Titration of the gap junction protein Connexin43 reduces atherogenesis.

Ubiquitous reduction of the gap junction protein Connexin43 (Cx43) in mice provides beneficial effects on progression and composition of atherosclerotic lesions. Cx43 is expressed in multiple atheroma-associated cells but its function in each cell type is not known. To examine specifically the role of Cx43 in immune cells, we have lethally irradiated low-density lipoprotein receptor-deficient mice and reconstituted with Cx43+/+, Cx43+/- or Cx43-/- haematopoietic fetal liver cells. Progression of atherosclerosis was significantly lower in aortic roots of Cx43+/- chimeras compared with Cx43+/+ and Cx43-/- chimeras, and their plaques contained significantly less neutrophils. The relative proportion of circulating leukocytes was similar between the three groups. Interestingly, the chemoattraction of neutrophils, which did not express Cx43, was reduced in response to supernatant secreted by Cx43+/- macrophages in comparison with the ones of Cx43+/+ and Cx43-/- macrophages. Cx43+/- macrophages did not differ from Cx43+/+ and Cx43-/- macrophages in terms of M1/M2 polarisation but show modified gene expression for a variety chemokines and complement components. In conclusion, titration of Cx43 expression in bone marrow-derived macrophages reduces atherosclerotic plaque formation and chemoattraction of neutrophils to the lesions.

Null mutation of connexin43 causes slow propagation of ventricular activation in the late stages of mouse embryonic development.

Connexin43 (Cx43) is the principal connexin isoform in the mouse ventricle, where it is thought to provide electrical coupling between cells. Knocking out this gene results in anatomic malformations that nevertheless allow for survival through early neonatal life. We examined electrical wave propagation in the left (LV) and right (RV) ventricles of isolated Cx43 null mutated (Cx43(-/-)), heterozygous (Cx43(+/)(-)), and wild-type (WT) embryos using high-resolution mapping of voltage-sensitive dye fluorescence. Consistent with the compensating presence of the other connexins, no reduction in propagation velocity was seen in Cx43(-/-) ventricles at postcoital day (dpc) 12.5 compared with WT or Cx43(+/)(-) ventricles. A gross reduction in conduction velocity was seen in the RV at 15.5 dpc (in cm/second, mean [1 SE confidence interval], WT 9.9 [8.7 to 11.2], Cx43(+/)(-) 9.9 [9.0 to 10.9], and Cx43(-/-) 2.2 [1.8 to 2.7; P<0.005]) and in both ventricles at 17.5 dpc (in RV, WT 8.4 [7.6 to 9.3], Cx43(+/)(-) 8.7 [8.1 to 9.3], and Cx43(-/-) 1.1 [0.1 to 1.3; P<0.005]; in LV, WT 10.1 [9.4 to 10.7], Cx43(+/)(-) 8.3 [7.8 to 8.9], and Cx43(-/-) 1.7 [1.3 to 2.1; P<0.005]) corresponding with the downregulation of Cx40. Cx40 and Cx45 mRNAs were detectable in ventricular homogenates even at 17.5 dpc, probably accounting for the residual conduction function. Neonatal knockout hearts were arrhythmic in vivo as well as ex vivo. This study demonstrates the contribution of Cx43 to the electrical function of the developing mouse heart and the essential role of this gene in maintaining heart rhythm in postnatal life.

The carboxyl terminal domain regulates the unitary conductance and voltage dependence of connexin40 gap junction channels.

