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Fig. 4. Multiple proteins modulate Shh spread in the neural tube. Schematic
of proteins that affect Shh spreading in the ventral neural tube. Several cell
surface proteins, including HSPGs, Ptch1, Hhip1, Gas1, Cdo and Boc, interact
with extracellular Shh, decreasing its spread (blue arrow) through the neural
tube and either enhancing or blocking signaling in a cell-autonomous manner.
HSPGs possibly bind extracellular Shh protein and slow its spread through
tissue. Ptch1 and Hhip1 both bind and sequester Shh protein, and act as
cell-autonomous inhibitors of signal transduction. Gas1, Cdo and Boc also bind
and sequester Shh, but act cell-autonomously to promote Shh signaling. Ptch1
and Hhip1 are transcriptional targets of Shh signaling and, consequently, are
upregulated in the ventral neural tube as development progresses. By contrast,
the positive regulators of signaling, Gas1, Cdo and Boc, are
transcriptionally repressed by Shh signaling. Unlike Boc, Gas1 and
Cdo are initially expressed in ventral regions of the neural tube,
but are subsequently downregulated in response to Shh.
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