Nedd4-2 functionally interacts with ClC-5: involvement in constitutive albumin endocytosis in proximal tubule cells

Constitutive albumin uptake by the proximal tubule is achieved by a receptor-mediated process in which the Cl(-) channel, ClC-5, plays an obligate role. Here we investigated the functional interaction between ClC-5 and ubiquitin ligases Nedd4 and Nedd4-2 and their role in albumin uptake in opossum kidney proximal tubule (OK) cells. In vivo immunoprecipitation using an anti-HECT antibody demonstrated that ClC-5 bound to ubiquitin ligases, whereas glutathione S-transferase pull-downs confirmed that the C terminus of ClC-5 bound both Nedd4 and Nedd4-2. Nedd4-2 alone was able to alter ClC-5 currents in Xenopus oocytes by decreasing cell surface expression of ClC-5. In OK cells, a physiological concentration of albumin (10 mug/ml) rapidly increased cell surface expression of ClC-5, which was also accompanied by the ubiquitination of ClC-5. Albumin uptake was reduced by inhibiting either the lysosome or proteasome. Total levels of Nedd4-2 and proteasome activity also increased rapidly in response to albumin. Overexpression of ligase defective Nedd4-2 or knockdown of endogenous Nedd4-2 with small interfering RNA resulted in significant decreases in albumin uptake. In contrast, pathophysiological concentrations of albumin (100 and 1000 mug/ml) reduced the levels of ClC-5 and Nedd4-2 and the activity of the proteasome to the levels seen in the absence of albumin. These data demonstrate that normal constitutive uptake of albumin by the proximal tubule requires Nedd4-2, which may act via ubiquitination to shunt ClC-5 into the endocytic pathway.     

The Rac1 C-terminal polybasic region regulates the nuclear localization and protein degradation of Rac1

We observed evolutionary conservation of canonical nuclear localization signal sequences (K(K/R)X(K/R)) in the C-terminal polybasic regions (PBRs) of some Rac and Rho isoforms. Canonical D-box sequences (RXXL), which target proteins for proteasome-mediated degradation, are also evolutionarily conserved near the PBRs of these small GTPases. We show that the Rac1 PBR (PVKKRKRK) promotes Rac1 nuclear accumulation, whereas the RhoA PBR (RRGKKKSG) keeps RhoA in the cytoplasm. A mutant Rac1 protein named Rac1 (pbrRhoA), in which the RhoA PBR replaces the Rac1 PBR, has greater cytoplasmic localization, enhanced resistance to proteasome-mediated degradation, and higher protein levels than Rac1. Mutating the D-box by substituting alanines at amino acids 174 and 177 significantly increases the protein levels of Rac1 but not Rac1(pbrRhoA). These results suggest that Rac1 (pbrRhoA) is more resistant than Rac1 to proteasome-mediated degradative pathways involving the D-box. The cytoplasmic localization of Rac1(pbrRhoA) provides the most obvious reason for its resistance to proteasome-mediated degradation, because we show that Rac1(pbrRhoA) does not greatly differ from Rac1 in its ability to stimulate membrane ruffling or to interact with SmgGDS and IQGAP1-calmodulin complexes. These findings support the model that nuclear localization signal sequences in the PBR direct Rac1 to the nucleus, where Rac1 participates in signaling pathways that ultimately target it for degradation.     

Visible fluorescent detection of proteins in polyacrylamide gels without staining

