Who cares about toxins?

Atheris matildae

Ever since humans made the link between interactions with toxic creatures and illness or death, both a fascination and fear of these animals developed. Both venomous and poisonous found in toxic creatures have long been used to give an evolutionary advantage and play a vital role in prey capture, immobilization and defence. As the species evolved so did the toxins they use providing an immense library of active proteins (Fox and Serrano, 2007).

Ranitomeya variablis

Toxins of all kinds have been a focal point of much research within the last 70 years, we have gained a lot of knowledge on how these toxins cause the effects they do on their prey or attackers (Fox and Serrano, 2007). These dramatic and highly specific effects led to the investigation of using these protein cocktails for therapeutic uses (Fox and Serrano, 2007). Despite this it has only become prevalent in the past two decades to mine these biological wonders for treatment, detection, diagnosis and research on an ever growing and seemingly endless list of diseases. While very much a developing field treatments for Hypertension, angina, coronary angioplasty, chronic pain, diabetes and many forms of cancer have been created and are currently being used globally (Takacs and Nathan, 2014). With an estimated 20 million toxins remaining unexplored these will no doubt produce many more (Takacs and Nathan, 2014). This extensive assemblage will be a major source of novel therapeutics in the years to come (Takacs and Nathan, 2014).

Maybe those who allow hate to cloud their vision of these creatures will one day see the benefit of their existence, not only their sheer biological elegance, efficiency and beauty but also in their medicinal benefits to all.

References
Fox, J.W. and Serrano, S.M., 2007. Approaching the Golden Age of Natural Product Pharmaceuticals from Venom Libraries: An Overview of Toxins and Toxin-Derivatives Currently Involved in Therapeutic or Diagnostic Applications. Current Pharmaceutical Design, Vol:13, No:28, pp.2927-2934.
Takacs, Z., Nathan, S., 2014. Animal Venoms in Medicine. In: Wexler, P. (Ed.), Encyclopedia of Toxicology, 3rd edition vol 1. Elsevier Inc., Academic Press, pp. 252-259.
Images
Ranitomeya variablis: https://www.flickr.com/photos/reptiles4all/14002572367/in/photolist-kGJBVB-prYMqt-pHUdZR-gmkwk6-kdcZhz-pqEcKP-rqpEkd-hFm8jU-uwcFQk-quhfDR-prsT4M-q1DjjH-8RiiWU-8SmGSj-qTELsq-qXmmTJ-qkN961-nkmQVv-oPjuA9-99CajC/
Atheris matildae: https://www.flickr.com/photos/reptiles4all/15570371547/in/photolist-kGJBVB-prYMqt-pHUdZR-gmkwk6-kdcZhz-pqEcKP-rqpEkd-hFm8jU-uwcFQk-quhfDR-prsT4M-q1DjjH-8RiiWU-8SmGSj-qTELsq-qXmmTJ-qkN961-nkmQVv-oPjuA9-99CajC

Antivenom

Australia is notorious for is venomous creatures in particular the venomous snakes. Taipans, brown snakes, tiger snakes and death adders strike fear into the hearts of many people. Despite these species harbouring some incredibly toxic venoms, snake bites in Australia resulting in fatalities is now a rare incidence. Why so?

Desert death adder (Acanthophis pyrrhus) a species with a strong postsynaptic toxin component.

Production of high quality and relatively easily sourced antivenom, education of the public and quality medical care all are essential in reducing fatalities (White & Meier, 1995).

Antivenom comes in two forms;
Polyvalent antivenoms are a fantastic idea conceptually. The idea of having a wide-ranging effective antivenom to cover most if not all species would truly change the medicinal treatment of bites. This level of effectiveness however, has not been reached (White & Meier, 1995). Polyvalents are often used as a last alternative as reactions, often worse than the response to the venom, are common (White & Meier, 1995). Many other side effects are common and effectively counteracting all components of the venom is far from certain.

Monovalent antivenoms are highly specific and are produced to counteract the venom from one species. These serums may still cause reaction but are far less common. Due to their specific nature they also target most if not all of the venom component. These factors make monovalent a much safer option to treat a patient with a known envenomation (White & Meier, 1995).

Red-headed krait (Bungarus flaviceps baluensis) a species with strong presynaptic neurotoxin components.

