The spatial and temporal arrangement of the radial glial scaffold suggests a role in axon tract formation in the developing spinal cord

Morphogenetic theory of mental and cognitive disorders: the role of neurotrophic and guidance molecules

Mental illness and cognitive disorders represent a serious problem for the modern society. Many studies indicate that mental disorders are polygenic and that impaired brain development may lay the ground for their manifestation. Neural tissue development is a complex and multistage process that involves a large number of distant and contact molecules. In this review, we have considered the key steps of brain morphogenesis, and the major molecule families involved in these process. The review provides many indications of the important contribution of the brain development process and correct functioning of certain genes to human mental health. To our knowledge, this comprehensive review is one of the first in this field. We suppose that this review may be useful to novice researchers and clinicians wishing to navigate the field.

Regulation of axon pathfinding by astroglia across genetic model organisms

Glia and neurons are intimately associated throughout bilaterian nervous systems, and were early proposed to interact for patterning circuit assembly. The investigations of circuit formation progressed from early hypotheses of intermediate guideposts and a "glia blueprint", to recent genetic and cell manipulations, and visualizations in vivo. An array of molecular factors are implicated in axon pathfinding but their number appears small relatively to circuit complexity. Comprehending this circuit complexity requires to identify unknown factors and dissect molecular topographies. Glia contribute to both aspects and certain studies provide molecular and functional insights into these contributions. Here, I survey glial roles in guiding axon navigation in vivo, emphasizing analogies, differences and open questions across major genetic models. I highlight studies pioneering the topic, and dissect recent findings that further advance our current molecular understanding. Circuits of the vertebrate forebrain, visual system and neural tube in zebrafish, mouse and chick, the Drosophila ventral cord and the C. elegans brain-like neuropil emerge as major contexts to study glial cell functions in axon navigation. I present astroglial cell types in these models, and their molecular and cellular interactions that drive axon guidance. I underline shared principles across models, conceptual or technical complications, and open questions that await investigation. Glia of the radial-astrocyte lineage, emerge as regulators of axon pathfinding, often employing common molecular factors across models. Yet this survey also highlights different involvements of glia in embryonic navigation or pioneer axon pathfinding, and unknowns in the molecular underpinnings of glial cell functions. Future cellular and molecular investigations should complete the comprehensive view of glial roles in circuit assembly.

Spinal cord tissue engineering using human primary neural progenitor cells and astrocytes

Neural progenitor cell (NPC) transplantation is a promising approach for repairing spinal cord injury (SCI). However, cell survival, maturation and integration after transplantation are still major challenges. Here, we produced a novel centimeter-scale human spinal cord neural tissue (hscNT) construct with human spinal cord neural progenitor cells (hscNPCs) and human spinal cord astrocytes (hscAS) on a linearly ordered collagen scaffold (LOCS). The hscAS promoted hscNPC adhesion, survival and neurite outgrowth on the LOCS, to form a linearly ordered spinal cord-like structure consisting of mature neurons and glia cells. When transplanted into rats with SCI, the hscNT created a favorable microenvironment by inhibiting inflammation and glial scar formation, and promoted neural and vascular regeneration. Notably, the hscNT promoted neural circuit reconstruction and motor functional recovery. Engineered human spinal cord implants containing astrocytes and neurons assembled on axon guidance scaffolds may therefore have potential in the treatment of SCI.

Single-cell analysis reveals dynamic changes of neural cells in developing human spinal cord

During central nervous system development, neurogenesis and gliogenesis occur in an orderly manner to create precise neural circuitry. However, no systematic dataset of neural lineage development that covers both neurogenesis and gliogenesis for the human spinal cord is available. We here perform single-cell RNA sequencing of human spinal cord cells during embryonic and fetal stages that cover neuron generation as well as astrocytes and oligodendrocyte differentiation. We also map the timeline of sensory neurogenesis and gliogenesis in the spinal cord. We further identify a group of EGFR-expressing transitional glial cells with radial morphology at the onset of gliogenesis, which progressively acquires differentiated glial cell characteristics. These EGFR-expressing transitional glial cells exhibited a unique position-specific feature during spinal cord development. Cell crosstalk analysis using CellPhoneDB indicated that EGFR glial cells can persistently interact with other neural cells during development through Delta-Notch and EGFR signaling. Together, our results reveal stage-specific profiles and dynamics of neural cells during human spinal cord development.

