Mutant alpha-synuclein exacerbates age-related decrease of neurogenesis

The Role of Alpha-Synuclein in Neurodevelopmental Diseases

Neurodevelopmental disorders are a group of diseases with cognitive, motor, and emotional development deficits. Alpha-synuclein (α-syn) is a synaptic protein involved in transmission and neurodevelopment. This protein was previously shown to be associated with several disorders, including Parkinson's disease. Furthermore, a close link between neurodevelopmental disorders and Parkinson's has also been found. Changes in synaptic function have been noticed in neurodevelopmental disorders, including autism spectrum disorder. Impaired neurogenesis and related cognitive problems have been associated with altered expression of α-syn. Various studies reported α-syn in different body fluids and tissues such as blood and serum. Alpha-synuclein can help in better understanding the pathogenesis of neurodevelopmental diseases and facilitating their early diagnosis. This review aims to go over the recent advances in the role of α-syn in the pathophysiology of neurodevelopmental disorders, including autism spectrum disorder, attention deficit hyperactivity disorder, and motor and social impairment, and its value as a diagnostic biomarker.

Subthalamic nucleus but not entopeduncular nucleus deep brain stimulation enhances neurogenesis in the SVZ-olfactory bulb system of Parkinsonian rats

Introduction

Deep brain stimulation (DBS) is a highly effective treatment option in Parkinson's disease. However, the underlying mechanisms of action, particularly effects on neuronal plasticity, remain enigmatic. Adult neurogenesis in the subventricular zone-olfactory bulb (SVZ-OB) axis and in the dentate gyrus (DG) has been linked to various non-motor symptoms in PD, e.g., memory deficits and olfactory dysfunction. Since DBS affects several of these non-motor symptoms, we analyzed the effects of DBS in the subthalamic nucleus (STN) and the entopeduncular nucleus (EPN) on neurogenesis in 6-hydroxydopamine (6-OHDA)-lesioned hemiparkinsonian rats.

Methods

In our study, we applied five weeks of continuous bilateral STN-DBS or EPN-DBS in 6-OHDA-lesioned rats with stable dopaminergic deficits compared to 6-OHDA-lesioned rats with corresponding sham stimulation. We injected two thymidine analogs to quantify newborn neurons early after DBS onset and three weeks later. Immunohistochemistry identified newborn cells co-labeled with NeuN, TH and GABA within the OB and DG. As a putative mechanism, we simulated the electric field distribution depending on the stimulation site to analyze direct electric effects on neural stem cell proliferation.

Results

STN-DBS persistently increased the number of newborn dopaminergic and GABAergic neurons in the OB but not in the DG, while EPN-DBS does not impact neurogenesis. These effects do not seem to be mediated via direct electric stimulation of neural stem/progenitor cells within the neurogenic niches.

Discussion

Our data support target-specific effects of STN-DBS on adult neurogenesis, a putative modulator of non-motor symptoms in Parkinson's disease.

Neurogenesis in primates versus rodents and the value of non-human primate models

Neurogenesis, the process of generating neurons from neural stem cells, occurs during both embryonic and adult stages, with each stage possessing distinct characteristics. Dysfunction in either stage can disrupt normal neural development, impair cognitive functions, and lead to various neurological disorders. Recent technological advancements in single-cell multiomics and gene-editing have facilitated investigations into primate neurogenesis. Here, we provide a comprehensive overview of neurogenesis across rodents, non-human primates, and humans, covering embryonic development to adulthood and focusing on the conservation and diversity among species. While non-human primates, especially monkeys, serve as valuable models with closer neural resemblance to humans, we highlight the potential impacts and limitations of non-human primate models on both physiological and pathological neurogenesis research.

Restoration of Adult Neurogenesis by Intranasal Administration of Gangliosides GD3 and GM1 in The Olfactory Bulb of A53T Alpha-Synuclein-Expressing Parkinson's-Disease Model Mice

Parkinson's disease (PD) is the second most common neurodegenerative disorder affecting the body and mind of millions of people in the world. As PD progresses, bradykinesia, rigidity, and tremor worsen. These motor symptoms are associated with the neurodegeneration of dopaminergic neurons in the substantia nigra. PD is also associated with non-motor symptoms, including loss of smell (hyposmia), sleep disturbances, depression, anxiety, and cognitive impairment. This broad spectrum of non-motor symptoms is in part due to olfactory and hippocampal dysfunctions. These non-motor functions are suggested to be linked with adult neurogenesis. We have reported that ganglioside GD3 is required to maintain the neural stem cell (NSC) pool in the subventricular zone (SVZ) of the lateral ventricles and the subgranular layer of the dentate gyrus (DG) in the hippocampus. In this study, we used nasal infusion of GD3 to restore impaired neurogenesis in A53T alpha-synuclein-expressing mice (A53T mice). Intriguingly, intranasal GD3 administration rescued the number of bromodeoxyuridine + (BrdU +)/Sox2 + NSCs in the SVZ. Furthermore, the administration of gangliosides GD3 and GM1 increases doublecortin (DCX)-expressing immature neurons in the olfactory bulb, and nasal ganglioside administration recovered the neuronal populations in the periglomerular layer of A53T mice. Given the relevance of decreased ganglioside on olfactory impairment, we discovered that GD3 has an essential role in olfactory functions. Our results demonstrated that intranasal GD3 infusion restored the self-renewal ability of the NSCs, and intranasal GM1 infusion promoted neurogenesis in the adult brain. Using a combination of GD3 and GM1 has the potential to slow down disease progression and rescue dysfunctional neurons in neurodegenerative brains.

Mesenchymal stem cell-derived exosomes altered neuron cholesterol metabolism via Wnt5a-LRP1 axis and alleviated cognitive impairment in a progressive Parkinson's disease model

Parkinson's disease (PD) is associated with abnormal metabolism of brain cholesterol, and the metabolites of neuronal cholesterol may also affect neurodegenerative progression. In this study, we aim to explore the therapeutic effect of BMSC derived exosomes on motor and cognitive deficits in α-synuclein (α-Syn) A53T transgenic mice, a progressive PD animal model. Results revealed that rotating rod performance of α-Syn A53T TG mice decreased by 45.4 %±8.6 % at the age of 12 months compared with wide-type (WT) mice. Striatum injection of BMSC quiescent exosomes (BMSC^quiescent-EXO) and BMSC induced exosomes (BMSC^induced-EXO) rescued the rotation behavior (BMSC^quiescent-EXO: 92.3 %±12.5 % P = 0.008; BMSC^induced-EXO: 102.3 %±16.7 %, P = 0.006). Although there was no difference in the escape latency within 5 days of Morris water maze learning between groups in the 12-month old mice. The exploration latency was shorter (p < 0.05) in BMSC^quiescent-EXO and BMSC^induced-EXO groups, the number of explorations and novel object recognition index were significantly increased (p < 0.05). More importantly, the total cholesterol level was increased (p < 0.05), while the content of 24S-hydroxycholesterol significantly decreased (p < 0.05) after intrastriatal injection BMSC^quiescent-EXO and BMSC^induced-EXO in A53T group. Liquid chromatography-mass spectrometry (LC/MS) was performed to profile phospholipid metabolites in lipid raft of hippocampal neurons, demonstrating that BMSC^quiescent-EXO injection caused the decreasing relative percentages of phosphatidylglycerol (PG) and phosphatidylethanolamine (PE) compared to those in A53T mice, while the relative percentages of phosphatidylinositol (PI), phosphatidylserine (PS), and phosphatidylcholine (PC) increased. The cholesterol content of lipid rafts was lower in BMSC^quiescent-EXO and BMSC^induced-EXO groups than that in A53T group (P < 0.05). In summary, exosomes isolated during BMSC dopaminergic neuron differentiation can significantly improve the motor, learning and memory ability of the progressive PD mice model, and its mechanism may be related to the change of altered phospholipid composition and cholesterol metabolism in hippocampal neurons.

Mitochondrial function and dynamics in neural stem cells and neurogenesis: Implications for neurodegenerative diseases

Mitochondria have been largely described as the powerhouse of the cell and recent findings demonstrate that this organelle is fundamental for neurogenesis. The mechanisms underlying neural stem cells (NSCs) maintenance and differentiation are highly regulated by both intrinsic and extrinsic factors. Mitochondrial-mediated switch from glycolysis to oxidative phosphorylation, accompanied by mitochondrial remodeling and dynamics are vital to NSCs fate. Deregulation of mitochondrial proteins, mitochondrial DNA, function, fission/fusion and metabolism underly several neurodegenerative diseases; data show that these impairments are already present in early developmental stages and NSC fate decisions. However, little is known about mitochondrial role in neurogenesis. In this Review, we describe the recent evidence covering mitochondrial role in neurogenesis, its impact in selected neurodegenerative diseases, for which aging is the major risk factor, and the recent advances in stem cell-based therapies that may alleviate neurodegenerative disorders-related neuronal deregulation through improvement of mitochondrial function and dynamics.

