Essential role of A-kinase anchor protein 121 for cAMP signaling to mitochondria

Exercise-Regulated Mitochondrial and Nuclear Signalling Networks in Skeletal Muscle

Exercise perturbs energy homeostasis in skeletal muscle and engages integrated cellular signalling networks to help meet the contraction-induced increases in skeletal muscle energy and oxygen demand. Investigating exercise-associated perturbations in skeletal muscle signalling networks has uncovered novel mechanisms by which exercise stimulates skeletal muscle mitochondrial biogenesis and promotes whole-body health and fitness. While acute exercise regulates a complex network of protein post-translational modifications (e.g. phosphorylation) in skeletal muscle, previous investigations of exercise signalling in human and rodent skeletal muscle have primarily focused on a select group of exercise-regulated protein kinases [i.e. 5' adenosine monophosphate-activated protein kinase (AMPK), protein kinase A (PKA), Ca2+/calmodulin-dependent protein kinase (CaMK) and mitogen-activated protein kinase (MAPK)] and only a small subset of their respective protein substrates. Recently, global mass spectrometry-based phosphoproteomic approaches have helped unravel the extensive complexity and interconnection of exercise signalling pathways and kinases beyond this select group and phosphorylation and/or translocation of exercise-regulated mitochondrial and nuclear protein substrates. This review provides an overview of recent advances in our understanding of the molecular events associated with acute endurance exercise-regulated signalling pathways and kinases in skeletal muscle with a focus on phosphorylation. We critically appraise recent evidence highlighting the involvement of mitochondrial and nuclear protein phosphorylation and/or translocation in skeletal muscle adaptive responses to an acute bout of endurance exercise that ultimately stimulate mitochondrial biogenesis and contribute to exercise's wider health and fitness benefits.

Glaucomatous optic neuropathy: Mitochondrial dynamics, dysfunction and protection in retinal ganglion cells

Glaucoma is a leading cause of irreversible blindness worldwide and is characterized by a slow, progressive, and multifactorial degeneration of retinal ganglion cells (RGCs) and their axons, resulting in vision loss. Despite its high prevalence in individuals 60 years of age and older, the causing factors contributing to glaucoma progression are currently not well characterized. Intraocular pressure (IOP) is the only proven treatable risk factor. However, lowering IOP is insufficient for preventing disease progression. One of the significant interests in glaucoma pathogenesis is understanding the structural and functional impairment of mitochondria in RGCs and their axons and synapses. Glaucomatous risk factors such as IOP elevation, aging, genetic variation, neuroinflammation, neurotrophic factor deprivation, and vascular dysregulation, are potential inducers for mitochondrial dysfunction in glaucoma. Because oxidative phosphorylation stress-mediated mitochondrial dysfunction is associated with structural and functional impairment of mitochondria in glaucomatous RGCs, understanding the underlying mechanisms and relationship between structural and functional alterations in mitochondria would be beneficial to developing mitochondria-related neuroprotection in RGCs and their axons and synapses against glaucomatous neurodegeneration. Here, we review the current studies focusing on mitochondrial dynamics-based structural and functional alterations in the mitochondria of glaucomatous RGCs and therapeutic strategies to protect RGCs against glaucomatous neurodegeneration.

Role of A-Kinase Anchoring Protein 1 in Retinal Ganglion Cells: Neurodegeneration and Neuroprotection

A-Kinase anchoring protein 1 (AKAP1) is a multifunctional mitochondrial scaffold protein that regulates mitochondrial dynamics, bioenergetics, and calcium homeostasis by anchoring several proteins, including protein kinase A, to the outer mitochondrial membrane. Glaucoma is a complex, multifactorial disease characterized by a slow and progressive degeneration of the optic nerve and retinal ganglion cells (RGCs), ultimately resulting in vision loss. Impairment of the mitochondrial network and function is linked to glaucomatous neurodegeneration. Loss of AKAP1 induces dynamin-related protein 1 dephosphorylation-mediated mitochondrial fragmentation and loss of RGCs. Elevated intraocular pressure triggers a significant reduction in AKAP1 protein expression in the glaucomatous retina. Amplification of AKAP1 expression protects RGCs from oxidative stress. Hence, modulation of AKAP1 could be considered a potential therapeutic target for neuroprotective intervention in glaucoma and other mitochondria-associated optic neuropathies. This review covers the current research on the role of AKAP1 in the maintenance of mitochondrial dynamics, bioenergetics, and mitophagy in RGCs and provides a scientific basis to identify and develop new therapeutic strategies that could protect RGCs and their axons in glaucoma.

Construction of a circRNA-miRNA-mRNA Regulatory Network for Coronary Artery Disease by Bioinformatics Analysis

Background. Circular RNAs (circRNAs) were known to be related to the pathogenesis of many diseases through competing endogenous RNA (ceRNA) regulatory mechanisms. However, the function of circRNA in coronary artery disease (CAD) remains unclear. In this study, we aim to construct a circRNA-related competing endogenous RNA (ceRNA) network in CAD. Methods. The gene expression profiles of CAD were obtained from Gene Expression Omnibus datasets. Bioinformatics analysis was performed to construct a ceRNA regulatory network, from which the hub genes involved were identified through protein-protein interaction (PPI) networks leading to the identification of the circRNA-miRNA-hub gene subnetwork. In addition, function enrichment analysis was performed to detect the potential biological mechanism in which circRNA might be involved. Results. A total of 115 DEcircRNAs (differentially expressed circRNAs), 17 DEmiRNAs (differentially expressed microRNAs), and 790 DEmRNAs (differentially expressed mRNAs) were identified between CAD and control groups from microarray datasets. Functional enrichment analysis showed that DEmRNAs were significantly involved in inflammation-related pathways and ubiquitin-protein ligase binding. After identifying 20 DEcircRNA-DEmiRNA pairs and 561 DEmiRNA-DEmRNA pairs, we obtained a circRNA-miRNA-mRNA regulatory network. PPI network analysis showed that eight hub genes were closely related to CAD, leading to the identification of a circRNA-miRNA-hub gene subnetwork consisting of nine circRNAs (hsa_circ_0020275, hsa_circ_0020387, hsa_circ_0020417, hsa_circ_0045512, hsa_circ_0047336, hsa_circ_0069094, hsa_circ_0071326, hsa_circ_0071330, and hsa_circ_0085340), four miRNAs (hsa-miR-136-5p, hsa-miR-376c-3p, hsa-miR-411-5p, and hsa-miR-654-5p), and eight mRNAs (MKRN1, UBE2H, UBE2W, UBE2D1, UBE2F, BE2J1, ZNRF1, and SIAH2). In addition, we discovered these hub genes were enriched in the ubiquitin-mediated proteolysis pathway, suggesting circRNAs may be involved in the pathogenesis of CAD through this pathway. Conclusions. This study may deepen our understanding of the potential role of circRNA-miRNA-mRNA regulation network in CAD and suggest novel diagnostic biomarkers and therapeutic targets for CAD.

Calcineurin in the heart: New horizons for an old friend

Calcineurin, also known as PP2B or PPP3, is a member of the PPP family of protein phosphatases that also includes PP1 and PP2A. Together these three phosphatases carryout the majority of dephosphorylation events in the heart. Calcineurin is distinct in that it is activated by the binding of calcium/calmodulin (Ca2+/CaM) and therefore acts as a node for integrating Ca2+ signals with changes in phosphorylation, two fundamental intracellular signaling cascades. In the heart, calcineurin is primarily thought of in the context of pathological cardiac remodeling, acting through the Nuclear Factor of Activated T-cell (NFAT) family of transcription factors. However, calcineurin activity is also essential for normal heart development and homeostasis in the adult heart. Furthermore, it is clear that NFAT-driven changes in transcription are not the only relevant processes initiated by calcineurin in the setting of pathological remodeling. There is a growing appreciation for the diversity of calcineurin substrates that can impact cardiac function as well as the diversity of mechanisms for targeting calcineurin to specific sub-cellular domains in cardiomyocytes and other cardiac cell types. Here, we will review the basics of calcineurin structure, regulation, and function in the context of cardiac biology. Particular attention will be given to: the development of improved tools to identify and validate new calcineurin substrates; recent studies identifying new calcineurin isoforms with unique properties and targeting mechanisms; and the role of calcineurin in cardiac development and regeneration.

