Cytochrome c binds to inositol (1,4,5) trisphosphate and ryanodine receptors in vivo after transient brain ischemia in gerbils

Tissue-specific regulation of cytochrome c by post-translational modifications: respiration, the mitochondrial membrane potential, ROS, and apoptosis

Cytochrome c (Cyt c) plays a vital role in the mitochondrial electron transport chain (ETC). In addition, it is a key regulator of apoptosis. Cyt c has multiple other functions including ROS production and scavenging, cardiolipin peroxidation, and mitochondrial protein import. Cyt c is tightly regulated by allosteric mechanisms, tissue-specific isoforms, and post-translational modifications (PTMs). Distinct residues of Cyt c are modified by PTMs, primarily phosphorylations, in a highly tissue-specific manner. These modifications downregulate mitochondrial ETC flux and adjust the mitochondrial membrane potential (ΔΨm), to minimize reactive oxygen species (ROS) production under normal conditions. In pathologic and acute stress conditions, such as ischemia-reperfusion, phosphorylations are lost, leading to maximum ETC flux, ΔΨm hyperpolarization, excessive ROS generation, and the release of Cyt c. It is also the dephosphorylated form of the protein that leads to maximum caspase activation. We discuss the complex regulation of Cyt c and propose that it is a central regulatory step of the mammalian ETC that can be rate limiting in normal conditions. This regulation is important because it maintains optimal intermediate ΔΨm, limiting ROS generation. We examine the role of Cyt c PTMs, including phosphorylation, acetylation, methylation, nitration, nitrosylation, and sulfoxidation and consider their potential biological significance by evaluating their stoichiometry.-Kalpage, H. A., Bazylianska, V., Recanati, M. A., Fite, A., Liu, J., Wan, J., Mantena, N., Malek, M. H., Podgorski, I., Heath, E. I., Vaishnav, A., Edwards, B. F., Grossman, L. I., Sanderson, T. H., Lee, I., Hüttemann, M. Tissue-specific regulation of cytochrome c by post-translational modifications: respiration, the mitochondrial membrane potential, ROS, and apoptosis.

Crosstalk Between Endoplasmic Reticulum Stress, Oxidative Stress, and Autophagy: Potential Therapeutic Targets for Acute CNS Injuries

Endoplasmic reticulum (ER) stress induces a variety of neuronal cell death pathways that play a critical role in the pathophysiology of stroke. ER stress occurs when unfolded/misfolded proteins accumulate and the folding capacity of ER chaperones exceeds the capacity of ER lumen to facilitate their disposal. As a consequence, a complex set of signaling pathways will be induced that transmit from ER to cytosol and nucleus to compensate damage and to restore the normal cellular homeostasis, collectively known as unfolded protein response (UPR). However, failure of UPR due to severe or prolonged stress leads to cell death. Following acute CNS injuries, chronic disturbances in protein folding and oxidative stress prolong ER stress leading to sustained ER dysfunction and neuronal cell death. While ER stress responses have been well studied after stroke, there is an emerging need to study the association of ER stress with other cell pathways that exacerbate neuronal death after an injury. In this review, we summarize the current understanding of the role for ER stress in acute brain injuries, highlighting the diverse molecular mechanisms associated with ER stress and its relation to oxidative stress and autophagy. We also discussed the existing and developing therapeutic options aimed to reduce ER stress to protect the CNS after acute injuries.

IP3, a small molecule with a powerful message

Research conducted over the past two decades has provided convincing evidence that cell death, and more specifically apoptosis, can exceed single cell boundaries and can be strongly influenced by intercellular communication networks. We recently reported that gap junctions (i.e. channels directly connecting the cytoplasm of neighboring cells) composed of connexin43 or connexin26 provide a direct pathway to promote and expand cell death, and that inositol 1,4,5-trisphosphate (IP3) diffusion via these channels is crucial to provoke apoptosis in adjacent healthy cells. However, IP3 itself is not sufficient to induce cell death and additional factors appear to be necessary to create conditions in which IP3 will exert proapoptotic effects. Although IP3-evoked Ca(2+) signaling is known to be required for normal cell survival, it is also actively involved in apoptosis induction and progression. As such, it is evident that an accurate fine-tuning of this signaling mechanism is crucial for normal cell physiology, while a malfunction can lead to cell death. Here, we review the role of IP3 as an intracellular and intercellular cell death messenger, focusing on the endoplasmic reticulum-mitochondrial synapse, followed by a discussion of plausible elements that can convert IP3 from a physiological molecule to a killer substance. Finally, we highlight several pathological conditions in which anomalous intercellular IP3/Ca(2+) signaling might play a role. This article is part of a Special Issue entitled:12th European Symposium on Calcium.

