Impaired recruitment of Grk6 and beta-Arrestin 2 causes delayed internalization and desensitization of a WHIM syndrome-associated CXCR4 mutant receptor

Unexpected diagnosis of WHIM syndrome in refractory autoimmune cytopenia

ABSTRACT

WHIM (warts, hypogammaglobulinemia, infections, and myelokathexis) syndrome is a rare primary immunodeficiency predominantly caused by heterozygous gain-of-function mutations in the C-terminus of the gene CXCR4. These CXCR4 variants display impaired receptor trafficking with persistence of the CXCR4 receptor on the surface, resulting in hyperactive downstream signaling after CXCL12 stimulation. In turn, this results in defective lymphoid differentiation, and reduced blood neutrophil and lymphocyte numbers. Here, we report a CXCR4 mutation that in 2 members of a kindred, led to life-long autoimmunity and lymphoid hypertrophy as the primary clinical manifestations of WHIM syndrome. We examine the functional effects of this mutation, and how these have affected phosphorylation, activation, and receptor internalization.

The complex nature of CXCR4 mutations in WHIM syndrome

Heterozygous autosomal dominant mutations in the CXCR4 gene cause WHIM syndrome, a severe combined immunodeficiency disorder. The mutations primarily affect the C-terminal region of the CXCR4 chemokine receptor, specifically several potential phosphorylation sites critical for agonist (CXCL12)-mediated receptor internalization and desensitization. Mutant receptors have a prolonged residence time on the cell surface, leading to hyperactive signaling that is responsible for some of the symptoms of WHIM syndrome. Recent studies have shown that the situation is more complex than originally thought, as mutant WHIM receptors and CXCR4 exhibit different dynamics at the cell membrane, which also influences their respective cellular functions. This review examines the functional mechanisms of CXCR4 and the impact of WHIM mutations in both physiological and pathological conditions.

Reduced G protein signaling despite impaired internalization and β-arrestin recruitment in patients carrying a CXCR4Leu317fsX3 mutation causing WHIM syndrome

WHIM syndrome is an inherited immune disorder caused by an autosomal dominant heterozygous mutation in CXCR4. The disease is characterized by neutropenia/leukopenia (secondary to retention of mature neutrophils in bone marrow), recurrent bacterial infections, treatment-refractory warts, and hypogammaglobulinemia. All mutations reported in WHIM patients lead to the truncations in the C-terminal domain of CXCR4, R334X being the most frequent. This defect prevents receptor internalization and enhances both calcium mobilization and ERK phosphorylation, resulting in increased chemotaxis in response to the unique ligand CXCL12. Here, we describe 3 patients presenting neutropenia and myelokathexis, but normal lymphocyte count and immunoglobulin levels, carrying what we believe to be a novel Leu317fsX3 mutation in CXCR4, leading to a complete truncation of its intracellular tail. The analysis of the L317fsX3 mutation in cells derived from patients and in vitro cellular models reveals unique signaling features in comparison with R334X mutation. The L317fsX3 mutation impairs CXCR4 downregulation and β-arrestin recruitment in response to CXCL12 and reduces other signaling events - including ERK1/2 phosphorylation, calcium mobilization, and chemotaxis - all processes that are typically enhanced in cells carrying the R334X mutation. Our findings suggest that, overall, the L317fsX3 mutation may be causative of a form of WHIM syndrome not associated with an augmented CXCR4 response to CXCL12.

The Role of G Protein-Coupled Receptor Kinase 6 Regulation in Inflammation and Pain

The G protein-coupled receptor kinase 6 is associated with inflammation and pathological pain. Impairment of GRK6 expression was described in chronic inflammatory diseases such as rheumatoid arthritis and this was shown to be accompanied by an imbalance of downstream signaling pathways. Here, we discuss novel aspects of GRK6 interaction and its impact upon hyperalgesia and inflammatory processes. In this review, we compile important findings concerning GRK6 regulation for a better pathophysiological understanding of the intracellular interaction in the context of inflammation and show clinical implications-for example, the identification of possible therapy goals in the treatment of chronic inflammatory hyperalgesia.

Characterization of a new WHIM syndrome mutant reveals mechanistic differences in regulation of the chemokine receptor CXCR4

WHIM syndrome is a rare immunodeficiency disorder that is characterized by warts, hypogammaglobulinemia, infections, and myelokathexis. While several gain-of-function mutations that lead to C-terminal truncations, frame shifts and point mutations in the chemokine receptor CXCR4 have been identified in WHIM syndrome patients, the functional effect of these mutations are not fully understood. Here, we report on a new WHIM syndrome mutation that results in a frame shift within the codon for Ser339 (S339fs5) and compare the properties of S339fs5 with wild-type CXCR4 and a previously identified WHIM syndrome mutant, R334X. The S339fs5 and R334X mutants exhibited significantly increased signaling compared to wild-type CXCR4 including agonist-promoted calcium flux and extracellular-signal-regulated kinase activation. This increase is at least partially due to a significant decrease in agonist-promoted phosphorylation, β-arrestin binding, and endocytosis of S339fs5 and R334X compared with wild-type CXCR4. Interestingly, there were also significant differences in receptor degradation, with S339fs5 having a very high basal level of degradation compared with that of R334X and wild-type CXCR4. In contrast to wild-type CXCR4, both R334X and S339fs5 were largely insensitive to CXCL12-promoted degradation. Moreover, while basal and agonist-promoted degradation of wild-type CXCR4 was effectively inhibited by the CXCR4 antagonist TE-14016, this had no effect on the degradation of the WHIM mutants. Taken together, these studies identify a new WHIM syndrome mutant, CXCR4-S339fs5, which promotes enhanced signaling, reduced phosphorylation, β-arrestin binding and endocytosis, and a very high basal rate of degradation that is not protected by antagonist treatment.

Design, Synthesis, and Characterization of 4-Aminoquinazolines as Potent Inhibitors of the G Protein-Coupled Receptor Kinase 6 (GRK6) for the Treatment of Multiple Myeloma

Both previous and additional genetic knockdown studies reported herein implicate G protein-coupled receptor kinase 6 (GRK6) as a critical kinase required for the survival of multiple myeloma (MM) cells. Therefore, we sought to develop a small molecule GRK6 inhibitor as an MM therapeutic. From a focused library of known kinase inhibitors, we identified two hits with moderate biochemical potencies against GRK6. From these hits, we developed potent (IC₅₀ < 10 nM) analogues with selectivity against off-target kinases. Further optimization led to the discovery of an analogue (18) with an IC₅₀ value of 6 nM against GRK6 and selectivity against a panel of 85 kinases. Compound 18 has potent cellular target engagement and antiproliferative activity against MM cells and is synergistic with bortezomib. In summary, we demonstrate that targeting GRK6 with small molecule inhibitors represents a promising approach for MM and identify 18 as a novel, potent, and selective GRK6 inhibitor.

MTSS1 inhibits colorectal cancer metastasis by regulating the CXCR4/CXCL12 signaling axis

The liver is the most common site of metastasis for colorectal cancer (CRC). Metastasis suppressor 1 (MTSS1), a potential tumor suppressor gene associated with tumor metastasis, has been reported to play an important role in cancer development. The present study aimed to investigate the effects and underlying mechanisms of MTSS1 on the biological behavior of CRC cells both in vitro and in vivo. A CRC mouse model with a high liver metastatic potential was established by injecting mice with SW1116 cells, and the association between MTSS1 expression levels and the metastatic potential of forming liver metastasis lesions was subsequently analyzed. MTSS1 gain‑ and loss‑of‑function experiments were performed by transfecting the CRC cell lines, SW1116 and DLD‑1, with Plvx‑IRES‑ZsGreen1‑MTSS1 plasmid and short hairpin RNA, respectively. Cell proliferation, migration, invasion and cell cycle distribution were analyzed by MTT, Transwell and flow cytometric assays, respectively. To further determine the underlying mechanisms of MTSS1 in CRC, the expression levels of cell surface chemokine C‑X‑C receptor 4 (CXCR4) and its downstream signaling factors, Rac and cell division cycle 42 (CDC42), were analyzed with or without C‑X‑C motif chemokine ligand 12 (CXCL12) stimulation. The results revealed that as the CRC metastatic potential increased, the expression levels of MTSS1 decreased. The overexpression of MTSS1 exerted an inhibitory effect on cell proliferation, migration and invasion, while the knockdown of MTSS1 exerted the opposite effects in vitro. Flow cytometric analysis and western blot analysis demonstrated that MTSS1 negatively regulated the expression levels of cell surface CXCR4 and its downstream signaling pathway activation. On the whole, the results of the present study indicate that MTSS1 may play an important negative role in CRC metastasis and the underlying mechanisms may involve the downregulation of the CXCR4/CXCL12 signaling axis.

