Calcium-binding analysis and molecular modeling reveal echis coagulation factor IX/factor X-binding protein has the Ca-binding properties and Ca ion-independent folding of other C-type lectin-like proteins

Philodryas (Serpentes: Dipsadidae) Envenomation, a Neglected Issue in Chile

Snakebite envenomation is considered a neglected tropical disease, although it also occurs outside the tropics. In this work, we analyzed the literature on Philodryas species in Chile (Philodryaschamissonis, P.simonsii, and P.tachymenoides) from 1834 to 2019, searching for epidemiological, clinical, and molecular aspects of envenomation. Ninety-one percent of the studies found regarded taxonomy, ecology, and natural history, suggesting that snakebites and venom toxins are a neglected issue in Chile. All snakebite cases reported and toxicological studies concerned the species Philodryaschamissonis. Using 185 distributional records from the literature and museum collections for this species, we show for the first time that the reported snakebite cases correlate with human population density, occurring in the Valparaiso and Metropolitan regions in Central Chile. The reduced number of snakebite cases, which were previously considered as having a low incidence in Chile, may be a consequence of under-reported cases, probably due to the inadequate publication or scarce research on this issue. Absence of information about official pharmacological treatment, post-envenoming sequels, clinical management of particular patient groups (e.g., with non-communicable diseases, pregnant women, and the elderly) was also detected. In conclusion, despite having over 185 years of literature on Chilean snakes, knowledge on the envenomation of Philodryas genus remains scarce, seriously affecting adequate medical handling during an ophidic accident. This review highlights the need to develop deep research in this area and urgent improvements to the management of this disease in Chile.

Identification and molecular characterization of five putative toxins from the venom gland of the snake Philodryas chamissonis (Serpentes: Dipsadidae)

Philodryas chamissonis is a rear-fanged snake endemic to Chile. Its bite produces mild to moderate symptoms with proteolytic and anti-coagulant effects. Presently, the composition of the venom, as well as, the biochemical and structural characteristics of its toxins, remains unknown. In this study, we cloned and reported the first full-length sequences of five toxin-encoding genes from the venom gland of this species: Type III snake venom metalloprotease (SVMP), snake venom serine protease (SVSP), Cysteine-rich secretory protein (CRISP), α and β subunits of C-type lectin-like protein (CLP) and C-type natriuretic peptide (NP). These genes are highly expressed in the venom gland and their sequences exhibited a putative signal peptide, suggesting that these are components of the venom. These putative toxins had different evolutionary relationships with those reported for some front-fanged snakes, being SVMP, SVSP and CRISP of P. chamissonis closely related to the toxins present in Elapidae species, while NP was more related to those of Viperidae species. In addition, analyses suggest that the α and β subunits of CLP of P. chamissonis might have a α-subunit scaffold in common with Viperidae species, whose highly variable C-terminal region might have allowed the diversification in α and β subunits. Our results provide the first molecular description of the toxins possibly implicated in the envenomation of prey and humans by the bite of P. chamissonis.

Factor IXa as a target for anticoagulation in thrombotic disorders and conditions

From acute coronary syndrome (ACS) to the prevention of cardioembolic events in patients with atrial fibrillation and thrombosis of mechanical heart valves, there is a quest to develop a new generation of anticoagulants. Perhaps the 'holy grail' of antithrombotic therapy is not only a drug that will prevent coagulation without promoting bleeding but also an anticoagulant that is easily reversible should the clinical need arise. Further, an optimally designed anticoagulant would have broad applications to include arterial, venous, hybrid conditions (atrial flutter and fibrillation) and nonbiological materials. Factor (F)IXa plays a pivotal part in tissue factor (TF)-mediated thrombin generation, and therefore represents a potentially promising target for drug development. FIXa activity has been targeted by multiple modalities, including oral inhibitors, RNA aptamers, monoclonal antibodies and synthetic active-site-blocking competitive inhibitors. Herein, we summarize the biochemistry of FIXa as it applies to thrombotic disorders and conditions, as well as the evolution of targeted therapies.

Factor IXa as a target for pharmacologic inhibition in acute coronary syndrome

Anticoagulant therapy, combined with platelet-directed inhibitors, represents a standard-of-care in the management of patients with acute coronary syndrome, particularly those who require percutaneous coronary interventions. While a vast clinical experience, coupled with large clinical trials have collectively provided guidance, an optimal anticoagulant drug and applied strategy, defined as one that reduces thrombotic and hemorrhagic events consistently, with minimal off-target effects and active control of systemic anticoagulation according to patient and clinical-setting specific need, remains at large. An advancing knowledge of coagulation, hemostasis, and thrombosis suggests that factor IXa, a protease that governs thrombin generation in common thrombotic disorders may represent a prime target for pharmacologic inhibition.

