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Samuel Lunenfeld Research Institute,3 Mount Sinai Hospital, Toronto, Ontario, Canada M5G 1X5
Departments of Physiology4
Obstetrics and Gynecology,5 University of Toronto, Toronto, Ontario, Canada M5S 1A1
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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parturition, pregnancy, uterus
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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and 
) and cytoplasmic (ß and ) actin isoforms [4]. The muscle isoforms are associated with the contractile filaments, whereas the cytoplasmic isoforms form the noncontractile cytoskeleton. The amount of different actin isoforms present in the muscle cells varies according to the source of the smooth muscle tissue. The dominant type of actin in the smooth muscle of mammalian visceral organs (chicken gizzard, uterus, intestine) is -actin, whereas the major actin type in vascular smooth muscle is -actin [2]. Smooth muscle -actin is also the first known marker of differentiated SMCs that is upregulated during vasculogenesis (reviewed in [5]). In contrast, in primary cultured vascular SMCs the expression of this actin isoform decreases with dedifferentiation [6]. It has been recently shown that visceral SMCs display an opposite pattern to that in vascular SMCs depending on the differentiation state. In differentiated visceral SMCs, the expression of -actin is decreased whereas the expression of -actin increased [7, 8], as has been shown to occur in the pregnant (compared to nonpregnant) human myometrium [9]. Uterine smooth muscle undergoes dramatic physiological adaptations during the course of pregnancy. Under the influence of high progesterone levels, mechanical tension induces myometrial growth and remodeling, which results in myocyte hypertrophy and an increased synthesis of interstitial matrix proteins [10]. The onset of labor requires triggering of the endocrine (the fetal hypothalamic-pituitary-adrenal-placental axis) and mechanical (stretch resulting from the fetal growth during pregnancy) signaling pathways within the myometrium to activate the synchronized and time-coordinated labor contractions of the uterine smooth muscle. The involvement of mechanical signals in the process of myometrial activation at the onset of labor requires a mechanism by which uterine myocytes can sense the mechanical signals. One potential mechanosensor of the hypertrophied myometrial cell is the cytoskeleton, which is built from three types of biopolymers, including the actin microfilaments, intermediate filaments, and microtubles [11]. There are many studies on the physiological changes in the pregnant uterus [10], however, surprisingly, there is relatively little information about the expression and localization of contractile proteins.
Our previous data has suggested a role for focal adhesion kinase (FAK), also known as protein tyrosine kinase 2, in the onset of labor [12]. Activation of FAK causes the recruitment and formation of the focal adhesion–cytoskeleton complex. We speculate that myometrial growth during pregnancy is associated with alteration in the structure and contractile apparatus characteristic of the uterine smooth muscles. Because actin filaments terminate at focal adhesions and because the actin cytoskeleton is thought to play a role in mechanotransduction, we sought to determine whether pregnancy affects the expression and localization of different actin isoforms in the myometrium. In the present study we used a reverse transcription-polymerase chain reaction (RT-PCR) technique, immunoblotting and standard immunohistological staining to investigate changes in two major smooth muscle actin isoforms ( and ) in the nonpregnant, pregnant, and postpartum rat myometrium. In addition, the effect of gravidity was investigated on the expression and localization of actin using a unilateral tubal-ligation rat model.
According to the Rat Genome Database, the standard genetic symbols for rat smooth muscle alpha-actin and gamma 2-actin (smooth muscle) are Acta2 and Actg2, respectively. Because of the similarity of these two symbols, we will use -SM-actin and -SM-actin throughout this paper to avoid confusion to readers.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Wistar rats (Charles River Co., St. Constance, PQ, Canada) were housed individually under standard environmental conditions (12L:12D cycle) and fed Purina Rat Chow (Ralston Purina, St. Louis, MO) and water ad libitum. Female virgin rats were mated with male Wistar rats. Day 1 of gestation was designated as the day a vaginal plug was observed. The average time of delivery was the morning of Day 23. The Samuel Lunenfeld Research Institute Animal Care Committee approved all animal experiments.
Experimental Design
Normal pregnancy and term labor Animals were killed by carbon dioxide inhalation and myometrial samples were collected on Gestational Days 0 (nonpregnant [NP]), 6, 8, 10, 12, 14, 15, 17, 19, 21, 22, and 23 (day of labor), and on Days 1 and 4 Postpartum (PP). Tissue was collected at 1200 h on all days with the exception of the labor (d23L) sample, which was collected once the animals had delivered at least one pup (n = 4).
