We have investigated the mechanism of mitochondrialnuclear crosstalk during cellular stress in mouse C2C12 myocytes. For this purpose, we used cells with reduced mitochondrial DNA (mtDNA) contents by ethidium bromide treatment or myocytes treated with known mitochondrial metabolic inhibitors, including carbonyl cyanide m-chlorophenylhydrazone (CCCP), antimycin, valinomycin and azide. Both genetic and metabolic stresses similarly affected mitochondrial membrane potential (Δψ m ) and electron transport-coupled ATP synthesis, which was also accompanied by an elevated steady-state cytosolic Ca 2⍣ level ([Ca 2⍣ ] i ). The mitochondrial stress resulted in: (i) an enhanced expression of the sarcoplasmic reticular ryanodine receptor-1 (RyR-1), hence potentiating the Ca 2⍣ release in response to its modulator, caffeine; (ii) enhanced levels of Ca 2⍣ -responsive factors calineurin, calcineurindependent NFATc (cytosolic counterpart of activated T-cell-specific nuclear factor) and c-Jun N-terminal kinase (JNK)-dependent ATF2 (activated transcription factor 2); (iii) reduced levels of transcription factor, NF-κB; and (iv) enhanced transcription of cytochrome oxidase Vb (COX Vb) subunit gene. These cellular changes, including the steady-state [Ca 2⍣ ] i were normalized in genetically reverted cells which contain near-normal mtDNA levels. We propose that the mitochondria-to-nucleus stress signaling occurs through cytosolic [Ca 2⍣ ] i changes, which are likely to be due to reduced ATP and Ca 2⍣ efflux. Our results indicate that the mitochondrial stress signal affects a variety of cellular processes, in addition to mitochondrial membrane biogenesis.

Two of the most commonly used immunosuppressants, cyclosporine A and tacrolimus (FK506), inhibit the activity of a ubiquitously expressed Ca 2؉ ͞calmodulin-sensitive phosphatase, calcineurin. Because both drugs also cause profound bone loss in humans and in animal models, we explored whether calcineurin played a role in regulating skeletal remodeling. We found that osteoblasts contained mRNA and protein for all isoforms of calcineurin A and B. TAT-assisted transduction of fusion protein TAT-calcineurin A␣ into osteoblasts resulted in the enhanced expression of the osteoblast differentiation markers Runx-2, alkaline phosphatase, bone sialoprotein, and osteocalcin. This expression was associated with a dramatic enhancement of bone formation in intact calvarial cultures. Calcineurin A␣ ؊/؊ mice displayed severe osteoporosis, markedly reduced mineral apposition rates, and attenuated colony formation in 10-day ex vivo stromal cell cultures. The latter was associated with significant reductions in Runx2, bone sialoprotein, and osteocalcin expression, paralleled by similar decreases in response to FK506. Together, the gain-and loss-of-function experiments indicate that calcineurin regulates bone formation through an effect on osteoblast differentiation.FK506 ͉ osteoporosis ͉ runx-2

Ryanodine receptors (RyRs) reside in microsomal membranes where they gate Ca2" release in response to changes in the cytosolic Ca2" concentration. In the osteoclast, a divalent cation sensor, the Ca2" receptor (CaR) Ca2" elevations induced by Ni2". In contrast, the responses to Ni2" were strongly potentiated by an antiserum Ab"2 raised to an epitope located within the channel-forming domain of the type II RyR. The antiserum also stained the surface of intact, unfixed, trypan blue-negative osteoclasts. Serial confocal sections and immunogold scanning electron microscopy confirmed a plasma membrane localization of this staining. Antiserum AbM directed to a putatively intracellular RyR epitope expectedly did not stain live osteoclasts nor did it potentiate CaR activation. It did, however, stain fixed, permeabilized cells in a distinctive cytoplasmic pattern. We conclude that an RyR-like molecule resides within the osteoclast plasma membrane and plays an important role in extracellular Ca2" sensing. (J. Clin. Invest. 1995.

We describe a physiologically significant mechanism through which interleukin-6 (IL-6) and a rising ambient Ca2+ interact to regulate osteoclastic bone resorption. VOXEL-based confocal microscopy of nonpermeabilized osteoclasts incubated with anti– IL-6 receptor antibodies revealed intense, strictly peripheral plasma membrane fluorescence. IL-6 receptor expression in single osteoclasts was confirmed by in situ reverse transcriptase PCR histochemistry. IL-6 (5 ng/l to 10 μg/l), but not IL-11 (10 and 100 μg/l), reversed the inhibition of osteoclastic bone resorption induced by high extracellular Ca2+ (15 mM). The IL-6 effect was abrogated by excess soluble IL-6 receptor (500 μg/l). Additionally, IL-6 (5 pg/l to 10 μg/l) inhibited cytosolic Ca2+ signals triggered by high Ca2+ or Ni2+. In separate experiments, osteoclasts incubated in 10 mM Ca2+ or on bone released more IL-6 than those in 1.25 mM Ca2+. Furthermore, IL-6 mRNA histostaining was more intense in osteoclasts in 10 or 20 mM Ca2+ than cells in 1.25 mM Ca2+. Similarly, IL-6 receptor mRNA histostaining was increased in osteoclasts incubated in 5 or 10 mM Ca2+. Thus, while high Ca2+ enhances IL-6 secretion, the released IL-6 attenuates Ca2+ sensing and reverses inhibition of resorption by Ca2+. Such an autocrine–paracrine loop may sustain osteoclastic activity in the face of an inhibitory Ca2+ level generated locally during resorption.