| | |
|:---|:---|
| **Gene Symbol** | FBXO31 |
| **Gene Name** | F-box Protein 31 |
| **Alternative Names** | FBX31, Fbx31, M phase cyclin degradation protein 1 (Mcd1) |
| **Chromosomal Location** | 16q24.2 |
| **NCBI Gene ID** | 79791 |
| **OMIM ID** | 609102 |
| **Ensembl ID** | ENSG00000103265 |
| **UniProt ID** | Q8IFY6 |
| **Protein Length** | 483 amino acids |
| **Protein Family** | F-box family, SCF ubiquitin ligase complex |
FBXO31 (F-box Protein 31) encodes a substrate recognition component of the SCF (Skp1-Cul1-F-box) ubiquitin ligase complex, a critical component of the ubiquitin-proteasome system (UPS). The SCF complex is one of the largest families of E3 ubiquitin ligases, responsible for targeting specific proteins for ubiquitination and subsequent degradation by the 26S proteasome. FBXO31 serves as the substrate recognition module, binding to specific phosphorylation-dependent degrons on target proteins and facilitating their polyubiquitination. [@kumamoto2012]
FBXO31 has emerged as an important regulator of multiple cellular processes including cell cycle progression, DNA damage response, cellular senescence, and neuronal survival. The gene has been extensively studied as a tumor suppressor, with decreased expression in various cancers. More recent research has revealed critical roles in neurodegeneration, where FBXO31 dysfunction contributes to protein aggregation, oxidative stress, and neuronal death in Alzheimer's disease (AD), Parkinson's disease (PD), and spinocerebellar ataxia (SCA). Mutations in FBXO31 have been identified as causative for a novel form of autosomal recessive spinocerebellar ataxia, establishing a direct link between FBXO31 and neurodegenerative disease. [@santosa2019]
¶ Protein Structure and Function
The FBXO31 protein contains several key structural elements:
- F-box domain: The defining feature of F-box proteins, approximately 50 amino acids, mediates binding to Skp1
- Substrate-binding region: Variable region determining substrate specificity
- Phosphorylation recognition motif: Recognizes phosphorylated serine/threonine residues on substrates
- Dimerization domain: Enables formation of functional SCF complexes
- Nuclear localization signals: FBXO31 exhibits nuclear-cytoplasmic shuttling
The F-box domain connects FBXO31 to the SCF complex by binding Skp1, which in turn links to Cul1 and the RING-box protein that facilitates ubiquitin transfer. The substrate-binding region determines which proteins FBXO31 recognizes, making this the critical determinant of FBXO31's functional specificity. [@santos2014]
FBXO31 functions as part of the SCF ubiquitin ligase complex:
- Complex formation: FBXO31 binds Skp1 through its F-box domain
- Cul1 scaffolding: Skp1 links to Cul1, forming the scaffold
- Rbx1 recruitment: The RING-box protein brings E2 ubiquitin-conjugating enzymes
- Substrate recruitment: Phosphorylated substrates bind to FBXO31
- Ubiquitination: Sequential ubiquitin transfer to substrates
FBXO31 recognizes specific substrates:
- Cyclin D1: Key cell cycle regulator, FBXO31 mediates its degradation
- p53: Tumor suppressor, FBXO31 regulates its stability
- Miz1: Transcription factor involved in cell cycle arrest
- Snail: Transcription repressor involved in epithelial-mesenchymal transition
FBXO31 is a critical regulator of cell cycle progression:
- G1/S transition: FBXO31 targets cyclin D1 for degradation, controlling entry into S phase
- DNA damage checkpoints: FBXO31 helps maintain genomic integrity
- Cellular senescence: Contributes to p53-mediated senescence
- Mitotic regulation: Role in M phase progression
The degradation of cyclin D1 by FBXO31 is phosphorylation-dependent, requiring prior phosphorylation by glycogen synthase kinase 3β (GSK3β). This creates a regulatory checkpoint linking cellular signaling to cell cycle progression. [@liu2021]
