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Expanding our scientific horizons: utilization of unique model organisms in biological research

2017; Springer Nature; Volume: 36; Issue: 16 Linguagem: Inglês

10.15252/embj.201797640

1460-2075

Angela K. Peter, Claudia Crocini, Leslie A. Leinwand,

Zebrafish Biomedical Research Applications

Commentary10 July 2017free access Expanding our scientific horizons: utilization of unique model organisms in biological research Angela K Peter Angela K Peter Department of Molecular, Cellular and Developmental Biology, Biofrontiers Institute, University of Colorado, Boulder, CO, USA Search for more papers by this author Claudia Crocini Claudia Crocini Department of Molecular, Cellular and Developmental Biology, Biofrontiers Institute, University of Colorado, Boulder, CO, USA Search for more papers by this author Leslie A Leinwand Leslie A Leinwand [email protected] Department of Molecular, Cellular and Developmental Biology, Biofrontiers Institute, University of Colorado, Boulder, CO, USA Search for more papers by this author Angela K Peter Angela K Peter Department of Molecular, Cellular and Developmental Biology, Biofrontiers Institute, University of Colorado, Boulder, CO, USA Search for more papers by this author Claudia Crocini Claudia Crocini Department of Molecular, Cellular and Developmental Biology, Biofrontiers Institute, University of Colorado, Boulder, CO, USA Search for more papers by this author Leslie A Leinwand Leslie A Leinwand [email protected] Department of Molecular, Cellular and Developmental Biology, Biofrontiers Institute, University of Colorado, Boulder, CO, USA Search for more papers by this author Author Information Angela K Peter1, Claudia Crocini1 and Leslie A Leinwand1 1Department of Molecular, Cellular and Developmental Biology, Biofrontiers Institute, University of Colorado, Boulder, CO, USA The EMBO Journal (2017)36:2311-2314https://doi.org/10.15252/embj.201797640 PDFDownload PDF of article text and main figures. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info During the past century, research studies using animal models have contributed to numerous scientific discoveries and have been vital for the understanding of numerous biological processes, including disease. Over the past decades, the scientific community has defined a small number of model organisms that includes a few mammals, fish (mainly zebrafish), birds (mainly chicken), frogs, flies, and nematodes. Rodents are by far the most commonly employed laboratory animals in biomedical research. Mice share many biological similarities to humans and can be genetically manipulated to express mutations linked to human diseases. Mice and rats reproduce relatively quickly and have a short life span, which allows scientists to study progressive disorders, including aging. A large range of inbred mice strains enables accurate and reproducible experiments by decreasing the variability often associated with animal models and biological systems in general. Finally, mice are cost-effective, small, and relatively easy to handle, transport, and house. All of these advantages combine to make mice the major species for recapitulating and studying human diseases. However, focusing exclusively on one or very few animal models may lead researchers to lose sight of other species with vastly different biology that might inform and affect our understanding of disease pathogenesis. Indeed, the notion of “classical” animal models was nearly nonexistent in the first decades of the 20th century. Prior to the advent of modern genetics, scientists studied an enormous variety of animal species to unlock numerous aspects of physiology. For example, our current understanding of action potential generation in neurons is based on pioneering work on the giant squid axon dating back to 1938. As another example, Emilio Veratti published a comprehensive study in 1902 on the fine structure of striated muscle fibers in mice, rats, cats, several species of bats, chicken, pigeon, lizard, green lizard, several ophidian, frog, triton, two species of fish, four species of insects, and two crustacean species. Horses, deer, elephants, snakes, turtles, fowl, and many other animals were not uncommon models in the scientific literature of the 1800s and the early 1900s (Fig 1). Figure 1. PubMed search results by publication date, from 1945 to 2015Search performed using the term “animal” excluding “nematode, worms, Drosophila, flies, and insects” (black columns) and including the terms “mouse and rat” (gray columns). The red arrows indicate the most important milestones for mouse and rat animal modeling. Download figure Download PowerPoint A conscious