Basic fibroblast growth factor increases junctional communication and connexin 43 expression in microvascular endothelial cells

Staphylococcus aureus impairs cutaneous wound healing by activating the expression of a gap junction protein, connexin-43 in keratinocytes

Chronic wounds have been considered as major medical problems that may result in expensive healthcare. One of the common causes of chronic wounds is bacterial contamination that leads to persistent inflammation and unbalanced host cell immune responses. Among the bacterial strains that have been identified from chronic wounds, Staphylococcus aureus is the most common strain. We previously observed that S. aureus impaired mouse cutaneous wound healing by delaying re-epithelialization. Here, we investigated the mechanism of delayed re-epithelialization caused by S. aureus infection. With the presence of S. aureus exudate, the migration of in vitro cultured human keratinocytes was significantly inhibited and connexin-43 (Cx43) was upregulated. Inhibition of keratinocyte migration by S. aureus exudate disappeared in keratinocytes where the expression of Cx43 knocked down. Protein kinase phosphorylation array showed that phosphorylation of Akt-S473 was upregulated by S. aureus exudate. In vivo study of Cx43 in S. aureus-infected murine splinted cutaneous wound model showed upregulation of Cx43 in the migrating epithelial edge by S. aureus infection. Treatment with a PI3K/Akt inhibitor reduced Cx43 expression and overcame the wound closure impairment by S. aureus infection in the mouse model. This may contribute to the development of treatment to bacterium-infected wounds.

The Functional Implications of Endothelial Gap Junctions and Cellular Mechanics in Vascular Angiogenesis

Angiogenesis—the sprouting and growth of new blood vessels from the existing vasculature—is an important contributor to tumor development, since it facilitates the supply of oxygen and nutrients to cancer cells. Endothelial cells are critically affected during the angiogenic process as their proliferation, motility, and morphology are modulated by pro-angiogenic and environmental factors associated with tumor tissues and cancer cells. Recent in vivo and in vitro studies have revealed that the gap junctions of endothelial cells also participate in the promotion of angiogenesis. Pro-angiogenic factors modulate gap junction function and connexin expression in endothelial cells, whereas endothelial connexins are involved in angiogenic tube formation and in the cell migration of endothelial cells. Several mechanisms, including gap junction function-dependent or -independent pathways, have been proposed. In particular, connexins might have the potential to regulate cell mechanics such as cell morphology, cell migration, and cellular stiffness that are dynamically changed during the angiogenic processes. Here, we review the implication for endothelial gap junctions and cellular mechanics in vascular angiogenesis.

Connexin Channels at the Glio-Vascular Interface: Gatekeepers of the Brain

Neuronal survival, electrical signaling and synaptic activity require a well-balanced micro-environment in the central nervous system. This is achieved by the blood-brain barrier (BBB), an endothelial barrier situated in the brain capillaries, that controls near-to-all passage in and out of the brain. The endothelial barrier function is highly dependent on signaling interactions with surrounding glial, neuronal and vascular cells, together forming the neuro-glio-vascular unit. Within this functional unit, connexin (Cx) channels are of utmost importance for intercellular communication between the different cellular compartments. Connexins are best known as the building blocks of gap junction (GJ) channels that enable direct cell-cell transfer of metabolic, biochemical and electric signals. In addition, beyond their role in direct intercellular communication, Cxs also form unapposed, non-junctional hemichannels in the plasma membrane that allow the passage of several paracrine messengers, complementing direct GJ communication. Within the NGVU, Cxs are expressed in vascular endothelial cells, including those that form the BBB, and are eminent in astrocytes, especially at their endfoot processes that wrap around cerebral vessels. However, despite the density of Cx channels at this so-called gliovascular interface, it remains unclear as to how Cx-based signaling between astrocytes and BBB endothelial cells may converge control over BBB permeability in health and disease. In this review we describe available evidence that supports a role for astroglial as well as endothelial Cxs in the regulation of BBB permeability during development as well as in disease states.

Targeting endothelial connexin40 inhibits tumor growth by reducing angiogenesis and improving vessel perfusion

Endothelial connexin40 (Cx40) contributes to regulate the structure and function of vessels. We have examined whether the protein also modulates the altered growth of vessels in tumor models established in control mice (WT), mice lacking Cx40 (Cx40-/-), and mice expressing the protein solely in endothelial cells (Tie2-Cx40). Tumoral angiogenesis and growth were reduced, whereas vessel perfusion, smooth muscle cell (SMC) coverage and animal survival were increased in Cx40-/- but not Tie2-Cx40 mice, revealing a critical involvement of endothelial Cx40 in transformed tissues independently of the hypertensive status of Cx40-/- mice. As a result, Cx40-/- mice bearing tumors survived significantly longer than corresponding controls, including after a cytotoxic administration. Comparable observations were made in WT mice injected with a peptide targeting Cx40, supporting the Cx40 involvement. This involvement was further confirmed in the absence of Cx40 or by peptide-inhibition of this connexin in aorta-sprouting, matrigel plug and SMC migration assays, and associated with a decreased expression of the phosphorylated form of endothelial nitric oxide synthase. The data identify Cx40 as a potential novel target in cancer treatment.

Biphasic Effect of Basic Fibroblast Growth Factor on Anterior Pituitary Folliculostellate TtT/GF Cell Coupling, and Connexin 43 Expression and Phosphorylation

Basic fibroblast growth factor (bFGF) is a mitogenic and differentiating cytokine. In the anterior pituitary, folliculostellate (FS) cells constitute the major source of bFGF. bFGF affects endocrine cell proliferation and secretion in the anterior pituitary. In addition, bFGF increases its own expression by acting directly on FS cells. FS cell Cx43-mediated gap junction intercellular communication allows the establishment of an intrapituitary network for the transmission of information. In the present study, we assessed how bFGF regulates FS cell coupling. Time course studies were carried out on the FS cell line TtT/GF. Short-term bFGF treatment induced a transient cell uncoupling and the phosphorylation in Ser368 of membrane-bound Cx43 without modifying Cx43 levels. We demonstrated the involvement of the protein kinase C (PKC) isoform α in the phosphorylation of Cx43 in S368. Moreover, we showed that bFGF induced PKCα activation by stimulating its expression, phosphorylation and association with the plasma membrane. The long-term incubation with bFGF increased TtT/GF cell coupling, total Cx43 levels and Cx43 accumulation at the cell membrane of cytoplasmic projections. The Cx43 level increase was a result of the stimulation of Cx43 gene transcription as mediated by the extracellular-regulated kinase 1/2 signalling pathway. Taken together, the data show that bFGF modulates TtT/GF cell coupling by activating different pathways that lead to opposite effects on Cx43 phosphorylation and expression depending on the duration of the exposure of the cells to bFGF. A short-term bFGF exposure reduces cell-to-cell communication as a mean of desynchronising FS cells. By contrast, long-term exposure to bFGF enhances cell-to-cell communication and facilitates coordination among FS cells.

