In Situ Imaging of Formaldehyde in Live Mice with High Spatiotemporal Resolution Reveals Aldehyde Dehydrogenase-2 as a Potential Target for Alzheimer’s Disease Treatment

Alterations in formaldehyde (FA) homeostasis are associated with the pathology of Alzheimer's disease (AD). In vivo tracking of FA flux is important for understanding the underlying molecular mechanisms, but is challenging due to the lack of sensitive probes favoring a selective, rapid, and reversible response toward FA. In this study, we re-engineered the promiscuous and irreversible phenylhydrazines to make them selective and reversible toward FA by tuning their nucleophilicity. This effort resulted in PFM309, a selective (selectivity coefficient K_{FA,methylglyoxal} = 0.06), rapid (t_1/2 = 32 s at [FA] = 200 μM), and reversible fluorogenic probe (K = 6.24 mM^-1) that tracks the FA flux in both live cells and live mice. In vivo tracking of the FA flux was realized by PFM309 imaging, which revealed the gradual accumulation of FA in the live mice brain during normal aging and its further increase in AD mice. We further identified the age-dependent loss of catabolism enzymes ALDH2 and ADH5 as the primary mechanism responsible for formaldehyde excess. Activating ALDH2 with the small molecular activator Alda1 significantly protected neurovascular cells from formaldehyde overload and consequently from impairment during AD progress both in vitro and in vivo. These findings revealed PFM309 as a robust tool to study AD pathology and highlight ALDH2 as a potential target for AD drug development.