| Gene Symbol | STK4 (MST1) |
| Full Name | Serine/Threonine Kinase 4 |
| Chromosomal Location | 20q13.12 |
| NCBI Gene ID | 6789 |
| Ensembl ID | ENSG00000101109 |
| UniProt ID | Q13043 |
| OMIM ID | 604472 |
| Protein Family | STE20 family, Hippo pathway |
| Aliases | MST1, KRS2 |
| Protein Length | 491 amino acids |
| Associated Diseases | Parkinson's Disease, Alzheimer's Disease, ALS, Cancer, Neurodevelopmental Disorders |
STK4 (Serine/Threonine Kinase 4), also known as MST1 (Mammalian Ste20-like Kinase 1), is a critical component of the Hippo signaling pathway. Located on chromosome 20q13.12, this gene encodes a serine/threonine kinase that plays essential roles in organ size control, cell proliferation, apoptosis, stem cell self-renewal, and neuronal survival. STK4 has emerged as a significant player in neurodegeneration, with dysregulated activity implicated in Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS). The kinase functions as both a tumor suppressor in peripheral tissues and a pro-apoptotic regulator in the nervous system .
¶ Protein Structure and Function
¶ Domain Architecture
STK4 contains several functional domains :
- N-terminal kinase domain: Catalytic serine/threonine kinase activity (~300 amino acids)
- C-terminal regulatory domain: Contains autoinhibitory sequences
- Scaffold interaction region: Bins to SAV1 and other pathway components
STK4 (MST1) is the founding member of the MST1/2 (STE20-like kinases 1 and 2) family. Within the Hippo pathway, STK4 functions as the upstream kinase that initiates the cascade :
- Activation — STK4 is activated by autophosphorylation and phosphorylation by upstream kinases
- Complex formation — STK4 forms a heterodimer with STK3 (MST2) and associates with SAV1 (Salvador)
- Downstream phosphorylation — Activated STK4 phosphorylates LATS1/2 kinases
- YAP/TAZ inhibition — LATS1/2 phosphorylate YAP/TAZ, preventing their nuclear translocation
Beyond its role in the Hippo pathway, STK4 has direct pro-apoptotic functions :
- Caspase activation: Direct phosphorylation of caspase-3 and caspase-9
- FOXO transcription factors: Phosphorylates FOXO1/FOXO3a, promoting nuclear translocation and transcriptional activation of pro-apoptotic genes
- BIM activation: Upregulates pro-apoptotic BIM protein
- Mitochondrial pathway: Promotes cytochrome c release
STK4 is expressed throughout the central nervous system :
- Hippocampus: High expression in CA1-CA3 regions and dentate gyrus
- Cortex: Pyramidal neurons in layers II-VI
- Cerebellum: Purkinje cells and granule cells
- Substantia nigra: Moderate expression in dopaminergic neurons
- Subventricular zone: Neural stem cells
- Cell types: Neurons, astrocytes, and neural progenitor cells
| Region |
Expression Level |
Cell Types |
| Hippocampus |
High |
Pyramidal neurons, granule cells |
| Cortex |
High |
Pyramidal neurons, interneurons |
| Cerebellum |
High |
Purkinje cells |
| Substantia nigra |
Moderate |
Dopaminergic neurons |
| Spinal cord |
Moderate |
Motor neurons |
Beyond the brain, STK4 is expressed in:
- Heart: High cardiac expression
- Liver: Hepatocytes
- Immune system: T lymphocytes and other immune cells
STK4 activation contributes to AD pathogenesis through multiple mechanisms :
- Apoptotic neuron loss: STK4 promotes apoptosis in hippocampal neurons
- Tau pathology: STK4 affects tau phosphorylation through cross-talk with other kinases
- Synaptic dysfunction: STK4 activation leads to synaptic loss
- Amyloid toxicity: Aβ-induced STK4 activation amplifies neuronal death
The kinase is activated in AD brain tissue, and inhibition of STK4 has shown neuroprotective effects in cellular models.
In PD, STK4 plays several roles :
- Dopaminergic neuron survival: STK4 promotes apoptosis of dopamine neurons
- Alpha-synuclein toxicity: STK4 is activated by α-synuclein aggregates
- Mitochondrial dysfunction: STK4 contributes to mitochondrial apoptosis
- Neuroinflammation: STK4 activation in glial cells promotes inflammation
STK4 inhibition has demonstrated protective effects in PD models, suggesting therapeutic potential.
