The reciprocal relationship between amyloid precursor protein and mitochondrial function.

Amyloid precursor protein (APP), secretase enzymes, and amyloid beta (Aß) have been extensively studied in the context of Alzheimer's disease (AD). Despite this, the function of these proteins and their metabolism is not understood. APP, secretase enzymes, and APP processing products (Aß and C-terminal fragments) localize to endosomes, mitochondria, endoplasmic reticulum (ER), and mitochondrial/ER contact sites. Studies implicate significant relationships between APP, secretase enzyme function, APP metabolism, and mitochondrial function. Mitochondrial dysfunction is a key pathological hallmark of AD and is intricately linked to proteostasis. Here, we review studies examining potential functions of APP, secretase enzymes, and APP metabolites in the context of mitochondrial function and bioenergetics. We discuss implications and limitations of studies and highlight knowledge gaps that remain in the field.

Mitochondrial function and Aß in Alzheimer's disease postmortem brain.

INTRODUCTION: Mitochondrial dysfunction is observed in Alzheimer's disease (AD). However, the relationship between functional mitochondrial deficits and AD pathologies is not well established in human subjects. METHODS: Post-mortem human brain tissue from 11 non-demented (ND) and 12 AD subjects was used to examine mitochondrial electron transport chain (ETC) function. Data were analyzed by neuropathology diagnosis and Apolipoprotein E (APOE) genotype. Relationships between AD pathology and mitochondrial function were determined. RESULTS: AD subjects had reductions in brain cytochrome oxidase (COX) function and complex II Vmax. APOE ε4 carriers had COX, complex II and III deficits. AD subjects had reduced expression of Complex I-III ETC proteins, no changes were observed in APOE ε4 carriers. No correlation between p-Tau Thr 181 and mitochondrial outcomes was observed, although brains from non-demented subjects demonstrated positive correlations between Aß concentration and COX Vmax. DISCUSSION: These data support a dysregulated relationship between brain mitochondrial function and Aß pathology in AD.

The Role of Bioenergetics in Neurodegeneration.

Bioenergetic and mitochondrial dysfunction are common hallmarks of neurodegenerative diseases. Decades of research describe how genetic and environmental factors initiate changes in mitochondria and bioenergetics across Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). Mitochondria control many cellular processes, including proteostasis, inflammation, and cell survival/death. These cellular processes and pathologies are common across neurodegenerative diseases. Evidence suggests that mitochondria and bioenergetic disruption may drive pathological changes, placing mitochondria as an upstream causative factor in neurodegenerative disease onset and progression. Here, we discuss evidence of mitochondrial and bioenergetic dysfunction in neurodegenerative diseases and address how mitochondria can drive common pathological features of these diseases.

O-GlcNAc impacts mitophagy via the PINK1-dependent pathway.

Background: The accumulation of dysfunctional mitochondria is an early feature of Alzheimer's disease (AD). The impaired turnover of damaged mitochondria increases reactive oxygen species production and lowers ATP generation, leading to cellular toxicity and neurodegeneration. Interestingly, AD exhibits a disruption in the global post-translational modification ß-N-acetylglucosamine (O-GlcNAc). O-GlcNAc is a ubiquitous single sugar modification found in the nuclear, cytoplasmic, and mitochondrial proteins. Cells maintain a homeostatic level of O-GlcNAc by cycling the addition and removal of the sugar by O-GlcNAc transferase (OGT) or O-GlcNAcase (OGA), respectively. Methods: We used patient-derived induced pluripotent stem cells, a transgenic mouse model of AD, SH-SY5Y neuroblastoma cell lines to examine the effect of sustained O-GlcNAcase inhibition by Thiamet-G (TMG) or OGT deficiency on mitophagy using biochemical analyses. Results: Here, we established an essential role for O-GlcNAc in regulating mitophagy (mitochondria-selective autophagy). Stimulating mitophagy using urolithin A (UA) decreases cellular O-GlcNAc and elevates mitochondrial O-GlcNAc. Sustained elevation in O-GlcNAcylation via pharmacologically inhibiting OGA using Thiamet-G (TMG) increases the mitochondrial level of mitophagy protein PTEN-induced kinase 1 (PINK1) and autophagy-related protein light chain 3 (LC3). Moreover, we detected O-GlcNAc on PINK1 and TMG increases its O-GlcNAcylation level. Conversely, decreasing cellular O-GlcNAcylation by knocking down OGT decreases both PINK1 protein expression and LC3 protein expression. Mitochondria isolated from CAMKII-OGT-KO mice also had decreased PINK1 and LC3. Moreover, human brain organoids treated with TMG showed significant elevation in LC3 compared to control. However, TMG-treated AD organoids showed no changes in LC3 expression. Conclusion: Collectively, these data demonstrate that O-GlcNAc plays a crucial role in the activation and progression of mitophagy, and this activation is disrupted in AD.

Amyloid precursor protein and mitochondria.

Amyloid Precursor Protein (APP) processing to amyloid beta (Aß) is a major hallmark of Alzheimer's disease (AD). The amyloid cascade hypothesis postulates that Aß accumulation and aggregation causes AD, however many therapeutics targeting Aß have failed recently. Decades of research describe metabolic deficits in AD. Mitochondrial dysfunction is observed in AD subjects within the brain and systemically. APP and γ-secretase are localized to mitochondria. APP can be processed within mitochondria and its localization to mitochondria affects function. Here we discuss the evidence showing APP and γ-secretase localize to mitochondria. We also discuss the implications for the function of APP and its cleavage products in regulating mitochondrial function.

