Recent work has demonstrated the pervasive role of mechanics in biology. The most prominent examples include matrix stiffness influencing stem cell
lineage and tumor progression, axonal tension regulating presynaptic vesicle clustering, and stiffness gradients guiding migrating cells.
Mechanotransduction, the mechanism underlying these cellular responses to mechanical stimuli, has been studied in detail for the past decade and
offers a new paradigm for directing the form and function of integrated cellular systems.
Over the past 5 years, we developed various microfluidic platforms for mimicking the three dimensional microenvironment and investigating the role of
mechanical stimuli, such as interstitial flow, cyclic strain, and ECM stiffness gradients, on cellular processes including cell migration, angiogenesis,
Recently, we have drawn upon our understanding of mechanobiology to direct the function of multicellular systems. For example, we extended our angiogenesis
model to build functional vascular networks in vitro, and we directed stem cell differentiation into cardiomyocytes by applying cyclic strain. As we
increase the complexity of synthetic modules toward building biological machines, mechanics will play a more significant role, particularly in the
engineering of neurons and myocytes for sensing and actuation. We will employ mechanical engineering as a tool to address this complexity while
simultaneously extending our understanding of mechanotransduction.
Formation of new blood vessel from an existing branch, by a regulated process known as angiogenesis, governs vascular patterning in the body and determines
the distribution of nutrients and oxygen supply. Angiogenesis has essential roles in development, reproduction and repair but also occurs in tumor formation
and in a variety of diseases [1, 2]. Our lab studies the angiogenic process by computational modeling across multiple scales [3, 4] and by in vitro
microfluidic experiments that mimics in vivo biophysical and biochemical microenvironment. We showed that angiogenic endothelial cells seeded in contact
with collagen gel can be induced to form nascent angiogenic sprouts in microfluidic which later develop into a vascular network [5, 6, 7].
To understand the single cell decisions in angiogenesis at the signaling level, we model individual cell as a decision making entities and follow individual
cell as they make decisions in angiogenic conditions . In collaboration with the Lauffenburger lab at MIT, we attempt to elucidate how such single cell
decision might be governed by an intracellular signaling by measuring the intracellular changes in signaling activities upon stimulating cells with potent
factors that induce and suppress sprout formation .
Tumor invasion has received a lot of attention as a critical step in metastatic disease for developing new cancer drugs. Current understanding of the role
of biophysical and cellular microenvironment in tumor invasion is limited, because of the lack of appropriate in vitro and in vivo models. We have adapted
our previous microfluidic platforms  for studying the role of the endothelium on tumor intravasation and the effects of interstitial flow on tumor cell
migration, along with the development of new hard plastic devices for commercial transition. Recent results from the tumor-endothelial interaction assay demonstrated the capability to form a 3D endothelium on collagen type I matrices, in the presence
of invading tumor cells in 3D (Figure 1). Upon stimulation with inflammatory cytokines we demonstrated an increase in diffusive permeability to fluorescent
dextrans, in agreement with a measured increase in the number of intravasation events. These results demonstrate the utility of this assay for studying the
role of the endothelial barrier function in tumor cell intravasation.
We developed a microfluidic system for investigating the role of interstitial flow in tumor cell migration (Figure 2). Tumor cells exposed to interstitial
flow preferentially migrated along streamlines, and the relative percentage of cells migrating upstream and downstream is a function of chemokine receptor
activity and cell density.
Interstitial flow stimulates downstream tumor cell migration through CCR7 autocrine signaling. However, flow also stimulates
upstream cell migration through a competing, mechanically mediated pathway, as evidenced by significantly increased FAK activation in devices with flow.
The relative strength of the autocrine and mechanical stimuli determines whether cells migrate upstream or downstream of the flow direction.
We applied the known commercially-viable manufacturing methods to a cyclic olefin copolymer (COC) material to fabricate a microfluidic device with controlled
surface properties and improved potential to serve high-volume applications. Culture of cells in the new hard plastic device indicated no adverse effects
of the COC material. Therefore, this transition of platform demonstrates a capability of using microfludic devices for 3D cell culture across the range from
the scientific research to applications with broad clinical impact.
Schematic of the angiogenesis process within a microfluidic device. A stochastic cell population model was used with experimental data gathered from the microfluidic platform to develop a closed-loop control system model for angiogenesis (From ref. 4).
