Western blotting is a widely used laboratory technique for detecting and analyzing specific proteins within a complex mixture. Imaging of western blots can be achieved by colorimetric and chemiluminescence techniques. The stripping procedure can be achieved using a stripping buffer containing sodium-dodecyl sulfate (SDS), β-mercaptoethanol, and washing buffer. There are a series of steps in detecting abundant protein in the stripped blot via chemiluminescence.

Stripping buffer disrupts the interaction between the antibody and the antigen and enables the removal of antibodies. Stripping strategies vary in intensity, from mild methods (mild stripping) that preserve both protein integrity and membrane structure to more aggressive approaches (harsh stripping) that effectively remove antibodies but may lead to protein loss.

Western blot stripping protocols

The general protocol for western blot stripping is simple, though different types of stripping may occur depending on the kind of stripping used, mild or harsh. This process enables sequential analysis of multiple proteins on a single blot, conserving samples and resources.  The standard procedure is as follows:

• Washing: After the initial western blot is run, the membrane should be adequately washed using washing buffer or PBS to remove excess detection reagents.
• Incubation: It should then be incubated in the stripping buffer at the recommended temperature, typically room temperature or 37°C. The incubation time may vary depending on the buffer, method, and the avidity of the antibodies, typically ranging from 10 minutes to 1 hour.
• Re-washing: After stripping, the membrane should be re-washed with washing buffer or PBS to remove any residual stripping buffer.
• Reprobing: The membrane is then ready to be reprobed with the next set of primary and secondary antibodies.

Protein retention considerations

High molecular weight proteins like titin or collagen, exceeding 300 kDa, are more susceptible to degradation or loss under harsh stripping conditions. Similarly, low-abundance proteins such as cytokines or transcription factors can be challenging to retain after multiple stripping cycles.

Researchers must find a balance between effectively removing antibodies and minimizing protein loss. When working with your proteins, it is essential to be cautious and choose stripping conditions that preserve the integrity of the protein. For example, milder stripping buffers may be used for sensitive or large proteins, while more robust methods can be reserved for less delicate proteins.

Researchers should also consider the number of stripping cycles, as repeated cycles can amplify the risk of protein loss, especially for proteins with weaker affinity to the membrane or those prone to degradation. Testing with control proteins or conducting preliminary trials can help identify the most effective and protein-preserving stripping conditions for specific experimental needs

It is recommended to optimize your stripping protocol for your specific proteins, starting with milder stripping conditions and assessing the removal of antibodies using your detection method of choice.

Mild vs. harsh stripping methods

• Mild stripping methods: Mild stripping is typically used when a gentle approach is required to remove antibodies from a membrane without disrupting protein integrity.

The membrane is incubated in the mild stripping buffer at room temperature for 5–10 minutes with agitation, ensuring the buffer fully covers the membrane. This step is followed by a brief rinse in buffers like PBS to remove any remaining stripping agents. This process is repeated once or twice, ensuring that the stripping is gentle enough to preserve protein structure for further probing or analysis. After stripping, the membrane is ready for blocking, making it suitable for reprobing with a different antibody.

This protocol often employs low-pH glycine or buffers that contain non-ionic detergents. Mild methods are not as harsh on proteins and membranes and can be very useful in reprobing with sensitive antibodies or detecting low-abundance proteins. They are, however, less effective in removing antibodies than the harsher methods.

• Harsh stripping methods: Harsh stripping is employed when a more aggressive removal of antibodies is required.

The harsh stripping buffer is typically warmed to 50°C before adding to the membrane, and the membrane is incubated with the stripping buffer for up to 45 minutes, with some agitation to ensure thorough stripping. After incubation, the solution is discarded, and the membrane is rinsed with water to remove any traces of the harsh chemicals, especially beta-mercaptoethanol, which can damage antibodies if not thoroughly washed away. The membrane is then washed in buffers such as PBS to ensure no residual buffer remains, and it is ready for reprobing or blocking.

These methods most often use SDS or heat (eg, 50-70°C) in combination with detergents. Harsh stripping methods are more efficient at removing antibodies but may result in increased loss or denaturation of proteins. This method is often used in cases where multiple stripping and reprobing cycles are needed or when the antibodies used have high affinities.

Optimization and best practices for stripping

Optimizing the stripping conditions is essential for the optimal removal of antibodies and minimizing the background signal. Researchers can optimize the following aspects of the protocol:

• Buffer composition: The pH, salt concentration, and detergent levels can be fine-tuned to achieve optimal stripping performance.
• Incubation time: Longer incubation times may increase the efficiency of antibody removal but can also lead to increased protein loss. A balance should be found to minimize both. High-affinity antibodies or saturated blots may require longer incubation times.
• Temperature: Incubation temperature is significant in stripping efficiency. For aggressive stripping processes, higher temperatures may be required to break the bonds of antibodies, but they also can degrade proteins.
• Control testing: After any stripping procedure, the blot can be tested to confirm the complete removal of detection reagents by incubating it with a secondary antibody alone and verifying the absence of any signal.

