Endoplasmic-reticulum calcium depletion and disease

Abstract

The endoplasmic reticulum (ER) as an intracellular Ca(2+) store not only sets up cytosolic Ca(2+) signals, but, among other functions, also assembles and folds newly synthesized proteins. Alterations in ER homeostasis, including severe Ca(2+) depletion, are an upstream event in the pathophysiology of many diseases. On the one hand, insufficient release of activator Ca(2+) may no longer sustain essential cell functions. On the other hand, loss of luminal Ca(2+) causes ER stress and activates an unfolded protein response, which, depending on the duration and severity of the stress, can reestablish normal ER function or lead to cell death. We will review these various diseases by mainly focusing on the mechanisms that cause ER Ca(2+) depletion.

Figure 1.

Normal and abnormal [Ca²⁺]_ER. A tight coordination between ER Ca²⁺-release and -refilling mechanisms enables proper Ca²⁺ signaling in response to physiological stimuli. Under these conditions, Ca²⁺ released from the ER stimulates mitochondrial activity and bioenergetics, leading to more ATP production. The initial decline in [Ca²⁺]_ER activates STIM, allowing for store-operated Ca²⁺ influx. Ca²⁺ is recycled via the SERCA pumps. Normal [Ca²⁺]_ER is restored and ER-related processes continue. In contrast, in pathological conditions, stress responses will occur and affect the Ca²⁺-signaling toolbox in various ways. Impaired mitochondrial activity, store-operated Ca²⁺ influx, or SERCA activity may all cause failure in restoring normal [Ca²⁺]_ER in response to ER Ca²⁺-signaling processes, and lead to a decreased [Ca²⁺]_ER. A chronic decrease in [Ca²⁺]_ER may also be because of an imbalance between the Ca²⁺-on and -off mechanisms as a result of increased IP₃R or RyR activity, decreased Ca²⁺ buffering, or increased ER Ca²⁺ leak.

Figure 2.

The UPR. At a normal [Ca²⁺]_ER the ER-stress sensors are scaffolded and inactivated by GRP78/BiP. Protein trafficking and quality-control mechanisms work normally. Polypeptides are translocated through Sec61 and glycosylated. This transport is facilitated by the molecular chaperone GRP78/BiP. Glucosidases then prepare the glycoprotein for binding to the ER lectins, calreticulin, and calnexin, whereas oxidoreductases catalyze disulfide-bond formation. ER-resident chaperones facilitate the proper folding of the nascent protein and prevent its aggregation. Further deglucosidation releases the ER lectins and once the protein is correctly folded and processed, the protein leaves the ER via the coat protein (COPII)-coated vesicles to the secretory pathway. Misfolded proteins, in contrast, associate with various chaperones, including GRP78/BiP, and are removed from the ER through ERAD. In contrast, when the [Ca²⁺]_ER is chronically decreased, the function of chaperones becomes disturbed and unfolded proteins accumulate and act as a sponge for luminal GRP78/BiP. As a consequence, ER-stress sensors are devoid of GRP78/BiP and become activated, yielding early adaptive responses promoting survival (indicated in green) or late responses promoting apoptosis under conditions of severe or on-going ER stress (indicated in red). Ire1 undergoes dimerization and activation of its kinase and endoribonuclease activity, thereby splicing XBP1 mRNA and yielding a potent transcriptional activator that induces the expression of genes involved in ERAD, protein folding (like GRP78/BiP), and lipid synthesis. ATF6 goes to the Golgi compartment, where it is proteolytically cleaved to yield a cytosolic fragment (p50) that migrates to the nucleus and activates the transcription of UPR genes, like GRP78/BiP and CHOP. PERK dimerizes, autophosphorylates, and phosphorylates eIF2α, thereby suppressing its activity and reducing the rate of translation initiation, while increasing the rate of translation of ATF4, a potent transcription factor that augments the expression of genes involved in antioxidative stress, amino acid metabolism, and protein chaperoning. During on-going ER stress or irreparable ER damage, apoptotic pathways are activated. Ire1 phosphorylates JNK, leading to inhibition of Bcl-2 activity and activation of Bim, and recruits, releases, and activates procaspases in the cytosol. Induction of CHOP via XBP1, ATF6 or ATF4, down-regulates prosurvival Bcl-2-family members, increases prodeath proteins (like Bim) and ROS, and decreases the levels of glutathione, a ROS scavenger. In the presence of ROS, Ca²⁺ transfer to the mitochondria leads to the release of cytochrome c. The balance between proapoptotic and antiapoptotic Bcl-2-family members is disturbed, with activation of the intrinsic apoptotic pathway.