Aquaporin-4 and transient receptor potential vanilloid 4 balance in early postnatal neurodevelopment.

In the adult brain, the water channel aquaporin-4 (AQP4) is expressed in astrocyte endfoot, in supramolecular assemblies, called "Orthogonal Arrays of Particles" (OAPs) together with the transient receptor potential vanilloid 4 (TRPV4), finely regulating the cell volume. The present study aimed at investigating the contribution of AQP4 and TRPV4 to CNS early postnatal development using WT and AQP4 KO brain and retina and neuronal stem cells (NSCs), as an in vitro model of astrocyte differentiation. Western blot analysis showed that, differently from AQP4 and the glial cell markers, TRPV4 was downregulated during CNS development and NSC differentiation. Blue native/SDS-PAGE revealed that AQP4 progressively organized into OAPs throughout the entire differentiation process. Fluorescence quenching assay indicated that the speed of cell volume changes was time-related to NSC differentiation and functional to their migratory ability. Calcium imaging showed that the amplitude of TRPV4 Ca2+ transient is lower, and the dynamics are changed during differentiation and suppressed in AQP4 KO NSCs. Overall, these findings suggest that early postnatal neurodevelopment is subjected to temporally modulated water and Ca2+ dynamics likely to be those sustaining the biochemical and physiological mechanisms responsible for astrocyte differentiation during brain and retinal development.

Human immunodeficiency virus-1/simian immunodeficiency virus infection induces opening of pannexin-1 channels resulting in neuronal synaptic compromise: A novel therapeutic opportunity to prevent NeuroHIV.

In healthy conditions, pannexin-1 (Panx-1) channels are in a close state, but in several pathological conditions, including human immunodeficiency virus-1 (HIV) and NeuroHIV, the channel becomes open. However, the mechanism or contribution of Panx-1 channels to the HIV pathogenesis and NeuroHIV is unknown. To determine the contribution of Panx-1 channels to the pathogenesis of NeuroHIV, we used a well-established model of simian immunodeficiency virus (SIV) infection in macaques (Macaca mulatta) in the presence of and absence of a Panx-1 blocker to later examine the synaptic/axonal compromise induced for the virus. Using Golgi's staining, we demonstrated that SIV infection compromised synaptic and axonal structures, especially in the white matter. Blocking Panx-1 channels after SIV infection prevented the synaptic and axonal compromise induced by the virus, especially by maintaining the more complex synapses. Our data demonstrated that targeting Panx-1 channels can prevent and maybe revert brain synaptic compromise induced by SIV infection.

Pannexin1 Channels Are Required for Chemokine-Mediated Migration of CD4+ T Lymphocytes: Role in Inflammation and Experimental Autoimmune Encephalomyelitis.

Pannexin1 (Panx1) channels are large high conductance channels found in all vertebrates that can be activated under several physiological and pathological conditions. Our published data indicate that HIV infection results in the extended opening of Panx1 channels (5-60 min), allowing for the secretion of ATP through the channel pore with subsequent activation of purinergic receptors, which facilitates HIV entry and replication. In this article, we demonstrate that chemokines, which bind CCR5 and CXCR4, especially SDF-1α/CXCL12, result in a transient opening (peak at 5 min) of Panx1 channels found on CD4(+) T lymphocytes, which induces ATP secretion, focal adhesion kinase phosphorylation, cell polarization, and subsequent migration. Increased migration of immune cells is key for the pathogenesis of several inflammatory diseases including multiple sclerosis (MS). In this study, we show that genetic deletion of Panx1 reduces the number of the CD4(+) T lymphocytes migrating into the spinal cord of mice subjected to experimental autoimmune encephalomyelitis, an animal model of MS. Our results indicate that opening of Panx1 channels in response to chemokines is required for CD4(+) T lymphocyte migration, and we propose that targeting Panx1 channels could provide new potential therapeutic approaches to decrease the devastating effects of MS and other inflammatory diseases.

The speed of swelling kinetics modulates cell volume regulation and calcium signaling in astrocytes: A different point of view on the role of aquaporins.

