On the structural response of eukaryotic cells

textThe actin, microtubule and intermediate filament cytoskeletal polymer assemblies, along with their accessory proteins, govern the mechanical or structural response of an eukaryotic cell to an external stress. Using statistical mechanics tools, this dissertation investigates the molecular properties, such as mesh size, persistence length and filament length, that determine the structural strength of the in vivo polymer networks, with an emphasis on the actin network or the cortex of cells. Our study of actin shows how the wide range of shear moduli from 1 Pa to 1 kPa that spans the viscous sol-like state to the elastic gel-like state witnessed in eukaryotic cells can be achieved through transient crosslinking and the spatial distribution of actin and actin crosslinking proteins alone. Thus, this gel-sol transition is achieved without the action of any severing or capping proteins that depolymerize the actin network. In order to understand how the microscopic quantities controlling the structural properties of these in vivo polymers are related to the deformation of a cell observed experimentally, a cell model is created by us. It starts with modeling the actin cortex as a thick shell and increases in complexity to include microtubules and the nucleus. Our cell model predicts that the structural response of the cell amplifies changes in molecular properties such as the in vivo actin concentration. Hence, the sensitivity of the structural response to cytoskeletal changes can be used to distinguish between different cells such as normal and cancer cells and can serve as an indicator of disease.Physic

On the designs of early phase oncology studies

This thesis focuses on the design, statistical operating characteristics and interpretation of early phase oncology clinical trials. Anti-cancer drugs are generally highly toxic and it is imperative to deliver a dose to the patient that is low enough to be safe but high enough to produce a clinically meaningful response. Thus, a study of dose limiting toxicities (DLTs) and a determination of the maximum tolerated dose (MTD) of a drug that can be used in later phase trials is the focus of most Phase I oncology trials. We first comprehensively compare the statistical operating characteristics of various early phase oncology designs, finding that all the designs examined select the MTD more accurately when there is a clear separation between the true DLT rate at the MTD and the rates at the dose levels immediately above and below. Among the rule-based designs studied, we found that the 3+3 design under-doses a large percentage of patients and is not accurate in selecting the MTD for all the cases considered. The 5+5 a design picks the MTD as accurately as the model based designs for the true DLT rates generated using the chosen log-logistic and linear dose-toxicity curves, but requires enrolling a larger number of patients. The model based designs examined, mTPI, TEQR, BOIN, CRM and EWOC designs, perform well on the whole, assign the maximum percentage of patients to the MTD, and pick the MTD fairly accurately. However, the limited sample size of these Phase I oncology trials makes it difficult to accurately predict the MTD. Hence, we next study the effect of sample size and cohort size on the accuracy of dose selection in early phase oncology designs, finding that an adequate sample size is crucial. We then propose some integrated Phase 1/2 oncology designs, namely the 20+20 accelerated titration design and extensions of the mTPI and TEQR designs, that consider both toxicity and efficacy in dose selection, utilizing a larger sample size. We demonstrate that these designs provide an improvement over the existing early phase designs.2019-12-01T00:00:00

Weak Force Stalls Protrusion at the Leading Edge of the Lamellipodium

AbstractProtrusion, the first step of cell migration, is driven by actin polymerization coupled to adhesion at the cell’s leading edge. Polymerization and adhesive forces have been estimated, but the net protrusion force has not been measured accurately. We arrest the leading edge of a moving fish keratocyte with a hydrodynamic load generated by a fluid flow from a micropipette. The flow arrests protrusion locally as the cell approaches the pipette, causing an arc-shaped indentation and upward folding of the leading edge. The effect of the flow is reversible upon pipette removal and dependent on the flow direction, suggesting that it is a direct effect of the external force rather than a regulated cellular response. Modeling of the fluid flow gives a surprisingly low value for the arresting force of just a few piconewtons per micrometer. Enhanced phase contrast, fluorescence, and interference reflection microscopy suggest that the flow does not abolish actin polymerization and does not disrupt the adhesions formed before the arrest but rather interferes with weak nascent adhesions at the very front of the cell. We conclude that a weak external force is sufficient to reorient the growing actin network at the leading edge and to stall the protrusion

