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1. Aggregation‐induced integrated stress response rejuvenates culture‐expanded human mesenchymal stem cells

   https://doi.org/10.1002/bit.27474  

   Bijonowski, Brent M. ; Fu, Qin ; Yuan, Xuegang ; Irianto, Jerome ; Li, Yan ; Grant, Samuel C. ; Ma, Teng ( July 2020 , Biotechnology and Bioengineering)

   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.

    

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2. Survival of aging CD264 + and CD264 − populations of human bone marrow mesenchymal stem cells is independent of colony‐forming efficiency

   https://doi.org/10.1002/bit.27195  

   Madsen, Sean D. ; Jones, Sean H. ; Tucker, H. Alan ; Giler, Margaret K. ; Muller, Dyllan C. ; Discher, Carson T. ; Russell, Katie C. ; Dobek, Georgina L. ; Sammarco, Mimi C. ; Bunnell, Bruce A. ; et al ( November 2019 , Biotechnology and Bioengineering)

   In vivo mesenchymal stem cell (MSC) survival is relevant to therapeutic applications requiring engraftment and potentially to nonengraftment applications as well. MSCs are a mixture of progenitors at different stages of cellular aging, but the contribution of this heterogeneity to the survival of MSC implants is unknown. Here, we employ a biomarker of cellular aging, the decoy TRAIL receptor CD264, to compare the survival kinetics of two cell populations in human bone marrow MSC (hBM‐MSC) cultures. Sorted CD264⁺hBM‐MSCs from two age‐matched donors have elevated β‐galactosidase activity, decreased differentiation potential and form in vitro colonies inefficiently relative to CD264⁻hBM‐MSCs. Counterintuitive to their aging phenotype, CD264⁺hBM‐MSCs exhibited comparable survival to matched CD264⁻hBM‐MSCs from the same culture during in vitro colony formation and in vivo when implanted ectopically in immunodeficient NIH III mice. In vitro and in vivo survival of these two cell populations were independent of colony‐forming efficiency. These findings have ramifications for the preparation of hBM‐MSC therapies given the prevalence of aging CD264⁺cells in hBM‐MSC cultures and the popularity of colony‐forming efficiency as a quality control metric in preclinical and clinical studies with MSCs.

    

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3. Tunneling nanotubes mediate the expression of senescence markers in mesenchymal stem/stromal cell spheroids

   https://doi.org/10.1002/stem.3056  

   Whitehead, Jacklyn ; Zhang, Jiali ; Harvestine, Jenna N. ; Kothambawala, Alefia ; Liu, Gang-yu ; Leach, J. Kent ( August 2019 , Stem Cells)

   The therapeutic potential of mesenchymal stem/stromal cells (MSCs) is limited by acquired senescence following prolonged culture expansion and high-passage numbers. However, the degree of cell senescence is dynamic, and cell-cell communication is critical to promote cell survival. MSC spheroids exhibit improved viability compared with monodispersed cells, and actin-rich tunneling nanotubes (TNTs) may mediate cell survival and other functions through the exchange of cytoplasmic components. Building upon our previous demonstration of TNTs bridging MSCs within these cell aggregates, we hypothesized that TNTs would influence the expression of senescence markers in MSC spheroids. We confirmed the existence of functional TNTs in MSC spheroids formed from low-passage, high-passage, and mixtures of low- and high-passage cells using scanning electron microscopy, confocal microscopy, and flow cytometry. The contribution of TNTs toward the expression of senescence markers was investigated by blocking TNT formation with cytochalasin D (CytoD), an inhibitor of actin polymerization. CytoD-treated spheroids exhibited decreases in cytosol transfer. Compared with spheroids formed solely of high-passage MSCs, the addition of low-passage MSCs reduced p16 expression, a known genetic marker of senescence. We observed a significant increase in p16 expression in high-passage cells when TNT formation was inhibited, establishing the importance of TNTs in MSC spheroids. These data confirm the restorative role of TNTs within MSC spheroids formed with low- and high-passage cells and represent an exciting approach to use higher-passage cells in cell-based therapies.

