Cholesterol and copper affect learning and memory in the rabbit

The Function of Transthyretin Complexes with Metallothionein in Alzheimer's Disease

Alzheimer’s disease (AD) is one of the most frequently diagnosed types of dementia in the elderly. An important pathological feature in AD is the aggregation and deposition of the β-amyloid (Aβ) in extracellular plaques. Transthyretin (TTR) can cleave Aβ, resulting in the formation of short peptides with less activity of amyloid plaques formation, as well as being able to degrade Aβ peptides that have already been aggregated. In the presence of TTR, Aβ aggregation decreases and toxicity of Aβ is abolished. This may prevent amyloidosis but the malfunction of this process leads to the development of AD. In the context of Aβplaque formation in AD, we discuss metallothionein (MT) interaction with TTR, the effects of which depend on the type of MT isoform. In the brains of patients with AD, the loss of MT-3 occurs. On the contrary, MT-1/2 level has been consistently reported to be increased. Through interaction with TTR, MT-2 reduces the ability of TTR to bind to Aβ, while MT-3 causes the opposite effect. It increases TTR-Aβ binding, providing inhibition of Aβ aggregation. The protective effect, assigned to MT-3 against the deposition of Aβ, relies also on this mechanism. Additionally, both Zn₇MT-2 and Zn₇MT-3, decrease Aβ neurotoxicity in cultured cortical neurons probably because of a metal swap between Zn₇MT and Cu(II)Aβ. Understanding the molecular mechanism of metals transfer between MT and other proteins as well as cognition of the significance of TTR interaction with different MT isoforms can help in AD treatment and prevention.

Critical Review of Exposure and Effects: Implications for Setting Regulatory Health Criteria for Ingested Copper

Decades of study indicate that copper oral exposures are typically not a human health concern. Ingesting high levels of soluble copper salts can cause acute gastrointestinal symptoms and, in uncommon cases, liver toxicity in susceptible individuals with repeated exposure. This focused toxicological review evaluated the current literature since the last comprehensive reviews (2007-2010). Our review identified limitations in the existing United States and international guidance for determining an oral reference dose (RfD) for essential metals like copper. Instead, an alternative method using categorical regression analysis to develop an optimal dose that considers deficiency, toxicity, and integrates information from human and animal studies was reviewed for interpreting an oral RfD for copper. We also considered subchronic or chronic toxicity from genetic susceptibility to copper dysregulation leading to rare occurrences of liver and other organ toxicity with elevated copper exposure. Based on this approach, an oral RfD of 0.04 mg Cu/kg/day would be protective of acute or chronic toxicity in adults and children. This RfD is also protective for possible genetic susceptibility to elevated copper exposure and allows for background dietary exposures. This dose is not intended to be protective of patients with rare genetic disorders for copper sensitivity within typical nutritional intake ranges, nor is it protective for those with excessive supplement intake. Less soluble mineral forms of copper in soil have reduced bioavailability as compared with more soluble copper in water and diet, which should be considered in using this RfD for risk assessments of copper.

A High-Cholesterol Diet Increases 27-Hydroxycholesterol and Modifies Estrogen Receptor Expression and Neurodegeneration in Rabbit Hippocampus

Hypercholesterolemia has been implicated in numerous health problems from cardiovascular disease to neurodegeneration. High serum cholesterol levels in midlife have been associated with an increased risk of developing Alzheimer's disease (AD) later in life which suggests that the pathways leading to AD pathology might be activated decades before the symptoms of the disease are detected. Cholesterol-fed animals, particularly cholesterol-fed rabbits, exhibit brain pathology similar to the changes found in brains of AD patients. Dietary cholesterol, which cannot pass the blood-brain barrier, is thought to influence central nervous system homeostasis by increased transport of its circulatory breakdown product, 27-hydroxycholesterol (27-OHC), into the brain. 27-OHC is an endogenous selective estrogen receptor modulator. Estrogen-mediated non-reproductive functions require estrogen receptors (ERs) and include modulation of mitochondrial function and structure, as well as regulation of synaptogenesis in the brain. ERs are located in brain areas affected early in AD pathogenesis, including the hippocampus. Here we report that increase in serum cholesterol, induced by feeding rabbits a high-cholesterol diet, is associated with higher levels of 27-OHC in the brain as well as increased levels of neurodegeneration in the hippocampus. Furthermore, these results are accompanied by changes in expression of ERs in the hippocampus as well as a decrease in hippocampal mitochondria. These findings provide an important insight into one of the possible mechanisms involved in the development of AD, and shed light on the processes that may antedate amyloid-β and tau phosphorylation changes currently hypothesized to cause AD symptomology and pathology.

