2.8 (Quantitative real‐time) Polymerase‐Chain‐Reaction
4.2 Project II: Aβ triggers Murine ApoE Fragmentation in Transgenic Mouse Models for Alzheimer’s Disease
4.2.6 Impact of Human Mutant Tau on Amyloid Pathology and Aβ‐induced Proteolysis of Murine ApoE in 5XFAD Mice
4.2.6.2 Neuronal Integrity Impairment, Hippocampal Atrophy and Inflammation are aggravated in 5XFAD/PS19 Mice
5XFAD/PS19 mice showed hippocampal shrinkage, significantly aggravated diminishment of synapto‐dendritic connections (Fig. 3.45), as well as drastically elevated astrocytosis (Fig.
3.46) compared to PS19 and 5XFAD parental lines, suggesting a clear impairment of neuronal integrity associated with increased tau pathology. Immunostaining using β3‐tubulin antibody revealed a drastic decrease of apical dendrites in the stratum radiatum of the hippocampus in 9‐month‐old 5XFAD/PS19 animals, whereas PS19 and 5XFAD littermates seemed to be not affected (Fig. 3.45). Under physiological conditions, tau is intraneuronally distributed with a spatial gradient, meaning higher concentrations in axons than in somato‐dendritic compartments (Buée et al. 2000). However, it has been demonstrated that tau gets
Discussion
aberrantly mislocalized to dendritic spines by hyperphosphorylation, resulting in a decreased expression of AMPA‐ and NMDA‐receptors and an impaired synaptic transmission (Hoover et al. 2010). In addition, intracerebral injection of adeno‐associated viral (AAV) mutant tauP301L in transgenic mice expressing the yellow fluorescent protein, led to a drastic degradation of apical dendrites of hippocampal CA1 neurons accompanied by accumulated, enlarged autophagic vacuoles and activated astrocytes (Jaworski et al. 2011). The appearance of hyperphosphorylated tau in apical dendrites in the hippocampus of 5XFAD/PS19 mice already at 3 months of age indicates that tau gets also mislocalized in somato‐dendritic compartments in this mouse model. Furthermore, missorted tau might induce dendritic diminishment, since the affected tau‐immunostained processes disappeared in aged animals (Fig. 3.44f, h), corresponding to a reduced amount of β3‐tubulin‐stained dendrites (Fig.
3.45f) and hippocampal atrophy. However, it has been reported that PS19 mice harbor reduced levels of endoplasmatic reticulum‐specific proteins in dendrites already at 3 months of age, as well as hippocampal shrinkage of approximately 25 % and 45 % at 9 and 12 months of age, respectively, compared to non‐transgenic controls (Yoshiyama et al. 2007).
Therefore, it cannot be excluded that PS19 transgenic mice also develop distorted apical dendrites in the hippocampal stratum radiatum in later ages, since tau pathology in 5XFAD/PS19 animals started earlier as well. Furthermore, immunostaining using synaptic markers like synapsin‐1 further confirms tau’s toxic function in damaging neuronal arborizations. Aged trigenic 5XFAD/PS19 mice harbored drastically reduced synapses in the hippocampal CA3 mossy fiber pathway compared to aged‐matched 5XFAD and PS19 mice, whereas the latter also had significantly less synapses than 5XFAD littermates (Fig. 3.45).
That the PS19 mouse model develops degenerated mossy fibers has been already demonstrated in a previous study, showing reduced neurofilament‐staining in the same region (Yoshiyama et al. 2007). The aggravated synaptic loss in the mossy fiber pathway in combination with drastically decreased apical dendrites in hippocampi of 5XFAD/PS19 suggests that the presence of human mutant APP and PS1, leading to high levels of Aβ, accelerates the degeneration of neuronal processes, which is putatively induced by early tau conformational changes.
5XFAD mice harbor chronic inflammation represented as micro‐ and astrogliosis that increases proportionally with Aβ42 levels (Oakley et al. 2006). Interestingly, in vitro data suggest that astrocytes are crucial for Aβ‐mediated toxicity and hyperphosphorylation of tau
Discussion
in neurons (Garwood et al. 2011). Furthermore, the application of conditioned medium from astrocytes, incubated with soluble Aβ peptides, induced tau phosphorylation and neuronal death of cultured hippocampal neurons (Saez et al. 2006). In the present work, 5XFAD mice also showed an increased inflammatory response in the cortex and thalamus compared to PS19 mice. However, the amounts of activated astrocytes were significantly elevated in 5XFAD/PS19 animals compared to the other genotypes (Fig. 3.46). This indicates that Aβ induced hyperphosphorylation of tau triggers astrocytosis and vice versa resulting in a subsequent aggravation of the phenotype.
Moreover, quantification of hippocampal CA1 neurons revealed a significant loss coinciding with overall hippocampal atrophy in 5XFAD/PS19 mice compared to PS19 and 5XFAD littermates at 9 months of age, with single transgenic PS19 animals showing a tendency towards reduced CA1 neuron number (Fig. 3.45). This finding is in good agreement with the demonstrated hippocampal atrophy in PS19 mice, described in a previous study (Yoshiyama et al. 2007). Notably, the 5XFAD model harbors neuron loss in the fifth cortical layer at 12 months of age, however, CA1 neurons, in which no intracellular Aβ is detectable, are not affected (Jawhar et al. 2012). These data suggest that the co‐expression of Aβ and tau aggravates the hippocampal atrophy as well as CA1 neuron loss in 5XFAD/PS19 mice, probably by Aβ‐triggered tau hyperphosphorylation and toxicity. Similar findings were observed in previous studies dealing with transgenic mice over‐expressing human mutant APP. Injection of AVV‐coupled mutant tau into the entorhinal cortex of TASTPM mice, expressing human mutant APPSw and PS1M146V, induced hyperphosphorylated tau pathology combined with neuritic dystrophy, neuron loss in the hippocampal CA1 region and entorhinal cortex, as well as increased inflammatory response, whereas no changes in Aβ pathology were observed (Dassie et al. 2013). Furthermore, a study using the same PS19 mouse model revealed that crossing these mice with the PDAPP mice, carrying human mutant APPV717F, resulted in the formation of NFT‐like structures and increased premature death rate, without any significant changes in amyloid pathology (Hurtado et al. 2010). Here, Hurtado and colleagues demonstrated that PS19‐PDAPP mice also develop neuron loss compared to PDAPP animals, but they did not consider the neuronal reduction in PS19 animals alone, which has been previously shown by Yoshiyama and colleagues (Yoshiyama et al. 2007).
Discussion
4.2.6.3 Aβ‐induced ApoE Proteolysis is a Possible Upstream Event of Tau