Differentiation of apical and basal dendrites in pyramidal cells and granule cells in dissociated hippocampal cultures

Perinatal compromise affects development, form, and function of the hippocampus part one; clinical studies

The hippocampus is a neuron-rich specialised brain structure that plays a central role in the regulation of emotions, learning and memory, cognition, spatial navigation, and motivational processes. In human fetal development, hippocampal neurogenesis is principally complete by mid-gestation, with subsequent maturation comprising dendritogenesis and synaptogenesis in the third trimester of pregnancy and infancy. Dendritogenesis and synaptogenesis underpin connectivity. Hippocampal development is exquisitely sensitive to perturbations during pregnancy and at birth. Clinical investigations demonstrate that preterm birth, fetal growth restriction (FGR), and acute hypoxic-ischaemic encephalopathy (HIE) are common perinatal complications that alter hippocampal development. In turn, deficits in hippocampal development and structure mediate a range of neurodevelopmental disorders, including cognitive and learning problems, autism, and Attention-Deficit/Hyperactivity Disorder (ADHD). In this review, we summarise the developmental profile of the hippocampus during fetal and neonatal life and examine the hippocampal deficits observed following common human pregnancy complications. IMPACT: The review provides a comprehensive summary of the developmental profile of the hippocampus in normal fetal and neonatal life. We address a significant knowledge gap in paediatric research by providing a comprehensive summary of the relationship between pregnancy complications and subsequent hippocampal damage, shedding new light on this critical aspect of early neurodevelopment.

Ultrasound neuromodulation of cultured hippocampal neurons

Ultrasound is becoming an emerging and promising method for neuromodulation due to its advantage of noninvasiveness and its high spatial resolution. However, the underlying principles of ultrasound neuromodulation have not yet been elucidated. We have herein developed a new in vitro setup to study the ultrasonic neuromodulation, and examined various parameters of ultrasound to verify the effective conditions to evoke the neural activity. Neurons were stimulated with 0.5 MHz center frequency ultrasound, and the action potentials were recorded from rat hippocampal neural cells cultured on microelectrode arrays. As the intensity of ultrasound increased, the neuronal activity also increased. There was a notable and significant increase in both the spike rate and the number of bursts at 50% duty cycle, 1 kHz pulse repetition frequency, and the acoustic intensities of 7.6 W/cm2 and 3.8 W/cm2 in terms of spatial-peak pulse-average intensity and spatial-peak temporal-average intensity, respectively. In addition, the impact of ultrasonic neuromodulation was assessed in the presence of a gamma-aminobutyric acid A (GABAA) receptor antagonist to exclude the effect of activated inhibitory neurons. Interestingly, it is noteworthy that the predominant neuromodulatory effects of ultrasound disappeared when the GABAA blocker was introduced, suggesting the potential of ultrasonic stimulation specifically targeting inhibitory neurons. The experimental setup proposed herein could serve as a useful tool for the clarification of the mechanisms underlying the electrophysiological effects of ultrasound.

Calcium signals tune AMPK activity and mitochondrial homeostasis in dendrites of developing neurons

Dendritic outgrowth in immature neurons is enhanced by neuronal activity and is considered one of the mechanisms of neural circuit optimization. It is known that calcium signals affect transcriptional regulation and cytoskeletal remodeling necessary for dendritic outgrowth. Here, we demonstrate that activity-dependent calcium signaling also controls mitochondrial homeostasis via AMP-activated protein kinase (AMPK) in growing dendrites of differentiating mouse hippocampal neurons. We found that the inhibition of neuronal activity induced dendritic hypotrophy with abnormally elongated mitochondria. In growing dendrites, AMPK is activated by neuronal activity and dynamically oscillates in synchrony with calcium spikes, and this AMPK oscillation was inhibited by CaMKK2 knockdown. AMPK activation led to phosphorylation of MFF and ULK1, which initiate mitochondrial fission and mitophagy, respectively. Dendritic mitochondria in AMPK-depleted neurons exhibited impaired fission and mitophagy and displayed multiple signs of dysfunction. Genetic inhibition of fission led to dendritic hypoplasia that was reminiscent of AMPK-deficient neurons. Thus, AMPK activity is finely tuned by the calcium-CaMKK2 pathway and regulates mitochondrial homeostasis by facilitating removal of damaged components of mitochondria in growing neurons during normal brain development.

