Higher iron in the red nucleus marks Parkinson's dyskinesia

Meta-analysis of brain iron levels of Parkinson's disease patients determined by postmortem and MRI measurements

Brain iron levels in patients of Parkinson's disease (PD) are usually measured in postmortem samples or by MRI imaging including R2* and SWI. In this study we performed a meta-analysis to understand PD-associated iron changes in various brain regions, and to evaluate the accuracy of MRI detections comparing with postmortem results. Databases including Medline, Web of Science, CENTRAL and Embase were searched up to 19th November 2015. Ten brain regions were identified for analysis based on data extracted from thirty-three-articles. An increase in iron levels in substantia nigra of PD patients by postmortem, R2* or SWI measurements was observed. The postmortem and SWI measurements also suggested significant iron accumulation in putamen. Increased iron deposition was found in red nucleus as determined by both R2* and SWI, whereas no data were available in postmortem samples. Based on SWI, iron levels were increased significantly in the nucleus caudatus and globus pallidus. Of note, the analysis might be biased towards advanced disease and that the precise stage at which regions become involved could not be ascertained. Our analysis provides an overview of iron deposition in multiple brain regions of PD patients, and a comparison of outcomes from different methods detecting levels of iron.

Influence of regional iron on the motor impairments of Parkinson's disease: A quantitative susceptibility mapping study

PURPOSE

Because the roles of striatal-thalamo-cortical and cerebello-thalamo-cortical circuits in the heterogeneous motor impairments of Parkinson's disease (PD) are becoming recognized, this study was designed to investigate the relationships between regional iron in the cardinal subcortical nuclei in these circuits and the different motor impairments.

MATERIALS AND METHODS

Sixty-two PD patients and 40 normal subjects were included and accepted for Enhanced T₂ -Star Weighted Angiography Scanning (3.0T). According to the Unified Parkinson's Disease Rating Scale, patients were divided into tremor-dominant (PD-TD) and akinetic/rigid-dominant groups (PD-AR). The intergroup differences of magnetic susceptibility in those cardinal nuclei were measured. Correlation analyses between magnetic susceptibility and motor impairments were performed in all patients.

RESULTS

Nigral magnetic susceptibility significantly increased for each PD group compared with controls (P < 0.001 for PD-TD; P = 0.001 for PD-AR). Magnetic susceptibility in the dentate nucleus (DN) and red nucleus (RN) for the PD-TD patients were significantly increased compared with controls (P < 0.001 and P = 0.004, respectively). Magnetic susceptibility in these regions was also significantly correlated with tremor severity (r = 0.444, P = 0.001 for DN; r = 0.418, P = 0.001 for RN). Significant correlation between caudate magnetic susceptibility and akinetic/rigid severity were observed (r = -0.322, P = 0.015).

CONCLUSION

This study provides evidence that nigral iron accumulation is a common characteristic in PD, while iron accumulation in the DN and RN is correlated with tremor symptoms. Our data also indicate that caudate iron content may be a potential marker for akinetic/rigid progression.

LEVEL OF EVIDENCE

3 J. MAGN. RESON. IMAGING 2017;45:1335-1342.

Imaging synucleinopathies

In this review the structural and functional imaging changes associated with the synucleinopathies PD, MSA, and dementias associated with Lewy bodies are reviewed. The role of imaging for supporting differential diagnosis, detecting subclinical disease, and following disease progression is discussed and its potential use for monitoring disease progression is debated. © 2016 International Parkinson and Movement Disorder Society.

Hemisphere, gender and age-related effects on iron deposition in deep gray matter revealed by quantitative susceptibility mapping

The purpose of this work was to investigate the effects of hemispheric location, gender and age on susceptibility value, as well as the association between susceptibility value and diffusional metrics, in deep gray matter. Iron content was estimated in vivo using quantitative susceptibility mapping. Microstructure was probed using diffusional kurtosis imaging. Regional susceptibility and diffusional metrics were measured for the putamen, caudate nucleus, globus pallidus, thalamus, substantia nigra and red nucleus in 42 healthy adults (age range 25-78 years). Susceptibility value was significantly higher in the left than the right side of the caudate nucleus (P = 0.043) and substantia nigra (P < 0.001). Women exhibited lower susceptibility values than men in the thalamus (P < 0.001) and red nucleus (P = 0.032). Significant age-related increases of susceptibility were observed in the putamen (P < 0.001), red nucleus (P < 0.001), substantia nigra (P = 0.004), caudate nucleus (P < 0.001) and globus pallidus (P = 0.017). The putamen exhibited the highest rate of iron accumulation with aging (slope of linear regression = 0.73 × 10(-3) ppm/year), which was nearly twice those in substantia nigra (slope = 0.40 × 10(-3) ppm/year) and caudate nucleus (slope = 0.39 × 10(-3) ppm/year). Significant positive correlations between the susceptibility value and diffusion measurements were observed for fractional anisotropy (P = 0.045) and mean kurtosis (P = 0.048) in the putamen without controlling for age. Neither correlation was significant after controlling for age. Hemisphere, gender and age-related differences in iron measurements were observed in deep gray matter. Notably, the putamen exhibited the highest rate of increase in susceptibility with aging. Correlations between susceptibility value and microstructural measurements were inconclusive. These findings could provide new clues for unveiling mechanisms underlying iron-related neurodegenerative diseases.

