Membrane tether formation from blebbing cells

Membrane tension has been proposed to be important in regulating cell funct ions such as endocytosis and cell motility. The apparent membrane tension h as been calculated from tether forces measured with laser tweezers. Both me mbrane-cytoskeleton adhesion and membrane tension contribute to the tether force. Separation of the plasma membrane from the cytoskeleton occurs in me mbrane blebs, which could remove the membrane-cytoskeleton adhesion term. I n renal epithelial cells, tether forces are significantly lower on blebs th an on membranes that are supported by cytoskeleton. Furthermore, the tether forces are equal on apical and basolateral blebs. In contrast, tether forc es from membranes supported by the cytoskeleton are greater in apical than in basolateral regions, which is consistent with the greater apparent cytos keletal density in the apical region. We suggest that the tether force on b lebs primarily contains only the membrane tension term and that the membran e tension may be uniform over the cell surface. Additional support for this hypothesis comes from observations of melanoma cells that spontaneously bl eb. In melanoma cells, tether forces on blebs are proportional to the radiu s of the bleb, and as large blebs form, there are spikes in the tether forc e in other cell regions. We suggest that an internal osmotic pressure infla tes the blebs, and the pressure calculated from the Law of Laplace is simil ar to independent measurements of intracellular pressures. When the membran e tension term is subtracted from the apparent membrane tension over the cy toskeleton, the membrane-cytoskeleton adhesion term can be estimated. In bo th cell systems, membrane-cytoskeleton adhesion was the major factor in gen erating the tether force.