J. Steibel

Brain dysmyelination and recovery assessment by noninvasive in vivo diffusion tensor magnetic resonance imaging.

Diffusion tensor magnetic resonance imaging (DT‐MRI) was applied for in vivo quantification of myelin loss and regeneration. A transgenic mouse line (Oligo‐TTK) expressing a truncated form of the herpes simplex virus 1 thymidine kinase gene (hsv1‐tk) in oligodendrocytes was studied along with two induced phenotypes of myelin pathology. Myelin loss and axonal abnormalities differentially affect values of DT‐MRI parameters in the brain of transgenic mice. Changes in the anisotropy of the white matter were assessed by calculating and mapping the radial (D⊥) and axial (D∥) water diffusion to axonal tracts and fractional anisotropy (FA). A significant increase in D⊥ attributed to the lack of myelin was observed in all selected brain white matter tracts in dysmyelinated mice. Lower D∥ values were consistent with the histological observation of axonal modifications, including reduced axonal caliber and overexpression of neurofilaments and III β‐tubulin. We show clearly that myelination and axonal changes play a role in the degree of diffusion anisotropy, because FA was significantly decreased in dysmyelinated brain. Importantly, myelin reparation during brain postnatal development induced a decrease in the magnitude of D⊥ and an increase in FA compared with the same brain before recovery. The progressive increase in D∥ values was attributed to the gain in normal axonal morphology. This regeneration was confirmed by the detection of enlarged oligodendrocyte population, newly formed myelin sheaths around additional axons, and a gradual increase in axonal caliber.

Recovery from Chronic Demyelination by Thyroid Hormone Therapy: Myelinogenesis Induction and Assessment by Diffusion Tensor Magnetic Resonance Imaging

The failure of the remyelination processes in multiple sclerosis contributes to the formation of chronic demyelinated plaques that lead to severe neurological deficits. Long-term cuprizone treatment of C57BL/6 mice resulted in pronounced white matter pathology characterized by oligodendrocyte depletion, irreversible demyelination and persistent functional deficits after cuprizone withdrawal. The use of a combination of in vivo diffusion tensor magnetic resonance imaging (DT-MRI) and histological analyses allowed for an accurate longitudinal assessment of demyelination. Injection of triiodothyronine (T3) hormone over a 3 week interval after cuprizone withdrawal progressively restored the normal DT-MRI phenotype accompanied by an improvement of clinical signs and remyelination. The effects of T3 were not restricted to the later stages of remyelination but increased the expression of sonic hedgehog and the numbers of Olig2+ and PSA-NCAM+ precursors and proliferative cells. Our findings establish a role for T3 as an inducer of oligodendrocyte progenitor cells in adult mouse brain following chronic demyelination.

Multi-exponential analysis of T2 images

Clinical images show that T2 contrast allows in certain cases a better discrimination between the healthy and pathological tissues. For instance, in Fig. 1 it can be seen that multiple sclerosis’ lesions are more clearly visible on the images obtained with late echoes than those obtained with the early echoes. On the other hand in vitro studies on perfused organs’ have shown that the T2 decay curve depends on the interval between 180” pulses: The decay curve is mono-exponential for 4-ms time intervals between 180” pulses, and bi-exponential for 0.4-ms time intervals (Fig. 2). For these reasons, we have tried to analyze the T2 decay curve by multi-exponential decomposition in order to improve tissue characterization. To obtain the T2 decay curve, we used the classical CPMG pulse sequence. By adding linear magnetic field gradients to this sequence it is possible to reconstruct a sample image for each echo. The T2 decay curve is constructed by taking the corresponding pixel from the echo image series, for instance, the first point corresponds to the first image, the second point to the second images, etc. Thus we obtain the T2 decay curve of the elementary volume defined by the pixel whose dimensions are the slice thickness (7 mm) and the spatial resolution (2-2.5 mm). It suffices, therefore, to repeat this operation pixel by pixel in order to obtain parametric T2 decay curves over the whole slice. A slice of the head is represented by about 6000 pixels, which means that we need to repeat the decomposition operation 6000 times. We therefore had to find an algorithm sufficiently rapid and not too memory consuming. The algorithm chosen was of the noniterative, Prony (algebraic) type. 3 It was then necessary to verify this method of analysis using first simulated image signals and thereafter simple test objects. Classically, the multi-exponential decomposition is an ill-posed problem. The “solution” is obtained by nonlinear least-squares optimization. With a noniterative algorithm we transform the nonlinear leastsquares optimization into two linear least-squares problems. The decomposition method may be presented in four steps:

