Senescent cells (SCs) accumulate with age and after genotoxic stress, such as total-body irradiation (TBI). Clearance of SCs in a progeroid mouse model using a transgenic approach delays several age-associated disorders, suggesting that SCs play a causative role in certain age-related pathologies. Thus, a 'senolytic' pharmacological agent that can selectively kill SCs holds promise for rejuvenating tissue stem cells and extending health span. To test this idea, we screened a collection of compounds and identified ABT263 (a specific inhibitor of the anti-apoptotic proteins BCL-2 and BCL-xL) as a potent senolytic drug. We show that ABT263 selectively kills SCs in culture in a cell type- and species-independent manner by inducing apoptosis. Oral administration of ABT263 to either sublethally irradiated or normally aged mice effectively depleted SCs, including senescent bone marrow hematopoietic stem cells (HSCs) and senescent muscle stem cells (MuSCs). Notably, this depletion mitigated TBI-induced premature aging of the hematopoietic system and rejuvenated the aged HSCs and MuSCs in normally aged mice. Our results demonstrate that selective clearance of SCs by a pharmacological agent is beneficial in part through its rejuvenation of aged tissue stem cells. Thus, senolytic drugs may represent a new class of radiation mitigators and anti-aging agents.

/pdf/clearance-of-senescent-cells-by-abt263-rejuvenates-aged-fzarmp1iu0.pdf

Clearance of senescent cells by ABT263 rejuvenates aged hematopoietic stem cells in mice.

Platelet α–granules: Basic biology and clinical correlates

As we age, the innate immune system becomes dysregulated and is characterized by persistent inflammatory responses that involve multiple immune and non-immune cell types and that vary depending on the cell activation state and tissue context. This ageing-associated basal inflammation, particularly in humans, is thought to be induced by several factors, including the reactivation of latent viral infections and the release of endogenous damage-associated ligands of pattern recognition receptors (PRRs). Innate immune cell functions that are required to respond to pathogens or vaccines, such as cell migration and PRR signalling, are also impaired in aged individuals. This immune dysregulation may affect conditions associated with chronic inflammation, such as atherosclerosis and Alzheimer's disease.

http://www.unizar.es/depfarfi/unidad_fisiologia/Temas%20Teoria/Biotecnologia%20Aplicada/T4_Age-innat%20immun.%20Nat%20Rev%20Immunol.pdf

Age-dependent dysregulation of innate immunity

Hematopoiesis: A Human Perspective

https://hal.archives-ouvertes.fr/hal-01596016/document

Genome regulation by Polycomb and Trithorax: 70 years and counting

Hematopoietic stem cell (HSC) populations change with aging, but the extent to which this is caused by qualitative versus quantitative alterations in HSC subtypes is unclear. Using clonal assays, in this study we show that the aging HSC compartment undergoes both quantitative and qualitative changes. We observed a variable increase of HSC pool size with age, accompanied by the accumulation of predominantly myeloid-biased HSCs that regenerate substantially fewer mature progeny than young myeloid-biased HSCs and exhibit reduced self-renewal activity as measured by long-term secondary transplantation. Old HSCs had a twofold reduction in marrow-homing efficiency and a similar decrease in functional frequency as measured using long-term transplantation assays. Similarly, old HSCs had a twofold reduced seeding efficiency and a significantly delayed proliferative response compared with young HSCs in long-term stromal cell co-cultures but were indistinguishable in suspension cultures. We show that these functional defects are characteristics of most or all old HSCs and are not indicative of a nonfunctional subset of cells that express HSC markers. Furthermore, we demonstrate that cells with functional properties of old HSCs can be generated directly from young HSCs by extended serial transplantation, which is consistent with the possibility that they arise through a process of cellular aging.

/pdf/clonal-analysis-reveals-multiple-functional-defects-of-aged-33k3n8774t.pdf

Clonal analysis reveals multiple functional defects of aged murine hematopoietic stem cells

The balance between self-renewal and differentiation of adult stem cells is essential for tissue homeostasis. Here we show that in the haematopoietic system this process is governed by polycomb chromobox (Cbx) proteins. Cbx7 is specifically expressed in haematopoietic stem cells (HSCs), and its overexpression enhances self-renewal and induces leukaemia. This effect is dependent on integration into polycomb repressive complex-1 (PRC1) and requires H3K27me3 binding. In contrast, overexpression of Cbx2, Cbx4 or Cbx8 results in differentiation and exhaustion of HSCs. ChIP-sequencing analysis shows that Cbx7 and Cbx8 share most of their targets; we identified approximately 200 differential targets. Whereas genes targeted by Cbx8 are highly expressed in HSCs and become repressed in progenitors, Cbx7 targets show the opposite expression pattern. Thus, Cbx7 preserves HSC self-renewal by repressing progenitor-specific genes. Taken together, the presence of distinct Cbx proteins confers target selectivity to PRC1 and provides a molecular balance between self-renewal and differentiation of HSCs.

/pdf/polycomb-cbx-family-members-mediate-the-balance-between-3zvi0mkl16.pdf

Sandra Olthof

Papers

Clonal analysis reveals multiple functional defects of aged murine hematopoietic stem cells

Polycomb Cbx family members mediate the balance between haematopoietic stem cell self-renewal and differentiation

Repression of BMI1 in normal and leukemic human CD34+ cells impairs self-renewal and induces apoptosis

Genetic screen identifies microRNA cluster 99b/let-7e/125a as a regulator of primitive hematopoietic cells

BMI1 collaborates with BCR-ABL in leukemic transformation of human CD34+ cells.