Pharmacological modulation of dietary lipid-induced cerebral capillary dysfunction: Considerations for reducing risk for Alzheimer's disease

Is type 2 diabetes associated dementia a microvascular early-Alzheimer's phenotype induced by aberrations in the peripheral metabolism of lipoprotein-amyloid?

There is increasing evidence of a positive association of type 2 diabetes with Alzheimer's disease (AD), the most prevalent form of dementia. Suggested pathways include cerebral vascular dysfunction; central insulin resistance, or exaggerated brain abundance of potentially cytotoxic amyloid-β (Aβ), a hallmark feature of AD. However, contemporary studies find that Aβ is secreted in the periphery by lipogenic organs and secreted as nascent triglyceride-rich lipoproteins (TRL's). Pre-clinical models show that exaggerated abundance in blood of TRL-Aβ compromises blood-brain barrier (BBB) integrity, resulting in extravasation of the TRL-Aβ moiety to brain parenchyme, neurovascular inflammation and neuronal degeneration concomitant with cognitive decline. Inhibiting secretion of TRL-Aβ by peripheral lipogenic organs attenuates the early-AD phenotype indicated in animal models, consistent with causality. Poorly controlled type 2 diabetes commonly features hypertriglyceridemia because of exaggerated TRL secretion and reduced rates of catabolism. Alzheimer's in diabetes may therefore be a consequence of heightened abundance in blood of lipoprotein-Aβ and accelerated breakdown of the BBB. This review reconciles the prevailing dogma of amyloid associated cytotoxicity as a primary risk factor in late-onset AD, with substantial evidence of a microvascular axis for dementia-in-diabetes. Consideration of potentially relevant pharmacotherapies to treat insulin resistance, dyslipidaemia and by extension plasma amyloidemia in type 2 diabetes are discussed.

Plasma Amyloid-β Homeostasis Is Associated with Body Mass Index and Weight Loss in People with Overweight and Obesity

BACKGROUND

Obesity is linked to a higher incidence of Alzheimer's disease (AD). Studies show that plasma amyloid-β (Aβ) dyshomeostasis, particularly low 42/40 ratio indicates a heightened risk for developing AD. However, the relationship between body mass index (BMI) and circulating plasma Aβ has not been extensively studied.

OBJECTIVE

We hypothesized that people with a high BMI have altered plasma Aβ homeostasis compared with people with a lower BMI. We also tested whether reducing BMI by calorie-restriction could normalize plasma concentrations of Aβ.

METHODS

Plasma concentrations of Aβ40, Aβ42, and Aβ42/40 ratio were measured in 106 participants with BMIs classified as lean, overweight, or obese. From this cohort, twelve participants with overweight or obese BMIs entered a 12-week calorie-restriction weight loss program. We then tested whether decreasing BMI affected plasma Aβ concentrations.

RESULTS

Plasma Aβ42/40 ratio was 17.54% lower in participants with an obese BMI compared to lean participants (p < 0.0001), and 11.76% lower compared to participants with an overweight BMI (p < 0.0001). The weight loss regimen decreased BMI by an average of 4.02% (p = 0.0005) and was associated with a 6.5% decrease in plasma Aβ40 (p = 0.0425). However, weight loss showed negligible correlations with plasma Aβ40, Aβ42, and Aβ42/40 ratio.

CONCLUSION

Obesity is associated with aberrant plasma Aβ homeostasis which may be associated with an increased risk for AD. Weight loss appears to lower Aβ40, but large-scale longitudinal studies in addition to molecular studies are required to elucidate the underlying mechanisms of how obesity and weight loss influence plasma Aβ homeostasis.

Cognitive Deficits in Type-1 Diabetes: Aspects of Glucose, Cerebrovascular and Amyloid Involvement

The evidence shows that individuals with type-1 diabetes mellitus (T1DM) are at greater risk of accelerated cognitive impairment and dementia. Although, to date the mechanisms are largely unknown. An emerging body of literature indicates that dysfunction of cerebral neurovascular network and plasma dyshomeostasis of soluble amyloid-β in association with impaired lipid metabolism are central to the onset and progression of cognitive deficits and dementia. However, the latter has not been extensively considered in T1DM. Therefore, in this review, we summarised the literature concerning altered lipid metabolism and cerebrovascular function in T1DM as an implication for potential pathways leading to cognitive decline and dementia.

