> A comprehensive comparison of cytoskeletal alterations across AD, PD, ALS, FTD, and HD, with emphasis on microtubule dysfunction, motor protein impairment, and therapeutic strategies
The cytoskeleton provides structural support, intracellular transport, and cellular dynamics in neurons. Cytoskeletal dysfunction disrupts axonal transport, leads to dendritic spine abnormalities, and contributes to neuronal death. The neuronal cytoskeleton consists of three major filament systems: microtubules, actin filaments (microfilaments), and intermediate filaments. Each system plays distinct yet interconnected roles in maintaining neuronal architecture, facilitating intracellular transport, and enabling dynamic remodeling essential for synaptic plasticity and signal transmission. Dysregulation of any component triggers a cascade of pathological changes that ultimately result in neurodegeneration [@gong2005][@takao2004].
> A comprehensive comparison of cytoskeletal alterations across AD, PD, ALS, FTD, and HD, with emphasis on microtubule dysfunction, motor protein impairment, and therapeutic strategies
The cytoskeleton provides structural support, intracellular transport, and cellular dynamics in neurons. Cytoskeletal dysfunction disrupts axonal transport, leads to dendritic spine abnormalities, and contributes to neuronal death. The neuronal cytoskeleton consists of three major filament systems: microtubules, actin filaments (microfilaments), and intermediate filaments. Each system plays distinct yet interconnected roles in maintaining neuronal architecture, facilitating intracellular transport, and enabling dynamic remodeling essential for synaptic plasticity and signal transmission. Dysregulation of any component triggers a cascade of pathological changes that ultimately result in neurodegeneration [@gong2005][@takao2004].
| Feature | AD | PD | ALS | FTD | HD |
|---------|-----|-----|------|------|-----|
| Microtubule Integrity | Tau hyperphosphorylation, MT depolymerization | alpha-Syn binds MT, disrupts transport | MT destabilization, dynactin mutations | Tau pathology, MT dysfunction | Mutant HTT disrupts MT motors |
| Motor Protein Function | Kinesin/dynactin impairment | Kinesin dysfunction | DYNC1H1, DCTN1 mutations | Tau affects transport | HTT cargo function disrupted |
| Axonal Transport Deficit | Early transport failure | Progressive transport failure | Early and severe transport failure | Transport deficits | Transport impairment |
| Actin Dynamics | Spine morphology changes | Actin binding by alpha-syn | Cytoskeletal remodeling | Actin abnormalities | Actin cytoskeleton altered |
| Intermediate Filaments | NF phosphorylation changes | NF aggregation | NF inclusions, peripherin | Cytoskeletal changes | NF abnormalities |
| Tau Pathology | Primary driver | Tau co-pathology | Tau in some cases | Primary in 3R/4R tau | Tau alterations |
Microtubules are hollow cylindrical polymers composed of alpha- and beta-tubulin heterodimers that assemble into 13 protofilaments forming a 25 nm diameter tube. In neurons, microtubules serve as tracks for long-range intracellular transport, with polarity determining direction: plus ends (axon) and minus ends (cell body). The microtubule network is particularly dense in axons where it spans the entire length from soma to synaptic terminals [@baas1991][@dixon1981].
Neurons express multiple tubulin isotypes (betaI, betaII, betaIII, betaIV) with distinct spatial distributions. Post-translational modifications including acetylation, detyrosination, polyglutamylation, and phosphorylation regulate microtubule stability and motor protein binding. Axonal microtubules are typically more stable (acetylated) while dendritic microtubules display mixed stability patterns.
Actin filaments form a submembranous network beneath the plasma membrane and populate dendritic spines, where they undergo rapid polymerization and depolymerization underlying synaptic plasticity. The spine actin cytoskeleton consists of branched networks (Arp2/3-mediated) and linear filaments (formin-mediated), with myosin motors generating tension and facilitating cargo transport in the spine cytoplasm. Actin dynamics are regulated by numerous binding proteins including cofilin, profilin, and capping proteins.
Neurofilaments (NF-L, NF-M, NF-H) constitute the major intermediate filament system in large projection neurons, with phosphorylation of the tail domains modulating assembly state and transport. Peripherin forms intermediate filaments in peripheral neurons, while vimentin and alpha-internexin are expressed during development. The intermediate filament network provides structural stability and contributes to axonal caliber determination.
Tau is a microtubule-associated protein (MAP) primarily expressed in neurons. In the CNS, six tau isoforms are produced by alternative splicing of the MAPT gene, containing 3 or 4 repeat domains (3R or 4R). Over 40 phosphorylation sites have been identified on tau, with pathological hyperphosphorylation driven by kinases (GSK-3beta, CDK5, MARK, CK1, CaMKII) and reduced phosphatase activity (PP2A accounts for ~70% of tau dephosphorylation) [@kahlson2021][@chun2003].
Key mechanisms:
Alpha-synuclein directly binds to microtubules through its N-terminal region, with pathological mutations (A53T, A30P) enhancing this interaction. The binding affinity increases with concentration and is modulated by Ser129 phosphorylation.
Key mechanisms:
Lewy Body Components: Contain neurofilament proteins (NF-L, NF-M, NF-H) in periphery, tubulin and MAPs co-aggregated with alpha-syn, and altered actin-binding proteins (cofilin, profilin).
ALS shows severe cytoskeletal dysfunction with multiple gene involvement.
TDP-43 and Cytoskeleton: TDP-43 regulates splicing of cytoskeletal genes. Over 30 cytoskeletal-related genes are TDP-43 targets including tropomyosin (actin-binding), ankyrins (membrane-cytoskeleton linkers), and spectrins (membrane stabilization). Loss of nuclear TDP-43 disrupts alternative splicing of these targets.
ALS-Specific Cytoskeletal Genes:
FTD encompasses multiple clinical syndromes with distinct cytoskeletal pathologies:
Wild-type huntingtin is a large scaffolding protein (~350 kDa) essential for vesicular transport.
Cargo Adaptor Function: HTT binds to kinesin and dynein via adaptor proteins, serving as a platform for transport complexes. Specific cargoes include BDNF vesicles, synaptic vesicles, mitochondria, endosomes, and autophagosomes.
Mutant HTT Effects on Transport:
| Strategy | Target | Disease | Status |
|----------|--------|---------|--------|
| Tau Aggregation Inhibitors | Tau oligomers | AD, FTD | Clinical trials |
| Alpha-Syn Aggregation Inhibitors | Alpha-syn oligomers | PD | Preclinical |
| HTT Lowering | Mutant HTT | HD | Clinical trials |
| Microtubule Stabilizers | MT integrity | AD, PD | Preclinical |
| Kinesin Activators | Motor function | Multiple | Research |
| Dynein Modulators | Retrograde transport | ALS | Research |
| Biomarker | Disease | Source | Status |
|-----------|---------|--------|--------|
| NfL in CSF | All | Axonal damage | Clinical |
| p-tau181 | AD, FTD | Tau pathology | Clinical |
| Alpha-syn oligomers | PD | Alpha-syn pathology | Research |
| TDP-43 fragments | ALS, FTD | TDP-43 pathology | Research |