Collier, Sharon (2017) Image based modelling of bleb site selection. PhD thesis, University of Warwick.

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Abstract

Cellular blebs are fast, pressure-driven protrusions of the cell membrane that are initially devoid of F-actin. Although blebs have often been overlooked as a functional part of cell motility, blebbing has been shown to play an important role in migration in 3D mechanically resistive environments, such as movement through densely packed tissues. The location of bleb nucleation sites is often assumed to be entirely stochastic, however, cells migrating using blebbing motility have been repeatedly observed to perform persistent, directional movement. Given the compelling evidence on the role of blebbing in directional cell migration, relatively little is known on the mechanisms of bleb site selection; how bleb sites are determined and directed to the cell front remains an open question.

Previously, Tyson et al. found that chemotaxing Dictyostelium cells preferentially bleb from concave regions, where membrane tension could facilitate membrane-cortex detachment. Based on this, a biophysical model for curvature dependent bleb nucleation was proposed, hinting at the possibility that polarised blebbing was due to physical forces alone.

We develop a novel image based modeling approach, using real cell contours from image data to initialise the model. This enables us to link quantitative experimental data and predictive modeling on the spatial distribution of blebs for the first time. We proceed to show that the extent to which cell geometry is a good predictor of bleb site selection is highly dependent on the degree of mechanical resistance the cells experience. For cells in highly resistive environments, where we observe the front and rear of the cell to be geometrically distinct, our novel modeling approach demonstrates that physical forces are sufficient to polarise blebbing activity in chemotaxing cells. For cells in low resistive environments however, where the front and rear of the cell are not geometrically distinct, we show that an additional mechanism is required to restrict blebbing to the cell front.

We propose this additional mechanism to be a front-to-rear gradient in the linker protein TalinA. Creation of a TalinA-mNeon construct allows us to experimentally confirm the existence of an asymmetric linker distribution. We incorporate the observed exponential linker gradient within the model. Inclusion of this mechanism increases the predictive power of the model, with regard to the spatial distribution of experimentally observed bleb sites, through efficiently directing blebbing activity to the cell front.

We have created a method which links quantitative experimental data and predictive modeling, this tool allows us to disentangle the role of physical forces and biological mechanisms in the prediction of bleb nucleation sites.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QH Natural history > QH301 Biology
Library of Congress Subject Headings (LCSH):	Cytology., Cell membranes., Dictyostelium.
Official Date:	December 2017
Dates:	Date Event December 2017 Submitted
Institution:	University of Warwick
Theses Department:	Molecular Organisation and Assembly in Cells
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Bretschneider, Till ; Kay, Robert R.
Sponsors:	Molecular Organisation and Assembly in Cells ; Engineering and Physical Sciences Research Council
Extent:	xiv, 159 leaves : illustrations (colour).
Language:	eng

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