Sin1-mTORC2 suppresses rag and il7r gene expression through Akt2 in B cells

Abstract

Mammalian target of rapamycin (mTOR) is an important mediator of phosphoinositol-3-kinase (PI3K) signaling. PI3K signaling regulates B cell development, homeostasis, and immune responses. However, the function and molecular mechanism of mTOR-mediated PI3K signaling in B cells has not been fully elucidated. Here we show that Sin1, an essential component of mTOR complex 2 (mTORC2), regulates B cell development. Sin1 deficiency results in increased IL-7 receptor (il7r) and RAG recombinase (rag1 and rag2) gene expression, leading to enhanced pro-B cell survival and augmented V(D)J recombinase activity. We further show that Akt2 specifically mediates the Sin1-mTORC2 dependent suppression of il7r and rag gene expression in B cells by regulating FoxO1 phosphorylation. Finally, we demonstrate that the mTOR inhibitor rapamycin induces rag expression and promotes V(D)J recombination in B cells. Our study reveals that the Sin1/mTORC2-Akt2 signaling axis is a key regulator of FoxO1 transcriptional activity in B cells.

Figure 1. Sin1 regulates B cell development

a) Bone marrow from Sin1^+/+ or Sin1^-/- chimeric mice was analyzed by flow cytometry and the percentage of CD19⁺ IgM^- and CD19⁺ IgM⁺ B cells is indicated. The data are representative of Sin1^+/+ (n=3) and Sin1^-/- (n=4) fetal liver chimeric mice. b) Bone marrow from a chimeric mouse containing a 1:1 ratio of wild type host and Sin1^-/- donor cells was analyzed by flow cytometry. The plots shown are pre-gated on CD45.1⁺ host cells or CD45.1^- Sin1^-/- donor cells and the percentage of CD19⁺IgM^- and CD19⁺IgM⁺ B cells is indicated. c) Bar graph illustrating the total number of Sin1^+/+ host or Sin1^-/- donor bone marrow cells from the chimeric mouse shown in (b). d) The proportion of B220⁺CD43^hiIgM^- bone marrow pro-B cells from wild type or Sin1^-/- fetal liver chimeric mice. The plots shown are pre-gated on CD45.2⁺ donor cells and are representative of Sin1^+/+ (n=3) and Sin1^-/- (n=4) fetal liver chimeric mice. e) Sin1^+/+ or Sin1^-/- pro-B cells were cultured in vitro on OP9 cells with or without IL-7 for 7 days and surface IgM and IgD expression was analyzed. The plots are representative of 4 independent experiments. f) Sin1^+/+ or Sin1^-/- pro-B cells were cultured on OP9 cells without IL-7 for 7 days, fixed, permeablized and stained for Igμ chain expression (shaded area). The proportion of Igμ^- pro-B cells is indicated. Rag1^-/- pro-B cells are a negative control for Igμ staining (dotted line). Representative plots are shown from 3 independent experiments.

Figure 2. Sin1^-/- pro-B cells exhibit increased il7r expression and enhanced IL-7 dependent survival

a) IL-7 receptor (il7r) mRNA levels in Sin1^+/+ (WT) or Sin1^-/- (KO) pro-B cells were measured by qPCR and normalized to GAPDH expression. Samples were run in triplicate and data are representative of two independent experiments. b) Expression of IL-7R (CD127) on Sin1^+/+ or Sin1^-/- pro-B cells was measured by flow cytometry. Shaded area is the isotype control staining. The plots are representative of 3 independent experiments. c) Sin1^+/+ or Sin1^-/- pro-B cells were cultured on OP9 cells without exogenous IL-7. The total number of live cells recovered at each time point was measured. Dead cells were excluded from the analysis by trypan blue staining. Data are representative of two independent experiments. d) Sin1^+/+ or Sin1^-/- pro-B cells were cultured without OP9 cells in medium supplemented with 0.1 ng/ml IL-7. The number of live cells at 10 and 24 hours after plating was determined by trypan blue dye exclusion. Each data point represents triplicate wells from one of two independent experiments. (* = p<0.01) e) Sin1^+/+ or Sin1^-/- pro-B cells were cultured without OP9 cells in medium supplemented with 5 ng/ml IL-7. The number of live cells at various time points after plating was determined by trypan blue dye exclusion. Each data point represents triplicate wells from one of two independent experiments. (* = p<0.01) Error bars indicate standard deviation.

