A major function of the B cell is the internalization of antigen through the BCR for processing and presentation to T cells. While there is evidence suggesting that lipid raft signaling may regulate internalization, the molecular machinery coordinating these two processes remains to be defined. Here we present a link between the B cell signaling and internalization machinery and show that Src-family kinase activity is required for inducible clathrin heavy chain phosphorylation, BCR colocalization with clathrin, and regulated internalization. An analysis of different B cell lines shows that BCR uptake occurs only when clathrin is associated with rafts and is tyrosine phosphorylated following BCR crosslinking. We therefore propose that lipid rafts spatially organize signaling cascades with clathrin to regulate BCR internalization.

B cell antigen receptor (BCR) association with lipid rafts, the actin cytoskeleton, and clathrin-coated pits influences B cell signaling and antigen presentation. Although all three cellular structures have been separately implicated in BCR internalization, the relationship between them has not been clearly defined. In this study, internalization pathways were characterized by specifically blocking each potential mechanism of internalization. BCR uptake was reduced by ϳ70% in B cells conditionally deficient in clathrin heavy chain expression. Actin or raft antagonists were both able to block the residual, clathrin-independent BCR internalization. These agents also affected clathrin-dependent internalization, indicating that clathrin-coated pits, in concert with mechanisms dependent on rafts and actin, mediate the majority of BCR internalization. Clustering G M1 gangliosides enhanced clathrin-independent BCR internalization, and this required actin. Thus, although rafts or actin independently did not mediate BCR internalization, they apparently cooperate to promote some internalization even in the absence of clathrin. Simultaneous inhibition of all BCR uptake pathways resulted in sustained tyrosine phosphorylation and activation of the extracellular signal-regulated kinase (ERK), strongly suggesting that downstream BCR signaling can occur without receptor translocation to endosomes and that internalization leads to signal attenuation. INTRODUCTIONThe B cell immune response is initiated by antigen binding to the B cell antigen receptor (BCR). This results in signaling, entry of B cells into the cell cycle, and antigen internalization leading to its presentation to helper T-cells (Lanzavecchia, 1985;Reth and Wienands, 1997). Although BCR internalization is a well-defined event during antigen processing, several different membrane traffic pathways have been implicated in this process. Here, we address the relative contributions of three cellular mechanisms to BCR internalization and the relationship of BCR internalization to downstream receptor signaling.Following engagement of the antigen receptor, the BCR can be observed in clathrin-coated pits and vesicles, suggesting a clathrin-mediated route of internalization (Salisbury et al., 1980;Brown and Song, 2001). The clathrin molecule has a triskelion or three-legged shape and is composed of three identical heavy chains, each with a tightly associated light chain. Clathrin triskelia, together with additional coat components, assemble into a polygonal lattice at the plasma membrane . The assembly of further triskelia accompanies invagination of the coated pit and transforms it into a clathrin-coated vesicle (CCV), which carries concentrated transmembrane receptors into the cell. In recent years, it has become increasingly apparent that internalization pathways are regulated by signaling cascades. For example, inhibitors or genetic deletions of protein tyrosine kinases prevent BCR internalization (Pure and Tardelli, 1992;Dykstra et al., 2001;Ma et al., 2001). Moreover, ...

The B lymphocyte adaptor molecule of 32 kDa (Bam32) is an adaptor that plays an indispensable role in BCR signaling. In this study, we found that upon BCR ligation, Bam32 is recruited to the plasma membrane where it associates with BCR complexes and redistributes and internalizes with BCRs. BCR ligation induced colocalization of Bam32 with lipid rafts, clathrin, and actin filaments. An inhibitor of Src family protein tyrosine kinases (PTKs) blocked both BCR-induced tyrosine phosphorylation of Bam32 and BCR internalization. Moreover, BCR internalization is impaired in Bam32−/− and Lyn−/− cells, and expression of Bam32 with a mutation of its tyrosine phosphorylation site (Y139F) inhibited BCR internalization. These data suggest that Bam32 functions downstream of Src family PTKs to regulate BCR internalization. Bam32 deficiency does not affect tyrosine phosphorylation of clathrin or the association of clathrin with lipid rafts upon BCR cross-linking. However, BCR-induced actin polymerization is impaired in Bam32−/− cells. Collectively, these findings indicate a novel role of Bam32 in connecting Src family PTKs to BCR internalization by an actin-dependent mechanism.

• Egr1 haploinsufficiency in cooperation with reduced Tp53 activity accelerates the development of hematologic disease in mice.• Loss of 1 copy of Egr1 and Apc in hematopoietic stem cells, in cooperation with Tp53 loss, results in myeloid neoplasms.An interstitial deletion of chromosome 5, del(5q), is the most common structural abnormality in primary myelodysplastic syndromes (MDS) and therapy-related myeloid neoplasms (t-MNs) after cytotoxic therapy. Loss of TP53 activity, through mutation or deletion, is highly associated with t-MNs with a del(5q). We previously demonstrated that haploinsufficiency of Egr1 and Apc, 2 genes lost in the 5q deletion, are key players in the progression of MDS with a del(5q). Using genetically engineered mice, we now show that reduction or loss of Tp53 expression, in combination with Egr1 haploinsufficiency, increased the rate of development of hematologic neoplasms and influenced the disease spectrum, but did not lead to overt myeloid leukemia, suggesting that altered function of additional gene(s) on 5q are likely required for myeloid leukemia development. Next, we demonstrated that cell intrinsic loss of Tp53 in hematopoietic stem and progenitor cells haploinsufficient for both Egr1 and Apc led to the development of acute myeloid leukemia (AML) in 17% of mice. The long latency (234-299 days) and clonal chromosomal abnormalities in the AMLs suggest that additional genetic changes may be required for full transformation. Thus, loss of Tp53 activity in cooperation with Egr1 and Apc haploinsufficiency creates an environment that is permissive for malignant transformation and the development of AML. (Blood. 2014;123(7):1069-1078)

