Functional analysis of selective interactions among rodent connexins

One consequence of the diversity in gap junction structural proteins is that cells expressing different connexins may come into contact and form intercellular channels that are mixed in connexin content. We have systematically examined the ability of adjacent cells expressing different connexins to communicate, and found that all connexins exhibit specificity in their interactions. Two extreme examples of selectivity were observed. Connexin40 (Cx40) was highly restricted in its ability to make heterotypic channels, functionally interacting with Cx37, but failing to do so when paired with Cx26, Cx32, Cx43, Cx46, and Cx50. In contrast, Cx46 interacted well with all connexins tested except Cx40. To explore the molecular basis of connexin compatibility and voltage gating, we utilized a chimera consisting of Cx32 from the N-terminus to the second transmembrane domain, fused to Cx43 from the middle cytoplasmic loop to the C-terminus. The chimeric connexin behaved like Cx43 with regard to selectivity and like Cx32 with regard to voltage dependence. Taken together, these results demonstrate that the second but not the first extracellular domain affects compatibility, whereas voltage gating is strongly influenced by sequences between the N-terminus and the second transmembrane domain.

Expression of a dominant negative inhibitor of intercellular communication in the early Xenopus embryo causes delamination and extrusion of cells

A chimeric construct, termed 3243H7, composed of fused portions of the rat gap junction proteins connexin32 (Cx32) and connexin43 (Cx43) has been shown to have selective dominant inhibitory activity when tested in the Xenopus oocyte pair system. Co-injection of mRNA coding for 3243H7 together with mRNAs coding for Cx32 or Cx43 completely blocked the development of channel conductances, while the construct was ineffective at blocking intercellular channel assembly when coinjected with rat connexin37 (Cx37). Injection of 3243H7 into the right anterodorsal blastomere of 8-cell-stage Xenopus embryos resulted in disadhesion and delamination of the resultant clone of cells evident by embryonic stage 8; a substantial number, although not all, of the progeny of the injected cell were eliminated from the embryo by stage 12. A second construct, 3243H8, differing from 3243H7 in the relative position of the middle splice, had no dominant negative activity in the oocyte pair assay, nor any detectable effects on Xenopus development, even when injected at four-fold higher concentrations. The 3243H7-induced embryonic defects could be rescued by coinjection of Cx37 with 3243H7. A blastomere reaggregation assay was used to demonstrate that a depression of dye-transfer could be detected in 3243H7-injected cells as early as stage 7; Lucifer yellow injections into single cells also demonstrated that injection of 3243H7 resulted in a block of intercellular communication. These experiments indicate that maintenance of embryonic cell adhesion with concomitant positional information requires gap junction-mediated intercellular communication.

Intercellular channels in teleosts: functional characterization of two connexins from Atlantic croaker

Gap junction channels, composed of protein subunits termed connexins, are believed to play a critical role in the process of oocyte differentiation and maturation. We have used the paired Xenopus oocyte assay to characterize functionally two connexin genes, connexin-32.2 and connexin-32.7, recently cloned from the ovary of the Atlantic croaker (Micropogonia undulatus), a species that has emerged as a useful model to study the process of maturation of the ovarian follicle. We have found that, while both connexin proteins were expressed at comparable levels in Xenopus oocytes, only one, connexin-32.2, was functionally competent to induce the formation of intercellular channels. Connexin-32.2 channels exhibited voltage-dependent closure that was similar to, but distinct from that of previously characterized mammalian connexins. In addition, the silent connexin-32.7 was unable to functionally interact with connexin-32.2, either in heterotypic channels or as dominant negative inhibitor. Because connexin-32.2 expression is strikingly regulated during oocyte maturation, these data provide further evidence for a role of intercellular channels in the control of oocyte-follicular cell interactions.

