Path: `entities/overview`
Introduction
The Entities section serves as a comprehensive reference for key biological entities, drug targets, biomarkers, and therapeutic concepts in neurodegenerative disease research. This overview provides context for navigating the 171+ entity pages in NeuroWiki, covering the molecular players, cellular components, and therapeutic interventions central to understanding Alzheimer's disease (AD), Parkinson's disease (PD), and related disorders.
Neurodegenerative diseases share common pathological mechanisms including protein aggregation, mitochondrial dysfunction, neuroinflammation, and impaired protein homeostasis. The entities documented in this section represent the key molecular actors in these processes, spanning from disease-causing genes and aggregated proteins to therapeutic targets and biomarkers[@mitochondrial][@tau_pathology].
Scope of Neurodegenerative Diseases Covered
The Entities section encompasses the following major disease categories:
Alzheimer's Disease (AD) — The most common cause of dementia, characterized by amyloid plaques and neurofibrillary tangles
Parkinson's Disease (PD) — Second most common neurodegenerative disorder, marked by dopaminergic neuron loss and Lewy bodies
Lewy Body Dementia (DLB) — Features both cortical Lewy bodies and variable AD-like pathology
Frontotemporal Dementia (FTD) — Heterogeneous group with behavioral and language variants
Amyotrophic Lateral Sclerosis (ALS) — Motor neuron disease with TDP-43 pathology
Multiple System Atrophy (MSA) — Oligodendrocyte pathology with autonomic failure
Progressive Supranuclear Palsy (PSP) — Tauopathy with vertical gaze palsy
Corticobasal Syndrome (CBS) — Asymmetric cortical and basal ganglia dysfunction
Huntington's Disease (HD) — CAG repeat disorder with mutant huntingtin aggregationWhy Entity Documentation Matters
Comprehensive documentation of biological entities serves multiple purposes:
- Understanding Disease Mechanisms: Each entity represents a piece of the mechanistic puzzle
- Identifying Therapeutic Targets: Drug development requires detailed understanding of molecular players
- Biomarker Discovery: Many entities serve as diagnostic or prognostic markers
- Genetic Risk Assessment: Gene variant interpretation guides clinical decision-making
- Research Translation: Basic science findings inform clinical applications
Entity Categories
Proteins & Peptides
The protein entities in NeuroWiki encompass the hallmark pathological proteins, their precursors, processing enzymes, and associated regulatory molecules.
Amyloid-Related Proteins:
- [Amyloid Precursor Protein (APP)](/entities/app-protein) — The transmembrane precursor protein that gives rise to amyloid beta through sequential proteolytic cleavage
- [Amyloid Beta (Aβ)](/proteins/amyloid-beta) — The 38-43 amino acid peptide that forms extracellular plaques in AD
- [BACE1](/entities/bace1) — Beta-secretase, the rate-limiting enzyme that initiates Aβ production
Tau and Kinases:
- [Tau Protein](/proteins/tau) — The microtubule-stabilizing protein that forms neurofibrillary tangles in AD
- [GSK3B](/genes/gsk3b) — Glyogen synthase kinase 3 beta, a major tau kinase
- [CDK5](/genes/cdk5) — Cyclin-dependent kinase 5, implicated in tau phosphorylation
Synucleinopathies:
- [Alpha-Synuclein](/proteins/alpha-synuclein) — The presynaptic protein that forms Lewy bodies in PD and DLB
Neurotrophic Factors:
- [BDNF](/entities/bdnf) — Brain-derived neurotrophic factor, critical for neuronal survival and plasticity
- [GDNF](/entities/gdnf) — Glial cell line-derived neurotrophic factor, supports dopaminergic neurons
Transporters and Channels:
- [VMAT2](/genes/vmat2) — Vesicular monoamine transporter 2, packages dopamine into synaptic vesicles
- [Dopamine Transporter](/entities/dopamine-transporter) — Regulates synaptic dopamine clearance
Genes & Genetic Factors
Genetic factors play a critical role in both familial and sporadic forms of neurodegenerative diseases.