Tubulin mutations in neurodevelopmental disorders as a tool to decipher microtubule function

Malformations of cortical development (MCDs) are a group of severe brain malformations associated with intellectual disability and refractory childhood epilepsy. Human missense heterozygous mutations in the 9 α-tubulin and 10 β-tubulin isoforms forming the heterodimers that assemble into microtubules (MTs) were found to cause MCDs. However, how a single mutated residue in a given tubulin isoform can perturb the entire microtubule population in a neuronal cell remains a crucial question. Here, we examined 85 MCD-associated tubulin mutations occurring in TUBA1A, TUBB2, and TUBB3 and their location in a three-dimensional (3D) microtubule cylinder. Mutations hitting residues exposed on the outer microtubule surface are likely to alter microtubule association with partners, while alteration of intradimer contacts may impair dimer stability and straightness. Other types of mutations are predicted to alter interdimer and lateral contacts, which are responsible for microtubule cohesion, rigidity, and dynamics. MCD-associated tubulin mutations surprisingly fall into all categories, thus providing unexpected insights into how a single mutation may impair microtubule function and elicit dominant effects in neurons.

Astroglia in Leukodystrophies

Leukodystrophies are genetically determined disorders affecting the white matter of the central nervous system. The combination of MRI pattern recognition and next-generation sequencing for the definition of novel disease entities has recently demonstrated that many leukodystrophies are due to the primary involvement and/or mutations in genes selectively expressed by cell types other than the oligodendrocytes, the myelin-forming cells in the brain. This has led to a new definition of leukodystrophies as genetic white matter disorders resulting from the involvement of any white matter structural component. As a result, the research has shifted its main focus from oligodendrocytes to other types of neuroglia. Astrocytes are the housekeeping cells of the nervous system, responsible for maintaining homeostasis and normal brain physiology and to orchestrate repair upon injury. Several lines of evidence show that astrocytic interactions with the other white matter cellular constituents play a primary pathophysiologic role in many leukodystrophies. These are thus now classified as astrocytopathies. This chapter addresses how the crosstalk between astrocytes, other glial cells, axons and non-neural cells are essential for the integrity and maintenance of the white matter in health. It also addresses the current knowledge of the cellular pathomechanisms of astrocytic leukodystrophies, and specifically Alexander disease, vanishing white matter, megalencephalic leukoencephalopathy with subcortical cysts and Aicardi-Goutière Syndrome.

Axoglial synapses are formed onto pioneer oligodendrocyte precursor cells at the onset of spinal cord gliogenesis

Virtually all oligodendrocyte precursors cells (OPCs) receive glutamatergic and/or GABAergic synapses that are lost upon their differentiation into oligodendrocytes in the postnatal and adult brain. Although OPCs are generated at mid-embryonic stages, several weeks before the onset of myelination, it remains unknown when and where OPCs receive their first synapses and become susceptible to the influence of neuronal activity. In the embryonic spinal cord, neuro-epithelial precursors in the pMN domain cease generating cholinergic motor neurons (MNs) to produce OPCs when the first synapses are formed in the ventral-lateral marginal zone. We discovered that when the first synapses form onto MNs, axoglial synapses also form onto the processes of neuro-epithelial precursors located in the marginal zone as they differentiate into OPCs. After leaving the neuro-epithelium, these pioneer OPCs preferentially accumulate in the marginal zone where they are contacted by functional glutamatergic and GABAergic synapses. Spontaneous activity of these axoglial synapses was significantly potentiated by cholinergic signaling acting through presynaptic nicotinic acetylcholine receptors. Moreover, we discovered that chronic nicotine treatment significantly increases early OPC proliferation and density in the marginal zone. Our results demonstrate that OPCs are contacted by functional synapses as soon as they emerge from their precursor domain and that embryonic spinal cord colonization by OPCs can be regulated by cholinergic signaling acting onto these axoglial synapses.

