Amyloid peptide toxicity and microtubule-stabilizing drugs

Power of Tyrosine Assembly in Microtubule Stabilization and Neuroprotection Fueled by Phenol Appendages

Microtubules play a crucial role in maintenance of structure, function, axonal extensions, cargo transport, and polarity of neurons. During neurodegenerative diseases, microtubule structure and function get severely damaged due to destabilization of its major structural proteins. Therefore, design and development of molecules that stabilize these microtubule networks have always been an important strategy for development of potential neurotherapeutic candidates. Toward this venture, we designed and developed a tyrosine rich trisubstituted triazine molecule (TY3) that stabilizes microtubules through close interaction with the taxol binding site. Detailed structural investigations revealed that the phenolic protons are the key interacting partners of tubulin. Interestingly, we found that this molecule is noncytotoxic in PC12 derived neurons, stabilizes microtubules against nocodazole induced depolymerization, and increases expression of acetylated tubulin (Ac-K40), an important marker of tubulin stability. Further, results show that TY3 significantly induces neurite sprouting as compared to the untreated control as well as the two other analogues (TS3 and TF3). It also possesses anti-Aβ fibrillation properties as confirmed by ThT assay, which leads to its neuroprotective effect against amyloidogenic induced toxicity caused through nerve growth factor (NGF) deprivation in PC12 derived neurons. Remarkably, our results reveal that it reduces the expression of TrkA (pY490) associated with NGF deprived amyloidogenesis, which further proves that it is a potent amyloid β inhibitor. Moreover, it promoted the health of the rat primary cortical neurons through higher expression of key neuronal markers such as MAP2 and Tuj1. Finally, we observed that it has good serum stability and has the ability to cross the blood-brain barrier (BBB). Overall, our work indicates the importance of phenolic -OH in promoting neuroprotection and its importance could be implemented in the development of future neurotherapeutics.

Crafting of Neuroprotective Octapeptide from Taxol-Binding Pocket of β-Tubulin

Microtubules play a crucial role in maintaining the shape and function of neurons. During progression of Alzheimer's disease (AD), severe destabilization of microtubules occurs, which leads to the permanent disruption of signal transduction processes and memory loss. Thus, microtubule stabilization is one of the key requirements for the treatment of AD. Taxol, a microtubule stabilizing anticancer drug, has been considered as a potential anti-AD drug but was never tested in AD patients, likely because of its' toxic nature and poor brain exposure. However, other microtubule-targeting agents such as epothilone D (BMS-241027) and TPI-287 (abeotaxane) and NAP peptide (davunetide) have entered in AD clinical programs. Therefore, the taxol binding pocket of tubulin could be a potential site for designing of mild and noncytotoxic microtubule stabilizing molecules. Here, we adopted an innovative strategy for the development of a peptide based microtubule stabilizer, considering the taxol binding pocket of β-tubulin, by using alanine scanning mutagenesis technique. This approach lead us to a potential octapeptide, which strongly binds to the taxol pocket of β-tubulin, serves as an excellent microtubule stabilizer, increases the expression of acetylated tubulin, and acts as an Aβ aggregation inhibitor and neuroprotective agent. Further, results revealed that this peptide is nontoxic against both PC12 derived neurons and primary cortical neurons. We believe that our strategy and discovery of peptide-based microtubule stabilizer will open the door for the development of potential anti-AD therapeutics in near future.

Microtubule-stabilizing peptides and small molecules protecting axonal transport and brain function: focus on davunetide (NAP)

This review focuses on the therapeutic effects and mechanisms of action of NAP (davunetide), an eight amino acid snippet derived from activity-dependent neuroprotective protein (ADNP) which was discovered in our laboratory. We have recently described the effects of NAP in neurodegenerative disorders, and we now review the beneficial effects of NAP and other microtubule-stabilizing agents on impairments in axonal transport. Experiments in animal models of microtubule-deficiency including tauopathy (spanning from drosophila to mammals) showed protection of axonal transport by microtubule-stabilizers and NAP, which was coupled to motor and cognitive protection. Clinical trials with NAP (davunetide) are reviewed paving the path to future developments.