Unilaterally pregnant rats Under general anesthesia, virgin female rats underwent tubal ligation through a flank incision to ensure that they subsequently became pregnant in only one horn [13]. Animals were allowed to recover from surgery for at least 7 days before mating. Pregnant myometrial samples from empty and gravid horns were collected on Days 15, 17, 19, 21, 22, 23 or 1PP (n = 3).
Tissue Collection
For RNA and protein extraction the uterine horns were placed into ice-cold PBS, bisected longitudinally, and dissected away from both pups and placentas. The endometrium was carefully removed from the myometrial tissue by mechanical scraping on ice, which we have previously shown to remove the entire luminal epithelium and the majority of the uterine stroma [14]. The myometrial tissue was flash-frozen in liquid nitrogen and stored at –70°C. For immunohistochemical studies the intact uterine horns were placed in ice-cold PBS buffer, cut into 3–10-mm segments using a scalpel blade, and fixed immediately in 4% paraformaldehyde solution at 4°C for 48 h. These segments were further cross-sectioned or sectioned longitudinally. For each day of gestation, tissue was collected from 3–4 different animals and processed for different experimental purposes.
Real-Time PCR Analysis
Total RNA was extracted from the frozen tissues using TRIZOL (Gibco BRL, Burlington, ON, Canada) according to the manufacturer's instructions. RNA samples were column-purified using RNeasy Mini Kit (Qiagen, Mississauga, ON, Canada), and treated with 2.5 µl DNase I (2.73 Kunitz unit/µl; Qiagen) to remove genomic DNA contamination. Reverse transcription (RT) and real-time PCR was performed to detect the mRNA expression of -SM-actin and -SM-actin in rat myometrium. Two µg of total RNA was primed with random hexamers to synthesize single-strand cDNAs in a total reaction volume of 100 µl using the TaqMan Reverse Transcription Kit (Applied Biosystems, Foster City, CA). The thermal cycling parameters of real-time were modified according to the Applied Biosystems manual. Hexamer incubation at 25°C for 10 min and RT at 42°C for 30 min was followed by reverse transcriptase inactivation at 95°C for 5 min. Twenty µg of cDNA from the previous step was subjected to real-time PCR using specific sets of primers (see the legends to Figs. 1 and 2) in a total reaction volume of 25 µl (Applied Biosystems). RT-PCR was performed in an optical 96-well plate with an ABI PRISM 7900 HT Sequence Detection System (Applied Biosystems), using the SYBR Green detection chemistry. The run protocol was as follows: initial denaturation stage at 95°C for 10 min, 40 cycles of amplification at 95°C for 15 sec and 60°C for 1 min. After PCR, a dissociation curve was constructed by increasing temperature from 65°C to 95°C for detection of PCR product specificity. In addition, a no-template control (H2O control) was analyzed for possible contamination in the master mix. A cycle threshold (Ct) value was recorded for each sample. PCR reactions were set up in triplicates and the mean of the 3 Cts was calculated. Relative quantitation of gene expression was the approach used to compare differences of gene expression across gestation. An arithmetic formula from the comparative Ct method (see ABI User Bulletin No. 2 available at http://dna-9.int-med.uiowa.edu/RealtimePCRdocs/Compar_Anal_Bulletin2.pdf) was applied to the raw Ct values to extract relative gene expression data. The mRNA level from each sample was normalized to ribosomal 18S mRNA. Validation experiments were performed to ensure that the PCR efficiencies between the target genes and 18S were approximately equal.
