Development of the aging cell surface: concanavalin A-mediated intercellular binding and the distribution of binding sites with progressive subcultivation of human embryo fibroblasts

The concept of telomeric non-reciprocal recombination (TENOR) applied to human fibroblasts grown in serial cultures: concordance with genealogical data

Since the discovery of the limited life span of human fibroblasts some 50 years ago, many genealogical studies have been undertaken to describe growth kinetics of fibroblasts in serial cultures by their individual division behavior. It is now accepted that proliferation capacities of human fibroblasts strongly depend on their telomere lengths and integrity. Telomeres shorten with each replication round, and there is a direct correlation between cell division capacity and telomere lengths; that is, the consumption of disposable telomeric DNA repeats during cell divisions progresses until critically short telomeres determining the replicative senescence of the cells are present. Recently, we have suggested that telomeres in fibroblasts can also become elongated during DNA replication by telomeric non-reciprocal recombination (TENOR). Here we discuss genealogical data collected over the last decades as well as more recent findings on the telomere-driven replicative senescence process, and we summarize both to give an integrated picture.

Changes in the cell surface of human diploid fibroblasts during cellular aging

The electrophoretic mobility of 13 human diploid cell strains, TIG-1, TIG-2, TIG-3, TIG-7, WI-38, IMR-90, MRC-5, MRC-9, TIG-1H, TIG-1L, TIG-2M, TIG-2B, and TIG-3S, which were established from different tissues of human embryos, was studied at different passages. The net negative surface charge of the cells was characteristic for each cell strain and decreased significantly during the in vitro aging of the cells. The decrease in the net negative charge of the cells correlated well with the decrease in cell density throughout the life span of the cells. A strict linear correlation between the electrophoretic mobility and the number of cells harvested at each passage was obtained for all the human diploid cell strains. Moreover, almost the same linear regression coefficient of the cells was obtained among these cell strains. Therefore, the net negative surface charge of human diploid cell strains could serve as a cell surface marker for in vitro cellular aging.

Changes in negative surface charge of human diploid fibroblasts, TIG-1, during in vitro aging

The electrophoretic mobility of human diploid fibroblasts, TIG-1, was studied at different passages. The net negative surface charge of the cells decreased from -1.658 +/- 0.108 micron/s/V/cm at an early passage (15 population doublings, PD) to -1.173 +/- 0.116 at the final passage (67 PD) in 1/15 M phosphate buffer supplemented with 5.4% glucose. The decrease was slow at 15-45 PD, but was rapid at 45-67 PD. The net negative surface charge of small cells in the late passage populations was not different from that of larger cells in this population, and was significantly lower than that of small cells in the middle passage populations. The distribution of the mobilities of cells in each passage was independent of the size of the individual cells, and the mean value was distinct for the passage number. The viability of the cells was retained during the assay of electrophoretic mobility under these conditions. These results indicate that the net negative surface charge of human diploid fibroblasts represents a cell surface maker for in vitro cellular age in the population.

Aging in vitro and D-glucose uptake kinetics of diploid human fibroblasts

By use of a rapid technique, initial rates of D-glucose transport were obtained during the lifespan in vitro of a commercially available strain of human embryo lung fibroblasts (Flow 2000). The apparent Km of the D-glucose carrier did not change during senescence in vitro: x = 1.8 mM (range 1.3-2.3) in phase II, x = 1.8 mM (range 1.5-2.2) in phase III. Transport rates remained constant in stationary phase II cultures, which had completed between 30% and 80% of their replicative lifespan. A wide variation, however, was observed in terminally differentiated cells (phase III), which showed a two- to threefold increase in average cell size and protein content. In some senescent cultures, glucose transport calculated on a per cell basis was also two- to threefold increased, while it was strongly decreased (-75%) in others. When calculated per unit of cell water, protein, and surface area, respectively, transport rates in phase III cultures ranged from values established for stationary phase II cultures down to very low values. Detaching cells flushed off from senescent cultures did not show measurable rates of glucose transport into the inulin impermeable cell space. Present evidence argues against the idea that an impairment of D-glucose transport might precede loss of replicative potential in aging human fibroblasts. Instead our data indicate that the transport capacity of cell membrane finally decreases during postreplicative senescence in terminally differentiated cells.

Variation in cytoskeletal assembly during spreading of progressively subcultivated human embryo fibroblasts (IMR-90)

The cytoskeletons of early and late passage IMR-90 human diploid fibroblasts have been directly imaged in replicas of Triton X-100 extracted cells during spreading following reseeding. All cells from both young and sensescent cultures exhibit a cytoskeletal network of actin microfilaments, intermediate (10 nm) filaments, microtubules, and interconnecting thin filaments (6-8 nm in diameter) which do not interact with heavy meromyosin. Early passage cells assemble linear aggregates of actin filaments within 1 h of spreading. By 4 h of incubation, these bundles establish a structural bond with the cell membrane which results in resistance by the plasmalemma to detergent extraction at these sites. Furthermore, these membrane regions are associated with developing stress fibers of well-spread cells. In contrast, late passage cells exhibit slower spreading which correlates with a retarded assembly of actin bundles. In addition, by 8 h of spreading, cells of older cultures do not exhibit the regions of membrane-actin interaction which impart detergent resistance to the plasmalemma. We conclude that the ability to reassemble actin-actin and actin-membrane association during cell spreading is reduced with increased serial subcultivation of cells.

