There are numerous organisms related to trypanosomes that live in the sap of plants (Phytomonas for example). All kinds of insects drink plant sap. Such parasites were possibly ingested by various insects. It is assumed that later, in the course of evolution, when insects were able to feed on the blood of land animals, a transmission to these animals took place and that Trypanosoma sp. developed. The parasites are pleomorphic in human blood. Some are elongated and slender ("slender trypomastigotes") and others are shorter and stumpy. Reproduction in man occurs via longitudinal binary cleavage.

The parasite has only one nucleus, is elongated, contains a giant mitochondrion and has a single flagellum. At the base of the flagellum is the basal body. This lies adjacent to the kinetoplast. The latter is a compact DNA (deoxyribonucleic acid) structure, located in the very long mitochondrion. This mitochondrion is almost as long as the entire trypanosome. The name of the Order to which the parasite belongs – Kinetoplastida - refers to this organelle. Between the basal body and the flagellum there is an undulating membrane which is required for movement. The microscopic recognition of all these structures is important, for example when in doubt about a suspect structure in a microscopy preparation. In a buffy coat and/or a fresh blood slide preparation the parasites can be seen to move rapidly ("trypanon" = to drill or bore and "soma" = body). In the form of the parasite such as it occurs in man (trypomastigote), the kinetoplast lies in a posterior position and the flagellum points towards the front, rather like a bowsprit on a large sailing vessel. The parasite occurs in the salivary glands of the tsetse fly as an epimastigote (kinetoplast located just in front of the nucleus). The varying location of the kinetoplast is possibly related to different metabolic requirements in the various hosts.

The DNA in the kinetoplast (kDNA) stains like that of the nucleus (recognizable on a smear). The structure of the DNA in this kinetoplast is very complex. There are numerous (about 40) large DNA loops ("maxicircles") and even more (some 5,000-10,000) small DNA loops ("minicircles"). These form a gigantic tangle. For replication the parasite requires a specific "disentanglement enzyme" (type II topoisomerase). This latter enzyme could be a target in the development of new drugs.

Several mitochondrial genes appear to be incomplete. In 1986 it was discovered that "editing" of the genetic information takes place in pre-messenger RNA (ribonucleic acid) after transcription of the maxicircle-DNA. Certain RNA-bases (uridines) are removed or inserted in order to form a "mature" mRNA. In 1990 it was discovered that very small, so-called guide-RNA or gRNA molecules play a major role in this editing. Most gRNAs are coded in the minicircles. After the discovery of kRNA-editing, RNA-editing was also found in other organisms.

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Another illustration of the importance of the mitochondrion in trypanosomes is found in Trypanosoma evansi. This trypanosome causes "surra", a disease in camels and horses. The kDNA of this parasite has a different structure in its mitochondrion (for example, it has no maxicircles). As the maxicircles play a crucial role in the functioning of the mitochondrion, it is reasonable to assume that T. evansi cannot go through a maturation cycle in an insect. A consequence of this may be that a change of host cannot occur here. Indeed, the parasite appears to be transmitted only by a mechanical vector, e.g. biting flies (Tabanidae) or vampire bats.

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More than a billion years ago the ancestor of trypanosomes probably merged with a type of green algae. This would have enabled it to harness the Sun's energy. This would have had a tremendous advantage. When trypanosomes became parasites, they no longer needed to photosynthesize. The symbiont degenerated and some of its genes passed to those of the trypanosome. At this moment, the genes are vital for the survival of trypanosomes. Several microorganisms, including the malaria parasite, seem to have absorbed others in the past. Both the cellular powerhouses called mitochondria and chloroplasts, which plants use to turn sunlight into chemical energy, are thought to have originally been free-living bacteria. The leftover plant genes were found by analysing the genomes of T. brucei. So far 16 genes have been found that have their closest relatives in plants. Researchers suspect that more wait to be discovered. Plants use the equivalent genes to photosynthesize, using carbon dioxide to make sugars. Trypanosomes use them to break sugars down, in a unique cellular system.

The parasites defend themselves against oxidative stress by means of a polyamine conjugate, trypanothione (two glutathione molecules linked by spermidine). For the parasite, this substance has the same function as glutathione has for man. Studies have been carried out to determine whether trypanothione can be a target for drugs. The polyamines spermidine, spermine and putrescine have various functions in the cell (including synthesis of trypanothione). Polyamines are organic substances that contain two or more amino groups. DFMO (di-fluoro-methyl-ornithine; Eflornitine®) is a therapeutic substance that interferes in the polyamine metabolism of the parasite. DFMO inhibits the conversion of ornithine into putrescine, a precursor of spermidine. Ultimately, the synthesis of trypanothione is disturbed and the parasite is damaged.