Figure 3. Sin1 is required for mTORC2 dependent Akt hydrophobic motif and Akt turn motif phosphorylation in B cells

a) The mTORC2 complex integrity in Sin1^+/- (WT) or Sin1^-/- (KO) Ab-MuLV pre-B cells was determined by immunoprecipitation (IP) of Rictor followed by immunoblotting (IB) of Sin1, Rictor and mTOR. b) Akt phosphorylation and expression was measured in primary Sin1^+/+ or Sin1^-/- pro-B cells cultured on OP9 cells with IL-7 and Ab-MuLV transformed Sin1^+/- or Sin1^-/- pre-B cells. Samples were normalized to total cellular protein and blotted as indicated. c) FoxO1 phosphorylation and expression was measured in Sin1^+/- or Sin1^-/- Ab-MuLV pre-B cells. Samples were normalized to total cellular protein and blotted for FoxO1 or FoxO1 p-Thr24. d) FoxO1 phosphorylation was measured in Sin1^+/+ or Sin1^-/- pro-B cells cultured with serum and IL-7. The samples were normalized to total FoxO1 protein then blotted for FoxO1 p-Thr24. e) FoxO1 expression was measured in Sin1^+/+ or Sin1^-/- pro-B cells. The samples were normalized to total cellular protein.

Figure 4. Increased RAG expression and V(D)J recombinase activity in developing Sin1^-/- B cells

a) Rag1 mRNA expression was measured by qPCR in Sin1^+/- (WT) or Sin1^-/- (KO) Ab-MuLV pre-B cells cultured for 5 hours in the presence or absence of Imatinib. Rag1 mRNA was normalized to GAPDH mRNA. ND: not detected. b) RAG1 protein in Sin1^+/- (WT), Sin1^-/- (KO) or rag1^-/- Ab-MuLV pre-B cells was measured by immunoblotting. The ERK2 protein is used as a loading control. c) Sin1^+/+ or Sin1^-/- pro-B cells were cultured on OP9 cells without exogenous IL-7 for 0, 20, or 48 hours. Rag1 expression was measured by qPCR. Rag1 expression was normalized to GAPDH mRNA. Samples were run in triplicate and are representative of two independent experiments. d) Sin1^+/+ or Sin1^-/- pro-B cells were differentiated in vitro on OP9 cells for 7 days, and rag1 or rag2 expression in pre-B and immature B cells obtained from these cultures was measured by qPCR as described in panel a). Samples were run in triplicate and are representative of two independent experiments. (* p<0.01) e) Sin1^-/- Ab-MuLV pre-B cells were infected with a retrovirus expressing human Sin1 or a control empty retrovirus, and the RAG1 expression was measured by immunoblotting. Data are representative of two independent experiments. f) Sin1^+/- or Sin1^-/- Ab-MuLV pre-B cells were infected with a retrovirus containing an EGFP RAG recombinase reporter. The infected Ab-MuLV pre-B cells were then grown under normal culture conditions and the EGFP⁺ cells were determined by flow cytometry 2 days and 11 days later. The plots shown are gated on infected cells (hCD4⁺ cells). The data are representative of three independent experiments. Error bars indicate standard deviation.

