Apolipoprotein E polymorphism influences the cerebral metabolic pattern in Alzheimer's disease

Alzheimer's disease and its treatment-yesterday, today, and tomorrow

Alois Alzheimer described the first patient with Alzheimer's disease (AD) in 1907 and today AD is the most frequently diagnosed of dementias. AD is a multi-factorial neurodegenerative disorder with familial, life style and comorbidity influences impacting a global population of more than 47 million with a projected escalation by 2050 to exceed 130 million. In the USA the AD demographic encompasses approximately six million individuals, expected to increase to surpass 13 million by 2050, and the antecedent phase of AD, recognized as mild cognitive impairment (MCI), involves nearly 12 million individuals. The economic outlay for the management of AD and AD-related cognitive decline is estimated at approximately 355 billion USD. In addition, the intensifying prevalence of AD cases in countries with modest to intermediate income countries further enhances the urgency for more therapeutically and cost-effective treatments and for improving the quality of life for patients and their families. This narrative review evaluates the pathophysiological basis of AD with an initial focus on the therapeutic efficacy and limitations of the existing drugs that provide symptomatic relief: acetylcholinesterase inhibitors (AChEI) donepezil, galantamine, rivastigmine, and the N-methyl-D-aspartate receptor (NMDA) receptor allosteric modulator, memantine. The hypothesis that amyloid-β (Aβ) and tau are appropriate targets for drugs and have the potential to halt the progress of AD is critically analyzed with a particular focus on clinical trial data with anti-Aβ monoclonal antibodies (MABs), namely, aducanumab, lecanemab and donanemab. This review challenges the dogma that targeting Aβ will benefit the majority of subjects with AD that the anti-Aβ MABs are unlikely to be the "magic bullet". A comparison of the benefits and disadvantages of the different classes of drugs forms the basis for determining new directions for research and alternative drug targets that are undergoing pre-clinical and clinical assessments. In addition, we discuss and stress the importance of the treatment of the co-morbidities, including hypertension, diabetes, obesity and depression that are known to increase the risk of developing AD.

The role of epsilon phenotype in brain glucose consumption in Alzheimer's disease

OBJECTIVE

The aim of our study was to investigate the impact of the epsilon phenotype in brain glucose consumption in a population with Alzheimer's disease.

METHODS

Statistical Parametric Mapping (SPM8) was used to investigate differences in brain glucose consumption (as detectable by means of 18F FDG-PET/CT) in the population examined. A total of 129 patients (72 females and 57 males) with a diagnosis of probable AD according to the NINCDS-ADRDA criteria underwent the PET/CT examination. The mean (SD) age of the patients was 70 (± 7) years; the mean Mini-Mental State Examination was 19(± 5.6). 59 expressed epsilon 4 phenotype (E4) and 70 expressed the epsilon 3 phenotype (E3). Cerebral spinal fluid amyloid, tau, and t-tau have been measured resulting equal to 367.4 (± 149.1), 584.7 (± 312.1), and 79.2(± 45.9) pg/ml, respectively. Patients with confirmed amyloid and Tau changes were classified as AD. Patients with amyloid changes but negative Tau, considered as high risk of AD, were classified as IAD. Age, sex, MMSE, scholarship, and CSF parameters were used as a covariate in the SPM analyses.

RESULTS

We did not find significant differences in age, gender, and MMSE and CSF parameters among groups. In the analysis of the AD group as compared to AD-E3, AD-E4 subjects show a significant reduction of brain glucose consumption in inferior frontal gyrus bilaterally (BA 45, BA 47). In the analysis of the IAD group as compared to IAD-E3, IAD-E4 subjects show a significant reduction of brain glucose consumption in right in medial, middle, and superior frontal gyrus (BA10, BA11), and in left medial and middle frontal gyrus (BA10, BA11). The differences between IAD-E3 and AD-E3 and between IAD-E4 and AD-E4 (and vice versa analysis) resulted not significant.

CONCLUSIONS

APO-e4 is related to a major involvement of the frontal cortex confirming its role of risk factor in AD, while APO-3 seems not related to a specific pattern, supporting the hypothesis of neutral/protective role in AD.

Vascular Dysfunction in Alzheimer's Disease: A Prelude to the Pathological Process or a Consequence of It?

