Review
Role of rho kinase in the functional and dysfunctional tonic smooth muscles

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Tonic smooth muscles play pivotal roles in the pathophysiology of debilitating diseases of the gastrointestinal and cardiovascular systems. Tonic smooth muscles differ from phasic smooth muscles in the ability to spontaneously develop myogenic tone. This ability has been primarily attributed to the local production of specific neurohumoral substances that can work in conjunction with calcium sensitization via signal transduction events associated with the Ras homolog gene family, member A (RhoA)/Rho-associated, coiled-coil containing protein kinase 2 (ROCK II) pathways. In this article, we discuss the molecular pathways involved in the myogenic properties of tonic smooth muscles, particularly the contribution of protein kinase C vs the RhoA/ROCK II pathway in the genesis of basal tone, pathophysiology and novel therapeutic approaches for certain gastrointestinal and cardiovascular diseases. Emerging evidence suggests that manipulation of RhoA/ROCK II activity through inhibitors or silencing of RNA interface techniques could represent a new therapeutic approach for various gastrointestinal and cardiovascular diseases.

Section snippets

Myogenic properties of tonic smooth muscles

Smooth muscles can be broadly classified according to the contractile patterns: phasic and tonic. Phasic smooth muscles contract transiently in response to neural stimulation and neurohumoral substances such as angiotensins and prostanoids. By contrast, tonic smooth muscles develop and sustain myogenic basal tone in the absence of an external stimulation. The term ‘myogenic’ implies that the stimulus for tone development and maintenance originates in the muscle itself, and its response is

Role of the Ca2+/calmodulin pathway in the smooth muscle motor response

The motor response in SMCs is controlled by the sliding of myosin and actin filaments over each other. This process requires chemical energy, which is provided by the hydrolysis of ATP (Figure 1). Myosin utilizes ATP for molecular conformational changes in its own structure to facilitate its attachment and interaction with actin filaments, leading to the formation of cross-bridges. Myosin attachment occurs through protruding globular heads in the myosin filaments that interact with actin

The RhoA/ROCK pathway

RhoA belongs to a family of small GTPases that include RhoA, RhoB and RhoC isoforms, of which RhoA is the best understood because of its important roles in the smooth muscle motility. RhoA might have high-affinity binding to guanosine trisphosphate (GTP) as well as guanosine diphosphate (GDP). However, the active form of RhoA is bound to GTP located in the cell membrane, whereas the GDP-bound form is inactive and located in the cytoplasm. Switching between the GDP/GTP-bound forms is controlled

Role of PKC vs RhoA/ROCK in the maintenance of basal tone

Both PKC and ROCK have been implicated in the inhibition of MLCP activity via phosphorylation of CPI-17 at threonine-38 (Thr38) residue (pThr38-CPI-17 or simply p-CPI-17). CPI-17 is an endogenous inhibitory protein of the catalytic subunit of MLCP 15, 49, 52, 53, 54. p-CPI-17 is ∼7000-fold more potent than nonphosphorylated CPI-17. Phosphorylation of CPI-17 by ROCK has also been suggested by in vivo studies 15, 55, 56. In these studies, increase in p-CPI-17 was specifically inhibited by the

Therapeutic potential of RhoA/ROCK inhibitors in GI diseases

Prototypes of tonic tissues, the LES and IAS, play major roles in the pathophysiology of several GI motility disorders. The LES maintains tone in the basal state and relaxes during swallowing to allow passage of food. Similarly, the IAS maintains spontaneous tone and relaxes to allow passage of processed food in response to the rectoanal inhibitory reflex that is initiated by rectal distension caused by the stool. Significant changes in the basal tone of the IAS and LES have been directly

Therapeutic potential of RhoA/ROCK inhibitors in cardiovascular diseases

As in GI smooth muscles, the contractile state of vascular SMCs (VSMCs) depends on the levels of p-MLC20 determined by the balance between the activities of Ca2+/CaM/MLCK-dependent (plus Ca2+-independent) and the MLCP pathways 19, 38, 43. Several neurohumoral agonists (e.g. Ang II and prostanoids) are known to modulate these pathways via GPCRs that lead not only to increases in the [Ca2+]i but also stimulate RhoA/ROCK. Thus, as outlined above, the inhibition of MLCP via p-MYPT1 and/or p-CPI-17

Therapeutic potential of RhoA/ROCK silencing with small interfering RNA (siRNA)

The main advantage of a siRNA approach over conventional inhibitors is that the conventional inhibitors such as Y27632 do not exhibit specificity for the ROCK isoforms. In addition, siRNAs are potentially ∼1000-fold more potent than the conventional ROCK inhibitors 13, 41, 94, 95, 96, 97, 98, 99, 100.

Because the pathophysiology of many diseases is based on the overexpression of certain genes, and intense research on gene function has allowed the identification of many putative target genes,

Concluding remarks

Basal tone in the smooth muscles of the GI sphincters (typified by the LES and IAS) and those of certain blood vessels of the cardiovascular system provide true representations of the sustained contraction in the absence of any exogenous stimulus or agonist. The basal tone in such smooth muscles is primarily myogenic. However, the molecular mechanisms underlying the myogenic control in relation to the external triggers (perhaps produced within the cells) and other modulatory neurohumoral

Acknowledgments

This work was supported by the National Institutes of Diabetes and Digestive and Kidney Diseases Grant DK-35385, and an institutional grant from Thomas Jefferson University.

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