Novel NIR-II Imaging Reveals Gut-First, Brain-Later Inorganic Mercury Signals with Structural Injury in Zebrafish

Abstract Mercury (Hg) neurotoxicity has generally been assessed by organic methylmercury, whereas inorganic mercury [Hg(II)] is often considered less likely to cause direct brain exposure. Conventional approaches cannot resolve the in vivo spatiotemporal distribution of Hg(II) across tissues. In this study, we first developed an Hg(II)-responsive aggregation-induced emission sensor for second near-infrared (NIR-II) fluorescence imaging, which was then employed to track Hg(II) signals in live zebrafish. The sensor produced enhanced NIR-II signals in the presence of Hg(II), allowing low-background, noninvasive, and real-time imaging. We observed that Hg(II)-responsive sensor signals first appeared in the intestinal region and later increased in the brain region, revealing an intestine-first and brain-later temporal pattern in living zebrafish. During a 48-h continuous exposure followed by depuration, brain-region Hg(II)-responsive sensor fluorescence increased with exposure concentration and time and remained detectable during the depuration phase. Expansion microscopy further showed that low-dose Hg(II) exposure reduced the brain cell density and average cell size, and disrupted the local tissue organization. These results provided in vivo evidence of Hg(II)-responsive sensor in the brain region and its association with structural brain injury during development. Thus, inorganic Hg should also be considered in the assessments of developmental brain exposure and neural structural injury.