Abstract Mammals express ∼20 different connexins, the main gap junction forming proteins in mammals, and 3 pannexins, homologs of innexins, the main gap junction forming proteins in invertebrates. In ...both classes of gap junction, each channel is formed by two hemichannels, one contributed by each of the coupled cells. There is now general, if not universal, agreement that hemichannels of both classes can open in response to various physiological and pathological stimuli when they are not apposed to another hemichannels and face the external milieu. Connexin (and likely pannexin) hemichannel permeability is consistent with that of the cell–cell channels and open hemichannels can be a release site for relatively large molecules such as ATP and glutamate, which can serve as transmitters between cells. Here we describe three experimental paradigms in which connexin and pannexin hemichannel signaling occurs. (1) In cultures of spinal astrocytes FGF-1 causes the release of ATP, and ATP causes opening of pannexin hemichannels, which then release further ATP. Subsequently, several hours later, connexin hemichannels are also opened by an unknown mechanism. Release of ATP appears to become self sustaining through action of P2X7 receptors to open pannexin hemichannels and then connexin hemichannels, both of which are ATP permeable. (2) Spinal cord injury by dropping a small weight on the exposed cord is followed by release of ATP in the region surrounding the primary lesion. This release is greatly reduced in a mouse in which Cx43 is knocked down in the astrocytes. Application of FGF-1 causes a similar release of ATP in the uninjured spinal cord, and an inhibitor of the FGF-1 receptor, PD173074, inhibits both FGF-1 and injury-induced release. Reduction in ATP release is associated with reduced inflammation and less secondary expansion of the lesion. (3) Cortical astrocytes in culture are permeabilized by hypoxia, and this effect is increased by high or zero glucose. The mechanism of permeabilization is opening of Cx43 hemichannels, which can lead to cell death. Activated microglia secrete TNF-α and IL-1β, which open connexin hemichannels in astrocytes. Astrocytes release ATP and glutamate which can kill neurons in co-culture through activation of neuronal pannexin hemichannels. These studies implicate two kinds of gap junction hemichannel in inflammatory responses and cell death. This article is part of a Special Issue entitled Electrical Synapses.
J. Neurochem. (2011) 118, 826–840.
Inflammation contributes to neurodegeneration in post‐ischemic brain, diabetes, and Alzheimer’s disease. Participants in this inflammatory response include ...activation of microglia and astrocytes. We studied the role of microglia treated with amyloid‐β peptide (Aβ) on hemichannel activity of astrocytes subjected to hypoxia in high glucose. Reoxygenation after 3 h hypoxia in high glucose induced transient astroglial permeabilization via Cx43 hemichannels and reduction in intercellular communication via Cx43 cell‐cell channels. Both responses were greater and longer lasting in astrocytes previously exposed for 24 h to conditioned medium from Aβ‐treated microglia (CM‐Aβ). The effects of CM‐Aβ were mimicked by TNF‐α and IL‐1β and were abrogated by neutralizing TNF‐α with soluble receptor and IL‐1β with a receptor antagonist. Astrocytes under basal conditions protected neurons against hypoxia, but exposure to CM‐Aβ made them toxic to neurons subjected to a sub‐lethal hypoxia/reoxygenation episode, revealing the additive nature of the insults. Astrocytes exposed to CM‐Aβ induced permeabilization of cortical neurons through activation of neuronal pannexin 1 (Panx1) hemichannels by ATP and glutamate released through astroglial Cx43 hemichannels. In agreement, inhibition of NMDA or P2X receptors only partially reduced the activation of neuronal Panx1 hemichannels and neuronal mortality, but simultaneous inhibition of both receptors completely prevented the neurotoxic response. Therefore, we suggest that responses to ATP and glutamate converge in activation of neuronal Panx1 hemichannels. Thus, we propose that blocking hemichannels expressed by astrocytes and/or neurons in the inflamed nervous system could represent a novel and alternative strategy to reduce neuronal loss in various pathological states including Alzheimer’s disease, diabetes and ischemia.
