Striatal modulation of cAMP-response-element-binding protein (CREB) after excitotoxic lesions: implications with neuronal vulnerability in Huntington's disease

Neuroimmune pathways involvement in neurodegeneration of R6/2 mouse model of Huntington's disease

Mechanisms of tissue damage in Huntington's disease (HD) involve excitotoxicity, mitochondrial damage, and neuroinflammation, including microglia activation. CD47 is a membrane protein that interacts with the inhibitory immunoreceptor SIRPα. Engagement of SIRPα by CD47 provides a downregulatory signal that inhibits host cell phagocytosis, promoting a "don't-eat-me" signal. These proteins are involved in the immune response and are downmodulated in inflammatory diseases. The involvement of inflammation and of the inflammasome in HD has already been described. In this study, we focused on other factors that can be involved in the unregulated inflammatory response that accelerates and exacerbate the neurodegenerative process in HD. Our results show that CD47 on striatal neurons decreased in HD mice, while it increased in wild type mice with age. SIRPα, on the other hand, was present in neurons in the wild type and increases in the R6/2 mice at all stages. Recruitment of SIRPα and binding to CD47 promotes the activation through phosphorylating events of non-receptor protein tyrosine phosphatase SHP-1 and SHP-2 in neurons and microglia. SHP phosphatases are able to curb the activity of NLRP3 inflammasome thereby reducing the detrimental effect of neuroinflammation. Such activity is mediated by the inhibition (dephosphorylation) of the proteins signal transducer and activator of transcription (STAT). We found that activated SHP-1 was present in microglia and neurons of WT mice at 5 and 13 weeks, increasing with time; while in R6/2 it was not localized in neurons but only in microglia, where it decreases with time. Consequently, STAT1 was overexpressed in neurons of R6/2 mice, as an effect of lack of modulation by SHP-1. Thus, our results shed light on the pathophysiology of neuronal damage, on one hand, paving the way toward a modulation of signal transducer proteins by specific inhibitors to achieve neuroprotection in HD, on the other.

Neurodegeneration, Mitochondria, and Antibiotics

Neurodegenerative diseases are characterized by the progressive loss of neurons, synapses, dendrites, and myelin in the central and/or peripheral nervous system. Actual therapeutic options for patients are scarce and merely palliative. Although they affect millions of patients worldwide, the molecular mechanisms underlying these conditions remain unclear. Mitochondrial dysfunction is generally found in neurodegenerative diseases and is believed to be involved in the pathomechanisms of these disorders. Therefore, therapies aiming to improve mitochondrial function are promising approaches for neurodegeneration. Although mitochondrial-targeted treatments are limited, new research findings have unraveled the therapeutic potential of several groups of antibiotics. These drugs possess pleiotropic effects beyond their anti-microbial activity, such as anti-inflammatory or mitochondrial enhancer function. In this review, we will discuss the controversial use of antibiotics as potential therapies in neurodegenerative diseases.

Failure to Thrive: Impaired BDNF Transport along the Cortical-Striatal Axis in Mouse Q140 Neurons of Huntington's Disease

Boosting trophic support to striatal neurons by increasing levels of brain-derived neurotrophic factor (BDNF) has been considered as a target for therapeutic intervention for several neurodegenerative diseases, including Huntington's disease (HD). To aid in the implementation of such a strategy, a thorough understanding of BDNF cortical-striatal transport is critical to help guide its strategic delivery. In this manuscript, we investigate the dynamic behavior of BDNF transport along the cortical-striatal axis in Q140 primary neurons, a mouse model for HD. We examine this by using single-molecule labeling of BDNF conjugated with quantum dots (QD-BDNF) to follow the transport along the cortical-striatal axis in a microfluidic chamber system specifically designed for the co-culture of cortical and striatal primary neurons. Using this approach, we observe a defect of QD-BDNF transport in Q140 neurons. Our study demonstrates that QD-BDNF transport along the cortical-striatal axis involves the impairment of anterograde transport within axons of cortical neurons, and of retrograde transport within dendrites of striatal neurons. One prominent feature we observe is the extended pause time of QD-BDNF retrograde transport within Q140 striatal dendrites. Taken together, these finding support the hypothesis that delinquent spatiotemporal trophic support of BDNF to striatal neurons, driven by impaired transport, may contribute to the pathogenesis of HD, providing us with insight into how a BDNF supplementation therapeutic strategy may best be applied for HD.

Mitochondria and Antibiotics: For Good or for Evil?

The discovery and application of antibiotics in the common clinical practice has undeniably been one of the major medical advances in our times. Their use meant a drastic drop in infectious diseases-related mortality and contributed to prolonging human life expectancy worldwide. Nevertheless, antibiotics are considered by many a double-edged sword. Their extensive use in the past few years has given rise to a global problem: antibiotic resistance. This factor and the increasing evidence that a wide range of antibiotics can damage mammalian mitochondria, have driven a significant sector of the medical and scientific communities to advise against the use of antibiotics for purposes other to treating severe infections. Notwithstanding, a notorious number of recent studies support the use of these drugs to treat very diverse conditions, ranging from cancer to neurodegenerative or mitochondrial diseases. In this context, there is great controversy on whether the risks associated to antibiotics outweigh their promising beneficial features. The aim of this review is to provide insight in the topic, purpose for which the most relevant findings regarding antibiotic therapies have been discussed.