   Extracellular vesicles (EVs) derived from mesenchymal stem/stromal cells (MSCs) have recently been explored in clinical trials for treatment of diseases with complex pathophysiologies. However, production of MSC EVs is currently hampered by donor‐specific characteristics and limited ex vivo expansion capabilities before decreased potency, thus restricting their potential as a scalable and reproducible therapeutic. Induced pluripotent stem cells (iPSCs) represent a self‐renewing source for obtaining differentiated iPSC‐derived MSCs (iMSCs), circumventing both scalability and donor variability concerns for therapeutic EV production. Thus, it is initially sought to evaluate the therapeutic potential of iMSC EVs. Interestingly, while utilizing undifferentiated iPSC EVs as a control, it is found that their vascularization bioactivity is similar and their anti‐inflammatory bioactivity is superior to donor‐matched iMSC EVs in cell‐based assays. To supplement this initial in vitro bioactivity screen, a diabetic wound healing mouse model where both the pro‐vascularization and anti‐inflammatory activity of these EVs would be beneficial is employed. In this in vivo model, iPSC EVs more effectively mediate inflammation resolution within the wound bed. Combined with the lack of additional differentiation steps required for iMSC generation, these results support the use of undifferentiated iPSCs as a source for therapeutic EV production with respect to both scalability and efficacy.

    

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4. Mesenchymal Stem Cell Aging and Senescence Associated Extracellular Matrix Contributions to Breast Cancer Progression

   Ghosh, Deepraj ; Quach, Nhat ; Pena, Carolina ; Xuan, Botai ; Lee, Amy ; Dawson, Michelle ( February 2019 , Gordon Research Conference: Physical Science of Cancer)

   Age is a leading risk factor for developing breast cancer. This may be in part to the time required for acquiring sufficient cancer mutations; however, stromal cells that accumulate in tissues and undergo senescence eventually develop a senescence-associated secretory phenotype that alters the microenvironment to promote cancer. Our focus is on mesenchymal stem cells (MSCs) – stromal cells recruited to tumors due to their natural tropism for inflammatory tissues; MSCs have been shown to enhance the metastatic potential of tumor cells through direct interactions or paracrine signaling within the tumor. In the tumor, MSCs can differentiate into carcinoma-associated fibroblasts that play a central role in tumor growth and matrix remodeling. We recently investigated the molecular and mechanical differences in pre- and post- senescent MSCs and how their interactions with MDA-MB-231 breast cancer cells contribute to malignancy. Our data show post-senescent MSCs are larger and less motile, with more homogeneous mechanical properties than pre-senescent MSCs. In-depth omics analysis revealed differentially regulated genes and peptides including factors related to inflammatory cytokines, cell adhesion to the extracellular matrix, and cytoskeletal regulation. A 3D co-culture model was used to assess the effects of pre- and post- senescent MSCs on collagen matrix remodeling. Although post-senescent MSCs were far less motile than pre-senescent MSCs and less contractile with the matrix, they profoundly altered matrix protein deposition and crosslinking, which resulted in local matrix stiffening effects. Post-senescent MSCs also induced an invasive breast cancer cell phenotype, characterized by increased proliferation and invasion of breast cancer cells. This invasive breast cancer cell behavior was further amplified when MDA-MB-231 was co-cultured with a mixture of pre- and post- senescent MSCs; this result was attributed to matrix remodeling and soluble factor secretion effects of post-senescent MSCs, which enhanced the migration of pre-senescent MSCs allowing them to form tracks in the collagen network for cancer cells to follow. Finally, molecular inhibitors targeting actomyosin contractility and adhesion were used to alter MSC interactions with breast cancer cells. Actin depolymerizing agent and focal adhesion kinase inhibitor were most efficient and completely able to block the effects of post-senescent MSCs on MDA-MB-231 invasion in collagen gels. This comprehensive approach can be used to identify molecular pathways regulating heterotypic interactions of post-senescent MSCs with other cells in the tumor. Furthermore, the local matrix stiffening effect of post-senescent MSCs may play a critical role in breast cancer progression. 

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