Amblyomin-X having a Kunitz-type homologous domain, is a noncompetitive inhibitor of FXa and induces anticoagulation in vitro and in vivo

Structural and functional properties of the Kunitz-type and C-terminal domains of Amblyomin-X supporting its antitumor activity

Amblyomin-X is a Kunitz-type FXa inhibitor identified through the transcriptome analysis of the salivary gland from Amblyomma sculptum tick. This protein consists of two domains of equivalent size, triggers apoptosis in different tumor cell lines, and promotes regression of tumor growth, and reduction of metastasis. To study the structural properties and functional roles of the N-terminal (N-ter) and C-terminal (C-ter) domains of Amblyomin-X, we synthesized them by solid-phase peptide synthesis, solved the X-Ray crystallographic structure of the N-ter domain, confirming its Kunitz-type signature, and studied their biological properties. We show here that the C-ter domain is responsible for the uptake of Amblyomin-X by tumor cells and highlight the ability of this domain to deliver intracellular cargo by the strong enhancement of the intracellular detection of molecules with low cellular-uptake efficiency (p15) after their coupling with the C-ter domain. In contrast, the N-ter Kunitz domain of Amblyomin-X is not capable of crossing through the cell membrane but is associated with tumor cell cytotoxicity when it is microinjected into the cells or fused to TAT cell-penetrating peptide. Additionally, we identify the minimum length C-terminal domain named F2C able to enter in the SK-MEL-28 cells and induces dynein chains gene expression modulation, a molecular motor that plays a role in the uptake and intracellular trafficking of Amblyomin-X.

Tick Salivary Kunitz-Type Inhibitors: Targeting Host Hemostasis and Immunity to Mediate Successful Blood Feeding

Kunitz domain-containing proteins are ubiquitous serine protease inhibitors with promising therapeutic potential. They target key proteases involved in major cellular processes such as inflammation or hemostasis through competitive inhibition in a substrate-like manner. Protease inhibitors from the Kunitz superfamily have a low molecular weight (18-24 kDa) and are characterized by the presence of one or more Kunitz motifs consisting of α-helices and antiparallel β-sheets stabilized by three disulfide bonds. Kunitz-type inhibitors are an important fraction of the protease inhibitors found in tick saliva. Their roles in inhibiting and/or suppressing host homeostatic responses continue to be shown to be additive or synergistic with other protease inhibitors such as cystatins or serpins, ultimately mediating successful blood feeding for the tick. In this review, we discuss the biochemical features of tick salivary Kunitz-type protease inhibitors. We focus on their various effects on host hemostasis and immunity at the molecular and cellular level and their potential therapeutic applications. In doing so, we highlight that their pharmacological properties can be exploited for the development of novel therapies and vaccines.

A Kunitz-type inhibitor from tick salivary glands: A promising novel antitumor drug candidate

Salivary glands are vital structures responsible for successful tick feeding. The saliva of ticks contains numerous active molecules that participate in several physiological processes. A Kunitz-type factor Xa (FXa) inhibitor, similar to the tissue factor pathway inhibitor (TFPI) precursor, was identified in the salivary gland transcriptome of Amblyomma sculptum ticks. The recombinant mature form of this Kunitz-type inhibitor, named Amblyomin-X, displayed anticoagulant, antiangiogenic, and antitumor properties. Amblyomin-X is a protein that inhibits FXa in the blood coagulation cascade and acts via non-hemostatic mechanisms, such as proteasome inhibition. Amblyomin-X selectively induces apoptosis in cancer cells and promotes tumor regression through these mechanisms. Notably, the cytotoxicity of Amblyomin-X seems to be restricted to tumor cells and does not affect non-tumorigenic cells, tissues, and organs, making this recombinant protein an attractive molecule for anticancer therapy. The cytotoxic activity of Amblyomin-X on tumor cells has led to vast exploration into this protein. Here, we summarize the function, action mechanisms, structural features, pharmacokinetics, and biodistribution of this tick Kunitz-type inhibitor recombinant protein as a promising novel antitumor drug candidate.

