Effects of 17-allylamino-17-demethoxygeldanamycin (17-AAG) in transgenic mouse models of frontotemporal lobar degeneration and Alzheimer’s disease

Abnormal aggregation of protein is a common theme among neurodegenerative diseases: Aβ peptide in Alzheimer’s Disease (AD), tau protein in AD and other tauopathies such as some forms of frontotemporal dementia (FTD), and α-synuclein in Parkinson’s disease and Lewy Body Disease [1‐4]. These serious diseases occur with a high prevalence, yet no treatment is available to halt their progression, and such treatments are urgently needed. Drugs interfering with protein misfolding may hold promise for slowing neurodegenerative disease, as well as other disorders of protein aggregation [5‐7].

Extracellular amyloid plaques and intracellular neurofibrillary tangles (NFTs) are the two core pathological lesions found in Alzheimer’s disease [3]. Amyloid plaques are mainly composed of Aβ, whereas NFTs consist of hyperphosphorylated tau. Aβ aggregation in the brain parenchyma may initiate a series of events that eventually results in AD dementia [2]. These events include activation of kinases that hyperphosphorylate tau, which may harm neurons by direct toxicity of abnormal tau or by decreased supply of tau to stabilize microtubules. Mutations in tau can also cause such dysfunctions, leading to frontotemporal lobar degeneration (FTLD) [1].

Tg2576 and JNPL3 are mouse models of Aβ and tau pathology, respectively [8, 9]. Tg2576 mice contain the human APP gene with the Swedish mutation, leading to five-fold and 14-fold higher than usual levels of Aβ1-40 and Aβ1-42, respectively. Cognitive impairment develops at 9 to 10 months, with the presence of Aβ plaques in cortical and limbic regions. JNPL3 mice contain human tau P301L mutant protein, with expression controlled by the murine prion (PrP) promoter [10]. Similar hyperphosphorylated tau products are generated in both human tauopathies and JNPL3 mice [11]. This mutation causes motor and behavioural deficits, plus NFTs, starting at 4.5 months of age, in amygdala, medulla, cerebellum, hypothalamus, pons and spinal cord [8].

Therapeutic approaches include reducing the brain concentrations of Aβ and tau or inhibiting their aggregation [4, 12, 13]. Immunotherapy is being investigated as a method for removing Aβ or tau, either in any form or in aggregated forms. For example, JNPL3 mice immunized against tau oligomers showed slower disease progression and fewer tau aggregates than untreated mice [14]. It is possible that removal of aggregation-prone proteins might be even more effective if conducted before aggregation occurs, when the proteins are still monomeric. But removing normal forms of these proteins may disrupt their important cellular functions. Aggregation-prone forms of proteins may be identified by their abnormally folded structure.

Heat shock proteins (HSPs) are chaperone proteins that have the ability to remodel misfolded proteins or direct them to the degradation pathway under either normal or stress conditions. Since proteins must fold into precise three-dimensional conformations to achieve their biological functions, HSPs assist cells to defend against external stress, such as accumulation of misfolded proteins or inflammation [15‐17].

Heat shock protein 90 (Hsp90) is one of the most abundant proteins in the cytosol of eukaryotic cells, accounting for 1-2% of the total proteins under non-stress conditions [18]. It has an essential role in the refolding of misfolded proteins under stress [19]. Despite this, inappropriate signal activation of Hsp90 could also occur during cellular stress and lead to toxic protein aggregation [20]. Hsp90 binds to a specific set of targets, which are known as Hsp90 client proteins. It interacts with its client proteins in a dynamic ATP dependent cycle to regulate folding and assembly of its clients, but the mechanism is poorly understood.

