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GEN Exclusives: January 10, 2017

Animal Models for Neurological Disorders

Dogs May Prove Man’s Best Friend in Alzheimer’s Research

  • Studies in animal models have proven extremely useful in understanding the mechanisms of many human diseases and in assessing new therapeutic strategies.

    But current animal models of human neurological diseases associated with aging, such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS), may not recapitulate key aspects of disease pathology. Recently, investigators have found that dog models of AD may present a truer, more translatable picture of the disease than genetic mouse models.

  • Mouse Models Do Not Recapitulate the Human Disease Spectrum

    Mice do develop age-dependent, progressive neuropathology and an accumulation of misfolded or aggregated proteins resulting from the expression of mutant genes. But, despite the widespread expression of mutant proteins throughout the body and brain, neuronal function appears to be selectively or preferentially affected in mice. Thus, these models have failed to anticipate subsequent clinical trial failures in humans.

    Frank M. LaFerla, Ph.D., and Kim N. Green, Ph.D., of the Institute for Memory Impairments and Neurological Disorders, department of neurobiology and behavior, University of California, Irvine, note that one of the most significant problems may be that many AD models to not reflect the extensive neuronal loss seen in the human condition.1

    According to these authors, because the etiology of idiopathic AD is unknown, animal models have been built to reflect genetic mutations associated with familial AD (fAD), with the rationale that the events downstream of the initial trigger are similar. While invaluable in determining the molecular mechanisms of disease progression and testing potential therapeutics, no single mouse model recapitulates all aspects of the disease spectrum. Each model allows for in-depth analysis of only one or two disease components.

    Transgenic mouse models of AD are produced by overexpressing mutant human amyloid precursor protein (APP) alone or combined with transgenic presenilin1 (PS1) and presenilin2 (PS2) genes in mice, which leads to amyloid-beta (Aβ) plaque formation. Unlike humans, however, transgenic mice show cellular and behavioral resilience to Aβ pathology and thus do not develop the extensive neuronal loss observed in the human AD brain. Human imaging and clinical-pathological studies show that patients with mild-to-moderate AD already have undergone brain atrophy, as well as extensive neuronal loss in several brain regions.

    Therapies pioneered in transgenic mouse models target primarily the pathologies modeled and do not address extensive neuronal loss. Many of these therapies, the authors say, may be effective at preventing or clearing the pathology, and hence the disease. But they are ineffective in patients in whom the pathology has already destroyed the requisite neurons for memory and cognition.

    These scientists and others believe that transgenic mice do not develop extensive neuronal loss like human AD patients, due to the amount of time needed for the loss to occur. In human AD, the disease progresses over decades, during which time synaptic disturbances, such as those measured in transgenic mice, could eventually lead to neuronal death. The two years for which most transgenic APP mice are kept may not be long enough for these changes to occur in mice.

  • Nonhuman Primate Models Do Not Develop AD

    Nonhuman primates are “somewhat paradoxical models of AD-like pathology,” says Eric Heuer, Ph.D., of the Yerkes National Primate Research Center, Emory University, and colleagues.2

    The molecular “key players” in the pathogenesis of AD—the Aβ protein and tau proteins—have significant homology among primates. With age, all nonhuman primates analyzed to-date develop senile Aβ plaques and cerebral Aβ angiopathy. In contrast, significant tauopathy is unusual in simians, and only humans manifest the profound tauopathy, neuronal degeneration, and cognitive impairment that characterize AD.

    Despite their biological closeness to humans, no nonhuman primate species has yet been shown to develop AD.3 Yet, investigators say that nonhuman primates do provide a good model of brain aging and Aβ-amyloidosis in the absence of neurodegenerative disease. The incomplete manifestation of AD-like changes in simians may provide clues to the unique susceptibility of the human brain to AD.

  • Marmosets Are a Potential Candidate for Modeling Neurodegenerative Diseases

    In the nonhuman primate world, some investigators have nominated the common marmoset (Callithrix jacchus), a New World monkey, as a good potential candidate for efficiently modeling human neurodegenerative diseases and aging and age-related disease.

    Suzette D. Tardif, Ph.D., of the Barshop Institute for Longevity and Aging Studies, University of Texas Health Science Center at San Antonio, and colleagues explain that with an average and maximum lifespan that is 30–40% that of commonly used Old World monkeys (e.g., macaques) aging studies can be accomplished in a fraction of the time required with larger, nonhuman primates.4 Marmosets display age-related spontaneous sensory and neurodegenerative changes, such as reduced neurogenesis, Aβ deposition in the cerebral cortex, loss of calbindin D28k binding, and evidence of senso-neuronal hearing loss.

    The spontaneous occurrence of these phenotypes in marmosets ranging in age from 7 to 10 years suggests that this animal may prove a valuable model for research on selected aspects of age-related neurodegenerative change. Similar to other nonhuman primates, the sequenced marmoset genome has high homology—about 93% with that of humans.

    Other proponents of marmosets as models of neurodegenerative diseases include Adam B. Salmon, Ph.D., also of the Barshop Institute. Marmosets display age-related changes in pathologies associated with diabetes, cardiac disease, cancer, and renal disease similar to those seen in humans, he says, providing a complement to the existing nonhuman primate models that are used to study aging and, in particular, a model in which effects on longevity might be assessed in a relatively timely manner.

