The 140-residue protein ␣-synuclein (AS) is able to form amyloid fibrils and as such is the main component of protein inclusions involved in Parkinson's disease. We have investigated the structure and dynamics of full-length AS fibrils by high-resolution solid-state NMR spectroscopy. Homonuclear and heteronuclear 2D and 3D spectra of fibrils grown from uniformly 13 C͞ 15 N-labeled AS and AS reverse-labeled for two of the most abundant amino acids, K and V, were analyzed. 13 C and 15 N signals exhibited linewidths of <0.7 ppm. Sequential assignments were obtained for 48 residues in the hydrophobic core region. We identified two different types of fibrils displaying chemical-shift differences of up to 13 ppm in the 15 N dimension and up to 5 ppm for backbone and side-chain 13 C chemical shifts. EM studies suggested that molecular structure is correlated with fibril morphology. Investigation of the secondary structure revealed that most amino acids of the core region belong to ␤-strands with similar torsion angles in both conformations. Selection of regions with different mobility indicated the existence of monomers in the sample and allowed the identification of mobile segments of the protein within the fibril in the presence of monomeric protein. At least 35 C-terminal residues were mobile and lacked a defined secondary structure, whereas the N terminus was rigid starting from residue 22. Our findings agree well with the overall picture obtained with other methods and provide insight into the amyloid fibril structure and dynamics with residue-specific resolution.EM ͉ protein structure ͉ amyloid ͉ Parkinson's disease ͉ protein aggregation T he ability of a protein to form amyloid fibrils is increasingly recognized as a general property of all polypeptide sequences (1). It is a phenomenon common to numerous neurodegenerative diseases such as Alzheimer's and Parkinson's diseases and spongiform encephalopathies (2, 3). Thus, the investigation of the mechanism(s) of protein misfolding as well as the detailed structures of amyloid fibrils is of paramount interest (4, 5). Parkinson's disease is defined by the presence of intracellular inclusions in dopaminergic neurons, the so-called Lewy bodies, which contain a high content of fibrils formed from the 140-aa cytoplasmic protein ␣-synuclein (AS) (6). The question of whether the mature fibrils themselves or rather protofilaments or folding intermediates are the neurotoxic species responsible for the cell death of dopaminergic neurons is still a subject of great controversy (7-9).AS belongs to the class of natively unfolded proteins, i.e., it lacks a well-defined secondary structure (10, 11), although long-range interactions have been shown to stabilize an aggregation-autoinhibited global protein architecture (12). Three regions of the protein can be classified. (i) The amphipathic N terminus (residues 1-60) consists of imperfect 11-mer repeats, with the consensus motif KTKEGV. (ii) The predominantly hydrophobic middle, also known as non-A␤ component region (residues 61-95...

Proton MRS (1H MRS) provides noninvasive, quantitative metabolite profiles of tissue and has been shown to aid the clinical management of several brain diseases. Although most modern clinical MR scanners support MRS capabilities, routine use is largely restricted to specialized centers with good access to MR research support. Widespread adoption has been slow for several reasons, and technical challenges toward obtaining reliable good‐quality results have been identified as a contributing factor. Considerable progress has been made by the research community to address many of these challenges, and in this paper a consensus is presented on deficiencies in widely available MRS methodology and validated improvements that are currently in routine use at several clinical research institutions. In particular, the localization error for the PRESS localization sequence was found to be unacceptably high at 3 T, and use of the semi‐adiabatic localization by adiabatic selective refocusing sequence is a recommended solution. Incorporation of simulated metabolite basis sets into analysis routines is recommended for reliably capturing the full spectral detail available from short TE acquisitions. In addition, the importance of achieving a highly homogenous static magnetic field (B0) in the acquisition region is emphasized, and the limitations of current methods and hardware are discussed. Most recommendations require only software improvements, greatly enhancing the capabilities of clinical MRS on existing hardware. Implementation of these recommendations should strengthen current clinical applications and advance progress toward developing and validating new MRS biomarkers for clinical use.

Mutations of arginine 132 (R132) in the enzyme isocitrate dehydrogenase-1 (IDH1) are present in up to 86% of grade II and III gliomas and secondary glioblastoma. R132 mutations in IDH1 result in excess production of the metabolite 2-hydroxyglutarate (2HG), which could be used as a biomarker for this subset of gliomas. Here, we use optimized spectral-editing and two-dimensional (2D) correlation magnetic resonance spectroscopy (MRS) methods to unambiguously detect 2HG non-invasively in glioma patients with IDH1 mutations. By comparison, fitting of conventional 1D MR spectra can provide false-positive readouts owing to spectral overlap of 2HG and chemically similar brain metabolites, such as glutamate and glutamine. 2HG has been found also by 2D high-resolution magic angle spinning MRS performed ex vivo on a separate set of glioma biopsy samples. 2HG detection by in vivo or ex vivo MRS enabled detailed molecular characterization of a clinically important subset of human gliomas. This has implications for diagnosis as well as monitoring of treatments targeting IDH mutations.

It is shown that molecular structure and dynamics of a uniformly labeled membrane protein can be studied under magic-angle-spinning conditions. For this purpose, dipolar recoupling experiments are combined with novel through-bond correlation schemes that probe mobile protein segments. These NMR schemes are demonstrated on a uniformly [13C,15N] variant of the 52-residue polypeptide phospholamban. When reconstituted in lipid bilayers, the NMR data are consistent with an alpha-helical trans-membrane segment and a cytoplasmic domain that exhibits a high degree of structural disorder.

