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Flip the switch: BTG2–PRMT1 protein complexes antagonize pre-B-cell proliferation to promote B-cell development

Cellular & Molecular Immunology volume 15, pages 808–811 (2018)Cite this article

BTG2 is an antiproliferative BTG/transducer of ErbB2 family member that inhibits the G1/S transition of the cell cycle.2 BTG2 binds and activates PRMT1, forming complexes that methylate protein substrates.3 Prior experiments suggested that BTG2 and PRMT1 each had a role in promoting B-cell development;4 however, the mechanisms by which these two proteins regulated B-cell differentiation were not explored. Dolezal et al.1 conducted experiments in mice and in cultured primary pre-B cells to determine the roles of BTG2 and PRMT1 in developing B cells. They found that B-lineage-specific deletion of Prmt1 resulted in a defect in early B-cell development as reflected by lower numbers of cells at the pro-B-cell stage onwards. Using a pre-B-cell line (1676) and ex vivo cultured primary pre-B cells, knockdown of Prmt1 mRNA in non-cycling cells resulted in a reduction of Igκ+ B cells, indicating that PRMT1 regulates pre-B-cell differentiation. Curiously, overexpression of Prmt1 did not enhance pre-B-cell development, suggesting other cellular factors limit PRMT1 activity. Given that PRMT1 and BTG2 complex in vitro,3 the authors quantified the BTG2 expression in developing B cells. They found that non-cycling 1676 pre-B cells exhibited elevated levels of Btg2 transcript and protein relative to cycling cells, mirroring the expression pattern of Prmt1. BTG2 contains a BoxC region that is required for BTG2 to bind and activate PRMT1.5 Overexpression of wild-type BTG2, but not a BTG2 mutant with a BoxC deletion (BTG2ΔBoxC), enhanced Igκ expression and pre-B-cell differentiation in 1676 and ex vivo cultured pre-B cells. These effects were dependent on PRMT1 as PRMT1 knockdown, even in the presence of overexpressed BTG2, reduced Igκ expression. BTG2 knockdown in 1676 and ex vivo cultured pre-B cells also reduced the frequency of Igκ+ cells. PRMT1 functions to asymmetrically dimethylate arginine residues (ADMA).6 Consistent with the cellular phenotype data, analysis of ADMA revealed extensive methylation in cells transduced with wild-type BTG2 but not the BTG2ΔBoxC mutant. Collectively, these data indicate that BTG2 and PRMT1 function together to promote pre-B-cell development.

Ig gene assembly occurs in G1-phase cells and requires the expression of both RAG1 and RAG2 proteins.7 Thus the authors next tested the effect of BTG2 on RAG1 and RAG2 expression in developing pre-B cells. 1676 pre-B cells transduced with wild-type BTG2 exhibited an increase in RAG2 protein as compared with control and BTG2ΔBoxC mutant cells. As RAG2 is degraded in S phase,8 Dolezal et al.1 reasoned that the increase in RAG2 protein in BTG2-transduced cells was due to G1 cell cycle arrest. Consistent with this notion, the authors found that 1676 pre-B cells expressing wild-type BTG2 had a higher fraction of cells arrested in G1 phase. Given that IL-7R signaling promotes Cyclin D3 expression to drive cells through G1 and into S phase,9 the authors asked whether BTG2 and PRMT1 reduce Cyclin D3 expression. Indeed, overexpression of wild-type BTG2, but not the BTG2ΔBoxC mutant, resulted in a drastic reduction in Cyclin D3 protein in a proteasome-dependent manner. Conversely, BTG2 or PRMT1 knockdown increased Cyclin D3 protein and drove cell cycle progression. These data suggest that BTG2 and PRMT1 cooperate to arrest pre-B cells in G1 by licensing Cyclin D3 degradation. As Cyclin D3 fuels cell cycle progression, the authors hypothesized that Cyclin D3 is an inhibitor of the pre-B-cell differentiation program. Indeed, 1676 pre-B cells transduced with Cyclin D3 exhibited a decrease in the frequency of Igκ+ B cells. Conversely, knockdown of Cyclin D3 increased the frequency of Igκ+ 1676 pre-B cells. Notably, combined overexpression of Cyclin D3 and BTG2 led to a defect in the development of Igκ+ B cells, indicating that Cyclin D3 is a potent inhibitor of pre-B-cell differentiation. Collectively, these data reveal a critical role for the BTG2–PRMT1 complex in promoting Cyclin D3 degradation to suppress cell cycle progression, stimulate RAG expression and thereby promote pre-B-cell development.

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This work was supported by the Immune System Development and Regulation T32-AI055428 (to GSW) and the National Institutes of Health R01 grants AI112621 and AI112621 (to CHB).

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  1. Immunology Graduate Group, Children’s Hospital of Philadelphia, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, 19104, PA, USA

    Glendon S Wu & Craig H Bassing

  2. Divsion of Cancer Pathobiology, Department of Pathology and Laboratory Medicine, Children’s Hospital of Philadelphia, Abramson Family Cancer Research Institute, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, 19104, PA, USA

    Glendon S Wu & Craig H Bassing

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Wu, G.S., Bassing, C.H. Flip the switch: BTG2–PRMT1 protein complexes antagonize pre-B-cell proliferation to promote B-cell development. Cell Mol Immunol 15, 808–811 (2018). https://doi.org/10.1038/cmi.2017.156

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