Neurogenesis in Huntington's disease: Can studying adult neurogenesis lead to the development of new therapeutic strategies?

Therapeutic advances in neural regeneration for Huntington's disease

Huntington's disease is a neurodegenerative disease caused by the expansion mutation of a cytosine-adenine-guanine triplet in the exon 1 of the HTT gene which is responsible for the production of the huntingtin (Htt) protein. In physiological conditions, Htt is involved in many cellular processes such as cell signaling, transcriptional regulation, energy metabolism regulation, DNA maintenance, axonal trafficking, and antiapoptotic activity. When the genetic alteration is present, the production of a mutant version of Htt (mHtt) occurs, which is characterized by a plethora of pathogenic activities that, finally, lead to cell death. Among all the cells in which mHtt exerts its dangerous activity, the GABAergic Medium Spiny Neurons seem to be the most affected by the mHtt-induced excitotoxicity both in the cortex and in the striatum. However, as the neurodegeneration proceeds ahead the neuronal loss grows also in other brain areas such as the cerebellum, hypothalamus, thalamus, subthalamic nucleus, globus pallidus, and substantia nigra, determining the variety of symptoms that characterize Huntington's disease. From a clinical point of view, Huntington's disease is characterized by a wide spectrum of symptoms spanning from motor impairment to cognitive disorders and dementia. Huntington's disease shows a prevalence of around 3.92 cases every 100,000 worldwide and an incidence of 0.48 new cases every 100,000/year. To date, there is no available cure for Huntington's disease. Several treatments have been developed so far, aiming to reduce the severity of one or more symptoms to slow down the inexorable decline caused by the disease. In this context, the search for reliable strategies to target the different aspects of Huntington's disease become of the utmost interest. In recent years, a variety of studies demonstrated the detrimental role of neuronal loss in Huntington's disease condition highlighting how the replacement of lost cells would be a reasonable strategy to overcome the neurodegeneration. In this view, numerous have been the attempts in several preclinical models of Huntington's disease to evaluate the feasibility of invasive and non-invasive approaches. Thus, the aim of this review is to offer an overview of the most appealing approaches spanning from stem cell-based cell therapy to extracellular vesicles such as exosomes in light of promoting neurogenesis, discussing the results obtained so far, their limits and the future perspectives regarding the neural regeneration in the context of Huntington's disease.

Can exercise benefits be harnessed with drugs? A new way to combat neurodegenerative diseases by boosting neurogenesis

Adult hippocampal neurogenesis (AHN) is affected by multiple factors, such as enriched environment, exercise, ageing, and neurodegenerative disorders. Neurodegenerative disorders can impair AHN, leading to progressive neuronal loss and cognitive decline. Compelling evidence suggests that individuals engaged in regular exercise exhibit higher production of proteins that are essential for AHN and memory. Interestingly, specific molecules that mediate the effects of exercise have shown effectiveness in promoting AHN and cognition in different transgenic animal models. Despite these advancements, the precise mechanisms by which exercise mimetics induce AHN remain partially understood. Recently, some novel exercise molecules have been tested and the underlying mechanisms have been proposed, involving intercommunications between multiple organs such as muscle-brain crosstalk, liver-brain crosstalk, and gut-brain crosstalk. In this review, we will discuss the current evidence regarding the effects and potential mechanisms of exercise mimetics on AHN and cognition in various neurological disorders. Opportunities, challenges, and future directions in this research field are also discussed.

Investigating Neuron Degeneration in Huntington's Disease Using RNA-Seq Based Transcriptome Study

Huntington's disease (HD) is a progressive neurodegenerative disorder caused due to a CAG repeat expansion in the huntingtin (HTT) gene. The primary symptoms of HD include motor dysfunction such as chorea, dystonia, and involuntary movements. The primary motor cortex (BA4) is the key brain region responsible for executing motor/movement activities. Investigating patient and control samples from the BA4 region will provide a deeper understanding of the genes responsible for neuron degeneration and help to identify potential markers. Previous studies have focused on overall differential gene expression and associated biological functions. In this study, we illustrate the relationship between variants and differentially expressed genes/transcripts. We identified variants and their associated genes along with the quantification of genes and transcripts. We also predicted the effect of variants on various regulatory activities and found that many variants are regulating gene expression. Variants affecting miRNA and its targets are also highlighted in our study. Co-expression network studies revealed the role of novel genes. Function interaction network analysis unveiled the importance of genes involved in vesicle-mediated transport. From this unified approach, we propose that genes expressed in immune cells are crucial for reducing neuron death in HD.

Transcriptional and Histone Acetylation Changes Associated with CRE Elements Expose Key Factors Governing the Regulatory Circuit in the Early Stage of Huntington's Disease Models

Huntington's disease (HD) is a disorder caused by an abnormal expansion of trinucleotide CAG repeats within the huntingtin (Htt) gene. Under normal conditions, the CREB Binding Protein interacts with CREB elements and acetylates Lysine 27 of Histone 3 to direct the expression of several genes. However, mutant Htt causes depletion of CBP, which in turn induces altered histone acetylation patterns and transcriptional deregulation. Here, we have studied a differential expression analysis and H3K27ac variation in 4- and 6-week-old R6/2 mice as a model of juvenile HD. The analysis of differential gene expression and acetylation levels were integrated into Gene Regulatory Networks revealing key regulators involved in the altered transcription cascade. Our results show changes in acetylation and gene expression levels that are related to impaired neuronal development, and key regulators clearly defined in 6-week-old mice are proposed to drive the downstream regulatory cascade in HD. Here, we describe the first approach to determine the relationship among epigenetic changes in the early stages of HD. We determined the existence of changes in pre-symptomatic stages of HD as a starting point for early onset indicators of the progression of this disease.