Uric Acid Enhances Neurogenesis in a Parkinsonian Model by Remodeling Mitochondria

Background

Adult neurogenesis is the process of generating new neurons to enter neural circuits and differentiate into functional neurons. However, it is significantly reduced in Parkinson’s disease (PD). Uric acid (UA), a natural antioxidant, has neuroprotective properties in patients with PD. This study aimed to investigate whether UA would enhance neurogenesis in PD.

Methods

We evaluated whether elevating serum UA levels in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonian mouse model would restore neurogenesis in the subventricular zone (SVZ). For a cellular model, we primary cultured neural precursor cells (NPCs) from post-natal day 1 rat and evaluated whether UA treatment promoted cell proliferation against 1-methyl-4-phenylpyridinium (MPP⁺).

Results

Uric acid enhanced neurogenesis in both in vivo and in vitro parkinsonian model. UA-elevating therapy significantly increased the number of bromodeoxyuridine (BrdU)-positive cells in the SVZ of PD animals as compared to PD mice with normal UA levels. In a cellular model, UA treatment increased the expression of Ki-67. In the process of modulating neurogenesis, UA elevation up-regulated the expression of mitochondrial fusion markers.

Conclusion

In MPTP-induced parkinsonian model, UA probably enhanced neurogenesis via regulating mitochondrial dynamics, promoting fusion machinery, and inhibiting fission process.

Neurogenesis in aging and age-related neurodegenerative diseases

Adult neurogenesis, the process by which neurons are generated in certain areas of the adult brain, declines in an age-dependent manner and is one potential target for extending cognitive healthspan. Aging is a major risk factor for neurodegenerative diseases and, as lifespans are increasing, these health challenges are becoming more prevalent. An age-associated loss in neural stem cell number and/or activity could cause this decline in brain function, so interventions that reverse aging in stem cells might increase the human cognitive healthspan. In this review, we describe the involvement of adult neurogenesis in neurodegenerative diseases and address the molecular mechanistic aspects of neurogenesis that involve some of the key aggregation-prone proteins in the brain (i.e., tau, Aβ, α-synuclein, …). We summarize the research pertaining to interventions that increase neurogenesis and regulate known targets in aging research, such as mTOR and sirtuins. Lastly, we share our outlook on restoring the levels of neurogenesis to physiological levels in elderly individuals and those with neurodegeneration. We suggest that modulating neurogenesis represents a potential target for interventions that could help in the fight against neurodegeneration and cognitive decline.

Recent advances in the pathology of prodromal non-motor symptoms olfactory deficit and depression in Parkinson's disease: clues to early diagnosis and effective treatment

Parkinson’s disease (PD) is a progressive neurodegenerative disease characterized by movement dysfunction due to selective degeneration of dopaminergic neurons in the substantia nigra pars compacta. Non-motor symptoms of PD (e.g., sensory dysfunction, sleep disturbance, constipation, neuropsychiatric symptoms) precede motor symptoms, appear at all stages, and impact the quality of life, but they frequently go unrecognized and remain untreated. Even when identified, traditional dopamine replacement therapies have little effect. We discuss here the pathology of two PD-associated non-motor symptoms: olfactory dysfunction and depression. Olfactory dysfunction is one of the earliest non-motor symptoms in PD and predates the onset of motor symptoms. It is accompanied by early deposition of Lewy pathology and neurotransmitter alterations. Because of the correlation between olfactory dysfunction and an increased risk of progression to PD, olfactory testing can potentially be a specific diagnostic marker of PD in the prodromal stage. Depression is a prevalent PD-associated symptom and is often associated with reduced quality of life. Although the pathophysiology of depression in PD is unclear, studies suggest a causal relationship with abnormal neurotransmission and abnormal adult neurogenesis. Here, we summarize recent progress in the pathology of the non-motor symptoms of PD, aiming to provide better guidance for its effective management.

Mitochondrial and Autophagic Regulation of Adult Neurogenesis in the Healthy and Diseased Brain

Adult neurogenesis is a highly regulated process during which new neurons are generated from neural stem cells in two discrete regions of the adult brain: the subventricular zone of the lateral ventricle and the subgranular zone of the dentate gyrus in the hippocampus. Defects of adult hippocampal neurogenesis have been linked to cognitive decline and dysfunction during natural aging and in neurodegenerative diseases, as well as psychological stress-induced mood disorders. Understanding the mechanisms and pathways that regulate adult neurogenesis is crucial to improving preventative measures and therapies for these conditions. Accumulating evidence shows that mitochondria directly regulate various steps and phases of adult neurogenesis. This review summarizes recent findings on how mitochondrial metabolism, dynamics, and reactive oxygen species control several aspects of adult neural stem cell function and their differentiation to newborn neurons. It also discusses the importance of autophagy for adult neurogenesis, and how mitochondrial and autophagic dysfunction may contribute to cognitive defects and stress-induced mood disorders by compromising adult neurogenesis. Finally, I suggest possible ways to target mitochondrial function as a strategy for stem cell-based interventions and treatments for cognitive and mood disorders.

PINK1 deficiency impairs adult neurogenesis of dopaminergic neurons

Recent evidence suggests neurogenesis is on-going throughout life but the relevance of these findings for neurodegenerative disorders such as Parkinson's disease (PD) is poorly understood. Biallelic PINK1 mutations cause early onset, Mendelian inherited PD. We studied the effect of PINK1 deficiency on adult neurogenesis of dopaminergic (DA) neurons in two complementary model systems. Zebrafish are a widely-used model to study neurogenesis in development and through adulthood. Using EdU analyses and lineage-tracing studies, we first demonstrate that a subset of ascending DA neurons and adjacent local-projecting DA neurons are each generated into adulthood in wild type zebrafish at a rate that decreases with age. Pink1-deficiency impedes DA neurogenesis in these populations, most significantly in early adult life. Pink1 already exerts an early effect on Th1+ progenitor cells rather than on differentiated DA neurons only. In addition, we investigate the effect of PINK1 deficiency in a human isogenic organoid model. Global neuronal differentiation in PINK1-deficient organoids and isogenic controls is similar, but PINK1-deficient organoids display impeded DA neurogenesis. The observation of impaired adult dopaminergic neurogenesis in Pink1 deficiency in two complementing model systems may have significant consequences for future therapeutic approaches in human PD patients with biallelic PINK1 mutations.

Intracellular A53T Mutant α-Synuclein Impairs Adult Hippocampal Newborn Neuron Integration

Dendritic dysfunction is an early event in α-synuclein (α-syn) mediated neurodegeneration. Altered postsynaptic potential and loss of dendritic spines have been observed in different in vitro and in vivo models of synucleinopathies. The integration of newborn neurons into the hippocampus offers the possibility to study dendrite and spine formation in an adult environment. Specifically, survival of hippocampal adult newborn neurons is regulated by synaptic input and was reduced in a mouse model transgenic for human A53T mutant α-syn. We thus hypothesized that dendritic integration of newborn neurons is impaired in the adult hippocampus of A53T mice. We analyzed dendritic morphology of adult hippocampal neurons 1 month after retroviral labeling. Dendrite length was unchanged in the dentate gyrus of A53T transgenic mice. However, spine density and mushroom spine density of newborn neurons were severely decreased. In this mouse model, transgenic α-syn was expressed both within newborn neurons and within their environment. To specifically determine the cell autonomous effects, we analyzed cell-intrinsic overexpression of A53T α-syn using a retrovirus. Since A53T α-syn overexpressing newborn neurons exhibited decreased spine density 1 month after labeling, we conclude that cell-intrinsic A53T α-syn impairs postsynaptic integration of adult hippocampal newborn neurons. Our findings further support the role of postsynaptic degeneration as an early feature in synucleinopathies and provide a model system to study underlying mechanisms.

Developmental deficits and early signs of neurodegeneration revealed by PD patient derived dopamine neurons

Parkinson's disease (PD) is the second most common neurodegenerative disease affecting millions of elder people due to the degeneration of dopamine neurons in the striatum and substantia nigra. The clinical manifestations of PD include tremor, rigidity, bradykinesia and postural instability. Studying PD is challenging due to two obstacles: 1) disease models such as primary neurons or animal models usually couldn't recapitulate the disease phenotype, and 2) accessibility of human autopsied brain samples is very limited if not impossible. Induced pluripotent stem cells (iPSCs)-derived neuronal cells from patients emerge as an ideal in vitro model for disease modeling and drug development. Here we describe a cell density-dependent method for preparing functional hiPSC-derived dopamine neurons (iDAs) with ~90% purity (TH-positive cells). iDAs derived from PD patient exhibit the disease-related phenotypes, for example, slowed morphogenesis, reduced dopamine release, impaired mitochondrial function, and α-synuclein accumulation as early as 35 days after induction. Furthermore, we found that the effects of cell density are different between iDA development stages, whereas high cell density increases stress for early neural progenitor cells (NPCs), but are neural-protective for mature iDAs, high density also favors morphogenesis. Hence, using stage and density-dependent strategies we can obtain high quality iDAs, which are critical for disease modeling, drug development and cell replacement therapy.

