The mouse gap junction gene connexin29 is highly expressed in sciatic nerve and regulated during brain development

Mechanisms of Diseases Associated with Mutation in GJC2/Connexin 47

Connexins are members of a family of integral membrane proteins that provide a pathway for both electrical and metabolic coupling between cells. Astroglia express connexin 30 (Cx30)-GJB6 and Cx43-GJA1, while oligodendroglia express Cx29/Cx31.3-GJC3, Cx32-GJB1, and Cx47-GJC2. Connexins organize into hexameric hemichannels (homomeric if all subunits are identical or heteromeric if one or more differs). Hemichannels from one cell then form cell-cell channels with a hemichannel from an apposed cell. (These are termed homotypic if the hemichannels are identical and heterotypic if the hemichannels differ). Oligodendrocytes couple to each other through Cx32/Cx32 or Cx47/Cx47 homotypic channels and they couple to astrocytes via Cx32/Cx30 or Cx47/Cx43 heterotypic channels. Astrocytes couple via Cx30/Cx30 and Cx43/Cx43 homotypic channels. Though Cx32 and Cx47 may be expressed in the same cells, all available data suggest that Cx32 and Cx47 cannot interact heteromerically. Animal models wherein one or in some cases two different CNS glial connexins have been deleted have helped to clarify the role of these molecules in CNS function. Mutations in a number of different CNS glial connexin genes cause human disease. Mutations in GJC2 lead to three distinct phenotypes, Pelizaeus Merzbacher like disease, hereditary spastic paraparesis (SPG44) and subclinical leukodystrophy.

Brain Disorders and Chemical Pollutants: A Gap Junction Link?

The incidence of brain pathologies has increased during last decades. Better diagnosis (autism spectrum disorders) and longer life expectancy (Parkinson's disease, Alzheimer's disease) partly explain this increase, while emerging data suggest pollutant exposures as a possible but still underestimated cause of major brain disorders. Taking into account that the brain parenchyma is rich in gap junctions and that most pollutants inhibit their function; brain disorders might be the consequence of gap-junctional alterations due to long-term exposures to pollutants. In this article, this hypothesis is addressed through three complementary aspects: (1) the gap-junctional organization and connexin expression in brain parenchyma and their function; (2) the effect of major pollutants (pesticides, bisphenol A, phthalates, heavy metals, airborne particles, etc.) on gap-junctional and connexin functions; (3) a description of the major brain disorders categorized as neurodevelopmental (autism spectrum disorders, attention deficit hyperactivity disorders, epilepsy), neurobehavioral (migraines, major depressive disorders), neurodegenerative (Parkinson's and Alzheimer's diseases) and cancers (glioma), in which both connexin dysfunction and pollutant involvement have been described. Based on these different aspects, the possible involvement of pollutant-inhibited gap junctions in brain disorders is discussed for prenatal and postnatal exposures.

Connexin Genes Variants Associated with Non-Syndromic Hearing Impairment: A Systematic Review of the Global Burden

Mutations in connexins are the most common causes of hearing impairment (HI) in many populations. Our aim was to review the global burden of pathogenic and likely pathogenic (PLP) variants in connexin genes associated with HI. We conducted a systematic review of the literature based on targeted inclusion/exclusion criteria of publications from 1997 to 2020. The databases used were PubMed, Scopus, Africa-Wide Information, and Web of Science. The protocol was registered on PROSPERO, the International Prospective Register of Systematic Reviews, with the registration number “CRD42020169697”. The data extracted were analyzed using Microsoft Excel and SPSS version 25 (IBM, Armonk, New York, United States). A total of 571 independent studies were retrieved and considered for data extraction with the majority of studies (47.8% (n = 289)) done in Asia. Targeted sequencing was found to be the most common technique used in investigating connexin gene mutations. We identified seven connexin genes that were associated with HI, and GJB2 (520/571 publications) was the most studied among the seven. Excluding PLP in GJB2, GJB6, and GJA1 the other connexin gene variants (thus GJB3, GJB4, GJC3, and GJC1 variants) had conflicting association with HI. Biallelic GJB2 PLP variants were the most common and widespread variants associated with non-syndromic hearing impairment (NSHI) in different global populations but absent in most African populations. The most common GJB2 alleles found to be predominant in specific populations include; p.Gly12ValfsTer2 in Europeans, North Africans, Brazilians, and Americans; p.V37I and p.L79Cfs in Asians; p.W24X in Indians; p.L56Rfs in Americans; and the founder mutation p.R143W in Africans from Ghana, or with putative Ghanaian ancestry. The present review suggests that only GJB2 and GJB3 are recognized and validated HI genes. The findings call for an extensive investigation of the other connexin genes in many populations to elucidate their contributions to HI, in order to improve gene-disease pair curations, globally.

Neonatal Seizures and Purinergic Signalling

Neonatal seizures are one of the most common comorbidities of neonatal encephalopathy, with seizures aggravating acute injury and clinical outcomes. Current treatment can control early life seizures; however, a high level of pharmacoresistance remains among infants, with increasing evidence suggesting current anti-seizure medication potentiating brain damage. This emphasises the need to develop safer therapeutic strategies with a different mechanism of action. The purinergic system, characterised by the use of adenosine triphosphate and its metabolites as signalling molecules, consists of the membrane-bound P1 and P2 purinoreceptors and proteins to modulate extracellular purine nucleotides and nucleoside levels. Targeting this system is proving successful at treating many disorders and diseases of the central nervous system, including epilepsy. Mounting evidence demonstrates that drugs targeting the purinergic system provide both convulsive and anticonvulsive effects. With components of the purinergic signalling system being widely expressed during brain development, emerging evidence suggests that purinergic signalling contributes to neonatal seizures. In this review, we first provide an overview on neonatal seizure pathology and purinergic signalling during brain development. We then describe in detail recent evidence demonstrating a role for purinergic signalling during neonatal seizures and discuss possible purine-based avenues for seizure suppression in neonates.

