CD4 dependence of gp120IIIB-CXCR4 interaction is cell-type specific

Discovery and characterization of novel small-molecule CXCR4 receptor agonists and antagonists

The chemokine CXCL12 (SDF-1) and its cognate receptor CXCR4 are involved in a large number of physiological processes including HIV-1 infectivity, inflammation, tumorigenesis, stem cell migration, and autoimmune diseases. While previous efforts have identified a number of CXCR4 antagonists, there have been no small molecule agonists reported. Herein, we describe the identification of a novel series of CXCR4 modulators, including the first small molecules to display agonist behavior against this receptor, using a combination of structure- and ligand-based virtual screening. These agonists produce robust calcium mobilization in human melanoma cell lines which can be blocked by the CXCR4-selective antagonist AMD3100. We also demonstrate the ability of these new agonists to induce receptor internalization, ERK activation, and chemotaxis, all hallmarks of CXCR4 activation. Our results describe a new series of biologically relevant small molecules that will enable further study of the CXCR4 receptor and may contribute to the development of new therapeutics.

Identification of a sustained neurogenic zone at the dorsal surface of the adult mouse hippocampus and its regulation by the chemokine SDF-1

We identified a previously unknown neurogenic region at the dorsal surface of the hippocampus; (the "subhippocampal zone," SHZ) in the adult brain. Using a reporter mouse in which SHZ cells and their progeny could be traced through the expression of EGFP under the control of the CXCR4 chemokine receptor promoter we observed the presence of a pool of EGFP expressing cells migrating in direction of the dentate gyrus (DG), which is maintained throughout adulthood. This population appeared to originate from the SHZ where cells entered a caudal migratory stream (aCMS) that included the fimbria, the meninges and the DG. Deletion of CXCR4 from neural stem cells (NSCs) or neuroinflammation resulted in the appearance of neurons in the DG, which were the result of migration of NSCs from the SHZ. Some of these neurons were ectopically placed. Our observations indicate that the SHZ is a neurogenic zone in the adult brain through migration of NSCs in the aCMS. Regulation of CXCR4 signaling in these cells may be involved in repair of the DG and may also give rise to ectopic granule cells in the DG in the context of neuropathology.

The chemokine CXCL12 and the HIV-1 envelope protein gp120 regulate spontaneous activity of Cajal-Retzius cells in opposite directions

Activation of the CXC chemokine receptor 4 (CXCR4) in Cajal–Retzius cells by CXC chemokine ligand 12 (CXCL12) is important for controlling their excitability. CXCR4 is also a co-receptor for the glycoprotein 120 (gp120) of the envelope of the human immunodeficiency virus type 1 (HIV-1), and binding of gp120 to CXCR4 may produce pathological effects. In order to study CXCR4-dependent modulation of membrane excitability, we recorded in cell-attached configuration spontaneous action currents from hippocampal stratum lacunosum-moleculare Cajal–Retzius cells of the CXCR4-EGFP mouse. CXCL12 (50 nM) powerfully inhibited firing independently of synaptic transmission, suggesting that CXCR4 regulates an intrinsic conductance. This effect was prevented by conditioning slices with BAPTA-AM (200 μM), and by blockers of the BK calcium-dependent potassium channels (TEA (1 mM), paxilline (10 μM) and iberiotoxin (100 nM)). In contrast, exposure to gp120 (pico- to nanomolar range, alone or in combination with soluble cluster of differentiation 4 (CD4)), enhanced spontaneous firing frequency. This effect was prevented by the CXCR4 antagonist AMD3100 (1 μM) and was absent in EGFP-negative stratum lacunosum-moleculare interneurons. Increased excitability was prevented by treating slices with BAPTA-AM or bumetanide, suggesting that gp120 activates a mechanism that is both calcium- and chloride-dependent. In conclusion, our results demonstrate that CXCL12 and gp120 modulate the excitability of Cajal–Retzius cells in opposite directions. We propose that CXCL12 and gp120 either generate calcium responses of different strength or activate distinct pools of intracellular calcium, leading to agonist-specific responses, mediated by BK channels in the case of CXCL12, and by a chloride-dependent mechanism in the case of gp120.

