A large number of mouse models have been engineered, characterized and used to advance biomedical research in Alzheimer disease (AD). Early models simply damaged the rodent brain through toxins or ...lesions. Later, the spread of genetic engineering technology enabled investigators to develop models of familial AD by overexpressing human genes such as those encoding amyloid precursor protein (APP) or presenilins (PSEN1 or PSEN2) carrying mutations linked to early-onset AD. Recently, more complex models have sought to explore the impact of multiple genetic risk factors in the context of different biological challenges. Although none of these models has proven to be a fully faithful reproduction of the human disease, models remain essential as tools to improve our understanding of AD biology, conduct thorough pharmacokinetic and pharmacodynamic analyses, discover translatable biomarkers and evaluate specific therapeutic approaches. To realize the full potential of animal models as new technologies and knowledge become available, it is critical to define an optimal strategy for their use. Here, we review progress and challenges in the use of AD mouse models, highlight emerging scientific innovations in model development, and introduce a conceptual framework for use of preclinical models for therapeutic development.
We show that a natural behavior, exploration of a novel environment, causes DNA double-strand breaks (DSBs) in neurons of young adult wild-type mice. DSBs occurred in multiple brain regions, were ...most abundant in the dentate gyrus, which is involved in learning and memory, and were repaired within 24 h. Increasing neuronal activity by sensory or optogenetic stimulation increased neuronal DSBs in relevant but not irrelevant networks. Mice transgenic for human amyloid precursor protein (hAPP), which simulate key aspects of Alzheimer's disease, had increased neuronal DSBs at baseline and more severe and prolonged DSBs after exploration. Interventions that suppress aberrant neuronal activity and improve learning and memory in hAPP mice normalized their levels of DSBs. Blocking extrasynaptic NMDA-type glutamate receptors prevented amyloid-β (Aβ)-induced DSBs in neuronal cultures. Thus, transient increases in neuronal DSBs occur as a result of physiological brain activity, and Aβ exacerbates DNA damage, most likely by eliciting synaptic dysfunction.
In light of the rising prevalence of Alzheimer’s disease (AD), new strategies to prevent, halt, and reverse this condition are needed urgently. Perturbations of brain network activity are observed in ...AD patients and in conditions that increase the risk of developing AD, suggesting that aberrant network activity might contribute to AD-related cognitive decline. Human amyloid precursor protein (hAPP) transgenic mice simulate key aspects of AD, including pathologically elevated levels of amyloid-β peptides in brain, aberrant neural network activity, remodeling of hippocampal circuits, synaptic deficits, and behavioral abnormalities. Whether these alterations are linked in a causal chain remains unknown. To explore whether hAPP/amyloid-β–induced aberrant network activity contributes to synaptic and cognitive deficits, we treated hAPP mice with different antiepileptic drugs. Among the drugs tested, only levetiracetam (LEV) effectively reduced abnormal spike activity detected by electroencephalography. Chronic treatment with LEV also reversed hippocampal remodeling, behavioral abnormalities, synaptic dysfunction, and deficits in learning and memory in hAPP mice. Our findings support the hypothesis that aberrant network activity contributes causally to synaptic and cognitive deficits in hAPP mice. LEV might also help ameliorate related abnormalities in people who have or are at risk for AD.
Environmental enrichment of laboratory animals influences brain plasticity, stimulates neurogenesis, increases neurotrophic factor expression, and protects against the effects of brain insult. ...However, these positive effects are not constantly observed, probably because standardized procedures of environmental enrichment are lacking. Therefore, we engineered an enriched cage (the Marlau™ cage), which offers: (1) minimally stressful social interactions; (2) increased voluntary exercise; (3) multiple entertaining activities; (4) cognitive stimulation (maze exploration), and (5) novelty (maze configuration changed three times a week). The maze, which separates food pellet and water bottle compartments, guarantees cognitive stimulation for all animals. Compared to rats raised in groups in conventional cages, rats housed in Marlau™ cages exhibited increased cortical thickness, hippocampal neurogenesis and hippocampal levels of transcripts encoding various genes involved in tissue plasticity and remodeling. In addition, rats housed in Marlau™ cages exhibited better performances in learning and memory, decreased anxiety-associated behaviors, and better recovery of basal plasma corticosterone level after acute restraint stress. Marlau™ cages also insure inter-experiment reproducibility in spatial learning and brain gene expression assays. Finally, housing rats in Marlau™ cages after severe status epilepticus at weaning prevents the cognitive impairment observed in rats subjected to the same insult and then housed in conventional cages. By providing a standardized enriched environment for rodents during housing, the Marlau™ cage should facilitate the uniformity of environmental enrichment across laboratories.
