Enhancement of anxiety, facilitation of avoidance behavior, and occurrence of adult-onset obesity in mice lacking mitochondrial cyclophilin D

Body weight modulates the impact of oxytocin on chronic cold-immobilization stress response

By activating the stress system, stress modulates various physiological parameters including food intake, energy consumption, and, consequently, body weight. The role of oxytocin in the regulation of stress and obesity cannot be disregarded. Based on these findings, we aimed to investigate the effect of intranasal oxytocin on stress response in high-fat-diet (HFD)--fed and control-diet-fed rats exposed to chronic stress. Cold-immobilization stress was applied for 5 consecutive days to male Sprague-Dawley rats fed either with a control diet (n=20) or HFD (n=20) for 6 weeks. Half of the animals in each group received oxytocin. Stress response was evaluated via plasma and salivary cortisol levels as well as elevated plus maze scores. Prefrontal cortex and hypothalamic oxytocin receptor (OxtR) expression levels were identified using western blot analysis. The results showed higher stress response in HFD-fed animals than in control animals both under basal and post-stress conditions. Oxytocin application had a prominent anxiolytic effect in the control group but an insignificant effect in the HFD group. While OxtR expression levels in the prefrontal cortex did not vary according to the body weight and oxytocin application, OxtR levels in the hypothalamus were higher in the HFD- and/or oxytocin-treated animals. Our results indicated that the peripheral and central effects of oxytocin vary with body weight. Moreover, obesity masks the anxiolytic effects of oxytocin, probably by reinforcing the stress condition via central OxtRs. In conclusion, elucidating the mechanisms underlying the central effect of oxytocin is important to cope with stress and obesity.

Cyclophilin D in Mitochondrial Dysfunction: A Key Player in Neurodegeneration?

Mitochondrial dysfunction plays a pivotal role in numerous complex diseases. Understanding the molecular mechanisms by which the "powerhouse of the cell" turns into the "factory of death" is an exciting yet challenging task that can unveil new therapeutic targets. The mitochondrial matrix protein CyPD is a peptidylprolyl cis-trans isomerase involved in the regulation of the permeability transition pore (mPTP). The mPTP is a multi-conductance channel in the inner mitochondrial membrane whose dysregulated opening can ultimately lead to cell death and whose involvement in pathology has been extensively documented over the past few decades. Moreover, several mPTP-independent CyPD interactions have been identified, indicating that CyPD could be involved in the fine regulation of several biochemical pathways. To further enrich the picture, CyPD undergoes several post-translational modifications that regulate both its activity and interaction with its clients. Here, we will dissect what is currently known about CyPD and critically review the most recent literature about its involvement in neurodegenerative disorders, focusing on Alzheimer's Disease and Parkinson's Disease, supporting the notion that CyPD could serve as a promising therapeutic target for the treatment of such conditions. Notably, significant efforts have been made to develop CyPD-specific inhibitors, which hold promise for the treatment of such complex disorders.

Latent toxoplasmosis impairs learning and memory yet strengthens short-term and long-term hippocampal synaptic plasticity at perforant pathway-dentate gyrus, and Schaffer collatterals-CA1 synapses

Investigating long-term potentiation (LTP) in disease models provides essential mechanistic insight into synaptic dysfunction and relevant behavioral changes in many neuropsychiatric and neurological diseases. Toxoplasma (T) gondii is an intracellular parasite causing bizarre changes in host's mind including losing inherent fear of life-threatening situations. We examined hippocampal-dependent behavior as well as in vivo short- and long-term synaptic plasticity (STP and LTP) in rats with latent toxoplasmosis. Rats were infected by T. gondii cysts. Existence of REP-529 genomic sequence of the parasite in the brain was detected by RT-qPCR. Four and eight weeks after infection, spatial, and inhibitory memories of rats were assessed by Morris water maze and shuttle box tests, respectively. Eight weeks after infection, STP was assessed in dentate gyrus (DG) and CA1 by double pulse stimulation of perforant pathway and Shaffer collaterals, respectively. High frequency stimulation (HFS) was applied to induce LTP in entorhinal cortex-DG (400 Hz), and CA3-CA1 (200 Hz) synapses. T. gondii infection retarded spatial learning and memory performance at eight weeks post-infection period, whereas inhibitory memory was not changed. Unlike uninfected rats that normally showed paired-pulse depression, the infected rats developed paired-pulse facilitation, indicating an inhibitory synaptic network disruption. T. gondii-infected rats displayed strengthened LTP of both CA1-pyramidal and DG-granule cell population spikes. These data indicate that T. gondii disrupts inhibition/excitation balance and causes bizarre changes to the post-synaptic neuronal excitability, which may ultimately contribute to the abnormal behavior of the infected host.

The mitochondrial permeability transition pore in Ca2+ homeostasis

The mitochondrial Permeability Transition Pore (PTP) can be defined as a Ca²⁺ activated mega-channel involved in mitochondrial damage and cell death, making its inhibition a hallmark for therapeutic purposes in many PTP-related paradigms. Although long-lasting PTP openings have been widely studied, the physiological implications of transient openings (also called "flickering" behavior) are still poorly understood. The flickering activity was suggested to play a role in the regulation of Ca²⁺ and ROS homeostasis, and yet this hypothesis did not reach general consensus. This state of affairs might arise from the lack of unquestionable experimental evidence, due to limitations of the available techniques for capturing transient PTP activity and to a still partial understanding of its molecular identity. In this review we will focus on possible implications of the PTP in physiology, in particular its role as a Ca²⁺ release pathway, discussing the consequences of its forced inhibition. We will also consider the recent hypothesis of the existence of more permeability pathways and their potential involvement in mitochondrial physiology.

Remodeling of Liver and Plasma Lipidomes in Mice Lacking Cyclophilin D

In recent years, several studies aimed to investigate the metabolic effects of non-functioning or absent cyclophilin D (CypD), a crucial regulatory component of mitochondrial permeability transition pores. It has been reported that the lack of CypD affects glucose and lipid metabolism. However, the findings are controversial regarding the metabolic pathways involved, and most reports describe the effect of a high-fat diet on metabolism. We performed a lipidomic analysis of plasma and liver samples of CypD-/- and wild-type (WT) mice to reveal the lipid-specific alterations resulting from the absence of CypD. In the CypD-/- mice compared to the WT animals, we found a significant change in 52% and 47% of the measured 225 and 201 lipid species in liver and plasma samples, respectively. The higher total lipid content detected in these tissues was not accompanied by abdominal fat accumulation assessed by nuclear magnetic resonance imaging. We also documented characteristic changes in the lipid composition of the liver and plasma as a result of CypD ablation with the relative increase in polyunsaturated membrane lipid species. In addition, we did not observe remarkable differences in the lipid distribution of hepatocytes using histochemistry, but we found characteristic changes in the hepatocyte ultrastructure in CypD-/- animals using electron microscopy. Our results highlight the possible long-term effects of CypD inhibition as a novel therapeutic consideration for various diseases.

Cyclophilin D regulation of the mitochondrial permeability transition pore

The opening of the mitochondrial permeability transition pore (PTP) has been proposed to play a critical role in activating cell death in many settings, including cardiac ischemia and reperfusion. Although the identity of pore forming unit of the PTP is still debated, it is generally agreed that cyclophilin D (CyPD) is a regulator of the PTP. This manuscript will focus on understanding how CyPD might regulate the PTP and how understanding CyPD might give insight about the identify and regulation of the PTP.

