Changes in the properties of gap junctions during neuronal differentiation of hippocampal progenitor cells

APOE ε4-associated downregulation of the IL-7/IL-7R pathway in effector memory T cells: Implications for Alzheimer's disease

INTRODUCTION

The apolipoprotein E (APOE) ε4 allele exerts a significant influence on peripheral inflammation and neuroinflammation, yet the underlying mechanisms remain elusive.

METHODS

The present study enrolled 54 patients diagnosed with late-onset Alzheimer's disease (AD; including 28 APOE ε4 carriers and 26 non-carriers). Plasma inflammatory cytokine concentration was assessed, alongside bulk RNA sequencing (RNA-seq) and single-cell RNA sequencing (scRNA-seq) analysis of peripheral blood mononuclear cells (PBMCs).

RESULTS

Plasma tumor necrosis factor α, interferon γ, and interleukin (IL)-33 levels increased in the APOE ε4 carriers but IL-7 expression notably decreased. A negative correlation was observed between plasma IL-7 level and the hippocampal atrophy degree. Additionally, the expression of IL-7R and CD28 also decreased in PBMCs of APOE ε4 carriers. ScRNA-seq data results indicated that the changes were mainly related to the CD4+ Tem (effector memory) and CD8+ Tem T cells.

DISCUSSION

These findings shed light on the role of the downregulated IL-7/IL-7R pathway associated with the APOE ε4 allele in modulating neuroinflammation and hippocampal atrophy.

HIGHLIGHTS

The apolipoprotein E (APOE) ε4 allele decreases plasma interleukin (IL)-7 and aggravates hippocampal atrophy in Alzheimer's disease. Plasma IL-7 level is negatively associated with the degree of hippocampal atrophy. The expression of IL-7R signaling decreased in peripheral blood mononuclear cells of APOE ε4 carriers Dysregulation of the IL-7/IL-7R signal pathways enriches T cells.

The Gap Junction Inhibitor Octanol Decreases Proliferation and Increases Glial Differentiation of Postnatal Neural Progenitor Cells

Neural precursor cells (NPCs) that persist in the postnatal/adult subventricular zone (SVZ) express connexins that form hemichannels and gap junctions. Gap junctional communication plays a role in NPC proliferation and differentiation during development, but its relevance on postnatal age remains to be elucidated. In this work we aimed to evaluate the effect of the blockade of gap junctional communication on proliferation and cell fate of NPCs obtained from the SVZ of postnatal rats. NPCs were isolated and expanded in culture as neurospheres. Electron microscopy revealed the existence of gap junctions among neurosphere cells. Treatment of cultures with octanol, a broad-spectrum gap junction blocker, or with Gap27, a specific blocker for gap junctions formed by connexin43, produced a significant decrease in bromodeoxyuridine incorporation. Octanol treatment also exerted a dose-dependent antiproliferative effect on glioblastoma cells. To analyze possible actions on NPC fate, cells were seeded in the absence of mitogens. Treatment with octanol led to an increase in the percentage of astrocytes and oligodendrocyte precursors, whereas the percentage of neurons remained unchanged. Gap27 treatment, in contrast, did not modify the differentiation pattern of SVZ NPCs. Our results indicate that general blockade of gap junctions with octanol induces significant effects on the behavior of postnatal SVZ NPCs, by reducing proliferation and promoting glial differentiation.

Inhibition of Gap Junction Formation Prior to Implantation of Bone Marrow-Derived Mesenchymal Cells Improves Function in the Ischemic Myocardium

Bone marrow-derived mesenchymal stem cells (BM-MSC) are reported to induce beneficial effects in the heart following ischemia, but a loss of these cells within hours of implantation could significantly diminish their long-term effect. We hypothesized that early coupling between BM-MSC and ischemic cardiomyocytes through gap junctions (GJ) may play an important role in stem cell survival and retention in the acute phase of myocardial ischemia. To determine the effect of GJ inhibition on murine BM-MSC in vivo, we induced ischemia in mice using 90 min left anterior descending coronary artery (LAD) occlusion followed by BM-MSC implantation and reperfusion. The inhibition of GJ coupling prior to BM-MSC implantation led to early improvement in cardiac function compared to mice in which GJ coupling was not inhibited. Our results with in vitro studies also demonstrated increased survival in BM-MSCs subjected to hypoxia after inhibition of GJ. While functional GJ are critical for the long-term integration of stem cells within the myocardium, early GJ communication may represent a novel paradigm whereby ischemic cardiomyocytes induce a "bystander effect" when coupled to newly transplanted BM-MSC and thus impair cell retention and survival.

Kv1.1 preserves the neural stem cell pool and facilitates neuron maturation during adult hippocampal neurogenesis

Despite decades of research on adult neurogenesis, little is known about the role of bioelectric signaling in this process. In this study, we describe how a voltage-gated potassium channel, K_v1.1, supports adult neurogenesis by maintaining the neural stem cell niche and facilitating newborn neuron development. Additionally, we show that deletion of K_v1.1 from adult neural stem cells contributes to modest impairments in hippocampus-dependent contextual fear learning and memory. Dysfunctional adult neurogenesis has been implicated in cognitive decline associated with aging and neurological disease. Therefore, understanding the role of K_v1.1 in adult neurogenesis represents an opportunity to identify new therapeutic targets to promote healthy neurogenesis and cognition.

Adult hippocampal neurogenesis is critical for learning and memory, and aberrant adult neurogenesis has been implicated in cognitive decline associated with aging and neurological diseases [J. T. Gonçalves, S. T. Schafer, F. H. Gage, Cell 167, 897–914 (2016)]. In previous studies, we observed that the delayed-rectifier voltage-gated potassium channel K_v1.1 controls the membrane potential of neural stem and progenitor cells and acts as a brake on neurogenesis during neonatal hippocampal development [S. M. Chou et al., eLife 10, e58779 (2021)]. To assess the role of K_v1.1 in adult hippocampal neurogenesis, we developed an inducible conditional knockout mouse to specifically remove K_v1.1 from adult neural stem cells via tamoxifen administration. We determined that K_v1.1 deletion in adult neural stem cells causes overproliferation and depletion of radial glia-like neural stem cells, prevents proper adult-born granule cell maturation and integration into the dentate gyrus, and moderately impairs hippocampus-dependent contextual fear learning and memory. Taken together, these findings support a critical role for this voltage-gated ion channel in adult neurogenesis.

GAP junctions: multifaceted regulators of neuronal differentiation

Gap junctions are intercellular membrane channels consisting of connexin proteins, which contribute to direct cytoplasmic exchange of small molecules, substrates and metabolites between adjacent cells. These channels play important roles in neuronal differentiation, maintenance, survival and function. Gap junctions regulate differentiation of neurons from embryonic, neural and induced pluripotent stem cells. In addition, they control transdifferentiation of neurons from mesenchymal stem cells. The expression and levels of several connexins correlate with cell cycle changes and different stages of neurogenesis. Connexins such as Cx36, Cx45, and Cx26, play a crucial role in neuronal function. Several connexin knockout mice display lethal or severely impaired phenotypes. Aberrations in connexin expression is frequently associated with various neurodegenerative disorders. Gap junctions also act as promising therapeutic targets for neuronal regenerative medicine, because of their role in neural stem cell integration, injury and remyelination.

