An enzyme in the kynurenine pathway that governs vulnerability to suicidal behavior by regulating excitotoxicity and neuroinflammation.

Emerging evidence suggests that inflammation has a key role in depression and suicidal behavior. The kynurenine pathway is involved in neuroinflammation and regulates glutamate neurotransmission. In the cerebrospinal fluid (CSF) of suicidal patients, levels of inflammatory cytokines and the kynurenine metabolite quinolinic acid (QUIN), an N-methyl-d-aspartate receptor agonist, are increased. The enzyme amino-ß-carboxymuconate-semialdehyde-decarboxylase (ACMSD) limits QUIN formation by competitive production of the neuroprotective metabolite picolinic acid (PIC). Therefore, decreased ACMSD activity can lead to excess QUIN. We tested the hypothesis that deficient ACMSD activity underlies suicidal behavior. We measured PIC and QUIN in CSF and plasma samples from 137 patients exhibiting suicidal behavior and 71 healthy controls. We used DSM-IV and the Montgomery-Åsberg Depression Rating Scale and Suicide Assessment Scale to assess behavioral changes. Finally, we genotyped ACMSD tag single-nucleotide polymorphisms (SNPs) in 77 of the patients and 150 population-based controls. Suicide attempters had reduced PIC and a decreased PIC/QUIN ratio in both CSF (P<0.001) and blood (P=0.001 and P<0.01, respectively). The reductions of PIC in CSF were sustained over 2 years after the suicide attempt based on repeated measures. The minor C allele of the ACMSD SNP rs2121337 was more prevalent in suicide attempters and associated with increased CSF QUIN. Taken together, our data suggest that increased QUIN levels may result from reduced activity of ACMSD in suicidal subjects. We conclude that measures of kynurenine metabolites can be explored as biomarkers of suicide risk, and that ACMSD is a potential therapeutic target in suicidal behavior.

Review: Spreading the word: precise animal models and validated methods are vital when evaluating prion-like behaviour of alpha-synuclein.

Synucleinopathies are characterized by abnormal proteinaceous aggregates, mainly composed of fibrillar α-synuclein (α-syn). It is now believed that α-syn can form small aggregates in a restricted number of cells, that propagate to neighbouring cells and seed aggregation of endogenous α-syn, in a 'prion-like manner'. This process could underlie the stereotypical progression of Lewy bodies described by Braak and colleagues across different stages of Parkinson's disease (PD). This prion-like behaviour of α-syn has been recently investigated in animal models of PD or multiple system atrophy (MSA). These models investigate the cell-to-cell transfer of α-syn seeds, or the induction and spreading of α-syn pathology in transgenic or wild-type rodent brain. In this review, we first outline the involvement of α-syn in Lewy body diseases and MSA, and discuss how 'prion-like' mechanisms can contribute to disease. Thereon, we debate the relevance of animal models used to study prion-like propagation. Finally, we review current main histological methods used to assess α-syn pathology both in animal models and in human samples and their relevance to the disease. Specifically, we discuss using α-syn phosphorylated at serine 129 as a marker of pathology, and the novel methods available that allow for more sensitive detection of early pathology, which has relevance for modelling synucleinopathies.

Adipocytes derived from PA6 cells reliably promote the differentiation of dopaminergic neurons from human embryonic stem cells.

The PA6 stromal cell line comprises a heterogeneous population of cells that can induce both mouse and human embryonic stem cells to differentiate into dopaminergic neurons. This ability of PA6 cells has been termed stromal cell-derived inducing activity (SDIA). The level of SDIA has been found to vary considerably between and within batches of PA6 cells. Not only are the molecular mechanisms that underlie SDIA unknown but also the cell type(s) within the heterogeneous PA6 cultures that underlie SDIA remain poorly defined. In this study, we reveal that adipocytes, which are present within the heterogeneous PA6 cell population, robustly release the factors mediating SDIA. Furthermore, we report that the coculture of human embryonic stem cells with PA6-derived adipocytes reliably induces their differentiation into midbrain dopaminergic neurons.

