Early Rat Brain Development Research Articles

FGF2/EGF contributes to brain neuroepithelial precursor proliferation and neurogenesis in rat embryos: the involvement of embryonic cerebrospinal fluid.

At the earliest stages of brain development, the neuroepithelium works as an interdependent functional entity together with cerebrospinal fluid, which plays a key regulatory role in neuroepithelial cell survival, replication and neurogenesis; however, the underlying mechanism remains unknown in mammals. We show the presence of fibroblast growth factor 2 (FGF2) and epidermal growth factor (EGF), in 13.5-day rat embryo cerebrospinal fluid (eCSF). Immunohistochemical detection of FGF2 expression localized this factor inside neuroepithelial precursors close to the neuroepithelial-CSF interface, suggesting that FGF2 from eCSF could originate in the neuroepithelium by apical secretion. The colocalization of FGFR1 and FGF2 in some neuroepithelial cells close to the ventricular surface suggests they are target cells for eCSF FGF2. Brain neuroepithelium EGF expression was negative. By using a neuroepithelial organotypic culture, we demonstrate that FGF2 and EGF from eCSF plays a specific role in triggering the self-renewal and are involved in neurogenetic induction of mesencephalic neuroepithelial precursor cells during rat development. We propose eCSF as an intercommunication medium for neuroepithelial precursor behavior control during early rat brain development, and the neuroepithelial regulation of FGF2 and EGF presence in eCSF, as a regulative mechanism controlling precursor proliferation and neurogenesis.

Fatty acid binding protein 7 and n-3 poly unsaturated fatty acid supply in early rat brain development.

Fatty acid binding protein 7 (FABP7), abundant in the embryonic brain, binds with the highest affinity to docosahexaenoic acid (DHA) and is expressed in the early stages of embryogenesis. Here, we have examined the consequences of the exposure to different DHA levels and of the in utero depletion of FABP7 on early rat brain development. Neurodevelopment was evaluated through the contents of two proteins, connexin 43 (Cx43) and cyclin-dependent kinase 5 (CDK5), both involved in neuroblast proliferation, differentiation, and migration. The dams were fed with diets presenting different DHA contents, from deficiency to supplementation. DHA brain embryos contents already differed at embryonic day 11.5 and the differences kept increasing with time. Cx43 and CDK5 contents were positively associated with the brain DHA levels. When FABP7 was depleted in vivo by injections of siRNA in the telencephalon, the enhancement of the contents of both proteins was lost in supplemented animals, but FABP7 depletion did not modify phospholipid compositions regardless of the diets. Thus, FABP7 is a necessary mediator of the effect of DHA on these proteins synthesis, but its role in DHA uptake is not critical, although FABP7 is localized in phospholipid-rich areas. Our study shows that high contents of DHA associated with FABP7 are necessary to promote early brain development, which prompted us to recommend DHA supplementation early in pregnancy.

Synthesis of two SAPAP3 isoforms from a single mRNA is mediated via alternative translational initiation

In mammalian neurons, targeting and translation of specific mRNAs in dendrites contribute to synaptic plasticity. After nuclear export, mRNAs designated for dendritic transport are generally assumed to be translationally dormant and activity of individual synapses may locally trigger their extrasomatic translation. We show that the long, GC-rich 5′-untranslated region of dendritic SAPAP3 mRNA restricts translation initiation via a mechanism that involves an upstream open reading frame (uORF). In addition, the uORF enables the use of an alternative translation start site, permitting synthesis of two SAPAP3 isoforms from a single mRNA. While both isoforms progressively accumulate at postsynaptic densities during early rat brain development, their levels relative to each other vary in different adult rat brain areas. Thus, alternative translation initiation events appear to regulate relative expression of distinct SAPAP3 isoforms in different brain regions, which may function to influence synaptic plasticity.

