Klhl6 Deficiency Impairs Transitional B Cell Survival and Differentiation.

Klhl6 belongs to the KLHL gene family, which is composed of an N-terminal BTB-POZ domain and four to six Kelch motifs in tandem. Several of these proteins function as adaptors of the Cullin3 E3 ubiquitin ligase complex. In this article, we report that Klhl6 deficiency induces, as previously described, a 2-fold reduction in mature B cells. However, we find that this deficit is centered on the inability of transitional type 1 B cells to survive and to progress toward the transitional type 2 B cell stage, whereas cells that have passed this step generate normal germinal centers (GCs) upon a T-dependent immune challenge. Klhl6-deficient type 1 B cells showed a 2-fold overexpression of genes linked with cell proliferation, including most targets of the anaphase-promoting complex/cyclosome complex, a set of genes whose expression is precisely downmodulated upon culture of splenic transitional B cells in the presence of BAFF. These results thus suggest a delay in the differentiation process of Klhl6-deficient B cells between the immature and transitional stage. We further show, in the BL2 Burkitt's lymphoma cell line, that KLHL6 interacts with Cullin3, but also that it binds to HBXIP/Lamtor5, a protein involved in cell-cycle regulation and cytokinesis. Finally, we report that KLHL6, which is recurrently mutated in B cell lymphomas, is an off-target of the normal somatic hypermutation process taking place in GC B cells in both mice and humans, thus leaving open whether, despite the lack of impact of Klhl6 deficiency on GC B cell expansion, mutants could contribute to the oncogenic process.

First-principles many-body study of the electronic and optical properties of CsK2Sb, a semiconducting material for ultra-bright electron sources.

We present a comprehensive first-principles investigation of the electronic and optical properties of CsK2Sb, a semiconducting material for ultra-bright electron sources for particle accelerators. Our study, based on density-functional theory and many-body perturbation theory, provides all the ingredients to model the emission of this material as a photocathode, including band gap, band dispersion, and optical absorption. An accurate description of these properties beyond the mean-field picture is relevant to take into account many-body effects. We discuss our results in the context of state-of-the-art electron sources for particle accelerators to set the stage towards improved modeling of quantum efficiency, intrinsic emittance, and other relevant quantities determining the macroscopic characteristics of photocathodes for ultra-bright beams.

Electronic structure and core electron fingerprints of caesium-based multi-alkali antimonides for ultra-bright electron sources.

The development of novel photocathode materials for ultra-bright electron sources demands efficient and cost-effective strategies that provide insight and understanding of the intrinsic material properties given the constraints of growth and operational conditions. To address this question, we propose a viable way to establish correlations between calculated and measured data on core electronic states of Cs-K-Sb materials. To do so, we combine first-principles calculations based on all-electron density-functional theory on the three alkali antimonides Cs3Sb, Cs2KSb, and CsK2Sb with x-ray photoemission spectroscopy (XPS) on Cs-K-Sb photocathode samples. Within the GW approximation of many-body perturbation theory, we obtain quantitative predictions of the band gaps of these materials, which range from 0.57 eV in Cs2KSb to 1.62 eV in CsK2Sb and manifest direct or indirect character depending on the relative potassium content. Our theoretical electronic-structure analysis also reveals that the core states of these systems have binding energies that depend only on the atomic species and their crystallographic sites, with largest shifts of the order of 2 eV and 0.5 eV associated to K 2p and Sb 3d states, respectively. This information can be correlated to the maxima in the XPS survey spectra, where such peaks are clearly visible. In this way, core-level shifts can be used as fingerprints to identify specific compositions of Cs-K-Sb materials and their relation with the measured values of quantum efficiency. Our results represent the first step towards establishing a robust connection between the experimental preparation and characterization of photocathodes, the ab initio prediction of their electronic structure, and the modeling of emission and beam formation processes.

The adaptor protein CIN85 assembles intracellular signaling clusters for B cell activation.

The adaptor molecule Cbl-interacting protein of 85 kD (CIN85) regulates signaling from a number of cell surface receptors, such as growth factor receptors and antigen receptors on lymphocytes. Because of its multidomain structure, CIN85 is thought to act as a classical adaptor protein that connects functionally distinct components of a given signaling pathway through diverse protein domains. However, we found that in B lymphocytes, CIN85 functions to oligomerize SLP-65, which is the central effector protein of the B cell receptor (BCR). Therefore, CIN85 trimerizes through a carboxyl-terminal, coiled-coil domain. The multiple Src homology 3 (SH3) domains of trimeric CIN85 molecules associated with multiple SLP-65 molecules, which recruited further CIN85 trimers, thereby perpetuating the oligomerization process. Formation of this oligomeric signaling complex in resting B cells rendered the cells poised for the efficient initiation of intracellular signaling upon BCR stimulation. Our data suggest that the functionality of signaling cascades does not rely solely on the qualitative linkage of their various components but requires a critical number of effectors to become concentrated in signaling complexes.

Macromolecular assembly of the adaptor SLP-65 at intracellular vesicles in resting B cells.

The traditional view of how intracellular effector proteins are recruited to the B cell antigen receptor (BCR) complex at the plasma membrane is based on the occurrence of direct protein-protein interactions, as exemplified by the recruitment of the tyrosine kinase Syk (spleen tyrosine kinase) to phosphorylated motifs in BCR signaling subunits. By contrast, the subcellular targeting of the cytosolic adaptor protein SLP-65 (Src homology 2 domain-containing leukocyte adaptor protein of 65 kD), which serves as a proximal Syk substrate, is unclear. We showed that SLP-65 activation required its association at vesicular compartments in resting B cells. A module of ~50 amino acid residues located at the amino terminus of SLP-65 anchored SLP-65 to the vesicles. Nuclear magnetic resonance spectroscopy showed that the SLP-65 amino terminus was structurally disordered in solution but could bind in a structured manner to noncharged lipid components of cellular membranes. Our finding that preformed vesicular signaling scaffolds are required for B cell activation indicates that vesicles may deliver preassembled signaling cargo to sites of BCR activation.