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Western Immunoblot Analysis
Frozen myometrial tissue was crushed under liquid nitrogen using a mortar and pestle. Crushed tissue was homogenized for 1 min in RIPA lysis buffer [50 mM Tris-HCl, pH 7.5, 150 mM NaCl, 1% (vol/vol) Triton X-100, 1% (vol/vol) sodium deoxycholate, and 0.1% (wt/vol) SDS, supplemented with 100 µM sodium orthovanadate and protease inhibitor cocktail tablets (CompleteTM Mini; Roche, Laval, QC, Canada). Samples were spun at 12 000 x g for 15 min at 4°C and the supernatant was transferred to a fresh tube to obtain a crude protein lysate. Protein concentrations were determined using the BioRad protein assay buffer (BioRad, Hercules, CA). Protein samples (40–50 µg) were resolved by electrophoresis on a 12–15% SDS-polyacrylamide gel. Proteins were transferred onto polyvinylidene difluoride (PVDF) membrane (Millipore, Bedford, MA) in 25 mM Tris-HCl, 250 mM glycine, 0.1% (wt/vol) SDS, pH 8.3, for 18 h at 30 V at 4°C. Protein expression levels of and actins were measured by Western analysis using anti-SM -actin (1:1000, Clone 1A4; DAKO Diagnostics, Mississauga, ON, Canada) mouse and anti--actin (1:1000; Chemicon International, Temecula, CA) sheep primary antibody. PVDF membranes were stripped and reprobed with anti-smooth muscle calponin (1:3000, clone hCP; Sigma-Aldrich, Oakville, ON, Canada) mouse primary antibodies to control the loading variations. Probed membranes were exposed to x-ray film (Kodak XAR; Eastman Kodak, Rochester, NY) and analyzed by densitometry. Calponin is constitutively expressed in nonpregnant and pregnant rat tissue under the same protein extraction conditions [15].
Immunohistochemistry
The fixed myometrial tissues were gradually dehydrated in ethanol and embedded in paraffin. Sections of 5 µm thickness were collected on Superfrost Plus slides (Fisher Scientific Ltd., Nepean, ON, Canada). Paraffin sections were deparaffinized and rehydrated. After immersion in 3% hydrogen peroxide (Fisher Scientific, Fair Lawn, NJ) the antigen retrieval was performed using 0.125% trypsin solution at RT for 15 min, followed by blocking with 5% normal horse serum (for -SM actin) or DAKO Protein Serum-Free Blocking solution (for -actin; DAKO Corporation, Carpinteria, CA) and incubation with primary antibodies overnight at 4°C. Primary antibodies used to label different actin isoforms were: sheep antisera against actin (1:6000; Chemicon International) and mouse anti-sera against -SM actin (1:50, Clone 1A4; Dako Diagnostics). For the negative controls, ChromPure nonspecific mouse or sheep IgGs (Jackson ImmunoResearch Laboratories, Inc., West Grove, PA) were used at the same concentration, and sections were also incubated with secondary antibodies in the absence of primary antibodies. Secondary antibodies used for detection were biotinylated anti-mouse (1:200; Vector, Burlingame, CA) and donkey anti-sheep-HRP-conjugated (1:1000; Serotec, Oxford, UK). Final visualization was achieved using Vectastain Elite Kit (Vector, Burlingame, CA). Counterstaining with Harris' Hematoxylin (Sigma Diagnostics, St. Louis, MO) was carried out before slides were mounted with Cytoseal XYL (Ricard-Allan Scientific, Kalamazoo, MI). Myometrial cells from each of the three tissue sets were observed on a Leica DMRXE microscope (Leica Microsystems, Richmond Hill, ON, Canada). A minimum of five fields were examined for each gestational day and uterine horn for each set of tissue, and representative tissue sections were photographed with Sony DXC-970 MD (Sony Ltd., Toronto, ON, Canada) 3CCD color video camera.
Statistical Analysis
Gestational profiles were subjected to a one-way analysis of variance (ANOVA) followed by pairwise multiple comparison procedures (Student-Newman-Keuls method) to determine differences between groups. Tubal ligation data were analyzed by two-way ANOVA followed by pairwise multiple comparison procedures as described above. Where required the data was transformed by the appropriate method to obtain a normal distribution. Statistical analysis was carried out using SigmaStat version 2.03 (Jandel Corp., San Rafael, CA) with the level of significance for comparison set at P < 0.05.
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RESULTS |
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-smooth muscle actin (-SM-actin) and -SM-actin gene transcripts in the myometrium throughout pregnancy relative to NP is shown in Figure 1, A and B. Pregnancy was associated with a significant increase in -actin mRNA abundance (8.83 fold increase at Day 6, P < 0.001) as compared to NP mRNA levels. There was a further significant (P < 0.001) increase in -actin gene expression in the second half of gestation (14.89-fold increase at Day 14 relative to NP), which peaked at Day 19 (31.78-fold increase relative to NP, P < 0.001). On the day of labor (Day 23) -actin levels were slightly decreased (15.21-fold increase relative to NP, P < 0.001). During the postpartum period (1PP), -actin gene expression returned to nonpregnant levels (Fig. 1A). In contrast, no statistical difference was found in mRNA levels for -SM-actin gene among gestational samples (Fig. 1B).
