TSH Pulses Finely Tune Thyroid Hormone Release and TSH Receptor Transduction

Detection of circulating TSH is a first-line test of thyroid dysfunction, a major health problem (affecting about 5% of the population) that, if untreated, can lead to a significant deterioration of quality of life and adverse effects on multiple organ systems. Human TSH levels display both pulsatile and (nonpulsatile) basal TSH secretion patterns; however, the importance of these in regulating thyroid function and their decoding by the thyroid is unknown. Here, we developed a novel ultra-sensitive ELISA that allows precise detection of TSH secretion patterns with minute resolution in mouse models of health and disease. We characterized the patterns of ultradian TSH pulses in healthy, freely behaving mice over the day-night cycle. Challenge of the thyroid axis with primary hypothyroidism because of iodine deficiency, a major cause of thyroid dysfunction worldwide, results in alterations of TSH pulsatility. Induction in mouse models of sequential TSH pulses that mimic ultradian TSH profiles in periods of minutes were more efficient than sustained rises in basal TSH levels at increasing both thyroid follicle cAMP levels, as monitored with a genetically encoded cAMP sensor, and circulating thyroid hormone. Hence, this mouse TSH assay provides a powerful tool to decipher how ultradian TSH pulses encode thyroid outcomes and to uncover hidden parameters in the TSH-thyroid hormone set-point in health and disease.

Plasma membrane preassociation drives β-arrestin coupling to receptors and activation

β-arrestin plays a key role in G protein-coupled receptor (GPCR) signaling and desensitization. Despite recent structural advances, the mechanisms that govern receptor-β-arrestin interactions at the plasma membrane of living cells remain elusive. Here, we combine single-molecule microscopy with molecular dynamics simulations to dissect the complex sequence of events involved in β-arrestin interactions with both receptors and the lipid bilayer. Unexpectedly, our results reveal that β-arrestin spontaneously inserts into the lipid bilayer and transiently interacts with receptors via lateral diffusion on the plasma membrane. Moreover, they indicate that, following receptor interaction, the plasma membrane stabilizes β-arrestin in a longer-lived, membrane-bound state, allowing it to diffuse to clathrin-coated pits separately from the activating receptor. These results expand our current understanding of β-arrestin function at the plasma membrane, revealing a critical role for β-arrestin preassociation with the lipid bilayer in facilitating its interactions with receptors and subsequent activation.

Temperature and friction fluctuations inside a harmonic potential

In this article we study the trapped motion of a molecule undergoing diffusivity fluctuations inside a harmonic potential. For the same diffusing-diffusivity process, we investigate two possible interpretations. Depending on whether diffusivity fluctuations are interpreted as temperature or friction fluctuations, we show that they display drastically different statistical properties inside the harmonic potential. We compute the characteristic function of the process under both types of interpretations and analyze their limit behavior. Based on the integral representations of the processes we compute the mean-squared displacement and the normalized excess kurtosis. In the long-time limit, we show for friction fluctuations that the probability density function (PDF) always converges to a Gaussian whereas in the case of temperature fluctuations the stationary PDF can display either Gaussian distribution or generalized Laplace (Bessel) distribution depending on the ratio between diffusivity and positional correlation times.

How Carvedilol activates β2-adrenoceptors

Carvedilol is among the most effective β-blockers for improving survival after myocardial infarction. Yet the mechanisms by which carvedilol achieves this superior clinical profile are still unclear. Beyond blockade of β1-adrenoceptors, arrestin-biased signalling via β2-adrenoceptors is a molecular mechanism proposed to explain the survival benefits. Here, we offer an alternative mechanism to rationalize carvedilol's cellular signalling. Using primary and immortalized cells genome-edited by CRISPR/Cas9 to lack either G proteins or arrestins; and combining biological, biochemical, and signalling assays with molecular dynamics simulations, we demonstrate that G proteins drive all detectable carvedilol signalling through β2ARs. Because a clear understanding of how drugs act is imperative to data interpretation in basic and clinical research, to the stratification of clinical trials or to the monitoring of drug effects on the target pathway, the mechanistic insight gained here provides a foundation for the rational development of signalling prototypes that target the β-adrenoceptor system.

ADRA1A-Gαq signalling potentiates adipocyte thermogenesis through CKB and TNAP

Noradrenaline (NA) regulates cold-stimulated adipocyte thermogenesis¹. Aside from cAMP signalling downstream of β-adrenergic receptor activation, how NA promotes thermogenic output is still not fully understood. Here, we show that coordinated α₁-adrenergic receptor (AR) and β₃-AR signalling induces the expression of thermogenic genes of the futile creatine cycle^2,3, and that early B cell factors, oestrogen-related receptors and PGC1α are required for this response in vivo. NA triggers physical and functional coupling between the α₁-AR subtype (ADRA1A) and Gα_q to promote adipocyte thermogenesis in a manner that is dependent on the effector proteins of the futile creatine cycle, creatine kinase B and tissue-non-specific alkaline phosphatase. Combined Gα_q and Gα_s signalling selectively in adipocytes promotes a continual rise in whole-body energy expenditure, and creatine kinase B is required for this effect. Thus, the ADRA1A-Gα_q-futile creatine cycle axis is a key regulator of facultative and adaptive thermogenesis.

Dimerization of GPCRs: Novel insight into the role of FLNA and SSAs regulating SST2 and SST5 homo- and hetero-dimer formation

The process of GPCR dimerization can have profound effects on GPCR activation, signaling, and intracellular trafficking. Somatostatin receptors (SSTs) are class A GPCRs abundantly expressed in pituitary tumors where they represent the main pharmacological targets of somatostatin analogs (SSAs), thanks to their antisecretory and antiproliferative actions. The cytoskeletal protein filamin A (FLNA) directly interacts with both somatostatin receptor type 2 (SST₂) and 5 (SST₅) and regulates their expression and signaling in pituitary tumoral cells. So far, the existence and physiological relevance of SSTs homo- and hetero-dimerization in the pituitary have not been explored. Moreover, whether octreotide or pasireotide may play modulatory effects and whether FLNA may participate to this level of receptor organization have remained elusive. Here, we used a proximity ligation assay (PLA)-based approach for the in situ visualization and quantification of SST₂/SST₅ dimerization in rat GH3 as well as in human melanoma cells either expressing (A7) or lacking (M2) FLNA. First, we observed the formation of endogenous SST₅ homo-dimers in GH3, A7, and M2 cells. Using the PLA approach combined with epitope tagging, we detected homo-dimers of human SST₂ in GH3, A7, and M2 cells transiently co-expressing HA- and SNAP-tagged SST₂. SST₂ and SST₅ can also form endogenous hetero-dimers in these cells. Interestingly, FLNA absence reduced the basal number of hetero-dimers (-36.8 ± 6.3% reduction of PLA events in M2, P < 0.05 vs. A7), and octreotide but not pasireotide promoted hetero-dimerization in both A7 and M2 (+20.0 ± 11.8% and +44.1 ± 16.3% increase of PLA events in A7 and M2, respectively, P < 0.05 vs. basal). Finally, immunofluorescence data showed that SST₂ and SST₅ recruitment at the plasma membrane and internalization are similarly induced by octreotide and pasireotide in GH3 and A7 cells. On the contrary, in M2 cells, octreotide failed to internalize both receptors whereas pasireotide promoted robust receptor internalization at shorter times than in A7 cells. In conclusion, we demonstrated that in GH3 cells SST₂ and SST₅ can form both homo- and hetero-dimers and that FLNA plays a role in the formation of SST₂/SST₅ hetero-dimers. Moreover, we showed that FLNA regulates SST₂ and SST₅ intracellular trafficking induced by octreotide and pasireotide.