Disease-Causing Genes:
- [APOE](/proteins/apoe) — Apolipoprotein E, the strongest genetic risk factor for late-onset AD
- [C9orf72](/entities/c9orf72) — Hexanucleotide repeat expansion causes ALS/FTD
- [GBA](/entities/gba) — Glucocerebrosidase, major risk factor for PD
- [LRRK2](/entities/lrrk2) — Leucine-rich repeat kinase 2, common genetic cause of familial PD
- [SNCA](/genes/snca) — Alpha-synuclein gene, first gene linked to familial PD
- [PSEN1](/entities/psen1) — Presenilin 1, causal gene for early-onset familial AD
- [PSEN2](/entities/psen2) — Presenilin 2, another AD-causing presenilin gene
Modifier Genes:
- [BIN1](/genes/bin1) — Bridging integrator 1, second-most significant AD risk locus
- [CLU](/genes/clu) — Clusterin, complement regulator linked to AD risk
- [PICALM](/genes/picalm) — Phosphatidylinositol binding clathrin assembly protein, affects APP trafficking
Rare Variants:
- [TREM2](/proteins/trem2) — Triggering receptor expressed on myeloid cells 2, rare variants increase AD risk 3-4x
- [PLD3](/genes/pld3) — Phospholipase D3, rare variant linked to AD
Cell Types & Cellular Components
Understanding the cellular context of neurodegeneration requires detailed knowledge of the affected cell types and their specialized structures.
Neuronal Cell Types:
- [Dopaminergic Neurons](/entities/dopaminergic-neurons) — The vulnerable neurons in the substantia nigra pars compacta that degenerate in PD
- [Cholinergic Neurons](/cell-types/cholinergic-neurons) — Basal forebrain neurons lost early in AD
- [Cortical Neurons](/cell-types/cortical-neurons) — Pyramidal neurons affected in AD and FTD
Glial Cell Types:
- [Astrocytes](/entities/astrocytes) — Support neurons, maintain homeostasis, become reactive in neurodegeneration
- [Microglia](/entities/microglia) — Resident immune cells, drive neuroinflammation[@microglia2023]
- [Oligodendrocytes](/entities/oligodendrocytes) — Myelin-producing cells affected in MSA
Cellular Structures:
- [Blood-Brain Barrier](/entities/blood-brain-barrier) — Protective barrier whose dysfunction contributes to disease
- [Synapse](/entities/synapse) — The site of neuronal communication, early target in neurodegeneration
- [Mitochondria](/entities/mitochondria) — Cellular powerhouses whose dysfunction is central to many neurodegenerative processes[@mitochondrial]
Biomarkers
Biomarkers enable early diagnosis, disease staging, and monitoring of therapeutic response.
Fluid Biomarkers:
- [Neurofilament Light Chain (NfL)](/biomarkers/neurofilament-light-chain-nfl) — Axonal damage marker elevated in blood and CSF
- [Tau](/proteins/tau) — Total and phosphorylated tau in CSF reflect neuronal injury
- [Alpha-Synuclein](/proteins/alpha-synuclein) — Seeded aggregation assays detect pathological forms
Imaging Biomarkers:
- [Amyloid PET](/entities/amyloid-pet) — Detects cerebral amyloid angiopathy and plaques
- [FDG PET](/entities/fdg-pet) — Measures cerebral glucose metabolism
- [Tau PET](/entities/tau-pet) — Visualizes neurofibrillary tangles in vivo
Drugs & Therapies
The therapeutic landscape for neurodegenerative diseases includes approved treatments and investigational agents.
