Targeting protein aggregation in neurodegeneration – lessons from polyglutamine disorders

Novel Multitarget Hydroxamic Acids with a Natural Origin CAP Group against Alzheimer's Disease: Synthesis, Docking and Biological Evaluation

Hydroxamic acids are one of the most promising and actively studied classes of chemical compounds in medicinal chemistry. In this study, we describe the directed synthesis and effects of HDAC6 inhibitors. Fragments of adamantane and natural terpenes camphane and fenchane, combined with linkers of various nature with an amide group, were used as the CAP groups. Accordingly, 11 original target compounds were developed, synthesized, and exposed to in vitro and in vivo biological evaluations, including in silico methods. In silico studies showed that all synthesized compounds were drug-like and could penetrate through the blood-brain barrier. According to the in vitro testing, hydroxamic acids 15 and 25, which effectively inhibited HDAC6 and exhibited anti-aggregation properties against β-amyloid peptides, were chosen as the most promising substances to study their neuroprotective activities in vivo. All in vivo studies were performed using 5xFAD transgenic mice simulating Alzheimer's disease. In these animals, the Novel Object Recognition and Morris Water Maze Test showed that the formation of hippocampus-dependent long-term episodic and spatial memory was deteriorated. Hydroxamic acid 15 restored normal memory functions to the level observed in control wild-type animals. Notably, this effect was precisely associated with the ability to restore lost cognitive functions, but not with the effect on motor and exploratory activities or on the level of anxiety in animals. Conclusively, hydroxamic acid 15 containing an adamantane fragment linked by an amide bond to a hydrocarbon linker is a possible potential multitarget agent against Alzheimer's disease.

Hydrogen Bond Surrogate-Constrained Dynamic Antiparallel β-Sheets

Antiparallel β-sheets are important secondary structures within proteins that equilibrate with random-coil states; however, little is known about the exact dynamics of this process. Here, the first dynamic β-sheet models that mimic this equilibrium have been designed by using an H-bond surrogate that introduces constraint and torque into a tertiary amide bond. 2D NMR data sufficiently reveal the structure, kinetics, and thermodynamics of the folding process, thereby leading the way to similar analysis in isolated biologically relevant β-sheets.

Promising Molecular Targets for Pharmacological Therapy of Neurodegenerative Pathologies

Drug development for the treatment of neurodegenerative diseases has to confront numerous problems occurring, in particular, because of attempts to address only one of the causes of the pathogenesis of neurological disorders. Recent advances in multitarget therapy research are gaining momentum by utilizing pharmacophores that simultaneously affect different pathological pathways in the neurodegeneration process. The application of such a therapeutic strategy not only involves the treatment of symptoms, but also mainly addresses prevention of the fundamental pathological processes of neurodegenerative diseases and the reduction of cognitive abilities. Neuroinflammation and oxidative stress, mitochondrial dysfunction, dysregulation of the expression of histone deacetylases, and aggregation of pathogenic forms of proteins are among the most common and significant pathological features of neurodegenerative diseases. In this review, we focus on the molecular mechanisms and highlight the main aspects, including reactive oxygen species, the cell endogenous antioxidant system, neuroinflammation triggers, metalloproteinases, α-synuclein, tau proteins, neuromelanin, histone deacetylases, presenilins, etc. The processes and molecular targets discussed in this review could serve as a starting point for screening leader compounds that could help prevent or slow down the development of neurodegenerative diseases.

PPARδ activation by bexarotene promotes neuroprotection by restoring bioenergetic and quality control homeostasis

Neurons must maintain protein and mitochondrial quality control for optimal function, an energetically expensive process. The peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that promote mitochondrial biogenesis and oxidative metabolism. We recently determined that transcriptional dysregulation of PPARδ contributes to Huntington's disease (HD), a progressive neurodegenerative disorder resulting from a CAG-polyglutamine repeat expansion in the huntingtin gene. We documented that the PPARδ agonist KD3010 is an effective therapy for HD in a mouse model. PPARδ forms a heterodimer with the retinoid X receptor (RXR), and RXR agonists are capable of promoting PPARδ activation. One compound with potent RXR agonist activity is the U.S. Food and Drug Administration-approved drug bexarotene. We tested the therapeutic potential of bexarotene in HD and found that bexarotene was neuroprotective in cellular models of HD, including medium spiny-like neurons generated from induced pluripotent stem cells (iPSCs) derived from patients with HD. To evaluate bexarotene as a treatment for HD, we treated the N171-82Q mouse model with the drug and found that bexarotene improved motor function, reduced neurodegeneration, and increased survival. To determine the basis for PPARδ neuroprotection, we evaluated metabolic function and noted markedly impaired oxidative metabolism in HD neurons, which was rescued by bexarotene or KD3010. We examined mitochondrial and protein quality control in cellular models of HD and observed that treatment with a PPARδ agonist promoted cellular quality control. By boosting cellular activities that are dysfunctional in HD, PPARδ activation may have therapeutic applications in HD and potentially other neurodegenerative diseases.