Is Disrupted Nucleotide-Substrate Cooperativity a Common Trait for Cushing's Syndrome Driving Mutations of Protein Kinase A?

Somatic mutations in the PRKACA gene encoding the catalytic α subunit of protein kinase A (PKA-C) are responsible for cortisol-producing adrenocortical adenomas. These benign neoplasms contribute to the development of Cushing's syndrome. The majority of these mutations occur at the interface between the two lobes of PKA-C and interfere with the enzyme's ability to recognize substrates and regulatory (R) subunits, leading to aberrant phosphorylation patterns and activation. Rarely, patients with similar phenotypes carry an allosteric mutation, E31V, located at the C-terminal end of the αA-helix and adjacent to the αC-helix, but structurally distinct from the PKA-C/R subunit interface mutations. Using a combination of solution NMR, thermodynamics, kinetic assays, and molecular dynamics simulations, we show that the E31V allosteric mutation disrupts central communication nodes between the N- and C- lobes of the enzyme as well as nucleotide-substrate binding cooperativity, a hallmark for kinases' substrate fidelity and regulation. For both orthosteric (L205R and W196R) and allosteric (E31V) Cushing's syndrome mutants, the loss of binding cooperativity is proportional to the density of the intramolecular allosteric network. This structure-activity relationship suggests a possible common mechanism for Cushing's syndrome driving mutations in which decreased nucleotide/substrate binding cooperativity is linked to loss in substrate fidelity and dysfunctional regulation.

Lipolysis drives expression of the constitutively active receptor GPR3 to induce adipose thermogenesis

Thermogenic adipocytes possess a therapeutically appealing, energy-expending capacity, which is canonically cold-induced by ligand-dependent activation of β-adrenergic G protein-coupled receptors (GPCRs). Here, we uncover an alternate paradigm of GPCR-mediated adipose thermogenesis through the constitutively active receptor, GPR3. We show that the N terminus of GPR3 confers intrinsic signaling activity, resulting in continuous Gs-coupling and cAMP production without an exogenous ligand. Thus, transcriptional induction of Gpr3 represents the regulatory parallel to ligand-binding of conventional GPCRs. Consequently, increasing Gpr3 expression in thermogenic adipocytes is alone sufficient to drive energy expenditure and counteract metabolic disease in mice. Gpr3 transcription is cold-stimulated by a lipolytic signal, and dietary fat potentiates GPR3-dependent thermogenesis to amplify the response to caloric excess. Moreover, we find GPR3 to be an essential, adrenergic-independent regulator of human brown adipocytes. Taken together, our findings reveal a noncanonical mechanism of GPCR control and thermogenic activation through the lipolysis-induced expression of constitutively active GPR3.

Spatiotemporal GLP-1 and GIP receptor signaling and trafficking/recycling dynamics induced by selected receptor mono- and dual-agonists

Objective

We assessed the spatiotemporal GLP-1 and GIP receptor signaling, trafficking, and recycling dynamics of GIPR mono-agonists, GLP-1R mono-agonists including semaglutide, and GLP-1/GIP dual-agonists MAR709 and tirzepatide.

Methods

Receptor G protein recruitment and internalization/trafficking dynamics were assessed using bioluminescence resonance energy transfer (BRET)-based technology and live-cell HILO microscopy.

Results

Relative to native and acylated GLP-1 agonists, MAR709 and tirzepatide showed preserved maximal cAMP production despite partial Gα_s recruitment paralleled by diminished ligand-induced receptor internalization at both target receptors. Despite MAR709's lower internalization rate, GLP-1R co-localization with Rab11-associated recycling endosomes was not different between MAR709 and GLP-1R specific mono-agonists.

Conclusions

Our data indicated that MAR709 and tirzepatide induce unique spatiotemporal GLP-1 and GIP receptor signaling, trafficking, and recycling dynamics relative to native peptides, semaglutide, and matched mono-agonist controls. These findings support the hypothesis that the structure of GLP-1/GIP dual-agonists confer a biased agonism that, in addition to its influence on intracellular signaling, uniquely modulates receptor trafficking.

•

GLP-1/GIP dual-agonists, MAR709 and tirzepatide, are partial effectors at multiple GLP-1R pathways, yet retain full cAMP agonism.

•

MAR709 elicits comparable GLP-1R incorporation into Rab11⁺ recycling endosomes relative to the native peptides and acyl-GLP-1.

•

At the GIPR, both dual-agonists exhibit full-agonism properties with limited receptor internalization/trafficking properties.

EJE AWARD 2020: Signalling by G protein-coupled receptors: why space and time matter

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors and major drug targets. They play a fundamental role in the endocrine system, where they mediate the effects of several hormones and neurotransmitters. As a result, alterations of GPCR signalling are a major cause of endocrine disorders such as congenital hypothyroidism or Cushing's syndrome. My group develops innovative optical methods such as fluorescence resonance energy transfer (FRET) and single-molecule microscopy, which allow us to investigate GPCR signalling in living cells with unprecedented spatiotemporal resolution. Using this innovative approach, we have contributed to elucidate some long-debated questions about the mechanisms of GPCR signalling and their involvement in human disease. Among other findings, these studies have led to the unexpected discovery that GPCRs are not only signalling at the cell surface, as previously assumed, but also at various intracellular sites. This has important implications to understand how hormones and neurotransmitters produce specific responses in our cells and might pave the way to innovative treatments for common diseases like diabetes or heart failure.

PKA Cα subunit mutation triggers caspase-dependent RIIβ subunit degradation via Ser114 phosphorylation

Mutations in the PRKACA gene are the most frequent cause of cortisol-producing adrenocortical adenomas leading to Cushing's syndrome. PRKACA encodes for the catalytic subunit α of protein kinase A (PKA). We already showed that PRKACA mutations lead to impairment of regulatory (R) subunit binding. Furthermore, PRKACA mutations are associated with reduced RIIβ protein levels; however, the mechanisms leading to reduced RIIβ levels are presently unknown. Here, we investigate the effects of the most frequent PRKACA mutation, L206R, on regulatory subunit stability. We find that Ser¹¹⁴ phosphorylation of RIIβ is required for its degradation, mediated by caspase 16. Last, we show that the resulting reduction in RIIβ protein levels leads to increased cortisol secretion in adrenocortical cells. These findings reveal the molecular mechanisms and pathophysiological relevance of the R subunit degradation caused by PRKACA mutations, adding another dimension to the deregulation of PKA signaling caused by PRKACA mutations in adrenal Cushing's syndrome.

