A cortical attractor network with Martinotti cells driven by facilitating synapses

The population of pyramidal cells significantly outnumbers the inhibitory interneurons in the neocortex, while at the same time the diversity of interneuron types is much more pronounced. One acknowledged key role of inhibition is to control the rate and patterning of pyramidal cell firing via negative feedback, but most likely the diversity of inhibitory pathways is matched by a corresponding diversity of functional roles. An important distinguishing feature of cortical interneurons is the variability of the short-term plasticity properties of synapses received from pyramidal cells. The Martinotti cell type has recently come under scrutiny due to the distinctly facilitating nature of the synapses they receive from pyramidal cells. This distinguishes these neurons from basket cells and other inhibitory interneurons typically targeted by depressing synapses. A key aspect of the work reported here has been to pinpoint the role of this variability. We first set out to reproduce quantitatively based on in vitro data the di-synaptic inhibitory microcircuit connecting two pyramidal cells via one or a few Martinotti cells. In a second step, we embedded this microcircuit in a previously developed attractor memory network model of neocortical layers 2/3. This model network demonstrated that basket cells with their characteristic depressing synapses are the first to discharge when the network enters an attractor state and that Martinotti cells respond with a delay, thereby shifting the excitation-inhibition balance and acting to terminate the attractor state. A parameter sensitivity analysis suggested that Martinotti cells might, in fact, play a dominant role in setting the attractor dwell time and thus cortical speed of processing, with cellular adaptation and synaptic depression having a less prominent role than previously thought.     

Enhancing protein crystallization through precipitant synergy

Suitable conditions for protein crystallization are commonly identified by screening combinations of independent factors that affect crystal formation. Because precipitating agents are prime determinants of crystallization, we investigated whether a systematic exploration of combinations of mechanistically distinct precipitants would enhance crystallization. A crystallization screen containing 64 precipitant mixtures was devised. Tests with ten HIV envelope-related proteins demonstrated that use of precipitant mixtures significantly enhanced both the probability of crystallization as well as the quality of optimized crystals. Tests with hen egg white lysozyme generated a novel C2 crystal from a salt/organic solvent mixture; structure solution at 2 A resolution revealed a lattice held together by both hydrophobic and electrostatic dyad interactions. The results indicate that mechanistically distinct precipitants can synergize, with precipitant combinations adding unique dimensions to protein crystallization.     

Concerted release of substrate domains from GroEL by ATP is demonstrated with FRET

The chaperonin GroEL assists protein folding by undergoing ATP-induced conformational changes that are concerted within each of its two back-to-back stacked rings. Here we examined whether concerted allosteric switching gives rise to all-or-none release and folding of domains in a chimeric fluorescent protein substrate, CyPet-YPet. Using this substrate, it was possible to determine the folding yield of each domain from its intrinsic fluorescence and that of the entire chimera by measuring Förster resonance energy transfer between the two domains. Hence, it was possible to determine whether release of one domain is accompanied by release of the other domain (concerted mechanism), or whether their release is not coupled. Our results show that the chimera's release tends to be concerted when folding is assisted by a wild-type GroEL variant, but not when assisted by the F44W/D155A mutant that undergoes a sequential allosteric switch. A connection between the allosteric mechanism of this molecular machine and its biological function in assisting folding is thus established.     

Natural selection on gene expression

Changes in genetic regulation contribute to adaptations in natural populations and influence susceptibility to human diseases. Despite their potential phenotypic importance, the selective pressures acting on regulatory processes in general and gene expression levels in particular are largely unknown. Studies in model organisms suggest that the expression levels of most genes evolve under stabilizing selection, although a few are consistent with adaptive evolution. However, it has been proposed that gene expression levels in primates evolve largely in the absence of selective constraints. In this article, we discuss the microarray-based observations that led to these disparate interpretations. We conclude that in both primates and model organisms, stabilizing selection is likely to be the dominant mode of gene expression evolution. An important implication is that mutations affecting gene expression will often be deleterious and might underlie many human diseases.     

Evaluating automated parameter constraining procedures of neuron models by experimental and surrogate data

Neuron models, in particular conductance-based compartmental models, often have numerous parameters that cannot be directly determined experimentally and must be constrained by an optimization procedure. A common practice in evaluating the utility of such procedures is using a previously developed model to generate surrogate data (e.g., traces of spikes following step current pulses) and then challenging the algorithm to recover the original parameters (e.g., the value of maximal ion channel conductances) that were used to generate the data. In this fashion, the success or failure of the model fitting procedure to find the original parameters can be easily determined. Here we show that some model fitting procedures that provide an excellent fit in the case of such model-to-model comparisons provide ill-balanced results when applied to experimental data. The main reason is that surrogate and experimental data test different aspects of the algorithm’s function. When considering model-generated surrogate data, the algorithm is required to locate a perfect solution that is known to exist. In contrast, when considering experimental target data, there is no guarantee that a perfect solution is part of the search space. In this case, the optimization procedure must rank all imperfect approximations and ultimately select the best approximation. This aspect is not tested at all when considering surrogate data since at least one perfect solution is known to exist (the original parameters) making all approximations unnecessary. Furthermore, we demonstrate that distance functions based on extracting a set of features from the target data (such as time-to-first-spike, spike width, spike frequency, etc.)—rather than using the original data (e.g., the whole spike trace) as the target for fitting—are capable of finding imperfect solutions that are good approximations of the experimental data.     

