Role of intestinal mucus and pancreatic proteases in the pathogenesis of trauma–hemorrhagic shock-induced gut barrier failure and multiple organ dysfunction syndrome

The intestinal mucus layer is a critical component of the gut barrier that is damaged during acute pancreatitis

Gut barrier failure has been implicated in the progression from single-organ injury to multiple-organ failure. The unstirred mucus layer is a major component of the physiological gut barrier; its role in acute pancreatitis (AP) is not clearly defined. Rats underwent biliopancreatic duct ligation-induced AP; two controls were used: biliopancreatic duct ligation with drainage and sham duct ligation. After 4.5 h, serum and ascitic amylase activity was measured. Mucus was analyzed for reactive nitrogen intermediate-mediated damage, reactive oxygen species-induced damage, and total antioxidant capacity. Mucus coverage and villous injury were assessed histologically. Ileum permeability was measured by diffusion of a fluorescent Dextran probe. Histology and morphology of the mucus layer were validated in a mouse AP model (intraductal taurocholate plus cerulein). Biliopancreatic duct ligation increased serum α-amylase, ascitic volume, and ascitic α-amylase. Intestinal permeability was increased, which was associated with loss of the unstirred mucus layer but not villous injury. These changes correlated with increased reactive oxygen species- and- reactive nitrogen intermediate-mediated mucus damage as well as decreased mucus total antioxidant capacity but were not present in the two control groups. Using a different model of AP in mice, the finding of mucus layer disruption was recapitulated at 6 h after AP, but by 24 h, rebound hypersecretion of inspissated mucus was seen. These results support the hypothesis that damage to the unstirred mucus layer with evidence of oxidative stress occurs during AP-induced gut barrier failure.

Intraluminal nonbacterial intestinal components control gut and lung injury after trauma hemorrhagic shock

OBJECTIVE

To test whether the mucus layer, luminal digestive enzymes, and intestinal mast cells are critical components in the pathogenesis of trauma shock-induced gut and lung injury.

BACKGROUND

Gut origin sepsis studies have highlighted the importance of the systemic component (ischemia-reperfusion) of gut injury, whereas the intraluminal component is less well studied.

METHODS

In rats subjected to trauma hemorrhagic shock (T/HS) or sham shock, the role of pancreatic enzymes in gut injury was tested by diversion of pancreatic enzymes via pancreatic duct exteriorization whereas the role of the mucus layer was tested via the enteral administration of a mucus surrogate. In addition, the role of mast cells was assessed by measuring mast cell activation and the ability of pharmacologic inhibition of mast cells to abrogate gut and lung injury. Gut and mucus injury was characterized functionally, morphologically, and chemically.

RESULTS

Pancreatic duct exteriorization abrogated T/HS-induced gut barrier loss and limited chemical mucus changes. The mucus surrogate prevented T/HS-induced gut and lung injury. Finally, pancreatic enzyme-induced gut and lung injury seems to involve mast cell activation because T/HS activates mast cells and pharmacologic inhibition of intestinal mast cells prevented T/HS-induced gut and lung injury.

CONCLUSIONS

These results indicate that gut and gut-induced lung injury after T/HS involves a complex process consisting of intraluminal digestive enzymes, the unstirred mucus layer, and a systemic ischemic-reperfusion injury. This suggests the possibility of intraluminal therapeutic strategies.

Dissolution of lipids from mucus: a possible mechanism for prompt disruption of gut barrier function by alcohol

Acute and/or chronic alcohol ingestion has been shown to exacerbate the morbidity and mortality rate associated with acute mechanical and/or thermal trauma. While alcohol ingestion can affect many organs and systems, clinical and preclinical studies indicate that alcohol ingestion can cause a 'leaky gut' syndrome which in turn contributes to infection and systemic organ dysfunction. This study investigated the acute effect of alcohol on gut barrier function. Using an in vivo isolated gut sac model of naïve male rats, each individual gut sac was injected with different concentrations (0, 5, 10, 20, and 40%, v/v) of alcohol. After different times of alcohol exposure, each isolated gut segment was harvested and intestinal permeability and mucosal surface hydrophobicity (a physiologic marker of mucus barrier function) were measured as well as luminal DNA, mucus, protein and free fatty acids. The results showed that alcohol caused dose-dependent and time-dependent increases in gut permeability and decreases in mucosal surface hydrophobicity, with significant changes to be observed 5 min after treatment with 10% alcohol. In addition, it is further found that these changes in permeability and hydrophobicity are more closely associated with increased intestinal luminal free fatty acids levels but not protein or DNA levels. These results suggest that alcohol may cause loss of gut barrier function by extracting and dissolving lipids from the mucus with a resultant decrease in mucosal surface hydrophobicity, which is a critical component of gut barrier function.

