Myometrial mechanoadaptation during pregnancy: implications for smooth muscle plasticity and remodelling

Evaluating aminophylline and progesterone combination treatment to modulate contractility and labor-related proteins in pregnant human myometrial tissues

Progesterone (P4) and cyclic adenosine monophosphate (cAMP) are regarded as pro-quiescent factors that suppress uterine contractions during pregnancy. We previously used human primary cells in vitro and mice in vivo to demonstrate that simultaneously enhancing myometrial P4 and cAMP levels may reduce inflammation-associated preterm labor. Here, we assessed whether aminophylline (Ami; phosphodiesterase inhibitor) and P4 can reduce myometrial contractility and contraction-associated proteins (CAPs) better together than individually; both agents are clinically used drugs. Myometrial tissues from pregnant non-laboring women were treated ex vivo with Ami acutely (while spontaneous contracting) or throughout 24-h tissue culture (±P4); isometric tension measurements, PKA assays, and Western blotting were used to assess tissue contractility, cAMP action, and inflammation. Acute (1 h) treatment with 250 and 750 μM Ami reduced contractions by 50% and 84%, respectively, which was not associated with a directly proportional increase in whole tissue PKA activity. Sustained myometrial relaxation was observed during 24-h tissue culture with 750 μM Ami, which did not require P4 nor reduce CAPs. COX-2 protein can be reduced by 300 nM P4 but this did not equate to myometrial relaxation. Ami (250 μM) and P4 (100 and 300 nM) co-treatment did not prevent oxytocin-augmented contractions nor reduce CAPs during interleukin-1β stimulation. Overall, Ami and P4 co-treatment did not suppress myometrial contractions more than either agent alone, which may be attributed to low specificity and efficacy of Ami; cAMP and P4 action at in utero neighboring reproductive tissues during pregnancy should also be considered.

Filament evanescence of myosin II and smooth muscle function

Smooth muscle is an integral part of hollow organs. Many of them are constantly subjected to mechanical forces that alter organ shape and modify the properties of smooth muscle. To understand the molecular mechanisms underlying smooth muscle function in its dynamic mechanical environment, a new paradigm has emerged that depicts evanescence of myosin filaments as a key mechanism for the muscle’s adaptation to external forces in order to maintain optimal contractility. Unlike the bipolar myosin filaments of striated muscle, the side-polar filaments of smooth muscle appear to be less stable, capable of changing their lengths through polymerization and depolymerization (i.e., evanescence). In this review, we summarize accumulated knowledge on the structure and mechanism of filament formation of myosin II and on the influence of ionic strength, pH, ATP, myosin regulatory light chain phosphorylation, and mechanical perturbation on myosin filament stability. We discuss the scenario of intracellular pools of monomeric and filamentous myosin, length distribution of myosin filaments, and the regulatory mechanisms of filament lability in contraction and relaxation of smooth muscle. Based on recent findings, we suggest that filament evanescence is one of the fundamental mechanisms underlying smooth muscle’s ability to adapt to the external environment and maintain optimal function. Finally, we briefly discuss how increased ROCK protein expression in asthma may lead to altered myosin filament stability, which may explain the lack of deep-inspiration–induced bronchodilation and bronchoprotection in asthma.

Tools, techniques, and future opportunities for characterizing the mechanobiology of uterine myometrium

The myometrium is the smooth muscle layer of the uterus that generates the contractions that drive processes such as menstruation and childbirth. Aberrant contractions of the myometrium can result in preterm birth, insufficient progression of labor, or other difficulties that can lead to maternal or fetal complications or even death. To investigate the underlying mechanisms of these conditions, the most common model systems have conventionally been animal models and human tissue strips, which have limitations mostly related to relevance and scalability, respectively. Myometrial smooth muscle cells have also been isolated from patient biopsies and cultured in vitro as a more controlled experimental system. However, in vitro approaches have focused primarily on measuring the effects of biochemical stimuli and neglected biomechanical stimuli, despite the extensive evidence indicating that remodeling of tissue rigidity or excessive strain is associated with uterine disorders. In this review, we first describe the existing approaches for modeling human myometrium with animal models and human tissue strips and compare their advantages and disadvantages. Next, we introduce existing in vitro techniques and assays for assessing contractility and summarize their applications in elucidating the role of biochemical or biomechanical stimuli on human myometrium. Finally, we conclude by proposing the translation of "organ on chip" approaches to myometrial smooth muscle cells as new paradigms for establishing their fundamental mechanobiology and to serve as next-generation platforms for drug development.

