Zonulin and gut barrier: the pathway in the body · Biomarker & Stool Analysis

This page shows the biochemical pathway behind the laboratory value Zonulin (stool): which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Zonulin is a protein that intestinal cells release to the outside after certain stimuli; it is the precursor of haptoglobin 2. From outside it binds receptors on those same cells and, through a signal inside the cell, loosens the junctions between them. The gap then widens.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Stimulus at gut wall → CXCR3 receptor Gliadin fragments bind to the CXCR3 receptor of the intestinal cell. The signal is passed inwards via the signalling protein MyD88. Without CXCR3 and MyD88, release does not occur. Source 1↑ supplies CXCR3 is the entry gate for gliadin: only binding there sets off the release of zonulin via MyD88. In cell culture and animal models, release does not occur without CXCR3 or MyD88. observed in studies Source 1
   ⚖ When the balance tips

   too much — If intestinal cells carry more CXCR3 on their surface, more gliadin fragments bind, and the signal to release zonulin is stronger.

   too little — If CXCR3 or MyD88 is missing, the signal does not reach the inside of the cell; in animal models the intestinal cells then release hardly any zonulin in response to gliadin.

   observed in studies · Source 1

2. CXCR3 receptor → Zonulin is released · MyD88 In response, the cell releases zonulin to the outside. Zonulin is the precursor of the protein haptoglobin 2, which is why it is also called pre-haptoglobin 2. From outside it acts on the junctions between the cells. Source 2, 1↑ supplies Zonulin is the link between stimulus and barrier: once released, it acts back on the same and neighbouring cells. Only the uncleaved form loosens the junctions; once cleaved, it loses this action. observed in studies Source 2, 1
   ⚖ When the balance tips

   too much — If a lot of zonulin is released, more receptors on the cell surface are occupied, and the junctions between the cells stay loosened for longer.

   too little — If little zonulin is released, the signal to loosen does not come, and the junctions between the cells stay largely closed.

   observed in studies · Source 1, 3

3. Zonulin is released → Zonulin in stool Part of the released zonulin is found in stool and in blood. Studies show that widely used test kits do not capture pre-haptoglobin 2 itself, but related proteins such as properdin. Source 5
4. PAR2 and EGFR receptors → Signal inside the cell Protein kinase C The receptor switches on protein kinase C. It attaches phosphate groups to the proteins of the junctions between two intestinal cells. This loosens the junctions. Source 1↕ both, depending on amount Protein kinase C alters the proteins of the junction by attaching phosphate groups. Depending on the type of kinase and the site of the phosphate group, the junction is loosened or built more tightly. observed in studies Source 1, 7
   ⚖ When the balance tips

   too much — If protein kinase C stays active for long, more phosphate groups remain on the junction proteins, and the actin scaffold stays contracted.

   too little — If the kinase is barely active, for example because little diacylglycerol and calcium are present, the junction proteins stay unchanged, and the zonulin signal does not reach the junction.

   observed in studies · Source 1, 6

5. Signal inside the cell → Tight junction opens · actin scaffold The junctions loosen and the actin scaffold contracts. As a result, the gap between two intestinal cells widens for a short time. If the pull persists, the gap stays open. Source 4, 1↕ both, depending on amount When the actin scaffold contracts, it pulls on the sealing proteins and widens the gap. Opened briefly, this is a reversible process; if the pull persists, the gap stays open too. observed in studies Source 1, 4
   ⚖ When the balance tips

   too much — If the seam stays loosened for long, more components of the gut contents cross into the gut wall and meet immune cells there.

   too little — If the seam barely opens, the route between the cells stays narrow; water and ions then pass mainly through the cells themselves via channels and pumps.

   observed in studies · Source 1, 4

6. Tight junction opens → Path between the cells Water, ions and small molecules pass between the cells along this route. When the phosphate groups are removed again, the gap closes. Open for long, it also lets larger molecules into the gut wall. Source 1, 3↕ both, depending on amount Water with ions passes along the opened route into the gut lumen and carries bacteria away. If it stays open for long, larger molecules such as food and bacterial components also reach the gut wall. observed in studies Source 1, 3
   ⚖ When the balance tips

   too much — If the route stays open for long, more food and bacterial components get beneath the cell layer; there they trigger a response from immune cells.

   too little — If the route is barely open, only little water and few ions pass between the cells, and the outflow into the gut lumen stays small.