Regulatory volume decrease (RVD) is a process by which cells restore their original volume in response to swelling. In this study, we have focused on the role played by two different Aquaporins (AQPs), Aquaporin-4 (AQP4), and Aquaporin-1 (AQP1), in triggering RVD and in mediating calcium signaling in astrocytes under hypotonic stimulus. Using biophysical techniques to measure water flux through the plasma membrane of wild-type (WT) and AQP4 knockout (KO) astrocytes and of an astrocyte cell line (DI TNC1) transfected with AQP4 or AQP1, we here show that AQP-mediated fast swelling kinetics play a key role in triggering and accelerating RVD. Using calcium imaging, we show that AQP-mediated fast swelling kinetics also significantly increases the amplitude of calcium transients inhibited by Gadolinium and Ruthenium Red, two inhibitors of the transient receptor potential vanilloid 4 (TRPV4) channels, and prevented by removing extracellular calcium. Finally, inhibition of TRPV4 or removal of extracellular calcium does not affect RVD. All together our study provides evidence that (1) AQP influenced swelling kinetics is the main trigger for RVD and in mediating calcium signaling after hypotonic stimulus together with TRPV4, and (2) calcium influx from the extracellular space and/or TRPV4 are not essential for RVD to occur in astrocytes.

Inhibitors of the 5-lipoxygenase pathway activate pannexin1 channels in macrophages via the thromboxane receptor.

A multitude of environmental signaling molecules influence monocyte and macrophage innate and adaptive immune responses, including ATP and prostanoids. Interestingly, purinergic (P2) and eicosanoid receptor signaling interact such that the activation of P2 receptors leads to prostanoid production, which can then interfere with P2Y-mediated macrophage migration. Recent studies suggest that blockade of 5-lipoxygenase (5-LOX) in macrophages can activate a permeation pathway involved in the influx of dye and the release of ATP. Here, we provide evidence that pannexin1 (Panx1) is a component of this pathway and present the intracellular signaling molecules linking the thromboxane (TP) receptor to Panx1-mediated dye influx and ATP release. Using pharmacological tools and transgenic mice deficient in Panx1, we show that two 5-LOX pathway inhibitors induce ATP release and influx of dye in a Panx1-dependent manner. Electrophysiological recordings performed in wild-type and Panx1-deficient macrophages confirmed that these 5-LOX pathway inhibitors activate currents characteristic of Panx1 channels. We found that the mechanism by which Panx1 channels are activated under this condition involves activation of the TP receptor that is mediated by the cAMP/PKA pathway. This is to our knowledge the first evidence for the involvement of Panx1 in the TP receptor signaling pathway. Future studies aimed to clarify the contribution of this TP-Panx1 signaling network to macrophage immune responses are likely to be important for targeting inflammatory and autoimmune diseases.

Neuronal Panx1 drives peripheral sensitization in experimental plantar inflammatory pain.

