Down syndrome cell adhesion molecule (Dscam)於近年來被認為是無脊椎動物類專一性免疫的關鍵分子，本研究鑑定出第一個蝦類Dscam，命名為白蝦Dscam (LvDscam)，其後進行此分子特性分析。研究結果顯示，LvDscam具有不同於其他無脊椎動物Dscam的獨特性，包括LvDscam可產生僅具有胞外域而缺少穿膜區及胞質尾之缺尾形式(tail-less form)；LvDscam主要於免疫相關組織中表現，而非如同其他無脊椎動物Dscam表現於神經組織；親源演化樹分析顯示，LvDscam在無脊椎動物Dscam分群中，被一獨立演化枝，與昆蟲綱及甲殼綱的水蚤分開。在LvDscam胞外區之功能性區域結構則與其他物種Dscam相同，皆具有10個immunoglobulin (Ig) domain及6個fibronectin type III (FNIII) domain，但較特別則為其具有一細胞吸附序列(RGD motif)。我們也針對白蝦在無WSSV感染、WSSV潛伏感染、WSSV急性感染等情況下，血球細胞表現Dscam之表現型態進行分析，在三組實驗組中共選殖62個clone進行定序，結果顯示，LvDscam在Ig2及Ig3的N端及整段Ig7至少具有23、30及13種isoform的存在，預估可產生8,970種不同組合型態，經觀測推論LvDscam之Ig2及Ig3或許在蝦類WSSV專一性免疫反應中比起Ig7扮演著更重要的功能。我們也推論不具尾端、分泌型的Dscam或許不具引領神經發育的功能，但在蝦類之類專一性免疫反應中可能扮演著重要的角色。

It has recently been suggested that Dscam (Down syndrome cell adhesion molecule), a member of the immunoglobulin-superfamily (IgSF), plays an essential role in the adaptive immune system of invertebrates. Here, we isolated and characterized the first shrimp Dscam from Litopenaeus vannamei. The LvDscam protein had an extracellular domain but lacked the expected transmembrane domain and cytoplasmic tail, both of which are found in all other members of the Dscam family (and may also be found in other L. vannamei Dscams that have not yet been isolated). In nervous tissue, expression levels of LvDscam were unexpectedly low. Phylogenetic analysis suggests that LvDscam is far from the Dscams found in other invertebrates. Nevertheless, the domain architecture of the extracellular region of LvDscam is similar to other invertebrate Dscams, and it exhibits the typical configuration of 10 immunoglobulin (Ig) domains, 6 fibronectin type 3 domains (FNIII) and one cell attachment sequence (RGD). Cloning and characterization of a total of 62 cDNAs from hemocytes collected from WSSV-free, WSSV-persistent and WSSV–acute-infected shrimp revealed 23 alternative amino acid sequences in the N-terminal of Ig2, 30 in the N-terminal of Ig3 and 13 in the Ig7 domain. This implies that LvDscam can potentially encode at least 8,970 unique isoforms. Further analysis suggested that the LvDscam Ig2 and Ig3 regions are more functionally important than Ig7 in the shrimp’s specific immune response against WSSV. We discuss how this tail-less, soluble Dscam can still play an active role in alternative adaptive immune response even while its axonal guidance functionality may be impaired.

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