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Summary

  1. Top of page
  2. Summary
  3. Wolbachia and filarial nematodes
  4. Intracellular niche and bacterial structure
  5. Population biology and dynamics
  6. Autophagy regulates Wolbachia populations
  7. Cell lineage tropism and route of transmission
  8. Molecular and cellular interactions between Wolbachia and their filarial hosts
  9. Exploitation of the Wolbachia filarial relationship as a therapeutic target
  10. Concluding remarks
  11. Acknowledgements
  12. References

Wolbachia pipientis is a widespread intracellular bacterial symbiont of arthropods and is common in insects. One of their more exotic and unexpected hosts is the filarial nematodes, notable for the parasites responsible for onchocerciasis (river blindness), lymphatic filariasis (elephantiasis) and dirofilariasis (heartworm). Wolbachia are only present in a subgroup of the filarial nematodes and do not extend to other groups of nematodes either parasitic or free-living. In the medically and veterinary important species that host Wolbachia, the symbiont has become an essential partner to key biological processes in the life of the nematode to the point where antibiotic elimination of the bacteria leads to a potent and effective anti-filarial drug treatment. We review the cellular and molecular basis of Wolbachia filarial interactions and highlight the key processes provided by the endosymbiont upon which the nematodes have become entirely dependent. This dependency is primarily restricted to periods of the lifecycle with heavy metabolic demands including growth and development of larval stages and embryogenesis in the adult female. Also, the longevity of filarial parasites is compromised following depletion of the symbiont, which for the first time has delivered a safe and effective treatment to kill adult parasites with antibiotics.


Wolbachia and filarial nematodes

  1. Top of page
  2. Summary
  3. Wolbachia and filarial nematodes
  4. Intracellular niche and bacterial structure
  5. Population biology and dynamics
  6. Autophagy regulates Wolbachia populations
  7. Cell lineage tropism and route of transmission
  8. Molecular and cellular interactions between Wolbachia and their filarial hosts
  9. Exploitation of the Wolbachia filarial relationship as a therapeutic target
  10. Concluding remarks
  11. Acknowledgements
  12. References

All filarial nematodes of medical and veterinary importance rely on Wolbachia symbiosis, with the exception of Loa loa (Taylor et al., 2005a). The species responsible for lymphatic filariasis (Wuchereria bancrofti and Brugia malayi), onchocerciasis (Onchocerca volvulus) and heartworm (Dirofilaria immitis) have been shown to host Wolbachia in all lifecycle stages and geographical isolates and show close co-evolutionary histories with their nematode host consistent with their mutualistic dependency. A recent study of a broader range of filarial nematode groups has shown that the symbiotic relationship is apparently absent from some individuals and species both within the Onchocercidae and in more ancestral groups infecting lizards, amphibians and birds (Ferri et al., 2011). Taken with the recent genomic analysis which highlights different roles for Wolbachia within its symbiosis with filarial nematodes (Darby et al., 2012), this suggests a broader range of symbiotic relationships exists between different Wolbachia clades.

Intracellular niche and bacterial structure

  1. Top of page
  2. Summary
  3. Wolbachia and filarial nematodes
  4. Intracellular niche and bacterial structure
  5. Population biology and dynamics
  6. Autophagy regulates Wolbachia populations
  7. Cell lineage tropism and route of transmission
  8. Molecular and cellular interactions between Wolbachia and their filarial hosts
  9. Exploitation of the Wolbachia filarial relationship as a therapeutic target
  10. Concluding remarks
  11. Acknowledgements
  12. References

Wolbachia are obligate endosymbiotic α-proteobacteria closely related to other rickettsial organisms such as Ehrlichia, Anaplasma and Rickettsia. They are pleiomorphic, ranging from 0.2 to 4 μm in size, and reside in an obligate intracellular niche, within host-derived vacuoles, throughout the syncytial hypodermal cord cells (Kozek, 1977; Taylor et al., 1999) and in the ovarian tissues, oogonia, oocytes and developing embryos within the uterus (Kozek, 1977; Taylor et al., 1999) (Figs 1 and 2). Individual bacteria reside in a single vacuole and during division the vacuole expands along the bacteria to form two separate vacuoles. During rapid periods of growth, clusters of Wolbachia are observed in a single vacuole. These bacteria are usually undergoing division and can assemble three to six bacteria per vacuole. This phenomenon appears unique to Wolbachia from filarial nematodes and is mostly observed in young adult worms (Fig. 1). Wolbachia generally have a moderately electron-dense matrix with distinct ribosomes and DNA. Some bacteria have a spore-like structure with very high electron density of the matrix, smaller size (0.5 μm), and are surrounded by a multi-membrane envelope (Fig. 1).

figure

Figure 1. Ultrastructural localization of Wolbachia within their filarial host.

