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Abstract Mesenchymal stem cell (MSC)-based therapy has shown great promises in various animal disease models. However, this therapeutic potency has not been well claimed when applied to human clinical trials. This is due to both the availability of MSCs at the time of administration and lack of viable expansion strategies. MSCs are very susceptible to in vitro culture environment and tend to adapt the microenvironment which could lead to cellular senescence and aging. Therefore, extended in vitro expansion induces loss of MSC functionality and its clinical relevance. To combat this effect, this work assessed a novel cyclical aggregation as a means of expanding MSCs to maintain stem cell functionality. The cyclical aggregation consists of an aggregation phase and an expansion phase by replating the dissociated MSC aggregates onto planar tissue culture surfaces. The results indicate that cyclical aggregation maintains proliferative capability, stem cell proteins, and clonogenicity, and prevents the acquisition of senescence. To determine why aggregation was responsible for this phenomenon, the integrated stress response pathway was probed with salubrial and GSK-2606414. Treatment with salubrial had no significant effect, while GSK-2606414 mitigated the effects of aggregation leading to in vitro aging. This method holds the potential to increase the clinical relevance of MSC therapeutic effects from small model systems (such as rats and mice) to humans, and may open the potential of patient-derived MSCs for treatment thereby removing the need for immunosuppression. 

Human mesenchymal stem cells (hMSCs) promote endogenous tissue regeneration and have become a promising candidate for cell therapy. However, in vitro culture expansion of hMSCs induces a rapid decline of stem cell properties through replicative senescence. Here, we characterize metabolic profiles of hMSCs during expansion. We show that alterations of cellular nicotinamide adenine dinucleotide (NAD + /NADH) redox balance and activity of the Sirtuin (Sirt) family enzymes regulate cellular senescence of hMSCs. Treatment with NAD + precursor nicotinamide increases the intracellular NAD + level and re-balances the NAD + /NADH ratio, with enhanced Sirt-1 activity in hMSCs at high passage, partially restores mitochondrial fitness and rejuvenates senescent hMSCs. By contrast, human fibroblasts exhibit limited senescence as their cellular NAD + /NADH balance is comparatively stable during expansion. These results indicate a potential metabolic and redox connection to replicative senescence in adult stem cells and identify NAD + as a metabolic regulator that distinguishes stem cells from mature cells. This study also suggests potential strategies to maintain cellular homeostasis of hMSCs in clinical applications.

 

Protein homeostasis is critical for cellular function, as loss of homeostasis is attributed to aging and the accumulation of unwanted proteins. Human mesenchymal stem cells (MSCs) have shown promising therapeutic potential due to their impressive abilities to secrete inflammatory modulators, angiogenic, and regenerative cytokines. However, there exists the problem of human MSC expansion with compromised therapeutic quality. Duringin vitro expansion, human MSCs are plated on stiff plastics and undergo culture adaptation, which results in aberrant proliferation, shifts in metabolism, and decreased autophagic activity. It has previously been shown that three‐dimensional (3D) aggregation can reverse some of these alterations by heightening autophagy and recovering the metabolic state back to a naïve phenotype. To further understand the proteostasis in human MSC culture, this study investigated the effects of 3D aggregation on the human MSC proteome to determine the specific pathways altered by aggregation. The 3D aggregates and 2D cultures of human MSCs derived from bone marrow (bMSC) and adipose tissue (ASC) were analyzed along with differentiated human dermal fibroblasts (FB). The proteomics analysis showed the elevated eukaryotic initiation factor 2 pathway and the upregulated activity of the integrated stress response (ISR) in 3D aggregates. Specific protein quantification further determined that bMSC and ASC responded to ISR, while FB did not. 3D aggregation significantly increased the ischemic survival of bMSCs and ASCs. Perturbation of ISR with small molecules salubrinal and GSK2606414 resulted in differential responses of bMSC, ASC, and FB. This study indicates that aggregation‐based preconditioning culture holds the potential for improving the therapeutic efficacy of expanded human MSCs via the establishment of ISR and homeostasis.