Super-resolution characterization of TCR-dependent signaling clusters

Multi-molecular signaling complexes drive the earliest events of immune cell activation via immunoreceptors with unexplained specificity and speed. Fluorescence microscopy has shown that these complexes form microclusters at the plasma membrane of activated T cells upon engagement of their antigen receptors (TCRs). Although crucial for cell function, much remains to be learned about the molecular content, fine structure, formation mechanisms, and function of these microclusters. Recent advancements in super-resolution microscopy have enabled the study of signaling microclusters at the single molecule level with resolution down to approximately 20 nm. These techniques have now helped to characterize the size distributions of signaling clusters at the plasma membrane of intact cells and to shed light on the formation mechanisms that govern their assembly. Surprisingly, dynamic and functional nanostructures have been identified within the signaling clusters. We expect that these novel methodologies, combined with older techniques, will shed new light on the nature of signaling clusters and their critical role in T-cell activation.

GTP-binding protein-like domain of AGAP1 is protein binding site that allosterically regulates ArfGAP protein catalytic activity

AGAPs are a subtype of Arf GTPase-activating proteins (GAPs) with 11 members in humans. In addition to the Arf GAP domain, the proteins contain a G-protein-like domain (GLD) with homology to Ras superfamily proteins and a PH domain. AGAPs bind to clathrin adaptors, function in post Golgi membrane traffic, and have been implicated in glioblastoma. The regulation of AGAPs is largely unexplored. Other enzymes containing GTP binding domains are regulated by nucleotide binding. However, nucleotide binding to AGAPs has not been detected. Here, we found that neither nucleotides nor deleting the GLD of AGAP1 affected catalysis, which led us to hypothesize that the GLD is a protein binding site that regulates GAP activity. Two-hybrid screens identified RhoA, Rac1, and Cdc42 as potential binding partners. Coimmunoprecipitation confirmed that AGAP1 and AGAP2 can bind to RhoA. Binding was mediated by the C terminus of RhoA and was independent of nucleotide. RhoA and the C-terminal peptide from RhoA increased GAP activity specifically for the substrate Arf1. In contrast, a C-terminal peptide from Cdc42 neither bound nor activated AGAP1. Based on these results, we propose that AGAPs are allosterically regulated through protein binding to the GLD domain.

Functional nanoscale organization of signaling molecules downstream of the T cell antigen receptor

Receptor-regulated cellular signaling often is mediated by formation of transient, heterogeneous protein complexes of undefined structure. We used single and two-color photoactivated localization microscopy to study complexes downstream of the T cell antigen receptor (TCR) in single-molecule detail at the plasma membrane of intact T cells. The kinase ZAP-70 distributed completely with the TCRζ chain and both partially mixed with the adaptor LAT in activated cells, thus showing localized activation of LAT by TCR-coupled ZAP-70. In resting and activated cells, LAT primarily resided in nanoscale clusters as small as dimers whose formation depended on protein-protein and protein-lipid interactions. Surprisingly, the adaptor SLP-76 localized to the periphery of LAT clusters. This nanoscale structure depended on polymerized actin and its disruption affected TCR-dependent cell function. These results extend our understanding of the mechanism of T cell activation and the formation and organization of TCR-mediated signaling complexes, findings also relevant to other receptor systems.

CD28 engagement promotes actin polymerization through the activation of the small Rho GTPase Cdc42 in human T cells

Engagement of the costimulatory molecule CD28 is an important step in the optimal activation of T cells. Nevertheless, the specific role of CD28 in the formation of the immunological synapse and cytoskeletal changes that occur upon TCR/CD3 complex engagement is still poorly understood. Using Ab-coated surfaces, we show that CD28 engagement in the absence of any other signal induced the formation of cytoplasmic elongations enriched in filamentous actin (F-actin), in this work called filopodia or microspikes. Such structures were specific for engagement of CD28 on mAb-coated surfaces because they could not be observed in surfaces coated with either poly(L-lysine) or anti-CD3 mAb. The signaling pathway coupling CD28 to cytoskeletal rearrangements required Src-related kinase activity and promoted Vav phosphorylation and Cdc42 activation independently of the zeta-chain-associated kinase (ZAP-70). CD28-induced filopodia required Cdc42 GTPase activity, but not the related Rho GTPase Rac1. Moreover, Cdc42 colocalized to areas of increased F-actin. Our results support a specific role for the activation of the small Rho GTPase Cdc42 in the actin reorganization mediated by CD28 in human T cells.

Intramuscular tenotomy of flexor digitorum superficialis in the distal forearm after surgical excision of dupuytren's disease

Contracture of the proximal interphalangeal joint after surgery to excise Dupuytren's disease, despite release of the contributory structures within the finger, can be caused by flexor digitorum superficialis (FDS) contracture. We describe five cases where FDS contracture was released by intramuscular tenotomy in the distal forearm. Standard postoperative therapy for Dupuytren's fasciectomy was used and clinical review showed improved finger extension with no loss of strength. We suggest that intramuscular tenotomy of FDS in the forearm can be used safely where indicated after excision of the Dupuytren's disease.

Arthrodesis of the knee using a custom-made intramedullary coupled device

Nine patients underwent arthrodesis of the knee using customised coupled nail (the Mayday arthrodesis nail), five after infected arthroplasty, one following failed arthrodesis, one for intractable anterior knee pain, one for Charcot instability and one after trauma. Comparison was made with 17 arthrodeses, eight undertaken using external fixation, four with dual compression plates, and five with long Kütntscher nails. Union was achieved in all patients (100%) at a mean time of ten months using the customised implant. There were no complications despite early weight-bearing. No further procedures were required. This contrasted with a rate of union of 53% and a complication rate of 76% with alternative techniques. Of this second group, 76% required a further operative procedure. We compared the Mayday arthrodesis nail with other techniques of arthrodesis of the knee. The differences in the need for further surgery and occurrence of complications were statistically significant (p < 0.001), and differences in the rate of nonunion and inpatient stay of less than three weeks were also significant (p < 0.05) using Fisher's exact test. We conclude that a customised coupled intramedullary nail can give excellent stability allowing early weight-bearing, and results in a high rate of union with minimal postoperative complications.

