Mitochondria Transfer in Bone Marrow Hematopoietic Activity

Abstract

Purpose of Review

The well-established crosstalk between hematopoietic stem cells (HSC) and bone marrow (BM) microenvironment is critical for the homeostasis and hematopoietic regeneration in response to blood formation emergencies. Past decade has witnessed that the intercellular communication mediated by the transfer of cytoplasmic material and organelles between cells can regenerate and/or repair the damaged cells. Mitochondria have recently emerged as a potential regulator of HSC fate. This review intends to discuss recent advances in the understanding of the mitochondrial dynamics, specifically focused on the role of mitochondrial transfer, in the maintenance of HSC activity with clear implications in stem cell transplantation and regenerative medicine.

Recent Findings

HSC are highly heterogeneous in their mitochondrial metabolism, and the quiescence and potency of HSC depend on the status of mitochondrial dynamics and the clearance of damaged mitochondria. Recent evidence has shown that in stress response, BM stromal cells transfer healthy mitochondria to HSC, facilitate HSC bioenergetics shift towards oxidative phosphorylation, and subsequently stimulate leukocyte expansion. Furthermore, metabolic rewiring following mitochondria transfer from HSPC to BM stromal cells likely to repair the damaged BM niche and accelerate limiting HSC transplantation post myeloablative conditioning.

Summary

The alteration of mitochondrial dynamics, damaged mitochondrial clearance, and intercellular mitochondria transfer, fine-tuned by BM niche factors and stress signaling, has considerable impacts on hematopoietic and BM microenvironment regeneration. Elucidating the differential BM niche component on mitochondria transfer, the trigger signal, mode of transfer, and the fate of transferred mitochondria are essential to underpin their application in regenerative medicine and transplantation settings.

Introduction

Hematopoietic stem cells (HSC) are quiescent, reside in a highly specialized microenvironment of bone marrow (BM), called “niche” and allow the sustained production of blood cells throughout the lifespan. These cells are self-renewing, multipotent, and undergo rapid expansion in response to stress stimuli [1–3]. Long-term HSC (LT-HSC), capable of long-term self-renewal and multipotential differentiation ability, rely on anaerobic glycolysis and exhibit low metabolic activity with low reactive oxygen species (ROS), mitochondrial membrane potential (MMP), and oxidative phosphorylation [4, 5•]. However, the metabolic requirement of HSC is cell cycle dependent. Activation and differentiation of LT-HSC into short-term HSC (ST-HSC) and downstream multipotent progenitors (MPP), deprived of lifelong self-renewal activity but indispensable for blood formation, favors the utilization of oxidative phosphorylation as a principal energy source to generate ATP in the mitochondria [6–8].

HSC transplantation is a key therapeutic strategy in many hematological disorders and inborn errors, and the regenerative potential of HSC depends on cell intrinsic, systemic as well as the microenvironmental cues. Studies using genetically encoded mitochondrial reporters demonstrated that HSC contain higher mitochondria than lineage committed progenitors and mature cells [9]; however, the mitochondrial respiration is more dispensable in these cells [6, 10]. It is more likely that restricted mitochondrial metabolism in quiescent HSC holds ROS at low level and consequently support their self-renewal and long-term repopulation ability [11, 12]. Subtle changes in intracellular ROS are however associated with HSC commitment and differentiation. There is compelling evidence that during myeloablative conditioning as required in stem cell transplantation, bone marrow-derived stromal cells and progenitors maintain HSC in a low ROS state by eliminating or scavenging the excessive ROS, aiming to facilitate hematopoietic reconstitution [13]. Indeed, HSC with similar low ROS level are heterogeneous in their mitochondrial metabolism, and HSC with low MMP are enriched with quiescent cells and display more robust reconstitution potential, while HSC with high MMP are mostly primed/activated [14••, 15••, 16. 17•]. Importantly, MMP low HSC contain small, punctate, and evenly dispersed mitochondria, and the lineages derived from these cells display balanced clonal composition. In contrast, HSC with high MMP display polarized, hyper-fused mitochondria, and their fate is biased towards myeloid differentiation [14••, 15••], features that phenocopy those ones identified in the process of HSC aging. This review intends to discuss the role of metabolic heterogeneity and mitochondrial dynamics in HSC quiescence and potency and highlight the significance of intercellular mitochondria donation in hematopoietic and BM microenvironment (BMME) regeneration.