Costameres, dense plaques and podosomes: the cell matrix adhesions in cardiovascular mechanosensing

The stiffness of the cardiovascular environment changes during ageing and in disease and contributes to disease incidence and progression. For instance, increased arterial stiffness can lead to atherosclerosis, while stiffening of the heart due to fibrosis can increase the chances of heart failure. Cells can sense the stiffness of the extracellular matrix through integrin adhesions and other mechanosensitive structures and in response to this initiate mechanosignalling pathways that ultimately change the cellular behaviour. Over the past decades, interest in mechanobiology has steadily increased and with this also our understanding of the molecular basis of mechanosensing and transduction. However, much of our knowledge about the mechanisms is derived from studies investigating focal adhesions in non-muscle cells, which are distinct in several regards from the cell-matrix adhesions in cardiomyocytes (costameres) or vascular smooth muscle cells (dense plaques or podosomes). Therefore, we will look here first at the evidence for mechanical sensing in the cardiovascular system, before comparing the different cytoskeletal arrangements and adhesion sites in cardiomyocytes and vascular smooth muscle cells and what is known about mechanical sensing through the various structures.

Mechanobiology of mice cervix: expression profile of mechano-related molecules during pregnancy

There is a known reciprocation between the chronic exertion of force on tissue and both increased tissue density (e.g., bone) and hypertrophy (e.g., heart). This can also be seen in cervical tissue where the excessive gravitational forces associated with multiple fetal pregnancies promote preterm births. While there is a well-known regulation of cervical remodeling (CR) by sex steroid hormones and growth factors, the role of mechanical force is less appreciated. Using proteome-wide technology, we previously provided evidence for the presence of and alteration in mechano-related signaling molecules in the mouse cervix during pregnancy. Here, we profile the expression of select cytoskeletal factors (filamin-A, gelsolin, vimentin, actinin-1, caveolin-1, transgelin, keratin-8, profilin-1) and their associated signaling molecules [focal adhesion kinase (FAK) and the Rho GTPases CDC42, RHOA, and RHOB] in cervices of pregnant mice by real-time PCR and confocal immunofluorescence microscopy. Messenger RNA and protein levels increased for each of these 12 factors, except for 3 (keratin-8, profilin-1, RHOA) that decreased during the course of pregnancy and this corresponded with an increase in gravitational force exerted by the fetus on the cervix. We therefore conclude that size or weight of the growing fetus likely plays a key role in CR through mechanotransduction processes.

Multifactorial Regulation of Myometrial Contractility During Pregnancy and Parturition

The steroid hormones progesterone (P₄) and estradiol-17β (E₂), produced by the placenta in humans and the ovaries in rodents, serve crucial roles in the maintenance of pregnancy, and the initiation of parturition. Because of their critical importance for species survival, the mechanisms whereby P₄ and its nuclear receptor (PR) maintain myometrial quiescence during pregnancy, and for the decline in P₄/PR and increase in E₂/estrogen receptor (ER) function leading to parturition, are multifaceted, cooperative, and redundant. These actions of P₄/PR include: (1) PR interaction with proinflammatory transcription factors, nuclear factor κB (NF-κB), and activating protein 1 (AP-1) bound to promoters of proinflammatory and contractile/contraction-associated protein (CAP) genes and recruitment of corepressors to inhibit NF-κB and AP-1 activation of gene expression; (2) upregulation of inhibitors of proinflammatory transcription factor activation (IκBα, MKP-1); (3) induction of transcriptional repressors of CAP genes (e.g., ZEB1). In rodents and most other mammals, circulating maternal P₄ levels remain elevated throughout most of pregnancy and decline precipitously near term. By contrast, in humans, circulating P₄ levels and myometrial PR levels remain elevated throughout pregnancy and into labor. However, even in rodents, wherein P₄ levels decline near term, P₄ levels remain higher than the K_d for PR binding. Thus, parturition is initiated in all species by a series of molecular events that antagonize the P₄/PR maintenance of uterine quiescence. These events include: direct interaction of inflammatory transcription factors (e.g., NF-κB, AP-1) with PR; increased expression of P₄ metabolizing enzymes; increased expression of truncated/inhibitory PR isoforms; altered expression of PR coactivators and corepressors. This article will review various mechanisms whereby P₄ acting through PR isoforms maintains myometrial quiescence during pregnancy as well as those that underlie the decline in PR function leading to labor. The roles of P₄- and E₂-regulated miRNAs in the regulation and integration of these mechanisms will also be considered.