Neuroprotective effects of roflumilast against quinolinic acid-induced rat model of Huntington's disease through inhibition of NF-κB mediated neuroinflammatory markers and activation of cAMP/CREB/BDNF signaling pathway

Huntington's disease (HD) is a progressive neurodegenerative and hyperkinetic movement disorder. Decreased activity of cAMP-responsive element-binding protein (CREB) is thought to contribute to the death of striatal medium spiny neurons in HD. The present study has been designed to explore the possible role of roflumilast against qunilonic acid (QA) induced neurotoxicity in rats intending to investigate whether it inhibits the neuroinflammatory response through activation of the cAMP/CREB/BDNF signaling pathway. QA was microinjected (200 nmol/2 µl, bilaterally) through the intrastriatal route in the stereotaxic apparatus. Roflumilast (0.5, 1, and 2 mg/kg, orally) once-daily treatment for 21 days significantly improved locomotor activity in actophotometer, motor coordination in rotarod, and impaired gait performance in narrow beam walk test. Moreover, roflumilast treatment significantly attenuated oxidative and nitrosative stress (p < 0.05) through attenuating lipid peroxidation nitrite concentration and enhancing reduced glutathione, superoxide dismutase, and catalase levels. Furthermore, roflumilast also significantly decreased elevated pro-inflammatory cytokines like TNF-α (p < 0.01), IL-6 (p < 0.01), IFN-γ (p < 0.05), NF-κB (p < 0.05) and significantly increased BDNF(p < 0.05) in the striatum and cortex of rat brain. The results further demonstrated that roflumilast effectively increased the gene expression of cAMP(p < 0.05), CREB(p < 0.05) and decreased the gene expression of PDE4 (p < 0.05) in qRT-PCR. These results conclusively depicted that roflumilast could be a potential candidate as an effective therapeutic agent in the management of HD through the cAMP/CREB/BDNF signaling pathway.

The CBP KIX domain regulates long-term memory and circadian activity

Background

CREB-dependent transcription necessary for long-term memory is driven by interactions with CREB-binding protein (CBP), a multi-domain protein that binds numerous transcription factors potentially affecting expression of thousands of genes. Identifying specific domain functions for multi-domain proteins is essential to understand processes such as cognitive function and circadian clocks. We investigated the function of the CBP KIX domain in hippocampal memory and gene expression using CBP^KIX/KIX mice with mutations that prevent phospho-CREB (Ser133) binding.

Results

We found that CBP^KIX/KIX mice were impaired in long-term memory, but not learning acquisition or short-term memory for the Morris water maze. Using an unbiased analysis of gene expression in the dorsal hippocampus after training in the Morris water maze or contextual fear conditioning, we discovered dysregulation of CREB, CLOCK, and BMAL1 target genes and downregulation of circadian genes in CBP^KIX/KIX mice. Given our finding that the CBP KIX domain was important for transcription of circadian genes, we profiled circadian activity and phase resetting in CBP^KIX/KIX mice. CBP^KIX/KIX mice exhibited delayed activity peaks after light offset and longer free-running periods in constant dark. Interestingly, CBP^KIX/KIX mice displayed phase delays and advances in response to photic stimulation comparable to wildtype littermates. Thus, this work delineates site-specific regulation of the circadian clock by a multi-domain protein.

Conclusions

These studies provide insight into the significance of the CBP KIX domain by defining targets of CBP transcriptional co-activation in memory and the role of the CBP KIX domain in vivo on circadian rhythms.

Graphical abstract

Ischemic injury precipitates neuronal vulnerability in Parkinson's disease: Insights from PINK1 mouse model study and clinical retrospective data

INTRODUCTION

Increasing evidence demonstrates the relevant association between Parkinson's disease (PD) and vascular diseases/risk factors, as well as a worse clinico-pathological progression in those patients with vascular comorbidity. The mechanisms underlying this relationship have not been clarified yet, although their comprehension is critical in a perspective of disease-modifying treatments development or prevention.

METHODS

We performed an experimental protocol of ischemic injury (glucose-oxygen deprivation, OGD) on PTEN-induced kinase 1 knockout (PINK1^-/-) mice, a well-established PD model, looking at both electrophysiological and morphological changes in basal ganglia. In addition, 253 PD patients were retrospectively analysed, to estimate the prevalence of vascular risk factors.

RESULTS

In PINK1^-/- mice, the OGD protocol induced electrophysiological (prolonged depolarization) and morphological alterations (picnotic cells, cellular loss and swelling, thickening of nuclear chromatin) in striatal medium spiny neurons and nigral dopaminergic neurons. Vascular comorbidity occurred in 75% of PD patients.

CONCLUSIONS

The ischemic injury precipitates neuronal vulnerability in basal ganglia of PINK1^-/- mice, probably through an impairment of mitochondrial metabolism and higher oxidative stress. These experimental data may provide a potential mechanistic explanation for both the association between vascular diseases and PD and their reciprocal interactions in determining the clinico-pathological burden of PD patients.

