Neurogenesis and morphogenesis in the cerebellar cortex

Propofol-Induced Developmental Neurotoxicity: From Mechanisms to Therapeutic Strategies

Propofol is the most commonly used intravenous general anesthetic in clinical anesthesia, and it is also widely used in general anesthesia for pregnant women and infants. Some clinical and preclinical studies have found that propofol causes damage to the immature nervous system, which may lead to neurodevelopmental disorders and cognitive dysfunction in infants and children. However, its potential molecular mechanism has not been fully elucidated. Recent in vivo and in vitro studies have found that some exogenous drugs and interventions can effectively alleviate propofol-induced neurotoxicity. In this review, we focus on the relevant preclinical studies and summarize the latest findings on the potential mechanisms and therapeutic strategies of propofol-induced developmental neurotoxicity.

Localization and Characterization of Major Neurogenic Niches in the Brain of the Lesser-Spotted Dogfish Scyliorhinus canicula

Adult neurogenesis is defined as the ability of specialized cells in the postnatal brain to produce new functional neurons and to integrate them into the already-established neuronal network. This phenomenon is common in all vertebrates and has been found to be extremely relevant for numerous processes, such as long-term memory, learning, and anxiety responses, and it has been also found to be involved in neurodegenerative and psychiatric disorders. Adult neurogenesis has been studied extensively in many vertebrate models, from fish to human, and observed also in the more basal cartilaginous fish, such as the lesser-spotted dogfish, Scyliorhinus canicula, but a detailed description of neurogenic niches in this animal is, to date, limited to the telencephalic areas. With this article, we aim to extend the characterization of the neurogenic niches of S. canicula in other main areas of the brain: we analyzed via double immunofluorescence sections of telencephalon, optic tectum, and cerebellum with markers of proliferation (PCNA) and mitosis (pH3) in conjunction with glial cell (S100β) and stem cell (Msi1) markers, to identify the actively proliferating cells inside the neurogenic niches. We also labeled adult postmitotic neurons (NeuN) to exclude double labeling with actively proliferating cells (PCNA). Lastly, we observed the presence of the autofluorescent aging marker, lipofuscin, contained inside lysosomes in neurogenic areas.

Added sugar intake during pregnancy: Fetal behavior, birth outcomes, and placental DNA methylation

Pregnancy is a critical time for the effects of environmental factors on children's development. The effect of added sugar intake on fetal development and pregnancy outcomes remains understudied despite increasing dietary intake in the United States. This study investigated the effect of added sugar on fetal programming by examining the association between maternal added sugar consumption, fetal movement, birth outcomes, and placental DNA methylation. Further, primary human fibroblasts were cultured under normal or high glucose conditions to assess the effect of high glucose exposure on cells' DNA methylation. We found that higher added sugar intake across pregnancy was associated with reduced 3rd-trimester fetal movement (p < .05) and shorter gestation (p < .01). Our sample size was not powered to detect the alteration of individual placental CpG with genome-wide significance. However, a secondary analysis suggested that added sugar consumption was associated with differential methylation of functionally related gene families across pregnancy. Consistent with this, high glucose exposure in primary cultured human fibroblasts altered the methylation of 17% of all CpGs, providing converging evidence for an effect of sugar on DNA methylation. Our results suggest that diets high in added sugar during pregnancy may have implications for offspring health via prenatal programming effects measurable before birth.

Propofol Exposure in Early Life Induced Developmental Impairments in the Mouse Cerebellum

Propofol is a widely used anesthetic in the clinic while several studies have demonstrated that propofol exposure may cause neurotoxicity in the developing brain. However, the effects of early propofol exposure on cerebellar development are not well understood. Propofol (30 or 60 mg/kg) was administered to mice on postnatal day (P)7; Purkinje cell dendritogenesis and Bergmann glial cell development were evaluated on P8, and granule neuron migration was analyzed on P10. The results indicated that exposure to propofol on P7 resulted in a significant reduction in calbindin-labeled Purkinje cells and their dendrite length. Furthermore, propofol induced impairments in Bergmann glia development, which might be involved in the delay of granule neuron migration from the external granular layer (EGL) to the internal granular layer (IGL) during P8 to P10 at the 60 mg/kg dosage, but not at the 30 mg/kg dosage. Several reports have suggested that the Notch signaling pathway plays instructive roles in the morphogenesis of Bergmann glia. Here, it was revealed that propofol treatment decreased Jagged1 and Notch1 protein levels in the cerebellum on P8. Taken together, exposure to propofol during the neonatal period impairs Bergmann glia development and may therefore lead to cerebellum development defects. Our results may aid in the understanding of the neurotoxic effects of propofol when administrated to infants.