Chemical regulation of connexin (Cx) 40 and Cx43 follows a ball-and-chain model, in which the carboxyl terminal (CT) domain acts as a gating particle that binds to a receptor affiliated with the pore. Moreover, Cx40 channels can be closed by a heterodomain interaction with the CT domain of Cx43 and vice versa. Here, we report similar interactions in the establishment of the unitary conductance and voltage-dependent profile of Cx40 in N2A cells. Two mean unitary conductance values ("lower conductance" and "main") were detected in wild-type Cx40. Truncation of the CT domain at amino acid 248 (Cx40tr248) caused the disappearance of the lower-conductance state. Coexpression of Cx40tr248 with the CT fragment of either Cx40 (homodomain interactions) or Cx43 (heterodomain interactions) rescued the unitary conductance profile of Cx40. In the N2A cells, the time course of macroscopic junctional current relaxation was best described by a biexponential function in the wild-type Cx40 channels, but it was reduced to a single-exponential function after truncation. However, macroscopic junctional currents recorded in the oocyte expression system were not significantly different between the wild-type and mutant channels. Concatenation of the CT domain of Cx43 to amino acids 1 to 248 of Cx40 yielded a chimeric channel with unitary conductance and voltage-gating profile indistinguishable from that of wild-type Cx40. We conclude that residence of Cx40 channels in the lower-conductance state involves a ball-and-chain type of interaction between the CT domain and the pore-forming region. This interaction can be either homologous (Cx40 truncation with Cx40CT) or heterologous (with the Cx43CT).

Coexpression of connexins 40 and 43 enhances the pH sensitivity of gap junctions: a model for synergistic interactions among connexins.

Gap junctions are formed by oligomerization of a protein called connexin. Most cells express more than one connexin isotype. Atrial myocytes, for example, coexpress connexin (Cx) 40 and Cx43. The consequence of connexin coexpression on the regulation of gap junctions is not well understood. In the present study, we show that cells coexpressing Cx40 and Cx43 are more susceptible to acidification-induced uncoupling than those cells expressing only one connexin isotype. Xenopus oocytes were injected with mRNA for Cx40, Cx43, or a combination of both. Intracellular pH and junctional conductance were simultaneously measured while cells were progressively acidified by superfusion with a bicarbonate-buffered solution gassed with increasing concentrations of carbon dioxide. The data show that the pKa (ie, the pH at which junctional conductance decreased to 50% from maximum) shifted from approximately 6.7 when cells expressed only Cx40 or only Cx43 to approximately 7.0 when one of the oocytes was coexpressing both connexins. Truncation of the carboxyl terminal domains of the connexins caused the loss of pH sensitivity even after coexpression. The data are interpreted on the basis of previous studies from our laboratory that demonstrated heterodomain interactions in the regulation of Cx40 and Cx43 gap junctions. The possible implications of these findings on the regulation of native gap junctions that express both connexins remain to be determined.

Identification of a protein kinase activity that phosphorylates connexin43 in a pH-dependent manner.

The carboxyl-terminal (CT) domain of connexin43 (Cx43) has been implicated in both hormonal and pH-dependent gating of the gap junction channel. An in vitro assay was utilized to determine whether the acidification of cell extracts results in the activation of a protein kinase that can phosphorylate the CT domain. A glutathione S-transferase (GST)-fusion protein was bound to Sephadex beads and used as a target for protein kinase phosphorylation. A protein extract produced from sheep heart was allowed to bind to the fusion protein-coated beads. The bound proteins were washed and then incubated with 32P-ATP. Phosphorylation was assessed after the proteins were resolved by SDS-PAGE. Incubation at pH 7.5 resulted in a minimal amount of phosphorylation while incubation at pH 6.5 resulted in significant phosphorylation reaction. Maximal activity was achieved when both the binding and kinase reactions were performed at pH 6.5. The protein kinase activity was stronger when the incubations were performed with manganese rather than magnesium. Mutants of Cx43 which lack the serines between amino acids 364-374 could not be phosphorylated in the in vitro kinase reaction, indicating that this is a likely target of this reaction. These results indicate that there is a protein kinase activity in cells that becomes more active at lower pH and can phosphorylate Cx43.

Proton and zinc effects on HERG currents.