Deletion of the Mitochondrial Matrix Protein CyclophilinD Prevents Parvalbumin Interneuron Dysfunctionand Cognitive Deficits in a Mouse Model of NMDA Hypofunction

Redox dysregulation and oxidative stress are final common pathways in the pathophysiology of a variety of psychiatric disorders, including schizophrenia. Oxidative stress causes dysfunction of GABAergic parvalbumin (PV)-positive interneurons (PVI), which are crucial for the coordination of neuronal synchrony during sensory and cognitive processing. Mitochondria are the main source of reactive oxygen species (ROS) in neurons and they control synaptic activity through their roles in energy production and intracellular calcium homeostasis. We have previously shown that in male mice transient blockade of NMDA receptors (NMDARs) during development [subcutaneous injections of 30 mg/kg ketamine (KET) on postnatal days 7, 9, and 11] results in long-lasting alterations in synaptic transmission and reduced PV expression in the adult prefrontal cortex (PFC), contributing to a behavioral phenotype that mimics multiple symptoms associated with schizophrenia. These changes correlate with oxidative stress and impaired mitochondrial function in both PVI and pyramidal cells. Here, we show that genetic deletion (Ppif-/-) of the mitochondrial matrix protein cyclophilin D (CypD) prevents perinatal KET-induced increases in ROS and the resulting deficits in PVI function, and changes in excitatory and inhibitory synaptic transmission in the PFC. Deletion of CypD also prevented KET-induced behavioral deficits in cognitive flexibility, social interaction, and novel object recognition (NOR). Taken together, these data highlight how mitochondrial activity may play an integral role in modulating PVI-mediated cognitive processes.SIGNIFICANCE STATEMENT Mitochondria are important modulators of oxidative stress and cell function, yet how mitochondrial dysfunction affects cell activity and synaptic transmission in psychiatric illnesses is not well understood. NMDA receptor (NMDAR) blockade with ketamine (KET) during development causes oxidative stress, dysfunction of parvalbumin (PV)-positive interneurons (PVI), and long-lasting physiological and behavioral changes. Here we show that mice deficient for the mitochondrial matrix protein cyclophilin D (CypD) show robust protection from PVI dysfunction following perinatal NMDAR blockade. Mitochondria serve as an essential node for a number of stress-induced signaling pathways and our experiments suggest that failure of mitochondrial redox regulation can contribute to PVI dysfunction.

Interactions of 17β-Hydroxysteroid Dehydrogenase Type 10 and Cyclophilin D in Alzheimer's Disease

The nucleus-encoded 17β-hydroxysteroid dehydrogenase type 10 (17β-HSD10) regulates cyclophilin D (cypD) in the mitochondrial matrix. CypD regulates opening of mitochondrial permeability transition pores. Both mechanisms may be affected by amyloid β peptides accumulated in mitochondria in Alzheimer's disease (AD). In order to clarify changes occurring in brain mitochondria, we evaluated interactions of both mitochondrial proteins in vitro (by surface plasmon resonance biosensor) and detected levels of various complexes of 17β-HSD10 formed in vivo (by sandwich ELISA) in brain mitochondria isolated from the transgenic animal model of AD (homozygous McGill-R-Thy1-APP rats) and in cerebrospinal fluid samples of AD patients. By surface plasmon resonance biosensor, we observed the interaction of 17β-HSD10 and cypD in a direct real-time manner and determined, for the first time, the kinetic parameters of the interaction (ka 2.0 × 105 M1s-1, kd 5.8 × 104 s-1, and KD 3.5 × 10-10 M). In McGill-R-Thy1-APP rats compared to controls, levels of 17β-HSD10-cypD complexes were decreased and those of total amyloid β increased. Moreover, the levels of 17β-HSD10-cypD complexes were decreased in cerebrospinal fluid of individuals with AD (in mild cognitive impairment as well as dementia stages) or with Frontotemporal lobar degeneration (FTLD) compared to cognitively normal controls (the sensitivity of the complexes to AD dementia was 92.9%, that to FTLD 73.8%, the specificity to AD dementia equaled 91.7% in a comparison with the controls but only 26.2% with FTLD). Our results demonstrate the weakened ability of 17β-HSD10 to regulate cypD in the mitochondrial matrix probably via direct effects of amyloid β. Levels of 17β-HSD10-cypD complexes in cerebrospinal fluid seem to be the very sensitive indicator of mitochondrial dysfunction observed in neurodegeneration but unfortunately not specific to AD pathology. We do not recommend it as the new biomarker of AD.

Delineating a role for the mitochondrial permeability transition pore in diabetic kidney disease by targeting cyclophilin D

Mitochondrial stress has been widely observed in diabetic kidney disease (DKD). Cyclophilin D (CypD) is a functional component of the mitochondrial permeability transition pore (mPTP) which allows the exchange of ions and solutes between the mitochondrial matrix to induce mitochondrial swelling and activation of cell death pathways. CypD has been successfully targeted in other disease contexts to improve mitochondrial function and reduced pathology. Two approaches were used to elucidate the role of CypD and the mPTP in DKD. Firstly, mice with a deletion of the gene encoding CypD (Ppif-/-) were rendered diabetic with streptozotocin (STZ) and followed for 24 weeks. Secondly, Alisporivir, a CypD inhibitor was administered to the db/db mouse model (5 mg/kg/day oral gavage for 16 weeks). Ppif-/- mice were not protected against diabetes-induced albuminuria and had greater glomerulosclerosis than their WT diabetic littermates. Renal hyperfiltration was lower in diabetic Ppif-/- as compared with WT mice. Similarly, Alisporivir did not improve renal function nor pathology in db/db mice as assessed by no change in albuminuria, KIM-1 excretion and glomerulosclerosis. Db/db mice exhibited changes in mitochondrial function, including elevated respiratory control ratio (RCR), reduced mitochondrial H2O2 generation and increased proximal tubular mitochondrial volume, but these were unaffected by Alisporivir treatment. Taken together, these studies indicate that CypD has a complex role in DKD and direct targeting of this component of the mPTP will likely not improve renal outcomes.

Oestrogen receptor ERα and ERβ agonists ameliorate oxidative brain injury and improve memory dysfunction in rats with an epileptic seizure

NEW FINDINGS

What is the central question of this study? Could different hormonally active substances, including oestrogen receptor (ER) agonists, protect against oxidative brain damage and memory impairment induced by a single epileptic seizure in rats? If so, which signalling mechanisms are involved in their anti-inflammatory effects? What is the main finding and its importance? Chronic administration of oestrogen, progesterone, ER modulators/agonists or blockade of testosterone exhibited anti-inflammatory and antioxidant actions on single seizure-induced neuronal injury, while ER agonists additionally improved memory function and up-regulated CREB signalling and hippocampal GABA(A)α1 receptor density, suggesting that ERα or ERβ receptor activation may be beneficial in protecting against seizure-related oxidative brain injury and cognitive dysfunction.

