Mesenchymal stem cell extracellular vesicle vascularization bioactivity and production yield are responsive to cell culture substrate stiffness

Abstract

Mesenchymal stem cell-derived extracellular vesicles (MSC EVs) are an attractive therapeutic option for regenerative medicine applications due to their inherently pro-angiogenic and anti-inflammatory properties. However, reproducible and cost-effective production of highly potent therapeutic MSC EVs is challenging, limiting their translational potential. Here, we investigated whether the well-characterized responsiveness of MSCs to their mechanical environment-specifically, substrate stiffness-could be exploited to generate EVs with increased therapeutic bioactivity without the need for biochemical priming or genetic manipulation. Using polydimethylsiloxane and bone marrow-derived MSCs (BM-MSCs), we show that decreasing the stiffness of MSC substrates to as low as 3 kPa significantly improves the pro-angiogenic bioactivity of EVs as measured by tube formation and gap closure assays. We also demonstrate that lower substrate stiffness improves EV production and overall yield, important for clinical translation. Furthermore, we establish the mechanoresponsiveness of induced pluripotent stem cell-derived MSC (iMSC) EVs and their comparability to BM-MSC EVs, again using tube formation and gap closure assays. With this data, we confirm iMSCs' feasibility as an alternative, renewable cell source for EV production with reduced donor variability. Overall, these results suggest that utilizing substrate stiffness is a promising, simple, and a potentially scalable approach that does not require exogenous cargo or extraneous reagents to generate highly potent pro-angiogenic MSC EVs.

Keywords: cell‐derived therapy; exosome; mesenchymal stromal cell.

FIGURE 1

Substrate and BM‐MSC EV characterization. (a) Elastic moduli of substrates made with varying ratios of Sylgard 184 base to crosslinker reagents. All values expressed as mean ± SD (n = 3). (b) Absorbance values indicating cell viability as determined by CCK8 assay over 5 days. All values expressed as mean ± SD (n = 3). (c) Size distribution from nanoparticle tracking analysis of EVs isolated from BM‐MSCs seeded on Sylgard 184 PDMS substrates with differing base to crosslinker reagent ratios (n = 3). (d) Representative Western blot of BM‐MSC EVs from each of the Sylgard 184 PDMS substrates and the corresponding cell lysates for EV‐positive markers ALIX, TSG101, and CD63 and cellular markers Calnexin and GAPDH (15 μg/lane). (e) Representative TEM images of BM‐MSC EVs from the softest Sylgard 184 PDMS substrates and collagen‐coated flasks. Statistical significance was determined by ANOVA; **p < 0.01, ***p < 0.001, and ****p < 0.0001.

FIGURE 2

Substrate stiffness influences BM‐MSC EV production and bioactivity. (a) EV production as quantified by EVs per cell from BM‐MSCs seeded on Sylgard 184 PDMS substrates with different base‐to‐crosslinker ratios. EVs used for this data were from 1 day of collection and isolated and counted separately from the conditioned media from the other 2 days. After media collection, cells were trypsinized and counted (n = 3). (b) After a scratch was induced, HUVECs were treated with BM‐MSC EVs from the different substrates or growth or basal media, and percent gap closure after 20 h was evaluated via microscopy (n = 3). (c) HUVECs were resuspended in EV treatments or growth or basal endothelial media, seeded in Matrigel‐coated wells, and tube formation after 3–6 h was quantified by the number of loops that had formed (n = 3). All values expressed as mean ± SD. All data are representative of at least three independent experiments (n = 3). Statistical significance was determined by ANOVA; *p < 0.05, **p < 0.01, ***p < 0.001, and ****p < 0.0001.

FIGURE 3

Softer 184:527 PDMS substrates improve the angiogenic bioactivity of BM‐MSC EVs. (a) EV production quantified as EV per cell from BM‐MSCs seeded on each substrate made with different ratios of Sylgard 184 and Sylgard 527 (n = 2). EVs used for this data were from 1 day of collection and isolated and counted separately from the conditioned media from the other 2 days. After media collection, cells were trypsinized and counted. (b) After a scratch was induced, HUVECs were treated with BM‐MSC EVs from the different substrates or growth or basal media, and percent gap closure after 20 h was evaluated via microscopy (n = 3). (c) HUVECs were resuspended in the different EV treatments or growth or basal endothelial basal media, and tube formation after 3–6 h was quantified by the number of loops that had formed (n = 3). All values expressed as mean ± SD. Statistical significance was determined by ANOVA; *p < 0.05, **p < 0.01, and ****p < 0.0001.

FIGURE 4

(a) EV production as quantified by EV per cell by EVs from iMSCs on different PDMS substrates. (b) EV size and concentration distribution from iMSCs cultured on different PDMS substrates as determined by nanoparticle tracking analysis. (c) iMSC proliferation/viability on PDMS substrates as measured by cell counting over 4 days. (d) Representative TEM images of F + C EVs and 527 EVs confirming morphology. (e) Western blot of EV markers CD63, ALIX, and TSG101, and EV‐negative marker, calnexin, on EVs from each PDMS substrate (12 μg/lane). (f) Western blot of MSC markers CD73, CD105, and CD90 and negative marker CD45 on iMSC lysate from each PDMS substrate. THP1 cell lysate was used as a positive control for CD45. (5 μg/lane). All values expressed as mean ± SD. Statistical significance was determined by ANOVA; **p < 0.01.

FIGURE 5

Substrate stiffness affects the pro‐angiogenic effect of iMSC EVs comparably to BM‐MSC EVs. (a) EVs isolated from iMSCs on different 184:527 PDMS substrates were used to treat HUVECs after a scratch had been induced, and percent gap closure after 20 h was evaluated via microscopy. (b) HUVECs were resuspended with the same EV groups and seeded, and tube formation after 3–6 h was quantified by the number of loops that had formed. All values expressed as mean ± SD. All data are representative of at least three independent experiments (n = 3). Statistical significance was determined by ANOVA; *p < 0.05, **p < 0.01, ***p < 0.001, and ****p < 0.0001.

FIGURE 6

HUVECs treated with 5E9 EVs/mL of soft substrate‐generated iMSC EVs result in the upregulation of certain angiogenesis‐related mRNAs. (a) mRNA array probing for expression of angiogenesis‐related genes in flask‐ and soft substrate (527 PDMS)‐generated EV‐treated HUVECs normalized to PBS‐treated HUVECs (n = 1). (b)–(e) Confirmation of upregulation of genes (ICAM1, HMOX1, PTGS2, and CCL2) selected from the initial array by independent qPCR experiments (n = 3). Statistical significance was determined by t‐test; *p < 0.05, **p < 0.01, and ***p < 0.001.

FIGURE 7

MiRNA qPCR array data of differentially expressed MSC EV‐associated miRNAs in flask vs. soft substrate‐generated EVs. (a) Fold change of miRNAs within EVs from iMSCs seeded on 527 PDMS normalized to the miRNA levels within EVs from iMSCs seeded on collagen‐coated flasks. (b) The same data represented as the log2 of the fold change, again normalized to the miRNA levels within EVs from iMSCs seeded on collagen‐coated flasks. All data are representative of at least three independent experiments (n = 3).