Direct effects of HIV-1 Tat on excitability and survival of primary dorsal root ganglion neurons: possible contribution to HIV-1-associated pain

The vast majority of people living with human immunodeficiency virus type 1 (HIV-1) have pain syndrome, which has a significant impact on their quality of life. The underlying causes of HIV-1-associated pain are not likely attributable to direct viral infection of the nervous system due to the lack of evidence of neuronal infection by HIV-1. However, HIV-1 proteins are possibly involved as they have been implicated in neuronal damage and death. The current study assesses the direct effects of HIV-1 Tat, one of potent neurotoxic viral proteins released from HIV-1-infected cells, on the excitability and survival of rat primary dorsal root ganglion (DRG) neurons. We demonstrated that HIV-1 Tat triggered rapid and sustained enhancement of the excitability of small-diameter rat primary DRG neurons, which was accompanied by marked reductions in the rheobase and resting membrane potential (RMP), and an increase in the resistance at threshold (R(Th)). Such Tat-induced DRG hyperexcitability may be a consequence of the inhibition of cyclin-dependent kinase 5 (Cdk5) activity. Tat rapidly inhibited Cdk5 kinase activity and mRNA production, and roscovitine, a well-known Cdk5 inhibitor, induced a very similar pattern of DRG hyperexcitability. Indeed, pre-application of Tat prevented roscovitine from having additional effects on the RMP and action potentials (APs) of DRGs. However, Tat-mediated actions on the rheobase and R(Th) were accelerated by roscovitine. These results suggest that Tat-mediated changes in DRG excitability are partly facilitated by Cdk5 inhibition. In addition, Cdk5 is most abundant in DRG neurons and participates in the regulation of pain signaling. We also demonstrated that HIV-1 Tat markedly induced apoptosis of primary DRG neurons after exposure for longer than 48 h. Together, this work indicates that HIV-1 proteins are capable of producing pain signaling through direct actions on excitability and survival of sensory neurons.

The neurotransmitter GABA is a potent inhibitor of the stromal cell-derived factor-1alpha induced migration of adult CD133+ hematopoietic stem and progenitor cells

The ability of hematopoietic stem and progenitor cells (HSPCs) to migrate is a prerequisite for bone marrow homing and tissue regeneration processes. Induction of HSPC migration is chiefly directed by stromal cell-derived factor-1alpha (SDF-1alpha). Considerably less is known about factors that terminate HSPC migration. Adult CD133(+) HSPCs were isolated from mobilized peripheral blood by immunomagnetic separation. Cell migration was assessed using the three-dimensional collagen matrix migration assay, which allows detailed migration analysis on a cell population and single-cell level. The SDF-1alpha-induced locomotory activity of CD133(+) cells was efficiently blocked by the neurotransmitter gamma-aminobutyric acid (GABA). GABA signaling was effected via the GABA(B)-receptor. This was verified by flow cytometry and cell migration studies using the specific GABA(A)-receptor and GABA(B)-receptor agonists isoguvacine and baclofen, respectively. Baclofen blocked SDF-1alpha-induced migration of CD133(+) cells. Flow cytometry-based calcium measurements revealed that GABA inhibits the SDF-1alpha-induced migration of CD133(+) cells by blocking the SDF-1alpha-induced calcium influx. Similar results were obtained with the specific calcium-release-activated calcium (CRAC) channel inhibitor BTP-2, which both blocked the SDF-1alpha-induced calcium influx and migration of CD133(+) cells. These results suggest that GABA(B)-receptor signaling modulates the activity of CRAC channels, whereby the mechanism in detail remains unclear. In summary, the neurotransmitter GABA is a potent blocker of the SDF-1alpha-induced migration of CD133(+) HSPCs from mobilized peripheral blood.

