Highlights d A mass departure of ER-resident proteins occurs in response to ER calcium depletion d KDELR2 and KDELR3, but not KDELR1, are unfolded protein response genes d KDELR2 and KDELR3 upregulation by ER calcium loss counteracts ER protein loss d Stabilizing calcium and increasing KDEL receptor expression have therapeutic potential

Depletion of ER calcium can lead to cell death and is implicated in numerous diseases. Secreted ER calcium-monitoring proteins (SERCaMPs) are secreted in response to ER calcium depletion. SERCaMPs identified ER calcium depletion in primary neurons exposed to glutamate, hyperthermia, and coxibs and in rat liver after a single exposure to thapsigargin.

Background & Aims Disruption to endoplasmic reticulum (ER) calcium homeostasis has been implicated in obesity, however, the ability to longitudinally monitor ER calcium fluctuations has been challenging with prior methodologies. We recently described the development of a Gaussia luciferase (GLuc)- based reporter protein responsive to ER calcium depletion (GLuc-SERCaMP) and investigated the effect of a high fat diet on ER calcium homeostasis. Methods A GLuc-based reporter cell line was treated with palmitate, a free fatty acid (FFA). Rats intrahepatically injected with GLuc-SERCaMP reporter were fed a cafeteria diet or high fat diet. The liver and plasma were examined for established markers of steatosis and compared to plasma levels of SERCaMP activity. Results Palmitate induced GLuc-SERCaMP release in vitro, indicating ER calcium depletion. Consumption of a cafeteria diet or high fat pellets correlated with alterations to hepatic ER calcium homeostasis in rats, as evidenced by increased GLuc-SERCaMP release. Access to ad lib high fat pellets also led to a corresponding decrease in microsomal calcium ATPase activity and increase in markers of hepatic steatosis. In addition to GLuc-SERCaMP, we have also identified endogenous proteins (endogenous SERCaMPs) with a similar response to ER calcium depletion. We demonstrated the release of an endogenous SERCaMP, thought to be a liver esterase, during access to a high fat diet. Attenuation of both GLuc-SERCaMP and endogenous SERCaMP was observed during dantrolene administration. Conclusions Here we describe the use of a reporter for in vitro and in vivo models of high fat diet. Our results support that dietary fat intake correlates with a decrease in ER calcium levels in the liver and suggest a high fat diet alters the ER proteome.