Métabolite | EPFL Graph Search

Un métabolite est un composé organique intermédiaire ou issu du métabolisme. On réserve ce terme en général aux petites molécules et aux monomères, par opposition aux macromolécules. Ainsi, le glucose est un métabolite, contrairement au glycogène, qui est un polysaccharide de poids moléculaire très élevé. Le toxicologue, l'écotoxicologue et l'écologue s'intéressent aux métabolites des toxiques inhalés ou ingérés ou dispersés dans l'environnement en tant qu'indicateurs (d'une contamination, d'une pollution) ou en raison de leur toxicité parfois plus aiguë que la molécule mère dans le cas de certaines molécules chimiques : le monométhylmercure (et plus encore le diméthylmercure) est ainsi beaucoup plus toxique et bioaccumulable que le mercure métallique pur, ce pourquoi on compare parfois des dosages de méthylmercure à un dosage de mercure total. Dans certains cas on ignore encore si des molécules problématiques retrouvées dans le sol, l'air, l'eau sont de simples résidus de dégradation

Methods to Enhance Nuclear Magnetic Resonance Sensitivity at High Magnetic Field

Claudia Christina Zanella

MR is a well-established technique routinely used in preclinical and clinical studies. This thesis focussed on the improvement of sensitivity of different methods in MR. Hyperpolarized 129Xe gas is used for studying pulmonary disease or cerebral perfusion. When hyperpolarization is conducted inside a polarizer, the achieved solid state polarization is currently limited to a few percent. One aim of this thesis was to counteract this bottleneck by investigating the effect of microwave frequency modulation for different spin systems. An increase in nuclear 129Xe spin polarization up to 14.5% was achieved compared to the polarization obtained without frequency modulation. Electron spin-lattice relaxation times T1S, as short as 5 ms, correlated directly with the gain in 129Xe nuclear polarization due to microwave frequency modulation, irrespective of the solvent or its deuteration. Simulations of the electron spin system confirmed that microwave frequency modulation could be an efficient method to considerably enhance nuclear spin polarization in short T1S matrices or in the presence of large linewidth radicals by promoting spectral diffusion. DNP matrices can also be hyperpolarized through non-persistent radicals generated by UV-irradiation. They have the advantage of quenching upon dissolution, yielding a hyperpolarized solution with no need for radical filtration. Radical precursors researched up to date either yielded low 13C polarization, contained toxic matrix components or interfered with metabolic processes. Therefore, the two novel endogenously-occurring precursors, alpha-ketovalerate (akV) and alpha-ketobutyrate (akB) were investigated. Liquid state polarization of the metabolic substrate glucose increased to 16.3% using akB compared with 13.3% obtained with pyruvate. [1-13C]butyric acid was polarized to 12.1% for akV and 12.9% for akB and used for in vivo hyperpolarized cardiac MRS. Hyperpolarized [1-13C]butyrate metabolism in the heart revealed label incorporation into a wide range of metabolites. This study demonstrated the potential of using UV-induced radicals generated in the endogenously-occurring metabolites akV and akB as polarizing agents, enabling high polarization without requiring radical filtration for radical-free hyperpolarized MRI. While MRI requires RF coils that generate high B1 homogeneity, MRS strongly depends on high sensitivity and potentially high B1 efficiency to allow for short-TE acquisitions. Two custom-made 1H coils resonating at 600 MHz, a single-channel saddle coil and an 8-leg quadrature birdcage coil, were investigated with respect to those criteria for applications at ultra-high field (14.1 T). The saddle coil yielded a 54% higher transmit field efficiency with a 20% higher SNR compared to the birdcage coil after full-FOV corrections. Using the saddle coil in glycoCEST imaging of a homogeneous phantom resulted in an MTR asymmetry coefficient of variation as small as 5.2% over an 11 mm diameter ROI. Overall, the saddle coil provided a good compromise between globally homogeneous excitation and high sensitivity. Finally, it was successfully used to map skeletal muscle glycogen content in vivo. This thesis focused on improving MR sensitivity by means of adequate custom-made RF coils as well as DNP through the implementation of a method capitalizing on electronic spin properties and the development of novel endogenous precursors for in vivo hyperpolarized MRI.

