FKBP52 overexpression accelerates hippocampal-dependent memory impairments in a tau transgenic mouse model.

Abnormal accumulation of hyperphosphorylated tau induces pathogenesis in neurodegenerative diseases, like Alzheimer's disease. Molecular chaperones with peptidyl-prolyl cis/trans isomerase (PPIase) activity are known to regulate these processes. Previously, in vitro studies have shown that the 52 kDa FK506-binding protein (FKBP52) interacts with tau inducing its oligomerization and fibril formation to promote toxicity. Thus, we hypothesized that increased expression of FKBP52 in the brains of tau transgenic mice would alter tau phosphorylation and neurofibrillary tangle formation ultimately leading to memory impairments. To test this, tau transgenic (rTg4510) and wild-type mice received bilateral hippocampal injections of virus overexpressing FKBP52 or GFP control. We examined hippocampal-dependent memory, synaptic plasticity, tau phosphorylation status, and neuronal health. This work revealed that rTg4510 mice overexpressing FKBP52 had impaired spatial learning, accompanied by long-term potentiation deficits and hippocampal neuronal loss, which was associated with a modest increase in total caspase 12. Together with previous studies, our findings suggest that FKBP52 may sensitize neurons to tau-mediated dysfunction via activation of a caspase-dependent pathway, contributing to memory and learning impairments.

Hsp90 activator Aha1 drives production of pathological tau aggregates.

The microtubule-associated protein tau (MAPT, tau) forms neurotoxic aggregates that promote cognitive deficits in tauopathies, the most common of which is Alzheimer's disease (AD). The 90-kDa heat shock protein (Hsp90) chaperone system affects the accumulation of these toxic tau species, which can be modulated with Hsp90 inhibitors. However, many Hsp90 inhibitors are not blood-brain barrier-permeable, and several present associated toxicities. Here, we find that the cochaperone, activator of Hsp90 ATPase homolog 1 (Aha1), dramatically increased the production of aggregated tau. Treatment with an Aha1 inhibitor, KU-177, dramatically reduced the accumulation of insoluble tau. Aha1 colocalized with tau pathology in human brain tissue, and this association positively correlated with AD progression. Aha1 overexpression in the rTg4510 tau transgenic mouse model promoted insoluble and oligomeric tau accumulation leading to a physiological deficit in cognitive function. Overall, these data demonstrate that Aha1 contributes to tau fibril formation and neurotoxicity through Hsp90. This suggests that therapeutics targeting Aha1 may reduce toxic tau oligomers and slow or prevent neurodegenerative disease progression.

Human cyclophilin 40 unravels neurotoxic amyloids.

The accumulation of amyloidogenic proteins is a pathological hallmark of neurodegenerative disorders. The aberrant accumulation of the microtubule associating protein tau (MAPT, tau) into toxic oligomers and amyloid deposits is a primary pathology in tauopathies, the most common of which is Alzheimer's disease (AD). Intrinsically disordered proteins, like tau, are enriched with proline residues that regulate both secondary structure and aggregation propensity. The orientation of proline residues is regulated by cis/trans peptidyl-prolyl isomerases (PPIases). Here we show that cyclophilin 40 (CyP40), a PPIase, dissolves tau amyloids in vitro. Additionally, CyP40 ameliorated silver-positive and oligomeric tau species in a mouse model of tau accumulation, preserving neuronal health and cognition. Nuclear magnetic resonance (NMR) revealed that CyP40 interacts with tau at sites rich in proline residues. CyP40 was also able to interact with and disaggregate other aggregating proteins that contain prolines. Moreover, CyP40 lacking PPIase activity prevented its capacity for disaggregation in vitro. Finally, we describe a unique structural property of CyP40 that may permit disaggregation to occur in an energy-independent manner. This study identifies a novel human protein disaggregase and, for the first time, demonstrates its capacity to dissolve intracellular amyloids.

Imbalances in the Hsp90 Chaperone Machinery: Implications for Tauopathies.

