In vivo axonal transport deficits in a mouse model of fronto-temporal dementia

Abstract

Background: Axonal transport is vital for neurons and deficits in this process have been previously reported in a few mouse models of Alzheimer's disease prior to the appearance of plaques and tangles. However, it remains to be determined whether axonal transport is defective prior to the onset of neurodegeneration. The rTg4510 mouse, a fronto-temporal dementia and parkinsonism-17 (FTDP-17) tauopathy model, over-express tau-P301L mutation found in familial forms of FTDP-17, in the forebrain driven by the calcium-calmodulin kinase II promoter. This mouse model exhibits tau pathology, neurodegeneration in the forebrain, and associated behavioral deficits beginning at 4-5 months of age.

Animal model: rTg4510 transgenic mice were used in these studies. Mice were given 2 μL of MnCl2 in each nostril 1 h prior to Magnetic Resonance Imaging (MRI). Following MnCl2 nasal lavage, mice were imaged using Manganese enhanced Magnetic Resonance Imaging (MEMRI) Protocol with TE = 8.5 ms, TR = 504 ms, FOV = 3.0 cm, matrix size = 128 × 128 × 128, number of cycles = 15 with each cycle taking approximately 2 min, 9 s, and 24 ms using Paravision software (BrukerBioSpin, Billerica, MA). During imaging, body temperature was maintained at 37.0 °C using an animal heating system (SA Instruments, Stony Brook, NY).

Data analysis: Resulting images were analyzed using Paravision software. Regions of interest (ROI) within the olfactory neuronal layer (ONL) and the water phantom consisting of one pixel (ONL) and 9 pixels (water) were selected and copied across each of the 15 cycles. Signal intensities (SI) of ONL and water phantom ROIs were measured. SI values obtained for ONL were then normalized the water phantom SI values. The correlation between normalized signal intensity in the ONL and time were assessed using Prism (GraphPad Software, San Diego, CA).

Results: Using the MEMRI technique on 1.5, 3, 5, and 10-month old rTg4510 mice and littermate controls, we found significant axonal transport deficits present in the rTg4510 mice beginning at 3 months of age in an age-dependent manner. Using linear regression analysis, we measured rates of axonal transport at 1.5, 3, 5, and 10 months of age in rTg4510 and WT mice. Axonal transport rates were observed in rTg4510 mice at 48% of WT levels at 3 months, 40% of WT levels at 5 months, and 30% of WT levels at 10 months of age. In order to determine the point at which tau appears in the cortex, we probed for phosphorylated tau levels, and found that pSer262 is present at 3 months of age, not earlier at 1.5 months of age, but observed no pathological tau species until 6 months of age, months after the onset of the transport deficits. In addition, we saw localization of tau in the ONL at 6 months of age.

Discussion: In our study, we identified the presence of age-dependent axonal transport deficits beginning at 3 months of age in rTg4510 mice. We correlated these deficits at 3 months to the presence of hyperphosphorylated tau in the brain and the presence within the olfactory epithelium. We observed tau pathology not only in the soma of these neurons but also within the axons and processes of these neurons. Our characterization of axonal transport in this tauopathy model provides a functional time point that can be used for future therapeutic interventions.

Keywords: Alzheimer's disease; Axonal transport; Fronto-temporal dementia; MEMRI; MRI; Tau.

Graphical abstract

Supplementary Table 1

The numbers of mice used throughout this study are displayed at 1.5, 3, and 10 months of age. WT is also referred to as NTg.

Fig. 1

Representative images from olfactory bulb slices from MEMRI scans of WT first cycle (A) and last cycle (B) and rTg4510 mice first cycle (C) and last cycle (D) 10 month old mice. The left side of the olfactory bulb contains an artifact that is present throughout all scans. Arrows on the right hand side indicate changes in signal intensity throughout multiple cycles of MEMRI scans.

Fig. 2

Axonal transport rates in non-transgenic (WT) and rTg4510 (tau +/tta +) are displayed at 3 months with n = 6 (WT) and n = 6 (rTg4510) (A), 5 months with n = 5 (WT) and n = 5 (rTg4510) (B), and 10 months with n = 5 (WT) and n = 8 (rTg4510) (C) of age. *p < 0.05, **p < 0.005.

Fig. 3

Western blots show the expression of various tau species (tau 5, AT8, PHF-1, pTauSer262), neurofilament, GFAP, and GAPDH in the cortex of control and rTg4510 mice at 1, 3, and 6 months of age. N = 3 per group.

Fig. 4

Representative images of human tau (HT7) and olfactory marker protein (OMP) staining in 6-month old rTg4510 mice. (A) Sagittal brain sections from NTg and rTg4510 mice were double-labeled with antibodies that recognize human tau (HT7, green) and OMP (red). Tau expression was detected in axonal projections from olfactory sensory neurons (OSNs) to the olfactory bulb (OB) in rTg4510 mice and colocalized with OMP (white arrowheads). (B) Expression of human tau and OMP in the olfactory epithelium of NTg and rTg4510. In rTg4510 mice, tau was expressed in the cell bodies of OSNs located with the olfactory epithelium (OE).