Apolipoprotein E Isoform-specific changes related to stress and trauma exposure

Gastrointestinal Dysfunction in Neurological and Neurodegenerative Disorders

Increasing research links the gut microbiome to neurodegenerative disorders. The gut microbiome communicates with the central nervous system via the gut-brain axis and affects behavioral and cognitive phenotypes. Dysbiosis (a dysfunctional microbiome) drives increased intestinal permeability and inflammation that can negatively affect the brain via the gut-brain axis. Healthier metabolic and lipid profiles and cognitive phenotypes are observed in individuals with more distinct microbiomes. In this review, we discuss the role of the gut microbiome and gut-brain axis in neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease and related animal models, in cancer and cancer treatments, and in metabolic syndrome. We also discuss strategies to improve the gut microbiome and ultimately brain function. Because healthier cognitive phenotypes are observed in individuals with more distinct microbiomes, increased efforts are warranted to develop therapeutic strategies for those at increased risk of developing neurological disorders and patients diagnosed with those disorders.

Role of white matter hyperintensity in effects of apolipoprotein E on cognitive injury

Magnetic Resonance Imaging (MRI) T2-weighted white matter hyperintensity (WMH) is a marker of small vessel cerebrovascular pathology and is of ischemic origin. The prevalence and severity of WMH is associated with cardiovascular risk factors, aging, and cognitive injury in mild cognitive impairment (MCI), vascular dementia, and Alzheimer's disease (AD). WMH especially affects executive function, with additional effects on memory and global cognition. Apolipoprotein E (apoE) plays a role in cholesterol metabolism and neuronal repair after injury. Human and animal studies support a role for apoE in maintaining white matter integrity. In humans, there are three major human apoE isoforms, E2, E3, and E4. Human apoE isoforms differ in risk to develop AD and in association with WMH. In this Mini Review, we propose an increased focus on the role of WMH in cognitive health and cognitive injury and the likely role of apoE and apoE isoform in modulating these effects. We hypothesize that apoE and apoE isoforms play a role in modulating WMH via apoE isoform-dependent effects on oxylipins and 7-ketocholesterol, as well as amyloid related vascular injury, as seen in cerebral amyloid angiopathy.

Behavioral and cognitive performance of humanized APOEε3/ε3 liver mice in relation to plasma apolipoprotein E levels

Plasma apolipoprotein E levels were previously associated with the risk of developing Alzheimer's disease (AD), levels of cerebrospinal fluid AD biomarkers, cognition and imaging brain measures. Outside the brain, the liver is the primary source of apoE and liver transplantation studies have demonstrated that liver-derived apoE does not cross the blood-brain-barrier. How hepatic apoE may be implicated in behavioral and cognitive performance is not clear. In the current study, we behaviorally tested FRGN mice with humanized liver harboring the ε3/ε3 genotype (E3-human liver (HL)) and compared their behavioral and cognitive performance with that of age-matched ε3/ε3 targeted replacement (E3-TR) mice, the latter produces human apoE3 throughout the body whereas the E3-HL mice endogenously produce human apoE3 only in the liver. We also compared the liver weights and plasma apoE levels, and assessed whether plasma apoE levels were correlated with behavioral or cognitive measures in both models. E3-HL were more active but performed cognitively worse than E3-TR mice. E3-HL mice moved more in the open field containing objects, showed higher activity levels in the Y maze, showed higher activity levels during the baseline period in the fear conditioning test than E3-TR mice, and swam faster than E3-TR mice during training to locate the visible platform in the water maze. However, E3-HL mice showed reduced spatial memory retention in the water maze and reduced fear learning and contextual and cued fear memory than E3-TR mice. Liver weights were greater in E3-HL than E3-TR mice and sex-dependent only in the latter model. Plasma apoE3 levels were similar to those found in humans and comparable in female and male E3-TR mice but higher in female E3-HL mice. Finally, we found correlations between plasma apoE levels and behavioral and cognitive measures which were predominantly model-dependent. Our study demonstrates mouse-model dependent associations between plasma apoE levels, behavior and cognition in an 'AD-neutral' setting and suggests that a humanized liver might be sufficient to induce mouse behavioral and cognitive phenotypes.

Insulin Resistance in Peripheral Tissues and the Brain: A Tale of Two Sites

The concept of insulin resistance has been around since a few decades after the discovery of insulin itself. To allude to the classic Charles Dicken's novel published 62 years before the discovery of insulin, in some ways, this is the best of times, as the concept of insulin resistance has expanded to include the brain, with the realization that insulin has a life beyond the regulation of glucose. In other ways, it is the worst of times as insulin resistance is implicated in devastating diseases, including diabetes mellitus, obesity, and Alzheimer's disease (AD) that affect the brain. Peripheral insulin resistance affects nearly a quarter of the United States population in adults over age 20. More recently, it has been implicated in AD, with the degree of brain insulin resistance correlating with cognitive decline. This has led to the investigation of brain or central nervous system (CNS) insulin resistance and the question of the relation between CNS and peripheral insulin resistance. While both may involve dysregulated insulin signaling, the two conditions are not identical and not always interlinked. In this review, we compare and contrast the similarities and differences between peripheral and CNS insulin resistance. We also discuss how an apolipoprotein involved in insulin signaling and related to AD, apolipoprotein E (apoE), has distinct pools in the periphery and CNS and can indirectly affect each system. As these systems are both separated but also linked via the blood-brain barrier (BBB), we discuss the role of the BBB in mediating some of the connections between insulin resistance in the brain and in the peripheral tissues.