Intracellular Nogo-A facilitates initiation of neurite formation in mouse midbrain neurons in vitro.

Nogo-A is a transmembrane protein originally discovered in myelin, produced by postnatal CNS oligodendrocytes. Nogo-A induces growth cone collapse and inhibition of axonal growth in the injured adult CNS. In the intact CNS, Nogo-A functions as a negative regulator of growth and plasticity. Nogo-A is also expressed by certain neurons. Neuronal Nogo-A depresses long-term potentiation in the hippocampus and modulates neurite adhesion and fasciculation during development in mice. Here we show that Nogo-A is present in neurons derived from human midbrain (Lund human mesencephalic (LUHMES) cell line), as well as in embryonic and postnatal mouse midbrain (dopaminergic) neurons. In LUHMES cells, Nogo-A was upregulated threefold upon differentiation and neurite extension. Nogo-A was localized intracellularly in differentiated LUHMES cells. Cultured midbrain (dopaminergic) neurons from Nogo-A knock-out mice exhibited decreased numbers of neurites and branches when compared with neurons from wild-type (WT) mice. However, this phenotype was not observed when the cultures from WT mice were treated with an antibody neutralizing plasma membrane Nogo-A. In vivo, neither the regeneration of nigrostriatal tyrosine hydroxylase fibers, nor the survival of nigral dopaminergic neurons after partial 6-hydroxydopamine lesions was affected by Nogo-A deletion. These results indicate that during maturation of cultured midbrain (dopaminergic) neurons, intracellular Nogo-A supports neurite growth initiation and branch formation.

The future of cell therapies and brain repair: Parkinson's disease leads the way.

During the past 40 years brain tissue grafting techniques have been used both to study fundamental neurobiological questions and to treat neurological diseases. Motor symptoms of Parkinson's disease are largely due to degeneration of midbrain dopamine neurones. Because the nigrostriatal pathology is relatively focused anatomically, Parkinson's disease is considered the ideal candidate for brain repair by neural grafting and dopamine neurone transplantation for it has led the way in the neural transplantation research field. In this mini-review, we briefly highlight four important areas of development. First, we describe marked functional benefits up to 18 years after transplantation surgery in patients with Parkinson's disease. This is proof-of-principle that, using optimal techniques and patient selection, grafted dopamine neurones can work in humans and the duration of the benefit exceeds placebo effects associated with surgery. Second, we describe that eventually protein aggregates containing α-synuclein, identical to Lewy bodies, develop inside foetal dopamine neurones transplanted to patients with Parkinson's disease. This gives clues about pathogenetic mechanisms operating in Parkinson's disease, and also raises the question whether neural graft function will eventually decline as the result of the disease process. Third, we describe new emerging sources of transplantable dopamine neurones derived from pluripotent stem cells or reprogrammed adult somatic cells. Fourth, we highlight an important European Union-funded multicentre clinical trial involving transplantation of foetal dopamine neurones in Parkinson's disease. We describe the design of this ongoing trial and how it can impact on the overall future of cell therapy in Parkinson's disease.

The executioners sing a new song: killer caspases activate microglia.

Activation of microglia and inflammation-mediated neurotoxicity are suggested to have key roles in the pathogenesis of several neurodegenerative disorders. We recently published an article in Nature revealing an unexpected role for executioner caspases in the microglia activation process. We showed that caspases 8 and 3/7, commonly known to have executioner roles for apoptosis, can promote microglia activation in the absence of death. We found these caspases to be activated in microglia of PD and AD subjects. Inhibition of this signaling pathway hindered microglia activation and importantly reduced neurotoxicity in cell and animal models of disease. Here we review evidence suggesting that microglia can have a key role in the pathology of neurodegenerative disorders. We discuss possible underlying mechanisms regulating their activation and neurotoxic effect. We focus on the provocative hypothesis that caspase inhibition can be neuroprotective by targeting the microglia rather than the neurons themselves.