Kv1.1 channels regulate early postnatal neurogenesis in mouse hippocampus via the TrkB signaling pathway

In the postnatal brain, neurogenesis occurs only within a few regions, such as the hippocampal sub-granular zone (SGZ). Postnatal neurogenesis is tightly regulated by factors that balance stem cell renewal with differentiation, and it gives rise to neurons that participate in learning and memory formation. The Kv1.1 channel, a voltage-gated potassium channel, was previously shown to suppress postnatal neurogenesis in the SGZ in a cell-autonomous manner. In this study, we have clarified the physiological and molecular mechanisms underlying Kv1.1-dependent postnatal neurogenesis. First, we discovered that the membrane potential of neural progenitor cells is highly dynamic during development. We further established a multinomial logistic regression model for cell-type classification based on the biophysical characteristics and corresponding cell markers. We found that the loss of Kv1.1 channel activity causes significant depolarization of type 2b neural progenitor cells. This depolarization is associated with increased tropomyosin receptor kinase B (TrkB) signaling and proliferation of neural progenitor cells; suppressing TrkB signaling reduces the extent of postnatal neurogenesis. Thus, our study defines the role of the Kv1.1 potassium channel in regulating the proliferation of postnatal neural progenitor cells in mouse hippocampus.

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.

Abrogation of atypical neurogenesis and vascular-derived EphA4 prevents repeated mild TBI-induced learning and memory impairments

Brain injury resulting from repeated mild traumatic insult is associated with cognitive dysfunction and other chronic co-morbidities. The current study tested the effects of aberrant neurogenesis in a mouse model of repeated mild traumatic brain injury (rmTBI). Using Barnes Maze analysis, we found a significant reduction in spatial learning and memory at 24 days post-rmTBI compared to repeated sham (rSham) injury. Cell fate analysis showed a greater number of BrdU-labeled cells which co-expressed Prox-1 in the DG of rmTBI-injured mice which coincided with enhanced cFos expression for neuronal activity. We then selectively ablated dividing neural progenitor cells using a 7-day continuous infusion of Ara-C prior to rSham or rmTBI. This resulted in attenuation of cFos and BrdU-labeled cell changes and prevented associated learning and memory deficits. We further showed this phenotype was ameliorated in EphA4f./f/Tie2-Cre knockout compared to EphA4f./f wild type mice, which coincided with altered mRNA transcript levels of MCP-1, Cx43 and TGFβ. These findings demonstrate that cognitive decline is associated with an increased presence of immature neurons and gene expression changes in the DG following rmTBI. Our data also suggests that vascular EphA4-mediated neurogenic remodeling adversely affects learning and memory behavior in response to repeated insult.

Pharmacological Transdifferentiation of Human Nasal Olfactory Stem Cells into Dopaminergic Neurons

The discovery of novel drugs for neurodegenerative diseases has been a real challenge over the last decades. The development of patient- and/or disease-specific in vitro models represents a powerful strategy for the development and validation of lead candidates in preclinical settings. The implementation of a reliable platform modeling dopaminergic neurons will be an asset in the study of dopamine-associated pathologies such as Parkinson's disease. Disease models based on cell reprogramming strategies, using either human-induced pluripotent stem cells or transcription factor-mediated transdifferentiation, are among the most investigated strategies. However, multipotent adult stem cells remain of high interest to devise direct conversion protocols and establish in vitro models that could bypass certain limitations associated with reprogramming strategies. Here, we report the development of a six-step chemically defined protocol that drives the transdifferentiation of human nasal olfactory stem cells into dopaminergic neurons. Morphological changes were progressively accompanied by modifications matching transcript and protein dopaminergic signatures such as LIM homeobox transcription factor 1 alpha (LMX1A), LMX1B, and tyrosine hydroxylase (TH) expression, within 42 days of differentiation. Phenotypic changes were confirmed by the production of dopamine from differentiated neurons. This new strategy paves the way to develop more disease-relevant models by establishing reprogramming-free patient-specific dopaminergic cell models for drug screening and/or target validation for neurodegenerative diseases.

Rat enteric glial cells express novel isoforms of Interleukine-7 regulated during inflammation

BACKGROUND

Neuroimmune interactions are essential to maintain gut homeostasis and prevent intestinal disorders but so far, the impact of enteric glial cells (EGC) on immune cells remains a relatively unexplored area of research. As a dysregulation of critical cytokines such as interleukine-7 (IL-7) was suggested to exacerbate gut chronic inflammation, we investigated whether EGC could be a source of IL-7 in the gastrointestinal tract.

METHODS

Expression of IL-7 in the rat enteric nervous system was analyzed by immunochemistry and Q-PCR. IL-7 variants were cloned and specific antibodies against rat IL-7 isoforms were raised to characterize their expression in the submucosal plexus. IL-7 isoforms were produced in vitro to analyze their impact on T-cell survival.

KEY RESULTS

Neurons and glial cells of the rat enteric nervous system expressed IL-7 at both mRNA and protein levels. Novel rat IL-7 isoforms with distinct C-terminal parts were detected. Three of these isoforms were found in EGC or in both enteric neurons and EGC. Exposure of EGC to pro-inflammatory cytokines (IL-1β and/or TNFα) induced an upregulation of all IL-7 isoforms. Interestingly, time-course and intensity of the upregulation varied according to the presence or absence of exon 5a in IL-7 variants. Functional analysis on T lymphocytes revealed that only canonical IL-7 protects T cells from cell death.

CONCLUSIONS AND INFERENCES

IL-7 and its variants are expressed by neurons and glial cells in the enteric nervous system. Their distinct expression and upregulation in inflammatory conditions suggest a role in gut homeostasis which could be critical in case of chronic inflammatory diseases.

Defined Geldrop Cultures Maintain Neural Precursor Cells

Distinct micro-environmental properties have been reported to be essential for maintenance of neural precursor cells (NPCs) within the adult brain. Due to high complexity and technical limitations, the natural niche can barely be studied systematically in vivo. By reconstituting selected environmental properties (adhesiveness, proteolytic degradability, and elasticity) in geldrop cultures, we show that NPCs can be maintained stably at high density over an extended period of time (up to 8 days). In both conventional systems, neurospheres and monolayer cultures, they would expand and (in the case of neurospheres) differentiate rapidly. Further, we report a critical dualism between matrix adhesiveness and degradability. Only if both features are functional NPCs stay proliferative. Lastly, Rho-associated protein kinase was identified as part of a pivotal intracellular signaling cascade controlling cell morphology in response to environmental cues inside geldrop cultures. Our findings demonstrate that simple manipulations of the microenvironment in vitro result in an important preservation of stemness features in the cultured precursor cells.