Oxidative modification of the intestinal mucus layer is a critical but unrecognized component of trauma hemorrhagic shock-induced gut barrier failure

Recent studies demonstrate that mechanisms underlying gut barrier failure include systemic processes and less studied luminal processes. We thus tested the hypothesis that mucus layer oxidation is a component of trauma/hemorrhagic shock-induced gut injury and dysfunction. Male Sprague-Dawley rats underwent trauma/hemorrhagic shock. Controls underwent trauma only. Mucus from the terminal 30 cm of the ileum was collected, processed, and analyzed for reactive nitrogen intermediates (RNI)-mediated damage, reactive oxygen species (ROS)-induced damage, and total antioxidant capacity. The distal ileum was stained to quantify the mucus layer; gut permeability was assessed physiologically. A time course study was conducted to determine the temporal sequence of mucus layer damage. The role of free radical-mediated damage to the gut barrier was investigated by the effect of the free radical scavenger dimethyl sulfoxide on trauma/hemorrhagic shock-induced changes on the mucus and on gut permeability. Trauma/hemorrhagic shock increased intestinal permeability, which was associated with evidence of loss of the unstirred mucus layer. These changes correlated with increased ROS- and RNI-mediated mucus damage and loss of mucus total antioxidant capacity. Based on the time course study, ROS-mediated mucus damage and loss of total antioxidant capacity were present immediately following shock, whereas RNI-mediated damage was delayed for 3 h. Dimethyl sulfoxide ameliorated gut barrier loss, ROS-mediated changes to the mucus layer, and loss of total antioxidant capacity. There was no change in RNI-induced changes to the mucus layer. These results support the hypothesis that trauma/hemorrhagic shock leads to mucus damage and gut dysfunction through the generation of free radical species.

The mucus layer is critical in protecting against ischemia-reperfusion-mediated gut injury and in the restitution of gut barrier function

It is well documented that the gut injury plays a critical role in the development of systemic inflammation and distant organ injury in conditions associated with splanchnic ischemia. Consequently, understanding the mechanisms leading to gut injury is important. In this context, recent work suggests a protective role for the intestinal mucus layer and an injury-inducing role for luminal pancreatic proteases. Thus, we explored the role of the mucus layer in gut barrier function by observing how the removal of the mucus layer affects ischemia-reperfusion-mediated gut injury in rats as well as the potential role of luminal pancreatic proteases in the pathogenesis of gut injury. Ischemia was induced by the ligation of blood vessels to segments of the ileum for 45 min, followed by up to 3 h of reperfusion. The ileal segments were divided into five groups. These included a nonischemic control, ischemic segments exposed to saline, the mucolytic N-acetylcysteine (NAC), pancreatic proteases, or NAC + pancreatic proteases. Changes in gut barrier function were assessed by the permeation of fluorescein isothiocyanate dextran (molecular weight, 4,000 d) in ileal everted sacs. Gut injury was measured morphologically and by the luminal content of protein, DNA, and hemoglobin. The mucus layer was assessed functionally by measuring its hydrophobicity and morphologically. Gut barrier function was promptly and effectively reestablished during reperfusion, which was accompanied by the restoration of the mucus layer. In contrast, treatment of the gut with the mucolytic NAC for 10 min during ischemia resulted in a failure of mucus restitution and further increases in gut permeability and injury. The presence of digestive proteases by themselves did not exacerbate gut injury, but in combination with NAC, they caused an even greater increase in gut injury and permeability. These results suggest that the mucus layer not only serves as a barrier between the luminal contents and gut surface epithelia, but also plays a critical role in the maintenance and restitution of gut barrier function.

