Role of mechanical signaling in stem cell self-renewal and differentiation
Cells feel subtle but constant pushes and tugs from their neighbors inside living organisms. Surprisingly, these tiny mechanical cues have a profound effect on how stem cells grow, divide, and turn into the many different cells that make up the human body. Based on recent findings in developmental, cancer, and stem cell biology, we hypothesize that proteins known as cadherins, which allow cells to adhere to one another, are critical to the ability of stem cells to sense and respond to mechanical force. We will use a new form of microscopy to directly visualize the mechanical forces experienced by cadherins in living cells. This information will allow us to determine how stem cells detect force, and how they convert mechanical inputs into changes in gene expression that drive growth and differentiation.
Our work addresses two major, unsolved issues in stem cell biology: the factors that allow stem cells to turn into any kind of cell in the body, and the mechanism by which mechanical cues guide this process. This research will advance biology and medicine by teaching us more about how cells talk to each other using mechanical force, a topic about which very little is known. This project will have a potentially transformative impact on regenerative medicine by providing fundamental knowledge that will be directly applicable to new stem cell treatments, for example for heart disease, and for engineering new tissues to repair or replace diseased tissues or even entire organs.
Cells feel subtle but constant pushes and tugs from their neighbors inside living organisms. Surprisingly, these tiny mechanical cues have a profound effect on how stem cells grow, divide, and turn into the many different cells that make up the human body. However, at present we know almost nothing about how the input provided by mechanical force is connected to changes in stem cell behavior. Our research aims to fill this fundamental gap in our knowledge of how stem cells work. Understanding this aspect of their basic biology will be critical in developing new stem-cell-based treatments for heart disease, and for engineering new tissues to repair or replace diseased tissues and organs.
In the past year we have used CIRM funding to discover how the physical properties of a stem cell’s surroundings affect its growth, proliferation, and ability to turn into other cell types. Previous research shows that stem cells grown on soft surfaces grow into fat cells, while stem cells grown on hard surfaces grow into muscle or bone cells. At present we do not know how stem cells sense the stiffness of their surroundings. We created a protein that changes color when it is stretched, and used this protein to measure the forces inside living stem cells for the first time. We are using this protein to discover how stem cells sense mechanical forces, both from the surrounding tissue and from neighboring cells. Understanding this scientific question will help scientists to grow large numbers of stem cells, a critical bottleneck in regenerative medicine. In addition, it will help tissue engineers understand how to construct tissues and organs to replace those damaged by disease or injury. We are grateful to the citizens of California for their support and look forward to exciting discoveries in the coming year.
The past year has seen excellent progress across all of our Specific Aims. Specifically:
i) We have made the first, to our knowledge, quantitative measurements of intercellular mechanical force transmission between human embryonic stem cells (hESCs). This powerful technology has the capacity to transform our understanding of the molecular and physical underpinnings of tissue and organ morphogenesis.
ii) We identified probable proteins that control stem cell proliferation and pluripotency maintenance. We used a high-throughput proteomics strategy to identify proteins that are likely to regulate the localization and activity of transcription factors that regulate cell proliferation specifically in hESCs. This experiment revealed both previously known and novel players in this complex signaling network. Excitingly, newly identified proteins suggest the presence of uninvestigated mechanically activated signaling pathways in hESCs.
iii) We created tunable stiffness substrates for hESC culture with completely defined molecular composition. This tool is required to recreate the physical environment of the early embryo, and is expected to be critical discovering the molecular pathways that underlie mechanically directed hESC differentiation.
iv) We found that the expression levels and conformation of a specific protein that regulates cell-cell adhesion influence hESC colony morphology and adhesiveness. Separate, non-CIRM funded research in the Dunn lab showed that this same protein is likely a master mechanosensor at cell-cell junctions (Buckley et al. Science 2014). Together, these data hint at an underlying molecular mechanism that may direct the formation and shape of living tissues.