Variations in the carotenoid content of Chlamydomonas reinhardii throughout the cell cycle

Lutein, zeaxanthin, and meso-zeaxanthin: The basic and clinical science underlying carotenoid-based nutritional interventions against ocular disease

The human macula uniquely concentrates three carotenoids: lutein, zeaxanthin, and meso-zeaxanthin. Lutein and zeaxanthin must be obtained from dietary sources such as green leafy vegetables and orange and yellow fruits and vegetables, while meso-zeaxanthin is rarely found in diet and is believed to be formed at the macula by metabolic transformations of ingested carotenoids. Epidemiological studies and large-scale clinical trials such as AREDS2 have brought attention to the potential ocular health and functional benefits of these three xanthophyll carotenoids consumed through the diet or supplements, but the basic science and clinical research underlying recommendations for nutritional interventions against age-related macular degeneration and other eye diseases are underappreciated by clinicians and vision researchers alike. In this review article, we first examine the chemistry, biochemistry, biophysics, and physiology of these yellow pigments that are specifically concentrated in the macula lutea through the means of high-affinity binding proteins and specialized transport and metabolic proteins where they play important roles as short-wavelength (blue) light-absorbers and localized, efficient antioxidants in a region at high risk for light-induced oxidative stress. Next, we turn to clinical evidence supporting functional benefits of these carotenoids in normal eyes and for their potential protective actions against ocular disease from infancy to old age.

Microbial carotenoids

Carotenoids are among the most widely distributed pigments in nature, and they are exclusively synthesized by plants and microorganisms. These compounds may serve a protective role against many chronic diseases such as cancers, age-related macular degeneration, and cardiovascular diseases and also act as an excellent antioxidant system within cells. Recent advances in the microbial genome sequences and increased understanding about the genes involved in the carotenoid biosynthetic pathways will assist industrial microbiologists in their exploration of novel microbial carotenoid production strategies. Here we present an overview of microbial carotenogenesis from biochemical, proteomic, and biotechnological points of view.

Coordinated regulation of gene expression for carotenoid metabolism in Chlamydomonas reinhardtii

Carotenoids are important plant pigments for both light harvesting and photooxidation protection. Using the model system of the unicellular green alga Chlamydomonas reinhardtii, we characterized the regulation of gene expression for carotenoid metabolism by quantifying changes in the transcript abundance of dxs, dxr and ipi in the plastidic methylerythritol phosphate pathway and of ggps, psy, pds, lcyb and bchy, directly involved in carotenoid metabolism, under different photoperiod, light and metabolite treatments. The expression of these genes fluctuated with light/dark shifting. Light treatment also promoted the accumulation of transcripts of all these genes. Of the genes studied, dxs, ggps and lcyb displayed the typical circadian pattern by retaining a rhythmic fluctuation of transcript abundance under both constant light and constant dark entrainments. The expression of these genes could also be regulated by metabolic intermediates. For example, ggps was significantly suppressed by a geranylgeranyl pyrophosphate supplement and ipi was upregulated by isopentenyl pyrophosphate. Furthermore, CrOr, a C. reinhardtii homolog of the recently characterized Or gene that accounts for carotenoid accumulation, also showed co-expression with carotenoid biosynthetic genes such as pds and lcyb. Our data suggest a coordinated regulation on carotenoid metabolism in C. reinhardtii at the transcriptional level.

Microbial xanthophylls

Xanthophylls are oxygenated carotenoids abundant in the human food supply. Lutein, zeaxanthin, and cryptoxanthin are major xanthophyll carotenoids in human plasma. The consumption of these xanthophylls is directly associated with reduction in the risk of cancers, cardiovascular disease, age-related macular degeneration, and cataract formation. Canthaxanthin and astaxanthin also have considerable importance in aquaculture for salmonid and crustacean pigmentation, and are of commercial interest for the pharmaceutical and food industries. Chemical synthesis is a major source for the heavy demand of xanthophylls in the consumer market; however, microbial producers also have potential as commercial sources. In this review, we discuss the biosynthesis, commercial utility, and major microbial sources of xanthophylls. We also present a critical review of current research and technologies involved in promoting microbes as potential commercial sources for mass production.