ABSTRACT

The susceptibility to epileptic seizures is dependent on sex as well as fluctuations in oestrogen levels, while exogenous oestrogen was shown to have no effect or to facilitate or to inhibit seizure activity. Oestrogen receptors (ERs) mediate antioxidant and anti-inflammatory actions in several inflammatory models, but the involvement of ERs in seizure-induced neuronal injury has not been evaluated previously. In order to assess the effects of resveratrol, progesterone, oestradiol (E2), an anti-testosterone (cyproterone acetate; CPA), a selective ER modulator (tamoxifen; TMX) and ERα/ERβ agonists (propyl pyrazole triol (PPT), diarylpropionitrile (DPN)) on oxidative brain damage and memory impairment due to epileptic seizure, male Wistar rats (n = 120) received one of the treatment choices either in drinking water or intraperitoneally for 31 days, and epileptic seizure was induced on the 28th day by injection of a single-dose of pentylenetetrazole (45 mg kg^-1 ). The results demonstrate that chronic pretreatment with resveratrol, progesterone, E2, CPA or TMX suppressed most of the inflammatory parameters indicative of oxidative neuronal injury, while treatment with the ER agonists DPN or PPT were found to be even more effective in limiting the oxidative damage. Treatment with DPN resulted in the up-regulation of cAMP response element-binding protein (CREB) and brain-derived neurotrophic factor (BDNF) expression, while PPT up-regulated expression of CREB without affecting BDNF levels. Moreover, both ER agonists provided protection against seizure-induced memory loss with a concomitant increase in hippocampal GABA(A)α1-positive cells. In conclusion, ER agonists, and more specifically ERβ agonist, appear to provide maximum protection against seizure-induced oxidative brain injury and associated memory dysfunction by up-regulating the expression of CREB, BDNF and GABA(A)α1 receptors.

Cyclophilin D, Somehow a Master Regulator of Mitochondrial Function

Cyclophilin D (CyPD) is an important mitochondrial chaperone protein whose mechanism of action remains a mystery. It is well known for regulating mitochondrial function and coupling of the electron transport chain and ATP synthesis by controlling the mitochondrial permeability transition pore (PTP), but more recent evidence suggests that it may regulate electron transport chain activity. Given its identification as a peptidyl-prolyl, cis-trans isomerase (PPIase), CyPD, is thought to be involved in mitochondrial protein folding, but very few reports demonstrate the presence of this activity. By contrast, CyPD may also perform a scaffolding function, as it binds to a number of important proteins in the mitochondrial matrix and inner mitochondrial membrane. From a clinical perspective, inhibiting CyPD to inhibit PTP opening protects against ischemia⁻reperfusion injury, making modulation of CyPD activity a potentially important therapeutic goal, but the lack of knowledge about the mechanisms of CyPD's actions remains problematic for such therapies. Thus, the important yet enigmatic nature of CyPD somehow makes it a master regulator, yet a troublemaker, for mitochondrial function.

Mitochondrial Permeability Transition: A Molecular Lesion with Multiple Drug Targets

Mitochondrial permeability transition, as the consequence of opening of a mitochondrial permeability transition pore (mPTP), is a cellular catastrophe. Initiating bioenergetic collapse and cell death, it has been implicated in the pathophysiology of major human diseases, including neuromuscular diseases of childhood, ischaemia-reperfusion injury, and age-related neurodegenerative disease. Opening of the mPTP represents a major therapeutic target, as it can be mitigated by a number of compounds. However, clinical studies have so far been disappointing. We therefore address the prospects and challenges faced in translating in vitro findings to clinical benefit. We review the role of mPTP opening in disease, discuss recent findings defining the putative structure of the mPTP, and explore strategies to identify novel, clinically useful mPTP inhibitors, highlighting key considerations in the drug discovery process.

Mitochondrial calcium transport and the redox nature of the calcium-induced membrane permeability transition

Mitochondria possess a Ca²⁺ transport system composed of separate Ca²⁺ influx and efflux pathways. Intramitochondrial Ca²⁺ concentrations regulate oxidative phosphorylation, required for cell function and survival, and mitochondrial redox balance, that participates in a myriad of signaling and damaging pathways. The interaction between Ca²⁺ accumulation and redox imbalance regulates opening and closing of a highly regulated inner membrane pore, the membrane permeability transition pore (PTP). In this review, we discuss the regulation of the PTP by mitochondrial oxidants, reactive nitrogen species, and the interactions between these species and other PTP inducers. In addition, we discuss the involvement of mitochondrial redox imbalance and PTP in metabolic conditions such as atherogenesis, diabetes, obesity and in mtDNA stability.

Zdhhc13-dependent Drp1 S-palmitoylation impacts brain bioenergetics, anxiety, coordination and motor skills

Protein S-palmitoylation is a reversible post-translational modification mediated by palmitoyl acyltransferase enzymes, a group of Zn²⁺-finger DHHC-domain-containing proteins (ZDHHC). Here, for the first time, we show that Zdhhc13 plays a key role in anxiety-related behaviors and motor function, as well as brain bioenergetics, in a mouse model (luc) carrying a spontaneous Zdhhc13 recessive mutation. At 3 m of age, mutant mice displayed increased sensorimotor gating, anxiety, hypoactivity, and decreased motor coordination, compared to littermate controls. Loss of Zdhhc13 in cortex and cerebellum from 3- and 24 m old hetero- and homozygous male mutant mice resulted in lower levels of Drp1 S-palmitoylation accompanied by altered mitochondrial dynamics, increased glycolysis, glutaminolysis and lactic acidosis, and neurotransmitter imbalances. Employing in vivo and in vitro models, we identified that Zdhhc13-dependent Drp1 S-palmitoylation, which acting alone or in concert, enables the normal occurrence of the fission-fusion process. In vitro and in vivo direct Zdhhc13-Drp1 protein interaction was observed, confirming Drp1 as a substrate of Zdhhc13. Abnormal fission-fusion processes result in disrupted mitochondria morphology and distribution affecting not only mitochondrial ATP output but neurotransmission and integrity of synaptic structures in the brain, setting the basis for the behavioral abnormalities described in the Zdhhc13-deficient mice.

Cyclophilin D regulates lifespan and protein expression of aging markers in the brain of mice

Cyclophilin D (cypD) modulates the properties of the permeability transition pore, a phenomenon implicated in the manifestation of many diseases including aging. Here, we examined the effects of partial or complete deletion of cypD on i) lifespan, ii) forebrain protein expression of 18 aging markers as well as regional expression of GFAP, mGluR1, and alpha-synuclein, and iii) behaviour of aged (>24month) male and female mice. Both male and female cypD heterozygous but not KO mice exhibited increased lifespans compared to WT littermates, associated with alterations in the protein expression of some markers, albeit without exhibiting changes in behaviour.

The Mitochondrial Permeability Transition Pore Regulator Cyclophilin D Exhibits Tissue-Specific Control of Metabolic Homeostasis

The mitochondrial permeability transition pore (mPTP) is a key regulator of mitochondrial function that has been implicated in the pathogenesis of metabolic disease. Cyclophilin D (CypD) is a critical regulator that directly binds to mPTP constituents to facilitate the pore opening. We previously found that global CypD knockout mice (KO) are protected from diet-induced glucose intolerance; however, the tissue-specific function of CypD and mPTP, particularly in the control of glucose homeostasis, has not been ascertained. To this end, we performed calcium retention capacity (CRC) assay to compare the importance of CypD in the liver versus skeletal muscle. We found that liver mitochondria are more dependent on CypD for mPTP opening than skeletal muscle mitochondria. To ascertain the tissue-specific role of CypD in metabolic homeostasis, we generated liver-specific and muscle-specific CypD knockout mice (LKO and MKO, respectively) and fed them either a chow diet or 45% high-fat diet (HFD) for 14 weeks. MKO mice displayed similar body weight gain and glucose intolerance compared with wild type littermates (WT), whereas LKO mice developed greater visceral obesity, glucose intolerance and pyruvate intolerance compared with WT mice. These findings demonstrate that loss of muscle CypD is not sufficient to alter whole body glucose metabolism, while the loss of liver CypD exacerbates obesity and whole-body metabolic dysfunction in mice fed HFD.