Human immunodeficiency virus gp120-induced apoptosis of human neuroblastoma cells in the absence of CXCR4 internalization

The chemokine receptor CXCR4 functions as human immunodeficiency virus (HIV)-1 coreceptor and is involved in acquired immunodeficiency virus (AIDS) neuropathogenesis. CXCR4 is expressed by most cell types in the brain, including microglia, astrocytes, and neurons. Studies have shown that the HIV envelope protein gp120 binds to neuronal CXCR4 and activates signal transduction pathways leading to apoptosis. However, the natural CXCR4 ligand (CXCL12) has been referred to induce both neuronal survival and death. Here the authors used flow cytometry to determine whether gp120 and CXCL12 differ in their ability to induce CXCR4 internalization in the human neuroblastoma cells SH-SY5Y, which constitutively express CXCR4. As expected, increasing concentration of CXCL12 reduced surface expression of CXCR4 in a time-and concentration-dependent manner. Conversely, gp120IIIB (monomeric or oligomeric, in presence or absence of soluble CD4) did not change CXCR4 membrane levels. Similar results were obtained in a murine lymphocyte cell line (300-19) stably expressing human CXCR4. Nevertheless, gp120IIIB was still able to activate intracellular signaling and proapoptotic pathways, via CXCR4. These results show that gp120IIIB toxicity and signaling do not require CXCR4 internalization in SH-SY5Y cells, and suggest that the viral protein may alter normal CXCR4 trafficking thus, interfering with activation of prosurvival pathways.

Chemokine CXCL12 induces binding of ferritin heavy chain to the chemokine receptor CXCR4, alters CXCR4 signaling, and induces phosphorylation and nuclear translocation of ferritin heavy chain

Chemokine receptor-initiated signaling plays critical roles in cell differentiation, proliferation, and migration. However, the regulation of chemokine receptor signaling under physiological and pathological conditions is not fully understood. In the present study, we demonstrate that the CXC chemokine receptor 4 (CXCR4) formed a complex with ferritin heavy chain (FHC) in a ligand-dependent manner. Our in vitro binding assays revealed that purified FHC associated with both the glutathione S-transferase-conjugated N-terminal and C-terminal domains of CXCR4, thereby suggesting the presence of more than one FHC binding site in the protein sequence of CXCR4. Using confocal microscopy, we observed that stimulation with CXCL12, the receptor ligand, induced colocalization of the internalized CXCR4 with FHC into internal vesicles. Furthermore, after CXCL12 treatment, FHC underwent time-dependent nuclear translocation and phosphorylation at serine residues. By contrast, a mutant form of FHC in which serine 178 was replaced by alanine (S178A) failed to undergo phosphorylation, suggesting that serine 178 is the major phosphorylation site. Compared with the wild type FHC, the FHC-S178A mutant exhibited reduced association with CXCR4 and constitutive nuclear translocation. We also found that CXCR4-mediated extracellular signal-regulated kinase 1/2 (ERK1/2) activation and chemotaxis were inhibited by overexpression of wild type FHC but not FHC-S178A mutant, and were prolonged by FHC knockdown. In addition to CXCR4, other chemokine receptor-initiated signaling appeared to be similarly regulated by FHC, because CXCR2-mediated ERK1/2 activation was also inhibited by FHC overexpression and prolonged by FHC knockdown. Altogether, our data provide strong evidence for an important role of FHC in chemokine receptor signaling and receptor-mediated cell migration.

