Neprilysin-sensitive amyloidogenic Aβ versus IDE-sensitive soluble Aβ: a probable mechanistic cause for sporadic Alzheimer’s disease

Neprilysin (NEP) and insulin-degrading enzyme (IDE) are considered the two major catabolic enzymes that degrade amyloid β peptide (Aβ), the primary cause of Alzheimer’s disease (AD). However, their roles in Aβ metabolism in vivo have never been compared in an impartial and side-by-side manner. Here, we crossbred single App knock-in mice with NEP (Mme) KO mice and with IDE (Ide) KO mice to generate double mutants that were analyzed for their biochemical and Aβ pathology properties. We found that NEP is responsible for the metabolism of amyloidogenic insoluble Aβ whereas IDE affects soluble Aβ. A deficiency of NEP, but not of IDE, augmented the formation of Aβ plaques, dystrophic neurites, and astrocytic and microglial activation, all of which are key pathological events in the development of AD. In addition, a deficiency of NEP had no significant impact on the levels of various neuropeptides (somatostatin, substance P, cholecystokinin, and neuropeptide Y), well known to be in vitro substrates for NEP, presumably because NEP is expressed in secretory vesicles and on the presynaptic membranes of excitatory neurons while most if not all neuropeptides are secreted from inhibitory neurons. This argues against the concern that NEP up-regulation for treatment of preclinical AD would reduce the levels of these neuropeptides. These findings indicate that NEP relatively selectively degrades Aβ in the brain. Whereas familial AD (FAD) is unambiguously caused by an increased anabolism of Aβ, and of Aβ 42 and Aβ 43 in particular, the anabolism of Aβ appears unaffected before its deposition in the brain that subsequently leads to the onset of sporadic AD (SAD). These observations thus suggest that NEP-sensitive amyloidogenic Aβ likely plays a primary pathogenic role in the etiology of SAD. Our findings are consistent with the aging-dependent decline of NEP expression in human brain and with recent genome-wide association studies (GWAS) indicating that variants of the gene encoding NEP (MME) are associated with the risk of SAD development. Taken together, our results imply that the aging-associated decrease in NEP expression is a primary cause of SAD and could thus be a target for the treatment of preclinical AD once other factors such as apolipoprotein E genotypes have also been considered.

Vascular mild cognitive impairment and its relationship to hemoglobin A1c levels and apolipoprotein E genotypes in the Dominican Republic

ABSTRACT. Dementia and vascular mild cognitive impairment (VaMCI) currently impose a tremendous human and economic burden on patients from aging populations and their families worldwide. Understanding the interplay of cardiometabolic risk factors and apolipoprotein E (APOE) may direct us to a more personalized medicine and preventative care in MCI and dementia. Objective: To evaluate the relationship of cardiometabolic risk factors with MCI and assess the APOE genotype’s role in an elderly cohort in the Dominican Republic. Methods: We studied a cohort of 180 participants 65 years of age and older using a combined assessment of cardiometabolic risk factors, neuropsychological battery tests, and APOE genotyping. We used the number of failed tests as a proxy to predict MCI. Results: We found that patients with the ε3-ε4 APOE genotype had 2.91 higher number of failed cognitive tests (p=0.027) compared to patients with the ε3-ε3 genotyped. The rate of test failures increased 10% (p=0.025) per unit increase in HbA1c percentage. Conclusions: Increased Hemoglobin A1c levels and ε3-ε4 APOE genotypes seem to have an association with the development of VaMCI.

Relationships of Alzheimer’s disease and apolipoprotein E genotypes with small RNA and protein cargo of brain tissue extracellular vesicles

ABSTRACTAlzheimer’s disease (AD) is a public health crisis that grows as populations age. Hallmarks of this neurodegenerative disease include aggregation of beta-amyloid peptides and hyperphosphorylated tau proteins in the brain. Variants of the APOE gene are the greatest known risk factors for sporadic AD. As emerging players in AD pathophysiology, extracellular vesicles (EVs) contain proteins, lipids, and RNAs and are involved in disposal of cellular toxins and intercellular communication. AD-related changes in the molecular composition of EVs may contribute to pathophysiology and lend insights into disease mechanisms. We recently adapted a method for separation of brain-derived EVs (bdEVs) from post-mortem tissues. Using this method, we isolated bdEVs from AD patients with different APOE genotypes (n=23) and controls (n=7). bdEVs were counted, sized, and subjected to parallel small RNA sequencing and proteomic analysis. Numerous bdEV-associated RNAs and proteins correlated with AD pathology and APOE genotype. Some of the identified entities have been implicated previously in important AD-related pathways, including amyloid processing, neurodegeneration, and metabolic functions. These findings provide further evidence that bdEVs and their molecular cargo modulate development and progression of AD.

SUN-593 Variants in Known Monogenic Causal Genes of Hypertriglyceridemia Are Not Major Contributors for Hypertriglyceridemia in Lipodystrophy Due to a LMNA Mutation

Background: Lipodystrophy is a heterogeneous disorder of adiposity, and one common lipid manifestation is hypertriglyceridemia (HTG). The LMNA gene, which encodes for nuclear envelope proteins, is a known causal gene for heritable lipodystrophy. At present, underlying mechanisms for each clinical manifestation of lipodystrophy due to a LMNA mutation are unknown. Hypothesis: A likely explanation for HTG in lipodystrophy is the paucity of adipose tissue where excess triglycerides (TGs) are normally stored, thus it may not be due to a specific defect in lipoprotein metabolism. Consequently, rare variants in HTG-associated genes would not be expected to be major contributors for HTG in lipodystrophy with LMNA mutations. Method: A proband and her father with a clinical diagnosis of lipodystrophy were recruited into an IRB-approved study investigating molecular etiologies of dyslipidemia at the University of Pennsylvania. Next-generation sequencing (NGS) with the LipidSeq panel, targeting causal genes for lipodystrophy, and monogenic HTG was performed, and confirmed by Sanger sequencing. Also, unweighted TG-polygenic scores by summing the number of TG-raising alleles from 14 single nucleotide polymorphisms (SNPs) associated with TG levels were assessed. Results: The proband and her father were diagnosed with lipodystrophy of two different subtypes, generalized in the daughter and partial in the father. The proband reported a gradual loss of subcutaneous fat starting around age 10. A highest reported TG in the proband was19,000 mg/dL with eruptive xanthomas, whereas TG in the father was never >500 mg/dL. Their BMI’s and DEXA body fat% were 12.9 kg/m2 and 7% in the proband, and 25.7 kg/m2 and 25% in the father, corresponding to their fat storage capacities. The molecular analyses revealed only a lipodystrophy causal mutation in LMNA, c.29C>T, T10I with no other significant findings in18 other lipodystrophy-related genes. No deletion or duplication was identified by a targeted array CGH of LMNA. As predicted, no rare monogenic variants in HTG-causal genes (LPL, GPIHBP1, APOA5, APOC2, LMF1, GPD1) were identified in either subject. However, TG-polygenic scores were 17/28 (95th %ile) in the proband, and 13/28 (50th %ile) in the father, the same trend as the level of HTG levels seen in them. Apolipoprotein E genotypes were non-contributory, (3/3) in the proband, and (3/4) in the father. Conclusion: Our findings support that the pathophysiology of HTG in lipodystrophy is likely to be due to lack of TG-storage space (adipose tissues), and is unlikely due to a defect in lipoprotein metabolism seen in patients with rare monogenic HTG-variants. Although the HTG-polygenic score was higher in the proband, and the accumulative effects of the at-risk alleles may be contributor to the HTG phenotype, it is unlikely to be the leading cause of severe HTG seen in the proband.