Ultrastructural localization of connexins (Cx36, Cx43, Cx45), glutamate receptors and aquaporin-4 in rodent olfactory mucosa, olfactory nerve and olfactory bulb

The physiological roles of anoctamin2/TMEM16B and anoctamin1/TMEM16A in chemical senses

Chemical senses allow animals to detect and discriminate a vast array of molecules. The olfactory system is responsible of the detection of small volatile molecules, while water dissolved molecules are detected by taste buds in the oral cavity. Moreover, many animals respond to signaling molecules such as pheromones and other semiochemicals through the vomeronasal organ. The peripheral organs dedicated to chemical detection convert chemical signals into perceivable information through the employment of diverse receptor types and the activation of multiple ion channels. Two ion channels, TMEM16B, also known as anoctamin2 (ANO2) and TMEM16A, or anoctamin1 (ANO1), encoding for Ca2+-activated Cl¯ channels, have been recently described playing critical roles in various cell types. This review aims to discuss the main properties of TMEM16A and TMEM16B-mediated currents and their physiological roles in chemical senses. In olfactory sensory neurons, TMEM16B contributes to amplify the odorant response, to modulate firing, response kinetics and adaptation. TMEM16A and TMEM16B shape the pattern of action potentials in vomeronasal sensory neurons increasing the interspike interval. In type I taste bud cells, TMEM16A is activated during paracrine signaling mediated by ATP. This review aims to shed light on the regulation of diverse signaling mechanisms and neuronal excitability mediated by Ca-activated Cl¯ channels, hinting at potential new roles for TMEM16A and TMEM16B in the chemical senses.

Assembly mechanisms of the neuronal gap junction channel connexin 36 elucidated by Cryo-EM

Electrical synapses are essential components of neural circuits. Neuronal signal transduction across electrical synapses is primarily mediated by gap junction channels composed of Connexin36 (Cx36), the lack of which causes impaired electrical coupling between certain neurons including cortical interneurons and thalamic reticular nucleus (TRN) neurons. However, the structural basis underlying Cx36 function and assembly remains elusive. Recently, Lee et al. reported cryo-EM structures of Cx36, thus provided first insights of its gating mechanism. Here, we report a consistent cryo-EM structure of Cx36 determined in parallel, and describe unique interactions underpinning its assembly mechanism in complementary to the competing work. In particular, we found non-canonical electrostatic interactions between protomers from opposing hemichannels and a steric complementary site between adjacent protomers within a hemichannel, which together provide a structural explanation for the assembly specificity in homomeric and heteromeric gap junction channels.

Shedding light on human olfaction: Electrophysiological recordings from sensory neurons in acute slices of olfactory epithelium

The COVID-19 pandemic brought attention to our limited understanding of human olfactory physiology. While the cellular composition of the human olfactory epithelium is similar to that of other vertebrates, its functional properties are largely unknown. We prepared acute slices of human olfactory epithelium from nasal biopsies and used the whole-cell patch-clamp technique to record electrical properties of cells. We measured voltage-gated currents in human olfactory sensory neurons and supporting cells, and action potentials in neurons. Additionally, neuronal inward current and action potentials responses to a phosphodiesterase inhibitor suggested a transduction cascade involving cAMP as a second messenger. Furthermore, responses to odorant mixtures demonstrated that the transduction cascade was intact in this preparation. This study provides the first electrophysiological characterization of olfactory sensory neurons in acute slices of the human olfactory epithelium, paving the way for future research to expand our knowledge of human olfactory physiology.

Long-term depression in neurons involves temporal and ultra-structural dynamics of phosphatidylinositol-4,5-bisphosphate relying on PIP5K, PTEN and PLC

Synaptic plasticity involves proper establishment and rearrangement of structural and functional microdomains. Yet, visualization of the underlying lipid cues proved challenging. Applying a combination of rapid cryofixation, membrane freeze-fracturing, immunogold labeling and electron microscopy, we visualize and quantitatively determine the changes and the distribution of phosphatidylinositol-4,5-bisphosphate (PIP₂) in the plasma membrane of dendritic spines and subareas thereof at ultra-high resolution. These efforts unravel distinct phases of PIP₂ signals during induction of long-term depression (LTD). During the first minutes PIP₂ rapidly increases in a PIP5K-dependent manner forming nanoclusters. PTEN contributes to a second phase of PIP₂ accumulation. The transiently increased PIP₂ signals are restricted to upper and middle spine heads. Finally, PLC-dependent PIP₂ degradation provides timely termination of PIP₂ cues during LTD induction. Together, this work unravels the spatial and temporal cues set by PIP₂ during different phases after LTD induction and dissects the molecular mechanisms underlying the observed PIP₂ dynamics.

Photobiomodulation Therapy and the Glymphatic System: Promising Applications for Augmenting the Brain Lymphatic Drainage System

The glymphatic system is a glial-dependent waste clearance pathway in the central nervous system, devoted to drain away waste metabolic products and soluble proteins such as amyloid-beta. An impaired brain glymphatic system can increase the incidence of neurovascular, neuroinflammatory, and neurodegenerative diseases. Photobiomodulation (PBM) therapy can serve as a non-invasive neuroprotective strategy for maintaining and optimizing effective brain waste clearance. In this review, we discuss the crucial role of the glymphatic drainage system in removing toxins and waste metabolites from the brain. We review recent animal research on the neurotherapeutic benefits of PBM therapy on glymphatic drainage and clearance. We also highlight cellular mechanisms of PBM on the cerebral glymphatic system. Animal research has shed light on the beneficial effects of PBM on the cerebral drainage system through the clearance of amyloid-beta via meningeal lymphatic vessels. Finally, PBM-mediated increase in the blood–brain barrier permeability with a subsequent rise in Aβ clearance from PBM-induced relaxation of lymphatic vessels via a vasodilation process will be discussed. We conclude that PBM promotion of cranial and extracranial lymphatic system function might be a promising strategy for the treatment of brain diseases associated with cerebrospinal fluid outflow abnormality.

Gap junctions and expression of Cx36, Cx43 and Cx45 in the posterodorsal medial amygdala of adult rats

The posterodorsal medial amygdala (MePD) has an adapted synaptic organization that dynamically modulates reproduction and other social behaviors in rats. Discrete gap junctions between glial cells were previously reported in the MePD neuropil. Connexins (Cx) are components of gap junctions and indicative of cellular electrical coupling. Here, we report the ultrastructural occurrence of gap junctions between neurons in the MePD and demonstrate the expression and immunofluorescent labeling of Cx36, Cx43 and Cx45 in this subcortical area of adult male rats. Few neuronal gap junctions were found in the MePD and, when identified, occurred between dendrites. On the other hand, there is a diffuse presence and distribution of punctate labelling for the tested Cxs. Puncta were visualized isolated or forming clusters in the same focal plane of cell bodies or along the MePD neuropil. The Cx36 puncta were found in neurons, Cx43 in astrocytes and Cx45 in both neurons and astrocytes. Our data indicate the presence of few gap junctions and different Cxs composition in the MePD. Because Cxs can assemble, form hemichannel units and/or serve as transcriptional regulator, it is likely that additional modulation of intercellular communication can occur besides the chemical transmission in the MePD of adult rats.

