Alzheimer's Mechanisms PDF

17 ALZHEIMER’S DISEASE 1  17. ALZHEIMERS’ I — CONTENTS — World Alzheimer’s Disease Neuroimaging Initiative (ADNI) Familial and Sporadic Alzheimer’s disease (AD) 1 Ab: Altered proteolytic processing of APP Tau: NeuroFibrillar Tang...

17 ALZHEIMER’S DISEASE 1  17. ALZHEIMERS’ I — CONTENTS — World Alzheimer’s Disease Neuroimaging Initiative (ADNI) Familial and Sporadic Alzheimer’s disease (AD) 1 Ab: Altered proteolytic processing of APP Tau: NeuroFibrillar Tangles (NFTs) 2 The Ab cascade hypothesis and UPR 3 Insulin in the brain: AD as diabetes type III 4 Impaired glucose metabolism in AD 5 Ab and oxidative stress 6 Perturbed Ca++ homeostasis and excitotoxicity 7 Inflammation in AD 8 Risk factors for AD: ApoE4, Maternal Transmission 2  17. ALZHEIMER’S DISEASE I Compare between familial- and sporadic Alzheimer’s disease Distinguish between amyloidogenic- and non-amyloidogenic pathways in APP processing and the secretases involved List the imaging probes to assess Ab load Explain the mechanism of NFT formation Discuss why Alzheimer’s is also viewed as diabetes type III Apply the knowledge on the role of insulin in the brain and the role of IDE (Insulin Degrading Enzyme) and the resulting outcomes Evaluate hypometabolism as assessed by [18F]-FDG-PET Discuss the mechanisms of excitotoxicity and their functional outcomes Explain the pathways that lead to inflammation in AD Recognize Apolipoprotein E4 as a risk factor for AD and by which mechanism 3  ALZHEIMER’S DISEASE ! Normal brain Alzheimer’s brain Alzheimer's disease patients lose their memory and their cognitive abilities, due to the progressive dysfunction and death of nerve cells responsible for storage and processing information. Although drugs can temporarily improve memory, at present there are no treatments that can stop or reverse the inexorable neurodegenerative process. Normal brain Alzheimer’s brain Brain regions involved in learning and ________________PET scans (glucose utilization) ________________ ! memory, including the temporal and frontal Healthy brain Severe Alzheimer’s lobes, are reduced in size in Alzheimer's disease patients as the result of degeneration of synapsis and death of neurons and major reductions in energy metabolism. 4  ALZHEIMER’S DISEASE World Alzheimer’s Disease Neuroimaging Initiative (ADNI) is a collaborative effort of scientists from around the world and is the umbrella organization for neuroimaging initiatives being carried out through continents and countries. ADNI is a global research effort that actively supports the investigation and development of treatments that slow or stop the progression of Alzheimer’s. World Wide ADNI (WW-ADNI) unites leading international investigators in a common effort to: Help predict and monitor the onset and progression of Alzheimer's disease Establish globally recognized standards to identify and diagnose Alzheimer's disease Document cognitive changes linked to physical changes Share data across the international research community http://adni.loni.usc.edu/ 5  ALZHEIMER’S DISEASE World-Wide Alzheimer’s Disease Neuroimaging Initiative North American ADNI Taiwan ADNI European ADNI Korea ADNI Japan ADNI China ADNI Australian ADNI Argentina ADNI 6  ALZHEIMER’S DISEASE Familial- and Sporadic Alzheimer’s disease Mutations in three genes are known to cause familial Alzheimer’s disease: the APP gene or Effect of FDA mutations/ApoE4 genotype in genes encoding presenilin 1 (PSEN1) or Dementia presenilin 2 (PSEN2) that are essential compo- nents of the g-secretase complex for cleavage and release of amyloid-b (Ab). The major effect of familial Alzheimer’s disease mutations (and the apolipoprotein E4 genotype) is to decrease the age of onset of the disease, without an effect on disease duration. Normal Familial Alzheimer’s disease accounts for a 20 C 40 Age (years) 60 D 80 Age of onset Death Age of onset Death small proportion of Alzheimer’s cases (1%). The majority of the cases are sporadic- or late- onset Alzheimer’s disease (LOAD). 7  ALZHEIMER’S DISEASE Definitive diagnosis of Alzheimer's disease requires postmortem examination of the brain, which must contain sufficient numbers of plaques and tangles to qualify as affected by Alzheimer's disease: Plaques are extracellular deposits of fibrils and aggregates of amyloid b- peptide (Ab). Brain regions with plaques typically exhibit reduced number of synapses. Tangles are intracellular fibrillar aggregates (NFTs) of the microtubule- associated protein tau, which exhibits hyperphosphorylation and oxidative modifications. 8  ALZHEIMER’S DISEASE Aspects of the pathophysiology of Alzheimer's disease that are important in order to understand different therapeutic / preventive approaches are: molecular and cell death synaptic cellular abnormalities (apoptosis?) dysfunction in the brain dysfunction  Altered proteolytic processing of the amyloid precursor protein (APP) resulting in the production and aggregation of Ab  The Ab cascade hypothesis and UPR  Insulin in the brain: Alzheimer’s as diabetes type III  Impaired glucose metabolism in Alzheimer's disease  Ab and oxidative stress  Perturbed calcium homeostasis and excitotoxicity  Inflammation in Alzheimer's disease  Risk Factors: ApoE4, Maternal Transmission 9   Ab PLAQUES ALTERED PROTEOLYTIC PROCESSING OF APP 10  ALZHEIMER’S DISEASE Definitive diagnosis of Alzheimer's disease requires postmortem examination of the brain, which must contain sufficient numbers of plaques and tangles to qualify as affected by Alzheimer's disease: Plaques are extracellular deposits of fibrils and aggregates of amyloid b- peptide (Ab). Brain regions with plaques typically exhibit reduced number of synapses. Tangles are intracellular fibrillar aggregates (NFTs) of the microtubule- associated protein tau, which exhibits hyperphosphorylation and oxidative modifications. 