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Enhanced survival of interneurons in the juvenile hippocampus a In situ hybridization on coronal sections of adult brains using a cRNA probe for GAD67 (GAD1) (top panels). Quantification of GAD67⁺ cells in the different hippocampal regions of control and mutant animals (bottom panel) showing no differences in the number of interneurons in distinct hippocampal regions (p = 0.6947 for CA1/CA2; p = 0.8548 for DG; p = 0.9578 for CA3, two-way ANOVA, Sidak’s multiple comparison test, n = 3 controls, n = 4 mutants). Dotted lines in a–d represent the subdivisions in different regions: CA1/CA2, CA3 and DG. b In situ hybridization on coronal sections of juvenile brains using a cRNA probe for GAD67 (top panels). Quantification of GAD67⁺ cells in the different hippocampal regions of control and mutant animals (bottom panel) showing increased number of interneurons in CA1/CA2 (p = 0.0118), but not CA3, DG or total number of interneurons in mutant animals (p > 0.9999; 0.8129 and 0.3429, respectively; two-way ANOVA, Sidak’s multiple comparison test; Mann–Whitney for total, n = 4 controls and n = 4 mutants). c In situ hybridization on coronal sections of juvenile brains using a cRNA probe for somatostatin (SST) (top panels). Quantification of SST⁺ cells of control and mutant animals (bottom panel) showing no differences in the number of interneurons in mutant animals (n = 3 controls and n = 3 mutants) in CA1/CA2, CA3 and DG or in their total numbers (p = 0.9655 for CA1/CA2; p = 0.9998 for DG; p > 0.9999 for CA3, two-way ANOVA, Sidak’s multiple comparison test; p > 0.9999 for total, Mann–Whitney). d Immunofluorescence for NPY on coronal sections of juvenile brains (top panels). Quantification of NPY⁺ cells of control and mutant animals showing increased number of interneurons in CA1/CA2 (p = 0.003), but not CA3 or DG (p = 0.9621 and p = 0.5889 respectively, two-way ANOVA, Sidak’s multiple comparison test), as well as an increase in the total number of NPY⁺ interneurons in mutant animals (p = 0.0017; Mann–Whitney; n = 4 controls and n = 4 mutants). High magnification of a CA1 portion (white square) (right panels) highlighting NPY⁺ neurons (yellow arrows). Scale bar: 100 μm. Scale bars in a–d low magnifications: 500 µm. Asterisks indicate statistical significance (*p < 0.05; **p < 0.01). All data are presented as mean ± SEM.
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Cajal-Retzius cells (CRs) are transient neurons, disappearing almost completely in the postnatal neocortex by programmed cell death (PCD), with a percentage surviving up to adulthood in the hippocampus. Here, we evaluate CR’s role in the establishment of adult neuronal and cognitive function using a mouse model preventing Bax-dependent PCD. CRs abn...
Citations
... This observation that immature dendrites of excitatory neurons project into a plexus of axons in a highly regulated fashion raises the question of the purpose of this projection. CRNs are spontaneously active [12][13][14]; however, Reelin secretion does not appear to be activity-dependent [15][16][17], and current models suggest Reelin or its proteolytic fragments might diffuse from the MZ [18], implying that close dendrite axon proximity is not required for Reelin signaling itself. While it is appealing to think that this projection anticipates future synapse and circuit formation, it is important to recognize that cortical CRNs are a transient cell class that dies off postnatally [19] and most L6 neurons do not maintain an apical dendrite in the MZ. ...
