A simple electronic volume cell sorter for clonogenicity assays

Quantitative criteria to benchmark new and existing bio-inks for cell compatibility

Recent advancements in 3D bioprinting have led to the fabrication of more complex, more precise, and larger printed tissue constructs. As the field continues to advance, it is critical to develop quantitative benchmarks to compare different bio-inks for key cell-biomaterial interactions, including (1) cell sedimentation within the ink cartridge, (2) cell viability during extrusion, and (3) cell viability after ink curing. Here we develop three simple protocols for quantitative analysis of bio-ink performance. These methods are used to benchmark the performance of two commonly used bio-inks, poly(ethylene glycol) diacrylate (PEGDA) and gelatin methacrylate (GelMA), against three formulations of a novel bio-ink, Recombinant-protein Alginate Platform for Injectable Dual-crosslinked ink (RAPID ink). RAPID inks undergo peptide-self-assembly to form weak, shear-thinning gels in the ink cartridge and undergo electrostatic crosslinking with divalent cations during curing. In the one hour cell sedimentation assay, GelMA, the RAPID inks, and PEGDA with xanthan gum prevented appreciable cell sedimentation, while PEGDA alone or PEGDA with alginate experienced significant cell settling. To quantify cell viability during printing, 3T3 fibroblasts were printed at a constant flow rate of 75 μl min-1 and immediately tested for cell membrane integrity. Less than 10% of cells were damaged using the PEGDA and GelMA bio-inks, while less than 4% of cells were damaged using the RAPID inks. Finally, to evaluate cell viability after curing, cells were exposed to ink-specific curing conditions for five minutes and tested for membrane integrity. After exposure to light with photoinitiator at ambient conditions, over 50% of cells near the edges of printed PEGDA and GelMA droplets were damaged. In contrast, fewer than 20% of cells found near the edges of RAPID inks were damaged after a 5 min exposure to curing in a 10 mM CaCl2 solution. As new bio-inks continue to be developed, these protocols offer a convenient means to quantitatively benchmark their performance against existing inks.

Characterization of the laser-based release of micropallets from arrays

The micropallet array system uses a pulsed laser to release pallets tens of microns to hundreds of micrometers in size from a larger array, enabling selective isolation of single cells adherent to the pallets. We characterize the laser-based release of pallets with respect to pallet array and laser parameters. The threshold laser energy required for pallet release increases linearly with the area of the pallet in contact with the underlying glass substrate. The spacing of the pallets within an array as well as the thickness or height of the pallet does not impact the energy required to release a pallet. Delivery of multiple laser pulses decreases the energy/pulse required for pallet release when the pallets were 100 microm or greater on a side. In addition to the square pallets, complex structures such as cantilevers and spirals could be released without damage using the pulsed laser. Identification of the pallet-array variables influencing the energy required for pallet release as well as strategies to minimize this energy will prove critical in optimizing the release of pallets with cells on the arrays.

Multiparameter analysis of progeny of individual cells by laser scanning cytometry

BACKGROUND

Effectiveness of antitumor drugs to suppress unrestricted proliferation of cancer cells is commonly measured by cell clonogenicity assays. Assays of clonogenicity are also used in studies of stem/progenitor cells and in analysis of carcinogenic transformation. The conventional assays are limited to providing information about frequency of colonies (cloning efficiency) and do not reveal the qualitative (phenotype) attributes of individual colonies that may yield clues on mechanisms by which cell proliferation was affected by the studied agent.

METHODS

Laser scanning cytometry (LSC) was adapted to identify and characterize size and phenotype of colonies of MCF-7 cells growing in microscope slide chambers, untreated and treated with the cytotoxic ribonuclease, onconase (Onc). Individual colonies were located and data representing each colony were segmented based on >650-nm fluorescence excited by a He-Ne laser of the cells whose protein was stained with BODIPY 630/650-X. The DNA of the cells was stained with propidium iodide (red fluorescence) whereas specific proteins (estrogen receptor [ER] or tumor suppressor p53) were detected immunocytochemically (green fluorescence), each excited by an Ar ion laser.

RESULTS

A plethora of attributes of individual colonies were measured, such as (a) morphometric features (area, circumference, area/circumference ratio, DNA or protein content per area ratio), (b) number of cells (nuclei), (c) DNA content, (d) protein content and protein/DNA ratio, and (e) expression of ER or p53 per colony, per total protein, per nucleus or per DNA, within a colony. Also cell cycle distribution within individual colonies and heterogeneity of colonies with respect to all the measured features could be assessed. The colonies growing in the presence of Onc had many of the above attributes different than the colonies from the untreated cultures.

CONCLUSIONS

Analysis of the features of cell colonies by LSC provides a wealth of information about the progeny of individual cells. Changes in colony size and phenotype, reflecting altered cell shape, cell size, colony protein/DNA ratio, and expression of individual proteins, may reveal mechanisms by which drugs suppress the proliferative capacity of the cells. This may include inducing growth imbalance and differentiation and modulating expression of the genes that may be associated with cell cycle, apoptosis, or differentiation in a progeny of individual cells. Extensions of LSC may make it applicable for automatic analysis of cloning efficiency and multiparameter analysis of cell colonies in soft agar. Such analyses may be useful in studies of the mechanisms and effectiveness of antitumor drugs, in the field of carcinogenesis, and for analyzing primary cultures and assessing tumor prognosis and drug sensitivity. The assay can also be adapted to analysis of microbial colonies.

Cell survival measurements at low doses using an automated image cytometry device

The statistical precision of cell survival data measured at low doses can be greatly improved if the number of cells assayed at each dose point is known exactly. This paper describes an image cytometry technique that has been developed for this purpose. Treated cells are plated into tissue culture flasks, and after attachment their locations are determined with 98% accuracy by automated scanning procedures. These locations are revisited after an appropriate incubation period to assay for colony formation. Colonies may be scored either manually or using image data collected by the device. Additional information, such as heterogeneity of colony size, can also be obtained.

Use of xantham gum to suspend large particles during flow cytometric analysis and sorting

In this report we describe the use of xantham gum as a biologically inert material for increasing the viscosity of a suspension of cells or particles during flow cytometric analysis and sorting. A 0.1% concentration of xantham gum in culture medium or saline will increase the viscosity approximately 9-fold. For suspensions of multicellular spheroids 100-400 microns in diameter the measured sedimentation velocity was approximately 9 times slower than that in medium alone. Thus, spheroids of 100 microns diameter remain in suspension in 0.1% xantham gum for 66 min, compared to 7.5 min in culture medium. This allows extended periods of sorting without stirring or agitating the sample suspension. The xantham gum solution is noncytotoxic for periods up to 8 h as measured by clonogenicity assay. Xantham gum has the added advantage that the viscosity is significantly reduced when the solution is subjected to shear stress, such as during flow. This technique should be applicable to extended sorting of suspensions of spheroids, plant cells, and other large particles, as well as for analyzing and sorting single cells for extended periods.