by Yuya Kunisaki, Jalal Ahmed, and Ali Zahalka
Hematopoietic stem cells (HSCs) are the precursors of all of the blood lineages. These distinct lineages confer immunity and help maintain homeostasis. A key characteristic of HSCs is their capacity for self-renewal. Harnessing these qualities of HSCs is what has made them promising areas of research in cancer therapy and regenerative medicine.
HSCs have been used therapeutically for many years, however, maximizing their purity and numbers continues to prove challenging. Expansion of HSCs with fully differentiating and self-renewal capacity can be useful clinically as a source of bone marrow for transplantation. Under homeostatic conditions, HSCs are largely found in a quiescent state and induction of the cell cycle often leads to their exhaustion (losing their ability to self-renew). Understanding how quiescence of HSCs is maintained inside the bone marrow may provide a clue on how one could expand HSCs.
A series of initial observations reported that HSCs are found near the endosteal surface and that osteoblasts were proposed as a niche. Other studies reported that most HSCs have been shown to be associated with sinusoids. Therefore, there has been a debate on whether distinct niches support quiescence and proliferation of HSCs.
To address this, we have developed a three-dimensional imaging system combined with computational modeling that allowed us to image the location of quiescent and proliferating HSCs and to analyze rigorously the relationships between HSCs and their microenvironments.
While we found that HSCs associate with bone marrow (BM) arterioles and sinusoids (36.8% and 67.1%, respectively), the dense sinusoidal network made it difficult to determine whether association with HSCs was specific or due to chance. To interpret the significance of these associations, we used a computer simulation of randomly placed HSCs on images on BM to define distance distributions for which there were no associations. By comparing the distances of HSC to various structures in situ to the distributions of randomly placed HSCs in silica (the computer generated model), we were able to conclude that HSCs are significantly associated with arterioles in the BM, but the associations observed in relation to sinusoids and osteoblasts was not distinguishable from random.
Furthermore, we discovered that quiescent Ki67- and dormant label retaining HSCs were significantly associated with BM arterioles. Additionally, 3D analyses of whole-mount BM samples using Nes-GFP transgenic mice led to the identification of two distinct types of Nestin+ cells associated with distinct vascular structures: Nes-GFPbright cells exhibiting a pericyte-like morphology were tightly associated with arterioles (Nesperi cells) whereas more abundant Nes-GFPdim cells exhibiting a reticular shape (Nesretic cells) were largely associated with sinusoids. Since these Nesperi cells expressed HSC maintenance genes, we tested their relevance as a niche-cell by depleting their numbers, and observed that not only did HSCs no longer associate with arterioles, but that the overall HSC pool was also significantly reduced. These data suggest that there are spatially distinct niches composed of distinct nestin+ cells for quiescent and proliferating HSCs in the bone marrow.
Gaining knowledge on the reciprocal regulation and functional correlation of the two vascular, arteriolar and sinusoidal, niches will thus helps us better understand the regeneration of stem cells in homeostasis and pathology.
To read more please check out the full article “Arteriolar niches maintain haematopoietic stem cell quiescence,” in the journal Nature.