Biochemical and ultrastructural evidence for the association of basic fibroblast growth factor with cardiac gap junctions

Inhibition of Cx43 attenuates ERK1/2 activation, enhances the expression of Cav‑1 and suppresses cell proliferation

In addition to being an important component of the gap junction, connexin 43 (Cx43) has been shown to regulate other cellular functions, including cell proliferation. This regulatory role of Cx43 may be important in therapeutic situations, including wound healing or ischemic injuries. Caveolin‑1 (Cav‑1) has been shown to regulate angiogenesis. The aim of the present study was to analyze whether Cx43 counter‑regulates Cav‑1 in controlling the proliferation and migration of endothelial cells. The inhibition of Cx43 with niflumic acid, flufenamic acid and 18‑α‑glycyrrhetinic acid in cultured human umbilical vein endothelial cells resulted in decreased phosphorylation of extracellular signal‑regulated kinase (ERK)1/2 and increased expression of Cav‑1, as shown by western blot analysis. Furthermore, the inhibition of Cx43 resulted in a 50±7% decrease in cell proliferation, determined using a crystal violet assay, a 48±5% decrease in migration, determined using a migration assay, and a 49±6% decrease in endothelial tube formation, determined using a Matrigel assay, compared with the control. Similar results were obtained following specific inhibition of Cx43 by mimetic peptides (Gap26 and Gap27). Inhibition of the mitogen‑activated protein kinase kinase/ERK pathway with PD‑98059 resulted in an increased expression of Cav‑1 and a reduction in the expression of Cx43. Furthermore, cell proliferation, migration and tube formation in endothelial cells were impaired. By contrast, downregulation of the protein expression of Cav‑1 by small interference RNA resulted in increased expression of Cx43 and phosphorylation of ERK1/2. Accordingly, the number of cells in the Cav‑1 treated‑group increased by 35±5% compared with the controls. The data of the present study showed that Cav‑1 suppressed cell proliferation by inhibiting the activity of Cx43, which is upstream of ERK1/2. The downregulation of Cav‑1 protein resulted in loss of the inhibitory activity of Cav‑1 on cell proliferation and led to increased cell proliferation. This counter‑regulatory effect of Cx43 may be of importance in therapeutic angiogenesis.

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.

Biological functions of the low and high molecular weight protein isoforms of fibroblast growth factor-2 in cardiovascular development and disease

Fibroblast growth factor 2 (FGF2) consists of multiple protein isoforms (low molecular weight, LMW, and high molecular weight, HMW) produced by alternative translation from the Fgf2 gene. These protein isoforms are localized to different cellular compartments, indicating unique biological activity. FGF2 isoforms in the heart have distinct roles in many pathological circumstances in the heart including cardiac hypertrophy, ischemia-reperfusion injury, and atherosclerosis. These studies suggest distinct biological activities of FGF2 LMW and HMW isoforms both in vitro and in vivo. Yet, due to the limitations that only the recombinant FGF2 LMW isoform is readily available and that the FGF2 antibody is nonspecific with regards to its isoforms, much remains to be determined regarding the role(s) of the FGF2 LMW and HMW isoforms in cellular behavior and in cardiovascular development and pathophysiology. This review summarizes the activities of LMW and HMW isoforms of FGF2 in cardiovascular development and disease.

Stimulation of Paracrine Pathways With Growth Factors Enhances Embryonic Stem Cell Engraftment and Host-Specific Differentiation in the Heart After Ischemic Myocardial Injury

Background— Growth factors play an essential role in organogenesis. We examine the potential of growth factors to enhance cell engraftment and differentiation and to promote functional improvement after transfer of undifferentiated embryonic stem cells into the injured heart.

Methods and Results— Green fluorescent protein (GFP)–positive embryonic stem cells derived from 129sv mice were injected into the ischemic area after left anterior descending artery ligation in allogenic (BALB/c) mice. Fifty nanograms of recombinant mouse vascular endothelial growth factor, fibroblast growth factor (FGF), and transforming growth factor (TGF) was added to the cell suspension. Separate control groups were formed in which only the growth factors were given. Echocardiography was performed 2 weeks later to evaluate heart function (fractional shortening [FS]), end-diastolic diameter, and left ventricular wall thickness). Hearts were harvested for histology (connexin 43, α-sarcomeric actin, CD3, CD11c, major histocompatability complex class I, hematoxylin-eosin). Degree of restoration (GFP-positive graft/infarct area ratio), expression of cardiac markers, host response, and tumorigenicity were evaluated. Cell transfer resulted in improved cardiac function. TGF-β led to better restorative effect and a stronger expression of connexin 43, α-sarcomeric actin, and major histocompatability complex class I. TGF-β and FGF retained left ventricular diameter. FS was better in the TGF-β, FGF, and embryonic stem cells-only group compared with left anterior descending artery-ligated controls. Growth factors with cells (TGF-β, FGF) resulted in higher FS and smaller end-diastolic diameter than growth factors alone.