Neuroprotective Effects of Doxycycline in the R6/2 Mouse Model of Huntington's Disease

Mechanisms of tissue damage in Huntington’s disease involve excitotoxicity, mitochondrial damage, and inflammation, including microglia activation. Immunomodulatory and anti-protein aggregation properties of tetracyclines were demonstrated in several disease models. In the present study, the neuroprotective and anti-inflammatory effects of the tetracycline doxycycline were investigated in the mouse model of HD disease R6/2. Transgenic mice were daily treated with doxycycline 20 mg/kg, starting from 4 weeks of age. After sacrifice, histological and immunohistochemical studies were performed. We found that doxycycline-treated R6/2 mice survived longer and displayed less severe signs of neurological dysfunction than the saline-treated ones. Primary outcome measures such as striatal atrophy, neuronal intranuclear inclusions, and the negative modulation of microglial reaction revealed a neuroprotective effect of the compound. Doxycycline provided a significantly increase of activated CREB and BDNF in the striatal neurons, along with a down modulation of neuroinflammation, which, combined, might explain the beneficial effects observed in this model. Our findings show that doxycycline treatment could be considered as a valid therapeutic approach 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.

Inhibition of phosphodiesterases as a strategy to achieve neuroprotection in Huntington's disease

Huntington's disease (HD) is a fatal neurodegenerative condition, due to a mutation in the IT15 gene encoding for huntingtin. Currently, disease-modifying therapy is not available for HD, and only symptomatic drugs are administered for the management of symptoms. In the last few years, preclinical and clinical studies have indicated that pharmacological strategies aimed at inhibiting cyclic nucleotide phosphodiesterase (PDEs) may develop into a novel therapeutic approach in neurodegenerative disorders. PDEs are a family of enzymes that hydrolyze cyclic nucleotides into monophosphate isoforms. Cyclic nucleotides are second messengers that transduce the signal of hormones and neurotransmitters in many physiological processes, such as protein kinase cascades and synaptic transmission. An alteration in their balance results in the dysregulation of different biological mechanisms (transcriptional dysregulation, immune cell activation, inflammatory mechanisms, and regeneration) that are involved in neurological diseases. In this review, we discuss the action of phosphodiesterase inhibitors and their role as therapeutic agents in HD.

Selective Sparing of Striatal Interneurons after Poly (ADP-Ribose) Polymerase 1 Inhibition in the R6/2 Mouse Model of Huntington's Disease

Poly (ADP-ribose) polymerases (PARPs) are enzymes that catalyze ADP-ribose units transfer from NAD to their substrate proteins. It has been observed that PARP-1 is able to increase both post-ischemic and excitotoxic neuronal death. In fact, we have previously shown that, INO-1001, a PARP-1 inhibitor, displays a neuroprotective effect in the R6/2 model of Huntington’s disease (HD). In this study, we investigated the effects of PARP-1-inhibition on modulation of phosphorylated c-AMP response element binding protein (pCREB) and CREB-binding protein (CBP) localization in the different striatal neuronal subsets. Moreover, we studied the neurodegeneration of those interneurons that are particularly vulnerable to HD such as parvalbuminergic and calretininergic, and of other subclasses of interneurons that are known to be resistant, such as cholinergic and somatostatinergic interneurons. Transgenic mice were treated with INO-1001 (10 mg/Kg daily) starting from 4 weeks of age. Double-label immunofluorescence was performed to value the distribution of CBP in ubiquitinated Neuronal intranuclear inclusions (NIIs) in the striatum. INO-1001-treated and saline-treated brain sections were incubated with: goat anti-choline acetyl transferase; goat anti-nitric oxide synthase; mouse anti-parvalbumin and mouse anti-calretinin. Morphometric evaluation and cell counts were performed. Our study showed that the PARP inhibitor has a positive effect in sparing parvalbumin and calretinin-containing interneurons of the striatum, where CREB was upregulated. Moreover, INO-1001 promoted CBP localization into the nuclei of the R6/2 mouse. The sum of our data corroborates the previous observations indicating PARP inhibition as a possible therapeutic tool to fight HD.

Synaptopathic mechanisms of neurodegeneration and dementia: Insights from Huntington's disease

Dementia encapsulates a set of symptoms that include loss of mental abilities such as memory, problem solving or language, and reduces a person's ability to perform daily activities. Alzheimer's disease is the most common form of dementia, however dementia can also occur in other neurological disorders such as Huntington's disease (HD). Many studies have demonstrated that loss of neuronal cell function manifests pre-symptomatically and thus is a relevant therapeutic target to alleviate symptoms. Synaptopathy, the physiological dysfunction of synapses, is now being approached as the target for many neurological and psychiatric disorders, including HD. HD is an autosomal dominant and progressive degenerative disorder, with clinical manifestations that encompass movement, cognition, mood and behaviour. HD is one of the most common tandem repeat disorders and is caused by a trinucleotide (CAG) repeat expansion, encoding an extended polyglutamine tract in the huntingtin protein. Animal models as well as human studies have provided detailed, although not exhaustive, evidence of synaptic dysfunction in HD. In this review, we discuss the neuropathology of HD and how the changes in synaptic signalling in the diseased brain lead to its symptoms, which include dementia. Here, we review and discuss the mechanisms by which the 'molecular orchestras' and their 'synaptic symphonies' are disrupted in neurodegeneration and dementia, focusing on HD as a model disease. We also explore the therapeutic strategies currently in pre-clinical and clinical testing that are targeted towards improving synaptic function in HD.

