Demao Chen

Alzheimer's Disease (AD)-Like Pathology in Aged Monkeys after Infantile Exposure to Environmental Metal Lead (Pb): Evidence for a Developmental Origin and Environmental Link for AD

The sporadic nature of Alzheimers disease (AD) argues for an environmental link that may drive AD pathogenesis; however, the triggering factors and the period of their action are unknown. Recent studies in rodents have shown that exposure to lead (Pb) during brain development predetermined the expression and regulation of the amyloid precursor protein (APP) and its amyloidogenic β-amyloid (Aβ) product in old age. Here, we report that the expression of AD-related genes [APP, BACE1 (β-site APP cleaving enzyme 1)] as well as their transcriptional regulator (Sp1) were elevated in aged (23-year-old) monkeys exposed to Pb as infants. Furthermore, developmental exposure to Pb altered the levels, characteristics, and intracellular distribution of Aβ staining and amyloid plaques in the frontal association cortex. These latent effects were accompanied by a decrease in DNA methyltransferase activity and higher levels of oxidative damage to DNA, indicating that epigenetic imprinting in early life influenced the expression of AD-related genes and promoted DNA damage and pathogenesis. These data suggest that AD pathogenesis is influenced by early life exposures and argue for both an environmental trigger and a developmental origin of AD.

Glucagon-like peptide-1 decreases endogenous amyloid-β peptide (Aβ) levels and protects hippocampal neurons from death induced by Aβ and iron

Glucagon‐like peptide‐1(7–36)‐amide (GLP‐1) is an endogenous insulinotropic peptide that is secreted from the gastrointestinal tract in response to food. It enhances pancreatic islet β‐cell proliferation and glucose‐dependent insulin secretion and lowers blood glucose and food intake in patients with type 2 diabetes mellitus. GLP‐1 receptors, which are coupled to the cyclic AMP second messenger pathway, are expressed throughout the brains of rodents and humans. It was recently reported that GLP‐1 and exendin‐4, a naturally occurring, more stable analogue of GLP‐1 that binds at the GLP‐1 receptor, possess neurotrophic properties and can protect neurons against glutamate‐induced apoptosis. We report here that GLP‐1 can reduce the levels of amyloid‐β peptide (Aβ) in the brain in vivo and can reduce levels of amyloid precursor protein (APP) in cultured neuronal cells. Moreover, GLP‐1 and exendin‐4 protect cultured hippocampal neurons against death induced by Aβ and iron, an oxidative insult. Collectively, these data suggest that GLP‐1 can modify APP processing and protect against oxidative injury, two actions that suggest a novel therapeutic target for intervention in Alzheimers disease.

The Experimental Alzheimer's Disease Drug Posiphen [(+)-Phenserine] Lowers Amyloid-β Peptide Levels in Cell Culture and Mice

Major characteristics of Alzheimers disease (AD) are synaptic loss, cholinergic dysfunction, and abnormal protein depositions in the brain. The amyloid β-peptide (Aβ), a proteolytic fragment of amyloid β precursor protein (APP), aggregates to form neuritic plaques and has a causative role in AD. A present focus of AD research is to develop safe Aβ-lowering drugs. A selective acetylcholinesterase inhibitor, phenserine, in current human trials lowers both APP and Aβ. Phenserine is dose-limited in animals by its cholinergic actions; its cholinergically inactive enantiomer, posiphen (+)-[phenserine], was assessed. In cultured human neuroblastoma cells, posiphen, like phenserine, dose- and time-dependently lowered APP and Aβ levels by reducing the APP synthesis rate. This action translated to an in vivo system. Posiphen administration to mice (7.5–75 mg/kg daily, 21 consecutive days) significantly decreased levels of total APP (tissue mass-adjusted) in a dose-dependent manner. Aβ40 and Aβ42 levels were significantly lowered by posiphen (≥15 mg/kg) compared with controls. The activities of α-, β-, and γ-secretases were assessed in the same brain samples, and β-secretase activity was significantly reduced. Posiphen, like phenserine, can lower Aβ via multiple mechanisms and represents an interesting drug candidate for AD treatment.