Mitochondria in Regulation of HSC Fate

Mitochondria are well-known metabolic sensors that bridge transcriptional signatures and cellular functions. Beyond ATP generation, mitochondria function as signaling organelles that control distinct biological outcomes, including HSC proliferation and differentiation [18–22]. Disruption of mitochondrial oxidative phosphorylation however blocks HSC differentiation, resulting in rapid hematopoietic failure [6, 23], suggesting that mitochondrial metabolism is an important regulator of HSC fate and function. Mitochondrial metabolism is in part controlled by the dynamic changes in their number and morphology in response to cell-intrinsic and cell-extrinsic changes.

Mitochondrial Dynamics in HSC Maintenance

Mitochondrial homeostasis is maintained by coordinated regulation of mitochondrial fission/fusion and mitophagy, and increasing evidences demonstrate that mitochondrial dynamics are the critical determinant of HSC fate decisions [5•, 24–28]. The mitochondrial fusion protein Mitofusin 2 (Mfn2) is crucial for the maintenance of HSC with extensive lymphoid potential [27••]. Conditional deletion of Mfn2 in the hematopoietic system attenuates the self-renewal and repopulation capacity of lymphoid-biased HSC [27••]. This capacity appears to be mediated through tethering of mitochondria to the endoplasmic reticulum; thereby, increased buffering of intracellular calcium, which prevents calcineurin-dependent aberrant nuclear factor of activated T cell (Nfat) activity, hence regulates clonal heterogeneity within the HSC pool [27••]. Indeed, the calcium-mitochondria axis is crucial for HSC fate decisions, and low intracellular calcium in quiescent HSC correlated with decreased mitochondrial motility and increased mitochondrial fusion [5, 29••]. In contrast, cycling HSC demonstrate activation of calcium-mitochondria pathway, which reversed in presence of adenosine A2 receptors agonist [5]. Within the phenotypically defined HSC, the intracellular calcium level is highly heterogeneous, and the lymphoid-biased CD150^low HSC display lower intracellular calcium than the myeloid-biased CD150^high HSCs [29••]. Remarkably, the transcriptional co-regulator, PR-domain-containing 16 (Prdm16), is critical for the establishment and maintenance of the HSC pool, while its selective deletion in HSC compartment induces mitochondrial fragmentation due to decreased level of Mfn2, ROS accumulation, and impairment of HSC long-term repopulation potential [27••, 30–32]. It is noteworthy that calcineurin activity positively regulates the mitochondrial fission protein, dynamin-related protein 1 (DRP1) [33], and recent studies have highlighted that DRP1 promotes reduction in MMP and regulates the HSC stemness [14••, 15••]. Increased levels of the active phosphorylated (pS616) form of DRP1 and its localization in close proximity to mitochondria are found more in MMP-low-HCS, suggesting that the DRP1-mediated mitochondrial fragmentation is partly associated with the suppression of mitochondrial activity in MMP-low quiescent HSC [15••]. In addition, these quiescent MMP low HSC are enriched in relatively large lysosomes, and the mitochondria of these cells display greater co-localization with PTEN-induced putative kinase 1 (PINK1) and PARKIN, the two proteins associated with mitochondrial clearance [15••]. Of note, dividing HSC asymmetrically segregate lysosomes, and the daughter cells receiving fewer lysosomes are more prone to differentiate [34]. Finally, Filippi’s group identified that the loss of the Drp1 activity in HSC enhances accumulation of aggregated mitochondria through asymmetric segregation and attenuates HSC regenerative potential [14••].