The cytoplasmic localization of ADNP through 14-3-3 promotes sex-dependent neuronal morphogenesis, cortical connectivity, and calcium signaling

Defective neuritogenesis is a contributing pathogenic mechanism underlying a variety of neurodevelopmental disorders. Single gene mutations in activity-dependent neuroprotective protein (ADNP) are the most frequent among autism spectrum disorders (ASDs) leading to the ADNP syndrome. Previous studies showed that during neuritogenesis, Adnp localizes to the cytoplasm/neurites, and Adnp knockdown inhibits neuritogenesis in culture. Here, we hypothesized that Adnp is localized in the cytoplasm during neurite formation and that this process is mediated by 14-3-3. Indeed, applying the 14-3-3 inhibitor, difopein, blocked Adnp cytoplasmic localization. Furthermore, co-immunoprecipitations showed that Adnp bound 14-3-3 proteins and proteomic analysis identified several potential phosphorylation-dependent Adnp/14-3-3 binding sites. We further discovered that knockdown of Adnp using in utero electroporation of mouse layer 2/3 pyramidal neurons in the somatosensory cortex led to previously unreported changes in neurite formation beginning at P0. Defects were sustained throughout development, the most notable included increased basal dendrite number and axon length. Paralleling the observed morphological aberrations, ex vivo calcium imaging revealed that Adnp deficient neurons had greater and more frequent spontaneous calcium influx in female mice. GRAPHIC, a novel synaptic tracing technology substantiated this finding, revealing increased interhemispheric connectivity between female Adnp deficient layer 2/3 pyramidal neurons. We conclude that Adnp is localized to the cytoplasm by 14-3-3 proteins, where it regulates neurite formation, maturation, and functional cortical connectivity significantly building on our current understanding of Adnp function and the etiology of ADNP syndrome.

Temperature-dependent structural plasticity of hippocampal synapses

The function of the central nervous system (CNS) is strongly affected by temperature. However, the underlying processes remain poorly understood. Here, we show that hypothermia and hyperthermia trigger bidirectional re-organization of presynaptic architecture in hippocampal neurons, resulting in synaptic strengthening, and weakening, respectively. Furthermore, hypothermia remodels inhibitory postsynaptic scaffold into enlarged, sparse synapses enriched in GABAA receptors. This process does not require protein translation, and instead is regulated by actin dynamics. Induction of hypothermia in vivo enhances inhibitory synapses in the hippocampus, but not in the cortex. This is confirmed by the proteomic analysis of cortical synapses, which reveals few temperature-dependent changes in synaptic content. Our results reveal a region-specific form of environmental synaptic plasticity with a mechanism distinct from the classic temperature shock response, which may underlie functional response of CNS to temperature.

Contraction of axonal and dendritic fields in Sox5-deficient cone bipolar cells is accompanied by axonal sprouting and dendritic hyper-innervation of pedicles

Multiple factors regulate the differentiation of neuronal morphology during development, including interactions with afferents, targets, and homotypic neighbors, as well as cell-intrinsic transcriptional regulation. Retinal bipolar cells provide an exemplary model system for studying the control of these processes, as there are 15 transcriptionally and morphologically distinct types, each extending their dendritic and axonal arbors in respective strata within the synaptic layers of the retina. Here we have examined the role of the transcription factor Sox5 in the control of the morphological differentiation of one type of cone bipolar cell (CBC), the Type 7 cell. We confirm selective expression of SOX5 in this single bipolar cell type, emerging at the close of the first post-natal week, prior to morphological differentiation. Conditional knockout mice were generated by crossing a bipolar cell-specific cre-expressing line with mice carrying floxed Sox5 alleles, as well as the Gustducin-gfp reporter which labels Type 7 CBCs. Loss of SOX5 was confirmed in the bipolar cell stratum, in GFP+ Type 7 cells. Such SOX5-deficient Type 7 cells differentiate axonal and dendritic arbors that are each reduced in areal extent. The axonal arbors exhibit sprouting in the inner plexiform layer (IPL), thereby extending their overall radial extent, while the dendritic arbors connect with fewer cone pedicles in the outer plexiform layer, showing an increase in the average number of dendritic contacts at each pedicle. SOX5-deficient Type 7 CBCs should therefore exhibit smaller receptive fields derived from fewer if now hyper-innervated pedicles, transmitting their signals across a broader depth through the IPL.