Cyclophilin D counteracts P53-mediated growth arrest and promotes Ras tumorigenesis

Mitochondrial alterations induced by oncogenes are known to be crucial for tumorigenesis. Ras oncogene leads to proliferative signals through a Raf-1/MEK/ERK kinase cascade, whose components have been found to be also associated with mitochondria. The mitochondrial pepdidyl-prolyl isomerase cyclophilin D (CypD) is an important regulator of the mitochondrial permeability transition and a key player in mitochondria physiology; however, its role in cancer is still unclear. Using cellular and in vivo mouse models, we demonstrated that CypD protein upregulation induced by oncogenic Ras through the Raf-1/MEK/ERK pathway has a deterministic role in tumor progression. In fact, targeting CypD gene expression clearly affected RasV12-induced transformation, as showed by in vitro data on murine NIH3T3 and human MCF10A mammary epithelial cells. In addition, studies in xenograft and K-Ras lung cancer mouse models demonstrated that genetic deletion or pharmacological suppression of CypD efficiently prevented Ras-dependent tumor formation. Furthermore, Erbb2-mediated breast tumorigenesis was similarly prevented by targeting CypD. From a mechanistic point of view, CypD expression was associated with a reduced induction of p21(WAF1/CIP1) and p53 functions, unraveling an antagonistic function of CypD on p21-p53-mediated growth suppression. CypD activity is p53 dependent. Interestingly, a physical association between p53 and CypD was detected in mitochondria of MCF10A cells; furthermore, both in vitro and in vivo studies proved that CypD inhibitor-based treatment was able to efficiently impair this interaction, leading to a tumor formation reduction. All together, these findings indicate that the countering effect of CypD on the p53-p21 pathway participates in oncogene-dependent transformation.

Knockout of Cyclophilin-D Provides Partial Amelioration of Intrinsic and Synaptic Properties Altered by Mild Traumatic Brain Injury

Mitochondria are central to cell survival and Ca²⁺ homeostasis due to their intracellular buffering capabilities. Mitochondrial dysfunction resulting in mitochondrial permeability transition pore (mPTP) opening has been reported after mild traumatic brain injury (mTBI). Cyclosporine A provides protection against the mPTP opening through its interaction with cyclophilin-D (CypD). A recent study has found that the extent of axonal injury after mTBI was diminished in neocortex in cyclophilin-D knockout (CypDKO) mice. Here we tested whether this CypDKO could also provide protection from the increased intrinsic and synaptic neuronal excitability previously described after mTBI in a mild central fluid percussion injury mice model. CypDKO mice were crossed with mice expressing yellow fluorescent protein (YFP) in layer V pyramidal neurons in neocortex to create CypDKO/YFP-H mice. Whole cell patch clamp recordings from axotomized (AX) and intact (IN) YFP+ layer V pyramidal neurons were made 1 and 2 days after sham or mTBI in slices from CypDKO/YFP-H mice. Both excitatory post synaptic currents (EPSCs) recorded in voltage clamp and intrinsic cellular properties, including action potential (AP), afterhyperpolarization (AHP), and depolarizing after potential (DAP) characteristics recorded in current clamp were evaluated. There was no significant difference between sham and mTBI for either spontaneous or miniature EPSC frequency, suggesting that CypDKO ameliorates excitatory synaptic abnormalities. There was a partial amelioration of intrinsic properties altered by mTBI. Alleviated were the increased slope of the AP frequency vs. injected current plot, the increased AP, AHP and DAP amplitudes. Other properties that saw a reversal that became significant in the opposite direction include the current rheobase and AP overshoot. The AP threshold remained depolarized and the input resistance remained increased in mTBI compared to sham. Additional altered properties suggest that the CypDKO likely has a direct effect on membrane properties, rather than producing a selective reduction of the effects of mTBI. These results suggest that inhibiting CypD after TBI is an effective strategy to reduce synaptic hyperexcitation, making it a continued target for potential treatment of network abnormalities.

Cyclophilin D regulates mitochondrial flashes and metabolism in cardiac myocytes

Cyclophilin D (CyP-D) is the mitochondrial-specific member of the evolutionally conserved cyclophilin family, and plays an important role in the regulation of mitochondrial permeability transition (MPT) under stress. Recently we have demonstrated that respiratory mitochondria undergo mitochondrial flash ("mitoflash") activity which is coupled with transient MPT under physiological conditions. However, whether and how CyP-D regulates mitoflashes remain incompletely understood. By using both loss- and gain-of-function approaches in isolated cardiomyocytes, beating hearts, and skeletal muscles in living mice, we revisited the role of CyP-D in the regulation of mitoflashes. Overexpression of CyP-D increased, and knockout of it halved, cardiac mitoflash frequency, while mitoflash amplitude and kinetics remained unaffected. However, CyP-D ablation did not alter mitoflash frequency, with mitoflash amplitude increased, in gastrocnemius muscles. This disparity was accompanied by 4-fold higher CyP-D expression in mouse cardiac than skeletal muscles. The mitochondrial maximal respiration rate and reserved capacity were reduced in CyP-D-null cardiomyocytes. These data indicate that CyP-D is a significant regulator of mitoflash ignition and mitochondrial metabolism in heart. In addition, tissue-specific CyP-D expression may partly explain the differential regulation of mitoflashes in the two types of striated muscles.

SPG7 Is an Essential and Conserved Component of the Mitochondrial Permeability Transition Pore

Mitochondrial permeability transition is a phenomenon in which the mitochondrial permeability transition pore (PTP) abruptly opens, resulting in mitochondrial membrane potential (ΔΨm) dissipation, loss of ATP production, and cell death. Several genetic candidates have been proposed to form the PTP complex, however, the core component is unknown. We identified a necessary and conserved role for spastic paraplegia 7 (SPG7) in Ca(2+)- and ROS-induced PTP opening using RNAi-based screening. Loss of SPG7 resulted in higher mitochondrial Ca(2+) retention, similar to cyclophilin D (CypD, PPIF) knockdown with sustained ΔΨm during both Ca(2+) and ROS stress. Biochemical analyses revealed that the PTP is a heterooligomeric complex composed of VDAC, SPG7, and CypD. Silencing or disruption of SPG7-CypD binding prevented Ca(2+)- and ROS-induced ΔΨm depolarization and cell death. This study identifies an ubiquitously expressed IMM integral protein, SPG7, as a core component of the PTP at the OMM and IMM contact site.

Traumatic brain injury-induced axonal phenotypes react differently to treatment

Injured axons with distinct morphologies have been found following mild traumatic brain injury (mTBI), although it is currently unclear whether they reflect varied responses to the injury or represent different stages of progressing pathology. This complicates evaluation of therapeutic interventions targeting axonal injury. To address this issue, we assessed axonal injury over time within a well-defined axonal population, while also evaluating mitochondrial permeability transition as a therapeutic target. We utilized mice expressing yellow fluorescent protein (YFP) in cortical neurons which were crossed with mice which lacked Cyclophilin D (CypD), a positive regulator of mitochondrial permeability transition pore opening. Their offspring were subjected to mTBI and the ensuing axonal injury was assessed using YFP expression and amyloid precursor protein (APP) immunohistochemistry, visualized by confocal and electron microscopy. YFP(+) axons initially developed a single, APP(+), focal swelling (proximal bulb) which progressed to axotomy. Disconnected axonal segments developed either a single bulb (distal bulb) or multiple bulbs (varicosities), which were APP(-) and whose ultrastructure was consistent with ongoing Wallerian degeneration. CypD knock-out failed to reduce proximal bulb formation but decreased the number of distal bulbs and varicosities, as well as a population of small, APP(+), callosal bulbs not associated with YFP(+) axons. The observation that YFP(+) axons contain several pathological morphologies points to the complexity of traumatic axonal injury. The fact that CypD knock-out reduced some, but not all, subtypes highlights the need to appropriately characterize injured axons when evaluating potential neuroprotective strategies.

