Citrate carrier links chromatin, metabolism and stemness upon ageing and exposure to high oxygen

Ageing is accompanied by a general decline in the function of many cellular pathways. Although the contribution of each individual pathway in ageing has been extensively studied over the last years, how these pathways crosstalk to regulate the development and progression of ageing remained elusive. Here, I sought to determine whether ageassociated changes in mitochondrial function, epigenetic modifications and stem cell activity are causally or functionally interconnected. Therefore, I studied the effects of mitochondrial–nuclear communication on stem cell function upon ageing. I found that aged mesenchymal stem cells isolated from the bone marrow (BM-MSCs) exhibit reduced chromatin accessibility and lower histone acetylation, particularly on promoters and enhancers of osteogenic genes. The reduced histone acetylation is due to impaired export of mitochondrial acetyl-CoA, owing to the lower levels of citrate carrier (CiC). I demonstrated that aged cells show enhanced lysosomal degradation of CiC, which is mediated via mitochondrial-derived vesicles. Strikingly, restoring cytosolic acetyl-CoA levels either by exogenous CiC expression or via acetate supplementation, remodels the chromatin landscape and rescues the osteogenesis defects of aged BM-MSCs. Collectively, my results establish a tight, age-dependent connection between mitochondrial quality control, chromatin and stem cell fate, which are altogether linked by CiC. The bone marrow stroma is characterized by low oxygen concentration (hypoxia), which is essential for the maintenance of BM-MSC stemness. However, in vitro BMMSC culture during stem cell therapies is performed under high oxygen conditions (normoxia), which could dramatically impact BM-MSC activity. Here, I explored how the metabolism-chromatin-stemness axis is affected by oxygen tension. I found that high oxygen impairs osteogenesis irreversibly, due to higher chromatin compaction and lower histone acetylation on promoters and enhancers of osteogenic genes. Although normoxia induces a metabolic switch which results in production of higher acetyl-CoA levels, I showed that this remains trapped inside the mitochondria, potentially due to lower CiC activity. Impressively, modulating CiC function impacts both the metabolic and the epigenetic profile of BM-MSCs, whereas exogenous supplementation with acetate restores the osteogenic differentiation capacity of normoxia-cultured cells

N1-acetylspermidine is a determinant of hair follicle stem cell fate

Stem cell differentiation is accompanied by increased mRNA translation. The rate of protein biosynthesis is influenced by the polyamines putrescine, spermidine and spermine, which are essential for cell growth and stem cell maintenance. However, the role of polyamines as endogenous effectors of stem cell fate and whether they act through translational control remains obscure. Here, we investigate the function of polyamines in stem cell fate decisions using hair follicle stem cell (HFSC) organoids. Compared to progenitor cells, HFSCs showed lower translation rates, correlating with reduced polyamine levels. Surprisingly, overall polyamine depletion decreased translation but did not affect cell fate. In contrast, specific depletion of natural polyamines mediated by spermidine/spermine N1-acetyltransferase (SSAT; also known as SAT1) activation did not reduce translation but enhanced stemness. These results suggest a translation-independent role of polyamines in cell fate regulation. Indeed, we identified N1-acetylspermidine as a determinant of cell fate that acted through increasing self-renewal, and observed elevated N1-acetylspermidine levels upon depilation-mediated HFSC proliferation and differentiation in vivo. Overall, this study delineates the diverse routes of polyamine metabolism-mediated regulation of stem cell fate decisions. This article has an associated First Person interview with the first author of the paper.Peer reviewe

FOXA2 controls the anti-oxidant response in FH-deficient cells

Hereditary leiomyomatosis and renal cell cancer (HLRCC) is a cancer syndrome caused by inactivating germline mutations in fumarate hydratase (FH) and subsequent accumulation of fumarate. Fumarate accumulation leads to profound epigenetic changes and the activation of an anti-oxidant response via nuclear translocation of the transcription factor NRF2. The extent to which chromatin remodeling shapes this anti-oxidant response is currently unknown. Here, we explored the effects of FH loss on the chromatin landscape to identify transcription factor networks involved in the remodeled chromatin landscape of FH-deficient cells. We identify FOXA2 as a key transcription factor that regulates anti-oxidant response genes and subsequent metabolic rewiring cooperating without direct interaction with the anti-oxidant regulator NRF2. The identification of FOXA2 as an anti-oxidant regulator provides additional insights into the molecular mechanisms behind cell responses to fumarate accumulation and potentially provides further avenues for therapeutic intervention for HLRCC

Long-lived macrophage reprogramming drives spike protein-mediated inflammasome activation in COVID-19

Innate immunity triggers responsible for viral control or hyperinflammation in COVID-19 are largely unknown. Here we show that the SARS-CoV-2 spike protein (S-protein) primes inflammasome formation and release of mature interleukin-1 beta (IL-1 beta) in macrophages derived from COVID-19 patients but not in macrophages from healthy SARS-CoV-2 naive individuals. Furthermore, longitudinal analyses reveal robust S-protein-driven inflammasome activation in macrophages isolated from convalescent COVID-19 patients, which correlates with distinct epigenetic and gene expression signatures suggesting innate immune memory after recovery from COVID-19. Importantly, we show that S-protein-driven IL-1 beta secretion from patient-derived macrophages requires non-specific monocyte pre-activation in vivo to trigger NLRP3-inflammasome signaling. Our findings reveal that SARS-CoV-2 infection causes profound and long-lived reprogramming of macrophages resulting in augmented immunogenicity of the SARS-CoV-2 S-protein, a major vaccine antigen and potent driver of adaptive and innate immune signaling