The Role of Alpha-Synuclein and Other Parkinson's Genes in Neurodevelopmental and Neurodegenerative Disorders

Neurodevelopmental and late-onset neurodegenerative disorders present as separate entities that are clinically and neuropathologically quite distinct. However, recent evidence has highlighted surprising commonalities and converging features at the clinical, genomic, and molecular level between these two disease spectra. This is particularly striking in the context of autism spectrum disorder (ASD) and Parkinson's disease (PD). Genetic causes and risk factors play a central role in disease pathophysiology and enable the identification of overlapping mechanisms and pathways. Here, we focus on clinico-genetic studies of causal variants and overlapping clinical and cellular features of ASD and PD. Several genes and genomic regions were selected for our review, including SNCA (alpha-synuclein), PARK2 (parkin RBR E3 ubiquitin protein ligase), chromosome 22q11 deletion/DiGeorge region, and FMR1 (fragile X mental retardation 1) repeat expansion, which influence the development of both ASD and PD, with converging features related to synaptic function and neurogenesis. Both PD and ASD display alterations and impairments at the synaptic level, representing early and key disease phenotypes, which support the hypothesis of converging mechanisms between the two types of diseases. Therefore, understanding the underlying molecular mechanisms might inform on common targets and therapeutic approaches. We propose to re-conceptualize how we understand these disorders and provide a new angle into disease targets and mechanisms linking neurodevelopmental disorders and neurodegeneration.

Angiomotin links ROCK and YAP signaling in mechanosensitive differentiation of neural stem cells

Mechanical cues regulate the function of a broad range of stem cells in culture and in tissue. For example, soft substrates promote the neuronal differentiation of neural stem cells (NSCs) by suppressing cytoskeletal contractility. However, the mechanisms that link cytoskeletal signaling to the transcriptional regulatory processes that ultimately govern stiffness-dependent NSC fate commitment are not fully understood. Here, we show that Angiomotin (AMOT), which can bind both F-actin and the neurosuppressive transcriptional coactivator Yes-associated protein (YAP), is critical for mechanotransduction in NSCs. On soft substrates, loss of AMOT substantially reduces neurogenesis, whereas on stiff substrates, loss of AMOT negates the rescue of neurogenesis normally induced by pharmacologic inhibition of myosin activity. Furthermore, overexpression of a phospho-mimetic S175E AMOT mutant, which has been established to enhance AMOT–YAP binding, increases β-catenin activity and rescues neurogenesis on stiff substrates. Together, our data identify AMOT as an important intermediate signal transducer that allows NSCs to sense and respond to extracellular stiffness cues.

The A30P α-synuclein mutation decreases subventricular zone proliferation

Parkinson's disease (PD) is associated with olfactory defects in addition to dopaminergic degeneration. Dopaminergic signalling is necessary for subventricular zone (SVZ) proliferation and olfactory bulb (OB) neurogenesis. Alpha-synuclein (α-syn or Snca) modulates dopaminergic neurotransmission, and SNCA mutations cause familial PD, but how α-syn and its mutations affect adult neurogenesis is unclear. To address this, we studied a bacterial artificial chromosome transgenic mouse expressing the A30P SNCA familial PD point mutation on an Snca-/- background. We confirmed that the SNCA-A30P transgene recapitulates endogenous α-syn expression patterns and levels by immunohistochemical detection of endogenous α-syn in a wild-type mouse and transgenic SNCA-A30P α-syn protein in the forebrain. The number of SVZ stem cells (BrdU+GFAP+) was decreased in SNCA-A30P mice, whereas proliferating (phospho-histone 3+) cells were decreased in Snca-/- and even more so in SNCA-A30P mice. Similarly, SNCA-A30P mice had fewer Mash1+ transit-amplifying SVZ progenitor cells but Snca-/- mice did not. These data suggest the A30P mutation aggravates the effect of Snca loss in the SVZ. Interestingly, calbindin+ and calretinin (CalR)+ periglomerular neurons were decreased in both Snca-/-, and SNCA-A30P mice but tyrosine hydroxylase+ periglomerular OB neurons were only decreased in Snca-/- mice. Cell death decreased in the OB granule layer of Snca-/- and SNCA-A30P mice. In the same region, CalR+ numbers increased in Snca-/- and SNCA-A30P mice. Thus, α-syn loss and human A30P SNCA decrease SVZ proliferation, cell death in the OB and differentially alter interneuron numbers. Similar disruptions in human neurogenesis may contribute to the olfactory deficits, which are observed in PD.

Early signs of colonic inflammation, intestinal dysfunction, and olfactory impairments in the rotenone-induced mouse model of Parkinson's disease

The factors that trigger the pathophysiology of Parkinson's disease (PD) are unknown. However, it is suggested that environmental factors, such as exposure to pesticides, play an important role, in addition to genetic predisposition and aging. Early signs of PD can appear in the gastrointestinal (GI) tract and in the olfactory system, preceding the onset of motor impairments by many years. The present study assessed the effects of oral rotenone administration (30 mg/kg) in inducing GI and olfactory dysfunctions associated with PD in mice. Here we show that rotenone transiently increased myeloperoxidase activity within 24 h of administration. Leucocyte infiltration in the colon, associated with histological damage and disrupted GI motility, were observed following treatment with rotenone for 7 days. Moreover, 7 days of treatment with rotenone disrupted olfactory discrimination in mice without affecting social recognition ability. The presence of specific deficits in olfactory function occurred with a concomitant decrease in tyrosine hydroxylase-positive neurons and an increase in serotonin (5-hydroxytryptamine) turnover in the olfactory bulb. These findings suggest that in Swiss mice, exposure to rotenone induces GI and olfactory dysfunction involving immunological and neurotransmitter alterations, similar to early signs of PD. This provides further evidence for the involvement of the gut-brain axis in PD.

Stress-induced precocious aging in PD-patient iPSC-derived NSCs may underlie the pathophysiology of Parkinson's disease

Parkinson’s disease (PD) is an aging-related degenerative disorder arisen from the loss of dopaminergic neurons in substantia nigra. Although many genetic mutations have been implicated to be genetically linked to PD, the low incidence of familial PD carried with mutations suggests that there must be other factors such as oxidative stress, mitochondrial dysfunction, accumulation of misfolded proteins, and enhanced inflammation, which are contributable to the pathophysiology of PD. The major efforts of current research have been devoted to unravel the toxic effect of multiple factors, which directly cause the degeneration of dopaminergic neurons in adulthood. Until recently, several studies have demonstrated that NSCs had compromised proliferation and differentiation capacity in PD animal models or PD patient-derived iPS models, suggesting that the pathology of PD may be rooted in some cellular aberrations at early developmental stage but the mechanism remains to be elusive. Based on the early-onset PD patient-specific iPSCs, we found that PD-patient iPSC-derived NSCs were more susceptible to stress and became functionally compromised by radiation or oxidative insults. We further unraveled that stress-induced SIRT1 downregulation leading to autophagic dysfunction, which were responsible for these deficits in PD-NSCs. Mechanistically, we demonstrated that stress-induced activation of p38 MAPK suppressed SIRT1 expression, which in turn augmented the acetylation of multiple ATG proteins of autophagic complex and eventually led to autophagic deficits. Our studies suggest that early developmental deficits may, at least partially, contribute to the pathology of PD and provide a new avenue for developing better therapeutic interventions to PD.

Synaptic Regulator α-Synuclein in Dopaminergic Fibers Is Essentially Required for the Maintenance of Subependymal Neural Stem Cells

Synaptic protein α-synuclein (α-SYN) modulates neurotransmission in a complex and poorly understood manner and aggregates in the cytoplasm of degenerating neurons in Parkinson's disease. Here, we report that α-SYN present in dopaminergic nigral afferents is essential for the normal cycling and maintenance of neural stem cells (NSCs) in the brain subependymal zone of adult male and female mice. We also show that premature senescence of adult NSCs into non-neurogenic astrocytes in mice lacking α-SYN resembles the effects of dopaminergic fiber degeneration resulting from chronic exposure to 1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine or intranigral inoculation of aggregated toxic α-SYN. Interestingly, NSC loss in α-SYN-deficient mice can be prevented by viral delivery of human α-SYN into their sustantia nigra or by treatment with l-DOPA, suggesting that α-SYN regulates dopamine availability to NSCs. Our data indicate that α-SYN, present in dopaminergic nerve terminals supplying the subependymal zone, acts as a niche component to sustain the neurogenic potential of adult NSCs and identify α-SYN and DA as potential targets to ameliorate neurogenic defects in the aging and diseased brain.SIGNIFICANCE STATEMENT We report an essential role for the protein α-synuclein present in dopaminergic nigral afferents in the regulation of adult neural stem cell maintenance, identifying the first synaptic regulator with an implication in stem cell niche biology. Although the exact role of α-synuclein in neural transmission is not completely clear, our results indicate that it is required for stemness and the preservation of neurogenic potential in concert with dopamine.

