Perivitellin-2 (PV2) is a pore-forming toxin present in the egg perivitelline fluid of the apple snails Pomacea maculata (PmPV2) and Pomacea canaliculata (PcPV2). This protein, called perivitellin, is massively accumulated in the eggs (~20 % total protein). As a toxin PV2 protects eggs from predators, but it also nourishes the developing snail embryos.[1]
| Pomacea canaliculata perivitellin-2 | |
|---|---|
| Identifiers | |
| Organism | |
| Symbol | PcPV2 |
| Alt. symbols | PV2 |
Structure and stability
editThese ~172-kDa proteins are dimers of AB toxins, each composed of a carbohydrate-binding protein of the tachylectin family (targeting module) disulfide-linked to a pore-forming protein of the Membrane Attack Complex and Perforin (MACPF) family (toxic unit).[2][3] Like most other studied perivitellins from Pomacea snails, PV2s are highly stable in a wide range of pH values and withstand gastrointestinal digestion, characteristics associated with an antinutritive defense system that deters predation by lowering the nutritional value of the eggs.[2][1][4]
Functions
editAs part of the perivitelline fluid, perivitellin-2 constitutes a nutrient source for the developing embryo, notably in the last stages where it is probably used as an endogenous source of energy and structural molecules during the transition to the free life.[5] PV2s also play a role in a complex defense system that protects the embryos against predation.[2][6][1]
| Pomacea maculata perivitellin-2 | |
|---|---|
 | |
| Identifiers | |
| Organism | |
| Symbol | PmPV2 |
| Alt. symbols | PV2 |
PV2s have both lectin and perforin activities, associated to the two subunits of their particular structures.[2][3] As a lectin, PV2s can agglutinate rabbit red blood cells and bind to the plasma membrane of intestinal cells both in vitro and in vivo.[2][1][7] As a perforin, PV2s are able to disrupt intestinal cells altering the plasma membrane conductance and to form large pores in artificial lipid bilayers.[3] An interesting issue with these perivitellins is that the combination of two immune proteins (lectin and perforin) gave rise to a new toxic entity, an excellent example of protein exaptation.[2][3] This binary structure includes PV2s within “AB-toxins”, a group of toxins mostly described in bacteria and plants. In PV2 toxins, the lectin would bind to target membranes through the recognition of specific glycans, acting as a delivery “B” subunit, and then the pore-forming “A” subunit would disrupt lipid bilayers forming large pores and leading to cell death, therefore constituting a true pore-forming toxin.[3]
Toxicity toward mammals
editPV2 toxins proved to be highly toxic to mice when it enters the bloodstream (LD50, 96 h 0.25 mg/kg, i.p.) and those receiving sublethal doses displayed neurological signs including weakness and lethargy, low head and bent down position (ortopneic), half-closed eyes, taquipnea, hirsute hair, extreme abduction of the rear limbs, paresia and were not able to support their body weight (tetraplegic), among others.[3][6] Histopathological analyses of affected mice showed that PV2 toxins affect the dorsal horn of the spinal cord, particularly on the 2nd and 3rd gray matter laminas, where alters the calcium metabolism and causes neuron apoptosis.[6] Apart from its neurotoxicity, it has been recently shown that PV2s are also enterotoxic to mice when ingested, a function that had never been ascribed to animal proteins.[1] At the cellular level, PV2 is cytotoxic to intestinal cells, on which it causes changes in their surface morphology increasing the membrane roughness. At the system level, oral administration of PV2 induces large morphological changes on mice intestine mucosa, reducing its absorptive surface. Additionally, PV2 reaches the Peyer's patches where it activates lymphoid follicles and triggers apoptosis.[1]
Evolution of a pore-forming toxin
edit
Proteomic analysis indicates that the MACPF and the Tachylectins are among the most abundant proteins in Pomacea eggs but are minor proteins in the genera laying eggs below the water.[9][10] According to the fossil record, some 3 MYA, when Pomacea diverged from Marisa and began laying eggs above the water, these two genes were subjected to extensive duplication and these unrelated proteins were combined by a covalent bond resulting in the dimerization into PV2 AB toxin that co-opted to new roles. This new structure rendered a novel toxin that is non-digestible, enterotoxic and neurotoxic.[3][1]
References
edit- ^ a b c d e f g Giglio ML, Ituarte S, Ibañez AE, Dreon MS, Prieto E, Fernández PE, Heras H (2020). "Novel Role for Animal Innate Immune Molecules: Enterotoxic Activity of a Snail Egg MACPF-Toxin". Frontiers in Immunology. 11: 428. doi:10.3389/fimmu.2020.00428. PMC 7082926. PMID 32231667.