Dendrite-selective redistribution of the chemokine receptor CXCR4 following agonist stimulation

The chemokine SDF-1 is a secreted protein that plays a critical role in several aspects of neuron development through interaction with its unique receptor CXCR4. A key mechanism that controls neuron responsiveness to extracellular signals during neuronal growth is receptor endocytosis. Since we previously reported that SDF-1 regulates axon development without affecting the other neurites, we asked whether this could correlate with a compartment-selective trafficking of CXCR4. We thus studied CXCR4 behavior upon SDF-1 exposure in rat hippocampus slices and in transfected neuron cultures. A massive agonist-induced redistribution of CXCR4 in endosomes was observed in dendrites whereas no modification was evidenced in axons. Our data suggest that CXCR4 trafficking may play a role in mediating selective effects of SDF-1 on distinct neuronal membrane subdomains.

Regulation of neuronal P53 activity by CXCR 4

Abnormal activation of CXCR 4 during inflammatory/infectious states may lead to neuronal dysfunction or damage. The major goal of this study was to determine the coupling of CXCR 4 to p53-dependent survival pathways in primary neurons. Neurons were stimulated with the HIV envelope protein gp120(IIIB) or the endogenous CXCR 4 agonist, SDF-1 alpha. We found that gp120 stimulates p53 activity and induces expression of the p53 pro-apoptotic target Apaf-1 in cultured neurons. Inhibition of CXCR 4 by AMD 3100 abrogates the effect of gp120 on both p53 and Apaf-1. Moreover, gp120 neurotoxicity is markedly reduced by the p53-inhibitor, pifithrin-alpha. The viral protein also regulates p53 phosphorylation and expression of other p53-responsive genes, such as MDM 2 and p21. Conversely, SDF-1 alpha, which can promote neuronal survival, increases p53 acetylation and p21 expression in neurons. Thus, the stimulation of different p53 targets could be instrumental in determining the outcome of CXCR 4 activation on neuronal survival in neuro-inflammatory disorders.

Differential involvement of p38 and JNK MAP kinases in HIV-1 Tat and gp120-induced apoptosis and neurite degeneration in striatal neurons

The role of p38 and c-jun-N-terminal kinases 1/2, members of the mitogen-activated protein kinase family, in mediating the toxic effects of human immunodeficiency virus-1 transactivator of transcription (Tat) and gp120 were explored in primary mouse striatal neurons in vitro. Both Tat and gp120 caused significant increases in p38 and c-jun-N-terminal kinase mitogen-activated protein kinase phosphorylation, caspase-3 activity, neurite losses and cell death in striatal neurons. Tat-induced increases in caspase-3 activity were significantly attenuated by an inhibitor of c-jun-N-terminal kinase (anthra[1,9-cd]pyrazol-6(2H)-one), but not by an inhibitor of p38 ([4-(4-fluorophenyl)-2-(4-methylsul-finylphenyl)-5-(4-pyridyl)1 H-imidazole]), mitogen-activated protein kinase. However, despite preventing increases in caspase-3 activity, c-jun-N-terminal kinase inhibition failed to avert Tat-induced neuronal losses suggesting that the reductions in caspase-3 activity were insufficient to prevent cell death caused by Tat. Alternatively, gp120-induced increases in caspase-3 activity, neurite losses and neuronal death were prevented by p38, but not c-jun-N-terminal kinase, mitogen-activated protein kinase inhibition. Our findings suggest that gp120 induces neuronal dysfunction and death through actions at p38 mitogen-activated protein kinase, while Tat kills neurons through actions that are independent of p38 or c-jun-N-terminal kinase mitogen-activated protein kinase, or through the concurrent activation of multiple proapoptotic pathways.

The HIV-1 coat protein gp120 regulates CXCR4-mediated signaling in neural progenitor cells

We demonstrate that hCD4-primed gp120IIIB interacts with CXCR4 receptors expressed by postnatal mouse neural progenitor cells and elicits robust Ca(2+) signals. The chemokine SDF-1 acted as a chemoattractant and a mitogenic stimulus for these neural progenitor cells. Although hCD4/gp120 was not able to produce chemoattraction or increase proliferation, it completely blocked the ability of SDF-1 to produce these effects. Thus, gp120 can act both as an agonist and de facto antagonist of CXCR4-mediated signaling in neural progenitor cells. It is possible that the ability of hCD4/gp120 to block SDF-1 signaling in neural progenitors may contribute to the neuropathological effects of HIV-1.

