ReviewAstrocytes in neuroprotection and neurodegeneration: The role of connexin43 and pannexin1
Introduction
Through evolutionary processes expansion from single-celled to multi-cellular organisms enabled the development of complex tissues and specialized organs. Intercellular channels called gap junctions (GJs) may have played a critical role in this process (Trosko, 2011), allowing cells to directly communicate with neighboring cells as a functioning syncytium. All multicellular animals, from hydra to humans, develop GJs (Fushiki et al., 2010). GJs consist of proteins called connexins (Cxs); 20–21 isoforms have been identified in vertebrates, and eleven of these are expressed in the vertebrate brain (Dermietzel et al., 1989, Bennett et al., 1991, Rash et al., 2001a, Rash et al., 2001b, Theis and Giaume, 2012). Cx isoforms are typically designated according to their respective molecular weights (Goodenough and Paul, 2009). Individual Cxs assemble into hexamers around a central pore to form transmembrane channels, termed connexons, which then couple with apposing connexons on neighboring cells and coalesce into dense GJ plaques that may contain thousands of channels (Fig. 1) (Unwin and Zampighi, 1980). These channels directly bridge the cytoplasm between coupled cells and permit the movement of ions and low molecular weight molecules (about 1–2 kDa) to neighboring cells (Loewenstein, 1981). In addition to GJ channels, connexons exist on their own as single membrane hemichannels that under certain conditions (primarily pathological) directly connect the cell cytoplasm to the extracellular milieu (Orellana et al., 2010, Orellana et al., 2011b, Giaume et al., 2013). However, Cx channels are not simply passive conduits through which molecules and ions move. For instance, GJ channels are gated by cell voltage and display several voltage-dependent conductance states (Goodenough and Paul, 2009). Furthermore, several groups have shown that Cxs interact with a diverse group of molecular factors, and participate in a host of physiological processes (Naus et al., 1991b, Nakase and Naus, 2004, Sin et al., 2008, Orellana et al., 2009, Abrams and Scherer, 2011, Rodriguez-Sinovas et al., 2011, Bond et al., 2012, Matsuuchi and Naus, 2012, Shao et al., 2012, Chen et al., 2013).
The discovery of another family of membrane channels, the pannexins (Panxs) (Panchin et al., 2000, Bruzzone et al., 2003), has added yet another layer of complexity. The Panxs were demonstrated to be orthologs of innexins, the gap junction proteins expressed in invertebrates (Baranova et al., 2004). Among the three Panx family members (Panx1, Panx2 and Panx3), Panx1 and Panx2 (Le Vasseur et al., 2014) are widely expressed (Baranova et al., 2004). Panx1 is a hexameric single membrane channel-forming protein with similar membrane topology but no sequence homology with Cxs (Fig. 2); it is permeable to ions and small signaling molecules like ATP and glucose (Bao et al., 2004, Bruzzone et al., 2005, Qu et al., 2011, Riquelme et al., 2013), and it is associated with neuronal ischemic injury, inflammation and apoptosis (Silverman et al., 2009, Chekeni et al., 2010, Murphy et al., 2011, Orellana et al., 2011a, Qu et al., 2011, Gulbransen et al., 2012, Sandilos et al., 2012). In contrast to the dual channel properties of Cxs (GJ vs. hemichannel), Panxs generally form large pore membrane channels, similar to hemichannels; this is attributed to the steric hindrance provided by the extracellular glycosylated arginine residue (Fig. 2) (Penuela et al., 2007, Boassa et al., 2008). The Panx1 C-terminal domain has been shown to interact with a host of intracellular factors under specific physiological and pathophysiological conditions (Bhalla-Gehi et al., 2010, Sandilos et al., 2012, Weilinger et al., 2012).