11   Ab PLAQUES: ALTERED PROTEOLYTIC PROCESSING OF APP APP is an integral membrane protein with a single membrane spanning domain, a large extracellular glycosylated N terminus, and a shorter cytosolic C terminus. A fundamental APP sAPPβ Extracellular Glycosylated abnormality that plays a pivotal role in the N-Terminus dysfunction and death of neurons in Alzheimer's disease is altered proteolytic processing of APP, Aβ which results in an increased production Aβ and mbrane Membrane panning Spanning accumulation in the brain of neurotoxic forms of domain domain β-secretase Ab. Cytosolic C-terminus 12   Altered proteolytic processing of APP The causes of APP mismetabolism and Ab deposition are not understood but include age-related increases in oxidative stress, impaired energy metabolism, and perturbed Ca++ homeostasis. Three different enzyme activities determine whether and which form of Ab is produced: APP sAPPβ Extracellular Aβ Glycosylated N-Terminus ✂ Aβ Aβ ✂ Membrane spanning domain β-secretase γ-secretase BACE-1 Cytosolic C-terminus The production of Ab requires sequential cleavages of the amyloid precursor protein by b- secretase at the N-terminus of Ab and by g- secretase at the C terminus of Ab. If a-secretase ✂ α-secretase cleaves APP within the Ab sequence, production of Ab is precluded. 13   Altered proteolytic processing of APP The product of b-secretase, containing the Ab and the shorter cytosolic C terminus can also be incorporated into the cell by endocytosis; a cytosolic g- secretase cleaves this complex into Ab and the cytosolic C terminus. APP sAPP! Extracellular A! Glycosylated N-Terminus ✂ A! A! Membrane spanning domain !-secretase "-secretase BACE-1 Cytosolic C-terminus endosome ✂ γ-secretase Aβ 14   Altered proteolytic processing of APP Depending on the activityNon-Amyloidogenic Amyloidogenic of secretases on APP, amyloidogenic and non-amyloidogenic Amyloid Peptide Precursor pathways can be distinguished: (APP) cleavage of APP by b-secretase and g-secretase leads to the Amyloidogenic Non-Amyloidogenic Amyloid Peptide Precursor formation of Ab peptides (amyloidogenic (APP) β-secretase α-secretase pathway), whereas cleavage of APP by a- secretase yields APPsa (non-amyloidogenic APPsβ β-secretase α-secretase APPsα pathway). γ-secretase APPsβ Under normal circumstances, Ab is cleared APPsα from the brain by an Ab transporter, LRP1 γ-secretase Aβ LRP1 Aβ Brain clearance (LDL receptor-related protein 1). Clearance clearance dysfunction is disturbed in Alzheimer’s disease, resulting accumulation/ aggregation LRP1 Brain Aβ Aβ clearance in the accumulation and aggregation of Ab clearance dysfunction accumulation/ leading to neuronal cell death. aggregation NEUROTOXICITY 15 Guo et al (2007) Proc Natl Acad Sci USA 104, 13444  IMAGING Ab Imaging Ab using [18F]flutematemol positron emission tomography (PET) Since the advent of the first Ab-specific PET ligand, 11C-Pittsburgh compound B, several 18F ligands have been developed that circumvent the limitations of [11C]PIB tied to its short half-life. Such compounds have been approved by the US and European regulatory bodies, including florbetapir, florbetaben, and flutemetamol. HO S 11CH 3 N H N 11C-PIB 18F HO S CH3 N H N 18F-Flutemetamol 16 Imaging Ab using [18F]flutematemol positron emission tomography (PET) [18F]flutemetamol helps diagnose positive AD subjects, negative amnestic, mild cognitive impair- ment (MCI) subjects, and positive amnestic mild cognitive impair- ment (MCI) subjects. Heurling et al (2015) Eur J Nucl Med Mol Imaging 17   TAU NEUROFIBRILLARY TANGLES (NFTS) 18  ALZHEIMER’S DISEASE Definitive diagnosis of Alzheimer's disease requires postmortem examination of the brain, which must contain sufficient numbers of plaques and tangles to qualify as affected by Alzheimer's disease: Plaques are extracellular deposits of fibrils and aggregates of amyloid b- peptide (Ab). Brain regions with plaques typically exhibit reduced number of synapses. Tangles are intracellular fibrillar aggregates (NFTs) of the microtubule- associated protein tau, which exhibits hyperphosphorylation and oxidative modifications. 19  TAU The human Tau (tubulin associated unit) is implicated in a wide range of neurodegenerative ‘tauopathy’ diseases, such as Alzheimer’s disease. There are six isoforms of the Tau protein generated by alternative mRNA splicing mechanisms. Tau pathology is manifested as the deposition of insoluble aggregated NFTs that results from hyperphosphorylation of normal tau, thus leading to a decreased tau affinity for microtubules. 20  Healthy neuron Damaged neuron Monomeric Tau Protein Tau Tangle Tau pathological condition Phosphorylation Other PMTs Upregulated kinases: Acetylation GSK3b, Cdk5, PKA Nitration Downregulated Glycation PP2A ubiquitination Tau monomer Paired helical Neurofibrillary Tau oligomer (hyperphosphorylated) filaments Tangles (NFTs) Boyarko, B. and Hook, V. (2021) Human Tau isoforms and proteolysis. Front. Neurosci. 