Cord blood stem cell expansion is permissive to epigenetic regulation and environmental cues

Reconstruction of bone marrow microenvironment for expansion of hematopoietic stem cells by a histone deacetylase inhibitor

Ex vivo expansion of hematopoietic stem cells (HSCs) is an approach for overcoming cell insufficiency for umbilical cord blood transplantation. It was suggested that in common ex vivo cultures, the stemness specificity of HSCs is rapidly reducing due to DNA hypermethylation. Here, Nicotinamide (NAM), a DNA methyltransferase and histone deacetylase inhibitor, is used with a bioengineered Bone Marrow-like niche (BLN) for HSC ex vivo expansion. The CFSE cell proliferation assay was used for tracking HSCs division. qRT-PCR was conducted to assay the HOXB4 mRNA expression levels. The morphology of BLN-cultured cells was analyzed using scanning electron microscopy (SEM). NAM boosted the induction of HSC proliferation in the BLN group compared to the control group. In addition, the ability of HSCs to colonize was more significant in the BLN group than in the control group. Our data suggest that the presence of NAM in bioengineered niches promotes HSC proliferation. The presented approach showed that small molecules could be used in the clinical setting to overcome the limited number of CD34+ cells in cord blood units.

Therapeutic targeting and HSC proliferation by small molecules and biologicals

Hematopoietic stem cells (HSCs) have considerably therapeutic value on autologous and allogeneic transplantation for many malignant/non-malignant hematological diseases, especially with improvement of gene therapy. However, acquirement of limited cell dose from HSC sources is the main handicap for successful transplantation. Therefore, many strategies based on the utilization of various cytokines, interaction of stromal cells, modulation of several extrinsic and intrinsic factors have been developed to promote ex vivo functional HSC expansion with high reconstitution ability until today. Besides all these strategies, small molecules become prominent with their ease of use and various advantages when they are translated to the clinic. In the last two decades, several small molecule compounds have been investigated in pre-clinical studies and, some of them were evaluated in different stages of clinical trials for their safety and efficiencies. In this chapter, we will present an overview of HSC biology, function, regulation and also, pharmacological HSC modulation with small molecules from pre-clinical and clinical perspectives.

Ex vivo expansion of hematopoietic stem cells: Finally transitioning from the lab to the clinic

Hematopoietic stem cells (HSCs) have been used for therapeutic purposes for decades in the form of autologous and allogeneic transplantation and are currently emerging as an attractive target for gene therapy. A low stem cell dose is a major barrier to the application of HSC therapy in several situations, primarily umbilical cord blood transplantation and gene modification. Strategies that promote ex vivo expansion of the numbers of functional HSCs could overcome this barrier, hence have been the subject of intense and prolonged research. Several ex vivo expansion strategies have advanced to evaluation clinical trials, which are showing favorable outcomes along with convincing safety signals. Preclinical studies have recently confirmed beneficial incorporation of ex vivo expansion into HSC gene modification protocols. Collectively, ex vivo HSC expansion holds promise for significantly broadening the availability of cord blood units for transplantation, and for optimizing gene therapy protocols to enable their clinical application.

Mechanistic Basis of ex Vivo Umbilical Cord Blood Stem Progenitor Cell Expansion

Umbilical cord blood (CB) transplantation has been used successfully in humans for three decades due to its rapid availability for patients lacking a suitable allogeneic donor, less stringent HLA matching requirements, and low rates of relapse and chronic graft-versus-host disease (GVHD). However, CB transplantation is associated with complications, such as delayed hematopoietic engraftment, graft failure, which increases infection and bleeding and causes longer hospital stays, and transplant-related mortality. The majority of these biological limitations are due to the unforeseeable functional potency of multipotent hematopoietic stem cells (HSCs), which reduce the predictability of successful transplantation; however, several strategies have been developed to increase the number of hematopoietic stem progenitor cells (HSPCs) infused during CB transplantation. This review primarily addresses the methods that promote ex vivo CB expansion within the context of symmetrical and asymmetrical HSC division and those that rely on epigenetic mechanisms, along with the reportedly most successful cytokine combinations. We also review recent clinical research on small molecules (StemRegenin-1, UM171, and nicotinamide) in ex vivo expanded CB and discuss yet unvalidated preclinical strategies. Expanding and transplanting CB graft enriched in HSPCs in a single CB unit is a particularly exciting prospect with the potential to improve the use and availability of CB grafts. Greater knowledge of optimal ex vivo expansion strategies, cell longevity, and graft potency will expand the scope of cellular therapies. Also the development of adequate ex vivo HSPC expansion strategies could bring expanded cord blood grafts to the forefront of transplant therapy and regenerative medicine.