The parasites replicate in humans by asexual mitosis. Experimental arguments for a possible sexual reproduction in T. brucei were first proposed in 1986. There is, however, as yet no definite proof of this (possibly artefacts). In the laboratory tsetse flies were infected with 2 different clones, after which hybrid parasites were isolated, which indicates exchange of genetic material. This could be important for a better understanding of the natural parasite populations, e.g. via the various iso-enzyme patterns that occur in nature. Even if these laboratory data were confirmed, it remains an open question how important this is in nature. Further studies are required.

When the parasite is present in an individual, it is covered with a thick monotonous layer of a single type of glycoprotein, VSG or Variant Surface Glycoprotein. These glycoproteins consist of 400-500 aminoacids and various saccharide groups and are anchored into the cell membrane with a so-called GPI-anchor (glycosyl-phosphatidyl-inositol). Ten percent of the proteins of the trypanosome consist of VSG. The entire VSG surface of a trypanosome is recycled every seven minutes by a process of VSG endocytosis and exocytosis. When the parasite is transferred to the tsetse fly, the VSG coating disappears within 4 hours and is replaced by an invariant glycoprotein ("procycline" or PARP). After the parasite has completed its cycle in the fly and arrives into the latter’s salivary glands, the VSG coating reappears. The VSG coating is of vital importance for the parasite when it is in the vertebrate host. This explains why only metacyclic trypanosomes (the mature forms in the salivary glands of the insect) are infectious. When an antigenically homogeneous population of parasites is in the human body, antibodies against the VSG of this population are produced. The immune system lyses the parasites (® fever episode). Infections with trypanosomes would be cured quickly, if the parasite population could not constantly change its surface antigens.

The parasite has about 1000 genes that code for different VSGs and thus has a vast repertoire of surface antigens. Most of these genes are located on some hundred minichromosomes in the nucleus of the parasite. The parasite also has about twenty chromosomes of "normal" size. These do not condense during mitosis. At any one time only one VSG gene per parasite is active. A few trypanosomes in a population have a different VSG (heterologous variants). After destruction of the first, dominant population by the immune system, the heterologous parasites increase in number until the variant VSG has induced antibodies and a new cycle of destruction begins. A third population of minority variants then emerges. This antigenic variation is a very important factor in the development of the disease and explains various symptoms (including its chronic course, fluctuating parasitaemia and fever episodes).

The order in which antigenic variants appear is partially pre-determined. Alteration of surface antigens is not induced by antibodies but occurs spontaneously. The genetic mechanism that the parasite uses for this is very complex. The gene that is to be expressed is duplicated from a locus on one of the chromosomes to a subtelomeric locus (close to one end of a chromosome). The mRNA originating from the gene on this latter locus is subsequently coupled ("trans-splicing") to a small mRNA fragment that is coded elsewhere. This small fragment (mini-exon) is the same for all VSGs. The mechanism for mutually exclusive activation of the VSG genes is still not known. Some other genetic mechanisms are linked to this. An unusual DNA-pyrimidine base ("J"; compare with A, T, G and C) is present in small quantities in the trypanosome genome and occurs more frequently in the telomeres. The unusual nucleotide is present only in the blood form, not in the procyclic form. The significance of this is not yet known.

It is important for the parasite to keep the population in the host as homogeneous as possible in order to use the VSGs economically. However, when parasites infect a host, diversity is advantageous. Hence, some 20 different variants can be present in the saliva of a fly. Any one of these variants is capable of infecting a host provided there are no antibodies present from a previous infection. If the host is immunologically naive to trypanosome antigens, the same VSG eventually dominates the first parasitic population.

