The role of divalent cations in the regulation of microtubule assembly. In vivo studies on microtubules of the heliozoan axopodium using the ionophore A23187.

Abstract

Low concentrations of calcium and magnesium ions have been shown to influence microtubule assembly in vitro. To test whether these cations also have an effect on microtubules in vivo, specimens of Actinosphaerium eichhorni were exposed to different concentrations of Ca ++ and Mg ++ and the divalent cation ionophore A23187. Experimental degradation and reformation of axopodia were studied by light and electron microscopy. In the presence of Ca ++ and the ionophore axopodia gradually shorten, the rate of shortening depending on the concentrations of Ca ++ and the ionophore used. Retraction of axopodia was observed with a concentration of Ca ++ as low as 0.01 mM. After transfer to a Ca++-free solution containing EGTA, axopodia re-extend; the initial length is reached after about 2 h. Likewise, reformation of axopodia of cold-treated organisms is observed only in solutions of EGTA or Mg ++, whereas it is completely inhibited in a Ca ++ solution. Electron microscope studies demonstrate degradation of the axonemal microtubular array in organisms treated with Ca ++ and A23187. No alteration was observed in organisms treated with Mg ++ or EGTA plus ionophore.The results suggest that, in the presence of the ionophore, formation of axonemal microtubules can be regulated by varying the Ca ++ concentration in the medium. Since A23187 tends to equilibrate the concentrations of divalent cations between external medium and cell interior, it is likely that microtubule formation in vivo is influenced by micromolar concentrations of Ca ++. These concentrations are low enough to be of physiological significance for a role in the regulation of microtubule assembly in vivo.Many functions with which microtubules are associated depend on their ordered assembly and disassembly. In vivo studies clearly show that formation and breakdown of microtubules are influenced by temperature, hydrostatic pressure, and D20 (for review, see reference 48). These

Evidence for an intracellular calcium store releasable by surface stimuli ifibroblasts (L cells).

“…W e c o n c l u d e f r o m the e l e c t r o p h y s i o l o g i c result that the f r e e i n t r a c e l l u l a r Ca ++ c o n c e n t r a t i o n was a l t e r e d very little d e s p i t e a p r e s u m a b l y substantial i n c r e a s e in the Ca ++ i n f l u x across the s u r f a c e m e m b r a n e as i n d i c a t e d by vacuole f o r m a t i o n . T h e p r e s e n c e o f n o r m a l n u m b e r s o f microtubules in these ionophore-treated cells also suggests that the cytoplasmic free Ca concentration may not be increasing substantially (Schliwa, 1976). Thus, intracellular systems for sequestration of Ca must compensate for the increased influx of Ca across the surface membrane induced by the ionophore.…”

“…The primary function of the release of calcium from intracellular stores in addition to the control of membrane permeability to other ions may be to control cellular motility by regulation of the interactions of actin and myosinlike proteins (reviewed by Hitchcock, 1977). The importance of" this Ca-regulating system probably extends beyond the control of motility, however, because Ca has been implicated as a second messenger or coupling factor in the regulation of a variety of other cellular processes (reviewed by Berridge, 1975 b;Rasmussen and Goodman, 1977) such as secretion (Douglas, 1968;Rubin, 1970), control of microtubule polymerization and function possibly including chromosome movement in mitosis (Gallin and Rosenthal, 1974;Schliwa, 1976;Harris, 1978), and control of cell proliferation (MacManus et al, 1975;Rebhun, 1977).…”

Calcium lability of cytoplasmic microtubules and its modulation by microtubule-associated proteins

“…The increase of Ca2+ to [1][2][3][4][5][6][7][8][9][10] ,uM by local and transient release from intracellular stores is believed to be the mechanism that activates many Ca2+-dependent functions in a variety of cells (26,27). With respect to microtubule regulation, fluctuations in cytoplasmic Ca2+ in the physiological range could either prevent or allow for microtubule assembly, thus regulating the state of the microtubule system in the cell.…”

Disturbance in Arrangement of Heliozoan Axopodia and Their Degradation with Concanavalin A Treatment<SUB>1</SUB>

“…It is known that axopodia are easily degraded by low temperature (28), hydrostatic pressure (7), colchicine (27,17), light metal ions, such as Ca'', Mg2+ and Sr2+ (19,23,24) and heavy metal ions, such as Hg2+, Cd2+ and Zn2+ (20) and Cu2+ and Ni2+ (16). But such effects of Con A have not been reported.…”