Hypothalamic neurogenesis persists in the aging brain and is controlled by energy-sensing IGF-I pathway

Potential role of tanycyte-derived neurogenesis in Alzheimer's disease

Tanycytes, specialized ependymal cells located in the hypothalamus, play a crucial role in the generation of new neurons that contribute to the neural circuits responsible for regulating the systemic energy balance. The precise coordination of the gene networks controlling neurogenesis in naive and mature tanycytes is essential for maintaining homeostasis in adulthood. However, our understanding of the molecular mechanisms and signaling pathways that govern the proliferation and differentiation of tanycytes into neurons remains limited. This article aims to review the recent advancements in research into the mechanisms and functions of tanycyte-derived neurogenesis. Studies employing lineage-tracing techniques have revealed that the neurogenesis specifically originating from tanycytes in the hypothalamus has a compensatory role in neuronal loss and helps maintain energy homeostasis during metabolic diseases. Intriguingly, metabolic disorders are considered early biomarkers of Alzheimer's disease. Furthermore, the neurogenic potential of tanycytes and the state of newborn neurons derived from tanycytes heavily depend on the maintenance of mild microenvironments, which may be disrupted in Alzheimer's disease due to the impaired blood-brain barrier function. However, the specific alterations and regulatory mechanisms governing tanycyte-derived neurogenesis in Alzheimer's disease remain unclear. Accumulating evidence suggests that tanycyte-derived neurogenesis might be impaired in Alzheimer's disease, exacerbating neurodegeneration. Confirming this hypothesis, however, poses a challenge because of the lack of long-term tracing and nucleus-specific analyses of newborn neurons in the hypothalamus of patients with Alzheimer's disease. Further research into the molecular mechanisms underlying tanycyte-derived neurogenesis holds promise for identifying small molecules capable of restoring tanycyte proliferation in neurodegenerative diseases. This line of investigation could provide valuable insights into potential therapeutic strategies for Alzheimer's disease and related conditions.

The Vestibular Nuclei: A Cerebral Reservoir of Stem Cells Involved in Balance Function in Normal and Pathological Conditions

In this review, we explore the intriguing realm of neurogenesis in the vestibular nuclei-a critical brainstem region governing balance and spatial orientation. We retrace almost 20 years of research into vestibular neurogenesis, from its discovery in the feline model in 2007 to the recent discovery of a vestibular neural stem cell niche. We explore the reasons why neurogenesis is important in the vestibular nuclei and the triggers for activating the vestibular neurogenic niche. We develop the symbiotic relationship between neurogenesis and gliogenesis to promote vestibular compensation. Finally, we examine the potential impact of reactive neurogenesis on vestibular compensation, highlighting its role in restoring balance through various mechanisms.

The neurobiology of insulin-like growth factor I: From neuroprotection to modulation of brain states

After decades of research in the neurobiology of IGF-I, its role as a prototypical neurotrophic factor is undisputed. However, many of its actions in the adult brain indicate that this growth factor is not only involved in brain development or in the response to injury. Following a three-layer assessment of its role in the central nervous system, we consider that at the cellular level, IGF-I is indeed a bona fide neurotrophic factor, modulating along ontogeny the generation and function of all the major types of brain cells, contributing to sculpt brain architecture and adaptive responses to damage. At the circuit level, IGF-I modulates neuronal excitability and synaptic plasticity at multiple sites, whereas at the system level, IGF-I intervenes in energy allocation, proteostasis, circadian cycles, mood, and cognition. Local and peripheral sources of brain IGF-I input contribute to a spatially restricted, compartmentalized, and timed modulation of brain activity. To better define these variety of actions, we consider IGF-I a modulator of brain states. This definition aims to reconcile all aspects of IGF-I neurobiology, and may provide a new conceptual framework in the design of future research on the actions of this multitasking neuromodulator in the brain.

Neural Progenitor Cells and the Hypothalamus

Neural progenitor cells (NPCs) are multipotent neural stem cells (NSCs) capable of self-renewing and differentiating into neurons, astrocytes and oligodendrocytes. In the postnatal/adult brain, NPCs are primarily located in the subventricular zone (SVZ) of the lateral ventricles (LVs) and subgranular zone (SGZ) of the hippocampal dentate gyrus (DG). There is evidence that NPCs are also present in the postnatal/adult hypothalamus, a highly conserved brain region involved in the regulation of core homeostatic processes, such as feeding, metabolism, reproduction, neuroendocrine integration and autonomic output. In the rodent postnatal/adult hypothalamus, NPCs mainly comprise different subtypes of tanycytes lining the wall of the 3^rd ventricle. In the postnatal/adult human hypothalamus, the neurogenic niche is constituted by tanycytes at the floor of the 3^rd ventricle, ependymal cells and ribbon cells (showing a gap-and-ribbon organization similar to that in the SVZ), as well as suprachiasmatic cells. We speculate that in the postnatal/adult human hypothalamus, neurogenesis occurs in a highly complex, exquisitely sophisticated neurogenic niche consisting of at least four subniches; this structure has a key role in the regulation of extrahypothalamic neurogenesis, and hypothalamic and extrahypothalamic neural circuits, partly through the release of neurotransmitters, neuropeptides, extracellular vesicles (EVs) and non-coding RNAs (ncRNAs).

Cell proliferation and glial cell marker expression in the wall of the third ventricle in the tuberal region of the male mouse hypothalamus during postnatal development

The third ventricle (3 V) wall of the tuberal hypothalamus is composed of two types of cells; specialized ependymoglial cells called tanycytes located ventrally and ependymocytes dorsally, which control the exchanges between the cerebrospinal fluid and the hypothalamic parenchyma. By regulating the dialogue between the brain and the periphery, tanycytes are now recognized as central players in the control of major hypothalamic functions such as energy metabolism and reproduction. While our knowledge of the biology of adult tanycytes is progressing rapidly, our understanding of their development remains very incomplete. To gain insight into the postnatal maturation of the 3 V ependymal lining, we conducted a comprehensive immunofluorescent study of the mouse tuberal region at four postnatal ages (postnatal day (P) 0, P4, P10, and P20). We analyzed the expression profile of a panel of tanycyte and ependymocyte markers (vimentin, S100, connexin-43 [Cx43], and glial fibrillary acidic protein [GFAP]) and characterized cell proliferation in the 3 V wall using the thymidine analog bromodeoxyuridine. Our results show that most changes in marker expression occur between P4 and P10, with a switch from a 3 V mostly lined by radial cells to the emergence of a tanycytic domain ventrally and an ependymocytic domain dorsally, a drop in cell proliferation and increased expression of S100, Cx43, and GFAP that acquire a mature profile at P20. Our study thus identifies the transition between the first and the second postnatal week as a critical time window for the postnatal maturation of the 3 V wall ependymal lining.

Proliferative Markers and Neural Stem Cells Markers in Tanycytes of the Third Cerebral Ventricle in Rats

The proliferative properties of tanycyte subpopulations during postnatal development and aging were studied. Using immunohistochemical markers, we described the distribution of proliferative markers and markers of neural stem cells (NSC) in 4 tanycyte subpopulations (α₁-, α₂-, β₁-, and β₂-tanycytes). During the first postnatal week, all tanycyte subpopulations exhibit proliferative activity. During aging, β-tanycytes lose their proliferative activity and retain a limited set of NSC markers, whereas α-tanycytes maintain both the ability to proliferate and the properties of NSC throughout the entire postnatal development including aging. The data obtained significantly improve modern understanding of the proliferative potential of tanycytes and their subpopulation differences in early postnatal period and during aging.

