Conserved neuron promoting activity in Drosophila and vertebrate laminin alpha1

Repairing sciatic nerve injury with self-assembling peptide nanofiber scaffold-containing chitosan conduit

Background

An increasing number of nerve guide scaffolds have been used to replace the “gold-standard” autologous nerve graft for repairing peripheral nerve defects, but nerve regeneration is usually far from complete.

Methods

Here, we designed and prepared two functionalized self-assembling peptides (SAP) with the IKVAV and KLT sequences, which were derived from the combination of laminin and VEGF, respectively. Their mixtures were also obtained to combine the effects of neuroprotective and neurotrophic and proangiogenic factors.

Results

The beneficial effect of peptide gels on nerve regeneration was evaluated in vitro using Schwann cells (SCs). As a useful intraluminal filling, a three-dimensional (3D) functionalized self-assembling peptide (SAP) nanofiber hydrogel was formed in the hollow lumen of chitosan conduits under physiological conditions. In vivo, the combination of the two functionalized SAP gels containing a chitosan nerve conduit significantly accelerated nerve healing and enhanced morphological repair.

Conclusion

Based on the current findings, the combined application of two functionalized SAP gels with chitosan nerve conduit is a promising therapy for the engineering of peripheral nerve regeneration.

Characterization of Drosophila Nidogen/entactin reveals roles in basement membrane stability, barrier function and nervous system patterning

Basement membranes (BMs) are specialized layers of extracellular matrix (ECM) mainly composed of Laminin, type IV Collagen, Perlecan and Nidogen/entactin (NDG). Recent in vivo studies challenged the initially proposed role of NDG as a major ECM linker molecule by revealing dispensability for viability and BM formation. Here, we report the characterization of the single Ndg gene in Drosophila. Embryonic Ndg expression was primarily observed in mesodermal tissues and the chordotonal organs, whereas NDG protein localized to all BMs. Although loss of Laminin strongly affected BM localization of NDG, Ndg-null mutants exhibited no overt changes in the distribution of BM components. Although Drosophila Ndg mutants were viable, loss of NDG led to ultrastructural BM defects that compromised barrier function and stability in vivo Moreover, loss of NDG impaired larval crawling behavior and reduced responses to vibrational stimuli. Further morphological analysis revealed accompanying defects in the larval peripheral nervous system, especially in the chordotonal organs and the neuromuscular junction (NMJ). Taken together, our analysis suggests that NDG is not essential for BM assembly but mediates BM stability and ECM-dependent neural plasticity during Drosophila development.

Building from the Ground up: Basement Membranes in Drosophila Development

Basement membranes (BMs) are sheetlike extracellular matrices found at the basal surfaces of epithelial tissues. The structural and functional diversity of these matrices within the body endows them with the ability to affect multiple aspects of cell behavior and communication; for this reason, BMs are integral to many developmental processes. The power of Drosophila genetics, as applied to the BM, has yielded substantial insight into how these matrices influence development. Here, we explore three facets of BM biology to which Drosophila research has made particularly important contributions. First, we discuss how newly synthesized BM proteins are secreted to and assembled exclusively on basal epithelial surfaces. Next, we examine how regulation of the structural properties of the BM mechanically supports and guides tissue morphogenesis. Finally, we explore how BMs influence development through the modulation of several major signaling pathways.

Drosophila growth cones advance by forward translocation of the neuronal cytoskeletal meshwork in vivo

In vitro studies conducted in Aplysia and chick sensory neurons indicate that in addition to microtubule assembly, long microtubules in the C-domain of the growth cone move forward as a coherent bundle during axonal elongation. Nonetheless, whether this mode of microtubule translocation contributes to growth cone motility in vivo is unknown. To address this question, we turned to the model system Drosophila. Using docked mitochondria as fiduciary markers for the translocation of long microtubules, we first examined motion along the axon to test if the pattern of axonal elongation is conserved between Drosophila and other species in vitro. When Drosophila neurons were cultured on Drosophila extracellular matrix proteins collected from the Drosophila Kc167 cell line, docked mitochondria moved in a pattern indicative of bulk microtubule translocation, similar to that observed in chick sensory neurons grown on laminin. To investigate whether the C-domain is stationary or advances in vivo, we tracked the movement of mitochondria during elongation of the aCC motor neuron in stage 16 Drosophila embryos. We found docked mitochondria moved forward along the axon shaft and in the growth cone C-domain. This work confirms that the physical mechanism of growth cone advance is similar between Drosophila and vertebrate neurons and suggests forward translocation of the microtubule meshwork in the axon underlies the advance of the growth cone C-domain in vivo. These results highlight the need for incorporating en masse microtubule translocation, in addition to assembly, into models of axonal elongation.