Bioengineering and semisynthesis of an optimized cyclophilin inhibitor for treatment of chronic viral infection

Inhibition of host-encoded targets, such as the cyclophilins, provides an opportunity to generate potent high barrier to resistance antivirals for the treatment of a broad range of viral diseases. However, many host-targeted agents are natural products, which can be difficult to optimize using synthetic chemistry alone. We describe the orthogonal combination of bioengineering and semisynthetic chemistry to optimize the drug-like properties of sanglifehrin A, a known cyclophilin inhibitor of mixed nonribosomal peptide/polyketide origin, to generate the drug candidate NVP018 (formerly BC556). NVP018 is a potent inhibitor of hepatitis B virus, hepatitis C virus (HCV), and HIV-1 replication, shows minimal inhibition of major drug transporters, and has a high barrier to generation of both HCV and HIV-1 resistance.

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Optimization and preclinical analysis of a bacterial natural product

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Combination of bioengineering and semisynthetic chemistry

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Preclinical analysis revealing potent antiviral activity

The mitochondrial permeability transition pore and its role in myocardial ischemia reperfusion injury

Ischemic heart disease (IHD) remains the leading cause of death and disability worldwide. For patients presenting with an acute myocardial infarction, the most effective treatment for limiting myocardial infarct (MI) size is timely reperfusion. However, in addition to the injury incurred during acute myocardial ischemia, the process of reperfusion can itself induce myocardial injury and cardiomyocyte death, termed 'myocardial reperfusion injury', the combination of which can be referred to as acute ischemia-reperfusion injury (IRI). Crucially, there is currently no effective therapy for preventing this form of injury, and novel cardioprotective therapies are therefore required to protect the heart against acute IRI in order to limit MI size and preserve cardiac function. The opening of the mitochondrial permeability transition pore (MPTP) in the first few minutes of reperfusion is known to be a critical determinant of IRI, contributing up to 50% of the final MI size. Importantly, preventing its opening at this time using MPTP inhibitors, such as cyclosporin-A, has been reported in experimental and clinical studies to reduce MI size and preserve cardiac function. However, more specific and novel MPTP inhibitors are required to translate MPTP inhibition as a cardioprotective strategy into clinical practice. In this article, we review the role of the MPTP as a mediator of acute myocardial IRI and as a therapeutic target for cardioprotection. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease".

The atomic resolution structure of human AlkB homolog 7 (ALKBH7), a key protein for programmed necrosis and fat metabolism

ALKBH7 is the mitochondrial AlkB family member that is required for alkylation- and oxidation-induced programmed necrosis. In contrast to the protective role of other AlkB family members after suffering alkylation-induced DNA damage, ALKBH7 triggers the collapse of mitochondrial membrane potential and promotes cell death. Moreover, genetic ablation of mouse Alkbh7 dramatically increases body weight and fat mass. Here, we present crystal structures of human ALKBH7 in complex with Mn(II) and α-ketoglutarate at 1.35 Å or N-oxalylglycine at 2.0 Å resolution. ALKBH7 possesses the conserved double-stranded β-helix fold that coordinates a catalytically active iron by a conserved HX(D/E) … Xn … H motif. Self-hydroxylation of Leu-110 was observed, indicating that ALKBH7 has the potential to catalyze hydroxylation of its substrate. Unlike other AlkB family members whose substrates are DNA or RNA, ALKBH7 is devoid of the "nucleotide recognition lid" which is essential for binding nucleobases, and thus exhibits a solvent-exposed active site; two loops between β-strands β6 and β7 and between β9 and β10 create a special outer wall of the minor β-sheet of the double-stranded β-helix and form a negatively charged groove. These distinct features suggest that ALKBH7 may act on protein substrate rather than nucleic acids. Taken together, our findings provide a structural basis for understanding the distinct function of ALKBH7 in the AlkB family and offer a foundation for drug design in treating cell death-related diseases and metabolic diseases.

Cell biology. Metabolic control of cell death

Beyond their contribution to basic metabolism, the major cellular organelles, in particular mitochondria, can determine whether cells respond to stress in an adaptive or suicidal manner. Thus, mitochondria can continuously adapt their shape to changing bioenergetic demands as they are subjected to quality control by autophagy, or they can undergo a lethal permeabilization process that initiates apoptosis. Along similar lines, multiple proteins involved in metabolic circuitries, including oxidative phosphorylation and transport of metabolites across membranes, may participate in the regulated or catastrophic dismantling of organelles. Many factors that were initially characterized as cell death regulators are now known to physically or functionally interact with metabolic enzymes. Thus, several metabolic cues regulate the propensity of cells to activate self-destructive programs, in part by acting on nutrient sensors. This suggests the existence of "metabolic checkpoints" that dictate cell fate in response to metabolic fluctuations. Here, we discuss recent insights into the intersection between metabolism and cell death regulation that have major implications for the comprehension and manipulation of unwarranted cell loss.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Endoplasmic reticulum stress contributes to heart protection induced by cyclophilin D inhibition

Preventing cyclophilin D (cypD) translocation to the inner mitochondrial membrane can limit lethal reperfusion injury through the inhibition of the opening of the mitochondrial permeability transition pore. Inhibition or loss of function of cypD may also result into an endoplasmic reticulum (ER) stress that has been shown to alter cell survival. We therefore questioned whether ER stress might play a role in the protection induced by CypD deficiency or inhibition. CypD-KO and NIM811 (a CypD inhibitor)-treated mice were subjected to a prolonged ischemia-reperfusion (I/R). Area at risk and infarct size was measured using blue dye and triphenyltetrazolium chloride staining. ER stress markers were measured in the hearts during the reperfusion phase. As expected, cypD-KO mice exhibited a decreased infarct size when compared to wild-type mice (8 ± 1 vs. 20 ± 4% of left ventricular weight; p < 0.01). CypD-deficient mice displayed an increased expression of ER stress proteins such as eukaryotic initiation factor 2α (eIF2α) or glucose regulated protein 78 (Grp78 or Bip). The ER stress inhibitor TUDCA prevented the infarct size reduction afforded by the loss of cypD function (mean infarct size averaged 21 ± 4% of LV weight, p < 0.01 vs. cypD-KO). Similar results were obtained when NIM811, an analog of cyclosporine A, was used to pharmacologically (instead of genetically) inhibit cypD function. This study suggests that the ER stress induced by the inhibition of cypD function plays a key role in protecting the heart against lethal ischemia-reperfusion injury.

Physiologic functions of cyclophilin D and the mitochondrial permeability transition pore

This review focuses on the role of cyclophilin D (CypD) as a prominent mediator of the mitochondrial permeability transition pore (MPTP) and subsequent effects on cardiovascular physiology and pathology. Although a great number of reviews have been written on the MPTP and its effects on cell death, we focus on the biology surrounding CypD itself and the non-cell death physiologic functions of the MPTP. A greater understanding of the physiologic functions of the MPTP and its regulation by CypD will likely suggest novel therapeutic approaches for cardiovascular disease, both dependent and independent of programmed necrotic cell death mechanisms.  