Developmental and functional relationships between hypothalamic tanycytes and embryonic radial glia

The hypothalamus is a key regulator of several homeostatic processes, such as circadian rhythms, energy balance, thirst, and thermoregulation. Recently, the hypothalamic third ventricle has emerged as a site of postnatal neurogenesis and gliogenesis. This hypothalamic neural stem potential resides in a heterogeneous population of cells known as tanycytes, which, not unlike radial glia, line the floor and ventrolateral walls of the third ventricle and extend a long process into the hypothalamic parenchyma. Here, we will review historical and recent data regarding tanycyte biology across the lifespan, focusing on the developmental emergence of these diverse cells from embryonic radial glia and their eventual role contributing to a fascinating, but relatively poorly characterized, adult neural stem cell niche.

Ontogeny of ependymoglial cells lining the third ventricle in mice

Introduction

During hypothalamic development, the germinative neuroepithelium gives birth to diverse neural cells that regulate numerous physiological functions in adulthood.

Methods

Here, we studied the ontogeny of ependymal cells in the mouse mediobasal hypothalamus using the BrdU approach and publicly available single-cell RNAseq datasets.

Results

We observed that while typical ependymal cells are mainly produced at E13, tanycyte birth depends on time and subtypes and lasts up to P8. Typical ependymocytes and β tanycytes are the first to arise at the top and bottom of the dorsoventral axis around E13, whereas α tanycytes emerge later in development, generating an outside-in dorsoventral gradient along the third ventricle. Additionally, α tanycyte generation displayed a rostral-to-caudal pattern. Finally, tanycytes mature progressively until they reach transcriptional maturity between P4 and P14.

Discussion

Altogether, this data shows that ependyma generation differs in time and distribution, highlighting the heterogeneity of the third ventricle.

Stereological Evidence of Non-Selective Hippocampal Neurodegeneration, IGF-1 Depletion, and Behavioral Deficit following Short Term Bilateral Adrenalectomy in Wistar Rats

The development of animal models to study cell death in the brain is a delicate task. One of the models, that was discovered in the late eighties, is the induction of neurodegeneration through glucocorticoid withdrawal by adrenalectomy in albino rats. Such a model is one of the few noninvasive models for studying neurodegeneration. In the present study, using stereological technique and ultrastructural examination, we aimed to investigate the impact of short-term adrenalectomy (2 weeks) on different hippocampal neuronal populations in Wistar rats. In addition, the underlying mechanism(s) of degeneration in these neurons were investigated by measuring the levels of insulin-like growth factor-1 (IGF-1) and β-nerve growth factor (β-NGF). Moreover, we examined whether the biochemical and histological changes in the hippocampus, after short-term adrenalectomy, have an impact on the cognitive behavior of Wistar rats. Stereological counting in the hippocampus revealed significant neuronal deaths in the dentate gyrus and CA4/CA3, but not in the CA2 and CA1 areas, 7 and 14 days post adrenalectomy. The ultrastructural examinations revealed degenerated and degenerating neurons in the dentate, as well as CA4, and CA3 areas, over the course of 3, 7 and 14 days. The levels of IGF-1 were significantly decreased in the hippocampus of ADX rats 24 h post adrenalectomy, and lasted over the course of two weeks. However, β-NGF was not affected in rats. Using a passive avoidance task, we found a cognitive deficit in the ADX compared to the SHAM operated rats over time (3, 7, and 14 days). In conclusion, both granule and pyramidal cells were degenerated in the hippocampus following short-term adrenalectomy. The early depletion of IGF-1 might play a role in hippocampal neuronal degeneration. Consequently, the loss of the hippocampal neurons after adrenalectomy leads to cognitive deficits.

Protective role of IGF-1 and GLP-1 signaling activation in neurological dysfunctions

Insulin-like growth factor-1 (IGF-1), a pleiotropic polypeptide, plays an essential role in CNS development and maturation. Glucagon-like peptide-1 (GLP-1) is an endogenous incretin hormone that regulates blood glucose levels and fatty acid oxidation in the brain. GLP-1 also exhibits similar functions and growth factor-like properties to IGF-1, which is likely how it exerts its neuroprotective effects. Recent preclinical and clinical evidence indicate that IGF-1 and GLP-1, apart from regulating growth and development, prevent neuronal death mediated by amyloidogenesis, cerebral glucose deprivation, neuroinflammation and apoptosis through modulation of PI3/Akt kinase, mammalian target of rapamycin (mTOR) and mitogen-activated protein kinase (MAPK/ERK). IGF-1 resistance and GLP-1 deficiency impair protective cellular signaling mechanisms, contributing to the progression of neurodegenerative diseases. Over the past decades, IGF-1 and GLP-1 have emerged as an essential component of the neuronal system and as potential therapeutic targets for several neurodegenerative and neuropsychiatric dysfunctions. There is substantial evidence that IGF-1 and GLP-1 analogues penetrate the blood-brain barrier (BBB) and exhibit neuroprotective functions, including synaptic formation, neuronal plasticity, protein synthesis, and autophagy. Conclusively, this review represents the therapeutic potential of IGF-1 and GLP-1 signaling target activators in ameliorating neurological disorders.

IGF-1 receptor regulates upward firing rate homeostasis via the mitochondrial calcium uniporter

An emerging hypothesis is that neuronal circuits homeostatically maintain a stable spike rate despite continuous environmental changes. This firing rate homeostasis is believed to confer resilience to neurodegeneration and cognitive decline. We show that insulin-like growth factor-1 receptor (IGF-1R) is necessary for homeostatic response of mean firing rate to inactivity, termed “upward firing rate homeostasis.” We show that its mechanism of action is to couple spike bursts with downstream mitochondrial Ca²⁺ influx via the mitochondrial calcium uniporter complex (MCUc). We propose that MCUc is a homeostatic Ca²⁺ sensor that triggers the integrated homeostatic response. Firing rate homeostasis may be the principal mechanism by which IGF-1R regulates aging and neurodevelopmental and neurodegenerative disorders.

Regulation of firing rate homeostasis constitutes a fundamental property of central neural circuits. While intracellular Ca²⁺ has long been hypothesized to be a feedback control signal, the molecular machinery enabling a network-wide homeostatic response remains largely unknown. We show that deletion of insulin-like growth factor-1 receptor (IGF-1R) limits firing rate homeostasis in response to inactivity, without altering the distribution of baseline firing rates. The deficient firing rate homeostatic response was due to disruption of both postsynaptic and intrinsic plasticity. At the cellular level, we detected a fraction of IGF-1Rs in mitochondria, colocalized with the mitochondrial calcium uniporter complex (MCUc). IGF-1R deletion suppressed transcription of the MCUc members and burst-evoked mitochondrial Ca²⁺ (mitoCa²⁺) by weakening mitochondria-to-cytosol Ca²⁺ coupling. Overexpression of either mitochondria-targeted IGF-1R or MCUc in IGF-1R–deficient neurons was sufficient to rescue the deficits in burst-to-mitoCa²⁺ coupling and firing rate homeostasis. Our findings indicate that mitochondrial IGF-1R is a key regulator of the integrated homeostatic response by tuning the reliability of burst transfer by MCUc. Based on these results, we propose that MCUc acts as a homeostatic Ca²⁺ sensor. Faulty activation of MCUc may drive dysregulation of firing rate homeostasis in aging and in brain disorders associated with aberrant IGF-1R/MCUc signaling.

