Radial migration in the cerebral cortex is enhanced by signals from thalamus

Perinatal exposure to fluoxetine increases anxiety- and depressive-like behaviours and alters glutamatergic markers in the prefrontal cortex and hippocampus of male adolescent rats: A comparison between Sprague-Dawley rats and the Wistar-Kyoto rat model of depression

BACKGROUND

With approximately 10% of pregnant women prescribed antidepressant drugs for the treatment of depressive disorders, there is growing concern regarding the potential long-term effects of this exposure on offspring. Research is needed in clinically relevant models to determine the effects on offspring behaviour and associated neurobiological systems.

AIM

The aim of this study was to determine the effects of maternal fluoxetine treatment on anxiety-like and depressive-like behaviours in adolescent offspring as well as associated glutamatergic markers, using a clinically relevant rodent model of depression.

METHODS

Wistar-Kyoto (model of innate depression) and Sprague-Dawley rats were treated with fluoxetine (10 mg/kg) from gestational day 0 to postnatal day 14. Male offspring underwent behavioural testing (open field, elevated plus maze, forced swim test) at adolescence followed by quantitative immuno-detection of glutamatergic markers in the prefrontal cortex and ventral hippocampus.

RESULTS

Perinatal fluoxetine exposure exacerbated the anxiety-like and depressive-like phenotype in Wistar-Kyoto offspring and induced an anxiety-like and depressive-like phenotype in Sprague-Dawley offspring. Wistar-Kyoto offspring showed reductions in NMDA receptor NR1, NR2A and NR2B subunits, as well as post-synaptic density 95 (PSD-95) and metabotropic glutamate receptor subtype 1 (mGluR1) in the prefrontal cortex; perinatal fluoxetine exposure further reduced NR1, NR2A, PSD-95 and mGluR1 expression in Wistar-Kyoto as well as Sprague-Dawley offspring. In the ventral hippocampus perinatal fluoxetine exposure reduced PSD-95 and increased metabotropic glutamate receptor subtype 5 (mGluR5) and Homer1b/c in both Sprague-Dawley and Wistar-Kyoto strains.

CONCLUSION

These findings suggest that maternal fluoxetine treatment exacerbates effects of underlying maternal depression on offspring behaviour, which may be mediated through alterations in the glutamatergic system. Further research investigating how to minimise these effects, whilst ensuring optimal treatment for mothers, is essential to move the field forward.

Comparison of two related lines of tauGFP transgenic mice designed for lineage tracing

Background

The tauGFP reporter fusion protein is produced nearly ubiquitously by the TgTP6.3 transgene in TP6.3 mice and its localisation to microtubules offers some advantages over soluble GFP as a lineage marker. However, TgTP6.3^Tg/Tg homozygotes are not viable and TgTP6.3^Tg/− hemizygotes are smaller than wild-type. TP6.4 mice carry the TgTP6.4 transgene, which was produced with the same construct used to generate TgTP6.3, so we investigated whether TgTP6.4 had any advantages over TgTP6.3.

Results

Although TgTP6.4^Tg/Tg homozygotes died before weaning, TgTP6.4^Tg/− hemizygotes were viable and fertile and only males were significantly lighter than wild-type. The TgTP6.4 transgene produced the tauGFP fusion protein by the 2-cell stage and it was widely expressed in adults but tauGFP fluorescence was weak or absent in several tissues, including some neural tissues. The TgTP6.4 transgene expression pattern changed over several years of breeding and mosaic transgene expression became increasingly common in all expressing tissues. This mosaicism was used to visualise clonal lineages in the adrenal cortex of TgTP6.4^Tg/− hemizygotes and these were qualitatively and quantitatively comparable to lineages reported previously for other mosaic transgenic mice, X-inactivation mosaics and chimaeras. Mosaicism occurred less frequently in TP6.3 than TP6.4 mice and was only observed in the corneal epithelium and adrenal cortex.

Conclusions

Mosaic expression makes the TgTP6.4 transgene unsuitable for use as a conventional cell lineage marker but such mosaicism provides a useful system for visualising clonal lineages that arise during development or maintenance of adult tissues. Differences in the occurrence of mosaicism between related transgenic lines, such as that described for lines TP6.3 and TP6.4, might provide a useful system for investigating the mechanism of transgene silencing.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-017-0149-x) contains supplementary material, which is available to authorized users.

