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Technical Program TRACK 4

Based on a purely geometric perspective, the degree of cellular uptake
monotically decreases with the aspect ratio of PEG-coated, non-targeting 4-6
nanorods. However, when active receptor-mediated targeting is enabled
via the attachment of DNA oligonucleotides, shorter nanorods can enter CELLULAR INTERACTIONS
the cell more readily than nanospheres and longer nanorods of the similar
nanoparticle volume. After cellular entry, most nanorods, independent of as- Bexar/Travis 11:30am - 1:10pm
pect ratio, traffic from the early endosome to the late endosome. Moreover,
nanorods of all aspect ratios will eventually reside in the late endosome but
not the lysosome. Intriguingly, sorting of nanoparticles to the late endosome Session Organizer: Ashutosh Agrawal, University of Houston,
appears to be faster for longer nanorods than their shorter counterparts. Houston, TX, United States
Our preliminary data will shed light on the effect of nanoparticle geometry 11:30am Mechanical Phase Transition of the Active Cytoskeleton
on cellular uptake and intracellular trafficking. They may also point to import-
ant materials design rules for drug delivery carriers based on anisotropic Technical Presentation. NEMB2016-6080
nanoparticles.

10:40am Can rigid proteins make a membrane softer? — the cu- Michael Mak, Massachusetts Institute of Technology, Cambridge,
rious case of HIV induced membrane softening MA, United States, Taeyoon Kim, Purdue University, West Lafayette,
IN, United States, Muhammad Zaman, Boston University, Boston,
Technical Presentation. NEMB2016-6011 MA, United States, Roger Kamm, Massachusetts Institute of Tech-
nology, Cambridge, MA, United States
Himani Agrawal, University of Houston, Houston, TX, United
The motor-driven actin cytoskeleton enables cells to perform mechanical
States, Liping Liu, Rutgers University, New Brunswick, NJ, United functions, such as migration, shape change, and force generation. These
States, Pradeep Sharma, University of Houston, Houston, TX, Unit- functions are important especially during development, when transient,
ed States pulsatile contractile events lead to the reorganization of tissues, e.g. Dro-
sophila gastrulation, and in cancer, in which metastatic cells can generate
A key step in the HIV infection process is the fusion of the virion membrane enhanced contractile forces potentially leading to mechanical remodeling
with the target cell membrane and the concomitant transfer of the viral RNA. of the extracellular matrix and promoting invasion. While the key molecules
Experimental evidence appears to suggest that the fusion is preceded by constituting the cytoskeleton are known, it is less clear how large numbers
considerable elastic softening and thinning of the cell membranes and the of these components interact and integrate in an active network to enable
formation of well-defined pores. What are the precise mechanisms under- diverse global morphologies and dynamic control of internal tension. Specif-
pinning the elastic softening of the membrane upon peptide insertion? A ically, the cytoskeleton is capable of maintaining homogeneous morpholo-
clear understanding of this could potentially pave the way for intelligent drug gies while generating a steady-state prestress, but can also undergo phase
design to combat the epidemic caused by this deadly virus. State-of-the-art separation and foci formation. It is not well known how these different global
experiments to understand the HIV peptide insertion with T-cell membranes mechanical states emerge spontaneously from a solution of randomly mixed
have been conducted recently. Using diffuse X-ray scattering, they deduced cytoskeletal proteins.
the bending modulus of the membranes upon HIV fusion peptide addition.
Depending on the type of membrane, they found that the bending modulus Through Brownian dynamics simulations, we demonstrate how vital nano-
(i.e. the property which dictates how resistant a membrane is to mechanical scopic features – the polymerization/depolymerization of actin filaments,
bending) can reduce between 3-13 times! This enormous mechanical soften- walking and local contractility of myosin II motors, and force-sensitive un-
ing greatly facilitates the subsequent fusion and infection process. While the binding of actin crosslinking proteins – regulate global mechanical states
experimental findings are quite interesting, very little atomistic insights were and drive nonequilibrium phase transition. In particular, our results show
gleaned. In short, modeling or simulations are necessary to interpret the that actin turnover, which enables the cytoskeletal network to self-regen-
aforementioned experiments and then provide guidelines for computation- erate, is a master regulator of cell mechanics, from the precise tuning of
ally driven rationale drug design. Predicated on the hypothesis that under- tension to driving the morphological phase transition from homogeneous
standing, at the atomistic level, the membrane softening due to HIV peptide to clustered actomyosin networks. Myosin activity and crosslinker density
insertion will enable counter-measures, we have conducted large-scale mo- shift the critical actin turnover rate required for phase transition. We present
lecular dynamics simulations on the interaction between HIV fusion peptide multidimensional phase maps revealing how the local biochemical reaction
and cell membrane. Such simulations require modeling millions of atoms kinetics and concentrations of these basic cytoskeletal components enable
that interact with each through a complicated set of forces. The dynamics of the emergence of diverse structure and functionality in the cytoskeleton.
such an ensemble was then studied and interpreted. For example, although Furthermore, we demonstrate that pulsations across phase transition enable
the experiments were able to measure the overall reduction in bending much larger global contractile forces, which would otherwise disconnect the
modulus of the membrane upon interaction with the HIV peptide—-the key percolated cytoskeleton, to be generated over prolonged periods. Finally,
physics lies in what is happening locally at the peptide-membrane insertion we complement our computational results with live-cell imaging studies of
interface. What exactly happens there that causes an overall softening of the the cytoskeleton and demonstrate the dynamics of reversible actin foci for-
membrane? In principle, insertion of rigid proteins or peptide in membranes mation due to the disruption of actin polymerization dynamics.
ought to stiffen the membrane not soften it thus rendering the experimental
observations even more perplexing. To this end, we have devised a numer- 11:30am Therapeutic Properties and Cellular Impact of pH-Re-
ical “experiment” which involves (computationally) sticking a needle into the sponsive Hybrid Nanoparticles
membrane region of interest. Through derived theoretical formulae, and ob-
servation of the response of the atoms in the simulation when subject to the
needle probe, we estimated the elastic behavior of a small and local patch Technical Presentation. NEMB2016-6060
of the membrane as opposed to the entire membrane itself. This, and the di-
rect observation of the atomic behavior, allowed us to understand precisely Alessandro Parodi, Claudia Corbo, Houston Methodist Research
what occurs at the peptide-membrane interface. Institute, Houston, TX, United States, Micheal Evangelopoulos,
Houston Methodist Research Institute, Hosuton, TX, United States,
Ennio Tasciotti, The Methodist Hospital Research Institute, Hous-
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ton, TX, United States