Reduced H+ channel activity disrupts pH homeostasis and calcification in coccolithophores at low ocean pH

Coccolithophores produce the bulk of ocean biogenic calcium carbonate but this process is predicted to be negatively affected by future ocean acidification scenarios. Since coccolithophores calcify intracellularly, the mechanisms through which changes in seawater carbonate chemistry affect calcification remain unclear. Here we show that voltage-gated H+ channels in the plasma membrane of Coccolithus braarudii serve to regulate pH and maintain calcification under normal conditions, but have greatly reduced activity in cells acclimated to low pH. This disrupts intracellular pH homeostasis and impairs the ability of C. braarudii to remove H+ generated by the calcification process, leading to specific coccolith malformations. These coccolith malformations can be reproduced by pharmacological inhibition of H+ channels. Heavily-calcified coccolithophore species such as C. braarudii, which make the major contribution to carbonate export to the deep ocean, have a large intracellular H+ load and are likely to be most vulnerable to future decreases in ocean pH.

Introducing hydrometeor orientation into all-sky microwave and submillimeter assimilation

Numerical weather prediction systems still employ many simplifications when assimilating microwave radiances under all-sky conditions (clear sky, cloudy, and precipitation). For example, the orientation of ice hydrometeors is ignored, along with the polarization that this causes. We present a simple approach for approximating hydrometeor orientation, requiring minor adaption of software and no additional calculation burden. The approach is introduced in the RTTOV (Radiative Transfer for TOVS) forward operator and tested in the Integrated Forecast System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). For the first time within a data assimilation (DA) context, this represents the ice-induced brightness temperature differences between vertical (V) and horizontal (H) polarization – the polarization difference (PD). The discrepancies in PD between observations and simulations decrease by an order of magnitude at 166.5 GHz, with maximum reductions of 10–15 K. The error distributions, which were previously highly skewed and therefore problematic for DA, are now roughly symmetrical. The approach is based on rescaling the extinction in V and H channels, which is quantified by the polarization ratio ρ. Using dual-polarization observations from the Global Precipitation Mission microwave imager (GMI), suitable values for ρ were found to be 1.5 and 1.4 at 89.0 and 166.5 GHz, respectively. The scheme was used for all the conical scanners assimilated at ECMWF, with a broadly neutral impact on the forecast but with an increased physical consistency between instruments that employ different polarizations. This opens the way towards representing hydrometeor orientation for cross-track sounders and at frequencies above 183.0 GHz where the polarization can be even stronger.

Structure and Function of Ion Channels Regulating Sperm Motility—An Overview

Sperm motility is linked to the activation of signaling pathways that trigger movement. These pathways are mainly dependent on Ca2+, which acts as a secondary messenger. The maintenance of adequate Ca2+ concentrations is possible thanks to proper concentrations of other ions, such as K+ and Na+, among others, that modulate plasma membrane potential and the intracellular pH. Like in every cell, ion homeostasis in spermatozoa is ensured by a vast spectrum of ion channels supported by the work of ion pumps and transporters. To achieve success in fertilization, sperm ion channels have to be sensitive to various external and internal factors. This sensitivity is provided by specific channel structures. In addition, novel sperm-specific channels or isoforms have been found with compositions that increase the chance of fertilization. Notably, the most significant sperm ion channel is the cation channel of sperm (CatSper), which is a sperm-specific Ca2+ channel required for the hyperactivation of sperm motility. The role of other ion channels in the spermatozoa, such as voltage-gated Ca2+ channels (VGCCs), Ca2+-activated Cl-channels (CaCCs), SLO K+ channels or voltage-gated H+ channels (VGHCs), is to ensure the activation and modulation of CatSper. As the activation of sperm motility differs among metazoa, different ion channels may participate; however, knowledge regarding these channels is still scarce. In the present review, the roles and structures of the most important known ion channels are described in regard to regulation of sperm motility in animals.

Introducing hydrometeor orientation into all-sky microwave/sub-millimeter assimilation

Numerical weather prediction systems still employ many simplifications when assimilating microwave radiances in all-sky conditions (clear sky, cloudy, and precipitation). For example, the orientation of ice hydrometeors is ignored, along with the polarization that this causes. We present a simple approach for approximating hydrometeor orientation, requiring minor adaption of software and no additional calculation burden. The approach is introduced in the RTTOV (Radiative Transfer for TOVS) forward operator and tested in the Integrated Forecast System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). For the first time within a data assimilation (DA) context, this represents the ice-induced brightness temperature differences between vertical (V) and horizontal (H) polarization, the polarization difference (PD). The discrepancies in PD between observations and simulations decrease by an order of magnitude at 166.5 GHz, with maximum reductions of 10–15 K. The error distributions, which were previously highly skewed and therefore problematic for DA, are now roughly symmetrical. The approach is based on rescaling the extinction in V- and H-channels, which is quantified by the polarization ratio ρ. Using dual polarization observations from Global Precipitation Mission microwave imager (GMI), suitable value for ρ was found to be 1.5 and 1.4 at 89.0 and 166.5 GHz, respectively. The scheme was used for all the conical scanners assimilated at ECMWF, with broadly neutral impact on the forecast, but with an increased physical consistency between instruments that employ different polarizations. This opens the way towards representing hydrometeor orientation for cross-track sounders, and at frequencies above 183.0 GHz where the polarization can be even stronger.