The proton and Zn2+ effects on the human ether-a-go-go related gene (HERG) channels were studied after expression in Xenopus oocytes and stable transfection in the mammalian L929 cell line. Experiments were carried out using the two-electrode voltage clamp at room temperature (oocytes) or the whole-cell patch clamp technique at 35 degrees C (L929 cells). In oocytes, during moderate extracellular acidification (pHo = 6.4), current activation was not shifted on the voltage axis, the time course of current activation was unchanged, but tail current deactivation was dramatically accelerated. At pHo < 6.4, in addition to accelerating deactivation, the time course of activation was slower and the midpoint voltage of current activation was shifted to more positive values. Protons and Zn2+ accelerated the kinetics of deactivation with apparent Kd values about one order of magnitude lower than for tail current inhibition. For protons, the Kd values for the effect on tail current amplitude versus kinetics were, respectively, 1.8 microM (pKa = 5.8) and 0.1 microM (pKa = 7.0). In the presence of Zn2+, the corresponding Kd values were, respectively, 1.2 mM and 169 microM. In L929 cells, acidification to pHo = 6.4 did not shift the midpoint voltage of current activation and had no effect on the time course of current activation. Furthermore, the onset and recovery of inactivation were not affected. However, the acidification significantly accelerated tail current deactivation. We conclude that protons and Zn2+ directly interact with HERG channels and that the interaction results, preferentially, in the regulation of channel deactivation mechanism.

ETS transcription factors regulate an enhancer activity in the third intron of TNF-alpha.

We describe an enhancer site in the third intron of tumor necrosis factor alpha (TNF-alpha). A reporter construct containing the 5'-flanking region of the mouse TNF-alpha gene displayed weak activity when transfected into RAW264.7 macrophage-like cells. The addition of the third intron of TNF-alpha to this construct resulted in an enhancement of CAT protein. This enhancement was eliminated if a conserved 20-bp sequence was removed from the intron or if a dominant-negative ets-binding factor was co-transfected with the reporter gene. Mutations of this site that destroyed potential ets transcription factor binding sites had reduced transcriptional activity. The major transcription factor that bound to the oligonucleotide was confirmed to be GABP by supershift and competition analysis. In RAW264.7 cells, the binding was constitutive, however, in bone marrow-derived macrophages binding activity was shown to be interferon-gamma inducible. This may imply a role for ets transcription factors in the production of TNF-alpha.

Formation of heteromeric gap junction channels by connexins 40 and 43 in vascular smooth muscle cells.

Connexin (Cx) 43 and Cx40 are coexpressed in several tissues, including cardiac atrial and ventricular myocytes and vascular smooth muscle. It has been shown that these Cxs form homomeric/homotypic channels with distinct permeability and gating properties but do not form functional homomeric/heterotypic channels. If these Cxs were to form heteromeric channels, they could display functional properties not well predicted by the homomeric forms. We assessed this possibility by using A7r5 cells, an embryonic rat aortic smooth muscle cell line that coexpresses Cxs 43 and 40. Connexons (hemichannels), which were isolated from these cells by density centrifugation and immunoprecipitated with antibody against Cx43, contained Cx40. Similarly, antibody against Cx40 coimmunoprecipitated Cx43 from the same connexon fraction but only Cx40 from Cx (monomer) fractions. These results indicate that heteromeric connexons are formed by these Cxs in the A7r5 cells. The gap junction channels formed in the A7r5 cells display many unitary conductances distinct from homomeric/homotypic Cx43 or Cx40 channels. Voltage-dependent gating parameters in the A7r5 cells are also quite variable compared with cells that express only Cx40 or Cx43. These data indicate that Cxs 43 and 40 form functional heteromeric channels with unique gating and conductance properties.

Hetero-domain interactions as a mechanism for the regulation of connexin channels.

