A BRET assay for monitoring insulin receptor interactions and ligand pharmacology

The insulin receptor (IR) belongs to the receptor tyrosine kinase super family and plays an important role in glucose homeostasis. The receptor interacts with several large docking proteins that mediate signaling from the receptor, including the insulin receptor substrate (IRS) family and Src homology-2-containing proteins (Src). Here, we applied the bioluminescence resonance energy transfer 2 (BRET2) technique to study the IR signaling pathways. The interaction between the IR and the substrates IRS1, IRS4 and Shc was examined in response to ligands with different signaling properties. The association between IR and the interacting partners could successfully be monitored when co-expressing green fluorescent protein 2 (GFP2) tagged substrates with Renilla reniformis luciferase 8 (Rluc8) tagged IR. Through additional optimization steps, we developed a stable and flexible BRET2 assay for monitoring the interactions between the IR and its substrates. Furthermore, the insulin analogue X10 was characterized in the BRET2 assay and was found to be 10 times more potent with respect to IRS1, IRS4 and Shc recruitment compared to human insulin. This study demonstrates that the BRET2 technique can be applied to study IR signaling pathways, and that this assay can be used as a platform for screening and characterization of IR ligands.

AT(1) receptor Gαq protein-independent signalling transcriptionally activates only a few genes directly, but robustly potentiates gene regulation from the β2-adrenergic receptor

The angiotensin II type 1 receptor (AT(1)R) is known to signal through heterotrimeric G proteins, and Gαq protein-independent signalling has only recently gained appreciation for profound impact on a diverse range of biological functions. β-Arrestins, among other central mediators of Gαq protein-independent signalling from the AT(1)R interact with transcriptional regulators and promote phosphorylation of nuclear proteins. However, the relative contribution of Gαq protein-independent signalling in AT(1)R mediated transcriptional regulation remains elusive. We here present a comprehensive comparative analysis of Gαq protein-dependent and -independent regulation of AT(1)R mediated gene expression. We found angiotensin II to regulate 212 genes, whereas Gαq-independent signalling obtained with the biased agonist, SII angiotensin II only regulated few genes. Interestingly, SII angiotensin II, like Ang II vastly potentiated β2-adrenergic receptor-stimulated gene expression. These novel findings indicate that the Gαq protein-independent signalling mainly modifies the transcriptional response governed by other signalling pathways, while direct induction of gene expression by the AT(1)R is dependent on classical Gαq protein activation.

Tracking of leptin, soluble leptin receptor, and the free leptin index during weight loss and regain in children

OBJECTIVE

To investigate changes in leptin and soluble leptin receptor (SLR) concentrations, and in the free leptin index (FLI) during weight loss and subsequent weight regain; and to ascertain whether these indices remain stable in the rank of the distribution in repeated measures (tracking) during perturbations of weight in obese children.

DESIGN AND MEASUREMENTS

In a longitudinal study, 115 obese children were examined during a 12- week weight loss programme and 28 months of follow-up. Height, weight, body composition, Tanner stages, testicular size, time of menarche, and concentrations of leptin and SLR were measured at baseline, on days 14, 33, and 82, and from months 10, 16, and 28.

RESULTS

During weight loss, leptin decreased and the SLR increased. During weight regain, leptin increased and the SLR decreased. The partial correlation coefficients expressing the relationship between leptin and SLR were significant in girls during both weight loss and weight regain, whereas in boys they were much weaker and not significant. Leptin, SLR and FLI exhibited individual-specific levels (tracking) during weight loss and regain in boys and girls. The observed tracking seemed stronger during weight loss than during weight regain. The observed tracking was independent of both baseline body mass index (BMI) standard deviation score (SDS) and pubertal development at baseline and of subsequent changes in BMI SDS and puberty stages.

CONCLUSION

Leptin and the SLR exhibit tracking during weight loss and regain, which indicates individual stability in the leptin system despite challenges of weight.

Biased signaling of the angiotensin II type 1 receptor can be mediated through distinct mechanisms

BACKGROUND

Seven transmembrane receptors (7TMRs) can adopt different active conformations facilitating a selective activation of either G protein or β-arrestin-dependent signaling pathways. This represents an opportunity for development of novel therapeutics targeting selective biological effects of a given receptor. Several studies on pathway separation have been performed, many of these on the Angiotensin II type 1 receptor (AT1R). It has been shown that certain ligands or mutations facilitate internalization and/or recruitment of β-arrestins without activation of G proteins. However, the underlying molecular mechanisms remain largely unresolved. For instance, it is unclear whether such selective G protein-uncoupling is caused by a lack of ability to interact with G proteins or rather by an increased ability of the receptor to recruit β-arrestins. Since uncoupling of G proteins by increased ability to recruit β-arrestins could lead to different cellular or in vivo outcomes than lack of ability to interact with G proteins, it is essential to distinguish between these two mechanisms.

