Molecular and cellular characterization of the glial roof plate of the spinal cord and optic tectum: a possible role for a proteoglycan in the development of an axon barrier

Keratan sulfate proteoglycan: putative template for neuroblast migratory and axonal fascicular pathways and fetal expression in globus pallidus, thalamus, and olfactory bulb

Keratan sulfate (KS) is a proteoglycan secreted in the fetal brain astrocytes and radial glia into extracellular parenchyma as granulofilamentous deposits. KS surrounds neurons except dendritic spines, repelling glutamatergic and facilitating GABAergic axons. The same genes are expressed in both neuroblast migration and axonal growth. This study examines timing of KS during morphogenesis of some normally developing human fetal forebrain structures. Twenty normal human fetal brains from 9-41 weeks gestational age were studied at autopsy. KS was examined by immunoreactivity in formalin-fixed paraffin sections, plus other markers including synaptophysin, S-100β protein, vimentin and nestin. Radial and tangential neuroblast migratory pathways from subventricular zone to cortical plate were marked by KS deposits as early as 9wk GA, shortly after neuroblast migration initiated. During later gestation this reactivity gradually diminished and disappeared by term. Long axonal fascicles of the internal capsule and short fascicles of intrinsic bundles of globus pallidus and corpus striatum also appeared as early as 9-12wk, as fascicular sleeves before axons even entered. Intense KS occurs in astrocytic cytoplasm and extracellular parenchyma at 9wk in globus pallidus, 15wk thalamus, 18wk corpus striatum, 22wk cortical plate, and hippocampus postnatally. Corpus callosum and anterior commissure do not exhibit KS at any age. Optic chiasm shows reactivity at the periphery but not around intrinsic subfasciculi. We postulate that KS forms a chemical template for many long and short axonal fascicles before axons enter and neuroblast migratory pathways at initiation of migration. Cross-immunoreactivity with aggrecan may render difficult molecular distinction.

Recovery of Forearm and Fine Digit Function After Chronic Spinal Cord Injury by Simultaneous Blockade of Inhibitory Matrix Chondroitin Sulfate Proteoglycan Production and the Receptor PTPσ

Spinal cord injuries (SCI), for which there are limited effective treatments, result in enduring paralysis and hypoesthesia, in part because of the inhibitory microenvironment that develops and limits regeneration/sprouting, especially during chronic stages. Recently, we discovered that targeted enzymatic removal of the inhibitory chondroitin sulfate proteoglycan (CSPG) component of the extracellular and perineuronal net (PNN) matrix via Chondroitinase ABC (ChABC) rapidly restored robust respiratory function to the previously paralyzed hemi-diaphragm after remarkably long times post-injury (up to 1.5 years) following a cervical level 2 lateral hemi-transection. Importantly, ChABC treatment at cervical level 4 in this chronic model also elicited improvements in gross upper arm function. In the present study, we focused on arm and hand function, seeking to highlight and optimize crude as well as fine motor control of the forearm and digits at lengthy chronic stages post-injury. However, instead of using ChABC, we utilized a novel and more clinically relevant systemic combinatorial treatment strategy designed to simultaneously reduce and overcome inhibitory CSPGs. Following a 3-month upper cervical spinal hemi-lesion using adult female Sprague Dawley rats, we show that the combined treatment had a profound effect on functional recovery of the chronically paralyzed forelimb and paw, as well as on precision movements of the digits. The regenerative and immune system related events that we describe deepen our basic understanding of the crucial role of CSPG-mediated inhibition via the PTPσ receptor in constraining functional synaptic plasticity at lengthy time points following SCI, hopefully leading to clinically relevant translational benefits.

Inter-axonal molecular crosstalk via Lumican proteoglycan sculpts murine cervical corticospinal innervation by distinct subpopulations

How CNS circuits sculpt their axonal arbors into spatially and functionally organized domains is not well understood. Segmental specificity of corticospinal connectivity is an exemplar for such regional specificity of many axon projections. Corticospinal neurons (CSN) innervate spinal and brainstem targets with segmental precision, controlling voluntary movement. Multiple molecularly distinct CSN subpopulations innervate the cervical cord for evolutionarily enhanced precision of forelimb movement. Evolutionarily newer CSN_BC-lat exclusively innervate bulbar-cervical targets, while CSN_medial are heterogeneous; distinct subpopulations extend axons to either bulbar-cervical or thoraco-lumbar segments. We identify that Lumican controls balance of cervical innervation between CSN_BC-lat and CSN_medial axons during development, which is maintained into maturity. Lumican, an extracellular proteoglycan expressed by CSN_BC-lat, non-cell-autonomously suppresses cervical collateralization by multiple CSN_medial subpopulations. This inter-axonal molecular crosstalk between CSN subpopulations controls murine corticospinal circuitry refinement and forelimb dexterity. Such crosstalk is generalizable beyond the corticospinal system for evolutionary incorporation of new neuron populations into preexisting circuitry.

Morphological and Functional Changes of Roof Plate Cells in Spinal Cord Development

The most dorsal region, or roof plate, is the dorsal organizing center of developing spinal cord. This region is also involved in development of neural crest cells, which are the source of migratory neural crest cells. During early development of the spinal cord, roof plate cells secrete signaling molecules, such as Wnt and BMP family proteins, which regulate development of neural crest cells and dorsal spinal cord. After the dorso-ventral pattern is established, spinal cord dynamically changes its morphology. With this morphological transformation, the lumen of the spinal cord gradually shrinks to form the central canal, a cavity filled with cerebrospinal fluid that is connected to the ventricular system of the brain. The dorsal half of the spinal cord is separated by a glial structure called the dorsal (or posterior) median septum. However, underlying mechanisms of such morphological transformation are just beginning to be understood. Recent studies reveal that roof plate cells dramatically stretch along the dorso-ventral axis, accompanied by reduction of the spinal cord lumen. During this stretching process, the tips of roof plate cells maintain contact with cells surrounding the shrinking lumen, eventually exposed to the inner surface of the central canal. Interestingly, Wnt expression remains in stretched roof plate cells and activates Wnt/β-catenin signaling in ependymal cells surrounding the central canal. Wnt/β-catenin signaling in ependymal cells promotes proliferation of neural progenitor and stem cells in embryonic and adult spinal cord. In this review, we focus on the role of the roof plate, especially that of Wnt ligands secreted by roof plate cells, in morphological changes occurring in the spinal cord.

From Neural Crest to Definitive Roof Plate: The Dynamic Behavior of the Dorsal Neural Tube

Research on the development of the dorsal neural tube is particularly challenging. In this highly dynamic domain, a temporal transition occurs between early neural crest progenitors that undergo an epithelial-to-mesenchymal transition and exit the neural primordium, and the subsequent roof plate, a resident epithelial group of cells that constitutes the dorsal midline of the central nervous system. Among other functions, the roof plate behaves as an organizing center for the generation of dorsal interneurons. Despite extensive knowledge of the formation, emigration and migration of neural crest progenitors, little is known about the mechanisms leading to the end of neural crest production and the transition into a roof plate stage. Are these two mutually dependent or autonomously regulated processes? Is the generation of roof plate and dorsal interneurons induced by neural tube-derived factors throughout both crest and roof plate stages, respectively, or are there differences in signaling properties and responsiveness as a function of time? In this review, we discuss distinctive characteristics of each population and possible mechanisms leading to the shift between the above cell types.

The Life of a Trailing Spouse

In 1981, I published a paper in the first issue of the Journal of Neuroscience with my postdoctoral mentor, Alan Pearlman. It reported a quantitative analysis of the receptive field properties of neurons in reeler mouse visual cortex and the surprising conclusion that although the neuronal somas were strikingly malpositioned, their receptive fields were unchanged. This suggested that in mouse cortex at least, neuronal circuits have very robust systems in place to ensure the proper formation of connections. This had the unintended consequence of transforming me from an electrophysiologist into a cellular and molecular neuroscientist who studied cell adhesion molecules and the molecular mechanisms they use to regulate axon growth. It took me a surprisingly long time to appreciate that your science is driven by the people around you and by the technologies that are locally available. As a professional puzzler, I like all different kinds of puzzles, but the most fun puzzles involve playing with other puzzlers. This is my story of learning how to find like-minded puzzlers to solve riddles about axon growth and regeneration.

