Inhibition of WNT signaling by G protein-coupled receptor (GPCR) kinase 2 (GRK2)

Reanalysis of Whole-Exome Sequencing Data of an Infant with Suspected Diagnosis of Jeune Syndrome Revealed a Likely Pathogenic Variant in GRK2: A Newly Associated Gene for Jeune Syndrome Phenotype

Introduction

Ciliopathies with major skeletal involvement embrace a group of heterogeneous disorders caused by pathogenic variants in a group of diverse genes. A narrow thorax with shortening of long bones inspires a clinical entity underlined by dysfunction of primary cilia. Currently, more than 23 genes are listed in the OMIM database corresponding to this clinical entity: WDR19/34/35/60, IFT43/52/80/81/140/172, DYNC2LI1, TTC21B, DYNLT2B, EVC2, EVC, INTU, NEK1, CEP120, DYNC2H1, KIAA0586, SRTD1, KIAA0753, and SRTD12. Recently, individuals with biallelic loss-of-function variants in GRK2 are shown to demonstrate a phenotype compatible with Jeune syndrome. Experimental evidence has shown that impaired function of GRK2 compromises cilia-based signaling of Hedgehog pathway as well as Wnt signaling, while cilia morphology remains intact. Hence, GRK2 is now considered an essential protein in regulation of the skeletogenesis.

Case Presentation

We presented a female infant born to a consanguineous marriage who was found to have a biallelic p.R474* alteration in GRK2 in reanalysis of the whole-exome sequencing (WES) data. The patient was exhibiting major clinical features of Jeune syndrome, such as shortened long bones, ribs, and narrow thorax.

Discussion

Our reanalysis of WES data revealed a likely pathogenic biallelic variant in the GRK2 which is probably responsible for the Jeune syndrome phenotype in the patient. Hence, our report supports the recently discovered association of GRK2 loss-of-function variants with Jeune syndrome phenotype and emphasizes the significance of reanalysis of WES data, notably in patients with phenotypes suggestive of a such discernible Mendelian disorder.

RAB3D/MDM2/β-catenin/c-MYC axis exacerbates the malignant behaviors of acute myeloid leukemia cells in vitro and in vivo

RAB3D, a small Ras-like GTPase involved in regulating secretory pathway, plays a cancer-promoting role in several solid tumors. However, its role in leukemogenesis remains unknown yet. Acute myeloid leukemia (AML) is a common acute leukemia with a high mortality. Here, we found the higher expression of RAB3D in bone marrow mononuclear cells derived from AML patients (n = 54) versus healthy participants (n = 20). The following loss- and gain-of-function experiments demonstrated that RAB3D promoted growth, enhanced colony formation and accelerated G1/S transition of U937, THP-1 and KG-1 AML cells. RAB3D silencing inhibited tumorigenesis of AML cells in vivo and delayed AML cells-induced death of mice. Interestingly, the expression of RAB3D is positively correlated with that of an oncogene mouse double minute 2 (MDM2) in bone marrow mononuclear cells of AML patients (r = 0.923, p < 0.001). Intracellular MDM2 was conjugated with more ubiquitins and degraded faster when RAB3D was silenced. A commonly therapeutic target of AML, β-catenin signaling, was activated by RAB3D overexpression, but deactivated after MDM2 was silenced. The RAB3D-induced proliferation acceleration and β-catenin activation were abolished by MDM2 knockdown, implying that RAB3D function by stabilizing MDM2. In addition, c-MYC, a β-catenin downstream effector, was recruited directly to the RAB3D gene promoter (-360/-349 and -136/-125 sites) and induced its transcription. Collectively, this study demonstrates that RAB3D may exacerbate the malignant behaviors of AML cells through forming a positive feedback loop with MDM2/β-catenin/c-MYC signaling. RAB3D might be a novel target of clinical AML treatment.

