Neuronal differentiation and growth control of neuro-2a cells after retroviral gene delivery of connexin43

The cytoplasmic localization of ADNP through 14-3-3 promotes sex-dependent neuronal morphogenesis, cortical connectivity, and calcium signaling

Defective neuritogenesis is a contributing pathogenic mechanism underlying a variety of neurodevelopmental disorders. Single gene mutations in activity-dependent neuroprotective protein (ADNP) are the most frequent among autism spectrum disorders (ASDs) leading to the ADNP syndrome. Previous studies showed that during neuritogenesis, Adnp localizes to the cytoplasm/neurites, and Adnp knockdown inhibits neuritogenesis in culture. Here, we hypothesized that Adnp is localized in the cytoplasm during neurite formation and that this process is mediated by 14-3-3. Indeed, applying the 14-3-3 inhibitor, difopein, blocked Adnp cytoplasmic localization. Furthermore, co-immunoprecipitations showed that Adnp bound 14-3-3 proteins and proteomic analysis identified several potential phosphorylation-dependent Adnp/14-3-3 binding sites. We further discovered that knockdown of Adnp using in utero electroporation of mouse layer 2/3 pyramidal neurons in the somatosensory cortex led to previously unreported changes in neurite formation beginning at P0. Defects were sustained throughout development, the most notable included increased basal dendrite number and axon length. Paralleling the observed morphological aberrations, ex vivo calcium imaging revealed that Adnp deficient neurons had greater and more frequent spontaneous calcium influx in female mice. GRAPHIC, a novel synaptic tracing technology substantiated this finding, revealing increased interhemispheric connectivity between female Adnp deficient layer 2/3 pyramidal neurons. We conclude that Adnp is localized to the cytoplasm by 14-3-3 proteins, where it regulates neurite formation, maturation, and functional cortical connectivity significantly building on our current understanding of Adnp function and the etiology of ADNP syndrome.

Nanomechanical Induction of Autophagy-Related Fluorescence in Single Cells with Atomic Force Microscopy

Mechanistic understanding of how living systems sense, transduce, and respond to mechanical cues has important implications in development, physiology, and therapy. Here, the authors use an integrated atomic force microscope (AFM) and brightfield/epifluorescent microscope platform to precisely simulate living single cells or groups of cells under physiological conditions, in real time, concomitantly measuring the single-cell autophagic response and its transmission to neighboring cells. Dual-color fluorescence monitoring of the cellular autophagic response reveals the dynamics of autophagosome formation, degradation, and induction in neighboring contacting and noncontacting cells. Autophagosome formation is dependent on both the applied force and contact area of the AFM tip. More importantly, the enhancement of the autophagic responses in neighboring cells via a gap junction-dependent mechanism is observed. This AFM-based nanoacupuncture platform can serve as a tool for elucidating the primary mechanism underlying mechanical stimulation of living systems and other biomechanical therapeutics.

Stress Induced Transcription Factors Transactivate the Herpes Simplex Virus 1 Infected Cell Protein 27 (ICP27) Transcriptional Enhancer

Following acute infection, herpes simplex virus 1 (HSV-1) establishes lifelong latency in neurons, including sensory neurons within trigeminal ganglia. During latency, lytic cycle viral gene expression is silenced. However, stressful stimuli can trigger reactivation from latency. The viral tegument protein, VP-16, transactivates all immediate early (IE) promoters during productive infection. Conversely, cellular factors are expected to trigger viral gene expression during early stages of reactivation from latency and in non-neuronal cells that do not support high levels of productive infection. The glucocorticoid receptor (GR), synthetic corticosteroid dexamethasone, and certain stress-induced transcription factors cooperatively transactivate infected cell protein 0 (ICP0) and ICP4 promoters. Since ICP27 protein expression is required for productive infection, we hypothesized that the ICP27 promoter is transactivated by stress-induced transcription factors. New studies have demonstrated that ICP27 enhancer sequences were transactivated by GR and Krüppel-like factor 15 (KLF15). Mutation of a consensus Sp1 binding site within ICP27 enhancer sequences impaired transactivation by GR and KLF15. Chromatin immunoprecipitation studies have demonstrated that GR and KLF15 occupy ICP27 promoter sequences during productive infection. Cells transfected with an ICP27 enhancer fragment revealed the GR and KLF15 occupancy of ICP27 enhancer sequences required the intact Sp1 binding site. Notably, GR and KLF15 form a feed-forward transcription loop in response to stress, suggesting these cellular factors promote viral replication following stressful stimuli.

Point Mutations of Nicotinic Receptor α1 Subunit Reveal New Molecular Features of G153S Slow-Channel Myasthenia

Slow-channel congenital myasthenic syndromes (SCCMSs) are rare genetic diseases caused by mutations in muscle nicotinic acetylcholine receptor (nAChR) subunits. Most of the known SCCMS-associated mutations localize at the transmembrane region near the ion pore. Only two SCCMS point mutations are at the extracellular domains near the acetylcholine binding site, α1(G153S) being one of them. In this work, a combination of molecular dynamics, targeted mutagenesis, fluorescent Ca²⁺ imaging and patch-clamp electrophysiology has been applied to G153S mutant muscle nAChR to investigate the role of hydrogen bonds formed by Ser 153 with C-loop residues near the acetylcholine-binding site. Introduction of L199T mutation to the C-loop in the vicinity of Ser 153 changed hydrogen bonds distribution, decreased acetylcholine potency (EC₅₀ 2607 vs. 146 nM) of the double mutant and decay kinetics of acetylcholine-evoked cytoplasmic Ca²⁺ rise (τ 14.2 ± 0.3 vs. 34.0 ± 0.4 s). These results shed light on molecular mechanisms of nAChR activation-desensitization and on the involvement of such mechanisms in channelopathy genesis.

Neuroprotective effect of heparin Trisulfated disaccharide on ischemic stroke

Cells undergoing hypoxia experience intense cytoplasmic calcium (Ca2+) overload. High concentrations of intracellular calcium ([Ca2+]i) can trigger cell death in the neural tissue, a hallmark of stroke. Neural Ca2+ homeostasis involves regulation by the Na+/Ca2+ exchanger (NCX). Previous data published by our group showed that a product of the enzymatic depolymerization of heparin by heparinase, the unsaturated trisulfated disaccharide (TD; ΔU, 2S-GlcNS, 6S), can accelerate Na+/Ca2+ exchange via NCX, in hepatocytes and aorta vascular smooth muscle cells. Thus, the objective of this work was to verify whether TD could act as a neuroprotective agent able to prevent neuronal cell death by reducing [Ca2+]i. Pretreatment of N2a cells with TD reduced [Ca2+]i rise induced by thapsigargin and increased cell viability under [Ca2+]I overload conditions and in hypoxia. Using a murine model of stroke, we observed that pretreatment with TD decreased cerebral infarct volume and cell death. However, when mice received KB-R7943, an NCX blocker, the neuroprotective effect of TD was abolished, strongly suggesting that this neuroprotection requires a functional NCX to happen. Thus, we propose TD-NCX as a new therapeutic axis for the prevention of neuronal death induced by [Ca2+]i overload.

Role of the Connexin C-terminus in skin pattern formation of Zebrafish

Background

Zebrafish display a striped skin pattern on their body; two types of connexins, namely, Connexin39.4 (Cx39.4) and Connexin41.8 (Cx41.8), are involved in stripe pattern formation. Herein, we investigated the role of the C-terminal (CT) domains of Cx39.4 and Cx41.8 in vivo and in vitro.

Methods

To investigate the role of CT domains in vivo, we established transgenic zebrafish lines expressing the CT-domain-modified connexin series in pigmented cells and observed skin patterns in fish. To investigate the role of the CT domains in vitro, we expressed the CT-domain modified connexin series in Neuro-2a (N2a) cells and calculated the plaque formation frequency.

