The study of model organisms such as Mus have proven to be insufficient in unlocking mechanisms by which biological functions cope with the complexity of environmental conditions in which life develops. Thus, scientists have turned to organisms that are not well characterized and cultivated as stable species in controlled laboratory conditions. These organisms are classed “non-model organisms”.
Since the discovery of their insect-like behaviours, naked mole-rats (NMRs) have gained interest from biologist across the world. NMRs are ideal animal subjects for medical studies that embrace broad principles in physiology, which in turn, opens important avenues of research such as aging, cancer and nociception research with the aim of improving human life.
NMRs have high levels of cysteine residues acting as a buffer of oxidative damage, which aids in their longevity thus; NMRs are exceptions to many current theories of aging including the widely accepted “Oxidative Damage/Stress Theory”. “Early Contact Inhibition” is a mechanism mediated by the interaction of high molecular mass hyaluronic acid (HMM-HA) with CD44–Neurofibromin 2 (NF2) to render cancer resistance in NMRs and although, they possess pain sensitive nerve fibers, NMRs are insensitive to pain and exhibit avoidance behaviours to noxious stimuli such as acid-induced pain. Furthermore, living underground in large colonies with low O2 levels, NMRs have evolved extreme resistance to hypoxia, a condition that causes neuronal damage and cellular death in humans.
Bat biology in particular, their physiology is advancing medical research in a number of different fields such as aging research but unlike NMRs, longevity in bats is due to mutations in genes involved in growth including growth hormone receptor and insulin-like growth factor 1 receptor, together with adaptations of hibernation and low reproductive rates. Certain bats also possess natural substances that is being exploited in medicine to effectively treat acute ischaemic stroke as a replacement for tissue-type plasminogen activator (tPA), which has significant negative effects that include neurotoxicity.
In this paper, I have summarized the latest developments of some non-model organisms used in research, and the impact of their study in medical research.
Model organisms such as Escherichia coli, Fruitfly Drosophila melanogaster, and Mus have been crucial in investigating cellular and molecular processes that underpin life. However, these organisms have proven to be insufficient in unlocking mechanisms by which biological functions cope with the complexity of environmental conditions in which life develops. Thus, to understand the mechanisms of adaptation of biological organisms to changing environmental conditions, it is important to increase the diversity of organisms used in medical research (1, 2).
To define a non-model organism, it is important to identify the characteristics that constitute a model organism. A model organism is an organism with a representational scope characterized by: easy cultivation as a stable isolated species in controlled laboratory conditions; its small size and short lifespan; description of its development and traits; the ability to easily be genetically manipulated (1, 3). Therefore, based on the characteristics that constitute a model organism, a non-model organism can be described as an organism that is not well characterized and cultivated as stable species in controlled laboratory conditions as they require more effort in protocol adaptation and method development and as a result make difficult experimental organisms (1, 3).
Non-model organisms are important organisms used in medical research not only because they differ from current models but their study also constitutes the heart of comparative biology, exposing mechanisms that underlie biological diversity. That is, the study of non-model organisms have provided an insight into certain processes and physiological changes that occur in life such as aging. Non-model organisms serve as great surrogates for human studies in environmental science because they are capable of surviving and coping with environmental alterations. Thus, studying the molecular mechanisms of adaptation of non-model organisms can be critical in developing drugs against certain diseases such as hypoxic related injury (1, 3).
There are over 30 different mole rats, however; the most common one is the naked mole-rat (NMR) (4). The NMR (Heterocephalus glaber) is a subterranean mammal of the family Bathyergidae endemic to semi-arid regions of East Africa mainly Kenya, Ethiopia and Somalia as shown in figure 1: the distribution of naked mole-rats (shaded blue) (5, 6). Mole rats including NMRs spend their entire life living underground in burrows (comprising mazes of tunnels with numerous chambers) where atmospheric conditions differ from those above ground, in that the relative humidity is higher and both hypoxic and hypercapnic conditions may be encountered. However, the conditions in the burrow are relatively stable because there is very little air exchange. With the burrow air remaining between 30 to 32°C year round, NMRs have very little need to control their body temperature. It is for this reason that researchers believe that NMRs are poikilothermic (5, 6).
