SummaryIn the adult mouse brain, the subventricular zone (SVZ) is a neurogenic stem cell niche only 4-5 cell diameters thick. Within this narrow zone, a unique microenvironment supports stem cell self-renewal, gliogenesis or neurogenesis lineage decisions and tangential migration of newly generated neurons out of the SVZ and into the olfactory bulb. However, with aging, SVZ neurogenesis declines. Here, we examine the dynamic interplay between SVZ cytoarchitecture and neurogenesis through aging. Assembly of high-resolution electron microscopy images of corresponding coronal sections from 2-, 10-and 22-month-old mice into photomontages reveal a thinning of the SVZ with age. Following a 2-h BrdU pulse, we detect a significant decrease in cell proliferation from 2 to 22 months. Neuroblast numbers decrease with age, as do transitory amplifying progenitor cells, while both SVZ astrocytes and adjacent ependymal cells remain relatively constant. At 22 months, only residual pockets of neurogenesis remain and neuroblasts become restricted to the anterior dorsolateral horn of the SVZ. Within this dorsolateral zone many key components of the younger neurogenic niche are maintained; however, in the aged SVZ, increased numbers of SVZ astrocytes are found interposed within the ependyma. These astrocytes co-label with markers to ependymal cells and astrocytes, form intercellular adherens junctions with neighboring ependymal cells, and some possess multiple basal bodies of cilia within their cytoplasm. Together, these data reveal an age-related, progressive restriction of SVZ neurogenesis to the dorsolateral aspect of the lateral ventricle with increased numbers of SVZ astrocytes interpolated within the ependyma.

Background: Genome wide association studies have not revealed any risk-conferring common genetic variants in Tourette syndrome (TS), requiring the adoption of alternative approaches to investigate the pathophysiology of this disorder. Methods: We obtained the basal ganglia transcriptome by RNA sequencing in the caudate and putamen of 9 TS and 9 matched normal controls. Results: We found 309 down-regulated and 822 up-regulated genes in the caudate and putamen (striatum) of TS individuals. Using data-driven gene network analysis, we identified seventeen gene co-expression modules associated with TS. The top-scoring down-regulated module in TS was enriched in striatal interneuron transcripts, which was confirmed by decreased numbers of cholinergic and GABAergic interneurons by immunohistochemistry in the same regions. The top-scoring up-regulated module was enriched in immune-related genes, consistent with activation of microglia in patients’ striatum. Genes implicated by copy number variants (CNV) in TS were enriched in the interneuron module as well as in a protocadherin module. Module clustering revealed that the interneuron module was correlated with a neuronal metabolism module. Conclusions: Convergence of differential expression, network analyses and module clustering, together with CNVs implicated in TS, strongly implicates disrupted interneuron signaling in the pathophysiology of severe TS, and suggests that metabolic alterations may be linked to their death or dysfunction.

Gilles de la Tourette syndrome (TS) is characterized by tics, which are transiently worsened by stress, acute administration of dopaminergic drugs, and by subtle deficits in motor coordination and sensorimotor gating. It represents the most severe end of a spectrum of tic disorders that, in aggregate, affect ∼5% of the population. Available treatments are frequently inadequate, and the pathophysiology is poorly understood. Postmortem studies have revealed a reduction in specific striatal interneurons, including the large cholinergic interneurons, in severe disease. We tested the hypothesis that this deficit is sufficient to produce aspects of the phenomenology of TS, using a strategy for targeted, specific cell ablation in mice. We achieved ∼50% ablation of the cholinergic interneurons of the striatum, recapitulating the deficit observed in patients postmortem, without any effect on GABAergic markers or on parvalbuminexpressing fast-spiking interneurons. Interneuron ablation in the dorsolateral striatum (DLS), corresponding roughly to the human putamen, led to tic-like stereotypies after either acute stress or D-amphetamine challenge; ablation in the dorsomedial striatum, in contrast, did not. DLS interneuron ablation also led to a deficit in coordination on the rotorod, but not to any abnormalities in prepulse inhibition, a measure of sensorimotor gating. These results support the causal sufficiency of cholinergic interneuron deficits in the DLS to produce some, but not all, of the characteristic symptoms of TS.Tourette sydrome | basal ganglia | interneurons | acetylcholine | animal models

Age-associated ventriculomegaly is typically attributed to neurodegeneration; however, additional factors might initiate or contribute to progressive ventricular expansion. By directly linking postmortem human MRI sequences with histological features of periventricular tissue, we show that substantial lateral ventricle surface gliosis is associated with ventriculomegaly. To examine whether loss of ependymal cell coverage resulting in ventricle surface glial scarring can lead directly to ventricle enlargement independent of any other injury or degenerative loss, we modeled in mice the glial scarring found along the lateral ventricle surface in aged humans. Neuraminidase, which cleaves glycosidic linkages of apical adherens junction proteins, was administered intracerebroventricularly to denude areas of ependymal cells. Substantial ependymal cell loss resulted in reactive gliosis rather than stem cell-mediated regenerative repair of the ventricle lining, and the gliotic regions showed morphologic and phenotypic characteristics similar to those found in aged humans. Increased levels of aquaporin-4, indicative of edema, observed in regions of periventricular gliosis in human tissue were also replicated in our mouse model. 3D modeling together with volume measurements revealed that mice with ventricle surface scarring developed expanded ventricles, independent of neurodegeneration. Through a comprehensive, comparative analysis of the lateral ventricles and associated periventricular tissue in aged humans and mouse, followed by modeling of surface gliosis in mice, we have demonstrated a direct link between lateral ventricle surface gliosis and ventricle enlargement. These studies highlight the importance of maintaining an intact ependymal cell lining throughout aging.

