Thesis
English
ID: <
10670/1.6jwhzj>
Abstract
summary Successful development of T cells in the thymus is dependent on the proliferation, differentiation and survival of precursor cells. Some of the key events of T cell ripening critically depend on the T cell receptor (TCR). At an early stage of development, T cell maturation is mediated by the pre-TCR, which depends on a functionally rearranged TCRβ chain. Additional factors unrelated to the TCR (like notch, SCF, IL7, TCF-1) are also crucial for linking decisions and normal T cell development. This thesis has addressed the role of the HMG (high mobility group) box transcription factor T cell factor-1 (TCF-1) for thymocyte development. TCF-1 has been identified as a nuclear effector of the canonical Wnt signing pathway. TCF-1 activates the transcription of Wnt responding target genes upon Interacting with the Wnt-signaling intermediate β-catenin. TCF-1 deficiency leads to separate thymocyte developmental blocks. At the most immature traineeship of thymocyte development (i.e. the CD4-CD8- cells) one block concerns the pre-TCR checkpoint where extensive thymocyte proliferation is normally covered. Here, we have added the previous role of TCF-1 at the pre-TCR traineeship to see what pre-TCR dependant and independent pathways co-operate to ensure thymocyte development. We found that TCF-1-deficient thymocytes are capable to generate a functional pre-TCR since cycling and differentiation were normally qualified. However, preTCR + TCF- 1-deficient thymocytes underwent rapid cell death upon growth factor withdrawal. TCF-1 thus proliferative separates from survival signals in immature pre-TCR + thymocytes. Moreover, we show that the catenin domain of TCF-1 is required for the survival of proliferating pre- TCR + thymocytes. However, thymocyte survival did not emerge on the putative co-activator γ-catenin, suggesting a major role for β-catenin. Active β-catenin (signing-competent) is present in immature thymocytes prior to pre-TCR signaling so active β-catenin arises independently from pre-TCR signaling, most likely via Wnt signals. On the induction of pre-TCR signaling, TCF-1 was upgraded as the level of signing-competent β-catenin did not increase. Thus, Wnt signals maintain a pool of stable β-catenin whereas pre- TCR signaling induces TCF-1. These findings suggest that TCF-1 mediates the cooperation of pre-TCR and canonical Wnt signals to ensure survival of pre-TCR + thymocytes during thymocyte development. Resumes The T precursor cells develop into mature cells within the thymus during proliferation and differentiation phenomena. Crucial events in the maturation of T lymphocytes depend on the antigen receptor in T cells, the TCR. At an early stage of development, an immature shape of the TCR, the pre-TCR allows this ripening. Other factors of a different nature to the TCR (notch, SCF, IL7, TCF-1) also play a decisive role in ensuring normal development of T lymphocytes. However, these mechanisms are still unclear. Thus, my thesis was to study the role of the transcription factor TCF-1 (T-cell factor 1) during the maturation of thymocytes. TCF-1 is a key nuclear effector of the Wnt signalling track. TCF-1 in combination with the β-catenine protein, the second messenger of the Wnt signalling route, allows the transcription of target genes. The development of T lymphocytes is defective in mice not expressing TCF-1, suggesting the importance of TCF-. At an early stage of development, the ripening of the T-lymphocytes is altered at the pre-TCR control point. In this study, we analysed the precise role of TCF-1 at this control point and assessed whether there was any relevance between the signalling routes dependent and independent of the pre-TCR. The results presented here show that, in the absence of TCF-1, immature thymocytes form a functional pre-TCR able to induce normal proliferation and differentiation of T lymphocytes, however, these cells have higher mortality. The TCF-1 factor therefore separates the proliferation signals from the survival signals normally provided by the pre-TCR. In addition, we have shown that the TCF-1 domain allowing binding to beta-catenine molecule is essential for thymocyte survival at the pre-TCR control point. At a differentiation stage prior to the pre-TCR control point, the active form of beta-catenine signalling is present. It is therefore not induced by the pre-TCR, but rather by Wnt signalling. However, pre-TCR signalling induces the expression of TCF-1 while the active form of β-catenine is not affected. These results show that the transcription factor TCF-1 allows cooperation between pre-TCR and Wnt signalling tracks ensuring thymocyte survival at the pre-TCR control point.