Mechanisms of astrocyte-directed neurite guidance

Astrocytes have recently become better recognized as playing vital roles in regulating the patterning of central nervous system neurites during development and following injury. In general, astrocytes have been shown to be supportive of neurite extension, but alterations in the biochemical properties of astrocytes in particular areas during development and in gliotic tissue may act to confine neurite outgrowth and thus provide guidance cues. In vivo studies indicate that restrictive astrocytes function through their altered expression of specific extracellular matrix molecules, including tenascin, chondroitin, and keratan sulfate proteoglycans. In addition, several in vitro models suggest that other cell surface molecules are utilized by restrictive astrocytes to direct neurite trajectories. Received: 5 May 1997 / Accepted: 6 June 1997     

Identification of extracellular matrix ligands for the heparan sulfate proteoglycan agrin.

Agrin is a major brain heparan sulfate proteoglycan which is expressed in nearly all basal laminae and in early axonal pathways of the developing central nervous system. To further understand agrin's function during nervous system development, we have examined agrin's ability to interact with several heparin-binding extracellular matrix proteins. Our data show that agrin binds FGF-2 and thrombospondin by a heparan sulfate-dependent mechanism, merosin and laminin by both heparan sulfate-dependent and -independent mechanisms, and tenascin solely via agrin's protein core. Furthermore, agrin's heparan sulfate side chains encode a specificity in interactions with heparin-binding molecules since fibronectin and the cell adhesion molecule L1 do not bind agrin. Surface plasmon resonance studies (BIAcore) reveal a high affinity for agrin's interaction with FGF-2 and merosin (2.5 and 1.8 nM, respectively). Demonstrating a biological significance for these interactions, FGF-2, laminin, and tenascin copurify with immunopurified agrin and immunohistochemistry reveals a partial codistribution of agrin and its ECM ligands in the chick developing visual system. These studies and our previous studies, showing that merosin and NCAM also colocalize with agrin, provide evidence that agrin plays a crucial role in the function of the extracellular matrix and suggest a role for agrin in axon pathway development.     

Extracellular matrix and the neural stem cell niche

Basal lamina is present in many stem cell niches, but we still have a poor understanding of the role of this and other extracellular matrix (ECM) components. Here, we review current knowledge regarding ECM expression and function in the neural stem cell niche, focusing on the subependymal zone of the adult CNS. An increasing complexity of ECM molecules has been described, and a number of receptors expressed on the stem cells identified. Experiments perturbing the niche using genetics or cytotoxic ablation of the rapidly dividing precursors, or using explant culture models to examine specific growth factors, have been influential in showing how changes in these ECM receptors might regulate neural stem cell behavior. However the role of changes in the matrix itself remains to be determined. The answers will be important, as they will point to the molecules required to engineer niches ex-vivo so as to provide tools for regenerative neuroscience.     

Glia unglued: how signals from the extracellular matrix regulate the development of myelinating glia

The health and function of the nervous system relies on glial cells that ensheath neuronal axons with a specialized plasma membrane termed myelin. The molecular mechanisms by which glial cells target and enwrap axons with myelin are only beginning to be elucidated, yet several studies have implicated extracellular matrix proteins and their receptors as being important extrinsic regulators. This review provides an overview of the extracellular matrix proteins and their receptors that regulate multiple steps in the cellular development of Schwann cells and oligodendrocytes, the myelinating glia of the PNS and CNS, respectively, as well as in the construction and maintenance of the myelin sheath itself. The first part describes the relevant cellular events that are influenced by particular extracellular matrix proteins and receptors, including laminins, collagens, integrins, and dystroglycan. The second part describes the signaling pathways and effector molecules that have been demonstrated to be downstream of Schwann cell and oligodendroglial extracellular matrix receptors, including FAK, small Rho GTPases, ILK, and the PI3K/Akt pathway, and the roles that have been ascribed to these signaling mediators. Throughout, we emphasize the concept of extracellular matrix proteins as environmental sensors that act to integrate, or match, cellular responses, in particular to those downstream of growth factors, to appropriate matrix attachment.     

