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Pannexin- and Connexin-Based Channels under Normal and Pathological Conditions

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A special issue of International Journal of Molecular Sciences (ISSN 1422-0067). This special issue belongs to the section "Biochemistry".

Deadline for manuscript submissions: closed (30 November 2020) | Viewed by 43493

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Special Issue Editors

Prof. Dr. Juan Carlos Saez

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Guest Editor

1. Centro Interdisciplinario de Neurociencia de Valparaíso CINV, Instituto de Neurociencia, Facultad de Ciencias, Universidad de Valparaíso, Chile
2. Alameda 340, Laboratorio de Fisiología, Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile, Santiago, Chile
Interests: Regulation and function of connexin- and pannexin-based channels
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Jean Jiang

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Department of Biochemistry and Structural Biology, University of Texas Health Science Center, San Antonio, TX 78229, USA
Interests: gap junctions and hemichannels; connexins; signaling transmission; breast cancer; osteosarcoma; metastasis
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Connexins and pannexins form non-selective membrane channels in most cell types. Few examples present only connexins or pannexins. Both channels are permeable to ions and small molecules and their expression and/or activity are elevated under physiological and pathological conditions. In several cases the first sign of an abnormal condition is the permeabilization of the cell membrane and reduction in membrane potential. To a great extent, these changes are explained by the upregulation or de novo expression of connexins and/or pannexins. Thus, channels formed by these proteins are potential therapeutic targets. In this Special Issue the role of these channels in the etiopathogenesis of several diseases is presented.

Prof. Dr. Juan Carlos Saez
Prof. Dr. Jean X. Jiang
Guest Editors

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Keywords

Non-selective channels in diseases

Published Papers (13 papers)

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Research

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18 pages, 3239 KiB

Open AccessArticle

Restraint of Human Skin Fibroblast Motility, Migration, and Cell Surface Actin Dynamics, by Pannexin 1 and P2X7 Receptor Signaling

by Carolina Flores-Muñoz, Jaime Maripillán, Jacqueline Vásquez-Navarrete, Joel Novoa-Molina, Ricardo Ceriani, Helmuth A. Sánchez, Ana C. Abbott, Caroline Weinstein-Oppenheimer, Donald I. Brown, Ana María Cárdenas, Isaac E. García and Agustín D. Martínez

Int. J. Mol. Sci. 2021, 22(3), 1069; https://doi.org/10.3390/ijms22031069 - 22 Jan 2021

Cited by 11 | Viewed by 3653

Abstract

Wound healing is a dynamic process required to maintain skin integrity and which relies on the precise migration of different cell types. A key molecule that regulates this process is ATP. However, the mechanisms involved in extracellular ATP management are poorly understood, particularly in the human dermis. Here, we explore the role, in human fibroblast migration during wound healing, of Pannexin 1 channels and their relationship with purinergic signals and in vivo cell surface filamentous actin dynamics. Using siRNA against Panx isoforms and different Panx1 channel inhibitors, we demonstrate in cultured human dermal fibroblasts that the absence or inhibition of Panx1 channels accelerates cell migration, increases single-cell motility, and promotes actin redistribution. These changes occur through a mechanism that involves the release of ATP to the extracellular space through a Panx1-dependent mechanism and the activation of the purinergic receptor P2X7. Together, these findings point to a pivotal role of Panx1 channels in skin fibroblast migration and suggest that these channels could be a useful pharmacological target to promote damaged skin healing. Full article

(This article belongs to the Special Issue Pannexin- and Connexin-Based Channels under Normal and Pathological Conditions)

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17 pages, 2017 KiB

Open AccessArticle

Stretch-Induced Activation of Pannexin 1 Channels Can Be Prevented by PKA-Dependent Phosphorylation

by Ximena López, Rosalba Escamilla, Paola Fernández, Yorley Duarte, Fernando González-Nilo, Nicolás Palacios-Prado, Agustín D. Martinez and Juan C. Sáez