15:702788 21  IMAGING TAU Both 18F-MK-6240 and 18F-flortaucipir are capable of quantifying signal in a common set of brain regions that develop tau pathology in Alzheimer disease, although 18F-MK-6240 showed a greater dynamic range, which may be an N H H advantage in detecting early tau N N 18F N pathology or performing longi- 18F N N N tudinal studies to detect small [18F]-Flortaucipir [18F]-MK-6240 interval changes. [18F]-MK-6240 [18F]-FTP No Cognitive Atypical AD2 AD3 AD4 Tau Normal AD patient Pathology Some Tau Increasing Tau pathology pathology with medial Temporal Tau Pathology 22  IMAGING TAU Both 18F-MK-6240 and 18F-flortaucipir can quantify signals in a common set of brain regions that develop tau pathology in Alzheimer's disease. N H H N 18F N N N N 18F N [18F]-Flortaucipir [18F]-MK-6240 CLINICAL VALUE OF TAU IMAGING IN THE DIAGNOSTIC OF PATIENTS WITH COGNITIVE SYMPTOMS Tau PET showed significant changes in diagnoses and patient treatment when Tau PET was added to an already extensive diagnostic that included cerebrospinal fluid AD biomarkers. The clinical use of Tau PET needs to be limited to populations with biomarkers indicating Ab positivity. Smith, R. et al (2023) JAMA Neurology 23  PHOSPHORYLATED TAU 217 p-tau217 correlates with brain atrophy and cognitive impairment in AD patients Antibody against p-tau217 attenuates pathology and neuronal loss in tauopathic mice p-tau217 antibody restores brain function and proteostasis in tauopathic mice An antibody targeting total tau leads to motor deficits in tauopathic mice  DIAGNOSTIC ACCURACY OF A PLASMA PHOSPHORYLATED TAU 217 IMMUNOASSAY FOR ALZHEIMER DISEASE PATHOLOGY IMPORTANCE Phosphorylated tau (p-tau) is a specific blood biomarker for Alzheimer disease (AD) pathology, with p-tau217 considered to have the most utility. However, availability of p- tau217 tests for research and clinical use has been limited. Expanding access to this highly accurate AD biomarker is crucial for wider evaluation and implementation of AD blood tests. OBJECTIVE To determine the utility of a novel and commercially available immune assay for plasma p-tau217 to detect AD pathology and evaluate reference ranges for abnormal amyloid b (Ab) and longitudinal change across 3 selected cohorts. CONCLUSION AND RELEVANCE. This study found that a commercially available plasma p- tau217 immunoassay accurately identified biological AD, comparable with results CSF biomarkers, with reproducible cut-offs across cohorts. It detected longitudinal changes, including at the preclinical stage. JAMA Neurol. Doi:10.1001/jamareurol.2-23.5319 Published online January 22, 2024 Diagnostics Accelerator (DxA) portfolio company ALZpath has recently announced a licensing agreement with Roche for use of the ALZpath pTau217 antibody to develop and commercialize an Alzheimer’s disease diagnostic blood test.  Tau Positron Emission Tomography for Predicting Dementia in Individuals with Mild Cognitive Impairment Colin Groot, PhD1,2; Ruben Smith, MD, PhD1,3; Lyduine E. Collij, PhD1,4,5; et al Author Affiliations Article Information JAMA Neurol. 2024;81(8):845-856. doi:10.1001/jamaneurol.2024.1612 Key Points Question How well do visual reads and quantitative assessments of tau positron emission tomography (PET) predict clinical progression from mild cognitive impairment (MCI) to dementia compared to amyloid-β (Aβ) PET and magnetic resonance imaging (MRI)? Findings In this cohort study, positivity on quantitative tau PET, but not Aβ PET or MRI, provided a better prediction of conversion from MCI to all-cause dementia when added to a base model including age, sex, education, and Mini-Mental State Examination score, while prediction of Alzheimer disease (AD) dementia was improved with quantitative tau PET as well as tau PET visual reads. The optimal set of neuroimaging biomarkers to predict all-cause and AD dementia included tau PET and MRI measures. Meaning These findings suggest that quantitative tau PET and tau PET visual reads show the greatest promise as a stand-alone prognostic marker for clinical progression to dementia among individuals with MCI, outperforming Aβ PET and MRI.  TAUTOPATHIES 1. Better Biomarkers 2. Diverse Patient Populations 1. Better Biomarkers 2. Diverse Patient Populations Earlier detection ethnicity Earlier detection & race ethnicity culture & race culture Diversity socioeconomic Diversity background socioeconomic gender & sexual AD background orientation gender & sexual AD orientation Improved precision PSP Improved precision PSP age geographic age background geographic CBS background CBS 3. Innovative Trial Design 3. Innovative Trial Design AD PSP CBS AD AD PSP CBS AD Anti-Tau Therapeutic Placebo Drug A Drug B Drug C Basket Anti-Tau Trial Therapeutic Placebo Drug A Umbrella Trial Drug B Drug C AD: Alzheimer’s disease Basket Trial Umbrella Trial PSP: Progressive supranuclear palsy CBS: Corticobasal syndrome Lane-Donovan, C. & Boxer, A.L. (2024) Neurotherapeutics 21 (2024) e00321 27   THE Ab HYPOTHESIS AND UNFOLDED PROTEIN RESPONSE (UPR) 28   THE Ab CASCADE HYPOTHESIS The Ab cascade hypothesis posits that the deposition of the Ab peptide in the brain is a crucial step that ultimately leads to Alzheimer’s. Autosomal