Longitudinal spatial mapping of lipid metabolites reveals pre-symptomatic changes in the hippocampi of Huntington's disease transgenic mice

In Huntington's disease (HD), a key pathological feature includes the development of inclusion-bodies of fragments of the mutant huntingtin protein in the neurons of the striatum and hippocampus. To examine the molecular changes associated with inclusion-body formation, we applied MALDI-mass spectrometry imaging and deuterium pulse labelling to determine lipid levels and synthesis rates in the hippocampus of a transgenic mouse model of HD (R6/1 line). The R6/1 HD mice lacked inclusions in the hippocampus at 6 weeks of age (pre-symptomatic), whereas inclusions were pervasive by 16 weeks of age (symptomatic). Hippocampal subfields (CA1, CA3 and DG), which formed the highest density of inclusion formation in the mouse brain showed a reduction in the relative abundance of neuron-enriched lipids that have roles in neurotransmission, synaptic plasticity, neurogenesis, and ER-stress protection. Lipids involved in the adaptive response to ER stress (phosphatidylinositol, phosphatidic acid, and ganglioside classes) displayed increased rates of synthesis in HD mice relative to WT mice at all the ages examined, including prior to the formation of the inclusion bodies. Our findings, therefore, support a role for ER stress occurring pre-symptomatically and potentially contributing to pathological mechanisms underlying HD.

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.

Chronic Administration of Thymoquinone Enhances Adult Hippocampal Neurogenesis and Improves Memory in Rats Via Regulating the BDNF Signaling Pathway

Thymoquinone is a pharmacologically active component of Nigella sativa Linn. seeds. Despite the diverse neuropharmacological attributes of TQ, limited reports related to adult neurogenesis and memory research are available. In this study, we investigated the effects of TQ on the proliferation and neural differentiation of cultured neural stem/progenitor cells (NSCs/NPCs). We also investigated the effect of TQ chronic administration on neurogenesis and memory in adult rats. Under proliferation conditions, TQ (0.05-0.3 μM) significantly increased NSCs/NPCs viability, neurosphere diameter, and cell count. TQ treatment under differentiation conditions increased the proportion of cells positive for Tuj1 (a neuronal marker). Furthermore, chronic oral administration of TQ (25 mg/kg/day for 12 weeks) to adult rats increased the number of bromodeoxyuridine (BrdU)-immunopositive cells double-stained with a mature neuronal marker, neuronal nuclei (NeuN), and a proliferation marker, doublecortin (Dcx), in the dentate gyrus of the hippocampus. TQ-administered rats showed a profound beneficial effect on avoidance-related learning ability, associated with an increase in the hippocampal mRNA and protein levels of brain-derived neurotrophic factor (BDNF), as measured by both real-time PCR and ELISA. Western blot analysis revealed that TQ stimulates the phosphorylation of cAMP-response element-binding protein (CREB), the upstream signaling molecule in the BDNF pathway. Furthermore, chronic administration of TQ decreased lipid peroxide and reactive oxygen species levels in the hippocampus. Taken together, our results suggest that TQ plays a role in memory improvement in adult rats and that the CREB/BDNF signaling pathways are involved in mediating the actions of TQ in hippocampal neurogenesis.

Stress-Related Dysfunction of Adult Hippocampal Neurogenesis-An Attempt for Understanding Resilience?

Newborn neurons in the adult hippocampus are regulated by many intrinsic and extrinsic cues. It is well accepted that elevated glucocorticoid levels lead to downregulation of adult neurogenesis, which this review discusses as one reason why psychiatric diseases, such as major depression, develop after long-term stress exposure. In reverse, adult neurogenesis has been suggested to protect against stress-induced major depression, and hence, could serve as a resilience mechanism. In this review, we will summarize current knowledge about the functional relation of adult neurogenesis and stress in health and disease. A special focus will lie on the mechanisms underlying the cascades of events from prolonged high glucocorticoid concentrations to reduced numbers of newborn neurons. In addition to neurotransmitter and neurotrophic factor dysregulation, these mechanisms include immunomodulatory pathways, as well as microbiota changes influencing the gut-brain axis. Finally, we discuss recent findings delineating the role of adult neurogenesis in stress resilience.

Glucocorticoid-mediated mechanisms of hippocampal damage: Contribution of subgranular neurogenesis

A comprehensive overview of the interplay between glucocorticoids (GCs) and adult hippocampal neurogenesis (AHN) is presented, particularly, in the context of a diseased brain. The effectors of GCs in the dentate gyrus neurogenic niche of the hippocampal are reviewed, and the consequences of the GC signaling on the generation and integration of new neurons are discussed. Recent findings demonstrating how GC signaling mediates impairments of the AHN in various brain pathologies are overviewed. GC-mediated effects on the generation and integration of adult-born neurons in the hippocampal dentate gyrus depend on the nature, severity, and duration of the acting stress factor. GCs realize their effects on the AHN primarily via specific glucocorticoid and mineralocorticoid receptors. Disruption of the reciprocal regulation between the hypothalamic-pituitary-adrenal (HPA) axis and the generation of the adult-born granular neurons is currently considered to be a key mechanism implicating the AHN into the pathogenesis of numerous brain diseases, including those without a direct hippocampal damage. These alterations vary from reduced proliferation of stem and progenitor cells to increased cell death and abnormalities in morphology, connectivity, and localization of young neurons. Although the involvement of the mutual regulation between the HPA axis and the AHN in the pathogenesis of cognitive deficits and mood impairments is evident, several unresolved critical issues are stated. Understanding the details of GC-mediated mechanisms involved in the alterations in AHN could enable the identification of molecular targets for ameliorating pathology-induced imbalance in the HPA axis/AHN mutual regulation to conquer cognitive and psychiatric disturbances.