Resveratrol Prevents Cytoarchitectural and Interneuronal Alterations in the Valproic Acid Rat Model of Autism

Autism spectrum disorder (ASD) is a prevalent neurodevelopmental disorder characterized by several alterations, including disorganized brain cytoarchitecture and excitatory/inhibitory (E/I) imbalance. We aimed to analyze aspects associated with the inhibitory components in ASD, using bioinformatics to develop notions about embryonic life and tissue analysis for postnatal life. We analyzed microarray and RNAseq datasets of embryos from different ASD models, demonstrating that regions involved in neuronal development are affected. We evaluated the effect of prenatal treatment with resveratrol (RSV) on the neuronal organization and quantity of parvalbumin-positive (PV+), somatostatin-positive (SOM+), and calbindin-positive (CB+) GABAergic interneurons, besides the levels of synaptic proteins and GABA receptors in the medial prefrontal cortex (mPFC) and hippocampus (HC) of the ASD model induced by valproic acid (VPA). VPA increased the total number of neurons in the mPFC, while it reduced the number of SOM+ neurons, as well as the proportion of SOM+, PV+, and CB+ neurons (subregion-specific manner), with preventive effects of RSV. In summary, metabolic alterations or gene expression impairments could be induced by VPA, leading to extensive damage in the late developmental stages. By contrast, due to its antioxidant, neuroprotective, and opposite action on histone properties, RSV may avoid damages induced by VPA.

Generation of dystrophin short product-specific tag-insertion mouse: distinct Dp71 glycoprotein complexes at inhibitory postsynapse and glia limitans

Duchenne muscular dystrophy (DMD), the most severe form of dystrophinopathies, is a fatal X-linked recessive neuromuscular disorder characterized by progressive muscle degeneration and various extents of intellectual disabilities. Physiological and pathological roles of the responsible gene, dystrophin, in the brain remain elusive due to the presence of multiple dystrophin products, mainly full-length dystrophin, Dp427, and the short product, Dp71. In this study, we generated a Dp71-specific hemagglutinin (HA) peptide tag-insertion mice to enable specific detection of intrinsic Dp71 expression by anti-HA-tag antibodies. Immunohistochemical detections in the transgenic mice demonstrated Dp71 expression not only at the blood-brain barrier, where astrocytic endfeet surround the microvessels, but also at the inhibitory postsynapse of hippocampal dentate granule neurons. Interestingly, hippocampal cornu ammonis (CA)1 pyramidal neurons were negative for Dp71, although Dp427 detected by anti-dystrophin antibody was clearly present at the inhibitory postsynapse, suggesting cell-type dependent dystrophin expressions. Precise examination using the primary hippocampal culture validated exclusive localization of Dp71 at the inhibitory postsynaptic compartment but not at the excitatory synapse in neurons. We further performed interactome analysis and found that Dp71 formed distinct molecular complexes, i.e. synapse-associated Dp71 interacted with dystroglycan (Dg) and dystrobrevinβ (Dtnb), whereas glia-associated Dp71 did with Dg and dystrobrevinα (Dtna). Thus, our data indicate that Dp71 and its binding partners are relevant to the inhibitory postsynaptic function of hippocampal granule neurons and the novel Dp71-transgenic mouse provides a valuable tool to understand precise physiological expressions and functions of Dp71 and its interaction proteins in vivo and in vitro.

