RDH10 Control of RA Synthesis Figure 6. Rdh10 is up-regulated by reduction of RA. (A) Histogram representing relative levels of Rdh10 mRNA from wild type, heterozygous Aldh1a2gri/+ and homozygous Aldh1a2gri/gri embryos, as assessed by quantitative RT-PCR, demonstrating elevated levels of Rdh10 transcript in homozygous Aldh1a2gri/gri mutants. (B) RNA in situ hybridization for expression of Rdh10 mRNA in E8.5 wild type and Aldh1a2gri/gri mutant embryos, demonstrating that the increased Rdh10 expression occurs in a pattern similar to the normal expression pattern for the gene (mutant embryos n = 5). (C) Model depicting that metabolic product RA feeds-back to reduce expression of membrane bound Rdh10 in mouse embryos. Pink lines represent that RDH10 and RDH reaction occur in membrane-bound compartment. Cytosolic ADH enzymes do not contribute to any physiologically relevant production of RA in the embryo. doi:10.1371/journal.pone.0030698.g006 Discussion Importance of RDH reaction in RA production Vertebrate embryonic development depends upon precise spatial and temporal control of RA signaling. Two sequential enzymatic steps are required for metabolism of retinol into RA, the RDH first reaction and the RALDH second reaction (Fig. 6C). Biochemically, either step could function as a control point for regulating synthesis of RA. However, because the RDH first reaction was initially attributed to ubiquitous or redundant members of the ADH family and was thought to occur in an unregulated fashion, the initial step of Vitamin A metabolism was interpreted as being unimportant for the control of RA synthesis. Over time this led to acceptance of the view that that control of Vitamin A metabolism and RA synthesis was driven predomi- nantly by the RALDH second step. PLoS ONE | www.plosone.org 8 February 2012 | Volume 7 | Issue 2 | e30698