A computational model of excitation and contraction in uterine myocytes from the pregnant rat

Aberrant uterine myometrial activities in humans are major health issues. However, the cellular and tissue mechanism(s) that maintain the uterine myometrium at rest during gestation, and that initiate and maintain long-lasting uterine contractions during delivery are incompletely understood. In this study we construct a computational model for describing the electrical activity (simple and complex action potentials), intracellular calcium dynamics and mechanical contractions of isolated uterine myocytes from the pregnant rat. The model reproduces variant types of action potentials - from spikes with a smooth plateau, to spikes with an oscillatory plateau, to bursts of spikes - that are seen during late gestation under different physiological conditions. The effects of the hormones oestradiol (via reductions in calcium and potassium selective channel conductance), oxytocin (via an increase in intracellular calcium release) and the tocolytic nifedipine (via a block of L-type calcium channels currents) on action potentials and contractions are also reproduced, which quantitatively match to experimental data. All of these results validated the cell model development. In conclusion, the developed model provides a computational platform for further investigations of the ionic mechanism underlying the genesis and control of electrical and mechanical activities in the rat uterine myocytes.

Mechanical strain induced phospho-proteomic signaling in uterine smooth muscle cells

Mechanical strain associated with the expanding uterus correlates with increased preterm birth rates. Mechanical signals result in a cascading network of protein phosphorylation events. These signals direct cellular activities and may lead to changes in contractile phenotype and calcium signaling. In this study, the complete phospho-proteome of uterine smooth muscle cells subjected to mechanical strain for 5 min was compared to un-strained controls. Statistically significant, differential phosphorylation events were annotated by Ingenuity Pathway Analysis to elucidate mechanically induced phosphorylation networks. Mechanical strain leads to the direct activation of ERK1/2, HSPB1, and MYL9, in addition to phosphorylation of PAK2, vimentin, DOCK1, PPP1R12A, and PTPN11 at previously unannotated sites. These results suggest a novel network reaction to mechanical strain and reveal proteins that participate in the activation of contractile mechanisms leading to preterm labor.

Fetal-to-maternal signaling in the timing of birth

Preterm birth remains the major cause of neonatal morbidity and mortality throughout the world. This is due, in part, to our incomplete understanding of the mechanisms that underlie the maintenance of pregnancy and the initiation of parturition at term. In this article, we review our current knowledge of the complex, interrelated and concerted mechanisms whereby progesterone maintains myometrial quiescence throughout most of pregnancy, as well as those that mediate the upregulation of the inflammatory response and decline in progesterone receptor function leading to parturition. Herein, we review findings that demonstrate a role of the fetus in the timing of birth. Specifically, we focus on our own studies indicating that maturation of the fetal lung and enhanced secretion of the surfactant components, surfactant protein A (SP-A) and the potent inflammatory glycerophospholipid, platelet-activating factor (PAF), initiate a signaling cascade culminating in parturition. Our studies suggest an essential role of steroid receptor coactivators, SRC-1 and SRC-2, which activate expression of genes encoding SP-A and LPCAT1. LPCAT1 is a key enzyme in the synthesis of PAF, as well as DPPC, a highly surface-active glycerophospholipid component of surfactant. Thus, we describe a novel pathway through which the fetus contributes to the initiation of labor by signaling the mother when its lungs have achieved sufficient maturity for survival in an aerobic environment.

Proteomics study reveals that the dysregulation of focal adhesion and ribosome contribute to early pregnancy loss

Purpose

Early pregnancy loss (EPL) affects 50–70% pregnant women in first trimester. The precise molecular mechanisms underlying EPL are far from being fully understood. Therefore, we aim to identify the molecular signaling pathways relating to EPL.

Experimental design

We performed proteomics and bioinformatics analysis of the placental villi in women who have undergone EPL and in normal pregnant women. The proteomics data were validated by Western blot analysis.

Results

We identified a total of 5952 proteins in placental villi, of which 588 proteins were differentially expressed in the EPL women. Bioinformatics analysis revealed that these differentially expressed proteins participated in a variety of signaling pathways, including the focal adhesion pathway and ribosome pathway. Moreover, results of the Western blot confirmed that Desmin, Lamin A/C, MMP‐9, and histone H4 were upregulated in EPL and the Lamin C/ Lamin A ratio decreased obviously in EPL. These proteins could be associated with the pathophysiology of EPL. The data have been deposited to the ProteomeXchange with identifier PXD002391.