STK4 dysregulation in ALS includes :
- Motor neuron apoptosis: Accelerated STK4-mediated cell death
- TDP-43 pathology: Interaction between STK4 and TDP-43 aggregates
- Axonal degeneration: STK4 promotes axonal retraction and degeneration
- Glial involvement: Activated STK4 in astrocytes and microglia
STK4 Activation in Neurodegeneration
│
├──► Stress Signals
│ ├──► Oxidative stress
│ ├──► ER stress
│ ├──► Mitochondrial dysfunction
│ └──► Protein aggregates
│
├──► Apoptotic Cascade
│ ├──► Caspase activation
│ ├──► FOXO phosphorylation
│ ├──► Mitochondrial permeabilization
│ └──► Cell death
│
└──► Synaptic Dysfunction
├──► Synaptic protein loss
├──► Dendritic spine reduction
└──► Neural connectivity impairment
STK4 functions as a tumor suppressor :
- Loss of STK4 promotes uncontrolled cell growth and tumor formation
- STK4 mutations are found in various cancers
- YAP/TAZ overactivation due to Hippo pathway inactivation promotes oncogenesis
Biallelic STK4 mutations cause:
- Autosomal recessive immunodeficiency 38 — combined immunodeficiency
- Neurodevelopmental abnormalities — developmental delay, intellectual disability
- Growth retardation — impaired somatic growth
- Stroke — MST1 mediates ischemic neuronal injury
- STK4 inhibitors: Pharmacological inhibition of kinase activity
- Caspase inhibitors: Downstream blockade of apoptotic pathway
- FOXO modulators: Prevent nuclear translocation of pro-apoptotic factors
- RNAi knockdown: Reduce STK4 expression
- ASO therapy: Antisense oligonucleotides targeting STK4
- CRISPR inhibition: CRISPRa to suppress STK4 transcription
- Antioxidants: Counteract oxidative stress that activates STK4
- Autophagy enhancers: Clear protein aggregates reducing STK4 activation
- Neurotrophic factors: Promote neuronal survival independent of STK4
¶ Interactions and Pathways
STK4 interacts with several key proteins:
- SAV1: Scaffold protein, forms functional complex
- STK3 (MST2): Kinase partner, forms heterodimer
- LATS1/2: Downstream kinases, phosphorylation targets
- FOXO1/3: Transcription factors, phosphorylation substrates
- Caspase-3/9: Executioner caspases, direct targets
STK4 activity is modulated by multiple pathways:
- PI3K/AKT: AKT phosphorylates and inhibits STK4
- MAPK pathways: Cross-talk with stress-activated kinases
- Wnt/β-catenin: Interaction with developmental pathways
- Notch signaling: Coordinate regulation of neural stem cells
- Stk4 knockout: Viable but with increased tumor susceptibility
- Conditional knockouts: Brain-specific deletion for neurodegeneration studies
- Transgenic models: Overexpression of activated STK4
- Kinase inhibitor testing
- Pathway validation in vivo
- Therapeutic intervention studies
STK4 activation (phosphorylated MST1) serves as a biomarker for:
- Neurodegenerative disease progression
- Cancer prognosis
- Treatment response to neuroprotective agents
STK4 plays a significant role in neuroinflammatory processes:
- Microglia: STK4 activation promotes microglial activation and pro-inflammatory cytokine release
- Astrocytes: STK4 modulates astrocyte reactivity in neurodegenerative conditions
- Inflammatory response: STK4 contributes to chronic neuroinflammation
STK4 affects production of:
- TNF-α
- IL-1β
- IL-6
- Chemokines
Targeting STK4 in neuroinflammation:
- Reduces glial activation
- Decreases cytokine production
- May slow disease progression
STK4 regulates synaptic function and plasticity:
- STK4 activation reduces spine density
- Affects synaptic stability
- Impairs learning and memory
- STK4 interferes with LTP formation
- Affects AMPA receptor trafficking
- Impairs synaptic strengthening
- STK4 activation in hippocampus impairs memory
- Inhibits CREB activation
- Affects immediate early gene expression
¶ STK4 and Mitochondrial Dynamics
STK4 directly impacts mitochondrial function:
- STK4 promotes PINK1/Parkin-independent mitophagy
- Affects mitochondrial quality control
- Contributes to neuronal energy deficits
- STK4 promotes mitochondrial outer membrane permeabilization
- Enhances cytochrome c release
- Activates intrinsic apoptotic pathway
- STK4 reduces ATP production
- Impairs mitochondrial membrane potential
- Contributes to metabolic dysfunction
- In vitro kinase assays with recombinant STK4
- Phospho-antibody detection of p-STK4
- ATP-based activity measurements
- Co-immunoprecipitation
- Yeast two-hybrid screening
- Mass spectrometry proteomics
- Stk4 knockout mice
- Conditional brain-specific knockouts
- Transgenic overexpression models
During development, STK4 plays important roles:
- Regulates proliferation of neural stem cells
- Controls cell cycle exit
- Affects differentiation decisions
- Modulates neuronal positioning
- Affects cortical layering
- Contributes to proper brain architecture
- STK4 influences axon pathfinding
- Regulates growth cone dynamics
- Affects circuit formation
- STK3 — MST2, paralog
- YAP1 — Downstream effector
- LATS1 — Downstream kinase
- TAZ — YAP partner