Host Immune Responses to Salivary Components - A Critical Facet of Tick-Host Interactions

Tick sialome is comprised of a rich cocktail of bioactive molecules that function as a tool to disarm host immunity, assist blood-feeding, and play a vibrant role in pathogen transmission. The adaptation of the tick's blood-feeding behavior has lead to the evolution of bioactive molecules in its saliva to assist them to overwhelm hosts' defense mechanisms. During a blood meal, a tick secretes different salivary molecules including vasodilators, platelet aggregation inhibitors, anticoagulants, anti-inflammatory proteins, and inhibitors of complement activation; the salivary repertoire changes to meet various needs such as tick attachment, feeding, and modulation or impairment of the local dynamic and vigorous host responses. For instance, the tick's salivary immunomodulatory and cement proteins facilitate the tick's attachment to the host to enhance prolonged blood-feeding and to modulate the host's innate and adaptive immune responses. Recent advances implemented in the field of "omics" have substantially assisted our understanding of host immune modulation and immune inhibition against the molecular dynamics of tick salivary molecules in a crosstalk between the tick-host interface. A deep understanding of the tick salivary molecules, their substantial roles in multifactorial immunological cascades, variations in secretion, and host immune responses against these molecules is necessary to control these parasites. In this article, we reviewed updated knowledge about the molecular mechanisms underlying host responses to diverse elements in tick saliva throughout tick invasion, as well as host defense strategies. In conclusion, understanding the mechanisms involved in the complex interactions between the tick salivary components and host responses is essential to decipher the host defense mechanisms against the tick evasion strategies at tick-host interface which is promising in the development of effective anti-tick vaccines and drug therapeutics.

The trypsin inhibitor-like domain is required for a serine protease inhibitor of Haemonchus contortus to inhibit host coagulation

Haemonchus contortus, a blood-feeding nematode, inhibits blood coagulation at the site of infection to facilitate blood-sucking and digesting for successful parasitism. However, the mechanism underlying anti-coagulation at the host-parasite interface is largely unknown. In the current study, Hc-spi-i8, which has two greatly different transcripts named Hc-spi-i8a and Hc-spi-i8b, respectively, was described. Hc-SPI-I8A was a serine protease inhibitor containing a trypsin inhibitor-like cysteine rich (TIL) domain, while Hc-SPI-I8B was not. Hc-SPI-I8A/B were primarily expressed in the hypodermis, intestines and gonads in the parasitic stages of H. contortus. Hc-SPI-I8A interacted with Ovis aries TSP1-containing protein (OaTSP1CP), which was determined by yeast two-hybrid, co-immunoprecipitation (Co-IP), pull down and co-localization experiments. The blood clotting time contributed by the TIL domain was prolonged by Hc-SPI-I8A. Hc-SPI-I8A is most likely interfering in the extrinsic coagulation cascade by interacting with OaTSP1CP through its TIL domain and intrinsic coagulation cascade by an unknown mechanism. These findings depict a crucial point in the host-parasite interaction during H. contortus colonization, which should contribute to drug discovery and vaccine development in fighting against this important parasite worldwide.

Changing the Recipe: Pathogen Directed Changes in Tick Saliva Components

Ticks are obligate hematophagous parasites and are important vectors of a wide variety of pathogens. These pathogens include spirochetes in the genus Borrelia that cause Lyme disease, rickettsial pathogens, and tick-borne encephalitis virus, among others. Due to their prolonged feeding period of up to two weeks, hard ticks must counteract vertebrate host defense reactions in order to survive and reproduce. To overcome host defense mechanisms, ticks have evolved a large number of pharmacologically active molecules that are secreted in their saliva, which inhibits or modulates host immune defenses and wound healing responses upon injection into the bite site. These bioactive molecules in tick saliva can create a privileged environment in the host’s skin that tick-borne pathogens take advantage of. In fact, evidence is accumulating that tick-transmitted pathogens manipulate tick saliva composition to enhance their own survival, transmission, and evasion of host defenses. We review what is known about specific and functionally characterized tick saliva molecules in the context of tick infection with the genus Borrelia, the intracellular pathogen Anaplasma phagocytophilum, and tick-borne encephalitis virus. Additionally, we review studies analyzing sialome-level responses to pathogen challenge.