Conclusion and clinical relevance

Our study demonstrated that the focal adhesion pathway and ribosome pathway are involved in EPL, and these findings might contribute to unveil the pathophysiology of EPL.

Electrophysiological Features of Telocytes

Telocytes (TCs) are interstitial cells described in multiple structures, including the gastrointestinal tract, respiratory tract, urinary tract, uterus, and heart. Several studies have indicated the possibility that TCs are involved in the pacemaker potential in these organs. It is supposed that TCs are interacting with the neighboring muscular cells and their network contributes to the initiation and propagation of the electrical potentials. In order to understand the contribution of TCs to various excitability mechanisms, it is necessary to analyze the plasma membrane proteins (e.g., ion channels) functionally expressed in these cells. So far, potassium, calcium, and chloride currents, but not sodium currents, have been described in TCs in primary cell culture from different tissues. Moreover, TCs have been described as sensors for mechanical stimuli (e.g., contraction, extension, etc.). In conclusion, TCs might play an essential role in gastrointestinal peristalsis, in respiration, in pregnant uterus contraction, or in miction, but further highlighting studies are necessary to understand the molecular mechanisms and the cell-cell interactions by which TCs contribute to the tissue excitability and pacemaker potentials initiation/propagation.

Progress in understanding electro-mechanical signalling in the myometrium

In this review, we give a state-of-the-art account of uterine contractility, focussing on excitation-contraction (electro-mechanical) coupling (ECC). This will show how electrophysiological data and intracellular calcium measurements can be related to more modern techniques such as confocal microscopy and molecular biology, to advance our understanding of mechanical output and its modulation in the smooth muscle of the uterus, the myometrium. This new knowledge and understanding, for example concerning the role of the sarcoplasmic reticulum (SR), or stretch-activated K channels, when linked to biochemical and molecular pathways, provides a clearer and better informed basis for the development of new drugs and targets. These are urgently needed to combat dysfunctions in excitation-contraction coupling that are clinically challenging, such as preterm labour, slow to progress labours and post-partum haemorrhage. It remains the case that scientific progress still needs to be made in areas such as pacemaking and understanding interactions between the uterine environment and ion channel activity.

Integrin upregulation and localization to focal adhesion sites in pregnant human myometrium

Focal adhesions are integrin-rich microdomains that structurally link the cytoskeleton to the extracellular matrix and transmit mechanical signals. In the pregnant uterus, increases in integrin expression and activation are thought to be critical for the formation of the mechanical syncytium required for labor. The aim of this study was to determine which integrins are upregulated and localized to focal adhesions in pregnant human myometrium. We used quantitative polymerase chain reaction, Western blotting, and confocal microscopy to determine the expression levels and colocalization with focal adhesion proteins. We observed increases in several integrin transcripts in pregnant myometrium. At the protein level, integrins such as α5-integrin (ITGA5), ITGA7, ITGAV, and ITGB3 were significantly increased during pregnancy. The integrins ITGA3, ITGA5, ITGA7, and ITGB1 colocalized with focal adhesion proteins in term human myometrium. These data suggest that integrins α3β1, α5β1, and α7β1 are the most likely candidates to transmit mechanical signals from the extracellular matrix through focal adhesions in pregnant human myometrium.

Mice deficient in surfactant protein A (SP-A) and SP-D or in TLR2 manifest delayed parturition and decreased expression of inflammatory and contractile genes

Previously we obtained compelling evidence that the fetus provides a critical signal for the initiation of term labor through developmental induction of surfactant protein (SP)-A expression by the fetal lung and secretion into amniotic fluid (AF). We proposed that interactions of AF macrophage (Mϕ) Toll-like receptors (TLRs) with SP-A, at term, or bacterial components, at preterm, result in their activation and migration to the pregnant uterus. Herein the timing of labor in wild-type (WT) C57BL/6 mice was compared with mice homozygous null for TLR2, SP-A, SP-D, or doubly deficient in SP-A and SP-D. Interestingly, TLR2(-/-) females manifested a significant (P < 0.001) delay in timing of labor compared with WT as well as reduced expression of the myometrial contraction-associated protein (CAP) gene, connexin-43, and Mϕ marker, F4/80, at 18.5 d postcoitum (dpc). Whereas in first pregnancies, SP-A(-/-), SP-D(-/-), and SP-A/D(-/-) females delivered at term (∼19.5 dpc), in second pregnancies, parturition was delayed by approximately 12 h in SP-A(-/-) (P = 0.07) and in SP-A/D(-/-) (P <0.001) females. Myometrium of SP-A/D(-/-) females expressed significantly lower levels of IL-1β, IL-6, and CAP genes, connexin-43, and oxytocin receptor at 18.5 dpc compared with WT. F4/80(+) AF Mϕs from TLR2(-/-) and SP-A/D(-/-) mice expressed significantly lower levels of both proinflammatory and antiinflammatory activation markers (e.g. IL-1β, IL-6, ARG1, YM1) compared with gestation-matched WT AF Mϕs. These novel findings suggest that the pulmonary collectins acting via TLR2 serve a modulatory role in the timing of labor; their relative impact may be dependent on parity.

