Figure 1: Collagen protein.

Collagen Target Introduction

Protein Function

• Collagens are the most abundant and essential proteins in the human body, with the most common forms being type I, II, III, IV, and V.
• Collagen is the main component of the extracellular matrix and can promote cell adhesion and migration.
• Collagen can be divided into six different categories: fibrillar collagens, network-forming collagens, fibril-associated collagens with interrupted triple helices (FACIT collagens), transmembrane collagens, multiplexins, and other collagens.
• Each collagen molecule comprises three α-chains, which assemble into homotrimeric or heterotrimeric triple helical initiators to form supramolecular structures with different functions.

Protein Expression

• Collagen is mainly found in bone, skin, and connective tissue (mainly type I collagen), cartilage (primarily type II collagen), and blood vessels (mostly type III collagen).

Protein Localization

• Collagen types XXV, XIII, XVII, and XXIII are transmembrane proteins.
• Collagen type I (Figure 2), collagen type II, and collagen type III (Figure 3) are all expressed in the extracellular matrix.

Figure 2: Collagen I ICC experimental result image, Anti-Collagen I antibody (ab270993). Green: Collagen I, Red: Tubulin, Blue: DAPI.

Figure 3: Collagen III ICC experimental result image, using Anti-Collagen III antibody [EPR17673] (ab184993). Green: Collagen III, Red: Tubulin, Blue: DAPI.

Experimental Guide

Extraction of Collagen Protein

There are usually three methods for extracting collagen protein, among which enzymatic extraction is the most effective (Figure 4):

• Salt precipitation extraction - gradually add sodium chloride in a neutral salt solution for extraction. (For example: 0.45 M NaCl, maintained at pH 7.5 under stirring conditions for 24 hours)
• Acid extraction - extract in organic acids such as acetic acid. (For example: 0.5 M acetic acid, maintained at pH 2.5 for 24 hours)
• Enzyme extraction - add pepsin in organic acid for extraction. (For example: 0.1% (w/v) pepsin in 0.5 M acetic acid, maintained at pH 2.5 for 48 hours)

Figure 4. Comparative data on the extraction of soluble collagen protein.

We recommend extracting collagen protein using pepsin, as it can remove the non-helical ends of collagen protein and increase its solubility, thereby improving extraction efficiency.

In addition, attention should be paid to the extraction of collagen protein:

• Collagen protein is insoluble in organic solvents and most of it is insoluble in water.
• Collagen protein is temperature-sensitive: extraction at 4°C/on ice can prevent collagen protein from degrading due to the unwinding of the triple helix.
• Acidic pH is crucial for the dissolution of collagen protein, and an acidic environment can effectively prevent collagen protein from polymerizing.
• Avoid the occurrence of polymerization due to high collagen protein concentration.

Collagen protein WB detection precautions

• Due to the instability of collagen protein compared to other proteins, WB experiments should be conducted at 4⁰C/on ice.
• For Collagen I protein detection (Figure 5), please use a 6% acrylamide gel, as it has a larger molecular weight.
• For Collagen IV, V, and VI, use a 10% acrylamide gel. Alternatively, a gradient gel can also be chosen.
• Adding 4M urea to the sample can improve the separation of collagen protein during electrophoresis.
• We recommend using 3% BSA for blocking and using PVDF membrane for transfer and subsequent procedures.

Figure 5. Detection of Collagen I expression in different human tissues

Lane 1: Human gastric tissue lysate
Lane 2: Human skin tissue lysate
Lane 3: Human adrenal tissue lysate

Collagen protein IHC detection precautions

• Due to the different types of collagen proteins, there may be significant differences in their cross-linking, so it is important to choose the appropriate antigen retrieval method to expose their epitopes.
• Intense antigen retrieval conditions may disrupt the tertiary structure of collagen proteins. We recommend using a pH 6.0 sodium citrate buffer for antigen retrieval, or you can try using an enzyme antigen retrieval method.
• If you need to observe the tertiary structure of collagen proteins, we recommend using frozen sections (IHC-Fr) for histological experiments.
• Picro Sirius Red (ab150681) can bind to the [gly-X-Y]n helical structure on fibrous collagen proteins (types I-V) and can be used to determine the degree of collagen protein cross-linking (Figure 6).

Figure 6. ab150681  (Picro Sirius Red stain) stains collagen protein (red), muscle fibers (yellow), cytoplasm (yellow), and red blood cells (yellow) in the normal human colon embedded in formalin-fixed paraffin.

Challenges faced by Collagen protein antibodies

Collagen protein, as a highly conserved protein, is characterized by uninterrupted glycine-X-Y triplet repeat sequences, which give them a triple helical structure. The close similarity of this triple helical structure in different types of collagen proteins makes it very difficult to prepare antibodies that recognize specific types of collagen proteins.

In order to make collagen protein antibodies specific, the immunogen used for antibody preparation must rely on undenatured three-dimensional collagen epitopes or shorter peptide sequences found only in specific types of collagen. Please choose the appropriate antibody and adjust the experimental protocol accordingly based on the type of collagen protein you need to detect.

If you are using collagen protein antibodies developed using short peptide sequences (such as ab138492), there is no need to carefully maintain the collagen protein's 3D structure. All of our RabMAb^® collagen protein antibodies (such as ab260043 for collagen protein I) are made using synthetic peptides with specific sequences for the target collagen protein.

Finally, attached is a table of vertebrate collagen protein distribution to help everyone succeed in their experiments.

References

1. R C Duhamel, E Meezan, K Brendel. Differential effect of urea on the migration of collagenous and non-collagenous polypeptides in SDS gels: basis for their separation by two-dimensional electrophoresis. Coll Relat Res. 1981 Feb;1(2):201-7.
2. Matthew D Shoulders, Ronald T Raines. Collagen structure and stability. Annu Rev Biochem. 2009;78:929-58.