The role of beta-carotene and its cleavage enzymes during vitamin A deficiency and mammalian embryonic development

Vitamin A deficiency during pregnancy is a widespread health problem, which can detrimentally affect embryonic development. Since most of the world has limited access to preformed vitamin A (retinoids), it is important to understand whether the more abundant dietary precursor to vitamin A (beta-carotene, bC) can adequately support embryonic development. bC is converted to vitamin A by its cleavage enzymes: the symmetric cleavage enzyme beta-carotene 15,15’-oxygenase (CMO1) which generates retinaldehyde, and the asymmetric cleavage enzyme beta-carotene 9’,10’-oxygenase (CMO2), which generates beta-apo-10’-carotenal. Both enzymes are expressed in mammalian embryos throughout development. We studied bC metabolism in a mouse model of severe vitamin A deficiency – the Lrat-/-Rbp-/- mice, which cannot store retinol via lecithin:retinol acyltransferase (LRAT) or mobilize retinol from the liver via retinol-binding protein (RBP), and therefore produce highly malformed embryos when deprived of dietary vitamin A. We found that bC supplementation during a critical window of organ development rescued ~40% of Lrat-/-Rbp-/- embryos from symptoms of vitamin A deficiency, while supplementation after organ development did not improve the embryonic phenotype. This study indicated that bC is a good source of retinoids during pregnancy, but cannot fully support embryonic development even after prolonged administration. We gained further insight into the ability to rescue embryos from vitamin A deficiency by studying Cmo1-/-Rbp-/- mice and our novel Cmo1-/-Rbp-/-Cmo2-/- strain. We found that spontaneous bC oxidation, rather than CMO2 activity, could generate small amounts of retinoids to improve the embryonic phenotype in the absence of CMO1. On the other hand, CMO2 deficiency on a background of vitamin A deficiency (Cmo2-/-Rbp-/- mice) caused a severe embryonic phenotype that could not be rescued by bC despite the potential to generate retinaldehyde via CMO1. Cmo2-/-Rbp-/- embryos were rescued following maternal supplementation with beta-apo-10’-carotenal, the apocarotenoid normally generated from bC by CMO2 action, which also can be converted to retinoids by CMO1. We found that beta-apo-10’-carotenal may influence mitochondrial energy homeostasis through a PKC signaling complex. Overall, these studies showed that beta-apo-10’-carotenal may be more effective than bC in supporting embryogenesis, and suggest that this apocarotenoid should be a recommended component of the human diet.Ph. D.Includes bibliographical referencesby Elizabeth Kuhn Spiegle