IGF-1 protects against diabetic features in an in vivo model of Huntington's disease.

Huntington's disease (HD) is the most prevalent polyglutamine expansion disorder. HD is caused by an expansion of CAG triplet in the huntingtin (HTT) gene, associated with striatal and cortical neuronal loss. Central and peripheral metabolic abnormalities and altered insulin-like growth factor-1 (IGF-1) levels have been described in HD. Thus, we hypothesized that restoration of IGF-1-mediated signaling pathways could rescue R6/2 mice from metabolic stress and behavioral changes induced by polyglutamine expansion. We analyzed the in vivo effect of continuous peripheral IGF-1 administration on diabetic parameters, body weight and motor behavior in the hemizygous R6/2 mouse model of HD. We used 9 week-old and age-matched wild-type mice, subjected to continuously infused recombinant IGF-I or vehicle, for 14 days. IGF-1 treatment prevented the age-related decrease in body weight in R6/2 mice. Although blood glucose levels were higher in R6/2 mice, they did not reach a diabetic state. Even though, IGF-1 ameliorated poor glycemic control in HD mice. This seemed to be associated with a decrease in blood insulin levels in R6/2 mice, which was increased following IGF-1 infusion. Similarly, blood IGF-1 levels decreased during aging in both wild-type and R6/2 mice, being significantly improved upon its continuous infusion. Although no significant differences were found in motor function in R6/2-treated mice, IGF-1 treatment highly improved paw clasping scores. In summary, these results suggest that IGF-1 has a protective role against HD-associated impaired glucose tolerance, by enhancing blood insulin levels.

A deadly spread: cellular mechanisms of α-synuclein transfer.

Classically, Parkinson's disease (PD) is linked to dopamine neuron death in the substantia nigra pars compacta. Intracytoplasmic protein inclusions named Lewy bodies, and corresponding Lewy neurites found in neuronal processes, are also key features of the degenerative process in the substantia nigra. The molecular mechanisms by which substantia nigra dopamine neurons die and whether the Lewy pathology is directly involved in the cell death pathway are open questions. More recently, it has become apparent that Lewy pathology gradually involves greater parts of the PD brain and is widespread in late stages. In this review, we first discuss the role of misfolded α-synuclein protein, which is the main constituent of Lewy bodies, in the pathogenesis of PD. We then describe recent evidence that α-synuclein might transfer between cells in PD brains. We discuss in detail the possible molecular mechanisms underlying the proposed propagation and the likely consequences for cells that take up α-synuclein. Finally, we focus on aspects of the pathogenic process that could be targeted with new pharmaceutical therapies or used to develop biomarkers for early PD detection.

Neurogenesis in the R6/2 mouse model of Huntington's disease is impaired at the level of NeuroD1.

Adult neurogenesis is impaired in the hippocampus of transgenic R6 mouse models of Huntington's disease (HD). The phenotypes of R6 transgenic mice mimic several symptoms and signs of the disease (Li et al., 2005). They exhibit neurological and endocrine changes resembling some symptoms seen in humans. The reduction in neurogenesis is only apparent in the dentate gyrus as the number of newborn neurons in the subventricular zone, and olfactory bulb, is normal in R6 mice. The mechanism(s) underlying the reduction in hippocampal neurogenesis is still not fully understood. Here we show that the number of neuroblasts, but not granule neuron progenitors, is greatly reduced in 11-week old transgenic mice compared with wild-type (WT) controls. We demonstrate that NeuroD1 expression is reduced in the hippocampus. This is coupled to a decreased expression of downstream markers doublecortin and calretinin in maturing neurons. Taken together, our results suggest that mutant huntingtin (Htt) causes alterations of proteins expression in hippocampal progenitors, which might contribute to cognitive deficits in Huntington's disease.

