Epidemilogical trends strongly suggest exposures as etiologic agents in the pathogenesis of sporadic Alzheimer's disease, diabetes mellitus, and non-alcoholic steatohepatitis

Correlation and dissociation of electrophysiology and histopathology in the assessment of toxic neuropathy

The evaluation of neurotoxic damage involves a unique set of challenges. Vulnerable structures, such as neocortex, hippocampus, spinal cord, and peripheral nerve are complex and sharply differentiated; deficits can result from insults to one or more element(s) in the system (e.g., myelin, axon, soma, synapse, or glia). In-life assessment of neurotoxic damage is complicated by the relative inaccessibility of structures in the brain and spinal cord, and recovery is severely limited. Histopathology and electrophysiology represent two of the most commonly used and valuable techniques in this field. This review outlines the strengths and limitations of these procedures and focuses on circumstances in which findings from these measures are dissociated. Electrophysiology is noninvasive and affords a longitudinal view of onset and progression of deficits; however, measures are generally weighted to large-diameter myelinated axons and to regions of primary sensory and motor processing. Histology is a highly validated biomarker, but it is restricted by sampling issues and is insensitive to some elements of neurotoxicity (e.g., altered channel function) associated with profound functional consequences. The central tenet of the discussion is that histology and electrophysiology offer complementary views of neurotoxic damage and, whenever possible, they should be used in concert.

The link between iron, metabolic syndrome, and Alzheimer's disease

Both Alzheimer's disease (AD), the most common form of dementia, and type-2 diabetes mellitus (T2DM), a disease associated with metabolic syndrome (MetS), affect a great number of the world population and both have increased prevalence with age. Recently, many studies demonstrated that pre-diabetes, MetS, and T2DM are risk factors in the development of AD and have many common mechanisms. The main focus of studies is the insulin resistance outcome found both in MetS as well as in brains of AD subjects. However, oxidative stress (OS)-related mechanisms, which are well known to be involved in AD, including mitochondrial dysfunction, elevated iron concentration, reactive oxygen species (ROS), and stress-related enzyme or proteins (e.g. heme oxygenase-1, transferrin, etc.), have not been elucidated in MetS or T2DM brains although OS and iron are involved in the degeneration of the pancreatic islet β cells. Therefore, this review sets to cover the current literature regarding OS and iron in MetS and T2DM and the similarities to mechanisms in AD both in human subjects as well as in animal models.

Early limited nitrosamine exposures exacerbate high fat diet-mediated type 2 diabetes and neurodegeneration

BACKGROUND

Type 2 diabetes mellitus (T2DM) and several types of neurodegeneration, including Alzheimer's, are linked to insulin-resistance, and chronic high dietary fat intake causes T2DM with mild neurodegeneration. Intra-cerebral Streptozotocin, a nitrosamine-related compound, causes neurodegeneration, whereas peripheral treatment causes DM.

HYPOTHESIS

Limited early exposures to nitrosamines that are widely present in the environment, enhance the deleterious effects of high fat intake in promoting T2DM and neurodegeneration.

METHODS

Long Evans rat pups were treated with N-nitrosodiethylamine (NDEA) by i.p. injection, and upon weaning, they were fed with high fat (60%; HFD) or low fat (5%; LFD) chow for 8 weeks. Cerebella were harvested to assess gene expression, and insulin and insulin-like growth factor (IGF) deficiency and resistance in the context of neurodegeneration.

RESULTS

HFD +/- NDEA caused T2DM, neurodegeneration with impairments in brain insulin, insulin receptor, IGF-2 receptor, or insulin receptor substrate gene expression, and reduced expression of tau and choline acetyltransferase (ChAT), which are regulated by insulin and IGF-1. In addition, increased levels of 4-hydroxynonenal and nitrotyrosine were measured in cerebella of HFD +/- NDEA treated rats, and overall, NDEA+HFD treatment reduced brain levels of Tau, phospho-GSK-3beta (reflecting increased GSK-3beta activity), glial fibrillary acidic protein, and ChAT to greater degrees than either treatment alone. Finally, pro-ceramide genes, examined because ceramides cause insulin resistance, oxidative stress, and neurodegeneration, were significantly up-regulated by HFD and/or NDEA exposure, but the highest levels were generally present in brains of HFD+NDEA treated rats.

CONCLUSIONS

Early limited exposure to nitrosamines exacerbates the adverse effects of later chronic high dietary fat intake in promoting T2DM and neurodegeneration. The mechanism involves increased generation of ceramides and probably other toxic lipids in brain.

Nitrosamine exposure exacerbates high fat diet-mediated type 2 diabetes mellitus, non-alcoholic steatohepatitis, and neurodegeneration with cognitive impairment

Background

The current epidemics of type 2 diabetes mellitus (T2DM), non-alcoholic steatohepatitis (NASH), and Alzheimer's disease (AD) all represent insulin-resistance diseases. Previous studies linked insulin resistance diseases to high fat diets or exposure to streptozotocin, a nitrosamine-related compound that causes T2DM, NASH, and AD-type neurodegeneration. We hypothesize that low-level exposure to nitrosamines that are widely present in processed foods, amplifies the deleterious effects of high fat intake in promoting T2DM, NASH, and neurodegeneration.