Insulin-like growth factor I sensitization rejuvenates sleep patterns in old mice

Sleep disturbances are common during aging. Compared to young animals, old mice show altered sleep structure, with changes in both slow and fast electrocorticographic (ECoG) activity and fewer transitions between sleep and wake stages. Insulin-like growth factor I (IGF-I), which is involved in adaptive changes during aging, was previously shown to increase ECoG activity in young mice and monkeys. Furthermore, IGF-I shapes sleep architecture by modulating the activity of mouse orexin neurons in the lateral hypothalamus (LH). We now report that both ECoG activation and excitation of orexin neurons by systemic IGF-I are abrogated in old mice. Moreover, orthodromical responses of LH neurons are facilitated by either systemic or local IGF-I in young mice, but not in old ones. As orexin neurons of old mice show dysregulated IGF-I receptor (IGF-IR) expression, suggesting disturbed IGF-I sensitivity, we treated old mice with AIK3a305, a novel IGF-IR sensitizer, and observed restored responses to IGF-I and rejuvenation of sleep patterns. Thus, disturbed sleep structure in aging mice may be related to impaired IGF-I signaling onto orexin neurons, reflecting a broader loss of IGF-I activity in the aged mouse brain.

Singular Adult Neural Stem Cells Do Not Exist

Adult neural stem cells (aNSCs) are the source for the continuous production of new neurons throughout life. This so-called adult neurogenesis has been extensively studied; the intermediate cellular stages are well documented. Recent discoveries have raised new controversies in the field, such as the notion that progenitor cells hold similar self-renewal potential as stem cells, or whether different types of aNSCs exist. Here, we discuss evidence for heterogeneity of aNSCs, including short-term and long-term self-renewing aNSCs, regional and temporal differences in aNSC function, and single cell transcriptomics. Reviewing various genetic mouse models used for targeting aNSCs and lineage tracing, we consider potential lineage relationships between Ascl1-, Gli1-, and Nestin-targeted aNSCs. We present a multidimensional model of adult neurogenesis that incorporates recent findings and conclude that stemness is a phenotype, a state of properties that can change with time, rather than a cell property, which is static and immutable. We argue that singular aNSCs do not exist.

Does Sertraline Affect Hypothalamic Food Intake Peptides in the Rat Experimental Model of Chronic Mild Stress-Induced Depression?

Depression is a chronic, recurrent and life-threatening disease affecting approximately 15% of the world population. Depression is responsible for neuropathologies like decreased neurogenesis and increased dendritic atrophy. Antidepressant treatments increase hippocampal neurogenesis and neurotrophic factor expression. Based on this information, it was aimed to investigate effect of sertraline on depression in rats with chronic mild stress (CMS) model and to determine how it affects cell proliferation and hypothalamic peptide levels in hypothalamus. 56 adult male Wistar albino; control, depression(D), depression + sertraline, sertraline were divided into groups. Various stressors were applied to D for 30 days. Open field test (OFT) and forced swimming test (FST) were conducted to check whether the animals were depressed. On the 16th day osmotic minipump was placed subcutaneously and sertraline (10 mg/kg/day) was administered for 15 days. Behavior tests were done. Hypothalamic peptide gene expression levels were analyzed by quantitative RT-PCR. Statistical evaluations were made using ANOVA. It caused a decrease in the percentage of movement in the D and control groups in the OFT, an increase in the immobility time in the D group in the FST, and an increase in the swimming behavior in the DS group. Animals did not show any anxiological behavior based on the elevated plus maze test results. CMS caused a decrease in GLUT2 and NPY gene expression in the hypothalamus of animals, an increase in POMC and FGFR2, and an increase in IGFIR and GLUT2 gene expression in the DS group. Sertraline has been shown to ameliorate the effects of CMS-induced depression. Sertraline is thought to have a positive regulatory effect on both the formation of neural precursor cells and the survival of newly formed neurons in the hypothalamus. Newly formed neurons in the hypothalamus express food intake-related NPY, POMC, GLUT2 neurons, and thus hypothalamic tanycytes may play a key role in the control of energy metabolism.

Lower hypothalamus subunit volumes link with impaired long-term body weight gain after preterm birth

INTRODUCTION

Preterm birth is associated with an increased risk for impaired body weight gain. While it is known that in prematurity several somatic and environmental factors (e.g., endocrine factors, nutrition) modulate short- and long-term body weight gain, the contribution of potentially impaired body weight control in the brain remains elusive. We hypothesized that the structure of hypothalamic nuclei involved in body weight control is altered after preterm birth, with these alterations being associated with aberrant body weight development into adulthood.

MATERIALS AND METHODS

We assessed 101 very preterm (i.e., <32 weeks of gestational age) and/or very low birth weight (i.e., <1500g; VP/VLBW) and 110 full-term born (FT) adults of the population-based Bavarian Longitudinal Study with T1-weighted MRI, deep learning-based hypothalamus subunit segmentation, and multiple body weight assessments from birth into adulthood.

RESULTS

Volumes of the whole hypothalamus and hypothalamus subunits relevant for body weight control were reduced in VP/VLBW adults and associated with birth variables (i.e., gestational age and intensity of neonatal treatment), body weight (i.e., weight at birth and adulthood), and body weight trajectories (i.e., trajectory slopes and cluster/types such as long-term catch-up growth). Particularly, VP/VLBW subgroups, whose individuals showed catch-up growth and/or were small for gestational age, were mostly associated with volumes of distinct hypothalamus subunits such as lateral or infundibular/ventromedial hypothalamus.

CONCLUSION

Results demonstrate lower volumes of body weight control-related hypothalamus subunits after preterm birth that link with long-term body weight gain. Data suggest postnatal development of body weight -related hypothalamic nuclei in VP/VLBW individuals that corresponds with distinct body weight trajectories into adulthood.

Selective Depletion of Adult GFAP-Expressing Tanycytes Leads to Hypogonadotropic Hypogonadism in Males

In adult mammals, neural stem cells are localized in three neurogenic regions, the subventricular zone of the lateral ventricle (SVZ), the subgranular zone of the dentate gyrus of the hippocampus (SGZ) and the hypothalamus. In the SVZ and the SGZ, neural stem/progenitor cells (NSPCs) express the glial fibrillary acidic protein (GFAP) and selective depletion of these NSPCs drastically decreases cell proliferation in vitro and in vivo. In the hypothalamus, GFAP is expressed by α-tanycytes, which are specialized radial glia-like cells in the wall of the third ventricle also recognized as NSPCs. To explore the role of these hypothalamic GFAP-positive tanycytes, we used transgenic mice expressing herpes simplex virus thymidine kinase (HSV-Tk) under the control of the mouse Gfap promoter and a 4-week intracerebroventricular infusion of the antiviral agent ganciclovir (GCV) which kills dividing cells expressing Tk. While GCV significantly reduced the number and growth of hypothalamus-derived neurospheres from adult transgenic mice in vitro, it causes hypogonadotropic hypogonadism in vivo. The selective death of dividing tanycytes expressing GFAP indeed results in a marked decrease in testosterone levels and testicular weight, as well as vacuolization of the seminiferous tubules and loss of spermatogenesis. Additionally, GCV-treated GFAP-Tk mice show impaired sexual behavior, but no alteration in food intake or body weight. Our results also show that the selective depletion of GFAP-expressing tanycytes leads to a sharp decrease in the number of gonadotropin-releasing hormone (GnRH)-immunoreactive neurons and a blunted LH secretion. Overall, our data show that GFAP-expressing tanycytes play a central role in the regulation of male reproductive function.