Transplantable living scaffolds comprised of micro-tissue engineered aligned astrocyte networks to facilitate central nervous system regeneration

Neurotrauma, stroke, and neurodegenerative disease may result in widespread loss of neural cells as well as the complex interconnectivity necessary for proper central nervous system function, generally resulting in permanent functional deficits. Potential regenerative strategies involve the recruitment of endogenous neural stem cells and/or directed axonal regeneration through the use of tissue engineered "living scaffolds" built to mimic features of three-dimensional (3-D) in vivo migratory or guidance pathways. Accordingly, we devised a novel biomaterial encasement scheme using tubular hydrogel-collagen micro-columns that facilitated the self-assembly of seeded astrocytes into 3-D living scaffolds consisting of long, cable-like aligned astrocytic networks. Here, robust astrocyte alignment was achieved within a micro-column inner diameter (ID) of 180μm or 300-350μm but not 1.0mm, suggesting that radius of curvature dictated the extent of alignment. Moreover, within small ID micro-columns, >70% of the astrocytes assumed a bi-polar morphology, versus ∼10% in larger micro-columns or planar surfaces. Cell-cell interactions also influenced the aligned architecture, as extensive astrocyte-collagen contraction was achieved at high (9-12×10(5)cells/mL) but not lower (2-6×10(5)cells/mL) seeding densities. This high density micro-column seeding led to the formation of ultra-dense 3-D "bundles" of aligned bi-polar astrocytes within collagen measuring up to 150μm in diameter yet extending to a remarkable length of over 2.5cm. Importantly, co-seeded neurons extended neurites directly along the aligned astrocytic bundles, demonstrating permissive cues for neurite extension. These transplantable cable-like astrocytic networks structurally mimic the glial tube that guides neuronal progenitor migration in vivo along the rostral migratory stream, and therefore may be useful to guide progenitor cells to repopulate sites of widespread neurodegeneration.

STATEMENT OF SIGNIFICANCE

This manuscript details our development of novel micro-tissue engineering techniques to generate robust networks of longitudinally aligned astrocytes within transplantable micro-column hydrogels. We report a novel biomaterial encasement scheme that facilitated the self-assembly of seeded astrocytes into long, aligned regenerative pathways. These miniature "living scaffold" constructs physically emulate the glial tube - a pathway in the brain consisting of aligned astrocytes that guide the migration of neuronal progenitor cells - and therefore may facilitate directed neuronal migration for central nervous system repair. The small size and self-contained design of these aligned astrocyte constructs will permit minimally invasive transplantation in models of central nervous system injury in future studies.

Gfap-positive radial glial cells are an essential progenitor population for later-born neurons and glia in the zebrafish spinal cord

Radial glial cells are presumptive neural stem cells (NSCs) in the developing nervous system. The direct requirement of radial glia for the generation of a diverse array of neuronal and glial subtypes, however, has not been tested. We employed two novel transgenic zebrafish lines and endogenous markers of NSCs and radial glia to show for the first time that radial glia are essential for neurogenesis during development. By using the gfap promoter to drive expression of nuclear localized mCherry we discerned two distinct radial glial-derived cell types: a major nestin+/Sox2+ subtype with strong gfap promoter activity and a minor Sox2+ subtype lacking this activity. Fate mapping studies in this line indicate that gfap+ radial glia generate later-born CoSA interneurons, secondary motorneurons, and oligodendroglia. In another transgenic line using the gfap promoter-driven expression of the nitroreductase enzyme, we induced cell autonomous ablation of gfap+ radial glia and observed a reduction in their specific derived lineages, but not Blbp+ and Sox2+/gfap-negative NSCs, which were retained and expanded at later larval stages. Moreover, we provide evidence supporting classical roles of radial glial in axon patterning, blood-brain barrier formation, and locomotion. Our results suggest that gfap+ radial glia represent the major NSC during late neurogenesis for specific lineages, and possess diverse roles to sustain the structure and function of the spinal cord. These new tools will both corroborate the predicted roles of astroglia and reveal novel roles related to development, physiology, and regeneration in the vertebrate nervous system. GLIA 2016;64:1170-1189.