Previous studies have shown that chemical regulation of connexin43 (Cx43) depends on the presence of the carboxyl terminal (CT) domain. A particle-receptor (or "ball-and-chain") model has been proposed to explain the mechanism of gating. We tested whether the CT region behaved as a functional domain for other members of the connexin family. The pH sensitivity of wild-type and Ct-truncated connexins was quantified by use of electrophysiological and optical techniques and the Xenopus oocyte system. The CT domain of Cx45 had no role in pH regulation, although a partial role was shown for Cx37 and Cx50. A prominent effect was observed for Cx40 and Cx43. In addition, we found that the CT domain of Cx40 that was expressed as a separate fragment rescued the pH sensitivity of the truncated Cx40 (Cx40tr), which was in agreement with a particle-receptor model. Because Cx40 and Cx43 often colocalize and possibly heteromerize, we tested the pH sensitivity of Cx40tr when coexpressed with the CT domain of Cx43 (hetero-domain interactions). We found that the CT domain of Cx43 enhanced the pH sensitivity of Cx40tr; similarly, the CT domain of Cx40 restored the pH sensitivity of the truncated Cx43. In addition, the CT domain of Cx43 granted insulin sensitivity to the otherwise insulin-insensitive Cx26 or Cx32 channels. These data show that the particle-receptor model is preserved in Cx40 and the regulatory domain of one connexin can specifically interact with a channel formed by another connexin. Hetero-domain interactions could be critical for the regulation of heteromeric channels.

Connexin diversity and gap junction regulation by pHi.

The molecular mechanisms controlling pH-sensitivity of gap junctions formed of two different connexins are yet to be determined. We used a proton-sensitive fluorophore and electrophysiological techniques to correlate changes in intracellular pH (pHi) with electrical coupling between connexin-expressing Xenopus oocytes. The pH sensitivities of alpha 3 (connexin46), alpha 2 (connexin38), and alpha 1 (connexin43) were studied when these proteins were expressed as: 1) nonjunctional hemichannels (for alpha 3 and alpha 2), 2) homotypic gap junctions, and 3) heterotypic gap junctions. We found that alpha 3 hemichannels are sensitive to changes in pHi within a physiological range (pKa = 7.13 +/- 0.03; Hill coefficient = 3.25 +/- 1.73; n = 8; mean +/- SEM); an even more alkaline pKa was obtained for alpha 2 hemichannels (pKa = 7.50 +/- 0.03; Hill coefficient = 3.22 +/- 0.66; n = 13). The pH sensitivity curves of alpha 2 and alpha 3 homotypic junctions were indistinguishable from those recorded from hemichannels of the same connexin. Based on a comparison of pKa values, both alpha 3 and alpha 2 gap junctions were more pHi-dependent than alpha 1. The pH sensitivity of alpha 2-containing heterotypic junctions could not be predicted from the behavior of the two connexons in the pair. When alpha 2 was paired with alpha 3, the pH sensitivity curve was similar to that obtained from alpha 2 homotypic pairs. Yet, pairing alpha 2 with alpha 1 shifted the curve similar to homotypic alpha 1 channels. Pairing alpha 2 with a less pH sensitive mutant of alpha 1 (M257) yielded the same curve as when alpha 1 was used. However, the pH sensitivity curve of alpha 3/alpha 1 channels was similar to alpha 3/alpha 3, while alpha 3/M257 was indistinguishable from alpha 3/alpha 1. Our results could not be consistently predicted by a probabilistic model of two independent gates in series. The data show that dissimilarities in the pH regulation of gap junctions are due to differences in the primary sequence of connexins. Moreover, we found that pH regulation is an intrinsic property of the hemichannels, but pH sensitivity is modified by the interactions between connexons. These interactions should provide a higher level of functional diversity to gap junctions that are formed by more than one connexin.

A particle-receptor model for the insulin-induced closure of connexin43 channels.