Astrocytes in neuroprotection and neurodegeneration: The role of connexin43 and pannexin1

The World Health Organization has predicted that by 2040 neurodegenerative diseases will overtake cancer to become the world's second leading cause of death after cardiovascular disease. This has sparked the development of several European and American brain research initiatives focusing on elucidating the underlying cellular and molecular mechanisms of neurodegenerative diseases. Connexin (Cx) and pannexin (Panx) membrane channel proteins are conduits through which neuronal, glial, and vascular tissues interact. In the brain, this interaction is highly critical for homeostasis and brain repair after injury. Understanding the molecular mechanisms by which these membrane channels function, in health and disease, might be particularly influential in establishing conceptual frameworks to develop new therapeutics against Cx and Panx channels. This review focuses on current insights and emerging concepts, particularly the impact of connexin43 and pannexin1, under neuroprotective and neurodegenerative conditions within the context of astrocytes.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Endothelial connexin32 enhances angiogenesis by positively regulating tube formation and cell migration

The gap junction proteins connexin32 (Cx32), Cx37, Cx40, and Cx43 are expressed in endothelial cells, and regulate vascular functions involving inflammation, vasculogenesis and vascular remodeling. Aberrant Cxs expression promotes the development of atherosclerosis which is modulated by angiogenesis; however the role played by endothelial Cxs in angiogenesis remains unclear. In this study, we determined the effects of endothelial Cxs, particularly Cx32, on angiogenesis. EA.hy926 cells that had been transfected to overexpress Cx32 significantly increased capillary length and the number on branches compared to Cx-transfectant cells over-expressing Cx37, Cx40, and Cx43 or mock-treated cells. Treatment via intracellular transfer of anti-Cx32 antibody suppressed tube formation of human umbilical vein endothelial cells (HUVECs) compared to controls. In vitro wound healing assays revealed that Cx32-transfectant cells significantly increased the repaired area while anti-Cx32 antibody-treated HUVECs reduced it. Ex vivo aorta ring assays and in vivo matrigel plaque assays showed that Cx32-deficient mice impaired both vascular sprouting from the aorta and cell migration into the implanted matrigel. Therefore endothelial Cx32 facilitates tube formation, wound healing, vascular sprouting, and cell migration. Our results suggest that endothelial Cx32 positively regulates angiogenesis by enhancing endothelial cell tube formation and cell migration.

Lucifer yellow - an angel rather than the devil

The fluorescent dye Lucifer yellow (LY) was introduced in 1978, and has been extremely useful in studying cell structure and communications. This dye has been used mostly for labelling cells by intracellular injection from microelectrodes. This review describes the numerous applications of LY, with emphasis on the enteric nervous system and interstitial cells of Cajal. Of particular importance is the dye coupling method, which enables the detection of cell coupling by gap junctions.

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.

Channel-independent influence of connexin 43 on cell migration

In this review we focus on the role of connexins, especially of Cx43, as modulators of migration - a fundamental process in embryogenesis and in physiologic functions of the adult organism. This impact of connexins is partly mediated by their function as intercellular channels but an increasing number of studies support the view that at least part of the effects are truly independent of the channel function. The channel-independent function comprises extrinsic guidance of migrating cells due to connexin mediated cell adhesion as well as intracellular processes. Cx43 has been shown to exert effects on migration by interfering with receptor signalling, cytoskeletal remodelling and tubulin dynamics. These effects are mainly dependent on the presence of the carboxyl tail of Cx43. The molecular basis of this channel-independent connexin function is still not yet fully understood but early results open an exciting view towards new functions of connexins in the cell. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

3,4,5,6-Tetrahydroxyxanthone preserves intercellular communication by reduction of the endogenous nitric oxide synthase inhibitor level

To observe the direct effects of 3,4,5,6-tetrahydroxyxanthone on connexin43 (Cx43) expression in cultured endothelial cells, cells were treated with lysophosphatidylcholine (LPC, 10 mg/l) for 24 h in the presence or absence of different concentrations of 3,4,5,6-tetrahydroxyxanthone (1, 3, or 10 μmol l(- 1)). The reactive oxygen species (ROS) production, cell viability, asymmetric dimethylarginine (ADMA) levels, and Cx43 expression were detected. 3,4,5,6-Tetrahydroxyxanthone significantly inhibited the increase in ROS production and ADMA level, increased cell viability and up-regulated Cx43 mRNA and protein expression induced by LPC. 3,4,5,6-Tetrahydroxyxanthone has protective effect in LPC-induced atherosclerotic lesions, which is at least partly related to the reduction of ADMA level and downregulation of Cx43 expression.

Asymmetric dimethylarginine damages connexin43-mediated endothelial gap junction intercellular communication

Asymmetric dimethylarginine (ADMA), a major endogenous inhibitor of nitric oxide synthase, is recently defined as a novel atherogenic factor. Communication via gap junction (GJIC) is involved in the regulation of a variety of endothelial activities, such as cell differentiation and senescence. The aim of this study is to explore the effects of ADMA on connexin43 (Cx43) mediated endothelial GJIC. Lysophosphatidylcholine (LPC) caused the downregulation of Cx43 expression and GJIC dysfunction in cultured human umbilical vein endothelial cells (HUVECs), which were significantly ameliorated by decreasing ADMA accumulation. Furthermore, we found that ADMA (10 µmol·L–1, 24 h) markedly downregulated Cx43 expression and damaged GJIC function in HUVECs. ADMA also increased production of intracellular reactive oxygen species (ROS) and induced phosphorylation of p38 MAPK. Furthermore, the inhibitory effect of ADMA on Cx43-mediated GJIC could be attenuated by NADPH oxidase inhibitor diphenyleneiodonium and apocynin as well as p38 MAPK inhibitor SB203580, respectively. In conclusion, our present results suggest that ADMA inhibits endothelial GJIC function via downregulating Cx43 expression, which suggesting a novel mechanism linking between elevated ADMA level and progression of atherosclerosis.

Intercellular Communication in Atherosclerosis

Cell-to-cell communication is a process necessary for physiological tissue homeostasis and appears often altered during disease. Gap junction channels, formed by connexins, allow the direct intercellular communication between adjacent cells. After a brief review of the pathophysiology of atherosclerosis, we will discuss the role of connexins throughout the different stages of the disease.

Connexin43 expression levels influence intercellular coupling and cell proliferation of native murine cardiac fibroblasts

Little is known about connexin expression and function in murine cardiac fibroblasts. The authors isolated native ventricular fibroblasts from adult mice and determined that although they expressed both connexin43 (Cx43) and connexin45 (Cx45), the relative abundance of Cx45 was greater than that of Cx43 in fibroblasts compared to myocytes, and the electrophoretic mobility of both Cx43 and Cx45 differed in fibroblasts and in myocytes. Increasing Cx43 expression by adenoviral infection increased intercellular coupling, whereas decreasing Cx43 expression by genetic ablation decreased coupling. Interestingly, increasing Cx43 expression reduced fibroblast proliferation, whereas decreasing Cx43 expression increased proliferation. These data demonstrate that native fibroblasts isolated from the mouse heart exhibit intercellular coupling via gap junctions containing both Cx43 and Cx45. Fibroblast proliferation is inversely related to the expression level of Cx43. Thus, connexin expression and remodeling is likely to alter fibroblast function, maintenance of the extracellular matrix, and ventricular remodeling in both normal and diseased hearts.