Region-specific disturbed iron distribution in early idiopathic Parkinson's disease measured by quantitative susceptibility mapping

In Parkinson's disease (PD), iron elevation in specific brain regions as well as selective loss of dopaminergic neurons is a major pathologic feature. A reliable quantitative measure of iron deposition is a potential biomarker for PD and may contribute to the investigation of iron-mediated PD. The primary purpose of this study is to assess iron variations in multiple deep grey matter nuclei in early PD with a novel MRI technique, quantitative susceptibility mapping (QSM). The inter-group differences of susceptibility and R2* value in deep grey matter nuclei, namely head of caudate nucleus (CN), putamen (PUT), global pallidus (GP), substantia nigra (SN), and red nucleus (RN), and the correlations between regional iron deposition and the clinical features were explored in forty-four early PD patients and 35 gender and age-matched healthy controls. Susceptibility values were found to be elevated within bilateral SN and RN contralateral to the most affected limb in early PD compared with healthy controls (HCs). The finding of increased susceptibility in bilateral SN is consistent with work on a subgroup of patients at the earliest clinical detectable state (Hoehn and Yahr [1967]: Neurology 17:427-442; Stage I). However, increased R2* values were only seen within SN contralateral to the most affected limb in the PD group when compared with controls. Furthermore, bilateral SN magnetic susceptibility positively correlated with disease duration and UPDRS-III scores in early PD. This finding supports the potential value of QSM as a non-invasive quantitative biomarker of early PD.

Noradrenergic modulation of glutamate-induced excitatory responses in single neurons of the red nucleus: an electrophysiological study

The effect induced by noradrenaline (NA) on the spiking activity evoked by glutamate (Glu) on single neurons of the mesencephalic red nucleus (RN) of the rat was studied extracellularly. Long-lasting microiontophoretic applications of the amine induced a significant and reversible depression of the responsiveness of RN neurons to Glu. This effect was mediated by noradrenergic alpha2 receptors since it was mimicked by application of clonidine, an alpha2 adrenoceptor agonist, and blocked or at least reduced by application of yohimbine, an antagonist of NA for the same receptors. The effect appears homogeneously throughout the nucleus and is independent of the effect of NA on baseline firing rate. Application of isoproterenol, a beta adrenoceptor agonist, either enhanced or depressed neuronal responses to Glu in a high percentage (86%) of the tested neurons. Moreover, application of timolol, a beta adrenoceptor antagonist, was able to strengthen the depressive effects induced by NA application on neuronal responsiveness to Glu. Although these data suggest some involvement of beta adrenergic receptors in the modulation of neuronal responsiveness to Glu, the overall results indicate a short-term depressive action of NA, mediated by alpha2 receptors, on the responsiveness of RN neurons and suggest that stress initially leads to an attenuation of the relay function of the RN.

A multicontrast approach for comprehensive imaging of substantia nigra

We characterize the contrast behavior of substantia nigra (SN) in both magnetization transfer (MT) imaging, which is believed to be sensitive to neuromelanin (NM), and susceptibility weighted imaging (SWI). Images were acquired with a MT prepared dual echo gradient echo sequence. The first echo was taken as the MT contrast image and the second was used to generate the SWI image. SN volumes were segmented from these two types of images using a thresholding method. The spatial and signal characteristics of the extracted SWI and MT volumes were compared. Both images showed the presence of SN but the volumes of the SN identified in the two are spatially incongruent. The MT volume was more caudal than the SWI volume and with only a 12% overlap between the two volumes. Considering the SN volumes in each hemisphere separately, the average distances between the centers of mass of the volumes from the two types images are 5.1±1.1mm and 4.1±1.2mm, respectively. The frequency offsets (homodyne filtered phase/echo time) for the volumes derived from MT (NM) images and SWI images are 0.09±0.32radians/s and -1.12±0.57radians/s (p<0.0001), respectively. The MT contrasts for the two volumes are 0.16±0.02 and 0.10±0.03 (p<0.001), respectively. Our results indicate that the two contrasts are sensitive to different portions of the SN, with MT seeing the more caudal portion of the SN than SWI, likely due to variations of NM and iron content in the SN. Despite the small overlap, these regions are complementary. Our results provide a new understanding of the contrast behavior of the SN in the two imaging approaches commonly used to image it and indicate that using both may yield a more comprehensive visualization of the SN.