Cytogenetic clonality in chronic myelomonocytic leukemia studied with fluorescence in situ hybridization

Chronic myelomonocytic leukemia (CMML) is a myelodysplastic syndrome (MDS) subtype, characterized by monocytosis, dysgranulocytosis and a low number of blast cells in the peripheral blood (PB). The clonal nature of MDS has been demonstrated by various techniques: the stem cell involved initially is capable of myeloid and lymphoid differentiation. Fluorescent in situ hybridization (FISH) is a technique which can be utilized without any pretreatment on whole interphase cells. In this study leukocytes of PB Wright-stained smears from four CMML patients with trisomy 8 (three cases) and 9 (one case) have been analyzed by FISH. Utilizing a probe for the centromere of chromosome 8 and for the heterochromatic region of chromosome 9, we observed the cells involved by trisomy. In each of the four cases neutrophils, eosinophils, basophils and monocytes may show trisomy 8 or 9, whereas lymphocytes resulted disomic. The comparison between leukocytes morphology and genotype suggests that the supernumerary chromosome does not influence cellular differentiation and maturation. We conclude that FISH analysis of PB leukocytes of patients with CMML is informative when studying the clonality of the disease. Chromosomal abnormalities seem to involve a hematopoietic cell committed to myeloid but not lymphoid differentiation. Trisomies 8 and 9 seem to confer some proliferative advantage without influencing the morphologic characteristics of leukocytes. Other causes will be investigated to explain dysmorphisms of neutrophils and monocytes typical of this disease.

Clonality study by fluorescence in situ hybridization of a patient with refractory anemia with ringed sideroblasts and monosomy 7

Myelodysplastic syndromes (MDS) are stem cell diseases but it is still controversial whether chromosomal abnormalities occurring in these disorders affect a multipotent stem cell or a committed progenitor. We studied a case of refractory anemia with ringed sideroblasts (RARS) and monosomy 7 in 100% of examined metaphases. Using the fluorescence in situ hybridization (FISH) technique with a probe specific for the centromeric region of chromosome 7, we demonstrated that 15% of BM cells fixed in acetic acid/methanol exhibited a normal diploid karyotype. Applying the FISH technique on PB cells smeared onto a slide, we observed that lymphocytes maintain two chromosomes 7, whereas other leukocytes exhibited monosomy 7. Our study confirms that chromosomal abnormalities found in MDS can occur in cells capable of differentiation along granulocytic and monocytic lineages, but not along the lymphocytic lineage.

Chromosome derived from translocation(1;17) retains alphoid sequences of both chromosomes involved

Examining a bone marrow (BM) karyotype of a patient with a refractory anemia with excess of blasts (RAEB), we detected a clone with an unbalanced translocation(1;17), resulting in monosomy of 17p and trisomy of 1q. Using fluorescence in situ hybridization (FISH) technique with alpha-satellite DNA probes specific for chromosomes 1 and 17, we observed that the chromosome derived from the translocation shows hybridization signals for both the centromeres of chromosomes 1 and 17. This finding suggests that the breakpoints of the two autosomes involved in the rearrangement occurred in the primary constriction. The case confirms the ability of ISH analysis to detect structural rearrangements in cancer cytogenetics.

Null mutations of connexin32 in patients with X-linked Charcot-Marie-Tooth disease

The X-linked form of Charcot-Marie-Tooth disease (CMTX) is associated with mutations in the gene encoding connexin32, a member of the family of proteins forming intercellular channels. We have compared the functional properties of three mutant connexin32 genes with those of the wild-type gene by testing their ability to form intercellular channels in the paired oocyte expression system. Whereas wild-type connexin32 induced the development of large junctional conductance between paired oocytes, no functional channels were detected between pairs expressing CMTX mutants. Furthermore, CMTX mutants selectively acted as dominant inhibitors of intercellular communication by interfering with the channel-forming ability of connexin26 but not with that of connexin40. These results demonstrate a functional loss in the product of a candidate gene for a demyelinating form of CMT.

Selective interactions among the multiple connexin proteins expressed in the vertebrate lens: the second extracellular domain is a determinant of compatibility between connexins