Connexin 26 and Connexin 43 in Canine Mammary Carcinoma

Incidence of canine mammary carcinoma is two times higher than the rate of human breast cancer. Mammary tumors are the most common type of cancer in intact female dogs and account for about half of all neoplasms in these dogs. Well-established models of breast cancer have shown that neoplastic cells often have a loss of intercellular communication, particularly gap junction proteins. Thus, the objective of this study is to explore the aspect of gap junction intercellular communication in canine mammary carcinoma, non-cancerous (CMEC) and cancerous (CMT12, CMT27, and CF41.Mg) cells, and patient-derived tumors. Both non-cancerous and cancerous mammary cells express connexins 26 and 43 using immunofluorescence; however, the level of expression is significantly different in quantitative analysis using western blot in which connexin 43 in both CMT12 and CMT27 is significantly decreased compared to CMEC. Furthermore, a decrease of gap junction capacity in CMT12 and CMT27 was observed compared to CMEC. Immunostaining of CMT27-xenograft tumors revealed positive Cx26 and negative Cx43 expression. Similarly, immunostaining of spontaneous canine mammary tumors revealed that Cx26 is present in all tumors while Cx43 is present in 25% of tumors. Overall, the study provides for the first time that a differential pattern of connexin expression exists between non-cancerous and cancerous mammary cells in dogs. This study will pave the path for further in vitro work of connexins in comparative canine models and possibly allow for novel therapeutics to be developed.

Olfactory ensheathing cells from the nasal mucosa and olfactory bulb have distinct membrane properties

Transplantation of olfactory ensheathing cells (OECs) is a potential therapy for the regeneration of damaged neurons. While they maintain tissue homeostasis in the olfactory mucosa (OM) and olfactory bulb (OB), their regenerative properties also support the normal sense of smell by enabling continual turnover and axonal regrowth of olfactory sensory neurons (OSNs). However, the molecular physiology of OECs is not fully understood, especially that of OECs from the mucosa. Here, we carried out whole-cell patch-clamp recordings from individual OECs cultured from the OM and OB of the adult rat, and from the human OM. A subset of OECs from the rat OM cultured 1-3 days in vitro had large weakly rectifying K⁺ currents, which were sensitive to Ba²⁺ and desipramine, blockers of Kir4-family channels. Kir4.1 immunofluorescence was detectable in cultured OM cells colabeled for the OEC marker S100, and in S100-labeled cells found adjacent to OSN axons in mucosal sections. OECs cultured from rat OB had distinct properties though, displaying strongly rectifying inward currents at hyperpolarized membrane potentials and strongly rectifying outward currents at depolarized potentials. Kir4.1 immunofluorescence was not evident in OECs adjacent to axons of OSNs in the OB. A subset of human OECs cultured from the OM of adults had membrane properties comparable to those of the rat OM that is dominated by Ba²⁺ -sensitive weak inwardly rectifying currents. The membrane properties of peripheral OECs are different to those of central OECs, suggesting they may play distinct roles during olfaction.

TMEM16A calcium-activated chloride currents in supporting cells of the mouse olfactory epithelium

Glial-like supporting (or sustentacular) cells are important constituents of the olfactory epithelium that are involved in several physiological processes such as production of endocannabinoids, insulin, and ATP and regulation of the ionic composition of the mucus layer that covers the apical surface of the olfactory epithelium. Supporting cells express metabotropic P2Y purinergic receptors that generate ATP-induced Ca²⁺ signaling through the activation of a PLC-mediated cascade. Recently, we reported that a subpopulation of supporting cells expresses also the Ca²⁺-activated Cl^- channel TMEM16A. Here, we sought to extend our understanding of a possible physiological role of this channel in the olfactory system by asking whether Ca²⁺ can activate Cl^- currents mediated by TMEM16A. We use whole-cell patch-clamp analysis in slices of the olfactory epithelium to measure dose-response relations in the presence of various intracellular Ca²⁺ concentrations, ion selectivity, and blockage. We find that knockout of TMEM16A abolishes Ca²⁺-activated Cl^- currents, demonstrating that TMEM16A is essential for these currents in supporting cells. Also, by using extracellular ATP as physiological stimuli, we found that the stimulation of purinergic receptors activates a large TMEM16A-dependent Cl^- current, indicating a possible role of TMEM16A in ATP-mediated signaling. Altogether, our results establish that TMEM16A-mediated currents are functional in olfactory supporting cells and provide a foundation for future work investigating the precise physiological role of TMEM16A in the olfactory system.

On the occurrence and enigmatic functions of mixed (chemical plus electrical) synapses in the mammalian CNS

Electrical synapses with diverse configurations and functions occur at a variety of interneuronal appositions, thereby significantly expanding the physiological complexity of neuronal circuitry over that provided solely by chemical synapses. Gap junctions between apposed dendritic and somatic plasma membranes form "purely electrical" synapses that allow for electrical communication between coupled neurons. In addition, gap junctions at axon terminals synapsing on dendrites and somata allow for "mixed" (dual chemical+electrical) synaptic transmission. "Dual transmission" was first documented in the autonomic nervous system of birds, followed by its detection in the central nervous systems of fish, amphibia, and reptiles. Subsequently, mixed synapses have been detected in several locations in the mammalian CNS, where their properties and functional roles remain undetermined. Here, we review available evidence for the presence, complex structural composition, and emerging functional properties of mixed synapses in the mammalian CNS.

Selective Cre-mediated gene deletion identifies connexin 43 as the main connexin channel supporting olfactory ensheathing cell networks

Many functions of glial cells depend on the formation of selective glial networks mediated by gap junctions formed by members of the connexin family. Olfactory ensheathing cells (OECs) are specialized glia associated with olfactory sensory neuron axons. Like other glia, they form selective networks, however, the connexins that support OEC connectivity in vivo have not been identified. We used an in vivo mouse model to selectively delete candidate connexin genes with temporal control from OECs and address the physiological consequences. Using this model, we effectively abolished the expression of connexin 43 (Cx43) in OECs in both juvenile and adult mice. Cx43-deleted OECs exhibited features consistent with the loss of gap junctions including reduced membrane conductance, largely reduced sensitivity to the gap junction blocker meclofenamic acid and loss of dye coupling. This indicates that Cx43, a typically astrocytic connexin, is the main connexin forming functional channels in OECs. Despite these changes in functional properties, the deletion of Cx43 deletion did not alter the density of OECs. The strategy used here may prove useful to delete other candidate genes to better understand the functional roles of OECs in vivo.

Connexin36 Expression in Primary Afferent Neurons in Relation to the Axon Reflex and Modality Coding of Somatic Sensation

Electrical coupling mediated by connexin36-containing gap junctions that form electrical synapses is known to be prevalent in the central nervous system, but such coupling was long ago reported also to occur between cutaneous sensory fibers. Here, we provide evidence supporting the capability of primary afferent fibers to engage in electrical coupling. In transgenic mice with enhanced green fluorescent protein (eGFP) serving as a reporter for connexin36 expression, immunofluorescence labeling of eGFP was found in subpopulations of neurons in lumbar dorsal root and trigeminal sensory ganglia, and in fibers within peripheral nerves and tissues. Immunolabeling of connexin36 was robust in the sciatic nerve, weaker in sensory ganglia than in peripheral nerve, and absent in these tissues from Cx36 null mice. Connexin36 mRNA was detected in ganglia from wild-type mice, but not in those from Cx36 null mice. Labeling of eGFP was localized within a subpopulation of ganglion cells containing substance P and calcitonin gene-releasing peptide, and in peripheral fibers containing these peptides. Expression of eGFP was also found in various proportions of sensory ganglion neurons containing transient receptor potential (TRP) channels, including TRPV1 and TRPM8. Ganglion cells labeled for isolectin B4 and tyrosine hydroxylase displayed very little co-localization with eGFP. Our results suggest that previously observed electrical coupling between peripheral sensory fibers occurs via electrical synapses formed by Cx36-containing gap junctions, and that some degree of selectivity in the extent of electrical coupling may occur between fibers belonging to subpopulations of sensory neurons identified according to their sensory modality responsiveness.

