Apolipoprotein E, A beta-amyloid, and the molecular pathology of Alzheimer's disease. Therapeutic implications

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.

AAVrh.10-Mediated APOE2 Central Nervous System Gene Therapy for APOE4-Associated Alzheimer's Disease

Alzheimer's disease (AD) is a progressive degenerative neurological disorder affecting nearly one in nine elderly people in the United States. Population studies have shown that an inheritance of the apolipoprotein E (APOE) variant APOE4 allele increases the risk of developing AD, whereas APOE2 homozygotes are protected from late-onset AD. It was hypothesized that expression of the "protective" APOE2 variant by genetic modification of the central nervous system (CNS) of APOE4 homozygotes could reverse or prevent progressive neurologic damage. To assess the CNS distribution and safety of APOE2 gene therapy for AD in a large-animal model, intraparenchymal, intracisternal, and intraventricular routes of delivery to the CNS of nonhuman primates of AAVrh.10hAPOE2-HA, an AAVrh.10 serotype coding for an HA-tagged human APOE2 cDNA sequence, were evaluated. To evaluate the route of delivery that achieves the widest extent of APOE2 expression in the CNS, the expression of APOE2 in the CNS was evaluated 2 months following vector administration for APOE2 DNA, mRNA, and protein. Finally, using conventional toxicology assays, the safety of the best route of delivery was assessed. The data demonstrated that while all three routes are capable of mediating ApoE2 expression in AD relevant regions, intracisternal delivery of AAVrh.10hAPOE2-HA safely mediated wide distribution of ApoE2 with the least invasive surgical intervention, thus providing the optimal strategy to deliver vector-mediated human APOE2 to the CNS.

Adiposity measures predict olfactory processing speed in older adult carriers of the apolipoprotein E ε4 allele

OBJECTIVE

The current study investigated the relationship between adiposity and P3 latency.

METHODS

Fifty-one adults in two age groups (18-25 and 65+) participated. Odor stimuli were delivered via olfactometer as participants focused on a computer screen. Each stimulus was followed by presentation on the screen of four odor identification choices. Participants attempted identification by button press. Olfactory event-related potentials (OERPs) were recorded. BMI and waist circumference were measured as indicators of adiposity.

RESULTS

In bivariate analyses with all participants included, positive correlations for P3 latency with both BMI and waist circumference were observed, indicating that as adiposity increased latencies also increased. When each age group was separately examined, correlations between adiposity measures and latency remained statistically significant for older adults. Furthermore, ApoE ε4 allele status was examined. Latencies remained positively correlated with adiposity in older adult ε4 carriers; but not in non-carriers.

CONCLUSIONS

This study indicates that adiposity predicts olfactory processing speed in older adults, specifically in ε4 carriers.

SIGNIFICANCE

The results suggest that olfactory processing speed may be a useful measure for detecting and following the effects of adiposity on brain integrity and cognitive function in those at genetic risk for AD.

The effect of hormone therapy on olfactory sensitivity is dependent on apolipoprotein E genotype

Patients with Alzheimer's disease (AD) show a deficit in olfactory threshold sensitivity. The Apolipoprotein E (ApoE) epsilon4 allele is associated with increased risk of AD and earlier symptom onset. Hormone therapy (HT) may exert neuroprotective effects on brain areas affected by AD. The current study investigated the effect of HT on performance on an olfactory threshold test in epsilon4 positive and epsilon4 negative non-hysterectomized, non-demented, elderly females and AD patients. Among the non-demented participants, epsilon4 positive females who had received HT performed 1) significantly better than those without HT, and 2) at levels similar to those of epsilon4 negative females. In contrast, those without HT who were epsilon4 positive performed significantly worse than those who were epsilon4 negative. HT had no effect on performance in AD patients regardless of epsilon4 status. These results suggest that HT may offer protection against loss of olfactory function in epsilon4 positive individuals in preclinical stages of AD. Future research is warranted in order to investigate further the neuroprotective role of HT on sensory and cognitive functions in non-demented aging individuals.