METHODOLOGY/PRINCIPAL FINDINGS

We studied five AT1R mutants previously published to display pathway separation: D74N, DRY/AAY, Y292F, N298A, and Y302F (Ballesteros-Weinstein numbering: 2.50, 3.49-3.51, 7.43, 7.49, and 7.53). We find that D74N, DRY/AAY, and N298A mutants are more prone to β-arrestin recruitment than WT. In contrast, receptor mutants Y292F and Y302F showed impaired ability to recruit β-arrestin in response to Sar1-Ile4-Ile8 (SII) Ang II, a ligand solely activating the β-arrestin pathway.

CONCLUSIONS/SIGNIFICANCE

Our analysis reveals that the underlying conformations induced by these AT1R mutants most likely represent principally different mechanisms of uncoupling the G protein, which for some mutants may be due to their increased ability to recruit β-arrestin2. Hereby, these findings have important implications for drug discovery and 7TMR biology and illustrate the necessity of uncovering the exact molecular determinants for G protein-coupling and β-arrestin recruitment, respectively.

beta-Arrestin 1 and 2 stabilize the angiotensin II type I receptor in distinct high-affinity conformations

BACKGROUND AND PURPOSE

The angiotensin II type 1 (AT(1)) receptor belongs to family A of 7 transmembrane (7TM) receptors. The receptor has important roles in the cardiovascular system and is commonly used as a drug target in cardiovascular diseases. Interaction of 7TM receptors with G proteins or beta-arrestins often induces higher binding affinity for agonists. Here, we examined interactions between AT(1A) receptors and beta-arrestins to look for differences between the AT(1A) receptor interaction with beta-arrestin1 and beta-arrestin2.

EXPERIMENTAL APPROACH

Ligand-induced interaction between AT(1A) receptors and beta-arrestins was measured by Bioluminescence Resonance Energy Transfer 2. AT(1A)-beta-arrestin1 and AT(1A)-beta-arrestin2 fusion proteins were cloned and tested for differences using immunocytochemistry, inositol phosphate hydrolysis and competition radioligand binding.

KEY RESULTS

Bioluminescence Resonance Energy Transfer 2 analysis showed that beta-arrestin1 and 2 were recruited to AT(1A) receptors with similar ligand potencies and efficacies. The AT(1A)-beta-arrestin fusion proteins showed attenuated G protein signalling and increased agonist binding affinity, while antagonist affinity was unchanged. Importantly, larger agonist affinity shifts were observed for AT(1A)-beta-arrestin2 than for AT(1A)-beta-arrestin1.

CONCLUSION AND IMPLICATIONS

beta-Arrestin1 and 2 are recruited to AT(1A) receptors with similar ligand pharmacology and stabilize AT(1A) receptors in distinct high-affinity conformations. However, beta-arrestin2 induces a receptor conformation with a higher agonist-binding affinity than beta-arrestin1. Thus, this study demonstrates that beta-arrestins interact with AT(1A) receptors in different ways and suggest that AT(1) receptor biased agonists with the ability to recruit either of the beta-arrestins selectively, would be possible to design.

Activation of the TASK-2 channel after cell swelling is dependent on tyrosine phosphorylation

