Surface topography of suspended tissue cells

Blebs promote cell survival by assembling oncogenic signalling hubs

Most human cells require anchorage for survival. Cell-substrate adhesion activates diverse signalling pathways, without which cells undergo anoikis-a form of programmed cell death1. Acquisition of anoikis resistance is a pivotal step in cancer disease progression, as metastasizing cells often lose firm attachment to surrounding tissue2,3. In these poorly attached states, cells adopt rounded morphologies and form small hemispherical plasma membrane protrusions called blebs4-11. Bleb function has been thoroughly investigated in the context of amoeboid migration, but it has been examined far less in other scenarios12. Here we show by three-dimensional imaging and manipulation of cell morphological states that blebbing triggers the formation of plasma membrane-proximal signalling hubs that confer anoikis resistance. Specifically, in melanoma cells, blebbing generates plasma membrane contours that recruit curvature-sensing septin proteins as scaffolds for constitutively active mutant NRAS and effectors. These signalling hubs activate ERK and PI3K-well-established promoters of pro-survival pathways. Inhibition of blebs or septins has little effect on the survival of well-adhered cells, but in detached cells it causes NRAS mislocalization, reduced MAPK and PI3K activity, and ultimately, death. This unveils a morphological requirement for mutant NRAS to operate as an effective oncoprotein. Furthermore, whereas some BRAF-mutated melanoma cells do not rely on this survival pathway in a basal state, inhibition of BRAF and MEK strongly sensitizes them to both bleb and septin inhibition. Moreover, fibroblasts engineered to sustain blebbing acquire the same anoikis resistance as cancer cells even without harbouring oncogenic mutations. Thus, blebs are potent signalling organelles capable of integrating myriad cellular information flows into concerted cellular responses, in this case granting robust anoikis resistance.

Mesenchymal stem cell mechanics from the attached to the suspended state

Human mesenchymal stem cells (hMSCs) are therapeutically useful cells that are typically expanded in vitro on stiff substrata before reimplantation. Here we explore MSC mechanical and structural changes via atomic force microscopy and optical stretching during extended passaging, and we demonstrate that cytoskeletal organization and mechanical stiffness of attached MSC populations are strongly modulated over >15 population doublings in vitro. Cytoskeletal actin networks exhibit significant coarsening, attendant with decreasing average mechanical compliance and differentiation potential of these cells, although expression of molecular surface markers does not significantly decline. These mechanical changes are not observed in the suspended state, indicating that the changes manifest themselves as alterations in stress fiber arrangement rather than cortical cytoskeleton arrangement. Additionally, optical stretching is capable of investigating a previously unquantified structural transition: remodeling-induced stiffening over tens of minutes after adherent cells are suspended. Finally, we find that optically stretched hMSCs exhibit power-law rheology during both loading and recovery; this evidence appears to be the first to originate from a biophysical measurement technique not involving cell-probe or cell-substratum contact. Together, these quantitative assessments of attached and suspended MSCs define the extremes of the extracellular environment while probing intracellular mechanisms that contribute to cell mechanical response.

Gamete interactions and the fate of sperm organelles in fertilized echinoderm eggs

Investigations of gamete fusion, sperm entry and the fate of the sperm nucleus, plasma membrane, mitochondrion, and axonemal complex in fertilized echinoderm eggs are reviewed. The timing of gamete fusion with respect to the onset of electrical activity characteristic of the activated egg and the affects of fixation conditions on the stability of fusing membranes are discussed. Observations from investigations using cationized ferritin labeled gametes and immunogold cytochemistry to demonstrate the mixing of sperm plasma membrane components within the egg plasma membrane, in particular along the surface of the fertilization cone, are compared with results from studies in somatic cells. Transformations of the sperm nucleus into a male pronucleus, consisting of sperm nuclear envelope breakdown, chromatin dispersion, and formation of a pronuclear envelope, are correlated with recent biochemical observation of similar processes in other cellular systems. Fates of the sperm mitochondrion and axonemal complex are examined.

