Effect of age and the APOE gene on metabolite concentrations in the posterior cingulate cortex.

Proton magnetic resonance spectroscopy (1H-MRS) has provided valuable information about the neurochemical profile of Alzheimer's disease (AD). However, its clinical utility has been limited in part by the lack of consistent information on how metabolite concentrations vary in the normal aging brain and in carriers of apolipoprotein E (APOE) ε4, an established risk gene for AD. We quantified metabolites within an 8cm3 voxel within the posterior cingulate cortex (PCC)/precuneus in 30 younger (20-40 years) and 151 cognitively healthy older individuals (60-85 years). All 1H-MRS scans were performed at 3T using the short-echo SPECIAL sequence and analyzed with LCModel. The effect of APOE was assessed in a sub-set of 130 volunteers. Older participants had significantly higher myo-inositol and creatine, and significantly lower glutathione and glutamate than younger participants. There was no significant effect of APOE or an interaction between APOE and age on the metabolite profile. Our data suggest that creatine, a commonly used reference metabolite in 1H-MRS studies, does not remain stable across adulthood within this region and therefore may not be a suitable reference in studies involving a broad age-range. Increases in creatine and myo-inositol may reflect age-related glial proliferation; decreases in glutamate and glutathione suggest a decline in synaptic and antioxidant efficiency. Our findings inform longitudinal clinical studies by characterizing age-related metabolite changes in a non-clinical sample

The relationship between Alzheimer's disease, inflammation, the APOE genotype and neuronal integrity

Includes abstract. Includes bibliographical references

APOE Genotype Modulates Proton Magnetic Resonance Spectroscopy Metabolites in the Aging Brain

Background: Proton magnetic resonance spectroscopy (H-1-MRS) studies on healthy aging have reported inconsistent findings and have not systematically taken into account the possible modulatory effect of APOE genotype. We aimed to quantify brain metabolite changes in healthy subjects in relation to age and the presence of the APOE E4 genetic risk factor for Alzheimer\u27s disease. Additionally, we examined these measures in relation to cognition. Methods: We studied a cohort of 112 normal adults between 50 and 86 years old who were genotyped for APOE genetic polymorphism. Measurements of H-1-MRS metabolites were obtained in the posterior cingulate and precuneus region. Measures of general cognitive functioning, memory, executive function, semantic fluency, and speed of processing were also obtained. Results: General linear model analysis demonstrated that older APOE E4 carriers had significantly higher choline/creatine and myoinositol/creatine ratios than APOE E3 homozygotes. Structural equation modeling resulted in a model with an excellent goodness of fit and in which the APOE x age interaction and APOE status each had a significant effect on H-1-MRS metabolites (choline/creatine and myo-inositol/creatine). Furthermore, the APOE x age variable modulation of cognition was mediated by H-1-MRS metabolites. Conclusions: In a healthy aging normal population, choline/creatine and myo-inositol/creatine ratios were significantly increased in APOE E4 carriers, suggesting the presence of neuroinflammatory processes and greater membrane turnover in older carriers. Structural equation modeling analysis confirmed these possible neurodegenerative markers and also indicated the mediator role of these metabolites on cognitive performance among older APOE E4 carriers

Evidence against altered excitatory/inhibitory balance in the posteromedial cortex of young adult APOE E4 carriers: a resting state 1H-MRS study

A strategy to gain insight into early changes that may predispose people to Alzheimer's disease (AD) is to study the brains of younger cognitively healthy people that are at increased genetic risk of AD. The Apolipoprotein (APOE) E4 allele is the strongest genetic risk factor for AD, and several neuroimaging studies comparing APOE E4 carriers with non-carriers at age ∼20–30 years have detected hyperactivity (or reduced deactivation) in posteromedial cortex (PMC), a key hub of the default network (DN), which has a high susceptibility to early amyloid deposition in AD. Transgenic mouse models suggest such early network activity alterations may result from altered excitatory/inhibitory (E/I) balance, but this is yet to be examined in humans. Here we test the hypothesis that PMC fMRI hyperactivity could be underpinned by altered levels of excitatory (glutamate) and/or inhibitory (GABA) neurotransmitters in this brain region. Forty-seven participants (20 APOE E4 carriers and 27 non-carriers) aged 18–25 years underwent resting-state proton magnetic resonance spectroscopy (1H-MRS), a non-invasive neuroimaging technique to measure glutamate and GABA in vivo. Metabolites were measured in a PMC voxel of interest and in a comparison voxel in the occipital cortex (OCC). There was no difference in either glutamate or GABA between the E4 carriers and non-carriers in either MRS voxel, or in the ratio of glutamate to GABA, a measure of E/I balance. Default Bayesian t-tests revealed evidence in support of this null finding. Our findings suggest that PMC hyperactivity in APOE E4 carriers is unlikely to be associated with, or possibly may precede, alterations in local resting-state PMC neurotransmitters, thus informing our understanding of the spatio-temporal sequence of early network alterations underlying APOE E4 related AD risk