Multi-parametric O₂ imaging in three-dimensional neural cell models with the phosphorescent probes

Recent progress in bio-imaging has allowed detailed mechanistic studies of neural cell function in complex 3D tissue models including multicellular aggregates, neurospheres, excised brain slices, ganglia, and organoids. Molecular oxygen (O2 ) is an important metabolite and an environmental parameter which determines the viability and physiological status of neural cells within tissue. Here we describe standard method for monitoring O2 in 3D tissue models using phosphorescence lifetime imaging microscopy (PLIM ) and cell-penetrating O2-sensing probes. The O2 probes can be multiplexed with many conventional fluorescence based live cell biomarkers and also end-point immunofluorescence staining. The multi-parametric O2 imaging method is particularly useful for areas such as stem cell development and differentiation , hypoxia research, neurodegenerative disorders, regeneration of brain tissue, evaluation of new drugs, and development of novel tissue models.

Tissue engineering is a promising method for the repair of spinal cord injuries (Review)

Spinal cord injury (SCI) may lead to a devastating and permanent loss of neurological function, which may place a great economic burden on the family of the patient and society. Methods for reducing the death of neuronal cells, inhibiting immune and inflammatory reactions, and promoting the growth of axons in order to build up synapses with the target cells are the focus of current research. Target cells are located in the damaged spinal cord which create a connect with the scaffold. As tissue engineering technology is developed for use in a variety of different areas, particularly the biomedical field, a clear understanding of the mechanisms of tissue engineering is important. This review establishes how this technology may be used in basic experiments with regard to SCI and considers its potential future clinical use.

Radial glial cells: key organisers in CNS development

Radial glia are elongated bipolar cells present in the CNS during development. Our understanding of the unique roles these cells play has significantly expanded in the last decade. Historically, radial glial cells were primarily thought to provide an architectural framework for neuronal migration. Recent research reveals that radial glia play a more dynamic and integrated role in the development of the brain and spinal cord. They represent a major progenitor pool during early development and can give rise to a small population of multipotent cells in neurogenic niches of the adult CNS. Radial glial cells are a heterogeneous population, with divergent and often poorly understood roles across different brain and spinal cord regions during development; this heterogeneity extends to specialised adult subtypes, such as tanycytes, Müller glial cells and Bergman glial cells which possess morphological similarities to radial glial but play distinct functional roles in the CNS.

The structural role of radial glial endfeet in confining spinal motor neuron somata is controlled by the Reelin and Notch pathways

Neuronal migration is a fundamental biological process that enables the precise positioning of neurons to form functional circuits. Cortical neurons migrate along glial scaffolds formed by radial glia guided by Reelin ligand. However, it is unclear whether the Reelin-directed behavior of radial glia is also critical for positioning the spinal neurons. Here we demonstrate a novel role of radial glia that confines motor neurons within the neural tube and is promoted by Reelin and Notch signaling. Spinal radial glia express the Dab1 adaptor for Reelin signaling and are surrounded by Reelin. In reeler mice, in which Reelin is absent, ectopic motor neurons are found outside the neural tube, although they appear to maintain their identity. Boundary cap (BC) cells, Schwann cell precursors and the basal lamina at motor exit points are intact, whereas the glia limitans of radial glia are disorganized and detached from the basement membrane. The sparse and irregular radial scaffold is wide enough to allow motor somata to pass. Forced activation of Notch signaling rescued the structural defects in radial glia in reeler mice and the appearance of extraspinal neurons. In the absence of Reelin, Notch intracellular domain (NICD) protein level was reduced. In addition, disrupting the radial glia scaffold by destroying its polarity induced ectopic motor neurons in chick embryos. These findings suggest that activation of the Notch pathways by Reelin is required to establish the radial glial scaffold, a structure that actively constrains motor neuron somata and specifies the CNS-PNS boundary.