Restoring cell polarity: an HSC fountain of youth

Autophagy-Sirt3 axis decelerates hematopoietic aging

Autophagy suppresses mitochondrial metabolism to preserve hematopoietic stem cells (HSCs) in mice. However, the mechanism by which autophagy regulates hematopoietic aging, in particular in humans, has largely been unexplored. Here, we demonstrate that reduction of autophagy in both hematopoietic cells and their stem cells is associated with aged hematopoiesis in human population. Mechanistically, autophagy delays hematopoietic aging by activating the downstream expression of Sirt3, a key mitochondrial protein capable of rejuvenating blood. Sirt3 is the most abundant Sirtuin family member in HSC‐enriched population, though it declines as the capacity for autophagy deteriorates with aging. Activation of autophagy upregulates Sirt3 in wild‐type mice, whereas in autophagy‐defective mice, Sirt3 expression is crippled in the entire hematopoietic hierarchy, but forced expression of Sirt3 in HSC‐enriched cells reduces oxidative stress and prevents accelerated hematopoietic aging from autophagy defect. Importantly, the upregulation of Sirt3 by manipulation of autophagy is validated in human HSC‐enriched cells. Thus, our results identify an autophagy‐Sirt3 axis in regulating hematopoietic aging and suggest a possible interventional solution to human blood rejuvenation via activation of the axis.

Biophysical regulation of hematopoietic stem cells

Blood is renewed throughout the entire life. The stem cells of the blood, called hematopoietic stem cells (HSCs), are responsible for maintaining a supply of all types of fresh blood cells. In contrast to other stem cells, the clinical application of these cells is well established and HSC transplantation is an established life-saving therapy for patients suffering from haematological disorders. Despite their efficient functionality throughout life in vivo, controlling HSC behaviour in vitro (including their proliferation and differentiation) is still a major task that has not been resolved with standard cell culture systems. Targeted HSC multiplication in vitro could be beneficial for many patients, because HSC supply is limited. The biology of these cells and their natural microenvironment - their niche - remain a matter of ongoing research. In recent years, evidence has come to light that HSCs are susceptible to physical stimuli. This makes the regulation of HSCs by engineering physical parameters a promising approach for the targeted manipulation of these cells for clinical applications. Nevertheless, the biophysical regulation of these cells is still poorly understood. This review sheds light on the role of biophysical parameters in HSC biology and outlines which knowledge on biophysical regulation identified in other cell types could be applied to HSCs.

The ageing haematopoietic stem cell compartment

Stem cell ageing underlies the ageing of tissues, especially those with a high cellular turnover. There is growing evidence that the ageing of the immune system is initiated at the very top of the haematopoietic hierarchy and that the ageing of haematopoietic stem cells (HSCs) directly contributes to changes in the immune system, referred to as immunosenescence. In this Review, we summarize the phenotypes of ageing HSCs and discuss how the cell-intrinsic and cell-extrinsic mechanisms of HSC ageing might promote immunosenescence. Stem cell ageing has long been considered to be irreversible. However, recent findings indicate that several molecular pathways could be targeted to rejuvenate HSCs and thus to reverse some aspects of immunosenescence.