A hallmark of neurodegenerative diseases is the reactive gliosis characterized by a phenotypic change in astrocytes and microglia. This glial response is associated with modifications in the ...expression and function of connexins (Cxs), the proteins forming gap junction channels and hemichannels. Increased Cx expression is detected in most reactive astrocytes located at amyloid plaques, the histopathological lesions typically present in the brain of Alzheimer's patients and animal models of the disease. The activity of Cx channels analyzed in vivo as well as in vitro after treatment with the amyloid β peptide is also modified and, in particular, hemichannel activation may contribute to neuronal damage. In this review, we summarize and discuss recent data that suggest glial Cx channels participate in the neurodegenerative process of Alzheimer's disease. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.
► Reactive gliosis is a hallmark of Alzheimer's disease. ► Glial connexin expression is affected in this pathology. ► Connexins, but also pannexins hemichannels, are activated in glia. ► Astroglial connexins may contribute to neuronal damages in neurodegenerative diseases.
The role of astrocytes in brain function has evolved over the last decade, from support cells to active participants in the neuronal synapse through the release of "gliotransmitters."Astrocytes ...express receptors for most neurotransmitters and respond to them through Ca(2+) intracellular oscillations and propagation of intercellular Ca(2+) waves. While such waves are able to propagate among neighboring astrocytes through gap junctions, thereby activating several astrocytes simultaneously, they can also trigger the release of gliotransmitters, including glutamate, d-serine, glycine, ATP, adenosine, or GABA. There are several mechanisms by which gliotransmitter release occurs, including functional hemichannels. These gliotransmitters can activate neighboring astrocytes and participate in the propagation of intercellular Ca(2+) waves, or activate pre- and post-synaptic receptors, including NMDA, AMPA, and purinergic receptors. In consequence, hemichannels could play a pivotal role in astrocyte-to-astrocyte communication and astrocyte-to-neuron cross-talk. Recent evidence suggests that astroglial hemichannels are involved in higher brain functions including memory and glucose sensing. The present review will focus on the role of hemichannels in astrocyte-to-astrocyte and astrocyte-to neuron communication and in brain physiology.
Spreading depression (SD) is an intriguing phenomenon characterized by massive slow brain depolarizations that affect neurons and glial cells. This phenomenon is repetitive and produces a metabolic ...overload that increases secondary damage. However, the mechanisms associated with the initiation and propagation of SD are unknown. Multiple lines of evidence indicate that persistent and uncontrolled opening of hemichannels could participate in the pathogenesis and progression of several neurological disorders including acute brain injuries. Here, we explored the contribution of astroglial hemichannels composed of connexin-43 (Cx43) or pannexin-1 (Panx1) to SD evoked by high-K.sup.+ stimulation in brain slices. Focal high-K.sup.+ stimulation rapidly evoked a wave of SD linked to increased activity of the Cx43 and Panx1 hemichannels in the brain cortex, as measured by light transmittance and dye uptake analysis, respectively. The activation of these channels occurs mainly in astrocytes but also in neurons. More importantly, the inhibition of both the Cx43 and Panx1 hemichannels completely prevented high K.sup.+-induced SD in the brain cortex. Electrophysiological recordings also revealed that Cx43 and Panx1 hemichannels critically contribute to the SD-induced decrease in synaptic transmission in the brain cortex and hippocampus. Targeting Cx43 and Panx1 hemichannels could serve as a new therapeutic strategy to prevent the initiation and propagation of SD in several acute brain injuries.
Cu(bipy)(C6F5) reacts with most aryl iodides to form heterobiphenyls by cross-coupling, but when Rf–I is used (Rf = 3,5-dicholoro-2,4,6-trifluorophenyl), homocoupling products are also formed. ...Kinetic studies suggest that, for the homocoupling reaction, a mechanism based on transmetalation from Cu(bipy)(C6F5) to Cu(III) intermediates formed in the oxidative addition step is at work. Density functional theory calculations show that the interaction between these Cu(III) species and the starting Cu(I) complex involves a Cu(I)–Cu(III) electron transfer concerted with the formation of an iodine bridge between the metals and that a fast transmetalation takes place in a dimer in a triplet state between two Cu(II) units.
