The Chemokine receptor CXCR4 is a 352 amino acid rhodopsin-like GPCR and selectively binds the CXC chemokine Stromal Cell-Derived Factor 1 (SDF-1) also known as CXCL12. Classically, two alternatively spliced isoforms of SDF have been identified. SDF-1Alpha is an 89 amino acid protein that is the predominantly expressed form of SDF-1 while SDF-1Beta contains a four amino acid extension at the carboxyl terminus. Recently, an additional four splice variants that contain 30 (SDF-1Gamma), 31 (SDF-1Delta), 1 (SDF-1Epsilon), and 51 (SDF-1phi) amino acid extensions at the carboxyl terminus compared to SDF-1Alpha have been identified (Ref.1).
CXCL12 is the sole ligand for the chemokine receptor CXCR4.This receptor has been described as undergoing dimerization after binding to CXCL12 or alternately in its unbound configuration. Upon binding of CXCL12 to CXCR4, the receptor is stabilized into a conformation that activates the heterotrimeric G-protein (G-alpha and G-beta gamma), G-alphaI and G-AlphaQ being the major components. G-alphaQ proteins activate phosphatidylinositol-specific phospholipases such as Phospholipase C-gamma (PLC-gamma), which hydrolyzes Phosphatidylinositol-4,5-biphosphate (PIP2) to generate Inositol triphosphate (IP3) and Diacylglyerol (DAG). These events lead to calcium flux and the activation of several PKC isoforms that have been shown to be important for SDF-1-induced chemotaxis. G-alphaI activates Phospholipases, phosphodiesterases, and the lipid kinase PI3K via Src-family kinases.Signaling through PI3K pathway leads to the activation of PAK and cell polarization, the first step in migration. PI3K and various tyrosine kinases that activate Akt and CDC42 are involved in actin polymerization. PLC¨Cmediated events, such as calcium release and protein kinase C activation, as well as focal adhesion kinase, Pyk2, Paxillin, and ERK are important in the adhesion process, leading to cell migration. In parallel, activation of Akt, ERK, and tyrosine kinases such as Src leads to transcription of genes involved in migration.G-alphaI activation inhibits adenylyl cyclases that in turn lead to a reduction in cAMP levels as well as the activation of phospholipases and phosphodiesterases (Ref.2 and 3).
G-beta gamma also activates PI3K. PI3K activation stimulates downstream targets such as protein kinase B (PKB/Akt), NF-kappaB, mitogen/extracellular signal-regulated kinase (MEK-1), and extracellular signal-regulated kinase (ERK1/2). PI3K also triggers the tyrosine phosphorylation of focal adhesion complex components such as proline-rich tyrosine kinase (Pyk2), Paxillin, Crk, and p130Cas. GTP-bound G-betagama stimulates guanine nucleotide exchange factors (GEFs) such as TIAM1 and PREX1 specific for the Rho family GTPases (Rac/CDC42/RhoA). These GTPases activate pathways regulating cytoskeleton: Rac activates p21-activated kinase (PAK), which then activates LIM kinase (LIMK), leading to cofilin phosporylation and actin polymerization. CDC42 promotes actin assembly through the Wiskott-Aldrich Syndrome family protein (WASP) and actin-nucleating protein ARP2/3.RhoA activates Rho kinase (ROCK) , leading to myosin light-chain (MLC) phosporylation and microtubule rearrangement. Janus-activated kinase/ signal transducers and activators of transcription (JAK/STAT) pathway is activated through CXCR4, partially independent of G-protein. The association of JAK with CXCR4 activates STAT proteins, which translocate to the nucleus and regulate transcription (Ref.2 and 3).
Down-regulation of the CXCR4 receptor is initiated by phosphorylation of its cytoplasmic tail. Subsequent to the binding of arrestin to the COOH terminus of CXCR4, the receptor is internalized through endocytosis. Degradation of CXCR4 occurs in the lysosome. The CXCR4¡¤CXCR7 heterodimer complex recruits Beta-arrestin, resulting in preferential activation of Beta-arrestin-linked signaling pathways over canonical G protein pathways, including ERK1/2, p38 MAPK, and SAPK. Stimulation of other G-protein coupled receptors may also down-regulate CXCR4 signaling through a process called heterologous desensitization (Ref. 3 and 4).
CXCR4 also acts as cell surface coreceptors of HIV-1 strains forming heterodimers with CD4, the principal HIV-1 receptor. On the virion surface, the viral envelope is arranged in spike©like structures formed by trimeric complexes of gp120, the external subunit that mediates virion attachment, and gp41, the transmembrane subunit that mediates the fusion process. For HIV-1 to enter a target cell, the viral envelope glycoprotein gp120 must interact with CD4 and CXCR4. This two©stage receptor©interaction strategy allows HIV©1 to maintain the highly conserved coreceptor©binding surface in a cryptic conformation, unraveling it only upon binding of gp120 to CD4. Binding of the HIV-1 envelope to its chemokine coreceptors mediates two major biological events: membrane fusion and signaling transduction (Ref.5 and 6). The CXCL12/CXCR4 pathway is a target for therapeutics that block CXCL12/CXCR4 interaction or inhibit downstream intracellular enzyme activities. Small molecular inhibitors of CXCR4, such as plerixafor or BKT140, and blocking antibodies toward CXCR4 or CXCL12 are being investigated in various cancer settings. CXCL12/CXCR4 disruption is essential for the egress of hematopoietic stem and/or progenitor cells from bone marrow into circulation. Conversely, CXCL12/CXCR4 function is essential for homing and/or engraftment of hematopoietic stem cells to the bone marrow after transplantation (Ref. 7).
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