CypD(-/-) hearts have altered levels of proteins involved in Krebs cycle, branch chain amino acid degradation and pyruvate metabolism

Cyclophilin D (CypD) is a mitochondrial chaperone that has been shown to regulate the mitochondrial permeability transition pore (MPTP). MPTP opening is a major determinant of mitochondrial dysfunction and cardiomyocyte death during ischemia/reperfusion (I/R) injury. Mice lacking CypD have been widely used to study regulation of the MPTP, and it has been shown recently that genetic depletion of CypD correlates with elevated levels of mitochondrial Ca(2+). The present study aimed to characterize the metabolic changes in CypD(-/-) hearts. Initially, we used a proteomics approach to examine protein changes in CypD(-/-) mice. Using pathway analysis, we found that CypD(-/-) hearts have alterations in branched chain amino acid metabolism, pyruvate metabolism and the Krebs cycle. We tested whether these metabolic changes were due to inhibition of electron transfer from these metabolic pathways into the electron transport chain. As we found decreased levels of succinate dehydrogenase and electron transfer flavoprotein in the proteomics analysis, we examined whether activities of these enzymes might be altered. However, we found no alterations in their activities. The proteomics study also showed a 23% decrease in carnitine-palmitoyltransferase 1 (CPT1), which prompted us to perform a metabolomics analysis. Consistent with the decrease in CPT1, we found a significant decrease in C4/Ci4, C5-OH/C3-DC, C12:1, C14:1, C16:1, and C20:3 acyl carnitines in hearts from CypD(-/-) mice. In summary, CypD(-/-) hearts exhibit changes in many metabolic pathways and caution should be used when interpreting results from these mice as due solely to inhibition of the MPTP.

New roles for OMA1 metalloprotease: From mitochondrial proteostasis to metabolic homeostasis

The mitochondrial quality control system is essential for the preservation and regulation of mitochondrial function. This system is formed by a complex machinery that controls and maintains protein function and regulates mitochondrial morphology through a coordinated system of continual fusion and fission events. Impairments in the mitochondrial quality control system through either mutation or deficiency in any of its components, can lead to mitochondrial dysfunction. However, the physiological consequences of these deficiencies remain unknown in most cases. Here, we briefly review the role of the OPA1-OMA1 system in mitochondrial biology, and summarize our recent report on the generation and phenotypic characterization of a model deficient in OMA1, an ATP-independent mitochondrial metalloprotease that participates in mitochondrial quality control. Interestingly, Oma1-deficient mice display an obesity phenotype, characterized by hepatic steatosis, decrease in energy expenditure and defective thermogenic regulation. In addition, our study has provided in vivo evidence of OMA1 function as a mitochondrial quality control protease, inactivating OPA1 under stress conditions and inhibiting mitochondrial fusion. Further, we have demonstrated the essential role of the OMA1-OPA1 system for brown adipose function and how this system regulates metabolic homeostasis in mice.

Tyr682 in the Aβ-precursor protein intracellular domain regulates synaptic connectivity, cholinergic function, and cognitive performance

Processing of Aβ-precursor protein (APP) plays an important role in Alzheimer's disease (AD) pathogenesis. The APP intracellular domain contains residues important in regulating APP function and processing, in particular the 682YENPTY687 motif. To dissect the functions of this sequence in vivo, we created an APP knock-in allele mutating Y682 to Gly (APP(YG/YG) mice). This mutation alters the processing of APP and TrkA signaling and leads to postnatal lethality and neuromuscular synapse defects when expressed on an APP-like protein 2 KO background. This evidence prompted us to characterize further the APP(YG/YG) mice. Here, we show that APP(YG/YG) mice develop aging-dependent decline in cognitive and neuromuscular functions, a progressive reduction in dendritic spines, cholinergic tone, and TrkA levels in brain regions governing cognitive and motor functions. These data are consistent with our previous findings linking NGF and APP signaling and suggest a causal relationship between altered synaptic connectivity, cholinergic tone depression and TrkA signaling deficit, and cognitive and neuromuscular decline in APP(YG/YG) mice. The profound deficits caused by the Y682 mutation underscore the biological importance of APP and indicate that APP(YG/YG) are a valuable mouse model to study APP functions in physiological and pathological processes.

Modulation of the mitochondrial permeability transition by cyclophilin D: moving closer to F(0)-F(1) ATP synthase?

Cyclophilin D was recently shown to mask an inhibitory site of the mitochondrial permeability transition pore (PTP) for phosphate, and to constitutively bind F(0)-F(1) ATP synthase resulting in the slowing of ATP synthesis and hydrolysis rates, thus regulating matrix adenine nucleotide levels. Here we review the striking similarities of the factors affecting the threshold for PTP induction, to those affecting binding of phosphate to formerly proposed sides on F(1)-ATPase affecting ATP hydrolytic activity, including critical arginine residues, matrix pH, [Mg(2+)], adenine nucleotides and proton motive force. Based on these similarities, we scrutinize the hypothesis that in depolarized mitochondria exhibiting reversal of F(0)-F(1) ATP synthase operation, the genetic ablation of cyclophilin D or its inhibition by cyclosporin A results in accelerated proton pumping by ATP hydrolysis, opposing a further decrease in membrane potential and promoting high matrix phosphate levels, both negatively affecting the probability of PTP opening.

Proteomics and metabolomics analysis of a trait anxiety mouse model reveals divergent mitochondrial pathways

BACKGROUND

Although anxiety disorders are the most prevalent psychiatric disorders, no molecular biomarkers exist for their premorbid diagnosis, accurate patient subcategorization, or treatment efficacy prediction. To unravel the neurobiological underpinnings and identify candidate biomarkers and affected pathways for anxiety disorders, we interrogated the mouse model of high anxiety-related behavior (HAB), normal anxiety-related behavior (NAB), and low anxiety-related behavior (LAB) employing a quantitative proteomics and metabolomics discovery approach.

METHODS

We compared the cingulate cortex synaptosome proteomes of HAB and LAB mice by in vivo (15)N metabolic labeling and mass spectrometry and quantified the cingulate cortex metabolomes of HAB/NAB/LAB mice. The combined data sets were used to identify divergent protein and metabolite networks by in silico pathway analysis. Selected differentially expressed proteins and affected pathways were validated with immunochemical and enzymatic assays.

RESULTS

Altered levels of up to 300 proteins and metabolites were found between HAB and LAB mice. Our data reveal alterations in energy metabolism, mitochondrial import and transport, oxidative stress, and neurotransmission, implicating a previously nonhighlighted role of mitochondria in modulating anxiety-related behavior.

CONCLUSIONS

Our results offer insights toward a molecular network of anxiety pathophysiology with a focus on mitochondrial contribution and provide the basis for pinpointing affected pathways in anxiety-related behavior.

Modulation of F0F1-ATP synthase activity by cyclophilin D regulates matrix adenine nucleotide levels

Cyclophilin D was recently shown to bind to and decrease the activity of F(0)F(1)-ATP synthase in submitochondrial particles and permeabilized mitochondria [Giorgio V et al. (2009) J Biol Chem, 284, 33982-33988]. Cyclophilin D binding decreased both ATP synthesis and hydrolysis rates. In the present study, we reaffirm these findings by demonstrating that, in intact mouse liver mitochondria energized by ATP, the absence of cyclophilin D or the presence of cyclosporin A led to a decrease in the extent of uncoupler-induced depolarization. Accordingly, in substrate-energized mitochondria, an increase in F(0)F(1)-ATP synthase activity mediated by a relief of inhibition by cyclophilin D was evident in the form of slightly increased respiration rates during arsenolysis. However, the modulation of F(0)F(1)-ATP synthase by cyclophilin D did not increase the adenine nucleotide translocase (ANT)-mediated ATP efflux rate in energized mitochondria or the ATP influx rate in de-energized mitochondria. The lack of an effect of cyclophilin D on the ANT-mediated adenine nucleotide exchange rate was attributed to the ∼ 2.2-fold lower flux control coefficient of the F(0)F(1)-ATP synthase than that of ANT, as deduced from measurements of adenine nucleotide flux rates in intact mitochondria. These findings were further supported by a recent kinetic model of the mitochondrial phosphorylation system, suggesting that an ∼ 30% change in F(0)F(1)-ATP synthase activity in fully energized or fully de-energized mitochondria affects the ADP-ATP exchange rate mediated by the ANT in the range 1.38-1.7%. We conclude that, in mitochondria exhibiting intact inner membranes, the absence of cyclophilin D or the inhibition of its binding to F(0)F(1)-ATP synthase by cyclosporin A will affect only matrix adenine nucleotides levels.