Expansion and preservation of the functional activity of adult hematopoietic stem cells cultured ex vivo with a histone deacetylase inhibitor

Attempts to expand ex vivo the numbers of human hematopoietic stem cells (HSCs) without compromising their marrow repopulating capacity and their ability to establish multilineage hematopoiesis has been the subject of intense investigation. Although most such efforts have focused on cord blood HSCs, few have been applied to adult HSCs, a more clinically relevant HSC source for gene modification. To date, the strategies that have been used to expand adult HSCs have resulted in modest effects or HSCs with lineage bias and a limited ability to generate T cells in vivo. We previously reported that culturing umbilical cord blood CD34+ cells in serum‐free media supplemented with valproic acid (VPA), a histone deacetylase inhibitor, and a combination of cytokines led to the expansion of the numbers of fully functional HSCs. In the present study, we used this same approach to expand the numbers of adult human CD34+ cells isolated from mobilized peripheral blood and bone marrow. This approach resulted in a significant increase in the numbers of phenotypically defined HSCs (CD34+CD45RA‐CD90+D49f+). Cells incubated with VPA also exhibited increased aldehyde dehydrogenase activity and decreased mitochondrial membrane potential, each functional markers of HSCs. Grafts harvested from VPA‐treated cultures were able to engraft in immune‐deficient mice and, importantly, to generate cellular progeny belonging to each hematopoietic lineage in similar proportion to that observed with unmanipulated CD34+ cells. These data support the utility of VPA‐mediated ex vivo HSC expansion for gene modification of adult HSCs.

Single-cell assessment of transcriptome alterations induced by Scriptaid in early differentiated human haematopoietic progenitors during ex vivo expansion

Priming haematopoietic stem/progenitor cells (HSPCs) in vitro with specific chromatin modifying agents and cytokines under serum-free-conditions significantly enhances engraftable HSC numbers. We extend these studies by culturing human CD133+ HSPCs on nanofibre scaffolds to mimic the niche for 5-days with the HDAC inhibitor Scriptaid and cytokines. Scriptaid increases absolute Lin−CD34+CD38−CD45RA−CD90+CD49f+ HSPC numbers, while concomitantly decreasing the Lin−CD38−CD34+CD45RA−CD90− subset. Hypothesising that Scriptaid plus cytokines expands the CD90+ subset without differentiation and upregulates CD90 on CD90− cells, we sorted, then cultured Lin−CD34+CD38−CD45RA−CD90− cells with Scriptaid and cytokines. Within 2-days and for at least 5-days, most CD90− cells became CD90+. There was no significant difference in the transcriptomic profile, by RNAsequencing, between cytokine-expanded and purified Lin−CD34+CD38−CD45RA−CD49f+CD90+ cells in the presence or absence of Scriptaid, suggesting that Scriptaid maintains stem cell gene expression programs despite expansion in HSC numbers. Supporting this, 50 genes were significantly differentially expressed between CD90+ and CD90− Lin−CD34+CD38−CD45RA−CD49f+ subsets in Scriptaid-cytokine- and cytokine only-expansion conditions. Thus, Scriptaid treatment of CD133+ cells may be a useful approach to expanding the absolute number of CD90+ HSC, without losing their stem cell characteristics, both through direct effects on HSC and potentially also conversion of their immediate CD90− progeny into CD90+ HSC.

