Olfactory Bulb Neurogenesis Research Articles

Enhanced odor discrimination learning in aged Bax-KO mice

Throughout life, new neurons are continuously generated from subventricular zone and added to the olfactory bulb (OB). Because a subset of mature OB neurons undergoes spontaneous cell death, adult OB neurogenesis serves for the replacement of this cell loss. Spontaneous cell turnover should alter the neuronal circuits, but the significance of cell turnover on olfactory learning is yet poorly understood. In this study, we explored the olfactory learning behaviors of model mice showing (1) absence of cell death and cell addition (aged Bax-KO mice); (2) absence of cell death but presence of cell addition (young Bax-KO mice); or (3) presence cell death but absence of cell addition (surgical lesion of rostral migratory stream of neuroblasts). Interestingly, aged Bax-KO mice with no cell replacement acquired the ability to discriminate odor differences faster than WT littermates, whereas other model mice exhibited virtually normal learning ability. These results suggest that the cell replacement is necessary for the normal olfactory learning behavior, and the chronic perturbation of cell replacement may result in the imbalance of neural circuits driving unexpected enhancement of olfactory learning ability.

Targeted Deletion of the ERK5 MAP Kinase Impairs Neuronal Differentiation, Migration, and Survival during Adult Neurogenesis in the Olfactory Bulb

Recent studies have led to the exciting idea that adult-born neurons in the olfactory bulb (OB) may be critical for complex forms of olfactory behavior in mice. However, signaling mechanisms regulating adult OB neurogenesis are not well defined. We recently reported that extracellular signal-regulated kinase (ERK) 5, a MAP kinase, is specifically expressed in neurogenic regions within the adult brain. This pattern of expression suggests a role for ERK5 in the regulation of adult OB neurogenesis. Indeed, we previously reported that conditional deletion of erk5 in adult neurogenic regions impairs several forms of olfactory behavior in mice. Thus, it is important to understand how ERK5 regulates adult neurogenesis in the OB. Here we present evidence that shRNA suppression of ERK5 in adult neural stem/progenitor cells isolated from the subventricular zone (SVZ) reduces neurogenesis in culture. By contrast, ectopic activation of endogenous ERK5 signaling via expression of constitutive active MEK5, an upstream activating kinase for ERK5, stimulates neurogenesis. Furthermore, inducible and conditional deletion of erk5 specifically in the neurogenic regions of the adult mouse brain interferes with cell cycle exit of neuroblasts, impairs chain migration along the rostral migratory stream and radial migration into the OB. It also inhibits neuronal differentiation and survival. These data suggest that ERK5 regulates multiple aspects of adult OB neurogenesis and provide new insights concerning signaling mechanisms governing adult neurogenesis in the SVZ-OB axis.

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.

Spatially regulated adult neurogenesis.

Adult neurogenesis has been the center of attention for decades. Neuroscientists hope to understand the mechanism underlying this phenomenon that might provide a unique perception of brain repair in future. Neurogenesis is referred to the process in which neuronal stem cells and progenitors generate new neurons in non-pathologic setting. Although there are some similarities between two neurogenetic regions including hippocampus and olfactory bulb, however there are some important differences. Regardless of the unique functional roles of ongoing neurogenesis in olfactory bulb and hippocampus, the differences are in terms of consequence of neurogenesis, origin of newly born neurons, responding receptors to nicotine exposure, neuronal migration and GABAergic input between two regions. In this paper, we have briefly reviewed the differences of adult neurogenesis between olfactory bulb and hippocampus.