BACKGROUND: The channel-forming protein Pannexin1 (Panx1) has been implicated in both human studies and animal models of chronic pain, but the underlying mechanisms remain incompletely understood. METHODS: Wild-type (WT, n = 24), global Panx1 KO (n = 24), neuron-specific Panx1 KO (n = 20), and glia-specific Panx1 KO (n = 20) mice were used in this study at Albert Einstein College of Medicine. The von Frey test was used to quantify pain sensitivity in these mice following complete Freund's adjuvant (CFA) injection (7, 14, and 21 d). The qRT-PCR was employed to measure mRNA levels of Panx1, Panx2, Panx3, Cx43, Calhm1, and ß-catenin. Laser scanning confocal microscopy imaging, Sholl analysis, and electrophysiology were utilized to evaluate the impact of Panx1 on neuronal excitability and morphology in Neuro2a and dorsal root ganglion neurons (DRGNs) in which Panx1 expression or function was manipulated. Ethidium bromide (EtBr) dye uptake assay and calcium imaging were employed to investigate the role of Panx1 in adenosine triphosphate (ATP) sensitivity. ß-galactosidase (ß-gal) staining was applied to determine the relative cellular expression levels of Panx1 in trigeminal ganglia (TG) and DRG of transgenic mice. RESULTS: Global or neuron-specific Panx1 deletion markedly decreased pain thresholds after CFA stimuli (7, 14, and 21 d; P < 0.01 vs. WT group), indicating that Panx1 was positively correlated with pain sensitivity. In Neuro2a, global Panx1 deletion dramatically reduced neurite extension and inward currents compared to the WT group (P < 0.05), revealing that Panx1 enhanced neurogenesis and excitability. Similarly, global Panx1 deletion significantly suppressed Wnt/ß-catenin dependent DRG neurogenesis following 5 d of nerve growth factor (NGF) treatment (P < 0.01 vs. WT group). Moreover, Panx1 channels enhanced DRG neuron response to ATP after CFA injection (P < 0.01 vs. Panx1 KO group). Furthermore, ATP release increased Ca2+ responses in DRGNs and satellite glial cells surrounding them following 7 d of CFA treatment (P < 0.01 vs. Panx1 KO group), suggesting that Panx1 in glia also impacts exaggerated neuronal excitability. Interestingly, neuron-specific Panx1 deletion was found to markedly reduce differentiation in cultured DRGNs, as evidenced by stunted neurite outgrowth (P < 0.05 vs. Panx1 KO group; P < 0.01 vs. WT group or GFAP-Cre group), blunted activation of Wnt/ß-catenin signaling (P < 0.01 vs. WT, Panx1 KO and GFAP-Cre groups), and diminished cell excitability (P < 0.01 vs. GFAP-Cre group) and response to ATP stimulation (P < 0.01 vs. WT group). Analysis of ß-gal staining showed that cellular expression levels of Panx1 in neurons are significantly higher (2.5-fold increase) in the DRG than in the TG. CONCLUSIONS: The present study revealed that neuronal Panx1 is a prominent driver of peripheral sensitivity in the setting of inflammatory pain through cell-autonomous effects on neuronal excitability. This hyperexcitability dependence on neuronal Panx1 contrasts with inflammatory orofacial pain, where similar studies revealed a prominent role for glial Panx1. The apparent differences in Panx1 expression in neuronal and non-neuronal TG and DRG cells are likely responsible for the distinct impact of these cell types in the two pain models.

Nature of plasmalemmal functional "hemichannels".

The molecular identity of the protein forming "hemichannels" at non-junctional membranes is disputed. The family of gap junction proteins, innexins, connexins, and pannexins share several common features, including permeability characteristics and sensitivity to blocking agents. Such overlap in properties renders the identification of which of these protein species actually establishes the non-junctional membrane conductance and permeability quite complicated, especially because in vertebrates pannexins and connexins have largely overlapping distributions in tissues. Recently, attempts to establish criteria to identify events that are "hemichannel" mediated and those to allow the distinction between connexin- from pannexin-mediated events have been proposed. Here, I present an update on that topic and discuss the most recent findings related to the nature of functional "hemichannels" focusing on connexin43 and pannexin1. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Differential activation of mouse and human Panx1 channel variants.

Pannexins are ubiquitously expressed in human and mouse tissues. Pannexin 1 (Panx1), the most thoroughly characterized member of this family, forms plasmalemmal membrane channels permeable to relatively large molecules, such as ATP. Although human and mouse Panx1 amino acid sequences are conserved in the presently known regulatory sites involved in trafficking and modulation of the channel, differences are reported in the N- and C-termini of the protein, and the mechanisms of channel activation by different stimuli remain controversial. Here we used a neuroblastoma cell line to study the activation properties of endogenous mPanx1 and exogenously expressed hPanx1. Dye uptake and electrophysiological recordings revealed that in contrast to mouse Panx1, the human ortholog is insensitive to stimulation with high extracellular [K+] but responds similarly to activation of the purinergic P2X7 receptor. The two most frequent Panx1 polymorphisms found in the human population, Q5H (rs1138800) and E390D (rs74549886), exogenously expressed in Panx1-null N2a cells revealed that regarding P2X7 receptor mediated Panx1 activation, the Q5H mutant is a gain of function whereas the E390D mutant is a loss of function variant. Collectively, we demonstrate differences in the activation between human and mouse Panx1 orthologs and suggest that these differences may have translational implications for studies where Panx1 has been shown to have significant impact.

