Gap junctions and their implications for neurogenesis and maturation of synaptic circuitry in the developing neocortex

Modulation of gap junction-associated Cx43 in neural stem/progenitor cells following traumatic brain injury

Restoration of learning and memory deficits following traumatic brain injury (TBI) is attributed, in part, to enhanced neural stem/progenitor cell (NSPCs) function. Recent findings suggest gap junction (GJ)-associated connexin 43 (Cx43) plays a key role in the cell cycle regulation and function of NSPCs and is modulated following TBI. Here, we demonstrate that Cx43 is up-regulated in the dentate gyrus following TBI and is expressed on vimentin-positive cells in the subgranular zone. To test the role of Cx43 on NSPCs, we exposed primary cultures to the α-connexin Carboxyl Terminal (αCT1) peptide which selectively modulates GJ-associated Cx43. Treatment with αCT1 substantially reduced proliferation and increased caspase 3/7 expression on NSPCs in a dose-dependent manner. αCT1 exposure also reduced overall expression of Cx43 and phospho (p)-Serine368. These findings demonstrate that Cx43 positively regulates adult NPSCs; the modulation of which may influence changes in the dentate gyrus following TBI.

Electrical coupling regulates layer 1 interneuron microcircuit formation in the neocortex

The coexistence of electrical and chemical synapses among interneurons is essential for interneuron function in the neocortex. However, it remains largely unclear whether electrical coupling between interneurons influences chemical synapse formation and microcircuit assembly during development. Here, we show that electrical and GABAergic chemical connections robustly develop between interneurons in neocortical layer 1 over a similar time course. Electrical coupling promotes action potential generation and synchronous firing between layer 1 interneurons. Furthermore, electrically coupled interneurons exhibit strong GABA-A receptor-mediated synchronous synaptic activity. Disruption of electrical coupling leads to a loss of bidirectional, but not unidirectional, GABAergic connections. Moreover, a reduction in electrical coupling induces an increase in excitatory synaptic inputs to layer 1 interneurons. Together, these findings strongly suggest that electrical coupling between neocortical interneurons plays a critical role in regulating chemical synapse development and precise formation of circuits.

Preferentially regulated expression of connexin 43 in the developing spiral ganglion neurons and afferent terminals in post-natal rat cochlea

The expression pattern of connexin 43 (Cx43) in the cochlea is not determined and is controversial. Since the presence of Cx43 is essential for hearing, we re-examined its distribution during post-natal development of rat cochlea. Cx43 protein was expressed in spiral ganglion neurons (SGNs) and their neurite terminals innervating the inner and outer hair cells (IHCs and OHCs) as early as birth (postnatal day 0, P0), and persisted until P14. Double immunofluorescence staining, using two antibodies against Cx43 and TUJ1, a marker for all SGNs and afferent terminals, showed that immunoreactivity for Cx43 and TUJ1 was perfectly colocalized in SGNs and afferent terminals associated with the IHCs and OHCs. However, beyond P14, Cx43 immunostaining could no longer be detected in the region of the synaptic terminals at the bases of IHCs and OHCs (P17, adult). In contrast, Cx43 maintained its expression in SGNs into adulthood. We further performed quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) to identify the presence of Cx43 mRNA in the modiolus (mainly containing SGNs). Cx43 mRNA was higher at P8, compared with P1, and subsequently decreased at P14. These results indicated that Cx43 correlated with cochlear synaptogenesis and establishment of auditory neurotransmission.

A novel method for inducing nerve growth via modulation of host resting potential: gap junction-mediated and serotonergic signaling mechanisms

A major goal of regenerative medicine is to restore the function of damaged or missing organs through the implantation of bioengineered or donor-derived components. It is necessary to understand the signals and cues necessary for implanted structures to innervate the host, as organs devoid of neural connections provide little benefit to the patient. While developmental studies have identified neuronal pathfinding molecules required for proper patterning during embryogenesis, strategies to initiate innervation in structures transplanted at later times or alternate locations remain limited. Recent work has identified membrane resting potential of nerves as a key regulator of growth cone extension or arrest. Here, we identify a novel role of bioelectricity in the generation of axon guidance cues, showing that neurons read the electric topography of surrounding cells, and demonstrate these cues can be leveraged to initiate sensory organ transplant innervation. Grafts of fluorescently labeled embryological eye primordia were used to produce ectopic eyes in Xenopus laevis tadpoles. Depolarization of host tissues through anion channel activation or other means led to a striking hyperinnervation of the body by these ectopic eyes. A screen of possible transduction mechanisms identified serotonergic signaling to be essential for hyperinnervation to occur, and our molecular data suggest a possible model of bioelectrical control of the distribution of neurotransmitters that guides nerve growth. Together, these results identify the molecular components of bioelectrical signaling among cells that regulates axon guidance, and suggest novel biomedical and bioengineering strategies for triggering neuronal outgrowth using ion channel drugs already approved for human use.