Troubleshooting

• Inadequate stripping of antibodies

  • Cause: Strong antibody-antigen interactions may prevent complete removal.
  • Solution: Use a more stringent stripping buffer, such as one containing higher concentrations of SDS or a reducing agent like 2-mercaptoethanol. Consider increasing the incubation temperature (eg, 50°C) and time to enhance stripping efficiency.

• Loss of antigen

  • Cause: Stripping conditions can lead to antigen loss from the membrane.
  • Solution: Start with mild stripping conditions to minimize antigen loss. If necessary, gradually increase the stringency only if complete antibody removal is not achieved.

• High background signal

  • Cause: Inadequate blocking or nonspecific binding of antibodies.
  • Solution: Optimize blocking conditions by increasing the concentration of the blocking agent or using a different blocking buffer. Ensure thorough washing between steps to reduce background noise.

• Weak or no signal after reprobing

  • Cause: Insufficient binding of antibodies or degradation of antigen during stripping.
  • Solution: Ensure that the primary antibodies used for reprobing are still active. Perform a dot blot to test antibody activity before use. Additionally, verify that sufficient antigen is present on the membrane using a stain such as Ponceau S solution.

• Membrane damage

  • Cause: Improper handling or drying out of membranes can affect results.
  • Solution: Handle membranes carefully, ensuring they remain wet throughout the process. Use clean tools and gloves to avoid contamination.

• Excessively strong signal from the chemiluminescent substrate

  • Cause: Overexposure or high substrate concentration can lead to saturation.
  • Solution: Reduce substrate concentration or shorten exposure time to film after incubation with substrate.

Types of stripping buffers and solutions

The formulation of the stripping buffer in western blotting plays a vital role in antibody removal while preserving protein integrity.

Mild stripping buffer: A mild stripping buffer typically consists of 15 g glycine, 1 g SDS, and 10 mL of a non-ionic detergent and surfactant (eg, tween 20), all diluted in 800 mL of distilled water.

The pH is adjusted to 2.2 using 0.5 M Tris-HCl (pH 6.8), and the solution is brought to a final volume of 1 L with deionized water. To enhance stripping efficiency, 0.8 mL of beta-mercaptoethanol is added under a fume hood. This mild buffer is typically the first choice, as it effectively removes antibodies while preserving the proteins on the membrane.

Harsh stripping buffer: The harsh stripping buffer consists of 20 mL of 10% SDS, 12.5 mL of Tris-HCl (pH 6.8), and 67.5 mL of distilled water, with the pH adjusted to 2.2 before bringing the final volume to 1 L.

This stronger formulation is more effective at removing antibodies and is typically used when milder methods do not adequately strip the membrane of antibodies.

Repeated stripping and reprobing, especially with harsh buffers, can reduce signal intensity and increase background noise, making it necessary for researchers to carefully assess the impact of each stripping cycle.

Commercial vs. in-house stripping buffers

Stripping buffers for western blotting are available commercially or can be prepared in the laboratory. Western blot stripping buffers are designed commercially for optimal performance with minimal protein loss, offering convenience and reliability, and a defined protocol.

These buffers are designed to strip antibodies quickly, often within 15 to 30 minutes, without damaging the target antigens. They come with the convenience of ready-to-use formulations and are rigorously tested, making them ideal for time-sensitive experiments or when consistent results are critical.

However, in-house buffers can be tailored to suit the specific experimental needs, providing more flexibility. These buffers are typically made from components like glycine, SDS, and tris hydrochloride and are often cost-effective for routine use.