KSHV-transformed primary effusion lymphoma cells induce a VEGF-dependent angiogenesis and establish functional gap junctions with endothelial cells

Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent of primary effusion lymphoma (PEL) and of Kaposi's sarcoma. PEL is an aggressive proliferation of B cells with poor prognosis. We evaluated both in vitro and in vivo the potential role of angiogenic factors secreted by PEL cells, that is, their interaction with endothelial cells and their implication in the invasive behavior of tumoral cells. In vitro, PEL-induced angiogenesis is dependent on vascular endothelial growth factor (VEGF) and VEGF receptors. However, although PEL cells produce VEGF and basic fibroblast growth factor (b-FGF) transcripts, they only secrete VEGF in vitro. In vivo, very high levels of both VEGF and b-FGF were found in the ascitic fluid of NOD/SCID mice injected with PEL cells. We then show evidence of cell adhesion and gap junction-mediated heterocellular communication between PEL cells and endothelial cells. Finally, we show that PEL cells extravasate through the endothelial barrier and that the specific tyrosine kinase inhibitor of VEGF receptors, PTK-787/ZK-222584, the anti-VEGF antibody, bevacizumab or the gap junction inhibitor 18-alpha-glycyrrhetinic acid, partially attenuate PEL cell extravasation. Angiogenesis, cell adhesion and communication likely contribute to the development of PEL and represent potential therapeutic targets.

Inhibition of endothelial cell proliferation, in vitro angiogenesis, and the down-regulation of cell adhesion-related genes by genistein. Combined with a cDNA microarray analysis

Antiangiogenesis is presently one of the powerful strategies for treating cancer, and endothelial cells play a pivotal role in the process of angiogenesis. Genistein, a tyrosine kinase inhibitor, a major isoflavone plentiful in soybeans, is known to inhibit both tumor growth and angiogenesis. However, the precise molecular mechanism(s) by which genistein affects endothelial cells has yet to be elucidated. In the present study, a cDNA microarray was performed to investigate the targeted genes of human umbilical vein endothelial cells (HUVECs) affected by 10 microM genistein. As a result, a total of 256 genes showed an altered expression of more than twofold. Among them were the genes related to cell proliferation, adhesion, transcription, translation, metabolism, cytoskeleton, apoptosis, kinases, and functionally unknown. The down-regulation of mRNA or the protein expression of cell adhesion-related genes, including VE-cadherin, gap junction protein alpha 1 (connexin 43), integrin alpha V, and multimerin, were confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR) or by immunofluorescence staining. The impaired cell-cell adhesion by genistein was also observed by electron microscopy. In addition, the antiangiogenesis role of genistein was confirmed on Matrigel using inverted microscopy and electron microscopy. In conclusion, genistein affects endothelial cells as a negative mediator of proliferation and angiogenesis in vitro, partially by down-regulating cell adhesion-related genes and impairing cell adhesions.

Expression of MAP kinases and connexins in the differentiation of rat mammary epithelial cells

Gap junctional intercellular communication (GJIC) is involved in the regulation of many cellular processes. MAP kinases are known to affect GJIC and phosphorylation of connexin (Cx). MAP kinases can also be a regulator of cell proliferation and growth. This study was undertaken to show the relevance between expression patterns of Cxs and MAP kinases in rat mammary epithelial cells (RMECs). In order to characterize the RMECs, they were stained with Peanut lectin, which indicates most alveolar epithelial cells, and Thy-1.1 was used as a marker of luminal epithelial cells or myoepithelial cells, respectively. We studied the expression patterns of major gap junction proteins, Cx26, 32, and 43 in RMECs. Western blot analysis demonstrated that Cx26 gradually decreased from day 2, while Cx32 was expressed constantly from day 1 to 14. Cx43 dramatically increased on day 5 and decreased thereafter. The expression patterns and phosphorylation of ERK1/2 and JNK were similar to Cx43, but expression of p38 was like that of Cx32. These results showed that the MAP kinases that comprise ERK1/2, p38, and JNK were involved in regulation of Cxs. Our data suggests that GJIC plays an important role during rat mammary differentiation and that MAP kinases may be closely related functionally to regulate the gap junction.

Enhancement of gap junctional intercellular communication of normal human dermal fibroblasts cultured on polystyrene dishes grafted with poly-N-isopropylacrylamide

Technology developed to allow recovery of cells without enzyme treatment, involving a dish grafted with a thermoreactive polymer gel of poly-N-isopropylacrylamide (PIPAAm), was found to significantly enhance gap junctional intercellular communication (GJIC) in normal human dermal fibroblasts (NHDF cells). NHDF cells were cultured for 4 days on PIPAAm-grafted dishes irradiated with various doses of electron beams, and GJIC was assayed by the scrape-loading dye transfer method. The area of dye transfer was greater in the PIPAAm-grafted dishes than in the control culture dishes, indicating that the PIPAAm-grafted dishes enhanced the GJIC of NHDF cells. Connexin-43 (Cx43) expression was analyzed because Cx43 is considered to be a main component of the gap junctional channel. PIPAAm-grafted dishes irradiated with 100, 250, or 500 kGy of electron beams showed significantly enhanced expression of Cx43-NP, Cx43-P1, and especially Cx43-P2. Enhanced expression of Cx43-P2, a functional transmembrane protein, may be related to the promotion of GJIC. These results suggest that the PIPAAm-grafted dish not only enables the enzyme-free recovery of a cell monolayer for use in the construction of a three-dimensional artificial tissue, but also significantly contributes to the enhancement of GJIC, which may partly promote tissue strength on the surface of the PIPAAm-grafted dish.

Increased connexin43 expression in human saphenous veins in culture is associated with intimal hyperplasia

OBJECTIVE

Intimal hyperplasia is a vascular remodelling process that occurs after a vascular injury. The mechanisms involved in intimal hyperplasia are proliferation, dedifferentiation, and migration of medial smooth muscle cells towards the subintimal space. We postulated that gap junctions, which coordinate physiologic processes such as cell growth and differentiation, might participate in the development of intimal hyperplasia. Connexin43 (Cx43) expression levels may be altered in intimal hyperplasia, and we therefore evaluated the regulated expression of Cx43 in human saphenous veins in culture in the presence or not of fluvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity.

METHODS

Segments of harvested human saphenous veins, obtained at the time of bypass graft, were opened longitudinally with the luminal surface uppermost and maintained in culture for 14 days. Vein fragments were then processed for histologic examination, neointimal thickness measurements, immunocytochemistry, RNA, and proteins analysis.

