| ISG15 |
|---|
| Gene Symbol | ISG15 |
| Full Name | Interferon-Stimulated Gene 15 |
| Chromosome | 1p36.33 |
| NCBI Gene ID | [9636](https://www.ncbi.nlm.nih.gov/gene/9636) |
| OMIM | 607117 |
| Ensembl ID | ENSG00000187608 |
| UniProt ID | [P51959](https://www.uniprot.org/uniprot/P51959) |
| Associated Diseases | [Parkinson's Disease](/diseases/parkinson-disease), [Amyotrophic Lateral Sclerosis](/diseases/amyotrophic-lateral-sclerosis), [Alzheimer's Disease](/diseases/alzheimer-disease), [Frontotemporal Dementia](/diseases/frontotemporal-dementia), [Multiple Sclerosis](/diseases/multiple-sclerosis) |
ISG15 (Interferon-Stimulated Gene 15) is a ubiquitin-like protein that is induced by type I interferons (IFN-α/β) and plays a critical role in the innate immune response. First discovered as an interferon-stimulated gene in 1979, ISG15 has emerged as a crucial regulator of protein function through a process called ISGylation—a covalent modification analogous to ubiquitination but with distinct biological outcomes [1][2].
ISG15 functions through two main mechanisms:
- ISGylation: Covalent conjugation to target proteins (similar to ubiquitination), modifying their function, localization, or stability
- Free ISG15: Secreted as a cytokine-like molecule that can modulate immune cell function
In the nervous system, ISG15 is involved in neuroinflammation regulation, protein quality control, response to viral infections, and modulation of microglial activation [3]. Dysregulated ISG15 expression has been implicated in Parkinson's disease (PD), Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and multiple sclerosis (MS), making it an important focus for neurodegenerative disease research.
¶ Gene Structure and Protein Architecture
The ISG15 gene is located on chromosome 1p36.33 and consists of 2 exons encoding a protein of 165 amino acids. The protein shares structural homology with ubiquitin and contains the characteristic ubiquitin-fold domain.
¶ Protein Domains
- Ubiquitin-like fold (residues 1-80): Three-dimensional structure similar to ubiquitin
- C-terminal LRLRGG motif (residues 155-165): Glycine residues required for conjugation to target proteins
- N-terminal extension: Contains interferon-stimulated response element (ISRE) promoter elements
ISG15 conjugation requires a enzymatic cascade analogous to ubiquitination:
- E1 enzyme: UBA7 (ubiquitin-like modifier-activating enzyme 7)
- E2 enzyme: UBCH8 (UbcH8)
- E3 enzymes: HERC5 (HECT and RLD domain containing E3 ubiquitin protein ligase 5) and others
- Deconjugating enzyme: USP18 (ubiquitin-specific peptidase 18)
ISGylation is a reversible post-translational modification that regulates protein function:
- Activation: UBA7 activates ISG15 in an ATP-dependent manner
- Conjugation: Activated ISG15 is transferred to UBCH8
- Ligation: HERC5 catalyzes isopeptide bond formation between ISG15 C-terminal glycine and target protein lysine residue
- Deconjugation: USP18 removes ISG15 from targets
Over 1000 proteins have been identified as ISGylation targets, including:
- Protein quality control: P62, LC3, Hsp70 family members
- Transcription factors: STAT3, NF-κB subunits
- Signal transducers: PI3K, MAPK components
- Cytoskeletal proteins: Actin, tubulin
- Neurodegeneration-related proteins: Alpha-synuclein, TDP-43
Free (non-conjugated) ISG15 functions as a cytokine-like molecule:
- Immune modulation: Enhances NK cell cytotoxicity
- Autocrine/paracrine signaling: Can be secreted and bind to receptors
- Interferon response amplification: Positive feedback in type I interferon signaling
Under normal (non-stimulated) conditions, ISG15 expression is:
- Low in most tissues: Basal levels are minimal
- High in immune cells: Constitutive expression in some lymphocyte populations
- Inducible by interferons: Dramatic upregulation in response to type I IFN
In the nervous system, ISG15 is expressed in:
- Neurons: Low baseline, inducible by interferon exposure
- Astrocytes: Moderate expression, increases with inflammation
- Microglia: High expression, particularly when activated
- Oligodendrocytes: Variable expression
Expression is typically low under basal conditions but is rapidly induced by viral infections, type I interferons, and inflammatory stimuli.
