From the Thomas C. Jenkins Department of Biophysics,
Johns Hopkins University, Baltimore, Maryland 21218, the
§ Department of Enzyme Technology, Gesellschaft für
Biotechnologische Forschung, Mascheroder Weg 1, 38124 Braunschweig,
Germany, and the
Department of Biochemistry, Yeungnam
University, 214-1 Daedong, Kyoungsan, Republic of Korea
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ABSTRACT |
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Intercellular connexin channels (gap junction channels) have long been thought to mediate molecular signaling between cells, but the nature of the signaling has been unclear. This study shows that connexin channels from native tissue have selective permeabilities, partially based on pore diameter, that discriminate among cytoplasmic second messenger molecules. Permeability was assessed by measurement of selective loss/retention of tracers from liposomes containing reconstituted connexin channels. The tracers employed were tritiated cyclic nucleotides and a series of oligomaltosaccharides derivatized with a small uncharged fluorescent moiety. The data define different size cut-off limits for permeability through homomeric connexin-32 channels and through heteromeric connexin-32/connexin-26 channels. Connexin-26 contributes to a narrowed pore. Both cAMP and cGMP were permeable through the homomeric connexin-32 channels. cAMP was permeable through only a fraction of the heteromeric channels. Surprisingly, cGMP was permeable through a substantially greater fraction of the heteromeric channels than was cAMP. The data suggest that isoform stoichiometry and/or arrangement within a connexin channel determines whether cyclic nucleotides can permeate, and which ones. This is the first evidence for connexin-specific selectivity among biological signaling molecules.
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INTRODUCTION |
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The molecular basis of the intercellular signaling mediated by gap junction channels is a key question in cell biology (cf. Ref. 1). Gap junction channels are formed by connexin protein, of which there are at least 15 isoforms (2, 3). Each junctional channel is composed of two anti-symmetric end-to-end hemichannels (connexons), which are hexamers of connexin (4-6). Recordings of single junctional channels show that each isoform generates channels with a characteristic "fingerprint" of conductances (7). A junctional channel typically has a maximal conductance and one or more subconductance states (cf. Ref. 8), the occupancy of which may be influenced by voltage (cf. Refs. 9 and 10) and/or phosphorylation (cf. Refs. 11-14). The number of connexin isoforms, their tissue-specific distributions, and their differential temporal expression suggest that the intercellular molecular signaling mediated by each isoform is unique. This is substantiated by the observation that genetic defects in each isoform produce distinct developmental and/or physiological defects (15, 16).
We report here investigation of molecular permeability of channels formed by connexin-32 and connexin-26. Defects in connexin-32 are responsible for a human demyelinating peripheral neuropathy (17). Suppression of connexin-26 function is associated with non-syndromic sensorineural human deafness (18). Both connexins seem to act as tumor suppressors (19-22).
Distinct isoform-specific channel conductances imply different pore permeation pathways, and therefore different molecular selectivities. Differences in molecular selectivity are likely to be responsible for key differences in molecular signaling. Understanding of intercellular signaling mediated by connexin channels will be greatly facilitated by discovery of the connexin-specific molecular selectivities and their physical bases.
Most comparative studies of connexin channel permeability have
documented junctional permeabilities either to ions (e.g., K+, Cl, NH4+)
much smaller than most biological signaling molecules, or to large,
charged fluorescent tracers (e.g. Lucifer
Yellow2
, DAPI2+) (cf. Refs.
23-26). For the former, the observed modest selectivities are
intriguing but may not reflect the selectivities among biological signaling molecules, which are larger and have more complex structures and chemistries. For the latter, it is difficult to distinguish the
roles played by size and charge in selectivity, and the consequent selectivity among cytoplasmic molecules must be inferred. The fact that
both ends of junctional pores are in cytoplasm has made it difficult to
obtain unambiguous and detailed in situ selectivity data.
A set of homologous uncharged fluorescent tracers of graded sizes was used to define the size selectivity (specifically independent of charge) of connexin channels purified from native tissue. The tracers were loaded into unilamellar liposomes, a significant fraction of which contained affinity-purified reconstituted functional connexin channels. By a transport-specific fractionation (TSF) assay (27-29), liposomes were separated into two populations: those with functional large channels and those without functional channels. Selective permeability among the tracers was determined by direct comparison of the tracers retained by the two populations of liposomes. The studies reported here used two connexin preparations, one containing channels of a single connexin isoform (Cx321; from rat liver), and one containing channels of mixed isoforms (Cx32/Cx26; from mouse liver). The results establish a connexin-specific difference in pore diameter and provide compelling evidence for heteromeric channels (channels composed of more than one isoform). These differences in selectivity were further explored using tritiated cyclic nucleotide tracers. The data show that connexin channel permeability to cyclic nucleotides, and selectivity among them, is influenced by isoform composition.