Notch signalling in adult neurons: a potential target for microtubule stabilization

Cytoskeletal dysfunction has been proposed during the last decade as one of the main mechanisms involved in the aetiology of several neurodegenerative diseases. Microtubules are basic elements of the cytoskeleton and the dysregulation of microtubule stability has been demonstrated to be causative for axonal transport impairment, synaptic contact degeneration, impaired neuronal function leading finally to neuronal loss. Several pathways are implicated in the microtubule assembly/disassembly process. Emerging evidence is focusing on Notch as a microtubule dynamics regulator. We demonstrated that activation of Notch signalling results in increased microtubule stability and changes in axonal morphology and branching. By contrast, Notch inhibition leads to an increase in cytoskeleton plasticity with intense neurite remodelling. Until now, several microtubule-binding compounds have been tested and the results have provided proof of concept that microtubule-binding agents or compounds with the ability to stabilize microtubules may have therapeutic potential for the treatment of Alzheimer's disease and other neurodegenerative diseases. In this review, based on its key role in cytoskeletal dynamics modulation, we propose Notch as a new potential target for microtubule stabilization.

NAP (davunetide) rescues neuronal dysfunction in a Drosophila model of tauopathy

Alzheimer's disease (AD) is a devastating neurodegenerative disease causing irreversible cognitive decline in the elderly. There is no disease-modifying therapy for this condition and the mechanisms underpinning neuronal dysfunction and neurodegeneration are unclear. Compromised cytoskeletal integrity within neurons is reported in AD. This is believed to result from loss-of-function of the microtubule-associated protein tau, which becomes hyper-phosphorylated and deposits into neurofibrillary tangles in AD. We have developed a Drosophila model of tauopathy in which abnormal human tau mediates neuronal dysfunction characterised by microtubule destabilisation, axonal transport disruption, synaptic defects and behavioural impairments. Here we show that a microtubule-stabilising drug, NAPVSIPQ (NAP), prevents as well as reverses these phenotypes even after they have become established. Moreover, it does not alter abnormal tau levels indicating that it by-passes toxic tau altogether. Thus, microtubule stabilisation is a disease-modifying therapeutic strategy protecting against tau-mediated neuronal dysfunction, which holds great promise for tauopathies like AD.

Microtubule stabilizing agents as potential treatment for Alzheimer's disease and related neurodegenerative tauopathies

The microtubule (MT) associated protein tau, which is highly expressed in the axons of neurons, is an endogenous MT-stabilizing agent that plays an important role in axonal transport. Loss of MT-stabilizing tau function, caused by misfolding, hyperphosphorylation, and sequestration of tau into insoluble aggregates, leads to axonal transport deficits with neuropathological consequences. Several in vitro and preclinical in vivo studies have shown that MT-stabilizing drugs can be utilized to compensate for the loss of tau function and to maintain/restore effective axonal transport. These findings indicate that MT-stabilizing compounds hold considerable promise for the treatment of Alzheimer disease and related tauopathies. The present article provides a synopsis of the key findings demonstrating the therapeutic potential of MT-stabilizing drugs in the context of neurodegenerative tauopathies, as well as an overview of the different classes of MT-stabilizing compounds.

Hyperdynamic microtubules, cognitive deficits, and pathology are improved in tau transgenic mice with low doses of the microtubule-stabilizing agent BMS-241027