- Type I interferons: IFN-α, IFN-β (strongest inducers)
- Type II interferon: IFN-γ (moderate induction)
- Viral infections: dsRNA, viral proteins
- Inflammatory cytokines: TNF-α, IL-1β
- Cellular stress: Oxidative stress, proteasome inhibition
ISG15 has been strongly implicated in PD pathogenesis [2][4][5]:
- Elevated ISG15 levels in PD substantia nigra
- Increased ISGylation in PD brain tissue
- Correlation with disease severity
- ISGylation affects alpha-synuclein aggregation [6]
- ISG15 colocalizes with Lewy bodies
- May influence prion-like spread of α-synuclein pathology
- Dysregulated protein quality control
- Impaired autophagy-lysosome pathway
- Altered interferon response
- Targeting ISGylation pathway
- Modulating interferon response
- Enhancing protein clearance
ISG15 is upregulated in AD brain [7][8]:
- Increased ISG15 expression in AD brain
- Regional distribution matching amyloid pathology
- Correlation with disease progression
- Part of type I interferon response in AD
- Microglial activation-associated
- May contribute to chronic neuroinflammation
¶ Amyloid and Tau Pathology
- Complex interactions with APP processing
- Potential effects on tau phosphorylation
- Role in protein aggregate clearance
- Cerebrospinal fluid ISG15 levels
- Peripheral blood mononuclear cell expression
- Potential disease progression marker
ISG15 is upregulated in ALS [9][10]:
- Increased ISG15 in ALS motor neurons
- Spinal cord ISGylation changes
- Correlation with disease progression
- ISGylation affects TDP-43 aggregation [11]
- TDP-43 is an ISGylation target
- Dysregulated ISGylation may contribute to cytoplasmic inclusions
- Chronic activation of type I interferon response
- Glial cell involvement
- Potential therapeutic target
- ISG15 dysregulation in FTD
- Overlap with ALS pathology
- TDP-43 involvement
- Elevated ISG15 in MS lesions [12]
- Demyelination-associated changes
- Role in immune cell regulation
ISG15 plays important roles in protein quality control [13][14]:
- ISGylation of autophagy proteins
- Modulation of autophagic flux
- Effects on protein aggregate clearance
- ISGylation can target proteins for degradation
- Cross-talk with ubiquitination
- Proteostasis maintenance
ISG15 is a key mediator of neuroinflammation [3][15]:
- ISG15 expression in activated microglia
- Regulation of pro-inflammatory cytokine production
- Modulation of neurotoxicity
- Chronic interferon response in neurodegeneration
- "Inflammaging" concept
- Therapeutic implications
ISG15 is involved in cellular stress responses [16]:
- Oxidative stress: ISGylation increases under oxidative conditions
- Proteotoxic stress: Response to protein aggregation
- DNA damage: ISG15 induction by genotoxic stress
- ER stress: Unfolded protein response involvement
ISG15 is part of the type I interferon response:
- IFN-α/β binding: Type I interferon receptor activation
- JAK-STAT signaling: STAT1/STAT2 phosphorylation
- IRF transcription factors: IRF3, IRF7 activation
- ISG15 transcription: IRF-dependent gene expression
- ISGylation: Post-translational protein modification
ISG15 and ubiquitin pathways intersect:
- Shared targets: Some proteins can be modified by both
- Competition: ISGylation can block ubiquitination
- Crosstalk: Coordinated regulation of protein function
Therapeutic strategies include [17][18]:
- USP18 inhibitors: Enhance ISGylation
- HERC5 modulators: Alter ISGylation levels
- Interferon pathway modulators: Reduce chronic interferon response
- Protein clearance enhancers: Overcome ISGylation-induced blocks
- CSF ISG15 as disease biomarker
- Peripheral blood mononuclear cell expression
- Disease progression monitoring
- Treatment response indicators
- Complex biology of ISGylation
- Cell type-specific effects
- Balancing immune response and neurodegeneration
ISG15 knockout mice exhibit:
- Enhanced viral susceptibility: Critical role in antiviral immunity
- Altered interferon responses: Dysregulated type I IFN signaling
- No major developmental defects: Compensatory mechanisms
- ISG15 overexpression: Neuron-specific
- Conditional deletion: Cell type-specific
- Disease model crosses: With α-synuclein, Tau, TDP-43 models
- Improved understanding of ISG15 in disease
- Testing therapeutic interventions
- Mechanistic insights
ISG15 is an interferon-stimulated, ubiquitin-like protein with critical roles in innate immunity and protein quality control. In the nervous system, ISG15 modulates neuroinflammation, protein aggregation, and cellular stress responses. Dysregulated ISG15 expression and ISGylation are implicated in Parkinson's disease, Alzheimer's disease, ALS, and other neurodegenerative conditions. The type I interferon-ISG15 axis represents a promising therapeutic target for developing disease-modifying treatments. Further research into ISG15's role in neurodegeneration will provide insights into disease mechanisms and identify novel therapeutic approaches.