Gap junctions are collections of intercellular channels composed of structural proteins called connexins (Cx). We have examined the functional interactions of the three rodent connexins present in the lens, Cx43, Cx46, and Cx50, by expressing them in paired Xenopus oocytes. Homotypic channels containing Cx43, Cx46, or Cx50 all developed high conductance. heterotypic channels composed of Cx46 paired with either Cx43 or Cx50 were also well coupled, whereas Cx50 did not form functional channels with Cx43. We also examined the functional response of homotypic and heterotypic channels to transjunctional voltage and cytoplasmic acidification. We show that all lens connexins exhibited sensitivity to cytoplasmic acidification as well as to voltage, and that voltage-dependent closure of heterotypic channels for a given connexin was dramatically influenced by its partner connexins in the adjacent cell. Based on the observation that Cx43 can discriminate between Cx46 and Cx50, we investigated the molecular determinants that specify compatibility by constructing chimeric connexins from portions of Cx46 and Cx50 and testing them for their ability to form channels with Cx43. When the second extracellular (E2) domain in Cx46 was replaced with the E2 of Cx50, the resulting chimera could no longer form heterotypic channels with Cx43. A reciprocal chimera, where the E2 of Cx46 was inserted into Cx50, acquired the ability to functionally interact with Cx43. Together, these results demonstrate that formation of intercellular channels is a selective process dependent on the identity of the connexins expressed in adjacent cells, and that the second extracellular domain is a determinant of heterotypic compatibility between connexins.

Expression of chimeric connexins reveals new properties of the formation and gating behavior of gap junction channels

Direct intercellular communication occurs through specialized channels, which are formed by the interaction of two half-channels, or connexons, contributed by each of the two participating cells. The ability to establish intercellular communication is specified, in part, by the expression of different structural proteins, termed connexins. Connexins can control the establishment of intercellular communication by selectively pairing with some but not other family members. To characterize the protein domains that allow connexins to recognize and discriminate between alternative partners, we have created chimeras composed of selected regions of rat connexin43, which forms channels with Xenopus connexin38, and rat connexin32, which cannot. Pairs of Xenopus oocytes were used to test the ability of the chimeras to form homotypic channels with themselves, and heterotypic channels with the parent connexins or with endogenous Xenopus connexin38. While all hybrid molecules tested were efficiently expressed by oocytes, most were devoid of functional activity. A chimera consisting of connexin32 from the N terminus to the second transmembrane domain, fused to connexin43 from the middle cytoplasmic loop to the C terminus, designated as 3243H4, was able to pair functionally with Xenopus connexin38 and one of its parent connexins, connexin43. Voltage-dependent closure of heterotypic channels containing 3243H4 was asymmetric, exhibited novel characteristics that were not predicted by the behavior of the parent connexins and was dependent on the type of connexin with which 3243H4 was paired. In contrast, 3243H4 was unable to form functional channels with either itself or the other parent, connexin32. Together, these results suggest that these connexins are not composed of functionally exchangeable regions and that multiple domains, namely the middle cytoplasmic portion and the second extracellular domain, can influence the interactions between connexins present in adjacent cells. Furthermore, they indicate that voltage gating is not strictly intrinsic behavior for a given connexin, but can be modulated by the partner connexins to which they are paired. Finally, the finding that 3243H4 is functional only in heterotypic configurations, and cannot form homotypic channels, suggests the existence of a novel form of selectivity: self-discrimination. The latter property may represent another mechanism that operates to control the extent of communication between cells.

Variant Philadelphia chromosome translocations are frequently associated with additional structural abnormalities

This work contains a cytogenetic analysis of 507 consecutive CML patients examined at diagnosis before any therapeutic treatment. Philadelphia chromosome translocations different from the standard t(9;22)(q34;q11) were observed in 28 patients (5.5%). Structural chromosomal abnormalities apparently unrelated to the Ph were found in six patients carrying variant Ph (6 of 28 = 21.4%) and in three patients carrying standard Ph translocations (3 of 472 = 0.6%). This finding confirms that structural abnormalities other than Ph are a rare event at diagnosis but that they occur with significantly different frequencies between the variant and standard Ph translocations, indicating that causes favoring the variant Ph formation may promote additional non-lethal DNA breakpoints.

Trisomy 8 detection in Ph+ CML patients using conventional cytogenetic and interphase fluorescence in situ hybridization techniques

We examined bone marrow (BM) cells from 6 Philadelphia chromosome positive (Ph+) chronic myeloid leukemia (CML) patients in advanced phase of the disease using conventional cytogenetic techniques and fluorescence in situ hybridization (FISH) for detection of an extra chromosome 8. All patients showed mosaicism for trisomy 8 as a secondary chromosome abnormality. For FISH, we used the D8Z5 probe specific for the centromeric region of chromosome 8 and analyzed 300 interphase nuclei and a variable number of mitoses for each patient. The percentages of metaphases carrying trisomy 8 were similar with both techniques, whereas the percentage of interphase nuclei showing three hybridization spots indicative of trisomy 8 was significantly lower than that in metaphases. This finding suggests that cells with a supernumerary chromosome 8 may have a cell cycle time shorter than that of disomic cells.