Functional analysis of a nonsyndromic hearing loss-associated mutation in the transmembrane II domain of the GJC3 gene

In a previous study, we identified a novel missense mutation, p.W77S, in the GJC3 gene encoding connexin30.2/connexin31.3 (CX30.2/CX31.3) from patients with hearing loss. The functional alteration of CX30.2/CX31.3 caused by the p.W77S mutant of GJC3 gene, however, remains unclear. In the current study, our result indicated that the p.W77 is localized at the second membrane-spanning segments (TM2) and near border of the E1 domain of the CX30.2/CX31.3 protein and highly conserved (Conseq score = 8~9) in all species. The p.W77S missense mutation proteins in the intracellular distribution are different CX30.2/CX31.3WT and an accumulation of the mutant protein in the endoplasmic reticulum (ER) of the HeLa cell. Furthermore, co-expression of WT and p.W77S mutant chimerae proteins showed that the heteromeric connexon accumulated in the cytoplasm, thereby impairing the WT proteins' expression in the cell membranes. In addition, we found that CX30.2/CX31.3W77S missense mutant proteins were degraded by lysosomes and proteosomes in the transfected HeLa cell. Based on these findings, we suggest that p.W77S mutant has a dominant negative effect on the formation and function of the gap junction. These results give a novel molecular elucidation for the mutation of GJC3 in the development of hearing loss.

GAP junctional communication in brain secondary organizers

Gap junctions (GJs) are integral membrane proteins that enable the direct cytoplasmic exchange of ions and low molecular weight metabolites between adjacent cells. They are formed by the apposition of two connexons belonging to adjacent cells. Each connexon is formed by six proteins, named connexins (Cxs). Current evidence suggests that gap junctions play an important part in ensuring normal embryo development. Mutations in connexin genes have been linked to a variety of human diseases, although the precise role and the cell biological mechanisms of their action remain almost unknown. Among the big family of Cxs, several are expressed in nervous tissue but just a few are expressed in the anterior neural tube of vertebrates. Many efforts have been made to elucidate the molecular bases of Cxs cell biology and how they influence the morphogenetic signal activity produced by brain signaling centers. These centers, orchestrated by transcription factors and morphogenes determine the axial patterning of the mammalian brain during its specification and regionalization. The present review revisits the findings of GJ composed by Cx43 and Cx36 in neural tube patterning and discuss Cx43 putative enrollment in the control of Fgf8 signal activity coming from the well known secondary organizer, the isthmic organizer.

Connexins and pannexins in neuronal development and adult neurogenesis

Connexins and pannexins share very similar structures and functions; they also exhibit overlapping expression in many stages of neuronal development. Here, we review evidence implicating connexin- and pannexin-mediated communication in the regulation of the birth and development of neurons, specifically Cx26, Cx30, Cx32, Cx36, Cx43, Cx45, Panx1, and Panx2. We begin by dissecting the involvement of these proteins in the generation and development of new neurons in the embryonic, postnatal, and adult brain. Next we briefly outline common mechanisms employed by both pannexins and connexins in these roles, including modulation of purinergic receptor signalling and signalling nexus functions. Throughout this review we highlight developing themes as well as important gaps in knowledge to be bridged.

Simvastatin Sodium Salt and Fluvastatin Interact with Human Gap Junction Gamma-3 Protein

Finding pleiomorphic targets for drugs allows new indications or warnings for treatment to be identified. As test of concept, we applied a new chemical genomics approach to uncover additional targets for the widely prescribed lipid-lowering pro-drug simvastatin. We used mRNA extracted from internal mammary artery from patients undergoing coronary artery surgery to prepare a viral cardiovascular protein library, using T7 bacteriophage. We then studied interactions of clones of the bacteriophage, each expressing a different cardiovascular polypeptide, with surface-bound simvastatin in 96-well plates. To maximise likelihood of identifying meaningful interactions between simvastatin and vascular peptides, we used a validated photo-immobilisation method to apply a series of different chemical linkers to bind simvastatin so as to present multiple orientations of its constituent components to potential targets. Three rounds of biopanning identified consistent interaction with the clone expressing part of the gene GJC3, which maps to Homo sapiens chromosome 7, and codes for gap junction gamma-3 protein, also known as connexin 30.2/31.3 (mouse connexin Cx29). Further analysis indicated the binding site to be for the N-terminal domain putatively 'regulating' connexin hemichannel and gap junction pores. Using immunohistochemistry we found connexin 30.2/31.3 to be present in samples of artery similar to those used to prepare the bacteriophage library. Surface plasmon resonance revealed that a 25 amino acid synthetic peptide representing the discovered N-terminus did not interact with simvastatin lactone, but did bind to the hydrolysed HMG CoA inhibitor, simvastatin acid. This interaction was also seen for fluvastatin. The gap junction blockers carbenoxolone and flufenamic acid also interacted with the same peptide providing insight into potential site of binding. These findings raise key questions about the functional significance of GJC3 transcripts in the vasculature and other tissues, and this connexin's role in therapeutic and adverse effects of statins in a range of disease states.

Diseases associated with leaky hemichannels

Hemichannels (HCs) and gap junction channels (GJCs) formed by protein subunits called connexins (Cxs) are major pathways for intercellular communication. While HCs connect the intracellular compartment with the extracellular milieu, GJCs allow the interchange of molecules between cytoplasm of two contacting cells. Under physiological conditions, HCs are mostly closed, but they can open under certain stimuli allowing the release of autocrine and paracrine molecules. Moreover, some pathological conditions, like ischemia or other inflammation conditions, significantly increase HCs activity. In addition, some mutations in Cx genes associated with human diseases, such as deafness or cataracts, lead to the formation of more active HCs or “leaky HCs.” In this article we will revise cellular and molecular mechanisms underlying the appearance of leaky HCs, and the consequences of their expression in different cellular systems and animal models, in seeking a common pattern or pathological mechanism of disease.

GJB1-associated X-linked Charcot-Marie-Tooth disease, a disorder affecting the central and peripheral nervous systems

Charcot-Marie-Tooth disease (CMT) is a group of inherited diseases characterized by exclusive or predominant involvement of the peripheral nervous system. Mutations in GJB1, the gene encoding Connexin 32 (Cx32), a gap-junction channel forming protein, cause the most common X-linked form of CMT, CMT1X. Cx32 is expressed in Schwann cells and oligodendrocytes, the myelinating glia of the peripheral and central nervous systems, respectively. Thus, patients with CMT1X have both central and peripheral nervous system manifestations. Study of the genetics of CMT1X and the phenotypes of patients with this disorder suggest that the peripheral manifestations of CMT1X are likely to be due to loss of function, while in the CNS gain of function may contribute. Mice with targeted ablation of Gjb1 develop a peripheral neuropathy similar to that seen in patients with CMT1X, supporting loss of function as a mechanism for the peripheral manifestations of this disorder. Possible roles for Cx32 include the establishment of a reflexive gap junction pathway in the peripheral and central nervous system and of a panglial syncitium in the central nervous system.