RESULTS

Of the four connexins (Cx37, 40, 43, and 45), we focused on Cx43 and Cx40, which we found by real-time polymerase chain reaction to be expressed in the saphenous vein because they are the predominant connexins expressed by smooth muscle cells and endothelial cells. After 14 days of culture, histomorphometric analysis showed a significant increase in the intimal thickness as observed during the process of intimal hyperplasia. A time-course analysis revealed a progressive upregulation of Cx43 to reach a maximal increase of sixfold to eightfold at both transcript and protein levels after 14 days in culture. In contrast, the expression of Cx40, abundantly expressed in the endothelial cells, was not altered. Immunofluorescence showed a large increase in Cx43 within smooth muscle cell membranes of the media layer. The development of intimal hyperplasia in vitro was decreased in presence of fluvastatin and was associated with reduced Cx43 expression.

CONCLUSIONS

These data show that Cx43 is increased in vitro during the process of intimal hyperplasia and that fluvastatin could prevent this induction, supporting a critical role for Cx43-mediated gap-junctional communication in the human vein during the development of intimal hyperplasia.

CLINICAL RELEVANCE

Stenosis due to intimal hyperplasia is the most common cause of failure of venous bypass grafts. To better understand the development of intimal hyperplasia, we used an ex vivo organ culture model to study saphenous veins harvested from patients undergoing a lower limb bypass surgery. In this model, the morphologic and functional integrity of the vessel wall is maintained and significant intimal hyperplasia development occurs after 14 days in culture. We have postulated that gap junctions, which coordinate physiologic processes such as cell growth and differentiation, may participate in the development of intimal hyperplasia. Indeed, intimal hyperplasia consists of proliferation and migration of smooth muscle cells into the subendothelial space. Intercellular communication is responsible for the direct transfer of ions and small molecules from one cell to the other through gap-junction channels found at cell-cell appositions. No study to date has evaluated whether gap junctional communication is involved in the process of intimal hyperplasia in humans. This assertion was investigated by using the aforementioned organ culture model of intimal hyperplasia in human saphenous veins, and our data support a critical role for Cx43-mediated gap junctional communication in human vein during the development of intimal hyperplasia.

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

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

Regulation of gap junctions by tyrosine protein kinases

Most of the gap junction proteins are regulated in part by post-translational phosphorylation. Phosphorylation has been shown to be important in gap junction assembly and turnover, and for channel function in the resting state. Connexin phosphorylation may be altered by the activation of intracellular signaling pathways in response to growth factors, tumor promoters, activated oncogenes, hormones and inflammatory mediators. In some instances altered phosphorylation has been associated with changes in connexin function and in other cases appears to be associated with changes in the levels of the connexin protein and/or mRNA. This review focuses on the role of tyrosine protein kinases in the regulation of gap junctions. The literature is most extensive for connexin43 and those studies are reviewed here. A great deal has been learned in recent years about how connexin43 is regulated by tyrosine kinase-dependent signaling pathways. These pathways are often complex and to some extent are cell type- and stimulus-dependent. Although considerable progress has been made in unraveling the cellular pathways that regulate connexin function, significant challenges remain to be addressed in identifying additional phosphorylation sites and determining the stoichiometries of the phosphorylation events that regulate connexin function and it's interaction with other cellular proteins.

Suppression of tumor growth and angiogenesis in vivo by a truncated form of 24-kd fibroblast growth factor (FGF)-2

Efforts to treat tumors have routinely depended on disruption of cell proliferation by a variety of methods, many involving stimulation of apoptosis. We have previously shown that a truncated form of 24-kd basic fibroblast growth factor consisting of the amino terminal 86 amino acids inhibits migration of tumor and endothelial cells in vitro. In the present study, this peptide was tested for its ability to suppress angiogenesis and tumor growth using the murine dorsal skin-fold chamber model in vivo. Treatment of MCF-7 breast carcinoma tumor spheroids with this peptide resulted in cessation of the angiogenic response and a significant reduction in tumor size. Blood vessels that did form were poorly developed. In addition to inhibiting angiogenesis, the peptide also inhibited migration of Lewis lung carcinoma cells away from the tumor core before onset of angiogenesis indicating that the peptide-mediated inhibition of migration affects both angiogenesis and tumor growth independently. Despite inhibition of tumor cell migration, the peptide had no effect on neutrophil or eosinophil chemotaxis. This study demonstrates that the truncated form of 24-kd basic fibroblast growth factor is effective in suppressing tumor development in vivo through inhibition of angiogenesis as well as inhibition of tumor cell migration without compromising other homeostatic events.

Effects of Aloe vera on gap junctional intercellular communication and proliferation of human diabetic and nondiabetic skin fibroblasts

OBJECTIVE

To evaluate the effects of Aloe vera on gap junctional intercellular communication (GJIC) and proliferation of human skin fibroblasts in the presence or absence of basic fibroblast growth factor (FGF-2).

DESIGN

In vitro study using human type II diabetic and nondiabetic skin fibroblast cell lines.

SETTING AND SUBJECTS

Diabetic (n = 4) and nondiabetic (n = 4) human skin fibroblast cell lines were purchased from Coriell Institute for Medical Research (Camden, NJ). The cells were cultured with or without Aloe vera extract in increasing concentrations (0%, 0.625%, 1.25%, 2.5%, 5%, 10%, and 20%; v/v) in culture medium and with or without FGF-2 (30 ng/mL).

MEASUREMENTS

GJIC was evaluated after 48-hour incubation with treatments by laser cytometry. Cells were counted after 72-hour incubation with treatments by using a Coulter counter.

RESULTS

The rate of GJIC was greater (p < 0.01) for diabetic than for nondiabetic fibroblasts (3.5 +/- 0.1 versus 3.0 +/- 0.1% per minute during the first 4 minutes after photobleaching). GJIC was increased ( p < 0.05) for diabetic fibroblasts in the presence of 2.5% and 5% of Aloe vera extract (4.2 +/- 0.1 and 4.0 +/- 0.2 versus 3.5 +/- 0.1% per minute for control, respectively). FGF-2 stimulated (p < 0.01) GJIC for diabetic (4.0 +/- 0.1 versus 3.5 +/- 0.1% per minute for control) and nondiabetic (3.5 +/- 0.1 versus 3.0 +/- 0.1% per minute for control) fibroblasts. Aloe vera extract did not affect GJIC of nondiabetic fibroblast cultured without FGF-2. However, Aloe vera extract decreased (p < 0.05) FGF-2 stimulatory effects on GJIC of diabetic and nondiabetic fibroblasts. Proliferation of diabetic fibroblasts was increased (p < 0.05) by 1.25% and 2.5% Aloe vera extract in medium. Proliferation of nondiabetic fibroblasts was not affected by Aloe vera extract. FGF-2 increased (p < 0.05) proliferation of nondiabetic fibroblasts and FGF-2 did not affect proliferation of diabetic fibroblasts. Aloe vera extract decreased (p < 0.05) FGF-2 stimulatory effects on proliferation of nondiabetic fibroblasts.

CONCLUSIONS

These data demonstrate that Aloe vera has the ability to stimulate GJIC and proliferation of human skin fibroblasts in diabetes mellitus. Furthermore, these results indicate that Aloe vera contains a compound(s) that neutralizes, binds with FGF-2 receptor, or otherwise alters signaling pathways for FGF-2. By affecting both GJIC and proliferation of diabetic fibroblasts, Aloe vera may improve wound healing in diabetes mellitus.