Interfacial interaction and lateral association of cross-seeding assemblies between hIAPP and rIAPP oligomers

Cross-sequence interactions between different amyloid peptides are important not only for fundamental understanding of amyloid aggregation and polymorphism mechanisms, but also for probing a potential molecular link between different amyloid diseases.

Experimental models for identifying modifiers of polyglutamine-induced aggregation and neurodegeneration

Huntington's disease (HD) typifies a class of inherited neurodegenerative disorders in which a CAG expansion in a single gene leads to an extended polyglutamine tract and misfolding of the expressed protein, driving cumulative neural dysfunction and degeneration. HD is invariably fatal with symptoms that include progressive neuropsychiatric and cognitive impairments, and eventual motor disability. No curative therapies yet exist for HD and related polyglutamine diseases; therefore, substantial efforts have been made in the drug discovery field to identify potential drug and drug target candidates for disease-modifying treatment. In this context, we review here a range of early-stage screening approaches based in in vitro, cellular, and invertebrate models to identify pharmacological and genetic modifiers of polyglutamine aggregation and induced neurodegeneration. In addition, emerging technologies, including high-content analysis, three-dimensional culture models, and induced pluripotent stem cells are increasingly being incorporated into drug discovery screening pipelines for protein misfolding disorders. Together, these diverse screening strategies are generating novel and exciting new probes for understanding the disease process and for furthering development of therapeutic candidates for eventual testing in the clinical setting.

Role of epigenetics in Alzheimer’s and Parkinson’s disease

Alzheimer’s disease (AD) and Parkinson’s disease (PD) are two common neurodegenerative diseases that result in the progressive damage or death of neurons. Environmental agents have the potential to damage the developing and mature nervous system, resulting in neurodegenerative diseases. Heritable changes in gene expression that do not involve coding sequence modifications are referred to as ‘epigenetic’. These modifications include DNA methylation and downstream modification of histones. Environmental factors, including heavy metals and dietary folate intake, perturb neurodegenerative genes by epigenetic means, leading to altered gene expression and late-onset neurodegenerative diseases. Research into the genetic control of DNA methylation indicates an allelic skewing in a significant proportion of genes. This phenomenon may determine how an individual’s genetic makeup can alter the effect an environmental factor has on their risk of developing neurodegeneration. Finally, preliminary evidence using cell culture and transgenic animal models suggests that whole classes of pan-epigenetic modifiers will have significant protective effects against common neurodegenerative diseases.

Inhibition of specific HDACs and sirtuins suppresses pathogenesis in a Drosophila model of Huntington's disease

Huntington's disease (HD) is associated with transcriptional dysregulation, and multiple studies with histone deacetylase (HDAC) inhibitors suggest that global approaches for restoring transcriptional balance and appropriate protein acetylation are therapeutically promising. To determine whether more targeted approaches might be effective, we have tested the impact of all the HDACs in Drosophila on Huntingtin (Htt)-induced pathology. Among the zinc-dependent or 'classic' HDACs, we find that neurodegeneration is most sensitive to levels of Rpd3. We also find that among the NAD(+)-dependent class III deacetylases, genetic or pharmacological reduction of either Sir2 or Sirt2 provides neuroprotection to Htt-challenged animals and that even greater neuroprotection is achieved when Rpd3 and Sir2 are simultaneously reduced. Our experiments suggest that longevity promoting strategies may be distinct from those that protect against neurodegeneration in Drosophila challenged with mutant human Htt. These results highlight a novel therapeutic approach for HD in the form of Sir2 inhibition and possible combinatorial inhibition of Sir2 and Rpd3.

Sodium dodecyl sulfate-insoluble oligomers are involved in polyglutamine degeneration

In polyglutamine (polyQ) degeneration, disease protein that carries an expanded polyQ tract is neurotoxic. Expanded polyQ protein exists in different conformations that display distinct solubility properties. In this study, an inducible transgenic Drosophila model is established to define the pathogenic form of polyQ protein at an early stage of degeneration in vivo. We show that microscopic polyQ aggregates are neither pathogenic nor protective. Further, no toxic effect of sodium dodecyl sulfate (SDS) -soluble polyQ protein is observed in our model. By means of filtration, 2 forms of SDS-insoluble protein species are identified according to their size. Coexpression of an ATPase-defective form of the molecular chaperone Hsc70 (Hsc70-K71S) selectively reduces the abundance of the large SDS-insoluble polyQ species, but such modulation has no modifying effects on degeneration. Notably, we detect a distinct Hsc70-K71S-resistant, small, SDS-insoluble polyQ oligomeric species that is closely correlated with degeneration. Our data highlight the toxic role of SDS-insoluble oligomers in polyQ degeneration in vivo.

