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 mi dface, 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 rel ation ship between these two causal agents in the genesis of the phenotype, Therefore, we used rat embryos exposed in utero to a distal inhibitor of c holesterol biosynthesis (AY9944) in which we analyzed different Shh-depende nt processes, as evaluated by the expression of eight target genes, In addi tion, to delineate between the impact of cholesterol shortage and/or sterol precursors accumulation on the Shh signaling cascade we exposed rat embryo s to AY9944 and we provided complementary diets rich in cholesterol and 7-D HC, At the early-somite stage we observed a reduction of Shh signaling in A Y9944 treated embryos,resulting in the definition of a narrower ventral dom ain. 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 sensi tive 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 c holesterol levels in embryonic tissue is sufficient. (C) 2001 Wiley-Liss, I nc.