Results

The overexpression of Cx39.4 lacking a CT domain produced skin patterns similar to that produced by full-length Cx39.4 in the cx39.4 ^-/- mutant and in cx39.4 and cx41.8 double-knockout mutant zebrafish. Fluorescence-protein-fused CT-domain-modified Cx39.4 formed gap junction plaques between N2a cells. The overexpression of CT-truncated Cx41.8 rescued the mutant phenotype in the cx41.8 ^-/- mutant but did not function in the double knockout zebrafish. Fluorescence-protein-fused CT-truncated Cx41.8 hardly formed plaques between N2a cells without Cx39.4 but formed gap junction plaques when co-expressed with Cx39.4.

Conclusions

The CT domain of Cx39.4 is not required for protein function, at least in the pigment cells of zebrafish. However, the need for the CT domain of Cx41.8 depends on Cx39.4 expression.

General significance

These results provide evidence for the interactions between Cx39.4 and Cx41.8 in pigment cells of zebrafish and suggest that at least one connexin must have a CT domain.

Ibuprofen enhances synaptic function and neural progenitors proliferation markers and improves neuropathology and motor coordination in Machado-Joseph disease models

Machado-Joseph disease or spinocerebellar ataxia type 3 is an inherited neurodegenerative disease associated with an abnormal glutamine over-repetition within the ataxin-3 protein. This mutant ataxin-3 protein affects several cellular pathways, leading to neuroinflammation and neuronal death in specific brain regions resulting in severe clinical manifestations. Presently, there is no therapy able to modify the disease progression. Nevertheless, anti-inflammatory pharmacological intervention has been associated with positive outcomes in other neurodegenerative diseases. Thus, the present work aimed at investigating whether ibuprofen treatment would alleviate Machado-Joseph disease. We found that ibuprofen-treated mouse models presented a significant reduction in the neuroinflammation markers, namely Il1b and TNFa mRNA and IKB-α protein phosphorylation levels. Moreover, these mice exhibited neuronal preservation, cerebellar atrophy reduction, smaller mutant ataxin-3 inclusions and motor performance improvement. Additionally, neural cultures of Machado-Joseph disease patients' induced pluripotent stem cells-derived neural stem cells incubated with ibuprofen showed increased levels of neural progenitors proliferation and synaptic markers such as MSI1, NOTCH1 and SYP. These findings were further confirmed in ibuprofen-treated mice that display increased neural progenitor numbers (Ki67 positive) in the subventricular zone. Furthermore, interestingly, ibuprofen treatment enhanced neurite total length and synaptic function of human neurons. Therefore, our results indicate that ibuprofen reduces neuroinflammation and induces neuroprotection, alleviating Machado-Joseph disease-associated neuropathology and motor impairments. Thus, our findings demonstrate that ibuprofen treatment has the potential to be used as a neuroprotective therapeutic approach in Machado-Joseph disease.

Protein kinases: master regulators of neuritogenesis and therapeutic targets for axon regeneration

Proper neurite formation is essential for appropriate neuronal morphology to develop and defects at this early foundational stage have serious implications for overall neuronal function. Neuritogenesis is tightly regulated by various signaling mechanisms that control the timing and placement of neurite initiation, as well as the various processes necessary for neurite elongation to occur. Kinases are integral components of these regulatory pathways that control the activation and inactivation of their targets. This review provides a comprehensive summary of the kinases that are notably involved in regulating neurite formation, which is a complex process that involves cytoskeletal rearrangements, addition of plasma membrane to increase neuronal surface area, coupling of cytoskeleton/plasma membrane, metabolic regulation, and regulation of neuronal differentiation. Since kinases are key regulators of these functions during neuromorphogenesis, they have high potential for use as therapeutic targets for axon regeneration after injury or disease where neurite formation is disrupted.

Identification of serine residues in the connexin43 carboxyl tail important for BCR-mediated spreading of B-lymphocytes

B-lymphocytes recognize antigen via B-cell antigen receptors (BCRs). This binding induces signaling, leading to B-cell activation, proliferation and differentiation. Early events of BCR signaling include reorganization of actin and membrane spreading, which facilitates increased antigen gathering. We have previously shown that the gap junction protein connexin43 (Cx43; also known as GJA1) is phosphorylated upon BCR signaling, and its carboxyl tail (CT) is important for BCR-mediated spreading. Here, specific serine residues in the Cx43 CT that are phosphorylated following BCR stimulation were identified. A chimeric protein containing the extracellular and transmembrane domains of CD8 fused to the Cx43 CT was sufficient to support cell spreading. Cx43 CT truncations showed that the region between amino acids 246-307 is necessary for B-cell spreading. Site-specific serine-to-alanine mutations (S255A, S262A, S279A and S282A) resulted in differential effects on both BCR signaling and BCR-mediated spreading. These serine residues can serve as potential binding sites for actin remodeling mediators and/or BCR signaling effectors; therefore, our results may reflect unique roles for each of these serines in terms of linking the Cx43 CT to actin remodeling.

Dihydrolipoic Acid-Gold Nanoclusters Regulate Microglial Polarization and Have the Potential To Alter Neurogenesis

Microglia-mediated neuroinflammation is one of the most significant features in a variety of central nervous system (CNS) disorders such as traumatic brain injury, stroke, and many neurodegenerative diseases. Microglia become polarized upon stimulation. The two extremes of the polarization are the neuron-destructive proinflammatory M1-like and the neuron-regenerative M2-like phenotypes. Thus, manipulating microglial polarization toward the M2 phenotype is a promising therapeutic approach for CNS repair and regeneration. It has been reported that nanoparticles are potential tools for regulating microglial polarization. Gold nanoclusters (AuNCs) could penetrate the blood-brain barrier and have neuroprotective effects, suggesting the possibility of utilizing AuNCs to regulate microglial polarization and improve neuronal regeneration in CNS. In the current study, AuNCs functionalized with dihydrolipoic acid (DHLA-AuNCs), an antioxidant with demonstrated neuroprotective roles, were prepared, and their effects on polarization of a microglial cell line (BV2) were examined. DHLA-AuNCs effectively suppressed proinflammatory processes in BV2 cells by inducing polarization toward the M2-like phenotype. This was associated with a decrease in reactive oxygen species and reduced NF-kB signaling and an improvement in cell survival coupled with enhanced autophagy and inhibited apoptosis. Conditioned medium from DHLA-AuNC-treated BV2 cells was able to enhance neurogenesis in both the neuronal cell line N2a and in an ex vivo brain slice stroke model. The direct treatment of brain slices with DHLA-AuNCs also ameliorated stroke-related tissue injury and reduced astrocyte activation (astrogliosis). This study suggests that by regulating neuroinflammation to improve neuronal regeneration, DHLA-AuNCs could be a potential therapeutic agent in CNS disorders.

The polyadenosine RNA-binding protein ZC3H14 interacts with the THO complex and coordinately regulates the processing of neuronal transcripts

The polyadenosine RNA-binding protein ZC3H14 is important in RNA processing. Although ZC3H14 is ubiquitously expressed, mutation of the ZC3H14 gene causes a non-syndromic form of intellectual disability. Here, we examine the function of ZC3H14 in the brain by identifying ZC3H14-interacting proteins using unbiased mass spectrometry. Through this analysis, we identified physical interactions between ZC3H14 and multiple RNA processing factors. Notably, proteins that comprise the THO complex were amongst the most enriched proteins. We demonstrate that ZC3H14 physically interacts with THO components and that these proteins are required for proper RNA processing, as loss of ZC3H14 or THO components leads to extended bulk poly(A) tail length. Furthermore, we identified the transcripts Atp5g1 and Psd95 as shared RNA targets of ZC3H14 and the THO complex. Our data suggest that ZC3H14 and the THO complex are important for proper processing of Atp5g1 and Psd95 RNA, as depletion of ZC3H14 or THO components leads to decreased steady-state levels of each mature transcript accompanied by accumulation of Atp5g1 and Psd95 pre-mRNA in the cytoplasm. Taken together, this work provides the first unbiased identification of nuclear ZC3H14-interacting proteins from the brain and links the functions of ZC3H14 and the THO complex in the processing of RNA.

