Study On The Mechanism Of Two Snake Venom C Type Lectin-like Proteins Assisting Vipers In Predation And Causing Thrombotic Microangiopathy

Snakebite is considered to be one of the most neglected diseases which causes a significant number of deaths and disabilities in tropical and subtropical rural areas.Snake venoms contain components that facilitate predation and defense either by neurotoxic or hemorrhagic effects.The venoms from elapids usually contain a high level of neurotoxins such as three-finger toxins(3FTxs)and phospholipases A2(PLA2),which block the signal transmission of the neuromuscular junction at the presynaptic or postsynaptic nerve terminals,thereby provoking flaccid paralysis,that lead to the rapid paralysis of their prey.While venoms from vipers always induce coagulopathy and platelet dysfunction,thus leading to bleeding disorders.The exact etiology or toxins responsible for thrombotic microangiopathy(TMA)after snake envenomation remain largely unknown.As one of the main nonenzymatic components in viper venoms,snake C-type lectin-like proteins(Snaclecs)usually have a heterodimeric structure with α and βsubunits,which are often oligomerized to form larger molecules,and have evolved to bind a wide range of physiologically important proteins such as GPIb,GPⅥ and integrins on platelets.Mucetin and stejnulxin are typical Snaclecs from the venom of Protobothrops mucrosquamatus and Trimeresurus stejnegeri,which are potent platelet agonists via activating GPIb and GPVI,respectively.however,the significance of Snaclecs for predation or defense and whether Snaclecs will lead to venom-induced consumption coagulopathy(VICC)or TMA has not been elucidated yet.Here,we show that the venoms from Protobothrops mucrosquamatus and Trimeresurus stejnegeri are comparable to those of Naja atra in prey immobilization.Snaclecs(mucetin and stejnulxin)from the venoms of P.mucrosquamatus and T.stejnegeri induce the aggregation of both mammalian platelets and avian thrombocytes at a concentration of 2 μg/mL,leading to acute cerebral ischemia at a concentration of 400μg/kg,and reduced animal locomotor activity and exploration in the open field test.Viper venoms in the absence of mucetin or stejnulxin fail to aggregate platelets and thrombocytes at a concentration of 2 μg/mL,and thus show an attenuated ability to cause cerebral ischemia and immobilization of their prey.Further studies indicate that intravenous injection of mucetin or stejnulxin in mice lead to TMA like symptoms,rapid thrombocytopenia and bleeding disorders with an acute consumption coagulopathy syndrome accompanying neurologic abnormality and other signs of organ injury through the activation of platelets.Meanwhile,we demonstrate that re-administration of mucetin or stejnulxin induces chronic immune thrombocytopenia mediated by platelet-associated IgG(PAIgG)in mice at different concentrations of 3.75,7.5 or 15μg/kg.Snaclecs,such as mucetin and stejnulxin,may play important roles in the predatory and defensive functions of snake venoms,as well as snake envenomation induced TMA symptoms,thrombocytopenia,and bleeding disorders.This work provides novel insights into the prey immobilization mechanism of Viperidae snakes.This experiment proves for the first time that Snaclecs are the key pathogenicity factors of viper envenomation-induced TMA symptoms,providing a potential target for the diagnosis and treatment of snakebite.