2',5'-Dihydroxychalcone down-regulates endothelial connexin43 gap junctions and affects MAP kinase activation

We examined the effect of 2',5'-dihydroxychalcone on connexin43 (Cx43) expression and gap-junctional communication in human umbilical vein endothelial cells (HUVEC). The result showed that expression of Cx43 is rapidly reduced by 2',5'-dihydroxychalcone in a dose-dependent manner, Concomitantly, the communication function, determined by fluorescence recovery after photobleaching (FRAP), is decreased. We further investigated whether the mitogen-activated protein (MAP) kinase and the degradation pathway of gap junctions are involved in these processes. Although the change of Cx43 is not affected by the level of fetal calf serum (FCS) used in the medium, activation of MAP kinase varies, depending on the FCS level. At a low level (0.5%), the chalcone inhibits the activation, like PD98059, a specific inhibitor of MAP kinase kinase. However, at a high level (20%), MAP kinase is activated. On the other hand, the chalcone's down-regulating effect on Cx43, while is totally blocked by protease inhibitors leupeptin and N-acetyl-leucyl-norleucinal (ALLN), persists in the presence of PD98059, We concluded that 2',5'-dihydroxychalcone down-regulates Cx43 expression and gap-junctional communication in the HUVEC via enhancement of the proteolysis pathway, and this compound possesses dual effects on MAP kinase activation.

Inhibition of cell migration and angiogenesis by the amino-terminal fragment of 24kD basic fibroblast growth factor

The 24-kDa form of basic fibroblast growth factor inhibits the migration of endothelial cells and mammary carcinoma cells while continuing to promote cell proliferation. This molecule consists of the 18-kDa fibroblast growth factor sequence plus an additional 55 amino acids at the amino-terminal end. Antibody neutralization studies suggested that the inhibition of migration is associated with these 55 amino acids, whereas the promotion of proliferation localizes to the 18-kDa domain. To determine whether 24kD basic fibroblast growth factor could be modified to eliminate its effect on cell proliferation but retain its inhibition of migration, portions of the carboxyl-terminal end of 24kD fibroblast growth factor were deleted, and the products were tested on MCF-7 and endothelial cells. A protein consisting of the 55 amino acids of the amino-terminal end and the first 31 amino acids of 18kD basic fibroblast growth factor (ATE+31) inhibited migration by 80% but did not promote cell growth. Arginine to alanine substitutions within the first 21 amino acids of the carboxyl-terminal end substantially reduced the efficacy of ATE+31, whereas substitutions in the remaining part of the molecule had no effect. Competition binding experiments showed that ATE+31 does not compete with 24kD basic fibroblast growth factor for binding to fibroblast growth factor receptor 1. In an in vivo matrigel plug assay, 150 nm ATE+31 peptide reduced angiogenesis by 80%. These studies demonstrate that the amino-terminal end of 24kD basic fibroblast growth factor is responsible for an activity that inhibits the migration rates of cultured cells as well as the angiogenic response in vivo.

Increase in gap junctional intercellular communication by high molecular weight hyaluronic acid associated with fibroblast growth factor 2 and keratinocyte growth factor production in normal human dermal fibroblasts

The effects of different molecular weights of hyaluronic acid (HA), a major component of extracellular matrix, on gap junctional intercellular communication (GJIC) in normal human dermal fibroblasts (NHDF cells) were investigated. NHDF cells were cultured for 4 days with different molecular weights of HA and then the extent of GJIC was assessed by the scrape-loading dye transfer method, using Lucifer yellow. The area of dye transfer was greater in the dishes coated with HA than in those to which HA was added. Thus, NHDF cells cultured on surfaces coated with high molecular weight (HMW) HA (MW, 800 kDa) showed greatly enhanced GJIC. Furthermore, another aim of this study was to evaluate the effects of different molecular weights of HA on the production of FGF-2 and KGF, because both are important cytokines produced by NHDF cells. When FGF-2 and KGF cultured levels of cell extracts and media were determined by ELISA, both levels were significantly enhanced when cells were grown on plates coated with HMW HA. This finding indicated that the function of gap junction channels in NHDF cells grown on plates coated with HMW HA may promote the biosynthesis of growth factors such as FGF-2 and KGF.

Increase in gap-junctional intercellular communications (GJIC) of normal human dermal fibroblasts (NHDF) on surfaces coated with high-molecular-weight hyaluronic acid (HMW HA)

Normal human dermal fibroblast (NHDF) cells were used to detect differences in gap-junctional intercellular communication (GJIC) by hyaluronic acid (HA), a linear polymer built from repeating disaccharide units that consist of N-acetyl-D-glucosamine (GlcNa) and D-glucuronic acid (GlcA) linked by a beta 1-4 glycosidic bond. The NHDF cells were cultured with different molecular weights (MW) of HA for 4 days. The rates of cell attachment in dishes coated with high-molecular-weight (HMW; 310 kDa or 800 kDa) HA at 2 mg/dish were significantly reduced at an early time point compared with low-molecular-weight (LMW; 4.8 kDa or 48 kDa) HA with the same coating amounts. HA-coated surfaces were observed by atomic force microscopy (AFM) under air and showed that HA molecules ran parallel in the dish coated with LMW HA and had an aggregated island structure in the dish coated with HMW HA surfaces. The cell functions of GJIC were assayed by a scrape-loading dye transfer (SLDT) method using a dye solution of Lucifer yellow. Promotion of the dye transfer was clearly obtained in the cell monolayer grown on the surface coated with HMW HA. These results suggest that HMW HA promotes the function of GJIC in NHDF cells. In contrast, when HMW HA was added to the monolayer of NHDF cells, the functions of GJIC clearly were lowered in comparison with the cells grown in the control dish or with those grown on the surface of HMW HA. Therefore it is concluded that the MW size of HA and its application method are important factors for generating biocompatible tissue-engineered products because of the manner in which the GJIC participates in cell differentiation and cell growth rate.

Differential expression of connexin43 in foetal, adult and tumour-associated human brain endothelial cells

Connexin43 (Cx43), the main protein constituting the gap junctions between astrocytes, has previously been demonstrated in endothelial cells of somatic vessels where the intercellular coupling that it provides plays a role in endothelial proliferation and migration. In this study, Cx43 expression was analysed in human brain microvascular endothelial cells of the cortical plate of 18-week foetal telencephalon, in adult cerebral cortex and glioma (astrocytomas). The study was carried out by immunocytochemistry utilizing a Cx43 monoclonal antibody and a polyclonal antibody anti-GLUT1 (glucose transporter isoform 1) to identify the endothelial cells and to localize Cx43. Endothelial Cx43 is differently expressed in the cortical plate, cerebral cortex and astrocytoma. Within the cortical plate and tumour, Cx43 is highly expressed in microvascular endothelial cells whereas it is virtually absent in the cerebral cortex microvessels. The high expression of the gap junction protein in developing brain, as well as in brain tumours, may be related to the growth status of the microvessels during brain and tumour angiogenesis. The lack of endothelial Cx43 in the cerebral cortex is in agreement with the characteristics of the mature brain endothelial cells that are sealed by tight junctions. In conclusion, the results indicate that endothelial Cx43 expression is developmentally regulated in the normal human brain and suggest that it is controlled by the microenvironment in both normal and tumour-related conditions.