Astrocyte and neuronal Panx1 support long-term reference memory in mice.

Pannexin 1 (Panx1) are ubiquitously expressed proteins that form plasma membrane channels permeable to anions and moderate sized signaling molecules (e.g., ATP, glutamate). In the nervous system, activation of Panx1 channels have been extensively shown to contribute to distinct neurological disorders (epilepsy, chronic pain, migraine, neuroAIDS, etc.) but knowledge of extent to which these channels have a physiological role remains restricted to three studies supporting their involvement in hippocampus dependent learning. Given that Panx1 channels may provide an important mechanism for activity-dependent neuron-glia interaction, we used Panx1 transgenic mice with global and cell-type specific deletions of Panx1 to interrogate their participation in working and reference memory. Using the 8-arm radial maze, we show that long-term spatial reference memory, but not spatial working memory, is deficient in Panx1-null mice and that both astrocyte and neuronal Panx1 contribute to the consolidation of long-term spatial memory. Field potential recordings in hippocampal slices of Panx1-null mice revealed an attenuation of both long-term potentiation (LTP) of synaptic strength and long-term depression (LTD) at Schaffer collateral - CA1 synapses without alterations basal synaptic transmission or pre-synaptic paired-pulse facilitation. Our results implicate both neuronal and astrocyte Panx1 channels as critical players for the development and maintenance of long-term spatial reference memory in mice.

Astrocyte and Neuronal Panx1 Support Long-Term Reference Memory in Mice.

Pannexin 1 (Panx1) is an ubiquitously expressed protein that forms plasma membrane channels permeable to anions and moderate-sized signaling molecules (e.g., ATP, glutamate). In the nervous system, activation of Panx1 channels has been extensively shown to contribute to distinct neurological disorders (epilepsy, chronic pain, migraine, neuroAIDS, etc.), but knowledge of the extent to which these channels have a physiological role remains restricted to three studies supporting their involvement in hippocampus dependent learning. Given that Panx1 channels may provide an important mechanism for activity-dependent neuron-glia interaction, we used Panx1 transgenic mice with global and cell-type specific deletions of Panx1 to interrogate their participation in working and reference memory. Using the eight-arm radial maze, we show that long-term spatial reference memory, but not spatial working memory, is deficient in Panx1-null mice and that both astrocyte and neuronal Panx1 contribute to the consolidation of long-term spatial memory. Field potential recordings in hippocampal slices of Panx1-null mice revealed an attenuation of both long-term potentiation (LTP) of synaptic strength and long-term depression (LTD) at Schaffer collateral-CA1 synapses without alterations of basal synaptic transmission or pre-synaptic paired-pulse facilitation. Our results implicate both neuronal and astrocyte Panx1 channels as critical players for the development and maintenance of long-term spatial reference memory in mice.

ATP signaling is deficient in cultured Pannexin1-null mouse astrocytes.

Pannexins (Panx1, 2, and 3) comprise a group of proteins expressed in vertebrates that share weak yet significant sequence homology with the invertebrate gap junction proteins, the innexins. In contrast to the other vertebrate gap junction protein family (connexin), pannexins do not form intercellular channels, but at least Panx1 forms nonjunctional plasma membrane channels. Panx1 is ubiquitously expressed and has been shown to form large conductance (500 pS) channels that are voltage dependent, mechanosensitive, and permeable to relatively large molecules such as ATP. Pharmacological and knockdown approaches have indicated that Panx1 is the molecular substrate for the so-called "hemichannel" originally attributed to connexin43 and that Panx1 is the pore-forming unit of the P2X(7) receptor. Here, we describe, for the first time, conductance and permeability properties of Panx1-null astrocytes. The electrophysiological and fluorescence imaging analyses performed on these cells fully support our previous pharmacological and Panx1 knockdown studies that showed profoundly lower dye uptake and ATP release than wild-type untreated astrocytes. As a consequence of decreased ATP paracrine signaling, intercellular calcium wave spread is altered in Panx1-null astrocytes. Moreover, we found that in astrocytes as in Panx1-expressing oocytes, elevated extracellular K(+) activates Panx1 channels independently of membrane potential. Thus, on the basis of our present findings and our previous report, we propose that Panx1 channels serve as K(+) sensors for changes in the extracellular milieu such as those occurring under pathological conditions.