Synchronized roles of pannexin and connexin in nasal mucosal epithelia

BACKGROUND

Nasal mucosal epithelial cells express connexins, the prototypical gap junction proteins, and pannexins, a new family of channel proteins homologous to the invertebrate gap junction proteins. The physiological and pathophysiological roles of these transmembrane proteins in nasal mucosa are largely still unknown.

PURPOSE

Pannexins participate in ATP release into the extracellular space in various tissues, and ATP plays important roles in mucociliary clearance, especially by regulating ciliary beat activity. Therefore, we focused on the functional relationship between connexins, pannexin-1, ATP release, and mucociliary clearance in nasal epithelia.

RESULTS AND CONCLUSIONS

Connexins participate in the generation of intercellular calcium waves, in which calcium-mediated signaling responses spread to contiguous cells through the gap junction formed by connexins to transmit calcium signaling throughout the airway epithelium. Pannexins in the nasal mucosa may contribute to not only ciliary beat modulation via ATP release, but also regulation of mucus blanket components via H2O efflux. The synchronized roles of pannexin and connexin may provide a new insight into effective mucociliary clearance systems in nasal mucosa.

A novel mechanism of depression: role for connexins

Major depressive disorder (MDD) is a chronic and debilitating illness that affects over 350 million people worldwide; however, current treatments have failed to cure or prevent the progress of depression. Increasing evidence suggests a crucial role for connexins in MDD. In this review, we have summarised recent accomplishments regarding the role of connexins, gap junctions, and hemichannels in the aetiology of MDD, and discussed the limitations of current research. A blockage of gap junctions or hemichannels induces depressive behaviour. Possible underlying mechanisms include the regulation of neurosecretory functions and synaptic activity by gap junctions and hemichannels. Gap junctions are functionally inhibited under stress conditions. Conversely, hemichannel permeability is increased. Antidepressants inhibit hemichannel permeability; however, they have contrasting effects on the function of gap junctions under normal conditions and can protect them against stress. In conclusion, the blockage of hemichannels concurrent with improvements in gap junction functionality might be potential targets for depression treatment.

Electrical synapses in mammalian CNS: Past eras, present focus and future directions

Gap junctions provide the basis for electrical synapses between neurons. Early studies in well-defined circuits in lower vertebrates laid the foundation for understanding various properties conferred by electrical synaptic transmission. Knowledge surrounding electrical synapses in mammalian systems unfolded first with evidence indicating the presence of gap junctions between neurons in various brain regions, but with little appreciation of their functional roles. Beginning at about the turn of this century, new approaches were applied to scrutinize electrical synapses, revealing the prevalence of neuronal gap junctions, the connexin protein composition of many of those junctions, and the myriad diverse neural systems in which they occur in the mammalian CNS. Subsequent progress indicated that electrical synapses constitute key elements in synaptic circuitry, govern the collective activity of ensembles of electrically coupled neurons, and in part orchestrate the synchronized neuronal network activity and rhythmic oscillations that underlie fundamental integrative processes. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Freeze fracture: new avenues for the ultrastructural analysis of cells in vitro

The ultrastructural analysis of biological membranes by freeze fracture has a 60-year tradition. In this review, we summarize the benefits of the freeze-fracture technique and review special structures analyzed by freeze fracture and by combined freeze-fracture replica immunogold labeling (FRIL) of cell cultures. In principle, every cellular membrane whether of cell suspensions, mono- or bilayers of cell cultures can be analyzed in freeze fracture. The combination of freeze fracture and immunogold labeling of the replica allows the ultrastructural identification of protein assemblies in combination with the molecular identification of their constituent proteins using specific antibodies. The analysis of fractured and labeled intramembrane particles enables determination of the arrangement and organization of proteins within the membrane due to the high resolution of the transmission electron microscope. Because of cell-specific ultrastructural features such as square arrays, identification of cell types can be performed in parallel. This review is aimed at presenting the possibilities of freeze fracture and FRIL in the high-resolution ultrastructural analysis of membrane proteins and their assembly in naïve, transfected or otherwise treated cultured cells. At the interface of molecular approaches and morphology, the application of FRIL in genetically modified cells provides a novel and intriguing aspect for their analysis.

miRNA Expression Profile and Effect of Wenxin Granule in Rats with Ligation-Induced Myocardial Infarction

Wenxin Granule (WXKL) is a traditional Chinese medicine used for treatment of myocardial infarction (MI) and arrhythmias. However, the genomic pathological mechanisms of MI and mechanisms of WXKL are largely unknown. This study aims to investigate a comprehensive miRNA expression profile, and the predicted correlation pathways to be targeted by differentially expressed miRNAs in MI, and mechanisms of WXKL from a gene level. MI rat model was established by a coronary artery ligation surgery. miRNA expression microarrays were performed and the data were deposited in Gene Expression Omnibus (GEO number GSE95855). And, pathway analysis was performed by using the DIANA-miRPath v3.0 online tool. The expressions of miR-1, miR-133, Cx43, and Cx45 were detected by quantitative real-time PCR. It was found that 35 differentially expressed miRNAs and 23 predicted pathways, including miR-1, miR-133, and gap junction pathway, are involved in the pathogenesis of MI. And, WXKL increased the expressions of miR-1 and miR-133, while also increased the mRNA levels of Cx43 and Cx45, and, especially, recovered the Cx43/Cx45 ratio near to normal level. The results suggest that regulatory effects on miR-1, miR-133, Cx43, and Cx45 might be a possible mechanism of WXKL in the treatment of MI at the gene level.

Pannexins in vision, hearing, olfaction and taste

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

Stream of consciousness: Quantum and biochemical assumptions regarding psychopathology

The accepted paradigms of mainstream neuropsychiatry appear to be incompletely adequate and in various cases offer equivocal analyses. However, a growing number of new approaches are being proposed that suggest the emergence of paradigm shifts in this area. In particular, quantum theories of mind, brain and consciousness seem to offer a profound change to the current approaches. Unfortunately these quantum paradigms harbor at least two serious problems. First, they are simply models, theories, and assumptions, with no convincing experiments supporting their claims. Second, they deviate from contemporary mainstream views of psychiatric illness and do so in revolutionary ways. We suggest a possible way to integrate experimental neuroscience with quantum models in order to address outstanding issues in psychopathology. A key role is played by the phenomenon called the "stream of consciousness", which can be linked to the so-called "Gamma Synchrony" (GS), which is clearly demonstrated by EEG data. In our novel proposal, a unipolar depressed patient could be seen as a subject with an altered stream of consciousness. In particular, some clues suggest that depression is linked to an "increased power" stream of consciousness. It is additionally suggested that such an approach to depression might be extended to psychopathology in general with potential benefits to diagnostics and therapeutics in neuropsychiatry.

KV1 channels identified in rodent myelinated axons, linked to Cx29 in innermost myelin: support for electrically active myelin in mammalian saltatory conduction

Saltatory conduction in mammalian myelinated axons was thought to be well understood before recent discoveries revealed unexpected subcellular distributions and molecular identities of the K(+)-conductance pathways that provide for rapid axonal repolarization. In this study, we visualize, identify, localize, quantify, and ultrastructurally characterize axonal KV1.1/KV1.2 channels in sciatic nerves of rodents. With the use of light microscopic immunocytochemistry and freeze-fracture replica immunogold labeling electron microscopy, KV1.1/KV1.2 channels are localized to three anatomically and compositionally distinct domains in the internodal axolemmas of large myelinated axons, where they form densely packed "rosettes" of 9-nm intramembrane particles. These axolemmal KV1.1/KV1.2 rosettes are precisely aligned with and ultrastructurally coupled to connexin29 (Cx29) channels, also in matching rosettes, in the surrounding juxtaparanodal myelin collars and along the inner mesaxon. As >98% of transmembrane proteins large enough to represent ion channels in these specialized domains, ∼500,000 KV1.1/KV1.2 channels define the paired juxtaparanodal regions as exclusive membrane domains for the voltage-gated K(+)conductance that underlies rapid axonal repolarization in mammals. The 1:1 molecular linkage of KV1 channels to Cx29 channels in the apposed juxtaparanodal collars, plus their linkage to an additional 250,000-400,000 Cx29 channels along each inner mesaxon in every large-diameter myelinated axon examined, supports previously proposed K(+)conductance directly from juxtaparanodal axoplasm into juxtaparanodal myeloplasm in mammalian axons. With neither Cx29 protein nor myelin rosettes detectable in frog myelinated axons, these data showing axon-to-myelin linkage by abundant KV1/Cx29 channels in rodent axons support renewed consideration of an electrically active role for myelin in increasing both saltatory conduction velocity and maximum propagation frequency in mammalian myelinated axons.

