Huub M. M. ten Eikelder

Synaptic Transmission from Horizontal Cells to Cones Is Impaired by Loss of Connexin Hemichannels

In the vertebrate retina, horizontal cells generate the inhibitory surround of bipolar cells, an essential step in contrast enhancement. For the last decades, the mechanism involved in this inhibitory synaptic pathway has been a major controversy in retinal research. One hypothesis suggests that connexin hemichannels mediate this negative feedback signal; another suggests that feedback is mediated by protons. Mutant zebrafish were generated that lack connexin 55.5 hemichannels in horizontal cells. Whole cell voltage clamp recordings were made from isolated horizontal cells and cones in flat mount retinas. Light-induced feedback from horizontal cells to cones was reduced in mutants. A reduction of feedback was also found when horizontal cells were pharmacologically hyperpolarized but was absent when they were pharmacologically depolarized. Hemichannel currents in isolated horizontal cells showed a similar behavior. The hyperpolarization-induced hemichannel current was strongly reduced in the mutants while the depolarization-induced hemichannel current was not. Intracellular recordings were made from horizontal cells. Consistent with impaired feedback in the mutant, spectral opponent responses in horizontal cells were diminished in these animals. A behavioral assay revealed a lower contrast-sensitivity, illustrating the role of the horizontal cell to cone feedback pathway in contrast enhancement. Model simulations showed that the observed modifications of feedback can be accounted for by an ephaptic mechanism. A model for feedback, in which the number of connexin hemichannels is reduced to about 40%, fully predicts the specific asymmetric modification of feedback. To our knowledge, this is the first successful genetic interference in the feedback pathway from horizontal cells to cones. It provides direct evidence for an unconventional role of connexin hemichannels in the inhibitory synapse between horizontal cells and cones. This is an important step in resolving a long-standing debate about the unusual form of (ephaptic) synaptic transmission between horizontal cells and cones in the vertebrate retina.

An equilibrium model for chiral amplification in supramolecular polymers

We describe a model that rationalizes amplification of chirality in cooperative supramolecular copolymerization. The model extends nucleation-elongation based equilibrium models for growth of supramolecular homopolymers to the case of two monomer and aggregate types. Using the principle of mass-balance for the two monomer types, we derive a set of two nonlinear equations, describing the thermodynamic equilibrium state of the system. These equations can be solved by numerical methods, but also analytical approximations are derived. The equilibrium model allows two-sided growth of the aggregates and can be applied to symmetric supramolecular copolymerizations, corresponding to the situation in which the monomers are enantiomerically related, as well as to the more general case of nonsymmetric supramolecular copolymerizations. In detail, so-called majority-rules phenomena in supramolecular systems with isodesmic as well as cooperative growth are analyzed. Comparison of model predictions with experimental data shows that the model gives a very good description of both titration and melting curves. When the system shows cooperative growth, the model leads to a phase diagram in which the presence of the various aggregate types is given as a function of composition and temperature.

Hemichannel-Mediated and pH-Based Feedback from Horizontal Cells to Cones in the Vertebrate Retina

Background Recent studies designed to identify the mechanism by which retinal horizontal cells communicate with cones have implicated two processes. According to one account, horizontal cell hyperpolarization induces an increase in pH within the synaptic cleft that activates the calcium current (Ca2+-current) in cones, enhancing transmitter release. An alternative account suggests that horizontal cell hyperpolarization increases the Ca2+-current to promote transmitter release through a hemichannel-mediated ephaptic mechanism. Methodology/Principal Findings To distinguish between these mechanisms, we interfered with the pH regulating systems in the retina and studied the effects on the feedback responses of cones and horizontal cells. We found that the pH buffers HEPES and Tris partially inhibit feedback responses in cones and horizontal cells and lead to intracellular acidification of neurons. Application of 25 mM acetate, which does not change the extracellular pH buffer capacity, does lead to both intracellular acidification and inhibition of feedback. Because intracellular acidification is known to inhibit hemichannels, the key experiment used to test the pH hypothesis, i.e. increasing the extracellular pH buffer capacity, does not discriminate between a pH-based feedback system and a hemichannel-mediated feedback system. To test the pH hypothesis in a manner independent of artificial pH-buffer systems, we studied the effect of interfering with the endogenous pH buffer, the bicarbonate/carbonic anhydrase system. Inhibition of carbonic anhydrase allowed for large changes in pH in the synaptic cleft of bipolar cell terminals and cone terminals, but the predicted enhancement of the cone feedback responses, according to the pH-hypothesis, was not observed. These experiments thus failed to support a proton mediated feedback mechanism. The alternative hypothesis, the hemichannel-mediated ephaptic feedback mechanism, was therefore studied experimentally, and its feasibility was buttressed by means of a quantitative computer model of the cone/horizontal cell synapse. Conclusion We conclude that the data presented in this paper offers further support for physiologically relevant ephaptic interactions in the retina.