Therapy-related myelodysplastic syndrome and acute myeloid leukemia (t-MDS/t-AML) are late complications of cytotoxic therapy used in the treatment of malignant diseases. The most common subtype of t-AML (~75% of cases) develops after exposure to alkylating agents, and is characterized by loss or deletion of chromosome 5 and/or 7 [−5/del(5q), −7/del(7q)], and a poor outcome (median survival 8 months). In the University of Chicago’s series of 386 patients with t-MDS/t-AML, 79 (20%) patients had abnormalities of chromosome 5, 95 (25%) patients had abnormalities of chromosome 7, and 85 (22%) patients had abnormalities of both chromosomes 5 and 7. t-MDS/t-AML with a −5/del(5q) is associated with a complex karyotype, characterized by trisomy 8, as well as loss of 12p, 13q, 16q22, 17p (TP53 locus), chromosome 18, and 20q. In addition, this subtype of t-AML is characterized by a unique expression profile (higher expression of genes involved in cell cycle control (CCNA2, CCNE2, CDC2), checkpoints (BUB1), or growth (MYC), loss of expression of IRF8, and overexpression of FHL2. Haploinsufficiency of the RPS14, EGR1, APC, NPM1, and CTNNA1 genes on 5q has been implicated in the pathogenesis of MDS/AML. In previous studies, we determined that Egr1 acts by haploinsufficiency and cooperates with mutations induced by alkylating agents to induce myeloid leukemias in the mouse. To identify mutations that cooperate with Egr1 haploinsufficiency, we used retroviral insertional mutagenesis. To date, we have identified two common integration sites involving genes encoding transcription factors that play a critical role in hematopoiesis (Evi1 and Gfi1b loci). Of note is that the EVI1 transcription factor gene is deregulated in human AMLs, particularly those with −7, and abnormalities of 3q. Identifying the genetic pathways leading to t-AML will provide new insights into the underlying biology of this disease, and may facilitate the identification of new therapeutic targets.

There is accumulating evidence that functional alteration(s) of the bone marrow (BM) microenvironment contribute to the development of some myeloid disorders, such as myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). In addition to a cell-intrinsic role of WNT activation in leukemia stem cells, WNT activation in the BM niche is also thought to contribute to the pathogenesis of MDS and AML. We previously showed that the -haploinsufficient mice ( ) model MDS induced by an aberrant BM microenvironment. We sought to determine whether Apc, a multifunctional protein and key negative regulator of the canonical β-catenin (Ctnnb1)/WNT-signaling pathway, mediates this disease through modulating WNT signaling, and whether inhibition of WNT signaling prevents the development of MDS in mice. Here, we demonstrate that loss of 1 copy of is sufficient to prevent the development of MDS in mice and that altered canonical WNT signaling in the microenvironment is responsible for the disease. Furthermore, the US Food and Drug Administration (FDA)-approved drug pyrvinium delays and/or inhibits disease in mice, even when it is administered after the presentation of anemia. Other groups have observed increased nuclear CTNNB1 in stromal cells from a high frequency of MDS/AML patients, a finding that together with our results highlights a potential new strategy for treating some myeloid disorders.

9-O-Acetylation is one of the most common modifications of sialic acids, and it can affect several sialic acidmediated recognition phenomena. We previously reported a cDNA encoding a lysosomal sialic acid-specific 9-O-acetylesterase, which traverses the endoplasmic reticulum-Golgi pathway and localizes primarily to lysosomes and endosomes. In this study, we report a variant cDNA derived from the same gene that contains a different 5 region. This cDNA has a putative open reading frame lacking a signal peptide-encoding sequence and is thus a candidate for the previously described cytosolic sialic acid 9-O-acetylesterase activity. Epitope-tagged constructs confirm that the new sequence causes the protein product to be targeted to the cytosol and has esterase activity. Using reverse transcription-polymerase chain reaction to distinguish the two forms of message, we show that although the lysosomal sialic acidspecific 9-O-acetylesterase message has a widespread pattern of expression in adult mouse tissues, this cytosolic sialic acid 9-O-acetylesterase form has a rather restricted distribution, with the strongest expression in the liver, ovary, and brain. Using a polyclonal antibody directed against the 69-amino acid region common to both proteins, we confirmed that the expression of glycosylated and nonglycosylated polypeptides occurred in appropriate subcellular fractions of normal mouse tissues. Rodent liver polypeptides reacting to the antibody also co-purify with previously described lysosomal sialic acid esterase activity and at least a portion of the cytosolic activity. Thus, two sialic acid 9-O-acetylesterases found in very different subcellular compartments can be encoded by a single gene by differential usage of a signal peptide-encoding exon at the N terminus. The 5-rapid amplification of cDNA ends results and the differences in tissue-specific expression suggest that expression of these two products may be differentially regulated by independent promoters.

Therapy-related myeloid neoplasm (t-MN) is a distinctive clinical syndrome occurring after exposure to chemotherapy or radiotherapy. t-MN arises in most cases from a multipotential hematopoietic stem cell or, less commonly, in a lineage committed progenitor cell. The prognosis for patients with t-MN is poor, as current forms of therapy are largely ineffective. Cytogenetic analysis, molecular analysis and gene expression profiling analysis of t-MN has revealed that there are distinct subtypes of the disease; however, our understanding of the genetic basis of t-MN is incomplete. Elucidating the genetic pathways and molecular networks that are perturbed in t-MNs, may facilitate the identification of therapeutic targets that can be exploited for the development of urgently-needed targeted therapies.