Int. J. Mol. Sci. 2020, 21(23), 9180; https://doi.org/10.3390/ijms21239180 - 2 Dec 2020

Cited by 19 | Viewed by 2571

Abstract

Pannexin 1 channels located in the cell membrane are permeable to ions, metabolites, and signaling molecules. While the activity of these channels is known to be modulated by phosphorylation on T198, T308, and S206, the possible involvement of other putative phosphorylation sites remains unknown. Here, we describe that the activity of Panx1 channels induced by mechanical stretch is reduced by adenosine via a PKA-dependent pathway. The mechanical stretch-induced activity—measured by changes in DAPI uptake—of Panx1 channels expressed in HeLa cell transfectants was inhibited by adenosine or cAMP analogs that permeate the cell membrane. Moreover, inhibition of PKA but not PKC, p38 MAPK, Akt, or PKG prevented the effects of cAMP analogs, suggesting the involvement of Panx1 phosphorylation by PKA. Accordingly, alanine substitution of T302 or S328, two putative PKA phosphorylation sites, prevented the inhibitory effect of cAMP analogs. Moreover, phosphomimetic mutation of either T302 or S328 to aspartate prevented the mechanical stretch-induced activation of Panx1 channels. A molecular dynamics simulation revealed that T302 and S328 are located in the water–lipid interphase near the lateral tunnel of the intracellular region, suggesting that their phosphorylation could promote conformational changes in lateral tunnels. Thus, Panx1 phosphorylation via PKA could be modulated by G protein-coupled receptors associated with the Gs subunit. Full article

(This article belongs to the Special Issue Pannexin- and Connexin-Based Channels under Normal and Pathological Conditions)

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17 pages, 2817 KiB

Open AccessArticle

Blockade of Hemichannels Normalizes the Differentiation Fate of Myoblasts and Features of Skeletal Muscles from Dysferlin-Deficient Mice

by Luis A. Cea, Gabriela Fernández, Guisselle Arias-Bravo, Mario Castillo-Ruiz, Rosalba Escamilla, María C. Brañes and Juan C. Sáez

Int. J. Mol. Sci. 2020, 21(17), 6025; https://doi.org/10.3390/ijms21176025 - 21 Aug 2020

Cited by 9 | Viewed by 2543

Abstract

Dysferlinopathies are muscle dystrophies caused by mutations in the gene encoding dysferlin, a relevant protein for membrane repair and trafficking. These diseases are untreatable, possibly due to the poor knowledge of relevant molecular targets. Previously, we have shown that human myofibers from patient biopsies as well as myotubes derived from immortalized human myoblasts carrying a mutated form of dysferlin express connexin proteins, but their relevance in myoblasts fate and function remained unknown. In the present work, we found that numerous myoblasts bearing a mutated dysferlin when induced to acquire myogenic commitment express PPARγ, revealing adipogenic instead of myogenic commitment. These cell cultures presented many mononucleated cells with fat accumulation and within 48 h of differentiation formed fewer multinucleated cells. In contrast, dysferlin deficient myoblasts treated with boldine, a connexin hemichannels blocker, neither expressed PPARγ, nor accumulated fat and formed similar amount of multinucleated cells as wild type precursor cells. We recently demonstrated that myofibers of skeletal muscles from blAJ mice (an animal model of dysferlinopathies) express three connexins (Cx39, Cx43, and Cx45) that form functional hemichannels (HCs) in the sarcolemma. In symptomatic blAJ mice, we now show that eight-week treatment with a daily dose of boldine showed a progressive recovery of motor activity reaching normality. At the end of this treatment, skeletal muscles were comparable to those of wild type mice and presented normal CK activity in serum. Myofibers of boldine-treated blAJ mice also showed strong dysferlin-like immunoreactivity. These findings reveal that muscle dysfunction results from a pathophysiologic mechanism triggered by mutated dysferlin and downstream connexin hemichannels expressed de novo lead to a drastic reduction of myogenesis and favor muscle damage. Thus, boldine could represent a therapeutic opportunity to treat dysfernilopathies. Full article