Loss of the intestinal mucus layer in the normal rat causes gut injury but not toxic mesenteric lymph nor lung injury

There is substantial evidence that gut barrier failure is associated with distant organ injury and systemic inflammation. After major trauma or stress, the factors and mechanisms involved in gut injury are unknown. Our primary hypothesis is that loss of the intestinal mucus layer will result in injury of the normal gut that is exacerbated by the presence of luminal pancreatic proteases. Our secondary hypothesis is that the injury produced in the gut will result in the production of biologically active mesenteric lymph and consequently distant organ (i.e., lung) injury. To test this hypothesis, five groups of rats were studied: 1) uninstrumented naive rats; 2) control rats in which a ligated segment of distal ileum was filled with saline; 3) rats with pancreatic proteases placed in their distal ileal segments; 4) rats with the mucolytic N-acetylcysteine (NAC) placed in their distal ileal segments; and 5) rats exposed to NAC and pancreatic proteases in their ileal segments. The potential systemic consequences of gut injury induced by NAC and proteases were assessed by measuring the biological activity of mesenteric lymph as well as gut-induced lung injury. Exposure of the normal intestine to NAC, but not saline or proteases, led to increased gut permeability, loss of mucus hydrophobicity, a decrease in the mucus layer, as well as morphological evidence of villous injury. Although proteases themselves did not cause gut injury, the combination of pancreatic proteases with NAC caused more severe injury than NAC alone, suggesting that once the mucus barrier is impaired, luminal proteases can injure the now vulnerable gut. Because comparable levels of gut injury caused by systemic insults are associated with gut-induced lung injury, which is mediated by biologically active factors in mesenteric lymph, we next tested whether this local model of gut injury would produce active mesenteric lymph or lead to lung injury. It did not, suggesting that gut injury by itself may not be sufficient to induce distant organ dysfunction. Therefore, loss of the intestinal mucus layer, especially in the presence of intraluminal pancreatic proteases, is sufficient to lead to injury and barrier dysfunction of the otherwise normal intestine but not to produce gut-induced distant organ dysfunction.

Intestinal mucus layer preservation in female rats attenuates gut injury after trauma-hemorrhagic shock

BACKGROUND

We tested the hypothesis that females are more resistant to trauma-hemorrhagic shock (T/HS)-induced gut injury than males, and this is related to better preservation of their intestinal mucus layer, which is influenced in turn by the estrus cycle stage at the time of injury.

METHODS

Male, proestrus and diestrus female rats underwent a laparotomy (trauma) and 90 minutes of shock ( approximately 35 mm Hg). At 3 hours after reperfusion, terminal ileum was harvested and stained with Carnoy's Alcian Blue for mucus assessment, hematoxylin and eosin, and periodic acid schiff for villous and goblet cell morphology and injury. Ileal permeability was measured in separate intestinal segments using the ex vivo everted gut sac technique.

RESULTS

When compared with males, proestrus female rats were significantly more resistant to T/HS-induced morphologic gut injury, as reflected in both a lower incidence of villous injury (14% vs. 22%; p < 0.05) and a lesser grade of injury (1.0 vs. 2.8; p < 0.05) as well as preservation of gut barrier function (17.9 vs. 32.2; p < 0.05). This resistance to gut injury was associated with significant preservation of the mucus layer (87% vs. 62%; p < 0.05) and was influenced by the estrus cycle stage of the female rats. There was a significant inverse correlation between mucus layer coverage and the incidence (r = 0.9; p < 0.0001) and magnitude (r = 0.89; p < 0.0001) of villous injury and gut permeability (r = 0.74; p < 0.001).

CONCLUSIONS

The resistance of female rats to T/HS-induced intestinal injury and dysfunction was associated with better preservation of the intestinal mucus barrier and was to some extent estrus cycle-dependent. Preservation of the mucus barrier may protect against shock-induced gut injury and subsequent distant organ injury by limiting the ability of luminal contents such as bacteria and digestive enzymes from coming into direct contact with the epithelium.