Excitatory/Inhibitory Synaptic Ratios in Polymicrogyria and Down Syndrome Help Explain Epileptogenesis in Malformations

BACKGROUND

The ratio between excitatory (glutamatergic) and inhibitory (GABAergic) inputs into maturing individual cortical neurons influences their epileptic potential. Structural factors during development that alter synaptic inputs can be demonstrated neuropathologically. Increased mitochondrial activity identifies neurons with excessive discharge rates.

METHODS

This study focuses on the neuropathological examinaion of surgical resections for epilepsy and at autopsy, in fetuses, infants, and children, using immunocytochemical markers, and electron microscopy in selected cases. Polymicrogyria and Down syndrome are highlighted.

RESULTS

Factors influencing afferent synaptic ratios include the following: (1) synaptic short-circuitry in fused molecular zones of adjacent gyri (polymicrogyria); (2) impaired development of dendritic spines decreasing excitation (Down syndrome); (3) extracellular keratan sulfate proteoglycan binding to somatic membranes but not dendritic spines may be focally diminished (cerebral atrophy, schizencephaly, lissencephaly, polymicrogyria) or augmented, ensheathing individual axons (holoprosencephaly), or acting as a barrier to axonal passage in the U-fiber layer. If keratan is diminished, glutamate receptors on the neuronal soma enable ectopic axosomatic excitatory synapses to form; (4) dysplastic, megalocytic neurons and balloon cells in mammalian target of rapamycin disorders; (5) satellitosis of glial cells displacing axosomatic synapses; (6) peri-neuronal inflammation (tuberous sclerosis) and heat-shock proteins.

CONCLUSIONS

Synaptic ratio of excitatory/inhibitory afferents is a major fundamental basis of epileptogenesis at the neuronal level. Neuropathology can demonstrate subcellular changes that help explain either epilepsy or lack of seizures in immature brains. Synaptic ratios in malformations influence postnatal epileptogenesis. Single neurons can be hypermetabolic and potentially epileptogenic.

Dorsal commissural axon guidance in the developing spinal cord

Commissural axons have been a key model system for identifying axon guidance signals in vertebrates. This review summarizes the current thinking about the molecular and cellular mechanisms that establish a specific commissural neural circuit: the dI1 neurons in the developing spinal cord. We assess the contribution of long- and short-range signaling while sequentially following the developmental timeline from the birth of dI1 neurons, to the extension of commissural axons first circumferentially and then contralaterally into the ventral funiculus.

Proteoglycan (Keratan Sulfate) Barrier in Developing Human Forebrain Isolates Cortical Epileptic Networks From Deep Heterotopia, Insulates Axonal Fascicles, and Explains Why Axosomatic Synapses Are Inhibitory

Axons from deep heterotopia do not extend through U-fibers, except transmantle dysplasias. Keratan sulfate (KS) in fetal spinal cord/brainstem median septum selectively repels glutamatergic axons while enabling GABAergic commissural axons. Immunocytochemical demonstration of KS in neocortical resections and forebrain at autopsy was studied in 12 fetuses and neonates 9-41 weeks gestational age (GA), 9 infants, children, and adolescents and 5 patients with focal cortical dysplasias (FCD1a). From 9 to 15 weeks GA, no KS is seen in the cortical plate; 19-week GA reactivity is detected in the molecular zone. By 28 weeks GA, patchy granulofilamentous reactivity appears in extracellular matrix and adheres to neuronal somata with increasing intensity in deep cortex and U-fibers at term. Perifascicular KS surrounds axonal bundles of both limbs of the internal capsule and within basal ganglia from 9 weeks GA. Thalamus and globus pallidus exhibit intense astrocytic reactivity from 9 weeks GA. In FCD1a, U-fiber reactivity is normal, discontinuous or radial. Ultrastructural correlates were not demonstrated; KS is not electron-dense. Proteoglycan barrier of the U-fiber layer impedes participation of deep heterotopia in cortical epileptic networks. Perifascicular KS prevents aberrant axonal exit from or entry into long and short tracts. KS adhesion to neuronal somatic membranes may explain inhibitory axosomatic synapses.

Crumbs2 mediates ventricular layer remodelling to form the spinal cord central canal

In the spinal cord, the central canal forms through a poorly understood process termed dorsal collapse that involves attrition and remodelling of pseudostratified ventricular layer (VL) cells. Here, we use mouse and chick models to show that dorsal ventricular layer (dVL) cells adjacent to dorsal midline Nestin(+) radial glia (dmNes+RG) down-regulate apical polarity proteins, including Crumbs2 (CRB2) and delaminate in a stepwise manner; live imaging shows that as one cell delaminates, the next cell ratchets up, the dmNes+RG endfoot ratchets down, and the process repeats. We show that dmNes+RG secrete a factor that promotes loss of cell polarity and delamination. This activity is mimicked by a secreted variant of Crumbs2 (CRB2S) which is specifically expressed by dmNes+RG. In cultured MDCK cells, CRB2S associates with apical membranes and decreases cell cohesion. Analysis of Crb2F/F/Nestin-Cre+/- mice, and targeted reduction of Crb2/CRB2S in slice cultures reveal essential roles for transmembrane CRB2 (CRB2TM) and CRB2S on VL cells and dmNes+RG, respectively. We propose a model in which a CRB2S-CRB2TM interaction promotes the progressive attrition of the dVL without loss of overall VL integrity. This novel mechanism may operate more widely to promote orderly progenitor delamination.

Keratan sulfate (KS)-proteoglycans and neuronal regulation in health and disease: the importance of KS-glycodynamics and interactive capability with neuroregulatory ligands

Compared to the other classes of glycosaminoglycans (GAGs), that is, chondroitin/dermatan sulfate, heparin/heparan sulfate and hyaluronan, keratan sulfate (KS), have the least known of its interactive properties. In the human body, the cornea and the brain are the two most abundant tissue sources of KS. Embryonic KS is synthesized as a linear poly-N-acetyllactosamine chain of d-galactose-GlcNAc repeat disaccharides which become progressively sulfated with development, sulfation of GlcNAc is more predominant than galactose. KS contains multi-sulfated high-charge density, monosulfated and non-sulfated poly-N-acetyllactosamine regions and thus is a heterogeneous molecule in terms of chain length and charge distribution. A recent proteomics study on corneal KS demonstrated its interactivity with members of the Slit-Robbo and Ephrin-Ephrin receptor families and proteins which regulate Rho GTPase signaling and actin polymerization/depolymerization in neural development and differentiation. KS decorates a number of peripheral nervous system/CNS proteoglycan (PG) core proteins. The astrocyte KS-PG abakan defines functional margins of the brain and is up-regulated following trauma. The chondroitin sulfate/KS PG aggrecan forms perineuronal nets which are dynamic neuroprotective structures with anti-oxidant properties and roles in neural differentiation, development and synaptic plasticity. Brain phosphacan a chondroitin sulfate, KS, HNK-1 PG have roles in neural development and repair. The intracellular microtubule and synaptic vesicle KS-PGs MAP1B and SV2 have roles in metabolite transport, storage, and export of neurotransmitters and cytoskeletal assembly. MAP1B has binding sites for tubulin and actin through which it promotes cytoskeletal development in growth cones and is highly expressed during neurite extension. The interactive capability of KS with neuroregulatory ligands indicate varied roles for KS-PGs in development and regenerative neural processes.

