Analysis of the hematopoietic stem cell niche

Visualizing Human Hematopoietic Stem Cell Trafficking In Vivo Using a Zebrafish Xenograft Model

Zebrafish is gaining increased popularity as a model organism to study stem cell biology. It also is widely used as model system to visualize human leukemic stem cells. However, xenotransplantation of primary human stem/progenitor cells has not been described. Here, we use casper pigmentation mutant fish that are transparent crossed to fli-GFP transgenic fish as recipients of red labeled human CD34(+) cells. We have investigated various conditions and protocols with the aim to monitor and visualize the fate of transplanted human CD34(+) cells. We here report successful use of casper mutant zebrafish embryos for the direct monitoring of human hematopoietic stem cell transplantation, differentiation, and trafficking in vivo.

Altered mesenchymal niche cells impede generation of normal hematopoietic progenitor cells in leukemic bone marrow

Degeneration of normal hematopoietic cells is a shared feature of malignant diseases in the hematopoietic system. Previous studies have shown the exhaustion of hematopoietic progenitor cells (HPCs) in leukemic marrow, whereas hematopoietic stem cells (HSCs) remain functional upon relocation to non-leukemic marrow. However, the underlying cellular mechanisms, especially the specific niche components that are responsible for the degeneration of HPCs, are unknown. In this study, we focused on murine bone mesenchymal stem cells (MSCs) and their supporting function for normal hematopoietic cells in Notch1-induced acute T-cell lymphocytic leukemia (T-ALL) mice. We demonstrate that the proliferative capability and differentiation potential of T-ALL MSCs were impaired due to accelerated cellular senescence. RNA-seq analysis revealed significant transcriptional alterations in leukemic MSCs. After co-cultured with the MSCs from T-ALL mice, a specific inhibitory effect on HPCs was defined, whereas in vivo repopulating potential of normal HSCs was not compromised. Furthermore, osteoprotegerin was identified as a cytokine to improve the function of T-ALL MSCs and to enhance normal HPC output via the p38/ERK pathway. Therefore, this study reveals a novel cellular mechanism underlying the inhibition of HPC generation in T-ALL. Leukemic MSCs may serve as a cellular target for improving normal hematopoietic regeneration therapeutically.

From seeing to believing: labelling strategies for in vivo cell-tracking experiments

Intravital microscopy has become increasingly popular over the past few decades because it provides high-resolution and real-time information about complex biological processes. Technological advances that allow deeper penetration in live tissues, such as the development of confocal and two-photon microscopy, together with the generation of ever-new fluorophores that facilitate bright labelling of cells and tissue components have made imaging of vertebrate model organisms efficient and highly informative. Genetic manipulation leading to expression of fluorescent proteins is undoubtedly the labelling method of choice and has been used to visualize several cell types in vivo. This approach, however, can be technically challenging and time consuming. Over the years, several dyes have been developed to allow rapid, effective and bright ex vivo labelling of cells for subsequent transplantation and imaging. Here, we review and discuss the advantages and limitations of a number of strategies commonly used to label and track cells at high resolution in vivo in mouse and zebrafish, using fluorescence microscopy. While the quest for the perfect label is far from achieved, current reagents are valuable tools enabling the progress of biological discovery, so long as they are selected and used appropriately.

Genotoxic consequences of endogenous aldehydes on mouse haematopoietic stem cell function

Haematopoietic stem cells (HSCs) regenerate blood cells throughout the lifespan of an organism. With age, the functional quality of HSCs declines, partly owing to the accumulation of damaged DNA. However, the factors that damage DNA and the protective mechanisms that operate in these cells are poorly understood. We have recently shown that the Fanconi anaemia DNA-repair pathway counteracts the genotoxic effects of reactive aldehydes. Mice with combined inactivation of aldehyde catabolism (through Aldh2 knockout) and the Fanconi anaemia DNA-repair pathway (Fancd2 knockout) display developmental defects, a predisposition to leukaemia, and are susceptible to the toxic effects of ethanol-an exogenous source of acetaldehyde. Here we report that aged Aldh2(-/-) Fancd2(-/-) mutant mice that do not develop leukaemia spontaneously develop aplastic anaemia, with the concomitant accumulation of damaged DNA within the haematopoietic stem and progenitor cell (HSPC) pool. Unexpectedly, we find that only HSPCs, and not more mature blood precursors, require Aldh2 for protection against acetaldehyde toxicity. Additionally, the aldehyde-oxidizing activity of HSPCs, as measured by Aldefluor stain, is due to Aldh2 and correlates with this protection. Finally, there is more than a 600-fold reduction in the HSC pool of mice deficient in both Fanconi anaemia pathway-mediated DNA repair and acetaldehyde detoxification. Therefore, the emergence of bone marrow failure in Fanconi anaemia is probably due to aldehyde-mediated genotoxicity restricted to the HSPC pool. These findings identify a new link between endogenous reactive metabolites and DNA damage in HSCs, and define the protective mechanisms that counteract this threat.

In vivo imaging of transplanted hematopoietic stem and progenitor cells in mouse calvarium bone marrow

In vivo imaging of transplanted hematopoietic stem and progenitor cells (HSPCs) was developed to investigate the relationship between HSPCs and components of their microenvironment in the bone marrow. In particular, it allows a direct observation of the behavior of hematopoietic cells during the first few days after transplantation, when the critical events in homing and early engraftment are occurring. By directly imaging these events in living animals, this method permits a detailed assessment of functions previously evaluated by crude assessments of cell counts (homing) or after prolonged periods (engraftment). This protocol offers a new means of investigating the role of cell-intrinsic and cell-extrinsic molecular regulators of hematopoiesis during the early stages of transplantation, and it is the first to allow the study of cell-cell interactions within the bone marrow in three dimensions and in real time. In this paper, we describe how to isolate, label and inject HSPCs, as well as how to perform calvarium intravital microscopy and analyze the resulting images. A typical experiment can be performed and analyzed in ∼1 week.

"Microenvironmental contaminations" induced by fluorescent lipophilic dyes used for noninvasive in vitro and in vivo cell tracking

Determining how normal and leukemic stem cells behave in vivo, in a dynamic and noninvasive way, remains a major challenge. Most optical tracking technologies rely on the use of fluorescent or bioluminescent reporter genes, which need to be stably expressed in the cells of interest. Because gene transfer in primary leukemia samples represents a major risk to impair their capability to engraft in a xenogenic context, we evaluated the possibility to use gene transfer-free labeling technologies. The lipophilic dye 3,3,3',3' tetramethylindotricarbocyanine iodide (DiR) was selected among 4 near-infrared (NIR) staining technologies. Unfortunately we report here a massive transfer of the dye occurring toward the neighbor cells both in vivo and in vitro. We further demonstrate that all lipophilic dyes tested in this study (1,1'-dioctadecyl-3,3,3',3'-tetramethylindotricarbocyanine perchlorate [DiI], DiD, DiR, and PKH26) can give rise to microenvironmental contamination, including when used in suboptimal concentration, after extensive washing procedures and in the absence of phagocytosis or marked cell death. This was observed from all cell types tested. Eventually, we show that this microenvironmental contamination is mediated by both direct cell-cell contacts and diffusible microparticles. We conclude that tracking of labeled cells using non-genetically encoded markers should always be accompanied by drastic cross validation using multimodality approaches.