Protein proximity networks and functional evaluation of the casein kinase 1 gamma family reveal unique roles for CK1γ3 in WNT signaling

Aberrant activation or suppression of WNT/β-catenin signaling contributes to cancer initiation and progression, neurodegeneration, and bone disease. However, despite great need and more than 40 years of research, targeted therapies for the WNT pathway have yet to be fully realized. Kinases are considered exceptionally druggable and occupy key nodes within the WNT signaling network, but several pathway-relevant kinases remain understudied and "dark." Here, we studied the function of the casein kinase 1γ (CSNK1γ) subfamily of human kinases and their roles in WNT signaling. miniTurbo-based proximity biotinylation and mass spectrometry analysis of CSNK1γ1, CSNK1γ2, and CSNK1γ3 revealed numerous components of the β-catenin-dependent and β-catenin-independent WNT pathways. In gain-of-function experiments, we found that CSNK1γ3 but not CSNK1γ1 or CSNK1γ2 activated β-catenin-dependent WNT signaling, with minimal effect on other signaling pathways. We also show that within the family, CSNK1γ3 expression uniquely induced low-density lipoprotein receptor-related protein 6 phosphorylation, which mediates downstream WNT signaling transduction. Conversely, siRNA-mediated silencing of CSNK1γ3 alone had no impact on WNT signaling, though cosilencing of all three family members decreased WNT pathway activity. Finally, we characterized two moderately selective and potent small-molecule inhibitors of the CSNK1γ family. We show that these inhibitors and a CSNK1γ3 kinase-dead mutant suppressed but did not eliminate WNT-driven low-density lipoprotein receptor-related protein 6 phosphorylation and β-catenin stabilization. Our data suggest that while CSNK1γ3 expression uniquely drives pathway activity, potential functional redundancy within the family necessitates loss of all three family members to suppress the WNT signaling pathway.

Mutations in GRK2 cause Jeune syndrome by impairing Hedgehog and canonical Wnt signaling

Mutations in genes affecting primary cilia cause ciliopathies, a diverse group of disorders often affecting skeletal development. This includes Jeune syndrome or asphyxiating thoracic dystrophy (ATD), an autosomal recessive skeletal disorder. Unraveling the responsible molecular pathology helps illuminate mechanisms responsible for functional primary cilia. We identified two families with ATD caused by loss-of-function mutations in the gene encoding adrenergic receptor kinase 1 (ADRBK1 or GRK2). GRK2 cells from an affected individual homozygous for the p.R158* mutation resulted in loss of GRK2, and disrupted chondrocyte growth and differentiation in the cartilage growth plate. GRK2 null cells displayed normal cilia morphology, yet loss of GRK2 compromised cilia-based signaling of Hedgehog (Hh) pathway. Canonical Wnt signaling was also impaired, manifested as a failure to respond to Wnt ligand due to impaired phosphorylation of the Wnt co-receptor LRP6. We have identified GRK2 as an essential regulator of skeletogenesis and demonstrate how both Hh and Wnt signaling mechanistically contribute to skeletal ciliopathies.

G protein-coupled receptor kinase 2 (GRK2) as a multifunctional signaling hub

Accumulating evidence indicates that G protein-coupled receptor kinase 2 (GRK2) is a versatile protein that acts as a signaling hub by modulating G protein-coupled receptor (GPCR) signaling and also via phosphorylation or scaffolding interactions with an extensive number of non-GPCR cellular partners. GRK2 multifunctionality arises from its multidomain structure and from complex mechanisms of regulation of its expression levels, activity, and localization within the cell, what allows the precise spatio-temporal shaping of GRK2 targets. A better understanding of the GRK2 interactome and its modulation mechanisms is helping to identify the GRK2-interacting proteins and its substrates involved in the participation of this kinase in different cellular processes and pathophysiological contexts.

GRK5 - A Functional Bridge Between Cardiovascular and Neurodegenerative Disorders

Complex aging-triggered disorders are multifactorial programs that comprise a myriad of alterations in interconnected protein networks over a broad range of tissues. It is evident that rather than being randomly organized events, pathophysiologies that possess a strong aging component such as cardiovascular diseases (hypertensions, atherosclerosis, and vascular stiffening) and neurodegenerative conditions (dementia, Alzheimer's disease, mild cognitive impairment, Parkinson's disease), in essence represent a subtly modified version of the intricate molecular programs already in place for normal aging. To control such multidimensional activities there are layers of trophic protein control across these networks mediated by so-called "keystone" proteins. We propose that these "keystones" coordinate and interconnect multiple signaling pathways to control whole somatic activities such as aging-related disease etiology. Given its ability to control multiple receptor sensitivities and its broad protein-protein interactomic nature, we propose that G protein coupled receptor kinase 5 (GRK5) represents one of these key network controllers. Considerable data has emerged, suggesting that GRK5 acts as a bridging factor, allowing signaling regulation in pathophysiological settings to control the connectivity between both the cardiovascular and neurophysiological complications of aging.

