Induction of Na+/myo-inositol cotransporter mRNA after focal cerebral ischemia: evidence for extensive osmotic stress in remote areas

Biomechanical properties of the hypoxic and dying brain quantified by magnetic resonance elastography

Respiratory arrest is a major life-threatening condition leading to cessation of vital functions and hypoxic-anoxic injury of the brain. The progressive structural tissue changes characterizing the dying brain biophysically are unknown. Here we use noninvasive magnetic resonance elastography to show that biomechanical tissue properties are highly sensitive to alterations in the brain in the critical period before death. Our findings demonstrate that brain stiffness increases after respiratory arrest even when cardiac function is still preserved. Within 5 min of cardiac arrest, cerebral stiffness further increases by up to 30%. This early mechanical signature of the dying brain can be explained by water accumulation and redistribution from extracellular spaces into cells. These processes, together, increase interstitial and intracellular pressure as revealed by magnetic resonance spectroscopy and diffusion-weighted imaging. Our data suggest that the fast response of cerebral stiffness to respiratory arrest enables the monitoring of life-threatening brain pathology using noninvasive in vivo imaging. STATEMENT OF SIGNIFICANCE: Hypoxia-anoxia is a life-threatening condition eventually leading to brain death. Therefore, monitoring vital brain functions in patients at risk is urgently required during emergency care or treatment of acute brain damage due to insufficient oxygen supply. In mouse model of hypoxia-anoxia, we have shown for the first time that biophysical tissue parameters such as brain stiffness changed markedly during the process of death.

Myo-inositol metabolism in appropriately grown and growth-restricted fetuses: a proton magnetic resonance spectroscopy study

OBJECTIVE

Myo-inositol (Myo-ins) is a marker of neuroglial cells, being present in the astrocytes of brain tissue, but also functions as an osmolyte. Numbers of astrocytes are known to increase following injury to the brain. Growth-restricted fetuses are at increased risk of later neurodevelopmental impairments even in the absence of overt lesions and despite preserved/increased cerebral blood flow. This study aims to investigate brain Myo-ins metabolism in fetuses with intrauterine growth restriction (IUGR) and evidence of cerebral redistribution using magnetic resonance spectroscopy (MRS) at a short echo time.

STUDY DESIGN

Biometry and Doppler assessment of blood flow was assessed using ultrasound in 28 fetuses with IUGR and 47 appropriately grown control subjects. MRI was used to exclude overt brain injury. Proton magnetic resonance spectroscopy of the fetal brain was then performed at an echo time of 42 ms to examine the Myo-ins:Choline (Cho), Myo-ins:Creatine (Cr) and Cho:Cr ratios.

RESULTS

No alterations in brain Myo-ins:Cho, Myo-ins:Cr or Cho:Cr ratios were detected between appropriately grown and growth restricted fetuses.

CONCLUSIONS

IUGR is not associated with a measureable difference in brain myo-inositol ratios. This may be due to the protective effects of preserved cerebral blood flow in growth restriction and comparable astrocyte numbers when compared to controls.

Deletion of TRAAK potassium channel affects brain metabolism and protects against ischemia

Cerebral stroke is a worldwide leading cause of disability. The two-pore domain K⁺ channels identified as background channels are involved in many functions in brain under physiological and pathological conditions. We addressed the hypothesis that TRAAK, a mechano-gated and lipid-sensitive two-pore domain K⁺ channel, is involved in the pathophysiology of brain ischemia. We studied the effects of TRAAK deletion on brain morphology and metabolism under physiological conditions, and during temporary focal cerebral ischemia in Traak⁻/⁻ mice using a combination of in vivo magnetic resonance imaging (MRI) techniques and multinuclear magnetic resonance spectroscopy (MRS) methods. We provide the first in vivo evidence establishing a link between TRAAK and neurometabolism. Under physiological conditions, Traak⁻/⁻ mice showed a particular metabolic phenotype characterized by higher levels of taurine and myo-inositol than Traak⁺/⁺ mice. Upon ischemia, Traak⁻/⁻ mice had a smaller infarcted volume, with lower contribution of cellular edema than Traak⁺/⁺ mice. Moreover, brain microcirculation was less damaged, and brain metabolism and pH were preserved. Our results show that expression of TRAAK strongly influences tissue levels of organic osmolytes. Traak⁻/⁻ mice resilience to cellular edema under ischemia appears related to their physiologically high levels of myo-inositol and of taurine, an aminoacid involved in the modulation of mitochondrial activity and cell death. The beneficial effects of TRAAK deletion designate this channel as a promising pharmacological target for the treatment against stroke.