Connexin43(Cx43) channels can be regulated by a variety of factors, including low pHi. Structure/function studies from this laboratory have demonstrated that pH gating follows a particle-receptor mechanism, similar to the "ball-and-chain" model of voltage-dependent inactivation of ion channels. The question whether the particle-receptor model is applicable only to pH gating or to other forms of Cx43 regulation as well remains. To address this question, we looked at the uncoupling effects of insulin and of insulin-like growth factor-1 (IGF) on Cx43 channels expressed in Xenopus oocytes. These agonists do not induce changes in pHi. Junctional conductance (Gj) was measured by the dual 2-electrode voltage-clamp technique. Control studies showed that relative Gj did not change spontaneously as a function of time. Continuous exposure of Cx43-expressing oocytes to insulin (10 micro/L) led to a decrease in Gj. After 80 minutes, Gj was 54+/-5% from control (n= 12). Exposure of oocytes to IGF (10 nmol/L) caused an even more pronounced change in Gj (37+/-4% of control, n=6). The time course of the IGF-induced uncoupling was similar to that observed after insulin exposure. The effect of insulin was abolished by truncation of the carboxyl-terminal domain of Cx43 at amino acid 257 (M257). Interestingly, as in the case of pH gating, coexpression of the carboxyl-terminal domain (amino acids 258 to 282) together with M257 rescued the ability of insulin to reduce coupling (Gj, 39+/-12% from control; n=6). Structure/function experiments using various deletion mutants of the carboxyl-terminal domain showed that insulin treatment does not modify Gj if amino acids 261 to 280 are missing from the Cx43 sequence. Our results suggest that a particle-receptor (or ball-and-chain) mechanism, similar to that described for pH gating, also applies to chemical regulation of Cx43 by other factors.

A 17mer peptide interferes with acidification-induced uncoupling of connexin43.

Structure/function analysis shows that the carboxyl terminal (CT) domain of connexin43 (Cx43) is essential for the chemical regulation of cell-cell communication. Of particular interest is the region between amino acids 260 and 300. Structural preservation of this region is essential for acidification-induced uncoupling (ie, pH gating). In this study, we report data showing that a 17mer peptide of the same sequence as amino acids 271 to 287 of Cx43 (CSSPTAPLSPMSPPGYK) can prevent pH gating of Cx43-expressing oocytes. Experiments were carried out in pairs of Xenopus oocytes previously injected with connexin38 antisense and expressing wild-type Cx43. Junctional conductance was measured electrophysiologically. pHi was determined from the light emission of the proton-sensitive dye dextran-seminaphthorhodafluor. Intracellular acidification was induced by superfusion with a bicarbonate-buffered solution gassed with a progressively increasing concentration of CO2. Injection of water alone into both oocytes of a Cx43-expressing pair or injection of a peptide from region 321 to 337 of Cx43 did not modify pH sensitivity. However, injection of a polypeptide corresponding to amino acids 241 to 382 of Cx43 interfered with the ability of gap junctions to close on acidification. Similar results were obtained when a 17mer peptide (region 271 to 287) was injected into both oocytes of the pair. Normal Cx43 pH gating was observed if (1) the amino acid sequence of the 17mer peptide was scrambled or (2) the N and the C ends of the 17mer peptide were not included in the sequence. This is the first demonstration of a molecule that can interfere with the chemical regulation of connexin channels in a cell pair. The data may lead to the development of small molecules that can be used in Cx43-expressing multicellular preparations to study the role of gap junction regulation in normal as well as diseased states.

Structure of connexin43 and its regulation by pHi.

Electrical coupling in the heart provides an effective mechanism for propagating the cardiac action potential efficiently throughout the entire heart. Cells within the heart are electrically coupled through specialized membrane channels called gap junctions. Studies have shown that gap junctions are dynamic, carefully regulated channels that are important for normal cardiogenesis. We have recently been interested in the molecular mechanisms by which intracellular acidification leads to gap junction channel closure. Previous results in this lab have shown that truncation of the carboxyl terminal (CT) of connexin43 (Cx43) does not interfere with functional channel expression. Further, the pH-dependent closure of Cx43 channels is significantly impaired by removal of this region of the protein. Other studies have shown that the CT is capable of interacting with its receptor even when not covalently attached to the channel protein. From these data we have proposed a particle-receptor model to explain the pH-dependent closure of Cx43 gap junction channels. Detailed analysis of the CT has revealed interesting new information regarding its possible structure. Here we review the most recent studies that have contributed to our understanding of the molecular mechanisms of regulation of the cardiac gap protein Cx43.

Two forms of NF-kappa B1 (p105/p50) in murine macrophages: differential regulation by lipopolysaccharide, interleukin-2, and interferon-gamma.