Reduced expression of insulin-like growth factor I receptors in MCF-7 breast cancer cells leads to a more metastatic phenotype

Several lines of evidence support an important role for the insulin-like growth factor system in breast cancer. Alterations in insulin-like growth factor I receptor (IGF-IR) have been associated with breast cancer metastasis; however, the specific role played by the IGF-IR in this process remains unclear. To address this issue, we evaluated MCF-7 breast cancer cells stably transfected either with an antisense construct to the IGF-IR, which reduced the expression of the IGF-IRs by approximately 50% (SX13 cells), or with the empty vector as control (NEO cells). Using functional assays for motility, attachment, and aggregation, we found a 3-fold increase in migration using both the wounding assay and the Boyden chamber migration assay. In addition, the SX13 cells attached less, and there was a reduction in cellular aggregation. These functional changes were accompanied by approximately 50% decrease in expression of E-cadherin and approximately 80% increase in p120 protein levels. Moreover, there was a significant reduction in p120 present in the E-cadherin-catenin-p120 complex. There was a 2-fold increase in active Rac1 and Cdc42 and a 35% decrease in active Rho in the SX13 cells. Our findings strongly suggest that the IGF-IR plays a role in the stabilization of the E-cadherin-catenin complex, thereby providing one possible explanation for the association between low levels of IGF-IR and a higher risk of mammary tumor metastasis.

Comparative study of "staples versus sutures" in skin closure following Dupuytren's surgery

This prospective, randomized controlled trial assessed the use of staples for closure of the palmar skin following Dupuytren's surgery. Although staples were significantly quicker to insert than sutures, patients experienced significantly more pain on removal of staples. There was no difference in the cosmetic appearance of the wound in the two groups. We recommend use of staples for palmar wound closure following long procedures.

Differential regulation of gene expression by insulin and IGF-1 receptors correlates with phosphorylation of a single amino acid residue in the forkhead transcription factor FKHR

The transcription factor FKHR is inhibited by phosphorylation in response to insulin and IGF-1 through Akt kinase. Here we show that FKHR phosphorylation in hepatocytes conforms to a hierarchical pattern in which phosphorylation of the Akt site at S(253), in the forkhead DNA binding domain, is a prerequisite for the phosphorylation of two additional potential Akt sites, T(24) and S(316). Using insulin receptor-deficient hepatocytes, we show that T(24) fails to be phosphorylated by IGF-1 receptors, suggesting that this residue is targeted by a kinase specifically activated by insulin receptors. Lack of T(24) phosphorylation is associated with the failure of IGF-1 to induce nuclear export of FKHR, and to inhibit expression of a reporter gene under the transcriptional control of the IGF binding protein-1 insulin response element. We propose that site-specific phosphorylation of FKHR is one of the mechanisms by which insulin and IGF-1 receptors exert different effects on gene expression.

Hyperleptinemia of pregnancy associated with the appearance of a circulating form of the leptin receptor

Leptin is a hormone produced in adipose cells that regulates energy expenditure, food intake, and adiposity. In mice, we observed that circulating leptin levels increase 20-40-fold during pregnancy. Pregnant ob/ob females had no detectable serum leptin, demonstrating that the heterozygous conceptus was not the source of the leptin. However, leptin RNA and protein levels in maternal adipose tissue were not elevated. The circulating leptin was in a high molecular weight complex, suggesting that the rise in leptin was due to expression of a binding protein. Indeed, quantitative assays of serum leptin binding capacity revealed a 40-fold increase, coincident with the rise in serum leptin. Leptin binding activity reached a capacity of 207 +/- 15 nmol/liter of serum at day 18 of gestation, and half-maximal binding was observed with approximately 3 nM leptin. The binding protein was purified and partially sequenced, revealing sequence identity to the extracellular domain of the leptin receptor. We found that the placenta produces large amounts of the OB-Re isoform of leptin receptor mRNA, which encodes a soluble binding protein. Thus, the extreme hyperleptinemia of late pregnancy is attributable to binding of the leptin by a secreted form of the leptin receptor made by the placenta.

Beta-lactam antibiotics increase the frequency of plasmid transfer in Staphylococcus aureus

The transfer of a plasmid specifying tetracycline resistance between different derivatives of Staphylococcus aureus by phage mediated conjugation was enhanced 100- to 1000-fold by exposure of the culture to subinhibitory concentrations of beta-lactam agents. A variety of other antibiotics, including vancomycin and teicoplanin, had no such effect. The enhanced frequency of transfer was probably due to the formation of large bacterial aggregates.

Intestinal parasites of well-cared-for dogs: an area revisited

A prior study (1970) of parasites in fecal samples of 846 dogs recovered from an area in the Edgewater and Rogers Park areas on the far North side of Chicago showed 13.7% of the fecal samples to be positive for one or more parasites. A pilot study of 200 fecal samples in the same area in 1979 revealed that 21.5% were positive. An additional 606 fecal samples were then observed for a total of 806 samples in 1979-1980. A prevalence of 23.4% was shown in the specimens collected in 1979. A study of 110 fecal samples from the Chicago dog pound revealed a prevalence of 71.8%.