Computational models aide with data interpretation and experimental design, and simulations can prove insight into biological mechanisms in instances where
experiments are not feasible. Modeling and simulation are integral parts to the Mechanobiology Lab, and we have developed models spanning length scales from
single molecules to cell populations. Furthermore, these models are not independent; we employ course-graining techniques to allow models developed at small
length scales to inform larger scale models. For example, the bulk properties of a material have been estimated by course-graining simulations of the
constitutive atoms, providing a quantitative link between the chemical composition and mechanics of biomaterials (1). The actin cytoskeleton contributes to the mechanical rigidity of cells, and dynamic reorganization of the intracellular actin network is required for key
cellular events such as proliferation and migration. Mechanical force plays a crucial role in governing the dynamics of the actin cytoskeleton, and we have
developed a computational model derived from Brownian dynamics simulations to study the mechanical properties of actin networks (2). The model well captures
experimentally observed viscoelastic properties of actin and provides novel insight into the contribution of molecular motors and actin crosslinking proteins
to the viscoelastic moduli of actin networks. Recently, we have extended this model to investigate the role of molecular motors in the generation of actin
stress fibers, molecular complexes under tension that provide direct mechanical connections to the extracellular environment.
In the MIT Mechanobiology Lab, our experiments are tightly coupled with computational models for investigating biological phenomena. The highly controlled
cell microenvironment enabled by our microfluidic platforms allows validation of our cell-level models, which in turn, provide insight into the mechanism
underlying experimentally observed cellular behavior. We have integrated the microfluidic platform for studying angiogenesis with a hybrid discrete-continuum
model to investigate the effects of matrix and growth factors on vascular network topology (3), and we have explored the link between matrix degrading
enzymes and angiogenic sprout structure through control theory (4). Recently we have developed finite element models for investigating the role of
interstitial flow in perturbing the tumor cell microenvironment (5), and we are adapting these models to aide in the development of patterned synthetic
tissues and biological machines.
Shear deformation of a computationally generated actin network. In silico actin networks were implemented to investigate the role of actin crosslinking proteins and molecular motors on the viscoelastic properties of physiologically relevant actin networks (From ref. 2).
Investigators: Michael Mak, Andrea Malandrino.
Simulation results from a hybrid continuum-discrete model show similar topology to neovascular networks generated in microfluidic devices (From ref. 3).
Actin is one of the primary protein components of the cellular cytoskeleton. By forming networks of filaments spanning considerable intracellular distances,
it provides the cell with structural support. However, actin also plays central roles in cell motility, cell division and force transmission through the
cell. Consequently, the dynamics of actin are pivotal to the initiation of mechanotransduction or the physio-chemical response of cells to mechanical
stimuli. The varied functions of actin also mean that it has tremendous implications in medicine and disease. The dynamics of actin filament polymerization
and the protein-protein interactions responsible for the regulation of the actin network have been implicated in the tumorigenisis, the pathogenesis of
cardiovascular disease, bacterial infections and viral entry. Moreover, actin filaments are of keen interest as a new platform for the delivery of gene
therapies and as a model material system for energy storage technology.
To understand better the remarkable behavior of filamentous actin, we investigate the mechanochemistry and dynamics of actin regulatory proteins using
optical microscopy and force spectroscopy.
Amyloid fibers formed by the NM domains from S. cerevisiae sup35 grown in-situ from the glass cover slip.
Specifically, we are interested in learning how these proteins use mechanical signals to regulate the
polymerization/de-polymerization of actin filaments at the single-molecule level. Amyloids are fibrous protein aggregates that are the basis for many diseases such as Alzheimer’s, type 2 diabetes, Parkinsons, Huntington’s, and scrapie,
among many others. It has been found however, that there are many instances of functional amyloids that are used by biology for structural purposes, such
as the E. coli curli proteins and spider silk; for sensing, such as HET-s from P. anserina; or as part of a system to adapt to new environments, such as the
yeast prions, eg: [PSI] (sup35).
A great deal of work has been done to characterize amyloids biochemically, genetically, and biophysically, but there is still quite a lot that is still
unknown regarding the mechanisms involved in assembly of amyloid fibers and the structure of the constituent proteins in the amyloid state. We are using
applied force via optical tweezers as a probe to better understand the organization of the monomers within the amyloid fibril, and to gain insight into the
structure of the monomers within the fiber. The overarching goal of this project is to determine if amyloids have similar mechanical properties, and thus
potentially similar organizations of the proteins within amyloid fibers.