Deep Imaging to Dissect Microvascular Contributions to White Matter Degeneration in Rodent Models of Dementia

Abstract

The increasing socio-economic burden of Alzheimer disease (AD) and AD-related dementias has created a pressing need to define targets for therapeutic intervention. Deficits in cerebral blood flow and neurovascular function have emerged as early contributors to disease progression. However, the cause, progression, and consequence of small vessel disease in AD/AD-related dementias remains poorly understood, making therapeutic targets difficult to pinpoint. Animal models that recapitulate features of AD/AD-related dementias may provide mechanistic insight because microvascular pathology can be studied as it develops in vivo. Recent advances in in vivo optical and ultrasound-based imaging of the rodent brain facilitate this goal by providing access to deeper brain structures, including white matter and hippocampus, which are more vulnerable to injury during cerebrovascular disease. Here, we highlight these novel imaging approaches and discuss their potential for improving our understanding of vascular contributions to AD/AD-related dementias.

Keywords: animal; arterioles; brain; cerebrovascular disease; hippocampus; imaging; white matter.

Figure 1.. In vivo imaging with deep 2PM and 3PM.

(A) Volume of tissue showing microvasculature in cortex, callosal white matter, and hippocampus, captured by deep 2P in vivo using Alexa 680-dextran dye. (B) Planar view at two imaging depths (Z = 0.40 mm and 0.95 mm below the brain surface) outlined by the red dashed lines in panel A. (C) Bar plots of average capillary RBC flux in the cerebral gray and white matter, during control conditions, mild hypercapnia and global cerebral hypoperfusion, respectively. Data are expressed as mean ± SD. *P < 0.05, Student’s t-test. Li et al., Journal of Cerebral Blood Flow and Metabolism (volume 40, issue 3), pp. 501–512, copyright ©2019 by (Sage Publications), reprinted by Permission of SAGE Publication. (D) Volume collected by 3PM showing GCaMP6s-labeled neurons in the mouse cortex and the hippocampus (green, GCaMP6 fluorescence; magenta, intrinsic third-harmonic signal (THG)). (E) Planar views showing cortical layer 6 (L6) and external capsule (EC) from panel D. THG visualizes blood vessels and myelinated axons in the EC. (F) Neuronal activity recording site in the hippocampus located at 984 μm. Traces on the right show spontaneous activity recorded from the labeled neurons indicated in panel F. Adapted from Ouzounov et al. with permission. Copyright ©2017, Springer Nature.

Figure 2.. In vivo imaging with OCTA and OCM.

(A) Volume of OCTA data collected in mouse cortex through a cranial window. Planar view maximum intensity projections (MIP) at different depths. WM: White matter. (B) Bi-directional axial velocity map generated by en face MIP of the 3D doppler OCTA data. Color bar represents the RBC axial velocity of the flow descending into (positive, green) and rising from (negative, red) the cortical surface. The 3D velocity signals are shown to the right of the projection image. (C) En face average intensity projection of the 3D capillary velocimetry dataset within a 300 μm thick region of cortex. Color represents a blood flow velocity range. A cross section at the white-dashed line position is shown to the right. Reprinted with permission from 76 © The Optical Society. Reprinted with permission from 80 © The Optical Society Used with permission of SPIE, from Optical coherence tomography angiography-based capillary velocimetry, Wang RK, 22, 2017; permission conveyed through Copyright Clearance Center, Inc. (D) OCM imaging of neuronal cell bodies with side view of tissue on the left and en face projection view on the right. Outline colors of the en face images correspond to z depth as indicated by the arrow colors on the side view. (E) Myelinated axons shown with side view of tissue on the left and en face projection at two selected z depths. Side view slice projection thickness: 190 μm. Axial projection thickness: 11.2 μm. Adapted from Zhu et al.

Figure 3.. In vivo imaging with ultrasound-based technologies.

(A) Photoacoustic computed tomography image providing coronal view of mouse brain through an intact skull. Used with permission of John Wiley & Sons, from High-resolution deep functional imaging of the whole mouse brain by photoacoustic computed tomography in vivo, Zhang P, 11, 2018; permission conveyed through Copyright Clearance Center, Inc. (B) Horizontal view showing oxygen saturation of blood in vasculature from dorsal surface of mouse brain, collected by ultra-fast wide-field photoacoustic microscopy. Images were collected after skull removal and implantation of a whole cortex window. Adapted from Zhu et al. (C,D) Entire depth of rat brain imaged through a bihemispheric transcranial window using fUS. Panel C shows power Doppler image and panel D shows axial blood velocity image revealing domains perfused by penetrating vessels. Adapted from Macé et al. with permission. Copyright ©2011, Springer Nature. (E,F) Magnified view of cortical microvessels imaged by ultrasound localization microscopy. Panel E shows microbubble density map and panel F shows flow velocity and flow direction map revealing individual cortical penetrating arterioles and ascending venules. Adapted from Errico et al. with permission. Copyright ©2015, Springer Nature.