dominant mutations that cause familial Alzheimer’s occur in 3 genes: presenilin 1 (PSEN1; g–secretase), PSEN2 and amyloid precursor protein (APP). There is no linear correlation between dementia APP and Ab accumulation. The Ab PSEN1/PSEN2 APP mutations mutations cascade hypothesis suggests that Aβ42 aggregation synaptotoxicity and neurotoxicity may be mediated by soluble soluble forms of ? deposited amyloid-β forms of Ab species. Given these oligomeric Aβ peptide uncertainties, the term aggregate aggregate stress stress seems more appropriate. Aggregate stress can be account- ed for by the accumulation and neuronal dysfunction and death aggregation of misfolded proteins because of a pathological Unfold- dementia ed Protein Response (UPR). Karran et al (2011) Nature Reviews Drug Discovery 10, 698 29   THE UNFOLDED PROTEIN RESPONSE (UPR) The Unfolded Protein Response (UPR) is a stress response of the endoplasmic reticulum (ER) to a disturbance in protein folding. Neurodegenerative disorders like Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), and Amyotrophic Lateral Sclerosis (ALS) are characterized by the accumulation and aggregation of misfolded proteins. Neurons have an extensive system for protein quality control, which serves to detect and remove aberrant proteins (misfolded proteins) and to prevent detrimental aggregation. The Endoplasmic Reticulum is a major site of protein synthesis. A key component of protein quality control in the ER is the Unfolded Protein Response (UPR), activated when protein homeostasis (proteostasis) ensues. 30   THE UNFOLDED PROTEIN RESPONSE (UPR) The Unfolded Protein Response (UPR) UPR Activation consists of several independent signaling pathways that are activated upon accumu- PERK P lation of misfolded proteins in the ER. One P of these pathways entails: The ER sensor protein kinase R (PKR)- eIF2α P like ER Kinase (PERK) Phosphorylation of Initation Factor 2a (eIF2a), essential for protein synthesis ATF4 Synthesis of UPR responsive genes is UPR Response Decreased mediated by the Activating Transcription Genes Protein synthesis Factor 4 (ATF4) Homeostatic control of UPR activation upon inhibition of PERK HOMEOSTASIS 31   THE UNFOLDED PROTEIN RESPONSE (UPR) The PERK pathway is activated in several neurodegenerative diseases (e.g., AD). The adaptive PERK pathway functions to restore ER proteostasis. In pathology, the prolonged activation of PERK leads to loss of regulatory feedback and turns into a UPR maladaptive response with accumulation of Ab and pTau. Adaptive Response Pathology Moderate activity Aberrant activation of UPR Re-establishment of homeostasis Loss of regulatory functions UPR Activation UPR Activation PERK P PERK P P P eIF2α P eIF2α P ATF4 X Loss of Synaptic Proteins Increased Aβ, pTau UPR Response Decreased Genes Protein synthesis neurodegeneration X HOMEOSTASIS HOMEOSTASIS 32   THE Ab CASCADE HYPOTHESIS AND UPR Ab is transported from the extracellular space into the cytosol through a b-amyloid channel and within the cell by ABAD (b-Amyloid Binding Alcohol Dehydrogenase). Ab crosses mitochondrial membranes by a mechanism involving the translocases of Outer mitochondrial Cell membrane membrane the outer (TOM40) and inner (TIM22) Extra Extracellular Cytosol Matrix Cellular membranes. Ab binds to ABAD in Aβ channel ABAD mitochondria. Binding of Ab to ABAD Aβ abolishes its cytoprotective activity and Aβ Aβ Aβ TOM40 TIM22 ABAD-dependent transport Aβ ABAD mitochondria become susceptible to the Aβ ANT reactive aldehydes and oxidants, thus lead- ing to mitochondria dysfunction. Inner mitochondrial Ab also binds to ANT (Adenine Nucleotide membrane Translocase), thereby activating the membrane permeability transition pore in Tillement, L., et al. Mitochondrion (2010) in press mitochondria leading to activation of the intrinsic pathway of apoptosis. Complexes IV and V are also targets of Ab. 33   INSULIN IN THE BRAIN ALZHEIMER’S AS DIABETES TYPE III? 34   INSULIN IN THE BRAIN: ALZHEIMER’S AS DIABETES TYPE III Impairments in the insulin signaling pathway in the periphery and in the brain have been implicated in Alzheimer’s disease, in which the age-related reduction insulin IGF1 IR in cerebral insulin levels seems to be accompanied by disturbances of the insulin receptor. Several players in insulin signaling are affected in IRS1 Alzheimer’s disease brains, thus leading to the proposal of the term Type III Diabetes. Brain insulin and its receptor are implicated in PI3K neuronal survival and synaptic plasticity. Brain insulin derives from peripheral insulin, transferred by a transporter through the brain blood barrier Akt (BBB). glycolysis Bad GSK3β FoxO 35   INSULIN IN THE BRAIN: ALZHEIMER’S AS DIABETES TYPE III Neuronal survival and synaptic plasticity are intertwined by a common signaling cascade: the PI3K route that promotes neuronal survival by inactivating apoptosis and stimulating glucose metabolism. Following activation of the insulin receptor by insulin, the docking protein IRS-1 is phosphorylated at the tyrosine residues and recruits PI3K, which leads to the subsequent activation of Akt. Akt phosphorylates several substrates at serine and/or threonine residues:  Inactivates (upon phosphorylation) the Akt Akt pro-apoptotic protein Bad, thus inhibit- GSK3β Bad active ing cell death. Inactivates GSK3b; one active