The swelling-activated K(+) currents (I(K,vol)) in Ehrlich ascites tumor cells (EATC) has been reported to be through the two-pore domain (K(2p)), TWIK-related acid-sensitive K(+) channel 2 (TASK-2). The regulatory volume decrease (RVD), following hypotonic exposure in EATC, is rate limited by I(K,vol) indicating that inhibition of RVD reflects inhibition of TASK-2. We find that in EATC the tyrosine kinase inhibitor genistein inhibits RVD by 90%, and that the tyrosine phosphatase inhibitor monoperoxo(picolinato)-oxo-vanadate(V) [mpV(pic)] shifted the volume set point for inactivation of the channel to a lower cell volume. Swelling-activated K(+) efflux was impaired by genistein and the Src kinase family inhibitor 4-amino-5-(4-chloro-phenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) and enhanced by the tyrosine phosphatase inhibitor mpV(pic). With the use of the TASK-2 inhibitor clofilium, it is demonstrated that mpV(pic) increased the volume-sensitive part of the K(+) efflux 1.3 times. To exclude K(+) efflux via a KCl cotransporter, cellular Cl(-) was substituted with NO(3)(-). Also under these conditions K(+) efflux was completely blocked by genistein. Thus tyrosine kinases seem to be involved in the activation of the volume-sensitive K(+) channel, whereas tyrosine phosphatases appears to be involved in inactivation of the channel. Overexpressing TASK-2 in human embryonic kidney (HEK)-293 cells increased the RVD rate and reduced the volume set point. TASK-2 has tyrosine sites, and precipitation of TASK-2 together with Western blotting and antibodies against phosphotyrosines revealed a cell swelling-induced, time-dependent tyrosine phosphorylation of the channel. Even though we found an inhibiting effect of PP2 on RVD, neither Src nor the focal adhesion kinase (FAK) seem to be involved. Inhibitors of the epidermal growth factor receptor tyrosine kinases had no effect on RVD, whereas the Janus kinase (JAK) inhibitor cucurbitacin inhibited the RVD by 40%. It is suggested that the cytokine receptor-coupled JAK/STAT pathway is upstream of the swelling-induced phosphorylation and activation of TASK-2 in EATC.

Quantitative phosphoproteomics dissection of seven-transmembrane receptor signaling using full and biased agonists

Seven-transmembrane receptors (7TMRs) signal through the well described heterotrimeric G proteins but can also activate G protein-independent signaling pathways of which the impact and complexity are less understood. The angiotensin II type 1 receptor (AT(1)R) is a prototypical 7TMR and an important drug target in cardiovascular diseases. "Biased agonists" with intrinsic "functional selectivity" that simultaneously blocks Galpha(q) protein activity and activates G protein-independent pathways of the AT(1)R confer important perspectives in treatment of cardiovascular diseases. In this study, we performed a global quantitative phosphoproteomics analysis of the AT(1)R signaling network. We analyzed ligand-stimulated SILAC (stable isotope labeling by amino acids in cell culture) cells by high resolution (LTQ-Orbitrap) MS and compared the phosphoproteomes of the AT(1)R agonist angiotensin II and the biased agonist [Sar(1),Ile(4),Ile(8)]angiotensin II (SII angiotensin II), which only activates the Galpha(q) protein-independent signaling. We quantified more than 10,000 phosphorylation sites of which 1183 were regulated by angiotensin II or its analogue SII angiotensin II. 36% of the AT(1)R-regulated phosphorylations were regulated by SII angiotensin II. Analysis of phosphorylation site patterns showed a striking distinction between protein kinases activated by Galpha(q) protein-dependent and -independent mechanisms, and we now place protein kinase D as a key protein involved in both Galpha(q)-dependent and -independent AT(1)R signaling. This study provides substantial novel insight into angiotensin II signal transduction and is the first study dissecting the differences between a full agonist and a biased agonist from a 7TMR on a systems-wide scale. Importantly, it reveals a previously unappreciated diversity and quantity of Galpha(q) protein-independent signaling and uncovers novel signaling pathways. We foresee that the amount and diversity of G protein-independent signaling may be more pronounced than previously recognized for other 7TMRs as well. Quantitative mass spectrometry is a promising tool for evaluation of the signaling properties of biased agonists to other receptors in the future.

Validation of the ICSD-2 criteria for CSF hypocretin-1 measurements in the diagnosis of narcolepsy in the Danish population

STUDY OBJECTIVES

The International Classification of Sleep Disorders (ICSD-2) criteria for low CSF hypocretin-1 levels (CSF hcrt-1) still need validation as a diagnostic tool for narcolepsy in different populations because inter-assay variability and different definitions of hypocretin deficiency complicate direct comparisons of study results.

DESIGN AND PARTICIPANTS

Interviews, polysomnography, multiple sleep latency test, HLA-typing, and CSF hcrt-1 measurements in Danish patients with narcolepsy with cataplexy (NC) and narcolepsy without cataplexy (NwC), CSF hcrt-1 measurements in other hypersomnias, neurological and normal controls. Comparisons of hypocretin deficiency and frequency of HLA-DQB1*0602-positivity in the Danish and eligible NC and NwC populations (included via MEDLINE search), by (re)calculation of study results using the ICSD-2 criterion for low CSF hcrt-1 (< 30% of normal mean).