Current understanding of the molecular mechanisms in Parkinson's disease: Targets for potential treatments

Gradual degeneration and loss of dopaminergic neurons in the substantia nigra, pars compacta and subsequent reduction of dopamine levels in striatum are associated with motor deficits that characterize Parkinson’s disease (PD). In addition, half of the PD patients also exhibit frontostriatal-mediated executive dysfunction, including deficits in attention, short-term working memory, speed of mental processing, and impulsivity. The most commonly used treatments for PD are only partially or transiently effective and are available or applicable to a minority of patients. Because, these therapies neither restore the lost or degenerated dopaminergic neurons, nor prevent or delay the disease progression, the need for more effective therapeutics is critical. In this review, we provide a comprehensive overview of the current understanding of the molecular signaling pathways involved in PD, particularly within the context of how genetic and environmental factors contribute to the initiation and progression of this disease. The involvement of molecular chaperones, autophagy-lysosomal pathways, and proteasome systems in PD are also highlighted. In addition, emerging therapies, including pharmacological manipulations, surgical procedures, stem cell transplantation, gene therapy, as well as complementary, supportive and rehabilitation therapies to prevent or delay the progression of this complex disease are reviewed.

Mutant Ataxin-1 Inhibits Neural Progenitor Cell Proliferation in SCA1

Spinocerebellar ataxia type 1 (SCA1) is a dominantly inherited neurodegenerative disease caused by the expansion of a polyglutamine (Q) repeat tract in the protein ataxin-1 (ATXN1). Beginning as a cerebellar ataxic disorder, SCA1 progresses to involve the cerebral cortex, hippocampus, and brainstem. Using SCA1 knock-in mice that mirror the complexity of the human disease, we report a significant decrease in the capacity of adult neuronal progenitor cells (NPCs) to proliferate. Remarkably, a decrease in NPCs proliferation can be observed in vitro, outside the degenerative milieu of surrounding neurons or glia, demonstrating that mutant ATXN1 acting cell autonomously within progenitor cells interferes with their ability to proliferate. Our findings suggest that compromised adult neurogenesis contributes to the progressive pathology of the disease particularly in areas such as the hippocampus and cerebral cortex where stem cells provide neurotropic factors and participate in adult neurogenesis. These findings not only shed light on the biology of the disease but also have therapeutic implications in any future stem cell-based clinical trials.

Synucleinopathies: common features and hippocampal manifestations

Parkinson's disease (PD), dementia with Lewy Bodies (DLB), and multiple system atrophy (MSA) are three major synucleinopathies characterized by α-synuclein-containing inclusions in the brains of patients. Because the cell types and brain structures that are affected vary markedly between the disorders, the patients have different clinical manifestations in addition to some overlapping symptoms, which are the basis for differential diagnosis. Cognitive impairment and depression associated with hippocampal dysfunction are frequently observed in these disorders. While various α-synuclein-containing inclusions are found in the hippocampal formation, increasing evidence supports that small α-synuclein aggregates or oligomers may be the real culprit, causing deficits in neurotransmission and neurogenesis in the hippocampus and related brain regions, which constitute the major mechanism for the hippocampal dysfunctions and associated neuropsychiatric manifestations in synucleinopathies.

Loss of PINK1 leads to metabolic deficits in adult neural stem cells and impedes differentiation of newborn neurons in the mouse hippocampus

Emerging evidence suggests that mitochondrial dynamics regulates adult hippocampal neurogenesis (AHN). Although abnormal AHN has been linked to depression, anxiety, and cognitive dysfunction, which are features of neurodegenerative conditions, including Parkinson's disease (PD), the impact of mitochondrial deficits on AHN have not been explored previously in a model of neurodegeneration. Here, we used PTEN-induced kinase 1-deficient (PINK1^-/- ) mice that lacked a mitochondrial kinase mutated in recessive familial PD. We show that mitochondrial defects, elevated glycolysis, and increased apoptosis are associated with impaired but not abrogated differentiation of PINK1-deficient neural stem cells (NSCs) in culture. In the dentate gyrus of PINK1^-/- mice, newly generated doublecortin-positive neurons show aberrant dendritic morphology, and their maturation is compromised compared with wild-type mice. In addition, in vivo labeling of NSCs with 5-ethynyl-2'-deoxyuridine shows that proliferating NSC numbers are normal, but the differentiation of NSCs to doublecortin-positive neuroblasts and mature NeuN⁺ neurons is impeded in PINK1^-/- mice. Finally, we demonstrate that home cage activity and corticosterone levels of PINK1^-/- mice are normal, thereby excluding reduced physical activity and increased stress as causes of neurogenesis defects. Our results reveal a new and important relationship between mitochondrial dysfunction and impaired AHN in a genetic PD model. Targeting mitochondrial function and metabolism to increase AHN may hold promise for the treatment of affective disorders and the mitigation of related symptoms in PD and other neurodegenerative conditions.-Agnihotri, S. K., Shen, R., Li, J., Gao, X., Büeler, H. Loss of PINK1 leads to metabolic deficits in adult neural stem cells and impedes differentiation of newborn neurons in the mouse hippocampus.

Distinct Pattern of Microgliosis in the Olfactory Bulb of Neurodegenerative Proteinopathies

The olfactory bulb (OB) shows early neuropathological hallmarks in numerous neurodegenerative diseases, for example, in Alzheimer's disease (AD) and Parkinson's disease (PD). The glomerular and granular cell layer of the OB is characterized by preserved cellular plasticity in the adult brain. In turn, alterations of this cellular plasticity are related to neuroinflammation such as microglia activation, implicated in the pathogenesis of AD and PD, as well as frontotemporal lobe degeneration (FTLD). To determine microglia proliferation and activation we analyzed ionized calcium binding adaptor molecule 1 (Iba1) expressing microglia in the glomerular and granular cell layer, and the olfactory tract of the OB from patients with AD, PD dementia/dementia with Lewy bodies (PDD/DLB), and FTLD compared to age-matched controls. The number of Iba1 and CD68 positive microglia associated with enlarged amoeboid microglia was increased particularly in AD, to a lesser extent in FTLD and PDD/DLB as well, while the proportion of proliferating microglia was not altered. In addition, cells expressing the immature neuronal marker polysialylated neural cell adhesion molecule (PSA-NCAM) were increased in the glomerular layer of PDD/DLB and FTLD cases only. These findings provide novel and detailed insights into differential levels of microglia activation in the OB of neurodegenerative diseases.

Alpha-Synuclein Suppresses Retinoic Acid-Induced Neuronal Differentiation by Targeting the Glycogen Synthase Kinase-3β/β-Catenin Signaling Pathway

Alpha-synuclein (α-SYN) is expressed during neuronal development and is mainly involved in the modulation of synaptic transmission. Missense mutations and amplifications of this gene have been associated with the pathogenesis of Parkinson's disease. Here, we evaluate whether α-SYN plays a detrimental role in the phenotypic and morphological regulation of neurons. We also identify the underlying mechanisms of this process in all-trans-retinoic acid (RA)-induced differentiated SH-SY5Y cells, which represents dopaminergic (DAergic) phenotype. Our results indicate that overexpression of wild-type or mutant A53T α-SYN attenuated the RA-induced upregulation of tyrosine hydroxylase and dopamine transporter as well as neurite outgrowth in SH-SY5Y cells. In addition, GSK-3β inactivation and downstream β-catenin stabilization were associated with RA-induced differentiation, which was attenuated by α-SYN. Moreover, protein phosphatase 2A was positively regulated by α-SYN and was implicated in the α-SYN-mediated interference with RA signaling. The results obtained from SH-SY5Y cells were verified in primary cultures of mesencephalic DAergic neurons from A53T α-SYN transgenic mice, which represent high levels of α-SYN and protein phosphatase 2A in the midbrain. The number and length of neurites in tyrosine hydroxylase-positive as well as Tau-positive cells from A53T α-SYN transgenic mice were significantly lower than those in littermate controls. The current results provide novel insight into the role of α-SYN in the regulation of neuronal differentiation, including DAergic neurons. Identifying the signaling pathway involved in the α-SYN-mediated dysregulation of neuronal differentiation could lead to a better understanding of the developmental processes underlying α-SYN-related pathologies and facilitate the discovery of specifically targeted therapeutics.

Neuropeptide Treatment with Cerebrolysin Enhances the Survival of Grafted Neural Stem Cell in an α-Synuclein Transgenic Model of Parkinson's Disease

Neuronal stem cell (NSC) grafts have been investigated as a potential neuro-restorative therapy in Parkinson’s disease (PD) but their use is compromised by the death of grafted cells. We investigated the use of Cerebrolysin (CBL), a neurotrophic peptide mixture, as an adjunct to NSC therapy in the α-synuclein (α-syn) transgenic (tg) model of PD. In vehicle-treated α-syn tg mice, there was decreased survival of NSCs. In contrast, CBL treatment enhanced the survival of NSCs in α-syn tg groups and ameliorated behavioral deficits. The grafted NSCs showed lower levels of terminal deoxynucleotidyl transferase dUTP nick end labeling positive cells in the CBL-treated mice when compared with vehicle-treated α-syn tg mice. No evidence of tumor growth was detected. Levels of α-syn were similar in the vehicle in CBL-treated tg mice. In conclusion, CBL treatment might be a potential adjuvant for therapeutic NSC grafting in PD.

