Heteromerization of chemokine (C-X-C motif) receptor 4 with α1A/B-adrenergic receptors controls α1-adrenergic receptor function

Extracellular Ubiquitin: Role in Myocyte Apoptosis and Myocardial Remodeling

Ubiquitin (UB) is a highly conserved low molecular weight (8.5 kDa) protein. It consists of 76 amino acid residues and is found in all eukaryotic cells. The covalent linkage of UB to a variety of cellular proteins (ubiquitination) is one of the most common posttranslational modifications in eukaryotic cells. This modification generally regulates protein turnover and protects the cells from damaged or misfolded proteins. The polyubiquitination of proteins serves as a signal for degradation via the 26S proteasome pathway. UB is present in trace amounts in body fluids. Elevated levels of UB are described in the serum or plasma of patients under a variety of conditions. Extracellular UB is proposed to have pleiotropic roles including regulation of immune response, anti-inflammatory, and neuroprotective activities. CXCR4 is identified as receptor for extracellular UB in hematopoietic cells. Heart failure represents a major cause of morbidity and mortality in western society. Cardiac remodeling is a determinant of the clinical course of heart failure. The components involved in myocardial remodeling include-myocytes, fibroblasts, interstitium, and coronary vasculature. Increased sympathetic nerve activity in the form of norepinephrine is a common feature during heart failure. Acting via β-adrenergic receptor (β-AR), norepinephrine is shown to induce myocyte apoptosis and myocardial fibrosis. β-AR stimulation increases extracellular levels of UB in myocytes, and UB inhibits β-AR-stimulated increases in myocyte apoptosis and myocardial fibrosis. This review summarizes intracellular and extracellular functions of UB with particular emphasis on the role of extracellular UB in cardiac myocyte apoptosis and myocardial remodeling.

Commercially available antibodies directed against α-adrenergic receptor subtypes and other G protein-coupled receptors with acceptable selectivity in flow cytometry experiments

Several previous reports suggested that many commercially available antibodies directed against G protein-coupled receptors (GPCR) lack sufficient selectivity. Accordingly, it has been proposed that receptor antibodies should be validated by at least one of several criteria, such as testing tissues or cells after knockout or silencing of the corresponding gene. Here, we tested whether 12 commercially available antibodies directed against α-adrenergic receptor (AR) subtypes (α1A/B/D, α2A/B/C), atypical chemokine receptor 3 (ACKR3), and vasopressin receptor 1A (AVPR1A) suffice these criteria. We detected in flow cytometry experiments with human vascular smooth muscle cells that the fluorescence signals from each of these antibodies were reduced by 46 ± 10 %-91 ± 2 % in cells treated with commercially available small interfering RNA (siRNA) specific for each receptor, as compared with cells that were incubated with non-targeting siRNA. The tested antibodies included anti-ACKR3 (R&D Systems, mab42273), for which specificity has previously been demonstrated. Staining with this antibody resulted in 72 ± 5 % reduction of the fluorescence signal after ACKR3 siRNA treatment. Furthermore, staining with anti-α1A-AR (Santa Cruz, sc1477) and anti-ACKR3 (Abcam, ab38089), which have previously been reported to be non-specific, resulted in 70 ± 19 % and 80 ± 4 % loss of the fluorescence signal after α1A-AR and ACKR3 siRNA treatment, respectively. Our findings demonstrate that the tested antibodies show reasonable selectivity for their receptor target under our experimental conditions. Furthermore, our observations suggest that the selectivity of GPCR antibodies depends on the method for which the antibody is employed, the species from which cells/tissues are obtained, and on the type of specimens (cell, tissue/cell homogenate, or section) tested.

G Protein-Coupled Receptor Heteromers

G protein-coupled receptors (GPCRs) compose one of the largest families of membrane proteins involved in intracellular signaling. They are involved in numerous physiological and pathological processes and are prime candidates for drug development. Over the past decade, an increasing number of studies have reported heteromerization between GPCRs. Many investigations in heterologous systems have provided important indications of potential novel pharmacology; however, the physiological relevance of these findings has yet to be established with endogenous receptors in native tissues. In this review, we focus on family A GPCRs and describe the techniques and criteria to assess their heteromerization. We conclude that advances in approaches to study receptor complex functionality in heterologous systems, coupled with techniques that enable specific examination of native receptor heteromers in vivo, are likely to establish GPCR heteromers as novel therapeutic targets.