Early deficits in olfaction are associated with structural and molecular alterations in the olfactory system of a Huntington disease mouse model

Olfactory dysfunction and altered neurogenesis are observed in several neurodegenerative disorders including Huntington disease (HD). These deficits occur early and correlate with a decline in global cognitive performance, depression and structural abnormalities of the olfactory system including the olfactory epithelium, bulb and cortices. However, the role of olfactory system dysfunction in the pathogenesis of HD remains poorly understood and the mechanisms underlying this dysfunction are unknown. We show that deficits in odour identification, discrimination and memory occur in HD individuals. Assessment of the olfactory system in an HD murine model demonstrates structural abnormalities in the olfactory bulb (OB) and piriform cortex, the primary cortical recipient of OB projections. Furthermore, a decrease in piriform neuronal counts and altered expression levels of neuronal nuclei and tyrosine hydroxylase in the OB are observed in the YAC128 HD model. Similar to the human HD condition, olfactory dysfunction is an early phenotype in the YAC128 mice and concurrent with caspase activation in the murine HD OB. These data provide a link between the structural olfactory brain region atrophy and olfactory dysfunction in HD and suggest that cell proliferation and cell death pathways are compromised and may contribute to the olfactory deficits in HD.

Manipulating Adult Neural Stem and Progenitor Cells with G-Quadruplex Ligands

G-quadruplexes are pervasive nucleic acid secondary structures in mammalian genomes and transcriptomes that regulate gene expression and genome duplication. Small molecule ligands that modify the stability of G-quadruplexes are widely studied in cancer, but whether G-quadruplex ligands can also be used to manipulate cell function under normal development and homeostatic conditions is largely unexplored. Here we show that two related G-quadruplex ligands (pyridostatin and carboxypyridostatin) can reduce proliferation of adult neural stem cell and progenitor cells derived from the adult mouse subventricular zone both in vitro and in vivo. Studies with neurosphere cultures show that pyridostatin reduces proliferation by a mechanism associated with DNA damage and cell death. By contrast, selectively targeting RNA G-quadruplex stability with carboxypyridostatin diminishes proliferation through a mechanism that promotes cell cycle exit and the production of oligodendrocyte progenitors. The ability to generate oligodendrocyte progenitors by targeting RNA G-quadruplex stability, however, is dependent on the cellular environment. Together, these findings show that ligands that can selectively stabilize RNA G-quadruplexes are an important, new class of molecular tool for neural stem and progenitor cell engineering, whereas ligands that target DNA G-quadruplexes have limited utility due to their toxicity.

Neuroinflammation and Neurogenesis in Alzheimer's Disease and Potential Therapeutic Approaches

In adult brain, new neurons are generated throughout adulthood in the subventricular zone and the dentate gyrus; this process is commonly known as adult neurogenesis. The regulation or modulation of adult neurogenesis includes various intrinsic pathways (signal transduction pathway and epigenetic or genetic modulation pathways) or extrinsic pathways (metabolic growth factor modulation, vascular, and immune system pathways). Altered neurogenesis has been identified in Alzheimer's disease (AD), in both human AD brains and AD rodent models. The exact mechanism of the dysregulation of adult neurogenesis in AD has not been completely elucidated. However, neuroinflammation has been demonstrated to alter adult neurogenesis. The presence of various inflammatory components, such as immune cells, cytokines, or chemokines, plays a role in regulating the survival, proliferation, and maturation of neural stem cells. Neuroinflammation has also been considered as a hallmark neuropathological feature of AD. In this review, we summarize current, state-of-the art perspectives on adult neurogenesis, neuroinflammation, and the relationship between these two phenomena in AD. Furthermore, we discuss the potential therapeutic approaches, focusing on the anti-inflammatory and proneurogenic interventions that have been reported in this field.

Mitochondria as central regulators of neural stem cell fate and cognitive function

Emerging evidence now indicates that mitochondria are central regulators of neural stem cell (NSC) fate decisions and are crucial for both neurodevelopment and adult neurogenesis, which in turn contribute to cognitive processes in the mature brain. Inherited mutations and accumulated damage to mitochondria over the course of ageing serve as key factors underlying cognitive defects in neurodevelopmental disorders and neurodegenerative diseases, respectively. In this Review, we explore the recent findings that implicate mitochondria as crucial regulators of NSC function and cognition. In this respect, mitochondria may serve as targets for stem-cell-based therapies and interventions for cognitive defects.

Decoding epigenetic cell signaling in neuronal differentiation

Neurogenesis is the process by which new neurons are generated in the brain. Neural stem cells (NSCs) are differentiated into neurons, which are integrated into the neural network. Nowadays, pluripotent stem cells, multipotent stem cells, and induced pluripotent stem cells can be artificially differentiated into neurons utilizing several techniques. Specific transcriptional profiles from NSCs during differentiation are frequently used to approach and observe phenotype alteration and functional determination of neurons. In this context, the role of non-coding RNA, transcription factors and epigenetic changes in neuronal development and differentiation has gained importance. Epigenetic elucidation has become a field of intense research due to distinct patterns of normal conditions and different neurodegenerative disorders, which can be explored to develop new diagnostic methods or gene therapies. In this review, we discuss the complexity of transcription factors, non-coding RNAs, and extracellular vesicles that are responsible for guiding and coordinating neural development.

