Response of tanycytes to aging in the median eminence of the rat

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.

Lipid droplets in the nervous system: involvement in cell metabolic homeostasis

Lipid droplets serve as primary storage organelles for neutral lipids in neurons, glial cells, and other cells in the nervous system. Lipid droplet formation begins with the synthesis of neutral lipids in the endoplasmic reticulum. Previously, lipid droplets were recognized for their role in maintaining lipid metabolism and energy homeostasis; however, recent research has shown that lipid droplets are highly adaptive organelles with diverse functions in the nervous system. In addition to their role in regulating cell metabolism, lipid droplets play a protective role in various cellular stress responses. Furthermore, lipid droplets exhibit specific functions in neurons and glial cells. Dysregulation of lipid droplet formation leads to cellular dysfunction, metabolic abnormalities, and nervous system diseases. This review aims to provide an overview of the role of lipid droplets in the nervous system, covering topics such as biogenesis, cellular specificity, and functions. Additionally, it will explore the association between lipid droplets and neurodegenerative disorders. Understanding the involvement of lipid droplets in cell metabolic homeostasis related to the nervous system is crucial to determine the underlying causes and in exploring potential therapeutic approaches for these diseases.

Role of lipid droplets in neurodegenerative diseases: From pathogenesis to therapeutics

Neurodegenerative diseases (NDDs) are a series of disorders characterized by the progressive loss of specific neurons, leading to cognitive and locomotor impairment. NDDs affect millions of patients worldwide but lack effective treatments. Dysregulation of lipids, particularly the accumulation of lipid droplets (LDs), is strongly implicated in the pathogenesis of NDDs. How LDs contribute to the occurrence and development of NDDs, and their potential as therapeutic targets remain to be addressed. In present review, we first introduce the processes of LDs formation, transportation and degradation. We then highlight how the accumulation of LDs contributes to the pathogenesis of NDDs in a cell type-specific manner. Moreover, we discuss currently available methods for detecting LDs and elaborate on LDs-based therapeutic strategies for NDDs. Lastly, we identify gaps that need to be filled to better leverage LD-based theranostics in NDDs and other diseases. We hope this review could shed light on the role of LDs in NDDs and facilitate the development of novel therapeutic strategies for NDDs.

Single-cell Transcriptomic Profiling of the Hypothalamic Median Eminence during Aging

Aging is a slow and progressive natural process that compromises the normal functions of cells, tissues, organs and systems. The aging of the hypothalamic median eminence (ME), a structural gate linking neural and endocrine systems, may impair hormone release, energy homeostasis and central sensing of circulating molecules, leading to systemic and reproductive aging. However, the molecular and cellular features of ME aging remain largely unknown. Here we describe the transcriptional landscape of young and middle-aged mouse ME at single-cell resolution, revealing the common and cell-type-specific transcriptional changes with age. The transcriptional changes in cell-intrinsic programs, cell-cell crosstalk and cell-extrinsic factors highlight five molecular features of ME aging and also implicate several potentially druggable targets at cellular, signaling and molecular levels. Importantly, our results suggest that vascular and leptomeningeal cells (VLMCs) may lead the asynchronized aging process among diverse cell types and drive local inflammation and cellular senescence via a unique secretome. Together, our study uncovers how intrinsic and extrinsic features of each cell type in the hypothalamic ME are changed by the aging process, which will facilitate our understanding of brain aging and provide clues for efficient anti-aging intervention at the middle-aged stage.

Quantitative, structural and molecular changes in neuroglia of aging mammals: A review

The neuroglia of the central and peripheral nervous systems undergo numerous changes during normal aging. Astrocytes become hypertrophic and accumulate intermediate filaments. Oligodendrocytes and Schwann cells undergo alterations that are often accompanied by degenerative changes to the myelin sheath. In microglia, proliferation in response to injury, motility of cell processes, ability to migrate to sites of neural injury, and phagocytic and autophagic capabilities are reduced. In sensory ganglia, the number and extent of gaps between perineuronal satellite cells – that leave the surfaces of sensory ganglion neurons directly exposed to basal lamina – increase significantly. The molecular profiles of neuroglia also change in old age, which, in view of the interactions between neurons and neuroglia, have negative consequences for important physiological processes in the nervous system. Since neuroglia actively participate in numerous nervous system processes, it is likely that not only neurons but also neuroglia will prove to be useful targets for interventions to prevent, reverse or slow the behavioral changes and cognitive decline that often accompany senescence.