Irx3 and Irx5 - Novel Regulatory Factors of Postnatal Hypothalamic Neurogenesis

The hypothalamus is a brain region that exhibits highly conserved anatomy across vertebrate species and functions as a central regulatory hub for many physiological processes such as energy homeostasis and circadian rhythm. Neurons in the arcuate nucleus of the hypothalamus are largely responsible for sensing of peripheral signals such as leptin and insulin, and are critical for the regulation of food intake and energy expenditure. While these neurons are mainly born during embryogenesis, accumulating evidence have demonstrated that neurogenesis also occurs in postnatal-adult mouse hypothalamus, particularly in the first two postnatal weeks. This second wave of active neurogenesis contributes to the remodeling of hypothalamic neuronal populations and regulation of energy homeostasis including hypothalamic leptin sensing. Radial glia cell types, such as tanycytes, are known to act as neuronal progenitors in the postnatal mouse hypothalamus. Our recent study unveiled a previously unreported radial glia-like neural stem cell (RGL-NSC) population that actively contributes to neurogenesis in the postnatal mouse hypothalamus. We also identified Irx3 and Irx5, which encode Iroquois homeodomain-containing transcription factors, as genetic determinants regulating the neurogenic property of these RGL-NSCs. These findings are significant as IRX3 and IRX5 have been implicated in FTO-associated obesity in humans, illustrating the importance of postnatal hypothalamic neurogenesis in energy homeostasis and obesity. In this review, we summarize current knowledge regarding postnatal-adult hypothalamic neurogenesis and highlight recent findings on the radial glia-like cells that contribute to the remodeling of postnatal mouse hypothalamus. We will discuss characteristics of the RGL-NSCs and potential actions of Irx3 and Irx5 in the regulation of neural stem cells in the postnatal-adult mouse brain. Understanding the behavior and regulation of neural stem cells in the postnatal-adult hypothalamus will provide novel mechanistic insights in the control of hypothalamic remodeling and energy homeostasis.

Adult-born proopiomelanocortin neurons derived from Rax-expressing precursors mitigate the metabolic effects of congenital hypothalamic proopiomelanocortin deficiency

OBJECTIVE

Proopiomelanocortin (POMC) neurons of the hypothalamic arcuate nucleus are essential regulators of energy balance. Selective loss of POMC production in these cells results in extreme obesity and metabolic comorbidities. Neurogenesis occurs in the adult hypothalamus, but it remains uncertain whether functional POMC neurons emerge in physiologically significant numbers during adulthood. Here, we tested whether Rax-expressing precursors generate POMC neurons in adult mice and rescue the metabolic phenotype caused by congenital hypothalamic POMC deficiency.

METHODS

Initially, we identified hypothalamic Rax-expressing cell types using wild-type and Rax-CreERT2:Ai34D mice. Then we generated compound Rax-CreERT2:ArcPomcloxTB/loxTB mice in which endogenous hypothalamic Pomc expression is silenced, but can be restored by tamoxifen administration selectively in neurons derived from Rax+ progenitors. The number of POMC neurons generated by Rax+ progenitors in adult mice and their axonal projections was determined. The metabolic effects of these neurons were assessed by measuring food intake, bodyweight, and body composition, along with glucose and insulin levels.

RESULTS

We found that Rax is expressed by tanycytes and a previously unrecognized cell type in the hypothalamic parenchyma of adult mice. Rax+ progenitors generated ~10% of the normal adult hypothalamic POMC neuron population within two weeks of tamoxifen treatment. The same rate and steady state of POMC neurogenesis persisted from young adult to aged mice. These new POMC neurons established terminal projections to brain regions that were involved in energy homeostasis. Mice with Rax+ progenitor-derived POMC neurons had reduced body fat mass, improved glucose tolerance, increased insulin sensitivity, and decreased bodyweight in proportion to the number of new POMC neurons.

CONCLUSIONS

These data demonstrate that Rax+ progenitors generate POMC neurons in sufficient numbers during adulthood to mitigate the metabolic abnormalities of hypothalamic POMC-deficient mice. The findings suggest that adult hypothalamic neurogenesis is a robust phenomenon in mice that can significantly impact energy homeostasis.

Adult hypothalamic neurogenesis and sleep-wake dysfunction in aging

In the mammalian brain, adult neurogenesis has been extensively studied in the hippocampal sub-granular zone and the sub-ventricular zone of the anterolateral ventricles. However, growing evidence suggests that new cells are not only "born" constitutively in the adult hypothalamus, but many of these cells also differentiate into neurons and glia and serve specific functions. The preoptic-hypothalamic area plays a central role in the regulation of many critical functions, including sleep-wakefulness and circadian rhythms. While a role for adult hippocampal neurogenesis in regulating hippocampus-dependent functions, including cognition, has been extensively studied, adult hypothalamic neurogenic process and its contributions to various hypothalamic functions, including sleep-wake regulation are just beginning to unravel. This review is aimed at providing the current understanding of the hypothalamic adult neurogenic processes and the extent to which it affects hypothalamic functions, including sleep-wake regulation. We propose that hypothalamic neurogenic processes are vital for maintaining the proper functioning of the hypothalamic sleep-wake and circadian systems in the face of regulatory challenges. Sleep-wake disturbance is a frequent and challenging problem of aging and age-related neurodegenerative diseases. Aging is also associated with a decline in the neurogenic process. We discuss a hypothesis that a decrease in the hypothalamic neurogenic process underlies the aging of its sleep-wake and circadian systems and associated sleep-wake disturbance. We further discuss whether neuro-regenerative approaches, including pharmacological and non-pharmacological stimulation of endogenous neural stem and progenitor cells in hypothalamic neurogenic niches, can be used for mitigating sleep-wake and other hypothalamic dysfunctions in aging.

Identification of Sox2 and NeuN Double-Positive Cells in the Mouse Hypothalamic Arcuate Nucleus and Their Reduction in Number With Aging

The hypothalamus plays a central role in homeostasis and aging. The hypothalamic arcuate nucleus (ARC) controls homeostasis of food intake and energy expenditure and retains adult neural stem cells (NSCs)/progenitor cells. Aging induces the loss of NSCs and the enhancement of inflammation, including the activation of glial cells in the ARC, but aging-associated alterations of the hypothalamic cells remain obscure. Here, we identified Sox2 and NeuN double-positive cells in a subpopulation of cells in the mouse ARC. These cells were reduced in number with aging, although NeuN-positive neuronal cells were unaltered in the total number. Diet-induced obesity mice fed with high-fat diet presented a similar hypothalamic alteration to aged mice. This study provides a new insight into aging-induced changes in the hypothalamus.