Rho Kinase Inhibition Is Essential During In Vitro Neurogenesis and Promotes Phenotypic Rescue of Human Induced Pluripotent Stem Cell-Derived Neurons With Oligophrenin-1 Loss of Function

: Rho-GTPases have relevant functions in various aspects of neuronal development, such as differentiation, migration, and synaptogenesis. Loss of function of the oligophrenin-1 gene (OPHN1) causes X-linked intellectual disability with cerebellar hypoplasia and leads to hyperactivation of the rho kinase (ROCK) pathway. ROCK mainly acts through phosphorylation of the myosin phosphatase targeting subunit 1, triggering actin-myosin contractility. We show that during in vitro neurogenesis, ROCK activity decreases from day 10 until terminal differentiation, whereas in OPHN1-deficient human induced pluripotent stem cells (h-iPSCs), the levels of ROCK are elevated throughout differentiation. ROCK inhibition favors neuronal-like appearance of h-iPSCs, in parallel with transcriptional upregulation of nuclear receptor NR4A1, which is known to induce neurite outgrowth. This study analyzed the morphological, biochemical, and functional features of OPHN1-deficient h-iPSCs and their rescue by treatment with the ROCK inhibitor fasudil, shedding light on the relevance of the ROCK pathway during neuronal differentiation and providing a neuronal model for human OPHN1 syndrome and its treatment.

SIGNIFICANCE

The analysis of the levels of rho kinase (ROCK) activity at different stages of in vitro neurogenesis of human induced pluripotent stem cells reveals that ROCK activity decreases progressively in parallel with the appearance of neuronal-like morphology and upregulation of nuclear receptor NR4A1. These results shed light on the role of the ROCK pathway during early stages of human neurogenesis and provide a neuronal stem cell-based model for the treatment of OPHN1 syndrome and other neurological disorders due to ROCK dysfunction.

Possible role of pineal allopregnanolone in Purkinje cell survival

It is believed that neurosteroids are produced in the brain and other nervous systems. Here, we show that allopregnanolone (ALLO), a neurosteroid, is exceedingly produced in the pineal gland compared with the brain and that pineal ALLO acts on the Purkinje cell, a principal cerebellar neuron, to prevent apoptosis in the juvenile quail. We first demonstrated that the pineal gland is a major organ of neurosteroidogenesis. A series of experiments using molecular and biochemical techniques has further demonstrated that the pineal gland produces a variety of neurosteroids de novo from cholesterol in the juvenile quail. Importantly, ALLO was far more actively produced in the pineal gland than in the brain. Pinealectomy (Px) decreased ALLO concentration in the cerebellum and induced apoptosis of Purkinje cells, whereas administration of ALLO to Px quail chicks prevented apoptosis of Purkinje cells. We further found that Px significantly increased the number of Purkinje cells that expressed active caspase-3, a key protease in apoptotic pathway, and daily injection of ALLO to Px quail chicks decreased the number of Purkinje cells expressing active caspase-3. These results indicate that the neuroprotective effect of pineal ALLO is associated with the decrease in caspase-3 activity during the early stage of neuronal development. We thus provide evidence that the pineal gland is an important neurosteroidogenic organ and that pineal ALLO may be involved in Purkinje cell survival during development. This is an important function of the pineal gland in the formation of neuronal circuits in the developing cerebellum.