Celia's Encephalopathy (BSCL2-Gene-Related): Current Understanding

Seipin, encoded by the BSCL2 gene, is a protein that in humans is expressed mainly in the central nervous system. Uniquely, certain variants in BSCL2 can cause both generalized congenital lipodystrophy type 2, upper and/or lower motor neuron diseases, or progressive encephalopathy, with a poor prognosis during childhood. The latter, Celia's encephalopathy, which may or may not be associated with generalized lipodystrophy, is caused by the c.985C >T variant. This cytosine to thymine transition creates a cryptic splicing zone that leads to intronization of exon 7, resulting in an aberrant form of seipin, Celia seipin. It has been proposed that the accumulation of this protein, both in the endoplasmic reticulum and in the nucleus of neurons, might be the pathogenetic mechanism of this neurodegenerative condition. In recent years, other variants in BSCL2 associated with generalized lipodystrophy and progressive epileptic encephalopathy have been reported. Interestingly, most of these variants could also lead to the loss of exon 7. In this review, we analyzed the molecular bases of Celia's encephalopathy and its pathogenic mechanisms, the clinical features of the different variants, and a therapeutic approach in order to slow down the progression of this fatal neurological disorder.

Structural and ultrastructural studies on the developing vomeronasal sensory epithelium in the grass snake Natrix natrix (Squamata: Colubroidea)

The sensory olfactory epithelium and the vomeronasal sensory epithelium (VSE) are characterized by continuous turnover of the receptor cells during postnatal life and are capable of regeneration after injury. The VSE, like the entire vomeronasal organ, is generally well developed in squamates and is crucial for detection of pheromones and prey odors. Despite the numerous studies on embryonic development of the VSE in squamates, especially in snakes, an ultrastructural analysis, as far as we know, has never been performed. Therefore, we investigated the embryology of the VSE of the grass snake (Natrix natrix) using electron microscopy (SEM and TEM) and light microscopy. As was shown for adult snakes, the hypertrophied ophidian VSE may provide great resolution of changes in neuron morphology located at various epithelial levels. The results of this study suggest that different populations of stem/progenitor cells occur at the base of the ophidian VSE during embryonic development. One of them may be radial glia-like cells, described previously in mouse. The various structure and ultrastructure of neurons located at different parts of the VSE provide evidence for neuronal maturation and aging. Based on these results, a few nonmutually exclusive hypotheses explaining the formation of the peculiar columnar organization of the VSE in snakes were proposed.

Lipid Droplets in Neurodegenerative Disorders

Knowledge of lipid droplets (LDs) has evolved from simple depots of lipid storage to dynamic and functionally active organelles involved in a variety of cellular functions. Studies have now informed significant roles for LDs in cellular signaling, metabolic disease, and inflammation. While lipid droplet biology has been well explored in peripheral organs such as the liver and heart, LDs within the brain are relatively understudied. The presence and function of these dynamic organelles in the central nervous system has recently gained attention, especially in the context of neurodegeneration. In this review, we summarize the current understanding of LDs within the brain, with an emphasis on their relevance in neurodegenerative diseases.

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.

Hypothalamic tanycytes-masters and servants of metabolic, neuroendocrine, and neurogenic functions

There is a resurgent interest in tanycytes, a radial glial-like cell population occupying the floor and ventro-lateral walls of the third ventricle (3V). Tanycytes reside in close proximity to hypothalamic neuronal nuclei that regulate appetite and energy expenditure, with a subset sending projections into these nuclei. Moreover, tanycytes are exposed to 3V cerebrospinal fluid and have privileged access to plasma metabolites and hormones, through fenestrated capillaries. Indeed, some tanycytes act as conduits for trafficking of these molecules into the brain parenchyma. Tanycytes can also act as neural stem/progenitor cells, supplying the postnatal and adult hypothalamus with new neurons. Collectively, these findings suggest that tanycytes regulate and integrate important trophic and metabolic processes and possibly endow functional malleability to neuronal circuits of the hypothalamus. Hence, manipulation of tanycyte biology could provide a valuable tool for modulating hypothalamic functions such as energy uptake and expenditure in order to tackle prevalent eating disorders such as obesity and anorexia.

