Exogenous Reelin modifies the migratory behavior of neurons depending on cortical location

Comparative Lipidomic Analyses Reveal Different Protections in Preterm and Term Breast Milk for Infants

Aim: Neonates are notably vulnerable, however they have improved outcomes if they are fed human milk. Human milk lipids constitute the primary constituents of human milk and serve a pivotal role in safeguarding infants from diseases. We assessed the lipid differences between preterm and term human milk and predicted the prospective impacts of these lipids on the development of neonates. Methods and results: We collected colostrum from healthy breast-feeding mothers who had delivered either term or preterm infants. We analyzed the lipid profiles of preterm, as well as term human milk using an LC-MS/MS metabolomics strategy. The orthogonal partial least-squares discriminant analysis score plots revealed remarkable distinction of lipids in preterm and term human milk. In total, 16 subclasses of 235 differential lipids (variable importance in projection > 1, P < 0.05) were identified. Notably, phosphatidylethanolamine and phosphatidylcholine were robustly increased in preterm human milk, while diacylglycerol and ceramide were markedly decreased in preterm human milk. Pathway analysis revealed that these dysregulated lipids are closely associated with glycerophospholipid metabolism, sphingolipid metabolism, Reelin signaling in neurons, and LXR/RXR activation. Conclusion: The results show that the lipids in preterm and term human colostrum vary, which may be critical for neonatal development.

Organotypic Cocultures of Human Pluripotent Stem Cell Derived-Neurons with Mammalian Inner Ear Hair Cells and Cochlear Nucleus Slices

Stem cells have been touted as a source of potential replacement neurons for inner ear degeneration for almost two decades now; yet to date, there are few studies describing the use of human pluripotent stem cells (hPSCs) for this purpose. If stem cell therapies are to be used clinically, it is critical to validate the usefulness of hPSC lines in vitro and in vivo. Here, we present the first quantitative evidence that differentiated hPSC-derived neurons that innervate both the inner ear hair cells and cochlear nucleus neurons in coculture, with significantly more new synaptic contacts formed on target cell types. Nascent contacts between stem cells and hair cells were immunopositive for both synapsin I and VGLUT1, closely resembling expression of these puncta in endogenous postnatal auditory neurons and control cocultures. When hPSCs were cocultured with cochlear nucleus brainstem slice, significantly greater numbers of VGLUT1 puncta were observed in comparison to slice alone. New VGLUT1 puncta in cocultures with cochlear nucleus slice were not significantly different in size, only in quantity. This experimentation describes new coculture models for assessing auditory regeneration using well-characterised hPSC-derived neurons and highlights useful methods to quantify the extent of innervation on different cell types in the inner ear and brainstem.

Reelin: Diverse roles in central nervous system development, health and disease

Over the past 20 years the structure and function of Reelin, an extracellular glycoprotein with a role in cell migration and positioning during development has been elucidated. Originally discovered in mice exhibiting a peculiar gait and hypoplastic cerebellar tissue, Reelin is secreted from Cajal-Retzius neurons during embryonic life and has been shown to act as a stop signal, guiding migrating radial neurons in a gradient-dependent manner. Reelin carries out its function by binding to the receptors, very low-density lipoprotein receptor (VLDLR) and apolipoprotein E receptor 2 (ApoER2) resulting in the phosphorylation of the intracellular protein Disabled-1 (Dab-1) which is essential for effective Reelin signaling. Abnormalities in the RELN gene can result in multiple unusual structural outcomes including disruption of cortical layers, heterotopia, polymicrogyria and lissencephaly. Recent research has suggested a potential role for Reelin in the pathogenesis of neurological diseases such as schizophrenia, autism and Alzheimer's disease. This short review will address the current understanding of the structure and function of this protein and its emerging role in the development of neurological disorders.

Correct setup of the substantia nigra requires Reelin-mediated fast, laterally-directed migration of dopaminergic neurons

Midbrain dopaminergic (mDA) neurons migrate to form the laterally-located substantia nigra pars compacta (SN) and medially-located ventral tegmental area (VTA), but little is known about the underlying cellular and molecular processes. Here we visualize the dynamic cell morphologies of tangentially migrating SN-mDA neurons in 3D and identify two distinct migration modes. Slow migration is the default mode in SN-mDA neurons, while fast, laterally-directed migration occurs infrequently and is strongly associated with bipolar cell morphology. Tangential migration of SN-mDA neurons is altered in absence of Reelin signaling, but it is unclear whether Reelin acts directly on migrating SN-mDA neurons and how it affects their cell morphology and migratory behavior. By specifically inactivating Reelin signaling in mDA neurons we demonstrate its direct role in SN-mDA tangential migration. Reelin promotes laterally-biased movements in mDA neurons during their slow migration mode, stabilizes leading process morphology and increases the probability of fast, laterally-directed migration.