Molecular mechanisms of node of Ranvier formation

Action potential propagation along myelinated nerve fibers requires high-density protein complexes that include voltage-gated Na(+) channels at the nodes of Ranvier. Several complementary mechanisms may be involved in node assembly including: (1) interaction of nodal cell adhesion molecules with the extracellular matrix; (2) restriction of membrane protein mobility by paranodal junctions; and (3) stabilization of ion channel clusters by axonal cytoskeletal scaffolds. In the peripheral nervous system, a secreted glial protein at the nodal extracellular matrix interacts with axonal cell adhesion molecules to initiate node formation. In the central nervous system, both glial soluble factors and paranodal axoglial junctions may function in a complementary manner to contribute to node formation.     

Novel roles for collagens in wiring the vertebrate nervous system

Wiring the vertebrate nervous system is a multi-step process that relies heavily upon the role of transmembrane and extracellular adhesion molecules. Despite the extensive attention focused on such molecules, collagens, a large family of structural adhesion molecules expressed in the vertebrate nervous system, have been largely overlooked for roles in neural circuit formation. Recently, however, several studies have unexpectedly identified novel roles of collagens and collagen-like molecules in the developing vertebrate nervous system. Here, contributions of these collagens and collagen-like molecules in neural circuit formation are reviewed.     

Amyotrophic lateral sclerosis as a protein level,non-genomic disease: Therapy with S2RM exosome released molecules

Amyotrophic lateral sclerosis(ALS) is a rapidly progressing neurodegenerative disease that leads to death. No effective treatments are currently available. Based on data from epidemiological, etiological, laboratory, and clinical studies, I offer a new way of thinking about ALS and its treatment. This paper describes a host of extrinsic factors, including the exposome, that disrupt the extracellular matrix and protein function such that a spreading, prionlike disease leads to neurodegeneration in the motor tracts. A treatment regimen is described using the stem cell released molecules from a number of types of adult stem cells to provide tissue dependent molecules that restore homeostasis, including proteostasis, in the ALS patient. Because stem cells themselves as a therapeutic are cumbersome and expensive, and when implanted in a host cause aging of the host tissue and often fail to engraft or remain viable, only the S2 RM molecules are used. Rebuilding of the extracellular matrix and repair of the dysfunctional proteins in the ALS patient ensues.     

Attachment to an endogenous laminin-like protein initiates sprouting by leech neurons

Leech neurons in culture sprout rapidly when attached to extracts from connective tissue surrounding the nervous system. Laminin-like molecules that promote sprouting have now been isolated from this extracellular matrix. Two mAbs have been prepared that react on immunoblots with a approximately equal to 220- and a approximately equal to 340-kD polypeptide, respectively. These antibodies have been used to purify molecules with cross-shaped structures in the electron microscope. The molecules, of approximately equal to 10(3) kD on nonreducing SDS gels, have subunits of approximately equal to 340, 220, and 160-180 kD. Attachment to the laminin-like molecules was sufficient to initiate sprouting by single isolated leech neurons in defined medium. This demonstrates directly a function for a laminin-related invertebrate protein. The mAbs directed against the approximately equal to 220-kD chains of the laminin-like leech molecule labeled basement membrane extracellular matrix in leech ganglia and nerves. A polyclonal antiserum against the approximately equal to 220-kD polypeptide inhibited neurite outgrowth. Vertebrate laminin did not mediate the sprouting of leech neurons; similarly, the leech molecule was an inert substrate for vertebrate neurons. Although some traits of structure, function, and distribution are conserved between vertebrate laminin and the invertebrate molecule, our results suggest that the functional domains differ.     

干细胞巢研究进展

干细胞巢即干细胞周围的微环境构成,一般包括干细胞的相邻细胞、粘附分子及基质等,但不同的干细胞有不同的巢结构。干细胞巢通过不同信号途径调控着干细胞的行为,使干细胞的自我更新和分化处于平衡状态。根据近年来有关干细胞巢的研究,本文从果蝇生殖系干细胞巢、哺乳动物造血干细胞巢、肠干细胞巢、毛囊表皮干细胞巢和神经干细胞巢等五个系统分别综述了干细胞巢的构成及其对干细胞的调节作用,探讨了干细胞巢作用于干细胞的内在机制。     

Expression of the cell adhesion molecule E-cadherin by the human bone marrow stromal cells and its probable role in CD34(+) stem cell adhesion.

