Tiagabine is neuroprotective in the N171-82Q and R6/2 mouse models of Huntington's disease

Endocytosis of GABA receptor: Signaling in nervous system

GABA (ᵞ-aminobutyric acid), is the principal neurotransmitter known for its inhibitory role in chemical synapses. Being localized primarily in the central nervous system (CNS) it maintains a balance between excitatory (regulated by another neurotransmitter, glutamate) and inhibitory impulses. GABA acts by binding to their specific receptors GABA_A and GABA_B when released into the post-synaptic nerve terminal. Both of these receptors are responsible for fast and slow inhibition of neurotransmission, respectively. GABA_A is a ligand-gated ionopore receptor which opens the Cl^- ion channel and decreases the resting potential of the membrane resulting into inhibition of the synapse. On the other hand, GABA_B is a metabotropic receptor which increases the K⁺ ion levels preventing Ca⁺ ion release inhibiting the release of other neurotransmitters into the presynaptic membrane. The internalization and trafficking of these receptors is also conducted through distinct pathways and mechanism, discussed in detail in the chapter. Without the desired levels of GABA in the body, the psychological and neurological states of brain get hard to maintain. Various neurodegenerative diseases/disorders have been associated to low levels of GABA, such as anxiety, mood disorders, fear, schizophrenia, hungtington's chorea, seizures, epilepsy, etc. The allosteric sites present on GABA receptors have been proved to be potent drug targets to pacify the pathological states of these brain related disorders to an extent. Further in depth studies focussing on the subtypes of GABA receptors and their comprehensive mechanism are required to explore new drug targets and therapeutic avenues for effectual management of GABA related neurological diseases.

Temporal Phenotypic Changes in Huntington's Disease Models for Preclinical Studies

Background:

Mouse models bearing genetic disease mutations are instrumental in the development of therapies for genetic disorders. Huntington’s disease (HD) is a late-onset lethal dominant genetic disorder due to a CAG repeat within exon 1 of the Huntingtin (Htt) gene. Several mice were developed to model HD through the expression of a transgenic fragment (exon 1 of the human HTT), the knock-in mutation of the CAG repeat in the context of the mouse Htt gene, or the full-length HTT human gene. The different mouse models present distinct onset, symptoms, and progression of the disease.

Objective:

The objective of this study is to advise on the best behavioral tests to assess disease progression in three HD mouse models.

Methods:

We tested N171-82Q transgenic mice, zQ175 knock-in mice, and BACHD full-length mice in a comprehensive behavior test battery in early, mid-, and late disease stages.

Results:

We contrast and compare the models and the emerging phenotypes with the available literature. These results suggest the most effective behavioral tests and appropriate sample sizes to detect treatment efficacy in each model at the different ages. We provide options for early detection of motor deficits while minimizing testing time and training.

Conclusion:

This information will inform researchers in the HD field as to which mouse model, tests and sample sizes can accurately and sensitively detect treatment efficacy in preclinical HD research.

Antiepileptic Drug Tiagabine Does Not Directly Target Key Cardiac Ion Channels Kv11.1, Nav1.5 and Cav1.2

Tiagabine is an antiepileptic drug used for the treatment of partial seizures in humans. Recently, this drug has been found useful in several non-epileptic conditions, including anxiety, chronic pain and sleep disorders. Since tachycardia-an impairment of cardiac rhythm due to cardiac ion channel dysfunction-is one of the most commonly reported non-neurological adverse effects of this drug, in the present paper we have undertaken pharmacological and numerical studies to assess a potential cardiovascular risk associated with the use of tiagabine. A chemical interaction of tiagabine with a model of human voltage-gated ion channels (VGICs) is described using the molecular docking method. The obtained in silico results imply that the adverse effects reported so far in the clinical cardiological of tiagabine could not be directly attributed to its interactions with VGICs. This is also confirmed by the results from the isolated organ studies (i.e., calcium entry blocking properties test) and in vivo (electrocardiogram study) assays of the present research. It was found that tachycardia and other tiagabine-induced cardiac complications are not due to a direct effect of this drug on ventricular depolarization and repolarization.

Alleviation of Huntington pathology in mice by oral administration of food additive glyceryl tribenzoate

Huntington's disease (HD) is a neurodegenerative disorder characterized by accumulation of mutant huntingtin protein and significant loss of neurons in striatum and cortex. Along with motor difficulties, the HD patients also manifest anxiety and loss of cognition. Unfortunately, the clinically approved drugs only offer symptomatic relief and are not free from side effects. This study underlines the importance of glyceryl tribenzoate (GTB), an FDA-approved food flavoring ingredient, in alleviating HD pathology in transgenic N171-82Q mouse model. Oral administration of GTB significantly reduced mutant huntingtin level in striatum, motor cortex as well as hippocampus and increased the integrity of viable neurons. Furthermore, we found the presence of sodium benzoate (NaB), a FDA-approved drug for urea cycle disorders and glycine encephalopathy, in the brain of GTB-fed HD mice. Accordingly, NaB administration also markedly decreased huntingtin level in striatum and cortex. Glial activation is found to coincide with neuronal death in affected regions of HD brains. Interestingly, both GTB and NaB treatment suppressed activation of glial cells and inflammation in the brain. Finally, neuroprotective effect of GTB and NaB resulted in improved motor performance of HD mice. Collectively, these results suggest that GTB and NaB may be repurposed for HD.

Overview of Huntington's Disease Models: Neuropathological, Molecular, and Behavioral Differences

Transgenic mouse models of Huntington's disease (HD), a neurodegenerative condition caused by a single gene mutation, have been transformative in their ability to reveal the molecular processes and pathophysiological mechanisms underlying the HD behavioral phenotype. Three model categories have been generated depending on the genetic context in which the mutation is expressed: truncated, full-length, and knock-in. No single model, however, broadly replicates the behavioral symptoms and massive neuronal loss that occur in human patients. The disparity between model and patient requires careful consideration of what each model has to offer when testing potential treatments. Although the translation of animal data to the clinic has been limited, each model can make unique contributions toward an improved understanding of the neurobehavioral underpinnings of HD. Thus, conclusions based on data obtained from more than one model are likely to have the most success in the search for new treatment targets. © 2018 by John Wiley & Sons, Inc.