Role of extracellular matrix and microenvironment in regulation of tumor growth and LAR-mediated invasion in glioblastoma

The cellular dispersion and therapeutic control of glioblastoma, the most aggressive type of primary brain cancer, depends critically on the migration patterns after surgery and intracellular responses of the individual cancer cells in response to external biochemical cues in the microenvironment. Recent studies have shown that miR-451 regulates downstream molecules including AMPK/CAB39/MARK and mTOR to determine the balance between rapid proliferation and invasion in response to metabolic stress in the harsh tumor microenvironment. Surgical removal of the main tumor is inevitably followed by recurrence of the tumor due to inaccessibility of dispersed tumor cells in normal brain tissue. In order to address this complex process of cell proliferation and invasion and its response to conventional treatment, we propose a mathematical model that analyzes the intracellular dynamics of the miR-451-AMPK- mTOR-cell cycle signaling pathway within a cell. The model identifies a key mechanism underlying the molecular switches between proliferative phase and migratory phase in response to metabolic stress in response to fluctuating glucose levels. We show how up- or down-regulation of components in these pathways affects the key cellular decision to infiltrate or proliferate in a complex microenvironment in the absence and presence of time delays and stochastic noise. Glycosylated chondroitin sulfate proteoglycans (CSPGs), a major component of the extracellular matrix (ECM) in the brain, contribute to the physical structure of the local brain microenvironment but also induce or inhibit glioma invasion by regulating the dynamics of the CSPG receptor LAR as well as the spatiotemporal activation status of resident astrocytes and tumor-associated microglia. Using a multi-scale mathematical model, we investigate a CSPG-induced switch between invasive and non-invasive tumors through the coordination of ECM-cell adhesion and dynamic changes in stromal cells. We show that the CSPG-rich microenvironment is associated with non-invasive tumor lesions through LAR-CSGAG binding while the absence of glycosylated CSPGs induce the critical glioma invasion. We illustrate how high molecular weight CSPGs can regulate the exodus of local reactive astrocytes from the main tumor lesion, leading to encapsulation of non-invasive tumor and inhibition of tumor invasion. These different CSPG conditions also change the spatial profiles of ramified and activated microglia. The complex distribution of CSPGs in the tumor microenvironment can determine the nonlinear invasion behaviors of glioma cells, which suggests the need for careful therapeutic strategies.

The 2016 Bernard Sachs Lecture: Timing in Morphogenesis and Genetic Gradients During Normal Development and in Malformations of the Nervous System

Nervous system development is quadradimensional. Both normal ontogenesis and developmental malformations are explained in the context of the fourth dimension, timing. Timing of the onset of either the genetic expression of a mutation or an epigenetic event that may be teratogenic is primordial in determining morphogenesis and the forms of malformations with their functional consequences. Multiple genotypes may cause similar phenotypes or a single genotype with different degrees of retained normal genetic expression may result in variable phenotypes. In this treatise, examples are presented of these principles, including both delayed and precocious maturation of processes such as synaptogenesis that may be out of synchrony with other simultaneous processes of neuronal maturation. In postzygotic somatic mosaicism, timing of onset determines not only the character but also the extent of a lesion; focal cortical dysplasia IIb and hemimegalencephaly are the same disease, both sharing activation of the mTOR pathway as the primary mechanism; the difference is timing of onset within the 33 mitotic cycles of the periventricular neuroepithelium. Genetic expression often follows gradients along the 3 axes of the neural tube. Defective gradients often can be identified by their morphological result without knowing the precise mutation. Upregulation in the vertical axis produces hyperplasia or duplication of either dorsal or ventral structures, whereas downregulation yields hypoplasia or fusion in the midline of bilateral structures. Disorders of segmentation or neuromere formation in the neural tube are increasingly recognized as another pathogenesis of cerebral dysgenesis. Our recent investigations show the participation of the U-fibre layer beneath FCD in epileptic networks because of neuronal dispersion with elaborate synaptic plexi and a barrier to deep heterotopia.

Flexible Roles for Proteoglycan Sulfation and Receptor Signaling

Proteoglycans (PGs) in the extracellular matrix (ECM) play vital roles in axon growth and navigation, plasticity, and regeneration of injured neurons. Different classes of PGs may support or inhibit cell growth, and their functions are determined in part by highly specific structural features. Among these, the pattern of sulfation on the PG sugar chains is a paramount determinant of a diverse and flexible set of outcomes. Recent studies of PG sulfation illustrate the challenges of attributing biological actions to specific sulfation patterns, and suggest ways in which highly similar molecules may exert opposing effects on neurons. The receptors for PGs, which have yet to be fully characterized, display a similarly nuanced spectrum of effects. Different classes of PG function via overlapping families of receptors and signaling pathways. This enables them to control axon growth and guidance with remarkable specificity, but it poses challenges for determining the precise binding interactions and downstream effects of different PGs and their assorted sulfated epitopes. This review examines existing and emerging evidence for the roles of PG sulfation and receptor interactions in determining how these complex molecules influence neuronal development, growth, and function.

Diverse roles for glycosaminoglycans in neural patterning

The nervous system coordinates the functions of most multicellular organisms and their response to the surrounding environment. Its development involves concerted cellular interactions, including migration, axon guidance, and synapse formation. These processes depend on the molecular constituents and structure of the extracellular matrices (ECM). An essential component of ECMs are proteoglycans, i.e., proteins containing unbranched glycan chains known as glycosaminoglycans (GAGs). A defining characteristic of GAGs is their enormous molecular diversity, created by extensive modifications of the glycans during their biosynthesis. GAGs are widely expressed, and their loss can lead to catastrophic neuronal defects. Despite their importance, we are just beginning to understand the function and mechanisms of GAGs in neuronal development. In this review, we discuss recent evidence suggesting GAGs have specific roles in neuronal patterning and synaptogenesis. We examine the function played by the complex modifications present on GAG glycans and their roles in regulating different aspects of neuronal patterning. Moreover, the review considers the function of proteoglycan core proteins in these processes, stressing their likely role as co-receptors of different signaling pathways in a redundant and context-dependent manner. We conclude by discussing challenges and future directions toward a better understanding of these fascinating molecules during neuronal development. Developmental Dynamics 247:54-74, 2018. © 2017 Wiley Periodicals, Inc.

Determinants of Axon Growth, Plasticity, and Regeneration in the Context of Spinal Cord Injury

The mechanisms that underlie recovery after injury of the central nervous system have rarely been definitively established. Axon regrowth remains the major prerequisite for plasticity, regeneration, circuit formation, and eventually functional recovery. The attributed functional relevance of axon regrowth, however, will depend on several subsequent conditional neurobiological modifications, including myelination and synapse formation, but also pruning of aberrant connectivity. Despite the ability to revamp axon outgrowth by altering an increasing number of extracellular and intracellular targets, disentangling which axons are responsible for the recovery of function from those that are functionally silent, or even contributing to aberrant functions, represents a pertinent void in our understanding, challenging the intuitive translational link between anatomical and functional regeneration. Anatomic hallmarks of regeneration are not static and are largely activity dependent. Herein, we survey mechanisms leading to the formation of dystrophic growth cone at the injured axonal tip, the subsequent axonal dieback, and the molecular determinants of axon growth, plasticity, and regeneration in the context of spinal cord injury.