PARP-1 Inhibition Is Neuroprotective in the R6/2 Mouse Model of Huntington's Disease

Poly (ADP-ribose) polymerase 1 (PARP-1) is a nuclear enzyme that is involved in physiological processes as DNA repair, genomic stability, and apoptosis. Moreover, published studies demonstrated that PARP-1 mediates necrotic cell death in response to excessive DNA damage under certain pathological conditions. In Huntington’s disease brains, PARP immunoreactivity was described in neurons and in glial cells, thereby suggesting the involvement of apoptosis in HD. In this study, we sought to determine if the PARP-1 inhibitor exerts a neuroprotective effect in R6/2 mutant mice, which recapitulates, in many aspects, human HD. Transgenic mice were treated with the PARP-1 inhibitor INO-1001 mg/Kg daily starting from 4 weeks of age. After transcardial perfusion, histological and immunohistochemical studies were performed. We found that INO 1001-treated R6/2 mice survived longer and displayed less severe signs of neurological dysfunction than the vehicle treated ones. Primary outcome measures such as striatal atrophy, morphology of striatal neurons, neuronal intranuclear inclusions and microglial reaction confirmed a neuroprotective effect of the compound. INO-1001 was effective in significantly increasing activated CREB and BDNF in the striatal spiny neurons, which might account for the beneficial effects observed in this model. Our findings show that PARP-1 inhibition could be considered as a valid therapeutic approach for HD.

Lysine acetyltransferases CBP and p300 as therapeutic targets in cognitive and neurodegenerative disorders

Neuropsychiatric pathologies, including neurodegenerative diseases and neurodevelopmental syndromes, are frequently associated with dysregulation of various essential cellular mechanisms, such as transcription, mitochondrial respiration and protein degradation. In these complex scenarios, it is difficult to pinpoint the specific molecular dysfunction that initiated the pathology or that led to the fatal cascade of events that ends with the death of the neuron. Among the possible original factors, epigenetic dysregulation has attracted special attention. This review focuses on two highly related epigenetic factors that are directly involved in a number of neurological disorders, the lysine acetyltransferases CREB-binding protein (CBP) and E1A-associated protein p300 (p300). We first comment on the role of chromatin acetylation and the enzymes that control it, particularly CBP and p300, in neuronal plasticity and cognition. Next, we describe the involvement of these proteins in intellectual disability and in different neurodegenerative diseases. Finally, we discuss the potential of ameliorative strategies targeting CBP/p300 for the treatment of these disorders.

The PDE1/5 Inhibitor SCH-51866 Does Not Modify Disease Progression in the R6/2 Mouse Model of Huntington's Disease

Huntington's disease is a neurodegenerative disorder caused by mutations in the CAG tract of huntingtin. Several studies in HD cellular and rodent systems have identified disturbances in cyclic nucleotide signaling, which might be relevant to pathogenesis and therapeutic intervention. To investigate whether selective phosphodiesterase (PDE) inhibitors can improve some aspects of disease pathogenesis in HD models, we have systematically evaluated the effects of a variety of cAMP and cGMP selective PDE inhibitors in various HD models. Here we present the lack of effect in a variety of endpoints of the PDE subtype selective inhibitor SCH-51866, a PDE1/5 inhibitor, in the R6/2 mouse model of HD, after chronic oral dosing.

Huntington's disease

Changes in the level and activity of brain-derived neurotrophic factor (BDNF) have been described in a number of neurodegenerative disorders since early 1990s. However, only in Huntington disease (HD) gain- and loss-of-function experiments have mechanistically linked these abnormalities with the genetic defect.In this chapter we will describe how huntingtin protein, whose mutation causes HD, is involved in the physiological control of BDNF synthesis and transport in neurons and how both processes are simultaneously disrupted in HD. We will describe the underlying molecular mechanisms and discuss pre-clinical data concerning the impact of the experimental manipulation of BDNF levels on HD progression. These studies have revealed that a major loss of BDNF protein in the brain of HD patients may contribute to the clinical manifestations of the disease. The experimental strategies under investigation to increase brain BDNF levels in animal models of HD will also be described, with a view to ultimately improving the clinical treatment of this condition.

A small molecule TrkB ligand reduces motor impairment and neuropathology in R6/2 and BACHD mouse models of Huntington's disease

Loss of neurotrophic support in the striatum caused by reduced brain-derived neurotrophic factor (BDNF) levels plays a critical role in Huntington's disease (HD) pathogenesis. BDNF acts via TrkB and p75 neurotrophin receptors (NTR), and restoring its signaling is a prime target for HD therapeutics. Here we sought to determine whether a small molecule ligand, LM22A-4, specific for TrkB and without effects on p75(NTR), could alleviate HD-related pathology in R6/2 and BACHD mouse models of HD. LM22A-4 was administered to R6/2 mice once daily (5-6 d/week) from 4 to 11 weeks of age via intraperitoneal and intranasal routes simultaneously to maximize brain levels. The ligand reached levels in the R6/2 forebrain greater than the maximal neuroprotective dose in vitro and corrected deficits in activation of striatal TrkB and its key signaling intermediates AKT, PLCγ, and CREB. Ligand-induced TrkB activation was associated with a reduction in HD pathologies in the striatum including decreased DARPP-32 levels, neurite degeneration of parvalbumin-containing interneurons, inflammation, and intranuclear huntingtin aggregates. Aggregates were also reduced in the cortex. Notably, LM22A-4 prevented deficits in dendritic spine density of medium spiny neurons. Moreover, R6/2 mice given LM22A-4 demonstrated improved downward climbing and grip strength compared with those given vehicle, though these groups had comparable rotarod performances and survival times. In BACHD mice, long-term LM22A-4 treatment (6 months) produced similar ameliorative effects. These results support the hypothesis that targeted activation of TrkB inhibits HD-related degenerative mechanisms, including spine loss, and may provide a disease mechanism-directed therapy for HD and other neurodegenerative conditions.