In addition to being poikilothermic, NMRs are also eusocial, which is defined by characteristics of cooperative brood care, overlapping generations within a colony of adults, and a division of labour into reproductive and non-reproductive groups. The division of labor creates specialized behavioral groups within an animal society, often called “castes”. A caste consists of up 20 to 300 individuals produced by a single breeding queen who suppresses the fertility of females, and most male members of the colony except for one to three male members (6, 7).
NMRs are about 8 to 9 centimeters (cm) long and weigh between 30 to 80 grams (g). Their skin is almost hairless and appears wrinkled because they have no fat under their skin. Like all rodents, two of their front protruding teeth grow continuously throughout their lives due to the continuous wear and tear during the process of digging. They have a cylindrical shaped body and small limbs that are suited for burrow living. They have tiny eyes that can only detect light and dark, and tiny ears with no external lobes as shown in the image above (figure 2) which illustrates some features of the NMR (7).
The image below (figure 3) is a summary of the underground lifestyle of NMRs.
Since the discovery of their insect-like behaviors, NMRs have gained interest from biologist across the world. Biologists have labeled NMRs unique and fascinating mammals because the molecular mechanisms that enable their unusual biology are now understood (5, 6). NMRs have a long lifespan, living over 30 years showing no signs of age related changes and aging. They have evolved numerous physiological mechanisms of adaptation that enables them to live and survive within extreme environmental conditions of low oxygen and high carbon dioxide levels. Thus, the wide spectrum of social organizations coupled with their adaptations to extreme living conditions makes NMRs ideal and suitable animal subjects for medical studies that embrace broad principles in physiology. This, in turn, opens important avenues of research that have implications for understanding these processes in humans and hence improving human life (5, 6, 7).
Human aging remains one of the most poorly understood biological phenomena, mainly because it is an integrative and complex process. Aging however, is a gradual, pervasive, and irresistible process that cause a decrease in function including physiological processes and reproductive capabilities such that overtime fit individuals become weak and turn into older individuals (as shown in figure 4 below, illustrating the physical changes associated with aging) with an increase in age-associated damage such as neurodegeneration (8, 9, 10).
Aging is a major factor that affects late life and healthy living. Thus, understanding why we age, how we age, and the physiological changes that occur during aging is a major medical research challenge (8, 9, 10). Most of the Information we know about the physiological processes and mechanism involved in human aging has been mainly gleaned using a variety of model organisms (11, 12). However, using model organisms in human aging research only provides a limited knowledge of the processes involved due to their short lifespan therefore; their study causes us to miss important factors that contribute to aging. Species with long extraordinary lifespans are more suitable for human aging research; one of such species is the NMR (10, 12).
NMRs are the longest-lived rodents known, remarkably; they appear to maintain good health for most of their lives (6, 13). At an age equivalent to a human age of 92 years, the level of activity and metabolic rate, as well as sustained muscle mass, bone density, cardiac health, and neuron numbers remain unchanged throughout the life of an NMR (13). NMRs eventually age, showing signs of muscle loss and cardiac dysfunction; however, they delay the onset of aging and compress the period of decline into a small fraction of their overall lifespan (13). These clear indications of both attenuated and delayed physiological aging in NMRs are signs of good maintenance of protein quality and gene expression levels, proven correct by Pérez et al. (2009) whereby they performed a comprehensive study of oxidation states of protein cysteines in mice and mole-rats. When compared to mice, proteins in NMRs have higher levels of cysteine residues, which act as a buffer of oxidative damage. While aging organisms accumulate proteins that exhibit both irreversibly oxidized cysteine and polyubiquitination, NMRs show no age-related changes in their overall low levels of either, which indicates that their proteins are kept in a healthy state throughout their life (14).