Nuclear factor of activated T cells 5 deficiency increases the severity of neuronal cell death in ischemic injury

Nuclear factor of activated T cells 5 (NFAT5) has been implicated in regulating several genes that are thought to be neuroprotective in ischemic injury. Because of the embryonic lethality of NFAT5 knockout (NFAT5(-/-)) mice, the heterozygous (NFAT5(+/-)) mice were used to study the in vivo role of NFAT5 in hypoxia/ischemia (H/I) condition. The NFAT5(+/-) mice exhibited more severe neurological deficits, larger infarct area and edema formation associated with increased aquaporin 4 expressions in the brain. Under in vitro H/I condition, increased apoptotic cell death was found in NFAT5(-/-) neurons. Moreover, SMIT, a downstream to NFAT5, was upregulated in NFAT5(+/+) neurons, while the SMIT level could not be upregulated in NFAT5(-/-) neurons under H/I condition. The elevation of reactive oxygen species generation in NFAT5(-/-) neurons under H/I condition further confirmed that NFAT5(-/-) neurons were more susceptible to oxidative stress. The present study demonstrated that activation of NFAT5 and its downstream SMIT induction is important in protecting neurons from ischemia-induced oxidative stress.

Magnetic resonance spectroscopy of the brain under mild hypothermia indicates changes in neuroprotection-related metabolites

Brain hypothermia has demonstrated pronounced neuroprotective effect in patients with cardiac arrest, ischemia and acute liver failure. However, its underlying neuroprotective mechanisms remain to be elucidated in order to improve therapeutic outcomes. Single voxel proton magnetic resonance spectroscopy ((1)H-MRS) was performed using a 7 Tesla MRI scanner on normal Sprague-Dawley rats (N=8) in the same voxel under normothermia (36.5 degrees C) and 30min mild hypothermia (33.5 degrees C). Levels of various brain proton metabolites were compared. The level of lactate (Lac) and myo-inositol (mI) increased in the cortex during hypothermia. In the thalamus, taurine (Tau), a cryogen in brain, increased and choline (Cho) decreased. These metabolic alterations indicated the onset of a number of neuroprotective processes that include attenuation of energy metabolism, excitotoxic pathways, brain osmolytes and thermoregulation, thus protecting neuronal cells from damage. These experimental findings demonstrated that (1)H-MRS can be applied to investigate the changes of specific metabolites and corresponding neuroprotection mechanisms in vivo noninvasively, and ultimately improve our basic understanding of hypothermia and ability to optimize its therapeutic efficacy.

High cerebral scyllo-inositol: a new marker of brain metabolism disturbances induced by chronic alcoholism

Cerebral metabolic changes that concur to motor and/or cognitive disorders in actively drinking alcoholics are not well established. We tested the hypothesis that chronic alcoholics exhibit profound alterations in the cerebral metabolism of scyllo-inositol. Brain metabolism was explored in nine actively drinking and 11 recently detoxified chronic alcoholics by in vivo brain (1)H-MRS and in vitro(1)H-MRS of blood serum and cerebrospinal fluid. The cohort was composed of individuals with acute, subacute or chronic encephalopathy or without any clinical encephalopathy. Chronic alcoholism is associated with a hitherto unrecognized accumulation of brain scyllo-inositol. Our results suggest that scyllo-inositol is produced within the central nervous system and shows a diffuse but heterogenous distribution in brain where it can persist several weeks after detoxification. Its highest levels were observed in subjects with a clinically symptomatic alcohol-related encephalopathy. When detected, brain scyllo-inositol takes part in a metabolic encephalopathy since it is associated with reduced N-acetylaspartate and increased creatine. High levels of cerebral scyllo-inositol are correlated with altered glial and neuronal metabolism. Our findings suggest that the accumulation of scyllo-inositol may precede and take part in the development of symptomatic alcoholic metabolic encephalopathy.