Amblyomma sculptum Salivary Protease Inhibitors as Potential Anti-Tick Vaccines

Amblyomma sculptum is the main tick associated with human bites in Brazil and the main vector of Rickettsia rickettsii, the causative agent of the most severe form of Brazilian spotted fever. Molecules produced in the salivary glands are directly related to feeding success and vector competence. In the present study, we identified sequences of A. sculptum salivary proteins that may be involved in hematophagy and selected three proteins that underwent functional characterization and evaluation as vaccine antigens. Among the three proteins selected, one contained a Kunitz_bovine pancreatic trypsin inhibitor domain (named AsKunitz) and the other two belonged to the 8.9 kDa and basic tail families of tick salivary proteins (named As8.9kDa and AsBasicTail). Expression of the messenger RNA (mRNA) encoding all three proteins was detected in the larvae, nymphs, and females at basal levels in unfed ticks and the expression levels increased after the start of feeding. Recombinant proteins rAs8.9kDa and rAsBasicTail inhibited the enzymatic activity of factor Xa, thrombin, and trypsin, whereas rAsKunitz inhibited only thrombin activity. All three recombinant proteins inhibited the hemolysis of both the classical and alternative pathways; this is the first description of tick members of the Kunitz and 8.9kDa families being inhibitors of the classical complement pathway. Mice immunization with recombinant proteins caused efficacies against A. sculptum females from 59.4% with rAsBasicTail immunization to more than 85% by immunization with rAsKunitz and rAs8.9kDa. The mortality of nymphs fed on immunized mice reached 70–100%. Therefore, all three proteins are potential antigens with the possibility of becoming a new tool in the control of A. sculptum.

Insights into the Role of Tick Salivary Protease Inhibitors during Ectoparasite-Host Crosstalk

Protease inhibitors (PIs) are ubiquitous regulatory proteins present in all kingdoms. They play crucial tasks in controlling biological processes directed by proteases which, if not tightly regulated, can damage the host organism. PIs can be classified according to their targeted proteases or their mechanism of action. The functions of many PIs have now been characterized and are showing clinical relevance for the treatment of human diseases such as arthritis, hepatitis, cancer, AIDS, and cardiovascular diseases, amongst others. Other PIs have potential use in agriculture as insecticides, anti-fungal, and antibacterial agents. PIs from tick salivary glands are special due to their pharmacological properties and their high specificity, selectivity, and affinity to their target proteases at the tick-host interface. In this review, we discuss the structure and function of PIs in general and those PI superfamilies abundant in tick salivary glands to illustrate their possible practical applications. In doing so, we describe tick salivary PIs that are showing promise as drug candidates, highlighting the most promising ones tested in vivo and which are now progressing to preclinical and clinical trials.

Sweet as honey, bitter as bile: Mitochondriotoxic peptides and other therapeutic proteins isolated from animal tissues, for dealing with mitochondrial apoptosis

Mitochondria are subcellular organelles involved in cell metabolism and cell life-cycle. Their role in apoptosis regulation makes them an interesting target of new drugs for dealing with cancer or rare diseases. Several peptides and proteins isolated from animal and plant sources are known for their therapeutic properties and have been tested on cancer cell-lines and xenograft murine models, highlighting their ability in inducing cell-death by triggering mitochondrial apoptosis. Some of those molecules have been even approved as drugs. Conversely, many other bioactive compounds are still under investigation for their proapoptotic properties. In this review we report about a group of peptides, isolated from animal venoms, with potential therapeutic properties related to their ability in triggering mitochondrial apoptosis. This class of compounds is known with different names, such as mitochondriotoxins or mitocans.

