(1/3250) Cell-type specific targeting of the alpha 2c-adrenoceptor. Evidence for the organization of receptor microdomains during neuronal differentiation of PC12 cells.
We have previously shown differences in the intracellular targeting of alpha2a (alpha(2A))- and alpha2c (alpha(2C))-adrenoreceptors expressed in the same cell line (von Zastrow, M., Link, R., Daunt, D. , Barsh, G., and Kobilka, B. (1993) J. Biol. Chem. 268, 763-766; Daunt, D. A., Hurt, C., Hein, L., Kallio, J., Feng, F., and Kobilka, B. K. (1997) Mol. Pharmacol. 51, 711-720). alpha(2A)-Adrenoreceptors reside primarily in the plasma membrane in HEK 293 cells, while co-expressed alpha(2C)-adrenoreceptors are found mainly in an intracellular compartment. Since alpha(2c)-adrenoreceptors are expressed primarily in the brain, we compared the intracellular targeting of alpha(2C)-adrenoreceptors in two neuroendocrine cell lines with the targeting in three epithelial cell lines and one fibroblast cell line. In transiently transfected COS7 cells, and in stably transfected normal rat kidney cells, Madin-Darby canine kidney cells, and Rat1 fibroblasts, a significant proportion of alpha(2C)-adrenoreceptor detected by immunocytochemistry co-localized with markers for both the endoplasmic reticulum and the cis/medial Golgi compartments. In contrast, both PC12 cells and AtT20 cells efficiently targeted alpha(2C)-adrenoreceptors to the plasma membrane. Ligand binding and Western blot analyses indicate that intracellular receptor in normal rat kidney cells is functional and undergoes normal post-translational processing. In PC12 cells the expressed alpha(2C)-adrenoreceptors become concentrated in neurite outgrowths in discrete regions of the plasma membrane having a high density of F-actin following treatment with nerve growth factor. These findings provide evidence for cell-type specific factors that facilitate the targeting of the G protein-coupled receptors to the plasma membrane. (+info)
(2/3250) The integrins alpha3beta1 and alpha6beta1 physically and functionally associate with CD36 in human melanoma cells. Requirement for the extracellular domain OF CD36.
Lateral association between different transmembrane glycoproteins can serve to modulate integrin function. Here we characterize a physical association between the integrins alpha(3)beta(1) and alpha(6)beta(1) and CD36 on the surface of melanoma cells and show that ectopic expression of CD36 by CD36-negative MV3 melanoma cells increases their haptotactic migration on extracellular matrix components. The association was demonstrated by co-immunoprecipitation, reimmunoprecipitation, and immunoblotting of surface-labeled cells lysed in Brij 96 detergent. Confocal microscopy illustrated the co-association of alpha(3) and CD36 in cell membrane projections and ruffles. A requirement for the extracellular domain of CD36 in this association was shown by co-immunoprecipitation experiments using surface-labeled MV3 melanoma or COS-7 cells that had been transiently transfected with chimeric constructs between CD36 and intercellular adhesion molecule 1 (ICAM-1) or with a truncation mutant of CD36. CD36 is known to engage in signal transduction and to localize to membrane microdomains or rafts in several cell types. Toward a mechanistic explanation for the functional effects of CD36 expression, we demonstrate that in fractionated Triton X-100 lysates of the MV3 cells stably transfected with CD36, CD36 was greatly enriched with the detergent-insoluble fractions that represent plasma membrane rafts. Significantly, when these fractionated lysates were reprobed for endogenous beta(1) integrin, it was found that a 4-fold increase in the proportion of the mature protein was contained within the detergent-insoluble fractions when extracted from the CD36-transfected cells compared with MV3 cells transfected with vector only. These results suggest that in melanoma cells CD36 expression may induce the sequestration of certain integrins into membrane microdomains and promote cell migration. (+info)
(3/3250) Detergent-resistant microdomains offer no refuge for proteins phosphorylated by the IgE receptor.
When the high affinity receptor for IgE and related receptors become aggregated, they emigrate to specialized microdomains of the plasma membrane that are enriched in certain lipids and lipid-anchored proteins. Among the latter are the kinases that initiate signaling cascade(s) by phosphorylating the receptors. In studying the IgE receptor, we explored whether, in addition to their potential role in enhancing the initiation of signaling by the kinase(s), the microdomains might augment the stimulation by excluding phosphatases. In vitro assessment of phosphatase activity, using either a relevant or irrelevant substrate, suggested that the microdomains were deficient in phosphatase activity, but, in vivo, proteins confined to the microdomains were found to be no less vulnerable to dephosphorylation than those outside such domains. In the course of our experiments, we observed that the procedures routinely used to isolate the detergent-resistant domains dissociated the receptor for IgE, thereby artificially accentuating the observed preferential distribution of phosphorylated subunits in the microdomains. (+info)
(4/3250) Actin cytoskeleton: thinking globally, actin' locally.
A class of proteins dubbed pipmodulins bind to and sequester the phospholipid PIP2 in the plasma membrane. Local release of PIP2 controls actin dynamics in specific subcellular regions and plays a critical role in regulating actin-based cell motility and morphogenesis. (+info)
(5/3250) Assembly of myelin by association of proteolipid protein with cholesterol- and galactosylceramide-rich membrane domains.
