Mechanically induced nuclear shuttling of β-catenin requires co-transfer of actin

Abstract

Mesenchymal stem cells (MSC) respond to environmental forces with both cytoskeletal re-structuring and activation of protein chaperones of mechanical information, β-catenin and Yes-Associated Protein 1 (YAP1). To function, MSCs must differentiate between dynamic forces such as cyclic strains of extracellular matrix due to physical activity and static strains due to ECM stiffening. To delineate how MSCs recognize and respond differently to both force types, we compared effects of dynamic (200 cycles x 2%) and static (1 × 2% hold) strain on nuclear translocation of β-catenin and YAP1 at 3h after force application. Dynamic strain induced nuclear accumulation of β-catenin, and increased cytoskeletal actin structure and cell stiffness, but had no effect on nuclear YAP1 levels. Critically, both nuclear actin and nuclear stiffness increased along with dynamic strain-induced β-catenin transport. Augmentation of cytoskeletal structure using either static strain or lysophosphatidic acid (LPA) did not increase nuclear content of β–catenin or actin, but induced robust nuclear increase in YAP1. As actin binds β-catenin, we considered whether β-catenin, which lacks a nuclear localization signal, was dependent on actin to gain entry to the nucleus. Knockdown of cofilin-1 (Cfl1) or importin-9 (Ipo9), which co-mediate nuclear transfer of G-actin, prevented dynamic strain-mediated nuclear transfer of both β-catenin and actin. In sum, dynamic strain induction of actin re-structuring promotes nuclear transport of G-actin, concurrently supporting nuclear access of β-catenin via mechanisms utilized for actin transport. Thus, dynamic and static strain activate alternative mechanoresponses reflected by differences in the cellular distributions of actin, β-catenin and YAP1.Significance statementCells integrate both static and dynamic mechanical signals through the actin cytoskeleton which is attached to the nuclear envelope, affecting nuclear transport of β-catenin and YAP1. Dynamic strain induces nuclear translocation of β-catenin, but not YAP1, while static strain causes nuclear translocation of YAP1, but not β-catenin. Importantly, nuclear transport of actin is induced by dynamic but not static force. Furthermore, nuclear import of β-catenin depends on cofilin/importin-9 dependent actin transport mechanisms. Thus the presence of β-catenin and YAP1 in the nucleus represent specific responses to regulatory mechanical signals.

YAP mechanotransduction under cyclic mechanical stretch loading for mesenchymal stem cell osteogenesis is regulated by ROCK

“…Our results provide additional evidence along with the study by the Mauck group that showed 3% strain and 1 Hz frequency cyclic stretching induced YAP nuclear transport in MSCs (Driscoll et al, 2015). A recent report from the Rubin's group showing a contrasting result-no YAP nuclear import in MSCs under cyclic stretching at 2% strain and 10 cycles/min (or 0.167 Hz) (Sen et al, 2022) suggests that either strain or frequency, or their combination, may affect YAP nuclear transport behavior under cyclic stretch loading. While these studies from Mauck and Rubin groups did not test the role of YAP on MSC osteogenic transcription, our results may suggest an implication for bone tissue engineering and skeletal regenerative medicine.…”

“…Robert Mauck's group showed that YAP nuclear transport was activated by cyclic stretching at 3% strain and 1 Hz frequency (Driscoll et al, 2015). On the other hand, Janet Rubin's group reported that cyclic stretching at 2% strain and 10 cycles/min did not induce YAP nuclear import (Sen et al, 2022). However, these two studies, both of which examined bone marrow MSCs, did not test the impact of YAP on MSC osteogenic transcription and differentiation.…”