Early appearance of dendritic alterations in neocortical pyramidal neurons of the Ts65Dn model of Down syndrome

Down syndrome (DS), which is due to triplication of chromosome 21, is constantly associated with intellectual disability (ID). ID can be ascribed to both neurogenesis impairment and dendritic pathology. These defects are replicated in the Ts65Dn mouse, a widely used model of DS. While neurogenesis impairment in DS is a fetal event, dendritic pathology occurs after the first postnatal months. Neurogenesis alterations across the lifespan have been extensively studied in the Ts65Dn mouse. In contrast, there is scarce information regarding dendritic alterations at early life stages in this and other models, although there is evidence for dendritic alterations in adult mouse models. Thus, the goal of the current study was to establish whether dendritic alterations are already present in the neonatal period in Ts65Dn mice. In Golgi-stained brains we quantified the dendritic arbors of layer II/III pyramidal neurons in the frontal cortex of Ts65Dn mice aged 2 (P2) and 8 (P8) days and their euploid littermates. In P2 Ts65Dn mice we found a moderate hypotrophy of the apical and collateral dendrites but a patent hypotrophy of the basal dendrites. In P8 Ts65Dn mice the distalmost apical branches were missing or reduced in number but there were no alterations in the collateral and basal dendrites. No genotype effects were detected on either somatic or dendritic spine density. This study shows dendritic branching defects that mainly involve the basal domain in P2 Ts65Dn mice, and the apical but not the other domains in P8 Ts65Dn mice. This suggests that dendritic defects may be related to dendritic compartment and age. The lack of a severe dendritic pathology in Ts65Dn pups is reminiscent of the delayed appearance of patent dendritic alterations in newborns with DS. This similarly highlights the usefulness of the Ts65Dn model for the study of the mechanisms underlying dendritic alterations in DS and the design of possible therapeutic interventions.

Glutathione S-transferase Pi (Gstp) proteins regulate neuritogenesis in the developing cerebral cortex

GSTP proteins are metabolic enzymes involved in the removal of oxidative stress and intracellular signaling and also have inhibitory effects on JNK activity. However, the functions of Gstp proteins in the developing brain are unknown. In mice, there are three Gstp proteins, Gstp1, 2 and 3, whereas there is only one GSTP in humans. By reverse transcription-polymerase chain reaction (RT-PCR) analysis, we found that Gstp1 was expressed beginning at E15.5 in the cortex, but Gstp2 and 3 started expressing at E18.5. Gstp 1 and 2 knockdown (KD) caused decreased neurite number in cortical neurons, implicating them in neurite initiation. Using in utero electroporation (IUE) to knock down Gstp1 and 2 in layer 2/3 pyramidal neurons in vivo, we found abnormal swelling of the apical dendrite at P3 and reduced neurite number at P15. Using time-lapse live imaging, we found that the apical dendrite orientation was skewed compared with the control. We explored the molecular mechanism and found that JNK inhibition rescued reduced neurite number caused by Gstp knockdown, indicating that Gstp regulates neurite formation through JNK signaling. Thus, we found novel functions of Gstp proteins in neurite initiation during cortical development. These findings not only provide novel functions of Gstp proteins in neuritogenesis during cortical development but also help us to understand the complexity of neurite formation.

Orai1 Channels Are Essential for Amplification of Glutamate-Evoked Ca2+ Signals in Dendritic Spines to Regulate Working and Associative Memory

Store-operated Orai1 calcium channels function as highly Ca²⁺-selective ion channels and are broadly expressed in many tissues including the central nervous system, but their contributions to cognitive processing are largely unknown. Here, we report that many measures of synaptic, cellular, and behavioral models of learning are markedly attenuated in mice lacking Orai1 in forebrain excitatory neurons. Results with focal glutamate uncaging in hippocampal neurons support an essential role of Orai1 channels in amplifying NMDA-receptor-induced dendritic Ca²⁺ transients that drive activity-dependent spine morphogenesis and long-term potentiation at Schaffer collateral-CA1 synapses. Consistent with these signaling roles, mice lacking Orai1 in pyramidal neurons (but not interneurons) exhibit striking deficits in working and associative memory tasks. These findings identify Orai1 channels as essential regulators of dendritic spine Ca²⁺ signaling, synaptic plasticity, and cognition.