The rabbit as a behavioral model system for magnetic resonance imaging

BACKGROUND

fMRI requires that subjects not move during image acquisition. This has been achieved by instructing people not to move, or by anesthetizing experimental animal subjects to induce immobility. We have demonstrated that a surgically implanted headbolt onto the skull of a rabbit allows their brain to be imaged comfortably while the animal is awake. This article provides a detailed method for the preparation.

NEW METHOD

We took advantage of the rabbit's tolerance for restraint to image the brain while holding the head at the standard stereotaxic angle. Visual stimulation was produced by flashing green LEDs and whisker stimulation was done by powering a small coil of wire attached to a fiber band. Blinking was recorded with an infrared emitter/detector directed at the eye with fiber-optic cabling.

RESULTS

Results indicate that a single daily session of habituation is sufficient to produce adequate immobility on subsequent days to avoid movement artifacts. Results include high resolution images in the stereotaxic plane of the rabbit.

COMPARISON WITH EXISTING METHOD(S)

We see no degradation or distortion of MR signal, and the headbolt provides a means for rapid realignment of the head in the magnet from day to day, and across subjects. The use of rabbits instead of rodents allows much shorter periods of habituation, and the rabbit allows behavior to be observed during the day while the animal is in its normal wake cycle.

CONCLUSIONS

The natural tolerance of the rabbit for restraint makes it a valuable subject for MRI studies of the brain.

Molecular events linking cholesterol to Alzheimer's disease and inclusion body myositis in a rabbit model

Alzheimer's disease (AD) is the most common neurodegenerative disorder, characterized by cognitive impairment and dementia, resulting from progressive synaptic dysfunction, loss and neuronal cell death. Inclusion body myositis (IBM) is a skeletal muscle degenerative disease, displaying progressive proximal and distal muscle weakness, in association with muscle fiber atrophy, degeneration and death. Studies have shown that the late onset version of AD (LOAD) and sporadic IBM (sIBM) in muscle share many pathological features, including the presence of extracellular plaques of β-amyloid peptides and intracellular tangles of hyperphosphorylated tau proteins. High blood cholesterol is suggested to be a risk factor for LOAD. Many neuropathological changes of LOAD can be reproduced by feeding rabbits a 2% enriched cholesterol diet for 12 weeks. The cholesterol fed rabbit model also simultaneously develops sIBM like pathology, which makes it an ideal model to study the molecular mechanisms common to the development of both diseases. In the present study, we determined the changes of gene expression in rabbit brain and muscle during the progression of LOAD and sIBM pathology using a custom rabbit nucleotide microarray, followed by qRT-PCR analyses. Out of 869 unique transcripts screened, 47 genes showed differential expression between the control and the cholesterol-treated group during the 12 week period and 19 changed transcripts appeared to be common to LOAD and sIBM. The most notable changes are the upregulation of the hemoglobin gene family and the downregulation of the genes required for mitochondrial oxidative phosphorylation in both brain and muscle tissues throughout the time course. The significant overlap on the changes of gene expression in the brain and muscle of rabbits fed with cholesterol-enriched diet supports the notion that LOAD and sIBM may share a common etiology.

The impact of hippocampal lesions on trace-eyeblink conditioning and forebrain-cerebellar interactions

Behavioral Neuroscience published a pivotal paper by Moyer, Deyo, and Disterhoft (1990) 25 years ago that described the impaired acquisition of trace-eyeblink conditioning in rabbits with complete removal of the hippocampus. As part of the Behavioral Neuroscience celebration commemorating the 30th anniversary of the journal, we reflect upon the impact of that study on understanding the role of the hippocampus, forebrain, and forebrain-cerebellar interactions that mediate acquisition and retention of trace-conditioned responses, and of declarative memory more globally. We discuss the expansion of the conditioning paradigm to species other than the rabbit, the heterogeneity of responses among hippocampal neurons during trace conditioning, the responsivity of hippocampal neurons following consolidation of conditioning, the role of awareness in conditioning, how blink conditioning can be used as a translational tool by assaying potential therapeutics for cognitive enhancement, how trace and delay classical conditioning may be used to investigate neurological disorders including Alzheimer's disease and schizophrenia, and how the 2 paradigms may be used to understand the relationship between declarative (explicit) and nondeclarative (implicit) memory systems.