Subarachnoid hemorrhage induces Na+/myo-inositol cotransporter in the rat brain

Neurons and glial cells respond to extracellular hyperosmolarity by accumulating small organic solutes, called "osmolytes." Na+/myo-inositol is one of the major organic osmolytes in the brain and Na+/myo-inositol cotransporter (SMIT) regulates extracellular Na+/myo-inositol content. Subarachnoid hemorrhage (SAH) is an osmotic stress-inducing event of the brain. The expression of SMIT messenger ribonucleic acid (mRNA) and protein was investigated with in situ hybridization and immunohistochemistry in rat brains with SAH induced by endovascular perforation. SMIT riboprobe was raised from a 490-bp rat SMIT complementary deoxyribonucleic acid. Anti-SMIT antibody was raised in rabbits. SMIT mRNA was expressed strongly in the cortex, hippocampus, and hypothalamus of the perforated side at 6 to 24 hours after SAH. Mild upregulation was noted in the contralateral cortex, hippocampus, and hypothalamus. The ventral aspect of the pons showed mild upregulation. Microautoradiography and immunostaining showed SMIT expression mainly in the neurons, but also in some non-neural cells in the hippocampus. The present results indicate that diffuse osmotic stress occurs in the host brain after SAH.

Intracerebral hemorrhage upregulates Na(+)/myo-inositol cotransporter in the rat brain

We examined the expression of Na(+)/myo-inositol cotransporter (SMIT) in the rat brain after intrastriatal hemorrhage. The expression of SMIT messenger RNA (mRNA) increased around hematoma 3 days after hemorrhage and it returned to control level as hematoma was absorbed. The expression of SMIT mRNA also increased at ipsilateral substantia nigra without blood-brain barrier disruption 7 days after hemorrhage and remained high until 42 days after hemorrhage. Immunohistochemistry revealed that macrophages around hematoma and microglias at ipsilateral substantia nigra were SMIT-positive. These results suggest that the expression of SMIT must be regulated not only by osmolality but also by unknown factors.

1H-NMR spectroscopy of cerebrospinal fluid of fetal sheep during hypoxia-induced acidemia and recovery

The purpose of the study was to investigate the sequence of processes occurring during and after hypoxia-induced acidemia. We used proton nuclear magnetic resonance spectroscopy, which provides an overview of metabolites in cerebrospinal fluid (CSF), reflecting neuronal metabolism and damage. The pathophysiological condition of acute fetal asphyxia was mimicked by reducing maternal uterine blood flow in 14 unanesthetized pregnant ewes. CSF metabolites were measured during hypoxia-induced acidemia, and during the following recovery period, including the periods at 24 and 48 h after the hypoxic insult. Maximum values of the following CSF metabolites were reached during severe hypoxia (pH <or= 7.00): glucose, lactate, pyruvate, hypoxanthine, alanine, beta-hydroxybutyrate, choline, creatine, myo-inositol, citrate, succinate, valine, and an unknown metabolite characterized by a resonance at 1.56 ppm in the proton nuclear magnetic resonance spectrum. Twenty-four hours after the hypoxic insult, myo-inositol was increased, and alanine was decreased 48 h after the hypoxic insult, both compared with control values. Choline levels in CSF had a linear relationship with arterial pH (r = 0.26, p < 0.005). During severe hypoxia, CSF levels of succinate and choline are increased. Increased CSF levels of succinate may indicate dysfunction of the mitochondrial respiratory chain, whereas elevated CSF choline levels may indicate disrupted cell membranes. The increase of the CSF myo-inositol level after 24 and 48 h may indicate osmolytic cell changes causing cell edema. Decreased alanine level may represent changes in the source of excitatory amino acid synthesis.