Connexin36 expression in major centers of the auditory system in the CNS of mouse and rat: Evidence for neurons forming purely electrical synapses and morphologically mixed synapses

Electrical synapses formed by gap junctions composed of connexin36 (Cx36) are widely distributed in the mammalian central nervous system (CNS). Here, we used immunofluorescence methods to document the expression of Cx36 in the cochlear nucleus and in various structures of the auditory pathway of rat and mouse. Labeling of Cx36 visualized exclusively as Cx36-puncta was densely distributed primarily on the somata and initial dendrites of neuronal populations in the ventral cochlear nucleus, and was abundant in superficial layers of the dorsal cochlear nucleus. Other auditory centers displaying Cx36-puncta included the medial nucleus of the trapezoid body (MNTB), regions surrounding the lateral superior olivary nucleus, the dorsal nucleus of the medial lemniscus, the nucleus sagulum, all subnuclei of the inferior colliculus, and the auditory cerebral cortex. In EGFP-Cx36 transgenic mice, EGFP reporter was detected in neurons located in each of auditory centers that harbored Cx36-puncta. In the ventral cochlear nuclei and the MNTB, many neuronal somata were heavily innervated by nerve terminals containing vesicular glutamate transporter-1 (vglut1) and Cx36 was frequently localized at these terminals. Cochlear ablation caused a near total depletion of vglut1-positive terminals in the ventral cochlear nuclei, with a commensurate loss of labeling for Cx36 around most neuronal somata, but preserved Cx36-puncta at somatic neuronal appositions. The results suggest that electrical synapses formed by Cx36-containing gap junctions occur in most of the widely distributed centers of the auditory system. Further, it appears that morphologically mixed chemical/electrical synapses formed by nerve terminals are abundant in the ventral cochlear nucleus, including those at endbulbs of Held formed by cochlear primary afferent fibers, and those at calyx of Held synapses on MNTB neurons.

Correlations of differentially expressed gap junction connexins Cx26, Cx30, Cx32, Cx43 and Cx46 with breast cancer progression and prognosis

BACKGROUND AND AIMS

Connexins and their cell membrane channels contribute to the control of cell proliferation and compartmental functions in breast glands and their deregulation is linked to breast carcinogenesis. Our aim was to correlate connexin expression with tumor progression and prognosis in primary breast cancers.

MATERIALS AND METHODS

Meta-analysis of connexin isotype expression data of 1809 and 1899 breast cancers from the Affymetrix and Illumina array platforms, respectively, was performed. Expressed connexins were also monitored at the protein level in tissue microarrays of 127 patients equally representing all tumor grades, using immunofluorescence and multilayer, multichannel digital microscopy. Prognostic correlations were plotted in Kaplan-Meier curves and tested using the log-rank test and cox-regression analysis in univariate and multivariate models.

RESULTS

The expression of GJA1/Cx43, GJA3/Cx46 and GJB2/Cx26 and, for the first time, GJA6/Cx30 and GJB1/Cx32 was revealed both in normal human mammary glands and breast carcinomas. Within their subfamilies these connexins can form homo- and heterocellular epithelial channels. In cancer, the array datasets cross-validated each other's prognostic results. In line with the significant correlations found at mRNA level, elevated Cx43 protein levels were linked with significantly improved breast cancer outcome, offering Cx43 protein detection as an independent prognostic marker stronger than vascular invasion or necrosis. As a contrary, elevated Cx30 mRNA and protein levels were associated with a reduced disease outcome offering Cx30 protein detection as an independent prognostic marker outperforming mitotic index and necrosis. Elevated versus low Cx43 protein levels allowed the stratification of grade 2 tumors into good and poor relapse free survival subgroups, respectively. Also, elevated versus low Cx30 levels stratified grade 3 patients into poor and good overall survival subgroups, respectively.

CONCLUSION

Differential expression of Cx43 and Cx30 may serve as potential positive and negative prognostic markers, respectively, for a clinically relevant stratification of breast cancers.

Heterotypic gap junctions at glutamatergic mixed synapses are abundant in goldfish brain

Gap junctions provide for direct intercellular electrical and metabolic coupling. The abundance of gap junctions at "large myelinated club ending (LMCE)" synapses on Mauthner cells (M-cells) of the teleost brain provided a convenient model to correlate anatomical and physiological properties of electrical synapses. There, presynaptic action potentials were found to evoke short-latency electrical "pre-potentials" immediately preceding their accompanying glutamate-induced depolarizations, making these the first unambiguously identified "mixed" (i.e., chemical plus electrical) synapses in the vertebrate CNS. We recently showed that gap junctions at these synapses exhibit asymmetric electrical resistance (i.e., electrical rectification), which we correlated with total molecular asymmetry of connexin composition in their apposing gap junction hemiplaques, with connexin35 (Cx35) restricted to axon terminal hemiplaques and connexin34.7 (Cx34.7) restricted to apposing M-cell plasma membranes. We now show that similarly heterotypic neuronal gap junctions are abundant throughout goldfish brain, with labeling exclusively for Cx35 in presynaptic hemiplaques and exclusively for Cx34.7 in postsynaptic hemiplaques. Moreover, the vast majority of these asymmetric gap junctions occur at glutamatergic axon terminals. The widespread distribution of heterotypic gap junctions at glutamatergic mixed synapses throughout goldfish brain and spinal cord implies that pre- vs. postsynaptic asymmetry at electrical synapses evolved early in the chordate lineage. We propose that the advantages of the molecular and functional asymmetry of connexins at electrical synapses that are so prominently expressed in the teleost CNS are unlikely to have been abandoned in higher vertebrates. However, to create asymmetric coupling in mammals, where most gap junctions are composed of connexin36 (Cx36) on both sides, would require some other mechanism, such as differential phosphorylation of connexins on opposite sides of the same gap junction or on asymmetric differences in the complement of their scaffolding and regulatory proteins.

Abundance of gap junctions at glutamatergic mixed synapses in adult Mosquitofish spinal cord neurons

"Dye-coupling", whole-mount immunohistochemistry for gap junction channel protein connexin 35 (Cx35), and freeze-fracture replica immunogold labeling (FRIL) reveal an abundance of electrical synapses/gap junctions at glutamatergic mixed synapses in the 14th spinal segment that innervates the adult male gonopodium of Western Mosquitofish, Gambusia affinis (Mosquitofish). To study gap junctions' role in fast motor behavior, we used a minimally-invasive neural-tract-tracing technique to introduce gap junction-permeant or -impermeant dyes into deep muscles controlling the gonopodium of the adult male Mosquitofish, a teleost fish that rapidly transfers (complete in <20 mS) spermatozeugmata into the female reproductive tract. Dye-coupling in the 14th spinal segment controlling the gonopodium reveals coupling between motor neurons and a commissural primary ascending interneuron (CoPA IN) and shows that the 14th segment has an extensive and elaborate dendritic arbor and more gap junctions than do other segments. Whole-mount immunohistochemistry for Cx35 results confirm dye-coupling and show it occurs via gap junctions. Finally, FRIL shows that gap junctions are at mixed synapses and reveals that >50 of the 62 gap junctions at mixed synapses are in the 14th spinal segment. Our results support and extend studies showing gap junctions at mixed synapses in spinal cord segments involved in control of genital reflexes in rodents, and they suggest a link between mixed synapses and fast motor behavior. The findings provide a basis for studies of specific roles of spinal neurons in the generation/regulation of sex-specific behavior and for studies of gap junctions' role in regulating fast motor behavior. Finally, the CoPA IN provides a novel candidate neuron for future studies of gap junctions and neural control of fast motor behaviors.