Chromatin-Modifying Agent-Expanded Human Cord Blood Cells Display Reduced Allostimulatory Capacity

The limited number of hematopoietic stem cells (HSC) within a single unit of human cord blood currently limits its use as an alternate graft source. However, we have developed a strategy using 5-aza-2'-deoxycytidine (5azaD) and trichostatin A (TSA), which expands transplantable HSC 7- to 10-fold. In our current studies, we have assessed the allostimulatory capacity of the 5azaD/TSA-expanded grafts. The coexpression of immunophenotypic dendritic cell (DC) markers, such as HLA-DR/CD86 and HLA-DR/CD11c as determined by flow cytometry, and the allostimulatory capacity of 5azaD/TSA-expanded cells as determined by MLC were both significantly lower than control. It has been previously demonstrated that STAT3 is indispensable for the differentiation of DC from HSC. Real-time quantitative PCR analysis revealed that 5azaD/TSA-expanded cells expressed more STAT3 transcript than control while also expressing increased transcripts for STAT3 inhibitors including SHP1, p21, and GATA1. Western blot analysis indicates that chromatin-modifying agent-expanded grafts displayed a reduced ratio of p-STAT3 to total STAT3 than control cultures, which is likely indicative of STAT3 inactivity in 5azD/TSA-expanded grafts. Culturing 5azaD/TSA-expanded cord blood cells in extended cultures reveals that they are still capable of generating DC. Notably, STAT3 inactivity was transient because the transcript levels of STAT3 and its inhibitors, including SHP1, were comparable between 5azaD/TSA and control cultures following extended culture. Taken together, our studies indicate that the reduced allostimulatory capacity of 5azaD/TSA-expanded cells is likely because of reversible inhibition of STAT3-dependent DC differentiation. These results suggest that a graft composed of 5azaD/TSA-expanded cells possesses relatively less allostimulatory response but is still capable of generating DC in permissive conditions.

Todralazine protects zebrafish from lethal effects of ionizing radiation: role of hematopoietic cell expansion

The Johns Hopkins Clinical Compound Library (JHCCL), a collection of Food and Drug Administration (FDA)-approved small molecules (1400), was screened in silico for identification of novel β2AR blockers and tested for hematopoietic stem cell (HSC) expansion and radioprotection in zebrafish embryos. Docking studies, followed by the capacity to hasten erythropoiesis, identified todralazine (Binding energy, -8.4 kcal/mol) as a potential HSC-modulating agent. Todralazine (5 μM) significantly increased erythropoiesis in caudal hematopoietic tissue (CHT) in wild-type and anemic zebrafish embryos (2.33- and 1.44-folds, respectively) when compared with untreated and anemic control groups. Todralazine (5 μM) treatment also led to an increased number of erythroid progenitors, as revealed from the increased expression of erythroid progenitor-specific genes in the CHT region. Consistent with these effects, zebrafish embryos, Tg(cmyb:gfp), treated with 5 μM todralazine from 24 to 36 hours post fertilization (hpf) showed increased (approximately two-folds) number of HSCs at the aorta-gonad-mesonephros region (AGM). Similarly, expression of HSC marker genes, runx1 (3.3-folds), and cMyb (1.41-folds) also increased in case of todralazine-treated embryos, further supporting its HSC expansion potential. Metoprolol, a known beta blocker, also induced HSC expansion (1.36- and 1.48-fold increase in runx1 and cMyb, respectively). Todralazine (5 μM) when added 30 min before 20 Gy gamma radiation, protected zebrafish from radiation-induced organ toxicity, apoptosis, and improved survival (80% survival advantage over 6 days). The 2-deoxyribose degradation test further suggested hydroxyl (OH) radical scavenging potential of todralazine, and the same is recapitulated in vivo. These results suggest that todralazine is a potential HSC expanding agent, which might be acting along with important functions, such as antioxidant and free radical scavenging, in manifesting radioprotection.