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EXPERIMENTAL PROCEDURES |
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Materials--
L--Phosphatidylcholine from
soybean (azolectin), egg phosphatidylcholine, bovine brain
phosphatidylserine, and lissamine rhodamine B-labeled egg PE were
purchased from Avanti Polar Lipids, Inc. Tween 20, nitro blue
tetrazolium, and diisopropyl fluorophosphate were obtained from Sigma.
Octylglucoside was from Calbiochem Corp. Bio-Gel (A-0.5m,
100-200-mesh, exclusion limit 500,000 Da) was purchased from Bio-Rad.
Alkaline phosphatase-conjugated goat anti-mouse IgG and
5-bromo-4-chloro-3-indolyl phosphate were purchased from Boehringer
Mannheim. CNBr-activated Sepharose beads were obtained from Pharmacia
Biotech Inc. and Immobilon-P transfer membrane from Millipore. Rats and
mice were obtained from Taconic Labs. [8,5
-3H]cGMP and
[2,8-3H]cAMP were obtained from NEN Life Science
Products.
Immunopurification of Connexin-- Connexin-32-containing structures were immunopurified from rodent livers by a published procedure (29). In brief, a crude membrane fraction was prepared from livers of ~3 75-100-g female Sprague-Dawley rats or ~16 10-12-week-old Swiss Webster fBR outbred mice. The crude membrane fraction was solubilized in 80 mM octylglucoside and the supernatant, following centrifugation at 100,000 × g for 45 min, was applied to a column containing Sepharose beads with an attached murine monoclonal antibody directed against Cx32 (M12.13), which does not cross-react with any other connexin (30). Following extensive rinsing, bound connexin was eluted from the antibody by brief exposure to pH 4 buffer. The eluent was rapidly neutralized by dropping directly into 1 M HEPES (pH 7.5). Final HEPES concentration was ~100 mM.
Gel Electrophoresis, Protein Blots, and Immunoblots-- These techniques were carried out as described in detail by Rhee et al. (29). Proteins were separated by discontinuous polyacrylamide gel electrophoresis (4% (w/v) stacking gel, 13% (w/v) separating gel) and electrotransferred to Immobilon polyvinylidene difluoride membrane. For staining of total protein, blots were blocked with 0.3% Tween 20 in PBS and stained with colloidal gold (0.01% (w/v)). For immunostaining, blots were blocked with PBS-Tween 20 (0.5% (v/v)) and then incubated with primary antibody. After washing with PBS-Tween 20 (0.05% (v/v)) blots were incubated with secondary antibody (alkaline phosphatase-conjugated goat anti-mouse IgG), washed, and developed in 0.1 mg/ml nitro blue tetrazolium and 0.05 mg/ml 5-bromo-4-chloro-3-indolyl phosphate.
Antibodies-- The monoclonal antibody used in the immunoaffinity purification and for specific staining of connexin-32 on Western blots (M12.13) is directed against a cytoplasmic domain of Cx32 (30). The antibody used to specifically detect Cx26 on Western blots was a sequence-specific polyclonal directed against the cytoplasmic loop of Cx26 (residues 108-122) (31). The antibody used to detect both connexins on Western blots was a sequence-specific polyclonal antibody directed against an extracellular domain (residues 116-185) conserved in Cx32 and Cx26 (32).
Purification of Plasma Membrane for Cross-linking Studies-- Plasma membranes were isolated from rat and mouse livers according to Goodenough (33) and Fallon and Goodenough (34). In brief, liver homogenate was pelleted at 11,000 × g and then rinsed and centrifuged at 6,000 × g to remove mitochondria and other small organelles. The final pellet was applied to a sucrose step gradient (55%, 50%, 45%, and 37% sucrose). Plasma membranes were collected at the 37%/45% interface and rinsed twice.