Molecular analysis of six variant Philadelphia chromosome translocations in chronic myeloid leukemia

In 420 Philadelphia positive (Ph+) chronic myeloid leukemia (CML) patients karyotyped at diagnosis in our laboratory, 26 Ph variants (6.2%) were observed. Twelve of them are reported. Five cases are "simple" variants without detectable involvement of band 9q34, and seven are "complex," since a third chromosomal band is involved in the Ph formation. Two translocations [t(7;22)(q36;q11) and t(9;22;12)(q34;q11;q11)] are reported for the first time. Six cases were characterized molecularly, and bcr-abl rearrangement was demonstrated, confirming involvement of 9q34 band also in the cases in which chromosomes 9 appear cytogenically normal. Chimeric mRNAs in which M-BCR exon 3 is joined to abl exon 2 (type b3-a2) were detected in four of six cases; one case showed a DNA breakpoint in zone III, which may also give rise to the same transcript. In one case, mRNA junction was b2-a2. The frequency of the b3-a2 junction occurs more frequently in CML patients with a Ph variant than in patients with the standard translocation, suggesting a preferential correlation between this type of transcript and the involvement of other chromosomes in Ph formation.

Connexin40, a component of gap junctions in vascular endothelium, is restricted in its ability to interact with other connexins

The cellular distribution of connexin40 (Cx40), a newly cloned gap junction structural protein, was examined by immunofluorescence microscopy using two different specific anti-peptide antibodies. Cx40 was detected in the endothelium of muscular as well as elastic arteries in a punctate pattern consistent with the known distribution of gap junctions. However, it was not detected in other cells of the vascular wall. By contrast, Cx43, another connexin present in the cardiovascular system, was not detected in endothelial cells of muscular arteries but was abundant in the myocardium and aortic smooth muscle. We have tested the ability of these connexins to interact functionally. Cx40 was functionally expressed in pairs of Xenopus oocytes and induced the formation of intercellular channels with unique voltage dependence. Unexpectedly, communication did not occur when oocytes expressing Cx40 were paired with those expressing Cx43, although each could interact with a different connexin, Cx37, to form gap junction channels in paired oocytes. These findings indicate that establishment of intercellular communication can be spatially regulated by the selective expression of different connexins and suggest a mechanism that may operate to control the extent of communication between cells.

Cytogenetic analysis of hemopoietic peripheral blood cells collected by leukapheresis after intensive chemotherapy in advanced phase Philadelphia-positive chronic myelogenous leukemia

Peripheral hemopoietic blood cells previously collected by leukapheresis were reinfused in advanced phase Philadelphia (Ph)-positive chronic myelogenous leukemic patients to promote the recovery of bone marrow function after intensive radiochemotherapy. Cytogenetic analysis was performed on these cells, induced to proliferate and to be mobilized by a first administration of marrow toxic drugs and collected when the white blood cell count was very low. In five patients only Ph-negative apparently normal cells were found. In five cases different proportions of Ph+/Ph- cells were observed and in the remaining five cases only Ph+ cells were present. Chromosomal abnormalities other than Ph, not detected in the cytogenetic analysis performed on bone marrow cells before chemotherapy treatment, were found in five cases. These findings confirm that Ph- cells can persist in the marrow of Ph+ patients in the advanced phase of disease and indicate that a high percentage of leukemic cells retain karyotype evolution not detectable using standard drawing and culture techniques.

