Molecular Probes: From Aβ Imaging to Phototherapy in Alzheimer’s Disease

ConspectusAlzheimer's disease (AD) is a common neurodegenerative disease, one of whose pathological characteristics is the abnormal deposition of amyloid beta (Aβ). The development of highly sensitive and specific Aβ imaging probes is of great significance for diagnosis, therapeutic discovery, and pathological mechanism studies of AD. In recent years, small molecule-based optical probes have shown significant potential for in vivo whole-brain imaging in small animals and high-resolution microscopic imaging of biological processes, advancing Aβ imaging from detection to molecular mechanisms study and drug discovery. Therefore, the rational design and efficient application of these small molecular probes in Aβ imaging and therapy remain active areas of research. In this regard, a comprehensive understanding of the design strategy of the Aβ probes is highly desirable for advancing and guiding future research directions.Over the past decade, our research has been focused on a trilogy of developing Aβ-based small molecules as imaging probes and therapeutics. In episode I, we invented a brand-new family of near-infrared fluorescent (NIRF) probes CRANAD-Xs, for in vivo selective imaging of Aβ species in AD mice. Considering that different Aβ species exhibit distinct neurotoxicities─with soluble oligomers regarded as the most toxic and insoluble plaques representing a less toxic, late stage of amyloidosis, we rationally designed the CRANAD-X series to cover the full spectrum of amyloid pathology, from low-toxicity plaques to highly toxic oligomers. Importantly, unlike most studies in this field that focus solely on probe characterization, we demonstrated that CRANAD-Xs can longitudinally monitor therapeutic efficacy in real time, supporting their use in drug discovery.Although in vivo NIRF imaging with CRANAD-Xs shows great promise, it remains severely limited by shallow tissue penetration, largely due to autofluorescence interference and a low signal-to-noise ratio (SNR). Consequently, achieving sufficient imaging depth in vivo continues to be a major challenge. In episode II, we pioneered the exploration of chemiluminescence probes (ADLumin-Xs) for detecting Aβ species to meet the needs of deep imaging. Due to high SNR and deep imaging with ADLumin-Xs, we demonstrate the first in vivo 3D whole-brain imaging using chemiluminescence probes, enabling precise localization of Aβ deposits. In addition, using chemiluminescence resonance energy transfer (CRET) with dual nonconjugated probes, we achieve dual-amplification of the Aβ signal in vivo whole-brain imaging.In episode III, we focus on molecularly produced light ("molecular light") for AD therapeutics. Molecular light, primarily including chemiluminescence and bioluminescence, owns a dual nature as both a deliverable molecule and intrinsic light source that enables limitless tissue penetration unattainable with naturally/physically produced light. This dual nature supports theranostic applications including imaging, photodynamic therapy, photooxidation, and photobiomodulation. Leveraging phototherapy that employs the synergistic effect of a photolabile Aβ ligand and molecular light could effectively slow Aβ accumulation in vivo in AD mice. Our studies support the feasibility of molecular light therapy. Overall, we believe that our research offers valuable scientific inspiration for researchers in the fields of chemistry, chemical biology, and biomedicine, particularly for neurodegenerative diseases.