A–C. Population dynamics of increasing bacterial numbers in the lateral cord cells of adult worms. Bacteria are distributed mostly on the inner side of the hypodermal cells oriented towards the pseudocoelom. Localization of bacteria (highlighted in green) in the cytoplasm of hypodermal cord and cells lining the uterus in a young adult female (A).

D and E. Wolbachia (green) in developing eggs (D) and a few Wolbachia (black asterisks) within a microfilaria (E).

F–H. Morphological features of Wolbachia in Brugia malayi. A single bacteria (black asterisks) each within a host vacuole (F); Wolbachia (white asterisk) with an electron-dense bacterial matrix and surrounded by a multi-membrane envelope, representing the morphology of spore-like structures (G); and several Wolbachia (black asterisks) are clustered in a shared single vacuole (H).

I and J. Specific localization of Wolbachia in hypodermal cells, which are oriented towards the pseudocoelom and close to a secretory canal. Short arrows show physical contact between the hypodermal cell and basal membrane of the uterus; long arrows represent the border between two hypodermal cells. Scale bar = 5 μm (A–E) and 1 μm (F–J).

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figure

Figure 2. Association of autophagy membrane protein (ATG8a, green) and Wolbachia (small red dots) in the hypodermal cord cells of the filarial nematode Brugia malayi (large red structures are nematode nuclei). Activation and recognition of Wolbachia by autophagy occurs during the periods of rapid bacterial expansion and growth. Image taken using 63× objective.

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Due to the obligate intracellular niche and lack of a requirement to infect cells, Wolbachia have lost much of the cell wall structures common to more typical Gram-negative bacteria. Peptidoglycan has not been reliably detected, and while the Wolbachia (wBm) genome predicts all the genes required for biosynthesis of lipid II, the monomer that will be polymerized into peptidoglycan (Foster et al., 2005), it also reveals that Wolbachia lack the transglycosylases that catalyse the formation of linear glycan chains of peptidoglycan (Foster et al., 2005; Henrichfreise et al., 2009). However, as shown in Chlamydia, lipid II biosynthesis does appear to be biologically important, and in Wolbachia this pathway has been shown to be sensitive to fosfomycin, an antibiotic that blocks synthesis of lipid II (Henrichfreise et al., 2009). Wolbachia may have a very atypical cell wall structure, which may provide an anchor point between the inner and outer membranes for outer membrane proteins; alternatively, it has been proposed that lipid II may be involved in cell division (Henrichfreise et al., 2009). Lipoproteins are essential structural and functional components of bacteria so are likely to be crucial in Wolbachia structure; analysis of the wBm genome indicates that the Wolbachia lipoproteins are diacylated since the necessary lgt and lspA genes are predicted, while the third gene responsible for triacylation (lnt) appears absent (Turner et al., 2009). Wolbachia peptidoglycan-associated lipoprotein (wBmPAL), which is abundantly expressed on the cell membranes, and is a potent stimulator of the innate and adaptive inflammatory responses associated with filarial pathogenesis, may play an important role in membrane structural integrity (Turner et al., 2009). Both the Wolbachia lipid II and lipoprotein pathways are being pursued as anti-filarial targets since they have been shown to be sensitive to specific inhibition (Henrichfreise et al., 2009; Johnston et al., 2010).

Population biology and dynamics

  1. Top of page
  2. Summary
  3. Wolbachia and filarial nematodes
  4. Intracellular niche and bacterial structure
  5. Population biology and dynamics
  6. Autophagy regulates Wolbachia populations
  7. Cell lineage tropism and route of transmission
  8. Molecular and cellular interactions between Wolbachia and their filarial hosts
  9. Exploitation of the Wolbachia filarial relationship as a therapeutic target
  10. Concluding remarks
  11. Acknowledgements
  12. References

