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Characterization and visualization of [125I] stromal cell-derived factor-1alpha binding to CXCR4 receptors in rat
G Banisadr1, E Dicou, T Berbar
1INSERM U. 339; Hôpital Saint-Antoine, 184 rue du Faubourg Saint-Antoine, 75571 Cedex 12, Paris, France. banisadr@st-antoine.inserm.fr
Abstract:
Stromal cell-Derived Factor-1 (SDF-1alpha), binds to the seven-transmembrane G protein-coupled CXCR4 receptor and modulates cell migration, differentiation, and proliferation. CXCR4 has been reported to be expressed in various tissues including brain. Moreover, CXCR4 has recently been shown to be one of the coreceptors for HIV-1 infection which could be implicated in HIV encephalitis. In the present study, the binding properties and autoradiographic distribution of [125I]SDF-1alpha binding to CXCR4 were characterized in the adult rat brain. SDF-1alpha binding and CXCR4 coupling system were also studied in human neuroblastoma cell line SK-N-SH. The binding of [125I]SDF-1alpha on rat brain sections was specific, time-dependent and reversible. The highest densities of CXCR4 were detected in the choroid plexus of the lateral and the dorsal third ventricle. Lower densities of [125I]SDF-1alpha binding sites were observed in various brain regions including cerebral cortex, anterior olfactory nuclei, hippocampal formation, thalamic nuclei, blood vessels and pituitary gland. In the choroid plexus, the IC(50) and K(d) of [125I]SDF-1alpha binding were respectively 0.6 nM and 0. 36 nM. Similar IC(50) values were obtained in other brain structures. A CXCR4 antagonist, bicyclam, competed with SDF-1alpha binding (30% inhibition at 10(-6) M). In SK-N-SH cells, [125I]SDF-1alpha bound to CXCR4 with a K(d) of 5.0 nM and a maximal binding capacity of 460 fmol/mg of protein. SDF-1alpha induced a rapid and transient intracellular calcium increase in SK-N-SH cells. These findings suggest that CXCR4 is highly expressed in some brain structures and have a regulatory role in the nervous system. The significance of this expression in the brain parenchyma and more specifically in the choroid plexus remains to be clarified in the normal as well as in the infected brain.
Insights
Stromal cell-Derived Factor-1 (SDF-1alpha) binds to the CXCR4 receptor, which is highly expressed in the adult rat brain, particularly the choroid plexus. This binding is specific and may play a regulatory role in the nervous system.
Area of Science:
- Neuroscience
- Molecular Biology
- Immunology
Background:
- Stromal cell-Derived Factor-1 (SDF-1alpha) is a chemokine that binds to the CXCR4 receptor, influencing cell functions.
- CXCR4 is expressed in the brain and acts as an HIV-1 coreceptor, potentially contributing to HIV encephalitis.
- The distribution and binding characteristics of SDF-1alpha/CXCR4 in the brain are not fully understood.
Purpose of the Study:
- To characterize the binding properties and autoradiographic distribution of [125I]SDF-1alpha binding to CXCR4 in the adult rat brain.
- To investigate SDF-1alpha binding and CXCR4 coupling in the human neuroblastoma cell line SK-N-SH.
Main Methods:
- Autoradiography of [125I]SDF-1alpha binding on adult rat brain sections.
- Biochemical assays to determine binding kinetics (IC50, Kd, Bmax) in brain tissue and SK-N-SH cells.
- Functional assays measuring intracellular calcium increase in SK-N-SH cells upon SDF-1alpha stimulation.
- Competition assays using a CXCR4 antagonist (bicyclam).
Main Results:
- Specific, time-dependent, and reversible binding of [125I]SDF-1alpha to CXCR4 was observed in the rat brain.
- Highest CXCR4 densities were found in the choroid plexus, with lower densities in the cerebral cortex, hippocampus, thalamus, and other regions.
- Binding affinity (Kd) in the choroid plexus was 0.36 nM, with similar IC50 values in other brain structures.
- In SK-N-SH cells, [125I]SDF-1alpha bound to CXCR4 with a Kd of 5.0 nM and induced intracellular calcium increase.
- A CXCR4 antagonist partially inhibited SDF-1alpha binding.
Conclusions:
- CXCR4 is highly expressed in specific brain structures, notably the choroid plexus, suggesting a regulatory role in the nervous system.
- The findings provide a basis for further research into CXCR4's function in the brain parenchyma and choroid plexus.
- Understanding CXCR4 expression is crucial for investigating its role in both normal brain function and conditions like HIV encephalitis.
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