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#vitamin A

8 public questions tagged with this topic.

Teratogens

Retinoic acid specifies:

Retinoic acid concentration gradient along embryo axis established by posterior Raldh2 synthesis and anterior Cyp26 degradation acts as morphogen instructing anterior-posterior identity via direct regulation of Hox clusters. Retinoic acid receptors heterodimerize with retinoid X receptor binding retinoic acid response elements upstream of Hox genes determining rhombomere boundaries in hindbrain and vertebral identities. Deficiency anteriorizes, excess posteriorizes development. While RA also influences limb patterning, its primary embryonic role is specifying anterior-posterior axis positioning of tissues through Hox gene collinear activation establishing body plan organization and segmental fate.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Retinoic acid specifies AP axis via Hox.

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Transgenic Plant -2

crtI gene used in Golden rice was obtained from:

crtI gene deployed in Golden Rice was sourced from soil bacterium Erwinia uredovora strain now taxonomically reclassified as Pantoea ananatis, a Gram-negative enterobacterium inhabiting plant surfaces capable of synthesizing carotenoids as photoprotective pigments. Gene encodes 489 amino acid phytoene desaturase with FAD binding domain catalyzing sequential introduction of four double bonds into phytoene backbone converting 15-cis-phytoene via phytofluene, zeta-carotene, neurosporene to all-trans lycopene with concomitant isomerization, effectively replacing plant PDS, ZDS, CRTISO functions. Bacterial enzyme simplified metabolic engineering reducing transgene count from three to two, avoiding homology-dependent gene silencing observed when introducing multiple plant desaturases sharing conserved domains. Codon usage adjusted for plant expression and fused to transit peptide for amyloplast import. In bacteria crtI operates without membrane association in cytosol, whereas in plants localized to plastid membranes where lipophilic substrates partition. Its broad substrate specificity and lack of feedback regulation increase flux. Isolation from bacteria exemplifies utilization of microbial modules to bypass plant regulatory constraints in biofortification.

Ref: Sandmann Eur J Biochem 1994 crtI bacterial; NCBI Gene crtI Pantoea; Beyer PNAS 2000.

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Transgenic Plant -2

Phytoene synthase gene in Golden rice was sourced from:

Original Golden Rice event employed phytoene synthase gene cloned from Narcissus pseudonarcissus, common daffodil, a monocot bulbous geophyte storing high carotenoid levels in chromoplasts of perianth flowers bright yellow. Daffodil psy cDNA was available, well characterized, and known to encode plastid-localized enzyme with high activity converting GGPP to phytoene. Its coding sequence fused to endosperm-specific glutelin promoter Gt1 and pea rbcS transit peptide sequence MSMA to direct import into plastids via Toc-Tic translocon machinery. Expression in rice endosperm complemented missing endogenous activity, initiating flux toward carotenoids. Later analysis of Golden Rice 1 showed psy transcript level limiting, with phytoene accumulation modest. Syngenta improved design replacing daffodil psy with Zea mays maize psy1 under same promoter, achieving 23-fold higher total carotenoids and preferential beta-carotene accumulation because maize enzyme better codon optimized for monocot translation, higher catalytic efficiency and association with metabolon channeling substrate. Daffodil origin highlights cross-species utility of orthologous enzymes from ornamental plants for cereal biofortification using conserved plastid pathways.

Ref: Ye et al. Science 2000 Daffodil psy; Paine et al. 2005 Maize psy improvement; Plant Physiol.

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Transgenic Plant -2

Genes introduced in Golden rice include:

Development of Golden Rice elegantly reduced complex plant carotenoid desaturation steps to two transgenes. Phytoene synthase psy catalyzes committed head-to-head condensation of two geranylgeranyl diphosphate GGPP molecules to colorless 15-cis-phytoene, rate limiting step strongly regulated by light and feedback. Phytoene requires four desaturation steps and isomerization to become all-trans lycopene red pigment. Higher plants use phytoene desaturase, zeta-carotene desaturase, carotene isomerase ZDS, CRTISO separately. Bacterial crtI from Erwinia uredovora performs all reactions in single polypeptide with FAD cofactor, converting phytoene directly to lycopene via poly-cis intermediates. Combined psy plus crtI sufficient because endogenous rice endosperm possesses residual lycopene beta-cyclase and other downstream enzymes induced by product accumulation. Vectors used rice glutelin Gt1 promoter for endosperm specificity ensuring seed expression, CaMV 35S for crtI, and pea Rubisco small subunit transit peptide targeting to amyloplasts where isoprenoid precursor GGPP generated via MEP pathway. This two-gene strategy minimized gene silencing and transgene load while restoring entire pathway up to beta-carotene.

Ref: Beyer et al. J Nutr 2002 Golden Rice pathway; NCBI Metabolic pathway psy crtI mechanism.

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Transgenic Plant -2

Beta-carotene is a precursor of:

Beta-carotene is tetraterpenoid C40 carotenoid consisting of two beta-ionone rings linked by eleven conjugated double bonds providing strong antioxidant scavenging singlet oxygen and light filtering. Most importantly it serves as provitamin A carotenoid because symmetrical cleavage at central 15,15' double bond by enzyme beta-carotene 15,15'-monooxygenase BCMO1 in intestinal enterocytes and hepatic stellate cells yields two molecules of retinaldehyde. Retinal can be reduced to retinol bound to retinol binding protein for systemic transport to retina for rhodopsin regeneration mediating scotopic vision, oxidized to retinoic acid activating gene transcription via nuclear receptors RAR and RXR heterodimers regulating embryonic development, epithelial differentiation, immune cell maturation, and spermatogenesis. Humans cannot synthesize carotenoid backbone requiring dietary intake. Vitamin A deficiency leads to xerophthalmia, night blindness, keratomalacia, impaired mucosal immunity increased measles mortality. Beta-carotene bioavailability enhanced by dietary lipids and food processing releasing crystalline aggregates, with conversion factor 12:1 to retinol activity equivalents in mixed diet.

Ref: Lodish Molecular Cell Biology Vitamins; NIH StatPearls Beta-carotene Vitamin A; Olson 1989.

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Transgenic Plant -2

Golden rice is engineered to synthesize:

Golden Rice is paradigm of metabolic engineering for nutritional biofortification to combat vitamin A deficiency blindness and mortality affecting 250 million children in Asia and Africa dependent on polished rice lacking provitamin A. Endosperm of Oryza sativa does not accumulate carotenoids because pathway blocked downstream of geranylgeranyl diphosphate due to absent psy expression and limited desaturase activity. Ingo Potrykus and Peter Beyer introduced minimal carotenogenic pathway: phytoene synthase condensing two GGPP to 15-cis-phytoene, phytoene desaturase performing four desaturations to all-trans lycopene, and lycopene beta-cyclase forming beta-carotene yellow pigment localized in amyloplasts as crystals requiring plastid transit peptide. Second-generation Golden Rice 2 using maize phytoene synthase under glutelin promoter accumulates up to 37 microgram per gram dry weight beta-carotene, turning endosperm golden. Conversion by intestinal BCMO1 cleavage yields retinol delivering estimated 30-50 percent RDA from 130 g rice per day, demonstrating how plant engineering can deliver essential micronutrients lacking in staple without agronomic penalty or yield loss.

Ref: Ye et al. Science 2000 287:303-305 Golden Rice; Paine et al. Nat Biotechnol 2005 GR2; WHO.

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