Concise Review: The Use of Stem Cells for Understanding and Treating Huntington's Disease

Two decades ago, researchers identified that a CAG expansion mutation in the huntingtin (HTT) gene was involved in the pathogenesis of Huntington's disease (HD). However, since the identification of the HTT gene, there has been no advance in the development of therapeutic strategies to prevent or reduce the progression of HD. With the recent advances in stem cell biology and human cell reprogramming technologies, several novel and exciting pathways have emerged allowing researchers to enhance their understanding of the pathogenesis of HD, to identify and screen potential drug targets, and to explore alternative donor cell sources for cell replacement therapy. This review will discuss the role of compensatory neurogenesis in the HD brain, the use of stem cell-based therapies for HD to replace or prevent cell loss, and the recent advance of cell reprogramming to model and/or treat HD. These new technologies, coupled with advances in genome editing herald a promising new era for HD research with the potential to identify a therapeutic strategy to alleviate this debilitating disorder. Stem Cells 2018;36:146-160.

Involvement of progranulin in modulating neuroinflammatory responses but not neurogenesis in the hippocampus of aged mice

It is well established that adult neurogenesis in the hippocampus declines with age. Our previous studies have suggested that progranulin (PGRN) has a facilitative effect on hippocampal neurogenesis. We have also shown that PGRN plays a role in suppressing excessive neuroinflammatory responses in the cortex and thalamus after brain injury and aging, respectively. However, the roles of PGRN in modulating neurogenesis and neuroinflammatory responses in the hippocampus of aged animals are not yet understood. In the present study, we investigated neurogenesis and neuroinflammation-related responses in the hippocampus of young (15-week-old) and old (135-week-old) wild-type and PGRN-deficient male mice. Neurogenesis in the dentate gyrus of the hippocampus markedly declined with age, and there was no significant difference between the genotype. The number of CD68-positive activated microglia and the expression of lysosomal genes in the hippocampus were significantly increased with age, and PGRN deficiency further increased them. The expression of pro-inflammatory genes was also increased with age, and PGRN deficiency significantly enhanced some of them. These results suggest that PGRN deficiency exacerbates neuroinflammatory responses related to activated microglia in aged animals, while PGRN may not counteract the decline of hippocampal neurogenesis with age.

Epigenetic and transcriptional modulation of WDR5, a chromatin remodeling protein, in Huntington's disease human induced pluripotent stem cell (hiPSC) model

DNA methylation (DNAm) changes are of increasing relevance to neurodegenerative disorders, including Huntington's disease (HD). We performed genome-wide screening of possible DNAm changes occurring during striatal differentiation in human induced pluripotent stem cells derived from a HD patient (HD-hiPSCs) as cellular model. We identified 240 differentially methylated regions (DMRs) at promoters in fully differentiated HD-hiPSCs. Subsequently, we focused on the methylation differences in a subcluster of genes related to Jumonji Domain Containing 3 (JMJD3), a demethylase that epigenetically regulates neuronal differentiation and activates neuronal progenitor associated genes, which are indispensable for neuronal fate acquisition. Noticeably among these genes, WD repeat-containing protein 5 (WDR5) promoter was found hypermethylated in HD-hiPSCs, resulting in a significant down-modulation in its expression and of the encoded protein. A similar WDR5 expression decrease was seen in a small series of HD-hiPSC lines characterized by different CAG length. The decrease in WDR5 expression was particularly evident in HD-hiPSCs compared to hESCs and control-hiPSCs from healthy subjects. WDR5 is a core component of the MLL/SET1 chromatin remodeling complexes essential for H3K4me3, previously reported to play an important role in stem cells self-renewal and differentiation. These results suggest the existence of epigenetic mechanisms in HD and the identification of genes, which are able to modulate HD phenotype, is important both for biomarker discovery and therapeutic interventions.

Exosome-Based Delivery of miR-124 in a Huntington's Disease Model

Objective

Huntington’s disease (HD) is a genetic neurodegenerative disease that is caused by abnormal CAG expansion. Altered microRNA (miRNA) expression also causes abnormal gene regulation in this neurodegenerative disease. The delivery of abnormally downregulated miRNAs might restore normal gene regulation and have a therapeutic effect.

Methods

We developed an exosome-based delivery method to treat this neurodegenerative disease. miR-124, one of the key miRNAs that is repressed in HD, was stably overexpressed in a stable cell line. Exosomes were then harvested from these cells using an optimized protocol. The exosomes (Exo-124) exhibited a high level of miR-124 expression and were taken up by recipient cells.

Results

When Exo-124 was injected into the striatum of R6/2 transgenic HD mice, expression of the target gene, RE1-Silencing Transcription Factor, was reduced. However, Exo-124 treatment did not produce significant behavioral improvement.

Conclusion

This study serves as a proof of concept for exosome-based delivery of miRNA in neurodegenerative diseases.

Hippocampal dysfunction defines disease onset in Huntington's disease

BACKGROUND

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterised by a triad of motor, psychiatric and cognitive deficits with the latter classically attributed to disruption of frontostriatal networks. However, emerging evidence from animal models of HD suggests that some of the early cognitive deficits may have a hippocampal basis. The objective of this study was to link previous rodent findings in this area to clinical practice.

METHODS

In this study, 94 participants included patients with early HD, premanifest HD and age-matched controls underwent hippocampal-based cognitive assessments. These included a virtual reality version of the Morris water maze, a task involved participants having to swim through a virtual pool to find a submerged platform using a joystick, and the Cambridge Neuropsychological Test Automated Battery (CANTAB) paired associates learning task, a test also known to rely on hippocampal integrity.

RESULTS

Patients with early HD showed impaired performance in both the virtual Morris water maze and the CANTAB paired associates learning. Such deficits were also correlated with estimated years to diagnosis in premanifest participants.

CONCLUSIONS

This study highlights the merit of using analogous tests in the laboratory and clinic and demonstrates that hippocampal impairments are an early feature of HD in patients as previously shown in rodent models of the disease. As such, they could be used not only to assist in the diagnosis of disease onset, but may also be useful as an outcome measure in future therapeutic trials.

