Biomedical Engineering Reference
In-Depth Information
Hh signaling is responsible for many developmental malformations (Ingham,
1998; Goodrich and Scott, 1998; McMahon, 2000; Lee et al., 1992; Kalderon,
2000). Hh was identified as a secreted signaling protein necessary for specifica-
tion of positional identity in the Drosophila embryonic segment and in pattern-
ing of imaginal disc-derived adult structures such as the appendages, the eye, and
the abdominal cuticle (Ingham, 1998; Goodrich and Scott, 1998; McMahon,
2000; Lee et al., 1992; Kalderon, 2000). The Hh signaling transduction cascade
commences upon the intramolecular cleavage and lipid modification of Hh. The
Hh protein precursor carboxyl-terminal portion catalyzes this reaction. The
resulting amino-terminal peptide, which is esterified on its C terminus to a
cholesterol molecule (HhNp), is responsible for most actions of the Hh protein.
In mammals ShhNp proteins undergo further palmitoylation at their N termini.
This additional process, which is dependent on prior addition of a cholesterol
group, augments the activity of the protein in certain cellular contexts (Porter
et al., 1996; Pepinsky et al., 1998). Intracellular Hh signaling, in contrast to other
pathways, occurs by chronological repressive interactions. Two transmembrane
proteins, the tumor-suppressor Patched (Ptch) and the proto-oncogene Smooth-
ened (Smo), regulate Hh initiation and transduction (Ingham, 1998; Goodrich
and Scott, 1998; Kalderon, 2000). Ptch is a 12-span transmembrane protein and
Smo is a member of the seven-transmembrane family of receptors resembling the
Fz receptor family in the Wnt pathway. During the 'off' state of the Hh pathway
Smo is suppressed by Ptch. Upon stimulation by Hh, the inhibition of Smo is
released, and Hh signaling commences. The heteromeric receptor model for the
Hh pathways proposes interaction between Hh and Ptch and between Ptch and
Smo; in this model Hh binds to Ptch within the Ptch/Smo complex, releasing the
activity of Smo without the disintegration of Ptch and Smo. Dissimilar in vivo
localization of Ptch and Smo, despite physiological relationships betweenHh and
Ptch, suggests that alternative models to explain this pathway must be explored
(Kalderon, 2000; Denef et al., 2000; Stone et al., 1996). The mechanisms through
which activation of Smo relates to some of the cytoplasmic components of the Hh
pathway, including the serine/threonine protein kinase Fused (Fu), Supressor of
Fused (Su(fu)), the kinesin-like protein Costal-2 (Cos2), and the transcription
factor Cubitus interruptus (Ci; Gli in mammals), remain to be understood
(Ingham, 1998; Goodrich and Scott, 1998; Denef et al., 2000; Taipale et al.,
2000; Aza-Blanc et al., 1997). In Drosophila these molecules aggregate upon
their interaction to form a microtubule-anchored cytoplasmic complex through
the action of Cos2. In the absence of Hh, protein kinase A phosphorylates Ci
(Ci155), which is then cleaved into anN-terminal transcriptional repressor (Ci75)
(Aza-Blanc et al., 1997). Hh signaling results in disintegration of the cytoplasmic
complex from the microtubules, followed by nuclear translocation of the full-
length Ci transcriptional activator, which leads to transcriptional activation of
Hh target genes (Robbins et al., 1997; Chen et al., 1999b), including Ptch. This in
turn results in an inhibitory feedback process (Freeman, 2000). Inmammals there
are three Hh genes: Sonic, Indian, and Desert hedgehog (Shh, Ihh, and Dhh).
Several but not all homologues of the Drosophila Hh cytoplasmic proteins have
