No abstract

Phox2b-expressing glutamatergic neurons of the retrotrapezoid nucleus (RTN) display properties expected of central respiratory chemoreceptors; they are directly activated by CO 2 /Hϩ via an unidentified pH-sensitive background K ϩ channel and, in turn, facilitate brainstem networks that control breathing. Here, we used a knock-out mouse model to examine whether TASK-2 (K2P5), an alkaline-activated background K ϩ channel, contributes to RTN neuronal pH sensitivity. We made patch-clamp recordings in brainstem slices from RTN neurons that were identified by expression of GFP (directed by the Phox2b promoter) or ␤-galactosidase (from the gene trap used for TASK-2 knock-out). Whereas nearly all RTN cells from control mice were pH sensitive (95%, n ϭ 58 of 61), only 56% of GFP-expressing RTN neurons from TASK-2 Ϫ/Ϫ mice (n ϭ 49 of 88) could be classified as pH sensitive (Ͼ30% reduction in firing rate from pH 7.0 to pH 7.8); the remaining cells were pH insensitive (44%). Moreover, none of the recorded RTN neurons from TASK-2 Ϫ/Ϫ mice selected based on ␤-galactosidase activity (a subpopulation of GFP-expressing neurons) were pH sensitive. The alkaline-activated background K ϩ currents were reduced in amplitude in RTN neurons from TASK-2 Ϫ/Ϫ mice that retained some pH sensitivity but were absent from pH-insensitive cells. Finally, using a working heart-brainstem preparation, we found diminished inhibition of phrenic burst amplitude by alkalization in TASK-2 Ϫ/Ϫ mice, with apneic threshold shifted to higher pH levels. In conclusion, alkaline-activated TASK-2 channels contribute to pH sensitivity in RTN neurons, with effects on respiration in situ that are particularly prominent near apneic threshold.

Key point• Fibroblasts play a major role in heart physiology. In pathological conditions, they can lead to cardiac fibrosis when they differentiate into myofibroblasts.• This differentiated status is associated with changes in expression profile leading to neo-expression of proteins such as ionic channels.• The present study investigates electrophysiological changes associated with fibroblast differentiation focusing on voltage-gated sodium channels in human atrial fibroblasts and myofibroblasts.• We show that human atrial fibroblast differentiation in myofibroblasts is associated with de novo expression of voltage gated sodium current. Multiple arguments support that this current is predominantly supported by the Na v 1.5 α-subunit which may generate a persistent sodium entry into myofibroblasts.• Our data revealed that Na v 1.5 α-subunit expression is not restricted to cardiac myocytes within the atrium. Since fibrosis is one of the fundamental mechanisms implicated in atrial fibrillation, it is of great interest to investigate how this channel could influence myofibroblasts function.Abstract Fibroblasts play a major role in heart physiology. They are at the origin of the extracellular matrix renewal and production of various paracrine and autocrine factors. In pathological conditions, fibroblasts proliferate, migrate and differentiate into myofibroblasts leading to cardiac fibrosis. This differentiated status is associated with changes in expression profile leading to neo-expression of proteins such as ionic channels. The present study investigates further electrophysiological changes associated with fibroblast differentiation focusing on the activity of voltage-gated sodium channels in human atrial fibroblasts and myofibroblasts. Using the patch clamp technique we show that human atrial myofibroblasts display a fast inward voltage gated sodium current with a density of 13.28 ± 2.88 pA pF −1 whereas no current was detectable in non-differentiated fibroblasts. Quantitative RT-PCR reveals a large amount of transcripts encoding the Na v 1.5 α-subunit with a fourfold increased expression level in myofibroblasts when compared to fibroblasts. Accordingly, half of the current was blocked by 1 μM of tetrodotoxin and immunocytochemistry experiments reveal the presence of Na v 1.5 proteins. Overall, this current exhibits similar biophysical characteristics to sodium currents found in cardiac myocytes except for the window current that is enlarged for potentials between −100 and −20 mV. Since fibrosis is one of the fundamental mechanisms implicated in atrial fibrillation, it is of great interest to investigate how this current could influence myofibroblast properties. Moreover, since several Na v 1.5 mutations are related to cardiac pathologies, this study offers a new avenue on the fibroblasts involvement of these mutations.

Cardiac fibrosis occurs in a number of cardiovascular diseases associated with a high incidence of arrhythmias. A critical event in the development of fibrosis is the transformation of fibroblasts into an active phenotype or myofibroblast. This transformation results in functional changes including increased proliferation and changes in the release of signaling molecules and extracellular matrix deposition. Traditionally fibroblasts have been considered to affect cardiac electrophysiology indirectly by physically isolating myocytes and creating conduction barriers. There is now increasing evidence that cardiac fibroblasts may play a direct role in modulating the electrophysiological substrate in diseased hearts. The purpose of this review is to summarize the functional changes associated with fibroblast activation, the membrane currents that have been identified in adult cardiac fibroblasts and describe recent studies of fibroblast-myocyte electrical interactions with emphasis on the changes that occur with cardiac injury. Further analysis of fibroblast membrane electrophysiology and their interactions with myocytes will lead to a more complete understanding of the arrhythmic substrate. These studies have the potential to generate new therapeutic approaches for the prevention of arrhythmias associated with cardiac fibrosis.