Most dangerous Australian snake’s venom is dominated by postsynaptic neurotoxins. Neurotoxins are classed into postsynaptic and presynaptic depending on what area of the neuron is impacted (White & Meier, 1995). Bites from presynaptic neurotoxins often take a long time to manifest with few symptoms initially, however, after a certain length of time the neuron is permanently damaged and antivenom is no longer effective (White & Meier, 1995). Postsynaptic neurotoxins work oppositely with a fast onset of symptoms but reacts much more effectively to antivenom even hours after the envenomation (White & Meier, 1995).

References
White, J. and Meier, J., 1(995). Handbook of clinical toxicology of animal venoms and poisons (Vol. 236). CRC Press.

Images by Nick Weigner

Snakes of the sea

Sea snakes occupy a huge amount of the worlds' oceans filling various niches from the pelagic going Yellow bellied sea snake (Pelamis platura) to the Australian beaked sea snake (Enhydrina zweifeli) that regularly ventures into freshwater rivers (Cogger, 2014). However, all ‘true’ sea snakes are in the subfamily Hydrophiinae and share the same reproductive strategy of viviparity. The Sea kraits (Laticaudae subfamily, not to be confused with true sea snakes or land kraits of the Bungarus genus) show superficial similarities to true sea snakes with their largely marine habit, flattened paddle-like tail and highly toxic venom. Their habit of terrestrial exploitation for mating, laying eggs and digestion led to their own subfamily classification. In fact sea snakes are more closely related to the terrestrial elapids than the sea kraits (Cogger, 2014).

Laticauda colubrina

Pelamis platura

Sea snake and sea kraits are a prime example of convergent evolution both developing the analogous characteristic of the paddle shaped tail to aid in movement through water (McDowell, 1969). The tail of sea kraits is a simple paddle supported by unmodified vertebrae unlike the highly modified vertebrae of the true sea snakes (Fig.1). Even within the true sea snakes the paddle tail has evolved separately many times (Sanders et al, 2012).

Fig.1 Sections of modified vertebrae to aid in support of paddle tail of six sea snake species (Sanders et al, 2012).

Both families belong to the family Elapidae and are proteroglyphs (Cogger, 2014). Venom from both Hydrophiinae and Laticaudae species are often potently neurotoxic and have caused many fatalities globally (Tamiya et al, 1983; Heatwole, 1969). Most deaths occur in S.E Asia where a high diversity of both subfamilies is found, a strong reliance on net fishing practices and poor medical facilities all contribute to a heightened incidence of fatalities (Heatwole, 1969). Most species of both Sea snakes and Sea krait are notoriously docile however the curious nature of some species is often mistaken for aggression and causes an inflammatory reaction from divers (Cogger 2014).



References
Cogger, H., (2014). “Reptiles and amphibians of Australia.” 7th ed. CSIRO Publishing.

Heatwole, H., (1999). “Sea snakes.” (No. ed. 2). Krieger Publishing Company.

McDowell, S.B., (1969). “Notes on the Australian sea-snake Ephalophis greyi M. Smith (Serpentes: Elapidae, Hydrophiinae) and the origin and classification of sea-snakes.” Zoological Journal of the Linnean Society, Vol: 48, No: 3, pp. 333–349.

Sanders, K.L., Rasmussen, A.R. and Elmberg, J., (2012). Independent innovation in the evolution of paddle-shaped tails in viviparous sea snakes (Elapidae: Hydrophiinae). Integrative and comparative biology, First published online May 24, 2012 doi:10.1093/icb/ics066.

Tamiya, N., Sato, A., Kim, H.S., Teruuchi, T., Takasaki, C., Ishikawa, Y., Guinea, M.L., McCoy, M., Heatwole, H. and Cogger, H.G., (1983). “Neurotoxins of sea snakes genus Laticauda.” Toxicon, Vol:21, No:3 pp.445-447.

Images
Pelamis platura: Cogger, H. (2014). “Reptiles and amphibians of Australia.” 7th ed. CSIRO Publishing.
Laticauda colubrina :https://www.robertharding.com/preview/860-282736/banded-sea-krait-surface-amatildecopydatildecopye-islet-new-caledonia/ 

Spitting Snakes

Naja sumatrana spraying venom.

Some animals have developed the ability to spray chemicals on perceived attackers as a form of defence. While many of the species abilities are very efficient such as the skunk and bombardier beetle (which sprays a liquid at 100°C; Eisner, 1958), the capability of some snake species to accurately spray venom up to 2m takes the evolutionary cake (Young et al, 2004).

