There is considerable evidence to suggest that drug actions at the κ-opioid receptor (KOR) may represent a means to control pain perception and modulate reward thresholds. As a G protein-coupled ...receptor (GPCR), the activation of KOR promotes Gαi/o protein coupling and the recruitment of β-arrestins. It has become increasingly evident that GPCRs can transduce signals that originate independently via G protein pathways and β-arrestin pathways; the ligand-dependent bifurcation of such signaling is referred to as “functional selectivity” or “signaling bias.” Recently, a KOR agonist, 6′-guanidinonaltrindole (6′-GNTI), was shown to display bias toward the activation of G protein-mediated signaling over β-arrestin2 recruitment. Therefore, we investigated whether such ligand bias was preserved in striatal neurons. Although the reference KOR agonist U69,593 induces the phosphorylation of ERK1/2 and Akt, 6′-GNTI only activates the Akt pathway in striatal neurons. Using pharmacological tools and β-arrestin2 knock-out mice, we show that KOR-mediated ERK1/2 phosphorylation in striatal neurons requires β-arrestin2, whereas Akt activation depends upon G protein signaling. These findings reveal a point of KOR signal bifurcation that can be observed in an endogenous neuronal setting and may prove to be an important indicator when developing biased agonists at the KOR.
Background: 6′-Guanidinonaltrindole (6′-GNTI) activates G protein coupling to κ-opioid receptors (KOR) without β-arrestin2 recruitment in transfected cells.
Results: In striatal neurons, 6′-GNTI activates Akt but not ERK1/2; U69,593 activates both kinases.
Conclusion: In neurons, U69,593-induced activation of ERK1/2 is β-arrestin2-dependent, whereas activation of Akt is G protein-mediated.
Significance: Identification of KOR signaling pathways in endogenous systems will inform the development of KOR-directed medications.
An extensive body of experimental and clinical evidence documents the negative impact of chronic psychological stress and depression on the immune system and health. Chronic stress and depression can ...result in general dysregulation of the immune system, of both cellular and humoral pathways, which may contribute to pathogenic infection and concomitant periodontal tissue destruction. In general, the evidence is consistent with the hypothesis that stress can modify the host defense and progression of periodontal infections in patients susceptible to periodontitis. However, substantial evidence also indicates that these conditions can mediate risk for disease, including periodontitis, through changes in health‐related behaviors, such as oral hygiene, smoking and diet. The unequivocal interpretation of studies has also been hampered, in part, by issues related to conceptualization of stress and depression, as well as commonly associated comorbidities, such as diabetes, that can modify the onset and progression of periodontal disease. In addition, stress and depression appear to fall into a spectrum, ranging from mild to severe, involving a complex interaction of genetic background, coping strategies and environment. Differences in the conceptualization of stress and depression are probably important in assessing associations with other biologic and clinical measures. Future studies are necessary to clarify the complex interactions of chronic stress and depression in periodontal diseases.
Preterm infants are at risk of developing intestinal dysbiosis with an increased proportion of Gammaproteobacteria. In this study, we sought the clinical determinants of the relative abundance of ...feces-associated Gammaproteobacteria in very low birth weight (VLBW) infants. Fecal microbiome was characterized at ≤ 2 weeks and during the 3rd and 4th weeks after birth, by 16S rRNA amplicon sequencing. Maternal and infant clinical characteristics were extracted from electronic medical records. Data were analyzed by linear mixed modeling and linear regression.
