Spatially coordinated dynamic gene transcription in living pituitary tissue

Abstract

Transcription at individual genes in single cells is often pulsatile and stochastic. A key question emerges regarding how this behaviour contributes to tissue phenotype, but it has been a challenge to quantitatively analyse this in living cells over time, as opposed to studying snap-shots of gene expression state. We have used imaging of reporter gene expression to track transcription in living pituitary tissue. We integrated live-cell imaging data with statistical modelling for quantitative real-time estimation of the timing of switching between transcriptional states across a whole tissue. Multiple levels of transcription rate were identified, indicating that gene expression is not a simple binary ‘on-off’ process. Immature tissue displayed shorter durations of high-expressing states than the adult. In adult pituitary tissue, direct cell contacts involving gap junctions allowed local spatial coordination of prolactin gene expression. Our findings identify how heterogeneous transcriptional dynamics of single cells may contribute to overall tissue behaviour.

DOI: http://dx.doi.org/10.7554/eLife.08494.001

eLife digest

Although humans have thousands of genes, only a fraction of these are expressed in any given cell. Each cell type expresses only the genes that are relevant to its particular job or that are necessary for general cell maintenance. Even these genes are not expressed all the time: most cells express genes in bursts, and the cells that make up a tissue produce these bursts at different times. This makes it easier for the tissue to respond to new conditions.

The pituitary gland, found at the base of the brain, is often studied to investigate changes in gene expression. The pituitary gland is found in all animals that have a backbone, and it makes and releases many different hormones. For example, one type of pituitary cell expresses the gene that encodes a hormone called prolactin. This hormone has a range of roles, including stimulating milk production and regulating fertility in mammals. The coordinated production of prolactin by pituitary cells is important for reproduction, but it is not clear how (or whether) individual prolactin-producing cells in the gland communicate to coordinate bursting patterns of expression of the prolactin gene.

Featherstone et al. marked the prolactin-encoding gene in the pituitary cells of rats with a gene that encodes a fluorescent protein; this enabled the gene’s activity to be observed in thin slices of living tissue using a microscope. Mathematical models were then used to analyse the recorded expression patterns.

The results showed that in a single cell, the bursts of expression of the prolactin gene are randomly timed. This means that although the expression activity of an individual cell is unpredictable, the overall activity of a group of cells can be precisely determined. The model also showed that cells coordinate when they express the prolactin gene to a greater extent with their near neighbours than with cells that are further away in the tissue. Featherstone et al. found that this coordination depends on structures (called gap junctions) that connect the cells and allow signalling between them, and this tissue organisation is established during early development.

The mechanisms underlying the timing of the bursts remain to be discovered. The timing for the prolactin gene seems to be dominated by a minimum delay that must occur before the next burst can be reactivated. Future challenges also include determining whether coordinated gene expression occurs in other tissues and whether this coordination is disrupted in disease.

DOI: http://dx.doi.org/10.7554/eLife.08494.002