The ATP-dependent 90 kDa heat shock protein, Hsp90, is a major regulator of protein triage, from assisting in nascent protein folding to refolding or degrading aberrant proteins. Tau, a microtubule associated protein, aberrantly accumulates in Alzheimer's disease (AD) and other neurodegenerative diseases, deemed tauopathies. Hsp90 binds to and regulates tau fate in coordination with a diverse group of co-chaperones. Imbalances in chaperone levels and activity, as found in the aging brain, can contribute to disease onset and progression. For example, the levels of the Hsp90 co-chaperone, FK506-binding protein 51 kDa (FKBP51), progressively increase with age. In vitro and in vivo tau models demonstrated that FKBP51 synergizes with Hsp90 to increase neurotoxic tau oligomer production. Inversely, protein phosphatase 5 (PP5), which dephosphorylates tau to restore microtubule-binding function, is repressed with aging and activity is further repressed in AD. Similarly, levels of cyclophilin 40 (CyP40) are reduced in the aged brain and further repressed in AD. Interestingly, CyP40 was shown to breakup tau aggregates in vitro and prevent tau-induced neurotoxicity in vivo. Moreover, the only known stimulator of Hsp90 ATPase activity, Aha1, increases tau aggregation and toxicity. While the levels of Aha1 are not significantly altered with aging, increased levels have been found in AD brains. Overall, these changes in the Hsp90 heterocomplex could drive tau deposition and neurotoxicity. While the relationship of tau and Hsp90 in coordination with these co-chaperones is still under investigation, it is clear that imbalances in these proteins with aging can contribute to disease onset and progression. This review highlights the current understanding of how the Hsp90 family of molecular chaperones regulates tau or other misfolded proteins in neurodegenerative diseases with a particular emphasis on the impact of aging.

Inhibition of Both Hsp70 Activity and Tau Aggregation in Vitro Best Predicts Tau Lowering Activity of Small Molecules.

Three scaffolds with inhibitory activity against the heat shock protein 70 (Hsp70) family of chaperones have been found to enhance the degradation of the microtubule associated protein tau in cells, neurons, and brain tissue. This is important because tau accumulation is linked to neurodegenerative diseases including Alzheimer's disease (AD) and chronic traumatic encephalopathy (CTE). Here, we expanded upon this study to investigate the anti-tau efficacy of additional scaffolds with Hsp70 inhibitory activity. Five of the nine scaffolds tested lowered tau levels, with the rhodacyanine and phenothiazine scaffolds exhibiting the highest potency as previously described. Because phenothiazines also inhibit tau aggregation in vitro, we suspected that this activity might be a more accurate predictor of tau lowering. Interestingly, the rhodacyanines did inhibit in vitro tau aggregation to a similar degree as phenothiazines, correlating well with tau-lowering efficacy in cells and ex vivo slices. Moreover, other Hsp70 inhibitor scaffolds with weaker tau-lowering activity in cells inhibited tau aggregation in vitro, albeit at lower potencies. When we tested six well-characterized tau aggregation inhibitors, we determined that this mechanism of action was not a better predictor of tau-lowering than Hsp70 inhibition. Instead, we found that compounds possessing both activities were the most effective at promoting tau clearance. Moreover, cytotoxicity and PAINS activity are critical factors that can lead to false-positive lead identification. Strategies designed around these principles will likely yield more efficacious tau-lowering compounds.

Noncontact Rotational Head Injury Produces Transient Cognitive Deficits but Lasting Neuropathological Changes.

Traumatic brain injury (TBI) caused by improvised explosive devices (IEDs) is a growing problem in military settings, but modeling this disease in rodents to pre-clinically evaluate potential therapeutics has been challenging because of inconsistency between models. Although the effects of primary blast wave injury have been extensively studied, little is known regarding the effects of noncontact rotational TBIs independent of the blast wave. To model this type of injury, we generated an air cannon system that does not produce a blast wave, but generates enough air pressure to cause rotational TBI. Mice exposed to this type of injury showed deficits in cognitive and motor task acquisition within 1-2 weeks post-injury, but mice tested 7-8 weeks post-injury did not retain any deficits. This suggests that the effects of a single, noncontact rotational TBI are not long lasting. Despite the transient nature of the behavioral deficits, increased levels of phosphorylated tau were observed at 2 and 8 weeks post-injury; however, this tau did not adopt typical pathological structures that have been observed in other TBI models that incorporate blast waves. This was possibly attributed to the fact that this injury was insufficient to induce changes in microglial activation, which was not affected at 2 or 8 weeks post-injury. Taken together, these data suggest that exposure to noncontact, rotational head injury only produces transient cognitive anomalies, but elicits some minor lasting neuropathological changes.