Modulation of stress and immune response by Amblyomin-X results in tumor cell death in a horse melanoma model

We have investigated Amblyomin-X-treated horse melanomas to better understand its mode of action through transcriptome analysis and the in vivo model. Amblyomin-X is a Kunitz-type homologous protein that selectively leads to the death of tumor cells via ER stress and apoptosis, currently under investigation as a new drug candidate for cancer treatment. Melanomas are immunogenic tumors, and a better understanding of the immune responses is warranted. Equine melanomas are spontaneous and not so aggressive as human melanomas are, as this study shows that the in vivo treatment of encapsulated horse melanoma tumors led to a significant reduction in the tumor size or even the complete disappearance of the tumor mass through intratumoral injections of Amblyomin-X. Transcriptome analysis identified ER- and mitochondria-stress, modulation of the innate immune system, apoptosis, and possibly immunogenic cell death activation. Interactome analysis showed that Amblyomin-X potentially interacts with key elements found in transcriptomics. Taken together, Amblyomin-X modulated the tumor immune microenvironment in different ways, at least contributing to induce tumor cell death.

Deciphering Biological Processes at the Tick-Host Interface Opens New Strategies for Treatment of Human Diseases

Ticks are obligatory blood-feeding ectoparasites, causing blood loss and skin damage in their hosts. In addition, ticks also transmit a number of various pathogenic microorganisms that cause serious diseases in humans and animals. Ticks evolved a wide array of salivary bioactive compounds that, upon injection into the host skin, inhibit or modulate host reactions such as hemostasis, inflammation and wound healing. Modulation of the tick attachment site in the host skin involves mainly molecules which affect physiological processes orchestrated by cytokines, chemokines and growth factors. Suppressing host defense reactions is crucial for tick survival and reproduction. Furthermore, pharmacologically active compounds in tick saliva have a promising therapeutic potential for treatment of some human diseases connected with disorders in hemostasis and immune system. These disorders are often associated to alterations in signaling pathways and dysregulation or overexpression of specific cytokines which, in turn, affect mechanisms of angiogenesis, cell motility and cytoskeletal regulation. Moreover, tick salivary molecules were found to exert cytotoxic and cytolytic effects on various tumor cells and have anti-angiogenic properties. Elucidation of the mode of action of tick bioactive molecules on the regulation of cell processes in their mammalian hosts could provide new tools for understanding the complex changes leading to immune disorders and cancer. Tick bioactive molecules may also be exploited as new pharmacological inhibitors of the signaling pathways of cytokines and thus help alleviate patient discomfort and increase patient survival. We review the current knowledge about tick salivary peptides and proteins that have been identified and functionally characterized in in vitro and/or in vivo models and their therapeutic perspective.

Chemical Equilibrium at the Tick-Host Feeding Interface:A Critical Examination of Biological Relevance in Hematophagous Behavior

Ticks secrete hundreds to thousands of proteins into the feeding site, that presumably all play important functions in the modulation of host defense mechanisms. The current review considers the assumption that tick proteins have functional relevance during feeding. The feeding site may be described as a closed system and could be treated as an ideal equilibrium system, thereby allowing modeling of tick-host interactions in an equilibrium state. In this equilibrium state, the concentration of host and tick proteins and their affinities will determine functional relevance at the tick-host interface. Using this approach, many characterized tick proteins may have functional relevant concentrations and affinities at the feeding site. Conversely, the feeding site is not an ideal closed system, but is dynamic and changing, leading to possible overestimation of tick protein concentration at the feeding site and consequently an overestimation of functional relevance. Ticks have evolved different possible strategies to deal with this dynamic environment and overcome the barrier that equilibrium kinetics poses to tick feeding. Even so, cognisance of the limitations that equilibrium binding place on deductions of functional relevance should serve as an important incentive to determine both the concentration and affinity of tick proteins proposed to be functional at the feeding site.

Preclinical evaluation of Amblyomin-X, a Kunitz-type protease inhibitor with antitumor activity

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Amblyomin-X is a recombinant protein with selective antitumor activity, causing tumor reduction in vivo.