The effects of maternal antidepressant use on offspring behaviour and brain development: Implications for risk of neurodevelopmental disorders

Approximately 10% of pregnant women are prescribed antidepressant drugs (ADDs), with selective serotonin reuptake inhibitors (SSRIs) the most widely prescribed. SSRIs bind to the serotonin transporter (SERT), blocking the reabsorption of serotonin by the presynaptic neuron and increasing serotonin levels in the synaptic cleft. The serotonergic system regulates a range of brain development processes including neuronal proliferation, migration, differentiation and synaptogenesis. Given the presence of SERT in early brain development, coupled with the ability of SSRIs to cross the placenta and also enter breast milk, concerns have been raised regarding the effects of SSRI exposure on the developing foetus and newborns. In this review, we evaluate preclinical and clinical studies that have examined the effects of maternal SSRI exposure and the risk for altered neurodevelopment and associated behaviours in offspring. While the current body of evidence suggests that maternal SSRI treatment may cause perturbations to the neurobiology, behaviour and ultimately risk for neurodevelopmental disorders in exposed offspring, conflicting findings do exist and the evidence is not conclusive. However, given the increasing incidence of depression and number of women prescribed ADDs during pregnancy, further investigation into this area is warranted.

Serotonin homeostasis and serotonin receptors as actors of cortical construction: special attention to the 5-HT3A and 5-HT6 receptor subtypes

Cortical circuits control higher-order cognitive processes and their function is highly dependent on their structure that emerges during development. The construction of cortical circuits involves the coordinated interplay between different types of cellular processes such as proliferation, migration, and differentiation of neural and glial cell subtypes. Among the multiple factors that regulate the assembly of cortical circuits, 5-HT is an important developmental signal that impacts on a broad diversity of cellular processes. 5-HT is detected at the onset of embryonic telencephalic formation and a variety of serotonergic receptors are dynamically expressed in the embryonic developing cortex in a region and cell-type specific manner. Among these receptors, the ionotropic 5-HT3A receptor and the metabotropic 5-HT6 receptor have recently been identified as novel serotonergic targets regulating different aspects of cortical construction including neuronal migration and dendritic differentiation. In this review, we focus on the developmental impact of serotonergic systems on the construction of cortical circuits and discuss their potential role in programming risk for human psychiatric disorders.

Cerebral cortical development in rodents and primates

Rodents and primates both show considerable variation in the overall size, the radial and tangential dimensions, folding and subdivisions into distinct areas of their cerebral cortex. Our current understanding of brain development is based on a handful of model systems. A detailed comparative analysis of the cellular and molecular mechanisms that regulate neural progenitor production, cell migration, and circuit assembly can provide much needed insights into the working of neocortical evolution. From the limited comparative data currently available, it is apparent that the emergence and variation of the neuronal progenitor cells have led to the production of increased neuronal populations and the evolution of the cortex. Further diversification and compartmentalization of the germinal zone together with changing proportions of radial glia in the ventricular zone and various intermediate progenitors in the subventricular zone may have been the driving force behind increased cell numbers in larger brains both in rodents and primates. Radial and tangential migratory patterns are both present in rodents and primates, but in different proportions. There are apparent differences between mouse and human in the generation and elaboration of the interneuronal subtypes and also in gene expression patterns associated with the appearance of distinct cortical areas. The increased cortical dimensions and the formation of a more elaborate cortical architecture in primates require a larger and more compartmentalized transient subplate zone during development. More comparative analysis in rodent and primate species with large, small, and smooth and folded brains is needed to reveal the biological significance of the alterations in these cortical developmental programs.

Compartmentalization of cerebral cortical germinal zones in a lissencephalic primate and gyrencephalic rodent

Previous studies of macaque and human cortices identified cytoarchitectonically distinct germinal zones; the ventricular zone inner subventricular zone (ISVZ), and outer subventricular zone (OSVZ). To date, the OSVZ has only been described in gyrencephalic brains, separated from the ISVZ by an inner fiber layer and considered a milestone that triggered increased neocortical neurogenesis. However, this observation has only been assessed in a handful of species without the identification of the different progenitor populations. We examined the Amazonian rodent agouti (Dasyprocta agouti) and the marmoset monkey (Callithrix jacchus) to further understand relationships among progenitor compartmentalization, proportions of various cortical progenitors, and degree of cortical folding. We identified a similar cytoarchitectonic distinction between the OSVZ and ISVZ at midgestation in both species. In the marmoset, we quantified the ventricular and abventricular divisions and observed similar proportions as previously described for the human and ferret brains. The proportions of radial glia, intermediate progenitors, and outer radial glial cell (oRG) populations were similar in midgestation lissencephalic marmoset as in gyrencephalic human or ferret. Our findings suggest that cytoarchitectonic subdivisions of SVZ are an evolutionary trend and not a primate specific feature, and a large population of oRG can be seen regardless of cortical folding.