Thyroid hormone and hypothalamic stem cells in seasonal functions

Seasonal rhythms are a pervasive feature of most living organisms, which underlie yearly timeliness in breeding, migration, hibernation or weight gain and loss. To achieve this, organisms have developed inner timing devices (circannual clocks) that endow them with the ability to predict then anticipate changes to come, usually using daylength as the proximate cue. In Vertebrates, daylength interpretation involves photoperiodic control of TSH production by the pars tuberalis (PT) of the pituitary, which governs a seasonal switch in thyroid hormone (TH) availability in the neighboring hypothalamus. Tanycytes, specialized glial cells lining the third ventricle (3V), are responsible for this TH output through the opposite, PT-TSH-driven, seasonal control of deiodinases 2/3 (Dio 2/3). Tanycytes comprise a photoperiod-sensitive stem cell niche and TH is known to play major roles in cell proliferation and differentiation, which suggests that seasonal control of tanycyte proliferation may be involved in the photoperiodic synchronization of seasonal rhythms. Here we review our current knowledge of the molecular and neuroendocrine pathway linking photoperiodic information to seasonal changes in physiological functions and discuss the potential implication of tanycytes, TH and cell proliferation in seasonal timing.

Tanycytes in the infundibular nucleus and median eminence and their role in the blood-brain barrier

The blood-brain barrier is generally attributed to endothelial cells. However, in circumventricular organs, such as the median eminence, tanycytes take over the barrier function. These ependymoglial cells form the wall of the third ventricle and send long extensions into the parenchyma to contact blood vessels and hypothalamic neurons. The shape and location of tanycytes put them in an ideal position to connect the periphery with central nervous compartments. In line with this, tanycytes control the transport of hormones and key metabolites in and out of the hypothalamus. They function as sensors of peripheral homeostasis for central regulatory networks. This chapter discusses current evidence that tanycytes play a key role in regulating glucose balance, food intake, endocrine axes, seasonal changes, reproductive function, and aging. The understanding of how tanycytes perform these diverse tasks is only just beginning to emerge and will probably lead to a more differentiated view of how the brain and the periphery interact.

Neurogenesis in the adult hypothalamus: A distinct form of structural plasticity involved in metabolic and circadian regulation, with potential relevance for human pathophysiology

The adult brain harbors specific niches where stem cells undergo substantial plasticity and, in some regions, generate new neurons throughout life. This phenomenon is well known in the subventricular zone of the lateral ventricles and the subgranular zone of the hippocampus and has recently also been described in the hypothalamus of several rodent and primate species. After a brief overview of preclinical studies illustrating the pathophysiologic significance of hypothalamic neurogenesis in the control of energy metabolism, reproduction, thermoregulation, sleep, and aging, we review current literature on the neurogenic niche of the human hypothalamus. A comparison of the organization of the niche between humans and rodents highlights some common features, but also substantial differences, e.g., in the distribution and extent of the hypothalamic neural stem cells. Exploring the full dynamics of hypothalamic neurogenesis in humans raises a formidable challenge however, given among others, inherent technical limitations. We close with discussing possible functional role(s) of the human hypothalamic niche, and how gaining more insights into this form of plasticity could be relevant for a better understanding of pathologies associated with disturbed hypothalamic function.

The FGF2-induced tanycyte proliferation involves a connexin 43 hemichannel/purinergic-dependent pathway

In the adult hypothalamus, the neuronal precursor role is attributed to the radial glia-like cells that line the third-ventricle (3V) wall called tanycytes. Under nutritional cues, including hypercaloric diets, tanycytes proliferate and differentiate into mature neurons that moderate body weight, suggesting that hypothalamic neurogenesis is an adaptive mechanism in response to metabolic changes. Previous studies have shown that the tanycyte glucosensing mechanism depends on connexin-43 hemichannels (Cx43 HCs), purine release, and increased intracellular free calcium ion concentration [(Ca²⁺ )_i ] mediated by purinergic P2Y receptors. Since, Fibroblast Growth Factor 2 (FGF2) causes similar purinergic events in other cell types, we hypothesize that this pathway can be also activated by FGF2 in tanycytes to promote their proliferation. Here, we used bromodeoxyuridine (BrdU) incorporation to evaluate if FGF2-induced tanycyte cell division is sensitive to Cx43 HC inhibition in vitro and in vivo. Immunocytochemical analyses showed that cultured tanycytes maintain the expression of in situ markers. After FGF2 exposure, tanycytic Cx43 HCs opened, enabling release of ATP to the extracellular milieu. Moreover, application of external ATP was enough to induce their cell division, which could be suppressed by Cx43 HC or P2Y1-receptor inhibitors. Similarly, in vivo experiments performed on rats by continuous infusion of FGF2 and a Cx43 HC inhibitor into the 3V, demonstrated that FGF2-induced β-tanycyte proliferation is sensitive to Cx43 HC blockade. Thus, FGF2 induced Cx43 HC opening, triggered purinergic signaling, and increased β-tanycytes proliferation, highlighting some of the molecular mechanisms involved in the cell division response of tanycyte. This article has an Editorial Highlight see https://doi.org/10.1111/jnc.15218.

The Lineage Before Time: Circadian and Nonclassical Clock Influences on Development

Diverse factors including metabolism, chromatin remodeling, and mitotic kinetics influence development at the cellular level. These factors are well known to interact with the circadian transcriptional-translational feedback loop (TTFL) after its emergence. What is only recently becoming clear, however, is how metabolism, mitosis, and epigenetics may become organized in a coordinated cyclical precursor signaling module in pluripotent cells prior to the onset of TTFL cycling. We propose that both the precursor module and the TTFL module constrain cellular identity when they are active during development, and that the emergence of these modules themselves is a key lineage marker. Here we review the component pathways underlying these ideas; how proliferation, specification, and differentiation decisions in both developmental and adult stem cell populations are or are not regulated by the classical TTFL; and emerging evidence that we propose implies a primordial clock that precedes the classical TTFL and influences early developmental decisions.

Hypothalamic plasticity in response to changes in photoperiod and food quality: An adaptation to support pre-migratory fattening in songbirds?

In latitudinal avian migrants, increasing photoperiods induce fat deposition and body mass increase, and subsequent night-time migratory restlessness in captive birds, but the underlying mechanisms remain poorly understood. We hypothesized that an enhanced hypothalamic neuronal plasticity was associated with the photostimulated spring migration phenotype. We tested this idea in adult migratory red-headed buntings (Emberiza bruniceps), as compared with resident Indian weaverbirds (Ploceus philippinus). Birds were exposed to a stimulatory long photoperiod (14L:10D, LP), while controls were kept on a short photoperiod (10L:14D, SP). Under both photoperiods, one half of birds also received a high calorie, protein- and fat-rich diet (SP-R, LP-R) while the other half stayed on the normal diet (SP-N, LP-N). Thirty days later, as expected, the LP had induced multiple changes in the behaviour and physiology in migratory buntings. Photostimulated buntings also developed a preference for the rich food diet. Most interestingly, the LP and the rich diet, both separately and in association, increased neurogenesis in the mediobasal hypothalamus (MBH), as measured by an increased number of cells immunoreactive for doublecortin (DCX), a marker of recently born neurons, in buntings, but not weaverbirds. This neurogenesis was associated with an increased density of fibres immunoreactive for the orexigenic neuropeptide Y (NPY). This hypothalamic plasticity observed in a migratory, but not in a non-migratory, species in response to photoperiod and food quality might represent an adaptation to the pre-migratory fattening, as required to support the extensive energy expenses that incur during the migratory flight.

