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Connexin and pannexin hemichannels in brain glial cells: properties, pharmacology, and roles.

Giaume C, Leybaert L, Naus CC, Sáez JC - Front Pharmacol (2013)

Bottom Line: They form multimeric membrane channels with pharmacology somewhat overlapping with that of Cx hemichannels.Such duality has led to several controversies in the literature concerning the identification of the molecular channel constituents (Cxs versus Panxs) in glia.In the present review, we update and discuss the knowledge of Cx hemichannels and Panx channels in glia, their properties and pharmacology, as well as the understanding of their contribution to neuroglial interactions in brain health and disease.

View Article: PubMed Central - PubMed

Affiliation: Collège de France, Center for Interdisciplinary Research in Biology/Centre National de la Recherche Scientifique, Unité Mixte de Recherche 7241/Institut National de la Santé et de la Recherche Médicale U1050 Paris, France ; University Pierre et Marie Curie Paris, France ; MEMOLIFE Laboratory of Excellence and Paris Science Lettre Research University Paris, France.

ABSTRACT
Functional interaction between neurons and glia is an exciting field that has expanded tremendously during the past decade. Such partnership has multiple impacts on neuronal activity and survival. Indeed, numerous findings indicate that glial cells interact tightly with neurons in physiological as well as pathological situations. One typical feature of glial cells is their high expression level of gap junction protein subunits, named connexins (Cxs), thus the membrane channels they form may contribute to neuroglial interaction that impacts neuronal activity and survival. While the participation of gap junction channels in neuroglial interactions has been regularly reviewed in the past, the other channel function of Cxs, i.e., hemichannels located at the cell surface, has only recently received attention. Gap junction channels provide the basis for a unique direct cell-to-cell communication, whereas Cx hemichannels allow the exchange of ions and signaling molecules between the cytoplasm and the extracellular medium, thus supporting autocrine and paracrine communication through a process referred to as "gliotransmission," as well as uptake and release of metabolites. More recently, another family of proteins, termed pannexins (Panxs), has been identified. These proteins share similar membrane topology but no sequence homology with Cxs. They form multimeric membrane channels with pharmacology somewhat overlapping with that of Cx hemichannels. Such duality has led to several controversies in the literature concerning the identification of the molecular channel constituents (Cxs versus Panxs) in glia. In the present review, we update and discuss the knowledge of Cx hemichannels and Panx channels in glia, their properties and pharmacology, as well as the understanding of their contribution to neuroglial interactions in brain health and disease.

No MeSH data available.


Peptide tools to interfere with Cx hemichannel function. Gap26 and Gap27 are composed of sequences located on the extracellular loops 1 (EL1) and 2 (EL2), respectively (amino acids 64–76 and 201–210, respectively) of Cx43 (mouse). These EL-mimetic peptides first inhibit hemichannels and with some delay also gap junction channels. The sequences they mimic are well conserved between different Cxs and peptides based on Cx43 sequences could also inhibit channels composed of other Cxs (e.g., Cx37). L2 peptide is identical to the L2 domain on the cytoplasmic loop (CL; amino acids 119–144). Gap19 is a nonapeptide located within the L2 domain (amino acids 128–136). L2 and Gap19 peptides block Cx43 hemichannels without blocking gap junctions and without blocking Cx40 hemichannels or Panx1 channels (other Cxs still need to be tested). CT10 and CT9 are the last 10 and last 9 amino acids of the C-terminal end (CT; amino acids 373–382 and 374–382, respectively). CT9/CT10 peptides remove the closure of Cx43 hemichannels with high micromolar cytoplasmic Ca2+ concentration and thus stimulate the opening of hemichannels. This particular effect of the CT-peptides is independent of the last amino acid (isoleucine 382) that is involved in linking Cx43 to scaffolding proteins involving ZO-1 interactions (the amino acid sequence 374–381 has the same effect as CT9/CT10; De Bock et al., 2012).
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Figure 2: Peptide tools to interfere with Cx hemichannel function. Gap26 and Gap27 are composed of sequences located on the extracellular loops 1 (EL1) and 2 (EL2), respectively (amino acids 64–76 and 201–210, respectively) of Cx43 (mouse). These EL-mimetic peptides first inhibit hemichannels and with some delay also gap junction channels. The sequences they mimic are well conserved between different Cxs and peptides based on Cx43 sequences could also inhibit channels composed of other Cxs (e.g., Cx37). L2 peptide is identical to the L2 domain on the cytoplasmic loop (CL; amino acids 119–144). Gap19 is a nonapeptide located within the L2 domain (amino acids 128–136). L2 and Gap19 peptides block Cx43 hemichannels without blocking gap junctions and without blocking Cx40 hemichannels or Panx1 channels (other Cxs still need to be tested). CT10 and CT9 are the last 10 and last 9 amino acids of the C-terminal end (CT; amino acids 373–382 and 374–382, respectively). CT9/CT10 peptides remove the closure of Cx43 hemichannels with high micromolar cytoplasmic Ca2+ concentration and thus stimulate the opening of hemichannels. This particular effect of the CT-peptides is independent of the last amino acid (isoleucine 382) that is involved in linking Cx43 to scaffolding proteins involving ZO-1 interactions (the amino acid sequence 374–381 has the same effect as CT9/CT10; De Bock et al., 2012).

