Manifold roles of ß-arrestins in GPCR signaling elucidated with siRNA and CRISPR/Cas9.

G protein-coupled receptors (GPCRs) use diverse mechanisms to regulate the mitogen-activated protein kinases ERK1/2. ß-Arrestins (ßArr1/2) are ubiquitous inhibitors of G protein signaling, promoting GPCR desensitization and internalization and serving as scaffolds for ERK1/2 activation. Studies using CRISPR/Cas9 to delete ßArr1/2 and G proteins have cast doubt on the role of ß-arrestins in activating specific pools of ERK1/2. We compared the effects of siRNA-mediated knockdown of ßArr1/2 and reconstitution with ßArr1/2 in three different parental and CRISPR-derived ßArr1/2 knockout HEK293 cell pairs to assess the effect of ßArr1/2 deletion on ERK1/2 activation by four Gs-coupled GPCRs. In all parental lines with all receptors, ERK1/2 stimulation was reduced by siRNAs specific for ßArr2 or ßArr1/2. In contrast, variable effects were observed with CRISPR-derived cell lines both between different lines and with activation of different receptors. For ß2 adrenergic receptors (ß2ARs) and ß1ARs, ßArr1/2 deletion increased, decreased, or had no effect on isoproterenol-stimulated ERK1/2 activation in different CRISPR clones. ERK1/2 activation by the vasopressin V2 and follicle-stimulating hormone receptors was reduced in these cells but was enhanced by reconstitution with ßArr1/2. Loss of desensitization and receptor internalization in CRISPR ßArr1/2 knockout cells caused ß2AR-mediated stimulation of ERK1/2 to become more dependent on G proteins, which was reversed by reintroducing ßArr1/2. These data suggest that ßArr1/2 function as a regulatory hub, determining the balance between mechanistically different pathways that result in activation of ERK1/2, and caution against extrapolating results obtained from ßArr1/2- or G protein-deleted cells to GPCR behavior in native systems.

A new inhibitor of the ß-arrestin/AP2 endocytic complex reveals interplay between GPCR internalization and signalling.

In addition to G protein-coupled receptor (GPCR) desensitization and endocytosis, ß-arrestin recruitment to ligand-stimulated GPCRs promotes non-canonical signalling cascades. Distinguishing the respective contributions of ß-arrestin recruitment to the receptor and ß-arrestin-promoted endocytosis in propagating receptor signalling has been limited by the lack of selective analytical tools. Here, using a combination of virtual screening and cell-based assays, we have identified a small molecule that selectively inhibits the interaction between ß-arrestin and the ß2-adaptin subunit of the clathrin adaptor protein AP2 without interfering with the formation of receptor/ß-arrestin complexes. This selective ß-arrestin/ß2-adaptin inhibitor (Barbadin) blocks agonist-promoted endocytosis of the prototypical ß2-adrenergic (ß2AR), V2-vasopressin (V2R) and angiotensin-II type-1 (AT1R) receptors, but does not affect ß-arrestin-independent (transferrin) or AP2-independent (endothelin-A) receptor internalization. Interestingly, Barbadin fully blocks V2R-stimulated ERK1/2 activation and blunts cAMP accumulation promoted by both V2R and ß2AR, supporting the concept of ß-arrestin/AP2-dependent signalling for both G protein-dependent and -independent pathways.

Nck1 deficiency improves pancreatic ß cell survival to diabetes-relevant stresses by modulating PERK activation and signaling.

Increasing evidence strongly supports a critical role for PERK in regulating pancreatic ß cell function. In agreement, we previously reported that enhancing PERK basal activity, by silencing the SH domain-containing adaptor protein Nck1 in pancreatic ß cells, increased insulin content in a PERK-dependent manner. Here we report that Nck1-deficient MIN6 cells display normal overall morphology while as expected increased number of secretory granules. Furthermore, we demonstrate that cell survival to diabetes-relevant stresses is increased, while cell viability in response to chemical endoplasmic reticulum (ER) stress inducers is not changed. In agreement, PERK activation in Nck1-depleted MIN6 cells exposed to palmitate was significantly reduced while it remained strongly induced by the ER stress inducer thapsigargin. Interestingly, silencing Nck1 in MIN6 cells results in increased PERK basal activity and expression of the PERK downstream target sestrin2, which promotes autophagy by attenuating mTORC1 activation through AMPK-dependent and -independent mechanisms. Accordingly, activated AMPK was increased, mTORC1 signaling decreased, and autophagy markers increased in Nck1-silenced MIN6 cells. Increased autophagy was recapitulated in Nck1(-/-) mice pancreatic ß cells. In addition, basal levels of the PERK substrate Nrf2 and its antioxidant gene targets (HO-1 and Nqo1) were upregulated in Nck1-silenced MIN6 cells, revealing an active PERK-Nrf2 signaling in these cells. Finally, Akt activation was increased in Nck1-silenced MIN6 cells. Altogether, this study demonstrates that Nck1 silencing in pancreatic ß cells promotes PERK activation and signaling to protect ß cells against pathological stresses. These findings further provide new perspectives to advance our understanding of molecular mechanisms and signaling systems regulating pancreatic ß cell fates.