Calpain-cleaved type 1 inositol 1,4,5-trisphosphate receptor (InsP(3)R1) has InsP(3)-independent gating and disrupts intracellular Ca(2+) homeostasis

The type 1 inositol 1,4,5-trisphosphate receptor (InsP(3)R1) is a ubiquitous intracellular Ca(2+) release channel that is vital to intracellular Ca(2+) signaling. InsP(3)R1 is a proteolytic target of calpain, which cleaves the channel to form a 95-kDa carboxyl-terminal fragment that includes the transmembrane domains, which contain the ion pore. However, the functional consequences of calpain proteolysis on channel behavior and Ca(2+) homeostasis are unknown. In the present study we have identified a unique calpain cleavage site in InsP(3)R1 and utilized a recombinant truncated form of the channel (capn-InsP(3)R1) corresponding to the stable, carboxyl-terminal fragment to examine the functional consequences of channel proteolysis. Single-channel recordings of capn-InsP(3)R1 revealed InsP(3)-independent gating and high open probability (P(o)) under optimal cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) conditions. However, some [Ca(2+)](i) regulation of the cleaved channel remained, with a lower P(o) in suboptimal and inhibitory [Ca(2+)](i). Expression of capn-InsP(3)R1 in N2a cells reduced the Ca(2+) content of ionomycin-releasable intracellular stores and decreased endoplasmic reticulum Ca(2+) loading compared with control cells expressing full-length InsP(3)R1. Using a cleavage-specific antibody, we identified calpain-cleaved InsP(3)R1 in selectively vulnerable cerebellar Purkinje neurons after in vivo cardiac arrest. These findings indicate that calpain proteolysis of InsP(3)R1 generates a dysregulated channel that disrupts cellular Ca(2+) homeostasis. Furthermore, our results demonstrate that calpain cleaves InsP(3)R1 in a clinically relevant injury model, suggesting that Ca(2+) leak through the proteolyzed channel may act as a feed-forward mechanism to enhance cell death.

Endoplasmic reticulum stress in brain damage

The efficient functioning of the ER is indispensable for most of the cellular activities and survival. Disturbances in the physiological functions of the ER result in the activation of a complex set of signaling pathways from the ER to the cytosol and nucleus, and these are collectively known as unfolded protein response (UPR), which is aimed to compensate damage and can eventually trigger cell death if ER stress is severe or persists for a longer period. The precise molecular mechanisms that facilitate this switch in brain damage have yet to be understood completely with multiple potential participants involved. The ER stress-associated cell death pathways have been recognized in the numerous pathophysiological conditions, such as diabetes, hypoxia, ischemia/reperfusion injury, and neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, and bipolar disorder. Hence, there is an emerging need to study the basic molecular mechanisms of ER stress-mediating multiple cell survival/death signaling pathways. These molecules that regulate the ER stress response would be potential drug targets in brain diseases.

The multiple functions of cytochrome c and their regulation in life and death decisions of the mammalian cell: From respiration to apoptosis

Cytochrome c (Cytc) is essential in mitochondrial electron transport and intrinsic type II apoptosis. Mammalian Cytc also scavenges reactive oxygen species (ROS) under healthy conditions, produces ROS with the co-factor p66(Shc), and oxidizes cardiolipin during apoptosis. The recent finding that Cytc is phosphorylated in vivo underpins a model for the pivotal role of Cytc regulation in making life and death decisions. An apoptotic sequence of events is proposed involving changes in Cytc phosphorylation, increased ROS via increased mitochondrial membrane potentials or the p66(Shc) pathway, and oxidation of cardiolipin by Cytc followed by its release from the mitochondria. Cytc regulation in respiration and cell death is discussed in a human disease context including neurodegenerative and cardiovascular diseases, cancer, and sepsis.

Endoplasmic reticulum Ca2+ signaling and mitochondrial Cyt c release in astrocytes following oxygen and glucose deprivation