pro- GSK3β P P Tau apoptotic Bad of the functions of GSK3b is to hyper- inactive inactive phosphorylate the protein Tau, a hall- P Tau P P mark of Alzheimer’s disease. hyper- phosphorylated 36   INSULIN IN THE BRAIN: ALZHEIMER’S AS DIABETES TYPE III  Akt phosphorylates FoxO (Forkhead box O), a FoxO P transcription factor with several members. Phospho- CYTOSOL rylation of FoxO by Akt leads to FoxO shuttling from P FoxO nucleus to cytosol and inactivation of transcription of Akt FoxO-related genes. In neurons, FoxO factors are linked FasL Bim FoxO (pro-apoptosis) to the induction of pro-apoptotic proteins such as Fas NUCLEUS ligand and Bim. glucose GLUT  Akt increases coupling of glucose metabolism to glucose glucose-6P oxidative phosphorylation via promotion of the VDAC- HK interaction (HK: hexokinase). Active Akt increases ATP HK distribution of hexokinase to mitochondria. Akt phos- ADP phorylates phosphofructo-kinase-2 (PFK) that generates VDAC ATP the main regulator of glycolysis, fructose-2,6-bisphos- phate. 37 Insulin Signaling (IIS) The PI3K-Akt pathway and the Grb2-Ras pathway of insulin signaling (IIS) activate survival signals and promote cell growth and differentiation. Insulin IGF1 IR IRS Grb2 Ras GDP PI3K PIP2 Ras GTP PTEN PIP3 Akt Erk cell growth survival survival cell growth differentiation neuroplasticity signals differentiation 38   INSULIN IN THE BRAIN: ALZHEIMER ’S AS DIABETES TYPE III Internalization Recycling Insulin-Degrading Enzyme (IDE) in the Pathogenesis of Alzheimer's disease – A candidate endosome that links insulin resistance and Alzheimer's disease is the insulin-degrading enzyme (IDE), IDE which is highly expressed in the brain, testis, muscle, and liver. IDE —degraded is predominantly insulin cytosolic. Substrates for IDE are insulin, IGF-1, IGF-II, and Ab. NORMAL DEFECTIVE IDE —Insulin —Insulin IR— IR— Internalization Recycling Internalization endosome endosome IDE IDE X —degraded insulin Levels of DEFECTIVE IDE protein IDE and transcripts are reduced in the hippocampi from Alzheimer's disease patients with an APO4-e4. Ab degradation is carried out extracellularly by IDE and —Insulin genetic disruption IR— of the IDE gene in mice causes increased levels of cerebral Ab and glucose intolerance with hyperinsulinemia. 39   INSULIN IN THE BRAIN: ALZHEIMER’S AS DIABETES TYPE III Insulin-Degrading Enzyme (IDE) in the Pathogenesis of Alzheimer's disease Because of the Ab-degrading ability of IDE, defects in IDE activity in the brain can be a trigger of Ab deposition to develop Alzheimer's disease. If IRs are unable to translocate to the plasma membrane, a reduction in the number of IR ensues that causes insulin resistance and IDE downregulation. This aggravates IDE IDE insufficiency and may be a insulin Aβ mechanism for the development of cognitive dysfunction and onset of IDE IDE Alzheimer's disease in diabetes degraded degraded patients. insulin Aβ IDE Because IDE has a high affinity for insulin, hyperinsulinemia results in binding of insulin to IDE (and catalysis) and IDE-dependent degradation of Ab is inhi- bited. 40  INSULIN IN THE BRAIN: ALZHEIMER’S AS DIABETES TYPE III Synaptic learning Plasticity memory synaptic AMPA-R plasticity NMDA-R PI3K Survival Ca2+ FoxO caspase-9 BBB INSULIN INSULIN PI3K Akt Bad Raf IDE GSK3β pancreas ERK1/2 Aβ Tau glucose Alzheimer’s Cell Cell Growth Growth Disease 41   IMPAIRED GLUCOSE METABOLISM IN ALZHEIMER’S DISEASE 42   IMPAIRED GLUCOSE METABOLISM IN ALZHEIMER’S DISEASE Preclinical diagnosis of Alzheimer's disease is one of the major challenges for the prevention of the disease. Alzheimer's disease biomarkers are needed not only to reveal preclinical pathological changes, but to monitor progression of therapeutics. Several neuroimaging modalities show promi- GLUT glucose sing results as early diagnostic tools and MRI OH plays an important role in the differential HO GLUT O diagnosis of AD from some other dementias P retained F in cell (vascular dementia). Functional imaging, 2- HO HO HO O glucose [18F]fluoro-2-deoxy-glucose (18FDG) PET, is a glucose-6P HO F HO radiolabeled modified glucose molecule that is ATP positron taken up by neurons but is not metabolized; it HK emitting provides qualitative and quantitative estimates ADP of the cerebral metabolic rate of glucose, an VDAC ATP index of synaptic functioning and density. 43   IMPAIRED GLUCOSE METABOLISM IN ALZHEIMER’S DISEASE FDG-PET studies in Alzheimer's disease show consistent and progressive cerebral decreased rate of glucose metabolism, whose Normal subject extent and topography correlate with symptom type and severity. In Alzheimer's patients glucose hypometa- bolism can be observed in different brain AD patient ! regions. The FDG-PET scan from an MCI (Mild Cognitive Impairment) subject shows moderate hypometabolism. MCI ! 44   IMPAIRED GLUCOSE METABOLISM IN ALZHEIMER’S DISEASE 1989 FDG-PET studies have shown that hypometabolism in the cortical areas 1992 could be a biomarker for asymptomatic 'high-risk' populations. The FDG-PET 1996 scans depict the progressive decline of 1998 glucose uptake in a patient that in 1998 ! was diagnosed with AD. 12 – clinically normal In 1989 and 1992 the patient is cli- 69 years clinically Glucose Metabolism normal nically normal but shows low glucose 8 – 72 years uptake. In 1996 the patient is diagnosed MCI DAT 76 years 77 years with MCI (Mild Cognitive Impairment) 4 – and in 1997 with DAT (Alzheimer’s Type Dementia); by 1998 glucose 0 – – – – – 1988 1990 1992 1994 1996 1998 uptake is very low. The patient died in Year 2004 with post-mortem diagnosis of definite Alzheimer's disease. 