Unique and shared functions of different connexins in mice.

BACKGROUND: Connexins are the protein subunits of intercellular gap junction channels. In mammals, they are encoded by a family of at least 15 genes, which show cell-type-specific but overlapping patterns of expression. Mice lacking connexin43 (Cx43) die postnatally from obstruction of the right ventricular outflow tract of the heart. To discriminate between the unique and shared functions of Cx43, Cx40 and Cx32, we generated two 'knock-in' mouse lines, Cx43KI32 and Cx43KI40, in which the coding region of the Cx43 gene was replaced, respectively, by the coding regions of Cx32 or Cx40. RESULTS: Heterozygous mutants were fertile and co-expressed the wild-type and the corresponding recombinant allele in all tissues analyzed. Heterozygous Cx43KI32, but not Cx43KI40, mutant mothers were unable to nourish their pups to weaning age, possibly reflecting a defect in milk ejection. Homozygous mutant males were sterile because of extensive germ-cell deficiency. The ovaries of homozygous Cx43KI32 neonates exhibited all stages of follicular development and ovulation. The hearts of homozygous Cx43KI32 neonates showed mild morphological defects, but the cardiac morphology of homozygous Cx43KI40 neonates was relatively normal. Spontaneous ventricular arrhythmias were observed in most Cx43KI40 and some Cx43KI32 mutant mice, suggesting increased ventricular vulnerability in these mice. CONCLUSIONS: The postnatal lethality of Cx43-deficient mice was rescued in Cx43KI32 or Cx43KI40 mice, indicating that Cx43, Cx40 and Cx32 share at least some vital functions. On the other hand, Cx43KI32 and Cx43KI40 mice differed functionally and morphologically from each other and from wild-type mice. Thus, these connexins also have unique functions.

Expression patterns of connexin genes in mouse retina.

To analyze the molecular basis of gap junctional communication in mouse retina, we examined the expression pattern of the following 13 connexin (Cx) genes: Cx26, Cx30, Cx30.3, Cx31, Cx31.1, Cx32, Cx36, Cx37, Cx40, Cx43, Cx45, Cx46, and Cx50. By using reverse transcriptase-polymerase chain reactions with primer oligonucleotides to murine connexin genes, we detected mRNAs of Cx26, Cx31, Cx32, Cx36, Cx37, Cx40, Cx43, Cx45, and Cx50. Retinae from heterozygous mice with targeted replacement of most of the Cx45 open reading frame by a lacZ reporter gene showed Cx45 promoter activity in somata of the ganglion cell layer and the inner nuclear layer. Immunoblot and immunofluorescence analyses with antibodies generated to murine connexin epitopes revealed the presence of Cx36, Cx37, Cx43, and Cx45 proteins: The outer and inner plexiform layer were immunopositive for Cx36 and Cx45. Cx37 immunoreactivity was found in blood vessels of the inner retina. Cx43 immunolabeling was detected in the ganglion cell layer and nerve fiber layer where it was largely colocalized with immunostaining of glial fibrillary acidic protein suggesting that Cx43-positive cells could be of glial origin. No Cx26 protein was detected in retina by using Cx26 antibodies for immunoblot analyses or confocal microscopy. Furthermore, comparative immunofluorescence analyses of retinae from mice deficient for Cx31, Cx32, or Cx40 with retinae of wild-type mice revealed no specific immunostaining. Our results demonstrate regional specificity in expression of connexin genes in mouse retina and, thus, provide a basis for future assignments of functional defects in connexin-deficient mice to cells in different regions of the retina.

Biological functions of connexin genes revealed by human genetic defects, dominant negative approaches and targeted deletions in the mouse.

