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Background Information
| Trypanosoma brucei |
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Pathogenesis of T. brucei. Human African trypanosomiasis (HAT), also known as African sleeping sickness is caused by the flagellated protozoan parasite, Trypanosoma brucei. This parasite is transmitted by the tsetse fly. HAT is endemic in 36 sub-Saharan African countries and infection has recently been reported in India (Brun, 2006). It is estimated that 50,000 to 70,000 people world-wide are currently infected, although 60 million are exposed to the tsetse fly (Figure 1). There have been several epidemics in Africa in the last century in 1896, 1906, 1920, and 1970. The 1920 epidemic was controlled because World Health Organization (WHO) volunteer teams organized the screening of millions of people at risk. By the mid 1960s, there were few new HAT infections. After the successful reduction in infections, surveillance was relaxed, and the disease reappeared in several areas over the last thirty years. Human African trypanosomiasis can be fatal if not treated and is often misdiagnosed and neglected (World Health Organization, 2006). |
Figure 1. Regions of Sub-Saharan Africa affected by HAT. Image courtesy of University of South Carolina.
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Symptoms associated with HAT. The first stage of the disease, known as the haemolymphatic phase, includes bouts of fever, headaches, joint pains, and itching at the site of infection (Figure 2). The second stage, known as the neurological phase, begins when the parasite crosses the blood-brain barrier and invades the central nervous system. During the neurological phase, the characteristic symptoms of HAT are observed in the host: confusion, disorientation, poor coordination, and disturbance of the sleep cycle (World Health Organization, 2006). |
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Figure 2. The site of the tsetse fly bite on the leg of a teenage girl with HAT. Image courtesy of WHO.
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Figure 3. Life cycle of T. brucei in Tsetse fly and human. Image courtesy of CDC. Click image to enlarge.
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The life cycle of T. brucei. An animation of the T. brucei life cycle in human and in tsetse fly is provided by Special Programme for Research and Training in Tropical Diseases. Human African trypanosomiasis is transmitted via the tsetse fly. The infected tsetse fly (genus Glossina) injects the trypanosome into human or animal skin tissue. The parasite enter the lymphatic system, passes into the bloodstream, and replicates by binary fission. Eventually, the parasite enters the central nervous system of the host. The tsetse fly obtains the parasite when it receives a blood meal from an infected mammalian host. The parasite replicates by binary fission in the insect, as it does in the human host (Figure 3) (Center for Disease Control 2006). |
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Current treatments for HAT. Treatment success for HAT is largely dependent on early identification. Drugs used for the haemolymphatic phase are less toxic, easier to administer, and more effective. Treatment in the neurological phase consists of drugs that can cross the blood-brain barrier to reach the parasite. Such drugs are quite toxic and complicated to administer. Four pharmaceuticals are registered for the treatment of sleeping sickness (for both the haemolymphatic phase and neurological phase): pentamidine, melarsoprol and eflornithine and Bayer AG (suramin). WHO provides these pharmaceuticals free of charge to endemic countries by via a private partnership with Sanofi-Aventis (World Health Organization, 2006). |
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Glycolysis |
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The glycolytic pathway in T. brucei is a promising chemotherapeutic target for African trypanosomiasis because the bloodstream form of T. brucei relies completely on glycolysis for its supply of ATP (Martinez-Oyanedel et al., 2007). Glycolysis is a metabolic process by which a 6-carbon glucose molecule is oxidized to two 3-carbon molecules of pyruvate. For every glucose oxidized, four ATP and NADH are produced at the expense of two ATP and two NADH. Glycolysis is conducted anaerobically, yet its products feed into both aerobic and anaerobic pathways. In some cells (and under some circumstances), the pyruvate and NADH made by glycolysis are used in other metabolic pathways, including fermentation and the citric acid cycle, to generate more ATP. However, glycolysis is the main source of energy (ATP, NADH) production in T. brucei (Nelson et al., 2006). |
Figure 4. Glycolytic pathway. Image courtesy of University of Miami. Click on image to enlarge.
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Evolution of T. brucei and phosphofructokinase |
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The glycosome. In T. brucei, glycolysis is compartmentalized in an organelle called the glycosome. Ninety percent of proteins in the glycosome are glycolytic enzymes. This compartmentalization appears to be essential for the regulation of glycolysis and enables cell growth under anaerobic conditions. In addition, these organelles contain enzymes for several other processes such as β-oxidation of fatty acids, purine salvage, pentose-phosphate pathway, and biosynthesis of pyrimidines (Parsons, 2006). |
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The evolutionary history of T. brucei. The glycosome is thought to be the remnant of an algal endosymbiont that was acquired by an ancestor of T. brucei. Recent studies show that trypanosomatid glycosomes contain several proteins that are homologous to proteins found in plant cells. Phosphofructokinase is found in the glycosome of trypanosomes and is more similar to the plant PFKs than it is to human PFK (Hannaert et al., 2002). Evolutionary lineage confers significant structural differences in PFK. These structural differences are essential in the development of T. brucei-specific drugs that are not toxic to the host organism. |
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Evolution of Phosphofructokinase (PFK). PFK is an essential enzyme in glycolysis because it catalyzes the first "committed" step. The reaction catalyzed by PFK is irreversible because a favorable energy change provided by breaking a high-energy phosphate bond prevents it from proceeding in the reverse direction. Another important feature of PFK is its allosteric regulation by intermediates of the citric acid cycle. The PFK of mammalian cells, yeast, some protozoa, and bacteria use ATP to provide energy for catalysis, while some protozoa and plants use pyrophosphate, or PPi, as a source of chemical energy for the reaction. ATP-dependent PFK and PPi-dependent PFK are two major evolutionary lineages of the enzyme and share a distant common ancestor. Amino acid sequence comparison of T. brucei PFK with ATP-dependent and PPi-dependent PFK of various cell types reveal that T. brucei belongs to the PPi-dependent PFK family. However, T. brucei actually relies on ATP for energy. Even so, T. brucei PFK is different from mammalian and other ATP-dependent PFK with respect to catalytic mechanism, regulation, and subunit size. These differences between T. brucei PFK and mammalian PFK provide the basis for species-specific drug design (Martinez-Oyanedel et al., 2007). |
Figure 5. Quaternary structure of PFK-1 from T. brucei. Each dimer is colored in cyan and dark blue. T. brucei has structural homology with the PPi-dependent PFKs, but uses ATP as a source of high-energy phosphate.
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