Evidence for structural and functional changes of subplate neurons in developing rat barrel cortex

Autism spectrum disorders pathogenesis: Toward a comprehensive model based on neuroanatomic and neurodevelopment considerations

Autism spectrum disorder (ASD) involves alterations in neural connectivity affecting cortical network organization and excitation to inhibition ratio. It is characterized by an early increase in brain volume mediated by abnormal cortical overgrowth patterns and by increases in size, spine density, and neuron population in the amygdala and surrounding nuclei. Neuronal expansion is followed by a rapid decline from adolescence to middle age. Since no known neurobiological mechanism in human postnatal life is capable of generating large excesses of frontocortical neurons, this likely occurs due to a dysregulation of layer formation and layer-specific neuronal migration during key early stages of prenatal cerebral cortex development. This leads to the dysregulation of post-natal synaptic pruning and results in a huge variety of forms and degrees of signal-over-noise discrimination losses, accounting for ASD clinical heterogeneities, including autonomic nervous system abnormalities and comorbidities. We postulate that sudden changes in environmental conditions linked to serotonin/kynurenine supply to the developing fetus, throughout the critical GW7 - GW20 (Gestational Week) developmental window, are likely to promote ASD pathogenesis during fetal brain development. This appears to be driven by discrete alterations in differentiation and patterning mechanisms arising from in utero RNA editing, favoring vulnerability outcomes over plasticity outcomes. This paper attempts to provide a comprehensive model of the pathogenesis and progression of ASD neurodevelopmental disorders.

Altered White Matter and Layer VIb Neurons in Heterozygous Disc1 Mutant, a Mouse Model of Schizophrenia

Increased white matter neuron density has been associated with neuropsychiatric disorders including schizophrenia. However, the pathogenic features of these neurons are still largely unknown. Subplate neurons, the earliest generated neurons in the developing cortex have also been associated with schizophrenia and autism. The link between these neurons and mental disorders is also not well established. Since cortical layer VIb neurons are believed to be the remnant of subplate neurons in the adult rodent brain, in this study, we aimed to examine the cytoarchitecture of neurons in cortical layer VIb and the underlying white matter in heterozygous Disc1 mutant (Het) mice, a mouse model of schizophrenia. In the white matter, the number of NeuN-positive neurons was quite low in the external capsule; however, the density of these cells was found increased (54%) in Het mice compared with wildtype (WT) littermates. The density of PV-positive neurons was unchanged in the mutants. In the cortical layer VIb, the density of CTGF-positive neurons increased (21.5%) in Het mice, whereas the number of Cplx3-positive cells reduced (16.1%) in these mutants, compared with WT mice. Layer VIb neurons can be classified by their morphological characters. The morphology of Type I pyramidal neurons was comparable between genotypes while the dendritic length and complexity of Type II multipolar neurons were significantly reduced in Het mice. White matter neurons and layer VIb neurons receive synaptic inputs and modulate the process of sensory information and sleep/arousal pattern. Aberrances of these neurons in Disc1 mutants implies altered brain functions in these mice.

Genetic Elimination of Connective Tissue Growth Factor in the Forebrain Affects Subplate Neurons in the Cortex and Oligodendrocytes in the Underlying White Matter

