MST4 Antibody

What is MST4 and what role does it play in cellular signaling?

MST4 (Mammalian Sterile 20-like kinase 4, also known as STK26) is a serine/threonine protein kinase belonging to the germinal center kinase (GCK) group III family, a subset of Ste20-like kinases. It functions as a mediator of cell growth and modulates apoptosis. MST4 plays critical roles in numerous signaling networks including the Hippo pathway, MAPK signaling, and cytoskeleton remodeling. It contains a C-terminal regulatory domain and an N-terminal kinase domain that requires full activation of the kinase . MST4 is part of the striatin-interacting phosphatase and kinase (STRIPAK) complexes, which have critical roles in protein (de)phosphorylation and regulate multiple signaling pathways involved in cell growth, differentiation, apoptosis, metabolism, and immune regulation .

What species reactivity should be considered when selecting an MST4 antibody?

When selecting an MST4 antibody, researchers should consider species homology and validated reactivity. According to available resources, most commercial MST4 antibodies demonstrate cross-reactivity with human, mouse, and rat samples . Tissue distribution analysis shows MST4 expression across multiple tissues in these species, though at varying levels . For instance, rabbit monoclonal antibody EP1864Y (ab52491) has been validated to react with human, mouse, rat, and recombinant fragment proteins . When working with less common species or specialized applications, validation of antibody cross-reactivity through positive controls is crucial, as MST4 is evolutionarily conserved but may have species-specific epitope variations.

What are the optimal conditions for using MST4 antibodies in Western blotting?

For optimal Western blot detection of MST4, researchers should:

• Sample preparation: Extract proteins using RIPA buffer supplemented with protease and phosphatase inhibitors to preserve MST4 phosphorylation status.

• Loading and separation: Load 10-30 μg of protein lysate on 5-20% SDS-PAGE gels for optimal separation (MST4 has a predicted molecular weight of 47 kDa but often appears at ~50-52 kDa on gels) .

• Transfer conditions: Transfer to nitrocellulose membranes at 150 mA for 50-90 minutes to ensure complete protein transfer .

• Blocking and antibody dilution: Block with 5% non-fat milk in TBS for 1.5 hours at room temperature. Primary antibody dilutions range from 1:1000 to 1:200,000 depending on the specific antibody (e.g., rabbit anti-MST4 polyclonal at 0.5 μg/mL, or Cell Signaling Technology's antibody at 1:1000) .

• Detection system: Use HRP-conjugated secondary antibodies and enhanced chemiluminescence for detection .

Positive controls should include HeLa, 293T, Jurkat (human) or PC-12 (rat) cell lysates, which consistently show strong MST4 expression .

How should MST4 antibodies be used for immunohistochemistry and immunocytochemistry?

For successful immunohistochemistry (IHC) and immunocytochemistry (ICC) with MST4 antibodies:

For IHC:

• Perform heat-mediated antigen retrieval with citrate buffer (pH 6.0) on formalin-fixed paraffin-embedded tissues .

• Use antibody dilutions between 1:100-1:500 with overnight incubation at 4°C.

• Human placenta tissue serves as an effective positive control .

• MST4 staining should be evaluated in relation to other markers, particularly when studying EMT (such as E-cadherin and vimentin) .

For ICC/IF:

• Fix cells with 100% methanol and permeabilize with 0.1% Triton X-100 .

• Use antibody at 1:500 dilution with appropriate fluorophore-conjugated secondary antibodies.

• Include DAPI counterstain for nuclear visualization.

• HeLa cells are recommended as positive controls .

For co-localization studies, MST4 can be paired with Golgi markers (GM130), as MST4 shows Golgi localization in some cell types .

What is the best approach for generating MST4 knockdown or overexpression models?

For effective MST4 manipulation in experimental models:

Knockdown approaches:

• RNA interference: The most validated shRNA target sequences for human MST4 are:

  • shMST4-1: 5'-GCTGCCAATGTCTTGCTCTCA-3'

  • shMST4-2: 5'-GCTGGTCAGCTGACAGATACA-3'

  • shMST4-3: 5'-GGCAGAAGGACACAGTGATGA-3'

• The shMST4-3 sequence has shown the highest knockdown efficiency in AGS cells and is recommended for experimental use .

• For transient knockdown, MST4 siRNA (HS01_00030410, Sigma-Aldrich) using Lipofectamine RNAiMax has proven effective .

Overexpression systems:

• Human MYC-tagged MST4 expression plasmid (EX-W0097-M43) has been successfully used with Lipofectamine 2000 in multiple cell lines .

• Lentiviral vectors encoding the human MST4 gene have been validated in MKN45 cells .

