Monoglyceride lipase deficiency causes desensitization of intestinal cannabinoid receptor type 1 and increased colonic μ-opioid receptor sensitivity

Background and Purpose

Monoglyceride lipase (MGL) degrades 2-arachidonoyl glycerol (2-AG), an endogenous agonist of cannabinoid receptors (CB_1/2). Because the CB₁ receptor is involved in the control of gut function, we investigated the effects of pharmacological inhibition and genetic deletion of MGL on intestinal motility. Furthermore, we determined whether defective 2-AG degradation affects μ-opioid receptor (μ receptor) signalling, a parallel pathway regulating gut motility.

Experimental Approach

Gut motility was investigated by monitoring Evans Blue transit and colonic bead propulsion in response to MGL inhibition and CB₁ receptor or μ receptor stimulation. Ileal contractility was investigated by electrical field stimulation. CB₁ receptor expression in ileum and colon was assessed by immunohistochemical analyses.

Key Results

Pharmacological inhibition of MGL slowed down whole gut transit in a CB₁ receptor-dependent manner. Conversely, genetic deletion of MGL did not affect gut transit despite increased 2-AG levels. Notably, MGL deficiency caused complete insensitivity to CB₁ receptor agonist-mediated inhibition of whole gut transit and ileal contractility suggesting local desensitization of CB₁ receptors. Accordingly, immunohistochemical analyses of myenteric ganglia of MGL-deficient mice revealed that CB₁ receptors were trapped in endocytic vesicles. Finally, MGL-deficient mice displayed accelerated colonic propulsion and were hypersensitive to μ receptor agonist-mediated inhibition of colonic motility. This phenotype was reproduced by chronic pharmacological inhibition of MGL.

Conclusion and Implications

Constantly elevated 2-AG levels induce severe desensitization of intestinal CB₁ receptors and increased sensitivity to μ receptor-mediated inhibition of colonic motility. These changes should be considered when cannabinoid-based drugs are used in the therapy of gastrointestinal diseases.

Genome-Wide Localization Study of Yeast Pex11 Identifies Peroxisome-Mitochondria Interactions through the ERMES Complex

Pex11 is a peroxin that regulates the number of peroxisomes in eukaryotic cells. Recently, it was found that a mutation in one of the three mammalian paralogs, PEX11β, results in a neurological disorder. The molecular function of Pex11, however, is not known. Saccharomyces cerevisiae Pex11 has been shown to recruit to peroxisomes the mitochondrial fission machinery, thus enabling proliferation of peroxisomes. This process is essential for efficient fatty acid β-oxidation. In this study, we used high-content microscopy on a genome-wide scale to determine the subcellular localization pattern of yeast Pex11 in all non-essential gene deletion mutants, as well as in temperature-sensitive essential gene mutants. Pex11 localization and morphology of peroxisomes was profoundly affected by mutations in 104 different genes that were functionally classified. A group of genes encompassing MDM10, MDM12 and MDM34 that encode the mitochondrial and cytosolic components of the ERMES complex was analyzed in greater detail. Deletion of these genes caused a specifically altered Pex11 localization pattern, whereas deletion of MMM1, the gene encoding the fourth, endoplasmic-reticulum-associated component of the complex, did not result in an altered Pex11 localization or peroxisome morphology phenotype. Moreover, we found that Pex11 and Mdm34 physically interact and that Pex11 plays a role in establishing the contact sites between peroxisomes and mitochondria through the ERMES complex. Based on these results, we propose that the mitochondrial/cytosolic components of the ERMES complex establish a direct interaction between mitochondria and peroxisomes through Pex11.

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Molecular function of Pex11, a protein with roles in metabolism and disease, is unknown.

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Genome-wide screening determined subcellular localization of Pex11-GFP in yeast.

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Mutants defective in components of the ERMES complex show altered Pex11 localization.

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Pex11 physically interacts with the ERMES complex component Mdm34.

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ERMES complex and Pex11 mediate interaction between mitochondria and peroxisomes.

Phospholipase C-dependent control of cardiac calcium homeostasis involves a TRPC3-NCX1 signaling complex

OBJECTIVE

Members of the classical transient receptor potential protein (TRPC) family are considered as key components of phospholipase C (PLC)-dependent Ca2+ signaling. Previous results obtained in the HEK 293 expression system suggested a physical and functional coupling of TRPC3 to the cardiac-type Na+/Ca2+ exchanger, NCX1 (sodium calcium exchanger 1). This study was designed to test for expression of TRPC3 (transient receptor potential channel 3) and for the existence of a native TRPC3/NCX1 signaling complex in rat cardiac myocytes.

METHODS

Protein expression and cellular distribution were determined by Western blot and immunocytochemistry. Protein-protein interactions were investigated by reciprocal co-immunoprecipitation and glutathione S-transferase (GST)-pulldown experiments. Recruitment of protein complexes into the plasma membrane was assayed by surface biotinylation. The functional role of TRPC3 was investigated by fluorimetric recording of angiotensin II-induced calcium signals employing a dominant negative knockdown strategy.

RESULTS

TRPC3 immunoreactivity was observed in surface plasma membrane regions and in an intracellular membrane system. Co-immunolabeling of TRPC3 and NCX1 indicated significant co-localization of the two proteins. Both co-immunoprecipitation and GST-pulldown experiments demonstrated association of TRPC3 with NCX1. PLC stimulation was found to trigger NCX-mediated Ca2+ entry, which was dependent on TRPC3-mediated Na+ loading of myocytes. This NCX-mediated Ca2+ signaling was significantly suppressed by expression of a dominant negative fragment of TRPC3. PLC stimulation was associated with increased membrane presentation of both TRPC3 and NCX1.

CONCLUSION

These results suggest a PLC-dependent recruitment of a TRPC3-NCX1 complex into the plasma membrane as a pivotal mechanism for the control of cardiac Ca2+ homeostasis.