Insights into GABAAergic system alteration in Huntington's disease

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disease that is characterized by a triad of motor, psychiatric and cognitive impairments. There is still no effective therapy to delay or halt the disease progress. The striatum and cortex are two particularly affected brain regions that exhibit dense reciprocal excitatory glutamate and inhibitory gamma-amino butyric acid (GABA) connections. Imbalance between excitatory and inhibitory signalling is known to greatly affect motor and cognitive processes. Emerging evidence supports the hypothesis that disrupted GABAergic circuits underlie HD pathogenesis. In the present review, we focused on the multiple defects recently found in the GABAergic inhibitory system, including altered GABA level and synthesis, abnormal subunit composition and distribution of GABA_A receptors and aberrant GABA_A receptor-mediated signalling. In particular, the important role of cation–chloride cotransporters (i.e. NKCC1 and KCC2) is discussed. Recent studies also suggest that neuroinflammation contributes significantly to the abnormal GABAergic inhibition in HD. Thus, GABA_A receptors and cation–chloride cotransporters are potential therapeutic targets for HD. Given the limited availability of therapeutic treatments for HD, a better understanding of GABAergic dysfunction in HD could provide novel therapeutic opportunities.

Cracking novel shared targets between epilepsy and Alzheimer's disease: need of the hour

Epilepsy and Alzheimer's disease (AD) are interconnected. It is well known that seizures are linked with cognitive impairment, and there are various shared etiologies between epilepsy and AD. The connection between hyperexcitability of neurons and cognitive dysfunction in the progression of AD or epileptogenesis plays a vital role for improving selection of treatment for both diseases. Traditionally, seizures occur less frequently and in later stages of age in patients with AD which in turn implies that neurodegeneration causes seizures. The role of seizures in early stages of pathogenesis of AD is still an issue to be resolved. So, it is well timed to analyze the common pathways involved in pathophysiology of AD and epilepsy. The present review focuses on similar potential underlying mechanisms which may be related to the causes of seizures in epilepsy and cognitive impairment in AD. The proposed review will focus on many possible newer targets like abnormal expression of various enzymes like GSK-3β, PP2A, PKC, tau hyperphosphorylation, MMPs, caspases, neuroinflammation and oxidative stress associated with number of neurodegenerative diseases linked with epilepsy. The brief about the prospective line of treatment of both diseases will also be discussed in the present review.

Structure, Function, and Modulation of γ-Aminobutyric Acid Transporter 1 (GAT1) in Neurological Disorders: A Pharmacoinformatic Prospective

γ-Aminobutyric acid (GABA) Transporters (GATs) belong to sodium and chloride dependent-transporter family and are widely expressed throughout the brain. Notably, GAT1 is accountable for sustaining 75% of the synaptic GABA concentration and entails its transport to the GABAA receptors to initiate the receptor-mediated inhibition of post-synaptic neurons. Imbalance in ion homeostasis has been associated with several neurological disorders related to the GABAergic system. However, inhibition of the GABA uptake by these transporters has been accepted as an effective approach to enhance GABAergic inhibitory neurotransmission in the treatment of seizures in epileptic and other neurological disorders. Here, we reviewed computational methodologies including molecular modeling, docking, and molecular dynamic simulations studies to underscore the structure and function of GAT1 in the GABAergic system. Additionally, various SAR and QSAR methodologies have been reviewed to probe the 3D structural features of inhibitors required to modulate GATs activity. Overall, present review provides an overview of crucial role of GAT1 in GABAergic system and its modulation to evade neurological disorders.

Activation of central GABAB receptors offsets the cyclosporine counteraction of endotoxic cardiovascular outcomes in conscious rats

We have previously shown that cyclosporine (CSA) counteracts cardiovascular manifestations induced by endotoxemia (lipopolysaccharide, LPS) such as hypotension and cardiac autonomic dysfunction in conscious rats. In this study, we investigated whether the facilitation of central γ-amino butyric acid (GABA) neurotransmission blunts these favorable influences of CSA. The LPS-CSA interaction was determined in the absence and presence of drugs that activate GABA_A or GABA_B receptors or elevate synaptic GABA levels in the central nervous system. The consequent i.v. administration of CSA (10 mg/kg) blunted the LPS-evoked hypotension, tachycardia, and reductions in time- and frequency-domain indices of heart rate variability (measures of cardiac autonomic control) evoked by LPS (10 mg/kg i.v.). The ability of CSA to reverse the LPS effects disappeared in rats treated intracisternally (i.c.) with baclofen (selective GABA_B agonist, 2 μg/rat) but not muscimol (selective GABA_A agonist, 1 μg/rat), indicating a preferential compromising action for central GABA_B receptors on the advantageous effects of CSA. Moreover, the improvement by CSA of LPS-evoked cardiovascular derangements was also eliminated after concurrent i.c. administration of vigabatrin (GABA transaminase inhibitor, 200 μg/rat) or tiagabine (GABA reuptake inhibitor, 100 μg/rat). These results demonstrate that the activation of central GABA_B receptors either directly via baclofen or indirectly following interventions that boost GABA levels in central synapses counterbalances the rectifying action of CSA on endotoxemia.