The G Protein-Coupled Receptor Kinases (GRKs) in Chemokine Receptor-Mediated Immune Cell Migration: From Molecular Cues to Physiopathology

Although G protein-coupled receptor kinases (GRKs) have long been known to regulate G protein-coupled receptor (GPCR) desensitization, their more recently characterized functions as scaffolds and signalling adapters underscore that this small family of proteins governs a larger array of physiological functions than originally suspected. This review explores how GRKs contribute to the complex signalling networks involved in the migration of immune cells along chemokine gradients sensed by cell surface GPCRs. We outline emerging evidence indicating that the coordinated docking of several GRKs on an active chemokine receptor determines a specific receptor phosphorylation barcode that will translate into distinct signalling and migration outcomes. The guidance cues for neutrophil migration are emphasized based on several alterations affecting GRKs or GPCRs reported to be involved in pathological conditions.

Aberrant CXCR4 Signaling at Crossroad of WHIM Syndrome and Waldenstrom's Macroglobulinemia

Given its pleiotropic functions, including its prominent role in inflammation, immune responses and cancer, the C-X-C chemokine receptor type 4 (CXCR4) has gained significant attention in recent years and has become a relevant target in drug development. Although the signaling properties of CXCR4 have been extensively studied, several aspects deserve deeper investigations. Mutations in the C-term tail of the CXCR4 gene cause WHIM syndrome, a rare congenital immunodeficiency associated by chronic leukopenia. Similar mutations have also been recently identified in 30% of patients affected by Waldenstrom’s macroglobulinaemia, a B-cell neoplasia with bone marrow accumulation of malignant cells. An ample body of work has been generated to define the impact of WHIM mutations on CXCR4 signaling properties and evaluate their role on pathogenesis, diagnosis, and response to therapy, although the identity of disease-causing signaling pathways and their relevance for disease development in different genetic variants are still open questions. This review discusses the current knowledge on biochemical properties of CXCR4 mutations to identify their prototypic signaling profile potentially useful to highlighting novel opportunities for therapeutic intervention.

A degradatory fate for CCR4 suggests a primary role in Th2 inflammation

CCR4 is the sole receptor for the chemokines CCL22 and CCL17. Clinical studies of asthmatic airways have shown levels of both ligands and CCR4⁺ Th2 cells to be elevated, suggestive of a role in disease. Consequently, CCR4 has aroused much interest as a potential therapeutic target and an understanding of how its cell surface expression is regulated is highly desirable. To this end, receptor expression, receptor endocytosis, and chemotaxis were assessed using transfectants expressing CCR4, CCR4⁺ human T cell lines, and human Th2 cells polarized in vitro. CCL17 and CCL22 drove rapid endocytosis of CCR4 in a dose-dependent manner. Replenishment at the cell surface was slow and sensitive to cycloheximide, suggestive of de novo synthesis of CCR4. Constitutive CCR4 endocytosis was also observed, with the internalized CCR4 found to be significantly degraded over a 6-h incubation. Truncation of the CCR4 C-terminus by 40 amino acids had no effect on cell surface expression, but resulted in significant impairment of ligand-induced endocytosis. Consequently, migration to both CCL17 and CCL22 was significantly enhanced. In contrast, truncation of CCR4 did not impair constitutive endocytosis or degradation, suggesting the use of alternative receptor motifs in these processes. We conclude that CCR4 cell surface levels are tightly regulated, with a degradative fate for endocytosed receptor. We postulate that this strict control is desirable, given that Th2 cells recruited by CCR4 can induce the further expression of CCR4 ligands in a positive feedback loop, thereby enhancing allergic inflammation.

How I treat warts, hypogammaglobulinemia, infections, and myelokathexis syndrome

Warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome is a genetic disease characterized by neutropenia, lymphopenia, susceptibility to infections, and myelokathexis, which describes degenerative changes of mature neutrophils and hyperplasia of bone marrow myeloid cells. Some patients present with hypogammaglobulinemia and/or refractory warts of skin and genitalia. Congenital cardiac defects constitute uncommon manifestations of the disease. The disorder, which is inherited as an autosomal dominant trait, is caused by heterozygous mutations of the chemokine receptor CXCR4. These mutations lead to an increased sensitivity of neutrophils and lymphocytes to the unique ligand CXCL12 and to an increased accumulation of mature neutrophils in the bone marrow. Despite greatly improved knowledge of the disease, therapeutic choices are insufficient to prevent some of the disease outcomes, such as development of bronchiectasis, anogenital dysplasia, or invasive cancer. The available therapeutic measures aimed at preventing the risk for infection in WHIM patients are discussed. We critically evaluate the diagnostic criteria of WHIM syndrome, particularly when WHIM syndrome should be suspected in patients with congenital neutropenia and lymphopenia despite the absence of hypogammaglobulinemia and/or warts. Finally, we discuss recent results of trials evaluating plerixafor, a selective antagonist of CXCR4, as a mechanism-oriented strategy for treatment of WHIM patients.

Pathogenesis, diagnosis and therapeutic strategies in WHIM syndrome immunodeficiency

21 INTRODUCTION

WHIM syndrome is a rare combined primary immunodeficiency disorder caused by autosomal dominant gain-of-function mutations in the chemokine receptor CXCR4. It is the only Mendelian condition known to be caused by mutation of a chemokine or chemokine receptor. As such, it provides a scientific opportunity to understand chemokine-dependent immunoregulation in humans and a medical opportunity to develop mechanism-based treatment and cure strategies.

22 AREAS COVERED

This review covers the clinical features, genetics, immunopathogenesis and clinical management of WHIM syndrome. Clinical trials of targeted therapeutic agents and potential cure strategies are also included.

23 EXPERT OPINION

WHIM syndrome may be particularly amenable to mechanism-based therapeutics for three reasons: 1) CXCR4 has been validated as the molecular target in the disease by Mendelian genetics; 2) the biochemical abnormality is excessive CXCR4 signaling; and 3) antagonists selective for CXCR4 have been developed. Plerixafor is FDA-approved for hematopoietic stem cell (HSC) mobilization and has shown preliminary safety and efficacy in phase I clinical trials in WHIM syndrome. Gene editing may represent a viable cure strategy, since chromothriptic deletion of the disease allele in HSCs resulted in clinical cure of a patient and because CXCR4 haploinsufficiency enhances engraftment of transplanted HSCs in mice.

CXCR4-Specific Nanobodies as Potential Therapeutics for WHIM syndrome

WHIM syndrome is a rare congenital immunodeficiency disease, named after its main clinical manifestations: warts, hypogammaglobulinemia, infections, and myelokathexis, which refers to abnormal accumulation of mature neutrophils in the bone marrow. The disease is primarily caused by C-terminal truncation mutations of the chemokine receptor CXCR4, giving these CXCR4-WHIM mutants a gain of function in response to their ligand CXCL12. Considering the broad functions of CXCR4 in maintaining leukocyte homeostasis, patients are panleukopenic and display altered immune responses, likely as a consequence of impairment in the differentiation and trafficking of leukocytes. Treatment of WHIM patients currently consists of symptom relief, leading to unsatisfactory clinical responses. As an alternative and potentially more effective approach, we tested the potency and efficacy of CXCR4-specific nanobodies on inhibiting CXCR4-WHIM mutants. Nanobodies are therapeutic proteins based on the smallest functional fragments of heavy chain antibodies. They combine the advantages of small-molecule drugs and antibody-based therapeutics due to their relative small size, high stability, and high affinity. We compared the potential of monovalent and bivalent CXCR4-specific nanobodies to inhibit CXCL12-induced CXCR4-WHIM-mediated signaling with the small-molecule clinical candidate AMD3100. The CXCR4-targeting nanobodies displace CXCL12 binding and bind CXCR4-wild type and CXCR4-WHIM (R334X/S338X) mutants and with (sub-) nanomolar affinities. The nanobodies' epitope was mapped to extracellular loop 2 of CXCR4, overlapping with the binding site of CXCL12. Monovalent, and in particular bivalent, nanobodies were more potent than AMD3100 in reducing CXCL12-mediated G protein activation. In addition, CXCR4-WHIM-dependent calcium flux and wound healing of human papillomavirus-immortalized cell lines in response to CXCL12 was effectively inhibited by the nanobodies. Based on these in vitro results, we conclude that CXCR4 nanobodies hold significant potential as alternative therapeutics for CXCR4-associated diseases such as WHIM syndrome.