These findings of sustained good health are surprising given that NMRs are exceptions to many current theories of aging, which elucidate mechanisms involved in human aging (13). For example, “The Telomerase Length Theory” states that telomeres, the repetitive DNA that caps the ends of chromosomes undergoes senescence and becomes shorter with consecutive cell division, is a biomarker of aging and will correlate with species lifespan (13, 15). However, compared to the much shorter-lived laboratory mouse, NMRs have shorter telomeres (13). Alternatively, cellular levels of telomerase, a ribonucleoprotein that adds a species-dependent telomere repeat sequence to the 3′ end of telomeres, correlate with species longevity but while telomerase activity has been measured in mole-rat skin cells in culture, it is generally very low and is limited to those tissues that are actively replicating such as testes, spleen, and skin. Thus, telomere length or maintenance is unlikely to explain the exceptional longevity of the naked mole-rat (13).
Another theory is the widely accepted “Oxidative Damage/Stress Theory of Aging” (11). The oxidative theory postulates that aging results from accumulated damage of reactive oxygen species (ROS), a by-product of aerobic respiration (13, 15). This theory is particularly appealing because NMRs in captivity, at a young age show high levels of oxidative damage, in other words, at a young age NMRs have increased levels of accumulated DNA oxidative damage yet their cellular functions are not impaired, and the animal is able to tolerate high levels of oxidative damage for over 20 years (13). Thus, NMRs possess mechanisms that enable them to reduce and resist oxidative damage as they grow which in turn contributes and increases their lifespan. The study of NMRs is advancing medical research in a sense that not only does it provide a better insight into the timing, mechanisms, and processes involved in how humans age but also helps shine a light on theories postulated to cause human aging (11-15).
NMRs also satisfy the principle of negligible senescence, characterized by the lack of age-related changes in reproductive and physiological functions. NMRs do not develop symptoms associated with aging such as neurodegeneration and cancer (6).
Therefore, due to their extraordinary lifespan coupled with their lack of age-related changes in reproductive and physiological function, NMRs possess outstanding anti-aging defenses. Thus, studying NMRs is advancing medical research in the sense that, despite subtle differences in physiological function, the mechanisms that enhance longevity and senescence impediment may be of great prevalence to provide an insight into the biology of human aging (11, 12).
According to Cancer Research UK and as shown in the table above (figure 5), there were more than 356,000 new cases of cancer and over 164,000 reported deaths in 2014 in the UK (16). These statistics point to the fact that we need to improve our knowledge and understanding of the biological mechanisms that cause cancer in order to identify new therapeutic targets (6).
NMRs are organisms that rarely develop cancer. Furthermore, subjecting NMRs to ionizing radiation does not induce much DNA damage as seen in other animals, nor does it result in the development of a tumor even 5 years later (13, 17). This was shown in a study by Liang et al. where upon injection of an oncogene (a retrovirus encoding SV40 large T antigen and oncogene Ras (G12V)) in NMRs cells, tumorigenesis failed to elicit, cells rather entered a state of crisis, a tumor suppressor mechanism in response to oncogene–expression (17, 18).
In addition to entering a state of crisis, NMRs have other mechanisms that researchers might be able to leverage for the treatment of cancer (6). For example, one common mechanism used to prevent cancer in humans is replicative senescence, a mechanism that limits the number of times a cell can divide accomplished by telomerase suppression in somatic cells during adult life as shown in figure 6 below; the mechanism of replicative senescence (6).
In rodents such as NMRs, this is not the case because they retain their telomerase activity in somatic cells, suggesting other mechanisms help them resist developing cancer (6). The mechanism that confers cancer resistance in NMR is a phenomenon termed “Early Contact Inhibition (ECI)”. ECI causes cell cycle arrest when cells encounter each other or the extracellular matrix (19). The effect of ECI is mediated via the retinoblastoma (Rb) pathway, a pathway that contains regulators of the cell cycle. The Rb pathway is backed up by an apoptotic response mediated by the p53 pathway, which is associated with the induction of p16 (6). NMRs also rely on the upregulation of p27 for contact inhibition, which causes cells to become dense and ultimately undergo apoptosis (6). Thus, as ECI is an important mechanism that enables NMRs to resist developing cancer, understanding what activates ECI could be a stepping-stone for developing drugs that can inhibit cancer in humans (6).