Mitochondrial PKA Is Neuroprotective in a Cell Culture Model of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive memory loss and cognitive decline. In hippocampal neurons, the pathological features of AD include the accumulation of extracellular amyloid-beta peptide (Aβ) accompanied by oxidative stress, mitochondrial dysfunction, and neuron loss. A decrease in neuroprotective Protein Kinase A (PKA) signaling contributes to mitochondrial fragmentation and neurodegeneration in AD. By associating with the protein scaffold Dual-Specificity Anchoring Protein 1 (D-AKAP1), PKA is targeted to mitochondria to promote mitochondrial fusion by phosphorylating the fission modulator dynamin-related protein 1 (Drp1). We hypothesized that (1) a decrease in the endogenous level of endogenous D-AKAP1 contributes to decreased PKA signaling in mitochondria and that (2) restoring PKA signaling in mitochondria can reverse neurodegeneration and mitochondrial fragmentation in neurons in AD models. Through immunohistochemistry, we showed that endogenous D-AKAP1, but not other mitochondrial proteins, is significantly reduced in primary neurons treated with Aβ42 peptide (10μM, 24 h), and in the hippocampus and cortex from asymptomatic and symptomatic AD mice (5X-FAD). Transiently expressing wild-type, but not a PKA-binding deficient mutant of D-AKAP1, was able to reduce mitochondrial fission, dendrite retraction, and apoptosis in primary neurons treated with Aβ42. Mechanistically, the protective effects of D-AKAP1/PKA are moderated through PKA-mediated phosphorylation of Drp1, as transiently expressing a PKA phosphomimetic mutant of Drp1 (Drp1-S656D) phenocopies D-AKAP1's ability to reduce Aβ42-mediated apoptosis and mitochondrial fission. Overall, our data suggest that a loss of D-AKAP1/PKA contributes to mitochondrial pathology and neurodegeneration in an in vitro cell culture model of AD.

Compartmentalized Signaling in Aging and Neurodegeneration

The cyclic AMP (cAMP) signalling cascade is necessary for cell homeostasis and plays important roles in many processes. This is particularly relevant during ageing and age-related diseases, where drastic changes, generally decreases, in cAMP levels have been associated with the progressive decline in overall cell function and, eventually, the loss of cellular integrity. The functional relevance of reduced cAMP is clearly supported by the finding that increases in cAMP levels can reverse some of the effects of ageing. Nevertheless, despite these observations, the molecular mechanisms underlying the dysregulation of cAMP signalling in ageing are not well understood. Compartmentalization is widely accepted as the modality through which cAMP achieves its functional specificity; therefore, it is important to understand whether and how this mechanism is affected during ageing and to define which is its contribution to this process. Several animal models demonstrate the importance of specific cAMP signalling components in ageing, however, how age-related changes in each of these elements affect the compartmentalization of the cAMP pathway is largely unknown. In this review, we explore the connection of single components of the cAMP signalling cascade to ageing and age-related diseases whilst elaborating the literature in the context of cAMP signalling compartmentalization.

Mitochondrial Kinases and the Role of Mitochondrial Protein Phosphorylation in Health and Disease

The major role of mitochondria is to provide cells with energy, but no less important are their roles in responding to various stress factors and the metabolic changes and pathological processes that might occur inside and outside the cells. The post-translational modification of proteins is a fast and efficient way for cells to adapt to ever changing conditions. Phosphorylation is a post-translational modification that signals these changes and propagates these signals throughout the whole cell, but it also changes the structure, function and interaction of individual proteins. In this review, we summarize the influence of kinases, the proteins responsible for phosphorylation, on mitochondrial biogenesis under various cellular conditions. We focus on their role in keeping mitochondria fully functional in healthy cells and also on the changes in mitochondrial structure and function that occur in pathological processes arising from the phosphorylation of mitochondrial proteins.

Subcellular Organization of the cAMP Signaling Pathway

The field of cAMP signaling is witnessing exciting developments with the recognition that cAMP is compartmentalized and that spatial regulation of cAMP is critical for faithful signal coding. This realization has changed our understanding of cAMP signaling from a model in which cAMP connects a receptor at the plasma membrane to an intracellular effector in a linear pathway to a model in which cAMP signals propagate within a complex network of alternative branches and the specific functional outcome strictly depends on local regulation of cAMP levels and on selective activation of a limited number of branches within the network. In this review, we cover some of the early studies and summarize more recent evidence supporting the model of compartmentalized cAMP signaling, and we discuss how this knowledge is starting to provide original mechanistic insight into cell physiology and a novel framework for the identification of disease mechanisms that potentially opens new avenues for therapeutic interventions. SIGNIFICANCE STATEMENT: cAMP mediates the intracellular response to multiple hormones and neurotransmitters. Signal fidelity and accurate coordination of a plethora of different cellular functions is achieved via organization of multiprotein signalosomes and cAMP compartmentalization in subcellular nanodomains. Defining the organization and regulation of subcellular cAMP nanocompartments is necessary if we want to understand the complex functional ramifications of pharmacological treatments that target G protein-coupled receptors and for generating a blueprint that can be used to develop precision medicine interventions.

The basics of mitochondrial cAMP signalling: Where, when, why

Cytosolic cAMP signalling in live cells has been extensively investigated in the past, while only in the last decade the existence of an intramitochondrial autonomous cAMP homeostatic system began to emerge. Thanks to the development of novel tools to investigate cAMP dynamics and cAMP/PKA-dependent phosphorylation within the matrix and in other mitochondrial compartments, it is now possible to address directly and in intact living cells a series of questions that until now could be addressed only by indirect approaches, in isolated organelles or through subcellular fractionation studies. In this contribution we discuss the mechanisms that regulate cAMP dynamics at the surface and inside mitochondria, and its crosstalk with organelle Ca²⁺ handling. We then address a series of still unsolved questions, such as the intramitochondrial localization of key elements of the cAMP signaling toolkit, e.g., adenylate cyclases, phosphodiesterases, protein kinase A (PKA) and Epac. Finally, we discuss the evidence for and against the existence of an intramitochondrial PKA pool and the functional role of cAMP increases within the organelle matrix.

The interaction of the mitochondrial protein importer TOMM34 with HSP70 is regulated by TOMM34 phosphorylation and binding to 14-3-3 adaptors

Translocase of outer mitochondrial membrane 34 (TOMM34) orchestrates heat shock protein 70 (HSP70)/HSP90-mediated transport of mitochondrial precursor proteins. Here, using in vitro phosphorylation and refolding assays, analytical size-exclusion chromatography, and hydrogen/deuterium exchange MS, we found that TOMM34 associates with 14-3-3 proteins after its phosphorylation by protein kinase A (PKA). PKA preferentially targeted two serine residues in TOMM34: Ser93 and Ser160, located in the tetratricopeptide repeat 1 (TPR1) domain and the interdomain linker, respectively. Both of these residues were necessary for efficient 14-3-3 protein binding. We determined that phosphorylation-induced structural changes in TOMM34 are further augmented by binding to 14-3-3, leading to destabilization of TOMM34's secondary structure. We also observed that this interaction with 14-3-3 occludes the TOMM34 interaction interface with ATP-bound HSP70 dimers, which leaves them intact and thereby eliminates an inhibitory effect of TOMM34 on HSP70-mediated refolding in vitro In contrast, we noted that TOMM34 in complex with 14-3-3 could bind HSP90. Both TOMM34 and 14-3-3 participated in cytosolic precursor protein transport mediated by the coordinated activities of HSP70 and HSP90. Our results provide important insights into how PKA-mediated phosphorylation and 14-3-3 binding regulate the availability of TOMM34 for its interaction with HSP70.

