Biology Reference
In-Depth Information
Looking at the situation in the context of the Darwinian model
of differentiation, if the cell could adjust the stability of its state,
i.e. the stability of the molecular interactions, depending on its
needs, that would mean its functioning would have considerably
developed; in other words, the cell would be in an unstable state
when it is in an unfavourable microenvironment and conversely, in
a stable state when in a favourable microenvironment. This would
increase the probability of it changing phenotype and adapting to
this microenvironment when necessary. In the example of the heap
of cells (Fig. 16B), those in its upper part are in an environment poor
in nutrients and their metabolism is slowed. It is therefore prefer-
able for them to be in an unstable state which would allow them to
change phenotype and adapt. On the other hand, the cells in the
lower part of the colony are in an environment rich in nutrients and
their metabolism is active. It is therefore preferable for them to be
in a stable state and not to change phenotype.
In the Darwinian model, the function of signals is to induce
these changes in stability depending on the needs of the cell
(Kupiec, 1996, 1997).
6.3.4
Mode of action of a signal
(
selector,
signal-food
)
The stability of a cell state depends on the stability of the bonds
between the molecules. This stability depends in turn on post-
translational or epigenetic modifications of the proteins. For example,
the phosphorylation state of proteins modifies their binding prop-
erties. This is a well established fact, especially as concerns gene
expression regulatory factors binding with their target sequences in
DNA (Li
et al
., 1994; Xu
et al
., 1994; Bourbon
et al
., 1995; Lefebvre
et al
., 1995; Takenaka
et al
., 1995). Gene expression and the cell
type that ensues from it are thus more or less stable depending on
the state of phosphorylation of these proteins. This state of phos-
phorylation itself depends on signalling pathway enzymes activated
by signals (see for example Hill and Treisman, 1995), so the latter
have a direct influence on the stability of intermolecular bonds in
