Shifting paradigms in Hedgehog signaling
Yu Wang1,2, Andrew P McMahon1and Benjamin L Allen1
Hedgehog (Hh) signaling proteins regulate multiple
developmental and adult homeostatic processes. A defining
feature of Hh signaling is that relatively small changes in the
concentration of Hh ligand elicit dramatically different cellular
responses. As a result, the processing, release and trafficking
of Hh ligands must be tightly regulated to ensure proper
signaling. In addition, sensitive and specific intracellular
signaling cascades areneeded tointerpret subtledifferences in
the level of Hh signal to execute an appropriate response. A
detailed understanding of the mechanisms that regulate these
responses is critical to shaping our view of this key regulatory
1Department of Molecular and Cellular Biology, Harvard University,
16 Divinity Avenue, Cambridge, MA 02138
2Department of Chemistry and Chemical Biology, Harvard University,
12 Oxford Street, Cambridge, MA 02138
Corresponding author: McMahon, Andrew P
Yu Wang and Benjamin L Allen contributed equally to this work.
Current Opinion in Cell Biology 2007, 19:159–165
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Studies of Hh over the past 25 years have identified many
basic components of the pathway (Figure 1; for review
see ). Briefly, Hh ligands are processed, released and
trafficked inaregulatedmanner fromproducingcells, and
1 (Ptch1). Activation of Ptch1 at the cell membrane
results in de-repression of Smoothened (Smo), a second
signaling inside cells. Smo initiates a signal transduction
cascade that culminates in activation of the Ci/Gli family
of transcription factors. These transcription factors both
activate and repress gene transcription in response to
Our review focuses on recent advances that affect our
understanding at each stage of Hh signaling, from ligand
processing and trafficking to activation of Hh signaling
and interpretation of those signals inside cells. Recent
attempts to understand Hh signaling on a genomic scale
are also discussed. Finally, the consequences of misregu-
lation of Hh signaling at multiple points within the path-
way will be examined in relation to cancer.
Shh processing, release and movement
One critical aspect of Hh biology is processing. Hh
ligands in both vertebrates and invertebrates are pro-
cessed from a precursor (45 kDa for Sonic Hedgehog
[Shh]) to a 19 kDa, N-terminal, dually lipidated protein
that mediates signaling. While precursor protein cleavage
and concurrent cholesterol modification are catalyzed by
the 25 kDa C-terminal portion of Hh, recent work has
shown that the C-terminal domain is also required for
proper Hh subcellular localization in Drosophila neural
signaling . Whether this represents a unique aspect of
axonal transport or of Drosophila biology, or a more gen-
eral property relevant to vertebrate signaling, remains to
Perhaps the most studied aspect of Hh trafficking is the
relationship between C-terminal cholesterol modification
and the range of action of Hh ligands. Initial studies in
Drosophila imaginal discs suggested that cholesterol
addition is required to restrict the range of Hh protein
, whereas mouse limb studies suggested a critical role
for cholesterol in long range signaling . New studies,
however, suggest different views. Studies of Hh action in
Drosophila point to a role for cholesterol modification in
establishing appropriate levels of signaling at given posi-
increased ligand distribution, but decreased activity
[5,6]. By contrast, a new allele of mouse Shh that differs
from a mutant previously associated with a loss of long-
range activity  actually results in enhanced long-range
action . Resolving these contradictory experimental
results will be critical to understand the precise function
of cholesterol in the trafficking and activity of Hh
The discovery that Hh co-purifies with lipoprotein
particles, and that lipoproteins are necessary for Hh
signaling in Drosophila [8?], suggests that lipophilic modi-
fications such as cholesterol serve, at least in part, to
localize Shh to particular regions of the plasma membrane
that are critical for proper Hh trafficking and activity. In
the mouse, Shh is reported to be released from cells of the
node in membranous fragments termed nodal vesicular
parcels (NVPs). Once released, NVPs are then trans-
ported via the action of cilia [9?] in conjunction with
the initiation of left–right asymmetry. While Hh signaling
in the node is poorly understood, and current studies do
Current Opinion in Cell Biology 2007, 19:159–165
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Shifting paradigms in Hedgehog signaling Wang, McMahon and Allen165
Current Opinion in Cell Biology 2007, 19:159–165