Bioengineering 6003Bioengineering 6003Cellular ElectrophysiologyCellular Electrophysiologyand Biophysicsand BiophysicsCardiac cellCardiac cell--cellcellCommunicationCommunicationPart 2Part 2Alonso P. Moreno D.Sc.CVRTI, Cardiology [email protected] of intercellular communication• It is simpleElectrically we evaluate gj or junction conductancegj= n * j* Pon = number of channelsj= unitary conductancePo = open probabilityRemodeling (long term ischemia or heart failure)Gating of gap junction channels• Gating by voltage– Transjunctional and transmembrane• Gating by intracellular pH• Gating by protein phosphorylationIV. Structure function relationship1. pH gating2. Fast voltage gating3. PhosphorylationTransjunctional voltage dependenceVoltage gating of Cx45Rectifying channels from murine embryonic isolated ventricle cellsThe formation of heterotypic channels can be demonstrated in this preparation, where gap junctions show gating rectification.Gap JunctionChannelsMultiple ConfigurationsMultiple expression of connexins in a tissueConnexins in the heartExample of the co-expression of connexinsCx30.2Heteromeric combinations modulate total and unitary conductancesElectrical propagation from the SA node to the musculatureControlUncoupledAdding connectionsGating by TransmembranevoltageEvaluation of changes in total conductance due to synchronous stimulation in both cellsElectrotonic conduction in cardiac fibersGating by pHM. Delmar et al. Current Topics in Membranes (2000)The reduction of intracellular pH causes a reduction in the conductance of the junction (Gj/Gmax). When the COOH tail is removed, there is no gating by pH.If the COOH tail is co-expressed, the gating by pH is re-established.Intra-molecular interactions rule pH gating for acidosisConnexins also gate for intracellular alkalosisHeterotypic channel formation reverts permeance properties without changing charge selectivityConnexin26 channel at 3.5Å resolutionDetails from the poreA mathematical model representing two cells coupled through a gap junction porePermeation modeling equations in a particle collision modelnemmcmuvnemmcmuvuuncˆ1ˆ1ˆ211222121121unnuuvˆˆˆ2ˆ122210ˆ..41rrqqFticElectrostaExternal ForcesParticle-ParticleInteractionsParticle DynamicsvKFBrBrownianˆ.qEFEFMiscEFBrownianticElectrostaFFFFF2021tatuss tauvmFa Wall- Particle interactionsVelocityAccelerationPositionTop viewLateral viewLateral view Axial viewviewTop viewview3D3DCollision model simulation of 16 Lucifer Yellow moleculesin a two cell model connected through a gap junction pore.Mondal et al, Biophys. J. 2010awhere, x is the position, n is the time step, k is the Boltzmann’s constant, T is the temperature (in Kelvin), D is the diffusion matrix of the particle, F is the net force acting on the particle, ∆t is the time step and ∆W is a random vector from Gaussian distribution with zero mean variance.Using Brownian Dynamics makes a more realistic modelComparison between simulated particle trajectories from two different mathematical modelsParticle collisionBrownian dynamicsConduction after infarction• Myocardial infarction (MI) healing process includes revascularization and formation of scar tissue.• Scar tissue is composed of fibroblast-like cells embedded in a collagen-fibrous matrix.• Scar formation causes complex changes in the organization of cells in the infarcted area.Sachse et al, Ann Biomed Eng, 37; 2009Modeling current interactions between myocytes and fibroblastsBuilding a FlexMEA as an alternative to regulate and study heterocellular communicationRecording system for the use of Perforated Flexible MEAsSummary • Gap junction channels could be formed of distinct isomeric forms.• Each isoform can form channels with unique gating for voltage and chemical agents.• Permeability of gap junctions depends on the isoforms involved in the formation of channels and on the properties of the crossing-molecules.• Heteromultimeric channels can become highly selective and are thought to be involved in various physiological and pathological processes.• Heterotypic communication can alter channel selectivity and create a preferential flux direction of metabolites across gap junction. One of the mechanisms to explain this preferential flux could be based in differences in pore mouth and charge. • Long standing electrophysiological phenomena related to the complex nature of electrograms in the infarct border region could be explained by changes in cardiac conduction due to hetero-cellular coupling.• Studies related to this mechanism of conduction could significantly impact the interpretation of cardiac electrograms, increase the understanding of arrhythmia substrates, and make a significant contribution on the development of more guided approaches to arrhythmia