Targeted methylation sequencing reveals dysregulated Wnt signaling in Parkinson disease

Parkinson disease (PD) is a progressive neurodegenerative movement disorder. Both environmental and genetic factors play important roles in PD etiology. A number of environmental toxins cause parkinsonism in human and animal models. Genetic studies of rare early onset familial PD cases resulted in identification of disease-linked mutations in multiple genes. Nevertheless, the potential interaction between environment and genetics in PD pathogenesis remains largely unknown. We hypothesized that environmental factors induce abnormal epigenetic regulation that is involved in the pathogenesis of both familial and sporadic PD. We determined the global methylation status of 80,000-110,000 CpG sites in each of the five sporadic PD patient brains and five age and postmodern interval matched control brains utilizing bisulfite padlock sequencing. Multiple genes involved in neurogenesis, particularly the ones in the Wnt signaling pathway, were hypermethylated in PD brains compared to their matched control brains. Consistent with the DNA methylation changes, marked reduction of protein expression was observed for four Wnt and neurogenesis related genes (FOXC1, NEURG2, SPRY1, and CTNNB1) in midbrain dopaminergic (DA) neurons of PD. The treatment of low concentration of 1-methyl-4-phenylpyridinium (MPP⁺) for cells resulted in downregulation of Wnt related genes. The study revealed an important link between the epigenetic disregulation of Wnt signaling and the pathogenesis and progression of PD.

Nestin-positive/SOX2-negative cells mediate adult neurogenesis of nigral dopaminergic neurons in mice

The primary clinical motor symptoms of Parkinson's disease (PD) result from loss of dopaminergic (DA) neurons in the substantia nigra (SN). Consequently, neurogenesis of this group of neurons in the adult brain has drawn considerable interest for the purpose of harnessing endogenous neurogenerative potential as well as devising better strategies for stem cell therapy for PD. However, the existence of adult neurogenesis for DA neurons within the SN remains controversial. To overcome technical and design limitations associated with previous studies, our group has developed a novel genetic mouse model for assessing adult nigral DA neurogenesis. This system utilizes transgenic mice that express a tamoxifen-activatable Cre recombinase (Cre(ERT2)) under the control of the neuronal progenitor cell promoters nestin or Sox2 leading to suppression of the DA neuron marker tyrosine hydroxylase (TH) via excision of exon 1 by flanking loxP sites in adult animals. This study reports that six months following initiation of a six week treatment with tamoxifen mice with nestin-mediated Th excision displayed a significant reduction in TH+ neurons in the SN. This finding indicates that nestin-expressing cells regenerate DA neurons within the SN of adult animals. Interestingly, no reduction was observed in TH+ cells following Sox2-mediated Th excision suggesting that a nestin+/SOX2- precursor cell population drives DA neurogenesis in the adult SN. This information represents a substantial leap in current knowledge of adult DA neurogenesis, will enable improved in vitro and in vivo modeling, as well as facilitate the harnessing of this process for therapeutic intervention for PD.

Altered machinery of protein synthesis is region- and stage-dependent and is associated with α-synuclein oligomers in Parkinson's disease

INTRODUCTION

Parkinson's disease (PD) is characterized by the accumulation of abnormal α-synuclein in selected regions of the brain following a gradient of severity with disease progression. Whether this is accompanied by globally altered protein synthesis is poorly documented. The present study was carried out in PD stages 1-6 of Braak and middle-aged (MA) individuals without alterations in brain in the substantia nigra, frontal cortex area 8, angular gyrus, precuneus and putamen.

RESULTS

Reduced mRNA expression of nucleolar proteins nucleolin (NCL), nucleophosmin (NPM1), nucleoplasmin 3 (NPM3) and upstream binding transcription factor (UBF), decreased NPM1 but not NPM3 nucleolar protein immunostaining in remaining neurons; diminished 18S rRNA, 28S rRNA; reduced expression of several mRNAs encoding ribosomal protein (RP) subunits; and altered protein levels of initiation factor eIF3 and elongation factor eEF2 of protein synthesis was found in the substantia nigra in PD along with disease progression. Although many of these changes can be related to neuron loss in the substantia nigra, selective alteration of certain factors indicates variable degree of vulnerability of mRNAs, rRNAs and proteins in degenerating sustantia nigra. NPM1 mRNA and 18S rRNA was increased in the frontal cortex area 8 at stage 5-6; modifications were less marked and region-dependent in the angular gyrus and precuneus. Several RPs were abnormally regulated in the frontal cortex area 8 and precuneus, but only one RP in the angular gyrus, in PD. Altered levels of eIF3 and eIF1, and decrease eEF1A and eEF2 protein levels were observed in the frontal cortex in PD. No modifications were found in the putamen at any time of the study except transient modifications in 28S rRNA and only one RP mRNA at stages 5-6. These observations further indicate marked region-dependent and stage-dependent alterations in the cerebral cortex in PD. Altered solubility and α-synuclein oligomer formation, assessed in total homogenate fractions blotted with anti-α-synuclein oligomer-specific antibody, was demonstrated in the substantia nigra and frontal cortex, but not in the putamen, in PD. Dramatic increase in α-synuclein oligomers was also seen in fluorescent-activated cell sorter (FACS)-isolated nuclei in the frontal cortex in PD.

CONCLUSIONS

Altered machinery of protein synthesis is altered in the substantia nigra and cerebral cortex in PD being the frontal cortex area 8 more affected than the angular gyrus and precuneus; in contrast, pathways of protein synthesis are apparently preserved in the putamen. This is associated with the presence of α-synuclein oligomeric species in total homogenates; substantia nigra and frontal cortex are enriched, albeit with different band patterns, in α-synuclein oligomeric species, whereas α-synuclein oligomers are not detected in the putamen.

Systemic Central Nervous System (CNS)-targeted Delivery of Neuropeptide Y (NPY) Reduces Neurodegeneration and Increases Neural Precursor Cell Proliferation in a Mouse Model of Alzheimer Disease

Neuropeptide Y (NPY) is one of the most abundant protein transmitters in the central nervous system with roles in a variety of biological functions including: food intake, cardiovascular regulation, cognition, seizure activity, circadian rhythms, and neurogenesis. Reduced NPY and NPY receptor expression is associated with numerous neurodegenerative disorders including Alzheimer disease (AD). To determine whether replacement of NPY could ameliorate some of the neurodegenerative and behavioral pathology associated with AD, we generated a lentiviral vector expressing NPY fused to a brain transport peptide (apoB) for widespread CNS delivery in an APP-transgenic (tg) mouse model of AD. The recombinant NPY-apoB effectively reversed neurodegenerative pathology and behavioral deficits although it had no effect on accumulation of Aβ. The subgranular zone of the hippocampus showed a significant increase in proliferation of neural precursor cells without further differentiation into neurons. The neuroprotective and neurogenic effects of NPY-apoB appeared to involve signaling via ERK and Akt through the NPY R1 and NPY R2 receptors. Thus, widespread CNS-targeted delivery of NPY appears to be effective at reversing the neuronal and glial pathology associated with Aβ accumulation while also increasing NPC proliferation. Overall, increased delivery of NPY to the CNS for AD might be an effective therapy especially if combined with an anti-Aβ therapeutic.

Impaired neurogenesis by HIV-1-Gp120 is rescued by genetic deletion of fatty acid amide hydrolase enzyme

Background and Purpose

The HIV-envelope glycoprotein Gp120 is involved in neuronal injury and is associated with neuro-AIDS pathogenesis in the brain. Endocannabinoids are important lipid ligands in the CNS regulating neural functions, and their degeneration is controlled by hydrolysing enzymes such as the fatty acid amide hydrolase (FAAH). Here, we examined whether in vivo genetic deletion of Faah gene prevents HIV-1 Gp120-mediated effects on neurogenesis.

Experimental Approach

We generated new GFAP/Gp120 transgenic (Tg) mice that have genetic deletion of Faah gene by mating glial fribillary acidic protein (GFAP)/Gp120 Tg mice with Faah−/− mice. Neurogenesis and cell death were assessed by immunocytochemical analysis.

Key Results

Endocannabinoid levels in the brain of the double GFAP/Gp120//Faah−/− mice were similar to those observed in Faah−/− mice. However, unlike the impaired neurogenesis observed in GFAP/Gp120 Tg mice and Faah−/− mice, these GFAP/Gp120//Faah-/ mice showed significantly improved neurogenesis in the hippocampus, indicated by a significant increase in neuroblasts and neuronal cells, an increase in BrdU⁺ cells and doublecortin positive cells (DCX⁺), and an increase in the number of PCNA. Furthermore, a significant decrease in astrogliosis and gliogenesis was observed in GFAP/Gp120//Faah−/−mice and neurogenesis was stimulated by neural progenitor cells (NPCs) and/or the newly formed NPC niches characterized by increased COX-2 expression and elevated levels of PGE₂.

Conclusions and Implications

In vivo genetic ablation of Faah, resulted in enhanced neurogenesis through modulation of the newly generated NPC niches in GFAP/Gp120//Faah−/− mice. This suggests a novel approach of using FAAH inhibitors to enhance neurogenesis in HIV-1 infected brain.

