Movements of intracellular particles in undifferentiated amebae of Dictyostelium discoideum

Abstract

We recorded live, undifferentiated amebae of Dicfyostelium discoideum by video microscopy and analyzed the behavior of cytoplasmic particles and granules. Cytoplasmic streaming and saltatory movements are the two major types of particle movements that occur in interphase amebae. Saltatory movements predominated in an area around the nucleus-associated body (NAB) and many were radial toward or away from it, the velocity being very similar in both directions. Some saltations were simple forward movements, and others were complex toand-fro movements with as many as seven turnabouts. For a given leg of movement the velocity was not uniform along the path. Small particles (< 1 pm) moved faster (X = 2.8 pm/s) than large (-1 pm; R = 2.1 pmis) and very large (> 1 pm; X = 1.4 pm/s) particles, but the smallest particles were visible only in the running image and could not be analyzed. Ultrastructurally, saltating particles are digestive vacuoles and vesicles of various sizes, appearances, and contents, which are numerous particularly in the vicinity of the NAB. Several lines of evidence pointed to a role of microtubules (MTs) in saltatory particle movements. Composites of particle tracks corresponded closely to MT arrays visualized by immunofluorescence. No saltations occurred in mitotic amebae that lack cytoplasmic MTs, but the movements resumed toward the end of division, concurrently with the rebuilding of the complex of cytoplasmic MTs. Nocodazole reduced and eventually stopped saltatory movements over a period of 3 h, when aberrant MT patterns were the rule. Saltations in slime mold amebae may be an eye-catching feature of intracellular transport functioning in endo-and exocytosis in the shuffling of vesicles containing factors involved in ameboid movement, and in the transduction of external signals to the cell center.

Morphology and Dynamics of the Endocytic Pathway inDictyostelium discoideum

“…By EM, such small vesicles of ϳ120 nm in diameter were connected to the microtubules by 35-40-nm-long tethers ( Figure 4B, higher magnification can be seen in the rightmost panel of Figure 4C). Overall, our findings are consistent with earlier observations of bidirectional movement of organelles in D. discoideum (Roos et al, 1987;Pollock et al, 1999).…”

Simple non-fluorescent polarity labeling of microtubules for molecular motor assays

“…In this manuscript we report the motion of spherical, refractile organelles which ranged 300800 nm in size. A majority of these organelles are likely to be vesicles (including vacuoles), based on earlier published electron micrographs (26), and as also reported in earlier work on the motility of Dictyostelium extracts (15,16). …”

cAMP-mediated inhibition of intracellular particle movement and actin reorganization in Dictyostelium.

“…Roos et al (29) measured particle movement in undifferentiated amoebae of Dictyostelium discoideum, and recorded average measurements of 2.8/zm/s for small particles, 2.1 #m/s for large particles and 2.1 #m/s for very large particles. They did not observe polar particle movement, but micrographs of their cells indicated that they lacked axial polarity (i.e., they were not elongate) (29), and, since they employed vegetative cells grown in the absence of bacteria, the cells were most likely not translocating in a rapid and persistent fashion (45). The rapid addition of 10 -6 M cAMP to a rapidly moving amoeba results in an almost immediate depression in the rate of particle movement.…”

“…The relationship of particle movement and the cytoskeleton has long been a focus of investigation (e.g., 2,11,28,29). Therefore, we tested whether parallel cytoskeletal events accompanied the depression in particle movement and cellular motility immediately after cAMP addition.…”