Advanced Glycation End Products Modulate Amyloidogenic APP Processing and Tau Phosphorylation: A Mechanistic Link between Glycation and the Development of Alzheimer's Disease

Advanced glycation end products (AGEs) are implicated in the pathology of Alzheimer's disease (AD), as they induce neurodegeneration following interaction with the receptor for AGE (RAGE). This study aimed to establish a mechanistic link between AGE-RAGE signaling and AD pathology. AGE-induced changes in the neuro2a proteome were monitored by SWATH-MS. Western blotting and cell-based reporter assays were used to investigate AGE-RAGE regulated APP processing and tau phosphorylation in primary cortical neurons. Selected protein expression was validated in brain samples affected by AD. The AGE-RAGE axis altered proteome included increased expression of cathepsin B and asparagine endopeptidase (AEP), which mediated an increase in Aβ_1-42 formation and tau phosphorylation, respectively. Elevated cathepsin B, AEP, RAGE, and pTau levels were found in human AD brain, coincident with enhanced AGEs. This study demonstrates that the AGE-RAGE axis regulates Aβ_1-42 formation and tau phosphorylation via increased cathepsin B and AEP, providing a new molecular link between AGEs and AD pathology.

cAMP response element-binding protein and Yes-associated protein form a feedback loop that promotes neurite outgrowth

The cAMP response element‐binding (CREB) protein is a member of the CREB/activating transcription factor family that is activated by various extracellular stimuli. It has been shown that CREB‐dependent transcription stimulation plays a key role in neuronal differentiation and plasticity, but the underlying mechanisms remain largely elusive. Here, we show that Yes‐associated protein (YAP) is a direct target induced by CREB upon retinoic acid (RA)‐induced neurite outgrowth stimuli in N2a cells. Interestingly, YAP knockout using the CRISPR/Cas9 system inhibits neuronal differentiation and reduced neurite length. We further show that YAP could directly bind to CREB via its N‐terminal region, and loss of YAP results in instability of phosphorylated CREB upon neurite outgrowth stimuli. Transient expression of YAP could largely restore CREB expression and neurite outgrowth in YAP knockout cells. Together, our results suggest that CREB and YAP form a positive feedback loop that is critical to maintain the stability of phosphorylated CREB and promote neurite outgrowth.

Long non-coding RNA Malat1 promotes neurite outgrowth through activation of ERK/MAPK signalling pathway in N2a cells

Accumulating evidence suggests that long non-coding RNAs (lncRNAs) are playing critical roles in neurogenesis, yet the underlying molecular mechanisms remain largely elusive. Neurite outgrowth is an early step in neuronal differentiation and regeneration. Using in vitro differentiation of neuroblastoma-derived Neuro-2a (N2a) cell as a model, we performed expression profiling to identify lncRNAs putatively relevant for neurite outgrowth. We identified that Metastasis-associated lung adenocarcinoma transcript 1 (Malat1) was one of the most significantly up-regulated lncRNAs during N2a cell differentiation. Malat1 knockdown resulted in defects in neurite outgrowth as well as enhanced cell death. To pinpoint signalling pathways perturbed by Malat1 depletion, we then performed a reporter-based screening to examine the activities of 50 signalling pathways in Malat1 knockdown cells. We found that Malat1 knockdown resulted in conspicuous inhibition of Mitogen-Activated Protein Kinase (MAPK) signaling pathway as well as abnormal activation of Peroxisome proliferator-activated receptor (PPAR) and P53 signalling pathway. Inhibition of ERK/MAPK pathway with PD98059 potently blocked N2a cell neurite outgrowth, whereas phorbol 12-myristate 13-acetate-induced ERK activation rescued defects in neurite outgrowth and cell death induced by Malat1 depletion. Together, our results established a critical role of Malat1 in the early step of neuronal differentiation through activating ERK/MAPK signalling pathway.

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.

Cx30 exhibits unique characteristics including a long half-life when assembled into gap junctions

In the present study we investigated the life-cycle, trafficking, assembly and cell surface dynamics of a poorly characterized connexin family member, connexin 30 (Cx30), which plays a critical role in skin health and hearing. Unexpectedly, Cx30 localization at the cell surface and gap junctional intercellular communication was not affected by prolonged treatments with the ER-Golgi transport inhibitor brefeldin-A or the protein synthesis inhibitor cycloheximide, whereas Cx43 was rapidly cleared. Fluorescent recovery after photobleaching revealed that Cx30 plaques were rebuilt from the outer edges in keeping with older channels residing in the inner core of the plaque. Expression of a dominant-negative form of Sar1 GTPase led to the accumulation of Cx30 within the ER in contrast to a report that Cx30 traffics via a Golgi-independent pathway. Co-expression of Cx30 with Cx43 revealed that these connexins segregate into distinct domains within common gap junction plaques suggesting their assembly is governed by different mechanisms. In summary, Cx30 was found to be an unusually stable, long-lived connexin (half-life >12 hrs), which may underlie its specific role in the epidermis and cochlea.

Connexins: junctional and non-junctional modulators of proliferation

Mounting evidence indicates that dysregulation of gap junctions and their structural subunits-connexins-often occurs in, and sometimes causes, a variety of proliferative disorders, including cancer. Connexin-mediated regulation of cell proliferation is complex and may involve modulation of gap junction intercellular communication (GJIC), hemichannel signalling, or gap junction-independent paths. However, the exact mechanisms linking connexins to proliferation remain poorly defined and a number of contradictory studies report both pro- and anti-proliferative effects, effects that often depend on the cell or tissue type or the microenvironment. The present review covers junctional and non-junctional regulation of proliferation by connexins, with a particular emphasis on their association with cancer.

Vitamin D3 regulates the formation and degradation of gap junctions in androgen-responsive human prostate cancer cells

1α-25(OH)2 vitamin D3 (1-25D), an active hormonal form of Vitamin D3, is a well-known chemopreventive and pro-differentiating agent. It has been shown to inhibit the growth of several prostate cancer cell lines. Gap junctions, formed of proteins called connexins (Cx), are ensembles of cell-cell channels, which permit the exchange of small growth regulatory molecules between adjoining cells. Cell-cell communication mediated by gap junctional channels is an important homeostatic control mechanism for regulating cell growth and differentiation. We have investigated the effect of 1-25D on the formation and degradation of gap junctions in an androgen-responsive prostate cancer cell line, LNCaP, which expresses retrovirally-introduced Cx32. Connexin32 is expressed by the luminal and well-differentiated cells of normal prostate and prostate tumors. Our results document that 1-25D enhances the expression of Cx32 and its subsequent assembly into gap junctions. Our results further show that 1-25D prevents androgen-regulated degradation of Cx32, post-translationally, independent of androgen receptor (AR)-mediated signaling. Finally, our findings document that formation of gap junctions sensitizes Cx32-expressing LNCaP cells to the growth inhibitory effects of 1-25D and alters their morphology. These findings suggest that the growth-inhibitory effects of 1-25D in LNCaP cells may be related to its ability to modulate the assembly of Cx32 into gap junctions.

The role of the gap junction protein connexin43 in B lymphocyte motility and migration

The gap junction family of proteins is widely expressed in mammalian cells and form intercellular channels between adjacent cells, as well as hemichannels, for transport of molecules between the cell and the surrounding environment. In addition, gap junction proteins have recently been implicated as important for the regulation of cell adhesion and migration in a variety of cell types. The gap junction protein connexin43 (Cx43) regulates B lymphocyte adhesion, BCR- and LFA-1-mediated activation of the GTPase Rap1, and cytoskeletal rearrangements resulting in changes to cell shape and membrane spreading. We demonstrate here that the actin cytoskeleton is important for the distribution of Cx43 in the B cell plasma membrane and for other cell processes involving the cytoskeleton. Using shRNA knockdown of Cx43 in B lymphoma cells we show that Cx43 is also necessary for chemokine-mediated Rap 1 activation, motility, CXCL12-directed migration, and movement across an endothelial cell monolayer. These results demonstrate that in addition to its role in B cell spreading, Cx43 is an important regulator of B-cell motility and migration, processes essential for normal B-cell development and immune responses.