The generation of oligodendroglial cells is preserved in the rostral migratory stream during aging

The subventricular zone (SVZ) is the largest source of newly generated cells in the adult mammalian brain. SVZ-derived neuroblasts migrate via the rostral migratory stream (RMS) to the olfactory bulb (OB), where they differentiate into mature neurons. Additionally, a small proportion of SVZ-derived cells contribute to the generation of myelinating oligodendrocytes. The production of new cells in the SVZ decreases during aging, affecting the incorporation of new neurons into the OB. However, the age-related changes that occur across the RMS are not fully understood. In this study we evaluate how aging affects the cellular organization of migrating neuroblast chains, the proliferation, and the fate of the newly generated cells in the SVZ-OB system. By using electron microscopy and immunostaining, we found that the RMS path becomes discontinuous and its cytoarchitecture is disorganized in aged mice (24-month-old mice). Subsequently, OB neurogenesis was impaired in the aged brain while the production of oligodendrocytes was not compromised. These findings provide new insight into oligodendrocyte preservation throughout life. Further exploration of this matter could help the development of new strategies to prevent neurological disorders associated with senescence.

Effects of melatonin on adult neurogenesis in the olfactory bulb as well as on learning and memory of renovascular hypertensive rat

Objective To investigate the possible abnormal changes of adult neurogenesis in the rostral migratory stream(RMS) , in the olfactory bulb and in learning or memory of renovascular hypertensive rat, and to evaluate the possible efficacies produced by intervention of exogenous melatonin. Methods Renovascular hypertensive rat model was established by clamping bilateral renal arteries. Rats were randomly divided into four groups with 10 rats per group: hypertensive, sham-operated, melatonin-treated and normal groups. Morris water maze was used to detect the abilities of learning and memory, and immunohistochemistry for bromodeoxyuridine(BrdU), bromodeoxyuridine plus glial fibrillary acidic protein (BrdU+ GFAP) and bromodeoxyuridine plus neurofilament (BrdU+ NT) was utilized to examine the changes of the adult neurogenesis in RMS and in the olfactory bulb of each rat. Results The escape latency of hypertensive group ((29.95±20.11) s) recorded in Morris water maze test was the longest in a significant comparison to those of normal group((20.58±19.18)s), sham-operated group ((22.11±20.28)s) and melatonin-treated group((23.81±22.25)s)(P<0.05); while the time spent by rats during swimming in the quadrant with being originally put platform of hypertensive((25.51±5.21)s) was the shortest by significant comparison to those of normal((34.67±4.53)s), sham-operated((33.56±6.12)s) and melatonin-treated group ((36.23±3.86)s)(P<0.05). Proliferation of neural stem cells(cells/field): the number of BrdU immunoreactive (BrdU-IR) cells of rats in RMS of hypertensive group(34.08±6.49) was the smallest, significantly compared with those of normal(43.53±7.56), sham-operated(46.11±4.75) and melatonin-treated group(44.30±8.29), (P<0.05). Differentiation of neural stem cells(cells/slice): the number of BrdU+ GFAP-IR cells in olfactory bulb of hypertensive group(2.45±1.32) was largest, significantly compared with those of normal(1.01±0.78), sham-operated(0.68±0.37) and melatonin-treated group(1.35±0.88)(P<0.05); while the number of BrdU+ NF-IR cells in olfactory bulb of hypertensive group(1.15±0.81) was the smallest, significantly compared with those of normal (1.89±0.98), sham-operated(1.63±1.01) and melatonin-treated group(2.30±1.22)(P<0.05). Conclusion Melatonin can ameliorate the abnormal cognitive behaviors induced by hypertension via a mechanism of reversing the progression of neurogenesis disorders both in RMS and in olfactory bulb, indicating that melatonin may prevent the development of dementia triggered by hypertension. Key words: Melatonin; Hypertension; Spatial memory; Rostral migratory stream; Olfactory bulb

A Subtype-Specific Critical Period for Neurogenesis in the Postnatal Development of Mouse Olfactory Glomeruli

