CyclinD1 and the mechanisms of thymic involution

描述（由申请人提供）：胸腺对T细胞的发育至关重要。胸腺上皮细胞（TECs）提供必要的生长、分化和存活信号，使胸腺细胞分化成熟为T细胞并向外周迁移。胸腺退化减少了na - ve T细胞的输出，这限制了TCR库并损害了对新遇到抗原的免疫反应。在衰老或细胞消融治疗后，TECs的消耗是导致胸腺退化的主要因素。我们发现，在表达角蛋白5 （K5）的TECs中，增强Cyclin D1的表达会导致胸腺增生，但功能正常，在衰老过程中不会发生退化。这些数据表明，持续高水平的Cyclin D1可增强TEC的增殖和分化，进而调节胸腺稳态和退化。本提案的一个主要目标是确定K5的机制。Cyclin D1转基因预防胸腺退化。虽然Cyclin D1是一种众所周知的细胞周期调节剂，但它也具有转录调节剂的功能。Cyclin D1b是一种选择性剪接的异构体，其中保留了Cyclin盒，但删除了外显子5。外显子5包含一个LxxLL基序，这是一个核受体相互作用域，通过招募转录共激活子来增强转录。我们发现胸腺在K5的正常时间范围内渐开线。CyclinD1b转基因小鼠。K5中不同的内翻表型。Cyclin D1和K5。Cyclin D1b小鼠暗示转录调控是Cyclin D1强制表达防止衰老的关键机制。与这个观点相一致，我们发现K5。Cyclin D1转基因提高了Foxn1的表达，Foxn1是TEC发展和维持所需的转录因子。我们将使用K5。CyclinD1和K5。CyclinD1b转基因细胞系鉴定内源性因子和途径，调节胸腺内化和幼稚T细胞输出。目标1和目标2将检验假设K5。CyclinD1通过控制细胞周期进程和转录调控影响胸腺稳态。目的3探讨预防胸腺内翻对外周血T细胞池组成的影响。由于我们发现年轻K5的脾脏中最近胸腺迁移（rte）的数量增加。在Cyclin D1小鼠中，我们将验证防止胸腺内翻维持RTE输出的假设，这反过来在衰老或细胞消融治疗后维持了不同的初始周围T细胞池。本研究将为制定预防或逆转胸腺退化的治疗策略提供合理依据。

DESCRIPTION (provided by applicant): The thymus is essential for T cell development. Thymic epithelial cells (TECs) supply essential growth, differentiation and survival signals as thymocytes differentiate mature to na¿ve T cells and emigrate to the periphery. Thymus involution reduces na¿ve T cell output, which restricts the TCR repertoire and impairs immune responses to newly encountered antigens. Depletion of TECs is a major factor contributing to thymus involution during aging or after cytoablative therapy. We find that enforcing Cyclin D1 expression in keratin 5 (K5) expressing TECs results in a hyperplastic, yet functional thymus that fails to undergo involution during aging. These data suggest that sustained high levels of Cyclin D1 enhances TEC proliferation and differentiation, which, in turn, regulates thymus homeostasis and involution. A major goal of this proposal is to determine the mechanism(s) by which the K5.Cyclin D1 transgene prevents thymus involution. Although Cyclin D1 is a well-known cell cycle regulator, it also functions as a transcriptional regulator. Cyclin D1b is an alternatively spliced isoform in which the cyclin box is retained, but exon 5 is deleted. Exon 5 contains an LxxLL motif, which is a nuclear receptor interaction domain that enhances transcription by recruiting transcriptional coactivators. We find that the thymus involutes within normal time frame in K5.CyclinD1b transgenic mice. The distinct involution phenotypes in K5.Cyclin D1 and K5.Cyclin D1b mice implicate transcriptional regulation as a key mechanism by which enforced expression of Cyclin D1 prevents involution. Consistent with this notion, we find that the K5.Cyclin D1 transgene enhances expression of Foxn1, a transcription factor required for TEC development and maintenance. We will use the K5.CyclinD1 and K5.CyclinD1b transgenic lines to identify endogenous factors and pathways that regulate thymus involution and naive T cell output. Aims 1 and 2 will test the hypothesis that K5.CyclinD1 influences thymus homeostasis both by control of cell cycle progression and by transcriptional regulation. Aim 3 explores the consequences of preventing thymus involution on the composition of the peripheral T cell pool. Since we find an increased number of recent thymic emigrants (RTEs) in spleens of young K5.Cyclin D1 mice, we will test the hypothesis that preventing thymus involution sustains RTE output, which in turn maintains a diverse pool of naive peripheral T cells during aging or after cytoablative therapy. This investigation will provid a rational basis for developing therapeutic strategies to prevent or reverse thymus involution.