Development of an enzyme-linked immunosorbent assay based on the murine leukemia virus p30 capsid protein

Retroviral vectors derived from the murine leukemia virus (MuLV) are widely used as the starting material in the development of vectors for gene therapy and critical in answering questions relating to viral pathogenesis. The p30 capsid (CA) is the major viral core protein and an internal group antigen in MuLV. In this study, an enzyme-linked immunosorbent assay (ELISA) was developed for quantitation of MuLV infectious particles with p30 CA core antigen protein. The ELISA was developed using several goat-polyclonal serum against MuLV p30 generated by the NCI as primary antibody and a rat-monoclonal antibody to CA available from ATCC. The MuLV p30 CA antigen was standardized against recombinant MuLV p30 CA expressed from bacteria. The assay is sensitive, accurate and linear within a defined concentration range of CA. Comparison with different MuLV quantitative methods including reporter gene transfer, reverse transcriptase activity assay, and viral RNA quantitative PCR, showed this ELISA protocol to be highly quantifiable within defined ranges, which can be correlated with infectious viral titer.

Autosomal recessive GJA1 (Cx43) gene mutations cause oculodentodigital dysplasia by distinct mechanisms

Oculodentodigital dysplasia (ODDD) is mainly an autosomal dominant human disease caused by mutations in the GJA1 gene, which encodes the gap junction protein connexin43 (Cx43). Surprisingly, there have been two autosomal recessive mutations reported that cause ODDD: a single amino acid substitution (R76H) and a premature truncation mutation (R33X). When expressed in either gap junctional intercellular communication (GJIC)-deficient HeLa cells or Cx43-expressing NRK cells, the R76H mutant trafficked to the plasma membrane to form gap junction-like plaques, whereas the R33X mutant remained diffusely localized throughout the cell, including the nucleus. As expected, the R33X mutant failed to form functional channels. In the case of the R76H mutant, dye transfer studies in HeLa cells and electrical conductance analysis in GJIC-deficient N2a cells revealed that this mutant could form functional gap junction channels, albeit with reduced macroscopic and single channel conductance. Alexa 350 dye transfer studies further revealed that the R76H mutant had no detectable negative effect on the function of co-expressed Cx26, Cx32, Cx37 or Cx40, whereas the R33X mutant exhibited significant dominant or trans-dominant effects on Cx43 and Cx40 as manifested by a reduction in wild-type connexin gap junction plaques. Taken together, our results suggest that the trans-dominant effect of R33X together with its complete inability to form a functional channel may explain why patients harboring this autosomal recessive R33X mutant exhibit greater disease burden than patients harboring the R76H mutant.

Contribution of intracellular calcium and pH in ischemic uncoupling of cardiac gap junction channels formed of connexins 43, 40, and 45: a critical function of C-terminal domain

Ischemia is known to inhibit gap junction (GJ) mediated intercellular communication. However the detail mechanisms of this inhibition are largely unknown. In the present study, we determined the vulnerability of different cardiac GJ channels formed of connexins (Cxs) 43, 40, and 45 to simulated ischemia, by creating oxygen glucose deprived (OGD) condition. 5 minutes of OGD decreased the junctional conductance (Gj) of Cx43, Cx40 and Cx45 by 53±3%, 64±1% and 85±2% respectively. Reduction of Gj was prevented completely by restricting the change of both intracellular calcium ([Ca(2+)]i) and pH (pHi) with potassium phosphate buffer. Clamping of either [Ca(2+)]i or pHi, through BAPTA (2 mM) or HEPES (80 mM) respectively, offered partial resistance to ischemic uncoupling. Anti-calmodulin antibody attenuated the uncoupling of Cx43 and Cx45 significantly but not of Cx40. Furthermore, OGD could reduce only 26±2% of Gj in C-terminus (CT) truncated Cx43 (Cx43-Δ257). Tethering CT of Cx43 to the CT-truncated Cx40 (Cx40-Δ249), and Cx45 (Cx45-Δ272) helped to resist OGD mediated uncoupling. Moreover, CT domain played a significant role in determining the junction current density and plaque diameter. Our results suggest; OGD mediated uncoupling of GJ channels is primarily due to elevated [Ca(2+)]i and acidic pHi, though the latter contributes more. Among Cx43, Cx40 and Cx45, Cx43 is the most resistant to OGD while Cx45 is the most sensitive one. CT of Cx43 has major necessary elements for OGD induced uncoupling and it can complement CT of Cx40 and Cx45.

P2X7 receptor-pannexin 1 hemichannel association: effect of extracellular calcium on membrane permeabilization

Activation of P2X(7) receptor (P2X(7)R) and pannexin have been implicated in membrane permeabilization associated with ischemic cell death and many other inflammatory processes. P2X(7)R has a unique property of forming large pore upon repeated or prolonged application of agonist like ATP or 2', 3'-(4-benzoyl) benzoyl ATP. It has been proposed that pannexin 1 (panx1) hemichannel associates with P2X(7)R to form large pore, though the actual mechanism is not yet understood. Calcium concentration in extracellular milieu drops in many patho-physiological conditions, e.g. ischemia, when P2X(7)R/pannexin is also known to be activated. Therefore, we hypothesize that extracellular calcium ([Ca(2+)](o)) plays an important role in the coupling of P2X(7)R-panx1 and subsequent membrane permeabilization. In this study we show that membrane permeability of the P2X(7)R and panx1 expressing N2A cell increases in ([Ca(2+)](o))-free solution. In [Ca(2+)](o)-free solution, fluorescent dye calcein trapped cells exhibited time-dependent dye leakage resulting in about 50% decrease of fluorescence intensity in 30 min. Control cells in 2 mM [Ca(2+)](o) did not show such leakage. Like N2A cells, mixed culture of neuron and glia, derived from hippocampal progenitor cells showed similar dye leakage. Dye leakage was blocked either by pannexin-specific blocker, carbenoxolone or P2X(7)R antagonists, Brilliant Blue G, and oxidized ATP. Furthermore P2X(7)R and panx1 were co-immunoprecipitated. The amount of P2X(7)R protein pulled-down with panx1, increased by twofold when cells were incubated 30 min in [Ca(2+)](o)-free buffer. Taken together, the results of this study demonstrate the activation and association of P2X(7)R-panx1, triggered by the removal of [Ca(2+)](o).

The gap junction protein Cx43 regulates B-lymphocyte spreading and adhesion

The gap junction protein connexin43 (Cx43) is widely expressed in mammalian cells and forms intercellular channels for the transfer of small molecules between adjacent cells, as well as hemichannels that mediate bidirectional transport of molecules between the cell and the surrounding environment. Cx43 regulates cell adhesion and migration in neurons and glioma cells, and we now show that Cx43 influences BCR-, LFA-1- and CXCL12-mediated activation of the Rap1 GTPase. Using shRNA knockdown of Cx43 in WEHI 231 cells, we show that Cx43 is required for sustained Rap1 activation and BCR-mediated spreading. To determine the domains of Cx43 that are important for this effect, Cx43-null J558 μm3 B cells (which express a wild-type IgM BCR) were transfected with wild-type Cx43-GFP or a C-terminal-truncated Cx43 (Cx43ΔT-GFP). Expression of wild-type Cx43-GFP, but not Cx43ΔT-GFP, was sufficient to restore sustained, BCR-mediated Rap1 activation and cell spreading. Cx43, and specifically the C-terminal domain, was also important for LFA-1- and CXCL12-mediated Rap1 activation, spreading and adhesion to an endothelial cell monolayer. These data show that Cx43 has an important and previously unreported role in B-cell processes that are essential to normal B-cell development and immune responses.

Lysophosphatidic acid induces neurite retraction in differentiated neuroblastoma cells via GSK-3β activation

Lysophosphatidic acid (LPA) is a lipid growth factor that exerts diverse biological effects, including rapid neurite retraction and cell migration. Alterations in cell morphology, including neurite retraction, in neurodegenerative disorders such as Alzheimer's disease involve hyperphosphorylation of the cytoskeletal protein tau. Since LPA has been shown to induce neurite retraction in various cultured neural cells and the detailed underlying molecular mechanisms have not yet been elucidated, we investigated whether LPA induced neurite retraction through taumediated signaling pathways in differentiated neuroblastoma cells. When Neuro2a cells differentiated with retinoic acid (RA) were exposed to LPA, cells exhibited neurite retraction in a time-dependent manner. The retraction of neurites was accompanied by the phosphorylation of tau. The LPA-induced neurite retraction and tau phosphorylation in differentiated Neuro2a cells were significantly abolished by the glycogen synthase kinase-3β (GSK-3β) inhibitor lithium chloride. Interestingly, the LPA-stimulated tau phosphorylation and neurite retraction were markedly prevented by the administration of H89, an inhibitor of both cyclic-AMP dependent protein kinase (PKA) and cyclic-AMP response element-binding protein (CREB). Transfection of the dominant-negative CREBs, K-CREB and A-CREB, failed to prevent LPA-induced tau phosphorylation and neurite retraction in differentiated Neuro2a cells. Taken together, these results suggest that GSK-3β and PKA, rather than CREB, play important roles in tau phosphorylation and neurite retraction in LPA-stimulated differentiated Neuro2a cells.

