The Congruency Effect in the Posterior Medial Frontal Cortex Is More Consistent with Time on Task than with Response Conflict

The posterior medial frontal cortex (pMFC) is thought to play a pivotal role in enabling the control of attention during periods of distraction. In line with this view, pMFC activity is ubiquitously greater in incongruent trials of response-interference (e.g., Stroop) tasks than in congruent trials. Nonetheless, the process underlying this congruency effect remains highly controversial. We therefore sought to distinguish between two competing accounts of the congruency effect. The conflict monitoring account posits the effect indexes a process that detects conflict between competing response alternatives, which is indexed by trial-specific reaction time (RT). The time on task account posits the effect indexes a process whose recruitment increases with time on task independent of response conflict (e.g., sustained attention, arousal, effort, etc.). To distinguish between these accounts, we used functional MRI to record brain activity in twenty-four healthy adults while they performed two tasks: a response-interference task and a simple RT task with only one possible response. We reasoned that demands on a process that detects response conflict should increase with RT in the response-interference task but not in the simple RT task. In contrast, demands on a process whose recruitment increases with time on task independent of response conflict should increase with RT in both tasks. Trial-by-trial analyses revealed that pMFC activity increased with RT in both tasks. Moreover, pMFC activity increased with RT in the simple RT task enough to fully account for the congruency effect in the response-interference task. These findings appear more consistent with the time on task account of the congruency effect than with the conflict monitoring account.     

Evolution of chromosome number in grasshoppers (Orthoptera: Caelifera: Acrididae)

Organisms Diversity & Evolution - Orthoptera have some of the largest genomes of all insects. At the same time, the architecture of their genomes remains poorly understood. Comparative...     

A membrane-proximal basic domain and cysteine cluster in the C-terminal tail of CCR5 constitute a bipartite motif critical for cell surface expression

We examined the structural requirements for cell surface expression, signaling, and human immunodeficiency virus co-receptor activity for the chemokine receptor, CCR5. Serial C-terminal truncation of CCR5 resulted in progressive loss of cell surface expression; mutants truncated at the 317th position and shorter were not detected at the cell surface. Alanine substitution of basic residues in the membrane-proximal domain (residues 314-322) in the context of a full-length C-tail resulted in severe reduction in surface expression. C-terminal truncation that excised the three cysteines in this domain reduced surface expression, but further truncation of upstream basic residue(s) abolished surface expression. Substituting the carboxyl-terminal domain of CXCR4 for that of CCR5 failed to rectify the trafficking defect of the tailless CCR5. In contrast, tailless CXCR4 or a CXCR4 chimera that exchanged the native cytoplasmic domain for that of wild type CCR5 was expressed at the cell surface. Deletion mutants that expressed at the cell surface responded to chemokine stimulation and mediated human immunodeficiency virus entry. Substitution of all serine and threonine residues in the C-terminal tail of CCR5 abolished chemokine-mediated receptor phosphorylation but preserved downstream signaling (Ca(2+) flux), while substitutions of tyrosine residues in the C-tail affected neither phenotype. CCR5 mutants that failed to traffic to the plasma membrane did not exhibit obvious changes in metabolic turnover and were retained in the Golgi or pre-Golgi compartments(s). Thus, the basic domain (-KHIAKRF-) and the cysteine cluster (-CKCC-) in the C-terminal tail of CCR5 function cooperatively for optimal surface expression.     

Assay for ubiquitin ligase activity: high-throughput screen for inhibitors of HDM2

An assay for the autoubiquitination activity of the E3 ligase HDM2 (Mdm2) was developed and adapted to a high-throughput format to identify inhibitors of this activity. The assay can also be used to measure the activity of other E3s and may be useful in finding both inhibitors and activators of a wide range of different ubiquitin ligases.     

Mechanism of prion propagation: amyloid growth occurs by monomer addition

Abundant nonfibrillar oligomeric intermediates are a common feature of amyloid formation, and these oligomers, rather than the final fibers, have been suggested to be the toxic species in some amyloid diseases. Whether such oligomers are critical intermediates for fiber assembly or form in an alternate, potentially separable pathway, however, remains unclear. Here we study the polymerization of the amyloidogenic yeast prion protein Sup35. Rapid polymerization occurs in the absence of observable intermediates, and both targeted kinetic and direct single-molecule fluorescence measurements indicate that fibers grow by monomer addition. A three-step model (nucleation, monomer addition, and fiber fragmentation) accurately accounts for the distinctive kinetic features of amyloid formation, including weak concentration dependence, acceleration by agitation, and sigmoidal shape of the polymerization time course. Thus, amyloid growth can occur by monomer addition in a reaction distinct from and competitive with formation of potentially toxic oligomeric intermediates.     

Depletion of host Langerhans cells before transplantation of donor alloreactive T cells prevents skin graft-versus-host disease

Skin is the most commonly affected organ in graft-versus-host disease (GVHD). To explore the role of Langerhans cells in GVHD, the principal dendritic cells of the skin, we studied the fate of these cells in mice transplanted with allogeneic bone marrow. In contrast to other dendritic cells, host Langerhans cells were replaced by donor Langerhans cells only when donor T cells were administered along with bone marrow, and the extent of Langerhans cell chimerism correlated with the dose of donor T cells injected. Donor T cells depleted host Langerhans cells through a Fas-dependent pathway and induced the production in skin of CCL20, which was required for the recruitment of donor Langerhans cells. Administration of donor T cells to bone marrow-chimeric mice with persistent host Langerhans cells, but not to mice whose Langerhans cells had been replaced, resulted in marked skin GVHD. These findings indicate a crucial role for donor T cells in host Langerhans cell replacement, and show that host dendritic cells can persist in nonlymphoid tissue for the duration of an animal's life and can trigger GVHD despite complete blood chimerism.     

Expression of Smad4 in the FaDu cell line partially restores TGF-beta growth inhibition but is not sufficient to regulate fibronectin expression or suppress tumorigenicity

Mutations of the Smad4 gene, a member of a group of TGF-beta signal transduction components, occur in several types of cancer suggesting that its inactivation significantly affects TGF-beta responsiveness in these tumors. To further investigate the role of Smad4 with respect to TGF-beta signaling and carcinogenesis, we re-expressed the Smad4 gene in the Smad4-deficient cancer cell line FaDu by microcell-mediated chromosome transfer (MMCT) and retroviral infection to closely approximate physiological protein levels. The Smad4-expressing FaDu clones were then evaluated for TGF-beta responsiveness to assess the role of Smad4 in TGF-beta-induced growth inhibition and target gene regulation. We found that the re-expression of the Smad4 gene by either method partially restored TGF-beta responsiveness in FaDu cells with respect to both growth inhibition and expression of p21WAF1/CIP1 and p15INK4B. However, only the microcell hybrids showed growth retardation in organotypic raft culture and an enhanced ability to upregulate fibronectin. In contrast, the re-expression of Smad4 by either method failed to suppress tumorigenicity. These results suggest that in addition to a homozygous deletion of Smad4, FaDu cells contain additional defects within the TGF-beta signaling pathway, thereby limiting the extent of TGF-beta responsiveness upon Smad4 re-expression and perhaps accounting for the inability to induce p15INK4B to a high level. They also demonstrate the advantages of providing a physiological extracellular environment, when assessing TGFbeta responsiveness.