Mechanoelectrical coupling enhances initiation and affects perpetuation of atrial fibrillation during acute atrial dilation

BACKGROUND Acute atrial dilation increases the susceptibility to atrial fibrillation (AF). However, the mechanisms by which atrial stretch may contribute to the initiation and perpetuation of AF remain to be determined. OBJECTIVE The purpose of this study was to use a novel multiscale model of atrial electromechanics and mechanoelectrical feedback to test the hypothesis that acute stretch increases vulnerability to AF by heterogeneous activation of stretch-activated channels. METHODS Human atria were represented by a triangular mesh obtained from magnetic resonance imaging data. Atrial trabecular bundle structure was incorporated by varying thicknesses of the atrial wall. Atrial membrane behavior was modeled by the Courtemanche-Ramirez-Nattel model with the addition of a nonselective stretch-activated cation current (I(sac)). Mechanical behavior was modeled by a series elastic, a contractile, and a parallel elastic element in which contractile force was related to intracellular concentration of free calcium and sarcomere length. RESULTS Acute atrial dilation was simulated by increasing stretch throughout the atrial wall. Stimulation near the pulmonary vein ostia at an interval of 600 ms induced AF at an overall stretch ratio of 1.10. Initiation and perpetuation of AF in our model were related to increased dispersion of effective refractory period, conduction slowing, and local conduction block, all related to heterogeneous activation of I(sac). Upon local contraction, mechanoelectrical coupling affects perpetuation of AF by temporarily changing local excitability. CONCLUSION During acute atrial dilation, heterogeneous activation of I(sac) enhances initiation and can affect perpetuation of AF.

Consequences of Cooperativity in Racemizing Supramolecular Systems

Saluting the sergeant: Phg-BTA (see scheme) cooperatively self-assembles into helical aggregates and shows unprecedented racemization behavior in the presence of base. In thermodynamically controlled conditions, the addition of a small amount of chiral auxiliary to this mixture results in a deracemization reaction and a final enantiomeric excess of 32 %. A theoretical model is presented to understand in detail the results obtained.

Mechanoelectric Feedback as a Trigger Mechanism for Cardiac Electrical Remodeling: A Model Study

Regional variation in ionic membrane currents causes differences in action potential duration (APD) and is proarrhythmic. After several weeks of ventricular pacing, AP morphology and duration are changed due to electrical remodeling of the transient outward potassium current (Ito) and the L-type calcium current (ICa,L). It is not clear what mechanism drives electrical remodeling. By modeling the cardiac muscle as a string of segments that are electrically and mechanically coupled, we investigate the hypothesis that electrical remodeling is triggered by changes in mechanical load. Contractile force generated by the sarcomeres depends on the calcium transient and on the sarcomere length. Stroke work is determined for each segment by simulating the cardiac cycle. Electrical remodeling is simulated by adapting ICa,L kinetics such that a homogeneous distribution of stroke work is obtained. With electrical remodeling, a more homogeneous shortening of the fiber is obtained, while heterogeneity in APD increases and the repolarization wave reverses. Our results are in agreement with experimentally observed homogeneity in mechanics and heterogeneity in electrophysiology. In conclusion, electrical remodeling is a possible mechanism to reduce heterogeneity in cardiomechanics induced by ventricular pacing.

Sequential and parallel local search algorithms for job shop scheduling

We describe a fast sequential tabu search algorithm for the job shop scheduling problem. The algorithm uses an efficient way to recompute the path lengths in the graph representation after a step to a neighbouring solution. For a parallel algorithm, that consists of independent runs of the sequential algorithm, we give a probabilistic analysis of the possible speed-up.

Chloride currents in cones modify feedback from horizontal cells to cones in goldfish retina.

•  The GABAergic pathway modulates feedback between retinal horizontal cells (HCs) and cone photoreceptors, but is not mediating negative feedback, as previously hypothesized. •  Opening of GABA‐gated chloride channels in cone photoreceptors reduces the amplitude of feedback responses generated by HCs. •  Activation of a different presynaptic chloride current, the calcium‐dependent chloride current, in individual cones has a similar effect on feedback as application of GABA. •  Modulation of the strength of feedback from HCs seems to be a general consequence of activation of presynaptic chloride currents in cones. •  This puts the functional role of these currents in a new perspective; GABA acts as a slow and global neuromodulator enhancing feedback in the light‐ and attenuating feedback in the dark‐adapted retina, whereas the calcium‐dependent chloride current modulates feedback fast and locally to tune the size of feedback to local light conditions.

Fragmentation and Coagulation in Supramolecular (Co)polymerization Kinetics

The self-assembly of molecular building blocks into one-dimensional supramolecular architectures has opened up new frontiers in materials science. Due to the noncovalent interactions between the monomeric units, these architectures are intrinsically dynamic, and understanding their kinetic driving forces is key to rationally programming their morphology and function. To understand the self-assembly dynamics of supramolecular polymerizations (SP), kinetic models based on aggregate growth by sequential monomer association and dissociation have been analyzed. However, fragmentation and coagulation events can also play a role, as evident from studies on peptide self-assembly and the fact that aggregations can be sensitive to mechanical agitations. Here, we analyze how fragmentation and coagulation events influence SP kinetics by theoretical analysis of self-assembling systems of increasing complexity. Our analysis starts with single-component systems in which aggregates are able to grow via an isodesmic or cooperative nucleation–elongation mechanism. Subsequently, equilibration dynamics in cooperative two-component supramolecular copolymerizations are investigated. In the final part, we reveal how aggregate growth in the presence of competing, kinetically controlled pathways is influenced by fragmentation and coagulation reactions and reveal how seed-induced growth can give rise to block copolymers. Our analysis shows how fragmentation and coagulation reactions are able to modulate SP kinetics in ways that are highly system dependent.

Stochastic Analysis of Amino Acid Substitution in Protein Synthesis

We present a formal analysis of amino acid replacement during mRNA translation. Building on an abstract stochastic model of arrival of tRNAs and their processing at the ribosome, we compute probabilities of the insertion of amino acids into the nascent polypeptide chain. To this end, we integrate the probabilistic model checker Prism in the Matlab environment. We construct the substitution matrix containing the probabilities of an amino acid replacing another. The resulting matrix depends on various parameters, including availability and concentration of tRNA species, as well as their assignment to individual codons. We draw a parallel with the standard mutation matrices like Dayhoff and PET91, and analyze the mutual replacement of biologically similar amino acids.