Cross-linking Studies-- For cross-linking of purified connexin, two lysine-specific cleavable cross-linking agents were used in combination: DSP, which is lipophilic, and DTSSP, which is water-soluble (Pierce). The cross-linkers were added to the connexin samples (which were ~10 mg/ml in 80 mM octylglucoside and 1 mg/ml phospholipid) from stock solutions (DSP in Me2SO, DTSSP in 0.1 M HEPES, pH 7.7) to final concentrations of 0.5-2 mM. Following incubation for 30 min at room temperature, the reaction was stopped by addition of one-sixth reaction volume of 0.5 M lysine in 0.1 M Tris, pH 8.2. Following 10 min of incubation, two-thirds volume of SDS-polyacrylamide gel electrophoresis sample buffer (without DTT) was added. To cleave the cross-linkers, sample buffer containing 100 mM DTT was used and the sample boiled prior to being loaded on the gel. The cross-linkers span a maximal distance of 12 Å.
Connexin in plasma membranes were cross-linked by addition of 0.1 ml of DSP (20 mM in Me2SO) and 0.1 ml of DTSSP (20 mM in 0.1 M HEPES, pH 7.7) to 2 ml of plasma membrane suspension at a protein concentration of 10 mg/ml, giving a final cross-linker concentration of 2 mM. Cross-linking proceeded for 45 min at room temperature, and was stopped as above. The membranes were then solubilized by addition of 1 volume of 160 mM octylglucoside in 50 mM sodium phosphate, 50 mM NaCl, 5 mM EDTA, 1 mMReconstitution of Purified Connexin into Unilamellar Phospholipid Liposomes-- Liposome formation and protein incorporation followed the protocol of Mimms et al. (35), as modified by Harris et al. (28) and Rhee et al. (29). Liposomes were formed by gel filtration of a 1 mg/ml mixture of phosphatidylcholine, phosphatidylserine, and rhodamine-labeled phosphatidylethanolamine at a molar ratio of 2:1:0.03 in urea buffer (see below) containing 80 mM octylglucoside and immunoaffinity-purified connexin. The protein-lipid-detergent mixture was applied to a Bio-Gel A-0.5m column pretreated with sonicated liposomes. The connexin-containing liposomes were collected in the void volume. The size distribution of liposomes was established by filtration over a calibrated TSK G6000PW HPLC column (36) to be highly monodisperse with an approximate mean diameter of 900 Å. The protein/lipid ratio of the liposomes was approximately 1:60 (w/w), corresponding to an amount of connexin equivalent to ~1 hemichannel/liposome. We intentionally worked at a protein/lipid ratio that produced some liposomes that contained functional channels and some that did not (for use as internal controls for tracer trapping and nonspecific leak).
Loading of Liposomes with Tracers-- Tracers were loaded into the liposomes by incubation with 5% (v/v) Me2SO, followed by removal of Me2SO and untrapped tracers by gel filtration. Control experiments showed that this treatment did not affect connexin channel activity or the behavior of liposomes in the TSF assay. A mixture of the oligosaccharide tracers was loaded into the liposomes at a nominal concentration of 10 mg/ml. Radiolabeled nucleotide tracers were dried from 50% ethanol (v/v) stocks under argon before loading into liposomes at a nominal concentration of 16 µM.
Transport-specific Fractionation (TSF)-- The procedure used to fractionate liposomes into two populations based on sucrose-permeability is described and fully characterized by Harris et al. (27, 28) and Rhee et al. (29). The principle of using a density shift to fractionate liposomes was adapted from Goldin and Rhoden (37). Linear iso-osmolar density gradients were formed from urea and sucrose buffers in 4.4-ml ultracentrifuge tubes using a Hoefer gradient maker. Urea buffer contained 10 mM KCl, 10 mM HEPES, 0.1 mM EDTA, 0.1 mM EGTA, 3 mM sodium azide, and 459 mM urea at pH 7.6. Sucrose buffer was identical to the urea buffer, except that an osmotically equivalent concentration of sucrose (400 mM) was substituted for the urea. Osmolality of urea and sucrose buffers was 500 mOsm/kg, and their specific gravities (D420) were measured to be 1.0056 and 1.0511, respectively, by refractive index and gravimetric methods.
