Hyperpolarized ¹³ C MR metabolic imaging can detect neuroinflammation in vivo in a multiple sclerosis murine model

Abstract

Proinflammatory mononuclear phagocytes (MPs) play a crucial role in the progression of multiple sclerosis (MS) and other neurodegenerative diseases. Despite advances in neuroimaging, there are currently limited available methods enabling noninvasive detection of MPs in vivo. Interestingly, upon activation and subsequent differentiation toward a proinflammatory phenotype MPs undergo metabolic reprogramming that results in increased glycolysis and production of lactate. Hyperpolarized (HP) 13 C magnetic resonance spectroscopic imaging (MRSI) is a clinically translatable imaging method that allows noninvasive monitoring of metabolic pathways in real time. This method has proven highly useful to monitor the Warburg effect in cancer, through MR detection of increased HP [1-13 C]pyruvate-tolactate conversion. However, to date, this method has never been applied to the study of neuroinflammation. Here, we questioned the potential of 13 C MRSI of HP [1-13 C]pyruvate to monitor the presence of neuroinflammatory lesions in vivo in the cuprizone mouse model of MS. First, we demonstrated that 13 C MRSI could detect a significant increase in HP [1-13 C]pyruvate-to-lactate conversion, which was associated with a high density of proinflammatory MPs. We further demonstrated that the increase in HP [1-13 C]lactate was likely mediated by pyruvate dehydrogenase kinase 1 up-regulation in activated MPs, resulting in regional pyruvate dehydrogenase inhibition. Altogether, our results demonstrate a potential for 13 C MRSI of HP [1-13 C]pyruvate as a neuroimaging method for assessment of inflammatory lesions. This approach could prove useful not only in MS but also in other neurological diseases presenting inflammatory components.hyperpolarized 13 C MR spectroscopy | multiple sclerosis | neuroinflammation | metabolism | macrophages M ultiple sclerosis (MS) is a multifaceted disorder of the CNS and is one of the most common causes of neurological disability in young adults (1, 2). Cortical and white-matter demyelination occurs early in MS pathogenesis and has been associated with disease progression and subsequent cognitive impairment (3). In the vast majority of cases, demyelinating lesions present a high inflammatory component, with elevated density of activated microglia/macrophages (mononuclear phagocytes, MPs) (4-6). Importantly, evidence suggests that proinflammatory MPs are one of the most abundant sources of reactive oxygen species (7), which mediate demyelination and axonal injury (8). Due to their central role in MS pathogenesis, noninvasive imaging of proinflammatory MPs would be of high importance for monitoring progression and response to antiinflammatory therapeutic approaches.Several recent studies have demonstrated that, upon activation and differentiation toward a proinflammatory phenotype, MPs undergo metabolic reprogramming toward enhanced glucose uptake and increased glycolysis (9-14). As for the Warburg effect observed in cancer cells, this augmented glycolysis takes place under aerobic conditions and results in the...

Dynamic metabolic imaging of copolarized [2‐13C]pyruvate and [1,4‐13C2]fumarate using 3D‐spiral CSI with alternate spectral band excitation

“…The complete spectral dispersion of resonances of copolarized mixture at 3T is divided into 2 sub-bands: the first subband comprises the substrate [1,4-13 C 2 ]Fum and the doublet of its product [1,[4][5][6][7][8][9][10][11][12][13]…”

“…Additionally, conversion of hyperpolarized 13 C-labeled fumarate (Fum) to malate (Mal) has been proposed as a biomarker for cell necrosis. 3 Metabolic imaging of hyperpolarized 13 Clabeled Pyr [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] and Fum 3,19-21 has shown great potential in characterizing multiple diseases and assessing response to treatment with Pyr already translated into the clinic. [8][9][10][11] Thus, performing their measurements with a single injection of copolarized substrates would be advantageous particularly for the clinical translation of this technology.…”

“…However, the small chemical shift (CS) dispersion of both substrates and metabolic products, e.g., the frequency difference between Fum and Ala is only 35 Hz at 3 Tesla (T), does not allow the use of SNR efficient differential flip angles for substrate and products 22 at clinical field strengths, potentially preventing the measurement of saturation kinetics that requires effective 90° excitation of the products. 23 The goal of this study is to investigate the feasibility of simultaneous metabolic imaging of copolarized [2-13 C]Pyr and [1,[4][5][6][7][8][9][10][11][12][13] Although, the large CS dispersion of resonances permits the desired differential flip angle scheme, it becomes more amenable to CS displacement artifacts for slice-selective acquisitions. Besides, this large frequency range is much beyond the practical spectral bandwidth limits of many fast MRSI methods and its encoding in interleaved manner significantly increases the imaging time, hence, severely limiting the temporal resolution in dynamic CSI.…”

“…24 The sequence used frequency-selective RF pulses to alternatingly excite 2 spectral sub-bands each one followed by a fast 3D-spCSI readout. In our application the first frequency sub-band contains resonances of [1,[4][5][6][7][8][9][10][11][12][13] C 2 ]Fum (175.4 ppm) and [1,[4][5][6][7][8][9][10][11][12][13]…”

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Purpose: Developing a method for simultaneous metabolic imaging of copolarized [2-13 C]pyruvate and [1,[4][5][6][7][8][9][10][11][12][13] C 2 ]fumarate without chemical shift displacement artifacts that also permits different excitation flip angles for substrates and their metabolic products. Methods: The proposed pulse sequence consists of 2 frequency-selective radiofrequency pulses to alternatingly excite 2 spectral sub-bands each one followed by a fast 3D spiral CSI (3D-spCSI) readout.

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