Nitric Oxide-Dependent Mitochondrial Biogenesis Generates Ca2+ Signaling Profile of Lupus T Cells

Abstract

Abnormal T cell activation and cell death underlie the pathology of systemic lupus erythematosus. Although mitochondrial hyperpolarization (MHP) represents an early and reversible checkpoint of T cell activation and apoptosis, lupus T cells exhibit persistent MHP. NO has recently been recognized as a key signal of mitochondrial biogenesis and mediator of MHP in human T lymphocytes. In this study, we show that persistent MHP was associated with increased mitochondrial mass (+47.7 ± 2.8%; p = 0.00017) and increased mitochondrial (+21.8 ± 4.1%; p = 0.016) and cytoplasmic Ca2+ content in T cells from 19 systemic lupus erythematosus patients with respect to 11 control donors (+38.0 ± 6.4%; p = 0.0023). Electron microscopy revealed that lupus lymphocytes contained 8.76 ± 1.0 mitochondria, while control donors contained 3.18 ± 0.28 mitochondria per cell (p = 0.0009). Increased mitochondrial mass in T cells was associated with 2.08 ± 0.09-fold enhanced NO production by lupus monocytes (p = 0.0023). Activation of T cells through the TCR initiates a biphasic elevation in cytosolic free Ca2+ concentration, a rapid initial peak observed within minutes, and a plateau phase lasting up to 48 h. In response to CD3/CD28 costimulation, rapid Ca2+ fluxing was enhanced while the plateau phase was diminished in lupus T cells. NO-induced mitochondrial biogenesis in normal T cells enhanced the rapid phase and reduced the plateau of Ca2+ influx upon CD3/CD28 costimulation, thus mimicking the Ca2+ signaling profile of lupus T cells. Mitochondria constitute major Ca2+ stores and NO-dependent mitochondrial biogenesis may account for altered Ca2+ handling by lupus T cells.

Nitric oxide, mitochondrial hyperpolarization, and T cell activation☆

“…Mitochondria can take up, store and release Ca 2+ , thereafter increased mitochondrial mass may account for altered Ca 2 handling [2]. Although SLE T cells and normal T cells produce comparable amount of NO, lupus monocytes were found to produce a significantly higher amount of NO than normal monocytes [43,65]. In comparison to control monocytes, lupus monocytes increased Δψ m and Ca 2+ of normal T cells after co-culture [43], suggesting that NO produced by monocytes is responsible for mitochondrial dysfunction and predisposition to necrosis by lupus T cells.…”

“…This operates through the cGMP-dependent peroxisome proliferator-activating receptor γ coactivator-1α, a master regulator of mitochondrial biogenesis [41]. NO was shown to induce mitochondrial biogenesis in brown adipocytes, U937 and HeLa cells and human lymphocytes [42,43]. Studies of T lymphocytes lacking eNOS will be important to establish an absolute requirement of this NOS isoform in T-cell activation.…”

“…Systemic lupus erythematosus (SLE) is a complex autoimmune disease characterized by production of antinuclear autoantibodies, clinical involvement in multiple organ systems and increased NO production in monocytes [43]. Lupus T cells exhibit persistent MHP or elevation of the mitochondrial transmembrane potential, increased ROI levels and ATP depletion [15].…”

“…Persistent MHP has been associated with increased mitochondrial mass of T lymphocytes in patients with SLE [43]. Mitochondria can take up, store and release Ca 2+ , thereafter increased mitochondrial mass may account for altered Ca 2 handling [2].…”

“…Although SLE T cells and normal T cells produce comparable amount of NO, lupus monocytes were found to produce a significantly higher amount of NO than normal monocytes [43,65]. In comparison to control monocytes, lupus monocytes increased Δψ m and Ca 2+ of normal T cells after co-culture [43], suggesting that NO produced by monocytes is responsible for mitochondrial dysfunction and predisposition to necrosis by lupus T cells. Release of necrotic materials from T cells activates monocytes and dendritic cells which may operate as a positive feed-back loop sustaining a proinflammatory state in SLE [2,18].…”