Approved Treatments:
- [Donepezil](/entities/donepezil) — Acetylcholinesterase inhibitor for AD
- [Memantine](/therapeutics/memantine) — NMDA receptor antagonist for moderate-to-severe AD
- [Levodopa](/therapeutics/levodopa) — Dopamine precursor for PD motor symptoms
Disease-Modifying Candidates:
- [Aducanumab](/entities/aduhelm) — Anti-amyloid antibody with FDA approval (Aduhelm)
- [Lecanemab](/entities/lecanemab) — Anti-amyloid antibody (Leqembi), full FDA approval
- [Donanemab](/entities/donanemab) — Anti-amyloid antibody (Kisunla), full FDA approval
Pipeline Agents:
- [BACE1 Inhibitors](/entities/bace1-inhibitors-alzheimers) — Attempted to block Aβ production
- [Tau Immunotherapy](/therapeutics/tau-immunotherapy) — Active and passive immunization against tau
Key Pathways Represented
1. Amyloid Cascade
The amyloid cascade hypothesis posits that Aβ accumulation initiates a cascade leading to tau pathology, synaptic loss, and cognitive decline. Key entities in this pathway include:
- APP undergoes sequential cleavage by [BACE1](/entities/bace1) and γ-secretase to produce Aβ
- Aβ42 (42 amino acid isoform) aggregates more readily than Aβ40
- Oligomeric Aβ species are thought to be the most neurotoxic form
- Therapeutic approaches include reducing production (BACE inhibitors), enhancing clearance (immunotherapy), and preventing aggregation (small molecules)
See: [Amyloid Cascade Hypothesis](/mechanisms/amyloid-cascade-hypothesis)
2. Tau Pathology
Tau protein forms neurofibrillary tangles (NFTs) that correlate with clinical severity better than amyloid burden[@tau_pathology]:
- Hyperphosphorylation of tau reduces its microtubule-binding capacity
- Tau seeds can propagate between neurons in a prion-like manner
- Tau pathology spreads in a predictable pattern (Braak stages)
- Multiple kinases (GSK3B, CDK5) and phosphatases (PP2A) regulate tau phosphorylation
See: [Tau Pathology Mechanisms](/mechanisms/tau-pathogenesis)
3. Alpha-Synucleinopathy
Alpha-synuclein aggregation characterizes PD, DLB, and MSA[@alpha_synuclein]:
- Lewy bodies are cytoplasmic inclusions rich in aggregated α-synuclein
- The protein adopts β-sheet conformations that seed further aggregation
- Pathological α-synuclein can spread between connected brain regions
- Mutations (A53T, A30P, E46K) cause familial forms of PD
See: [Alpha-Synuclein Aggregation](/mechanisms/alpha-synuclein-aggregation)
4. Neuroinflammation
Chronic neuroinflammation is a hallmark of all neurodegenerative diseases[@neuroinflammation]:
- [Microglia](/entities/microglia) become persistently activated, producing pro-inflammatory cytokines
- The [Complement System](/entities/complement-system) contributes to synaptic elimination
- TREM2 on microglia senses Aβ and triggers protective responses
- Anti-inflammatory therapies have not yet demonstrated clinical benefit
See: [Neuroinflammation Mechanisms](/mechanisms/neuroinflammation)
5. Mitochondrial Dysfunction
Mitochondrial impairment is central to neurodegeneration[@mitochondrial]:
- PINK1/Parkin pathway detects and eliminates damaged mitochondria through mitophagy
- Complex I deficiency is well-documented in PD substantia nigra
- Mitochondrial DNA mutations accumulate in neurons with age
- Energy failure leads to calcium dysregulation and excitotoxicity
See: [Mitochondrial Dysfunction](/mechanisms/mitochondrial-dysfunction-neurodegeneration)
6. Protein Homeostasis
The [UPS](/mechanisms/ubiquitin-proteasome-system) and autophagy-lysosome system maintain proteostasis[@protein_homeostasis]:
- Ubiquitin tags proteins for degradation by the proteasome
- Autophagy removes larger aggregates and damaged organelles
- chaperone proteins assist folding and prevent aggregation
- Impairment leads to accumulation of toxic protein aggregates
See: [Protein Quality Control](/mechanisms/protein-quality-control-network)
Molecular Interaction Networks
APP Processing Network
Mermaid diagram (expand to render)
Alpha-Synuclein Lifecycle
Mermaid diagram (expand to render)
Shared Disease Mechanisms
Neurodegenerative diseases, despite their clinical diversity, share fundamental biological processes that represent key therapeutic targets.
