Dictyostelium Discoideum - Use As A Model Organism

Use As A Model Organism

Because of the simple life cycle of D. discoideum, it is commonly used as a model organism. It can be observed at organismic, cellular, and molecular levels primarily because of their restricted number of cell types, behaviors, and their rapid growth. It is used to study cell differentiation, chemotaxis and programmed cell death, which are all normal cellular processes. It is also used to study other aspects of development including cell sorting, pattern formation, phagocytosis, motility, and signal transduction. These processes and aspects of development are either absent or too difficult to view in other model organisms. D. discoideum is closely related to higher metazoans. It carries similar genes and pathways making it a good candidate for gene knockout.

Cell differentiation is the process that occurs when a cell becomes more specialized to develop into a multicellular organism. Changes in size, shape, metabolic activities, and responsiveness can occur as a result of adjustments in gene expression. Cell diversity and differentiation, in this species, involves decisions made from cell-cell interactions in pathways to either stalk cells or spore cells. These cell fates depend on their environment and pattern formation. Therefore, the organism is an excellent model for studying cell differentiation.

Chemotaxis is defined as a passage of an organism toward or away from a chemical stimulus along a chemical concentration gradient. Certain organisms demonstrate chemotaxis when they move toward a supply of nutrients. In D. discoideum, the amoeba secretes the signal, cAMP, out of the cell attracting other amoebas to migrate toward the source. Every amoeba moves toward a central amoeba, the one dispensing the greatest amount of cAMP secretions. The secretion of the cAMP is then exhibited by all amoebas and is a call for the amoebas to begin aggregation. These chemical emissions and amoeba movement occur every six minutes. The amoebas move toward the concentration gradient for sixty seconds and stop until the next secretion is sent out. This behavior of individual cells tends to cause oscillations in a group of cells, and chemical waves of varying cAMP concentration propagate through the group in spirals.

The use of cAMP as a chemotactic agent is not established in any other organism. In developmental biology, this is one of the comprehensible examples of chemotaxis.

Thermotaxis is movement along a gradient of temperature. The slugs have been shown to migrate along extremely shallow gradients of only 0.05C/cm. But the direction chosen is complicated; it seems to be away from a temperature about 2C below the temperature to which they had been acclimated. This complicated behavior has been analyzed by computer modeling of the behavior and the periodic pattern of temperature changes in soil caused by daily changes in air temperature. The conclusion is that the behavior moves slugs a few centimeters below the soil surface up to the surface. This is an amazingly sophisticated behavior by a primitive organism with no sense of gravity.

Programmed cell death (apoptosis) is a normal part of species development. Apoptosis is necessary for the proper spacing and sculpting of complex organs. Around 20% of cells in D. discoideum altruistically sacrifice themselves in the formation of the mature fruiting body. During the pseudoplasmodium (slug or grex) stage of its life cycle, the organism has formed three main types of cells: prestalk, prespore, and anterior-like cells. During culmination, the prestalk cells secrete a cellulose coat and extend as a tube through the grex. As they differentiate, they form vacuoles and enlarge lifting up the prespore cells. The stalk cells undergo apoptosis and die as the prespore cells are lifted high above the substrate. The prespore cells then become spore cells; each one becoming a new myxamoeba upon dispersal. This is an example of how apoptosis is used in the formation of a reproductive organ, the mature fruiting body.

A recent major contribution from Dictyostelium research has come from new techniques allowing the activity of individual genes to be visualised in living cells. This has shown that transcription occurs in "bursts" or "pulses" (see transcriptional bursting) rather than following simple probabilistic or continuous behaviour. Bursting transcription now appears to be conserved between bacteria and humans. Another remarkable feature of the organism is that it has sets of DNA repair enzymes found in human cells which are lacking from many other popular metazoan model systems. Defects in DNA repair leads to devastating human cancers, so the ability to study human repair proteins in a simple tractable model will prove invaluable.