In B. malayi, Wolbachia have been detected in all lifecycle stages, individual adult worms and isolates from different endemic regions (Kozek, 1977; Taylor et al., 1999; Fenn and Blaxter, 2004; McGarry et al., 2004). Unlike the situation in most arthropods, the presence of Wolbachia within a filarial species generally appears fixed with all individual nematodes infected (Bandi et al., 1998; Taylor et al., 2005a), suggestive of a mutualistic relationship. However, the level of infection varies considerably during filarial development, illustrating dynamic growth of the bacterial population at certain stages of the nematodes lifecycle (Fenn and Blaxter, 2004; McGarry et al., 2004; Fischer et al., 2011). Wolbachia numbers are at their lowest (∼ 70–100) and constant in microfilariae (Mf) and the insect-borne larval stages (L2 and L3). Shortly after transmission to the mammalian host a dramatic increase in the bacterial population occurs (McGarry et al., 2004) and this appears to play an essential role in larval development and establishment, as demonstrated by the arrested larval growth and development in response to antibiotic treatment (reviewed in Taylor and Hoerauf, 2001; Taylor et al., 2012). This rapid multiplication continues throughout L4 development, so that the major Wolbachia population growth occurs within the first month following infection of the mammalian host (Fenn and Blaxter, 2004; McGarry et al., 2004; Fischer et al., 2011). This proliferation serves to populate the hypodermal cords of adult worms with ∼ 2–5 × 105 bacteria in adult male and 4–14 × 106 in female worms (McGarry et al., 2004). In female worms, Wolbachia numbers continue to increase as the worms mature and the ovary and embryonic stages become infected (McGarry et al., 2004), supportive of their role in oogenesis and/or embryogenesis and consistent with the observed effects of antibiotic treatment on these processes. While the studies described above demonstrate dynamic changes in Wolbachia load in filarial lifecycle stages, there also appears to be considerable variation in bacterial numbers of individual worms (McGarry et al., 2004). Higher levels of Wolbachia infection within a worm may potentially confer selective advantages in terms of worm development or fecundity.

The periods of rapid growth of Wolbachia coinciding with larval and embryonic growth and development suggests that the fundamental role of Wolbachia is the provision of nutrients or metabolites essential for these metabolically demanding processes. This accounts for the rapid arrestment of larval growth and embryogenesis seen soon after antibiotic depletion. Nevertheless, other biological processes become compromised much later following loss of the symbiont including the partial block in transmission of the parasite in the insect vector (Albers et al., 2012) and the macrofilaricidal effects which occur between 12 and 24 months post treatment (Taylor et al., 2010).

Autophagy regulates Wolbachia populations

  1. Top of page
  2. Summary
  3. Wolbachia and filarial nematodes
  4. Intracellular niche and bacterial structure
  5. Population biology and dynamics
  6. Autophagy regulates Wolbachia populations
  7. Cell lineage tropism and route of transmission
  8. Molecular and cellular interactions between Wolbachia and their filarial hosts
  9. Exploitation of the Wolbachia filarial relationship as a therapeutic target
  10. Concluding remarks
  11. Acknowledgements
  12. References

In order to understand the process by which the host nematode regulates the population growth of Wolbachia at a sufficient level to maintain the symbiosis, yet to avoid fitness costs or the pathological consequences of bacterial overgrowth, the role of autophagy, a conserved intracellular defence mechanism and regulator of cell homeostasis, was investigated (Voronin et al., 2012). Activation of autophagy coincided with the onset of rapid bacterial growth and expansion, which shows that in spite of their mutualistic association, the nematode's immune system recognizes Wolbachia as a ‘pathogen’. Genetic and chemical modulation of autophagy activation or suppression resulted in a corresponding decrease or increase in bacterial populations. Indeed, activation of autophagy produced a bactericidal activity of similar magnitude to antibiotic elimination, which could be exploited as a novel mode of action for bactericidal drugs. In order to maintain its population, Wolbachia must evade or circumvent autophagic destruction, which may be achieved by a balance between the rate of bacterial growth and the rate of autophagic elimination, possibly through modification or mimicry of components of the autophagy pathway (Voronin et al., 2012).

Cell lineage tropism and route of transmission

  1. Top of page
  2. Summary
  3. Wolbachia and filarial nematodes
  4. Intracellular niche and bacterial structure
  5. Population biology and dynamics
  6. Autophagy regulates Wolbachia populations
  7. Cell lineage tropism and route of transmission
  8. Molecular and cellular interactions between Wolbachia and their filarial hosts
  9. Exploitation of the Wolbachia filarial relationship as a therapeutic target
  10. Concluding remarks
  11. Acknowledgements
  12. References

Work investigating the localization of Wolbachia during embryogenesis has given an insight into the cell lineage of B. malayi and the journey Wolbachia take to result in the restricted tissue tropism observed in adult worms. Using the completely defined lineage of Caenorhabditis elegans as a framework and whole mount immunofluorescent techniques, Landmann et al. (2010) followed Wolbachia through fertilization, the initial zygotic divisions and embryogenesis, as well as examining adult worms. It was observed that Wolbachia become enriched at the posterior pole of the egg following fertilization as early as the pronuclei migration step. In the early zygotic divisions, Wolbachia maintain this asymmetric localization at the posterior which allows them to first preferentially segregate into the P1 blastomere and, following its division into EMS and P2 blastomeres, into the posterior P2. The majority of the Wolbachia then segregate into the C blastomere, which eventually gives rise to their presence in hypodermal cord cells. Furthermore it was observed that not only did the Wolbachia titres throughout the hypodermal cords vary between worms, confirming previous work conducted using qPCR (McGarry et al., 2004), but they also varied within an individual worm. In some instances, only one of the cords was infected, while in others only half of each cord contained Wolbachia. It was suggested that Wolbachia spreads throughout the hypodermal cord during fusion of the individual hypodermal cord cells to produce the syncytium (Landmann et al., 2010).