Another crucial regulator is the mitochondrial carrier homolog 2 (MTCH2) that crucially controls the process of HSC activation and differentiation. Mice conditionally deleted for Mcth2 show attenuated Drp1 translocation to mitochondria, resulting in mitochondrial hyper-fusion, increased oxidative phosphorylation, ROS accumulation, and impairment of HSC repopulation potential [35]. These findings suggest that mitochondrial dynamics by modulating the mitochondrial activity influences HSC quiescence, self-renewal, and differentiation through numerous mechanisms and warrant further studies to dissect the precise molecular mechanism (Fig. 1).

Fig. 1.

Mitochondrial dynamics and fate in stem cells and progenitors. Quiescent HSC exhibit an immature mitochondrial network characterized by fragmented, punctate mitochondria with primitive cristae. These cells rely on glycolysis as the major energy source and have low levels of ATP, oxidative phosphorylation, and ROS levels. Balanced mitochondrial dynamics (fission and fusion) and basal mitophagy regulates the maintenance of HSC. During transition from quiescent to active cycling, HSC increases their mitochondrial activity and potential to meet the increased demands of cycling cells. Suppression of the mitochondrial activity and activation of the p53 pathway however retain the self-renewing potential of HSC. In differentiated cells, mature mitochondrial network with the tubular mitochondria filled with a high number of regular cristae was noted. Correspondingly, mitochondria fusion increases with the accumulation of hyper-fused mitochondria. Moreover, these cells exhibit high mitochondrial oxidative phosphorylation accompanied by high membrane potential, ATP and ROS level

Mitochondrial Autophagy in HSC Maintenance

Mitochondria sustain cumulative damage during cell divisions, and clearance of damaged mitochondria by activation of PPAR (peroxisome proliferator-activated receptor)-fatty acid oxidation pathway is a key process required for self-renewal and expansion of Tie2⁺ HSC [25•]. In general, basal autophagy actively suppresses HSC metabolism by clearing active, healthy mitochondria through mitophagy to maintain HSC quiescence and stemness [36•]. Interestingly, conditional deletion of the autophagy gene Atg7 in the hematopoietic system displayed accumulation of mitochondria, elevated mitochondrial ROS that lead to DNA damage resulting in expansion of multipotent progenitors, but an almost complete loss of LT-HSC and ST-HSC [37]. Of note, NAD⁺-boosting agent nicotinamide riboside reduces mitochondrial activity and improves hematopoiesis via induction of mitophagy and increased asymmetric LT-HSC divisions [17•]. In contrast, Jin et al. have observed that AAA+-ATPase (“ATPase associated with diverse cellular activities”) Atad3a by attenuating the Pink1-dependent mitophagy regulates the HSPC differentiation and maintenance [38]. Conditional deficiency of Atad3a in HSC, however, induces hyperactivated mitophagy, enlarged HSC pool, and impaired erythroid and B-lymphoid differentiation resulting in bone marrow failure [38]. These studies suggest that mitophagy must be controlled precisely to ensure the maintenance of HSC and their appropriate differentiation. Further work is needed to provide deeper insight on how removal of active mitochondria influences HSC fate and identifies molecular targets in mitophagy for HSC engineering.

Mitochondria Transfer

Mesenchymal stromal cells and progenitors (MSC/P) are critical components of the hematopoietic niche of the BM (reviewed in [39]). Intercellular communication between hematopoietic and non-hematopoietic cells of the BMME is critical for homeostasis, normal cellular functions, and response to stress stimulus. A previous study from our group has shown that hematopoietic regeneration and efficient blood formation after myeloablation depend on the transfer of damaging ROS from HSPC to BMME, and hematopoietic Cx43, a gap junction protein facilitates this transfer and prevents ROS-p38MAPK-p16/INK4a-mediated HSC senescence [13]. Deficiency of Cx43 in HSPC compartment however associated with ROS accumulation and impairment of HSC repopulation potential, which was significantly prevented in presence of antioxidant, N-acetyl cysteine (NAC) [13]. Importantly, Cx43 deficiency in BM MSC/P results in a reduction of functional HSC and progenitor cells in the fetal liver and impairs BM HSC proliferation and ST-HSC regeneration upon myeloablation [40, 41]. These studies underscore the importance of heterocellular interactions between HSPC and BMME for an adequate regenerative response. Since mitochondria are the most important source of ROS in nucleated cells, these findings further raise the question whether stress-induced hematopoietic regeneration following ROS transfer depends on trafficking of mitochondria between HSPC and the surrounding niche.

