Ixodes scapularis dystroglycan-like protein promotes Borrelia burgdorferi migration from the gut

Tissue-specific localization of tick-borne pathogens in ticks collected from camels in Kenya: insights into vector competence

Background

Tick-borne pathogen (TBP) surveillance studies often use whole-tick homogenates when inferring tick-pathogen associations. However, localized TBP infections within tick tissues (saliva, hemolymph, salivary glands, and midgut) can inform pathogen transmission mechanisms and are key to disentangling pathogen detection from vector competence.

Methods

We screened 278 camel blood samples and 504 tick tissue samples derived from 126 camel ticks sampled in two Kenyan counties (Laikipia and Marsabit) for Anaplasma, Ehrlichia, Coxiella, Rickettsia, Theileria, and Babesia by PCR-HRM analysis.

Results

Candidatus Anaplasma camelii infections were common in camels (91%), but absent in all samples from Rhipicephalus pulchellus, Amblyomma gemma, Hyalomma dromedarii, and Hyalomma rufipes ticks. We detected Ehrlichia ruminantium in all tissues of the four tick species, but Rickettsia aeschlimannii was only found in Hy. rufipes (all tissues). Rickettsia africae was highest in Am. gemma (62.5%), mainly in the hemolymph (45%) and less frequently in the midgut (27.5%) and lowest in Rh. pulchellus (29.4%), where midgut and hemolymph detection rates were 17.6% and 11.8%, respectively. Similarly, in Hy. dromedarii, R. africae was mainly detected in the midgut (41.7%) but was absent in the hemolymph. Rickettsia africae was not detected in Hy. rufipes. No Coxiella, Theileria, or Babesia spp. were detected in this study.

Conclusions

The tissue-specific localization of R. africae, found mainly in the hemolymph of Am. gemma, is congruent with the role of this tick species as its transmission vector. Thus, occurrence of TBPs in the hemolymph could serve as a predictor of vector competence of TBP transmission, especially in comparison to detection rates in the midgut, from which they must cross tissue barriers to effectively replicate and disseminate across tick tissues. Further studies should focus on exploring the distribution of TBPs within tick tissues to enhance knowledge of TBP epidemiology and to distinguish competent vectors from dead-end hosts.

Pathogenicity and virulence of Borrelia burgdorferi

Infection with Borrelia burgdorferi often triggers pathophysiologic perturbations that are further augmented by the inflammatory responses of the host, resulting in the severe clinical conditions of Lyme disease. While our apprehension of the spatial and temporal integration of the virulence determinants during the enzootic cycle of B. burgdorferi is constantly being improved, there is still much to be discovered. Many of the novel virulence strategies discussed in this review are undetermined. Lyme disease spirochaetes must surmount numerous molecular and mechanical obstacles in order to establish a disseminated infection in a vertebrate host. These barriers include borrelial relocation from the midgut of the feeding tick to its body cavity and further to the salivary glands, deposition to the skin, haematogenous dissemination, extravasation from blood circulation system, evasion of the host immune responses, localization to protective niches, and establishment of local as well as distal infection in multiple tissues and organs. Here, the various well-defined but also possible novel strategies and virulence mechanisms used by B. burgdorferi to evade obstacles laid out by the tick vector and usually the mammalian host during colonization and infection are reviewed.

Induced Transient Immune Tolerance in Ticks and Vertebrate Host: A Keystone of Tick-Borne Diseases?

Ticks and tick transmitted infectious agents are increasing global public health threats due to increasing abundance, expanding geographic ranges of vectors and pathogens, and emerging tick-borne infectious agents. Greater understanding of tick, host, and pathogen interactions will contribute to development of novel tick control and disease prevention strategies. Tick-borne pathogens adapt in multiple ways to very different tick and vertebrate host environments and defenses. Ticks effectively pharmacomodulate by its saliva host innate and adaptive immune defenses. In this review, we examine the idea that successful synergy between tick and tick-borne pathogen results in host immune tolerance that facilitates successful tick infection and feeding, creates a favorable site for pathogen introduction, modulates cutaneous and systemic immune defenses to establish infection, and contributes to successful long-term infection. Tick, host, and pathogen elements examined here include interaction of tick innate immunity and microbiome with tick-borne pathogens; tick modulation of host cutaneous defenses prior to pathogen transmission; how tick and pathogen target vertebrate host defenses that lead to different modes of interaction and host infection status (reservoir, incompetent, resistant, clinically ill); tick saliva bioactive molecules as important factors in determining those pathogens for which the tick is a competent vector; and, the need for translational studies to advance this field of study. Gaps in our understanding of these relationships are identified, that if successfully addressed, can advance the development of strategies to successfully disrupt both tick feeding and pathogen transmission.