Actin network architecture and elasticity in lamellipodia of melanoma cells

Cell migration is an essential element in the immune response on the one hand and in cancer metastasis on the other hand. The architecture of the actin network in lamellipodia determines the elasticity of the leading edge and contributes to the regulation of migration. We have implemented a new method for the analysis of actin network morphology in the lamellipodia of B16F1 mouse melanoma cells. This method is based on fitting multilayer geometrical models to electron microscopy images of lamellipodial actin networks. The chosen model and F-actin concentrations are thereby deterministic parameters. Using this approach, we identified distinct structural features of actin networks in lamellipodia. The mesh size which defines the elasticity of the lamellipodium was determined as 34 and 78 nm for a two-layer networ

The Forces Behind Cell Movement

Cell movement is a complex phenomenon primarily driven by the actin network beneath the cell membrane, and can be divided into three general components: protrusion of the leading edge of the cell, adhesion of the leading edge and deadhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each of these steps is driven by physical forces generated by unique segments of the cytoskeleton. This review examines the specific physics underlying these phases of cell movement and the origins of the forces that drive locomotion.</p

A schematic of the three stages of cell movement, based on ,: after determining its direction of motion, the cell extends a protusion in this direction by actin polymerization at the leading edge

<p><b>Copyright information:</b></p><p>Taken from "The Forces Behind Cell Movement"</p><p></p><p>International Journal of Biological Sciences 2007;3(5):303-317.</p><p>Published online 1 Jun 2007</p><p>PMCID:PMC1893118.</p><p>© Ivyspring International Publisher. This is an open-access article distributed under the terms of the Creative Commons License (http://creativecommons.org/licenses/by-nc-nd/3.0/). Reproduction is permitted for personal, noncommercial use, provided that the article is in whole, unmodified, and properly cited.</p> It then adheres its leading edge to the surface on which it is moving and de-adheres at the cell body and rear. Finally, it pulls the whole cell body forward by contracile forces generated at the cell body and rear of the cell

A schematic depicting two phenomena that can cause retrograde actin flow based on

<p><b>Copyright information:</b></p><p>Taken from "The Forces Behind Cell Movement"</p><p></p><p>International Journal of Biological Sciences 2007;3(5):303-317.</p><p>Published online 1 Jun 2007</p><p>PMCID:PMC1893118.</p><p>© Ivyspring International Publisher. This is an open-access article distributed under the terms of the Creative Commons License (http://creativecommons.org/licenses/by-nc-nd/3.0/). Reproduction is permitted for personal, noncommercial use, provided that the article is in whole, unmodified, and properly cited.</p> Retrograde flow is postulated to occur either due to release of the molecular clutch and resultant slippage, as seen in Figure a) (where the yellow and green parts of the clutch do not fit), or due to adhesion raking as shown in Figure b) (where the clutch is engaged (yellow and green parts fit) but there is raking of the cytoskeleton against the substrate (blue and white parts))

A schematic (based on a figure in ) showing how the cell adheres to the substrate

<p><b>Copyright information:</b></p><p>Taken from "The Forces Behind Cell Movement"</p><p></p><p>International Journal of Biological Sciences 2007;3(5):303-317.</p><p>Published online 1 Jun 2007</p><p>PMCID:PMC1893118.</p><p>© Ivyspring International Publisher. This is an open-access article distributed under the terms of the Creative Commons License (http://creativecommons.org/licenses/by-nc-nd/3.0/). Reproduction is permitted for personal, noncommercial use, provided that the article is in whole, unmodified, and properly cited.</p> Cell-substrate attachments are formed when actin bundles connect to the substrate at certain sites via adhesion molecules such as vinculin, talin and integrin