Disease Modifying Effects of the Spider Toxin Parawixin2 in the Experimental Epilepsy Model

(1) Background: Temporal lobe epilepsy (TLE) is the most common type of epilepsy in adults. It is also the one with the highest percentage of drug-resistance to the current available anti-epileptic drugs (AED). Additionaly, most antiepileptic drugs are only able to control seizures in epileptogenesis, but do not decrease the hippocampal neurodegenerative process. TLE patients have a reduced population of interneuronal cells, which express Parvalbumin (PV) proteins. This reduction is directly linked to seizure frequency and severity in the chronic period of epilepsy. There is therefore a need to seek new therapies with a disease-modifying profile, and with efficient antiepileptic and neuroprotective properties. Parawixin2, a compound isolated from the venom of the spider Parawixia bistriata, has been shown to inhibit GABA transporters (GAT) and to have acute anticonvulsant effects in rats. (2) Methods: In this work, we studied the effects of Parawixin2 and Tiagabine (an FDA- approved GAT inhibitor), and compared these effects in a TLE model. Rats were subjected to lithium-pilocarpine TLE model and the main features were evaluated over a chronic period including: (a) spontaneous recurrent seizures (SRS), (b) neuronal loss, and (c) PV cell density in different regions of the hippocampus (CA1, CA3, DG and Hilus). (3) Results: Parawixin2 treatment reduced SRS frequency whereas Tiagabine did not. We also found a significant reduction in neuronal loss in CA3 and in the hilus regions of the hippocampus, in animals treated with Parawixin2. Noteworthy, Parawixin2 significantly reversed PV cell loss observed particularly in DG layers. (4) Conclusions: Parawixin2 exerts a promising neuroprotective and anti-epileptic effect and has potential as a novel agent in drug design.

2,4 DNP improves motor function, preserves medium spiny neuronal identity, and reduces oxidative stress in a mouse model of Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the first exon of the gene huntingtin. There is no treatment to prevent or delay the disease course of HD currently. Oxidative stress and mitochondrial dysfunction have emerged as key determinants of the disease progression in HD. Therefore, counteracting mutant huntingtin (mHtt)-induced oxidative stress and mitochondrial dysfunction appears as a new approach to treat this devastating disease. Interestingly, mild mitochondrial uncoupling improves neuronal resistance to stress and facilitates neuronal survival. Mild mitochondrial uncoupling can be induced by the proper dose of 2,4-dinitrophenol (DNP), a proton ionophore that was previously used for weight loss. In this study, we evaluated the effects of chronic administration of DNP at three doses (0.5, 1, 5mg/kg/day) on mHtt-induced behavioral deficits and cellular abnormalities in the N171-82Q HD mouse model. DNP at a low dose (1mg/kg/day) significantly improved motor function and preserved medium spiny neuronal marker DARPP32 and postsynaptic protein PSD95 in the striatum of HD mice. Further mechanistic study suggests that DNP at this dose reduced oxidative stress in HD mice, which was indicated by reduced levels of F2-isoprostanes in the brain of HD mice treated with DNP. Our data indicated that DNP provided behavioral benefit and neuroprotective effect at a weight neutral dose in HD mice, suggesting that the potential value of repositioning DNP to HD treatment is warranted in well-controlled clinical trials in HD.

Stereochemical Control of Enzymatic Carbon-Carbon Bond-Forming Michael-Type Additions by "Substrate Engineering"

The enzyme 4-oxalocrotonate tautomerase (4-OT) promiscuously catalyzes the Michael-type addition of acetaldehyde to β-nitrostyrene derivatives to yield chiral γ-nitroaldehydes, which are important precursors for pharmaceutically active γ-aminobutyric acids. In this study, we investigated the effect of different substituents at the aromatic ring of the Michael acceptor on the catalytic efficiency and stereoselectivity of the 4-OT-catalyzed acetaldehyde addition reactions. Highly enantioenriched (R)- and (S)-γ-nitroaldehydes and 4-substituted chroman-2-ol could be obtained in good to excellent yields by applying different substituents at appropriate positions of the aromatic substrate. Stereochemical control of these enzymatic Michael-type additions by "substrate engineering" allowed the enantioselective synthesis of valuable γ-aminobutyric acid precursors. In addition, the results suggest a novel enzymatic synthesis route towards precursors for chromans and derivatives, which are valuable scaffolds for preparing biologically active natural products.

Recent trends in the transdermal delivery of therapeutic agents used for the management of neurodegenerative diseases

With the increasing proportion of the global geriatric population, it becomes obvious that neurodegenerative diseases will become more widespread. From an epidemiological standpoint, it is necessary to develop new therapeutic agents for the management of Alzheimer's disease, Parkinson's disease, multiple sclerosis and other neurodegenerative disorders. An important approach in this regard involves the use of the transdermal route. With transdermal drug delivery systems (TDDS), it is possible to modulate the pharmacokinetic profiles of these medications and improve patient compliance. Transdermal drug delivery has also been shown to be useful for drugs with short half-life and low or unpredictable bioavailability. In this review, several transdermal drug delivery enhancement technologies are being discussed in relation to the delivery of medications used for the management of neurodegenerative disorders.

Expression of brain-derived neurotrophic factor in astrocytes - Beneficial effects of glatiramer acetate in the R6/2 and YAC128 mouse models of Huntington's disease

Glatiramer acetate (GA) is a FDA-approved drug which is licensed for the treatment of relapsing-remitting multiple sclerosis and which may exert neuroprotective effects via brain-derived neurotrophic factor (BDNF). In this study, we investigate effects of GA on BDNF expression especially in astrocytes in vitro and in vivo in brains of R6/2 and YAC128 transgenic mouse models of Huntington's disease (HD) where a pathogenic role of astroglial cells has recently been shown. We show that GA increases the expression of functionally active BDNF in astrocyte culture and in astrocytes of GA treated HD mice. In the brains of these mice, GA decreases neurodegeneration and restores BDNF levels. The beneficial effect of GA in R6/2 mice also comprises reduced weight loss and prolonged life span and, for both models, also improved motor performance. Further studies with this safe and effective drug in HD are warranted.