Some species of Naja (cobras) and the Rinkhals of Hemachatus genus have separately evolved unique fang structure to produce an accurate ranged defence. Some vipers, primarily the Mangshan Viper (Zhaoermia mangshanensis) of China have been reported to “spit” venom. These cases are put down to a larger venom yield and hissing action, they have not been shown to employ “intentionally aimed spitting” unlike the Spitting Cobras and Rinkhals (Young et al, 2004).Spitting is only used as a defence mechanism and not for hunting. These snakes still use their fangs normally to deliver a fatal bite whilst hunting (Wüster & Thorpe, 1992).

Fig 1. Fang structure of Spitting and Non-spitting cobras. Note the size and shape of the exit orifice
(Wüster & Thorpe, 1992).

Venom is produced as per normal in the venom glands but the ability comes from the small elliptical exit point at the terminal end of the fang (Fig 1.). The venom is pushed into the fang under great pressure, formed by the contraction of muscles around the venom gland (Young et al, 2004; Wüster & Thorpe, 1992).The head is tilted back and often with a shaking motion of the head the venom is pushed out the smaller rounder exit hole (Wüster & Thorpe, 1992).

 The venoms from most of the spitting Naja contain Neurotoxins and Cytotoxins (Wüster & Thorpe, 1992).

References

Eisner, T. (1958). "The protective role of the spray mechanism of the bombardier beetle, Brachynus ballistarius Lec." Journal of Insect Physiology,Vol:2, No:3, pp.215-220.

Wüster, W. and Thorpe, R.S. (1992). "Dentitional phenomena in cobras revisited: spitting and fang structure in the Asiatic species of Naja (Serpentes: Elapidae)." Herpetologica, Vol: 48, No: 4,  pp.424-434.

Young, B.A., Dunlap, K., Koenig, K. and Singer, M. (2004). "The buccal buckle: the functional morphology of venom spitting in cobras." Journal of Experimental Biology, Vol: 207, No: 20, pp.3483-3494.

Image

Naja sumatrana, Nick Weigner

Ontogenetic Shifts in Venom Composition

When venomous snakes are born they already possess the apparatus and venom to deliver a toxic bite. It is an essential ability in order for the young to feed and defend themselves immediately as this is when they are most susceptible to predation. As growth from young to old is closely linked to preferential prey it is likely a that this change corresponds to more accurately target its prey at that age (Gibbs et al. 2011).

Venoms are an efficient and targeted tool to aid in feeding, this specialisation of venoms leads to the diverse forms we see today. As the diet of the snakes change so must its venom to ensure it is effective. Investigations into the composition shifts in venoms over a snakes lifetime has been carried out on many species particularly the Crotaline snakes (rattle snakes and relatives). Many of these snakes feed on frogs or small lizards when young. In the case of Crotalus oreganus a higher percentage of myotoxins are found in the venom of older snakes that have progressed to feeding on small mammals.The younger specimens showed more neurotoxic venom while still at a similar overall toxicity to the adults (Mackessy et al. 2003). 

This adaptation can cause greater difficulty in producing an effective treatment for bites. The most effective antivenom, polyvalent antivenom, is produced to target specific component of the venom.Should the venom be varied due to ontogenetic shifts a bite from a juvenile may not be effectively treated by an antivenom produced for adults venom. Natural variation in populations and between populations in different localities and conditions also presents the same concern (McCue. 2006).


References 

Mackessy, S.P., Williams, K. and Ashton, K.G., (2003). “Ontogenetic variation in venom composition and diet of Crotalus oreganus concolor: a case of venom paedomorphosis?.” Copeia, Vol: 2003 No: 4, pp.769-782.
McCue M.D. (2006). “Cost of Producing Venom in Three North American Pitviper Species.” Copeia, Vol: 2006, No: 4, pp. 818-825.
Gibbs, H.L., Sanz, L., Chiucchi, J.E., Farrell, T.M. and Calvete, J.J., (2011). “Proteomic analysis of ontogenetic and diet-related changes in venom composition of juvenile and adult Dusky Pigmy rattlesnakes (Sistrurus miliarius barbouri).” Journal of proteomics, Vol: 74 No:10, pp.2169-2179.

Image 
http://www.stuartdahnephotography.com/keyword/crotalus%20tigris%20the%20tiger%20rattler/i-xttv4cn accessed: 13/4/16

Toxicofera

Phylogenetic of the venomous reptiles is a broadly debated. Many different classifications previously presented used morphological features to define clades. The Toxicofera theory was born after genetic testing of the Squamata order showed serious discrepancies between this morphological grouping and their genetic relationships (Vidal & Hedges. 2005). Further testing of this group gave greater support for the new classification of toxicofera based on the genetic similarities, primarily the presence of venom coding genes within three of the Squamata groupings (Fry et al. 2009a). 

