The mitochondrial sodium/calcium exchanger NCLX (Slc8b1) in B lymphocytes

Abstract

Antigen receptor stimulation triggers cytosolic Ca²⁺ signals, which activate transcriptional and metabolic programs critical for immune function. B-cell receptor (BCR) engagement causes rapid cytosolic Ca²⁺ rise through the ubiquitous store-operated calcium entry (SOCE) pathway. Slc8b1, which encodes the mitochondrial Na⁺/Ca²⁺ exchanger (NCLX), extrudes Ca²⁺ out of the mitochondria and maintains optimal SOCE activity. Inhibition of NCLX in DT40 and A20 B lymphocyte lines was recently shown to impair cytosolic Ca²⁺ transients in response to antigen-receptor stimulation, however the downstream functional consequences of this impairment remain unclear. Here, we generated Slc8b1 knockout A20 B-cell lines using CRISPR/Cas9 technology and B-cell specific Slc8b1 knockout mice. Surprisingly, while loss of Slc8b1 in B lymphocytes led to reduction in SOCE, it had a marginal effect on mitochondrial Ca²⁺ extrusion, suggesting that NCLX is not the major mitochondrial Ca²⁺ extrusion mechanism in B cells. Furthermore, endoplasmic reticulum (ER) Ca²⁺ content and rates of ER depletion and refilling remained unaltered in Slc8b1 knockout B cells. Slc8b1 deficiency increased mitochondrial production of oxidants, reduced mitochondrial bioenergetics and altered mitochondrial ultrastructure. B-cell specific Slc8b1 knockout mice showed reduced germinal center B cell responses following foreign antigen and pathogen driven immune responses. Our studies provide novel insights into the function of Slc8b1 in germinal center B cells and its contribution to B-cell signaling and effector function.

Keywords: B lymphocytes; Ca(2+) signaling; Germinal Center; Mitochondrial Ca(2+); NCLX; SOCE; Slc8b1.