The hypothalamic median eminence and its role in reproductive aging

The median eminence at the base of the hypothalamus serves as an interface between the neural and peripheral endocrine systems. It releases hypothalamic-releasing hormones into the portal capillary bed for transport to the anterior pituitary, which provides further signals to target endocrine systems. Of specific relevance to reproduction, a group of about 1000 neurons in mammals release the gonadotropin-releasing hormone (GnRH) peptide from neuroterminals in the median eminence. During the life cycle, there are dramatic changes in reproductive demands, and we focus this review on how GnRH terminals in the median eminence change during reproductive senescence. We discuss morphological and functional properties of the median eminence, and how relationships among GnRH terminals and their microenvironment of nerve terminals, glial cells, and the portal capillary vasculature determine the ability of GnRH peptide to be secreted and to reach its target in the anterior pituitary gland.

Gonadotropin-releasing hormone neuroterminals and their microenvironment in the median eminence: effects of aging and estradiol treatment

The GnRH decapeptide controls reproductive function through its release from neuroendocrine terminals in the median eminence, a site where there is a convergence of numerous nerve terminals and glial cells. Previous work showed dynamic changes in the GnRH-glial-capillary network in the median eminence under different physiological conditions. Because aging in rats is associated with a diminution of GnRH release and responsiveness to estradiol feedback, we examined effects of age and estradiol treatment on these anatomical interactions. Rats were ovariectomized at young (4 months), middle-aged (11 months), or old (22-23 months) ages, allowed 4 wk to recover, and then treated with vehicle or estradiol for 72 h followed by perfusion. Immunofluorescence of GnRH was measured, and immunogold electron microscopic analyses were performed to study the ultrastructural properties of GnRH neuroterminals and their microenvironment. Although the GnRH immunofluorescent signal showed no significant changes with age and estradiol treatment, we found that the median eminence underwent both qualitative and quantitative structural changes with age, including a disorganization of cytoarchitecture with aging and a decrease in the apposition of GnRH neuroterminals to glia with age and estradiol treatment. Thus, although GnRH neurons can continue to synthesize and transport peptide, changes in the GnRH neuroterminal-glial-capillary machinery occur during reproductive senescence in a manner consistent with a disconnection of these elements and a potential dysregulation of GnRH neurosecretion.

Brain inflammatory response induced by intracerebroventricular infusion of lipopolysaccharide: an immunohistochemical study

Inflammatory processes may play a critical role in the pathogenesis of the degenerative changes associated with Alzheimer's disease (AD). In the present study, we used an animal model of brain inflammation in order to study a possible mechanism involved in AD. Lipopolysaccharide (LPS) was used to produce global microglial reactivity within the brain of young rats. Time-dependent changes in the inflammatory reaction and the participation of glial cells after acute injection of LPS (50 or 100 microg) into the lateral ventricle or the fourth ventricle were compared with the chronic infusion of LPS (0.15, 0.5, 1.5 or 5.0 microg/h) into the fourth ventricle (14 days). Several immunohistochemical markers were used to characterize the microglial response. Acute and chronic exposure to LPS induced major histocompatibility complex class II (MHC II) antigen expression, detected with OX-6 antibody, in a sub-population of microglial cells in defined brain areas. The morphological features and distribution of OX-6 positive cells observed in the proximity of the cannula track after LPS injection into the lateral ventricle suggested the recruitment of monocytes/macrophages from the periphery. The activation of the resident microglial cells was delayed and mainly concentrated within the temporal lobe regions and the limbic system. Chronic infusion to LPS into the fourth ventricle induced a comparable activation of microglial cells. Quantitative analysis of OX-6 positive cells showed a dose-dependent response to LPS exposure.

Age-related alterations in tanycytes of the mediobasal hypothalamus of the male rat

By means of semiquantitative immunocytochemistry, possible age-related changes in dopamine and cyclic AMP-regulated phosphoprotein mr 32 (DARPP-32) and glial fibrillary acidic protein (GFAP) immunoreactivities (IR) were investigated in tanycytes of the arcuate nucleus. These two markers showed opposite changes during aging. DARPP-32 IR decreased by around 70%, whereas GFAP IR increased by around 300% in 24-month-old vs. 3-month-old rats. These changes were accompanied by a progressive loss in the number of tanycytes, measured by counting of their long processes in the arcuate nucleus. No significant age-related change was observed either in GFAP IR in astrocytic populations of the mediobasal hypothalamus or in tyrosine hydroxylase IR in dopaminergic neurons of the dorsal arcuate nucleus. These observations indicate that the tanycytic population of the arcuate nucleus undergoes important modifications during aging, which include cell loss, impairment in the intracellular signalling cascade linked to DARPP-32, and hypertrophy. These changes may be related to the alterations in the neuroendocrine systems known to occur during aging.