Woo E, Patten A, Yau SY, Gil-Mohapel J. Beyond the Hippocampus and the SVZ: Adult Neurogenesis Throughout the Brain

Convincing evidence has repeatedly shown that new neurons are produced in the mammalian brain into adulthood. Adult neurogenesis has been best described in the hippocampus and the subventricular zone (SVZ), in which a series of distinct stages of neuronal development has been well characterized. However, more recently, new neurons have also been found in other brain regions of the adult mammalian brain, including the hypothalamus, striatum, substantia nigra, cortex, and amygdala. While some studies have suggested that these new neurons originate from endogenous stem cell pools located within these brain regions, others have shown the migration of neurons from the SVZ to these regions. Notably, it has been shown that the generation of new neurons in these brain regions is impacted by neurologic processes such as stroke/ischemia and neurodegenerative disorders. Furthermore, numerous factors such as neurotrophic support, pharmacologic interventions, environmental exposures, and stem cell therapy can modulate this endogenous process. While the presence and significance of adult neurogenesis in the human brain (and particularly outside of the classical neurogenic regions) is still an area of debate, this intrinsic neurogenic potential and its possible regulation through therapeutic measures present an exciting alternative for the treatment of several neurologic conditions. This review summarizes evidence in support of the classic and novel neurogenic zones present within the mammalian brain and discusses the functional significance of these new neurons as well as the factors that regulate their production. Finally, it also discusses the potential clinical applications of promoting neurogenesis outside of the classical neurogenic niches, particularly in the hypothalamus, cortex, striatum, substantia nigra, and amygdala.

Fibroblast growth factor 10 is a negative regulator of postnatal neurogenesis in the mouse hypothalamus

New neurons are generated in the postnatal rodent hypothalamus, with a subset of tanycytes in the third ventricular (3V) wall serving as neural stem/progenitor cells. However, the precise stem cell niche organization, the intermediate steps and the endogenous regulators of postnatal hypothalamic neurogenesis remain elusive. Quantitative lineage-tracing in vivo revealed that conditional deletion of fibroblast growth factor 10 (Fgf10) from Fgf10-expressing β-tanycytes at postnatal days (P)4-5 results in the generation of significantly more parenchymal cells by P28, composed mostly of ventromedial and dorsomedial neurons and some glial cells, which persist into adulthood. A closer scrutiny in vivo and ex vivo revealed that the 3V wall is not static and is amenable to cell movements. Furthermore, normally β-tanycytes give rise to parenchymal cells via an intermediate population of α-tanycytes with transient amplifying cell characteristics. Loss of Fgf10 temporarily attenuates the amplification of β-tanycytes but also appears to delay the exit of their α-tanycyte descendants from the germinal 3V wall. Our findings suggest that transience of cells through the α-tanycyte domain is a key feature, and Fgf10 is a negative regulator of postnatal hypothalamic neurogenesis.

Summary: Generation of new hypothalamic neurons after birth is a multistep process involving cell division and cell movements that are controlled by Fgf10.

Norepinephrine is a negative regulator of the adult periventricular neural stem cell niche

The limited proliferative capacity of neuroprogenitor cells (NPCs) within the periventricular germinal niches (PGNs) located caudal of the subventricular zone (SVZ) of the lateral ventricles together with their high proliferation capacity after isolation strongly implicates cell-extrinsic humoral factors restricting NPC proliferation in the hypothalamic and midbrain PGNs. We comparatively examined the effects of norepinephrine (NE) as an endogenous candidate regulator of PGN neurogenesis in the SVZ as well as the periventricular hypothalamus and the periaqueductal midbrain. Histological and neurochemical analyses revealed that the pattern of NE innervation of the adult PGNs is inversely associated with their in vivo NPC proliferation capacity with low NE levels coupled to high NPC proliferation in the SVZ but high NE levels coupled to low NPC proliferation in hypothalamic and midbrain PGNs. Intraventricular infusion of NE decreased NPC proliferation and neurogenesis in the SVZ-olfactory bulb system, while pharmacological NE inhibition increased NPC proliferation and early neurogenesis events in the caudal PGNs. Neurotoxic ablation of NE neurons using the Dsp4-fluoxetine protocol confirmed its inhibitory effects on NPC proliferation. Contrarily, NE depletion largely impairs NPC proliferation within the hippocampus in the same animals. Our data indicate that norepinephrine has opposite effects on the two fundamental neurogenic niches of the adult brain with norepinephrine being a negative regulator of adult periventricular neurogenesis. This knowledge might ultimately lead to new therapeutic approaches to influence neurogenesis in hypothalamus-related metabolic diseases or to stimulate endogenous regenerative potential in neurodegenerative processes such as Parkinson's disease.

Adult Neurogenesis in the Context of Brain Repair and Functional Relevance

Urodeles and some fishes possess a remarkable capacity to regenerate their limbs/fins, a property that correlates with their additional ability to regenerate large areas of the brain and/or produce a variety of new neurons during adulthood. In contrast, neurogenesis in adult mammals is apparently restricted to two main regions, the subventricular zone of lateral ventricles and the subgranular zone of the hippocampus. There, astrocyte-like neural stem cells (NSCs) reside and derive into new neurons. Although it is becoming apparent that other brain regions carry out neurogenesis, in many cases, its functional significance is controversial, particularly, because very few putative NSCs capable of deriving into new neurons have been found. Hence, is renewal of certain neurons a requirement for a healthy brain? Are there specific physiological conditions that stimulate neurogenesis in a particular region? Does the complexity of the brain demand reduced neurogenesis? In this study, we review the production of new neurons in the vertebrate adult brain in the context of a possible functional relevance. In addition, we consider the intrinsic properties of potential cellular sources of new neurons, as well as the contribution of the milieu surrounding them to estimate the reparative capacity of the brain upon injury or a neurodegenerative condition. The conclusion of this review should bring into debate the potential and convenience of promoting neuronal regeneration in the adult human brain.

Lifestyle mediates the role of nutrient-sensing pathways in cognitive aging: cellular and epidemiological evidence

Aging induces cellular and molecular changes including modification of stem cell pools. In particular, alterations in aging neural stem cells (NSCs) are linked to age-related cognitive decline which can be modulated by lifestyle. Nutrient-sensing pathways provide a molecular basis for the link between lifestyle and cognitive decline. Adopting a back-translation strategy using stem cell biology to inform epidemiological analyses, here we show associations between cellular readouts of NSC maintenance and expression levels of nutrient-sensing genes following NSC exposure to aging human serum as well as morphological and gene expression alterations following repeated passaging. Epidemiological analyses on the identified genes showed associations between polymorphisms in SIRT1 and ABTB1 and cognitive performance as well as interactions between SIRT1 genotype and physical activity and between GRB10 genotype and adherence to a Mediterranean diet. Our study contributes to the understanding of neural stem cell molecular mechanisms underlying human cognitive aging and hints at lifestyle modifiable factors.

Quiescent Neural Stem Cells for Brain Repair and Regeneration: Lessons from Model Systems

Neural stem cells (NSCs) are multipotent progenitors that are responsible for producing all of the neurons and macroglia in the nervous system. In adult mammals, NSCs reside predominantly in a mitotically dormant, quiescent state, but they can proliferate in response to environmental inputs such as feeding or exercise. It is hoped that quiescent NSCs could be activated therapeutically to contribute towards repair in humans. This will require an understanding of quiescent NSC heterogeneities and regulation during normal physiology and following brain injury. Non-mammalian vertebrates (zebrafish and salamanders) and invertebrates (Drosophila) offer insights into brain repair and quiescence regulation that are difficult to obtain using rodent models alone. We review conceptual progress from these various models, a first step towards harnessing quiescent NSCs for therapeutic purposes.

