Endogenous Phospholipase A2 Inhibitors in Snakes: a Brief Overview
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Campos et al. Journal of Venomous Animals and Toxins including Tropical Diseases (2016) 22:37 DOI 10.1186/s40409-016-0092-5 REVIEW Open Access Endogenous phospholipase A2 inhibitors in snakes: a brief overview Patrícia Cota Campos†, Lutiana Amaral de Melo†, Gabriel Latorre Fortes Dias and Consuelo Latorre Fortes-Dias* Abstract The blood plasma of numerous snake species naturally comprises endogenous phospholipase A2 inhibitors, which primarily neutralize toxic phospholipases A2 that may eventually reach their circulation. This inhibitor type is generally known as snake blood phospholipase A2 inhibitors (sbPLIs). Most, if not all sbPLIs are oligomeric glycosylated proteins, although the carbohydrate moiety may not be essential for PLA2 inhibition in every case. The presently known sbPLIs belong to one of three structural classes – namely sbαPLI, sbβPLI or sbγPLI – depending on the presence of characteristic C-type lectin-like domains, leucine-rich repeats or three-finger motifs, respectively. Currently, the most numerous inhibitors described in the literature are sbαPLIs and sbγPLIs, whereas sbβPLIs are rare. When the target PLA2 is a Lys49 homolog or an Asp49 myotoxin, the sbPLI is denominated a myotoxin inhibitor protein (MIP). In this brief overview, the most relevant data on sbPLIs will be presented. Representative examples of sbαPLIs and sbγPLIs from two Old World – Gloydius brevicaudus and Malayopython reticulatus – and two New World – Bothrops alternatus and Crotalus durissus terrificus – snake species will be emphasized. Keywords: PLA2 inhibitor, Phospholipase A2, Snake blood, Natural resistance, Snakes Background domain of Ca2+-dependent lectins, and preferentially in- A number of venomous and nonvenomous snake species hibit acidic PLA2s. Beta-type inhibitors (sbβPLIs) exhibit are naturally resistant to the deleterious actions of snake tandem leucine-rich repeats (LRRs), and specifically inhibit venom components, in many cases due to the presence basic PLA2s. Gamma inhibitors (sbγPLIs) display a three- of specific antitoxins in their circulating blood [1–10]. finger pattern and are less specific than the aforementioned These antitoxins were identified as liver-secreted pro- classes, therefore inhibiting neutral, acidic and basic PLA2s teins, which prevent any possible damage from toxins from snake venoms. The structural classification of sbPLIs that might have reached the snake’s blood stream [11]. has been adopted by most authors working on the subject, Among these inhibitors, phospholipase A2 inhibitors or but the selectivity concept is not absolute [13–16]. In gene- snake blood phospholipase A2 inhibitors (sbPLIs) play a ral, α and γ sbPLIs simultaneously occur in several snake key role in this type of endogenous resistance. species, while sbβPLIs have only been reported in three During the 80’s and 90’s, a number of sbPLIs were snake species. purified from different snake species. The first authors Native sbPLIs are usually homo- or heterooligomers of to identify various sbPLIs in a single snake species – glycosylated and/or non-glycosylated subunits. Carbohy- Gloydius brevicaudus, formerly Agkistrodon blomhoffii drates do not seem essential for the inhibition of PLA2 siniticus – proposed a classification based on the presence by sbPLIs, since some of them remain functional in the of characteristic domains of known mammalian proteins in absence of this moiety [16–20]. When the target PLA2s their structure and on variations in their PLA2 selectivity are Lys49 homologues or Asp49 myotoxins, the sbPLIs [12]. Alpha sbPLIs (sbαPLIs) have a C-type lectin-like do- are specifically called myotoxin inhibitor proteins (MIPs) main that is highly similar to the carbohydrate recognition [13, 14, 16, 21, 22]. The following sections present the most relevant char- * Correspondence: [email protected] † acteristics of the three classes of sbPLIs. Subsequently, Equal contributors α γ Laboratory of Enzymology, Center of Research and Development, Ezequiel examples of sb PLIs and sb PLIs from two Old World Dias Foundation (FUNED), Rua Conde Pereira Carneiro, 80, Gameleira, Belo snake species — Gloydius brevicaudus and Malayopython Horizonte, MG CEP 30510-010, Brazil © The Author(s). 