MicroRNAs in neuronal communication

MicroRNAs (miRNAs) are short nucleotides sequences that regulate the expression of genes in different eukaryotic cell types. A tremendous amount of knowledge on miRNAs has rapidly accumulated over the last few years, revealing the growing interest in this field of research. On the other hand, clarifying the physiological regulation of gene expression in the central nervous system is important for establishing a reference for comparison to the diseased state. It is well known that the fine tuning of neuronal networks relies on intricate molecular mechanisms, such as the adjustment of the synaptic transmission. As determined by recent studies, regulation of neuronal interactions by miRNAs has critical consequences in the development, adaptation to ambient demands, and degeneration of the nervous system. In contrast, activation of synaptic receptors triggers downstream signaling cascades that generate a vast array of effects, which includes the regulation of novel genes involved in the control of the miRNA life cycle. In this review, we have examined the hot topics on miRNA gene-regulatory activities in the broad field of neuronal communication-related processes. Furthermore, in addition to indicating the newly described effect of miRNAs on the regulation of specific neurotransmitter systems, we have pointed out how these systems affect the expression, transport, and stability of miRNAs. Moreover, we discuss newly described and under-investigation mechanisms involving the intercellular transfer of miRNAs, aided by exosomes and gap junctions. Thus, in the current review, we were able to highlight recent findings related to miRNAs that indisputably contributed towards the understanding of the nervous system in health and disease.

Gap junctions

Gap junctions are essential to the function of multicellular animals, which require a high degree of coordination between cells. In vertebrates, gap junctions comprise connexins and currently 21 connexins are known in humans. The functions of gap junctions are highly diverse and include exchange of metabolites and electrical signals between cells, as well as functions, which are apparently unrelated to intercellular communication. Given the diversity of gap junction physiology, regulation of gap junction activity is complex. The structure of the various connexins is known to some extent; and structural rearrangements and intramolecular interactions are important for regulation of channel function. Intercellular coupling is further regulated by the number and activity of channels present in gap junctional plaques. The number of connexins in cell-cell channels is regulated by controlling transcription, translation, trafficking, and degradation; and all of these processes are under strict control. Once in the membrane, channel activity is determined by the conductive properties of the connexin involved, which can be regulated by voltage and chemical gating, as well as a large number of posttranslational modifications. The aim of the present article is to review our current knowledge on the structure, regulation, function, and pharmacology of gap junctions. This will be supported by examples of how different connexins and their regulation act in concert to achieve appropriate physiological control, and how disturbances of connexin function can lead to disease.

Re-evaluation of connexins associated with motoneurons in rodent spinal cord, sexually dimorphic motor nuclei and trigeminal motor nucleus

Electrical synapses formed by neuronal gap junctions composed of connexin36 (Cx36) are a common feature in mammalian brain circuitry, but less is known about their deployment in spinal cord. It has been reported based on connexin mRNA and/or protein detection that developing and/or mature motoneurons express a variety of connexins, including Cx26, Cx32, Cx36 and Cx43 in trigeminal motoneurons, Cx36, Cx37, Cx40, Cx43 and Cx45 in spinal motoneurons, and Cx32 in sexually dimorphic motoneurons. We re-examined the localization of these connexins during postnatal development and in adult rat and mouse using immunofluorescence labeling for each connexin. We found Cx26 in association only with leptomeninges in the trigeminal motor nucleus (Mo5), Cx32 only with oligodendrocytes and myelinated fibers among motoneurons in this nucleus and in the spinal cord, and Cx37, Cx40 and Cx45 only with blood vessels in the ventral horn of spinal cord, including those among motoneurons. By freeze-fracture replica immunolabeling, > 100 astrocyte gap junctions but no neuronal gap junctions were found based on immunogold labeling for Cx43, whereas 16 neuronal gap junctions at postnatal day (P)4, P7 and P18 were detected based on Cx36 labeling. Punctate labeling for Cx36 was localized to the somatic and dendritic surfaces of peripherin-positive motoneurons in the Mo5, motoneurons throughout the spinal cord, and sexually dimorphic motoneurons at lower lumbar levels. In studies of electrical synapses and electrical transmission between developing and between adult motoneurons, our results serve to focus attention on mediation of this transmission by gap junctions composed of Cx36.

Connexin36 (Cx36) expression and protein detection in the mouse carotid body and myenteric plexus

Although connexin36 (Cx36) has been studied in several tissues, it is notable that no data are available on Cx36 expression in the carotid body and the intestine. The present study was undertaken to evaluate using immunohistochemistry, PCR and Western blotting procedures, whether Cx36 was expressed in the mouse carotid body and in the intestine at ileum and colon level. In the carotid body, Cx36 was detected as diffuse punctate immunostaining and as protein by Western blotting and mRNA by RT-PCR. Cx36 punctate immunostaining was also evident in the intestine with localization restricted to the myenteric plexus of both the ileum and the colon, and this detection was also confirmed by Western blotting and RT-PCR. All the data obtained were validated using Cx36 knockout mice. Taken together the present data on localization of Cx36 gap-junctions in two tissues of neural crest-derived neuroendocrine organs may provide an anatomical basis for future functional investigations.

Burst firing versus synchrony in a gap junction connected olfactory bulb mitral cell network model

A key player in olfactory processing is the olfactory bulb (OB) mitral cell (MC). We have used dual whole-cell patch-clamp recordings from the apical dendrite and cell soma of MCs to develop a passive compartmental model based on detailed morphological reconstructions of the same cells. Matching the model to traces recorded in experiments we find: C_m = 1.91 ± 0.20 μF cm⁻², R_m = 3547 ± 1934 Ω cm² and R_i = 173 ± 99 Ω cm. We have constructed a six MC gap-junction (GJ) network model of morphologically accurate MCs. These passive parameters (PPs) were then incorporated into the model with Na⁺, Kdr, and KA conductances and GJs from Migliore et al. (2005). The GJs were placed in the apical dendrite tuft (ADT) and their conductance adjusted to give a coupling ratio between MCs consistent with experimental findings (~0.04). Firing at ~50 Hz was induced in all six MCs with continuous current injections (0.05–0.07 nA) at 20 locations to the ADT of two of the MCs. It was found that MCs in the network synchronized better when they shared identical PPs rather than using their own PPs for the fit suggesting that the OB may have populations of MCs tuned for synchrony. The addition of calcium-activated potassium channels (iKCa) and L-type calcium channels (iCa(L)) (Bhalla and Bower, 1993) to the model enabled MCs to generate burst firing. However, the GJ coupling was no longer sufficient to synchronize firing. When cells were stimulated by a continuous current injection there was an initial period of asynchronous burst firing followed after ~120 ms by synchronous repetitive firing. This occurred as intracellular calcium fell due to reduced iCa(L) activity. The kinetics of one of the iCa(L) gate variables, which had a long activation time constant (τ ~ range 18–150 ms), was responsible for this fall in iCa(L). The model makes predictions about the nature of the kinetics of the calcium current that will need experimental verification.