Western immunoblot analysis was performed using anti--actin antibody that recognized protein of both forms of -actin (smooth muscle and cytoplasmic) and anti--actin antibody that recognized only smooth muscle-specific -actin isoform (Fig. 2, A and B). Both proteins were detected as single bands of the same size (42 kDa) on the Western blots; therefore, analysis of - and -actin isoforms was performed using different PVDF membranes to avoid cross-reactivity between antibodies. Although the expression of -SM-actin did not change across gestation (Fig. 2B), statistically significant changes were detected in the expression of -actin protein in pregnant myometrium. After normalization to calponin, -actin protein levels increased 10.2-fold starting from Day 17 (P < 0.001) until Day 23 of gestation (16.7-fold increase relative to NP, P < 0.001), followed by a decrease postpartum (Fig. 2A). Thus, the pattern of -actin protein expression measured by immunoblotting was similar to the pattern of mRNA expression measured by real-time PCR (Figs. 1A and 2A).
To determine whether -actin gene expression was affected by the stretch of the myometrium during late pregnancy, real-time PCR reaction was performed using RNA isolated from empty and gravid uterine horns of unilaterally pregnant rats. Expression of the -actin gene in the empty horn remained relatively constant at late gestation. Our data (Fig. 3) indicated that the increase in -actin gene expression during late pregnancy was significantly greater in gravid horns than in nongravid horns (P < 0.05). Interestingly, the expression of -actin mRNA was dramatically decreased on Day 1 PP in both empty and gravid horns, compared to midpregnant levels. The gravid horn mRNA profile of unilaterally pregnant rats agreed with that of normal pregnant rats at late gestation (Figs. 1A and 3). In contrast to the significantly elevated expression of -actin mRNA in the gravid horn, protein levels were only slightly increased and did not reach statistical significance (data not shown).
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Immunohistochemical staining for -actin and -SM actin revealed changes in the localization and the temporal distribution of actins in rat myometrium at various stages of pregnancy (Figs. 4– 7). Immunostaining of - and -SM actin was detected in both (longitudinal and circular) muscle layers of the gravid horn myometrium. Strong immunoreactivity of -actin in SMCs in the longitudinal layer was observed throughout gestation in all myometrial samples examined and no marked changes were detected. In contrast, in the circular muscle layer of gravid rat uterus we observed a temporal alteration of -actin staining. Immunostaining of -actin in circular uterine muscle layer of nonpregnant and early pregnant (Days 6–10) animals was extremely weak compared with that in the longitudinal layer (Fig. 4, A and B). However, starting from midgestation, -actin levels in the circular myometrial layer were dramatically upregulated (Fig. 4, C–E). This high level of protein expression in circular uterine muscle was maintained until labor (Day 23; Fig. 4E), and then declined by 4 days postpartum to almost undetectable levels (Fig. 4F). As with the PCR and Western blotting data, these changes were primarily limited to the gravid horns of unilaterally pregnant rats. Expression of -actin was very low in the circular muscle layers of empty uterine horns at early pregnancy and no increase was observed at mid- to late gestation (Fig. 5). These data demonstrate differential regulation of -actin expression between the muscle layers that would result in enhancement of contractile activity in the circular muscle of the gravid horn during late pregnancy. The spatial distribution of -actin immunoreactivity within myometrial SMCs was similar in both uterine muscle layers. Positive signals for -actin were detected throughout the cytoplasm and in the regions closely associated with plasma membrane of myometrial SMCs (Figs. 4 and 6). At early pregnancy and midpregnancy, the distribution of -actin was rather diffuse. In contrast, during late pregnancy, significant amount of -actin was found to concentrate close to the cell membrane making a continuous ring of actin staining localized around the SMC periphery (Fig. 6).