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Acute and subchronic toxicity studies shows low toxicity in healthy mice, with reversible adverse effects.

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Amblyomin-X toxicity profile was defined.

Amblyomin-X, a Kunitz-type protease inhibitor, is a recombinant protein that selectively induces apoptosis in tumor cells and promotes tumor reduction in vivo in melanoma animal models. Furthermore, Amblyomin-X was able to drastically reduce lung metastasis in a mice orthotopic kidney tumor model. Due to its antitumor activity, Amblyomin-X potential to become a new drug is currently under investigation, therefore the aim of the present study was to perform preclinical assays to evaluate Amblyomin-X toxicity in healthy mice. Exploratory toxicity assays have shown that treatment with 512 mg/kg of Amblyomin-X lead to animal mortality, therefore two groups of treatment were evaluated in the present work: in the acute toxicity assay, animals were injected once with doses ranging from 4 to 256 mg/kg of Amblyomin-X, while in the subacute toxicity assay, animals were injected with 0.25, 0.57 and 1 mg/kg of Amblyomin-X daily, during 28 days. Following this treatment regimens, Amblyomin-X did not cause any mortality; moreover, toxicity signs were discrete, reversible and observed only at the higher doses, thus establishing a safety profile for administration in mice, which can be further used to determine the dose translation of this novel drug candidate for treatment in other species.

Amblyomin-X, a recombinant Kunitz-type inhibitor, regulates cell adhesion and migration of human tumor cells

In a tumor microenvironment, endothelial cell migration and angiogenesis allow cancer to spread to other organs causing metastasis.  Indeed, a number of molecules that are involved in cytoskeleton re-organization and intracellular signaling have been investigated for their effects on tumor cell growth and metastasis. Alongside that, Amblyomin-X, a recombinant Kunitz-type protein, has been shown to reduce metastasis and tumor growth in in vivo experiments. In the present report, we provide a mechanistic insight to these antitumor effects, this is,  Amblyomin-X modulates Rho-GTPases and uPAR signaling, and reduces the release of MMPs, leading to disruption of the actin cytoskeleton and decreased cell migration of tumor cell lines. Altogether, our data support a role for Amblyomin-X as a novel potential antitumor drug.

ABBREVIATIONS

Amb-X: Amblyomin-X; ECGF: endotelial cell growth factor; ECM: extracellular matrix; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; HUVEC: human umbilical vein endothelial cell; LRP1: low-density lipoprotein receptor-related protein; MMP: matrix metalloproteinase; HPI-4: hedgehog pathway inhibitor 4; PAI-1: plasminogen activator inhibitor 1; PMA: phorbol 12-myristate-13-acetate; TFPI: tissue factor pathway inhibitor; uPA: urokinase plasminogen activator; uPAR: uPA receptor.

The Essential Role of Tick Salivary Glands and Saliva in Tick Feeding and Pathogen Transmission

As long-term pool feeders, ticks have developed myriad strategies to remain discreetly but solidly attached to their hosts for the duration of their blood meal. The critical biological material that dampens host defenses and facilitates the flow of blood-thus assuring adequate feeding-is tick saliva. Saliva exhibits cytolytic, vasodilator, anticoagulant, anti-inflammatory, and immunosuppressive activity. This essential fluid is secreted by the salivary glands, which also mediate several other biological functions, including secretion of cement and hygroscopic components, as well as the watery component of blood as regards hard ticks. When salivary glands are invaded by tick-borne pathogens, pathogens may be transmitted via saliva, which is injected alternately with blood uptake during the tick bite. Both salivary glands and saliva thus play a key role in transmission of pathogenic microorganisms to vertebrate hosts. During their long co-evolution with ticks and vertebrate hosts, microorganisms have indeed developed various strategies to exploit tick salivary molecules to ensure both acquisition by ticks and transmission, local infection and systemic dissemination within the vertebrate host.