Central GABAA receptors are involved in inflammatory and cardiovascular consequences of endotoxemia in conscious rats

γ-Aminobutyric acid (GABA), the principal brain inhibitory neurotransmitter, modulates inflammatory and neurodegenerative disease. Here, we tested the hypothesis that central GABAergic neurotransmission mediates the detrimental inflammatory, hemodynamic, and cardiac autonomic actions of endotoxemia. The effects of drugs that block GABA receptors or interfere with GABA uptake or degradation on blood pressure (BP), heart rate (HR), and HR variability (HRV) responses elicited by i.v. lipopolysaccharide (LPS) were assessed in conscious rats. The hypotensive effect of LPS (10 mg/kg) was blunted after intracisternal (i.c.) administration of bicuculline (GABAA receptor antagonist) or saclofen (GABAB receptor antagonist). By contrast, the concomitant LPS-evoked tachycardia and decreases in time domain and frequency domain indices of HRV (measures of cardiac autonomic control) were abolished upon treatment with bicuculline but not saclofen. Increases in serum tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) caused by LPS disappeared in the presence of bicuculline or saclofen, whereas LPS-evoked increases in serum nitric oxide metabolites (NOx) were counteracted by bicuculline only. None of the endotoxemia effects was altered in rats treated with i.c. tiagabine (GABA reuptake inhibitor) or vigabatrin (GABA transaminase inhibitor). These data suggest a major role for central GABAA receptors in the inflammatory and cardiovascular effects of endotoxemia.

Tiagabine Protects Dopaminergic Neurons against Neurotoxins by Inhibiting Microglial Activation

Microglial activation and inflammation are associated with progressive neuronal apoptosis in neurodegenerative disorders such as Parkinson's disease (PD). γ-Aminobutyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system, has recently been shown to play an inhibitory role in the immune system. Tiagabine, a piperidine derivative, enhances GABAergic transmission by inhibiting GABA transporter 1 (GAT 1). In the present study, we found that tiagabine pretreatment attenuated microglial activation, provided partial protection to the nigrostriatal axis and improved motor deficits in a methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD. The protective function of tiagabine was abolished in GAT 1 knockout mice that were challenged with MPTP. In an alternative PD model, induced by intranigral infusion of lipopolysaccharide (LPS), microglial suppression and subsequent neuroprotective effects of tiagabine were demonstrated. Furthermore, the LPS-induced inflammatory activation of BV-2 microglial cells and the toxicity of conditioned medium toward SH-SY5Y cells were inhibited by pretreatment with GABAergic drugs. The attenuation of the nuclear translocation of nuclear factor κB (NF-κB) and the inhibition of the generation of inflammatory mediators were the underlying mechanisms. Our results suggest that tiagabine acts as a brake for nigrostriatal microglial activation and that it might be a novel therapeutic approach for PD.

Ectopic expression of the striatal-enriched GTPase Rhes elicits cerebellar degeneration and an ataxia phenotype in Huntington's disease

Huntington's disease (HD) is caused by an expansion of glutamine repeats in the huntingtin protein (mHtt) that invokes early and prominent damage of the striatum, a region that controls motor behaviors. Despite its ubiquitous expression, why certain brain regions, such as the cerebellum, are relatively spared from neuronal loss by mHtt remains unclear. Previously, we implicated the striatal-enriched GTPase, Rhes (Ras homolog enriched in the striatum), which binds and SUMOylates mHtt and increases its solubility and cellular cytotoxicity, as the cause for striatal toxicity in HD. Here, we report that Rhes deletion in HD mice (N171-82Q), which express the N-terminal fragment of human Htt with 82 glutamines (Rhes(-/-)/N171-82Q), display markedly reduced HD-related behavioral deficits, and absence of lateral ventricle dilatation (secondary to striatal atrophy), compared to control HD mice (N171-82Q). To further validate the role of GTPase Rhes in HD, we tested whether ectopic Rhes expression would elicit a pathology in a brain region normally less affected in HD. Remarkably, ectopic expression of Rhes in the cerebellum of N171-82Q mice, during the asymptomatic period led to an exacerbation of motor deficits, including loss of balance and motor incoordination with ataxia-like features, not apparent in control-injected N171-82Q mice or Rhes injected wild-type mice. Pathological and biochemical analysis of Rhes-injected N171-82Q mice revealed a cerebellar lesion with marked loss of Purkinje neuron layer parvalbumin-immunoreactivity, induction of caspase 3 activation, and enhanced soluble forms of mHtt. Similarly reintroducing Rhes into the striatum of Rhes deleted Rhes(-/-)Hdh(150Q/150Q) knock-in mice, elicited a progressive HD-associated rotarod deficit. Overall, these studies establish that Rhes plays a pivotal role in vivo for the selective toxicity of mHtt in HD.

Amitriptyline improves motor function via enhanced neurotrophin signaling and mitochondrial functions in the murine N171-82Q Huntington disease model

Huntington disease (HD) is a neurodegenerative disorder characterized by progressive motor impairment and cognitive alterations. Hereditary HD is primarily caused by the expansion of a CAG trinucleotide repeat in the huntingtin (Htt) gene, which results in the production of mutant huntingtin protein (mHTT) with an expanded amino-terminal polyglutamine (poly(Q)) stretch. Besides pathological mHTT aggregation, reduced brain-derived neurotrophic factor (BDNF) levels, impaired neurotrophin signaling, and compromised mitochondrial functions also contribute to the deleterious progressive etiology of HD. As a well tolerated Food and Drug Administration-approved antidepressant, amitriptyline (AMI) has shown efficacy in treating neurodegenerative murine models via potentiation of BDNF levels and amelioration of alterations in neurotrophin signaling pathways. In this study, we observed profound improvements in the motor coordination of AMI-treated N171-82Q HD model mice. The beneficial effects of AMI treatment were associated with its ability to reduce mHTT aggregation, potentiation of the BDNF-TrkB signaling system, and support of mitochondrial integrity and functionality. Our study not only provides preclinical evidence for the therapeutic potency of AMI in treating HD, but it also represents an important example of the usefulness of additional pharmacogenomic profiling of pre-existing drugs for novel therapeutic effects with often intractable pathological scenarios.