Researchers have pinpointed at a natural substance that causes the activation of ECI (6). ECI is therefore, thought to be triggered by the secretion of HMM-HA (6, 20). HMM-HA is an unbranched disaccharide glucuronic acid/N- acetylglucosamine polymer of the extracellular matrix that accumulates abundantly in the tissues owing to a decreased activity of HA-degrading enzymes and the unique sequence of hyaluronan synthase 2 (HAS2) (19). The HMM-HA produced in NMRs is five times larger than the HA produced in humans and as in humans, HMM-HA acts via CD44 receptor which also interacts with NF2 encoding merlin to activate the induction of p16 which coincides with ECI, which is shown in figure 7. Thus, ECI is mediated by the interaction of HMM-HA with CD44–NF2 (5, 19, 20). Furthermore, NMRs also produce a tumor suppressor protein, which is an isoform of INK4 in response to HMM-HA stimulation, which leads to the induction of contact inhibition, and thus, renders cancer resistance (21).
Cancer is a disease that generally affects most mammalian species, and believed to be an unavoidable accompaniment of aging. Therefore, understanding the basis for cancer resistance in the NMR could be important in defining causes of cancer susceptibility and resistance in humans (21).
The ability to detect pain is crucial to an organism’s well-being. The neurobiology of pain is, therefore, of fundamental interest to medical researchers (22, 23). Pain is a complex experience triggered by real or perceived tissue damage, which in turn, causes manifestations of certain autonomic, psychological, and behavioural reactions (24). Pain can be acute or chronic but chronic pain is more severe and serves no survival benefit with as much as 19% of the adult human population expected to experience chronic pain at some point in their life (6, 23).
Recent studies using NMRs in pain research has helped identify some molecular mechanisms that drive pain (6). In humans and most mammals, the sensory nerve cells that detect and respond to pain are nociceptors, which are either myelinated (Aδ) fibers or unmyelinated C-fibres (22). The unmyelinated C-fibres express substance P (SP) and calcitonin gene-related peptide (CGRP). SP and CGRP are signaling neuropeptides found in the brain and spinal cord responsible for mediating the effect of pain by binding to neurokinin-1 receptors and G-protein coupled receptors respectively (6). However, in NMRs, although they possess nerves that contain unmyelinated C-fibres, their C-fibres completely lack SP and CGRP (6). Thus, NMRs are insensitive to pain or exhibit avoidance behaviours to noxious stimuli. This effect was demonstrated in a study by laVinka PC et al. and Smith ES et al. where they revealed that NMRs are insensitive to histamine-induced itch, capsaicin, and ammonia fumes, which activate C-fibers to mediate nociception in other organisms such as mice (25, 26). In others studies by Park TJ et al., laVinka PC et al., and Smith ES et al. they demonstrated that neurons in NMRs are also, insensitive to acidic-induced pain (27, 28). They concluded this to be due to the actions of their voltage-gated sodium channels (NaV1.7), which undergoes mutations to cause them to shut down under acidic conditions thereby, strongly inhibiting and avoiding pain signals which are normally transduced along nerve fibers (23, 27, 28).
As mentioned previously, NMRs are eusocial mammals living in underground burrows with poor air circulation resulting in high carbon dioxide levels and low oxygen levels. Concerning high CO2, elevated CO2 in most mammals drives tissue acidosis and pain. However, NMRs have a number of putative adaptive mechanisms to live under chronic these conditions as explained previously and summarised in figure 8 (pain free in the NMR) (23, 27, 28).
Thus, one interesting evolutionary question is, why might this have evolved in NMRs? Alternatively, why do NMRs appear to possess redundant mechanisms that render insensitivity? To answer this, the obvious reason for this evolution is due to living in chronically high CO2/acidic environments. In other words, because of the selection pressure arising from the extremity of their normal habitat (27, 28).
Therefore, further investigations of the biology of pain and the processes that confer alterations of pain in NMRs may provide a significant insight into the components necessary for transduction of painful stimuli. This, in turn, could provide vital implications for the treatment of chronic pain in humans such as inflammatory, joint, and muscle pain etc. (29).
