Increased N-acetylaspartate in model mouse of pelizaeus-merzbacher disease

Magnetic Resonance Imaging and Spectroscopy Analysis in a Pelizaeus-Merzbacher Disease Rat Model

Pelizaeus−Merzbacher disease (PMD) is an X-linked recessive disorder of the central nervous system. We performed 7 Tesla magnetic resonance imaging of the brain in Tama rats, a rodent PMD model, and control rats, as well as evaluated the diagnostic values. In the white matter of the Tama rats, the T2 values were prolonged, which is similar to that observed in patients with PMD (60.7 ± 1.8 ms vs. 51.6 ± 1.3 ms, p < 0.0001). The apparent diffusion coefficient values in the white matter of the Tama rats were higher than those of the control rats (0.68 ± 0.03 × 10−3 mm2/s vs. 0.64 ± 0.03 × 10−3 mm2/s, p < 0.05). In proton magnetic resonance spectroscopy, the N-acetylaspartate (6.97 ± 0.12 mM vs. 5.98 ± 0.25 mM, p < 0.01) and N-acetylaspartate + N-acetylaspartylglutamate values of the Tama rats were higher (8.22 ± 0.17 mM vs. 7.14 ± 0.35 mM, p < 0.01) than those of the control rats. The glycerophosphocholine + phosphocholine values of the Tama rats were lower than those of the control rats (1.04 ± 0.09 mM vs. 1.45 ± 0.04 mM, p < 0.001). By using Luxol fast blue staining, we confirmed dysmyelination in the Tama rats. These results are similar to those of patients with PMD and other PMD animal models.

Striatal N-Acetylaspartate Synthetase Shati/Nat8l Regulates Depression-Like Behaviors via mGluR3-Mediated Serotonergic Suppression in Mice

BACKGROUND

Several clinical studies have suggested that N-acetylaspartate and N-acetylaspartylglutamate levels in the human brain are associated with various psychiatric disorders, including major depressive disorder. We have previously identified Shati/Nat8l, an N-acetyltransferase, in the brain using an animal model of psychosis. Shati/Nat8l synthesizes N-acetylaspartate from L-aspartate and acetyl-coenzyme A. Further, N-acetylaspartate is converted into N-acetylaspartylglutamate, a neurotransmitter for metabotropic glutamate receptor 3.

METHODS

Because Shati/Nat8l mRNA levels were increased in the dorsal striatum of mice following the exposure to forced swimming stress, Shati/Nat8l was overexpressed in mice by the microinjection of adeno-associated virus vectors containing Shati/Nat8l gene into the dorsal striatum (dS-Shati/Nat8l mice). The dS-Shati/Nat8l mice were further assessed using behavioral and neurochemical tests.

RESULTS

The dS-Shati/Nat8l mice exhibited behavioral despair in the forced swimming and tail suspension tests and social withdrawal in the 3-chamber social interaction test. These depression-like behaviors were attenuated by the administration of a metabotropic glutamate receptor 2/3 antagonist and a selective serotonin reuptake inhibitor. Furthermore, the metabolism of N-acetylaspartate to N-acetylaspartylglutamate was decreased in the dorsal striatum of the dS-Shati/Nat8l mice. This finding corresponded with the increased expression of glutamate carboxypeptidase II, an enzyme that metabolizes N-acetylaspartylglutamate present in the extracellular space. Extracellular serotonin levels were lower in the dorsal striatum of the dS-Shati/Nat8l and normal mice that were repeatedly administered a selective glutamate carboxypeptidase II inhibitor.

CONCLUSIONS

Our findings indicate that the striatal expression of N-acetylaspartate synthetase Shati/Nat8l plays a role in major depressive disorder via the metabotropic glutamate receptor 3-mediated functional control of the serotonergic neuronal system.