Hsp90 consists of three domains, a highly conserved N-terminal ATP-binding domain, a middle domain, and a carboxyl domain. Both the N-terminal and C-terminal domains play a role in preventing aggregation of denatured proteins in vitro. By inhibiting the N-terminal ATP binding domain, the activity of Hsp90 is reduced [21], suggesting that substrate binding is nucleotide regulated. Since Hsp90 has a unique N-terminal ATP binding site, high specificity inhibitors have been developed to inhibit the ATP-dependent chaperone activities of Hsp90 [22].

17-N-Allylamino-17-demethoxygeldanamycin (17-AAG), or tanespimycin, is a novel HSP90 inhibitor that has been extensively studied for targeting oncoproteins [23]. It is a less toxic derivative of geldanamycin (GA), a benzoquinoid ansamycin compound produced by Streptomyces hygroscopicus that has growth inhibitory activity against tumours [24]. However, due to undesired toxicity, GA was not suitable for clinical studies. Instead, 17-AAG has been or is now in clinical trials for a wide range of cancers. It is a candidate for treating neurodegenerative disorders.

17-AAG has two mechanisms of action. First, 17-AAG disrupts the function of Hsp90 by binding to its ATP binding pocket, inhibiting the formation of the stabilizing form and shifting the complex to the proteosome-targeting form, which leads to proteosome degradation of client protein [25]. Second, it inhibits the association of Hsp90 with heat shock factor-1, which results in transcription of chaperones such as Hsp70 and Hsp40 [20, 26, 27]. These two molecular chaperones act as protein quality control units, refolding misfolded proteins or leading them to destruction. They use the energy gained from ATP hydrolysis to bind and remodel substrates under conditions either with or without stress. Hsp70 requires assistance from Hsp40 to facilitate substrate recognition and stimulate the hydrolysis of ATP [28]. Induction of Hsp40 and Hsp70 by geldanamycin successfully suppressed mutant huntingtin in a Huntington’s disease cell model [20], suggesting the possibility of geldanamycin or 17-AAG as treatments for other neurodegenerative diseases involving protein misfolding. Waza et al. [15] tested 17-AAG to decrease mutant androgen receptor (AR), a client protein of Hsp90 that causes spinal and bulbar muscular atrophy (SBMA). Mutant AR products in cell models were drastically decreased after being treated by 17-AAG, even with complete inhibition of Hsp70 and Hsp40, which was achieved by protein synthesis inhibitor.

17-AAG might be used to block Aβ or tau aggregation. Hsp70 helps prevent early stages of protein aggregation, including that of Aβ [17]. It can function extracellularly [29] and therefore might promote disaggregation of extracellular Aβ. Hsp90 may play a role in Aβ peptide aggregation [17], and a non-toxic Hsp90 inhibitor significantly reduced Aβ neurotoxicity in embryonic primary neurons [30]. By targeting Hsp90, client proteins which are normally stabilized by Hsp90 will be targeted for degradation [31]. Therefore, 17-AAG could potentially act as a neuroprotective agent that counters the toxic effects of Aβ by disrupting its aggregation and promoting its clearance.

Inhibiting the action of Hsp90 may affect tau by several possible mechanisms. Tau is a Hsp90 client protein [32]. Hyperphosphorylated tau binds with higher affinity to Hsp90 than to normal tau [33]. Binding of hyperphosphorylated tau to Hsp90 enhances stabilization and aggregation [34, 35], thus Hsp90 inhibition may reduce tau aggregation. Extracellular signal-regulated protein kinase (ERK) is an Hsp90 client protein, and 17-AAG is able to reduce the amount of and aggregation of phosphorylated tau, possibly by inhibiting the action of ERK [36]. 17-AAG is capable of inducing Hsp40 and Hsp70, of which Hsp70 was seen to be able to reduce the level of tau [33, 37]. In light of the above evidence, 17-AAG is worth investigating as a therapeutic agent for tauopathies such as FTLD.