Ex vivo expansion of human mobilized peripheral blood stem cells using epigenetic modifiers

BACKGROUND

Epigenetic modifications likely control the fate of hematopoietic stem cells (HSCs). The chromatin-modifying agents (CMAs), 5-aza-2'-deoxycytidine (5azaD) and trichostatin A (TSA), have previously been shown to expand HSCs from cord blood and marrow. Here we assessed whether CMA can also expand HSCs present in growth factor-mobilized human peripheral blood (MPB).

STUDY DESIGN AND METHODS

5azaD and TSA were sequentially added to CD34+ MPB cells in the presence of cytokines, and the cells were cultured for 9 days.

RESULTS

After culture, a 3.6 ± 0.5-fold expansion of CD34+CD90+ cells, a 10.1 ± 0.5-fold expansion of primitive colony-forming unit (CFU)-mix, and a 2.2 ± 0.5-fold expansion of long-term cobblestone-area-forming cells (CAFCs) was observed in 5azaD/TSA-expanded cells. By contrast, cells cultured in cytokines without 5azaD/TSA displayed no expansion; rather, a reduction in CD34+CD90+ cells (0.7 ± 0.1-fold) and CAFCs (0.3 ± 0.1-fold) from their initial numbers was observed. Global hypomethylation corresponding with increased transcript levels of several genes implicated in HSC self-renewal, including HOXB4, GATA2, and EZH2, was observed in 5azaD/TSA-expanded MPB cells in contrast to controls. 5azaD/TSA-expanded MPB cells retained in vivo hematopoietic engraftment capacity.

CONCLUSION

MPB CD34+ cells from donors can be expanded using 5azaD/TSA, and these expanded cells retain in vivo hematopoietic reconstitution capacity. This strategy may prove to be potentially useful to augment HSC numbers for patients who fail to mobilize.

Differential effects of epigenetic modifiers on the expansion and maintenance of human cord blood stem/progenitor cells

Epigenetic therapies, including DNA methyltransferase and histone deacetylase (HDAC) inhibitors, are increasingly being considered to treat hematological malignancies, but their effects on normal hematopoietic stem cells (HSCs) remain largely unexplored. We compared the effects of several HDAC inhibitors, including valproic acid (VPA) and trichostatin A (TSA), alone or in combination with 5-aza-2'-deoxycytidine (5azaD) on the expansion of HSCs. VPA induced the highest expansion of CD34+CD90+ cells and progenitor cells compared with other HDAC inhibitors or the sequential addition of 5azaD/TSA in culture. Xenotransplantation studies demonstrated that VPA prevents HSC loss, whereas 5azaD/TSA treatment leads to a net expansion of HSCs that retain serial transplantation ability. 5azaD/TSA-mediated HSC expansion was associated with increased histone acetylation and transient DNA demethylation, which corresponded with higher gene transcript levels. However, some genes with increased transcript levels lacked changes in methylation. Importantly, a global microarray analysis revealed a set of differentially expressed genes in 5azaD/TSA- and VPA-expanded CD34+ cells that might be involved in the expansion and maintenance of transplantable HSCs, respectively. In summary, our data indicate that treatment of HSCs with different chromatin-modifying agents results in either the expansion or maintenance of HSCs, an observation of potential therapeutic importance.

Tumor stiffness is unrelated to myosin light chain phosphorylation in cancer cells

Many tumors are stiffer than their surrounding tissue. This increase in stiffness has been attributed, in part, to a Rho-dependent elevation of myosin II light chain phosphorylation. To characterize this mechanism further, we studied myosin light chain kinase (MLCK), the main enzyme that phosphorylates myosin II light chains. We anticipated that increases in MLCK expression and activity would contribute to the increased stiffness of cancer cells. However, we find that MLCK mRNA and protein levels are substantially less in cancer cells and tissues than in normal cells. Consistent with this observation, cancer cells contract 3D collagen matrices much more slowly than normal cells. Interestingly, inhibiting MLCK or Rho kinase did not affect the 3D gel contractions while blebbistatin partially and cytochalasin D maximally inhibited contractions. Live cell imaging of cells in collagen gels showed that cytochalasin D inhibited filopodia-like projections that formed between cells while a MLCK inhibitor had no effect on these projections. These data suggest that myosin II phosphorylation is dispensable in regulating the mechanical properties of tumors.