In macrophages, nuclear factor kappa B (NF-kappa B) has been shown to transactivate the promoters of many cytokines, including tumor necrosis factor-alpha (TNF-alpha). We have used the -510 kappa B binding site from the murine TNF-alpha promoter to assay the induction of NF-kappa B in murine macrophages by various stimuli. A basal level of NF-kappa B activity in murine macrophages was detectable, and this activity was enhanced by treatment of these cells with lipopolysaccharide (LPS) or interleukin-2 (IL-2). Interferon-gamma (IFN-gamma), an important regulator of macrophage gene expression, significantly enhanced NF-kappa B activity and altered the apparent molecular weight of the NF-kappa B1-like proteins in LPS-stimulated and IL-2-stimulated murine macrophages. The NRD (NF-kappa B/Rel/Dorsal) complexes induced by LPS and IFN-gamma were further characterized by addition of antisera to electrophoretic mobility shift assay (EMSA) reaction mixtures. NF-kappa B1/p50 was a component of all complexes, whereas RelA/p65 was present in the IFN-gamma/LPS-stimulated activity. IFN-gamma priming or treatment with LPS for 19 h resulted in an upregulation of the larger species of NF-kappa B1/p50. In addition, regulation of the two pools of NF-kappa B1/p50 by IFN-gamma was confirmed by Western immunoblot analysis of cytosolic and nuclear extracts. This is the first demonstration of the presence of two pools of NF-kappa B1/p50 differentially regulated in response to cytokine activation of macrophages.

PH regulation of connexin43: molecular analysis of the gating particle.

Gap junction channels allow for the passage of ions and small molecules between neighboring cells. These channels are formed by multimers of an integral membrane protein named connexin. In the heart and other tissues, the most abundant connexin is a 43-kDa, 382-amino acid protein termed connexin43 (Cx43). A characteristic property of connexin channels is that they close upon acidification of the intracellular space. Previous studies have shown that truncation of the carboxyl terminal of Cx43 impairs pH sensitivity. In the present study, we have used a combination of optical, electrophysiological, and molecular biological techniques and the oocyte expression system to further localize the regions of the carboxyl terminal that are involved in pH regulation of Cx43 channels. Our results show that regions 261-300 and 374-382 are essential components of a pH-dependent "gating particle," which is responsible for acidification-induced uncoupling of Cx43-expressing cells. Regions 261-300 and 374-382 seem to be interdependent. The function of region 261-300 may be related to the presence of a poly-proline repeat between amino acids 274 and 285. Furthermore, site-directed mutagenesis studies show that the function of region 374-382 is not directly related to its net balance of charges, although mutation of only one amino acid (aspartate 379) for asparagine impairs pH sensitivity to the same extent as truncation of the carboxyl terminal domain (from amino acid 257). The mutation in which serine 364 is substituted for proline, which has been associated with some cases of cardiac congenital malformations in humans, also disrupts the pH gating of Cx43, although deletion of amino acids 364-373 has no effect on acidification-induced uncoupling. These results provide new insight into the molecular mechanisms responsible for acidification-induced uncoupling of gap junction channels in the heart and in other Cx43-expressing structures.

Intramolecular interactions mediate pH regulation of connexin43 channels.