The Neuroglial Dialog Between Cannabinoids and Hemichannels

The formation of gap junctions was initially thought to be the central role of connexins, however, recent evidence had brought to light the high relevance of unopposed hemichannels as an independent mechanism for the selective release of biomolecules during physiological and pathological conditions. In the healthy brain, the physiological opening of astrocyte hemichannels modulates basal excitatory synaptic transmission. At the other end, the release of potentially neurotoxic compounds through astroglial hemichannels and pannexons has been insinuated as one of the functional alterations that negatively affect the progression of multiple brain diseases. Recent insights in this matter have suggested encannabinoids (eCBs) as molecules that could regulate the opening of these channels during diverse conditions. In this review, we discuss and hypothesize the possible interplay between the eCB system and the hemichannel/pannexon-mediated signaling in the inflamed brain and during event of synaptic plasticity. Most findings indicate that eCBs seem to counteract the activation of major neuroinflammatory pathways that lead to glia-mediated production of TNF-α and IL-1β, both well-known triggers of astroglial hemichannel opening. In contrast to the latter, in the normal brain, eCBs apparently elicit the Ca²⁺-activation of astrocyte hemichannels, which could have significant consequences on eCB-dependent synaptic plasticity.

Multifaceted Roles of Connexin 43 in Stem Cell Niches

Purpose of Review

Considerable progress has been made in the field of stem cell research; nonetheless, the use of stem cells for regenerative medicine therapies, for either endogenous tissue repair or cellular grafts post injury, remains a challenge. To better understand how to maintain stem cell potential in vivo and promote differentiation ex vivo, it is fundamentally important to elucidate the interactions between stem cells and their surrounding partners within their distinct niches.

Recent Findings

Among the vast array of proteins depicted as mediators for cell-to-cell interactions, connexin-comprised gap junctions play pivotal roles in the regulation of stem cell fate both in vivo and in vitro.

Summary

This review summarizes and illustrates the current knowledge regarding the multifaceted roles of Cx43, specifically, in various stem cell niches.

Connexin-Dependent Neuroglial Networking as a New Therapeutic Target

Astrocytes and neurons dynamically interact during physiological processes, and it is now widely accepted that they are both organized in plastic and tightly regulated networks. Astrocytes are connected through connexin-based gap junction channels, with brain region specificities, and those networks modulate neuronal activities, such as those involved in sleep-wake cycle, cognitive, or sensory functions. Additionally, astrocyte domains have been involved in neurogenesis and neuronal differentiation during development; they participate in the “tripartite synapse” with both pre-synaptic and post-synaptic neurons by tuning down or up neuronal activities through the control of neuronal synaptic strength. Connexin-based hemichannels are also involved in those regulations of neuronal activities, however, this feature will not be considered in the present review. Furthermore, neuronal processes, transmitting electrical signals to chemical synapses, stringently control astroglial connexin expression, and channel functions. Long-range energy trafficking toward neurons through connexin-coupled astrocytes and plasticity of those networks are hence largely dependent on neuronal activity. Such reciprocal interactions between neurons and astrocyte networks involve neurotransmitters, cytokines, endogenous lipids, and peptides released by neurons but also other brain cell types, including microglial and endothelial cells. Over the past 10 years, knowledge about neuroglial interactions has widened and now includes effects of CNS-targeting drugs such as antidepressants, antipsychotics, psychostimulants, or sedatives drugs as potential modulators of connexin function and thus astrocyte networking activity. In physiological situations, neuroglial networking is consequently resulting from a two-way interaction between astrocyte gap junction-mediated networks and those made by neurons. As both cell types are modulated by CNS drugs we postulate that neuroglial networking may emerge as new therapeutic targets in neurological and psychiatric disorders.

Connexins and pannexins: At the junction of neuro-glial homeostasis & disease

In the central nervous system (CNS), connexin (Cx)s and pannexin (Panx)s are an integral component of homeostatic neuronal excitability and synaptic plasticity. Neuronal Cx gap junctions form electrical synapses across biochemically similar GABAergic networks, allowing rapid and extensive inhibition in response to principle neuron excitation. Glial Cx gap junctions link astrocytes and oligodendrocytes in the pan-glial network that is responsible for removing excitotoxic ions and metabolites. In addition, glial gap junctions help constrain excessive excitatory activity in neurons and facilitate astrocyte Ca²⁺ slow wave propagation. Panxs do not form gap junctions in vivo, but Panx hemichannels participate in autocrine and paracrine gliotransmission, alongside Cx hemichannels. ATP and other gliotransmitters released by Cx and Panx hemichannels maintain physiologic glutamatergic tone by strengthening synapses and mitigating aberrant high frequency bursting. Under pathological depolarizing and inflammatory conditions, gap junctions and hemichannels become dysregulated, resulting in excessive neuronal firing and seizure. In this review, we present known contributions of Cxs and Panxs to physiologic neuronal excitation and explore how the disruption of gap junctions and hemichannels lead to abnormal glutamatergic transmission, purinergic signaling, and seizures.

Connexin 50 modulates Sox2 expression in spinal-cord-derived ependymal stem/progenitor cells

Ion channels included in the family of Connexins (Cx) have been reported to influence the secondary expansion of traumatic spinal cord injury (SCI) and neuropathic pain following SCI. However, Cxs also contribute to spinal cord neurogenesis during the remyelinating process and functional recovery after SCI. Certain Cxs have been recently related to the control of cell proliferation and the differentiation of neuronal progenitors. Adult spinal-cord-derived ependymal stem progenitor cells (epSPC) show high expression levels of Cx50 in non-pathological conditions and lower expression when they actively proliferate after injury (epSPCi). We explore the role of Cx50 in the ependymal population in the modulation of Sox2, a crucial factor of neural progenitor self-renewal and a promising target for promoting neuronal-cell-fate induction for neuronal tissue repair. Short-interfering-RNA ablation or over-expression of Cx50 regulates the expression of Sox2 in both epSPC and epSPCi. Interestingly, Cx50 and Sox2 co-localize at the nucleus indicating a potential role for this ion channel beyond cell-to-cell communication in the spinal cord. In vivo and in vitro experiments with Clotrimazole, a specific pharmacological modulator of Cx50, show the convergent higher expression of Cx50 and Sox2 in the isolated epSPC/epSPCi and in spinal cord tissue. Therefore, the pharmacological modulation of Cx50 might constitute an interesting mechanism for Sox2 induction to modulate the endogenous regenerative potential of neuronal tissue with a potential application in regenerative therapies.

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.

Connexin 50 Expression in Ependymal Stem Progenitor Cells after Spinal Cord Injury Activation

Ion channels included in the family of Connexins (Cx) help to control cell proliferation and differentiation of neuronal progenitors. Here we explored the role of Connexin 50 (Cx50) in cell fate modulation of adult spinal cord derived neural precursors located in the ependymal canal (epSPC). epSPC from non-injured animals showed high expression levels of Cx50 compared to epSPC from animals with spinal cord injury (SCI) (epSPCi). When epSPC or epSPCi were induced to spontaneously differentiate in vitro we found that Cx50 favors glial cell fate, since higher expression levels, endogenous or by over-expression of Cx50, augmented the expression of the astrocyte marker GFAP and impaired the neuronal marker Tuj1. Cx50 was found in both the cytoplasm and nucleus of glial cells, astrocytes and oligodendrocyte-derived cells. Similar expression patterns were found in primary cultures of mature astrocytes. In addition, opposite expression profile for nuclear Cx50 was observed when epSPC and activated epSPCi were conducted to differentiate into mature oligodendrocytes, suggesting a different role for this ion channel in spinal cord beyond cell-to-cell communication. In vivo detection of Cx50 by immunohistochemistry showed a defined location in gray matter in non-injured tissues and at the epicenter of the injury after SCI. epSPCi transplantation, which accelerates locomotion regeneration by a neuroprotective effect after acute SCI is associated with a lower signal of Cx50 within the injured area, suggesting a minor or detrimental contribution of this ion channel in spinal cord regeneration by activated epSPCi.

