Early Central Nervous System Development Research Articles

Serotonergic gene variation: implications for personality traits and psychopathology.

Serotonin 5-hydroxytryptamine (5-HT) is an important regulator of morphogenetic activities during early central nervous system development, including cell proliferation, migration, and differentiation as well as synapto-genesis. Serotonergic raphe neurons diffusely project to a variety of brain regions (e.g. cortex, amygdala, hippocampus) and play known roles in integrating emotion, cognition, motor function as well as in food intake, sleep, pain, and sexual activity. The diversity of physiologic functions is due to the fact that 5-HT acts as a master control neurotransmitter within a highly complex system of neural communication mediated by multiple pre- and postsynaptic 5-HT receptors, thus orchestrating the activity and interaction of several other neurotransmitter systems. Since proteins involved in the regulation of central serotonergic activity (e.g. enzymes, receptors, transporter) play pivotal role in brain 5-HT homeostasis, polymorphisms in the regulatory regions of their genes resulting in variation of expression and function are likely to influence complex traits, such as temperament/personality and psychopathology.

Genetically driven variation in serotonin uptake: is there a link to affective spectrum, neurodevelopmental, and neurodegenerative disorders?

Serotonin (5-HT) is an important regulator of morphogenetic activities during early central nervous system development, including cell proliferation, migration, and differentiation. The 5-HT transporter (5-HTT) plays a pivotal role in brain 5-HT homeostasis. It is also the initial target for both antidepressant drugs and drugs of abuse, some of which are potent neurotoxins. A polymorphism in the 5′-flanking regulatory region of the 5-HTT gene that results in allelic variation of 5-HTT expression is associated with anxiety-related personality traits and may influence the risk of developing affective disorders. Progress in 5-HTT gene inactivation studies are also changing views of the relevance of adaptive 5-HT uptake function in brain development and plasticity as well as processes underlying drug dependence and neurodegeneration. Despite evidence for a potential role of the 5-HTT in the integration of synaptic connections in the mammalian brain during development, adult life, and old age, detailed knowledge of the molecular mechanisms involved in these fine-tuning processes is just beginning to emerge. Integration of various strategies, including molecular genetic, transgenic, and gene transfer techniques, will allow elucidation of the 5-HTT’s role in brain development, plasticity, and degeneration as well as in affective illness, drug abuse, and dementia.

Distribution of mRNA for the α4 subunit of the nicotinic acetylcholine receptor in the human fetal brain

Neuronal nicotinic acetylcholine receptors (nAChRs) present in the central nervous system (CNS), are multimeric proteins constituted of two different subunits, α and β, with different subtype arrangements and different pharmacological and functional properties. By in situ hybridization, we studied the distribution of the mRNA for the α4 subunit of nAChRs in brains of human 25-week old normal and fragile X fetuses. A strong hybridization signal was detected throughout the thalamus, cortex, pyramidal layer of the Ammon's horn, and the granular layer of the dentate gyrus. Several other areas including the claustrum, caudate nucleus, putamen, globus pallidus, subthalamic nucleus, subiculum, entorhinal cortex, and Purkinje cell layer displayed a low to moderate radiosignal. With few exceptions, our data in the human brain agree those previously reported in the rat. Also, our data indicate that the α4 subunit mRNA is produced early in the development, in the more differentiated cells, and in a site-specific manner. Additionally, the α4 mRNA is produced in the brain of fragile X fetuses with the same pattern and same intensity than in the normal fetal brain suggesting that α4 subunit mRNA production is not altered in the fragile X syndrome. High levels of α4 subunit mRNA in human fetal brain support the hypothesis of a morphogenic role of nAChRs during the early CNS development.

Cellular expression of the immediate early transcription factors Nurr1 and NGFI-B suggests a gene regulatory role in several brain regions including the nigrostriatal dopamine system

Nurr1 and NGFI-B are closely related orphan members of the steroid-thyroid hormone receptor family involved in immediate early responses to stimuli such as growth factors. In-situ hybridization in the developing and adult mouse and rat demonstrated Nurr1 mRNA in several regions during early central nervous system (CNS) development. Expression persisted through the pre- and postnatal periods and was also found in several areas in the adult CNS. Positive areas include the olfactory bulb, parts of the cortex, the hippocampal formation and substantia nigra where Nurr1 and tyrosine hydroxylase mRNAs were co-expressed. 6-Hydroxydopamine-induced degeneration of mesencephalic dopamine neurons led to a corresponding loss of Nurr1 mRNA, demonstrating a link between Nurr1 and dopaminergic neurons. NGFI-B mRNA was not found in the prenatal CNS but was highly expressed in the adult brain in many areas including the olfactory bulb, cortex, basal ganglia and hippocampus. The spatiotemporal distribution of Nurr1 and NGFI-B mRNAs suggests that these transcription factors are involved in the development and maturation of specific sets of CNS neurons. The experimental data imply that one of these functions may be to control gene regulatory events important for development and function of those neurons that degenerate in patients with Parkinson's disease.