A cis-element in the Notch1 locus is involved in the regulation of gene expression in interneuron progenitors

Interneurons comprise approximately one third of the total cortical neurons in the mammalian cerebral cortex. Studies have revealed many details in the generation of this cell type. However, the mechanism that defines interneuron-lineage specific gene expression is not well understood. Gene regulatory elements, e.g., promoters, enhancers, and trans-acting factors, are essential for the proper control of gene expression. Here, we report that a novel evolutionarily conserved cis-element in the second intron of the Notch1 locus plays an important role in regulating gene expression in interneuron progenitors. The spatiotemporal activity of the cis-element in the developing central nervous system (CNS) was determined by both transient reporter expression in the developing chick and a transgenic mouse model. Its activity is well correlated with neurogenesis in both the chick and mouse and restricted to neural progenitor cells in the ganglionic eminence that are fated to differentiate into GABAergic interneurons of the neocortex. We further demonstrate that the cis-element activity requires the binding motif for trans-acting factors Gsh1/Barx2/Brn3. Deletion of this binding motif abolishes reporter gene expression. Together, these data provide new insights into the regulatory mechanisms of interneuron development in the vertebrate CNS.

Radial glial cells and the lamination of the cerebellar cortex

Radial glial cells are stem cells that play an important role in neuronal migration and proliferation in the developing brain. However, how radial glial cells contribute to the lamination of the cerebellar cortex is not well understood. We therefore used immunohistochemistry and BrdU labeling to follow radial glial cell differentiation, cell migration and cerebellar cortex development in mice from embryonic day 8 to postnatal day 180. We report that radial glial cells represent the stem cell population of the neuroepithelium of the neural tube, and act as progenitors for both neurons and neuroglia. In addition, radial glial cells not only give rise to the principal cells of the cerebellar cortex, the Purkinje and granule cells, but they also provide a scaffold for the migration of these cells. We conclude that radial glial cells play a pivotal role in establishing the laminar structure of the cerebellar cortex.

Neuronal apoptosis in the developing cerebellum

The following study analysed apoptosis in proliferative cells and migrating neurons of the developing cerebellum. The external granular layer, Purkinje cell layer and internal granular layer in the developing mouse cerebellar cortex were analysed by active caspase-3 immunohistochemistry, Hoechst 33258 staining and Western blot analysis. Immunocytochemistry results indicated that the peak of apoptosis appeared at postnatal days P8, P5 and P9 in the external granular layer, Purkinje cell layer and internal granular layer, respectively. Subsequently, in each region, the rate of apoptosis decreased with increasing age. In contrast, Western blot results demonstrated the highest expression of activated caspase-3 in the cerebellum at P5, followed by a subsequent decline and disappearance of expression by P14. Activated caspase-8 was expressed maximally at P10, and subsequently disappeared by P30. The results of this study suggest that the key period of neuronal apoptosis in the cerebellar cortex is between P0 and P14, indicating that this developmental period could be susceptible to treatment for congenital neurodegenerative diseases.

The genesis of cerebellar interneurons and the prevention of neural DNA damage require XRCC1

Defective responses to DNA single strand breaks underlie various neurodegenerative diseases. However, the exact role of this repair pathway during the development and maintenance of the nervous system is unclear. Using murine neural-specific inactivation of Xrcc1, a factor that is critical for the repair of DNA single strand breaks, we found a profound neuropathology that is characterized by the loss of cerebellar interneurons. This cell loss was linked to p53-dependent cell cycle arrest and occurred as interneuron progenitors commenced differentiation. Loss of Xrcc1 also led to the persistence of DNA strand breaks throughout the nervous system and abnormal hippocampal function. Collectively, these data detail the in vivo link between DNA single strand break repair and neurogenesis and highlight the diverse consequences of specific types of genotoxic stress in the nervous system.

