The Transmembrane Isoform of <em>Plasmodium falciparum</em> MAEBL Is Essential for the Invasion of <em>Anopheles</em> Salivary Glands

Malaria transmission depends on infective stages in the mosquito salivary glands. Plasmodium sporozoites that mature in midgut oocysts must traverse the hemocoel and invade the mosquito salivary glands in a process thought to be mediated by parasite ligands. MAEBL, a homologue of the transmembrane EBP ligands essential in merozoite invasion, is expressed abundantly in midgut sporozoites. Alternative splicing generates different MAEBL isoforms and so it is unclear what form is functionally essential. To identify the MAEBL isoform required for P. falciparum (NF54) sporozoite invasion of salivary glands, we created knockout and allelic replacements each carrying CDS of a single MAEBL isoform. Only the transmembrane form of MAEBL is essential and is the first P. falciparum ligand validated as essential for invasion of Anopheles salivary glands. MAEBL is the first P. falciparum ligand experimentally determined to be essential for this important step in the life cycle where the vector becomes infectious for transmitting sporozoites to people. With an increasing emphasis on advancing vector-based transgenic methods for suppression of malaria, it is important that this type of study, using modern molecular genetic tools, is done with the agent of the human disease. Understanding what P. falciparum sporozoite ligands are critical for mosquito transmission will help validate targets for vector-based transmission-blocking strategies.疟疾传播依赖于蚊子唾液腺中的感染期虫体。在中肠卵囊中成熟的疟原虫子孢子，需穿过血腔并侵袭蚊子唾液腺，这一过程被认为由疟原虫配体介导。MAEBL作为裂殖子侵袭所必需的跨膜EBP配体（transmembrane EBP ligands）的同源物，在中肠子孢子中大量表达。可变剪接可产生多种MAEBL亚型，因此目前尚不明确哪种亚型具备功能必要性。为鉴定恶性疟原虫（Plasmodium falciparum，NF54株）子孢子侵袭唾液腺所必需的MAEBL亚型，我们构建了仅携带单一MAEBL亚型编码区（CDS）的敲除株与等位基因替换株。实验结果显示，仅MAEBL的跨膜形式是必需的，这也是首个被证实对恶性疟原虫侵袭按蚊（Anopheles）唾液腺至关重要的配体。MAEBL亦是首个经实验确定的、对恶性疟原虫生命周期中该关键步骤必需的配体——该步骤使媒介昆虫获得将子孢子传播给人类的感染性。当前基于媒介转基因的疟疾抑制策略日益受到重视，因此利用现代分子遗传学工具开展针对人类致病疟原虫的此类研究具有重要意义。阐明恶性疟原虫子孢子中哪些配体对蚊子传播过程至关重要，将有助于验证基于媒介的传播阻断策略的靶标。