Dynamics of stimulus-expression coupling as revealed by monitoring of prolactin promoter-driven reporter activity in individual, living mammotropes

Single-cell paradigms have greatly expanded our knowledge about stimulus-se cretion coupling, but the understanding of stimulus-gene expression couplin g has lagged behind for lack of a dynamic model sufficiently sensitive to p rovide single-cell resolution. In the present study we made continuous indi rect measurements within individual, living cells of expression dynamics bo th before and after treatment with a gene-activating secretagogue. This was accomplished by transfecting (via microinjection) individual, primary mamm otropes with a PRL promoter-driven luciferase reporter plasmid, and then qu antifying the rate of photonic emissions (reflective of endogenous gene act ivity). We found that individual cells exhibit spontaneous, random, short-t erm fluctuations of basal reporter activity and are extremely heterogeneous in terms of responses to a stimulatory agent (TRH). In addition, we found that responses are affected by several factors including the secretory stat us of the pituitary donor, the manner in which the stimulus is presented, a nd by the initial level of reporter activity, Moreover, the responsiveness of an individual cell can fluctuate dramatically over time. These results i nvite speculation that a given cell can "sense" its gene activation state a nd regulate its response accordingly to satisfy requirements for the corres ponding secretory product.