Evolution of cerebral cortical development

Understanding how the human cerebral cortex evolved to its present complex state is a fascinating topic for neuroscience, genetics, bioinformatics and comparative biology. To gain further insights into the origins of the mammalian neocortex and to understand how the cortex evolved to be able to serve more complex cognitive functions, we study the development of various extant species. Our aim is to correlate cortical cell numbers and neuronal cell types with the elaboration of cortical progenitor populations and their modes of proliferation in different species. There are several progenitors, i.e. the ventricular radial glia, the subventricular intermediate progenitors and subventricular (outer) radial glia types, but the contribution of each to cortical layers and cell types through specific lineages is not fully understood. Recent comparisons of the proportions of these progenitors in various species during embryonic neurogenesis have revealed the elaboration and cytoarchitectonic compartmentalization of the germinal zone, with alterations in the proportions of various types that can be included among the intermediate progenitors. Across species, larger and more diverse intermediate progenitor populations correlate with brain size and cortical cell diversity. Understanding the molecular and cellular interactions regulating the divisions of these intermediate progenitors not only has implications for cortical evolution but also relates to stem cell biology and illuminates the pathomechanisms of several cortical developmental disorders.

The role of Sonic hedgehog of neural origin in thalamic differentiation in the mouse

The specification of the intricate neuronal assemblies that characterize the forebrain is not well understood. The ventral spinal cord is specified through a concentration gradient of Sonic hedgehog (Shh) protein secreted by the notochord. Shh is expressed also in the forebrain neuroepithelium (neural Shh) and the underlying notochord and prechordal plate. Neural Shh is essential for the development of the prethalamus (ventral thalamus), but its effects on the thalamus (dorsal thalamus) are still unclear. We hypothesized that neural Shh would act on a previously regionalized dorsal diencephalic region to promote the emergence of specific thalamic nuclear and histological traits. To find out, we generated a conditional mouse mutant line specifically lacking Shh expression in the diencephalic neuroepithelium. We show that the transcription factor Gbx2, required for thalamic development downstream Shh, is expressed in our mutant in a restricted thalamic region and is necessary and sufficient for the differentiation of the medial and intralaminar thalamic nuclei. In the rest of the thalamus, neural Shh is required to promote neuronal aggregation into nuclei as well as axonal extension. In this way, the individual thalamic nuclei show differential dependence on Shh, Gbx2, or both for their differentiation. Additionally, Gbx2 is required for the survival of thalamic neurons.

A molecular neuroanatomical study of the developing human neocortex from 8 to 17 postconceptional weeks revealing the early differentiation of the subplate and subventricular zone

We have employed immunohistochemistry for multiple markers to investigate the structure and possible function of the different compartments of human cerebral wall from the formation of cortical plate at 8 postconceptional weeks (PCW) to the arrival of thalamocortical afferents at 17 PCW. New observations include the subplate emerging as a discrete differentiated layer by 10 PCW, characterized by synaptophysin and vesicular gamma-aminobutyric acid transporter expression also seen in the marginal zone, suggesting that these compartments may maintain a spontaneously active synaptic network even before the arrival of thalamocortical afferents. The subplate expanded from 13 to 17 PCW, becoming the largest compartment and differentiated further, with NPY neurons located in the outer subplate and KCC2 neurons in the inner subplate. Glutamate decarboxylase and calretinin-positive inhibitory neurons migrated tangentially and radially from 11.5 PCW, appearing in larger numbers toward the rostral pole. The proliferative zones, marked by Ki67 expression, developed a complicated structure by 12.5 PCW reflected in transcription factor expression patterns, including TBR2 confined to the inner subventricular and outer ventricular zones and TBR1 weakly expressed in the subventricular zone (SVZ). PAX6 was extensively expressed in the proliferative zones such that the human outer SVZ contained a large reservoir of PAX6-positive potential progenitor cells.