Tau is a microtubule (MT)-stabilizing protein that is altered in Alzheimer's disease (AD) and other tauopathies. It is hypothesized that the hyperphosphorylated, conformationally altered, and multimeric forms of tau lead to a disruption of MT stability; however, direct evidence is lacking in vivo. In this study, an in vivo stable isotope-mass spectrometric technique was used to measure the turnover, or dynamicity, of MTs in brains of living animals. We demonstrated an age-dependent increase in MT dynamics in two different tau transgenic mouse models, 3xTg and rTg4510. MT hyperdynamicity was dependent on tau expression, since a reduction of transgene expression with doxycycline reversed the MT changes. Treatment of rTg4510 mice with the epothilone, BMS-241027, also restored MT dynamics to baseline levels. In addition, MT stabilization with BMS-241027 had beneficial effects on Morris water maze deficits, tau pathology, and neurodegeneration. Interestingly, pathological and functional benefits of BMS-241027 were observed at doses that only partially reversed MT hyperdynamicity. Together, these data suggest that tau-mediated loss of MT stability may contribute to disease progression and that very low doses of BMS-241027 may be useful in the treatment of AD and other tauopathies.

Current experimental therapy for Alzheimer's disease

In the past decade, enormous efforts have been devoted to understand the genetics and molecular pathogenesis of Alzheimer's disease (AD), which has been transferred into extensive experimental approaches aimed at reversing disease progression. The trend in future AD therapy has been shifted from traditional anti-acetylcholinesterase treatment to multiple mechanisms-based therapy targeting amyloid plaques formation and amyloid peptides (Abeta)-mediated cytotoxicity, and neurofibrillary tangles generation. This review will cover current experimental studies with the focus on secretases-based drug development, immunotherapy, and anti-neurofibrillary tangles intervention. The outcome of these on-going studies may provide high hope that AD can be cured in the future.

(3R,5S,7as)-(3,5-Bis(4-fluorophenyl)tetrahydro-1H-oxazolo[3,4-c]oxazol-7a-yl)methanol, a novel neuroprotective agent

Compounds that interact with microtubules, such as paclitaxel, have been shown to possess protective properties against beta-amyloid (Abeta) induced neurodegeneration associated with Alzheimer's disease. In this work, the novel agent (3R,5S,7as)-(3,5-bis(4-fluorophenyl)tetrahydro-1H-oxazolo[3,4-c]oxazol-7a-yl)methanol was investigated for effectiveness in protecting neurons against several toxic stimuli and its interaction with the microtubule network. Exposure of neuronal cultures to Abeta peptide in the presence of 5 nM (3R,5S,7as)-(3,5-bis(4-fluorophenyl)tetrahydro-1H-oxazolo[3,4-c]oxazol-7a-yl)methanol resulted in a 50% increase in survival. Neuronal cultures treated with other toxic stimuli such as staurosporine, thapsigargin, paraquat, and H(2)O(2) showed significantly enhanced survival in the presence of (3R,5S,7as)-(3,5-bis(4-fluorophenyl)tetrahydro-1H-oxazolo[3,4-c]oxazol-7a-yl)methanol. Microtubule binding and tubulin assembly studies revealed differences compared to paclitaxel but confirmed the interaction of (3R,5S,7as)-(3,5-bis(4-fluorophenyl)tetrahydro-1H-oxazolo[3,4-c]oxazol-7a-yl)methanol with microtubules. Furthermore, in vitro studies using bovine brain microvessel endothelial cells experiments suggest that (3R,5S,7as)-(3,5-bis(4-fluorophenyl)tetrahydro-1H-oxazolo[3,4-c]oxazol-7a-yl)methanol can readily cross the blood-brain barrier in a passive manner.

NAP: research and development of a peptide derived from activity-dependent neuroprotective protein (ADNP)