Recording and spectrum analysis of the planarian electroencephalogram

Many animals produce continuous brainwaves, known as the electroencephalogram (EEG), but it is not known at what point in evolution the EEG developed. Planarians possess the most primitive form of brain, but still exhibit learning and memory behaviors. Here, we observed and characterized the EEG waveform of the planarian. We inserted a monopole electrode into the head of a planarian on a cold stage, and were able to observe the EEG at sub-microvolt amplitudes. The EEG had a continuous waveform, similar to that of evolutionarily advanced animals with more developed brains. Occasional myogenic potential spikes were observed in the EEG due to sticking of the electrode, but this was markedly diminished by cooling the sample, which enabled us to investigate the intrinsic character of the continuous EEG waveform. The frequency spectrum of the EEG was observed in the range of 0.1-5 Hz, showing a broad rise below 0.5 Hz and a monotonic decrease above 1 Hz, apparently following the 1/f law. The intensity of the total EEG diminished during anesthesia by cooling to 2-3 degrees C, and recovered when the sample was warmed to about 10 degrees C. The EEG signal was sustained for 30-40 min, and gradually weakened as the animal died. Stimulation of the planarian with water vibration at 0.5-2 Hz induced chaotic resonance with a broad peak spectrum of around the stimulation frequency. Strong illumination suppressed the EEG signals for several minutes, with the degree of suppression positively correlating with the intensity of the light. This provides evidence that the EEG responds to optical signals, although there were no synchronous reactions to light flashes. The continuous EEG waveform suggests the existence of feedback loop circuits in the neural network of the planarian, which was supposed in electric shock memory experiments [McConnell JV, Cornwell P, Clay M (1960) An apparatus for conditioning planaria. Am J Psychol 73:618-622]. However, because of the broad band character of chaotic resonance observed, these loops appear to be loose couplings between ganglia.

Timing neurogenesis and differentiation: insights from quantitative clonal analyses of cerebellar granule cells

The cerebellum is an excellent model system to study how developmental programs give rise to exquisite neuronal circuits in the adult brain. Here, we describe our findings regarding granule cell neurogenesis and differentiation using the MADM method (mosaic analysis with double markers) in mice. By following the development of individual granule cell clones, we show that (1) granule cell precursors (GCPs) undergo predominantly symmetric division during postnatal development; (2) clonally related granule cells (GCs) exit the cell cycle within a narrow time window and stack their axons in the molecular layer in chronological order from deep to superficial sublayers; and (3) whereas the average GCP proliferation in the external granular layer is progressively slower as development proceeds, there is a rapid expansion of GCPs shortly before clonally related GCs exit the cell cycle. These properties produce GC clones that are distinct, each having a restricted axonal projection, but that are on average similar in cell number. We discuss possible developmental mechanisms and functional implications of these findings.

Aligned neurite bundles of granule cells regulate orientation of Purkinje cell dendrites by perpendicular contact guidance in two-dimensional and three-dimensional mouse cerebellar cultures

To identify structures that determine the 90 degree orientation of thin espalier dendritic trees of Purkinje cells with respect to parallel fibers (axonal neurite bundles of granule cells) in the cerebellar cortex, we designed five types of two-dimensional and three-dimensional cell and tissue cultures of cerebella from postnatal mice and analyzed the orientation of Purkinje cell dendrites with respect to neurite bundles and astrocyte fibers by immunofluorescence double or triple staining. We cultured dissociated cerebellar cells on micropatterned substrates and preformed neurite bundles of a microexplant culture two-dimensionally and in matrix gels three-dimensionally. Dendrites, but not axons, of Purkinje cells extended toward the neurites of granule cells and oriented at right angles two-dimensionally to aligned neurite bundles in the three cultures. In a more organized explant proper of the microexplant culture, Purkinje cell dendrites extended toward thin aligned neurite bundles not only consistently at right angles but also two-dimensionally. However, in the "organotypic microexplant culture," in which three-dimensionally aligned thick neurite bundles mimicking parallel fibers were produced, Purkinje cell dendrites often oriented perpendicular to the thick bundles three-dimensionally. Astrocytes were abundant in all cultures, and there was no definite correlation between the presence of and orientation to Purkinje cell dendrites, although their fibers were frequently associated in parallel with dendrites in the organotypic microexplant culture. Therefore, Purkinje cells may grow their dendrites to the newly produced neurite bundles of parallel fibers in the cerebellar cortex and be oriented at right angles three-dimensionally mainly via "perpendicular contact guidance."