Mitochondria Transfer from BM Stromal Cells and Its Consequences on Recipient Cells

Stem cells are recognized as unexceptionable mitochondria donor cells, and cumulative evidences suggest that transportation of mitochondria from one cell to another through tunneling nanotubes (TNT), gap junctions, and/or microvesicles favors recipient cell bioenergetics and consequently maintains tissue homeostasis (Fig. 2, Table 1) [67–72]. Islam et al. using a lipopolysaccharide (LPS)-induced acute lung injury mouse model demonstrated that BM-derived MSC/P protect against acute lung injury by restituting alveolar oxidative phosphorylation and ATP production through Cx43 dependent alveolar attachment, calcium buffering, and mitochondrial transfer [42]. The protective effect of mitochondria transfer was however significantly prevented in presence of BM-MSC/P containing dysfunctional mitochondria or following disruption of gap junction by carbenoxolone (CBX, a glycyrrhetinic acid derivative) or by Cx43 inhibitory peptide, Gap26 [42, 73]. The membrane architecture of mitochondria is prerequisite for efficient respiration and ATP generation, and imaging analyses have revealed that calcium-binding mitochondrial Rho-GTPase, Miro1, and Miro2 are indispensable for the maintenance of normal mitochondrial cristae architecture and intracellular mitochondrial movement [74]. Miro1 also act as an essential mediator of microtubule-based mitochondrial motility and regulates the transfer of mitochondrial between the cells [74]. Ahmad et al. has demonstrated that Miro1 facilitates TNT-mediated transfer of mitochondria from MSC to alveolar epithelial cells in both rotenone-induced airway injury and allergic airway inflammation mouse models, which subsequently improves cellular bioenergetics and ameliorates epithelial cell injury [43]. Meanwhile, it has been shown that iPSC-derived MSC/P, expressing elevated levels of Miro1 transfer higher amount of mitochondria to the injured alveolar epithelial cells compared to BM-MSC/P, and consequently induce more proficient tissue repair in chronic obstructive pulmonary disease and asthma mouse models [43, 44]. Of note, mitochondria transfer from MSC/P to cardiomyocytes through nanotubes and even at a higher extent from iPSC-derived MSC/P to cardiomyocytes (due to TNF-α-induced nanotube formation and upregulated Miro1 expression) protects against cell death induced by sterile inflammatory ischemia/reperfusion insults [47, 48, 75]. Acquistapace et al. further identified that mitochondria transferred from MSC/P have the capacity to reprogram cardiomyocytes back to a progenitor-like state [49] and draw an extension to transplant stem cell-derived mitochondria to injured cells as a novel strategy for stem cell-based therapy.

Fig. 2.

Different mode of intercellular mitochondrial transfer. Intercellular mitochondria transfer between two spatially separated cells occur through (1) plasma-membrane fusion with TNT; (2) Gap junction channels; (3) extracellular vesicles; and/or (4) by cellular fusion. TNT contains cytoskeletal elements such as actin and microtubules depending on the cell types. Myosin and Miro1/2 are the dynamic protein involved in mitochondria transfer. Extracellular vesicles ranging from 0.1 to 1 μm containing mitochondria can be released from donor cells and engulfed by the recipient cells. Cells expressing connexin 43 proteins initially closely contact with the target cells, followed by formation of gap junction channel and mitochondria transfer. Transferred mitochondria result in the improvement in mitochondrial respiration, ATP level, cellular metabolism, restoration of cell function, and/or transcriptional reprogramming in the recipient cells. Mitochondria transfer from stromal cells to macrophages (MΦ) results in improved cellular bioenergetics and phagocytosis, while stromal cells mitochondria uptake by leukemic cells results in higher drug resistance and relapse. The local microenvironment of an injured recipient cell releases physiological cues including mitochondrial DNA (mtDNA), ROS, NADPH oxidase-2 (NOX2), Ca²⁺, and CD38, among others that trigger transfer of mitochondria from donor stem cells. TNT, tunneling nanotubes; ATP, adenosine triphosphate; Miro1/2, mitochondrial Rho-GTPase 1/2; ER, endoplasmic reticulum

Table 1.

