Mialostatin, a Novel Midgut Cystatin from Ixodes ricinus Ticks: Crystal Structure and Regulation of Host Blood Digestion

Tick cysteine protease inhibitors suppress immune responses in mannan-induced psoriasis-like inflammation

Protease inhibitors regulate various biological processes and prevent host tissue/organ damage. Specific inhibition/regulation of proteases is clinically valuable for treating several diseases. Psoriasis affects the skin in the limbs and scalp of the body, and the contribution of cysteine and serine proteases to the development of skin inflammation is well documented. Cysteine protease inhibitors from ticks have high specificity, selectivity, and affinity to their target proteases and are efficient immunomodulators. However, their potential therapeutic effect on psoriasis pathogenesis remains to be determined. Therefore, we tested four tick cystatins (Sialostatin L, Sialostatin L2, Iristatin, and Mialostatin) in the recently developed, innate immunity-dependent mannan-induced psoriasis model. We explored the effects of protease inhibitors on clinical symptoms and histological features. In addition, the number and percentage of immune cells (dendritic cells, neutrophils, macrophages, and γδT cells) by flow cytometry, immunofluorescence/immunohistochemistry and, the expression of pro-inflammatory cytokines (TNF-a, IL-6, IL-22, IL-23, and IL-17 family) by qPCR were analyzed using skin, spleen, and lymph node samples. Tick protease inhibitors have significantly decreased psoriasis symptoms and disease manifestations but had differential effects on inflammatory responses and immune cell populations, suggesting different modes of action of these inhibitors on psoriasis-like inflammation. Thus, our study demonstrates, for the first time, the usefulness of tick-derived protease inhibitors for treating skin inflammation in patients.

An Unusual Two-Domain Thyropin from Tick Saliva: NMR Solution Structure and Highly Selective Inhibition of Cysteine Cathepsins Modulated by Glycosaminoglycans

The structure and biochemical properties of protease inhibitors from the thyropin family are poorly understood in parasites and pathogens. Here, we introduce a novel family member, Ir-thyropin (IrThy), which is secreted in the saliva of Ixodes ricinus ticks, vectors of Lyme borreliosis and tick-borne encephalitis. The IrThy molecule consists of two consecutive thyroglobulin type-1 (Tg1) domains with an unusual disulfide pattern. Recombinant IrThy was found to inhibit human host-derived cathepsin proteases with a high specificity for cathepsins V, K, and L among a wide range of screened cathepsins exhibiting diverse endo- and exopeptidase activities. Both Tg1 domains displayed inhibitory activities, but with distinct specificity profiles. We determined the spatial structure of one of the Tg1 domains by solution NMR spectroscopy and described its reactive center to elucidate the unique inhibitory specificity. Furthermore, we found that the inhibitory potency of IrThy was modulated in a complex manner by various glycosaminoglycans from host tissues. IrThy was additionally regulated by pH and proteolytic degradation. This study provides a comprehensive structure-function characterization of IrThy-the first investigated thyropin of parasite origin-and suggests its potential role in host-parasite interactions at the tick bite site.

Tick innate immune responses to hematophagy and Ehrlichia infection at single-cell resolution

Introduction

Ticks rely on robust cellular and humoral responses to control microbial infection. However, several aspects of the tick's innate immune system remain uncharacterized, most notably that of the immune cells (called hemocytes), which are known to play a significant role in cellular and humoral responses. Despite the importance of hemocytes in regulating microbial infection, our understanding of their basic biology and molecular mechanisms remains limited. Therefore, we believe that a more detailed understanding of the role of hemocytes in the interactions between ticks and tick-borne microbes is crucial to illuminating their function in vector competence and to help identify novel targets for developing new strategies to block tick-borne pathogen transmission.

Methods

This study examined hemocytes from the lone star tick (Amblyomma americanum) at the transcriptomic level using the 10X genomics single-cell RNA sequencing platform to analyze hemocyte populations from unfed, partially blood-fed, and Ehrlichia chaffeensis-infected ticks. The functional role of differentially expressed hemocyte markers in hemocyte proliferation and Ehrlichia dissemination was determined using an RNA interference approach.

Results and discussion

Our data exhibit the identification of fourteen distinct hemocyte populations. Our results uncover seven distinct lineages present in uninfected and Ehrlichia-infected hemocyte clusters. The functional characterization of hemocytin, cystatin, fibronectin, and lipocalin demonstrate their role in hemocyte population changes, proliferation, and Ehrlichia dissemination.

Conclusion

Our results uncover the tick immune responses to Ehrlichia infection and hematophagy at a single-cell resolution. This work opens a new field of tick innate immunobiology to understand the role of hemocytes, particularly in response to prolonged blood-feeding (hematophagy), and tick-microbial interactions.