PAKs inhibitors ameliorate schizophrenia-associated dendritic spine deterioration in vitro and in vivo during late adolescence

Drug discovery in psychiatry has been limited to chemical modifications of compounds originally discovered serendipitously. Therefore, more mechanism-oriented strategies of drug discovery for mental disorders are awaited. Schizophrenia is a devastating mental disorder with synaptic disconnectivity involved in its pathophysiology. Reduction in the dendritic spine density is a major alteration that has been reproducibly reported in the cerebral cortex of patients with schizophrenia. Disrupted-in-Schizophrenia-1 (DISC1), a factor that influences endophenotypes underlying schizophrenia and several other neuropsychiatric disorders, has a regulatory role in the postsynaptic density in association with the NMDA-type glutamate receptor, Kalirin-7, and Rac1. Prolonged knockdown of DISC1 leads to synaptic deterioration, reminiscent of the synaptic pathology of schizophrenia. Thus, we tested the effects of novel inhibitors to p21-activated kinases (PAKs), major targets of Rac1, on synaptic deterioration elicited by knockdown expression of DISC1. These compounds not only significantly ameliorated the synaptic deterioration triggered by DISC1 knockdown but also partially reversed the size of deteriorated synapses in culture. One of these PAK inhibitors prevented progressive synaptic deterioration in adolescence as shown by in vivo two-photon imaging and ameliorated a behavioral deficit in prepulse inhibition in adulthood in a DISC1 knockdown mouse model. The efficacy of PAK inhibitors may have implications in drug discovery for schizophrenia and related neuropsychiatric disorders in general.

Mouse models of polyglutamine diseases in therapeutic approaches: review and data table. Part II

Mouse models of human diseases are created both to understand the pathogenesis of the disorders and to find successful therapies for them. This work is the second part in a series of reviews of mouse models of polyglutamine (polyQ) hereditary disorders and focuses on in vivo experimental therapeutic approaches. Like part I of the polyQ mouse model review, this work is supplemented with a table that contains data from experimental studies of therapeutic approaches in polyQ mouse models. The aim of this review was to characterize the benefits and outcomes of various therapeutic strategies in mouse models. We examine whether the therapeutic strategies are specific to a single disease or are applicable to more than one polyQ disorder in mouse models. In addition, we discuss the suitability of mouse models in therapeutic approaches. Although the majority of therapeutic studies were performed in mouse models of Huntington disease, similar strategies were also used in other disease models.

Temporal separation of aggregation and ubiquitination during early inclusion formation in transgenic mice carrying the Huntington's disease mutation

Abnormal insoluble ubiqitinated protein aggregates are found in the brains of Huntington’s disease (HD) patients and in mice transgenic for the HTT mutation. Here, we describe the earliest stages of visible NII formation in brains of R6/2 mice killed between 2 and 6 weeks of age. We found that huntingtin-positive aggregates formed rapidly (within 24–48 hours) in a spatiotemporal manner similar to that we described previously for ubiquitinated inclusions. However, in most neurons, aggregates are not ubiquitinated when they first form. It has always been assumed that mutant huntingtin is recognised as ‘foreign’ and consequently ubiquitinated and targeted for degradation by the ubiquitin-proteasome system pathway. Our data, however, suggest that aggregation and ubiquitination are separate processes, and that mutant huntingtin fragment is not recognized as ‘abnormal’ by the ubiquitin-proteasome system before aggregation. Rather, mutant Htt appears to aggregate before it is ubiquitinated, and then either aggregated huntingtin is ubiquitinated or ubiquitinated proteins are recruited into aggregates. Our findings have significant implications for the role of the ubiquitin-proteasome system in the formation of aggregates, as they suggest that this system is not involved until after the first aggregates form.

Network organization of the huntingtin proteomic interactome in mammalian brain

We used affinity-purification mass spectrometry to identify 747 candidate proteins that are complexed with Huntingtin (Htt) in distinct brain regions and ages in Huntington's disease (HD) and wild-type mouse brains. To gain a systems-level view of the Htt interactome, we applied Weighted Correlation Network Analysis to the entire proteomic data set to unveil a verifiable rank of Htt-correlated proteins and a network of Htt-interacting protein modules, with each module highlighting distinct aspects of Htt biology. Importantly, the Htt-containing module is highly enriched with proteins involved in 14-3-3 signaling, microtubule-based transport, and proteostasis. Top-ranked proteins in this module were validated as Htt interactors and genetic modifiers in an HD Drosophila model. Our study provides a compendium of spatiotemporal Htt-interacting proteins in the mammalian brain and presents an approach for analyzing proteomic interactome data sets to build in vivo protein networks in complex tissues, such as the brain.

Protein misfolding detected early in pathogenesis of transgenic mouse model of Huntington disease using amyloid seeding assay

Huntington disease (HD) is one of several fatal neurodegenerative disorders associated with misfolded proteins. Here, we report a novel method for the sensitive detection of misfolded huntingtin (HTT) isolated from the brains of transgenic (Tg) mouse models of HD and humans with HD using an amyloid seeding assay (ASA), which is based on the propensity of misfolded proteins to act as a seed and shorten the nucleation-associated lag phase in the kinetics of amyloid formation in vitro. Using synthetic polyglutamine peptides as the substrate for amyloid formation, we found that partially purified misfolded HTT obtained from end-stage brain tissue of two Tg HD mouse models and brain tissue of post-mortem human HD patients was capable of specifically accelerating polyglutamine amyloid formation compared with unseeded reactions and controls. Alzheimer and prion disease brain tissues did not do so, demonstrating the specificity of the ASA. It is unclear whether early intermediates or later conformational species in the protein misfolding process act as seeds in the ASA for HD. However, we were able to detect misfolded protein in the brains of YAC128 mice early in disease pathogenesis (11 weeks of age), whereas large inclusion bodies have not been observed in the brains of these mice by histology until 78 weeks of age, much later in the pathogenic process. The sensitive detection of misfolded HTT protein early in the disease pathogenesis in the YAC128 Tg mouse model strengthens the argument for a causative role of protein misfolding in HD.