The apical dendrite of a cortical projection neuron (CPN) is generated from the leading process of the migrating neuron as the neuron completes migration. This transformation occurs in the cortical marginal zone (MZ), a layer that contains the Cajal-Retzius neurons and their axonal projections. Cajal-Retzius neurons (CRNs) are well known for their critical role in secreting Reelin, a glycoprotein that controls dendritogenesis and cell positioning in many regions of the developing brain. In this study, we examine the possibility that CRNs in the MZ may provide additional signals to arriving CPNs, that may promote the maturation of CPNs and thus shape the development of the cortex. We use whole embryonic hemisphere explants and multiphoton microscopy to confirm that CRNs display intracellular calcium transients of <1-min duration and high amplitude during early corticogenesis. In contrast, developing CPNs do not show high-amplitude calcium transients, but instead show a steady increase in intracellular calcium that begins at the time of dendritic initiation, when the leading process of the migrating CPN is encountering the MZ. The possible existence of CRN to CPN communication was revealed by the application of veratridine, a sodium channel activator, which has been shown to preferentially stimulate more mature cells in the MZ at an early developmental time. Surprisingly, veratridine application also triggers large calcium transients in CPNs, which can be partially blocked by a cocktail of antagonists that block glutamate and glycine receptor activation. These findings outline a model in which CRN spontaneous activity triggers the release of glutamate and glycine, neurotransmitters that can trigger intracellular calcium elevations in CPNs. These elevations begin as CPNs initiate dendritogenesis and continue as waves in the post-migratory cells. Moreover, we show that the pharmacological blockade of glutamatergic signaling disrupts migration, while forced expression of a bacterial voltage-gated calcium channel (CavMr) in the migrating neurons promotes dendritic growth and migration arrest. The identification of CRN to CPN signaling during early development provides insight into the observation that many autism-linked genes encode synaptic proteins that, paradoxically, are expressed in the developing cortex well before the appearance of synapses and the establishment of functional circuits.
Epilepsy, a prevalent neurological disorder, necessitates precise and reliable electrophysiological recording for accurate study and diagnosis. Although traditional stereo‐electroencephalography and micro‐wire probes are widely used in epilepsy research, they are limited by recording site numbers and mechanical properties. This study explores the use of high‐density, ultra‐flexible neural probes in epilepsy monitoring, highlighting their advantages in terms of recording performance, scalability, and tissue compatibility. This work validates the effectiveness of the probes in detecting characteristic epileptic signals across various stages, including resting, preictal, and ictal stages, in two different mouse models of epilepsy. Additionally, the high spatial resolution of the probes allows to capture fine spatial propagation of epilepsy‐associated high‐frequency oscillations. Furthermore, the flexible probes exhibit superior biocompatibility, reducing inflammation and neuronal apoptosis compared to rigid electrodes. The results underscore the promising potential of ultra‐flexible neural probes for advancing epilepsy research, providing a powerful technological platform for future studies in the field.
Cajal-Retzius (CR) cells are a transient neuron type that populate the postnatal hippocampus. To understand how CR cells’ persistence influences the maturation of hippocampal circuits, we combined a specific transgenic mouse line with viral vector injection to selectively ablate CR cells from the postnatal hippocampus. We observed layer-specific changes in the dendritic complexity and spine density of CA1 pyramidal cells. In addition, transcriptomic analysis highlighted significant changes in the expression of synaptic related genes across development. Finally, we were able to identify significant changes in the expression levels of Latrophilin-2, a postsynaptic guidance molecule known for its role in the entorhinal-hippocampal connectivity. Those findings were supported by changes in the synaptic proteomic content in CA1 stratum lacunosum-moleculare. Our results reveal a critical role of CR cells in the establishment of the hippocampal network.
Cajal–Retzius (CR) cells are transient neurons with long-lasting effects on the architecture and circuitry of the neocortex and hippocampus. Contrary to the prevailing assumption that CR cells completely disappear in rodents shortly after birth, a substantial portion of these cells persist in the hippocampus throughout adulthood. The role of these surviving CR cells in the adult hippocampus is largely unknown, partly because of the paucity of suitable tools to dissect their functions in the adult versus the embryonic brain. Here, we show that genetic crosses of the Δ Np73-Cre mouse line, widely used to target CR cells, to reporter mice induce reporter expression not only in CR cells, but also progressively in postnatal dentate gyrus granule neurons. Such a lack of specificity may confound studies of CR cell function in the adult hippocampus. To overcome this, we devise a method that not only leverages the temporary CR cell-targeting specificity of the Δ Np73-Cre mice before the first postnatal week, but also capitalizes on the simplicity and effectiveness of freehand neonatal intracerebroventricular injection of adeno-associated virus. We achieve robust Cre-mediated recombination that remains largely restricted to hippocampal CR cells from early postnatal age to adulthood. We further demonstrate the utility of this method to manipulate neuronal activity of CR cells in the adult hippocampus. This versatile and scalable strategy will facilitate experiments of CR cell-specific gene knockdown and/or overexpression, lineage tracing, and neural activity modulation in the postnatal and adult brain.