Targeting Tanycytes: Balance between Efficiency and Specificity

Tanycytes are peculiar ependymoglial cells lining the bottom and the lateral wall of the third ventricle. For a decade, the utilization of molecular genetic approaches allowed us to make important discoveries about their diverse physiological functions. Here, I review the current methods used to target tanycytes, focusing on their specificity, their efficiency, their limitations, as well as their potential future improvements.

An integrative view of mammalian seasonal neuroendocrinology

Seasonal neuroendocrine cycles that govern annual changes in reproductive activity, energy metabolism and hair growth are almost ubiquitous in mammals that have evolved at temperate and polar latitudes. Changes in nocturnal melatonin secretion regulating gene expression in the pars tuberalis (PT) of the pituitary stalk are a critical common feature in seasonal mammals. The PT sends signal(s) to the pars distalis of the pituitary to regulate prolactin secretion and thus the annual moult cycle. The PT also signals in a retrograde manner via thyroid-stimulating hormone to tanycytes, which line the ventral wall of the third ventricle in the hypothalamus. Tanycytes show seasonal plasticity in gene expression and play a pivotal role in regulating local thyroid hormone (TH) availability. Within the mediobasal hypothalamus, the cellular and molecular targets of TH remain elusive. However, two populations of hypothalamic neurones, which produce the RF-amide neuropeptides kisspeptin and RFRP3 (RF-amide related peptide 3), are plausible relays between TH and the gonadotrophin-releasing hormone-pituitary-gonadal axis. By contrast, the ways by which TH also impinges on hypothalamic systems regulating energy intake and expenditure remain unknown. Here, we review the neuroendocrine underpinnings of seasonality and identify several areas that warrant further research.

Lack of Brain Serotonin Affects Feeding and Differentiation of Newborn Cells in the Adult Hypothalamus

Serotonin (5-HT) is a crucial signal in the neurogenic niche microenvironment. Dysregulation of the 5-HT system leads to mood disorders but also to changes in appetite and metabolic rate. Tryptophan hydroxylase 2-deficient (Tph2^-/-) mice depleted of brain 5-HT display alterations in these parameters, e.g., increased food consumption, modest impairment of sleep and respiration accompanied by a less anxious phenotype. The newly discovered neural stem cell niche of the adult hypothalamus has potential implications of mediating stress responses and homeostatic functions. Using Tph2^-/- mice, we explore stem cell behavior and cell genesis in the adult hypothalamus. Specifically, we examine precursor cell proliferation and survival in Tph2^-/- mice at baseline and following Western-type diet (WD). Our results show a decline in BrdU numbers with aging in the absence of 5-HT. Furthermore, wild type mice under dietary challenge decrease cell proliferation and survival in the hypothalamic niche. In contrast, increased high-calorie food intake by Tph2^-/- mice does not come along with alterations in cell numbers. However, lack of brain 5-HT results in a shift of cell phenotypes that was abolished under WD. We conclude that precursor cells in the hypothalamus retain fate plasticity and respond to environmental challenges. A novel link between 5-HT signaling and cell genesis in the hypothalamus could be exploited as therapeutic target in metabolic disease.

Tanycytes: A rich morphological history to underpin future molecular and physiological investigations

Tanycytes are located at the base of the brain and retain characteristics from their developmental origins, such as radial glial cells, throughout their life span. With transport mechanisms and modulation of tight junction proteins, tanycytes form a bridge connecting the cerebrospinal fluid with the external limiting basement membrane. They also retain the powers of self-renewal and can differentiate to generate neurones and glia. Similar to radial glia, they are a heterogeneous family with distinct phenotypes. Although the four subtypes so far distinguished display distinct characteristics, further research is likely to reveal new subtypes. In this review, we have re-visited the work of the pioneers in the field, revealing forgotten work that is waiting to inspire new research with today's cutting-edge technologies. We have conducted a systematic ultrastructural study of α-tanycytes that resulted in a wealth of new information, generating numerous questions for future study. We also consider median eminence pituicytes, a closely-related cell type to tanycytes, and attempt to relate pituicyte fine morphology to molecular and functional mechanism. Our rationale was that future research should be guided by a better understanding of the early pioneering work in the field, which may currently be overlooked when interpreting newer data or designing new investigations.

The Surface Proteome of Adult Neural Stem Cells in Zebrafish Unveils Long-Range Cell-Cell Connections and Age-Related Changes in Responsiveness to IGF

In adult stem cell populations, recruitment into division is parsimonious and most cells maintain a quiescent state. How individual cells decide to enter the cell cycle and how they coordinate their activity remains an essential problem to be resolved. It is thus important to develop methods to elucidate the mechanisms of cell communication and recruitment into the cell cycle. We made use of the advantageous architecture of the adult zebrafish telencephalon to isolate the surface proteins of an intact neural stem cell (NSC) population. We identified the proteome of NSCs in young and old brains. The data revealed a group of proteins involved in filopodia, which we validated by a morphological analysis of single cells, showing apically located cellular extensions. We further identified an age-related decrease in insulin-like growth factor (IGF) receptors. Expressing IGF2b induced divisions in young brains but resulted in incomplete divisions in old brains, stressing the role of cell-intrinsic processes in stem cell behavior.

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The cell-surface proteome of an intact adult neural stem cell population was identified

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Zebrafish adult neural stem cells harbor filopodia on their apical surface

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Aging neural stem cells display an altered mitotic response to IGF ligands

Tanycyte Gene Expression Dynamics in the Regulation of Energy Homeostasis

Animal survival relies on a constant balance between energy supply and energy expenditure, which is controlled by several neuroendocrine functions that integrate metabolic information and adapt the response of the organism to physiological demands. Polarized ependymoglial cells lining the floor of the third ventricle and sending a single process within metabolic hypothalamic parenchyma, tanycytes are henceforth described as key components of the hypothalamic neural network controlling energy balance. Their strategic position and peculiar properties convey them diverse physiological functions ranging from blood/brain traffic controllers, metabolic modulators, and neural stem/progenitor cells. At the molecular level, these functions rely on an accurate regulation of gene expression. Indeed, tanycytes are characterized by their own molecular signature which is mostly associated to their diverse physiological functions, and the detection of variations in nutrient/hormone levels leads to an adequate modulation of genetic profile in order to ensure energy homeostasis. The aim of this review is to summarize recent knowledge on the nutritional control of tanycyte gene expression.

Metabolism and adult neurogenesis: Towards an understanding of the role of lipocalin-2 and iron-related oxidative stress

The process of generating new functional neurons in the adult mammalian brain occurs from the local neural stem and progenitor cells and requires tight control of the progenitor cell's activity. Several signaling pathways and intrinsic/extrinsic factors have been well studied over the last years, but recent attention has been given to the critical role of cellular metabolism in determining the functional properties of progenitor cells. Here, we review recent advances in the current understanding of when and how metabolism affects neural stem cell (NSC) behavior and subsequent neuronal differentiation and highlight the role of lipocalin-2 (LCN2), a protein involved in the control of oxidative stress, as a recently emerged regulator of NSC activity and neuronal differentiation.