Connexin and pannexin signaling pathways, an architectural blueprint for CNS physiology and pathology?

The central nervous system (CNS) is composed of a highly heterogeneous population of cells. Dynamic interactions between different compartments (neuronal, glial, and vascular systems) drive CNS function and allow to integrate and process information as well as to respond accordingly. Communication within this functional unit, coined the neuro-glio-vascular unit (NGVU), typically relies on two main mechanisms: direct cell-cell coupling via gap junction channels (GJCs) and paracrine communication via the extracellular compartment, two routes to which channels composed of transmembrane connexin (Cx) or pannexin (Panx) proteins can contribute. Multiple isoforms of both protein families are present in the CNS and each CNS cell type is characterized by a unique Cx/Panx portfolio. Over the last two decades, research has uncovered a multilevel platform via which Cxs and Panxs can influence different cellular functions within a tissue: (1) Cx GJCs enable a direct cell-cell communication of small molecules, (2) Cx hemichannels and Panx channels can contribute to autocrine/paracrine signaling pathways, and (3) different structural domains of these proteins allow for channel-independent functions, such as cell-cell adhesion, interactions with the cytoskeleton, and the activation of intracellular signaling pathways. In this paper, we discuss current knowledge on their multifaceted contribution to brain development and to specific processes in the NGVU, including synaptic transmission and plasticity, glial signaling, vasomotor control, and blood-brain barrier integrity in the mature CNS. By highlighting both physiological and pathological conditions, it becomes evident that Cxs and Panxs can play a dual role in the CNS and that an accurate fine-tuning of each signaling mechanism is crucial for normal CNS physiology.

Can Archival Tissue Reveal Answers to Modern Research Questions?: Computer-Aided Histological Assessment of Neuroblastoma Tumours Collected over 60 Years

Despite neuroblastoma being the most common extracranial solid cancer in childhood, it is still a rare disease. Consequently, the unavailability of tissue for research limits the statistical power of studies. Pathology archives are possible sources of rare tissue, which, if proven to remain consistent over time, could prove useful to research of rare disease types. We applied immunohistochemistry to investigate whether long term storage caused any changes to antigens used diagnostically for neuroblastoma. We constructed and quantitatively assessed a tissue microarray containing neuroblastoma archival material dating between 1950 and 2007. A total of 119 neuroblastoma tissue cores were included spanning 6 decades. Fourteen antibodies were screened across the tissue microarray (TMA). These included seven positive neuroblastoma diagnosis markers (NB84, Chromogranin A, NSE, Ki-67, INI1, Neurofilament Protein, Synaptophysin), two anticipated to be negative (S100A, CD99), and five research antibodies (IL-7, IL-7R, JAK1, JAK3, STAT5). The staining of these antibodies was evaluated using Aperio ImageScope software along with novel pattern recognition and quantification algorithms. This analysis demonstrated that marker signal intensity did not decrease over time and that storage for 60 years had little effect on antigenicity. The construction and assessment of this neuroblastoma TMA has demonstrated the feasibility of using archival samples for research.

Cross-regulation of Connexin43 and β-catenin influences differentiation of human neural progenitor cells

Connexin43 (Cx43) is the most widely and abundantly expressed gap junction (GJ) protein and it is strongly associated with the regulation of cell cycle progression. Emerging roles for Cx43 in cell adhesion and migration during neural differentiation have also been recently recognized, and this has emphasized the involvement of Cx43 in different physiological process beyond its role as a GJ protein. In this study, we explore the function of Cx43 in the differentiation of human neural progenitor cells (hNPCs) using viral vectors that mediate the overexpression or knockdown of the protein. Results showed that in the absence of this protein fetal cortex-derived hNPCs differentiated toward a neuronal phenotype at expenses of a glial phenotype. Furthermore, the silencing of Cx43 did not affect hNPC proliferation rate or numbers of apoptotic cells. The increase in the number of neurons was not recapitulated when GJ intercellular communications were pharmacologically blocked, and this suggested that Cx43 was influencing hNPCs differentiation with a GJ-independent effect. In addition, Cx43 knockdown significantly increased β-catenin signaling, which has been shown to regulate the transcription of pro-neuronal genes during embryonic neural development. Our results add further support to the hypothesis that Cx43 protein itself regulates key signaling pathways during development and neurogenesis beyond its role as GJ protein.

Neuronal differentiation requires a biphasic modulation of gap junctional intercellular communication caused by dynamic changes of connexin43 expression

It was suggested that gap junctional intercellular communication (GJIC) and connexin (Cx) proteins play a crucial role in cell proliferation and differentiation. However, the mechanisms of cell coupling in regulating cell fate during embryonic development are poorly understood. To study the role of GJIC in proliferation and differentiation, we used a human neural progenitor cell line derived from the ventral mesencephalon. Fluorescence recovery after photobleaching (FRAP) showed that dye coupling was extensive in proliferating cells but diminished after the induction of differentiation, as indicated by a 2.5-fold increase of the half-time of fluorescence recovery. Notably, recovery half-time decreased strongly (five-fold) in the later stage of differentiation. Western blot analysis revealed a similar time-dependent expression profile of Cx43, acting as the main gap junction-forming protein. Interestingly, large amounts of cytoplasmic Cx43 were retained mainly in the Golgi network during proliferation but decreased when differentiation was induced. Furthermore, down-regulation of Cx43 by small interfering RNA reduced functional cell coupling, which in turn resulted in a 50% decrease of both the proliferation rate and neuronal differentiation. Our findings suggest a dual function of Cx43 and GJIC in the neural development of ReNcell VM197 human progenitor cells. GJIC accompanied by high Cx43 expression is necessary (1) to maintain cells in a proliferative state and (2) to complete neuronal differentiation, including the establishment of a neural network. However, uncoupling of cells is crucial in the early stage of differentiation during cell fate commitment.