Targeted strategies directed at the molecular defect: Toward precision medicine for select primary immunodeficiency disorders

Primary immunodeficiency disorders (PIDs) represent a range of genetically determined diseases that typically have increased susceptibility to infections and in many cases also have evidence of immune dysregulation that often presents as autoimmunity. Most recently, the concept of gain-of-function mutations associated with PIDs has become well recognized and adds a new dimension to the understanding of this group of disorders, moving beyond the more commonly seen loss-of-function mutations. The rapidly expanding genetic defects that have been identified in patients with previously uncharacterized PIDs has opened up the potential for targeted therapy directed at the specific disease-causing abnormality. This has been driven by linking PID-specific genetic defects to the associated unique abnormalities in cellular signaling pathways amenable to directed therapies. These include agents that either block overactive or enhance underresponsive cellular pathways. Selected primary immunodeficiencies were chosen, the genetic defects of which have been recently characterized and are amenable to targeted therapy, as a reflection of the power of precision medicine.

G Protein-Coupled Receptor Kinases in the Inflammatory Response and Signaling

G protein-coupled receptor kinases (GRKs) are serine/threonine kinases that regulate a large and diverse class of G protein-coupled receptors (GPCRs). Through GRK phosphorylation and β-arrestin recruitment, GPCRs are desensitized and their signal terminated. Recent work on these kinases has expanded their role from canonical GPCR regulation to include noncanonical regulation of non-GPCR and nonreceptor substrates through phosphorylation as well as via scaffolding functions. Owing to these and other regulatory roles, GRKs have been shown to play a critical role in the outcome of a variety of physiological and pathophysiological processes including chemotaxis, signaling, migration, inflammatory gene expression, etc. This diverse set of functions for these proteins makes them popular targets for therapeutics. Role for these kinases in inflammation and inflammatory disease is an evolving area of research currently pursued in many laboratories. In this review, we describe the current state of knowledge on various GRKs pertaining to their role in inflammation and inflammatory diseases.

G Protein-Coupled Receptor Kinase 3 and Protein Kinase C Phosphorylate the Distal C-Terminal Tail of the Chemokine Receptor CXCR4 and Mediate Recruitment of β-Arrestin

Phosphorylation of G protein-coupled receptors (GPCRs) is a key event for cell signaling and regulation of receptor function. Previously, using tandem mass spectrometry, we identified two phosphorylation sites at the distal C-terminal tail of the chemokine receptor CXCR4, but were unable to determine which specific residues were phosphorylated. Here, we demonstrate that serines (Ser) 346 and/or 347 (Ser-346/7) of CXCR4 are phosphorylated upon stimulation with the agonist CXCL12 as well as a CXCR4 pepducin, ATI-2341. ATI-2341, a Gα_iβγ heterotrimer-biased CXCR4 agonist, induced more robust phosphorylation of Ser-346/7 compared with CXCL12. Knockdown of G protein-coupled receptor kinase (GRK) 2, GRK3, or GRK6 reduced CXCL12-induced phosphorylation of Ser-346/7 with GRK3 knockdown having the strongest effect, while inhibition of the conventional protein kinase C (PKC) isoforms, particularly PKCα, reduced phosphorylation of Ser-346/7 induced by either CXCL12 or ATI-2341. The loss of GRK3- or PKC-mediated phosphorylation of Ser-346/7 impaired the recruitment of β-arrestin to CXCR4. We also found that a pseudo-substrate peptide inhibitor for PKCζ effectively inhibited CXCR4 phosphorylation and signaling, most likely by functioning as a nonspecific CXCR4 antagonist. Together, these studies demonstrate the role Ser-346/7 plays in arrestin recruitment and initiation of receptor desensitization and provide insight into the dysregulation of CXCR4 observed in patients with various forms of WHIM syndrome.

Missing-in-metastasis protein downregulates CXCR4 by promoting ubiquitylation and interaction with small Rab GTPases

Surface expression of chemokine receptor CXCR4 is downregulated by missing-in-metastasis protein (MIM; also known as MTSS1), a member of the inverse BAR (I-BAR)-domain protein family that recognizes and generates membranes with negative curvature. Yet, the mechanism for the regulation is unknown. Here, we show that MIM forms a complex with CXCR4 by binding to E3 ubiquitin ligase AIP4 (also known as ITCH) in response to stromal cell-derived factor 1 (SDF-1; also known as CXCL12). Overexpression of MIM promoted CXCR4 ubiquitylation, inhibited cellular response to SDF-1, caused accumulation and aggregation of multivesicular bodies (MVBs) in the cytoplasm, and promoted CXCR4 sorting into MVBs in a manner depending on binding to AIP4. In response to SDF-1, MIM also bound transiently to the small GTPase Rab5 at 5 min and to Rab7 at 30 min. Binding to Rab7 requires an N-terminal coiled-coil motif, deletion of which abolished MIM-mediated MVB formation and CXCR4 internalization. Our results unveil a previously unknown property of MIM that establishes the linkage of protein ubiquitylation with Rab-guided trafficking of CXCR4 in endocytic vesicles.

VEGF gene therapy cooperatively recruits molecules from the immune system and stimulates cell homing and angiogenesis in refractory angina

BACKGROUND

New vessels are formed in response to stimuli from angiogenic factors, a process in which paracrine signaling is fundamental.

OBJECTIVE

To investigate the cooperative paracrine signaling profile in response to Vascular Endothelial Growth Factor (VEGF) gene therapy in patients with coronary artery disease (CAD) and refractory angina.

METHOD

A cohort study was conducted in which plasma was collected from patients who underwent gene therapy with a plasmid expressing VEGF 165 (10) and from surgical procedure controls (4). Blood samples were collected from both groups prior to baseline and on days 3, 9 and 27 after the interventions and subjected to systemic analysis of protein expression (Interleukin-6, IL-6; Tumor Necrosis Factor-α, TNF-α; Interleukin-10, IL-10; Stromal Derived Factor-1 α, SDF-1α; VEGF; Angiopoietin-1, ANGPT-1; and Endothelin-1, ET-1) using the enzyme-linked immunosorbent assay (ELISA).

RESULTS

Analysis showed an increase in proinflammatory IL-6 (p=0.02) and ET-1 (p=0.05) on day 3 after gene therapy and in VEGF (p=0.02) on day 9. A strong positive correlation was found between mobilization of endothelial progenitor cells and TNF-α on day 9 (r=0.71; p=0.03). Furthermore, a strong correlation between β-blockers, antiplatelets, and vasodilators with SDF-1α baseline in the group undergoing gene therapy was verified (r=0.74; p=0.004).

CONCLUSION

Analysis of cooperative paracrine signaling after VEGF gene therapy suggests that the immune system cell and angiogenic molecule expression as well as the endothelial progenitor cell mobilization are time-dependent, influenced by chronic inflammatory process and continuous pharmacological treatment.

Prognostic value of decreased GRK6 expression in lung adenocarcinoma

BACKGROUND

In recent years, G protein-coupled receptor kinases (GRKs) have been implicated in cancer metastasis through phosphorylation of the activated form of G protein-coupled receptors. However, little is known of GRK6 expression in lung adenocarcinoma (LADC) and its potential prognostic value in LADC.