Mentions: A more specific targeting of Cx channels is to be expected, at least in principle, from mimetic peptides of Cx proteins. Peptides identical to certain sequences on the Cx protein have been extensively used to interfere with the Cx channel function (Figure 2). The first Cx mimetic peptides that were introduced in the 1990s were identical to specific domains on the extracellular loops (first or second extracellular loop) of the Cx protein. The sequences mimicked were located in domains thought to be involved in the docking of two hemichannels during formation of a full gap junction channel (Warner et al., 1995). Exogenous addition of peptides mimicking parts of these domains were hypothesized to interact with yet unknown extracellular loop domains, thereby preventing extracellular loop interactions of apposed hemichannels during docking and thus hindering gap junction channel formation. These domains, known as Gap26 and Gap27, are present on the first and second extracellular loops, respectively. Accordingly, Gap26 and Gap27 peptides were indeed found to inhibit gap junctional coupling (Evans and Boitano, 2001). Interestingly, subsequent work demonstrated that Gap26 and Gap27 peptides also inhibited unapposed hemichannels with which these peptides are supposed to interact (Braet et al., 2003; Evans et al., 2006, 2012; Desplantez et al., 2012; Wang et al., 2012). Currently, clear evidence that these peptides indeed interact with the extracellular loops is only available for Gap26 (Liu et al., 2006). Nevertheless, both Gap26 and Gap27 rapidly inhibit hemichannels within minutes, followed by a somewhat delayed inhibition of gap junction channels, often in the range of hours, with some exceptions (Matchkov et al., 2006). The exact mechanism of hemichannel block is currently not known. It is known that Gap26/27 influence voltage-dependent gating (Wang et al., 2012) but this does not explain their inhibitory action on hemichannels opened by triggers other than voltage. It has been suggested that these peptides inhibit hemichannels by just blocking the pore because of steric hindrance effects (Wang et al., 2007). However, this has been carefully checked and it was found that this only occurs at very high (1 mM and above) concentrations (Wang et al., 2012). Although Gap26/27 peptides proved to be interesting tools to inhibit Cx hemichannels, their delayed inhibition of gap junction channels remains a serious drawback. Moreover, Gap26/27 peptides also show rather limited specificity toward different Cx types. Indeed, the extracellular loop domains mimic sequences that show pronounced homology between different Cx proteins. Thus, Gap27 directed against Cx43 in astrocytes is also known to inhibit channels or hemichannels composed of Cx37, a vascular Cx present in brain blood vessels (Martin et al., 2005).