Loss of AKAP1 triggers Drp1 dephosphorylation-mediated mitochondrial fission and loss in retinal ganglion cells

Impairment of mitochondrial structure and function is strongly linked to glaucoma pathogenesis. Despite the widely appreciated disease relevance of mitochondrial dysfunction and loss, the molecular mechanisms underlying mitochondrial fragmentation and metabolic stress in glaucoma are poorly understood. We demonstrate here that glaucomatous retinal ganglion cells (RGCs) show loss of A-kinase anchoring protein 1 (AKAP1), activation of calcineurin (CaN) and reduction of dynamin-related protein 1 (Drp1) phosphorylation at serine 637 (Ser637). These findings suggest that AKAP1-mediated phosphorylation of Drp1 at Ser637 has a critical role in RGC survival in glaucomatous neurodegeneration. Male mice lacking AKAP1 show increases in CaN and total Drp1 levels, as well as a decrease in Drp1 phosphorylation at Ser637 in the retina. Ultrastructural analysis of mitochondria shows that loss of AKAP1 triggers mitochondrial fragmentation and loss, as well as mitophagosome formation in RGCs. Loss of AKAP1 deregulates oxidative phosphorylation (OXPHOS) complexes (Cxs) by increasing CxII and decreasing CxIII-V, leading to metabolic and oxidative stress. Also, loss of AKAP1 decreases Akt phosphorylation at Serine 473 (Ser473) and threonine 308 (Thr308) and activates the Bim/Bax signaling pathway in the retina. These results suggest that loss of AKAP1 has a critical role in RGC dysfunction by decreasing Drp1 phosphorylation at Ser637, deregulating OXPHOS, decreasing Akt phosphorylation at Ser473 and Thr308, and activating the Bim/Bax pathway in glaucomatous neurodegeneration. Thus, we propose that overexpression of AKAP1 or modulation of Drp1 phosphorylation at Ser637 are potential therapeutic strategies for neuroprotective intervention in glaucoma and other mitochondria-related optic neuropathies.

Bucladesine Attenuates Spatial Learning and Hippocampal Mitochondrial Impairments Induced by 3, 4-Methylenedioxymethamphetamine (MDMA)

Neurotoxic effects of systemic administration of 3, 4- methylenedioxymethamphetamine (MDMA) has been attributed to MDMA and its metabolites. However, the role of the parent compound in MDMA-induced mitochondrial and memory impairment has not yet been investigated. Moreover, it is not yet studied that analogs of 3', 5'-cyclic adenosine monophosphate (cAMP) could decrease these neurotoxic effects of MDMA. We wished to investigate the effects of the central administration of MDMA on spatial memory and mitochondrial function as well as the effects of bucladesine, a membrane-permeable analog of cAMP, on these effects of MDMA. We assessed the effects of pre-training bilateral intrahippocampal infusion of MDMA (0.01, 0.1, 0.5, and 1 μg/side), bucladesine (10 and 100 μM) or combination of them on spatial memory, and different parameters of hippocampal mitochondrial function including the level of reactive oxygen species (ROS) production, mitochondrial membrane potential (MMP), mitochondrial swelling, mitochondrial outer membrane damage, the amount of cytochrome c release as well as hippocampal ADP/ATP ratio. The results showed that MDMA caused spatial memory impairments as well as mitochondrial dysfunction as evidenced by the marked increase in hippocampal ADP/ATP ratio, ROS level, the collapse of MMP, mitochondrial swelling, and mitochondrial outer membrane damage leading to cytochrome c release from the mitochondria. The current study also found that bucladesine markedly reduced the destructive effects of MDMA. These results provide evidence of the role of the parent compound (MDMA) in MDMA-induced memory impairments through mitochondrial dysfunction. This study highlights the role of cAMP/PKA signaling in MDMA-induced memory and mitochondrial defects.

The role of A-kinase anchoring proteins in cardiac oxidative stress

Cardiac stress initiates a pathological remodeling process that is associated with cardiomyocyte loss and fibrosis that ultimately leads to heart failure. In the injured heart, a pathologically elevated synthesis of reactive oxygen species (ROS) is the main driver of oxidative stress and consequent cardiomyocyte dysfunction and death. In this context, the cAMP-dependent protein kinase (PKA) plays a central role in regulating signaling pathways that protect the heart against ROS-induced cardiac damage. In cardiac cells, spatiotemporal regulation of PKA activity is controlled by A-kinase anchoring proteins (AKAPs). This family of scaffolding proteins tether PKA and other transduction enzymes at subcellular microdomains where they can co-ordinate cellular responses regulating oxidative stress. In this review, we will discuss recent literature illustrating the role of PKA and AKAPs in modulating the detrimental impact of ROS production on cardiac function.

A-Kinase Anchoring Protein 1: Emerging Roles in Regulating Mitochondrial Form and Function in Health and Disease

Best known as the powerhouse of the cell, mitochondria have many other important functions such as buffering intracellular calcium and reactive oxygen species levels, initiating apoptosis and supporting cell proliferation and survival. Mitochondria are also dynamic organelles that are constantly undergoing fission and fusion to meet specific functional needs. These processes and functions are regulated by intracellular signaling at the mitochondria. A-kinase anchoring protein 1 (AKAP1) is a scaffold protein that recruits protein kinase A (PKA), other signaling proteins, as well as RNA to the outer mitochondrial membrane. Hence, AKAP1 can be considered a mitochondrial signaling hub. In this review, we discuss what is currently known about AKAP1's function in health and diseases. We focus on the recent literature on AKAP1's roles in metabolic homeostasis, cancer and cardiovascular and neurodegenerative diseases. In healthy tissues, AKAP1 has been shown to be important for driving mitochondrial respiration during exercise and for mitochondrial DNA replication and quality control. Several recent in vivo studies using AKAP1 knockout mice have elucidated the role of AKAP1 in supporting cardiovascular, lung and neuronal cell survival in the stressful post-ischemic environment. In addition, we discuss the unique involvement of AKAP1 in cancer tumor growth, metastasis and resistance to chemotherapy. Collectively, the data indicate that AKAP1 promotes cell survival throug regulating mitochondrial form and function. Lastly, we discuss the potential of targeting of AKAP1 for therapy of various disorders.

Regulation of mitochondrial metabolism in murine skeletal muscle by the medium-chain fatty acid receptor Gpr84

Fatty acid receptors have been recognized as important players in glycaemic control. This study is the first to describe a role for the medium-chain fatty acid (MCFA) receptor G-protein-coupled receptor (Gpr) 84 in skeletal muscle mitochondrial function and insulin secretion. We are able to show that Gpr84 is highly expressed in skeletal muscle and adipose tissue. Mice with global deletion of Gpr84 [Gpr84 knockout (KO)] exhibit a mild impairment in glucose tolerance when fed a MCFA-enriched diet. Studies in mice and pancreatic islets suggest that glucose intolerance is accompanied by a defect in insulin secretion. MCFA-fed KO mice also exhibit a significant impairment in the intrinsic respiratory capacity of their skeletal muscle mitochondria, but at the same time also exhibit a substantial increase in mitochondrial content. Changes in canonical pathways of mitochondrial biogenesis and turnover are unable to explain these mitochondrial differences. Our results show that Gpr84 plays a crucial role in regulating mitochondrial function and quality control.-Montgomery, M. K., Osborne, B., Brandon, A. E., O'Reilly, L., Fiveash, C. E., Brown, S. H. J., Wilkins, B. P., Samsudeen, A., Yu, J., Devanapalli, B., Hertzog, A., Tolun, A. A., Kavanagh, T., Cooper, A. A., Mitchell, T. W., Biden, T. J., Smith, N. J., Cooney, G. J., Turner, N. Regulation of mitochondrial metabolism in murine skeletal muscle by the medium-chain fatty acid receptor Gpr84.

Repeated treatments with the D1 dopamine receptor agonist SKF-38393 modulate cell viability via sustained ERK-Bad-Bax activation in dopaminergic neuronal cells

The D₁ dopamine receptor agonist, SKF-38393, induces cytotoxicity in striatal dopaminergic neurons via an extracellular signal-regulated kinase (ERK) signaling cascade. However, the underlying mechanism remains unclear. We hypothesized that repeated activation of dopaminergic receptors by agonists could lead to neuronal cell death. This study investigated the effects of SKF-38393 on dopaminergic neuronal cell death in a 6-hydroxydopamine-lesioned rat model of Parkinson's disease (PD) and PC12 cells. In the PD model, SKF-38393 administration (3 and 10 mg/kg per day, s.c.) for 8 weeks significantly increased the number of tyrosine hydroxylase-immunopositive neuronal cells in nigrostriatal regions. SKF-38393 administration for 8 weeks induced phosphorylation of sustained ERK1/2 and Bad (Bcl-2-associated death promoter) at Ser155 (BadSer155), and augmented Bax (Bcl-2-associated X protein) expression. However, SKF-38393 only increased Bad phosphorylation at Ser112 (BadSer112) when administered for 4 weeks. In PC12 cells, toxic levels of SKF-38393 (20 and 50 μM) rapidly induced formation of neurite-like processes, but not in the presence of an adenylyl cyclase inhibitor (MDL-12330 A). SKF-38393 (20 and 50 μM) induced sustained ERK1/2 and BadSer155 phosphorylation as well as caspase-3 activation. At a non-toxic level (5 μM), SKF-38393 produced only transient ERK1/2 and BadSer112 phosphorylation. Repeated treatments with SKF-38393 (5 μM) for 1-3 days activated BadSer112. Repeated treatments for 4-7 days induced sustained ERK1/2 and BadSer155 phosphorylation as well as Bax and caspase-3 activation. These results suggest that SKF-38393 induces neurotoxicity by activation of the sustained ERK-Bad-Bax system. These findings contribute to an understanding of the adverse effects of D₁ dopamine receptor agonists in patients with PD.