Sodium imaging of climbing fiber innervation fields in developing mouse Purkinje cells

Maturation of specific neuronal connections in the mature nervous system includes elimination of redundant synapses formed earlier during development. In the cerebellum of adult animals, each Purkinje cell (PC) is innervated by a single climbing fiber (CF). In early postnatal development each PC is innervated by multiple CFs and elimination of synapses formed by supernumerary CFs occurs until monoinnervation is established at around postnatal day 20 (P20) in mice. It is not clear whether multiple CFs, or only a single CF, translocate from the cell body of immature PCs to the developing dendrite and, in case several CFs translocate, whether they share or segregate their innervation fields. To localize CF innervation fields, we imaged changes in postsynaptic sodium concentration resulting from CF-mediated postsynaptic currents. We found that more than one CF translocates from an innervation field on the cell body of the PC to the developing dendrite and that these CFs share rather than segregate their innervation fields. We concluded that both the soma and the proximal dendrite of the PC are territories of competition for the developing CFs and that the overlapping of their termination fields may be the prerequisite for a local process of elimination of all but one CF, as previously demonstrated in the developing neuromuscular junction.

Pharmacological and functional implications of developmentally-regulated changes in GABA(A) receptor subunit expression in the cerebellum

The cerebellum undergoes many morphological, pharmacological, and electrophysiological changes during the first 3 weeks of postnatal development. The purpose of this review is to present the most up to date synopsis of the pharmacological and functional changes in, gamma-aminobutyric acid (GABA) type A receptors during this time of cerebellar maturation. Since most of the diversity in cerebellar, GABA(A) receptor pharmacology lies within the granule cell layer, research groups have focused on this area of the cerebellum to study the developmental changes in GABA(A) receptor subunit expression and the neurodifferentiating factors involved in regulating this expression. Thus, it is important to note that developmental changes in GABA(A) receptor composition and its corresponding pharmacology will be essential for determining the type of GABA-mediated transmission that occurs between neuronal contacts in the neonatal and subsequently in the mature cerebellum.

Observations on the primary sensory ending of tenuissimus muscle spindles in the cat

The arrangement of preterminal and terminal axon branches in the primary sensory endings of cat tenuissimus muscle spindles was studied using whole-mount and serial-section techniques. Although in every case one first-order preterminal branch was supplied exclusively to the bag1 type of intrafusal muscle fibre, the preterminal branching patterns differed considerably in detail. Terminals varied widely in size and location. Their precise form varied according to their position on the intrafusal muscle fibres rather than their relationship to preterminal branches. Terminals derived from separate preterminal branches remained separate and did not fuse with themselves or each other. Individually bag1 fibres had most terminals, chain fibres least. The surface of the muscle fibres were differentially indented by the terminals, least in bag1 fibres and most in chain fibres. The results are discussed in relation to mechanosensory transduction and to the factors involved in determining the form of the primary ending.

A quantitative electron microscopic study of the effect of glucocorticoids in vivo on the early postnatal differentiation of paraneuronal cells in the carotid body and the adrenal medulla of the rat

The postnatal differentiation of carotid body chief cells and endocrine adrenal medullary cells was comparatively examined during ontogenesis and in rats which were treated with dexamethasone for 7 days after birth. Ultrastructure and innervation of carotid body chief cells are mature in neonates according to the functional requirements of chemoreception. By the end of the first postnatal week, only an increase in number of dense core vesicles can be noticed, the concentration of which then will reach the adult level. Under the effect of dexamethasone most of the heterochromatin is transformed into finely dispersed euchromatin within the nuclei of carotid body chief cells. In the cytoplasm, the Golgi apparatus becomes larger and the granular endoplasmic reticulum hypertrophic. The number of catecholamines storing dense core vesicles increases considerably. The innervation density remains constant. In contrast to the carotid body chief cells, the adrenal medullary cells have not reached their definitive maturity at the time of birth. Besides phenotypes of adrenaline-cells, noradrenaline-cells and small granules containing cells, pheochromoblasts and intermediary cells can be seen as well. Their cytoplasm is sparse, the concentration of dense core vesicles and the innervation density very low. After 8 days of postnatal ontogenesis, pheochromoblasts and intermediary cells are no longer present in the adrenal medulla. In adrenaline-cells and noradrenaline-cells, important processes of growth can be noticed, the cytoplasm has grown in extent, the number of dense core vesicles doubled and the innervation density of single cells triplicated. Only the few small granules containing cells remain small. Under the effect of dexamethasone also in the nuclei of chromaffin cells a transformation of heterochromatin into euchromatin occurs. The increase in number of dense core vesicles is relatively lower than in carotid body chief cells. The significant growth of innervation density during the first postnatal week was inhibited. Our observations suggest that dexamethasone stimulates the synthesis of catecholamines in adrenal medullary cells of newborn rats less pronouncedly than in carotid body chief cells. This could be attributed to the inhibited formation of synapses of growing chromaffin cells and to the in vivo active endocrine counter-regulation.

