There are several ways to assess the presence or concentration of a protein. In the second module, you used a colorimetric Coomassie-based assay to measure the concentration of protein expressed by your bacteria. Because you were purifying a His-tagged protein from bacteria induced primarily to express said protein, you could assume that the protein concentration that you measured was primarily inverse pericam. In contrast, today you are trying to measure the concentration of a specific protein that is only one among many in a complex mixture.
A great way to identify proteins is to exploit antibodies – also called immunoglobulins – whether in a Western blot or by ELISA (enzyme-linked immunosorbent assay). In native physiological settings (such as your own body), antibodies are secreted by B cells in response to pathogens. A given antibody is highly specific (KD ~ nM) for its binding partner, called an antigen, and the entire antibody population for a given person is incredibly diverse (>107 unique antibodies). Diversity is maintained by recombination processes at the DNA level, and specificity entailed by protein structure.
Antibody proteins comprise constant (C) and variable (V) regions, on both their heavy and light chains. The C regions determine antibody effector functions, such as antibody-dependent killing of infected cells. The three hypervariable portions of the V region together make up the antigen-recognition site. Only a small portion of an antigen, called an epitope, is recognized by its cognate antibody. This ~10 amino acid region may be linear, or it may be made up of linearly distant regions and thus recognized only when the antigen is in its native conformation. For example, conformation-dependent antibodies are useful for distinguishing different collagen types.
Schematics of indirect and sandwich ELISA.
Triangles indicate the protein of interest, and * indicates a conjugated enzyme for later detection. (Blocking step not shown.)
Antibodies can be raised in animals, special cell lines, and even genetically engineered. Polyclonal antibodies (pools of antibodies that recognize distinct epitopes on the same antigen) can be obtained from animal serum. The animal is infected with the antigen of interest in the presence of a costimulatory signal, usually multiple times, and then bled. In this case, a large fraction of the antibodies obtained will not be against the antigen of interest. In contrast, monoclonal antibodies can be made both highly specific and pure. In this process, normal antibody-producing B cells are fused with immortalized B cells derived from myelomas, and the two cell types are fused by chemical treatment with a limited efficiency. To select only heterogeneously fused cells, the cultures are maintained in medium in which myeloma cells alone cannot survive (often HAT medium). Normal B cells will naturally die out over time with no intervention, so ultimately only the fused cells, called hybridomas, remain. Genetic engineering can be used to combine a human antibody ‘frame’ (all of the C and part of the V region) with an antigen-recognition site discovered in another species (e.g., murine). When antibodies are used as therapeutics, this decreases the possibility that the patient’s body will treat them as foreign, compared to an antibody produced from only mouse genes. Normally, injecting an antibody from species X into an animal of species Y will cause production of anti-X antibodies, called secondary antibodies. These can be very useful in technical assays, as you will see below.
Today you will use antibodies against collagen in an indirect ELISA assay. Both indirect and sandwich ELISA are shown in the figure at right – can you see why sandwich ELISA might be the superior assay with respect to sensitivity and specificity? In indirect ELISA, the first step is to bind protein extracts, obtained from your two different culture conditions, to well plates. Next you will add a primary antibody that recognizes a particular antigen – namely, epitopes on collagen I or collagen II – to the relevant wells. (Actually, before adding the antibody you will "block" the plate with milk protein to prevent non-specific binding of the antibody.) Next, any excess antibody must be washed away with a mild detergent. Finally, a secondary antibody – namely one that recognizes the primary antibody – must be added. The secondary antibody is conjugated to alkaline phosphatase, which will undergo a colorimetric reaction in the presence of its substrate. Thus, the relative quantity of protein can be assessed by absorbance spectroscopy following substrate addition. To quantify the absolute amount of protein, you will run dilutions of a collagen standard in parallel with your culture samples. During your ELISA incubation steps, you can run the cDNAs you prepared last time out on a gel, and begin some analysis.
Reference: Abbas, A.K. & Lichtman, A.H. (2005). Cellular and Molecular Immunology (5th ed.). Philedelphia: Elsevier Saunders.
Changes for 2011: focus on setting up qPCR plate --> will be delicate operation; but still need to start ELISA (wait for DMB till next time)
Part 1: Day 1 of ELISA
Optional: Measure whole protein concentration
- If you wish, you can test your protein extracts using the Bradford assay from Module 2, then normalize the amount of total protein that you add per sample.
