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Analytical technique used in molecular biology

The western blot (sometimes called the protein immunoblot), or western blotting, is a widely used analytical technique in molecular biology and immunogenetics to detect specific proteins in a sample of tissue homogenate or extract. Besides detecting the proteins, this technique is also utilized to visualize, distinguish, and quantify the different proteins in a complicated protein combination. Western blot technique uses three elements to achieve its task of separating a specific protein from a complex: separation by size, transfer of protein to a solid support, and marking target protein using a primary and secondary antibody to visualize. A secondary antibody is added which recognizes and binds to the primary antibody. The secondary antibody is visualized through various methods such as staining, immunofluorescence, and radioactivity, allowing indirect detection of the specific target protein. The confirmatory HIV test employs a western blot to detect anti HIV antibody in a human serum sample. Proteins from known HIV infected cells are separated and blotted on a membrane as above. Then, the serum to be tested is applied in the primary antibody incubation step; free antibody is washed away, and a secondary anti human antibody linked to an enzyme signal is added. The stained bands then indicate the proteins to which the patient's serum contains antibody. A western blot is also used as the definitive test for variant Creutzfeldt–Jakob disease, a type of prion disease linked to the consumption of contaminated beef from cattle with bovine spongiform encephalopathy (BSE, commonly referred to as 'mad cow disease'). Another application is in the diagnosis of tularemia. An evaluation of the western blot's ability to detect antibodies against F. tularensis revealed that its sensitivity is almost 100% and the specificity is 99.6%. Some forms of Lyme disease testing employ western blotting. A western blot can also be used as a confirmatory test for Hepatitis B infection and HSV 2 (Herpes Type 2) infection. In veterinary medicine, a western blot is sometimes used to confirm FIV+ status in cats. Further applications of the western blot technique include its use by the World Anti Doping Agency (WADA). Blood doping is the misuse of certain techniques and/or substances to increase one's red blood cell mass, which allows the body to transport more oxygen to muscles and therefore increase stamina and performance. There are three widely known substances or methods used for blood doping, namely, erythropoietin (EPO), synthetic oxygen carriers and blood transfusions. Each is prohibited under WADA's List of Prohibited Substances and Methods. The western blot technique was used during the 2014 FIFA World Cup in the anti doping campaign for that event. In total, over 1000 samples were collected and analysed by Reichel, et al. With the adoption of the horizontal SAR PAGE in combination with the precast film supported Velum SAR gels the discriminatory capacity of micro dose application of rEPO was significantly enhanced. In addition to the application of western blot in scientific research, it is also utilized in clinical research areas. Since it can be applied to the direct protein identification process, western blot is regarded as a powerful diagnostic tool that is frequently used in the clinic setting. WB and protein detection techniques can be used to find disease biomarkers like specific proteins or antibodies. It is thought to be a viable method for identifying particular proteins during the diagnosis of diseases like cancer, autoimmune disease, and prion disorders. The detection of several biomarkers used in the diagnosis of neurological and oncological illnesses by Western blotting is a common procedure. For instance, it is widely believed that the advent of multidrug resistance (MDR) has made effective cancer therapy extremely challenging. Therefore, early, accurate, and sensitive MDR mechanism discovery is essential, as is the search for more effective chemotherapeutic approaches for application in clinical settings. The expression of MDR1/P glycoprotein in the P388/ADR, P388 and HCT 15 cell lines is examined using the WB technique. WB has also identified MRP1 levels. On the other hand, as western blot has the potential to distinguish different protein isoforms, it may be used to diagnose prion and protein isoform related diseases, such as cancer. For instance, the western blotting analysis of the isoform pattern of 14 3 3 proteins in cerebral fluid can identify Creutzfeldt Jakob disease. Also, Farmers lung disease is a pulmonary condition brought on by breathing antigenic particles, and studies have indicated that western blot may be a useful option for identifying immunoreactive proteins related to farmers lung disease. western blot is used to assess the levels of FSTL1 protein expression in individuals with knee osteoarthritis, which serves as a potential biomarker of articular damage. Additionally, it is used to identify proteins in synovial fluid and serum, enabling the diagnosis of osteoarthritis and rheumatoid arthritis clinical symptoms. It is used to assess the levels of FSTL1 protein expression in individuals with knee osteoarthritis, which serves as a potential biomarker of articular damage.

For medication development, the identification of therapeutic targets, and biological research, it is essential to comprehend where proteins are located within a cell. The subcellular locations of proteins inside the cell and their functions are closely related. The relationship between protein function and localization suggests that when proteins move, their functions may change or acquire new characteristics. A protein's subcellular placement can be determined using a variety of methods. Numerous efficient and reliable computational tools and strategies have been created and used to identify protein subcellular localization. With the aid of subcellular fractionation methods, WB continues to be an important fundamental method for the investigation and comprehension of protein localization.

