Laura K. Hamilton

Widespread deficits in adult neurogenesis precede plaque and tangle formation in the 3xTg mouse model of Alzheimer's disease.

Alzheimer’s disease (AD) affects cognitive modalities that are known to be regulated by adult neurogenesis, such as hippocampal‐ and olfactory‐dependent learning and memory. However, the relationship between AD‐associated pathologies and alterations in adult neurogenesis has remained contentious. In the present study, we performed a detailed investigation of adult neurogenesis in the triple transgenic (3xTg) mouse model of AD, a unique model that generates both amyloid plaques and neurofibrillary tangles, the hallmark pathologies of AD. In both neurogenic niches of the brain, the hippocampal dentate gyrus and forebrain subventricular zone, we found that 3xTg mice had decreased numbers of (i) proliferating cells, (ii) early lineage neural progenitors, and (iii) neuroblasts at middle age (11 months old) and old age (18 months old). These decreases correlated with major reductions in the addition of new neurons to the respective target areas, the dentate granule cell layer and olfactory bulb. Within the subventricular zone niche, cytological alterations were observed that included a selective loss of subependymal cells and the development of large lipid droplets within the ependyma of 3xTg mice, indicative of metabolic changes. Temporally, there was a marked acceleration of age‐related decreases in 3xTg mice, which affected multiple stages of neurogenesis and was clearly apparent prior to the development of amyloid plaques or neurofibrillary tangles. Our findings indicate that AD‐associated mutations suppress neurogenesis early during disease development. This suggests that deficits in adult neurogenesis may mediate premature cognitive decline in AD.

Mammalian Target of Rapamycin Signaling Is a Key Regulator of the Transit-Amplifying Progenitor Pool in the Adult and Aging Forebrain

Adult forebrain neurogenesis is dynamically regulated. Multiple families of niche-derived cues have been implicated in this regulation, but the precise roles of key intracellular signaling pathways remain vaguely defined. Here, we show that mammalian target of rapamycin (mTOR) signaling is pivotal in determining proliferation versus quiescence in the adult forebrain neural stem cell (NSC) niche. Within this niche, mTOR complex-1 (mTORC1) activation displays stage specificity, occurring in transiently amplifying (TA) progenitor cells but not in GFAP+ stem cells. Inhibiting mTORC1 depletes the TA progenitor pool in vivo and suppresses epidermal growth factor (EGF)-induced proliferation within neurosphere cultures. Interestingly, mTORC1 inhibition induces a quiescence-like phenotype that is reversible. Likewise, mTORC1 activity and progenitor proliferation decline within the quiescent NSC niche of the aging brain, while EGF administration reactivates the quiescent niche in an mTORC1-dependent manner. These findings establish fundamental links between mTOR signaling, proliferation, and aging-associated quiescence in the adult forebrain NSC niche.

Aberrant Lipid Metabolism in the Forebrain Niche Suppresses Adult Neural Stem Cell Proliferation in an Animal Model of Alzheimer’s Disease

Lipid metabolism is fundamental for brain development and function, but its roles in normal and pathological neural stem cell (NSC) regulation remain largely unexplored. Here, we uncover a fatty acid-mediated mechanism suppressing endogenous NSC activity in Alzheimers disease (AD). We found that postmortem AD brains and triple-transgenic Alzheimers disease (3xTg-AD) mice accumulate neutral lipids within ependymal cells, the main support cell of the forebrain NSC niche. Mass spectrometry and microarray analyses identified these lipids as oleic acid-enriched triglycerides that originate from niche-derived rather than peripheral lipid metabolism defects. In wild-type mice, locally increasing oleic acid was sufficient to recapitulate the AD-associated ependymal triglyceride phenotype and inhibit NSC proliferation. Moreover, inhibiting the rate-limiting enzyme of oleic acid synthesis rescued proliferative defects in both adult neurogenic niches of 3xTg-AD mice. These studies support a pathogenic mechanism whereby AD-induced perturbation of niche fatty acid metabolism suppresses the homeostatic and regenerative functions of NSCs.

Central canal ependymal cells proliferate extensively in response to traumatic spinal cord injury but not demyelinating lesions.