Gap junction channels and hemichannels in the CNS: regulation by signaling molecules

Coordinated interaction among cells is critical to develop the extremely complex and dynamic tasks performed by the central nervous system (CNS). Cell synchronization is in part mediated by connexins and pannexins; two different protein families that form gap junction channels and hemichannels. Whereas gap junction channels connect the cytoplasm of contacting cells and coordinate electric and metabolic activities, hemichannels communicate intra- and extra-cellular compartments and serve as diffusional pathways for ions and small molecules. Cells in the CNS depend on paracrine/autocrine communication via several extracellular signaling molecules, such as, cytokines, growth factors, transmitters and free radical species to sense changes in microenvironment as well as to adapt to them. These signaling molecules modulate crucial processes of the CNS, including, cellular migration and differentiation, synaptic transmission and plasticity, glial activation, cell viability and microvascular blood flow. Gap junction channels and hemichannels are affected by different signaling transduction pathways triggered by these paracrine/autocrine signaling molecules. Most of the modulatory effects induced by these signaling molecules are specific to the cell type and the connexin and pannexin subtype expressed in different brain areas. In this review, we summarized and discussed most of the relevant and recently published information on the effects of signaling molecules on connexin or pannexin based channels and their possible relevance in CNS physiology and pathology. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

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.

Biological role of connexin intercellular channels and hemichannels

Gap junctions (GJ) and hemichannels (HC) formed from the protein subunits called connexins are transmembrane conduits for the exchange of small molecules and ions. Connexins and another group of HC-forming proteins, pannexins comprise the two families of transmembrane proteins ubiquitously distributed in vertebrates. Most cell types express more than one connexin or pannexin. While connexin expression and channel activity may vary as a function of physiological and pathological states of the cell and tissue, only a few studies suggest the involvement of pannexin HC in acquired pathological conditions. Importantly, genetic mutations in connexin appear to interfere with GJ and HC function which results in several diseases. Thus connexins could serve as potential drug target for therapeutic intervention. Growing evidence suggests that diseases resulting from HC dysfunction might open a new direction for development of specific HC reagents. This review provides a comprehensive overview of the current studies of GJ and HC formed by connexins and pannexins in various tissue and organ systems including heart, central nervous system, kidney, mammary glands, ovary, testis, lens, retina, inner ear, bone, cartilage, lung and liver. In addition, present knowledge of the role of GJ and HC in cell cycle progression, carcinogenesis and stem cell development is also discussed.

Large Pore Ion and Metabolite-Permeable Channel Regulation of Postnatal Ventricular Zone Neural Stem and Progenitor Cells: Interplay between Aquaporins, Connexins, and Pannexins?

The birth of new neurons from unspecialized neural stem and progenitor cells surrounding the lateral ventricles occurs throughout postnatal life. This process, termed neurogenesis, is complex and multistepped, encompassing several types of cellular behaviours, such as proliferation, differentiation, and migration. These behaviours are influenced by numerous factors present in the unique, permissive microenvironment. A major cellular mechanism for sensing the plethora of environmental cues directing this process is the presence of different channel forming proteins spanning the plasma membrane. So-called large pore membrane channels, which are selective for the passage of specific types of small molecules and ions, are emerging as an important subgroup of channel proteins. Here, we focus on the roles of three such large pore channels, aquaporin 4, connexin 43, and pannexin 1. We highlight both their independent functions as well as the accumulating evidence for crosstalk between them.

Structure of the gap junction channel and its implications for its biological functions

Gap junctions consist of arrays of intercellular channels composed of integral membrane proteins called connexin in vertebrates. Gap junction channels regulate the passage of ions and biological molecules between adjacent cells and, therefore, are critically important in many biological activities, including development, differentiation, neural activity, and immune response. Mutations in connexin genes are associated with several human diseases, such as neurodegenerative disease, skin disease, deafness, and developmental abnormalities. The activity of gap junction channels is regulated by the membrane voltage, intracellular microenvironment, interaction with other proteins, and phosphorylation. Each connexin channel has its own property for conductance and molecular permeability. A number of studies have tried to reveal the molecular architecture of the channel pore that should confer the connexin-specific permeability/selectivity properties and molecular basis for the gating and regulation. In this review, we give an overview of structural studies and describe the structural and functional relationship of gap junction channels.

Connexin 36 expression regulates neuronal differentiation from neural progenitor cells

BACKGROUND

Gap junction communication has been shown in glial and neuronal cells and it is thought they mediate inter- and intra-cellular communication. Connexin 36 (Cx36) is expressed extensively in the developing brain, with levels peaking at P14 after which its levels fall and its expression becomes entirely neuronal. These and other data have led to the hypothesis that Cx36 may direct neuronal coupling and neurogenesis during development.

METHODOLOGY/PRINCIPAL FINDINGS

To investigate Cx36 function we used a neurosphere model of neuronal cell development and developed lentiviral Cx36 knockdown and overexpression strategies. Cx36 knockdown was confirmed by western blotting, immunocytochemistry and functionally by fluorescence recovery after photobleaching (FRAP). We found that knockdown of Cx36 in neurosphere neuronal precursors significantly reduced neuronal coupling and the number of differentiated neurons. Correspondingly, the lentiviral mediated overexpression of Cx36 significantly increased the number of neurons derived from the transduced neurospheres. The number of oligodendrocytes was also significantly increased following transduction with Cx36 indicating they may support neuronal differentiation.

CONCLUSIONS/SIGNIFICANCE

Our data suggests that astrocytic and neuronal differentiation during development are governed by mechanisms that include the differential expression of Cx36.

Mechanisms explaining transitions between tonic and phasic firing in neuronal populations as predicted by a low dimensional firing rate model

Several firing patterns experimentally observed in neural populations have been successfully correlated to animal behavior. Population bursting, hereby regarded as a period of high firing rate followed by a period of quiescence, is typically observed in groups of neurons during behavior. Biophysical membrane-potential models of single cell bursting involve at least three equations. Extending such models to study the collective behavior of neural populations involves thousands of equations and can be very expensive computationally. For this reason, low dimensional population models that capture biophysical aspects of networks are needed. The present paper uses a firing-rate model to study mechanisms that trigger and stop transitions between tonic and phasic population firing. These mechanisms are captured through a two-dimensional system, which can potentially be extended to include interactions between different areas of the nervous system with a small number of equations. The typical behavior of midbrain dopaminergic neurons in the rodent is used as an example to illustrate and interpret our results. The model presented here can be used as a building block to study interactions between networks of neurons. This theoretical approach may help contextualize and understand the factors involved in regulating burst firing in populations and how it may modulate distinct aspects of behavior.

Role of tau protein on neocortical and hippocampal oscillatory patterns

Tau is a neuronal microtubule-associated protein implicated in microtubules stabilization, axonal establishment and elongation during neuronal morphogenesis. Because of its elevated expression in neocortical regions and hippocampus, tau might play a role in sculpting collective neural responses underlying slow and fast brain oscillations and/or long-range synchronization patterns between hippocampus and neocortex. To test this hypothesis, local field potentials were recorded in tau-deficient (tau(-/-) ) and wild-type mice from different neocortical regions and from the hippocampus during spontaneous motor exploratory behavior. We found that tau(-/-) mice showed hippocampal theta slowing and reduced levels of gamma long-range synchronization involving the frontal cortex. We hypothesize that the lack of normal phosphorylated tau during early stages of development might influence the maturation of parvalbumin interneurons affecting the spatiotemporal structure of long-range gamma synchronization. Also, the proper functioning of gap-junction channels might be compromised by the absence of tau in hippocampal networks. Altogether, these results provide novel insights into the functional role of tau protein in the formation of collective neural responses and emergence of neocortical-hippocampal interactions in the mammalian brain.