The smaller number of Wolbachia that do not follow the route to the hypodermal cords was also followed. Those that initially reside in the AB blastomere instead of P1 become diluted out without replicating, but the final destination of the minority of Wolbachia that end up in the P3 blastomere instead of C remains unclear. Wolbachia are vertically transmitted from mother to offspring and therefore a cell lineage-specific pattern of segregation from the P3 blastomere, which leads to the Z2/Z3 germline cells, would ensure that Wolbachia are thereby transmitted to the next generation. However, it was observed that not all embryos contained Wolbachia in their P4 blastomeres, indicating that there is a loss at some point along the P3-P4-Z2/Z3 route in some cases. This drew the authors to propose that those embryos with Wolbachia in their P4 blastomeres would become female and male worms respectively (Landmann et al., 2010). However, a recent study demonstrated that Wolbachia are absent from both male and female reproductive tissues from at least third-stage larvae to young adult worms (Fischer et al., 2011) indicating that the germline is not infected from the outset. In young female adult worms, the germinative zones in the ovaries, which contain the oogonia, were found to be free of Wolbachia. Over a period of 3 weeks following the L4 to L5 molt, these germinative zones become populated with Wolbachia and it was suggested that this is a result of invading bacteria from the adjacent hypodermal cords. Further investigations by Landmann et al. (2012), examining late embryogenesis through to adult, have demonstrated Wolbachia crossing the basal membrane of the cords in both male and female L4 (8–10 days post infection) and subsequently invading the distal gonad of the female worms, thereby confirming that invasion does occur. Overall it appears that Wolbachia reach the female reproductive tissue to coincide with oocyte development to ensure transmission to the next generation, although the precise mechanisms used by Wolbachia to cross tissue zones and invade the germline remain to be defined.

Molecular and cellular interactions between Wolbachia and their filarial hosts

  1. Top of page
  2. Summary
  3. Wolbachia and filarial nematodes
  4. Intracellular niche and bacterial structure
  5. Population biology and dynamics
  6. Autophagy regulates Wolbachia populations
  7. Cell lineage tropism and route of transmission
  8. Molecular and cellular interactions between Wolbachia and their filarial hosts
  9. Exploitation of the Wolbachia filarial relationship as a therapeutic target
  10. Concluding remarks
  11. Acknowledgements
  12. References

The sequencing of the genomes of both Wolbachia (wBm) (Foster et al., 2005) and its B. malayi host (Ghedin et al., 2007) has provided genomic clues and allowed for an in silico comparison of the biosynthetic capacities of the nematode and its endosymbiont, to highlight potential complementing metabolic capabilities that might contribute to their mutualistic coexistence. This revealed that the nematodes have become dependent on their endosymbionts for a diverse range of biological processes (reviewed in Slatko et al., 2010), such as synthesis of metabolites including haem, riboflavin, flavin adenine dinucleotide and nucleotides, which are provided by Wolbachia to the nematode, which cannot synthesize these molecules de novo (Foster et al., 2005; Ghedin et al., 2007).

Recently molecular analysis of Wolbachia from the cattle filarial nematode, Onchocerca ochengi (wOo), by Darby et al. (2012), involving sequencing of the genome and analysis of the transcriptome, has revealed certain dissimilarities between wOo and wBm in terms of metabolic capability, the density of insertion sequences and the range of repeat motif-containing proteins (Darby et al., 2012). This work highlights potential roles for wOo in both energy production in the somatic tissue, where Wolbachia may have a mitochondrion-like function generating ATP for the host, and modulation of the mammalian immune response, but interestingly does not provide strong support for the provisioning of vitamins or cofactors by this strain (Darby et al., 2012), as shown in wBm (Foster et al., 2005).

Intriguingly, evidence of lateral gene transfer (LGT) of Wolbachia DNA into the nematode genome has been shown in Wolbachia-infected filariae (Dunning Hotopp et al., 2007; Slatko et al., 2010), and fragments of Wolbachia genes have also been identified in the nematode nuclear genomes of filarial species free of Wolbachia indicating LGT occurred prior to the secondary loss of Wolbachia (McNulty et al., 2010). It remains to be demonstrated whether these LGT events have complemented the nematode genome with essential Wolbachia genes or alternatively represent genetic ‘ghosts’ of a parasitic relationship in the past.

