The triggering receptor expressed on myeloid cells 2 (TREM-2) delivers intracellular signals through the adaptor DAP12 to regulate myeloid cell function both within and outside the immune system. The role of TREM-2 in immunity has been obscured by the failure to detect expression of the TREM-2 protein in vivo. In this study, we show that TREM-2 is expressed on macrophages infiltrating the tissues from the circulation and that alternative activation with IL-4 can induce TREM-2. TREM-2 expression is abrogated by macrophage maturation with LPS of IFN-γ. Using TREM-2−/− mice, we find that TREM-2 functions to inhibit cytokine production by macrophages in response to the TLR ligands LPS, zymosan, and CpG. Furthermore, we find that TREM-2 completely accounts for the increased cytokine production previously reported by DAP12−/− macrophages. Taken together, these data show that TREM-2 is expressed on newly differentiated and alternatively activated macrophages and functions to restrain macrophage activation.

TREM proteins are a family of cell surface receptors that participate in diverse cell processes, including inflammation, bone homeostasis, neurological development and coagulation. TREM-1, the first to be identified, acts to amplify inflammation. Other TREM proteins regulate the differentiation and function of macrophages, microglia, dendritic cells, osteoclasts and platelets. Here we discuss the state of the field, putative ligands of TREM proteins and the challenges that remain in understanding TREM biology.

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Background Burnout is a complex syndrome of emotional distress that can disproportionately affect individuals who work in healthcare professions. Study Design For a national survey of burnout in US general surgery residents, we asked all Accreditation Council for Graduate Medical Education-accredited general surgery program directors to email their general surgery residents an invitation to complete an anonymous, online survey. Burnout was assessed with the Maslach Burnout Inventory; total scores for Emotional Exhaustion (EE), Depersonalization (DP), and Personal Accomplishment (PA) subscales were calculated. Burnout was defined as having a score in the highest tertile for EE or DP or lowest tertile for PA. Chi-square tests and one-way analyses of variance were used to test associations between burnout tertiles for each subscale and various resident and training-program characteristics as appropriate. Results From April–December, 2014, 665 residents actively engaged in clinical training had data for analysis; 69% met the criterion for burnout on at least one subscale. Higher burnout on each subscale was reported by residents planning private practice compared with academic careers. A greater proportion of women than men reported burnout on EE and PA. Higher burnout on EE and DP was associated with greater work hours per week. Having a structured mentoring program was associated with lower burnout on each subscale. Conclusions The high rates of burnout among general surgery residents are concerning given the potential impact of burnout on the quality of patient care. Efforts to identify at-risk populations and to design targeted interventions to mitigate burnout in surgical trainees are warranted.

Macrophage colony stimulating factor (MCSF) influences proliferation and survival of mononuclear phagocytes through the CSF-1 receptor. The DAP12 adaptor protein, which transduces signals emanating from various myeloid receptors, is critical for mononuclear phagocyte function. DAP12-mutant mice and humans show defects in osteoclasts and microglia and exhibit brain and bone abnormalities. Here, we demonstrated that DAP12 deficiency impairs MCSF-induced macrophage proliferation and survival in vitro. In addition, DAP12-deficient mice show fewer microglia in defined central nervous system areas, and DAP12-deficient progenitors regenerate myeloid cells inefficiently following BM transplantation. MCSF-CSF1-R signaling induced stabilization and nuclear translocation of β-catenin, which activates cell cycle genes. DAP12 was essential for phosphorylation and nuclear accumulation of β-catenin. These results outline a mechanistic explanation for the multiple defects in DAP12-deficient mononuclear phagocytes.

When associated with different receptors, the signalling adaptor DAP12 has been shown to both potentiate and attenuate the activation of leukocytes. But how can a protein with a single signalling motif elicit qualitatively different cellular responses? We describe a model of DAP12 function, whereby the quality of the cellular response (activation or inhibition) is modulated by the avidity of the interaction between the DAP12-associated receptor and its ligand. This model extends from previous studies of inhibitory signalling mediated by other adaptors, such as the Fc-receptor gamma-chain and CD3zeta, and provides a potential mechanism for the conflicting phenotypes observed in studies of DAP12-deficient mice.

The topology of most eukaryotic polytopic membrane proteins is established cotranslationally in the endoplasmic reticulum (ER) through a series of coordinated translocation and membrane integration events. For the human aquaporin water channel AQP1, however, the initial four-segment-spanning topology at the ER membrane differs from the mature six-segment-spanning topology at the plasma membrane. Here we use epitope-tagged AQP1 constructs to follow the transmembrane (TM) orientation of key internal peptide loops in Xenopus oocyte and cell-free systems. This analysis revealed that AQP1 maturation in the ER involves a novel topological reorientation of three internal TM segments and two peptide loops. After the synthesis of TMs 4-6, TM3 underwent a 180-degree rotation in which TM3 C-terminal flanking residues were translocated from their initial cytosolic location into the ER lumen and N-terminal flanking residues underwent retrograde translocation from the ER lumen to the cytosol. These events convert TM3 from a type I to a type II topology and reposition TM2 and TM4 into transmembrane conformations consistent with the predicted six-segment-spanning AQP1 topology. AQP1 topological reorientation was also associated with maturation from a protease-sensitive conformation to a protease-resistant structure with water channel function. These studies demonstrate that initial protein topology established via cotranslational translocation events in the ER is dynamic and may be modified by subsequent steps of folding and/or maturation.

The incidence and mortality of sepsis increase with age, consequently, 80% of the clinical mortality from sepsis occurs in patients over age 65. Despite this aged clinical population, most research models of sepsis use 6- to 16-week-old mice as patient surrogates. This age range of mice corresponds to human ages 10 to 17 years. To assess the influence of age on rodent CLP and on antibiotic therapy, we studied young (4 month), mature (12 month), and aged (24 month) mice. Male C57BL/6 mice (n = 27-30 in each age group) were subjected to cecal ligation and puncture (CLP), two punctures with a 25-gauge needle. Mice were observed untreated for 10 days. Young mice had 20% mortality, mature mice had 70% mortality (P = 0.0013 vs. young), and aged mice had 75% mortality (P = 0.0001 vs. young). To assess the effects of age on antibiotic therapy, mice were subjected to CLP as above (n = 38-40 in each age group). Mice were then randomized to treatment with intraperitoneal injections of ceftriaxone and metronidazole or normal saline. Therapy was initiated 12 h after CLP, and injections were repeated every 12 h for 7 days. Young mice saw a 56% decrease in mortality from CLP with antibiotic therapy (P = 0.001), and mature mice had a 30% decrease in mortality (P = 0.06). Aged mice saw no benefit from antibiotic therapy. We also compared plasma cytokine levels between young and aged mice after CLP. When compared with young mice, aged mice had higher levels of IL-6 and TNF-alpha 24 h after CLP. However, high IL-6 was predictive of mortality at any age. Mice appear to have age-dependent responses to intra-abdominal sepsis and to appropriate therapy.