Expression of pannexin family of proteins in the retina

P2X7 receptors and pannexin1 hemichannels shape presynaptic transmission

Over the last decades, since the discovery of ATP as a transmitter, accumulating evidence has been reported about the role of this nucleotide and purinergic receptors, in particular P2X7 receptors, in the modulation of synaptic strength and plasticity. Purinergic signaling has emerged as a crucial player in orchestrating the molecular interaction between the components of the tripartite synapse, and much progress has been made in how this neuron-glia interaction impacts neuronal physiology under basal and pathological conditions. On the other hand, pannexin1 hemichannels, which are functionally linked to P2X7 receptors, have appeared more recently as important modulators of excitatory synaptic function and plasticity under diverse contexts. In this review, we will discuss the contribution of ATP, P2X7 receptors, and pannexin hemichannels to the modulation of presynaptic strength and its impact on motor function, sensory processing, synaptic plasticity, and neuroglial communication, with special focus on the P2X7 receptor/pannexin hemichannel interplay. We also address major hypotheses about the role of this interaction in physiological and pathological circumstances.

Skin in the game: pannexin channels in healthy and cancerous skin

The skin is a highly organized tissue composed of multiple layers and cell types that require coordinated cell to cell communication to maintain tissue homeostasis. In skin cancer, this organized structure and communication is disrupted, prompting the malignant transformation of healthy cells into melanoma, basal cell carcinoma or squamous cell carcinoma tumours. One such family of channel proteins critical for cellular communication is pannexins (PANX1, PANX2, PANX3), all of which are present in the skin. These heptameric single-membrane channels act as conduits for small molecules and ions like ATP and Ca2+ but have also been shown to have channel-independent functions through their interacting partners or action in signalling pathways. Pannexins have diverse roles in the skin such as in skin development, aging, barrier function, keratinocyte differentiation, inflammation, and wound healing, which were discovered through work with pannexin knockout mice, organotypic epidermis models, primary cells, and immortalized cell lines. In the context of cutaneous cancer, PANX1 is present at high levels in melanoma tumours and functions in melanoma carcinogenesis, and both PANX1 and PANX3 expression is altered in non-melanoma skin cancer. PANX2 has thus far not been implicated in any skin cancer. This review will discuss pannexin isoforms, structure, trafficking, post-translational modifications, interactome, and channel activity. We will also outline the expression, localization, and function of pannexin channels within the diverse cell types of the epidermis, dermis, hypodermis, and adnexal structures of the skin, and how these properties are exploited or abrogated in instances of skin cancer.

ATP transporters in the joints

Extracellular adenosine triphosphate (ATP) plays a central role in a wide variety of joint diseases. ATP is generated intracellularly, and the concentration of the extracellular ATP pool is determined by the regulation of its transport out of the cell. A variety of ATP transporters have been described, with connexins and pannexins the most commonly cited. Both form intercellular channels, known as gap junctions, that facilitate the transport of various small molecules between cells and mediate cell-cell communication. Connexins and pannexins also form pores, or hemichannels, that are permeable to certain molecules, including ATP. All joint tissues express one or more connexins and pannexins, and their expression is altered in some pathological conditions, such as osteoarthritis (OA) and rheumatoid arthritis (RA), indicating that they may be involved in the onset and progression of these pathologies. The aging of the global population, along with increases in the prevalence of obesity and metabolic dysfunction, is associated with a rising frequency of joint diseases along with the increased costs and burden of related illness. The modulation of connexins and pannexins represents an attractive therapeutic target in joint disease, but their complex regulation, their combination of gap-junction-dependent and -independent functions, and their interplay between gap junction and hemichannel formation are not yet fully elucidated. In this review, we try to shed light on the regulation of these proteins and their roles in ATP transport to the extracellular space in the context of joint disease, and specifically OA and RA.

Panx1b Modulates the Luminance Response and Direction of Locomotion in the Zebrafish

Pannexin1 (Panx1) can form ATP-permeable channels that play roles in the physiology of the visual system. In the zebrafish two ohnologs of Panx1, Panx1a and Panx1b, have unique and shared channel properties and tissue expression patterns. Panx1a channels are located in horizontal cells of the outer retina and modulate light decrement detection through an ATP/pH-dependent mechanisms and adenosine/dopamine signaling. Here, we decipher how the strategic localization of Panx1b channels in the inner retina and ganglion cell layer modulates visually evoked motor behavior. We describe a panx1b knockout model generated by TALEN technology. The RNA-seq analysis of 6 days post-fertilization larvae is confirmed by real-time PCR and paired with testing of locomotion behaviors by visual motor and optomotor response tests. We show that the loss of Panx1b channels disrupts the retinal response to an abrupt loss of illumination and it decreases the larval ability to follow leftward direction of locomotion in low light conditions. We concluded that the loss of Panx1b channels compromises the final output of luminance as well as motion detection. The Panx1b protein also emerges as a modulator of the circadian clock system. The disruption of the circadian clock system in mutants suggests that Panx1b could participate in non-image forming processes in the inner retina.

Effects of miR-224-5p-enhanced downregulation of pannexin-1 on docetaxel-induced apoptosis in amoeboid-like CD44high oral cancer cells

We previously found that microRNAs play major roles in the maintenance of amoeboid-like oral squamous cell carcinoma (OSCC) cells with high expression of CD44 (CD44^high ). However, the roles of microRNAs in chemotherapeutic resistance exhibited by CD44^high amoeboid-like OSCC cells are unclear. Here, docetaxel-induced apoptosis was examined in CD44^high OSCC cells (CD44^high OM-1 cells) cultured on laminin-coated silicone gel. Amoeboid-like CD44^high OSCC cells exhibited robust resistance to docetaxel-induced apoptosis and significant upregulation of miR-224-5p expression compared with epithelial-like CD44^high OSCC cells and mesenchymal-like CD44^high OSCC cells. The expression of pannexin-1 (PANX1), a channel-forming protein that regulates the release of ATP, was significantly upregulated following transfection of amoeboid-like CD44^high OSCC cells with an miR-224-5p inhibitor. These results suggest that miR-224-5p inhibits PANX1 expression. Furthermore, miR-224-5p inhibitor-transfected amoeboid-like CD44^high OSCC cells exhibited significant enhancement of the proportion of apoptotic cells; however, this effect was significantly inhibited by knockdown of PANX1 with PANX1 small interfering RNA. Additionally, the miR-224-5p inhibitor-enhanced extracellular ATP levels were significantly reduced by PANX1 knockdown. These findings imply that miR-224-5p plays a vital role in the resistance to docetaxel-induced apoptosis by attenuating PANX1-induced ATP discharge. Moreover, amoeboid-like CD44^high OSCC cells may be involved in chemotherapeutic resistance of OSCC.

Pannexin-1 Channels as Mediators of Neuroinflammation

Neuroinflammation is a major component of central nervous system (CNS) injuries and neurological diseases, including Alzheimer’s disease, multiple sclerosis, neuropathic pain, and brain trauma. The activation of innate immune cells at the damage site causes the release of pro-inflammatory cytokines and chemokines, which alter the functionality of nearby tissues and might mediate the recruitment of leukocytes to the injury site. If this process persists or is exacerbated, it prevents the adequate resolution of the inflammation, and ultimately enhances secondary damage. Adenosine 5′ triphosphate (ATP) is among the molecules released that trigger an inflammatory response, and it serves as a chemotactic and endogenous danger signal. Extracellular ATP activates multiple purinergic receptors (P2X and P2Y) that have been shown to promote neuroinflammation in a variety of CNS diseases. Recent studies have shown that Pannexin-1 (Panx1) channels are the principal conduits of ATP release from dying cells and innate immune cells in the brain. Herein, we review the emerging evidence that directly implicates Panx-1 channels in the neuroinflammatory response in the CNS.

