Isolation and Characterization of Bone Marrow Mesenchymal Stromal Cell Subsets in Culture Based on Aldehyde Dehydrogenase Activity

Functional single-cell analyses of mesenchymal stromal cell proliferation and differentiation using ALDH-activity and mitochondrial ROS content

BASKGROUND

Previous research has unveiled a stem cell-like transcriptome enrichment in the aldehyde dehydrogenase-expressing (ALDHhigh) mesenchymal stromal cell (MStroC) fraction. However, considering the heterogeneity of MStroCs, with only a fraction of them presenting bona fide stem cells (MSCs), the actual potency of ALDH as an MSC-specific selection marker remains an issue.

METHODS

To address this, the proliferative and differentiation potential of individual ALDHhigh and ALDHlow MStroCs incubated at low oxygen concentrations, estimated to mimic stem cell niches (0.1% O2), were assayed using single-cell clonal analysis, compared to standard conditions (20% O2).

RESULTS

We confirm that a high proliferative capacity and multi-potent MSCs are enriched in the ALDHhigh MStroC population, especially when cells are cultured at 0.1% O2. Measurements of reduced/oxidized glutathione and mitochondrial superoxide anions with MitoSoX (MSX) indicate that this advantage induced by low oxygen is related to a decrease in the oxidative and reactive oxygen species (ROS) levels in the stem cell metabolic setup. However, ALDH expression is neither specific nor exclusive to MSCs, as high proliferative capacity and multi-potent cells were also found in the ALDHlow fraction. Furthermore, single-cell assays performed after combined cell sorting based on ALDH and MSX showed that the MSXlow MStroC population is enriched in stem/progenitor cells in all conditions, irrespective of ALDH expression or culture oxygen concentration. Importantly, the ALDHhighMSXlow MStroC fraction exposed to 0.1% O2 was almost exclusively composed of genuine MSCs. In contrast, neither progenitors nor stem cells (with a complete absence of colony-forming ability) were detected in the MSXhigh fraction, which exclusively resides in the ALDHlow MStroC population.

CONCLUSION

Our study reveals that ALDH expression is not exclusively associated with MSCs. However, cell sorting using combined ALDH expression and ROS content can be utilized to exclude MStroCs lacking stem/progenitor cell properties.

Insight into Hypoxia Stemness Control

Recently, the research on stemness and multilineage differentiation mechanisms has greatly increased its value due to the potential therapeutic impact of stem cell-based approaches. Stem cells modulate their self-renewing and differentiation capacities in response to endogenous and/or extrinsic factors that can control stem cell fate. One key factor controlling stem cell phenotype is oxygen (O₂). Several pieces of evidence demonstrated that the complexity of reproducing O₂ physiological tensions and gradients in culture is responsible for defective stem cell behavior in vitro and after transplantation. This evidence is still worsened by considering that stem cells are conventionally incubated under non-physiological air O₂ tension (21%). Therefore, the study of mechanisms and signaling activated at lower O₂ tension, such as those existing under native microenvironments (referred to as hypoxia), represent an effective strategy to define if O₂ is essential in preserving naïve stemness potential as well as in modulating their differentiation. Starting from this premise, the goal of the present review is to report the status of the art about the link existing between hypoxia and stemness providing insight into the factors/molecules involved, to design targeted strategies that, recapitulating naïve O₂ signals, enable towards the therapeutic use of stem cell for tissue engineering and regenerative medicine.

Homogeneity and heterogeneity of biological characteristics in mesenchymal stem cells from human umbilical cords and exfoliated deciduous teeth

Mesenchymal stem cells (MSCs) have proven powerful potential for cell-based therapy both in regenerative medicine and disease treatment. Human umbilical cords and exfoliated deciduous teeth are the main sources of MSCs with no donor injury or ethical issues. The goal of this study was to investigate the differences in the biological characteristics of human umbilical cord mesenchymal stem cells (UCMSCs) and stem cells from human exfoliated deciduous teeth (SHEDs). UCMSCs and SHEDs were identified by flow cytometry. The proliferation, differentiation, migration, chemotaxis, paracrine, immunomodulatory, neurite growth-promoting capabilities, and acetaldehyde dehydrogenase (ALDH) activity were comparatively studied between these two MSCs in vitro. The results showed that both SHEDs and UCMSCs expressed cell surface markers characteristic of MSCs. Furthermore, SHEDs exhibited better capacity for proliferation, migration, promotion of neurite growth, and chondrogenic differentiation. Meanwhile, UCMSCs showed more outstanding adipogenic differentiation and chemotaxy. Additionally, there were no significant differences in osteogenic differentiation, immunomodulatory capacity, and the proportion of ALDH^Bright compartment. Our findings indicate that although both UCMSCs and SHEDs are mesenchymal stem cells and presented some similar biological characteristics, they also have differences in many aspects, which might be helpful for developing future clinical cellular therapies.