title: Pivotal role of mitochondrial Na⁺₋Ca²⁺ exchange in antigen receptor mediated Ca²⁺ signalling in DT40 and A20 B lymphocytes.
abstract: Cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) increases upon activation of antigen-receptor in lymphocytes. Mitochondria have been suggested to regulate the [Ca(2+)](i) response, but the molecular mechanisms and the roles are poorly understood. To clarify them, we carried out a combination study of mathematical simulations and knockout or knockdown of NCLX, a gene candidate for the mitochondrial Na(+)-Ca(2+) exchanger (NCX(mit)), in B lymphocytes. A mathematical model of Ca(2+) dynamics in B lymphocytes demonstrated that NCX(mit) inhibition reduces basal Ca(2+) content of endoplasmic reticulum (ER) and suppresses B-cell antigen receptor (BCR)-mediated [Ca(2+)](i) rise. The predictions were validated in DT40 B lymphocytes of heterozygous NCLX knockout (NCLX(+/-)). In NCLX(+/-) cells, mitochondrial Ca(2+) efflux via NCX(mit) was strongly decelerated, suggesting NCLX is a gene responsible for NCX(mit) in B lymphocytes. Consistent with the predictions, ER Ca(2+) content declined and [Ca(2+)](i) hardly rose upon BCR activation in NCLX(+/-) cells. ER Ca(2+) uptake was reduced to ∼58% of the wild-type (WT), while it was comparable to WT when mitochondrial respiration was disturbed. Essentially the same results were obtained by a pharmacological inhibition or knockdown of NCLX by siRNA in A20 B lymphocytes. Unexpectedly, ER Ca(2+) leak was augmented and co-localization of mitochondria with ER was lower in NCLX(+/-) and NCLX silenced cells. Taken together, we concluded that NCLX is a key Ca(2+) provider to ER, and that NCLX-mediated Ca(2+) recycling between mitochondria and ER is pivotal in B cell responses to antigen.
authors.affiliation: Center for Innovation in Immunoregulative Technology and Therapeutics, Kyoto University, Yoshida-konoe, Sakyo-ku, Kyoto 606-8501, Japan.
authors.forename: Bongju
authors.initials: B
authors.lastname: Kim
authors.forename: Ayako
authors.initials: A
authors.lastname: Takeuchi
authors.forename: Orie
authors.initials: O
authors.lastname: Koga
authors.forename: Masaki
authors.initials: M
authors.lastname: Hikida
authors.forename: Satoshi
authors.initials: S
authors.lastname: Matsuoka
mesh_terms: D000818:Animals; D001402:B-Lymphocytes / Q000502:physiology*; D002118:Calcium / Q000276:immunology*; D002460:Cell Line; D002645:Chickens; D004721:Endoplasmic Reticulum / Q000276:immunology; D055785:Gene Knockdown Techniques; D007075:Immunoglobulin M / Q000276:immunology; D053078:Membrane Potential, Mitochondrial; D051379:Mice; D008928:Mitochondria / Q000502:physiology; D008954:Models, Biological; D041681:NIH 3T3 Cells; D034741:RNA, Small Interfering / Q000235:genetics; D011947:Receptors, Antigen, B-Cell / Q000276:immunology*; D015398:Signal Transduction; D019831:Sodium-Calcium Exchanger / Q000235:genetics / Q000276:immunology*
chemical_list: D007075:Immunoglobulin M; D034741:RNA, Small Interfering; D011947:Receptors, Antigen, B-Cell; D019831:Sodium-Calcium Exchanger; D002118:Calcium