The oligodendroglial precursor cell line Oli-neu represents a cell culture system to examine functional expression of the mouse gap junction gene connexin29 (Cx29)

The potential gap junction forming mouse connexin29 (Cx29) protein is concomitantly expressed with connexin32 (Cx32) in peripheral myelin forming Schwann cells and together with both Cx32 and connexin47 (Cx47) in oligodendrocytes of the CNS. To study the genomic structure and functional expression of Cx29, either primary cells or cell culture systems might be selected, from which the latter are easier to cultivate. Both structure and expression of Cx29 is still not fully understood. In the mouse sciatic nerve, brain and the oligodendroglial precursor cell line Oli-neu the Cx29 gene is processed in two transcript isoforms both harboring a unique reading frame. In contrast to Cx32 and Cx47, only Cx29 protein is abundantly expressed in undifferentiated as well as differentiated Oli-neu cells but the absence of Etbr dye transfer after microinjection concealed the function of Cx29-mediated gap junction communication between those cells. Although HeLa cells stably transfected with Cx29 or Cx29-eGFP neither demonstrated any permeability for Lucifer yellow nor for neurobiotin, blocking of Etbr uptake from the media by gap junction blockers does suppose a role of Cx29 in hemi-channel function. Thus, we conclude that, due to its high abundance of Cx29 expression and its reproducible culture conditions, the oligodendroglial precursor cell line Oli-neu might constitute an appropriate cell culture system to study molecular mechanisms or putative extracellular stimuli to functionally open Cx29 channels or hemi-channels.

Microscopic anatomy: normal structure

A peripheral nerve trunk is composed of nerve fascicles supported in a fibrous collagenous sheath and defined by concentric layers of cells (the perineurium) that separate the contents (the endoneurium) from its fibrous collagen support (the epineurium). In the endoneurium are myelinated and unmyelinated fibers that are axons combined with their supporting Schwann cells to provide physical and electrical connections with end-organs such as muscle fibers and sensory endings. Axons are tubular neuronal extensions with a cytoskeleton of neurotubules and tubulin along which organelles and proteins can travel between the neuronal cell body and the axon terminal. During development some axons enlarge and are covered by a chain of Schwann cells each associated with just one axon. As the axons grow in diameter, the Schwann cells wrap round them to produce a myelin sheath. This consists of many layers of compacted Schwann cell membrane plus some additional proteins. Adjacent myelin segments connect at highly specialized structures, the nodes of Ranvier. Myelin insulates the axon so that the nerve impulse can jump from one node to the next. The region adjacent to the node, the paranodal segment, is the site of myelin terminations on the axolemma. There are connections here between the Schwann cell and the axon via a complex chain of proteins. The Schwann cell cytoplasm in the adjacent segment, the juxtaparanode, contains most of the Schwann cell mitochondria. In addition to the node, continuity of myelin lamellae is broken at intervals along the internode by helical regions of decompaction known as Schmidt-Lanterman incisures; these are seen as paler conical segments in suitably stained microscopical preparations and provide a pathway between the adaxonal and abaxonal cytoplasm. Smaller axons without a myelin sheath conduct very much more slowly and have a more complex relationship with their supporting Schwann cells that has important implications for repair.

Gap junctions in inherited human disorders of the central nervous system

CNS glia and neurons express connexins, the proteins that form gap junctions in vertebrates. We review the connexins expressed by oligodendrocytes and astrocytes, and discuss their proposed physiologic roles. Of the 21 members of the human connexin family, mutations in three are associated with significant central nervous system manifestations. For each, we review the phenotype and discuss possible mechanisms of disease. Mutations in GJB1, the gene for connexin 32 (Cx32) cause the second most common form of Charcot-Marie-Tooth disease (CMT1X). Though the only consistent phenotype in CMT1X patients is a peripheral demyelinating neuropathy, CNS signs and symptoms have been found in some patients. Recessive mutations in GJC2, the gene for Cx47, are one cause of Pelizaeus-Merzbacher-like disease (PMLD), which is characterized by nystagmus within the first 6 months of life, cerebellar ataxia by 4 years, and spasticity by 6 years of age. MRI imaging shows abnormal myelination. A different recessive GJC2 mutation causes a form of hereditary spastic paraparesis, which is a milder phenotype than PMLD. Dominant mutations in GJA1, the gene for Cx43, cause oculodentodigital dysplasia (ODDD), a pleitropic disorder characterized by oculo-facial abnormalities including micropthalmia, microcornia and hypoplastic nares, syndactyly of the fourth to fifth fingers and dental abnormalities. Neurologic manifestations, including spasticity and gait difficulties, are often but not universally seen. Recessive GJA1 mutations cause Hallermann-Streiff syndrome, a disorder showing substantial overlap with ODDD. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and functions.

Human connexin30.2/31.3 (GJC3) does not form functional gap junction channels but causes enhanced ATP release in HeLa cells

Gap junctional intercellular communication has numerous functions, each of which meets the particular needs of organs, tissues, or groups of cells. Connexins (CXs) are homologous four-transmembrane-domain proteins that are the major components of gap junctions. CX30.2/CX31.3 (GJC3) is a relatively new member of the CX protein family. Until now, however, the functional characteristics of CX30.2/CX31.3 have been unclear. To elucidate the properties of CX30.2/CX31.3 channels, their subcellular localization in HeLa cells, their effectiveness in dye transfer, and function on channels were investigated. In the immunofluorescent assay, cells that express CX30.2/CX31.3-GFP exhibited continuous fluorescence along the apposed cell membranes, rather than punctated fluorescence in contacting membranes between two cells. Surprisingly, dyes that can be capable of being permeated by CX26 GJ, according to a scrape loading dye transfer assay in previous studies, are impermeated by CX30.2/CX31.3 GJ, suggesting a difference between the characteristics of CX30.2/CX31.3 GJ and CX26 GJ. Furthermore, a significant amount of ATP was released from the HeLa cells that stably expressed CX30.2/CX31.3, in a medium with low calcium ion concentration, suggesting a hemichannel-based function for CX30.2/CX31.3. Based on these findings, we suggest that CX30.2/CX31.3 shares functional properties with pannexin (hemi) channels rather than gap junction channels of other CXs.

Cx32 and Cx47 mediate oligodendrocyte:astrocyte and oligodendrocyte:oligodendrocyte gap junction coupling

In addition to the extensive gap junction coupling between astrocytes themselves, oligodendrocytes are thought to be exclusively coupled to astrocytes (O:A coupling) via heterotypic gap junctions composed of Cx47:Cx43 and Cx32:Cx30. We used fluorescent dyes to examine functional coupling in acute slices from the cerebra of mice lacking Cx32 and/or Cx47. In the corpus callosum, unexpectedly, oligodendrocytes appeared to be directly and exclusively coupled to other oligodendrocytes (O:O coupling), and electron microscopy revealed gap junctions between adjacent oligodendrocytes. O:O coupling was more affected in mice lacking Cx32 than in mice lacking Cx47. In the neocortex, oligodendrocytes appeared to be directly and exclusively coupled to astrocytes; Cx47, but not Cx32, was required for O:A coupling.

