All hematopoiesis cells develop from multipotent progenitor cells. Hematopoietic stem cells (HSC) have the ability to develop into all blood lineages but also maintain their stemness. Different molecular mechanisms have been identified that are crucial for regulating quiescence and self‐renewal to maintain the stem cell pool and for inducing proliferation and lineage differentiation. The stem cell niche provides the microenvironment to keep HSC in a quiescent state. Furthermore, several transcription factors and epigenetic modifiers are involved in this process. These create modifications that regulate the cell fate in a more or less reversible and dynamic way and contribute to HSC homeostasis. In addition, HSC respond in a unique way to DNA damage. These mechanisms also contribute to the regulation of HSC function and are essential to ensure viability after DNA damage. How HSC maintain their quiescent stage during the entire life is still matter of ongoing research. Here we will focus on the molecular mechanisms that regulate HSC function.

In complex organisms, stem cells are key for tissue maintenance and regeneration. Adult stem cells replenish continuously dividing tissues of the epithelial and connective types, whereas in non-growing muscle and nervous tissues, they are mainly activated upon injury or stress. In addition to replacing deteriorated cells, adult stem cells have to prevent their exhaustion by self-renewal. There is mounting evidence that both differentiation and self-renewal are impaired upon aging, leading to tissue degeneration and functional decline. Understanding the molecular pathways that become deregulate in old stem cells is crucial to counteract aging-associated tissue impairment. In this review, we focus on the epigenetic mechanisms governing the transition between quiescent and active states, as well as the decision between self-renewal and differentiation in three different stem cell types, i.e., spermatogonial stem cells, hematopoietic stem cells, and muscle stem cells. We discuss the epigenetic events that channel stem cell fate decisions, how this epigenetic regulation is altered with age, and how this can lead to tissue dysfunction and disease. Finally, we provide short prospects of strategies to preserve stem cell function and thus promote healthy aging.

RNA interference can be applied to different target genes for treating a variety of diseases, but an appropriate delivery system is necessary to ensure the transport of intact siRNAs to the site of action. In this study, cellulose was dually modified to create a non-viral vector for HDAC3 short interfering RNA (siRNA) transfer into cells. A guanidinium group introduced positive charges into the cellulose to allow complexation of negatively charged genetic material. Furthermore, a biotin group fixed by a polyethylene glycol (PEG) spacer was attached to the polymer to allow, if required, the binding of targeting ligands. The resulting polyplexes with HDAC3 siRNA had a size below 200 nm and a positive zeta potential of up to 15 mV. For N/P ratio 2 and higher, the polymer could efficiently complex siRNA. Nanoparticles, based on this dually modified derivative, revealed a low cytotoxicity. Only minor effects on the endothelial barrier integrity and a transfection efficiency in HEK293 cells higher than Lipofectamine 2000^TM were found. The uptake and release of the polyplexes were confirmed by immunofluorescence imaging. This study indicates that the modified biopolymer is an auspicious biocompatible non-viral vector with biotin as a promising moiety.

Bringing light into gene regulation in hematopoietic stem cells by the Mediator complex

Liver-specific genes regulating homeostatic functions are markedly repressed following an inflammatory stimulus. Conversely, transcriptional resources are reallocated to boost acute-phase genes, driving the systemic immune reaction termed the acute phase response (APR). While the liver-enriched transcription factor hepatocyte nuclear factor 4 alpha (HNF4A) directs expression of more than 50% of hepatic genes, its role in the APR is poorly investigated. To analyze inflammatory gene profile in hepatocytes, we adopted an in vitro APR model of LPS-treated differentiated HepaRG (dHepaRG) cells. RNA sequencing and transcription factor enrichment of the downregulated gene set directed our research focus to HNF4A. The APR resulted in a rapid repression of HNF4A RNA and protein levels. Consequently, reduced binding of HNF4A to regulatory regions of acute-phase genes serum amyloid A (SAA) and haptoglobin (HP) was followed by a chromatin-opening and derepression of these genes. Vice versa, HNF4A overexpression potently inhibited activation of acute-phase genes by maintaining a repressive chromatin state. This result was reproducible in a liver organoid model, where HNF4A overexpression protected from severe loss of liver function while limiting the APR. We demonstrate that a rapid repression of HNF4A levels and activity is required to enable a transcriptomic switch during the hepatic acute-phase response. HNF4A overexpression impaired acute-phase gene expression while preserving liver functionality, offering potential for the treatment of liver dysfunction during systemic inflammation or sepsis.