7 ESSENTIAL FATTY ACID PROFILE IN THE EARLY CHOLESTASIS OF INFANCY

Long-chain polyunsaturated fatty acids seem to be essential for early central nervous system development not only in premature but also in term infants. Since cholestatic infants suffer from fat malabsorption and disturbed lipid metabolism, they are prone to develop polyunsaturated fatty acid deficiency in early life. To test this hypothesis, thirty- four cholestatic infants (biliary atresia, 13; Alagille syndrome, 1; α1-ATD, 3; galactosemia, 1; CMV hepatitis, 2; intrahepatic cholestasis of unknown origin, 14) aged 1 to 7 months (median 2.3 m) were investigated. We took 24h dietary records, determined midarm, subscapular and supraspinal skin folds, bilirubin levels, activities of ALAT, AP and γ-GT, prothrombin time, serum concentrations of albumin, bile acids, total lipids, phospholipids, cholesterol and vitamins A and E, as well as the fatty acid composition of plasma phospholipids [%wt/wt] and plasma lipid peroxides expressed as TBARS (thiobarbiturate reactive substance, nmol/ml). Compared to 12 age-matched control infants, the patients studied had very low percentage values of phospholipid essential fatty acids, particularly linoleic acid (18:2ω-6, median 14.86 vs 20.76 in patients and controls, P<0.001) and arachidonic acid (20:4ω-6, 6.79 vs 7.87, P=0.004) of the ω-6 series. The poor essential fatty acid status of the patients was also indicated by increased levels of the non-essential fatty acids, Mead acid (20:3ω-9, 0.74 vs 0.21, P<0.001) and palmitoleic acid (16:1ω-7, 2.20 vs 0.43, P<0.001). There were no differences in polyunsaturated fatty acid profiles between groups of infants with biliary atresia and intrahepatic cholestasis as well as groups of infants with severe malnutrition (all skin folds below the 10th percentile) and mild malnutrition (at least one between the 10th and 25th percentile). Fatty acid values did not correlate with results of clinical chemistry, vit E/total lipid ratios and TBARS. TBARS were significantly increased in cholestatic patients compared to controls (2.74 vs 0.85, P<0.001) and correlated with direct bilirubin (R=0.41, P=0.02) and total bilirubin. Breast fed cholestatic infants (N=6) had higher ω-3 LCP docosapentaenoic (22:5ω-3, 0.47 vs 0.28, P=0.0006) and docosahexanoic acid (22:6ω-3, 2.39 vs 1.73, P=0.01) levels than formula fed infants, while parameters of clinical chemistry were similar. The daily linoleic acid intake per kg body weight correlated with the sum of all ω-6 fatty acids in plasma (R=0.42, P=0.02) and was inversely related to the sum of ω-3 fatty acids (R=-0.52, P=0.003). Conclusions: 1. Cholestatic infants have a high risk of essential fatty acid deficiency. 2. Lipid peroxidation is enhanced in these patients and related to bilirubin concentration. 3.Dietary factors have a significant influence on polyunsaturated fatty acid status in cholestatic infants.

Expression and activity of L-Myc in normal mouse development.

To determine the role of L-Myc in normal mammalian development and its functional relationship to other members of the Myc family, we determined the normal patterns of L-myc gene expression in the developing mouse by RNA in situ hybridization and assessed the phenotypic impact of L-Myc deficiency produced through standard gene targeting methodology. L-myc transcripts were detected in the developing kidney and lung as well as in both the proliferative and the differentiative zones of the brain and neural tube. Despite significant expression of L-myc in developing mouse tissue, homozygous null L-myc mice were found to be viable, reproductively competent, and represented in expected frequencies from heterozygous matings. A detailed histological survey of embryonic and adult tissues, characterization of an embryonic neuronal marker, and measurement of cellular proliferation in situ did not reveal any congenital abnormalities. The lack of an apparent phenotype associated with L-Myc deficiency indicates that L-Myc is dispensable for gross morphological development and argues against a unique role for L-Myc in early central nervous system development as had been previously suggested. Although overlapping expression patterns among myc family members raise the possibility of complementation of L-Myc deficiency by other Myc oncoproteins, compensatory changes in the levels of c- and/or N-myc transcripts were not detected in homozygous null L-myc mice.

