Mammalian genes display pulsatile transcription dynamics with bursts of expression occurring with variable duration and frequency. Using the human prolactin gene as a model of tissue specific gene regulation, we have characterised the transcription dynamics of this gene in cell lines, primary cells and pituitary tissue slices. Our data indicate that the tissue environment may have an important influence on cellular transcription activity. Cultures of cells from enzymatically dispersed tissue show greater transcriptional pulsatility than cells maintained in their native tissue environment. Temporal transcription patterns of the prolactin gene also change during pituitary development with transient pulses of expression replaced by more stable expression patterns as the pituitary develops.
We have performed spatio-temporal analyses of prolactin gene transcription dynamics in pituitary tissue to assess the nature of tissue influence on gene activity. In adult pituitary tissue, cells display a co-ordinated increase in expression over the first 24 h of culture, potentially due to the removal of the pituitary from the inhibitory influence of hypothalamic dopamine. Patterns of transcription activity from individual cells showed increased correlation over short, one to two cell distances, suggesting that local cellular communication affects transcription activity. Consistent with a cell signalling influence on transcription, limited trypsin digestion, which maintains cells in a tissue structure whilst facilitating the degradation of extracellular proteins, abolished the local increase in correlated activity. In developing pituitary tissue, where lactotroph cell density is reduced, correlation of transcription activity was independent of cellular distance, suggesting that cell communication, if established, is incapable of influencing transcription activity. We are currently investigating whether gap junction mediated transfer of small signalling molecules directly between cells mediates the short range co-ordination of cellular activity that we have detected.