Peripheral modulation of smell: fact or fiction?

Despite studies dating back 30 or more years showing modulation of odorant responses at the level of the olfactory epithelium, most descriptions of the olfactory system infer that odorant signals make their way from detection by cilia on olfactory sensory neurons to the olfactory bulb unaltered. Recent identification of multiple subtypes of microvillar cells and identification of neuropeptide and neurotransmitter expression in the olfactory mucosa add to the growing body of literature for peripheral modulation in the sense of smell. Complex mechanisms including perireceptor events, modulation of sniff rates, and changes in the properties of sensory neurons match the sensitivity of olfactory sensory neurons to the external odorant environment, internal nutritional status, reproductive status, and levels of arousal or stress. By furthering our understanding of the players mediating peripheral olfaction, we may open the door to novel approaches for modulating the sense of smell in both health and disease.

Patterns of heterogeneous expression of pannexin 1 and pannexin 2 transcripts in the olfactory epithelium and olfactory bulb

Pannexins form membrane channels that release biological signals to communicate with neighboring cells. Here, we report expression patterns of pannexin 1 (Panx1) and pannexin 2 (Panx2) in the olfactory epithelium and olfactory bulb of adult mice. In situ hybridization revealed that mRNAs for Panx1 and Panx2 were both expressed in the olfactory epithelium and olfactory bulb. Expression of Panx1 and Panx2 was mainly found in cell bodies below the sustentacular cell layer in the olfactory epithelium, indicating that Panx1 and Panx2 are expressed in mature and immature olfactory neurons, and basal cells. Expression of Panx2 was observed in sustentacular cells in a few locations of the olfactory epithelium. In the olfactory bulb, Panx1 and Panx2 were expressed in spatial patterns. Many mitral cells, tufted cells, periglomerular cells and granule cells were Panx1 and Panx2 positive. Mitral cells located at the dorsal and lateral portions of the olfactory bulb showed weak Panx1 expression compared with those in the medial side. However, the opposite was true for the distribution of Panx2 positive mitral cells. There were more Panx2 mRNA positive mitral cells and granule cells compared to those expressing Panx1. Our findings on pannexin expression in the olfactory system of adult mice raise the novel possibility that pannexins play a role in information processing in the olfactory system. Demonstration of expression patterns of pannexins in the olfactory system provides an anatomical basis for future functional studies.

The role of gap junction channels during physiologic and pathologic conditions of the human central nervous system

Gap junctions (GJs) are expressed in most cell types of the nervous system, including neuronal stem cells, neurons, astrocytes, oligodendrocytes, cells of the blood brain barrier (endothelial cells and astrocytes) and under inflammatory conditions in microglia/macrophages. GJs connect cells by the docking of two hemichannels, one from each cell with each hemichannel being formed by 6 proteins named connexins (Cx). Unapposed hemichannels (uHC) also can be open on the surface of the cells allowing the release of different intracellular factors to the extracellular space. GJs provide a mechanism of cell-to-cell communication between adjacent cells that enables the direct exchange of intracellular messengers, such as calcium, nucleotides, IP(3), and diverse metabolites, as well as electrical signals that ultimately coordinate tissue homeostasis, proliferation, differentiation, metabolism, cell survival and death. Despite their essential functions in physiological conditions, relatively little is known about the role of GJs and uHC in human diseases, especially within the nervous system. The focus of this review is to summarize recent findings related to the role of GJs and uHC in physiologic and pathologic conditions of the central nervous system.

Compensatory redistribution of neuroligins and N-cadherin following deletion of synaptic β1-integrin

β1-containing integrins are required for persistent synaptic potentiation in hippocampus and regulate hippocampal-dependent learning. Based largely on indirect evidence, there is a prevailing assumption that β1-integrins are localized at synapses, where they contribute to synapse adhesion and signaling, but this has not been examined directly. Here we investigate the fine localization of β1-integrin in adult mouse hippocampus using high-resolution immunogold labeling, with a particular emphasis on synaptic labeling patterns. We find that β1-integrins localize to synapses in CA1 and are concentrated postsynaptically. At the postsynaptic membrane, β1-integrins are found more commonly clustered near active zone centers rather than at the peripheral edges. In mice harboring a conditional deletion of β1-integrins, labeling for N-cadherin and neuroligins increases. Western blots show increased levels of N-cadherin in total lysates and neuroligins increase selectively in synaptosomes. These data suggest there is a dynamic, compensatory adjustment of synaptic adhesion. Such adjustment is specific only for certain cell adhesion molecules (CAMs), because labeling for SynCAM is unchanged. Together, our findings demonstrate unequivocally that β1-integrin is an integral synaptic adhesion protein, and suggest that adhesive function at the synapse reflects a cooperative and dynamic network of multiple CAM families.

Connexin and AMPA receptor expression changes over time in the rat olfactory bulb

Circadian rhythms affect olfaction by an unknown molecular mechanism. Independent of the suprachiasmatic nuclei, the mammalian olfactory bulb (OB) has recently been identified as a circadian oscillator. The electrical activity in the OB was reported to be synchronized to a daily rhythm and the clock gene, Period1, was oscillatory in its expression pattern. Because gap junctions composed of connexin36 and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) have been reported to work together to synchronize firing of action potentials in the OB, we hypothesized that circadian electrical oscillations could be synchronized by daily changes in the expression of connexins and AMPAR subunits (GluR1-4). We examined the OB for the presence of clock genes by polymerase chain reaction (PCR) and whether Period2, connexins, and AMPARs fluctuated across the light/dark cycle by quantitative PCR or SDS-PAGE/Western blot analysis. We observed significant changes in the messenger RNA and protein expression of our targets across 24 or 48 h. Whereas most targets were rhythmic by some measures, only GluR1 mRNA and protein were both rhythmic by the majority of our tests of rhythmicity across all time scales. Differential expression of these synaptic proteins over the light/dark cycle may underlie circadian synchronization of action potential firing in the OB or modify synaptic interactions that would be predicted to impact olfactory coding, such as alteration of granule cell inhibition, increased number of available AMPARs to bind glutamate, or an increased gap junction conductance between mitral/tufted cells.

Connexin composition in apposed gap junction hemiplaques revealed by matched double-replica freeze-fracture replica immunogold labeling

Despite the combination of light-microscopic immunocytochemistry, histochemical mRNA detection techniques and protein reporter systems, progress in identifying the protein composition of neuronal versus glial gap junctions, determination of the differential localization of their constituent connexin proteins in two apposing membranes and understanding human neurological diseases caused by connexin mutations has been problematic due to ambiguities introduced in the cellular and subcellular assignment of connexins. Misassignments occurred primarily because membranes and their constituent proteins are below the limit of resolution of light microscopic imaging techniques. Currently, only serial thin-section transmission electron microscopy and freeze-fracture replica immunogold labeling have sufficient resolution to assign connexin proteins to either or both sides of gap junction plaques. However, freeze-fracture replica immunogold labeling has been limited because conventional freeze fracturing allows retrieval of only one of the two membrane fracture faces within a gap junction, making it difficult to identify connexin coupling partners in hemiplaques removed by fracturing. We now summarize progress in ascertaining the connexin composition of two coupled hemiplaques using matched double-replicas that are labeled simultaneously for multiple connexins. This approach allows unambiguous identification of connexins and determination of the membrane "sidedness" and the identities of connexin coupling partners in homotypic and heterotypic gap junctions of vertebrate neurons.