Extracellular K⁺ and astrocyte signaling via connexin and pannexin channels.

Astrocytes utilize two major pathways to achieve long distance intercellular communication. One pathway involves direct gap junction mediated signal transmission and the other consists of release of ATP through pannexin channels and excitation of purinergic receptors on nearby cells. Elevated extracellular potassium to levels occurring around hyperactive neurons affects both gap junction and pannexin1 channels. The action on Cx43 gap junctions is to increase intercellular coupling for a period that long outlasts the stimulus. This long term increase in coupling, termed "LINC", is mediated through calcium and calmodulin dependent activation of calmodulin dependent kinase (CaMK). Pannexin1 can be activated by elevations in extracellular potassium through a mechanism that is quite different. In this case, potassium shifts activation potentials to more physiological range, thereby allowing channel opening at resting or slightly depolarized potentials. Enhanced activity of both these channel types by elevations in extracellular potassium of the magnitude occurring during periods of high neuronal activity likely has profound effects on intercellular signaling among astrocytes in the nervous system.

Activity and Stability of Panx1 Channels in Astrocytes and Neuroblastoma Cells Are Enhanced by Cholesterol Depletion.

Pannexin1 (Panx1) is expressed in both neurons and glia where it forms ATP-permeable channels that are activated under pathological conditions such as epilepsy, migraine, inflammation, and ischemia. Membrane lipid composition affects proper distribution and function of receptors and ion channels, and defects in cholesterol metabolism are associated with neurological diseases. In order to understand the impact of membrane cholesterol on the distribution and function of Panx1 in neural cells, we used fluorescence recovery after photobleaching (FRAP) to evaluate its mobility and electrophysiology and dye uptake to assess channel function. We observed that cholesterol extraction (using methyl-ß-cyclodextrin) and inhibition of its synthesis (lovastatin) decreased the lateral diffusion of Panx1 in the plasma membrane. Panx1 channel activity (dye uptake, ATP release and ionic current) was enhanced in cholesterol-depleted Panx1 transfected cells and in wild-type astrocytes compared to non-depleted or Panx1 null cells. Manipulation of cholesterol levels may, therefore, offer a novel strategy by which Panx1 channel activation might modulate various pathological conditions.

The carboxyl-terminal domain of connexin43 is a negative modulator of neuronal differentiation.

Connexin43 (Cx43) is widely expressed in embryonic brain, and its expression becomes restricted mainly to astrocytes as the central nervous system matures. Recent studies have indicated that Cx43 plays important, nonchannel, roles during central nervous system development by affecting neuronal cell migration. Here, we evaluated the effects of Cx43 on neuronal differentiation. For that we used an in vitro model of neural cell development (neurospheres) to evaluate, through immunocytochemistry, electrophysiology, and molecular biology, the degree of neuronal maturation from neurospheres derived from wild-type (WT) and Cx43-null mice. Our results indicate that Cx43 is a negative modulator of neuronal differentiation. The percent neurospheres containing differentiated neurons and the number of cells displaying inward currents were significantly higher in Cx43-null than in WT littermate neurospheres. Knockdown of Cx43 with small interfering RNA increased the number of WT neurospheres generating differentiated neurons. Blockade of gap junctional communication with carbenoxolone did not induce neuronal differentiation in WT neurospheres. Transfection of Cx43-null neurospheres with Cx43 mutants revealed that Cx43 carboxyl terminus prevents neuronal maturation. In agreement with these in vitro data, in situ analysis of embryonic day 16 brains revealed increased beta-III-tubulin expression in germinal zones of Cx43-null compared with that of WT littermates. These results indicate that Cx43, and specifically its carboxyl terminus, is crucial for signaling mechanisms preventing premature neuronal differentiation during embryonic brain development.

The Contribution of Astrocyte and Neuronal Panx1 to Seizures Is Model and Brain Region Dependent.