Adult Neurogenesis and Gliogenesis: Possible Mechanisms for Neurorestoration

The subgranular zone (SGZ) and subventricular zone (SVZ) are developmental remnants of the germinal regions of the brain, hence they retain the ability to generate neuronal progenitor cells in adult life. Neurogenesis in adult brain has an adaptive function because newly produced neurons can integrate into and modify existing neuronal circuits. In contrast to the SGZ and SVZ, other brain regions have a lower capacity to produce new neurons, and this usually occurs via parenchymal and periventricular cell genesis. Compared to neurogenesis, gliogenesis occurs more prevalently in the adult mammalian brain. Under certain circumstances, interaction occurs between neurogenesis and gliogenesis, facilitating glial cells to transform into neuronal lineage. Therefore, modulating the balance between neurogenesis and gliogenesis may present a new perspective for neurorestoration, especially in diseases associated with altered neurogenesis and/or gliogenesis, cell loss, or disturbed homeostasis of cellular constitution. The present review discusses important neuroanatomical features of adult neurogenesis and gliogenesis, aiming to explore how these processes could be modulated toward functional repair of the adult brain.

Time-Course Analysis of Protein and Lipid Oxidation in the Brains of Yac128 Huntington's Disease Transgenic Mice

Huntington's disease (HD) is caused by an expansion of cytosine-adenine-guanine (CAG) repeats within the coding region of the HD gene, which expresses the protein huntingtin and is characterized by selective degeneration of specific neuronal populations, mainly in the striatum and the cortex. The mechanisms that account for this selective neuronal death are multifaceted, but oxidative stress might play an important role in this process. To determine whether changes in the intracellular redox state will result in oxidative damage to cellular macromolecules with disease progression, we analyzed levels of lipid peroxidation (with the thiobarbituric acid reactive substances [TBARS] assay) and protein carbonyl formation (using the 2,4-dinitrophenylhydrazine reaction) in the cerebellum, cerebral cortex, prefrontal cortex, striatum, and hippocampus of the YAC128 HD mouse model at 3, 6, and 12 months of age. With the exception of a transient increase in protein carbonyl levels in the YAC128 prefrontal cortex at 6 months of age, levels of lipid peroxidation and protein oxidation were not significantly different between YAC128 mice and their age-matched wild-type counterparts in any of the brain regions analyzed up to 12 months of age. However, age-related increases in oxidative stress were observed in various brain regions. These results suggest that lipid and protein oxidative damage is not a major contributor to neurodegeneration in the YAC128 brain up to 12 months of age.

Immune and neurotrophin stimulation by electroconvulsive therapy: is some inflammation needed after all?

A low-grade inflammatory response is commonly seen in the peripheral blood of major depressive disorder (MDD) patients, especially those with refractory and chronic disease courses. However, electroconvulsive therapy (ECT), the most drastic intervention reserved for these patients, is closely associated with an enhanced haematogenous as well as neuroinflammatory immune response, as evidenced by both human and animal studies. A related line of experimental evidence further shows that inflammatory stimulation reinforces neurotrophin expression and may even mediate dramatic neurogenic and antidepressant-like effects following exposure to chronic stress. The current review therefore attempts a synthesis of our knowledge on the neurotrophic and immunological aspects of ECT and other electrically based treatments in psychiatry. Perhaps contrary to contemporary views, we conclude that targeted potentiation, rather than suppression, of inflammatory responses may be of therapeutic relevance to chronically depressed patients or a subgroup thereof.

Recent prospective of surface engineered Nanoparticles in the management of Neurodegenerative disorders

Clinically, the therapeutic outcomes in neurodegenerative disorders (NDs) by drug treatment are very limited, and the most insurmountable obstacle in the treatment of NDs is the blood-brain barrier (BBB), which provides the highest level of protection from xenobiotics. A great deal of attention still needs to be paid to overcome these barriers, and surface-engineered polymeric nanoparticles are emerging as innovative tools that are able to interact with the biological system at a molecular level for the desired response. The present review covers the potential importance of surface-structure-engineered nanoparticles to overcome the BBB for good bioavailability, and the evaluation of drug therapy in NDs.

Discovery of a novel 2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione series promoting neurogenesis of human neural progenitor cells

A novel neurogenic compound (1), discovered from a mouse neural progenitor cell (NPC) screen, showed profound neurogenic effect on human NPCs. Synthesis and SAR of this novel 2,3,11,11a-tetrahydro-1H-pyrazino[1,2-b]isoquinoline-1,4(6H)-dione series are described. Compound 20 is brain penetrable in rodents, and promotes neurogenesis in wild type mice, therefore it is a good tool molecule to study neurogenesis induction as a potential treatment for conditions associated with neurogenesis impairment diseases.

Stem cell therapy in neurodegenerative diseases: From principles to practice

The lack of curative therapies for neurodegenerative diseases has high economic impact and places huge burden on the society. The contribution of stem cells to cure neurodegenerative diseases has been unraveled and explored extensively over the past few years. Beyond substitution of the lost neurons, stem cells act as immunomodulators and neuroprotectors. A large number of preclinical and a small number of clinical studies have shown beneficial outcomes in this context. In this review, we have summarized the current concepts of stem cell therapy in neurodegenerative diseases and the recent advances in this field, particularly between 2010 and 2012. Further studies should be encouraged to resolve the clinical issues and vague translational findings for maximum optimization of the efficacy of stem cell therapy in neurodegenerative diseases.

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.

Targets for future clinical trials in Huntington's disease: what's in the pipeline?