2016 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Campos et al. Journal of Venomous Animals and Toxins including Tropical Diseases (2016) 22:37 Page 2 of 7 reticulatus — and two New World ones — Bothrops these homologs failed to inhibit all tested snake venom alternatus and Crotalus durissus terrificus — will be PLA2s [26, 27]. introduced. The sbαPLI from Asian Gloydius brevicaudus (GbαPLI) The sbαPLI from G. brevicaudus (formerly Agkistrodon Alpha class of sbPLIs (sbαPLIs) blomhoffii siniticus) is a homotrimer, in which the α- Members of this class of inhibitors are found in solution helical coiled-coil neck subunit forms a central pore that as homo- or heterooligomers, with molecular masses constitutes the binding site for the target PLA2s[28–30]. between 75 kDa and 120 kDa (Table 1). The C-type lectin-like domain was discarded as respon- In addition to the typical C-type lectin-like domain, sible for PLA2 binding [30]. sbαPLI monomers present two other highly conserved The correct configuration of the central pore in GbαPLI regions in their structure: a hydrophobic core at their is controlled by the primary structures of the α-helical carboxy-terminus and an α-helical coiled-coil neck com- coiled-coil neck in the formation of subunits. Chimeric prising the 13th to 36th amino acid segment in the mature constructions of GbαPLI and the non-functional sbαPLI protein [23, 24]. The last amino acid stretch corresponds to homolog from E. quadrivirgata allowed the mapping of the exon 3 reported for the gene of the sbPLI from Protobo- important amino acids for PLA2 inhibition in the 13–36 throps flavovoridis (formerly Trimeresurus flavoviridis) [25]. segment, which are expected to be located in the heli- Besides the functional sbαPLIs, non-functional homologs cal neck of the GbαPLI trimer based on the three- were purified from the blood serum of two nonvenomous dimensional structural model constructed by homology species, Elaphe quadrivirgata and E. climacophora.Despite modeling [29, 30]. The trimerization occurs only among displaying not only molecular masses, but also primary subunits having the same α-helical motif in the regions and quaternary structures comparable to classical sbαPLIs, 13–36 and the oligomer is structurally stabilized by inter- molecular electrostatic interactions. Two charged resi- dues, E23 and K28, have been found specifically responsible Table 1 Snake blood PLA2 inhibitors in the alpha structural class (sbαPLIs) for these essential interactions between the forming sub- Family, species Common name Reference units in the trimer. The contribution of each subunit to or subspecies the total inhibitory activity of trimeric GbαPLI has also Colubridae been investigated. In the trimer, the inhibitory action is driven by one subunit with the highest affinity and is not Elaphe climacophora Japanese ratsnake [26] affected by the number of subunits of this type [29]. Elaphe quadrivirgata Japanese four-lined [27] α ratsnake Gb PLI displays lower affinities (about 2000-fold less) for neutral or basic PLA s from the homologous venom Viperidae 2 compared to acidic PLA2s. In the absence of carbohy- Bothrops alternatus Urutu (Portuguese) [15, 31]a,[23] drates, the inhibition of acidic and neutral PLA2shas Bothrops asper Fer-de-lance, Terciopelo [62] been reported to remain unchanged, while the inhibition (Spanish) of basic PLA2s is affected [19]. The possibility of different Bothrops erythromelas Caatinga lancehead [23] inhibition mechanisms, depending on the ionic character Bothrops jararaca Jararaca (Port.) [23] of the target PLA2, has been attributed to GbαPLI and Bothrops jararacussu Jararacussu (Port.) [14, 23] other sbαPLIs, but further studies are required to clarify Bothrops moojeni Brazilian lancehead [16] this issue. Bothrops neuwiedi Jararaca pintada (Port.) [23] The sbαPLI from Latin American Bothrops alternatus Cerrophidion godmani Honduras montane [21] pit viper (BaltMIP) This inhibitor was purified from the blood serum of Crotalus durissus South American rattlesnake, [23] terrificus tropical rattlesnake Bothrops alternatus snakes by affinity chromatography a using bothropstoxin I – a basic Lys49 PLA from the Gloydius brevicaudus Short-tailed mamushi, [19] ,[63] 2 Japanese or Chinese homologous venom – as the immobilized ligand. The mamushi, monomer of BaltMIP is composed of a single polypep- Lachesis muta South American [23] tide chain with apparent molecular mass of 24 kDa. The bushmaster native molecule is able to inhibit myotoxicity and cyto- a Protobothrops Habu [25, 64, 65], [66] toxicity caused by both Lys49 and Asp49 PLA2s, possibly flavoviridis by different mechanisms depending on the type of en- Protobothrops elegans Sakishima habu [41] zyme to be inhibited [15]. Amino acid residues