Biophysical regulation of cell function : the yin and yang of the microenvironment

Abstract: From embryonic development to tissue regeneration and disease progression, the human body is continuously subject to mechanical stresses. Physical forces are increasingly recognized as major microenvironmental cues that control tensional homeostasis in tissues. Cells constantly receive and translate physical cues into biological messages, which in turn dictate cell shape, state and function. While much is known about biochemical signaling, many of the mechanisms that drive cell outcome in response to biophysical influences remain to be uncovered. Here we have investigated biophysical regulation of cell function. The goal was to gain a deeper understanding of fundamental principles that govern cell behavior in response to physical stimuli. To carefully recapitulate signaling in the in vivo microenvironment, we utilized a battery of tools that stem from the field of bioengineering. We used conjugated polymers to develop a novel neural stem cell culture substrate with anchored growth factors to promote cell self-renewal. Upon an electrochemical switch, growth factor presentation was reversed, which initiated cellular differentiation along the neuronal lineages. This electroactive material allowed for temporal control of growth factor presentation, increased growth factor stability and a closer reflection of biological signaling during brain development in vivo. In addition to temporal changes in growth factor presentation, mechanical stiffness of tissues is also dynamically altered over time. Cells sense and respond to the mechanics of their substrate - be it the extracellular matrix, neighboring cells or artificial matrix in cell culture. Using biologically relevant elastic substrates to study cell function in vitro has proven beneficial, as the in vivo microenvironment usually is much softer than rigid plastic dishes. Stiffened tumor stroma is a hallmark of cancer and understanding mechanosensitive pathways involved in the onset of cancer is key in identifying strategies for cancer treatment. We have therefore investigated the role of matrix stiffness in Notch signaling in breast cancer cells. This signaling pathway is a highly conserved cell-to-cell communication system that regulates cell fate in development and disease. Aberrant Notch signaling in breast cancer has been found to correlate with invasion and patient outcome. Our results show that we can tune cell stiffness and migration by regulating Notch activity and matrix stiffness. We propose an opportunity to target the cancer cell/microenvironment interface instead of the Notch pathway itself in the development of cancer therapies. Finally, we have studied the role of nanoarchitecture of ephrin ligands in Eph receptor activation. Eph/ephrin signaling is a cell-to-cell communication pathway, which regulates cell migration and proliferation. Dysregulation of this pathway has been associated with a multitude of human diseases, including breast cancer. Here, we developed a new tool based on DNA origami, which allows for precise positioning of ephrin ligands on DNA at the nanoscale. We found that Eph receptor activation and downstream events are regulated by ephrin spatial distribution. This work contributes to our understanding of how physical cues in the form of ligand presentation impact breast cancer cell behavior. Ultimately, elucidating the mechanisms involved in biophysical regulation of cell function is necessary to understand cellular dysfunction and diseases.