Cajal-Retzius neurons are required for the development of the human hippocampal fissure

Differential contribution of P73+ Cajal-Retzius cells and Reelin to cortical morphogenesis

Cajal-Retzius cells (CRs) are peculiar neurons in the developing mammalian cerebral cortex. They robustly secrete Reln, a glycoprotein essential for the establishment of cortical layers through the control of radial migration. We previously identified Gmnc as a crucial fate determinant for P73+ CR subtypes. In Gmnc-/- mutants, P73+ CRs are initially produced and cover the telencephalic vesicle but undergo massive apoptosis resulting in their complete depletion at mid-corticogenesis. Here, we investigated the consequences of such a CR depletion on dorsal cortex lamination and hippocampal morphogenesis. We found that preplate splitting normally occurs in Gmnc-/- mutants but is followed by defective radial migration arrest in the dorsal cortex, an altered cellular organization in the lateral cortex, aberrant hippocampal CA1 folding and lack of vasculature development in the hippocampal fissure. We then performed conditional Reln deletion in P73+ CRs to evaluate its relative contribution and found that only radial migration defects were recapitulated. We concluded that at mid-corticogenesis, CR-derived Reln is required for radial migration arrest and additionally identified Reln-independent functions for CRs in the control of hippocampal fissure formation and CA1 folding.

Lmx1a is a master regulator of the cortical hem

Development of the nervous system depends on signaling centers - specialized cellular populations that produce secreted molecules to regulate neurogenesis in the neighboring neuroepithelium. In some cases, signaling center cells also differentiate to produce key types of neurons. The formation of a signaling center involves its induction, the maintenance of expression of its secreted molecules, and cell differentiation and migration events. How these distinct processes are coordinated during signaling center development remains unknown. By performing studies in mice, we show that Lmx1a acts as a master regulator to orchestrate the formation and function of the cortical hem (CH), a critical signaling center that controls hippocampus development. Lmx1a co-regulates CH induction, its Wnt signaling, and the differentiation and migration of CH-derived Cajal-Retzius neurons. Combining RNAseq, genetic, and rescue experiments, we identified major downstream genes that mediate distinct Lmx1a-dependent processes. Our work revealed that signaling centers in the mammalian brain employ master regulatory genes and established a framework for analyzing signaling center development.

Early Regional Patterning in the Human Prefrontal Cortex Revealed by Laminar Dynamics of Deep Projection Neuron Markers

Early regional patterning and laminar position of cortical projection neurons is determined by activation and deactivation of transcriptional factors (TFs) and RNA binding proteins (RBPs) that regulate spatiotemporal framework of neurogenetic processes (proliferation, migration, aggregation, postmigratory differentiation, molecular identity acquisition, axonal growth, dendritic development, and synaptogenesis) within transient cellular compartments. Deep-layer projection neurons (DPN), subplate (SPN), and Cajal-Retzius neurons (CRN) are early-born cells involved in the establishment of basic laminar and regional cortical architecture; nonetheless, laminar dynamics of their molecular transcriptional markers remain underexplored. Here we aimed to analyze laminar dynamics of DPN markers, i.e., transcription factors TBR1, CTIP2, TLE4, SOX5, and RBP CELF1 on histological serial sections of the human frontal cortex between 7.5-15 postconceptional weeks (PCW) in reference to transient proliferative, migratory, and postmigratory compartments. The subtle signs of regional patterning were seen during the late preplate phase in the pattern of sublaminar organization of TBR1+/Reelin+ CRN and TBR1+ pioneering SPN. During the cortical plate (CP)-formation phase, TBR1+ neurons became radially aligned, forming continuity from a well-developed subventricular zone to CP showing clear lateral to medial regional gradients. The most prominent regional patterning was seen during the subplate formation phase (around 13 PCW) when a unique feature of the orbitobasal frontal cortex displays a "double plate" pattern. In other portions of the frontal cortex (lateral, dorsal, medial) deep portion of CP becomes loose and composed of TBR1+, CTIP2+, TLE4+, and CELF1+ neurons of layer six and later-born SPN, which later become constituents of the expanded SP (around 15 PCW). Overall, TFs and RBPs mark characteristic regional laminar dynamics of DPN, SPN, and CRN subpopulations during remarkably early fetal phases of the highly ordered association cortex development.

The cortical hem lacks stem cell potential despite expressing SOX9 and HOPX

The adult dentate gyrus (DG) of rodents hosts a neural stem cell (NSC) niche capable of generating new neurons throughout life. The embryonic origin and molecular mechanisms underlying formation of DG NSCs are still being investigated. We performed a bulk transcriptomic analysis on mouse developing archicortex conditionally deleted for Sox9, a SoxE transcription factor controlling both gliogenesis and NSC formation, and identified Hopx, a recently identified marker of both prospective adult DG NSCs and astrocytic progenitors, as being downregulated. We confirm SOX9 is required for HOPX expression in the embryonic archicortex. In particular, we found that both NSC markers are highly expressed in the cortical hem (CH), while only weakly in the adjacent dentate neuroepithelium (DNE), suggesting a potential CH embryonic origin for DG NSCs. However, we demonstrate both in vitro and in vivo that the embryonic CH, as well as its adult derivatives, lacks stem cell potential. Instead, deletion of Sox9 in the DNE affects both HOPX expression and NSC formation in the adult DG. We conclude that HOPX expression in the CH is involved in astrocytic differentiation downstream of SOX9, which we previously showed regulates DG development by inducing formation of a CH-derived astrocytic scaffold. Altogether, these results suggest that both proteins work in a dose-dependent manner to drive either astrocytic differentiation in CH or NSC formation in DNE.