(This article belongs to the Special Issue Pannexin- and Connexin-Based Channels under Normal and Pathological Conditions)

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22 pages, 4706 KiB

Open AccessArticle

Apocynin Treatment Prevents Cardiac Connexin 43 Hemichannels Hyperactivity by Reducing Nitroso-Redox Stress in Mdx Mice

by Alejandra Z. Vielma, Mauricio P. Boric and Daniel R. Gonzalez

Int. J. Mol. Sci. 2020, 21(15), 5415; https://doi.org/10.3390/ijms21155415 - 30 Jul 2020

Cited by 14 | Viewed by 3476

Abstract

Duchenne muscular dystrophy (DMD) is a fatal disease that causes cardiomyopathy and is associated with oxidative stress. In the heart, oxidative stress interferes with the location of connexin 43 (Cx43) to the intercalated discs causing its lateralization to the plasma membrane where Cx43 forms hemichannels. We tested the hypothesis that in DMD cardiomyopathy, increased oxidative stress is associated with the formation and activation of Cx43 hemichannels. For this, we used mdx mice as a DMD model and evaluated cardiac function, nitroso-redox changes and Cx43 hemichannels permeability. Mdx hearts presented increased NADPH oxidase-derived oxidative stress and increased Cx43 S-nitrosylation compared to controls. These redox changes were associated with increased Cx43 lateralization, decreased cardiac contractility and increased arrhythmic events. Pharmacological inhibition of NADPH oxidase using apocynin (one month) reduced systemic oxidative stress and reversed the aforementioned changes towards normal, except Cx43 lateralization. Opening of Cx43 hemichannels was blocked by apocynin treatment and by acute hemichannel blockade with carbenoxolone. NADPH oxidase inhibition also prevented the occurrence of apoptosis in mdx hearts and reversed the ventricular remodeling. These results show that NADPH oxidase activity in DMD is associated with S-nitrosylation and opening of Cx43 hemichannels. These changes lead to apoptosis and cardiac dysfunction and were prevented by NADPH oxidase inhibition. Full article

(This article belongs to the Special Issue Pannexin- and Connexin-Based Channels under Normal and Pathological Conditions)

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9 pages, 2141 KiB

Open AccessCommunication

Cx43 in Neural Progenitors Promotes Glioma Invasion in a 3D Culture System

by Kanika Khosla, Christian C. Naus and Wun Chey Sin

Int. J. Mol. Sci. 2020, 21(15), 5216; https://doi.org/10.3390/ijms21155216 - 23 Jul 2020

Cited by 7 | Viewed by 2188

Abstract

The environment that envelops the cancer cells intimately affects the malignancy of human cancers. In the case of glioma, an aggressive adult brain cancer, its high rate of recurrence after total resection is responsible for a poor prognosis. Connexin43 (Cx43) is a gap junction protein with a prominent presence in glioma-associated normal brain cells, specifically in the reactive astrocytes. We previously demonstrated that elimination of Cx43 in these astrocytes reduces glioma invasion in a syngeneic mouse model. To further our investigation in human glioma cells, we developed a scaffold-free 3D platform that takes into account both the tumor and its interaction with the surrounding tissue. Using cell-tracking dyes and 3D laser scanning confocal microscopy, we now report that the elimination of Cx43 protein in neural progenitor spheroids reduced the invasiveness of human brain tumor-initiating cells, confirming our earlier observation in an intact mouse brain. By investigating the glioma invasion in a defined multicellular system with a tumor boundary that mimics the intact brain environment, our findings strengthen Cx43 as a candidate target for glioma control. Full article

(This article belongs to the Special Issue Pannexin- and Connexin-Based Channels under Normal and Pathological Conditions)