Alzheimer's disease (AD) is the most prevalent form of dementia. Despite decades of research following several theoretical and clinical lines, all existing treatments for the disorder are purely symptomatic. AD research has traditionally been focused on neuronal and glial dysfunction. Although there is a wealth of evidence pointing to a significant vascular component in the disease, this angle has been relatively poorly explored. In this review, we consider the various aspects of vascular dysfunction in AD, which has a significant impact on brain metabolism and homeostasis and the clearance of β-amyloid and other toxic metabolites. This may potentially precede the onset of the hallmark pathophysiological and cognitive symptoms of the disease. Pathological changes in vessel haemodynamics, angiogenesis, vascular cell function, vascular coverage, blood-brain barrier permeability and immune cell migration may be related to amyloid toxicity, oxidative stress and apolipoprotein E (APOE) genotype. These vascular deficits may in turn contribute to parenchymal amyloid deposition, neurotoxicity, glial activation and metabolic dysfunction in multiple cell types. A vicious feedback cycle ensues, with progressively worsening neuronal and vascular pathology through the course of the disease. Thus, a better appreciation for the importance of vascular dysfunction in AD may open new avenues for research and therapy.

Proteomic analysis of mitochondria in APOE transgenic mice and in response to an ischemic challenge

Apolipoprotein E (APOE)-ɛ4 is associated with a deleterious outcome after ischemic brain injury, which may involve abnormal regulation of mitochondrial function. We have assessed the mitochondrial proteomic response of APOE-ɛ3 and APOE-ɛ4 transgenic mice to transient global ischemic injury in the hippocampus. A genotype-dependent increase in ApoE levels in mitochondria was observed after ischemia, with APOE-ɛ4 mice showing significantly greater increases than APOE-ɛ3 mice. Quantitative analysis of the mitochondria-enriched fractions was performed using liquid-chromatography mass spectrometry coupled to label-free analysis. Of the 1,067 identified proteins, 274 were mitochondria associated. Mitochondrial protein expression was significantly different between genotypes under basal conditions as well as in response to global ischemia. A total of 12 mitochondrial proteins (including respiratory chain proteins NDUFA11, NDUFS3, NDUF5B, ATP5J, as well as ETFA, CYB5B, ATP6V1A, HSPA1B, OXR1, GLUL, IARS2, and PHYHIPL) were significantly altered with respect to genotype, global ischemia, or their interaction (P<0.01). A compelling interactome, created using proteins found to be significantly modulated by global ischemia (P<0.05), involved proteins that regulate energy production and oxidative stress. Thus, APOE genotype has a differential effect on the mitochondrial protein expression in the absence and presence of an injury, which may underlie the differing genotype susceptibility.

Early-onset versus late-onset Alzheimer's disease: the case of the missing APOE ɛ4 allele

Some patients with early-onset Alzheimer's disease (AD) present with a distinct phenotype. Typically, the first and most salient characteristic of AD is episodic memory impairment. A few patients, however, present with focal cortical, non-memory symptoms, such as difficulties with language, visuospatial, or executive functions. These presentations are associated with specific patterns of atrophy and frequently with a young age at onset. Age is not, however, the only determinant of phenotype; underlying factors, especially genetic factors, seem also to affect phenotype and predispose patients to younger or older age at onset. Importantly, patients with atypical early-onset disease seldom carry the APOE ɛ4 allele, which is the most important risk factor for lowering the age of onset in patients with AD. Additionally, theAPOE ɛ4 genotype seems to predispose patients to vulnerability in the medial temporal areas, which leads to memory loss. Conversely, patients negative for the APOE ɛ4 allele and with early-onset AD are more likely to be predisposed to vulnerability of cerebral networks beyond the medial temporal lobes. Other factors are probably involved in determining the pattern of atrophy, but these are currently unknown.

Effect of apolipoprotein E on biomarkers of amyloid load and neuronal pathology in Alzheimer disease

OBJECTIVE

To study the effect of apolipoprotein E epsilon4 status on biomarkers of neurodegeneration (atrophy on magnetic resonance imaging [MRI]), neuronal injury (cerebrospinal fluid [CSF] t-tau), and brain Abeta amyloid load (CSF Abeta(1-42)) in cognitively normal subjects (CN), amnestic subjects with mild cognitive impairment (aMCI), and patients with Alzheimer disease (AD).

METHODS

We included all 399 subjects (109 CN, 192 aMCI, 98 AD) from the Alzheimer's Disease Neuroimaging Initiative study with baseline CSF and MRI scans. Structural Abnormality Index (STAND) scores, which reflect the degree of AD-like anatomic features on MRI, were computed for each subject.