The known genetic cause of Huntington's disease (HD) has fueled considerable progress in understanding its pathobiology and the development of therapeutic approaches aimed at correcting specific changes linked to the causative mutation. Among the most promising is reducing expression of mutant huntingtin protein (mHTT) with RNA interference or antisense oligonucleotides; human trials are now being planned. Zinc-finger transcriptional repression is another innovative method to reduce mHTT expression. Modulation of mHTT phosphorylation, chaperone upregulation, and autophagy enhancement represent attempts to alter cellular homeostasis to favor removal of mHTT. Inhibition of histone deacetylases (HDACs) remains of interest; recent work affirms HDAC4 as a target but questions the assumed centrality of its catalytic activity in HD. Phosphodiesterase inhibition, aimed at restoring synaptic function, has progressed rapidly to human trials. Deranged cellular signaling provides several tractable targets, but specificity and complexity are challenges. Restoring neurotrophic support in HD remains a key potential therapeutic approach. with several approaches being pursued, including brain-derived neurotrophic factor (BDNF) mimesis through tyrosine receptor kinase B (TrkB) agonism and monoclonal antibodies. An increasing understanding of the role of glial cells in HD has led to several new therapeutic avenues, including kynurenine monooxygenase inhibition, immunomodulation by laquinimod, CB2 agonism, and others. The complex metabolic derangements in HD remain under study, but no clear therapeutic strategy has yet emerged. We conclude that many exciting therapeutics are progressing through the development pipeline, and combining a better understanding of HD biology in human patients, with concerted medicinal chemistry efforts, will be crucial for bringing about an era of effective therapies.

Exercise and environment as an intervention for neonatal alcohol effects on hippocampal adult neurogenesis and learning

Neonatal alcohol exposure impairs cognition and learning in adulthood and permanently damages the hippocampus. Wheel running (WR) improves hippocampus-associated learning and memory and increases the genesis and survival of newly generated neurons in the hippocampal dentate gyrus. WR significantly increases proliferation of newly generated dentate granule cells in alcohol-exposed (AE) and control rats on Postnatal Day (PD) 42 but only control rats show an increased number of surviving cells thirty days after WR (Helfer et al., 2009b). The present studies examined whether proliferation-promoting WR followed by survival-enhancing environmental complexity (EC) during adolescence could increase survival of new neurons in AE rats. On PD 4-9, pups were intubated with alcohol in a binge-like manner (5.25g/kg/day, AE), were sham-intubated (SI), or were reared normally (suckle control, SC). On PD 30 animals were assigned to WR (PD 30-42) followed by EC (PD 42-72; WR/EC) or were socially housed (SH/SH) for the duration of the experiment. All animals were injected with 200mg/kg bromodeoxyuridine (BrdU) on PD 41. In Experiment 1, survival of newly generated cells was significantly enhanced in the AE-WR/EC group in comparison with AE-SH/SH group. Experiment 2A examined trace eyeblink conditioning. In the SH/SH condition, AE impaired trace eyeblink conditioning relative to SI and SC controls. In the WR/EC condition, AE rats performed as well as controls. In Experiment 2B, the same intervention was examined using the context preexposure facilitation effect (CPFE); a hippocampus-dependent variant of contextual fear conditioning. Again, the WR/EC intervention reversed the deficit in conditioned fear to the context that was evident in the SH/SH condition. Post-weaning environmental manipulations promote cell survival and reverse learning deficits in rats that were exposed to alcohol during development. These manipulations may provide a basis for developing interventions that ameliorate learning impairments associated with human fetal alcohol spectrum disorders.

Physical exercise-induced adult neurogenesis: a good strategy to prevent cognitive decline in neurodegenerative diseases?

Cumulative evidence has indicated that there is an important role for adult hippocampal neurogenesis in cognitive function. With the increasing prevalence of cognitive decline associated with neurodegenerative diseases among the ageing population, physical exercise, a potent enhancer of adult hippocampal neurogenesis, has emerged as a potential preventative strategy/treatment to reduce cognitive decline. Here we review the functional role of adult hippocampal neurogenesis in learning and memory, and how this form of structural plasticity is altered in neurodegenerative diseases known to involve cognitive impairment. We further discuss how physical exercise may contribute to cognitive improvement in the ageing brain by preserving adult neurogenesis, and review the recent approaches for measuring changes in neurogenesis in the live human brain.

Huntingtin acts non cell-autonomously on hippocampal neurogenesis and controls anxiety-related behaviors in adult mouse

Huntington's disease (HD) is a fatal neurodegenerative disease, characterized by motor defects and psychiatric symptoms, including mood disorders such as anxiety and depression. HD is caused by an abnormal polyglutamine (polyQ) expansion in the huntingtin (HTT) protein. The development and analysis of various mouse models that express pathogenic polyQ-HTT revealed a link between mutant HTT and the development of anxio-depressive behaviors and various hippocampal neurogenesis defects. However, it is unclear whether such phenotype is linked to alteration of HTT wild-type function in adults. Here, we report the analysis of a new mouse model in which HTT is inducibly deleted from adult mature cortical and hippocampal neurons using the CreER(T2)/Lox system. These mice present defects in both the survival and the dendritic arborization of hippocampal newborn neurons. Our data suggest that these non-cell autonomous effects are linked to defects in both BDNF transport and release upon HTT silencing in hippocampal neurons, and in BDNF/TrkB signaling. The controlled deletion of HTT also had anxiogenic-like effects. Our results implicate endogenous wild-type HTT in adult hippocampal neurogenesis and in the control of mood disorders.

Huntingtin mediates anxiety/depression-related behaviors and hippocampal neurogenesis

Huntington disease (HD) is associated with early psychiatric symptoms including anxiety and depression. Here, we demonstrate that wild-type huntingtin, the protein mutated in HD, modulates anxiety/depression-related behaviors according to its phosphorylation at serines 1181 and 1201. Genetic phospho-ablation at serines 1181 and 1201 in mouse reduces basal levels of anxiety/depression-like behaviors. We observe that the reduction in anxiety/depression-like phenotypes is associated with increased adult hippocampal neurogenesis. By improving the attachment of molecular motors to microtubules, huntingtin dephosphorylation increases axonal transport of BDNF, a crucial factor for hippocampal adult neurogenesis. Consequently, the huntingtin-mediated increased BDNF dynamics lead to an increased delivery and signaling of hippocampal BDNF. These results support the notion that huntingtin participates in anxiety and depression-like behavior and is thus relevant to the etiology of mood disorders and anxiety/depression in HD.

