Cells' G protein-coupled receptor signaling system is capable to pass more than 2 bits of information per one interaction with external stimuli. It means cells receptors could recognize different concentrations - at least four - of incoming signal, rather than transmit just "yes-or-no" information regarding the received signal as it was estimated earlier.
These findings, made by Vladimir L. Katanaev of Far Eastern Federal University (FEFU, Russia), his research team of the University of Lausanne, and his collaborators at the Swiss Institute of Bioinformatics, are crucial for biology. Considering the fact that more than half of the existing drugs target G protein-coupled receptors (GPCRs), understanding of the principles of GPCR signaling could pave the new ways drug discovery, including development of novel anti-cancer agents. The article devoted to these findings is published in Nature Communications.
During their experiments, the researchers have also revealed that every single cell responds individually to the external stimuli, such that some cells detect well relatively low levels of the stimulus, but are unable to discriminate among different high concentrations, while the others are insensitive to the low concentrations, yet reliably read different high levels of the stimulation. This conclusion sheds light on such well-established biological processes as tissue origin and development (morphogenesis) when cells transform in different ways depending on the concentration of a signal received.
Using the information theory (a field of mathematics and engineering), computer modelling, and thanks to the establishment of a novel experimental approach, the researchers have found that cellular signaling systems can be compared to the information transmission "highways" - channels with more than 2 bits of the capacity of reliable information transmission. These "highways" could further have branches with lower capacity. These "regional routes" are the matter for future research.
The research flow
To quantify the channel capacity of GPCR signaling, the researchers selected HEK293 cells, which are among the most popular servants in cell biology experimentation. The scientists have initially conducted a series of experiments during which they, firstly, found that different cells respond with different strengths to same stimulus concentration. Secondly, it has become evident that these different cellular responses are reproducible to >20 repeated stimulations, indicating low noise in the cellular response. Despite this reproducibility, the analyses have also revealed that the response strength shows a tendency to decrease with the every new stimulation, indicating cellular adaptation to repeated stimuli.
These results laid the ground for designing the experimental setup by means of which the scientists made the GPCR channel capacity estimation.
The researchers have preloaded the cells with the fluorescent dye (Fura-2 AM) which glows the more strongly the more charged calcium particles are released in the cytoplasm. Application of the GPCR ligand acetylcholine to the cells caused an increase in cells' calcium concentration and therefore the intensity of the dye. The researchers have applied their experimental setup to hundreds of cells in dozens of independent experiments. In each experiment, 10-30 cells have been stimulated with an escalating dose of acetylcholine, each concentration of the ligand being applied multiple times; each ligand application is followed by a washout phase using the microfluidics device. Recording individual cell responses in this manner permits quantification of the channel capacity of the GPCR signaling system starting from the ligand interaction with the receptor and ending at opening of the intracellular calcium stores.
Using this approach, the researchers have discovered that cellular GPCR signaling systems possess the channel capacity strongly exceeding the previous estimations, revealing the average channel capacity is >2 bits. Importantly, this is still an underestimation, and future experiments will measure how high the real channel capacity of GPCR signaling exceeds the 2 bits.
The researchers further note that in the future, it may be insightful to move beyond the initial concept of channel capacity and consider the amount of information transmitted by adaptive cellular pathways. These findings provide insights into the basic principles of GPCR signaling and will inspire further work on the molecular mechanisms of the intracellular signal transduction, uncovering further mysteries of biology of the healthy and pathological states of the cell.