Purinergic signaling in nervous system health and disease: Focus on pannexin 1

Purinergic signaling encompasses the cycle of adenosine 5' triphosphate (ATP) release and its metabolism into nucleotide and nucleoside derivatives, the direct release of nucleosides, and subsequent receptor-triggered downstream intracellular pathways. Since the discovery of nerve terminal and glial ATP release into the neuropil, purinergic signaling has been implicated in the modulation of nervous system development, function, and disease. In this review, we detail our current understanding of the roles of the pannexin 1 (PANX1) ATP-release channel in neuronal development and plasticity, glial signaling, and neuron-glial-immune interactions. We additionally provide an overview of PANX1 structure, activation, and permeability to orientate readers and highlight recent research developments. We identify areas of convergence between PANX1 and purinergic receptor actions. Additional highlights include data on PANX1's participation in the pathophysiology of nervous system developmental, degenerative, and inflammatory disorders. Our aim in combining this knowledge is to facilitate the movement of our current understanding of PANX1 in the context of other nervous system purinergic signaling mechanisms one step closer to clinical translation.

Human Pannexin 1 channel: Insight in structure-function mechanism and its potential physiological roles

Pannexins, large non-gap junction super family exists in vertebrates, play multiple roles in different cellular functions through their ATP release. Panx1-mediated adenosine 5'-triphosphate (ATP) release plays a vital role in physiological and pathophysiological conditions and is known major extracellular molecule in purinergic signaling. To modulate their function in vivo, a proper regulation of channel is necessary. Post-translational modifications are considered to be some regulating mechanisms for PANX1, while PANX2, PANX3 have been uncharacterized to date. Through their significant evidences, PANXs exclude from gap junction and conduits ATP release and other cellular molecules from cells by various mechanisms. PANX1 is most extensive characterized and implicated in ATP signaling and inflammatory processes. Despite the constant advances, much significance of PANX1 in physiological processes remains elusive. Recently, various research groups along with our group have reported the Cryo-EM structure of Panx1 channel and uncovered the hidden functions in structure-function mechanism as well as to provide the clear understanding in physiological and pathophysiological roles. These research groups reported the novel heptameric structure with contains 4 transmembrane helices (TM), two extracellular loops and one intracellular loop with N and C terminus located at the intracellular side. In addition, the structure contains a large pore of which an inhibitor CBX act as a plug that blocking the passage of substrate. In this context, this review will present current mechanistic understanding in structure and function together with significant physiological roles particularly ATP release in health and disease. As such, this review emphasizes on recent functional properties associated with novel heptameric channel and demystifies channel-mediated ATP release function.

Connexin therapeutics: blocking connexin hemichannel pores is distinct from blocking pannexin channels or gap junctions

Compounds that block the function of connexin and pannexin protein channels have been suggested to be valuable therapeutics for a range of diseases. Some of these compounds are now in clinical trials, but for many of them, the literature is inconclusive about the molecular effect on the tissue, despite evidence of functional recovery. Blocking the different channel types has distinct physiological and pathological implications and this review describes current knowledge of connexin and pannexin protein channels, their function as channels and possible mechanisms of the channel block effect for the latest therapeutic compounds. We summarize the evidence implicating pannexins and connexins in disease, considering their homeostatic versus pathological roles, their contribution to excesive ATP release linked to disease onset and progression.

Inflammasome Activation Induces Pyroptosis in the Retina Exposed to Ocular Hypertension Injury

Mechanical stress and hypoxia during episodes of ocular hypertension (OHT) trigger glial activation and neuroinflammation in the retina. Glial activation and release of pro-inflammatory cytokines TNFα and IL-1β, complement, and other danger factors was shown to facilitate injury and loss of retinal ganglion cells (RGCs) that send visual information to the brain. However, cellular events linking neuroinflammation and neurotoxicity remain poorly characterized. Several pro-inflammatory and danger signaling pathways, including P2X7 receptors and Pannexin1 (Panx1) channels, are known to activate inflammasome caspases that proteolytically activate gasdermin D channel-formation to export IL-1 cytokines and/or induce pyroptosis. In this work, we used molecular and genetic approaches to map and characterize inflammasome complexes and detect pyroptosis in the OHT-injured retina. Acute activation of distinct inflammasome complexes containing NLRP1, NLRP3 and Aim2 sensor proteins was detected in RGCs, retinal astrocytes and Muller glia of the OHT-challenged retina. Inflammasome-mediated activation of caspases-1 and release of mature IL-1β were detected within 6 h and peaked at 12–24 h after OHT injury. These coincided with the induction of pyroptotic pore protein gasdermin D in neurons and glia in the ganglion cell layer (GCL) and inner nuclear layer (INL). The OHT-induced release of cytokines and RGC death were significantly decreased in the retinas of Casp1^−/−Casp4(11)^del, Panx1^−/− and in Wild-type (WT) mice treated with the Panx1 inhibitor probenecid. Our results showed a complex spatio-temporal pattern of innate immune responses in the retina. Furthermore, they indicate an active contribution of neuronal NLRP1/NLRP3 inflammasomes and the pro-pyroptotic gasdermin D pathway to pathophysiology of the OHT injury. These results support the feasibility of inflammasome modulation for neuroprotection in OHT-injured retinas.

Synaptic Functions of Hemichannels and Pannexons: A Double-Edged Sword

The classical view of synapses as the functional contact between presynaptic and postsynaptic neurons has been challenged in recent years by the emerging regulatory role of glial cells. Astrocytes, traditionally considered merely supportive elements are now recognized as active modulators of synaptic transmission and plasticity at the now so-called "tripartite synapse." In addition, an increasing body of evidence indicates that beyond immune functions microglia also participate in various processes aimed to shape synaptic plasticity. Release of neuroactive compounds of glial origin, -process known as gliotransmission-, constitute a widespread mechanism through which glial cells can either potentiate or reduce the synaptic strength. The prevailing vision states that gliotransmission depends on an intracellular Ca²⁺/exocytotic-mediated release; notwithstanding, growing evidence is pointing at hemichannels (connexons) and pannexin channels (pannexons) as alternative non-vesicular routes for gliotransmitters efflux. In concurrence with this novel concept, both hemichannels and pannexons are known to mediate the transfer of ions and signaling molecules -such as ATP and glutamate- between the cytoplasm and the extracellular milieu. Importantly, recent reports show that glial hemichannels and pannexons are capable to perceive synaptic activity and to respond to it through changes in their functional state. In this article, we will review the current information supporting the "double edge sword" role of hemichannels and pannexons in the function of central and peripheral synapses. At one end, available data support the idea that these channels are chief components of a feedback control mechanism through which gliotransmitters adjust the synaptic gain in either resting or stimulated conditions. At the other end, we will discuss how the excitotoxic release of gliotransmitters and [Ca²⁺]_i overload linked to the opening of hemichannels/pannexons might impact cell function and survival in the nervous system.

Purinergic signaling in the retina: From development to disease

Retinal injuries and diseases are major causes of human disability involving vision impairment by the progressive and permanent loss of retinal neurons. During development, assembly of this tissue entails a successive and overlapping, signal-regulated engagement of complex events that include proliferation of progenitors, neurogenesis, cell death, neurochemical differentiation and synaptogenesis. During retinal damage, several of these events are re-activated with both protective and detrimental consequences. Purines and pyrimidines, along with their metabolites are emerging as important molecules regulating both retinal development and the tissue's responses to damage. The present review provides an overview of the purinergic signaling in the developing and injured retina. Recent findings on the presence of vesicular and channel-mediated ATP release by retinal and retinal pigment epithelial cells, adenosine synthesis and release, expression of receptors and intracellular signaling pathways activated by purinergic signaling in retinal cells are reported. The pathways by which purinergic receptors modulate retinal cell proliferation, migration and death of retinal cells during development and injury are summarized. The contribution of nucleotides to the self-repair of the injured zebrafish retina is also discussed.