Spinal RacGAP α-Chimaerin Is Required to Establish the Midline Barrier for Proper Corticospinal Axon Guidance

In the developing CNS, the midline barrier, which comprises guidance molecule-expressing midline glial somata and processes, plays a pivotal role in midline axon guidance. Accumulating evidence has revealed the molecular mechanisms by which the midline barrier ensures proper midline guidance for axons. In contrast, the mechanisms for establishing the midline barrier remain obscure. Here, we report that Rac-specific GTPase-activating protein (RacGAP) α-chimaerin is required for both axonal repulsion at and establishment of the midline barrier in the spinal cord. We generated cortex-specific and spinal-cord-specific α-chimaerin gene (Chn1) knock-out mice (Cx-Chn1KO and Sp-Chn1KO mice, respectively) and found that both showed aberrant corticospinal tract (CST) axon midline crossing in the spinal cord. Strikingly, Sp-Chn1KO mice had breaks (holes) in the ephrinB3(+) spinal midline barrier and EphA4(+) CST axons aberrantly crossed the midline through these holes. During normal embryonic development, EphA4(+) spinal cells are located in juxta-midline areas but are excluded from the midline. In contrast, in Chn1KO embryos, several EphA4(+) cells were aberrantly relocated into the midline and the midline barrier was broken around these cells. Similarly, the spinal cord midline of Epha4KO mice was invaded by juxta-midline EphA4 cells (i.e., Epha4 promoter-active cells) during the embryonic stage and holes were formed in the midline barrier. Juxta-midline EphA4 cells in the spinal cord expressed α-chimaerin. We propose that spinal α-chimaerin aids in establishing an intact spinal midline barrier by mediating juxta-midline EphA4(+) cell repulsion, thus preventing these cells from breaking into the ephrinB3(+) midline barrier.SIGNIFICANCE STATEMENT The midline barrier plays a critical role in midline axon guidance, which is fundamental to the formation of neural circuits that are responsible for proper left-right coordination of the body. Studies have revealed some of the mechanisms underlying how the midline barrier navigates axons. In contrast, the establishment of the midline barrier during embryonic development remains unclear. In this study, we determined that α-chimaerin is required for the formation of an intact midline barrier. Spinal-cord-specific α-chimaerin knock-out mice had spinal midline barriers with numerous breaks (holes), through which corticospinal axons aberrantly crossed the midline. We propose that α-chimaerin protects the midline barrier by mediating cell-repulsive signaling in juxta-midline cells, which prevents these cells from invading the midline.

Chondroitinase ABC Treatment Enhances Synaptogenesis between Transplant and Host Neurons in Model of Retinal Degeneration

Although recent studies revealed chondroitinase ABC (ChABC), an enzyme that degrades chondroitin sulfate proteoglycans, promotes CNS regeneration in vivo, the usefulness of its application for transplantation is not clear. We investigated if treatment with ChABC can promote synapse formation between graft and host neurons following retinal transplantation. Dissociated retinal cells were prepared from neonatal Nrl-GFP transgenic mice in which rod photoreceptors and their progenitor cells are labeled with GFP. Each cell suspension with or without ChABC (Nrl/ChABC group and Nrl group, respectively) was injected subretinally into the eyes of mice following chemically induced photoreceptor degeneration. The survival and functional integration of the transplanted photoreceptors were examined by histologically and electrophysio-logically. Up to 4 weeks after transplantation, almost all the grafted GFP+ photoreceptor cells were widely distributed at the outer margin of the host retina where the photoreceptor layer was located originally. In the Nrl/ChABC group, 33.6% of the GFP+ photoreceptors elaborated neurites horizontally or vertically, and 4.6% elaborated neurites toward the retina. These neurites extended over the glial seal at the graft–host interface, and established synaptic contacts with neurons in the host retina as determined by confocal microscopy and three-dimensional analysis. Although 30.7% cells (p = 0.68) elaborated neurites in the Nrl group, only 1.2% cells (p < 0.05) projected neurites towards the host tissue and synaptic contacts were rare. Our results illustrate the potential utility of ChABC for enhancing synaptogenesis between transplanted neurons and host retina.

"Targeting astrocytes in CNS injury and disease: A translational research approach"

Astrocytes are a major constituent of the central nervous system. These glia play a major role in regulating blood-brain barrier function, the formation and maintenance of synapses, glutamate uptake, and trophic support for surrounding neurons and glia. Therefore, maintaining the proper functioning of these cells is crucial to survival. Astrocyte defects are associated with a wide variety of neuropathological insults, ranging from neurodegenerative diseases to gliomas. Additionally, injury to the CNS causes drastic changes to astrocytes, often leading to a phenomenon known as reactive astrogliosis. This process is important for protecting the surrounding healthy tissue from the spread of injury, while it also inhibits axonal regeneration and plasticity. Here, we discuss the important roles of astrocytes after injury and in disease, as well as potential therapeutic approaches to restore proper astrocyte functioning.

Putative Inhibitory Extracellular Matrix Molecules Do Not Prevent Dorsal Root Regeneration into Fetal Spinal Cord Transplants

We examined the distribution of several extracellular matrix molecules (ECM) and their relationship to regenerating axons in embryonic day 14 spinal cord transplants 1 to 12 weeks after transplantation into adult rats. We used immunocytochemical tech niques to label chondroitin sulfate proteoglycans (CSPGs) and tenascin-C in adjacent sections. Synthesis of these molecules by astrocytes is thought to be one mechanism by which astrocytes inhibit regeneration in the central nervous system (CNS); glial fibrillary acidic protein antibody was used to label astrocytes and examine their rela tionship to both the ECM molecules and regenerating calcitonin gene-related pep tide (CORP)-contammg dorsal roots. We also compared the expression and distribu tion of these five markers in transplants with normal spinal cord development.

Deletion of Crmp4 attenuates CSPG-induced inhibition of axonal growth and induces nociceptive recovery after spinal cord injury

The capacity for regeneration in the injured adult mammalian central nervous system (CNS) is largely limited by potent inhibitory barriers. Chondroitin sulfate proteoglycans (CSPGs) are major inhibitors of axonal regeneration/sprouting and accumulate at lesion sites after CNS trauma. Despite extensive research during the two decades since their discovery, the molecular mechanisms remain elusive, including intracellular phosphorylation events. Collapsin response mediator protein 4 (CRMP4) is known to directly regulate cytoskeletal dynamics and neurite extension, while phosphorylated CRMP4 loses its binding affinity for cytoskeletal proteins. We have previously found that spinal cord injury (SCI) induces CRMP4 upregulation and phosphorylation and that CRMP4 knockout (Crmp4-/-) mice show behavioral recovery of locomotor function after SCI. However, the role of CRMP4 in the recovery of other forms of physiological function such as sensation remains largely unknown. We here have demonstrated CRMP4 involvement in CSPG-induced inhibitory signaling and nociceptive recovery in Crmp4-/- mice after SCI. We cultured dorsal root ganglion (DRG) neurons on CSPG-coated dishes; Crmp4 deletion overrode CSPG-induced inhibition of axon growth in vitro. CRMP4 levels were increased in DRGs in vivo after SCI. Crmp4-/- mice exhibited axonal growth of sensory neurons and recovery of nociceptive function after spinal transection. These results support Crmp4 deletion as a therapeutic target in the treatment of SCI.

Review : Glial Boundaries and Scars: Programs for Normal Development and Wound Healing in the Brain

Early studies of glial boundaries, which are composed of immature astrocytes and extracellular matrix mol ecules (which they express), initially offered insight into the partitioning that occurs in the developing nervous system. More recently, however, it has been suggested that similar "boundaries" may have important roles in other processes occurring in the brain, including repair after traumatic brain injury. As more is understood about the expression and function of boundary molecules and glia, their potential importance is becoming apparent in numerous neuropathological conditions, including neurodegeneration and neuroregeneration in Alzheimer's and Huntington's diseases as well as in brain neoplasms. Furthermore, before we can hope to fully understand and facilitate regeneration in the compromised brain, our knowledge of the glial boundary, both during development and in the adult, must be more complete. The Neuroscientist 1:142-154, 1995

Sensory and spinal inhibitory dorsal midline crossing is independent of Robo3

Commissural neurons project across the midline at all levels of the central nervous system (CNS), providing bilateral communication critical for the coordination of motor activity and sensory perception. Midline crossing at the spinal ventral midline has been extensively studied and has revealed that multiple developmental lineages contribute to this commissural neuron population. Ventral midline crossing occurs in a manner dependent on Robo3 regulation of Robo/Slit signaling and the ventral commissure is absent in the spinal cord and hindbrain of Robo3 mutants. Midline crossing in the spinal cord is not limited to the ventral midline, however. While prior anatomical studies provide evidence that commissural axons also cross the midline dorsally, little is known of the genetic and molecular properties of dorsally-crossing neurons or of the mechanisms that regulate dorsal midline crossing. In this study, we describe a commissural neuron population that crosses the spinal dorsal midline during the last quarter of embryogenesis in discrete fiber bundles present throughout the rostrocaudal extent of the spinal cord. Using immunohistochemistry, neurotracing, and mouse genetics, we show that this commissural neuron population includes spinal inhibitory neurons and sensory nociceptors. While the floor plate and roof plate are dispensable for dorsal midline crossing, we show that this population depends on Robo/Slit signaling yet crosses the dorsal midline in a Robo3-independent manner. The dorsally-crossing commissural neuron population we describe suggests a substrate circuitry for pain processing in the dorsal spinal cord.

