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BOX
BMP AND WNT: AN INTRODUCTION
TO SIGNALING IN DEVELOPMENT
cell. However, when wnt binds to frizzled, a protein called
Disheveled blocks the degradation of the complex and
causes the b-catenin to accumulate. As the b-catenin
accumulates, some of it moves to the nucleus, where
it forms a complex with a different protein, TCF. The
b-catenin/TCF complex can bind to DNA at specific
sequences and activate target genes.
There are many similarities between the wnt and BMP
signaling pathways. Both rely on cell-surface receptors
to cause a change in cytoplasmic components of the cell
that eventually reach the nucleus to cause a change in
gene transcription. In addition, there are several natural
inhibitors of these pathways; for the BMP pathway, follis-
tatin, noggin, and chordin can interfere with the activation
of the pathway by blocking BMP from binding to the recep-
tor, and for the wnt pathway, cerberus , FrzB, and Dkk
prevent activation, most likely by blocking the wnt from
accessing the receptor. Throughout this topic we will see
that different signal-receptor systems are involved in
nearly all developmental events, and while the types of
proteins and details of the transduction cascades may
vary, the fundamental features of all these pathways are
similar.
BMPs
P
P
P
Smad Smad
Type II
Type I
Smad
Co-Smads
Smad
P
Smad
P
P
Smad
Complex
Smad
Nucleus
P
Smad
P
P
Smad
Transcription
Smad-interacting protein called Sip1 is activated in the
neural plate by FGFs (Akai and Storey, 2004).
The model of neural induction that has emerged
from many lines of investigation is quite gratifying and
at the same time somewhat surprising. Despite all of
the experiments that were done to study the embryol-
ogy of neural induction by a great many investigators,
it has only been recently that we have appreciated that
the development of neural tissue in the ectoderm of a
gastrulating embryo is actively inhibited by BMPs.
Neural “induction” is actually the reversal of this inhi-
bition (Figure 1.23). What has also emerged from these
studies is that the process of neural induction is
coupled to the process of axis specification, and the
“neural inducers” have more general effects on defin-
ing the dorsal axis of the embryo. Moreover, despite the
apparent redundant requirement for the BMP antago-
nists in CNS induction, in all of the single and com-
pound knockout animals that have been analyzed to
date, the posterior nervous system develops relatively
normally. Therefore, it is possible that antagonism of
BMP in more posterior regions of the embryo may
require other types of inhibition, such as antagonism of
the downstream signaling cascade or Smad pathway.
INTERACTIONS AMONG THE
ECTODERMAL CELLS IN
CONTROLLING NEUROBLAST
SEGREGATION
The generation of neurons from the neurogenic
region of both vertebrates and invertebrates typically
involves an intermediate step: a neural precursor cell
is first produced, and this cell goes on to produce many
neurons. The neural precursor is capable of mitotic
divisions, whereas the neuron itself is usually a termi-
nally postmitotic cell. In the previous section, we saw
that in both Drosophila and Xenopus the antagonism of
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