Stressed Hematopoiesis Induces Mitochondrial Trafficking to Hematopoietic Stem Cells

Introduction

The bone marrow microenvironment regulates the production of both hematopoietic and non-hematopoietic cells for the maintenance of blood production under normal and stress conditions. Intercellular mitochondrial transfer has recently been reported in an acute myeloid leukemia as well as models of lung inflammation. In the context of acute bacterial infection the hematopoietic system needs to drive the granulocytic response necessary for host survival. Therefore, we hypothesis that under stressed hematopoiesis, mitochondria move from the non-hematopoietic cells of the bone marrow microenvironment to the hematopoietic stem/progenitor cells (HSPC) to rapidly support and sustain the host response to bacterial infection.

Methods

C57/BL6 mice were injected with lipopolysaccharide (LPS) for 16 hours or infected with salmonella for 72 hours. The mice were sacrificed after 16 hours and the BM harvested and analyzed using flow cytometry for HSPC mitochondrial content using MitoTracker Green FM. Human cord blood CD34+ cells were engrafted into 3 to 4 week old NSG mice (hu-NSG). After 3 months engraftment of human cord blood CD34+ cells was verified by flow cytometry. Hu-NSG mice were then injected with LPS. HSPC populations were isolated by cell sorting for human CD34, CD38, CD45RA, CD90 and CD49f. Quantification of mitochondrial DNA (mtDNA) transfer was undertaken using Taqman qPCR with species or strain specific probes. Human CD34+ cells obtained from patient cord blood, with informed consent and under approval from the United Kingdom (UK) National Health Service Health Research Authority. Animal experiments were conducted with approval from the UK Home Office and University of East Anglia Animal Welfare and Ethical Review Board

Results

First, we quantified mitochondria content in the HSPC populations before and after LPS treatment or salmonella infection and quantified multipotent progenitor cells (MPP), total hematopoietic stem cell (HSC), long-term (LT-HSC) and short-term (ST-HSC) hematopoietic stem cells. MPP, HSC and ST HSC, but not the LT-HSC population, have increased mitochondria content in response to LPS treatment or salmonella infection. This corresponded with an increase frequency of MPP, HSC, and ST HSC. Next, we quantified levels of mitochondrial transfer within the bone marrow by analyzing the percentage of mouse mitochondria in the human HSC and MPP population in response to LPS treatment. Results show that mouse mtDNA was detected in the human HSC and human MPP populations of hu-NSG animals treated with LPS. No mouse mtDNA was detected in human HSC and human MPP populations in untreated hu-NSG animals.

Conclusion

Here we show that acute bacterial infection and LPS drive mitochondrial transfer from the bone marrow microenvironment to HSPC populations. We do not observe this occurring in comparator baseline unstressed hematopoiesis.