Inducible expression of hyperactive Syk in B cells activates Blimp-1-dependent terminal differentiation

The non-receptor protein tyrosine kinase Syk (spleen tyrosine kinase) is an important mediator of signal transduction in B cells. By acting downstream of the B-cell antigen receptor, Syk promotes signaling pathways involved in proliferation, differentiation and survival of B cells. To study the oncogenic potential of Syk, we generated a mouse model for the inducible expression of the leukemia-derived TEL-Syk fusion protein exhibiting constitutive kinase activity. To achieve B-cell-specific expression of TEL-Syk in adult mice, we used a tamoxifen-inducible Cre mouse line. This study shows that inducible expression of TEL-Syk in B cells leads to transient proliferation and subsequent plasma cell differentiation. However, it does not lead to B-cell transformation. Instead, Syk activation induces the tumor suppressor B-lymphocyte-induced maturation protein-1 (Blimp-1), which interferes with the expression of the antiapoptotic protein Bcl-2. Combined induction of TEL-Syk with transgenic expression of Bcl-2 results in a severe phenotype and plasma cell expansion. Our results suggest that deregulated Syk activity by itself is not sufficient for the transformation of B cells, as downstream effectors, such as Blimp-1, limit the survival and expansion of the activated B cell.

Isolation of the first metallo-beta-lactamase producing Klebsiella pneumoniae in Lebanon

INTRODUCTION

A 58 year-old man was admitted to the Saint Joseph Hospital-Raymond and Aida Najjar polyclinic in Beirut on July 17, 2007 to undergo surgery for a moderately differentiated colonic adenocarcinoma (T3N0). Following several discharges and re-admissions, an extended spectrum beta-lactamase (ESBL) producing Escherichia coli susceptible to imipenem was isolated. The patient was put on imipenem and metronidazole. Three weeks later, imipenem (IMP) resistant Klebsiella pneumoniae was isolated.

METHODS AND RESULTS

The antimicrobial susceptibility profile of the imipenem-resistant Klebsiella pneumoniae strain and related minimum inhibitory concentrations of antibiotics were determined. Hydrolysis of IMP was evaluated and production of metallo-beta-lactamase (MBL) was detected by a double disk-synergy test, ethylene diamine tetraacetic acid (EDTA) inhibited the imipenemase activity, whereas clavulanate and tazobactam did not, this suggesting the production of a metallo-beta-lactamase. Isoelectric focusing analysis was performed and indicated the presence of a cefotaximase (blaCTX-M-15). Polymerase chain reaction (PCR) was used and detected the presence of blaIMP-1 and blaCTX-M genes.

CONCLUSIONS

During the last decade, many hospital outbreaks caused by ESBL-producing Enterobacteriaceae spp. have been reported in Lebanon. To our knowledge, this is the first report of a clinical isolate of K. pneumoniae producing an MBL in Lebanon.

Testing gene function early in the B cell lineage in mb1-cre mice

The mb1 gene encodes the Ig-alpha signaling subunit of the B cell antigen receptor and is expressed exclusively in B cells beginning at the very early pro-B cell stage in the bone marrow. We examine here the efficacy of the mb1 gene as a host locus for cre recombinase expression in B cells. We show that by integrating a humanized cre recombinase into the mb1 locus we obtain extraordinarily efficient recombination of loxP sites in the B cell lineage. The results from a variety of reporter genes including the splicing factor SRp20 and the DNA methylase Dnmt1 suggest that mb1-cre is probably the best model so far described for pan-B cell-specific cre expression. The availability of a mouse line with efficient cre-mediated recombination at an early developmental stage in the B lineage provides an opportunity to study the role of various genes specifically in B cell development and function.

Chloroplast import of four carotenoid biosynthetic enzymes in vitro reveals differential fates prior to membrane binding and oligomeric assembly

The precursor proteins of the carotenogenic enzymes geranylgeranyl diphosphate synthase, phytoene synthase, phytoene desaturase and lycopene cyclase were imported into isolated pea chloroplasts. Geranylgeranyl diphosphate synthase remained soluble in the stroma in a free form and phytoene synthase associated to thylakoid membranes upon import, both as expected. Surprisingly, phytoene desaturase and lycopene cyclase, which strongly depend on membrane association for enzymatic activity, also remained soluble in the chloroplast stroma. The soluble forms of these enzymes were, however, still competent for membrane-association, e.g. with protein-free liposomal membranes. Indeed the soluble forms of phytoene synthase, phytoene desaturase and lycopene cyclase occurred as ATP- and cold-sensitive high-molecular-mass complexes. Gel-filtration experiments and blue native-PAGE plus autoradiography and western blot analysis indicated a participation of the chloroplast 60-kDa chaperonin (Cpn60) in the soluble high-molecular-mass complexes of imported carotenogenic enzymes. Finally, it was inferred that a membrane-bound regulatory factor plays a decisive role in membrane-binding.