The role of G protein-coupled receptor kinases in the pathology of malignant tumors

G protein-coupled receptor kinases (GRKs) constitute seven subtypes of serine/threonine protein kinases that specifically recognize and phosphorylate agonist-activated G protein-coupled receptors (GPCRs), thereby terminating the GPCRs-mediated signal transduction pathway. Recent research shows that GRKs also interact with non-GPCRs and participate in signal transduction in non-phosphorylated manner. Besides, GRKs activity can be regulated by multiple factors. Changes in GRKs expression have featured prominently in various tumor pathologies, and they are associated with angiogenesis, proliferation, migration, and invasion of malignant tumors. As a result, GRKs have been intensively studied as potential therapeutic targets. Herein, we review evolving understanding of the function of GRKs, the regulation of GRKs activity and the role of GRKs in human malignant tumor pathophysiology.

Grking the Smoothened signal

The kinase GRK2 has been linked to the clinically important Hedgehog (HH) signaling pathway, where it is paradoxically required for signal transduction yet also promotes internalization and degradation of the critical HH signal transducer Smoothened. Two reports by Li et al and Pusapati et al in this issue of Science Signaling provide new insights into the role of GRK2 in HH signaling.

Downregulation of RIKP by miR-200a promotes the invasive ability of esophageal cancer cells by upregulating the expression of LIN28 and MMP-14

Esophageal cancer (EC) is one of common digestive tract malignant tumors which morbidity and mortality were increased year by year. This study was aimed to investigate the role of microRNA (miR)-200a in EC. Human esophageal squamous cell carcinoma (ESCC) cells TE3 was transfected with miR-200a mimic or scramble control. Cell viability and invasion were assessed by MTT and Transwell assay, respectively. Binding effect of miR-200a on 3'UTR of RKIP was verified by luciferase activity assay. RKIP expression in miR-200a mimic transfected cells was measured. RKIP was overexpressed in miR-200a transfected cells and cell viability and invasion were measured. The expressions of Raf1, ERK, MMP-14, LIN28 and GRK-2 were also measured by qRT-PCR and Western blot analysis, respectively. Results showed that miR-200a mimic transfection increased cell viability and invasion of TE3 cells in vitro. miR-200a binding with 3'UTR of RKIP negatively regulated RKIP expression. RKIP overexpression inhibited effects of miR-200a on cell viability and invasion, as well as the increased phosphorylation levels of Raf1 and ERK. miR-200a increased expressions of MMP-14, LIN28 and GRK-2 in TE3 cells, and the up-regulations were inhibited by RKIP overexpression. In conclusion, the up-regulation of miR-200a in TE3 cells promoted cell viability and invasion via negatively regulating RKIP expression. RKIP was a direct target of miR-200a. miR-200a might be involved in activation of MAPK/ERK signaling pathway and expression of MMP-14, LIN28 and GRK-2 which were important factors of intracellular information transduction. Our findings demonstrated that miR-200a regulated ESCC cells via regulating RKIP expression.

Expression of GRK2 and IGF1R in hepatocellular carcinoma: clinicopathological and prognostic significance

AIM

It has been shown that G-protein-coupled receptor kinase 2 (GRK2) negatively regulates the insulin-like growth factor 1 receptor (IGF1R) signalling pathway in hepatocellular carcinoma (HCC). The aim of this study was to evaluate the clinicopathological and prognostic significance of GRK2 and IGF1R in HCC.

METHODS

Expression of GRK2 and IGF1R was first detected by tissue microarray-based immunohistochemistry in 156 patients with HCC. Staining results, termed the H-score, were then correlated with clinicopathological variables and patient survival. Finally, the prognostic value of GRK2 and IGF1R was validated in the publically available TCGA (The Cancer Genome Atlas) RNA-sequencing database.

