Phospho-FLT3 (Tyr842) Antibody

What is the biological significance of FLT3 phosphorylation at Tyr842?

Phosphorylation of FLT3 at tyrosine 842 represents a critical event in the activation mechanism of this receptor tyrosine kinase. FLT3 is activated when its ligand (FLT3L) binds to the extracellular domain, inducing homodimer formation in the plasma membrane. This leads to autophosphorylation of several tyrosine residues, including Tyr842, which is located in the activation loop of the kinase domain . While studies have shown that Tyr842 phosphorylation is not essential for initial activation, it is critical for downstream signaling pathways involved in hematopoietic cell proliferation, differentiation, and survival .

The importance of this specific phosphorylation site is underscored by its involvement in pathological conditions. Mutations affecting FLT3 activity, including those influencing Tyr842 phosphorylation status, have been identified in approximately 30-40% of acute myeloid leukemia (AML) patients . Specifically, constitutive phosphorylation at this site contributes to dysregulated signaling cascades involving PI3K/AKT and MAPK pathways, promoting leukemic cell proliferation and survival .

What are the recommended applications for Phospho-FLT3 (Tyr842) antibody?

Phospho-FLT3 (Tyr842) antibodies have been validated for several experimental applications, each with specific advantages depending on your research question:

For optimal results in Western blotting, the recommended protocol involves:

• Lysing cells in buffer containing phosphatase inhibitors to preserve phosphorylation status

• Using 4-12% Bis-TRIS gels for resolution

• Transferring to nitrocellulose or PVDF membranes

• Blocking with 5% BSA in TBST rather than milk (which contains phosphatases)

• Incubating with primary antibody overnight at 4°C

For detection of phosphorylated FLT3 in samples with low expression, immunoprecipitation with total FLT3 antibody followed by Western blotting with phospho-specific antibody increases sensitivity .

How do FLT3 inhibitors affect detection of Phospho-FLT3 (Tyr842)?

FLT3 inhibitors significantly impact the detection of Phospho-FLT3 (Tyr842) in experimental systems, providing valuable tools for validating antibody specificity and studying signaling dynamics:

Tyrosine kinase inhibitors (TKIs) such as quizartinib (AC220) and midostaurin (PKC412) modulate FLT3 phosphorylation status by:

• Decreasing phosphorylation at Tyr842 in a dose-dependent manner

• Affecting subcellular localization of FLT3, particularly FLT3-ITD mutant forms

• Interrupting downstream signaling cascades including STAT5, AKT, and ERK pathways

When using these inhibitors as experimental controls, researchers should consider:

• Time-dependent responses: Rapid dephosphorylation occurs within 15-60 minutes of inhibitor treatment

• Concentration effects: Quizartinib has been shown to effectively inhibit phosphorylation at nanomolar concentrations (1-10 nM)

• Differential effects on wild-type versus mutant FLT3: FLT3-ITD shows altered sensitivity profiles compared to wild-type FLT3

In experimental design, inclusion of treated and untreated samples creates essential positive and negative controls for validating antibody specificity. This approach helps distinguish specific Phospho-FLT3 (Tyr842) signal from background or non-specific binding .

What is the relationship between Phospho-FLT3 (Tyr842) and activation of downstream signaling pathways?

Phosphorylation of FLT3 at Tyr842 serves as a critical node for activation of multiple downstream signaling cascades, with distinct pathway engagement depending on subcellular localization and mutation status:

The compartmentalization of signaling is particularly significant in mutant forms of FLT3. While wild-type FLT3 primarily signals from the plasma membrane after ligand stimulation, FLT3-ITD can activate downstream pathways from intracellular compartments, notably the endoplasmic reticulum and Golgi apparatus . This spatial organization of signaling contributes to the oncogenic potential of FLT3-ITD.

Experimentally, this relationship can be assessed through:

• Co-immunoprecipitation of Phospho-FLT3 (Tyr842) with downstream signaling components

• Inhibitor studies targeting specific nodes in each pathway

• Phospho-specific antibodies against key mediators (e.g., phospho-STAT5, phospho-AKT, phospho-ERK1/2)

How can I validate the specificity of Phospho-FLT3 (Tyr842) antibody in my experimental system?

