ABSTRACT
BACKGROUND: Five different G protein-coupled sphingosine-1-phosphate (S1P) receptors (S1P1-S1P5) regulate a variety of physiologic and pathophysiologic processes, including lymphocyte circulation, multiple sclerosis (MS), and cancer. Although B-lymphocyte circulation plays an important role in these processes and is essential for normal immune responses, little is known about S1P receptors in human B cells. OBJECTIVE: To explore their function and signaling, we studied B-cell lines and primary B cells from control subjects, patients with leukemia, patients with S1P receptor inhibitor-treated MS, and patients with primary immunodeficiencies. METHODS: S1P receptor expression was analyzed by using multicolor immunofluorescence microscopy and quantitative PCR. Transwell assays were used to study cell migration. S1P receptor internalization was visualized by means of time-lapse imaging with fluorescent S1P receptor fusion proteins expressed by using lentiviral gene transfer. B-lymphocyte subsets were characterized by means of flow cytometry and immunofluorescence microscopy. RESULTS: Showing that different B-cell populations express different combinations of S1P receptors, we found that S1P1 promotes migration, whereas S1P4 modulates and S1P2 inhibits S1P1 signals. Expression of CD69 in activated B lymphocytes and B cells from patients with chronic lymphocytic leukemia inhibited S1P-induced migration. Studying B-cell lines, normal B lymphocytes, and B cells from patients with primary immunodeficiencies, we identified Bruton tyrosine kinase, ß-arrestin 2, LPS-responsive beige-like anchor protein, dedicator of cytokinesis 8, and Wiskott-Aldrich syndrome protein as critical signaling components downstream of S1P1. CONCLUSION: Thus S1P receptor signaling regulates human B-cell circulation and might be a factor contributing to the pathology of MS, chronic lymphocytic leukemia, and primary immunodeficiencies.
Subject(s)
B-Lymphocyte Subsets/metabolism , Common Variable Immunodeficiency/metabolism , Leukemia, Lymphocytic, Chronic, B-Cell/metabolism , Multiple Sclerosis/metabolism , Receptors, Lysosphingolipid/metabolism , Adaptor Proteins, Signal Transducing/genetics , Adaptor Proteins, Signal Transducing/immunology , Adaptor Proteins, Signal Transducing/metabolism , Agammaglobulinaemia Tyrosine Kinase , Antigens, CD/genetics , Antigens, CD/immunology , Antigens, CD/metabolism , Antigens, Differentiation, T-Lymphocyte/genetics , Antigens, Differentiation, T-Lymphocyte/immunology , Antigens, Differentiation, T-Lymphocyte/metabolism , Arrestins/genetics , Arrestins/immunology , Arrestins/metabolism , B-Lymphocyte Subsets/immunology , B-Lymphocyte Subsets/pathology , Cell Line , Cell Movement , Common Variable Immunodeficiency/genetics , Common Variable Immunodeficiency/immunology , Common Variable Immunodeficiency/pathology , Gene Expression Regulation , Guanine Nucleotide Exchange Factors/genetics , Guanine Nucleotide Exchange Factors/immunology , Guanine Nucleotide Exchange Factors/metabolism , Humans , Lectins, C-Type/genetics , Lectins, C-Type/immunology , Lectins, C-Type/metabolism , Leukemia, Lymphocytic, Chronic, B-Cell/genetics , Leukemia, Lymphocytic, Chronic, B-Cell/immunology , Leukemia, Lymphocytic, Chronic, B-Cell/pathology , Multiple Sclerosis/genetics , Multiple Sclerosis/immunology , Multiple Sclerosis/pathology , Primary Cell Culture , Protein Isoforms/genetics , Protein Isoforms/immunology , Protein Isoforms/metabolism , Protein-Tyrosine Kinases/genetics , Protein-Tyrosine Kinases/immunology , Protein-Tyrosine Kinases/metabolism , Receptors, Lysosphingolipid/genetics , Receptors, Lysosphingolipid/immunology , Signal Transduction , Time-Lapse Imaging , Wiskott-Aldrich Syndrome Protein/genetics , Wiskott-Aldrich Syndrome Protein/immunology , Wiskott-Aldrich Syndrome Protein/metabolism , beta-Arrestin 2 , beta-ArrestinsABSTRACT
Lateral roots are initiated postembryonically in response to environmental cues, enabling plants to explore efficiently their underground environment. However, the mechanisms by which the environment determines the position of lateral root formation are unknown. In this study, we demonstrate that in Arabidopsis thaliana lateral root initiation can be induced mechanically by either gravitropic curvature or by the transient bending of a root by hand. The plant hormone auxin accumulates at the site of lateral root induction before a primordium starts to form. Here we describe a subcellular relocalization of PIN1, an auxin transport protein, in a single protoxylem cell in response to gravitropic curvature. This relocalization precedes auxin-dependent gene transcription at the site of a new primordium. Auxin-dependent nuclear signaling is necessary for lateral root formation; arf7/19 double knock-out mutants normally form no lateral roots but do so upon bending when the root tip is removed. Signaling through arf7/19 can therefore be bypassed by root bending. These data support a model in which a root-tip-derived signal acts on downstream signaling molecules that specify lateral root identity.
