Smooth Muscle–Specific Myosin Phosphatase Target Subunit 1 (MYPT1): An Important Piece of the Puzzle

Dear Sir: We have read with a great interest a recent article by He et al in the June issue of Gastroenterology.1 The studies provide strong evidence in favor of the concept that smooth muscle–specific myosin phosphatase target subunit 1 (MYPT1) of myosin light chain phosphatase (MLCP) plays a critical role in the agonist-induced contraction/relaxation of the smooth muscle. This was shown in their studies using animals with knocked out MYPT1−/−. The investigators employed the Cre-loxP system in which they used the promoter region and exon 1 of Mypt1 flanked by 2 loxP sites to establish Mypt1-floxed mice. These mice were crossed with SMA-Cre transgenic mice to generate smooth muscle–specific MYPT1 knockout mice (Mypt1SMKO). The authors demonstrated that the phasic responses (to acetylcholine [ACh] and K+-depolarization) of the jejunal and ileal smooth muscles of the mutant mice in comparison with the control mice are converted into a tonic type with sustained force. The converted responses had reduced rates of shortening velocity and relaxation because of higher levels of phospho-MLC20 (p-MLC20). The authors observed no apparent abnormality in the intestinal motility in the mutant mice, although there was a definite trend toward the decrease in the intestinal transit (a predicted effect). The lack of significance for the abnormal intestinal motility may have been because of the huge variability among the mutant mice reflected by 49.9 ±13.9 SEM percent transit values. The issue of intestinal motility abnormality in the Mypt1SMKO may be resolved by studying a larger number of animals with a group beyond 16 weeks old, and by detailed examinations of the entire gut from the esophagus to the anorectum. The present studies had primarily focused on the limited regions of the gastrointestinal tract. A critical role of MYPT1 in sustained contraction has been previously demonstrated in the phenotypically tonic versus the phasic smooth muscles of the gastrointestinal tract, in the basal state.2,3 Additionally, there seem to be certain similarities between Mypt1SMKO animals and the spontaneously hypertensive rats: An increase in blood pressure and in the intestinal smooth muscle contractility with a corresponding decrease in MYPT1.1,4,5 An increase in the neurotransmitter (ACh)-mediated amplitude and sustained contraction of the intestinal smooth muscle in MYPT1SMKO is suggestive of dysfunctional smooth muscle typified in the diffuse esophageal spasm in response to swallowing. It has been proposed that defective inhibitory neurotransmission mediated by nitric oxide and vasoactive intestinal polypeptide, unopposed excitatory neurotransmitters’ (ACh; substance P) contractile actions, and increased smooth muscle sensitivity may be responsible for the uncoordinated often hypertensive contractions, failure of the descending inhibition, and achalasic/hypertensive sphincteric smooth muscles.6 Present data with greater sensitivity of the smooth muscle in response to the excitatory agonists in the presence of similar concentrations of intracellular Ca2+, suggest the role of Ca2+- sensitization via inhibition of MLCP via MYPT1, the primary target for RhoA/ROCK. Also, there are studies to show significantly higher levels of endogenous inhibitory protein CPI-17 (originally named so because of its targeting PKC, protein-kinase C potentiated inhibitor) in the tonic versus phasic smooth muscles. Recently, it is becoming evident that RhoA/ROCK contributes to Ca2+ sensitization not only by targeting MYPT1 but also by targeting CPI-17, so that CPI-17 is not exclusively targeted by PKC.3,7 Those data from humans and animals show significantly higher levels of CPI-17 in the spontaneously tonic smooth muscle versus the phasic, and specific decreases in the phospho-CPI-17 after selective RhoA/ROCK inhibitors. The bimodal effect of RhoA/ROCK on MYPT1 and CPI-17, however, was not appropriately discussed in the paper by He et al. In the view of a critical role of MLCK/MYPT1-MLCP/p-MLC20 in smooth muscle relaxation/contraction, it is important to determine the significance of MYPT1 in the region-specific pathophysiology in response to the corresponding reflexes, for example, swallowing in the case of esophagus and rectoanal inhibitory (defecation) reflex in the anorectum. In this regard, the potential of MYPT1 gene-deleted animal models similar to that of Mypt1SMKO (but without compensatory genetic and adaptive physiologic responses) may go beyond the investigation of the molecular mechanisms for the agonist-induced smooth muscle contraction. Such molecular insights may further reveal the pathophysiology of certain motility disorders, with or without characteristic dysfunctional inhibitory and excitatory neurotransmissions, as discussed.6 These disorders may involve MYPT1-associated dearranged signal transduction cascade for the smooth muscle contraction/relaxation to explain disturbed changes in the latency gradient for the sequential contractions, a hallmark of the normal progression of the food and ingesta leading to the expulsion of waste.8