Generation of functional neural artificial tissue from human umbilical cord blood stem cells

Stem cell-based regenerative neurology is an emerging concept for treatment of diseases of central nervous system. Among variety of proposed procedures, one of the most promising is refilling of cystic cavities of injured brain parenchyma with artificial neural tissue. Recent studies revealed that after allogenic transplantation in rodents these tissue-engineered entities were shown efficient in repair of hypoxic/ischemic brain injury. Human umbilical cord blood (HUCB) was recognized to be an efficient and noncontroversial source of neural stem cells (NSC). The main purpose of this study was to generate HUCB-derived neural artificial tissue and investigate their functional properties. Neural organoids formed on human-originated biodegradable scaffolds within 3 weeks and resembled niche structure where immature stem cells (Oct4+ and Sox2+) and proliferating neuroblasts (Nestin+, GFAP+, and Ki67+) were present. Such aggregates were placed on multi-electrode chips and differentiated toward mature neurons (TUJ1+ and MAP2+). These three-dimensional aggregates in contrast to two-dimensional cultures formed functional circuits and generated spontaneous field/action potentials. Our results indicate that three-dimensional environment facilitates maturation of HUCB-derived NSC what should be considered regarding regenerative medicine application.

Innexins in the lobster stomatogastric nervous system: cloning, phylogenetic analysis, developmental changes and expression within adult identified dye and electrically coupled neurons

Gap junctions play a key role in the operation of neuronal networks by enabling direct electrical and metabolic communication between neurons. Suitable models to investigate their role in network operation and plasticity are invertebrate motor networks, which are built of comparatively few identified neurons, and can be examined throughout development; an excellent example is the lobster stomatogastric nervous system. In invertebrates, gap junctions are formed by proteins that belong to the innexin family. Here, we report the first molecular characterization of two crustacean innexins: the lobster Homarus gammarus innexin 1 (Hg-inx1) and 2 (Hg-inx2). Phylogenetic analysis reveals that innexin gene duplication occurred within the arthropod clade before the separation of insect and crustacean lineages. Using in situ hybridization, we find that each innexin is expressed within the adult and developing lobster stomatogastric nervous system and undergoes a marked down-regulation throughout development within the stomatogastric ganglion (STG). The number of innexin expressing neurons is significantly higher in the embryo than in the adult. By combining in situ hybridization, dye and electrical coupling experiments on identified neurons, we demonstrate that adult neurons that express at least one innexin are dye and electrically coupled with at least one other STG neuron. Finally, two STG neurons display no detectable amount of either innexin mRNAs but may express weak electrical coupling with other STG neurons, suggesting the existence of other forms of innexins. Altogether, we provide evidence that innexins are expressed within small neuronal networks built of dye and electrically coupled neurons and may be developmentally regulated.

Involvement of gap junctions in the development of the neocortex

Gap junctions play an important role during the development of the mammalian brain. In the neocortex, gap junctions are already expressed at very early stages of development and they seem to be involved in many processes like neurogenesis, migration and synapse formation. Gap junctions are found in all cell types including progenitor cells, glial cells and neurons. These direct cell-to-cell connections form clusters consisting of a distinct number of cells of a certain type. These clusters can be considered as communication compartments in which the information transfer is mediated electrically by ionic currents and/or chemically by, e.g., small second messenger molecules. Within the neocortex, four such communication compartments can be identified: (1) gap junction-coupled neuroblasts of the ventricular zone and gap junctions in migrating cells and radial glia, (2) gap junction-coupled glial cells (astrocytes and oligodendrocytes), (3) gap junction-coupled pyramidal cells (only during the first two postnatal weeks) and (4) gap junction-coupled inhibitory interneurons. These compartments can consist of sub-compartments and they may overlap to some degree. The compartments 1 and 3 disappear with ongoing develop, whereas compartments 2 and 4 persist in the mature neocortex. Gap junction-mediated coupling of glial cells seems to be important for stabilization of the extracellular ion homeostasis, uptake of neurotransmitters, migration of neurons and myelination of axons. Electrical synapses between inhibitory interneurons facilitate the synchronization of pyramidal cells. In this way, they contribute to the generation of oscillatory network activity correlated with higher cortical functions. The role of gap junctions present in neuroblasts of the ventricular zone as well as the role of gap junctions found in pyramidal cells during the early postnatal stages is less clear. It is assumed that they might help to form precursors of the functional columns observed in the mature neocortex. Although recent developments of new techniques led to the solution of many problems concerning gap junction-coupling between neurons and glial cells in the neocortex, there are many open questions which need to be answered before we can achieve a comprehensive understanding of the role of gap junctions in the development of the neocortex.