Implications of pannexin 1 and pannexin 3 for keratinocyte differentiation

Pannexin (Panx) 1 and Panx3 are integral membrane proteins that share some sequence homology with the innexin family of invertebrate gap junctions. They are expressed in mammalian skin. Pannexins have been reported to form functional mechanosensitive single-membrane channels, but their importance in regulating cellular function is poorly understood. In this study, Panx1 and Panx3 were detected in the epidermis of 13.5 day embryonic mice. Compared with newborn mice, there was less Panx1 expression in both thin and thick murine skin, whereas Panx3 expression was unchanged. To investigate the role of pannexins in keratinocyte differentiation, we employed rat epidermal keratinocytes (REKs) that have the capacity to differentiate into organotypic epidermis, and engineered them to overexpress Panx1, Panx1-GFP or Panx3. The expression of Panx1 or Panx3 resulted in the increased ability of REKs to take up dye, suggesting that cell-surface channels were formed. Compared with monolayer REKs, endogenous Panx1 levels remained unchanged, whereas the 70 kDa immunoreactive species of Panx3 was greatly increased in the organotypic epidermis. In monolayer cultures, ectopic Panx1 and Panx1-GFP localized to the plasma membrane, whereas Panx3 displayed both intracellular and plasma-membrane profiles. Although both pannexins reduced cell proliferation, only Panx1 disrupted the architecture of the organotypic epidermis and markedly dysregulated cytokeratin 14 expression and localization. Furthermore, ectopic expression of only Panx1 reduced the vital layer thickness of the organotypic epidermis. In summary, Panx1 and Panx3 are coexpressed in the mammalian epidermis, and the regulation of Panx1 plays a key role in keratinocyte differentiation.

Gain and loss of function of ALS-related mutations of TARDBP (TDP-43) cause motor deficits in vivo

TDP-43 has been found in inclusion bodies of multiple neurological disorders, including amyotrophic lateral sclerosis, frontotemporal dementia, Parkinson's disease and Alzheimer's disease. Mutations in the TDP-43 encoding gene, TARDBP, have been subsequently reported in sporadic and familial ALS patients. In order to investigate the pathogenic nature of these mutants, the effects of three consistently reported TARDBP mutations (A315T, G348C and A382T) were tested in cell lines, primary cultured motor neurons and living zebrafish embryos. Each of the three mutants and wild-type (WT) human TDP-43 localized to nuclei when expressed in COS1 and Neuro2A cells by transient transfection. However, when expressed in motor neurons from dissociated spinal cord cultures these mutant TARDBP alleles, but less so for WT TARDBP, were neurotoxic, concomitant with perinuclear localization and aggregation of TDP-43. Finally, overexpression of mutant, but less so of WT, human TARDBP caused a motor phenotype in zebrafish (Danio rerio) embryos consisting of shorter motor neuronal axons, premature and excessive branching as well as swimming deficits. Interestingly, knock-down of zebrafisfh tardbp led to a similar phenotype, which was rescued by co-expressing WT but not mutant human TARDBP. Together these approaches showed that TARDBP mutations cause motor neuron defects and toxicity, suggesting that both a toxic gain of function as well as a novel loss of function may be involved in the molecular mechanism by which mutant TDP-43 contributes to disease pathogenesis.

NT-polyplex: a new tool for therapeutic gene delivery to neuroblastoma tumors

Neurotensin (NT)-polyplex is a nonviral system for the targeted gene delivery to cells that express and internalize the high-affinity NT receptor (NTSR1). In hemiparkinsonian rats, we previously demonstrated the morphological and functional recovery from dopaminergic neurodegeneration using the NT-polyplex as a vehicle to transfect a neurotrophic gene. The main objective of this work was to demonstrate the feasibility of NT-polyplex to transfect reporter or therapeutic genes into neuroblastoma tumors through the blood stream or by intratumoral injection. N1E-115 cells known to express NTSR1 were allografted into athymic mice to generate the neuroblastoma tumor model. Both routes of administration allowed the NT-polyplex to reach and transfect tumoral cells. A low transgene expression was also detected in intestinal tract cells only after the injection into the blood stream. The transfection of the thymidine kinase (HSVTK) suicide gene followed by ganciclovir (GCV) treatment decreased the size and weight of neuroblastoma tumors by 30-50% and increased apoptosis compared to controls. This study shows the potential of the NT-polyplex as specific gene-transfer system for NTSR1 cancer cells.

Caveolin-1 and -2 interact with connexin43 and regulate gap junctional intercellular communication in keratinocytes

Connexin43 (Cx43) has been reported to interact with caveolin (Cav)-1, but the role of this association and whether other members of the caveolin family bind Cx43 had yet to be established. In this study, we show that Cx43 coimmunoprecipitates and colocalizes with Cav-1 and Cav-2 in rat epidermal keratinocytes. The colocalization of Cx43 with Cav-1 was confirmed in keratinocytes from human epidermis in vivo. Our mutation and Far Western analyses revealed that the C-terminal tail of Cx43 is required for its association with Cavs and that the Cx43/Cav-1 interaction is direct. Our results indicate that newly synthesized Cx43 interacts with Cavs in the Golgi apparatus and that the Cx43/Cavs complex also exists at the plasma membrane in lipid rafts. Using overexpression and small interfering RNA approaches, we demonstrated that caveolins regulate gap junctional intercellular communication (GJIC) and that the presence of Cx43 in lipid raft domains may contribute to the mechanism modulating GJIC. Our results suggest that the Cx43/Cavs association occurs during exocytic transport, and they clearly indicate that caveolin regulates GJIC.

Connexin43 enhances glioma invasion by a mechanism involving the carboxy terminus

Gliomas are particularly difficult to cure owing largely to their invasive nature. The neoplastic changes of astrocytes which give rise to these tumors frequently include a reduction of connexin43 (Cx43), the most abundant connexin isoform expressed in astrocytes. Cx43 is a subunit of gap junctions (GJ), intercellular channels which directly link the cytosol of adjacent cells and allow the regulated passage of ions and small molecules. To examine the role of Cx43 in glioma motility, we identified two variant C6 cell lines which endogenously express high (C6-H) or low (C6-L) levels of Cx43. In wound healing and transwell assays, C6-H cells were more motile than C6-L cells. To deduce whether Cx43 mediated these differences, assays were conducted on C6-H cells retrovirally transduced with Cx43 shRNA. Coincident with the stable knockdown of endogenous Cx43, a decrease in motility and invasion was observed. Gap junctional intercellular communication was also decreased, however motility assays conducted in the presence of GJ inhibitors did not reveal significant differences in cell motility. C6 cells transfected with full length or C-terminal truncated Cx43 (Cx43DeltaCT) were subjected to the aforementioned motility assays to expose alternate mechanisms of Cx43-mediated motility. Cells expressing full length Cx43 exhibited increased motility while cells expressing Cx43DeltaCT did not. This report, the first in which RNAi has been employed to reduce Cx43 expression in gliomas, indicates that the downregulation of Cx43 decreases motility of C6 cells. Furthermore, it is the first report to suggest that the Cx43 CT plays an important role in glioma motility.