Genome-wide loss of 5-hmC is a novel epigenetic feature of Huntington's disease

5-Hydroxymethylcytosine (5-hmC) may represent a new epigenetic modification of cytosine. While the dynamics of 5-hmC during neurodevelopment have recently been reported, little is known about its genomic distribution and function(s) in neurodegenerative diseases such as Huntington's disease (HD). We here observed a marked reduction of the 5-hmC signal in YAC128 (yeast artificial chromosome transgene with 128 CAG repeats) HD mouse brain tissues when compared with age-matched wild-type (WT) mice, suggesting a deficiency of 5-hmC reconstruction in HD brains during postnatal development. Genome-wide distribution analysis of 5-hmC further confirmed the diminishment of the 5-hmC signal in striatum and cortex in YAC128 HD mice. General genomic features of 5-hmC are highly conserved, not being affected by either disease or brain regions. Intriguingly, we have identified disease-specific (YAC128 versus WT) differentially hydroxymethylated regions (DhMRs), and found that acquisition of DhmRs in gene body is a positive epigenetic regulator for gene expression. Ingenuity pathway analysis (IPA) of genotype-specific DhMR-annotated genes revealed that alternation of a number of canonical pathways involving neuronal development/differentiation (Wnt/β-catenin/Sox pathway, axonal guidance signaling pathway) and neuronal function/survival (glutamate receptor/calcium/CREB, GABA receptor signaling, dopamine-DARPP32 feedback pathway, etc.) could be important for the onset of HD. Our results indicate that loss of the 5-hmC marker is a novel epigenetic feature in HD, and that this aberrant epigenetic regulation may impair the neurogenesis, neuronal function and survival in HD brain. Our study also opens a new avenue for HD treatment; re-establishing the native 5-hmC landscape may have the potential to slow/halt the progression of HD.

Striatal oligodendrogliogenesis and neuroblast recruitment are increased in the R6/2 mouse model of Huntington's disease

The subventricular zone (SVZ) is one of the two major neurogenic regions in the adult mammalian brain. Its close proximity to the striatum suggests that a cell-based therapeutic strategy for the treatment of Huntington's disease (HD) is possible. To achieve this, it is important to understand how adult cell production, migration and differentiation may be altered in the HD brain. In this study, we quantified the number of adult-born striatal cells and characterized their fate in the R6/2 transgenic mouse model of HD. We found that the number of new striatal cells was approximately two-fold greater in R6/2 vs. wild type mice, while SVZ cell proliferation was not affected. Using cell-type specific markers, we demonstrated that the majority of new striatal cells were mature oligodendrocytes or oligodendroglial precursors that were intrinsic to the striatum. We also detected a significant increase in the number of migrating neuroblasts that appeared to be recruited from the SVZ to the striatum. However, these neuroblasts did not mature into neurons and most were lost between 1 and 2 weeks of cell age. Crossing the R6/2 mice with mice the over-expressing brain-derived neurotrophic factor in the striatum increased the numbers of neuroblasts that survived to 2 weeks, but did not promote their differentiation. Together, our data indicate that the potential treatment of HD based on manipulating endogenous progenitor cells should take into consideration the apparent enhancement in striatal oligodendrogliogenesis and the limited ability of recruited SVZ neuroblasts to survive long-term and differentiate in the diseased striatum.

Small-molecule TrkB receptor agonists improve motor function and extend survival in a mouse model of Huntington's disease

Huntington's disease (HD) is a fatal neurodegenerative disease characterized by abnormal motor coordination, cognitive decline and psychiatric disorders. This disease is caused by an expanded CAG trinucleotide repeat in the gene encoding the protein huntingtin. Reduced levels of brain-derived neurotrophic factor (BDNF) in the brain, which results from transcriptional inhibition and axonal transport deficits mediated by mutant huntingtin, have been suggested as critical factors underlying selective neurodegeneration in both HD patients and HD mouse models. BDNF activates its high-affinity receptor TrkB and promotes neuronal survival; restoring BDNF signaling is thus of particular therapeutic interest. In the present study, we evaluated the ability of a small-molecule TrkB agonist 7,8-dihydroxyflavone (7,8-DHF) and its synthetic derivative 4'-dimethylamino-7,8- dihydroxyflavone (4'-DMA-7,8-DHF) to protect neurons in the well-characterized N171-82Q HD mouse model. We found that chronic administration of 7, 8-DHF (5 mg/kg) or 4'-DMA-7,8-DHF (1 mg/kg) significantly improved motor deficits, ameliorated brain atrophy and extended survival in these N171-82Q HD mice. Moreover, 4'-DMA-7,8-DHF preserved DARPP32 levels in the striatum and rescued mutant huntingtin-induced impairment of neurogenesis in the N171-82Q HD mice. These data highlight consideration of TrkB as a therapeutic target in HD and suggest that small-molecule TrkB agonists that penetrate the brain have high potential to be further tested in clinical trials of HD.