δ-Catenin engages the autophagy pathway to sculpt the developing dendritic arbor

The development of the dendritic arbor in pyramidal neurons is critical for neural circuit function. Here, we uncovered a pathway in which δ-catenin, a component of the cadherin-catenin cell adhesion complex, promotes coordination of growth among individual dendrites and engages the autophagy mechanism to sculpt the developing dendritic arbor. Using a rat primary neuron model, time-lapse imaging, immunohistochemistry, and confocal microscopy, we found that apical and basolateral dendrites are coordinately sculpted during development. Loss or knockdown of δ-catenin uncoupled this coordination, leading to retraction of the apical dendrite without altering basolateral dendrite dynamics. Autophagy is a key cellular pathway that allows degradation of cellular components. We observed that the impairment of the dendritic arbor resulting from δ-catenin knockdown could be reversed by knockdown of autophagy-related 7 (ATG7), a component of the autophagy machinery. We propose that δ-catenin regulates the dendritic arbor by coordinating the dynamics of individual dendrites and that the autophagy mechanism may be leveraged by δ-catenin and other effectors to sculpt the developing dendritic arbor. Our findings have implications for the management of neurological disorders, such as autism and intellectual disability, that are characterized by dendritic aberrations.

Metoprolol prevents neuronal dendrite remodeling in a canine model of chronic obstructive sleep apnea

Obstructive sleep apnea (OSA) is closely associated with central nervous system diseases and could lead to autonomic nerve dysfunction, which is often seen in neurodegenerative diseases. Previous studies have shown that metoprolol prevents several chronic OSA-induced cardiovascular diseases through inhibiting autonomic nerve hyperactivity. It remains unclear whether chronic OSA can lead to dendritic remodeling in the brain, and whether metoprolol affects the dendritic remodeling. In this study we investigated the effect of metoprolol on dendrite morphology in a canine model of chronic OSA, which was established in beagles through clamping and reopening the endotracheal tube for 4 h every other day for 12 weeks. OSA beagles were administered metoprolol (5 mg· kg⁻¹· d⁻¹). The dendritic number, length, crossings and spine density of neurons in hippocampi and prefrontal cortices were assessed by Golgi staining. And the protein levels of hypoxia-inducible factor-1α (HIF-1α) and brain-derived neurotrophic factor (BDNF) were measured by Western blotting. We showed that chronic OSA successfully induced significant brain hypoxia evidenced by increased HIF-1α levels in CA1 region and dentate gyrus of hippocampi, as well as in prefrontal cortex. Furthermore, OSA led to markedly decreased dendrite number, length and intersections, spine loss as well as reduced BDNF levels. Administration of metoprolol effectively prevented the dendritic remodeling and spine loss induced by chronic OSA. In addition, administration of metoprolol reversed the decreased BDNF, which might be associated with the metoprolol-induced neuronal protection. In conclusion, metoprolol protects against neuronal dendritic remodeling in hippocampi and prefrontal cortices induced by chronic OSA in canine.

Procedures for Culturing and Genetically Manipulating Murine Hippocampal Postnatal Neurons

Neuronal hippocampal cultures are simple and valuable models for studying neuronal function. While embryonic cultures are widely used for different applications, mouse postnatal cultures are still challenging, lack reproducibility and/or exhibit inappropriate neuronal activity. Yet, postnatal cultures have major advantages such as allowing genotyping of pups before culture and reducing the number of experimental animals. Herein we describe a simple and fast protocol for culturing and genetically manipulating hippocampal neurons from P0 to P3 mice. This protocol provides reproducible cultures exhibiting a consistent neuronal development, normal excitatory over inhibitory neuronal ratio and a physiological neuronal activity. We also describe simple and efficient procedures for genetic manipulation of neurons using transfection reagent or lentiviral particles. Overall, this method provides a detailed and validated protocol allowing to explore cellular mechanisms and neuronal activity in postnatal hippocampal neurons in culture.