RESULTS

A clear epsilon4 allele dose effect was seen on CSF Abeta(1-42) levels within each clinical group. In addition, the proportion of the variability in Abeta(1-42) levels explained by APOE epsilon4 dose was significantly greater than the proportion of the variability explained by clinical diagnosis. On the other hand, the proportion of the variability in CSF t-tau and MRI atrophy explained by clinical diagnosis was greater than the proportion of the variability explained by APOE epsilon4 dose; however, this effect was only significant for STAND scores.

INTERPRETATION

Low CSF Abeta(1-42) (surrogate for Abeta amyloid load) is more closely linked to the presence of APOE epsilon4 than to clinical status. In contrast, MRI atrophy (surrogate for neurodegeneration) is closely linked with cognitive impairment, whereas its association with APOE epsilon4 is weaker. The data in this paper support a model of AD in which CSF Abeta(1-42) is the earliest of the 3 biomarkers examined to become abnormal in both APOE carriers and noncarriers.

APOE-4 genotype and neurophysiological vulnerability to Alzheimer's and cognitive aging

Many years before receiving a clinical diagnosis of Alzheimer's disease (AD), patients experience evidence of cognitive decline. Recent studies using a variety of brain imaging technologies have detected subtle changes in brain structure and function in normal adults with a genetic risk for AD; these brain changes have similar pathological features as AD, and some appear to be predictive of future cognitive decline. This review examines the most recent data on brain changes in genetic risk for AD and discusses the benefits and potential risks of detecting individuals at risk.

Functional imaging of cognition in Alzheimer's disease using positron emission tomography

Positron emission tomography in Alzheimer's disease (AD) demonstrates a metabolic decrease, predominantly in associative posterior cortices (comprising the posterior cingulate cortex), and also involving medial temporal structures and frontal regions at a lesser degree. The level of activity in this wide network is roughly correlated with dementia severity, but several confounds (such as age, education or subcortical ischemic lesions) may influence the brain-behaviour relationship. Univariate analyses allow one to segregate brain regions that are particularly closely related to specific neuropsychological performances. For example, a relationship was established between the activity in lateral associative cortices and semantic performance in AD. The role of semantic capacities (subserved by temporal or parietal regions) in episodic memory tasks was also emphasized. The residual activity in medial temporal structures was related to episodic memory abilities, as measured by free recall performance, cued recall ability and recognition accuracy. More generally, AD patients' performance on episodic memory tasks was correlated with the metabolism in several structures of Papez's circuit (including the medial temporal and posterior cingulate regions). Multivariate analyses should provide complementary information on impaired metabolic covariance in functional networks of brain regions and the consequences for AD patients' cognitive performance. More longitudinal studies are being conducted that should tell us more about the prognostic value of initial metabolic impairment and the neural correlates of progressive deterioration of cognitive performance in AD.

Altered medial temporal lobe responses during visuospatial encoding in healthy APOE*4 carriers

The apolipoprotein varepsilon4 allele (APOE*4) is a major genetic risk factor for Alzheimer's disease (AD) and has been associated with altered cortical activation as assessed by functional neuroimaging in cognitively normal younger and older carriers. We chose to evaluate medial temporal lobe (MTL) activation during encoding and recognition using a perspective-dependent (route or survey) visuospatial memory task by monitoring the blood-oxygen-level-dependent (BOLD) fMRI response in older, non-demented APOE*4 carriers (APOE*4+) and non-carriers (APOE*4-). During encoding, the APOE*4- group had greater average task-associated BOLD responses in ventral visual pathways, including the MTLs, as compared to the APOE*4+ group. Furthermore, MTL activation was greater during route encoding than survey encoding on average in APOE*4-, but not APOE*4+, subjects. During recognition, both groups performed similarly and no BOLD signal differences were found. Finally, within-group analysis revealed MTL activation during encoding was correlated with recognition performance in APOE*4-, but not APOE*4+ subjects. Reduced task-associated MTL activation that does not correlate with either visuospatial perspective or task performance suggests that MTL dysregulation occurs prior to clinical symptoms of dementia in APOE*4 carriers.