Histomorphometric analysis of the effects of creatine on rat myometrium

OBJECTIVE

To analyze the myometrial thickness of rats subjected to creatine (Cr) ingestion.

STUDY DESIGN

A total of 14 rats was equally divided into the control group (ConGr) receiving 1 ml potable water and the creatine group (CrGr) subjected to the ingestion of 1.6 g/kg Cr diluted in 1 ml potable water. At the end of 8 weeks, the animals were anesthetized (xylazine and ketamine) and sacrificed, the uteri and ovaries stained with hematoxylin and eosin, the thickness of both the myometrium and the epithelium measured and the follicles counted.

RESULTS

Analysis revealed a significant increase in thickness of the myometrium in the CrGr (272.26 ± 66.71μm) contrasted with that from the ConGr (160.76 ± 35.65μm), CrGr > ConGr (p < 0001).

CONCLUSION

Our data suggest that Cr changed myometrial morphology in rats by enhancing myometrial thickness, but its action mechanism in the smooth muscle is still unclear.

What do we know about what happens to myometrial function as women age?

Much has been written about the effects of aging on reproductive function, especially female fertility. Much less is known about how aging may affect the contractility of the smooth muscle within the uterus, the myometrium. The myometrium is active through a woman’s entire life, not just during pregnancy. Here we will discuss briefly the contractile functions of the uterus and the changes it undergoes throughout the stages of a woman’s life from menstruation and the menopause, before evaluating the evidence for any changes in myometrial contractility and responses as women age, with a particular focus on women of advanced maternal age. We present original contractility analysis for the widest data set for human myometrium so far examined, and determine inherent spontaneous activity as well as responses to depolarisation and stimulation with oxytocin. Our data show that in the non-pregnant state there is a significant decrease in contractility for both spontaneous and depolarised-induced contractions, with age. We suggest that muscle atrophy and down regulation of Ca channels may account for this. Interestingly in pregnant myometrium we found a wide range of contractile ability between women and little evidence for decreased spontaneous activity between the ages of 25–40. Oxytocin responses appear to be more affected by aging, a finding that is consistent with previously reported clinical findings, and may partly be the result of membrane lipids such as cholesterol, increasing as women age. The marked differences between the age-related decline of force beyond age 30 in non-pregnant uterus, and the lack of difference in the pregnant state over this period, shows that the uterus retains its ability to respond to gestational hormones. The growth of the pregnant uterus and increase in content of myofibrillar proteins, may abolish any previous age-related force deficit. This finding is consistent with what is apparent for postmenopausal women in their 50s and 60s; that with the appropriate hormonal stimulation the uterus can allow an embryo to implant, and then without further intervention, carry the foetus to term. It is tempting therefore to speculate that unlike other well documented declines in female reproductive functions with age, the myometrium remains able to function into a woman’s 7th decade.

Carbachol-induced volume adaptation in mouse bladder and length adaptation via rhythmic contraction in rabbit detrusor

The length-tension (L-T) relationships in rabbit detrusor smooth muscle (DSM) are similar to those in vascular and airway smooth muscles and exhibit short-term length adaptation characterized by L-T curves that shift along the length axis as a function of activation and strain history. In contrast to skeletal muscle, the length-active tension (L-T(a)) curve for rabbit DSM strips does not have a unique peak tension value with a single ascending and descending limb. Instead, DSM can exhibit multiple ascending and descending limbs, and repeated KCl-induced contractions at a particular muscle length on an ascending or descending limb display increasingly greater tension. In the present study, mouse bladder strips with and without urothelium exhibited KCl-induced and carbachol-induced length adaptation, and the pressure-volume relationship in mouse whole bladder displayed short-term volume adaptation. Finally, prostaglandin-E(2)-induced low-level rhythmic contraction produced length adaptation in rabbit DSM strips. A likely role of length adaptation during bladder filling is to prepare DSM cells to contract efficiently over a broad range of volumes. Mammalian bladders exhibit spontaneous rhythmic contraction (SRC) during the filling phase and SRC is elevated in humans with overactive bladder (OAB). The present data identify a potential physiological role for SRC in bladder adaptation and motivate the investigation of a potential link between short-term volume adaptation and OAB with impaired contractility.