Reprint of "Hypomyelinating disorders: An MRI approach

In recent years, the concept of hypomyelinating disorders has been proposed as a group of disorders with varying systemic manifestations that are identified by MR findings of absence or near absence of the T2 hypointensity that develops in white matter as a result of myelination. Initially proposed as a separate group because they were the largest single category of undiagnosed leukodystrophies, their separation as a distinct group that can be recognized by looking for a specific MRI feature has resulted in a marked increase in their diagnosis and a better understanding of the different causes of hypomyelination. This review will discuss the clinical presentations, imaging findings on standard MRI, and new MRI-related techniques that allow a better understanding of these disorders and proposed methods for quantifying the myelination as a potential means of assessing disease course and the effects of proposed treatments. Disorders with hypomyelination of white matter, or hypomyelinating disorders (HMDs), represent the single largest category among undiagnosed genetic leukoencephalopathies (Schiffmann and van der Knaap, 2009; Steenweg et al., 2010). This group of inborn errors of metabolism is characterized by a magnetic resonance imaging (MRI) appearance of reduced or absent myelin development: delay in the development of T2 hypointensity and, often, T1 hyperintensity in the white matter of the brain. The concept of hypomyelination was first conceptualized by (Schiffmann and van der Knaap, 2009; Steenweg et al., 2010; Schiffmann et al., 1994) in a series of papers that showed that these MRI characteristics were easily recognized, were different from the MRI characteristics of dysmyelinating and demyelinating disorders, and that the combination of these imaging findings with specific other clinical and imaging features could be used to make diagnoses with some confidence. In this manuscript, we will discuss the physiologic and genetic bases of hypomyelinating disorders, as well as their classification, clinical manifestations and imaging characteristics.

Hypomyelinating disorders: An MRI approach

In recent years, the concept of hypomyelinating disorders has been proposed as a group of disorders with varying systemic manifestations that are identified by MR findings of absence or near absence of the T2 hypointensity that develops in white matter as a result of myelination. Initially proposed as a separate group because they were the largest single category of undiagnosed leukodystrophies, their separation as a distinct group that can be recognized by looking for a specific MRI feature has resulted in a marked increase in their diagnosis and a better understanding of the different causes of hypomyelination. This review will discuss the clinical presentations, imaging findings on standard MRI, and new MRI-related techniques that allow a better understanding of these disorders and proposed methods for quantifying the myelination as a potential means of assessing disease course and the effects of proposed treatments. Disorders with hypomyelination of white matter, or hypomyelinating disorders (HMDs), represent the single largest category among undiagnosed genetic leukoencephalopathies (Schiffmann and van der Knaap, 2009; Steenweg et al., 2010). This group of inborn errors of metabolism is characterized by a magnetic resonance imaging (MRI) appearance of reduced or absent myelin development: delay in the development of T2 hypointensity and, often, T1 hyperintensity in the white matter of the brain. The concept of hypomyelination was first conceptualized by (Schiffmann and van der Knaap, 2009; Steenweg et al., 2010; Schiffmann et al., 1994) in a series of papers that showed that these MRI characteristics were easily recognized, were different from the MRI characteristics of dysmyelinating and demyelinating disorders, and that the combination of these imaging findings with specific other clinical and imaging features could be used to make diagnoses with some confidence. In this manuscript, we will discuss the physiologic and genetic bases of hypomyelinating disorders, as well as their classification, clinical manifestations and imaging characteristics.

Neurochemistry of Hypomyelination Investigated with MR Spectroscopy

Proton magnetic resonance spectroscopy (MRS) allows the noninvasive exploration of tissue metabolism in vivo, providing neurophysiological and neurochemical information. N-acetylaspartate (NAA) is generally considered to be a marker of neurons and axons, and many neurodegenerative disorders, including demyelinating disorders, exhibit a decrease in total NAA (tNAA). MRS in human hypomyelination disorders, such as Pelizaeus-Merzbacher disease (PMD), is characterized by normal to elevated tNAA, elevated myo-inositol and creatine (Cr), and normal to decreased choline (Cho). MRS in the thalamus of a hypomyelinating mouse model, a myelin synthesis-deficient (msd) mouse, a model of connatal PMD with mutation of the Plp1 gene, revealed increased tNAA and Cr and decreased Cho. That of a shiverer mouse with an autosomal recessive mutation of the Mbp gene showed decreased Cho with normal tNAA and Cr. Accordingly, the reduction of Cho on MRS might be a common marker for hypomyelinating disorders. tNAA concentrations range from normal to increased, probably depending upon the underlying pathology of oligodendrocytes. tNAA may be increased in hypomyelination with a reduced number of mature oligodendrocytes, such as PMD.

