Blood-brain barrier transport of amyloid beta peptides in efflux pump knock-out animals evaluated by in vivo optical imaging

Synthetic human Aβ_1-40 or scrambled Aβ_40-1 peptides were labeled with the near-infrared fluorescent tracer, Cy5.5. The free tracer or Cy5.5-labeled peptides were injected intravenously into Abcb1^-KO or Abcg2^-KO mice or their corresponding wild-type controls. The animals were imaged prospectively at different time points over a period of 8 hours using eXplore Optix small animal imager. At the end of the observation, animals were sacrificed by perfusion, their brains were imaged ex-vivo and sectioned for immunofluorescence analyses.

After appropriate circulation time, the fluorescence concentration in the head ROI measured in vivo was close to background values in both wild-type and Abcb1^-KO or Abcg2^-KO mice injected with either free dye or scrambled Aβ_40-1-Cy5.5. In animals injected with Aβ_1-40-Cy5.5, the deficiency in either Abcb1 or Abcg2 resulted in significant increases in fluorescence concentration in the head ROIs 2 hours after injection compared to wild-type animals. Fluorescence decay (elimination rate) over 2–8 hours after injection was similar between wild-type (t_1/2 = 1.97 h) and Abcg2^-KO (t_1/2 = 2.34 h) and was slightly faster (t_1/2 = 1.38 h) in Abcb1^-KO mice. In vivo time-domain imaging method allows prospective, dynamic analyses of brain uptake/elimination of fluorescently-labeled compounds, including Aβ. Deficiency of either of the two major efflux pumps, Abcb1 and Abcg2, implicated in Aβ trafficking across the BBB, resulted in increased accumulation of peripherally-injected Aβ_1-40 in the brain.