Physiologic uterine inflammation and labor onset: integration of endocrine and mechanical signals

The mechanisms underlying the preparation of the uterus for labor are not fully understood. Our previous studies have shown that during pregnancy, the uterine muscle (myometrium) undergoes dramatic phenotypic modulation culminating in term labor. The current review will discuss the cellular mechanisms involved in the regulation of myometrial activity and its modulation by endocrine signals and by mechanical stimulation of the uterus by the growing fetus. In particular, the contribution of uterine inflammation to the onset of labor will be described. We provide evidence that increased production of cytokines/chemokines in pregnant myometrium is associated with uterine occupancy and regulated by progesterone, suggesting the integration of mechanical and endocrine signals. Myometrial cells can actively participate in the inflammatory process in the uterus through the release of multiple proinflammatory cytokines and chemokines, providing a strong signal for activation of immune cells, their subsequent infiltration into pregnant uterus, and the initiation of labor.

Focal adhesion kinase regulation of mechanotransduction and its impact on endothelial cell functions

Vascular endothelial cells lining the blood vessels form the interface between the bloodstream and the vessel wall and as such they are continuously subjected to shear and cyclic stress from the flowing blood in the lumen. Additional mechanical stimuli are also imposed on these cells in the form of substrate stiffness transmitted from the extracellular matrix components in the basement membrane, and additional mechanical loads imposed on the lung endothelium as the result of respiration or mechanical ventilation in clinical settings. Focal adhesions (FAs) are complex structures assembled at the abluminal endothelial plasma membrane which connect the extracellular filamentous meshwork to the intracellular cytoskeleton and hence constitute the ideal checkpoint capable of controlling or mediating transduction of bidirectional mechanical signals. In this review we focus on focal adhesion kinase (FAK), a component of FAs, which has been studied for a number of years with regards to its involvement in mechanotransduction. We analyzed the recent advances in the understanding of the role of FAK in the signaling cascade(s) initiated by various mechanical stimuli with particular emphasis on potential implications on endothelial cell functions.

Length oscillation mimicking periodic individual deep inspirations during tidal breathing attenuates force recovery and adaptation in airway smooth muscle

Airway smooth muscle (ASM) is able to generate maximal force under static conditions, and this isometric force can be maintained over a large length range due to length adaptation. The increased force at short muscle length could lead to excessive narrowing of the airways. Prolonged exposure of ASM to submaximal stimuli also increases the muscle's ability to generate force in a process called force adaptation. To date, the effects of length and force adaptation have only been demonstrated under static conditions. In the mechanically dynamic environment of the lung, ASM is constantly subjected to periodic stretches by the parenchyma due to tidal breathing and deep inspiration. It is not known whether force recovery due to muscle adaptation to a static environment could occur in a dynamic environment. In this study the effect of length oscillation mimicking tidal breathing and deep inspiration was examined. Force recovery after a length change was attenuated in the presence of length oscillation, except at very short lengths. Force adaptation was abolished by length oscillation. We conclude that in a healthy lung (with intact airway-parenchymal tethering) where airways are not allowed to narrow excessively, large stretches (associated with deep inspiration) may prevent the ability of the muscle to generate maximal force that would occur under static conditions irrespective of changes in mean length; mechanical perturbation on ASM due to tidal breathing and deep inspiration, therefore, is the first line of defense against excessive bronchoconstriction that may result from static length and force adaptation.

Retinoic acid metabolizing enzyme CYP26A1 is implicated in rat embryo implantation

BACKGROUND

The retinoic acid metabolizing enzyme Cyp26a1 plays a pivotal role in vertebrate embryo development. Cyp26a1 was characterized previously as a differentially expressed gene in peri-implantation rat uteri via suppressive subtracted hybridization analysis. However, the role of Cyp26a1 in rat embryo implantation remained elusive.

