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Cell-cell interactions in cell fate determination in Caenorhabditis
What is the role of induction in cell fate determination?
C. elegans has similar advantages to Drosophila as a system for the study
of development: the exploitation of the power of genetics. Furthermore the fates of each
and every cell during development to the adult are known (Sulston et al., 1983).
Thus, cell fate studies can be done very precisely. We shall examine the determination of
pharyngeal fate development as an example of the kinds of elegant developmental analyses
that are possible with C. elegans..
The first cleavage division generates a large anterior AB blastomere and a smaller
posterior P1 blastomere (Evans et al., 1994, Fig. 1). Descendants of both AB and P1
produce pharyngeal cells during development. However, they differ in their potential to
produce these cells. P1 has an intrinsic ability to produce posterior pharyngeal cells
when it is isolated from AB. However, AB descendants only produce anterior pharyngeal
cells as a result of interactions with P1 descendants, in a process that requires maternal
expression of the glp-1 gene.
The GLP-1 protein is a transmembrane protein that is thought to function as a receptor for
extracellular signals. In embryos from homozygous glp-1 mutant mothers, P1 produces
pharyngeal cells, but AB does not, indicating that a glp-1-dependent signaling
event is necessary for AB to produce pharyngeal cells. This observation demonstrates how
genetic tools can be used to answer questions that would otherwise require manipulations
The larger AB blastomere divides to produce ABa and ABp, whereas P1 divides to yield EMS
and P2. Of the two products of P1 division, only EMS retains the ability to produce
posterior pharyngeal cells. The cells at this stage are situated in the egg case such that
P2 is adjacent to ABp and not ABa (see Fig. 1). Thus, P2 and ABp can engage in cell-cell
interactions that require glp-1.
At the next division, EMS divides to produce E and MS. The MS cell engages in an inductive
interaction that causes descendents of ABa to form anterior pharynx. This interaction also
requires glp-1 . Because GLP-1 is a thought to be receptor protein, it should be
produced in descendants of the anterior blastomere, which are the targets of these glp-1-dependent
Although the glp-1 mRNA is present in both the oocyte and
embryo, the GLP-1 protein is not synthesized in oocytes. Is its synthesis generalized, or
is it restricted to the anterior blastomeres? The immunolocalization results shown in
Figure 2 reveal that the protein is strictly restricted to the anterior blastomeres. This
localization occurs in spite of the uniform distribution of the mRNA. Thus, the mRNA is
subject to spatially-restricted translation.
Because of the frequent involvement of 3' UTRs on translational regulation, Evans et
al. tested whether the 3' UTR of glp-1 is responsible for its regulation. A
reporter construct was made that had a ß-galactosidase coding sequence and nuclear
localization signal located upstream of a glp-1 3' UTR. (The nuclear localization
signal allowed for the ß-gal to accumulate in nuclei and be more readily detectable.) A
control reporter that lacked the glp-1 3' UTR was also used. RNA was synthesized in
vitro from either construct and injected into the germline.
As shown in Figure 5 (Evans et al., 1994), embryos derived from animals injected
with control RNA had lacZ staining throughout the embryo, whereas those derived from
animals injected with lacZglp-1 RNA had ß-gal staining only in anterior blastomeres.
However, the RNA itself was evenly distributed within the embryos (Fig. 6). Thus, the glp-1
3' UTR is sufficient to restrict translation to the anterior blastomeres. A deletion
analysis was conducted to identify the elements in the 3' UTR that regulate glp-1
translation. As shown in Table 1, a 125 nt element at the 3' end of the UTR is necessary
and sufficient for temporal regulation, whereas a 61 nucleotide region in the middle of
the 3' UTR is required for spatial regulation. (See also Fig. 5G-5I.) These results
indicate that the localization of GLP-1 protein is a consequence of translational
regulation of glp-1 mRNA.
The role of induction in determination of cell fate in C. elegans has only recently
been appreciated. It had previously been thought that cell fate was entirely a consequence
of intrinsic differences among cells due to localization of determinants. These inductive
interactions operate at the level of single cells, whereas inductive interactions in
higher organisms function at the tissue level.
- Diagram cleavage to the 8-cell stage. Label each cell and place on your diagram the
interactions that are necessary for the formation of the anterior pharynx.
- What is the role of GLP-1?
- What is the phenotype of embryos derived from homozygous glp-1 mothers?
- How is synthesis of GLP-1 regulated? Describe experimental evidence.
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