Human T-cell lymphotropic virus type 1-transformed cells induce angiogenesis and establish functional gap junctions with endothelial cells

The role of angiogenesis in the growth and metastasis of solid tumors is well established. However, the role of angiogenesis in hematologic malignancies was only recently appreciated. We show that HTLV-I-transformed T cells, but not HTLV-I-negative CD4(+) T cells, secrete biologically active forms of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) and, accordingly, induce angiogenesis in vitro. Furthermore, fresh ATL leukemic cells derived from patients with acute ATL produce VEGF and bFGF transcripts and proteins. The viral transactivator Tax activates the VEGF promoter, linking the induction of angiogenesis to viral gene expression. Angiogenesis is associated with the adhesion of HTLV-I-transformed cells to endothelial cells and gap junction-mediated heterocellular communication between the 2 cell types. Angiogenesis, cell adhesion, and communication likely contribute to the development of adult T-cell leukemia-lymphoma and represent potential therapeutic targets.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.

Suppressive effect of irsogladine maleate on N-methyl-N-nitro-N-nitrosoguanidine (MNNG)-initiated and glyoxal-promoted gastric carcinogenesis in rats

The modifying effect of irsogladine maleate (IRG) on N-methyl-N-nitro-N-nitrosoguanidine (MNNG)-initiated and glyoxal-promoted gastric carcinogenesis was examined in male Wistar rats. Six-week-old rats were divided into ten groups. Groups 1 through 6 were given MNNG (100 mg/l in drinking water) for 25 weeks from the start of the experiment, whereas groups 7 through 10 received distilled water in the initiation phase as the vehicle treatment. Groups 1 and 8 were kept on the basal diet and distilled water throughout the experiment (55 weeks). Groups 2-8 were given 0.5% glyoxal in the drinking water for 30 weeks from 26th week of the experiment. Group 3 was fed the diet mixed with 100 ppm IRG for 25 weeks from the start of experiment. Groups 4 and 8 were fed the diet mixed with 100 ppm IRG for 30 weeks from 26th week of experiment. Groups 5 and 9 or 6 were given 100 or 25 ppm IRG containing diet, respectively throughout the experiment. Group 10 was given the basal diet and distilled water as the vehicle treated control. Tumors of upper digestive tracts (stomach and duodenum) were developed in groups: 1 (12/17 rats, 71%), 2 (11/12 rats, 92%), 3 (9/16 rats, 56%), 4 (5/12 rats, 42%), 5 (6/15 rats, 40%) and 6 (7/12 rats, 58%). High dose of IRG in initiation and/or promotion phase significantly reduced the incidence of tumors of the upper digestive tracts. The average numbers of the digestive tracts neoplasms in groups 3,5 and 6 given glyoxal and IRG were less than those in group 2 which received only glyoxal. These results suggest that IRG could be a preventive agent against the occurrence of neoplasms of the upper digestive tract.

A novel role for FGF and extracellular signal-regulated kinase in gap junction-mediated intercellular communication in the lens

Gap junction-mediated intercellular coupling is higher in the equatorial region of the lens than at either pole, a property believed to be essential for lens transparency. We show that fibroblast growth factor (FGF) upregulates gap junctional intercellular dye transfer in primary cultures of embryonic chick lens cells without detectably increasing either gap junction protein (connexin) synthesis or assembly. Insulin and insulin-like growth factor 1, as potent as FGF in inducing lens cell differentiation, had no effect on gap junctions. FGF induced sustained activation of extracellular signal-regulated kinase (ERK) in lens cells, an event necessary and sufficient to increase gap junctional coupling. We also identify vitreous humor as an in vivo source of an FGF-like intercellular communication-promoting activity and show that FGF-induced ERK activation in the intact lens is higher in the equatorial region than in polar and core fibers. These findings support a model in which regional differences in FGF signaling through the ERK pathway lead to the asymmetry in gap junctional coupling required for proper lens function. Our results also identify upregulation of intercellular communication as a new function for sustained ERK activation and change the current paradigm that ERKs only negatively regulate gap junction channel activity.

Inhibition of endothelial wound repair by dominant negative connexin inhibitors

Wounding of endothelial cells is associated with altered direct intercellular communication. To determine whether gap junctional communication participates to the wound repair process, we have compared connexin (Cx) expression, cell-to-cell coupling and kinetics of wound repair in monolayer cultures of PymT-transformed mouse endothelial cells (clone bEnd.3) and in bEnd.3 cells expressing different dominant negative Cx inhibitors. In parental bEnd.3 cells, mechanical wounding increased expression of Cx43 and decreased expression of Cx37 at the site of injury, whereas Cx40 expression was unaffected. These wound-induced changes in Cx expression were associated with functional changes in cell-to-cell coupling, as assessed with different fluorescent tracers. Stable transfection with cDNAs encoding for the chimeric connexin 3243H7 or the fusion protein Cx43-betaGal resulted in perturbed gap junctional communication between bEnd.3 cells under both basal and wounded conditions. The time required for complete repair of a defined wound within a confluent monolayer was increased by ~50% in cells expressing the dominant negative Cx inhibitors, whereas other cell properties, such as proliferation rate, migration of single cells, cyst formation and extracellular proteolytic activity, were unaltered. These findings demonstrate that proper Cx expression is required for coordinated migration during repair of an endothelial wound.

Examining new models for the study of autocrine and paracrine mechanisms of angiogenesis through FGF2-transfected endothelial and tumour cells

Angiogenesis is the process of generating new capillary blood vessels. Uncontrolled endothelial cell proliferation is observed in tumour neovascularization. Several growth factors and cytokines have been shown to stimulate endothelial cell proliferation in vitro and in vivo and among them FGF2 was one of the first to be characterised. FGF2 is a Mr 18,000 heparin-binding cationic polypeptide that induces proliferation, migration, and protease production in endothelial cells in culture and neovascularization in vivo. FGF2 interacts with endothelial cells through two distinct classes of receptors, the high affinity tyrosine-kinase receptors (FGFRs) and low affinity heparan sulfate proteoglycans (HSPGs) present on the cell surface and in the extracellular matrix. Besides experimental evidence for paracrine mode of action for FGF2, some observations raise the hypothesis that FGF2 may also play an autocrine role in endothelial cells. FGF2 may therefore represent a target for anti-angiogenic therapies. In order to assess the angiostatic potential of different classes of compounds, novel experimental models have been developed based on the autocrine and/or the paracrine capacity of FGF2.