Mechanisms and consequences of intercellular mitochondria transfer described in different cell types

Donor cells	Recipient cells	Regulators of mitochondria transfer	Mode of transfer	Outcomes in recipient cells	Ref.
BM-MSC	Alveolar epithelial cells	LPS	TNT, MV, connexin 43, Ca2+	Alveolar OXPHOS ↑ ATP ↑ Protection against acute lung injury	[42]
BM-MSC	Alveolar epithelial cells	Rotenone-induced airway injury and allergic airway inflammation	TNT, Miro1	Improved cellular bioenergetics Epithelial cells survival	[43]
iPSC-MSC	Alveolar epithelial cells	Cigarette smoke	TNT, Miro1	Improved cellular bioenergetics Rescue cigarette smoke-induced epithelial cells damage	[44]
iPSC-MSC	Corneal epithelial Cells	Rotenone-induced oxidative stress	TNT	Protection of corneal epithelial cells against alkali burn	[45]
Renal tubular cells	BM-MSC	In vitro co-culture	TNT	MSC differentiation towards renal cells	[46]
BM-MSC	Cardiomyocytes	Inflammatory I/R	TNT	Repair damaged cardiomyocytes Epithelial cells survival	[47]
iPSC-MSC	Cardiomyocytes	Anthracycline-induced cardiomyopathy	TNT, Miro1	Repair damaged cardiomyocytes Cell survival	[48]
hMADS cells	Cardiomyocytes		Cell fusion	Reprogram adult cardiac cells towards progenitor like state	[49]
MSC	Macrophage	Mouse model of ARDS and sepsis	TNT	Phagocytosis ↑ OXPHOS ↑ ATP ↑	[50]
hBM-MSC	Macrophage	LPS-induced lung injury model	MV	Phagocytosis ↑ Differentiation towards M2 phenotype OXPHOS ↑ ATP ↑	[51, 52]
Stem cell-educated platelets	Islet β cells	Diabetes model	MV	↑ Replication and rescue of function of pancreatic islet β cells	[53]
BM MSC	AML cells	NOX-2 and NOX-2 derived ROS	TNT, EV	↑ OXPHOS and ATP ↑Tumorigenic potential and drug resistance	[54–59]
BM MSC	MM cells	CD38	TNT	↑ Mitochondrial bioenergetics and ATP	[60]
ALL	Stromal cells		Exosomes	Promote disease progression Induce the transition of stromal cells into cancer-associated fibroblasts	[61]
BM MSC	Human CD34+ HSPC	Oxidative stress		N.D.	[55]
BM MSC	HSC	Acute bacterial infection	Connexin 43	↑OXPHOS and ATP ↑HSPC proliferation Leukocyte expansion	[62•]
Platelet-derived mitochondria	Human CD34+ HSC			↑ OXPHOS and ATP HSPC differentiation and leukocyte expansion	[63, 64]
HSPC	BM-MSC	TBI	Connexin 43	Improved BM-MSC bioenergetics Hematopoietic and BM niche regeneration	[65, 66••]

ALL acute lymphoblastic leukemia, AML acute myeloid leukemia, ARDS acute respiratory distress syndrome, BM bone marrow; hMADS human multipotent adipose-derived stem cells, iPSC induced pluripotent stem cells, IR ischemia/reperfusion, Miro1 mitochondrial Rho-GTPase 1; MM multiple myeloma, MSC mesenchymal stromal cells, MV microvesicles, OXPHOS oxidative phosphorylation, NOX-2 NADPH oxidase-2, TNT tunneling nanotubes