RESULTS

The H-score of GRK2 staining (which was significantly lower in tumour than non-tumour tissue) was negatively associated with that of IGF1R with a reverse trend. No clinicopathological significance of the proteins was found except for a relationship between tumoral IGF1R expression and tumour-node-metastasis stage. In univariate analysis, high IGF1R expression predicted poor overall and disease-free survival, whereas GRK2 was not prognostic. In multivariate analysis, IGF1R was significant for overall survival. Furthermore, IGF1R was also of prognostic value in the TCGA database.

CONCLUSIONS

Our data indicate that GRK2 and IGF1R show a negative correlation in HCC. IGF1R could be a potential marker of poor prognosis for this malignancy.

G-protein-coupled receptor kinase 2 in pancreatic cancer: clinicopathologic and prognostic significance

G-protein-coupled receptor kinase 2 (GRK2) was found to regulate biological behaviors in some cancers, including pancreatic cancer (PC). However, its clinicopathologic and prognostic implications in cancer remain unclear. This study was designed to address the issues in PC. Expression of GRK2 was measured by Western blotting and tissue microarray-based immunohistochemical staining in 3 and 171 patients with PC, respectively. The H-score was used to evaluate the staining results. In addition, GRK2 expression was correlated with clinicopathologic variables and overall survival. Finally, the prognostic value of GRK2 was validated in a publically available PC dataset, GSE21501. It was suggested that GRK2 expression was highly up-regulated in 2 out of 3 tumor samples, in contrast to corresponding non-tumor ones. Furthermore, H-score of GRK2 staining was significantly higher in tumor than in non-tumor tissues. Tumoral expression of GRK2 was significantly associated with T stage. Univariate analysis showed that high GRK2 expression in tumor tissues was predictive for poor overall survival of PC. However, GRK2 expression was not identified as an independent prognostic marker in multivariate Cox regression test, although close to the statistical significance. In dataset GSE21501, GRK2 was also revealed to be prognostic. Our data establish that GRK2 is overexpressed in PC, and might serve as a potential indicator of unfavorable prognosis.

Multiple functions of G protein-coupled receptor kinases

Desensitization is a physiological feedback mechanism that blocks detrimental effects of persistent stimulation. G protein-coupled receptor kinase 2 (GRK2) was originally identified as the kinase that mediates G protein-coupled receptor (GPCR) desensitization. Subsequent studies revealed that GRK is a family composed of seven isoforms (GRK1–GRK7). Each GRK shows a differential expression pattern. GRK1, GRK4, and GRK7 are expressed in limited tissues. In contrast, GRK2, GRK3, GRK5, and GRK6 are ubiquitously expressed throughout the body. The roles of GRKs in GPCR desensitization are well established. When GPCRs are activated by their agonists, GRKs phosphorylate serine/threonine residues in the intracellular loops and the carboxyl-termini of GPCRs. Phosphorylation promotes translocation of β-arrestins to the receptors and inhibits further G protein activation by interrupting receptor-G protein coupling. The binding of β-arrestins to the receptors also helps to promote receptor internalization by clathrin-coated pits. Thus, the GRK-catalyzed phosphorylation and subsequent binding of β-arrestin to GPCRs are believed to be the common mechanism of GPCR desensitization and internalization. Recent studies have revealed that GRKs are also involved in the β-arrestin-mediated signaling pathway. The GRK-mediated phosphorylation of the receptors plays opposite roles in conventional G protein- and β-arrestin-mediated signaling. The GRK-catalyzed phosphorylation of the receptors results in decreased G protein-mediated signaling, but it is necessary for β-arrestin-mediated signaling. Agonists that selectively activate GRK/β-arrestin-dependent signaling without affecting G protein signaling are known as β-arrestin-biased agonists. Biased agonists are expected to have potential therapeutic benefits for various diseases due to their selective activation of favorable physiological responses or avoidance of the side effects of drugs. Furthermore, GRKs are recognized as signaling mediators that are independent of either G protein- or β-arrestin-mediated pathways. GRKs can phosphorylate non-GPCR substrates, and this is found to be involved in various physiological responses, such as cell motility, development, and inflammation. In addition to these effects, our group revealed that GRK6 expressed in macrophages mediates the removal of apoptotic cells (engulfment) in a kinase activity-dependent manner. These studies revealed that GRKs block excess stimulus and also induce cellular responses. Here, we summarized the involvement of GRKs in β-arrestin-mediated and G protein-independent signaling pathways.

