Yoanne M. Clovis, Ph.D.
Non-mammalian model organisms are typically used in early research to deliver fast answers to a discovery problem. The most popular model organisms in biological and biomedical research are the fruit fly Drosophila melanogaster, the zebrafish and the nematode C. elegans. In this review, we provide an overview of the advantages and limitations of these organisms as models for human diseases.
Key-words: C.elegans, Drosophila, zebrafish, invertebrates, genetic models, human diseases
- Age-related and neurodegenerative diseases
Neurodegenerative diseases are caused by the progressive loss of specific neurons. These diseases are often age-related, with significant pathological and clinical similarities. Patients who will develop neurodegenerative diseases are generally asymptomatic during the development of the nervous system. Many late-onset neurodegenerative diseases, including Parkinson’s disease and Huntington’s chorea, are associated with the aggregation of toxic proteins (Taylor et al., 2002). The identification of mutations associated with familial cases of many of these neurodegenerative diseases has allowed investigators to develop in vitro and in vivo model systems to determine the cellular and molecular mechanisms underlying neurodegenerative diseases. These models proved instrumental in determining the biochemical and genetic alterations in neuronal tissues and understanding how mutant proteins cause damage to specific sets of neurons, leading to distinct clinical phenotypes. Despite the significant contribution of human genetic studies in the identification of new genes associated with familial forms of neurodegenerative diseases, studies on human patients are of limited use for elucidating the signaling pathways and cellular processes underlying the neurodegenerative process. In addition, both ethical and technical problems pose a limit on the types of genetic analysis that can be performed in human patients to delineate signaling pathways. Most human neuropathological investigations use postmortem brain and spinal cord tissues that almost never reflect the earliest pathologic events at the presymptomatic stage.
Motor neuron diseases (MNDs) constitute an expanding, heterogeneous group of developmental and neurodegenerative disorders. Recently, smaller model organisms have been used to decipher the neurobiological basis of MNDs and predict successful treatment strategies (Bebee et al., 2012; Grice et al., 2011; Hirth, 2010; Lanson and Pandey, 2012; McGoldrick et al., 2013; Sleigh et al., 2011).
C. elegans provides a convenient model for studying genes involved in aging, age-related and feeding disorders. Screening for compounds that affect these mechanisms could provide important applications in developing treatments for diabetes, obesity and neurodegenerative diseases.
The nervous system of C. elegans is very simple and uses most of the neurotransmitters presents in the mammalian brain (Loer and Rand, Worm Atlas). These characteristics make it a desirable model for studying the neuronal connectome and cellular mechanisms involved in complex disorders. In fact, several biotechnology companies have been taking advantage of this powerful model. Syngenta is using C. elegans as a tool to predict chemical activity on mammalian development and identify mechanisms influencing toxicological outcome (Harlow et al., 2016). Union Biometrica has fashioned a particle dispenser useful for automatic manipulation and analysis of C. elegans. NemaMetrix has developed an automated microfluidics platform for assaying pharyngeal pumping in C. elegans using the electrical activity generated by the pharynx.
C. elegans models for Alzheimer’s, Huntington’s and Parkinson’s disease are a powerful tool for testing therapeutic agents for these diseases. In a recent groundbreaking study, C. elegans has been used to demonstrate the protective role of A-beta plaques against age-progressive paralysis caused by infection from Candida Albicans. Interestingly, the result, an increase of survival of the infected worms that expresses the 1–42 residue human A-beta isoform, was duplicated in the Alzheimer’s disease mouse model 5XFAD (Kumar et al., 2016).
Drosophila. One of the main advantages of Drosophila for the study of neurodegenerative diseases is the presence of a centralized brain capable of mediating a repertoire of complex behaviors. Alzheimer’s disease (AD) is the most common neurodegenerative disease and is characterized by progressive impairments in memory and cognitive abilities. Most of the genes implicated in AD pathogenesis have Drosophila homologs; e.g., the fly homolog of human APP is known as APP-like or APPL. Flies deficient for APPL demonstrate a behavioral abnormality that can be strongly suppressed by expression of a human APP transgene, indicating functional conservation between Drosophila APPL and human APP (Luo et al., 1992, Prüßing et al., 2013). Other models have been developed in flies to study Parkinson’s and polyglutamine diseases such as Huntington’s chorea (Bonini and fortini., 2003; Iijima et al., 2004; Iijima and Iijima-Ando 2008).
There are limitations to using flies to study neurodegeneration. Drosophila models often show striking phenotypes at early developmental stages, such as the larval, pupal, or early adult, in contrast to the human neurodegenerative diseases modeled, which mostly have an onset in late adulthood.
