RETINOID BIOACTIVATION and FUNCTION
The Napoli lab studies regulation of the molecular apparatus that generates the hormone all-trans-retinoic acid (atRA) and the autocoid 9-cis-retinoic acid (9cRA) from retinol (vitamin A), and the function of atRA and 9cRA in the nervous system and in modulating intermediary metabolism (energy balance), thereby affecting memory, adiposity, and blood glucose control. This lab uses a combination of analytical chemistry, bioimaging, biochemistry and molecular and cell biology to determine the physiological effects of RAs.
The nervous system
[Hippocampus neurons in primary culture images -RA vs +RA] The hippocampus serves as the center of memory formation and spatial navigation. Damage to the hippocampus prevents new memories from fo rming and causes spatial disorientation. For example, loss of short-term memory and disorientation noted in early onset Alzheimer’s disease reflects hippocampus damage. Likewise, low atRA or loss of atRA signaling also prevents spatial learning and memory, because the hippocampus requires atRA for neurogenesis and synaptogenesis. This lab was the first to show that atRA promotes synaptogenesis in the hippocampus by relieving translation suppression of an AMPA receptor subunit and CAMKIIα kinase, two proteins essential for synaptogenesis. Our ensuing studies were the first to show that a nuclear transcription factor regulated by atRA, RAR, also functions as a translation suppresser in RNA granules of hippocampus neurons. The lab continues to study the mechanisms of generating atRA in the hippocampus and its mechanisms of action (see also the connection to fetal alcohol spectrum disorder, below).
Fetal alcohol spectrum disorder (FASD)
[FASD Chart] The ability of ethanol to cause liver to mobilize its substantial stores of vitamin A has been known for many decades. Not understood have been the mechanisms of the mobilization, the disposition of the retinol, and most importantly, the biological consequences of ethanol’s interaction with retinoid signaling, although many studies have addressed these issues. We developed highly sensitive and specific analytical assays based on LC/MS/MS to resolve and quantify the geometric isomers of RA that occur in vivo, and HPLC/UV assays to quantify retinol and its storage form retinyl esters (RE), and used the assays to determine the impact of ethanol on the concentrations of retinoids in tissues. We confirmed that ethanol causes a massive decrease in RE and retinol in liver, but surprising found that ethanol prompts increases of atRA in testis, hippocampus and cortex—three tissues that require measured amounts of atRA for normal function. The increases seemed sufficient to contribute to the harmful effects of ethanol consumption, as 2-fold increases in atRA can be toxic. After dam ethanol consumption, increases in embryo hippocampus and cortex were as high as 50-fold. We are studying the impact of this increase on RA-regulated genes and processes and in post-natal behavior.
[Rdh 1-null vs.WT] We created a “designer mouse” to determine the precise physiological function(s) of one of the retinol dehydrogenases, Rdh1. Retinol dehydrogenases catalyze the first and rate-limiting step in RA biosynthesis. Cells express multiple Rdh homologs. Each Rdh catalyzes the same reaction, suggesting that each generates RA for a specific physiological process. This hypothesis was confirmed when the Rdh1-knockout mouse showed a very precise phenotype—it became fatter than a wild-type mouse, despite not eating more food and not decreasing its activity, but did not show the developmental effects associated with low atRA. In contrast to many models of adiposity, which require a high-fat diet to induce abnormal weight gain, the Rdh1-null mouse gains excess weight when fed a normal diet. This makes the Rdh1-null mouse a unique model capable of producing insight into the gradual weight gain as human age, and into weight gain that may be related to regulating heat production, as opposed to energy expenditure vs. fat production. We are now determining the mechanisms of this weight gain to generate insight into the function of RA in maintaining normal weight, the causes of aging related weight gain, and the function of RA in regulating heat production.