Role of regulator of G protein signaling proteins in bone

Regulators of G protein signaling (RGS) proteins are a family with more than 30 proteins that all contain an RGS domain. In the past decade, increasing evidence has indicated that RGS proteins play crucial roles in the regulation of G protein coupling receptors (GPCR), G proteins, and calcium signaling during cell proliferation, migration, and differentiation in a variety of tissues. In bone, those proteins modulate bone development and remodeling by influencing various signaling pathways such as GPCR-G protein signaling, Wnt, calcium oscillations and PTH. This review summarizes the recent advances in the understanding of the regulation of RGS gene expression, as well as the functions and mechanisms of RGS proteins, especially in regulating GPCR-G protein signaling, Wnt signaling, calcium oscillations signaling and PTH signaling during bone development and remodeling. This review also highlights the regulation of different RGS proteins in osteoblasts, chondrocytes and osteoclasts. The knowledge from the recent advances of RGS study summarized in the review would provide the insights into new therapies for bone diseases.

Mechanisms of the proteinuria induced by Rho GTPases

Podocytes are highly differentiated cells that play an important role in maintaining glomerular filtration barrier integrity; a function regulated by small GTPase proteins of the Rho family. To investigate the role of Rho A in podocyte biology, we created transgenic mice expressing doxycycline-inducible constitutively active (V14 Rho) or dominant-negative Rho A (N19 Rho) in podocytes. Specific induction of either Rho A construct in podocytes caused albuminuria and foot process effacement along with disruption of the actin cytoskeleton as evidenced by decreased expression of the actin-associated protein synaptopodin. The mechanisms of these adverse effects, however, appeared to be different. Active V14 Rho enhanced actin polymerization, caused a reduction in nephrin mRNA and protein levels, promoted podocyte apoptosis, and decreased endogenous Rho A levels. In contrast, the dominant-negative N19 Rho caused a loss of podocyte stress fibers, did not alter the expression of either nephrin or Rho A, and did not cause podocyte apoptosis. Thus, our findings suggest that Rho A plays an important role in maintaining the integrity of the glomerular filtration barrier under basal conditions, but enhancement of Rho A activity above basal levels promotes podocyte injury.

Technical phosphoproteomic and bioinformatic tools useful in cancer research

Reversible protein phosphorylation is one of the most important forms of cellular regulation. Thus, phosphoproteomic analysis of protein phosphorylation in cells is a powerful tool to evaluate cell functional status. The importance of protein kinase-regulated signal transduction pathways in human cancer has led to the development of drugs that inhibit protein kinases at the apex or intermediary levels of these pathways. Phosphoproteomic analysis of these signalling pathways will provide important insights for operation and connectivity of these pathways to facilitate identification of the best targets for cancer therapies. Enrichment of phosphorylated proteins or peptides from tissue or bodily fluid samples is required. The application of technologies such as phosphoenrichments, mass spectrometry (MS) coupled to bioinformatics tools is crucial for the identification and quantification of protein phosphorylation sites for advancing in such relevant clinical research. A combination of different phosphopeptide enrichments, quantitative techniques and bioinformatic tools is necessary to achieve good phospho-regulation data and good structural analysis of protein studies. The current and most useful proteomics and bioinformatics techniques will be explained with research examples. Our aim in this article is to be helpful for cancer research via detailing proteomics and bioinformatic tools.

G protein-coupled receptor kinases: more than just kinases and not only for GPCRs

G protein-coupled receptor (GPCR) kinases (GRKs) are best known for their role in homologous desensitization of GPCRs. GRKs phosphorylate activated receptors and promote high affinity binding of arrestins, which precludes G protein coupling. GRKs have a multidomain structure, with the kinase domain inserted into a loop of a regulator of G protein signaling homology domain. Unlike many other kinases, GRKs do not need to be phosphorylated in their activation loop to achieve an activated state. Instead, they are directly activated by docking with active GPCRs. In this manner they are able to selectively phosphorylate Ser/Thr residues on only the activated form of the receptor, unlike related kinases such as protein kinase A. GRKs also phosphorylate a variety of non-GPCR substrates and regulate several signaling pathways via direct interactions with other proteins in a phosphorylation-independent manner. Multiple GRK subtypes are present in virtually every animal cell, with the highest expression levels found in neurons, with their extensive and complex signal regulation. Insufficient or excessive GRK activity was implicated in a variety of human disorders, ranging from heart failure to depression to Parkinson's disease. As key regulators of GPCR-dependent and -independent signaling pathways, GRKs are emerging drug targets and promising molecular tools for therapy. Targeted modulation of expression and/or of activity of several GRK isoforms for therapeutic purposes was recently validated in cardiac disorders and Parkinson's disease.