While intercellular coupling via GJs occurs between neurons, oligodendrocytes, and ependymal cells, the main cell type in the brain coupled by GJs is the astrocyte (Giaume et al., 1991, Spray et al., 1999, Nakase and Naus, 2004). In astrocytes, GJs are composed primarily of the channel protein, connexin43 (Cx43), and to a lesser extent Cx30 and Cx26 (Giaume et al., 1991, Naus et al., 1991a, Giaume, 1996, Nagy et al., 2003). Panx1 has also been identified in astrocytes, under in vitro conditions (Iglesias et al., 2009, Silverman et al., 2009, Garre et al., 2010, Karpuk et al., 2011, Suadicani et al., 2012, Jackson et al., 2014, Le et al., 2014, Wei et al., 2014). Astrocytes were initially thought to act as a ‘glue’ that held the cellular structures of the central nervous system (CNS) together, as indicated by Virchow in 1859. Astrocytes are now known as a very heterogeneous population of cells that exhibit different structural, functional, chemical, and molecular characteristics and play distinct roles in different brain regions. Astrocytes express different GJ proteins (Belliveau and Naus, 1994, Wallraff et al., 2004), neurotransmitter receptors, transporters (Zhou and Kimelberg, 2001, Matthias et al., 2003), and ion channels (Verkhratsky and Steinhauser, 2000). These specific molecular characteristics, expressed in different subtypes of astrocytes, allow these cells to perform a range of homeostatic functions including modulating neurotransmission, Ca2+/K+ and glucose homeostasis (Nedergaard et al., 2003, Peters et al., 2003, Wallraff et al., 2006, Rouach et al., 2008, Montero and Orellana, 2014). Astrocytes are critical to neuronal survival and repair (Chen and Swanson, 2003). After brain injury, astrocytes upregulate glial fibrillary acidic protein (GFAP), an intermediate filament associated with astrocyte reactivity (Li et al., 2008, Theodoric et al., 2012). In situations such as brain insults (e.g. stroke, trauma) or conditions (e.g. Alzheimer’s, Parkinson’s), reactive astrocytes minimize brain damage by clearing glutamate and ions released from injured neurons, and clearing metabolic byproducts (Chen and Swanson, 2003). Astrocytes also secrete neurotrophic factors that promote neuronal survival, and minimize damage to neighboring cells by the formation of the glial scar (Fawcett and Asher, 1999). Cx43 membrane channels have been linked to several of these astrocytic processes that impact both brain homeostasis and repair after injury (Giaume et al., 1991, Giaume, 1996, Bani-Yaghoub et al., 1999, Orellana et al., 2011a, Pannasch et al., 2011, Ezan et al., 2012, Le et al., 2014, Montero and Orellana, 2014). A recent study demonstrated that Cx43 expression is directly correlated with reactive astrocytes in brain injury (Theodoric et al., 2012). In addition, Panx1 has been implicated as a potential factor in astrocyte survival under pathological conditions (Scemes et al., 2007, Silverman et al., 2009, Garre et al., 2010, Suadicani et al., 2012, Jackson et al., 2014). The objective of this review is to underline the impact of Cx43 and Panx1 on astrocytes and their implication in neurodegeneration and neuroprotection.
Section snippets
The role of Cx43 and Panx1 membrane channels in neurodegenerative diseases
Traditionally, research in neurodegenerative diseases has revolved around a neurocentric approach. This has been attributed to the fact that neurodegenerative disorders, such as Alzheimer’s disease (AD) and Parkinson’s disease (PD), are primarily linked with the selective loss of specific neuronal subtypes. However, a defining hallmark of neurodegenerative diseases is the increase in reactive astrocytes. Converging lines of evidence illustrate the importance of the astrocytic environment,
Promises and challenges for the treatment of human neurodegeneration and stroke
Understanding the molecular mechanism regulating the function of Cx43 and Panx1 under normal physiological and pathological conditions may lead to therapeutics with very precise modes of action. Lampe’s group has delineated specific residues in the C-terminal tail of Cx43 which exhibit phosphorylation sites as well as identified several interacting factors that affect GJ trafficking, communication, turnover, assembly and gating (Lampe et al., 2000, Lampe et al., 2001, Cooper and Lampe, 2002,
The effects of Cx43 and Panx1 on astrocytes in the neurovascular unit (NVU) and its implication in neurodegeneration
The NVU is comprised of brain endothelial cells and surrounding astrocytes, pericytes, neurons, and ECM of the basement membrane or basal lamina, together forming a functional unit. The spatial relationship of each of the different cells of the NVU is an important anatomical feature that is critical to its function (Abbott et al., 2006). Disturbances in the NVU have been implicated in triggering or propelling neurodegeneration and increasing the risk for stroke (Neuwelt et al., 2011, Zlokovic,
Conclusion
Taken together, these studies provide insights and a vision for future research that would integrate the influence of Cx43 and Panx1 under neurodegenerative conditions. Many questions are still unanswered. For example, is Panx1 expressed in astrocytes in vivo, and if so what are the cellular and molecular conditions that trigger expression and channel opening? Considering that some neurological conditions such as AD, PD and stroke disproportionately affect one gender, are Cx43 and Panx1
Disclosure/conflict of interest
No duality of interest to declare.