Validating antibody specificity is critical for ensuring reliable results. For Phospho-FLT3 (Tyr842) antibodies, several complementary approaches are recommended:

• Phosphatase treatment control:

  • Treat half of your sample with lambda phosphatase before immunoblotting

  • Disappearance of the signal confirms phospho-specificity

• Peptide competition assay:

  • Pre-incubate antibody with phospho-peptide used as immunogen

  • Signal should be blocked with phospho-peptide but not non-phosphorylated peptide

  • Commercial antibodies often provide evidence of this validation

• Kinase inhibitor treatment:

  • Treat cells with FLT3 inhibitors (quizartinib or midostaurin)

  • Reduction in signal confirms specificity for FLT3 phosphorylation

• Stimulation/activation experiments:

  • Compare unstimulated versus FLT3-ligand stimulated cells

  • Increased signal after stimulation confirms sensitivity to phosphorylation changes

• Knockout/knockdown validation:

  • Use FLT3 knockout cells or siRNA knockdown

  • Absence of signal confirms target specificity

Example validation data shows Western blot analysis from HepG2 cells treated with EGF, demonstrating specific band detection at ~170kDa that disappears when the sample is treated with the antigen-specific peptide .

What are the differences in detecting phosphorylation of wild-type FLT3 versus FLT3-ITD mutant?

Detection of phosphorylation patterns between wild-type FLT3 and FLT3-ITD presents several critical differences that researchers must account for in experimental design:

Methodological considerations for accurate detection:

• Sample preparation: For wild-type FLT3, stimulation with FLT3L (typically 100-500 ng/mL for 5-15 minutes) is necessary to detect robust Tyr842 phosphorylation. For FLT3-ITD, phosphorylation is detectable without stimulation .

• Cellular fractionation: To distinguish compartment-specific phosphorylation, particularly in FLT3-ITD samples, subcellular fractionation protocols can separate plasma membrane, ER, and Golgi fractions prior to immunoblotting .

• Detection methods: For wild-type FLT3, enrichment by immunoprecipitation before probing with phospho-specific antibodies may be necessary. For FLT3-ITD, direct immunoblotting is often sufficient due to higher phosphorylation levels .

• Controls: Inclusion of both wild-type and FLT3-ITD expressing cell lines (e.g., MOLM-13, MV4-11 for FLT3-ITD; RS4;11 for wild-type) provides important reference points for comparative studies .

How do the crystal structure findings of FLT3 impact our understanding of Tyr842 phosphorylation?

The crystal structure of FLT3 kinase domain has provided crucial insights into the structural basis of Tyr842 phosphorylation and its role in kinase activity regulation:

Crystallographic studies of FLT3 bound to inhibitors like quizartinib have revealed that Tyr842 is located in the activation loop of the kinase domain . This positioning is critical for understanding several key aspects of FLT3 biology:

• Activation mechanism: The activation loop undergoes significant conformational changes upon phosphorylation. In the inactive conformation, the activation loop folds against the kinase domain. Phosphorylation at Tyr842 stabilizes the active conformation by:

  • Creating new electrostatic interactions that maintain the activation loop in an extended conformation

  • Enabling proper alignment of catalytic residues required for substrate binding and phosphate transfer

• Inhibitor binding interactions: Structural studies show that type II inhibitors like quizartinib stabilize an inactive conformation of FLT3 where:

  • The DFG motif adjacent to Tyr842 adopts an "out" conformation

  • The activation loop collapses toward the active site

  • Phosphorylation at Tyr842 would destabilize this inhibitor-induced conformation, explaining why dephosphorylation often precedes inhibitor binding

• Resistance mechanism: Mutations near Tyr842, including the Y842S mutation, confer resistance to inhibitors by:

  • Altering activation loop dynamics and stability

  • Enabling alternative conformations that prevent inhibitor binding while maintaining kinase activity

  • Creating multiple conformational states that reduce inhibitor efficacy

Molecular dynamics simulations demonstrate that FLT3 adopts a meta-stable state that is stabilized by inhibitor binding. When the inhibitor is removed, the activation loop collapses into the active site, and Phe830 in the DFG motif forms hydrophobic interactions with the gatekeeper residue Phe691 .

