Wang Kailai, del Castillo Carmelo, Corre Erwan, Pales Espinosa Emmanuelle, Allam Bassem
School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY, 11794-5000, USA.
Analyses and Bioinformatics for Marine Science, Station Biologique de Roscoff, 29688, Roscoff Cedex, France.
BMC Genomics. 2016 Feb 27;17:146. doi: 10.1186/s12864-016-2493-9.
The hard clam Mercenaria mercenaria is an important seafood species widely exploited along the eastern coasts of the United States and play a crucial role in coastal ecology and economy. Severe hard clam mortalities have been associated with the protistan parasite QPX (Quahog Parasite Unknown). QPX infection establishes in pallial organs with the lesions typically characterized as nodules, which represent inflammatory masses formed by hemocyte infiltration and encapsulation of parasites. QPX infection is known to induce host changes on both the whole-organism level and at specific lesion areas, which imply systemic and focal defense responses, respectively. However, little is known about the molecular mechanisms underlying these alterations.
RNA-seq was performed using Illumina Hiseq 2000 (641 Million 100 bp reads) to characterize M. mercenaria focal and systemic immune responses to QPX. Transcripts were assembled and the expression levels were compared between nodule and healthy tissues from infected clams, and between these and tissues from healthy clams. De novo assembly reconstructed a consensus transcriptome of 62,980 sequences that was functionally-annotated. A total of 3,131 transcripts were identified as differentially expressed in different tissues. Results allowed the identification of host immune factors implicated in the systemic and focal responses against QPX and unraveled the pathways involved in parasite neutralization. Among transcripts significantly modulated upon host-pathogen interactions, those involved in non-self recognition, signal transduction and defense response were over-represented. Alterations in pathways regulating hemocyte focal adhesion, migration and apoptosis were also demonstrated.
Our study is the first attempt to thoroughly characterize M. mercenaria transcriptome and identify molecular features associated with QPX infection. It is also one of the first studies contrasting focal and systemic responses to infections in invertebrates using high-throughput sequencing. Results identified the molecular signatures of clam systemic and focal defense responses, to collectively mediate immune processes such as hemocyte recruitment and local inflammation. These investigations improve our understanding of bivalve immunity and provide molecular targets for probing the biological bases of clam resistance towards QPX.
硬壳蛤（Mercenaria mercenaria）是一种重要的海鲜物种，在美国东海岸被广泛捕捞，在沿海生态和经济中发挥着关键作用。硬壳蛤的严重死亡与原生生物寄生虫QPX（圆蛤寄生虫未知）有关。QPX感染发生在鳃腔器官中，病变通常表现为结节，这些结节代表由血细胞浸润和寄生虫包囊形成的炎症肿块。已知QPX感染会在整个生物体水平和特定病变区域引起宿主变化，分别意味着全身和局部防御反应。然而，对于这些改变背后的分子机制知之甚少。
使用Illumina Hiseq 2000（6.41亿条100碱基对读数）进行RNA测序，以表征硬壳蛤对QPX的局部和全身免疫反应。对转录本进行组装，并比较感染蛤的结节组织与健康组织之间以及这些组织与健康蛤组织之间的表达水平。从头组装重建了一个由62980个序列组成的共有转录组，并进行了功能注释。总共鉴定出3131个在不同组织中差异表达的转录本。结果使得能够识别参与针对QPX的全身和局部反应的宿主免疫因子，并揭示了参与寄生虫中和的途径。在宿主-病原体相互作用后显著调节的转录本中，参与非自我识别、信号转导和防御反应的转录本占比过高。还证明了调节血细胞局部黏附、迁移和凋亡的途径发生了改变。
我们的研究是首次全面表征硬壳蛤转录组并识别与QPX感染相关的分子特征的尝试。这也是首批使用高通量测序对比无脊椎动物对感染的局部和全身反应的研究之一。结果确定了蛤全身和局部防御反应 的分子特征，以共同介导血细胞募集和局部炎症等免疫过程。这些研究增进了我们对双壳贝类免疫的理解，并为探究蛤对QPX抗性的生物学基础提供了分子靶点。