Most mammalian brains including those of humans suffer serious damage after just 3 to 4 minutes of oxygen deprivation as shown in figure 9; which illustrates the time it takes for brain damage (29). Cells in the brain primarily require oxygen during aerobic metabolism to produce adenosine triphosphate (ATP), which in turn, is used to maintain the concentration gradient and membrane potential of cells during synaptic transmission (6). Therefore, a decrease in ATP due to oxygen deprivation results in disruption of ion and neurotransmitter homeostasis, ultimately causing neuronal damage and cellular death (6).
“Hypoxia” is the term used to describe lack of oxygen supply to cells in the body and as a result is involved in many pathological conditions including neurodegenerative disorders such as Huntington’s disease. Hypoxia is also a major concern for victims of heart attack and stroke, where, the blood supply to the brain is interrupted (29).
The use of hypoxic sensitive models organisms such as mice has to an extent advanced our understanding of some mechanisms that contribute to hypoxia-induced neuronal damage and has enabled us to develop some therapeutic targets. However, studying organisms that have evolved effective physiological, molecular, and cellular strategies to survive hypoxic insults would enable us to have a better understanding and thus, aid in discovering new targets for preventing and treating hypoxia-associated damages. Organisms with subterranean lifestyles, NMRs have yet again received great attention for their ability to cope with hypoxic insults (6).
In NMRs, the brain tissues remain functional even with low or no oxygen supply. In addition, when the levels of oxygen are oxygen restored the brain tissues recover fully even after several minutes of inactivity (29).
The ability of NMRs to tolerate hypoxic insults is due to their low basal metabolic rate, further reduced to cause them to; slowly consume oxygen so that the supply matches the demand when pools of ATP are decreased during hypoxic conditions. NMRs also possess haemoglobin with a high affinity for oxygen, in other words, their blood is better at capturing what little oxygen there is in their environment (6, 29, 30).
In addition to the physiological adaptations explained above, NMRs are also tolerant to hypoxia at the neuronal level where their brain tissues possess adaptive characteristics that enable them to survive chronic hypoxic conditions (29, 30). NMRs have a mechanism at the neuronal level that mirrors that used by the brain of an infant in the womb (29). Infant babies somehow cope with and tolerate low levels of oxygen where calcium is a key factor. Starving nerve cells of oxygen as mentioned earlier results in lack of energy formed to regulate the amount of calcium entering the cells, in turn causing an influx of calcium, which poisons the cell and ultimately causes neuronal death. In infants, calcium channels close during lack of oxygen, which protects the brain cells from calcium overdose in the womb (29). However, after the baby is born these channels are replaced by ones that open in response to oxygen deprivation thus; can cause neuronal damage and cellular death (29).
In a recent study by Peterson et al. on NMRs, they showed that NMRs retain an infant style of calcium channels into adulthood (31). NMRs avoid hypoxic insults by reducing signals that function to transport calcium into the cell thus, avoiding the resulting neurotoxicity. However, they do this by modulating the GluN2D subunit of their neuronal N-Methyl-D-aspartate receptor (NMDAR). This makes NMRs one of a few organisms known to modulate their glutamatergic activity to effectively deal with hypoxic insults (6, 30).
Therefore, investigating and understanding the strategies by which NMRs cope with and survive hypoxic insults will help us improve our knowledge of the mechanisms involved in reducing neurotoxicity during hypoxia, which in turn, would help in identifying new therapeutic targets for preventing and effectively tackling hypoxic associated injuries such as heart attacks, stroke, and epilepsy etc.
Bats are a unique group of beneficial yet misunderstood mammals mainly because they are routinely feared and deemed as sinister denizens of the night, in other words, certain popular cultures view bats as evil and bloodsucking creatures that brings about death but bats are far from being our enemy and now are friends (32). Bats are mammals that belong to the order Chiroptera, grouped into two suborders the fruit-eating megabats and the microbats that prey on insects, blood, and nectar. Bats are extremely diverse, they represent about a fifth (20%) of all classified mammalian species worldwide (33, 34).
