Live-cell imaging reveals nutrient-dependent dynamics of ER-mitochondria contact formation via PDZD8

Cells adapt to changes in nutrient conditions by reorganizing organelle activities. Mitochondria-endoplasmic reticulum contact sites (MERCS) have been proposed to play an essential role in this adaptive response. However, the lack of suitable tools for detecting these contact sites in living cells has hindered the studies of the mechanisms regulating MERCS in response to nutritional changes. Here, we establish a novel cell line, MERCdRED, which expresses a ddFP-based fluorescent MERCS sensor. Using correlative light-electron microscopy, we show that the MERCdRED signal accurately labels the contact sites as defined by electron microscopy. Live imaging of MERCdRED cells revealed that large MERCS are more stable than smaller ones. Furthermore, in combination with knockout of the ER-mitochondria tethering protein PDZD8, we revealed that nutrient deprivation reduces MERCS area in a PDZD8-dependent manner. This new tool, together with our findings, will contribute to a better understanding of the molecular basis of cellular metabolism. Summary StatementWe established a novel cell line enabling live imaging of MERCS, which revealed their dynamic nature and demonstrated nutrient deprivation-induced MERCS reduction along with its underlying mechanisms.