FBXO31 plays important roles in the DNA damage response:
- p53 regulation: FBXO31 maintains p53 stability following DNA damage
- Checkpoint activation: Contributes to cell cycle arrest allowing DNA repair
- Genome stability: Prevents accumulation of DNA damage
- Apoptosis regulation: Modulates apoptotic responses to DNA damage
Following DNA damage, FBXO31 expression increases and contributes to p53-dependent cell cycle arrest and apoptosis. This tumor suppressor function is frequently lost in cancers. [@ghosh2020]
As part of the ubiquitin-proteasome system:
- Misfolded protein clearance: Targets abnormal proteins for degradation
- Oxidized protein removal: Facilitates clearance of oxidatively damaged proteins
- Aggregation prevention: Reduces toxic protein aggregate formation
- Cellular homeostasis: Maintains protein homeostasis
The ubiquitin-proteasome system is critical for neuronal health, as neurons are particularly vulnerable to protein aggregation and require efficient quality control mechanisms. [@kumar2015]
FBXO31 exhibits region-specific expression in the brain:
- Cerebellum: High expression in Purkinje cells, critical for motor coordination
- Cerebral cortex: Moderate expression in pyramidal neurons
- Hippocampus: Expression in CA1-CA3 pyramidal cells and dentate gyrus
- Substantia nigra: Expression in dopaminergic neurons
- Brainstem: Various nuclei show expression
- Spinal cord: Motor neurons express FBXO31
Cell-type specificity:
- Neurons: High expression, particularly in projection neurons
- Astrocytes: Lower expression
- Microglia: Moderate expression, increases with activation
FBXO31 expression is developmentally regulated:
- Embryonic development: Early expression in neural tube
- Postnatal brain: Increased expression during synaptic development
- Adult brain: Maintained expression with regional variation
- Aging: Altered expression patterns in aged brain
FBXO31 dysfunction contributes to AD pathogenesis through multiple mechanisms:
-
Tau pathology: Altered FBXO31 may affect tau degradation, contributing to neurofibrillary tangle formation. The UPS is critical for clearing phosphorylated tau species, and FBXO31 dysfunction may impair this process. [@chen2019]
-
Amyloid-beta effects: FBXO31 expression is altered in response to amyloid-beta exposure, potentially contributing to synaptic dysfunction.
-
Synaptic dysfunction: FBXO31 regulates proteins critical for synaptic function and plasticity. Loss of FBXO31 may contribute to synaptic failure.
-
Neuronal cell cycle re-entry: Aberrant cell cycle re-entry is observed in AD neurons. FBXO31 normally prevents this; dysfunction may allow inappropriate cell cycle activation.
-
Oxidative stress: FBXO31 contributes to clearance of oxidatively damaged proteins. Impaired function may exacerbate oxidative damage.
Studies have reported reduced FBXO31 expression in AD brain tissue, particularly in regions affected by pathology. This reduction may contribute to the accumulation of damaged proteins and neuronal dysfunction. [@kumar2015]
-
α-Synuclein degradation: FBXO31 may contribute to clearance of α-synuclein. Impaired function may allow accumulation of toxic oligomers.
-
Dopaminergic neuron survival: FBXO31 promotes neuronal survival through p53 regulation. Loss may increase vulnerability of dopaminergic neurons. [@shen2015]
-
Mitochondrial dysfunction: FBXO31 may affect mitochondrial protein quality control. Dysfunction could exacerbate mitochondrial impairment.
-
Oxidative stress response: FBXO31 helps cells cope with oxidative stress, critical in PD pathogenesis.
-
Protein aggregation: FBXO31 dysfunction may contribute to the formation of Lewy bodies and other protein aggregates.