The carboxyl-terminal domain of connexin43 is a negative modulator of neuronal differentiation

Connexin43 (Cx43) is widely expressed in embryonic brain, and its expression becomes restricted mainly to astrocytes as the central nervous system matures. Recent studies have indicated that Cx43 plays important, nonchannel, roles during central nervous system development by affecting neuronal cell migration. Here, we evaluated the effects of Cx43 on neuronal differentiation. For that we used an in vitro model of neural cell development (neurospheres) to evaluate, through immunocytochemistry, electrophysiology, and molecular biology, the degree of neuronal maturation from neurospheres derived from wild-type (WT) and Cx43-null mice. Our results indicate that Cx43 is a negative modulator of neuronal differentiation. The percent neurospheres containing differentiated neurons and the number of cells displaying inward currents were significantly higher in Cx43-null than in WT littermate neurospheres. Knockdown of Cx43 with small interfering RNA increased the number of WT neurospheres generating differentiated neurons. Blockade of gap junctional communication with carbenoxolone did not induce neuronal differentiation in WT neurospheres. Transfection of Cx43-null neurospheres with Cx43 mutants revealed that Cx43 carboxyl terminus prevents neuronal maturation. In agreement with these in vitro data, in situ analysis of embryonic day 16 brains revealed increased beta-III-tubulin expression in germinal zones of Cx43-null compared with that of WT littermates. These results indicate that Cx43, and specifically its carboxyl terminus, is crucial for signaling mechanisms preventing premature neuronal differentiation during embryonic brain development.

Analysis of stem cell lineage progression in the neonatal subventricular zone identifies EGFR+/NG2- cells as transit-amplifying precursors

In the adult subventricular zone (SVZ), astroglial stem cells generate transit-amplifying precursors (TAPs). Both stem cells and TAPs form clones in response to epidermal growth factor (EGF). However, in vivo, in the absence of sustained EGF receptor (EGFR) activation, TAPs divide a few times before differentiating into neuroblasts. The lack of suitable markers has hampered the analysis of stem cell lineage progression and associated functional changes in the neonatal germinal epithelium. Here we purified neuroblasts and clone-forming precursors from the neonatal SVZ using expression levels of EGFR and polysialylated neural cell adhesion molecule (PSANCAM). As in the adult SVZ, most neonatal clone-forming precursors did not express the neuroglia proteoglycan 2 (NG2) but displayed characteristics of TAPs, and only a subset exhibited antigenic characteristics of astroglial stem cells. Both precursors and neuroblasts were PSANCAM(+); however, neuroblasts also expressed doublecortin and functional voltage-dependent Ca(2+) channels. Neuroblasts and precursors had distinct outwardly rectifying K(+) current densities and passive membrane properties, particularly in precursors contacting each other, because of the contribution of gap junction coupling. Confirming the hypothesis that most are TAPs, cell tracing in brain slices revealed that within 2 days the majority of EGFR(+) cells had exited the cell cycle and differentiated into a progenitor displaying intermediate antigenic and functional properties between TAPs and neuroblasts. Thus, distinct functional and antigenic properties mark stem cell lineage progression in the neonatal SVZ.

Connexin 33 impairs gap junction functionality by accelerating connexin 43 gap junction plaque endocytosis

Connexin 33 (Cx33) is a testis-specific gap junction protein. We previously reported that Cx33 exerts dominant-negative effect on gap junction intercellular communication by sequestering Cx43 within early endosomes in Sertoli cells. However, the molecular mechanisms that drive this process are unknown. The present study analyzed: (i) the trafficking of Cx33 and Cx43 in wild-type Sertoli cells transfected with Cx33-DsRed2 and Cx43-green fluorescent protein vectors; (ii) the formation of heteromeric Cx33/Cx43 hemi-channels and their incorporation into gap junction plaques. Fluorescence lifetime imaging microscopy-fluorescence resonance energy transfer and videomicroscopy studies demonstrated that Cx33 and Cx43 associated to form heteromeric oligomers that trafficked along microtubules to the plasma membrane. However, the plaques containing Cx33 were not functional. Immunoprecipitation experiments revealed that zonula occludens-1 (ZO-1), a scaffold protein proposed to secure Cx in gap junction plaques at the cell-cell boundary, associated with Cx33 in testis extracts. In cells expressing Cx33, Cx33 and ZO-1 specifically interacted with P(1) phosphorylated and P(0) unphosphorylated isoforms of Cx43, and the ZO-1 membranous signal level was reduced. It is suggested that alteration of Cx43/ZO-1 association by Cx33 could be one mechanism by which Cx33 exerts its dominant-negative effect on gap junction plaque.

Gap junctions: multifaceted regulators of embryonic cortical development

The morphological development of the cerebral cortex from a primitive neuroepithelium into a complex laminar structure underlying higher cognition must rely on a network of intercellular signaling. Gap junctions are widely expressed during embryonic development and provide a means of cell-cell contact and communication. We review the roles of gap junctions in regulating the proliferation of neural progenitors as well as the migration and differentiation of young neurons in the embryonic cerebral cortex. There is substantial evidence that although gap junctions act in the classical manner coupling neural progenitors, they also act as hemichannels mediating the spread of calcium waves across progenitor cell populations and as adhesive molecules aiding neuronal migration. Gap junctions are thus emerging as multifaceted regulators of cortical development playing diverse roles in intercellular communication.

Modulation of astrocyte P2Y1 receptors by the carboxyl terminal domain of the gap junction protein Cx43

Gap junction proteins, connexins, provide intercellular channels that allow ions and small signaling molecules to be transmitted to adjacent coupled cells. Besides this function, it is becoming apparent that connexins also exert channel-independent effects, which are likely mediated by processes involving protein-protein interactions. Although a number of connexin interacting proteins have been identified, only little is known about the functional consequences of such interactions. We have previously shown that deletion of the astrocytic gap junction protein, connexin43 (Cx43) causes a right-ward shift in the dose-response curve to P2Y1R agonists and decreased P2Y1R expression levels. To evaluate whether these changes were due to reduced gap junctional communication or to protein-protein interactions, Cx43-null astrocytes were transfected with full-length Cx43 and Cx43 domains, and P2Y1R function and expression levels evaluated. Results indicate that restoration of P2Y1R function is independent of gap junctional communication and that the Cx43 carboxyl terminus spanning the SH3 binding domain (260-280) participates in the rescue of P2Y1R pharmacological behavior (shifting to the left the P2Y1R dose-response curve) without affecting its expression levels. These results suggest that the Cx43 carboxyl-terminus domain provides a binding site for an intracellular molecule, most likely a member of the c-Src tyrosine kinase family, which affects P2Y1R-induced calcium mobilization. It is here proposed that a nonchannel function of Cx43 is to serve as a decoy for such kinases. Such modulation of P2Y1R is expected to influence several neural cell functions, especially under inflammation and neurodegenerative disorders where expression levels of Cx43 are decreased.