Pannexin 1 regulates adipose stromal cell differentiation and fat accumulation

Pannexin 1 (Panx1) is a channel-forming glycoprotein important in paracrine signaling and cellular development. In this study, we discovered that mice globally lacking Panx1 (KO) have significantly greater total fat mass and reduced lean mass compared to wild type (WT) mice under a normal diet. Despite having higher fat content, Panx1 KO mice on a high fat diet exhibited no differences in weight gain and blood markers of obesity as compared to WT controls, except for an increase in glucose and insulin levels. However, metabolic cage data revealed that these Panx1 KO mice display significantly increased activity levels, higher ambulatory activity, and reduced sleep duration relative to their WT littermates on a high-fat diet. To uncover the cellular mechanism responsible for the increased fat content in the KO, we isolated primary cultures of adipose-derived stromal cells (ASCs) from WT and KO fat pads. In WT ASCs we observed that Panx1 protein levels increase upon induction into an adipogenic lineage. ASCs isolated from Panx1 KO mice proliferate less but demonstrate enhanced adipogenic differentiation with increased intracellular lipid accumulation, glycerol-3-phosphate dehydrogenase (GPDH) enzyme activity, and adipokine secretion, as compared to WT ASCs. This was consistent with the increased adipocyte size and decreased adipocyte numbers observed in subcutaneous fat of the Panx1 KO mice compared to WT. We concluded that Panx1 plays a key role in adipose stromal cells during the early stages of adipogenic proliferation and differentiation, regulating fat accumulation in vivo.

N-Glycosylation Regulates Pannexin 2 Localization but Is Not Required for Interacting with Pannexin 1

Pannexins (Panx1, 2, 3) are channel-forming glycoproteins expressed in mammalian tissues. We previously reported that N-glycosylation acts as a regulator of the localization and intermixing of Panx1 and Panx3, but its effects on Panx2 are currently unknown. Panx1 and Panx2 intermixing can regulate channel properties, and both pannexins have been implicated in neuronal cell death after ischemia. Our objectives were to validate the predicted N-glycosylation site of Panx2 and to study the effects of Panx2 glycosylation on localization and its capacity to interact with Panx1. We used site-directed mutagenesis, enzymatic de-glycosylation, cell-surface biotinylation, co-immunoprecipitation, and confocal microscopy. Our results showed that N86 is the only N-glycosylation site of Panx2. Panx2 and the N86Q mutant are predominantly localized to the endoplasmic reticulum (ER) and cis-Golgi matrix with limited cell surface localization was seen only in the presence of Panx1. The Panx2 N86Q mutant is glycosylation-deficient and tends to aggregate in the ER reducing its cell surface trafficking but it can still interact with Panx1. Our study indicates that N-glycosylation may be important for folding and trafficking of Panx2. We found that the un-glycosylated forms of Panx1 and 2 can readily interact, regulating their localization and potentially their channel function in cells where they are co-expressed.

The two faces of pannexins: new roles in inflammation and repair

Pannexins belong to a family of ATP-release channels expressed in almost all cell types. An increasing body of literature on pannexins suggests that these channels play dual and sometimes contradictory roles, contributing to normal cell function, as well as to the pathological progression of disease. In this review, we summarize our understanding of pannexin "protective" and "harmful" functions in inflammation, regeneration and mechanical signaling. We also suggest a possible basis for pannexin's dual roles, related to extracellular ATP and K⁺ levels and the activation of various types of P2 receptors that are associated with pannexin. Finally, we speculate upon therapeutic strategies related to pannexin using eyes, lacrimal glands, and peripheral nerves as examples of interesting therapeutic targets.

Pannexin-1 in Human Lymphatic Endothelial Cells Regulates Lymphangiogenesis

The molecular mechanisms governing the formation of lymphatic vasculature are not yet well understood. Pannexins are transmembrane proteins that form channels which allow for diffusion of ions and small molecules (<1 kDa) between the extracellular space and the cytosol. The expression and function of pannexins in blood vessels have been studied in the last few decades. Meanwhile, no studies have been conducted to evaluate the role of pannexins during human lymphatic vessel formation. Here we show, using primary human dermal lymphatic endothelial cells (HDLECs), pharmacological tools (probenecid, Brilliant Blue FCF, mimetic peptides [¹⁰Panx]) and siRNA-mediated knockdown that Pannexin-1 is necessary for capillary tube formation on Matrigel and for VEGF-C-induced invasion. These results newly identify Pannexin-1 as a protein highly expressed in HDLECs and its requirement during in vitro lymphangiogenesis.

Knockout of Pannexin-1 Induces Hearing Loss

Mutations of gap junction connexin genes induce a high incidence of nonsyndromic hearing loss. Pannexin genes also encode gap junctional proteins in vertebrates. Recent studies demonstrated that Pannexin-1 (Panx1) deficiency in mice and mutation in humans are also associated with hearing loss. So far, several Panx1 knockout (KO) mouse lines were established. In general, these Panx1 KO mouse lines demonstrate consistent phenotypes in most aspects, including hearing loss. However, a recent study reported that a Panx1 KO mouse line, which was created by Genentech Inc., had no hearing loss as measured by the auditory brainstem response (ABR) threshold at low-frequency range (<24 kHz). Here, we used multiple auditory function tests and re-examined hearing function in the Genentech Panx1 (Gen-Panx1) KO mouse. We found that ABR thresholds in the Gen-Panx1 KO mouse were significantly increased, in particular, in the high-frequency region. Moreover, consistent with the increase in ABR threshold, distortion product otoacoustic emission (DPOAE) and cochlear microphonics (CM), which reflect active cochlear amplification and auditory receptor current, respectively, were significantly reduced. These data demonstrated that the Gen-Panx1 KO mouse has hearing loss and further confirmed that Panx1 deficiency can cause deafness.

A Potential Compensatory Role of Panx3 in the VNO of a Panx1 Knock Out Mouse Model

Pannexins (Panx) are integral membrane proteins, with Panx1 being the best-characterized member of the protein family. Panx1 is implicated in sensory processing, and knockout (KO) animal models have become the primary tool to investigate the role(s) of Panx1 in sensory systems. Extending previous work from our group on primary olfaction, the expression patterns of Panxs in the vomeronasal organ (VNO), an auxiliary olfactory sense organ with a role in reproduction and social behavior, were compared. Using qRT-PCR and Immunohistochemistry (IHC), we confirmed the loss of Panx1, found similar Panx2 expression levels in both models, and a significant upregulation of Panx3 in mice with a global ablation of Panx1. Specifically, Panx3 showed upregulated expression in nerve fibers of the non-sensory epithelial layer in juvenile and adult KO mice and in the sensory layer of adults, which overlaps with Panx1 expression areas in WT populations. Since both social behavior and evoked ATP release in the VNO was not compromised in KO animals, we hypothesized that Panx3 could compensate for the loss of Panx1. This led us to compare Panx1 and Panx3 channels in vitro, demonstrating similar dye uptake and ATP release properties. Outcomes of this study strongly suggest that Panx3 may functionally compensate for the loss of Panx1 in the VNO of the olfactory system, ensuring sustained chemosensory processing. This finding extends previous reports on the upregulation of Panx3 in arterial walls and the skin of Panx1 KO mice, suggesting that roles of Panx1 warrant uncharacterized safeguarding mechanisms involving Panx3.

