Doctoral Defense
MECHANOCHEMICAL SIGNALING DIRECTS CELL STATE: A
MECHANICS OF MATERIALS FOUNDATION FOR CELL BIOLOGY
Mr. Adam J. Engler, Ph.D. Candidate
Advisor: Professor Dennis E. Discher
Department of Mechanical Engineering
University of Pennsylvania
Abstract
Many different cell types respond to substrate
elasticity as sensitively as more well studied soluble or
immobilized ligands, yet mechanisms by which mechanical
cues are transduced to cells have been far less explored.
Sufficient substrate stiffness, i.e. – Young’s
Modulus E, appears important to anchorage-dependent, contractile
cells, relying on finite resistance to cell-generated forces
to induce outside-in mechanical signals and maintain cell
function. Muscle cells, in particular, transmit large actomyosin
contractions to surrounding extracellular matrix (ECM),
appearing mechanochemically sensitive very early as cell
adhesion depends on substrate compliance and adhesive ligand
density; limited spreading on soft gels (E ~ 1kPa) is surprisingly
insensitive to adhesive ligand density. On rigid matrices
or substrates, cells produce isometric contractile efforts
not conducive to cell function; in contrast, a more compliant
matrix, i.e. – a collagen or polyacrylamide gel, permits
cell contractions. Thus, longer-term cultures indicate that
muscle cell striation and cardiomyocyte function is stiffness
sensitive, with an intermediate range similar to passive
muscle (EMuscle ~ 8 – 12kPa) being most favorable.
Remarkably, mesenchymal stem cell differentiation also appears
mechanosensitive, with cells adopting a myoblast-like elongation
and expressing myogenic markers on substrates only near
ESk Muscle, while matrices 10-fold softer or several fold
stiffer generate cells which commit to neuronal or osteogenic
lineages, respectively. Incomplete expression by mechanosensing
is augmented by growth factors thus inducing full differentiation;
chemical or physical stimulus alone cannot, and when altered,
as in fibrotic scarring post-myocardial infarct, cell therapies
are ineffective at regenerating functional tissue as the
physical mircoenvironment inhibits contraction.
Underlying these mechano-sensitive observations
are key signaling ‘sets-points.’ Cytoskeletal
proteins expression and phosphorylation, myosin II contractility
and organization, and Rho-GTPases and their affectors when
perturbed, alter cell function and can be fit to novel biphasic
mechanochemical functions to explain the coupling observed
in muscle and stem cells. Gene expression arrays and site
specific amino acid labels from these cells implicate several
mechanically active proteins and signaling pathways. Overall,
our data implies that, in addition to soluble or immobilized
ligands, tissue and/or matrix elasticity and structure is
critical for musculoskeletal cell signaling, development,
and subsequent therapies.
Tuesday, July 11h
2 PM, 5000 Vagelos Labs