Expression of connexin genes in the human retina

BACKGROUND

Gap junction channels allow direct metabolically and electrical coupling between adjacent cells in various mammalian tissues. Each channel is composed of 12 protein subunits, termed connexins (Cx). In the mouse retina, Cx43 could be localized mostly between astroglial cells whereas expression of Cx36, Cx45 and Cx57 genes has been detected in different neuronal subtypes. In the human retina, however, the expression pattern of connexin genes is largely unknown.

METHODS

Northern blot hybridizations, RT-PCR as well as immunofluorescence analyses helped to explore at least partially the expression pattern of the following human connexin genes GJD2 (hCx36), GJC1 (hCx45), GJA9 (hCx59) and GJA10 (hCx62) in the human retina.

RESULTS

Here we report that Northern blot hybridization signals of the orthologuous hCx36 and hCx45 were found in human retinal RNA. Immunofluorescence signals for both connexins could be located in both inner and outer plexiform layer (IPL, OPL). Expression of a third connexin gene denoted as GJA10 (Cx62) was also detected after Northern blot hybridization in the human retina. Interestingly, its gene structure is similar to that of Gja10 (mCx57) being expressed in mouse horizontal cells. RT-PCR analysis suggested that an additional exon of about 25 kb further downstream, coding for 12 amino acid residues, is spliced to the nearly complete reading frame on exon2 of GJA10 (Cx62). Cx59 mRNA, however, with high sequence identity to zebrafish Cx55.5 was only weakly detectable by RT-PCR in cDNA of human retina.

CONCLUSION

In contrast to the neuron-expressed connexin genes Gjd2 coding for mCx36, Gjc1 coding for mCx45 and Gja10 coding for mCx57 in the mouse, a subset of 4 connexin genes, including the unique GJA9 (Cx59) and GJA10 (Cx62), could be detected at least as transcript isoforms in the human retina. First immunofluorescence analyses revealed a staining pattern of hCx36 and hCx45 expression both in the IPL and OPL, partially reminiscent to that in the mouse, although additional post-mortem material is needed to further explore their sublamina-specific distribution. Appropriate antibodies against Cx59 and Cx62 protein will clarify expression of these proteins in future studies.

Attenuation of MCP-1/CCL2 expression ameliorates neuropathy in a mouse model for Charcot-Marie-Tooth 1X

The chemokine monocyte chemoattractant protein-1 (MCP-1/CCL2) has been previously shown to be an important mediator of macrophage-related neural damage in models of two distinct inherited neuropathies, Charcot-Marie-Tooth (CMT) 1A and 1B. In mice deficient in the gap junction protein connexin 32 (Cx32def), an established model for the X-chromosome-linked dominant form of CMT (CMT1X), we investigated the role of the chemokine in macrophage immigration and neural damage by crossbreeding the Cx32def mice with MCP-1 knockout mutants. In Cx32def mutants typically expressing increased levels of MCP-1, macrophage numbers were strongly elevated, caused by an MCP-1-mediated influx of haematogenous macrophages. Curiously, the complete genetic deletion of MCP-1 did not cause reduced macrophage numbers in the nerves due to compensatory proliferation of resident macrophages. In contrast, and as already seen in other CMT models, heterozygous deletion of MCP-1 led to reduced numbers of phagocytosing macrophages and an alleviation of demyelination. Whereas alleviated demyelination was transient, axonal damage was persistently improved and even robust axonal sprouting was detectable at 12 months. Other axon-related features were alleviated electrophysiological parameters, reduced muscle denervation and atrophy, and increased muscle strength. Similar to models for CMT1A and CMT1B, we identified MEK-ERK signalling as mediating MCP-1 expression in Cx32-deficient Schwann cells. Blocking this pathway by the inhibitor CI-1040 caused reduced MCP-1 expression, attenuation of macrophage increase and amelioration of myelin- and axon-related alterations. Thus, attenuation of MCP-1 upregulation by inhibiting ERK phosphorylation might be a promising approach to treat CMT1X and other so far untreatable inherited peripheral neuropathies in humans.

Modulation of brain hemichannels and gap junction channels by pro-inflammatory agents and their possible role in neurodegeneration

In normal brain, neurons, astrocytes, and oligodendrocytes, the most abundant and active cells express pannexins and connexins, protein subunits of two families forming membrane channels. Most available evidence indicates that in mammals endogenously expressed pannexins form only hemichannels and connexins form both gap junction channels and hemichannels. Whereas gap junction channels connect the cytoplasm of contacting cells and coordinate electric and metabolic activity, hemichannels communicate the intra- and extracellular compartments and serve as a diffusional pathway for ions and small molecules. A subthreshold stimulation by acute pathological threatening conditions (e.g., global ischemia subthreshold for cell death) enhances neuronal Cx36 and glial Cx43 hemichannel activity, favoring ATP release and generation of preconditioning. If the stimulus is sufficiently deleterious, microglia become overactivated and release bioactive molecules that increase the activity of hemichannels and reduce gap junctional communication in astroglial networks, depriving neurons of astrocytic protective functions, and further reducing neuronal viability. Continuous glial activation triggered by low levels of anomalous proteins expressed in several neurodegenerative diseases induce glial hemichannel and gap junction channel disorders similar to those of acute inflammatory responses triggered by ischemia or infectious diseases. These changes are likely to occur in diverse cell types of the CNS and contribute to neurodegeneration during inflammatory process.

Human oligodendrocytes express Cx31.3: function and interactions with Cx32 mutants

Murine oligodendrocytes express the gap junction (GJ) proteins connexin32 (Cx32), Cx47, and Cx29. CNS phenotypes in patients with X-linked Charcot-Marie-Tooth disease may be caused by dominant effects of Cx32 mutations on other connexins. Here we examined the expression of Cx31.3 (the human ortholog of murine Cx29) in human brain and its relation to the other oligodendrocyte GJ proteins Cx32 and Cx47. Furthermore, we investigated in vitro whether Cx32 mutants with CNS manifestations affect the expression and function of Cx31.3. Cx31.3 was localized mostly in the gray matter along small myelinated fibers similar to Cx29 in rodent brain and was co-expressed with Cx32 in a subset of human oligodendrocytes. In HeLa cells Cx31.3 was localized at the cell membrane and appeared to form hemichannels but no GJs. Cx32 mutants with CNS manifestations were retained intracellularly, but did not alter the cellular localization or function of co-expressed Cx31.3. Thus, Cx31.3 shares many characteristics with its ortholog Cx29. Cx32 mutants with CNS phenotypes do not affect the trafficking or function of Cx31.3, and may have other toxic effects in oligodendrocytes.