METHODS

In this study, protein expression of GRK6 was determined in LADC tissues (n = 122) and normal lung tissues (n = 45) by immunohistochemistry (IHC) analysis on tissue microarray (TMA). In addition, mRNA level of GRK6 in matched pairs of cancerous and non-cancerous fresh frozen tissues from 20 LADC patients was investigated using real-time quantitative PCR (qPCR). Furthermore, protein expression level of GRK6 was evaluated in matched pairs of cancerous and non-cancerous fresh frozen tissues from another 18 LADC patients. Univariate and multivariate analyses based on Cox proportional hazards regression models were performed to investigate the correlation between GRK6 expression and overall survival of LADC patients.

RESULTS

According to the IHC analysis on TMA, GRK6 expression was significantly down-regulated in LADC patients, but high in normal lung tissue (P < 0.001). Besides, our qPCR and western blot results confirmed GRK6 down-regulation in both mRNA and protein levels in LADC tissues as compared to matched adjacent non-cancerous tissues (all P < 0.001). Additionally, For TMA slides, protein expression of GRK6 was significantly associated with staging (P = 0.030), pathology grade (P = 0.036). Consistent with the associated poor clinicopathologic features, patients with GRK6 low expression tumors had a worse overall survival compared to patients with GRK6 high expression tumors. Further multivariate analysis using the Cox proportional hazards model revealed that GRK6 expression level (P = 0.004) was an independent prognostic factor for overall survival.

CONCLUSION

These findings indicate for the first time that decreased expression of GRK6 may serve as an independent predictor of overall survival in LADC patients.

A New Anti-CXCR4 Antibody That Blocks the CXCR4/SDF-1 Axis and Mobilizes Effector Cells

The type IV C-X-C-motif chemokine receptor (CXCR4) is expressed in a large variety of human cancers, including hematologic malignancies, and this receptor and its ligand, stromal cell-derived factor-1 (SDF-1), play a crucial role in cancer progression. We generated a humanized immunoglobulin G1 mAb, hz515H7, which binds human CXCR4, efficiently competes for SDF-1 binding, and induces a conformational change in CXCR4 homodimers. Furthermore, it inhibits both CXCR4 receptor-mediated G-protein activation and β-arrestin-2 recruitment following CXCR4 activation. The binding of the hz515H7 antibody to CXCR4 inhibits the SDF-1-induced signaling pathway, resulting in reduced phosphorylation of downstream effectors, such as Akt, Erk1/2, p38, and GSK3β. Hz515H7 also strongly inhibits cell migration and proliferation and, while preserving normal blood cells, induces both antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity against neoplastic cells. In mouse xenograft models, hz515H7 displays antitumor activities with multiple hematologic tumor cell lines, with its Fc-mediated effector functions proving essential in this context. Furthermore, hz515H7 binds to primary tumor cells from acute myeloid leukemia and multiple myeloma patients. Collectively, our results demonstrate two major mechanisms of action, making hz515H7 unique in this regard. Its potential as a best-in-class molecule is currently under investigation in a phase I clinical trial. Mol Cancer Ther; 15(8); 1890-9. ©2016 AACR.

Overexpression of GRK6 attenuates neuropathic pain via suppression of CXCR2 in rat dorsal root ganglion

G protein-coupled kinase (GRK) 6 is a member of the GRK family that mediates agonist-induced desensitization and signaling of G protein-coupled receptors (GPCRs), thus involving in a wide variety of processes including inflammation and nociception. Recent studies have indicated that chemokines play an important role in chronic pain via increased expression of respective GPCRs. This study was designed to investigate the role of GRK6 and its interaction with substrate chemokine receptors in dorsal root ganglion (DRG) in a rat model of neuropathic pain induced by chronic constriction injury (CCI). Following induction of CCI, GRK6 expression was significantly downregulated in rat DRGs at L4-L6 segments. Overexpression of GRK6 using lentiviral-mediated production strategy via sciatic nerve injection markedly attenuated mechanical allodynia and thermal hyperalgesia in CCI rats. Overexpression of GRK6 also drastically reversed the hyperexcitability of DRG neurons innervating the hind paw and suppressed the enhanced expression of CXCR2 in DRGs of CCI rats. In addition, co-immunoprecipitation, immunofluorescence, and correlation analysis supported the interaction between GRK6 and CXCR2. These results suggest that GRK6 might be a key molecular involved in peripheral mechanism of neuropathic pain and that overexpression of GRK6 might be a potential strategy for treatment for neuropathic pain through inhibition of CXCR2 signal pathway.

Allosteric Modulation of Chemoattractant Receptors

Chemoattractants control selective leukocyte homing via interactions with a dedicated family of related G protein-coupled receptor (GPCR). Emerging evidence indicates that the signaling activity of these receptors, as for other GPCR, is influenced by allosteric modulators, which interact with the receptor in a binding site distinct from the binding site of the agonist and modulate the receptor signaling activity in response to the orthosteric ligand. Allosteric modulators have a number of potential advantages over orthosteric agonists/antagonists as therapeutic agents and offer unprecedented opportunities to identify extremely selective drug leads. Here, we resume evidence of allosterism in the context of chemoattractant receptors, discussing in particular its functional impact on functional selectivity and probe/concentration dependence of orthosteric ligands activities.

MIM regulates the trafficking of bone marrow cells via modulating surface expression of CXCR4

Missing in metastasis (MIM) is abundantly expressed in hematopoietic cells. Here we characterized the impact of MIM deficiency on murine bone marrow (BM) cells. Although MIM^-/- cells proliferated similarly to wild type (WT), they exhibited stronger response to chemokine SDF-1, increase in surface expression of CXCR4, impaired CXCR4 internalization and constitutive activation of Rac, Cdc42 and p38. Transplantation of MIM^-/- BM cells into lethally irradiated mice showed enhanced homing to BM, which was abolished when mice were pretreated with a p38 antagonist. Interestingly, MIM^-/- BM cells, including hematopoietic stem and progenitor cells (HSPCs), showed 2 to 5-fold increase in mobilization into the peripheral blood upon treatment with AMD3100. In vitro, MIM^-/- leukocytes were susceptible to AMD3100 and maintained increased response to AMD3100 for mobilization even after transfer into wild type mice. MIM^-/- mice had also a higher level of SDF-1 in the circulation. Our data highlighted an unprecedented role of MIM in the homoeostasis of BM cells, including HSPCs, through modulation of the CXCR4/SDF-1 axis and interactions of BM leukocytes with their microenvironments.

Aberrant GRK6 promoter methylation is associated with poor prognosis in hypopharyngeal squamous cell carcinoma

Hypopharyngeal squamous cell carcinoma (HSCC) is one of the most common head and neck cancers with high invasiveness and poor prognosis. To identify targeted therapeutics against metastasis, a better understanding of the regulation of HSCC cell invasion is needed. In recent years, G protein-coupled receptor kinases (GRKs) have been implicated in cancer metastasis through phosphorylation of the activated form of G protein coupled receptors (GPCRs). However, there is little information regarding GRKs expression in HSCC. In the present study, we examined GRK6 expression in HSCC and also assessed the possible cause of its aberrant expression, as well as its clinical significance. Real-time quantitative PCR (qPCR) and western blotting were performed to analyze the expression of GRK6 in HSCC tissues and corresponding non-malignant tissues. Subsequently, paired HSCC and corresponding non-malignant tissues were evaluated for the methylation status of GRK6 gene promoter using methylation-specific PCR (MSP). Furthermore, we investigated the methylation status and the clinicopathological significance of GRK6 in 45 paired HSCC and corresponding non-malignant tissues. The suppression of GRK6 in hypopharyngeal cell line FaDu by GRK6-shRNA lentivirus transfection was utilized to detect the role of GRK6 in hypopharyngeal cancer progression. Our results showed that the expression of GRK6 mRNA and protein was significantly lower in HSCC than in corresponding adjacent non-tumor tissues, and this downregulation was found to be in accordance with aberrant methylation of the gene. Hypermethylation of the gene was observed in 77.8% (35/45) of the HSCC tissues, while it was found in only 42.2% (19/45) of the corresponding non-malignant tissues. GRK6 methylation was related to depth of tumor invasion and TNM stage. Upon treatment with 5-aza-2'-deoxycytidine, GRK6 expression was upregulated in FaDu cells, and cell invasion was signinficantly inhibited. Furthermore, the suppression of GRK6 by shRNA transfection enhanced FaDu cells invasion. Our results indicate that the aberrant methylation of GRK6 gene promoter may underlie its downregulation in HSCC, and may play an important role in the metastasis of HSCC.