Functional-biochemical relationships in the posterior cingulate cortex, and their application to the genetic risk of Alzheimer’s disease

A major challenge in Alzheimer’s disease (AD) research is to improve understanding of the earliest brain changes associated with AD, in order to develop treatments to combat this disease. One strategy potentially able to inform this need is the study of brain regions associated with AD in young adults at genetic risk of AD. A key region of increasing interest to AD researchers is the posterior cingulate cortex (PCC). Despite being implicated in AD pathogenesis, its function is not well understood. Neuroimaging studies in young adults who carry the strongest genetic risk factor for AD, the Apolipoprotein (APOE) E4 allele, have detected glucose metabolism, activity and functional connectivity alterations in the PCC. A better understanding of PCC function would aid interpretation of these changes, to improve insight into AD pathogenesis. This thesis applied functional magnetic resonance imaging (fMRI) and magnetic resonance spectroscopy (MRS) to study PCC activity correlates with brain metabolites in vivo (Chapters 3 and 4), prior to investigating whether these metabolites might be altered in APOE-E4 carriers aged 18-25 years (Chapter 5). Chapters 3 and 4 detected a category-sensitive fMRI response and fMRI-MRS relationship for the scene condition of a perception task and a working memory task, specifically a positive correlation between PCC activity and the metabolite N-acetyl- aspartate (tNAA), a marker of neuronal integrity associated with neuronal mitochondrial metabolism. The scene conditions of these paradigms previously elicited an altered pattern of PCC activity in young APOE-E4 carriers. Chapter 5 therefore compared PCC tNAA and other MRS metabolites between young APOE-E4 carriers and non-carriers. No MRS differences, however, were evident between APOE groups. These findings contribute to our understanding of the biochemistry underpinning PCC activity, but suggest that an alteration in such a pathway may not be linked with the activity alterations detected in young APOE-E4 carriers

EVALUATING THE MICROBIOME TO BOOST RECOVERY FROM STROKE: THE EMBRS STUDY

Accumulating evidence suggests that gut microbes modulate brain plasticity via the bidirectional gut-brain axis and may play a role in stroke rehabilitation. A severely imbalanced microbial community has been shown to occur following stroke, causing a systemic flood of neuro- and immunomodulatory substances due to increased gut permeability and decreased gut motility. Here we measure post-stroke increased gut dysbiosis and how it correlates with gut permeability and subsequent cognitive impairment. We recruited 12 participants with acute stroke, 12 healthy control participants, and 18 participants who had risk factors for stroke, but had not had a stroke. We measured the gut microbiome with whole shotgun sequencing on stool samples. We measured cognitive and emotional health with MRI imaging and the NIH toolbox. We normalized all variables and used linear regression methods to identify gut microbial levels associations with cognitive and emotional assessments. Beta diversity analysis revealed that the bacteria populations of the stroke group were statistically dissimilar from the risk factors and healthy control groups. Relative abundance analysis revealed notable decreases in butyrate-producing microbial taxa. The stroke group had higher levels of the leaky gut marker alpha-1-antitrypsin than the control groups, and roseburia species were negatively correlated with alpha-1-antitrypsin. Several Actinobacteria species were associated with cerebral blood flow and white matter integrity in areas of the brain responsible for language, learning, and memory. Stroke participants scored lower on the picture vocabulary and list sorting tests than those in the control groups. Stroke participants who had higher levels of roseburia performed better on the picture vocabulary task. We found that microbial communities are disrupted in a stroke population. Many of the disrupted bacteria have previously been reported to have correlates to health and disease. This preparatory study will lay the foundation for the development of therapeutics targeting the gut following stroke

\u3cem\u3eAPOE\u3c/em\u3e and Alzheimer’s Disease: Neuroimaging of Metabolic and Cerebrovascular Dysfunction

Apolipoprotein E4 (ApoE4) is the strongest genetic risk factor for late onset Alzheimer’s Disease (AD), and is associated with impairments in cerebral metabolism and cerebrovascular function. A substantial body of literature now points to E4 as a driver of multiple impairments seen in AD, including blunted brain insulin signaling, mismanagement of brain cholesterol and fatty acids, reductions in blood brain barrier (BBB) integrity, and decreased cerebral glucose uptake. Various neuroimaging techniques, in particular positron emission topography (PET) and magnetic resonance imaging (MRI), have been instrumental in characterizing these metabolic and vascular deficits associated with this important AD risk factor. In the current mini-review article, we summarize the known effects of APOE on cerebral metabolism and cerebrovascular function, with a special emphasis on recent findings via neuroimaging approaches

