Thomas Su

PKC-β controls IκB kinase lipid raft recruitment and activation in response to BCR signaling

NF-κB signaling is required for the maintenance of normal B lymphocytes, whereas dysregulated NF-κB activation contributes to B cell lymphomas. The events that regulate NF-κB signaling in B lymphocytes are poorly defined. Here, we demonstrate that PKC-β is specifically required for B cell receptor (BCR)-mediated NF-κB activation. B cells from protein kinase C-β (PKC-β)-deficient mice failed to recruit the IκB kinase (IKK) complex into lipid rafts, activate IKK, degrade IκB or up-regulate NF-κB–dependent survival signals. Inhibition of PKC-β promoted cell death in B lymphomas characterized by exaggerated NF-κB activity. Together, these data define an essential role for PKC-β in BCR survival signaling and highlight PKC-β as a key therapeutic target for B-lineage malignancies.

Cell-type-specific isolation of ribosome-associated mRNA from complex tissues

Gene profiling techniques allow the assay of transcripts from organs, tissues, and cells with an unprecedented level of coverage. However, most of these approaches are still limited by the fact that organs and tissues are composed of multiple cell types that are each unique in their patterns of gene expression. To identify the transcriptome from a single cell type in a complex tissue, investigators have relied upon physical methods to separate cell types or in situ hybridization and immunohistochemistry. Here, we describe a strategy to rapidly and efficiently isolate ribosome-associated mRNA transcripts from any cell type in vivo. We have created a mouse line, called RiboTag, which carries an Rpl22 allele with a floxed wild-type C-terminal exon followed by an identical C-terminal exon that has three copies of the hemagglutinin (HA) epitope inserted before the stop codon. When the RiboTag mouse is crossed to a cell-type-specific Cre recombinase-expressing mouse, Cre recombinase activates the expression of epitope-tagged ribosomal protein RPL22ha, which is incorporated into actively translating polyribosomes. Immunoprecipitation of polysomes with a monoclonal antibody against HA yields ribosome-associated mRNA transcripts from specific cell types. We demonstrate the application of this technique in brain using neuron-specific Cre recombinase-expressing mice and in testis using a Sertoli cell Cre recombinase-expressing mouse.

Transitional B lymphocyte subsets operate as distinct checkpoints in murine splenic B cell development.

Signaling through the Ag receptor is required for peripheral B lymphocyte maturation and maintenance. Defects in components of the B cell receptor (BCR) signalosome result in developmental blocks at the transition from immature (heat-stable Ag (HSA)high) to mature (HSAlow) B cells. Recent studies have subdivided the immature, or transitional, splenic B cells into two subsets, transitional 1 (T1) and transitional 2 (T2) cells. T1 and T2 cells express distinct surface markers and are located in distinct anatomic locations. In this report, we evaluated the BCR signaling capacity of T1 and T2 B cell subsets. In response to BCR engagement, T2 cells rapidly entered cell cycle and resisted cell death. In contrast, T1 cells did not proliferate and instead died after BCR stimulation. Correlating with these results, T2 cells robustly induced expression of the cell cycle regulator cyclin D2 and the antiapoptotic factors A1/Bfl-1 and Bcl-xL and exhibited activation of Akt. In contrast, T1 cells failed to up-regulate these markers. BCR stimulation of T2 cells also led to down-regulation of CD21 and CD24 (HSA) expression, resulting in a mature B cell phenotype. In addition, T2 cells from Bruton’s tyrosine kinase-deficient Xid mice failed to generate these proliferative and survival responses, suggesting a requirement for the BCR signalosome specifically at the T2 stage. Taken together, these data clearly demonstrate that T2 immature B cells comprise a discrete developmental subset that mediates BCR-dependent proliferative, prosurvival, and differentiation signals. Their distinct BCR-dependent responses suggest unique roles for T1 vs T2 cells in peripheral B cell selection.

Signaling in transitional type 2 B cells is critical for peripheral B-cell development.