effort by the scientific community to understand and embrace unique organisms will therefore help to increase our knowledge about biology in general and may lead to new applications in biomedicine. Exploring unique model systems will help researchers to study complex questions in animals with unique traits and stimulate new perspectives and ideas. The utility or even groundbreaking nature of studies employing “atypical” animal models is apparent from the examples which we describe below. The gila monster, a large venomous lizard native to the southwestern USA, produces venom that exerts excruciating pain. However, gila monsters are infrequent feeders that primarily prey on eggs from bird nests and newborn mammals such as rabbits and squirrels. It is therefore unlikely that it uses venom to incapacitate its prey. However, being one of only a few venomous lizard species does have an evolutionary advantage. Gila monsters are often viewed as sluggish, slow moving, lizards and therefore seem to an easy prey—the excruciating pain which accompanies a gila monster bite is an extremely effective defense mechanism. It was known since the 1980s that gila monster venom, as well as the venom of other snakes and lizards, causes inflammation of the pancreas. Based on this initial observation, John Eng at the Bronx Veterans Administration Medical Center in New York set out to determine the individual factors in the venom of the gila monster and whether any of these have an effect on pancreatic cells. One of the two proteins he identified is a hormone, exendin-4, which is similar in both structure and function to GLP-1, a human hormone that stimulates insulin production in the pancreas. After a meal, glucose levels increase, as do levels of GLP-1. However, within minutes, GLP-1 activity diminishes along with falling insulin levels in healthy persons. Gila monsters in contrast are infrequent eaters, and exendin-4 remains active for hours which suggested a use as a long-acting, effective treatment in diabetes patients. Synthetic versions of exendin-4 (Byetta or exenatide) are currently being used to stabilize glucose levels in patients diagnosed with type II diabetes and help them to loose weight. Exenatide stimulates glucose-induced insulin secretion in the pancreas, and, unlike other therapeutics currently used to treat type II diabetes, no increased risk for developing heart failure has been observed (for a review, see Scheen, 2017). Thus, by studying components of the gila monster's poisonous saliva, researchers developed a type II diabetic therapeutic more suitable for long-term use in patients with a high risk of cardiovascular disease. The deadly tropical cone snail, one of the most venomous animals on the planet, produces a dazzling array of neurotoxins to incapacitate its prey. While cone snail venom causes symptoms such as numbness, muscle paralysis, respiratory failure, and death, one thing that at least human victims do not seem to experience is excruciating pain owing to potent analgesic components. While most venoms contain only a few individual toxins, cone snail venom can include 100–1,000 different toxins. Most of these are peptides targeting different ion channels. Many of these peptide toxins were subsequently tested in mice models to identify their cellular targets. Baldomero Olivera's research group at the University of Utah discovered that one particular toxin, MVIIA, works by blocking a calcium channel that has been implicated in chronic neuropathic pain caused by damage to the nervous system (McIntosh et al, 1982). Since this initial discovery, the synthetic form of MVIIA, ziconotide, is now being tested in advanced clinical trials in patients with cancer or AIDS who are suffering from pain that cannot be relieved by opiate-based analgesics. In addition to MVIIA, several other toxins isolated from different species of cone snails are in various stages of clinical trials to treat various neurological disorders ranging from chronic pain to epilepsy. While the vampire bat does not employ venom or toxins to incapacitate prey like the gila monster or the cone snail, its saliva contains a unique anticoagulant protein coined Draculin, which prevents the victims’ blood from coagulating while the vampire bat continues to drink. Subsequent analysis of the vampire bat saliva revealed that the post-translational glycosylation of the native Draculin protein is responsible for its anticoagulation properties (Fernandez et al, 1998). The reversible nature of Draculin, and its long-lasting effectiveness of up to 9 h, has made this protein an interesting prospective therapeutic for treating stroke and heart attack. In addition, it could possibly be marketed