A quadrupolar two-photon fluorescent probe for <i>in vivo</i> imaging of amyloid-β plaques

Abstract

The formation of beta amyloid (Aβ) plaques in specific brain regions is one of the early pathological hallmarks of Alzheimer's disease (AD). To enable the early detection of AD and related applications, a method for real-time, clear 3D visualization of Aβ plaques <i>in vivo</i> is highly desirable. Two-photon microscopy (TPM) which utilizes two near-infrared photons is an attractive tool for such applications. However, this technique needs a sensitive and photostable two-photon (TP) probe possessing bright TP exited fluorescence to impart high signal-to-noise (S/N) visualization of Aβ plaques. Herein, we report a quadrupolar TP fluorescent probe (<b>QAD1</b>) having large TP action cross section (<i>Φδ</i>_max = 420 GM) and its application for <i>in vivo</i> TPM imaging of Aβ plaques. This probe, designed with a centrosymmetric D-A-D motif with a cyclic conjugating bridge and solubilizing unit, displays bright TP excited fluorescence, appreciable water solubility, robust photostability, and high sensitivity and selectivity for Aβ plaques. Using the real-time TPM imaging of transgenic 5XFAD mice after intravenous injection of <b>QAD1</b>, we show that this probe readily enters the blood brain barrier and provides high S/N ratio images of individual Aβ plaques <i>in vivo</i>. We also used <b>QAD1</b> in dual-color TPM imaging for 3D visualization of Aβ plaques along with blood vessels and cerebral amyloid angiopathy (CAA) inside living mouse brains. These findings demonstrate that this probe will be useful in biomedical applications including early diagnosis and treatments of AD.