Frequency-dependent neural activity in Parkinson's disease

The brainstem and basal ganglia are important in the pathophysiology of Parkinson's disease (PD). Reliable and sensitive detection of neural activity changes in these regions should be helpful in scientific and clinical research on PD. In this study, we used resting state functional MRI and amplitude of low frequency fluctuation (ALFF) methods to examine spontaneous neural activity in 109 patients with PD. We examined activity in two frequency bands, slow-4 (between 0.027 and 0.073 Hz) and slow-5 (0.010-0.027 Hz). Patients had decreased ALFF in the striatum and increased ALFF in the midbrain, and changes were more significant in slow-4. Additionally, changes in slow-4 in both basal ganglia and midbrain correlated with the severity of the parkinsonism. The ALFF in the caudate nucleus positively correlated with the dose of levodopa, while the ALFF in the putamen negatively correlated with the disease duration in both slow-4 and slow-5 bands. In addition, the ALFF in the rostral supplementary motor area negatively correlated with bradykinesia subscale scores. Our findings show that with a large cohort of patients and distinguishing frequency bands, neural modulations in the brainstem and striatum in PD can be detected and may have clinical relevance. The physiological interpretation of these changes needs to be determined.

The role of iron in brain ageing and neurodegenerative disorders

SUMMARY

In the CNS, iron in several proteins is involved in many important processes such as oxygen transportation, oxidative phosphorylation, myelin production, and the synthesis and metabolism of neurotransmitters. Abnormal iron homoeostasis can induce cellular damage through hydroxyl radical production, which can cause the oxidation and modification of lipids, proteins, carbohydrates, and DNA. During ageing, different iron complexes accumulate in brain regions associated with motor and cognitive impairment. In various neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease, changes in iron homoeostasis result in altered cellular iron distribution and accumulation. MRI can often identify these changes, thus providing a potential diagnostic biomarker of neurodegenerative diseases. An important avenue to reduce iron accumulation is the use of iron chelators that are able to cross the blood-brain barrier, penetrate cells, and reduce excessive iron accumulation, thereby affording neuroprotection.

Association between increased magnetic susceptibility of deep gray matter nuclei and decreased motor function in healthy adults

In the human brain, iron is more prevalent in gray matter than in white matter, and deep gray matter structures, particularly the globus pallidus, putamen, caudate nucleus, substantia nigra, red nucleus, and dentate nucleus, exhibit especially high iron content. Abnormally elevated iron levels have been found in various neurodegenerative diseases. Additionally, iron overload and related neurodegeneration may also occur during aging, but the functional consequences are not clear. In this study, we explored the correlation between magnetic susceptibility--a surrogate marker of brain iron--of these gray matter structures with behavioral measures of motor and cognitive abilities, in 132 healthy adults aged 40-83 years. Latent variables corresponding to manual dexterity and executive functions were obtained using factor analysis. The factor scores for manual dexterity declined significantly with increasing age. Independent of gender, age, and global cognitive function, increasing magnetic susceptibility in the globus pallidus and red nuclei was associated with decreasing manual dexterity. This finding suggests the potential value of magnetic susceptibility, a non-invasive quantitative imaging marker of iron, for the study of iron-related brain function changes.

The role of iron in brain ageing and neurodegenerative disorders

In the CNS, iron in several proteins is involved in many important processes such as oxygen transportation, oxidative phosphorylation, myelin production, and the synthesis and metabolism of neurotransmitters. Abnormal iron homoeostasis can induce cellular damage through hydroxyl radical production, which can cause the oxidation and modification of lipids, proteins, carbohydrates, and DNA. During ageing, different iron complexes accumulate in brain regions associated with motor and cognitive impairment. In various neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease, changes in iron homoeostasis result in altered cellular iron distribution and accumulation. MRI can often identify these changes, thus providing a potential diagnostic biomarker of neurodegenerative diseases. An important avenue to reduce iron accumulation is the use of iron chelators that are able to cross the blood-brain barrier, penetrate cells, and reduce excessive iron accumulation, thereby affording neuroprotection.

Eryptosis as a marker of Parkinson's disease

A major trend in recent Parkinson's disease (PD) research is the investigation of biological markers that could help in identifying at-risk individuals or to track disease progression and response to therapies. Central to this is the knowledge that inflammation is a known hallmark of PD and of many other degenerative diseases. In the current work, we focus on inflammatory signalling in PD, using a systems approach that allows us to look at the disease in a more holistic way. We discuss cyclooxygenases, prostaglandins, thromboxanes and also iron in PD. These particular signalling molecules are involved in PD pathophysiology, but are also very important in an aberrant coagulation/hematology system. We present and discuss a hypothesis regarding the possible interaction of these aberrant signalling molecules implicated in PD, and suggest that these molecules may affect the erythrocytes of PD patients. This would be observable as changes in the morphology of the RBCs and of PD patients relative to healthy controls. We then show that the RBCs of PD patients are indeed rather dramatically deranged in their morphology, exhibiting eryptosis (a kind of programmed cell death). This morphological indicator may have useful diagnostic and prognostic significance.