ApoER2 Controls Not Only Neuronal Migration in the Intermediate Zone But Also Termination of Migration in the Developing Cerebral Cortex

Neuronal migration contributes to the establishment of mammalian brain. The extracellular protein Reelin sends signals to various downstream molecules by binding to its receptors, the apolipoprotein E receptor 2 (ApoER2) and very low-density lipoprotein receptor and exerts essential roles in the neuronal migration and formation of the layered neocortex. However, the cellular and molecular functions of Reelin signaling in the cortical development are not yet fully understood. Here, to gain insight into the role of Reelin signaling during cortical development, we examined the migratory behavior of Apoer2-deficient neurons in the developing brain. Stage-specific labeling of newborn neurons revealed that the neurons ectopically invaded the marginal zone (MZ) and that neuronal migration of both early- and late-born neurons was disrupted in the intermediate zone (IZ) in the Apoer2 KO mice. Rescue experiments showed that ApoER2 functions both in cell-autonomous and noncell-autonomous manners, that Rap1, integrin, and Akt are involved in the termination of migration beneath the MZ, and that Akt also controls neuronal migration in the IZ downstream of ApoER2. These data indicate that ApoER2 controls multiple processes in neuronal migration, including the early stage of radial migration and termination of migration beneath the MZ in the developing neocortex.

Cell Polarity in Cerebral Cortex Development-Cellular Architecture Shaped by Biochemical Networks

The human cerebral cortex is the seat of our cognitive abilities and composed of an extraordinary number of neurons, organized in six distinct layers. The establishment of specific morphological and physiological features in individual neurons needs to be regulated with high precision. Impairments in the sequential developmental programs instructing corticogenesis lead to alterations in the cortical cytoarchitecture which is thought to represent the major underlying cause for several neurological disorders including neurodevelopmental and psychiatric diseases. In this review article we discuss the role of cell polarity at sequential stages during cortex development. We first provide an overview of morphological cell polarity features in cortical neural stem cells and newly-born postmitotic neurons. We then synthesize a conceptual molecular and biochemical framework how cell polarity is established at the cellular level through a break in symmetry in nascent cortical projection neurons. Lastly we provide a perspective how the molecular mechanisms applying to single cells could be probed and integrated in an in vivo and tissue-wide context.

Control of Neuronal Migration and Aggregation by Reelin Signaling in the Developing Cerebral Cortex

The mammalian cerebral neocortex has a well-organized laminar structure, achieved by the highly coordinated control of neuronal migration. During cortical development, excitatory neurons born near the lateral ventricle migrate radially to reach their final positions to form the cortical plate. During this process, dynamic changes are observed in the morphologies and migration modes, including multipolar migration, locomotion, and terminal translocation, of the newborn neurons. Disruption of these migration processes can result in neuronal disorders such as lissencephaly and periventricular heterotopia. The extracellular protein, Reelin, mainly secreted by the Cajal-Retzius neurons in the marginal zone during development, plays a crucial role in the neuronal migration and neocortical lamination. Reelin signaling, which exerts essential roles in the formation of the layered neocortex, is triggered by the binding of Reelin to its receptors, ApoER2 and VLDLR, followed by phosphorylation of the Dab1 adaptor protein. Accumulating evidence suggests that Reelin signaling controls multiple steps of neuronal migration, including the transition from multipolar to bipolar neurons, terminal translocation, and termination of migration beneath the marginal zone. In addition, it has been shown that ectopically expressed Reelin can cause neuronal aggregation via an N-cadherin-mediated manner. This review attempts to summarize our knowledge of the roles played by Reelin in neuronal migration and the underlying mechanisms.

Cell migration in schizophrenia: Patient-derived cells do not regulate motility in response to extracellular matrix

Schizophrenia is a highly heritable psychiatric disorder linked to a large number of risk genes. The function of these genes in disease etiology is not fully understood but pathway analyses of genomic data suggest developmental dysregulation of cellular processes such as neuronal migration and axon guidance. Previous studies of patient-derived olfactory cells show them to be more motile than control-derived cells when grown on a fibronectin substrate, motility that is dependent on focal adhesion kinase signaling. The aim of this study was to investigate whether schizophrenia patient-derived cells are responsive to other extracellular matrix (ECM) proteins that bind integrin receptors. Olfactory neurosphere-derived cells from nine patients and nine matched controls were grown on ECM protein substrates at increasing concentrations and their movement was tracked for 24h using automated high-throughput imaging. Control-derived cells increased their motility as the ECM substrate concentration increased, whereas patient-derived cell motility was little affected by ECM proteins. Patient and control cells had appropriate integrin receptors for these ECM substrates and detected them as shown by increases in focal adhesion number and size in response to ECM proteins, which also induced changes in cell morphology and cytoskeleton. These observations indicate that patient cells failed to translate the detection of ECM proteins into appropriate changes in cell motility. In a sense, patient cells act like a moving car whose accelerator is jammed, moving at the same speed without regard to the external environment. This focuses attention on cell motility regulation rather than speed as key to impairment of neuronal migration in the developing brain in schizophrenia.

