Intracellular magnesium-dependent modulation of gap junction channels formed by neuronal connexin36.

Gap junction (GJ) channels composed of Connexin36 (Cx36) are widely expressed in the mammalian CNS and form electrical synapses between neurons. Here we describe a novel modulatory mechanism of Cx36 GJ channels dependent on intracellular free magnesium ([Mg(2+)]i). We examined junctional conductance (gj) and its dependence on transjunctional voltage (Vj) at different [Mg(2+)]i in cultures of HeLa or N2A cells expressing Cx36. We found that Cx36 GJs are partially inhibited at resting [Mg(2+)]i. Thus, gj can be augmented or reduced by lowering or increasing [Mg(2+)]i, respectively. Similar changes in gj and Vj-gating were observed using MgATP or K2ATP in pipette solutions, which increases or decreases [Mg(2+)]i, respectively. Changes in phosphorylation of Cx36 or in intracellular free calcium concentration were not involved in the observed Mg(2+)-dependent modulation of gj. Magnesium ions permeate the channel and transjunctional asymmetry in [Mg(2+)]i resulted in asymmetric Vj-gating. The gj of GJs formed of Cx26, Cx32, Cx43, Cx45, and Cx47 was also reduced by increasing [Mg(2+)]i, but was not increased by lowering [Mg(2+)]i; single-channel conductance did not change. We showed that [Mg(2+)]i affects both open probability and the number of functional channels, likely through binding in the channel lumen. Finally, we showed that Cx36-containing electrical synapses between neurons of the trigeminal mesencephalic nucleus in rat brain slices are similarly affected by changes in [Mg(2+)]i. Thus, this novel modulatory mechanism could underlie changes in neuronal synchronization under conditions in which ATP levels, and consequently [Mg(2+)]i, are modified.

Gap junction-mediated electrical transmission: regulatory mechanisms and plasticity.

The term synapse applies to cellular specializations that articulate the processing of information within neural circuits by providing a mechanism for the transfer of information between two different neurons. There are two main modalities of synaptic transmission: chemical and electrical. While most efforts have been dedicated to the understanding of the properties and modifiability of chemical transmission, less is still known regarding the plastic properties of electrical synapses, whose structural correlate is the gap junction. A wealth of data indicates that, rather than passive intercellular channels, electrical synapses are more dynamic and modifiable than was generally perceived. This article will discuss the factors determining the strength of electrical transmission and review current evidence demonstrating its dynamic properties. Like their chemical counterparts, electrical synapses can also be plastic and modifiable. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Synergy between electrical coupling and membrane properties promotes strong synchronization of neurons of the mesencephalic trigeminal nucleus.

Electrical synapses are known to form networks of extensively coupled neurons in various regions of the mammalian brain. The mesencephalic trigeminal (MesV) nucleus, formed by the somata of primary afferents originating in jaw-closing muscles, constitutes one of the first examples supporting the presence of electrical synapses in the mammalian CNS; however, the properties, functional organization, and developmental emergence of electrical coupling within this structure remain unknown. By combining electrophysiological, tracer coupling, and immunochemical analysis in brain slices of rat and mouse, we found that coupling is mostly restricted to pairs or small clusters of MesV neurons. Electrical transmission is supported by connexin36 (Cx36)-containing gap junctions at somato-somatic contacts where only a small proportion of channels appear to be open (∼0.1%). In marked contrast with most brain structures, coupling among MesV neurons increases with age, such that it is absent during early development and appears at postnatal day 8. Interestingly, the development of coupling parallels the development of intrinsic membrane properties responsible for repetitive firing in these neurons. We found that, acting together, sodium and potassium conductances enhance the transfer of signals with high-frequency content via electrical synapses, leading to strong spiking synchronization of the coupled neurons. Together, our data indicate that coupling in the MesV nucleus is restricted to mostly pairs of somata between which electrical transmission is supported by a surprisingly small fraction of the channels estimated to be present, and that coupling synergically interacts with specific membrane conductances to promote synchronization of these neurons.

Electrical transmission between mammalian neurons is supported by a small fraction of gap junction channels.

Electrical synapses formed by gap junctions between neurons create networks of electrically coupled neurons in the mammalian brain, where these networks have been found to play important functional roles. In most cases, interneuronal gap junctions occur at remote dendro-dendritic contacts, making difficult accurate characterization of their physiological properties and correlation of these properties with their anatomical and morphological features of the gap junctions. In the mesencephalic trigeminal (MesV) nucleus where neurons are readily accessible for paired electrophysiological recordings in brain stem slices, our recent data indicate that electrical transmission between MesV neurons is mediated by connexin36 (Cx36)-containing gap junctions located at somato-somatic contacts. We here review evidence indicating that electrical transmission between these neurons is supported by a very small fraction of the gap junction channels present at cell-cell contacts. Acquisition of this evidence was enabled by the unprecedented experimental access of electrical synapses between MesV neurons, which allowed estimation of the average number of open channels mediating electrical coupling in relation to the average number of gap junction channels present at these contacts. Our results indicate that only a small proportion of channels (~0.1 %) appear to be conductive. On the basis of similarities with other preparations, we postulate that this phenomenon might constitute a general property of vertebrate electrical synapses, reflecting essential aspects of gap junction function and maintenance.