Stage-specific roles for CXCR4 signaling in murine hematopoietic stem/progenitor cells in the process of bone marrow repopulation

Hematopoietic cell transplantation has proven beneficial for various intractable diseases, but it remains unclear how hematopoietic stem/progenitor cells (HSPCs) home to the bone marrow (BM) microenvironment, initiate hematopoietic reconstitution, and maintain life-long hematopoiesis. The use of newly elucidated molecular determinants for overall HSPC engraftment should benefit patients. Here, we report that modification of C-X-C chemokine receptor type 4 (Cxcr4) signaling in murine HSPCs does not significantly affect initial homing/lodging events, but leads to alteration in subsequent BM repopulation kinetics, with observations confirmed by both gain- and loss-of-function approaches. By using C-terminal truncated Cxcr4 as a gain-of-function effector, we demonstrated that signal augmentation likely led to favorable in vivo repopulation of primitive cell populations in BM. These improved features were correlated with enhanced seeding efficiencies in stromal cell cocultures and altered ligand-mediated phosphorylation kinetics of extracellular signal-regulated kinases observed in Cxcr4 signal-augmented HSPCs in vitro. Unexpectedly, however, sustained signal enhancement even with wild-type Cxcr4 overexpression resulted in impaired peripheral blood (PB) reconstitution, most likely by preventing release of donor hematopoietic cells from the marrow environment. We thus conclude that timely regulation of Cxcr4/CXCR4 signaling is key in providing donor HSPCs with enhanced repopulation potential following transplantation, whilst preserving the ability to release HSPC progeny into PB for improved transplantation outcomes.

Functional consequences of perturbed CXCL12 signal processing: analyses of immature hematopoiesis in GRK6-deficient mice

Hematopoietic stem and progenitor cells (HSPCs) reside in bone marrow (BM) in an environment rich in CXCL12, the ligand for CXCR4, which is constitutively expressed on all immature hematopoietic cells in BM. This ligand-receptor pair critically controls HSPC retention and (relative) quiescence in BM. Interestingly, in a chemokine-abundant environment, CXCR4 surface expression and CXCL12 sensitivity of BM-residing HSPCs are continuously maintained. The mechanisms underlying this peculiar pattern of G-protein signal integration by BM-HSPCs are unknown. G-protein receptor kinases (GRKs) control receptor function by phosphorylating the intracellular domains upon ligand-induced activation, which results in receptor internalization and transient refractoriness. Using, therefore, a GRK6-deficient (GRK6(-/-)) mouse, we sought to address how perturbed ligand-induced CXCR4 (in)activation affects HSPC behavior in vitro and in vivo. In vitro, GRK6(-/-) HSPCs were characterized by hyper-responsiveness to CXCL12, as expected. In vivo, GRK6(-/-) immature hematopoiesis was characterized by a marked expansion of immature hematopoiesis in spleens and a modest repopulation defect in serial competitive transplantation. Enforced mobilization with granulocyte colony-stimulating factor (G-CSF) and AMD3100 was normal, as was hematopoietic regeneration after noncompetitive transplantation or pharmacological myelosuppression. These observations illustrate that GRK-mediated restriction of CXCR4 signal input after ligand engagement is largely dispensable for BM-resident HSPCs, which may explain how continuous CXCL12 responsiveness of BM-HSPCs can be maintained.

Biased ligands at G-protein-coupled receptors: promise and progress

Drug discovery targeting G protein-coupled receptors (GPCRs) is no longer limited to seeking agonists or antagonists to stimulate or block cellular responses associated with a particular receptor. GPCRs are now known to support a diversity of pharmacological profiles, a concept broadly referred to as functional selectivity. In particular, the concept of ligand bias, whereby a ligand stabilizes subsets of receptor conformations to engender novel pharmacological profiles, has recently gained increasing prominence. This review discusses how biased ligands may deliver safer, better tolerated, and more efficacious drugs, and highlights several biased ligands that are in clinical development. Biased ligands targeting the angiotensin II type 1 receptor and the μ opioid receptor illustrate the translation of the biased ligand concept from basic biology to clinical drug development.

A phase 1 clinical trial of long-term, low-dose treatment of WHIM syndrome with the CXCR4 antagonist plerixafor

Warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome is a rare immunodeficiency disorder caused by gain-of-function mutations in the G protein-coupled chemokine receptor CXCR4. The CXCR4 antagonist plerixafor, which is approved by the US Food and Drug Administration (FDA) for stem cell mobilization in cancer and administered for that indication at 0.24 mg/kg, has been shown in short-term (1- to 2-week) phase 1 dose-escalation studies to correct neutropenia and other cytopenias in WHIM syndrome. However, long-term safety and long-term hematologic and clinical efficacy data are lacking. Here we report results from the first long-term clinical trial of plerixafor in any disease, in which 3 adults with WHIM syndrome self-injected 0.01 to 0.02 mg/kg (4% to 8% of the FDA-approved dose) subcutaneously twice daily for 6 months. Circulating leukocytes were durably increased throughout the trial in all patients, and this was associated with fewer infections and improvement in warts in combination with imiquimod; however, immunoglobulin levels and specific vaccine responses were not fully restored. No drug-associated side effects were observed. These results provide preliminary evidence for the safety and clinical efficacy of long-term, low-dose plerixafor in WHIM syndrome and support its continued study as mechanism-based therapy in this disease. The ClinicalTrials.gov identifier for this study is NCT00967785.

Genetics on a WHIM

We initially described the WHIM syndrome based on the combination of Warts, Hypogammaglobulinaemia, Infections and Myelokathexis (neutrophil retention in the bone marrow). Translational research led to the discovery that this rare immunodeficiency disease is caused by a heterozygous mutation in the CXCR4 gene. Recently, Plerixafor has been suggested as a treatment for WHIM syndrome due to its efficacy as a CXCR4 antagonist, closing the translational research loop. In this review, we will focus on the clinical manifestations, pathophysiology, diagnosis and possible therapies for this rare entity.

Heparin-binding epidermal growth factor-like growth factor/diphtheria toxin receptor in normal and neoplastic hematopoiesis

Heparin-binding EGF-like growth factor (HB-EGF) belongs to the EGF family of growth factors. It is biologically active either as a molecule anchored to the membrane or as a soluble form released by proteolytic cleavage of the extracellular domain. HB-EGF is involved in relevant physiological and pathological processes spanning from proliferation and apoptosis to morphogenesis. We outline here the main activities of HB-EGF in connection with normal or neoplastic differentiative or proliferative events taking place primitively in the hematopoietic microenvironment.

G protein-coupled receptor kinase-3-deficient mice exhibit WHIM syndrome features and attenuated inflammatory responses

Chemokine receptor interactions coordinate leukocyte migration in inflammation. Chemokine receptors are GPCRs that when activated, are phosphorylated by GRKs to turn off G protein-mediated signaling yet recruit additional signaling machinery. Recently, GRK3 was identified as a negative regulator of CXCL12/CXCR4 signaling that is defective in human WHIM syndrome. Here, we report that GRK3-/- mice exhibit numerous features of human WHIM, such as impaired CXCL12-mediated desensitization, enhanced CXCR4 signaling to ERK activation, altered granulocyte migration, and a mild myelokathexis. Moreover, GRK3-/- protects mice from two acute models of inflammatory arthritis (K/BxN serum transfer and CAIA). In these granulocyte-dependent disease models, protection of GRK3-/- mice is mediated by retention of cells in the marrow, fewer circulating granulocytes in the peripheral blood, and reduced granulocytes in the joints during active inflammation. In contrast to WHIM, GRK3-/- mice have minimal hypogammaglobulinemia and a peripheral leukocytosis with increased lymphocytes and absent neutropenia. Thus, we conclude that the loss of GRK3-mediated regulation of CXCL12/CXCR4 signaling contributes to some, but not all, of the complete WHIM phenotype and that GRK3 inhibition may be beneficial in the treatment of inflammatory arthritis.

