Independent expression of the metacyclic and bloodstream variable antigen repertoires of Trypanosoma brucei rhodesiense

Activation of endocytosis as an adaptation to the mammalian host by trypanosomes

Immune evasion in African trypanosomes is principally mediated by antigenic variation, but rapid internalization of surface-bound immune factors may contribute to survival. Endocytosis is upregulated approximately 10-fold in bloodstream compared to procyclic forms, and surface coat remodeling accompanies transition between these life stages. Here we examined expression of endocytosis markers in tsetse fly stages in vivo and monitored modulation during transition from bloodstream to procyclic forms in vitro. Among bloodstream stages nonproliferative stumpy forms have endocytic activity similar to that seen with rapidly dividing slender forms, while differentiation of stumpy forms to procyclic forms is accompanied by rapid down-regulation of Rab11 and clathrin, suggesting that modulation of endocytic and recycling systems accompanies this differentiation event. Significantly, rapid down-regulation of endocytic markers occurs upon entering the insect midgut and expression of Rab11 and clathrin remains low throughout subsequent development, which suggests that high endocytic activity is not required for remodeling the parasite surface or for survival within the fly. However, salivary gland metacyclic forms dramatically increase expression of clathrin and Rab11, indicating that emergence of mammalian infective forms is coupled to reacquisition of a high-activity endocytic-recycling system. These data suggest that high-level endocytosis in Trypanosoma brucei is an adaptation required for viability in the mammalian host.

A novel selection regime for differentiation defects demonstrates an essential role for the stumpy form in the life cycle of the African trypanosome

A novel selection scheme has been developed to isolate bloodstream forms of Trypanosoma brucei, which are defective in their ability to differentiate to the procyclic stage. Detailed characterization of one selected cell line (defective in differentiation clone 1 [DiD-1]) has demonstrated that these cells are indistinguishable from the wild-type population in terms of their morphology, cell cycle progression, and biochemical characteristics but are defective in their ability to initiate differentiation to the procyclic form. Although a small proportion of DiD-1 cells remain able to transform, deletion of the genes for glycophosphatidyl inositol-phospholipase C demonstrated that this enzyme was not responsible for this inefficient differentiation. However, the attenuated growth of the Delta-glycophosphatidyl inositol-phospholipase C DiD-1 cells in mice permitted the expression of stumpy characteristics in this previously monomorphic cell line, and concomitantly their ability to differentiate efficiently was restored. Our results indicate that monomorphic cells retain expression of a characteristic of the stumpy form essential for differentiation, and that this is reduced in the defective cells. This approach provides a new route to dissection of the cytological and molecular basis of life cycle progression in the African trypanosome.

Activity of a trypanosome metacyclic variant surface glycoprotein gene promoter is dependent upon life cycle stage and chromosomal context

African trypanosomes evade the mammalian host immune response by antigenic variation, the continual switching of their variant surface glycoprotein (VSG) coat. VSG is first expressed at the metacyclic stage in the tsetse fly as a preadaptation to life in the mammalian bloodstream. In the metacyclic stage, a specific subset (<28; 1 to 2%) of VSG genes, located at the telomeres of the largest trypanosome chromosomes, are activated by a system very different from that used for bloodstream VSG genes. Previously we showed that a metacyclic VSG (M-VSG) gene promoter was subject to life cycle stage-specific control of transcription initiation, a situation unique in Kinetoplastida, where all other genes are regulated, at least partly, posttranscriptionally (S. V. Graham and J. D. Barry, Mol. Cell. Biol. 15:5945-5956, 1985). However, while nuclear run-on analysis had shown that the ILTat 1.22 M-VSG gene promoter was transcriptionally silent in bloodstream trypanosomes, it was highly active when tested in bloodstream-form transient transfection. Reasoning that chromosomal context may contribute to repression of M-VSG gene expression, here we have integrated the 1.22 promoter, linked to a chloramphenicol acetyltransferase (CAT) reporter gene, back into its endogenous telomere or into a chromosomal internal position, the nontranscribed spacer region of ribosomal DNA, in both bloodstream and procyclic trypanosomes. Northern blot analysis and CAT activity assays show that in the bloodstream, the promoter is transcriptionally inactive at the telomere but highly active at the chromosome-internal position. In contrast, it is inactive in both locations in procyclic trypanosomes. Both promoter sequence and chromosomal location are implicated in life cycle stage-specific transcriptional regulation of M-VSG gene expression.

