MICU1 occludes the mitochondrial calcium uniporter in divalent-free conditions

Mitochondrial Ca2+ uptake is mediated by the mitochondrial uniporter complex (mtCU) that includes a tetramer of the pore-forming subunit, MCU, a scaffold protein, EMRE, and the EF-hand regulatory subunit, MICU1 either homodimerized or heterodimerized with MICU2/3. MICU1 has been proposed to regulate Ca2+ uptake via the mtCU by physically occluding the pore and preventing Ca2+ flux at resting cytoplasmic [Ca2+] (free calcium concentration) and to increase Ca2+ flux at high [Ca2+] due to cooperative activation of MICUs EF-hands. However, mtCU and MICU1 functioning when its EF-hands are unoccupied by Ca2+ is poorly studied due to technical limitations. To overcome this barrier, we have studied the mtCU in divalent-free conditions by assessing the Ru265-sensitive Na+ influx using fluorescence-based measurement of mitochondrial matrix [Na+] (free sodium concentration) rise and the ensuing depolarization and swelling. We show an increase in all these measures of Na+ uptake in MICU1KO cells as compared to wild-type (WT) and rescued MICU1KO HEK cells. However, mitochondria in WT cells and MICU1 stable-rescued cells still allowed some Ru265-sensitive Na+ influx that was prevented by MICU1 in excess upon acute overexpression. Thus, MICU1 restricts the cation flux across the mtCU in the absence of Ca2+, but even in cells with high endogenous MICU1 expression such as HEK, some mtCU seem to lack MICU1-dependent gating. We also show rearrangement of the mtCU and altered number of functional channels in MICU1KO and different rescues, and loss of MICU1 during mitoplast preparation, that together might have obscured the pore-blocking function of MICU1 in divalent-free conditions in previous studies.

Сlass II histone deacetylases in the post-stroke recovery period-expression, cellular, and subcellular localization-promising targets for neuroprotection

Histone deacetylases (HDAC) inhibitors can protect nerve cells after a stroke, but it is unclear which HDAC isoform is involved in this effect. We studied cellular and intracellular rearrangement of class II HDACs at late periods after photothrombotic infarct (PTI) in the mouse sensorimotor cortex in the tissue surrounding the ischemia core and in the corresponding region of the contralateral hemisphere. We observed a decrease in HDAC4 in cortical neurons and an increase in its nuclear translocation. HDAC6 expression in neurons was also increased. Moreover, HDAC6-positive cells had elevated apoptosis. Tubostatin A (Tub A)-induced decrease in the activity of HDAC6 restored acetylation of α-tubulin during the early poststroke recovery period and reduced apoptosis of nerve cells thus protecting the brain tissue. Selective inhibition of HDAC6 elevated expression of growth-associated protein-43 (GAP43), which remained high up to 14 days after stroke and promoted axogenesis and recovery from the PTI-induced neurological deficit. Selective HDAC6 inhibitor Tub A markedly reduced neuronal death and increased acetylation of α-tubulin and the level of GAP43. Thus, HDAC6 inhibition could be a promising strategy for modulation of brain recovery as it can increase the intensity and reduce the duration of reparation processes in the brain after stroke.

Photo-Induced Oxidative Stress Impairs Mitochondrial Metabolism in Neurons and Astrocytes

Photodynamic therapy is selective destruction of cells stained with a photosensitizer upon irradiation with light at a specific wavelength in the presence of oxygen. Cell death upon photodynamic treatment is known to occur mainly due to free radical production and subsequent development of oxidative stress. During photodynamic therapy of brain tumors, healthy cells are also damaged; considering this, it is important to investigate the effect of the treatment on normal neurons and glia. We employed live-cell imaging technique to investigate the cellular mechanism of photodynamic action of radachlorin (200 nM) on neurons and astrocytes in primary rat cell culture. We found that the photodynamic effect of radachlorin increases production of reactive oxygen species measured by dihydroethidium and significantly decrease mitochondrial membrane potential. Mitochondrial depolarization was independent of opening of mitochondrial permeability transition pore and was insensitive to blocker of this pore cyclosporine A. However, irradiation of cells with radachlorin dramatically decreased NADH autofluorescence and also reduced mitochondrial NADH pool suggesting inhibition of mitochondrial respiration by limitation of substrate. This effect could be prevented by inhibition of poly (ADP-ribose) polymerase (PARP) with DPQ. Thus, irradiation of neurons and astrocytes in the presence of radachlorin leads to activation of PARP and decrease in NADH that leads to mitochondrial dysfunction.