Pannexin 1 sustains the electrophysiological responsiveness of retinal ganglion cells

Pannexin 1 (Panx1) forms ATP-permeable membrane channels that play a key role in purinergic signaling in the nervous system in both normal and pathological conditions. In the retina, particularly high levels of Panx1 are found in retinal ganglion cells (RGCs), but the normal physiological function in these cells remains unclear. In this study, we used patch clamp recordings in the intact inner retina to show that evoked currents characteristic of Panx1 channel activity were detected only in RGCs, particularly in the OFF-type cells. The analysis of pattern electroretinogram (PERG) recordings indicated that Panx1 contributes to the electrical output of the retina. Consistently, PERG amplitudes were significantly impaired in the eyes with targeted ablation of the Panx1 gene in RGCs. Under ocular hypertension and ischemic conditions, however, high Panx1 activity permeated cell membranes and facilitated the selective loss of RGCs or stably transfected Neuro2A cells. Our results show that high expression of the Panx1 channel in RGCs is essential for visual function in the inner retina but makes these cells highly sensitive to mechanical and ischemic stresses. These findings are relevant to the pathophysiology of retinal disorders induced by increased intraocular pressure, such as glaucoma.

Pannexin 1 Modulates Axonal Growth in Mouse Peripheral Nerves

The pannexin family of channels consists of three members—pannexin-1 (Panx1), pannexin-2 (Panx2), and pannexin-3 (Panx3) that enable the exchange of metabolites and signaling molecules between intracellular and extracellular compartments. Pannexin-mediated release of intracellular ATP into the extracellular space has been tied to a number of cellular activities, primarily through the activity of type P2 purinergic receptors. Previous work indicates that the opening of Panx1 channels and activation of purinergic receptors by extracellular ATP may cause inflammation and apoptosis. In the CNS (central nervous system) and PNS (peripheral nervous system), coupled pannexin, and P2 functions have been linked to peripheral sensitization (pain) pathways. Purinergic pathways are also essential for other critical processes in the PNS, including myelination and neurite outgrowth. However, whether such pathways are pannexin-dependent remains to be determined. In this study, we use a Panx1 knockout mouse model and pharmacological inhibitors of the Panx1 and the ATP-mediated signaling pathway to fill gaps in our understanding of Panx1 localization in peripheral nerves, roles for Panx1 in axonal outgrowth and myelination, and neurite extension. Our data show that Panx1 is localized to axonal, myelin, and vascular compartments of the peripheral nerves. Knockout of Panx1 gene significantly increased axonal caliber in vivo and axonal growth rate in cultured dorsal root ganglia (DRG) neurons. Furthermore, genetic knockout of Panx1 or inhibition of components of purinergic signaling, by treatment with probenecid and apyrase, resulted in denser axonal outgrowth from cultured DRG explants compared to untreated wild-types. Our findings suggest that Panx1 regulates axonal growth in the peripheral nervous system.

Mechanisms of pannexin1 channel gating and regulation

Pannexins are a family of integral membrane proteins with distinct post-translational modifications, sub-cellular localization and tissue distribution. Panx1 is the most studied and best-characterized isoform of this gene family. The ubiquitous expression, as well as its function as a major ATP release and nucleotide permeation channel, makes Panx1 a primary candidate for participating in the pathophysiology of CNS disorders. While many investigations revolve around Panx1 functions in health and disease, more recently, details started emerging about mechanisms that control Panx1 channel activity. These advancements in Panx1 biology have revealed that beyond its classical role as an unopposed plasma membrane channel, it participates in alternative pathways involving multiple intracellular compartments, protein complexes and a myriad of extracellular participants. Here, we review recent progress in our understanding of Panx1 at the center of these pathways, highlighting its modulation in a context specific manner. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Pannexins in vision, hearing, olfaction and taste

In mammals, the pannexin gene family consists of three members (Panx1, 2, 3), which represent a class of integral membrane channel proteins sharing some structural features with chordate gap junction proteins, the connexins. Since their discovery in the early 21st century, pannexin expression has been detected throughout the vertebrate body including eye, ear, nose and tongue, making the investigation of the roles of this new class of channel protein in health and disease very appealing. The localization in sensory organs, coupled with unique channel properties and associations with major signaling pathways make Panx1, and its relative's, significant contributors for fundamental functions in sensory perception. Until recently, cell-based studies were at the forefront of pannexin research. Lately, the availability of mice with genetic ablation of pannexins opened new avenues for testing pannexin functions and behavioural phenotyping. Although we are only at the beginning of understanding the roles of pannexins in health and disease, this review summarizes recent advances in elucidating the various emerging roles pannexins play in sensory systems, with an emphasis on unresolved conflicts.

Transcriptional and post-translational regulation of pannexins

Pannexins are a 3-membered family of proteins that form large pore ion and metabolite channels in vertebrates. The impact of pannexins on vertebrate biology is intricately tied to where and when they are expressed, and how they are modified, once produced. The purpose of this review is therefore to outline our current understanding of transcriptional and post-translational regulation of pannexins. First, we briefly summarize their discovery and characteristics. Next, we describe several aspects of transcriptional regulation, including cell and tissue-specific expression, dynamic expression over development and disease, as well as new insights into the underlying molecular machinery involved. Following this, we delve into the role of post-translational modifications in the regulation of trafficking and channel properties, highlighting important work on glycosylation, phosphorylation, S-nitrosylation and proteolytic cleavage. Embedded throughout, we also highlight important knowledge gaps and avenues of future research. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Calcium dynamics and regulation in horizontal cells of the vertebrate retina: lessons from teleosts

Horizontal cells (HCs) are inhibitory interneurons of the vertebrate retina. Unlike typical neurons, HCs are chronically depolarized in the dark, leading to a constant influx of Ca2+ Therefore, mechanisms of Ca2+ homeostasis in HCs must differ from neurons elsewhere in the central nervous system, which undergo excitotoxicity when they are chronically depolarized or stressed with Ca2+ HCs are especially well characterized in teleost fish and have been used to unlock mysteries of the vertebrate retina for over one century. More recently, mammalian models of the retina have been increasingly informative for HC physiology. We draw from both teleost and mammalian models in this review, using a comparative approach to examine what is known about Ca2+ pathways in vertebrate HCs. We begin with a survey of Ca2+-permeable ion channels, exchangers, and pumps and summarize Ca2+ influx and efflux pathways, buffering, and intracellular stores. This includes evidence for Ca2+-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and N-methyl-d-aspartate receptors and for voltage-gated Ca2+ channels. Special attention is given to interactions between ion channels, to differences among species, and in which subtypes of HCs these channels have been found. We then discuss a number of unresolved issues pertaining to Ca2+ dynamics in HCs, including a potential role for Ca2+ in feedback to photoreceptors, the role for Ca2+-induced Ca2+ release, and the properties and functions of Ca2+-based action potentials. This review aims to highlight the unique Ca2+ dynamics in HCs, as these are inextricably tied to retinal function.

Retinal glial responses to optic nerve crush are attenuated in Bax-deficient mice and modulated by purinergic signaling pathways

Background

Retinal ganglion cell (RGC) soma death is a consequence of optic nerve damage, including in optic neuropathies like glaucoma. The activation of the innate immune network in the retina after nerve damage has been linked to RGC pathology. Since the eye is immune privileged, innate immune functions are the responsibility of the glia, specifically the microglia, astrocytes, and Müller cells that populate the retina. Glial activation, leading to the production of inflammatory cytokines, is a hallmark feature of retinal injury resulting from optic nerve damage and purported to elicit secondary degeneration of RGC somas.

Methods

A mouse model of optic nerve crush (ONC) was used to study retinal glial activation responses. RGC apoptosis was blocked using Bax-deficient mice. Glial activation responses were monitored by quantitative PCR and immunofluorescent labeling in retinal sections of activation markers. ATP signaling pathways were interrogated using P2X receptor agonists and antagonists and Pannexin 1 (Panx1)-deficient mice with RGC-specific deletion.

Results

ONC induced activation of both macroglia and microglia in the retina, and both these responses were dramatically muted if RGC death was blocked by deletion of the Bax gene. Macroglial, but not microglial, activation was modulated by purinergic receptor activation. Release of ATP after optic nerve damage was not mediated by PANX1 channels in RGCs.

Conclusions

RGC death in response to ONC plays a principal stimulatory role in the retinal glial activation response.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0558-y) contains supplementary material, which is available to authorized users.