Sensory input is essential for the normal development of sensory centers in the brain, such as the somatosensory, visual, auditory, and olfactory systems. Visual deprivation during a specific developmental stage, called the critical period, results in severe and irreversible functional impairments in the primary visual cortex. Olfactory deprivation in the early postnatal period also causes significant developmental defects in the olfactory bulb, the primary center for olfaction. Olfactory bulb interneurons are continuously generated from neural stem cells in the ventricular-subventricular zone, suggesting that the olfactory system has plasticity even in adulthood. Here, we investigated the effect of transient neonatal olfactory deprivation on the addition of interneurons to the glomerular layer of the adult mouse olfactory bulb. We found that the addition of one subtype of interneurons was persistently inhibited even after reopening the naris. BrdU pulse-chase experiments revealed that the neonatal olfactory deprivation predominantly affected an early phase in the maturation of this neuronal subtype in the olfactory bulb. Subjecting the mice to odor stimulation for 6 weeks after naris reopening resulted in significant recovery from the histological and functional defects caused by the olfactory deprivation. These results suggest that a subtype-specific critical period exists for olfactory bulb neurogenesis, but that this period is less strict and more plastic compared with the critical periods for other systems. This study provides new insights into the mechanisms of postnatal neurogenesis and a biological basis for the therapeutic effect of olfactory training.

Photoperiod Mediated Changes in Olfactory Bulb Neurogenesis and Olfactory Behavior in Male White-Footed Mice (Peromyscus leucopus)

Brain plasticity, in relation to new adult mammalian neurons generated in the subgranular zone of the hippocampus, has been well described. However, the functional outcome of new adult olfactory neurons born in the subventricular zone of the lateral ventricles is not clearly defined, as manipulating neurogenesis through various methods has given inconsistent and conflicting results in lab mice. Several small rodent species, including Peromyscus leucopus, display seasonal (photoperiodic) brain plasticity in brain volume, hippocampal function, and hippocampus-dependent behaviors; plasticity in the olfactory system of photoperiodic rodents remains largely uninvestigated. We exposed adult male P. leucopus to long day lengths (LD) and short day lengths (SD) for 10 to 15 weeks and then examined olfactory bulb cell proliferation and survival using the thymidine analog BrdU, olfactory bulb granule cell morphology using Golgi-Cox staining, and behavioral investigation of same-sex conspecific urine. SD mice did not differ from LD counterparts in granular cell morphology of the dendrites or in dendritic spine density. Although there were no differences due to photoperiod in habituation to water odor, SD mice rapidly habituated to male urine, whereas LD mice did not. In addition, short day induced changes in olfactory behavior were associated with increased neurogenesis in the caudal plexiform and granule cell layers of the olfactory bulb, an area known to preferentially respond to water-soluble odorants. Taken together, these data demonstrate that photoperiod, without altering olfactory bulb neuronal morphology, alters olfactory bulb neurogenesis and olfactory behavior in Peromyscus leucopus.

Impaired olfactory bulb neurogenesis depends on the presence of human wild-type alpha-synuclein

Synucleinopathies including Parkinson’s disease (PD) are characterized by the accumulation of alpha-synuclein (α-syn) within neural cell bodies and their processes. Transgenic mice overexpressing human wild-type or mutant forms of α-syn under the control of different promoters were developed to analyse the underlying neuropathology of PD. One of the earliest clinical symptoms associated with PD is olfactory impairment. The generation of new neurons persists up to adulthood in mammals, in particular the olfactory bulb (OB). In order to assess this process in relation to α-syn accumulation, we used mice overexpressing human wild-type α-syn under the regulatable control (tet-off) of the calcium/calmodulin-dependent protein kinase IIα-promoter (CaMKII). We observed a decrease in OB neurogenesis in transgenic animals compared to controls using 5-bromo-2′-deoxyuridine (BrdU) to label newly generated cells (neuron-specific nuclear protein; NeuN). After cessation of transgene expression we detected an increase in newly generated cells both in granular (GCL) and glomerular (GLOM) layers of the OB. This led to a rescue of newly generated neurons (BrdU+/NeuN+) within the GLOM with a distinct specificity for the dopaminergic subpopulation. In contrast, we did not detect a cell-specific rescue of neuronal cells in the GCL suggesting diverse effects of alpha-synucleinopathy in both interneuronal layers of the OB. Colabelling of BrdU with glial markers showed that a differentiation into neither astroglia nor microglia attributed to the observed phenotype in the GCL. In particular, BrdU+ particles located within microglial cells were predominantly associated close to the membrane therefore the resembling phagocytosed nuclear fragments of BrdU+ cells. Thus, our study further contributes insights into α-syn accumulation as a causative player in the impairment of adult neurogenesis and emphasizes its diverse role in cell renewal of distinct OB cell layers.

