In our research (https://bam.lab.mcgill.ca), we try to understand how cells generate and respond to forces. Diverse previous works have shown that forces also influence stem cell differentiation, selecting between general adipocyte or osteocyte lineage; however, the central mechanosensors that regulate this process have remained challenging to parse. Nevertheless, the nuclear localization and activity of Yes-associated protein (YAP) has remained a central player in regulating diverse cell biology, including differentiation. A publication from our lab last year (https://www.pnas.org/doi/10.1073/pnas.2301285120) revealed that nuclear compression, as opposed to substrate stiffness, is a key director of YAP activity.
To examine how this may influence stem cell differentiation, our paper describes a new way to compress nuclei in a systematic fashion by plating cells on micropatterned adhesive lines, implemented with an Alvéole PRIMO system (https://www.alveolelab.com). In the cover image of the May 21 issue of Biophysical Journal, we have two mesenchymal stem cells cultured on glass with identical cell culture media. The red fluorescence reveals the localization of YAP, whereas the blue fluorescence illustrates the extent of the nucleus. The cell on the left is plated on a thin (1.5-µm) line, whereas the cell on the right is plated on a thick (8-µm) line. When cells are on narrow patterns, actin stress fibers laterally compress the nucleus into a narrower cigar shape, whereas on thicker patterns the nuclei are significantly less compressed. Examining the extent of YAP fluorescence, one can see much higher nuclear localization in the compressed nucleus, which we subsequently show leads to osteogenic bone cell differentiation, whereas the cells with less-compressed nuclei and correspondingly less nuclear YAP develop into adipogenic fat cells.
Key to YAP localization in the nucleus is its ability to transit the nuclear membrane; in this particular image, we also examined how drugs that modulate nuclear pore activity influence YAP translocation. Here, we found that both active and passive transport contribute to YAP nuclear localization.
In the remainder of the study, we also investigated what aspect of nuclear deformation leads to YAP entry. By comparing variably deformed nuclei volume and surface curvature, we identified that nuclear curvature, or how strongly the nuclear envelope is bent, is the key determinant of YAP nuclear entry.
The research described has significant implications across various fields. Understanding how mechanical forces influence stem cell differentiation, particularly through nuclear compression directing YAP activity, offers potential for novel therapies in bone regeneration and fracture healing. Insights also extend to regenerative medicine, in which manipulation of mechanical cues may enhance tissue engineering for organ transplantation. In addition, in cancer biology, uncovering the role of YAP in nuclear localization sheds light on its involvement in cancer progression and metastasis, suggesting new avenues for targeted therapies. Moreover, in drug discovery, identifying drugs affecting nuclear pore activity and YAP translocation holds promise for treating diseases like cancer and fibrosis. Lastly, in biomedical engineering, using microfabrication techniques to manipulate mechanical cues opens doors to innovative tools for studying mechanobiology and tissue-engineering applications, showcasing the diverse real-world impacts of research in mechanotransduction and stem cell mechanobiology.
— Ajinkya Ghagre, Alice Delarue, Luv Kishore Srivastava, Newsha Koushki, and Allen Ehrlicher