Connexin and pannexin hemichannels in brain glial cells: properties, pharmacology, and roles.

Giaume C, Leybaert L, Naus CC, Sáez JC - Front Pharmacol (2013)

Peptide tools to interfere with Cx hemichannel function. Gap26 and Gap27 are composed of sequences located on the extracellular loops 1 (EL1) and 2 (EL2), respectively (amino acids 64–76 and 201–210, respectively) of Cx43 (mouse). These EL-mimetic peptides first inhibit hemichannels and with some delay also gap junction channels. The sequences they mimic are well conserved between different Cxs and peptides based on Cx43 sequences could also inhibit channels composed of other Cxs (e.g., Cx37). L2 peptide is identical to the L2 domain on the cytoplasmic loop (CL; amino acids 119–144). Gap19 is a nonapeptide located within the L2 domain (amino acids 128–136). L2 and Gap19 peptides block Cx43 hemichannels without blocking gap junctions and without blocking Cx40 hemichannels or Panx1 channels (other Cxs still need to be tested). CT10 and CT9 are the last 10 and last 9 amino acids of the C-terminal end (CT; amino acids 373–382 and 374–382, respectively). CT9/CT10 peptides remove the closure of Cx43 hemichannels with high micromolar cytoplasmic Ca2+ concentration and thus stimulate the opening of hemichannels. This particular effect of the CT-peptides is independent of the last amino acid (isoleucine 382) that is involved in linking Cx43 to scaffolding proteins involving ZO-1 interactions (the amino acid sequence 374–381 has the same effect as CT9/CT10; De Bock et al., 2012).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3713369&req=5

Figure 2: Peptide tools to interfere with Cx hemichannel function. Gap26 and Gap27 are composed of sequences located on the extracellular loops 1 (EL1) and 2 (EL2), respectively (amino acids 64–76 and 201–210, respectively) of Cx43 (mouse). These EL-mimetic peptides first inhibit hemichannels and with some delay also gap junction channels. The sequences they mimic are well conserved between different Cxs and peptides based on Cx43 sequences could also inhibit channels composed of other Cxs (e.g., Cx37). L2 peptide is identical to the L2 domain on the cytoplasmic loop (CL; amino acids 119–144). Gap19 is a nonapeptide located within the L2 domain (amino acids 128–136). L2 and Gap19 peptides block Cx43 hemichannels without blocking gap junctions and without blocking Cx40 hemichannels or Panx1 channels (other Cxs still need to be tested). CT10 and CT9 are the last 10 and last 9 amino acids of the C-terminal end (CT; amino acids 373–382 and 374–382, respectively). CT9/CT10 peptides remove the closure of Cx43 hemichannels with high micromolar cytoplasmic Ca2+ concentration and thus stimulate the opening of hemichannels. This particular effect of the CT-peptides is independent of the last amino acid (isoleucine 382) that is involved in linking Cx43 to scaffolding proteins involving ZO-1 interactions (the amino acid sequence 374–381 has the same effect as CT9/CT10; De Bock et al., 2012).
Mentions: A more specific targeting of Cx channels is to be expected, at least in principle, from mimetic peptides of Cx proteins. Peptides identical to certain sequences on the Cx protein have been extensively used to interfere with the Cx channel function (Figure 2). The first Cx mimetic peptides that were introduced in the 1990s were identical to specific domains on the extracellular loops (first or second extracellular loop) of the Cx protein. The sequences mimicked were located in domains thought to be involved in the docking of two hemichannels during formation of a full gap junction channel (Warner et al., 1995). Exogenous addition of peptides mimicking parts of these domains were hypothesized to interact with yet unknown extracellular loop domains, thereby preventing extracellular loop interactions of apposed hemichannels during docking and thus hindering gap junction channel formation. These domains, known as Gap26 and Gap27, are present on the first and second extracellular loops, respectively. Accordingly, Gap26 and Gap27 peptides were indeed found to inhibit gap junctional coupling (Evans and Boitano, 2001). Interestingly, subsequent work demonstrated that Gap26 and Gap27 peptides also inhibited unapposed hemichannels with which these peptides are supposed to interact (Braet et al., 2003; Evans et al., 2006, 2012; Desplantez et al., 2012; Wang et al., 2012). Currently, clear evidence that these peptides indeed interact with the extracellular loops is only available for Gap26 (Liu et al., 2006). Nevertheless, both Gap26 and Gap27 rapidly inhibit hemichannels within minutes, followed by a somewhat delayed inhibition of gap junction channels, often in the range of hours, with some exceptions (Matchkov et al., 2006). The exact mechanism of hemichannel block is currently not known. It is known that Gap26/27 influence voltage-dependent gating (Wang et al., 2012) but this does not explain their inhibitory action on hemichannels opened by triggers other than voltage. It has been suggested that these peptides inhibit hemichannels by just blocking the pore because of steric hindrance effects (Wang et al., 2007). However, this has been carefully checked and it was found that this only occurs at very high (1 mM and above) concentrations (Wang et al., 2012). Although Gap26/27 peptides proved to be interesting tools to inhibit Cx hemichannels, their delayed inhibition of gap junction channels remains a serious drawback. Moreover, Gap26/27 peptides also show rather limited specificity toward different Cx types. Indeed, the extracellular loop domains mimic sequences that show pronounced homology between different Cx proteins. Thus, Gap27 directed against Cx43 in astrocytes is also known to inhibit channels or hemichannels composed of Cx37, a vascular Cx present in brain blood vessels (Martin et al., 2005).