Summary:  Splenic peripheral B‐cell development and the events regulating this functionally significant but relatively poorly defined developmental process have become a major focus in recent studies in B‐cell immunology. Following the exit from the bone marrow, peripheral B cells develop through transitional type 1 (T1) and transitional type 2 (T2) B‐cell stages. Emerging data suggest that the T2 subset is the immediate precursor of the mature B‐cell populations present in the spleen. In this review, we first elaborate on the evidence describing the unique properties of CD21hiCD24hiCD23hiIgMhiIgDhi T2 B cells. T2 cells uniquely activate a proliferative, pro‐survival, and differentiation program in response to B‐cell antigen receptor (BCR) engagement. The potential mechanisms leading to the differential BCR responsiveness of T1 versus T2 B cells are discussed. We also review evidence that distinguishes key BCR‐dependent signaling pathways operative in T2 and mature B cells. These signaling cascades include a protein kinase Cβ (PKCβ)‐dependent cell‐survival pathway and a second PKCβ‐independent pathway essential for BCR‐driven differentiation. Finally, we discuss recent intriguing results suggesting that the type of signal(s) encountered by T2 cells leads to their differential maturation toward the follicular mature versus marginal zone mature B‐cell populations. These combined observations suggest important implications with regard to B‐cell selection and tolerance, potential novel therapeutic targets for B‐cell lymphomas, and how the intricate balance of commensal organisms and other microenvironmental signals interact to promote the generation of ‘innate‐like’ versus adaptive effector B‐cell populations.

Dysregulated TCL1 promotes multiple classes of mature B cell lymphoma

The TCL1 protooncogene is overexpressed in many mature B cell lymphomas, especially from AIDS patients. To determine whether aberrant expression promotes B cell transformation, we generated a murine model in which a TCL1 transgene was overexpressed at similar levels in both B and T cells. Strikingly, transgenic mice developed Burkitt-like lymphoma (BLL) and diffuse large B cell lymphoma (DLBCL) with attendant Bcl-6 expression and mutated JH gene segments at a very high penetrance beginning at 4 months of age. In contrast, only one mouse developed a T cell malignancy at 15 months, consistent with a longer latency for transformation of T cells by TCL1. Activation of premalignant splenic B cells by means of B cell antigen receptor (BCR) engagement resulted in significantly increased proliferation and augmented AKT-dependent signaling, including increased S6 ribosomal protein phosphorylation. Transgenic spleen cells also survived longer than wild-type spleen cells in long-term culture. Together these data demonstrate that TCL1 is a powerful oncogene that, when overexpressed in both B and T cells, predominantly yields mature B cell lymphomas.

Mutations in AKAP5 Disrupt Dendritic Signaling Complexes and Lead to Electrophysiological and Behavioral Phenotypes in Mice

AKAP5 (also referred to as AKAP150 in rodents and AKAP79 in humans) is a scaffolding protein that is highly expressed in neurons and targets a variety of signaling molecules to dendritic membranes. AKAP5 interacts with PKA holoenzymes containing RIIα or RIIβ as well as calcineurin (PP2B), PKC, calmodulin, adenylyl cyclase type V/VI, L-type calcium channels, and β-adrenergic receptors. AKAP5 has also been shown to interact with members of the MAGUK family of PSD-scaffolding proteins including PSD95 and SAP97 and target signaling molecules to receptors and ion channels in the postsynaptic density (PSD). We created two lines of AKAP5 mutant mice: a knockout of AKAP5 (KO) and a mutant that lacks the PKA binding domain of AKAP5 (D36). We find that PKA is delocalized in both the hippocampus and striatum of KO and D36 mice indicating that other neural AKAPs cannot compensate for the loss of PKA binding to AKAP5. In AKAP5 mutant mice, a significant fraction of PKA becomes localized to dendritic shafts and this correlates with increased binding to microtubule associated protein-2 (MAP2). Electrophysiological and behavioral analysis demonstrated more severe deficits in both synaptic plasticity and operant learning in the D36 mice compared with the complete KO animals. Our results indicate that the targeting of calcineurin or other binding partners of AKAP5 in the absence of the balancing kinase, PKA, leads to a disruption of synaptic plasticity and results in learning and memory defects.

PKC-|[beta]| controls I|[kappa]|B kinase lipid raft recruitment and activation in response to BCR signaling

NF-κB signaling is required for the maintenance of normal B lymphocytes, whereas dysregulated NF-κB activation contributes to B cell lymphomas. The events that regulate NF-κB signaling in B lymphocytes are poorly defined. Here, we demonstrate that PKC-β is specifically required for B cell receptor (BCR)-mediated NF-κB activation. B cells from protein kinase C-β (PKC-β)-deficient mice failed to recruit the IκB kinase (IKK) complex into lipid rafts, activate IKK, degrade IκB or up-regulate NF-κB–dependent survival signals. Inhibition of PKC-β promoted cell death in B lymphomas characterized by exaggerated NF-κB activity. Together, these data define an essential role for PKC-β in BCR survival signaling and highlight PKC-β as a key therapeutic target for B-lineage malignancies.