G protein-coupled receptor-G protein interactions: a single-molecule perspective

G protein-coupled receptors (GPCRs) regulate many cellular and physiological processes, responding to a diverse range of extracellular stimuli including hormones, neurotransmitters, odorants, and light. Decades of biochemical and pharmacological studies have provided fundamental insights into the mechanisms of GPCR signaling. Thanks to recent advances in structural biology, we now possess an atomistic understanding of receptor activation and G protein coupling. However, how GPCRs and G proteins interact in living cells to confer signaling efficiency and specificity remains insufficiently understood. The development of advanced optical methods, including single-molecule microscopy, has provided the means to study receptors and G proteins in living cells with unprecedented spatio-temporal resolution. The results of these studies reveal an unexpected level of complexity, whereby GPCRs undergo transient interactions among themselves as well as with G proteins and structural elements of the plasma membrane to form short-lived signaling nanodomains that likely confer both rapidity and specificity to GPCR signaling. These findings may provide new strategies to pharmaceutically modulate GPCR function, which might eventually pave the way to innovative drugs for common diseases such as diabetes or heart failure.

Autocrine negative feedback regulation of lipolysis through sensing of NEFAs by FFAR4/GPR120 in WAT

OBJECTIVES

Long-chain fatty acids (LCFAs) released from adipocytes inhibit lipolysis through an unclear mechanism. We hypothesized that the LCFA receptor, FFAR4 (GPR120), which is highly expressed in adipocytes, may be involved in this feedback regulation.

METHODS AND RESULTS

Liquid chromatography mass spectrometry (LC-MS) analysis of conditioned media from isoproterenol-stimulated primary cultures of murine and human adipocytes demonstrated that most of the released non-esterified free fatty acids (NEFAs) are known agonists for FFAR4. In agreement with this, conditioned medium from isoproterenol-treated adipocytes stimulated signaling strongly in FFAR4 transfected COS-7 cells as opposed to non-transfected control cells. In transfected 3T3-L1 cells, FFAR4 agonism stimulated Gi- and Go-mini G protein binding more strongly than Gq, effects which were blocked by the selective FFAR4 antagonist AH7614. In primary cultures of murine white adipocytes, the synthetic, selective FFAR4 agonist CpdA inhibited isoproterenol-induced intracellular cAMP accumulation in a manner similar to the antilipolytic control agent nicotinic acid acting through another receptor, HCAR2. In vivo, oral gavage with the synthetic, specific FFAR4 agonist CpdB decreased the level of circulating NEFAs in fasting lean mice to a similar degree as nicotinic acid. In agreement with the identified anti-lipolytic effect of FFAR4, plasma NEFAs and glycerol were increased in FFAR4-deficient mice as compared to littermate controls despite having elevated insulin levels, and cAMP accumulation in primary adipocyte cultures was augmented by treatment with the FFAR4 antagonist conceivably by blocking the stimulatory tone of endogenous NEFAs on FFAR4.

CONCLUSIONS

In white adipocytes, FFAR4 functions as an NEFA-activated, autocrine, negative feedback regulator of lipolysis by decreasing cAMP though Gi-mediated signaling.

Heterotrimeric G Protein Subunit Gαq Is a Master Switch for Gβγ-Mediated Calcium Mobilization by Gi-Coupled GPCRs

Mechanisms that control mobilization of cytosolic calcium [Ca²⁺]_i are key for regulation of numerous eukaryotic cell functions. One such paradigmatic mechanism involves activation of phospholipase Cβ (PLCβ) enzymes by G protein βγ subunits from activated Gα_i-Gβγ heterotrimers. Here, we report identification of a master switch to enable this control for PLCβ enzymes in living cells. We find that the Gα_i-Gβγ-PLCβ-Ca²⁺ signaling module is entirely dependent on the presence of active Gα_q. If Gα_q is pharmacologically inhibited or genetically ablated, Gβγ can bind to PLCβ but does not elicit Ca²⁺ signals. Removal of an auto-inhibitory linker that occludes the active site of the enzyme is required and sufficient to empower "stand-alone control" of PLCβ by Gβγ. This dependence of Gi-Gβγ-Ca²⁺ on Gα_q places an entire signaling branch of G-protein-coupled receptors (GPCRs) under hierarchical control of Gq and changes our understanding of how Gi-GPCRs trigger [Ca²⁺]_i via PLCβ enzymes.

PRKACB variants in skeletal disease or adrenocortical hyperplasia: effects on protein kinase A

Genetic variants in components of the protein kinase A (PKA) enzyme have been associated with various defects and neoplasms in the context of Carney complex (CNC) and in isolated cases, such as in primary pigmented nodular adrenocortical disease (PPNAD), cortisol-producing adrenal adenomas (CPAs), and various cancers. PRKAR1A mutations have been found in subjects with impaired cAMP-dependent signaling and skeletal defects; bone tumors also develop in both humans and mice with PKA abnormalities. We studied the PRKACB gene in 148 subjects with PPNAD and related disorders, who did not have other PKA-related defects and identified two subjects with possibly pathogenic PRKACB gene variants and unusual bone and endocrine phenotypes. The first presented with bone and other abnormalities and carried a de novo c.858_860GAA (p.K286del) variant. The second subject carried the c.899C>T (p.T300M or p.T347M in another isoform) variant and had a PPNAD-like phenotype. Both variants are highly conserved in the PRKACB gene. In functional studies, the p.K286del variant affected PRKACB protein stability and led to increased PKA signaling. The p.T300M variant did not affect protein stability or response to cAMP and its pathogenicity remains uncertain. We conclude that PRKACB germline variants are uncommon but may be associated with phenotypes that resemble those of other PKA-related defects. However, detailed investigation of each variant is needed as PRKACB appears to be only rarely affected in these conditions, and variants such as p.T300M maybe proven to be clinically insignificant, whereas others (such as p.K286del) are clearly pathogenic and may be responsible for a novel syndrome, associated with endocrine and skeletal abnormalities.

Author Correction: Super-resolution microscopy compatible fluorescent probes reveal endogenous glucagon-like peptide-1 receptor distribution and dynamics

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Genomic and sequence variants of protein kinase A regulatory subunit type 1β (PRKAR1B) in patients with adrenocortical disease and Cushing syndrome

Purpose:

Protein kinase A (PKA) subunit defects (in PRKAR1A and PRKACA) are known to contribute to adrenal tumor pathogenesis. We studied the PRKAR1B gene for any genetic changes in bilateral adrenocortical hyperplasia (BAH) and cortisol-producing adrenal adenomas (CPA).

Methods:

Exome sequencing and PRKAR1B copy number variant (CNV) analysis were performed in 74 patients with BAH and 21 with CPA. PKA activity was studied in tumors with defects; sequence variants were investigated in vitro.