Identification of Tick Ixodes ricinus Midgut Genes Differentially Expressed During the Transmission of Borrelia afzelii Spirochetes Using a Transcriptomic Approach

Lyme borreliosis is an emerging tick-borne disease caused by spirochetes Borrelia burgdorferi sensu lato. In Europe, Lyme borreliosis is predominantly caused by Borrelia afzelii and transmitted by Ixodes ricinus. Although Borrelia behavior throughout tick development is quite well documented, specific molecular interactions between Borrelia and the tick have not been satisfactorily examined. Here, we present the first transcriptomic study focused on the expression of tick midgut genes regulated by Borrelia. By using massive analysis of cDNA ends (MACE), we searched for tick transcripts expressed differentially in the midgut of unfed, 24h-fed, and fully fed I. ricinus nymphs infected with B. afzelii. In total, we identified 553 upregulated and 530 downregulated tick genes and demonstrated that B. afzelii interacts intensively with the tick. Technical and biological validations confirmed the accuracy of the transcriptome. The expression of five validated tick genes was silenced by RNA interference. Silencing of the uncharacterized protein (GXP_Contig_30818) delayed the infection progress and decreased infection prevalence in the target mice tissues. Silencing of other genes did not significantly affect tick feeding nor the transmission of B. afzelii, suggesting a possible role of these genes rather in Borrelia acquisition or persistence in ticks. Identification of genes and proteins exploited by Borrelia during transmission and establishment in a tick could help the development of novel preventive strategies for Lyme borreliosis.

Interactions between Borrelia burgdorferi and its hosts across the enzootic cycle

The bacterial pathogen Borrelia burgdorferi is the causative agent of Lyme disease and is transmitted to humans through an Ixodes tick vector. B. burgdorferi is able to survive in both mammalian and tick hosts through careful modulation of its gene expression. This allows B. burgdorferi to adapt to the environmental and nutritional changes that occur when it is transmitted between the two hosts. Distinct interactions between the spirochete and its host occur at every step of the enzootic cycle and dictate the ability of the spirochete to survive until the next stage of the cycle. Studying the interface between B. burgdorferi, the Ixodes tick vector and the natural mammalian reservoirs has been made significantly more feasible through the complete genome sequences of the organisms and the advent of high throughput screening technologies. Ultimately, a thorough investigation of the interplay between the two domains (and two phyla within one domain) is necessary in order to completely understand how the pathogen is transmitted.

Interactions Between Ticks and Lyme Disease Spirochetes

Borrelia burgdorferi sensu lato causes Lyme borreliosis in a variety of animals and humans. These atypical bacterial pathogens are maintained in a complex enzootic life cycle that primarily involves a vertebrate host and Ixodes spp. ticks. In the Northeastern United States, I. scapularis is the main vector, while wild rodents serve as the mammalian reservoir host. As B. burgdorferi is transmitted only by I. scapularis and closely related ticks, the spirochete-tick interactions are thought to be highly specific. Various borrelial and arthropod proteins that directly or indirectly contribute to the natural cycle of B. burgdorferi infection have been identified. Discrete molecular interactions between spirochetes and tick components also have been discovered, which often play critical roles in pathogen persistence and transmission by the arthropod vector. This review will focus on the past discoveries and future challenges that are relevant to our understanding of the molecular interactions between B. burgdorferi and Ixodes ticks. This information will not only impact scientific advancements in the research of tick- transmitted infections but will also contribute to the development of novel preventive measures that interfere with the B. burgdorferi life cycle.

Interactions between Borrelia burgdorferi and ticks

Borrelia burgdorferi is the causative agent of Lyme disease and is transmitted to vertebrate hosts by Ixodes spp. ticks. The spirochaete relies heavily on its arthropod host for basic metabolic functions and has developed complex interactions with ticks to successfully colonize, persist and, at the optimal time, exit the tick. For example, proteins shield spirochaetes from immune factors in the bloodmeal and facilitate the transition between vertebrate and arthropod environments. On infection, B. burgdorferi induces selected tick proteins that modulate the vector gut microbiota towards an environment that favours colonization by the spirochaete. Additionally, the recent sequencing of the Ixodes scapularis genome and characterization of tick immune defence pathways, such as the JAK–STAT, immune deficiency and cross-species interferon-γ pathways, have advanced our understanding of factors that are important for B. burgdorferi persistence in the tick. In this Review, we summarize interactions between B. burgdorferi and I. scapularis during infection, as well as interactions with tick gut and salivary gland proteins important for establishing infection and transmission to the vertebrate host.

Borrelia burgdorferi has a complex life cycle with several different hosts, causing Lyme disease when it infects humans. In this Review, Fikrig and colleagues discuss how B. burgdorferi infects and interacts with its tick vector to ensure onward transmission.