Loss-of-function (LOF) variants of TREM2, an immune receptor expressed in microglia, increase Alzheimer’s disease risk. TREM2 senses lipids and mediates myelin phagocytosis, but its role in ...microglial lipid metabolism is unknown. Combining chronic demyelination paradigms and cell sorting with RNA sequencing and lipidomics, we find that wild-type microglia acquire a disease-associated transcriptional state, while TREM2-deficient microglia remain largely homeostatic, leading to neuronal damage. TREM2-deficient microglia phagocytose myelin debris but fail to clear myelin cholesterol, resulting in cholesteryl ester (CE) accumulation. CE increase is also observed in APOE-deficient glial cells, reflecting impaired brain cholesterol transport. This finding replicates in myelin-treated TREM2-deficient murine macrophages and human iPSC-derived microglia, where it is rescued by an ACAT1 inhibitor and LXR agonist. Our studies identify TREM2 as a key transcriptional regulator of cholesterol transport and metabolism under conditions of chronic myelin phagocytic activity, as TREM2 LOF causes pathogenic lipid accumulation in microglia.
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•Upon demyelination, Trem2–/– microglia fail to upregulate lipid metabolism genes•Trem2–/– microglia accumulate cholesteryl ester derived from myelin cholesterol•Accumulation is rescued by ACAT1 inhibitor and LXR agonist•Apoe–/– glia defective in cholesterol transport also accumulate cholesteryl ester
TREM2 and APOE are implicated in late-onset Alzheimer’s disease. Here, Nugent et al. report that TREM2 upregulates Apoe and other damage-associated microglial genes upon chronic demyelination. Loss of either Trem2 or Apoe causes dysregulated cholesterol transport and metabolism in microglia.
Objective
Reducing levels of the microtubule‐associated protein tau has shown promise as a potential treatment strategy for diseases with secondary epileptic features such as Alzheimer disease. We ...wanted to determine whether tau reduction may also be of benefit in intractable genetic epilepsies.
Methods
We studied a mouse model of Dravet syndrome, a severe childhood epilepsy caused by mutations in the human SCN1A gene encoding the voltage‐gated sodium channel subunit Nav1.1. We genetically deleted 1 or 2 Tau alleles in mice carrying an Nav1.1 truncation mutation (R1407X) that causes Dravet syndrome in humans, and examined their survival, epileptic activity, related hippocampal alterations, and behavioral abnormalities using observation, electroencephalographic recordings, acute slice electrophysiology, immunohistochemistry, and behavioral assays.
Results
Tau ablation prevented the high mortality of Dravet mice and reduced the frequency of spontaneous and febrile seizures. It reduced interictal epileptic spikes in vivo and drug‐induced epileptic activity in brain slices ex vivo. Tau ablation also prevented biochemical changes in the hippocampus indicative of epileptic activity and ameliorated abnormalities in learning and memory, nest building, and open field behaviors in Dravet mice. Deletion of only 1 Tau allele was sufficient to suppress epileptic activity and improve survival and nesting performance.
Interpretation
Tau reduction may be of therapeutic benefit in Dravet syndrome and other intractable genetic epilepsies. Ann Neurol 2014;76:443–456
Effective delivery of protein therapeutics to the central nervous system (CNS) has been greatly restricted by the blood-brain barrier (BBB). We describe the development of a BBB transport vehicle ...(TV) comprising an engineered Fc fragment that exploits receptor-mediated transcytosis for CNS delivery of biotherapeutics by binding a highly expressed brain endothelial cell target. TVs were engineered using directed evolution to bind the apical domain of the human transferrin receptor (hTfR) without the use of amino acid insertions, deletions, or unnatural appendages. A crystal structure of the TV-TfR complex revealed the TV binding site to be away from transferrin and FcRn binding sites, which was further confirmed experimentally in vitro and in vivo. Recombinant expression of TVs fused to anti-β-secretase (BACE1) Fabs yielded antibody transport vehicle (ATV) molecules with native immunoglobulin G (IgG) structure and stability. Peripheral administration of anti-BACE1 ATVs to hTfR-engineered mice and cynomolgus monkeys resulted in substantially improved CNS uptake and sustained pharmacodynamic responses. The TV platform readily accommodates numerous additional configurations, including bispecific antibodies and protein fusions, yielding a highly modular CNS delivery platform.
