Synaptic mitochondria: a brain mitochondria cluster with a specific proteome

The synapse is a particularly important compartment of neurons. To reveal its molecular characteristics we isolated whole brain synaptic (sMito) and non-synaptic mitochondria (nsMito) from the mouse brain with purity validated by electron microscopy and fluorescence activated cell analysis and sorting. Two-dimensional differential gel electrophoresis and mass spectrometry based proteomics revealed 22 proteins with significantly higher and 34 proteins with significantly lower levels in sMito compared to nsMito. Expression differences in some oxidative stress related proteins, such as superoxide dismutase [Mn] (Sod2) and complement component 1Q subcomponent-binding protein (C1qbp), as well as some tricarboxylic acid cycle proteins, including isocitrate dehydrogenase subunit alpha (Idh3a) and ATP-forming β subunit of succinyl-CoA ligase (SuclA2), were verified by Western blot, the latter two also by immunohistochemistry. The data suggest altered tricarboxylic acid metabolism in energy supply of synapse while the marked differences in Sod2 and C1qbp support high sensitivity of synapses to oxidative stress. Further functional clustering demonstrated that proteins with higher synaptic levels are involved in synaptic transmission, lactate and glutathione metabolism. In contrast, mitochondrial proteins associated with glucose, lipid, ketone metabolism, signal transduction, morphogenesis, protein synthesis and transcription were enriched in nsMito. Altogether, the results suggest a specifically tuned composition of synaptic mitochondria.

BIOLOGICAL SIGNIFICANCE

Neurons communicate with each other through synapse, a compartment metabolically isolated from the cell body. Mitochondria are concentrated in presynaptic terminals by active transport to provide energy supply for information transfer. Mitochondrial composition in the synapse may be different than in the cell body as some examples have demonstrated altered mitochondrial composition with cell type and cellular function in the muscle, heart and liver. Therefore, we posed the question whether protein composition of synaptic mitochondria reflects its specific functions. The determined protein difference pattern was in accordance with known functional specialties of high demand synaptic mitochondria. The data also suggest specifically tuned metabolic fluxes for energy production by means of interaction with glial cells surrounding the synapse. These findings provide possible mechanisms for dynamically adapting synaptic mitochondrial output to actual demand. In turn, an increased vulnerability of synaptic mitochondria to oxidative stress is implied by the data. This is important from theoretical but potentially also from therapeutic aspects. Mitochondria are known to be affected in some neurodegenerative and psychiatric disorders, and proteins with elevated level in synaptic mitochondria, e.g. C1qbp represent targets for future drug development, by which synaptic and non-synaptic mitochondria can be differentially affected.

Parsing glucose entry into the brain: novel findings obtained with enzyme-based glucose biosensors

Extracellular levels of glucose in brain tissue reflect dynamic balance between its gradient-dependent entry from arterial blood and its use for cellular metabolism. In this work, we present several sets of previously published and unpublished data obtained by using enzyme-based glucose biosensors coupled with constant-potential high-speed amperometry in freely moving rats. First, we consider basic methodological issues related to the reliability of electrochemical measurements of extracellular glucose levels in rats under physiologically relevant conditions. Second, we present data on glucose responses induced in the nucleus accumbens (NAc) by salient environmental stimuli and discuss the relationships between local neuronal activation and rapid glucose entry into brain tissue. Third, by presenting data on changes in NAc glucose induced by intravenous and intragastric glucose delivery, we discuss other mechanisms of glucose entry into the extracellular domain following changes in glucose blood concentrations. Lastly, by showing the pattern of NAc glucose fluctuations during glucose-drinking behavior, we discuss the relationships between “active” and “passive” glucose entry to the brain, its connection to behavior-related metabolic activation, and the possible functional significance of these changes in behavioral regulation. These data provide solid experimental support for the “neuronal” hypothesis of neurovascular coupling, which postulates the critical role of neuronal activity in rapid regulation of vascular tone, local blood flow, and entry of glucose and oxygen to brain tissue to maintain active cellular metabolism.

