Regulation of gastrointestinal motility--insights from smooth muscle biology

Abstract

Gastrointestinal motility results from coordinated contractions of the tunica muscularis, the muscular layers of the alimentary canal. Throughout most of the gastrointestinal tract, smooth muscles are organized into two layers of circularly or longitudinally oriented muscle bundles. Smooth muscle cells form electrical and mechanical junctions between cells that facilitate coordination of contractions. Excitation-contraction coupling occurs by Ca(2+) entry via ion channels in the plasma membrane, leading to a rise in intracellular Ca(2+). Ca(2+) binding to calmodulin activates myosin light chain kinase; subsequent phosphorylation of myosin initiates cross-bridge cycling. Myosin phosphatase dephosphorylates myosin to relax muscles, and a process known as Ca(2+) sensitization regulates the activity of the phosphatase. Gastrointestinal smooth muscles are 'autonomous' and generate spontaneous electrical activity (slow waves) that does not depend upon input from nerves. Intrinsic pacemaker activity comes from interstitial cells of Cajal, which are electrically coupled to smooth muscle cells. Patterns of contractile activity in gastrointestinal muscles are determined by inputs from enteric motor neurons that innervate smooth muscle cells and interstitial cells. Here we provide an overview of the cells and mechanisms that generate smooth muscle contractile behaviour and gastrointestinal motility.

Figure 1

Regulation of gastrointestinal motility. Major factors involved in regulation of the tunica muscularis of the gastrointestinal tract. SMCs are shown in close contact with ICC and PDGFRα⁺ cells. SMCs form gap junctions with both classes of interstitial cells, creating an electrical syncytium (SIP syncytium). Excitatory (orange) and inhibitory (blue) enteric motor neurons, with cell bodies in the myenteric plexus, innervate ICC, PDGFRα⁺ and SMCs, so motor responses are integrated by all three cell types. Information is sent by afferent (sensory) neurons from the gut to the enteric ganglia, the CNS, and to autonomic ganglia (not shown). Information is sent by efferent neurons from the CNS to enteric ganglia. Hormones, reaching the gut via the circulation, and paracrine and immune factors, produced by mast cells and resident macrophages, also affect motor output. Abbreviations: ICC, interstitial cells of Cajal; SMC, smooth muscle cell; SIP, syncytium of neuro-effector cells consisting of SMCs, ICC and PDGFRα⁺ cells.

Figure 2

Major cellular mechanisms controlling contraction in gastrointestinal smooth muscle cells. Mechanisms leading to enhanced contraction are depicted in red, and pathways linked to decreased contraction are shown in blue. Ca²⁺ required for excitation–contraction coupling can enter cells through VDCC or NSCC. The open probability of VDCC is enhanced (circle with + sign) by depolarization caused by opening of NSCC and influx of Na⁺ or Ca²⁺. Openings of VDCC are decreased by a variety of K⁺ channels expressed by SMCs; most inhibitory agonists regulate Ca²⁺ influx by activating K⁺ channels. Ca²⁺ entry can also be supplemented by release of Ca²⁺ from IP₃ receptor-operated Ca²⁺ channels in the sarcoplasmic reticulum membrane. Ca²⁺ release from sacroplasmic reticulum can also occur through ryanodine receptors (not shown). IP₃ is synthesized by PLCβ in response to agonist binding to G-protein-coupled receptors and coupling through Gα _q/11. [Ca²⁺]_i binds to calmodulin and activates myosin light chain kinase, which phosphorylates MLC20 to facilitate cross-bridge formation. Phosphorylation of MLC20 is balanced by the action of MLCP. Dephosphorylation of MLC20 reduces cross-bridge cycling and leads to muscle relaxation. Factors that lead to inhibition of MLCP increase contraction and, in effect, enhance Ca²⁺ sensitivity of the contractile apparatus. The opposite is true for factors that activate MLCP. A pathway that increases Ca²⁺ sensitization (and therefore increases contraction) occurs through binding of G-protein-coupled (Gα_q/11 or Gα_12/13) receptors and regulation of the GDP-GTP exchange factor (Rho-GEF), RhoA and activation of Rho kinase. Rho kinase and protein kinase C can phosphorylate CPI-17 (at T38), a protein that when phosphorylated inhibits the catalytic subunit of MLCP (PPlc; circle with negative sign). Rho kinase can also phosphorylate the regulatory subunit of MLCP (MYPT) at T696 and T853. Phosphorylation of MYPT decreases the activity of MLCP, preserving phosphorylation of MLC20. ZIPK also phosphorylates CPI-17 and MYPT. Cyclic nucleotide-dependent pathways decrease Ca²⁺ sensitivity. NO, for example, binds to guanylyl cyclase (composed of GCα and GCβ subunits) and generates cGMR cGMP activates cGMP-dependent protein kinase (PKG), which can phosphorylate RhoA and reduce activation of Rho kinase, thus reducing Ca²⁺ sensitization, or it can phosphorylate telokin (S13), which stimulates MLCP (circle with + sign). Binding of receptors coupled through Gα_s activates adenylate cyclase and production of cAMP. PKA can also phosphorylate telokin and increase MLCP activity. Abbreviations; [Ca²⁺]_i, cytoplasmic Ca²⁺; DAG, diacylglycerol; IP₃, inositol 1,4,5-trisphosphate; MLC20, 20kDA light chain of myosin; MLCP, myosin light chain phosphatase; NO, nitric oxide; NSCC, nonselective cation channels; PKA, cAMP-dependent protein kinase; PKG, cGMP-dependent protein kinase; PLCβ, phospholipase Cβ; VDCC, voltage-dependent Ca²⁺ channels; ZIPK, zipper-interacting protein kinase. Permission obtained Wiley © Sanders, K. M. Neurogastroenterol. Motil. 20 (Suppl. 1), 39–53 (2008).

Figure 3

miRNA-mediated SMC remodelling. SMC fate and plasticity are regulated by differentiation-specific miRNAs (orange) and proliferation-specific miRNAs (purple). Differentiation-specific miRNAs promote expression of SMC contractile genes (red lines) and repress expression of proliferative genes (blue lines) by targeting transcriptional regulators. Conversely, proliferation-specific miRNAs promote expression of proliferative genes (red lines) and repress expression of contractile genes (blue lines). Differentiation-specific miRNAs miR-143 and miR-145 promote SMC contractile gene expression through SRF-MYOCD binding to CArG boxes found in the promoter regions of SMC genes whilst suppressing the transcriptional activators ELK1, KLF4, S0×2, and 0CT4 of proliferative genes. miR-1 and miR-133a also promote SMC differentiation by repressing PIM1 and SP1, respectively. Conversely, proliferation-specific miRNAs miR-214 and miR-199a suppress contractile gene expression by repressing SRF, but promote proliferative gene expression by enhancing ELK1 expression. miR-21 also promotes proliferative gene expression by repressing PTEN and PDCD4. Abbreviations: miRNA. microRNA: SMC. smooth muscle cell: SRF. serum response factor.