Interactions of pannexin 1 with NMDA and P2X7 receptors in central nervous system pathologies: Possible role on chronic pain

Pannexin 1 (Panx1) is a glycoprotein that acts as a membrane channel in a wide variety of tissues in mammals. In the central nervous system (CNS) Panx1 is expressed in neurons, astrocytes and microglia, participating in the pathophysiology of some CNS diseases, such as epilepsy, anoxic depolarization after stroke and neuroinflammation. In these conditions Panx1 acts as an important modulator of the neuroinflammatory response, by secreting ATP, by interacting with the P2X7 receptor (P2X7R), and as an amplifier of NMDA receptor (NMDAR) currents, particularly in conditions of pathological neuronal hyperexcitability. Here, we briefly reviewed the current evidences that support the interaction of Panx1 with NMDAR and P2X7R in pathological contexts of the CNS, with special focus in recent data supporting that Panx1 is involved in chronic pain signaling by interacting with NMDAR in neurons and with P2X7R in glia. The participation of Panx1 in chronic pain constitutes a novel topic for research in the field of clinical neurosciences and a potential target for pharmacological interventions in chronic pain.

Nonvesicular release of ATP from rat retinal glial (Müller) cells is differentially mediated in response to osmotic stress and glutamate

Retinal glial (Müller) cells release ATP upon osmotic stress or activation of metabotropic glutamate receptors. ATP inhibits the osmotic Müller cell swelling by activation of P2Y1 receptors. In the present study, we determined the molecular pathways of the ATP release from Müller cells in slices of the rat retina. Administration of the ATP/ADPase apyrase induced a swelling of Müller cells under hypoosmotic conditions, and prevented the swelling-inhibitory effect of glutamate, suggesting that swelling inhibition is mediated by extracellular ATP. A hypoosmotic swelling of Müller cells was also observed in the presence of a blocker of multidrug resistance channels (MK-571), a CFTR inhibitor (glibenclamide), and connexin hemichannel blockers (18-α-glycyrrhetinic acid, 100 µM carbenoxolone). The swelling-inhibitory effect of glutamate was prevented by MK-571, the connexin hemichannel blockers, and a pannexin-1 hemichannel blocker (5 µM carbenoxolone). The p-glycoprotein blocker verapamil had no effect. As revealed by single-cell RT-PCR, subpopulations of Müller cells expressed mRNAs for pannexin-1 and -2, and connexins 30, 30.3, 32, 43, 45, and 46. The data may suggest that rat Müller cells release ATP by multidrug resistance channels, CFTR, and connexin hemichannels in response to osmotic stress, while glutamate induces a release of ATP via multidrug resistance channels, connexin hemichannels, and pannexin-1.

Pannexin 2 protein expression is not restricted to the CNS

Pannexins (Panx) are proteins homologous to the invertebrate gap junction proteins called innexins (Inx) and are traditionally described as transmembrane channels connecting the intracellular and extracellular compartments. Three distinct Panx paralogs (Panx1, Panx2 and Panx3) have been identified in vertebrates but previous reports on Panx expression and functionality focused primarily on Panx1 and Panx3 proteins. Several gene expression studies reported that Panx2 transcript is largely restricted to the central nervous system (CNS) hence suggesting that Panx2 might serve an important role in the CNS. However, the lack of suitable antibodies prevented the creation of a comprehensive map of Panx2 protein expression and Panx2 protein localization profile is currently mostly inferred from the distribution of its transcript. In this study, we characterized novel commercial monoclonal antibodies and surveyed Panx2 expression and distribution at the mRNA and protein level by real-time qPCR, Western blotting and immunofluorescence. Panx2 protein levels were readily detected in every tissue examined, even when transcriptional analysis predicted very low Panx2 protein expression. Furthermore, our results indicate that Panx2 transcriptional activity is a poor predictor of Panx2 protein abundance and does not correlate with Panx2 protein levels. Despite showing disproportionately high transcript levels, the CNS expressed less Panx2 protein than any other tissues analyzed. Additionally, we showed that Panx2 protein does not localize at the plasma membrane like other gap junction proteins but remains confined within cytoplasmic compartments. Overall, our results demonstrate that the endogenous expression of Panx2 protein is not restricted to the CNS and is more ubiquitous than initially predicted.

Pannexin channels and their links to human disease

In less than a decade, a small family of channel-forming glycoproteins, named pannexins, have captured the interest of many biologists, in large part due to their association with common diseases, ranging from cancers to neuropathies to infectious diseases. Although the pannexin family consists of only three members (Panx1, Panx2 and Panx3), one or more of these pannexins are expressed in virtually every mammalian organ, implicating their potential role in a diverse array of pathophysiologies. Panx1 is the most extensively studied, but features of this pannexin must be cautiously extrapolated to the other pannexins, as for example we now know that Panx2, unlike Panx1, exhibits unique properties such as a tendency to be retained within intracellular compartments. In the present review, we assess the biochemical and channel features of pannexins focusing on the literature which links these unique molecules to over a dozen diseases and syndromes. Although no germ-line mutations in genes encoding pannexins have been linked to any diseases, many cases have shown that high pannexin expression is associated with disease onset and/or progression. Disease may also occur, however, when pannexins are underexpressed, highlighting that pannexin expression must be exquisitely regulated. Finally, we discuss some of the most pressing questions and controversies in the pannexin field as the community seeks to uncover the full biological relevance of pannexins in healthy organs and during disease.

Emerging functions of pannexin 1 in the eye

Pannexin 1 (Panx1) is a high-conductance, voltage-gated channel protein found in vertebrates. Panx1 is widely expressed in many organs and tissues, including sensory systems. In the eye, Panx1 is expressed in major divisions including the retina, lens and cornea. Panx1 is found in different neuronal and non-neuronal cell types. The channel is mechanosensitive and responds to changes in extracellular ATP, intracellular calcium, pH, or ROS/nitric oxide. Since Panx1 channels operate at the crossroad of major signaling pathways, physiological functions in important autocrine and paracrine feedback signaling mechanisms were hypothesized. This review starts with describing in depth the initial Panx1 expression and localization studies fostering functional studies that uncovered distinct roles in processing visual information in subsets of neurons in the rodent and fish retina. Panx1 is expressed along the entire anatomical axis from optical nerve to retina and cornea in glia, epithelial and endothelial cells as well as in neurons. The expression and diverse localizations throughout the eye points towards versatile functions of Panx1 in neuronal and non-neuronal cells, implicating Panx1 in the crosstalk between immune and neural cells, pressure related pathological conditions like glaucoma, wound repair or neuronal cell death caused by ischemia. Summarizing the literature on Panx1 in the eye highlights the diversity of emerging Panx1 channel functions in health and disease.

Investigation of olfactory function in a Panx1 knock out mouse model

Pannexin 1 (Panx1), the most extensively investigated member of a channel-forming protein family, is able to form pores conducting molecules up to 1.5 kDa, like ATP, upon activation. In the olfactory epithelium (OE), ATP modulates olfactory responsiveness and plays a role in proliferation and differentiation of olfactory sensory neurons (OSNs). This process continuously takes place in the OE, as neurons are replaced throughout the whole lifespan. The recent discovery of Panx1 expression in the OE raises the question whether Panx1 mediates ATP release responsible for modulating chemosensory function. In this study, we analyzed pannexin expression in the OE and a possible role of Panx1 in olfactory function using a Panx1^−/− mouse line with a global ablation of Panx1. This mouse model has been previously used to investigate Panx1 functions in the retina and adult hippocampus. Here, qPCR, in-situ hybridization, and immunohistochemistry (IHC) demonstrated that Panx1 is expressed in axon bundles deriving from sensory neurons of the OE. The localization, distribution, and expression of major olfactory signal transduction proteins were not significantly altered in Panx1^−/− mice. Further, functional analysis of Panx1^−/− animals does not reveal any major impairment in odor perception, indicated by electroolfactogram (EOG) measurements and behavioral testing. However, ATP release evoked by potassium gluconate application was reduced in Panx1^−/− mice. This result is consistent with previous reports on ATP release in isolated erythrocytes and spinal or lumbar cord preparations from Panx1^−/− mice, suggesting that Panx1 is one of several alternative pathways to release ATP in the olfactory system.