Conclusions— Growth factors can promote in vivo organ-specific differentiation of early embryonic stem cells and improve myocardial function after cell transfer into an area of ischemic lesion. TGF-β should be considered as an adjuvant for myocardial restoration with the use of embryonic stem cells.

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.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Protective effect of basic fibroblast growth factor against myocyte death and arrhythmias in acute myocardial infarction in rats

The present study in rats investigated whether basic fibroblast growth factor (bFGF) plays an important role in cardioprotection against myocardial cell death and arrhythmias in acute myocardial infarction (AMI). After ligating the left coronary artery in 62 Wistar rats, 20 Eg of human recombinant bFGF was injected into the infarcted myocardium in 33 rats (group F), while saline was used for 29 control rats (group C). The development of ventricular tachyarrhythmias was assessed during the first 30 min of ischemia. After 24 h occlusion, the hearts of the surviving rats (group F: n=13, group C: n=10) were excised to assess minimum infarct wall thickness and infarct size, determine the number of TUNEL-positive cardiomyocytes and to analyze Bcl-2 and Bax expression by immunohistochemical staining and Western blotting. The incidence of ventricular tachycardia was higher in group C than in group F (p<0.05). The thinning ratio was higher in group F than in group C (p<0.05). There were fewer TUNEL-positive cardiomyocytes in the infarct border area in group F than in group C (p<.0001). Western blot analysis showed greater expression of Bcl-2 in group F than in group C (p<0.05), but similar expression of Bax in the 2 groups. In conclusion, intramyocardial administration of bFGF prevented ischemia-induced myocardial cell death and arrhythmias.

Age- and myopathy-dependent changes in connexins of normal and cardiomyopathic syrian hamster ventricular myocardium

The conduction of cardiac action potentials depends on the flow of excitation through gap junctions, which are hexameric protein associations of connexins (Cxs). The major Cx reported in the heart is Cx43, although some Cx40 and Cx45 are also present. There is some evidence for altered Cx content in heart failure. In heart failure, conduction is depressed and slowed conduction may contribute to arrhythmogenesis and (or) the maintenance of arrhythmia. Cx content and distribution were determined in ventricular tissues from normal and cardiomyopathic Syrian hamsters, an animal model of heart failure which has reproducible age-specific cardiomyopathy resulting in heart failure and age-matched controls in three groups: young (3-5 weeks), adult (13-18 weeks), and old (>45 weeks). Frozen, unfixed sections of ventricular tissues were immunofluorescently stained using antibodies against Cx43, Cx40, and Cx45. Cx43 was the predominant Cx detected in all samples. In normal hamsters, Cx43 was localized predominantly at the intercalated disc region, while in myopathic myocytes, it was scattered. In Western blots, Cx43 content of normal hamster hearts was highest in the adult hearts compared with young and old hamster hearts. In contrast, Cx43 content was significantly lower in adult cardiomyopathic hamster hearts compared with all other groups. The alterations of content and distribution of gap junction Cx43 may contribute to diminished conduction, pump function, and arrhythmogenesis in heart failure.Key words: connexin, gap junction, heart failure, arrhythmia, confocal microscopy.

5-Hydroxytryptamine2B receptors stimulate Ca2+ increases in cultured astrocytes from three different brain regions

The expression of 5-hydroxytryptamine-2B (5-HT2B) receptor mRNA has recently been shown in cultured astrocytes. Here the expression of functional 5-HT2B receptors has been studied in cultured astrocytes from rat cerebral cortex, hippocampus, and brain stem. Fluo-3- and fura-2-based microspectrofluorometry was used for measuring changes in intracellular free calcium concentrations ([Ca2+]i). The 5-HT2B agonist alpha-methyl 5-HT (40 nM) produced rapid transient increases in [Ca2+]i in astrocytes from all three brain regions studied, and these responses were blocked by the selective 5-HT2B antagonist rauwolscine (1 microM). The specificity of the responses to alpha-methyl 5-HT was further demonstrated by the failure of 4-(4-fluorobenzoyl)-1-(4-phenylbutyl)-piperidine oxalate (1 microM), a specific 5-HT2A/5-HT2C antagonist, to block these responses. The 5-HT2B-induced increases in [Ca2+]i persisted in Ca2+-free buffer, indicating that the increase in [Ca2+]i results from mobilization of intracellular Ca2+ stores. The expression of 5-HT2B receptors on astroglial cells was further verified immunohistochemically and by Western blot analysis. These results provide evidence of the existence of 5-HT2B receptors on astrocytes in primary culture.

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.

Identification of connexin-interacting proteins: application of the yeast two-hybrid screen

Protein-protein interactions are recognized as one of the fundamental mechanisms for relaying the intra- and intercellular signals that are required for normal cellular activities affecting growth, development, and maintenance of homeostasis in tissues and organs. The yeast two-hybrid screen has become a valuable tool for identifying protein-protein interactions. The gap junction protein connexin 43 (Cx43) has been implicated in a number of biological processes including development and cellular growth control. To further advance our understanding of the ways in which Cx43 may influence these cellular activities, and to extend our knowledge of the regulation of Cx43 function and/or processing, we have employed the yeast two-hybrid screen technique to identify Cx43-interacting proteins. We present detailed methods for the yeast two-hybrid screen of a mouse embryonic cDNA library using the C terminus of Cx43 as "bait." We also describe additional methods to confirm the interactions between Cx43 and the identified proteins. These methods include in vitro binding assays, coimmunoprecipitation, and subcellular localization using immunofluorescence microscopy.