We have previously proposed that acidification-induced regulation of the cardiac gap junction protein connexin43 (Cx43) may be modeled as a particle-receptor interaction between two separate domains of Cx43: the carboxyl terminal (acting as a particle), and a region including histidine 95 (acting as a receptor). Accordingly, intracellular acidification would lead to particle-receptor binding, thus closing the channel. A premise of the model is that the particle can bind its receptor, even if the particle is not covalently bound to the rest of the protein. The latter hypothesis was tested in antisense-injected Xenopus oocyte pairs coexpressing mRNA for a pH-insensitive Cx43 mutant truncated at amino acid 257 (i.e., M257) and mRNA coding for the carboxyl terminal region (residues 259-382). Intracellular pH (pHo) was recorded using the dextran form of the proton-sensitive dye seminaphthorhodafluor (SNARF). Junctional conductance (Gj) was measured with the dual voltage clamp technique. Wild-type Cx43 channels showed their characteristic pH sensitivity. M257 channels were not pH sensitive (pHo tested: 7.2 to 6.4). However, pH sensitivity was restored when the pH-insensitive channel (M257) was coexpressed with mRNA coding for the carboxyl terminal. Furthermore, coexpression of the carboxyl terminal of Cx43 enhanced the pH sensitivity of an otherwise less pH-sensitive connexin (Cx32). These data are consistent with a model of intramolecular interactions in which the carboxyl terminal acts as an independent domain that, under the appropriate conditions, binds to a separate region of the protein and closes the channel. These interactions may be direct (as in the ball-and-chain mechanism of voltage-dependent gating of potassium channels) or mediated through an intermediary molecule. The data further suggest that the region of Cx43 that acts as a receptor for the particle is conserved among connexins. A similar molecular mechanism may mediate chemical regulation of other channel proteins.

Lipoarabinomannans derived from different strains of Mycobacterium tuberculosis differentially stimulate the activation of NF-kappa B and KBF1 in murine macrophages.

The inflammatory cytokine tumor necrosis factor alpha (TNF-alpha) is rapidly induced in macrophages after exposure to Mycobacterium tuberculosis. Recently it was shown that lipoarabinomannan (LAM) derived from an attenuated (H37Ra) strain of M. tuberculosis (AraLAM) was capable of macrophage activation and induction of TNF-alpha production, whereas LAM derived from the virulent Erdman strain (ManLAM) was considerably reduced in this activity. A critical component in the regulation of many genes central to immune function is the transcription factor NF-kappa B. Lipopolysaccharide (LPS)-mediated induction of TNF-alpha expression in murine macrophages has been demonstrated to be regulated in part by NF-kappa B. In this study, we demonstrate that AraLAM is capable of rapid activation of NF-kappa B- and KBF1-binding activities in C3H/HeN bone marrow-derived macrophages and the J774.A and RAW264.7 murine macrophagelike cell lines, whereas ManLAM is considerably less potent at stimulating NF-kappa B. Treatment of RAW264.7 cells with AraLAM or LPS results in the stimulation of DNA binding of both forms within 7.5 min, which peaks within 30 min and 1 h, respectively. Interestingly, treatment of RAW264.7 macrophage-like cells with AraLAM, LPS, or ManLAM for greater than 2 h resulted in significant accumulation of KBF1. Inhibition of protein synthesis blocked the transient nature of NF-kappa B activation as well as the accumulation of KBF1. Using Western immunodetection of the NF kappa B1 p50 subunit, we also show that AraLAM and LPS stimulate the loss of the NF kappa B1 p105 precursor. These results demonstrate that NF-kappa B and KBF1 are rapidly induced in response to AraLAM and may play a role in avirulent M. tuberculosis activation of TNF-alpha expression in macrophages. The differential temporal regulation of kappa B element DNA-binding activities and the transient stimulation of NF kappa B followed by the sustained accumulation of KBF1 may serve as a feedback switch ensuring transient induction of TNF-alpha transcription.

Characterization of an E4031-sensitive potassium current in quiescent AT-1 cells.