Pannexin1 (Panx1) is an ATP release channel expressed in neurons and astrocytes that plays important roles in CNS physiology and pathology. Evidence for the involvement of Panx1 in seizures includes the reduction of epileptiform activity and ictal discharges following Panx1 channel blockade or deletion. However, very little is known about the relative contribution of astrocyte and neuronal Panx1 channels to hyperexcitability. To this end, mice with global and cell type specific deletion of Panx1 were used in one in vivo and two in vitro seizure models. In the low-Mg2+in vitro model, global deletion but not cell-type specific deletion of Panx1 reduced the frequency of epileptiform discharges. This reduced frequency of discharges did not impact the overall power spectra obtained from local field potentials. In the in vitro KA model, in contrast, global or cell type specific deletion of Panx1 did not affect the frequency of discharges, but reduced the overall power spectra. EEG recordings following KA-injection in vivo revealed that although global deletion of Panx1 did not affect the onset of status epilepticus (SE), SE onset was delayed in mice lacking neuronal Panx1 and accelerated in mice lacking astrocyte Panx1. EEG power spectral analysis disclosed a Panx1-dependent cortical region effect; while in the occipital region, overall spectral power was reduced in all three Panx1 genotypes; in the frontal cortex, the overall power was not affected by deletion of Panx1. Together, our results show that the contribution of Panx1 to ictal activity is model, cell-type and brain region dependent.

Cx43 carboxyl terminal domain determines AQP4 and Cx30 endfoot organization and blood brain barrier permeability.

The neurovascular unit (NVU) consists of cells intrinsic to the vessel wall, the endothelial cells and pericytes, and astrocyte endfeet that surround the vessel but are separated from it by basement membrane. Endothelial cells are primarily responsible for creating and maintaining blood-brain-barrier (BBB) tightness, but astrocytes contribute to the barrier through paracrine signaling to the endothelial cells and by forming the glia limitans. Gap junctions (GJs) between astrocyte endfeet are composed of connexin 43 (Cx43) and Cx30, which form plaques between cells. GJ plaques formed of Cx43 do not diffuse laterally in the plasma membrane and thus potentially provide stable organizational features to the endfoot domain, whereas GJ plaques formed of other connexins and of Cx43 lacking a large portion of its cytoplasmic carboxyl terminus are quite mobile. In order to examine the organizational features that immobile GJs impose on the endfoot, we have used super-resolution confocal microscopy to map number and sizes of GJ plaques and aquaporin (AQP)-4 channel clusters in the perivascular endfeet of mice in which astrocyte GJs (Cx30, Cx43) were deleted or the carboxyl terminus of Cx43 was truncated. To determine if BBB integrity was compromised in these transgenic mice, we conducted perfusion studies under elevated hydrostatic pressure using horseradish peroxide as a molecular probe enabling detection of micro-hemorrhages in brain sections. These studies revealed that microhemorrhages were more numerous in mice lacking Cx43 or its carboxyl terminus. In perivascular domains of cerebral vessels, we found that density of Cx43 GJs was higher in the truncation mutant, while GJ size was smaller. Density of perivascular particles formed by AQP4 and its extended isoform AQP4ex was inversely related to the presence of full length Cx43, whereas the ratio of sizes of the particles of the AQP4ex isoform to total AQP4 was directly related to the presence of full length Cx43. Confocal analysis showed that Cx43 and Cx30 were substantially colocalized in astrocyte domains near vasculature of truncation mutant mice. These results showing altered distribution of some astrocyte nexus components (AQP4 and Cx30) in Cx43 null mice and in a truncation mutant, together with leakier cerebral vasculature, support the hypothesis that localization and mobility of gap junction proteins and their binding partners influences organization of astrocyte endfeet which in turn impacts BBB integrity of the NVU.

Generation and Characterization of Immortalized Mouse Cortical Astrocytes From Wildtype and Connexin43 Knockout Mice.