Selection and validation:

• Use 3.0 μg/mL puromycin for selection of stable transfectants (48 hours post-transfection) .

• Always confirm knockdown or overexpression efficiency by both qPCR and Western blot.

• For functional rescue experiments, consider using MST4 kinase-dead mutants (K53R) or constitutively active phosphomimetic mutants (T178E) to study phosphorylation-dependent activities .

Why does MST4 appear at a different molecular weight than predicted in Western blots?

MST4 has a predicted molecular weight of approximately 47 kDa, but frequently appears at 50-52 kDa on Western blots . This discrepancy can be attributed to several factors:

• Post-translational modifications: MST4 undergoes phosphorylation, particularly at Thr178, which is critical for its activation. These phosphorylation events can increase the apparent molecular weight .

• Protein structure and charges: The tertiary structure and charge distribution of MST4 may cause it to migrate differently than expected based solely on amino acid composition.

• Expression tags: When using recombinant MST4, fusion tags (such as MYC, HA, or GST) will increase the observed molecular weight.

To confirm band specificity:

• Include positive controls (HeLa or 293T cell lysates) alongside samples .

• Use MST4 knockout or knockdown samples as negative controls .

• For phospho-specific detection, include samples treated with alkaline phosphatase to demonstrate specificity of phosphorylation-dependent mobility shifts.

• In case of multiple bands, validate with at least two different MST4 antibodies recognizing distinct epitopes.

How can background be reduced in MST4 immunostaining?

To minimize background and improve signal-to-noise ratio in MST4 immunostaining:

• Blocking optimization:

  • Extend blocking time to 1-2 hours at room temperature with 5-10% normal serum from the same species as the secondary antibody.

  • Add 0.1-0.3% Triton X-100 to blocking solution for better penetration and reduced non-specific binding.

• Antibody dilution and incubation:

  • Titrate antibody concentrations (starting with manufacturer recommendations and adjusting as needed).

  • Extend primary antibody incubation to overnight at 4°C instead of shorter incubations at room temperature.

  • Wash thoroughly with PBS containing 0.1% Tween-20 (PBST) between antibody incubations (minimum 3×5 minutes).

• Tissue-specific considerations:

  • For tissues with high endogenous peroxidase activity, include a hydrogen peroxide quenching step (3% H₂O₂ in methanol for 10 minutes).

  • For tissues with high background, use buffers containing 0.1-0.5% BSA or 0.5% non-fat milk.

• Autofluorescence reduction (for IF):

  • Treat sections with 0.1% Sudan Black B in 70% ethanol for 20 minutes before mounting.

  • Include a photo-bleaching step if tissue autofluorescence is problematic.

• Controls:

  • Always include a negative control by omitting primary antibody .

  • Use MST4 knockdown tissues/cells as specificity controls.

What are the key considerations for phospho-specific MST4 detection?

Detecting phosphorylated MST4 requires special considerations:

• Sample preparation:

  • Use phosphatase inhibitors (sodium fluoride, sodium orthovanadate, β-glycerophosphate) in lysis buffers.

  • Process samples quickly and maintain cold temperatures throughout to prevent dephosphorylation.

  • Avoid multiple freeze-thaw cycles which can degrade phospho-epitopes.

• Antibody selection:

  • Use phospho-specific antibodies that recognize MST4 phosphorylated at Thr178, which is the critical regulatory phosphorylation site for MST4 activation .

  • Validate antibody specificity using phosphatase treatments or phospho-mimetic mutants (T178E) and phospho-resistant mutants (T178A).

• Experimental conditions:

  • For studying MST4 activation, serum starvation can be used as it affects MST4 phosphorylation status .

  • When studying associations with other signaling pathways, consider relevant stimuli (growth factors, stress inducers) that may affect MST4 phosphorylation.

• Detection methods:

  • For Western blotting, use PVDF membranes rather than nitrocellulose for better retention of phosphorylated proteins.

  • For IHC/IF of phosphorylated proteins, tyramide signal amplification can improve detection sensitivity.

• Controls and validation:

  • Include phosphatase-treated samples as negative controls.

  • Consider using kinase assays with recombinant MST4 and [γ-³²P]ATP for direct assessment of phosphorylation activity .

How can MST4 antibodies be applied in studying the role of MST4 in EMT and cancer metastasis?

To investigate MST4's role in epithelial-mesenchymal transition (EMT) and cancer metastasis:

• Multi-marker immunohistochemistry analysis:

  • Perform serial section IHC staining for MST4 alongside EMT markers (E-cadherin, vimentin, N-cadherin, ZEB1/2, Snail/Slug).

  • Use Spearman correlation analysis to establish relationships between MST4 expression and EMT marker expression in patient samples .