Differences in acid-base regulation of haploid and diploid life-cycle stages of Coccolithus braarudii and their consequences for the sensitivity towards ocean acidification

Coccolithophores are calcifying microalgae that carry characteristic calcite platelets (coccoliths) on their surfaces. Most coccolithophore species exhibit diploid and haploid life cycle stages, each adjusted to different environmental conditions. The diploid life cycle stage of the coccolithophore C. braarudii is heavily calcifying with calcification rates that exceed the rates of photosynthesis. Haploid life-cycle stages are often weakly calcifying, generating significantly less H+ from the intracellular calcification reaction. We show how these different cellular “H+ burdens” require substantially different physiological molecular strategies to regulate intracellular pH under changing environmental conditions. Voltage-gated H+ channels (Hv) have been shown to play a role in the release of H+ in the diploid life cycle previously (Taylor et al. 2011). Combining scanning electron microscopy, electrophysiology, gene expression approaches and physiological measurements, we here show a direct link between the function of proton channels and coccolith formation of the diploid but not the haploid life-cycle stage. Our data also indicate how the different mechanisms for acid-base regulation of the diploid and haploid life-cycle stages may result in different sensitivities towards ocean acidification.

Somatodendritic HCN channels in hippocampal OLM cells revealed by a convergence of computational models and experiments

Determining details of spatially extended neurons is a challenge that needs to be overcome. The oriens-lacunosum/moleculare (OLM) interneuron has been implicated as a critical controller of hippocampal memory making it essential to understand how its biophysical properties contribute to function. We previously used computational models to show that OLM cells exhibit theta spiking resonance frequencies that depend on their dendrites having hyperpolarization-activated cation channels (h-channels). However, whether OLM cells have dendritic h-channels is unknown. We performed a set of whole-cell recordings of OLM cells from mouse hippocampus and constructed multi-compartment models using morphological and electrophysiological parameters extracted from the same cell. The models matched experiments only when dendritic h-channels were present. Immunohistochemical localization of the HCN2 subunit confirmed dendritic expression. These models can be used to obtain insight into hippocampal function. Our work shows that a tight integration of model and experiment tackles the challenge of characterizing spatially extended neurons.

β3-Adrenergic receptor-dependent modulation of the medium afterhyperpolarization in rat hippocampal CA1 pyramidal neurons

Action potential firing in hippocampal pyramidal neurons is regulated by generation of an afterhyperpolarization (AHP). Three phases of AHP are recognized, with the fast AHP regulating action potential firing at the onset of a burst and the medium and slow AHPs supressing action potential firing over hundreds of milliseconds and seconds, respectively. Activation of β-adrenergic receptors suppresses the slow AHP by a protein kinase A-dependent pathway. However, little is known regarding modulation of the medium AHP. Application of the selective β-adrenergic receptor agonist isoproterenol suppressed both the medium and slow AHPs evoked in rat CA1 hippocampal pyramidal neurons recorded from slices maintained in organotypic culture. Suppression of the slow AHP was mimicked by intracellular application of cAMP, with the suppression of the medium AHP by isoproterenol still being evident in cAMP-dialyzed cells. Suppression of both the medium and slow AHPs was antagonized by the β-adrenergic receptor antagonist propranolol. The effect of isoproterenol to suppress the medium AHP was mimicked by two β3-adrenergic receptor agonists, BRL37344 and SR58611A. The medium AHP was mediated by activation of small-conductance calcium-activated K+ channels and deactivation of H channels at the resting membrane potential. Suppression of the medium AHP by isoproterenol was reduced by pretreating cells with the H-channel blocker ZD7288. These data suggest that activation of β3-adrenergic receptors inhibits H channels, which suppresses the medium AHP in CA1 hippocampal neurons by utilizing a pathway that is independent of a rise in intracellular cAMP. This finding highlights a potential new target in modulating H-channel activity and thereby neuronal excitability. NEW & NOTEWORTHY The noradrenergic input into the hippocampus is involved in modulating long-term synaptic plasticity and is implicated in learning and memory. We demonstrate that activation of functional β3-adrenergic receptors suppresses the medium afterhyperpolarization in hippocampal pyramidal neurons. This finding provides an additional mechanism to increase action potential firing frequency, where neuronal excitability is likely to be crucial in cognition and memory.

Mammalian skeletal muscle does not express functional voltage-gated H+ channels

High metabolic activity and existence of a large transmembrane inward electrochemical gradient for H+ at rest promote intracellular acidification of skeletal muscle. Exchangers and cotransports efficiently contend against accumulation of intracellular H+ and associated deleterious effects on muscle functions. Voltage-gated H+ channels have also been found to represent another H+ extrusion pathway in cultured muscle cells. Up to now, the skeletal muscle cell was therefore the unique vertebrate excitable cell in which voltage-gated H+ currents have been described. In this study, we show that, unlike cultured cells, single mouse muscle fibers do not generate H+ currents in response to depolarization. In contrast, expression of human voltage-gated H+ channels in mouse muscle gives rise to robust outward voltage-gated H+ currents. This result excludes that inappropriate experimental conditions may have failed to reveal voltage-gated H+ currents in control muscle. This work therefore demonstrates that fully differentiated mammalian muscle fibers do not express functional voltage-gated H+ channels and consequently can no longer be considered as the only vertebrate excitable cells exhibiting voltage-gated H+ currents.