Hierarchical organization of multi-site phosphorylation at the CXCR4 C terminus

The chemokine receptor CXCR4 regulates cell migration during ontogenesis and disease states including cancer and inflammation. Upon stimulation by the endogenous ligand CXCL12, CXCR4 becomes phosphorylated at multiple sites in its C-terminal domain. Mutations in the CXCR4 gene affecting C-terminal phosphorylation sites are a hallmark of WHIM syndrome, a genetic disorder characterized by a gain-of-CXCR4-function. To better understand how multi-site phosphorylation of CXCR4 is organized and how perturbed phosphorylation might affect CXCR4 function, we developed novel phosphosite-specific CXCR4 antibodies and studied the differential regulation and interaction of three C-terminal phosphorylation sites in human embryonic kidney cells (HEK293). CXCL12 promoted a robust phosphorylation at S346/347 which preceded phosphorylation at S324/325 and S338/339. After CXCL12 washout, the phosphosites S338/339 and S324/325 were rapidly dephosphorylated whereas phosphorylation at S346/347 was long-lasting. CXCL12-induced phosphorylation at S346/347 was staurosporine-insensitive and mediated by GRK2/3. WHIM syndrome-associated CXCR4 truncation mutants lacking the S346/347 phosphosite and the recently identified E343K WHIM mutant displayed strongly impaired phosphorylation at S324/325 and S338/339 as well as reduced CXCL12-induced receptor internalization. Relevance of the S346-S348 site was confirmed by a S346-348A mutant showing strongly impaired CXCL12-promoted phosphorylation at S324/325 and S338/339, defective internalization, gain of calcium mobilization, and reduced desensitization. Thus, the triple serine motif S346-S348 contains a major initial CXCR4 phosphorylation site and is required for efficient subsequent multi-site phosphorylation and receptor regulation. Hierarchical organization of CXCR4 phosphorylation explains why small deletions at the extreme CXCR4 C terminus typically associated with WHIM syndrome severely alter CXCR4 function.

Growth factor receptor-Src-mediated suppression of GRK6 dysregulates CXCR4 signaling and promotes medulloblastoma migration

BACKGROUND

Metastasis in medulloblastoma (MB) is associated with poor survival. Recent genetic studies revealed MB to comprise distinct molecular subgroups, including the sonic hedgehog (SHH) subgroup that exhibits a relatively high rate of progression. To identify targeted therapeutics against metastasis, a better understanding of the regulation of MB cell migration is needed. G protein-coupled receptor kinases (GRKs) have been implicated in cancer metastasis through their regulation of G-protein coupled receptors (GPCRs) involved in growth factor (GF)-mediated cell migration. However, the specific roles and regulation of GRKs in MB have not been investigated.

METHODS

Microarray mRNA analysis was performed for GRKs, GPCRs, and GFs in 29 human MB, and real time RT-PCR was used to detect GRK6 expression in MB cells. Lenti- or retro-virus infection, and siRNA or shRNA transfection, of MB cells was used to overexpress and knockdown target genes, respectively. Western blot was used to confirm altered expression of proteins. The effect of altered target protein on cell migration was determined by Boyden chamber assay and xCELLigence migration assays.

RESULTS

We observed co-overexpression of PDGFRA, CXCR4, and CXCL12 in the SHH MB subtype compared to non-SHH MB (5, 7, and 5-fold higher, respectively). GRK6, which typically acts as a negative regulator of CXCR4 signaling, is downregulated in MB, relative to other GRKs, while the percentage of GRK6 expression is lower in MB tumors with metastasis (22%), compared to those without metastasis (43%). In SHH-responsive MB cells, functional blockade of PDGFR abolished CXCR4-mediated signaling. shPDGFR transfected MB cells demonstrated increased GRK6 expression, while PDGF or 10% FBS treatment of native MB cells reduced the stability of GRK6 by inducing its proteosomal degradation. Overexpression or downregulation of Src, a key mediator of GF receptor/PDGFR signaling, similarly inhibited or induced GRK6 expression, respectively. siRNA downregulation of GRK6 enhanced CXCR4 signaling and promoted MB migration, while lentiviral-GRK6 overexpression suppressed CXCR4 signaling, potentiated the effect of AMD3100, a CXCR4 antagonist, and impaired migration.

CONCLUSIONS

Our findings demonstrate a novel mechanism of GF receptor/PDGFR-Src-mediated dysregulation of CXCR4 signaling that promotes MB cell migration, which could potentially be exploited for therapeutic targeting in SHH MB.

β-Arrestins in the immune system

β-Arrestins regulate G protein-coupled receptors through receptor desensitization while also acting as signaling scaffolds to facilitate numerous effector pathways. Recent studies have provided evidence that β-arrestins play a key role in inflammatory responses. Here, we summarize these advances on the roles of β-arrestins in immune regulation and inflammatory responses under physiological and pathological conditions, with an emphasis on translational implications of β-arrestins on human diseases.

The loss of RGS protein-Gα(i2) interactions results in markedly impaired mouse neutrophil trafficking to inflammatory sites

Neutrophils are first responders rapidly mobilized to inflammatory sites by a tightly regulated, nonredundant hierarchy of chemoattractants. These chemoattractants engage neutrophil cell surface receptors triggering heterotrimeric G-protein Gα(i) subunits to exchange GDP for GTP. By limiting the duration that Gα(i) subunits remain GTP bound, RGS proteins modulate chemoattractant receptor signaling. Here, we show that neutrophils with a genomic knock in of a mutation that disables regulator of G-protein signaling (RGS)-Gα(i2) interactions accumulate in the bone marrow and mobilize poorly to inflammatory sites. These defects are attributable to enhanced sensitivity to background signals, prolonged chemoattractant receptor signaling, and inappropriate CXCR2 downregulation. Intravital imaging revealed a failure of the mutant neutrophils to accumulate at and stabilize sites of sterile inflammation. Furthermore, these mice could not control a nonlethal Staphylococcus aureus infection. Neutrophil RGS proteins establish a threshold for Gα(i) activation, helping to coordinate desensitization mechanisms. Their loss renders neutrophils functionally incompetent.

Regulation of neutrophil trafficking from the bone marrow

Neutrophils are an essential component of the innate immune response and a major contributor to inflammation. Consequently, neutrophil homeostasis in the blood is highly regulated. Neutrophil number in the blood is determined by the balance between neutrophil production in the bone marrow and release from the bone marrow to blood with neutrophil clearance from the circulation. This review will focus on mechanisms regulating neutrophil release from the bone marrow. In particular, recent data demonstrating a central role for the chemokines CXCL12 and CXCL2 in regulating neutrophil egress from the bone marrow will be discussed.

CXCL12 and [N33A]CXCL12 in 5637 and HeLa cells: regulating HER1 phosphorylation via calmodulin/calcineurin

In the human neoplastic cell lines 5637 and HeLa, recombinant CXCL12 elicited, as expected, downstream signals via both G-protein-dependent and β-arrestin-dependent pathways responsible for inducing a rapid and a late wave, respectively, of ERK1/2 phosphorylation. In contrast, the structural variant [N33A]CXCL12 triggered no β-arrestin-dependent phosphorylation of ERK1/2, and signaled via G protein-dependent pathways alone. Both CXCL12 and [N33A]CXCL12, however, generated signals that transinhibited HER1 phosphorylation via intracellular pathways. 1) Prestimulation of CXCR4/HER1-positive 5637 or HeLa cells with CXCL12 modified the HB-EGF-dependent activation of HER1 by delaying the peak phosphorylation of tyrosine 1068 or 1173. 2) Prestimulation with the synthetic variant [N33A]CXCL12, while preserving CXCR4-related chemotaxis and CXCR4 internalization, abolished HER1 phosphorylation. 3) In cells knockdown of β-arrestin 2, CXCL12 induced a full inhibition of HER1 like [N33A]CXCL12 in non-silenced cells. 4) HER1 phosphorylation was restored as usual by inhibiting PCK, calmodulin or calcineurin, whereas the inhibition of CaMKII had no discernable effect. We conclude that both recombinant CXCL12 and its structural variant [N33A]CXCL12 may transinhibit HER1 via G-proteins/calmodulin/calcineurin, but [N33A]CXCL12 does not activate β-arrestin-dependent ERK1/2 phosphorylation and retains a stronger inhibitory effect. Therefore, we demonstrated that CXCL12 may influence the magnitude and the persistence of signaling downstream of HER1 in turn involved in the proliferative potential of numerous epithelial cancer. In addition, we recognized that [N33A]CXCL12 activates preferentially G-protein-dependent pathways and is an inhibitor of HER1.