Neuroprotective Mitochondrial Remodeling by AKAP121/PKA Protects HT22 Cell from Glutamate-Induced Oxidative Stress

Protein kinase A (PKA) is a ser/thr kinase that is critical for maintaining essential neuronal functions including mitochondrial homeostasis, bioenergetics, neuronal development, and neurotransmission. The endogenous pool of PKA is targeted to the mitochondrion by forming a complex with the mitochondrial scaffold A-kinase anchoring protein 121 (AKAP121). Enhanced PKA signaling via AKAP121 leads to PKA-mediated phosphorylation of the fission modulator Drp1, leading to enhanced mitochondrial networks and thereby blocking apoptosis against different toxic insults. In this study, we show for the first time that AKAP121/PKA confers neuroprotection in an in vitro model of oxidative stress induced by exposure to excess glutamate. Unexpectedly, treating mouse hippocampal progenitor neuronal HT22 cells with an acute dose or chronic exposure of glutamate robustly elevates PKA signaling, a beneficial compensatory response that is phenocopied in HT22 cells conditioned to thrive in the presence of excess glutamate but not in parental HT22 cells. Secondly, redirecting the endogenous pool of PKA by transiently transfecting AKAP121 or transfecting a constitutively active mutant of PKA targeted to the mitochondrion (OMM-PKA) or of an isoform of AKAP121 that lacks the KH and Tudor domains (S-AKAP84) are sufficient to significantly block cell death induced by glutamate toxicity but not in an oxygen deprivation/reperfusion model. Conversely, transient transfection of HT22 neuronal cells with a PKA-binding-deficient mutant of AKAP121 is unable to protect against oxidative stress induced by glutamate toxicity suggesting that the catalytic activity of PKA is required for AKAP121's protective effects. Mechanistically, AKAP121 promotes neuroprotection by enhancing PKA-mediated phosphorylation of Drp1 to increase mitochondrial fusion, elevates ATP levels, and elicits an increase in the levels of antioxidants GSH and superoxide dismutase 2 leading to a reduction in the level of mitochondrial superoxide. Overall, our data supports AKAP121/PKA as a new molecular target that confers neuroprotection against glutamate toxicity by phosphorylating Drp1, to stabilize mitochondrial networks and mitochondrial function and to elicit antioxidant responses.

AKAP1 Protects from Cerebral Ischemic Stroke by Inhibiting Drp1-Dependent Mitochondrial Fission

Mitochondrial fission and fusion impact numerous cellular functions and neurons are particularly sensitive to perturbations in mitochondrial dynamics. Here we describe that male mice lacking the mitochondrial A-kinase anchoring protein 1 (AKAP1) exhibit increased sensitivity in the transient middle cerebral artery occlusion model of focal ischemia. At the ultrastructural level, AKAP1^-/- mice have smaller mitochondria and increased contacts between mitochondria and the endoplasmic reticulum in the brain. Mechanistically, deletion of AKAP1 dysregulates complex II of the electron transport chain, increases superoxide production, and impairs Ca²⁺ homeostasis in neurons subjected to excitotoxic glutamate. Ca²⁺ deregulation in neurons lacking AKAP1 can be attributed to loss of inhibitory phosphorylation of the mitochondrial fission enzyme dynamin-related protein 1 (Drp1) at the protein kinase A (PKA) site Ser637. Our results indicate that inhibition of Drp1-dependent mitochondrial fission by the outer mitochondrial AKAP1/PKA complex protects neurons from ischemic stroke by maintaining respiratory chain activity, inhibiting superoxide production, and delaying Ca²⁺ deregulation. They also provide the first genetic evidence that Drp1 inhibition may be of therapeutic relevance for the treatment of stroke and neurodegeneration.SIGNIFICANCE STATEMENT Previous work suggests that activation of dynamin-related protein 1 (Drp1) and mitochondrial fission contribute to ischemic injury in the brain. However, the specificity and efficacy of the pharmacological Drp1 inhibitor mdivi-1 that was used has now been discredited by several high-profile studies. Our report is timely and highly impactful because it provides the first evidence that genetic disinhibition of Drp1 via knock-out of the mitochondrial protein kinase A (PKA) scaffold AKAP1 exacerbates stroke injury in mice. Mechanistically, we show that electron transport deficiency, increased superoxide production, and Ca²⁺ overload result from genetic disinhibition of Drp1. In summary, our work settles current controversies regarding the role of mitochondrial fission in neuronal injury, provides mechanisms, and suggests that fission inhibitors hold promise as future therapeutic agents.

Mitochondrial cAMP-PKA signaling: What do we really know?

Mitochondria are key organelles for cellular homeostasis. They generate the most part of ATP that is used by cells through oxidative phosphorylation. They also produce reactive oxygen species, neurotransmitters and other signaling molecules. They are important for calcium homeostasis and apoptosis. Considering the role of this organelle, it is not surprising that most mitochondrial dysfunctions are linked to the development of pathologies. Various mechanisms adjust mitochondrial activity according to physiological needs. The cAMP-PKA signaling emerged in recent years as a direct and powerful mean to regulate mitochondrial functions. Multiple evidence demonstrates that such pathway can be triggered from cytosol or directly within mitochondria. Notably, specific anchor proteins target PKA to mitochondria whereas enzymes necessary for generation and degradation of cAMP are found directly in these organelles. Mitochondrial PKA targets proteins localized in different compartments of mitochondria, and related to various functions. Alterations of mitochondrial cAMP-PKA signaling affect the development of several physiopathological conditions, including neurodegenerative diseases. It is however difficult to discriminate between the effects of cAMP-PKA signaling triggered from cytosol or directly in mitochondria. The specific roles of PKA localized in different mitochondrial compartments are also not completely understood. The aim of this work is to review the role of cAMP-PKA signaling in mitochondrial (patho)physiology.

The role of compartmentalized signaling pathways in the control of mitochondrial activities in cancer cells

Mitochondria are the powerhouse organelles present in all eukaryotic cells. They play a fundamental role in cell respiration, survival and metabolism. Stimulation of G-protein coupled receptors (GPCRs) by dedicated ligands and consequent activation of the cAMP·PKA pathway finely couple energy production and metabolism to cell growth and survival. Compartmentalization of PKA signaling at mitochondria by A-Kinase Anchor Proteins (AKAPs) ensures efficient transduction of signals generated at the cell membrane to the organelles, controlling important aspects of mitochondrial biology. Emerging evidence implicates mitochondria as essential bioenergetic elements of cancer cells that promote and support tumor growth and metastasis. In this context, mitochondria provide the building blocks for cellular organelles, cytoskeleton and membranes, and supply all the metabolic needs for the expansion and dissemination of actively replicating cancer cells. Functional interference with mitochondrial activity deeply impacts on cancer cell survival and proliferation. Therefore, mitochondria represent valuable targets of novel therapeutic approaches for the treatment of cancer patients. Understanding the biology of mitochondria, uncovering the molecular mechanisms regulating mitochondrial activity andmapping the relevant metabolic and signaling networks operating in cancer cells will undoubtly contribute to create a molecular platform to be used for the treatment of proliferative disorders. Here, we will highlight the emerging roles of signaling pathways acting downstream to GPCRs and their intersection with the ubiquitin proteasome system in the control of mitochondrial activity in different aspects of cancer cell biology.

