Snake or honeybee envenomation can cause substantial morbidity and mortality, and it has been proposed that the activation of mast cells by snake or insect venoms can contribute to these effects. We show, in contrast, that mast cells can significantly reduce snake-venom–induced pathology in mice, at least in part by releasing carboxypeptidase A and possibly other proteases, which can degrade venom components. Mast cells also significantly reduced the morbidity and mortality induced by honeybee venom. These findings identify a new biological function for mast cells in enhancing resistance to the morbidity and mortality induced by animal venoms.

5′-AMP-activated protein kinase (AMPK) is a metabolic stress sensor present in all eukaryotes. A dominant missense mutation (R225Q) in pig PRKAG3, encoding the muscle-specific γ3 isoform, causes a marked increase in glycogen content. To determine the functional role of the AMPK γ3 isoform, we generated transgenic mice with skeletal muscle-specific expression of wild type or mutant (225Q) mouse γ3 as well as Prkag3 knockout mice. Glycogen resynthesis after exercise was impaired in AMPK γ3 knock-out mice and markedly enhanced in transgenic mutant mice. An AMPK activator failed to increase skeletal muscle glucose uptake in AMPK γ3 knock-out mice, whereas contraction effects were preserved. When placed on a high fat diet, transgenic mutant mice but not knock-out mice were protected against excessive triglyceride accumulation and insulin resistance in skeletal muscle. Transfection experiments reveal the R225Q mutation is associated with higher basal AMPK activity and diminished AMP dependence. Our results validate the muscle-specific AMPK γ3 isoform as a therapeutic target for prevention and treatment of insulin resistance.

Chymases, serine proteases exclusively expressed by mast cells, have been implicated in various pathological conditions. However, the basis for these activities is not known, i.e. the in vivo substrate(s) for mast cell chymase has not been identified. In this study we show that mice lacking the chymase mouse mast cell protease 4 (mMCP-4) fail to process pro-matrix metalloprotease 9 (pro-MMP-9) to its active form in vivo, whereas both the pro and active form of MMP-9 was found in tissues of wild type mice. Moreover, the processing of pro-MMP-2 into active enzyme was markedly defective in mMCP-4 null animals. Histological analysis revealed an increase in collagen in the ear tissue of mMCP-4-deficient animals accompanied by increased ear thickness and a higher content of hydroxyproline. Furthermore, both lung and ear tissue from the knock-out animals showed a markedly increased staining for fibronectin. MMP-9 and MMP-2 are known to have a range of important activities, but the mechanisms for their activation in vivo have not been clarified previously. The present study thus indicates a key role for mast cell chymase in the regulation of pro-MMP-2 and -9 activities. Moreover, the results suggest an important role for mast cell chymase in regulating connective tissue homeostasis.

The glycosaminoglycan, heparan sulfate (HS), binds proteins to modulate signaling events in embryogenesis. All identified protein-binding HS epitopes contain l-iduronic acid (IdoA). We report that targeted disruption of the murine d-glucuronyl C5-epimerase gene results in a structurally altered HS lacking IdoA. The corresponding phenotype is lethal, with renal agenesis, lung defects, and skeletal malformations. Unexpectedly, major organ systems, including the brain, liver, gastrointestinal tract, skin, and heart, appeared normal. We find that IdoA units are essential for normal kidney, lung, and skeletal development, albeit with different requirement for 2-O-sulfation. By contrast, major early developmental events known to critically depend on heparan sulfate apparently proceed normally even in the absence of IdoA.

Altered intestinal barrier function is postulated to be a central predisposing factor to intestinal diseases, including inflammatory bowel diseases and food allergies. However, the mechanisms involved in maintaining homeostatic intestinal barrier integrity remain undefined. In this study, we demonstrate that mice deficient in mast cells ( Kit ^W-sh/W-sh [Wsh]) or mast cell chymase (Mcpt4 ^−/− ) have significantly decreased basal small intestinal permeability compared with wild-type (WT) mice. Altered intestinal barrier function was linked to decreased intestinal epithelial cell migration along the villus/crypt axis, altered intestinal morphology, and dysregulated claudin-3 crypt expression. Remarkably, engraftment of Wsh mice with WT but not Mcpt4 ^−/− mast cells restored intestinal epithelial cell migration, morphology, and intestinal epithelial barrier function. Collectively, these findings identify a mechanism by which mast cells regulate homeostatic intestinal epithelial migration and barrier function.