Chondroitin sulfate proteoglycans: Key modulators in the developing and pathologic central nervous system

Chondroitin Sulfate Proteoglycans (CSPGs) are a major component of the extracellular matrix in the central nervous system (CNS) and play critical role in the development and pathophysiology of the brain and spinal cord. Developmentally, CSPGs provide guidance cues for growth cones and contribute to the formation of neuronal boundaries in the developing CNS. Their presence in perineuronal nets plays a crucial role in the maturation of synapses and closure of critical periods by limiting synaptic plasticity. Following injury to the CNS, CSPGs are dramatically upregulated by reactive glia which form a glial scar around the lesion site. Increased level of CSPGs is a hallmark of all CNS injuries and has been shown to limit axonal plasticity, regeneration, remyelination, and conduction after injury. Additionally, CSPGs create a non-permissive milieu for cell replacement activities by limiting cell migration, survival and differentiation. Mounting evidence is currently shedding light on the potential benefits of manipulating CSPGs in combination with other therapeutic strategies to promote spinal cord repair and regeneration. Moreover, the recent discovery of multiple receptors for CSPGs provides new therapeutic targets for targeted interventions in blocking the inhibitory properties of CSPGs following injury. Here, we will provide an in depth discussion on the impact of CSPGs in normal and pathological CNS. We will also review the recent preclinical therapies that have been developed to target CSPGs in the injured CNS.

Contributions of chondroitin sulfate proteoglycans to neurodevelopment, injury, and cancer

Chondroitin sulfate proteoglycans (CSPGs) are a diverse family of extracellular matrix (ECM) molecules that make significant contributions to the patterning and routing of migrating neural cells and extending axons. Three distinct modes of migration mediation result from the relative abundance and positioning of expressed CSPGs, the profile of CSPG receptors expressed by the motile cell types, and the overall way in which the CSPGs integrate into and stabilize the neural ECM. Here we discuss recent findings that help to clarify the molecular mechanisms that underlie these distinct migration-regulating properties as they pertain to neural development, CNS injury, and gliomagenesis.

Epidermal growth factor receptor inhibitor ameliorates excessive astrogliosis and improves the regeneration microenvironment and functional recovery in adult rats following spinal cord injury

BACKGROUND

Astrogliosis is a common phenomenon after spinal cord injury (SCI). Although this process exerts positive effects on axonal regeneration, excessive astrogliosis imparts negative effects on neuronal repair and recovery. Epidermal growth factor receptor (EGFR) pathway is critical to the regulation of reactive astrogliosis, and therefore is a potential target of therapeutics to better control the response. In this report, we aim to investigate whether blocking EGFR signaling using an EGFR tyrosine kinase specific inhibitor can attenuate reactive astrogliosis and promote functional recovery after a traumatic SCI.

METHOD

The astrocyte scratch injury model in vitro and the weight-drop SCI model in vivo were used as model systems. PD168393 was used to inhibit EGFR signaling activation. Astrocytic activation and phosphorylated EGFR (pEGFR) were observed after immunofluorescence staining and Western blot analysis. The rate of proliferation was determined by immunofluorescence detection of BrdU-incorporating cells located next to the wound. The levels of TNF-α, iNOS, COX-2 and IL-1β in the culture medium under different conditions were assayed by ELISA. Western blot was performed to semi-quantify the expression of EGFR/pEGFR, glial fibrillary acid protein (GFAP) and chondroitin sulfate proteoglycans (CSPGs). Myelin was stained by Luxol Fast Blue Staining. Cresyl violet eosin staining was performed to analyze the lesion cavity volume and neuronal survival following injury. Finally, functional scoring and residual urine recording were performed to show the rats' recovery.

RESULTS

EGFR phosphorylation was found to parallel astrocyte activation, and EGFR inhibitor PD168393 potently inhibited scratch-induced reactive astrogliosis and proinflammatory cytokine/mediator secretion of reactive astrocytes in vitro. Moreover, local administration of PD168393 in the injured area suppressed CSPGs production and glial scar formation, and resulted in reduced demyelination and neuronal loss, which correlated with remarkable hindlimb motor function and bladder improvement in SCI rats.

CONCLUSIONS

The specific EGFR inhibitor PD168393 can ameliorate excessive reactive astrogliosis and facilitate a more favorable environment for axonal regeneration after SCI. As such, EGFR inhibitor may be a promising therapeutic intervention in CNS injury.

Functional regeneration beyond the glial scar

Astrocytes react to CNS injury by building a dense wall of filamentous processes around the lesion. Stromal cells quickly take up residence in the lesion core and synthesize connective tissue elements that contribute to fibrosis. Oligodendrocyte precursor cells proliferate within the lesion and entrap dystrophic axon tips. Here we review evidence that this aggregate scar acts as the major barrier to regeneration of axons after injury. We also consider several exciting new interventions that allow axons to regenerate beyond the glial scar, and discuss the implications of this work for the future of regeneration biology.

Chondroitin sulfate proteoglycans potently inhibit invasion and serve as a central organizer of the brain tumor microenvironment

Glioblastoma (GBM) remains the most pervasive and lethal of all brain malignancies. One factor that contributes to this poor prognosis is the highly invasive character of the tumor. GBM is characterized by microscopic infiltration of tumor cells throughout the brain, whereas non-neural metastases, as well as select lower grade gliomas, develop as self-contained and clearly delineated lesions. Illustrated by rodent xenograft tumor models as well as pathological human patient specimens, we present evidence that one fundamental switch between these two distinct pathologies--invasion and noninvasion--is mediated through the tumor extracellular matrix. Specifically, noninvasive lesions are associated with a rich matrix containing substantial amounts of glycosylated chondroitin sulfate proteoglycans (CSPGs), whereas glycosylated CSPGs are essentially absent from diffusely infiltrating tumors. CSPGs, acting as central organizers of the tumor microenvironment, dramatically influence resident reactive astrocytes, inducing their exodus from the tumor mass and the resultant encapsulation of noninvasive lesions. Additionally, CSPGs induce activation of tumor-associated microglia. We demonstrate that the astrogliotic capsule can directly inhibit tumor invasion, and its absence from GBM presents an environment favorable to diffuse infiltration. We also identify the leukocyte common antigen-related phosphatase receptor (PTPRF) as a putative intermediary between extracellular glycosylated CSPGs and noninvasive tumor cells. In all, we present CSPGs as critical regulators of brain tumor histopathology and help to clarify the role of the tumor microenvironment in brain tumor invasion.