Connexin Levels Regulate Keratinocyte Differentiation in the Epidermis

To understand the role of connexin43 (Cx43) in epidermal differentiation, we reduced Cx43 levels by RNA-mediated interference knockdown and impaired its functional status by overexpressing loss-of-function Cx43 mutants associated with the human disease oculodentodigital dysplasia (ODDD) in rat epidermal keratinocytes. When Cx43 expression was knocked down by 50-75%, there was a coordinate 55-65% reduction in Cx26 level, gap junction-based dye coupling was reduced by 60%, and transepithelial resistance decreased. Importantly, the overall growth and differentiation of Cx43 knockdown organotypic epidermis was severely impaired as revealed by alterations in the levels of the differentiation markers loricrin and involucrin and by reductions in vital and cornified layer thicknesses. Conversely, although the expression of Cx43 mutants reduced the coupling status of rat epidermal keratinocytes by approximately 80% without altering the levels of endogenous Cx43 or Cx26, their ability to differentiate was not altered. In addition, we used a mouse model of ODDD and found that newborn mice harboring the loss-of-function Cx43(G60S) mutant had slightly reduced Cx43 levels, whereas Cx26 levels, epidermis differentiation, and barrier function remained unaltered. This properly differentiated epidermis was maintained even when Cx43 and Cx26 levels decreased by more than 70% in 3-week-old mutant mice. Our studies indicate that Cx43 and Cx26 collectively co-regulate epidermal differentiation from basal keratinocytes but play a more minimal role in the maintenance of established epidermis. Altogether, these studies provide an explanation as to why the vast majority of ODDD patients, where Cx43 function is highly compromised, do not suffer from skin disease.

Differentiation of Organotypic Epidermis in the Presence of Skin Disease-Linked Dominant-Negative Cx26 Mutants and Knockdown Cx26

In this study, we chose a differentiation-competent rat epidermal keratinocyte (REK) cell line to examine the role of Cx26 and disease-linked Cx26 mutants in organotypic epidermal differentiation. First, we generated stable REK cell lines expressing three skin disease-linked mutants (G59A, D66H and R75W). Second, we used an RNAi approach to knock down the expression of Cx26 in REKs. Interestingly, the three-dimensional (3D) architecture of the organotypic epidermis altered the intracellular spatial distribution of the mutants in comparison to 2D cultured REKs, highlighting the importance of using organotypic cultures. Unexpectedly, the presence of disease-linked mutants or the overexpression of wild-type Cx26 had little effect on the differentiation of the organotypic epidermis as determined by the architecture of the epidermis, expression of molecular markers indicative of epidermis differentiation (keratin 10, keratin 14, involucrin, loricrin) and stratification/cornification of the epidermis. Likewise, organotypic epidermis continued to differentiate normally upon Cx26 knockdown. While Cx26 has been reported to be upregulated during wound healing, no reduction in wound closure was observed in 2D REK cultures that expressed loss-of-function, dominant Cx26 mutants. In conclusion, we demonstrate that gain or loss of Cx26 function does not disrupt organotypic epidermal differentiation and offer insights into why patients harboring Cx26 mutations do not frequently present with more severe disease that encompasses thin skin.

Functional characterization of oculodentodigital dysplasia-associated Cx43 mutants

Oculodentodigital dysplasia (ODDD) is associated with at least 28 connexin43 (Cx43) mutations. We characterized four of these mutants; Q49K, L90V, R202H, and V216L. Populations of these GFP-tagged mutants were transported to the cell surface in Cx43-negative HeLa cells and Cx43-positive NRK cells. Dual patch-clamp functional analysis in N2A cells demonstrated that channels formed by each mutant have dramatically reduced conductance. Dye-coupling analysis revealed that each mutant exhibits a dominant-negative effect on wild-type Cx43. Since ODDD patients display skeletal abnormalities, we examined the effect of three other Cx43 mutants previously shown to exert dominant-negative effects on wild-type Cx43 (G21R, G138R, and G60S) in neonatal calvarial osteoblasts. Differentiation was unaltered by expression of these mutants as alkaline phosphatase activity and extent of culture mineralization were unchanged. This suggests that loss-of-function Cx43 mutants are insufficient to deter committed osteoblasts from their normal function in vitro. Thus, we hypothesize that the bone phenotype of ODDD patients may result from disrupted gap junctional intercellular communication earlier in development or during bone remodeling.

Activation of the canonical Wnt pathway by the antipsychotics haloperidol and clozapine involves dishevelled-3

Protein kinase B (Akt), glycogen synthase kinase-3 (GSK-3) and members of the Wnt signal transduction pathway were recently found to be altered in schizophrenia and targeted by antipsychotic drugs. In the current study, selected Wnt signalling proteins were investigated to determine if they are altered by the antipsychotics clozapine or haloperidol in the rat prefrontal cortex. Pheochromocytoma (PC12) and neuroblastoma (SH-SY5Y) cells were also used to elucidate how antipsychotics generated the pattern of changes observed in vivo. Western blotting (WB) revealed that treatment with haloperidol or clozapine caused an up-regulation of Wnt-5a, dishevelled-3, Axin, total and phosphorylated GSK-3 and beta-catenin protein levels. Treatment of PC12 and SH-SY5Y cells with a variety of pharmacological agents as well as the over-expression of several Wnt related proteins failed to mimic the pattern observed in vivo following antipsychotic treatment. However, the over-expression of dishevelled-3 nearly perfectly duplicated the changes observed in vivo. Immunoprecipitations (IP) conducted using protein isolated from the rat prefrontal cortex indicated that dishevelled-3 is associated with the D2 dopamine receptor thereby suggesting that antipsychotics may act on dishevelled-3 via D2 dopamine receptors to initiate a cascade of downstream changes involving Axin, GSK-3 and beta-catenin that may help to alleviate psychosis in schizophrenic patients.

Tumor-suppressive effects of pannexin 1 in C6 glioma cells

Mammalian gap junction proteins, connexins, have long been implicated in tumor suppression. Recently, a novel family of proteins named pannexins has been identified as the mammalian counterpart of the invertebrate gap junction proteins, innexins. To date, pannexin 1 (Panx1) and pannexin 2 (Panx2) mRNAs are reported to be expressed in the brain. Most neoplastic cells, including rat C6 gliomas, exhibit reduced connexin expression, aberrant gap junctional intercellular communication (GJIC), and an increased proliferation rate. When gap junctions are up-regulated by transfecting C6 cells with connexin43, GJIC is restored and the proliferation is reduced. In this study, we examined the tumor-suppressive effects of Panx1 expression in C6 cells. Reverse transcription-PCR analysis revealed that C6 cells do not express any of the pannexin transcripts, whereas its nontumorigenic counterpart, rat primary astrocytes, exhibited mRNAs for all three pannexins. On generation of stable C6 transfectants with tagged Panx1 [myc or enhanced green fluorescent protein (EGFP)], a localization of Panx1 expression to the Golgi apparatus and plasma membrane was observed. In addition, Panx1 transfectants exhibited a flattened morphology, which differs greatly from the spindle-shaped control cells (EGFP only). Moreover, Panx1 expression increased gap junctional coupling as shown by the passage of sulforhodamine 101. Finally, we showed that stable expression of Panx1 in C6 cells significantly reduced cell proliferation in monolayers, cell motility, anchorage-independent growth, and in vivo tumor growth in athymic nude mice. Altogether, we conclude that the loss of pannexin expression may participate in the development of C6 gliomas, whereas restoration of Panx1 plays a tumor-suppressive role.

Over-expression of AhR (aryl hydrocarbon receptor) induces neural differentiation of Neuro2a cells: neurotoxicology study

BACKGROUND

Dioxins and related compounds are suspected of causing neurological disruption in human and experimental animal offspring following perinatal exposure during development and growth. The molecular mechanism(s) of the actions in the brain, however, have not been fully investigated. A major participant in the process of the dioxin-toxicity is the dioxin receptor, namely the aryl hydrocarbon receptor (AhR). AhR regulates the transcription of diverse genes through binding to the xenobiotic-responsive element (XRE). Since the AhR has also been detected in various regions of the brain, the AhR may play a key role in the developmental neurotoxicity of dioxins. This study focused on the effect of AhR activation in the developing neuron.