Expression of Class I Histone Deacetylases in Ipsilateral and Contralateral Hemispheres after the Focal Photothrombotic Infarction in the Mouse Brain

Histone acetylation and deacetylation are among the most important epigenetic processes that regulate gene expression. Nonselective inhibitors of histone deacetylases (HDAC) can protect brain cells during ischemia and stroke. However, which HDAC isoform is involved in this effect is unknown. Some isoforms of histone deacetylases (HDACs) protect brain cells after ischemia, whereas others can promote their death. Most studies consider early periods (1-24 h) after stroke, whereas little is known on the involvement of HDACs during recovery after stroke. In this study, cellular and intracellular rearrangement of class I HDACs (HDAC1, HDAC2, HDAC3, HDAC8) was investigated at late periods after photothrombotic infarction (PTI) of the mouse sensorimotor cortex in intact tissue that surrounds the ischemia core, in the corresponding region of the contralateral hemisphere, and in the hippocampus. Each HDAC isoform had a specific pattern of expression and intracellular distribution in neurons and astrocytes at different periods after the ischemia. We did not observe ischemia-induced changes in the subcellular localization of HDACs under study. Three days after the PTI, the expression of HDAC2 was increased in neurons of the damaged hemisphere. The activity of HDAC2 and HDAC8 was elevated 7 days after the ischemia both in neurons and astrocytes of the studied brain structures; the activity of HDAC8 was also increased 14 days after the ischemia. It is notable that the expression of class I HDACs in the intact hemisphere changes in the same way as their expression in the living tissue of the damaged hemisphere. HDAC1 was found both in the nuclei and cytoplasm of the brain cells; HDAC2 was predominantly localized in the nuclei, and HDAC8 was predominantly observed in the cytoplasm. This in addition to the regulation of gene transcription indicates nontranscriptional activity of HDAC1 and HDAC8 during recovery of the brain tissue after the ischemia. HDAC2 and HDAC8 were identified as potential mediators in an early recovery period after stroke, suggesting that selective inhibitors and activators of HDACs can be considered for therapeutic approaches in this period.

Protection of the Crayfish Mechanoreceptor Neuron and Glial Cells from Photooxidative Injury by Modulators of Diverse Signal Transduction Pathways

Oxidative stress is the reason of diverse neuropathological processes. Photodynamic therapy (PDT), an effective inducer of oxidative stress, is used for cancer treatment, including brain tumors. We studied the role of various signaling pathways in photodynamic injury and protection of single neurons and satellite glial cells in the isolated crayfish mechanoreceptor. It was photosensitized with alumophthalocyanine Photosens in the presence of inhibitors or activators of various signaling proteins. PDT eliminated neuronal activity and killed neurons and glial cells. Inhibitory analysis showed the involvement of protein kinases Akt, glycogen synthase kinase-3β (GSK-3β), mammalian target of rapamycin (mTOR), mitogen-activated protein kinase kinases 1 and 2 (MEK1/2), calmodulin, calmodulin-dependent kinase II (CaMKII), adenylate cyclase, and nuclear factor NF-κB in PDT-induced necrosis of neurons. Nitric oxide (NO) and glial cell-derived neurotrophic factor (GDNF) reduced neuronal necrosis. In glial cells, protein kinases Akt, calmodulin, and CaMKII; protein kinases C and G, adenylate cyclase, and p38; and nuclear transcription factor NF-κB also mediated PDT-induced necrosis. In contrast, NO and neurotrophic factors nerve growth factor (NGF) and GDNF demonstrated anti-necrotic activity. Phospholipase Cγ, protein kinase C, GSK-3β, mTOR, NF-κB, mitochondrial permeability transition pores, and NO synthase mediated PDT-induced apoptosis of glial cells, whereas protein kinase A, tyrosine phosphatases, and neurotrophic factors NGF, GDNF, and neurturin were involved in protecting glial cells from photoinduced apoptosis. Signaling pathways that control cell survival and death differed in neurons and glia. Inhibitors or activators of some signaling pathways may be used as potential protectors of neurons and glia from photooxidative stress and following death.

On involvement of transcription factors nuclear factor kappa-light-chain-enhancer of activated B cells, activator protein-1 and signal transducer and activator of transcription-3 in photodynamic therapy-induced death of crayfish neurons and satellite glial cells

Photodynamic therapy (PDT) is currently used in the treatment of brain tumors. However, not only malignant cells but also neighboring normal neurons and glial cells are damaged during PDT. In order to study the potential role of transcription factors-nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), activator protein (AP-1), and signal transducer and activator of transcription-3 (STAT-3)-in photodynamic injury of normal neurons and glia, we photosensitized the isolated crayfish mechanoreceptor consisting of a single sensory neuron enveloped by glial cells. Application of different inhibitors and activators showed that transcription factors NF-κB (inhibitors caffeic acid phenethyl ester and parthenolide, activator betulinic acid), AP-1 (inhibitor SR11302), and STAT-3 (inhibitors stattic and cucurbitacine) influenced PDT-induced death and survival of neurons and glial cells in different ways. These experiments indicated involvement of NF-κB in PDT-induced necrosis of neurons and apoptosis of glial cells. However, in glial cells, it played the antinecrotic role. AP-1 was not involved in PDT-induced necrosis of neurons and glia, but mediated glial apoptosis. STAT-3 was involved in PDT-induced apoptosis of glial cells and necrosis of neurons and glia. Therefore, signaling pathways that regulate cell death and survival in neurons and glial cells are different. Using various inhibitors or activators of transcription factors, one can differently influence the sensitivity and resistance of neurons and glial cells to PDT.