Gap junction channels in mammalian organs can be built up of at least 13 different connexin proteins, most of which are expressed in only few cell types, although many cells express more than one connexin protein. Recently, the consequences of missing or defective connexin proteins were studied in human patients with defects in connexin32 (Cx32; beta 1; X-linked Charcot-Marie-Tooth disease) or in Cx26 (beta 2; non-syndromic sensorineural deafness), and in mice with targeted deletions in the Cx26, Cx32, Cx37 (alpha 4), Cx43 (alpha 1), Cx46 (alpha 3) or Cx50 (alpha 8) genes. Some effects of dominant negative mutations in connexin genes have been characterized in Xenopus oocytes and transfected mammalian cells in culture. Here we review results of these different experimental approaches and report new findings regarding the characterization of Cx40 (alpha 5)- and Cx31 (beta 3)-deficient mice. The phenotypic alterations, caused by different defective connexin genes in mice or humans, are divergent, although in most known cases the viability is not affected. When more than one connexin gene, coexpressed in the same cell, is inactivated, development or maturation can be more severely affected at an earlier stage. Some connexin proteins, if present in the same cell, can partially replace each other in certain functions. Thus, the diversity of connexin proteins in mammalian cells may provide functional overlap and complementation.

The effects of selected drugs on the in vitro protein binding of repaglinide in human plasma.

This study investigated plasma protein binding by the novel oral hypoglycemic agent, repaglinide, and assessed the influence of other protein-bound drugs upon this process. Varying concentrations of [3H]-repaglinide (0.01 to 100 micrograms/ml) were incubated in solutions of plasma proteins (human serum albumin, HSA; alpha 1-acid glycoprotein, AAGP), or human plasma in the absence or presence of several test drugs. Protein binding was assessed using an ultrafiltration technique. At all concentrations tested, the mean binding of repaglinide in plasma was 98.5%, binding to HSA averaged 98.6%, and the binding to AAGP was saturable and remained below 50%. Warfarin 10 micrograms/ml, furosemide 0.2 microgram/ml, and tolbutamide 100 micrograms/ml, significantly reduced in vitro binding of repaglinide at 1 and 100 micrograms/ml versus control (p < 0.05), producing an 18-36% increase in free repaglinide. No reduction was found using 0.1 microgram/ml repaglinide. Diazepam, glibenclamide and nicardipine hydrochloride had no significant effects on the in vitro protein binding of repaglinide. These data suggest that the binding of repaglinide to HSA in human plasma has potential clinical significance, and that within the therapeutic range for repaglinide, the presence of the test drugs has no clinically relevant effects on repaglinide binding to plasma proteins.

[Cardiac arrhythmias in targeted connexin deficient mice: significance for the arrhythmia field]. / Herzrhythmusstörungen bei gezielt Connexin-defizienten Mäusen: Bedeutung für die Rhythmologie.

Intercellular communication can be mediated by gap junction channels. One channel is composed of two hexameric hemichannels which consist of six polypeptide subunits called connexines (Cx). Three different connexines were documented in the cardiac myocytes: Cx40, Cx43 and Cx45. The labeling by number represents the rounded, molecular mass of the amino acid sequences given in kD. Identical connexons form homotypic channels different connexons can form heterotypic channels. Each channel type has specific properties regarding permeability and electrical conductance. Beside a typical age-dependent alignment of gap junction channels on the surface of the cardiac myocytes, regional distribution of the different connexins is different at distinct parts of the mouse heart. The ventricular working myocardium is characterized by Cx43, whereas Cx40 and Cx45 were not found in this region. In the atria as well as in the conduction system, Cx40 is the most frequently expressed. Cx45 appears to form a border zone between conductive and the surrounding working myocardium. In line with the localization and the conduction properties of distinct homotypic gap junction channels, the Cx43 deficient mouse is suitable for analysis of ventricular arrhythmias and the Cx40 deficient mouse primarily for studies of atrial arrhythmias. Increased ventricular conduction velocity and increased ventricular vulnerability were observed in the presence of a decreased number and density of Cx43 gap junction channels. This observation, however, is controversially discussed. Cx40 deficiency induces an impairment of the sinuatrial, intraatrial and atrioventricular conduction properties and is associated with an increased atrial vulnerability. Transgenic mouse models and new mapping techniques for detection of the electrical wavefront propagation provide new insights into the mechanisms of arrhythmogenesis. Geneticists, clinicians and basic researchers need to collaborate in order to explore the clinical relevance of the new mouse models and their use for molecular medicine.

