Ultramicroscopy reveals axonal transport impairments in cortical motor neurons at prion disease

Abstract

The functional imaging of neuronal circuits of the central nervous system is crucial for phenotype screenings or investigations of defects in neurodegenerative disorders. Current techniques yield either low penetration depth, yield poor resolution, or are restricted by the age of the animals. Here, we present a novel ultramicroscopy protocol for fluorescence imaging and three-dimensional reconstruction in the central nervous system of adult mice. In combination with tracing as a functional assay for axonal transport, retrogradely labeled descending motor neurons were visualized with >4 mm penetration depth. The analysis of the motor cortex shortly before the onset of clinical prion disease revealed that >80% neurons have functional impairments in axonal transport. Our study provides evidence that prion disease is associated with severe axonal transport defects in the cortical motor neurons and suggests a novel mechanism for prion-mediated neurodegeneration.

Figure 1

(A) Prions inoculated into the right sciatic nerve spread along axonal projections into the RN and the MC. Application of AAV-REx tracer into the spinal cord resulted in retrograde labeling of motor neurons in CNS centers. The areas analyzed in this study are shown on the images of coronar sections taken from www.hms.harvard.edu/ and http://biology.clc.uc.edu/, respectively. To visualize defects in axonal transport, the tracer was applied shortly before the onset of the disease (onset). (B) For the ultramicroscopy, the specimens placed into the chamber with CS were moved through light sheet obtained with cylindrical lens. Emitted light was collected with 10× objective located at 90° to the light sheet, let through optical filter allowing only REx to efficiently pass and imaged with an EMCCD camera. (C–E) 3D reconstructions of ultramicroscopy optical stacks showing REx-positive cells in spinal cord, 0.3 mm-thick stack, 225-day-old Tga20 mouse (C); RN, 0.2 mm-thick stack, 244-day-old Tga20 mouse (D), and MC, 4.2 mm-thick stack, 242-day-old wt mouse (E). Scale bars, 100 μm.

Figure 2

(A) Ultramicroscopy image from 0.2 mm-thick stack from RN of 396-day-old wt mock mouse. The image quality is comparable to the confocal imaging (B). (B) Confocal image of the RN, 70-day-old wt mock mouse. (C) 3D reconstruction of 0.2 mm-thick ultramicroscopy RN stack, the same specimen as in A. The ultramicroscopy enables 3D reconstruction of superior quality and containing lower background as compared to the confocal one (D). (D) 3D reconstruction of six serial confocal stacks from RN. Because of cryosectioning, the reconstruction contains high background. (E) 3D reconstruction of ultramicroscopy stacks from prion-challenged Tga20 mouse. The side of the RN contralateral to prion inoculation contains reduced amount of REx-positive cells (arrows). Scale bars, 100 μm.

Figure 3

(A) 3D reconstruction of 3.8 mm-thick ultramicroscopy stacks showing REx-positive cells in the MC of 273-day-old wt mouse challenged with prions i.n. (AAV-REx application at 136 days post infection). (B) Confocal image of the MC from 264-day-old prion-infected wt mouse (AAV-REx application at 136 days post infection). (C and D) First (C) and last (D) ultramicroscopy images from MC stack done with prion-infected wt mouse (the same as in A). Graph coordinates indicate the distances in micrometers. Scale bars, 100 μm.

Figure 4

(A) Number of REx-positive neurons is reduced bilaterally in the MC of i.n. prion-infected mice as compared to mock control. Confocal imaging done with 288-day-old mock and 264-day-old prion-challenged wt mice (AAV-REx application at 136 days post infection). Scale bar, 100 μm. (B) 3D reconstructions of 0.2 mm-thick ultramicroscopy stacks also reveal the reduction of traced neurons in the MC upon prion challenge. The imaging was done on 244-day-old mock and 225-day-old i.n. prion-challenged Tga20 mice (AAV-REx application at 54 days post infection). Scale bar, 100 μm. (C) 3D cell position and image analysis of the data from Fig. 3C are shown in the MC of mock and prion-challenged mice (the same specimens as in B).

Figure 5

(A) REx deposits colocalize with NeuN-positive neurons in the MC of mock-challenged mouse. The deposit sizes allow distinguishing the REx-positive neurons from background fluorescence. Prion-infected animals show dramatically less labeled neurons. Scale bars, 100 μm. (B) Graph of the REx-positive neuron quantification. Despite the difference in the incubation times (168.8 days post infection, dpi and 67.9 dpi for wt and Tga20 mice, respectively), both mouse lines showed similar reduction of REx-positive neurons. Wt demonstrates 96% ± 4% impaired neurons on the contralateral side and 92% ± 11% on the ipsilateral side as evaluated on confocal images, (n = 4), and 93% ± 7% and 87 ± 6%, respectively, as evaluated on 0.25 mm-thick ultramicroscopy stacks (n = 5). Tga20 shows 98% ± 0.1% and 78% ± 4% impaired neurons, on the contra- and ipsilateral sides, respectively, as evaluated on 0.25 mm-thick ultramicroscopy stacks (n = 3). ^∗∗∗p < 0.0001 ^∗∗p = 0.0005 (unpaired t-test).