HSC Fate Is Tethered to Mitochondria

Mitochondria Dictate Function and Fate of HSCs and T Cells

Hematopoietic stem cells (HSC) and T cells are intimately related, lineage-dependent cell populations that are extensively used as therapeutic products for the treatment of hematologic malignancies and certain types of solid tumors. These cellular therapies can be life-saving treatments; however, their efficacies are often limited by factors influencing their activity and cellular properties. Among these factors is mitochondrial metabolism, which influences the function and fate commitment of both HSCs and T cells. Mitochondria, besides being the "cellular powerhouse," provide metabolic intermediates that are used as substrates for epigenetic modifications and chromatin remodeling, thus, driving cell fate decisions during differentiation. Moreover, mitochondrial fitness and mitochondrial quality control mechanisms are closely related to cellular function, and impairment of these mitochondrial properties associates with cellular dysfunction due to factors such as T-cell exhaustion and aging. Here, we give an overview of the role of mitochondria in shaping the behavior of these lineage-related cell populations. Moreover, we discuss the potential of novel mitochondria-targeting strategies for enhancing HSC- and T cell-based cancer immunotherapies and highlight how design and application of such approaches requires consideration of the metabolic similarities and differences between HSCs and T cells. See related article on p. 1302.

Ex vivo reprogramming of human hematopoietic stem cells is accompanied by increased transcripts of genes regulating metabolic integrity

The regenerative potential of human hematopoietic stem cells (HSCs) is functionally defined by their ability to provide life-long blood cell production and to repopulate myeloablated allogeneic transplant recipients. The expansion of HSC numbers is dependent not only on HSC divisions but also on a coordinated adaptation of HSCs to metabolic stress. These variables are especially critical during the ex vivo culture of HSCs with cytokine combinations, which frequently results in HSC exhaustion. We have previously reported that human CD34⁺ hematopoietic stem and progenitor cells (HSPCs) can be efficiently reprogrammed ex vivo and that the number of phenotypic HSCs with long-term repopulation capacity is expanded in the presence of a combination of cytokines and an epigenetic modifier. Here, we present evidence that ex vivo HSC reprogramming and maintenance is accompanied by increased transcripts of genes regulating metabolic integrity, including SIRT1 and SIRT3.

A Conserved Role for Asrij/OCIAD1 in Progenitor Differentiation and Lineage Specification Through Functional Interaction With the Regulators of Mitochondrial Dynamics

Mitochondria are highly dynamic organelles whose activity is an important determinant of blood stem and progenitor cell state. Mitochondrial morphology is maintained by continuous fission and fusion and affects stem cell proliferation, differentiation, and aging. However, the mechanism by which mitochondrial morphology and dynamics regulate cell differentiation and lineage choice remains incompletely understood. Asrij/OCIAD1 is a conserved protein that governs mitochondrial morphology, energy metabolism and human embryonic stem cell (hESC) differentiation. To investigate the in vivo relevance of these properties, we compared hESC phenotypes with those of Drosophila hematopoiesis, where Asrij is shown to regulate blood progenitor maintenance by conserved mechanisms. In concordance with hESC studies, we found that Drosophila Asrij also localizes to mitochondria of larval blood cells and its depletion from progenitors results in elongated mitochondria. Live imaging of asrij knockdown hemocytes and of OCIAD1 knockout hESCs showed reduced mitochondrial dynamics. Since key regulators of mitochondrial dynamics actively regulate mitochondrial morphology, we hypothesized that mitochondrial fission and fusion may control progenitor maintenance or differentiation in an Asrij-dependent manner. Knockdown of the fission regulator Drp1 in Drosophila lymph gland progenitors specifically suppressed crystal cell differentiation whereas depletion of the fusion regulator Marf (Drosophila Mitofusin) increased the same with concomitant upregulation of Notch signaling. These phenotypes were stronger in anterior progenitors and were exacerbated by Asrij depletion. Asrij is known to suppress Notch signaling and crystal cell differentiation. Our analysis reveals that synergistic interactions of Asrij with Drp1 and Marf have distinct impacts on lymph gland progenitor mitochondrial dynamics and crystal cell differentiation. Taken together, using invertebrate and mammalian model systems we demonstrate a conserved role for Asrij/OCIAD1 in linking mitochondrial dynamics and progenitor differentiation. Our study sets the stage for deciphering how regulators of mitochondrial dynamics may contribute to functional heterogeneity and lineage choice in vertebrate blood progenitors.

Biomechanical Regulation of Hematopoietic Stem Cells in the Developing Embryo

PURPOSE OF REVIEW

The contribution of biomechanical forces to hematopoietic stem cell (HSC) development in the embryo is a relatively nascent area of research. Herein, we address the biomechanics of the endothelial-to-hematopoietic transition (EHT), impact of force on organelles, and signaling triggered by extrinsic forces within the aorta-gonad-mesonephros (AGM), the primary site of HSC emergence.

RECENT FINDINGS

Hemogenic endothelial cells undergo carefully orchestrated morphological adaptations during EHT. Moreover, expansion of the stem cell pool during embryogenesis requires HSC extravasation into the circulatory system and transit to the fetal liver, which is regulated by forces generated by blood flow. Findings from other cell types also suggest that forces external to the cell are sensed by the nucleus and mitochondria. Interactions between these organelles and the actin cytoskeleton dictate processes such as cell polarization, extrusion, division, survival, and differentiation.