Pharmacokinetics of the rapid-acting insulin analog, insulin aspart, in rats, dogs, and pigs, and pharmacodynamics of insulin aspart in pigs.

The objective of this study was to compare the pharmacokinetics and pharmacodynamics of insulin aspart (IA), a rapidly acting insulin analog, with those of human soluble (regular) insulin (HI) in animal models after s.c. and i.v. dosing. Single doses of IA and HI were administered i.v. and s.c. to rats and dogs at three dose levels, and at one dose level to pigs; rats and dogs also underwent repeated s.c. dosing for 1 week. Plasma insulin levels were assessed at predetermined time points after dosing; plasma glucose levels were measured in pigs only. There were no significant pharmacokinetic differences between IA and HI after a single s.c. or i.v. dose in rats or dogs, and no differences were observed after repeated s.c. dosing, implying there was no accumulation. In pigs, there was a strong trend toward more rapid absorption of IA compared with HI after s.c. dosing, whereas there were no differences after i.v. administration. After s.c. dosing in pigs, IA produced significantly lower plasma glucose levels compared with HI during the period 30 to 75 min after dosing (P <.05). In conclusion, IA was more rapidly absorbed than HI after s.c. administration only in the pig; this difference was reflected in earlier and more pronounced effects on plasma glucose levels.

General or cell type-specific deletion and replacement of connexin-coding DNA in the mouse.

Here we describe several gene targeting approaches currently used in our laboratory for the generation of deletion or replacement mutants of connexin genes in the mouse and discuss the advantage of the double-replacement strategy for the generation of conditional mutants. For the analysis of complementary functions of connexins, it will be necessary to generate mice with mutations in several connexin genes. We also report how this can be effectively accomplished. The replacement of targeted connexin-coding DNA with a reporter gene, to mimic expression of the deleted gene product, is currently being used in several laboratories. The use of different reporter genes or their differently localized gene products could allow distinction of promoter activity in double or triple connexin mutant mice.

Validity and use of a non-parallel insulin assay for pharmacokinetic studies of the rapid-acting insulin analogue, insulin aspart.

A radioimmunoassay (RIA) for insulin was validated for reliable measurement of the human insulin analogue, insulin aspart, by correction of non-linear measurements. Specificity was equivalent for several species of insulin, except insulin aspart. A non-linear hyperbolic model fitted insulin aspart with a correction formula for non-linearity of: z = 1,503y/ (1,398 - y), where y denotes measured concentration and z denotes true concentration. Matrix-effects were insignificant for human, porcine, and canine heparin-plasma and for human and porcine serum. The coefficient of variation was below 15% for 80-800 pmol/L human and porcine insulin and for 80-600 pmol/L insulin aspart. The limit of detection for insulin aspart was 11.5 pmol/L with a lower limit of quantification of 17.5 pmol/ L. Dilution of serum with Pharmacia dilution media introduced no significant error. In conclusion, this paper demonstrates that a non-parallel radioimmunoassay can be used to estimate accurate concentrations of insulin aspart.

[Electrophysiological characterization of connexin 40 deficient hearts--in vivo studies in mice]. / Elektrophysiologische Charakterisierung Connexin 40-defizienter Herzen--In vivo-Untersuchungen an Mäusen.