Prss56 expression in the rodent hypothalamus: Inverse correlation with pro-opiomelanocortin suggests oscillatory gene expression in adult rat tanycytes

We recently reported that the number of hypothalamic tanycytes expressing pro-opiomelanocortin (Pomc) is highly variable among brains of adult rats. While its cause and significance remain unknown, identifying other variably expressed genes in tanycytes may help understand this curious phenomenon. In this in situ hybridization study, we report that the Prss56 gene, which encodes a trypsin-like serine protease and is expressed in neural stem/progenitor cells, shows a similarly variable mRNA expression in tanycytes of adult rats and correlates inversely with tanycyte Pomc mRNA. Prss56 was expressed in α1, β1, subsets of α2, and some median eminence γ tanycytes, but virtually absent from β2 tanycytes. Prss56 was also expressed in vimentin positive tanycyte-like cells in the parenchyma of the ventromedial and arcuate nuclei, and in thyrotropin beta subunit-expressing cells of the pars tuberalis of the pituitary. In contrast to adults, Prss56 expression was uniformly high in tanycytes in adolescent rats. In mice, Prss56-expressing tanycytes and parenchymal cells were also observed but fewer in number and without significant variations. The results identify Prss56 as a second gene that is expressed variably in tanycytes of adult rats. We propose that the variable, inversely correlating expression of Prss56 and Pomc reflect periodically oscillating gene expression in tanycytes rather than stable expression levels that vary between individual rats. A possible functional link between Prss56 and POMC, and Prss56 as a potential marker for migrating tanycytes are discussed.

The Versatile Tanycyte: A Hypothalamic Integrator of Reproduction and Energy Metabolism

The fertility and survival of an individual rely on the ability of the periphery to promptly, effectively, and reproducibly communicate with brain neural networks that control reproduction, food intake, and energy homeostasis. Tanycytes, a specialized glial cell type lining the wall of the third ventricle in the median eminence of the hypothalamus, appear to act as the linchpin of these processes by dynamically controlling the secretion of neuropeptides into the portal vasculature by hypothalamic neurons and regulating blood-brain and blood-cerebrospinal fluid exchanges, both processes that depend on the ability of these cells to adapt their morphology to the physiological state of the individual. In addition to their barrier properties, tanycytes possess the ability to sense blood glucose levels, and play a fundamental and active role in shuttling circulating metabolic signals to hypothalamic neurons that control food intake. Moreover, accumulating data suggest that, in keeping with their putative descent from radial glial cells, tanycytes are endowed with neural stem cell properties and may respond to dietary or reproductive cues by modulating hypothalamic neurogenesis. Tanycytes could thus constitute the missing link in the loop connecting behavior, hormonal changes, signal transduction, central neuronal activation and, finally, behavior again. In this article, we will examine these recent advances in the understanding of tanycytic plasticity and function in the hypothalamus and the underlying molecular mechanisms. We will also discuss the putative involvement and therapeutic potential of hypothalamic tanycytes in metabolic and fertility disorders.

Are Tanycytes the Missing Link Between Type 2 Diabetes and Alzheimer's Disease?

Tanycytes are highly specialized bipolar ependymal cells that line the ventrolateral wall and the floor of the third ventricle in the brain and form a blood-cerebrospinal fluid barrier at the level of the median eminence. They play a pivotal role in regulating metabolic networks that control body weight and energy homeostasis. Due to the glucosensing function of tanycytes, they could be considered as a critical player in the pathogenesis of type 2 diabetes. Genetic fate mapping studies have established the role of tanycytes for the newly detected adult hypothalamic neurogenesis with important implications for metabolism as well as pathophysiology of various neurodegenerative diseases. We believe that a comprehensive understanding of the physiological mechanisms underlying their neuroplasticity, glucosensing, and cross talk with endothelial cells will enable us to achieve metabolic homeostasis in type 2 diabetes patients and possibly delay the progression of Alzheimer's disease and hopefully improve cognitive function.

Hormonal and nutritional regulation of postnatal hypothalamic development

The hypothalamus is a key centre for regulation of vital physiological functions, such as appetite, stress responsiveness and reproduction. Development of the different hypothalamic nuclei and its major neuronal populations begins prenatally in both altricial and precocial species, with the fine tuning of neuronal connectivity and attainment of adult function established postnatally and maintained throughout adult life. The perinatal period is highly susceptible to environmental insults that, by disrupting critical developmental processes, can set the tone for the establishment of adult functionality. Here, we review the most recent knowledge regarding the major postnatal milestones in the development of metabolic, stress and reproductive hypothalamic circuitries, in the rodent, with a particular focus on perinatal programming of these circuitries by hormonal and nutritional influences. We also review the evidence for the continuous development of the hypothalamus in the adult brain, through changes in neurogenesis, synaptogenesis and epigenetic modifications. This degree of plasticity has encouraging implications for the ability of the hypothalamus to at least partially reverse the effects of perinatal mal-programming.

Regulation and function of neurogenesis in the adult mammalian hypothalamus

Over the past two decades, evidence has accumulated that neurogenesis can occur in both the juvenile and adult mammalian hypothalamus. Levels of hypothalamic neurogenesis can be regulated by dietary, environmental and hormonal signals. Since the hypothalamus has a central role in controlling a broad range of homeostatic physiological processes, these findings may have far ranging behavioral and medical implications. However, many questions in the field remain unresolved, including the cells of origin of newborn hypothalamic neurons and the extent to which these cells actually regulate hypothalamic-controlled behaviors. In this manuscript, we conduct a critical review of the literature on postnatal hypothalamic neurogenesis in mammals, lay out the main outstanding controversies in the field, and discuss how best to advance our knowledge of this fascinating but still poorly understood process.

A comparative study of the neural stem cell niche in the adult hypothalamus of human, mouse, rat and gray mouse lemur (Microcebus murinus)

The adult brain contains niches of neural stem cells that continuously add new neurons to selected circuits throughout life. Two niches have been extensively studied in various mammalian species including humans, the subventricular zone of the lateral ventricles and the subgranular zone of the hippocampal dentate gyrus. Recently, studies conducted mainly in rodents have identified a third neurogenic niche in the adult hypothalamus. In order to evaluate whether a neural stem cell niche also exists in the adult hypothalamus in humans, we performed multiple immunofluorescence labeling to assess the expression of a panel of neural stem/progenitor cell (NPC) markers (Sox2, nestin, vimentin, GLAST, GFAP) in the human hypothalamus and compared them with the mouse, rat and a non-human primate species, the gray mouse lemur (Microcebus murinus). Our results show that the adult human hypothalamus contains four distinct populations of cells that express the five NPC markers: (a) a ribbon of small stellate cells that lines the third ventricular wall behind a hypocellular gap, similar to that found along the lateral ventricles, (b) ependymal cells, (c) tanycytes, which line the floor of the third ventricle in the tuberal region, and (d) a population of small stellate cells in the suprachiasmatic nucleus. In the mouse, rat and mouse lemur hypothalamus, co-expression of NPC markers is primarily restricted to tanycytes, and these species lack a ventricular ribbon. Our work thus identifies four cell populations with the antigenic profile of NPCs in the adult human hypothalamus, of which three appear specific to humans.