Amoeboid Locomotion Unsolved Problems (and partly solved problems)

A) There are about a dozen fundamentally different mechanisms of amoeboid locomotion. Here are ten. Any case in which cells change shape while moving, and are not being propelled by cilia or flagella, most biology textbooks and most scientists (those who do not specialize in the subject) will say that the cells "move by amoeboid locomotion", as if there were just one mechanism by which cells crawl. (And as if it were an explanation to say that something moves by amoeboid locomotion. That was the impression that I got in high school and college - until I worked one summer at the University of Miami Marine lab, specifically on a certain kind of amoeba. I owe Prof. Sam Meyers, and everybody at the U. of Miami a lot of gratitude for giving me my first chance to do real research on cells).

B) Most of these different kinds of amoeboid locomotion get their propulsive force from actin and myosin. The form of amoeboid locomotion used by nematode sperm is the only known example that doesn't use acto-myosin. (In fact, they contain almost no myosin at all.) Instead, they use a special cytoplasmic protein found nowhere else, which polymerizes into fibers and somehow exerts the propulsive force. This was discovered by Sam Ward and his successors at Florida State University. The molecules of this special protein have reflection symmetry in their long axis, which is unusual for cytoskeletal proteins, and is specially surprising for a protein that exerts directional forces.

C) One kind of "amoeba" (called "Labyrinthula") secretes large areas of membrane-like sheets of lipid and protein (with the same molecular structure as cell membranes). They have actin and myosin in the fluid between their plasma membranes and these external membrane-like sheets. These sheets extend outward ahead of the cells, which are shaped like footballs, are stiff, and slide (or are pulled?) rapidly along inside the extracellular membrane-like sheets. These sheets were called "slime trails". At the time I was studying these organisms, there were two competing theories about their structure:

1) Either the organisms were syncytia, with many cells inside the same plasma membrane. (In other words that what seemed to be cells, and really are cells, supposedly being nuclei, and the extracellular membranes being ordinary plasma membranes.)

2) Alternatively, that the extracellular membranes were non-living, extracellular secretions.

My two research contributions were (a) to discover that the slime trails move actively, even where there are no cells, and (b) that these cells have DNA only in their nuclei, in other words, that these structures are not themselves nuclei, but contain nuclei separate from their cytoplasm. These results were inconsistent with both the competing paradigms (so my boss thought I couldn't be right, and didn't continue or publish the results). If he had published my work, I would have stayed in marine biology and done my PhD at Miami. Five years later, researchers at Harvard (David Porter and Keith Porter) used electron microscopy and time lapse photography to prove the same two conclusions, which were considered a major discovery: extracellular membranes!

Incidentally, Labyrinthula is an extremely common organism. You can culture it from pieces of "sea grass" (genus Zostera in North Carolina and northward; genus Thalassia in Florida). It is a major pathogen for sea grass, and caused epidemics along the US coast in the 1930s, greatly changing offshore ecology and indirectly causing scallops to become much less common (which was an economic disaster).

The special behaviors of Labyrinthula membranes might become useful for studying membrane structure, membrane synthesis and physical connections between membranes and acto-myosin fibers. Other interesting questions include how these creatures absorb nutrients out of the sea grass, and how they digest holes through cell walls.

D) "Shelled Amoebae". Those of the genus Arcella secrete hard shells, with an opening at one end, out of which long, thin pseudopods are protruded to capture food. Another genus, Difflugia, make hollow shells by gluing together hundreds of tiny grains of sand. They extend and contract long narrow pseudopods, which pick up bits of food and more sand grains.

Every time one of these pseudopods adheres to anything, the cytoplasm inside it becomes birefringent. The explanation is believed to be that contact induces polymerization of acto-myosin fibers, which is interesting.

E) "Radiolaria" are a large group of amoeboid organisms that secrete beautifully complicated shells. More scientists should try to discover the mechanisms by which they move and secrete their shells.

F) Diatoms . Those species of diatoms that have circular shells (= "centric diatoms") have no method of locomotion. Most (or all) of the elongate species of diatoms can crawl slowly by means of almost invisible tubes of cytoplasm and membrane that protrude out a series of tiny holes through the rigid silicon oxide shells. This is not traditionally considered to be a case of amoeboid locomotion, but perhaps for no better reason than that the cell as a whole can't change shape because of the rigid shell.

The video at the following URL shows locomotion by many diatoms, and also some flagellated bacteria and "gliding locomotion of blue green algae.

G) Rolling Amoebae (for lack of any better name for this subgroup of amoebae). Their plasma membrane slides forward across the top of each amoeba, rolls downward at the front end, and moves rearward across the bottom, just like the tread of a tank. If you put small particles on the top of an amoeba of this kind, each particle gets carried forward over the top of the amoebae at exactly twice the speed as the speed of the amoeba itself.

Much has been learned by this method of attaching particles, antibodies, or other markers to the outside surfaces of amoebae and other motile cells. For example, in the case of Amoeba proteus, attached particles get carried forward over both the top and the bottom of pseudopodia. In contrast, tissue culture cells carry attached particles rearward across both the top and the bottom.

An important topic of research is how actin transmits forces tangentially through plasma membranes. Presumably, force transmission works differently in one category of amoeboid locomotion as compared with another.

H) The Amoeba proteus type of amoeboid locomotion (also occurs in the giant amoeba Chaos carolinense that was discovered by Wilson, during a teaching laboratory at this university. The species name "carolinense" was chosen by Wilson in honor of this university, and not the state. The genus name "Chaos" is also appropriate. These are the kind with the dramatic and rapid cytoplasmic flow. Much research has been more about what causes the flow, rather than how the flow contributes to forward displacement of the cell as a whole. They rapidly phagocytize paramecia and other protozoa, by means of a sudden change in the flow pattern.

I) The Dictyostelium type of amoeboid locomotion. These are closest to tissue culture cells of all kinds of amoebae. A not-yet-answered question is whether multicellular aggregations ("slugs") of their individual cells crawl by the same basic mechanism as the individual cells use.

J) The Physarum type of amoeboid locomotion. These are multinucleate, and can be a inch or more long. Cytoplasmic flow alternates in direction every minute or so (I have got to check this time), and the cells increase their force of contractility at regular time intervals.


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