  • Dogs and Humans Share Identical Aß Amino Acid Sequence

    Elizabeth Head, Ph.D., of the department of molecular and biomedical pharmacology of the University of Kentucky, and others propose that canine models of AD complement existing animal models and will prove helpful in developing new vaccine approaches to slowing or preventing Aβ pathology.

    The aged dog, she points out,5 develops cognitive decline in learning and memory and shows human-like individual variability in the aging process. And unlike mice, dogs show effects of treatments designed to slow or reverse AD-like pathological changes that can be replicated in humans. Dr. Head and colleagues point out that aged dogs accumulate human-type Aβ in diffuse plaques in the brain with parallel declines in cognitive function.6

    One of the key reasons, Dr. Head and colleagues note, that the canine brain has been examined extensively for Aβ neurologic disorders is that dogs and humans share an identical amino acid sequence of the protein, thought to be a causative factor for AD in humans. The observation of brain Aβ first stimulated interest in the use of the dog to model human aging and disease.

    Diffuse plaques are the predominant subtype in the aging dog brain, with specific brain regions showing differential vulnerability to Aβ. When cortical regions are sampled for Aβ deposition, each region shows a different age of Aβ onset.

    Dr. Head and her colleagues reasoned that reduction of Aβ in a therapeutic treatment study of aged dogs with preexisting Aβ pathology and cognitive deficits would lead to cognitive improvements.6 The investigators immunized beagles (ages 8.4 to 12.4 years) with fibrillar Aβ for 2.4 years, totaling 25 vaccinations.

    While cognitive testing during this time indicated that no improvement in measures of learning, spatial attention, or spatial memory occurred, after 22 vaccinations the investigators observed maintenance of prefrontal-dependent reversal of learning ability. In the animals’ brains, soluble and insoluble Aβ and the extent of diffuse plaque accumulation decreased significantly in several cortical regions, with preferential reductions in the prefrontal cortex, which is associated with a maintenance of cognition.

    However, the amount of soluble oligomers remained unchanged. The extent of prefrontal Aβ was correlated with frontal function and serum anti-Aβ antibody titers. Thus, the investigators noted, reducing total Aβ alone may be of limited therapeutic benefit to recovery of cognitive decline in a higher mammalian model of human brain aging and disease.

    They further concluded that immunization of animals prior to extensive Aβ deposition and cognitive decline to prevent oligomeric or fibrillar formation may have a greater impact on cognition and also more directly evaluate the role of Aβ in cognition in canines.

    In a more recent study,7 Dr. Head and colleagues extended these findings by exploring the potential benefits of combining the Aβ immunotherapy strategy with behavioral enrichment, which has been previously shown to improve cognition independently of Aβ reduction in canines. The investigators immunized beagles (ages 10.5 to 13.6 years) with fibrillar Aβ for 2 years, with some groups receiving behavioral enrichment alone or in combination with the immunotherapy.

    The combination treatment resulted in a significant enhancement of maintenance of learning over time, reduced Aβ, and increased brain-derived neurotrophic factor mRNA; however, there were no benefits to memory. They concluded that benefits could be elicited, even in aged canines with extensive Aβ deposition and cognitive dysfunction at the time of initiation of therapy. In the future, prevention studies using a combination-therapy approach in middle-aged canines may result in memory maintenance, which is promising for patients with mild-moderate AD.

    Ted Barrett, Ph.D., director, translational research/pharmacology, Lovelace Biomedical, and a collaborator in this research, told GEN that Lovelace’s extensive work with dogs, particularly beagles, for neurodegenerative diseases has “been ongoing for 20 years.” Lovelace Biomedical, a not-for-profit preclinical contract research organization, offers toxicology, pharmacology, and other services to pharmaceutical and biotechnology companies.

    Dogs offer a “natural host model” for human neurodegenerative diseases, says Dr. Barrett. “You don’t need to make a transgenic animal and you can see the complexities of human disease in a natural setting.” He added that, “the Aβ amino acid sequence is identical in dogs and humans. While the aged dogs don’t develop the neurofibrillary tangles of human AD, they share many features with mild or moderate AD and do show elevated tau.”

    GEN asked Dr. Barrett whether any early-adopter AD drug developers have considered the dog as a more predictive model than rodents.

    “One of the challenges with the dog has been that in relation to rodent studies dog studies are expensive and lengthy. But the great advantage is that dogs more closely model human disease and decline. And the vaccine study results follow what has happened in human clinical trials, showing a limited effect on preserving or improving cognitive function with immunotherapy alone.”

    Dr. Barrett says Lovelace is “trying to make a concerted push, especially in light of clinical trials not meeting expectations. We think the time might be right for us to make the effort to further engage with the pharma and biotech industries to see if there is renewed interest in using alternative models, like the dog, to supplement other preclinical work with transgenic mouse models for neurocognitive disorders, such as AD.” 

    But he cautions that drug development companies always push for tight cost and development timelines, which have historically made rodent models more attractive. However, in diseases in which a long-term horizon is key to pathogenesis, a model with a much longer lifespan than a mouse may be needed.