Age-related alteration of gap junction distribution and connexin expression in rat aortic endothelium

We investigated endothelial gap junctions and their three component connexins, connexin37 (Cx37), Cx40, and Cx43, during growth and senescence in rat aorta by en face immunoconfocal microscopy and electron microscopy. Gap junction spots labeled by specific antisera against Cx37, Cx40, and Cx43 were quantified at 1 day, 7 days, 28 days, 16 months, and > or =20 months of age, and the relationship between the connexins was examined by co-localization analysis. At birth, all three connexins were abundantly expressed; the number and total area of connexin spots then declined within 1 week (p<0.05 for each connexin). From 1 week, each connexin showed a distinct temporal expression pattern. Whereas Cx43 signal decreased progressively, Cx37 signal fluctuated in a downward trend. By contrast, Cx40 maintained an abundant level until > or =20 months of age (> or =20 months vs. 28 days, p<0.05 for number and total connexin signal area). These patterns were associated with changes in endothelial cell morphology. Double-label analysis showed that the extent of co-localization of connexins to the same gap junctional spot was age-dependent [>70% at birth and 28 days old; <70% at later stages (p<0.05)]. We conclude that expression of the three connexins in aortic endothelium is age-related, implying specific intercellular communication requirements during different stages after birth.

Multiple connexin expression in regenerating arterial endothelial gap junctions

Endothelial cells form gap junctions that, according to vessel type, may be composed of up to 3 types of connexin, connexin37, connexin40, and connexin43. Although changes in connexin expression have been linked to growth and injury in cultured endothelial cells, information on connexin expression in regenerating endothelium in situ is lacking. We investigated gap junction distribution and expression of all 3 endothelial connexins during healing in rat carotid artery after denudation injury. En face viewing of the vascular luminal surface by means of immunoconfocal microscopy was used to examine the spatial and temporal expression pattern of the endothelial connexins. Gap junction spots labeled by specific antisera against connexin37, connexin40, and connexin43 were quantified 7, 14, and 28 days after injury, and the relations among the connexins were examined by using colocalization analysis. Complementary electron microscopy was also conducted. After injury, the regenerating endothelium initially expressed small, sparse gap junctions, the numbers of which progressively increased to values equivalent to those of controls. Although connexin40 gap-junctional spot size and area returned to uninjured levels by 28 days after injury, connexin37 and connexin43 spot size and area exceeded those of the uninjured artery (P<0.05). Double-label analysis showed that even though colocalization of connexins to the same gap-junctional spot is a common feature, the extent of colocalization was time dependent (>80% in the intact artery at postinjury day 28 and <70% at postinjury days 7 and 14, P<0.01). We conclude that distinct alterations in expression of the 3 connexins are associated with regeneration of the arterial endothelium in situ, implying different intercellular communication requirements during the various phases of the healing process.

Connexins and gap junctions of astrocytes and oligodendrocytes in the CNS

This review article summarizes early and recent literature on the structure, distribution and composition of gap junctions between astrocytes and oligodendrocytes, and the differential expression of glial connexins in adult and developing mammalian CNS. In addition to an overview of the topic, discussion is focused on the organization of homologous gap junctional interactions between astrocytes and between oligodendrocytes as well as on heterologous junctional coupling between astrocytes and oligodendrocytes. The homotypic and heterotypic nature of these gap junctions is related to the connexins known to be produced by glial cells in the intact brain and spinal cord. Emphasis is placed on the ultrastructural level of analysis required to attribute gap junction and connexin deployment to particular cell types and subcellular locations. Our aim is to provide a firm basis for consideration of anticipated rapid advances in understanding of structural relationships of gap junctions and connexins within the glial gap junctional syncytium. Conclusions to date suggest that the glial syncytium is more complex than previously appreciated and that glial pathways of junctional communication may not only be determined by the presence of gap junctions, but also by the connexin composition and conductance regulation of junctional channels.

A non-transmembrane form of Jagged-1 regulates the formation of matrix-dependent chord-like structures

Jagged-Notch interactions regulate a transmembrane ligand-receptor signaling pathway involved in the regulation of cell fate determination as well as myoblast and endothelial cell differentiation. To further examine the role of the transmembrane ligand, Jagged-1, in the regulation of cell differentiation, we stably transfected NIH 3T3 cells with a truncated form of Jagged(J)-1, which results in the secretion of a soluble(s) form of the protein. Comparison of gene expression by serial analysis demonstrated that among the 227 transcripts differentially regulated in the sJ-1 transfectants, the expression of the pro-alpha-2(I) collagen transcript and pro-alpha-1(I) collagen translation product was predominantly repressed in sJ-1 transfectants. When plated on extracellular matrices, sJ-1 transfectants formed prominent chord-like structures on type I collagen but not on fibrin, fibronectin, or vitronectin. While the sJ-1 transfectants exhibited growth kinetics similar to control cells and were unable to grow in soft agar, the cells were less sensitive to contact inhibition of growth in vitro and sJ-1 allografts formed tissue masses in nude mice after a prolonged latency period and exhibited an abundance of host-derived microvascular endothelial cells. These data suggest that J-1 may be able to modulate, in a matrix-dependent manner, the organization of cell to cell interactions including its ability to promote the development of chord-like structures.

Cells of the carotid body express connexin43 which is up-regulated by cAMP

We identified a gap junction protein subunit, connexin43 (Cx43) by immunofluorescence and immunoblotting, in cultured rat carotid body cells and in whole organs. In 1-week-old cultures, all cells were flat but after 3 h exposure to 8Br-cAMP (1 mM), tyrosine hydroxylase (TH) positive cells (chemoreceptors), but not TH negative cells, adopted a round body with multiple thin arborization processes. The incidence of dye coupling between cultured cells of the same type increased from 26% in controls to 73% after treatment with 8Br-cAMP. In control cultures, Cx43 immunoreactivity showed a diffuse perinuclear distribution and after 8Br-cAMP treatment, it was also found at cell-cell contacts. Both 8Br-cAMP-induced dye coupling and cellular redistribution of Cx43 were blocked by pretreatment with actinomycin D (5 microM), a mRNA transcription blocker. Moreover, 3 h exposure to 8Br-cAMP increased the levels of Cx43 in entire organs. We suggest that conditions that promote a sustained increase in cytosolic cAMP up-regulate coupling between carotid body cells in a transcription-dependent manner. The possible functional significance of these findings is discussed.

Mechanical stretching of human saphenous vein grafts induces expression and activation of matrix-degrading enzymes associated with vascular tissue injury and repair

After coronary artery bypass surgery, saphenous vein graft occlusion occurs through tissue remodeling. Although a likely trigger, the role of preparative mechanical injury incurred by the graft is not yet understood. We studied the early effects of simple mechanical injury on human saphenous vein grafts by exposing them to longitudinal stretch, a deformation which potentially occurs during surgery. We then maintained ex vivo for up to 7 days matched pairs of experimentally stretched and nonstretched (control) vein segments and examined the expression and activation of matrix metalloproteinases (MMPs) and integrin alphav, molecules implicated in vascular remodeling. At peak expression on day 3, stretched vein secreted 177 +/- 16% active MMP-2 (P < 0.01), 161 +/- 36% (P < 0.05) pro-MMP-9, and contained 206 +/- 18% (P < 0.01) alphav, a receptor for active MMP-2, compared to control. In situ gelatinase activity was present in the intima and adventitia of stretched veins, but not of control, and correlated spatially with expression of alphav. Stretch also increased severalfold cell proliferation (1.27 +/- 0.4 vs. 0.23 +/- 0.05% in control, P < 0.05), as assessed by bromodeoxyuridine incorporation. Furthermore, we found that cell proliferation colocalized with gelatinase activity and alphav in the adventitia. Our results show that a single longitudinal stretch of vein grafts produces significant changes in the expression and activation of key molecules in vascular remodeling. We also found support for the notion that the adventitial layer contributes to vein graft remodeling.