Evidence of the Importance of the First Intracellular Loop of Prokineticin Receptor 2 in Receptor Function

Prokineticin receptors (PROKR) are G protein-coupled receptors (GPCR) that regulate diverse biological processes, including olfactory bulb neurogenesis and GnRH neuronal migration. Mutations in PROKR2 have been described in patients with varying degrees of GnRH deficiency and are located in diverse functional domains of the receptor. Our goal was to determine whether variants in the first intracellular loop (ICL1) of PROKR2 (R80C, R85C, and R85H) identified in patients with hypogonadotropic hypogonadism interfere with receptor function and to elucidate the mechanisms of these effects. Because of structural homology among GPCR, clarification of the role of ICL1 in PROKR2 activity may contribute to a better understanding of this domain across other GPCR. The effects of the ICL1 PROKR2 mutations on activation of signal transduction pathways, ligand binding, and receptor expression were evaluated. Our results indicated that the R85C and R85H PROKR2 mutations interfere only modestly with receptor function, whereas the R80C PROKR2 mutation leads to a marked reduction in receptor activity. Cotransfection of wild-type (WT) and R80C PROKR2 showed that the R80C mutant could exert a dominant negative effect on WT PROKR2 in vitro by interfering with WT receptor expression. In summary, we have shown the importance of Arg80 in ICL1 for PROKR2 expression and demonstrate that R80C PROKR2 exerts a dominant negative effect on WT PROKR2.

Identification of radial glia-like cells in the adult mouse olfactory bulb

Immature neurons migrate tangentially within the rostral migratory stream (RMS) to the adult olfactory bulb (OB), then radially to their final positions as granule and periglomerular neurons; the controls over this transition are not well understood. Using adult transgenic mice with the human GFAP promoter driving expression of enhanced GFP, we identified a population of radial glia-like cells that we term adult olfactory radial glia-like cells (AORGs). AORGs have large, round somas and simple, radially oriented processes. Confocal reconstructions indicate that AORGs variably express typical radial glial markers, only rarely express mouse GFAP, and do not express astroglial, oligodendroglial, neuronal, or tanycyte markers. Electron microscopy provides further supporting evidence that AORGs are not immature neurons. Developmental analyses indicate that AORGs are present as early as P1, and are generated through adulthood. Tracing studies show that AORGs are not born in the SVZa, suggesting that they are born either in the RMS or the OB. Migrating immature neurons from the adult SVZa are closely apposed to AORGs during radial migration in vivo and in vitro. Taken together, these data indicate a newly-identified population of radial glia-like cells in the adult OB that might function uniquely in neuronal radial migration during adult OB neurogenesis.

Spatiotemporal Changes to the Subventricular Zone Stem Cell Pool through Aging

Through adulthood, the rodent subventricular zone (SVZ) stem cell niche generates new olfactory bulb interneurons. We had previously reported that the number of new neurons produced in the SVZ declines through aging; however, age-related changes attributable specifically to the SVZ neural stem cell (NSC) population have not been fully characterized. Here, we conducted a spatiotemporal evaluation of adult SVZ NSCs. We assessed ventricle-contacting NSCs, which together with ependymal cells form regenerative units (pinwheels) along the lateral wall of the lateral ventricle. Based on their apical GFAP-expressing process, individual NSCs were identified across the ventricle surface using serial reconstruction of the SVZ. We observed an 86% decline in total NSCs/mm² of intact ependyma in 2-year old versus 3-month-old mice, with fewer NSC processes within each aged pinwheel. This resulted in an associated 78% decline in total pinwheel units/mm². Regional analysis along the lateral ventricle surface revealed that the age-dependent decline of NSCs and pinwheels is spatially uniform and ultimately maintains the conserved ratio of olfactory bulb interneuron subtypes generated in young mice. However, the overall neurogenic output of the aged SVZ is reduced. Surprisingly, we found no significant change in the number of actively proliferating NSCs per mm² of ventricle surface. Instead, our data reveal that, although the total NSC number, pinwheel units and NSCs per pinwheel decline with age, the percentage of actively, mitotic NSCs increases, indicating that age-related declines in SVZ-mediated olfactory bulb neurogenesis occur downstream of NSC proliferation.

