IP3 receptors (IP3Rs) release Ca2+ from the ER when they bind IP3 and Ca2+. The spatial organization of IP3Rs determines both the propagation of Ca2+ signals between IP3Rs and the selective regulation of cellular responses. Here we use gene editing to fluorescently tag endogenous IP3Rs, and super-resolution microscopy to determine the geography of IP3Rs and Ca2+ signals within living cells. We show that native IP3Rs cluster within ER membranes. Most IP3R clusters are mobile, moved by diffusion and microtubule motors. Ca2+ signals are generated by a small population of immobile IP3Rs. These IP3Rs are licensed to respond, but they do not readily mix with mobile IP3Rs. The licensed IP3Rs reside alongside ER-plasma membrane junctions where STIM1, which regulates store-operated Ca2+ entry, accumulates after depletion of Ca2+ stores. IP3Rs tethered close to ER-plasma membrane junctions are licensed to respond and optimally placed to be activated by endogenous IP3 and to regulate Ca2+ entry.

SummaryInositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) allow extracellular stimuli to redistribute Ca2+ from the ER to cytosol or other organelles. We show, using small interfering RNA (siRNA) and vacuolar H+-ATPase (V-ATPase) inhibitors, that lysosomes sequester Ca2+ released by all IP3R subtypes, but not Ca2+ entering cells through store-operated Ca2+ entry (SOCE). A low-affinity Ca2+ sensor targeted to lysosomal membranes reports large, local increases in cytosolic [Ca2+] during IP3-evoked Ca2+ release, but not during SOCE. Most lysosomes associate with endoplasmic reticulum (ER) and dwell at regions populated by IP3R clusters, but IP3Rs do not assemble ER-lysosome contacts. Increasing lysosomal pH does not immediately prevent Ca2+ uptake, but it causes lysosomes to slowly redistribute and enlarge, reduces their association with IP3Rs, and disrupts Ca2+ exchange with ER. In a “piston-like” fashion, ER concentrates cytosolic Ca2+ and delivers it, through large-conductance IP3Rs, to a low-affinity lysosomal uptake system. The involvement of IP3Rs allows extracellular stimuli to regulate Ca2+ exchange between the ER and lysosomes.

Background: GBA2 and GBA are both ␤-glucosidases that degrade glucosylceramide. Results: Conduritol B epoxide inactivates both GBA and GBA2, whereas the imino sugar NB-DGJ selectively inhibits GBA2. Conclusion: NB-DGJ is a suitable reagent to distinguish GBA2 from GBA. Significance: This study redefines GBA2 activity, which is relevant for clinical GBA2 measurements and imino sugar pharmacology.

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About 50 million of the U.S. adult population suffer from chronic pain. It is a complex disease in its own right for which currently available analgesics have been deemed woefully inadequate since ∼20% of the sufferers derive no benefit. Vitamin D, known for its role in calcium homeostasis and bone metabolism, is thought to be of clinical benefit in treating chronic pain without the side-effects of currently available analgesics. A strong correlation between hypovitaminosis D and incidence of bone pain is known. However, the potential underlying mechanisms by which vitamin D might exert its analgesic effects are poorly understood. In this review, we discuss pathways involved in pain sensing and processing primarily at the level of dorsal root ganglion (DRG) neurons and the potential interplay between vitamin D, its receptor (VDR) and known specific pain signaling pathways including nerve growth factor (NGF), glial-derived neurotrophic factor (GDNF), epidermal growth factor receptor (EGFR), and opioid receptors. We also discuss how vitamin D/VDR might influence immune cells and pain sensitization as well as review the increasingly important topic of vitamin D toxicity. Further in vitro and in vivo experimental studies will be required to study these potential interactions specifically in pain models. Such studies could highlight the potential usefulness of vitamin D either alone or in combination with existing analgesics to better treat chronic pain.