Epidermal growth factor down-regulates connexin-43 expression in cultured rat cortical astrocytes

Gap junction-mediated cell-to-cell communication in oral development and oral diseases: a concise review of research progress

Homoeostasis depends on the close connection and intimate molecular exchange between extracellular, intracellular and intercellular networks. Intercellular communication is largely mediated by gap junctions (GJs), a type of specialized membrane contact composed of variable number of channels that enable direct communication between cells by allowing small molecules to pass directly into the cytoplasm of neighbouring cells. Although considerable evidence indicates that gap junctions contribute to the functions of many organs, such as the bone, intestine, kidney, heart, brain and nerve, less is known about their role in oral development and disease. In this review, the current progress in understanding the background of connexins and the functions of gap junctions in oral development and diseases is discussed. The homoeostasis of tooth and periodontal tissues, normal tooth and maxillofacial development, saliva secretion and the integrity of the oral mucosa depend on the proper function of gap junctions. Knowledge of this pattern of cell-cell communication is required for a better understanding of oral diseases. With the ever-increasing understanding of connexins in oral diseases, therapeutic strategies could be developed to target these membrane channels in various oral diseases and maxillofacial dysplasia.

A novel serum free primary astrocyte culture method that mimic quiescent astrocyte phenotype

BACKGROUND

Primary astrocyte cultures have been used for decades to study astrocyte functions in health and disease. The current primary astrocyte cultures are mostly maintained in serum-containing medium which produces astrocytes with a reactive phenotype as compared to in vivo quiescent astrocytes. The aim of this study was to establish a serum-free astrocyte culture medium that maintains primary astrocytes in a quiescent state.

NEW METHOD

Serum free astrocyte base medium (ABM) supplemented with basic fibroblast growth factor 2 (FGF2) and epidermal growth factor (EGF) (ABM-FGF2-EGF) or serum supplemented DMEM (MD-10%FBS) was used to culture primary astrocytes isolated from cerebral cortex of postnatal day 1 C57BL/6 mice.

RESULTS

Compared to astrocytes cultured in MD-10%FBS medium, astrocytes in ABM-FGF2-EGF had higher process bearing morphologies similar to in vivo astrocytes. Western blot, immunostaining, quantitative polymerase chain reaction and metabolic assays revealed that astrocytes maintained in ABM-FGF2-EGF had enhanced glycolytic metabolism, higher glycogen content, lower GFAP expression, increased glutamine synthase, and glutamate transporter-1 mRNA levels as compared to astrocytes cultured in MD-10% FBS medium.

COMPARISON TO EXISTING METHODS

These observations suggest that astrocytes cultured in ABM-FGF2-EGF media compared to the usual FBS media promote quiescent and biosynthetic phenotype similar to in vivo astrocytes.

CONCLUSION

This media provides a novel method for studying astrocytes functions in vitro under physiological and pathological conditions.

Docosahexaenoic Acid (DHA) Induced Morphological Differentiation of Astrocytes Is Associated with Transcriptional Upregulation and Endocytosis of β2-AR

Docosahexaenoic acid (DHA), an important ω-3 fatty acid, is abundantly present in the central nervous system and is important in every step of brain development. Much of this knowledge has been based on studies of the role of DHA in the function of the neurons, and reports on its effect on the glial cells are few and far between. We have previously reported that DHA facilitates astrocyte differentiation in primary culture. We have further explored the signaling mechanism associated with this event. It was observed that a sustained activation of the extracellular signal-regulated kinase (ERK) appeared to be critical for DHA-induced differentiation of the cultured astrocytes. Prior exposure to different endocytic inhibitors blocked both ERK activation and differentiation of the astrocytes during DHA treatment suggesting that the observed induction of ERK-2 was purely endosomal. Unlike the β₁-adrenergic receptor (β₁-AR) antagonist, atenolol, pre-treatment of the cells with the β₂-adrenergic receptor (β₂-AR) antagonist, ICI-118,551 inhibited the DHA-induced differentiation process, indicating a downstream involvement of β₂-AR in the differentiation process. qRT-PCR and western blot analysis demonstrated a significant induction in the mRNA and protein expression of β₂-AR at 18-24 h of DHA treatment, suggesting that the induction of β₂-AR may be due to transcriptional upregulation. Moreover, DHA caused activation of PKA at 6 h, followed by activation of downstream cAMP response element-binding protein, a known transcription factor for β₂-AR. Altogether, the observations suggest that DHA upregulates β₂-AR in astrocytes, which undergo endocytosis and signals for sustained endosomal ERK activation to drive the differentiation process.

