Emergent properties of electrically coupled smooth muscle cells

Asynchronous and synchronous calcium oscillations occur in a variety of cells. A well-established pathway for intercellular communication is provided by gap junctions which connect adjacent cells and can mediate electrical and chemical coupling. Several experimental studies report that cells presenting only a transient increase when freshly dispersed may oscillate when they are coupled. Such observations suggest that the role of gap junctions is not only to coordinate calcium oscillations of adjacent cells. Gap junctions may also be important to generate oscillations. Here we illustrate the emergent properties of electrically coupled smooth muscle cells using a model that we recently proposed. A bifurcation analysis in the case of two cells reveals that synchronous and asynchronous calcium oscillations can be induced by electrical coupling. In a larger population of smooth muscle cells, electrical coupling may result in the creation of groups of cells presenting synchronous calcium oscillations. The elements of one group may be distant from each other. Moreover, our results highlight a general mechanism by which gap junctional electrical coupling can give rise to out of phase calcium oscillations in smooth muscle cells that are non-oscillating when uncoupled. All these observations remain true in the case of non-identical cells, except that the solution corresponding to synchronous calcium oscillations disappears and that the formation of groups is sensitive to the degree of heterogeneity. © 2005 Society for Mathematical Biology. Published by Elsevier Ltd. All rights reserved.

A theoretical model of the cellular calcium dynamics and vasomotion in muscular arteries

Michèle Koenigsberger

Muscular arteries possess the ability to control actively their lumen by altering the tone of the smooth muscle cells of the arterial wall, a property which is vital for a large number of the hemodynamical functions of the body. Arterial contraction is due to an increase in the smooth muscle cytosolic calcium concentration. Abnormalities in the contractile mechanisms of arteries contribute to a variety of cardiovascular diseases such as hypertension. The understanding of the mechanisms of vascular smooth muscle functions can therefore contribute to a better diagnosis and treatment of such diseases. Muscular arteries, arterioles and capillaries display also slow rhythmic diameter variations, called vasomotion, independent on other rhythms in the body (cardiac, respiratory, circadian). It is well established that vasomotion is due to the contractile activity of vascular smooth muscle cells, but its underlying mechanisms are not well understood. The aim of the present thesis is to develop a theoretical model to get a better understanding of vasomotion in muscular arteries and arterioles. Many experimental studies have shown that arterial smooth muscle cells respond with cytosolic calcium rises to stimulation by substances causing arterial contraction (vasoconstrictors). A low vasoconstrictor concentration gives rise to asynchronous "all or none" calcium rises (calcium flashes) in few cells. With a higher vasoconstrictor concentration, the number of smooth muscle cells responding in this way increases (recruitment). When all cells are recruited, synchronous calcium oscillations are observed that generate arterial contraction and vasomotion. We show that these phenomena of recruitment and synchronization naturally emerge from a theoretical model of a population of smooth muscle cells coupled through their gap junctions. The effects of electrical, calcium and inositol 1,4,5-trisphosphate couplings are studied. In our model, the asynchronous calcium flashes arise from stochastic opening of channels. The calcium oscillations result from a dynamic system instability and can be synchronized by gap junctional coupling. Our model is validated by published in vitro experiments obtained on rat mesenteric arterial segments. Several experimental studies report that cells presenting only a transient calcium increase when freshly dispersed may present calcium oscillations when they are coupled. Such observations suggest that the role of gap junctions is not only to coordinate calcium oscillations of adjacent cells. Gap junctions may also be important to generate oscillations. To address this point, we study in more detail the properties of electrically coupled smooth muscle cells. We show that the effect of electrical coupling may not only be to synchronize the calcium oscillations in smooth muscle cells, as intuitively expected. A bifurcation analysis in the case of two cells reveals that electrical coupling can cause the calcium oscillations to be synchronous or asynchronous. In a larger population of smooth muscle cells, electrical coupling may result in the formation of groups of cells presenting synchronous calcium oscillations. The elements of one group may be distant from each other. Moreover, our results highlight a general mechanism by which gap junctional electrical coupling can give rise to out of phase calcium oscillations in smooth muscle cells that are non oscillating when uncoupled. Even though it is now established that vasomotion is induced by synchronous calcium oscillations of smooth muscle cells, the role of the endothelium for vasomotion is still unclear and controversial. Some experimental studies claim that the endothelium is necessary for synchronization and vasomotion, whereas others report rhythmic diameter oscillations in absence of endothelium. Moreover, endothelium derived factors have been shown to abolish vasomotion by desynchronizing the calcium signals in smooth muscle cells. By modeling the calcium dynamics of a population of smooth muscle cells coupled to a population of endothelial cells, we analyze the effects of a smooth muscle vasoconstrictor stimulation on endothelial cells and the feedback effects of endothelium derived factors. Our results show that the endothelium essentially decreases the calcium level in smooth muscle cells and may move these cells from a steady state to an oscillatory domain, and vice versa. In the oscillatory domain, a population of coupled smooth muscle cells exhibits synchronous calcium oscillations. Outside this domain, the coupled smooth muscle cells present only irregular calcium flashes. Our findings provide explanations for the published contradictory experimental observations. Smooth muscle and endothelial cells in the arterial wall are exposed to mechanical stress. Indeed, blood flow induces intraluminal pressure variations and shear stress. An increase in pressure may induce a vessel contraction, a phenomenon known as the myogenic response. Vasomotion has also been shown to be modulated by pressure changes. To get a better understanding of the effect of stress and in particular pressure on vasomotion, we propose a model of a blood vessel describing the calcium dynamics in a coupled population of smooth muscle cells and endothelial cells and the consequent vessel diameter variations. We show that a rise in pressure increases the calcium concentration. This may either induce or abolish vasomotion, or increase its frequency depending on the initial conditions. In our model the myogenic response is less pronounced for large arteries than for small arteries and occurs at higher values of pressure if the arterial wall is thickened. Our results are in agreement with experimental observations concerning a broad range of vessels. In vitro different techniques are used to study the calcium dynamics and contraction/relaxation mechanisms on arteries. Most experimental studies use either an isometric (i.e. the arterial diameter is kept constant) or an isobaric (i.e. the intraluminal pressure is kept constant) setup. However none of these setups correspond to the in vivo situation. In vivo, a blood vessel is neither isobaric nor isometric nor isotonic (i.e. the wall tension is kept constant), since it is continuously submitted to intraluminal pressure variations arising from heart beat. We determine theoretically whether results may be considerably different depending on the experimental setup (isometric, isobaric, or isotonic) used. We show that the vasoconstrictor sensitivity is higher in isometric than isobaric or isotonic conditions, in agreement with experimental observations. The threshold vasoconstrictor concentration necessary for vasomotion is therefore higher in isobaric than isometric setups. The model suggests that isometric conditions may generate the coexistence of multiple stable states. The contraction is less pronounced in isotonic than in isobaric conditions, in agreement with experimental findings.