Convergence of genetic and environmental factors on parvalbumin-positive interneurons in schizophrenia

Schizophrenia etiology is thought to involve an interaction between genetic and environmental factors during postnatal brain development. However, there is a fundamental gap in our understanding of the molecular mechanisms by which environmental factors interact with genetic susceptibility to trigger symptom onset and disease progression. In this review, we summarize the most recent findings implicating oxidative stress as one mechanism by which environmental insults, especially early life social stress, impact the development of schizophrenia. Based on a review of the literature and the results of our own animal model, we suggest that environmental stressors such as social isolation render parvalbumin-positive interneurons (PVIs) vulnerable to oxidative stress. We previously reported that social isolation stress exacerbates many of the schizophrenia-like phenotypes seen in a conditional genetic mouse model in which NMDA receptors (NMDARs) are selectively ablated in half of cortical and hippocampal interneurons during early postnatal development (Belforte et al., 2010). We have since revealed that this social isolation-induced effect is caused by impairments in the antioxidant defense capacity in the PVIs in which NMDARs are ablated. We propose that this effect is mediated by the down-regulation of PGC-1α, a master regulator of mitochondrial energy metabolism and anti-oxidant defense, following the deletion of NMDARs (Jiang et al., 2013). Other potential molecular mechanisms underlying redox dysfunction upon gene and environmental interaction will be discussed, with a focus on the unique properties of PVIs.

Systemic delivery of recombinant brain derived neurotrophic factor (BDNF) in the R6/2 mouse model of Huntington's disease

Loss of huntingtin-mediated BDNF gene transcription has been shown to occur in HD and thus contribute to the degeneration of the striatum. Several studies have indicated that an increase in BDNF levels is associated with neuroprotection and amelioration of neurological signs in animal models of HD. In a recent study, an increase in BDNF mRNA and protein levels was recorded in mice administered recombinant BDNF peripherally. Chronic, indwelling osmotic mini-pumps containing either recombinant BDNF or saline were surgically placed in R6/2 or wild-type mice from 4 weeks of age until euthanasia. Neurological evaluation (paw clasping, rotarod performance, locomotor activity in an open field) was performed. After transcardial perfusion, histological and immunohistochemical studies were performed. We found that BDNF- treated R6/2 mice survived longer and displayed less severe signs of neurological dysfunction than the vehicle treated ones. Primary outcome measures such as brain volume, striatal atrophy, size and morphology of striatal neurons, neuronal intranuclear inclusions and microglial reaction confirmed a neuroprotective effect of the compound. BDNF was effective in increasing significantly the levels of activated CREB and of BDNF the striatal spiny neurons. Moreover, systemically administered BDNF increased the synthesis of BDNF as demonstrated by RT-PCR, and this might account for the beneficial effects observed in this model.

Therapeutic Strategies for Huntington's Disease

Huntington's disease (HD) is a devastating autosomal dominant neurodegenerative disease, caused by expansion of the CAG repeat in the huntingtin (HTT) gene and characterized pathologically by the loss of pyramidal neurons in several cortical areas, of striatal medium spiny neurons, and of hypothalamic neurons. Clinically, a distinguishing feature of the disease is uncontrolled involuntary movements (chorea, dyskensias) accompanied by progressive cognitive, motor, and psychiatric impairment. This review focuses on the current state of therapeutic development for the treatment of HD, including the preclinical and clinical development of small molecules and molecular therapies.

Phosphodiesterase 10A (PDE10A) localization in the R6/2 mouse model of Huntington's disease

In Huntington's disease (HD) mutant huntingtin protein impairs the function of several transcription factors, in particular the cAMP response element-binding protein (CREB). CREB activation can be increased by targeting phosphodiesterases such as phospohodiesterase 4 (PDE4) and phosphodiesterase 10A (PDE10A). Indeed, both PDE4 inhibition (DeMarch et al., 2008) and PDE10A inhibition (Giampà et al., 2010) proved beneficial in the R6/2 mouse model of HD. However, Hebb et al. (2004) reported PDE10A decline in R6/2 mice. These findings raise the issue of how PDE10A inhibition is beneficial in HD if such enzyme is lost. R6/2 mice and their wild type littermates were treated with the PDE10A inhibitor TP10 (a gift from Pfizer) or saline, sacrificed at 5, 9, and 13 weeks of age, and single and double label immunohistochemistry and western blotting were performed. PDE10A increased dramatically in the spiny neurons of R6/2 compared to the wild type mice. Conversely, in the striatal cholinergic interneurons, PDE10A was lower and it did not change significantly with disease progression. In the other subsets of striatal interneurons (namely, parvalbuminergic, somatostatinergic, and calretininergic interneurons) PDE10A immunoreactivity was higher in the R6/2 compared to the wild-type mice. In the TP10 treated R6/2, PDE10A levels were lower than in the saline treated mice in the medium spiny neurons, whereas they were higher in all subsets of striatal interneurons except for the cholinergic ones. However, in the whole striatum densitometry studies, PDE10A immunoreactivity was lower in the R6/2 compared to the wild-type mice. Our study demonstrates that PDE10A is increased in the spiny neurons of R6/2 mice striatum. Thus, the accumulation of PDE10A in the striatal projection neurons, by hydrolyzing greater amounts of cyclic nucleotides, is likely to contribute to cell damage in HD. Consequently, the beneficial effect of TP10 in HD models (Giampà et al., 2009, 2010) is explained by the efficiency of such compound in counteracting this phenomenon and therefore increasing the availability of cyclic nucleotides.