These structural insights have implications for antibody recognition of phosphorylated Tyr842, as conformational changes in the activation loop may affect epitope accessibility in different activation states.

What methodological approaches can detect transient versus sustained FLT3 Tyr842 phosphorylation?

Distinguishing between transient and sustained phosphorylation at Tyr842 requires specialized techniques that capture temporal dynamics of FLT3 activation:

Methodological implementation for detecting different phosphorylation dynamics:

• For transient phosphorylation (typical of wild-type FLT3 after ligand stimulation):

  • Precise timing of sample collection is critical (0, 2, 5, 10, 15, 30, 60 minutes)

  • Rapid sample processing with immediate lysis in buffer containing phosphatase inhibitors

  • Consider using HTRF assays which allow kinetic measurements

  • Example protocol: Stimulate cells with 500 ng/mL FLT3L, lyse at indicated timepoints, and process immediately for detection

• For sustained phosphorylation (characteristic of FLT3-ITD):

  • Extended timepoints (hours to days) may be necessary

  • Inhibitor washout experiments can reveal re-phosphorylation kinetics

  • Cell-based ELISA methods allow multiple samples/timepoints

  • Example approach: Treat FLT3-ITD expressing cells with inhibitor, wash out inhibitor, and monitor re-emergence of phosphorylation signal over 24-48 hours

The AlphaLISA SureFire Ultra Human Phospho-FLT3 (Tyr842) assay and HTRF Human Phospho-FLT3 (Tyr842) Detection Kit are specifically designed for quantitative detection of phospho-FLT3 (Tyr842) in cellular lysates, enabling high-throughput kinetic measurements with high sensitivity .

How does DEP-1 phosphatase regulation of FLT3 Tyr842 impact experimental design?

Research has identified DEP-1 (Density-Enhanced Phosphatase-1/PTPRJ) as a critical negative regulator of FLT3 phosphorylation, with particular relevance to Tyr842. This relationship has important implications for experimental design:

The interaction between DEP-1 and FLT3 creates several considerations for researchers:

• Variable baseline phosphorylation levels: DEP-1 expression levels vary across cell types, creating differential basal phosphorylation of FLT3 Tyr842. This variance necessitates careful selection of cellular models and controls .

• Substrate-trapping studies: DEP-1 has been shown to directly interact with FLT3 through "substrate trapping" experiments. DEP-1 mutants (D1205A or C1239S) associate with FLT3 by co-immunoprecipitation, indicating direct dephosphorylation of FLT3 by DEP-1 .

• Phosphorylation site specificity: DEP-1 demonstrates preferential dephosphorylation of specific FLT3 phosphorylation sites. Studies show Tyr589, Tyr591, and particularly Tyr842 involved in FLT3 ligand-mediated activation are hyperphosphorylated most significantly when DEP-1 is depleted .

Methodological approaches to account for DEP-1 regulation:

Example experimental design:

• Compare FLT3 Tyr842 phosphorylation in matched cell lines with DEP-1 knockdown versus control

• Include both unstimulated and FLT3L-stimulated conditions

• Assess downstream signaling (ERK, STAT5) in parallel

• Perform time-course analysis to capture both immediate and sustained phosphorylation changes

This approach provides comprehensive analysis of how DEP-1-mediated regulation affects FLT3 Tyr842 phosphorylation dynamics and downstream signaling consequences.

What are the most common technical challenges when detecting Phospho-FLT3 (Tyr842)?

Detecting Phospho-FLT3 (Tyr842) presents several technical challenges that can impact experimental reliability and interpretation:

For cell-based experiments specifically:

• Cell density effects: Overgrown cultures can show reduced FLT3 phosphorylation. Maintain cells at 60-80% confluence for consistent results .