Tanycytes in the sunfish brain: NADPH-diaphorase histochemistry and regional distribution

NADPH-diaphorase histochemistry has been shown to be a useful method for identifying cells that synthesize and release nitric oxide, which is implicated in the modulation of a variety of neural functions, including synaptic transmission, cerebral blood flow, and excitotoxicity. In the sunfish brain, NADPH-diaphorase histochemistry stains tanycytes specifically and almost exclusively, allowing for a thorough examination of the morphology and distribution of this type of cell. Tanycytes are nonciliated, process-bearing ependymal and extraependymal cells that contact the ventricular surface via apical processes, and the pial surface via basal processes. Ependymal tanycytes are located at the ventricular surface, and project basal processes into the parenchyma of the brain. Extraependymal tanycytes are found away from the ventricular matrix. Some extraependymal tanycytes are small, bipolar, and tend to be associated with bundles of basal processes. Isolated extraependymal tanycytes are larger, darkly stained, and multipolar. Their basal processes terminate in specialized endfeet on blood vessels, neuronal somata, or the pial surface. Specialized types of tanycytes are found in the optic tectum, the epineurial septum between axonal bundles along the midline in the medulla, and in restricted regions on the pial surface in the medulla. The only NADPH-diaphorase-positive neurons are found in the commissural nucleus of area ventralis telencephali. Injection of horseradish peroxidase into the ventricles shows that tanycytes lining the third and fourth ventricles are capable of taking up the tracer and transporting it into their basal processes. Tanycytes are unevenly distributed in the brain. There is a rough rostrocaudal gradient of cell density: tanycytes are sparse in the telencephalon and dense in the isthmus and medulla, although cell density is low in the spinal cord. Not all ventricular linings contain tanycytes: cell density is low in the medial ventricle of the telencephalon and in the infundibular recess, and high along the fourth ventricle. The function of tanycytes in the sunfish is not known. The association of tanycytes with both the ventricles and blood vessels raises the possibility that they play some role in sampling the biochemical constituents of both compartments and communicating the information to neural elements. It is proposed that tanycytes react to the biochemical composition in the ventricle and plasma by increasing or decreasing nitric oxide synthesis and release, which in turn influence neuronal activity or cerebral blood flow.

Ultrastructural changes in magnocellular neurons from the supraoptic nucleus of aged rats

Magnocellular neurosecretory neurons in the supraoptic nucleus were examined in aged and young subjects to assess the ultrastructural correlates of cellular activity including cell, nuclear and nucleolar size, as well as the percentage of the cell occupied by Golgi, mitochondrial and rough endoplasmic reticulum compartments. Morphometric analysis was performed on arbitrarily selected cell profiles using a computerized morphometry protocol. Approximately 50% of those neurons examined from aged subjects exhibited a statistically significant increase relative to the young rats in total cell area and all cellular organelles examined except nuclear area. Moreover, an increase in the number and density (per unit membrane area) of synaptic contacts onto those neurons was observed. These findings suggest an increase in synthetic activity by a subpopulation of supraoptic magnocellular neurons, and support the concept of neuronal plasticity in aged rats.

Gap junctional intercellular communication between cultured ependymal cells, revealed by lucifer yellow CH transfer and freeze-fracture

In order to analyze intercellular communication between ependymal cells in mammalian brain, we have studied gap junctional communication of ependymal and glial cells in long term primary cultures derived from fetal mouse or rat hypothalamus and choroid plexus obtained in serum supplemented media with two complementary methods: 1) dye transfer of Lucifer Yellow CH after intracellular microinjection of the different cellular types, and 2) freeze-fracture of the same cultured ependymal cells. In our culture conditions, we have shown that the GJIC capacity to transfer dye was very different according to cellular types microinjected with Lucifer Yellow CH in the following respects: 1) in ependymal cells, GJIC was always important: ciliated ependymal cells, which are numerous in hypothalamic ependymal cultures (10-120 coupled cells), choroidal ependymocytes in plexus cultures (15-250 coupled cells), and non-choroidal ependymocytes in diencephalic roof cultures (10-30 coupled cells), and 2) in astroglial cells found in these primary cultures, no GJIC was observed in spite of the presence of well-differentiated gap junctions revealed by freeze-fracture replicas. All these results show a strong GJIC in ependymal cells and indicate the very good functional state of these cells in vitro.