The melatonin MT1 receptor axis modulates mutant Huntingtin-mediated toxicity

Melatonin mediates neuroprotection in several experimental models of neurodegeneration. It is not yet known, however, whether melatonin provides neuroprotection in genetic models of Huntington's disease (HD). We report that melatonin delays disease onset and mortality in a transgenic mouse model of HD. Moreover, mutant huntingtin (htt)-mediated toxicity in cells, mice, and humans is associated with loss of the type 1 melatonin receptor (MT1). We observe high levels of MT1 receptor in mitochondria from the brains of wild-type mice but much less in brains from HD mice. Moreover, we demonstrate that melatonin inhibits mutant htt-induced caspase activation and preserves MT1 receptor expression. This observation is critical, because melatonin-mediated protection is dependent on the presence and activation of the MT1 receptor. In summary, we delineate a pathologic process whereby mutant htt-induced loss of the mitochondrial MT1 receptor enhances neuronal vulnerability and potentially accelerates the neurodegenerative process.

Worsening of Huntington disease phenotype in CB1 receptor knockout mice

Huntington's disease (HD) is a progressive neurodegenerative genetic disorder which leads to motor, cognitive and psychiatric disturbances. The primary neuropathological hallmark is atrophy of the striatum. Cannabinoid CB1 receptors (CB1Rs) are particularly enriched in the striatum and previous works indicate their early loss of expression in HD, even before symptom occurrence. However, pathophysiological significance of this loss of expression remains unclear. In addition, whether specific modulation of CB1R is able to mitigate striatal neuron fate in HD remains currently controversial. In order to gain further insights on the potential role of CB1R in HD physiopathology, we evaluated the pathophysiological consequences of a genetic deletion of CB1R in the N171-82Q transgenic model and following 3-nitropropionic (3NP) intoxication. Taken together our data demonstrate that CB1R knockout (1) worsens motor performances in N171-82Q mice and (2) increases mouse susceptibility to 3NP. These results suggest that functional changes in CB1R may contribute to the physiopathological development of HD.

In silico repositioning of approved drugs for rare and neglected diseases

One approach to speed up drug discovery is to examine new uses for existing approved drugs, so-called 'drug repositioning' or 'drug repurposing', which has become increasingly popular in recent years. Analysis of the literature reveals many examples of US Food and Drug Administration-approved drugs that are active against multiple targets (also termed promiscuity) that can also be used to therapeutic advantage for repositioning for other neglected and rare diseases. Using proof-of-principle examples, we suggest here that with current in silico technologies and databases of the structures and biological activities of chemical compounds (drugs) and related data, as well as close integration with in vitro screening data, improved opportunities for drug repurposing will emerge for neglected or rare/orphan diseases.

Efficacy of fumaric acid esters in the R6/2 and YAC128 models of Huntington's disease

Huntington's disease (HD) is an autosomal dominantly inherited progressive neurodegenerative disease. The exact sequel of events finally resulting in neurodegeneration is only partially understood and there is no established protective treatment so far. Some lines of evidence speak for the contribution of oxidative stress to neuronal tissue damage. The fumaric acid ester dimethylfumarate (DMF) is a new disease modifying therapy currently in phase III studies for relapsing-remitting multiple sclerosis. DMF potentially exerts neuroprotective effects via induction of the transcription factor “nuclear factor E2-related factor 2” (Nrf2) and detoxification pathways. Thus, we investigated here the therapeutic efficacy of DMF in R6/2 and YAC128 HD transgenic mice which mimic many aspects of HD and are characterized by an enhanced generation of free radicals in neurons. Treatment with DMF significantly prevented weight loss in R6/2 mice between postnatal days 80–90. At the same time, DMF treatment led to an attenuated motor impairment as measured by the clasping score. Average survival in the DMF group was 100.5 days vs. 94.0 days in the placebo group. In the histological analysis on day 80, DMF treatment resulted in a significant preservation of morphologically intact neurons in the striatum as well as in the motor cortex. DMF treatment resulted in an increased Nrf2 immunoreactivity in neuronal subpopulations, but not in astrocytes. These beneficial effects were corroborated in YAC128 mice which, after one year of DMF treatment, also displayed reduced dyskinesia as well as a preservation of neurons. In conclusion, DMF may exert beneficial effects in mouse models of HD. Given its excellent side effect profile, further studies with DMF as new therapeutic approach in HD and other neurodegenerative diseases are warranted.

Huntington's disease: from molecular pathogenesis to clinical treatment

Huntington's disease is a progressive, fatal, neurodegenerative disorder caused by an expanded CAG repeat in the huntingtin gene, which encodes an abnormally long polyglutamine repeat in the huntingtin protein. Huntington's disease has served as a model for the study of other more common neurodegenerative disorders, such as Alzheimer's disease and Parkinson's disease. These disorders all share features including: delayed onset; selective neuronal vulnerability, despite widespread expression of disease-related proteins during the whole lifetime; abnormal protein processing and aggregation; and cellular toxic effects involving both cell autonomous and cell-cell interaction mechanisms. Pathogenic pathways of Huntington's disease are beginning to be unravelled, offering targets for treatments. Additionally, predictive genetic testing and findings of neuroimaging studies show that, as in some other neurodegenerative disorders, neurodegeneration in affected individuals begins many years before onset of diagnosable signs and symptoms of Huntington's disease, and it is accompanied by subtle cognitive, motor, and psychiatric changes (so-called prodromal disease). Thus, Huntington's disease is also emerging as a model for strategies to develop therapeutic interventions, not only to slow progression of manifest disease but also to delay, or ideally prevent, its onset.

A2A receptor knockout worsens survival and motor behaviour in a transgenic mouse model of Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative genetic disorder that leads to motor, cognitive, and psychiatric disturbances. The primary neuropathological hallmark is atrophy of the striatum. HD preferentially affects efferent striato-pallidal neurons that express enkephalin as well as dopamine D2 and A(2A) adenosine receptors (A(2A)Rs). Expression and function of A(2A)Rs are altered in HD but, despite being an important modulator of the striato-pallidal function, the subsequent pathophysiological consequence of such changes remains unclear. Whether blockade of A(2A)Rs is of therapeutic interest in HD remains ill-defined. In the present work, we aimed to determine the pathophysiological consequences of genetic deletion of A(2A)Rs in HD by crossing A(2A)R knockout mice with the N171-82Q HD transgenic model. Our data demonstrate that knockout of A(2A)Rs moderately but significantly worsens motor performances and survival of N171-82Q mice and leads to a decrease in striatal enkephalin expression. These results support that early and chronic blockade of A(2A)Rs might not be beneficial in HD.