Further research in preclinical models of acute respiratory distress syndrome (ARDS) and sepsis identified that mitochondrial transfer from MSC/P via TNT greatly improved alveolar macrophage bioenergetics and enhance their phagocytic activity [50]. Inhibition of TNT by cytochalasin B however only partially attenuates the mitochondrial transfer, suggesting the involvement of other cell contact-independent mechanisms [50]. Study by Phinney et al. and Morrison et al. elegantly demonstrated that mitochondria conveyed by MSC/P through arrestin domain-containing protein-1-mediated microvesicles (ARMMs) improve recipient macrophages oxidative phosphorylation and promote their differentiation towards a M2 anti-inflammatory and highly phagocytic macrophage phenotype [51, 52]. Intriguingly, mitochondrial transfer also improved the donor MSC/P survival by allowing MSC/P to get rid of partially depolarized mitochondria [51]. In a recent study, Mahrouf et al. described the bidirectional exchanges of mitochondria between damaged cells and MSC/P [76]. In particular, mitochondria released from the damaged cells were engulfed and degraded by MSC/P, leading to the induction of heme oxygenase-1 (HO-1). HO-1 enhances the MSC/P mitochondrial biogenesis resulting in increased functional mitochondria amenable to be transferred to injured cells and improvement of the effectiveness of MSC/P-based therapies [76]. Zhao et al. have shown that mitochondria released from circulating platelets can migrate to pancreatic islets and be up-taken by islet β cells, leading to the improvement of islet β-cell proliferation and function in diabetic patients [53]. While these findings require rigorous confirmation in relevant translational models and human clinical trials, these results suggest that platelets (endogenous or transfused) may be major mediators of mitochondrial transfer and exchange between tissues and organs, contributing to tissue homeostasis and repair, amenable to translational approaches in regenerative medicine. Collectively, all these studies suggest that stress signals originating from damaged cells trigger mitochondria donation from MSC/P and signify the potential benefits of mitochondria transfer in restoration of recipient cell bioenergetics and fate.

However, the mitochondria transfer not always exerts beneficial/protective effects. Several studies provide evidence that mitochondrial donation by MSC/P selectively boost cancer cell metabolism and provide a survival advantage following chemotherapy [54–56, 60, 67, 68, 77, 78]. AML progression is enabled by the transfer of functional mitochondria from BM stromal cells to AML blasts via tumor-derived TNT [54, 55, 57]. In particular, NADPH oxidase-2 (NOX2)-derived ROS overproduction in AML blasts and/or peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α)-driven mitochondrial biogenesis in BM MSC/P stimulates mitochondrial transfer to acute myelogenous leukemia (AML) cells, which associates with improved cellular bioenergetics and partly supports leukemia cell survival and chemoresistance [54, 55, 57]. Interestingly, MSC/P isolated from B cell precursor ALL (B-ALL) patient’s BM specimens adopt an activated, high pro-inflammatory cytokine secreting cancer-associated fibroblast like phenotype, and the TNT mediated mitochondria transferred from these activated MSC/P promote leukemogenesis by perturbing chemotherapy-induced ROS [56, 79]. Of note, a higher number of nestin⁺ MSC/P found in AML patient’s BM and in murine xenograft model of ALL represent cell-to-cell contact-dependent ROS-detoxifying mechanism and allow mitochondria transfer to facilitate chemoresistance and relapse [56, 58, 59]. Similarly, increased mitochondrial bioenergetics and ATP production in multiple myeloma is associated with mitochondria transfer from BM MSC/P via tumor-derived TNT, and tumor cell CD38 supports the formation of TNT [60]. In a recent study, Bajzikova et al. have shown that after acquisition of mitochondria from the host stroma, tumorigenesis depends on oxidative phosphorylation-linked dihydroorotate dehydrogenase-driven pyrimidine biosynthesis, while mitochondrial ATP is dispensable for this process [80]. Nonetheless, treatment targeting cancer-associated fibroblast activation by corticosteroids, disruption of TNT formation by microtubule damaging drug, vincristine and ROS detoxification by N-acetyl cysteine (NAC), glutathione, and diphenyleneiodonium (DPI) selectively diminished the leukemia burden and improved survival [54–59]. In another study, Mistry et al. have shown that inhibition of CD38 by daratumumab associated with improved animal survival via a mechanism that blocks mitochondrial transfer from BM MSC/P to AML blasts, resulting in inhibition of pro-tumoral oxidative phosphorylation and subsequent reduced leukemia growth [81]. Therefore, the therapeutic approach targeting mitochondrial transfer from MSC/P to leukemic cells and subsequent check on leukemic cells metabolism could be used as an intriguing approach to eradicate the minimal residual disease.