Assessment of metabolic changes within normal appearing gray and white matter in children with growth hormone deficiency: magnetic resonance spectroscopy and hormonal correlation

OBJECTIVE

The pathogenesis of idiopathic growth hormone deficiency (GHD) in children, including possible cerebral metabolic alterations, remains unclear. The aim of the study was to evaluate metabolic changes within the normal appearing brain in children with GHD using MR spectroscopy (MRS) and to correlate MRS measurements with hormonal concentrations and with pituitary gland size.

METHODS

Seventy children with GHD (mean age 7.8 yrs) and 11 healthy controls (mean age 8.4 yrs) were enrolled in the study. The MRS examinations were performed on a 1.5T scanner. Voxels were located in the posterior cingulate gyrus (PCG) and the left parietal white matter (PWM). The NAA/Cr, Cho/Cr and mI/Cr ratios were analyzed. The metabolite ratios, pituitary gland size and hormonal concentrations: growth hormone (GH) in two stimulation tests and GH during the night, as well as IGF-1 (insulin-like growth factor) and IGFBP3 (insulin-like growth factor-binding protein) levels were also correlated.

RESULTS

There was a significant (p < 0.05) decrease of the NAA/Cr ratios in PCG and PWM in children with GHD compared to the normal subjects. Other metabolite ratios showed no significant differences. We also found significant positive correlations between NAA/Cr ratio in PWM and IGFBP3 level, as well as with GH concentration in a stimulation test with glucagon.

CONCLUSIONS

The reduction of NAA/Cr ratios may suggest loss of neuronal activity within normal appearing gray and white matters in children with GHD. MRS could be a sensitive marker of cerebral metabolic disturbances associated with GHD and maybe used as an additional indicator for therapy with recombinant GH.

Isoform-selective regulation of glycogen phosphorylase by energy deprivation and phosphorylation in astrocytes

Glycogen phosphorylase (GP) is activated to degrade glycogen in response to different stimuli, to support both the astrocyte's own metabolic demand and the metabolic needs of neurons. The regulatory mechanism allowing such a glycogenolytic response to distinct triggers remains incompletely understood. In the present study, we used siRNA-mediated differential knockdown of the two isoforms of GP expressed in astrocytes, muscle isoform (GPMM), and brain isoform (GPBB), to analyze isoform-specific regulatory characteristics in a cellular setting. Subsequently, we tested the response of each isoform to phosphorylation, triggered by incubation with norepinephrine (NE), and to AMP, increased by glucose deprivation in cells in which expression of one GP isoform had been silenced. Successful knockdown was demonstrated on the protein level by Western blot, and on a functional level by determination of glycogen content showing an increase in glycogen levels following knockdown of either GPMM or GPBB. NE triggered glycogenolysis within 15 min in control cells and after GPBB knockdown. However, astrocytes in which expression of GPMM had been silenced showed a delay in response to NE, with glycogen levels significantly reduced only after 60 min. In contrast, allosteric activation of GP by AMP, induced by glucose deprivation, seemed to mainly affect GPBB, as only knockdown of GPBB, but not of GPMM, delayed the glycogenolytic response to glucose deprivation. Our results indicate that the two GP isoforms expressed in astrocytes respond to different physiological triggers, therefore conferring distinct metabolic functions of brain glycogen.

(13)C-labelled microdialysis studies of cerebral metabolism in TBI patients

Human brain chemistry is incompletely understood and better methodologies are needed. Traumatic brain injury (TBI) causes metabolic perturbations, one result of which includes increased brain lactate levels. Attention has largely focussed on glycolysis, whereby glucose is converted to pyruvate and lactate, and is proposed to act as an energy source by feeding into neurons’ tricarboxylic acid (TCA) cycle, generating ATP. Also reportedly upregulated by TBI is the pentose phosphate pathway (PPP) that does not generate ATP but produces various molecules that are putatively neuroprotective, antioxidant and reparative, in addition to lactate among the end products.

