SH2B1 enhances leptin signaling by both Janus kinase 2 Tyr813 phosphorylation-dependent and -independent mechanisms

Leptin controls body weight by activating its long form receptor (LEPRb). LEPRb binds to Janus kinase 2 (JAK2), a cytoplasmic tyrosine kinase that mediates leptin signaling. We previously reported that genetic deletion of SH2B1 (previously known as SH2-B), a JAK2-binding protein, results in severe leptin-resistant and obese phenotypes, indicating that SH2B1 is a key endogenous positive regulator of leptin sensitivity. Here we show that SH2B1 regulates leptin signaling by multiple mechanisms. In the absence of leptin, SH2B1 constitutively bound, via its non-SH2 domain region(s), to non-tyrosyl-phosphorylated JAK2, and inhibited JAK2. Leptin stimulated JAK2 phosphorylation on Tyr(813), which subsequently bound to the SH2 domain of SH2B1. Binding of the SH2 domain of SH2B1 to phospho-Tyr(813) in JAK2 enhanced leptin induction of JAK2 activity. JAK2 was required for leptin-stimulated phosphorylation of insulin receptor substrate 1 (IRS1), an upstream activator of the phosphatidylinositol 3-kinase pathway. Overexpression of SH2B1 enhanced both JAK2- and JAK2(Y813F)-mediated tyrosine phosphorylation of IRS1 in response to leptin, even though SH2B1 did not enhance JAK2(Y813F) activation. Leptin promoted the interaction of SH2B1 with IRS1. These data suggest that constitutive SH2B1-JAK2 interaction, mediated by the non-SH2 domain region(s) of SH2B1 and the non-Tyr(813) region(s) in JAK2, increases the local concentration of SH2B1 close to JAK2 and inhibits JAK2 activity. Leptin-stimulated SH2B1-JAK2 interaction, mediated by the SH2 domain of SH2B1 and phospho-Tyr(813) in JAK2, promotes JAK2 activation, thus globally enhancing leptin signaling. SH2B1-IRS1 interaction facilitates IRS1 phosphorylation by recruiting IRS1 to JAK2 and/or by protecting IRS1 from dephosphorylation, thus specifically enhancing leptin stimulation of the phosphatidylinositol 3-kinase pathway.     

The T210M Substitution in the HLA-a*02:01 gp100 Epitope Strongly Affects Overall Proteasomal Cleavage Site Usage and Antigen Processing

MHC class I-restricted epitopes, which carry a tumor-specific mutation resulting in improved MHC binding affinity, are preferred T cell receptor targets in innovative adoptive T cell therapies. However, T cell therapy requires efficient generation of the selected epitope. How such mutations may affect proteasome-mediated antigen processing has so far not been studied. Therefore, we analyzed by in vitro experiments the effect on antigen processing and recognition of a T210M exchange, which previously had been introduced into the melanoma gp100_209–217tumor epitope to improve the HLA-A*02:01 binding and its immunogenicity. A quantitative analysis of the main steps of antigen processing shows that the T210M exchange affects proteasomal cleavage site usage within the _mutgp100_201–230 polypeptide, leading to the generation of an unique set of cleavage products. The T210M substitution qualitatively affects the proteasome-catalyzed generation of spliced and non-spliced peptides predicted to bind HLA-A or -B complexes. The T210M substitution also induces an enhanced production of the _mutgp100_209–217 epitope and its N-terminally extended peptides. The T210M exchange revealed no effect on ERAP1-mediated N-terminal trimming of the precursor peptides. However, mutant N-terminally extended peptides exhibited significantly increased HLA-A*02:01 binding affinity and elicited CD8⁺ T cell stimulation in vitro similar to the _wtgp100_209–217 epitope. Thus, our experiments demonstrate that amino acid exchanges within an epitope can result in the generation of an altered peptide pool with new antigenic peptides and in a wider CD8⁺ T cell response also towards N-terminally extended versions of the minimal epitope.     

Rem uncouples excitation–contraction coupling in adult skeletal muscle fibers

In skeletal muscle, excitation–contraction (EC) coupling requires depolarization-induced conformational rearrangements in L-type Ca²⁺ channel (Ca_V1.1) to be communicated to the type 1 ryanodine-sensitive Ca²⁺ release channel (RYR1) of the sarcoplasmic reticulum (SR) via transient protein–protein interactions. Although the molecular mechanism that underlies conformational coupling between Ca_V1.1 and RYR1 has been investigated intensely for more than 25 years, the question of whether such signaling occurs via a direct interaction between the principal, voltage-sensing α_1S subunit of Ca_V1.1 and RYR1 or through an intermediary protein persists. A substantial body of evidence supports the idea that the auxiliary β_1a subunit of Ca_V1.1 is a conduit for this intermolecular communication. However, a direct role for β_1a has been difficult to test because β_1a serves two other functions that are prerequisite for conformational coupling between Ca_V1.1 and RYR1. Specifically, β_1a promotes efficient membrane expression of Ca_V1.1 and facilitates the tetradic ultrastructural arrangement of Ca_V1.1 channels within plasma membrane–SR junctions. In this paper, we demonstrate that overexpression of the RGK protein Rem, an established β subunit–interacting protein, in adult mouse flexor digitorum brevis fibers markedly reduces voltage-induced myoplasmic Ca²⁺ transients without greatly affecting Ca_V1.1 targeting, intramembrane gating charge movement, or releasable SR Ca²⁺ store content. In contrast, a β_1a-binding–deficient Rem triple mutant (R200A/L227A/H229A) has little effect on myoplasmic Ca²⁺ release in response to membrane depolarization. Thus, Rem effectively uncouples the voltage sensors of Ca_V1.1 from RYR1-mediated SR Ca²⁺ release via its ability to interact with β_1a. Our findings reveal Rem-expressing adult muscle as an experimental system that may prove useful in the definition of the precise role of the β_1a subunit in skeletal-type EC coupling.     