Fibroblast growth factor-2 mediates pressure-induced hypertrophic response

In vitro, fibroblast growth factor-2 (FGF2) has been implicated in cardiomyocyte growth and reexpression of fetal contractile genes, both markers of hypertrophy. However, its in vivo role in cardiac hypertrophy during pressure overload is not well characterized. Mice with or without FGF2 (Fgf2(+/+) and Fgf2(-/-), respectively) were subjected to transverse aortic coarctation (AC). Left ventricular (LV) mass and wall thickness were assessed by echocardiography preoperatively and once a week postoperatively for 10 weeks. In vivo LV function during dobutamine stimulation, cardiomyocyte cross-sectional area, and recapitulation of fetal cardiac genes were also measured. AC Fgf2(-/-) mice develop significantly less hypertrophy (4-24% increase) compared with AC Fgf2(+/+) mice (41-52% increase). Cardiomyocyte cross-sectional area is significantly reduced in AC Fgf2(-/-) mice. Noncoarcted (NC) and AC Fgf2(-/-) mice have similar beta-adrenergic responses, but those of AC Fgf2(+/+) mice are blunted. A lack of mitotic growth in both AC Fgf2(+/+) and Fgf2(-/-) hearts indicates a hypertrophic response of cardiomyocytes. Consequently, FGF2 plays a major role in cardiac hypertrophy. Comparison of alpha- and beta-cardiac myosin heavy chain mRNA and protein levels in NC and AC Fgf2(+/+) and Fgf2(-/-) mice indicates that myosin heavy chain composition depends on hemodynamic stress rather than on FGF2 or hypertrophy, and that isoform switching is transcriptionally, not posttranscriptionally, regulated.

Immunorecognition, ultrastructure and phosphorylation status of astrocytic gap junctions and connexin43 in rat brain after cerebral focal ischaemia

Gap junctions between astrocytes support a functional syncytium that is thought to play an important role in neural homeostasis. In order to investigate regulation of this syncytium and of connexin43 (Cx43), a principal astrocytic gap junction protein, we determined the sequelae of gap junction and Cx43 disposition in a rat cerebral focal ischaemia model with various ischaemia/reperfusion times using sequence-specific anti-Cx43 antibodies (designated 13-8300, 18A, 16A and 71-0700) that exhibit differential recognition of Cx43, perhaps reflecting functional aspects of gap junctions. Antibody 13-8300 specifically detects only an unphosphorylated form of Cx43 in both Western blots and tissue sections. In hypothalamus after brief (15 min) ischaemic injury, Cx43 at intact gap junctions undergoes dephosphorylation, accompanied by reduced epitope recognition by antibodies 16A and 71-0700. Tissue examined 24 h after reperfusion showed that these effects were reversible. Astrocytic gap junction internalization occurring 1 h after ischaemia was accompanied by decreased immunodetection with 13-8300. At this time, gap junctions were absent in the ischaemic core, coinciding with a loss of Cx43 recognition with 18A and 13-8300, but elevated labelling of internalized Cx43 with 16A and 71-0700. Unphosphorylated Cx43 persisted at intact gap junctions confined to a thin corridor at the ischaemic penumbra which contained presumptive apoptotic cell profiles. Similar results were obtained in ischaemic striatum and cerebral cortex, though with a delayed time course that depended on the severity of the ischaemic insult. These results demonstrate that astrocytic Cx43 epitope masking, dephosphorylation and cellular redistribution occur after ischaemic brain injury, proceed as a temporally and spatially ordered sequence of events and culminate in differential patterns of Cx43 modification and sequestration at the lesion centre and periphery. These observations suggest an attempt by astrocytes in the vicinity of injury to remodel the junctional syncytium according to altered tissue homeostatic requirements.

Cardiac myocyte membrane wounding in the abruptly pressure-overloaded rat heart under high wall stress

The potential role of transient sarcolemmal membrane wounding as a signal transduction event for cardiomyocyte hypertrophy was evaluated in rats with short-term pressure overload caused by banding of the proximal aorta. This procedure resulted in significant increases in left ventricular systolic (1.5-fold) and end-diastolic (2.6-fold) pressures and wall stresses that were associated with significant wall thinning and cavitary enlargement. Quantitative image analysis of frozen sections of the stressed ventricles obtained 60 minutes after banding demonstrated a 6- to 10-fold increase in cytosolic staining with a horseradish peroxidase-labeled anti-albumin antibody compared with sham-operated controls, indicating that an increase in transient sarcolemmal membrane permeability (wounding) is an early response to an abrupt increase in hemodynamic load in vivo. We conclude that an intense hemodynamic stress in vivo can result in histologically detectable cardiomyocyte wounding.