The western blot method is composed of gel electrophoresis to separate native proteins by 3 D structure or denatured proteins by the length of the polypeptide, followed by an electrophoretic transfer onto a membrane (mostly PVDF or nitrocellulose) and an immunostaining procedure to visualize a certain protein on the blot membrane. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS PAGE) is generally used for the denaturing electrophoretic separation of proteins. Sodium dodecyl sulfate (SDS) is generally used as a buffer (as well as in the gel) in order to give all proteins present a uniform negative charge, since proteins can be positively, negatively, or neutrally charged. Prior to electrophoresis, protein samples are often boiled to denature the proteins present. This ensures that proteins are separated based on size and prevents proteases (enzymes that break down proteins) from degrading samples. Following electrophoretic separation, the proteins are transferred to a membrane (typically nitrocellulose or PVDF). The membrane is often then stained with Ponceau S in order to visualize the proteins on the blot and ensure a proper transfer occurred. Next the proteins are blocked with milk (or other blocking agents) to prevent non specific antibody binding, and then stained with antibodies specific to the target protein. Besides that, the ideal lysis buffer is needed to acquire substantial amounts of target protein content because the buffer is leading the process of protein solubilization and preventing protein degradation. After completing the sample preparation, the protein content is ready to be separated by the utilization of gel electrophoresis. At least seven different approaches for total protein staining have been described for western blot normalization: Ponceau S, stain free techniques, Sypro Ruby, Epicocconone, Coomassie R 350, Amido Black, and Cy5.

Since the membrane has been chosen for its ability to bind protein and as both antibodies and the target are proteins, steps must be taken to prevent the interactions between the membrane and the antibody used for detection of the target protein. Blocking of non specific binding is achieved by placing the membrane in a dilute solution of protein – typically 3–5% bovine serum albumin (BSA) or non fat dry milk (both are inexpensive) in tris buffered saline (TBS) or I Block, with a minute percentage (0.1%) of detergent such as Tween 20 or Triton X 100. Although non fat dry milk is preferred due to its availability, an appropriate blocking solution is needed as not all proteins in milk are compatible with all the detection bands. or epicocconone. This practice ensures correction for the amount of total protein on the membrane in case of errors or incomplete transfers. (see western blot normalization)

The colorimetric detection method depends on incubation of the western blot with a substrate that reacts with the reporter enzyme (such as peroxidase) that is bound to the secondary antibody. This converts the soluble dye into an insoluble form of a different colour that precipitates next to the enzyme and thereby stains the membrane. Development of the blot is then stopped by washing away the soluble dye. Protein levels are evaluated through densitometry (how intense the stain is) or spectrophotometry.

Chemiluminescent detection methods depend on incubation of the western blot with a substrate that will luminesce when exposed to the reporter on the secondary antibody. The light is then detected by CCD cameras which capture a digital image of the western blot or photographic film. The use of film for western blot detection is slowly disappearing because of non linearity of the image (non accurate quantification). The image is analysed by densitometry, which evaluates the relative amount of protein staining and quantifies the results in terms of optical density. Newer software allows further data analysis such as molecular weight analysis if appropriate standards are used.

Radioactive labels do not require enzyme substrates, but rather, allow the placement of medical X ray film directly against the western blot, which develops as it is exposed to the label and creates dark regions which correspond to the protein bands of interest (see image above). The importance of radioactive detections methods is declining due to its hazardous radiation , because it is very expensive, health and safety risks are high, and ECL (enhanced chemiluminescence) provides a useful alternative.

The fluorescently labeled probe is excited by light and the emission of the excitation is then detected by a photosensor such as a CCD camera equipped with appropriate emission filters which captures a digital image of the western blot and allows further data analysis such as molecular weight analysis and a quantitative western blot analysis. Fluorescence is considered to be one of the best methods for quantification but is less sensitive than chemiluminescence.

One major difference between nitrocellulose and PVDF membranes relates to the ability of each to support "stripping" antibodies off and reusing the membrane for subsequent antibody probes. While there are well established protocols available for stripping nitrocellulose membranes, the sturdier PVDF allows for easier stripping, and for more reuse before background noise limits experiments. Another difference is that, unlike nitrocellulose, PVDF must be soaked in 95% ethanol, isopropanol or methanol before use. PVDF membranes also tend to be thicker and more resistant to damage during use.

In order to ensure that the results of Western blots are reproducible, it is important to report the various parameters mentioned above, including specimen preparation, the concentration of protein used for loading, the percentage of gel and running condition, various transfer methods, attempting to block conditions, the concentration of antibodies, and identification and quantitative determination methods. Many of the articles that have been published don't cover all of these variables. Hence, it is crucial to describe different experimental circumstances or parameters in order to increase the repeatability and precision of WB. To increase WB repeatability, a minimum reporting criteria is thus required.