Aging is the principal demographic risk factor for Alzheimer disease (AD), the most common neurodegenerative disorder. Klotho is a key modulator of the aging process and, when overexpressed, extends ...mammalian lifespan, increases synaptic plasticity, and enhances cognition. Whether klotho can counteract deficits related to neurodegenerative diseases, such as AD, is unknown. Here we show that elevating klotho expression decreases premature mortality and network dysfunction in human amyloid precursor protein (hAPP) transgenic mice, which simulate key aspects of AD. Increasing klotho levels prevented depletion of NMDA receptor (NMDAR) subunits in the hippocampus and enhanced spatial learning and memory in hAPP mice. Klotho elevation in hAPP mice increased the abundance of the GluN2B subunit of NMDAR in postsynaptic densities and NMDAR-dependent long-term potentiation, which is critical for learning and memory. Thus, increasing wild-type klotho levels or activities improves synaptic and cognitive functions, and may be of therapeutic benefit in AD and other cognitive disorders.
Alzheimer's disease (AD) is the most frequent and costly neurodegenerative disorder. Although diverse lines of evidence suggest that the amyloid precursor protein (APP) is involved in its causation, ...the precise mechanisms remain unknown and no treatments are available to prevent or halt the disease. A favorite hypothesis has been that APP contributes to AD pathogenesis through the cerebral accumulation of the amyloid-β peptide (Aβ), which is derived from APP through sequential proteolytic cleavage by BACE1 and γ-secretase. However, inhibitors of these enzymes have failed in clinical trials despite clear evidence for target engagement.
To further elucidate the roles of APP and its metabolites in AD pathogenesis, we analyzed transgenic mice overexpressing wildtype human APP (hAPP) or hAPP carrying mutations that cause autosomal dominant familial AD (FAD), as well as App knock-in mice that do not overexpress hAPP but have two mouse App alleles with FAD mutations and a humanized Aβ sequence.
Although these lines of mice had marked differences in cortical and hippocampal levels of APP, APP C-terminal fragments, soluble Aβ, Aβ oligomers and age-dependent amyloid deposition, they all developed cognitive deficits as well as non-convulsive epileptiform activity, a type of network dysfunction that also occurs in a substantive proportion of humans with AD. Pharmacological inhibition of BACE1 effectively reduced levels of amyloidogenic APP C-terminal fragments (C99), soluble Aβ, Aβ oligomers, and amyloid deposits in transgenic mice expressing FAD-mutant hAPP, but did not improve their network dysfunction and behavioral abnormalities, even when initiated at early stages before amyloid deposits were detectable.
hAPP transgenic and App knock-in mice develop similar pathophysiological alterations. APP and its metabolites contribute to AD-related functional alterations through complex combinatorial mechanisms that may be difficult to block with BACE inhibitors and, possibly, also with other anti-Aβ treatments.
Previous studies suggested that the cellular prion protein (PrP(c)) plays a critical role in the pathogenesis of Alzheimer's disease (AD). Specifically, amyloid-β (Aβ) oligomers were proposed to ...cause synaptic and cognitive dysfunction by binding to PrP(c). To test this hypothesis, we crossed human amyloid precursor protein (hAPP) transgenic mice from line J20 onto a PrP(c)-deficient background. Ablation of PrP(c) did not prevent the premature mortality and abnormal neural network activity typically seen in hAPPJ20 mice. Furthermore, hAPPJ20 mice with or without PrP(c) expression showed comparably robust abnormalities in learning and memory and in other behavioral domains at 6-8 months of age. Notably, these abnormalities are not refractory to therapeutic manipulations in general: they can be effectively prevented by interventions that prevent Aβ-dependent neuronal dysfunction also in other lines of hAPP transgenic mice. Thus, at least in this model, PrP(c) is not an important mediator of Aβ-induced neurological impairments.
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