Thrombopoietin from hepatocytes promotes hematopoietic stem cell regeneration after myeloablation

Pregnane X receptor activation promotes hematopoiesis during liver regeneration by inducing proliferation of hematopoietic stem and progenitor cells in mice

Liver regeneration is a complex process that involves the recruitment of bone marrow (BM)-derived hematopoietic stem and progenitor cells (HSPCs). Pregnane X receptor (PXR), also known as NR1I2, is an important regulator for liver enlargement and regeneration. However, the role of PXR activation in hematopoiesis during liver regeneration remains unclear. This study investigates the effects of PXR activation on HSPCs and hematopoiesis during liver regeneration, as well as the underlying mechanisms involved. Using a 70 % partial hepatectomy (PHx) on C57BL/6 wild-type (WT) and Pxr-null mice, we observed a significant correlation between the changes in HSPCs numbers in BM and the process of liver regeneration. PXR activation significantly increased the population of Lineage- Sca-1+ c-Kit+ (LSK) cells in the BM, which are key HSPCs involved in hematopoiesis. Additionally, PXR activation increased serum levels of thrombopoietin (TPO) and erythropoietin (EPO), factors known to support HSPCs proliferation and hematopoiesis in the process of liver regeneration. PXR activation does not affect the hematopoietic function of normal mice. Furthermore, mice subjected to irradiation or busulfan-induced hematopoietic dysfunction exhibited impaired liver regeneration, which was alleviated by PXR activation. Importantly, in Pxr-null mice, the promotive effects of PXR activation on liver regeneration and increase of HSPCs were markedly diminished. Moreover, liver-specific Pxr silencing using AAV-Pxr shRNA attenuated the PXR activation-mediated liver regeneration and increase in BM LSK cells, confirming the critical role of hepatic PXR in hematopoiesis during liver regeneration. Collectively, these findings reveal that PXR activation promotes HSPCs proliferation and hematopoiesis during liver regeneration, providing new insights into the molecular mechanisms underlying the role of PXR in liver regeneration and hematopoiesis.

Bone marrow niches for hematopoietic stem cells: life span dynamics and adaptation to acute stress

ABSTRACT

Hematopoietic stem cells (HSCs) are instrumental for organismal survival because they are responsible for lifelong production of mature blood lineages in homeostasis and response to external stress. To fulfill their function, HSCs rely on reciprocal interactions with specialized tissue microenvironments, termed HSC niches. From embryonic development to advanced aging, HSCs transition through several hematopoietic organs in which they are supported by distinct extrinsic cues. Here, we describe recent discoveries on how HSC niches collectively adapt to ensure robust hematopoietic function during biological aging and after exposure to acute stress. We also discuss the latest strategies leveraging niche-derived signals to revert aging-associated phenotypes and enhance hematopoietic recovery after myeloablation.

Systemic and local regulation of hematopoietic homeostasis in health and disease

Hematopoietic stem cells (HSCs) generate all blood cell lineages responsible for tissue oxygenation, life-long hematopoietic homeostasis and immune protection. In adulthood, HSCs primarily reside in the bone marrow (BM) microenvironment, consisting of diverse cell types that constitute the stem cell 'niche'. The adaptability of the hematopoietic system is required to respond to the needs of the host, whether to maintain normal physiology or during periods of physical, psychosocial or environmental stress. Hematopoietic homeostasis is achieved by intricate coordination of systemic and local factors that orchestrate the function of HSCs throughout life. However, homeostasis is not a static process; it modulates HSC and progenitor activity in response to circadian rhythms coordinated by the central and peripheral nervous systems, inflammatory cues, metabolites and pathologic conditions. Here, we review local and systemic factors that impact hematopoiesis, focusing on the implications of aging, stress and cardiovascular disease.

