Nitric Oxide in the Control of the in vitro Proliferation and Differentiation of Human Hematopoietic Stem and Progenitor Cells.

Hematopoietic stem and progenitor cell (HSPC) transplantation is the best-studied cellular therapy and successful in vitro control of HSPCs has wide clinical implications. Nitric oxide (NO) is a central signaling molecule in vivo and has been implicated in HSPC mobilization to the blood stream in mice. The influence of NO on HSPC behavior in vitro is, however, largely obscure due to the variety of employed cell types, NO administration systems, and used concentration ranges in the literature. Additionally, most studies are based on murine cells, which do not necessarily mimic human HSPC behavior. Thus, the aim of the present study was the systematic, concentration-dependent evaluation of NO-mediated effects on human HSPC behavior in vitro. By culture in the presence of the long-term NO donor diethylenetriamine/nitric oxide adduct (DETA/NO) in a nontoxic concentration window, a biphasic role of NO in the regulation of HSPC behavior was identified: Low DETA/NO concentrations activated classical NO signaling, identified via increased intracellular cyclic guanosine monophosphate (cGMP) levels and proteinkinases G (PKG)-dependent vasodilator-stimulated phosphoprotein (VASP) phosphorylation and mediated a pro-proliferative response of HSPCs. In contrast, elevated NO concentrations slowed cell proliferation and induced HSPC differentiation. At high concentrations, s-nitrosylation levels were elevated, and myeloid differentiation was increased at the expense of lymphoid progenitors. Together, these findings hint at a central role of NO in regulating human HSPC behavior and stress the importance and the potential of the use of adequate NO concentrations for in vitro cultures of HSPCs, with possible implications for clinical application of in vitro expanded or differentiated HSPCs for cellular therapies.

Microfluidic Shear Force Assay to Determine Cell Adhesion Forces.

Cell adhesion is implicated in many physiological settings such as the retention of hematopoietic stem cells (HSCs) in their bone marrow niches or their migration into the bloodstream. During HSC mobilization these adhesion sites are cleaved and have to be newly formed during HSC homing and engraftment. To determine the adhesive properties of HSCs on different extracellular matrix (ECM) molecules, we present a microfluidic shear force assay, where a laminar flow is used to detach a semi-adherent cell population, the HSC model cell line KG-1a, from an ECM protein-coated substrate. This technique combines the high throughput of population-based assays with the ability to observe cell detachment in real time. Additionally, it is suitable for weakly adherent cells, as the setup allows cell incubation on various substrates and application of shear stress ranging several orders of magnitude in one setup without additional washing or transfer steps. As a measure for the adhesion strength of the studied cell population on the substrate, the critical shear force τ50 is determined which is required to remove 50% of the initially adherent cell fraction.

Development of Endothelial Cell Networks in 3D Tissues by Combination of Melt Electrospinning Writing with Cell-Accumulation Technology.

A remaining challenge in tissue engineering approaches is the in vitro vascularization of engineered constructs or tissues. Current approaches in engineered vascularized constructs are often limited in the control of initial vascular network geometry, which is crucial to ensure full functionality of these constructs with regard to cell survival, metabolic activity, and potential differentiation ability. Herein, the combination of 3D-printed poly-ε-caprolactone scaffolds via melt electrospinning writing with the cell-accumulation technique to enable the formation and control of capillary-like network structures is reported. The cell-accumulation technique is already proven itself to be a powerful tool in obtaining thick (50 µm) tissues and its main advantage is the rapid production of tissues and its ease of performance. However, the applied combination yields tissue thicknesses that are doubled, which is of outstanding importance for an improved handling of the scaffolds and the generation of clinically relevant sample volumes. Moreover, a correlation of increasing vascular endothelial growth factor secretion to hypoxic conditions with increasing pore sizes and an assessment of the formation of neovascular like structures are included.