Stress fibers of the aortic smooth muscle cells in tissues do not align with the principal strain direction during intraluminal pressurization

Mechanical Trapping of the Cell Nucleus Into Microgroove Concavity But Not On Convexity Induces Cell Tissue Growth and Vascular Smooth Muscle Differentiation

Introduction

Vascular smooth muscle cells (VSMCs) in the normal aortic wall regulate vascular contraction and dilation. VSMCs change their phenotype from contractile to synthetic and actively remodel the aortic wall under pathological conditions. Findings on the differentiation mechanism of VSMCs have been reported in many in vitro studies; however, the mechanical environments in vivo aortic walls are quite different from those of in vitro culture conditions: VSMCs in vivo exhibit an elongated shape and form a tissue that aligns with the circumferential direction of the walls, whereas VSMCs in vitro spread randomly and form irregular shapes during cultivation on conventional flat culture dishes and dedifferentiate into a synthetic phenotype. To clarify the mechanisms underlying the VSMC differentiation, it is essential to develop a cell culture model that considers the mechanical environment of in vivo aortic walls.

Methods

We fabricated a polydimethylsiloxane-based microgrooved substrate with 5, 10, and 20 μm of groove width and 5 μm of groove depth to induce VSMC elongation and alignment as observed in vivo. We established a coating method to control cell adhesion proteins only on the surface of groove concavities and investigated the effects of mechanical trapping of the cell nucleus in microgroove concavities on the morphology of intracellular nuclei, cell proliferation and motility, and VSMC differentiation.

Results

We found that VSMCs adhering to the concavities formed a uniform cell tissue and allowed remarkable elongation. In particular, the microgrooves with 5 μm of groove width and depth facilitated a significant nuclear deformation and volume reduction of the nucleus due to a lateral compression by the side wall of the groove concavities that is relatively similar to a sandwich-like arrangement of in vivo elastic lamellae, resulting in the drastic inhibition of cell motility and proliferation, and the significant improvement of VSMC differentiation.

Conclusions

The results indicate that mechanical trapping of the cell nucleus into microgroove concavity but not on convexity induces cell tissue growth and VSMC differentiation. Our cell culture model with microgrooved substrates can be useful for studying the mechanisms of VSMC differentiation, considering the in vivo vascular mechanical environment.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-024-00827-w.

Stress fiber strain is zero in normal aortic smooth muscle, elevated in hypertensive stretch, and minimal in wall thickening rats

Hypertension causes aortic wall thickening until the original wall stress is restored. We hypothesized that this regulation involves stress fiber (SF) tension transmission to the nucleus in smooth muscle cells (SMCs) and investigated the strain in the SF direction as a condition required for this transmission. Thoracic aortas from Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHRs) were examined. SFs in aortic SMCs were fluorescently labeled and observed under a confocal microscope while stretched along the circumferential (θ) axis. Three conditions were studied: WKY physiological (WKYphys; blood pressure changes from diastolic to systolic for WKY), high-strain state (WKYhigh; diastolic to hypertensive level for WKY simulating initial hypertension), and SHR physiological (SHRphys; diastolic to systolic for SHR simulating after wall-thickening). SF strain and direction were measured. The SF inclination angle from the θ axis was 18° ± 3° in WKYphys, 13° ± 2° in WKYhigh, and 20° ± 1° in SHRphys. SF strain was 0.01 ± 0.02 in WKYphys, 0.20 ± 0.04 in WKYhigh, and 0.02 ± 0.02 SHRphys. SF strain was minimal in WKYphys, significantly increased in WKYhigh, and reduced to approximately zero in SHRphys. These findings support SFs function as mechanosensors in response to hypertension.

In situ FRET measurement of cellular tension using conventional confocal laser microscopy in newly established reporter mice expressing actinin tension sensor

FRET-based sensors are utilized for real-time measurements of cellular tension. However, transfection of the sensor gene shows low efficacy and is only effective for a short period. Reporter mice expressing such sensors have been developed, but sensor fluorescence has not been measured successfully using conventional confocal microscopy. Therefore, methods for spatiotemporal measurement of cellular tension in vivo or ex vivo are still limited. We established a reporter mouse line expressing FRET-based actinin tension sensors consisting of EGFP as the donor and mCherry as the acceptor and whose FRET ratio change is observable with confocal microscopy. Tension-induced changes in FRET signals were monitored in the aorta and tail tendon fascicles, as well as aortic smooth muscle cells isolated from these mice. The pattern of FRET changes was distinctive, depending on tissue type. Indeed, aortic smooth muscle cells exhibit different sensitivity to macroscopic tensile strain in situ and in an isolated state. This mouse strain will enable novel types of biomechanical investigations of cell functions in important physiological events.