The Structure of a CXCR4:Chemokine Complex

Identification of a conserved chemokine receptor motif that enables ligand discrimination

Extensive ligand-receptor promiscuity in the chemokine signaling system balances beneficial redundancy and specificity. However, this feature poses a major challenge to selectively modulate the system pharmacologically. Here, we identified a conserved cluster of three aromatic receptor residues that anchors the second extracellular loop (ECL2) to the top of receptor transmembrane helices (TM) 4 and 5 and enables recognition of both shared and specific characteristics of interacting chemokines. This cluster was essential for the activation of several chemokine receptors. Furthermore, characteristic motifs of the ß₁ strand and 30s loop make the two main CC-chemokine subgroups-the macrophage inflammatory proteins (MIPs) and monocyte chemoattractant proteins (MCPs)-differentially dependent on this cluster in the promiscuous receptors CCR1, CCR2, and CCR5. The cluster additionally enabled CCR1 and CCR5 to discriminate between closely related MIPs based on the N terminus of the chemokine. G protein signaling and β-arrestin2 recruitment assays confirmed the importance of the conserved cluster in receptor discrimination of chemokine ligands. This extracellular site may facilitate the development of chemokine-related therapeutics.

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.

WHIM Syndrome: from Pathogenesis Towards Personalized Medicine and Cure

WHIM syndrome is a rare combined primary immunodeficiency disease named by acronym for the diagnostic tetrad of warts, hypogammaglobulinemia, infections, and myelokathexis. Myelokathexis is a unique form of non-cyclic severe congenital neutropenia caused by accumulation of mature and degenerating neutrophils in the bone marrow; monocytopenia and lymphopenia, especially B lymphopenia, also commonly occur. WHIM syndrome is usually caused by autosomal dominant mutations in the G protein-coupled chemokine receptor CXCR4 that impair desensitization, resulting in enhanced and prolonged G protein- and β-arrestin-dependent responses. Accordingly, CXCR4 antagonists have shown promise as mechanism-based treatments in phase 1 clinical trials. This review is based on analysis of all 105 published cases of WHIM syndrome and covers current concepts, recent advances, unresolved enigmas and controversies, and promising future research directions.

Viral GPCR US28 can signal in response to chemokine agonists of nearly unlimited structural degeneracy

Human cytomegalovirus has hijacked and evolved a human G-protein-coupled receptor into US28, which functions as a promiscuous chemokine 'sink’ to facilitate evasion of host immune responses. To probe the molecular basis of US28’s unique ligand cross-reactivity, we deep-sequenced CX3CL1 chemokine libraries selected on ‘molecular casts’ of the US28 active-state and find that US28 can engage thousands of distinct chemokine sequences, many of which elicit diverse signaling outcomes. The structure of a G-protein-biased CX3CL1-variant in complex with US28 revealed an entirely unique chemokine amino terminal peptide conformation and remodeled constellation of receptor-ligand interactions. Receptor signaling, however, is remarkably robust to mutational disruption of these interactions. Thus, US28 accommodates and functionally discriminates amongst highly degenerate chemokine sequences by sensing the steric bulk of the ligands, which distort both receptor extracellular loops and the walls of the ligand binding pocket to varying degrees, rather than requiring sequence-specific bonding chemistries for recognition and signaling.

Chemokines from a Structural Perspective

Chemokines are a family of small, highly conserved cytokines that mediate various biological processes, including chemotaxis, hematopoiesis, and angiogenesis, and that function by interacting with cell surface G-Protein Coupled Receptors (GPCRs). Because of their significant involvement in various biological functions and pathologies, chemokines and their receptors have been the focus of therapeutic discovery for clinical intervention. There are several sub-families of chemokines (e.g., CXC, CC, C, and CX3C) defined by the positions of sequentially conserved cysteine residues. Even though all chemokines also have a highly conserved, three-stranded β-sheet/α-helix tertiary structural fold, their quarternary structures vary significantly with their sub-family. Moreover, their conserved tertiary structures allow for subunit swapping within and between sub-family members, thus promoting the concept of a “chemokine interactome”. This review is focused on structural aspects of CXC and CC chemokines, their functional synergy and ability to form heterodimers within the chemokine interactome, and some recent developments in structure-based chemokine-targeted drug discovery.

Epithelial chemokine CXCL14 synergizes with CXCL12 via allosteric modulation of CXCR4

The chemokine receptor, CXC chemokine receptor 4 (CXCR4), is selective for CXC chemokine ligand 12 (CXCL12), is broadly expressed in blood and tissue cells, and is essential during embryogenesis and hematopoiesis. CXCL14 is a homeostatic chemokine with unknown receptor selectivity and preferential expression in peripheral tissues. Here, we demonstrate that CXCL14 synergized with CXCL12 in the induction of chemokine responses in primary human lymphoid cells and cell lines that express CXCR4. Combining subactive concentrations of CXCL12 with 100–300 nM CXCL14 resulted in chemotaxis responses that exceeded maximal responses that were obtained with CXCL12 alone. CXCL14 did not activate CXCR4-expressing cells (i.e., failed to trigger chemotaxis and Ca²⁺ mobilization, as well as signaling via ERK1/2 and the small GTPase Rac1); however, CXCL14 bound to CXCR4 with high affinity, induced redistribution of cell-surface CXCR4, and enhanced HIV-1 infection by >3-fold. We postulate that CXCL14 is a positive allosteric modulator of CXCR4 that enhances the potency of CXCR4 ligands. Our findings provide new insights that will inform the development of novel therapeutics that target CXCR4 in a range of diseases, including cancer, autoimmunity, and HIV.—Collins, P. J., McCully, M. L., Martínez-Muñoz, L., Santiago, C., Wheeldon, J., Caucheteux, S., Thelen, S., Cecchinato, V., Laufer, J. M., Purvanov, V., Monneau, Y. R., Lortat-Jacob, H., Legler, D. F., Uguccioni, M., Thelen, M., Piguet, V., Mellado, M., Moser, B. Epithelial chemokine CXCL14 synergizes with CXCL12 via allosteric modulation of CXCR4.