Rescuing the Corticostriatal Synaptic Disconnection in the R6/2 Mouse Model of Huntington's Disease: Exercise, Adenosine Receptors and Ampakines

In the R6/2 mouse model of Huntington's disease (HD) we examined the effects of a number of behavioral and pharmacological manipulations aimed at rescuing the progressive loss of synaptic communication between cerebral cortex and striatum. Two cohorts of transgenic mice with ~110 and 210 CAG repeats were utilized. Exercise prevented the reduction in striatal medium-sized spiny neuron membrane capacitance but did not reestablish synaptic communication. Activation of adenosine A2A type receptors renormalized postsynaptic activity to some extent. Finally, the ampakine Cx614, which has been shown to prevent α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor desensitization, slow deactivation, and facilitate glutamate release, induced significant increases in synaptic activity, albeit the effect was somewhat reduced in fully symptomatic, compared to control mice. With some limitations, each of these strategies can be used to delay and partially rescue phenotypic progression of HD in this model.

Synergy of combined doxycycline/TUDCA treatment in lowering Transthyretin deposition and associated biomarkers: studies in FAP mouse models

Familial Amyloidotic Polyneuropathy (FAP) is a disorder characterized by the extracellular deposition of fibrillar Transthyretin (TTR) amyloid, with a special involvement of the peripheral nerve. We had previously shown that doxycycline administered for 3 months at 40 mg/Kg/ml in the drinking water, was capable of removing TTR amyloid deposits present in stomachs of old TTR-V30M transgenic mice; the removal was accompanied by a decrease in extracellular matrix remodeling proteins that accompany fibrillar deposition, but not of non-fibrillar TTR deposition and/or markers associated with pre-fibrillar deposits. On the other hand, Tauroursodeoxycholic acid (TUDCA), a biliary acid, administrated to the same mouse model was shown to be effective at lowering deposited non-fibrillar TTR, as well as the levels of markers associated with pre-fibrillar TTR, but only at young ages.

In the present work we evaluated different doxycycline administration schemes, including different periods of treatment, different dosages and different FAP TTR V30M animal models. Evaluation included CR staining, immunohistochemistry for TTR, metalloproteinase 9 (MMP-9) and serum amyloid P component (SAP). We determined that a minimum period of 15 days of treatment with a 8 mg/Kg/day dosage resulted in fibril removal. The possibility of intermittent treatments was also assessed and a maximum period of 15 days of suspension was determined to maintain tissues amyloid-free. Combined cycled doxycycline and TUDCA administration to mice with amyloid deposition, using two different concentrations of both drugs, was more effective than either individual doxycycline or TUDCA, in significantly lowering TTR deposition and associated tissue markers. The observed synergistic effect of doxycycline/TUDCA in the range of human tolerable quantities, in the transgenic TTR mice models prompts their application in FAP, particularly in the early stages of disease.

Polyglutamine toxicity in non-neuronal cells

The neurodegenerative polyglutamine diseases are caused by an expansion of unstable polyglutamine repeats in various disease proteins. Although these mutant proteins are expressed ubiquitously in neuronal and non-neuronal cells, they cause selective degeneration of specific neuronal populations. Recently, increasing evidence shows that polyglutamine disease proteins also affect non-neuronal cells. However, it remains unclear how the expression of polyglutamine proteins in non-neuronal cells contributes to the course of the polyglutamine diseases. Here, we discuss recent findings about the expression of mutant polyglutamine proteins in non-neuronal cells and their influence on neurological symptoms. Understanding the contribution of non-neuronal polyglutamine proteins to disease progression will help elucidate disease mechanisms and also help in the development of new treatment options.

Probing the metabolic aberrations underlying mutant huntingtin toxicity in yeast and assessing their degree of preservation in humans and mice

Metabolomics is a powerful multiparameter tool for evaluating phenotypic traits associated with disease processes. We have used (1)H NMR metabolome profiling to characterize metabolic aberrations in a yeast model of Huntington's disease that are attributable to the mutant huntingtin protein's gain-of-toxic-function effects. A group of 11 metabolites (alanine, acetate, galactose, glutamine, glycerol, histidine, proline, succinate, threonine, trehalose, and valine) exhibited significant concentration changes in yeast expressing the N-terminal fragment of a mutant human huntingtin gene. Correspondence analysis was used to compare results from our yeast model to data reported from transgenic mice expressing a mutant huntingtin gene fragment and Huntington's disease patients. This technique enabled us to identify a variety of both model-specific (pertaining to a single species) and conserved (observed in multiple species) biomarkers related to mutant huntingtin's toxicity. Among the 59 metabolites identified, four compounds (alanine, glutamine, glycerol, and valine) changed significantly in concentration in all three Huntington's disease systems. We propose that alanine, glutamine, glycerol, and valine should be considered as promising biomarkers for evaluating new Huntington's disease therapies, as well as for providing unique insight into the mechanisms associated with mutant huntingtin toxicity.