APOE AS A METABOLIC REGULATOR IN HUMANS, MICE, AND ASTROCYTES

Altered metabolic pathways appear to play central roles in the pathophysiology of late-onset Alzheimer’s disease (AD). Carrier status of the E4 allele of the APOE gene is the strongest genetic risk factor for late-onset AD, and increasing evidence suggests that E4 carriers may be at an increased risk for neurodegeneration based on inherent metabolic impairments. A new appreciation is forming for the role of APOE in cerebral metabolism, and how nutritional factors may impact this role. In chapter 1, the literature on nutritional interventions in E4 carriers aimed at mitigating disease risk is reviewed. Studies investigating the mechanism by which E4 increases disease risk have focused primarily on the association of E4 with the neuropathological hallmarks. While these studies have aided in our understanding of the role of E4 in late-disease pathology, investigating metabolic signatures of E4 carriers who have not yet developed neuropathology gives insight into the potential earlier mechanisms of E4 as a risk factor for AD. For example, an early and consistent biological hallmark of AD is cerebral glucose hypometabolism as observed by fluorodeoxyglucose positron emission tomography (FDG-PET). Interestingly, E4 carriers also display an AD-like pattern of decreased glucose metabolism by FDG-PET far before clinical symptomology. Since glucose hypometabolism occurs early in AD and early in E4 carriers, it may represent a critical prodromal phase of AD. Beyond this brain imaging finding, it is unclear if APOE has any other discernable metabolic effects in cognitively unimpaired young people. In chapter 2 we bridge this knowledge gap in the field. We utilized indirect calorimetry (IC) as a method for assessing metabolism in young and middle aged volunteers with and without the E4 allele. While IC is commonly used in clinical settings to assess nutritional status, it has never been used to assess risk for cognitive decline. Thus, repurposing IC to study the metabolic effects of an AD risk factor such as E4 represents a simple, cost-effective, and innovative new approach. We found that young female E4 carriers show a lower resting energy expenditure compared to non-carriers. We also tested how E4 carriage affects response to a glucose challenge by administering a glucose rich beverage in conjunction with IC measurements and plasma metabolomics. We found that female E4 carriers were unable to increase oxygen consumption relative to non-carriers, reflecting an impairment in glucose oxidation. Additionally, the plasma metabolome of E4 carriers showed increased lactate and an overall metabolic profile consistent with aerobic glycolysis. We translated these findings to mice expressing the human alleles of APOE. We found that E4 mice on a normal chow diet have lower energy expenditure than E3 mice, a difference further exacerbated by high carbohydrate diet feeding. Stable isotope tracing in mice whole brains and astrocytes implicate increased utilization of aerobic glycolysis as a mechanism for altered glucose handling in E4 carriers. Another pathological feature of the Alzheimer’s brain is glial lipid accumulation. The mechanism for this is largely unknown. In chapter 3, the literature pertaining to lipid droplets (LD) in the brain is reviewed. We show that LDs are much more than simple fat depots, playing critical roles in metabolism, inflammation, and various neurodegenerative diseases. In chapter 4, the effect of the E4 allele on astrocyte LD accumulation and turnover is assessed. Using an in vitro model of APOE we probed the storage and oxidation capacity of fatty acids in E3 and E4 astrocytes. We observed that E4 astrocytes exhibit greater storage of fatty acids as LDs under control and lipid loaded conditions compared to E3 astrocytes. Furthermore, we found that E4 astrocytes rely on these LDs as a source of fuel for oxidation. Therefore, APOE appears to regulate whole body energy expenditure, cerebral glucose oxidation, astrocyte LD metabolism, and risk for a host of metabolic diseases. In chapter 5, the evolutionary history of APOE is presented to posit a hypothesis for why E4 may be disadvantageous in modern times compared to its prior advantages in the pre-historic era. These results point toward a larger role for APOE in the regulation of metabolism than previously understood and advocates for alternative nutritional approaches including calorie reduction and intermittent fasting as plausible interventions to mitigate disease risk in E4 carriers

Early detection of Alzheimer’s disease - Twin study on episodic memory and imaging biomarkers of neuroinflammation and β-amyloid