Transcriptional regulation of metacyclic variant surface glycoprotein gene expression during the life cycle of Trypanosoma brucei

In antigenic variation in African trypanosomes, switching of the variant surface glycoprotein (VSG) allows evasion of the mammalian host immune response. Trypanosomes first express the VSG in the tsetse fly vector, at the metacyclic stage, in preparation for transfer into the mammal. In this life cycle stage, a small, specific subset (1 to 2%) of VSGs are activated, and we have shown previously that the system of activation and expression of metacyclic VSG (M-VSG) genes is very different from that used for bloodstream VSG genes (S.V. Graham, K.R. Matthews, P.G. Shiels, and J.D. Barry, Parasitology 101:361-367, 1990). Now we show that unlike other trypanosome genes including bloodstream VSG genes, M-VSG genes are expressed from promoters subject to exclusively transcriptional regulation in a life cycle stage-dependent manner. We have located an M-VSG gene promoter, and we demonstrate that it is specifically up-regulated at the metacyclic stage. This is the first demonstration of gene expression being regulated entirely at the level of transcription among the Kinetoplastida; all other protein-coding genes examined in these organisms are, at least partly, under posttranscriptional control. The distinctive mode of expression of M-VSG genes may be due to a stochastic mechanism for metacyclic VSG activation.

Mechanisms of stage-regulated gene expression in kinetoplastida

During their life cycle, trypanosomatid parasites of mammals encounter substantially different environments in their hosts and insect vectors, to which they must adapt by undergoing a series of differentiation processes. At the molecular level, these processes must be the direct result of an elaborate series of changes in stage-regulated expression of a wide range of gene products. How are these changes accomplished? In this review, Sheila Graham discusses some recent advances in understanding the mechanisms of gene expression in trypanosomatids, and examines some clues to some intriguingly complex means of regulating life cycle stage-specific gene expression.

A monocistronic transcript for a trypanosome variant surface glycoprotein

Many protein-encoding genes of African trypanosomes are transcribed as large polycistronic pre-mRNAs that are processed into individual mRNAs containing a 5' spliced leader and 3' poly(A). The 45- to 60-kb pre-mRNAs encoding some variant surface glycoproteins (VSGs) contain as many as eight unrelated coding regions. Here we identify the promoter for a metacyclic VSG gene that is expressed without duplication in a bloodstream trypanosome clone. This 70-bp promoter is located 2 kb upstream of the telomere-linked VSG gene and directs the synthesis of a monocistronic VSG pre-mRNA lacking the 5' spliced leader. Its sequence only slightly resembles those of other known trypanosome promoters, but it does cross-hybridize with several related sequences elsewhere in the genome. These results suggest that a new class of trypanosome promoters has been found, whose function is to initiate monocistronic transcription of those VSG genes normally expressed during the metacyclic stage.

A monocistronic transcript for a trypanosome variant surface glycoprotein

Many protein-encoding genes of African trypanosomes are transcribed as large polycistronic pre-mRNAs that are processed into individual mRNAs containing a 5' spliced leader and 3' poly(A). The 45- to 60-kb pre-mRNAs encoding some variant surface glycoproteins (VSGs) contain as many as eight unrelated coding regions. Here we identify the promoter for a metacyclic VSG gene that is expressed without duplication in a bloodstream trypanosome clone. This 70-bp promoter is located 2 kb upstream of the telomere-linked VSG gene and directs the synthesis of a monocistronic VSG pre-mRNA lacking the 5' spliced leader. Its sequence only slightly resembles those of other known trypanosome promoters, but it does cross-hybridize with several related sequences elsewhere in the genome. These results suggest that a new class of trypanosome promoters has been found, whose function is to initiate monocistronic transcription of those VSG genes normally expressed during the metacyclic stage.