Transfer of Mitochondria to BM HSPC

Elucidating the niche regulatory mechanisms that maintain HSC homeostasis and promote HSC expansion under stress conditions is of major importance. BM stromal cells enhance hematopoiesis in response to different insults including inflammation and chemotherapy, and it by various secretory (such as CXCL12, SCF, IL-7, osteopontin, angiopoietin-1, and pleiotrophin) and cell-to-cell contract-dependent mechanisms (such as activation of Notch ligands, CXCL12 secretion) supports the replenishment of innate effector cells and prevents the exhaustion of the HSPC pool [82–91]. Although many experimental studies highlighted potential benefits of BM MSC/P-derived mitochondria transfer in restoration of recipient cells bioenergetics and fate, the involvement of BM MSC/P-derived mitochondria in steady-state and stress hematopoiesis is poorly understood. Marlein et al. using human CD34⁺ HSPC demonstrated that BM MSC/P could transfer mitochondria to HSPC only under oxidative stress condition, while this process does not occur under baseline conditions [55]. In a recent study, Mistry et al. have shown that in response to acute bacterial infection BM MSC/P transfer healthy mitochondria to HSC, facilitates the HSC bioenergetics shift towards oxidative phosphorylation and subsequently stimulate leukocyte expansion [62•]. This study further accessed the BMME heterogeneity in terms of mitochondrial transfer and identified that BM MSC/P but not osteoblasts or macrophages donate their mitochondria to HSC in response to infection. They observed that during infection ROS-induced oxidative stress activates PI3K-Akt signaling, which drive transfer of mitochondria from the BM MSC/P to HSC populations through opening of Cx43 channels [62•]. This data is important because it highlights the notion that BM MSC/P are necessary intermediates for mitochondrial transfer. Xu et al. have recently demonstrated the utility of transposase accessible mitochondrial DNA for single-cell lineage tracing [92]. In these experiments, investigators demonstrated that the direct transfer of mitochondria between isolated, co-cultured hematopoietic progenitors is rare or absent [92]. Interestingly, it has been reported that the uptake of platelet-derived mitochondria by adult peripheral blood insulin-producing cells reprograms them into functional CD34⁺ HSC with differential potential, leading to the production of blood cells including granulocytes, megakaryocytes, monocytes, RBC, and lymphocytes [63, 64]. While provocative, these findings are to be confirmed in rigorous transplantation experiments set up, in translational preclinical settings and ultimately human clinical trials. Time will tell whether these findings can be successfully implemented in human stem cell protocols. Overall, these data support the notion that mitochondrial transfer-mediated bioenergetics changes within the hematopoietic system facilitate the rapid onset of emergency hematopoiesis and can be implicated as a specific strategy to improve hematopoietic reconstitution following bone marrow transplantation.