Results:

Three PRKAR1B germline variants (p.I40V, p.A67V, p.A300T) were identified among 74 patients with BAH. PRKAR1B copy number gains (CNG) were found in 3 of 21 CPAs, one in a tumor carrying a somatic PRKACA “hot-spot” pathogenic variant p.L206R. CPAs bearing PRKAR1B CNGs showed higher PRKAR1B mRNA levels and reduced PKA activity. Baseline PKA activity was also decreased for p.A67V and p.A300T in vitro, and mutant PRKAR1β bound PRKACα in FRET recordings of co-transfected HEK293 cells stronger than normal.

Conclusion:

PRKAR1B is yet another PKA subunit that may potentially contribute to adrenal tumor formation. Its involvement in adrenocortical disease may be different from that of other subunits, because PRKAR1B variants and PRKAR1B CNG were associated with decreased (rather than increased) overall PKA activity in vitro.

Selective and Wash-Resistant Fluorescent Dihydrocodeinone Derivatives Allow Single-Molecule Imaging of μ-Opioid Receptor Dimerization

μ-Opioid receptors (μ-ORs) play a critical role in the modulation of pain and mediate the effects of the most powerful analgesic drugs. Despite extensive efforts, it remains insufficiently understood how μ-ORs produce specific effects in living cells. We developed new fluorescent ligands based on the μ-OR antagonist E-p-nitrocinnamoylamino-dihydrocodeinone (CACO), that display high affinity, long residence time and pronounced selectivity. Using these ligands, we achieved single-molecule imaging of μ-ORs on the surface of living cells at physiological expression levels. Our results reveal a high heterogeneity in the diffusion of μ-ORs, with a relevant immobile fraction. Using a pair of fluorescent ligands of different color, we provide evidence that μ-ORs interact with each other to form short-lived homodimers on the plasma membrane. This approach provides a new strategy to investigate μ-OR pharmacology at single-molecule level.

Super-resolution imaging reveals the nanoscale organization of metabotropic glutamate receptors at presynaptic active zones

G protein-coupled receptors (GPCRs) play a fundamental role in the modulation of synaptic transmission. A pivotal example is provided by the metabotropic glutamate receptor type 4 (mGluR4), which inhibits glutamate release at presynaptic active zones (AZs). However, how GPCRs are organized within AZs to regulate neurotransmission remains largely unknown. Here, we applied two-color super-resolution imaging by direct stochastic optical reconstruction microscopy (dSTORM) to investigate the nanoscale organization of mGluR4 at parallel fiber AZs in the mouse cerebellum. We find an inhomogeneous distribution, with multiple nanodomains inside AZs, each containing, on average, one to two mGluR4 subunits. Within these nanodomains, mGluR4s are often localized in close proximity to voltage-dependent Ca_V2.1 channels and Munc-18-1, which are both essential for neurotransmitter release. These findings provide previously unknown insights into the molecular organization of GPCRs at AZs, suggesting a likely implication of a close association between mGluR4 and the secretory machinery in modulating synaptic transmission.

Super-resolution microscopy compatible fluorescent probes reveal endogenous glucagon-like peptide-1 receptor distribution and dynamics

The glucagon-like peptide-1 receptor (GLP1R) is a class B G protein-coupled receptor (GPCR) involved in metabolism. Presently, its visualization is limited to genetic manipulation, antibody detection or the use of probes that stimulate receptor activation. Herein, we present LUXendin645, a far-red fluorescent GLP1R antagonistic peptide label. LUXendin645 produces intense and specific membrane labeling throughout live and fixed tissue. GLP1R signaling can additionally be evoked when the receptor is allosterically modulated in the presence of LUXendin645. Using LUXendin645 and LUXendin651, we describe islet, brain and hESC-derived β-like cell GLP1R expression patterns, reveal higher-order GLP1R organization including membrane nanodomains, and track single receptor subpopulations. We furthermore show that the LUXendin backbone can be optimized for intravital two-photon imaging by installing a red fluorophore. Thus, our super-resolution compatible labeling probes allow visualization of endogenous GLP1R, and provide insight into class B GPCR distribution and dynamics both in vitro and in vivo.

Glucagon-like peptide-1 receptor is an important regulator of appetite and glucose homeostasis. Here the authors describe super-resolution microscopy and in vivo imaging compatible fluorescent probes, which reveal endogenous glucagon-like peptide-1 receptor distribution and dynamics in islets and brain.

G Protein–Coupled Receptor Pharmacology at the Single-Molecule Level

G protein–coupled receptors (GPCRs) mediate the effects of numerous hormones and neurotransmitters and are major pharmacological targets. Classical studies with crude cell lysates or membrane preparations have identified the main biochemical steps involved in GPCR signaling. Moreover, recent studies on purified proteins have provided astounding details at the atomic level of the 3-D structures of receptors in multiple conformations, including in complex with G proteins and β-arrestins. However, several fundamental questions remain regarding the highly specific effects and rapid nature of GPCR signaling. Recent developments in single-molecule microscopy are providing important contributions to answering these questions. Overall, single-molecule studies have revealed unexpected levels of complexity, with receptors existing in different conformations and dynamically interacting among themselves, their signaling partners, and structural elements of the plasma membrane to produce highly localized signals in space and time. These findings may provide a new basis to develop innovative strategies to modulate GPCR function for pharmacological purposes.

Cytoskeleton Protein Filamin A Is Required for Efficient Somatostatin Receptor Type 2 Internalization and Recycling through Rab5 and Rab4 Sorting Endosomes in Tumor Somatotroph Cells

The high expression of somatostatin receptor 2 (SST2) in growth hormone (GH)-secreting tumors represents the rationale for the clinical use of somatostatin analogs (SSAs) in acromegaly. Recently, the cytoskeletal protein Filamin A (FLNA) has emerged as key modulator of the responsiveness of GH-secreting pituitary tumors to SSAs by regulating SST2 signaling and expression. The aim of this study was to explore FLNA involvement in SST2 intracellular trafficking in tumor somatotroph cells. By biotinylation assay, we found that FLNA silencing abolished octreotide-mediated SST2 internalization in rat GH3 cell line (28.0 ± 2.7 vs. 4 ± 4.3% SST2 internalization, control versus FLNA small interfering RNAs (siRNA) cells, respectively, p < 0.001) and human GH-secreting primary cultured cells (70.3 ± 21.1 vs. 24 ± 19.2% SST2 internalization, control versus FLNA siRNA cells, respectively, p < 0.05). In addition, confocal imaging revealed impaired SST2 recycling to the plasma membrane in FLNA silenced GH3 cells. Coimmunoprecipitation and immunofluorescence experiments showed that FLNA, as well as β-arrestin2, is timely dependent recruited to octreotide-stimulated SST2 receptors both in rat and human tumor somatotroph cells. Although FLNA expression knock down did not prevent the formation of β-arrestin2-SST2 complex in GH3 cells, it significantly impaired efficient SST2 loading into cytosolic vesicles positive for the early endocytic and recycling markers Rab5 and 4, respectively (33.7 ± 8.9% down to 25.9 ± 6.9%, p < 0.05, and 28.4 ± 7.4% down to 17.6 ± 5.7%, p < 0.01, for SST2-Rab5 and SST2-Rab4 colocalization, respectively, in control versus FLNA siRNA cells). Altogether these data support an important role for FLNA in the mediation of octreotide-induced SST2 trafficking in GH-secreting pituitary tumor cells through Rab5 and 4 sorting endosomes.