Interaction of Nck1 and PERK phosphorylated at Y56¹ negatively modulates PERK activity and PERK regulation of pancreatic ß-cell proinsulin content.

PERK, the PKR-like endoplasmic reticulum (ER) kinase, is an ER transmembrane serine/threonine protein kinase activated during ER stress. In this study, we provide evidence that the Src-homology domain-containing adaptor Nck1 negatively regulates PERK. We show that Nck directly binds to phosphorylated Y(561) in the PERK juxtamembrane domain through its SH2 domain. We demonstrate that mutation of Y(561) to a nonphosphorylatable residue (Y561F) promotes PERK activity, suggesting that PERK phosphorylation at Y(561) (pY(561)PERK) negatively regulates PERK. In agreement, we show that pY(561)PERK delays PERK activation and signaling during ER stress. Compatible with a role for PERK in pancreatic ß-cells, we provide strong evidence that Nck1 contributes to PERK regulation of pancreatic ß-cell proteostasis. In fact, we demonstrated that down-regulation of Nck1 in mouse insulinoma MIN6 cells results in faster dephosphorylation of pY(561)PERK, which correlates with enhanced PERK activation, increased insulin biosynthesis, and PERK-dependent increase in proinsulin content. Furthermore, we report that pancreatic islets in whole-body Nck1-knockout mice contain more insulin than control littermates. Together our data strongly suggest that Nck1 negatively regulates PERK by interacting with PERK and protecting PERK from being dephosphorylated at its inhibitory site pY(561) and in this way affects pancreatic ß-cell proinsulin biogenesis.

Deletion of Nck1 attenuates hepatic ER stress signaling and improves glucose tolerance and insulin signaling in liver of obese mice.

Obesity has been shown to create stress in the endoplasmic reticulum (ER), and that initiates the activation of the unfolded protein response (UPR). This has been reported to cause insulin resistance in selective tissues through activation of the inositol-requiring enzyme 1α (IRE1α)-c-Jun NH(2)-terminal kinase (JNK) pathway, which results in the phosphorylation of the insulin receptor substrate-1 (IRS-1) at an inhibitory site and blocks insulin receptor signaling. In this study, we report that the Src homology domain-containing adaptor protein Nck1, previously shown to modulate the UPR, is of functional importance in obesity-induced ER stress signaling and inhibition of insulin actions. We have examined obese Nck1(-/-) and Nck1(+/+) mice for glucose tolerance, insulin sensitivity, and signaling as well as for ER stress markers and IRS-1 phosphorylation at Ser(307). Our findings show that obese Nck1-deficient mice display improved glucose disposal accompanied by enhanced insulin signaling in liver. This correlates with attenuated IRE1α and JNK activation and IRS-1 phosphorylation at Ser(307) compared with obese wild-type mice. Consistent with our in vivo data, we report that downregulation of Nck1 using siRNA in HepG2 cells results in decreased thapsigargin-induced IRE1α activation and signaling and IRS-1 phosphorylation at Ser(307), whereas it markedly enhances insulin signaling. Overall, in liver and in cultured cells, we show that depletion of Nck1 attenuates the UPR signal and its inhibitory action on insulin signaling. Taken all together, our findings implicate Nck1 in regulating the UPR, which secondary to obesity impairs glucose homeostasis and insulin actions.

Transcobalamin in cultured fibroblasts from patients with inborn errors of vitamin B12 metabolism.

Derivatives of vitamin B(12) (cobalamin, Cbl) are required for activity of the mitochondrial enzyme L-methylmalonyl-CoA mutase and the cytoplasmic enzyme methionine synthase in human cells. We recently described a putative novel Cbl-binding protein in crude mitochondrial fractions isolated from cultured fibroblasts. The amount of Cbl bound to this protein varied in fibroblasts from patients with different genetic defects affecting cobalamin metabolism. We have now identified this protein as the cobalamin transport protein transcobalamin (TC) by its binding to anti-TC antibodies and mass spectrometry, and suggest that its presence in crude mitochondrial fractions was the result of lysosomal contamination. Increased Cbl bound TC levels were confirmed in whole cell extracts in at least one cell line from both the cblB and mut classes of inborn errors of cobalamin metabolism.