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14 pages, 1945 KiB

Open AccessArticle

Connexin Hemichannels Contribute to the Activation of cAMP Signaling Pathway and Renin Production

by Jingru Hong and Jian Yao

Int. J. Mol. Sci. 2020, 21(12), 4462; https://doi.org/10.3390/ijms21124462 - 23 Jun 2020

Cited by 2 | Viewed by 2094

Abstract

Connexin hemichannels play an important role in the control of cellular signaling and behaviors. Given that lowering extracellular Ca²⁺, a condition that activates hemichannels, is a well-characterized stimulator of renin in juxtaglomerular cells, we, therefore, tested a potential implication of hemichannels in the regulation of renin in As4.1 renin-secreting cells. Lowering extracellular Ca²⁺ induced hemichannel opening, which was associated with cAMP signaling pathway activation and increased renin production. Blockade of hemichannels with inhibitors or downregulation of Cxs with siRNAs abrogated the activation of cAMP pathway and the elevation of renin. Further analysis revealed that cAMP pathway activation was blocked by adenylyl cyclase inhibitor SQ 22536, suggesting an implication of adenyl cyclase. Furthermore, the participation of hemichannels in the activation of the cAMP signaling pathway was also observed in a renal tubular epithelial cell line NRK. Collectively, our results characterized the hemichannel opening as a presently unrecognized molecular event involved in low Ca²⁺-elicited activation of cAMP pathway and renin production. Our findings thus provide novel mechanistic insights into the low Ca²⁺-initiated cell responses. Given the importance of cAMP signaling pathway in the control of multiple cellular functions, our findings also highlight the importance of Cx-forming channels in various pathophysiological situations. Full article

(This article belongs to the Special Issue Pannexin- and Connexin-Based Channels under Normal and Pathological Conditions)

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19 pages, 6814 KiB

Open AccessArticle

Vitamin E Blocks Connexin Hemichannels and Prevents Deleterious Effects of Glucocorticoid Treatment on Skeletal Muscles

by Elisa Balboa, Fujiko Saavedra, Luis A. Cea, Valeria Ramírez, Rosalba Escamilla, Aníbal A. Vargas, Tomás Regueira and Juan C. Sáez

Int. J. Mol. Sci. 2020, 21(11), 4094; https://doi.org/10.3390/ijms21114094 - 8 Jun 2020

Cited by 14 | Viewed by 3197

Abstract

Glucocorticoids are frequently used as anti-inflammatory and immunosuppressive agents. However, high doses and/or prolonged use induce undesired secondary effects such as muscular atrophy. Recently, de novo expression of connexin43 and connexin45 hemichannels (Cx43 HCs and Cx45 HCs, respectively) has been proposed to play a critical role in the mechanism underlying myofiber atrophy induced by dexamethasone (Dex: a synthetic glucocorticoid), but their involvement in specific muscle changes promoted by Dex remains poorly understood. Moreover, treatments that could prevent the undesired effects of glucocorticoids on skeletal muscles remain unknown. In the present work, a 7-day Dex treatment in adult mice was found to induce weight loss and skeletal muscle changes including expression of functional Cx43/Cx45 HCs, elevated atrogin immunoreactivity, atrophy, oxidative stress and mitochondrial dysfunction. All these undesired effects were absent in muscles of mice simultaneously treated with Dex and vitamin E (VitE). Moreover, VitE was found to rapidly inhibit the activity of Cx HCs in freshly isolated myofibers of Dex treated mice. Exposure to alkaline pH induced free radical generation only in HeLa cells expressing Cx43 or Cx45 where Ca²⁺ was present in the extracellular milieu, response that was prevented by VitE. Besides, VitE and two other anti-oxidant compounds, Tempol and Resveratrol, were found to inhibit Cx43 HCs in HeLa cells transfectants. Thus, we propose that in addition to their intrinsic anti-oxidant potency, some antioxidants could be used to reduce expression and/or opening of Cx HCs and consequently reduce the undesired effect of glucocorticoids on skeletal muscles. Full article

