3D Imaging of Optically Cleared Spheroids in Corning® Spheroid Microplate
The use of three-dimensional (3D) cell cultures for in vitro drug discovery assays has increased dramatically in recent years because 3D cell culture models more accurately mimic the in vivo environment compared to traditional two-dimensional (2D) monolayer cultures. However, current imaging-based analysis of these 3D cultures relies upon techniques originally developed for 2D cell culture, and as such, has significant limitations. Specifically, the light scattering inherent with thick microscopy specimens prevents imaging the entirety of a 3D spheroid, which are typically >100 µm in diameter. This technical limitation introduces a sampling bias in imaging analysis in which only the exterior cells of a spheroid can be imaged. To accurately survey the cellular environment and response of the spheroid, the field needs new techniques to overcome the sampling bias. Accordingly, we sought to remedy the light scattering caused by 3D spheroid opacity by employing an optical clearing agent designed specifically for spheroids, Visikol® HISTO-M™. Here, we demonstrate the utility of Visikol HISTO-M tissue clearing with HepG2 spheroids generated in Corning 96-well spheroid microplates in a high-content screening approach.
Materials and Methods
Generation of Spheroids in Corning Spheroid Microplates
HepG2 cells (Sigma Cat. No. 85011430) were routinely cultured in DMEM supplemented with 10% FBS. To form spheroids, HepG2 (1 x 103 cells/well) were seeded into wells of 96-well spheroid microplates (Corning Cat. No. 4515) and incubated in a humified incubator in 5% CO2 at 37°C for 2 days.
Treatment of Spheroids with Antiproliferative Compound
A 1 mM stock solution of the antiproliferative compound paclitaxel (Alfa Aesar Cat. No. J62734MC) was prepared in DMSO. Working solutions (100X) were prepared as 10-fold serial dilutions of the DMSO stock and then diluted to final assay concentration (1 µM, 500 nM, 100 nM, 10 nM, 1 nM) in growth media. Spheroids were treated with paclitaxel on day 0 and again on day 3.
Fixation and Immunolabeling
On day 5, spheroids were fixed in situ using 10% neutral buffered formalin (NBF, Thermo Fisher Cat. No. SF100-20), followed by washing in phosphate-buffered saline (PBS) to remove fixative. Spheroids were permeabilized with methanol, followed by 20% DMSO/methanol to improve penetration of antibodies and stains and then blocked in 10% donkey serum (Jackson Immuno Research Labs Cat. No. 017000121). Spheroids were incubated in 1:150 rabbit anti-Ki67 antibody (Thermo Fisher Cat. No. RB1510P) to label proliferating cells followed by Alexa Fluor® 488 conjugated Anti Rabbit IgG secondary (Invitrogen/Thermo Fisher Cat. No. A32731). Nuclei were stained with DAPI (Thermo Fisher Cat. No. D1306).
Clearing and High Throughput Imaging of Spheroids
Spheroids were cleared with Visikol HISTO-M. Briefly, this is accomplished through dehydration of the spheroids by exchange with methanol (200 µL/well for 15 min.) and subsequent exchange and transfer to the one-step immersion clearing formula, HISTO-M, for immersion clearing (200 µL/well for 30 min.). Spheroids were imaged in HISTO-M in the spheroid microplate with an IN Cell Analyzer 6000 (GE Healthcare). Z-stacks (~150 µm) were collected at 10X magnification in 5 µm steps. Images were processed using ImageJ, and DAPI+ and Ki67+ cells were counted using CellProfiler.
Results and Discussion
Visikol HISTO-M Enables Visualization of Spheroid Interior
Figure 1A (top rows) feature images representative of the obstacles encountered when imaging 3D cell cultures. The interior of the spheroid appears dark, as light scattering drastically reduces fluorescence signal due to the opacity of the spheroids. Signal is only detected from the outermost cells of the spheroid. In contrast, light scattering was greatly reduced by using Visikol HISTO-M clearing agent, as evidenced by more visible nuclear fluorescence in the cleared spheroids (Figure 1A, bottom rows). This allows for comprehensive profiling of the interior of the spheroids.
Visikol HISTO-M Increases Detectable Cells in Interior of 3D Structure
CellProfiler was used for automated cell counting of the confocal image stacks. The data presented in Figure 1B quantifies the problems inherent with imaging non-cleared spheroids. As the image stack progresses through the non-cleared spheroid, fewer and fewer cells are detected in each plane, until only the periphhery is detectable. Light scattering caused by the opacity of non- 2 cleared spheroids limits imaging to approximately 20 to 50 µm, even with confocal microscopy. By clearing the spheroids, cells are detectable across the entire image plane, deep into the spheroid. On average, 3-fold more cells were detected on each plane of the cleared spheroid than the non-cleared spheroid (Figure 1B). The effect is more dramatic deeper in the spheroid; at 120 µm, 7-fold more cells were detectable.