Fibronectin from aged fibroblasts is defective in promoting cellular adhesion

Late passage fibroblasts show decreased cell-substrate adhesion. We provide evidence that the reduced adhesion is due to a defect in the adhesive glycoprotein fibronectin. Late passage cells become more adhesive in culture media that has been conditioned by the growth of early passage cells. Analysis of fibronectins purifed from early and late passage cell conditioned media indicates that there are striking differences in their abilities to promote cell adhesion. Young cell fibronectin supports the maximal adhesion of both young and old cells. However, old cells require quantitatively more fibronectin. In contrast, old cell fibronectin is less effective in supporting the adhesion of either cell type. In addition, neither cell type achieves a normal morphology in the presence of old cell fibronectin. The results support the conclusion that the fibronectin released by late passage cells i defective and does not support normal cell-substrate interactions.

Age-related cell surface changes in human diploid fibroblasts revealed by lectin-mediated red blood cell adsorption assay: a lectin survey

Two types of age-related cell surface changes could be demonstrated in human diploid fibroblasts with the two methods of the lectin-mediated red blood cell (RBC) adsorption assay: the fibroblast coating method (in which RBCs are adsorbed to lectin-coated fibroblasts) and the RBC coating method (in which lectin-coated RBCs are adsorbed to fibroblasts). With the fibroblast coating method, concanavalin A and agglutinin L from Phaseolus vulgaris gave a change in RBC adsorption which did not occur throughout the phase II period, but increased with the advance of the phase III period (type I). With the RBC coating method, these lectins gave another type of change in RBC adsorption which increased continuously from early phases of cell passage up through cell senescence (type II). Ricinus communis agglutinin 120 also gave the type I change in RBC adsorption with the fibroblast coating method. On the other hand, even with the fibroblast coating method, Phaseolus vulgaris agglutinin E and wheat-germ agglutinin gave the type II change in RBC adsorption. Soy bean agglutinin and Bauhinia purpurea agglutinin gave only a restricted amount of RBC adsorption. Lens culinaris agglutin, pokeweed mitogen, Dolichos biflorus agglutinin, Lotus tetragonolobus agglutinin, Limulus polyhemus agglutinin and divalent succinylated concanavalin A did not give any RBC adsorption throughout the life span of human diploid fibroblasts.

A new cell surface marker of aging in human diploid fibroblasts

The relationship of cell surface changes to proliferative decline of human diploid fibroblasts was investigated using the concanavalin A-mediated red blood cell adsorption assay. The amount of the red blood cells adsorbed to human diploid fibroblasts via concanavalin A increased continuously from the early phases of cell passage up through cell senescence, while the amount of 3H-concanavalin A binding did not change to a significant extent. The red blood cell adsorption is not a function of cell cycle phase and time spent in culture. Cocultivation of young cells with old cells also did not affect the adsorption capacity of respective cells. Thus, the concanavalin A-mediated red blood cell adsorption can be expected to serve as a new cell surface marker for aging in vitro. Using this marker, it was revealed that transient cell size or 3H-thymidine incorporating capacity di not have a direct relationship with the division age of a cell. Small rapidly dividing cells in old populations resemble large slowly dividing or nondividing cells of the same populations and differ from small rapidly dividing cells in young populations, in terms of cell surface properties.

Fine structure of IMR-90 cells in culture as examined by scanning and transmission electron microscopy

Cells from the new strain IMR-90 were examined by scanning (SEM) and transmission (TEM) electron microscopy at early, middle, and late population doubling levels. The cells are characteristically flattened and elongated and arranged in clusters from 1 to several cells thick. Long thin processes extend from the poles and sides of the cells. The number of blebs and microvilli on the cell surface varies. In later population doubling level (PDL) cultures, a larger number of cells have greater quantities of microvilli on their surface. It is suggested that the increased number of microvilli might represent an increased level of differentiation. By TEM the cells typically have elongated to oval shaped nuclei which are sometimes deeply invaginated. The cytoplasm contains a well developed Golgi region, elongated mitochondria, microtubules, filaments, a variety of vesicles, vacuoles and dense bodies and large amounts of RNA in the form of granular endoplasmic reticulum and ribosomes. Cytoplasmic appearance, particularly the number of dense bodies, varies widely at all PDL. With increasing PDL, cells tend to have nuclei with more condensed chromatin, and a cytoplasm containing less mitochondria and granular endoplasmic reticulum and more dense bodies. Also at later PDL there is a higher frequency of cells containing long, thin dense mitochondria as well as bizarre shaped mitochondria. In older populations there are many cells in a state of filamentous degeneration. Cells with large numbbers of surface projections (microvilli) tend to be correlated with an osmiophilic cytoplasm containing many filaments and numerous dense bodies.