Apoptotic and antiapoptotic effects of CXCR4: is it a matter of intrinsic efficacy? Implications for HIV neuropathogenesis

CXCR4, the specific receptor for the chemokine SDF-1 alpha that also binds CXCR4-using HIV gp120s, affects survival of different cell types, including neurons. However, current data show that the outcome of CXCR4 activation on neuronal survival may vary depending on the ligand and/or the cellular conditions. In this study, we have systematically compared the effects of SDF-1 alpha and gp120(IIIB) (with or without CD4) on several intracellular pathways involved in cell survival, including MAP kinases and Akt-dependent pathways. Our data show that gp120(IIIB) and SDF-1 alpha are both potent activators of MAP kinases in neuronal and non-neuronal cells, though the kinetic of these responses is slightly different. Furthermore, unlike SDF-1 alpha, and independently of CD4, gp120(IIIB) is unable to stimulate Akt and some of its antiapoptotic targets (NF-kappa B and MDM2)--despite its ability to activate other signaling pathways in the same conditions. Finally, the viral protein is more efficient in recruiting some effectors (e.g., JNK) than others in comparison with SDF-1 alpha (EC(50) = 0.1 vs. 0.6 nM). We conclude that the intrinsic efficacy of the two ligands is significantly different and is pathway dependent. These findings have important implications for our understanding of CXCR4-mediated responses in the CNS, as well as the role of this coreceptor in HIV neuropathogenesis.

Activation of c-Jun N-terminal kinase mediates gp120IIIB- and nucleoside analogue-induced sensory neuron toxicity

Peripheral neuropathy is the most common neurological symptom in patients with acquired immunodeficiency syndrome (AIDS). Here, we examine possible mechanisms of gp120 and nucleoside reverse transcriptase inhibitors (NRTIs) in the pathogenesis of AIDS peripheral neuropathy. Neonatal dorsal root ganglion (DRG) neurons were found to undergo apoptosis in response to chronic treatment with gp120IIIB, an effect enhanced by the co-application of hCD4, as well as upon exposure to the nucleoside reverse transcriptase inhibitor (NRTI), 2',3'-dideoxyinosine (ddI). DRG neurons were rescued from the neurotoxic effects of these agents by CEP-1347, an inhibitor of the mixed lineage kinases (MLKs), upstream activators of the c-Jun N-terminal kinase (JNK) signaling pathway. In addition, gp120- or ddI-mediated toxicity were also inhibited by neuronal expression of dominant negative versions of the MLKs. Our results suggest that both gp120 and the NRTIs cause sensory neuron apoptosis through the activation of the JNK pathway, and that CEP-1347-like compounds may serve as a therapeutic option in patients with AIDS-associated peripheral neuropathy.

Stromal cell-derived factor 1-mediated CXCR4 signaling in rat and human cortical neural progenitor cells

Stromal cell-derived factor 1 (SDF-1) and the chemokine receptor CXCR4 are highly expressed in the nervous system. Knockout studies have suggested that both SDF-1 and CXCR4 play essential roles in cerebellar, hippocampal, and neocortical neural cell migration during embryogenesis. To extend these observations, CXCR4 signaling events in rat and human neural progenitor cells (NPCs) were examined. Our results show that CXCR4 is expressed in abundance on rat and human NPCs. Moreover, SDF-1alpha induced increased NPCs levels of inositol 1,4,5-triphosphate, extracellular signal-regulated kinases 1/2, Akt, c-Jun N-terminal kinase, and intracellular calcium whereas it diminished cyclic adenosine monophosphate. Finally, SDF-1alpha can induce human NPC chemotaxis in vitro, suggesting that CXCR4 plays a functional role in NPC migration. Both T140, a CXCR4 antagonist, and pertussis toxin (PTX), an inactivator of G protein-coupled receptors, abrogated these events. Ultimately, this study suggested that SDF-1alpha can influence NPC function through CXCR4 and that CXCR4 is functional on NPC.