Acknowledgments
Special thanks to Alastair Williams (supported by Canadian Stroke Network Summer Studentship Program) and Destiny Lu-Cleary for technical assistance. MAF was supported by a fellowship from the Heart and Stroke Foundation of Canada and through the CIHR Team Grant on “Vascular Cognitive Impairment: Animal Models of Co-morbidity.” CCN holds a Canada Research Chair in Gap Junctions and Neurological Disorders.
References (186)
- et al.
Pannexin-1 channels do not regulate alpha1-adrenoceptor-mediated vasoconstriction in resistance arteries
Eur J Pharmacol
(2015) - et al.
Pannexin membrane channels are mechanosensitive conduits for ATP
FEBS Lett
(2004) - et al.
Pannexin1 drives multicellular aggregate compaction via a signaling cascade that remodels the actin cytoskeleton
J Biol Chem
(2012) - et al.
The mammalian pannexin family is homologous to the invertebrate innexin gap junction proteins
Genomics
(2004) - et al.
Gap junctions: new tools, new answers, new questions
Neuron
(1991) - et al.
Pannexin1 and pannexin3 delivery, cell surface dynamics, and cytoskeletal interactions
J Biol Chem
(2010) - et al.
Pannexin 1 in the regulation of vascular tone
Trends Cardiovasc Med
(2012) - et al.
Intercellular signaling in glial cells: calcium waves and oscillations in response to mechanical stimulation and glutamate
Neuron
(1991) - et al.
Connexin multi-site phosphorylation: mass spectrometry-based proteomics fills the gap
Biochim Biophys Acta
(2013) - et al.
Casein kinase 1 regulates connexin-43 gap junction assembly
J Biol Chem
(2002)
Expression and function of astrocytic gap junctions in aging
Brain Res
Presenilins and Alzheimer’s disease: biological functions and pathogenic mechanisms
Prog Neurobiol
Pannexin: from discovery to bedside in 11+/−4 years?
Brain Res
Attempts to define functional domains of gap junction proteins with synthetic peptides
Biophys J
A new angle on blood-CNS interfaces: a role for connexins?
FEBS Lett
The glial scar and central nervous system repair
Brain Res Bull
Upregulation in astrocytic connexin 43 gap junction levels may exacerbate generalized seizures in mesial temporal lobe epilepsy
Brain Res
Inhibition of cytokine-induced connexin43 hemichannel activity in astrocytes is neuroprotective
Mol Cell Neurosci
Gap junctions in cultured astrocytes: single-channel currents and characterization of channel-forming protein
Neuron
Evidence that disruption of connexon particle arrangements in gap junction plaques is associated with inhibition of gap junctional communication by a glycyrrhetinic acid derivative
Exp Cell Res
Ischemia-induced cellular redistribution of the astrocytic gap junctional protein connexin43 in rat brain
Brain Res
Future directions in Alzheimer’s disease from risk factors to prevention
Biochem Pharmacol
Modulation of connexin 43 in rotenone-induced model of Parkinson’s disease
Neuroscience
Glial connexin expression and function in the context of Alzheimer’s disease
Biochim Biophys Acta
Cerebral ischemic injury is enhanced in a model of oculodentodigital dysplasia
Neuropharmacology
Gap junction intercellular communication mediated by connexin43 in astrocytes is essential for their resistance to oxidative stress
J Biol Chem
Acidic and basic fibroblast growth factor mRNAs are increased in striatum following MPTP-induced dopamine neurofiber lesion: assay by quantitative PCR
Brain Res Mol Brain Res
Disruption of neuronal-glial-vascular units in the hippocampus of ovariectomized mice injected with D-galactose
Neuroscience
Pannexin1 is part of the pore forming unit of the P2X(7) receptor death complex
FEBS Lett
Global and regional mortality from 235 causes of death for 20 age groups in 1990 and 2010: a systematic analysis for the Global Burden of Disease Study 2010
Lancet
Astroglial connexin immunoreactivity is specifically altered at beta-amyloid plaques in beta-amyloid precursor protein/presenilin1 mice
Neuroscience
Astrocyte-endothelial interactions at the blood–brain barrier
Nat Rev Neurosci
Gap junctions in inherited human disorders of the central nervous system