An aliquot of the liposomes (typically 200 µl) was layered on each gradient. Gradients were typically centrifuged at 300,000 × g for 3-5 h in a swinging bucket rotor (Sorvall TST 60.4, DuPont) at 4 °C. Liposome bands were recovered by aspiration. The distribution of the liposomes in the gradient was calculated from the specific intensity of rhodamine fluorescence (Perkin-Elmer 650-10S spectrofluorometer; 560 nm excitation and 590 nm emission) and the volume of each collected band. During the centrifugation, liposomes not permeable to sucrose move into the gradient a short distance, being buoyed by the (lighter) entrapped urea buffer and form a band in the upper part of the gradient. Permeable liposomes continuously equilibrate their internal solution with the external solution, and move to a position in the lower part of the gradient corresponding to the density of the liposome phospholipid membrane. Equilibration of extraliposomal and intraliposomal osmolytes through pores the dimensions of hemichannels is very rapid (<1 ms for these liposomes). Therefore, even a channel that opens infrequently will mediate full exchange of osmolytes and cause liposome movement to the characteristic lower position. The protein/lipid ratios used yielded functional channels in 30-50% of the liposomes.Oligosaccharide Tracers--
A maltooligosaccharide mixture
consisting chiefly of maltose ((1
4)Glc2) through
maltopentaose (
(1
4)Glc5) was fluorescently labeled by
pyridylamination according to Hase et al. (38). The fluorescent adducts are labeled at the reducing ends of the
oligosaccharides and result in linearization of the reducing-end
glucose. Stocks were frozen for later use after HPLC chromatograms were
run to verify purity.
Recovery of PA-labeled Sugars from Liposomes-- Aliquots from bands of vesicles recovered from spun density gradients were diluted 1:1 with HPLC-grade methanol and vortexed to lyse vesicles and disperse lipid as micelles. The mixture was slowly passed through a tC18 Sep-Pak Vac solid phase extraction cartridge (Waters Corp.) pre-equilibrated in methanol, followed by 50% methanol in water to remove lipid. The eluent containing the sugars was next incubated for 30 min with vortexing at room temperature with 50 mg of H-form AG 50W-X2 biotechnology grade cation exchange resin (Bio-Rad) to bind the PA-sugars (positively charged at low pH of resin) as well as other cations. The resin was transferred to a small fritted column and washed with water. Material was eluted from the resin by treatment with 2.5 M ammonium hydroxide for 1 min with agitation, followed by a second wash in ammonia and vacuum recovery of the sample. The samples were speed-vacuumed to dryness and brought up in water for fluorescence quantitation.
Measurement of Tracers-- The PA-sugars were quantitatively analyzed on a normal-phase amide-80 HPLC column by monitoring PA fluorescence. 2-PA, 3-PA, 4-PA, and 5-PA (abbreviation: n-PA, where n is the number of saccharide units) were eluted at 6.8, 8.3, 10.6, and 13.5 min, respectively, with a 70% to 60% gradient of acetonitrile in 3% aqueous triethylammonium acetate, pH 7.3 (modified from Ref. 38). Quantitation was by performed by calculation of areas under Gaussians simultaneously fit to eluted peaks. The mole fractions of the entrapped PA tracers were as follows: 2-PA, 0.18; 3-PA, 0.35; 4-PA, 0.16; 5-PA, 0.31.
To determine loss of radiolabeled tracers, the [3H activity]/[rhodamine fluorescence] ratios were measured for the upper and lower TSF bands in each experiment, and normalized to give the fractional retention of tracers in the lower band relative to that of the upper band. Liposome bands were recovered in approximately 0.75 ml, added to 5 ml of liquid scintillation mixture (Ultima Gold, Packard), thoroughly mixed by vortexing, and the 3H activity measured in either a Packard TriCarb model 2250CA or model 2700TR liquid scintillation counter (counting window, 0-18.6 keV). Blank controls of 0.75 ml of aqueous buffer and 5 ml of mixture were counted between samples. Counting times were 30 min. The minimum detectable activity, the net signal that can be detected at the 95% confidence level, is approximately given by ![]() |
RESULTS |
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Co-purification of Connexin-32 and Connexin-26-- Connexin was immunoaffinity-purified from octylglucoside-solubilized plasma membrane of rat or mouse liver using a Cx32-specific monoclonal antibody (29). Gel filtration studies showed that the purified connexin was predominantly in structures the size of single hemichannels (hexamers of connexin) (29). The liposomes were unilamellar (35), and, for a given protein/lipid ratio, the number of permeable liposomes was too great for the functional channels to be formed by dodecamers (29). We infer that the permeabilities described below are those of hemichannel-sized connexin structures (there is strong evidence that connexin hemichannels function in the plasma membrane of cells that express connexin endogenously (40-42) or heterologously (cf. Refs. 43-48)). Rat liver gap junctions contain predominantly Cx32 and a small amount of Cx26. Mouse liver junctions contain a much higher ratio of Cx26 to Cx32 (49, 50). Both connexins can be found in the same junctional plaques (32, 50-54).