Spatial control of membrane traffic in neuronal dendrites

Neuronal dendrites are highly branched and specialized compartments with distinct structures and secretory organelles (e.g., spines, Golgi outposts), and a unique cytoskeletal organization that includes microtubules of mixed polarity. Dendritic membranes are enriched with proteins, which specialize in the formation and function of the post-synaptic membrane of the neuronal synapse. How these proteins partition preferentially in dendrites, and how they traffic in a manner that is spatiotemporally accurate and regulated by synaptic activity are long-standing questions of neuronal cell biology. Recent studies have shed new insights into the spatial control of dendritic membrane traffic, revealing new classes of proteins (e.g., septins) and cytoskeleton-based mechanisms with dendrite-specific functions. Here, we review these advances by revisiting the fundamental mechanisms that control membrane traffic at the levels of protein sorting and motor-driven transport on microtubules and actin filaments. Overall, dendrites possess unique mechanisms for the spatial control of membrane traffic, which might have specialized and co-evolved with their highly arborized morphology.

Synapse Dysfunction of Layer V Pyramidal Neurons Precedes Neurodegeneration in a Mouse Model of TDP-43 Proteinopathies

TDP-43 is a major protein component of pathological neuronal inclusions that are present in frontotemporal dementia and amyotrophic lateral sclerosis. We report that TDP-43 plays an important role in dendritic spine formation in the cortex. The density of spines on YFP+ pyramidal neurons in both the motor and somatosensory cortex of Thy1-YFP mice, increased significantly from postnatal day 30 (P30), to peak at P60, before being pruned by P90. By comparison, dendritic spine density was significantly reduced in the motor cortex of Thy1-YFP::TDP-43A315T transgenic mice prior to symptom onset (P60), and in the motor and somatosensory cortex at symptom onset (P90). Morphological spine-type analysis revealed that there was a significant impairment in the development of basal mushroom spines in the motor cortex of Thy1-YFP::TDP-43A315T mice compared to Thy1-YFP control. Furthermore, reductions in spine density corresponded to mislocalisation of TDP-43 immunoreactivity and lowered efficacy of synaptic transmission as determined by electrophysiology at P60. We conclude that mutated TDP-43 has a significant pathological effect at the dendritic spine that is associated with attenuated neural transmission.

Nesprins and opposing microtubule motors generate a point force that drives directional nuclear motion in migrating neurons

Nuclear migration of newly born neurons is essential for cortex formation in the brain. The nucleus is translocated by actin and microtubules, yet the actual force generated by the interplay of these cytoskeletons remains elusive. High-resolution time-lapse observation of migrating murine cerebellar granule cells revealed that the nucleus actively rotates along the direction of its translocation, independently of centrosome motion. Pharmacological and molecular perturbation indicated that spin torque is primarily generated by microtubule motors through the LINC complex in the absence of actomyosin contractility. In contrast to the prevailing view that microtubules are uniformly oriented around the nucleus, we observed that the perinuclear microtubule arrays are of mixed polarity and both cytoplasmic dynein complex and kinesin-1 are required for nuclear rotation. Kinesin-1 can exert a point force on the nuclear envelope via association with nesprins, and loss of kinesin-1 causes failure in neuronal migration in vivo Thus, microtubules steer the nucleus and drive its rotation and translocation via a dynamic, focal interaction of nesprins with kinesin-1 and dynein, and this is necessary for neuronal migration during brain development.

Atmospheric pressure mass spectrometric imaging of live hippocampal tissue slices with subcellular spatial resolution

We report a high spatial resolution mass spectrometry (MS) system that allows us to image live hippocampal tissue slices under open-air atmospheric pressure (AP) and ambient temperature conditions at the subcellular level. The method is based on an efficient desorption process by femtosecond (fs) laser assisted with nanoparticles and a subsequent ionization step by applying nonthermal plasma, termed AP nanoparticle and plasma assisted laser desorption ionization (AP-nanoPALDI) MS method. Combining the AP-nanoPALDI with microscopic sample scanning, MS imaging with spatial resolution of 2.9 µm was obtained. The observed AP-nanoPALDI MS imaging clearly revealed the differences of molecular composition between the apical and basal dendrite regions of a hippocampal tissue. In addition, the AP-nanoPALDI MS imaging showed the decrease of cholesterol in hippocampus by treating with methyl β-cyclodextrin, which exemplifies the potential of AP-nanoPALDI for live tissue imaging for various biomedical applications without any chemical pretreatment and/or labeling process.

