Endocrine Abstracts

Technical advances frequently lead to surprises. Studies of gene expression have used reporter genes since the 1980s to assess how mammalian gene promoters are activated or repressed. Two of the most widely used reporters are firefly luciferase and green fluorescent protein, and both have the advantage that their expression can be seen, and their bioluminescence or fluorescence readily measured. In studying how hormone genes were controlled in the pituitary, we used quantitative microscopic imaging of cell lines transfected with either luciferase or destabilised EGFP linked to the human prolactin gene locus. We expected to find that gene expression varied in response to standard stimuli, but to our surprise discovered that gene expression was dramatically pulsatile, fluctuating from hour to hour. We used mathematical modelling to evaluate the pulses initially as if they were binary on-off events, and estimated the length of active and inactive periods. In order to study normal cells we created transgenic rats expressing prolactin-reporter genes in the anterior pituitary. This allowed us to look at the patterns of pulsing gene expression in normal pituitary cells in the context of intact tissue. We found that the characteristic timing of transcriptional pulses were not circadian, and changed during development through fetal, neonatal and adult life. It also became clear that gap junctional communication in a lactotroph cell network was necessary for coordination in the timing of transcriptional pulses between nearby cells. The discovery that gene transcription was pulsatile was unexpected, and means that the behaviour of living cells in real time is more complex than we had imagined from earlier biochemical investigations using cell extracts. Biological timing of many cellular processes, including gene transcription, is a vital aspect of our understanding different physiological and pathological states, and likely to underlie the resilience and adaptation of the endocrine system.