Hyperpolarized ¹³C magnetic resonance spectroscopy detects toxin‐induced neuroinflammation in mice

Abstract

Lipopolysaccharide (LPS) is a commonly used agent for induction of neuroinflammation in preclinical studies. Upon injection, LPS causes activation of microglia and astrocytes, whose metabolism alters to favor glycolysis. Assessing in vivo neuroinflammation and its modulation following therapy remains challenging, and new noninvasive methods allowing for longitudinal monitoring would be highly valuable. Hyperpolarized (HP) 13C magnetic resonance spectroscopy (MRS) is a promising technique for assessing in vivo metabolism. In addition to applications in oncology, the most commonly used probe of [1–13C] pyruvate has shown potential in assessing neuroinflammation‐linked metabolism in mouse models of multiple sclerosis and traumatic brain injury. Here, we aimed to investigate LPS‐induced neuroinflammatory changes using HP [1–13C] pyruvate and HP 13C urea. 2D chemical shift imaging following simultaneous intravenous injection of HP [1–13C] pyruvate and HP 13C urea was performed at baseline (day 0) and at days 3 and 7 post‐intracranial injection of LPS (n = 6) or saline (n = 5). Immunofluorescence (IF) analyses were performed for Iba1 (resting and activated microglia/macrophages), GFAP (resting and reactive astrocytes) and CD68 (activated microglia/macrophages). A significant increase in HP [1–13C] lactate production was observed at days 3 and 7 following injection, in the injected (ipsilateral) side of the LPS‐treated mouse brain, but not in either the contralateral side or saline‐injected animals. HP 13C lactate/pyruvate ratio, without and with normalization to urea, was also significantly increased in the ipsilateral LPS‐injected brain at 7 days compared with baseline. IF analyses showed a significant increase in CD68 and GFAP staining at 3 days, followed by increased numbers of Iba1 and GFAP positive cells at 7 days post‐LPS injection. In conclusion, we can detect LPS‐induced changes in the mouse brain using HP 13C MRS, in alignment with increased numbers of microglia/macrophages and astrocytes. This study demonstrates that HP 13C spectroscopy has substantial potential for providing noninvasive information on neuroinflammation.

Metabolic needs of brain‐infiltrating leukocytes and microglia in multiple sclerosis

“…During inflammation, microglia up-regulate their glycolytic genes, in particular hexokinase, resulting in accumulation of acetyl-CoA, which facilitates protein acetylation and activation of IL-1β genetic program . Similarly, when LPS was intracranially injected in mice, lactate accumulation (as measured by MRSI) in ipsilateral brain correlated with activated microglia and astrocytes as noted by histology (Le Page et al, 2019). However, microglia exhibit remarkable flexibility in its metabolic needs.…”

Non-Invasive Differentiation of M1 and M2 Activation in Macrophages Using Hyperpolarized 13C MRS of Pyruvate and DHA at 1.47 Tesla

“…An investigative tool for assessment of in vivo metabolism exists in the form of 13 C magnetic resonance spectroscopy (MRS) combined with dissolution dynamic nuclear polarization (dDNP) of 13 C-labeled probes [ 23 ], so-called hyperpolarized (HP) 13 C MRS. This technique has been used to noninvasively monitor metabolic impairment in multiple preclinical models, including cancer, neuroinflammation, and cardiomyopathy [ 24 , 25 , 26 ]. Importantly, the use of this technology is now expanding into the clinic, with ongoing clinical trials on patients with brain tumor, traumatic brain injury, and prostate cancer, among others [ 27 , 28 , 29 ].…”

Hyperpolarized 13C magnetic resonance imaging for noninvasive assessment of tissue inflammation

“…Finally, stimulation of the pro‐inflammatory phenotype using LPS injection was applied to a mouse model of intracranial inflammation to examine the metabolic changes associated with directly induced neuroinflammation (Figure 4). Increased hyperpolarized [1‐ 13 C]pyruvate flux through LDH produced a significantly higher lactate/pyruvate ratio and was associated with significantly increased microglial cell numbers 89 . Collectively, these studies highlight a growing body of evidence that upregulated glycolysis, as detected by increased [1‐ 13 C]lactate production, is a potent indicator of increased macrophage function in pathological states, and that hyperpolarized [1‐ 13 C]pyruvate can be applied to measure neuroinflammation.…”

“…Heatmaps of hyperpolarized [1‐ 13 C]lactate, [1‐ 13 C]pyruvate and [1‐ 13 C]urea in rodent model of neuroinflammation induced by intracranial injection of lipopolysaccharide 89 …”