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

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

Glaucomatous Optic Neuropathy: When Glia Misbehave

The loss of vision in the human eye disease, glaucoma, is due to degeneration of the axons of the retinal ganglion cells. In glaucoma, reactive astrocytes in the optic nerve head contain inducible nitric oxide synthase, which apparently produces excessive nitric oxide that damages the axons. The astrocytes respond to the elevated intraocular pressure that is characteristic of the disease. An important signal transduction pathway for the induction of nitric oxide synthase in response to pressure is the epidermal growth factor receptor tyrosine kinase. Pharmacological inhibition of the activity or the induction of inducible nitric oxide synthase may provide neuroprotection for the treatment of glaucoma.

Regulation of connexin 43 and microRNA expression via β2-adrenoceptor signaling in 1321N1 astrocytoma cells

Connexin 43 (Cx43) is the main gap junction protein in astrocytes and exerts the same effects on growth inhibition in astrocytoma and glioma as microRNA-146a (miR-146a) in glioma. β2-adrenergic receptor (AR) signaling modulates Cx43 expression in myocytes via components downstream of protein kinase A (PKA) and exchange protein directly activated by cAMP (Epac). However, it remains to be elucidated how expression of Cx43 is modulated in astrocytes. In the present study, 1321N1 astrocytoma cells were treated with β2-AR signaling agents in order to evaluate the expression of Cx43 and miRNAs. RNA and protein were extracted from the cells for use in reverse transcription-quantitative polymerase chain reaction and western blot analysis, respectively. The results revealed that clenbuterol increased miR-146a level and upregulated Cx43 expression via cAMP/PKA at the mRNA and protein level. Pre-inhibition of adenyl cyclase decreased expression of Cx43 and miR-146a. PKA activation and overexpression of miR-146a in A-1321N1 cells increased the expression of Cx43. β2-AR stimulation and 6Bnz, a PKA activator, suppressed oncomiRs miR-155 and miR-27a, while 8-(4-chlorophenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate, an Epac activator, increased their levels. The current findings demonstrated that β2-AR signaling has growth inhibitory effects via modulation of the cAMP/PKA pathway in A-1321N1 cells through increasing the expression level of Cx43 and miR-146a as well as decreasing miR-155 and miR-27a levels. Thus, stimulation of the β2-AR and PKA signaling pathway may be a useful approach for astrocytoma therapy.

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'.

Modulation of gap junction channels and hemichannels by growth factors

Gap junction hemichannels and cell-cell channels have roles in coordinating numerous cellular processes, due to their permeability to extra and intracellular signaling molecules. Another mechanism of cellular coordination is provided by a vast array of growth factors that interact with relatively selective cell membrane receptors. These receptors can affect cellular transduction pathways, including alteration of intracellular concentration of free Ca(2+) and free radicals and activation of protein kinases or phosphatases. Connexin and pannexin based channels constitute recently described targets of growth factor signal transduction pathways, but little is known regarding the effects of growth factor signaling on pannexin based channels. The effects of growth factors on these two channel types seem to depend on the cell type, cell stage and connexin and pannexin isoform expressed. The functional state of hemichannels and gap junction channels are affected in opposite directions by FGF-1 via protein kinase-dependent mechanisms. These changes are largely explained by channels insertion in or withdrawal from the cell membrane, but changes in open probability might also occur due to changes in phosphorylation and redox state of channel subunits. The functional consequence of variation in cell-cell communication via these membrane channels is implicated in disease as well as normal cellular responses.