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Expression of -SM-actin was detected in both (longitudinal and circular) rat myometrial layers throughout pregnancy and did not show any gestational changes (Fig. 7). As with -actin, -SM-actin immunostaining was present exclusively in the cytoplasm of uterine myocytes throughout pregnancy.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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-actin (the major isoform of visceral muscle) is markedly upregulated during mid and late pregnancy, but that expression of -actin (the major actin isoform in vascular smooth muscle) is relatively unchanged during pregnancy. In addition, the increase in -actin during late pregnancy was limited to the circular muscle layer of the gravid horn, whereas this isoform was highly expressed in longitudinal muscle throughout pregnancy in both gravid and nongravid horns. Together these data indicate a complex spatial and temporal regulation of actin isoforms within the myometrium and suggest that the elevated expression of -actin in the circular SMCs contributes to the ability of this layer to generate optimal contractile activity during labor.
The suggestion that -actin is the major myometrial actin isoform during pregnancy is supported by the report of Cavaille [16], who found that total -actin protein was increased in human myometrial samples at term compared to samples from nonpregnant uterus. Two types of -actin, smooth muscle and cytoplasmic, have been reported based on amino acid [2] and cDNA sequencing [17]. Whereas Cavaille did not distinguish between these isoforms, our mRNA analysis (which used primers specific for the smooth muscle form) suggests that it is the smooth muscle isoform that is predominantly increased in the late pregnant rat myometrium. This may in part explain the limited changes in protein expression found by this study, in that the antibody used does not differentiate between the two isoforms of -actin.
This observation suggests that the mRNA and protein data may not be directly comparable. Whereas the primers for PCR recognize only smooth-muscle-specific -actin the antibodies for Western blotting recognized both smooth-muscle and cytoplasmic isoforms. We would suggest that the cytoplasmic -actin, that is a component of stable myometrial cytoskeletal stress fibrils, may represent a part of total -actin protein and, therefore, may account for the maintained high protein expression in postpartum tissues. Moreover, studies of aortic smooth muscle documented a switch of actin isoform from -smooth muscle actin in normal quiescent state to nonmuscle actins (including cytoplasmic -actin) during injury and tissue repair. These led us to believe that cytoplasmic -actin protein might be expressed higher in the postpartum myometrium when there is tremendous tissue involution and repair (reviewed in [18]).
Although -actin expression was already elevated in early pregnancy, the predominant increase was observed in mid- to late pregnancy, a time when the myometrial cells have been shown to exhibit a hypertrophic phenotype. A similar positive relationship between hypertrophy and increased expression of -actin has been reported in bladder (visceral) [8] and in hepatic portal vein (vascular) muscle [19]. Importantly, as in our analysis of the myometrium, these authors found no increase in expression of -SM-actin in association with muscle hypertrophy. This linkage of actin isoform expression with the differentiation state of smooth muscle has been reported widely in other muscles, with the differentiated state being associated with increased -actin and decreased -actin and the dedifferentiated state associated with a reversal of this pattern of actin expression [6]. Moreover, when mice deficient in cardiac-specific -actin were rescued by the ectopic expression of visceral SM -actin, their hearts were enlarged and hypertrophied [20]. Together these results strongly suggest that increase in -actin contributes to myometrial smooth muscle hypertrophy during pregnancy.
Our previous in vivo data in unilaterally pregnant rats suggest that myometrial hypertrophy (increase in protein: DNA ratio) during mid- to late pregnancy is controlled by mechanical (because of stretch of the uterine wall by the growing fetus) and hormonal (progesterone) signals [21]. Stretch has also been reported to increase hypertrophy of the nonpregnant [22] and to increase protein synthesis in the pregnant uterus [23]. Our finding that the increase in -actin occurred predominantly in the gravid uterine horn during midpregnancy suggested that stretch played a more important role in controlling the expression of this actin isoform than did elevated levels of progesterone, because there was no significant change in -actin levels of empty horns. Interestingly, mechanical stretch has been reported to decrease the expression of -SM actin mRNA and protein [24] and to induce degradation of the -SM actin filaments in vascular smooth muscle cells [25]. This is consistent with our observation that expression of this isoform does not increase (and may even decrease) when labor approaches. Other studies [26, 27] have reported that estrogen can induce increased expression of cytoplasmic actin isoforms (ß- and -actin) in cultured uterine smooth muscle cells. It is possible that some of the increase in -actin mRNA (but not protein) expression observed in early pregnancy may be caused by the elevated levels of estrogen present at this time.