Memantine lowers amyloid-β peptide levels in neuronal cultures and in APP/PS1 transgenic mice

Memantine is a moderate‐affinity, uncompetitive N‐methyl‐D‐aspartate (NMDA) receptor antagonist that stabilizes cognitive, functional, and behavioral decline in patients with moderate to severe Alzheimers disease (AD). In AD, the extracellular deposition of fibrillogenic amyloid‐β peptides (Aβ) occurs as a result of aberrant processing of the full‐length Aβ precursor protein (APP). Memantine protects neurons from the neurotoxic effects of Aβ and improves cognition in transgenic mice with high brain levels of Aβ. However, it is unknown how memantine protects cells against neurodegeneration and affects APP processing and Aβ production. We report the effects of memantine in three different systems. In human neuroblastoma cells, memantine, at therapeutically relevant concentrations (1–4 μM), decreased levels of secreted APP and Aβ1–40. Levels of the potentially amylodogenic Aβ1–42 were undetectable in these cells. In primary rat cortical neuronal cultures, memantine treatment lowered Aβ1–42 secretion. At the concentrations used, memantine treatment was not toxic to neuroblastoma or primary cultures and increased cell viability and/or metabolic activity under certain conditions. In APP/presenilin‐1 (PS1) transgenic mice exhibiting high brain levels of Aβ1–42, oral dosing of memantine (20 mg/kg/day for 8 days) produced a plasma drug concentration of 0.96 μM and significantly reduced the cortical levels of soluble Aβ1–42. The ratio of Aβ1–40/Aβ1–42 increased in treated mice, suggesting effects on the γ‐secretase complex. Thus, memantine reduces the levels of Aβ peptides at therapeutic concentrations and may inhibit the accumulation of fibrillogenic Aβ in mammalian brains. Memantines ability to preserve neuronal cells against neurodegeneration, to increase metabolic activity, and to lower Aβ level has therapeutic implications for neurodegenerative disorders.

Aluminum and copper in drinking water enhance inflammatory or oxidative events specifically in the brain

Inflammatory and oxidative events are up-regulated in the brain of AD patients. It has been reported that in animal models of AD, exposure to aluminum (Al) or copper (Cu) enhanced oxidative events and accumulation of amyloid beta (Abeta) peptides. The present study was designed to evaluate the effect of a 3-month exposure of mice to copper sulfate (8 microM), aluminum lactate (10 or 100 microM), or a combination of the salts. Results suggest that although Al or Cu may independently initiate inflammatory or oxidative events, they may function cooperatively to increase APP levels.

High levels of Alzheimer beta-amyloid precursor protein (APP) in children with severely autistic behavior and aggression

Autism is characterized by restricted, repetitive behaviors and impairment in socialization and communication. Although no neuropathologic substrate underlying autism has been found, the findings of brain overgrowth via neuroimaging studies and increased levels of brain-derived neurotrophic factor (BDNF) in neuropathologic and blood studies favor an anabolic state. We examined acetylcholinesterase, plasma neuronal proteins, secreted beta-amyloid precursor protein (APP), and amyloid-beta 40 and amyloid-beta 42 peptides in children with and without autism. Children with severe autism and aggression expressed secreted beta-amyloid precursor protein at two or more times the levels of children without autism and up to four times more than children with mild autism. There was a trend for children with autism to show higher levels of secreted beta-amyloid precursor protein and nonamyloidogenic secreted beta-amyloid precursor protein and lower levels of amyloid-beta 40 compared with controls. This favors an increased α-secretase pathway in autism (anabolic), opposite to what is seen in Alzheimer disease. Additionally, a complex relationship between age, acetylcholinesterase, and plasma neuronal markers was found. (J Child Neurol 2006;21:444—449; DOI 10.2310/7010.2006.00130).

Dietary supplementation with melatonin reduces levels of amyloid beta-peptides in the murine cerebral cortex

Abstract:  Melatonin levels decrease with aging in mice. Dietary supplementation with melatonin has recently been shown to result in a significant rise in levels of endogenous melatonin in the serum and all other tissue samples tested. Herein, the effects of dietary melatonin on brain levels of nitric oxide synthase, synaptic proteins and amyloid beta‐peptides (Aβ) were determined in mice. Melatonin supplementation did not significantly change cerebral cortical levels of nitric oxide synthase or synaptic proteins such as synaptophysin and SNAP‐25. Increased brain melatonin concentrations however, led to a significant reduction in levels of toxic cortical Aβ of both short and long forms which are involved in amyloid depositions and plaque formation in Alzheimers diseases. Thus, melatonin supplementation may retard neurodegenerative changes associated with brain aging. Depletion of melatonin in the brain of aging mice may in part account for this adverse change.

