Collagen I antibody and matrix and adhesion reagents: detecting the insoluble

Getting matrix and junction stains to work

1. Decide native or denatured before buying. Fibrillar matrix proteins present very different epitopes assembled and unfolded. A reagent validated on a denatured chain may see nothing in tissue, and one raised against the assembled form may see nothing on a blot.
2. Extract for the compartment. Cross linked matrix and junctional complexes are not solubilised by a standard buffer. Sequential extraction, or a strong denaturing buffer with the loss of native epitopes that implies, is what the blot requires.
3. Choose retrieval per reagent, not per tissue. Junction proteins in fixed tissue frequently need retrieval to be visible at all, and the optimum differs between reagents. Optimise on a tissue known to be positive rather than on the experimental material.
4. Read junction stains as continuity, not intensity. What matters at a junction is usually whether the staining is continuous along the contact, not how bright it is. Scoring intensity where continuity is the biology misses the effect.
5. Confirm the matrix is yours. Cultured cells sit on a coating and in serum containing medium, both of which contribute matrix protein. A no cell control on the same coating shows how much of the signal came with the plate.

Insolubility is the defining property

The biological point of matrix is that it does not dissolve. Every technical difficulty in measuring it follows from that: incomplete extraction, epitopes destroyed by the conditions needed to extract, and quantitation that depends on how hard you tried.

State the extraction method with any matrix result. Two laboratories using different buffers are not measuring the same fraction, and comparing their numbers is not meaningful.

Adhesion is a complex, not a protein

Adhesion structures assemble and disassemble, and their components are present whether or not they are assembled. Total protein levels therefore report little; what reports is localisation into discrete structures, which is an imaging measurement.

Where adhesion dynamics is the question, image it. Where composition is the question, a blot of an enriched fraction is informative, provided the enrichment is shown to have worked.

A heparan sulfate antibody and a sugar rather than a protein

The epitope here is a sulfation pattern on a polysaccharide, not a peptide sequence, so a heparan sulfate antibody is raised against a structure that varies between tissues and clones differ in which pattern they see. Enzymatic digestion with the matching lyase is the negative control that proves specificity, and that control is what separates a real stain from matrix background.

A hyaluronic acid antibody and the binding protein alternative

Hyaluronan carries no sulfation and no protein core, which makes raising an antibody against it hard, so much of the field uses a labelled binding protein instead and a hyaluronic acid antibody has to demonstrate specificity by digestion with hyaluronidase. Molecular size is the property that matters biologically, and neither reagent reports it, which is why a separation step is added.

An mgp antibody and a vitamin K dependent modification

Matrix Gla protein exists in carboxylated and uncarboxylated forms with opposite meanings for calcification, so an mgp antibody has to state which it detects, and conformation specific clones exist for exactly that reason. It is small and secreted, so conditioned medium or a tissue extract is the sample, and the modification state is lost by harsh denaturation.

A collagen ii antibody and a cartilage restricted type

Type II collagen is confined to cartilage and related tissue, so a collagen ii antibody used elsewhere is expected to be negative and that expectation is the control. As with every collagen, the chain and the conformation decide the clone: a clone raised on a denatured fragment reads a pepsin digest but may miss the intact triple helix in a section.

An involucrin antibody and terminal differentiation

Involucrin appears as keratinocytes commit to terminal differentiation, so an involucrin antibody reads the upper layers of an epidermis and nothing below, and that stratified pattern is the internal control. It is crosslinked into the cornified envelope by transglutaminase, which makes the most differentiated pool insoluble and invisible to a standard lysate.

An occludin antibody and a junction read by location

Tight junction proteins are informative as a continuous line between cells rather than as a level, so an occludin antibody is an imaging reagent and a diffuse cytoplasmic signal usually means the junctions were disrupted before fixation. It is phosphorylated during barrier changes, so apparent mass shifts, and a barrier measurement is the functional companion to the stain.

A cx43 antibody and the phosphorylated forms it resolves

Connexin 43 appears as several bands because phosphorylation tracks its assembly into gap junctions, so a cx43 antibody that resolves them reports the state while a single band suggests the blot was run too fast. Punctate staining at cell borders is the pattern in an image, and a dye transfer experiment is what turns a stain into a statement about coupling.

Common questions

Why does my matrix protein not appear on a blot?

Because it was not solubilised. Cross linked matrix stays in the pellet through standard lysis, and the sample that matters is the one most people discard. Sequential or denaturing extraction is required.

Do junction proteins, such as a PECAM-1 antibody target, need retrieval?

Usually, in cross linked fixed tissue, and the optimal conditions differ between reagents. A weak junction stain is far more often a retrieval problem than a reagent problem.

How do I know the matrix signal is from my cells?

Run the coating and the medium without cells through the same protocol. Serum and coatings contribute matrix proteins, and that background is frequently substantial in culture experiments.

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