Myelin is a specialized membrane enriched in glycosphingolipids and cholesterol that contains a limited spectrum of proteins. We investigated the assembly of myelin components by oligodendrocytes and analyzed the role of lipid-protein interactions in this process. Proteolipid protein (PLP), the major myelin protein, was recovered from cultured oligodendrocytes from a low-density CHAPS-insoluble membrane fraction (CIMF) enriched in myelin lipids. PLP associated with the CIMF after leaving the endoplasmic reticulum but before exiting the Golgi apparatus, suggesting that myelin lipid and protein components assemble in the Golgi complex. The specific association of PLP with myelin lipids in CIMF was supported by the finding that it was efficiently cross-linked to photoactivable cholesterol, but not to phosphatidylcholine, which is underrepresented in both myelin and CIMF. Furthermore, depletion of cholesterol or inhibition of sphingolipid synthesis in oligodendrocytes abolished the association of PLP with CIMF. Thus, PLP may be recruited to myelin rafts, represented by CIMF, via lipid-protein interactions. In contrast to oligodendrocytes, after transfection in BHK cells, PLP is absent from isolated CIMF, suggesting that PLP requires specific lipids for raft association. In mice deficient in the enzyme ceramide galactosyl transferase, which cannot synthesize the main myelin glycosphingolipids, a large fraction of PLP no longer associates with rafts. Formation of a cholesterol- and galactosylceramide-rich membrane domain (myelin rafts) may be critical for the sorting of PLP and assembly of myelin in oligodendrocytes. (+info)
(6/3250) Measles virus assembly within membrane rafts.
During measles virus (MV) replication, approximately half of the internal M and N proteins, together with envelope H and F glycoproteins, are selectively enriched in microdomains rich in cholesterol and sphingolipids called membrane rafts. Rafts isolated from MV-infected cells after cold Triton X-100 solubilization and flotation in a sucrose gradient contain all MV components and are infectious. Furthermore, the H and F glycoproteins from released virus are also partly in membrane rafts (S. N. Manie et al., J. Virol. 74:305-311, 2000). When expressed alone, the M but not N protein shows a low partitioning (around 10%) into rafts; this distribution is unchanged when all of the internal proteins, M, N, P, and L, are coexpressed. After infection with MGV, a chimeric MV where both H and F proteins have been replaced by vesicular stomatitis virus G protein, both the M and N proteins were found enriched in membrane rafts, whereas the G protein was not. These data suggest that assembly of internal MV proteins into rafts requires the presence of the MV genome. The F but not H glycoprotein has the intrinsic ability to be localized in rafts. When coexpressed with F, the H glycoprotein is dragged into the rafts. This is not observed following coexpression of either the M or N protein. We propose a model for MV assembly into membrane rafts where the virus envelope and the ribonucleoparticle colocalize and associate. (+info)
(7/3250) Galectin-1 induces partial TCR zeta-chain phosphorylation and antagonizes processive TCR signal transduction.
Galectin-1 is an endogenous lectin with known T cell immunoregulatory activity, though the molecular basis by which galectin-1 influences Ag specific T cell responses has not been elucidated. Here, we characterize the ability of galectin-1 to modulate TCR signals and responses by T cells with well defined hierarchies of threshold requirements for signaling distinct functional responses. We demonstrate that galectin-1 antagonizes TCR responses known to require costimulation and processive protein tyrosine phosphorylation, such as IL-2 production, but is permissive for TCR responses that only require partial TCR signals, such as IFN-gamma production, CD69 up-regulation, and apoptosis. Galectin-1 binding alone or together with Ag stimulation induces partial phosphorylation of TCR-zeta and the generation of inhibitory pp21zeta. Galectin-1 antagonizes Ag induced signals and TCR/costimulator dependent lipid raft clustering at the TCR contact site. We propose that galectin-1 functions as a T cell "counterstimulator" to limit required protein segregation and lipid raft reorganization at the TCR contact site and, thus, processive and sustained TCR signal transduction. These findings support the concept that TCR antagonism can arise from the generation of an inhibitory pp21zeta-based TCR signaling complex. Moreover, they demonstrate that TCR antagonism can result from T cell interactions with a ligand other than peptide/MHC. (+info)
(8/3250) Arsenite induces apoptosis of murine T lymphocytes through membrane raft-linked signaling for activation of c-Jun amino-terminal kinase.
Because of its dual roles in acute toxicity and in therapeutic application in cancer treatment, arsenic has recently attracted a renewed attention. In this study, we report NaAsO(2)-induced signal cascades from the cell surface to the nucleus of murine thymic T lymphocytes that involve membrane rafts as an initial signal transducer. NaAsO(2) induced apoptosis through fragmentation of DNA, activation of caspase, and reciprocal regulation of Bcl-2/Bax with the concomitant reduction of membrane potential. We demonstrated that NaAsO(2)-induced caspase activation is dependent on curcumin-sensitive c-Jun amino-terminal kinase and barely dependent on SB203580-sensitive p38 kinase or PD98059-sensitive extracellular signal-regulated kinase. Additionally, staurosporine, which severely inhibited the activation of mitogen-activated protein (MAP) family kinases and c-Jun, partially blocked the NaAsO(2)-mediated signal for poly(ADP-ribose) polymerase (PARP) degradation. Potentially as the initial cell surface event for intracellular signaling, NaAsO(2) induced aggregation of GPI-anchored protein Thy-1 and superoxide production. This Thy-1 aggregation and subsequent activation of MAP family kinase and c-Jun and the degradation of PARP induced by NaAsO(2) were all inhibited by DTT, suggesting the requirement of interaction between arsenic and protein sulfhydryl groups for those effects. beta cyclodextrin, which sequestrates cholesterol from the membrane rafts, inhibited NaAsO(2)-induced activation of protein tyrosine kinases and MAP family kinases, degradation of PARP, and production of superoxide. In addition, beta cyclodextrin dispersed NaAsO(2)-induced Thy-1 clustering. These results suggest that a membrane raft integrity-dependent cell surface event is a prerequisite for NaAsO(2)-induced protein tyrosine kinase/c-Jun amino-terminal kinase activation, superoxide production, and downstream caspase activation. (+info)