Inhibition of sonic hedgehog autoprocessing in cultured mammalian cells by sterol deprivation

A freely diffusible form of Sonic hedgehog mediates long-range signalling

The secreted protein Sonic hedgehog (Shh) exerts many of its patterning effects through a combination of short- and long-range signalling. Three distinct mechanisms, which are not necessarily mutually exclusive, have been proposed to account for the long-range effects of Shh: simple diffusion of Shh, a relay mechanism in which Shh activates secondary signals, and direct delivery of Shh through cytoplasmic extensions, termed cytonemes. Although there is much data (using soluble recombinant Shh (ShhN)) to support the simple diffusion model of long-range Shh signalling, there has been little evidence to date for a native form of Shh that is freely diffusible and not membrane-associated. Here we provide evidence for a freely diffusible form of Shh (s-ShhNp) that is cholesterol modified, multimeric and biologically potent. We further demonstrate that the availability of s-ShhNp is regulated by two functional antagonists of the Shh pathway, Patched (Ptc) and Hedgehog-interacting protein (Hip). Finally, we show a gradient of s-ShhNp across the anterior-posterior axis of the chick limb, demonstrating the physiological relevance of s-ShhNp.

The origins and roles of cholesterol and fatty acids in the fetus

The fetus grows at a rate that is unparalleled by that at any other stage of life. Significant amounts of cholesterol and fatty acids are required to maintain membrane growth. Recent studies have shown that these lipids are also necessary mediators of processes that are essential for proper development.

Expression of Sonic Hedgehog downstream genes is modified in rat embryos exposed in utero to a distal inhibitor of cholesterol biosynthesis

Holoprosencephaly is a common developmental anomaly of the forebrain and midface, that has been associated with mutations in the Sonic Hedgehog gene, and with perturbations of cholesterol synthesis and metabolism in mammalian embryos. The study presented here was aimed to evaluate the functional relationship between these two causal agents in the genesis of the phenotype. Therefore, we used rat embryos exposed in utero to a distal inhibitor of cholesterol biosynthesis (AY9944) in which we analyzed different Shh-dependent processes, as evaluated by the expression of eight target genes. In addition, to delineate between the impact of cholesterol shortage and/or sterol precursors accumulation on the Shh signaling cascade we exposed rat embryos to AY9944 and we provided complementary diets rich in cholesterol and 7-DHC. At the early-somite stage we observed a reduction of Shh signaling in AY9944 treated embryos, resulting in the definition of a narrower ventral domain. Later in development this reduction of Shh signaling led to a complete interruption of the pathway in the rostral hindbrain and caudal midbrain. Other regions such as the forebrain and the spinal cord appeared less sensitive to the reduction of Shh signaling and interruption of the pathway was only observed in a subset of embryos. Finally, we did provide evidence that 7-DHC accumulation is compatible with normal activity of Shh, as long as cholesterol levels in embryonic tissue is sufficient.