Role of endothelial cell extracellular signal-regulated kinase1/2 in urokinase-type plasminogen activator upregulation and in vitro angiogenesis by fibroblast growth factor-2

Downstream signaling triggered by the binding of fibroblast growth factor-2 (FGF2) to its tyrosine-kinase receptors involves the activation of mitogen-activated protein kinase kinase (MEK) with consequent phosphorylation of extracellular signal-regulated kinases (ERKs). Here we demonstrate that FGF2 induces ERK1/2 activation in bovine aortic endothelial (BAE) cells and that the continuous presence of the growth factor is required for sustained ERK1/2 phosphorylation. This is prevented by the MEK inhibitors PD 098059 and U0126, which also inhibit FGF2-mediated upregulation of urokinase-type plasminogen activator (uPA) and in vitro formation of capillary-like structures in three-dimensional type I collagen gel. Various FGF2 mutants originated by deletion or substitution of basic amino acid residues in the amino terminus or in the carboxyl terminus of FGF2 retained the capacity to induce a long-lasting activation of ERK1/2 in BAE cells. Among them, K128Q/R129Q-FGF2 was also able to stimulate uPA production and morphogenesis whereas R129Q/K134Q-FGF2 caused uPA upregulation only. In contrast, K27, 30Q/R31Q-FGF2, K128Q/K138Q-FGF2 and R118,129Q/K119,128Q-FGF2 exerted a significant uPA-inducing and morphogenic activity in an ERK1/2-dependent manner only in the presence of heparin. Furthermore, no uPA upregulation and morphogenesis was observed in BAE cells treated with the deletion mutant (delta)27-32-FGF2 even in the presence of soluble heparin. Thus, mutational analysis of FGF2 dissociates the capacity of the growth factor to induce a persistent activation of ERK1/2 from its ability to stimulate uPA upregulation and/or in vitro angiogenesis. In conclusion, the data indicate that ERK1/2 phosphorylation is a key step in the signal transduction pathway switched on by FGF2 in endothelial cells. Nevertheless, a sustained ERK1/2 activation is not sufficient to trigger uPA upregulation and morphogenesis. FGF2 mutants may represent useful tools to dissect the signal transduction pathway(s) mediating the complex response elicited by an angiogenic stimulus in endothelial cells.

Connexin make-up of endothelial gap junctions in the rat pulmonary artery as revealed by immunoconfocal microscopy and triple-label immunogold electron microscopy

Integration of vascular endothelial function relies on multiple signaling mechanisms, including direct cell-cell communication through gap junctions. Gap junction proteins expressed in the endothelium include connexin37, connexin40, and connexin43. To investigate whether individual endothelial cells in vivo express all three connexin types and, if so, whether multiple connexins are assembled into the same gap junction plaque, we used affinity-purified connexin-specific antibodies raised in three different species to permit multiple-label immunoconfocal and immunoelectron microscopy in the rat main pulmonary artery. Immunoconfocal microscopy showed a high incidence of co-localization between connexin43 and connexin40, but lower incidences of co-localization between connexin37 and connexin40 or connexin43. Immunoelectron microscopy revealed that 83% of gap junction profiles contained all three connexins, with the proportion of connexin40 labeling being significantly higher than that of connexin37 or connexin43. The presence of three different connexin types of distinct properties in vitro provides potential for complex regulation and functional differentiation of endothelial intercellular communication properties in vivo.

Gap junction-mediated communication in the developing and adult cerebral cortex

Recent cell biological and electrophysiological studies have shown that gap junctional coupling and the proteins that mediate this form of communication are present in the developing cerebral cortex from early in corticogenesis to the later stage of neuronal circuit formation. We have used electron microscopy to visualize gap junctions in the developing rat cerebral cortex, and studied the expression patterns and cellular localizations of connexin26 (Cx26; beta 2), Cx32 (beta 1) and Cx43 (alpha 1), which take part in their formation. We found that these connexins are expressed differentially during development, and their patterns of expression are correlated with important developmental events such as cell proliferation, migration and formation of cortical neuronal circuits. We also observed that gap junctions and their constituent connexins were abundant in the adult cerebral cortex. Junctions were predominantly between glial cells or between neurons and glia. The frequency and distribution of gap junctions varied in different regions of the adult cortex, possibly reflecting differences in the cellular and functional organization of these cortical areas.

Dual activities of 22-24 kDA basic fibroblast growth factor: inhibition of migration and stimulation of proliferation

Basic fibroblast growth factor (FGF2) is synthesized as four isoforms with molecular weights of 24, 22.5, 22, and 18 kDa, with each of the three higher molecular weight forms (hmwFGF2) produced by the initiation of translation at one of three upstream CUG codons. We have shown that bovine arterial endothelial cells export the high molecular weight forms of FGF2 (hmwFGF2) in a 17beta-estradiol-dependent manner (Piotrowicz et al., 1997, J Biol Chem 272:7042-7047). To determine whether the hmwFGF2 forms affected cell behavior after release, we evaluated the effect of recombinant hmwFGF2 on the growth and migration of endothelial cells and mammary carcinoma cells (MCF-7). Treatment with the recombinant protein resulted in the inhibition of endothelial cell migration by 45% and MCF-7 cell migration by 70%. HmwFGF2-dependent inhibition was observed when endothelial cell migration was stimulated by 18 kDa FGF2 or vascular endothelial growth, and MCF cell migration was stimulated with insulin-like growth factor. In each case, inclusion of an antibody against the 55 amino acid amino terminal end of 24 kDa FGF2 abrogated the inhibition of migration, while antibodies to the 18 kDa FGF2 domain had no effect. When endothelial cells were cultured under conditions which promoted export of hmwFGF2, a 40% decrease in motility was observed which was reversed by the antibodies to the 24 kDa FGF2. Thus, both recombinant and endogenously produced hmw-FGF2 are capable of inhibiting migration. In contrast to the ubiquitous effect on migration, hmwFGF2 had no effect on endothelial cell growth but stimulated MCF-7 growth equally as well as the 18 kDa FGF2 (threefold). Antibodies to the 18 kDa domain of 24 kDa FGF2 blocked the growth-promoting activity of hmwFGF2, but those to the amino terminal end were ineffective. These data suggest that hmwFGF2 has dual activities, an inhibitory effect on cell migration and a growth-stimulating effect. The two activities can be localized to different parts of hmwFGF2: inhibitory activity to the amino terminal 55 amino acids (which are absent from the 18 kDa FGF2) and growth-promoting activity to the 18 kDa domain. Therefore, the ratio of hmwFGF2 and 18 kDa FGF2 in the extracellular space may provide a mechanism of control for angiogenesis and mammary tumor development.