Activity-dependent neuroprotective protein (ADNP) is essential for brain formation. Peptide activity scanning identified NAP (NAPVSIPQ) as a small active fragment of ADNP that provides neuroprotection at very low concentrations. In cell culture, NAP has demonstrated protection against toxicity associated with the beta-amyloid peptide, N-methyl-D-aspartate, electrical blockade, the envelope protein of the AIDS virus, dopamine, H2O2, nutrient starvation and zinc overload. NAP has also provided neuroprotection in animal models of apolipoprotein E deficiency, cholinergic toxicity, closed head injury, stroke, middle aged anxiety and cognitive dysfunction. NAP binds to tubulin and facilitates microtubule assembly leading to enhanced cellular survival that is associated with fundamental cytoskeletal elements. A liquid-chromatography, mass spectrometry assay demonstrated that NAP reaches the brain after either intravenous or intranasal administration. In a battery of toxicological tests including repeated dose toxicity in rats and dogs, cardiopulmonary tests in dogs, and functional behavioral assays in rats, no adverse side effects were observed with NAP concentrations that were approximately 500-fold higher than the biologically active dose. A Phase Ia clinical trial in the US assessed the tolerability and pharmacokinetics of intranasal administration of NAP in sequential ascending doses. The results supported the safety and tolerability of a single dose of NAP administered at up to 15 mg intranasally. Furthermore, dosing was recently completed for a second Phase I clinical trial in healthy adults and elderly volunteers with an intravenous formulation of NAP. NAP is poised for further clinical development targeting several indications, including Alzheimer's disease.

Disturbed assembly of human cerebral microtubules in Alzheimer’s disease

It is shown for the first time that microtubular proteins isolated from the brain of patients with Alzheimer's disease can in vitro polymerize into microtubules with abnormal structure.

Neuroprotective strategies in Alzheimer's disease

In addition to strategies designed to decrease amyloid beta (A beta) levels, it is likely that successful Alzheimer's disease (AD) therapeutic regimens will require the concomitant application of neuroprotective agents. Elucidation of pathophysiological processes occurring in AD and identification of the molecular targets mediating these processes point to potential high-yield neuroprotective strategies. Candidate neuroprotective agents include those that interact specifically with neuronal targets to inhibit deleterious intraneuronal mechanisms triggered by A beta and other toxic stimuli. Strategies include creating small molecules that block A beta interactions with cell surface and intracellular targets, down-regulate stress kinase signaling cascades, block activation of caspases and expression of pro-apoptotic proteins, and inhibit enzymes mediating excessive tau protein phosphorylation. Additional potential neuroprotective compounds include those that counteract loss of cholinergic function, promote the trophic state and plasticity of neurons, inhibit accumulation of reactive oxygen species, and block excitotoxicity. Certain categories of compounds, such as neurotrophins or neurotrophin small molecule mimetics, have the potential to alter neuronal signaling patterns such that several of these target actions might be achieved by a single agent.

Stabilization of the cyclin-dependent kinase 5 activator, p35, by paclitaxel decreases beta-amyloid toxicity in cortical neurons

One hallmark of Alzheimer's disease (AD) is the formation of neurofibrillary tangles, aggregated paired helical filaments composed of hyperphosphorylated tau. Amyloid-beta (Abeta) induces tau hyperphosphorylation, decreases microtubule (MT) stability and induces neuronal death. MT stabilizing agents have been proposed as potential therapeutics that may minimize Abeta toxicity and here we report that paclitaxel (taxol) prevents cell death induced by Abeta peptides, inhibits Abeta-induced activation of cyclin-dependent kinase 5 (cdk5) and decreases tau hyperphosphorylation. Taxol did not inhibit cdk5 directly but significantly blocked Abeta-induced calpain activation and decreased formation of the cdk5 activator, p25, from p35. Taxol specifically inhibited the Abeta-induced activation of the cytosolic cdk5-p25 complex, but not the membrane-associated cdk5-p35 complex. MT-stabilization was necessary for neuroprotection and inhibition of cdk5 but was not sufficient to prevent cell death induced by overexpression of p25. As taxol is not permeable to the blood-brain barrier, we assessed the potential of taxanes to attenuate Abeta toxicity in adult animals using a succinylated taxol analog (TX67) permeable to the blood-brain barrier. TX67, but not taxol, attenuated the magnitude of both basal and Abeta-induced cdk5 activation in acutely dissociated cortical cultures prepared from drug treated adult mice. These results suggest that MT-stabilizing agents may provide a therapeutic approach to decrease Abeta toxicity and neurofibrillary pathology in AD and other tauopathies.