Applied to rat liver, this procedure yielded Cx32 without detectable non-connexin protein (Fig. 1A). In some purifications, a small amount of Cx26 was recovered. In this study, only rat liver connexin preparations (ratCx) that contained no detectable Cx26 were used.
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Transport-specific Fractionation (TSF)-- The molecular selectivities of channels formed by ratCx and mouseCx were explored and compared by a tracer-flux technique used in combination with transport-specific liposome fractionation. Purified connexin was incorporated into unilamellar liposomes by gel filtration of octylglucoside-solubilized lipid and connexin (28, 35). Liposomes containing functional channels were separated from liposomes without functional channels by TSF achieved by centrifugation through an iso-osmotic density gradient formed by urea and sucrose solutions (see "Experimental Procedures") (27, 28). In this assay, liposomes that do not contain functional channels (i.e. are not permeable to the density-conferring osmolytes of the gradient, urea and sucrose) migrate to an equilibrium position in the upper part of the gradient, determined by their density. This density is a volume-weighted sum of the densities of the entrapped urea buffer and the more dense phospholipid membrane. For liposomes that contain large open pores, the osmolytes exchange across the liposome membrane through the open channels. These liposomes migrate to a lower equilibrium position, determined by lipid density.
Limiting Pore Diameters-- The limiting dimensions of the channel pores were examined using a series of oligomaltosaccharides (two to five glucose units) derivatized with an aminopyridyl group (PA), a small, highly fluorescent moiety uncharged at physiological pH. X-ray studies, solution NMR studies, and computer modeling show that, with increasing oligomeric number, these compounds form rigid helical structures with a full turn requiring at least six saccharide units (60-62). Thus, the axial cross-sectional areas of this series of tracers increase with oligomeric number (Fig. 2).
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Permeability to Cyclic Nucleotides-- Using radiolabeled tracers, permeabilities to two cyclic monophosphate nucleotides were compared. Tritiated cAMP or cGMP was loaded into the liposomes, and their permeabilities through the reconstituted channels determined as for the PA sugars. As before, tracer retained by liposomes without functional channels served as an internal control. For homomeric Cx32 liposomes, no cAMP or cGMP label above background levels was retained by liposomes with functional channels (Fig. 4A). This demonstrates permeability of cAMP and cGMP through Cx32 channels. However, for the heteromeric Cx32/26 channels, the liposome population with functional channels lost 26% of the entrapped cAMP, and 72% of the entrapped cGMP (Fig. 4B).
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DISCUSSION |
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Different connexin isoforms have been shown to intermingle within a junctional plaque (53, 54, 64). Formation of heteromeric junctional channels seems firmly established from biochemical and physiological studies on native (65, 66) and transfected cells (67-69). Scanning electron microscopic measurements on purified rodent hemichannel plaques failed to reveal hemichannel masses indicative of heteromeric structures (53), but they may have been masked to some extent by statistical variation in the data.
The cross-linking data presented here support the existence of heteromeric hemichannels in native tissue. Lateral interhemichannel linkages within the same plasma membrane are unlikely except under maximal cross-linking conditions (6) because, even in condensed junctions, the interchannel spacing is wider than the cross-linker length (12 Å) (70-72). Moreover, lateral collisional cross-linking in other membrane protein arrays is rare (73-75). It is possible that interhemichannel linkages could form across the intercellular "gap" if the cross-linker could penetrate this intercellular space. The "gap" is known to be penetrated only by heavy metal stains and sucrose, which are smaller than the cross-linkers (5, 76). Additionally, due to the relative surface areas available for interaction, under mild cross-linking conditions intrahemichannel linkages would be favored over interhemichannel linkages (77).
