Extracellular matrix and neuronal movement

During brain development, both neuronal migration and axon guidance are influenced by extracellular matrix molecules present in the environment of the migrating neuronal cell bodies and nerve fibers. Glial laminin is an extracellular matrix protein which these early brain cells preferentially attach to. Extracellular glycosaminoglycans are suggested to function in restricting neuronal cell bodies and axons from certain brain areas. Since laminin is deposited along the radial glial fibers and along the developing nerve pathways in punctate form, the punctate assemblies may be one of the key factors in routing the developing neurons in vivo. This review discusses the role of laminin in neuronal movement given the present concept of the extracellular matrix molecules and their proposed interactions.     

Mechanisms of glial-guided neuronal migration in vitro and in vivo

Summary Our laboratory has developed an in vitro model system in which glial-guided neuronal migration can be observed in real time. Cerebellar granule neurons migrate on astroglial fibers by apposing their cell soma against the glial arm, forming a specialized migration junction, and extending a motile leading process in the direction of migration. In vitro assays indicate that the neuronal antigen astrotactin functions as a neuron-glia ligand, and is likely to play a role in the movement of neurons along glial fibers. In heterotypic recombinations of neurons and glia from mouse cerebellum and rat hippocampus, neurons migrate on heterotypic glial processes with a cytology, speed and mode of movement identical to that of neuronal migration on homotypic glial fibers, suggesting that glial fibers provide a permissive pathway for neuronal migration in developing brain. In vivo analyses of developing cerebellum demonstrate a close coordination of afferent axon ingrowth relative to target cell migration. These studies indicate that climbing fibers contact immature Purkinje neurons during the migration and settling of Purkinje cells, implicating a role for afferents in the termination of migration.     

Mechanisms of glial-guided neuronal migration in vitro and in vivo

Our laboratory has developed an in vitro model system in which glial-guided neuronal migration can be observed in real time. Cerebellar granule neurons migrate on astroglial fibers by apposing their cell soma against the glial arm, forming a specialized migration junction, and extending a motile leading process in the direction of migration. In vitro assays indicate that the neuronal antigen astrotactin functions as a neuron-glia ligand, and is likely to play a role in the movement of neurons along glial fibers. In heterotypic recombinations of neurons and glia from mouse cerebellum and rat hippocampus, neurons migrate on heterotypic glial processes with a cytology, speed and mode of movement identical to that of neuronal migration on homotypic glial fibers, suggesting that glial fibers provide a permissive pathway for neuronal migration in developing brain. In vivo analyses of developing cerebellum demonstrate a close coordination of afferent axon ingrowth relative to target cell migration. These studies indicate that climbing fibers contact immature Purkinje neurons during the migration and settling of Purkinje cells, implicating a role for afferents in the termination of migration.     

Extracellular matrix components in intestinal development

Intestinal morphogenesis and differentiation are dependent on heterotypic cell interactions between embryonic epithelial cells (endoderm) and stromal cells (mesenchyme). Extracellular matrix molecules represent attractive candidates for regulators of these interactions. The structural and functional diversity of the extracellular matrix as intestinal development proceeds is demonstrated by 1) spatio-temporal specific expression of the classically described constituents, 2) the finding of laminin and collagen IV variants, 3) changes in the ratio of individual constituent chains, and 4) a stage-specific regulation of basement membrane molecule production, in particular by glucocorticoids. The orientation/assembly of these extracellular matrix molecules could direct precise cellular functions through interactions via integrin molecules. The involvement of extracellular matrix, and in particular basement membrane molecules in heterotypic cell interactions leading to epithelial cell differentiation, has been highlighted by the use of experimental models such as cocultures, hybrid intestines and antisense approaches. These models allowed us to conclude that a correct elaboration and assembly of the basement membrane, following close contacts between epithelial and fibroblastic cells, is necessary for the expression of differentiation markers such as digestive enzymes.     