Overcoming neurite-inhibitory chondroitin sulfate proteoglycans in the astrocyte matrix

Acute trauma to the central nervous system (CNS) can result in permanent damage and loss of function related to the poor regeneration of injured axons. Injured axons encounter several barriers to regeneration, such as the glial scar at the injury site. The glial scar contains extracellular matrix (ECM) macromolecules deposited by reactive astrocytes in response to injury. The scar ECM is rich in chondroitin sulfate proteoglycans (CSPGs), macromolecules that inhibit axonal growth. CSPGs consist of a core protein with attachment sites for glycosaminoglycan (GAG) chains. An extensive literature demonstrates that enzymatic removal of the GAG chains by chondroitinase ABC permits some axonal regrowth; however, the remaining intact core proteins also possess inhibitory domains. Because metalloproteinases can degrade core proteins of CSPGs, we have evaluated five matrix metalloproteinases (MMPs) and a related protease-a disintegrin and metalloproteinase with thrombospondin motifs-4 (ADAMTS-4)-for their capacity to overcome CSPG inhibition of neuritic growth in culture. The metalloproteinases were selected for their known expression after CNS injuries. Of the MMPs, MMP-3, -7 and -8 reduced or abolished inhibition of neurite outgrowth on a purified CSPG substrate and on an astrocyte-derived ECM. ADAMTS-4 also attenuated CSPG inhibition of neurites and had the additional benefits of neither degrading laminin nor causing neurotoxicity. The efficacy of ADAMTS-4 matched that of blocking the EGFR signaling previously reported to mediate CSPG inhibition. These findings highlight ADAMTS-4 as a superior protease for overcoming CSPG inhibition of axonal regeneration in the CNS.

Comparison of sensory neuron growth cone and filopodial responses to structurally diverse aggrecan variants, in vitro

Following spinal cord injury, a regenerating neurite encounters a glial scar enriched in chondroitin sulfate proteoglycans (CSPGs), which presents a major barrier. There are two points at which a neurite makes contact with glial scar CSPGs: initially, filopodia surrounding the growth cone extend and make contact with CSPGs, then the peripheral domain of the entire growth cone makes CSPG contact. Aggrecan is a CSPG commonly used to model the effect CSPGs have on elongating or regenerating neurites. In this study, we investigated filopodia and growth cone responses to contact with structurally diverse aggrecan variants using the common stripe assay. Using time-lapse imaging with 15-s intervals, we measured growth cone area, growth cone width, growth cone length, filopodia number, total filopodia length, and the length of the longest filopodia following contact with aggrecan. Responses were measured after both filopodia and growth cone contact with five different preparations of aggrecan: two forms of aggrecan derived from bovine articular cartilage (purified and prepared using different techniques), recombinant aggrecan lacking chondroitin sulfate side chains (produced in CHO-745 cells) and two additional recombinant aggrecan preparations with varying lengths of chondroitin sulfate side chains (produced in CHO-K1 and COS-7 cells). Responses in filopodia and growth cone behavior differed between the structurally diverse aggrecan variants. Mutant CHO-745 aggrecan (lacking chondroitin sulfate chains) permitted extensive growth across the PG stripe. Filopodia contact with the CHO-745 aggrecan caused a significant increase in growth cone width and filopodia length (112.7% ± 4.9 and 150.9% ± 7.2 respectively, p<0.05), and subsequently upon growth cone contact, growth cone width remained elevated along with a reduction in filopodia number (121.9% ± 4.2; 72.39% ± 6.4, p<0.05). COS-7 derived aggrecan inhibited neurite outgrowth following growth cone contact. Filopodia contact produced an increase in growth cone area and width (126.5% ± 8.1; 150.3% ± 13.31, p<0.001), and while these parameters returned to baseline upon growth cone contact, a reduction in filopodia number and length was observed (73.94% ± 5.8, 75.3% ± 6.2, p<0.05). CHO-K1 derived aggrecan inhibited neurite outgrowth following filopodia contact, and caused an increase in growth cone area and length (157.6% ± 6.2; 117.0% ± 2.8, p<0.001). Interestingly, the two bovine articular cartilage aggrecan preparations differed in their effects on neurite outgrowth. The proprietary aggrecan (BA I, Sigma-Aldrich) inhibited neurites at the point of growth cone contact, while our chemically purified aggrecan (BA II) inhibited neurite outgrowth at the point of filopodia contact. BA I caused a reduction in growth cone width following filopodia contact (91.7% ± 2.5, p<0.05). Upon growth cone contact, there was a further reduction in growth cone width and area (66.4% ± 2.2; 75.6% ± 2.9; p<0.05), as well as reductions in filopodia number, total length, and max length (75.9% ± 5.7, p<0.05; 68.8% ± 6.0; 69.6% ± 3.5, p<0.001). Upon filopodia contact, BA II caused a significant increase in growth cone area, and reductions in filopodia number and total filopodia length (115.9% ± 5.4, p<0.05; 72.5% ± 2.7; 77.7% ± 3.2, p<0.001). In addition, filopodia contact with BA I caused a significant reduction in growth cone velocity (38.6 nm/s ± 1.3 before contact, 17.1 nm/s ± 3.6 after contact). These data showed that neuron morphology and behavior are differentially dependent upon aggrecan structure. Furthermore, the behavioral changes associated with the approaching growth cone may be predictive of inhibition or growth.

Stretching morphogenesis of the roof plate and formation of the central canal

Background

Neurulation is driven by apical constriction of actomyosin cytoskeleton resulting in conversion of the primitive lumen into the central canal in a mechanism driven by F-actin constriction, cell overcrowding and buildup of axonal tracts. The roof plate of the neural tube acts as the dorsal morphogenetic center and boundary preventing midline crossing by neural cells and axons.

Methodology/Principal Findings

The roof plate zebrafish transgenics expressing cytosolic GFP were used to study and describe development of this structure in vivo for a first time ever. The conversion of the primitive lumen into the central canal causes significant morphogenetic changes of neuroepithelial cells in the dorsal neural tube. We demonstrated that the roof plate cells stretch along the D–V axis in parallel with conversion of the primitive lumen into central canal and its ventral displacement. Importantly, the stretching of the roof plate is well-coordinated along the whole spinal cord and the roof plate cells extend 3× in length to cover 2/3 of the neural tube diameter. This process involves the visco-elastic extension of the roof place cytoskeleton and depends on activity of Zic6 and the Rho-associated kinase (Rock). In contrast, stretching of the floor plate is much less extensive.

Conclusions/Significance

The extension of the roof plate requires its attachment to the apical complex of proteins at the surface of the central canal, which depends on activity of Zic6 and Rock. The D–V extension of the roof plate may change a range and distribution of morphogens it produces. The resistance of the roof plate cytoskeleton attenuates ventral displacement of the central canal in illustration of the novel mechanical role of the roof plate during development of the body axis.

Corneal sulfated glycosaminoglycans and their effects on trigeminal nerve growth cone behavior in vitro: roles for ECM in cornea innervation

PURPOSE

Sensory trigeminal nerve growth cones innervate the cornea in a highly coordinated fashion. The purpose of this study was to determine if extracellular matrix glycosaminoglycans (ECM-GAGs), including keratan sulfate (KS), dermatan sulfate (DS), and chondroitin sulfate A (CSA) and C (CSC), polymerized in developing eyefronts, may provide guidance cues to nerves during cornea innervation.

METHODS

Immunostaining using antineuron-specific-β-tubulin and monoclonal antibodies for KS, DS, and CSA/C was performed on eyefronts from embryonic day (E) 9 to E14 and staining visualized by confocal microscopy. Effects of purified GAGs on trigeminal nerve growth cone behavior were tested using in vitro neuronal explant cultures.

RESULTS

At E9 to E10, nerves exiting the pericorneal nerve ring grew as tight fascicles, advancing straight toward the corneal stroma. In contrast, upon entering the stroma, nerves bifurcated repeatedly as they extended anteriorly toward the epithelium. KS was localized in the path of trigeminal nerves, whereas DS and CSA/C-rich areas were avoided by growth cones. When E10 trigeminal neurons were cultured on different substrates comprised of purified GAG molecules, their neurite growth cone behavior varied depending on GAG type, concentration, and mode of presentation (immobilized versus soluble). High concentrations of immobilized KS, DS, and CSA/C inhibited neurite growth to varying degrees. Neurites traversing lower, permissive concentrations of immobilized DS and CSA/C displayed increased fasciculation and decreased branching, whereas KS caused decreased fasciculation and increased branching. Enzymatic digestion of sulfated GAGs canceled their effects on trigeminal neurons.