We characterize the landscape of somatic mutations—mutations occurring after fertilization—in the human brain using ultra-deep (~250X) whole-genome sequencing of prefrontal cortex from 59 autism spectrum disorder (ASD) cases and 15 controls. We observe a mean of 26 somatic single nucleotide variants (sSNVs) per brain present in ≥4% of cells, with enrichment of mutations in coding and putative regulatory regions. Our analysis reveals that the first cell division after fertilization produces ~3.4 mutations, followed by 2–3 mutations in subsequent generations. This suggests that a typical individual possesses ~80 sSNVs present in ≥2% of cells—comparable to the number of de novo germline mutations per generation—with about half of individuals having at least one potentially function-altering somatic mutation somewhere in the cortex. ASD brains show an excess of somatic mutations in neural enhancer sequences compared to controls, suggesting that mosaic enhancer mutations may contribute to ASD risk.

Nucleus accumbens is involved in several aspects of instrumental behavior, motivation and learning. Recent studies showed that dopamine (DA) release in the accumbens shell was significantly increased on the first day of training on a fixed ratio (FR) 5 schedule (i.e. the transition from FR1 to FR5) compared with those rats that continued FR1 training, even though the rats on their first day of FR5 training received less food reinforcement than rats continuing on the FR1 schedule. Additionally, the second day of FR5 responding was marked by a significant increase in DA release in accumbens core. The present studies employed immunohistochemical methods to characterize the changes in cellular markers of accumbens and neostriatal neural activity that occur during various stages of food‐reinforced FR5 training. c‐Fos and DARPP‐32 immunoreactivity in accumbens shell was significantly increased on the first day of FR5 training, while core c‐Fos and DARPP‐32 expression showed large increases on the second day of FR5 training. Additional studies showed that c‐Fos and DARPP‐32 expression in neostriatum increased after more extensive training. Double‐labeling studies with immunofluorescence methods indicated that increases in accumbens c‐Fos and DARPP‐32 expression were primarily seen in substance‐P‐positive neurons. These increases in accumbens c‐Fos and DARPP‐32 immunoreactivity seen during the initial phases of FR training may reflect several factors, including novelty, learning, stress or the presentation of a work‐related challenge to the organism. Moreover, it appears that the separate subregions of the striatal complex are differentially activated at distinct phases of instrumental training.

Coordinated regulation of the adult neurogenic subventricular zone (SVZ) is accomplished by a myriad of intrinsic and extrinsic factors. The neurotransmitter dopamine is one regulatory molecule implicated in SVZ function. Nigrostriatal and ventral tegmental area (VTA) midbrain dopamine neurons innervate regions adjacent to the SVZ, and dopamine synapses are found on SVZ cells. Cell division within the SVZ is decreased in humans with Parkinson’s disease and in animal models of Parkinson’s disease following exposure to toxins that selectively remove nigrostriatal neurons, suggesting that dopamine is critical for SVZ function and nigrostriatal neurons are the main suppliers of SVZ dopamine. However, when we examined the aphakia mouse, which is deficient in nigrostriatal neurons, we found no detrimental effect to SVZ proliferation or organization. Instead, dopamine innervation of the SVZ tracked to neurons at the ventrolateral boundary of the VTA. This same dopaminergic neuron population also innervated the SVZ of control mice. Characterization of these neurons revealed expression of proteins indicative of VTA neurons. Furthermore, exposure to the neurotoxin MPTP depleted neurons in the ventrolateral VTA and resulted in decreased SVZ proliferation. Together, these results reveal that dopamine signaling in the SVZ originates from a population of midbrain neurons more typically associated with motivational and reward processing.

The adult subventricular zone (SVZ) supports a population of cells that display the hallmarks of stem cells: they are self-renewing and multipotent-capable of generating neurons, oligodendrocytes, and astrocytes. In vivo, these adult neural stem cells (aNSCs) are fated primarily for a gamma-amino butyric acid (GABA)-ergic lineage of olfactory bulb interneurons, a small subpopulation of which is dopaminergic. Here, we investigate the plasticity of aNSCs in vitro, in particular, their ability to generate a specific neuronal lineage, midbrain dopamine neurons. Previous work using mouse embryonic stem (ES) cells showed that introduction of early developmental inductive cues, sonic hedgehog (SHH) and fibroblast growth factor-8 (FGF-8), directed ES cell-derived neuroepithelial cells to generate midbrain dopaminergic neurons, those lost in Parkinson's disease. Placing aNSCs under similar culture conditions, immunocytochemistry and RT-PCR analysis revealed early dopaminergic neuron specification. However, aNSC-derived neurons remained morphologically immature, exhibiting concurrent nestin and tyrosine hydroxylase (TH) expression, with cell death occurring in the final differentiation stage. High-performance liquid chromatography (HPLC) analysis revealed that while aNSC-derived neurons released dopamine, release was not significantly increased following depolarization with K+. In contrast, ES cell-generated TH+ neurons expressed the mature markers MAP2 and NeuN and showed K+-evoked release of dopamine. Reduced culture time of aNSC-derived nestin+ progenitors in FGF-2-containing medium improved survival of TH+ neurons. However, these neurons exhibited characteristics of forebrain dopamine neurons and also expressed low levels of midbrain transcription factors. Together, our data indicate that when presented with in vitro conditions that promote midbrain-specific dopamine neuron specification, aNSCs instead generate forebrain-like dopamine neurons, demonstrating their restricted and prescribed nature.