Laminins during muscle development and in muscular dystrophies

Cellular interactions with the extracellular matrix during muscle formation and in muscular dystrophy have received increased interest during the past years. Laminins constitute a growing family of proteins with complex expression patterns in forming basement membranes during muscle development. In skeletal muscle, laminins constitute major ligands for cell surface receptors involved in the transmission of force from the cell interior, but laminins might also influence signal transmission events during muscle formation and in muscle regeneration. During myogenesis the laminin alpha1 chain is present around the epithelial somite; but later, in forming muscle, the laminin alpha1 chain is restricted to the myotendinous junction. The laminin alpha2, alpha4 and alpha5 chains are major laminin chains in the muscle basement membrane during muscle formation, but laminin alpha4 and alpha5 chains are absent in adult muscle. The importance of laminins for muscle integrity is manifested in congenital muscular dystrophies with defects in the laminin alpha2 chain. There is no good evidence for the presence of laminin alpha1 chain in dystrophic muscle, but some other fetal muscle laminins can be detected in dystrophic muscle. Characterization of laminin expression patterns in muscular dystrophies might be of diagnostic and therapeutic value. In this paper, we review the recent publications on the biological functions of muscle laminins and discuss their roles in skeletal muscle.     

Extracellular matrix components associated with remodeling processes in brain

In the central nervous system, various extracellular matrix components have been identified which are strongly expressed during development and in most areas of the brain down-regulated during maturation. Examples are tenascin-C, neurocan and hyaluronan. While tenascin-C is well known to be associated with morphogenic events and the active contribution of hyaluronan to various physiological processes is increasingly acknowledged, neurocan belongs to a class of molecules thought to be generally more associated with barrier functions: chondroitin sulfate proteoglycans. Consideration of these and related molecules and their processing in the context of the general organization of the brain extracellular matrix, their changes during brain maturation and their implication in different types of remodeling processes in adult brain, like normal and pathological synaptic plasticity, inflammatory and dementia-associated diseases and gliomas, may indicate that components of the extracellular matrix could provide valuable early information about the pathological state of the brain.Received 29 January 2004; received after revision 25 March 2004; accepted 2 April 2004     

Immunofluorescent localization of collagen types I,III, IV,V, fibronectin,laminin, entactin,and heparan sulphate proteoglycan in human immature placenta

The distribution of eight components of the extracellular matrix in immature human placenta was studied by an indirect immunofluorescence method with monospecific antibodies. In the stroma of the term chorionic villi, collagen types I, III, IV, V, and fibronectin formed a mesh of fibers and conglomerates. Heparan sulphate proteoglycan formed multiple conglomerates, whereas laminin comprised small, scanty, discrete granules. Collagen type IV, laminin, entactin, and heparan sulphate proteoglycan were confined to the basement membrane of the trophoblast. Sometimes, only collagen type IV was identified in fetal vascular basement membrane.     

Lecticans: organizers of the brain extracellular matrix

Lecticans are a family of chondroitin sulfate proteoglycans, encompassing aggrecan, versican, neurocan and brevican. These proteoglycans are characterized by the presence of ahyaluronan-binding domain and a C-type lectin domain in their core proteins. Through these domains, lecticans interact with carbohydrate and protein ligands in the extracellular matrix and act as linkers of these extracellular matrix molecules. In adult brain, lecticans are thought to interact with hyaluronan and tenascin-R to form a ternary complex. We propose that the hyaluronan-lectican-tenascin-R complex constitutes the core assembly of the adult brain extracellular matrix, which is found mainly in pericellular spaces of neurons as ‘perineuronal nets’. Received 27 September 1999; accepted 26 October 1999     

Role of laminin-nidogen complexes in basement membrane formation during embryonic development

Laminin and nidogen (entactin) are major glycoprotein components of basement membranes. At least seven different isoforms of laminin have been identified. Laminin and nidogen form high affinity complexes in basement membranes by specific binding between the laminin γ1 chain and the G3 globule of nidogen. Additional interactions between nidogen and collagen IV, perlecan and other basement membrane components result in the formation of ternary complexes between these matrix components. Nidogen is highly susceptible to proteolytic cleavage, and binding to laminin protects nidogen from degradation. Nidogen is considered to have a crucial role as a link protein in the assembly of basement membranes. Basement membrane components are synthesized at high levels during tissue growth and development, and sites of morphogenesis correlate with localized remodelling of basement membranes. The formation of distinct basement membrane matrices in the developing embryo is influenced by the laminin isoforms produced and by whether laminin and nidogen are co-expressed and secreted as a complex or are produced by cooperation between two cell layers. The potential roles of laminin-nidogen complexes, cell-matrix interactions, and other intermolecular interactions within the matrix in basement membrane assembly and stability are discussed in this review.     

