Ito Katsuhiko, Shimura Sachiko, Katsuma Susumu, Tsuda Yasuhiro, Kobayashi Jun, Tabunoki Hiroko, Yokoyama Takeshi, Shimada Toru, Kadono-Okuda Keiko
Department of Science of Biological Production, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan.
Insect Mimetics Research Unit, National Institute of Agrobiological Sciences, 1-2 Owashi, Tsukuba, Ibaraki 305-8634, Japan.
J Invertebr Pathol. 2016 May;136:50-6. doi: 10.1016/j.jip.2016.03.005. Epub 2016 Mar 4.
Bombyx mori bidensovirus (BmBDV), which causes fatal flacherie disease in the silkworm, replicates only in midgut columnar cells. The viral resistance expressed by some silkworm strains, which is characterized as non-susceptibility irrespective of the viral dose, is determined by a single gene, nsd-2. We previously identified nsd-2 by positional cloning and found that this gene encodes a putative amino acid transporter that might function as a receptor for BmBDV. In this study, we investigated the relationship between the part of the midgut expressing nsd-2 (resistance gene), +(nsd-2) (susceptibility gene) and BmBDV propagation. Quantitative RT-PCR (qRT-PCR) analysis using total RNA isolated from the anterior, middle, and posterior parts of the midgut showed that nsd-2 and +(nsd-2) were strongly expressed in the posterior part of the midgut. The expression levels of both genes were very low in the anterior and middle parts. The qRT-PCR analysis showed that the expression levels of BmBDV-derived transcripts were correlated with the levels of +(nsd-2) expression. However, BmBDV-derived transcripts were clearly detected in all parts of the midgut. These results suggest that the infectivity of BmBDV depends mainly on the expression level of +(nsd-2) in the midgut and that viral infection is supported even by very faint expression of +(nsd-2). By contrast, the expression levels of +(nsd-2) were exceedingly low or undetectable in the middle part of the midgut, indicating that BmBDV infection might occur via another mechanism, independent of +(nsd-2), in the middle part of the midgut.
家蚕双顺反子病毒（BmBDV）可在家蚕中引发致命的空头性软化病，且仅在中肠柱状细胞中复制。一些家蚕品系所表现出的病毒抗性，其特征为无论病毒剂量如何均不敏感，由单个基因nsd - 2决定。我们之前通过定位克隆鉴定出了nsd - 2，并发现该基因编码一种假定的氨基酸转运蛋白，可能作为BmBDV的受体发挥作用。在本研究中，我们调查了中肠表达nsd - 2的部分（抗性基因）、+(nsd - 2)（敏感基因）与BmBDV增殖之间的关系。使用从中肠前部、中部和后部分离的总RNA进行的定量逆转录聚合酶链反应（qRT - PCR）分析表明，nsd - 2和+(nsd - 2)在中肠后部强烈表达。这两个基因在前部和中部的表达水平非常低。qRT - PCR分析表明，BmBDV衍生转录本的表达水平与+(nsd - 2)的表达水平相关。然而，在中肠的所有部分均能清楚地检测到BmBDV衍生转录本。这些结果表明，BmBDV的感染性主要取决于中肠中+(nsd - 2)的表达水平，并且即使+(nsd - 2)的表达非常微弱也能支持病毒感染。相比之下，+(nsd - 2)在中肠中部的表达水平极低或无法检测到，这表明BmBDV感染可能通过另一种独立于+(nsd - 2)的机制在中肠中部发生。