Ambient mass spectrometry-based approaches have found application in biology and medicine. Here the authors report a mass spectrometric imaging method (ambient nanoPALDI) for live hippocampal tissues, based on gold nanorodassisted femtosecond laser desorption and subsequent non-thermal plasma induced ionization.

BIG1 is required for the survival of deep layer neurons, neuronal polarity, and the formation of axonal tracts between the thalamus and neocortex in developing brain

BIG1, an activator protein of the small GTPase, Arf, and encoded by the Arfgef1 gene, is one of candidate genes for epileptic encephalopathy. To know the involvement of BIG1 in epileptic encephalopathy, we analyzed BIG1-deficient mice and found that BIG1 regulates neurite outgrowth and brain development in vitro and in vivo. The loss of BIG1 decreased the size of the neocortex and hippocampus. In BIG1-deficient mice, the neuronal progenitor cells (NPCs) and the interneurons were unaffected. However, Tbr1+ and Ctip2+ deep layer (DL) neurons showed spatial-temporal dependent apoptosis. This apoptosis gradually progressed from the piriform cortex (PIR), peaked in the neocortex, and then progressed into the hippocampus from embryonic day 13.5 (E13.5) to E17.5. The upper layer (UL) and DL order in the neocortex was maintained in BIG1-deficient mice, but the excitatory neurons tended to accumulate before their destination layers. Further pulse-chase migration assay showed that the migration defect was non-cell autonomous and secondary to the progression of apoptosis into the BIG1-deficient neocortex after E15.5. In BIG1-deficient mice, we observed an ectopic projection of corticothalamic axons from the primary somatosensory cortex (S1) into the dorsal lateral geniculate nucleus (dLGN). The thalamocortical axons were unable to cross the diencephalon-telencephalon boundary (DTB). In vitro, BIG1-deficient neurons showed a delay in neuronal polarization. BIG1-deficient neurons were also hypersensitive to low dose glutamate (5 μM), and died via apoptosis. This study showed the role of BIG1 in the survival of DL neurons in developing embryonic brain and in the generation of neuronal polarity.

Anisotropically organized three-dimensional culture platform for reconstruction of a hippocampal neural network

In native tissues, cellular and acellular components are anisotropically organized and often aligned in specific directions, providing structural and mechanical properties for actuating biological functions. Thus, engineering alignment not only allows for emulation of native tissue structures but might also enable implementation of specific functionalities. However, achieving desired alignment is challenging, especially in three-dimensional constructs. By exploiting the elastomeric property of polydimethylsiloxane and fibrillogenesis kinetics of collagen, here we introduce a simple yet effective method to assemble and align fibrous structures in a multi-modular three-dimensional conglomerate. Applying this method, we have reconstructed the CA3–CA1 hippocampal neural circuit three-dimensionally in a monolithic gel, in which CA3 neurons extend parallel axons to and synapse with CA1 neurons. Furthermore, we show that alignment of the fibrous scaffold facilitates the establishment of functional connectivity. This method can be applied for reconstructing other neural circuits or tissue units where anisotropic organization in a multi-modular structure is desired.

Alignment or anisotropic organisation within and between cells enables biological function but is challenging to engineer. Here, the authors align collagen fibres in a pre-strained polydimethylsiloxane mould to generate a 3D scaffold that guides hippocampal neuron axon growth to form CA3–CA1 neural circuits.

mTOR kinase is needed for the development and stabilization of dendritic arbors in newly born olfactory bulb neurons

Neurogenesis is the process of neuron generation, which occurs not only during embryonic development but also in restricted niches postnatally. One such region is called the subventricular zone (SVZ), which gives rise to new neurons in the olfactory bulb (OB). Neurons that are born postnatally migrate through more complex territories and integrate into fully functional circuits. Therefore, differences in the differentiation of embryonic and postnatally born neurons may exist. Dendritogenesis is an important process for the proper formation of future neuronal circuits. Dendritogenesis in embryonic neurons cultured in vitro was shown to depend on the mammalian target of rapamycin (mTOR). Still unknown, however, is whether mTOR could regulate the dendritic arbor morphology of SVZ‐derived postnatal OB neurons under physiological conditions in vivo. The present study used in vitro cultured and differentiated SVZ‐derived neural progenitors and found that both mTOR complex 1 and mTOR complex 2 were required for the dendritogenesis of SVZ‐derived neurons. Furthermore, using a combination of in vivo electroporation of neural stem cells in the SVZ and genetic and pharmacological inhibition of mTOR, it was found that mTOR was crucial for the growth of basal and apical dendrites in postnatally born OB neurons under physiological conditions and contributed to the stabilization of their basal dendrites. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1308–1327, 2016