Somatic PRKACA Mutations: Association With Transition From Pituitary-Dependent to Adrenal-Dependent Cushing Syndrome

CONTEXT

Prolonged adrenal stimulation by corticotropin, as in long-standing Cushing disease (CD), leads to diffuse to nodular hyperplasia. Adrenal functional autonomy has been described in a subset of patients with CD, leading to the hypothesis of transition from ACTH-dependent to ACTH-independent hypercortisolism.

OBJECTIVE

With the consideration that the catalytic α subunit of protein kinase A (PKA; PRKACA) somatic mutations are the most common finding in adrenal adenomas associated with ACTH-independent Cushing syndrome, our aim was to analyze PRKACA mutations in adrenals of patients with persistent/long-standing CD.

DESIGN

Cross-sectional.

SETTING

University hospital.

PATIENTS

Two patients with long-standing CD and suspicion of coexistence of autonomous adrenal hyperfunction, according to pre and postoperative evaluations, were selected for this study, following an intensive literature search and patient-chart reviewing.

INTERVENTION

Clinical data were analyzed. DNA was extracted from adrenal tissue for PRKACA sequencing. PKA activity was assayed.

MAIN OUTCOME MEASURE

PRKACA somatic mutations.

RESULTS

Both patients showed mutations of PRKACA in the macronodule in the context of micronodular adrenal hyperplasia. One patient harbored the previously described p.Leu206Arg substitution, whereas a p.Ser213Arg missense variation was detected in the adrenal nodule of the second patient. No mutations were detected in the adjacent adrenal cortex of the second patient. In silico analysis predicts that p.Ser213Arg can interfere with the interaction between the regulatory and catalytic subunits of PKA.

CONCLUSIONS

Our study shows that PRKACA somatic mutations can be found in adrenal nodules of patients with CD. These genetic alterations could represent a possible mechanism underlying adrenal nodule formation and autonomous cortisol hyperproduction in a subgroup of patients with long-standing CD.

Agonist-induced membrane nanodomain clustering drives GLP-1 receptor responses in pancreatic beta cells

The glucagon-like peptide-1 receptor (GLP-1R), a key pharmacological target in type 2 diabetes (T2D) and obesity, undergoes rapid endocytosis after stimulation by endogenous and therapeutic agonists. We have previously highlighted the relevance of this process in fine-tuning GLP-1R responses in pancreatic beta cells to control insulin secretion. In the present study, we demonstrate an important role for the translocation of active GLP-1Rs into liquid-ordered plasma membrane nanodomains, which act as hotspots for optimal coordination of intracellular signaling and clathrin-mediated endocytosis. This process is dynamically regulated by agonist binding through palmitoylation of the GLP-1R at its carboxyl-terminal tail. Biased GLP-1R agonists and small molecule allosteric modulation both influence GLP-1R palmitoylation, clustering, nanodomain signaling, and internalization. Downstream effects on insulin secretion from pancreatic beta cells indicate that these processes are relevant to GLP-1R physiological actions and might be therapeutically targetable.

Nanodomain segregation and clustering of the glucagon-like peptide-1 receptor, a key target for type 2 diabetes therapy, is regulated by agonist binding, leading to compartmentalization of downstream signaling and clathrin-dependent internalization and impacting pancreatic beta cell responses.

Cushing's syndrome driver mutation disrupts protein kinase A allosteric network, altering both regulation and substrate specificity

Genetic alterations in the PRKACA gene coding for the catalytic α subunit of the cAMP-dependent protein kinase A (PKA-C) are linked to cortisol-secreting adrenocortical adenomas, resulting in Cushing's syndrome. Among those, a single mutation (L205R) has been found in up to 67% of patients. Because the x-ray structures of the wild-type and mutant kinases are essentially identical, the mechanism explaining aberrant function of this mutant remains under active debate. Using NMR spectroscopy, thermodynamics, kinetic assays, and molecular dynamics simulations, we found that this single mutation causes global changes in the enzyme, disrupting the intramolecular allosteric network and eliciting losses in nucleotide/pseudo-substrate binding cooperativity. Remarkably, by rewiring its internal allosteric network, PKA-CL205R is able to bind and phosphorylate non-canonical substrates, explaining its changes in substrate specificity. Both the lack of regulation and change in substrate specificity reveal the complex role of this mutated kinase in the formation of cortisol-secreting adrenocortical adenomas.

SUN-381 Somatic PRKACA Mutations In Patients With Transition From Pituitary-dependent To Adrenal-dependent Cushing’S Syndrome

Background. Transition from ACTH-dependent to ACTH-independent hypercortisolism has been described in patients with long-standing Cushing´s disease (CD). The underlying molecular mechanisms are unknown. Clinical case.Case 1. A 41-years old woman was referred for Cushing’s syndrome (cortisol after 1-mg dexamethasone suppression test (DST) 410 nmol/L, urinary free cortisol (UFC) 2xULN, midnight cortisol 440 nmol/L). High-dose DST: no suppression. CRH test: no ACTH/cortisol increase. Basal ACTH: 18 pmol/L. Pituitary MRI showed a 7-mm adenoma. The inferior petrosal sinus sampling showed central source of ACTH. She was treated by trans-sphenoidal surgery (pituitary adenoma confirmed histologically) followed by clinical remission. Cushing’s syndrome recurred 1 year after. ACTH was undetectable and pituitary MRI normal. Abdominal CT-scan showed a left adrenal macronodule. Left adrenalectomy was performed, followed by glucocorticoid replacement therapy (1 year) and clinical/biochemical remission at last follow-up (15 years). Histopathology showed a 35-mm adrenal nodule and micronodular hyperplasia. We found a p.L206R missense mutation of PRKACA in the adrenal nodule. The functional implication of the mutation has been described (1). Case 2. A 31-years old woman was referred for Cushing´s syndrome (cortisol after 1-mg DST 579 nmol/L, UFC 4xULN, midnight cortisol 773 nmol/L). Basal ACTH: 18 pmol/L. CRH and desmopressin tests: indicative of CD. High-dose DST: no suppression. A pituitary MRI revealed a microadenoma, treated by trans-sphenoidal surgery (confirmed by histology). She had persistence of Cushing’s syndrome, undetectable basal ACTH and negative pituitary MRI. An abdominal CT-scan showed bilateral adrenal enlargement with a left nodule, treated by bilateral adrenalectomy. Histology showed a macronodule in micronodular hyperplasia. We found a p.S213R somatic missense mutation of PRKACA in the adrenal macronodule. Functional analyses showed that mutant cells had higher cAMP-independent PKA activity than wild-type, and similar to the L206R mutation. Conclusion. This is the first evidence showing that PRKACA somatic mutations can be found in adrenal nodules of patients with CD, possibly explaining the mechanism underlying transition from ACTH-dependent to ACTH-independent hypercortisolism. References. 1. Calebiro D et al. PKA catalytic subunit mutations in adrenocortical Cushing's adenoma impair association with the regulatory subunit. Nat Commun. 2014;5:5680.