In the present study, we investigated changes of cytosolic Ca2+([Ca2+](cyt)), endoplasmic reticulum Ca2+([Ca2+](ER)) and mitochondrial Ca2+(Ca2+(m)) in astrocytes following oxygen/glucose deprivation and reoxygenation (OGD/REOX). Two hours OGD did not cause changes in [Ca2+](cyt), but led to a significant increase in [Ca2+](ER). The elevation in [Ca2+](ER) continued and reached a peak level (130 +/- 2 microM) by 90 min REOX. An abrupt release of Ca2+(ER) occurred during 1.5-2.5 h REOX, which was accompanied with a delayed and sustained rise in [Ca2+](cyt). Moreover, Ca2+(m) content was increased significantly within 15 min REOX followed by a secondary rise (approximately 4.5-fold) and a release of mitochondrial cytochrome c (Cyt c). Astrocytes exhibited translocation of Cyt c from mitochondria to endoplasmic reticulum (ER) and up regulation of ER stress protein p-eIF2alpha. Blocking Na+-K+-Cl(-) cotransporter isoform 1 activity, either by its potent inhibitor bumetanide or genetic ablation, abolished release of ER Ca2+, delayed rise in [Ca2+](cyt) and Ca2+(m). Inhibition of the reverse mode operation of the Na+/Ca2+ exchanger significantly attenuated OGD/REOX-mediated Cyt c release. In summary, this study illustrates that OGD/REOX triggers a time-dependent loss of Ca2+ homeostasis in cytosol and organelles (ER and mitochondria) in astrocytes. Collective stimulation of Na+-K+-Cl(-) cotransporter isoform 1 and reverse mode function of Na+/Ca2+ exchanger contributes to these changes.

Association of protein kinase C delta and phospholipid scramblase 3 in hippocampal mitochondria correlates with neuronal vulnerability to brain ischemia

Recent findings support the idea that mitochondrial integrity plays an important role in the propagation of excitotoxic ischemic signal and PKC is implicated in the regulation of mitochondrial membranes properties. One of the targets of PKC delta is phospholipid scramblase 3 (PLSCR3), an enzyme responsible for cardiolipin translocation from the inner to outer mitochondrial membrane. To get an insight into in vivo mechanism by which PKC delta mediates ischemia/reperfusion injury of hippocampal neurons, we examined the effects of transient brain ischemia in gerbil on association of PKC delta with mitochondria isolated from ischemia-vulnerable (CA1) and ischemia-resistant regions, and interactions between PKC delta and PLSCR3. Postischemic, biphasic and brain region-specific translocation of PKC delta to mitochondria was observed. First peak was at 30-60 min of reperfusion and the second was observed after 72-96 h following ischemia. PKC delta was translocated to mitochondria only in CA1 region. The PLSCR3 mRNA and protein was detected in brain by RT-PCR and sequence analysis, Western blotting and immunocytochemistry in electron microscopy (EM). Co-immunoprecipitation and double-labeled immuno-EM showed association of PKC delta and PLSCR3 in postischemic CA1 mitochondria. Additionally, the amount of tBid associated with mitochondria was elevated 96 h following ischemia. Our data suggest that in the postischemic brain PKC delta co-localizes with PLSCR3 in mitochondria and this event might influence the mitochondrial membranes architecture and delayed neurons degeneration.

Ion transporters and ischemic mitochondrial dysfunction

Ischemia-induced ionic imbalance leads to the activation of numerous events including mitochondrial dysfunction and eventual cell death. Dysregulation of mitochondrial Ca(2+) (Ca(2+)(m)) plays a critical role in cell damage under pathological conditions including traumatic brain injury and stroke. High Ca(2+)(m) levels can induce the persistent opening of the mitochondrial permeability transition pore and trigger mitochondrial membrane depolarization, Ca(2+) release, cessation of oxidative phosphorylation, matrix swelling and eventually outer membrane rupture with release of cytochrome c and other apoptogenic proteins. Thus, the dysregulation of mitochondrial Ca(2+) homeostasis is now recognized to play a crucial role in triggering mitochondrial dysfunction and subsequent apoptosis. Recent studies show that some secondary active transport proteins, such as Na(+)-dependent chloride transporter and Na(+)/Ca(2+) exchanger, contribute to ischemia-induced dissipation of ion homeostasis including Ca(2+)(m).

