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

Pawig L, Klasen C, Weber C, Bernhagen J, Noels H - Front Immunol (2015)

Bottom Line: This has complicated the understanding of CXCR4-mediated signaling and associated biological processes.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.

View Article: PubMed Central - PubMed

Affiliation: Institute of Molecular Cardiovascular Research (IMCAR), RWTH Aachen University , Aachen , Germany.

ABSTRACT
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.

No MeSH data available.


Related in: MedlinePlus

Three-dimensional structure of the chemokine receptor CXCR4 and its ligands CXCL12 and MIF. (A) Crystal structure of CXCR4 [Protein data bank identifier 3ODU; (24)]. Included in the crystal structure are amino acids (aa) 27 to 319 of the 352 residues of CXCR4. CXCR4 can exist as a homodimer; here, only chain A is depicted. The N-terminal aa 27–31 are depicted in blue, extracellular loop 1 (aa 100–104) in red, extracellular loop 2 (aa 174–192) in purple, and extracellular loop 3 (aa 267–273) in orange. Intracellular loops (ICL) 1 (aa 65–71), ICL2 (aa 140–144), and ICL3 (aa 225–230) are depicted in cyan. The C-terminal aa 315–319 are shown in yellow. The degradation motif SSLKILSKGK (aa 324–333) cannot be depicted here due to the shortened sequence in the crystal structure. (B) Crystal structure of CXCL12 isoform α [Protein data bank identifier 3GV3; (26)]. Included in the crystal structure are aa 4–67 (with aa counting referring to the mature CXCL12). Here, a CXCL12 monomer is depicted with the N-terminal aa 4–7 in blue. The RFFESH motif (aa 12–17), which is the most important binding motif for CXCR4 in the CXCL12 core, is depicted in red. Each monomer includes a three-stranded β-sheet and one α-helix. (C) Crystal structure of MIF [Protein data bank identifier 3DJH; (27)]. Included in the crystal structure are aa 2–114 (with aa counting including Met-1). A MIF monomer is depicted. The N-terminal aa 2–5 are shown in orange. Depicted in red is the pseudo-(E)LR motif (Asp45–X–Arg12), the N-like loop (aa 48–57) is depicted in blue. Each monomer includes two antiparallel α-helices and a four-stranded β-sheet. Two additional, short β-strands (aa 48–50 and aa 107–109) can be found in each monomer stabilized by trimer formation, and are indicated with “β-strand*.”
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4543903&req=5

Figure 1: Three-dimensional structure of the chemokine receptor CXCR4 and its ligands CXCL12 and MIF. (A) Crystal structure of CXCR4 [Protein data bank identifier 3ODU; (24)]. Included in the crystal structure are amino acids (aa) 27 to 319 of the 352 residues of CXCR4. CXCR4 can exist as a homodimer; here, only chain A is depicted. The N-terminal aa 27–31 are depicted in blue, extracellular loop 1 (aa 100–104) in red, extracellular loop 2 (aa 174–192) in purple, and extracellular loop 3 (aa 267–273) in orange. Intracellular loops (ICL) 1 (aa 65–71), ICL2 (aa 140–144), and ICL3 (aa 225–230) are depicted in cyan. The C-terminal aa 315–319 are shown in yellow. The degradation motif SSLKILSKGK (aa 324–333) cannot be depicted here due to the shortened sequence in the crystal structure. (B) Crystal structure of CXCL12 isoform α [Protein data bank identifier 3GV3; (26)]. Included in the crystal structure are aa 4–67 (with aa counting referring to the mature CXCL12). Here, a CXCL12 monomer is depicted with the N-terminal aa 4–7 in blue. The RFFESH motif (aa 12–17), which is the most important binding motif for CXCR4 in the CXCL12 core, is depicted in red. Each monomer includes a three-stranded β-sheet and one α-helix. (C) Crystal structure of MIF [Protein data bank identifier 3DJH; (27)]. Included in the crystal structure are aa 2–114 (with aa counting including Met-1). A MIF monomer is depicted. The N-terminal aa 2–5 are shown in orange. Depicted in red is the pseudo-(E)LR motif (Asp45–X–Arg12), the N-like loop (aa 48–57) is depicted in blue. Each monomer includes two antiparallel α-helices and a four-stranded β-sheet. Two additional, short β-strands (aa 48–50 and aa 107–109) can be found in each monomer stabilized by trimer formation, and are indicated with “β-strand*.”

