Marc Freichel (Heidelberg)

Organellar Ca²⁺ regulators (OCaRs) encoded by TMEM63 proteins determining NAADP-mediated Ca²⁺ release from acidic intracellular store

NAADP is a potent second messenger triggering Ca²⁺ release of from intracellular stores in numerous cell types e.g. immune cells, acinar cells and cardiomyocytes. NAADP targets include the ryanodine receptor RYR1 in the endoplasmic reticulum (ER), two-pore channels TPC1 and TPC2 as well as TRPML1 in endo-lysosomes.

TMEM63 proteins (TMEM63A, TMEM63B, TMEM63C) are the closest homologues of OSCAs, a family of plasma membrane proteins that can form hyperosmolality-gated calcium-permeable channels activated by hyperosmotic shock.^[1] TMEM63A and TMEM63B are ubiquitously expressed. TMEM63A is localized in the membrane of lysosomes and secretory granules in pancreatic acinar cells as revealed by microscopic analysis in cells of TMEM63A-YFP knock add-on mice and high-resolution organellar proteomics analysis, indicating its dynamic subcellular (re-)distribution during endo-/exocytosis.^[2,3] Because of its subcellular localization and function we dubbed TMEM63A Organellar Calcium Regulator protein 1 (OCaR1). Using genetically encoded Ca²⁺ indicator targeted directly to TPC2-containing vesicles, it could be shown that TMEM63A/OCaR1 controls Ca²⁺ release from acidic Ca²⁺ stores via functional inhibition of endo-lysosomal TPC1 and TPC2 channels. Accordingly, OCaR1 deletion leads to extensive Ca²⁺ release from NAADP-responsive, acidic Ca²⁺ stores under basal conditions and upon stimulation of GPCR receptors leading to exacerbation of the disease phenotype in murine models of severe and chronic pancreatitis.^[3] This study thus established OCaR1 acts as a gatekeeper of NAADP-mediated Ca²⁺ release via TPC channels and led to the concept that OCaR1 endows exocytotic vesicles with an autoregulatory competence that enables the vesicles to regulate their own exocytosis.

In my CRC1425 lecture, I will provide evidence that OCaR proteins are also acting as the central orchestrator of agonist-evoked Ca²⁺ release that originates from NAADP-sensitive acidic organelles mediated via TPC channels in cardiomyocytes and platelets.

In conjunction with other recently published studies investigating genetic inactivation of TMEM63/OCaR proteins in mice a concept was established that these proteins form cation channels in the membrane of intracellular organelles e.g. acidic endo-lysosomes and secretory granules. Here, TMEM63/OCaR proteins regulate the Ca²⁺ release either after hormonal stimulation or mechano-activation and control exocytosis that is critical for vitally important body functions and development for life-threatening diseases.^[1] During exocytosis, a small portion of TMEM63a/OCaR1 proteins can be translocated to the plasma membrane. However, several questions remain:
(i) how exactly do TMEM63/OCaR proteins contribute to the cytosolic Ca²⁺ rise, either directly by conducting Ca²⁺ or indirectly, e.g. by membrane depolarization, or by activating other Ca²⁺ permeable channel;
(ii) how are changes in membrane tension acting on the cell surface from the outside transmitted to mechano-sensing TMEM63/OCaR proteins in the membrane of intracellular organelles;
(iii) in which way are changes in membrane tension involved in the gating and activation of TMEM63/OCaR channels following stimulation of the cells by agonists or other chemical signals, including changes in the metabolism in acinar cells or neuro(endocrine) cells as well as many other cells in which they are expressed.

1. Freichel M et al. The many facets of TMEM63/OCaR proteins as mechanosensitive channels in lysosomes, NAADP signaling and beyond. Cell Calcium 2024/123:102942
2. Tsvilovskyy V et al. OCaR1 confers exocytotic vesicles an autoregulatory competence to prevent Ca²⁺ release, exocytosis and pancreatic disorder. In: J. Mackrill (Ed.) 16th International Meeting of the European Calcium Society, European Calcium Society, University College Cork, UK, 2022, pp. 34.
3. Tsvilovskyy V et al. OCaR1 endows exocytic vesicles with autoregulatory competence by preventing uncontrolled Ca²⁺ release, exocytosis, and pancreatic tissue damage. J Clin Invest 2024/134:e169428

Link to Speaker

This will be a hybrid seminar held at the IEKM seminar room, 1st floor, Elsässer Str. 2Q.
Prior registration is required – please contact info@sfb1425.uni-freiburg.de.