Calcium ions in neuronal degeneration

Neuronal Ca(2+) homeostasis and Ca(2+) signaling regulate multiple neuronal functions, including synaptic transmission, plasticity, and cell survival. Therefore disturbances in Ca(2+) homeostasis can affect the well-being of the neuron in different ways and to various degrees. Ca(2+) homeostasis undergoes subtle dysregulation in the physiological ageing. Products of energy metabolism accumulating with age together with oxidative stress gradually impair Ca(2+) homeostasis, making neurons more vulnerable to additional stress which, in turn, can lead to neuronal degeneration. Neurodegenerative diseases related to aging, such as Alzheimer's disease, Parkinson's disease, or Huntington's disease, develop slowly and are characterized by the positive feedback between Ca(2+) dyshomeostasis and the aggregation of disease-related proteins such as amyloid beta, alfa-synuclein, or huntingtin. Ca(2+) dyshomeostasis escalates with time eventually leading to neuronal loss. Ca(2+) dyshomeostasis in these chronic pathologies comprises mitochondrial and endoplasmic reticulum dysfunction, Ca(2+) buffering impairment, glutamate excitotoxicity and alterations in Ca(2+) entry routes into neurons. Similar changes have been described in a group of multifactorial diseases not related to ageing, such as epilepsy, schizophrenia, amyotrophic lateral sclerosis, or glaucoma. Dysregulation of Ca(2+) homeostasis caused by HIV infection or by sudden accidents, such as brain stroke or traumatic brain injury, leads to rapid neuronal death. The differences between the distinct types of Ca(2+) dyshomeostasis underlying neuronal degeneration in various types of pathologies are not clear. Questions that should be addressed concern the sequence of pathogenic events in an affected neuron and the pattern of progressive degeneration in the brain itself. Moreover, elucidation of the selective vulnerability of various types of neurons affected in the diseases described here will require identification of differences in the types of Ca(2+) homeostasis and signaling among these neurons. This information will be required for improved targeting of Ca(2+) homeostasis and signaling components in future therapeutic strategies, since no effective treatment is currently available to prevent neuronal degeneration in any of the pathologies described here.

Molecular mechanisms of apoptosis in cerebral ischemia: multiple neuroprotective opportunities

Cerebral ischemia/reperfusion (I/R) injury triggers multiple and distinct but overlapping cell signaling pathways, which may lead to cell survival or cell damage. There is overwhelming evidence to suggest that besides necrosis, apoptosis do contributes significantly to the cell death subsequent to I/R injury. Both extrinsic and intrinsic apoptotic pathways play a vital role, and upon initiation, these pathways recruit downstream apoptotic molecules to execute cell death. Caspases and Bcl-2 family members appear to be crucial in regulating multiple apoptotic cell death pathways initiated during I/R. Similarly, inhibitor of apoptosis family of proteins (IAPs), mitogen-activated protein kinases, and newly identified apoptogenic molecules, like second mitochondrial-activated factor/direct IAP-binding protein with low pI (Smac/Diablo), omi/high-temperature requirement serine protease A2 (Omi/HtrA2), X-linked mammalian inhibitor of apoptosis protein-associated factor 1, and apoptosis-inducing factor, have emerged as potent regulators of cellular apoptotic/antiapoptotic machinery. All instances of cell survival/death mechanisms triggered during I/R are multifaceted and interlinked, which ultimately decide the fate of brain cells. Moreover, apoptotic cross-talk between major subcellular organelles suggests that therapeutic strategies should be optimally directed at multiple targets/mechanisms for better therapeutic outcome. Based on the current knowledge, this review briefly focuses I/R injury-induced multiple mechanisms of apoptosis, involving key apoptotic regulators and their emerging roles in orchestrating cell death programme. In addition, we have also highlighted the role of autophagy in modulating cell survival/death during cerebral ischemia. Furthermore, an attempt has been made to provide an encouraging outlook on emerging therapeutic approaches for cerebral ischemia.

IP3 receptors in cell survival and apoptosis: Ca2+ release and beyond

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) serve to discharge Ca(2+) from ER stores in response to agonist stimulation. The present review summarizes the role of these receptors in models of Ca(2+)-dependent apoptosis. In particular we focus on the regulation of IP(3)Rs by caspase-3 cleavage, cytochrome c, anti-apoptotic proteins and Akt kinase. We also address the evidence that some of the effects of IP(3)Rs in apoptosis may be independent of their ion-channel function. The role of IP(3)Rs in delivering Ca(2+) to the mitochondria is discussed from the perspective of the factors determining inter-organellar dynamics and the spatial proximity of mitochondria and ER membranes.

Requirement of biphasic calcium release from the endoplasmic reticulum for Fas-mediated apoptosis

Fas receptor is a member of the tumor necrosis factor-α family of death receptors that mediate physiologic apoptotic signaling. To investigate the molecular mechanisms regulating calcium mobilization during Fas-mediated apoptosis, we have analyzed the sequential steps leading to altered calcium homeostasis and cell death in response to activation of the Fas receptor. We show that Fas-mediated apoptosis requires endoplasmic reticulum–mediated calcium release in a mechanism dependent on phospholipase C-γ1 (PLC-γ1) activation and Ca²⁺ release from inositol 1,4,5-trisphosphate receptor (IP₃R) channels. The kinetics of Ca²⁺ release were biphasic, demonstrating a rapid elevation caused by PLC-γ1 activation and a delayed and sustained increase caused by cytochrome c binding to IP₃R. Blocking either phase of Ca²⁺ mobilization was cytoprotective, highlighting PLC-γ1 and IP₃R as possible therapeutic targets for disorders associated with Fas signaling.