Mentions: CXCR4 is an evolutionary conserved protein with 89% similarity between the human [352 aa] and mouse (359 aa) ortholog (22). Ubiquitous expression of CXCR4 has been detected in bone marrow, lymph nodes, liver, lung, brain, heart, kidney, thymus, stomach, pancreas, spleen, ovary, and small intestine (23). Similar to other GPCRs, CXCR4 has an extracellular N-terminus (34 aa for CXCR4), seven transmembrane alpha helices connected by three extracellular and three intracellular loops (ICL), and a C-terminus that is located in the cytoplasm (Figure 1A). However, analysis of crystal structures of CXCR4 in complex with the small-molecular antagonist IT1t and the cyclic peptide antagonist CVX15 in 2010 revealed specific differences in the orientation of the alpha helices compared with other available GPCR structures, mostly at the extracellular side (24). One important difference is the extension of the extracellular end of helix VII, which is two helical turns longer as in other GPCR structures and allows a disulfide bond of Cys274 in helix VII with Cys28 in the N-terminal region. This disulfide bond, together with and among chemokine receptors highly conserved disulfide bond between extracellular loop 2 (ECL2, Cys186) and the extracellular end of alpha helix III (Cys109), is essential for CXCL12 binding by shaping the entrance to the ligand-binding pocket (24). Compared to the extracellular half of CXCR4, which overall substantially differs from other GPCRs, the intracellular part of CXCR4 is structurally fairly similar to the intracellular half of other GPCRs. As exception, the intracellular part of helix VII of CXCR4 is one turn shorter compared to other available GPCR structures. Furthermore, the CXCR4 crystal structure lacks the short alpha helix VIII present in many GPCRs and also lacks in its C-terminus a palmitoylation site, which in many GPCRs attaches to the lipid membrane (24). Crystallization studies revealed CXCR4 to exist as a homodimer, however, left open the possibility of CXCL12/CXCR4 binding in a 1:1 or 1:2 ligand:receptor stoichiometry (24). Recently, a combination of computational, functional, as well as biophysical methods supported a 1:1 over a 1:2 CXCL12:CXCR4 binding stoichiometry (25). This was recently further supported by the crystal structure of CXCR4 in complex with the high-affinity antagonist vMIP-II encoded by Kaposi’s sarcoma-associated herpesvirus (as discussed in more detail later). CXCR4 interacted as a dimer with vMIP-II in a 1:1 stoichiometry (11). In addition to homodimerization, CXCR4 can also form dimers with ACKR3 and other receptors, as discussed in detail later.


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

Pawig L, Klasen C, Weber C, Bernhagen J, Noels H - Front Immunol (2015)

Three-dimensional structure of the chemokine receptor CXCR4 and its ligands CXCL12 and MIF. (A) Crystal structure of CXCR4 [Protein data bank identifier 3ODU; (24)]. Included in the crystal structure are amino acids (aa) 27 to 319 of the 352 residues of CXCR4. CXCR4 can exist as a homodimer; here, only chain A is depicted. The N-terminal aa 27–31 are depicted in blue, extracellular loop 1 (aa 100–104) in red, extracellular loop 2 (aa 174–192) in purple, and extracellular loop 3 (aa 267–273) in orange. Intracellular loops (ICL) 1 (aa 65–71), ICL2 (aa 140–144), and ICL3 (aa 225–230) are depicted in cyan. The C-terminal aa 315–319 are shown in yellow. The degradation motif SSLKILSKGK (aa 324–333) cannot be depicted here due to the shortened sequence in the crystal structure. (B) Crystal structure of CXCL12 isoform α [Protein data bank identifier 3GV3; (26)]. Included in the crystal structure are aa 4–67 (with aa counting referring to the mature CXCL12). Here, a CXCL12 monomer is depicted with the N-terminal aa 4–7 in blue. The RFFESH motif (aa 12–17), which is the most important binding motif for CXCR4 in the CXCL12 core, is depicted in red. Each monomer includes a three-stranded β-sheet and one α-helix. (C) Crystal structure of MIF [Protein data bank identifier 3DJH; (27)]. Included in the crystal structure are aa 2–114 (with aa counting including Met-1). A MIF monomer is depicted. The N-terminal aa 2–5 are shown in orange. Depicted in red is the pseudo-(E)LR motif (Asp45–X–Arg12), the N-like loop (aa 48–57) is depicted in blue. Each monomer includes two antiparallel α-helices and a four-stranded β-sheet. Two additional, short β-strands (aa 48–50 and aa 107–109) can be found in each monomer stabilized by trimer formation, and are indicated with “β-strand*.”
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4543903&req=5