17-AAG has excellent bioavailability after intraperitoneal injection in mice [38], but oral bioavailability is poor. It undergoes extensive hepatic metabolism by cytochrome P450 (CYP) 3A4 and is widely distributed in body tissues [38, 39]. Preclinical toxicology studies showed that 17-AAG has dose-limiting hepatic and gallbladder toxicity, but it is less toxic than geldanamycin [40]. Studies on the tissue distribution of 17-AAG in mice showed that it penetrates the blood brain barrier [41, 42].

In this study, we investigated the effect of 17-AAG on Aβ aggregation in the Tg2576 transgenic mouse model of AD and tau aggregation in the JNPL3 mouse model of FTLD. We quantified the effects on several outcomes: mortality, motor function, and plaque and tangle pathology.

Many mice in the study died during treatment. However, mortality did not significantly differ between mice treated with or without 17-AAG, thus 17-AAG was not the cause of the high death rate. Perhaps repeated intraperitoneal injections, totaling 39 during the three month study, killed mice due to needle injury. However, other studies involving many intraperitoneal injections did not report adverse health effects on mice.

Alternatively, DMSO, which was used as the vehicle to dissolve 17-AAG, might have caused the deaths. As an effective solvent for a wide range of hydrophilic and hydrophobic compounds, DMSO readily penetrates animal tissue and is widely used as a solvent in research [49]. At injection sites, undiluted DMSO can irritate, inflame, and cause intravascular thrombosis [50]. For mice that completed the three month study, the cumulative dose of DMSO was 7.8% (v/w), quite a large proportion of body weight. Autopsies showed that the abdominal cavities of some of the mice contained liquid, purple or reddish-purple for mice receiving 17-AAG and clear for control mice, and revealed abnormal adhesions between membranes and muscles, which might have been caused by the solubilizing or inflammatory effect of exposure to accumulations of DMSO. An additional control group, using injections without either 17-AAG or DMSO, could have helped explore whether DMSO contributed to the early mortality we observed, however we did not anticipate this problem and thus did not plan a control group without DMSO.

DMSO was used because 17-AAG has limited water solubility. A potential alternative is Cremophor EL, but it may induce hypersensitivity reactions and anaphylaxis. Thus, patients require treatment with steroids and antihistamines before the administration of 17-AAG with Cremophor EL. When 17-AAG was used in a phase II clinical trial as an anti-tumour agent, only a limited effect of 17-AAG against tumors was observed, perhaps due to DMSO toxicity [51]. Thus, another clinical trial was conducted using Cremophor instead of DMSO, resulting in minimal toxicity [52]. Therefore, Cremophor, or safer delivery methods, may be considered for future studies.

Treatment with 17-AAG tended to preserve motor function in JNPL3 mice (p = 0.14), as measured by the rotarod test. Whether this tendency represents a real effect would require further studies, perhaps with a safer method of administration, larger numbers of mice, longer treatment, and treatment at different age ranges. It is possible that treatment began too late or did not last long enough to display effects to their greatest potential.

In this experiment, treatment with 17-AAG did not reduce Aβ deposition in a transgenic mouse model of Alzheimer’s disease. There are several possible explanations for this lack of an effect.

We had hypothesized that 17-AAG could up-regulate the expression of Hsp40 and Hsp70, which work synergistically to disaggregate and degrade misfolded proteins, and hence may suppress aggregation of Aβ. But 17-AAG had limited ability to induce Hsp40 and Hsp70 in vivo[20, 53]. Induction of these heat shock proteins may have been insufficient.

Another possibility is that Aβ may not be a Hsp90 client protein. Hsp90 assists protein folding and stabilization of its client protein, and 17-AAG is an Hsp90 inhibitor. Although Hsp90 co-localized in vivo with amyloid filaments, indicating that Hsp90 may have a role in either promoting or suppressing the refolding of misfolded amyloid proteins [17, 54], the overall effect of Hsp90 on Aβ aggregation is not clear. Since the action of Hsp90 is ATP-dependent, and since the increase of reactive oxygen species induced by Aβ may interfere with cellular mitochondrial energy metabolism, thereby decreasing the supply of ATP [55], Aβ might partially inhibit its own aggregation by inhibiting Hsp90. When taking into account the possible contradictory effects on protein aggregation, it is hard to predict whether 17-AAG would raise or lower the level of protein aggregates.