Borrelia burgdorferi chemotaxis toward tick protein Salp12 contributes to acquisition

Lyme disease is a common tick-borne infection caused by the spirochete Borrelia burgdorferi sensu stricto (s.s.). B. burgdorferi s.s. may utilize chemotaxis, the directional migration towards or away from a chemical stimulus, for transmission, acquisition, and infection. However, the specific signals recognized by the spirochete for these events have not been defined. In this study, we identify an Ixodes scapularis salivary gland protein, Salp12, that is a chemoattractant for the spirochete. We demonstrate that Salp12 is expressed in the I. scapularis salivary glands and midgut and expression is not impacted by B. burgdorferi s.s. infection. Knockdown of Salp12 in the salivary glands or passive immunization against Salp12 reduces acquisition of the spirochete by ticks but acquisition is not completely prevented. Knockdown does not impact transmission of B. burgdorferi s.s. This work suggests a new role for chemotaxis in acquisition of the spirochete and suggests that recognition of Salp12 contributes to this phenomenon.

Counterattacking the tick bite: towards a rational design of anti-tick vaccines targeting pathogen transmission

Hematophagous arthropods are responsible for the transmission of a variety of pathogens that cause disease in humans and animals. Ticks of the Ixodes ricinus complex are vectors for some of the most frequently occurring human tick-borne diseases, particularly Lyme borreliosis and tick-borne encephalitis virus (TBEV). The search for vaccines against these diseases is ongoing. Efforts during the last few decades have primarily focused on understanding the biology of the transmitted viruses, bacteria and protozoans, with the goal of identifying targets for intervention. Successful vaccines have been developed against TBEV and Lyme borreliosis, although the latter is no longer available for humans. More recently, the focus of intervention has shifted back to where it was initially being studied which is the vector. State of the art technologies are being used for the identification of potential vaccine candidates for anti-tick vaccines that could be used either in humans or animals. The study of the interrelationship between ticks and the pathogens they transmit, including mechanisms of acquisition, persistence and transmission have come to the fore, as this knowledge may lead to the identification of critical elements of the pathogens' life-cycle that could be targeted by vaccines. Here, we review the status of our current knowledge on the triangular relationships between ticks, the pathogens they carry and the mammalian hosts, as well as methods that are being used to identify anti-tick vaccine candidates that can prevent the transmission of tick-borne pathogens.

A bite so sweet: the glycobiology interface of tick-host-pathogen interactions

Vector-borne diseases constitute 17% of all infectious diseases in the world; among the blood-feeding arthropods, ticks transmit the highest number of pathogens. Understanding the interactions between the tick vector, the mammalian host and the pathogens circulating between them is the basis for the successful development of vaccines against ticks or the tick-transmitted pathogens as well as for the development of specific treatments against tick-borne infections. A lot of effort has been put into transcriptomic and proteomic analyses; however, the protein-carbohydrate interactions and the overall glycobiology of ticks and tick-borne pathogens has not been given the importance or priority deserved. Novel (bio)analytical techniques and their availability have immensely increased the possibilities in glycobiology research and thus novel information in the glycobiology of ticks and tick-borne pathogens is being generated at a faster pace each year. This review brings a comprehensive summary of the knowledge on both the glycosylated proteins and the glycan-binding proteins of the ticks as well as the tick-transmitted pathogens, with emphasis on the interactions allowing the infection of both the ticks and the hosts by various bacteria and tick-borne encephalitis virus.

Disruption of blood meal-responsive serpins prevents Ixodes scapularis from feeding to repletion

Serine protease inhibitors (serpins) are thought to mediate the tick's evasion of the host's serine protease-mediated defense pathways such as inflammation and blood clotting. This study describes characterization and target validation of 11 blood meal-responsive serpins that are associated with nymph and adult Ixodes scapularis tick feeding as revealed by quantitative (q)RT-PCR and RNAi silencing analyses. Given the high number of targets, we used combinatorial (co) RNAi silencing to disrupt candidate serpins in two groups (G): seven highly identical and four non-identical serpins based on amino acid identities, here after called GI and GII respectively. We show that injection of both GI and GII co-dsRNA into unfed nymph and adult I. scapularis ticks triggered suppression of cognate serpin mRNA. We show that disruption of GII, but not GI serpins significantly reduced feeding efficiency of both nymph and adult I. scapularis ticks. Knockdown of GII serpin transcripts caused significant respective mortalities of ≤40 and 71% of nymphal and adult ticks that occurred within 24-48 h of attachment. This is significant, as the observed lethality preceded the tick feeding period when transmission of tick borne pathogens is predominant. We suspect that some of the GII serpins (S9, S17, S19 and S32) play roles in the tick detachment process in that upon detachment, mouthparts of GII co-dsRNA injected were covered with a whitish gel-like tissue that could be the tick cement cone. Normally, ticks do not retain tissue on their mouthparts upon detachment. Furthermore, disruption of GII serpins reduced tick blood meal sizes and the adult tick's ability to convert the blood meal to eggs. We discuss our data with reference to tick feeding physiology and conclude that some of the GII serpins are potential targets for anti-tick vaccine development.