Figure 1: Three-dimensional structure of the chemokine receptor CXCR4 and its ligands CXCL12 and MIF. (A) Crystal structure of CXCR4 [Protein data bank identifier 3ODU; (24)]. Included in the crystal structure are amino acids (aa) 27 to 319 of the 352 residues of CXCR4. CXCR4 can exist as a homodimer; here, only chain A is depicted. The N-terminal aa 27–31 are depicted in blue, extracellular loop 1 (aa 100–104) in red, extracellular loop 2 (aa 174–192) in purple, and extracellular loop 3 (aa 267–273) in orange. Intracellular loops (ICL) 1 (aa 65–71), ICL2 (aa 140–144), and ICL3 (aa 225–230) are depicted in cyan. The C-terminal aa 315–319 are shown in yellow. The degradation motif SSLKILSKGK (aa 324–333) cannot be depicted here due to the shortened sequence in the crystal structure. (B) Crystal structure of CXCL12 isoform α [Protein data bank identifier 3GV3; (26)]. Included in the crystal structure are aa 4–67 (with aa counting referring to the mature CXCL12). Here, a CXCL12 monomer is depicted with the N-terminal aa 4–7 in blue. The RFFESH motif (aa 12–17), which is the most important binding motif for CXCR4 in the CXCL12 core, is depicted in red. Each monomer includes a three-stranded β-sheet and one α-helix. (C) Crystal structure of MIF [Protein data bank identifier 3DJH; (27)]. Included in the crystal structure are aa 2–114 (with aa counting including Met-1). A MIF monomer is depicted. The N-terminal aa 2–5 are shown in orange. Depicted in red is the pseudo-(E)LR motif (Asp45–X–Arg12), the N-like loop (aa 48–57) is depicted in blue. Each monomer includes two antiparallel α-helices and a four-stranded β-sheet. Two additional, short β-strands (aa 48–50 and aa 107–109) can be found in each monomer stabilized by trimer formation, and are indicated with “β-strand*.”
Mentions: CXCR4 is an evolutionary conserved protein with 89% similarity between the human [352 aa] and mouse (359 aa) ortholog (22). Ubiquitous expression of CXCR4 has been detected in bone marrow, lymph nodes, liver, lung, brain, heart, kidney, thymus, stomach, pancreas, spleen, ovary, and small intestine (23). Similar to other GPCRs, CXCR4 has an extracellular N-terminus (34 aa for CXCR4), seven transmembrane alpha helices connected by three extracellular and three intracellular loops (ICL), and a C-terminus that is located in the cytoplasm (Figure 1A). However, analysis of crystal structures of CXCR4 in complex with the small-molecular antagonist IT1t and the cyclic peptide antagonist CVX15 in 2010 revealed specific differences in the orientation of the alpha helices compared with other available GPCR structures, mostly at the extracellular side (24). One important difference is the extension of the extracellular end of helix VII, which is two helical turns longer as in other GPCR structures and allows a disulfide bond of Cys274 in helix VII with Cys28 in the N-terminal region. This disulfide bond, together with and among chemokine receptors highly conserved disulfide bond between extracellular loop 2 (ECL2, Cys186) and the extracellular end of alpha helix III (Cys109), is essential for CXCL12 binding by shaping the entrance to the ligand-binding pocket (24). Compared to the extracellular half of CXCR4, which overall substantially differs from other GPCRs, the intracellular part of CXCR4 is structurally fairly similar to the intracellular half of other GPCRs. As exception, the intracellular part of helix VII of CXCR4 is one turn shorter compared to other available GPCR structures. Furthermore, the CXCR4 crystal structure lacks the short alpha helix VIII present in many GPCRs and also lacks in its C-terminus a palmitoylation site, which in many GPCRs attaches to the lipid membrane (24). Crystallization studies revealed CXCR4 to exist as a homodimer, however, left open the possibility of CXCL12/CXCR4 binding in a 1:1 or 1:2 ligand:receptor stoichiometry (24). Recently, a combination of computational, functional, as well as biophysical methods supported a 1:1 over a 1:2 CXCL12:CXCR4 binding stoichiometry (25). This was recently further supported by the crystal structure of CXCR4 in complex with the high-affinity antagonist vMIP-II encoded by Kaposi’s sarcoma-associated herpesvirus (as discussed in more detail later). CXCR4 interacted as a dimer with vMIP-II in a 1:1 stoichiometry (11). In addition to homodimerization, CXCR4 can also form dimers with ACKR3 and other receptors, as discussed in detail later.

Bottom Line: This has complicated the understanding of CXCR4-mediated signaling and associated biological processes.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.

View Article: PubMed Central - PubMed

Affiliation: Institute of Molecular Cardiovascular Research (IMCAR), RWTH Aachen University , Aachen , Germany.

ABSTRACT
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.

No MeSH data available.


Related in: MedlinePlus