Expression of connexins in embryonic mouse neocortical development

During embryonic development, young neurons migrate from the ventricular zone to the cortical plate of the cerebral cortex. Disturbances in this neuronal migration have been associated with numerous diseases such as mental retardation, double cortex, Down syndrome, and epilepsy. One possible cause of these neuropathologies is an aberration in normal gap junctional communication. At least 20 connexin (Cx) genes encode gap junction proteins in mice and humans. A proper understanding of the role of specific connexins in the developing brain requires the characterization of their spatial and temporal pattern of expression. In the current study we performed all the experiments on mouse developing cortex at embryonic days (E) 14, 16, and 18, timepoints that are highly active with regard to cortical development. Using reverse transcription-polymerase chain reaction, Western blot analysis, and immunohistochemistry, we found that among the family of gap junction proteins, Cx26, Cx36, Cx37, Cx43, and Cx45 were expressed in the developing cortex of mice, Cx30 and Cx32 were absent, while Cx40 was expressed at a very low level. Our results demonstrate that Cx26 and Cx37 were evenly distributed in the cortical layers of developing brain, while Cx36 and Cx43 were more abundant in the ventricular zone and cortical plate. Cx45 distribution appeared to be more abundant at E18 compared to the other timepoints (E14 and E16). Thus, the present study provides identification and the distribution pattern for Cxs associated with cortical development during normal neuronal migration.

Gap junctional intercellular communication in hypoxia-ischemia-induced neuronal injury

Brain hypoxia-ischemia is a relatively common and serious problem in neonates and in adults. Its consequences include long-term histological and behavioral changes and reduction in seizure threshold. Gap junction intercellular communication is pivotal in the spread of hypoxia-ischemia related injury and in mediating its long-term effects. This review provides a comprehensive and critical review of hypoxia-ischemia and hypoxia in the brain and the potential role of gap junctions in the spread of the neuronal injury induced by these insults. It also presents the effects of hypoxia-ischemia and of hypoxia on the state of gap junctions in vitro and in vivo. Understanding the mechanisms involved in gap junction-mediated neuronal injury due to hypoxia will lead to the development of novel therapeutic strategies.

smedinx-11 is a planarian stem cell gap junction gene required for regeneration and homeostasis

The largely unknown mechanisms that regulate adult stem cells probably involve signals from neighboring differentiated cells. Gap junction channels providing direct cell-cell communication via small molecules are a crucial component of morphogenesis and normal physiology. However, no specific gap junction protein has yet been functionally linked to adult/somatic stem cell behavior in vivo or to organ regeneration. We report the identification and characterization of smedinx-11--an innexin gap junction channel gene expressed in the adult stem cells (neoblasts) of the planarian Schmidtea mediterranea. smedinx-11 RNAi treatment inhibits regeneration and abrogates neoblast maintenance. Moreover, smedinx-11 expression is enriched in an irradiation-sensitive subpopulation (;X2') and is required for proper expression of other stem cell-specific markers. Analyses of the smedinx-11 downregulation phenotype revealed a striking anterior-posterior neoblast gradient. Our data demonstrate a novel role for gap junction proteins and suggest gap junction-mediated signaling as a new and tractable control point for adult, somatic stem cell regulation.

Structure and function of gap junctions in the developing brain

Gap-junction-dependent neuronal communication is widespread in the developing brain, and the prevalence of gap-junctional coupling is well correlated with specific developmental events. We summarize here our current knowledge of the contribution of gap junctions to brain development and propose that they carry out this role by taking advantage of the full complement of their functional properties. Thus, hemichannel activation may represent a key step in the initiation of Ca(2+) waves that coordinate cell cycle events during early prenatal neurogenesis, whereas both hemichannels and/or gap junctions may control the division and migration of cohorts of precursor cells during late prenatal neurogenesis. Finally, the recent discovery that pannexins, a novel group of proteins prominently expressed in the brain, are able to form both hemichannels and gap-junction channels suggests that we need to seek more than just connexins with respect to these junctions.

What is the role of the vasculature in the neural stem cell niche?

Evaluation of: Ramirez-Castillejo C, Sanchez-Sanchez F, Andreu-Agullo C et al.: Pigment epithelium-derived factor is a niche signal for neural stem cell renewal. Nat. Neurosci. 9(3), 331–339 (2006) [3] . Vascular cells are essential components of the cytoarchitecture of multiple stem cell niches, although their exact role(s) in mediating cell–cell, cell–matrix or paracrine interactions are not clearly defined. In the neural stem cell niche, vascular endothelial cells are thought to secrete soluble factors and extracellular matrix proteins that can modulate the proliferation and potential of neural stem cells and progenitors within their microenvironment. Current studies, such as those reported by Ramirez-Castillejo and colleagues focus on elucidating the molecular regulation of neural stem cell phenotype and function by one such endothelial cell-derived effector, pigment epithelium-derived factor. This work and its implications are discussed herein.

Electrical synapses--gap junctions in the brain

In the nervous system, interneuronal communication can occur via indirect or direct transmission. The mode of indirect communication involves chemical synapses, in which transmitters are released into the extracellular space to subsequently bind to the postsynaptic cell membrane. Direct communication is mediated by electrical synapses, and will be the focus of this review. The most prevalent group of electrical synapses are neuronal gap junctions (both terms are used interchangeably in this article), which directly connect the intracellular space of two cells by gap junction channels. The structural components of gap junction channels in the nervous system are connexin proteins, and, as recently identified, pannexin proteins. Connexin gap junction channels enable the intercellular, bidirectional transport of ions, metabolites, second messengers and other molecules smaller than 1 kD. More than 20 connexin genes have been found in the mouse and human genome. With the cloning of connexin36 (Cx36), a connexin protein with predominantly neuronal expression, the biochemical correlate of electrotonic transmission between neurons was identified. We outline the distribution of Cx36 as well as two other neuronal connexins (Cx57 and Cx45) in the nervous system, describing their spatial and temporal expression patterns. One focus in this review was the retina, as it shows many and diverse electrical synapses whose connexin components have been identified in fish and mammals. In view of the function of neuronal gap junctions, the network of inhibitory interneurons will be reviewed in detail, focussing on the hippocampus. Although in vivo data on pannexin proteins are still restricted to information on mRNA expression, electrophysiological data and the expression pattern in the nervous system have been included.

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.

Gap junctional communication in the male reproductive system

Male fertility is a highly controlled process that allows proliferation, meiosis and differentiation of male germ cells in the testis, final maturation in the epididymis and also requires functional male accessory glands: seminal vesicles, prostate and corpus cavernosum. In addition to classical endocrine and paracrine controls, mainly by gonadotropins LH and FSH and steroids, there is now strong evidence that all these processes are dependent upon the presence of homocellular or heterocellular junctions, including gap junctions and their specific connexins (Cxs), between the different cell types that structure the male reproductive tract. The present review is focused on the identification of Cxs, their distribution in the testis and in different structures of the male genital tract (epididymis, seminal vesicle, prostate, corpus cavernosum), their crucial role in the control of spermatogenesis and their implication in the function of the male accessory glands, including functional smooth muscle tone. Their potential dysfunctions in some testis (spermatogenic arrest, seminoma) and prostate (benign hyperplasia, adenocarcinoma) diseases and in the physiopathology of the human erectile function are also discussed.