as a blood thinner for preventing heart attacks. However, because the effectiveness of this protein largely depends on the level of glycosylation, many studies were necessary before a synthetic version of Draculin (desmoteplase) could be tested in clinical trials. However, Lundbeck, the company that developed desmoteplase eventually made the decision to pull the compound from development and terminate an ongoing phase III trial after initial results failed to see significant improvements in clinical outcomes above that of placebo. Prairie voles mate for life—as opposed to laboratory rodents—and are now being used to study how monogamy is mediated by secreted hormones, such as oxytocin. The knowledge gained by this line of research could inspire research into the role of oxytocin in humans, and how to advance from understanding the molecules of monogamy to treating disorders characterized by abnormal social behavior, such as autism (Carter et al, 1995). Another unique animal model used to study neuronal pathways in order to provide valuable insights into human stuttering and speech-related disorders and conditions, including autism and stroke, is the zebra finch. Pioneering research performed at the University of California, Los Angeles, has linked an area of the zebra finch brain, Area X, with vocalizations in male zebra finches. By studying genes activated in Area X, the researchers identified two especially important genetic links between Area X and human disease. One is FoxP2, a transcription factor that is critical for both human speech and birdsong: Its mRNA expression patterns are strikingly similar between developing humans and songbirds. The second genetic factor of interest identified, Reelin, has been shown to play a role in a spectrum of neurological disorders such as schizophrenia, bipolar disorder, major depression, lissencephaly, and autism. Autism is often associated with delays in language development. By continuing to study both Reelin and FoxP2, researchers hope to better understand the development of human speech and how defects in these genes, or the resulting protein expression and alterations in downstream signaling pathways, may contribute to autism (Hilliard et al, 2012). The naked mole rat is a tiny African subterranean rodent with a surprising long life span of nearly 30 years, which is over 10 times longer than the average life span of a laboratory rat or mouse. Another astounding observation from these animals is that they appear to be resistant against cancer unlike their close relatives, mice and rats, which commonly develop numerous cancerous lesions within their short life span. Recently, a team of researchers from the University of Rochester in New York and the University of Haifa discovered a key molecule to the naked mole rat's cancer resistance: a sugar called high-molecular-mass Hyaluronan (HMM-HA). When HMM-HA is secreted from cells, it prevents overcrowding within a given cell population, which is key to cancer development. Owing to a more robust synthesis of HMM-HA and decreased activity of HA-degrading enzymes, the naked mole rat is able to secrete extremely high-molecular-mass HA, more than five times larger than that of the human or mouse. Subsequent protein comparison of the enzyme responsible for HA synthesis between the naked mole rat and other mammals revealed that two asparagines, which are 100% conserved among most mammals, were replaced by two serine residues. In addition, the naked mole rat cells display a twofold higher affinity for HA than mouse or human cells. Higher molecular weight HA and a higher cellular affinity likely increase the sensitivity of naked mole rat cells to HA signaling, ultimately reducing cancer risk and increasing life span (Tian et al, 2013). Similar to the toxins of the gila monster and the cone snail or the protein in the saliva of the vampire bat, harnessing the therapeutic potential of HMM-HA may pave the way for future cancer therapeutics. Our own laboratory exploits characteristics of infrequently feeding constricting snakes, such as the Burmese python, in order to study the extraordinary postprandial physiological changes such as increased oxygen consumption, increased metabolic rate, and significant physiological organ growth that are unique to these animals. The rapid growth of the python heart after feeding provides a unique opportunity not only to examine the mechanisms responsible for adaptive cardiac hypertrophy, but also to examine pathways involved in cardiac hypertrophy (Riquelme et al, 2011). If we are fortunate enough to discover key pathways involved in cardiac regression, perhaps therapeutics could be developed to stimulate these physiological