In Vivo Pre-Instructed HSCs Robustly Execute Asymmetric Cell Divisions In Vitro

Decoding Clonal Hematopoiesis: Emerging Themes and Novel Mechanistic Insights

Clonal hematopoiesis (CH), the relative expansion of mutant clones, is derived from hematopoietic stem cells (HSCs) with acquired somatic or cytogenetic alterations that improve cellular fitness. Individuals with CH have a higher risk for hematological and non-hematological diseases, such as cardiovascular disease, and have an overall higher mortality rate. Originally thought to be restricted to a small fraction of elderly people, recent advances in single-cell sequencing and bioinformatics have revealed that CH with multiple expanded mutant clones is universal in the elderly population. Just a few years ago, phylogenetic reconstruction across the human lifespan and novel sensitive sequencing techniques showed that CH can start earlier in life, decades before it was thought possible. These studies also suggest that environmental factors acting through aberrant inflammation might be a common theme promoting clonal expansion and disease progression. However, numerous aspects of this phenomenon remain to be elucidated and the precise mechanisms, context-specific drivers, and pathways of clonal expansion remain to be established. Here, we review our current understanding of the cellular mechanisms driving CH and specifically focus on how pro-inflammatory factors affect normal and mutant HSC fates to promote clonal selection.

Linking cell cycle to hematopoietic stem cell fate decisions

Hematopoietic stem cells (HSCs) have the properties to self-renew and/or differentiate into any blood cell lineages. In order to balance the maintenance of the stem cell pool with supporting mature blood cell production, the fate decisions to self-renew or to commit to differentiation must be tightly controlled, as dysregulation of this process can lead to bone marrow failure or leukemogenesis. The contribution of the cell cycle to cell fate decisions has been well established in numerous types of stem cells, including pluripotent stem cells. Cell cycle length is an integral component of hematopoietic stem cell fate. Hematopoietic stem cells must remain quiescent to prevent premature replicative exhaustion. Yet, hematopoietic stem cells must be activated into cycle in order to produce daughter cells that will either retain stem cell properties or commit to differentiation. How the cell cycle contributes to hematopoietic stem cell fate decisions is emerging from recent studies. Hematopoietic stem cell functions can be stratified based on cell cycle kinetics and divisional history, suggesting a link between Hematopoietic stem cells activity and cell cycle length. Hematopoietic stem cell fate decisions are also regulated by asymmetric cell divisions and recent studies have implicated metabolic and organelle activity in regulating hematopoietic stem cell fate. In this review, we discuss the current understanding of the mechanisms underlying hematopoietic stem cell fate decisions and how they are linked to the cell cycle.

New Insights into Hematopoietic Stem Cell Expansion to Stimulate Repopulation of the Adult Blood System for Transplantation

Successful engraftment of hematopoietic stem cells (HSCs) and progenitor cells (HSPCs) may be considered as a basis for the repopulation of the blood cells after transplantation in adults. Therefore, in vivo and ex vivo expansion of HSCs holds great promise for clinical applications. In this review, the mechanisms of HSC expansion will be discussed, considering the previous studies and works of literature. This is aimed to identify the signaling pathways that regulate HSC expansion and improve the application of engraftment in disease management. The following aspects will be included: (i) Stimulation of HSCs growth in vivo through gene regulation and cytokines activation; (ii) direct or indirect induction of HSC expansion by regulating signaling pathways; (iii) addition to assisting cells to help in the proliferation of HSCs; (iv) changing of living environment in the HSCs cultures via adjusting components and forms of cultures; (v) enhancement of HSC expansion by incorporating substances, such as extracellular vesicles (EVs), UM171, among others. In this review, recent new findings that provide us with new insights into HSC expansion methods have been summarized. Furthermore, these findings will also provide more possibilities for the development of some novel strategies for expanding and engrafting HSCs applied for treatments of some hematopoietic disorders.

Symmetric and asymmetric activation of hematopoietic stem cells

PURPOSE OF REVIEW

Hematopoietic stem cells (HSCs) are in an inactive quiescent state for most of their life. To replenish the blood system in homeostasis and after injury, they activate and divide. HSC daughter cells must then decide whether to return to quiescence and metabolic inactivity or to activate further to proliferate and differentiate and replenish lost blood cells. Although the regulation of HSC activation is not well understood, recent discoveries shed new light on involved mechanisms including asymmetric cell division (ACD).

RECENT FINDINGS

HSC metabolism has emerged as a regulator of cell fates. Recent evidence suggests that cellular organelles mediating anabolic and catabolic processes can be asymmetrically inherited during HSC divisions. These include autophagosomes, mitophagosomes, and lysosomes, which regulate HSC quiescence. Their asymmetric inheritance has been linked to future metabolic and translational activity in HSC daughters, showing that ACD can regulate the balance between HSC (in)activity.

SUMMARY

We discuss recent insights and remaining questions in how HSCs balance activation and quiescence, with a focus on ACD.