We have developed a novel combination of ¹³C-labelled cerebral microdialysis both to deliver ¹³C-labelled substrates into brains of TBI patients and recover the ¹³C-labelled metabolites, with high-resolution ¹³C NMR analysis of the microdialysates. This methodology has enabled us to achieve the first direct demonstration in humans that the brain can utilise lactate via the TCA cycle. We are currently using this methodology to make the first direct comparison of glycolysis and the PPP in human brain.

In this article, we consider the application of ¹³C-labelled cerebral microdialysis for studying brain energy metabolism in patients. We set this methodology within the context of metabolic pathways in the brain, and ¹³C research modalities addressing them.

[Cortical spreading depolarization phenomena in patients with traumatic and ischemic brain injuries. Results of a pilot study]

OBJECTIVES

To determine the frequency and duration of cortical spreading depolarization (CSD) and CSD-like episodes in patients with traumatic brain injury (TBI) and malignant middle cerebral artery infarction (MMCAI) requiring craniotomy.

DESIGN

A descriptive observational study was carried out during 19 months.

SETTING

Neurocritical patients.

PATIENTS

Sixteen patients were included: 9 with MMCAI and 7 with moderate or severe TBI, requiring surgical treatment.

INTERVENTIONS

A 6-electrode subdural electrocorticographic (ECoG) strip was placed onto the perilesional cortex.

MAIN VARIABLES OF INTEREST

An analysis was made of the time profile and the number and duration of CSD and CSD-like episodes recorded from the ECoGs.

RESULTS

Of the 16 patients enrolled, 9 presented episodes of CSD or CSD-like phenomena, of highly variable frequency and duration.

CONCLUSIONS

Episodes of CSD and CSD-like phenomena are frequently detected in the ischemic penumbra and/or traumatic cortical regions of patients with MMCAI who require decompressive craniectomy or of patients with contusional TBI.

Mapping phosphorylation rate of fluoro-deoxy-glucose in rat brain by (19)F chemical shift imaging

(19)F magnetic resonance spectroscopy (MRS) studies of 2-fluoro-2-deoxy-d-glucose (FDG) and 2-fluoro-2-deoxy-d-glucose-6-phosphate (FDG-6P) can be used for directly assessing total glucose metabolism in vivo. To date, (19)F MRS measurements of FDG phosphorylation in the brain have either been achieved ex vivo from extracted tissue or in vivo by unusually long acquisition times. Electrophysiological and functional magnetic resonance imaging (fMRI) measurements indicate that FDG doses up to 500 mg/kg can be tolerated with minimal side effects on cerebral physiology and evoked fMRI-BOLD responses to forepaw stimulation. In halothane-anesthetized rats, we report localized in vivo detection and separation of FDG and FDG-6P MRS signals with (19)F 2D chemical shift imaging (CSI) at 11.7 T. A metabolic model based on reversible transport between plasma and brain tissue, which included a non-saturable plasma to tissue component, was used to calculate spatial distribution of FDG and FDG-6P concentrations in rat brain. In addition, spatial distribution of rate constants and metabolic fluxes of FDG to FDG-6P conversion were estimated. Mapping the rate of FDG to FDG-6P conversion by (19)F CSI provides an MR methodology that could impact other in vivo applications such as characterization of tumor pathophysiology.