hypertrophic pathways and regression pathways in maladaptive cardiac hypertrophy as well. The Burmese python is not the only unique model system for studying cardiac physiology. Deep hibernators, such as ground squirrels and Grizzly bears, undergo significant cardiac functional changes as well as skeletal muscle sparing during hibernation. Grizzly bears are currently being studied owing to their unique ability to spend long periods of time, 4–5 months, with very low heart rates. During hibernation, the heart rate of the Grizzly bear decreases from around 80–90 beats per minute to anywhere between 8 and 24 beats per minute. The ground squirrel undergoes an even more dramatic shift from a heart rate of 300 beats per minute while alert, to < 10 beats per minute while hibernating. At such low heart rates, a human would likely develop congestive heart failure within a matter of weeks. However, hibernating bears and squirrels show no signs of congestion. If researchers can unlock the mechanism(s) by which Grizzly bears and ground squirrels maintain cardiac function without developing heart disease or undergoing significant skeletal muscle atrophy, these same pathways could be targeted in patients at high risk for developing congestive heart failure (Nelson et al, 2003; Nelson & Rourke, 2013). The arctic wood frog takes surviving winter months to an extreme. Similar to the Grizzly bear and ground squirrel, it undergoes significant physiological changes during winter months. Although deep hibernators significantly reduce their heart rate as described above, they still fall short of the drastic changes observed in the arctic wood frog. Incredibly, wood frogs stop breathing and their hearts stop beating entirely for days to weeks at a time. These drastic changes are accompanied by an abrupt end of metabolic activity and waste production. Researchers at Notre Dame and the University of Alaska began studying these animals after observing that these frogs survive multiple freeze/thaw cycles each winter. The key to the arctic wood frog's ability to survive freezing temperatures is the level of cryoprotectants in the tissues of these animals. Glucose, one of the cryoprotectants identified by this group of researchers, was found to be elevated 13-fold, 10-fold, and 3.3-fold in the muscle, heart, and liver, respectively, compared to the same species of frog from more temperate climates (Larson et al, 2014). The arctic wood frog is not alone in its ability to withstand subzero temperatures. Researchers in Germany showed that Antarctic octopods can survive extreme water temperatures from −1.8 to 35.6°C with the help of a specialized protein in the blood called hemocyanin. This pigment contains copper, giving it a blue appearance, and is responsible for binding oxygen. By adjusting oxygen's affinity for hemocyanin, the arctic octopus is able to counter the adverse effect of lower temperature on oxygen binding and manages to supply sufficient oxygen to tissues and organs (Oellermann et al, 2015). By understanding the basic biology of animals capable of surviving extreme temperatures, we may be able to better understand and improve tissue preservation. This research has therefore potential implications for human organ transplants in which the time frame to find an appropriate recipient and transport organs is extremely limited. Currently, there are no procedures in place to freeze and oxygenate human tissues even for a short period of time. If cryoprotective procedures were to be developed allowing transport of frozen, or semi-frozen, tissues, donor-matching procedures could be improved. Occasionally, studies on atypical laboratory models may not have immediate impact on human health but may trigger a revolutionary idea later. The most illustrative example is the identification and utilization of the green fluorescent protein (GFP). Osamu Shimomura was the first to isolate GFP from the jellyfish Aequorea victoria in 1962. It took another 30 years for Roger Tsien, Martin Chalfie, and Douglas Prasher to harness GFP's potential as a reporter for gene expression. Since then, GFP and all of its variants have revolutionized science, and they have been used in thousands of publications to assess numerous aspects of biology. Another example is the discovery of a light-gated ion channel, channelrhodopsin, from algae. In 1978, a study published by Litvin, Sineshchekov, and Sineshchekov determined that an algae light-gated receptor was able to produce a photocurrent, which ultimately controlled flagellar movements, thus mediating phototaxis. Decades later, Karl Deisseroth's research group exogenously expressed channelrhodopsin in mammalian neurons allowing them to control