Bats are extraordinary in a number of ways; they are nocturnal animals active at twilight. Their forelimbs form webbed wings that enable them to fly (33). Bats are shy creatures thus; their social organization of living varies. That is, some bats live solitary whiles others live in caves colonized by millions of bats. Bats are also among the few mammals that use specialized highly sophisticated echolocation systems similar to a sonar to navigate and locate food (33).
Bats are important mammals not only because of their ecological role in pollinating flowers and consuming masses of insect pests thereby; reducing the need for pesticides but also, because studying their biology, in particular, their physiology is advancing medical research in a number of different fields, which are briefly discussed below (33).
Just like mole-rats for example, the NMR, bats have an exceptionally long lifespan and a low incidence of tumorigenesis. The Brandt’s bat (Myotis brandtii) has been reported to holds the record with regard to lifespan (reported lifespan of 41 years) among the bats (34). Brandt’s bats exist in temperate regions throughout Europe and Asia, and they hibernate during the winter (34). However, unlike NMRs, Seim et al. showed that mutations in genes involved in growth including growth hormone receptor (GHR), and insulin-like growth factor 1 receptor (IGF1R), together with adaptations of hibernation and low reproductive rates confer longevity in the Brandt’s bat (34).
Acute ischaemic stroke (AIS) is a serious condition caused by thrombotic or embolic occlusion of a cerebral artery. AIS is normally treated with tissue-type plasminogen activator (tPA), a serine protease found in endothelial cells involved in the breakdown of blood clots by catalysing the conversion of plasminogen to plasmin, the major enzyme responsible for clot breakdown. Because it works on the clotting system, tPA is used to treat embolic or thrombotic stroke and although proven effective, this drug has significant negative effects that include systemic plasminogen consumption, fibrinogenolysis and causes neurotoxicity by triggering overreaction on N-Methyl-D-aspartic acid (NMDA) receptors, which in turn, hastens neuronal and cellular death (35). Thus, because of these negative effects another new option for treating stroke under clinical studies (studies by Brott et al. (1992) and Hacke et al. (2004)) is desmoteplase or Desmodus rotundus plasminogen activator (DSPA), an enzyme found in the saliva of vampire bats (Desmodus rotundus) that targets a mass of coagulated blood and quickly breaks it down (36, 37). When the vampire bat preys, this enzyme goes to work and ensures that there is a continuous flow of blood so that it can feed until it is satisfied (31, 32). Thus, in studying bats scientists are exploring the potential use of an anticoagulant compound that can be effective in treating blood clot or ischaemic disorders such as stroke.
Another vital aspect of the physiology of bats is the ability of their hearts to cope and function during low temperatures. The human heart is incapable of surviving below the body temperatures of 20°C. However, the heart of a bat can continue to function even at low temperatures approaching 0°C. Most bat species use a process known as “torpor”, a state of decreased physiological activity to drop their body temperature on a daily basis, which in turn, reduces the energy requirement of tissues and render their body to survive in conditions of little or no blood flow to the extremities (38).
Since the 1950s, surgeons have adopted this method during open-heart surgery by inducing surgical hypothermia. Unfortunately, there are still major risks associated with surgical hypothermia such as reperfusion injuries (38). Studies on hibernators such as bats have shown that certain chemicals activate receptors in the myocardial tissues during low temperatures. Thus, studying the mechanisms that confer a bats heart to function during hypothermia is advancing medical research in the field of cardiac surgery, in the sense that scientists may be able to produce a drug that would enable the human heart to stay functional during surgical hypothermia and therefore, reduce possible risk factors such as reperfusion injury (38).
Clearly, studies involving non-model organism has highlighted many key facets of their biology that is directly relevant to medical research. Indeed, these studies have yielded critical information regarding mechanisms and processes of aging, cancer, and the harmful effects that occur when oxygen levels are low. Thus in sum, Non-model organisms are important organisms used in medical research, not just, because of the fact that they differ from current models, but their study constitutes the heart of comparative biology, exposing mechanisms that underlie biological diversity. Therefore, it will be exciting to be a part of the continued research on these incredible and fascinating species that are likely to reveal novel drug targets for a variety of human ailments.