Calcineurin (CN) activation promotes apoptosis of glomerular podocytes both in vitro and in vivo

To determine the role of Gq signaling and calcineurin (CN) activation in promoting apoptosis of glomerular podocytes, constitutively active Gq [Gq(+)] or CN [CN(+)] proteins were introduced into cultured podocytes using protein transduction by tagging the proteins with the transactivator of transcription peptide. To investigate the role of CN in promoting podocyte apoptosis in vivo, a genetic model of type 1 diabetes mellitus (Akita mice) was treated with the CN inhibitor FK506. In cultured podocytes, Gq(+) stimulated nuclear translocation of nuclear factor of activated T cells (NFAT) family members, activated an NFAT reporter construct, and enhanced podocyte apoptosis in a CN-dependent fashion. CN(+) similarly promoted podocyte apoptosis, and apoptosis induced by either angiotensin II or endothelin-1 was blocked by FK506. Induction of apoptosis required NFAT-induced gene transcription because apoptosis induced by either Gq(+) or CN(+) was blocked by an inhibitor that prevented CN-dependent NFAT activation without affecting CN phosphatase activity. Podocyte apoptosis was mediated, in part, by the NFAT-responsive gene cyclooxygenase 2 (COX2) and prostaglandin E(2) generation because apoptosis induced by Gq(+) was attenuated by either COX2 inhibition or blockade of the Gq-coupled E-series prostaglandins receptor. The findings appeared relevant to podocyte apoptosis in diabetic nephropathy because apoptosis was significantly reduced in Akita mice by treatment with FK506. These data suggest that Gq stimulates CN and promotes podocyte apoptosis both in vitro and in vivo. Apoptosis requires NFAT-dependent gene transcription and is mediated, in part, by CN-dependent COX2 induction, prostaglandin E(2) generation, and autocrine activation of the Gq-coupled E-series prostaglandins receptor.

Novel regulatory mechanism of canonical Wnt signaling by dopamine D2 receptor through direct interaction with beta-catenin

Classical G protein-coupled receptors (GPCRs) and canonical Wnt pathways were believed to use distinct signaling pathways. However, recent studies have shown that these two pathways interact each other by sharing several intermediate signaling components. Recent in vivo studies showed that antipsychotic drugs, which block dopamine D2-like receptors, increase the cellular levels of downstream signaling components of canonical Wnt pathways, such as dishevelled (Dvl), glycogen synthase kinase 3β (GSK3β), and β-catenin. These results suggest that some functional interactions might exist between Wnt pathway and D2-like receptors. In this study, we show that among five different dopamine receptor subtypes, D(2) receptor (D(2)R) selectively inhibited the Wnt signaling, which was measured by lymphoid enhancing factor-1 (LEF-1)-dependent transcriptional activities. D(2)R-mediated inhibition of Wnt signaling was agonist- and G protein-independent and did not require receptor phosphorylation or endocytosis. D(2)R inhibited the LEF-1-dependent transcriptional activities, and this inhibitory activity was not affected by the inhibition of GSK-3β, suggesting that D(2)R inhibited the Wnt signaling by acting on the downstream of GSK3β. D(2)R directly interacted with β-catenin through the second and third loops, leading to a reduction of β-catenin distribution in the nucleus, resulting in an inhibition of LEF-1-dependent transcription. This is a novel mechanism for the regulation of canonical Wnt signaling by GPCRs, in which receptor proteins recruit β-catenin from cytosol to the plasma membrane, resulting in the decrement of the β-catenin/LEF-1-dependent transcription in the nucleus.