The extracellular matrix of the hematopoietic microenvironment

The bone marrow microenvironment plays an important role in promoting hematopoietic progenitor cell proliferation and differentiation and the controlled egress of these developing hematopoietic cells. The establishment of long-term bone marrow cultures, which are thought to mimic hematopoiesis in vitro, and various stromal cell lines has greatly facilitated the analysis of the functions of this microenvironment. Extracellular matrix (ECM) molecules of all three categories (collagens, proteoglycans and glycoproteins) have been identified as part of this microenvironment and have been shown to be involved in, different biological functions such as cell adhesion and anti-adhesion, binding and presentation of various cytokines and regulation of cell growth. It is suggested that these matrix molecules in combination with cytokines are crucial for compartmentalization of the bone marrow. Although many cell adhesion molecules have been characterized on the surface of hematopoietic progenitor cells, the nature of cellular receptors for the ECM components is less well defined. During leukemia, many immature blood cells are released from bone marrow, but it is not yet known whether these abnormalities in hematopoiesis are also caused by an altered microenvironment or altered composition of its extracellular matrix. The elucidation of the involvement of specific ECM-isoforms and as yet not characterized ECM components and their receptors in the bone marrow will certainly help towards a better understanding of these phenomena.     

Netrins and netrin receptorsRID="†"ID="†" Review

The formation of precise connections between neurons and their targets during development is dependent on extracellular guidance cues that allow growing axons to navigate to their targets. One family of such guidance molecules. conserved across all species examined, is that of the netrin/UNC-6 proteins. Netrins act to both attract and repel the growing axons of a broad range of neuronal cell types during development and are also involved in controling neuronal cell migration. These actions are mediated by specific receptor complexes containing either the colorectal cancer (DCC) or neogenin protein, in the case of the attractive receptor, or UNC-5-related proteins, in the case of the repellent receptor. Recent work has identified a key role for intracellular cyclic nucleotide levels in regulating the nature of the response of the growing axon to netrins as either attractive or repulsive. Netrin-DCC signaling has also been shown to regulate cell death in epithelial cells in vitro, raising the interesting possibility that netrins may also regulate cell death in the developing nervous system.     

Immunoreactive luteinizing hormone-containing neurons in the brain of the white-footed mouse, Peromyscus leucopus

The distribution of immunoreactive LH in the brain of the white-footed mouse (Peromyscus leucopus) was determined using immunocytochemical procedures. Immunoreactive fibers are located in the hypothalamus, preoptic area, septum and amygdala. Stained cell bodies are seen in the arcuate nucleus and preoptic area. Gonadectomy enhances staining for LH in the brain.     

Cell lineage and cell migration in the developing cerebral cortex

Summary Modern techniques which trace lineages of individual progenitor cells have provided some clues about the processes that determine cell fate in the brain, and have also given us some information about migratory patterns of clonally related cells. In many parts of the central nervous system, progenitors are multipotent; single clones can contain multiple neuronal types or even mixtures of neurons and glia. In addition, one can observe a wide distribution in clone size, even when marking is done in a narrow time window. This suggests that progenitor cells may be fairly plastic and responsive to environmental signals. In the developing cortex, clonally related cells are initially grouped near each other, as in the retina and tectum. However, the subsequent migration of these cells from the ventricular zone to the cortex along glial fibers is accompanied by a progressive dispersion of clonally related neurons.     

Laminin: loss-of-function studies

Laminin, one of the most widely expressed extracellular matrix proteins, exerts many important functions in multiple organs/systems and at various developmental stages. Although its critical roles in embryonic development have been demonstrated, laminin’s functions at later stages remain largely unknown, mainly due to its intrinsic complexity and lack of research tools (most laminin mutants are embryonic lethal). With the advance of genetic and molecular techniques, many new laminin mutants have been generated recently. These new mutants usually have a longer lifespan and show previously unidentified phenotypes. Not only do these studies suggest novel functions of laminin, but also they provide invaluable animal models that allow investigation of laminin’s functions at late stages. Here, I first briefly introduce the nomenclature, structure, and biochemistry of laminin in general. Next, all the loss-of-function mutants/models for each laminin chain are discussed and their phenotypes compared. I hope to provide a comprehensive review on laminin functions and its loss-of-function models, which could serve as a reference for future research in this understudied field.     