Joannsin, a novel Kunitz-type FXa inhibitor from the venom of Prospirobolus joannsi

The repugnatorial glands of millipedes release various defensive chemical secretions. Although varieties of such defensive secretions have been studied, none of them is protein or peptide. Herein, a novel factor Xa (FXa) inhibitor named joannsin was identified and characterised from repugnatorial glands of Prospirobolus joannsi. Joannsin is composed of 72 amino acid residues including six cysteines, which form three intra-molecular disulfide bridges. It is a member of Kunitz-type protease inhibitor family, members of which are also found in the secretory glands of other arthropods. Recombinant joannsin exhibited remarkable inhibitory activity against trypsin and FXa with a Ki of 182.7 ± 14.6 and 29.5 ± 4.7 nM, respectively. Joannsin showed strong anti-thrombosis functions in vitro and in vivo. Joannsin is the first peptide component in millipede repugnatorial glands to be identified and is a potential candidate and/or template for the development of anti-thrombotic agents. These results also indicated that there is Kunitz-type protease inhibitor toxin in millipede repugnatorial glands as in other arthropods secretory glands.

Serine Protease Inhibitors in Ticks: An Overview of Their Role in Tick Biology and Tick-Borne Pathogen Transmission

New tick and tick-borne pathogen control approaches that are both environmentally sustainable and which provide broad protection are urgently needed. Their development, however, will rely on a greater understanding of tick biology, tick-pathogen, and tick-host interactions. The recent advances in new generation technologies to study genomes, transcriptomes, and proteomes has resulted in a plethora of tick biomacromolecular studies. Among these, many enzyme inhibitors have been described, notably serine protease inhibitors (SPIs), whose importance in various tick biological processes is only just beginning to be fully appreciated. Among the multiple active substances secreted during tick feeding, SPIs have been shown to be directly involved in regulation of inflammation, blood clotting, wound healing, vasoconstriction and the modulation of host defense mechanisms. In light of these activities, several SPIs were examined and were experimentally confirmed to facilitate tick pathogen transmission. In addition, to prevent coagulation of the ingested blood meal within the tick alimentary canal, SPIs are also involved in blood digestion and nutrient extraction from the meal. The presence of SPIs in tick hemocytes and their involvement in tick innate immune defenses have also been demonstrated, as well as their implication in hemolymph coagulation and egg development. Considering the involvement of SPIs in multiple crucial aspects of tick-host-pathogen interactions, as well as in various aspects of the tick parasitic lifestyle, these molecules represent highly suitable and attractive targets for the development of effective tick control strategies. Here we review the current knowledge regarding this class of inhibitors in tick biology and tick-borne pathogen transmission, and their potential as targets for future tick control trials.

Revisiting antithrombotic therapeutics; sculptin, a novel specific, competitive, reversible, scissile and tight binding inhibitor of thrombin

Thrombin is a multifunctional enzyme with a key role in the coagulation cascade. Its functional modulation can culminate into normal blood coagulation or thrombosis. Thus, the identification of novel potent inhibitors of thrombin are of immense importance. Sculptin is the first specific thrombin inhibitor identified in the transcriptomics analysis of tick’s salivary glands. It consists of 168 residues having four similar repeats and evolutionary diverged from hirudin. Sculptin is a competitive, specific and reversible inhibitor of thrombin with a K_i of 18.3 ± 1.9 pM (k_on 4.04 ± 0.03 × 10⁷ M⁻¹ s⁻¹ and k_off 0.65 ± 0.04 × 10⁻³ s⁻¹). It is slowly consumed by thrombin eventually losing its activity. Contrary, sculptin is hydrolyzed by factor Xa and each polypeptide fragment is able to inhibit thrombin independently. A single domain of sculptin alone retains ~45% of inhibitory activity, which could bind thrombin in a bivalent fashion. The formation of a small turn/helical-like structure by active site binding residues of sculptin might have made it a more potent thrombin inhibitor. In addition, sculptin prolongs global coagulation parameters. In conclusion, sculptin and its independent domain(s) have strong potential to become novel antithrombotic therapeutics.