Electrical synapse formation disrupts calcium-dependent exocytosis, but not vesicle mobilization

Electrical coupling exists prior to the onset of chemical connectivity at many developing and regenerating synapses. At cholinergic synapses in vitro, trophic factors facilitated the formation of electrical synapses and interfered with functional neurotransmitter release in response to photolytic elevations of intracellular calcium. In contrast, neurons lacking trophic factor induction and electrical coupling possessed flash-evoked transmitter release. Changes in cytosolic calcium and postsynaptic responsiveness to acetylcholine were not affected by electrical coupling. These data indicate that transient electrical synapse formation delayed chemical synaptic transmission by imposing a functional block between the accumulation of presynaptic calcium and synchronized, vesicular release. Despite the inability to release neurotransmitter, neurons that had possessed strong electrical coupling recruited secretory vesicles to sites of synaptic contact. These results suggest that the mechanism by which neurotransmission is disrupted during electrical synapse formation is downstream of both calcium influx and synaptic vesicle mobilization. Therefore, electrical synaptogenesis may inhibit synaptic vesicles from acquiring a readily releasable state. We hypothesize that gap junctions might negatively interact with exocytotic processes, thereby diminishing chemical neurotransmission.

Ethanol inhibits gap-junctional coupling between P19 cells

BACKGROUND

Gap junctions are plaques of multiple intercellular channels that connect the cytoplasm of adjacent cells. They provide both electrical and metabolic coupling and are an essential element in normal growth, development, and physiology. Little research exists on the relationship between alcohol administration and gap-junctional function or expression. This study looks at the function and expression of gap junctions after incubation and withdrawal of ethanol with P19 cell cultures.

METHODS

Gap-junctional communication was assessed after 24 and 48 hr of exposure to 20 and 40 mM ethanol and after a 24-hr withdrawal period. The seeding technique was used, and diacyl-3,3'-indocarbocyanine iodide/calcein-stained donor cells were seeded on an unstained monolayer and then reviewed by confocal microscope and counted by flow cytometry. Analysis of connexin (Cx) proteins was performed by Western blot, gel electrophoresis, and immunoblots with antibodies for Cx26 and Cx43.

RESULTS

All treatment regimens produced similar results, reducing dye coupling by more than 50% without recovery after a 24-hr withdrawal period. Exposing the cells to 20 mM ethanol for 48 hr did not significantly change the levels of Cx26 protein, but ethanol significantly decreased the levels of Cx43 in cultured P19 cells.

CONCLUSIONS

This study illustrates that ethanol can inhibit gap-junction function in the P19 cell line. Chronic exposure to 20 mM ethanol selectively decreased the levels of Cx43 protein in the membrane fraction of the cell cultures.

Gap junctions in developing neocortex: a review

Several independent lines of evidence indicate that gap junctional coupling is widespread and functionally important in early cortical development. The extensive expression of many connexins, the clusters of coupled neuroblasts or neurons revealed by tracer injections or the functional coactivation of neurons shown by calcium imaging can be seen as pieces of the same puzzle. At the same time, there is a lack of electrophysiological experiments at early developmental stages that directly demonstrate coupling between pairs of cells. The function of this gap junctional coupling is still mysterious, though it may be involved in controlling neurogenesis, differentiation or the formation of columnar microcircuits.

Modulation of muscarinic facilitation of epileptiform discharges in immature rat neocortex

We examined the cholinergic effects on epileptiform discharge generation in immature (postnatal days 10-20) rat neocortex. Evoked and spontaneous field potentials were recorded from the deep layers of neocortical slices during GABA(A) receptor blockade by bicuculline methiodide (BMI, 50 microM). The anticholinesterase eserine (10 microM) as well as the ACh-analog carbamylcholine chloride (CCh, 25 microM) decreased the amplitude and duration of evoked field potentials and in parallel, increased significantly the rate of occurrence of spontaneous discharges. These effects were reversed by the muscarinic antagonist atropine (2.5 microM, n = 20), but not by the nicotinic receptor antagonist hexamethonium (50 microM, n = 3). The M1 subtype-selective muscarinic antagonist pirenzepine (1 microM, n = 12) blocked spontaneous discharges in 8/12 slices, while muscarinic antagonists of the M2 (AFDX 116 n = 4), M3 (4-DAMP n = 4) and M4 (gallamine n = 5, tropicamide n = 6) type, all at 1 microM, only reduced their frequency. CCh-induced spontaneous discharges were blocked by the combination of the glutamate receptor antagonists AP5 and CNQX (both at 10 microM; n = 11). Gap junction blockers abolished them (halothane, n = 7) or reduced their frequency by 65% (carbenoxolone, n = 8). Inhibiting Ca2+ release from intracellular stores by dantrolene (100 microM, n = 5) or thapsigargin (1 microM, n = 5) also depressed their frequencies by 55-65%. By contrast, their rates were not altered by perfusion with high Ca2+ (7 mM; n = 6) medium, a manipulation suppressing polysynaptic connections. These findings demonstrate that activation of muscarinic receptors, notably of the M1 type, in immature rat neocortex facilitates the generation of glutamatergic epileptiform discharges. These discharges are strongly inhibited by gap junction blockers, and are also partly mediated by the, presumably muscarinic receptor-dependent, mobilization of intracellular calcium.