The Role of Mycobacterium tuberculosis in Experimental Allergic Encephalomyelitis

Various detergents used in preparative membrane protein chemistry were added to a complete Freunds adjuvant/water emulsion in order to increase the solubility and/or immunologic availability of the Mycobacterium tuberculosis membrane and to explain its role in blood-brain barrier (BBB) permeability. Magnetic resonance imaging was used for in vivo determination of the BBB breakdown and cerebral edema. The results showed that with 1% 10 tridecyl ether, which increases emulsion stability, abundant BBB breakdown and cerebral edema were observed, similar to those encountered in experimental allergic encephalomyelitis (EAE). We suggest that the immunologic response triggered off by M. tuberculosis largely contributes to the BBB permeability changes observed during EAE, probably by an action on the endothelial cells of the cerebral blood vessels.

Antibody directed against mannan of the Mycobacterium tuberculosis cell envelope provokes blood-brain barrier breakdown.

Previous studies demonstrated that blood-brain barrier (BBB) breakdown observed at the cerebral level during experimental allergic encephalomyelitis (EAE) arises from the presence of Mycobacterium tuberculosis in Freunds adjuvant and not only from the encephalitogenic antigens. The main objective of this study was to check if the mannan moiety of lipoarabinomannan present in the mycobacterial cell envelope is responsible for an immune response provoking a BBB breakdown. The results showed that: firstly, the complete Freunds adjuvant (CFA) contains a high release of polysaccharides; secondly, the rats immunized with the CFA present important serum concentration of anti-mannan antibody; and finally, a single 2-mg dose of anti-mannan antibody injected intravenously in naive rats provokes an immediate and reversible BBB breakdown. These results suggest that mannan arising from the solubilization of the mycobacterial cell wall in Freunds adjuvant induces a high production of anti-mannan antibody, which, in turn, provokes a BBB breakdown and possibly facilitates the induction of EAE.

Proton nuclear magnetic resonance relaxation rates in aqueous solutions of amino acids

Variations of water-proton NMR relaxation rates induced by amino acids are very different for longitudinal and transverse relaxations. The presence of amino acids increases the longitudinal relaxation rate only slightly, when compared with the increase induced by the oxygen content of the solution. The measured transverse relaxation rate is dependent on the pulse delay of a Carr-Purcell-Meiboom-Gill sequence. Transverse proton relaxation dispersions are shown to be consistent with rapid proton exchange between water and the NH+ 3 groups of the amino acids. The mean exchange time is about 1 ms. The variation of the exchange time with temperature yields the enthalpy and the entropy of the apparent first-order reaction of this exchange. Changes of pH, temperature and oxygen content give rise to opposite effects on relaxation rates of amino acid solutions. The discussion here underlines the fact that, even in such a simple system, relaxation-rate measurements give valuable information only if the physico-chem...

Effect of interpulse delay on NMR transverse relaxation rate of tissues

INTRODUCTION The nuclear spin systems have relaxation times varying from a few microseconds to several seconds. The time during which the precession maintains phase memory has been called spin-spin or transverse relaxation time ( T, ). The Carr-Purcell-Meiboom-Gil1 (CPMG) sequence’p2 provides a good method for measuring the spin-spin relaxation time with the classical sequence 90” (7, 180”, 7, record), . While Hahn’s method’ (90, 7, 90, 7, record, 5Tl), is sensitive to diffusion effects in nonviscous samples and permits accurate measurements of diffusion constants, the CPMG sequence partially eliminates the influence of diffusion on echo decay. As shown by Carr and Purcell’ the echo amplitude, detected at time t = 2nr is given by quencies. The exchange process will transfer nuclei from one environment to the other, with an accompanying switch of their resonance frequency. These frequency changes increase the linewidth of the NMR spectrum. An increase of the exchange rate causes the complex broad pattern to narrow to a single line. Finally, if the jumping frequency reaches values comparable to the Larmor frequency, the jumping becomes a mechanism for the spin relaxation. The general theory for calculating the effect6s7 of chemical exchange and spin coupling on echo amplitudes in CPMG sequences was developed by Gutowsky et a/.* As a first-order approximation Luz and Meiboom’ calculated the decay rate of the echoes:

Chemical and molecular exchange effects on T2 relaxation of living tissues: a pulse spacing dependence study.