Hybrid Composite Coatings for Durable and Efficient Solar Hydrogen Generation under Diverse Operating Conditions

Safe and practical solar‐driven hydrogen generators must be capable of efficient and stable operation under diurnal cycling with full separation of gaseous H₂ and O₂ products. In this study, a novel architecture that fulfills all of these requirements is presented. The approach is inherently scalable and provides versatility for operation under diverse electrolyte and lighting conditions. The concept is validated using a 1 cm² triple‐junction photovoltaic cell with its illuminated photocathode protected by a composite coating comprising an organic encapsulant with an embedded catalytic support. The device is compatible with operation under conditions ranging from 1 m H₂SO₄ to 1 m KOH, enabling flexibility in selection of semiconductor, electrolyte, membrane, and catalyst. Stable operation at a solar‐to‐hydrogen conversion efficiency of >10% is demonstrated under continuous operation, as well as under diurnal light cycling for at least 4 d, with simulated sunlight. Operational characteristics are validated by extended time outdoor testing. A membrane ensures products are separated, with nonexplosive gas streams generated for both alkaline and acidic systems. Analysis of operational characteristics under different lighting conditions is enabled by comparison of a device model to experimental data.     

Serum histones as biomarkers of the severity of heatstroke in dogs

Heatstroke is associated with systemic inflammatory response syndrome, leading to multiple organ dysfunction and death. Currently, there is no specific treatment decreasing hyperthermia-induced inflammatory/hemostatic derangements. Emerging studies indicate that histones leaking from damaged cells into the extracellular space are toxic, pro-inflammatory, and pro-thrombotic. We therefore hypothesize that serum histones (sHs) are elevated during heatstroke and are associated with the severity of the disease. Sixteen dogs with heatstroke and seven healthy controls were included in the study. Median serum histones (sHs) upon admission in dogs with heatstroke were significantly higher (P = 0.043) compared to that in seven controls (13.2 vs. 7.3 ng/mL, respectively). sHs level was significantly higher among non-survivors and among dogs with severe hemostatic derangement (P = 0.049, median 21.4 ng/mL vs. median 8.16 ng/mL and P = 0.038, 19.0 vs. 7.0 ng/mL, respectively). There were significant positive correlation between sHs and urea (r = 0.8, P = 0.02); total CO2 (r = 0.661, P = 0.05); CK (r = 0.678, P = 0.04); and prothrombin time (PT) 12 h post presentation (r = 0.888, P = 0.04). The significant positive correlation between sHs and other heatstroke severity biomarkers, and significant increase among severely affected dogs, implies its role in inflammation/oxidation/coagulation during heatstroke. sHs, unlike other prognostic and severity biomarkers in heatstroke, can be pharmacologically manipulated, offering a potential therapeutic target.     

Effects of flash flooding on mosquito and community dynamics in experimental pools

Pulsed disturbances of larval mosquito sites are likely to have a direct negative effect on mosquitoes but may also have indirect effects due to the alteration of community structure. These altered communities may become attractive to gravid mosquitoes searching for oviposition sites when the disturbances decrease the abundance of mosquito antagonists such as competitors, which often results in an increase in mosquito food resources. However, flash flood disturbances in intermittent riverbeds can also remove mosquito food resources such as algae, so that the net effect of flash floods could be either to increase or decrease mosquito abundance. We conducted an outdoor mesocosm experiment to assess the effects of flash floods on mosquito oviposition habitat selection and larval abundance during the post‐disturbance period of community recovery. Mesocosms were artificially flooded. Mosquito oviposition, immature abundance, invertebrate species diversity, chlorophyll a, and abiotic parameters were monitored. Our results showed that the flash flood negatively affected phytoplankton and zooplankton, leading to a decrease of mosquito oviposition in flooded mesocosms compared to non‐flooded mesocosms. More broadly, this study indicates how disturbances influence mosquito oviposition habitat selection due to the loss of food resources in ephemeral pools, and it highlights the importance of considering the effects of disturbances in management, habitat restoration, and biodiversity conservation in temporary aquatic habitats.     

Ypt and Rab GTPases: insight into functions through novel interactions.

Ypt/Rab GTPases are key regulators of vesicular transport in eukaryotic cells. During the past two years, a number of new Ypt/Rab-interacting proteins have been identified and shown to serve as either upstream regulators or downstream effectors. Proteins that interact with these regulators and effectors of Ypt/Rabs have also been identified, and together they provide new insights into Ypt/Rab mechanisms of action. The picture that emerges from these studies suggests that Ypt/Rabs function in multiple and diverse aspects of vesicular transport. In addition, not only are Ypt/Rabs highly conserved, but their functions and interactions are as well. Interestingly, crosstalk among Ypt/Rabs and between Ypt/Rabs and other signaling factors, suggest the possibility of coordination of the individual vesicular transport steps and of the protein transport machinery with other cellular processes.