Hematopoietic stem cell niche generation and maintenance are distinguishable by an epitranscriptomic program

The niche is typically considered as a pre-established structure sustaining stem cells. Therefore, the regulation of its formation remains largely unexplored. Whether distinct molecular mechanisms control the establishment versus maintenance of a stem cell niche is unknown. To address this, we compared perinatal and adult bone marrow mesenchymal stromal cells (MSCs), a key component of the hematopoietic stem cell (HSC) niche. MSCs exhibited enrichment in genes mediating m6A mRNA methylation at the perinatal stage and downregulated the expression of Mettl3, the m6A methyltransferase, shortly after birth. Deletion of Mettl3 from developing MSCs but not osteoblasts led to excessive osteogenic differentiation and a severe HSC niche formation defect, which was significantly rescued by deletion of Klf2, an m6A target. In contrast, deletion of Mettl3 from MSCs postnatally did not affect HSC niche. Stem cell niche generation and maintenance thus depend on divergent molecular mechanisms, which may be exploited for regenerative medicine.

Thrombopoietin mimetic stimulates bone marrow vascular and stromal niches to mitigate acute radiation syndrome

BACKGROUND

Acute radiation syndrome (ARS) manifests after exposure to high doses of radiation in the instances of radiologic accidents or incidents. Facilitating regeneration of the bone marrow (BM), namely the hematopoietic stem and progenitor cells (HSPCs), is key in mitigating ARS and multi-organ failure. JNJ-26366821, a PEGylated thrombopoietin mimetic (TPOm) peptide, has been shown as an effective medical countermeasure (MCM) to treat hematopoietic-ARS (H-ARS) in mice. However, the activity of TPOm on regulating BM vascular and stromal niches to support HSPC regeneration has yet to be elucidated.

METHODS

C57BL/6J mice (9-14 weeks old) received sublethal or lethal total body irradiation (TBI), a model for H-ARS, by 137Cs or X-rays. At 24 h post-irradiation, mice were subcutaneously injected with a single dose of TPOm (0.3 mg/kg or 1.0 mg/kg) or PBS (vehicle). At homeostasis and on days 4, 7, 10, 14, 18, and 21 post-TBI with and without TPOm treatment, BM was harvested for histology, BM flow cytometry of HSPCs, endothelial (EC) and mesenchymal stromal cells (MSC), and whole-mount confocal microscopy. For survival, irradiated mice were monitored and weighed for 30 days. Lastly, BM triple negative cells (TNC; CD45-, TER-119-, CD31-) were sorted for single-cell RNA-sequencing to examine transcriptomics after TBI with or without TPOm treatment.

RESULTS

At homeostasis, TPOm expanded the number of circulating platelets and HSPCs, ECs, and MSCs in the BM. Following sublethal TBI, TPOm improved BM architecture and promoted recovery of HSPCs, ECs, and MSCs. Furthermore, TPOm elevated VEGF-C levels in normal and irradiated mice. Following lethal irradiation, mice improved body weight recovery and 30-day survival when treated with TPOm after 137Cs and X-ray exposure. Additionally, TPOm reduced vascular dilation and permeability. Finally, single-cell RNA-seq analysis indicated that TPOm increased the expression of collagens in MSCs to enhance their interaction with other progenitors in BM and upregulated the regeneration pathway in MSCs.

CONCLUSIONS

TPOm interacts with BM vascular and stromal niches to locally support hematopoietic reconstitution and systemically improve survival in mice after TBI. Therefore, this work warrants the development of TPOm as a potent radiation MCM for the treatment of ARS.

Cellular plasticity of the bone marrow niche promotes hematopoietic stem cell regeneration

Hematopoietic stem cells (HSCs) regenerate after myeloablation, a procedure that adversely disrupts the bone marrow and drives leptin receptor-expressing cells, a key niche component, to differentiate extensively into adipocytes. Regeneration of the bone marrow niche is associated with the resolution of adipocytes, but the mechanisms remain poorly understood. Using Plin1-creER knock-in mice, we followed the fate of adipocytes in the regenerating niche in vivo. We found that bone marrow adipocytes were highly dynamic and dedifferentiated to leptin receptor-expressing cells during regeneration after myeloablation. Bone marrow adipocytes could give rise to osteolineage cells after skeletal injury. The cellular fate of steady-state bone marrow adipocytes was also plastic. Deletion of adipose triglyceride lipase (Atgl) from bone marrow stromal cells, including adipocytes, obstructed adipocyte dedifferentiation and led to severely compromised regeneration of HSCs as well as impaired B lymphopoiesis after myeloablation, but not in the steady state. Thus, the regeneration of HSCs and their niche depends on the cellular plasticity of bone marrow adipocytes.