POLARIZATION OF CELL LOCOMOTION IN VITRO

Chemotaxis of Amoeba proteus in the developing pH gradient within a pocket-like chamber studied with the computer assisted method

A new "U" shaped, pocket-like chamber was used to observe the chemotactic responses of individual cells. This method permits monitoring of both the development of the concentration gradient of a tested substance and cell locomotion. We investigated the chemotactic responses of Amoeba proteus and observed that the amoebae moved in positively and negatively developing [H+] gradients towards the solution of lower pH in a pH range 5.75-7.75. The chemotactic response of amoebae to [H+] gradients required the presence of extracellular calcium ions. It was blocked and random locomotion was restored by the replacement of calcium with magnesium in the cell medium. Time-lapse video recording and data processing were accomplished with computer-assisted methods. This made it possible to compare selected methods of data presentation and analysis for cells locomoting in isotropic and anisotropic conditions. The cell trajectories were determined and displayed in circular diagrams, lengths of cell tracks and final cell displacements were estimated and a few parameters characterizing cell locomotion were computed.

Calcium destabilizes and causes conformational changes in hyaluronic acid

The physical properties of proteoglycans found in the extracellular matrix influence the behavior of resident macromolecules, micro-ions, and cells. The forces stabilizing structured regions of hyaluronic acid (HA), a major component of extracellular matrix, were measured. The free energy of stability associated with structured regions within human HA was determined by potentiometric titration. A plot of free energy versus temperature demonstrated that the secondary structure was most stable at 25 degrees C and was destabilized at temperatures near 50 degrees C. In the presence of 5 mM CaCl2, the free energy of stability of the structured regions was reduced by 30 cal/mole. Classical light scattering and viscosity measurements confirmed the impact of calcium on HA. CaCl2 (5 mM) reduced the radius of gyration of HA. Above concentrations of 3 mM calcium, the viscosity of 0.1% HA solutions was reduced sharply. The ability of cells to secrete calcium, combined with the exquisite sensitivity of HA to calcium, may permit cell-mediated modification of the extracellular matrix and thus may facilitate cell motility.

pH and population density in the regulation of animal cell multiplication

Sparse and dense cultures of chick embryo cells were affected differently by pH. The rates of cell multiplication and of thymidine-(3)H incorporation into DNA of dense cultures were increased as the pH was increased from 6.6 to 7.6. At pH higher than 7.6 the rate of multiplication decreased slightly in the dense cultures, but the rate of thymidine-(3)H incorporation continued to increase. The discrepancy was due in part to cell death and detachment at very high pH, and in part to a more rapid uptake of thymidine-(3)H at very high pH. Sparse cultures were much less sensitive to pH reduction and, when a suitably conditioned medium was used to minimize cell damage, very sparse cultures grew almost as well at pH 6.7 as at higher pH. The rates of cell multiplication and thymidine-(3)H incorporation at low pH decreased in the initially sparse cultures before they reached confluent cell densities. There was no microscope evidence of direct contact between plasma membranes of cells at these densities although the parallel orientation indicated that the cells were influencing locally each other's behavior. Even at much higher cell densities, electron microscopy revealed large intercellular gaps partly filled with a fragmentary electron-opaque material suspected to be glycoprotein. Wounding experiments showed that pH affected cell migration in a manner similar to its effects on cell multiplication. Low pH inhibited cell migration, but those cells which migrated into the denuded region multiplied as rapidly at low pH as at high pH. The effects of pH on growth were correlated with effects on the uptake of 2-deoxyglucose-(3)H. Dense populations of cells inhibited by low pH were stimulated to incorporate thymidine-(3)H by the addition of small amounts of diethylaminoethyl-dextran. Rous sarcoma cells at high cell density were less sensitive to pH than were normal cells at the same density, but were more sensitive than sparse normal cultures. The results suggest that cell growth is inhibited through the combined effects of both lowered pH and high cell density on cell surface permeability.