In addition to highlighting metabolites provided by Wolbachia to its host, comparisons of the genomes can also identify metabolites provided to the endosymbiont by the host nematode as incomplete biochemical pathways in Wolbachia suggest a metabolic dependency on the nematode host. Wolbachia appear unable to perform de novo synthesis of several vitamins and cofactors such as coenzyme A, nicotinamide adenine dinucleotide, biotin, ubiquinone, folate, lipoic acid and pyridoxal phosphate (Foster et al., 2005).

Many filarial genes show expression changes in response to Wolbachia clearance following tetracycline treatment (Ghedin et al., 2009; Strubing et al., 2010; Rao et al., 2012). Several nuclearly encoded pathway transcripts were increased in response to elimination of Wolbachia in adult B. malayi, including those involved in protein synthesis and the stress response (Ghedin et al., 2009), and this has recently been shown to involve specific regulatory elements present in the promoters of these genes (Liu et al., 2011). Changes in the expression levels of specific filarial genes such as phosphate permease (Heider et al., 2006) and heat shock protein 60 (Pfarr et al., 2008) following Wolbachia clearance have also been shown, suggesting that the encoded proteins are involved at some level. However, whether these changes are specific to a disruption in the balance of the Wolbachia–host interaction or due to indirect effects such as antibiotic toxicity or a nematode stress response is unclear and needs to be further investigated.

Although the molecular details of the Wolbachia processes essential for the symbiosis remain to be defined, studies on the cellular consequences of symbiont elimination have provided an important insight into the cellular mechanism at the basis of the symbiotic relationship. Soon after antibiotic elimination of the bacteria extensive apoptosis in the adult germline, and in the somatic cells of the embryos, microfilariae and L4 occurs (Landmann et al., 2011). Apoptosis extends to uninfected cells, suggesting an indirect provision of products from the hypodermal population is required to prevent cells from undergoing cell death. This cellular mechanism does not extend to all somatic cells, including those of the hypodermal cord cells, where the bacteria reside, although the cytoskeletal arrangement is disrupted. The pattern of apoptosis activation correlates closely with the stages most vulnerable to antibiotic depletion and provides a mechanism to account for the rapid anti-filarial effects of antibiotic treatment.

Exploitation of the Wolbachia filarial relationship as a therapeutic target

  1. Top of page
  2. Summary
  3. Wolbachia and filarial nematodes
  4. Intracellular niche and bacterial structure
  5. Population biology and dynamics
  6. Autophagy regulates Wolbachia populations
  7. Cell lineage tropism and route of transmission
  8. Molecular and cellular interactions between Wolbachia and their filarial hosts
  9. Exploitation of the Wolbachia filarial relationship as a therapeutic target
  10. Concluding remarks
  11. Acknowledgements
  12. References

The symbiotic relationship between Wolbachia and their host nematode has been exploited with great success using doxycycline to treat filariae-infected individuals in several field studies (Taylor et al., 2005b; Debrah et al., 2006; 2007; Hoerauf et al., 2008; 2009; Supali et al., 2008; Mand et al., 2009; Turner et al., 2010). A 4–6 week course of doxycycline leads to permanent sterilization of female worms and sustained loss of microfilariae with the eventual death of the adult worms, providing superior efficacy to existing anti-filarial drugs and the added benefit of improvements in filarial pathology (Taylor et al., 2010). However, the known contraindications of doxycycline and the logistical constraints which prevent its widespread use in mass drug administration (MDA) control programmes have prompted the formation of the A·WOL consortium which aims to overcome these obstacles by discovering new anti-Wolbachia drugs (http://www.a-wol.com/).

The A·WOL screening strategy uses a pipeline of approaches optimized to identify and validate anti-Wolbachia compounds by screening of both focused anti-infective and diversity-based libraries of existing and novel drugs and natural products. Early in the consortium's inception, a cell-based Wolbachia screening assay was developed utilizing a Wolbachia-containing Aedes albopictus cell line (C6/36 Wp) (Turner et al., 2006), in a 96-well format, with a quantitative PCR (qPCR) readout to quantify the Wolbachia 16S rRNA gene copy number following treatment (Johnston et al., 2010). This validated assay was first used to screen 2664 drugs of the human pharmacopoeia for potential repurposing and identified 121 hits, 70 of which were orally available thus satisfying the target product profile defined by A·WOL. The hits crossed over several drug classes and several have progressed further down the screening pipeline into in vitro nematode assays and in vivo screening models. As the compounds tested in this screen were already registered for use in humans, this screening strategy represents an opportunity to quickly develop an alternative chemotherapy for filariasis.