Chronic Consumption of Bovine Dairy Milk Attenuates Dietary Saturated Fatty Acid-Induced Blood-Brain Barrier Dysfunction

Ingestion of Western-diets enriched in long chain saturated fatty acids (LCSFA) are associated with increased risk of blood-brain barrier (BBB) dysfunction and neurovascular inflammation. Potential mechanisms include vascular insult as a consequence of metabolic aberrations, or changes in capillary permeability resulting in brain parenchymal extravasation of pro-inflammatory molecules. Bovine dairy milk (BDM) is potentially a significant source of dietary LCSFA, however, BDM contains an array of bioactive molecules purported to have vascular anti-inflammatory properties. This study investigated the effects of full cream (4% total fat) and delipidated (skim) BDM on BBB integrity and neuroinflammation in wild-type mice. Mice consuming substantial amounts of full cream or skim BDM with LCSFA-enriched chow were dyslipidemic compared to control mice provided with standard chow and water. However, there was no evidence of BBB dysfunction or neuroinflammation indicated by parenchymal abundance of immunoglobulin G and microglial recruitment, respectively. Positive control mice maintained on an LCSFA-enriched diet derived from cocoa-butter and water, had marked BBB dysfunction, however, co-provision of both full cream and skim milk solutions effectively attenuated LCSFA-induced BBB dysfunction. In mice provided with low-fat chow and full cream BDM drinking solutions, there were substantial favorable changes in the concentration of plasma anti-inflammatory cytokines. This study suggests that consumption of BDM may confer potent vascular benefits through the neuroprotective properties exuded by the milk-fat globule membrane moiety of BDM.

Probucol prevents blood-brain barrier dysfunction and cognitive decline in mice maintained on pro-diabetic diet

An emerging body of evidence consistently suggests that compromised blood-brain barrier integrity may be causally associated with cognitive decline induced by type-2 diabetes. Our previous studies demonstrated that selected anti-inflammatory/anti-oxidative agents can preserve the integrity of blood-brain barrier and prevent neuroinflammation in mouse models of dysfunctional blood-brain barrier. Therefore, we have tested whether the previously proven blood-brain barrier protective agent, probucol, can prevent blood-brain barrier breakdown and cognitive decline in a dietary-induced murine model of diabetic insulin resistance. After 6-month chronic ingestion of a diet high in fat and fructose, the mice became insulin resistant. The high-fat and high-fructose-fed mice showed significant cognitive decline assessed by Morris water maze, concomitant with significant elevations in cortical and hippocampal glial acidic fibrillary protein and Fluoro Jade-C staining, indicating heightened neuroinflammation and neurodegeneration, respectively. The integrity of blood-brain barrier in high-fat and high-fructose-fed mice was substantially compromised, and this showed a significant association with heightened neurodegeneration. Co-provision of probucol with high-fat and high-fructose diet completely prevented the cognitive decline and blood-brain barrier dysfunction. Similarly, metformin was able to restore the cognitive function in high-fat and high-fructose-fed mice, while its blood-brain barrier protective effects were modest. These data suggest that probucol may prevent cognitive decline induced by insulin resistance by preserving the integrity of blood-brain barrier, whereas metformin's neuroprotective effects may be mediated through a separate pathway.

Resveratrol Restores Neuronal Tight Junction Proteins Through Correction of Ammonia and Inflammation in CCl4-Induced Cirrhotic Mice

Systemic inflammation and ammonia (hyperammonemia) act synergistically in the pathogenesis of hepatic encephalopathy (HE), the neurobehavioral sequelae of advanced liver disease. In cirrhotic patients, we have recently observed elevated levels of circulating neuronal tight junction (TJ) protein, zonula occludens 1 (ZO-1), reflective of a change to blood-brain barrier (BBB) integrity. Moreover, ZO-1 levels positively correlated with hyperammonemia, although any potential relationship remains unclear. Using a carbon tetrachloride (CCl₄)-induced mouse model of cirrhosis, we primarily looked to explore the relationship between neuronal TJ protein expression and hyperammonemia. Secondarily, we assessed the potential role of a natural antioxidant, resveratrol, on neuronal TJ protein expression and hyperammonemia. Over 12 weeks, male Swiss mice were randomized (n = 8/group) to either naïve controls or induced cirrhosis, using two doses of intraperitoneal CCl₄ (0.5 ml/kg/week). After 12 weeks, naïve and cirrhotic mice were randomized to receive either 2 weeks of par-oral resveratrol (10 mg/kg). Plasma samples were analyzed for ammonia, liver biochemistry (ALT, AST, albumin, and bilirubin), and pro-inflammatory cytokines (TNF-α and IL-1β), and brain tissue for brain water content, TJ protein expression (e.g., ZO-1, claudin 5, and occludin), and tissue oxidative stress and inflammatory markers (NF-κB and iNOS) using western blotting. Compared to naïve mice, cirrhosis significantly increased circulating ammonia, brain water, ALT, AST, TNF-α, IL-1β, 4HNE, NF-κB, and iNOS levels, with a concomitant reduction in all TJ proteins (P < 0.05, respectively). In cirrhotic mice, resveratrol treatment ameliorated these changes significantly (P < 0.05, respectively). Our findings provide evidence for a causal association between hyperammonemia and inflammation in cirrhosis linked to TJ protein alterations, BBB disruption, and HE predilection. Moreover, this is the first report of a potential role for resveratrol as a novel therapeutic approach to managing neurological sequelae complicating cirrhosis.