ErbB receptor signaling in astrocytes: a mediator of neuron-glia communication in the mature central nervous system

Astrocytes are now recognized as active players in the developing and mature central nervous system. Each astrocyte contacts vascular structures and thousands of synapses within discrete territories. These cells receive a myriad of inputs and generate appropriate responses to regulate the function of brain microdomains. Emerging evidence has implicated receptors of the ErbB tyrosine kinase family in the integration and processing of neuronal inputs by astrocytes: ErbB receptors can be activated by a wide range of neuronal stimuli; they control critical steps of glutamate-glutamine metabolism; and they regulate the biosynthesis and release of various glial-derived neurotrophic factors, gliomediators and gliotransmitters. These key properties of astrocytic ErbB signaling in neuron-glia interactions have significance for the physiology of the mature central nervous system, as exemplified by the central control of reproduction within the hypothalamus, and are also likely to contribute to pathological situations, since both dysregulation of ErbB signaling and glial dysfunction occur in many neurological disorders.

Adenovirus-mediated delivery of bFGF small interfering RNA increases levels of connexin 43 in the glioma cell line, U251

BACKGROUND

bFGF is an important growth factor for glioma cell proliferation and invasion, while connexin 43 is implicated in the suppression of glioma growth. Correspondingly, gliomas have been shown to have reduced, or compromised, connexin 43 expression.

METHODS

In this study, a bFGF-targeted siRNA was delivered to the glioma cell line, U251, using adenovirus (Ad-bFGF-siRNA) and the expression of connexin 43 and its phosphorylation state were evaluated. U251 cells were infected with Ad-bFGF-siRNA (100, 50, or 25 MOI), and infection with adenovirus expressing green fluorescent protein (Ad-GFP) at 100 MOI served as a control. Western blotting and immunofluorescence were used to detect the expression levels, phosphorylation, and localization of connexin 43 in U251 cells infected, and not infected, with Ad-bFGF-siRNA.

RESULTS

Significantly higher levels of connexin 43 were detected in U251 cells infected with Ad-bFGF-siRNA at 100 and 50 MOI than in cells infected with Ad-GFP, and the same amount of connexin 43 was detected in Ad-GFP-infected and uninfected U251 cells. Connexin 43 phosphorylation did not differ between Ad-bFGF-siRNA-infected and uninfected U251 cells. However, the ratio of phosphorylated to unphosphorylated connexin 43 in Ad-bFGF-siRNA cells was lower, and connexin 43 was predominantly localized to the cytoplasm. Using a scrape loading dye transfer assay, more Lucifer Yellow was transferred to neighboring cells in the Ad-bFGF-siRNA treated group than in the control group.

CONCLUSION

To our knowledge, this is the first description of a role for connexin 43 in the inhibition of U251 growth using Ad-bFGF-siRNA.

Thyroid hormone-induced morphological differentiation and maturation of astrocytes involves activation of protein kinase A and ERK signalling pathway

Thyroid hormone (TH) has a profound effect on astrocyte differentiation and maturation. Astrocytes cultured under TH-deficient conditions fail to transform from flat polygonal morphology to mature, process-bearing, stellate cells. Supplementation of physiological concentrations of TH initiate gradual transformation of the cells and the process takes approximately 48 h to complete. The signal transduction pathways associated with TH-mediated maturation of astrocytes have been investigated. TH treatment caused an initial activation of protein kinase A (PKA), with a peak activity at 2 h which fell back to basal level there after. Although there was no visible change in morphology of the cells during the observed activation of PKA, it was sufficient to drive the process of transformation to completion, suggesting the involvement of downstream regulators of PKA. PKA inhibitors as well as the MEK inhibitor PD098059 attenuated the TH-induced morphological transformation. Further studies showed that TH treatment resulted in a biphasic response on the cellular phospho-MAP kinase (p-MAPK or p-ERK) level: an initial decline in the p-ERK level followed by an induction at 18-24 h, both of which could be blocked by a PKA inhibitor. Such sustained activation of p-ERK levels by TH at this later stage coincided with initiation of morphological differentiation of the astrocytes and appeared to be critical for the transformation of astrocytes. The nitric oxide synthase (NOS) inhibitor 7-NI inhibited this induction of p-ERK activity. Moreover, the induction was accompanied by a parallel increase in phospho-CREB activity which, however, persisted at the end of the transformation of the astroglial cells.