The chemokine CXCL12 is essential for the clearance of the filaria Litomosoides sigmodontis in resistant mice

Litomosoides sigmodontis is a cause of filarial infection in rodents. Once infective larvae overcome the skin barrier, they enter the lymphatic system and then settle in the pleural cavity, causing soft tissue infection. The outcome of infection depends on the parasite's modulatory ability and also on the immune response of the infected host, which is influenced by its genetic background. The goal of this study was to determine whether host factors such as the chemokine axis CXCL12/CXCR4, which notably participates in the control of immune surveillance, can influence the outcome of the infection. We therefore set up comparative analyses of subcutaneous infection by L. sigmodontis in two inbred mouse strains with different outcomes: one susceptible strain (BALB/c) and one resistant strain (C57BL/6). We showed that rapid parasite clearance was associated with a L. sigmodontis-specific CXCL12-dependent cell response in C57BL/6 mice. CXCL12 was produced mainly by pleural mesothelial cells during infection. Conversely, the delayed parasite clearance in BALB/c mice was neither associated with an increase in CXCL12 levels nor with cell influx into the pleural cavity. Remarkably, interfering with the CXCL12/CXCR4 axis in both strains of mice delayed filarial development, as evidenced by the postponement of the fourth molting process. Furthermore, the in vitro growth of stage 4 filariae was favored by the addition of low amounts of CXCL12. The CXCL12/CXCR4 axis thus appears to have a dual effect on the L. sigmodontis life cycle: by acting as a host-cell restriction factor for infection, and as a growth factor for worms.

Proper desensitization of CXCR4 is required for lymphocyte development and peripheral compartmentalization in mice

Desensitization controls G protein-dependent signaling of chemokine receptors. We investigate the physiologic implication of this process for CXCR4 in a mouse model harboring a heterozygous mutation of the Cxcr4 gene, which engenders a desensitization-resistant receptor. Such anomaly is linked to the warts, hypogammaglobulinemia, infections, myelokathexis (WHIM) syndrome, a human rare combined immunodeficiency. Cxcr4(+/mutant(1013)) mice display leukocytes with enhanced responses to Cxcl12 and exhibit leukopenia as reported in patients. Treatment with CXCL12/CXCR4 antagonists transiently reverses blood anomalies, further demonstrating the causal role of the mutant receptor in the leukopenia. Strikingly, neutropenia occurs in a context of normal bone marrow architecture and granulocyte lineage maturation, indicating a minor role for Cxcr4-dependent signaling in those processes. In contrast, Cxcr4(+/1013) mice show defective thymopoiesis and B-cell development, accounting for circulating lymphopenia. Concomitantly, mature T and B cells are abnormally compartmentalized in the periphery, with a reduction of primary follicles in the spleen and their absence in lymph nodes mirrored by an unfurling of the T-cell zone. These mice provide a model to decipher the role of CXCR4 desensitization in the homeostasis of B and T cells and to investigate which manifestations of patients with WHIM syndrome may be overcome by dampening the gain of CXCR4 function.

Identification of anti-malarial compounds as novel antagonists to chemokine receptor CXCR4 in pancreatic cancer cells

Despite recent advances in targeted therapies, patients with pancreatic adenocarcinoma continue to have poor survival highlighting the urgency to identify novel therapeutic targets. Our previous investigations have implicated chemokine receptor CXCR4 and its selective ligand CXCL12 in the pathogenesis and progression of pancreatic intraepithelial neoplasia and invasive pancreatic cancer; hence, CXCR4 is a promising target for suppression of pancreatic cancer growth. Here, we combined in silico structural modeling of CXCR4 to screen for candidate anti-CXCR4 compounds with in vitro cell line assays and identified NSC56612 from the National Cancer Institute's (NCI) Open Chemical Repository Collection as an inhibitor of activated CXCR4. Next, we identified that NSC56612 is structurally similar to the established anti-malarial drugs chloroquine and hydroxychloroquine. We evaluated these compounds in pancreatic cancer cells in vitro and observed specific antagonism of CXCR4-mediated signaling and cell proliferation. Recent in vivo therapeutic applications of chloroquine in pancreatic cancer mouse models have demonstrated decreased tumor growth and improved survival. Our results thus provide a molecular target and basis for further evaluation of chloroquine and hydroxychloroquine in pancreatic cancer. Historically safe in humans, chloroquine and hydroxychloroquine appear to be promising agents to safely and effectively target CXCR4 in patients with pancreatic cancer.

Carboxy-terminus of CXCR7 regulates receptor localization and function

Chemokine receptor CXCR7 is essential for normal development, and this receptor promotes initiation and progression of diseases including cancer and autoimmunity. To understand normal and pathologic functions of CXCR7 and advance development of therapeutic agents, there is a need to define structural domains that regulate this receptor. We generated mutants of CXCR7 with deletion of different lengths of the predicted intracellular tail and analyzed effects on CXCR7 signaling and function in cell-based assays. While wild-type CXCR7 predominantly localized to intracellular vesicles, progressive deletion of the carboxy terminus redistributed the receptor to the plasma membrane. Truncating the intracellular tail of CXCR7 did not alter binding to CXCL12, but mutant receptors had reduced scavenging of this chemokine. Using a firefly luciferase complementation system, we established that deletions of the carboxy terminus decreased basal interactions and eliminated ligand-dependent recruitment of the scaffolding protein β-arrestin-2 to receptors. Deleting the carboxy terminus of CXCR7 impaired constitutive internalization of the receptor and reduced activation of ERK1/2 by CXCL12-CXCR7. Inhibiting dynamin, a molecule required for internalization of CXCR7, increased ligand-dependent association of the receptor with β-arrestin-2 and enhanced activation of ERK1/2. These studies establish mechanisms of action for CXCR7 and establish the intracellular tail of CXCR7 as a critical determinant of receptor trafficking, chemokine scavenging, and signaling.

MicroRNA126 contributes to granulocyte colony-stimulating factor-induced hematopoietic progenitor cell mobilization by reducing the expression of vascular cell adhesion molecule 1

BACKGROUND

Mobilization of hematopoietic stem/progenitor cells from the bone marrow to the peripheral blood by granulocyte colony-stimulating factor is the primary means to acquire stem cell grafts for hematopoietic cell transplantation. Since hematopoietic stem/progenitor cells represent a minority of all blood cells mobilized by granulocyte colony-stimulating factor, the underlying mechanisms need to be understood in order to develop selective drugs.

DESIGN AND METHODS

We analyzed phenotypic, biochemical and genetic changes in bone marrow cell populations from granulocyte colony-stimulating factor-mobilized and control mice, and linked such changes to effective mobilization of hematopoietic stem/progenitor cells.

RESULTS

We show that granulocyte colony-stimulating factor indirectly reduces expression of surface vascular cell adhesion molecule 1 on bone marrow hematopoietic stem/progenitor cells, stromal cells and endothelial cells by promoting the accumulation of microRNA-126 (miR126)-containing microvescicles in the bone marrow extracellular compartment. We found that hematopoietic stem/progenitor cells, stromal cells and endothelial cells readily incorporate these miR126-loaded microvescicles, and that miR126 represses vascular cell adhesion molecule 1 expression on bone marrow hematopoietic stem/progenitor cells, stromal cells and endothelial cells. In line with this, miR126-null mice displayed a reduced mobilization response to granulocyte colony-stimulating factor.