Methods

Long Evans rat pups were treated with N-nitrosodiethylamine (NDEA) by i.p. Injection, and upon weaning, they were fed with high fat (60%; HFD) or low fat (5%; LFD) chow for 6 weeks. Rats were evaluated for cognitive impairment, insulin resistance, and neurodegeneration using behavioral, biochemical, molecular, and histological methods.

Results

NDEA and HFD ± NDEA caused T2DM, NASH, deficits in spatial learning, and neurodegeneration with hepatic and brain insulin and/or IGF resistance, and reductions in tau and choline acetyltransferase levels in the temporal lobe. In addition, pro-ceramide genes, which promote insulin resistance, were increased in livers and brains of rats exposed to NDEA, HFD, or both. In nearly all assays, the adverse effects of HFD+NDEA were worse than either treatment alone.

Conclusions

Environmental and food contaminant exposures to low, sub-mutagenic levels of nitrosamines, together with chronic HFD feeding, function synergistically to promote major insulin resistance diseases including T2DM, NASH, and AD-type neurodegeneration. Steps to minimize human exposure to nitrosamines and consumption of high-fat content foods are needed to quell these costly and devastating epidemics.

Insulin resistance and neurodegeneration: roles of obesity, type 2 diabetes mellitus and non-alcoholic steatohepatitis

Recent studies have linked obesity, type 2 diabetes mellitus (T2DM) or non-alcoholic steatohepatitis (NASH) to insulin resistance in the brain, cognitive impairment and neurodegeneration. Insulin resistance compromises cell survival, metabolism and neuronal plasticity, and increases oxidative stress, cytokine activation and apoptosis. T2DM/NASH has been demonstrated to be associated with increased ceramide generation, suggesting a mechanistic link between peripheral insulin resistance and neurodegeneration because ceramides mediate insulin resistance and can cross the blood-brain barrier (BBB). Peripheral insulin resistance diseases may potentially cause brain insulin resistance via a liver-brain axis of neurodegeneration as a result of the trafficking of ceramides across the BBB. Therapy that includes insulin-sensitizing agents may help prevent brain insulin resistance-mediated cognitive impairment.

Insulin resistance and Alzheimer's disease

Emerging data demonstrate pivotal roles for brain insulin resistance and insulin deficiency as mediators of cognitive impairment and neurodegeneration, particularly Alzheimer's disease (AD). Insulin and insulin-like growth factors (IGFs) regulate neuronal survival, energy metabolism, and plasticity, which are required for learning and memory. Hence, endogenous brain-specific impairments in insulin and IGF signaling account for the majority of AD-associated abnormalities. However, a second major mechanism of cognitive impairment has been linked to obesity and Type 2 diabetes (T2DM). Human and experimental animal studies revealed that neurodegeneration associated with peripheral insulin resistance is likely effectuated via a liver-brain axis whereby toxic lipids, including ceramides, cross the blood brain barrier and cause brain insulin resistance, oxidative stress, neuro-inflammation, and cell death. In essence, there are dual mechanisms of brain insulin resistance leading to AD-type neurodegeneration: one mediated by endogenous, CNS factors; and the other, peripheral insulin resistance with excess cytotoxic ceramide production.

Nitrosamine exposure causes insulin resistance diseases: relevance to type 2 diabetes mellitus, non-alcoholic steatohepatitis, and Alzheimer's disease

The current epidemics of type 2 diabetes mellitus (T2DM), non-alcoholic steatohepatitis (NASH), and Alzheimer's disease (AD) all represent insulin-resistance diseases. Previous studies showed that streptozotocin, a nitrosamine-related com-pound, causes insulin resistance diseases including, T2DM, NASH, and AD-type neurodegeneration. We hypothesize that chronic human exposure to nitrosamine compounds, which are widely present in processed foods, contributes to the pathogenesis of T2DM, NASH, and AD. Long Evans rat pups were treated with N-nitrosodiethylamine (NDEA) by i.p. (x3) or i.c. (x1) injection, and 2-4 weeks later, they were evaluated for cognitive-motor dysfunction, insulin resistance, and neurodegeneration using behavioral, biochemical, and molecular approaches. NDEA treatment caused T2DM, NASH, deficits in motor function and spatial learning, and neurodegeneration characterized by insulin resistance and deficiency, lipid peroxidation, cell loss, increased levels of amyloid-beta protein precursor/amyloid-beta, phospho-tau, and ubiquitin immunoreactivities, and upregulated expression of pro-inflammatory cytokine and pro-ceramide genes, which together promote insulin resistance. In conclusion, environmental and food contaminant exposures to nitrosamines play critical roles in the pathogenesis of major insulin resistance diseases including T2DM, NASH, and AD. Improved detection and prevention of human exposures to nitrosamines will lead to earlier treatments and eventual quelling of these costly and devastating epidemics.