2,2,2-Trichloroethanol (TCE) incorporated into polyacrylamide gels before polymerization provides fluorescent visible detection of proteins in less than 5min of total processing time. The tryptophans in proteins undergo an ultraviolet light-induced reaction with trihalocompounds to produce fluorescence in the visible range so that the protein bands can be visualized on a 300-nm transilluminator. In a previous study trichloroacetic acid or chloroform was used to stain polyacrylamide gel electrophoresis (PAGE) gels for protein visualization. This study shows that placing TCE in the gel before electrophoresis can eliminate the staining step. The gel is removed from the electrophoresis apparatus and placed on a transilluminator and then the protein bands develop their fluorescence in less than 5min. In addition to being rapid this visualization method provides detection of 0.2microg of typical globular proteins, which for some proteins is slightly more sensitive than the standard Coomassie brilliant blue (CBB) method. Integral membrane proteins, which do not stain well with CBB, are visualized well with the TCE in-gel method. After TCE in-gel visualization the same gel can then be CBB stained, allowing for complementary detection of proteins. In addition, visualization with TCE in the gel is compatible with two-dimensional PAGE, native PAGE, Western blotting, and autoradiography.     

Phosphorylation of human vitamin D receptor serine-182 by PKA suppresses 1,25(OH)2D3-dependent transactivation

The human vitamin D receptor (hVDR), which is a substrate for several protein kinases, mediates the actions of its 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) ligand to regulate gene expression. To determine the site, and functional impact, of cAMP-dependent protein kinase (PKA)-catalyzed phosphorylation of hVDR, we generated a series of C-terminally truncated and point mutant receptors. Incubation of mutant hVDRs with PKA and [gamma-32P]ATP, in vitro, or overexpressing them in COS-7 kidney cells labeled with [32P]orthophosphate, revealed that serine-182 is the predominant residue in hVDR phosphorylated by PKA. An aspartate substituted mutant (S182D), incorporating a negative charge to mimic phosphorylation, displayed only 50% of the transactivation capacity in response to 1,25(OH)2D3 of either wild-type or an S182A-altered hVDR. When the catalytic subunit of PKA was overexpressed, a similar reduction in wild-type but not S182D hVDR transactivity was observed. In a mammalian two-hybrid system, S182D bound less avidly than wild-type or S182A hVDR to the retinoid X receptor (RXR) heterodimeric partner that co-mediates vitamin D responsive element recognition and transactivation. These data suggest that hVDR serine-182 is a primary site for PKA phosphorylation, an event that leads to an attenuation of both RXR heterodimerization and resultant transactivation of 1,25(OH)2D3 target genes.     

Effects of glucocorticoids on the respiratory burst of Chlamydia-primed THP-1 cells

We previously observed that the respiratory burst of human monocytes (THP-1 cell line) triggered by phorbol myristate acetate was strongly enhanced by a priming of the cells by Chlamydia pneumoniae [Biochem. Biophys. Res. Commun. 287 (2001) 781]. We describe here the modifications of the responses of Chlamydia-primed THP-1 cells to hydrocortisone (HCT) and methylprednisolone (MPL). HCT and MPL inhibited the production of the cytokines TNFα and IL-8. But HCT, which inhibited the respiratory burst in LPS-primed monocytes, paradoxically stimulated the phenomenon in Chlamydia-primed cells; MPL exerted no significant effect. Both glucocorticoids did not significantly modify the triggering effect of Chlamydia on NF-κB binding activity. On the expression of p22^phox, a protein subunit of the NADPH oxidase, HCT had an increasing and MPL a decreasing effect. Glucocorticoids thus had unexpected effects on the inflammatory response of Chlamydia-primed monocytes.     

Phosphorylation of H2AX at short telomeres in T cells and fibroblasts

Eukaryotic cells undergo arrest and enter apoptosis in response to short telomeres. T cells from late generation mTR(-/-) mice that lack telomerase show increased apoptosis when stimulated to enter the cell cycle. The increased apoptosis was not inhibited by colcemid, indicating that the response did not result from breakage of dicentric chromosomes at mitosis. The damage response protein gamma-H2AX localized to telomeres in metaphases from T cells and fibroblasts from mTR(-/-) cells with short telomeres. These data suggest that the major mechanism for induction of apoptosis in late generation mTR(-/-) cells is independent of chromosome segregation and that loss of telomere function through progressive telomere shortening in the absence of telomerase leads to recognition of telomeres as DNA breaks.     