Despite these limitations, Drosophila has recently been used as a star model organism for drug screening. Today, several biotechnology companies combine genomics with massive selective drug screening using Drosophila. Parkure (Zografos et al., 2016) analyzes and exploits the genetics of Drosophila and human whole genomes to uncover a cure for Parkinson disease. Genescient (Matsagas et al., 2009) tests for chronic side effects during fail-early drug screens.
Zebrafish is the smallest vertebrate model organism is biomedical research. This feature makes it a more relevant model than Drosophila and C. elegans to study neuromuscular degeneration in the context of human diseases.
However, it is important to note that most studies in the zebrafish model are limited to the embryonic phase, while neuromuscular diseases usually have an adult onset in humans.
Moreover, zebrafish as a disease model is in its early developmental stages. During the last decade, this model organism has mainly been used to establish a link with highly penetrant mutations caused either by loss of function or by knockdown of a gene’s transcript.
There are several studies using the zebrafish as an in vivo model for investigating the pathogenesis of amyotrophic lateral sclerosis (ALS). As in mammals, mutations in zebrafish sod1, an enzyme implicated in ALS, have been shown to cause MN loss (Lemmens et al., 2007). In addition, transient overexpression of mutant SOD1 via microinjection of mRNA in developing zebrafish embryos confers a toxic-gain-of-function of this gene characterized by abnormal branching and shortening of MN axons (Lemmens et al., 2007; Sakowski et al., 2012). Transgenic zebrafish overexpressing mutant SOD1 also displayed the hallmarks of ALS, including loss of MNs, muscle degeneration, loss of neuromuscular connectivity and defective motor performance, and increased stress in the MNs (McGown et al., 2013; Ramesh et al., 2010; Sakowski et al., 2012).
Like C. elegans and Drosophila, zebrafish has been gaining traction as a model organism for drug testing. ZeClinics uses zebrafish for safety and efficacy screenings of novel chemical molecules. Early in 2016, Evotek added the zebrafish screening operations of Summit Corporation plc to its high-quality drug discovery platform portfolio.
- Cardiovascular diseases
Cardiovascular diseases are the leading global cause of death (World Health Organization Fact Sheet N°310, 2014). Currently, we only know the causative mutations for about 50% of the cases of cardiac disorders. Despite knowing that mutations in so many different muscle-specific proteins result in cardiac disease, the exact molecular mechanisms by which these mutations result in clinical heart failure in unknown.
Recently, the use of small, organisms with well understood architecture that allow easy access to cardiac or heart-like structures as allowed researchers to gain insight into of the genetic and molecular mechanisms underlying cardiovascular diseases.
C. elegans. The pharynx of nematodes is a rhythmic muscular pump involved in feeding. Although nematodes have no heart or defined circulatory system, evidence suggests that the pharynx shares functional and molecular similarities with the heart in other species. First, pharyngeal muscle function, like that of vertebrate cardiac muscle, does not require nervous system input (Avery et al., 1989). Second, C. elegans’ pharynx and vertebrates’ heart rely on similar types of channels such as the LQT potassium channels and L-type voltage-gated calcium channels (Avery and Horvitz, 1989; Raizen and Avery, 1994; Salkoff et al., 2005). Third, both organs rely on NKX transcription factors for their formation: ceh-22 for the pharynx and Nkx2.5 for the zebrafish heart (Chen and Fishman, 1996; Okkema and Fire, 1994; Okkema et al., 1997). Interestingly, Nkx2.5 can activate the CEH-22 target gene myo-2 when expressed in C. elegans body wall muscles, and Nkx2.5 can rescue ceh-22 mutants when expressed under control of the ceh-22 promoter (Haun et al., 1998). These characteristics make the pharynx an interesting model to study the mechanisms involved in cardiac diseases in a simple, easy to access model.
Moreover, mutations leading to molecular, structural and functional changes in C. elegans’ pharynx can be screened easily and in an unbiased manner by observing the general and cellular anatomy of the pharynx through the transparent cuticle and by studying the patterns of electrical activity generated by the pumping pharynx. Such features permit mechanistic studies of the interactions relevant to human cardiomyopathies. Therefore, future work in C. elegans muscle holds great promise in uncovering new mechanisms involved in cardiac disorders.
Drosophila is the only invertebrate model with a heart developmentally homologous to the vertebrate heart, including that of humans (Bodmer et al., 1995). Despite the fact that the mature Drosophila and vertebrate hearts are quite different morphologically, the initial developmental processes and the basic functional properties and molecular constituents are remarkably conserved (Ocorr et al., 2007; Bodmer et al., 1995; Cammarato et al., 2011). In fact, the fly’s heart functions in an manner analogous to a vertebrate’s heart to pump the blood (haemolymph) through the body cavity in an open circulatory system (Rizki et al., 1978).