A Molecular Survey of Tick-Borne Pathogens from Ticks Collected in Central Queensland, Australia

Central Queensland (CQ) is a large and isolated, low population density, remote tropical region of Australia with a varied environment. The region has a diverse fauna and several species of ticks that feed upon that fauna. This study examined 518 individual ticks: 177 Rhipicephalus sanguineus (brown dog tick), 123 Haemaphysalis bancrofti (wallaby tick), 102 Rhipicephalus australis (Australian cattle tick), 47 Amblyomma triguttatum (ornate kangaroo tick), 57 Ixodes holocyclus (paralysis tick), 9 Bothriocroton tachyglossi (CQ short-beaked echidna tick), and 3 Ornithodoros capensis (seabird soft tick). Tick midguts were pooled by common host or environment and screened for four genera of tick-borne zoonoses by PCR and sequencing. The study examined a total of 157 midgut pools of which 3 contained DNA of Coxiella burnetii, 13 Rickettsia gravesii, 1 Rickettsia felis, and 4 other Rickettsia spp. No Borrelia spp. or Babesia spp. DNA were recovered.

Tick-Pathogen Interactions and Vector Competence: Identification of Molecular Drivers for Tick-Borne Diseases

Ticks and the pathogens they transmit constitute a growing burden for human and animal health worldwide. Vector competence is a component of vectorial capacity and depends on genetic determinants affecting the ability of a vector to transmit a pathogen. These determinants affect traits such as tick-host-pathogen and susceptibility to pathogen infection. Therefore, the elucidation of the mechanisms involved in tick-pathogen interactions that affect vector competence is essential for the identification of molecular drivers for tick-borne diseases. In this review, we provide a comprehensive overview of tick-pathogen molecular interactions for bacteria, viruses, and protozoa affecting human and animal health. Additionally, the impact of tick microbiome on these interactions was considered. Results show that different pathogens evolved similar strategies such as manipulation of the immune response to infect vectors and facilitate multiplication and transmission. Furthermore, some of these strategies may be used by pathogens to infect both tick and mammalian hosts. Identification of interactions that promote tick survival, spread, and pathogen transmission provides the opportunity to disrupt these interactions and lead to a reduction in tick burden and the prevalence of tick-borne diseases. Targeting some of the similar mechanisms used by the pathogens for infection and transmission by ticks may assist in development of preventative strategies against multiple tick-borne diseases.

An evaluation of the evolution of the gene structure of dystroglycan

Background

Dystroglycan (DG) is an adhesion receptor complex composed of two non-covalently associated subunits, transcribed from a single gene. The extracellular α-DG is highly and heterogeneously glycosylated and binds with high affinity to laminins, and the transmembrane β-DG binds intracellular dystrophin. Multiple cellular functions have been proposed for DG, notwithstanding that its role in skeletal muscle appears central as demonstrated by both primary and secondary severe muscular dystrophic phenotypes collectively known as dystroglycanopathies. We recently analysed the molecular phylogeny of the DG core protein and identified the α/β interface, transmembrane and cytoplasmic domains of β-DG as the most conserved region. It was also identified that the IG2_MAT_NU region has been independently duplicated in multiple lineages.

Results

To understand the evolution of dystroglycan in more depth, we investigated dystroglycan gene structure in 35 species representative of the phyla in which dystroglycan has been identified (i.e., all metazoan phyla except Ctenophora). The gene structure of three exons and two introns is remarkably conserved. However, additional lineage-specific introns were identified, which interrupt the coding sequence at distinct points, were identified in multiple metazoan groups, most prominently in ecdysozoans.

Conclusions

A coding DNA sequence (CDS) intron that interrupts the encoding of the IG1 domain is universally conserved and this intron is longer in gnathostomes (jawed vertebrates) than in other metazoans. Lineage-specific gain of additional introns has occurred notably in ecdysozoans, where multiple introns interrupt the large 3′ exon. More limited intron gain has also occurred in placozoa, cnidarians, urochordates and the DG paralogues of lamprey and teleost fish.

Electronic supplementary material

The online version of this article (doi:10.1186/s13104-016-2322-x) contains supplementary material, which is available to authorized users.