Selective monoclonal antibody recognition and cellular localization of an unphosphorylated form of connexin43

A sequence-specific monoclonal antibody directed against the gap junction protein connexin43 (Cx43) is shown here to be specific for the unphosphorylated form of this protein. In tissues and cultured cells containing different phosphorylated and unphosphorylated forms of Cx43, the antibody detected only the latter as shown by Western blotting of native and alkaline phosphatase-treated samples. Immunohistochemically, this monoclonal antibody did not recognize gap junctions in the vast majority of cultured cardiac myocytes, where nearly all detectable Cx43 is phosphorylated. In contrast, it was able to detect some intracellular Cx43 in tracheal smooth muscle cells and an epithelial cell line (Cl-9 cells), producing patterns of labeling consistent with those seen using a polyclonal antibody that recognizes both phosphorylated and unphosphorylated forms of Cx43. Immunostaining of gap junctions in the cultured cells indicates that both phosphorylated and unphosphorylated Cx43 are present in some assembled gap junctions, suggesting that assembled junctions do not contain exclusively the phosphorylated form of the protein. Annular gap junctions, believed to form as part of the pathway for internalization and degradation of gap junctions, were only occasionally and sparsely labeled by the monoclonal antibody, indicating that complete protein dephosphorylation is not required for uptake and degradation of gap junctions. Furthermore, the ability of this antibody to recognize only unphosphorylated Cx43, and not any of the phosphorylated forms present in the tissues and cell types examined, suggests that a unique phosphorylation site, perhaps present in the epitope recognized by this antibody, must be phosphorylated prior to phosphorylation of Cx43 at other sites.

Evidence for the co-localization of another connexin with connexin-43 at astrocytic gap junctions in rat brain

Gap junctions between astrocytes as well as between astrocytes and oligodendrocytes in rat brain were immunohistochemically labelled with a monoclonal and an affinity-purified polyclonal antibody generated against connexin-26. By light microscopy, the immunolabelling patterns obtained were, with a few exceptions, remarkably similar to previously described distribution patterns of the gap junctional protein connexin-43, which is expressed by astrocytes and is localized at astrocytic gap junctions. By electron microscopy, immunoreactivity with these two anti-connexin-26 antibodies was restricted to astrocytes; inter-astrocytic gap junctional membranes were symmetrically labelled, heterologous oligo-astrocytic junctional membranes were asymmetrically labelled only on the astrocyte side and oligo-oligodendrocyte junctions were unlabelled. Two additional anti-connexin-26 antibodies that were found to produce punctate labelling in leptomeninges and liver failed to do so in brain parenchyma, consistent with reports indicating the absence of authentic connexin-26 in this tissue. Antibodies that labelled astrocytic gap junctions exhibited no cross-reaction with connexin-43 or connexin-32, as demonstrated by western blotting, but recognized liver connexin-26 as well as several brain proteins, including an approximately 32000 mol. wt protein that did not correspond to connexin-32 and a 26000 mol. wt protein that co-migrated with liver connexin-26. These results suggest that connexin-26, or more likely a protein having sequence homology with connexin-26, is targeted to astrocytic gap junctions and raise the possibility of the existence of connexins that may be co-expressed with connexin-43 in most, but perhaps not all, astrocytes.

Modulation of intercellular communication between smooth muscle cells by growth factors and cytokines

We recently reported that tumor necrosis factor alpha is able to cause a dose-dependent and persistent reduction in gap junctional intercellular communication between primary human smooth muscle cells. In order to study whether this observed persistent reduction in gap junctional intercellular communication is a unique feature for tumor necrosis factor alpha, the present study focuses on the effects of other growth factors and cytokines on gap junctional intercellular communication. Platelet-derived growth factor AA and BB (PDGF-AA, PDGF-BB), basic fibroblast growth factor (bFGF), interleukin-6 and interferon-gamma were able to modulate gap junctional intercellular communication between primary human smooth muscle cells in vitro. However, our results demonstrate that the magnitude and nature of the observed effects are growth factor- and cytokine-specific. PDGF-AA, PDGF-BB and interleukin-6 caused a transient reduction in gap junctional intercellular communication, while bFGF induced a transient increase in gap junctional intercellular communication. Interferon-gamma was shown to be capable of causing a persistent reduction in gap junctional intercellular communication. In addition, PDGF-AA, PDGF-BB, bFGF, interleukin-6, interferon-gamma and tumor necrosis factor alpha all stimulated smooth muscle cell proliferation. These observations suggest a more complex relationship between modulation of gap junctional intercellular communication and cell proliferation than current hypotheses imply. The implications of the observed effects of growth factors and cytokines on gap junctional intercellular communication between smooth muscle cells in relation to the process of atherosclerosis is discussed.