HSPC Mitochondria Donation to Other BM Cells

Lethal or sublethal total body irradiation (TBI) is widely used as conditioning regimen before BM transplantation, which severely impairs the BM niche and HSC regeneration. Therefore novel strategies are needed to enhance HSC regeneration in irradiated BM. In a recent study, severe and colleagues using mass cytometry (CyTOF)-based single-cell protein analysis have demonstrated that under an irradiation-based conditioning regimen, only 3 out of 28 BM stromal subsets previously identified in homeostatic conditions persisted, suggesting a deep remodeling of the BMME following a genotoxic insult [93•]. Surprisingly, the LeptinR⁺ and Nestin⁺ stromal cell subsets, the two previously described critical components of MSC niche and most relevant for hematopoiesis were lost post-irradiation. This study further identified CD73⁺ NGFR^high stromal cells, which are retained after irradiation, serving as a candidate mediator of HSPC engraftment and acute hematopoietic regeneration post irradiation [93•]. An additional study by Chen et al. described that Apelin-expressing (Apln⁺) BM endothelial cells undergoing angiogenesis are indispensable for physiological homeostasis and hematopoietic reconstitution after myeloablation [83]. These Apln⁺ endothelial cells expand substantially post-irradiation and, in response to VEGF-A provided by transplanted BM cells, in particular by HSPC and LSK cells, promote vascular generation and BM transplantation [83]. The ionizing radiation also altered mitochondrial length, electron transport chain super-complexes, and mitochondrial metabolic response in MSC/P. During short-term adaptation, mitochondrial remodeling increases metabolic efficiency and ATP production, while in the longer term, the ionizing radiation induced damage results in a metabolic shift towards glycolysis, which may limit the damaging ROS production from mitochondria [94]. Taken together, these studies clearly indicate toxic effect of TBI on host BM stromal cells bioenergetics and fate, where the restoration of BMME is a prerequisite for efficient and successful BM engraftment. Gillette et al. identified that the intercellular contact between HSPC-osteoblast cells acts as a critical determinant of BM niche remodeling. Specifically, intercellular transfer of a portion of HSPC uropod membrane to osteoblasts downregulates Smad signaling and increases production of SDF-1 in osteoblasts, thereby promoting niche reconstitution and in vivo HSC expansion [95]. In a recent study, our group, in collaboration with Dr. Tsvee Lapidot’s group (Weizmann Institute, Rehovot, Israel), has shown that following TBI and HSPC transplantation, transplanted HSPC through transfer of part of their mitochondria to the irradiated host stromal cells improves the metabolic recovery of recipient BM stromal cells, which in turn increase the hematopoietic reconstitution [65, 66••]. This mitochondrial transfer is cell contact dependent and mediated by HSPC Cx43. We further identified that the increased intracellular level of ATP in HSPC activates the purinergic receptor P2RX7 that leads to low adenosine monophosphate kinase (AMPK) activity in the hematopoietic progenitors and facilitates of mitochondria from HSPC to damaged BM MSC [65, 66••]. These observations suggest that the understanding of the mechanisms regulating stromal recovery following myeloablative stress is of high clinical interest to optimize bone marrow transplant.

Conclusions and Perspectives

HSPC transplantation is curative in many hematopoietic disorders; however, the major limitation is a limited availability of HSC and challenges associated with the preservation of the short-term and long-term engraftment properties of HSPC when complex processes of gene addition or editing are undertaken.

Current evidence emphasized in this review implicates mitochondrial dynamics and metabolic heterogeneity as a critical determinant of HSPC regenerative potential and fate making decisions. However, how mitochondrial fate and transfer influence regenerative BM remains to be understood. Future research demonstrating the molecular mechanisms of mitochondrial remodeling in HSC in response to regenerative demands and their interconnections with BMME will expand our understanding of how metabolic reprogramming controls the emergency granulopoiesis. Although bioenergetic changes following transfer of mitochondria from BMME to HSC have been evidenced in acute bacterial infection, their precise role in steady state and emergency hematopoiesis is still not well defined. BMME differentially interacts with and affects HSC, and differential effect of myeloablative conditioning has been observed in BM niche cells; further studies are warranted to understand the role of differential BMME cellular component on mitochondria transfer, fate of transferred mitochondria, and their application in regenerative medicine and transplantation settings. It will be important to understand the upstream trigger signal(s) that activate mitochondria transfer and how transferred mitochondria accelerate blood recovery after lethal irradiation and translational models of limiting HSC transplantation. Several clinical data suggest that bone mesenchymal lineage diseases due to germinal mutations are ameliorated by HSC transplant despite the inability of BM MSC/P to engraft robustly in the recipient bone tissues. These findings further open the question, whether the transplanted stem cells permit interventions to repair the damaged BMME, which in turn improves hematopoietic recovery post chemo- and radiotherapy.