The extant taxa forming this clade includes all Ophidia (snakes), Anguimorpha (lizards including monitors) and the Iguanians (lizards including dragons, chameleons and of course iguanas). All the known venomous reptiles belong to this group, however some families in the group are not venomous. It is thought that the non-venomous members have simply lost the venom production ability to some degree (Fry et al. 2009a).

New evidence is constantly being uncovered to support this clade. A study on anguimorphs showed a toxin homologous with those of snakes and functional venom glands have been discovered in the jaw of Komodo dragons Varanus komodoensis dispelling the previously held belief that it was only bacteria that forms the toxic bite by these gargantuous lizard (Fry et al. 2010; Fry et al. 2009b).

The common ancestor to all toxicoferan had a host of core venom genes. Along with other toxin recruitment, these are the original genes which have diversified into the venoms we see in many species today (Fry et al. 2009a).

References 

Fry, BG, Winter, K, Norman, JA, Roelants, K, Nabuurs, RJA, et al. 2010, ‘Functional and structural diversification of the Anguimorpha lizard venom system’, Molecular & Cellular Proteomics, Vol: 9, No: 11, pp. 2369-2390.

Fry, BG, Vidal, N, van der Weerd, L, Kochva, E & Renjifo, C. 2009a, ‘Evolution and diversification of the Toxicofera reptile venom system’, Journal of Proteomics, Vol: 72, No: 2, pp. 127-136.

Fry, BG, Wroe, S, Teeuwisse, W, van Osch, MJP, Moreno, et al. 2009b, ‘A central role for venom in predation by Varanus komodoensis (Komodo Dragon) and the extinct giant Varanus (Megalania) priscus’, Proceedings of the National Academy of Sciences of the United States of America, Vol:106, No: 22, pp. 8969-8974.

Vidal, N & Hedges, SB. 2005, ‘The phylogeny of squamate reptiles (lizards, snakes, and amphsbaenians) inferred from nine nuclear protein-coding genes’, Comptes rendus – Biologies, Vol: 328, No: 10, pp. 1000-1008.

Image

http://www.bbc.com/earth/story/20160226-the-islands-where-dragons-are-real, accessed 5/4/2015 

Tiger Keelback

Image of Rhabdophis tigrinus including insets of feed behaviour and chemical structure of  commonly sequestered toxin.

The primary goal for most organisms is to reproduce. However passing on genes to the next generation is pointless if they don’t survive the onslaught of predators in their new world. Animals that are pregnant and support the development of young internally also become susceptible to predation due to a greater size and changed behavior, eg sun baking for longer (Mori & Burghardt. 2001).
The Tiger Keelback (Rhabdophis tigrinus) along with others of the same genus have developed an unusual strategy to enhance survive rates of both it and its young by having the best of both worlds being venomous and poisonous (for an explanation see Venomous Anurans ;Hutchinson  et al. 2007).
The Keelbacks (Rhabdophis genus) of Asia have developed a row of paired glands along the back of the neck. These glands sit just below the surface and release their toxin if pressure is applied. The poison is variable as it is sequestered by the snake from toads which they feed on (Hutchinson et al. 2007).

These snakes are oviparous meaning they produce eggs. It has been shown that pregnant snakes with ready access to the toxin producing toads in their diet consume a higher proportion of these toads.This leads to an increase in poison availability which provides two benefits;

-A female often left more susceptible to predation now has a better defensive capability.

-The female is also enabled to pass on the poison to the young, which can in turn use them for defence. This gives them a greater chance for survival after birth when they are most vulnerable.

The Keelbacks also biosynthesise (as apposed to sequester) as venom which is delivered through the rear mouth fangs.The venom is produced in the Duvernoy's gland and the most significant effects of envenomation include coagulopathy and renal failure.


Reference

Hutchinson, D.A., Mori, A., Savitzky, A.H., Burghardt, G.M., Wu, X., Meinwald, J. and Schroeder, F.C., 2007."Dietary Sequestration of Defensive Steroids in Nuchal Glands of the Asian Snake Rhabdophis tigrinus." Proceedings of the National Academy of Sciences, Vol:104 No:7, pp.2265-2270.

Mori, A. and Burghardt, G.M., 2001."Temperature Effects on Anti‐Predator Behaviour in Rhabdophis tigrinus, a Snake with Toxic Nuchal Glands." Ethology, Vol;107, No:9, pp.795-811.

Image

http://modernsteroid.blogspot.com.au/2016/01/sequestration-of-defensive.html