Molecular cloning, characterization, and expression of pannexin genes in chicken

Pannexins (Panx) are a family of proteins that share sequences with the invertebrate gap junction proteins, innexins, and have a similar structure to that of the vertebrate gap junction proteins, connexins. To date, the Panx family consists of 3 members, but their genetic sequences have only been completely determined in a few vertebrate species. Moreover, expression of the Panx family has been reported in several rodent tissues: Panx1 is ubiquitously expressed in mammals, whereas Panx2 and Panx3 expressions are more restricted. Although members of the Panx family have been detected in mammals, their genetic sequences in avian species have not yet been fully elucidated. Here, we obtained the full-length mRNA sequences of chicken PANX genes and evaluated the homology of the amino acids from these sequences with those of other species. Furthermore, PANX gene expression in several chicken tissues was investigated based on mRNA levels. PANX1 was detected in the brain, cochlea, chondrocytes, eye, lung, skin, and intestine, and PANX2 was expressed in the brain, eye, and intestine. PANX3 was observed in the cochlea, chondrocytes, and bone. In addition, expression of PANX3 was higher than PANX1 in the cochlea. Immunofluorescent staining revealed PANX1 in hair cells, as well as the supporting cells, ganglion neurons, and the tegmentum vasculosum in chickens, whereas PANX3 was only detected in the bone surrounding the cochlea. Overall, the results of this study provide the first identification and characterization of the sequence and expression of the PANX family in an avian species, and fundamental data for confirmation of Panx function.

The pannexins: past and present

The pannexins (Panxs) are a family of chordate proteins homologous to the invertebrate gap junction forming proteins named innexins. Three distinct Panx paralogs (Panx1, Panx2, and Panx3) are shared among the major vertebrate phyla, but they appear to have suppressed (or even lost) their ability to directly couple adjacent cells. Connecting the intracellular and extracellular compartments is now widely accepted as Panx's primary function, facilitating the passive movement of ions and small molecules along electrochemical gradients. The tissue distribution of the Panxs ranges from pervasive to very restricted, depending on the paralog, and are often cell type-specific and/or developmentally regulated within any given tissue. In recent years, Panxs have been implicated in an assortment of physiological and pathophysiological processes, particularly with respect to ATP signaling and inflammation, and they are now considered to be a major player in extracellular purinergic communication. The following is a comprehensive review of the Panx literature, exploring the historical events leading up to their discovery, outlining our current understanding of their biochemistry, and describing the importance of these proteins in health and disease.

Molecular pathways of pannexin1-mediated neurotoxicity

Pannexin1 (Panx1) forms non-selective membrane channels, structurally similar to gap junction hemichannels, and are permeable to ions, nucleotides, and other small molecules below 900 Da. Panx1 activity has been implicated in paracrine signaling and inflammasome regulation. Recent studies in different animal models showed that overactivation of Panx1 correlates with a selective demise of several types of neurons, including retinal ganglion cells, brain pyramidal, and enteric neurons. The list of Panx1 activators includes extracellular ATP, glutamate, high K(+), Zn(2+), fibroblast growth factors (FGFs),pro-inflammatory cytokines, and elevation of intracellular Ca(2+). Most of these molecules are released following mechanical, ischemic, or inflammatory injury of the CNS, and rapidly activate the Panx1 channel. Prolonged opening of Panx1 channel induced by these "danger signals" triggers a cascade of neurotoxic events capable of killing cells. The most vulnerable cell type are neurons that express high levels of Panx1. Experimental evidence suggests that Panx1 channels mediate at least two distinct neurotoxic processes: increased permeability of the plasma membrane and activation of the inflammasome in neurons and glia. Importantly, both pharmacological and genetic inactivation of Panx1 suppresses both these processes, providing a marked protection in several disease and injury models. These findings indicate that external danger signals generated after diverse types of injuries converge to activate Panx1. In this review we discuss molecular mechanisms associated with Panx1 toxicity and the crosstalk between different pathways.

Regulation of pannexin channels by post-translational modifications

The large-pore channels formed by the pannexin family of proteins have been implicated in many physiological and pathophysiological functions, mainly through their ATP release function. However, a tight regulation of channel opening is necessary to modulate their function in vivo. Post-translational modifications have been postulated as some of the regulating mechanisms for Panx1, while Panx2 and Panx3 have not been as well characterized. Positive regulators include caspase cleavage to open Panx1 channels in apoptotic cells, and activation by Src family kinases via ionotropic receptors in neurons and macrophages. S-nitrosylation of cysteines has been shown to both inhibit and activate the Panx1 channel in different cell types. All three pannexins are N-glycosylated but to different levels of modification. Their diverse glycosylation appears to regulate cellular localization, intermixing, and may restrict their ability to function as inter-cellular channels. It is clear that our understanding of pannexin post-translational modification and their role in channel function regulation is still in its infancy even a decade after their discovery.

Pore positioning: current concepts in Pannexin channel trafficking

Pannexins (Panxs) are a multifaceted family of ion and metabolite channels that play key roles in a number of physiological and pathophysiological settings. These single membrane large-pore channels exhibit a variety of tissue, cell type, and subcellular distributions. The lifecycles of Panxs are complex, yet must be understood to accurately target these proteins for future therapeutic use. Here we review the basics of Panx function and localization, and then analyze the recent advances in knowledge regarding Panx trafficking. We examine several intrinsic features of Panxs including specific post-translational modifications, the divergent C-termini, and oligomerization, all of which contribute to Panx anterograde transport pathways. Further, we examine the potential influence of extrinsic factors, such as protein-protein interactions, on Panx trafficking. Finally, we highlight what is currently known with respect to Panx internalization and retrograde transport, and present new data illustrating Panx1 internalization following an activating stimulus.

Retinal characteristics of the congenital disorder of glycosylation PMM2-CDG

The congenital disorder of glycosylation, PMM2-CDG, is associated with progressive photoreceptor degeneration, which causes a pigmentary retinopathy. We identified a sibling pair, mildly affected with PMM2-CDG, who showed preserved photoreceptor function, but profound deficits of the 'on-pathway' in the retina. This localises the site of early, or initial, retinal dysfunction in PMM2-CDG to the synapse in the outer plexiform layer between bipolar cells, photoreceptors and horizontal cells. We sought wider evidence to support this novel finding by reviewing retrospectively the case notes of eight patients, diagnosed with PMM2-CDG between the ages of 7 months to 16 years. We compared the clinical presentation and electroretinograms, (ERGs), of these patients with the sibling pair. We found that five of eight patients showed characteristic ERG features of on-pathway dysfunction in the form of reduced ERG b-wave amplitude. The remaining three patients had significant photoreceptor dysfunction by the time of ERG recording, and both a- and b-wave amplitudes were markedly attenuated. We conclude that ERG signs of on-pathway dysfunction can be detected in the early stages of PMM2-CDG. Referral for electroretinography evidence of this specific on-pathway deficit, with preservation of oscillatory potentials, can help establish the diagnosis of infants with developmental delay or failure to thrive in whom a glycosylation defect is suspected. Also by increasing our understanding of the interaction of N-glycoproteins at this synapse we may be able to design future therapeutic intervention to prevent or ameliorate the progressive visual loss associated with PMM2-CDG.

