Yang
Laboratory

SUMOylation is a branch of ubiquitination-like (Ubl) posttranslational modifications (PTMs) that conjugate monomeric SUMO (an ~100 aa protein tag) or polySUMO chains to target proteins, with a strong connection to stress responses and aging. A major function of mono sumoylation is to recruit sumo-interacting motifs (SIMs)-containing proteins to form protein complexes. In contrast, polysumoylation commonly destabilizes target proteins by promoting their ubiquitination, resulting in complex disassembly. Such versatility in protein complexes assembly/disassembly makes sumoylation important for cells to respond and adapt to changes and stresses. Additionally, sumoylation can mask other PTM sites or alter protein spatial conformation. SUMO modifications can be deconjugated by SENP family desumoylases, most of which remove only mono-sumoylation, whereas SENP6 and SENP7 can edit/remove poly-SUMO chains. The sumoylation/desumoylation equilibrium regulates many cellular processes, including DNA damage response, mitochondrial dynamics, epigenetic regulation, cell proliferation, senescence and apoptosis.

After SNPs in the SENP6 gene locus were identified as a top genetic risk factor for osteoarthritis, we began exploring the role of sumoylation in skeletal development and diseases. We found that losing SENP6 in mice leads to skeletal development defects and premature aging, partly due to elevated apoptosis and senescence in the progenitor compartments. Mechanistically, SENP6 desumoylates TRIM28, a multifaceted protein involved in chromatin silencing and p53 inhibition. Senp6 loss results in excessive SUMOylation of TRIM28; this weakens TRIM28’s function as a p53 suppressor, causing p53 hyperactivation and elevated senescence. Because chondrocyte senescence, joint aging and osteoarthritis (OA) susceptibility function in a feed-forward loop, SENP6 and its downstream pathways may be used as therapeutic targets. We are currently working in this direction. In addition, we are studying SENP6’s role in osteoporosis in various cellular contexts.

We also found that a sumoylation inhibitor, ginkgolide acid, overrides mesenchymal stem cells’ fate from osteoblast toward adipocyte, even under osteogenic differentiation conditions. Meanwhile, we have developed a tracking tool to dynamically visualize polysumoylated proteins in live cells, providing a new approach to the sumoylation research community.

As with other stem cells, lineage-specific transcription factors and the microenvironment cooperatively regulate skeletal stem cell (SSC) maintenance and differentiation, but their effectiveness depends on the permissiveness of the epigenetic landscape. The epigenetic landscape of a cell reflects its history of lineage progression and predetermines its future identity. In the mammalian skeletal system, appendicular bones and most axial bones develop from mesoderm-derived skeletal precursors/stem cells (SSCs). In contrast, most craniofacial bones arise from ectoderm-derived neural crest cells (NCCs). NCCs have a default neural/glial and epidermal fate but acquire an osteochondrogenic potential through transient reactivation of pluripotency. We were curious whether and how chromatin silencing marks contribute to establishing SSC identity and guide SSC lineage progression for skeletogenesis. TRIM28 is an adaptor protein that recruits specific enzymes to promote H3K9me3 and DNA methylation, which makes it ideal for addressing this question. Our study found that loss of TRIM28 in mesoderm-derived cells expands the SSC pool, but weakens SSC osteochondrogenic potential while endowing SSCs with properties of ectoderm-derived NCCs, leading to severe defects of skeletogenesis. TRIM28 preferentially silences chromatin regions that are more accessible in NCCs; loss of this silencing upregulates neural gene expression and enhances neurogenic potential. Moreover, TRIM28 loss causes hyperexpression of GREM1, an extracellular signaling factor promoting SSC self-renewal and neurogenic potential by activating AKT/mTORC1 signaling. This work suggests that TRIM28-mediated chromatin silencing establishes a barrier to securing the SSC lineage trajectory and prevents a transition to ectodermal fate by regulating intrinsic and microenvironment cues.

The other branch of our research focuses on how epigenetic mechanisms contribute to the development of age-related skeletal diseases. With age, cells gradually lose chromatin silencing marks, such as DNA methylation or histone methylation. Loss of these marks leads to the reactivation of transposable elements (TEs), which comprise about 50% of the human genome. Reciprocally, the host organism develops chromatin-silencing mechanisms to counteract/silence TE activity.  Endogenous retroviruses (ERVs), a major type of TE, are genetic remnants of past retrovirus infections in the host genome throughout evolution. Currently, most ERV elements are not transposed. However, they still contribute to cis-regulatory function and genetic diversity in the hosts or produce double-stranded RNAs (dsRNAs) that stimulate inflammation response. Our lab found that the chromatin accessibility and transcription of ERVs, particularly from evolutionarily “intermediate age” ERV families, were enriched and elevated in OA cartilage. We also found that global H3K9me3 levels were reduced in osteoarthritic or aging cartilage. This study unveils the intricate relationship between epigenetic alterations, ERV activation and OA, paving the way for potential therapeutic interventions by targeting these pathogenic mechanisms. Our lab seeks a comprehensive understanding of the interaction between aging, ERV activation and OA pathogenesis, with the goal of using this knowledge to alleviate OA development.

In collaboration with the Krawczyk Lab at VAI, we are studying how an X-linked lysine demethylase governs sex-dimorphic bone mass regulation by modulating energy metabolism in osteoclast lineage. The outcome of this work may lead to new approaches to enhance women’s bone health.

We study signaling dysregulation and cellular stress-related skeletal disorders. We are also exploring innovative approaches to alleviate OA symptoms and regenerate articular cartilage in the OA joints through stem cell and gene therapies.