Clinical data and fecal microbiome profiles of 45 VLBW infants (gestational age 27.9 ± 2.2 weeks; birth weight 1126 ± 208 g) were studied. Three stool samples were analyzed for each infant at mean postnatal ages of 9.9 ± 3, 20.7 ± 4.1, and 29.4 ± 4.9 days. The average relative abundance of Gammaproteobacteria was 42.5% (0-90%) at ≤ 2 weeks, 69.7% (29.9-86.9%) in the 3rd, and 75.5% (54.5-86%) in the 4th week (p < 0.001). Hierarchical and K-means clustering identified two distinct subgroups: cluster 1 started with comparatively low abundance that increased with time, whereas cluster 2 began with a greater abundance at ≤ 2 weeks (p < 0.001) that decreased over time. Both groups resembled each other by the 3rd week. Single variants of Klebsiella and Staphylococcus described variance in community structure between clusters and were shared between all infants, suggesting a common, hospital-derived source. Fecal Gammaproteobacteria was positively associated with vaginal delivery and antenatal steroids.
We detected a dichotomy in gut microbiome assembly in preterm infants: some preterm infants started with low relative gammaproteobacterial abundance in stool that increased as a function of postnatal age, whereas others began with and maintained high abundance. Vaginal birth and antenatal steroids were identified as predictors of Gammaproteobacteria abundance in the early (≤ 2 weeks) and later (3rd and 4th weeks) stool samples, respectively. These findings are important in understanding the development of the gut microbiome in premature infants.
Morphine and other opiates mediate their effects through activation of the μ-opioid receptor (MOR), and regulation of the MOR has been shown to critically affect receptor responsiveness. Activation ...of the MOR results in receptor phosphorylation, β-arrestin recruitment, and internalization. This classical regulatory process can differ, depending on the ligand occupying the receptor. There are two forms of β-arrestin, β-arrestin1 and β-arrestin2 (also known as arrestin2 and arrestin3, respectively); however, most studies have focused on the consequences of recruiting β-arrestin2 specifically. In this study, we examine the different contributions of β-arrestin1- and β-arrestin2-mediated regulation of the MOR by comparing MOR agonists in cells that lack expression of individual or both β-arrestins. Here we show that morphine only recruits β-arrestin2, whereas the MOR-selective enkephalin d-Ala2,N-Me-Phe4,Gly5-olenkephalin (DAMGO), recruits either β-arrestin. We show that β-arrestins are required for receptor internalization and that only β-arrestin2 can rescue morphine-induced MOR internalization, whereas either β-arrestin can rescue DAMGO-induced MOR internalization. DAMGO activation of the receptor promotes MOR ubiquitination over time. Interestingly, β-arrestin1 proves to be critical for MOR ubiquitination as modification does not occur in the absence of β-arrestin1 nor when morphine occupies the receptor. Moreover, the selective interactions between the MOR and β-arrestin1 facilitate receptor dephosphorylation, which may play a role in the resensitization of the MOR and thereby contribute to overall development of opioid tolerance.
G protein-coupled receptor desensitization and trafficking are important regulators of opioid receptor signaling that can dictate overall drug responsiveness in vivo. Furthermore, different mu-opioid ...receptor (muOR) ligands can lead to varying degrees of receptor regulation, presumably because of distinct structural conformations conferred by agonist binding. For example, morphine binding produces a muOR with low affinity for beta-arrestin proteins and limited receptor internalization, whereas enkephalin analogs promote robust trafficking of both beta-arrestins and the receptors. Here, we evaluate muOR trafficking in response to activation by a novel mu-selective agonist derived from the naturally occurring plant product, salvinorin A. It is interesting that this compound, termed herkinorin, does not promote the recruitment of beta-arrestin-2 to the muOR and does not lead to receptor internalization. Moreover, whereas G protein-coupled receptor kinase overexpression can promote morphine-induced beta-arrestin interactions and muOR internalization, such manipulations do not promote herkinorin-induced trafficking. Studies in mice have shown that beta-arrestin-2 plays an important role in the development of morphine-induced tolerance, constipation, and respiratory depression. Therefore, drugs that can activate the receptor without recruiting the arrestins may be a promising step in the development of opiate analgesics that distinguish between agonist activity and receptor regulation and may ultimately lead to therapeutics designed to provide pain relief without the adverse side effects normally associated with the opiate narcotics.
PDF