As well as registered drugs, the A·WOL cell-based screen has also been used to screen focused drug libraries selected based on known and bio-informatically predicted essential gene targets (Holman et al., 2009). Focused anti-infective library screening has, thus far, involved A·WOL in vitro screening of 7144 novel compounds from eight chemical libraries. To date this has generated 460 diverse hit compounds, a number of which have progressed further into the screening funnel. Notably, the ability to identify hit compounds from these focused libraries, which are effective at reducing Wolbachia load and have improved efficacy over doxycycline, is highly supportive of the long-term goal to identify A·WOL new chemical entities (NCEs).

In order to expand the capacity to discover NCEs, diversity-based library screening has also been a part of A·WOL's activities. Three diversity-based chemical libraries consisting in total of 60 000+ compounds have entered the primary cell-based assay screen. Hits from these libraries will be tested for narrow-spectrum activity against Wolbachia in order to produce targeted treatments that do not overlap with other anti-bacterial treatment domains.

Overall, the A·WOL consortium has made significant progress both in optimizing current regimes of existing anti-Wolbachia treatments and in discovering potential new therapies through screening. It is hoped that these discoveries will ultimately lead to the delivery of an alternative complementary strategy, which can achieve the goal of eliminating lymphatic filariasis and onchocerciasis.

Concluding remarks

  1. Top of page
  2. Summary
  3. Wolbachia and filarial nematodes
  4. Intracellular niche and bacterial structure
  5. Population biology and dynamics
  6. Autophagy regulates Wolbachia populations
  7. Cell lineage tropism and route of transmission
  8. Molecular and cellular interactions between Wolbachia and their filarial hosts
  9. Exploitation of the Wolbachia filarial relationship as a therapeutic target
  10. Concluding remarks
  11. Acknowledgements
  12. References

As discussed in this review, it is clear that Wolbachia play an obligatory role in the biology of medically important filarial nematodes involving a number of cellular and molecular interactions. The essentiality of the symbiosis is primarily associated with periods of growth and development with high metabolic demands, but extends to other biological processes including transmission through the vector and longevity of the adult worms. Further investigation into the nature of these interactions should serve to enhance our ability to exploit this dependency and potentially lead to novel treatments for lymphatic filariasis and onchocerciasis.