Analysis of connexin expression during mouse Schwann cell development identifies connexin29 as a novel marker for the transition of neural crest to precursor cells

Connexins are transmembrane proteins forming gap junction channels for direct intercellular and, for example in myelinating glia cells, intracellular communication. In mature myelin-forming Schwann cells, expression of multiple connexins, i.e. connexin (Cx) 43, Cx29, Cx32, and Cx46 (after nerve injury) has been detected. However, little is known about connexin protein expression during Schwann cell development. Here we use histochemical methods on wildtype and Cx29lacZ transgenic mice to investigate the developmental expression of connexins in the Schwann cell lineage. Our data demonstrate that in the mouse Cx43, Cx29, and Cx32 protein expression is activated in a developmental sequence that is clearly correlated with major developmental steps in the lineage. Only Cx43 was expressed from neural crest cells onwards. Cx29 protein expression was absent from neural crest cells but appeared as neural crest cells generated precursors (embryonic day 12) both in vivo and in vitro. This identifies Cx29 as a novel marker for cells of the defined Schwann cell lineage. The only exception to this were dorsal roots, where the expression of Cx29 was delayed four days relative to ventral roots and spinal nerves. Expression of Cx32 commenced postnatally, coinciding with the onset of myelination. Thus, the coordinated expression of connexin proteins in cells of the embryonic and postnatal Schwann cell lineage might point to a potential role in peripheral nerve development and maturation.

The effects of a dominant connexin32 mutant in myelinating Schwann cells

Mutations in GJB1, the gene encoding the gap junction protein connexin32 (Cx32), cause X-linked Charcot-Marie-Tooth disease, an inherited demyelinating peripheral neuropathy. We generated transgenic mice that express the R142W mutation in myelinating Schwann cells. The R142W mutant protein was aberrantly localized to the Golgi, indicating that it does not traffic properly, but the molecular organization of the myelin sheath, including the localization of Cx29, another connexin expressed by myelinating Schwann cells, was not disrupted. In a wild type background, this mutation dramatically decreased the level of wild type mouse Cx32 in immunoblots of sciatic nerve and caused demyelination. The expression of wild type human Cx32 with the same transgenic construct had different effects-increased amounts of Cx32, normal localization of Cx32 at nodes and incisures, and split myelin sheaths. Thus, the R142W mutant protein has dominant effects that are distinct from overexpression.

The structural and functional integrity of peripheral nerves depends on the glial-derived signal desert hedgehog

We show that desert hedgehog (dhh), a signaling molecule expressed by Schwann cells, is essential for the structural and functional integrity of the peripheral nerve. Dhh-null nerves display multiple abnormalities that affect myelinating and nonmyelinating Schwann cells, axons, and vasculature and immune cells. Myelinated fibers of these mice have a significantly increased (more than two times) number of Schmidt-Lanterman incisures (SLIs), and connexin 29, a molecular component of SLIs, is strongly upregulated. Crossing Dhh-null mice with myelin basic protein (MBP)-deficient shiverer mice, which also have increased SLI numbers, results in further increased SLIs, suggesting that Dhh and MBP control SLIs by different mechanisms. Unmyelinated fibers are also affected, containing many fewer axons per Schwann cell in transverse profiles, whereas the total number of unmyelinated axons is reduced by approximately one-third. In Dhh-null mice, the blood-nerve barrier is permeable and neutrophils and macrophage numbers are elevated, even in uninjured nerves. Dhh-null nerves also lack the largest-diameter myelinated fibers, have elevated numbers of degenerating myelinated axons, and contain regenerating fibers. Transected dhh nerves degenerate faster than wild-type controls. This demonstrates that a single identified glial signal, Dhh, plays a critical role in controlling the integrity of peripheral nervous tissue, in line with its critical role in nerve sheath development (Parmantier et al., 1999). The complexity of the defects raises a number of important questions about the Dhh-dependent cell-cell signaling network in peripheral nerves.

Connexin29 is highly expressed in cochlear Schwann cells, and it is required for the normal development and function of the auditory nerve of mice

Connexins (Cxs) are a family of protein subunits constituting gap junctions, which facilitate exchanges of molecules important for cellular signaling and metabolic activities intercellularly or between different regions of the cytoplasm in the same cells. Mutations in Cxs are the major cause of nonsyndromic childhood deafness, which are mostly found in Cx26 and Cx30 expressed in cochlear supporting cells and fibrocytes. So far, little is known about the functional contribution of Cxs in other types of cochlear cells. Here, we show that Cx29 was highly expressed in the cochlea. The developmental expression time course of Cx29 was similar to that of a myelin marker [myelin associate glycoprotein (MAG)]. Immunolabeling identified Cx29 exclusively in the Schwann cells myelinating the soma and fiber of spiral ganglion (SG) neurons. The absence of the Cx29 gene in mice (Cx29(-/-) mice), with a penetrance of approximately 50%, caused a delay in the maturation of hearing thresholds, an early loss of high-frequency sensitivities, a prolongation in latency and distortion in the wave I of the auditory brainstem responses, and elevated sensitivity to noise damages. The morphology of sensory hair cells and otoacoustic emissions that depend on the integrity of hair cells were normal in Cx29(-/-) mice. In contrast, decreases in MAG expression and severe demyelination at the soma of SG neurons were found in Cx29(-/-) mice. Our findings demonstrated the requirement of Cx29 for normal cochlear functions and suggest that Cx29 is a new candidate gene for studying the auditory neuropathy.

The detection of hamster connexins: a comparison of expression profiles with wild-type mouse and the cancer-prone Min mouse

The open reading frames of 17 connexins from Syrian hamster (using tissues) and 16 connexins from the Chinese hamster cell line V79, were fully (Cx30, Cx31, Cx37, Cx43 and Cx45) or partially sequenced. We have also detected, and partially sequenced, seven rat connexins that previously were unavailable. The expression of connexin genes was examined in some hamster organs and cultured hamster cells, and compared with wild-type mouse and the cancer-prone Min mouse. Although the expression patterns were similar for most organs and connexins in hamster and mouse, there were also some prominent differences (Cx29 and 30.3 in testis; Cx31.1 and 32 in eye; Cx46 in brain, kidney and testis; Cx47 in kidney). This suggests that some connexins have species-specific expression profiles. In contrast, there were minimal differences in expression profiles between wild type and Min mice. Species-specific expression profiles should be considered in attempts to make animal models of human connexin-associated diseases.