Immunoreactive luteinizing hormone-containing neurons in the brain of the white-footed mouse,Peromyscus leucopus

Summary The distribution of immunoreactive LH in the brain of the white-footed mouse (Peromyscus leucopus) was determined using immunocytochemical procedures. Immunoreactive fibers are located in the hypothalamus, preoptic area, septum and amygdala. Stained cell bodies are seen in the arcuate nucleus and preoptic area. Gonadectomy enhances staining for LH in the brain.We wish to thank the National Institutes of Arthritis, Metabolism, and Digestive Diseases (NIAMDD) for the gift of the antiserum to LH.     

Comparative ultrastructural and cytochemical analysis of the cephalopod systemic heart and its innervation

Summary The present knowledge of the morphology of cephalopod central hearts is presented. The cytological characteristics of the epicardial, myocardial and endothelial tissue layers are reviewed. Myocardial cells are characterized as obliquely striated myocytes with a high level of oxidative metabolism. The voluminous myocardium is intensively penetrated by nerve fibers controlling the myogenic heart rhythm by different chemical transmitter systems. Catecholaminergic fluorophores and acetylcholinesterase activity could be localized by means of histochemical and cytochemical investigations. A glio-interstitial cell system is shown to be present in connection with nerve fibers and also uncombined between heart muscle cells. Its content of large different-sized inclusions is described and their function discussed.     

Neurocan: a brain chondroitin sulfate proteoglycan

Neurocan is a chondroitin sulfate proteoglycan of the lectican family and a component of the extracellular matrix of the central nervous system. It is mainly expressed during modeling and remodeling stages of this tissue. Neurocan can bind to various structural extracellular matrix components, such as hyaluronan, heparin, tenascin-C and tenascin-R, and the growth and mobility factors FGF-2, HB-GAM, and amphoterin. Neurocan can also interact with several cell surface molecules, such as N-CAM, L1/Ng-CAM, TAG-1/axonin-1, and an N-cadherin-binding N-acetyl-galactosamine-phosphoryl-transferase, and in vitro studies have shown that neurocan is able to modulate the cell-binding and neurite outgrowth promoting activites of these molecules. Current analysis of the molecular structures and substructures involved in homophilic and heterophilic interactions of these molecules and complementary loss-of-function mutations might shed some light on the roles played by neurocan and interacting molecules in the fine tuning of the nervous system.     

Collagens in the developing and diseased nervous system

Collagens are extracellular proteins characterized by a structure in triple helices. There are 28 collagen types which differ in size, structure and function. Their architectural and functional roles in connective tissues have been widely assessed. In the nervous system, collagens are rare in the vicinity of the neuronal soma, occupying mostly a “marginal” position, such as the meninges, the basement membranes and the sensory end organs. In neural development, however, where various ECM molecules are known to be determinant, recent studies indicate that collagens are no exception, participating in axonal guidance, synaptogenesis and Schwann cell differentiation. Insights on collagens function in the brain have also been derived from neural pathophysiological conditions. This review summarizes the significant advances which underscore the function and importance of collagens in the nervous system. Received 09 September 2008; received after revision 24 October 2008; accepted 28 October 2008     

Nitric oxide stimulates human neural progenitor cell migration via cGMP-mediated signal transduction

Neuronal migration is one of the most critical processes during early brain development. The gaseous messenger nitric oxide (NO) has been shown to modulate neuronal and glial migration in various experimental models. Here, we analyze a potential role for NO signaling in the migration of fetal human neural progenitor cells. Cells migrate out of cultured neurospheres and differentiate into both neuronal and glial cells. The neurosphere cultures express neuronal nitric oxide synthase and soluble guanylyl cyclase that produces cGMP upon activation with NO. By employing small bioactive enzyme activators and inhibitors in both gain and loss of function experiments, we show NO/cGMP signaling as a positive regulator of migration in neurosphere cultures of early developing human brain cells. Since NO signaling regulates cell movements from developing insects to mammalian nervous systems, this transduction pathway may have evolutionary conserved functions.