Proteostasis Failure
The ability to maintain proper protein folding and clearance is compromised in all neurodegenerative diseases:
Protein Misfolding: Genetic mutations, environmental stressors, and aging can cause proteins to adopt abnormal conformations. Misfolded proteins expose hydrophobic regions that promote aggregation into oligomers, fibrils, and eventually insoluble inclusions.
Impaired Clearance: The ubiquitin-proteasome system (UPS) and autophagy-lysosome pathway are both affected in neurodegeneration. Proteasome activity declines with age, and autophagy becomes less efficient. Lysosomal function is compromised by lipid accumulation and cellular stress.
Seeded Aggregation: Pathological proteins can act as "seeds" that template the misfolding of normal proteins, creating a prion-like propagation. This explains the spread of pathology through connected brain regions[@alpha_synuclein].
Cellular Energy Crisis
Neurons have exceptionally high energy demands but limited capacity for glycolysis, making them vulnerable to mitochondrial dysfunction:
Oxidative Phosphorylation Defects: Complex I deficiency is well-documented in PD, and similar impairments occur in AD. Decreased ATP production compromises ion gradient maintenance, neurotransmitter recycling, and cellular homeostasis.
Calcium Dysregulation: Mitochondrial dysfunction leads to impaired calcium buffering. Elevated cytosolic calcium activates destructive pathways including proteases, phosphatases, and apoptotic cascades.
Reactive Oxygen Species: Electron leakage from damaged mitochondria generates superoxide and other ROS. Neurons are particularly vulnerable because of high lipid content and limited antioxidant capacity.
See: [Mitochondrial Dysfunction](/mechanisms/mitochondrial-dysfunction-neurodegeneration)
Neuroinflammatory Cascade
Chronic neuroinflammation is both a cause and consequence of neurodegeneration[@neuroinflammation]:
Microglial Activation: Resting microglia become activated by pathological proteins (Aβ, α-synuclein, tau), damage-associated molecular patterns (DAMPs), and complement components. Once activated, they produce pro-inflammatory cytokines (IL-1β, TNF-α, IL-6) that create a toxic environment.
Complement Activation: The complement system tags synapses for elimination by microglia. Excessive complement activation contributes to synaptic loss, which correlates best with cognitive decline.
Peripheral Immune System: Blood-derived monocytes and T cells can enter the CNS in neurodegeneration. The blood-brain barrier becomes more permeable, allowing peripheral immune cells to contribute to pathology.
Synaptic Failure
Synapse loss is the strongest correlate of cognitive decline:
Early Vulnerability: Synapses are early targets in both AD and PD. Synaptic proteins are reduced in vulnerable brain regions before neuron loss is evident.
Excitotoxicity: Excessive glutamate receptor activation leads to calcium influx and downstream damage. NMDA receptor overactivation is particularly harmful.
Protein Misfolding at Synapses: Aβ oligomers bind directly to synapses, impairing function. α-Synuclein aggregates at presynaptic terminals, disrupting neurotransmitter release.
Axonal Transport Defects
Long neurons require efficient transport systems:
Microtubule Disruption: Tau pathology disrupts microtubule stability, impairing transport of organelles, proteins, and RNA between cell body and synapse.
Motor Protein Dysfunction: Kinesins and dyneins that walk along microtubules are affected by multiple disease mechanisms. Transport deficits precede neurodegeneration.
Energy-Dependent Process: Axonal transport requires ATP, so mitochondrial dysfunction directly impairs this critical function.