Figure 5. Akt2 regulates Rag and IL-7R expression in pro-B cells

a) Wild type (WT), Sin1^-/- and Akt deficient pro-B cells were cultured on OP9 cells with IL-7. Rag1 mRNA levels were measured by qPCR and normalized to GAPDH mRNA. Samples were run in triplicate and are representative of two independent experiments. (* = p<0.01) b) RAG1 protein was measured in primary pro-B cells cultured on OP9 cells with IL-7. Total Akt expression was measured with a pan-Akt antibody. Samples were normalized to total cellular protein. c) Il7r mRNA expression in wild type, Akt1^-/-, Akt2^-/- or Akt1^-/- Akt2^-/- (DKO) pro-B cells cultured on OP9 cells with IL-7 was measured by quantitative RT-PCR and normalized to GAPDH mRNA. Samples were run in triplicate and are representative of two independent experiments. (* = p<0.01) d) FoxO1 phosphorylation was measured in wild type, Akt2^-/-, or Akt1^-/- pro-B cells. Samples were normalized to total cellular protein. e) Cytoplasmic and nuclear Akt1 and Akt2 was determined in Sin1^+/+ (WT) or Sin1^-/- (KO) pro-B cells cultured on OP9 cells with IL-7. ERK2 and H2B proteins were used to verify the quality of the cytoplasmic and nuclear fractions. Three times more nuclear proteins (based on the total cell numbers) than cytosolic proteins were loaded in this assay to enhance the nuclear signals. Standard deviation is indicated by all error bars.

Figure 6. Rapamycin induces RAG gene expression and inhibits Akt Ser473 phosphorylation in B cells

a) Rag mRNA levels were measured by qPCR in Sin1^+/- Ab-MuLV pre-B cells treated with rapamycin for the indicated periods of time. Rag1 and rag2 expression was normalized to GAPDH mRNA. Samples were run in triplicate and are representative of two independent experiments. (* = p<0.01) b) Sin1^+/- Ab-MuLV pre-B cells were cultured in the presence or absence of rapamycin for 25 hours and RAG1 or FoxO1 protein was measured. ERK2 is used as a loading control. c) Sin1^+/- Ab-MuLV pre-B cells were vehicle or rapamycin treated for 5 or 25 hours. Cell lysates were normalized to total protein and immunoblotted as indicated. d) Sin1^+/- Ab-MuLV pre-B cells were infected with a retrovirus containing an EGFP RAG recombinase reporter. The infected cells were continuously cultured in the presence (Rapa.) or absence (vehicle) of rapamycin for 6 days. The EGFP⁺ cells were determined by flow cytometry at 1 day and 6 days after infection. The plots are gated on infected cells (hCD4⁺ cells) and are representative of three independent experiments. e) Sin1^+/+ (WT) or Sin1^-/- (KO) pro-B cells cultured on OP9 cells with IL-7 were treated with rapamycin or vehicle only for 25 hours and rag1 expression was measured by qPCR. Samples were normalized to GAPDH expression. Samples were run in triplicate and the data are representative of two independent experiments. f) Resting Sin1^+/- splenic B cells were enriched by negative selection and cultured in vitro in the presence (Rapa) or absence (Vehicle) of rapamycin for 25 hours. Rag1 expression was measured by qPCR and normalized to GAPDH expression. Samples were run in triplicate and the data are representative of two independent experiments. g) Sin1^-/- Ab-MuLV pre-B cells were infected with retrovirus expressing human Akt with a null HM mutation (Ser473Ala) or a phospho-mimetic HM mutation (Ser473Asp). Rag1 mRNA expression was measured by qPCR and normalized to GAPDH. RAG1 protein levels were measured by immunoblotting and the HA blotting verifies expression of the virally expressed Akt. Quantitative RT-PCR samples were run in triplicate and the data are representative of two independent experiments. Standard deviation is indicated by all error bars.

Figure 7

A model illustrating the regulation of il7r and rag expression by the Sin1/mTORC2-Akt2 signaling axis in developing B cells. PDK1 and Sin1/mTORC2 mediate the PI3K signals to phosphorylate Akt2 at Thr308 and Ser473, respectively. Activated Akt2 then phosphorylates FoxO1, resulting in FoxO1 degradation and down regulation of il7r and rag gene expression. In the absence of Sin1 or during chronic rapamycin treatment, mTORC2 is disrupted and Akt2 is not fully activated resulting in FoxO1 hypophosphorylation and accumulation; this leads to the augmented il7r and rag gene expression.