The polymerization of the microtubule-associated protein tau into paired helical filaments (PHFs) represents one of the hallmarks of Alzheimer's disease. We employed solid-state nuclear magnetic resonance (NMR) to investigate the structure and dynamics of PHFs formed in vitro by the three-repeat-domain (K19) of protein tau, representing the core of Alzheimer PHFs. While N and C termini of tau monomers in PHFs are highly dynamic and solvent-exposed, the rigid segment consists of three major beta-strands. Combination of through-bond and through-space ssNMR transfer methods with water-edited ((15)N, (13)C) and ((13)C, (13)C) correlation experiments suggests the existence of a fibril core that is largely built by repeat unit R3, flanked by surface-exposed units R1 and R4. Solid-state NMR, circular dichroism, and the fibrillization behavior of a K19 mutant furthermore indicate that electrostatic interactions play a central role in stabilizing the K19 PHFs.

No abstract

In population groups where head pose cannot be assumed to be constant during a magnetic resonance spectroscopy examination or in difficult-to-shim regions of the brain, realtime volume of interest, frequency, and shim optimization may be necessary. We investigate the effect of pose change on the B 0 homogeneity of a (2 cm) 3 volume and observe typical first-order shim changes of 1 mT/m per 1°rotation (chin down to up) in four different volumes of interest in a single volunteer. An echo planar imaging volume navigator was constructed to measure and apply in real-time within each pulse repetition time: volume of interest positioning, frequency adjustment, and first-order shim adjustment. This volume navigator is demonstrated in six healthy volunteers and achieved a mean linewidth of 4.4 Hz, similar to that obtained by manual shim adjustment of 4.9 Hz. Furthermore, this linewidth is maintained by the volume navigator at 4.9 Hz in the presence of pose change. By comparison, a mean linewidth of 7.5 Hz was observed, when no correction was applied. Single voxel spectroscopy (SVS) relies on a homogeneous B 0 , a consistent frequency, and assumes that the localization remains valid for the duration of the scan. For a restless subject, who is unable to maintain a consistent pose during the scan, these do not hold true. We present a method that provides real-time (once every pulse repetition time [TR]) B 0 and frequency measurements in addition to real-time correction of the volume of interest (VOI) position.Current motion and artifact correction methods in magnetic resonance spectroscopy can be divided into two categories: phase and frequency adjustments and localization correction. Phase and frequency adjustments refer to a group of techniques that measure the signal phase and frequency by using either the residual water signal (1-4) or a secondary navigator (5-7). These methods correct both a velocity-induced phase error and frequency changes that result from either scanner drift or pose change. Phase and frequency adjustments can be applied both retrospectively and prospectively, but only prospective methods are able to correct the change in water saturation frequency.Localization correction techniques in magnetic resonance spectroscopy have been demonstrated using an optical tracking system (7) and an imaging navigator technique called PROspective MOtion correction (PROMO) (8). The technique presented by Zaitsev et al. (7) provides both frequency and localization correction by combining optical tracking with navigator based frequency correction in addition to reacquisition of free induction decays (FIDs) with velocity induced phase errors. The disadvantages of an optical device are that they require: additional hardware, a marker to be rigidly affixed to the head, a clear line of sight between camera and marker, and the calibration of a camera to scanner transform.There are several navigator-based motion tracking methods available, which take advantage of the k-space properties of rigid body transforms to subsample ks...

Gamma-aminobutyric acid (GABA) and glutamate (Glu) are the major neurotransmitters in the brain. They are crucial for the functioning of healthy brain and their alteration is a major mechanism in the pathophysiology of many neuro-psychiatric disorders. Magnetic resonance spectroscopy (MRS) is the only way to measure GABA and Glu non-invasively in vivo. GABA detection is particularly challenging and requires special MRS techniques. The most popular is MEscher-GArwood (MEGA) difference editing with single-voxel Point RESolved Spectroscopy (PRESS) localization. This technique has three major limitations: a) MEGA editing is a subtraction technique, hence is very sensitive to scanner instabilities and motion artifacts. b) PRESS is prone to localization errors at high fields (≥3T) that compromise accurate quantification. c) Single-voxel spectroscopy can (similar to a biopsy) only probe average GABA and Glu levels in a single location at a time. To mitigate these problems, we implemented a 3D MEGA-editing MRS imaging sequence with the following three features: a) Real-time motion correction, dynamic shim updates, and selective reacquisition to eliminate subtraction artifacts due to scanner instabilities and subject motion. b) Localization by Adiabatic SElective Refocusing (LASER) to improve the localization accuracy and signal-to-noise ratio. c) K-space encoding via a weighted stack of spirals provides 3D metabolic mapping with flexible scan times. Simulations, phantom and in vivo experiments prove that our MEGA-LASER sequence enables 3D mapping of GABA+ and Glx (Glutamate + Gluatmine), by providing 1.66 times larger signal for the 3.02 ppm multiplet of GABA+ compared to MEGA-PRESS, leading to clinically feasible scan times for 3D brain imaging. Hence, our sequence allows accurate and robust 3D-mapping of brain GABA+ and Glx levels to be performed at clinical 3T MR scanners for use in neuroscience and clinical applications.