Regulation of carotenoid biosynthesis genes in response to light in Chlamydomonas reinhardtii

Carotenoids are ubiquitous and essential components of photosynthetic tissues in plants, algae and cyanobacteria. They participate in the light harvesting process and prevent photooxidative damage of the photosynthetic apparatus. Although de-etiolation and growth under different light conditions were reported to have pronounced effects on carotenoid contents in higher plants and algae, very little is known about the light regulation of carotenogenesis on a molecular level. In the present study, we chose the unicellular green alga Chlamydomonas reinhardtii to investigate the regulation of carotenoid biosynthesis genes in response to light. The carotenoid genes phytoene synthase and phytoene desaturase were selected for gene expression studies. Both phytoene synthase and phytoene desaturase revealed a fast up-regulation in response to light, which seemed to be due to transcriptional control. Only blue light was effective whereas illumination with red light did not lead to elevated transcript levels of phytoene synthase and phytoene desaturase. The inhibition of photosynthesis did not abolish the light induction of carotenoid genes. Comparison with published results showed that the carotenoid genes are simultaneously expressed with other genes involved in chlorophyll biosynthesis and light harvesting. This simultaneous expression may represent one mechanism for the coordinated biosynthesis of carotenoids, chlorophylls and the proteins of the photosynthetic apparatus.

Diurnal rhythm of beta-carotene in photosynthetic alga Gonyaulax polyedra

Carotenoids comprise one of the most widespread classes of pigments found in nature. Polyene pigments from the dinoflagellate Gonyaulax polyedra were extracted every hour over a 24 hour period and the levels of beta-carotene during the day-phase were found to be twice as high as during the night-phase period. Using the thermal decomposition of 1,4-dimethylnaphthalene endoperoxide, the overall quenching ability of beta-carotene and G. polyedra carotenoids of singlet oxygen were found to be 2.04 x 10(9) M-1 s-1 and 4.75 x 10(9) M-1 s-1 respectively. Such a high quenching rate constant suggest a possible role for carotenoids as effective singlet oxygen quenchers in G. polyedra. The dose response curve for light-induced carotenoid synthesis shows a linear response up to 45 minutes of light exposure, after which night phase cells contain the same levels of beta-carotene as day-phase cells. Cells exposed to light pulses at different times during the night period display the highest beta-carotene induction in the middle of the night.

Pigment and protein composition of reconstituted light-harvesting complexes and effects of some protein modifications

The structure and heterogeneity of LHC II were studied by in vitro reconstitution of apoproteins with pigments (Plumley and Schmidt 1987, Proc Natl Acad Sci 84: 146-150). Reconstituted CP 2 complexes purified by LDS-PAGE were subsequently characterized and shown to have spectroscopic properties and pigment-protein compositions and stoichiometries similar to those of authentic complexes. Heterologous reconstitutions utilizing pigments and light-harvesting proteins from spinach, pea and Chlamydomonas reinhardtii reveal no evidence of specialized binding sites for the unique C. reinhardtii xanthophyll loroxanthin: lutein and loroxanthin are interchangeable for in vitro reconstitution. Proteins modified by the presence of a transit peptide, phosphorylation, or proteolytic removal of the NH2-terminus could be reconstituted. Evidence suggests that post-translational modification are not responsible for the presence of six electrophoretic variants of C. reinhardtii CP 2. Reconstitution is blocked by iodoacetamide pre-treatment of the apoproteins suggesting a role for cysteine in pigment ligation and/or proper folding of the pigment-protein complex. Finally, no effect of divalent cations on pigment reassembly could be detected.

Thylakoid membrane biogenesis in Chlamydomonas reinhardtii 137+. II. Cell-cycle variations in the synthesis and assembly of pigment

Synthesis of the chlorophyll and the major carotenoid pigments and their assembly into thylakoid membrane have been studied throughout the 12-h light/12-h dark vegetative cell cycle of synchronous Chlamydomonas reinhardtii 137+ (wild-type). Pulse exposure of cells to radioactive acetate under conditions in which labeling accurately reflects lipogenesis, followed by cellular fractionation to purify thylakoid membrane, allowed direct analysis of the pigment synthesis and assembly attendant to thylakoid biogenesis. All pigments are synthesized and assembled into thylakoids continuously, but differentially, with respect to cell-cycle time. Highest synthesis and assembly rates are confined to the photoperiod (mid-to-late G1) and support chlorophyll and carotenoid accretion before M-phase. The lower levels at which these processes take place during the dark period (S, M, and early-to-mid G1) have been ascribed to pigment turnover. Within this general periodic pattern, pigment synthesis and assembly occur in a "multi-step" manner, i.e., by a temporally-ordered, stepwise integration of the various pigments into the thylakoid membrane matrix. The cell-cycle kinetics of pigment assembly at the subcellular level mirror the kinetics of pigment synthesis at the cellular level, indicating that pigment synthesis not only provides chlorophyll and carotenoid for thylakoid biogenesis but may also serve as a critical rate-determinant to pigment assembly.