Statistical testing approach for fractional anomalous diffusion classification

Taking advantage of recent single-particle tracking data, we compare the popular standard mean-squared displacement method with a statistical testing hypothesis procedure for three testing statistics and for two particle types: membrane receptors and the G proteins coupled to them. Each method results in different classifications. For this reason, more rigorous statistical tests are analyzed here in detail. The main conclusion is that the statistical testing approaches might provide good results even for short trajectories, but none of the proposed methods is "the best" for all considered examples; in other words, one needs to combine different approaches to get a reliable classification.

The subcellular dynamics of GPCR signaling

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors and mediate the effects of a multitude of extracellular cues, such as hormones, neurotransmitters, odorants and light. Because of their involvement in numerous physiological and pathological processes and their accessibility, they are extensively exploited as pharmacological targets. Biochemical and structural biology investigations have clarified the molecular basis of GPCR signaling to a high level of detail. In spite of this, how GPCRs can efficiently and precisely translate extracellular signals into specific and well-orchestrated biological responses in the complexity of a living cell or organism remains insufficiently understood. To explain the high efficiency and specificity observed in GPCR signaling, it has been suggested that GPCR might signal in discrete nanodomains on the plasma membrane or even form stable complexes with G proteins and effectors. However, directly testing these hypotheses has proven a major challenge. Recent studies taking advantage of innovative optical methods such as fluorescence resonance energy transfer (FRET) and single-molecule microscopy have begun to dig into the organization of GPCR signaling in living cells on the spatial (nm) and temporal (ms) scales on which cell signaling events are taking place. The results of these studies are revealing a complex and highly dynamic picture, whereby GPCRs undergo transient interaction with their signaling partners, membrane lipids and the cytoskeleton to form short-lived signaling nanodomains both on the plasma membrane and at intracellular sites. Continuous exchanges among such nanodomains via later diffusion as well as via membrane trafficking might provide a highly sophisticated way of controlling the timing and location of GPCR signaling. Here, we will review the most recent advances in our understanding of the organization of GPCR signaling in living cells, with a particular focus on its dynamics.

Alterations in Protein Kinase A Substrate Specificity as a Potential Cause of Cushing Syndrome

Cushing syndrome is a severe endocrine disorder of cortisol excess associated with major metabolic and cardiovascular sequelae. We recently identified somatic mutations in PRKACA, the gene encoding the catalytic (C) α subunit of protein kinase A (PKA), as being responsible for cortisol-producing adrenocortical adenomas (CPAs), which are a major cause of Cushing syndrome. In spite of previous studies on the two initially identified mutations (L206R, 199_200insW), the mechanisms of action of the clinically highly relevant PRKACA mutations remain poorly understood. Here, by investigating a large panel of PRKACA mutations, including all those identified so far in Cushing syndrome, we unexpectedly found that not all mutations interfere with the binding of regulatory (R) subunits as previously hypothesized. Because several mutations lie in a region of PKA Cα involved in substrate recognition, we investigated their consequences on substrate specificity by quantitative phosphoproteomics. We found that all three mutations analyzed (L206R, 200_201insV, and d244-248+E249Q) cause major changes in the preference of PKA for its targets, leading to hyperphosphorylation of several PKA substrates, most notably including histone H1.4 at Ser36, which is required for and promotes mitosis. This is reflected by a ninefold hyperphosphorylation of H1.4 in CPAs carrying the L206R mutation. Thus, our findings suggest that in addition to hampering binding to R subunits, PRKACA mutations act by altering PKA substrate specificity. These findings shed light on the molecular events leading to Cushing syndrome and illustrate how mutations altering substrate specificity of a protein kinase may cause human disease.

Hot spots for GPCR signaling: lessons from single-molecule microscopy

G protein-coupled receptors (GPCRs) are among the best-studied membrane receptors, mainly due to their central role in human physiology, involvement in disease and relevance as drug targets. Although biochemical and pharmacological studies have characterized the main steps in GPCR signaling, how GPCRs produce highly specific responses in our cells remains insufficiently understood. New developments in single-molecule microscopy have made it possible to study the protein-protein interactions at the basis of GPCR signaling in previously inconceivable detail. Using this approach, it was recently possible to follow individual receptors and G proteins as they diffuse, interact and signal on the surface of living cells. This has revealed hot spots on the plasma membrane, where receptors and G proteins undergo transient interactions to produce rapid and local signals. Overall, these recent findings reveal a high degree of dynamicity and complexity in signaling by GPCRs, which provides a new basis to understand how these important receptors produce specific effects and might pave the way to innovative pharmacological approaches.

Single-Molecule Microscopy Reveals Dynamic FLNA Interactions Governing SSTR2 Clustering and Internalization

The cytoskeletal protein filamin A (FLNA) has been suggested to play an important role in the responsiveness of GH-secreting pituitary tumors to somatostatin receptor subtype 2 (SSTR2) agonists by regulating SSTR2 expression and signaling. However, the underlying mechanisms are unknown. In this study, we use fast multicolor single-molecule microscopy to image individual SSTR2 and FLNA molecules at the surface of living cells with unprecedented spatiotemporal resolution. We find that SSTR2 and FLNA undergo transient interactions, which occur preferentially along actin fibers and contribute to restraining SSTR2 diffusion. Agonist stimulation increases the localization of SSTR2 along actin fibers and, subsequently, SSTR2 clustering and recruitment to clathrin-coated pits (CCPs). Interfering with FLNA-SSTR2 binding with a dominant-negative FLNA fragment increases SSTR2 mobility, hampers the formation and alignment of SSTR2 clusters along actin fibers, and impairs both SSTR2 recruitment to CCPs and SSTR2 internalization. These findings indicate that dynamic SSTR2-FLNA interactions critically control the nanoscale localization of SSTR2 at the plasma membrane and are required for coupling SSTR2 clustering to internalization. These mechanisms explain the critical role of FLNA in the control of SSTR2 expression and signaling and suggest the possibility of targeting SSTR2-FLNA interactions for the therapy of pharmacologically resistant GH-secreting pituitary tumors.

Activating PRKACB somatic mutation in cortisol-producing adenomas

Mutations in the gene encoding the protein kinase A (PKA) catalytic subunit α have been found to be responsible for cortisol-producing adenomas (CPAs). In this study, we identified by whole-exome sequencing the somatic mutation p.S54L in the PRKACB gene, encoding the catalytic subunit β (Cβ) of PKA, in a CPA from a patient with severe Cushing syndrome. Bioluminescence resonance energy transfer and surface plasmon resonance assays revealed that the mutation hampers formation of type I holoenzymes and that these holoenzymes were highly sensitive to cAMP. PKA activity, measured both in cell lysates and with recombinant proteins, based on phosphorylation of a synthetic substrate, was higher under basal conditions for the mutant enzyme compared with the WT, while maximal activity was lower. These data suggest that at baseline the PRKACB p.S54L mutant drove the adenoma cells to higher cAMP signaling activity, probably contributing to their autonomous growth. Although the role of PRKACB in tumorigenesis has been suggested, we demonstrated for the first time to our knowledge that a PRKACB mutation can lead to an adrenal tumor. Moreover, this observation describes another mechanism of PKA pathway activation in CPAs and highlights the particular role of residue Ser54 for the function of PKA.