METHODS

The influence of the AhR on the developing neuron was assessed using the Neuro2a-AhR transfectant. The undifferentiated murine neuroblastoma Neuro2a cell line (ATCC) was stably transfected with AhR cDNA and the established cell line was named N2a-Ralpha. The activation of exogenous AhR in N2a-Ralpha cells was confirmed using RNAi, with si-AhR suppressing the expression of exogenous AhR. The neurological properties of N2a-Ralpha based on AhR activation were evaluated by immunohistochemical analysis of cytoskeletal molecules and by RT-PCR analysis of mRNA expression of neurotransmitter-production related molecules, such as tyrosine hydroxylase (TH).

RESULTS

N2a-Ralpha cells exhibited constant activation of the exogenous AhR. CYP1A1, a typical XRE-regulated gene, mRNA was induced without the application of ligand to the culture medium. N2a-Ralpha cells exhibited two significant functional features. Morphologically, N2a-Ralpha cells bore spontaneous neurites exhibiting axon-like properties with the localization of NF-H. In addition, cdc42 expression was increased in comparison to the control cell line. The other is the catecholaminergic neuron-like property. N2a-Ralpha cells expressed tyrosine hydroxylase (TH) mRNA as a functional marker of catecholaminergic neurotransmitter production. Thus, exogenous AhR induced catecholaminergic differentiation in N2a-Ralpha cells.

CONCLUSION

The excessive activation of AhR resulted in neural differentiation of Neuro2a cells. This result revealed that dioxins may affect the nervous system through the AhR-signaling pathway. Activated AhR may disrupt the strictly regulated brain formation with irregular differentiation occurring rather than cell death.

Enhanced neurite outgrowth in PC12 cells mediated by connexin hemichannels and ATP

Gap junctions have traditionally been described as transmembrane channels that facilitate intercellular communication via the passage of small molecules. Connexins, the basic building blocks of gap junctions, are expressed in most mammalian tissues including the developing and adult central nervous system. During brain development, connexins are temporally and spatially regulated suggesting they play an important role in the proper formation of the central nervous system. In the current study, connexins 32 and 43 were overexpressed in PC12 cells to determine whether connexins are involved in neuronal differentiation. Both connexin 32 and 43 were appropriately trafficked to the cell membrane following overexpression and resulted in the formation of functional gap junctions. Connexin overexpression was found to cause enhanced neurite outgrowth in PC12 cells treated with nerve growth factor to initiate neuritogenesis. Surprisingly, however, enhanced neurite outgrowth was found to be the consequence of functional hemichannel formation as opposed to traditional intercellular communication. Additional analysis revealed that ATP was released into the media likely through hemichannels and acted on purinergic receptors to cause enhanced neurite outgrowth. Collectively, the results of the current study suggest that connexins may play an important role in neuronal differentiation by non-traditional mechanisms.

Role of Sox2 in the development of the mouse neocortex

The mammalian neocortex is established from neural stem and progenitor cells that utilize specific transcriptional and environmental factors to create functional neurons and astrocytes. Here, we examined the mechanism of Sox2 action during neocortical neurogenesis and gliogenesis. We established a robust Sox2 expression in neural stem and progenitor cells within the ventricular zone, which persisted until the cells exited the cell cycle. Overexpression of constitutively active Sox2 in neural progenitors resulted in upregulation of Notch1, recombination signal-sequence binding protein-J (RBP-J) and hairy enhancer of split 5 (Hes5) transcripts and the Sox2 high mobility group (HMG) domain seemed sufficient to confer these effects. While Sox2 overexpression permitted the differentiation of progenitors into astroglia, it inhibited neurogenesis, unless the Notch pathway was blocked. Moreover, neuronal precursors engaged a serine protease(s) to eliminate the overexpressed Sox2 protein and relieve the repression of neurogenesis. Glial precursors and differentiated astrocytes, on the other hand, maintained Sox2 expression until they reached a quiescent state. Sox2 expression was re-activated by signals that triggered astrocytic proliferation (i.e., injury, mitogenic and gliogenic factors). Taken together, Sox2 appears to act upstream of the Notch signaling pathway to maintain the cell proliferative potential and to ensure the generation of sufficient cell numbers and phenotypes in the developing neocortex.

Nerve growth factor increases connexin43 phosphorylation and gap junctional intercellular communication

The function of gap junctions is regulated by the phosphorylation state of their connexin subunits. Numerous growth factors are known to regulate connexin phosphorylation; however, the effect of nerve growth factor on gap junction function is not understood. The phosphorylation of connexin subunits is a key event during many aspects of the lifecycle of a connexin, including open/close states, assembly/trafficking, and degradation, and thus affects the functionality of the channel. PC12 cells infected with connexin43 (Cx43) retrovirus were used as a neuronal model to characterize the signal transduction pathways activated by nerve growth factor (NGF) that potentially affect the functional state of Cx43. Immunoblot analysis demonstrated that Cx43 and the mitogen-activated protein kinase (MAPK), ERK-1/2, were phosphorylated in response to TrkA activation via NGF and that phosphorylation could be prevented by treatment with the MEK-1/2 inhibitor U0126. The effects of NGF on gap junction intercellular communication were examined by monitoring fluorescence recovery after photobleaching PC12-Cx43 cells preloaded with calcein. Fluorescence recovery in the photobleached area increased after NGF treatment and decreased when pretreated with the MEK-1/2 inhibitor U0126. These data are the first to show a direct signaling link between neurotrophins and the phosphorylation of connexin proteins through the MAPK pathway resulting in increased gap junctional intercellular communication. Neurotrophic regulation of connexin activity provides a novel mechanism of regulating intercellular communication between neurons during nervous system development and repair.

Connexin over-expression differentially suppresses glioma growth and contributes to the bystander effect following HSV-thymidine kinase gene therapy

Neoplastic transformation is frequently associated with a loss of gap junctional intercellular communication and reduced expression of connexins. The introduction of connexin genes into tumor cells reverses the proliferative characteristics of such cells. However, there is very little comparative information on the effects of different connexins on cancer cell growth. We hypothesized that Cx26, Cx32, or Cx43 would display differential growth suppression of C6 glioma cells and uniquely modulate the bystander effect following transduction of C6 cells with HSVtk followed by suicide gene therapy. The bystander phenomenon is the death of a greater number of tumor cells than are expressing the HSVtk gene, presumably due to the passage of toxic molecules through gap junction channels. To test this hypothesis, we used retroviral vectors to infect C6 glioma cells producing connexin-expressing and HSVtk-expressing cell lines. All three connexin-expressing cell lines grew significantly slower than GFP-infected or native C6 cells. Cx32 and Cx26 were significantly more effective at mediating the bystander effect in cocultures of C6-connexin cells with C6-HSVtk cells. These studies indicate that connexins have unique properties that contribute to their tumor suppressive function.

Mechanisms of Cx43 and Cx26 transport to the plasma membrane and gap junction regeneration

Previous reports have suggested that Cx26 exhibits unique intracellular transport pathways en route to the cell surface compared with other members of the connexin family. To directly examine and compare nascent and steady-state delivery of Cx43 and Cx26 to the plasma membrane and gap junction biogenesis we expressed fluorescent-protein-tagged Cx43 and Cx26 in BICR-M1Rk and NRK cells. Static and time-lapse imaging revealed that both connexins were routed through the Golgi apparatus prior to being transported to the cell surface, a process inhibited in the presence of brefeldin A (BFA) or the expression of a dominant-negative form of Sar1 GTPase. During recovery from BFA, time-lapse imaging of nascent connexin Golgi-to-plasma membrane delivery revealed many dynamic post-Golgi carriers (PGCs) originating from the distal side of the Golgi apparatus consisting of heterogeneous vesicles and long, tubular-like extensions. Vesicles and tubular extensions were also observed in HBL-100 cells expressing a human, disease-linked, Golgi-localized Cx26 mutant, D66H-GFP. A diffuse cell surface rim of fluorescent-protein-tagged wild-type connexins was observed prior to the appearance of punctate gap junctions, which suggests that random fusion of PGCs occurred with the plasma membrane followed by lateral diffusion of connexins into clusters. Fluorescence recovery after photobleaching studies revealed that Cx26-YFP was more mobile within gap junction plaques compared with Cx43-GFP. Intriguingly, Cx43-GFP delivery and gap junction regeneration was inhibited by BFA and nocodazole, whereas Cx26-GFP delivery was prevented by BFA but not nocodazole. Collectively, these studies suggest that during gap junction biogenesis two phylogenetically distinct members of the connexin family, Cx43 and Cx26, share common secretory pathways, types of transport intermediates and turnover dynamics but differ in their microtubule-dependence and mobility within the plasma membrane, which might reflect differences in binding to protein scaffolds.