Cajal-Retzius cells (CRs) are key players in cerebral cortex development, and they display a unique transcriptomic identity. Here, we use scRNA-seq to reconstruct the differentiation trajectory of mouse hem-derived CRs, and we unravel the transient expression of a complete gene module previously known to control multiciliogenesis. However, CRs do not undergo centriole amplification or multiciliation. Upon deletion of Gmnc, the master regulator of multiciliogenesis, CRs are initially produced but fail to reach their normal identity resulting in their massive apoptosis. We further dissect the contribution of multiciliation effector genes and identify Trp73 as a key determinant. Finally, we use in utero electroporation to demonstrate that the intrinsic competence of hem progenitors as well as the heterochronic expression of Gmnc prevent centriole amplification in the CR lineage. Our work exemplifies how the co-option of a complete gene module, repurposed to control a distinct process, may contribute to the emergence of novel cell identities.
Leucine restriction (LR) improves insulin resistance and promotes white adipose tissue browning. However, the effect of LR on obesity-associated cognitive impairment remains unclear. The present study found that an 8-week LR dramatically improved high-fat diet (HFD)-induced cognitive decline by preventing synaptic dysfunction, increasing the expressions of neurotrophic factors, and inhibiting neuroinflammation in memory-related brain regions. Moreover, LR notably reshaped the structure of gut microbiota, which was manifested by downregulating the Firmicutes/Bacteroidetes ratio, reducing the relative abundance of inflammation-related bacteria including Acetatifactor, Helicobacter, Mucispirillum, and Oscillibacter but increasing short-chain fatty acid (SCFA)-producing bacterial genera including Alistipes, Allobaculum, Odoribacter, and Olsenella. Notably, HFD-caused SCFA reduction, gut barrier damage, and LPS leakage were recovered by LR. Our findings suggested that LR could serve as an effective approach to attenuate obesity-induced cognitive deficits, which may be achieved by balancing gut microbiota homeostasis and enhancing SCFA production.
Reelin is a large extracellular matrix protein abundantly expressed in the developing neocortex of mammals. During embryonic and early postnatal stages in mice, Reelin is secreted by a transient neuronal population, the Cajal-Retzius neurons (CRs), and is mostly known to insure the inside-out migration of neurons and the formation of cortical layers. During the first 2 postnatal weeks, CRs disappear from the neocortex and a subpopulation of GABAergic neurons takes over the expression of Reelin, albeit in lesser amounts. Although Reelin expression requires a tight regulation in a time- and cell-type specific manner, the mechanisms regulating the expression and secretion of this protein are poorly understood. In this study, we establish a cell-type specific profile of Reelin expression in the marginal zone of mice neocortex during the first 3 postnatal weeks. We then investigate whether electrical activity plays a role in the regulation of Reelin synthesis and/or secretion by cortical neurons during the early postnatal period. We show that increased electrical activity promotes the transcription of reelin via the brain-derived neurotrophic factor/TrkB pathway, but does not affect its translation or secretion. We further demonstrate that silencing the neuronal network promotes the translation of Reelin without affecting the transcription or secretion. We conclude that different patterns of activity control various stages of Reelin synthesis, whereas its secretion seems to be constitutive.
Cajal–Retzius cells (CRs) are a class of transient neurons in the mammalian cortex that play a critical role in cortical development. Neocortical CRs undergo almost complete elimination in the first two postnatal weeks in rodents and the persistence of CRs during postnatal life has been detected in pathological conditions related to epilepsy. However, it is unclear whether their persistence is a cause or consequence of these diseases. To decipher the molecular mechanisms involved in CR death, we investigated the contribution of the PI3K/AKT/mTOR pathway as it plays a critical role in cell survival. We first showed that this pathway is less active in CRs after birth before massive cell death. We also explored the spatio-temporal activation of both AKT and mTOR pathways and reveal area-specific differences along both the rostro–caudal and medio–lateral axes. Next, using genetic approaches to maintain an active pathway in CRs, we found that the removal of either PTEN or TSC1, two negative regulators of the pathway, lead to differential CR survivals, with a stronger effect in the Pten model. Persistent cells in this latter mutant are still active. They express more Reelin and their persistence is associated with an increase in the duration of kainate-induced seizures in females. Altogether, we show that the decrease in PI3K/AKT/mTOR activity in CRs primes these cells to death by possibly repressing a survival pathway, with the mTORC1 branch contributing less to the phenotype.