Glucose control and diabetes
[Glucose control and diabetes chart]
Beta-cells in pancreas islets react to increasing blood glucose by releasing preformed insulin into the blood. Insulin then enables muscle and adipose to absorb and metabolize blood glucose into energy and to store any excess as fat. The process of glucose stimulated insulin secretion (GSIS), although essential to health, is not completely understood despite a century of study. Recently, this lab identified an autocoid in pancreas (and so far only in pancreas) 9cRA, which acts in opposition to glucose to arrest GSIS. 9cRA has both rapid (non-genomic) and longer-term actions (transcription regulation) that regulate insulin production and release. Abnormal increases in pancreas 9cRA accompany diseases characterized by glucose intolerance, such as diet-induced obesity and diabetes.
Selected Recent Publications
M.A. Kane, A.E. Folias, A. Pingitore, M. Perri, C.R. Krois, J-Y. Ryu, E. Cione, J.L. Napoli, CrbpI modulates glucose homeostasis and pancreas 9-cis-retinoic acid concentrations, Mol. Cell. Biol. 16, 3277-3285 (2011)
C. Wang, M.A. Kane, J.L. Napoli. Multiple retinol and retinal dehydrogenases catalyze all-trans-retinoic acid biosynthesis in astrocytes. J. Biol. Chem. 268, 6542-6553 (2011)
M.A. Kane, A.E. Folias, A. Pingitore, M. Perri, K. Obrochta, C.R. Krois, E. Cione, J.-Y. Ryu, J.L. Napoli, Identification of 9-cis-retinoic acid as a pancreas-specific autacoid that attenuates glucose-stimulated insulin secretion, Proc. Natl. Acad. Sci. U.S.A., 107, 21884-21889 (2010)
J. M. Starkey, Y. Zhao, R.G. Sadygov, S.J. Haidacher, W.S. LeJeune, N. Dey, B.A. Luxon, M.A. Kane, J.L. Napoli, L. Denner, R.G. Tilton, Altered Retinoic Acid Metabolism in Diabetic Mouse Kidney Identified by 18O Isotopic Labeling and 2D Mass Spectrometry. Plos One, 5, e11095 (2010).
N. Sidell, Y. Feng, L. Hao, J. Wu, J. Yu, M.A. Kane, J.L. Napoli, R.N. Taylor. Retinoic acid is a co-factor in the translational regulation of Vascular Endothelial Growth Factor in human endometrial stromal cells. Mol. Endocrinol. 24, 148-160 (2010).
M.A. Kane, A.E. Folias, C. Wang, J.L. Napoli, Ethanol elevates physiological all-trans-retinoic acid levels in select loci through altering retinoid metabolism in multiple loci: a potential mechanism of ethanol toxicity. FASEB J. 24, 823-832 (2010)
J.A. Siegenthaler, A.M. Ashique, K. Zarbalis, K.P. Patterson, J.H. Hecht, M.A. Kane, A.E. Folias, Y. Choe, S.R. May, T. Kume, J.L. Napoli, A.S. Peterson, S.J. Pleasure. Retinoic Acid from the Meninges Regulates Cortical Neuron Generation, Cell 139, 597-609 (2009).
N. Chen, B. Onisko, J.L. Napoli . (2008). The nuclear transcription factor RARα associates with neuronal RNA granules and suppresses translation. J. Biol. Chem. 283, 20841-20847.
M.A. Kane, A.E. Folias, C. Wang, J.L. Napoli. (2008). Quantitative profiling of endogenous retinoic acid in vivo and in vitro by tandem mass spectrometry. Anal. Chem. 80, 1702-1708.
N. Chen, J.L. Napoli (2008). All-trans-retinoic acid stimulates translation and induces spine formation in hippocampal neurons through a membrane-associated RARα. FASEB J. 22, 236-245.
K. M. Kransler, D. A. Tonucci, B. P. McGarrigle, J. L. Napoli, J. R. Olson. (2007). Gestational Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin alters retinoid homeostasis in maternal and perinatal tissues of the Holtzman rat. Toxicol. Appl. Pharmacol. 224, 29-38.
M. Zhang, P. Hu, C.R. Krois, M.A. Kane, J.L. Napoli (2007). Altered vitamin A homeostasis and increased size and adiposity in the rdh1-null mouse. FASEB J. 21, 2886-2896.