Intrastriatal manganese chloride exposure causes acute locomotor impairment as well as partial activation of substantia nigra GABAergic neurons

Our previous studies showed chronic exposure to manganese chloride (Mn) causes locomotor impairment and lesion of dopaminergic neurons in substantia nigra (SN). But effects of acute Mn exposure on locomotor ability, SN dopaminergic and GABAergic neurons were not clear. In the current study, Mn was injected into the striatum of GAD(67)-GFP mice. Twenty-four hours after injection, locomotor ability was quantitatively evaluated with behavioral tests (rotarod test and open field test). Meanwhile, the numbers of dopaminergic and GABAergic neurons were counted through immunofluorescent staining for TH and GFP respectively, and activations of dopaminergic and GABAergic neurons were evaluated by double immunofluorescent labeling for TH/Fos and GFP/Fos, respectively. Behavioral tests showed a significant locomotor impairment 24h after Mn injection. The numbers of SN dopaminergic and GABAergic neurons were not altered significantly 24h after Mn injection; however, some of SN GABAergic neurons were activated and dopaminergic neurons were left inactivated. In addition, there were still a large number of Mn-activated neurons that fell into neither dopaminergic nor GABAergic criteria. Our data suggested that activation of SN GABAergic neurons but not lesion of dopaminergic neurons, which was found to play an important role in the Mn-induced chronic neurotoxicity in our previous studies, contributed partially to Mn-induced acute locomotor impairment. Therefore we come to the conclusion that Mn exposure can induce acute or chronic neurotoxicity via different neuronal elements.

Therapeutic implications of down-regulation of cyclophilin D in bipolar disorder

We previously reported that neuron-specific mutant Polg1 (mitochondrial DNA polymerase) transgenic (Tg) mice exhibited bipolar disorder (BD)-like phenotypes such as periodic activity change and altered circadian rhythm. In this study, we re-evaluated two datasets resulting from DNA microarray analysis to estimate a biological pathway associated with the disorder. The gene lists were derived from the comparison between post-mortem brains of BD patients and control subjects, and from the comparison between the brains of Tg and wild-type mice. Gene ontology analysis showed that 16 categories overlapped in the altered gene expression profiles of BD patients and the mouse model. In the brains of Tg mice, 33 genes showed similar changes in the frontal cortex and hippocampus compared to wild-type mice. Among the 33 genes, SFPQ and PPIF were differentially expressed in post-mortem brains of BD patients compared to control subjects. The only gene consistently down-regulated in both patients and the mouse model was PPIF, which encodes cyclophilin D (CypD), a component of the mitochondrial permeability transition pore. A blood-brain barrier-permeable CypD inhibitor significantly improved the abnormal behaviour of Tg mice at 40 mg/kg.d. These findings collectively suggest that CypD is a promising target for a new drug for BD.

Cyclophilin D in mitochondrial pathophysiology

Cyclophilins are a family of peptidyl-prolyl cis-trans isomerases whose enzymatic activity can be inhibited by cyclosporin A. Sixteen cyclophilins have been identified in humans, and cyclophilin D is a unique isoform that is imported into the mitochondrial matrix. Here we shall (i) review the best characterized functions of cyclophilin D in mitochondria, i.e. regulation of the permeability transition pore, an inner membrane channel that plays an important role in the execution of cell death; (ii) highlight new regulatory interactions that are emerging in the literature, including the modulation of the mitochondrial F1FO ATP synthase through an interaction with the lateral stalk of the enzyme complex; and (iii) discuss diseases where cyclophilin D plays a pathogenetic role that makes it a suitable target for pharmacologic intervention.

Targeting cyclophilin D and the mitochondrial permeability transition enhances beta-cell survival and prevents diabetes in Pdx1 deficiency

Mutations of the pancreatic duodenal homeobox gene-1, Pdx1, cause heritable diabetes in humans and mice. A central abnormality with Pdx1 deficiency is increased death of beta-cells, leading to decreased beta-cell mass. We show that lentiviral suppression of Pdx1 increases death of mouse insulinoma MIN6 beta-cells associated with dissipation of the mitochondrial inner membrane electrochemical gradient, Deltapsi(m). Preventing mitochondrial permeability transition pore opening with the cyclophilin D inhibitor cyclosporin A restored Deltapsi(m) and rescued cell viability. Reduced beta-cell mass, markers of beta-cell apoptosis, necrosis, and decreased proliferation are present in Pdx1 haploinsufficient mice. Genetic ablation of the Ppif gene, encoding cyclophilin D, restored beta-cell mass and decreased TUNEL and complement complex labeling without affecting beta-cell proliferation. In adult mice maintained on a high-fat diet, Ppif ablation normalized fasting glucose and glucose and insulin responses to acute glucose challenge. Thus, cyclophilin D and the mitochondrial permeability transition are critical regulators of beta-cell death caused by Pdx1 insufficiency.

Signal transduction to the permeability transition pore

The permeability transition pore (PTP) is an inner mitochondrial membrane channel that has been thoroughly characterized functionally, yet remains an elusive molecular entity. The best characterized PTP-regulatory component, cyclophilin (CyP) D, is a matrix protein that favors pore opening. CyP inhibitors, CyP-D null animals, and in situ PTP readouts have established the role of PTP as an effector mechanism of cell death, and the growing definition of PTP signalling mechanisms. This review briefly covers the functional features of the PTP and the role played by its dysregulation in disease pathogenesis. Recent progress on PTP modulation by kinase/phosphatase signal transduction is discussed, with specific emphasis on hexokinase and on the Akt-ERK-GSK3 axis, which might modulate the PTP through CyP-D phosphorylation.

Brain distribution of genes related to changes in locomotor activity

The relationship between genes and behavior, and particularly the hyperactive behavior, is clearly not linear nor monotonic. To address this problem, a database of the locomotor behavior obtained from thousands of mutant mice has been previously retrieved from the literature. Data showed that the percent of genes in the genome related to locomotor hyperactivity is probably more than 1.56%. These genes do not belong to a single neurotransmitter system or biochemical pathway. Indeed, they are probably required for the correct development of a specific neuronal network necessary to decrease locomotor activity. The present paper analyzes the brain expression pattern of the genes whose deletion is accompanied by changes in locomotor behavior. Using literature data concerning knockout mice, 46 genes whose deletion was accompanied by increased locomotor behavior, 24 genes related to decreased locomotor behavior and 23 genes not involved in locomotor behavior (but important for other brain functions) have been identified. These three groups of genes belonged to overlapping neurotransmitter systems or cellular functions. Therefore, we postulated that a better predictor of the locomotor behavior resulting from gene deletion might be the brain expression pattern. To this aim we correlated the brain expression of the genes and the locomotor activity resulting from the deletion of the same genes, using two databases (Allen Brain Atlas and SymAtlas). The results showed that the deletion of genes with higher expression level in the brain had higher probability to be accompanied by increased behavioral activity. Moreover the genes that were accompanied by locomotor hyperactivity when deleted, were more expressed in the cerebral cortex, amygdala and hippocampus compared to the genes unrelated to locomotor activity. Therefore, the prediction of the behavioral effect of a gene should take into consideration its brain distribution. Moreover, data confirmed that genes highly expressed in the brain are more likely to induce hyperactivity when deleted. Finally, it is suggested that gene mutations linked to specific behavioral abnormalities (e.g. inattention) might probably be associated to hyperactivity if the same gene has elevated brain expression.