Long-lasting effects of prenatal MAM treatment on water maze performance in rats:: Associations with altered brain development and neurotrophin levels

We previously reported that prenatal methylazoxymethanol (MAM) administered on days 11 and 12 of rat pregnancy induces structural changes in the cytoarchitecture of the hippocampal–entorhinal axis. We also showed that young and middle-aged prenatally treated MAM animals displayed changes in brain neurotrophin levels [Neurosci. Lett. 309 (2001) 113; Physiol. Behav. 71 (2000) 57.]. To continue this line of investigation, the working hypothesis adopted was that prenatal MAM administration, by interfering with limbic neurogenesis, could impair learning and memory ability of aged animals in the water maze. It was found that injection of MAM during early rat brain development induced deficits in both the acquisition and retention phases of the Morris maze. These behavioral changes were associated with significant changes in brain nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), reduced choline acetyltransferase (ChAT) immunoreactivity in forebrain cholinergic neurons and loss of neuropeptide Y (NPY) immunodistribution in cells of the entorhinal cortex. This finding, as well as confirming previous studies showing that injection of prenatal MAM administration induces significant changes in hippocampal–entorhinal axis neurogenesis and marked behavioral deficits in adult life, provides additional experimental evidence supporting the hypothesis that loss of NGF and/or BDNF-receptive or producing cells can co-occur at the onset of neurodevelopmental disorders.

Patterns of nitric oxide synthase at the messenger RNA and protein levels during early rat brain development

There is substantial evidence that the intra- and intercellular messenger nitric oxide, generated enzymatically from l-arginine by nitric oxide synthase in different isoforms, is involved in the development of nervous tissue. In this study we investigated the nitric oxide expression in the pre- and postnatally developing rat brain. With regard to messenger RNA, all of the basic nitric oxide synthase isoforms (neuronal, endothelial and macrophage nitric oxide synthase) were expressed already at embryonic day 10 and showed a temporary decrease at embryonic day 17. Western blot analysis of the three isoform proteins revealed a time pattern that was different from those of messenger RNAs. Although the endothelial nitric oxide synthase isoform was also expressed at embryonic day 10, no quantitative changes were observed over the whole time period studied. Protein amounts of brain and inducible nitric oxide synthase were first detectable at embryonic day 15, with a tendency to rise. A parallel time pattern was found for the NADPH-diaphorase activity in our light microscopic studies, whereas ultrastructurally the reaction product was seen in the brain pallium even of 13-day-old embryos. The data indicate a permanent presence of the transcripts for all nitric oxide synthase isoforms in the rat central nervous system from embryonic day 10 onwards, although the expression of respective proteins and staining patterns may differ.

Developmental Expression of Proteolipid Protein and DM20 mRNAs and Proteins in the Rat Brain

Proteolipid protein (PLP) and DM20, two abundant proteins of myelin, are produced from alternatively spliced mRNAs from the primary PLP gene transcript. Recent studies on the mouse, bovine and human central nervous systems have found that DM20 protein is expressed prior to PLP during development. As development proceeds, however, PLP becomes the predominant protein. This implies that there must be some form of regulation controlling the ratio of these proteins during development. If this regulation occurs during transcription or splicing, the PLP/DM20 mRNA ratio should mimic the PLP/DM20 protein ratio during development. In order to study these closely related mRNAs, we developed a method that used oligonucleotide probes to specifically identify PLP or DM20 mRNA. In this study, we established that (1) as with other species studied, the DM20 protein is present at higher or equivalent levels to PLP during early rat brain development, and (2) PLP and DM20 mRNAs have essentially identical developmental profiles in the rat, although the DM20 mRNA is expressed at lower levels than PLP mRNA. Since the PLP/DM20 protein ratio in young rats does not reflect the PLP/DM20 mRNA ratio, the mechanism of regulation responsible for altering the PLP/DM20 protein ratio during development must occur after transcription and splicing. Possible posttranscriptional mechanisms controlling the ratio of these proteins during development are discussed.