Connective tissue growth factor (CTGF) is a secreted extracellular matrix-associated protein, which play a role in regulating various cellular functions. Although the expression of CTGF has been reported in the cortical subplate, its function is still not clear. Thus, to explore the significance of CTGF in the brain, we created a forebrain-specific Ctgf knockout (FbCtgf KO) mouse model. By crossing Ctgf^fl/fl mice with Emx1-Cre transgenic mice, in which the expression of Cre is prenatally initiated, the full length Ctgf is removed in the forebrain structures. In young adult (2–3 months old) FbCtgf KO mice, subplate markers such as Nurr1 and Cplx3 are still expressed in the cortical layer VIb; however, the density of the subplate neurons is increased. Interestingly, in these mutants, we found a reduced structural complexity in the subplate neurons. The distribution patterns of neurons and glial cells, examined by immunohistochemistry, are comparable between genotypes in the somatosensory cortex. However, increased densities of mature oligodendrocytes, but not immature ones, were noticed in the external capsule underneath the cortical layer VIb in young adult FbCtgf KO mice. The features of myelinated axons in the external capsule were then examined using electron microscopy. Unexpectedly, the thickness of the myelin sheath was reduced in middle-aged (>12 months old), but not young adult FbCtgf KO mice. Our results suggest a secretory function of the subplate neurons, through the release of CTGF, which regulates the density and dendritic branching of subplate neurons as well as the maturation and function of nearby oligodendrocytes in the white matter.

The Superior Function of the Subplate in Early Neocortical Development

During early development the structure and function of the cerebral cortex is critically organized by subplate neurons (SPNs), a mostly transient population of glutamatergic and GABAergic neurons located below the cortical plate. At the molecular and morphological level SPNs represent a rather diverse population of cells expressing a variety of genetic markers and revealing different axonal-dendritic morphologies. Electrophysiologically SPNs are characterized by their rather mature intrinsic membrane properties and firing patterns. They are connected via electrical and chemical synapses to local and remote neurons, e.g., thalamic relay neurons forming the first thalamocortical input to the cerebral cortex. Therefore SPNs are robustly activated at pre- and perinatal stages by the sensory periphery. Although SPNs play pivotal roles in early neocortical activity, development and plasticity, they mostly disappear by programmed cell death during further maturation. On the one hand, SPNs may be selectively vulnerable to hypoxia-ischemia contributing to brain damage, on the other hand there is some evidence that enhanced survival rates or alterations in SPN distribution may contribute to the etiology of neurological or psychiatric disorders. This review aims to give a comprehensive and up-to-date overview on the many functions of SPNs during early physiological and pathophysiological development of the cerebral cortex.

Neocortical Layer 6B as a Remnant of the Subplate - A Morphological Comparison

The fate of the subplate (SP) is still a matter of debate. The SP and layer 6 (which is ontogenetically the oldest and innermost neocortical lamina) develop coincidentally. Yet, the function of sublamina 6B is largely unknown. It has been suggested that it consists partly of neurons from the transient SP, however, experimental evidence for this hypothesis is still missing. To obtain first insights into the neuronal complement of layer 6B in the somatosensory rat barrel cortex, we used biocytin stainings of SP neurons (aged 0-4 postnatal days, PND) and layer 6B neurons (PND 11-35) obtained during in vitro whole-cell patch-clamp recordings. Neurons were reconstructed for a quantitative characterization of their axonal and dendritic morphology. An unsupervised cluster analysis revealed that the SP and layer 6B consist of heterogeneous but comparable neuronal cell populations. Both contain 5 distinct spine-bearing cell types whose relative fractions change with increasing age. Pyramidal cells were more prominent in layer 6B, whereas non-pyramidal neurons were less frequent. Because of the high morphological similarity of SP and layer 6B neurons, we suggest that layer 6B consists of persistent non-pyramidal neurons from the SP and cortical L6B pyramidal neurons.

Sustained synchronized neuronal network activity in a human astrocyte co-culture system