45   Ab AND OXIDATIVE STRESS 46   Ab AND OXIDATIVE STRESS Cells in the brains of Alzheimer's disease patients exhibit abnormally high amounts of oxidatively-modified proteins, lipids, and DNA; such oxidative damage is prominent in the environment of plaques and in neurofibrillary tangle-bearing neurons. A major source of oxidative stress in the brain in Alzheimer's disease is the transition metals Cu and Fe when bound to Ab. The metal ion coordinates with three histidine residues in Ab and participates in Fenton reactions. histidine residues histidine residues Aβ–Cu+ Aβ–Cu++ A!–Cu+ A!–Cu++ sites of coordination sites of with metals coordination Aβ with metals A! H2O2 HO Amyloid β H2O2 HO (Aβ) Amyloid ! sites of (A!) coordination Aβox with metals A!ox (cytosol) (cytosol) Associatedassociated with dementia with dementia(cytosol) associated with dementia 47   Ab AND OXIDATIVE STRESS Oxidative stress induced by Ab impairs the function of ion-motive ATPases, glucose and glutamate transporters, and GTP-binding proteins, thus rendering neurons vulnerable to excitotoxicity and apoptosis. The dysfunction and degeneration of synapses in Alzheimer's disease involves Ab-induced oxidative stress. Cu+ Fe++ Aβ H2O2 transporter receptor ion channel H2O2 Cu+ Fe++ Aβ 48   PERTURBED CALCIUM HOMEOSTASIS AND EXCITOTOXICITY 49   PERTURBED CA++ HOMEOSTASIS AND EXCITOTOXICITY Ca2+ plays fundamental roles in learning and memory. Dysregulation of Ca2+ homeostasis is a feature of AD pathogenesis and is involved in neuron dysfunction and death. Ab perturbs Ca2+ regulation by inducing oxidative stress, which, in turn, impairs membrane RECEPTOR ION CHANNEL TRANSPORTER Ca2+ pumps and enhances Ca2+ influx GPCR through voltage-dependent channels and glutamate receptors. In neurons stimulat- ed with glutamate, the amount of Ca2+ Gq GT P Ca2+ released from the ER increases; likewise, ligand-dependent activation of receptors IP3 that lead to IP3 production, affects Ca2+ Ca2+ homeostasis. This is an abnormality in both familial and sporadic forms of Alzheimer’s disease. endoplasmic reticulum 50   PERTURBED CA++ HOMEOSTASIS AND EXCITOTOXICITY Excitotoxicity –defined as excessive exposure to the neurotransmitter glutamate or over-stimulation of the N-methyl-D-aspartate (NMDA)-type glutamate receptors– is one of the key factors contributing to neuronal injury and death in Alzheimer's disease. Overstimulation of the NMDA receptor leads excessive Ca2+ influx and production of free radicals. NR1 NR2 Na+ Ca2+ SNO Gly Glu Zn +++ out NR1 NR2 ––– in Mg++ Na+ Ca2+ ! 51   PERTURBED CA++ HOMEOSTASIS AND EXCITOTOXICITY Excitotoxicity NMDA receptors are made of different subunits: NR1 and NR2A-D. NR1 and NR2 are the binding sites for glycine (required co-agonist) and glutamate (the endogenous agonist), respectively. When glutamate and glycine bind and the cell is depolarized to remove the Mg2+ block, the NMDA receptor opens with influx of Ca2+ and Na+. The two regulatory sites of the NMDA receptor are the Mg2+ site in the NR1 NR2 Na+ Ca2+ SNO the ion channel and an S-nitrosylation Gly Glu site located in the extracellular region Zn +++ out of the receptor. S-nitrosylation of NR1 NR2 ––– in cysteine protein in the receptor decre- Mg++ ases channel activity associated with Na+ Ca2+ stimulation of the NMDA receptor. ! 52   PERTURBED CA++ HOMEOSTASIS AND EXCITOTOXICITY Excessive release of glutamate or overstimulation of the NMDA receptor causes excitotoxic cell death also known as apoptotic-like excitotoxicity. Ca2+ influx following activation of the NMDA receptor triggers a variety of processes than can lead to apoptosis: Ca2+ NMDA-R  activation of the p38 MAPK pathway that contributes to cell death..  activation of nNOS with formation of NO and nNOS subsequent toxic effects. ❷  activation of the mitochondrial permeability ❸ Ca2+ ❶ transition with formation of reactive oxygen MTP p38 species and release of proapoptotic factors, such as cytochrome c and subsequent activa- cyt c ROS MEF2 NO ↓ ↓ ↓ ↓ tion of the caspase cascade CELL DEATH 53   INFLAMMATION IN ALZHEIMER’S DISEASE 54   INFLAMMATION IN ALZHEIMER’S Alzheimer's disease is an example of chronic sterile inflammation, in which Chronic sterile inflammation Ab activates the NLRP3 inflammasome in microglia (as an example, Nlrp3-knockout mice are protected in a mouse model of Alzheimer's Amyloid-β disease). The inflammasome is located in microglia and macrophages. NLRP3 microglial macrophage! cell! 55   CENTRAL ROLE OF INFLAMMATION IN ALZHEIMER’S DISEASE — SPECIFIC CYTOKINE SIGNALING — Inflammation in Alzheimer’s disease has emerged as a central pathology that plays a role in onset and progression of the disease. Sustained inflammation in the brain accelerates other core pathologies, making inflammatory mechanisms viable targets for therapeutic development. PRO-INFLAMMATORY SIGNALING ANTI-INFLAMMATORY SIGNALING TNF-a IL-10 IL-1b TGF-b1 IL-6 NFkB 56   INFLAMMATION IN ALZHEIMER’S Alzheimer's is characterized by the accumulation of Ab the brain. Microglial cells Aβ TLR2 express Pattern Recognition Receptors TLR6 