There are different ways to query cells. Some of these use standard light microscopy observations of dye uptake or morphology, size, adherence, or mobility.
Others rely on fluorescence microscopy to track labeled antibodies or reporter-driven GFP expression, or on specialized spectrophotometers to examine enzyme-driven color or light intensity changes in the culture medium. And still others make use of nonoptical properties such as heat flow or electrical impedance to detect activity.
Especially in drug discovery, most such assays have been geared toward a particular reaction—whether it is recognition of a cell surface marker, triggering endocytosis, or exciting or inhibiting a phosphorylation cascade. Scientists have been working with very specific assays for very specific targets for years, explained Magnus Jansson, Ph.D., CSO of SymCel Sverige. “I think that the trend is turning back now toward a screening for more of a specific phenotypic effect.” He said there was a huge interest in label-free assays among the participants of the recent SMi Conference on “Cell Based Assays” in London, where he discussed SymCel’s pending entry into the field.
The company has developed a calorimetry-based assay system that can monitor the total metabolic activity by measuring the heat flow in and out of a cell culture well in real time, down to the microwatt level. “It’s not necessary to know anything about your system with regard to the receptor, nor what kind of assay you’re going to run to monitor that activity,” he pointed out.
Say you have a G-protein coupled receptor (GPCR) of unknown function—with this assay you can determine whether it effects a stimulatory or inhibitory response, or induces necrosis under your experimental conditions, Dr. Jansson proposed as an example. “You really don’t have to be that advanced or that long into your research of that novel target—you can pinpoint the result of your chemical, or try to elucidate a pathway without previous knowledge about it.”
SymCel’s calPlate assay is independent of cell type and media conditions. Three-dimensional cultures, adherent or suspension cells, bacteria, yeast, and even small parasites such as malaria can be queried. Likewise, clear, colored, and turbid media can be used. Because the assays require no labels they are noninvasive and nondestructive to the cells, inexpensive, simple, and quick to set up and use.
The calPlate, which is currently in beta testing at several European universities, charts a continuous readout of heat exchange throughout the assay, generally about three to six hours. Because it is based on a sealed system that needs to equilibrate with a heat sink, it is not designed to acquire rapid measurements. And due to lack of gas exchange there is an upper limit on the length of an assay as well—at least in aerobic cultures. After being removed from the instrument, cultures can be manipulated and returned for another run, however.
The calPlate uses a standardized 48-well plate format—with 32 of the stainless steel wells available for the user, the remainder used by the system for thermodynamic internal controls—making it amenable to automation. While this allows it to offer a higher throughput than existing calorimetry equipment, Dr. Jansson sees the system as having a role more in secondary screening and validation than high-throughput screening (HTS).