Abstract
Amoebae of Naegleria gruberi differentiate to temporary flagellates that have a regular, asymmetric, streamlined body contour. During the hour-long differentiation, amoeboid movement gradually ceases and as a consequence the cells round up. Subsequent elongation to flagellate shape includes the formation of a microtubular cytoskeleton. Both the loss of amoeboid motility and the formation of the flagellate shape require prior transcription and translation, suggesting the possibility that specific syntheses of RNA and protein may be required for each shape change. Flagellates can “revert” to motile amoebae within 20 sec after a suitable stimulus, indicating that the amoeboid motility system remains latent in flagellates. A cell-produced chemical factor extracted from Naegleria, Ψ, triggers a reproducible sequence of rapid shape changes in flagellates when added to their environment. Cells respond to the presence of external Ψ only “transiently,” and the reaction of flagellates to added Ψ requires extracellular Ca+2. Ionophore A23187 produces shape changes in flagellates similar to those produced by Ψ, supporting the conclusion that Ψ is involved in the movement of Ca+2. Normally Ψ is intracellular, and the intracellular distribution of Ψ changes during differentiation.
These results lead to and support a working hypothesis to explain the rapid changes in shape and motility in Naegleria. Four elements are postulated: Ca+2; an actin-based amoeboid motility system that depends on free Ca+2 for functioning; a tubulin-based cytoskeleton that assembles and remains assembled only when free Ca+2 is low; and Ψ. The factor Ψ is postulated to regulate the intracellular release of Ca+2. According to the hypothesis, intracellular free Ca+2 is constantly swept up into Ca-reservoirs. Motility of amoebae depends on local release of Ca+2 from these reservoirs, which in turn is caused by the intracellular release of Ψ. During differentiation, Ψ is “compartmentalized” as part of the developmental program, and as a consequence intracellular Ca+2 is swept up into Ca-reservoirs but not released. As free Ca+2 becomes limiting, amoeboid movement stops, and the cells round up. Subsequently, in a process that depends on low free Ca+2, the microtubular cytoskeleton is assembled, and the flagellate shape is formed. During reversion of flagellates to amoebae, release of Ψ from its “compartments” permits local release of Ca+2, which then causes both disassembly of the flagellate cytoskeleton and immediate resumption of amoeboid movement. This testable hypothesis has implications for the study of cell shape, motility, and differentiation.