The connexin gene family in mammals

Unannotated mammalian genome databases (dog, cow, opossum) were searched for candidate connexin genes, using sequences from annotated genomes (man, mouse). All 18 'multi-species' connexin genes, i.e., orthologs of connexin26 , 29/31.3 (duplicated in opossum), 30, 30.2/31.9, 30.3, 31, 31.1, 32, 36, 37, 39/40.1, 40, 43, 45, 44/46, 47, 50, and 57/62 , were found in dog, cow and opossum. Connexin25 and 58 have been considered specific for man, but evident orthologs of connexin25 were found in dog, cow and opossum, and orthologs of connexin58 were found in dog and cow. Moreover, a connexin43 -like sequence (approx. 80% identical to connexin43 ) was found in man, chimpanzee, dog and cow. In the three former species, the sequences were located on the X chromosome. In man, chimpanzee and cow, there were stop codons in all reading frames; these sequences are therefore judged as pseudogenes, called here Cx43pX . In the dog, the sequence contained an open reading frame for a protein of 35.7 kDa (connexin35.7). We suggest that these sequences are orthologs of connexin33 , previously considered as a rodent-specific connexin gene. Thus, connexin25 , 33 and 58 are not species-specific genes. However, the opossum may possess a candidate, connexin39.2 , without obvious orthologs in other mammals. Furthermore, pseudogenes of primate connexin31.3 and opossum connexin35 (one of the two orthologs of primate connexin31.3) were detected. These results suggest that the structure of the mammalian connexin gene family should be revised, especially with regard to the so-called 'species-specific' connexins .

New insights into the expression and function of neural connexins with transgenic mouse mutants

Gap junctions represent direct intercellular conduits between contacting cells. The subunit proteins of these conduits are called connexins. To date, 20 and 21 connexin genes have been described in the mouse and human genome, respectively, many of them represent sequence-orthologous pairs. Targeted deletion of connexin genes in the mouse genome opened new insights into the biological function of these channel forming proteins, which, in some cases, could be correlated to phenotypic abnormalities in humans, suffering from inherited diseases caused by mutations in the corresponding orthologous connexin gene. Replacing the connexin coding DNA by an appropriate reporter gene has clarified in several cases its cell type specific expression in mouse brain. Various studies demonstrated that connexin36 is mainly expressed in interneurons of retina and brain. Targeted deletion of connexin36 evoked a loss of electrical signal transduction and interferes with synchrony which probably leads to defects in visual transmission and memory. Deletion of connexin43 in astrocytes of mouse brain resulted in increased spreading depression consistent with the notion of altered "spatial buffering" of K(+) ions and glutamate secreted by active neurons. General connexin30-deficiency led to hearing impairment and apoptosis of hair cells, similar to that observed in mice with cochlea specific deletion of connexin26. Reporter gene expression in connexin30-deficient mice indicated that astrocytes in certain brain regions and leptomeningeal as well as ependymal cells are labelled. Reporter gene expression in connexin45- and connexin47-deficient mice was used to reassign connexin45 expression to certain CNS neurons and connexin47 expression to oligodendrocytes.

Role of connexin-based gap junction channels and hemichannels in ischemia-induced cell death in nervous tissue

Gap junction channels and hemichannels formed of connexin subunits are found in most cell types in vertebrates. Gap junctions connect cells via channels not open to the extracellular space and permit the passage of ions and molecules of approximately 1 kDa. Single connexin hemichannels, which are connexin hexamers, are present in the surface membrane before docking with a hemichannel in an apposed membrane. Because of their high conductance and permeability in cell-cell channels, it had been thought that connexin hemichannels remained closed until docking to form a cell-cell channel. Now it is clear that at least some hemichannels can open to allow passage of molecules between the cytoplasm and extracellular space. Here we review evidence that gap junction channels may allow intercellular diffusion of necrotic or apoptotic signals, but may also allow diffusion of ions and substances from healthy to injured cells, thereby contributing to cell survival. Moreover, opening of gap junction hemichannels may exacerbate cell injury or mediate paracrine or autocrine signaling. In addition to the cell specific features of an ischemic insult, propagation of cell damage and death within affected tissues may be affected by expression and regulation of gap junction channels and hemichannels formed by connexins.

Cochlear gap junctions coassembled from Cx26 and 30 show faster intercellular Ca2+signaling than homomeric counterparts

The importance of connexins (Cxs) in cochlear functions has been demonstrated by the finding that mutations in Cx genes cause a large proportion of sensorineural hearing loss cases. However, it is still unclear how Cxs contribute to the cochlear function. Recent data ( 33 ) obtained from Cx30 knockout mice showing that a reduction of Cx diversity in assembling gap junctions is sufficient to cause deafness suggest that functional interactions of different subtypes of Cxs may be essential in normal hearing. In this work we show that the two major forms of Cxs (Cx26 and Cx30) in the cochlea have overlapping expression patterns beginning at early embryonic stages. Cx26 and Cx30 were colocalized in most gap junction plaques in the cochlea, and their coassembly was tested by coimmunoprecipitation. To compare functional differences of gap junctions with different molecular configurations, homo- and heteromeric gap junctions composed of Cx26 and/or Cx30 were reconstituted by transfections in human embryonic kidney-293 cells. The ratio imaging technique and fluorescent tracer diffusion assays were used to assess the function of reconstituted gap junctions. Our results revealed that gap junctions with different molecular configurations show differences in biochemical coupling, and that intercellular Ca2+signaling across heteromeric gap junctions consisting of Cx26 and Cx30 was at least twice as fast as their homomerically assembled counterparts. Our data suggest that biochemical permeability and the dynamics of intercellular signaling through gap junction channels, in addition to gap junction-mediated intercellular ionic coupling, may be important factors to consider for studying functional roles of gap junctions in the cochlea.