CD14 (PRRs) that sense and respond to DAMPs MyD88— —DD and PAMPs. Such PRRs include Toll-like IkB IKK receptor 2 (TLR2) and TLR4. Ab ligation to TLR2 and TLR4 leads to pro-inflammatory NFκB P signaling via the MyD88 – NFkB pathway ubiquitination proteasome and the NLRP3 inflammasome, which proteasome GSK3! Thr295–P NFκB Thr295–P degraded generates mature cytokines from pro-forms PGC-1" PGC-1" PGC-1" IκB ATP ADP nuclear through activation of caspase 1. membrane nucleus NUCLEUS co-activator NLRP3 protein NFκB IL-1β DNA IL-18 57   INFLAMMATORY CYTOKINES: INFLAMMASOME ACTIVATION Most of the signaling pathways for NLRP3 inflammasome activators involve reactive oxygen species (ROS). Amyloid-b through lysosome rupture activates the inflamma- some. Activator Proposed signaling pathway _____________________________________________________________________________ PAMPs Candida albicans ROS Saccharomyces cerevisiae ROS Staphylococcus aureus ROS Bacterial pore-forming toxins ROS DAMPs Extracellular ATP ROS + channel formation Glucose ROS Amyloid-β Lysosome rupture Environmental irritants Skin irritants ROS Silica ROS + lysosome rupture Asbestos ROS Alum + lysosome rupture 58 All NLRP3 agonists that have been tested trigger the production of reactive oxygen species   INFLAMMATION IN ALZHEIMER’S In Alzheimer’s, CD36 mediates the internalization of soluble Ab and its intracellular conversion to fibrillary Ab. This leads to disruption of the phagolysosome and activation of the NLRP3 inflammasome due to cathepsin B release. soluble Aβ fibrillary CD36— Aβ pro-IL-1β IL-1β phagolysosome cathepsin B oligomerization recruitment pro-caspase 1 ACS inflammasome 59   INFLAMMATION IN ALZHEIMER’S MICROGLIA TREM2 (Triggering Receptor Expressed on Myeloid cells 2) TREM2 is a receptor belonging to the Ig family expressed in microglia, the immune cells in the central nervous system. TREM2 stimulates phagocytosis and inhibits cytokine production and inflammation. TREM2 directly binds to β-amyloid (Aβ) and protects from AD by enabling microglia to surround and alter Aβ plaque structure, thereby limiting neuritic damage. TREM2 protects against Tau hyperphosphorylation upon Aβ activation of the PI3K/Akt/GSK3b pathway. ApoE Tau Ligands of TREM2 LDL Ab Tau DAP12 ApoE P P Syk LDL Phagocytosis Proliferation Inflammation /Survival 60   INFLAMMATION IN ALZHEIMER’S MICROGLIA – TREM2 SIGNALING Binding of a ligand to TREM2 recruits DAP12 and results in its autophosphorylation. Syk (Spleen Tyrosine Kinase) is the docking protein leading to activation of the PI3K, Grb2, and PLCg signaling pathways. Aβ ApoE Tau LDL PI3K Akt GSK3β DAP12 P P Syk Grb2/SOS Ras-ERK docking protein PLCγ DAG IP3 Ca2+ adaptor proteins 61   INFLAMMATION IN ALZHEIMER’S TREM2 VARIANTS INCREASE RISK OF LATE ONSET ALZHEIMER’S DISEASE (LOAD) Activated Activated microglia microglia Filamentous Filamentous amyloid amyloid Oligomeric Oligomeric AbAb NFT plaque plaque NFT Compact Compact amyloid amyloid plaque plaque Dystrophic Dystrophic neurite neurite Homeostatic Homeostatic microglia microglia Functional TREM2 allows microglia acti- TREM2 variants result in TREM2 partial loss vation (by NFTs and Ab), promotes micro- of function, thus abolishing microglia glia clustering around plaques, plaque clustering around plaques and phagocytic compaction through oligomeric Ab. activity. Consequences are that filamentous plaques associate with increasing dystrophic neurites. Gatuze et al. (2018) Mol. Neurodegener. 13(1):66 62  THE P75 NEUROTROPHIN RECEPTOR The p75 Neurotrophin Receptor (p75NTR) NT p75NPR signaling pathways substantially overlap with degenerative networks active in Alzheimer’s disease (AD). p75NTR is expressed in astrocytes, microglia, and oligodendrocytes. Signaling through the p75NTR receptor proceeds with JNK NFκB activation of JNK (a death MAPK) and NFkB leading to cell degeneration and neuroinflamma- tion, respectively. Cell Degeneration NeuroInflammation 63  THE P75 NEUROTROPHIN RECEPTOR Modulation of p75NTR with the first-in-class LM11A-31 small molecule LM11A-31 mitigates amyloid- induced and pathological tau-induced synaptic X loss in preclinical models. In a 26-week rando- p75NPR mized, placebo-controlled, double-blinded phase 2a safety and exploratory endpoint trial of LM11A-31 in 242 participants with mild to X X moderate AD. Many of the biomarkers that exhibited a significant AD-slowing drug effect JNK NFκB (Inflammatory responses, sMRI and [18F]- X X FDG PET) correlated highly with cognitive function at baseline in the trial cohort. Cell Degeneration NeuroInflammation Shanks et al (2024) Nature Medicine https://doi.org/10.1038/s41591-024-02977-w 64   RISK FACTORS 65   RISK FACTORS FOR ALZHEIMER’S: APOLIPOPROTEIN E4 LDL receptors and LDL clearance There are three isoforms of ApoE: ApoE-2, ApoE-3, and ApoE-4; these isoforms have slight amino acid variations but pronounced functional differences: __________________________________________________________________________________________________ Isoform Amino Acid Binding to Associated disorders Variation LDL receptor 112 158 __________________________________________________________________________________________________ ApoE2 Cys Cys Low Type III hyperlipoproteinemia ApoE3 Cys Arg High Unknown ‘Normal’ ApoE4 Arg Arg High Alzheimer’s disease Atherosclerosis _________________________________________________________________________________________________ 66   RISK FACTORS FOR ALZHEIMER’S: APOLIPOPROTEIN E4 Genetic, pathological, and functional studies show that an imbalance between production and clearance of Ab peptides in the brain results in accumulation of Ab. Impairment of Ab clearance is the major contributor to Alzheimer’s pathogenesis and, genetically, the e4 allele of the apolipoprotein E (APOE) gene is the strongest risk factor for Alzheimer’s disease. polymorphic worldwide alleles frequency ε2 8.4% Human APOE gene ε3 77.9% Aβ ε4 13.7% clearance Aβ production frequency dramatically increased (40%) in AD patients 67   RISK FACTORS FOR ALZHEIMER’S: APOLIPOPROTEIN E4 ApoE in CNS is mainly produced by astrocytes and lipidated by the ABCA1 trans- porter; ApoE transports cholesterol to neurons via Apo-E receptors, members of the low-density lipo- protein receptor (LDL-R) family. ApoE also binds Aβ Aβ clearance to Ab, thus facilitating transport and recognition by production receptors in neurons. The single amino acid difference between the ApoE isoforms affects their structure and influences their ability to bind lipids, receptors, and Ab. LDL-R LRP1 ApoE ApoE Aβ ASTROCYTE NEURON 68   RISK FACTORS FOR ALZHEIMER’S: APOLIPOPROTEIN E4 Major Ab clearance pathways in the brain include:  receptor-mediated uptake by neurons (LDL-R, LDL-related receptor protein (LRP1),  drainage into interstitial fluid or through BBB  proteolytic degradation by IDE and neprilysin (a metalloprotease in microglia) Impaired Ab clearance triggers formation of Ab oligomers and plaques. Perivascular Ab accumulation disrupts blood vessel function. ❸ MICROGLIA ❸ IDE LDL-R ❶ LRP1 ApoE ApoE NEURON Aβ ASTROCYTE ASTROCYTE NEURON Aβ oligomer ENDOTHELIAL CELLS ❷ BLOOD VESSEL Aβ 69   RISK FACTORS FOR ALZHEIMER’S: APOLIPOPROTEIN E4 ApoE affects Ab clearance, aggregation, and deposition in an isoform-dependent manner e4 allele of the ApoE gene is the main genetic risk factor for Alzheimer’s ApoE e4 carriers have enhanced Alzheimer’s pathology, accelerated age-dependent cognitive decline, and worse memory performance ApoE4 contributes to Alzheimer’s pathogenesis by Ab-independent mechanisms that involve synaptic plasticity, cholesterol homeostasis, neurovascular functions, and neuro- inflammation The effect of APOE e4 on Alzheimer’s frequency and age at onset __________________________________________________________________________________ Characteristic _______________ APOEe4_______________ noncarrier heterozygous homozygous AD frequency (%) 20 47 91 Mean age clinical onset (years) 84 76 68 70   RISK FACTORS FOR ALZHEIMER’S: APOLIPOPROTEIN E4 ApoE4 and Lipid Metabolism ApoE4 impairs the ability to synthesize and secrete cholesterol in astrocytes, thus contributing to an age-dependent cerebral cholesterol deficit. delivers less cholesterol to neurons, thus taxing the acetyl-CoA pool towards cholesterol synthesis and decreasing the expression of memory genes. decreases ABCA1 levels, resulting in impaired ApoE lipidation and lipid trafficking. decreases lipid droplet trafficking from neurons to astrocytes, thus resulting in neuronal lipotoxicity and energy deficits in astrocytes. 71   RISK FACTORS FOR ALZHEIMER’S: APOLIPOPROTEIN E4 18F-FDG PET imaging (brain glucose uptake): cognitively normal APOE e4 carriers have lower glucose metabolism than do non-carriers. APOE e4 carriers show increase cerebral Ab deposition. 80 APOE e4+ Normal memory (APOE e4 non-carrier) PIB+ (%) Normal memory 40 (APOE e4 carrier) Dementia APOE e4– 0 MRI Baseline 45-59 60-69 70-79 80-89 PET ___________ Age (years) ___________ 72   RISK FACTORS FOR ALZHEIMER’S: MATERNAL TRANSMISSION β-Amyloid in Cognitively impaired Individuals— Blame Mom? Dena B. Dubal, MD, PhD1,2; Holly C. Elser, MD, PhD3,4 Author Affiliations Article Information JAMA Neurol. 2024;81(8):795-797. doi:10.1001/jamaneurol.2024.1748 Family history is one of the biggest risk factors—behind advanced age—for developing late-onset Alzheimer disease (AD). This is especially true if the family history involves a first-degree relative, such as a parent, increasing AD risk by at least 2- to 4-fold.1 Several questions arise. Do parents transmit AD risk? If so, who is to blame? Why does it matter? A history of parents with AD increases risk in offspring for reasons that may be biological, sociocultural, or both. In late-onset AD, mounting studies support a preferential risk of developing AD with a maternal, but not paternal, history of the disease. Maternal transmission of AD may be rooted in biological origins related to passing on the maternal X chromosome, mitochondria, and specific genomic imprinting (or silencing of genes) to offspring. More maternal history of AD in a family could also result from gender disparities and secular trends. For example, in studies to date, the generations of women with AD had less access to and systematically experienced less formal education compared with men, potentially decreasing brain reserve. Thus, it may seem justified, although not fair, to blame your mom for part of your AD risk. 73 Biology of Maternal Transmission Mothers pass on an X chromosome, mitochondria, and specific imprinting (or silencing) of the genome to offspring. Any or all of these factors could be factors in the inheritance of Alzheimer’s disease risk.