Mouse Cx50, a functional member of the connexin family of gap junction proteins, is the lens fiber protein MP70

The crystalline lens is an attractive system to study the biology of intercellular communication; however, the identity of the structural components of gap junctions in the lens has been controversial. We have cloned a novel member of the connexin family of gap junction proteins, Cx50, and have shown that it is likely to correspond to the previously described lens fiber protein MP70. The N-terminal amino acid sequence of MP70 closely matches the sequence predicted by the clone. Cx50 mRNA is detected only in the lens, among the 12 organs tested, and this distribution is indistinguishable from that of MP70 protein. A monoclonal antibody directed against MP70 and an anti-Cx50 antibody produced against a synthetic peptide identify the same proteins on western blots and produce identical patterns of immunofluorescence on frozen sections of rodent lens. We also show that expression of Cx50 in paired Xenopus oocytes induces high levels of voltage-dependent conductance. This indicates that Cx50 is a functional member of the connexin family with unique physiological properties. With the cloning of Cx50, all known participants in gap junction formation between various cell types in the lens are available for study and reconstitution in experimental systems.

Four novel members of the connexin family of gap junction proteins. Molecular cloning, expression, and chromosome mapping

We have used low stringency hybridization and polymerase chain reaction (PCR) amplification with degenerate oligonucleotides to identify four new members of the rat connexin gene family. On the basis of their predicted molecular mass, these proteins have been designated connexin (Cx) 40 (Cx40), Cx37, Cx33, and Cx31.1. The new connexins exhibit all of the conserved structural features of the connexin family, including highly similar extracellular and transmembrane domains but divergent major cytoplasmic domains. On the basis of primary sequence similarity, the connexin family may be divided into two classes. Cx40, Cx37, and Cx33 are similar to the previously characterized Cx43 and Cx46. Cx31.1 is similar to Cx26, Cx31, and Cx32. Cx37 and Cx40 mRNAs are expressed in a wide variety of adult organs and tissues, with particular abundance in lung. However, their relative levels are different in many organs and thus their distribution is not completely coincident. Cx33 and Cx31.1 genes exhibit a much more restricted pattern of expression; mRNAs are detected only in testes and skin, respectively. Chromosomal mapping studies indicate that Cx26 and Cx46 are tightly linked on chromosome 14, and Cx37 and Cx31.1 are linked on chromosome 4, while the rest of the connexin genes are dispersed.

Karyotype evolution of Ph positive chronic myelogenous leukemia patients relapsed in advanced phases of the disease after allogeneic bone marrow transplantation

Sixty-eight patients affected by Philadelphia chromosome (Ph) positive chronic myelogenous leukemia (CML) underwent allogeneic bone marrow transplantation (BMT) and were successfully studied from a cytogenetic point of view, before and after the BMT. Nineteen had evidence of cytogenetic and clinical relapse. Cytogenetic analyses of 14 patients who, after the relapse, showed progression to the accelerated or blastic phase of the disease, are presented. Five of these cases had only the Ph chromosome without karyotype evolution; in one case Ph duplication without other anomalies was detected, while in the remaining eight cases cytogenetic analysis showed apparently random clonal structural abnormalities (translocations, inversions, deletions, and marker formations). Therefore, the classical "non-random" abnormalities (+8, i(17q), +Ph, +19, +21) were not as common as in conventionally treated Ph+ CML. From our data, karyotype evolution during advanced phases in Ph+ CML patients after BMT differs from the evolution seen in conventionally treated patients, by the presence of numerous structural unusual abnormalities, possibly related to radiochemotherapy conditioning to BMT. Therefore, BMT treatment is not always able to eradicate the Ph+ clone but can reduce the incidence of the formation and/or expansion of Ph+ clones with additional non-random abnormalities.

The molecular basis of enzyme secretion

Acinar cells are one of the best studied models of exocytotic secretion. A number of different hormones and neurotransmitters interact with specific membrane receptors, and it is commonly held that pancreatic secretagogues stimulate enzyme release via the elevation of either cytosolic free Ca2+ or cellular cyclic adenosine monophosphate. The discovery of the pivotal role played by phospholipid metabolism in the chain of events leading to secretion, together with the introduction of sensitive techniques to monitor cytosolic free Ca2+, has generated a series of studies that have challenged this classical model. Thus, several observations in pancreatic acini as well as other cell types have argued against the notion that a generalized increase in cytosolic free Ca2+ represents a sufficient and necessary stimulus for exocytosis in nonexcitable cells. Furthermore, the demonstration that a single agonist activates multiple transduction pathways has served to refute the schematic view that receptor agonists activate only one second messenger system. The aim of this article is to review the recent advances in understanding the molecular and cellular mechanisms of signal transduction, with particular emphasis on the inositol lipid pathway, and to integrate this information into a new working model of enzyme secretion from acinar cells.