Brain injury activates microglia that induce neural stem cell proliferation ex vivo and promote differentiation of neurosphere-derived cells into neurons and oligodendrocytes.

Brain damage, such as ischemic stroke, enhances proliferation of neural stem/progenitor cells (NSPCs) in the subventricular zone (SVZ). To date, no reliable in vitro systems, which can be used to unravel the potential mechanisms underlying this lesion-induced effect, have been established. Here, we developed an ex vivo method to investigate how the proliferation of NSPCs changes over time after experimental stroke or excitotoxic striatal lesion in the adult rat brain by studying the effects of microglial cells derived from an injured brain on NSPCs. We isolated NSPCs from the SVZ of brains with lesions and analyzed their growth and differentiation when cultured as neurospheres. We found that NSPCs isolated from the brains 1-2 weeks following injury consistently generated more and larger neurospheres than those harvested from naive brains. We attributed these effects to the presence of microglial cells in NSPC cultures that originated from injured brains. We suggest that the effects are due to released factors because we observed increased proliferation of NSPCs isolated from non-injured brains when they were exposed to conditioned medium from cultures containing microglial cells derived from injured brains. Furthermore, we found that NSPCs derived from injured brains were more likely to differentiate into neurons and oligodendrocytes than astrocytes. Our ex vivo system reliably mimics what is observed in vivo following brain injury. It constitutes a powerful tool that could be used to identify factors that promote NSPC proliferation and differentiation in response to injury-induced activation of microglial cells, by using tools such as proteomics and gene array technology.

Priming for L-DOPA-induced abnormal involuntary movements increases the severity of amphetamine-induced dyskinesia in grafted rats.

In some patients, graft-induced dyskinesia develops following intrastriatal transplantation of embryonic neural tissue for the treatment of Parkinson's disease. The mechanisms underlying these involuntary movements need to be clarified before this approach to clinical cell therapy can be developed further. We previously found that rats with 6-OHDA lesions, primed with L-DOPA treatment and that have subsequently undergone intrastriatal graft surgery exhibit involuntary movements when subjected to amphetamine. This model of amphetamine-induced AIMs reflects a pattern of post-graft behaviours that in the absence of robust spontaneous GID in the rat is the closest approximation that we currently have available. We now show that they are associated with the chronic administration of L-DOPA prior to the transplantation surgery. We also demonstrate that neither changes in c-fos nor FosB/DeltaFosB expression in the lateral striatum are associated with the expression of these behaviours. Taken together, these data reveal that the severity of abnormal movements elicited by amphetamine in grafted animals may relate to previous L-DOPA exposure and dyskinesia development, but they develop through mechanisms that are independent of FosB/DeltaFosB upregulation.

Weight loss in Huntington disease increases with higher CAG repeat number.

OBJECTIVE: Huntington disease (HD) is a hereditary neurodegenerative disorder caused by an expanded number of CAG repeats in the huntingtin gene. A hallmark of HD is unintended weight loss, the cause of which is unknown. In order to elucidate the underlying mechanisms of weight loss in HD, we studied its relation to other disease characteristics including motor, cognitive, and behavioral disturbances and CAG repeat number. METHODS: In 517 patients with early stage HD, we applied mixed-effects model analyses to correlate weight changes over 3 years to CAG repeat number and various components of the Unified Huntington's Disease Rating Scale (UHDRS). We also assessed the relation between CAG repeat number and body weight and caloric intake in the R6/2 mouse model of HD. RESULTS: In patients with HD, mean body mass index decreased with -0.15 units per year (p < 0.001). However, no single UHDRS component, including motor, cognitive, and behavioral scores, was independently associated with the rate of weight loss. Patients with HD with a higher CAG repeat number had a faster rate of weight loss. Similarly, R6/2 mice with a larger CAG repeat length had a lower body weight, whereas caloric intake increased with larger CAG repeat length. CONCLUSIONS: Weight loss in Huntington disease (HD) is directly linked to CAG repeat length and is likely to result from a hypermetabolic state. Other signs and symptoms of HD are unlikely to contribute to weight loss in early disease stages. Elucidation of the responsible mechanisms could lead to effective energy-based therapeutics.

