A CD36-related Transmembrane Protein Is Coordinated with an Intracellular Lipid-binding Protein in Selective Carotenoid Transport for Cocoon Coloration

Abstract

The transport pathway of specific dietary carotenoids from the midgut lumen to the silk gland in the silkworm, Bombyx mori, is a model system for selective carotenoid transport because several genetic mutants with defects in parts of this pathway have been identified that manifest altered cocoon pigmentation. In the wild-type silkworm, which has both genes, Yellow blood (Y) and Yellow cocoon (C), lutein is transferred selectively from the hemolymph lipoprotein to the silk gland cells where it is accumulated into the cocoon. The Y gene encodes an intracellular carotenoid-binding protein (CBP) containing a lipid-binding domain known as the steroidogenic acute regulatory protein-related lipid transfer domain. Positional cloning and transgenic rescue experiments revealed that the C gene encodes Cameo2, a transmembrane protein gene belonging to the CD36 family genes, some of which, such as the mammalian SR-BI and the fruit fly ninaD, are reported as lipoprotein receptors or implicated in carotenoid transport for visual system. In C mutant larvae, Cameo2 expression was strongly repressed in the silk gland in a specific manner, resulting in colorless silk glands and white cocoons. The developmental profile of Cameo2 expression, CBP expression, and lutein pigmentation in the silk gland of the yellow cocoon strain were correlated. We hypothesize that selective delivery of lutein to specific tissue requires the combination of two components: 1) CBP as a carotenoid transporter in cytosol and 2) Cameo2 as a transmembrane receptor on the surface of the cells.All organisms exposed to light contain carotenoids, which are yellow to red C 40 hydrophobic isoprenoid pigments. Carotenoids play pivotal roles in living organisms as precursors of vitamin A, antioxidants, and colorants (1). Their potential roles in medicine have recently been investigated. For example, macular accumulation of the carotenoids lutein and zeaxanthin is associated with a decreased risk of age-related macular degeneration (2), the leading cause of blindness in the developed world. Although plants, certain fungi, and bacteria synthesize carotenoids, animals appear to be incapable of synthesizing these molecules de novo. Therefore, animals must acquire carotenoids from dietary sources, and subsequently transport them to cells of target tissues.The delivery of lipids, including carotenoids, to cells can be divided into three categories: 1) enzyme-mediated processes, such as the action of lipoprotein lipase on very low density lipoproteins, which converts a lipoprotein-bound lipid, triacylglycerol, into a water-soluble product, fatty acid, which diffuses into cells and leaves behind in the blood a lipoprotein product depleted in triacylglycerol (3); 2) receptor-mediated endocytosis, such as the uptake of low density lipoproteins by low density lipoprotein receptor, in which the entire lipoprotein particle is taken into the cell and metabolized (4); and 3) the delivery of specific lipids to specific tissues devoid of lipoprotein degradation, called selective lip...

Polymorphism identification of carotenoid binding protein gene transcription in the silkworm, <i>Bombyx mori</i>

“…Tissue distribution pattern of CR31 mRNA products were similar to that of CBP, while CR42 mRNA was expressed mainly in the gonad (see Figure 3). Similar to CBP expression patterns [10,13], expression of these transcripts was repressed in silk gland during the fourth instar (data not shown), suggesting a common mechanism of expression regulation on these genes and thus giving this special spatial-temporal expression pattern.…”

Diversity in Copy Number and Structure of a Silkworm Morphogenetic Gene as a Result of Domestication

“…For the B. mandarina individuals, the CBP copy number was found to be $1, with the exception of two Taiwanese individuals that were calculated to have copy numbers of $2. Our calculation also showed that the two Taiwanese individuals had approximately two copies of dib (Niwa et al 2005) and Cameo2 (Sakudoh et al 2010) (Figure 2, C and E), the control nuclear genes used for copy number quantification. Therefore, we concluded that B. mandarina most likely had only one copy of CBP, and the apparent duplication in the Taiwanese individuals may be due to an unexplained 50% decrease in the copy number of OR2 (Krieger et al 2003;Sakurai et al 2004), which was used for the normalization of all copy numbers in this study (for details of the normalization process, see materials and methods).…”

“…We speculate that a tendency of human nature to prefer unambiguous or pure colors may have caused the diversification of CBP structure from a duplicated allele to a null allele, resulting in the production of both deep yellow and clear white B. mori cocoons. On the other hand, it is noteworthy that cocoon color is also affected by genes other than CBP, such as Cameo2 (Sakudoh et al 2010) or UGT10286 (Daimon et al 2010). To properly evaluate the contribution of the changes of CBP structure to cocoon color, elucidation of the genetic diversity and domestication history of other cocoon color genes would be useful.…”

Disruption of an N-acetyltransferase gene in the silkworm reveals a novel role in pigmentation

“…In the candidate region, C12, another NAT gene, is located adjacent to the candidate in mln, which raises the question of whether a defect of C12 might be responsible for the observed mln phenotype. We initially excluded C12 because of the lack of a coding mutation, but recent published work has shown evidence for regulatory mutation in the silkworm trait (Sakudoh et al, 2010). Comparative analysis indicated that the expression levels of C12 at the sites where melanism was seen (such as the larval head, the larval tail spot and the whole body of the adult) were essentially the same in the wild type and mln mutant (see Fig.…”