METHODS

The expression of Cyp26a1 in the uteri of early pregnancy, pseudopregnancy and artificial decidualization was detected by northern blotting, real time-PCR, in situ hybridization, western blotting and immunofluorescent staining. The effect of Cyp26a1 on apoptosis of endometrial stromal cells (ESCs) isolated from rat uteri was determined by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) and Hoechst staining. Apoptosis-related proteins in ESCs were detected by western blotting.

RESULTS

Cyp26a1 showed distinctive expression patterns in embryos and uteri during the peri-implantation period, with a remarkable increase (P < 0.01 versus Days 4-5) in mRNA and protein in the implantation phase (Days 5.5-6.5 of pregnancy). CYP26A1 was specifically localized in glandular epithelium, luminal epithelium and decidua basalis. The level of CYP26A1 protein was significantly increased in uteri of artificial decidualization (P < 0.01 versus control). Forced Cyp26a1 overexpression significantly reduced the sensitivity of ESCs to etoposide-induced apoptosis, with reductions in p53 (P < 0.01) and Fas (P < 0.05) proteins versus control, while in contrast, FasL (P < 0.01) and proliferating cell nuclear antigen (P < 0.05) proteins increased.

CONCLUSIONS

Cyp26a1 is spatiotemporally expressed in the uterus during embryo implantation and decidualization. Overexpression of Cyp26a1 attenuates the process of uterine stromal cell apoptosis, probably via down-regulating the expression of p53 and FasL.

Dense-body aggregates as plastic structures supporting tension in smooth muscle cells

The wall of hollow organs of vertebrates is a unique structure able to generate active tension and maintain a nearly constant passive stiffness over a large volume range. These properties are predominantly attributable to the smooth muscle cells that line the organ wall. Although smooth muscle is known to possess plasticity (i.e., the ability to adapt to large changes in cell length through structural remodeling of contractile apparatus and cytoskeleton), the detailed structural basis for the plasticity is largely unknown. Dense bodies, one of the most prominent structures in smooth muscle cells, have been regarded as the anchoring sites for actin filaments, similar to the Z-disks in striated muscle. Here, we show that the dense bodies and intermediate filaments formed cable-like structures inside airway smooth muscle cells and were able to adjust the cable length according to cell length and tension. Stretching the muscle cell bundle in the relaxed state caused the cables to straighten, indicating that these intracellular structures were connected to the extracellular matrix and could support passive tension. These plastic structures may be responsible for the ability of smooth muscle to maintain a nearly constant tensile stiffness over a large length range. The finding suggests that the structural plasticity of hollow organs may originate from the dense-body cables within the smooth muscle cells.

Smoothelin-like 1 protein regulates myosin phosphatase-targeting subunit 1 expression during sexual development and pregnancy

Pregnancy coordinately alters the contractile properties of both vascular and uterine smooth muscles reducing systemic blood pressure and maintaining uterine relaxation. The precise molecular mechanisms underlying these pregnancy-induced adaptations have yet to be fully defined but are likely to involve changes in the expression of proteins regulating myosin phosphorylation. Here we show that smoothelin like protein 1 (SMTNL1) is a key factor governing sexual development and pregnancy induced adaptations in smooth and striated muscle. A primary target gene of SMTNL1 in these muscles is myosin phosphatase-targeting subunit 1 (MYPT1). Deletion of SMTNL1 increases expression of MYPT1 30-40-fold in neonates and during development expression of both SMTNL1 and MYPT1 increases over 20-fold. Pregnancy also regulates SMTNL1 and MYPT1 expression, and deletion SMTNL1 greatly exaggerates expression of MYPT1 in vascular smooth muscle, producing a profound reduction in force development in response to phenylephrine as well as sensitizing the muscle to acetylcholine. We also show that MYPT1 is expressed in Type2a muscle fibers in mice and humans and its expression is regulated during pregnancy, suggesting unrecognized roles in mediating skeletal muscle plasticity in both species. Our findings define a new conserved pathway in which sexual development and pregnancy mediate smooth and striated muscle adaptations through SMTNL1 and MYPT1.