neuronal electrical activity by means of light with unprecedented spatio-temporal precision. Channelrhodopsin is now extensively utilized in neuroscience and cardiac research and has radically transformed the field of electrophysiology. Many animals alive today have evolved elaborate systems for surviving in extreme environments, and some of these have already led to therapeutics in human disease as described above. However, these breakthroughs come with inherent drawbacks. One of the biggest downsides of using non-conventional research models is that fewer research facilities are capable of housing these organisms. This often requires researchers to maintain the organisms outside of common animal research facilities. Similarly, laboratory equipment used to house or study common laboratory animals often needs to be modified to accommodate “unique” model organisms, which can increase the cost associated with purchasing the equipment. Some animals, for example, the Burmese python, have been declared as invasive species and require specific permits. In addition, finding suppliers willing to sell uncommon model organisms to research laboratories can prove difficult. Alternatively, little may be known about specimens collected from the wild. Unlike common laboratory animals, very few “inbred” strains of unique model organisms exist which invariably leads to increased variability in results ultimately increasing the sample size required to obtain significant results. Very few commercially available products, such as antibodies, are designed to cross-react with uncommon model organisms. This often means that researcher has to develop their own research tools, such as custom antibodies, in order to complete a given research study. Again, this can contribute to increased costs. While there are some limitations associated with studying unique organisms, the examples in this commentary illustrate their enormous value. Every organism has its limits, and for all the practical reasons mentioned above, rodents are likely to remain the most commonly used laboratory animal model in biomedical research. Yet, given the few examples highlighted above, it seems we have many more opportunities to increase our knowledge by expanding our research beyond the most common laboratory models. Even physics is drawing inspiration from “atypical” biological models. The brilliant whiteness of ultrathin beetle scales has puzzled and fascinated physicists for many years, and the incredible adhesion capability of geckos has inspired physicists and engineers to develop gecko-like synthetic adhesives. While “classical” laboratory models will likely always be employed for biomedical research, a more inclusive approach where “atypical” models contribute knowledge in a complementary fashion would greatly benefit our understanding of biology and, perhaps, lead to new therapeutics. As scientists, we should always question our choices in laboratory models we utilize for research and have the courage to explore the unconventional. Acknowledgements This work is supported by National Institutes of Health grant HL119937 awarded to L.A.L. This work was supported by Human Frontier Science Program Fellowship LT001449/2017-L awarded to C.C. The picture of the Burmese python was graciously provided by Bob Clark (Bob Clark Reptiles, Oklahoma City, Oklahoma). 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Nature 499: 346–349CrossrefCASPubMedWeb of Science®Google Scholar Previous ArticleNext Article Read MoreAbout the coverClose modalView large imageVolume 36,Issue 16,15 August 2017Cover: Glutamine supplies the majority of carbons in the tricyclic acid (TCA) cycle of endothelial cells (ECs) and is required for cataplerosis and as a nitrogen source in order to sustain EC proliferation. The glutamine metabolism serves multiple metabolic functions but is especially interlinked with asparagine synthesis. The cover shows an image of a mosaic spheroid (upper left corner) consisting of a 1:1 mixture of mCherry +‐control (red) and GFP +‐control (green) endothelial cells combined with an image of cultured human umbilical endothelial cells (HUVECs) stained for F‐actin (red), mitochondria (green) and nucleus (blue). From Hongling Huang, Guy Eelen, Peter Carmeliet and colleagues: Role of glutamine and interlinked asparagine metabolism in vessel formation, and from Boa Kim, Jia Li, Cholsoon Jang and Zoltan Arany: Glutamine fuels proliferation but not migration of endothelial cells. For details, see the Articles on p 2334 and 2321. Also highlighted by Andrade and Potente on p 2315. (Artistic rendition by Uta Mackensen) Volume 36Issue 1615 August 2017In this issue FiguresReferencesRelatedDetailsLoading ...