Calcineurin differentially regulates fast myosin heavy chain genes in oxidative muscle fibre type conversion

In skeletal muscle, calcineurin is crucial for myocyte differentiation and in the determination of the slow oxidative fibre phenotype, both processes being important determinants of muscle performance, metabolic health and meat-animal production. Fibre type is defined by the isoform identity of the skeletal myosin heavy chain (MyHC). We have examined the responses of the major MyHC genes to calcineurin signalling during fibre formation of muscle C2C12 cells. We have found that calcineurin acts as a signal to up-regulate the fast-oxidative MyHC2a gene and to down-regulate the faster MyHC2x and MyHC2b genes in a manner that appears to be NFAT-independent. Contrary to expectation, the up-regulation of MyHCslow by calcineurin seems to be time-dependent and is only detectable once the initial differential expression of the post-natal fast MyHC genes has been established. The simultaneous elevated expression of MyHC2a and the repression of MyHC2x and MyHC2b expression indicate that both processes (elevation and repression) are actively coordinated during oxidative fibre conversion. We have further determined that muscle LIM protein (MLP), a calcineurin-binding Z-line co-factor, is induced by calcineurin and that its co-expression with calcineurin has an additive effect on MyHCslow expression. Hence, post-natal fast MyHCs are important early effector targets of calcineurin, whereas MyHCslow up-regulation is mediated in part by calcineurin-induced MLP.

Porcine congenital splayleg is characterised by muscle fibre atrophy associated with relative rise in MAFbx and fall in P311 expression

BACKGROUND

Porcine congenital splayleg (PCS) is the most important congenital condition of piglets, associated with lameness and immobility, of unknown aetiology and pathogenesis, hence the need to better understand the condition by defining, in the first instance, its histopathology and molecular pathology.

RESULTS

Semitendinosus, longissimus dorsi, and gastrocnemius muscles were removed from 4 sets of 2-day-old splayleg piglets, each with a corresponding normal litter mate. Based on immunohistochemistry and histological image analysis, PCS piglets showed significantly smaller fibre size without any accompanying sign of inflammation. Although there was no dramatic change in fibre type composition in affected muscles, several structural myosin heavy chain genes were significantly down-regulated. MAFbx, a major atrophy marker, was highly up-regulated in nearly all PCS muscles, in comparison with controls from normal litter mates. In contrast, P311, a novel 8 kDa protein, was relatively down-regulated in all the PCS muscles. To investigate a functional role of P311 in skeletal muscle, its full-length cDNA was over-expressed in murine C2C12 muscle cells, which resulted in enhanced cell proliferation with reduced myotube formation. Hence, reduced P311 expression in PCS piglets might contribute to atrophy through reduced muscle cell proliferation. P311, predictably, was down-regulated by the over-expression of calcineurin, a key signalling factor of muscle differentiation.

CONCLUSION

We demonstrated that PCS is a condition characterised by extensive fibre atrophy and raised fibre density, and propose that the combined differential expression of MAFbx and P311 is of potential in the diagnosis of subclinical PCS.

An immunohistochemical study of the development of sensorimotor components of the early fetal human spinal cord

Sections from all spinal cord levels from 20 human fetuses, age range 7.5-17 gestational weeks (GW) were immunostained for non-phosphorylated neurofilaments (to reveal motoneurones, spinocerebellar neurones and other large neurones), the calcium-binding protein parvalbumin (large proprioreceptive afferents), growth-associated protein 43 kDa (growing axons), glial fibrillary acidic protein (radial glia), synaptophysin (synaptic terminals) the cell-cell recognition molecule ephrin A4 (EphA4) and the ETS transcription factor Er81 (subclasses of motoneurone and proprioreceptive neurone). Muscle afferents crossed the dorsal horn by 7.5 GW and innervated motoneurones by 9 GW. An alignment of glial fibres guided them from dorsal columns to ventral horn, at right angles to the radial glia. They continued to provide a dense innervation of motoneurone pools up to 17 GW. By 13 GW motoneurones were segregated into distinct columns, all of which expressed EphA4 although only certain lateral groups expressed Er81. However, Er81 expression was more widespread amongst dorsal root ganglion neurones. From 9 GW Clarke's column neurones were identified and by 14 GW were heavily innervated by parvalbumin-positive afferents whilst their efferent axons could be traced to the lateral funiculus. This investigation contributes towards a timetable for the functional development of human motor control and makes comparisons with well-studied rodent models.