Cytovillin and other microvillar proteins of human choriocarcinoma cells

Microvilli were isolated from cultured human JEG-3 choriocarcinoma cells using a gentle shearing method. The protein components of the isolated microvilli were examined by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting. The major Mr 42,000 and Mr 100,000 polypeptide bands reacted with anti-actin and anti-alpha-actinin antisera, respectively. Extraction of the isolated JEG-3 microvilli with Triton X-100 left an insoluble cytoskeletal residue containing mainly actin, alpha-actin, and polypeptides of Mr 200,000, 55,000 and 35,000. The Mr 35,000 polypeptide remained insoluble only at high concentrations of free Ca2+. Immunoblotting analysis of the JEG-3 microvilli indicated that they were devoid of tropomyosin, although the total JEG-3 protein lysates gave a strong positive reaction with anti-tropomyosin antiserum. The different subcellular localization of cytovillin and tropomyosin was also shown by indirect immunofluorescence microscopy. Cytovillin, an Mr 75,000 microvillus-specific membrane protein of JEG-3 cells, existed in an oligomeric form (dimer or trimer) as shown by gel filtration of Triton X-100 solubilized microvillar proteins and by native polyacrylamide gel electrophoresis of purified cytovillin. Disulfide bridges were not involved in the aggregation, because the mobility of cytovillin was similar under reducing and nonreducing conditions in SDS-PAGE. Cytovillin was shown to be closely related to ezrin, a minor component of chicken intestinal brush border microvilli.

Attachment and spreading of human gingival fibroblasts on potentially bioactive glasses in vitro

The attachment and spreading of cultured fibroblasts on potentially bioactive glasses (bioglasses) of ten different compositions were studied. Human gingival fibroblasts were allowed to attach and spread on bio-glasses for 1-72 h. Unreactive silica glass and cell culture polystyrene served as controls. The attachment and spreading of cells were examined by 3H-thymidine labeling of cells, planimetric analysis, cytological staining, immunocytochemistry, and scanning electron microscopy. The cell attachment to bioglasses and silica glass and the cell spreading on bioglasses were slower and cell morphology more elongated compared to control plastic. In spite of great differences in bioglass compositions no great differences in cell behavior on these surfaces were detected. Thus the initial events in the tissue-implant interface might be independent on the bioglass composition, and furthermore the differences in the organization of the tissue-implant interface in vivo might depend on the nature of the surrounding tissues and subsequent changes of the implant surface and the extracellular environment.

Hydrocortisone is associated with cell surface blebbing in cultures of human embryonic lung fibroblasts

Treatment with hydrocortisone (HC) is associated with increased cell surface blebbing of proliferatively active WI-38 and IMR-90 fibroblasts at low and high population doubling levels within the first 24 h after seeding. Time course studies show a marked decrease in blebbing after 2 days. In contrast, very old cells near the end of the population life span (greater than 96% life span completed) show blebbing which is independent of HC treatment. Since the time of increased HC-associated blebbing in cycling cell populations coincides with the time during which HC-treated cells condition the medium with a growth-stimulating (autocrine) factor, it is possible that blebbing may represent extrusion of this factor from the cells at these times.

Surface changes at fertilization: integration of sea urchin (Arbacia punctulata) sperm and oocyte plasma membranes

To examine the integration and fate of the sperm plasma membrane following its incorporation into the oocyte plasma membrane, we have fertilized sea urchin (Arbacia punctulata) gametes reciprocally labeled with cationized ferritin. When unlabeled oocytes were inseminated with labeled sperm, cationized ferritin acceptors moved laterally from the sperm plasma membrane into the fertilization cone and surrounding microvilli, mixing with components of the oocyte plasmalemma. Labeled oocytes inseminated with unlabeled sperm produced extremely large fertilization cones, completely devoid of cationized ferritin, while the remainder of the oocyte surface remained heavily labeled. Surface area measurements indicated that if all the sperm plasmalemma were utilized to delimit a fertilization cone it would provide less than 10% of the total surface membrane. Evidence is presented indicating that a principal source of membrane to the expanding fertilization cone of inseminated oocytes is from microvilli, i.e., microvilli are retracted to accommodate fertilization cone formation. Membrane delimiting the fertilization cone has a much lower affinity for agents (cationized ferritin and concanavalin A) that stain negatively charged and carbohydrate moieties compared to other regions of the oocyte surface. These ultrastructural observations indicate that significant rearrangements occur in the oocyte and sperm plasma membranes following gamete fusion which give rise to asymmetries in membrane topography; components of both membranes are redistributed within the bilayer adjacent to and delimiting the fertilization cone.