The cellular hydration state: a critical determinant for cell death and survival

Alterations in cellular hydration not only contribute to metabolic regulation, but also critically determine the cellular response to different kinds of stress. Whereas cell swelling triggers anabolic pathways and protects cells from heat and oxidative challenge, cellular dehydration contributes to insulin resistance and catabolism and increases the cellular susceptibility to stress-induced damage. Intracellular accumulation of organic osmolytes, cell cycle delay and the expression of heat shock proteins provide cellular tolerance to hyperosmolarity and protect against stressors under dehydrating conditions. This article discusses some mechanisms by which alterations in cell hydration contribute to cytoprotection or cell damage. In addition, the close relationship between osmotic and oxidative stress and the contribution of isoosmotic shrinkage to apoptotic cell death are considered.

Peg3/Pw1 is involved in p53-mediated cell death pathway in brain ischemia/hypoxia

Emerging evidence has shown that tumor suppressor p53 expression is enhanced in response to brain ischemia/hypoxia and that p53 plays a critical role in the cell death pathway in such an acute neurological insult. However the mechanism remains unclear. Recently it was reported that Peg3/Pw1, originally identified as a paternally expressed gene, plays a pivotal role in the p53-mediated cell death pathway in mouse fibroblast cell lines. In this study, we found that Peg3/Pw1 expression is enhanced in peri-ischemic neurons in rat stroke model by in situ hybridization analysis, where p53 expression was also induced by immunohistochemical analysis. Moreover, we found that p53 was co-localized with Peg3/Pw1 in brain ischemia/hypoxia by double staining analysis. In human neuroblastoma-derived SK-N-SH cells, Peg3/Pw1 mRNA expression is enhanced remarkably at 24 h post-hypoxia, when p53 protein expression was also enhanced at high levels. Subcellular localization of Peg3/Pw1 was observed in the nucleus. Adenovirus-mediated high dose p53 overexpression induced Peg3/Pw1 mRNA expression. Overexpression of Peg3/Pw1 reduced cell viability under hypoxic conditions, whereas that of the C-terminal-deleted mutant and anti-sense Peg3/Pw1 inhibited hypoxia-induced cell death. These results suggest that Peg3/Pw1 is involved in the p53-mediated cell death pathway as a downstream effector of p53 in brain ischemia/hypoxia.

Early increases in brain myo-inositol measured by proton magnetic resonance spectroscopy in term infants with neonatal encephalopathy

Our aim was to assess brain myo-inositol/creatine plus phosphocreatine (Cr) in the first week in term infants with neonatal encephalopathy using localized short echo time proton magnetic resonance spectroscopy and to relate this to measures of brain injury, specifically lactate/Cr in the first week, basal ganglia changes on magnetic resonance imaging (MRI), and neurodevelopmental outcome at 1 y. Fourteen term infants with neonatal encephalopathy of gestational age (mean +/- SD) 39.6 +/- 1.6 wk, birth weight 3270 +/- 490 g, underwent MRI and magnetic resonance spectroscopy at 3.5 +/- 2.1 d. Five infants were entered in a pilot study of treatment with moderate whole-body hypothermia for neonatal encephalopathy; two were being cooled at the time of the scan. T(1)- and T(2)-weighted transverse magnetic resonance images were graded as normal or abnormal according to the presence or absence of the normal signal intensity of the posterior limb of the internal capsule and signal intensity changes in the basal ganglia. Localized proton magnetic resonance spectroscopy data were obtained from an 8-cm(3) voxel in the basal ganglia using echo times of 40 and 270 ms, and the peak area ratios of myo-inositol/Cr and lactate/Cr were measured. Outcome was scored using Griffith's development scales and neurodevelopmental examination at 1 y. MRI and outcome were normal in six infants and abnormal in eight. myo-Inositol/Cr and lactate/Cr were higher in infants with abnormal MRI and outcome (p < 0.01, p < 0.01, respectively). myo-Inositol/Cr and lactate/Cr were correlated (p < 0.01) and were both correlated to the Griffith's developmental scales (p < 0.01, p < 0.01, respectively). In conclusion, these preliminary data suggest that early increases in brain basal ganglia myo-inositol/Cr in infants with neonatal encephalopathy are associated with increased lactate/Cr, MRI changes of severe injury, and a poor neurodevelopmental outcome at 1 y.