MEASUREMENTS AND RESULTS

In Danes, low CSF hcrt-1 was present in 40/46 NC, 3/14 NwC and 0/106 controls (P < 0.0001). Thirty-nine of 41 NC and 4/13 NwC patients were HLA-DQB1*0602-positive (P < 0.01). Hypocretin-deficient NC patients had higher frequency of cataplexy, shorter mean sleep latency, more sleep onset REM periods (P < 0.05) and more awakenings (NS) than did NC patients with normal CSF hcrt-1. Across populations, low CSF hcrt-1 and HLA-DQB1*0602-positivity characterized the majority of NC (80% to 100%, P = 0.53; 80% to 100%, P = 0.11) but a minority of NwC patients (11% to 29%, P = 0.75; 29% to 89%, P = 0.043).

CONCLUSION

The study provides evidence that hypocretin deficiency causes a more severe NC phenotype. The ICSD-2 criterion for low CSF hcrt-1 (< 30% of normal mean) is valid for diagnosing NC, but not NwC. HLA-typing should precede CSF hcrt-1 measurements because hypocretin deficiency is rare in HLA-DQB1*0602-negative patients.

Longitudinal analysis of leptin variation during weight regain after weight loss in obese children

OBJECTIVE

This study assessed if lower than predicted serum leptin concentrations seen during weight loss persisted during weight regain, with possible implications for weight control.

METHODS

115 children were investigated during a 12-week weight loss program. 90 children completed the program, and 68 children entered a follow-up program spanning 28 months. Measurements were performed at baseline and day 82 as well as at months 10, 16, and 28. Height, weight, body composition, Tanner stages, testicular size, and serum concentrations of leptin, and insulin were measured at all time points.

RESULTS

Children with the greatest increases in BMI standard deviation score (SDS) exhibited the largest leptin increments. The disproportionate reduction of leptin seen during weight loss recovered after weight loss. Leptin increases mirrored increases in BMI SDS during weight regain, and the leptin-BMI SDS relationship seen during follow-up resembled the baseline leptin-BMI SDS relationship.

CONCLUSION

Proportional increases of leptin and BMI SDS during weight regain suggests an intact leptin response during re-accumulation of fat. Following the pronounced reduction of leptin during weight loss, leptin levels were restored during weight regain to an extent where leptin levels were comparable with those at baseline, which is indicative of an inefficient lipostatic control exerted by leptin during weight regain.

Variation within the PPARG gene is associated with residual beta-cell function and glycemic control in children and adolescents during the first year of clinical type 1 diabetes

CONTEXT

Conflicting evidence exists as to whether the Pro12Ala single nucleotide polymorphism of the type 2 diabetes susceptibility gene peroxisome proliferator-activated receptor gamma (PPARG) also confers risk for type 1 diabetes (T1D).

OBJECTIVE

The objective of this study was to investigate the PPARG gene in relation to residual beta-cell function and glycemic control in newly diagnosed T1D.

DESIGN

Prospective, non-interventional, 12-month follow-up study, conducted in 18 centers in 15 countries.

PATIENTS

Two hundred and fifty-seven children and adolescents (aged <16 yr) with newly diagnosed T1D.

MAIN OUTCOME MEASURES

Beta-cell function was determined as 90-min meal-stimulated C-peptide (Boost test) 1, 6, and 12 months after diagnosis. Hemoglobin A1c (HbA1c) and daily insulin dose (IU/kg/d) were recorded at 1, 3, 6, 9, and 12 months after diagnosis. Haplotypes within PPARG were estimated by SNPHap program. Statistical analyses were performed in a repeated measurements model.

RESULTS

Five haplotypes within PPARG were generated (h1, 68.4%; h2, 16.3%; h3, 8.3%; h4, 3.5%; and hx, 3.5%). Compared with the most frequent h1 haplotype, the haplotypes h3 and h4 of the PPARG associated with residual beta-cell function during the first year of clinical disease: h3 with a 27% lower C-peptide (p = 0.02) and h4 with a 39% lower C-peptide (p = 0.01). Haplotype h4 also associated with a 0.86% (absolute) higher HbA1c, after adjustment for the insulin dose (p = 0.02).

CONCLUSION

Variation in the PPARG locus may influence disease progression during the first year after the presentation of T1D.

