RDH10 is the primary enzyme responsible for the first step of embryonic Vitamin A metabolism and retinoic acid synthesis

Abstract

Retinoic acid (atRA) signaling is essential for regulating embryonic development, and atRA levels must be tightly controlled in order to prevent congenital abnormalities and fetal death which can result from both excessive and insufficient atRA signaling. Cellular enzymes synthesize atRA from Vitamin A, which is obtained from dietary sources. Embryos express multiple enzymes that are biochemically capable of catalyzing the initial step of Vitamin A oxidation, but the precise contribution of these enzymes to embryonic atRA synthesis remains unknown. Using Rdh10trex-mutant embryos, dietary supplementation of retinaldehyde, and retinol dehydrogenase (RDH) activity assays, we demonstrate that RDH10 is the primary RDH responsible for the first step of embryonic Vitamin A oxidation. Moreover, we show that this initial step of atRA synthesis occurs predominantly in a membrane-bound cellular compartment, which prevents inhibition by the cytosolic cellular retinol binding protein (RBP1). These studies reveal that widely-expressed cytosolic enzymes with RDH activity play a very limited role in embryonic atRA synthesis under normal dietary conditions. This provides a breakthrough in understanding the precise cellular mechanisms that regulate Vitamin A metabolism and the synthesis of the essential embryonic regulatory molecule atRA.

Enzymology of retinoic acid biosynthesis and degradation

“…To date, neither of the human SDRs characterized in our laboratory, including RDH10 and all of the SDR9C and SDR7C enzymes, was found to utilize CRBPI-bound all-trans -retinol in vitro. Others have come to a similar conclusion by showing that the addition of CRBPI to reactions containing microsomes with RDH10 inhibits rather than stimulates the oxidation of retinol ( 22 ).…”

“…The current consensus appears to be that the ADH enzymes may contribute to the oxidation of retinol postnatally in specifi c tissues during vitamin A excess (ADH1) or defi ciency (ADH4) but that they are not essential for atRA biosynthesis from a physiologically relevant supply of vitamin A during embryogenesis or adulthood (20)(21)(22). Thus, ADHs appear to play a role as backup enzymes under extreme dietary conditions.…”

Retinoid Machinery in Distinct Neural Stem Cell Populations with Different Retinoid Responsiveness

“…1) catalyzing the first and second steps of ROL4RAL/RA conversion, respectively. ROL dehydrogenase 10 (Rdh10), a prominent member of the Rdh family [12,13], was present at the mRNA level in both noninduced and differentiated neural stem cells regardless of their origin (Fig. 5A, F).…”

“…Once within the cell, ROL can either be stored after conversion to retinyl esters (RE) by lecithin retinol acyltransferase (LRAT, [11]) or it can be directed towards RA synthesis. In the latter case, ROL is reversibly converted to retinaldehyde (RAL) by ROL/alcohol dehydrogenases (RDHs/ADHs), among which RDH10 seems to play a primary role [12,13]. RAL is irreversibly oxidized to all-trans RA by RALDH1-3 retinaldehyde dehydrogenases, while RALDH4 was shown to be involved in the biosynthesis of 9-cis RA [14].…”

The Retinol Dehydrogenase Rdh10 Localizes to Lipid Droplets during Acyl Ester Biosynthesis

“…Subsequently, Rdh10 was correlated with multiple sites of atRA biosynthesis in the mouse embryo (12). The Rdh10-null mouse is embryonic lethal, and hypomorphs fail to develop a normal cortex, because of decreased but not absent atRA (13)(14)(15)(16). Rdh10 also produces atRA to regulate genes in human epidermis (17).…”