Junctional channels generally behave as two hemichannels in series from data where properties of hemichannels are directly determined and compared with those of the corresponding junctional channels (45, 46, 48). However, the voltage-dependent gating behavior of heterotypic junctional channels cannot always be predicted from the hemichannel properties inferred from studies of homotypic junctional channels (78). There are no data that address whether hemichannel selectivities are altered when hemichannels form junctional channels. The fluorescent tracers developed for the present study can likely be used in conjunction with maltase inhibitors in cellular studies to directly address this issue.
Inspection of the PA-sugar structures (Fig. 2) shows that with increasing oligomeric number the axial cross-section becomes more circular. Therefore, the impermeability of a tracer may not necessarily be due to a larger cross-sectional major axis, but due to a larger minor axis. Additionally, because of structural differences between the two isoforms, the packing of connexin monomers within heteromeric channels may not be radially symmetric, and may contribute to the selectivities described here. Therefore, the molecular selectivity of homomeric Cx26 channels cannot be inferred; homomeric Cx26 channels may not necessarily have narrower pores than homomeric Cx32 channels. Molecular flexibility, hydration, and other factors in permeation prevent a precise determination of the cross-sectional dimensions of the pores from these studies. However, the differential permeability of the uncharged tracers does serve as an index of relative pore diameter and permits comparison with other potential permeants and other connexin channel pores.
The permeability studies with the PA-sugars do not permit discrimination between a localized size selectivity filter and a decrease in diameter along the entire pore length. The unitary conductance of Cx32 homomeric channels is less than that of homomeric Cx26 channels (79, 80). If the pores were right cylinders, the data would suggest that the Cx26-induced narrowing is localized, and not along the full length of the pore (i.e. the structure that produces the size selectivity does not dominate the pore conductance). However, there are data suggesting that connexin pores are not simple right cylinders (23). If selectivity among charged permeants arises from straightforward lumenal surface charge considerations (as may be the case for voltage-dependent anion channel; Ref. 81), a narrowed pore would enhance charge selectivity, due to closer approach of charged residues in the pore by permeants.
Cellular and liposome studies are consistent with the data obtained
here for uncharged tracers. Data from HeLa cells expressing single
murine connexin isoforms probed with several nonhomologous charged
tracers (Lucifer Yellow2, DAPI2+,
neurobiotin
+, propidium2+,
ethidium+) can be explained by Cx26 having an uncharged
pore, and Cx32 having a cationic pore the same size or larger (25).
Other studies involving differential permeability through Cx26 and Cx32
channels support the idea of a cationic (i.e.
anion-selective) Cx32 channel, but do not bear directly on limiting
diameter (82, 83). A smaller diameter for Cx26 channels relative to
that of Cx32 channels is supported by comparative data using Lucifer
Yellow (83). Studies using a different type of permeability assay and
different uncharged probes (raffinose, Nycodenz, metrizamide) supports
Cx26 contributing to a narrowed pore (84, 85). Elucidation of the structural basis for the difference in limiting dimension must await
identification of the pore-lining domains of connexin channels. Recent
data suggest that the first transmembrane domain contributes to the
pore lumen (86, 87), but inspection of the relevant sequences is not
overtly informative as to the basis of the difference in
selectivity.
From a biophysical perspective, it will be satisfying to obtain complementary selectivity data for homomeric Cx26 channels. A focus of the present study was to investigate the molecular selectivity (and thereby, the signaling properties) of connexin channels found in native tissue. It is possible that homomeric Cx26 could be purified from murine liver by a Cx26-specific antibody or from transfected cells (88).
Gap junction permeability to cAMP is strongly suspected in several cellular systems (89-91). The connexins involved in those systems have not been positively identified, but the connexins studied here are not among the likely candidates (which are Cx35, Cx37, and Cx43). This is the first study to directly assess permeability to cyclic nucleotides in channels formed by Cx32 or Cx32/Cx26. The cAMP- and cGMP-permeable heteromeric channels may compose separate populations. On the other hand, some cAMP-permeable heteromeric channels may also be permeable to cGMP (i.e. the cAMP-permeable channels may be a subset, in whole or in part, of the cGMP-permeable channels). The data do not distinguish between these possibilities.
It would be desirable to have a quantitative model that accounts for the relation between Cx26:Cx32 ratio and cyclic nucleotide permeability. The binomial distribution was used to calculate a possible distribution of channel stoichiometries. The corresponding permeabilities were predicted assuming that (a) stoichiometry alone controls permeability, and (b) the relation between stoichiometry and change in permeability is linear. The prediction did not obviously match the relations shown in Fig. 5, indicating that the simple assumptions above are invalid. Further analysis of this system will require explicit determination of the distribution of stoichiometries in a channel population. It is surprising that such a small change in the proportion of Cx26 present (8%) generates such a significant change in the number of channels permeable to cGMP (25%).