Adult neural progenitor cells from Huntington's disease mouse brain exhibit increased proliferation and migration due to enhanced calcium and ROS signals

OBJECTIVES

Huntington's disease (HD) is an inherited human neurodegenerative disorder characterized by uncontrollable movement, psychiatric disturbance and cognitive decline. Impaired proliferative/differentiational potentials of adult neural progenitor cells (ANPCs) have been thought to be a pathogenic mechanism involved in it. In this study, we aimed to elucidate intrinsic properties of ANPCs subjected to neurodegenerative condition in YAC128 HD mice.

MATERIALS AND METHODS

ANPCs were isolated from the SVZ regions of 4-month-old WT and YAC128 mice. Cell proliferation, migration and neuronal differentiation in vitro were compared between these two genotypes with/without Ca(2+) inhibitors or ROS scavenger treatments. Differences in ANPC proliferation and differentiation capabilities in vivo between the two genotypes were evaluated using Ki-67 and Doublecortin (DCX) immunofluorescence respectively.

RESULTS

Compared to WT ANPCs, YAC128 ANPCs had significantly enhanced cell proliferation, migration and neuronal differentiation in vitro, accompanied by increased Ca(2+) and ROS signals. Raised proliferation and migration in YAC128 ANPCs were abolished by Ca(2+) signalling antagonists and ROS scavenging. However, in vivo, HD ANPCs failed to show any elevated proliferation or differentiation.

CONCLUSIONS

Increased Ca(2+) signalling and higher level of ROS conferred HD ANPC enhancement of proliferation and migration potentials. However, the in vivo micro-environment did not support endogenous ANPCs to respond appropriately to neuronal loss in these YAC128 mouse brains.

Noncoding RNAs and neurobehavioral mechanisms in psychiatric disease

The human genome project has revolutionized our understanding of the underlying mechanisms in psychiatric disease. It is now abundantly clear that neurobehavioral phenotypes are epigenetically controlled by noncoding RNAs (ncRNAs). The microRNA (miRNA) class of ncRNAs are ubiquitously expressed throughout the brain and govern all major neuronal pathways. The attractive therapeutic potential of miRNAs is underscored by their pleiotropic capacities, putatively targeting multiple pathways within a single neuron. Many psychiatric diseases stem from a multifactorial origin, thus conventional drug targeting of single proteins may not prove most effective. In this exciting post-genome sequencing era, many new epigenetic targets are emerging for therapeutic investigation. Here we review the reported roles of miRNAs, as well as other ncRNA classes, in the pathology of psychiatric disorders; there are both common and unique ncRNA mechanisms that influence the various diagnoses. Collectively, these potent epigenetic regulators may clarify the disrupted signaling networks in psychiatric phenotypes.

The temporal expression pattern of alpha-synuclein modulates olfactory neurogenesis in transgenic mice

Background

Adult neurogenesis mirrors the brain´s endogenous capacity to generate new neurons throughout life. In the subventricular zone/ olfactory bulb system adult neurogenesis is linked to physiological olfactory function and has been shown to be impaired in murine models of neuronal alpha-Synuclein overexpression. We analyzed the degree and temporo-spatial dynamics of adult olfactory bulb neurogenesis in transgenic mice expressing human wild-type alpha-Synuclein (WTS) under the murine Thy1 (mThy1) promoter, a model known to have a particularly high tg expression associated with impaired olfaction.

Results

Survival of newly generated neurons (NeuN-positive) in the olfactory bulb was unchanged in mThy1 transgenic animals. Due to decreased dopaminergic differentiation a reduction in new dopaminergic neurons within the olfactory bulb glomerular layer was present. This is in contrast to our previously published data on transgenic animals that express WTS under the control of the human platelet-derived growth factor β (PDGF) promoter, that display a widespread decrease in survival of newly generated neurons in regions of adult neurogenesis, resulting in a much more pronounced neurogenesis deficit. Temporal and quantitative expression analysis using immunofluorescence co-localization analysis and Western blots revealed that in comparison to PDGF transgenic animals, in mThy1 transgenic animals WTS is expressed from later stages of neuronal maturation only but at significantly higher levels both in the olfactory bulb and cortex.

Conclusions

The dissociation between higher absolute expression levels of alpha-Synuclein but less severe impact on adult olfactory neurogenesis in mThy1 transgenic mice highlights the importance of temporal expression characteristics of alpha-Synuclein on the maturation of newborn neurons.

Young systemic factors as a medicine for age-related neurodegenerative diseases

It is widely known that neurogenesis, brain function and cognition decline with aging. Increasing evidence suggests that cerebrovascular dysfunction is a major cause of cognitive impairment in the elderly but is also involved in age-related neurodegenerative diseases. Finding ways and molecules that reverse this process and ameliorate age- and disease-related cognitive impairment by targeting vascular and neurogenic deterioration would be of great therapeutic value. In Katsimpardi et al. we reported that young blood has a dual beneficial effect in the aged brain by restoring age-related decline in neurogenesis as well as inducing a striking remodeling of the aged vasculature and restoring blood flow to youthful levels. Additionally, we identified a youthful systemic factor, GDF11 that recapitulates these beneficial effects of young blood. We believe that the identification of young systemic factors that can rejuvenate the aged brain opens new roads to therapeutic intervention for neurodegenerative diseases by targeting both neural stem cells and neurogenesis as well as at the vasculature.

Neural stem cells in Parkinson's disease: a role for neurogenesis defects in onset and progression

Parkinson's disease (PD) is the second most common neurodegenerative disorder, leading to a variety of motor and non-motor symptoms. Interestingly, non-motor symptoms often appear a decade or more before the first signs of motor symptoms. Some of these non-motor symptoms are remarkably similar to those observed in cases of impaired neurogenesis and several PD-related genes have been shown to play a role in embryonic or adult neurogenesis. Indeed, animal models deficient in Nurr1, Pitx3, SNCA and PINK1 display deregulated embryonic neurogenesis and LRRK2 and VPS35 have been implicated in neuronal development-related processes such as Wnt/β-catenin signaling and neurite outgrowth. Moreover, adult neurogenesis is affected in both PD patients and PD animal models and is regulated by dopamine and dopaminergic (DA) receptors, by chronic neuroinflammation, such as that observed in PD, and by differential expression of wild-type or mutant forms of PD-related genes. Indeed, an increasing number of in vivo studies demonstrate a role for SNCA and LRRK2 in adult neurogenesis and in the generation and maintenance of DA neurons. Finally, the roles of PD-related genes, SNCA, LRRK2, VPS35, Parkin, PINK1 and DJ-1 have been studied in NSCs, progenitor cells and induced pluripotent stem cells, demonstrating a role for some of these genes in stem/progenitor cell proliferation and maintenance. Together, these studies strongly suggest a link between deregulated neurogenesis and the onset and progression of PD and present strong evidence that, in addition to a neurodegenerative disorder, PD can also be regarded as a developmental disorder.

Modulation of neurogenesis by targeting epigenetic enzymes using small molecules: an overview

Neurogenesis consists of a plethora of complex cellular processes including neural stem cell (NSC) proliferation, migration, maturation or differentiation to neurons, and finally integration into the pre-existing neural circuits in the brain, which are temporally regulated and coordinated sequentially. Mammalian neurogenesis begins during embryonic development and continues in postnatal brain (adult neurogenesis). It is now evident that adult neurogenesis is driven by extracellular and intracellular signaling pathways, where epigenetic modifications like reversible histone acetylation, methylation, as well as DNA methylation play a vital role. Epigenetic regulation of gene expression during neural development is governed mainly by histone acetyltransferases (HATs), histone methyltransferase (HMTs), DNA methyltransferases (DNMTs), and also the enzymes for reversal, like histone deacetylases (HDACs), and many of these have also been shown to be involved in the regulation of adult neurogenesis. The contribution of these epigenetic marks to neurogenesis is increasingly being recognized, through knockout studies and small molecule modulator based studies. These small molecules are directly involved in regeneration and repair of neurons, and not only have applications from a therapeutic point of view, but also provide a tool to study the process of neurogenesis itself. In the present Review, we will focus on small molecules that act predominantly on epigenetic enzymes to enhance neurogenesis and neuroprotection and discuss the mechanism and recent advancements in their synthesis, targeting, and biology.

Thoracic rat spinal cord contusion injury induces remote spinal gliogenesis but not neurogenesis or gliogenesis in the brain

After spinal cord injury, transected axons fail to regenerate, yet significant, spontaneous functional improvement can be observed over time. Distinct central nervous system regions retain the capacity to generate new neurons and glia from an endogenous pool of progenitor cells and to compensate neural cell loss following certain lesions. The aim of the present study was to investigate whether endogenous cell replacement (neurogenesis or gliogenesis) in the brain (subventricular zone, SVZ; corpus callosum, CC; hippocampus, HC; and motor cortex, MC) or cervical spinal cord might represent a structural correlate for spontaneous locomotor recovery after a thoracic spinal cord injury. Adult Fischer 344 rats received severe contusion injuries (200 kDyn) of the mid-thoracic spinal cord using an Infinite Horizon Impactor. Uninjured rats served as controls. From 4 to 14 days post-injury, both groups received injections of bromodeoxyuridine (BrdU) to label dividing cells. Over the course of six weeks post-injury, spontaneous recovery of locomotor function occurred. Survival of newly generated cells was unaltered in the SVZ, HC, CC, and the MC. Neurogenesis, as determined by identification and quantification of doublecortin immunoreactive neuroblasts or BrdU/neuronal nuclear antigen double positive newly generated neurons, was not present in non-neurogenic regions (MC, CC, and cervical spinal cord) and unaltered in neurogenic regions (dentate gyrus and SVZ) of the brain. The lack of neuronal replacement in the brain and spinal cord after spinal cord injury precludes any relevance for spontaneous recovery of locomotor function. Gliogenesis was increased in the cervical spinal cord remote from the injury site, however, is unlikely to contribute to functional improvement.