Bone marrow stroma is the physical basis of the haematopoietic microenvironment and regulates several key features of stem cell proliferation and differentiation. It plays a crucial role in maintaining haematopoietic homeostasis. Earlier studies have shown that this is achieved through interactions with the extracellular matrix and specific molecules called the cell adhesion molecules (CAMs). In this paper, we show that E-cadherin, a cell adhesion molecule which plays a crucial role in cell-cell aggregation during development, is also present in the bone marrow stroma. The expression of the CAM can also be demonstrated on a subset of CD34(+)stem cells. Stromal expression of E-cadherin is decreased when treated with lymphokine mixture, phytohaemagglutinin-treated-leukocyte-conditioned medium (PHA-LCM). This is the reverse of ICAM-I expression, which increases with PHA-LCM treatment. E-cadherin shows homotypic and homophilic interaction and its presence on a subset of CD34(+)cells leads to speculation on whether this CAM has a role in adherence of primitive stem cells to the marrow stroma.     

Development of the mammalian enteric nervous system.

The mammalian enteric nervous system is derived from neural crest cells which invade the foregut and hindgut mesenchyme. It has been established that signalling molecules produced by the mesenchyme of the gut wall play a critical role in the development of the mammalian enteric nervous system. Recent studies have characterised further the role of such molecules and have identified novel extracellular and intracellular signals that are critical for enteric ganglia formation.     

Nervous system pathology: the fibrin perspective

Studies of extracellular matrix (ECM) biology in the nervous system have mainly focused on laminin, fibronectin and tenascin-R, proteins that are present during nervous system development and normal function. However, during disease, fibrin, which physiologically is not present in the nervous tissue, is detected at nervous tissue lesions. This review summarizes evidence that correlates fibrin deposition with neuropathology and presents recent findings on cellular mechanisms and intracellular signaling pathways regulated by fibrin that might contribute to nervous system disease.     

Glycans and neural cell interactions

Carbohydrate-carrying molecules in the nervous system have important roles during development, regeneration and synaptic plasticity. Carbohydrates mediate interactions between recognition molecules, thereby contributing to the formation of a complex molecular meshwork at the cell surface and in the extracellular matrix. The tremendous structural diversity of glycan chains allows for immense combinatorial possibilities that might underlie the fine-tuning of cell-cell and cell-matrix interactions.     

Matrix metalloproteinases and their inhibitors,modulators of neuro-immune interactions and of pathophysiological processes in the nervous system

The matrix metalloproteinases (MMPs) belong to a growing family of Zn2+-dependent endopeptidases, secreted or membrane-bound (MT-MMP), that regulate or degrade by proteolytic cleavage protein components of the extracellular matrix, cytokines, chemokines, cell adhesion molecules and a variety of membrane receptors. MMP activity is counterbalanced by their physiological inhibitors, the tissue inhibitors of MMPs (TIMPs), a family of 4 secreted multifunctional proteins that have growth promoting activities. In physiological conditions MMP activity is tightly regulated and altered MMP regulation is associated with pathological processes including inflammation, cell proliferation, cell death and tissue remodeling. The MMP/TIMP system is involved in the development and function of cells of the immune system by promoting their differentiation, activation, migration across basement membranes and tissues. In the last years, data has accumulated indicating that the MMP/TIMP system is expressed in the nervous system where it regulates neuro-immune interactions and plays a major role in pathophysiological processes. In this review, we present recent in vivo and in vitro studies that highlight the contribution of the MMP/TIMP system to various diseases of the nervous system, involving blood brain barrier breakdown, neuroinflammation, glial reactivity, neuronal death, reactive plasticity, and to developmental and physiological processes including cell migration, axonal sprouting and neuronal plasticity. This review also alludes to the beneficial effects of synthetic MMP inhibitors in different animal models of neuropathology. In all, a further understanding of the role of MMPs and TIMPs in the nervous system should contribute to unravel mechanisms of neuronal plasticity and pathology and set the basis of new therapeutic strategies in nervous system disorders based on the development of synthetic MMP inhibitors.     

