Convergence of presenilin- and tau-mediated pathways on axonal trafficking and neuronal function

Abstract

Alzheimer's disease (AD) is a significant and growing health problem in the aging population. Although definitive mechanisms of pathogenesis remain elusive, genetic and histological clues have implicated the proteins presenilin (PS) and tau as key players in AD development. PS mutations lead to familial AD, and although tau is not mutated in AD, tau pathology is a hallmark of the disease. Axonal transport deficits are a common feature of several neurodegenerative disorders and may represent a point of intersection of PS and tau function. To investigate the contribution of wild-type, as opposed to mutant, tau to axonal transport defects in the context of presenilin loss, we used a mouse model postnatally deficient for PS (PS cDKO) and expressing wild-type human tau (WtTau). The resulting PS cDKO;WtTau mice exhibited early tau pathology and axonal transport deficits that preceded development of these phenotypes in WtTau or PS cDKO mice. These deficits were associated with reduced neurotrophin signaling, defective learning and memory and impaired synaptic plasticity. The combination of these effects accelerated neurodegeneration in PS cDKO;WtTau mice. Our results strongly support a convergent role for PS and tau in axonal transport and neuronal survival and function and implicate their misregulation as a contributor to AD pathogenesis.

Figure 1.

Tau pathology in 6-month-old PS cDKO;WtTau mice. A, Representative images from the dentate gyrus showing CP13 (left) and MC-1 (right) immunostaining of PS2 KO, WtTau, PS cDKO, and PS cDKO;WtTau mice. B, Representative images of CP13 and MC-1 immunostaining of a region including both the dorsolateral septal nucleus (upper right of each panel) and the medial septum (lower left of each panel). High-magnification insets show characteristic somatodendritic immunoreactivity in PS cDKO;WtTau neurons. Scale bars, 10 μm for inset images, 50 μm for all others. C, Representative Western blots of phosphorylated (PHF-1 and AT8) and total (Tau 5) tau in hippocampal tissue homogenates dissected from 6-month-old mice show increased tau phosphorylation in PS cDKO;WtTau animals. H, human transgenic tau; M, mouse endogenous tau.

Figure 2.

Age-dependent reduction in axonal transport rates of PS cDKO;WtTau mice. A, Representative MEMRI color map images from PS2 KO controls and PS cDKO;WtTau mice at 6 months of age showing signal enhancement (emphasized by rectangular outline) between the first image (left) and the last image (right) of the series. B, Axonal transport rates are normal in 2-month-old PS2 KO, WtTau, PS cDKO, and PS cDKO;WtTau mice. C, Axonal transport rates at 6 months are selectively reduced in PS cDKO;WtTau mice. D, Linear regression of all data points from each genotype of 6-month-old mice. Given slopes are graphically represented and statistically analyzed in C). ΔSI/t = change in normalized signal intensity over time = rate of axonal transport. ***p < 0.001.

Figure 3.

Decreased synaptosomal BDNF levels and Erk1/2 activation in the hippocampus of 6-month-old PS cDKO;WtTau animals. A, Representative Western blots of total BDNF and BDNF from synaptosome fractions (BDNF). B, Quantification of normalized BDNF band densities relative to control (PS2 KO). C, Representative Western blots of total and phospho-Erk1/2. D, Quantification of normalized Erk1/2 band densities relative to control. Loading controls were as follows: synaptotagmin (Syt) for synaptosomal BDNF and γ-tubulin for all others. *p < 0.05, **p < 0.01.

Figure 4.

Hippocampal-dependent memory and synaptic plasticity impairments in PS cDKO;WtTau mice at 6 months of age. A, Freezing behaviors during various phases of fear conditioning. Immediate freezing occurs during the initial exploratory period of training. Context test freezing represents the amount of time the mice froze during second exposure to the training chamber 24 h after training. Normalized cued test freezing is the difference between freezing during second exposure to the conditioned stimulus (CS) and freezing before CS in a novel chamber 24 h after training. B, Following 10 min of baseline recording, theta burst stimulation (TBS) was delivered to the Schaffer collateral pathway. When compared with PS2 KO controls, PS cDKO;WtTau mice demonstrate significantly impaired LTP induction and maintenance at all time points tested; however, the difference between PS cDKO;WtTau and PS cDKO mice reaches significance only at the initial 3 and last 40 min. No significant difference is observed between PS cDKO and PS2 KO mice. Insets (a–c), Example fEPSP traces taken before and after TBS from PS2 KO, PS cDKO, and PS cDKO;WtTau mice, respectively (calibration, 1 mV, 5 ms). C, Increased paired-pulse ratio in PS cDKO;WtTau mice at 10 and 20 ms interpulse intervals indicating altered presynaptic function in the hippocampus of PS cDKO;WtTau mice. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 5.

Increased cortical neurodegeneration in PS cDKO;WtTau compared with PS cDKO mice. A, Volumes of cerebellum, cerebral cortex, and ventricles are normal in 2-month-old PS2 KO, PS cDKO, and PS cDKO;WtTau mice. Quantification of volumes (left) and representative 3D MRI images (right) show no obvious differences in structures between genotypes. B, Cerebral cortical atrophy of PS cDKO mice is exacerbated in PS cDKO;WtTau animals at 6 months. Quantification of volumes (left) and representative 3D MRI images (right) show a decrease in cortical volume with compensatory increase in ventricular volume (top) in PS cDKO mice and, to a further degree, in PS cDKO;WtTau mice. No change in cerebellar volumes was detected (bottom). C, Representative images from cresyl violet (Nissl)-stained frozen brain sections demonstrate cortical atrophy in 6-month-old PS cDKO and PS cDKO;WtTau mice as quantified in panel 5B. Vertical lines are provided to assist with visual assessment of cortical thickness at sites that are comparable between sections. Scale bar, 500 μm, ***p < 0.001.

Figure 6.

Model of neuronal dysfunction and neurodegeneration in PS cDKO;WtTau mice. Presenilin loss has been shown previously to increase tau phosphorylation, reduce TrkB receptor maturation and synaptic localization, and result in defective Ca²⁺ homeostasis (dotted lines; see Discussion for references). In combination with presenilin loss, expression of WtTau accelerates tau hyperphosphorylation, leading to impaired anterograde axonal transport of synaptic proteins such as BDNF. This decrease in BDNF at the synapse reduces downstream signaling, which, along with loss of presenilin, can directly affect neuronal survival. In addition, specific reduction of Erk phosphorylation/activation impairs hippocampal-dependent memory and long-term potentiation, phenotypes to which presenilin loss likely contributes through mechanisms outside the scope of this study. Boxed results were obtained from in vivo studies, with the exception of LTP.