Postsynaptic degeneration as revealed by PSD-95 reduction occurs after advanced Aβ and tau pathology in transgenic mouse models of Alzheimer's disease

Abstract

Impairment of synaptic plasticity underlies memory dysfunction in Alzheimer's disease (AD). Molecules involved in this plasticity such as PSD-95, a major postsynaptic scaffold protein at excitatory synapses, may play an important role in AD pathogenesis. We examined the distribution of PSD-95 in transgenic mice of amyloidopathy (5XFAD) and tauopathy (JNPL3) as well as in AD brains using double-labeling immunofluorescence and confocal microscopy. In wild type control mice, PSD-95 primarily labeled neuropil with distinct distribution in hippocampal apical dendrites. In 3-month-old 5XFAD mice, PSD-95 distribution was similar to that of wild type mice despite significant Aβ deposition. However, in 6-month-old 5XFAD mice, PSD-95 immunoreactivity in apical dendrites markedly decreased and prominent immunoreactivity was noted in neuronal soma in CA1 neurons. Similarly, PSD-95 immunoreactivity disappeared from apical dendrites and accumulated in neuronal soma in 14-month-old, but not in 3-month-old, JNPL3 mice. In AD brains, PSD-95 accumulated in Hirano bodies in hippocampal neurons. Our findings support the notion that either Aβ or tau can induce reduction of PSD-95 in excitatory synapses in hippocampus. Furthermore, this PSD-95 reduction is not an early event but occurs as the pathologies advance. Thus, the time-dependent PSD-95 reduction from synapses and accumulation in neuronal soma in transgenic mice and Hirano bodies in AD may mark postsynaptic degeneration that underlies long-term functional deficits.

Figure 1. PSD-95 decreases in apical dendrites in 6 month old, but not in 3 month old, 5XFAD mice

a. PSD-95 (green) and Aβ (red) immunoreactivity with TO-PRO 3 (blue) nuclear counterstain in hippocampus of wild type (WT), 3 month old (3M) and 6 month old (6M) 5XFAD mice. PSD-95 immunoreactivity highlights regions of apical dendrites in CA1, CA3 and DG (dentate gyrus). Boxes indicate the portion of CA1 region illustrated in b and c. Bar for all = 500 μm. b. High power images show PSD-95 and Aβ double labeling. PSD-95 immunoreactivity is prominent in apical dendrites and weak in soma in WT. Similar PSD-95 distribution is seen in 3M 5XFAD, despite of the presence of Aβ plaques. In contrast, PSD-95 immunoreactivity is weak in apical dendrites and prominent in soma (arrow) in 6M 5XFAD. Bar for all = 50 μm. c. Images of 100 μm × 100 μm areas show TO-PRO 3 stained nuclei (upper panel) and PSD-95 labeled apical dendrites (lower panel) that were analyzed in d and e respectively. Bar for all = 50 μm. d. Numbers of neuronal nuclei counted in CA1 pyramidal layer do not differ among the 3 groups: WT = 24.0±0.4; 3M = 23.5 ±0.5; 6M = 23.0±0.6 (p > 0.05, ANOVA). e. PSD-95 intensity measured in CA1 apical dendrites is reduced in 6M 5XFAD by 39.6±1.3% and 41.8±1.4% compared with WT and 3M 5XFAD mice, respectively (*: p < 0.001, post hoc Tukey test).

Figure 2. PSD-95 decreases in apical dendrites in hippocampus in 14 month old, but not in 3 month old, JNPL3 mice

a. PSD-95 (green) and tau (red) immunoreactivity with TO-PRO 3 (blue) nuclear counterstain in hippocampus of wild type (WT), 3 month old (3M) and 14 month old (14M) JNPL3 mice. Boxes indicate the portion of CA1 region illustrated in b and c. Bar for all = 500 μm. b. High power images show PSD-95 and tau double-labeling. In WT, PSD-95 immunoreactivity labels apical dendrites and no tau positive neurons are present. In 3M JNPL3, PSD-95 immunoreactivity also labels apical dendrites, with a number of tau positive neurons present. In 14M JNPL3, PSD-95 immunoreactivity disappears from apical dendrites and appears in a number of neuronal soma (arrow), some of which localize with tau (arrowhead). Bar for all = 50 μm. c. Images of 100 μm × 100 μm areas show TO-PRO 3 stained nuclei (upper panel) and PSD-95 labeled apical dendrites (lower panel) that were analyzed in d and e respectively. Bar for all = 50 μm. d. Numbers of neuronal nuclei counted from CA1 pyramidal layer do not differ among the 3 groups: WT = 24.2±0.7, 3M = 23.6 ±0.3, 14M = 23.2±0.2 (p > 0.05, ANOVA). e. PSD-95 intensity measured from CA1 apical dendrites is significantly reduced in 14M by 57.7±0.9% and 57.4±5.1% compared to WT and 3M JNPL3 mice, respectively (*: p < 0.001, post hoc Tukey test).

Fig. 3. PSD-95 immunoreactivity in Hirano bodies in AD brains

a. PSD-95 (green) and tau (red) immunoreactivity with TO-PRO 3 counterstain (blue) in CA1 region of a control brain. PSD-95 immunoreactivity labels neuropil, while no tau positivity is detected. b. CA1 region of an AD brain shows tau positive neurofibrillary tangles (NFT, arrowhead) and dystrophic neurons (DN) and PSD-95 positive spindle-like Hirano bodies (HB, arrows). c. H&E stain shows an eosinophilic HB (arrow) in association with a neuron. b. A HB (arrow) is labeled with PSD-95 antibody with immunoproxidase-DAB reaction. e. A HB (arrow) is double-labeled with antibodies to PSD-95 and MAP2, a somatodendritic marker. f. A HB (arrow) is double-labeled with antibodies to PSD-95 and actin filament. Bar in b for a and b = 50 μm. Bar in f for c, d, e, and f = 20μm.