Smaller Cardiac Organoids Separate Doxorubicin’s Functional Effects From Cell Death
A human stem-cell-derived cardiac culture seeded with 20,000 cells had less apoptosis than the larger comparison at seven days. In subsequent doxorubicin tests, its beating response changed at lower concentrations than its viability, reversing the ordering observed in two-dimensional cultures.
Choosing a Model Before Testing the Drug
A University of Texas at Austin study examined how initial cell number changes the condition of self-assembled cardiac organoids. The investigators then used the selected formulation to compare functional and viability responses to doxorubicin. The work connects model preparation with the interpretation of a drug-safety endpoint: a culture can continue beating even while its overall cellular health deteriorates.
The cells came from the human WTC11 induced pluripotent stem cell line carrying a fluorescent calcium reporter. Differentiation produced a mixed population: approximately 22.3% cardiomyocytes, 76.4% fibroblasts and 1.3% endothelial cells. The resulting organoids were therefore dominated by cardiomyocyte–fibroblast interactions, with little endothelial contribution and no established vascular network.
Two starting populations contained 20,000 or 80,000 cells per organoid. The team followed their size, cardiac and fibroblast-associated gene expression, calcium transients, apoptosis and mitochondrial staining. Parallel two-dimensional cultures supplied a comparator.
Persistent Beating Did Not Mean Equivalent Cellular Health
Both organoid groups compacted during culture. At seven days, mean effective diameters were approximately 421.5 μm for the 20,000-cell group and 590.7 μm for the 80,000-cell group. Size measurements involved many individual organoids, with separate sample counts for each time point and group.
The smaller formulation at seven days combined a more favorable cardiac-marker profile with lower fibrotic-marker expression. Its cleaved-caspase-3-positive fraction was approximately 0.075, versus 0.212 in the larger group, based on nine and ten organoids respectively. These values were presented as means, with SEM error bars. Much of the detected apoptosis occurred in non-cardiomyocytes rather than cardiomyocytes.
Calcium imaging nevertheless showed continuing beating in both groups. Overall beating rates and the main kinetic comparisons were not significantly different across the tested seeding-density and time conditions. Mitochondrial staining was higher in the smaller organoids, while within-group mitochondrial activity did not change significantly between days seven and 14. These findings show why beating alone did not capture the differences in model condition.
By day 14, apoptosis had increased in both groups, and fibroblast-associated changes became more evident. The authors selected the smaller formulation for the drug comparison, rather than treating the largest or longest-cultured organoid as inherently the most mature or healthiest model.
Function and Viability Gave Different Doxorubicin Thresholds
The doxorubicin comparison exposed organoids and two-dimensional cultures to 0.1–10 μM for 48 hours. Calcium imaging tracked changes in beating rate relative to each model’s baseline, while live/dead staining supplied a separate viability ratio. Figure 5 reports 15 two-dimensional measurements and 13 organoids per concentration across four biological replicates.
Organoids were less susceptible to overt cell loss in this assay. The viability-based half-maximal inhibitory concentration was 3.61 μM, compared with 0.69 μM in two-dimensional cultures. The 95% confidence intervals were 2.31–4.39 and 0.58–0.81 μM, respectively.
The functional comparison ran in the opposite direction. The concentration associated with a half-maximal beating-rate change was 0.84 μM in organoids, with a 95% confidence interval of 0.47–1.40 μM. In the two-dimensional cultures it was 2.41 μM, with a wider interval of 0.50–5.80 μM.
The investigators divided the viability-based concentration by the functional concentration to calculate a selectivity index. This yielded 4.30 for organoids and 0.29 for two-dimensional cultures. In this experiment, the three-dimensional model therefore displayed functional disturbance before the concentration required for marked viability loss, whereas the surviving cells in a damaged monolayer could still contribute a beating signal.
The comparison involved one stem-cell origin and one drug. The authors discuss adding vascular and other cardiac cell populations, because diffusion constraints and missing cell types influence the existing cultures. Their suggestions for using larger, more apoptotic organoids as disease-like models remain proposed applications; those disease-specific drug responses were not tested here.