Intercellular communication is not only mediated by extracellular transmitters, but also directly by gap junction channels. One channel is composed of two hexameric hemichannels which consist of six polypeptide subunits called connexines (Cx). In the mammalian heart the following connexines have been documented: Cx37, Cx40, Cx43, Cx45, Cx46, Cx50 and Cx57. The labeling by number represents the rounded, molecular mass of the amino acid sequences given in kD. If identical connexin-isotypes form both connexons of a gap junction channel, homotypic coupling exists and a homomeric gap junction channel is formed. Different connexin-isotypes within both connexons cause form heterotypic coupling and heteromeric gap junction channels. Each channel type has specific properties regarding permeability and electrical conductance. Beside a typical age-dependent alignment of gap junction channels on the surface of the cardiac myocytes, regional distribution of the different connexins is different at distinct parts of the mouse heart. Cx40 is not found in the ventricular working myocardium of mice. In the atria as well as in the conduction system, Cx40 is the most frequently expressed. In line with the localization and the conduction properties of distinct homotypic gap junction channels, the Cx40 deficient mouse is suitable for analysis of atrial arrhythmias. Cx40-deficiency in the mouse heart results in characteristic ECG changes like first degree atrioventricular block and prolongation of the QRS duration. Thus, an impairment of the sinuatrial, intraatrial and atrioventricular conduction properties is documented in Cx40 deficient mice. These observations are associated with an increased atrial vulnerability. The Cx40 deficient mouse provides a good example of the relevance of transgenic mouse models to clarify the mechanisms of arrhythmogenesis. The clinical impact of future transgenic mouse models depends on the cooperation of geneticists, basic researchers and clinicians.

Connexin31-deficiency in mice causes transient placental dysmorphogenesis but does not impair hearing and skin differentiation.

Mutations in the human GJB3 gene that codes for Connexin31 (Cx31), a protein subunit of gap junction channels, have recently been reported to cause deafness and the skin disorder erythrokeratodermia variabilis. To study the function of this gene in mice, we generated animals with targeted replacement of the Cx31 gene (Gjb3) by a lacZ reporter gene. Although homozygous Cx31-deficient adult mice (Gjb3(-/-)) were found among the offspring of heterozygous Cx31-deficient parents (Gjb3(+/-)), 60% of the animals expected according to Mendelian inheritance were lost between ED 10.5 and 13.5. Placentas of Gjb3(-/-) embryos at ED 9.5 were smaller than controls as a result of severely reduced labyrinth and spongiotrophoblast size. From ED 10.5 onward, placentas of surviving Gjb3(-/-) embryos recovered progressively and reached normal size and morphology by ED 18.5. This corresponds to a time period in which another connexin isoform, Connexin43, is upregulated in spongiotrophoblast cells of Cx31-deficient and control placentas. No morphological or functional defects of skin or inner ear were observed in surviving adult Gjb3(-/-) mice. We conclude that Cx31 is essential for early placentation but can be compensated for by other connexins in the embryo proper and adult mouse.

Defective vascular development in connexin 45-deficient mice.

In order to reveal the biological function(s) of the gap-junction protein connexin 45 (Cx45), we generated Cx45-deficient mice with targeted replacement of the Cx45-coding region with the lacZ reporter gene. Heterozygous Cx45(+/)(-) mice showed strong expression of the reporter gene in vascular and visceral smooth muscle cells. Cx45-deficient embryos exhibited striking abnormalities in vascular development and died between embryonic day (E) 9.5 and 10.5. Differentiation and positioning of endothelial cells appeared to be normal, but subsequent development of blood vessels revealed impaired formation of vascular trees in the yolk sac, impaired allantoic mesenchymal ingrowth and capillary formation in the labyrinthine part of the placenta, and arrest of arterial growth, including a failure to develop a smooth muscle layer surrounding the major arteries of the embryo proper. As a consequence, the hearts of most Cx45-deficient embryos were dilated. The abnormal development of the vasculature in the yolk sac of Cx45(-)(/)(-) embryos could be caused by defective TGFbeta signalling, as the amount of TGF beta1 protein in the epithelial layer of the yolk sac was largely decreased in the E9.5 Cx45(-)(/)(-) embryo, compared with the wild-type embryo. The defective vascular development was accompanied by massive apoptosis, which began in some embryos at E8.5 and was abundant in virtually all tissues of the embryos at E9.5. We conclude that in Cx45(-)(/)(-) embryos, vasculogenesis was normal, but subsequent transformation into mature vessels was interrupted. Development of different types of vessels was impaired to a varying extent, which possibly reflects the complementation by other connexin(s).