Somatostatin receptor 1 and 5 double knockout mice mimic neurochemical changes of Huntington's disease transgenic mice

BACKGROUND

Selective degeneration of medium spiny neurons and preservation of medium sized aspiny interneurons in striatum has been implicated in excitotoxicity and pathophysiology of Huntington's disease (HD). However, the molecular mechanism for the selective sparing of medium sized aspiny neurons and vulnerability of projection neurons is still elusive. The pathological characteristic of HD is an extensive reduction of the striatal mass, affecting caudate putamen. Somatostatin (SST) positive neurons are selectively spared in HD and Quinolinic acid/N-methyl-D-aspartic acid induced excitotoxicity, mimic the model of HD. SST plays neuroprotective role in excitotoxicity and the biological effects of SST are mediated by five somatostatin receptor subtypes (SSTR1-5).

METHODS AND FINDINGS

To delineate subtype selective biological responses we have here investigated changes in SSTR1 and 5 double knockout mice brain and compared with HD transgenic mouse model (R6/2). Our study revealed significant loss of dopamine and cAMP regulated phosphoprotein of 32 kDa (DARPP-32) and comparable changes in SST, N-methyl-D-aspartic acid receptors subtypes, calbindin and brain nitric oxide synthase expression as well as in key signaling proteins including calpain, phospho-extracellular-signal-regulated kinases1/2, synapsin-IIa, protein kinase C-α and calcineurin in SSTR1/5(-/-) and R6/2 mice. Conversely, the expression of somatostatin receptor subtypes, enkephalin and phosphatidylinositol 3-kinases were strain specific. SSTR1/5 appears to be important in regulating NMDARs, DARPP-32 and signaling molecules in similar fashion as seen in HD transgenic mice.

CONCLUSIONS

This is the first comprehensive description of disease related changes upon ablation of G- protein coupled receptor gene. Our results indicate that SST and SSTRs might play an important role in regulation of neurodegeneration and targeting this pathway can provide a novel insight in understanding the pathophysiology of Huntington's disease.

Transforming Growth Factor-Beta Signaling in the Neural Stem Cell Niche: A Therapeutic Target for Huntington's Disease

The neural stem cell niches possess the regenerative capacity to generate new functional neurons in the adult brain, suggesting the possibility of endogenous neuronal replacement after injury or disease. Huntington disease (HD) is a neurodegenerative disease and characterized by neuronal loss in the basal ganglia, leading to motor, cognitive, and psychological disabilities. Apparently, in order to make use of the neural stem cell niche as a therapeutic concept for repair strategies in HD, it is important to understand the cellular and molecular composition of the neural stem cell niche under such neurodegenerative conditions. This paper mainly discusses the current knowledge on the regulation of the hippocampal neural stem cell niche in the adult brain and by which mechanism it might be compromised in the case of HD.

Inhibition of the striatal specific phosphodiesterase PDE10A ameliorates striatal and cortical pathology in R6/2 mouse model of Huntington's disease

BACKGROUND

Huntington's disease is a devastating neurodegenerative condition for which there is no therapy to slow disease progression. The particular vulnerability of striatal medium spiny neurons to Huntington's pathology is hypothesized to result from transcriptional dysregulation within the cAMP and CREB signaling cascades in these neurons. To test this hypothesis, and a potential therapeutic approach, we investigated whether inhibition of the striatal-specific cyclic nucleotide phosphodiesterase PDE10A would alleviate neurological deficits and brain pathology in a highly utilized model system, the R6/2 mouse.

METHODOLOGY/PRINCIPAL FINDINGS

R6/2 mice were treated with the highly selective PDE10A inhibitor TP-10 from 4 weeks of age until euthanasia. TP-10 treatment significantly reduced and delayed the development of the hind paw clasping response during tail suspension, deficits in rotarod performance, and decrease in locomotor activity in an open field. Treatment prolonged time to loss of righting reflex. These effects of PDE10A inhibition on neurological function were reflected in a significant amelioration in brain pathology, including reduction in striatal and cortical cell loss, the formation of striatal neuronal intranuclear inclusions, and the degree of microglial activation that occurs in response to the mutant huntingtin-induced brain damage. Striatal and cortical levels of phosphorylated CREB and BDNF were significantly elevated.

CONCLUSIONS/SIGNIFICANCE

Our findings provide experimental support for targeting the cAMP and CREB signaling pathways and more broadly transcriptional dysregulation as a therapeutic approach to Huntington's disease. It is noteworthy that PDE10A inhibition in the R6/2 mice reduces striatal pathology, consistent with the localization of the enzyme in medium spiny neurons, and also cortical pathology and the formation of neuronal nuclear inclusions. These latter findings suggest that striatal pathology may be a primary driver of these secondary pathological events. More significantly, our studies point directly to an accessible new therapeutic approach to slow Huntington's disease progression, namely, PDE10A inhibition. There is considerable activity throughout the pharmaceutical industry to develop PDE10A inhibitors for the treatment of basal ganglia disorders. The present results strongly support the investigation of PDE10A inhibitors as a much needed new treatment approach to Huntington's disease.