The Age of Olfactory Bulb Neurons in Humans

Continuous turnover of neurons in the olfactory bulb is implicated in several key aspects of olfaction. There is a dramatic decline postnatally in the number of migratory neuroblasts en route to the olfactory bulb in humans, and it has been unclear to what extent the small number of neuroblasts at later stages contributes new neurons to the olfactory bulb. We have assessed the age of olfactory bulb neurons in humans by measuring the levels of nuclear bomb test-derived (14)C in genomic DNA. We report that (14)C concentrations correspond to the atmospheric levels at the time of birth of the individuals, establishing that there is very limited, if any, postnatal neurogenesis in the human olfactory bulb. This identifies a fundamental difference in the plasticity of the human brain compared to other mammals.

Human Adult Olfactory Bulb Neurogenesis? Novelty Is the Best Policy

There is ongoing controversy as to whether the understanding of adult mammalian neurogenesis gained from rodent studies is applicable to humans. In this issue of Neuron, Bergmann et al. (2012) propose that adult human olfactory bulb neurogenesis with long-term neuronal survival is extremely limited.

An Endogenous Vitamin K-Dependent Mechanism Regulates Cell Proliferation in the Brain Subventricular Stem Cell Niche

Neural stem cells (NSC) persist in the adult mammalian brain, within the subventricular zone (SVZ). The endogenous mechanisms underpinning SVZ stem and progenitor cell proliferation are not fully elucidated. Vitamin K-dependent proteins (VKDPs) are mainly secreted factors that were initially discovered as major regulators of blood coagulation. Warfarin ((S(−)-3-acetonylbenzyl)-4-hydroxycoumarin)), a widespread anticoagulant, is a vitamin K antagonist that inhibits the production of functional VKDP. We demonstrate that the suppression of functional VKDPs production, in vitro, by exposure of SVZ cell cultures to warfarin or, in vivo, by its intracerebroventricular injection to mice, leads to a substantial increase in SVZ cell proliferation. We identify the anticoagulant factors, protein S and its structural homolog Gas6, as the two only VKDPs produced by SVZ cells and describe the expression and activation pattern of their Tyro3, Axl, and Mer tyrosine kinase receptors. Both in vitro and in vivo loss of function studies consisting in either Gas6 gene invalidation or in endogenous protein S neutralization, provided evidence for an important novel regulatory role of these two VKDPs in the SVZ neurogenic niche. Specifically, we show that while a loss of Gas6 leads to a reduction in the numbers of stem-like cells and in olfactory bulb neurogenesis, endogenous protein S inhibits SVZ cell proliferation. Our study opens up new perspectives for investigating further the role of vitamin K, VKDPs, and anticoagulants in NSC biology in health and disease. Stem Cells 2012; 30:719–731

Targeted Deletion of ERK5 MAP Kinase in the Developing Nervous System Impairs Development of GABAergic Interneurons in the Main Olfactory Bulb and Behavioral Discrimination between Structurally Similar Odorants

ERK5 MAP kinase is highly expressed in the developing nervous system and has been implicated in promoting the survival of immature neurons in culture. However, its role in the development and function of the mammalian nervous system has not been established in vivo. Here, we report that conditional deletion of the erk5 gene in mouse neural stem cells during development reduces the number of GABAergic interneurons in the main olfactory bulb (OB). Our data suggest that this is due to a decrease in proliferation and an increase in apoptosis in the subventricular zone and rostral migratory stream of ERK5 mutant mice. Interestingly, ERK5 mutant mice have smaller OB and are impaired in odor discrimination between structurally similar odorants. We conclude that ERK5 is a novel signaling pathway regulating developmental OB neurogenesis and olfactory behavior.

