A new mouse model has achieved a long-sought pathological prize—the production of hyperphosphorylated and aggregated tau proteins in a background of elevated Aβ. The trick to linking Aβ to tau’s tangles, it turns out, is to get rid of nitric oxide (NO). In a paper published August 14 in PNAS online, Hana Dawson and colleagues from Duke University, Durham, North Carolina, report that knocking out the gene for inducible nitric oxide synthase (NOS2) in an APP transgenic mouse leads to the hyperphosphorylation and aggregation of endogenous mouse tau protein and neuronal death.
The results, showing that the decrease in NOS exacerbates Aβ production and generates pathological tau species, run contrary to previous work finding NOS knockouts are protected against amyloid pathology (Nathan et al., 2005). In general, NO has been considered harmful to neurons (see ARF related news story and ARF news story), but the new results suggest that under some circumstances, NO may actually protect them.
The new mouse, generated by first author Carol Colton and coworkers, resulted from crossing the APPsw transgenic line Tg2576 with a NOS2 knockout. The progeny lacked the iNOS protein and displayed total NOS activity of about one-third that of APPsw mice. Hyperphosphorylated tau appeared in the soma and dendrites of cortical and hippocampal neurons in the APPsw/NOS-/- animals—phospho-tau was not seen in the APPsw or NOS-/- mice. Aggregated tau was detected by immunostaining or electron microscopy after filtration of brain lysates, and in intact tissue using thioflavin F staining.
Knockout of NOS2 also enhanced amyloid pathology in the mice. Total brain Aβ levels were five to six times higher in APPsw/NOS2-/- compared to APPsw littermates. The increase was mostly due to insoluble Aβ peptides. These results suggest that NO might influence Aβ generation or clearance by an unknown mechanism.
Finally, the mice showed neuron loss, which is not normally seen in APPsw mice. In three of four APPsw/NOS2-/- animals, cortical neurodegeneration was apparent after staining with Fluoro-Jade C, an anionic fluorescein derivative that specifically stains degenerating neurons (Schmued et al., 2005). Some neurons appeared to be undergoing apoptosis, as activated caspase-3 was detected in hippocampal neurons. Tau cleavage was also elevated in the same neurons compared to those in the APPsw mice.
The mechanism by which loss of NO promotes the accumulation of phospho-tau remains a mystery, but the authors propose two possibilities. Nitrosylation of tau inhibits tangle formation, so lack of NO might contribute to tau aggregation. Also, NO activates the Akt kinase, which inhibits tau phosphorylation via GSK3, and loss of NOS might release this inhibition.
In contrast to these results, NOS2 knockout in a double transgenic APPsw and mutant human presenilin background was previously reported to result in fewer plaques, longer lifespan, and less microglial activation. The authors of the current report speculate that the presenilin mutation, which has been implicated in NO-mediated toxicity (Hashimoto et al., 2004), could account for the different results.
As NO is generally accepted as a danger to neurons (see ARF related news story), one contribution of this work is to shade the view a bit to consider that the levels and timing of NO production may determine its impact. In light of the results that lowering NO over the life of a mouse causes the appearance of tau pathology, it could be that the induction of iNOS documented in Alzheimer disease (see Meda et al., 1995) might even represent a protective response, the authors speculate.—Pat McCaffrey