Chronic suppression of phosphodiesterase 10A alters striatal expression of genes responsible for neurotransmitter synthesis, neurotransmission, and signaling pathways implicated in Huntington's disease

Inhibition of phosphodiesterase 10A (PDE10A) promotes cyclic nucleotide signaling, increases striatal activation, and decreases behavioral activity. Enhanced cyclic nucleotide signaling is a well established route to producing changes in gene expression. We hypothesized that chronic suppression of PDE10A activity would have significant effects on gene expression in the striatum. A comparison of the expression profile of PDE10A knockout (KO) mice and wild-type mice after chronic PDE10A inhibition revealed altered expression of 19 overlapping genes with few significant changes outside the striatum or after administration of a PDE10A inhibitor to KO animals. Chronic inhibition of PDE10A produced up-regulation of mRNAs encoding genes that included prodynorphin, synaptotagmin10, phosphodiesterase 1C, glutamate decarboxylase 1, and diacylglycerol O-acyltransferase and a down-regulation of mRNAs encoding choline acetyltransferase and Kv1.6, suggesting long-term suppression of the PDE10A enzyme is consistent with altered striatal excitability and potential utility as a antipsychotic therapy. In addition, up-regulation of mRNAs encoding histone 3 (H3) and down-regulation of histone deacetylase 4, follistatin, and claspin mRNAs suggests activation of molecular cascades capable of neuroprotection. We used lentiviral delivery of cAMP response element (CRE)-luciferase reporter constructs into the striatum and live animal imaging of 2-{4-[-pyridin-4-yl-1-(2,2,2-trifluoro-ethyl)-1H-pyrazol-3-yl]-phenoxymethyl}-quinoline succinic acid (TP-10)-induced luciferase activity to further demonstrate PDE10 inhibition results in CRE-mediated transcription. Consistent with potential neuroprotective cascades, we also demonstrate phosphorylation of mitogen- and stress-activated kinase 1 and H3 in vivo after TP-10 treatment. The observed changes in signaling and gene expression are predicted to provide neuroprotective effects in models of Huntington's disease.

Phosphodiesterase type IV inhibition prevents sequestration of CREB binding protein, protects striatal parvalbumin interneurons and rescues motor deficits in the R6/2 mouse model of Huntington's disease

The phosphodiesterase type IV inhibitor rolipram increases cAMP response element-binding protein (CREB) phosphorylation and exerts neuroprotective effects in both the quinolinic acid rat model of Huntington's disease (DeMarch et al., 2007) and the R6/2 mouse including sparing of striatal neurons, prevention of neuronal intranuclear inclusion formation and attenuation of microglial reaction (DeMarch et al., 2008). In this study, we sought to determine if rolipram has a beneficial role in the altered distribution of CREB binding protein in striatal spiny neurons and in the motor impairments shown by R6/2 mutants. Moreover, we investigated whether rolipram treatment altered the degeneration of parvalbuminergic interneurons typical of Huntington's disease (Fusco et al., 1999). Transgenic mice and their wild-type controls from a stable colony maintained in our laboratory were treated with rolipram (1.5 mg/kg) or saline daily starting from 4 weeks of age. The cellular distribution of CREB binding protein in striatal spiny neurons was assessed by immunofluorescence, whereas parvalbuminergic neuron degeneration was evaluated by cell counts of immunohistochemically labeled tissue. Motor coordination and motor activity were also examined. We found that rolipram was effective in preventing CREB binding protein sequestration into striatal neuronal intranuclear inclusions, sparing parvalbuminergic interneurons of R6/2 mice, and rescuing their motor coordination and motor activity deficits. Our findings demonstrate the possibility of reversing pharmacologically the behavioral and neuropathological abnormalities of symptomatic R6/2 mice and underline the potential therapeutic value of phosphodiesterase type IV inhibitors in Huntington's disease.

Phosphodiesterase 10A inhibition is associated with locomotor and cognitive deficits and increased anxiety in mice

