Biology Reference
In-Depth Information
biochemical assays, and there is some question as to
whether the same rules apply in vivo, or whether organisms
have evolved tight controls to maintain modularity and
prevent promiscuous cross talk.
More recently, newer findings from in vivo genetic
studies have put the linear and simplistic model of
signaling pathways increasingly at odds even in lower or
simple metazoan model organisms. For example, during
dorsal closure of the Drosophila embryo, the JNK, small
GTPase and TGF-
b
pathways may act together or
sequentially
[6]
. In addition, components that initially
were thought to be unique to one pathway have now
become implicated in others. For example, GSK3
b
and
CK1
a
act as important regulators of both the Wingless and
Hedgehog pathways in Drosophila
[7]
. Further, the Hippo
pathway has recently been found to restrict Wingless/
b
-
catenin signaling by promoting interaction between
a canonical Hippo pathway target, the transcription factor
TAZ/Yorkie, and Dishevelled, a canonical cytoplasmic
component of the Wingless pathway
[8]
. In addition, the
serine/threonine kinase Fused (Fu), a component of the
canonical Hedgehog pathway, functions together with
the E3 ligase Smurf to regulate the ubiquitylation and
subsequent degradation of Thickveins (Tkv), a BMP
receptor, during Drosophila oogenesis
[9]
. Altogether, an
increasing number of examples escape the canonical view
of linear signaling cassettes but rather argue in favor of
more elaborate signaling mechanisms in which variations
in both content and molecular interactions are a general
feature of and between signaling pathways. In summary,
our knowledge of the organization of signaling pathways
is still rudimentary despite our extensive understanding of
some of the players involved in signal transduction.
Recognizing the flexible and interconnected nature of
signaling cascades will promote a more systematic study
of complex cellular signaling, which in turn may greatly
improve our understanding of the origin of signaling
versatility in development and pathology.
in that gene. The systematic application of genetics has led
to a wealth of knowledge in processes such as pattern
formation during development, and signal transduction. For
example, saturation screens (see
Box 5.1
for definition) have
led to a global understanding of pattern formation in the
early Drosophila embryo, and to the identification of the key
genes involved in the process
[10]
. A major result of these
seminal studies was that genes exhibiting the same or
similar morphological mutant phenotypes were often found
to be part of the same signaling pathway.
An important consideration to keep in mind when
taking a genetic approach to deduce gene function is that
one studies the global response of the organism to a genetic
perturbation. Thus, the endpoint phenotype may be telling
us more about the way an organism responds to a genetic
perturbation rather than about the wild-type function of the
gene itself. A telling example is found in the context of
Wingless (Wg/Wnt) signal transduction in Drosophila.
There, the seven transmembrane protein DFz2 (Drosophila
frizzled 2), which encodes the Wg receptor, regulates
the activities of the Dishevelled (Dsh), Glycogen Syn-
thase Kinase 3 (GSK3) and
b
-catenin proteins
[11]
. In the
absence of DFz2, a related receptor encoded by Frizzled
(Fz) can substitute for DFz2, suggesting that these two
related receptors can act redundantly. However, in the
presence of DFz2, Fz does not appear to regulate the
activity of the Dsh/GSK3/
b
-catenin pathway, but instead is
involved in the regulation of the Planar Cell Polarity (PCP)
pathway. Although Fz does not transmit the Wg signal in
the wild-type context, the structure of the signaling network
allows the activity of Fz to be hijacked to compensate for
the absence of DFz2. In this case, the analysis of mutations
in DFz2 failed to reveal the bona fide physiological func-
tion of DFz2 in Wingless signaling.
This simple example illustrates a critical but often
overlooked concept: when interpreting the results of
a genetic approach, there is the danger that our conclusions
about the purported wild-type function of a single gene
product might be obscured by our existing (but incomplete)
knowledge of the signaling network of which it is
a component. This is analogous to Plato's powerful Alle-
gory of the Cave, which argues that our interpretation of the
world around us is limited by observations made from our
vantage point and current knowledge. Thus, in theory, the
best way to fully evaluate the function of a single gene
product would be to first have a global understanding of the
cellular network in which they operate, then remove that
component from the network and conclude about
GENETIC DISSECTION OF SIGNAL
TRANSDUCTION PATHWAYS
In 1958, G. Beadle and E. Tatum received the Nobel Prize
in Physiology and Medicine for demonstrating that 'body
substances are synthesized in the individual cell step-by-
step in long chains of chemical reactions, and that genes
control these processes by regulating definite steps in the
synthesis chain (http: //
www.nobel.se
). Since the realiza-
tion half a century ago that genes encode the building
blocks that make up cells, identifying their functions has
become a priority in the life sciences.
Historically, identifying gene function has relied on
genetic approaches whereby the function(s) of a given gene
is inferred from the phenotype(s) associated with a mutation
the
function of that gene based on 'network knowledge'.
Access to the full repertoire of genes encoded by
different genomes has made it possible to design new
systems-level approaches based on the principles of 'reverse
genetics' to construct such global cellular networks.
Reverse genetics is an approach to discover the function of