The multiple facets of Cajal-Retzius neurons

Cajal-Retzius neurons (CRs) are among the first-born neurons in the developing cortex of reptiles, birds and mammals, including humans. The peculiarity of CRs lies in the fact they are initially embedded into the immature neuronal network before being almost completely eliminated by cell death at the end of cortical development. CRs are best known for controlling the migration of glutamatergic neurons and the formation of cortical layers through the secretion of the glycoprotein reelin. However, they have been shown to play numerous additional key roles at many steps of cortical development, spanning from patterning and sizing functional areas to synaptogenesis. The use of genetic lineage tracing has allowed the discovery of their multiple ontogenetic origins, migratory routes, expression of molecular markers and death dynamics. Nowadays, single-cell technologies enable us to appreciate the molecular heterogeneity of CRs with an unprecedented resolution. In this Review, we discuss the morphological, electrophysiological, molecular and genetic criteria allowing the identification of CRs. We further expose the various sources, migration trajectories, developmental functions and death dynamics of CRs. Finally, we demonstrate how the analysis of public transcriptomic datasets allows extraction of the molecular signature of CRs throughout their transient life and consider their heterogeneity within and across species.

An early Sox2-dependent gene expression programme required for hippocampal dentate gyrus development

The hippocampus is a brain area central for cognition. Mutations in the human SOX2 transcription factor cause neurodevelopmental defects, leading to intellectual disability and seizures, together with hippocampal dysplasia. We generated an allelic series of Sox2 conditional mutations in mouse, deleting Sox2 at different developmental stages. Late Sox2 deletion (from E11.5, via Nestin-Cre) affects only postnatal hippocampal development; earlier deletion (from E10.5, Emx1-Cre) significantly reduces the dentate gyrus (DG), and the earliest deletion (from E9.5, FoxG1-Cre) causes drastic abnormalities, with almost complete absence of the DG. We identify a set of functionally interconnected genes (Gli3, Wnt3a, Cxcr4, p73 and Tbr2), known to play essential roles in hippocampal embryogenesis, which are downregulated in early Sox2 mutants, and (Gli3 and Cxcr4) directly controlled by SOX2; their downregulation provides plausible molecular mechanisms contributing to the defect. Electrophysiological studies of the Emx1-Cre mouse model reveal altered excitatory transmission in CA1 and CA3 regions.

Expression of reelin with age in the human hippocampal formation

Reelin plays a key role in neuronal migration and lamination in the cortex and hippocampus. Animal studies have shown that reelin expression decreases with age. The aim of this study was to evaluate the expression of reelin in all layers of the human hippocampal formation across three age groups. We used immunohistochemistry in formalin fixed and paraffin embedded hippocampal tissue from infants (1-10 months; n = 9), children (4-10 years; n = 4), and adults (45-60 years; n = 6) to stain for reelin. Expression was quantified (measured as the number of positive reelin cells/mm² ) in the granule cell layer of the dentate gyrus (DG), the molecular layer of the dentate gyrus (ML), the hippocampal fissure (HF), stratum lacunosum moleculare (SLM), CA4/Hilus and the stratum pyramidale layer of CA3, CA2, and CA1. Expression of reelin was highest in the HF irrespective of age, followed by the SLM and ML. Minimal to no expression was seen in the stratum pyramidale layer of CA1-3. With age, reelin expression decreased and was statistically significant from infancy to childhood in the HF (p = .02). This study confirms that reelin expression decreases with age in the human hippocampus, and shows for the first time that the major decrease occurs between infancy and early childhood.

Cell cycle during neuronal migration and neocortical lamination

OBJECTIVES

In order to understand the relationships between neocortical lamination and cell cycle, various cells, such as neural stem cell, migrating postmitotic neuron, Cajal-Retzius (CR) cell, and mature pyramidal cell in various cell phases were investigated in mouse cortices.

METHODS

With mouse neocortex and hippocampus, the immunofluorescent labeling, BrdU assay, and DiI tracing technique were implemented in the study.

RESULTS

(1) During mouse development, the neocortex expressed different proteins, such as FOXP2, CDP, and Nestin, which could be used as the markers for cortical lamination. (2) The neural stem cells were mainly located in the subventricular zone, with the expressions of Nestin, Cyclin A2, Cyclin E1, and CDT1, suggesting that they were in the repeated cell cycle. Furthermore, the migrating neurons in the neocortex were Cyclin D1- (G1 phase-specific marker) positive, suggesting that they were in the G1 phase. However, Pyramidal cells that developed from postmitotic migrating neurons and settled in the cortical plate were Cyclin D1- negative, suggesting that they were in the G0 phase. (3) Reelin positive CR cells appeared in the molecular layer of the neocortex in early embryonic day (E10), which could express Cyclin A2, Cyclin E1, and CDT1 as pyramidal cells, but not Cyclin D1, suggesting that they may have exited the cell cycle and entered the G0 phase.

CONCLUSION

The neural migration, neural proliferation, and cell cycle alterations play an important role during cortical lamination. During the cortical development and lamination, the neural stem cells and migrating postmitotic neurons are in different cell cycle phases, but pyramidal cells and CR cells have exited the cell cycle.

From cancer to rejuvenation: incomplete regeneration as the missing link (part II: rejuvenation circle)

In the first part of our study, we substantiated that the embryonic reontogenesis and malignant growth (disintegrating growth) pathways are the same, but occur at different stages of ontogenesis, this mechanism is carried out in opposite directions. Cancer has been shown to be epigenetic-blocked redifferentiation and unfinished somatic embryogenesis. We formulated that only this approach of aging elimination has real prospects for a future that is fraught with cancer, as we will be able to convert this risk into a rejuvenation process through the continuous cycling of cell dedifferentiation-differentiation processes (permanent remorphogenesis). Here, we continue to develop the idea of looped ontogenesis and formulate the concept of the rejuvenation circle.