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Fig. 1. Vertebrate Shh signal transduction. A summary of Shh signal
transduction in vertebrates in (A) the absence and (B) the
presence of Shh. (A) Patched 1 (Ptch1), a twelve-pass transmembrane protein
that contains a sterol-sensing domain that binds Shh
(Ingham and McMahon, 2001 ),
represses the activity of a seven-pass transmembrane protein, smoothened
(Smo), in the absence of ligand. (B) When bound by Shh, Ptch1 relieves its
inhibition of Smo, allowing Smo to transduce Shh signaling intracellularly
(Alcedo et al., 1996). (A)
Several small sterol-like molecules inhibit or activate Smo independently of
Shh. These findings, together with the similarity between Ptch1 and the RND
family of bacterial transmembrane transporters, indicate that Ptch1 regulates
Smo activity by moving a regulatory small molecule in or out of the cell.
Cholesterol and vitamin D derivatives are possible candidates for endogenous
Ptch1/Smo regulation (Bijlsma et al.,
2006; Corcoran and Scott,
2006; Dwyer et al.,
2007), but confirmation of this awaits further analyses. By
contrast, Drosophila Smo is insensitive to the small-molecule
modulators of vertebrate Smo (Chen et al.,
2002a) and the structure of its C terminus is considerably
different to vertebrate Smo, suggesting that significant differences in the
mechanism of signaling have arisen during evolution
(Varjosalo et al., 2006).
Several additional cell surface-expressed molecules also bind to Shh,
including Hhip1, which blocks pathway activation, and Cdo, Boc and Gas1, which
enhance pathway activation, perhaps by increasing the presentation of Shh to
Ptch1. Changes in the subcellular location of Ptch1 and Smo possibly regulate
the activity of these proteins. (A) In the absence of Shh, Ptch1 localizes to
cilia, and Smo is not present in cilia. (B) Upon Shh exposure, Ptch1 leaves
the cilia, leading to an accumulation of Smo and to the activation of
signaling. The function of cilia in hedgehog signaling is unique to
vertebrates (reviewed by Huangfu and
Anderson, 2006) and is essential for Shh signal transduction in
the neural tube. Downstream of Smo, several proteins, including suppressor of
fused (Sufu), protein kinase A (PKA) and possibly costal 2 (Cos2), are
implicated in signal transduction (reviewed by
Huangfu and Anderson, 2006).
The exact involvement of these and other factors in vertebrates remains
unclear. Although the mechanism remains to be elucidated, Smo signal
transduction culminates in the regulation of Gli transcription factors, by
promoting Gli activity and/or blocking the formation of Gli transcriptional
repressor forms. Three Gli transcriptional regulators (Gli1, 2 and 3) are
present and expressed in the neural tube, where Gli3 expression is
repressed at high Shh signaling levels
(Matise and Joyner, 1999).
Gli3 is a bifunctional transcriptional repressor and activator. In the absence
of Shh signaling, Gli3 is proteolytically processed to generate a
transcriptional repressor (GliR). Similarly, Gli2 also undergoes proteolytic
processing in the absence of Shh signaling, but in contrast to Gli3, Gli2 is
mostly completely degraded (yellow spots)
(Pan et al., 2006). Finally,
Gli1 expression is completely dependent on Gli2/3 activator (GliA)
function. Gli1 is also trafficked from the nucleus in the absence of active
signaling (Sheng et al.,
2006). Therefore, Shh signaling not only induces Gli1
expression, but also regulates its nuclear accumulation and thereby its
function.
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loop2, which does not bind Shh and which inhibits signal
transduction, results in a cell-autonomous blockade of signaling (compared
with control, dotted line) that inhibits the generation of ventral identities.
This also reduces the upregulation of Ptch1 and Hhip1, which would normally
sequester Shh protein, thus increasing its spread beyond the cluster of
transfected cells. (c) By contrast, the forced expression of Cdo or Gas1,
which bind Shh and promote signaling, results in a cell-autonomous enhancement
of responses. This increases Shh sequestration, which produces a
non-autonomous reduction in the spread of Shh beyond the cluster of
transfected cells, which then receive a lower amount of Shh, when compared
with control electroporation (dotted line). (C) Shh spreads from
ventral to dorsal in the neural tube to establish a gradient. (a) Within the
target field, punctae of Shh protein (Shh-GFP; green) are observed apically
that accumulate over time in the neural tube; first in volume v1 then in v2
and v3. Shh punctae appear to be associated with the basal bodies (marked by
-tubulin, red) of the apically located primary cilia of
neuroepithelial cells. (b,c) In neuroepithelial cells abutting the notochord
(asterisk), punctae of Shh protein are observed in close proximity to
microtubule fibers. The significance of this distribution remains to be
determined. Adapted, with permission, from Chamberlain et al.
(Chamberlain et al.,
2008