Bottom Line: They form multimeric membrane channels with pharmacology somewhat overlapping with that of Cx hemichannels.Such duality has led to several controversies in the literature concerning the identification of the molecular channel constituents (Cxs versus Panxs) in glia.In the present review, we update and discuss the knowledge of Cx hemichannels and Panx channels in glia, their properties and pharmacology, as well as the understanding of their contribution to neuroglial interactions in brain health and disease.

View Article: PubMed Central - PubMed

Affiliation: Collège de France, Center for Interdisciplinary Research in Biology/Centre National de la Recherche Scientifique, Unité Mixte de Recherche 7241/Institut National de la Santé et de la Recherche Médicale U1050 Paris, France ; University Pierre et Marie Curie Paris, France ; MEMOLIFE Laboratory of Excellence and Paris Science Lettre Research University Paris, France.

ABSTRACT
Functional interaction between neurons and glia is an exciting field that has expanded tremendously during the past decade. Such partnership has multiple impacts on neuronal activity and survival. Indeed, numerous findings indicate that glial cells interact tightly with neurons in physiological as well as pathological situations. One typical feature of glial cells is their high expression level of gap junction protein subunits, named connexins (Cxs), thus the membrane channels they form may contribute to neuroglial interaction that impacts neuronal activity and survival. While the participation of gap junction channels in neuroglial interactions has been regularly reviewed in the past, the other channel function of Cxs, i.e., hemichannels located at the cell surface, has only recently received attention. Gap junction channels provide the basis for a unique direct cell-to-cell communication, whereas Cx hemichannels allow the exchange of ions and signaling molecules between the cytoplasm and the extracellular medium, thus supporting autocrine and paracrine communication through a process referred to as "gliotransmission," as well as uptake and release of metabolites. More recently, another family of proteins, termed pannexins (Panxs), has been identified. These proteins share similar membrane topology but no sequence homology with Cxs. They form multimeric membrane channels with pharmacology somewhat overlapping with that of Cx hemichannels. Such duality has led to several controversies in the literature concerning the identification of the molecular channel constituents (Cxs versus Panxs) in glia. In the present review, we update and discuss the knowledge of Cx hemichannels and Panx channels in glia, their properties and pharmacology, as well as the understanding of their contribution to neuroglial interactions in brain health and disease.

No MeSH data available.