Piezo1 activation by stretching of uterine myometrium supports pregnancy and prevents preterm labour

During pregnancy, the uterus undergoes significant distention to accommodate the growing fetus. Myometrium, the inner lining of the uterus that supports the fetus, comprises 90% smooth muscles, followed by endothelial cells, fibroblasts, stromal cells and other cell types. Interestingly, though the smooth muscles are highly stretched during the entire gestational period of 40 weeks, they remain in a quiescent state for the entire duration. This quiescent state of the myometrium is clinically vital for both the mother and the child as initiation of untimely contractions leads to preterm labour (PTL). Often PTL leads to preterm birth, which is the leading cause of death in children under 5 years of age. Notably, children who are born preterm often have poor health and weak immunity that persists for life. PTL-induced preterm delivery (PTD) is a serious health risk for the mother and the offspring, besides being a heavy strain on the healthcare and financial system globally. It is notable that despite efforts at developing tocolytic drugs to prevent PTL and PTD, we still lack drugs that can extend the gestational period to complete term. This reinforces the importance of the quiescence mechanism of the myometrium in preventing PTL and associated complications. The mechanism involved in maintaining this unique quiescence feature of uterine muscles is largely unknown. Motor nerves do not innervate the human myometrium, while the identity and role of pacemaker cell types is contentious. Therefore, it is logical to speculate that the mechanical force generated due to distention of the myometrium is itself responsible for the quiescence. The mechanosensitive channels present in the myometrium likely sense this mechanical stretch and their activation triggers a mechanism that sustains quiescence. In a recent study published in The Journal of Physiology, Barnett et al. set out to characterize the expression pattern and (patho)physiological role of the mechanosensitive channel PIEZO1 in quiescence and PTL mechanisms of the human myometrium (Barnett et al., 2022). They found that PIEZO1 levels are comparable in the myometrium of non-pregnant women and women at term or preterm but without initiation of labour. Interestingly, myometrium from women experiencing term labour (TL) has four-fold higher expression of PIEZO1. On the contrary, myometrium from women experiencing PTL has 14-fold lower expression of Piezo1. This observation indicates a direct correlation between the protein expression levels of PIEZO1 and initiation of term or preterm labour. The primary human myometrium endothelial cells show three-fold higher expression of PIEZO1 together with three-fold higher calcium influx in response to pharmacological activation of PIEZO1 by Yoda1 in comparison to the primary human uterine smooth muscle cells. Therefore, physiologically the effect of myometrium stretching is likely to be more pronounced in the myometrium endothelial cells, triggering calcium influx-induced activation of endothelial nitric oxide synthase (eNOS). Nitric oxide produced by eNOS is known to induce vascular smooth muscle relaxation (Grange et al., 2001). Myometrium obtained from non-labouring women at term was simulated with oxytocin to mimic labouring condition. Contractile recordings of this primed myometrium exposed to the PIEZO1 activator Yoda1 revealed that myometrium tissue becomes more relaxed with increasing concentration of Yoda1. While Yoda1 treatment alone reduced the contractions of the myometrium to 17% of interested control, competitive inhibition by Dooku1 rescued oxytocin-induced contractions to 90%, indicating a direct effect of PIEZO1 activation in relaxation of human myometrium. Pre-incubation with protein kinase A (PKA) or AKT inhibitors prevented Yoda1-induced relaxation of the primed myometrium, indicating that both PKA and AKT are activated downstream of Piezo1 activation to maintain quiescence. eNOS inhibition was partially effective in blocking Yoda1-induced relaxation of the primed myometrium. Inhibition of PKA, AKT or eNOS resulted in significant increase in peak tension of the myometrium, despite Yoda1 stimulation (Fig. 1). Notably, combination of Yoda1 treatment with activation or inhibition of BKCa channels showed neither more relaxation nor a significant effect on peak tension. None of the above treatments had any effect on the contractile frequency of the myometrium, when compared to Yoda1 treatment alone. This indicates that naturally distended myometrium activates a fraction of its PIEZO1 channels to trigger signalling pathways that keep it in a quiescent state. However, this also leaves room for further pharmacological activation of the remaining PIEZO1 channels to get additional relaxation of the smooth muscle. PTD is harmful for the mother's long-term health as well as the child's. Women who deliver preterm are at two-fold higher risk of future cardiovascular diseases (CVD) (Tanz et al., 2017). Higher CVD risk in women with a history of PTD is linked with pre-pregnancy risk factors such as smoking, sedentary lifestyle, food habits, body mass index (BMI) and genetics. It is not known to what extent each risk factor accounts for the development of CVD risk factors after PTD. Nevertheless, women with PTD and preeclampsia experience up to 8.12-fold higher risk of CVD death, independent of lifestyle or other socioeconomic variations. However, the impact of traditional CVD risk biomarkers such as lipid profile and glucose in the context of PTD and subsequent future maternal CVD risk warrants further investigation. Limited studies have shown that the abnormal vascular response to pregnancy and subsequent delivery may be an early warning sign for the development of future maternal CVD. Some studies found higher vascular stiffness and lower smooth muscle tone in women with medically indicated PTD than spontaneous PTD and term delivery. Furthermore, biomarkers of impaired endothelial function such as soluble E-selectin, soluble intercellular adhesion molecule-1 (sICAM-1) and soluble vascular adhesion molecule-1 (sVCAM-1) are increased in women with PTD. Literature suggests vascular function accounts for pregnancy and subsequent labour. In fact, it is often considered that overdistention of the myometrium along with higher inflammation triggers PTL (Adams et al., 2015). Though the involvement of immune cells and their inflammatory status cannot be ruled out in this context, Barnett et al. have provided compelling evidence for PIEZO1 as a tocolytic target against PTL and PTD. The importance of PIEZO1 is well-known in vascular function including endothelial shear stress sensing, NO generation, vascular tone, angiogenesis, vascular remodelling and blood pressure regulation (Beech & Kalli, 2019). Barnett et al. showed that PIEZO1 expression levels in human myometrium modulate gestational age. The vascular health is tightly interlinked with gestational age, and PIEZO1 functions as a modulator in this setting. The cumulative evidence suggests the PIEZO1 expression levels can be used as a biomarker to predict the gestational period, and it is a specific therapeutic target to reduce the number of PTDs. Nevertheless, women with lower levels of PIEZO1 expression levels may show higher risk of PTD and susceptibility to CVD. It seems women with a history of PTD are likely to develop CVD later in their life, due to the underlying risk factors of vascular malfunction as the result of lower level of functional PIEZO1 levels. Considering the ubiquitous role of PKA and AKT in physiological signalling mechanisms, it is unlikely that their inhibitors can provide a desired relaxation of myometrium without showing adverse effects at a systemic level. The alternative tocolytic approach of using reagents that undergo biotransformation resulting in formation and release of nitric oxide has failed to provide desired results. Discovery of a non-invasive method to measure PIEZO1 expression levels or activity in pregnant women might provide a screening test to segregate women with higher risk of PTD. Furthermore, developing a pharmaceutical agent that specifically and precisely activates PI may be a promising therapeutic paradigm. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. None. Both authors have read and approved the final version of this manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed. S.P. has received funding from Carl von Ossietzky University of Oldenburg. R.K.M. has received funding from the European Union's Horizon 2020 research and innovation programme under Marie Sklodowska-Curie grant agreement No-801133.