In the brain, neurons establish bona fide synapses onto oligodendrocyte precursor cells (OPCs), but the function of these neuron-glia synapses remains unresolved. A leading hypothesis suggests that these synapses regulate OPC proliferation and differentiation. However, a causal link between synaptic activity and OPC cellular dynamics is still missing. In the developing somatosensory cortex, OPCs receive a major type of synapse from GABAergic interneurons that is mediated by postsynaptic γ2-containing GABA receptors. Here we genetically silenced these receptors in OPCs during the critical period of cortical oligodendrogenesis. We found that the inactivation of γ2-mediated synapses does not impact OPC proliferation and differentiation or the propensity of OPCs to myelinate their presynaptic interneurons. However, this inactivation causes a progressive and specific depletion of the OPC pool that lacks γ2-mediated synaptic activity without affecting the oligodendrocyte production. Our results show that, during cortical development, the γ2-mediated interneuron-to-OPC synapses do not play a role in oligodendrogenesis and suggest that these synapses finely tune OPC self-maintenance capacity. They also open the interesting possibility that a particular synaptic signaling onto OPCs plays a specific role in OPC function according to the neurotransmitter released, the identity of presynaptic neurons or the postsynaptic receptors involved.

Myelination of projection neurons by oligodendrocytes is key to optimize action potential conduction over long distances. However, a large fraction of myelin enwraps the axons of parvalbumin-positive fast-spiking interneurons (FSI), exclusively involved in local cortical circuits. Whether FSI myelination contributes to the fine‐tuning of intracortical networks is unknown. Here we demonstrate that FSI myelination is required for the establishment and maintenance of the powerful FSI-mediated feedforward inhibition of cortical sensory circuits. The disruption of GABAergic synaptic signaling of oligodendrocyte precursor cells prior to myelination onset resulted in severe FSI myelination defects characterized by longer internodes and nodes, aberrant myelination of branch points and proximal axon malformation. Consequently, high-frequency FSI discharges as well as FSI-dependent postsynaptic latencies and strengths of excitatory neurons were reduced. These dysfunctions generated a strong excitation-inhibition imbalance that correlated with whisker-dependent texture discrimination impairments. FSI myelination is therefore critical for the function of mature cortical inhibitory circuits.

Human mutations of the GRID1 gene encoding the orphan delta1 glutamate receptor-channel (GluD1) are associated with schizophrenia but the explicit role of GluD1 in brain circuits is unknown. Based on the known function of its paralog GluD2 in cerebellum, we searched for a role of GluD1 in slow glutamatergic transmission mediated by metabotropic receptor mGlu1 in midbrain dopamine neurons, whose dysfunction is a hallmark of schizophrenia. We found that an mGlu1 agonist elicits a slow depolarizing current in HEK cells co-expressing mGlu1 and GluD1, but not in cells expressing mGlu1 or GluD1 alone. This current is abolished by additional co-expression of a dominant-negative GluD1 dead pore mutant. We then characterized mGlu1-dependent currents in dopamine neurons from midbrain slices. Both the agonist-evoked and the slow postsynaptic currents are abolished by expression of the dominant-negative GluD1 mutant, pointing to the involvement of native GluD1 channels in these currents. Likewise, both mGlu1-dependent currents are suppressed in GRID1 knockout mice, which reportedly display endophenotypes relevant for schizophrenia. It is known that mGlu1 activation triggers the transition from tonic to burst firing of dopamine neurons, which signals salient stimuli and encodes reward prediction. In vivo recordings of dopamine neurons showed that their spontaneous burst firing is abolished in GRID1 knockout mice or upon targeted expression of the dominant-negative GluD1 mutant in wild-type mice. Our results de-orphanize GluD1, unravel its key role in slow glutamatergic transmission and provide insights into how GRID1 gene alterations can lead to dopaminergic dysfunctions in schizophrenia.

The first wave of oligodendrocyte precursor cells (firstOPCs) and most GABAergic interneurons share common embryonic origins. Cortical firstOPCs are thought to be replaced by other OPC populations shortly after birth, maintaining a consistent OPC density and making postnatal interactions between firstOPCs and ontogenetically-related interneurons unlikely. Challenging these ideas, we show that a cortical firstOPC subpopulation survives and forms functional cell clusters with lineage-related interneurons. Favored by a common embryonic origin, these clusters display unexpected preferential synaptic connectivity and are anatomically maintained after firstOPCs differentiate into myelinating oligodendrocytes. While the concomitant rescue of interneurons and firstOPCs committed to die causes an exacerbated neuronal inhibition, it abolishes interneuron-firstOPC high synaptic connectivity. Further, the number of other oligodendroglia populations increases through a non-cell-autonomous mechanism, impacting myelination. These findings demonstrate unprecedented roles of interneuron and firstOPC apoptosis in regulating lineage-related cell interactions and the homeostatic oligodendroglia density.