Although a large body of evidence supports some level of interaction between presenilin and tau in AD pathogenesis, the precise contributions of each have yet to be fully ascertained. Specifically, the role of wild-type human tau has been largely passed over in favor of study of mutant tau-expressing mouse models, which more readily develop tau pathology but do not accurately reflect the genetics of tau in AD. Here we have shown that expression of WtTau in PS cDKO mice can impair multiple cellular functions that are significant to the development of AD (). In these mice, tau hyperphosphorylation and pathological conformation were detected as early as 6 months of age in neurons of the septal nuclei of the basal forebrain and the dentate gyrus of the hippocampus. We determined that anterograde axonal transport was impaired specifically in PS cDKO;WtTau mice at this time point, suggesting a role for tau hyperphosphorylation and/or aggregation in this deficit. In the hippocampus, reduced synaptosomal BDNF in the context of normal levels of total BDNF provided additional support for the hypothesis that axonal transport is impaired in areas with tau pathology. The finding that Erk activation was also diminished in the hippocampus of PS cDKO;WtTau mice implied that this level of synaptic BDNF reduction was sufficient to decrease its downstream signaling. Deficient BDNF/Erk signaling had important consequences for contextual learning and memory, and long-term potentiation as well, as impairments in both were observed to a greater degree in PS cDKO mice expressing WtTau. Finally, the combination of these deficits led to an increase in cortical atrophy in PS cDKO;WtTau mice when compared with PS cDKO mice, suggesting that multiple mechanisms underlie the neurodegeneration in these mice, and possibly in AD.
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 (more ...)
The presence of tau pathology in the septal nuclei and dentate gyrus of PS cDKO;WtTau mice could have several implications for neuronal function in the hippocampus. Septal neurons are known to project to the dentate gyrus in addition to other hippocampal and extrahippocampal regions. Lesion of septal cholinergic neurons or pharmacological inhibition of septohippocampal cholinergic signaling impairs hippocampal-dependent learning and memory, a deficit that can be rescued by administration of acetylcholine to the hippocampus (for review, see Parent and Baxter, 2004
). This corresponds with the loss of cholinergic neurons in AD, which results in memory impairment that can be partially allayed by treatment with acetylcholinesterase inhibitors (Kasa et al., 1997
; Seltzer, 2006
). In addition, pharmacological inhibition of cholinergic signaling by scopolamine has been shown to reduce protein levels of BDNF in the rodent hippocampus (Kotani et al., 2006
). Although we did not detect a change in total levels of BDNF in the hippocampus, tau phosphorylation of a subset of neurons in the septohippocampal pathway would perturb the system to a significantly lesser degree than global inhibition of cholinergic signaling. This limited tau phosphorylation also explains our inability to detect with western blots the tau pathology observed with immunohistochemistry. Ultimately, the finding that interruption of cholinergic signaling can lead to reductions in downstream BDNF levels provides support for our model that disruption of septohippocampal signaling through tau hyperphosphorylation leads to reduced synaptic BDNF in the hippocampus. Furthermore, strong involvement of this pathway in hippocampal-dependent learning and memory implies a potential mechanism for the specific contextual memory deficit we observed in PS cDKO;WtTau mice.
The development of tau pathology and impaired axonal transport in PS cDKO;WtTau mice are likely mediated by changes in the activity of tau kinases. Levels of p25, a potent activator of the tau kinase Cdk5, have been shown to increase in PS cDKO mice by 9 months of age (Saura et al., 2004
). Interestingly, loss of presenilin has also been shown to increase GSK3β kinase activity, leading to GSK3β-mediated phosphorylation of the motor protein kinesin to promote its release from its membrane-bound cargoes during anterograde axonal transport (Morfini et al., 2002
; Pigino et al., 2003
). GSK3β misregulation due to presenilin loss could also lead to tau phosphorylation, suggesting two possible mechanisms for kinase-mediated axonal transport defects in PS cDKO mice: directly through kinesin phosphorylation, and/or indirectly through tau phosphorylation. Because PS cDKO mice did not demonstrate impairment of axonal transport in our studies, tau phosphorylation appears to exert a more profound effect on trafficking than kinesin phosphorylation. Other kinases that phosphorylate tau could also be involved, and it is possible that a combination of small increases in multiple kinases, rather than large increases in a single kinase’s activity, could be responsible for the tau phenotypes observed in our study.
