Biology Reference
In-Depth Information
stages of this process they adhere to each other in a Ca
2+
-independent
manner. Once the mound is formed, differentiation and morphogenesis begin,
culminating in the formation of a mature fruiting body comprised of a stalk
and a spore head. The reappearance of food results in the germination of the
spores and re-emergence of amoebae.
The adhesion systems in Dictyostelium amoebae are, in some ways, specific to
this organism as in their native environments, such as the leaf litter of the forest
floor, they encounter a wide range of surfaces along which to move or bacteria
to ingest. Therefore, a given Dictyostelium amoebae extracellular receptor may
have a wide range of ligands in order to provide the cell with maximal adhesion.
Similarly, the molecules known to play a role in cell-cell adhesion during
streaming and morphogenesis are specific for Dictyostelium but they do share
general features with adhesion molecules in higher eukaryotes (Cornillon et al.,
2000; Fey et al., 2002). Furthermore, adherens junctions have been observed
close to the top of the stalk in mature fruiting bodies (Grimson et al., 2000).
These junctions appear to have actin filaments emanating from either side and a
b-catenin homologue, aardvark, is localized to these structures. The b-catenin
null mutant exhibits altered junctions and the fruiting bodies lack mechanical
stability (Grimson et al., 2000). Additionally, a novel Dictyostelium protein,
AmpA, that possesses both disintegrin and ornitin domains appears to act as an
anti-adhesion molecule, modulating cell-cell contacts that must occur during
morphogenesis and late development (Varney et al., 2002). Thus, while
Dictyostelium employs organism-specific adhesion and anti-adhesion molecules
throughout its life cycle, the general themes of cell-cell adhesion/de-adhesion,
signalling and junction formation are found indicating a general conservation
of mechanism throughout phylogeny.
Dictyostelium cell surface receptors are linked to cytoskeletal proteins, as
observed for higher eukaryotes. Support for this comes from the observation
that mutant strains, which lack various actin-binding proteins, have reduced
adhesion (Niewo
¨
hner et al., 1997; Tuxworth et al., 2001; Han et al., 2002). The
loss of adhesion not only alters the motility of these cells but in many cases
(but not all) phagocytosis is also impaired. This reflects the fact that
pseudopod and phagocytic cup extension share the same general mechanism
and machinery - engagement of cell surface receptors stimulates directed
actin polymerization along a surface. In fact, the two behaviours appear to be
mutually exclusive as it has been observed that cells typically stop to eat,
recruiting leading edge components to the phagocytic cup (Maniak et al.,
1995). Mutants in VASP, talin and myosin VII (DdM7) are each defective in
adhesion during motility and the talin and DdM7 mutants additionally show
reduced phagocytic activity (Niewo¨ hner et al., 1997; Tuxworth et al., 2001;
Han et al., 2002). Interestingly, both talin and M7 are linked to adhesion in
higher eukaryotes, thus their function appears to be conserved throughout
phylogeny.