Blood-Brain Barrier Dysfunction Precedes Cognitive Decline and Neurodegeneration in Diabetic Insulin Resistant Mouse Model: An Implication for Causal Link

Diabetic insulin resistance and pro-diabetic diet are reported to increase dementia risk through unknown mechanisms. Emerging evidence suggests that the integrity of blood-brain barrier (BBB) is central to the onset and progression of neurodegeneration and cognitive impairment. Therefore, the current study investigated the effect of pro-diabetic diets on cognitive dysfunction in association to BBB integrity and its putative mechanisms. In C57BL/6J mice chronically ingested with a diet enriched in fat and fructose (HFF), Morris Water Maze (MWM) test indicated no significant cognitive decline after 4 weeks of HFF feeding compared to low-fat (LF) fed control. However, at this stage, BBB dysfunction accompanied by heightened neuroinflammation in cortex and hippocampal regions was already evident. After 24 weeks, HFF fed mice showed significantly deteriorated cognitive function concomitant with substantial neurodegeneration, which both showed significant associations with increased BBB permeability. In addition, the data indicated that the loss of BBB tight junctions was significantly associated with heightened inflammation and leukocyte infiltration. The data collectively suggest that in mice maintained on pro-diabetic diet, the dysfunctional BBB associated to inflammation and leukocyte recruitment precedes the neurodegeneration and cognitive decline, possibly indicating causal association.

Activation of the Nrf2/ARE signaling pathway by probucol contributes to inhibiting inflammation and neuronal apoptosis after spinal cord injury

The nuclear erythroid 2-related factor 2 (Nrf2)/antioxidant response element (ARE) signaling pathway plays an essential role in the cellular antioxidant and anti-inflammatory responses. Spinal cord injury (SCI) results in a massive release of inflammatory factors and free radicals, which seriously compromise nerve recovery and axon regeneration. In this study, we examined the efficacy of probucol on anti-inflammatory responses and functional recovery after SCI by activating the Nrf2/ARE signaling pathway. We also investigated the mechanism by which inflammation is inhibited in this process. We found that treatment of injured rats with probucol significantly increased levels of Nrf2, heme oxygenase-1 (HO-1) and NAD(P)H:quinone oxidoreductase-1 (NQO1), while levels of inflammatory cytokines, interleukin-1β (IL-1β), interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) were decreased. This was associated with a reduction in neural cell apoptosis and promotion of nerve function recovery. These results demonstrate that the neuroprotective effects of probucol after SCI are mediated by activation of the Nrf2/ARE signaling pathway. These findings indicate that the anti-inflammatory effects of probucol represent a viable treatment for improving functional recovery following SCI.

1'-Acetoxychavicol acetate ameliorates age-related spatial memory deterioration by increasing serum ketone body production as a complementary energy source for neuronal cells

1'-Acetoxychavicol acetate (ACA) is naturally obtained from the rhizomes and seeds of Alpinia galangal. Here, we examined the effect of ACA on learning and memory in senescence-accelerated mice prone 8 (SAMP8). In mice that were fed a control diet containing 0.02% ACA for 25 weeks, the learning ability in the Morris water maze test was significantly enhanced in comparison with mice that were fed the control diet alone. In the Y-maze test, SAMP8 mice showed decreased spontaneous alterations in comparison with senescence-accelerated resistant/1 (SAMR1) mice, a homologous control, which was improved by ACA pretreatment. Serum metabolite profiles were obtained by GC-MS analysis, and each metabolic profile was plotted on a 3D score plot. Based upon the diagram, it can be seen that the distribution areas for the three groups were completely separate. Furthermore, the contents of β-hydroxybutyric acid and palmitic acid in the serum of SAMP8-ACA mice were higher than those of SAMP8-control mice and SAMR1-control mice. We also found that SAMR1 mice did not show histological abnormalities, whereas histological damage in the CA1 region of the hippocampus in SAMP8-control mice was observed. However, SAMP8-ACA mice were observed in a similar manner as SAMR1 mice. These findings confirm that ACA increases the serum concentrations of β-hydroxybutyric acid and palmitic acid levels and thus these fuels might contribute to the maintenance of the cognitive performance of SAMP8 mice.