Tara Bryant-Thomas

Hypercholesterolemia accelerates the Alzheimer's amyloid pathology in a transgenic mouse model.

Recent data suggest that cholesterol metabolism is linked to susceptibility to Alzheimers disease (AD). However, no direct evidence has been reported linking cholesterol metabolism and the pathogenesis of AD. To test the hypothesis that amyloid β-peptide (Aβ) deposition can be modulated by diet-induced hypercholesterolemia, we used a transgenic-mouse model for AD amyloidosis and examined the effects of a high-fat/high-cholesterol diet on central nervous system (CNS) Aβ accumulation. Our data showed that diet-induced hypercholesterolemia resulted in significantly increased levels of formic acid-extractable Aβ peptides in the CNS. Furthermore, the levels of total Aβ were strongly correlated with the levels of both plasma and CNS total cholesterol. Biochemical analysis revealed that, compared with control, the hypercholesterolemic mice had significantly decreased levels of sAPPα and increased levels of C-terminal fragments (β-CTFs), suggesting alterations in amyloid precursor protein processing in response to hypercholesterolemia. Neuropathological analysis indicated that the hypercholesterolemic diet significantly increased β-amyloid load by increasing both deposit number and size. These data demonstrate that high dietary cholesterol increases Aβ accumulation and accelerates the AD-related pathology observed in this animal model. Thus, we propose that diet can be used to modulate the risk of developing AD.

Mild hypercholesterolemia is an early risk factor for the development of Alzheimer amyloid pathology.

Background: Epidemiologic and experimental data suggest that cholesterol may play a role in the pathogenesis of AD. Modulation of cholesterolemia in transgenic animal models of AD strongly alters amyloid pathology. Objective: To determine whether a relationship exists between amyloid deposition and total cholesterolemia (TC) in the human brain. Methods: The authors reviewed autopsy cases of patients older than 40 years and correlated cholesterolemia and presence or absence of amyloid deposition (amyloid positive vs amyloid negative subjects) and cholesterolemia and amyloid load. Amyloid load in human brains was measured by immunohistochemistry and image analysis. To remove the effect of apoE isoforms on cholesterol levels, cases were genotyped and duplicate analyses were performed on apoE3/3 subjects. Results: Cholesterolemia correlates with presence of amyloid deposition in the youngest subjects (40 to 55 years) with early amyloid deposition (diffuse type of senile plaques) (p = 0.000 for all apoE isoforms; p = 0.009 for apoE3/3 subjects). In this group, increases in cholesterolemia from 181 to 200 almost tripled the odds for developing amyloid, independent of apoE isoform. A logistic regression model showed consistent results (McFadden ρ2 = 0.445). The difference in mean TC between subjects with and without amyloid disappeared as the age of the sample increased (>55 years: p = 0.491), possibly reflecting the effect of cardiovascular deaths among other possibilities. TC and amyloid load were not linearly correlated, indicating that there are additional factors involved in amyloid accumulation. Conclusions: Serum hypercholesterolemia may be an early risk factor for the development of AD amyloid pathology.

Melatonin increases survival and inhibits oxidative and amyloid pathology in a transgenic model of Alzheimer's disease

Increased levels of a 40–42 amino‐acid peptide called the amyloid β protein (Aβ) and evidence of oxidative damage are early neuropathological markers of Alzheimers disease (AD). Previous investigations have demonstrated that melatonin is decreased during the aging process and that patients with AD have more profound reductions of this hormone. It has also been recently shown that melatonin protects neuronal cells from Aβ‐mediated oxidative damage and inhibits the formation of amyloid fibrils in vitro. However, a direct relationship between melatonin and the biochemical pathology of AD had not been demonstrated. We used a transgenic mouse model of Alzheimers amyloidosis and monitored over time the effects of administering melatonin on brain levels of Aβ, abnormal protein nitration, and survival of the mice. We report here that administration of melatonin partially inhibited the expected time‐dependent elevation of β‐amyloid, reduced abnormal nitration of proteins, and increased survival in the treated transgenic mice. These findings may bear relevance to the pathogenesis and therapy of AD.

The neuroprotective activities of melatonin against the Alzheimer β‐protein are not mediated by melatonin membrane receptors

Exposure of neuronal cells to the Alzheimers amyloid β protein (Aβ) results in extensive oxidative damage of bio‐molecules that are profoundly harmful to neuronal homeostasis. It has been demonstrated that melatonin protects neurons against Aβ‐mediated neurotoxicity, including cell death and a spectrum of oxidative lesions. We undertook the current study to determine whether melatonin membrane receptors are involved in the mechanism of neuroprotection against Aβ neurotoxicity. For this purpose, we characterized the free‐radical scavenging potency of several compounds exhibiting various affinities for melatonin membrane receptors (MLT 1a and 1b). Aβ‐mediated neurotoxicity was assessed in human neuroblastoma cells and in primary hippocampal neurons. In sharp contrast with melatonin, no neuroprotection against Aβ toxicity was observed when we used melatonin membrane receptor agonists that were devoid of antioxidant activity. In contrast, the cells were fully protected in parallel control experiments when either melatonin, or the structurally unrelated free‐radical scavenger phenyl‐N‐t‐butyl nitrone (PBN), were added to Aβ‐containing culture media. This study demonstrates that the neuroprotective properties of melatonin against Aβ‐mediated toxicity does not require binding of melatonin to a membrane receptor and is likely the result of the antioxidant and antiamyloidogenic features of the agent.

Cholesterol, oxidative stress, and Alzheimer’s disease: expanding the horizons of pathogenesis

Recent epidemiological, clinical, and experimental data suggest that cholesterol may play a role in Alzheimers disease (AD). We have recently shown that cholesterolemia has a profound effect in the development and modulation of amyloid pathology in a transgenic model of AD. This review summarizes recent advancements in our understanding of the potential role of cholesterol and the amyloid beta protein in initiating the generation of free radicals and points out their role in a chain of events that causes damage of essential macromolecules in the central nervous system and culminates in neuronal dysfunction and loss. Experimental data links cholesterol and oxidative stress with some neurodegenerative aspects of AD.

Deficiency of chaperonin 60 in Down's syndrome

Patients with Down syndrome (DS) and Alzheimers disease (AD) share a number of characteristic neuropathologic lesions. Several lines of evidence suggest that mitochondria and the oxidative stress response are involved in the pathogenesis of both conditions. In the process of investigating the stress response in DS, we discovered a defective basal expression of a major mitochondrial heat shock protein, chaperonin 60 (Cpn60) in non-transformed dermal fibroblast cell lines from DS individuals. Such a defect was not present in control cells that had been cultured under identical physiological growth conditions. A quantitative analysis by Western blots showed a marked reduction of Cpn60 per equal amount of total protein in DS cells to an average of 35% of normal. Northern blot studies confirmed the defect and also showed a marked reduction of the mRNA signal for Cpn60 in all the DS cell lines. To gain further information, experiments were conducted to study the rate of de-novo synthesis of Cpn60 at normal and supraoptimal temperatures in DS and controls. Results showed no significant differences between the two study groups. HSP60 is important in mitochondrial function and defects in these organelles have been reported in DS and AD. Thus, the findings may have potential implications in the neuropathology of DS.