Therapeutic Target Categories
Disease-Modifying Targets
| Target Category | Key Entities | Therapeutic Approach |
|-----------------|--------------|---------------------|
| Amyloid Reduction | BACE1, APP, γ-secretase | Small molecule inhibitors |
| Amyloid Removal | Aβ peptides | Active/passive immunotherapy |
| Tau Modification | GSK3B, CDK5, PP2A | Kinase inhibitors, phosphatase activators |
| Tau Clearance | Tau protein | Immunotherapy |
| α-Synuclein | SNCA, GBA | Immunotherapy, gene silencing |
| Neuroinflammation | TREM2, CD33, complement | Antibody therapy, small molecules |
| Mitochondrial Function | PINK1, Parkin, TFAM | Gene therapy, small molecules |
| Protein Homeostasis | UPS, autophagy machinery | Enhancers of clearance |
Symptomatic Targets
| Symptom | Target | Therapeutic Agent |
|---------|--------|-------------------|
| Cognitive Decline | Acetylcholinesterase | Donepezil, Rivastigmine, Galantamine |
| Cognitive Decline | NMDA Receptor | Memantine |
| Motor Symptoms | Dopamine Receptor | Pramipexole, Ropinirole |
| Motor Symptoms | MAO-B | Selegiline, Rasagiline |
| Motor Fluctuations | COMT | Entacapone, Tolcapone |
| Neuropsychiatric | 5-HT2A | Pimavanserin |
Emerging Concepts
1. Protein Co-Pathology
Recent research reveals that neurodegenerative diseases commonly involve multiple protein aggregates:
- Many AD brains contain α-synuclein (Lewy bodies) in addition to amyloid and tau
- Tau pathology is frequently found in PD brains (tauopathy co-occurrence)
- The presence of multiple pathologies correlates with more severe clinical presentation
2. Genetic Risk Architecture
Genome-wide association studies (GWAS) have identified numerous risk loci:
- Over 40 AD risk loci identified (APOE, CLU, PICALM, BIN1, SORL1, etc.)
- Over 20 PD risk loci identified (GBA, LRRK2, SNCA, PARK16, etc.)
- Many risk genes converge on common biological pathways (endocytosis, immune response)
3. Epigenetic Mechanisms
Environmental factors influence disease through epigenetic modifications:
- DNA methylation patterns change with age and disease
- Histone modifications affect expression of neurodegeneration-related genes
- Non-coding RNAs (miRNAs, lncRNAs) regulate gene expression post-transcriptionally
4. Cellular Senescence
Cellular senescence contributes to neurodegeneration:
- Senescent neurons and glia accumulate with age
- Senescence-associated secretory phenotype (SASP) drives inflammation
- Senolytics are being explored as potential therapeutics
Navigation and Resources
- All Entities (A-Z): [Entities Index](/entities) — Complete alphabetical listing
- Genes: [Genes Section](/genes) — Focused gene pages with variant information
- Proteins: [Proteins Section](/proteins) — Detailed protein structure and function
- Biomarkers: [Biomarkers Section](/biomarkers) — Diagnostic and prognostic markers
- Mechanisms: [Mechanisms Section](/mechanisms) — Process pathways and diagrams
Statistics
| Category | Count | Reference |
|----------|-------|-----------|
| Total Entity Pages | 171+ | |
| Protein Entities | ~60 | |
| Gene Entities | ~40 | |
| Drug/Treatment Entities | ~35 | |
| Cell Type/Component Entities | ~20 | |
| Other Entities | ~16 | |
Cross-Linking and Connectivity
The entities in this section are interconnected through multiple pathways:
- Gene-Protein-Disease Triangles: Each disease-linked gene encodes a protein involved in disease mechanisms
- Pathway Membership: Many entities participate in multiple overlapping pathways
- Therapeutic Targeting: Drug entities modulate the function of disease-related proteins
- Biomarker Relationships: Many biomarkers directly measure disease-related proteins or their fragments