Gap junction-mediated electrical transmission: regulatory mechanisms and plasticity

The term synapse applies to cellular specializations that articulate the processing of information within neural circuits by providing a mechanism for the transfer of information between two different neurons. There are two main modalities of synaptic transmission: chemical and electrical. While most efforts have been dedicated to the understanding of the properties and modifiability of chemical transmission, less is still known regarding the plastic properties of electrical synapses, whose structural correlate is the gap junction. A wealth of data indicates that, rather than passive intercellular channels, electrical synapses are more dynamic and modifiable than was generally perceived. This article will discuss the factors determining the strength of electrical transmission and review current evidence demonstrating its dynamic properties. Like their chemical counterparts, electrical synapses can also be plastic and modifiable. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.

Panglial gap junctional communication is essential for maintenance of myelin in the CNS

In this study, we have investigated the contribution of oligodendrocytic connexin47 (Cx47) and astrocytic Cx30 to panglial gap junctional networks as well as myelin maintenance and function by deletion of both connexin coding DNAs in mice. Biocytin injections revealed complete disruption of oligodendrocyte-to-astrocyte coupling in the white matter of 10- to 15-d-old Cx30/Cx47 double-deficient mice, while oligodendrocyte-to-oligodendrocyte coupling was maintained. There were no quantitative differences regarding cellular networks in acute brain slices obtained from Cx30/Cx47 double-null mice and control littermates, probably caused by the upregulation of oligodendrocytic Cx32 in Cx30/Cx47 double-deficient mice. We observed early onset myelin pathology, and ∼40% of Cx30/Cx47 double-deficient animals died within 42 to 90 d after birth, accompanied by severe motor impairments. Histological and ultrastructural analyses revealed severe vacuolization and myelination defects in all white matter tracts of the CNS. Furthermore, Cx30/Cx47 double-deficient mice exhibited a decreased number of oligodendrocytes, severe astrogliosis, and microglial activation in white matter tracts. Although less affected concerning motor impairment, surviving double-knock-out (KO) mice showed behavioral alterations in the open field and in the rotarod task. Vacuole formation and thinner myelin sheaths were evident also with adult surviving double-KO mice. Since interastrocytic coupling due to Cx43 expression and interoligodendrocytic coupling because of Cx32 expression are still maintained, Cx30/Cx47 double-deficient mice demonstrate the functional role of both connexins for interastrocytic, interoligodendrocytic, and panglial coupling, and show that both connexins are required for maintenance of myelin.

Mixed Electrical-Chemical Synapses in Adult Rat Hippocampus are Primarily Glutamatergic and Coupled by Connexin-36

Dendrodendritic electrical signaling via gap junctions is now an accepted feature of neuronal communication in mammalian brain, whereas axodendritic and axosomatic gap junctions have rarely been described. We present ultrastructural, immunocytochemical, and dye-coupling evidence for “mixed” (electrical/chemical) synapses on both principal cells and interneurons in adult rat hippocampus. Thin-section electron microscopic images of small gap junction-like appositions were found at mossy fiber (MF) terminals on thorny excrescences of CA3 pyramidal neurons (CA3pyr), apparently forming glutamatergic mixed synapses. Lucifer Yellow injected into weakly fixed CA3pyr was detected in MF axons that contacted four injected CA3pyr, supporting gap junction-mediated coupling between those two types of principal cells. Freeze-fracture replica immunogold labeling revealed diverse sizes and morphologies of connexin-36-containing gap junctions throughout hippocampus. Of 20 immunogold-labeled gap junctions, seven were large (328–1140 connexons), three of which were consistent with electrical synapses between interneurons; but nine were at axon terminal synapses, three of which were immediately adjacent to distinctive glutamate receptor-containing postsynaptic densities, forming mixed glutamatergic synapses. Four others were adjacent to small clusters of immunogold-labeled 10-nm E-face intramembrane particles, apparently representing extrasynaptic glutamate receptor particles. Gap junctions also were on spines in stratum lucidum, stratum oriens, dentate gyrus, and hilus, on both interneurons and unidentified neurons. In addition, one putative GABAergic mixed synapse was found in thin-section images of a CA3pyr, but none were found by immunogold labeling, suggesting the rarity of GABAergic mixed synapses. Cx36-containing gap junctions throughout hippocampus suggest the possibility of reciprocal modulation of electrical and chemical signals in diverse hippocampal neurons.

Regulation of connexin 36 expression during development

In the mammalian CNS, the expression of neuronal gap junction protein, connexin 36 (Cx36), increases during the first 2 weeks of postnatal development and then decreases during the following 2 weeks. Recently we showed that the developmental increase in Cx36 expression is augmented by chronic (2 weeks) activation of group II metabotropic glutamate receptors (mGluR), prevented by chronic receptor inactivation, and the receptor-dependent increase in Cx36 expression is regulated via transcriptional control of the Cx36 gene activity. We demonstrate here that acute (60 min) activation of group II mGluRs in developing cortical neuronal cultures causes transient increase in Cx36 protein expression with decrease during the following 24h. However, there is no change in Cx36 mRNA expression. In addition, the data indicate that transient increase in Cx36 expression is due to new protein synthesis. The results suggest that, during development, acute activation of group II mGluRs causes up-regulation of Cx36 via post-transcriptional mechanisms. However, if the receptor activation is sustained, transcriptional activation of the Cx36 gene occurs.

Epithelial membrane transporters expression in the developing to adult mouse vomeronasal organ and olfactory mucosa

To contribute clarifying mechanisms operating in nose chemosensory epithelia and their developmental patterns, we analyzed the expression of different epithelial membrane transporters as well as the Clara cell secretory protein, CC26 in the olfactory, vomeronasal organ (VNO), and respiratory epithelia of embryonic (E13-E19) and postnatal (P1-P60) mice by means of immunohistochemistry and reverse transcriptase-polymerase chain reaction. Results showed that CC26, cAMP-activated chloride channel (CFTR), and the water channel protein aquaporin 2, 3, 4, and 5 (AQP2, AQP3, AQP4, and AQP5) are expressed in developing to adult chemosensory epithelia with differential timing; moreover, their pattern of expression is not identical in VNO and olfactory epithelia as well as the corresponding associated glands; co-localization experiments using olfactory marker protein showed that CFTR, CC26, and AQP4 are not expressed in olfactory neurones. CFTR is expressed in sustentacular cells of the VNO and olfactory epithelium as well as blood vessels of the underlying mucosa, and VNO (but not Bowman's) glands; a similar pattern (excluding blood vessels) is present for AQP2; AQP4 is found in the two chemosensory epithelia and in Bowman's glands. AQP3 is expressed in the olfactory epithelium and the associated Bowman's glands, but not in the VNO chemosensory epithelium and glands. AQP5 is expressed in the olfactory epithelium and both Bowman's and VNO glands. These results indicate that water/ions handling as well as antioxidant mechanisms operating at the surface and/or inside the nose chemosensory epithelia start developing in utero and are maintained up to sexual maturity.

Post-Golgi supramolecular assembly of aquaporin-4 in orthogonal arrays

The supramolecular assembly of aquaporin-4 (AQP4) in orthogonal arrays of particles (OAPs) involves N-terminus interactions of the M23-AQP4 isoform. We found AQP4 OAPs in cell plasma membranes but not in endoplasmic reticulum (ER) or Golgi, as shown by: (i) native gel electrophoresis of brain and AQP4-transfected cells, (ii) photobleaching recovery of green fluorescent protein-AQP4 chimeras in live cells and (iii) freeze-fracture electron microscopy (FFEM). We found that AQP4 OAP formation in plasma membranes, but not in the Golgi, was not related to AQP4 density, pH, membrane lipid composition, C-terminal PDZ domain interactions or α-syntrophin expression. Remarkably, however, fusion of AQP4-containing Golgi vesicles with (AQP4-free) plasma membrane vesicles produced OAPs, suggesting the involvement of plasma membrane factor(s) in AQP4 OAP formation. In investigating additional possible determinants of OAP assembly we discovered membrane curvature-dependent OAP assembly, in which OAPs were disrupted by extrusion of plasma membrane vesicles to ∼110 nm diameter, but not to ∼220 nm diameter. We conclude that AQP4 supramolecular assembly in OAPs is a post-Golgi phenomenon involving plasma membrane-specific factor(s). Post-Golgi and membrane curvature-dependent OAP assembly may be important for vesicle transport of AQP4 in the secretory pathway and AQP4-facilitated astrocyte migration, and suggests a novel therapeutic approach for neuromyelitis optica.