The myometrium is the muscular part of the uterus that is responsible for generating the force to expel the fetus at term. It is composed of three well-defined layers. The circumferentially oriented SMCs of the circular muscle of the myometrium surround the endometrium. The SMCs of the outer longitudinal muscle layer of the myometrium align along the long axis of the uterus. Separating these two muscle layers is the vascular plexus, which is an extensive network of blood vessels that nourish the muscle layers. In order to investigate the distribution of -actin in the myometrial tissue throughout gestation, we have analyzed all three layers of myometrium. The differential expression of -actin in circular and longitudinal uterine smooth muscle is intriguing. During pregnancy the uterus initially grows longitudinally, with circumferential growth occurring from midpregnancy onwards in association with fetal growth. It would be expected that tension on the uterine wall at this time would be predominantly on the circular smooth muscle layer. Such data provide additional support for a role of mechanical signals in the expression of -actin. These muscle layers exhibit quite different phenotypes during pregnancy. Our previous studies have shown expression of the gap junction protein, connexin43 (also known as gap junction membrane channel protein alpha 1), is limited to uterine circular muscle [28], whereas others have reported differential production of prostaglandins [29], as well as different responses to stretch, noradrenaline and estrogen stimulation in circular versus longitudinal muscle [28, 30]. Such data imply specific roles for these layers during late pregnancy and labor. The circular muscle would provide the primary contractile response, whereas the role of the longitudinal layer is to shorten the uterus upon expulsion of each fetus.
The smooth muscle cells of the uterine vasculature express -actin. Our visual observation of -actin staining in vasculature of nonpregnant and early and late pregnant myometrium indicated no marked changes in the intensity of -actin immunostaining, whereas there was a dramatic change in the intensity of uterine muscle -actin staining. Moreover, we found equally high immunostaining in myometrial vessels from empty and gravid myometrial horns of unilaterally pregnant rats. The presence of intense -actin immunostaining in nonpregnant myometrium as well as in both the empty and gravid horns of pregnant rats suggested that -actin expression in myometrial vasculature is not a function of pregnancy. However, we visually detected an increase in actual size of myometrial blood vessels throughout pregnancy, although we did not find a difference in a number of blood vessels in individual fields of view. Because in our study we used whole myometrium to generate proteins, our Western blot results would include a component of vascular -actin. Therefore, we cannot exclude some contribution of vascular -actin to the Western blot data.
Our immunohistologic analysis revealed a change in the cellular distribution of -actin from a diffuse, cytoplasmic localization during early pregnancy to midpregnancy to a more membrane-associated distribution during late pregnancy. Because antibody used in this study does not discriminate between smooth muscle and cytoplasmic -actin isoforms it is not possible to conclude which isoform is involved in this redistribution. Myometrial cells are very rich in actin fibers, which generally appear as parallel bundles along the longest axis of the cells [31], and their presence is consistent with the contractile function of these cells. The cytoskeleton is known to act as a cell volume sensor and a structure primarily involved in linking protein filaments to each other and to anchoring sites in the adhesion plaques (reviewed in [31]). We have previously reported a marked increase in FAK activity in the myometrium during late pregnancy, together with increased expression of proteins within the focal adhesion complex adjacent to the cell membrane [12]. We speculate that the increased expression of -actin and its translocation to the membrane may facilitate smooth muscle contraction by 1) increasing the contractile protein content in a tissue and 2) supporting the interaction of myocytes with the surrounding collagen matrix, which is especially important at late gestation and during coordinated labor contractions.
In conclusion, our data demonstrated an intricate regulation of smooth muscle actin isoforms within the pregnant myometrium. The and isoforms exhibit differential spatial and temporal expression within the myometrium and our data imply complex interactions in the control of -actin (but not -actin) by hormonal and mechanical signals. We speculate that the increase in the expression and cellular distribution of -actin protein in circular smooth muscle cells of gravid horns during the latter half of gestation supports myometrial hypertrophy and, as labor approaches, contributes to the restoration of a contractile phenotype of myometrial smooth muscle cells.
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FOOTNOTES |
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2 Correspondence: Stephen J. Lye, Samuel Lunenfeld Research Institute at Mount Sinai, 600 University Avenue, Suite 870, Toronto, Ontario, Canada M5G 1X5. FAX: 416 586 8740; lye{at}mshri.on.ca
Received: 19 January 2005.
First decision: 3 February 2005.
Accepted: 20 June 2005.
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