- ^ a b c d e f Dreon MS, Frassa MV, Ceolín M, Ituarte S, Qiu JW, Sun J, et al. (2013-05-30). "Novel animal defenses against predation: a snail egg neurotoxin combining lectin and pore-forming chains that resembles plant defense and bacteria attack toxins". PLOS ONE. 8 (5): e63782. Bibcode:2013PLoSO...863782D. doi:10.1371/journal.pone.0063782. PMC 3667788. PMID 23737950.
- ^ a b c d e f g Giglio ML, Ituarte S, Milesi V, Dreon MS, Brola TR, Caramelo J, et al. (August 2020). "Exaptation of two ancient immune proteins into a new dimeric pore-forming toxin in snails". Journal of Structural Biology. 211 (2): 107531. doi:10.1016/j.jsb.2020.107531. hdl:11336/143650. PMID 32446810. S2CID 218873723.
- ^ Frassa MV, Ceolín M, Dreon MS, Heras H (July 2010). "Structure and stability of the neurotoxin PV2 from the eggs of the apple snail Pomacea canaliculata". Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics. 1804 (7): 1492–1499. doi:10.1016/j.bbapap.2010.02.013. PMID 20215051.
- ^ Heras H, Garin CF, Pollero RJ (1998). "Biochemical composition and energy sources during embryo development and in early juveniles of the snail Pomacea canaliculata (Mollusca: Gastropoda)". Journal of Experimental Zoology. 280 (6): 375–383. doi:10.1002/(SICI)1097-010X(19980415)280:6<375::AID-JEZ1>3.0.CO;2-K. ISSN 1097-010X.
- ^ a b c Heras H, Frassa MV, Fernández PE, Galosi CM, Gimeno EJ, Dreon MS (September 2008). "First egg protein with a neurotoxic effect on mice". Toxicon. 52 (3): 481–488. doi:10.1016/j.toxicon.2008.06.022. PMID 18640143.
- ^ Dreon MS, Fernández PE, Gimeno EJ, Heras H (June 2014). "Insights into embryo defenses of the invasive apple snail Pomacea canaliculata: egg mass ingestion affects rat intestine morphology and growth". PLOS Neglected Tropical Diseases. 8 (6): e2961. doi:10.1371/journal.pntd.0002961. PMC 4063725. PMID 24945629.
- ^ Sun J, Mu H, Ip JC, Li R, Xu T, Accorsi A, et al. (July 2019). Russo C (ed.). "Signatures of Divergence, Invasiveness, and Terrestrialization Revealed by Four Apple Snail Genomes". Molecular Biology and Evolution. 36 (7): 1507–1520. doi:10.1093/molbev/msz084. PMC 6573481. PMID 30980073.
- ^ Sun J, Zhang H, Wang H, Heras H, Dreon MS, Ituarte S, et al. (August 2012). "First proteome of the egg perivitelline fluid of a freshwater gastropod with aerial oviposition". Journal of Proteome Research. 11 (8): 4240–4248. doi:10.1021/pr3003613. hdl:11336/94414. PMID 22738194.
- ^ Mu H, Sun J, Heras H, Chu KH, Qiu JW (February 2017). "An integrated proteomic and transcriptomic analysis of perivitelline fluid proteins in a freshwater gastropod laying aerial eggs". Journal of Proteomics. 155: 22–30. doi:10.1016/j.jprot.2017.01.006. hdl:11336/100499. PMID 28095328. S2CID 19632393.