Gap junction assembly: roles for the formation plaque and regulation by the C-terminus of connexin43

Gap junction (GJ) “formation plaques” are distinct membrane domains with GJ precursors; they assemble by means of a series of defined steps. The C-terminus of Cx43 is required for normal progression of assembly, normal aggregation of 10-nm particles into small GJs, and negative regulation of assembly involving protein kinase C.

Using an established gap junction (GJ) assembly system with experimentally reaggregated cells, we analyzed “formation plaques” (FPs), apparent sites of GJ assembly. Employing freeze-fracture electron microscopy methods combined with filipin labeling of sterols and immunolabeling for connexin43 (Cx43), we demonstrated that FPs constitute distinct membrane “domains” and that their characteristic 10-nm particles contain connexin43, thus representing precursors (i.e., GJ hemichannels) engaged in assembly. Analysis of FPs in new systems—HeLa and N2A cells—resolved questions surrounding several key but poorly understood steps in assembly, including matching of FP membranes in apposed cells, reduction in the separation between FP membranes during assembly, and the process of particle aggregation. Findings also indicated that “docking” of GJ hemichannels occurs within FP domains and contributes to reduction of intermembrane separation between FPs. Other experiments demonstrated that FPs develop following a major C-terminal truncation of Cx43 (M257), although assembly was delayed. Particle aggregation also occurred at lower densities, and densities of particles within developing GJ aggregates failed to achieve full-length levels. With regard to regulation, inhibition of assembly following protein kinase C activation failed to occur in the M257 truncation mutants, as measured by intercellular dye transfer. However, several C-terminal serine mutations failed to disrupt inhibition.

Expression of connexin 57 in the olfactory epithelium and olfactory bulb

In the visual system, deletion of connexin 57 (Cx57) reduces gap junction coupling among horizontal cells and results in smaller receptive fields. To explore potential functions of Cx57 in olfaction, in situ hybridization and immunohistochemistry methods were used to investigate expression of Cx57 in the olfactory epithelium and olfactory bulb. Hybridization signal was stronger in the olfactory epithelial layer compared to the connective tissue underneath. Within the sensory epithelial layer, hybridization signal was visible in sublayers containing cell bodies of basal cells and olfactory neurons but not evident at the apical sublayer comprising cell bodies of sustentacular cells. These Cx57 positive cells were clustered into small groups to form different patterns in the olfactory epithelium. However, individual patterns did not associate with specific regions of olfactory turbinates or specific olfactory receptor zones. Patched distribution of hybridization positive cells was also observed in the olfactory bulb and accessory olfactory bulb in layers where granule cells, mitral cells, and juxtaglomerular cells reside. Immunostaining was observed in the cell types described above but the intensity was weaker than that in the retina. This study has provided anatomical basis for future studies on the function of Cx57 in the olfactory system.

Interplay of chemical neurotransmitters regulates developmental increase in electrical synapses

Coupling of neurons by electrical synapses (gap junctions) transiently increases in the mammalian CNS during development. We report here that the developmental increase in neuronal gap junction coupling and expression of connexin 36 (Cx36; neuronal gap junction protein) are regulated by an interplay between the activity of group II metabotropic glutamate receptors (mGluRs) and GABA(A) receptors. Specifically, using dye coupling, electrotonic coupling, Western blots and small interfering RNA in the rat and mouse hypothalamus and cortex in vivo and in vitro, we demonstrate that activation of group II mGluRs augments, and inactivation prevents, the developmental increase in neuronal gap junction coupling and Cx36 expression. However, changes in GABA(A) receptor activity have the opposite effects. The regulation by group II mGluRs is via cAMP/PKA-dependent signaling, and regulation by GABA(A) receptors is via Ca(2+)/PKC-dependent signaling. Furthermore, the receptor-mediated upregulation of Cx36 requires a neuron-restrictive silencer element in the Cx36 gene promoter, and the downregulation involves the 3'-untranslated region of the Cx36 mRNA, as shown using reverse-transcription quantitative real-time PCR and luciferase reporter activity analysis. In addition, the methyl thiazolyl tetrazolium analysis indicates that mechanisms for the developmental increase in neuronal gap junction coupling directly control the death/survival mechanisms in developing neurons. Together, the results suggest a multitiered strategy for chemical synapses in developmental regulation of electrical synapses.

Microvillous cells expressing IP3 receptor type 3 in the olfactory epithelium of mice

Microvillous cells of the main olfactory epithelium have been described variously as primary olfactory neurons, secondary chemosensory cells or non-sensory cells. Here we generated an IP3R3(tm1(tauGFP)) mouse in which the coding region for a fusion protein of tau and green fluorescent protein replaces the first exon of the Itpr3 gene. We provide immunohistochemical and functional characterization of the cells expressing IP3 receptor type 3 in the olfactory epithelium. These cells bear microvilli at their apex, and we therefore termed them IP3R3 MV cells. The cell body of these IP3R3 MV cells lies in the upper third of the main olfactory epithelium; a long thick basal process projects towards the base of the epithelium without penetrating the basal lamina. Retrograde labeling and unilateral bulbectomy corroborated that these IP3R3 MV cells do not extend axons to the olfactory bulb and therefore are not olfactory sensory neurons. The immunohistochemical features of IP3R3 MV cells varied, suggesting either developmental stages or the existence of subsets of these cells. Thus, for example, subsets of the IP3R3 MV cells make contact with substance P fibers or express the purinergic receptor P2X3. In addition, in recordings of intracellular calcium, these cells respond to ATP and substance P as well as to a variety of odors. The characterization of IP3R3 MV cells as non-neuronal chemoresponsive cells helps to explain the differing descriptions of microvillous cells in the literature.

A male poecillid's sexually dimorphic body plan, behavior, and nervous system

Here we review the literature of a male poecillid's sexually dimorphic body plan, behavior, and nervous system, including work dating from the mid 1800s to the mid 1990s as well as work in press or in preparation for publication. Rosa-Molinar described the remodeling of the sexually dimorphic anal fin appendicular support, confirmed earlier claims about the development of the male and female secondary sex characteristics in the Western Mosquitofish, Gambusia affinis and provided for the first time direct embryonic evidence suggesting that remodeling of the sexually dimorphic anal fin appendicular support is biphasic. The first process begins in embryos and proceeds similarly in immature males and females; the second process occurs only in males and results in the anterior transposition of the anal fin and its appendicular support to the level of vertebra 11 [Rosa-Molinar E, Hendricks SE, Rodriguez-Sierra JF, Fritzsch B. 1994. Development of the anal fin appendicular support in the western mosquitofish, Gambusia affinis (Baird and Girard, 1854): a reinvestigation and reinterpretation. Acta Anat 151:20-35.] and the formation of a gonopodium used for internal fertilization. Studies using high-speed video cameras confirmed and extended Peden's and others' observations of copulatory behavior. The cameras showed that circumduction is a complex movement combining in a very fast sequence abduction, extension and pronation, S-start-type fast-start (defined as torque-thrust), and adduction movements. Recent work on the nervous system demonstrated dye-coupling between motor neurons and interneurons via gap junctions, suggesting an attractive substrate for the rapid motions involved in poecillid copulatory reflexes.