In their mammalian host the parasites have a simple metabolism. They use several biochemical substrates of their host, but this poses a practical problem. Since the parasite is coated with a thick monotonous layer of glycoproteins, how can it absorb the necessary metabolites? At the base of the flagellum there is a small invagination ("flagellar pocket"). This site is accessible to macromolecules, but not to macrophages. Endocytosis can take place here. The receptors in this pocket are possibly also variable. This invagination, since it is the site of an important interaction with the host, may turn out a weak spot of the parasite which can be exploited therapeutically. Trypanosomes have no receptors for the uptake of albumin, but do have receptors for the uptake of LDL (Low Density Lipoprotein). LDL is essential for the parasite. Suramin binds LDL. This may explain the action of this medicament (interference with the normal LDL uptake in the flagellar pocket) and perhaps also explain the concentration of this molecule in the parasite via this receptor-mediated mechanism.

T. b. brucei cannot infect humans since it undergoes lysis in human serum. There are two distinct circulating trypanosome lytic factors (TLFs) in human blood. TLF1 is a lipid-rich subtype of high density lipiprotein (HDL), whereas TLF2 is a lipid-poor particle complexed with IgM. TLF2 is a lipoprotein composed of apolipoprotein A-I, haptoglobin-related protein (Hpr) and IgM and is composed of less than 1% lipid. Persons suffering from Tangier disease (analphalipoproteinaemia; HDL deficiency) do not produce TLF1.

The hpr-gene is present in the genomes of humans, Old World monkeys and chimpanzees. Of these primates, only chimpanzee serum is not trypanolytic. A functional Hpr is not synthetised by the chimpanzee because the chimp hpr-gene has a single-base deletion that results in a frameshift. Hpr is 94% identical to haptaglobin. Its physiological function, aside from a role in trypanolysis, is not known. TLF1 is completely inhibited by haptaglobin, whereas TLF2 is not. Therefore, it is possible that TLF2 may be the only active factor in vivo, considering that normal serum levels of haptaglobin are likely to completely inhibit endogenous TLF1 activity. Haptoglobin is a heterotetrameric protein consisting of two alpha-subunits and two glycosylated beta-subunits. There are 3 common types of haptaglobin in human populations, namely Hp 1:1, Hp 2:1 and Hp 2:2. All are inhibitory for TLF1. Haptoglobin binds free haemoglobin in human plasma. In individuals with severe haemolysis, haptaglobin decreases in plasma with the formation of haptaglobin-haemoglobin complexes that are cleared via the liver. In contrast, Hpr does not bind haemoglobin and Hpr serum levels remain constant at 25-50 µg/ml in normal and haemolytic sera. Individuals who have a haemolytic disease have high levels of TLF1 activity, which has been revealed as a consequence of depleted haptaglobin.

Parasite lysis by TLF1 and probably also TLF2 requires their uptake by receptor-mediated endocytosis in the flagellar pocket. After uptake, both lytic factors apparently need to enter an intracellular acidic vesicle to be activated. Lysis is inhibited by reagents that prevent acidification of lysosomes, even though TLFs are still delivered to the vesicles. The mechanism by which trypanosomes resist lysis by normal human serum is not known in detail, but it is thought to be associated with a defect in uptake and processing of TLF1 and TLF2. This may be the result of altered TLF receptors or other mechanisms.

Trypanosomes harbour numerous genes sharing apparent common ancestry with bacteria and/or plants. Many of these horizontally acquired genes seem to function in the glycosome. The glycosome is being studied as a possible therapeutic drug target. How did the trypanosomes acquire these "foreign" genes? Trypanosomes fall within the Phylum Euglenozoa, which includes the beautiful euglenid algae. These algae have chloroplasts, green plastids surrounded by three membranes. This is thought to reflect an engulfed eukaryotic algal endosymbiont similar to the apicoplast in Plasmodium parasites. Chloroplasts themselves are thought to have arisen form an endosymbiotic merger of a cyanobacterial prokaryote with an eukaryote. Although there is no evidence of a plastid in trypanosomes, the presence of such genes suggest that lateral gene transfer from some photosynthetic organism(s) occurred in the distant past. It is possible that the ancestors of the currect trypanosomes had plastids, but that they lost their passengers during evolution. There are known examples of plastid loss, e.g. in oomycetes and possibly in ciliates. Bodonids are free-living bacteriovorous kinetoplastids. Some kinetoplastids contain their own bacterial endosymbionts (e.g. Crithidia oncopelti). It is conceivable that independent endosymbionts could have been an alternative source for some of the "foreign" genes. If further phylogenetic analyses of lateral transferred genes consistently point to a single genetic source, the endosymbiont hypothesis will be supported. If the genes derive from multiple independent lineages, multiple independent gene transfers from ingested food items will be more probable.