Elastic Properties of Pore-Spanning Apical Cell Membranes Derived from MDCK II Cells

Abstract

The mechanical response of adherent, polarized cells to indentation is frequently attributed to the presence of an endogenous actin cortex attached to the inner leaflet of the plasma membrane. Here, we scrutinized the elastic properties of apical membranes separated from living cells and attached to a porous mesh in the absence of intracellular factors originating from the cytosol, organelles, the substrate, neighbors, and the nucleus. We found that a tension-based model describes the data very well providing essentially the prestress of the shell generated by adhesion of the apical membrane patches to the pore rim and the apparent area compressibility modulus, an intrinsic elastic modulus modulated by the surface excess stored in membrane reservoirs. Removal of membrane-associated proteins by proteases decreases the area compressibility modulus, whereas fixation and cross-linking of proteins with glutaraldehyde increases it.

Stiffness of MDCK II Cells Depends on Confluency and Cell Size

“…Deformation of cells either globally or locally with a sharp indenter is usually interpreted either in terms of a contact mechanics model, providing the Young's modulus, or in terms of a tension model, in which cortical tension prevails at low strain, whereas area dilatation dominates at large strain (11,12). Here, we focus on the apical mechanics of adherent epithelial cells in terms of contact mechanics, in which studies have already shown that the thin actomyosin cortex, together with the plasma membrane of such cells, dominates the repulsive forces that are experienced by nanoindenters (13,14). In epithelial cells, mechanical homeostasis is of major importance and must continue even in challenging environments, e.g., high or low osmolarity, to ensure layer integrity (15).…”

Viscoelasticity of basal plasma membranes and cortices derived from MDCK II cells

“…While cells maintain a constant volume during deformation, the basal membrane sheets are physically and chemically attached to the pore rims -the strength of attachment given by equation (24). 19,40,41 The measured area compressibility modulus of the basolateral membrane sheets contains contributions from the rather inexten-sible membrane and the actin mesh. Albeit the outstretched plasma membrane is almost inextensible exhibiting considerable large K A values of 0.1 -0.5 N/m depending on the lipid composition, 42 excess membrane area A ex can be recruited from wrinkles and folds during indentation.…”

“…In contrast to our previous publication that simplified the theoretical treatment by assuming that the AFM-indenter can be modelled by a cylindrical flat punch with a small radius of a few nanometers, 28 I now consider a conical indenter and also abandoned the small gradient approximation, which is typically used to simplify the description of the force response. 9,[29][30][31][32] The following treatment is partly inspired by the work of Powers et al 33 who dealt with the formation of tethers pulled from a planar membrane. The free energy of the membrane is given by 34,35…”

“…On the basal side, stress fibers and focal adhesions emerge, responsible for attaching cells to the extracellular matrix. As for confluent polar epithelial cells, our understanding of cell mechanics comes mainly from indentation experiments probing the apical side facing the culture medium, [4][5][6][7][8][9][10][11][12] whereas very few studies have addressed the elastic properties of the basal side let alone dissipative properties. [13][14][15] It is generally believed that the response of cells to deformation originates predominately from the cellular cortex consisting of a thin, contractile and transiently cross-linked actin mesh connected to the plasma membrane.…”

Phase separation of biomimetic membranes:

“…This fitting neglects the bending of the membrane, because the pore radius is greater than the membrane thickness by a factor of >100. [155]…”