Prostaglandin E2 enhances long-term repopulation but does not permanently alter inherent stem cell competitiveness

Hematopoietic stem cell (HSC) transplantation is a lifesaving therapy for malignant and nonmalignant hematologic diseases and metabolic disorders. Although successful, hematopoietic transplantation can be hindered by inadequate stem cell number or poor engrafting efficiency. To overcome these deficits, we and others have previously reported the HSC-enhancing ability of a short-term exposure of prostaglandin E2 (PGE2); this strategy has now progressed to phase 1 clinical trials in double cord blood transplantation. To further analyze the short- and long-term effects of HSC exposure to PGE2, we followed the repopulation kinetics of PGE2-treated hematopoietic grafts through 5 serial transplantations and compared inherent long-term competitiveness in a HSC head-to-head secondary transplantation model. Treatment with PGE2 did not result in a long-term increase in HSC competitiveness, lineage bias, or enhanced proliferative potential, demonstrating that pulse exposure to PGE2 results in transient increases in HSC homing and engraftment potential.

Inhibition of p38 MAPK activity promotes ex vivo expansion of human cord blood hematopoietic stem cells

Ex vivo expansion of hematopoietic stem cells (HSCs) depends on HSC self-renewing proliferation and functional maintenance, which can be negatively affected by HSC differentiation, apoptosis, and senescence. Therefore, inhibition of HSC senescence may promote HSC expansion. To test this hypothesis, we examined the effect of inhibition of p38 mitogen-activated protein kinase (p38) on the expansion of human umbilical cord blood (hUCB) CD133(+) cells because activation of p38 has been implicated in the induction of HSC senescence under various physiological and pathological conditions. Our results showed that ex vivo expansion of hUCB CD133(+) cells activated p38, which was abrogated by the p38 specific inhibitor SB203580 (SB). Inhibition of p38 activity with SB promoted the expansion of CD133(+) cells and CD133(+)CD38(-) cells. In addition, hUCB CD133(+) cells expanded in the presence of SB for 7 days showed about threefold increase in the clonogenic function of HSCs and engraftment in non-obese diabetic/severe combined immunodeficient mice after transplantation compared to the input cells. In contrast, the cells expanded without SB exhibited a significant reduction in these HSC functions. The enhancement of ex vivo expansion of hUCB HSCs is primarily attributable to SB-mediated inhibition of HSC senescence. In addition, inhibition of HSC apoptosis and upregulation of CXCR4 may also contribute to the enhancement. However, p38 inhibition had no significant effect on HSC differentiation and proliferation. These findings suggest that inhibition of p38 activation may represent a novel strategy to promote ex vivo expansion of hUCB HSCs.

Ex vivo expansion of human hematopoietic stem cells by garcinol, a potent inhibitor of histone acetyltransferase

Background

Human cord blood (hCB) is the main source of hematopoietic stem and progenitor cells (HSCs/PCs) for transplantation. Efforts to overcome relative shortages of HSCs/PCs have led to technologies to expand HSCs/PCs ex vivo. However, methods suitable for clinical practice have yet to be fully established.

Methodology/Principal Findings

In this study, we screened biologically active natural products for activity to promote expansion of hCB HSCs/PCs ex vivo, and identified Garcinol, a plant-derived histone acetyltransferase (HAT) inhibitor, as a novel stimulator of hCB HSC/PC expansion. During a 7-day culture of CD34⁺CD38^– HSCs supplemented with stem cell factor and thrombopoietin, Garcinol increased numbers of CD34⁺CD38^– HSCs/PCs more than 4.5-fold and Isogarcinol, a derivative of Garcinol, 7.4-fold. Furthermore, during a 7-day culture of CD34⁺ HSCs/PCs, Garcinol expanded the number of SCID-repopulating cells (SRCs) 2.5-fold. We also demonstrated that the capacity of Garcinol and its derivatives to expand HSCs/PCs was closely correlated with their inhibitory effect on HAT. The Garcinol derivatives which expanded HSCs/PCs inhibited the HAT activity and acetylation of histones, while inactive derivatives did not.