References

  1. Top of page
  2. Summary
  3. Wolbachia and filarial nematodes
  4. Intracellular niche and bacterial structure
  5. Population biology and dynamics
  6. Autophagy regulates Wolbachia populations
  7. Cell lineage tropism and route of transmission
  8. Molecular and cellular interactions between Wolbachia and their filarial hosts
  9. Exploitation of the Wolbachia filarial relationship as a therapeutic target
  10. Concluding remarks
  11. Acknowledgements
  12. References
  • Albers, A., Esum, M.E., Tendongfor, N., Enyong, P., Klarmann, U., Wanji, S., et al. (2012) Retarded Onchocerca volvulus L1 to L3 larval development in the Simulium damnosum vector after anti-wolbachial treatment of the human host. Parasit Vectors 5: e12.
  • Bandi, C., Anderson, T.J., Genchi, C., and Blaxter, M.L. (1998) Phylogeny of Wolbachia in filarial nematodes. Proc Biol Sci 265: 24072413.
  • Darby, A.C., Armstrong, S.D., Bah, G.S., Kaur, G., Hughes, M.A., Kay, S.M., et al. (2012) Analysis of gene expression from the Wolbachia genome of a filarial nematode supports both metabolic and defensive roles within the symbiosis. Genome Res 22: 24672477.
  • Debrah, A.Y., Mand, S., Specht, S., Marfo-Debrekyei, Y., Batsa, L., Pfarr, K., et al. (2006) Doxycycline reduces plasma VEGF-C/sVEGFR-3 and improves pathology in lymphatic filariasis. PLoS Pathog 2: e92.
  • Debrah, A.Y., Mand, S., Marfo-Debrekyei, Y., Batsa, L., Pfarr, K., Buttner, M., et al. (2007) Macrofilaricidal effect of 4 weeks of treatment with doxycycline on Wuchereria bancrofti. Trop Med Int Health 12: 14331441.
  • Dunning Hotopp, J.C., Clark, M.E., Oliveira, D.C., Foster, J.M., Fischer, P., Munoz Torres, M.C., et al. (2007) Widespread lateral gene transfer from intracellular bacteria to multicellular eukaryotes. Science 317: 17531756.
  • Fenn, K., and Blaxter, M. (2004) Quantification of Wolbachia bacteria in Brugia malayi through the nematode lifecycle. Mol Biochem Parasitol 137: 361364.
  • Ferri, E., Bain, O., Barbuto, M., Martin, C., Lo, N., Uni, S., et al. (2011) New insights into the evolution of Wolbachia infections in filarial nematodes inferred from a large range of screened species. PLoS ONE 6: e20843.
  • Fischer, K., Beatty, W.L., Jiang, D., Weil, G.J., and Fischer, P.U. (2011) Tissue and stage-specific distribution of Wolbachia in Brugia malayi. PLoS Negl Trop Dis 5: e1174.
  • Foster, J., Ganatra, M., Kamal, I., Ware, J., Makarova, K., Ivanova, N., et al. (2005) The Wolbachia genome of Brugia malayi: endosymbiont evolution within a human pathogenic nematode. PLoS Biol 3: e121.
  • Ghedin, E., Wang, S., Spiro, D., Caler, E., Zhao, Q., Crabtree, J., et al. (2007) Draft genome of the filarial nematode parasite Brugia malayi. Science 317: 17561760.
  • Ghedin, E., Hailemariam, T., DePasse, J.V., Zhang, X., Oksov, Y., Unnasch, T.R., and Lustigman, S. (2009) Brugia malayi gene expression in response to the targeting of the Wolbachia endosymbiont by tetracycline treatment. PLoS Negl Trop Dis 3: e525.
  • Heider, U., Blaxter, M., Hoerauf, A., and Pfarr, K.M. (2006) Differential display of genes expressed in the filarial nematode Litomosoides sigmodontis reveals a putative phosphate permease up-regulated after depletion of Wolbachia endobacteria. Int J Med Microbiol 296: 287299.
  • Henrichfreise, B., Schiefer, A., Schneider, T., Nzukou, E., Poellinger, C., Hoffmann, T.J., et al. (2009) Functional conservation of the lipid II biosynthesis pathway in the cell wall-less bacteria Chlamydia and Wolbachia: why is lipid II needed? Mol Microbiol 73: 913923.
  • Hoerauf, A., Specht, S., Buttner, M., Pfarr, K., Mand, S., Fimmers, R., et al. (2008) Wolbachia endobacteria depletion by doxycycline as antifilarial therapy has macrofilaricidal activity in onchocerciasis: a randomized placebo-controlled study. Med Microbiol Immunol (Berl) 197: 295311.
  • Hoerauf, A., Specht, S., Marfo-Debrekyei, Y., Buttner, M., Debrah, A.Y., Mand, S., et al. (2009) Efficacy of 5-week doxycycline treatment on adult Onchocerca volvulus. Parasitol Res 104: 437447.
  • Holman, A.G., Davis, P.J., Foster, J.M., Carlow, C.K., and Kumar, S. (2009) Computational prediction of essential genes in an unculturable endosymbiotic bacterium, Wolbachia of Brugia malayi. BMC Microbiol 9: e243.
  • Johnston, K.L., Wu, B., Guimaraes, A., Ford, L., Slatko, B.E., and Taylor, M.J. (2010) Lipoprotein biosynthesis as a target for anti-Wolbachia treatment of filarial nematodes. Parasit Vectors 3: e99.
  • Kozek, W.J. (1977) Transovarially-transmitted intracellular microorganisms in adult and larval stages of Brugia malayi. J Parasitol 63: 9921000.
  • Landmann, F., Foster, J.M., Slatko, B., and Sullivan, W. (2010) Asymmetric Wolbachia segregation during early Brugia malayi embryogenesis determines its distribution in adult host tissues. PLoS Negl Trop Dis 4: e758.