  • Connect these correlations with clinical parameters (lymph node metastasis, lymphovascular invasion).

• Functional validation in cell models:

  • Create stable MST4 knockdown and overexpression cell lines.

  • Assess morphological changes and cytoskeletal reorganization through F-actin staining .

  • Quantify EMT marker expression changes via qRT-PCR and Western blot.

  • Perform cell-ECM adhesion assays and cell-cell adhesion assays to evaluate EMT-related functional changes .

• In vivo metastasis models:

  • Use tail vein injection of MST4-modulated cancer cells to quantify lung metastasis formation .

  • Employ orthotopic injection models to assess local invasion and distant metastasis.

  • Perform IHC on metastatic lesions to confirm MST4 and EMT marker status.

• Pathway analysis:

  • Investigate MST4's interaction with Ezrin through co-immunoprecipitation and phosphorylation studies .

  • Examine downstream effectors using phospho-specific antibodies against MAPK/ERK pathway components.

• Therapeutic implications:

  • Evaluate MST4 inhibition as a potential strategy to reverse EMT and prevent metastasis in preclinical models.

  • Perform combination studies with existing EMT-targeting drugs to identify synergistic effects.

What methods are effective for studying MST4 interactions with its binding partners?

To effectively study MST4 protein-protein interactions:

• Co-immunoprecipitation (Co-IP):

  • Use anti-MST4 antibodies for immunoprecipitation followed by Western blotting for suspected binding partners.

  • Conversely, immunoprecipitate binding partners and probe for MST4.

  • Cross-validate interactions using different antibodies and cell types.

  • For weaker interactions, consider using crosslinking agents before lysis.

• Proximity ligation assay (PLA):

  • Employ MST4 antibodies alongside antibodies against potential binding partners (e.g., Ezrin, YAP).

  • This technique enables visualization of protein interactions in situ with subcellular localization information.

  • Quantify PLA signals to compare interaction strength under different conditions.

• FRET/BRET analysis:

  • Generate fluorescent protein-tagged MST4 constructs alongside tagged binding partners.

  • Measure energy transfer to confirm direct interactions and provide dynamic interaction information.

• Mass spectrometry-based approaches:

  • Perform immunoprecipitation with MST4 antibodies followed by mass spectrometry to identify novel binding partners.

  • Use SILAC or TMT labeling to compare interaction profiles under different conditions.

  • Validate identified interactions using Co-IP or PLA.

• In vitro kinase assays:

  • For kinase-substrate relationships, perform in vitro kinase assays using recombinant MST4.

  • Use [γ-³²P]ATP labeling or thiophosphate ester antibodies to detect phosphorylation .

  • Identify specific phosphorylation sites through point mutation studies and mass spectrometry.

• Domain mapping:

  • Generate truncated versions of MST4 to map interaction domains.

  • Use GST pull-down assays with recombinant protein fragments to confirm direct binding regions.

How can researchers study the relationship between MST4 and the Hippo pathway?

To investigate MST4's role in Hippo pathway regulation:

• YAP phosphorylation and localization analysis:

  • Use MST4 knockdown or overexpression systems to examine YAP phosphorylation status via Western blotting with phospho-specific antibodies .

  • Perform immunofluorescence to track YAP nuclear/cytoplasmic localization in response to MST4 modulation .

  • Quantify the nuclear/cytoplasmic ratio of YAP under various conditions (serum starvation, glucose deprivation, energy stress, F-actin disruption) .

• Site-specific phosphorylation studies:

  • Generate phospho-mimetic (T83E) and phospho-resistant (T83A) YAP mutants to investigate the functional significance of MST4-mediated phosphorylation .

  • Confirm direct phosphorylation using in vitro kinase assays with recombinant proteins.

  • Use mass spectrometry to identify and validate phosphorylation sites.

• Transcriptional reporter assays:

  • Employ TEAD luciferase reporters to measure YAP-dependent transcriptional activity in the context of MST4 manipulation .

  • Analyze expression of YAP target genes (CTGF, CYR61, ANKRD1) via qRT-PCR and Western blotting.

• Functional relationships in tumorigenesis:

  • Develop MST4 knockout mouse models to assess YAP activation and gastric tumorigenesis .

  • Perform rescue experiments combining MST4 deletion with YAP inhibition to confirm pathway relationships.

  • Use patient-derived tumor organoids to validate findings in a clinically relevant system.

• Upstream regulation analysis:

  • Investigate how various cellular stresses affect MST4 activity and consequent YAP regulation.

  • Examine potential cross-talk between MST4 and canonical Hippo pathway components (MST1/2, LATS1/2).

