Non-equilibration of hydrostatic pressure in blebbing cells

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Original Entry by Holly McIlwee, AP225 Fall 09

Overview

Non-equilibration of hydrostatic pressure in blebbing cells, G. Charras, J. Yarrow, M. Horton, L. Mahadevan and T. Mitchison, Nature, 435, 365-69. 2005.

Abstract

Blebs are large protrusions on a surface of a cell. The mechanism by which blebs occur is assigned to the separation of the plasma membrane from the cytoskeleton and subsequent swelling and retraction. The process of bleb formation and retraction lasts on the order of tens of seconds, over about ten microns of the cell surface and typically occurs during and in aid of apoptosis, cytokinesis, and cell movement.

In plant cells, cell membrane protrusions are governed by hydrostatic pressure. Current models of animal cells dictate that protrusions are governed by local regulation of actin biochemistry and therefore treat models of the cytoskeleton as a incompressible, viscoelastic species. The problem with this assumption is that this relies on the fact that the hydrostatic pressure equilibrates immediately across the entire cell. The author of this manuscript expresses unease with this assumption and sets out herein to create a model for non-equilibrium hydrostatic pressure to explain the blebbing phenomena.

Keywords

Cell blebbing, Poroelasticity, Hydrostatic pressure

Soft Matter

Cells are great examples of heterogeneous structured fluids. Their autonomous nature makes them particularly interesting to study for synthetic mimickry. With current interests in using polymer systems for tissue engineering and drug delivery, cells are an obvious system to look to for inspiration. Cell blebbing for instance occurs during apoptosis when the cytoplasm is breaking up and the cell is dismantling itself. A programmed separation of a heterogeneous polymeric system would be very desirable for use in implantable tissue engineering models for a variety of reasons. Understanding an optimized system such as the cell is an important step in advancing polymer engineering, and engineering of soft matter.

Here the author sets out to change current methodology for thinking about a model of the cell cytoplasm. Very simply, the mechanism of blebbing is explained with respect to the hydrostatic pressure within the cell system.

Blebbing essentially is a result of non-equilibrium of the cell membrane. There is currently a discrepancy as to whether the non-equilibrium is a result of globally uniform hydrostatic pressure with local nucleation of membrane dettachment from the cytoskeleton, or from globally non-equilibrium hydrostatic pressure leading to dettachment in areas of high pressure.

First, a detailed description of blebbing dynamics is established through confirmations made using fluourescence microscopy. When the coritcal acto-myosin in a cell contracts, a hydrostatic pressure is generated causing a section of the plasma membrance to dettach from the cytoskeleton. After separation, the space quickly swells with cytosol and separated across a larger area at the base resulting in a growing bleb. When swelling slows, actin filaments attach to the plasma membrane and myosin II heads move along the filaments. This forms a contractile cortex which works to bring the bleb back toward the cell body. It was confirmed that volume is constant during cell blebbing. The action of the actin and myosin II is displayed in Figure 1.

Cell Bleb.
Figure 1. Localization of GFP-actin and GFP-MRLC in blebs shows that expansion is a passive process and retraction is an active process necessitating actin and MRLC. A. GFP-actin transfected cell B. GFP-MRLC transfected cell.

By using drugs affecting membrance rigity, osmotic pressure, myosin, and actin function, local disruptions of cortical contraction were studied in cells that bleb profusely. It was found that the side of the cell that was treated with drugs does not swell preferentially, or inhibit blebbing on the untreated side. This is a clue to the fact that hydrostatic pressure does not equilibrate across entire cells in the time scale of blebbing, which is about ten seconds.

The new model of the cytoplasm, based on poroelasticity, assumes that the membrane is porous, actively contractile, and elastic. Hydrostatic pressure does not immediately propogate through the network, but instead diffuses over a length:

Hmcilwee Eq 1.JPG

with D, Diffusion constant, which determines the time needed for the effect of a local contractile force to be felt in other parts of the cell:

Hmcilwee Eq 2.JPG

where:

K = elastic bulk modulus k = hydraulic permeability of the network o = local volume fraction of fluid

Using reasonable values, x is found to be about 15-30 microns.

This once again proves that the cell can be in equilibrium over the time and space that is relevant for blebbing. Meaning that opposite sides of a cell can do not effect one another's probabilty of blebbing. This work presents that the phenomena of cell blebbing is not only a biochemical action, but a physical action.

References

Non-equilibration of hydrostatic pressure in blebbing cells, G. Charras, J. Yarrow, M. Horton, L. Mahadevan and T. Mitchison, Nature, 435, 365-69. 2005.