Increase in pancreatic exocrine secretion during uncoupling: evidence for a protein kinase C-independent effect

It has been demonstrated that blockade of the normal communication between pancreatic acinar cells leads to an increase in amylase release. Although the physiological mechanisms that regulate the gating of gap junction channels are unknown, the involvement of protein kinase C (PKC) in the inhibition of cell coupling has been reported in various cell lines. Since the activation of PKC also stimulates amylase secretion of pancreatic acinar cells, we sought to determine whether blockers of gap junctions and activators of PKC modify basal secretion by a similar mechanism. Thus, we have studied the effects of heptanol and of 12-O-tetradecanoylphorbol-13-acetate (TPA) on the subcellular distribution of PKC, dye coupling, and amylase release of dispersed pancreatic acini. The data show that TPA activates PKC and stimulates amylase secretion without affecting the extensive dye coupling of acinar cells. By contrast, heptanol inhibits cell-to-cell coupling and increases enzyme output without altering the subcellular distribution of PKC. Heptanol also enhances significantly the secretion evoked by TPA. These results indicate that the stimulation of amylase release caused by uncoupling of acinar cells occurs by a mechanism(s) that does not involve the activation of PKC.

Effects of n-alcohols on junctional coupling and amylase secretion of pancreatic acinar cells

We have tested the effects of alcohols differing by their alkyl chain length on the membrane channels and amylase secretion of rat pancreatic acinar cells. In intact acini, alcohols with a chain of seven, eight, or nine carbons (C-7, C-8, and C-9) induced dye uncoupling and increased basal amylase release. These effects were readily reversible after alcohol removal. By contrast, an alcohol with a chain of 15 carbons (C-15) and several alcohols with chains of fewer than six carbons (C-2, C-4, and C-6) did not uncouple acinar cells and had no effects of amylase secretion. Neither did alkanes and oxidized derivatives of C-7 and C-8 alcohols did not affect dye coupling. Double patch-clamp experiments on pairs of acinar cells, under conditions of strong cytosolic Ca2+ and pH buffering, showed that C-7, C-8, and C-9 alcohols blocked completely and reversibly the electrical conductance of junctional channels. Furthermore, studies of single voltage-clamped acinar cells revealed that the uncoupling alcohols did not affect the resting nonjunctional membrane conductances. Thus the alcohols that did not affect acinar cells coupling did not affect amylase secretion, whereas the alcohols that caused uncoupling increased secretion. The latter effect was not mediated by changes in the conductance of nonjunctional membrane, cytosolic Ca2+, and pH and, as revealed by an immunological hemolytic plaque assay for amylase, had a time course consistent with the rapid (within 1 min) inhibition of coupling. These data provide new support for the view that the regulation of cell-to-cell communications is correlated with that of digestive enzyme secretion.

Mechanism of action of bombesin on amylase secretion. Evidence for a Ca2+-independent pathway

The mode of action of bombesin on amylase secretion was investigated in rat pancreatic acini. Bombesin induced a dose-dependent increase in inositol 1,4,5-trisphosphate and cytosolic free Ca2+. The threshold concentration capable of inducing both effects was 0.1 nM and the half-maximal dose of the peptide for Ca2+ mobilization was approximately 0.6 nM. By contrast, amylase release was approximately 30 times more sensitive than inositol 1,4,5-trisphosphate production and Ca2+ mobilization to bombesin action, with 1 pM being the first stimulatory concentration and a half-maximal effect at approximately 20 pM. The ability of low bombesin doses to trigger enzyme secretion was unaffected by chelation of extracellular Ca2+ with EGTA. In order to test whether the stimulation of amylase release was truly a Ca2+-independent response, the intracellular Ca2+ stores were depleted by pretreating acini with EGTA plus ionomycin, the Ca2+ ionophore. Under these conditions bombesin was still capable of eliciting a significant twofold enhancement of the secretory activity. These results indicate that bombesin, an agonist thought to activate secretion mainly through mobilization of Ca2+ from intracellular stores, elicits amylase release at low concentrations, independently of a concomitant rise in cytosolic free Ca2+. The relevance of these findings to the physiological regulation of pancreatic exocrine secretion is discussed.