Comparison of the effects of immune killing mechanisms on Trypanosoma brucei parasites of slender and stumpy morphology

Trypanosoma brucei slender forms predominate over stumpy forms as the parasite population grows but at the peak of a parasitaemic wave and during remission of infection stumpy forms predominate. To determine whether this change in predominance might be caused by selective killing of slender forms, the fates of slender and stumpy form trypanosomes in two in vitro assays of immune-mediated killing were compared. Parasite populations in which > 90% of cells were of slender morphology were observed to be killed by antibody-dependent complement-mediated lysis approximately five times faster than populations in which < 15% of cells were slender and most were of intermediate or stumpy morphology. Quantification of the relationship between the proportion of slender forms in the population and the rate of lysis indicated that slender forms were killed approximately 7.3 times faster than other forms. In an opsonization assay, no differences were observed between slender and stumpy forms in the extent to which they attached to macrophages in an antibody-dependent manner. These results suggest that the change in proportions of slender and stumpy forms at the peak of a parasitaemic wave results from slender forms being more susceptible to complement-mediated killing as the antibody response develops.

Point mutations are associated with a gene duplication leading to the bloodstream reexpression of a trypanosome metacyclic VSG

African trypanosomes evade the immune response of their hosts by sequentially expressing different variant surface glycoproteins (VSGs). We isolated a bloodstream trypanosome clone of Trypanosoma brucei rhodesiense that expresses a VSG normally present during the metacyclic stage of the parasite in the insect vector. Associated with the bloodstream reexpression of this metacyclic VSG is a gene conversion in which the duplicated, expressed gene of 1650 nt contains 11 scattered point mutations when compared with its donor gene. Analysis of an uncloned population of bloodstream trypanosomes revealed another VSG reexpressor of the same donor gene in which the coding region had undergone 24 point mutations. The mutations are unique to the duplicated gene and appear to be nontemplated. The generation of these mutations provides a way for the trypanosome to increase further its antigenic diversity.

Metacyclic form-specific variable surface glycoprotein-encoding genes of Trypanosoma (Nannomonas) congolense

A complementary DNA expression library in phage lambda gt11 was synthesized using mRNA from in vitro-produced metacyclic forms of a clone of Trypanosoma (Nannomonas) congolense. The unamplified library was screened with antiserum from a goat immune to infection with metacyclic (m)-forms of T. congolense ILRAD Nannomonas antigen repertoire 2(ILNaR2). Of the 100 antiserum-reactive phage clones identified, 22 were analyzed further: 21 of the clones contained overlapping portions of a single transcript, while one other contained a different transcript. Northern blot analyses indicated that the sequences contained in the clones were transcribed only by m-forms of ILNaR2. Immunological and sequence analyses indicated that the two different cloned sequences encode m-form-specific variable surface glycoproteins.

Duplicative activation mechanisms of two trypanosome telomeric VSG genes with structurally simple 5' flanks

In the mammalian bloodstream, African trypanosomes express variant surface glycoprotein (VSG) genes from a family of long and complex telomeric expression sites. VSG switching generally occurs by the duplication of different VSG genes into these sites by gene conversion involving a series of 70 base pair (70bp) repeats in the 5' flank. In contrast, when VSG is first synthesised by trypanosomes in the tsetse fly at the metacyclic stage, a separate set of telomeric expression sites is activated. These latter telomeres appear not to act as recipients in gene conversion. We have found that the structure of two such expression sites is simple, with very short 70bp repeat regions and very little other sequence in common with bloodstream expression sites. However, the two telomeres readily act as donors in VSG gene conversion in the bloodstream and we show for one a consistent association of the conversion 5' end point with the short 70bp repeat region. These findings help explain why a very predictable set of VSGs is expressed in the tsetse fly and have implications for VSG gene conversion mechanisms.