ApoE epsilon4 genotype is accompanied by lower metabolic activity in nucleus basalis of Meynert neurons in Alzheimer patients and controls as indicated by the size of the Golgi apparatus

We previously found apolipoprotein (apoE) epsilon4-dependent lower metabolic activity in nucleus basalis of Meynert (NBM) neurons in Alzheimer disease (AD). In the present study we examined the metabolic activity in the NBM of 39 mentally intact control subjects with different APOE genotype. The control subjects had either no AD pathology (Braak stage 0) or the very beginning of AD pathology (Braak stage I-II). We used the Golgi apparatus (GA) size as a measure of neuronal metabolic activity. Control subjects carrying an apoE epsilon4 allele showed reduced neuronal metabolism; they had significantly more neurons with smaller GA sizes compared to control subjects not carrying an apoE epsilon4 allele. Only control subjects not carrying an apoE epsilon4 allele had increased neuronal metabolism in Braak I-II subjects. They had more neurons with larger GA sizes compared to Braak 0 subjects, which may reflect a compensatory mechanism. Our data indicate that APOE epsilon4 may act by a lower neuronal metabolism as a risk factor for cognitive impairment in normal aging and early prodromal AD. As the disease progresses into later stages of AD (Braak V-VI) neuronal metabolism strongly diminishes, resulting in neurons with extremely small GA sizes, irrespective of APOE genotype.

Genetics of human prefrontal function

Evolution of the prefrontal cortex was an essential precursor to civilization. During the past decade, it became increasingly obvious that human prefrontal function is under substantial genetic control. In particular, heritability studies of frontal lobe-related neuropsychological function, electrophysiology and neuroimaging have greatly improved our insight. Moreover, the first genes that are relevant for prefrontal function such as catechol-O-methyltransferase (COMT) are currently discovered. In this review, we summarize the present knowledge on the genetics of human prefrontal function. For historical reasons, we discuss the genetics of prefrontal function within the broader concept of general cognitive ability (intelligence). Special emphasis is also given to methodological concerns that need to be addressed when conducting research on the genetics of prefrontal function in humans.

Functional brain abnormalities in young adults at genetic risk for late-onset Alzheimer's dementia

Fluorodeoxyglucose positron emission tomography (PET) studies have found that patients with Alzheimer's dementia (AD) have abnormally low rates of cerebral glucose metabolism in posterior cingulate, parietal, temporal, and prefrontal cortex. We previously found that cognitively normal, late-middle-aged carriers of the apolipoprotein E epsilon4 allele, a common susceptibility gene for late-onset Alzheimer's dementia, have abnormally low rates of glucose metabolism in the same brain regions as patients with probable AD. We now consider whether epsilon4 carriers have these regional brain abnormalities as relatively young adults. Apolipoprotein E genotypes were established in normal volunteers 20-39 years of age. Clinical ratings, neuropsychological tests, magnetic resonance imaging, and PET were performed in 12 epsilon4 heterozygotes, all with the epsilon3/epsilon4 genotype, and 15 noncarriers of the epsilon4 allele, 12 of whom were individually matched for sex, age, and educational level. An automated algorithm was used to generate an aggregate surface-projection map that compared regional PET measurements in the two groups. The young adult epsilon4 carriers and noncarriers did not differ significantly in their sex, age, educational level, clinical ratings, or neuropsychological test scores. Like previously studied patients with probable AD and late-middle-aged epsilon4 carriers, the young epsilon4 carriers had abnormally low rates of glucose metabolism bilaterally in the posterior cingulate, parietal, temporal, and prefrontal cortex. Carriers of a common Alzheimer's susceptibility gene have functional brain abnormalities in young adulthood, several decades before the possible onset of dementia.

ApoE isoforms affect neuronal N-methyl-d-aspartate calcium responses and toxicity via receptor-mediated processes

Apolipoprotein E (apoE) alters the pathophysiology of Alzheimer's disease, but its mechanism is not fully understood. We examined the effects of recombinant human apoE3 and apoE4 on the neuronal calcium response to N-methyl-D-aspartate (NMDA), and compared them to their toxicity. ApoE4 (100 nM) significantly increased the resting calcium (by 70%) and the calcium response to NMDA (by 185%), whereas similar changes were not obtained in apoE3-treated neurons. ApoE4, but not apoE3, also significantly increased neurotoxicity, as evidenced by enhanced lactate dehydrogenase release (by 53%) and reduced 3-(4,5-dimethylthiazol-2-yl)-2,5,diphenyltetrazolium bromide levels (by 32%). ApoE4-induced changes in the calcium response to NMDA and associated neurotoxicity were blocked by coincubation with MK-801. Both the receptor-associated protein, which inhibits interaction of apoE with members of the LDL receptor family, including the low-density lipoprotein receptor-related protein (LRP), and activated alpha2-macroglobulin, another LRP ligand, prevented apoE4-induced enhancement of the calcium response to NMDA, resting calcium levels, and neurotoxicity. A tandem apoE peptide (100 nM) containing only the receptor binding region residues also eliminated the enhanced calcium signaling and neurotoxicity by apoE4. Taken together, our data demonstrate that differential effects of apoE3 and apoE4 on the calcium signaling in neurons correlate with their effect on neurotoxicity, which are secondary to receptor binding.