Impaired neuronal network function is a hallmark of neurodevelopmental and neurodegenerative disorders such as autism, schizophrenia, and Alzheimer's disease and is typically studied using genetically modified cellular and animal models. Weak predictive capacity and poor translational value of these models urge for better human derived in vitro models. The implementation of human induced pluripotent stem cells (hiPSCs) allows studying pathologies in differentiated disease-relevant and patient-derived neuronal cells. However, the differentiation process and growth conditions of hiPSC-derived neurons are non-trivial. In order to study neuronal network formation and (mal)function in a fully humanized system, we have established an in vitro co-culture model of hiPSC-derived cortical neurons and human primary astrocytes that recapitulates neuronal network synchronization and connectivity within three to four weeks after final plating. Live cell calcium imaging, electrophysiology and high content image analyses revealed an increased maturation of network functionality and synchronicity over time for co-cultures compared to neuronal monocultures. The cells express GABAergic and glutamatergic markers and respond to inhibitors of both neurotransmitter pathways in a functional assay. The combination of this co-culture model with quantitative imaging of network morphofunction is amenable to high throughput screening for lead discovery and drug optimization for neurological diseases.

Development of Synaptic Boutons in Layer 4 of the Barrel Field of the Rat Somatosensory Cortex: A Quantitative Analysis

Understanding the structural and functional mechanisms underlying the development of individual brain microcircuits is critical for elucidating their computational properties. As synapses are the key structures defining a given microcircuit, it is imperative to investigate their development and precise structural features. Here, synapses in cortical layer 4 were analyzed throughout the first postnatal month using high-end electron microscopy to generate realistic quantitative 3D models. Besides their overall geometry, the size of active zones and the pools of synaptic vesicles were analyzed. At postnatal day 2 only a few shaft synapses were found, but spine synapses steadily increased with ongoing corticogenesis. From postnatal day 2 to 30 synaptic boutons significantly decreased in size whereas that of active zones remained nearly unchanged despite a reshaping. During the first 2 weeks of postnatal development, a rearrangement of synaptic vesicles from a loose distribution toward a densely packed organization close to the presynaptic density was observed, accompanied by the formation of, first a putative readily releasable pool and later a recycling and reserve pool. The quantitative 3D reconstructions of synapses will enable the comparison of structural and functional aspects of signal transduction thus leading to a better understanding of networks in the developing neocortex.

Morphology and dendritic maturation of developing principal neurons in the rat basolateral amygdala

The basolateral nucleus of the amygdala (BLA) assigns emotional valence to sensory stimuli, and many amygdala-dependent behaviors undergo marked development during postnatal life. We recently showed principal neurons in the rat BLA undergo dramatic changes to their electrophysiological properties during the first postnatal month, but no study to date has thoroughly characterized changes to morphology or gene expression that may underlie the functional development of this neuronal population. We addressed this knowledge gap with reconstructions of biocytin-filled principal neurons in the rat BLA at postnatal days 7 (P7), 14, 21, 28, and 60. BLA principal neurons underwent a number of morphological changes, including a twofold increase in soma volume from P7 to P21. Dendritic arbors expanded significantly during the first postnatal month and achieved a mature distribution around P28, in terms of total dendritic length and distance from soma. The number of primary dendrites and branch points were consistent with age, but branch points were found farther from the soma in older animals. Dendrites of BLA principal neurons at P7 had few spines, and spine density increased nearly fivefold by P21. Given the concurrent increase in dendritic material, P60 neurons had approximately 17 times as many total spines as P7 neurons. Together, these developmental transitions in BLA principal neuron morphology help explain a number of concomitant electrophysiological changes during a critical period in amygdala development.

Lsamp is implicated in the regulation of emotional and social behavior by use of alternative promoters in the brain