EPFL2006

Ca2+ dynamics in a population of smooth muscle cells: modeling the recruitment and synchronization

Jean-Louis Bény, Michèle Koenigsberger, Mathieu Lamboley, Jean-Jacques Meister, Roger Sauser

Many experimental studies have shown that arterial smooth muscle cells respond with cytosolic calcium rises to vasoconstrictor stimulation. A low vasoconstrictor concentration gives rise to asynchronous spikes in the calcium concentration in a few cells (asynchronous flashing). With a greater vasoconstrictor concentration, the number of smooth muscle cells responding in this way increases (recruitment) and calcium oscillations may appear. These oscillations may eventually synchronize and generate arterial contraction and vasomotion. We show that these phenomena of recruitment and synchronization naturally emerge from a model of a population of smooth muscle cells coupled through their gap junctions. The effects of electrical, calcium, and inositol 1,4,5-trisphosphate coupling are studied. A weak calcium coupling is crucial to obtain a synchronization of calcium oscillations and the minimal required calcium permeability is deduced. Moreover, we note that an electrical coupling can generate oscillations, but also has a desynchronizing effect. Inositol 1,4,5-trisphosphate diffusion does not play an important role to achieve synchronization. Our model is validated by published in vitro experiments obtained on rat mesenteric arterial segments.

Elsevier2004

Coordination of Myofibroblast Contraction in Vitro

Lysianne Follonier Castella

The aim of this thesis is to elucidate the common mechanisms that regulate myofibroblast contraction and intercellular communication, with a particular focus on the role of Ca2+ signalling. Synthesis and remodelling of collagenous matrix by myofibroblasts are key elements in the healing process of injured organs. Deregulation of myofibroblast activity and excessive remodelling result in hypertrophic scarring and organ fibrosis. Myofibroblasts are specialised fibroblasts with particularly contractile stress fibres and features that resemble smooth muscle cells. Although myofibroblast contraction is acknowledged being responsible for wound closure and tissue contracture, the underlying regulation mechanisms are unclear. In particular, it remains elusive to what extent changes in the cytosolic Ca2+, a central regulator of smooth muscle contraction, regulate myofibroblast activities. Previous studies have revealed that cultured fibroblasts and myofibroblasts exhibit spontaneous cytosolic Ca2+ oscillations but a link with cell contraction has not been established. In the first part of this thesis, we are testing the hypothesis that cytosolic Ca2+ oscillations regulate myofibroblast contraction. For this, we use analysis of Ca2+ dynamics with fluorescent indicators, tracking of stress-fibre-linked microbeads on the myofibroblast surface and quantification of subcellular pulling events with atomic force microscopy. This combined approach demonstrates that myofibroblasts exhibit periodic (∼100 seconds) Ca2+ oscillations that control small (∼400 nm) and weak (∼100 pN) contractions. Using deformable culture substrate to assess cell isometric tension, we further demonstrate that myofibroblast contraction is regulated at two levels: variations of intracellular Ca2+ mediate contractions of dorsal stress fibres whereas the Ca2+-independent Rho kinase pathway is responsible for maintaining overall isometric cell tension. In the second part of this thesis, we investigate whether the observed subcellular contractions and Ca2+ oscillations may play a role in coordinating the activities between directly contacting myofibroblasts. Rational for this hypothesis is that myofibroblasts in densely populated wound granulation tissue form two types of intercellular junctions: adherens junctions that mechanically couple stress fibres between cells, and gap junctions that provide electrochemical coupling. Both junctions have been shown to play a role in modulating the contraction of wound tissue and of myofibroblast collagen gel cultures but their precise roles are still elusive. To fill this gap in knowledge, we analyse the coordination of Ca2+ oscillations between adjacent fibroblasts and myofibroblasts, respectively. Intercellular communication is modulated with pharmacological treatments acting on either mechanical or electrochemical coupling, as well as on the cell's contractile apparatus and mechanosensitive membrane channels. Our results demonstrate that stress-fibre associated adherens junctions mechanically coordinate myofibroblast activities within a cell population. This mechanism implies that the transmission of contractile events from one cell induces a Ca2+ influx through mechanosensitive ion channels in its neighbour and there triggers a new contraction. Based on these results we suggest that, at the tissue level, myofibroblasts remodel collagen by Ca2+-dependent micro-contractile events that add up to macroscopic tissue contracture, whereas Rho kinase maintains cell isometric tension. Mechanical coupling via adherens junctions may simultaneously improve the remodelling of cell-dense tissue by coordinating the activity of myofibroblasts.