UC Berkeley

All–trans–retinoic acid stimulates neurogenesis, dendritic growth of hippocampal neurons and higher cognitive functions, such as spatial learning and memory formation. Although astrocyte–derived atRA has been considered a key factor in neurogenesis, little direct evidence identifies hippocampus cell types and the enzymes that biosynthesize atRA. Nor has any factor been reported to regulate atRA biosynthesis in adult CNS. Here we show that primary rat astrocytes, but not neurons, biosynthesize atRA using multiple retinol dehydrogenases (Rdh) of the short–chain dehydrogenase/reductase (SDR) gene family and retinaldehyde dehydrogenases (Raldh). Astrocytes secrete atRA into their medium; neurons sequester atRA. The first step, conversion of retinol into retinal, is rate–limiting and the second step, conversion of retinal to atRA, is much more active and usually affected in response to different stimulis. Both neurons and astrocytes can synthesize retinyl esters and reduce retinal into retinol. siRNA knockdown indicates that Rdh10, Rdh2 (mRdh1), and Raldh 1, 2 and 3 contribute to atRA production. Knockdown of the Rdh Dhrs9 increased atRA synthesis ∼40% by increasing Raldh1 expression. Immunocytochemistry revealed cytosolic and nuclear expression of Raldh1 and cytosol and perinuclear expression of Raldh2. atRA autoregulated its concentrations by inducing retinyl ester synthesis via lecithin:retinol acyltransferase and stimulating its catabolism via inducing Cyp26B1. Raldh1, Raldh2, Rdh2, Rdh10 and Dhrs9 increase their expression as the elongation of in vitro culture time. Rdh1 null and CrbpI null mice astrocytes showed similar changes of retinoids metabolism except RE formation and partially overlap genes compensation. Though shown a broad and strong expression pattern in pure cultured astrocytes, Raldh1 expression dramatically dropped in astrocytes mixed cultured with neurons or in hippocampus in vivo. In contrast, Raldh1 is widely expressed in cultured neurons, with special intense signals on axons. CA1 neurons and mossy fibers enriched Raldh1 expression pattern was confirmed as a postnatal development process by immunohistochemistry. As a proinflammatory cytokine, TNFα oppositely down regulate Raldh1 expression via JNK and MAPK pathway and up regulate Raldh3 expression partially through P38 pathway, which resulted in different overall effect on atRA biosynthesis in young and old astrocytes. These data show that adult hippocampus astrocytes rely on multiple Rdh and Raldh to provide a paracrine source of atRA to neurons, and atRA regulates its own biosynthesis in astrocytes directing flux of retinol. Besides redundancy, different Rdh and Raldh may have unique function according to cell types and/or subcellular locations. Cross talk between first and second step dehydrogenation indicate a novel regulation mechanism that control the atRA homeostasis in astrocytes.