Early Activation of Microglia Triggers Long-Lasting Impairment of Adult Neurogenesis in the Olfactory Bulb

Microglia, the innate immune cells of the brain, engulf and eliminate cellular debris during brain injury and disease. Recent observations have extended their roles to the healthy brain, but the functional impact of activated microglia on neural plasticity has so far been elusive. To explore this issue, we investigated the role of microglia in the function of the adult olfactory bulb network in which both sensory afferents and local microcircuits are continuously molded by the arrival of adult-born neurons. We show here that the adult olfactory bulb hosts a large population of resident microglial cells. Deafferentation of the olfactory bulb resulted in a transient activation of microglia and a concomitant reduction of adult olfactory bulb neurogenesis. One day after sensory deafferentation, microglial cells proliferate in the olfactory bulb, and their numbers peaked at day 3, and reversed at day 7 after lesion. Similar lesions performed on immunodeficient mice demonstrate that the both innate and adaptive lymphocyte responses are dispensable for the lesion-induced microglial proliferation and activation. In contrast, when mice were treated with an antiinflammatory drug to prevent microglial activation, olfactory deafferentation did not reduce adult neurogenesis, showing that activated microglial cells per se, and not the lack of sensory experience, relates to the survival of adult-born neurons. We conclude that the status of the resident microglia in the olfactory bulb is an important factor directly regulating the survival of immature adult-born neurons.

Growth hormone responsive neural precursor cells reside within the adult mammalian brain

The detection of growth hormone (GH) and its receptor in germinal regions of the mammalian brain prompted our investigation of GH and its role in the regulation of endogenous neural precursor cell activity. Here we report that the addition of exogenous GH significantly increased the expansion rate in long-term neurosphere cultures derived from wild-type mice, while neurospheres derived from GH null mice exhibited a reduced expansion rate. We also detected a doubling in the frequency of large (i.e. stem cell-derived) colonies for up to 120 days following a 7-day intracerebroventricular infusion of GH suggesting the activation of endogenous stem cells. Moreover, gamma irradiation induced the ablation of normally quiescent stem cells in GH-infused mice, resulting in a decline in olfactory bulb neurogenesis. These results suggest that GH activates populations of resident stem and progenitor cells, and therefore may represent a novel therapeutic target for age-related neurodegeneration and associated cognitive decline.

Dynamic expression of the pro-dopaminergic transcription factors Pax6 and Dlx2 during postnatal olfactory bulb neurogenesis

Olfactory bulb (OB) neurogenesis generates neurons that use GABA or dopamine as their neurotransmitters throughout life. Regionalized stem cell populations in the periventricular zone (PVZ) of the lateral ventricles (LVs) have been shown to be at the basis of neuronal diversity in the system. For example dopaminergic neurons arise predominantly from neural stem cells (NSCs) residing in the dorsal PVZ and depend on the expression of the transcription factors Pax6 and Dlx2 for their specification. In addition, Dlx2 is required for neurogenesis in general. Using targeted in vivo electroporation combined with immuno-fluorescence imaging and microarray analysis, we provide here detailed spatial and temporal expression data with cellular resolution in this system. We find that all along the neurogenic process Pax6 expression remains restricted to the dorsal PVZ, whereas nearly all neuroblasts express Dlx2, including those of the dorsal lineage, which are switched on for Dlx2 when they enter the rostral migratory stream (RMS). These data allow to explain and precise the functions of these two genes in postnatal OB neurogenesis.

Cell transplantation: relevance in understanding brain development and prospects in brain repair

Cell transplantation: relevance in understanding brain development and prospects in brain repair