An exploratory study of the relationship of symptom domains and diagnostic severity to PET scan imaging in borderline personality disorder

The purpose of this report is to describe the relationship between clinical rating assessments of borderline personality disorder (BPD) and regional brain metabolism as measured by positron emission tomography with fluorodeoxyglucuse-F18 (PET-FDG). Fourteen women with BPD underwent PET-FDG scanning in a medication-free state. Correlations were performed on a voxel-by-voxel basis with Buss-Durkee Hostility Index (BDHI) and the Zanarini Rating Scale for Borderline Personality Disorder (ZAN-BPD) which provides a score for BPD severity. There was a significant negative correlation between glucose metabolism in frontal brain areas and the BDHI. Correlations of brain metabolic changes and diagnostic behavioral rating scale scores (ZAN-BPD) were small and seen mostly in posterior areas. The assessment of the statistical relationship of the BDHI to brain regions was substantially more robust than the correlations of the total ZAN-BPD. This exploratory study illustrates regional metabolic values that are highly related to hostile behavior. Our findings replicate some prior studies that have identified a negative relationship between frontal metabolism and aggression in personality disorders. We have also identified a range of other areas that relate to both positive (representing increased drive) and negative (representing impaired control) hostility scores. The substantially greater correlations of the BDHI compared with the ZAN-BPD provide information about the neural underpinnings of BPD.

Multimodal invasive monitoring in status epilepticus: what is the evidence it has a place?

The underlying pathophysiology of status epilepticus (SE) remains mostly invisible to the clinician in the intensive care unit (ICU) setting. In animal studies associated hemodynamic and brain neurochemical changes have been well described. In the last decade, bedside invasive neuromonitoring techniques allow the assessments of changes in focal and global cerebral physiology associated with ictal activity on the tissue level in humans. Recent studies demonstrate that laboratory research insufficiently replicates the complexity of the human condition. Herein we summarize the current knowledge gained from human studies integrating cortical electrographic and brain tissue metabolic and hemodynamic information into the current pathophysiologic concept of SE in humans. With increasing experience gained by the use of extended neuromonitoring, we are more and more able to understand associated hemodynamic and brain neurochemical changes in patients with SE. In the future, this information can potentially provide integrated pathophysiologic end points into SE treatment concepts.

Brain metabolism and oxygenation in healthy pigs receiving hypoventilation and hyperoxia

Modulation in ventilatory settings is one of the approaches and interventions used to treat and prevent secondary brain damage after traumatic brain injury (TBI). Here we investigate the effect of hyperoxia in combination with hypoventilation on brain oxygenation, metabolism and intracranial pressure. Twelve pigs were divided into three groups; group1-100% hyperoxia (n=4), group 2-100% hyperoxia and 20% decrease in minute volume (MV) (n=4) and group 3-100% hyperoxia and 50% decrease in MV (n=4). Neither of the ventilator settings affected the lactate/pyruvate ratio significantly. However, there was a significant decrease of brain lactate (2.6±1.7 to 1.8±1.6mM) and a rapid and marked increase in brain oxygenation (7.9±0.7 to 61.3±17.6mmHg) in group 3. Intracranial pressure (ICP) was not significantly affected in this group, however, the ICP increased significantly in group 2 with 100% hyperoxia plus 20% reduction in minute volume. We conclude that hyperoxia in combination with 50% decrease in MV showed pronounced increase in partial brain oxygen tension (pbrO2) and decrease in brain lactate. The ventilatory modification, used in this study should be considered for further investigation as a possible therapeutic intervention for TBI patients.

Magnetic resonance spectroscopy as a predictor of conversion of mild cognitive impairment to dementia

BACKGROUND AND PURPOSE

Proton magnetic resonance spectroscopy ((1)H-MRS) is one of the imaging techniques that could be potentially useful for identification of patients with mild cognitive impairment (MCI) who are at increased risk of developing dementia. The aim of the study was to investigate if there is a difference in brain metabolism between stable MCI patients and converters to dementia and if a use of (1)H-MRS can predict the conversion from MCI to dementia.

