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Drosophila development
Segment polarity genes and cell-cell signalling during development.
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The transcription factor proteins encoded by the gap and pair rule genes divide the
embryo in consecutively smaller units (segments/parasegments), ending with stripes of
expression of the segment polarity genes in each segment. Up until now transcription
factors were able to interact with each other [a nucleus can be influenced by two such
proteins, even if neither protein was produced (as a transcript) in that nucleus], cell
membranes are formed at the time the segment polarity genes become expressed.
All segment polarity mutants show similar patterning defects in each segment, repeated
along the trunk of the (dead) larva. Consistent with the fact that the proteins encoded by
the segment polarity genes are required for patterning of the segment (evident from the
mutant phenotypes), the genes are expressed repeated in each segment. In addition, in the
now cellularised animal, cell-cell signalling is needed to ensure proper cell differentiation
and indeed segment polarity genes encode for signalling-type proteins.
Do the genetic findings tell us anything about how the system works? In a wingless
mutant embryo, all cells that normally secrete naked cuticle (about half the segment) are
lost or re-patterned. The wingless gene is however only expressed in a one cell wide
stripe, one per segment. From such simple experiments but not necessarily knowing
anything else, we can conclude that Wingless protein function is required over several
cell diameters away. Indeed when the gene was sequenced it appeared that the wingless
gene encoded for a secreted protein that is apparently able to travel several cell diameters
from its source and influence patterning at a distance. The protein was also found to be a
direct homologue of a gene already known in human and mouse to cause mammary
tumours.
Normal segment
wingless mutant
Using both molecular (looking at expression of genes in mutant backgrounds) and genetic
means (combining mutants into double mutant combinations; leading to the elucidation
of genetic hierarchies, a process called epistasis in genetic terms), the role of most of the
segment polarity genes has been eluded. Their hierarchical interactions were thus
“known” even before the proteins were biochemically characterised
Pair rule gene activity is required to establish the expression of the segment
polarity genes. The anterior boundary of engrailed and hedgehog expression (in same
cells) is precisely put down by pair rule genes, this is the future “middle” of the segment
(or also known as parasegment boundary). On the anterior side of the parasegment
boundary a row of cells expresses wingless; i.e. anterior to the engrailed/hedgehog
domain.
It appears that two secreted signalling proteins control the patterning of the
segments: these are encoded by the segment polarity genes wingless and hedgehog. Most
of the other segment polarity genes encode for proteins that are required in the signalling
pathways of either Hedgehog or Wingless. This was established first by genetic analysis
and later (sometimes) confirmed by biochemical analysis. Both signalling pathways lead
to transcriptional activation of targets. It was found that in early stages of segment
polarity function, the transcriptional target of Wingless, is the hedgehog gene and vice
versa. This reciprocal signalling makes sure that the segment is maintained as a unit and
it also provides a reference point within the segment (i.e. the parasegment boundary).
Some not so straightforward interactions between genes were found early on. For
instance, the patched mutant leads to a phenotype that is almost the opposite of the
hedgehog phenotype. When the hedgehog gene is over-expressed in an embryo (using
transgenic flies, easy to make), a phenotype like patched is generated. This indicated that
patched works to inhibit Hedgehog signalling (loss of patched = gain of Hedgehog). The
double mutant of hedgehog and patched looks like patched. Patched therefore works
downstream of hedgehog (see figure below). This genetic analysis has been confirmed by
biochemical methods: Hedgehog protein binds to Patched protein and inactivates it.
Besides genes that have been discovered as required downstream of Wingless or
Hedgehog, some might be required “upstream” (for instance for diffusion of the
signalling protein or secretion from producing cells).
normal
patched
mutant
hedgehog
patched hedgehog
The Drosophila segment can be seen as a paradigm for the patterning of fields of
cells (one-dimensional in this case: anterior-posterior). A dual signalling mechanism
provides a simple though reliable/stable and regenerating system. Such patterning
of a group of naïve cells tells us something about for instance regeneration processes
as well as giving us information on for instance stem cell biology.
In both cases (Wingless and Hedgehog) a “distinct” signalling pathway has been
identified through the work on the Drosophila segment polarity genes.
However both hedgehog and wingless belong to large protein families that are highly
conserved in evolution. Wingless was originally discovered as an oncogene. In fact, many
proteins encoded by the segment polarity genes are proto-oncogenes or tumour
suppressor genes in certain contexts.
A lot of work has established that both the wingless and hedgehog gene families are
required for patterning and development in vertebrates (humans). The signalling
pathways as described in the Drosophila segment are conserved at all these sites. For
instance, the sonic hedgehog gene is required for patterning at many sites. One of these is
in the patterning of the neural tube. It is expressed in the notochord and patterns the
ventral side of the tube (floor plate and motoneurons). Its absence leads to midline
defects. However later in development required for patterning of muscle, arms,/legs,
neurons etc etc.
Text books:
Chapter 5, Wolpert, Principles of Development; pages 150-161 (1st edition).170-181 (2nd
edition).
Chapter 11, Wolpert, Principles of Development, pages 346-347 (1st edition); 383-386
(2nd edition).
Chapter 14, Gilbert, Developmental Biology; pages 565-569 (5th edition), 282-285 (6th
edition), 283-285 (7th edition).
Chapter 14, Gilbert, Developmental Biology; pages 659-660.(5th edition), 151-152 (6th
and 7th edition).
Literature:
Ingham, P. W. (1988) Nature, 335, 25-34.
Ingham and McMahon (2001), Genes and Development, 15, 3059-87.
Look at website: http://www.stanford.edu/~rnusse/wntwindow.html