   observed in studies · Source 1, 3

Further stations

• Stimulus at gut wall — bacteria, gliadin fragments Certain bacteria in the small intestine and fragments of the wheat protein gliadin reach the surface of the intestinal cells. They respond by releasing zonulin. Source 1↑ supplies Bacteria and gliadin are the two described triggers to which intestinal cells respond by releasing zonulin. The water flow into the gut lumen that follows carries bacteria away. observed in studies Source 1
  ⚖ When the balance tips

  too much — If gliadin fragments or bacteria keep reaching the gut wall, the cells release zonulin continuously, and the junctions between them stay loosened for longer.

  too little — If few such stimuli arrive, the intestinal cells release hardly any zonulin, and the junctions between them stay largely closed.

  observed in studies · Source 1

• PAR2 and EGFR receptors — on the cell surface From the outside, zonulin binds to the PAR2 and EGFR receptors of the same or neighbouring cells. This is where its action at the barrier begins. EGFR is switched on via PAR2 as well. Source 3, 1↑ supplies When zonulin binds to PAR2, EGFR is switched on at the same time via this receptor; both pass the signal to loosen into the cell. In cell culture and animal models, the action does not occur without PAR2. observed in studies Source 2, 3
  ⚖ When the balance tips

  too much — If many receptors are occupied by zonulin, the signal into the cell is triggered more often, and the junctions loosen further.

  too little — If the receptors are barely occupied or PAR2 is missing, the signal does not come; in cell culture and animal models the barrier then stays tight.

  observed in studies · Source 2, 3

• ZO-1, occludin, claudins — junction proteins The junction consists of several proteins: claudins and occludin form the seal, and ZO-1 anchors it to the actin scaffold. The claudins determine how tight the seam is. Source 4↑ supplies Claudins determine which ions and how much water pass through the gap; occludin and ZO-1 hold the seal to the actin scaffold. Depending on the type of claudin, the seam is tighter or more permeable. established physiology Source 4, 7
  ⚖ When the balance tips

  too much — If many sealing claudins are built in, the gap narrows, and fewer ions and less water pass between the cells.

  too little — If few junction proteins are built in or ZO-1 is not anchored to actin, no tight seal forms, and more substances pass between the cells.

  established physiology · Source 4

Cofactors in this pathway

• Calcium — Messenger inside the cell; together with protein kinase C it sets how the junctions are assembled Source 7, 1In the ORY catalogue as a laboratory value: Calcium (intrazellulär)
• Diacylglycerol — Lipid messenger that frees protein kinase C from its own inhibition and so sets off the loosening of the junctions Source 6
• ATP — Phosphate donor of protein kinase C; it also sits bound within the actin scaffold; without ATP no phosphorylation Source 6, 8
• Magnesium — Binds together with ATP inside actin, the scaffold beneath the junctions; needed to build actin filaments Source 8In the ORY catalogue as a laboratory value: Magnesium

Sources

1. Fasano A. Zonulin and its regulation of intestinal barrier function: the biological door to inflammation, autoimmunity, and cancer. Physiol Rev 2011 · PubMed 21248165
2. Tripathi A, Lammers KM, Goldblum S et al. Identification of human zonulin, a physiological modulator of tight junctions, as prehaptoglobin-2. Proc Natl Acad Sci U S A 2009 · PubMed 19805376
3. Sturgeon C, Fasano A. Zonulin, a regulator of epithelial and endothelial barrier functions, and its involvement in chronic inflammatory diseases. Tissue Barriers 2016 · PubMed 28123927
4. Zihni C, Mills C, Matter K et al. Tight junctions: from simple barriers to multifunctional molecular gates. Nat Rev Mol Cell Biol 2016 · PubMed 27353478
5. Scheffler L, Crane A, Heyne H et al. Widely Used Commercial ELISA Does Not Detect Precursor of Haptoglobin2, but Recognizes Properdin as a Potential Second Member of the Zonulin Family. Front Endocrinol (Lausanne) 2018 · PubMed 29459849
6. Newton AC. Protein kinase C: perfectly balanced. Crit Rev Biochem Mol Biol 2018 · PubMed 29513138
7. Denker BM, Nigam SK. Molecular structure and assembly of the tight junction. Am J Physiol 1998 · PubMed 9458817
8. Kabsch W, Holmes KC. The actin fold. FASEB J 1995 · PubMed 7781919

Related pathways

• HbA1c and insulin — Calcium (intrazellulär), Magnesium
• CRP — Calcium (intrazellulär), Magnesium
• Vitamin D — Calcium (intrazellulär), Magnesium

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert approval pending
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