Neurochemical Metabolomics Reveals Disruption to Sphingolipid Metabolism Following Chronic Haloperidol Administration

Haloperidol is an effective antipsychotic drug for treatment of schizophrenia, but prolonged use can lead to debilitating side effects. To better understand the effects of long-term administration, we measured global metabolic changes in mouse brain following 3 mg/kg/day haloperidol for 28 days. These conditions lead to movement-related side effects in mice akin to those observed in patients after prolonged use. Brain tissue was collected following microwave tissue fixation to arrest metabolism and extracted metabolites were assessed using both liquid and gas chromatography mass spectrometry (MS). Over 300 unique compounds were identified across MS platforms. Haloperidol was found to be present in all test samples and not in controls, indicating experimental validity. Twenty-one compounds differed significantly between test and control groups at the p < 0.05 level. Top compounds were robust to analytical method, also being identified via partial least squares discriminant analysis. Four compounds (sphinganine, N-acetylornithine, leucine and adenosine diphosphate) survived correction for multiple testing in a non-parametric analysis using false discovery rate threshold < 0.1. Pathway analysis of nominally significant compounds (p < 0.05) revealed significant findings for sphingolipid metabolism (p = 0.015) and protein biosynthesis (p = 0.024). Altered sphingolipid metabolism is suggestive of disruptions to myelin. This interpretation is supported by our observation of elevated N-acetyl-aspartyl-glutamate in the haloperidol-treated mice (p = 0.004), a marker previously associated with demyelination. This study further demonstrates the utility of murine neurochemical metabolomics as a method to advance understanding of CNS drug effects.

Overexpression of Shati/Nat8l, an N-acetyltransferase, in the nucleus accumbens attenuates the response to methamphetamine via activation of group II mGluRs in mice

A novel N-acetyltransferase, Shati/Nat8l, was identified in the nucleus accumbens (NAc) of mice with methamphetamine (METH) treatment. Previously we reported that suppression of Shati/Nat8l enhanced METH-induced behavioral alterations via dopaminergic neuronal regulation. However, the physiological mechanisms of Shati/Nat8l on the dopaminergic system in the brain are unclear. In this study, we injected adeno-associated virus (AAV) vector containing Shati/Nat8l into the NAc or dorsal striatum (dS) of mice, to increase Shati/Nat8l expression. Overexpression of Shati/Nat8l in the NAc, but not in the dS, attenuated METH-induced hyperlocomotion, locomotor sensitization, and conditioned place preference in mice. Moreover, the Shati/Nat8l overexpression in the NAc attenuated the elevation of extracellular dopamine levels induced by METH in in vivo microdialysis experiments. These behavioral and neurochemical alterations due to Shati/Nat8l overexpression in the NAc were inhibited by treatment with selective group II metabotropic glutamate receptor type 2 and 3 (mGluR2/3) antagonist LY341495. In the AAV vector-injected NAc, the tissue contents of both N-acetylaspartate and N-acetylaspartylglutamate (NAAG), endogenous mGluR3 agonist, were elevated. The injection of peptidase inhibitor of NAAG or the perfusion of NAAG itself reduced the basal levels of extracellular dopamine in the NAc of naive mice. These results indicate that Shati/Nat8l in the NAc, but not in the dS, plays an important suppressive role in the behavioral responses to METH by controlling the dopaminergic system via activation of group II mGluRs.

