pTau231 Mouse mAb: Protocol & Usage Guidelines

pTau231 Mouse mAb is a phosphorylation-specific monoclonal antibody designed to detect tau protein phosphorylated at threonine 231, serving as a critical research tool for studying Alzheimer's disease pathology and related tauopathies.

Product SKU

Ab349228

Antibody Type

Mouse Monoclonal Antibody (mAb)

Target Epitope

Tau protein phosphorylated at Threonine 231 (pThr231)

Species Reactivity

Human (cross-reactivity with other species should be experimentally verified)

Recommended Applications

Western Blotting (WB), Immunohistochemistry (IHC), Immunofluorescence (IF)

Storage Recommendation

Store at -20°C; avoid repeated freeze-thaw cycles

Background: Tau Phosphorylation at Thr231 in Neurodegeneration

Tau is a microtubule-associated protein predominantly expressed in neurons, where it stabilizes microtubules and supports axonal transport. In Alzheimer's disease (AD) and other tauopathies, tau undergoes abnormal hyperphosphorylation at multiple serine and threonine residues, leading to its dissociation from microtubules, aggregation into neurofibrillary tangles (NFTs), and ultimately neuronal dysfunction and death.

Phosphorylation at threonine 231 (pThr231) is among the earliest and most disease-relevant post-translational modifications of tau. Elevated pTau231 levels have been detected in cerebrospinal fluid (CSF) and plasma of AD patients, making this epitope a valuable research biomarker for early-stage pathology detection and therapeutic target investigation. The pTau231 Mouse mAb (Ab349228) provides high specificity for this phospho-epitope, enabling precise differentiation of pathologically phosphorylated tau from its non-phosphorylated form.

Recommended Protocols by Application

Western Blotting (WB)

1. Sample Preparation: Lyse cells or tissue in RIPA buffer supplemented with phosphatase inhibitors (e.g., sodium orthovanadate, sodium fluoride) and protease inhibitors. Phosphatase inhibitors are critical to preserve the pThr231 modification during extraction.
2. Gel Electrophoresis: Resolve 20–40 µg total protein per lane on a 10–12% SDS-PAGE gel. Tau isoforms typically migrate at 55–75 kDa depending on isoform and phosphorylation state.
3. Transfer: Transfer to PVDF or nitrocellulose membrane using standard wet or semi-dry transfer protocols.
4. Blocking: Block for 1 hour at room temperature in 5% BSA in TBST (BSA is preferred over non-fat dry milk for phospho-antibodies to minimize background).
5. Primary Antibody Incubation: Dilute pTau231 Mouse mAb (Ab349228) in 5% BSA/TBST at the manufacturer-recommended dilution. Incubate overnight at 4°C with gentle agitation.
6. Washing: Wash 3 × 10 minutes with TBST.
7. Secondary Antibody: Incubate with an HRP-conjugated anti-mouse secondary antibody for 1 hour at room temperature.
8. Detection: Develop using ECL substrate and image on a chemiluminescence system.

Immunohistochemistry (IHC)

1. Tissue Preparation: Use formalin-fixed, paraffin-embedded (FFPE) sections at 4–6 µm thickness, or fresh-frozen sections.
2. Antigen Retrieval: For FFPE sections, perform heat-induced epitope retrieval (HIER) using citrate buffer (pH 6.0) or EDTA buffer (pH 9.0). Verify optimal retrieval conditions empirically.
3. Blocking: Block endogenous peroxidase with 3% H₂O₂ for 10 minutes, then block non-specific binding with 5–10% normal serum matching secondary antibody host species.
4. Primary Antibody: Apply diluted pTau231 Mouse mAb (Ab349228) and incubate overnight at 4°C in a humidified chamber.
5. Detection: Use an appropriate HRP-polymer detection system with DAB chromogen. Counterstain with hematoxylin.

Immunofluorescence (IF)

1. Fixation: Fix cells or cryosections in 4% paraformaldehyde for 15–20 minutes; permeabilize with 0.1–0.3% Triton X-100 in PBS.
2. Blocking: Block with 5% normal serum or 1% BSA in PBS for 1 hour.
3. Primary Antibody: Incubate with pTau231 Mouse mAb (Ab349228) overnight at 4°C.
4. Secondary Antibody: Use an Alexa Fluor- or FITC-conjugated anti-mouse secondary antibody. Incubate 1 hour at room temperature, protected from light.
5. Counterstain & Mount: Counterstain nuclei with DAPI, mount with anti-fade mounting medium, and image by fluorescence microscopy.

Troubleshooting Common Issues

The following table summarizes frequent technical challenges encountered when using phospho-specific tau antibodies and recommended corrective actions:

• No signal or weak signal: Ensure phosphatase inhibitors were included during sample preparation. Increase primary antibody concentration or extend incubation time. Verify that the target protein is expressed and phosphorylated in your cell/tissue model.
• High background: Switch blocking agent from milk to 5% BSA. Increase washing stringency (more washes or add 0.1% Tween-20). Titrate primary and secondary antibody concentrations down.
• Non-specific bands (WB): Confirm specificity using a phosphatase-treated control sample. Non-phosphorylated tau should show reduced or absent signal. Use a positive control lysate from AD brain tissue or tau-overexpressing cells treated with phosphatase inhibitors.
• High background in IHC: Optimize antigen retrieval method and duration. Ensure complete endogenous peroxidase blocking. Use species-appropriate blocking serum.
• Signal loss after storage: Avoid repeated freeze-thaw cycles by preparing single-use aliquots. Store at -20°C long term; working aliquots may be stored at 4°C for up to one month.