Adenosine deaminase in rodent median eminence: detection by antibody to the mouse enzyme and co-localization with adenosine deaminase-complexing protein (CD26)

Adenosine deaminase in the hypothalamic tuberomammillary nucleus and median eminence of rat and mouse brains was investigated with two different antibodies generated against the enzyme derived from either calf or mouse. Both antibodies labelled neurons in the tuberomammillary nucleus and, as determined in rat, they immunolabelled the same neurons. In the median eminence, immunopositive fibres and terminals were detected with anti-mouse adenosine deaminase in both rat and mouse, while no such staining was seen in either species with antibody against the calf enzyme. These fibres were most concentrated in the external median eminence, had a more restricted distribution than those containing either galanin or tyrosine hydroxylase and only partially overlapped with oxytocin-positive fibres. By electron microscopy, adenosine deaminase was found in terminals containing both small, clear vesicles with diameters of 35 to 45 nm and large dense-core vesicles with diameters of 100 to 140 nm. Preadsorption of antibodies with purified enzyme derived from the species against which they were directed eliminated all staining in rat, while antibody adsorptions across species were less effective. Preadsorption of anti-mouse adenosine deaminase antibody with the mouse deaminase led to increased labelling in mouse median eminence, suggesting an interaction between tissue components and antibody-linked enzyme. Tests for the presence of adenosine deaminase-complexing protein (CD26) with an antibody against this protein gave positive labelling in the median eminence of both species and this labelling was co-distributed with that seen for adenosine deaminase. These results confirm the expression of adenosine deaminase in restricted populations of neurons in rodent brain as revealed with a novel antibody, suggest the presence of a distinct form or localization of the enzyme in the median eminence, and raise the possibility that it contributes, perhaps along with CD26, to purinergic regulation of hormone secretion in this structure.

Connections with connexins: the molecular basis of direct intercellular signaling

Adjacent cells share ions, second messengers and small metabolites through intercellular channels which are present in gap junctions. This type of intercellular communication permits coordinated cellular activity, a critical feature for organ homeostasis during development and adult life of multicellular organisms. Intercellular channels are structurally more complex than other ion channels, because a complete cell-to-cell channel spans two plasma membranes and results from the association of two half channels, or connexons, contributed separately by each of the two participating cells. Each connexon, in turn, is a multimeric assembly of protein subunits. The structural proteins comprising these channels, collectively called connexins, are members of a highly related multigene family consisting of at least 13 members. Since the cloning of the first connexin in 1986, considerable progress has been made in our understanding of the complex molecular switches that control the formation and permeability of intercellular channels. Analysis of the mechanisms of channel assembly has revealed the selectivity of inter-connexin interactions and uncovered novel characteristics of the channel permeability and gating behavior. Structure/function studies have begun to provide a molecular understanding of the significance of connexin diversity and demonstrated the unique regulation of connexins by tyrosine kinases and oncogenes. Finally, mutations in two connexin genes have been linked to human diseases. The development of more specific approaches (dominant negative mutants, knockouts, transgenes) to study the functional role of connexins in organ homeostasis is providing a new perception about the significance of connexin diversity and the regulation of intercellular communication.

High and low molecular weight fibroblast growth factor-2 increase proliferation of neonatal rat cardiac myocytes but have differential effects on binucleation and nuclear morphology. Evidence for both paracrine and intracrine actions of fibroblast growth factor-2

Basic fibroblast growth factor (FGF-2) plays a vital role in the growth and differentiation of cardiac myocytes. It exists in high and low molecular weight forms because of the use of alternative initiation codons in the same mRNA. Higher levels of high molecular weight forms (molecular mass of 22 and 21.5 kD) are present in the rat heart during the neonatal stage, whereas the low molecular weight form (molecular mass of 18 kD) is predominant in the adult heart, suggesting different roles in development. Rat FGF-2 cDNAs that can preferentially express high or low molecular weight forms were introduced into neonatal rat ventricular myocyte cultures. Significant and comparable increases in overall cardiac myocyte DNA synthesis and proliferation were seen with 22/21.5- and 18-kD FGF-2 expression. A significantly higher mitotic index was seen in the vicinity of cardiac myocytes overexpressing high or low molecular weight forms of FGF-2 compared with nonoverexpressing cells. This increase was inhibited in the presence of neutralizing antibodies to FGF-2, pointing to a proximity-dependent paracrine effect of 22/21.5- and 18-kD FGF-2 on mitosis. By contrast, overexpression of high but not low molecular weight FGF-2 was associated with a significant increase in binucleation (approximately 36% of cardiac myocytes overexpressing 22/21.5-kD FGF-2 were binucleated compared with 9% of cardiac myocytes overexpressing 18-kD FGF-2), which was not affected by neutralizing antibodies to FGF-2. These results suggest that 22/21.5-kD FGF-2 and 18-kD FGF-2 have similar paracrine effects on proliferation but that 22-21.5-kD FGF-2 exerts a distinct intracrine effect on binucleation.