Enduring consequences of maternal obesity for brain inflammation and behavior of offspring

Obesity is well characterized as a systemic inflammatory condition, and is also associated with cognitive disruption, suggesting a link between the two. We assessed whether peripheral inflammation in maternal obesity may be transferred to the offspring brain, in particular, the hippocampus, and thereby result in cognitive dysfunction. Rat dams were fed a high-saturated-fat diet (SFD), a high-trans-fat diet (TFD), or a low-fat diet (LFD) for 4 wk prior to mating, and remained on the diet throughout pregnancy and lactation. SFD/TFD exposure significantly increased body weight in both dams and pups compared to controls. Microglial activation markers were increased in the hippocampus of SFD/TFD pups at birth. At weaning and in adulthood, proinflammatory cytokine expression was strikingly increased in the periphery and hippocampus following a bacterial challenge [lipopolysaccharide (LPS)] in the SFD/TFD groups compared to controls. Microglial activation within the hippocampus was also increased basally in SFD rats, suggesting a chronic priming of the cells. Finally, there were marked changes in anxiety and spatial learning in SFD/TFD groups. These effects were all observed in adulthood, even after the pups were placed on standard chow at weaning, suggesting these outcomes were programmed early in life.

The role of cyclophilin D in learning and memory

Cyclophilin D (Cyp D) is implicated in cell death pathway and blockade of Cyp D could be a potent therapeutic strategy for degenerative disorders such as Alzheimer's disease, ischemia, and multiple sclerosis, but physiological role of Cyp D remains elusive. Here, we investigated the ability of learning and memory in several behavioral tasks in mice that lacked Cyp D (Cyp D(-/-)) and the relationship between ability of learning and memory and hippocampal architecture or neuronal transmission in Cyp D(-/-) mice. Cyp D(-/-) mice showed impairments of short-term memory in the Y-maze, object recognition memory in the novel-object recognition test, reference memory in the water maze test, and associative learning in the conditioned fear learning test. Hippocampal infusion of Cyclosporine A, which binds to Cyp D, replicated the defect in hippocampus-dependent cognition observed in Cyp D(-/-) mice. The Cyp D(-/-) mice did not show histopathological abnormalities upon Nissl staining and GFAP immunostaining or irregular expression of neuronal and glial marker proteins on Western blotting. However, release of glutamate and acetylcholine was decreased from the hippocampus in response to high-potassium treatment in the Cyp D(-/-) mice than in the wild-type mice. These results suggest a physiological role for Cyp D in learning and memory via the regulation of neurotransmission.

Mitochondrial permeability transition pore opening as a promising therapeutic target in cardiac diseases

In addition to their central role in ATP synthesis, mitochondria play a critical role in cell death. Oxidative stress accompanied by calcium overload, ATP depletion, and elevated phosphate levels induces mitochondrial permeability transition (MPT) with formation of nonspecific MPT pores (MPTP) in the inner mitochondrial membrane. Pore opening results in mitochondrial dysfunction with uncoupled oxidative phosphorylation and ATP hydrolysis, ultimately leading to cell death. For the past 20 years, three proteins have been accepted as key structural components of the MPTP: adenine nucleotide translocase (ANT) in the inner membrane, cyclophilin D (CyP-D) in the matrix, and the voltage-dependent anion channel (VDAC) in the outer membrane. However, most recent studies have questioned the molecular identity of the pores. Genetic studies have eliminated the VDAC as an essential component of MPTP and attributed a regulatory (rather than structural) role to ANT. Currently, the phosphate carrier appears to play a crucial role in MPTP formation. MPTP opening has been examined extensively in cardiac pathological conditions, including ischemia/reperfusion as well as heart failure. Accordingly, MPTP is accepted as a therapeutic target for both pharmacological and conditional strategies to block pore formation by direct interaction with MPTP components or indirectly by decreasing MPTP inducers. Inhibition of MPTP opening by reduction of CyP-D activity by nonimmunosuppressive analogs of cyclosporine A or sanglifehrin A, as well as attenuation of reactive oxygen species accumulation through mitochondria-targeted antioxidants, is the most promising. This review outlines our current knowledge of the structure and function of the MPTP and describes possible approaches for cardioprotection.

What is the mitochondrial permeability transition pore?

Under conditions of mitochondrial calcium overload, especially when accompanied by oxidative stress, elevated phosphate concentrations and adenine nucleotide depletion, a non-specific pore, the mitochondrial permeability transition pore (MPTP), opens in the inner mitochondrial membrane. MPTP opening enables free passage into the mitochondria of molecules of <1.5 kDa including protons. The resulting uncoupling of oxidative phosphorylation leads to ATP depletion and necrotic cell death and it is now widely recognised that MPTP opening is a major cause of reperfusion injury and an effective target for cardioprotection. The properties of the MPTP are well defined, but despite extensive research in many laboratories, its exact molecular identity remains uncertain. Knockout studies have confirmed a role for cyclophilin-D (CyP-D), probably mediated by its peptidyl-prolyl cis-trans isomerase activity facilitating a conformational change of an inner membrane protein. However, the identity of the membrane component(s) remains controversial. Knockout studies have eliminated an essential role for either the voltage dependent anion channel (VDAC) or the adenine nucleotide translocase (ANT), although a regulatory role for the ANT was confirmed. Our own studies implicate the mitochondrial phosphate carrier (PiC) in MPTP formation and are consistent with a calcium-triggered conformational change of the PiC, facilitated by CyP-D, inducing pore opening. We propose that this is enhanced by an association of the PiC with the "c" conformation of the ANT. Agents that modulate pore opening may act on either or both the PiC and the ANT. However, knockdown and reconstitution studies are awaited to confirm or refute this model.

Honokiol, a multifunctional antiangiogenic and antitumor agent

Honokiol is a small-molecule polyphenol isolated from the genus Magnolia. It is accompanied by other related polyphenols, including magnolol, with which it shares certain biologic properties. Recently, honokiol has been found to have antiangiogenic, antiinflammatory, and antitumor properties in preclinical models, without appreciable toxicity. These findings have increased interest in bringing honokiol to the clinic as a novel chemotherapeutic agent. In addition, mechanistic studies have tried to find the mechanism(s) of action of honokiol, for two major reasons. First, knowledge of the mechanisms of action may assist development of novel synthetic analogues. Second, mechanistic actions of honokiol may lead to rational combinations with conventional chemotherapy or radiation for enhanced response to systemic cancers. In this review, we describe the findings that honokiol has two major mechanisms of action. First, it blocks signaling in tumors with defective p53 function and activated ras by directly blocking the activation of phospholipase D by activated ras. Second, honokiol induces cyclophilin D, thus potentiating the mitochondrial permeability transition pore, and causing death in cells with wild-type p53. Knowledge of the dual activities of honokiol can assist with the development of honokiol derivatives and the design of clinical trials that will maximize the potential benefit of honokiol in the patient setting.