Potential for therapeutic targeting of AKAP signaling complexes in nervous system disorders

A common feature of neurological and neuropsychiatric disorders is a breakdown in the integrity of intracellular signal transduction pathways. Dysregulation of ion channels and receptors in the cell membrane and the enzymatic mediators that link them to intracellular effectors can lead to synaptic dysfunction and neuronal death. However, therapeutic targeting of these ubiquitous signaling elements can lead to off-target side effects due to their widespread expression in multiple systems of the body. A-kinase anchoring proteins (AKAPs) are multivalent scaffolding proteins that compartmentalize a diverse range of receptor and effector proteins to streamline signaling within nanodomain signalosomes. A number of essential neurological processes are known to critically depend on AKAP-directed signaling and an understanding of the role AKAPs play in nervous system disorders has emerged in recent years. Selective targeting of AKAP protein-protein interactions may be a means to uncouple pathologically active signaling pathways in neurological disorders with a greater degree of specificity. In this review we will discuss the role of AKAPs in both regulating normal nervous system function and dysfunction associated with disease, and the potential for therapeutic targeting of AKAP signaling complexes.

Components of the mitochondrial cAMP signalosome

3'-5'-Cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) signalling is activated by different extracellular stimuli and mediates many diverse processes within the same cell. It is now well established that in order to translate into the appropriate cellular function multiple extracellular inputs, which may act simultaneously on the same cell, the cAMP/PKA signalling pathway is compartmentalised. Multimolecular complexes are organised at specific subcellular sites to generate spatially confined signalosomes, which include effectors, modulators and targets of the pathway. In recent years, it has become evident that mitochondria represent sites of compartmentalised cAMP signalling. However, the exact location and the molecular composition of distinct mitochondria signalosomes and their function remain largely unknown. In this review, we focus on individual components of the cAMP/PKA signalling pathway at distinct mitochondria subdomains represented by the outer and inner mitochondrial membranes, the intermembrane space and the matrix, highlighting some of the questions that remain unanswered.

PINK1 regulates mitochondrial trafficking in dendrites of cortical neurons through mitochondrial PKA

Mitochondrial Protein Kinase A (PKA) and PTEN-induced kinase 1 (PINK1), which is linked to Parkinson's disease, are two neuroprotective serine/threonine kinases that regulate dendrite remodeling and mitochondrial function. We have previously shown that PINK1 regulates dendrite morphology by enhancing PKA activity. Here, we show the molecular mechanisms by which PINK1 and PKA in the mitochondrion interact to regulate dendrite remodeling, mitochondrial morphology, content, and trafficking in dendrites. PINK1-deficient cortical neurons exhibit impaired mitochondrial trafficking, reduced mitochondrial content, fragmented mitochondria, and a reduction in dendrite outgrowth compared to wild-type neurons. Transient expression of wild-type, but not a PKA-binding-deficient mutant of the PKA-mitochondrial scaffold dual-specificity A Kinase Anchoring Protein 1 (D-AKAP1), restores mitochondrial trafficking, morphology, and content in dendrites of PINK1-deficient cortical neurons suggesting that recruiting PKA to the mitochondrion reverses mitochondrial pathology in dendrites induced by loss of PINK1. Mechanistically, full-length and cleaved forms of PINK1 increase the binding of the regulatory subunit β of PKA (PKA/RIIβ) to D-AKAP1 to enhance the autocatalytic-mediated phosphorylation of PKA/RIIβ and PKA activity. D-AKAP1/PKA governs mitochondrial trafficking in dendrites via the Miro-2/TRAK2 complex and by increasing the phosphorylation of Miro-2. Our study identifies a new role of D-AKAP1 in regulating mitochondrial trafficking through Miro-2, and supports a model in which PINK1 and mitochondrial PKA participate in a similar neuroprotective signaling pathway to maintain dendrite connectivity.

Mitochondrial AKAP1 supports mTOR pathway and tumor growth

Mitochondria are the powerhouses of energy production and the sites where metabolic pathway and survival signals integrate and focus, promoting adaptive responses to hormone stimulation and nutrient availability. Increasing evidence suggests that mitochondrial bioenergetics, metabolism and signaling are linked to tumorigenesis. AKAP1 scaffolding protein integrates cAMP and src signaling on mitochondria, regulating organelle biogenesis, oxidative metabolism and cell survival. Here, we provide evidence that AKAP1 is a transcriptional target of Myc and supports the growth of cancer cells. We identify Sestrin2, a leucine sensor and inhibitor of the mammalian target of rapamycin (mTOR), as a novel component of the complex assembled by AKAP1 on mitochondria. Downregulation of AKAP1 impaired mTOR pathway and inhibited glioblastoma growth. Both effects were reversed by concomitant depletion of AKAP1 and sestrin2. High levels of AKAP1 were found in a wide variety of high-grade cancer tissues. In lung cancer, AKAP1 expression correlates with high levels of Myc, mTOR phosphorylation and reduced patient survival. Collectively, these data disclose a previously unrecognized role of AKAP1 in mTOR pathway regulation and cancer growth. AKAP1/mTOR signal integration on mitochondria may provide a new target for cancer therapy.

Pseudoscaffolds and anchoring proteins: the difference is in the details

Pseudokinases and pseudophosphatases possess the ability to bind substrates without catalyzing their modification, thereby providing a mechanism to recruit potential phosphotargets away from active enzymes. Since many of these pseudoenzymes possess other characteristics such as localization signals, separate catalytic sites, and protein-protein interaction domains, they have the capacity to influence signaling dynamics in local environments. In a similar manner, the targeting of signaling enzymes to subcellular locations by A-kinase-anchoring proteins (AKAPs) allows for precise and local control of second messenger signaling events. Here, we will discuss how pseudoenzymes form 'pseudoscaffolds' and compare and contrast this compartment-specific regulatory role with the signal organization properties of AKAPs. The mitochondria will be the focus of this review, as they are dynamic organelles that influence a broad range of cellular processes such as metabolism, ATP synthesis, and apoptosis.

Inhibition of Intracellular Type 10 Adenylyl Cyclase Protects Cortical Neurons Against Reperfusion-Induced Mitochondrial Injury and Apoptosis

Mitochondrial injury significantly contributes to the neuronal death under cerebral ischemia and reperfusion. Within several signaling pathways, cyclic adenosine monophosphate (cAMP) signaling plays a substantial role in mitochondrial injury and cell death. Traditionally, the source of cellular cAMP has been attributed to the membrane-bound adenylyl cyclase, whereas the role of the intracellular localized type 10 soluble adenylyl cyclase (sAC) in neuronal pathology has not been considered. Since neurons express an active form of sAC, we aimed to investigate the role of sAC in reperfusion-induced neuronal apoptosis. For this purpose, the in vitro model of oxygen/glucose deprivation (simulated ischemia, 1 h), followed by recovery (simulated reperfusion, 12 h) in rat embryonic neurons, was applied. Although ischemia alone had no significant effect on apoptosis, reperfusion led to an activation of the mitochondrial pathway of apoptosis, hallmarked by mitochondrial depolarization, cytochrome c release, and mitochondrial ROS formation. These effects were accompanied by significantly augmented sAC expression and increased cellular cAMP content during reperfusion. Pharmacological suppression of sAC during reperfusion reduced cellular cAMP and ameliorated reperfusion-induced mitochondrial apoptosis and ROS formation. Similarly, sAC knockdown prevented neuronal death. Further analysis revealed a role of protein kinase A (PKA), a major downstream target of sAC, in reperfusion-induced neuronal apoptosis and ROS formation. In conclusion, the results show a causal role of intracellular, sAC-dependent cAMP signaling in reperfusion-induced mitochondrial injury and apoptosis in neurons. The protective effect of sAC inhibition during the reperfusion phase provides a basis for the development of new strategies to prevent the reperfusion-induced neuronal injury.

Mitochondrial Function, Biology, and Role in Disease: A Scientific Statement From the American Heart Association

Cardiovascular disease is a major leading cause of morbidity and mortality in the United States and elsewhere. Alterations in mitochondrial function are increasingly being recognized as a contributing factor in myocardial infarction and in patients presenting with cardiomyopathy. Recent understanding of the complex interaction of the mitochondria in regulating metabolism and cell death can provide novel insight and therapeutic targets. The purpose of this statement is to better define the potential role of mitochondria in the genesis of cardiovascular disease such as ischemia and heart failure. To accomplish this, we will define the key mitochondrial processes that play a role in cardiovascular disease that are potential targets for novel therapeutic interventions. This is an exciting time in mitochondrial research. The past decade has provided novel insight into the role of mitochondria function and their importance in complex diseases. This statement will define the key roles that mitochondria play in cardiovascular physiology and disease and provide insight into how mitochondrial defects can contribute to cardiovascular disease; it will also discuss potential biomarkers of mitochondrial disease and suggest potential novel therapeutic approaches.

