Sphingolipid biosynthesis in the parasitic apicomplexan protozoa: divergent enzymes in key host:pathogen interactions

顶复合体是一大类相关的单细胞微生物，可在人类和经济上重要的家畜中引起一系列疾病（包括疟疾）。其中之一，刚地弓形虫，可以感染大多数温血动物，引起一种叫做弓形虫病的疾病。最近，弓形虫病作为一种严重的疾病，在那些对传染性微生物的防御（免疫反应）已经受损的病人中，例如那些患有获得性免疫缺陷综合症（艾滋病）和正在接受抗癌化疗的病人中，已经得到了突出的重视。此外，在正常妊娠期间，未出生婴儿感染弓形虫是人类流产和先天性缺陷的主要原因。同样，弓形虫会导致未出生羔羊的自然流产，导致英国羊农每年损失数百万英镑。问题人类和其他动物通常在摄入受感染猫的粪便或通过受污染的食物后感染弓形虫。随后，寄生虫可以侵入人体的几乎所有细胞。然而，在大多数情况下，这种感染不会引起严重的疾病，相反，弓形虫是由人类或动物的免疫反应控制的，并退回到肌肉组织或大脑的囊肿中。尽管一些研究已经将这些囊肿与人类的行为改变和精神疾病联系起来，但这些囊肿可以在宿主的一生中一直存在，而不会引起任何身体问题。然而，受损的（艾滋病或癌症患者）或未发育的（未出生的人类或动物）免疫反应无法以这种方式控制寄生虫，从而发生严重的疾病，通常导致严重的脑损伤。不幸的是，用于治疗由弓形虫和其他顶复合体寄生虫引起的疾病的少数现有药物显示出严重的有效性问题和严重的副作用，这意味着迫切需要为人类和动物健康发现新的治疗方法。在人类或动物细胞内，弓形虫制造了许多生长所需的分子，但它也能够清除用于自身目的的各种物质。鞘脂是弓形虫质膜（将寄生虫细胞内部与外部环境分开的脂肪屏障）的重要组成部分。我们之前的工作表明，尽管寄生虫可以从宿主动物细胞中清除鞘脂，但这一过程对生长和感染的传播不是必需的。这表明弓形虫合成必需鞘脂的机制可能对其生存和致病能力至关重要。我们已经确定了该机制的关键酶成分，本研究计划旨在通过以下方式利用这些发现：[i]证明弓形虫鞘脂合成对寄生虫至关重要；和[ii]表征已识别的酶，鞘脂机制的组成部分。所产生的数据将有助于未来的工作，以利用弓形虫（和其他顶端复合体寄生虫）鞘脂合成机制作为新的，急需的药物的目标。

IntroductionThe apicomplexans are a large group of related, single-celled microscopic organisms that cause a range of diseases (including malaria) in both humans and economically important domestic animals. One of these, Toxoplasma gondii, can infect most species of warm-blooded animal causing a disease known as toxoplasmosis. In recent times toxoplasmosis has come to prominence as a serious disease in patients whose defences (immune response) against infectious microbes have been damaged, such as those suffering from Acquired ImmunoDeficiency Syndrome (AIDS) and those undergoing anti-cancer chemotherapy. Furthermore, during normal pregnancy infection of the unborn child with Toxoplasma is a major cause of miscarriage and congenital defects in humans. Similarly, Toxoplasma causes the spontaneous abortion of unborn lambs, leading to annual multi-million pound losses to UK sheep farmers.The ProblemHumans and other animals usually become infected with Toxoplasma following ingestion of faeces from the infected cats, or via contaminated food. Subsequently, the parasites can invade almost any of the cells that make up the body. However, in most cases this infection does not cause serious disease, instead Toxoplasma is controlled by the immune response of the human or animal and retreats into cysts in muscle tissues or the brain. These cysts can remain throughout the host's life without causing any physical problems, although some studies have linked the cysts to behavioral changes and mental illness in humans. However, a damaged (in an AIDS or cancer patient) or undeveloped (in an unborn human or animal) immune response is unable to control the parasite in this way and serious disease occurs, commonly leading to severe brain damage. Unfortunately, the few available drugs to treat the diseases caused by Toxoplasma and other apicomplexan parasites show severe problems with effectiveness and major side-effects, meaning that there is an urgent need to discover new therapies for both human and animal health.The BackgroundWithin the human or animal cell Toxoplasma manufactures many molecules it needs for growth, however it is also able to scavenge various materials that it uses for its own purposes. Sphingolipids are essential components of the Toxoplasma plasma membrane (the fatty barrier that separates the inside of the parasite cell from the external environment). Our previous work has shown that although the parasite can scavenge sphingolipid from the host animal cell, this process is not essential for growth and the spread of infection. This indicated that the machinery that Toxoplasma possesses to synthesize the essential sphingolipids is likely to be crucial for its survival and ability to cause disease. The AimsWe have identified key enzyme components of this machinery and this research proposal aims to exploit these findings by: [i] Demonstrating that Toxoplasma sphingolipid synthesis is essential for the parasite; and [ii] Characterizing the identified enzymes, the components of the sphingolipid machinery. The data generated will facilitate future work to exploit the Toxoplasma (and other apicomplexan parasites') sphingolipid synthesis machinery as a target for new, much needed, drugs.