Tissue-specific PKA inhibition using a chemical genetic approach and its application to studies on sperm capacitation

Studies on cAMP signaling and protein kinase A (PKA) function in vivo are limited by the lack of highly specific inhibitors that can be used in primary cell culture and whole animals. Previously we reported that a mutation in the ATP binding pocket of a catalytic subunit (Cα) of PKA confers sensitivity to the pyrazolo[3,4-d]pyrimidine inhibitor, 1NM-PP1. We have now engineered the mouse Pkraca gene such that after Cre-mediated recombination in vivo, the CαM120A mutant protein is expressed and the wild-type Cα is turned off. We demonstrate the utility of this approach by examining the requirement for PKA activity during capacitation of sperm from mice that express CαM120A mutant protein. For CαM120A sperm, 10 μM of 1NM-PP1 prevented PKA-dependent phosphorylation and the activation of motility that are both rapidly (<90 s) evoked by the HCO3− anion. A continuous (90 min) inhibition with 10 μM of 1NM-PP1 prevented the protein tyrosine phosphorylation of late-stage capacitation. Delayed application of 1NM-PP1 demonstrated that PKA activity was required for at least the initial 30 min of capacitation to produce subsequent protein tyrosine phosphorylation. Acute application of 1NM-PP1 rapidly slowed the accelerated beat of activated motility but did not affect the established waveform asymmetry of hyperactivated sperm. Our results demonstrate that PKA in CαM120A mutant sperm is rapidly and reversibly inhibited by 1NM-PP1 and that this blockade has selective and time-dependent effects on multiple aspects of capacitation. The conditional CαM120A-expressing mouse lines will be valuable tools for studying PKA function in vivo.

Cardiomyocytes from AKAP7 knockout mice respond normally to adrenergic stimulation

Protein kinase A (PKA) is activated during sympathetic stimulation of the heart and phosphorylates key proteins involved in cardiac Ca2+ handling, including the L-type Ca2+ channel (CaV1.2) and phospholamban (PLN). This results in acceleration and amplification of the beat-to-beat changes in cytosolic Ca2+ in cardiomyocytes and, in turn, an increased rate and force of contraction. PKA is held in proximity to its substrates by protein scaffolds called A kinase anchoring proteins (AKAPs). It has been suggested that the short and long isoforms of AKAP7 (also called AKAP15/18) localize PKA in complexes with CaV1.2 and PLN, respectively. We generated an AKAP7 KO mouse in which all isoforms were deleted and tested whether Ca2+ current, intracellular Ca2+ concentration, or Ca2+ reuptake were impaired in isolated adult ventricular cardiomyocytes following stimulation with the β-adrenergic agonist isoproterenol. KO cardiomyocytes responded normally to adrenergic stimulation, as measured by whole-cell patch clamp or a fluorescent intracellular Ca2+ indicator. Phosphorylation of CaV1.2 and PLN were also unaffected by genetic deletion of AKAP7. Immunoblot and RT-PCR revealed that only the long isoforms of AKAP7 were detectable in ventricular cardiomyocytes. The results indicate that AKAP7 is not required for regulation of Ca2+ handling in mouse cardiomyocytes.

Resident enteric microbiota and CD8+ T cells shape the abundance of marginal zone B cells

Since enteric microbial composition is a distinctive and stable individual trait, microbial heterogeneity may confer lifelong, non‐genetic differences between individuals. Here we report that C57BL/6 mice bearing restricted flora microbiota, a distinct but diverse resident enteric microbial community, are numerically and functionally deficient in marginal zone (MZ) B cells. Surprisingly, MZ B‐cell levels are minimally affected by germ‐free conditions or null mutations of various TLR signaling molecules. In contrast, MZ B‐cell depletion is exquisitely dependent on cytolytic CD8+ T cells, and includes targeting of a cross‐reactive microbial/endogenous MHC class 1B antigen. Thus, members of certain enteric microbial communities link with CD8+ T cells as a previously unappreciated mechanism that shapes innate immunity dependent on innate‐like B cells.