Distinct, developmental stage-specific activation mechanisms of trypanosome VSG genes

The metacyclic form of African trypanosomes is the first to express genes for the Variant Surface Glycoprotein (VSG) and it uses an unusually predictable subset of the VSG gene repertoire. We have developed a model system for the analysis of metacyclic VSG (M-VSG) gene expression and have used this to demonstrate that, for two M-VSG genes, different modes of expression operate in the insect and mammalian phases of the life-cycle. In metacyclic-derived clones, these genes are expressed in situ, whereas they are routinely activated by duplication in bloodstream trypanosomes. The expression loci for both M-VSG genes studied are structurally simple and we present a model, based on this, for the maintenance of a separate M-VSG repertoire and expression system.

A comparison of multiplication rates in primary and challenge infections of Trypanosoma brucei bloodstream forms

The hypothesis that division of Trypanosoma brucei slender bloodstream forms is dependent upon the availability of a host-derived growth factor has been tested by superimposing challenge doses of slender-form trypanosomes onto preexisting infections at a time during the primary infection when stumpy forms predominated. The challenge populations grew in the doubly-infected mice indicating that depletion of a putative growth factor by the expanding population of the primary infection had not prevented division of the trypanosomes although slight reductions in multiplication rates were observed. This effect was independent of the variable antigen type (VAT) of the trypanosomes and of their stock of origin.

The use of experimental artefacts in African trypanosome research

When trypanosomes are removed from the field and maintained in laboratory conditions, phenotypic changes commonly occur such that the lines used by many investigators in routine work show several differences from the populations that affect humans and cattle in Africa. Whether these differences are important or irrelevant of course depends on the purpose of each particular experiment, but an awareness of what the differences are can be a useful aid in the interpretation of results. Furthermore, trypanosomes can be manipulated in the laboratory to possess particular characteristics that aid in the testing of hypotheses that are difficult to test using 'wild-type' trypanosomes. In this article, Mike Turner describes how some defined trypanosome lines have been created, how they differ from one another and several of their uses.

High frequency of antigenic variation in Trypanosoma brucei rhodesiense infections

Rates at which Trypanosoma brucei rhodesiense trypanosomes switch from expression of one variable antigen type (VAT) to that of another have been determined in cloned populations that have been recently tsetse-fly transmitted. Switching rates have been determined between several, specific pairs of VATs in each population. High rates of switching were observed in 2 cloned trypanosome lines, each derived from a separate cyclical transmission of the same parental stock and each expressing a different major VAT. Five estimates of the switching rate between one particular pair of VATs were consistently high (approximately 1 x 10(-3) switches/cell/generation). These high switching rates were similar both in bloodstream populations of mice and in populations confined to subcutaneously implanted growth chambers in mice, thus indicating that the interaction of the bloodstream population with other trypanosome populations in the lymphatics or extravascular sites in systemic infections did not influence the estimates of the rate of switching. Fourteen estimates were made of VAT-specific switching rates in bloodstream infections involving 8 combinations from among 6 VATs. Switching rate estimates were VAT-specific and showed considerable variation between different combinations of VATs--from 1.9 x 10(-6) to 6.9 x 10(-3) switches/cell/generation. The rates of switching to different metacyclic-VATs were, however, very similar. Summation of between 3 and 5 VAT-specific switching rate values in each of 4 experiments conducted in bloodstream infections has provided minimum estimates of the overall rate of antigenic variation: 2.0-9.3 x 10(-3) switches/cell/generation. These values are between 20 and 66,000-fold higher than previously published estimates.(ABSTRACT TRUNCATED AT 250 WORDS)