The extent and strength of electrical coupling between inferior olivary neurons is heterogeneous.

Gap junctions constitute the only form of synaptic communication between neurons in the inferior olive (IO), which gives rise to the climbing fibers innervating the cerebellar cortex. Although its exact functional role remains undetermined, electrical coupling was shown to be necessary for the transient formation of functional compartments of IO neurons and to underlie the precise timing of climbing fibers required for cerebellar learning. So far, most functional considerations assume the existence of a network of permanently and homogeneously coupled IO neurons. Contrasting this notion, our results indicate that coupling within the IO is highly variable. By combining tracer-coupling analysis and paired electrophysiological recordings, we found that individual IO neurons could be coupled to a highly variable number of neighboring neurons. Furthermore, a given neuron could be coupled at remarkably different strengths with each of its partners. Freeze-fracture analysis of IO glomeruli revealed the close proximity of glutamatergic postsynaptic densities to connexin 36-containing gap junctions, at distances comparable to separations between chemical transmitting domains and gap junctions in goldfish mixed contacts, where electrical coupling was shown to be modulated by the activity of glutamatergic synapses. On the basis of structural and molecular similarities with goldfish mixed synapses, we speculate that, rather than being hardwired, variations in coupling could result from glomerulus-specific long-term modulation of gap junctions. This striking heterogeneity of coupling might act to finely influence the synchronization of IO neurons, adding an unexpected degree of complexity to olivary networks.

The use of non- invasive ventilation in asthma exacerbation - a two year retrospective analysis of outcomes.

Background: The use of Non-Invasive Ventilation (NIV) in acute asthma exacerbation remains controversial. Comparative data on patient characteristics that benefit from NIV in asthma exacerbation to those patients that fail NIV remains limited. Our study compares some of these patient characteristics and examines if NIV is safe and effective in carefully selected patients. Methods: Following institutional review board approval, we extracted from the electronic medical record and conducted a retrospective chart-based review of those patients who received NIV in the emergency room for a diagnosis of asthma exacerbation from January 2017 to December 2018. Results and Conclusion: The rate of failure of NIV overall was low, at 9.17%, with younger patients more likely to fail NIV (P = 0.03) and need invasive mechanical ventilation. Surprisingly, baseline asthma severity did not impact NIV failure rate, and neither did body mass index, smoking history, and a host of clinical characteristics. Understandably, the length of stay was significantly longer in the group of patients that failed NIV. There were no adverse events, such as an increased rate of barotrauma events in either group. In conclusion, this study contributes to the growing body of evidence that NIV is a safe and effective adjunct to routine care in the management of patients with asthma exacerbation.

Empiric use of anticoagulation in hospitalized patients with COVID-19: a propensity score-matched study of risks and benefits.

BACKGROUND: Hospitalized patients with COVID-19 demonstrate a higher risk of developing thromboembolism. Anticoagulation (AC) has been proposed for high-risk patients, even without confirmed thromboembolism. However, benefits and risks of AC are not well assessed due to insufficient clinical data. We performed a retrospective analysis of outcomes from AC in a large population of COVID-19 patients. METHODS: We retrospectively reviewed 1189 patients hospitalized for COVID-19 between March 5 and May 15, 2020, with primary outcomes of mortality, invasive mechanical ventilation, and major bleeding. Patients who received therapeutic AC for known indications were excluded. Propensity score matching of baseline characteristics and admission parameters was performed to minimize bias between cohorts. RESULTS: The analysis cohort included 973 patients. Forty-four patients who received therapeutic AC for confirmed thromboembolic events and atrial fibrillation were excluded. After propensity score matching, 133 patients received empiric therapeutic AC while 215 received low dose prophylactic AC. Overall, there was no difference in the rate of invasive mechanical ventilation (73.7% versus 65.6%, p = 0.133) or mortality (60.2% versus 60.9%, p = 0.885). However, among patients requiring invasive mechanical ventilation, empiric therapeutic AC was an independent predictor of lower mortality (hazard ratio [HR] 0.476, 95% confidence interval [CI] 0.345-0.657, p < 0.001) with longer median survival (14 days vs 8 days, p < 0.001), but these associations were not observed in the overall cohort (p = 0.063). Additionally, no significant difference in mortality was found between patients receiving empiric therapeutic AC versus prophylactic AC in various subgroups with different D-dimer level cutoffs. Patients who received therapeutic AC showed a higher incidence of major bleeding (13.8% vs 3.9%, p < 0.001). Furthermore, patients with a HAS-BLED score of ≥2 had a higher risk of mortality (HR 1.482, 95% CI 1.110-1.980, p = 0.008), while those with a score of ≥3 had a higher risk of major bleeding (Odds ratio: 1.883, CI: 1.114-3.729, p = 0.016). CONCLUSION: Empiric use of therapeutic AC conferred survival benefit to patients requiring invasive mechanical ventilation, but did not show benefit in non-critically ill patients hospitalized for COVID-19. Careful bleeding risk estimation should be pursued before considering escalation of AC intensity.