Connexin 43 (Cx43) is expressed in kidney tissue where it forms hemichannels and gap junction channels. However, the possible functional relationship between these membrane channels and their role in ...damaged renal cells remains unknown. Here, analysis of ethidium uptake and thiobarbituric acid reactive species revealed that treatment with TNF-α plus IL-1β increases Cx43 hemichannel activity and oxidative stress in MES-13 cells (a cell line derived from mesangial cells), and in primary mesangial cells. The latter was also accompanied by a reduction in gap junctional communication, whereas Western blotting assays showed a progressive increase in phosphorylated MYPT (a target of RhoA/ROCK) and Cx43 upon TNF-α/IL-1β treatment. Additionally, inhibition of RhoA/ROCK strongly antagonized the TNF-α/IL-1β-induced activation of Cx43 hemichannels and reduction in gap junctional coupling. We propose that activation of Cx43 hemichannels and inhibition of cell–cell coupling during pro-inflammatory conditions could contribute to oxidative stress and damage of mesangial cells via the RhoA/ROCK pathway.
Recent in vitro evidence indicates that astrocytes can modulate synaptic plasticity by releasing neuroactive substances (gliotransmitters). However, whether gliotransmitter release from astrocytes is ...necessary for higher brain function in vivo, particularly for memory, as well as the contribution of connexin (Cx) hemichannels to gliotransmitter release, remain elusive. Here, we microinfused into the rat basolateral amygdala (BLA) TAT‐Cx43L2, a peptide that selectively inhibits Cx43‐hemichannel opening while maintaining synaptic transmission or interastrocyte gap junctional communication. In vivo blockade of Cx43 hemichannels during memory consolidation induced amnesia for auditory fear conditioning, as assessed 24 h after training, without affecting short‐term memory, locomotion, or shock reactivity. The amnesic effect was transitory, specific for memory consolidation, and was confirmed after microinfusion of Gap27, another Cx43‐hemichannel blocker. Learning capacity was recovered after coinfusion of TAT‐Cx43L2 and a mixture of putative gliotransmitters (glutamate, glutamine, lactate, d‐serine, glycine, and ATP). We propose that gliotransmitter release from astrocytes through Cx43 hemichannels is necessary for fear memory consolidation at the BLA. Thus, the present study is the first to demonstrate a physiological role for astroglial Cx43 hemichannels in brain function, making these channels a novel pharmacological target for the treatment of psychiatric disorders, including post‐traumatic stress disorder.—Stehberg, J., Moraga‐Amaro, R., Salazar, C., Becerra, A., Echeverría, C., Orellana, J. A., Bultynck, G., Ponsaerts, R., Leybaert, L., Simon, F., Sáez, J. C., Retamal, M. A. Release of gliotransmitters through astroglial connexin 43 hemichannels is necessary for fear memory consolidation in the basolateral amygdala. FASEB J. 26, 3649–3657 (2012). www.fasebj.org
Coordinated interaction among cells is critical to develop the extremely complex and dynamic tasks performed by the central nervous system (CNS). Cell synchronization is in part mediated by connexins ...and pannexins; two different protein families that form gap junction channels and hemichannels. Whereas gap junction channels connect the cytoplasm of contacting cells and coordinate electric and metabolic activities, hemichannels communicate intra- and extra-cellular compartments and serve as diffusional pathways for ions and small molecules. Cells in the CNS depend on paracrine/autocrine communication via several extracellular signaling molecules, such as, cytokines, growth factors, transmitters and free radical species to sense changes in microenvironment as well as to adapt to them. These signaling molecules modulate crucial processes of the CNS, including, cellular migration and differentiation, synaptic transmission and plasticity, glial activation, cell viability and microvascular blood flow. Gap junction channels and hemichannels are affected by different signaling transduction pathways triggered by these paracrine/autocrine signaling molecules. Most of the modulatory effects induced by these signaling molecules are specific to the cell type and the connexin and pannexin subtype expressed in different brain areas. In this review, we summarized and discussed most of the relevant and recently published information on the effects of signaling molecules on connexin or pannexin based channels and their possible relevance in CNS physiology and pathology.
This article is part of the Special Issue Section entitled ‘Current Pharmacology of Gap Junction Channels and Hemichannels’.
► Gap junction channels and hemichannels mediate cell interaction and synchronization. ► Gap junction channels connect the cytoplasm of contacting cells. ► Hemichannels communicate intra- and extra-cellular compartments. ► Signaling molecules modulate gap junction channels and hemichannels in the CNS. ► Connexins and pannexins are targets for therapeutic intervention in brain diseases.
PDF