Neuroinflammation in the generation of post-transplantation dyskinesia in Parkinson's disease.

The observation that neural grafts can induce dyskinesias has severely hindered the development of a transplantation therapy for Parkinson's disease (PD). We addressed the hypothesis that inflammatory responses within and around an intrastriatal graft containing dopamine neurons can trigger dyskinetic behaviors. We subjected rats to unilateral nigrostriatal lesions with 6-hydroxydopamine (6-OHDA) and treated them with L-DOPA for 21 days in order to induce abnormal involuntary movements (AIMs). Subsequently, we grafted the rats with allogeneic embryonic ventral mesencephalic tissue in the dopamine-denervated striatum. In agreement with earlier studies, the grafted rats developed dyskinesia-like AIMs in response to amphetamine. We then used two experimental approaches to induce an inflammatory response and examined if the amphetamine-induced AIMs worsened or if spontaneous AIMs developed. In one experiment, we challenged the neural graft hosts immunologically with an orthotopic skin allograft of the same genetic origin as the intracerebral neural allograft. In another experiment, we infused the pro-inflammatory cytokine interleukin 2 (IL-2) adjacent to the intrastriatal grafts using osmotic minipumps. The skin allograft induced rapid rejection of the mesencephalic allografts, leading to disappearance of the amphetamine-induced AIMs. Contrary to our hypothesis, the rejection process itself did not elicit AIMs. Likewise, the IL-2 infusion did not induce spontaneous AIMs, nor did it alter L-DOPA-induced AIMs. The IL-2 infusions did, however, elicit the predicted marked striatal inflammation, as evidenced by the presence of activated microglia and IL2Ralpha-positive cells. These results indicate that an inflammatory response in and around grafted dopaminergic neurons is not sufficient to evoke dyskinetic behaviors in experimental models of PD.

Mutant huntingtin can paradoxically protect neurons from death.

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a mutation in the gene huntingtin and characterized by motor, cognitive and psychiatric symptoms. Huntingtin contains a CAG repeat in exon 1. An expansion of this CAG repeat above 35 results in misfolding of Huntingtin, giving rise to protein aggregates and neuronal cell death. There are several transgenic HD mouse models that reproduce most of the features of the human disorder, for example protein inclusions, some neurodegeneration as well as motor and cognitive symptoms. At the same time, a subgroup of the HD transgenic mouse models exhibit dramatically reduced susceptibility to excitotoxicity. The mechanism behind this is unknown. Here, we review the literature regarding this phenomenon, attempt to explain what protein domains are crucial for this phenomenon and point toward a putative mechanism. We suggest, that the C-terminal domain of exon 1 Huntingtin, namely the proline rich domain, is responsible for mediating a neuroprotective effect against excitotoxicity. Furthermore, we point out the possible importance of this mechanism for future therapies in neurological disorders that have been suggested to be associated with excitotoxicity, for example Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis.

High-fat diet impairs hippocampal neurogenesis in male rats.

High fat diets and obesity pose serious health problems, such as type II diabetes and cardiovascular disease. Impaired cognitive function is also associated with high fat intake. In this study, we show that just 4 weeks of feeding a diet rich in fat ad libitum decreased hippocampal neurogenesis in male, but not female, rats. There was no obesity, but male rats fed a diet rich in fat exhibited elevated serum corticosterone levels compared with those fed standard rat chow. These data indicate that high dietary fat intake can disrupt hippocampal neurogenesis, probably through an increase in serum corticosterone levels, and that males are more susceptible than females.

Working memory training decreases hippocampal neurogenesis.