CONCLUSIONS

Our results implicate miR126 in the regulation of hematopoietic stem/progenitor cell trafficking between the bone marrow and peripheral sites, clarify the role of vascular cell adhesion molecule 1 in granulocyte colony-stimulating factor-mediated mobilization, and have important implications for improved approaches to selective mobilization of hematopoietic stem/progenitor cells.

Suppression of G-protein-coupled receptor kinase 3 expression is a feature of classical GBM that is required for maximal growth

G-protein-coupled receptor kinases (GRK) regulate the function of G-protein-coupled receptors (GPCR). Previously, we found that GPCR (CXCR4)-mediated astrocytoma growth was dependent upon abnormally sustained CXCR4 signaling and was correlated with decreased GRK-mediated receptor phosphorylation. As CXCR4 has also been implicated in the stimulation of high-grade glioma growth, we sought to determine whether dysregulation of GRK expression and/or function might also be present in high-grade gliomas. In an analysis of data from The Cancer Genome Atlas, we found that GRK3 expression is frequently decreased in glioblastoma (GBM) of the classical subtype, which possesses signature amplification or mutational activation of the epidermal growth factor (EGF) receptor. We tested the correlation between GRK3 expression and GBM subtypes, as well as the relationship between the activation of the EGF and other growth factor receptor pathways and GRK expression. In analyses of primary GBM tissue and RNA specimens, we found that GRK3 expression is correlated with established criteria for GBM subtyping including expression of EGF receptor, platelet-derived growth factor receptor (PDGFR)α, NF1, PTEN, CDKN2A, and neurofilament. We also found that established drivers of gliomagenesis, the EGF, PDGF, and TGF-β pathways, all regulate GRK expression. Coculture experiments, designed to mimic critical interactions between tumor and brain microvascular endothelial cells, showed that specifically increasing GRK3 expression reduced the trophic effect of endothelial cells on tumor cells. Together, these experiments show that GRK3 is a negative regulator of cell growth whose expression is preferentially reduced in GBM of the classical subtype as a consequence of activity in primary gliomagenic pathways.

The CXCR4 antagonist plerixafor corrects panleukopenia in patients with WHIM syndrome

WHIM syndrome is a rare congenital immunodeficiency disorder characterized by warts, hypogammaglobulinemia, infections, and myelokathexis (neutropenia because of impaired egress from the BM); most patients also have severe panleukopenia. Because WHIM syndrome is caused by mutations in the chemokine receptor CXCR4 that result in increased agonist-dependent signaling, we hypothesized that the CXCR4 antagonist plerixafor (Mozobil [Genyzme Corporation], AMD3100), might be an effective treatment. To test this, we enrolled 3 unrelated adult patients with the most common WHIM mutation, CXCR4(R334X), in a phase 1 dose-escalation study. Plerixafor increased absolute lymphocyte, monocyte, and neutrophil counts in blood to normal without significant side effects in all 3 patients. Peak responses occurred at 3-12 hours after injection and waned by 24 hours after injection which tracked the drug's pharmacokinetics. All 3 cell types increased in a dose-dependent manner with the rank order of responsiveness absolute lymphocyte > monocyte > neutrophil. These data provide the first pharmacologic evidence that panleukopenia in WHIM syndrome is caused by CXCL12-CXCR4 signaling-dependent leukocyte sequestration, and support continued study of plerixafor as mechanism-based therapy in this disease. This study is registered at http://www.clinicaltrials.gov as NCT00967785.

[CXCR4, a therapeutic target in rare immunodeficiencies?]

Currently, more than 200 primary immunodeficiency diseases have been discovered. In most cases, genetic defects affect the expression or the function of proteins involved in immune development and homeostasis. Some orphan immuno-hematological disorders are characterized by an abnormal leukocyte trafficking, a notion predictive of an anomaly of the chemokine/chemokine receptor system. In this review, we focus on recent advances in the characterization of dysfunctions of the CXCL12 (SDF-1)/CXCR4 signaling axis in two rare human immunodeficiencies, one associated with a loss of CXCR4 function, the Idiopathic CD4(+) T-cell Lymphocytopenia, and the other with a gain of CXCR4 function, the WHIM syndrome.

CXCR4 expression on activated B cells is downregulated by CD63 and IL-21

CXCR4 expression is critical for localization of centroblasts in the dark zone of germinal centers (GCs), and centrocytes downregulate CXCR4 and thus leave the dark zone to reside in the light zone. However, mechanisms governing CXCR4 downregulation on centrocytes are not known. In this study, we show that the amount of intracellular CXCR4 in centroblasts was similar to that in centrocytes, suggesting differential control of CXCR4 protein expression in these GC B cells. Restimulation of activated B cells with IL-21, which is a major cytokine produced by T follicular helper cells, accelerated CXCR4 internalization by inducing endocytosis-related GRK6 expression. Although CXCR4 expression was downregulated on GC B cells by IL-21 stimulation, CXCR4(low) centrocytes developed in the spleens of IL-21R-deficient mice, suggesting other mechanisms for downregulation. The level of CD63 (which recruits CXCR4 to late endosome in CD4 T cells) in centrocytes was more than that in centroblasts and was strikingly elevated in activated Bcl6-deficient B cells. Bcl6, a transcriptional repressor, was detected on the chromatin of the CD63 gene in resting B cells, therefore CD63 is a molecular target of Bcl6. Downregulation of CD63 mRNA in activated Bcl6-deficient B cells by small interfering RNA upregulated CXCR4 expression on the B cells. Furthermore, addition of Bcl6 inhibitor to activated B cell cultures increased CD63 mRNA expression in (and downregulated CXCR4 expression on) those activated B cells. Thus, CXCR4 can be downregulated on activated B cells by IL-21-induced endocytosis and CD63-mediated endosomal recruitment, and these mechanisms may contribute to downregulation of CXCR4 on centrocytes.

[Dysfunctions of the CXCL12 (SDF-1)/CXCR4 signaling axis in the WHIM syndrome and the idiopathic CD4(+) T-cell lymphocytopenia]

Chemokines are small cytokine-like secreted proteins that govern migration of leukocytes to their specific niches in lymphoid organs and to inflammatory sites. They mediate their functions by binding to and activating chemokine receptors, which belong to the heptahelical G protein-coupled receptor family. The CXC chemokine Stromal cell Derived Factor-1 (SDF-1/CXCL12) is the sole natural ligand for the broadly expressed CXCR4 receptor and acts as a chemoattractant for many leukocyte subsets. The CXCL12/CXCR4 axis exerts critical activities in homeostatic processes such as organogenesis, hematopoiesis and leukocyte trafficking. Dysregulations of CXCR4 signaling and/or expression are associated with several infectious, inflammatory, autoimmune and malignant conditions. In light of recent data, we review here CXCR4 dysfunctions unveiled in two rare human immunodeficiency disorders, one characterized by a gain of CXCR4 function, the WHIM syndrome, and the other by a loss of CXCR4 function, the idiopathic CD4(+) T-cell lymphocytopenia.

An essential role of the cytoplasmic tail of CXCR4 in G-protein signaling and organogenesis

CXCR4 regulates cell proliferation, enhances cell survival and induces chemotaxis, yet molecular mechanisms underlying its signaling remain elusive. Like all other G-protein coupled receptors (GPCRs), CXCR4 delivers signals through G-protein-dependent and -independent pathways, the latter involving its serine-rich cytoplasmic tail. To evaluate the signaling and biological contribution of this G-protein-independent pathway, we generated mutant mice that express cytoplasmic tail-truncated CXCR4 (ΔT) by a gene knock-in approach. We found that ΔT mice exhibited multiple developmental defects, with not only G-protein-independent but also G-protein-dependent signaling events completely abolished, despite ΔT's ability to still associate with G-proteins. These results reveal an essential positive regulatory role of the cytoplasmic tail in CXCR4 signaling and suggest the tail is crucial for mediating G-protein activation and initiating crosstalk between G-protein-dependent and G-protein-independent pathways for correct GPCR signaling.