While most neurotrophins undergo primarily retrograde axonal transport from the synapse to the cell body, anterograde transport of BDNF is seen in multiple pathways within the brain, including septal and cortical projections to the hippocampus and intrahippocampal circuits (reviewed in Schindowski et al., 2008
). Along with our observation that synaptic localization of BDNF is reduced, this supports the hypothesis that the defects in axonal transport detected in the olfactory system of PS cDKO;WtTau mice might extend to other brain regions, notably the hippocampus. Decreased Erk signaling in the hippocampus and functional deficits in hippocampal-dependent memory and synaptic plasticity in PS cDKO;WtTau animals further support this notion. In addition to a role for impaired axonal transport in reduced BDNF/Erk signaling, presenilin loss may contribute to this phenotype through other mechanisms. Loss of presenilin in primary neurons results in decreased maturation and membrane localization of TrkB, the primary receptor for BDNF (Naruse et al., 1998
). This reduction in mature TrkB at the synaptic membrane could exacerbate an Erk signaling impairment caused by reduced synaptic BDNF. Although PS cDKO mice did not exhibit a statistically significant reduction in Erk activation, a downward trend was observed in these mice, supporting the existence of more than one mechanism underlying this phenotype. In fact, this could be the case with each of the phenotypes exhibited by PS cDKO mice that were exacerbated in PS cDKO;WtTau animals (i.e. contextual memory deficits and neurodegeneration).
Although LTP defects were not clearly demonstrated in PS cDKO mice, a trend toward reduced potentiation did exist. As with Erk signaling, other effects of presenilin loss could be contributing to this phenotype. LTP and other measures of synaptic plasticity require Ca2+ signaling, which is regulated by presenilin and is defective in familial AD mutants or PS-deficient mice (reviewed in Mattson, 2010). A recent study genetically and pharmacologically dissected the role of presenilin in Schaffer collateral neurotransmitter release and found that presynaptic loss of presenilin impairs theta-burst induced LTP through impairment of calcium release (Zhang et al., 2009). This corresponds with our observation that in addition to LTP, paired-pulse facilitation is altered in PS cDKO;WtTau mice, indicating a presynaptic defect. Although these alterations in Ca2+ release could also affect Mn2+, our administration of saturating doses of MnCl2 and the lack of axonal transport impairment in PS cDKO mice make it unlikely that such a phenomenon occurred in our study. Our inability to detect LTP deficits in PS cDKO mice could be explained by differences in experimental setup or variation in mouse strains and transgenes between our study and previous reports. Because of the trend toward a decrease in PS cDKO vs. PS2 KO mice in our present study, however, it is possible that a statistically significant difference would be observed with an increased sample size or a modified stimulation protocol.
Taken together, our data support a combined role for presenilin and tau in AD pathogenesis. Substantial evidence supports the notion that familial AD mutations in presenilin genes primarily result in PS loss of function, particularly with regard to γ-secretase cleavage of Notch, cadherins, and APP (Chen et al., 2002
; Moehlmann et al., 2002
; Wiley et al., 2005
). In PS mutants, the cleavage of APP generates β-amyloid species in a ratio that favors the pathogenic Aβ42 over Aβ40, but this could result from decreased production of Aβ40, not excess Aβ42 (reviewed in De Strooper, 2007
). Presenilin mutations can also impair γ-secretase-independent functions, leading to defects in Wnt signaling and Ca2+
homeostasis (reviewed in Shen and Kelleher, 2007
). This evidence supports the use of PS cDKO mice to study the roles of presenilin and tau in AD pathogenesis. We have established that loss of presenilin alone results in a phenotype that can be enhanced and even expanded by the presence of wild-type human tau, leading to impairment of axonal trafficking, neurotrophin signaling, learning and memory, and synaptic plasticity, all of which culminate in neurodegeneration. These findings provide evidence for the convergence of multiple pathways in the progression of AD and indicate that subtle differences in protein composition (i.e. human vs. mouse tau) can dramatically affect cellular processes governing neuronal function and survival.