Conclusions/Significance

Our findings identify Garcinol as the first natural product acting on HSCs/PCs and suggest the inhibition of HAT to be an alternative approach for manipulating HSCs/PCs.

Human cord blood CD34+ progenitor cells acquire functional cardiac properties through a cell fusion process

The efficacy of cardiac repair by stem cell administration relies on a successful functional integration of injected cells into the host myocardium. Safety concerns have been raised about the possibility that stem cells may induce foci of arrhythmia in the ischemic myocardium. In a previous work (36), we showed that human cord blood CD34(+) cells, when cocultured on neonatal mouse cardiomyocytes, exhibit excitation-contraction coupling features similar to those of cardiomyocytes, even though no human genes were upregulated. The aims of the present work are to investigate whether human CD34(+) cells, isolated after 1 wk of coculture with neonatal ventricular myocytes, possess molecular and functional properties of cardiomyocytes and to discriminate, using a reporter gene system, whether cardiac differentiation derives from a (trans)differentiation or a cell fusion process. Umbilical cord blood CD34(+) cells were isolated by a magnetic cell sorting method, transduced with a lentiviral vector carrying the enhanced green fluorescent protein (EGFP) gene, and seeded onto primary cultures of spontaneously beating rat neonatal cardiomyocytes. Cocultured EGFP(+)/CD34(+)-derived cells were analyzed for their electrophysiological features at different time points. After 1 wk in coculture, EGFP(+) cells, in contact with cardiomyocytes, were spontaneously contracting and had a maximum diastolic potential (MDP) of -53.1 mV, while those that remained isolated from the surrounding myocytes did not contract and had a depolarized resting potential of -11.4 mV. Cells were then resuspended and cultured at low density to identify EGFP(+) progenitor cell derivatives. Under these conditions, we observed single EGFP(+) beating cells that had acquired an hyperpolarization-activated current typical of neonatal cardiomyocytes (EGFP(+) cells, -2.24 ± 0.89 pA/pF; myocytes, -1.99 ± 0.63 pA/pF, at -125 mV). To discriminate between cell autonomous differentiation and fusion, EGFP(+)/CD34(+) cells were cocultured with cardiac myocytes infected with a red fluorescence protein-lentiviral vector; under these conditions we found that 100% of EGFP(+) cells were also red fluorescent protein positive, suggesting cell fusion as the mechanism by which cardiac functional features are acquired.

Systematic delineation of optimal cytokine concentrations to expand hematopoietic stem/progenitor cells in co-culture with mesenchymal stem cells

The major obstacle to the widespread use of umbilical cord blood (UCB) in hematopoietic stem/progenitor (HSC) cell therapy is the low cell dose available. A cytokine cocktail for the ex vivo expansion of UCB HSC, in co-culture with a bone marrow (BM) mesenchymal stem cells (MSC)-derived stromal layer was optimized using an experimental design approach. Proliferation of total cells (TNC), stem/progenitor cells (CD34(+)) and colony-forming units (CFU) was assessed after 7 days in culture, while sole and interactive effects of each cytokine on HSC expansion were statistically determined using a two-level Face-Centered Cube Design. The optimal cytokine cocktail obtained for HSC-MSC co-cultures was composed by SCF, Flt-3L and TPO (60, 55 and 50 ng mL(-1), respectively), resulting in 33-fold expansion in TNC, 17-fold in CD34(+) cells, 3-fold in CD34(+)CD90(+) cells and 21-fold in CFU-MIX. More importantly, these short-term expanded cells preserved their telomere length and extensively generated cobblestone area-forming cells (CAFCs) in vitro. The statistical tools used herein contributed for the rational delineation of the cytokine concentration range, in a cost-effective way, while systematically addressing complex cytokine-to-cytokine interactions, for the efficient HSC expansion towards the generation of clinically significant cell numbers for transplantation.