The adult mammalian spinal cord has limited regenerative capacity in settings such as spinal cord injury (SCI) and multiple sclerosis (MS). Recent studies have revealed that ependymal cells lining the central canal possess latent neural stem cell potential, undergoing proliferation and multi-lineage differentiation following experimental SCI. To determine whether reactive ependymal cells are a realistic endogenous cell population to target in order to promote spinal cord repair, we assessed the spatiotemporal dynamics of ependymal cell proliferation for up to 35 days in three models of spinal pathologies: contusion SCI using the Infinite Horizon impactor, focal demyelination by intraspinal injection of lysophosphatidylcholine (LPC), and autoimmune-mediated multi-focal demyelination using the active experimental autoimmune encephalomyelitis (EAE) model of MS. Contusion SCI at the T9–10 thoracic level stimulated a robust, long-lasting and long-distance wave of ependymal proliferation that peaked at 3 days in the lesion segment, 14 days in the rostral segment, and was still detectable at the cervical level, where it peaked at 21 days. This proliferative wave was suppressed distal to the contusion. Unlike SCI, neither chemical- nor autoimmune-mediated demyelination triggered ependymal cell proliferation at any time point, despite the occurrence of demyelination (LPC and EAE), remyelination (LPC) and significant locomotor defects (EAE). Thus, traumatic SCI induces widespread and enduring activation of reactive ependymal cells, identifying them as a robust cell population to target for therapeutic manipulation after contusion; conversely, neither demyelination, remyelination nor autoimmunity appears sufficient to trigger proliferation of quiescent ependymal cells in models of MS-like demyelinating diseases.

Aging and neurogenesis in the adult forebrain: what we have learned and where we should go from here

In the brains of adult vertebrates, including humans, neurogenesis occurs in restricted niches where it maintains cellular turnover and cognitive plasticity. In virtually all species, however, aging is associated with a significant decline in adult neurogenesis. Moreover, an acceleration of neurogenic defects is observed in models of Alzheimers disease and other neurodegenerative diseases, suggesting an involvement in aging‐ and disease‐associated cognitive deficits. To gain insights into when, how and why adult neurogenesis decreases in the aging brain, we critically reviewed the scientific literature on aging of the rodent subventricular zone, the neurogenic niche of the adult forebrain. Our analysis revealed that deficits in the neurogenic pathway are largely established by middle age, but that there remains striking ambiguity in the underlying mechanisms, especially at the level of stem and progenitor cells. We identify and discuss several challenging issues that have contributed to these key gaps in our current knowledge. In the future, addressing these issues should help untangle the interactions between neurogenesis, aging and aging‐associated diseases.

Bone morphogenetic protein dominantly suppresses epidermal growth factor-induced proliferative expansion of adult forebrain neural precursors

A single asymmetric division by an adult neural stem cell (NSC) ultimately generates dozens of differentiated progeny, a feat made possible by the proliferative expansion of transit-amplifying progenitor cells (TAPs). Although NSC activation and TAP expansion is determined by pro- and anti-proliferative signals found within the niche, remarkably little is known about how these cells integrate simultaneous conflicting signals. We investigated this question focusing on the subventricular zone (SVZ) niche of the adult murine forebrain. Using primary cultures of SVZ cells, we demonstrate that Epidermal Growth Factor (EGF) and Bone Morphogenetic Protein (BMP)-2 are particularly powerful pro- and anti-proliferative factors for SVZ-derived neural precursors. Dose-response experiments showed that when simultaneously exposed to both signals, BMP dominantly suppressed EGF-induced proliferation; moreover, this dominance extended to all parameters of neural precursor behavior tested, including inhibition of proliferation, modulation of cell cycle, promotion of differentiation, and increase of cell death. BMPs anti-proliferative effect did not involve inhibition of mTORC1 or ERK signaling, key mediators of EGF-induced proliferation, and had distinct stage-specific consequences, promoting TAP differentiation but NSC quiescence. In line with these in vitro data, in vivo experiments showed that exogenous BMP limits EGF-induced proliferation of TAPs while inhibition of BMP-SMAD signaling promotes activation of quiescent NSCs. These findings clarify the stage-specific effects of BMPs on SVZ neural precursors, and support a hierarchical model in which the anti-proliferative effects of BMP dominate over EGF proliferation signaling to constitutively drive TAP differentiation and NSC quiescence.