Hypomyelinating leukodystrophies: translational research progress and prospects

Hypomyelinating leukodystrophies represent a genetically heterogeneous but clinically overlapping group of heritable disorders. Current management approaches in the care of the patient with a hypomyelinating leukodystrophy include use of serial magnetic resonance imaging (MRI) to establish and monitor hypomyelination, molecular diagnostics to determine a specific etiology, and equally importantly, careful attention to neurologic complications over time. Emerging research in oligodendrocyte biology and neuroradiology with bedside applications may result in the possibility of clinical trials in the near term, yet there are significant gaps in knowledge in disease classification, characterization, and outcome measures in this group of disorders. Here we review the biological background of myelination, the clinical and genetic variability in hypomyelinating leukodystrophies, and the insights that can be obtained from current MRI techniques. In addition, we discuss ongoing research approaches to define potential outcome markers for future clinical trials.

Age-related changes in a patient with Pelizaeus-Merzbacher disease determined by repeated 1H-magnetic resonance spectroscopy

A boy with Pelizaeus-Merzbacher disease underwent repeated evaluations by 3-Tesla (1)H-magnetic resonance spectroscopy (MRS). The patient showed overlap of the PLP1. Individuals selected as normal controls for (1)H-magnetic resonance spectroscopy consisted of healthy age-matched children. For (1)H-magnetic resonance spectroscopy, the center of a voxel was positioned in the right parietal lobe. (1)H-magnetic resonance spectroscopy was performed when the patient was 2, 6, 14, and 25 months old. γ-Aminobutyric acid concentration in early childhood was increased compared with that in normal controls. However, the γ-aminobutyric acid concentration in the Pelizaeus-Merzbacher disease patient was normalized at 14 and 25 months. No remarkable changes were observed in choline-containing compounds concentration at any time. These results suggest that the changes in metabolite concentrations during growth can reflect the pathological condition of Pelizaeus-Merzbacher disease. Furthermore, the lack of change in the choline-containing compounds concentration can be useful for differentiating Pelizaeus-Merzbacher disease from other white matter disorders.

MR spectroscopy in 18q(-) syndrome suggesting other than hypomyelination

We reported a 5-year-old boy with 18q(-) syndrome who showed typical magnetic resonance imaging (MRI) findings of high signal intensity on T2-weighted imaging, and a slightly high but lower than normal signal on T1-weighted imaging of the white matter. MR spectroscopy (MRS) revealed increased concentrations of creatine, myoinositol and choline with a normal N-acetylaspartate one. The cerebral white matter lesions observed on MRI in patients with 18q(-) syndrome have been considered to reflect hypomyelination due to a decrease in myelin basic protein so far, however, MRS suggested reactive astrocytic gliosis and accelerated myelin turnover, which are compatible with recent pathological reports of 18q(-) syndrome.

Neuroradiologic correlates of clinical disability and progression in the X-linked leukodystrophy Pelizaeus-Merzbacher disease

OBJECTIVE

To determine whether quantitative measure of magnetic resonance imaging data from patients with the inherited leukodystrophy, Pelizaeus-Merzbacher disease (PMD) correlates with clinical severity or progression.

METHODS

In our current work we have analyzed the clinical phenotypes and MRI scans of 51 male patients with PMD and 10 female carriers for whom the PLP1 genotype had been determined. In addition, we developed a 32-point functional disability scoring (FDS) system for PMD, and validated it for inter-rater reliability. Using conventional T1- and T2-weighted MRI images of the whole brain, we measured white matter and total brain volume (WMV and TBV), inter-caudate ratio (ICR), and corpus callosum area.

RESULTS

There was a significant positive correlation of FDS with white matter fraction (WMV/TBV) and corpus callosum area. Also, when applying a median split based on FDS, patients with lower FDS showed reduced white matter fraction and corpus callosum area, and increased ICR compared to patients with relatively higher FDS, regardless of age.

CONCLUSION

Although this patient population is heterogeneous, with multiple genetic and molecular mechanisms causing PMD, these data imply that white matter atrophy is a major pathological determinant of the clinical disability in most patients. Development of reliable non-invasive quantitative biomarkers of disease activity would be useful not only for following the natural history of the disease, but also raising the potential for evaluating future therapies.