Table_3_cAMP-Dependent Signaling Pathways as Potential Targets for Inhibition of Plasmodium falciparum Blood Stages.XLSX

We review the role of signaling pathways in regulation of the key processes of merozoite egress and red blood cell invasion by Plasmodium falciparum and, in particular, the importance of the second messengers, cAMP and Ca2+, and cyclic nucleotide dependent kinases. cAMP-dependent protein kinase (PKA) is comprised of cAMP-binding regulatory, and catalytic subunits. The less well conserved cAMP-binding pockets should make cAMP analogs attractive drug leads, but this approach is compromised by the poor membrane permeability of cyclic nucleotides. We discuss how the conserved nature of ATP-binding pockets makes ATP analogs inherently prone to off-target effects and how ATP analogs and genetic manipulation can be useful research tools to examine this. We suggest that targeting PKA interaction partners as well as substrates, or developing inhibitors based on PKA interaction sites or phosphorylation sites in PKA substrates, may provide viable alternative approaches for the development of anti-malarial drugs. Proximity of PKA to a substrate is necessary for substrate phosphorylation, but the P. falciparum genome encodes few recognizable A-kinase anchor proteins (AKAPs), suggesting the importance of PKA-regulatory subunit myristylation and membrane association in determining substrate preference. We also discuss how Pf14-3-3 assembles a phosphorylation-dependent signaling complex that includes PKA and calcium dependent protein kinase 1 (CDPK1) and how this complex may be critical for merozoite invasion, and a target to block parasite growth. We compare altered phosphorylation levels in intracellular and egressed merozoites to identify potential PKA substrates. Finally, as host PKA may have a critical role in supporting intracellular parasite development, we discuss its role at other stages of the life cycle, as well as in other apicomplexan infections. Throughout our review we propose possible new directions for the therapeutic exploitation of cAMP-PKA-signaling in malaria and other diseases caused by apicomplexan parasites.

本综述探讨了信号通路在恶性疟原虫（Plasmodium falciparum）裂殖子（merozoite）逸出与红细胞入侵关键过程中的调控作用，重点阐述了第二信使（second messengers）环磷酸腺苷（cAMP）、钙离子（Ca²+）以及环核苷酸依赖性激酶（cyclic nucleotide dependent kinases）的重要性。环磷酸腺苷依赖性蛋白激酶（cAMP-dependent protein kinase，简称PKA）由cAMP结合调节亚基与催化亚基构成。保守性较低的cAMP结合口袋（cAMP-binding pockets）使得cAMP类似物成为极具潜力的药物先导物，但环核苷酸较差的膜通透性使得该研发思路受到制约。本文讨论了ATP结合口袋（ATP-binding pockets）的保守性为何使ATP类似物（ATP analogs）天然易引发脱靶效应（off-target effects），以及ATP类似物与基因操作（genetic manipulation）如何作为有效的研究工具用于相关机制探究。我们提出，靶向PKA相互作用伴侣（PKA interaction partners）及其底物，或基于PKA相互作用位点或PKA底物的磷酸化位点（phosphorylation sites）开发抑制剂，或许可为抗疟药物（anti-malarial drugs）的研发提供可行的替代方案。PKA靠近底物是底物磷酸化（substrate phosphorylation）的必要条件，但恶性疟原虫基因组中可识别的A激酶锚定蛋白（A-kinase anchor proteins，简称AKAPs）数量极少，这提示PKA调节亚基的肉豆蔻酰化（myristylation）与膜结合（membrane association）在决定底物偏好性（substrate preference）中具有关键意义。我们还探讨了恶性疟原虫14-3-3蛋白（Pf14-3-3）如何组装包含PKA与钙依赖性蛋白激酶1（calcium dependent protein kinase 1，简称CDPK1）的磷酸化依赖性信号复合物（phosphorylation-dependent signaling complex），以及该复合物为何可能对裂殖子入侵至关重要并可作为阻断寄生虫增殖的靶点。我们对比了细胞内裂殖子与逸出的裂殖子的磷酸化水平差异，以识别潜在的PKA底物。最后，鉴于宿主PKA可能在支持寄生虫胞内发育中发挥关键作用，我们讨论了其在疟原虫生命周期（life cycle）其他阶段以及其他顶复门寄生虫感染（apicomplexan infections）中的作用。本综述全程探讨了利用cAMP-PKA信号通路治疗疟疾及其他顶复门寄生虫引发疾病的潜在新方向。