Quantitative immunoconfocal analysis of human myometrial gap junction connexin43 in relation to steroid hormone concentrations at term labour

Pro-labour myometrial gene expression: are preterm labour and term labour the same?

Preterm labour (PTL) is the most important cause of neonatal morbidity and mortality. While some causes have been identified, the mechanisms involved remain elusive. This study investigates whether term labour (TL) is an appropriate model for PTL by examining pro-labour gene expression, using quantitative rtPCR, and protein synthesis, using Western analysis, in preterm and term myometrial samples obtained from the upper and lower uterine segments before and after the onset of labour. In the lower segment, the levels of prostaglandin H synthase type-2 (PGHS-2), interleukin-1beta (IL-1beta), IL-6 and IL-8 mRNA expression were significantly higher in TL compared with PTL samples. Compared with non-labour controls, the expression of IL-1beta and IL-8 mRNA was increased in both PTL and TL samples and the expression of PGHS-2 and IL-6 mRNA was increased in TL samples only. In the upper segment, there were no differences between PTL and TL samples and the mRNA expression of PGHS-2 and IL-1beta was increased in TL compared with term no labour samples. No effect of PTL or TL was seen on either oxytocin receptor or connexin-43 mRNA expression or protein levels. The multiple regression analysis and studies in primary cultures of uterine myocytes suggest that the inflammatory cytokines, IL-1beta and tumour necrosis factor-alpha, are the most important regulators of PGHS-2 and IL-8. Our data show that preterm and term labouring myometrium are significantly different and that the most marked labour-induced changes in gene expression are in the lower segment. These changes may occur in response to the release of inflammatory cytokines by the labour-associated inflammatory infiltration.

Connexin 43 expression in normal versus dysfunctional labor

OBJECTIVE

We sought to determine whether connexin 43 (Cx43), the major myometrial gap junction protein, is differentially expressed in normal versus dysfunctional labor.

STUDY DESIGN

Myometrial biopsies were obtained from 28 patients undergoing cesarean section and grouped into the following categories: (1) no labor, (2) dysfunctional labor, and (3) normal labor. Northern and Western analyses were performed to determine Cx43 messenger RNA (mRNA) and protein expression, respectively. Localization of Cx43 protein was determined by immunohistochemistry.

RESULTS

Labor was associated with increased Cx43 mRNA expression (P <.05). This association was not true for protein. There was no difference in mRNA or protein expression between patients with normal labor versus those with dysfunctional labor. The extent of Cx43 immunohistochemical staining was not significantly different among the groups (P >.05).

CONCLUSION

Dysfunctional labor is not associated with aberrant Cx43 mRNA or protein expression or with a reduction in immunodetectable Cx43 gap junctions.

Multiple connexins localized to individual gap-junctional plaques in human myometrial smooth muscle

The synchronous contractions of the uterus in labour depend on electrical coupling of myometrial smooth muscle cells by gap junctions. In the human myometrium, gap junctions are scarce in the non-pregnant uterus, but become abundant at term in preparation for labour. We have previously demonstrated that in the human myometrium at term, three different gap-junctional proteins are expressed, connexins 43, 45, and 40. These connexins are known to have distinctive functional capacities in in vitro expression systems but whether, in the human myometrium in vivo, they are co-assembled into the same gap junction or form different types of gap junction has previously been unclear. By applying triple immunogold labelling to sections of Lowicryl-embedded tissue for electron microscopy, together with complementary immunoconfocal microscopy, we demonstrate here that connexins 43, 45, and 40 are commonly present as mixtures within the same gap-junctional plaque. While all gap junctions contain connexin43, the relative signal for each connexin type varies between individual junctions. The presence within single gap-junctional plaques of three different connexins, each with the potential for conferring distinctive channel properties, suggests an inherent versatility for modulation of smooth muscle cell intercellular communication properties during human parturition.