Transgenic neural plate contributes neuronal cells that survive greater than one year when transplanted into the adylt mouse central nervous system

Neural plate cells from the early embryo may have a number of important advantages as donor material for the delivery of foreign genes into the diseased adult central nervous system (CNS). Mesencephalic neural plate from transgenic GT4-2 mice was used as a source of marked donor cells to determine whether transgene-expressing embryonic CNS progenitor cells can be used as donor material for implantation into the adult mouse brain. Transgenic mouse embryos from this line express the Escherichia coli β-galactosidase (β-gal) gene throughout early CNS development. At the early somite stage (Embryonic Day 8.5), mesencephalic neural plate tissue from heterozygous embryos was dissected out and either transferred into culture for characterization or immediately implanted into the striatum or lateral ventricle of adult wild-type CD-1 mice. Explants of neural plate tissue possessed intense β-gal activity and produced extensive outgrowth of neurofilament-positive processes after 6 days in vitro. Many β-gal-positive cells migrated away from the explanted tissue mass. Grafts of transgenic neural plate tissue in the normal adult mouse striatum, sampled 2 weeks to 1 year after implantation, possessed healthy β-gal-positive cells. More detailed analysis of grafts 3 months after implantation indicated that most β-gal-positive cells were also immunoreactive for neurofilament and microtubule-associated proteins, two neuron-specific markers. In addition, extensive neurofilament-positive axonal tangles were evident within the grafts among the β-gal-positive cells. Electron microscopic (EM) findings of implanted tissue stained with Bluo-Gal revealed many β-gal-positive neurons received synaptic contacts from other cells. A few donor-derived astrocytes were also found in the grafts by EM analysis. No obvious signs of immunological rejection, or of significant decrease in graft volume, were observed at any age. Some β-gal-positive cells were observed to he up to 230 μm away from the main graft mass in both striatal and intraventricular implantations. These data suggest that the neural plate can contribute a long-surviving population of neuronal and astrocytic cells when transplanted into the adult CNS.

Mouse Wnt genes exhibit discrete domains of expression in the early embryonic CNS and limb buds.

Mutation and expression studies have implicated the Wnt gene family in early developmental decision making in vertebrates and flies. In a detailed comparative analysis, we have used in situ hybridization of 8.0- to 9.5-day mouse embryos to characterize expression of all ten published Wnt genes in the central nervous system (CNS) and limb buds. Seven of the family members show restricted expression patterns in the brain. At least three genes (Wnt-3, Wnt-3a, and Wnt-7b) exhibit sharp boundaries of expression in the forebrain that may predict subdivisions of the region later in development. In the spinal cord, Wnt-1, Wnt-3, and Wnt-3a are expressed dorsally, Wnt-5a, Wnt-7a, and Wnt-7b more ventrally, and Wnt-4 both dorsally and in the floor plate. In the forelimb primordia, Wnt-3, Wnt-4, Wnt-6 and Wnt-7b are expressed fairly uniformly throughout the limb ectoderm. Wnt-5a RNA is distributed in a proximal to distal gradient through the limb mesenchyme and ectoderm. Along the limb's dorsal-ventral axis, Wnt-5a is expressed in the ventral ectoderm and Wnt-7a in the dorsal ectoderm. We discuss the significance of these patterns of restricted and partially overlapping domains of expression with respect to the putative function of Wnt signalling in early CNS and limb development.

Decreased Plasma Taurine in Aged Rats

Taurine (TAU) is thought to be a neuromodulator or an inhibitory neurotransmitter in the mammalian brain. In the periphery, TAU is involved in a wide spectrum of physiological functions. One of the most important putative physiological function of TAU is the modulation of cardiovascular activity including blood pressure control. TAU also appears to be important for the development of an organism, especially early central nervous system development. Regional TAU content was measured in brains of adult (6-month) and aged (23- to 26-month) Fischer-344 (F-344) male rats. To assess the putative neurotransmitter role of TAU, release due to potassium depolarization was studied using slices from frontal cortex of F-344 rats. Circulating levels of TAU were also determined in male F-344 rats (6 and 24 months of age) and female Long-Evans rats (LE, 6 and 30 months old). We found no age-related change in TAU stores from the brain regions examined, nor did we find any indication of TAU release due to potassium depolarization in the release paradigm utilized. There was, however, a significant reduction (28 and 42%) in plasma TAU concentration in F-344 and LE rats, respectively. This age-related reduction in plasma TAU may have important consequences both peripherally and centrally with respect to regulation of blood pressure, cardiovascular function, and cardioprotection, as well as possible CNS complications.