CONCLUSIONS

Data herein suggest that GAGs may direct the movement of trigeminal nerve growth cones innervating the cornea.

Toward libraries of biotinylated chondroitin sulfate analogues: from synthesis to in vivo studies

Chondroitin sulfate-E (CS-E) oligosaccharidic analogues (di to hexa) were prepared from lactose. In these compounds, the 2-acetamido group was replaced by a hydroxyl group. This modification speeded up the synthesis, and large oligosaccharides were constructed in a few steps from a lactose-originated block. The protecting groups used were as follows; Fmoc for hydroxyl groups to be glycosylated, allyl group for anomeric position protection, and trichoroacetimidate leaving groups were used to prepare up to octasaccharides. We took advantage of the presence of allyl group to develop a click biotinylation, through its transformation into a 3-azido-2-hydroxyl propyl group in two steps (epoxidation and sodium azide epoxide opening). The biotinylating agent was a water-soluble propargylated and biotinylated triethylene glycol (PEG). By using surface plasmon resonance (SPR), it was shown that the di-, tetra-, and hexasaccharides display a binding affinity and selectivity toward HSF/GSF and CXCL12 similar to that of CS-E. A parallel study confirmed their mimicry of natural compounds, based on the hexasaccharide interaction with Otx2, a homeodomain protein involved in brain maturation, thus validating our simplification approach to synthesize bioactive GAG.

The subcommissural organ and the development of the posterior commissure

Growing axons navigate through the developing brain by means of axon guidance molecules. Intermediate targets producing such signal molecules are used as guideposts to find distal targets. Glial, and sometimes neuronal, midline structures represent intermediate targets when axons cross the midline to reach the contralateral hemisphere. The subcommissural organ (SCO), a specialized neuroepithelium located at the dorsal midline underneath the posterior commissure, releases SCO-spondin, a large glycoprotein belonging to the thrombospondin superfamily that shares molecular domains with axonal pathfinding molecules. Several evidences suggest that the SCO could be involved in the development of the PC. First, both structures display a close spatiotemporal relationship. Second, certain mutants lacking an SCO present an abnormal PC. Third, some axonal guidance molecules are expressed by SCO cells. Finally, SCO cells, the Reissner's fiber (the aggregated form of SCO-spondin), or synthetic peptides from SCO-spondin affect the neurite outgrowth or neuronal aggregation in vitro.

Glial scar size, inhibitor concentration, and growth of regenerating axons after spinal cord transection

A mathematical model has been formulated in accordance with cell chemotaxis and relevant experimental data. A three-dimensional lattice Boltzmann method was used for numerical simulation. The present study observed the effects of glial scar size and inhibitor concentration on regenerative axonal growth following spinal cord transection. The simulation test comprised two parts: (1) when release rates of growth inhibitor and promoter were constant, the effects of glial scar size on axonal growth rate were analyzed, and concentrations of inhibitor and promoters located at the moving growth cones were recorded. (2) When the glial scar size was constant, the effects of inhibitor and promoter release rates on axonal growth rate were analyzed, and inhibitor and promoter concentrations at the moving growth cones were recorded. Results demonstrated that (1) a larger glial scar and a higher release rate of inhibitor resulted in a reduced axonal growth rate. (2) The axonal growth rate depended on the ratio of inhibitor to promoter concentrations at the growth cones. When the average ratio was < 1.5, regenerating axons were able to grow and successfully contact target cells.

Pioneering studies on the mechanisms of axonal regeneration

While ultimately, focus must be placed on experimentation using adult systems, vastly important clues to regeneration can be found in the study of the embryonic nervous system. In embryonic systems, axonal regeneration is successful before a critical period, and numerous advances have resulted from the study of isolated cells and tissues in vitro. Studies over many decades from the laboratory of Paul C. Letourneau have probed the cellular and molecular phenomena involved in axon outgrowth and guidance in the embryonic central and peripheral nervous system and have laid the framework for many current advances in regeneration research. Letourneau’s pioneering work related to growth cone behavior, guidance, and regeneration has resulted in considerable contributions toward our understanding not only of cellular mechanisms that underlie axon growth, but also of the specific areas of study that require attention to accomplish future breakthroughs. The present article summarizes some of the major contributions from Paul Letourneau and his team in the area of axonal regeneration.

The role of well-defined patterned substrata on the regeneration of DRG neuron pathfinding and integrin expression dynamics using chondroitin sulfate proteoglycans

Injured neurons intrinsically adapt to and partially overcome inhibitory proteoglycan expression in the central nervous system by upregulating integrin expression. It remains unclear however, to what extent varying proteoglycan concentrations influence the strength of this response, how rapidly neurons adapt to proteoglycans, and how pathfinding dynamics are altered over time as integrin expression is modulated in response to proteoglycan signals. To investigate these quandaries, we created well-defined substrata in which postnatal DRG neuron pathfinding dynamics and growth cone integrin expression were interrogated as a function of proteoglycan substrata density. DRGs responded by upregulating integrin expression in a proteoglycan dose dependent fashion and exhibited robust outgrowth over all proteoglycan densities at initial time frames. However, after prolonged proteoglycan exposure, neurons exhibited decreasing velocities associated with increasing proteoglycan densities, while neurons growing on low proteoglycan levels exhibited robust outgrowth at all time points. Additionally, DRG outgrowth over proteoglycan density step boundaries, and a brief β1 integrin functional block proved that regeneration was integrin dependent and that DRGs exhibit delayed slowing and loss in persistence after even transient encounters with dense proteoglycan boundaries. These findings demonstrate the complexity of proteoglycan regulation on integrin expression and regenerative pathfinding.

Keratan sulfate restricts neural plasticity after spinal cord injury

Chondroitin sulfate (CS) proteoglycans are strong inhibitors of structural rearrangement after injuries of the adult CNS. In addition to CS chains, keratan sulfate (KS) chains are also covalently attached to some proteoglycans. CS and KS sometimes share the same core protein, but exist as independent sugar chains. However, the biological significance of KS remains elusive. Here, we addressed the question of whether KS is involved in plasticity after spinal cord injury. Keratanase II (K-II) specifically degraded KS, i.e., not CS, in vivo. This enzyme digestion promoted the recovery of motor and sensory function after spinal cord injury in rats. Consistent with this, axonal regeneration/sprouting was enhanced in K-II-treated rats. K-II and the CS-degrading enzyme chondroitinase ABC exerted comparable effects in vivo and in vitro. However, these two enzymes worked neither additively nor synergistically. These data and further in vitro studies involving artificial proteoglycans (KS/CS-albumin) and heat-denatured or reduced/alkylated proteoglycans suggested that all three components of the proteoglycan moiety, i.e., the core protein, CS chains, and KS chains, were required for the inhibitory activity of proteoglycans. We conclude that KS is essential for, and has an impact comparable to that of CS on, postinjury plasticity. Our study also established that KS and CS are independent requirements for the proteoglycan-mediated inhibition of axonal regeneration/sprouting.