The disease process of Alzheimer’s disease (AD) causes damage to the brain for several years leading to the development of mild cognitive impairment (MCI) and finally to dementia which interferes with independent living. The early detection of AD disease process is key for the prevention and treatment of disease. The aim of this thesis was to improve the assessment of episodic memory (EM) and cognitive performance with a telephone interview and neuroimaging of early AD. The study population belonged to the older Finnish Twin Cohort study. 2631 twins (856 pairs) participated in the telephone interview (TELE, TICS) during 1999–2007 and 1817 twins (559 pairs) participated in the interview (TELE, TICS, TICS-m) during 2013– 2017. Cognitively discordant twin pairs were asked to participate in more detailed examinations. 11 twin pairs participated in [11C]PBR28 positron emission tomography (PET) imaging measuring neuroinflammation during 2014–2017 and 45 twin pairs participated in [11C]PiB PET imaging measuring β-amyloid (Aβ) deposits during 2005–2017. Twins who had co-twins with dementia (n=101) performed poorer than average in a word list learning test. When using the telephone interview TICS-m, the education‐adjusted classification resulted in a higher proportion of apolipoprotein (APOE) ε4 allele carriers among those identified as having MCI. Twins with poorer EM performance (n=10) had higher cortical [11C]PBR28 uptake compared to their better-performing co-twins. In addition, higher cortical [11C]PiB uptake was associated with poorer EM performance. The results from the telephone interview studies indicate that poorer word list learning performance may be an early marker of dementia risk and that the use of education‐adjustment may increase the accuracy of MCI classification. The twin pair setting controlling for genetic and environmental effects indicated that brain Aβ load and neuroinflammation have a negative association with EM performance.Alzheimerin taudin varhainen havaitseminen – Kaksostutkimus episodisesta muistista ja neuroinflammaation ja β-amyloidin kuvantamisbiomarkkereista Alzheimerin taudin (AT) prosessi vaurioittaa aivoja vuosien ajan ja johtaa lievään kognitiiviseen heikentymiseen (MCI) ja lopulta itsenäistä selviytymistä häiritsevään dementiaan. AT:n dementiaan johtavan prosessin varhainen havaitseminen on avainasemassa ehkäisyn ja hoidon kannalta. Tämän väitöskirjatutkimuksen tavoitteena oli kehittää puhelinhaastattelun käyttöä episodisen muistin (EM) ja muiden tiedonkäsittely- eli kognitiivisten toimintojen arvioimisessa sekä AT:n varhaista kuvantamista. Tutkimusjoukko kuului vanhempaan suomalaisen kaksoskohorttitutkimukseen. 2631 kaksosta (856 paria) osallistui puhelinhaastatteluun (TELE, TICS) 1999–2007 aikana ja 1817 kaksosta (559 paria) osallistui haastatteluun (TELE, TICS, TICS-m) 2013–2017 aikana. Kognitiivisesti diskordantit kaksosparit kutsuttiin tarkempiin jatkotutkimuksiin. 11 kaksosparia osallistui neuroinflammaatiota mittaavaan [11C]PBR28-merkkiaineen positroniemissiotomografia (PET) - kuvaukseen 2014–2017 aikana ja 45 kaksosparia osallistui aivojen β-amyloidikertymää mittaavaan [11C]PiB-merkkiaineen PET-kuvaukseen 2005–2017 aikana. Sellaisten kognitiivisesti normaalien ikääntyneiden kaksosten (n=101), joiden sisaruksella oli dementia, havaittiin suoriutuvan keskimääräistä heikommin sanalistan oppimista mittaavassa testissä. Käytettäessä TICS-m-puhelinhaastattelua koulutuskorjauksen käyttäminen johti siihen, että MCI:tä sairastavien joukossa oli suurempi osuus apolipoproteiini E:n (APOE) ε4-alleelin kantajia. Kaksosilla (n=10), jotka suoriutuivat heikommin EM-testeissä, oli suurempi aivokuoren [11C]PBR28-kertymä verrattuna paremmin suoriutuviin sisaruksiinsa. Myös suurempi aivokuoren [11C]PiB-kertymä oli yhteydessä heikompaan EM-suoritukseen. Puhelinhaastattelujen tulokset viittaavat siihen, että sanalistan oppiminen voi olla dementiariskistä kertova varhainen merkki ja että koulutuskorjauksen käyttö voi lisätä MCI-luokittelun tarkkuutta. Kaksosasetelma, joka kontrolloi geneettisten ja ympäristötekijöiden vaikutusta, osoitti, että aivojen β-amyloidikertymä ja neuroinflammaatio ovat negatiivisessa yhteydessä EM:n toiminnan kanssa