SUMMARY

Despite challenges of measuring and modeling biophysical cues in the embryonic HSC niche, the past decade has revealed critical roles for mechanotransduction in governing HSC fate decisions. Lessons learned from the study of the embryonic hematopoietic niche promise to provide critical insights that could be leveraged for improvement in HSC generation and expansion ex vivo.

Mitochondria: A Galaxy in the Hematopoietic and Leukemic Stem Cell Universe

Mitochondria are the main fascinating energetic source into the cells. Their number, shape, and dynamism are controlled by the cell’s type and current behavior. The perturbation of the mitochondrial inward system via stress response and/or oncogenic insults could activate several trafficking molecular mechanisms with the intention to solve the problem. In this review, we aimed to clarify the crucial pathways in the mitochondrial system, dissecting the different metabolic defects, with a special emphasis on hematological malignancies. We investigated the pivotal role of mitochondria in the maintenance of hematopoietic stem cells (HSCs) and their main alterations that could induce malignant transformation, culminating in the generation of leukemic stem cells (LSCs). In addition, we presented an overview of LSCs mitochondrial dysregulated mechanisms in terms of (1) increasing in oxidative phosphorylation program (OXPHOS), as a crucial process for survival and self-renewal of LSCs,(2) low levels of reactive oxygen species (ROS), and (3) aberrant expression of B-cell lymphoma 2 (Bcl-2) with sustained mitophagy. Furthermore, these peculiarities may represent attractive new “hot spots” for mitochondrial-targeted therapy. Finally, we remark the potential of the LCS metabolic effectors to be exploited as novel therapeutic targets.

Proteostatic and Metabolic Control of Stemness

Adult stem cells, particularly those resident in tissues with little turnover, are largely quiescent and only activate in response to regenerative demands, while embryonic stem cells continuously replicate, suggesting profoundly different regulatory mechanisms within distinct stem cell types. In recent years, evidence linking metabolism, mitochondrial dynamics, and protein homeostasis (proteostasis) as fundamental regulators of stem cell function has emerged. Here, we discuss new insights into how these networks control potency, self-renewal, differentiation, and aging of highly proliferative embryonic stem cells and quiescent adult stem cells, with a focus on hematopoietic and muscle stem cells and implications for anti-aging research.

Endothelial Cell Metabolism

Endothelial cells (ECs) are more than inert blood vessel lining material. Instead, they are active players in the formation of new blood vessels (angiogenesis) both in health and (life-threatening) diseases. Recently, a new concept arose by which EC metabolism drives angiogenesis in parallel to well-established angiogenic growth factors (e.g., vascular endothelial growth factor). 6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3-driven glycolysis generates energy to sustain competitive behavior of the ECs at the tip of a growing vessel sprout, whereas carnitine palmitoyltransferase 1a-controlled fatty acid oxidation regulates nucleotide synthesis and proliferation of ECs in the stalk of the sprout. To maintain vascular homeostasis, ECs rely on an intricate metabolic wiring characterized by intracellular compartmentalization, use metabolites for epigenetic regulation of EC subtype differentiation, crosstalk through metabolite release with other cell types, and exhibit EC subtype-specific metabolic traits. Importantly, maladaptation of EC metabolism contributes to vascular disorders, through EC dysfunction or excess angiogenesis, and presents new opportunities for anti-angiogenic strategies. Here we provide a comprehensive overview of established as well as newly uncovered aspects of EC metabolism.

Magnesium isoglycyrrhizinate promotes the activated hepatic stellate cells apoptosis via endoplasmic reticulum stress and ameliorates fibrogenesis in vitro and in vivo

Varied pathogenetic elements have been touched upon the liver fibrosis, including inflammatory, stress, apoptosis and unfolded proteins aggregation. Magnesium Isoglycyrrhizinate (MgIG) has been accepted to be a neuroprotective effect, hepatoprotective and anti-inflammatory molecule. In our vitro researches, MgIG was considered to activate hepatic stellate cells (HSCs) apoptosis by promoting endoplasmic reticulum stress (ERS) detrimental response to a certain extent. Consequently, MgIG showed its potential therapeutic capacity in fibrogenesis and counteracted the pathogenetic aspects, which were involved in integrating current treatments correcting liver fibrosis. In addition, we further verificated the behavior and pathogenic mechanisms in the CCl₄ -induced liver fibrosis in male mice. What surprised us was that with the treatment of MgIG caused the activation of ERS and resisted the activated HSCs in the protective effects on liver damage. We found MgIG significantly promoted the apoptosis of activated HSCs and protected the CCl₄ -induced liver fibrosis. Main molecules came down to the unfolded protein response signaling pathway. Furthermore, MgIG inhibited the levels of the downstream inflammatory cytokines, which were triggered by CCl₄ -induced liver fibrosis. Here, we reported that MgIG improved behavioral impairments induced by intraperitoneal injection of CCl₄ and decreased the expression of proinflammatory factor, which indicated the preserving effects on liver fibrosis. © 2017 BioFactors, 43(6):836-846, 2017.