Another possible reason may be that the mice were too old when they began treatment. Inhibition of intracellular Hsp90 may prevent misfolding of Aβ monomers, but might not be effective to reverse extracellular aggregation of Aβ once it has occurred. An earlier treatment may attenuate the Aβ aggregation before pathology begins, but mice of younger ages would be needed to test this.

Alternatively, the number of mice may have been insufficient. Detection of a small treatment effect would require many mice.

Finally, the method of quantification might have obscured a treatment effect. By selecting the microscope fields with the highest concentrations of plaques, a treatment effect that decreased maximum or overall plaque density would be detected, but an effect only on areas of lower plaque density would be missed, as would an effect on Aβ oligomers but not on plaques.

Tau is a member of the Hsp90 client protein family, and mutant or hyperphosphorylated tau binds to Hsp90 at high affinity compared with normal tau [32]; therefore, theoretically, Hsp90 inhibitor might promote degradation of hyperphosphorylated tau and hence reduce neurotoxicity and decrease NFT formation. However, among all mice in our study, 17-AAG did not significantly reduce tau deposition.

As for NFT quantification, the method used to quantify NFTs might have obscured a treatment effect. By selecting the microscope fields with the highest concentrations of NFTs, a treatment effect that decreased overall or maximum NFT density would be detected, but an effect only on areas of lower NFT density would be missed, as would an effect on tau oligomers but not on NFTs. Quantifying total tau rather than only hyperphosphorylated tau may also have provided additional data, however hyperphosphorylated tau is a marker of dysfunction and aggregation in tauopathies and is thus more informative.

It is notable that, although 17-AAG did not decrease NFTs when all mice were examined together, when mice were analyzed by sex, male but not female mice exhibited a tendency toward a decrease in NFTs with the high dose of 17-AAG. A possible explanation could be that female JNPL3 mice tend to develop phenotypic features earlier than male JNPL3 mice, as found in previous research [56] and in the current study, in which 11-month old female mice displayed more NFT than did males of the same age. Sex differences have also been found in other amyloidoses [57‐59]. If treatment with 17-AAG is effective at early stages of tauopathy, it would be more beneficial in male than female mice of the same age. A possible explanation for the difference in tauopathy progression between male and female mice is the difference in hormonal states. The primary female steroid sex hormones are estrogens, which occur as three types: estrone, estradiol, and estriol, of which estradiol is the most potent. Numerous studies, including both human and animal, have shown that estrogens can be neuroprotective and neurotrophic [60‐62], though studies also suggest estrogens may either reduce or increase AD risk [63]. Estradiol can decrease tau phosphorylation [64]. Yet female JNPL3 mice display more NFTs than males of the same age. The mechanism for this difference remains unknown.

Luo et al. [34] tested the effect of an Hsp90 inhibitor on tau reduction, both in vitro and in vivo. As in the present study, they used Hsp90 inhibitors and JNPL3 mice. Their mice were heterozygous females aged 6.5 months, treated five times per week for 30 days. The Hsp90 inhibitors, PU24FC1 and PU-DZ8, reduced tau expression and phosphorylation (p = 0.0034). The team concluded that Hsp90 inhibitors can prevent and reverse tau aggregation. However, we found no effect of Hsp90 inhibition on tau pathology in female JNPL3 mice. Differences in the mice between the two studies might explain the discrepancy. The Luo mice had milder pathology because they were younger (7.5 months vs. 11 months at the end) and expressed less mutant tau (hemizygous vs. homozygous). Consistent with our finding of a tendency toward a protective effect of 17-AAG in the subset of mice with milder pathology—males—the drug might be useful against tauopathies at early stages of disease. Further studies may explore this possibility.