Regulation of CXCR4 receptor dimerization by the chemokine SDF-1alpha and the HIV-1 coat protein gp120: a fluorescence resonance energy transfer (FRET) study

Both the chemokine SDF-1alpha and the human immunodeficiency virus-1 (HIV-1) coat protein gp120 can bind to CXCR4 chemokine receptors but with different signaling consequences. To understand the molecular basis for these differences, we tagged the rat CXCR4 receptor with enhanced cyan (ECFP) and yellow (EYFP) derivatives of the green fluorescent protein and investigated CXCR4 receptor dimerization in human embryonic kidney (HEK)-tsA201 cells using fluorescence resonance energy transfer (FRET). Elevated FRET was detected under basal conditions from EYFP-CXCR4 and ECFP-CXCR4 receptor-transfected cells indicating a high level of CXCR4 receptor dimerization. In comparison, EYFP-CXCR4 and ECFP-mu-opioid receptor-cotransfected cells displayed a much lower FRET signal. The FRET signal resulting from EYFP-CXCR4- and ECFP-CXCR4-expressing cells could be attenuated by coexpressing nontagged CXCR4 receptors suggesting competition with fluorophore-tagged receptors in the membrane. Nontagged mu-opioid, kappa-opioid, and muscarinic receptors also decreased the FRET between the tagged CXCR4 receptor pairs but to a lesser extent. Application of the CXCR4 receptor agonist SDF-1alpha (50 nM) further increased the FRET signal from tagged CXCR4 receptors, an effect that was inhibited by the CXCR4 antagonist AMD3100. SDF-1alpha had no effect when EYFP-CXCR4 and ECFP-mu-opioid receptors were coexpressed. The effect of gp120IIIB on CXCR4 FRET was dependent on the coexpression of human CD4 (hCD4) when it increased the FRET signal, and this was decreased by AMD3100 pretreatment. FRET analysis of tagged hCD4 constructs demonstrated that there was significant association of hCD4 and CXCR4, as well as hCD4 dimerization. These data suggest that CXCR4 dimerization is involved in SDF-1alpha- and gp120-induced signaling events.

The effect of human tissue factor pathway inhibitor-2 on the growth and metastasis of fibrosarcoma tumors in athymic mice

Human tissue factor pathway inhibitor-2 (TFPI-2) is a matrix-associated Kunitz inhibitor that inhibits the plasmin- and trypsin-mediated activation of zymogen matrix metalloproteinases involved in tumor progression, invasion, and metastasis. To directly assess its role in tumor growth and metastasis in vivo, we stably transfected HT-1080 fibrosarcoma cells expressing either fully active wild-type human TFPI-2 (WT) or inactive R24Q TFPI-2 (QT) and examined their ability to form tumors and metastasize in athymic mice in comparison to mock-transfected cells (MT). MT and QT fibrosarcoma tumors grew 2 to 3 times larger than WT tumors. Tumor metastasis was confined to the lung and was observed in 75% of mice treated with either MT or QT cells, whereas only 42% of mice treated with WT cells developed lung metastases. Real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analyses of each tumor group revealed 3- to 6-fold lower levels of murine vascular endothelial growth factor gene expression in WT tumors in relation to either MT or QT tumors. Comparative tumor gene expression analysis revealed that several human genes implicated in oncogenesis, invasion, metastasis, apoptosis, and angiogenesis had significantly altered levels of expression in WT tumors. Our collective data demonstrate that secretion of inhibitory TFPI-2 by a highly metastatic tumor cell markedly inhibits its growth and metastasis in vivo by regulating pericellular extracellular matrix (ECM) remodeling and angiogenesis. (Blood. 2004;103:1069-1077)