; 377: 449–62 2 Ratz PH, Berg KM, Urban NH, Miner AS. Regulation of smooth muscle calcium sensitivity: KCl as a calcium-sensitizing stimulus. Am J Physiol 2005; 288: C769–83 3 Hayashi M, Kajioka S, Itsumi M et al. Actions of cyclic 30 50 -adenosine monophosphate (cAMP) on calcium sensitisation in human detrusor smooth muscle contraction. BJU Int 2015; 117: 179–91 4 Cernecka H, Kersten K, Maarsingh H et al. b3-Adrenoceptor-mediated relaxation of rat and human urinary bladder: roles of BKCa channels and rho kinase. Naunyn-Schmiedeberg’s Arch Pharmacol 2015; 386: 749–59 5 Michel MC, Sand C. Effect of pre-contraction on b-adrenoceptormediated relaxation of rat urinary bladder. World J Urol 2009; 27: 711–5
Correspondence: Martin C. Michel, Department of Pharmacology, Johannes Gutenberg University, Obere Zahlbacher Str. 67, 51101 Mainz, Germany. e-mail: [email protected] Abbreviations: MLCK, myosin light chain kinase; MLCP, myosin light chain phosphatase.
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