The complex G protein-coupled receptor kinase 2 (GRK2) interactome unveils new physiopathological targets

GRK2 is a ubiquitous member of the G protein-coupled receptor kinase (GRK) family that appears to play a central, integrative role in signal transduction cascades. GRKs participate together with arrestins in the regulation of G protein-coupled receptors (GPCR), a family of hundreds of membrane proteins of key physiological and pharmacological importance, by triggering receptor desensitization from G proteins and GPCR internalization, and also by helping assemble macromolecular signalosomes in the receptor environment acting as agonist-regulated adaptor scaffolds, thus contributing to signal propagation. In addition, emerging evidence indicates that GRK2 can phosphorylate a growing number of non-GPCR substrates and associate with a variety of proteins related to signal transduction, thus suggesting that this kinase could also have diverse 'effector' functions. We discuss herein the increasing complexity of such GRK2 'interactome', with emphasis on the recently reported roles of this kinase in cell migration and cell cycle progression and on the functional impact of the altered GRK2 levels observed in several relevant cardiovascular, inflammatory or tumour pathologies. Deciphering how the different networks of potential GRK2 functional interactions are orchestrated in a stimulus, cell type or context-specific way is critical to unveil the contribution of GRK2 to basic cellular processes, to understand how alterations in GRK2 levels or functionality may participate in the onset or development of several cardiovascular, tumour or inflammatory diseases, and to assess the feasibility of new therapeutic strategies based on the modulation of the activity, levels or specific interactions of GRK2.

Wnt11 promotes cardiomyocyte development by caspase-mediated suppression of canonical Wnt signals

Specification and early patterning of the vertebrate heart are dependent on both canonical and noncanonical wingless (Wnt) signal pathways. However, the impact of each Wnt pathway on the later stages of myocardial development and differentiation remains controversial. Here, we report that the components of each Wnt signal conduit are expressed in the developing and postnatal heart, yet canonical/β-catenin activity is restricted to nonmyocardial regions. Subsequently, we observed that noncanonical Wnt (Wnt11) enhanced myocyte differentiation while preventing stabilization of the β-catenin protein, suggesting active repression of canonical Wnt signals. Wnt11 stimulation was synonymous with activation of a caspase 3 signal cascade, while inhibition of caspase activity led to accumulation of β-catenin and a dramatic reduction in myocyte differentiation. Taken together, these results suggest that noncanonical Wnt signals promote myocyte maturation through caspase-mediated inhibition of β-catenin activity.

Regulation of Lrp6 phosphorylation

The Wnt/beta-catenin signaling pathway plays important roles in embryonic development and tissue homeostasis, and is implicated in human disease. Wnts transduce signals via transmembrane receptors of the Frizzled (Fzd/Fz) family and the low density lipoprotein receptor-related protein 5/6 (Lrp5/6). A key mechanism in their signal transduction is that Wnts induce Lrp6 signalosomes, which become phosphorylated at multiple conserved sites, notably at PPSPXS motifs. Lrp6 phosphorylation is crucial to beta-catenin stabilization and pathway activation by promoting Axin and Gsk3 recruitment to phosphorylated sites. Here, we summarize how proline-directed kinases (Gsk3, PKA, Pftk1, Grk5/6) and non-proline-directed kinases (CK1 family) act upon Lrp6, how the phosphorylation is regulated by ligand binding and mitosis, and how Lrp6 phosphorylation leads to beta-catenin stabilization.

Development of small molecules targeting the Wnt pathway for the treatment of colon cancer: a high-throughput screening approach

Wnt proteins play major roles in development and differentiation, and abnormalities in their regulation are believed to contribute to the formation of many cancers, including colorectal malignancies. As a result, there has been an interest in identifying small molecule inhibitors of Wnt signaling as tool compounds for research or as precursors to new generations of anticancer drugs. Advancements in robotic technology along with reductions in the costs of equipment, chemical libraries, and information handling have made high-throughput drug discovery programs possible in an academic setting. In this minireview we discuss the most plausible protein targets for inhibiting Wnt signaling in colon cancer therapy, list small molecule Wnt inhibitors that have been identified through recent drug discovery efforts, and provide our laboratory's strategy for identifying novel Wnt signaling antagonists using high-throughput screening. In particular, we summarize the results of a screen of over 1,200 drug and druglike compounds we recently completed in which niclosamide was identified as a Wnt pathway antagonist.