Cell type and sex specific mitochondrial phenotypes in iPSC derived models of Alzheimer's disease.

Introduction: Mitochondrial dysfunction is observed in Alzheimer's disease (AD). Altered mitochondrial respiration, cytochrome oxidase (COX) Vmax, and mitophagy are observed in human subjects and animal models of AD. Models derived from induced pluripotent stem cells (iPSCs) may not recapitulate these phenotypes after reprogramming from differentiated adult cells. Methods: We examined mitochondrial function across iPSC derived models including cerebral organoids, forebrain neurons, and astrocytes. iPSCs were reprogrammed from fibroblasts either from the University of Kansas Alzheimer's Disease Research Center (KU ADRC) cohort or purchased from WiCell. A total of four non-demented and four sporadic AD iPSC lines were examined. Models were subjected to mitochondrial respiration analysis using Seahorse XF technology, spectrophotometric cytochrome oxidase (COX) Vmax assays, fluorescent assays to determine mitochondrial mass, mitochondrial membrane potential, calcium, mitochondrial dynamics, and mitophagy levels. AD pathological hallmarks were also measured. Results: iPSC derived neurons and cerebral organoids showed reduced COX Vmax in AD subjects with more profound defects in the female cohort. These results were not observed in astrocytes. iPSC derived neurons and astrocytes from AD subjects had reduced mitochondrial respiration parameters with increased glycolytic flux. iPSC derived neurons and astrocytes from AD subjects showed sex dependent effects on mitochondrial membrane potential, mitochondrial superoxide production, and mitochondrial calcium. iPSC derived neurons from AD subjects had reduced mitochondrial localization in lysosomes with sex dependent effects on mitochondrial mass, while iPSC derived astrocytes from female AD subjects had increased mitochondrial localization to lysosomes. Both iPSC derived neurons and astrocytes from AD subjects showed altered mitochondrial dynamics. iPSC derived neurons had increased secreted Aß, and sex dependent effects on total APP protein expression. iPSC derived astrocytes showed sex dependent changes in GFAP expression in AD derived cells. Conclusion: Overall, iPSC derived models from AD subjects show mitochondrial phenotypes and AD pathological hallmarks in a cell type and sex dependent manner. These results highlight the importance of sex as a biological variable in cell culture studies.

Apolipoprotein E and Alzheimer's disease.

Genetic variation in apolipoprotein E (APOE) influences Alzheimer's disease (AD) risk. APOE ε4 alleles are the strongest genetic risk factor for late onset sporadic AD. The AD risk is dose dependent, as those carrying one APOE ε4 allele have a 2-3-fold increased risk, while those carrying two ε4 alleles have a 10-15-fold increased risk. Individuals carrying APOE ε2 alleles have lower AD risk and those carrying APOE ε3 alleles have neutral risk. APOE is a lipoprotein which functions in lipid transport, metabolism, and inflammatory modulation. Isoform specific effects of APOE within the brain include alterations to Aß, tau, neuroinflammation, and metabolism. Here we review the association of APOE with AD, the APOE isoform specific effects within brain and periphery, and potential therapeutics.

Mitochondrial Membrane Potential Influences Amyloid-ß Protein Precursor Localization and Amyloid-ß Secretion.

BACKGROUND: Amyloid-ß (Aß), which derives from the amyloid-ß protein precursor (AßPP), forms plaques and serves as a fluid biomarker in Alzheimer's disease (AD). How Aß forms from AßPP is known, but questions relating to AßPP and Aß biology remain unanswered. AD patients show mitochondrial dysfunction, and an Aß/AßPP mitochondria relationship exists. OBJECTIVE: We considered how mitochondrial biology may impact AßPP and Aß biology. METHODS: SH-SY5Y cells were transfected with AßPP constructs. After treatment with FCCP (uncoupler), Oligomycin (ATP synthase inhibitor), or starvation Aß levels were measured. ß-secretase (BACE1) expression was measured. Mitochondrial localized full-length AßPP was also measured. All parameters listed were measured in ρ0 cells on an SH-SY5Y background. iPSC derived neurons were also used to verify key results. RESULTS: We showed that mitochondrial depolarization routes AßPP to, while hyperpolarization routes AßPP away from, the organelle. Mitochondrial AßPP and cell Aß secretion inversely correlate, as cells with more mitochondrial AßPP secrete less Aß, and cells with less mitochondrial AßPP secrete more Aß. An inverse relationship between secreted/extracellular Aß and intracellular Aß was observed. CONCLUSION: Our findings indicate mitochondrial function alters AßPP localization and suggest enhanced mitochondrial activity promotes Aß secretion while depressed mitochondrial activity minimizes Aß secretion. Our data complement other studies that indicate a mitochondrial, AßPP, and Aß nexus, and could help explain why cerebrospinal fluid Aß is lower in those with AD. Our data further suggest Aß secretion could serve as a biomarker of cell or tissue mitochondrial function.