Intracellular localisation and innate immune responses following Francisella noatunensis infection of Atlantic cod (Gadus morhua) macrophages

The facultative intracellular bacterium Francisella noatunensis causes francisellosis in Atlantic cod (Gadus morhua), but little is known about its survival strategies or how these bacteria evade the host immune response. In this study we show intracellular localisation of F. noatunensis in cod macrophages using indirect immunofluorescence techniques and green fluorescent labelled bacteria. Transmission electron microscopy revealed that F. noatunensis was enclosed by a phagosomal membrane during the initial phase of infection. Bacteria were at a later stage of the infection found in large electron-lucent zones, apparently surrounded by a partially intact or disintegrated membrane. Immune electron microscopy demonstrated the release of bacterial derived vesicles from intracellular F. noatunensis, an event suspected of promoting phagosomal membrane degradation and allowing escape of the bacteria to cytoplasm. Studies of macrophages infected with F. noatunensis demonstrated a weak activation of the inflammatory response genes as measured by increased expression of the Interleukin (IL)-1β and IL-8. In comparison, a stronger induction of gene expression was found for the anti-inflammatory IL-10 indicating that the bacterium exhibits a role in down-regulating the inflammatory response. Expression of the p40 subunit of IL-12/IL-17 genes was highly induced during infection suggesting that F. noatunensis promotes T cell polarisation. The host macrophage responses studied here showed low ability to distinguish between live and inactivated bacteria, although other types of responses could be of importance for such discriminations. The immunoreactivity of F. noatunensis lipopolysaccharide (LPS) was very modest, in contrast to the strong capacity of Escherichia coli LPS to induce inflammatory responsive genes. These results suggest that F. noatunensis virulence mechanisms cover many strategies for intracellular survival in cod macrophages.     

Monitoring of biofilms grown on differentially structured metallic surfaces using confocal laser scanning microscopy

Imaging of biofilms on opaque surfaces is a challenge presented to researchers especially considering pathogenic bacteria, as those typically grow on living tissue, such as mucosa and bone. However, they can also grow on surfaces used in industrial applications such as food production, acting as a hindrance to the process. Thus, it is important to understand bacteria better in the environment they actually have relevance in. Stainless steel and titanium substrata were line structured and dotted surface topographies for titanium substrata were prepared to analyze their effects on biofilm formation of a constitutively green fluorescent protein (GFP)‐expressing Escherichia coli strain. The strain was batch cultivated in a custom built flow cell initially for 18 h, followed by continuous cultivation for 6 h. Confocal laser scanning microscopy (CLSM) was used to determine the biofilm topography. Biofilm growth of E. coli GFPmut2 was not affected by the type of metal substrate used; rather, attachment and growth were influenced by variable shapes of the microstructured titanium surfaces. In this work, biofilm cultivation in flow cells was coupled with the most widely used biofilm analytical technique (CLSM) to study the time course of growth of a GFP‐expressing biofilm on metallic surfaces without intermittent sampling or disturbing the natural development of the biofilm.     

Serine residues 286, 288, and 293 within the CIITA: a mechanism for down-regulating CIITA activity through phosphorylation

CIITA is the primary factor activating the expression of the class II MHC genes necessary for the exogenous pathway of Ag processing and presentation. Strict control of CIITA is necessary to regulate MHC class II gene expression and induction of an immune response. We show in this study that the nuclear localized form of CIITA is a predominantly phosphorylated form of the protein, whereas cytoplasmic CIITA is predominantly unphosphorylated. Novel phosphorylation sites were determined to be located within a region that contains serine residues 286, 288, and 293. Double mutations of these residues increased nuclear CIITA, indicating that these sites are not required for nuclear import. CIITA-bearing mutations of these serine residues significantly increased endogenous MHC class II expression, but did not significantly enhance trans-activation from a MHC class II promoter, indicating that these phosphorylation sites may be important for gene activation from intact chromatin rather than artificial plasmid-based promoters. These data suggest a model for CIITA function in which phosphorylation of these specific sites in CIITA in the nucleus serves to down-regulate CIITA activity.