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

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