Cellular sublocalization of Cx43 and the establishment of functional coupling in IMR-32 neuroblastoma cells

Neuroblastoma (NB) is the most common solid pediatric tumor. IMR-32 cells are a highly malignant human NB cell line with uncontrolled proliferation but with the potential to be differentiated under specific conditions. Preliminary research indicated that connexin 43 (Cx43), the most widespread of the Cx family, is aberrantly located in IMR-32 cells, which renders these cells incapable of gap junction (GJ) intercellular communication. Functioning GJ intercellular communication has been strongly associated with growth control and a decrease in tumorigenicity. 8-br-cAMP, known to initiate the differentiation process in cancer cells, was used to examine changes in Cx43 localization and expression via immunocytochemistry, Western blot analysis, and flow cytometry. Exposure of IMR-32 cells to 8-br-cAMP decreased cell proliferation, restored the abnormally localized Cx43 from around the nucleus to the cell membrane, increased de novo Cx43 protein expression, and appeared to phosphorylate Cx43 on serine (Ser) 255 and Ser262. Forskolin, an activator of cAMP dependent protein kinase (PKA), produced identical results to 8-br-cAMP demonstrating the effect that was not unique to a cAMP analog. The use of a PKA inhibitor further confirmed the specificity of 8-br-cAMP and forskolin's effect on Cx43. The cellular relocation of Cx43, combined with the increased protein expression, established first ever GJ intercellular communication between IMR-32 cells as revealed by scrape loading. These results suggest that the GJ-mediated return of growth control, as a prerequisite for further differentiation, offers a new therapeutic avenue in the treatment of NB.

Neurogenesis and neuronal communication on micropatterned neurochips

Neural networks are formed by accurate connectivity of neurons and glial cells in the brain. These networks employ a three-dimensional bio-surface that both assigns precise coordinates to cells during development and facilitates their connectivity and functionality throughout life. Using specific topographic and chemical features, we have taken steps towards the development of poly(dimethylsiloxane; PDMS) neurochips that can be used to generate and study synthetic neural networks. These neurochips have micropatterned structures that permit adequate cell positioning and support cell survival. Within days of plating, cells differentiate into neurons displaying excitability and communication, as evidenced by intracellular calcium oscillations and action potentials. The structural and functional capacities of such simple neural networks open up new opportunities to study synaptic communication and plasticity.

CCN3 (NOV) interacts with connexin43 in C6 glioma cells: possible mechanism of connexin-mediated growth suppression

Many tumor cells exhibit aberrant gap junctional intercellular communication, which can be restored by transfection with connexin genes. We have previously discovered that overexpression of connexin43 (Cx43) in C6 glioma cells not only reduces proliferation but also leads to production of soluble growth-inhibitory factors. We identified that several members of the CCN (Cyr61/connective tissue growth factor/nephroblastoma-overexpressed) family are up-regulated following Cx43 expression, including CCN3 (NOV). We now report evidence for an association between CCN3 and Cx43. Western blot analysis demonstrated that the 48-kDa full-length CCN3 protein was present in the lysate and conditioned medium of growth-suppressed C6-Cx43 cells, as well as primary astrocytes, but not in C6 parental and human glioma cells. Immunocytochemical examination of CCN3 revealed diffuse localization in parental C6 cells, whereas transfection of C6 cells with Cx43 (C6-Cx43) or with a modified Cx43 tagged to green fluorescent protein on its C terminus (Cx43-GFP) resulted in punctate staining, suggesting that CCN3 co-localizes with Cx43 in plaques at the plasma membrane. In cells expressing a C-terminal truncation of Cx43 (Cx43Delta244-382), this co-localization was lost. Glutathione S-transferase pull-down assay and co-immunoprecipitation demonstrated that CCN3 was able to physically interact with Cx43. In contrast, CCN3 was not found to associate with Cx43Delta244-382. Similar experiments revealed that CCN3 did not co-localize or associate with other connexins, including Cx40 or Cx32. Taken together, these data support an interaction of CCN3 with the C terminus of Cx43, which could play an important role in mediating growth control induced by specific gap junction proteins.

Selective assembly of connexin37 into heterocellular gap junctions at the oocyte/granulosa cell interface

Studies of mice with targeted disruptions of specific connexin genes have revealed that at least two connexins, connexin37 (Cx37) and connexin43 (Cx43), play essential roles in ovarian follicle development. To explore the respective roles of these two connexins in gap-junctional communication between the developing murine oocyte and its surrounding cumulus granulosa cells, we used confocal immunofluorescence microscopy and oocyte preloading functional assays. Immunofluorescence microscopy located Cx37 within gap-junction plaques between granulosa cells and the oocyte, and Cx43 between surrounding granulosa cells. Preloading assays combining denuded oocytes and cultured granulosa cells expressing or lacking Cx37 or Cx43 revealed that Cx37 must be present in both cell types for the establishment of heterocellular gap-junctional coupling. Furthermore, immunofluorescence microscopy of cultured granulosa cells after incubation with denuded oocytes showed that the oocyte induces the formation of gap junctions containing Cx37 at the surface of granulosa cells. Continuous Cx37 expression in granulosa cells was confirmed using RT-PCR. Together, these results indicate that the growing murine oocyte is functionally coupled with granulosa cells by homotypic gap junctions composed of Cx37, and that the formation and/or stabilization of Cx37 junctions is selectively induced at the oocyte-granulosa interface by cell contact.

Electrical synapses: a dynamic signaling system that shapes the activity of neuronal networks

Gap junctions consist of intercellular channels dedicated to providing a direct pathway for ionic and biochemical communication between contacting cells. After an initial burst of publications describing electrical coupling in the brain, gap junctions progressively became less fashionable among neurobiologists, as the consensus was that this form of synaptic transmission would play a minimal role in shaping neuronal activity in higher vertebrates. Several new findings over the last decade (e.g. the implication of connexins in genetic diseases of the nervous system, in processing sensory information and in synchronizing the activity of neuronal networks) have brought gap junctions back into the spotlight. The appearance of gap junctional coupling in the nervous system is developmentally regulated, restricted to distinct cell types and persists after the establishment of chemical synapses, thus suggesting that this form of cell-cell signaling may be functionally interrelated with, rather than alternative to chemical transmission. This review focuses on gap junctions between neurons and summarizes the available data, derived from molecular, biological, electrophysiological, and genetic approaches, that are contributing to a new appreciation of their role in brain function.

Functional Domain Mapping and Selective Trans-dominant Effects Exhibited by Cx26 Disease-causing Mutations

Mutations in Cx26 are a major cause of autosomal dominant and recessive forms of sensorineural deafness. Some mutations in Cx26 are associated not only with deafness but also with skin disease. We examined the subcellular localization and function of two green fluorescent protein (GFP)-tagged Cx26 point mutants that exhibit both phenotypes, G59A-GFP and D66H-GFP. D66H-GFP was retained within the brefeldin A-insensitive trans-Golgi network, whereas a population of G59A-GFP was transported to the cell surface. Neither G59A nor D66H formed gap junctions that were permeable to small fluorescent dyes, suggesting they are loss-of-function mutations. When co-expressed with wild-type Cx26, both G59A and D66H exerted dominant-negative effects on Cx26 function. G59A also exerted a trans-dominant negative effect on co-expressed wild type Cx32 and Cx43, whereas D66H exerted a trans-dominant negative effect on Cx43 but not Cx32. We propose that the severity of the skin disease is dependent on the specific nature of the Cx26 mutation and the trans-dominant selectivity of the Cx26 mutants on co-expressed connexins. Additional systematic mutations at residue D66, in which the overall charge of this motif was altered, suggested that the first extracellular loop is critical for Cx26 transport to the cell surface as well as function of the resulting gap junction channels.