Pannexin1 channel proteins in the zebrafish retina have shared and unique properties

In mammals, a single pannexin1 gene (Panx1) is widely expressed in the CNS including the inner and outer retinae, forming large-pore voltage-gated membrane channels, which are involved in calcium and ATP signaling. Previously, we discovered that zebrafish lack Panx1 expression in the inner retina, with drPanx1a exclusively expressed in horizontal cells of the outer retina. Here, we characterize a second drPanx1 protein, drPanx1b, generated by whole-genome duplications during teleost evolution. Homology searches strongly support the presence of pannexin sequences in cartilaginous fish and provide evidence that pannexins evolved when urochordata and chordata evolution split. Further, we confirm Panx1 ohnologs being solely present in teleosts. A hallmark of differential expression of drPanx1a and drPanx1b in various zebrafish brain areas is the non-overlapping protein localization of drPanx1a in the outer and drPanx1b in the inner fish retina. A functional comparison of the evolutionary distant fish and mouse Panx1s revealed both, preserved and unique properties. Preserved functions are the capability to form channels opening at resting potential, which are sensitive to known gap junction and hemichannel blockers, intracellular calcium, extracellular ATP and pH changes. However, drPanx1b is unique due to its highly complex glycosylation pattern and distinct electrophysiological gating kinetics. The existence of two Panx1 proteins in zebrafish displaying distinct tissue distribution, protein modification and electrophysiological properties, suggests that both proteins fulfill different functions in vivo.

From mechanosensitivity to inflammatory responses: new players in the pathology of glaucoma

PURPOSE OF THE STUDY

Many blinding diseases of the inner retina are associated with degeneration and loss of retinal ganglion cells (RGCs). Recent evidence implicates several new signaling mechanisms as causal agents associated with RGC injury and remodeling of the optic nerve head. Ion channels such as Transient receptor potential vanilloid isoform 4 (TRPV4), pannexin-1 (Panx1) and P2X7 receptor are localized to RGCs and act as potential sensors and effectors of mechanical strain, ischemia and inflammatory responses. Under normal conditions, TRPV4 may function as an osmosensor and a polymodal molecular integrator of diverse mechanical and chemical stimuli, whereas P2X7R and Panx1 respond to stretch- and/or swelling-induced adenosine triphosphate release from neurons and glia. Ca(2+) influx, induced by stimulation of mechanosensitive ion channels in glaucoma, is proposed to influence dendritic and axonal remodeling that may lead to RGC death while (at least initially) sparing other classes of retinal neuron. The secondary phase of the retinal glaucoma response is associated with microglial activation and an inflammatory response involving Toll-like receptors (TLRs), cluster of differentiation 3 (CD3) immune recognition molecules associated with the T-cell antigen receptor, complement molecules and cell type-specific release of neuroactive cytokines such as tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β). The retinal response to mechanical stress thus involves a diversity of signaling pathways that sense and transduce mechanical strain and orchestrate both protective and destructive secondary responses.

CONCLUSIONS

Mechanistic understanding of the interaction between pressure-dependent and independent pathways is only beginning to emerge. This review focuses on the molecular basis of mechanical strain transduction as a primary mechanism that can damage RGCs. The damage occurs through Ca(2+)-dependent cellular remodeling and is associated with parallel activation of secondary ischemic and inflammatory signaling pathways. Molecules that mediate these mechanosensory and immune responses represent plausible targets for protecting ganglion cells in glaucoma, optic neuritis and retinal ischemia.

Expression of Pannexin1 in the outer plexiform layer of the mouse retina and physiological impact of its knockout

Pannexin1 (Panx1) belongs to a class of vertebrate proteins that exhibits sequence homology to innexins, the invertebrate gap junction proteins, and which also shares topological similarities with vertebrate gap junction proteins, the connexins. Unlike gap junctional channels, Panx1 forms single-membrane channels, whose functional role in neuronal circuits is still unsettled. We therefore investigated the subcellular distribution of Panx1 in the mouse retina of wildtype and Panx1-null mice by reverse-transcription polymerase chain reaction (RT-PCR), immunohistochemistry, and electron microscopy. Use of Panx1-deficient mice served as a model to assess the physiological role of Panx1 by electroretinographic recordings and also to ensure the specificity of the anti-Panx1 antibody labeling. Expression of Panx1 was found in type 3a OFF bipolar cells and in dendrites and axonal processes of horizontal cells. Panx1 was also found in horizontal cell dendrites representing the lateral elements of the triad synapse at cone and rod terminals. In vivo electroretinography of Panx1 knockout mice indicated an increased a- and b-wave compared to Panx1 wildtype mice under scotopic conditions. The effect on the b-wave was confirmed by in vitro electroretinograms from the inner retina. These results suggest that Panx1 channels serve as sinks for extracellular current flow making them possible candidates for the mediation of feedback from horizontal cells to photoreceptors.

Regulation of connexin- and pannexin-based channels by post-translational modifications

Connexin (Cx) and pannexin (Panx) proteins form large conductance channels, which function as regulators of communication between neighbouring cells via gap junctions and/or hemichannels. Intercellular communication is essential to coordinate cellular responses in tissues and organs, thereby fulfilling an essential role in the spreading of signalling, survival and death processes. The functional properties of gap junctions and hemichannels are modulated by different physiological and pathophysiological stimuli. At the molecular level, Cxs and Panxs function as multi-protein channel complexes, regulating their channel localisation and activity. In addition to this, gap junctional channels and hemichannels are modulated by different post-translational modifications (PTMs), including phosphorylation, glycosylation, proteolysis, N-acetylation, S-nitrosylation, ubiquitination, lipidation, hydroxylation, methylation and deamidation. These PTMs influence almost all aspects of communicating junctional channels in normal cell biology and pathophysiology. In this review, we will provide a systematic overview of PTMs of communicating junction proteins and discuss their effects on Cx and Panx-channel activity and localisation.

The ATP required for potentiation of skeletal muscle contraction is released via pannexin hemichannels

During repetitive stimulation of skeletal muscle, extracellular ATP levels raise, activating purinergic receptors, increasing Ca2+ influx, and enhancing contractile force, a response called potentiation. We found that ATP appears to be released through pannexin1 hemichannels (Panx1 HCs). Immunocytochemical analyses and function were consistent with pannexin1 localization to T-tubules intercalated with dihydropyridine and ryanodine receptors in slow (soleus) and fast (extensor digitorum longus, EDL) muscles. Isolated myofibers took up ethidium (Etd+) and released small molecules (as ATP) during electrical stimulation. Consistent with two glucose uptake pathways, induced uptake of 2-NBDG, a fluorescent glucose derivative, was decreased by inhibition of HCs or glucose transporter (GLUT4), and blocked by dual blockade. Adult skeletal muscles apparently do not express connexins, making it unlikely that connexin hemichannels contribute to the uptake and release of small molecules. ATP release, Etd+ uptake, and potentiation induced by repetitive electrical stimulation were blocked by HC blockers and did not occur in muscles of pannexin1 knockout mice. MRS2179, a P2Y1R blocker, prevented potentiation in EDL, but not soleus muscles, suggesting that in fast muscles ATP activates P2Y1 but not P2X receptors. Phosphorylation on Ser and Thr residues of pannexin1 was increased during potentiation, possibly mediating HC opening. Opening of Panx1 HCs during repetitive activation allows efflux of ATP, influx of glucose and possibly Ca2+ too, which are required for potentiation of contraction. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

A comparative antibody analysis of pannexin1 expression in four rat brain regions reveals varying subcellular localizations

Pannexin1 (Panx1) channels release cytosolic ATP in response to signaling pathways. Panx1 is highly expressed in the central nervous system. We used four antibodies with different Panx1 anti-peptide epitopes to analyze four regions of rat brain. These antibodies labeled the same bands in Western blots and had highly similar patterns of immunofluorescence in tissue culture cells expressing Panx1, but Western blots of brain lysates from Panx1 knockout and control mice showed different banding patterns. Localizations of Panx1 in brain slices were generated using automated wide field mosaic confocal microscopy for imaging large regions of interest while retaining maximum resolution for examining cell populations and compartments. We compared Panx1 expression over the cerebellum, hippocampus with adjacent cortex, thalamus, and olfactory bulb. While Panx1 localizes to the same neuronal cell types, subcellular localizations differ. Two antibodies with epitopes against the intracellular loop and one against the carboxy terminus preferentially labeled cell bodies, while an antibody raised against an N-terminal peptide highlighted neuronal processes more than cell bodies. These labeling patterns may be a reflection of different cellular and subcellular localizations of full-length and/or modified Panx1 channels where each antibody is highlighting unique or differentially accessible Panx1 populations. However, we cannot rule out that one or more of these antibodies have specificity issues. All data associated with experiments from these four antibodies are presented in a manner that allows them to be compared and our claims thoroughly evaluated, rather than eliminating results that were questionable. Each antibody is given a unique identifier through the NIF Antibody Registry that can be used to track usage of individual antibodies across papers and all image and metadata are made available in the public repository, the Cell Centered Database, for on-line viewing, and download.