(This article belongs to the Special Issue Pannexin- and Connexin-Based Channels under Normal and Pathological Conditions)

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22 pages, 3988 KiB

Open AccessArticle

Permeation of Molecules through Astroglial Connexin 43 Hemichannels Is Modulated by Cytokines with Parameters Depending on the Permeant Species

by Juan C. Sáez, Aníbal A. Vargas, Diego E. Hernández, Fernando C. Ortiz, Christian Giaume and Juan A. Orellana

Int. J. Mol. Sci. 2020, 21(11), 3970; https://doi.org/10.3390/ijms21113970 - 1 Jun 2020

Cited by 12 | Viewed by 2656

Abstract

Recent studies indicate that connexin hemichannels do not act as freely permeable non-selective pores, but they select permeants in an isoform-specific manner with cooperative, competitive and saturable kinetics. The aim of this study was to investigate whether the treatment with a mixture of IL-1β plus TNF-α, a well-known pro-inflammatory condition that activates astroglial connexin 43 (Cx43) hemichannels, could alter their permeability to molecules. We found that IL-1β plus TNF-α left-shifted the dye uptake rate vs. dye concentration relationship for Etd and 2-NBDG, but the opposite took place for DAPI or YO-PRO-1, whereas no alterations were observed for Prd. The latter modifications were accompanied of changes in K_d (Etd, DAPI, YO-PRO-1 or 2-NBDG) and Hill coefficients (Etd and YO-PRO-1), but not in alterations of V_max. We speculate that IL-1β plus TNF-α may distinctively affect the binding sites to permeants in astroglial Cx43 hemichannels rather than their number in the cell surface. Alternatively, IL-1β plus TNF-α could induce the production of endogenous permeants that may favor or compete for in the pore-lining residues of Cx43 hemichannels. Future studies shall elucidate whether the differential ionic/molecule permeation of Cx43 hemichannels in astrocytes could impact their communication with neurons in the normal and inflamed nervous system. Full article

(This article belongs to the Special Issue Pannexin- and Connexin-Based Channels under Normal and Pathological Conditions)

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23 pages, 10574 KiB

Open AccessArticle

HIV gp120 Protein Increases the Function of Connexin 43 Hemichannels and Pannexin-1 Channels in Astrocytes: Repercussions on Astroglial Function

by Rosario Gajardo-Gómez, Cristian A. Santibañez, Valeria C. Labra, Gonzalo I. Gómez, Eliseo A. Eugenin and Juan A. Orellana

Int. J. Mol. Sci. 2020, 21(7), 2503; https://doi.org/10.3390/ijms21072503 - 3 Apr 2020

Cited by 21 | Viewed by 3156

Abstract

At least half of human immunodeficiency virus (HIV)-infected individuals suffer from a wide range of cognitive, behavioral and motor deficits, collectively known as HIV-associated neurocognitive disorders (HAND). The molecular mechanisms that amplify damage within the brain of HIV-infected individuals are unknown. Recently, we described that HIV augments the opening of connexin-43 (Cx43) hemichannels in cultured human astrocytes, which result in the collapse of neuronal processes. Whether HIV soluble viral proteins such as gp120, can regulate hemichannel opening in astrocytes is still ignored. These channels communicate the cytosol with the extracellular space during pathological conditions. We found that gp120 enhances the function of both Cx43 hemichannels and pannexin-1 channels in mouse cortical astrocytes. These effects depended on the activation of IL-1β/TNF-α, p38 MAP kinase, iNOS, cytoplasmic Ca²⁺ and purinergic signaling. The gp120-induced channel opening resulted in alterations in Ca²⁺ dynamics, nitric oxide production and ATP release. Although the channel opening evoked by gp120 in astrocytes was reproduced in ex vivo brain preparations, these responses were heterogeneous depending on the CA1 region analyzed. We speculate that soluble gp120-induced activation of astroglial Cx43 hemichannels and pannexin-1 channels could be crucial for the pathogenesis of HAND. Full article