Update on connexins and gap junctions in neurons and glia in the mammalian nervous system

Among the 20 proposed members of the connexin family of proteins that form gap junctional intercellular communication (GJIC) channels in mammalian tissues, over half are reported to be expressed in the nervous system. There have been conflicting observations, however, concerning the particular connexins expressed by astrocytes, oligodendrocytes, Schwann cells and neurons. Identification of the several connexin proteins at gap junctions between each neuronal and glial cell type is essential for the rational design of investigations into the functions of GJIC between glial cells and into the functional contributions of electrical and "mixed" (chemical plus electrical) synapses to communication between neurons in the mammalian nervous system. In this report, we provide a summary of recent findings regarding the localization of connexins in gap junctions between glial cells and between neurons. Attention is drawn to technical considerations involved in connexin localization by light and electron microscope immunohistochemistry and to limitations of physiological methods and approaches currently used to analyze neuronal and glial coupling. Early physiological studies that provided evidence for the presence of gap junctions and electrical synapses in isolated regions of the mammalian brain and spinal cord are reexamined in light of recent evidence for widely expressed neuron-specific connexins and for the existence of several newly discovered types of gap junctions linking neurons.

Pathogenesis of X-linked Charcot-Marie-Tooth disease: differential effects of two mutations in connexin 32

X-linked Charcot-Marie-Tooth disease is an inherited peripheral neuropathy arising in patients with mutations in the gene encoding connexin 32 (Cx32). Cx32 is expressed at the paranodes and Schmidt-Lantermann incisures of myelinating Schwann cells in which it is believed to form a reflexive pathway between the abaxonal and adaxonal cytoplasmic domains. Patients with the Val181Ala (V181A) mutation have a severe peripheral neuropathy. Experiments using a nude mouse xenograft system show that Schwann cells expressing only this mutant form of Cx32 are profoundly impaired in their ability to support the earliest stages of regeneration of myelinated fibers. Coupling between paired Xenopus oocytes expressing V181A is reduced compared with the coupling between oocytes expressing wild-type human Cx32 (32WT), and protein levels assayed by Western blot are substantially lower. Immunocytochemisty shows that Neuro2a cells expressing the V181A mutant have very few gap junction plaques compared with cells expressing 32WT; Cx32 protein levels are lower in these cells than in those expressing 32WT. Because failure of normal regeneration is evident before formation of myelin, loss of function of Cx32 may impact on the function of precursors of the myelinating Schwann cell before the formation of the hypothesized reflexive pathway. The Glu102Gly (E102G) mutation leads to a milder phenotype. Early regeneration is normal in grafts with Schwann cells expressing the E102G mutant. The only abnormality detected in the behavior of its channel is increased sensitivity to acidification-induced closure, a property that may lead to reduced gap junction coupling during periods of metabolic stress. This restricted functional abnormality may explain the relatively mild phenotype seen in the xenograft model and in E102G patients.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Connexin29 and connexin32 at oligodendrocyte and astrocyte gap junctions and in myelin of the mouse central nervous system

The cellular localization, relation to other glial connexins (Cx30, Cx32, and Cx43), and developmental expression of Cx29 were investigated in the mouse central nervous system (CNS) with an anti-Cx29 antibody. Cx29 was enriched in subcellular fractions of myelin, and immunofluorescence for Cx29 was localized to oligodendrocytes and myelinated fibers throughout the brain and spinal cord. Oligodendrocyte somata displayed minute Cx29-immunopositive puncta around their periphery and intracellularly. In developing brain, Cx29 levels increased during the first few postnatal weeks and were highest in the adult brain. Immunofluorescence labeling for Cx29 in oligodendrocyte somata was intense at young ages and was dramatically shifted in localization primarily to myelinated fibers in mature CNS. Labeling for Cx32 also was localized to oligodendrocyte somata and myelin and absent in Cx32 knockout mice. Cx29 and Cx32 were minimally colocalized on oligodendrocytes somata and partly colocalized along myelinated fibers. At gap junctions on oligodendrocyte somata, Cx43/Cx32 and Cx30/Cx32 were strongly associated, but there was minimal association of Cx29 and Cx43. Cx32 was very sparsely associated with astrocytic connexins along myelinated fibers. With Cx26, Cx30, and Cx43 expressed in astrocytes and Cx29, Cx32, and Cx47 expressed in oligodendrocytes, the number of connexins localized to gap junctions of glial cells is increased to six. The results suggested that Cx29 in mature CNS contributes minimally to gap junctional intercellular communication in oligodendrocyte cell bodies but rather is targeted to myelin, where it, with Cx32, may contribute to connexin-mediated communication between adjacent layers of uncompacted myelin.

Spatiotemporal transcription of connexin45 during brain development results in neuronal expression in adult mice

Characterization of the expression pattern of connexins in neural tissue is a necessary prerequisite for understanding the functional relevance of the corresponding gap junction channels in brain. Here we describe the cell type-specific expression of connexin45 in the CNS and the spatiotemporal expression pattern from embryonic day 19.5 to adult brain using a recently described connexin45 LacZ-reporter mouse. The connexin45 gene is highly expressed during embryogenesis and up to 2 weeks after birth in nearly all brain regions. Afterward its expression is restricted to the thalamus, the CA3 region of hippocampus and the cerebellum. In adult mouse brain, the pattern of LacZ-staining in combination with the analysis of different neuronal and glial marker proteins strongly suggests that connexin45 is expressed in neurons, but presumably not in astrocytes or mature oligodendrocytes. Expression of the LacZ/connexin45 reporter gene in subsets of neurons, such as cerebral cortical, hippocampal and thalamic neurons as well as basket and stellate cells of cerebellum should be corroborated by functional investigations of connexin45 protein in electrical synapses. Based on its expression pattern during development, we suggest that the connexin45-containing gap junction channels have a rather ubiquitous role during brain development and may contribute to functional specification in certain subsets of neurons in the adult brain.

Oligodendrocyte progenitor enrichment in the connexin32 null-mutant mouse

Before the establishment of chemical synapses, neural progenitors are often coupled by connexin-mediated gap junctions providing a robust mechanism for cell-cell communication in developing brain. The present study was undertaken to determine whether alterations in junctional coupling also affect neural progenitor proliferation, survival, and differentiation in adult brain. We localized the connexin32 gap junction protein to a subset of NG2+ and platelet-derived growth factor alpha receptor+ early oligodendrocyte progenitors in the dentate gyrus of adult mice. In connexin32-deficient mice, we found an increase in the total number of proliferating nestin+ and NG2+ progenitors in the subgranular zone, hilus, and polymorphonuclear layer of the dentate gyrus in vivo and in the total number of nestin+ progenitors capable of clonogenic expansion in vitro. By bromodeoxyuridine labeling, lineage analysis, and terminal deoxynucleotidyl nick end labeling, we demonstrate that turnover of these cells is constitutively enhanced in the connexin32 knock-out dentate gyrus reflecting a dynamic defect in oligodendrogenesis in this population. Analyses of surviving bromodeoxyuridine-labeled cells at 1, 3, 7, and 28 d after injection demonstrate that this transient amplifying population fails to terminally differentiate and is deleted by an apoptotic-like mechanism within 3 d of labeling. These data provide empirical evidence to support the hypothesis that connexin expression influences adult progenitor number and specifically implicate connexin32-mediated signaling in the activation, survival, and differentiation of a subset of early oligodendrocyte progenitors in postnatal brain.