INTRODUCTION: A cardiac culture cell line (AT-1) recently has been generated from transgenic mice. Initial studies have yielded opposing results as to the nature of the major repolarizing current(s) in these cells. The present study describes the ion selectivity, voltage dependence, and E4031 sensitivity of the major time-dependent outward current present in AT-1 cells. In addition, we have determined whether an outward current with the characteristics we observed could be capable of modulating action potential duration in a frequency-dependent manner (for stimulation cycle lengths between 250 and 1000 msec). METHODS AND RESULTS: Action potentials and membrane currents were recorded from nonconfluent cultures of quiescent AT-1 cells using the "perforated patch" technique. AT-1 cells showed a round appearance 1 or 2 days after plating. An E4031-insensitive transient outward current seemed to be absent in these cells. The main time-dependent outward current was a rapidly activating and rectifying potassium current with properties similar to those of IKr. Most of the potassium current was sensitive to the benzenesulfonamide E4031 (5 microM). The same concentration of E4031 led to a 38% increase in action potential duration. Action potential parameters were independent of the stimulation cycle length within the range of 250 to 1000 msec, thus suggesting that the membrane currents involved in the action potential of AT-1 cells are completely reset within a diastolic interval of approximately 150 msec. CONCLUSION: AT-1 cells present a unique electrophysiologic phenotype, which is clearly different from that reported for freshly dissociated adult atrial or ventricular myocytes from other species. AT-1 cells may be a good model to study IKr, since there seems to be minimal contamination by other outward conductances (such as IKs). In addition, the feasibility of culturing AT-1 cells provides us with a system where electrophysiologic experiments on IKr currents could be combined with biochemical or molecular biological studies requiring significant periods of incubation in a cell culture system.

Role of histidine 95 on pH gating of the cardiac gap junction protein connexin43.

We have studied the role of histidine 95 (H95) on the pH gating of the cardiac gap junction protein connexin43 (Cx43). Wild-type and mutant rat cardiac Cx43 channels were expressed in antisense-injected Xenopus oocytes. Junctional conductance was measured using the dual voltage-clamp technique, and intracellular acidification was induced by superfusion with a sodium acetate-containing solution balanced at a pH of 6.2. H95 was substituted by other amino acids by use of oligonucleotide-directed site-specific mutagenesis. Replacing H95 for the hydrophobic residues methionine or phenylalanine, for the charged basic residue arginine, or for the noncharged residue glutamine (H95Q) yielded nonfunctional channels. Functional expression of H95Q was rescued by placing a histidine residue in position 93 (H95Q-L93H), 94 (H95Q-A94H), or 97 (H95Q-F97H) but not in position 96. Further experiments showed that replacing H95 with either aspartate (an acidic residue) or tyrosine (a polar uncharged residue) led to the expression of functional channels with a reduced susceptibility to acidification-induced uncoupling, whereas lysine (a basic residue) was more susceptible to uncoupling than the wild-type protein. The susceptibility to acidification-induced uncoupling was enhanced for the H95Q-A94H mutant when compared with the wild-type mutant, but it was significantly reduced when histidine was placed at position 93 (H95Q-L93H). Our data indicate that a properly placed histidine residue is an important structural element for functional expression as well as for pH regulation of Cx43. The results suggest that the importance of H95 on pH gating may be associated with a possible protonation of this residue on acidification of the intracellular environment.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipopolysaccharide stimulates both nuclear localization of the nuclear factor kappa B 50-kDa subunit and loss of the 105-kDa precursor in RAW264 macrophage-like cells.

Nuclear factor kappa B (NF-kappa B) is an important regulator of gene expression in cells of the immune system. One such gene, tumor necrosis factor, is induced by bacterial lipopolysaccharide (LPS) in macrophages, and this induction has been shown to be mediated in part by NF-kappa B activation in murine macrophages. In this study, immunochemical analysis was used to follow LPS activation of the NF-kappa B 50-kDa subunit in the RAW264 macrophage-like cell line. The recombinant NF-kappa B 50-kDa subunit was used as an immunogen to produce a rabbit antiserum, which was then affinity-purified using a portion of the NF-kappa B 50-kDa subunit that does not have homology to other members of the c-rel gene family. Untreated macrophages had little NF-kappa B in the nucleus as detected by Western immunoblotting. The protein was predominantly localized in the cytoplasmic fraction. Interestingly, NF-kappa B was found as the 50-kDa mature protein and 105-kDa precursor. After LPS treatment, there was a rapid nuclear translocation of NF-kappa B as detected by immunoblot analysis. There was also a rapid decrease in the amount of the cytoplasmic 105-kDa protein. This may indicate that the 105-kDa protein is a reservoir for the 50-kDa protein and that one of the actions of LPS is to increase the rate of 105-kDa precursor processing.