Alterations in cortical excitation and inhibition in genetic mouse models of Huntington's disease

Previously, we identified progressive alterations in spontaneous EPSCs and IPSCs in the striatum of the R6/2 mouse model of Huntington's disease (HD). Medium-sized spiny neurons from these mice displayed a lower frequency of EPSCs, and a population of cells exhibited an increased frequency of IPSCs beginning at approximately 40 d, a time point when the overt behavioral phenotype begins. The cortex provides the major excitatory drive to the striatum and is affected during disease progression. We examined spontaneous EPSCs and IPSCs of somatosensory cortical pyramidal neurons in layers II/III in slices from three different mouse models of HD: the R6/2, the YAC128, and the CAG140 knock-in. Results revealed that spontaneous EPSCs occurred at a higher frequency, and evoked EPSCs were larger in behaviorally phenotypic mice whereas spontaneous IPSCs were initially increased in frequency in all models and subsequently decreased in R6/2 mice after they displayed the typical R6/2 overt behavioral phenotype. Changes in miniature IPSCs and evoked IPSC paired-pulse ratios suggested altered probability of GABA release. Also, in R6/2 mice, blockade of GABA(A) receptors induced complex discharges in slices and seizures in vivo at all ages. In conclusion, altered excitatory and inhibitory inputs to pyramidal neurons in the cortex in HD appear to be a prevailing deficit throughout the development of the disease. Furthermore, the differences between synaptic phenotypes in cortex and striatum are important for the development of future therapeutic approaches, which may need to be targeted early in the development of the phenotype.

Identification and characterization of ambroxol as an enzyme enhancement agent for Gaucher disease

Gaucher disease (GD), the most prevalent lysosomal storage disease, is caused by a deficiency of glucocerebrosidase (GCase). The identification of small molecules acting as agents for enzyme enhancement therapy is an attractive approach for treating different forms of GD. A thermal denaturation assay utilizing wild type GCase was developed to screen a library of 1,040 Food and Drug Administration-approved drugs. Ambroxol (ABX), a drug used to treat airway mucus hypersecretion and hyaline membrane disease in newborns, was identified and found to be a pH-dependent, mixed-type inhibitor of GCase. Its inhibitory activity was maximal at neutral pH, found in the endoplasmic reticulum, and undetectable at the acidic pH of lysosomes. The pH dependence of ABX to bind and stabilize the enzyme was confirmed by monitoring the rate of hydrogen/deuterium exchange at increasing guanidine hydrochloride concentrations. ABX treatment significantly increased N370S and F213I mutant GCase activity and protein levels in GD fibroblasts. These increases were primarily confined to the lysosome-enriched fraction of treated cells, a finding confirmed by confocal immunofluorescence microscopy. Additionally, enhancement of GCase activity and a reduction in glucosylceramide storage was verified in ABX-treated GD lymphoblasts (N370S/N370S). Hydrogen/deuterium exchange mass spectrometry revealed that upon binding of ABX, amino acid segments 243-249, 310-312, and 386-400 near the active site of GCase are stabilized. Consistent with its mixed-type inhibition of GCase, modeling studies indicated that ABX interacts with both active and non-active site residues. Thus, ABX has the biochemical characteristics of a safe and effective enzyme enhancement therapy agent for the treatment of patients with the most common GD genotypes.

Nonallele-specific silencing of mutant and wild-type huntingtin demonstrates therapeutic efficacy in Huntington's disease mice

Huntington's disease (HD) is a fatal neurodegenerative disease caused by mutant huntingtin (htt) protein, and there are currently no effective treatments. Recently, we and others demonstrated that silencing mutant htt via RNA interference (RNAi) provides therapeutic benefit in HD mice. We have since found that silencing wild-type htt in adult mouse striatum is tolerated for at least 4 months. However, given the role of htt in various cellular processes, it remains unknown whether nonallele-specific silencing of both wild-type and mutant htt is a viable therapeutic strategy for HD. Here, we tested whether cosilencing wild-type and mutant htt provides therapeutic benefit and is tolerable in HD mice. After treatment, HD mice showed significant reductions in wild-type and mutant htt, and demonstrated improved motor coordination and survival. We performed transcriptional profiling to evaluate the effects of reducing wild-type htt in adult mouse striatum. We identified gene expression changes that are concordant with previously described roles for htt in various cellular processes. Also, several abnormally expressed transcripts associated with early-stage HD were differentially expressed in our studies, but intriguingly, those involved in neuronal function changed in opposing directions. Together, these encouraging and surprising findings support further testing of nonallele-specific RNAi therapeutics for HD.

Mitochondria in neuroplasticity and neurological disorders

Mitochondrial electron transport generates the ATP that is essential for the excitability and survival of neurons, and the protein phosphorylation reactions that mediate synaptic signaling and related long-term changes in neuronal structure and function. Mitochondria are highly dynamic organelles that divide, fuse, and move purposefully within axons and dendrites. Major functions of mitochondria in neurons include the regulation of Ca(2+) and redox signaling, developmental and synaptic plasticity, and the arbitration of cell survival and death. The importance of mitochondria in neurons is evident in the neurological phenotypes in rare diseases caused by mutations in mitochondrial genes. Mitochondria-mediated oxidative stress, perturbed Ca(2+) homeostasis, and apoptosis may also contribute to the pathogenesis of prominent neurological diseases including Alzheimer's, Parkinson's, and Huntington's diseases; stroke; amyotrophic lateral sclerosis; and psychiatric disorders. Advances in understanding the molecular and cell biology of mitochondria are leading to novel approaches for the prevention and treatment of neurological disorders.

The R6 lines of transgenic mice: a model for screening new therapies for Huntington's disease

Huntington's disease (HD) is a hereditary neurodegenerative disorder caused by an expanded CAG repeat in the HD gene that results in cortical and striatal degeneration, and mutant huntingtin aggregation. Current treatments are unsatisfactory. R6 transgenic mice replicate many features of the human condition, show early onset of symptoms and fast disease progression, being one of the most used models for therapy screening. Here we review the therapies that have been tested in these mice: environmental enrichment, inhibition of histone deacetylation and methylation, inhibition of misfolding and oligomerization, transglutaminase inhibition, rescue of metabolic impairment, amelioration of the diabetic phenotype, use of antioxidants, inhibition of excitotoxicity, caspase inhibition, transplantation, genetic manipulations, and restoration of neurogenesis. Although many of these treatments were beneficial in R6 mice, they may not be as effective in HD patients, and thus the search for a combination of therapies that will rescue the human condition continues.