A reexamination on the deficiency of riboflavin accumulation in Malpighian tubules in larval translucent mutants of the silkworm, Bombyx mori

Abstract

A variety of insects accumulate high contents of riboflavin (vitamin B) in their Malpighian tubules (MTs). Although this process is known to be genetically controlled, the mechanism is not known. In the 1940s and the 1950s, several studies showed that riboflavin contents were low in the MTs of some Bombyx mori (silkworm) mutants with translucent larval skin mutations (e.g., w-3, od, oa, and otm) and that genes responsible for these translucent mutations also affected riboflavin accumulation in the MTs. Since the 2000s, it has been shown that the w-3 gene encodes an ABC transporter, whereas genes responsible for od, oa, and otm mutations encode for the biogenesis of lysosome-related organelles. These findings suggest that some genes of ABC transporters and biogenesis of lysosome-related organelles may control the accumulation of riboflavin in MTs. Therefore, we reexamined the effects that translucent mutations have on the accumulation of riboflavin in MTs by using the translucent and wild-type segregants in mutant strains to measure the specific effect that each gene has on riboflavin accumulation (independent of genomic background). We used nine translucent mutations (w-3, oa, od, otm, Obs, oy, or, oh, and obt) even though the genes responsible for some of these mutations (Obs, oy, or, oh, and obt) have not yet been isolated. Through observation of larval MTs and measurements of riboflavin content using high-performance liquid chromatography, we found that the oa, od, otm, and or mutations were responsible for low contents of riboflavin in MTs, whereas the Obs and oy mutations did not affect riboflavin accumulation. This indicates that the molecular mechanism for riboflavin accumulation is similar but somewhat different than the mechanism responsible for uric acid accumulation in epidermal cells. We found that the genes responsible for oa, od, and otm mutations were consistent with those already established for uric acid accumulation in larval epidermis. This suggests that these three genes control riboflavin accumulation in MTs through a mechanism similar to that of uric acid accumulation, although we do not yet know why the or mutation also controls riboflavin accumulation.

5′-Nucleotidase Plays a Key Role in Uric Acid Metabolism of Bombyx mori

“…Our joint metabolome/transcriptome analysis showed that purine metabolites are tightly linked with the ABC transporter gene expression. This phenomenon has been described previously in w-3, od, oa, and otm silkworm mutants, but the underlying mechanism is unknown [14]. The expression level of ABG2, ABG3, ABG4, ABG5, ABG8, and Bmwh3 were significantly down-regulated in ∆Bm5 N. Among these down regulated genes, we obtained ∆BmABCG5 larval that had translucent integument.…”

“…In Bombyx mori , a lepidopteran model insect, UA granules accumulate in larval integument epidermal cells and cause the white skin, which protects against the adverse effects of sunlight [ 12 , 13 ]. Mutations in several genes have been shown to cause translucent larval integument in silkworm and are, thus, implicated in UA metabolism [ 14 ]. The proteins encoded by BmXDH (xanthine dehydrogenase), BmMOCO (molybdenum cofactor sulfurase), and BmBLOS2 (biogenesis of lysosome-related organelles complex-1, subunit 2) have functions in purine metabolism similar to homologous proteins in humans [ 15 , 16 , 17 , 18 ].…”

RNAi-induced knockdown of white gene in the southern green stink bug (Nezara viridula L.)

“…For example, the level of riboflavin was strongly reduced in testes of a white-eyed mutant strain of E. kühniella 99 and Anopheles gambiae Giles 23 , and the testes and Malpighian tubules of white-eyed mutants of Drosophila 4 , 6 , 21 , 80 . More recently, investigations on mutant strains of the silkworm Bombyx mori L. showed that some mutations of the white and brown orthologs prevented an accumulation of riboflavin in Malpighian tubules 102 , 103 . The reproductive organs of more mature wild-type SGSB adults, such as testes and vas deferens of males and ovarioles of females are normally yellow or orange-yellow 104 .…”

The road to lysosome‐related organelles: Insights from Hermansky‐Pudlak syndrome and other rare diseases

“…B mori larvae with mutations in subunits of the HPS‐associated BLOC‐1 and BLOC‐2 are translucent, suggesting that these granules are LROs. In addition, riboflavin accumulation in needle‐like granules in the Malpighian tubules has been observed to be reduced in mutants with translucent skin, including mutants in BLOC1 and BLOC2 subunits and in ABC transporters w‐3 and Bm‐brown that are homologous to the Drosophila white gene and brown gene, respectively. These organelles and their control by BLOC‐1 subunits are conserved in other silkworms such as Samia ricini …”