Microtubule Dynamics in Neuronal Development, Plasticity, and Neurodegeneration

Neurons are the basic information-processing units of the nervous system. In fulfilling their task, they establish a structural polarity with an axon that can be over a meter long and dendrites with a complex arbor, which can harbor ten-thousands of spines. Microtubules and their associated proteins play important roles during the development of neuronal morphology, the plasticity of neurons, and neurodegenerative processes. They are dynamic structures, which can quickly adapt to changes in the environment and establish a structural scaffold with high local variations in composition and stability. This review presents a comprehensive overview about the role of microtubules and their dynamic behavior during the formation and maturation of processes and spines in the healthy brain, during aging and under neurodegenerative conditions. The review ends with a discussion of microtubule-targeted therapies as a perspective for the supportive treatment of neurodegenerative disorders.

Reelin Prevents Apical Neurite Retraction during Terminal Translocation and Dendrite Initiation

The mechanisms controlling cortical dendrite initiation and targeting are poorly understood. Multiphoton imaging of developing mouse cortex reveals that apical dendrites emerge by direct transformation of the neuron's leading process during the terminal phase of neuronal migration. During this ∼110 min period, the dendritic arbor increases ∼2.5-fold in size and migration arrest occurs below the first stable branch point in the developing arbor. This dendritic outgrowth is triggered at the time of leading process contact with the marginal zone (MZ) and occurs primarily by neurite extension into the extracellular matrix of the MZ. In reeler cortices that lack the secreted glycoprotein Reelin, a subset of neurons completed migration but then retracted and reorganized their arbor in a tangential direction away from the MZ soon after migration arrest. For these reeler neurons, the tangential oriented primary neurites were longer lived than the radially oriented primary neurites, whereas the opposite was true of wild-type (WT) neurons. Application of Reelin protein to reeler cortices destabilized tangential neurites while stabilizing radial neurites and stimulating dendritic growth in the MZ. Therefore, Reelin functions as part of a polarity signaling system that links dendritogenesis in the MZ with cellular positioning and cortical lamination.

SIGNIFICANCE STATEMENT

Whether the apical dendrite emerges by transformation of the leading process of the migrating neuron or emerges de novo after migration is completed is unclear. Similarly, it is not clear whether the secreted glycoprotein Reelin controls migration and dendritic growth as related or separate processes. Here, multiphoton microscopy reveals the direct transformation of the leading process into the apical dendrite. This transformation is coupled to the successful completion of migration and neuronal soma arrest occurs below the first stable branch point of the nascent dendrite. Deficiency in Reelin causes the forming dendrite to avoid its normal target area and branch aberrantly, leading to improper cellular positioning. Therefore, this study links Reelin-dependent dendritogenesis with migration arrest and cortical lamination.

Differential regulation of apical-basolateral dendrite outgrowth by activity in hippocampal neurons

Hippocampal pyramidal neurons have characteristic dendrite asymmetry, characterized by structurally and functionally distinct apical and basolateral dendrites. The ability of the neuron to generate and maintain dendrite asymmetry is vital, since synaptic inputs received are critically dependent on dendrite architecture. Little is known about the role of neuronal activity in guiding maintenance of dendrite asymmetry. Our data indicate that dendrite asymmetry is established and maintained early during development. Further, our results indicate that cell intrinsic and global alterations of neuronal activity have differential effects on net extension of apical and basolateral dendrites. Thus, apical and basolateral dendrite extension may be independently regulated by cell intrinsic and network neuronal activity during development, suggesting that individual dendrites may have autonomous control over net extension. We propose that regulated individual dendrite extension in response to cell intrinsic and neuronal network activity may allow temporal control of synapse specificity in the developing hippocampus.