Nicotine Reduces the Secretion of Alzheimer's β‐Amyloid Precursor Protein Containing β‐Amyloid Peptide in the Rat without Altering Synaptic Proteins

Abstract: Alzheimers disease (AD) is characterized by cerebrovascular deposition of the amyloid β‐peptide (Aβ), which is derived from a larger β‐amyloid precursor protein (βAPP). Altered metabolism of βAPP, resulting in increased Aβ production, appears central in the neuropathology of AD. The processing of the holoprotein βAPP by different “secretase” enzymes results in three major carboxyl‐truncated species. One species, which results from the cleavage of βAPP by γ‐secretase, is secreted into the cerebrospinal fluid (CSF) and is called sAPPγ as it contains an intact Aβ domain. Moreover, AD is characterized by cholinergic dysfunction and the loss of synaptic proteins. Reports of an inverse relation between nicotine intake, due to cigarette smoking, and the incidence of AD prompted us to investigate the effects of nicotine on βAPP processing and synaptic proteins in rats and in cell culture. Nicotine, 1 and 8 mg/kg/day, doses commensurate with cigarette smoking, and a higher but well tolerated dose, respectively, was administered over 14 days to rats. Levels of sAPP in the CSF sample were evaluated by Western blot analysis. The higher dose significantly increased levels of total sAPP; however, both doses significantly reduced sAPPγ, which contains the amyloidogenic portion of Aβ. These actions were blocked by nicotinic receptor antagonism. Nicotinic antagonists alone had no effect on either total sAPP or sAPPγ levels in CSF. Nicotine did not significantly change the intracellular levels of total βAPP in rat brain extracts, which is consistent with neuronal cell culture data. Similarly, levels of vesicular protein, such as synaptophysin, and presynaptic terminal protein SNAP‐25 were unaffected by nicotine treatment both in vivo and in cell culture experiments. Taken together, these results suggest that nicotine modifies βAPP processing away from the formation of potentially amyloidogenic products, without altering the levels of synaptic proteins, and that this can potentially offer therapeutic potential for Alzheimers disease.

Identification of Novel Small Molecule Inhibitors of Amyloid Precursor Protein Synthesis as a Route to Lower Alzheimer's Disease Amyloid-β Peptide

A wealth of independent research with transgenic mice, antibodies, and vaccines has pointed to a causative role of the amyloid-β peptide (Aβ) in Alzheimers disease (AD). Based on these and earlier associative studies, Aβ represents a promising target for development of therapeutics focused on AD disease progression. Interestingly, a cholinesterase inhibitor currently in clinical trials, phenserine, has been shown to inhibit production of both amyloid precursor protein (APP) and Aβ. We have shown that this inhibition occurs at the post-transcriptional level with a specific blocking of the synthesis of APP relative to total protein synthesis (Shaw et al., 2001). However, the dose of phenserine necessary to block APP production is far higher than that needed to elicit its anticholinesterase activity, and it is these latter actions that are dose limiting in vivo. The focus of this study was to screen 144 analogs of phenserine to identify additional small molecules that inhibit APP protein synthesis, and thereby Aβ production, without possessing potent acetylcholinesterase (AChE) inhibitory activity. An enzyme-linked immunosorbent assay was used to identify analogs capable of suppressing APP production following treatment of human neuroblastoma cells with 20 μM of compound. Eight analogs were capable of dose dependently reducing APP and Aβ production without causing cell toxicity in further studies. Several of these analogs had little to no AChE activities. Translation of APP and Aβ actions to mice was demonstrated with one agent. They thus represent interesting lead molecules for assessment in animal models, to define their tolerance and utility as potential AD therapeutics.

Does Nitric Oxide Synthase Contribute to the Pathogenesis of Alzheimer's Disease?

Abstract: Oxidative stress is a risk factor for Alzheimers disease (AD) whose major hallmark includes brain depositions of the amyloid beta peptide (Aβ) derived from the β‐amyloid precursor protein (APP). Our aim was to determine whether or not excessive Aβ deposition would alter nitric oxide synthase (NOS) activity, and thereby affect NOS‐mediated superoxide formation. We compared NOS activity in brain extracts between Tg mice (expressing APP Swedish double mutation plus presenilin [PS‐1] and nontransgenic [nTg] mice. Five brain regions, including cerebral cortex, hippocampus, cerebellum, and striatum from both nTg and Tg mice showed a detectable level of neuronal (n) NOS activity. Cerebellar extracts from both nTg and Tg mice displayed the highest level of nNOS activity, which was fourfold higher than cortical extracts. Although there was an increase in nNOS activity in Tg brain extracts, this did not attain statistical significance. A similar result was obtained for inducible NOS levels. Our results suggest that excess levels of Aβ failed to both trigger NOS activity and change NOS levels.