Free and centrosome-attached microtubules: Quantitative analysis and modeling of two-component system

In cultivated in vitro interphase animal cells, microtubules form a network whose density is highest in the central cell area, in the region of centrosome, and decreases towards the cell periphery. Since identification of individual microtubules in the central cell area is significantly difficult and more often is impossible, there are several approaches to studying microtubules in the internal cell cytoplasm. These approaches are based on a decrease of microtubule density—both real, due to their partial depolymerization (by the action of cold temperatures or cytostatics), or apparent, due to a decrease of cell thickness (by photobleaching of preexisting microtubules and analysis of newly formed ones). In the present work, we propose a method based on the determination of optical density which allows evaluation of the state of the cytoplasmic microtubule system as a whole. The method consists of a comparison of the dependences describing changes of the microtubule optical density from the cell center to the periphery in controls and in experiments. Analysis of living cells by the proposed method has shown that the character of curves describing the decrease of optical density from the cell center to its periphery is different for various cell types; the dependence can be described both as an exponential regression (the CHO cell line) and as a linear regression (the NIH-3T3 and REF cell lines). Our previous studies have allowed the suggestion that the character of the dependence is determined by the ratio of free and centrosome-attached microtubules and by the position of their ends in the cell cytoplasm. To test this hypothesis, we considered model systems with all microtubules assumed to be in a straight orientation and divergent radially from the centrosome, but with different arrangements of plus-and minus-ends. In the model system, in which all the microtubule minus-ends are attached to the centrosome while the plus-ends are at different distances from it, the microtubule density is described by the exponential (f(x) = ae ^?bx ). Introduction of free microtubules into the system leads to a change of the character of this dependence, and the system in which the concentration of free microtubules with minus ends located at different distances from the cytoplasm is 5 times higher than that of the centrosome-attached microtubules is described by the linear regression equation (f(x) = k ^* x + b), which corresponds to the experimentally obtained dependences for 3T3 and REF cells. Thus, we believe that even in cells with a radial microtubule system, free microtubules may constitute the majority.