Diffuse brain injury induces local expression of Na+/myo-inositol cotransporter in the rat brain

We studied expression of an osmoprotective gene, sodium/myo-inositol cotransporter (SMIT) in Marmarou's animal model for human diffuse brain injury by in situ hybridization and immunohistochemistry. In rats with diffuse brain injury, transient upregulation of SMIT mRNA was exclusively observed in the lateral area of pyramidal tract in lower brainstem. The expression was induced at 1 h after injury, peaked at 24 h, and returned to almost control levels at 48 h. Upregulated expression was found mainly in small glia-like cells. By immunohistochemistry using antibodies to phosphorylated mitogen-activated protein (MAP) kinases, inductions of phosphorylated p44/42 MAP kinase were also observed after diffuse brain injury. Interestingly, the distribution patterns of induced phosphorylated p44/42 MAP kinase were completely coincident with those of upregulated SMIT mRNA after diffuse brain injury. These results suggest that diffuse brain injury induces local expression of SMIT by activation of p44/42 MAP kinase cascade. The confined SMIT induction may reflect regional differences of damage and/or cellular differences in sensitivity to neuropathological stresses caused by this injury.

Characterization of cerebral white matter damage in preterm infants using 1H and 31P magnetic resonance spectroscopy

The biochemical characteristics of white matter damage (WMD) in preterm infants were assessed using magnetic resonance spectroscopy (MRS). The authors hypothesized that preterm infants with WMD at term had a persisting cerebral lactic alkalosis and reduced N-acetyl aspartate (NAA)/ creatine plus phosphocreatine (Cr), similar to that previously documented in term infants weeks after perinatal hypoxiaischemia (HI). Thirty infants (gestational age 27.9 +/- 3.1 weeks, birth weight 1,122 +/- 445 g) were studied at postnatal age of 9.8 +/- 4.1 weeks (corrected age 40.3 +/- 3.9 weeks). Infants were grouped according to the presence or absence of WMD on magnetic resonance (MR) images. The peak area ratios of lactate/Cr, NAA/Cr, myo-inositol/Cr, and choline (Cho)/Cr were measured from an 8-cm3 voxel in the posterior periventricular white matter (WM) using proton MRS. Intracellular pH (pHi) was calculated using phosphorus MRS. Eighteen infants had normal WM on MR imaging; 12 had WMD. For infants with WMD, lactate/Cr and myo-inositol/Cr were related (P < 0.01); lactate/Cr and pHi were not (P = 0.8). In the WMD group, mean lactate/Cr and myo-inositol/Cr were higher (P < 0.001, P < 0.05, respectively) than the normal WM group. There was no difference in the NAA/Cr, Cho/Cr, or pHi between the two groups, although pHi was not measured in all infants. These findings suggest that WMD in the preterm infant at term has a different biochemical profile compared with the term infant after perinatal HI.

Induction of aquaporin-4 water channel mRNA after focal cerebral ischemia in rat

Aquaporin-4 (AQP4) is a member of a water-selective channel aquaporin-family and mainly expressed in the several structures of the brain and in the collecting duct of the kidney. Here we show its functional involvement in the water homeostasis of the ischemic brain. The expression of AQP4-mRNA is increased in the peri-infarcted cortex during the observation period ( approximately 7 days) after MCA-occlusion, maximally on day 3. The change corresponds to the generation and resolution of brain edema monitored by MRI. The signals for the mRNA are predominantly observed in glial cells in the molecular and outer granular layer of the peri-infarcted cortex. These results indicate that AQP4 plays a role in post-ischemic edema formation.

Kainic acid-induced seizure upregulates Na(+)/myo-inositol cotransporter mRNA in rat brain

A major organic osmolyte, myo-inositol protects cells from perturbing effects of high intracellular concentrations of electrolytes. Myo-inositol is accumulated into cells through Na(+)/myo-inositol cotransporter (SMIT). In order to investigate the regulation of SMIT in generalized seizure, we employed Northern blot analysis and in situ hybridization to study the changes in SMIT mRNA expression in kainic acid-injected rats. Northern blot analysis demonstrated that SMIT mRNA began to increase in the brain 2 h after onset of seizure, and peaked at 12 h. In situ hybridization revealed rapid increase of SMIT mRNA (2 h of seizure) in the CA3 hippocampal pyramidal cells and in the dentate granular cells. Then, at 4-6 h SMIT mRNA expression was observed in the other limbic structure such as amygdala and piriform cortex. Finally, in neocortex and in CA1 pyramidal cells, SMIT mRNA was slowly increased and peaked at 12 h. Microautoradiogram demonstrated that cells expressed SMIT mRNA were mainly neurons. These results suggest that SMIT mRNA is upregulated by kainic acid-induced seizure primarily in structures involved in seizure activity.