Internalization of G-protein-coupled receptors: Implication in receptor function, physiology and diseases

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors and mediate the effects of numerous hormones and neurotransmitters. The nearly 1000 GPCRs encoded by the human genome regulate virtually all physiological functions and are implicated in the pathogenesis of prevalent human diseases such as thyroid disorders, hypertension or Parkinson's disease. As a result, 30-50% of all currently prescribed drugs are targeting these receptors. Once activated, GPCRs induce signals at the cell surface. This is often followed by internalization, a process that results in the transfer of receptors from the plasma membrane to membranes of the endosomal compartment. Internalization was initially thought to be mainly implicated in signal desensitization, a mechanism of adaptation to prolonged receptor stimulation. However, several unexpected functions have subsequently emerged. Most notably, accumulating evidence indicates that internalization can induce prolonged receptor signaling on intracellular membranes, which is apparently required for at least some biological effects of hormones like TSH, LH and adrenaline. These findings reveal an even stronger connection between receptor internalization and signaling than previously thought. Whereas new studies are just beginning to reveal an important physiological role for GPCR signaling after internalization and ways to exploit it for therapeutic purposes, future investigations will be required to explore its involvement in human disease.

cAMP Signals in Drosophila Motor Neurons Are Confined to Single Synaptic Boutons

The second messenger cyclic AMP (cAMP) plays an important role in synaptic plasticity. Although there is evidence for local control of synaptic transmission and plasticity, it is less clear whether a similar spatial confinement of cAMP signaling exists. Here, we suggest a possible biophysical basis for the site-specific regulation of synaptic plasticity by cAMP, a highly diffusible small molecule that transforms the physiology of synapses in a local and specific manner. By exploiting the octopaminergic system of Drosophila, which mediates structural synaptic plasticity via a cAMP-dependent pathway, we demonstrate the existence of local cAMP signaling compartments of micrometer dimensions within single motor neurons. In addition, we provide evidence that heterogeneous octopamine receptor localization, coupled with local differences in phosphodiesterase activity, underlies the observed differences in cAMP signaling in the axon, cell body, and boutons.

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Boutons, axon, and cell body are independent cAMP signaling compartments

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Receptors and PDEs are responsible for the compartmentalization of cAMP

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cAMP does not propagate from the bouton to the cell body

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Local cAMP increases provides a basis for site-specific control of synaptic plasticity

Somatostatin Receptor Type 2 (SSTR2) Internalization and Intracellular Trafficking in Pituitary GH-Secreting Adenomas: Role of Scaffold Proteins and Implications for Pharmacological Resistance

Somatostatin receptor type 2 (SSTR2), together with SSTR5, represents the main target of medical treatment for growth hormone (GH)-secreting pituitary tumors, since it is expressed in most of these tumors and exerts both antiproliferative and cytostatic effects, and reduces hormone secretion, as well. However, clinical practice indicates a great variability in the frequency and entity of favorable responses of acromegalic patients to long-acting somatostatin analogues (SSAs), but the molecular mechanisms regulating this pharmacological resistance are not completely understood. So far, several potentially implied mechanisms have been suggested, including impaired expression of SSTRs, or post-receptor signal transduction alterations. More recently, new studies exploited the molecular factors involved in SSTRs intracellular trafficking regulation, this being a critical point for the modulation of the available active G-coupled receptors (GPCRs) amount at the cell surface. In this respect, the role of the scaffold proteins such as β-arrestins, and the cytoskeleton protein Filamin A (FLNA), have become of relevant importance for GH-secreting pituitary tumors. In fact, β-arrestins are linked to SSTR2 desensitization and internalization, and FLNA is able to regulate SSTR2 trafficking and stability at the plasma membrane. Therefore, the present review will summarize emerging evidence highlighting the role of β-arrestins and FLNA, as possible novel players in the modulation of agonist activated-SSTR2 receptor trafficking and response in GH-secreting pituitary tumors.

Mechanisms of Aberrant PKA Activation by Cα Subunit Mutations

Somatic mutations in PRKACA, coding for the catalytic α subunit of protein kinase A (PKA), have been recently identified as the most frequent genetic alteration in cortisol-secreting adrenocortical adenomas, which are responsible for adrenal Cushing's syndrome. The mutations identified so far lie at the interface between the catalytic (C) and regulatory (R) subunit of PKA. Detailed functional studies of the most frequent of these mutations (L206R) as well as of another one in the same region of the C subunit (199_200insW) have revealed that these mutations cause constitutive activation of PKA and lack of regulation by cAMP. This is due to interference with the binding of the R subunit, which keeps the C subunit inactive in the absence of cyclic AMP. Here, we review these recent findings, with a particular focus on the mechanisms of action of PRKACA mutations.

Differential expression of the protein kinase A subunits in normal adrenal glands and adrenocortical adenomas

Somatic mutations in protein kinase A catalytic α subunit (PRKACA) were found to be causative for 30-40% of cortisol-producing adenomas (CPA) of the adrenal gland, rendering PKA signalling constitutively active. In its resting state, PKA is a stable and inactive heterotetramer, consisting of two catalytic and two regulatory subunits with the latter inhibiting PKA activity. The human genome encodes three different PKA catalytic subunits and four different regulatory subunits that are preferentially expressed in different organs. In normal adrenal glands all regulatory subunits are expressed, while CPA exhibit reduced protein levels of the regulatory subunit IIβ. In this study, we linked for the first time the loss of RIIβ protein levels to the PRKACA mutation status and found the down-regulation of RIIβ to arise post-transcriptionally. We further found the PKA subunit expression pattern of different tumours is also present in the zones of the normal adrenal cortex and demonstrate that the different PKA subunits have a differential expression pattern in each zone of the normal adrenal gland, indicating potential specific roles of these subunits in the regulation of different hormones secretion.

Genetic Landscape of Sporadic Unilateral Adrenocortical Adenomas Without PRKACA p.Leu206Arg Mutation

CONTEXT

Adrenocortical adenomas (ACAs) are among the most frequent human neoplasias. Genetic alterations affecting the cAMP/protein kinase A signaling pathway are common in cortisol-producing ACAs, whereas activating mutations in the gene encoding β-catenin (CTNNB1) have been reported in a subset of both benign and malignant adrenocortical tumors. However, the molecular pathogenesis of most ACAs is still largely unclear.

OBJECTIVE

The aim of the study was to define the genetic landscape of sporadic unilateral ACAs.

DESIGN AND SETTING

Next-generation whole-exome sequencing was performed on fresh-frozen tumor samples and corresponding normal tissue samples.