Pharmacology of gap junctions. New pharmacological targets for treatment of arrhythmia, seizure and cancer?

Intercellular communication in many organs is maintained via intercellular gap junction channels composed of connexins, a large protein family with a number of isoforms. This gap junction intercellular communication (GJIC) allows the propagation of action potentials (e.g., in brain, heart), and the transfer of small molecules which may regulate cell growth, differentiation and function. The latter has been shown to be involved in cancer growth: reduced GJIC often is associated with increased tumor growth or with de-differentiation processes. Disturbances of GJIC in the heart can cause arrhythmia, while in brain electrical activity during seizures seems to be propagated via gap junction channels. Many diseases or pathophysiological conditions seem to be associated with alterations of gap junction protein expression. Thus, depending on the target disease opening or closure of gap junctions may be of interest, or alteration of connexin expression. GJIC can be affected acutely by changing gap junction conductance or--more chronic--by altering connexin expression and membrane localisation. This review gives an overview on drugs affecting GJIC.

Cardiomyocyte hypertrophy and degradation of connexin43 through spatially restricted autocrine/paracrine heparin-binding EGF

Growth factor signaling can affect tissue remodeling through autocrine/paracrine mechanisms. Recent evidence indicates that EGF receptor transactivation by heparin-binding EGF (HB-EGF) contributes to hypertrophic signaling in cardiomyocytes. Here, we show that HB-EGF operates in a spatially restricted circuit in the extracellular space within the myocardium, revealing the critical nature of the local microenvironment in intercellular signaling. This highly localized microenvironment of HB-EGF signaling was demonstrated with 3D morphology, consistent with predictions from a computational model of EGF signaling. HB-EGF secretion by a given cardiomyocyte in mouse left ventricles led to cellular hypertrophy and reduced expression of connexin43 in the overexpressing cell and in immediately adjacent cells but not in cells farther away. Thus, HB-EGF acts as an autocrine and local paracrine cardiac growth factor that leads to loss of gap junction proteins within a spatially confined microenvironment. These findings demonstrate how cells can coordinate remodeling with their immediate neighboring cells with highly localized extracellular EGF signaling.

Connexin phosphorylation as a regulatory event linked to gap junction internalization and degradation

Gap junction proteins, connexins, are dynamic polytopic membrane proteins that exhibit unprecedented short half-lives of only a few hours. Consequently, it is well accepted that in addition to channel gating, gap junctional intercellular communication is regulated by connexin biosynthesis, transport and assembly as well as the formation and removal of gap junctions from the cell surface. At least nine members of the 20-member connexin family are known to be phosphorylated en route or during their assembly into gap junctions. For some connexins, notably Cx43, evidence exists that phosphorylation may trigger its internalization and degradation. In recent years it has become apparent that the mechanisms underlying the regulation of connexin turnover are quite complex with the identification of many connexin binding molecules, a multiplicity of protein kinases that phosphorylate connexins and the involvement of both lysosomal and proteasomal pathways in degrading connexins. This paper will review the evidence that connexin phosphorylation regulates, stimulates or triggers gap junction disassembly, internalization and degradation.