Proteasomes cleave at multiple sites within polyglutamine tracts: activation by PA28gamma(K188E)

Eukaryotic proteasomes have been reported to cleave only once within polyglutamine tracts and then only after the N-terminal glutamine (Venkatraman, P., Wetzel, R., Tanaka, M., Nukina, N., and Goldberg, A. L. (2004) Mol. Cell 14, 95-104). We have obtained results that directly conflict with that report. In the presence of the proteasome activator PA28gamma(K188E) human red cell proteasomes progressively degraded fluorescein-GGQ(10)RR or fluorescein-HPHQ(10)RR into small fragments as shown by size exclusion chromatography and mass spectrometry. MALDI-TOF mass spectrometry revealed that proteolytic products arose from cleavage after every glutamine in fluorescein-HPHQ(10)RR, and mass accuracy rules out deamidation of glutamine to glutamic acid as an explanation for peptide degradation. Moreover, degradation cannot be attributed to a contaminating protease because peptide hydrolysis was completely blocked by the proteasome-specific inhibitors, lactacystin and epoxomicin. We conclude that proteasomes cleave repetitively anywhere within a stretch of ten glutamine residues. Thus our results cast doubt on the idea that mammalian proteasomes cannot degrade glutamine-expanded regions within pathogenic polyQ-expanded proteins, such as Huntingtin.

Akt blocks ligand binding and protects against expanded polyglutamine androgen receptor toxicity

Spinal and bulbar muscular atrophy (SBMA) is a progressive neurodegenerative disease caused by an expansion of the polyglutamine tract in the androgen receptor (AR). Here, we investigated the regulation of AR phosphorylation in order to understand factors that may modify SBMA disease progression. We show that expanded polyglutamine AR is phosphorylated by Akt. Substitution of the AR at two Akt consensus sites, S215 and S792, with aspartate, which mimics phosphorylation, reduces ligand binding, ligand-dependent nuclear translocation, transcriptional activation and toxicity of expanded polyglutamine AR. Co-expression of constitutively active Akt and the AR has similar consequences, which are blocked by alanine substitutions at residues 215 and 792. Furthermore, in motor neuron-derived MN-1 cells toxicity associated with polyglutamine-expanded AR is rescued by co-expression with Akt. Insulin-like growth factor-1 (IGF-1) stimulation, which activates several cell survival promoting pathways, also reduces toxicity of the expanded polyglutamine AR in MN-1 cells, in a manner dependent upon phospho-inositol-3-kinase. IGF-1 rescue of AR toxicity is diminished by alanine substitutions at the Akt consensus sites. These results highlight potential targets for therapeutic intervention in SBMA.

The interplay between PolyQ and protein context delays aggregation by forming a reservoir of protofibrils

Polyglutamine (polyQ) diseases are inherited neurodegenerative disorders caused by the expansion of CAG codon repeats, which code for polyQ in the corresponding gene products. These diseases are associated with the presence of amyloid-like protein aggregates, induced by polyQ expansion. It has been suggested that the soluble aggregates rather than the mature fibrillar aggregates are the toxic species, and that the aggregation properties of polyQ can be strongly modulated by the surrounding protein context. To assess the importance of the protein carrier in polyQ aggregation, we have studied the misfolding pathway and the kinetics of aggregation of polyQ of lengths above (Q41) and below (Q22) the pathological threshold fused to the well-characterized protein carrier glutathione S-transferase (GST). This protein, chosen as a model system, is per se able to misfold and aggregate irreversibly, thus mimicking the behaviour of domains of naturally occurring polyQ proteins. We prove that, while it is generally accepted that the aggregation kinetics of polyQ depend on its length and are faster for longer polyQ tracts, the presence of GST alters the polyQ aggregation pathway and reverses this trend. Aggregation occurs through formation of a reservoir of soluble intermediates whose populations and kinetic stabilities increase with polyQ length. Our results provide a new model that explains the toxicity of expanded polyQ proteins, in which the interplay between polyQ regions and other aggregation-prone domains plays a key role in determining the aggregation pathway.