PATIENTS

Ninety-nine patients with ACAs (74 cortisol-producing and 25 endocrine inactive) negative for p.Leu206Arg PRKACA mutation.

MAIN OUTCOME MEASURES

Identification of known and/or new genetic alterations potentially involved in adrenocortical tumorigenesis and autonomous hormone secretion, genotype-phenotype correlation.

RESULTS

A total of 706 somatic protein-altering mutations were detected in 88 of 99 tumors (median, six per tumor). We identified several mutations in genes of the cAMP/protein kinase A pathway, including three novel mutations in PRKACA, associated with female sex and Cushing's syndrome. We also found genetic alterations in different genes involved in the Wnt/β-catenin pathway, associated with larger tumors and endocrine inactivity, and notably, in many genes of the Ca(2+)-signaling pathway. Finally, by comparison of our genetic data with those available in the literature, we describe a comprehensive genetic landscape of unilateral ACAs.

CONCLUSIONS

This study provides the largest sequencing effort on ACAs to date. We thereby identified somatic alterations affecting known and novel pathways potentially involved in adrenal tumorigenesis.

Recurrent EZH1 mutations are a second hit in autonomous thyroid adenomas

Autonomous thyroid adenomas (ATAs) are a frequent cause of hyperthyroidism. Mutations in the genes encoding the TSH receptor (TSHR) or the Gs protein α subunit (GNAS) are found in approximately 70% of ATAs. The involvement of other genes and the pathogenesis of the remaining cases are presently unknown. Here, we performed whole-exome sequencing in 19 ATAs that were paired with normal DNA samples and identified a recurrent hot-spot mutation (c.1712A>G; p.Gln571Arg) in the enhancer of zeste homolog 1 (EZH1) gene, which codes for a catalytic subunit of the polycomb complex. Targeted screening in an independent cohort confirmed that this mutation occurs with high frequency (27%) in ATAs. EZH1 mutations were strongly associated with known (TSHR, GNAS) or presumed (adenylate cyclase 9 [ADCY9]) alterations in cAMP pathway genes. Furthermore, functional studies revealed that the p.Gln571Arg EZH1 mutation caused increased histone H3 trimethylation and increased proliferation of thyroid cells. In summary, this study revealed that a hot-spot mutation in EZH1 is the second most frequent genetic alteration in ATAs. The association between EZH1 and TSHR mutations suggests a 2-hit model for the pathogenesis of these tumors, whereby constitutive activation of the cAMP pathway and EZH1 mutations cooperate to induce the hyperproliferation of thyroid cells.

PRKACA Somatic Mutations Are Rare Findings in Aldosterone-Producing Adenomas

CONTEXT

Somatic mutations have been found causative for endocrine autonomy in aldosterone-producing adenomas (APAs). Whereas mutations of PRKACA (catalytic subunit of protein kinase A) have been identified in cortisol-producing adenomas, the presence of PRKACA variants in APAs is unknown, especially in those that display cosecretion of cortisol.

OBJECTIVE

The objective of the study was to investigate PRKACA somatic variants identified in APA cases.

DESIGN

Identification of PRKACA somatic variants in APAs by whole-exome sequencing followed by in vitro analysis of the enzymatic activity of PRKACA variants and functional characterization by double immunofluorescence of CYP11B2 and CYP11B1 expression in the corresponding tumor tissues.

SETTING AND PATIENTS

APA tissues were collected from 122 patients who underwent unilateral adrenalectomy for primary aldosteronism between 2005 and 2015 at a single institution.

RESULTS

PRKACA somatic mutations were identified in two APA cases (1.6%). One APA carried a newly identified p.His88Asp variant, whereas in a second case, a p.Leu206Arg mutation was found, previously described only in cortisol-producing adenomas with overt Cushing's syndrome. Functional analysis showed that the p.His88Asp variant was not associated with gain of function. Although CYP11B2 was strongly expressed in the p.His88Asp-mutated APA, the p.Leu206Arg carrying APA predominantly expressed CYP11B1. Accordingly, biochemical Cushing's syndrome was present only in the patient with the p.Leu206Arg mutation. After adrenalectomy, both patients improved with a reduced number of antihypertensive medications and normalized serum potassium levels.

CONCLUSIONS

We describe for the first time PRKACA mutations as rare findings associated with unilateral primary aldosteronism. As cortisol cosecretion occurs in a subgroup of APAs, other molecular mechanisms are likely to exist.

Persistent cAMP Signaling by Internalized LH Receptors in Ovarian Follicles

A crucial event in female reproduction occurs at midcycle, when a LH peak induces the final maturation of ovarian follicles. LH signals via a G protein-coupled receptor selectively expressed in the outermost follicular cell layers. However, how LH signals are relayed inside these cells and finally to the oocyte is incompletely understood. Here, we monitored LH signaling in intact ovarian follicles of transgenic mice expressing a fluorescent cAMP sensor. We found that LH stimulation induces 2 phases of cAMP signaling in all cell layers surrounding the oocyte. Interfering with LH receptor internalization abolished the second, persistent cAMP phase and partially inhibited oocyte meiosis resumption. These data suggest that persistent cAMP signals from internalized LH receptors contribute to transmitting LH effects inside follicle cells and ultimately to the oocyte. Thus, this study indicates that the recently proposed paradigm of cAMP signaling by internalized G protein-coupled receptors is implicated in receptor function and is physiologically relevant.

Landscape of somatic mutations in sporadic GH-secreting pituitary adenomas

CONTEXT

Alterations in the cAMP signaling pathway are common in hormonally active endocrine tumors. Somatic mutations at GNAS are causative in 30-40% of GH-secreting adenomas. Recently, mutations affecting the USP8 and PRKACA gene have been reported in ACTH-secreting pituitary adenomas and cortisol-secreting adrenocortical adenomas respectively. However, the pathogenesis of many GH-secreting adenomas remains unclear.

AIM

Comprehensive genetic characterization of sporadic GH-secreting adenomas and identification of new driver mutations.

DESIGN

Screening for somatic mutations was performed in 67 GH-secreting adenomas by targeted sequencing for GNAS, PRKACA, and USP8 mutations (n=31) and next-generation exome sequencing (n=36).

RESULTS

By targeted sequencing, known activating mutations in GNAS were detected in five cases (16.1%), while no somatic mutations were observed in both PRKACA and USP8. Whole-exome sequencing identified 132 protein-altering somatic mutations in 31/36 tumors with a median of three mutations per sample (range: 1-13). The only recurrent mutations have been observed in GNAS (31.4% of cases). However, seven genes involved in cAMP signaling pathway were affected in 14 of 36 samples and eight samples harbored variants in genes involved in the calcium signaling or metabolism. At the enrichment analysis, several altered genes resulted to be associated with developmental processes. No significant correlation between genetic alterations and the clinical data was observed.

CONCLUSION

This study provides a comprehensive analysis of somatic mutations in a large series of GH-secreting adenomas. No novel recurrent genetic alterations have been observed, but the data suggest that beside cAMP pathway, calcium signaling might be involved in the pathogenesis of these tumors.