Lamin B partecipates in cartilage senescence and degeneration

It has been demonstrated that nuclear lamins play a key role in a wide range of nuclear functions, including transcription, DNA replication and repair, cell proliferation and differentiation of specific lineages during development and adult life. Given the role of lamins in aging as well as disease (the premature aging disease, Hutchinson-Gilford Progeria Syndrome, HGPS, is the most intensively studied laminopathy), direct and indirect approaches to understanding the significance of the differential expression of lamins could be important to human health. More recently, lamins have been linked to stem cell niche function suggesting that the nucleoplasmic lamin pool and its associated proteins may have important function in chromatin organization, cell signaling and cell cycle control in adult tissue stem cells. Consequently, tissue homeostasis based on stem cell activity could be disrupted by different mutations and expression levels of lamins that can lead to premature senescence and tissue degeneration. This notion is supported by recent demonstration of the relationship between lamins, tissue stiffness and environmental signals that drive differentiation from progenitor cells. Therefore, a defect of adult stem cell functions, coupled with a potentially increased mechanical sensitivity, could result in an inefficient repair of damaged tissues in laminopathies. The information gathered to date on the lamins can be relevant not only for the understanding of molecular mechanisms underlying the aging process, but also for the developing of more effective regenerative medicine techniques and better drug targets. It has been recently shown that overexpression of lamin A occurs in chondrocytes from osteoarthritis (OA) leading to cellular senescence. OA is a common complex degenerative joint disease whose prominent risk factor for its development is aging. In this study, we wondered whether the process of de-differentiation and senescence in OA chondrocytes may resemble what happens in the joint of individuals with premature aging. At the same time we are interested in understanding whether an OA in vitro model may help to understand the mechanisms that may occur in premature ageing syndromes. As experimental models we used human primary chondrocytes from healthy subjects and from OA patients. A first set of experiments demonstrated that OA chondrocytes strongly resemble the behavior of de-differentiated chondrocytes obtained through prolonged passages in culture and mesenchymal stem cells (MSCs) as they express very low levels of typical chondrogenic markers including collagen type II, aggrecan, Sox9 and TRPS1. Conversely, they express high level of Slug transcription factor, a negative regulator of chondrogenesis. For the first time, a marked Slug expression was observed in a sample of HGPS patient derived fibroblasts associated with a strong nuclear localization, suggesting that Slug may be a new potential marker of senescence. Consistent with these observations, cartilage histological sections and chondrocytes from OA showed an increase of both Lamin A and Lamin B1 expression, and a co-localization of lamin B1 and Slug expression. This prompted us to further this correlation. After bioinformatics analysis showing the existence of putative Slug consensus sites (E boxes) in the LMNB1 promoter, we demonstrated an in vivo recruitment of Slug at Lamin B1 promoter by chromatin immunoprecipitation (ChIP analysis). This suggests for the first time that Lamin B1 expression is dependent on Slug regulation and that this may play a critical role in the process of cartilage degeneration. Considering that cellular senescence is emerging as an important issue in stem cell–based therapies, our data may be relevant for two main issues: i. improvement of information about the regulation of lamin B gene expression, and ii. characterization of a new cellular approach to study senescence and connective tissue degeneration processes.