For this reason, the fruit fly has emerged as a useful model for developmental and adult onset cardiac diseases. In particular, the larval stage of Drosophila development has been a fruitful model for isolating the effects of various genetic alterations on current formation and propagation of currents involved in electrical stimulation of cardiac contractions and maintenance of rhythmic control (Piazza et al., 2011, Sanghinetti et al., 2006, Ocorr et al., 2007). Meanwhile, the recent development of effective techniques to study adult cardiac performance in the fly allows Drosophila to be used to study long-term alterations in adult cardiac performance caused by factors such as diet, exercise and aging. In 2001, Paternostro and colleagues successfully used adult flies to model cardiac functional disturbance. They observed that the heart rate was increased when flies were exposed to increased temperature or to external currents and that older flies entered cardiac arrest when exposed to similar conditions to those tolerated easily by younger flies. They therefore determined that the ability of the heart to tolerate externally-induced increases in heart rate is an age-dependent phenomenon (Paternostro et al., 2001).
Zebrafish as a genetic and embryonic model system for cardiovascular disease offers several distinct advantages. Zebrafish embryos are not completely dependent on a functional cardiovascular system for their development and in embryos that lack blood circulation, oxygen can still reach all tissues by passive diffusion, allowing them to survive the initial phase of embryonic development. This particularity makes zebrafish embryos particularly well suited for studying gene function during cardiovascular development as it allows the analysis of embryos with severe cardiovascular defects. In addition, the optical transparency of zebrafish embryos has given new insights into various cellular aspects of cardiovascular development.
Human cardiomyopathies are diseases that primarily affect the myocardium. The two most prevalent forms are dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM). DCM is a syndrome characterized by enlargement of one or both ventricles of the heart, accompanied by diminished myocardial contractility, while HCM is defined as thickening of the myocardium in the left and/or right ventricle in the absence of any other diseases that cause myocardial hypertrophy, such as high blood pressure or storage diseases. Several zebrafish mutants have been identified that display heart phenotypes resembling the features of human cardiomyopathies, and their characterization has provided a better understanding of the human disease. In addition, identification of the genes affected in these mutants has provided novel candidate genes that allow further characterization of the genetics of human cardiomyopathies.
In the past several decades, efforts to understand and manipulate the genomes of model organisms resulted in great advances in biomedical research. Smaller organisms have increasingly been evaluated and recognized as models for many diseases and conditions. Their common evolutionary heritage with mammals, including humans, makes it possible to use genetically tractable organisms to model important aspects of human medical disorders such as developmental, neurodegenerative and cardiovascular diseases and aging in systems amenable to rapid and powerful experimentation.
Non-mammalian model organisms are typically used in early research to deliver fast answers to a discovery problem, such as the function of a gene, or to define novel therapeutic entry points. The most popular model organisms in biological and biomedical research are the fruit fly Drosophila melanogaster, the zebrafish and the nematode C. elegans. These organisms present the advantage of being highly prolific reproducers with relatively short generation time. They are easily grown in laboratory setting and by far less expensive than murine models and more powerful than cell culture systems. In addition, thanks to the vast number of molecular and genetic tools available to study Drosophila, C.elegans and zebrafish, coupled with similarities in development and behavior, these organisms serve as powerful alternatives to mammalian model to study human genetics and diseases.
- elegans models have many advantages over more complex model systems for use in biological and biomedical studies. The worms are anatomically simple, and have a fully mapped nervous system. In addition, humans and C. elegans utilize many similar genetic and molecular mechanisms. These features lead C. elegans to be heavily used as a model for aging, neurodegenerative disorders. Moreover, C.elegans is the multicellular organism the most amenable to ultra-high throughput screenings and drug discovery and is likely to become the organism of choice for in-vivo testing of new drugs.
Recently, the zebrafish has been used to study mechanisms leading to human diseases such as ALS and DCM. Predictably, this field will grow rapidly in the coming years owing to the increase in sequencing efforts, the growing interest in cardiac diseases, and the improved availability of the zebrafish model for clinical and basic researchers interested in studying cardiac diseases.
Because of the close similarities and the conservation of key genes between Drosophila and vertebrate cardiogenesis, the fly’s heart serves as a powerful model for cardiac development and disease. Study of fly organogenesis has led to an increased understanding of the mechanisms that underlie several cardiac and neurodevelopmental diseases.
Applying model systems to understand the mechanisms involved in human diseases allows researchers to identify common genes, proteins, and processes that underlie human medical conditions. It also permits systematic deciphering of the gene-gene and gene-environment interactions that influence complex multigenic disorders.
By developing new and more specialized assays, small model organisms will continue to contribute new insights to the mechanisms underlying complex human diseases.
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