Adult hippocampal neurogenesis in Parkinson's disease: impact on neuronal survival and plasticity

In Parkinson's disease (PD) and other synucleinopathies, chronic neurodegeneration occurs within different areas of the central nervous system leading to progressive motor and nonmotor symptoms. The symptomatic treatment options that are currently available do not slow or halt disease progression. This highlights the need of a better understanding of disease mechanisms and disease models. The generation of newborn neurons in the adult hippocampus and in the subventricular zone/olfactory bulb system is affected by many different regulators and possibly involved in memory processing, depression, and olfaction, symptoms which commonly occur in PD. The pathology of the adult neurogenic niches in human PD patients is still mostly elusive, but different preclinical models have shown profound alterations of adult neurogenesis. Alterations in stem cell proliferation, differentiation, and survival as well as neurite outgrowth and spine formation have been related to different aspects in PD pathogenesis. Therefore, neurogenesis in the adult brain provides an ideal model to study disease mechanisms and compounds. In addition, adult newborn neurons have been proposed as a source of endogenous repair. Herein, we review current knowledge about the adult neurogenic niches in PD and highlight areas of future research.

Chronic administration of cholesterol oximes in mice increases transcription of cytoprotective genes and improves transcriptome alterations induced by alpha-synuclein overexpression in nigrostriatal dopaminergic neurons

Cholesterol-oximes TRO19622 and TRO40303 target outer mitochondrial membrane proteins and have beneficial effects in preclinical models of neurodegenerative diseases leading to their advancement to clinical trials. Dopaminergic neurons degenerate in Parkinson's disease (PD) and are prone to oxidative stress and mitochondrial dysfunction. In order to provide insights into the neuroprotective potential of TRO19622 and TRO40303 for dopaminergic neurons in vivo, we assessed their effects on gene expression in laser captured nigrostriatal dopaminergic neurons of wildtype mice and of mice that over-express alpha-synuclein, a protein involved in both familial and sporadic forms of PD (Thy1-aSyn mice). Young mice were fed the drugs in food pellets or a control diet from 1 to 4months of age, approximately 10months before the appearance of striatal dopamine loss in this model. Unbiased weighted gene co-expression network analysis (WGCNA) of transcriptional changes revealed effects of cholesterol oximes on transcripts related to mitochondria, cytoprotection and anti-oxidant response in wild-type and transgenic mice, including increased transcription of stress defense (e.g. Prdx1, Prdx2, Glrx2, Hspa9, Pink1, Drp1, Trak1) and dopamine-related (Th, Ddc, Gch1, Dat, Vmat2, Drd2, Chnr6a) genes. Even at this young age transgenic mice showed alterations in transcripts implicated in mitochondrial function and oxidative stress (e.g. Bcl-2, Bax, Casp3, Nos2), and both drugs normalized about 20% of these alterations. Young Thy1-aSyn mice exhibit motor deficits that differ from parkinsonism and are established before the onset of treatment; these deficits were not improved by cholesterol oximes. However, high doses of TRO40303 improved olfaction and produced the same effects as dopamine agonists on a challenging beam test, specifically an increase in footslips, an observation congruent with its effects on transcripts involved in dopamine synthesis. High doses of TRO19622 increased alpha-synuclein aggregates in the substantia nigra; this effect, not seen with TRO40303 was inconsistent and may represent a protective mechanism as in other neurodegenerative diseases. Overall, the results suggest that cholesterol oximes, while not improving early effects of alpha-synuclein overexpression on motor behavior or pathology, may ameliorate the function and resilience of dopaminergic neurons in vivo and support further studies of neuroprotection in models with dopaminergic cell loss.

Olfactory bulb involvement in neurodegenerative diseases

Olfactory dysfunction is a common and early symptom of many neurodegenerative diseases, particularly of Parkinson's disease and other synucleinopathies, Alzheimer's disease (AD), and mild cognitive impairment heralding its progression to dementia. The neuropathologic changes of olfactory dysfunction in neurodegenerative diseases may involve the olfactory epithelium, olfactory bulb/tract, primary olfactory cortices, and their secondary targets. Olfactory dysfunction is related to deposition of pathological proteins, α-synuclein, hyperphosphorylated tau protein, and neurofilament protein in these areas, featured by neurofibrillary tangles, Lewy bodies and neurites inducing a complex cascade of molecular processes including oxidative damage, neuroinflammation, and cytosolic disruption of cellular processes leading to cell death. Damage to cholinergic, serotonergic, and noradrenergic systems is likely involved, since such damage is most marked in those diseases with severe anosmia. Recent studies of olfactory dysfunction have focused its potential as an early biomarker for the diagnosis of neurodegenerative disorders and their disease progression. Here, we summarize the current knowledge on neuropathological and pathophysiological changes of the olfactory system in the most frequent neurodegenerative diseases, in particular AD and synucleinopathies. We also present neuropathological findings in the olfactory bulb and tract in a large autopsy cohort (n = 536, 57.8 % female, mean age 81.3 years). The severity of olfactory bulb HPτ, Aβ, and αSyn pathology correlated and increased significantly (P < 0.001) with increasing neuritic Braak stages, Thal Aβ phases, and cerebral Lewy body pathology, respectively. Hence, further studies are warranted to investigate the potential role of olfactory biopsies (possibly restricted to the olfactory epithelium) in the diagnostic process of neurodegenerative diseases in particular in clinical drug trials to identify subjects showing early, preclinical stages of neurodegeneration and to stratify clinically impaired cohorts according to the underlying cerebral neuropathology.

A30P α-Synuclein interferes with the stable integration of adult-born neurons into the olfactory network

Impaired olfaction is an early symptom in Parkinson disease (PD), although the exact cause is as yet unknown. Here, we investigated the link between PD-related mutant α-Synuclein (α-SYN) pathology and olfactory deficit, by examining the integration of adult-born neurons in the olfactory bulb (OB) of A30P α-SYN overexpressing mice. To this end, we chose to label one well-known vulnerable subpopulation of adult-born cells, the dopaminergic neurons. Using in vivo two-photon imaging, we followed the dynamic process of neuronal turnover in transgenic A30P α-SYN and wild-type mice over a period of 2.5 months. Our results reveal no difference in the number of cells that reach, and possibly integrate into, the glomerular layer in the OB. However, in mutant transgenic mice these new neurons have a significantly shortened survival, resulting in an overall reduction in the addition of neurons to the glomerular layer over time. We therefore propose unstable integration and impaired homeostasis of functional new neurons as a likely contributor to odour discrimination deficits in mutant α-SYN mice.

Smad3 is required for the survival of proliferative intermediate progenitor cells in the dentate gyrus of adult mice

Background

New neurons are continuously being generated in the adult hippocampus, a phenomenon that is regulated by external stimuli, such as learning, memory, exercise, environment or stress. However, the molecular mechanisms underlying neuron production and how they are integrated into existing circuits under such physiological conditions remain unclear. Indeed, the intracellular modulators that transduce the extracellular signals are not yet fully understood.

Results

We show that Smad3, an intracellular molecule involved in the transforming growth factor (TGF)-β signaling cascade, is strongly expressed by granule cells in the dentate gyrus (DG) of adult mice, although the loss of Smad3 in null mutant mice does not affect their survival. Smad3 is also expressed by adult progenitor cells in the subgranular zone (SGZ) and more specifically, it is first expressed by Type 2 cells (intermediate progenitor cells). Its expression persists through the distinct cell stages towards that of the mature neuron. Interestingly, proliferative intermediate progenitor cells die in Smad3 deficiency, which is associated with a large decrease in the production of newborn neurons in Smad3 deficient mice. Smad3 signaling appears to influence adult neurogenesis fulfilling distinct roles in the rostral and mid-caudal regions of the DG. In rostral areas, Smad3 deficiency increases proliferation and promotes the cell cycle exit of undifferentiated progenitor cells. By contrast, Smad3 deficiency impairs the survival of newborn neurons in the mid-caudal region of the DG at early proliferative stages, activating apoptosis of intermediate progenitor cells. Furthermore, long-term potentiation (LTP) after high frequency stimulation (HFS) to the medial perforant path (MPP) was abolished in the DG of Smad3-deficient mice.

Conclusions

These data show that endogenous Smad3 signaling is central to neurogenesis and LTP induction in the adult DG, these being two forms of hippocampal brain plasticity related to learning and memory that decline with aging and as a result of neurological disorders.

