No / Thymic epithelial cells (TECs) are the main component of the thymic stroma, which supports T-cell proliferation and repertoire selection. Here, we demonstrate that Cbx4, a Polycomb protein that is highly expressed in the thymic epithelium, has an essential and non-redundant role in thymic organogenesis. Targeted disruption of Cbx4 causes severe hypoplasia of the fetal thymus as a result of reduced thymocyte proliferation. Cell-specific deletion of Cbx4 shows that the compromised thymopoiesis is rooted in a defective epithelial compartment. Cbx4-deficient TECs exhibit impaired proliferative capacity, and the limited thymic epithelial architecture quickly deteriorates in postnatal mutant mice, leading to an almost complete blockade of T-cell development shortly after birth and markedly reduced peripheral T-cell populations in adult mice. Furthermore, we show that Cbx4 physically interacts and functionally correlates with p63, which is a transcriptional regulator that is proposed to be important for the maintenance of the stemness of epithelial progenitors. Together, these data establish Cbx4 as a crucial regulator for the generation and maintenance of the thymic epithelium and, hence, for thymocyte development.

Human epidermis relies on a population of adult stem cells to maintain its homeostasis. Stem cells transit from a dormant to an active state and undergo a tightly regulated process of differentiation that replenishes the tissue according to its needs. This process either replaces cells that get shed away, or contributes to tissue healing upon injuries, such as wounding. Distinct molecular mechanisms are required to keep human epidermal stem cells localized in their niche and for their active proliferation and mobilization, while others regulate their differentiation status. However, little is known about the proper global chromatin modifications that ensure the correct transition between these stem cell states. This work shows that Cbx4, a Polycomb Repressive Complex-1 (PRC1)-associated protein, maintains human epidermal stem cells slow-cycling and undifferentiated, while protecting them from senescence. Interestingly, abrogating the polycomb activity of Cbx4 impairs its anti-senescent function without affecting stem cell differentiation, indicating that differentiation and senescence are independent processes in human epidermis. Conversely, Cbx4 inhibits stem cell activation and differentiation through its SUMO ligase activity. Global transcriptome and chromatin occupancy analyses indicate that Cbx4 regulates modulators of epidermal homeostasis and represses factors, such as Ezh2, Dnmt1, and Bmi1, to prevent the active stem cell state. Interestingly, Cbx4 also represses genes required for neuronal fate repression, suggesting that it might have a role in ectoderm patterning during development. Cbx proteins are differently expressed during epidermal differentiation and the activity of Cbx4 towards promoting human epidermal stem cell quiescence is unique among the Cbx proteins. This suggests that different Polycomb complexes are assembled, based on the availability of its core member, and balance epidermal stem cell dormancy and activation, while continually preventing senescence and differentiation. / La homeostasis de la epidermis humana depende de una población de células troncales adultas (CTAs). Las CTAs alternan ciclos de quiescencia y actividad, seguidos por una regulación estricta de su diferenciación, según las necesidades celulares del tejido. Este proceso es esencial para repoblar el tejido de células envejecidas o dañadas. Cada estadío por el que transita una CTA está regulado por procesos moleculares específicos. Sin embargo, aún sabemos poco sobre los procesos que regulan la reorganización de la cromatina necesarios para mediar dichas transiciones en la población de las CTAs. Estos resultados demuestran que la proteina Cbx4, pertenciente al complejo Polycomb Repressive Complex-1 (PRC1), es necesaria para mantener a las CTAs de la epidermis humana quiescentes, indiferenciadas, y protegidas de la senescencia. A nivel molecular, la actividad polycomb de Cbx4 es únicamente necesaria para su función antisenescente, pero es dispensable para la regulación de la proliferación y diferenciación de las CTAs. La inhibición de la proliferación y diferenciación celular sin embargo depende de la activdad E3 SUMO ligasa de Cbx4. Analisis del transcriptoma global y de unión a la cromatina (ChIP), demuestran que Cbx4 regula la expresión de moduladores esenciales de la homeostasis de la epidermis, y reprime la expresión de factores necesarios para la activación de las CTAs, tales como Ezh2, Dnmt1 y Bmi1. Cabe destacar que Cbx4 también reprime la expresión de genes que determinam el linage neuronal, lo que sugiere que Cbx4 pueda ser importante para separar el neuroectodermo entre ectodermo y neuronas, durante el desarrollo embrionario. Cbx4 es la única proteina Cbx capaz de inducir entrada en quiescencia de las CTAs, y el resto de proteinas Cbx se expresa de forma diferente durante la diferenciación en la epidermis. Por lo tanto, nuestros estudios sugieren que la actividad de distintos complejos Polycomb actúa en los sucesivos estadíos de quiescencia, proliferación y diferenciación de las CTAs, a la vez que impiden su senescencia de forma constante.