Catecholamine biosynthetic enzymes are expressed in replicating cells of the peripheral but not the central nervous system

We sought to determine whether the precursors of catecholamine-containing neurons in the developing peripheral and central nervous systems of chickens and rats express the biosynthetic enzymes tyrosine hydroxylase [THase; tyrosine 3-monooxygenase; L-tyrosine, tetrahydropteridine: oxygen oxidoreductase (3-hydroxylating), EC 1.14.16.2] or dopamine beta-hydroxylase [DBHase; 3,4-dihydroxyphenylethylamine, ascorbate:oxygen oxidoreductase (beta-hydroxylating), EC 1.14.17.1], prior to the time they withdraw from the cell cycle. Chicken embryos (stages 26-27) were injected with [3H-thymidine and 4 hr later were prepared for the simultaneous demonstration of radioautographically labeled nuclei in immunoreactive THase cells. The brains and sympathetic chains of rat fetuses (days E12-E14), exposed for 2 hr to [3H]thymidine, were treated similarly except that peripheral tissues were stained with a specific antibody to DBHase as well as anti-THase. In the peripheral nervous system of both chicken and rat, nuclei of THase-containing cells were radioautographically labeled. DBHase-containing cells in the peripheral nervous system of rats were also labeled and thus are noradrenergic. THase was localized in cells of the brain of the same rat fetuses beginning on day E12 (no THase was detected on day E11 or E11.5) in the mantle layer of the ventral mesencephalic and rostrolateral rhombocephalic cellular groups; however. THase-containing cells in the central nervous system did not incorporate [3H]thymidine. We conclude that, during development, the adrenergic neuronal precursors of the peripheral nervous system but not of the central, have the capacity to synthesize catecholamines before they withdraw from the cell cycle. Differences in the maturation of peripheral and central neurons may be related to differences in their embryological origin.

Quantification of cell death in developing cerebellum by a 14C tracer method

To study the question of whether or not cell death contributes significantly to normal or stressed postnatal brain development in a way which is biochemically quantifiable, we carried out an experiment to assess the amount of cell death in developing cerebellum. By measuring the loss of DNA content and the loss of 14C from labelled thymidine previously incorporated into the DNA fraction (DNAF) in X-irradiated neonatal animals, shown by histological methods to have cell death to the degree of degranulating the external granular layer (EGL), we showed that when cells die both label and DNA content are greatly decreased in the cerebellum. Experiments on both normal and malnourished animals showed that cell death does not contribute significantly to cerebellar development in either malnutrition-stressed or normal animals. Here, we present a biochemical tool for assessing cell death and evidence that cell death does not contribute significantly to cerebellar development.

Autoradiographic studies of cerebellar histogenesis in the bullfrog tadpole during metamorphosis: the external granular layer

Spontaneously metamorphosing bullfrog tadpoles and those induced to metamorphose by injections of thyroid stimulating hormone (TSH) were given a single intraperitoneal injection of thymidine-3H (10 muCi/g body weight). The brains were dissected out at various periods 3 hours to 14 days later, and processed for autoradiography. At the 3-hour interval after thymidine-3H injection, ependymal cells were labelled, but not the external granular layer (EGL) cells. Furthermore, in all the metamorphosing tadpoles intense labelling was restricted to the ependyma of the marginal region of the cerebellar plate. At 48 hours, labelled cells were seen in the EGL of the marginal region. At the 4-day interval, most of the EGL was labelled, and labelled cells were also seen migrating from the EGL into the internal granular layer (IGL). By 14 days, several labelled cells were seen in the IGL. Although the sequence of cerebellar histogenesis in the frog is similar to the general pattern described in other vertebrate groups, the results indicate that the EGL of the frog cerebellum does not serve the function of a secondary proliferating zone.