The narrow range of Cx26:Cx32 ratios in the purified protein may be because each purification of connexin from mouse liver used tissue from 16 animals, possibly averaging over substantial individual variations in connexin-26 content. The use of channels from native tissues, although helpful in establishing the properties of in situ channels, places constraints on the range of available isoform ratios. These studies could be extended by the use of heterologous co-expression systems, using fewer animals per preparation (i.e. obtaining greater preparation-to-preparation variation in Cx26:Cx32 ratio), or by preparative fractionation of purified channels on the basis of subunit stoichiometry.
The nonselective permeability of cAMP and cGMP through homomeric Cx32 channels is consistent with their wider pore dimensions established by the PA-sugar studies. Similarly, the impermeability of the nucleotides through some of the heteromeric channels does not require any basis other than a narrower pore, also established by the PA-sugar studies. The basis of the selectivity among these nucleotides by heteromeric channels, however, is not likely to be size alone (the size difference between cAMP and cGMP is minimal). The nucleotides have the same net charge, but the charge contours of the purine ring systems differ. Additionally, cAMP can form two hydrogen bonds while cGMP can form three. The selectivity is likely to derive from the different chemistries of the molecules, suggesting the presence of selective binding sites for signaling molecules within connexin pores.
The isoform-dependent molecular selectivity of connexin channels in native tissues demonstrated here, particularly that among second messengers, may be physiologically important. Differential regulation of Cx26 and Cx32 expression occurs in regenerating and differentiating liver, in hepatic tumorigenesis, and in cellular growth control (20, 92-97). The data reported here suggest that dynamic regulation of junctional channel composition in vivo could effectively and efficiently modulate intercellular signaling mediated by cyclic nucleotides.
Altered or disrupted permeability of cyclic nucleotides through connexin channels is likely to have important consequences. Cx32-deficient mice have increased rates of spontaneous liver tumors and greater sensitivity to carcinogens (22). In humans, genetic defects in Cx32 cause the X-linked form of Charcot-Marie-Tooth disease (a peripheral demyelinating neuropathy) (98). Intriguingly, it has been shown recently that the full disease phenotype can be caused by a point mutation in Cx32 whose only observable effect is to narrow the pore, perhaps sufficiently to exclude cyclic nucleotides, while remaining permeable to salts (87). The distinct roles of each connexin in developmental and pathological processes may arise from differences in molecular signaling mediated, in part, by the selectivities measured and described here.
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ACKNOWLEDGEMENTS |
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We thank Yuan-Chuan Lee and Jian-Qian Fan for their crucial assistance in the synthesis and HPLC analysis of the PA-sugar tracers, and William H. Biggley for assistance with high accuracy nuclear counting of 3H-labeled tracers. We also thank Daniel Goodenough and David Paul for providing the hybridoma secreting the Cx32-specific monoclonal antibody, and for providing the Cx26-specific affinity-purified antiserum, and Bruce Nicholson for providing affinity-purified antiserum that recognized both Cx32 and Cx26. Use and care of animals was according to institutional guidelines.
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FOOTNOTES |
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* This work was supported in part by National Institutes of Health Grant GM36044 and Office of Naval Research Grant N00014-90-J-1960 (to A. L. H.)The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
¶ Present address: Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie, 53175 Bonn, Germany.
** To whom correspondence should be addressed.
1
The abbreviations used are: Cx, connexin;
(1
4)Glcn,
(1
4)-linked glucose linear oligomer,
where n is the number of glucose units; DSP,
dithiobis(succinimidyl propionate); DTSSP, dithiobis(sulfosuccinimidyl
propionate); DTT, dithiothreitol; HPLC, high performance liquid
chromatography; mouseCx, connexin immunopurified from mouse liver
(Cx32/Cx26); PA; aminopyridyl moiety; PA-sugar; oligomaltose labeled
with an aminopyridyl group; n-PA, PA-sugar where
n is the number of saccharide units; PBS, phosphate-buffered
saline; ratCx, connexin immunopurified from rat liver (Cx32); TSF,
transport-specific fractionation of liposomes.
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REFERENCES |
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