Oestriol and oestradiol increase cell to cell communication and connexin43 protein expression in human myometrium

Oestradiol increases the protein expression of connexin43 (Cx43) gap junctions in myometrium but the effect of oestriol on gap junction expression has not been described previously. Oestriol is the most abundant free oestrogen in pregnant women and there is a marked surge in oestriol concentrations before term and idiopathic preterm labour. In order to determine whether oestriol may have a physiological action on the myometrium, cultured human myometrial cells obtained from non-pregnant hysterectomy specimens were exposed to 10 nmol/l oestradiol or oestriol. Intercellular communication between myometrial cells was investigated by microinjection of confluent cultured cells with the gap junction-permeant tracer Cascade Blue. There was a progressive increase in coupling after exposure to oestradiol or oestriol (P < 0.0005). An increase in Cx43 protein expression was demonstrated by immunocytochemistry after 1 h (P < 0.01) and 3 days (P < 0.01) exposure, and by Western blotting after 1 h (P < 0.01) and 3 days (P < 0.05) exposure, to both oestradiol and to oestriol. We conclude that oestriol increases gap junction communication in human myometrium by increasing gap junction expression. Elevated oestriol concentrations may thus play a role in the initiation of labour in women, by increasing cell-cell communication in the myometrium.

Modulation of potassium current characteristics in human myometrial smooth muscle by 17beta-estradiol and progesterone

The K(+) channel currents are important modulators of smooth muscle membrane potential and excitability. We assessed whether voltage-gated K(+) currents from human myometrium are regulated by placental steroid hormones during pregnancy and labor. Pregnant human myometrial cells were isolated from samples obtained at cesarean section. Primary cultured cells were treated with 100 nM 17beta-estradiol, 1 microM progesterone, or both hormones in combination for 24 h. Acute effects of the two hormones were also determined. The K(+) currents were recorded using the standard whole-cell, patch-clamp technique. Primary cultures possessed both delayed rectifier (I(KV)) and A-like (I(KA)) voltage-gated K(+) currents. The 24-h 17beta-estradiol treatment caused a hyperpolarizing shift in the steady-state inactivation of both I(KV) and I(KA). Progesterone treatment also shifted the inactivation of I(KA) and increased I(KV) amplitude by 60%-110%. Conversely, the combined treatment had no effect on these currents. Neither 17beta-estradiol (0.1-1 microM) nor progesterone (1-5 microM) had any effect on the K(+) current when applied acutely. These results show that 17beta-estradiol should inhibit myometrial K(+) channel activity, whereas progesterone is likely to have the opposite effect. These results are consistent with the respective procontractile and proquiescence roles for 17beta-estradiol and progesterone in human uterus during pregnancy.

Connexin43 in porcine myocardium and non-pregnant myometrium

It is generally accepted that connexin43 (Cx43) is a major constituent of heart and myometrial gap junctions. However, the presence of Cx43 gap junctions in non-pregnant myometrium is still poorly documented. Tissue sections of porcine heart and non-pregnant uterus and myometrial smooth muscle cell cultures were immunostained with monoclonal antibody against Cx43. In the heart, intensive immunostaining was confined to the intercalated discs as previously reported. In the non-pregnant uterus, punctuate immunostaining of Cx43 was seen throughout the myometrium along cell interfaces between myocytes. The expression of Cx43 was sustained in cultured smooth muscle cells isolated from non-pregnant myometrium. Western blotting has detected single isoform of Cx43 in both, cardiac and myometrial tissues. The electrophoretic mobility of porcine heart Cx43 was similar to that of myometrial isoform but different from the pattern of mobility of Cx43 of the rat heart. Hence, porcine myometrium may provide attractive model for studying cellular mechanisms triggering expression of gap junction protein in normal (non-pregnant) uterus.

Expression of connexin-26, -32, and -43 gap junction proteins in the porcine cervix and uterus during pregnancy and relaxin-induced growth

Connexin (CX) proteins participate in growth, differentiation, and tissue remodeling. Relaxin-stimulated reproductive tissue growth and remodeling may be facilitated by enhanced intracellular communication. This study was an examination of the effects of relaxin in vivo on expression of CX-26, CX-32, and CX-43 in the cervix and uterus of prepubertal pigs. In addition, expression of these proteins was monitored in the sow uterus during pregnancy. Relaxin was administered to prepubertal gilts every 6 h for 54 h. CX expression was characterized by immunoblotting and localized by immunofluorescence. Significant increases in all three CXs were observed in the cervix following relaxin treatment (P < 0.05). Uterine CX proteins were also significantly higher (P < 0.05) in relaxin-treated animals compared to controls. The CX protein level in relaxin-treated animals was similar to that observed during the second half of pregnancy, but below levels found in mature, nonpregnant sows. This is the first evidence for specific CX expression in the porcine cervix, and the first study to show that relaxin increases the expression of CX proteins in the porcine uterus and cervix. The data show that CX proteins are differentially regulated in the uterus of the pig during pregnancy. These data support a role for CX-mediated communication during relaxin-induced reproductive tissue growth and remodeling.