Normal levels of cerebrospinal fluid hypocretin-1 and daytime sleepiness during attacks of relapsing-remitting multiple sclerosis and monosymptomatic optic neuritis

There is emerging evidence that multiple sclerosis (MS), the hypothalamic sleep-wake regulating neuropeptide hypocretin-1 (hcrt-1) and the sleep disorder narcolepsy may be connected. Thus, the major pathophysiological component of narcolepsy is lack of hcrt-1. Dysfunction of the hypocretin system has been reported in MS case reports with attacks of hypothalamic lesions, undetectable cerebrospinal fluid (CSF) hcrt-1 and hypersomnia, but not found during remission in small samples. Finally, daytime sleepiness, the major symptom of narcolepsy, is reported in several MS populations, and there are case reports of co-existent narcolepsy and MS. However, it is unknown whether hcrt-1 and daytime sleepiness generally change during MS attacks. We therefore analyzed whether daytime sleepiness (using the Epworth Sleepiness Scale (ESS)) and CSF hcrt-1 levels differed between MS attack and remission, in 48 consecutively referred patients with relapsing–remitting MS (RRMS) or monosymptomatic optic neuritis (MON). Twenty-seven patients were in attack and 21 in remission. ESS was normal both during attacks (5.4 ± 3.0) and remission (5.8 ± 2.6), and mean CSF hcrt-1 was normal (456 ± 41 pg/ml). No statistically significant differences were found between attack and remission. MRI scans revealed no hypothalamic lesions. The results show that the hypocretin system is intact and sleepiness is not typical in RRMS and MON without hypothalamic lesions on MRI.

Differential extracellular signal-regulated kinases 1 and 2 activation by the angiotensin type 1 receptor supports distinct phenotypes of cardiac myocytes

The angiotensin II (AngII) type 1 receptor (AT(1)R) is a seven-transmembrane receptor well established to activate extracellular signal-regulated kinases 1 and 2 (ERK1/2) by discrete G protein-dependent and beta-arrestin2-dependent pathways. The biological importance of this, however, remains obscure. Application of the modified analogue [Sar(1), Ile(4), Ile(8)]-AngII ([SII] AngII) allowed us to dissect the two pathways of ERK1/2 activation in native cardiac myocytes. Although cytosol-retained, the beta-arrestin2-bound pool of ERK1/2 represents an active signalling component that phosphorylates p90 Ribosomal S6 Kinase, a ubiquitous and versatile mediator of ERK1/2 signal transduction. Moreover, the beta-arrestin2-dependent ERK1/2 signal supports intact proliferation of cardiac myocytes. In contrast to G(q)-activated ERK1/2, and in keeping with its failure to translocate to the nucleus, the beta-arrestin2-scaffolded pool of ERK1/2 does not phosphorylate the transcription factor Elk-1, induces no increased transcription of the immediate-early gene c-Fos, and does not entail myocyte hypertrophy. These results clearly demonstrate the biological significance of differential signalling by the AT(1)R. The opportunity to separate desirable cardiac myocyte division from detrimental hypertrophy holds promise that novel pharmacological approaches will allow targeting of pathway-specific actions.

The angiotensin type 1 receptor activates extracellular signal-regulated kinases 1 and 2 by G protein-dependent and -independent pathways in cardiac myocytes and langendorff-perfused hearts

The angiotensin II (AngII) type 1 receptor (AT(1)R) has been shown to activate extracellular signal-regulated kinases 1 and 2 (ERK1/2) through G proteins or G protein-independently through beta-arrestin2 in cellular expression systems. As activation mechanisms may greatly influence the biological effects of ERK1/2 activity, differential activation of the AT(1)R in its native cellular context could have important biological and pharmacological implications. To examine if AT(1)R activates ERK1/2 by G protein-independent mechanisms in the heart, we used the [Sar(1), Ile(4), Ile(8)]-AngII ([SII] AngII) analogue in native preparations of cardiac myocytes and beating hearts. We found that [SII] AngII does not activate G(q)-coupling, yet stimulates the beta-arrestin2-dependent ERK1/2. The G(q)-activated pool of ERK1/2 rapidly translocates to the nucleus, while the beta-arrestin2-scaffolded pool remains in the cytosol. Similar biased agonism was achieved in Langendorff-perfused hearts, where both agonists elicit ERK1/2 phosphorylation, but [SII] AngII induces neither inotropic nor chronotropic effects.