Membrane properties of retinal stem cells/progenitors

The membrane properties of cells help integrate extrinsic information relayed through growth factors, chemokines, extracellular matrix, gap junctions and neurotransmitters towards modulating cell-intrinsic properties, which in turn determine whether cells remain quiescent, proliferate, differentiate, establish contact with other cells or remove themselves by activating programmed cell death. This review highlights some of the membrane properties of early and late retinal stem cells/progenitors, which are likely to be helpful in the identification and enrichment of these cells and in understanding mechanisms underlying their maintenance and differentiation. Understanding of membrane properties of retinal stem cells/progenitors is essential for the successful formulation of approaches to treat retinal degeneration and diseases by cell therapy.

Sensitivity of the brain transcriptome to connexin ablation

Extensive studies on mice with total or partial disruption of either connexin43 (Cx43) or connexin32 (Cx32) have detected only subtle changes in central nervous system structure, growth, development, or function. We have used high density cDNA arrays to analyze the regulation, control, and coordination of the abundances of 7446 distinct transcripts in four brains, each of Cx43 null (K43), Cx43 heterozygous (H43), and Cx32 null (K32) mice as compared to the brains of wildtype (W) mice. The use of multiple samples allowed the determination of the statistical significance of gene regulation. Significantly regulated genes encoded proteins of all functional categories, extending beyond those that might be expected to depend on junctional communication. Moreover, we found a high degree of similarity between genes regulated in the K43 and H43 brains and a remarkable overlap between gene regulation in brains of K43 and K32. The regulated genes in both K43 and H43 brains showed an outstanding inverse coordination with the levels of expression of Cx43 in W brain, indicating that the regulated genes are largely predictable from their co-variance with Cx43 in the wildtype samples. These findings lead to the hypothesis that connexin expression may represent a central node in the regulation of gene expression patterns in brain.

In search of "stemness"

Stem cells have been identified and characterized in a variety of tissues. In this review we examine possible shared properties of stem cells. We suggest that irrespective of their lineal origin, stem cells have to respond in similar ways to regulate self-renewal and differentiation and it is likely that cell-cycle control, asymmetry/differentiation controls, cellular protective and DNA repair mechanisms, and associated apoptosis/senescence signaling pathways all might be expected to be more highly regulated in stem cells, likely by similar mechanisms. We review the literature to suggest a set of candidate stemness genes that may serve as universal stem cell markers. While we predict many similarities, we also predict that differences will exist between stem cell populations and that when transdifferentiation is considered genes expected to be both similar and different need to be examined.

The complex of ciliary neurotrophic factor-ciliary neurotrophic factor receptor alpha up-regulates connexin43 and intercellular coupling in astrocytes via the Janus tyrosine kinase/signal transducer and activator of transcription pathway

Cytokines regulate numerous cell processes, including connexin expression and gap junctional coupling. In this study, we examined the effect of ciliary neurotrophic factor (CNTF) on connexin43 (Cx43) expression and intercellular coupling in astrocytes. Murine cortical astrocytes matured in vitro were treated with CNTF (20 ng/ml), soluble ciliary neurotrophic factor receptor alpha (CNTFRalpha) (200 ng/ml), or CNTF-CNTFRalpha. Although CNTF and CNTFRalpha alone had no effect on Cx43 expression, the heterodimer CNTF-CNTFRalpha significantly increased both Cx43 mRNA and protein levels. Cx43 immunostaining correlated with increased intercellular coupling as determined by dye transfer analysis. By using the pharmacological inhibitor alpha-cyano-(3,4-dihydroxy)-N-benzylcinnamide (AG490), the increase in Cx43 was found to be dependent on the Janus tyrosine kinase/signal transducer and activator of transcription (JAK/STAT) pathway. Immunocytochemical analysis revealed that CNTF-CNTFRalpha treatment produced nuclear localization of phosphorylated STAT3, whereas CNTF treatment alone did not. Transient transfection of constructs containing various sequences of the Cx43 promoter tagged to a LacZ reporter into ROS 17/2.8 cells confirmed that the promoter region between -838 to -1693 was deemed necessary for CNTF-CNTFRalpha to induce heightened expression. CNTF-CNTFRalpha did not alter Cx30 mRNA levels, suggesting selectivity of CNTF-CNTFRalpha for connexin signaling. Together in the presence of soluble receptor, CNTF activates the JAK/STAT pathway leading to enhanced Cx43 expression and intercellular coupling.

Dominant negative effect of connexin33 on gap junctional communication is mediated by connexin43 sequestration

Gap junctional intercellular communication is involved in the control of cell proliferation and differentiation. Connexin33, a member of the multi-gene family of gap junction proteins, exerts an inhibitory effect on intercellular communication when injected into Xenopus oocytes. However, the molecular mechanisms involved remain to be elucidated. Our results show that connexin33 was only expressed within the seminiferous tubules in the testis. In contrast to the majority of connexins, connexin33 was unphosphorylated. Immunoprecipitation experiments revealed that connexin33 physically interacted with connexin43, mainly with the phosphorylated P1 isoform of connexin43 but not with connexin26 and connexin32, two other connexins expressed in the tubular compartment. In Sertoli cells and COS-7 cells, connexin43 was located at the plasma membrane, whereas in connexin33 transfected cells, the specific association of connexin33/43 was sequestered in the intracellular compartment. High-resolution fluorescent deconvolution microscopy indicated that the connexin33/43 complex was mainly found within early endosomes. Sequestration of connexin33/43 complex was associated with a complete inhibition of the gap junctional coupling between adjacent cells. These findings provide the first evidence of a new mechanistic model by which a native connexin, exerting a dominant negative effect, can inhibit gap junctional intercellular communication. In the testis, connexin33 could exert a specific role on germ cell proliferation by suppressing the regulatory effect of connexin43.

Gap junctional communication is required to maintain mouse cortical neural progenitor cells in a proliferative state

The mechanisms that determine whether neural stem cells remain in a proliferative state or differentiate into neurons or glia are largely unknown. Here we establish a pivotal role for gap junction-mediated intercellular communication in determining the proliferation and survival of mouse neural progenitor cells (NPCs). When cultured in the presence of basic fibroblast growth factor (bFGF), NPCs express the gap junction protein connexin 43 and are dye-coupled. Upon withdrawal of bFGF, levels of connexin 43 and dye coupling decrease, and the cells cease proliferating and differentiate into neurons; the induction of gap junctions by bFGF is mediated by p42/p44 mitogen-activated protein kinases. Inhibition of gap junctions abolishes the ability of bFGF to maintain NPCs in a proliferative state resulting in cell differentiation or cell death, while overexpression of connexin 43 promotes NPC self-renewal in the absence of bFGF. In addition to promoting their proliferation, gap junctions are required for the survival of NPCs. Gap junctional communication is therefore both necessary and sufficient to maintain NPCs in a self-renewing state.