Brain morphometry and the neurobiology of levodopa-induced dyskinesias: current knowledge and future potential for translational pre-clinical neuroimaging studies

Dopamine replacement therapy in the form of levodopa results in a significant proportion of patients with Parkinson’s disease developing debilitating dyskinesia. This significantly complicates further treatment and negatively impacts patient quality of life. A greater understanding of the neurobiological mechanisms underlying levodopa-induced dyskinesia (LID) is therefore crucial to develop new treatments to prevent or mitigate LID. Such investigations in humans are largely confined to assessment of neurochemical and cerebrovascular blood flow changes using positron emission tomography and functional magnetic resonance imaging. However, recent evidence suggests that LID is associated with specific morphological changes in the frontal cortex and midbrain, detectable by structural MRI and voxel-based morphometry. Current human neuroimaging methods however lack sufficient resolution to reveal the biological mechanism driving these morphological changes at the cellular level. In contrast, there is a wealth of literature from well-established rodent models of LID documenting detailed post-mortem cellular and molecular measurements. The combination therefore of advanced neuroimaging methods and rodent LID models offers an exciting opportunity to bridge these currently disparate areas of research. To highlight this opportunity, in this mini-review, we provide an overview of the current clinical evidence for morphological changes in the brain associated with LID and identify potential cellular mechanisms as suggested from human and animal studies. We then suggest a framework for combining small animal MRI imaging with rodent models of LID, which may provide important mechanistic insights into the neurobiology of LID.

Volumetric analysis of the subthalamic and red nuclei based on magnetic resonance imaging in patients with Parkinson's disease

Parkinson's disease (PD) is associated with degeneration of the dopaminergic neurons in the substantia nigra. The subthalamic nucleus (STN) plays a pivotal role in the pathogenesis. However, there is not much known about the morphological changes in the STN. The red nucleus (RN) has many connections with the motor coordinating pathways although it is not primarily involved in the pathogenesis. In this study we aimed to compare the volumes of the STN and RN measured by magnetic resonance imaging in PD patients and controls to investigate how these structures are affected at the morphological level. Twenty patients with PD and twenty age/sex matched controls were enrolled in this study. Severity score was determined by Hoehn & Yahr staging: 6 at stage II and 14 at stage III in med-off state. Imaging was performed by a 1.5 Tesla (T) MR scanner. Measurements of total brain and normalized STN and RN volumes were performed by manual planimetry using Image J software. No statistically significant differences were observed between two groups based on age or gender and disease stage and nuclei volumes. The total estimated brain volumes were not different between PD patients and controls. However, normalized volumes of the STN and RN were 14% and 16% larger, respectively, in PD patients compared to the controls (p < 0.05). Our findings suggest that the volumes of the STN and RN are increased in patients with PD. These changes possibly reflect the altered metabolic activity of these regions demonstrated by neurophysiological studies.

Age at onset influences neurodegenerative processes underlying PD with levodopa-induced dyskinesias

PURPOSE

Recently, we demonstrated that PD patients with levodopa-induced dyskinesias are characterized by neuroanatomical and functional changes involving the prefrontal cortex. When compared with non-dyskinetic PD patients, dyskinetic PD patients showed increased volume of the inferior frontal cortex and a dysfunctional imbalance between this region and the supplementary motor area during motor task. In the current study, we investigated the impact of age at onset of the disease on the neuroanatomical characteristics of dyskinetic patients, because it is well known that early-onset PD patients usually develop dyskinesias sooner with respect to late-onset PD.

METHODS

Whole-brain voxel-wise investigations of gray matter volume and cortical thickness were carried out in dyskinetic (n = 33), non-dyskinetic PD patients (n = 33) and in age-sex-matched healthy controls (n = 40). Neuroimaging analyses were performed separately according to the age at onset (early < 50 y > late).

RESULTS

Independent of age at onset, dyskinetic PD patients showed altered morphology in the inferior frontal cortex when compared with non-dyskinetic patients. Moreover, additional significant abnormalities emerged in the early- and late-onset PD patients when compared to controls. In fact, early-onset dyskinetic patients showed increased volume in a large cluster of the midbrain encompassing substantia nigra and red nucleus, whereas late-onset dyskinetic patients were characterized by abnormal gray matter increase in the supplementary motor area.

DISCUSSION

Our findings demonstrate different patterns of brain abnormalities in patients with LID according to age at onset, highlighting the role of the nigral pathology in early-onset and of the cortical pathology in late-onset patients with PD.