Lymphocyte interactions with extracellular matrix

To mediate an immune response, lymphocytes must be able to interact with and respond to the surrounding extracellular environment. In addition to cell surface molecules that facilitate adhesion of lymphocytes to other cells, recent studies have demonstrated that lymphocytes interact with glycoproteins and glycosaminoglycans that are major components of the extracellular matrix (ECM). Although many receptors mediating the effects of ECM components on lymphocyte function remain poorly defined, a number of lymphocyte ECM receptors have recently been identified; these include members of the integrin family of adhesion molecules as well as structurally unrelated molecules such as CD44 and CD26. Furthermore, as lymphocytes must be able to move between various microenvironments in vivo, they have proved to be an excellent cell type in which to identify and analyze various modes of regulation of cell-ECM interactions. As with other cell types, the ECM has been shown to have multiple effects on lymphocytes; functional analysis reveals effects of the ECM on lymphocyte migration, recognition/activation, and differentiation. These studies emphasize: 1) the importance of lymphocytes as a model system for identifying and analyzing ECM receptor expression and function, and 2) the multiple roles that the ECM plays in the function of the immune system in vivo.     

Collagen Type I Improves the Differentiation of Human Embryonic Stem Cells towards Definitive Endoderm

Human embryonic stem cells have the ability to generate all cell types in the body and can potentially provide an unlimited source of cells for cell replacement therapy to treat degenerative diseases such as diabetes. Current differentiation protocols of human embryonic stem cells towards insulin producing beta cells focus on soluble molecules whereas the impact of cell-matrix interactions has been mainly unattended. In this study almost 500 different extracellular matrix protein combinations were screened to systemically identify extracellular matrix proteins that influence differentiation of human embryonic stem cells to the definitive endoderm lineage. The percentage of definitive endoderm cells after differentiation on collagen I and fibronectin was >85% and 65%, respectively. The cells on collagen I substrates displayed different morphology and gene expression during differentiation as assessed by time lapse studies compared to cells on the other tested substrates. Global gene expression analysis showed that cells differentiated on collagen I were largely similar to cells on fibronectin after completed differentiation. Collectively, the data suggest that collagen I induces a more rapid and consistent differentiation of stem cells to definitive endoderm. The results shed light on the importance of extracellular matrix proteins for differentiation and also points to a cost effective and easy method to improve differentiation.     

Fibrillin-2 is dispensable for peripheral nerve development,myelination and regeneration

The extracellular matrix of peripheral nerve is formed from a diverse set of macromolecules, including glycoproteins, collagens and proteoglycans. Recent studies using knockout animal models have demonstrated that individual components of the extracellular matrix play a vital role in peripheral nerve development and regeneration. In this study we identified fibrillin-1 and fibrillin-2, large modular structural glycoproteins, as components of the extracellular matrix of peripheral nerve. Previously it was found that fibrillin-2 null mice display joint contractures, suggesting a possible defect of the peripheral nervous system in these animals. Close examination of the peripheral nerves of fibrillin-2 deficient animals described here revealed some structural abnormalities in the perineurium, while general structure of the nerve and molecular composition of nerve extracellular matrix remained unchanged. We also found that in spite of the obvious motor function impairment, fibrillin-2 null mice failed to display changes of nerve conduction properties or nerve regeneration capacity. Based on the data obtained we can conclude that peripheral neuropathy should be excluded as the cause of the impairment of locomotory function and joint contractures observed in fibrillin-2 deficient animals.     

Glycobiology of the synapse

Synapses are the fundamental units of connectivity that link together the nervous system. Lectin studies from 30 years ago suggested that specific glycans are concentrated at neuromuscular synapses in the peripheral nervous system and at excitatory synapses in the brain. Subsequent studies have confirmed that particular glycan structures are localized at these synapses, including polysialic acid, high mannose, the cytotoxic T cell antigen, and forms of heparan sulfate. Though the role of these molecules in synapse formation and function is still poorly understood, there is increasing evidence that the function of agrin, a synaptogenic factor in neuromuscular formation, is modulated by several glycans. In addition, the recent generation of ST8SiaIV null mice strongly suggests a role for polysialic acid in synaptic plasticity in the some regions of the central nervous system.