Quantitative kinetic analysis of nucleolar breakdown and reassembly during mitosis in live human cells

One of the great mysteries of the nucleolus surrounds its disappearance during mitosis and subsequent reassembly at late mitosis. Here, the relative dynamics of nucleolar disassembly and reformation were dissected using quantitative 4D microscopy with fluorescent protein-tagged proteins in human stable cell lines. The data provide a novel insight into the fates of the three distinct nucleolar subcompartments and their associated protein machineries in a single dividing cell. Before the onset of nuclear envelope (NE) breakdown, nucleolar disassembly started with the loss of RNA polymerase I subunits from the fibrillar centers. Dissociation of proteins from the other subcompartments occurred with faster kinetics but commenced later, coincident with the process of NE breakdown. The reformation pathway also follows a reproducible and defined temporal sequence but the order of reassembly is shown not to be dictated by the order in which individual nucleolar components reaccumulate within the nucleus after mitosis.     

Persistence of apoptotic cells without autoimmune disease or inflammation in CD14-/- mice

Interaction of macrophages with apoptotic cells involves multiple steps including recognition, tethering, phagocytosis, and anti-inflammatory macrophage responses. Defective apoptotic cell clearance is associated with pathogenesis of autoimmune disease. CD14 is a surface receptor that functions in vitro in the removal of apoptotic cells by human and murine macrophages, but its mechanism of action has not been defined. Here, we demonstrate that CD14 functions as a macrophage tethering receptor for apoptotic cells. Significantly, CD14(-/-) macrophages in vivo are defective in clearing apoptotic cells in multiple tissues, suggesting a broad role for CD14 in the clearance process. However, the resultant persistence of apoptotic cells does not lead to inflammation or increased autoantibody production, most likely because, as we show, CD14(-/-) macrophages retain the ability to generate anti-inflammatory signals in response to apoptotic cells. We conclude that CD14 plays a broad tethering role in apoptotic cell clearance in vivo and that apoptotic cells can persist in the absence of proinflammatory consequences.     

Ionophore-resistant mutant of Toxoplasma gondii reveals involvement of a sodium/hydrogen exchanger in calcium regulation

Calcium is a critical mediator of many intracellular processes in eukaryotic cells. In the obligate intracellular parasite Toxoplasma gondii, for example, a rise in [Ca2+] is associated with significant morphological changes and rapid egress from host cells. To understand the mechanisms behind such dramatic effects, we isolated a mutant that is altered in its responses to the Ca2+ ionophore A23187 and found the affected gene encodes a homologue of Na+/H+ exchangers (NHEs) located on the parasite's plasma membrane. We show that in the absence of TgNHE1, Toxoplasma is resistant to ionophore-induced egress and extracellular death and amiloride-induced proton efflux inhibition. In addition, the mutant has increased levels of intracellular Ca2+, which explains its decreased sensitivity to A23187. These results provide direct genetic evidence of a role for NHE1 in Ca2+ homeostasis and important insight into how this ubiquitous pathogen senses and responds to changes in its environment.     

A peptide that antagonizes TCR-mediated reactions with both syngeneic and allogeneic agonists: functional and structural aspects

We identify and consider some characteristics of a peptide antagonist for the Ag-specific receptor on 2C cells (the 2C TCR). The peptide, GNYSFYAL (called GNY), binds to H-2K(b), and a very high-resolution crystal structure of the GNY-K(b) complex at 1.35 A is described. Although the GNY peptide does not bind to L(d), the potency of GNY-K(b) as an antagonist is evident from its ability to specifically inhibit 2C TCR-mediated reactions to an allogenic agonist complex (QLSPFPFDL-L(d)), as well as to a syngeneic agonist complex (SIYRYYGL-K(b)). The crystal structure and the activities of alanine-substituted peptide variants point to the properties of the peptide P4 side chain and the conformation of the Tyr-P6 side chain as the structural determinants of GNYSFYAL antagonist activity.