Impaired basal and running-induced hippocampal neurogenesis coincides with reduced Akt signaling in adult R6/1 HD mice

Huntington's disease (HD) is a fatal neurodegenerative disorder affecting a range of cellular and molecular functions in the brain. Deficits in adult hippocampal neurogenesis (AHN) have been documented in the R6/1 mouse model of HD. Here we examined basal and running-induced neuronal precursor proliferation in adult female and male R6/1 HD mice. We further tested whether sequential delivery of voluntary running followed by environmental enrichment could synergistically enhance functional AHN in female R6/1 HD mice. R6/1 HD mice engaged in significantly reduced levels of voluntary running, with males showing a more severe deficit. Basal neural precursor proliferation in the hippocampal sub-granular zone remained unchanged between female and male R6/1 HD mice and neither sex significantly responded to running-induced proliferation. While discrete provision of running wheels and enriched environments doubled AHN in adult female R6/1 HD mice it did not reflect the significant 3-fold increase in female wildtypes. Nevertheless, triple-label c-Fos/BrdU/NeuN immunofluorescence and confocal microscopy provided evidence that the doubling of AHN in female R6/1 HD mice was functional. Intrinsic cellular dysfunction mediated by protein aggregates containing mutant huntingtin (mHtt) did not appear to coincide with AHN deficits. In the hippocampus of female R6/1 HD mice, proliferating precursors and 6 week old adult-generated neurons were devoid of mHtt immuno-reactive aggregates, as were endothelial, microglial and astroglial cells populating the neurogenic niche. Serum transforming growth factor-β concentrations remained unaltered in female R6/1 HD mice as did the hippocampal levels of proliferating microglia and glial fibrillarly acidic protein expression. Examining the growth hormone/insulin-like growth factor 1 (GH/IGF-1) axis showed no change in base-line serum GH between genotypes. However, despite a reduced distance, acute running increases serum GH in both female wildtype and R6/1 HD mice. Serum IGF-1 levels were increased in female R6/1 HD mice compared to wildtypes during daytime inactive period, while hippocampal levels of the IGF-1 receptor remained unchanged. Running induced Akt phosphorylation in the hippocampus of female wildtype mice, which was not reflected in R6/1 HD mice. Total Akt levels were decreased in the hippocampus of both control and running R6/1 HD mice. Our results show adult-generated hippocampal neurons in female R6/1 HD mice express c-Fos and that running and Akt signaling deficits may mediate reduced basal and running-induced AHN levels.

Extensive changes in DNA methylation are associated with expression of mutant huntingtin

The earliest stages of Huntington disease are marked by changes in gene expression that are caused in an indirect and poorly understood manner by polyglutamine expansions in the huntingtin (HTT) protein. To explore the hypothesis that DNA methylation may be altered in cells expressing mutated HTT, we use reduced representation bisulfite sequencing (RRBS) to map sites of DNA methylation in cells carrying either wild-type or mutant HTT. We find that a large fraction of the genes that change in expression in the presence of mutant huntingtin demonstrate significant changes in DNA methylation. Regions with low CpG content, which have previously been shown to undergo methylation changes in response to neuronal activity, are disproportionately affected. On the basis of the sequence of regions that change in methylation, we identify AP-1 and SOX2 as transcriptional regulators associated with DNA methylation changes, and we confirm these hypotheses using genome-wide chromatin immunoprecipitation sequencing (ChIP-Seq). Our findings suggest new mechanisms for the effects of polyglutamine-expanded HTT. These results also raise important questions about the potential effects of changes in DNA methylation on neurogenesis and cognitive decline in patients with Huntington disease.

Repair of the CNS using endogenous and transplanted neural stem cells

Restoration of the damaged central nervous system is a vast challenge. However, there is a great need for research into this topic, due to the prevalence of central nervous system disorders and the devastating impact they have on people's lives. A number of strategies are being examined to achieve this goal, including cell replacement therapy, enhancement of endogenous plasticity and the recruitment of endogenous neurogenesis. The current chapter reviews this topic within the context of Parkinson's disease, Huntington's disease and stroke. For each disease exogenous cell therapies are discussed including primary (foetal) cell transplants, neural stem cells, induced pluripotent stem cells and marrow stromal cells. This chapter highlights the different mechanistic approaches of cell replacement therapy versus cells that deliver neurotropic factors, or enhance the endogenous production of these factors. Evidence of exogenously transplanted cells functionally integrating into the host brain, replacing cells, and having a behavioural benefit are discussed, along with the ability of some cell sources to stimulate endogenous neuroprotective and restorative events. Alongside exogenous cell therapy, the role of endogenous neurogenesis in each of the three diseases is outlined and methods to enhance this phenomenon are discussed.

Hippocampal neurogenesis, cognitive deficits and affective disorder in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a tandem repeat expansion encoding a polyglutamine tract in the huntingtin protein. HD involves progressive psychiatric, cognitive, and motor symptoms, the selective pathogenesis of which remains to be mechanistically elucidated. There are a range of different brain regions, including the cerebral cortex and striatum, known to be affected in HD, with evidence for hippocampal dysfunction accumulating in recent years. In this review we will focus on hippocampal abnormalities, in particular, deficits of adult neurogenesis. We will discuss potential molecular mechanisms mediating disrupted hippocampal neurogenesis, and how this deficit of cellular plasticity may in turn contribute to specific cognitive and affective symptoms that are prominent in HD. The generation of transgenic animal models of HD has greatly facilitated our understanding of disease mechanisms at molecular, cellular, and systems levels. Transgenic HD mice have been found to show progressive behavioral changes, including affective, cognitive, and motor abnormalities. The discovery, in multiple transgenic lines of HD mice, that adult hippocampal neurogenesis and synaptic plasticity is disrupted, may help explain specific aspects of cognitive and affective dysfunction. Furthermore, these mouse models have provided insight into potential molecular mediators of adult neurogenesis deficits, such as disrupted serotonergic and neurotrophin signaling. Finally, a number of environmental and pharmacological interventions which are known to enhance adult hippocampal neurogenesis have been found to have beneficial affective and cognitive effects in mouse models, suggesting common molecular targets which may have therapeutic utility for HD and related diseases.