Phosphodiesterase 10A (PDE10A) mRNA and protein levels decline in the striatum of R6/1 and R6/2 Huntington's disease (HD) mice prior to motor symptom development. In human HD, PDE10A protein levels are significantly decreased in the caudate-putamen of patients with grade 3 HD compared to age-matched controls. To test whether the loss of PDE10A activity in the striatum was detrimental to normal brain function, we treated wild-type (WT) mice with chronic administration of papaverine, which is a specific inhibitor of PDE10A. At 7 weeks of age, mice were introduced to a weekly battery of motor tests, including assessment of weight, locomotion, gait, and coordination. Beginning at 8 weeks of age, mice received 0, 5, 10 or 20 mg/kg papaverine once daily until the completion of behavioral testing. Following 14 days of papaverine injections, mice were assessed for deficits in cognitive performance as measured in the Morris water maze (MWM). All behavioral tests occurred either immediately prior to or 30 min following a subcutaneous papaverine challenge dose. Twenty-four hours following completion of the 2-3 week MWM protocol, mice were given a dose of papaverine and 30 min later psychological function assessed in the Light-Dark (LD) Test. Chronic administration of papaverine for 42 days was associated with distinct motor perturbations, mild cognitive disturbance and anxiety-like behaviors. Subsequently, we assessed the effect of 14 days papaverine (i.e. sub-chronic) treatment on psychological function of WT and R6/1 HD mice. While sub-chronic papaverine induced anxiety-like behavior in WT mice, it appeared to have little effect on the behavior of R6/1 HD mice. Finally, a separate group of 6-week old WT and R6/2 HD mice were treated for 21 days with saline or 10 mg/kg fluoxetine, an agent with anxiolytic and anti-depressant effects, in order to compare the effects of papaverine and fluoxetine on anxiety-like behavior in the LD test. CREB and PDE10A protein levels in striatum and hippocampus were determined by western blot. While papaverine treatment reduced CREB protein levels in the hippocampus and striatum, fluoxetine increased CREB in the hippocampus. These data suggest that papaverine and fluoxetine may produce quite different effects on behavior; these behaviors may be linked to CREB expression in brain regions associated with motor and cognitive functions. PDE10A protein levels were decreased by both papaverine and fluoxetine. Chronic PDE10A inhibition produced a variety of behavioral and central neurochemical deficits and these effects were exacerbated by stress. The unique localization of PDE10A and its apparent role in basal ganglia function may underlie its role in psychiatric and neurological disorders involving the basal ganglia.

Beneficial effects of rolipram in the R6/2 mouse model of Huntington's disease

We have previously showed that rolipram, a phosphodiesterase type IV inhibitor, displays a neuroprotective effect in a rat quinolinic acid model of HD [DeMarch Z., Giampa C., Patassini S., Martorana A., Bernardi G. and Fusco F.R., (2007) Beneficial effects of rolipram in a quinolinic acid model of striatal excitotoxicity. Neurobiol. Dis. 25:266-273.]. In this study, we sought to determine if rolipram exerts a neuroprotective effect in R6/2 mutant mice, which recapitulates, in many aspects, human HD [Mangiarini L., Sathasivam K., Seller M., Cozens B., Harper A., Hetherington C., Lawton M., Trottier Y., Lehrach H., Davies S.W. and Bates G.P. (1996) Exon 1 of the HD gene with an expanded CAG repeat is sufficient to cause a progressive neurological phenotype in transgenic mice. Cell. 87:493-506]. Transgenic mice were treated with rolipram 1.5 mg/kg daily starting from 4 weeks of age. After transcardial perfusion, histological and immunohistochemical studies were performed. We found that rolipram-treated R6/2 mice survived longer and displayed less severe signs of neurological dysfunction than the vehicle treated ones. Primary outcome measures such as brain volume, striatal atrophy, size and morphology of striatal neurons, neuronal intranuclear inclusions and microglial reaction confirmed a neuroprotective effect of the compound. Rolipram was effective in increasing significantly the levels of activated CREB and of BDNF the striatal spiny neurons, which might account for the beneficial effects observed in this model. Our findings show that rolipram could be considered as a valid therapeutic approach for HD.

BDNF and the diseased nervous system: a delicate balance between adaptive and pathological processes of gene regulation

It is clear that brain-derived neurotrophic factor (BDNF) plays a crucial role in organizing the response of the genome to dynamic changes in the extracellular environment that enable brain plasticity. BDNF has emerged as one of the most important signaling molecules for the developing nervous system as well as the impaired nervous system, and multiple diseases, such as Alzheimer's, Parkinson's, Huntington's, epilepsy, Rett's syndrome, and psychiatric depression, are linked by their association with potential dysregulation of BDNF-driven signal transduction programs. These programs are responsible for controlling the amount of activated transcription factors, such as cAMP response element binding protein, that coordinate the expression of multiple brain proteins, like ion channels and early growth response factors, whose job is to maintain the balance of excitation and inhibition in the nervous system. In this review, we will explore the evidence for BDNF's role in gene regulation side by side with its potential role in the etiology of neurological diseases. It is hoped that by bringing the datasets together in these diverse fields we can help develop the foundation for future studies aimed at understanding basic principles of gene regulation in the nervous system and how they can be harnessed to develop new therapeutic opportunities.

Beneficial effects of rolipram in a quinolinic acid model of striatal excitotoxicity

Activity of c-AMP responsive element-binding protein (CREB) is decreased in Huntington's disease (HD). Such decrease was also described by our group in the quinolinic acid lesion model of striatal excitotoxicity. The phosphodiesterase type IV inhibitor rolipram increases CREB phosphorylation. Such drug has a protective effect in global ischaemia and embolism in rats. In this study, we sought to determine whether rolipram displays a neuroprotective effect in our rat model of HD. Animals were surgically administered QA and subsequently treated with rolipram daily up to 2 and 8 weeks respectively. After these time points, rats were sacrificed and immunohistochemical studies were performed in the striata. In the rolipram-treated animals, striatal lesion size was about 62% smaller that in the vehicle-treated ones at 2 weeks time point. Moreover, the surviving cell number was several times higher in the rolipram-treated animals than in the vehicle group at both time points. Rolipram also showed to be effective in increasing significantly the levels of activated CREB in the striatal spiny neurons, which accounts mostly for its beneficial effect in our rodent model of excitotoxicity. Our findings show that rolipram could be considered as a valid therapeutic approach for HD.