Distribution and dynamics of endoplasmic reticulum-plasma membrane junctions in pancreatic acinar and pancreatic cancer cells

Endoplasmic reticulum (ER)-plasma membrane (PM) junctions are found in many different cell types. They function as important calcium (Ca2+) signalling structures, as sites of store-operated calcium entry (SOCE) and play many other crucial roles. Their distribution in polarised migrating pancreatic cancer cells, and in the polarised pancreatic acinar cells, was the subject of this investigation. In migrating cells, the specific spatial pattern of Ca2+ signalling has been shown to be of paramount importance, but is produced by an unknown mechanism that I aimed to investigate. In acinar cells, Ca2+ influx (via the SOCE complex) occurs basolaterally, whilst Ca2+ signals are generated apically, physically separating the two processes. The presence of the Ca2+ influx channel normally present in SOCE complexes (Orai1) in the apical region therefore, is a conundrum this investigation set out to untangle. I found that ER-PM junctions and SOCE complexes localise to the leading edge of migrating cells and are ideally positioned for Ca2+-dependent regulation of actin dynamics and focal adhesion turnover – processes crucial to migration. The appearance of junctions at the leading edge was saltatory, suggesting that it develops as a result of the formation of new junctions, not from a relocation of junctions from a more interior position. I also saw long distance translocation and dissolution of junctions in the retracting tails of migrating cells, in a manner similar to the dynamics of focal adhesions. In pancreatic acinar cells, PM-localised Orai1 was found on basal and lateral membranes, whilst apical Orai1 was found to be absent from the PM and instead in a structure just beneath it, co-localising with ER-located Ca2+-release channels