Correlating notch signaling with thymocyte maturation

Abstract

The Notch receptor and its ligands are involved in many developmental processes. They are highly expressed in the thymus and have been implicated in the CD4 versus CD8 lineage decision. We identified the constitutively active intracellular fragment of murine Notch-1 as capable of rendering thymomas resistant to glucocorticoid-induced apoptosis. This effect was confirmed in other T cell lines and in CD4+ CD8+ DP thymocytes. Activation of the Notch signaling pathway also upregulated a number of other markers that, like steroid resistance, correlate with DP maturation into both the CD4 and CD8 lineages. These results suggest that Notch signaling is critically involved in the maturation of DP thymocytes into both CD4+ and CD8+ SP thymocytes.

Figure 1

Identification of NotchIC as a Mediator of Resistance to Glucocorticoids in T Cells (A) Schematic representation of the mNotch-1 amino acid sequence. Labeled are the transmembrane domain (TM), residues 1724–1746; the RAM domain, residues 1751–1806; the ankyrin domain, residues 1865–2075; and the PEST domain, residues 2481–2503 (del Amo et al., 1993; Tamura et al., 1995). The majority of the extracellular domain has been omitted. (B) Schematic representation of the coding regions of mNotch-1 cDNAs identified by expression cloning. Numbers indicating the amino- and carboxyl-terminal amino acid residues are based on conceptual translation of the cDNA sequences. The initiation methionine for these cDNAs has not been determined. RH, US, and OT refer to clones derived from these respective thymocyte cDNA libraries. (C) Schematic representation of the regions of mNotch-1 cloned into the pMI retroviral vector and transferred into the AKR1010 and 2B4.11 T cell lines for functional assays. NotchIC begins at the first amino acid of the RAM domain. NotchIC/mM2-2 contains a mutation in the RAM domain that disrupts RBP/Su(H) binding (Tamura et al., 1995). NotchIC/ΔAnk begins immediately amino terminal to the third ankyrin repeat of Notch.

Figure 2

Expression of NotchIC Confers Resistance to Glucocorticoid-Induced Apoptosis and Cell Cycle Arrest in T Cells (A) Expression of NotchIC in AKR1010 and 2B4.11 confers resistance to dexamethasone. AKR1010 and 2B4.11 were infected with retrovirus derived from the pMI/NotchIC vector. Following infection, transgene-expressing cells were identified by FACS analysis for IRES-driven hCD2 expression. The cells were then cultured in medium alone or in medium containing 10⁻⁷ M dexamethasone for 7 days followed by staining for hCD2 expression. Numbers represent the percentage of transgene-expressing cells in the indicated gates. (B) Expression of NotchIC confers resistance to glucocorticoid-induced apoptosis and cell-cycle arrest in AKR1010. Control cells and polyclonal AKR1010 cells expressing NotchIC were cultured in the presence of 10⁻⁷ M dexamethasone for the indicated times and analyzed for DNA content by FACS. Numbers represent the percentage of cells in the indicated gates. (C) Expression of NotchIC confers resistance to glucocorticoid-induced apoptosis and cell cycle arrest in 2B4.11. Control cells and polyclonal 2B4.11 cells expressing NotchIC were assayed for sensitivity to glucocorticoid-induced apoptosis and cell cycle arrest as in (B).

Figure 3

NotchIC-Mediated Resistance to Glucocorticoids Correlates with Activation of the Notch Signaling Pathway and Maps to the RAM Domain of Notch (A) Expression of NotchIC upregulates endogenous mNotch-1 expression in AKR1010 and 2B4.11. Vector control cells and polyclonal AKR1010 and 2B4.11 cells expressing NotchIC were analyzed for endogenous mNotch-1 expression by Western blot analysis. (B) Resistance to glucocorticoids in T cells is mediated by the RAM and ankyrin domains of mNotch-1. Polyclonal 2B4.11 cell lines expressing the different NotchIC constructs diagrammed in Figure 1C were incubated in increasing concentrations of dexamethasone for 24 hr and their proliferation measured by ³H thymidine incorporation during an additional 12 hr of culture. Expressed is the percentage of proliferation relative to cells cultured in the absence of dexamethasone. Counts incorporated in the absence of dexamethasone are 62158 ± 2686 cpm for NotchIC; 65778 ± 3759 cpm for NotchIC/mM2-2; 72343 ± 4864 cpm for NotchIC/ΔAnk; and 89784 ± 2942 cpm for vector alone.

Figure 4

Western Blot Analysis of Bcl-2 and Bcl-X_L Expression in AKR1010 and 2B4.11 Cells Expressing NotchIC Lysates from the indicated cell lines were analyzed by Western blot analysis for Bcl-2 expression. The blots were then stripped and reprobed for Bcl-X_L. Analysis of two additional dexamethasone-resistant AKR1010 clones isolated by expressing cloning gave identical results to the RH4 clone.

Figure 5

Expression of NotchIC Upregulates TCR Expression (A) Expression of NotchIC in AKR1010 induces TCR surface expression. AKR1010 was infected with retrovirus prepared from control vector or the pMI/NotchIC vector. Two days following infection, the cells were assayed for IRES-driven hCD2 expression and CD3 expression by FACS analysis. Gates on uninfected and infected cells were established based on hCD2 expression (top panels). Cells falling in these gates were then analyzed for CD3 expression (lower panels). Identical results were obtained using the H57-597 MAb directed against the αβTCR. (B) Dose-dependent relationship between the level of NotchIC expression and TCR expression. Vector control and polyclonal AKR1010 cell lines expressing NotchIC were stained simultaneously for hCD2 expression and CD3 expression. Gates for different levels of hCD2 expression were established, and cells within these were analyzed for CD3 expression. The dotted line represents background staining levels. (C) Upregulation of TCR expression maps to the RAM domain of Notch. Polyclonal AKR1010 cell lines expressing NotchIC, NotchIC/mM2-2, or control vector were analyzed for CD3 expression as in (B). The graph represents the percentage of cells expressing CD3 relative to the level of transgene expression.

Figure 6

Deltex Is Positively Regulated by Notch Signaling and Is Expressed at High levels in DN and CD4⁺ and CD8⁺ SP Thymocytes and Low Levels in DP Thymocytes (A) NotchIC expression upregulates Deltex expression. AKR1010, the RH4 dexamethasone-resistant clone, and polyclonal AKR1010 expressing NotchIC were analyzed for Deltex expression by Northern blot analysis. (B) Expression of Deltex in thymocyte subpopulations. cDNA prepared from FACS-sorted thymocyte subpopulations was serially diluted at 3-fold and amplified by PCR with primers specific for Deltex. The PCR products were analyzed by Southern blot using a Deltex-specific probe. Amplification of the same cDNA using primers specific for HPRT was performed to control for template quality.

Figure 7

Expression of NotchIC Confers Resistance to Glucocorticoids in DP Thymocytes from Transgenic Mice Thymocytes from NotchIC-9 and littermate control mice were incubated in medium alone or medium supplemented with 10⁻⁸ M dexamethasone for the indicated times and analyzed by FACS for viability and CD4 and CD8 expression. The graph shows the percentage of viable DP thymocytes in cultures containing dexamethasone relative to viable DP thymocytes in cultures containing medium alone. Similar results were obtained in three independent experiments.