- Because you are ultimately interested in the ratio of collagen II to collagen I amounts, this step is not strictly necessary. However, it will give you more information than you otherwise have.
We will run this assay in a 96-well microtiter plate, as we did for the fluorescence titration curves in Module 1. In ELISA, we will be testing for absorbance at a particular wavelength, rather than emission.
- Label one 96-well plate as your collagen I assay, and one as your collagen II assay. (Why might we want to use separate plates?)
- The first step in indirect ELISA is to adsorb all your samples to the wells. You will also need to prepare standard samples in the same plate, which get treated just the same as your test samples. These standards will be used as a reference for protein concentration. Both standards and unknown samples will be run in duplicate (see figure at right).
Suggested ELISA plan.
This plan can be used for both your collagen I and your collagen II plate. In each case, columns 1 and 2 are duplicates of the collagen standards, and column 3 contains your experimental samples and a few wells (labeled BLANK) to measure background.
- You will be given 250 μL aliquots of collagens I and II at 10 μg/mL each. Prepare your standards as follows:
- Ultimately, you want to pipet 50 μL per well of each standard concentration, two wells per standard.
- Option 1:
- Prepare 7 eppendorf tubes with 120 μL of PBS each.
- Add 120 μL of the 10 μg/mL collagen to the first eppendorf tube, and vortex.
- Now take 120 μL of that standard (now 5 μg/mL) and add it to the next eppendorf tube.
- When you are all done or as you go, pipet the standards into the appropriate wells.
- Option 2:
- Pipet 50 μL of PBS into wells 1 and 2 of rows B-H (skip A!).
- Pipet 100 μL of the 10 μg/mL collagen into the appropriate wells (A1 and A2).
- Using a regular or multichannel pipet, transfer 50 μL of these solutions to the next wells down (B1 and B2), and mix with the PBS.
- Repeat, now moving 50 μL of the 5 μg/mL solution in the B wells down to the C wells.
- Either of these methods is called making doubling dilutions. Which way do you think introduces less error?
- Now add 50 μL of your samples to the appropriate wells. For the blank wells you should add PBS.
- Cover each plate (CN I and CNII) when you are done, wrap around it with parafilm to better prevent evaporation, and allow the samples to sit for 80 minutes. In the meantime, set up your agarose gel (Part 2).
- After the incubation time has passed, you will wash and then block your plate.
- First, flick the solutions in the plate into the sink.
- Using the multichannel pipet and a reservoir, add 200 μL of Wash Buffer to each well, then gently swirl the plate (by hand) for a few seconds.
- Flick the solutions out again, and then blot the plate against paper towels. You can smack the plates pretty hard, but it is possible to break them!
- Repeat the wash one more time.
- Finally, add 200 μL of Block Buffer to the plate. Wait another 60-90 minutes. In the meantime, work on your analysis (Part 3).
- Repeat the wash step that you performed above, again with two rinses.
- When you are ready, ask the teaching faculty for some primary anti-collagen antibodies (these should be diluted at the last minute). Add 100 μL of diluted antibody per well.
- Your samples will be left overnight in antibody solution, then moved back to block buffer by the teaching faculty.
Part 2: Set up qPCR reaction
Three different master mixes for three different primer sets... made for you because sensitive/ensure reproducibility...
Dilute cDNA in water to...
Part 3: Continue viability analysis (optional)
For next time
1. The final draft of your Module 2 research article is due by 11 AM next time. Please name your files according to the following convention: Firstinitial_Lastname_LabSection_Mod2-Rev.doc and email them to firstname.lastname@example.org
- ELISA solutions
- PBS reconstituted from EMD tablets
- 140 mM NaCl
- 10 mM phosphate buffer
- 3 mM KCl
- pH 7.4
- Wash buffer
- Block buffer
- ELISA collagen-specific reagents
- All from GeneTex
- Collagen I standard, diluted from 1 mg/mL in PBS
- Collagen II standard, diluted from 1 mg/mL in PBS
- Collagen I antibody, diluted 1:4000 from 1 mg/mL in PBS
- Collagen II antibody, diluted 1:4000 from 1 mg/mL in PBS