The extracellular matrix molecule tenascin C modulates expression levels and territories of key patterning genes during spinal cord astrocyte specification

The generation of astrocytes during the development of the mammalian spinal cord is poorly understood. Here, we demonstrate for the first time that the extracellular matrix glycoprotein tenascin C regulates the expression of key patterning genes during late embryonic spinal cord development, leading to a timely maturation of gliogenic neural precursor cells. We first show that tenascin C is expressed by gliogenic neural precursor cells during late embryonic development. The loss of tenascin C leads to a sustained generation and delayed migration of Fgfr3-expressing immature astrocytes in vivo. Consistent with an increased generation of astroglial cells, we documented an increased number of GFAP-positive astrocytes at later stages. Mechanistically, we could demonstrate an upregulation and domain shift of the patterning genes Nkx6.1 and Nkx2.2 in vivo. In addition, sulfatase 1, a known downstream target of Nkx2.2 in the ventral spinal cord, was also upregulated. Sulfatase 1 regulates growth factor signalling by cleaving sulphate residues from heparan sulphate proteoglycans. Consistent with this function, we observed changes in both FGF2 and EGF responsiveness of spinal cord neural precursor cells. Taken together, our data implicate Tnc in the regulation of proliferation and lineage progression of astroglial progenitors in specific domains of the developing spinal cord.

Crystal structure of the Nogo-receptor-2

The inhibition of axon regeneration upon mechanical injury is dependent on interactions between Nogo receptors (NgRs) and their myelin-derived ligands. NgRs are composed of a leucine-rich repeat (LRR) region, thought to be structurally similar among the different isoforms of the receptor, and a divergent "stalk" region. It has been shown by others that the LRR and stalk regions of NgR1 and NgR2 have distinct roles in conferring binding affinity to the myelin associated glycoprotein (MAG) in vivo. Here, we show that purified recombinant full length NgR1 and NgR2 maintain significantly higher binding affinity for purified MAG as compared to the isolated LRR region of either NgR1 or NgR2. We also present the crystal structure of the LRR and part of the stalk regions of NgR2 and compare it to the previously reported NgR1 structure with respect to the distinct signaling properties of the two receptor isoforms.

Strategies for regenerating injured axons after spinal cord injury - insights from brain development

Axonal regeneration does not occur easily after an adult central nervous system (CNS) injury. Various attempts have partially succeeded in promoting axonal regeneration after the spinal cord injury (SCI). Interestingly, several recent therapeutic concepts have emerged from or been tightly linked to the researches on brain development. In a developing brain, remarkable and dynamic axonal elongation and sprouting occur even after the injury; this finding is essential to the development of a therapy for SCI. In this review, we overview the revealed mechanism of axonal tract formation and plasticity in the developing brain and compare the differences between a developing brain and a lesion site in an adult brain. One of the differences is that mature glial cells participate in the repair process in the case of adult injuries. Interestingly, these cells express inhibitory molecules that impede axonal regeneration such as myelin-associated proteins and the repulsive guidance molecules found originally in the developing brain for navigating axons to specific routes. Some reports have clearly elucidated that any treatment designed to suppress these inhibitory cues is beneficial for promoting regeneration and plasticity after an injury. Thus, understanding the developmental process will provide us with an important clue for designing therapeutic strategies for recovery from SCI.

Targeted downregulation of N-acetylgalactosamine 4-sulfate 6-O-sulfotransferase significantly mitigates chondroitin sulfate proteoglycan-mediated inhibition

Chondroitin sulfate-4,6 (CS-E) glycosaminoglycan (GAG) upregulation in astroglial scars is a major contributor to chondroitin sulfate proteoglycan (CSPG)-mediated inhibition [Gilbert et al. (2005) Mol Cell Neurosci 29:545–558]. However, the role of N-acetylgalactosamine 4-sulfate 6-O-sulfotransferase (GalNAc4S6ST) catalyzed sulfation of CS-E, and its contribution to CSPG-mediated inhibition of CNS regeneration remains to be fully elucidated. Here, we used in situ hybridization to show localized upregulation of GalNAc4S6ST mRNA after CNS injury. Using in vitro spot assays with immobilized CS-E, we demonstrate dose-dependent inhibition of rat embryonic day 18 (E18) cortical neurons. To determine whether selective downregulation of CS-E affected the overall inhibitory character of extracellular matrix produced by reactive astrocytes, single [against (chondroitin 4) sulfotransferase 11 (C4ST1) or GalNAc4S6ST mRNA] or double [against C4ST1 and GalNAc4S6ST mRNA] siRNA treatments were conducted and assayed using quantitative real-time polymerase chain reaction and high-performance liquid chromatography to confirm the specific downregulation of CS-4S GAG (CS-A) and CS-E. Spot and Bonhoeffer stripe assays using astrocyte-conditioned media from siRNA-treated rat astrocytes showed a significant decrease in inhibition of neuronal attachment and neurite extensions when compared with untreated and TGF-treated astrocytes. These findings reveal that selective attenuation of CS-E via siRNA targeting of GalNAc4S6ST significantly mitigates CSPG-mediated inhibition of neurons, potentially offering a novel intervention strategy for CNS injury.

Upregulation of EphA4 on astrocytes potentially mediates astrocytic gliosis after cortical lesion in the marmoset monkey

Glial scar formation occurs in response to brain injury in mammalian models and inhibits axonal growth. Identification of molecules that may mediate reactivity of astrocytes has become a leading therapeutic goal in the field of neurotrauma. In adult rodent brain and spinal cord, many of the Eph receptors and their ephrin ligands have been demonstrated to be upregulated on reactive astrocytes at the injury site; however, little is known about the expression of these molecules in nonhuman primate injury models. This study examines the role of the tyrosine kinase EphA4 receptor, which predominantly binds most ephrin ligands, after injury in marmoset monkey brain. Following lesioning of the primary visual cortex (V1) in the adult marmoset, EphA4 is strongly upregulated on reactive astrocytes around the lesion site, which secrete extracellular matrix molecules such as chondroitin sulfate proteoglycans, which are known for their inhibitory effect on axonal growth and regeneration. This astrocyte reactivity was also associated with neuronal death in the area adjacent to the lesion site. EphA4 activation induced by clustered ephrin A5-Fc-mediated astrocyte proliferation and glial fibrillary acidic protein expression in vitro, as demonstrated by closure of scratched wound and MTT assays, occurs via two potential signaling pathways, the mitogen-activated protein kinase and Rho pathways. These results in a nonhuman primate model highlight the importance of developing pharmacotherapeutic approaches to block these molecules following brain injury.

Engineered in vitro/in silico models to examine neurite target preference

Research on spinal cord injury (SCI) repair focuses on developing mechanisms to allow neurites to grow past an injury site. In this article, we observe that numerous divergent paths (i.e., spinal roots) are present along the spinal column, and hence guidance strategies must be devised to ensure that regrowing neurites reach viable targets. Therefore, we have engineered an in vitro micropatterned model in which cultured E7 dorsal root ganglia (DRG) explants may enter alternate pathways (?roots?) along a branching micropattern. Alongside this in vitro model, we have developed an in silico simulation that we validate by comparison with independent experiments. We find in both in silico and in vitro models that the probability of a neurite entering a given root decreases exponentially with respect to the number of roots away from the DRG; consequently, the likelihood of neurites reaching a distant root can be vanishingly small. This result represents a starting point for future strategies to optimize the likelihood that neurites will reach appropriate targets in the regenerating nervous system, and provides a new computational tool to evaluate the feasibility and expected success of neurite guidance in complex geometries.

Basal enrichment within neuroepithelia suggests novel function(s) for Celsr1 protein

A characteristic of the 7TM-cadherins, Flamingo and Celsr1, is their asymmetric protein distribution and polarized activity at neighboring epithelial cell interfaces along defined axes of planar cell polarity. Here, we describe a novel distribution of Celsr1 protein to the basal surface of neuroepithelial cells within both the early neural tube and a less well-defined group of ventricular zone cells at the midline of the developing spinal cord. Importantly, this basal enrichment is lost in embryos homozygous for a mutant Celsr1 allele. We also demonstrate an intimate association between basal enrichment of Celsr1 protein and dorsal sensory tract morphogenesis, an intriguing spatio-temporal organization of Celsr1 protein along the apico-basal neuroepithelial axis suggestive of multiple Celsr1 protein isoforms and the existence of distinct cell surface Celsr1 protein species with direct signaling potential. Together, these data raise compelling new questions concerning the role of Celsr1 during neural development.