(This article belongs to the Special Issue Pannexin- and Connexin-Based Channels under Normal and Pathological Conditions)

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Review

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19 pages, 1272 KiB

Open AccessReview

Mast Cell and Astrocyte Hemichannels and Their Role in Alzheimer’s Disease, ALS, and Harmful Stress Conditions

by Paloma A. Harcha, Polett Garcés, Cristian Arredondo, Germán Fernández, Juan C. Sáez and Brigitte van Zundert

Int. J. Mol. Sci. 2021, 22(4), 1924; https://doi.org/10.3390/ijms22041924 - 15 Feb 2021

Cited by 21 | Viewed by 5532

Abstract

Considered relevant during allergy responses, numerous observations have also identified mast cells (MCs) as critical effectors during the progression and modulation of several neuroinflammatory conditions, including Alzheimer’s disease (AD) and amyotrophic lateral sclerosis (ALS). MC granules contain a plethora of constituents, including growth factors, cytokines, chemokines, and mitogen factors. The release of these bioactive substances from MCs occurs through distinct pathways that are initiated by the activation of specific plasma membrane receptors/channels. Here, we focus on hemichannels (HCs) formed by connexins (Cxs) and pannexins (Panxs) proteins, and we described their contribution to MC degranulation in AD, ALS, and harmful stress conditions. Cx/Panx HCs are also expressed by astrocytes and are likely involved in the release of critical toxic amounts of soluble factors—such as glutamate, adenosine triphosphate (ATP), complement component 3 derivate C3a, tumor necrosis factor (TNFα), apoliprotein E (ApoE), and certain miRNAs—known to play a role in the pathogenesis of AD, ALS, and other neurodegenerative disorders. We propose that blocking HCs on MCs and glial cells offers a promising novel strategy for ameliorating the progression of neurodegenerative diseases by reducing the release of cytokines and other pro-inflammatory compounds. Full article

(This article belongs to the Special Issue Pannexin- and Connexin-Based Channels under Normal and Pathological Conditions)

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22 pages, 4759 KiB

Open AccessReview

Cardiac Connexin-43 Hemichannels and Pannexin1 Channels: Provocative Antiarrhythmic Targets

by Katarina Andelova, Tamara Egan Benova, Barbara Szeiffova Bacova, Matus Sykora, Natalia Jorgelina Prado, Emiliano Raul Diez, Peter Hlivak and Narcis Tribulova

Int. J. Mol. Sci. 2021, 22(1), 260; https://doi.org/10.3390/ijms22010260 - 29 Dec 2020

Cited by 35 | Viewed by 3829

Abstract

Cardiac connexin-43 (Cx43) creates gap junction channels (GJCs) at intercellular contacts and hemi-channels (HCs) at the peri-junctional plasma membrane and sarcolemmal caveolae/rafts compartments. GJCs are fundamental for the direct cardiac cell-to-cell transmission of electrical and molecular signals which ensures synchronous myocardial contraction. The HCs and structurally similar pannexin1 (Panx1) channels are active in stressful conditions. These channels are essential for paracrine and autocrine communication through the release of ions and signaling molecules to the extracellular environment, or for uptake from it. The HCs and Panx1 channel-opening profoundly affects intracellular ionic homeostasis and redox status and facilitates via purinergic signaling pro-inflammatory and pro-fibrotic processes. These conditions promote cardiac arrhythmogenesis due to the impairment of the GJCs and selective ion channel function. Crosstalk between GJCs and HCs/Panx1 channels could be crucial in the development of arrhythmogenic substrates, including fibrosis. Despite the knowledge gap in the regulation of these channels, current evidence indicates that HCs and Panx1 channel activation can enhance the risk of cardiac arrhythmias. It is extremely challenging to target HCs and Panx1 channels by inhibitory agents to hamper development of cardiac rhythm disorders. Progress in this field may contribute to novel therapeutic approaches for patients prone to develop atrial or ventricular fibrillation. Full article