Antidepressants reduce neuroinflammatory responses and astroglial alpha-synuclein accumulation in a transgenic mouse model of multiple system atrophy

Multiple system atrophy (MSA) is a neurodegenerative disease characterized by the pathological accumulation of alpha-synuclein (α-syn) within oligodendroglial cells. This accumulation is accompanied by neuroinflammation with astrogliosis and microgliosis, that leads to neuronal death and subsequent parkinsonism and dysautonomia. Antidepressants have been explored as neuroprotective agents as they normalize neurotrophic factor levels, increase neurogenesis and reduce neurodegeneration, but their anti-inflammatory properties have not been fully characterized. We analyzed the anti-inflammatory profiles of three different antidepressants (fluoxetine, olanzapine and amitriptyline) in the MBP1-hα-syn transgenic (tg) mouse model of MSA. We observed that antidepressant treatment decreased the number of α-syn-positive cells in the basal ganglia of 11-month-old tg animals. This reduction was accompanied with a similar decrease in the colocalization of α-syn with astrocyte markers in this brain structure. Consistent with these results, antidepressants reduced astrogliosis in the hippocampus and basal ganglia of the MBP1-hα-syn tg mice, and modulated the expression levels of key cytokines that were dysregulated in the tg mouse model, such as IL-1β. In vitro experiments in the astroglial cell line C6 confirmed that antidepressants inhibited NF-κB translocation to the nucleus and reduced IL-1β protein levels. We conclude that the anti-inflammatory properties of antidepressants in the MBP1-hα-syn tg mouse model of MSA might be related to their ability to inhibit α-syn propagation from oligodendrocytes to astroglia and to regulate transcription factors involved in cytokine expression. Our results suggest that antidepressants might be of interest as anti-inflammatory and α-syn-reducing agents for MSA and other α-synucleinopathies.

Glial A30P alpha-synuclein pathology segregates neurogenesis from anxiety-related behavior in conditional transgenic mice

In Parkinson's disease (PD) patients, alpha-synuclein (α-syn) pathology advances in form of Lewy bodies and Lewy neurites throughout the brain. Clinically, PD is defined by motor symptoms that are predominantly attributed to the dopaminergic cell loss in the substantia nigra. However, motor deficits are frequently preceded by smell deficiency or neuropsychological symptoms, including increased anxiety and cognitive dysfunction. Accumulating evidence indicates that aggregation of α-syn impairs synaptic function and neurogenic capacity that may be associated with deficits in memory, learning and mood. Whether and how α-syn accumulation contributes to neuropathological events defining these earliest signs of PD is presently poorly understood. We used a tetracycline-suppressive (tet-off) transgenic mouse model that restricts overexpression of human A30P α-syn to neurons owing to usage of the neuron-specific CaMKIIα promoter. Abnormal accumulation of A30P correlated with a decreased survival of newly generated neurons in the hippocampus and olfactory bulb. Furthermore, when A30P α-syn expression was suppressed, we observed reduction of the human protein in neuronal soma. However, residual dox resistant A30P α-syn was detected in glial cells within the hippocampal neurogenic niche, concomitant with the failure to fully restore hippocampal neurogenesis. This finding is indicative to a potential spread of pathology from neuron to glia. In addition, mice expressing A30P α-syn show increased anxiety-related behavior that was reversed after dox treatment. This implies that glial A30P α-synucleinopathy within the dentate gyrus is part of a process leading to impaired hippocampal neuroplasticity, which is, however, not a sole critical event for circuits implicated in anxiety-related behavior.

Neurodegeneration in Parkinson's disease: interactions of oxidative stress, tryptophan catabolites and depression with mitochondria and sirtuins

The biological underpinnings to the etiology and course of neurodegeneration in Parkinson's disease are an area of extensive research that has yet to produce an early biological marker or disease-slowing or preventative treatment. Recent conceptualizations of Parkinson's disease have integrated immuno-inflammation and oxidative and nitrosative stress occurring in depression, somatization and peripheral inflammation into the course of Parkinson's disease. We review the data showing the importance of immuno-inflammatory processes and oxidative and nitrosative stress in such classically conceived 'comorbidities', suggesting that lifetime, prodromal and concurrent depression and somatization may be intricately involved in the etiology and course of Parkinson's disease, rather than psychiatric comorbidities. This produces a longer term developmental perspective of Parkinson's disease, which incorporates tryptophan catabolites (TRYCATs), lipid peroxidation, sirtuins, cyclic adenosine monophosphate, aryl hydrocarbon receptor, and circadian genes. This integrates wider bodies of data pertaining to neuronal loss in Parkinson's disease, emphasizing how these interact with susceptibility genes to drive changes in mitochondria, blood-brain barrier permeability and intercellular signalling. We review this data here in the context of neurodegeneration in Parkinson's disease and to the future directions indicated for slowing disease progression.

Resident adult neural stem cells in Parkinson's disease--the brain's own repair system?

One important pathological process in the brain of Parkinson disease (PD) patients is the degeneration of the dopaminergic neurons in the substantia nigra, which leads to a decline in striatal dopamine levels and motor dysfunction. A major clinical problem is that this degenerative process currently cannot be stopped or reversed. Expectations from the restorative capacity of neural stem cells (NSCs) are high, as these cells can potentially replace the degenerating neurons. The discovery of the presence of NSCs in the adult human brain has instigated research into the potential of these cells as a resource to promote brain repair in neurodegenerative diseases. Neural stem and progenitor cells reside in the subventricular zone (SVZ), which is closely situated to the striatum, which is affected in PD. Therefore, restoring the dopamine levels in the striatum of PD patients through stimulating endogenous NSCs in the nearby SVZ to migrate into the striatum and differentiate into dopaminergic neurons might thus be an attractive future therapeutic approach. We will review the reported changes in NSCs in the SVZ of PD animal models and PD patients, which are due to a lack of striatal dopamine. Furthermore, we will summarise the reports that describe efforts to stimulate NSCs to replace dopaminergic cells in the SN and restore striatal dopamine levels. In our opinion, mobilizing the endogenous SVZ NSCs to replenish striatal dopamine is an attractive approach to alleviate the motor symptoms in PD patients, without the ethical and immunological challenges of transplantation of NSCs and foetal brain tissue.

Rasagiline ameliorates olfactory deficits in an alpha-synuclein mouse model of Parkinson's disease

Impaired olfaction is an early pre-motor symptom of Parkinson's disease. The neuropathology underlying olfactory dysfunction in Parkinson's disease is unknown, however α-synuclein accumulation/aggregation and altered neurogenesis might play a role. We characterized olfactory deficits in a transgenic mouse model of Parkinson's disease expressing human wild-type α-synuclein under the control of the mouse α-synuclein promoter. Preliminary clinical observations suggest that rasagiline, a monoamine oxidase-B inhibitor, improves olfaction in Parkinson's disease. We therefore examined whether rasagiline ameliorates olfactory deficits in this Parkinson's disease model and investigated the role of olfactory bulb neurogenesis. α-Synuclein mice were progressively impaired in their ability to detect odors, to discriminate between odors, and exhibited alterations in short-term olfactory memory. Rasagiline treatment rescued odor detection and odor discrimination abilities. However, rasagiline did not affect short-term olfactory memory. Finally, olfactory changes were not coupled to alterations in olfactory bulb neurogenesis. We conclude that rasagiline reverses select olfactory deficits in a transgenic mouse model of Parkinson's disease. The findings correlate with preliminary clinical observations suggesting that rasagiline ameliorates olfactory deficits in Parkinson's disease.

Alpha-synuclein promotes early neurite outgrowth in cultured primary neurons

We previously showed that alpha-synuclein (α-Syn), a protein implicated in the pathogenesis of several neurodegenerative diseases, is a microtubule-associated protein (MAP), facilitating the polymerization of tubulin into microtubules. Therefore, we hypothesized that α-Syn might promote neurite outgrowth, a process that requires microtubule assembly. To test this hypothesis, recombinant human wild type (WT) and mutant (A30P and A53T) α-Syn proteins were added to cultured primary rat cortical neurons, and their effects on early neurite outgrowth were observed. The WT and mutant α-Syn proteins entered the neurons after 1-4 h of incubation. However, a significant increase in neurite outgrowth was observed only in neurons treated with WT α-Syn. MES23.5 dopaminergic neuronal cells overexpressing WT α-Syn also exhibited enhanced neurite outgrowth, indicating that the ability of α-Syn to promote neurite outgrowth was not due to a direct action on the cell membrane or by the membrane translocation process. Co-immunoprecipitation demonstrated that the recombinant human α-Syn was bound to tubulin. In addition, the α-Syn-treated neurons displayed increased levels of polymerized tubulin. Because α-Syn's MAP functionality is mediated by specific domains, we generated N-terminal (a.a. 1-65), non-amyloid-β (non-Aβ) component (NAC) (a.a. 61-95) and C-terminal (a.a. 96-140) fragments and added them to the primary neurons. After 1-4 h of incubation, the various α-Syn fragments had entered the neurons. However, only the NAC and C-terminal fragments, which have been previously shown to mediate MAP functionality, promoted neurite outgrowth. These results suggest that α-Syn promotes neurite outgrowth by facilitating the polymerization of tubulin into microtubules.