Neuroprotective role of Na+/myo-inositol cotransporter against veratridine cytotoxicity

Na+/myo-inositol cotransporter has been shown to protect cells from the perturbing effects of hypertonic stress by the accumulation of myo-inositol. Here we report a regulatory mechanism for the cotransporter. Induction of myo-inositol cotransporter mRNA was observed after exposure to veratridine, a voltage-gated sodium channel opener. The veratridine-elicited induction was inhibited when Na+ was eliminated from the bath, although calcium chelation failed to modify the gene expression. Veratridine evoked an accumulation of Na+ in the cells, which paralleled the abundance of the mRNA. These results strongly suggested that an increase in Na+ influx due to sodium channel opening affected transcription of the cotransporter gene. Activity of the myo-inositol cotransporter was also up-regulated after veratridine exposure. To clarify the possible roles of myoinositol accumulation under veratridine exposure, we next examined the neurotoxic effects of veratridine when myo-inositol uptake was blocked. Neither 30 microM veratridine nor 500 microM 2-O,C-methylene myo-inositol, a competitive inhibitor of myo-inositol, elicited apparent cytotoxicity. However, a combination of these agents markedly increased cytotoxicity in culture, suggesting that an adequate amount of myo-inositol was necessary when the cells were stimulated with veratridine.

Differential regulation of adenine nucleotide translocators by hypertonicity in the brain

To determine the gene(s) induced by hypertonicity in the brain, we performed a differential display analysis using RNA isolated from isotonic and hypertonic rat astrocytes. One cDNA rapidly up-regulated by hypertonicity was isolated, and the DNA sequence revealed that it was identical to adenine nucleotide translocator (ANT)2. ANT2 protein exchanges intramitochondrial ATP for cytoplasmic ADP. Among three ANT isoforms, only ANT2 mRNA was up-regulated markedly from 1 to 4 h after exposure to hypertonicity. Induction of the mRNA did not require de novo protein synthesis. Furthermore, ADP translocase activity in mitochondria of astrocytes was increased significantly by hypertonicity. To see the localization and regulation of ANT2 mRNA in the brain, we performed in situ hybridization of rat brain after intraperitoneal injection of a high concentration of NaCl. Although there were only weak signals in the control, intense hybridization signals were seen in hypertonic rat whole brain. Microscopic examination showed that ANT2 signals were present in the neurons, as well as glial cells. These results suggest that ANT2 may play a role in brain cells to adapt to the hypertonic environment.

Changes in brain organic osmolytes in experimental cerebral ischemia

The cell volume is regulated not only by inorganic ions, but also by organic osmolytes, such as amino acids, methylamines, and polyhydric alcohols (polyols). Using proton nuclear magnetic resonance spectroscopy (1H-NMR), we measured the tissue concentrations of amino acids (alanine, aspartate, gamma-aminobutyric acid (GABA), glutamate, glutamine, N-acetyl-aspartate (NAA), taurine), methylamines (glycerophosphorylcholine (GPC), creatine+phosphocreatine (total creatine, tCr)), and polyols (myo-inositol) in the rat brain after middle cerebral artery occlusion (incomplete focal ischemia) or after decapitation (complete global ischemia). The total osmolytes expressed as a sum of total amino acids, total methylamines, and total polyols were significantly decreased at 24 h of focal ischemia (58.7% of control value, P=0.0025) whereas they were not changed following decapitation. The water content was increased from control value of 77.9%-84.1% after focal ischemia (P<0.0001) but not after decapitation. These results suggest that the brain organic osmolytes are involved in the process of edema formation following focal cerebral ischemia. Further elucidation of the cellular mechanisms regulating these organic osmolytes in cerebral ischemia may promote greater understanding of the pathophysiology involved in the evolution of brain edema.