(This article belongs to the Special Issue Pannexin- and Connexin-Based Channels under Normal and Pathological Conditions)

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10 pages, 2270 KiB

Open AccessReview

Connexin Mutants Compromise the Lens Circulation and Cause Cataracts through Biomineralization

by Viviana M. Berthoud, Junyuan Gao, Peter J. Minogue, Oscar Jara, Richard T. Mathias and Eric C. Beyer

Int. J. Mol. Sci. 2020, 21(16), 5822; https://doi.org/10.3390/ijms21165822 - 13 Aug 2020

Cited by 31 | Viewed by 2851

Abstract

Gap junction-mediated intercellular communication facilitates the circulation of ions, small molecules, and metabolites in the avascular eye lens. Mutants of the lens fiber cell gap junction proteins, connexin46 (Cx46) and connexin50 (Cx50), cause cataracts in people and in mice. Studies in mouse models have begun to elucidate the mechanisms by which these mutants lead to cataracts. The expression of the dominant mutants causes severe decreases in connexin levels, reducing the gap junctional communication between lens fiber cells and compromising the lens circulation. The impairment of the lens circulation results in several changes, including the accumulation of Ca²⁺ in central lens regions, leading to the formation of precipitates that stain with Alizarin red. The cataract morphology and the distribution of Alizarin red-stained material are similar, suggesting that the cataracts result from biomineralization within the organ. In this review, we suggest that this may be a general process for the formation of cataracts of different etiologies. Full article

(This article belongs to the Special Issue Pannexin- and Connexin-Based Channels under Normal and Pathological Conditions)

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12 pages, 1145 KiB

Open AccessReview

The Role of Connexin Channels in the Response of Mechanical Loading and Unloading of Bone

by Manuel A. Riquelme, Eduardo R. Cardenas, Huiyun Xu and Jean X. Jiang

Int. J. Mol. Sci. 2020, 21(3), 1146; https://doi.org/10.3390/ijms21031146 - 9 Feb 2020

Cited by 36 | Viewed by 4830

Abstract

The skeleton adapts to mechanical loading to promote bone formation and remodeling. While most bone cells are involved in mechanosensing, it is well accepted that osteocytes are the principal mechanosensory cells. The osteocyte cell body and processes are surrounded by a fluid-filled space, forming an extensive lacuno-canalicular network. The flow of interstitial fluid is a major stress-related factor that transmits mechanical stimulation to bone cells. The long dendritic processes of osteocytes form a gap junction channel network connecting not only neighboring osteocytes, but also cells on the bone surface, such as osteoblasts and osteoclasts. Mechanosensitive osteocytes also form hemichannels that mediate the communication between the cytoplasmic and extracellular microenvironment. This paper will discuss recent research progress regarding connexin (Cx)-forming gap junctions and hemichannels in osteocytes, osteoblasts, and other bone cells, including those richly expressing Cx43. We will then cover the recent progress regarding the regulation of these channels by mechanical loading and the role of integrins and signals in mediating Cx43 channels, and bone cell function and viability. Finally, we will summarize the recent studies regarding bone responses to mechanical unloading in Cx43 transgenic mouse models. The osteocyte has been perceived as the center of bone remodeling, and connexin channels enriched in osteocytes are a likely major player in meditating the function of bone. Based on numerous studies, connexin channels may present as a potential new therapeutic target in the treatment of bone loss and osteoporosis. This review will primarily focus on Cx43, with some discussion in other connexins expressed in bone cells. Full article

(This article belongs to the Special Issue Pannexin- and Connexin-Based Channels under Normal and Pathological Conditions)

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