Factors to consider in choosing a stripping buffer

• pH: A low pH, usually around 2, is commonly used for antibody removal. However, the pH of the buffer must be balanced to avoid damaging proteins or the membrane.
• Salt concentration: Buffers with high salt concentrations help in the efficient removal of antibodies but might compromise protein retention.
• Compatibility between protein/antibody: The type of protein and antibody being investigated determines the stripping buffer. For example, phospho-specific antibodies should be stripped with mild buffer conditions to prevent loss of signal.
• Stripping time and temperature: Stripping duration and temperature must be optimized to balance antibody removal and protein preservation. Milder conditions with shorter times and lower temperatures are best for sensitive proteins to avoid damage.
• Membrane type: Polyvinylidene fluoride (PVDF) membranes retain proteins better than nitrocellulose, making them suitable for multiple reprobing cycles. Nitrocellulose is typically used for single-use applications due to reduced protein retention after repeated stripping.
• Detergent type and concentration: Non-ionic detergents (eg, tween-20) are gentler on proteins and preserve integrity, while ionic detergents (eg, SDS) more effectively remove antibodies but can cause greater protein loss, especially for sensitive proteins.
• Presence of reducing agents: Reducing agents like 2-mercaptoethanol break disulfide bonds, aiding antibody removal but potentially altering protein structure. This requires careful consideration, especially for proteins with disulfide linkages, which may affect analysis.
• Proper containers: Using shallow trays or containers that allow for even distribution of the stripping buffer can enhance the efficiency of antibody removal.
• Temperature-controlled incubators: Utilizing incubators that maintain precise temperatures can prevent fluctuations that might affect protein stability during stripping.
• Experimental goals: Stripping buffer selection should align with experimental goals. For quantitative analysis, milder conditions are preferred to preserve protein integrity, while more aggressive methods may be acceptable for qualitative studies.

PVDF vs. nitrocellulose membranes

The two most commonly used membrane types in western blotting are PVDF and nitrocellulose. Each membrane type presents unique advantages and challenges that must be considered to ensure successful antibody removal without compromising the integrity of the proteins. Both can be stripped, but the methodologies and efficiency of stripping between these two materials differ.

• PVDF: PVDF membranes are highly effective for capturing low-abundance proteins due to their high protein-binding capacity, typically ranging from 170 to 200 µg/cm². Their hydrophobic nature enhances protein retention compared to nitrocellulose membranes. Additionally, PVDF membranes can endure harsher stripping conditions, making them suitable for removing high-affinity antibodies through more aggressive methods.

However, this durability may also result in increased protein carryover after stripping, requiring careful optimization of stripping conditions.

• Nitrocellulose membrane: Nitrocellulose membranes are usually easier to strip but liable to undergo protein loss or membrane destruction, specifically under harsh stripping processes.

Nitrocellulose is often favored for its simplicity and effectiveness in binding proteins. However, it is more fragile than PVDF and can suffer from structural integrity issues under rigorous stripping conditions.

The risk of losing significant amounts of protein during stripping is higher with nitrocellulose membranes, especially when subjected to aggressive buffers or prolonged incubation times.

Membrane integrity preservation

Maintaining membrane integrity during the stripping process is essential for successful reprobing. Using moderate buffers with careful control of incubation time and temperature while avoiding harsh reagents can help preserve this integrity. This ensures that the protein structure remains intact for subsequent antibody probing.

Benefits of stripping

• Cost and time efficiency: Reusing a stripped and reprobed membrane eliminates the need for multiple gels and separate membranes for each detection, enhancing research efficiency and conserving resources.
• Multiple protein detection: Detecting multiple proteins sequentially from a single sample enhances experimental throughput while maximizing the use of a limited sample source.
• Sample conservation: The reuse of membranes means preserving experimental samples, mainly when dealing with rare or expensive materials.
• Flexibility in antibody use: Stripping provides the flexibility to use different antibodies on the same membrane, enabling the correction of errors by replacing an incorrect antibody without discarding the membrane. This process minimizes resource wastage and reduces the need to repeat the entire experiment, saving both time and effort.
• Enhanced quantitative analysis: Stripping and reprobing enable researchers to compare the relative abundance of proteins under the same experimental conditions, improving confidence in the results. While absolute quantification may be limited, this technique still allows for robust quantitative analysis, making it a valuable tool for studying protein expression variations.
• Versatility in experimental design: Stripping allows for dynamic experimental setups by enabling the assessment of loading controls after initial protein detection. This ensures more accurate normalization of results, contributing to reliable and reproducible findings.
• Multiple reprobing opportunities: Membranes can undergo multiple cycles of stripping and reprobing - up to 10 times in certain instances - allowing researchers to extend their analyses without requiring new samples. However, it is generally recommended to limit stripping to no more than three cycles and to optimize the number of reuse cycles based on the specific requirements of your experiments.

Specific considerations

Phospho-specific antibodies and low-abundance proteins require specialized stripping practices.

Phosphorylated proteins require gentle stripping to preserve their phosphorylation signal. Harsh stripping conditions may lead to dephosphorylation, making them undetectable with phospho-specific antibodies during subsequent reprobing.

In such circumstances, milder stripping buffers are frequently utilized to keep the delicate changes intact while removing the primary antibody.

These buffers often have a lower pH and contain milder reducing agents or detergents than traditional harsher stripping solutions. For example, protocols suggest using Tris-HCl buffers at a slightly acidic pH (6.8) with 2% SDS and β-mercaptoethanol, combined with incubation at 50°C for 30 minutes under agitation to effectively remove bound antibodies while preserving membrane integrity.