We transduced mouse cortical astrocytes cultured from four litters of embryonic wildtype (WT) and connexin43 (Cx43) null mouse pups with lentiviral vector encoding hTERT and measured expression of astrocyte-specific markers up to passage 10 (p10). The immortalized cell lines thus generated (designated IWCA and IKOCA, respectively) expressed biomarkers consistent with those of neonatal astrocytes, including Cx43 from wildtype but not from Cx43-null mice, lack of Cx30, and presence of Cx26. AQP4, the water channel that is found in high abundance in astrocyte end-feet, was expressed at moderately high levels in early passages, and its mRNA and protein declined to low but still detectable levels by p10. The mRNA levels of the astrocyte biomarkers aldehyde dehydrogenase 1L1 (ALDH1L1), glutamine synthetase (GS) and glial fibrillary acidic protein (GFAP) remained relatively constant during successive passages. GS protein expression was maintained while GFAP declined with cell passaging but was still detectable at p10. Both mRNA and protein levels of glutamate transporter 1 (GLT-1) declined with passage number. Immunostaining at corresponding times was consistent with the data from Western blots and provided evidence that these proteins were expressed at appropriate intracellular locations. Consistent with our goal of generating immortalized cell lines in which Cx43 was either functionally expressed or absent, IWCA cells were found to be well coupled with respect to intercellular dye transfer and similar to primary astrocyte cultures in terms of time course of junction formation, electrical coupling strength and voltage sensitivity. Moreover, barrier function was enhanced in co-culture of the IWCA cell line with bEnd.3 microvascular endothelial cells. In addition, immunostaining revealed oblate endogenous Cx43 gap junction plaques in IWCA that were similar in appearance to those plaques obtained following transfection of IKOCA cells with fluorescent protein tagged Cx43. Re-expression of Cx43 in IKOCA cells allows experimental manipulation of connexins and live imaging of interactions between connexins and other proteins. We conclude that properties of these cell lines resemble those of primary cultured astrocytes, and they may provide useful tools in functional studies by facilitating genetic and pharmacological manipulations in the context of an astrocyte-appropriate cellular environment.

Pannexin-1 channel opening is critical for COVID-19 pathogenesis.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) rapidly rampaged worldwide, causing a pandemic of coronavirus disease (COVID -19), but the biology of SARS-CoV-2 remains under investigation. We demonstrate that both SARS-CoV-2 spike protein and human coronavirus 229E (hCoV-229E) or its purified S protein, one of the main viruses responsible for the common cold, induce the transient opening of Pannexin-1 (Panx-1) channels in human lung epithelial cells. However, the Panx-1 channel opening induced by SARS-CoV-2 is greater and more prolonged than hCoV-229E/S protein, resulting in an enhanced ATP, PGE2, and IL-1ß release. Analysis of lung lavages and tissues indicate that Panx-1 mRNA expression is associated with increased ATP, PGE2, and IL-1ß levels. Panx-1 channel opening induced by SARS-CoV-2 spike protein is angiotensin-converting enzyme 2 (ACE-2), endocytosis, and furin dependent. Overall, we demonstrated that Panx-1 channel is a critical contributor to SARS-CoV-2 infection and should be considered as an alternative therapy.

Pannexin 1: the molecular substrate of astrocyte "hemichannels".

Purinergic signaling plays distinct and important roles in the CNS, including the transmission of calcium signals between astrocytes. Gap junction hemichannels are among the mechanisms proposed by which astrocytes might release ATP; however, whether the gap junction protein connexin43 (Cx43) forms these "hemichannels" remains controversial. Recently, a new group of proteins, the pannexins, have been shown to form nonselective, high-conductance plasmalemmal channels permeable to ATP, thereby offering an alternative for the hemichannel protein. Here, we provide strong evidence that, in cultured astrocytes, pannexin1 (Panx1) but not Cx43 forms hemichannels. Electrophysiological and fluorescence microscope recordings performed in wild-type and Cx43-null astrocytes did not reveal any differences in hemichannel activity, which was mostly eliminated by treating Cx43-null astrocytes with Panx1-short interfering RNA [Panx1-knockdown (Panx1-KD)]. Moreover, quantification of the amount of ATP released from wild-type, Cx43-null, and Panx1-KD astrocytes indicates that downregulation of Panx1, but not of Cx43, prevented ATP release from these cells.