Dietary cholesterol concentration affects synaptic plasticity and dendrite spine morphology of rabbit hippocampal neurons

Previous studies have shown dietary cholesterol can enhance learning but retard memory which may be partly due to increased cholesterol levels in hippocampus and reduced afterhyperpolarization (AHP) amplitude of hippocampal CA1 neurons. This study explored the dose-dependent effect of dietary cholesterol on synaptic plasticity of rabbit hippocampal CA1 neurons and spine morphology, the postsynaptic structures responsible for synaptic plasticity. Field potential recordings revealed a low concentration of dietary cholesterol increased long-term potentiation (LTP) expression while high concentrations produced a pronounced reduction in LTP expression. Dietary cholesterol facilitated basal synaptic transmission but did not influence presynaptic function. DiI staining showed dietary cholesterol induced alterations in dendrite spine morphology characterized by increased mushroom spine density and decreased thin spine density, two kinds of dendritic spines that may be linked to memory consolidation and learning acquisition. Dietary cholesterol also modulated the geometric measures of mushroom spines. Therefore, dietary cholesterol dose-dependently modulated both synaptic plasticity and dendrite spine morphologies of hippocampal CA1 neurons that could mediate learning and memory changes previously seen to result from feeding a cholesterol diet.

Are microRNAs the Molecular Link Between Metabolic Syndrome and Alzheimer's Disease?

Alzheimer's disease (AD) is the most common cause of dementia in people over 65 years of age. At present, treatment options for AD address only its symptoms, and there are no available treatments for the prevention or delay of the disease process. Several preclinical and epidemiological studies have linked metabolic risk factors such as hypertension, obesity, dyslipidemia, and diabetes to the pathogenesis of AD. However, the molecular mechanisms that underlie this relationship are not fully understood. Considering that less than 1% of cases of AD are attributable to genetic factors, the identification of new molecular targets linking metabolic risk factors to neuropathological processes is necessary for improving the diagnosis and treatment of AD. The dysregulation of microRNAs (miRNAs), small non-coding RNAs that regulate several biological processes, has been implicated in the development of different pathologies. In this review, we summarize some of the relevant evidence that points to the role of miRNAs in metabolic syndrome (MetS) and AD and propose that miRNAs may be a molecular link in the complex relationship between both diseases.

Identification of microRNAs involved in Alzheimer's progression using a rabbit model of the disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder characterized by the presence of extracellular plaques of β-amyloid peptides and intracellular tangles of hyperphosphorylated tau proteins in the brain. The vast majority of cases are late onset AD (LOAD), which are genetically heterogeneous and occur sporadically. High blood cholesterol is suggested to be a risk factor for this disease. Several neuropathological changes of LOAD can be reproduced by supplementing a rabbit's diet with 2% cholesterol for 12 weeks. Accumulating data in the literature suggest that microRNAs (miRNA) participate in the development of AD pathology. The present study focuses on the survey of changes of miRNA expression in rabbit brains during the progression of AD-like pathology using microarray followed by Taq-Man qRT-PCR analyses. Out of 1769 miRNA probes used in the experiments, 99 miRNAs were found to be present in rabbit brain, 57 were newly identified as miRNAs from rabbit brain. Eleven miRNAs showed significant changes over AD-like pathology progression. Among them, the changes of miR-125b, miR-98, miR-107, miR-30, along with 3 members of the let-7 family were similar to those observed in human AD samples, whereas the expression patterns of miR-15a, miR-26b, miR-9 and miR-576-3p were unique to this rabbit LOAD model. The significant up regulation of miR-26b is consistent with the decrease of leptin levels in the brains of cholesterol fed rabbit model for AD, confirming that miR-26b is indeed regulated by leptin and that both leptin and miR-26b may be involved in cholesterol induced AD-like pathology.

Dietary cholesterol degrades rabbit long term memory for discrimination learning but facilitates acquisition of discrimination reversal

We have shown previously that feeding dietary cholesterol before learning can improve acquisition whereas feeding cholesterol after learning can degrade long term memory. To examine these different findings within a single paradigm, we fed groups of rabbits 2% cholesterol or normal chow with or without 0.12 ppm copper added to the drinking water following two-tone discrimination learning of the nictitating membrane response in which a 8-kHz tone (conditioned stimulus, CS+) was followed by air puff and a 1-kHz tone (CS-) was not. After eight weeks on the diet, we assessed the rabbits' conditioned responding during testing and retraining. We then reversed the two-tone discrimination and assessed responding to the 1-kHz tone CS+ and the 8-kHz CS-. During testing, rabbits given cholesterol without copper had lower levels of responding to CS+ than rabbits in the other groups suggesting they did not retain the discrimination as well. However, during a brief discrimination retraining session, their response levels to the CS+ returned to the level of the other groups, demonstrating a return of the memory of the original discrimination. At the end of discrimination reversal, these same rabbits exhibited superior discrimination indexed by lower response levels to CS- but similar levels to CS+, suggesting they were better able to acquire the new relationship between the two tones by inhibiting CS- responses. These results add to our previous data by showing cholesterol diet-induced degradation of an old memory and facilitation of a new memory can both be demonstrated within a discrimination reversal paradigm. Given discrimination reversal is a hippocampally-dependent form of learning, the data support the role of cholesterol in modifying hippocampal function as we have shown previously with in vitro brain slice recordings.