  • Landmann, F., Voronin, D., Sullivan, W., and Taylor, M.J. (2011) Anti-filarial activity of antibiotic therapy is due to extensive apoptosis after Wolbachia depletion from filarial nematodes. PLoS Pathog 7: e1002351.
  • Landmann, F., Bain, O., Martin, C., Uni, S., Taylor, M.J., and Sullivan, W. (2012) Both asymmetric mitotic segregation and cell-to-cell invasion are required for stable germline transmission of Wolbachia in filarial nematodes. Biology Open 1: 536547.
  • Liu, C., Kelen, P.V., Ghedin, E., Lustigman, S., and Unnasch, T.R. (2011) Analysis of transcriptional regulation of tetracycline responsive genes in Brugia malayi. Mol Biochem Parasitol 180: 106111.
  • McGarry, H.F., Egerton, G., and Taylor, M.J. (2004) Population dynamics of Wolbachia bacterial endosymbionts in Brugia malayi. Mol Biochem Parasitol 135: 5767.
  • McNulty, S.N., Foster, J.M., Mitreva, M., Dunning Hotopp, J.C., Martin, J., Fischer, K., et al. (2010) Endosymbiont DNA in endobacteria-free filarial nematodes indicates ancient horizontal genetic transfer. PLoS ONE 5: e11029.
  • Mand, S., Pfarr, K., Sahoo, P.K., Satapathy, A.K., Specht, S., Klarmann, U., et al. (2009) Macrofilaricidal activity and amelioration of lymphatic pathology in bancroftian filariasis after 3 weeks of doxycycline followed by single-dose diethylcarbamazine. Am J Trop Med Hyg 81: 702711.
  • Pfarr, K.M., Heider, U., Schmetz, C., Buttner, D.W., and Hoerauf, A. (2008) The mitochondrial heat shock protein 60 (HSP60) is up-regulated in Onchocerca volvulus after the depletion of Wolbachia. Parasitology 135: 529538.
  • Rao, R.U., Huang, Y., Abubucker, S., Heinz, M., Crosby, S.D., Mitreva, M., and Weil, G.J. (2012) Effects of doxycycline on gene expression in Wolbachia and Brugia malayi adult female worms in vivo. J Biomed Sci 19: e21.
  • Slatko, B.E., Taylor, M.J., and Foster, J.M. (2010) The Wolbachia endosymbiont as an anti-filarial nematode target. Symbiosis 51: 5565.
  • Strubing, U., Lucius, R., Hoerauf, A., and Pfarr, K.M. (2010) Mitochondrial genes for heme-dependent respiratory chain complexes are up-regulated after depletion of Wolbachia from filarial nematodes. Int J Parasitol 40: 11931202.
  • Supali, T., Djuardi, Y., Pfarr, K.M., Wibowo, H., Taylor, M.J., Hoerauf, A., et al. (2008) Doxycycline treatment of Brugia malayi-infected persons reduces microfilaremia and adverse reactions after diethylcarbamazine and albendazole treatment. Clin Infect Dis 46: 13851393.
  • Taylor, M.J., and Hoerauf, A. (2001) A new approach to the treatment of filariasis. Curr Opin Infect Dis 14: 727731.
  • Taylor, M.J., Bilo, K., Cross, H.F., Archer, J.P., and Underwood, A.P. (1999) 16S rDNA phylogeny and ultrastructural characterization of Wolbachia intracellular bacteria of the filarial nematodes Brugia malayi, B. pahangi, and Wuchereria bancrofti. Exp Parasitol 91: 356361.
  • Taylor, M.J., Bandi, C., and Hoerauf, A. (2005a) Wolbachia bacterial endosymbionts of filarial nematodes. Adv Parasitol 60: 245284.
  • Taylor, M.J., Makunde, W.H., McGarry, H.F., Turner, J.D., Mand, S., and Hoerauf, A. (2005b) Macrofilaricidal activity after doxycycline treatment of Wuchereria bancrofti: a double-blind, randomised placebo-controlled trial. Lancet 365: 21162121.
  • Taylor, M.J., Hoerauf, A., and Bockarie, M. (2010) Lymphatic filariasis and onchocerciasis. Lancet 376: 11751185.
  • Taylor, M.J., Ford, L., Hoerauf, A., Pfarr, K., Foster, J., Kumar, S., and Slatko, B. (2012) Drugs and targets to perturb the symbiosis of Wolbachia and filarial nematodes. In Parasitic Helminths. Targets, Screens, Drugs, and Vaccines. Caffrey, C.R. (ed.). Weinheim: Wiley-VCH, pp. 251266.
  • Turner, J.D., Langley, R.S., Johnston, K.L., Egerton, G., Wanji, S., and Taylor, M.J. (2006) Wolbachia endosymbiotic bacteria of Brugia malayi mediate macrophage tolerance to TLR- and CD40-specific stimuli in a MyD88/TLR2-dependent manner. J Immunol 177: 12401249.
  • Turner, J.D., Langley, R.S., Johnston, K.L., Gentil, K., Ford, L., Wu, B., et al. (2009) Wolbachia lipoprotein stimulates innate and adaptive immunity through toll-like receptors 2 and 6 (TLR2/6) to induce disease manifestations of filariasis. J Biol Chem 284: 2236422378.
  • Turner, J.D., Tendongfor, N., Esum, M., Johnston, K.L., Langley, R.S., Ford, L., et al. (2010) Macrofilaricidal activity after doxycycline only treatment of Onchocerca volvulus in an area of Loa loa co-endemicity: a randomized controlled trial. PLoS Negl Trop Dis 4: e660.
  • Voronin, D., Cook, D.A., Steven, A., and Taylor, M.J. (2012) Autophagy regulates Wolbachia populations across diverse symbiotic associations. Proc Natl Acad Sci USA 109: E1638E1646.