Two dimensional SDS PAGE uses the principles and techniques outlined above. 2 D SDS PAGE, as the name suggests, involves the migration of polypeptides in 2 dimensions. For example, in the first dimension, polypeptides are separated according to isoelectric point, while in the second dimension, polypeptides are separated according to their molecular weight. The isoelectric point of a given protein is determined by the relative number of positively (e. g. lysine, arginine) and negatively (e. g. glutamate, aspartate) charged amino acids, with negatively charged amino acids contributing to a low isoelectric point and positively charged amino acids contributing to a high isoelectric point. Samples could also be separated first under nonreducing conditions using SDS PAGE, and under reducing conditions in the second dimension, which breaks apart disulfide bonds that hold subunits together. SDS PAGE might also be coupled with urea PAGE for a 2 dimensional gel. In principle, this method allows for the separation of all cellular proteins on a single large gel. A major advantage of this method is that it often distinguishes between different isoforms of a particular protein – e. g. a protein that has been phosphorylated (by addition of a negatively charged group). Proteins that have been separated can be cut out of the gel and then analysed by mass spectrometry, which identifies their molecular weight.

There may be a weak or absent signal in the band for a number of reasons related to the amount of antibody and antigen used. This problem might be resolved by using the ideal antigen and antibody concentrations and dilutions specified in the supplier's data sheet. Increasing the exposition period in the detection system's software can address weak bands caused by lower sample and antibody concentrations.

The single cell WB (scWB), in addition to the conventional WB, is regarded as a breakthrough in the study of protein subcellular localization and in the evaluation of single cell protein. When measuring the levels and conditions of protein expression from one cell to the next, it is used. With the aid of single cell WB, the western blot's selectivity and specificity were expanded to include single cell protein analysis. The limitations of antibody accuracy and sensitivity are overcome by this technique. Furthermore, because of its versatility, it may be utilized to measure numerous target proteins concurrently from different cell lines and single cells.

The development known as quantifiable fluorescence based WB (QFWB) enables researchers to carry out comparative expression analysis with better sensitivity and precision than ever before. Quantifiable in QFWB refers to genuinely quantitative with increased sensitivity. This method is employed to identify the minute expression variations between various samples. With the aid of a secondary antibody that has been fluorescently tagged, QFWB produces a linear detection profile. Modern QFWB techniques enable simultaneous dual labeling and are more sensitive to identify minute variations.

Quantitative computerized western blot analyses the reactivity of individual antibodies to specific antigens to identify immunodominant and immunorecessive determinants using two measures, such as net band intensity and total lane intensity of the WB. The creation of quick serodiagnostic tests and efficient vaccines is made possible by the identification of certain immunodominant antigens. The study looks for serological markers for the early diagnosis of cancer, viral, and autoimmune illnesses using quantitative computerized western blot.

It is a technique that combines conventional SDS PAGE protein resolution with a bead based microarray platform that immobilizes proteins on microspheres. This combination of protein separation, uniformity, and sensitivity allows for the quick quantification of a number of different protein targets as well as their changes. The benefit of DigiWest is that western blot is carried out using beads based microarrays, allowing for the simultaneous detection and analysis of hundreds of distinct proteins and their changes using a wide range of varied antibodies.

In order to detect many proteins on a single microfluidic chip, microfluidic western blot is carried out using a number of processes, including sample enrichment, protein size, deposition of protein, and then in situ antibody probing. A photoreactive (UV light) polyacrylamide gel and a photopatternable (blue light) surface are the foundation of this multistep procedure. Due to improvements in analytical performance, WB may now be completed in 10–60 minutes while maintaining high sensitivity detection limits (50 picomoles) and multiplexed component detection levels (femtograms). Therefore, by fusing superb specificity and the high throughput benefits of multiplexing, WB creates a cornerstone for quick proteomics.

A enhanced WB technique called multistrip WB is based on the simultaneous transfer of different proteins from a number of polyacrylamide gel strips to a single polyvinylidene difluoride or nitrocellulose membrane. Multistrip WB allows for the simultaneous monitoring of up to nine separate proteins from the same loading of the sample and up to a tenfold increase in data output for a single WB cycle. Systems biology, cell signaling research, and biomedical diagnostics would all benefit from using this technique.

The capillary and microchip electrophoresis based western blot was created to reduce the quantity of protein samples and the amount of time it takes to execute western blot. It contributes to the more sensitive and accurate measurement of various protein targets from any single cell lysate carried out on a microchip. 400 nanograms of cell lysate are all that is needed to identify and quantify eleven different proteins.