Schizophrenia patient-derived olfactory neurosphere-derived cells do not respond to extracellular reelin

Reelin expression is reduced in various regions in the post-mortem brain of schizophrenia patients but the exact role of reelin function in the neurobiology of schizophrenia remains elusive. Absence of reelin in knockout mouse causes inverted lamination of the neocortex due to aberrant neuronal migration. The aim of this study was to utilize patient-derived olfactory neurosphere-derived (ONS) cells to investigate whether extracellular reelin alters cell motility in schizophrenia patient-derived cells. ONS cells from nine patients were compared with cells from nine matched healthy controls. Automated high-throughput imaging and analysis were used to track motility of individual living cells on reelin-coated surfaces produced from reelin secreted into the medium by HEK293FT cells transfected with the full-length reelin plasmid pCrl. Automated assays were used to quantify intracellular cytoskeleton composition, cell morphology, and focal adhesions. Expression of reelin and components of the reelin signaling pathway were measured by western blot and flow cytometry. Reelin inhibited the motility of control cells but not patient cells, and increased the number and size of focal adhesions in control cells but not patient cells. Patient and control cells expressed similar levels of the reelin receptors and the reelin signaling protein, Dab1, but patient cells expressed less reelin. Patient cells were smaller than control cells and had less actin and acetylated α-tubulin, components of the cytoskeleton. These findings are the first direct evidence that cellular responses to reelin are impaired in schizophrenia and are consistent with the role of reelin in cytoarchitectural deficits observed in schizophrenia patient brains.

Reelin receptors ApoER2 and VLDLR are expressed in distinct spatiotemporal patterns in developing mouse cerebral cortex

In mammalian developing brain, neuronal migration is regulated by a variety of signaling cascades, including Reelin signaling. Reelin is a glycoprotein that is mainly secreted by Cajal-Retzius neurons in the marginal zone, playing essential roles in the formation of the layered neocortex via its receptors, apolipoprotein E receptor 2 (ApoER2) and very low density lipoprotein receptor (VLDLR). However, the precise mechanisms by which Reelin signaling controls the neuronal migration process remain unclear. To gain insight into how Reelin signaling controls individual migrating neurons, we generated monoclonal antibodies against ApoER2 and VLDLR and examined the localization of Reelin receptors in the developing mouse cerebral cortex. Immunohistochemical analyses revealed that VLDLR is localized to the distal portion of leading processes in the marginal zone (MZ), whereas ApoER2 is mainly localized to neuronal processes and the cell membranes of multipolar cells in the multipolar cell accumulation zone (MAZ). These different expression patterns may contribute to the distinct actions of Reelin on migrating neurons during both the early and late migratory stages in the developing cerebral cortex.

An in silico agent-based model demonstrates Reelin function in directing lamination of neurons during cortical development

The characteristic six-layered appearance of the neocortex arises from the correct positioning of pyramidal neurons during development and alterations in this process can cause intellectual disabilities and developmental delay. Malformations in cortical development arise when neurons either fail to migrate properly from the germinal zones or fail to cease migration in the correct laminar position within the cortical plate. The Reelin signalling pathway is vital for correct neuronal positioning as loss of Reelin leads to a partially inverted cortex. The precise biological function of Reelin remains controversial and debate surrounds its role as a chemoattractant or stop signal for migrating neurons. To investigate this further we developed an in silico agent-based model of cortical layer formation. Using this model we tested four biologically plausible hypotheses for neuron motility and four biologically plausible hypotheses for the loss of neuron motility (conversion from migration). A matrix of 16 combinations of motility and conversion rules was applied against the known structure of mouse cortical layers in the wild-type cortex, the Reelin-null mutant, the Dab1-null mutant and a conditional Dab1 mutant. Using this approach, many combinations of motility and conversion mechanisms can be rejected. For example, the model does not support Reelin acting as a repelling or as a stopping signal. In contrast, the study lends very strong support to the notion that the glycoprotein Reelin acts as a chemoattractant for neurons. Furthermore, the most viable proposition for the conversion mechanism is one in which conversion is affected by a motile neuron sensing in the near vicinity neurons that have already converted. Therefore, this model helps elucidate the function of Reelin during neuronal migration and cortical development.