When working with phospho-specific antibodies, it is also essential to optimize the blocking and washing steps. Using phosphate-free buffers, such as tris-buffered saline (TBS), during blocking and washing can minimize background signals and improve the specificity of phospho-antibody detection. Furthermore, adding phosphatase inhibitors like sodium fluoride (NaF) and sodium orthovanadate (Na3VO4) to the buffers can help prevent the dephosphorylation of proteins during the stripping process.

Similarly, when working with low-abundance proteins, it is vital to adopt moderate stripping procedures to preserve protein integrity and prevent additional loss, allowing for identification during reprobing. Repeated stripping and reprobing can gradually reduce the amount of target protein on the membrane, potentially leading to weaker signals or even false-negative results.

To minimize antigen loss, transfer conditions should be optimized to ensure efficient protein binding to the membrane. PVDF membranes are generally preferred over nitrocellulose due to their higher protein-binding capacity and greater durability during stripping.

When assessing low-abundance proteins, it is recommended to probe these proteins first before stripping. Minimizing incubation time with the stripping buffer and lowering the temperature can help mitigate protein loss.

Additionally, the membrane must be thoroughly washed after stripping to remove any residual stripping buffer, which could interfere with subsequent antibody binding.

Alternative stripping methods, such as enzymatic stripping using enzymes like papain or trypsin, have been explored to selectively cleave antibodies without affecting the target protein. However, these methods require careful optimization to avoid excessive protein degradation. The selection of a stripping procedure should be tailored to the specific target proteins, antibodies, and experimental objectives, requiring careful evaluation and optimization for each application.

Detection methods after stripping

Detection approaches, such as chemiluminescence or fluorescence detection, can significantly improve the sensitivity of western blotting after stripping. Chemiluminescence is a widely used method that relies on light emission resulting from enzyme-substrate reactions.

Horseradish peroxidase (HRP)-conjugated antibodies are commonly used to catalyze the oxidation of substrates like luminol, generating a transient light signal for protein detection. This highly sensitive method enables the detection of proteins at femtogram levels, making it well-suited for identifying low-abundance proteins.

A major advantage of chemiluminescence is its broad dynamic range, enabling accurate quantification across diverse protein concentrations. Multiple exposures can be captured to optimize the signal-to-noise ratio, allowing researchers to fine-tune their results while preserving membrane integrity. Furthermore, since light emission occurs only during the enzymatic reaction, the same blot can be reprobed with different antibodies after stripping, enhancing resource efficiency.

In contrast, fluorescence provides a more versatile detection method. By attaching fluorophores to secondary antibodies, multiple proteins can be visualized simultaneously on the same blot. Each fluorophore emits light at distinct wavelengths when excited by specific light sources (UV, visible, or infrared), allowing for accurate quantification and comparison of different target proteins. This capability eliminates the need for repeated stripping and probing, which not only saves time but also reduces the potential loss of target proteins during multiple rounds of processing.

The use of fluorescence-based approaches also offers greater stability over time compared to chemiluminescence. Fluorescent signals remain consistent and linear throughout the imaging process, improving accuracy in quantification. Advanced imaging systems equipped with appropriate filters and high-intensity LED lighting facilitate rapid imaging of multiple fluorescent proteins. Software tools can automatically adjust imaging conditions based on the specific dyes used, simplifying and enhancing the analysis.

In summary, both chemiluminescence and fluorescence can be used to detect proteins post-stripping in western blotting. While chemiluminescence excels in sensitivity and simplicity for single protein detection, fluorescence enables multiplexing capabilities that enhance experimental efficiency and data richness. The choice between these methods ultimately depends on your specific research needs and goals.

FAQs

Can you use the same stripping buffer for both nitrocellulose and PVDF membranes?

Yes, the same stripping buffer can frequently be used on both nitrocellulose and PVDF membranes. However, there are some limitations. Nitrocellulose membranes are more sensitive and may be harmed by strong stripping conditions. Hence, a gentler stripping buffer is advised. PVDF membranes are more robust and can withstand harsher temperatures.

However, repeated stripping cycles may cause protein carryover. As a result, changing the buffer strength according to membrane type and stripping frequency is critical for preserving protein integrity and membrane quality. To optimize results for both membrane types, always test the buffer conditions before applying them on a broad scale.

How often can a western blot membrane be stripped and reprobed?

Depending on the membrane type, stripping circumstances, and target proteins, the western blot membrane can be stripped and reprobed several times. For example,

• Nitrocellulose membranes are more delicate and may only withstand 1-2 stripping cycles before their integrity is destroyed, particularly in severe environments.
• PVDF membranes are more durable and can resist numerous stripping and reprobing cycles, generally up to 3 to 5 times without sustaining substantial damage.