Pannexin1 and Pannexin3 exhibit distinct localization patterns in human skin appendages and are regulated during keratinocyte differentiation and carcinogenesis

Having shown that Panx1 and Panx3 are expressed in the epidermis, we investigated their distribution in human skin adnexal structures and skin cancer. Both proteins were found in hair follicles, sebaceous and eccrine glands, as well as blood vessels. Panx1 was detected as punctate or diffuse intracellular labeling, while Panx3 was only observed as diffuse intracellular staining, suggesting different functions. We also identified the Panx3 immunoreactive ~70 kD species modulated during keratinocyte differentiation as Panx3. Since our data indicate that pannexins are regulated during keratinocyte differentiation, we assessed whether their levels are altered under circumstances in which keratinocyte differentiation is compromised. We found that Panx1 and Panx3 levels are highly reduced in human keratinocyte tumors, thus showing for the first time that both pannexins are dysregulated in human cancers. Altogether, these data suggest that Panx1 and Panx3 have distinct and unique functions within the skin in health and disease.

Connexin- and pannexin-based channels in normal skeletal muscles and their possible role in muscle atrophy

Precursor cells of skeletal muscles express connexins 39, 43 and 45 and pannexin1. In these cells, most connexins form two types of membrane channels, gap junction channels and hemichannels, whereas pannexin1 forms only hemichannels. All these channels are low-resistance pathways permeable to ions and small molecules that coordinate developmental events. During late stages of skeletal muscle differentiation, myofibers become innervated and stop expressing connexins but still express pannexin1 hemichannels that are potential pathways for the ATP release required for potentiation of the contraction response. Adult injured muscles undergo regeneration, and connexins are reexpressed and form membrane channels. In vivo, connexin reexpression occurs in undifferentiated cells that form new myofibers, favoring the healing process of injured muscle. However, differentiated myofibers maintained in culture for 48 h or treated with proinflammatory cytokines for less than 3 h also reexpress connexins and only form functional hemichannels at the cell surface. We propose that opening of these hemichannels contributes to drastic changes in electrochemical gradients, including reduction of membrane potential, increases in intracellular free Ca(2+) concentration and release of diverse metabolites (e.g., NAD(+) and ATP) to the extracellular milieu, contributing to multiple metabolic and physiologic alterations that characterize muscles undergoing atrophy in several acquired and genetic human diseases. Consequently, inhibition of connexin hemichannels expressed by injured or denervated skeletal muscles might reduce or prevent deleterious changes triggered by conditions that promote muscle atrophy.

Neurons respond directly to mechanical deformation with pannexin-mediated ATP release and autostimulation of P2X7 receptors

Mechanical deformation produces complex effects on neuronal systems, some of which can lead to dysfunction and neuronal death. While astrocytes are known to respond to mechanical forces, it is not clear whether neurons can also respond directly. We examined mechanosensitive ATP release and the physiological response to this release in isolated retinal ganglion cells. Purified ganglion cells released ATP upon swelling. Release was blocked by carbenoxolone, probenecid or peptide (10)panx, implicating pannexin channels as conduits. Mechanical stretch of retinal ganglion cells also triggered a pannexin-dependent ATP release. Whole cell patch clamp recording demonstrated that mild swelling induced the activation of an Ohmic cation current with linear kinetics. The current was inhibited by removal of extracellular ATP with apyrase, by inhibition of the P2X(7) receptor with A438079, zinc, or AZ 10606120, and by pannexin blockers carbenoxolone and probenecid. Probenecid also inhibited the regulatory volume decrease observed after swelling isolated neurons. Together, these observations indicate mechanical strain triggers ATP release directly from retinal ganglion cells and that this released ATP autostimulates P2X(7) receptors. Since extracellular ATP levels in the retina increase with elevated intraocular pressure, and stimulation of P2X(7) receptors on retinal ganglion cells can be lethal, this autocrine response may impact ganglion cells in glaucoma. It remains to be determined whether the autocrine stimulation of purinergic receptors is a general response to a mechanical deformation in neurons, or whether preventing ATP release through pannexin channels and blocking activation of the P2X(7) receptor, is neuroprotective for stretched neurons.

Genetic ablation of Pannexin1 protects retinal neurons from ischemic injury

Pannexin1 (Panx1) forms large nonselective membrane channel that is implicated in paracrine and inflammatory signaling. In vitro experiments suggested that Panx1 could play a key role in ischemic death of hippocampal neurons. Since retinal ganglion cells (RGCs) express high levels of Panx1 and are susceptible to ischemic induced injury, we hypothesized that Panx1 contributes to rapid and selective loss of these neurons in ischemia. To test this hypothesis, we induced experimental retinal ischemia followed by reperfusion in live animals with the Panx1 channel genetically ablated either in the entire mouse (Panx1 KO), or only in neurons using the conditional knockout (Panx1 CKO) technology. Here we report that two distinct neurotoxic processes are induced in RGCs by ischemia in the wild type mice but are inactivated in Panx1KO and Panx1 CKO animals. First, the post-ischemic permeation of RGC plasma membranes is suppressed, as assessed by dye transfer and calcium imaging assays ex vivo and in vitro. Second, the inflammasome-mediated activation of caspase-1 and the production of interleukin-1β in the Panx1 KO retinas are inhibited. Our findings indicate that post-ischemic neurotoxicity in the retina is mediated by previously uncharacterized pathways, which involve neuronal Panx1 and are intrinsic to RGCs. Thus, our work presents the in vivo evidence for neurotoxicity elicited by neuronal Panx1, and identifies this channel as a new therapeutic target in ischemic pathologies.

Pannexin protein expression in the rat middle cerebral artery

BACKGROUND

Connexin proteins are well known to participate in cell-to-cell communication within the cerebral vasculature. Pannexins are a recently discovered family of proteins that could potentially be involved in cell-to-cell communication. Herein, we sought to determine whether pannexins are expressed in rat middle cerebral artery (MCA).

METHODS

A combination of RT-PCR, immunoblotting and immunohistochemistry techniques was used to characterize the expression pattern of pannexins in rat MCA. A fluorescent dye uptake approach in cultured smooth muscle cells was used to determine whether these cells have functional hemichannels.

RESULTS

We report for the first time that pannexins are expressed in the cerebral vasculature. We reveal that pannexin 1 is expressed in smooth muscle but not in endothelium and pannexin 2 is expressed in both endothelium and smooth muscle. Fluorescent dye entered cultured smooth muscle cells in the absence of extracellular calcium or when the cells were depolarized, which was prevented by the putative hemichannel blocker carbenoxolone.

CONCLUSIONS

The identification of pannexins in rat MCA indicates that pannexin expression is not restricted to neuronal cells. Dye uptake in cultured smooth muscle cells exhibited properties similar to those of connexin and pannexin hemichannels, which may represent another form of cell-to-cell communication within the vasculature.