Entity Relationship Examples
APOE → Amyloid → Alzheimer's Disease
- [APOE](/proteins/apoe) (gene/protein) affects Aβ clearance and aggregation
- [Amyloid Beta](/proteins/amyloid-beta) is the pathological protein
- [Alzheimer's Disease](/diseases/alzheimers-disease) is the clinical phenotype
GBA → Alpha-Synuclein → Parkinson's Disease
- [GBA](/entities/gba) mutations increase PD risk 5-10x
- [Alpha-Synuclein](/proteins/alpha-synuclein) aggregation is enhanced
- [Parkinson's Disease](/diseases/parkinsons-disease) manifests with Lewy bodies
LRRK2 → Tau → Neurodegeneration
- [LRRK2](/entities/lrrk2) mutations cause familial PD
- Pathogenic LRRK2 affects tau phosphorylation
- Leads to dopaminergic neuron degeneration
Entity Documentation Standards
All entity pages in NeuroWiki follow consistent documentation standards:
Required Elements
Title and Description: Clear naming convention with descriptive summary
Category Tags: Kind (gene/protein/drug/etc.), section, state
Cross-Links: Links to related entities within NeuroWiki
External References: Hyperlinked DOIs and PMIDs for primary literature
Last Updated: Timestamp for version trackingQuality Benchmarks
- Minimum Length: 500 words for standalone entity pages
- References: At least 5 PubMed references for key entities
- Diagrams: Mermaid.js flowcharts for pathway entities
- Cross-Links: Minimum 10 internal links to other NeuroWiki pages
Future Directions
The Entities section continues to evolve with emerging research:
Expanding Coverage
- Epigenetic Regulators: Adding pages for DNA methyltransferases, histone modifiers
- Non-Coding RNAs: miRNAs and lncRNAs implicated in neurodegeneration
- Metabolomic Entities: Key metabolites and metabolic pathways
- Single-Cell Markers: Cell type-specific markers for finer categorization
Integration with Other Sections
- Mechanisms: Deeper cross-linking to pathway documentation
- Clinical: Connection to clinical trials and therapeutic approaches
- Research: Linking to researcher profiles and institutional resources
See Also
- [Alzheimer's Disease](/diseases/alzheimers-disease)
- [Parkinson's Disease](/diseases/parkinsons-disease)
- [Mechanisms Overview](/mechanisms)
- [Genes Section](/genes)
- [Proteins Section](/proteins)
- [Biomarkers Section](/biomarkers)
External Links
- [PubMed](https://pubmed.ncbi.nlm.nih.gov/)
- [KEGG Pathways](https://www.genome.jp/kegg/pathway.html)
- [Allen Human Brain Atlas](https://brain-map.org/)
- [GWAS Catalog](https://www.ebi.ac.uk/gwas/)
Summary Statistics
- Total Pages: 171+ entity pages in this section
- Last Updated: This section is actively maintained and expanded
- Coverage: Comprehensive documentation of molecular players in neurodegeneration
- Quality: All entity pages include proper references with hyperlinked DOIs and PMIDs
References
[Alzheimer's Disease: The Role of Aβ and Tau in Neurodegeneration](https://doi.org/10.1002/alz.12318)
[Alpha-Synuclein in Parkinson's Disease: From Pathogenesis to Therapeutics](https://doi.org/10.1038/s41583-023-00700-z)
[Microglia in Alzheimer's Disease: Emerging Concepts (2023)](https://doi.org/10.1016/j.neuron.2023.05.012)
[Mitochondrial Dysfunction in Neurodegenerative Diseases](https://doi.org/10.1038/s41583-022-00592-5)
[Tau Pathology in Alzheimer's Disease — From Mechanisms to Therapy (2024)](https://doi.org/10.1038/s41583-024-00800-9)
[Alpha-Synuclein Propagation in Parkinson's Disease (2024)](https://doi.org/10.1038/s41583-024-00777-4)
[Neuroinflammation in Neurodegeneration (2024)](https://doi.org/10.1038/s41583-024-00357-7)
[Protein Homeostasis in Neurodegenerative Diseases (2024)](https://doi.org/10.1038/s41583-024-00289-2)