• Stimulation conditions: For wild-type FLT3, stimulation with FLT3L requires optimization:

  • Concentration: 100-500 ng/mL is typically effective

  • Duration: 5-15 minutes is optimal for peak Tyr842 phosphorylation

  • Serum starvation: 4-16 hours before stimulation reduces background

• Sample handling: Phosphorylation status can change rapidly during processing:

  • Maintain samples at 4°C during preparation

  • Process samples immediately after collection

  • Consider using phosphorylation stabilizing reagents or direct lysis

A systematic approach to troubleshooting involves:

• Testing multiple antibody dilutions (1:500, 1:1000, 1:2000)

• Including both positive and negative controls (FLT3L-stimulated vs. unstimulated; inhibitor-treated samples)

• Validating with alternative methods (ELISA vs. Western blot)

How can multiplexed detection systems improve Phospho-FLT3 (Tyr842) analysis?

Multiplexed detection approaches offer significant advantages for comprehensive analysis of FLT3 phosphorylation and associated signaling events:

Conventional single-target methods like Western blotting limit the ability to simultaneously assess multiple phosphorylation sites and downstream effects. Modern multiplexed systems overcome these limitations:

• AlphaLISA/AlphaSCREEN technology:

  • Allows simultaneous detection of total FLT3 and phospho-FLT3 (Tyr842)

  • Sample-efficient: requires only 10 μL per assay point

  • Enables high-throughput screening of FLT3 inhibitors

  • Offers superior sensitivity compared to traditional ELISAs

• HTRF (Homogeneous Time-Resolved Fluorescence):

  • Based on FRET between europium cryptate (donor) and d2 (acceptor)

  • Allows ratio-metric detection that normalizes for sample variability

  • Compatible with both one-plate and two-plate assay formats

  • Provides both kinetic and endpoint measurement options

• Phospho-flow cytometry:

  • Enables single-cell resolution analysis

  • Can simultaneously measure multiple phosphorylation sites

  • Allows correlation of FLT3 phosphorylation with cellular phenotypes

  • Requires specific protocol optimizations for intracellular epitopes

Implementation strategies for multiplexed analysis:

Example experimental design using AlphaLISA SureFire Ultra technology:

• Prepare cells in 96-well format with various treatment conditions

• Process samples according to kit protocol (typically 30-minute lysis)

• Run parallel assays for Phospho-FLT3 (Tyr842), Phospho-STAT5, and Phospho-ERK

• Analyze data to establish quantitative relationships between receptor phosphorylation and downstream signaling

This multiplexed approach enables more comprehensive analysis of FLT3 signaling dynamics while minimizing experimental variability, sample requirements, and processing time.

What considerations should guide selection between polyclonal and monoclonal Phospho-FLT3 (Tyr842) antibodies?

The choice between polyclonal and monoclonal antibodies for Phospho-FLT3 (Tyr842) detection has significant implications for experimental outcomes:

Research applications guiding selection:

• For exploratory studies and initial characterization:

  • Polyclonal antibodies may offer advantages due to their ability to recognize multiple epitopes around the phosphorylation site

  • Examples include the rabbit polyclonal antibodies offered by multiple vendors

  • These antibodies are typically raised against synthetic phosphopeptides derived from the region surrounding Tyr842

• For standardized assays and reproducible protocols:

  • Monoclonal antibodies provide consistent performance

  • The Cell Signaling Technology Phospho-FLT3 (Tyr842) (10A8) Rabbit mAb represents this category

  • Particularly valuable for longitudinal studies requiring consistent reagent performance

• For specific detection methods:

  • Western blot: Both antibody types work well, with polyclonals sometimes offering greater sensitivity

  • Immunoprecipitation: Monoclonals may provide cleaner results with less background

  • Multiplexed assays: Specialized monoclonal pairs are often optimized for technologies like HTRF and AlphaLISA

Technical considerations for optimal performance:

• Verify the exact immunogen sequence used to generate the antibody

• Test multiple antibody concentrations to determine optimal working dilution

• Include appropriate controls to validate specificity for the phosphorylated form

• Consider the species cross-reactivity requirements (human, mouse, rat)

Most commercially available Phospho-FLT3 (Tyr842) antibodies are generated using synthetic phosphopeptides containing the sequence around Tyr842 (commonly S-N-Y(p)-V-V) conjugated to carrier proteins like KLH . This immunogen design is critical for ensuring phospho-specificity of the resulting antibodies.