  • Study how MST4 integrates into STRIPAK complexes, which are known Hippo pathway regulators.

How should researchers interpret contradictory findings about MST4's role in different cancer types?

Research has revealed seemingly contradictory roles for MST4 in different cancer contexts. To properly interpret these findings:

• Context-dependent analysis:

  • Acknowledge that MST4 can function as both an oncogene and tumor suppressor depending on cancer type and stage.

  • In gastric cancer, MST4 has been reported to both promote metastasis through EMT and suppress tumorigenesis by limiting YAP activation .

  • Compare methodologies, cell lines, and animal models used in contradictory studies.

• Pathway cross-talk considerations:

  • Analyze the status of interconnected pathways (Hippo, MAPK, Ezrin) in different experimental systems.

  • Consider that MST4 may prioritize different downstream targets depending on cellular context and existing genetic alterations.

  • Evaluate the balance between competing functions (e.g., MST4's role in both apoptosis regulation and EMT promotion).

• Technical reconciliation approaches:

  • Use multiple cell lines representing different cancer stages/subtypes.

  • Employ both gain-of-function and loss-of-function approaches.

  • Validate findings through multiple techniques (in vitro, in vivo, patient samples).

  • Consider temporal aspects of MST4 function during cancer progression.

• Comprehensive biomarker analysis:

  • Correlate MST4 expression with multiple clinical parameters and molecular markers.

  • Perform multivariate analyses to identify contextual factors that may influence MST4's role.

  • Use TCGA and other public databases to validate findings across larger patient cohorts with detailed molecular characterization .

• Experimental design for resolving contradictions:

  • Design experiments that can directly test competing hypotheses.

  • Use rescue experiments with specific functional mutants (kinase-dead, phospho-mimetic).

  • Consider conditional knock-out/knock-in models to evaluate stage-specific effects.

What factors should be considered when comparing phosphorylation results in MST4 research papers?

When comparing phosphorylation data across MST4 research papers, consider:

• Experimental conditions that affect phosphorylation status:

  • Cell culture conditions (serum levels, confluence, starvation protocols).

  • Tissue processing methods (fixation time, phosphatase inhibitor usage).

  • Time course considerations (transient vs. sustained signaling).

• Detection method differences:

  • Antibody specificity and validation methods used.

  • Direct detection (³²P labeling, mass spectrometry) vs. antibody-based methods.

  • Relative quantification approaches (normalization methods, loading controls).

• Phosphorylation site specificity:

  • T178 is the primary regulatory site for MST4 activation .

  • Different phosphorylation sites may have distinct functional consequences.

  • Autophosphorylation vs. trans-phosphorylation events should be distinguished.

• Upstream regulators and downstream effectors:

  • Status of pathways that regulate MST4 (e.g., PDCD10, GM130 binding) .

  • Consequences for different downstream targets (Ezrin, YAP) .

  • Feedback loops that may affect interpretation of results.

• Reconciliation strategies:

  • Use multiple phospho-specific antibodies targeting different sites.

  • Employ both in vitro and cellular phosphorylation assays.

  • Validate with phospho-mimetic and phospho-resistant mutants.

  • Consider phosphatase activities that may counterbalance kinase functions.

How can researchers address tissue-specific variation in MST4 expression when designing experiments?

To account for tissue-specific variation in MST4 expression:

• Comprehensive expression profiling:

  • Analyze MST4 expression across multiple tissues at both mRNA and protein levels .

  • Consider public databases (TCGA, GTEx) alongside experimental validation.

  • Note that MST4 protein abundance in mouse liver is lower compared to brown adipose tissue, heart, muscle, and kidney, while mRNA may be detected in all tissues .

• Selection of appropriate controls:

  • Use tissue-matched controls in all experiments.

  • Include a panel of tissues/cell lines with known MST4 expression levels as reference standards.

  • For cancer studies, always compare with adjacent normal tissue from the same patient .

• Subcellular localization considerations:

  • Note that MST4 shows homogenous expression in some tissues (liver, adipose, heart, muscle) but restricted expression patterns in others (kidney tubulointerstitial region) .

  • Use subcellular fractionation or high-resolution imaging to document localization differences.

• Experimental design adaptations:

  • Adjust antibody concentrations for different tissues based on endogenous expression levels.

  • Consider tissue-specific knockdown/overexpression models rather than global approaches.

  • For multi-tissue studies, normalize data to account for baseline expression differences.

• Functional significance assessment:

  • Investigate whether different expression levels correlate with different functional outputs.

  • Determine tissue-specific binding partners that may modify MST4 function.

  • Consider compensatory mechanisms involving related kinases (MST1/2/3) in tissues with low MST4 expression.