Sample Preparation & Controls

Critical Sample Preparation Notes: Because pTau231 detection depends on the presence of a labile phosphate group, sample handling is critical. Always add phosphatase inhibitor cocktails immediately upon tissue disruption or cell lysis. For tissue samples, snap-freeze in liquid nitrogen immediately after harvest and store at -80°C until use. Avoid prolonged incubation at room temperature.

Positive Controls: Suitable positive controls include: (1) human AD brain tissue lysate or sections, which contain abundant pTau231-positive neurofibrillary tangles; (2) lysates from tau-transfected neuronal cell lines treated with okadaic acid or other phosphatase inhibitors to induce tau hyperphosphorylation.

Negative Controls: Include a no-primary-antibody control to assess non-specific secondary antibody binding. A phosphatase-treated sample control (incubate lysate with lambda protein phosphatase prior to electrophoresis) should abolish the pTau231 signal, confirming phospho-specificity of the antibody.

Loading Controls: For WB, probe a parallel membrane with total tau antibody (recognizing all tau isoforms regardless of phosphorylation) to confirm equal loading and verify the ratio of phosphorylated to total tau.

Storage, Handling & Safety

Storage Conditions: Store pTau231 Mouse mAb (Ab349228) at -20°C upon receipt. For long-term storage, aliquot to avoid repeated freeze-thaw cycles, which can degrade antibody performance. Working aliquots may be kept at 4°C for short-term use (up to 4 weeks). Do not store at room temperature for extended periods.

Handling: Centrifuge briefly before opening to collect contents. Handle on ice during experimental setup. Avoid contamination by using sterile pipette tips. Do not dilute the entire stock; prepare only the volume needed for each experiment.

Carrier Protein & Preservatives: Refer to the product datasheet for specific formulation details including carrier protein (e.g., BSA) concentration and preservative (e.g., sodium azide) content. If sodium azide is present, take appropriate precautions — it is toxic and can inhibit HRP-based detection systems; remove by dialysis if necessary.

Safety: Handle according to standard laboratory safety protocols for biological reagents. Refer to the Safety Data Sheet (SDS) provided by Aladdin Scientific for full hazard information.

Frequently asked questions

Why must phosphatase inhibitors be included during sample preparation for pTau231 detection?

The pTau231 epitope is defined by a phosphate group on threonine 231. Cellular and tissue phosphatases are highly active and will rapidly dephosphorylate this site upon cell lysis if inhibitors are not present, resulting in loss of signal. Including a phosphatase inhibitor cocktail (e.g., sodium orthovanadate, sodium fluoride, or commercial phosphatase inhibitor tablets) immediately during lysis preserves the phospho-epitope and ensures reliable detection.

Why is BSA recommended over non-fat dry milk for blocking when using pTau231 Mouse mAb?

Non-fat dry milk contains casein, which is itself a phosphoprotein. Using milk as a blocking agent can compete with and mask phospho-epitopes on the target protein, significantly reducing signal from phospho-specific antibodies. BSA is a non-phosphorylated protein and therefore does not interfere with phospho-epitope detection, making it the preferred blocking agent for pTau231 and all other phospho-specific antibodies.

How can I confirm that the signal detected by pTau231 Mouse mAb is truly phospho-specific and not a cross-reaction with non-phosphorylated tau?

The gold-standard approach is to include a phosphatase-treated control: incubate a parallel aliquot of your lysate with lambda protein phosphatase (and appropriate buffer/cofactors) prior to running the gel. This dephosphorylates tau at all sites, including Thr231. If the antibody is phospho-specific, the band should be absent or greatly diminished in the phosphatase-treated lane compared to the untreated lane. Additionally, running an untreated cell lysate alongside an AD brain positive control increases confidence in band identity.

What is the significance of pTau231 as a research biomarker compared to other tau phosphorylation sites?

Phosphorylation at Thr231 is considered one of the earliest tau modifications detectable in Alzheimer's disease pathology, appearing prior to the formation of mature neurofibrillary tangles. Research studies have shown that CSF and plasma pTau231 levels can distinguish AD from other neurodegenerative conditions and correlate with amyloid burden and cognitive decline. This early-stage relevance makes pTau231 particularly valuable for mechanistic studies of tau pathology initiation and progression.

Can pTau231 Mouse mAb be used in co-immunoprecipitation (Co-IP) experiments?

While the primary validated applications for Ab349228 are WB, IHC, and IF, use in Co-IP may be possible but requires empirical optimization. Key considerations include maintaining phosphatase inhibitors throughout the IP procedure, using non-denaturing lysis conditions to preserve protein-protein interactions, and verifying that the antibody binds its epitope under native conditions. Users should perform pilot experiments to assess feasibility before committing to large-scale Co-IP studies.

References

1. Malia TJ, et al. Epitope mapping and structural basis for the recognition of phosphorylated tau by the AT series of antibodies. Proteins. 2016.
2. Barthélemy NR, et al. A soluble phosphorylated tau signature links tau, amyloid and the evolution of stages of dominantly inherited Alzheimer's disease. Nat Med. 2020.
3. Karikari TK, et al. Blood phosphorylated tau 181 as a biomarker for Alzheimer's disease: a diagnostic performance and prediction modelling study using data from four prospective cohorts. Lancet Neurol. 2020.
4. Aladdin Scientific. pTau231 Mouse mAb, Product No. Ab349228. Product Datasheet.

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.