Binding of Ca2+ and Zn2+ to factor IX/X-binding protein from venom of Agkistrodon halys Pallas: stabilization of the structure during GdnHCl-induced and thermally induced denaturation

Coagulation factor IX/coagulation factor X binding protein from the venom of Agkistrodon halys Pallas (AHP IX/X-bp) is a unique coagulation factor IX/coagulation factor X binding protein (IX/X-bp). Among all IX/X-bps identified, only AHP IX/X-bp is a Ca(2+)- and Zn(2+)-binding protein. The binding properties of Ca(2+) and Zn(2+) ions binding to apo-AHP IX/X-bp and their effects on the stability of the protein have been investigated by isothermal titration calorimetry, fluorescence spectroscopy, and differential scanning calorimetry. The results show that AHP IX/X-bp has two metal binding sites, one specific for Ca(2+) with lower affinity for Zn(2+) and one specific for Zn(2+) with lower affinity for Ca(2+). The bindings of Ca(2+) and Zn(2+) in the two sites are entropy- and enthalpy-driven. The binding affinity of AHP IX/X-bp for Zn(2+) is 1 order of magnitude higher than for Ca(2+) for either high-affinity binding or low-affinity binding, which accounts for the existence of one Zn(2+) in the purified AHP IX/X-bp. Guanidine hydrochloride (GdnHCl)-induced and thermally induced denaturations of Ca(2+)-Ca(2+)-AHP IX/X-bp, Zn(2+)-Zn(2+)-AHP IX/X-bp, and Ca(2+)-Zn(2+)-AHP IX/X-bp are all a two-state processes with no detectable intermediate state(s), indicating the Ca(2+)/Zn(2+)-induced tight packing of the protein. Ca(2+) and Zn(2+) increase the structural stability of AHP IX/X-bp against GdnHCl or thermal denaturation to a similar extent. Although Ca(2+) and Zn(2+) have no obvious effect on the secondary structure of AHP IX/X-bp, they induce different rearrangements in local conformation. The Zn(2+)-stabilized specific conformation of AHP IX/X-bp may be helpful to its recognition of the structure of coagulation factor IX. This work suggests that in vitro, Ca(2+) plays a structural rather than an active role in the anticoagulation of AHP IX/X-bp, whereas Zn(2+) plays both structural and active roles in the anticoagulation. In blood, Ca(2+) binds to AHP IX/X-bp and stabilizes its structure, whereas Zn(2+) cannot bind to AHP IX/X-bp owing to the low Zn(2+) concentration. AHP IX/X-bp prolongs the clotting time in vivo through its binding only with coagulation factor X/activated coagulation factor X.

New insights into the functions and N-glycan structures of factor X activator from Russell's viper venom

The coagulation factor X activator from Russell's viper venom (RVV-X) is a heterotrimeric glycoprotein. In this study, its three subunits were cloned and sequenced from the venom gland cDNAs of Daboia siamensis. The deduced heavy chain sequence contained a C-terminal extension with four additional residues to that published previously. Both light chains showed 77-81% identity to those of a homologous factor X activator from Vipera lebetina venom. Far-western analyses revealed that RVV-X could strongly bind protein S, in addition to factors X and IX. This might inactivate protein S and potentiate the disseminated intravascular coagulation syndrome elicited by Russell's viper envenomation. The N-glycans released from each subunit were profiled and sequenced by MALDI-MS and MS/MS analyses of the permethyl derivatives. All the glycans, one on each light chain and four on the heavy chain, showed a heterogeneous pattern, with a combination of variable terminal fucosylation and sialylation on multiantennary complex-type sugars. Amongst the notable features were the presence of terminal Lewis and sialyl-Lewis epitopes, as confirmed by western blotting analyses. As these glyco-epitopes have specific receptors in the vascular system, they possibly contribute to the rapid homing of RVV-X to the vascular system, as supported by the observation that slower and fewer fibrinogen degradation products are released by desialylated RVV-X than by native RVV-X.

Bothrojaracin, a Bothrops jararaca Snake Venom-Derived (Pro)Thrombin Inhibitor, as an Anti-Thrombotic Molecule

Bothrojaracin (BJC) is a selective and potent thrombin inhibitor (KD = 0.6 nM) which also binds to prothrombin on proexosite I (KD = 175 nM). Incubation of BJC with human or rat plasma produced a band that co-migrates with purified prothrombin-BJC complex. We further analyzed the in vivo anti-thrombotic effect of BJC on a venous thrombosis model in rats that combines stasis and hypercoagulability. The administration of 1 mg/kg (i.v.) doses of BJC decreased thrombus weight by approximately 95%. Evaluation of the in vivo effect of BJC in mice using a pulmonary thromboembolism model induced by thrombin showed that BJC protects 100% of mice from death. Altogether, our data show that BJC is a potent anti-thrombotic agent that could further help the development of new prothrombin-directed drugs.