Connexin genes in the mouse and human genome

Gap junctions serve for direct intercellular communication by docking of two hemichannels in adjacent cells thereby forming conduits between the cytoplasmic compartments of adjacent cells. Connexin genes code for subunit proteins of gap junction channels and are members of large gene families in mammals. So far, 17 connexin (Cx) genes have been described and characterized in the murine genome. For most of them, orthologues in the human genome have been found (see White and Paul 1999; Manthey et al. 1999; Teubner et al. 2001; Söhl et al. 2001). We have recently performed searches for connexin genes in murine and human gene libraries available at EMBL/Heidelberg, NCBI and the Celera company that have increased the number of identified connexins to 19 in mouse and 20 in humans. For one mouse connexin gene and two human connexin genes we did not find orthologues in the other genome. Here we present a short overview on distinct connexin genes which we found in the mouse and human genome and which may include all members of this gene family, if no further connexin gene will be discovered in the remaining non-sequenced parts (about 1-5%) of the genomes.

Diverse trafficking abnormalities of connexin32 mutants causing CMTX

Mutations in GJB1, the gene encoding the gap junction protein connexin32 (Cx32), cause X-linked Charcot-Marie-Tooth disease (CMTX). We compared the localization of CMTX mutants that affect different domains of Cx32, by expressing them in HeLa cells. Mutants were localized to the endoplasmic reticulum (M34K, N205I, and Y211x), in the Golgi apparatus without reaching the cell membrane (M34T, V38M, A40V, R75Q, R75P, R75W, and C217x), in the Golgi apparatus but also forming rare small gap junction-like plaques (M34I, M34V, and V37M), or mainly on the cell membrane, forming gap junction-like plaques (V35M, I213V, R219C, R219H, R220G, R230C, R230L, R238H, L239I, and S281x). Selected mutants expressed in cultured rat Schwann cells showed localization similar to that in HeLa cells. Thus, many CMTX mutants have trafficking abnormalities, whereas the carboxy-terminus mutants reach the cell membrane and probably cause disease through other mechanisms.

Connexin29 is uniquely distributed within myelinating glial cells of the central and peripheral nervous systems

Although both Schwann cells and oligodendrocytes express connexin32 (Cx32), the loss of this connexin causes demyelination only in the PNS. To determine whether oligodendrocytes might express another connexin that can function in place of Cx32, we searched for novel CNS-specific connexins using reverse transcriptase-PCR and degenerate primers. We identified Cx29, whose transcript was restricted to brain, spinal cord, and sciatic nerve. Developmental expression of Cx29 mRNA in the CNS paralleled that of other myelin-related mRNAs, including Cx32. In the CNS, Cx29 antibodies labeled the internodal and juxtaparanodal regions of small myelin sheaths, whereas Cx32 staining was restricted to large myelinated fibers. In the PNS, Cx29 expression preceded that of Cx32 and declined to lower levels than Cx32 in adulthood. In adult sciatic nerve, Cx29 was primarily localized to the innermost aspects of the myelin sheath, the paranode, the juxtaparanode, and the inner mesaxon. Cx29 displayed a striking coincidence with Kv1.2 K(+) channels, which are localized in the axonal membrane. Both Cx29 and Cx32 were found in the incisures. Cx29 expressed in N2A cells did not induce intercellular conductances but did participate in the formation of active channels when coexpressed with Cx32. Together, these data show that Cx29 and Cx32 are expressed by myelinating glial cells with distinct distributions.

Connexin29 expression, immunocytochemistry and freeze-fracture replica immunogold labelling (FRIL) in sciatic nerve

The recently discovered connexin29 (Cx29) was reported to be present in the central and peripheral nervous systems (CNS and PNS), and its mRNA was found in particular abundance in peripheral nerve. The expression and localization of Cx29 protein in sciatic nerve were investigated using an antibody against Cx29. The antibody recognized Cx29 in HeLa cells transfected with Cx29 cDNA, while nontransfected HeLa cells were devoid of Cx29. Immunoblotting of sciatic nerve homogenate revealed monomeric and possibly higher molecular weight forms of Cx29. These were distinguished from connexin32 (Cx32), which also is expressed in peripheral nerve. Double immunofluorescence labelling for Cx29 and Cx32 revealed only partial colocalization of the two connexins, with codistribution at intermittent, conical-shaped striations along nerve fibers. By freeze-fracture replica immunogold labelling (FRIL), Cx32 was found in gap junctions in the outermost layers of myelin, whereas Cx29-immunogold labelling was found only in the innermost layer of myelin in close association with hexagonally arranged intramembrane particle (IMP) 'rosettes' and gap junction-like clusters of IMPs. Although both Cx32 and Cx29 were detected in myelin of normal mice, only Cx29 was present in Schwann cell membranes in Cx32 knockout mice. The results confirm that Cx29 is a second connexin expressed in Schwann cells of sciatic nerve. In addition, Cx29 is present in distinctive IMP arrays in the inner most layer of myelin, adjacent to internodal axonal plasma membranes, where this connexin may have previously unrecognized functions.

Structural and functional diversity of connexin genes in the mouse and human genome

Gap junctions are clustered channels between contacting cells through which direct intercellular communication via diffusion of ions and metabolites can occur. Two hemichannels, each built up of six connexin protein subunits in the plasma membrane of adjacent cells, can dock to each other to form conduits between cells. We have recently screened mouse and human genomic data bases and have found 19 connexin (Cx) genes in the mouse genome and 20 connexin genes in the human genome. One mouse connexin gene and two human connexin genes do not appear to have orthologs in the other genome. With three exceptions, the characterized connexin genes comprise two exons whereby the complete reading frame is located on the second exon. Targeted ablation of eleven mouse connexin genes revealed basic insights into the functional diversity of the connexin gene family. In addition, the phenotypes of human genetic disorders caused by mutated connexin genes further complement our understanding of connexin functions in the human organism. In this review we compare currently identified connexin genes in both the mouse and human genome and discuss the functions of gap junctions deduced from targeted mouse mutants and human genetic disorders.