Inflammation, Aging and Hematopoiesis: A Complex Relationship.

All vertebrate blood cells descend from multipotent hematopoietic stem cells (HSCs), whose activity and differentiation depend on a complex and incompletely understood relationship with inflammatory signals. Although homeostatic levels of inflammatory signaling play an intricate role in HSC maintenance, activation, proliferation, and differentiation, acute or chronic exposure to inflammation can have deleterious effects on HSC function and self-renewal capacity, and bias their differentiation program. Increased levels of inflammatory signaling are observed during aging, affecting HSCs either directly or indirectly via the bone marrow niche and contributing to their loss of self-renewal capacity, diminished overall functionality, and myeloid differentiation skewing. These changes can have significant pathological consequences. Here, we provide an overview of the current literature on the complex interplay between HSCs and inflammatory signaling, and how this relationship contributes to age-related phenotypes. Understanding the mechanisms and outcomes of this interaction during different life stages will have significant implications in the modulation and restoration of the hematopoietic system in human disease, recovery from cancer and chemotherapeutic treatments, stem cell transplantation, and aging.

Chemotherapy-induced transposable elements activate MDA5 to enhance haematopoietic regeneration.

Haematopoietic stem cells (HSCs) are normally quiescent, but have evolved mechanisms to respond to stress. Here, we evaluate haematopoietic regeneration induced by chemotherapy. We detect robust chromatin reorganization followed by increased transcription of transposable elements (TEs) during early recovery. TE transcripts bind to and activate the innate immune receptor melanoma differentiation-associated protein 5 (MDA5) that generates an inflammatory response that is necessary for HSCs to exit quiescence. HSCs that lack MDA5 exhibit an impaired inflammatory response after chemotherapy and retain their quiescence, with consequent better long-term repopulation capacity. We show that the overexpression of ERV and LINE superfamily TE copies in wild-type HSCs, but not in Mda5-/- HSCs, results in their cycling. By contrast, after knockdown of LINE1 family copies, HSCs retain their quiescence. Our results show that TE transcripts act as ligands that activate MDA5 during haematopoietic regeneration, thereby enabling HSCs to mount an inflammatory response necessary for their exit from quiescence.

New tools for 'ZEBRA-FISHING'.

Zebrafish has been established as a classical model for developmental studies, yet in the past years, with the explosion of novel technological methods, the use of zebrafish as a model has expanded. One of the prominent fields that took advantage of zebrafish as a model organism early on is hematopoiesis, the process of blood cell generation from hematopoietic stem and progenitor cells (HSPCs). In zebrafish, HSPCs are born early during development in the aorta-gonad-mesonephros region and then translocate to the caudal hematopoietic tissue, where they expand and finally take residence in the kidney marrow. This journey is tightly regulated at multiple levels from extracellular signals to chromatin. In order to delineate the mechanistic underpinnings of this process, next-generation sequencing techniques could be an important ally. Here, we describe genome-wide approaches that have been undertaken to delineate zebrafish hematopoiesis.

Repetitive Elements Trigger RIG-I-like Receptor Signaling that Regulates the Emergence of Hematopoietic Stem and Progenitor Cells.

Inflammatory signaling is required for hematopoietic stem and progenitor cell (HSPC) development. Here, we studied the involvement of RIG-I-like receptors (RLRs) in HSPC formation. Rig-I or Mda5 deficiency impaired, while Lgp2 deficiency enhanced, HSPC emergence in zebrafish embryos. Rig-I or Mda5 deficiency reduced HSPC numbers by inhibiting inflammatory signals that were in turn enhanced in Lgp2 deficient embryos. Simultaneous reduction of Lgp2 and either Rig-I or Mda5 rescued inflammatory signals and HSPC numbers. Modulating the expression of the signaling mediator Traf6 in RLR deficient embryos restored HSPC numbers. Repetitive element transcripts could be detected in hemogenic endothelial cells and HSPCs, suggesting a role as RLR ligands. Indeed, ectopic expression of repetitive elements enhanced HSPC formation in wild-type, but not in Rig-I or Mda5 deficient embryos. Manipulation of RLR expression in mouse fetal liver HSPCs indicated functional conservation among species. Thus, repetitive elements transcribed during development drive RLR-mediated inflammatory signals that regulate HSPC formation.