Mechanical stretch regulates hypertrophic phenotype of the myometrium during pregnancy

The adaptive growth of the uterus is a critical event that involves changes in cellular phenotypes throughout pregnancy. In early pregnancy, uterine growth is due to hyperplasia of uterine smooth muscle cells (SMCs) within the myometrium; however, the major component of myometrial growth occurs after mid-gestation. This study sought to test the hypothesis that increase in myometrial growth seen during late pregnancy is due to SMC hypertrophy caused by mechanical stretch of uterine tissue by a growing fetus(es) by providing direct measurements of individual SMC size. We employed a stereological approach to calculate the average cell volumes of uterine myocytes through diameter measurements using the Stereoinvestigator statistical software. Uterine tissues were collected from nonpregnant Wistar rats, as well as from gravid and nongravid horns of unilaterally pregnant animals on gestational days (d) 8 (early gestation), 14 (mid-gestation), 19 (late gestation), 22 (term), and 4 days post partum. Anti-caveolin-1 immunostaining was used to clearly delineate SMC boundaries. The stereological analysis revealed that the dramatic increase in myometrial growth seen during late gestation (d19-22) is due to a threefold increase in the size of uterine myocytes. A significant increase in SMC volumes was detected in the gravid uterine horn as compared with the corresponding empty horn of unilateral term pregnant animals (day 22, mean cell volume 1114 vs 361 microm(3), P<0.05), indicating the effect of uterine occupancy. The restriction of the hypertrophy to cells within the gravid horn suggests that it may be a response to the biological mechanical stretch of uterine walls by the growing fetus(es) and placenta(s).

Stretch activates human myometrium via ERK, caldesmon and focal adhesion signaling

An incomplete understanding of the molecular mechanisms responsible for myometrial activation from the quiescent pregnant state to the active contractile state during labor has hindered the development of effective therapies for preterm labor. Myometrial stretch has been implicated clinically in the initiation of labor and the etiology of preterm labor, but the molecular mechanisms involved in the human have not been determined. We investigated the mechanisms by which gestation-dependent stretch contributes to myometrial activation, by using human uterine samples from gynecologic hysterectomies and Cesarean sections. Here we demonstrate that the Ca requirement for activation of the contractile filaments in human myometrium increases with caldesmon protein content during gestation and that an increase in caldesmon phosphorylation can reverse this inhibitory effect during labor. By using phosphotyrosine screening and mass spectrometry of stretched human myometrial samples, we identify 3 stretch-activated focal adhesion proteins, FAK, p130Cas, and alpha actinin. FAK-Y397, which signals integrin engagement, is constitutively phosphorylated in term human myometrium whereas FAK-Y925, which signals downstream ERK activation, is phosphorylated during stretch. We have recently identified smooth muscle Archvillin (SmAV) as an ERK regulator. A newly produced SmAV-specific antibody demonstrates gestation-specific increases in SmAV protein levels and stretch-specific increases in SmAV association with focal adhesion proteins. Thus, whereas increases in caldesmon levels suppress human myometrium contractility during pregnancy, stretch-dependent focal adhesion signaling, facilitated by the ERK activator SmAV, can contribute to myometrial activation. These results suggest that focal adhesion proteins may present new targets for drug discovery programs aimed at regulation of uterine contractility.

Myometrial interstitial cells and the coordination of myometrial contractility

A strict regulation of contractility in the uterus and fallopian tube is essential for various reproductive functions. The uterus contributes, through either increased contractility or periods of relative quiescence, to: (i) expulsion of menstrual debris, (ii) sperm transport, (iii) adequate embryo placement during implantation, (iv) enlarging its capacity during pregnancy and (v) parturition. The dominant cell population of the uterine wall consists of smooth muscle cells that contain the contractile apparatus responsible for the generation of contractile force. Recent interest has focused on a new population of cells located throughout the myometrium on the borders of smooth muscle bundles. These cells are similar to interstitial cells of Cajal (ICC) in the gut that are responsible for the generation of electrical slow waves that control peristalsis. A precise role for myometrial Cajal-like interstitial cells (m-ICLC) has not been identified. m-ICLC express the c-kit receptor, involved in creating and maintaining the ICC phenotype in the gastrointestinal tract. However, both acute and prolonged inhibition of this receptor with the c-kit antagonist imatinib mesylate does not appear to affect the spontaneous contractility of myometrium. Calcium imaging of live tissue slices suggests that contractile signalling starts on the borders of smooth muscle bundles where m-ICLC are located and recently the possible role of extracellular ATP signalling from m-ICLC has been studied. This manuscript reviews the evidence regarding tissue-level signalling in the myometrium with a particular emphasis on the anatomical and possible functional aspects of m-ICLC as new elements of the contractile mechanisms in the uterus.