Biochim Biophys Acta
The connexin43 mimetic peptide Gap19 inhibits hemichannels without altering gap junctional communication in astrocytes
Front Cell Neurosci
Neurodegeneration in Parkinson’s disease: interactions of oxidative stress, tryptophan catabolites and depression with mitochondria and sirtuins
Mol Neurobiol
Gap junction blockage interferes with neuronal and astroglial differentiation of mouse P19 embryonal carcinoma cells
Dev Genet
Pannexins in ischemia-induced neurodegeneration
Proc Natl Acad Sci USA
Cortical type 2 astrocytes are not dye coupled nor do they express the major gap junction genes found in the central nervous system
Glia
Pannexin1 regulates alpha1-adrenergic receptor- mediated vasoconstriction
Circ Res
Endothelins regulate astrocyte gap junctions in rat hippocampal slices
Eur J Neurosci
Trafficking dynamics of glycosylated pannexin 1 proteins
Cell Commun Adhes
Pannexin 1 ohnologs in the teleost lineage
J Membr Biol
Pannexins, a family of gap junction proteins expressed in brain
Proc Natl Acad Sci USA
Pharmacological properties of homomeric and heteromeric pannexin hemichannels expressed in Xenopus oocytes
J Neurochem
Pannexin protein expression in the rat middle cerebral artery
J Vasc Res
Astrocytic modulation of blood brain barrier: perspectives on Parkinson’s disease
Front Cell Neurosci
Intercellular calcium signaling via gap junctions in glioma cells
J Cell Biol
Peptides homologous to extracellular loop motifs of connexin 43 reversibly abolish rhythmic contractile activity in rabbit arteries
J Physiol
Pannexin 1 channels mediate ‘find-me’ signal release and membrane permeability during apoptosis
Nature
Astrocytes and brain injury
J Cereb Blood Flow Metab
Cited by (53)
A biocompatible and injectable hydrogel to boost the efficacy of stem cells in neurodegenerative diseases treatment
2021, Life SciencesCitation Excerpt :Hence, NDs are a serious public health concern, being associated with tremendous human suffering and high socio-economic impact. Importantly, their prevalence and incidence are rising dramatically, with the World Health Organization predicting that they will surpass cancer and become the second leading cause of death in the world by 2040 [4]. Currently, there is no effective treatment for NDs, but advanced therapy medicinal products (ATMPs), and, particularly, mesenchymal stem cells (MSCs) show great potential for the development of innovative and healing therapies [5,6].
Astrocyte responses to nanomaterials: Functional changes, pathological changes and potential applications
2021, Acta BiomaterialiaCitation Excerpt :In summary, the use of BBB co-culture models to study the related effects and mechanisms of NMs traversing the BBB is an effective research strategy for understanding CNS nanotoxicology and the efficiency and long-term biological safety of using NMs for targeted drug delivery within the CNS. The influence of NPs on the cellular network formed by astrocytes, microglia, and neurons can be more effectively investigated using a coculture model versus individual cell types [150]. Such research has shown that astrocytes can mediate the neuronal toxic effects of NMs, with one study reporting that Ag-NPs can induce H2O2 release by astrocytes and NO release by microglia, leading to neuronal apoptosis [151].
The role and therapeutic potential of connexins, pannexins and their channels in Parkinson's disease
2019, Cellular SignallingCitation Excerpt :The presence of excess ATP in the extracellular space generates a sustained inflammatory condition that might influence resident neurons and glial cells in a negative manner [70]. A parallel mechanism involving Panx1 channels and FGF1 might also take place in the striatum in PD [71]. P2X7 receptors act as important targets of endogenous ATP in astrocytes in physiological conditions.
Correction to: Neuroimmune response in ischemic preconditioning
2018, NeurotherapeuticsAstroglial Hmgb1 regulates postnatal astrocyte morphogenesis and cerebrovascular maturation
2023, Nature Communications