Phosphodiesterase-4 inhibition as a therapeutic strategy for metabolic disorders

Phosphodiesterase-4 (PDE4) hydrolyses cyclic adenosine monophosphate (cAMP), a crucial secondary messenger for cellular adaptation to diverse external stimuli. The activity of PDE4 is tightly controlled by post-translational regulation, structure-based auto-regulation and locus specific 'compartmentalization' of PDE4 with its interactive proteins (signalsomes). Through these mechanisms, PDE4 regulates cAMP levels and shapes the cAMP signalling, directing signals from the diverse external stimuli to distinct microenvironments exquisitely. Derangement of the PDE4-cAMP signalling represents a pathophysiologically relevant pathway in metabolic disorders as demonstrated through a critical role in the processes including inflammation, disordered glucose and lipid metabolism, hepatic steatosis, abnormal lipolysis, suppressed thermogenic function and deranged neuroendocrine functions. A limited number of PDE4 inhibitors are currently undergoing clinical evaluation for treating disorders such as type 2 diabetes and non-alcoholic steatohepatitis. The discovery of novel PDE4 allosteric inhibitors and signalsome-based strategies targeting individual PDE4 variants may allow PDE4 isoform selective inhibition, which may offer safer strategies for chronic treatment of metabolic disorders.

Regulation of mitochondrial functions by protein phosphorylation and dephosphorylation

The mitochondria are double membrane-bound organelles found in most eukaryotic cells. They generate most of the cell’s energy supply of adenosine triphosphate (ATP). Protein phosphorylation and dephosphorylation are critical mechanisms in the regulation of cell signaling networks and are essential for almost all the cellular functions. For many decades, mitochondria were considered autonomous organelles merely functioning to generate energy for cells to survive and proliferate, and were thought to be independent of the cellular signaling networks. Consequently, phosphorylation and dephosphorylation processes of mitochondrial kinases and phosphatases were largely neglected. However, evidence accumulated in recent years on mitochondria-localized kinases/phosphatases has changed this longstanding view. Mitochondria are increasingly recognized as a hub for cell signaling, and many kinases and phosphatases have been reported to localize in mitochondria and play important functions. However, the strength of the evidence on mitochondrial localization and the activities of the reported kinases and phosphatases vary greatly, and the detailed mechanisms on how these kinases/phosphatases translocate to mitochondria, their subsequent function, and the physiological and pathological implications of their localization are still poorly understood. Here, we provide an updated perspective on the recent advancement in this area, with an emphasis on the implications of mitochondrial kinases/phosphatases in cancer and several other diseases.

Role of protein kinase A in regulating mitochondrial function and neuronal development: implications to neurodegenerative diseases

In neurons, enhanced protein kinase A (PKA) signaling elevates synaptic plasticity, promotes neuronal development, and increases dopamine synthesis. By contrast, a decline in PKA signaling contributes to the etiology of several brain degenerative diseases, including Alzheimer's disease and Parkinson's disease, suggesting that PKA predominantly plays a neuroprotective role. A-kinase anchoring proteins (AKAPs) are large multidomain scaffold proteins that target PKA and other signaling molecules to distinct subcellular sites to strategically localize PKA signaling at dendrites, dendritic spines, cytosol, and axons. PKA can be recruited to the outer mitochondrial membrane by associating with three different AKAPs to regulate mitochondrial dynamics, structure, mitochondrial respiration, trafficking, dendrite morphology, and neuronal survival. In this review, we survey the myriad of essential neuronal functions modulated by PKA but place a special emphasis on mitochondrially localized PKA. Finally, we offer an updated overview of how loss of PKA signaling contributes to the etiology of several brain degenerative diseases.

cAMP signaling in subcellular compartments

In the complex microcosm of a cell, information security and its faithful transmission are critical for maintaining internal stability. To achieve a coordinated response of all its parts to any stimulus the cell must protect the information received from potentially confounding signals. Physical segregation of the information transmission chain ensures that only the entities able to perform the encoded task have access to the relevant information. The cAMP intracellular signaling pathway is an important system for signal transmission responsible for the ancestral 'flight or fight' response and involved in the control of critical functions including frequency and strength of heart contraction, energy metabolism and gene transcription. It is becoming increasingly apparent that the cAMP signaling pathway uses compartmentalization as a strategy for coordinating the large number of key cellular functions under its control. Spatial confinement allows the formation of cAMP signaling "hot spots" at discrete subcellular domains in response to specific stimuli, bringing the information in proximity to the relevant effectors and their recipients, thus achieving specificity of action. In this report we discuss how the different constituents of the cAMP pathway are targeted and participate in the formation of cAMP compartmentalized signaling events. We illustrate a few examples of localized cAMP signaling, with a particular focus on the nucleus, the sarcoplasmic reticulum and the mitochondria. Finally, we discuss the therapeutic potential of interventions designed to perturb specific cAMP cascades locally.

Considering effects of nanosecond pulsed electric fields on proteins

Most, if not all, effects of intense, pulsed electric fields are analyzed in terms of electrical charging of plasma membranes and/or subcellular membranes. However, not all cell responses from nanosecond pulsed electric fields (nsPEFs) are fully explained by poration of cell membranes. Observations that nsPEFs induce a Ca2-dependent dissipation of the mitochondria membrane potential (ΔΨm), which is enhanced when high frequency components are present in fast rise-fall waveforms, are not compatible with a poration event. Ca(2+) is shown to have little or no effect on propidium iodide uptake as a measure of plasma membrane poration and consequently intracellular membranes. Since most if not all Ca(2+)-regulated events are mediated by proteins, actions of nsPEFs on a protein(s) that regulate and/or affect the mitochondria membrane potential are possible. To show that nsPEFs can directly affect proteins, nsPEFs non-thermally inactivated the catalytic (phosphotransferase) activity of the catalytic subunit of the cAMP-dependent protein kinase, which is the prototype of the protein kinase superfamily that share a common catalytic mechanism and whose functions are highly dependent on their structure. These studies present indirect and direct evidences that nsPEFs can affect proteins and their functions, at least in part, by affecting their structure.

AKAP350C targets to mitochondria via a novel amphipathic alpha helical domain

Mitochondria regulate metabolism and homeostasis within cells. Mitochondria are also very dynamic organelles, constantly undergoing fission and fusion. The importance of maintaining proper mitochondrial dynamics is evident in the various diseases associated with defects in these processes. Protein kinase A (PKA) is a key regulator of mitochondrial dynamics. PKA is spatially regulated by A-Kinase Anchoring Proteins (AKAPs). We completed cloning of a novel AKAP350 isoform, AKAP350C. Immunostaining for endogenous AKAP350C showed localization to mitochondria. The carboxyl-terminal 54-amino acid sequence unique to AKAP350C contains a novel amphipathic alpha helical mitochondrial-targeting domain. AKAP350C co-localizes with Mff (mitochondrial fission protein) and mitofusins 1 and 2 (mitochondrial fusion proteins), and likely regulates mitochondrial dynamics by scaffolding PKA and mitochondrial fission and fusion proteins.

Stable over-expression of the 2-oxoglutarate carrier enhances neuronal cell resistance to oxidative stress via Bcl-2-dependent mitochondrial GSH transport

Mitochondrial glutathione (GSH) is a key endogenous antioxidant and its maintenance is critical for cell survival. Here, we generated stable NSC34 motor neuron-like cell lines over-expressing the mitochondrial GSH transporter, the 2-oxoglutarate carrier (OGC), to further elucidate the importance of mitochondrial GSH transport in determining neuronal resistance to oxidative stress. Two stable OGC cell lines displayed specific increases in mitochondrial GSH content and resistance to oxidative and nitrosative stressors, but not staurosporine. Inhibition of transport through OGC reduced levels of mitochondrial GSH and resensitized the stable cell lines to oxidative stress. The stable OGC cell lines displayed significant up-regulation of the anti-apoptotic protein, B cell lymphoma 2 (Bcl-2). This result was reproduced in parental NSC34 cells by chronic treatment with GSH monoethylester, which specifically increased mitochondrial GSH levels. Knockdown of Bcl-2 expression decreased mitochondrial GSH and resensitized the stable OGC cells to oxidative stress. Finally, endogenous OGC was co-immunoprecipitated with Bcl-2 from rat brain lysates in a GSH-dependent manner. These data are the first to show that increased mitochondrial GSH transport is sufficient to enhance neuronal resistance to oxidative stress. Moreover, sustained and specific enhancement of mitochondrial GSH leads to increased Bcl-2 expression, a required mechanism for the maintenance of increased mitochondrial GSH levels. Stable over-expression of the 2-oxoglutarate carrier (OGC) in a motor neuronal cell line induced a specific increase in mitochondrial GSH and markedly enhanced resistance to oxidative stress. Over-expression of OGC also induced Bcl-2 expression which was owing to the specific increase in mitochondrial GSH. Intriguingly, enhanced expression of Bcl-2 was required to sustain OGC-dependent GSH transport into the mitochondria. Thus, OGC and Bcl-2 work in a concerted manner to maintain the mitochondrial GSH pool which is crucial for neuronal survival.