Trypanosome sociology and antigenic variation

Survival of the trypanosome (Trypanosoma brucei) population in the mammalian body depends upon paced stimulation of the host's humoral immune response by different antigenic variants and serial sacrifice of the dominant variant (homotype) so that minority variants (heterotypes) can continue the infection and each become a homotype in its turn. New variants are generated by a spontaneous switch in gene expression so that the trypanosome puts on a surface coat of a glycoprotein differing in antigenic specificity from its predecessor. Homotypes appear in a characteristic order for a given trypanosome clone but what determines this order and the pacing of homotype generation so that the trypanosome does not quickly exhaust its repertoire of variable antigens, is not clear. The tendency of some genes to be expressed more frequently than others may reflect the location within the genome and mode of expression of the genes concerned and may influence homotype succession. Differences in the doubling time of different variants or in the rate at which trypanosomes belonging to a particular variant differentiate into non-dividing (vector infective) stumpy forms have also been invoked to explain how a heterotype's growth characteristics may determine when it becomes a homotype. Recent estimations of the frequency of variable antigen switching in trypanosome populations after transmission through the tsetse fly vector, however, suggest a much higher figure (0.97-2.2 x 10(-3) switches per cell per generation) than that obtained for syringe-passed infections (10(-5)-10(-7) switches per cell per generation) and it seems probable that most of the variable antigen genes are expressed as minority variable antigen types very early in the infection. Instability of expression is a feature of trypanosome clones derived from infective tsetse salivary gland (metacyclic) trypanosomes and it is suggested that high switching rates in tsetse-transmitted infections may delay the growth of certain variants to homotype status until later in the infection.

Loss of variable antigen during transformation of Trypanosoma brucei rhodesiense from bloodstream to procyclic forms in the tsetse fly

A pleomorphic line of Trypanosoma brucei rhodesiense expressing a single variable antigen was used to quantify the rate of loss of the surface coat from bloodstream forms transforming to procyclics in the tsetse fly, Glossina morsitans, and in in vitro culture. Loss of variable antigen occurred at similar rates in the crop and anterior portion of the midgut of tsetse flies and in in vitro culture, but in the posterior portion of the fly midgut it occurred 2-3 times faster. The posterior portion of the midgut is the most important site for transformation of bloodstream-form trypanosomes to procyclics, and the dynamics of at least one component of this process are therefore not accurately paralleled in vitro.

Ontogenetic changes in helminth membrane function

During their life-cycle many parasites experience a wide range of environments including free living and those provided by a variety of intermediate and final hosts. The nutritional requirements of parasites are met by physiological processes adapted to exploit the physicochemical characteristics provided by different hosts. In helminth parasites these adaptations are frequently expressed on the tegumentary surface. As an example of adaptations within the Trematoda, the control of monosaccharide transport in Proterometra sp. is described. Environmental sodium, although not directly involved in the uptake process, nevertheless regulates the expression of transport capabilities. In the Cestoda, the uptake of monosaccharides and amino acids is described for Hymenolepis diminuta. The metacestode of this tapeworm inhabits the blood system of an arthropod, and the adult the gut of a mammal. There are quantitative and qualitative differences in the amino acids and monosaccharides in these two environments and these are reflected in the transport mechanisms exhibited by the two forms of the life-cycle. In Echinococcus granulosus the transfer of amino acids, sugars and macromolecules across the membranes of hydatid cysts and protoscoleces is described. The major difference between these two stages in the life-cycle relates to the ability of hydatid cysts to absorb macromolecules, whereas protoscoleces are impermeable to these compounds. The potential for future work is emphasized.

Characterization of genes coding for two major metacyclic surface antigens in Trypanosoma brucei

In African trypanosomes, only a very small fraction of the total repertoire of variable antigen types (VATs) is expressed by the metacyclic form. In Trypanosoma brucei stock EATRO 1125, the VATs AnTat 1.30 and 1.45 are reproducibly present in about 15% and 4% of the metacyclic population, respectively. The genes encoding the corresponding antigens or variant surface glycoproteins (VSGs) are in telomeres of large chromosomes, as are some non-metacyclic VSG genes from the same stock. Their activation mechanism has been studied in seven independent clones, 3 of which, referred to as 'first wave' metacyclic VATs (M-VATs), have been cloned from the first wave of parasitemia after cyclic transmission. In all these clones, activation of the antigen gene was linked to the transposition of an expression linked copy (ELC) of the gene to a telomeric expression site. For first wave M-VATs, this site seems variable, although restricted to large chromosomes, and it can be re-used for VSG gene expression in the bloodstream form. In 'late bloodstream' M-VATs, isolated from established chronic infections, the active expression site, at the end of a 200 kb chromosome, is the one preferred for the expression of late antigen types. It can be concluded that no characteristic feature in the genomic location and expression mechanism can distinguish metacyclic antigen genes from those expressed in the bloodstream forms, although the control of their expression must clearly be different.