Imaging neurochemical endophenotypes: promises and pitfalls

A large number of polymorphisms in genes coding for neurotransmitter receptors and transporters have been associated with neuropsychiatric conditions, although few of these associations have been consistently replicated. These proteins are critical targets of psychoactive drugs and the clarification of the functional significance of these polymorphisms might offer important leads for drug development and therapeutic applications. Brain imaging techniques such as single photon emission computed tomography (SPECT) and positron emission tomography (PET) provide the means to monitor the expression and function of many of these proteins in the living human brain. This paper reviews brain imaging studies designed to evaluate the significance of polymorphisms in genes coding for important drug targets (e.g., the serotonin transporter [SERT], the dopamine transporter [DAT] and the dopamine D(2) receptor) in terms of expression or function. These studies illustrate the unique opportunities, as well as the pitfalls, generated by combining genetic analysis with brain imaging studies.

Single photon emission computed tomography and apolipoprotein E in Alzheimer's disease: impact of the epsilon4 allele on regional cerebral blood flow

The aim of this study was to examine the impact of the apolipoprotein E (APOE) epsilon4 allele on semiquantitative regional cerebral blood flow (rCBF) in Alzheimer's disease. Single photon emission computed tomography technetium (SPECT) with (99m)Tc d,l-hexamethyl propylenamine oxine was used to determine rCBF in 41 consecutive patients (18 males/23 females) with probable Alzheimer's disease according to the National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's Disease and Related Disorders Association criteria (mean age 71.0 years; range 54-85). The mean Mini-Mental State Examination (MMSE) score was 20.4 (range 10-30). After normalization of CBF to mean blood flow in the cerebellum, values for rCBF in several cortical regions of interest, side-to-side asymmetry indices, and anterior-posterior ratios were calculated. Determination of the APOE genotype from blood samples was performed using restriction enzyme polymerase chain reaction technique. Multivariate regression analyses and the Wilcoxon rank-sum test for unpaired data (Mann-Whitney) were used for statistical analysis. The patients comprised 27APOE epsilon4-positive and 14APOE epsilon4-negative individuals. Five patients were APOE epsilon4 homozygotes. APOE epsilon4-positive patients had significantly reduced rCBF in the right frontal and left occipital lobes. On nonparametric analysis, the most prominent differences between epsilon4-negative and epsilon4-positive patients were demonstrated in subregions representing the frontal association cortex (Mann-Whitney, P < .01). Age-stratified analysis suggested that these findings could be demonstrated predominantly in the elderly patients. The results of this study suggest that the APOE genotype in itself may have an impact on the pattern of rCBF deficits in Alzheimer's disease. The more pronounced reduction of rCBF in frontal association cortex observed in elderly APOE epsilon4-positive patients might predict clinical progression.

Decreased hippocampal metabolic activity in Alzheimer patients is not reflected in the immunoreactivity of cytochrome oxidase subunits

In the present study we have compared histochemically determined cytochrome oxidase activity with the levels of immunocytochemically stained cytochrome oxidase subunits (CO II and CO IV) and ATP synthase in the human hippocampus in relation with Alzheimer's disease. Cytochrome oxidase activity was significantly reduced in all hippocampal areas of Alzheimer patients. The protein levels of subunits II and IV were not different between control subjects and Alzheimer patients. Additionally, it was observed that the active cytochrome oxidase is evenly distributed over both cell bodies and neuropil, while a relatively large pool of inactive enzyme or precursors is limited to the neuronal somata. Further, in Alzheimer patients the CO IV immunoreactivity decreased with age, whereas in control subjects it increased with age. Our results suggest that the assembly of cytochrome oxidase or the processing of its subunits may be impaired.

How does the apolipoprotein E genotype modulate the brain in aging and in Alzheimer's disease? A review of neuroimaging studies

The epsilon 4 allele of apolipoprotein E is a risk factor for Alzheimer's disease, but also a modulator of its clinical picture. In this paper, recent research in neuroimaging of aging and Alzheimer's disease in relation to apolipoprotein E is reviewed, emphasizing the advances but also the controversies. Further, the possible clinicopathological implications of these findings are discussed.