Neural stem cells and adult brain fatty acid metabolism: Lessons from the 3xTg model of Alzheimer's disease: Fatty acid metabolism in the NSC niche during AD

Neural stem cell (NSC) activity and adult neurogenesis are physiologically relevant regulators of adult brain structure, function and repair. Given these roles, the NSC impairments observed in a wide range of neurodegenerative and psychiatric conditions likely factor into the overall cognitive dysfunction in these conditions. We investigated NSC regulation in the context of Alzheimers disease (AD) using the well‐characterised triple transgenic (3xTg) model of AD. In this review, we describe our recent findings that link 3xTg‐AD neurogenesis impairments to AD‐associated abnormalities in brain fatty acid metabolism. Notably, we identified an accumulation of triglycerides rich in oleic acid, a mono‐unsaturated fatty acid, within the forebrain NSC niche in AD. Inhibiting the local conversion of saturated to mono‐unsaturated fatty acids within the brain was sufficient to counteract the loss of NSC activity in 3xTg‐AD mice (Hamilton et al., 2015). We place these findings within the context of recent evidence that dynamic changes in lipid metabolism occur during the transition from NSC quiescence to activation. The picture that emerges is that the critical NSC quiescence‐to‐activation decision is sensitive to the local levels of specific fatty acids and can be impaired by a disease‐associated shift in brain fatty acid balance.

Fate Through Fat: Neutral Lipids as Regulators of Neural Stem Cells

Lipids constitute the majority of the brain’s dry mass, yet little is known about their influences on neural stem cell (NSC) biology during development, adulthood, and disease. This review focuses on “neutral” or simple lipids, most prominently fatty acids, and examines the emerging concept that neutral lipid metabolism plays a prominent role in regulating NSC behaviour under physiological and pathological conditions. We also outline technical approaches for advancing the study of lipid metabolism in the brain.

Acceleration of aging-dependent changes in adult neural stem cell populations in the 3XTg mouse model of Alzheimer's disease

neurotransmitter in learning and memory processes. Alterations in 5-HT neurotransmission have been recently implicated in AD, however, it is not clear how hippocampal 5-HT innervation is modified. Methods: We studied the hippocampal 5-HT input by analyzing (i) the expression, density and distribution of serotonin transporter immunoreactive fibres (SERT-IRF); (ii) the specific morphological characteristics of serotonergic fibres and their relation to amyloid plaques; (iii) the distribution and synaptic connectivity of serotonergic terminals and unmyelinated axons (SERT-Te/Ax) and (iv) the total number of serotonin neurones within the raphe nuclei in the triple transgenic mouse model of Alzheimer’s disease (3xTg-AD). We used quantitative light and electron microscopy immunohistochemistry to analyze the differences between 3xTg-AD and non-transgenic animals (non-Tg) at different ages (3, 6, 9, 12 and 18 months). Results: 3xTg-AD showed a significant increase in SERT-IRF density in the hippocampus in a subfield, strata and age specific manner. The increase in SERT-IRF was specific to the CA1 stratum lacunosum moleculare. Increase in SERT-IRF in 3xTg-AD was observed at 3 months (61%) and at 18 months (74%) when compared to non-Tg. Increased SERT-IRF density was more pronounced adjacent to amyloid plaques. Ultrastructural studies also revealed that the 3xTg-AD animals have an specific concomitant increase of SERT-Te/Ax in the CA1. This Increase in SERTTe/Ax in transgenic animals was observed also at 3 months (146 %) and at 18 months (153 %) of non-Tg values. In addition, SERTTe/Ax had a significant increased surface area these ages (67% and 50% respectively). However, no changes were found in the total number of raphe serotonin neurones at any age. Conclusions: Our results indicate that triple transgenic mice display increased SERT-IRF sprouting and increased SERT-Te density which may account for imbalanced serotonergic neurotransmission associated with Alzheimer’s disease cognitive impairment.