Upregulation of connexin43 gap junctions during early stages of human coronary atherosclerosis

Interactions between cells form the framework for understanding the pathogenesis of atherosclerosis, but little information is available on the role of direct intercellular communication via gap junctions in this process. To investigate gap junction expression in the pathogenesis of human atherosclerosis, lesions representing different stages of the disease were obtained from coronary arteries of hearts removed from patients undergoing cardiac transplantation. Twelve hearts, each providing 1 to 3 segments of artery, were used in the study. Sections were examined by confocal laser scanning microscopy after immunofluorescent labeling with a specific antibody against connexin43, the major gap-junctional protein of smooth muscle cells, to permit high-definition visualization of immunolabeled gap junctions through the depth of the specimen. Double labeling using anti-connexin43 and cell type-specific antibodies demonstrated colocalization of gap junctions with smooth muscle cells but not with macrophages, a relationship confirmed by electron microscopy. Regions of intimal thickening and early atheromatous lesions showed markedly increased expression of connexin43 gap junctions between intimal smooth muscle cells compared with the undiseased vessels. This increase in gap junctions was most marked in regions of intimal thickening, semiquantitative analysis of the confocal digital images revealing a > 10-fold increase compared with the undiseased vessel. The quantity of labeled gap junctions in early atheromatous lesions, although higher than that of the undiseased vessel, was lower than that of intimal thickenings, and this trend toward reduced levels of gap junction immunolabeling with lesion progression continued, the value observed in the most advanced atheromatous lesions being lower than that of the undiseased vessel. As the quantity of gap junctions declined, their distribution became more patchy and the sizes of individual junctions larger. The results suggest that enhanced expression of gap junctions between smooth muscle cells may play a role in maintaining the synthetic phenotype during early growth of the atherosclerotic plaque.

Characterization of fibroblast growth factor receptor 1 RNA expression in the embryonic mouse heart

We used reverse transcriptase-polymerase chain reaction (RT-PCR) to clone fibroblast growth factor receptor (FGFR) 1 isoforms from embryonic mouse heart and as a more sensitive method to characterize FGFR1 RNA expression in embryonic and adult mouse hearts. We describe the cloning of both full-length short (2259 base pairs) and long (2526 base pairs) FGFR1 isoform cDNAs which generated 86 and 102 kilodalton proteins, respectively, following in vitro translation. An assessment of FGFR1 RNA indicates that FGFR1-IIIc is the major form in both the embryonic and adult heart but there is an approximately 8.5-fold decrease in RNA levels in the adult. Differential RNA blotting as well as RT-PCR analyses are consistent with a switch in the relative expression of the short versus long FGFR1 isoforms during heart development. The long isoforms are more abundant in the embryo and the short isoforms predominate in the adult. This may be important in the regulation of growth and development of the heart.

Basic fibroblast growth factor is cardioprotective in ischemia-reperfusion injury

To examine whether basic fibroblast growth factor (bFGF) administered to the heart by perfusion can improve cardiac resistance to injury we employed an isolated rat heart model of ischemia-reperfusion injury and determined the extent of functional recovery in bFGF-treated and control hearts. Global ischemia was simulated by interruption of flow for 60 min. Recovery of developed force of contraction (DF), recorded after reestablishment of flow for 30 min, reached 63.8 +/- 1.5% and 96.5 +/- 3.5% of preischemic levels in control and bFGF-treated hearts (10 micrograms/heart), respectively, indicating that bFGF induced significantly improved recovery of mechanical function. Recoveries of the rates of contraction or relaxation were also significantly improved in bFGF-treated hearts. Extent of myocardial injury, assessed by determination of phosphocreatine kinase in the effluent, was reduced as a result of bFGF treatment. As a first step towards understanding the mechanism and direct cellular target(s) of bFGF-induced cardioprotection, we investigated its fate after perfusion. Perfusion of 10 micrograms bFGF/heart resulted in a 4-fold increase in bFGF associated with the heart compared to control levels, as estimated by biochemical fractionation and immunoblotting. Immunofluorescent staining of the bFGF-perfused hearts revealed intense anti-bFGF staining in association with blood vessels as well as the periphery of cardiomyocytes, suggesting that the latter may be a target for direct bFGF action. In conclusion, our findings of bFGF-induced increases in cardiac resistance to, and improved functional recovery from, ischemia-reperfusion injury indicate that bFGF may have clinical applications in the treatment of ischemic heart disease.

Basic fibroblast growth factor stimulates connexin-43 expression and intercellular communication of cardiac fibroblasts

Gap junctions (GJ) are membrane specializations responsible for intercellular communication and for ensuring electrical and/or metabolic coupling between cells. They are composed of connexins, a family of related proteins. Connexin-43 (Cx43) is a major connexin of the rat heart, expressed by myocytes as well as non-muscle cells. In this communication we have examined expression of Cx43 by cardiac fibroblasts and regulation of its expression by an endogenous mitogen, basic fibroblast growth factor (bFGF). Recombinant human bFGF, administered to cultured cells which had been maintained in 0.5% serum for 48 h, induced dose-dependent and statistically significant increases in Cx43 mRNA as well as protein accumulation, at 6 h after addition. Intercellular communication was also increased at 6 h but not 30 min after bFGF treatment, as assessed using a scrape-loading protocol. It is concluded that the bFGF-induced stimulation of Cx43 expression caused increased coupling between cardiac fibroblasts. This would be of importance in injured myocardium, the increased bFGF content of which might stimulate electrical coupling involving fibroblasts of the scar tissue.

