Imaging toxin-induced neuroinflammation in mice using hyperpolarized ¹³C magnetic resonance spectroscopy

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 non-invasive methods allowing for longitudinal monitoring would be greatly 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 wished 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), day 3 and day 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 in the injected (ipsilateral) side at 3 and 7 days 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 to baseline. IF analyses showed a significant increase in CD68 and GFAP 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 holds much potential for providing non-invasive information on neuroinflammation.