Development of complex multicellular organisms relies on highly regulated tissue growth and cell fate specification. One molecule that coordinately performs these functions is Sonic Hedgehog (Shh). Understanding how Shh achieves this requires the dissection of Shh structure, and how this structure relates to biological activities elicited by this morphogen. Within the Shh sequence, the Cardin-Weintraub motif/domain is responsible for interactions with heparan sulfate proteoglycans (HSPGs). Altering this interaction results in functional consequences at cellular, molecular and systems levels. We find abrogation of Shh-HSPG interactions perturbs Shh's mitogenic, but not patterning functions. Also, these interactions influence Shh's localization to mitogenic niches. Shh-HSPG interactions act at the single cell to modulate the duration of signaling, promoting a gene expression program important for mitogenesis. At the molecular level, the complex structures of biological macromolecules encode information that instructs biological responses. Structure-activity relationships for polysaccharides, however, are not yet fully understood. We find Shh-dependent neural precursor proliferation requires a proteoglycan partner, wherein the glycosaminoglycan chain has a non-reducing end 2- O -sulfated iduronic acid residue.
This motif localizes Shh at the tissue level and amplifies Shh-dependent signals. At the systems level, in a mouse model, mutations in the Cardin-Weintraub motif have the effect of reducing olfactory bulb size. We asked if these gross changes were accompanied by alterations in the structure of glomeruli, which are anatomical-functional units of the olfactory system. Mutant glomeruli are fewer and larger. Functionally, their olfactory discriminatory ability may be impaired. Given that functions of intrinsic inhibitory circuits are essential for such discrimination, we asked if newly-born neurons of these circuits were affected. Their numbers are reduced during development and throughout life. In juvenile mutants, we see changes in gross and glomerular morphology similar to changes seen in adults, suggesting Shh influences olfactory system development. These studies support an important role for Sonic Hedgehog in the organization and proper functioning of the olfactory system. Taken together, these results demonstrate that Shh's organizational role impacts function at several levels. In addition, we find structural changes in Shh's interacting partner, heparan sulfate proteoglycans, alter encoded information, and subsequently, instructions that direct Shh responses.