EPFL2021

Mitochondrial respiratory chain dysfunction disrupts intracellular cholesterol homeostasis

Christopher Tadhg James Wall

Mitochondrial diseases are rare and severe conditions with debilitating symptoms. Biochemical defects in mitochondria however are common. The difference between these two frequencies is suspected to lie in the capability of the cells to adapt to the homeostatic disbalance. Cells activate retrograde signalling pathways in response to mitochondrial dysfunction, activating cellular transcriptional programs to correct for the disbalance. The full extent of the retrograde signals that are induced by mitochondrial dysfunctions still remains unknown. Studying these retrograde signalling pathways can lead to insights how to make the difference between restoring the homeostasis and a detrimental disease. Investigating the transcriptional responses to artificially induced mitochondrial dysfunction has proven to be lucrative in unveiling novel responses to mitochondrial defects. In this work we studied the transcriptional responses to three chemical models of mitochondrial respiratory chain dysfunction. In line with recently published work, we found a commonly upregulated stress response in all models. Interestingly, shared between all the models was a downregulation of genes enriched in cholesterol biosynthesis pathways. Upon closer examination this encompassed, almost without exception, all transcripts of the mevalonate pathway. This coordinated downregulation of transcripts had functional consequences on the output of the pathway, lowering the abundance of cholesterol precursor metabolites. Other metabolites in more upstream regions of the pathway were changed differently depending on the complex affected, suggesting other regulatory influences involved. Upon closer mechanistic investigation to the cause of the transcriptional suppression we found the main transcription factor responsible for mevalonate pathway gene transcription, the Sterol Response Element Binding Protein 2 (SREBP2), to be inhibited during complex I impairment. This was most evident in sterol depleted conditions where SREBP2 did not activate to normal levels. A decreased activation of SREBP2 is signature for an elevated concentration of ER-cholesterol. Indeed, other ER-bound cholesterol sensitive proteins confirmed this, showing a similar decreased abundance following complex I impairment in both normal and low lipid conditions. To investigate the source of this cholesterol we measured various cholesterol levels. No changes in the levels of total cholesterol (free cholesterol plus esterified cholesterol) were detected. Also, the fraction of free cholesterol was not significantly affected by complex I inhibition. Using the fluorescent cholesterol stain filipin, we were able to measure a modest but reproducible increase in intracellular free cholesterol. This provided novel mechanistic insight into the inhibition of ER sterol sensors and mevalonate pathway activity by mitochondrial dysfunction. Several well-known retrograde signalling pathways AMPK, ISR and mTOR did not explain the changes observed. The exact mechanism through which mitochondria incite an increase in intracellular free cholesterol remains unknown. Taken together this work can provide a mechanistic link between mitochondrial function and intracellular cholesterol homeostasis and could possibly link mitochondrial function to circulating lipoprotein homeostasis. Further work should focus on elucidating the possible mechanism, where differences in complex functions could provide a hint in the right direction.

EPFL2022

Radical-free hyperpolarized MRI using endogenously occurring pyruvate analogues and UV-induced nonpersistent radicals

Lionel Pierre Arn, Josefina Adriana Maria Bastiaansen, Andrea Capozzi, Rolf Gruetter, Alice Radaelli, Hikari Ananda Infinity Yoshihara, Claudia Christina Zanella

It was recently demonstrated that nonpersistent radicals can be generated in frozen solutions of metabolites such as pyruvate by irradiation with UV light, enabling radical-free dissolution dynamic nuclear polarization. Although pyruvate is endogenous, the presence of pyruvate may interfere with metabolic processes or the detection of pyruvate as a metabolic product, making it potentially unsuitable as a polarizing agent. Therefore, the aim of the current study was to characterize solutions containing endogenously occurring alternatives to pyruvate as UV-induced nonpersistent radical precursors for in vivo hyperpolarized MRI. The metabolites alpha-ketovalerate (alpha kV) and alpha-ketobutyrate (alpha kB) are analogues of pyruvate and were chosen as potential radical precursors. Sample formulations containing alpha kV and alpha kB were studied with UV-visible spectroscopy, irradiated with UV light, and their nonpersistent radical yields were quantified with electron spin resonance and compared with pyruvate. The addition of C-13-labeled substrates to the sample matrix altered the radical yield of the precursors. Using alpha kB increased the C-13-labeled glucose liquid-state polarization to 16.3% +/- 1.3% compared with 13.3% +/- 1.5% obtained with pyruvate, and 8.9% +/- 2.1% with alpha kV. For [1-C-13]butyric acid, polarization levels of 12.1% +/- 1.1% for alpha kV, 12.9% +/- 1.7% for alpha kB, 1.5% +/- 0.2% for OX063 and 18.7% +/- 0.7% for Finland trityl, were achieved. Hyperpolarized [1-C-13]butyrate metabolism in the heart revealed label incorporation into [1-C-13]acetylcarnitine, [1-C-13]acetoacetate, [1-C-13]butyrylcarnitine, [5-C-13]glutamate and [5-C-13]citrate. This study demonstrates the potential of alpha kV and alpha kB as endogenous polarizing agents for in vivo radical-free hyperpolarized MRI. UV-induced, nonpersistent radicals generated in endogenous metabolites enable high polarization without requiring radical filtration, thus simplifying the quality-control tests in clinical applications.

WILEY2021