Structure--function relationships in gap junctions

Gap junctions are metabolic and electrotonic pathways between cells and provide direct cooperation within and between cellular nets. They are among the cellular structures most frequently investigated. This chapter primarily addresses aspects of the assembly of the gap junction channel, considering the insertion of the protein into the membrane, the importance of phosphorylation of the gap junction proteins for coupling modulation, and the formation of whole channels from two hemichannels. Interactions of gap junctions with the subplasmalemmal cytoplasm on the one side and with tight junctions on the other side are closely considered. Furthermore, reviewing the significance and alterations of gap junctions during development and oncogenesis, respectively, including the role of adhesion molecules, takes up a major part of the chapter. Finally, the literature on gap junctions in the central nervous system, especially between astrocytes in the brain cortex and horizontal cells in the retina, is summarized and new aspects on their structure-function relationship included.

Fibroblast growth factors and their receptors: an information network controlling tissue growth, morphogenesis and repair

The stimulation of cellular metabolism by the nine fibroblast growth factors (FGFs) is mediated by a dual-receptor system. This comprises a family of four receptor tyrosine kinases (FGFR) and heparan sulphate proteoglycans (HSPG). The stimulation of cell division by FGFs has an obligate requirement for both partners of the dual-receptor system. The binding of the nine FGFs to the FGFRs is marked by a pattern of overlapping specificity despite alternative splicing events generating a large number of FGFR proteins. Thus many of the FGFR isoforms bind several FGFs. It is likely that each FGF requires a different pattern of sulphation within the heparan sulphate chains for binding. Therefore, the HSPG receptors may provide additional specificity, allowing a cell to fine tune its response to the FGFs present in the extracellular milieu. The HSPG receptors also control the availability of FGFs and hence regulate the transport of FGFs within a tissue. FGF-stimulated cell division would appear to have a mandatory requirement for the FGFs to be translocated to the nucleus via the cytosol after interacting with the dual-receptor system. The consequences of the potential direct action of FGFs in stimulating cell division are examined in the light of current models of signal transduction.

Increased basic fibroblast growth factor (bFGF) accumulation and distinct patterns of localization in isoproterenol-induced cardiomyocyte injury

Basic FGF is a multifunctional protein which promotes regeneration in several tissues. To investigate involvement in cardiac injury-repair, bFGF accumulation and localization was examined in hearts of rats injected with a single high dose of isoproterenol. The bFGF content of cardiac extracts was analyzed at 6 and 24 hours as well as 1, 4, and 6 weeks by western blotting of heparin-sepharose-bound fractions. The 18 kilodalton bFGF species showed an approximately 2-fold increase in extracts from treated animals compared to non-treated controls. A transient rise in a 21-23 kilodalton bFGF species was seen at 24 hours after treatment. An induction of bFGF mRNA was also observed in treated animals. To localize bFGF in vivo, immunofluorescent labelling with specific antibodies was used at 4-24 hours and 1-4 weeks after treatment. Simultaneous labelling for the cytoskeletal proteins vinculin or vimentin was employed to identify viable myocytes or non-muscle interstitial cells, respectively. Necrotic myocytes, identified by loss of vinculin, displayed a pronounced increase in cytoplasmic anti-bFGF staining compared to adjacent normal myocytes. This increase occurred prior to and may play a role in promoting mobile cell migration and proliferation in areas of necrosis. Viable cardiomyocytes adjacent to fibrotic regions displayed strong pericellular anti-bFGF staining and, occasionally, were also stained by anti-vimentin antibodies, suggesting reexpression of an embryonic phenotype and thus an attempt for regeneration. These data showing increased accumulation and distinct patterns of localization of bFGF in the hearts of isoproterenol-treated animals suggest that this growth factor plays a role in short-term as well as long term response of the myocardium to injury.

Endogenous and exogenous modulation of gap junctional intercellular communication: toxicological and pharmacological implications

During the evolution of single-celled organisms to multicellular metazoans, a family of highly conserved genes coding for proteins (connexins), which as hexameric units (connexins), has evolved to form intercellular channels (gap junctions). These gap junctions allow ions and small molecular weight molecules to flow between coupled cells, thereby facilitating synchronization of electrotonic or metabolic cooperation. Control of cell proliferation, cell differentiation and adaptive responses of differentiated cells have been speculated to be biological roles of gap junctions. The regulation of these gap junctions can occur at the transcriptional, translational and posttranslational levels. Transient downregulation by endogenous or exogenous chemicals can bring about adaptive or maladaptive consequences depending on circumstances. Stable abnormal regulation of gap junction function has been associated with the activation of several oncogenes. Several tumor suppressor genes have also been associated with the up-regulation of gap junction function. Since gap junctions exist in all organs of the multi-cellular organisms, the dysfunction of these gap junctions by various toxic chemicals which have cell type/tissue/organ specificity could bring about very distinct clinical consequences, such as embryo lethality or teratogenesis, reproductive dysfunction in the gonads, neurotoxicity of the CNS system, hyperplasia of the skin, and tumor promotion of initiated tissue. Understanding how many non-mutagenic chemicals might alter normal gap junction function should form the basis of "epigenetic" toxicology. On the other hand, restoring normal gap junction function to cells which have dysfunctional intercellular communication could be the basis for a new approach for therapeutic pharmaceuticals.