Limbic system-associated membrane protein (LSAMP) is a neural cell adhesion molecule involved in neurite formation and outgrowth. The purpose of the present study was to characterize the distribution of alternatively transcribed Lsamp isoforms in the mouse brain and its implications on the regulation of behavior. Limbic system-associated membrane protein 1b transcript was visualized by using a mouse strain expressing beta-galactosidase under the control of Lsamp 1b promoter. The distribution of Lsamp 1a transcript and summarized expression of the Lsamp transcripts was investigated by non-radioactive in situ RNA hybridization analysis. Cross-validation was performed by using radioactive in situ hybridization with oligonucleotide probes. Quantitative RT-PCR was used to study correlations between the expression of Lsamp isoforms and behavioral parameters. The expression pattern of two promoters differs remarkably from the developmental initiation at embryonic day 12.5. Limbic system-associated membrane protein 1a promoter is active in “classic” limbic structures where the hippocampus and amygdaloid area display the highest expression. Promoter 1b is mostly active in the thalamic sensory nuclei and cortical sensory areas, but also in areas that regulate stress and arousal. Higher levels of Lsamp 1a transcript had significant correlations with all of the measures indicating higher trait anxiety in the elevated plus-maze test. Limbic system-associated membrane protein transcript levels in the hippocampus and ventral striatum correlated with behavioral parameters in the social interaction test. The data are in line with decreased anxiety and alterations in social behavior in Lsamp-deficient mice. We propose that Lsamp is involved in emotional and social operating systems by complex regulation of two alternative promoters.

Phenotypic characterization of C57BL/6J mice carrying the Disc1 gene from the 129S6/SvEv strain

Disruption of disrupted-in-schizophrenia 1 (DISC1), a candidate susceptibility gene for schizophrenia, was first identified in a large Scottish family in which many members suffered from various psychiatric disorders, including schizophrenia. To model the Scottish DISC1 truncation, we established a Disc1 mutant mouse line in which the 129S6/SvEv 25-bp deletion variant was transferred into the C57BL/6J strain by backcrossing. A battery of behavioral tasks was conducted to evaluate the basic behaviors and cognitive function of these mice. In heterozygote and homozygote Disc1 mutant (Het and Homo) mice, behavioral impairments were noted in working memory test which is thought to be mediated by the function of the medial prefrontal cortex (mPFC). The properties of mPFC neurons were characterized in both morphological and physiological aspects. The dendritic diameters were decreased in layer II/III mPFC pyramidal neurons of Het and Homo mice, whereas a significant reduction in spine density was observed in Homo mice. Neuronal excitability was declined in layer II/III mPFC pyramidal neurons of Het and Homo mice, yet increased transmitter release was identified in Homo mice. Thus, the structural and functional alterations of the mPFC in Het and Homo mice might account for their cognitive impairment. Since most of the gene knockout mice are generated from 129 substrain-derived embryonic stem cells, potential Disc1 deficiency should be considered.

Contamination of current-clamp measurement of neuron capacitance by voltage-dependent phenomena

Measuring neuron capacitance is important for morphological description, conductance characterization, and neuron modeling. One method to estimate capacitance is to inject current pulses into a neuron and fit the resulting changes in membrane potential with multiple exponentials; if the neuron is purely passive, the amplitude and time constant of the slowest exponential give neuron capacitance (Major G, Evans JD, Jack JJ. Biophys J 65: 423-449, 1993). Golowasch et al. (Golowasch J, Thomas G, Taylor AL, Patel A, Pineda A, Khalil C, Nadim F. J Neurophysiol 102: 2161-2175, 2009) have shown that this is the best method for measuring the capacitance of nonisopotential (i.e., most) neurons. However, prior work has not tested for, or examined how much error would be introduced by, slow voltage-dependent phenomena possibly present at the membrane potentials typically used in such work. We investigated this issue in lobster (Panulirus interruptus) stomatogastric neurons by performing current clamp-based capacitance measurements at multiple membrane potentials. A slow, voltage-dependent phenomenon consistent with residual voltage-dependent conductances was present at all tested membrane potentials (-95 to -35 mV). This phenomenon was the slowest component of the neuron's voltage response, and failure to recognize and exclude it would lead to capacitance overestimates of several hundredfold. Most methods of estimating capacitance depend on the absence of voltage-dependent phenomena. Our demonstration that such phenomena make nonnegligible contributions to neuron responses even at well-hyperpolarized membrane potentials highlights the critical importance of checking for such phenomena in all work measuring neuron capacitance. We show here how to identify such phenomena and minimize their contaminating influence.