Contrast in magnetic resonance imaging depends principally on the longitudinal relaxation (R1) and the transverse relaxation rate (R2) of the observed nuclei, most often the protons. The spin-spin relaxation rate (R2) is the result of several mechanisms. The dependence of the interpulse delay of the Carr-Purcell-Meiboom-Gill sequence on the transverse relaxation rate of the water was studied in rat organs in vitro. It gives an insight into the exchange mechanisms involved. The increase of the interpulse delay from 0.2 ms to 5 ms gives an R2 increase of 23, 15, 3, and 2 s-1 for the heart, the liver, the spleen and the brain, respectively. These increases are compared to the R2 increases obtained in 17O-enriched water, amino acid and albumin solutions atomic exchange takes place. The concentration of these materials in organs cannot explain the R2 increase of the organs with the interpulse delay. Water exchange between intra and extracellular compartments is proposed to explain the R2 increase with interpulse delays in organs like the heart and the liver.

Magnetic resonance imaging of PLP-induced experimental allergic encephalomyelitis in Lewis rats

An in vivo magnetic resonance (MR) imaging study was performed on experimental allergic encephalomyelitis (EAE) induced in Lewis rats through proteolipid protein (PLP). PLP was solubilized in water or in an aqueous solution of 1% 10-tridecyl ether (TDE), a non-ionic detergent used in membrane protein research. All 16 rats immunized with 500 microg of TDE-solubilized PLP developed clinical signs and MR abnormalities fully comparable to those observed in MBP-induced EAE. Total paraplegia was observed in 12.5% of rats, mild or moderate paraparesis in 68.8% of rats and tail paralysis in the remaining 18.7% of rats. Whereas only 37.5% of the eight rats immunized with 500 microg of water-solubilized PLP developed minor clinical signs (tail weakness or paralysis). Our observations confirm that the difficulties encountered when trying to induce EAE by means of PLP arise from the highly hydrophobic nature of this protein. Accordingly, if a reproducible model is to be developed, it seems more judicious to use non-ionic detergents in both the extraction and solubilization phases of PLP preparation, this would allow maximal solubilization of the protein while avoiding aggregates, which may otherwise form during either of the PLP preparation.

Synthesis and characterization of a highly stable dendritic catechol-tripod bearing technetium-99m

The synthesis and preliminary biological tests (in vitro toxicity, in vitro stability) of new Tc(III)-radiolabelled dendro-chelates are presented. A dendritic 99mTc chelate 1 derived from a pre-organized tripodal tris-catecholamide exhibits a kinetic stability by far more important than its corresponding diethylenetriamine pentaacetic acid (DTPA) homologue 2. This permitted an assessment of the real impact of the pre-organized tripodal structure on kinetic inertness (and thus toxicity), an important issue to address when considering in vivo applications. Radiolabelling was performed using the stannous chloride reduction method; while DTPA-homologue 2 showed a high radiolabelling efficiency (96% radiolabelling yield after 30 min), tripodal complex 1 induced a 93% complexation yield after 45 min. In contrast, radiocomplex 1 derived from the most rigid and organized structure has a higher kinetic stability than 2. Indeed, while dissociation of 2 reached 50% after 1 h 30 min in physiological media like phosphate buffer saline (PBS) and bovine serum albumin (BSA), over 80% of 1 remained stable during the half-life of the radionucleide (6.02 h for 99mTc). Measurements of the cell leakage resulting from membrane damage of neuronal cells treated with increasing concentrations of dendritic ligand 16, together with pictures of treated neurons after staining, showed no detectable toxicity.