Ultrastructure of the pituitary intermediate lobe in aging rats

The pituitary intermediate lobe, a source of pro-opiomelanocortin (POMC) peptides, was examined with light and electron microscopic techniques in Sprague-Dawley rats aged 5-18 months and Zucker rats aged 18 months. Cysts were common in the intermediate lobe in the Zucker animals, a finding also noted in human pituitary glands by other investigations. The nuclear envelopes were often indented in cells of aging rats, while those of young animals were generally smooth. Lipid droplets and lysosomes, rarely seen in tissue from young animals, were frequently observed in endocrine cells of older rats. Most cells had an abundance of secretory granules, suggestive of enhanced storage of peptides in the cytoplasm. Nerve terminals which were present among endocrine cells contained myelin figures in some of the old rats, and may indicate degenerative changes, while other terminals appeared normal. These morphologic findings suggest that the aging phenomena in intermediate lobe tissue have characteristics in common with nervous tissue, and may also reflect a diminished inhibitory neuroregulation.

An immunohistochemical study of the germinal layer in the late gestation human fetal brain

In the cerebral lateral ventricle of the human fetus, the embryonic ventricular and subventricular zones (VZ and SVZ) persist into the latter half of gestation, particularly in the lateral wall. The SVZ is usually referred to as the germinal layer at this stage. The VZ is gradually replaced by ependyma, a single epithelial layer composed of tanycytes and ciliated columnar cells. In the prematurely born infant, the germinal layer is frequently the site of haemorrhage, the incidence of which diminishes with increasing maturity. There are many contributory pathogenetic factors but the structure of the germinal layer itself is considered important. It contains numerous, thin walled vessels in a cellular matrix which demonstrates little fibrillary background. Immunohistochemical evidence of glial differentiation in the germinal layer was sought in 21 preterm brains, using antibody to glial fibrillary acidic protein (GFAP). Early immunoreactivity was due to GFAP positive tanycyte fibres. Subsequently, associated with astrocyte differentiation, there was progressive development of a glial fibre network. It is suggested that the increase in glial fibres may be a significant factor in capillary stabilization, and in the inverse relationship between gestation of the infant and the risk of intracerebral haemorrhage. The possible structural significance of the tanycyte is also highlighted.

Tanycytes in the medial habenular nucleus of the rat

Tanycytes with foot processes contacting capillary basal membranes were identified in the rat medial habenular nucleus. They constitute a relatively small but constant population of cells among the conventional ependymal cells. In contrast to tanycytes in most circumventricular organs, habenular tanycytes possess cilia. Superimposed upon the cells are nerve fibers belonging to the serotonergic supraependymal axon plexus. Their ultrastructure differs in many respects from that of hypothalamic tanycytes.

Toxic effects of adriamycin on the central nervous system. Ultrastructural changes in some circumventricular organs of the mouse after intravenous administration of the drug

Recent experimental studies have shown that the cytotoxic antibiotic adriamycin (doxorubicin) after systemic administration can enter the so-called circumventricular organs (CVO) of the brain of the mouse. The present experiments were performed to find out whether such penetration of the brain is associated with signs of neurotoxic injury. For this purpose, light- and electron-microscopic observations were carried out on three of these organs: the neurohypophysis (NH), median eminence (ME), and postremal area (PA). Pronounced widening of the extracellular space indicating the presence of edema was present in all the regions, particularly in animals examined within 3 days of injection of the drug. Many degenerated axon terminals were observed in the NH and ME. The glial cells within these regions showed rarefaction of the nuclear chromatin, nucleolar segregation, and also cytoplasmic changes. The PA presented marked cellular changes resulting in degeneration of neurons, which was most evident 30 days after the injection. Hence, regions of the CNS outside the blood-brain barrier can be reached by adriamycin after systemic administration, and the drug can induce morphological changes there. The doses of the drug used in the present experiments were comparable to those given to patients for the treatment of malignant tumors.