MATERIAL AND METHODS

Forty-one amnestic MCI patients and 35 cognitively unimpaired controls were examined by (1)H-MRS (TE - 35 ms) at baseline. Metabolite ratios (NAA/Cr, Cho/Cr, mI/Cr, Glx/Cr, NAA/Cho) were estimated in four different brain regions: posterior cingulate gyrus (PCG), left hippocampus (LH), cortical area of right (RPL) and left parietal lobe (LPL). Participants were followed up for a period of 12 months.

RESULTS

Twelve subjects with MCI progressed to Alzheimer's disease (AD) after one year. Analysis showed that the NAA/Cr ratio in the LH was significantly lower in MCI patients than in controls (p=0.008), but there were no differences in metabolite ratios at baseline between MCI converters and stable subjects. mI/Cr ratio in RPL predicted the conversion to AD with sensitivity 70% and specificity 85% (p<0.0004). Coexistence of diabetes improved prediction, yielding 70% sensitivity and 96% specificity (p<0.0001).

CONCLUSIONS

(1)H-MRS in MCI can be a predictor of cognitive decline and conversion to dementia, especially in MCI patients with coexisting diabetes. Further studies are needed to confirm this finding.

Insulin prevents mitochondrial generation of H₂O₂ in rat brain

The mitochondrial electron transport system (ETS) is a main source of cellular ROS, including hydrogen peroxide (H₂O₂). The production of H₂O₂ also involves the mitochondrial membrane potential (ΔΨm) and oxygen consumption. Impaired insulin signaling causes oxidative neuronal damage and places the brain at risk of neurodegeneration. We evaluated whether insulin signaling cross-talks with ETS components (complexes I and F₀F₁ATP synthase) and ΔΨm to regulate mitochondrial H₂O₂ production, in tissue preparations from rat brain. Insulin (50 to 100 ng/mL) decreased H₂O₂ production in synaptosomal preparations in high Na(+) buffer (polarized state), stimulated by glucose and pyruvate, without affecting the oxygen consumption. In addition, insulin (10 to 100 ng/mL) decreased H₂O₂ production induced by succinate in synaptosomes in high K(+) (depolarized state), whereas wortmannin and LY290042, inhibitors of the PI3K pathway, reversed this effect; heated insulin had no effect. Insulin decreased H₂O₂ production when complexes I and F₀F₁ATP synthase were inhibited by rotenone and oligomycin respectively suggesting a target effect on complex III. Also, insulin prevented the generation of maximum level of ∆Ψm induced by succinate. The PI3K inhibitors and heated insulin maintained the maximum level of ∆Ψm induced by succinate in synaptosomes in a depolarized state. Similarly, insulin decreased ROS production in neuronal cultures. In mitochondrial preparations, insulin neither modulated H2O2 production or oxygen consumption. In conclusion, the normal downstream insulin receptor signaling is necessary to regulate complex III of ETS avoiding the generation of maximal ∆Ψm and increased mitochondrial H2O2 production.

Regional brain metabolism in schizophrenia: An FDG-PET study

BACKGROUND

Recent technological advances have established beyond any doubt the biological nature of schizophrenia. Functional neuroimaging using FDG-PET forms an important technique in understanding the biological underpinnings of psychopathology of schizophrenia.

METHODS

Eighteen male patients diagnosed as having schizophrenia and having active psychosis as determined by PANSS were subjected to FDG-PET scanning under resting conditions. The glucose uptake in selected regions of interest was studied across the spectrum of schizophrenia.

RESULTS

Chronicity and severity of illness did not influence cerebral glucose metabolism. Participants with negative schizophrenia had significantly decreased metabolism in all regions of the brain as compared to the positive type. The positive syndrome of schizophrenia was associated with significantly increased glucose metabolism in the medial temporal regions, basal ganglia and left thalamic regions. Hypometabolism was also noted in the cerebellum.

CONCLUSION

While a number of brain areas can be identified as potential causative regions and hypotheses regarding putative mechanisms can be formed, the considerable heterogeneity of schizophrenia poses a great challenge in the precise delineation of the disease process.