pH-Dependent Structural Changes at Ca2+-binding Sites of Coagulation Factor IX-binding Protein

Coagulation factor IX-binding protein, isolated from Trimeresurus flavoviridis (IX-bp), is a C-type lectin-like protein. It is an anticoagulant consisting of homologous subunits, A and B. Each subunit has a Ca(2+)-binding site with a unique affinity (K(d) values of 14muM and 130muM at pH 7.5). These binding characteristics are pH-dependent and, under acidic conditions, the Ca(2+) binding of the low-affinity site was reduced considerably. In order to identify which site has high affinity and to investigate the pH-dependent Ca(2+) release mechanism, we have determined the crystal structures of IX-bp at pH 6.5 and pH 4.6 (apo form), and compared the Ca(2+)-binding sites with each other and with those of the solved structures under alkaline conditions; pH 7.8 and pH 8.0 (complexed form). At pH 6.5, Glu43 in the Ca(2+)-binding site of subunit A displayed two conformations. One (minor) is that in the alkaline state, and the other (major) is that at pH 4.6. However, the corresponding Gln43 residue of subunit B is in only a single conformation, which is almost identical with that in the alkaline state. At pH 4.6, Glu43 of subunit A adopts a conformation similar to that of the major conformer observed at pH 6.5, while Gln43 of subunit B assumes a new conformation, and both Ca(2+) positions are occupied by water molecules. These results showed that Glu43 of subunit A is much more sensitive to protonation than Gln43 of subunit B, and the conformational change of Glu43 occurs around pH6.5, which may correspond to the step of Ca(2+) release.

Structures and functions of snake venom CLPs (C-type lectin-like proteins) with anticoagulant-, procoagulant-, and platelet-modulating activities

C-type lectin-like proteins (CLPs) have a variety of biological activities, including anticoagulant- and platelet-modulating activities but have no lectin activity. CLPs are made up of heterodimers or oligomers of heterodimers, while C-type lectins from snake venom are composed exclusively of homodimers or homooligomers. In the last decade, numerous CLPs, such as blood coagulation factor IX/X-binding protein and botrocetin, have been isolated from various snake venoms, sequenced, and characterized. In addition, RVV-X (factor X activator) and carinactivase-1 (prothrombin activator) are metalloproteases composed of two C-type lectin-like domains that recognize the Gla domain of factor X and prothrombin, respectively. The basic structures of these CLPs include two homologous subunits: subunit alpha (A chain) of 14-15 kDa and subunit beta (B chain) of 13-14 kDa. CLPs occur in a variety of oligomeric forms, including alphabeta, (alphabeta)(2), and (alphabeta)(4). The basic homologous dimer (alphabeta) of these CLPs is formed by three-dimensional (3D) domain swapping. The CLPs constitute a new protein family and are useful tools for elucidating the mechanisms involved in clotting and platelet activation as well as the structure-function relationships of both blood clotting factors and platelet glycoproteins.

Novel sequences encoding venom C-type lectins are conserved in phylogenetically and geographically distinct Echis and Bitis viper species

Envenoming by Echis saw scaled vipers and Bitis arietans puff adders is the leading cause of death and morbidity in Africa due to snake bite. Despite their medical importance, the composition and constituent functionality of venoms from these vipers remains poorly understood. Here, we report the cloning of cDNA sequences encoding seven clusters or isoforms of the haemostasis-disruptive C-type lectin (CTL) proteins from the venom glands of Echis ocellatus, E. pyramidum leakeyi, E. carinatus sochureki and B. arietans. All these CTL sequences encoded the cysteine scaffold that defines the carbohydrate-recognition domain of mammalian CTLs. All but one of the Echis and Bitis CTL sequences showed greater sequence similarity to the beta than alpha CTL subunits in venoms of related Asian and American vipers. Four of the new CTL clusters showed marked inter-cluster sequence conservation across all four viper species which were significantly different from that of previously published viper CTLs. The other three Echis and Bitis CTL clusters showed varying degrees of sequence similarity to published viper venom CTLs. Because viper venom CTLs exhibit a high degree of sequence similarity and yet exert profoundly different effects on the mammalian haemostatic system, no attempt was made to assign functionality to the new Echis and Bitis CTLs on the basis of sequence alone. The extraordinary level of inter-specific and inter-generic sequence conservation exhibited by the Echis and Bitis CTLs leads us to speculate that antibodies to representative molecules should neutralise the biological function of this important group of venom toxins in vipers that are distributed throughout Africa, the Middle East and the Indian subcontinent.