The relationship between adult hippocampal neurogenesis and cognition appears more complex than suggested by early reports. We aimed to determine if the duration and task demands of spatial memory training differentially affect hippocampal neurogenesis. Adult male rats were trained in the Morris water maze in a reference memory task for 4 days, or alternatively working memory for either 4 or 14 days. Four days of maze training did not impact neurogenesis regardless of whether reference or working memory paradigms were used. Interestingly, 2 weeks of working memory training using a hidden platform resulted in fewer newborn hippocampal neurons compared with controls that received either cue training or no maze exposure. Stress is a well-established negative regulator of hippocampal neurogenesis. We found that maze training in general, and a working memory task in particular, increased levels of circulating corticosterone after 4 days of training. Our study indicates that working memory training over a prolonged period of time reduces neurogenesis, and this reduction may partially be mediated by increased stress.

Therapeutic potential of controlled drug delivery systems in neurodegenerative diseases.

Several compounds that exhibit a therapeutic effect in experimental models of neurodegenerative diseases have been identified over recent years. Safe and effective drug delivery to the central nervous system is still one of the main obstacles in translating these experimental strategies into clinical therapies. Different approaches have been developed to enable drug delivery in close proximity to the desired site of action. In this review, we describe biodegradable polymeric systems as drug carriers in models of neurodegenerative diseases. Biomaterials described for intracerebral drug delivery are well tolerated by the host tissue and do not exhibit cytotoxic, immunologic, carcinogenic or teratogenic effects even after chronic exposure. Behavioral improvement and normalization of brain morphology have been observed following treatment using such biomaterials in animal models of Parkinson's, Alzheimer's and Huntington's diseases. Application of these devices for neuroactive drugs is still restricted due to the relatively small volume of tissue exposed to active compound. Further development of polymeric drug delivery systems will require that larger volumes of brain tissue are targeted, with a controlled and sustained drug release that is carefully controlled so it does not cause damage to the surrounding tissue.

The impact of graft size on the development of dyskinesia following intrastriatal grafting of embryonic dopamine neurons in the rat.

Intrastriatal transplants of embryonic ventral mesencephalon can cause dyskinesia in patients with Parkinson's disease (PD). We assessed the impact of transplant size on the development of graft-induced dyskinesia. Rats with unilateral 6-hydroxydopamine lesions were primed to exhibit L-DOPA-induced dyskinesia. They were then intrastriatally grafted with different quantities of embryonic ventral mesencephalic tissue to give small and large grafts. Without drug treatment, discrete dyskinetic-like movements were observed in most rats with large grafts 2-6 weeks after transplantation, but disappeared later. Amphetamine evoked severe abnormal involuntary movements (AIMs) in grafted animals, which were more striking with large grafts. The AIMs coincided with contralateral rotation, but displayed a different temporal profile and pharmacological properties. Thus, selective dopamine uptake blockade elicited rotational behavior, whereas coadministration of both dopamine and serotonin uptake blockers was required to evoke significant orolingual and limb AIMs. In conclusion, robust and reproducible AIMs were evoked in rats with large grafts by blockade of monoamine reuptake. These AIMs may provide a new tool for assessing dyskinetic effects of neural grafting.

Synaptic dysfunction in Huntington's disease: a new perspective.

Huntington's disease (HD) is caused by a polyglutamine expansion in the protein huntingtin and is characterized by intraneuronal inclusions and widespread neuronal death at the late stage of the disease. In research, most of the emphasis has been on understanding the cell death and its mechanisms. Until recently, it was believed that the vast majority, if not all, of the symptoms in HD are a direct consequence of neurodegeneration. However, increasing evidence shows that subtle alterations in synaptic function could underlie the early symptoms. It is of particular interest to understand the nature of this neuronal dysfunction. Normal huntingtin interacts with various cytoskeletal and synaptic vesicle proteins that are essential for exocytosis and endocytosis. Altered interactions of mutant huntingtin with its associated partners could contribute to abnormal synaptic transmission in HD. This review describes recent advances in understanding synaptic dysfunction in HD.