Our study demonstrates that overexpression of P301L-mutant hTau in the EC is insufficient to cause cognitive deficits in mice up to 16 months of age, even though it causes extensive hyperphosphorylation and abnormal folding of tau as well as tau aggregation and synaptic abnormalities. These results contrast with our recent findings that overexpression of mutant APP/Aβ in the EC causes not only synaptic deficits, but also age-dependent cognitive and behavioral abnormalities 
. Taken together, these studies suggest a predominant role of APP/Aβ in the pathogenesis of early neuronal dysfunction in the entorhinal-hippocampal network. They do not exclude critical roles of tau in the pathogenesis of AD-related dysfunction of other brain regions or in the loss of neurons that occurs as the disease progresses in humans. Evidence for such roles of tau has been obtained in multiple lines of transgenic mice with widespread neuronal expression of hTau and may involve disruptions of axonal transport, destabilization of microtubules, mislocalization of tau into dendritic spines, and changes in neurotransmission 
. It should also be noted that hAPP transgenic mice that lack endogenous tau are protected from developing many of the synaptic, network and cognitive alterations seen in hAPP mice with wildtype tau levels 
, suggesting a critical role of endogenous wildtype tau in Aβ-induced neuronal dysfunction.
Although no tau mutations have been discovered in AD patients, many transgenic lines of mice with neuronal overexpression of FTLD-mutant or wildtype forms of hTau simulate typical AD pathologies, including hyperphosphorylation, aggregation and missorting of tau, and neuronal loss in vulnerable brain regions 
. Cognitive deficits have also been reported in tau transgenic mice with widespread transgene expression 
. However, since these transgenic mice overexpress hTau throughout most of the brain, it is difficult to know whether the resulting cognitive deficits are due specifically to tau-induced dysfunction of AD-vulnerable brain regions (e.g. entorhinal-hippocampal areas), as opposed to brain regions vulnerable to other tauopathies with cognitive symptoms (e.g. frontal cortex).
In contrast to these models, EC-hTau mice at ages up to 16 months were not cognitively impaired relative to their littermate controls. We calculated statistical power curves to ensure that the lack of differences between EC-hTau mice and control groups in our behavioral tests was not spurious. As reported in the Results
section, our current sample sizes and variances were such that we could have detected statistically significant learning deficits in EC-hTau mice if their average latencies in the Morris water maze had increased by 6.2–9.4 sec over average latencies found in control groups. Such increases are smaller or approximately equal to those found in transgenic lines with widespread hAPP expression 
and with EC-restricted expression of mutant APP 
, supporting the conclusion that the absence of detectable behavioral deficits in the EC-hTau line is real. However, more subtle deficits could still have been missed.
It is important to consider in this regard whether repeated exposure and training in the same behavioral tests might have helped EC-hTau mice overcome subtle deficits. However, behaviorally naïve 8-month-old EC-hTau mice were also unimpaired in the Morris water maze. Furthermore, EC-hTau mice were tested for the first time in the passive avoidance paradigm at 12 months and in the novel place recognition paradigm at 16 months and showed no deficits in either paradigm as compared to age-matched NTG controls. Lastly, because the levels of tau dimers and insoluble tau in the EC and DG were clearly lower in EC-hTau mice than in rTg4510 mice, it is possible that the levels of functionally relevant abnormal tau assemblies were simply not high enough in EC-hTau mice to cause significant behavioral impairments.
The EC and its connections to the hippocampus have an established role in spatial navigation memory 
. We were therefore surprised that EC-hTau mice with overt tau pathology within the EC and pathological alterations of EC to GC synapses did not display any measurable cognitive deficits, particularly since the same paradigms revealed obvious deficits in transgenic mice expressing APP/Aβ in a similar distribution 
. The EC-hTau mice tested here were F1 hybrids between C57Bl6 and FVBN strains instead of congenic C57Bl6 as our EC-APP mice 
and, thus, might have benefited from hybrid vigor. However, hAPPJ20 mice on the same FVBN/C57Bl6 background did display cognitive deficits relative to their NTG littermates 
. Interestingly, virus-mediated overexpression of wildtype hTau produced tau pathology within the hippocampus but was also not sufficient to cause cognitive impairment 
. Extensive aggregation of tau in both EC and hippocampal regions may be required to cause cognitive decline 
Whereas several transgenic lines with widespread neuronal expression of P301L-mutant hTau develop cognitive deficits 
, others do not 
, or have improved cognition at younger ages before developing deficits at old ages 
. The P301L-mutant hTau line reported by Kimura et al. (2007) had no cognitive deficits but extensive tau pathology and neuronal loss, whereas a complementary line expressing wildtype hTau had cognitive deficits but less tau pathology and no neuronal loss 
. Phosphorylation of wildtype tau in the EC of these mice correlated with synaptic loss, impairment of neuronal activity, and cognitive deficits 
. Thus, it is possible that wildtype, but not P301L-mutant, hTau can cause neuronal dysfunction of the EC.
At first glance, the EC-hTau model may seem suitable for testing recent hypotheses on “prion-like” spread of tau pathology between cells and interconnected brain regions 
, because tTA in the neuropsin-tTA line is expressed in presynaptic EC cells but not in postsynaptic DG GCs 
and this expression pattern was confirmed in EC-APP mice 
. However, the tet-hTau singly transgenic line has some level of “leaky” transgene expression in the absence of the transactivator, including in GCs of the DG (
and this study). Consequently, hTau is weakly expressed in the DG of EC-hTau mice, albeit at much lower levels than in the EC. These findings are consistent with those reported by others in similar EC-hTau mice 
. In our opinion, the expression of hTau in GCs of tet-hTau mice makes it impossible to definitely conclude that hTau was transferred from EC into DG neurons in the EC-hTau model. Using a similar model, another group recently showed that GCs containing hTau protein were devoid of hTau mRNA 
and concluded that hTau protein must have been transferred into these cells from other neurons. Another possible interpretation is that high levels of pathological hTau protein in GCs caused a reduction in mRNA synthesis, as has been reported for tangle-bearing neurons 
Notwithstanding these caveats, we did find pathological forms of tau in GC bodies in EC-hTau mice, as also observed by 
, but not in tet-hTau singly transgenic mice. Therefore, the “leaky” expression of hTau in GCs cannot account for the accumulation of pathological tau in GCs of EC-hTau mice. These findings raise two possibilities. First, pathological tau may have been transferred from EC to DG neurons, causing the accumulation of pathological tau within GCs, possibly enabled or promoted by low levels of corresponding hTau “templates” in GCs (indicating a prion-like behavior). In support of this hypothesis, hTau was found to co-aggregate with endogenous mouse tau 
. However, in P301L FTD patients, mutant tau actually does not seem to sequester wildtype tau 
, possibly because “seeds” of P301L tau induce the assembly of tau filaments from P301L-mutant, but not wildtype, tau 
. Second, overexpression and accumulation of tau in the EC may have indirectly caused the abnormal localization and conformational changes of hTau expressed by GCs, perhaps through alterations in network activity and afferent inputs. Additional studies are needed to distinguish between these possibilities.
The transgenic tau model presented here recapitulates both the topological pattern of tau pathology and the lack of cognitive deficits in AD patients with early Braak stages. It could be utilized to investigate how additional factors such as Aβ, apolipoprotein E4, α-synuclein or TDP-43, may advance the progression of AD beyond these early stages. A better understanding of this transition could provide additional avenues for therapeutic intervention to prevent loss of memory and other cognitive functions.