Diversity and Inter-Connections in the CXCR4 Chemokine Receptor/Ligand Family: Molecular Perspectives

CXCR4 and its ligand CXCL12 mediate the homing of progenitor cells in the bone marrow and their recruitment to sites of injury, as well as affect processes such as cell arrest, survival, and angiogenesis. CXCL12 was long thought to be the sole CXCR4 ligand, but more recently the atypical chemokine macrophage migration inhibitory factor (MIF) was identified as an alternative, non-cognate ligand for CXCR4 and shown to mediate chemotaxis and arrest of CXCR4-expressing T-cells. This has complicated the understanding of CXCR4-mediated signaling and associated biological processes. Compared to CXCL12/CXCR4-induced signaling, only few details are known on MIF/CXCR4-mediated signaling and it remains unclear to which extent MIF and CXCL12 reciprocally influence CXCR4 binding and signaling. Furthermore, the atypical chemokine receptor 3 (ACKR3) (previously CXCR7) has added to the complexity of CXCR4 signaling due to its ability to bind CXCL12 and MIF, and to evoke CXCL12- and MIF-triggered signaling independently of CXCR4. Also, extracellular ubiquitin (eUb) and the viral protein gp120 (HIV) have been reported as CXCR4 ligands, whereas viral chemokine vMIP-II (Herpesvirus) and human β3-defensin (HBD-3) have been identified as CXCR4 antagonists. This review will provide insight into the diversity and inter-connections in the CXCR4 receptor/ligand family. We will discuss signaling pathways initiated by binding of CXCL12 vs. MIF to CXCR4, elaborate on how ACKR3 affects CXCR4 signaling, and summarize biological functions of CXCR4 signaling mediated by CXCL12 or MIF. Also, we will discuss eUb and gp120 as alternative ligands for CXCR4, and describe vMIP-II and HBD-3 as antagonists for CXCR4. Detailed insight into biological effects of CXCR4 signaling und underlying mechanisms, including diversity of CXCR4 ligands and inter-connections with other (chemokine) receptors, is clinically important, as the CXCR4 antagonist AMD3100 has been approved as stem cell mobilizer in specific disease settings.

The triplet puzzle theory indicates extensive formation of heteromers between opioid and chemokine receptor subtypes

Biochemical studies had previously demonstrated examples of heteromerization between opioid and chemokine receptors. Based on the triplet puzzle theory, it has been discovered that opioid receptors are structurally more closely related to chemokine receptors than to other class A G-protein-coupled receptors. Their similarity is established in terms of the number of triplet homologies Asn-Leu-Ala, Thr-Leu-Pro, and Tyr-Ala-Phe in the amino acid code of extensive numbers of members of these two receptor groups. Such widespread similarities probably mean that many opioid and chemokine receptor subtypes utilize some of these mutual triplets to form heteromers. The findings underline that heteromerization among these two receptor groups can represent a major general mechanism for significant interactions between opioid peptides and chemokines in pain and neuroinflammation within the neural-glial networks of the CNS including immune cells.

Localized CCR2 Activation in the Bone Marrow Niche Mobilizes Monocytes by Desensitizing CXCR4

Inflammatory (classical) monocytes residing in the bone marrow must enter the bloodstream in order to combat microbe infection. These monocytes express high levels of CCR2, a chemokine receptor whose activation is required for them to exit the bone marrow. How CCR2 is locally activated in the bone marrow and how their activation promotes monocyte egress is not understood. Here, we have used double transgenic lines that can visualize CCR2 activation in vivo and show that its chemokine ligand CCL2 is acutely released by stromal cells in the bone marrow, which make direct contact with CCR2-expressing monocytes. These monocytes also express CXCR4, whose activation immobilizes cells in the bone marrow, and are in contact with stromal cells expressing CXCL12, the CXCR4 ligand. During the inflammatory response, CCL2 is released and activates the CCR2 on neighboring monocytes. We demonstrate that acutely isolated bone marrow cells co-express CCR2 and CXCR4, and CCR2 activation desensitizes CXCR4. Inhibiting CXCR4 by a specific receptor antagonist in mice causes CCR2-expressing cells to exit the bone marrow in absence of inflammatory insults. Taken together, these results suggest a novel mechanism whereby the local activation of CCR2 on monocytes in the bone marrow attenuates an anchoring signalling provided by CXCR4 expressed by the same cell and mobilizes the bone marrow monocyte to the blood stream. Our results also provide a generalizable model that cross-desensitization of chemokine receptors fine-tunes cell mobility by integrating multiple chemokine signals.