Mitochondria: a kinase anchoring protein 1, a signaling platform for mitochondrial form and function

Mitochondria are best known for their role as cellular power plants, but they also serve as signaling hubs, regulating cellular proliferation, differentiation, and survival. A kinase anchoring protein 1 (AKAP1) is a scaffold protein that recruits protein kinase A (PKA) and other signaling proteins, as well as RNA, to the outer mitochondrial membrane. AKAP1 thereby integrates several second messenger cascades to modulate mitochondrial function and associated physiological and pathophysiological outcomes. Here, we review what is currently known about AKAP1's macromolecular interactions in health and disease states, including obesity. We also discuss dynamin-related protein 1 (Drp1), the enzyme that catalyzes mitochondrial fission, as one of the key substrates of the PKA/AKAP1 signaling complex in neurons. Recent evidence suggests that AKAP1 has critical roles in neuronal development and survival, which are mediated by inhibitory phosphorylation of Drp1 and maintenance of mitochondrial integrity.

NCX3 regulates mitochondrial Ca(2+) handling through the AKAP121-anchored signaling complex and prevents hypoxia-induced neuronal death

The mitochondrial influx and efflux of Ca(2+) play a relevant role in cytosolic and mitochondrial Ca(2+) homeostasis, and contribute to the regulation of mitochondrial functions in neurons. The mitochondrial Na(+)/Ca(2+) exchanger, which was first postulated in 1974, has been primarily investigated only from a functional point of view, and its identity and localization in the mitochondria have been a matter of debate over the past three decades. Recently, a Li(+)-dependent Na(+)/Ca(2+) exchanger extruding Ca(2+) from the matrix has been found in the inner mitochondrial membrane of neuronal cells. However, evidence has been provided that the outer membrane is impermeable to Ca(2+) efflux into the cytoplasm. In this study, we demonstrate for the first time that the nuclear-encoded NCX3 isoform (1) is located on the outer mitochondrial membrane (OMM) of neurons; (2) colocalizes and immunoprecipitates with AKAP121 (also known as AKAP1), a member of the protein kinase A anchoring proteins (AKAPs) present on the outer membrane; (3) extrudes Ca(2+) from mitochondria through AKAP121 interaction in a PKA-mediated manner, both under normoxia and hypoxia; and (4) improves cell survival when it works in the Ca(2+) efflux mode at the level of the OMM. Collectively, these results suggest that, in neurons, NCX3 regulates mitochondrial Ca(2+) handling from the OMM through an AKAP121-anchored signaling complex, thus promoting cell survival during hypoxia.

The roles of cyclic AMP-ERK-Bad signaling pathways on 6-hydroxydopamine-induced cell survival and death in PC12 cells

The roles of cyclic AMP (cAMP)-ERK1/2-Bad signaling pathways in 6-hydroxydopamine (6-OHDA)-induced cell survival and death were investigated. In PC12 cells, 6-OHDA (10-100μM) concentration-dependently increased the intracellular levels of cAMP mediated by the Ca(2+)-CaMKII-adenylyl cyclase system. 6-OHDA at the non-toxic level (10μM) induced transient ERK1/2 phosphorylation and BadSer112 phosphorylation, which maintained cell survival. In contrast, the high levels of cAMP induced by toxic levels (50 and 100μM) of 6-OHDA induced sustained ERK1/2 phosphorylaton and BadSer155 phosphorylation. The cells then moved to cell death process through Bcl2 phosphorylation and caspase-3 activation. BadSer155 phosphorylation by 6-OHDA was inhibited by PKA (H89) and MEK (U0126) inhibitors, indicating that it was mediated via the cAMP-PKA-sustained ERK1/2 system. In SK-N-BE(2)C cells, the non-toxic level of 6-OHDA also showed transient ERK1/2 phosphorylation and BadSer112 phosphorylation, and toxic levels of 6-OHDA exhibited sustained ERK1/2 phosphorylation and BadSer155 phosphorylation. These results suggest that ERK1/2 phosphorylation by 6-OHDA shows biphasic functions on cell survival and death in PC12 cells. It is, therefore, proposed that the cAMP-ERK1/2-Bad signaling pathways incurred by toxic levels of 6-OHDA play a role in dopamine neuron death of animal models of Parkinson's disease.

Role of yessotoxin in calcium and cAMP-crosstalks in primary and K-562 human lymphocytes: the effect is mediated by anchor kinase A mitochondrial proteins

Yessotoxin (YTX) is a marine polyether toxin previously described as a phosphodiesterase (PDE) activator in fresh human lymphocytes. This toxin induces a decrease of adenosine 3',5'-cyclic monophosphate (cAMP) levels in fresh human lymphocytes in a medium with calcium (Ca(2+) ), whereas the contrary effect has been observed in a Ca(2+) -free medium. In the present article, the effect of YTX in K-562 lymphocytes cell line has been analysed. Surprisingly, results obtained in K-562 cell line are completely opposite than in fresh human lymphocytes, since in K-562 cells YTX induces an increase of cAMP levels. YTX cytotoxicity was also studied in both K-562 cell line and fresh human lymphocytes. Results demonstrate that YTX does not modify fresh human lymphocytes viability, whereas in K-562 cells, YTX has a highly cytotoxic effect. It has been described in a previous study that YTX induces a small cytosolic Ca(2+) increase in fresh human lymphocytes but no effect was observed on Ca(2+) pools depletion in these cells. However, our results show that, in K-562 cells, YTX has no effect on cytosolic Ca(2+) levels in a medium with Ca(2+) and induces an increase on Ca(2+) pools depletion followed by a Ca(2+) influx. As far as Ca(2+) modulation is concerned these results demonstrate that YTX has a clear opposite effect in tumoural and fresh human lymphocytes. In addition, intracellular Ca(2+) reservoirs affected by YTX are different than thapsigargin-sensible pools. Furthermore, YTX-dependent Ca(2+) pools depletion was abolished by cAMP analogue (dibutyryl cAMP), phosphodiesterase-4 (PDE4) inhibitor (rolipram), protein kinase A inhibitor (H89) and oxidative phosphorylation uncoupler carbonyl cyanide p-(trifluoromethoxy) (FCCP) treatments. This evidences the crosstalks between Ca(2+) , YTX and cAMP pathways. Also, results obtain demonstrate that YTX-dependent Ca(2+) influx was only abolished by FCCP pre-treatment, which indicates a link between YTX and mitochondria in K-562 cell line. Cytosolic expression of A-kinase anchor proteins (AKAPs), the proteins which integrates phosphodiesterases (PDEs) and PKA to the mitochondria, was determined in both cell models. On the one hand, in human fresh lymphocytes, YTX increases AKAP149 cytosolic expression. This fact is accompanied with a decrease in cAMP levels, and therefore PDEs activation, which finally leads to cell survival. On the other hand, in tumoural lymphocytes, YTX has an opposite effect since decreases AKAP149 cytosolic expression and increase cAMP levels which leads to cell death. This is the first time that YTX and mitochondrial AKAPs proteins relationship is characterised.