A morphogenetic model for the development of cortical convolutions

The convolutions of the mammalian cortex are one of its most intriguing characteristics. Their pattern is very distinctive for different species, and there seems to be a remarkable relationship between convolutions and the architectonic and functional regionalization of the cerebral cortex. Yet the mechanisms behind the development of convolutions and their association with the cortical regionalization are poorly understood. Here we propose a morphogenetic model for the development of cortical convolutions based on the structure of the cortex as a closed surface with glial and axonal fibres pulling radially, the fundamental mechanical properties of cortex and fibres (elasticity and plasticity), and the growth of the cortical surface. The computer simulations of this model suggest that convolutions are a natural consequence of cortical growth. The model reproduces several aspects of convolutional development, such as the relationship between cortical surface and brain volume among mammals, the period of compensation in the degree of convolution observed in gyrencephalic brains and the dependence of the degree of convolution on cortical thickness. We have also studied the effect of early cortical regionalization on the development of convolutions by introducing geometric, mechanic and growth asymmetries in the model. The morphogenetic model is thus able to reproduce the gradients in the degree of convolution, the development of primary, secondary and tertiary convolution, and the overproduction of sulci observed in animals with altered afferent cortical connections.

Postnatal development of the afferent projections from the dorsomedial thalamic nucleus to the frontal cortex in mice

The postnatal development of mediodorsal thalamic projections to the dorsomedial frontal cortex of mice was assessed by means of the retrograde peroxidase-colloidal gold complex tract tracing system. The tracer was injected into the dorsomedial frontal cortex from the day of birth (P0) to 60 days of postnatal age (P60). Since birth, a dense retrograde labeling has been found in the mediodorsal nucleus, which increased progressively from P4 to P8 and began to decrease at P10 until P13 (67.37% vs. the maximal average, P4). After P16, the mean average remains stable up to P60.

Inhibition of SRC family kinases and non-classical protein kinases C induce a reeler-like malformation of cortical plate development

During development, most cortical neurons migrate to the cortical plate (CP) radially. CP development is abnormal in reeler and other mutant mice with defective Reelin signaling. Reelin is secreted by Cajal-Retzius cells and binds to the very low density lipoprotein receptor and apolipoprotein E receptor type 2 receptors on the surface of CP cells, inducing tyrosine phosphorylation of the intracellular Dab1 adapter. As with Reelin receptors, the identification of Reelin signaling partners is hampered by genetic redundancy. Using a new in vitro embryonic slice culture system, we demonstrate that chemical inhibitors of Src family kinases and Abl, but not inhibitors of Abl alone, generate a reeler-like malformation and that inhibitors of protein kinases C induce a malformation of cortical development that is also reminiscent of reeler. Our observations demonstrate a key role for these enzymes in radial migration to the cortical plate, possibly via interference with Reelin signaling.

Early development of voltage-gated ion currents and firing properties in neurons of the mouse cerebral cortex

Voltage- and current-clamp recordings were made from acute slices of mouse cerebral cortex from embryonic day 14 to postnatal day 17. We targeted cells in the migratory population of the embryonic intermediate zone (IZ) and in deep layers of embryonic and postnatal cortical plate (CP). IZ neurons maintain fairly consistent properties through the embryonic period, all expressing high-input resistance, inward Na(+) currents and outward K(+) currents, and none showing any hyperpolarization-activated currents. In CP neurons, several changes in physiological properties occur in the late embryonic and early postnatal period: inward Na(+) current density is strongly upregulated while outward K(+) current density remains almost unchanged, input resistance drops dramatically, and a hyperpolarization-activated current resembling I(h) appears. As a result of these changes, the action potential becomes larger, shorter in duration, and its threshold shifts to more negative potentials. In addition, CP cells become capable of firing repetitively and an increasing fraction show spontaneous action potentials. This coordinated development of ion channel properties may help to time the occurrence of developmentally relevant spontaneous activity in the immature cortex.

The role of serotonin in early cortical development

The cerebral cortex is widely innervated by serotonin (5-HT)-containing axons originating from neurons in the raphe nuclei. The early development of this monoamine system in the cortex prompted speculation long ago that it has important functions in cortical maturation and plasticity. Here we review evidence, derived from a plethora of studies and from our recent unpublished work, that supports an important role for 5-HT in a number of major events in the developing cortex, especially at the early stages. This evidence points to a regulatory role for 5-HT in neuronal proliferation, migration and differentiation, and in preventing apoptotic cell death.