Activation of estrogen receptor alpha increases and estrogen receptor beta decreases apolipoprotein E expression in hippocampus in vitro and in vivo

Previous evidence indicates that, in carriers of apolipoprotein E4 (ApoE4), estrogen therapy increased the risk of late-onset Alzheimer's disease (AD), whereas in individuals carrying ApoE2/3, estrogen therapy reduced the risk of AD [Cauley JA, Zmuda JM, Yaffe K, Kuller LH, Ferrell RE, Wisniewski SR, Cummings SR (1999) J Bone Miner Res 14:1175-1181; Yaffe K, Haan M, Byers A, Tangen C, Kuller L (2000) Neurology 54:1949-1954]. Estrogen mechanisms of action are mediated by two estrogen receptors (ERs), ERalpha and ERbeta. In this study, we determined the relationship between ER subtype and estrogen regulation of ApoE expression in HT-22 cells ectopically transfected with ERalpha or ERbeta, in primary cultured rat hippocampal neurons in vitro and in rat hippocampus in vivo by both molecular biological and pharmacological analyses. Results of these analyses demonstrated that activation of ERalpha either by 17beta-estradiol or a specific-agonist, propylpyrazole triol, up-regulated ApoE mRNA and protein expression. In contrast, the ERbeta-selective agonist, diarylpropionitrile, down-regulated ApoE mRNA and protein expression. These results demonstrate that, in vitro and in vivo, ApoE expression can be differentially regulated depending on activation of ER subtypes. These data suggest that use of ER-selective ligands could provide therapeutic benefit to reduce the risk of AD by increasing ApoE expression in ApoE2/3 allele carriers and decreasing ApoE expression in ApoE4 allele carriers.

Current concepts in therapeutic strategies targeting cognitive decline and disease modification in Alzheimer's disease

Alzheimer's disease is a progressive neurodegenerative disorder and the leading cause of dementia in the Western world. Postmortem, it is characterized neuropathologically by the presence of amyloid plaques, neurofibrillary tangles, and a profound gray matter loss. Neurofibrillary tangles are composed of an abnormally hyperphosphorylated intracellular protein called tau, tightly wound into paired helical filaments and thought to impact microtubule assembly and protein trafficking, resulting in the eventual demise of neuronal viability. The extracellular amyloid plaque deposits are composed of a proteinacious core of insoluble aggregated amyloid-beta (Abeta) peptide and have led to the foundation of the amyloid hypothesis. This hypothesis postulates that Abeta is one of the principal causative factors of neuronal death in the brains of Alzheimer's patients. With multiple drugs now moving through clinical development for the treatment of Alzheimer's disease, we will review current and future treatment strategies aimed at improving both the cognitive deficits associated with the disease, as well as more novel approaches that may potentially slow or halt the deadly neurodegenerative progression of the disease.

Experimental approaches and drugs in development for the treatment of dementia

Treatment of dementia can be divided as symptomatic treatment of cognitive or non-cognitive symptoms and the treatment of underlying pathology. In the last decade the thrust of symptomatic treatment of Alzheimer's disease (AD) has been enhancement of cholinergic transmission. Besides the acetycholinesterase inhibitors (AChE-I) currently in use, cholinergic agonists and enhancers are in development. Other therapeutic approaches directed towards neurotransmitter substitution or modulation include serotoninergic, noradrenergic substances, neuropeptides and those acting via excitatory amino acid receptors, such as ampakines or NMDA antagonists. Introduction of atypical neuroleptics represents the most recent development in the treatment of behavioural symptoms. Efforts to treat the underlying pathology are based on modulation of APP processing in order to decrease the accumulation of beta-amyloid, those to decrease tau hyperphosphorylation, use of nerve growth factors and those based on Apo-E modulation. Potential use of oestrogens and NSAIDs are also under investigation. Recently, vaccination with amyloid-beta peptide has been reported to be effective in an animal model of AD, this putative vaccine is now in clinical trials. Likewise, recent studies suggest that some statins may have a prophylactic effect.