Insight Into the Dynamics of the Ixodes ricinus Nymphal Midgut Proteome

Ticks are ectoparasites that feed on blood and have an impressive ability to consume and process enormous amounts of host blood, allowing extremely long periods of starvation between blood meals. The central role in the parasitic lifestyle of ticks is played by the midgut. This organ efficiently stores and digests ingested blood and serves as the primary interface for the transmission of tick-borne pathogens. In this study, we used a label-free quantitative approach to perform a novel dynamic proteomic analysis of the midgut of Ixodesricinus nymphs, covering their development from unfed to pre-molt stages. We identified 1534 I. ricinus-specific proteins with a relatively low proportion of host proteins. This proteome dataset, which was carefully examined by manual scrutiny, allowed precise annotation of proteins important for blood meal processing and their dynamic changes during nymphal ontogeny. We focused on midgut molecules related to lipid hydrolysis, storage, and transport, opening a yet unexplored avenue for studying lipid metabolism in ticks. Further dynamic profiling of the tick's multi-enzyme digestive network, protease inhibitors, enzymes involved in redox homeostasis and detoxification, antimicrobial peptides, and proteins responsible for midgut colonization by Borrelia spirochetes promises to uncover new targets for targeting tick nymphs, the most critical life stage for transmission the pathogens that cause tick-borne diseases.

Protease-bound structure of Ricistatin provides insights into the mechanism of action of tick salivary cystatins in the vertebrate host

Tick saliva injected into the vertebrate host contains bioactive anti-proteolytic proteins from the cystatin family; however, the molecular basis of their unusual biochemical and physiological properties, distinct from those of host homologs, is unknown. Here, we present Ricistatin, a novel secreted cystatin identified in the salivary gland transcriptome of Ixodes ricinus ticks. Recombinant Ricistatin inhibited host-derived cysteine cathepsins and preferentially targeted endopeptidases, while having only limited impact on proteolysis driven by exopeptidases. Determination of the crystal structure of Ricistatin in complex with a cysteine cathepsin together with characterization of structural determinants in the Ricistatin binding site explained its restricted specificity. Furthermore, Ricistatin was potently immunosuppressive and anti-inflammatory, reducing levels of pro-inflammatory cytokines IL-6, IL-1β, and TNF-α and nitric oxide in macrophages; IL-2 and IL-9 levels in Th9 cells; and OVA antigen-induced CD4+ T cell proliferation and neutrophil migration. This work highlights the immunotherapeutic potential of Ricistatin and, for the first time, provides structural insights into the unique narrow selectivity of tick salivary cystatins determining their bioactivity.

A longitudinal transcriptomic analysis from unfed to post-engorgement midguts of adult female Ixodes scapularis

The hematophagy behavior has evolved independently several times within the Arthropoda phylum. Interestingly, the process of acquiring a blood meal in ticks is considerably distinct from that observed in other blood-feeding arthropods. Instead of taking seconds to minutes to complete a blood meal, an adult female Ixodes scapularis tick can remain attached to its host for numerous days. During this extended feeding period, the tick undergoes drastic morphological changes. It is well established that the tick midgut plays a pivotal role not only in blood meal digestion but also in pathogen acquisition and transmission. However, our understanding of the underlying molecular mechanisms involved in these events remains limited. To expedite tick research, we conducted a comprehensive longitudinal RNA-sequencing of the tick midgut before, during, and after feeding. By collecting ticks in different feeding stages (unfed, slow feeding, rapid feeding, and early post-detached), we obtained a comprehensive overview of the transcripts present in each stage and the dynamic transcriptional changes that occur between them. This provides valuable insights into tick physiology. Additionally, through unsupervised clustering, we identified transcripts with similar patterns and stage-specific sequences. These findings serve as a foundation for selecting targets in the development of anti-tick control strategies and facilitate a better understanding of how blood feeding and pathogen infection impact tick physiology.

Editorial: Special Issue on the "Molecular Biology of Disease Vectors"

Arthropod disease vectors not only transmit malaria but many other serious diseases, many of which are, to a greater or lesser degree, neglected [...].

An evolutionary molecular adaptation of an unusual stefin from the liver fluke Fasciola hepatica redefines the cystatin superfamily

Fasciolosis is a worldwide parasitic disease of ruminants and an emerging human disease caused by the liver fluke Fasciola hepatica. The cystatin superfamily of cysteine protease inhibitors is composed of distinct families of intracellular stefins and secreted true cystatins. FhCyLS-2 from F. hepatica is an unusual member of the superfamily, where our sequence and 3D structure analyses in this study revealed that it combines characteristics of both families. The protein architecture demonstrates its relationship to stefins, but FhCyLS-2 also contains the secretion signal peptide and disulfide bridges typical of true cystatins. The secretion status was confirmed by detecting the presence of FhCyLS-2 in excretory/secretory products, supported by immunolocalization. Our high-resolution crystal structure of FhCyLS-2 showed a distinct disulfide bridging pattern and functional reactive center. We determined that FhCyLS-2 is a broad specificity inhibitor of cysteine cathepsins from both the host and F. hepatica, suggesting a dual role in the regulation of exogenous and endogenous proteolysis. Based on phylogenetic analysis that identified several FhCyLS-2 homologues in liver/intestinal foodborne flukes, we propose a new group within the cystatin superfamily called cystatin-like stefins.