Acidic fibroblast growth factor and heart development. Role in myocyte proliferation and capillary angiogenesis

Proliferative growth of the ventricular myocyte (cardiomyocyte) is primarily limited to fetal and early neonatal periods of development. In concert with the neonatal "transition" from proliferative to hypertrophic growth, ventricular remodeling of the nonmyocyte compartment is characterized by increased extracellular matrix synthesis/deposition and capillary angiogenesis. A role for locally generated and bioactive ventricular acidic fibroblast growth factor (aFGF) in these processes is proposed and substantiated by the following: 1) colocalization of aFGF peptide and fibroblast growth factor receptor (flg) transcripts to the developing fetal cardiomyocyte by immunohistochemistry, immunoelectron microscopy, and in situ hybridization, 2) continued localization of aFGF peptide and transcripts to the neonatal/mature cardiomyocyte, and 3) localization of flg immunoreactivity and transcripts to specific neonatal ventricular nonmuscle cell types. Specific ventricular cell types at distinct developmental stages appear to be responsive to ventricular myocyte-derived aFGF (myocytes in the fetal heart and nonmyocytes/endothelial cells in the neonatal heart). These data indicate that expression of aFGF and one of its receptors (flg) are most pronounced in the fetal to early neonatal ventricle, the presence of both suggesting an autocrine/paracrine growth regulatory function. As the animal matures, ventricular capillary angiogenesis may be facilitated by "release" of cardiomyocyte-derived fibroblast growth factors into the surrounding extracellular space/matrix functioning as a "paracrine" angiogenic stimuli. Therefore, the results of our study suggest that myocyte-derived aFGF may function to increase the fetal ventricular cardiomyocyte population in absolute number as well as to facilitate the subsequent increase in capillary angiogenesis that occurs during cardiomyocyte maturation and ventricular remodeling.

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

We have analyzed the effect of basic fibroblast growth factor (bFGF) on junctional communication (coupling) and connexin 43 (Cx43) expression in bovine microvascular endothelial (BME) cells. In control confluent cultures, the incidence of coupling, as assessed by the intercellular transfer of microinjected Lucifer Yellow, was limited to 13% of injected cells, and decreased to 0% with time in culture. After exposure to bFGF (3ng/ml), the incidence of coupling was increased in a time-dependent manner, reaching a maximum of 38% of microinjected cells after 10-12 hours. The extent of coupling, as assessed by scrape loading, was maximally increased 2.1-fold 8-9 hours after addition of bFGF. bFGF also induced a 2-fold increase in Cx43 as assessed by Western blotting, and increased Cx43 immunolabelling at contacting interfaces of adjacent BME cells. Cx43 mRNA was likewise increased after exposure to bFGF in a time- and dose-dependent manner, with a maximal 6-7-fold increase after a 4 hour exposure to 3-10ng/ml. Finally, the increase in coupling and Cx43 mRNA expression observed after mechanically wounding a confluent monolayer of BME cells was markedly reduced by antibodies to bFGF, which have previously been shown to inhibit migration. Taken together, these results indicate that exogenous and endogenous bFGF increase intercellular communication and Cx43 expression in microvascular endothelial cells. We propose that the bFGF-mediated increase in coupling is necessary for the coordination of endothelial cells during angiogenesis and other vessel wall functions.

Identification of basic fibroblast growth factor-like proteins in African trypanosomes and Leishmania

Basic fibroblast growth factor (bFGF) is a multifunctional, heparin-binding, mitogenic polypeptide found in all tissues or cells of multicellular organisms so far examined. Here we report that Trypanosoma brucei rhodesiense procyclic culture forms (PCF) and Leishmania donovani promastigotes grown in serum-containing and serum-free medium, contained peptides of 15-34 kDa which bound heparin-sepharose with high affinity and which reacted in immunoblots with several preparations of antibodies specific for bovine brain bFGF. Similar peptides were not detectable in foetal bovine serum. Immunofluorescence studies showed bFGF-like molecules to have a cytoplasmic distribution in both species growing in serum-free media. A nuclear and/or perinuclear distribution of immunoreactivity was also observed in parasites which had been grown in the presence of serum. The data indicate that both species of parasites synthesize their own bFGF-like molecules. Association of an ubiquitous growth factor with parasitic protozoa may play an important role in parasite multiplication and in host-parasite interactions.