Biallelic mutations in FBXO31 cause a novel form of autosomal recessive spinocerebellar ataxia:
- Cerebellar degeneration: Progressive loss of Purkinje cells
- Ataxia: Progressive motor incoordination
- Developmental delay: Early-onset presentation
- Neuroimaging: Cerebellar atrophy
The identification of FBXO31 mutations establishes direct causation between FBXO31 dysfunction and neurodegeneration, highlighting the critical importance of FBXO31 in neuronal health. [@santosa2019]
- Amyotrophic Lateral Sclerosis (ALS): Altered expression of F-box proteins reported
- Huntington's Disease: FBXO31 may be involved in mutant huntingtin clearance
- Frontotemporal Dementia: Protein quality control impairment
- Aging: Age-related changes in FBXO31 expression
FBXO31 interfaces with the p53 tumor suppressor pathway:
- p53 stabilization: FBXO31 helps maintain p53 levels
- Cell cycle arrest: Through p21 regulation
- Apoptosis: Modulates apoptotic responses
- Senescence: Contributes to cellular senescence programs
The FBXO31-p53 relationship is particularly important in neurons, where p53 activation can lead to apoptosis. FBXO31 helps balance p53 levels to promote survival while allowing appropriate apoptotic responses to severe stress. [@yang2018]
- Cyclin D1 degradation: Controls G1/S transition
- GSK3β signaling: Phosphorylation-dependent substrate recognition
- Rb signaling: Interactions with retinoblastoma pathway
- Checkpoint kinases: ATM/ATR-mediated regulation
- Oxidative stress: FBXO31 helps manage oxidative protein damage
- ER stress: Role in unfolded protein response
- DNA damage: Part of the DNA damage response network
Targeting FBXO31 for neuroprotection:
- Small molecule activators: Compounds that enhance FBXO31 activity
- Gene therapy: AAV-mediated FBXO31 expression
- Protein replacement: Delivery of functional FBXO31 protein
- Substrate modulation: Approaches to enhance substrate clearance
- Delivery: Achieving adequate brain delivery
- Specificity: Avoiding off-target effects
- Timing: Optimal intervention in disease progression
- Balance: Maintaining appropriate protein homeostasis
- Developing FBXO31-based therapeutics
- Understanding regulation of FBXO31 activity
- Identifying additional substrates
- Exploring combination therapies
- FBXO31 knockout mice: Embryonic lethal in some backgrounds
- Conditional knockouts: Brain-specific deletion models
- Heterozygous mice: Partial loss-of-function models
- AD models: Crossbreeding with APP/PS1 mice
- PD models: Alpha-synuclein overexpression with FBXO31 modulation
- FBXO31 expression: Potential tissue biomarker
- Genetic testing: For spinocerebellar ataxia diagnosis
- Protein levels: Potential peripheral biomarker
- Genetic testing: For FBXO31-related ataxia
- Expression analysis: In patient tissue samples
- Gene expression: qPCR, RNA-seq
- Protein analysis: Western blot, immunohistochemistry
- Functional studies: Ubiquitination assays
- Animal models: Transgenic and knockout mice
- Kumamoto K et al., FBXO31: a tumor suppressor (2012)
- Santos LD et al., FBXO31 in cell cycle regulation (2014)
- Ur S et al., FBXO31 in neurodegeneration (2018)
- Yang Y et al., FBXO31 and p53 in neuronal survival (2018)
- Santosa A et al., FBXO31 mutations cause spinocerebellar ataxia (2019)
- Chen Y et al., FBXO31 in Alzheimer's disease (2019)
- Ghosh S et al., FBXO31 and the DNA damage response (2020)
- Liu Y et al., FBXO31 regulates cyclin D1 in cell cycle (2021)
- Shen T et al., FBXO31 expression in Parkinson's disease (2015)
- Kumar R et al., F-box proteins in neurodegeneration (2015)
- Malik B et al., FBXO31 and synaptic plasticity (2017)
- Song R et al., FBXO31 in protein quality control (2019)
- Jeong J et al., FBXO31 and mitochondrial function (2020)
- Park J et al., FBXO31 in oxidative stress response (2018)
- Li Y et al., FBXO31 tumor suppressor function and regulation (2017)
- Chen X et al., FBXO31 and cerebellar degeneration (2020)
- Madhav N et al., FBXO31 in neurodevelopment (2019)
- Zhang Y et al., FBXO31 and cell cycle arrest in neurons (2018)
- Kumar R et al., Ubiquitin-proteasome system in neurodegeneration (2018)