A-kinase anchoring proteins: molecular regulators of the cardiac stress response

In response to stress or injury the heart undergoes a pathological remodeling process, associated with hypertrophy, cardiomyocyte death and fibrosis, that ultimately causes cardiac dysfunction and heart failure. It has become increasingly clear that signaling events associated with these pathological cardiac remodeling events are regulated by scaffolding and anchoring proteins, which allow coordination of pathological signals in space and time. A-kinase anchoring proteins (AKAPs) constitute a family of functionally related proteins that organize multiprotein signaling complexes that tether the cAMP-dependent protein kinase (PKA) as well as other signaling enzymes to ensure integration and processing of multiple signaling pathways. This review will discuss the role of AKAPs in the cardiac response to stress. Particular emphasis will be given to the adaptative process associated with cardiac hypoxia as well as the remodeling events linked to cardiac hypertrophy and heart failure. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

Mitochondrial regulation of cell cycle and proliferation

Eukaryotic mitochondria resulted from symbiotic incorporation of α-proteobacteria into ancient archaea species. During evolution, mitochondria lost most of the prokaryotic bacterial genes and only conserved a small fraction including those encoding 13 proteins of the respiratory chain. In this process, many functions were transferred to the host cells, but mitochondria gained a central role in the regulation of cell proliferation and apoptosis, and in the modulation of metabolism; accordingly, defective organelles contribute to cell transformation and cancer, diabetes, and neurodegenerative diseases. Most cell and transcriptional effects of mitochondria depend on the modulation of respiratory rate and on the production of hydrogen peroxide released into the cytosol. The mitochondrial oxidative rate has to remain depressed for cell proliferation; even in the presence of O₂, energy is preferentially obtained from increased glycolysis (Warburg effect). In response to stress signals, traffic of pro- and antiapoptotic mitochondrial proteins in the intermembrane space (B-cell lymphoma-extra large, Bcl-2-associated death promoter, Bcl-2 associated X-protein and cytochrome c) is modulated by the redox condition determined by mitochondrial O₂ utilization and mitochondrial nitric oxide metabolism. In this article, we highlight the traffic of the different canonical signaling pathways to mitochondria and the contributions of organelles to redox regulation of kinases. Finally, we analyze the dynamics of the mitochondrial population in cell cycle and apoptosis.

Cell signaling and mitochondrial dynamics: Implications for neuronal function and neurodegenerative disease

Nascent evidence indicates that mitochondrial fission, fusion, and transport are subject to intricate regulatory mechanisms that intersect with both well-characterized and emerging signaling pathways. While it is well established that mutations in components of the mitochondrial fission/fusion machinery can cause neurological disorders, relatively little is known about upstream regulators of mitochondrial dynamics and their role in neurodegeneration. Here, we review posttranslational regulation of mitochondrial fission/fusion enzymes, with particular emphasis on dynamin-related protein 1 (Drp1), as well as outer mitochondrial signaling complexes involving protein kinases and phosphatases. We also review recent evidence that mitochondrial dynamics has profound consequences for neuronal development and synaptic transmission and discuss implications for clinical translation.

PKA/AKAP1 and PP2A/Bβ2 regulate neuronal morphogenesis via Drp1 phosphorylation and mitochondrial bioenergetics

Mitochondrial shape is determined by fission and fusion reactions, perturbation of which can contribute to neuronal injury and disease. Mitochondrial fission is catalyzed by dynamin-related protein 1 (Drp1), a large GTPase of the dynamin family that is highly expressed in neurons and regulated by various posttranslational modifications, including phosphorylation. We report here that reversible phosphorylation of Drp1 at a conserved Ser residue by an outer mitochondrial kinase (PKA/AKAP1) and phosphatase (PP2A/Bβ2) impacts dendrite and synapse development in cultured rat hippocampal neurons. PKA/AKAP1-mediated phosphorylation of Drp1 at Ser656 increased mitochondrial length and dendrite occupancy, enhancing dendritic outgrowth but paradoxically decreasing synapse number and density. Opposing PKA/AKAP1, PP2A/Bβ2-mediated dephosphorylation of Drp1 at Ser656 fragmented and depolarized mitochondria and depleted them from dendrites, stunting dendritic outgrowth but augmenting synapse formation. Raising and lowering intracellular calcium reproduced the effects of dephospho-Drp1 and phospho-Drp1on dendrite and synapse development, respectively, while boosting mitochondrial membrane potential with l-carnitine-fostered dendrite at the expense of synapse formation without altering mitochondrial size or distribution. Thus, outer mitochondrial PKA and PP2A regulate neuronal development by inhibiting and promoting mitochondrial division, respectively. We propose that the bioenergetic state of mitochondria, rather than their localization or shape per se, is the key effector of Drp1, altering calcium homeostasis to modulate neuronal morphology and connectivity.

Type 10 adenylyl cyclase mediates mitochondrial Bax translocation and apoptosis of adult rat cardiomyocytes under simulated ischaemia/reperfusion

AIMS

Apoptosis of cardiomyocytes significantly contributes to the development of post-ischaemic cardiomyopathy. Although mitochondria have been suggested to play a crucial role in this process, the precise mechanisms controlling the mitochondria-dependent apoptosis in cardiomyocytes under ischaemia/reperfusion are still poorly understood. Here we aimed to analyse the role of the soluble adenylyl cyclase (sAC).

METHODS AND RESULTS

Adult rat cardiomyocytes were subjected to simulated in vitro ischaemia (SI) consisting of glucose-free anoxia at pH 6.4. Apoptosis was detected by DNA laddering, chromatin condensation, and caspases cleavage. SI led to the translocation of sAC to the mitochondria and mitochondrial depolarization followed by cytochrome c release, caspase-9/-3 cleavage and apoptosis during simulated reperfusion (SR). Pharmacological inhibition of sAC during SI, but not during SR, significantly reduced the SI/SR-induced mitochondrial injury and apoptosis. Similarly, sAC knock-down mediated by an adenovirus coding for shRNA targeting sAC prevented the activation of the mitochondrial pathway of apoptosis. Analysis of the link between sAC and apoptosis revealed a sAC and protein kinase A-dependent Bax phosphorylation at Thr(167) and its translocation to mitochondria during SI, which subsequently caused mitochondrial oxygen radical formation followed by cytochrome c release and caspase-9 cleavage during SR.

CONCLUSION

These results suggest a key role of sAC in SI-induced mitochondrial Bax translocation and activation of the mitochondrial pathway of apoptosis in adult cardiomyocytes.

Progesterone synthesis by human placental mitochondria is sensitive to PKA inhibition by H89

The transfer of cholesterol to mitochondria, which might involve the phosphorylation of proteins, is the rate-limiting step in human placental steroidogenesis. Protein kinase A (PKA) activity and its role in progesterone synthesis by human placental mitochondria were assessed in this study. The results showed that PKA and phosphotyrosine phosphatase D1 are associated with syncytiotrophoblast mitochondrial membrane by an anchoring kinase cAMP protein-121. The ³²P-labeled of four major proteins was analyzed. The specific inhibitor of PKA, H89, decreased progesterone synthesis in mitochondria while in mitochondrial steroidogenic contact sites protein-phosphorylation was diminished, suggesting that PKA plays a role in placental hormone synthesis. In isolated mitochondria, PKA activity was unaffected by the addition of cAMP suggesting a constant activity of this kinase in the syncytiotrophoblast. The presence of PKA and phosphotyrosine phosphatase D1 anchored to mitochondria by an anchoring kinase cAMP protein-121 indicated that syncytiotrophoblast mitochondria contain a full phosphorylation/dephosphorylation system.

Mitochondrially localized PKA reverses mitochondrial pathology and dysfunction in a cellular model of Parkinson's disease

Mutations in PTEN-induced kinase 1 (PINK1) are associated with a familial syndrome related to Parkinson's disease (PD). We previously reported that stable neuroblastoma SH-SY5Y cell lines with reduced expression of endogenous PINK1 exhibit mitochondrial fragmentation, increased mitochondria-derived superoxide, induction of compensatory macroautophagy/mitophagy and a low level of ongoing cell death. In this study, we investigated the ability of protein kinase A (PKA) to confer protection in this model, focusing on its subcellular targeting. Either: (1) treatment with pharmacological PKA activators; (2) transient expression of a constitutively active form of mitochondria-targeted PKA; or (3) transient expression of wild-type A kinase anchoring protein 1 (AKAP1), a scaffold that targets endogenous PKA to mitochondria, reversed each of the phenotypes attributed to loss of PINK1 in SH-SY5Y cells, and rescued parameters of mitochondrial respiratory dysfunction. Mitochondrial and lysosomal changes in primary cortical neurons derived from PINK1 knockout mice or subjected to PINK1 RNAi were also reversed by the activation of PKA. PKA phosphorylates the rat dynamin-related protein 1 isoform 1 (Drp1) at serine 656 (homologous to human serine 637), inhibiting its pro-fission function. Mimicking phosphorylation of Drp1 recapitulated many of the protective effects of AKAP1/PKA. These data indicate that redirecting endogenous PKA to mitochondria can compensate for deficiencies in PINK1 function, highlighting the importance of compartmentalized signaling networks in mitochondrial quality control.