Role of gap junctional intercellular communication (GJIC) through p38 and ERK1/2 pathway in the differentiation of rat neuronal stem cells

Gap junctional intercellular communications (GJIC) contributes to neural function in development and differentiation of CNS. In this study, we have investigated the expression of GJIC during the differentiation of neuronal stem cells and 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced neuronal stem cell-derived cells from rat brain. During neuronal stem cell differentiation, expressions of Cx43 and 32 were increased for the duration of 72 hr, however the effect were decreased on the 7d. In the neuronal stem cell-derived cells, pretreatments with p38 MAP kinase inhibitor, SB203580, and MEK inhibitor, PD98059, could protect GJIC against TPA-induced inhibition of GJIC. Our data suggest that GJIC plays an important role during neuronal stem cell differentiation, and ERK1/2 and p38 MAP kinase signaling pathway may be closely related functionally to regulate gap junction in rat neuronal stem cell-derived cells.

A novel secretory factor, Neurogenesin-1, provides neurogenic environmental cues for neural stem cells in the adult hippocampus

Neurogenesis occurs in restricted regions in the adult mammalian brain, among which the neurogenesis in the hippocampal dentate gyrus plays the crucial role in learning and memory. To date, little is known about neurogenic cues, which result in the neuronal fate adoption of neural stem cells residing in neurogenic regions, especially neurogenic cues in adult hippocampal neurogenesis. In the present study, we show that hippocampal astrocytes and also dentate granule cells adjacent to neural stem cells secrete a newly cloned novel secretory factor, Neurogenesin-1. This protein contains three cysteine-rich domains and a unique sequence and contributes to neuronal differentiation of neural stem cells in the adult brain by preventing the adoption of a glial fate. Furthermore, the neurogenic activity detected in the hippocampal culture medium was markedly suppressed by the administration of an anti-Neurogenesin-1 antibody. These findings suggest endogenous mechanisms that induce adult hippocampal neurogenesis and propose an innovative treatment for the neurodegenerative diseases that cause loss of hippocampal neurons.

Delayed but sustained induction of mitogen-activated protein kinase activity is associated with β-adrenergic receptor-mediated morphological differentiation of astrocytes

Astroglial beta-adrenergic receptors (beta-ARs) are functionally linked to regulate cellular morphology. In primary cultures, the beta-AR agonist isoproterenol (ISP) can transform flat polygonal astrocytes into process-bearing, mature stellate cells by 48 h, an effect that can be blocked by the beta-AR antagonist, propranolol. ISP induced immediate activation of protein kinase A (PKA) which persisted up to 2 h, with no visible change in cell morphology. However, activation of PKA was sufficient to drive the process of transformation to completion, suggesting the involvement of downstream regulators of PKA. In addition to PKA inhibitors, the mitogen-activated protein kinase (MAPK) kinase inhibitor PD098059 also blocked ISP-induced morphological transformation. ISP treatment resulted in a biphasic response of cellular phosphorylated MAPK (phosphorylated extracellular signal-regulated kinase; p-ERK) level: an initial decline in p-ERK level followed by a sustained induction at 12-24 h, both of which were blocked by PKA inhibitor. The induction in pERK level coincided with initiation of morphological differentiation of the astrocytes and nuclear translocation of p-ERK. A long-lasting activation of p-ERK activity by ISP, at a later stage, appears to be critical for the transformation of astrocytes.

Gap junctions, homeostasis, and injury

Gap junctions (Gj) play an important role in the communication between cells of many tissues. They are composed of channels that permit the passage of ions and low molecular weight metabolites between adjacent cells, without exposure to the extracellular environment. These pathways are formed by the interaction between two hemichannels on the surface of opposing cells. These hemichannels are formed by the association of six identical subunits, named connexins (Cx), which are integral membrane proteins. Cell coupling via Gj is dependent on the specific pattern of Cx gene expression. This pattern of gene expression is altered during several pathological conditions resulting in changes of cell coupling. The regulation of Cx gene expression is affected at different levels from transcription to post translational processes during injury. In addition, Gj cellular communication is regulated by gating mechanisms. The alteration of Gj communication during injury could be rationalized by two opposite theories. One hypothesis proposes that the alteration of Gj communication attenuates the spread of toxic metabolites from the injured area to healthy organ regions. The alternative proposition is that a reduction of cellular communication reduces the loss of important cellular metabolisms, such as ATP and glucose.