Extracellular matrix and its receptors in Drosophila neural development

Extracellular matrix (ECM) and matrix receptors are intimately involved in most biological processes. The ECM plays fundamental developmental and physiological roles in health and disease, including processes underlying the development, maintenance, and regeneration of the nervous system. To understand the principles of ECM-mediated functions in the nervous system, genetic model organisms like Drosophila provide simple, malleable, and powerful experimental platforms. This article provides an overview of ECM proteins and receptors in Drosophila. It then focuses on their roles during three progressive phases of neural development: (1) neural progenitor proliferation, (2) axonal growth and pathfinding, and (3) synapse formation and function. Each section highlights known ECM and ECM-receptor components and recent studies done in mutant conditions to reveal their in vivo functions, all illustrating the enormous opportunities provided when merging work on the nervous system with systematic research into ECM-related gene functions.

Drosophila growth cones: a genetically tractable platform for the analysis of axonal growth dynamics

The formation of neuronal networks, during development and regeneration, requires outgrowth of axons along reproducible paths toward their appropriate postsynaptic target cells. Axonal extension occurs at growth cones (GCs) at the tips of axons. GC advance and navigation requires the activity of their cytoskeletal networks, comprising filamentous actin (F-actin) in lamellipodia and filopodia as well as dynamic microtubules (MTs) emanating from bundles of the axonal core. The molecular mechanisms governing these two cytoskeletal networks, their cross-talk, and their response to extracellular signaling cues are only partially understood, hindering our conceptual understanding of how regulated changes in GC behavior are controlled. Here, we introduce Drosophila GCs as a suitable model to address these mechanisms. Morphological and cytoskeletal readouts of Drosophila GCs are similar to those of other models, including mammals, as demonstrated here for MT and F-actin dynamics, axonal growth rates, filopodial structure and motility, organizational principles of MT networks, and subcellular marker localization. Therefore, we expect fundamental insights gained in Drosophila to be translatable into vertebrate biology. The advantage of the Drosophila model over others is its enormous amenability to combinatorial genetics as a powerful strategy to address the complexity of regulatory networks governing axonal growth. Thus, using pharmacological and genetic manipulations, we demonstrate a role of the actin cytoskeleton in a specific form of MT organization (loop formation), known to regulate GC pausing behavior. We demonstrate these events to be mediated by the actin-MT linking factor Short stop, thus identifying an essential molecular player in this context.

Drosophila as a genetic and cellular model for studies on axonal growth

One of the most fascinating processes during nervous system development is the establishment of stereotypic neuronal networks. An essential step in this process is the outgrowth and precise navigation (pathfinding) of axons and dendrites towards their synaptic partner cells. This phenomenon was first described more than a century ago and, over the past decades, increasing insights have been gained into the cellular and molecular mechanisms regulating neuronal growth and navigation. Progress in this area has been greatly assisted by the use of simple and genetically tractable invertebrate model systems, such as the fruit fly Drosophila melanogaster. This review is dedicated to Drosophila as a genetic and cellular model to study axonal growth and demonstrates how it can and has been used for this research. We describe the various cellular systems of Drosophila used for such studies, insights into axonal growth cones and their cytoskeletal dynamics, and summarise identified molecular signalling pathways required for growth cone navigation, with particular focus on pathfinding decisions in the ventral nerve cord of Drosophila embryos. These Drosophila-specific aspects are viewed in the general context of our current knowledge about neuronal growth.

Tandem Sp1/Sp3 sites together with an Ets-1 site cooperate to mediate α11 integrin chain expression in mesenchymal cells

Alpha11beta1 integrin is a collagen receptor, which is expressed in a highly regulated manner in a specific subset of ectomesenchymally and mesodermally derived cells. We previously established that a 3 kb region upstream of the transcription start site of the ITGA11 gene efficiently induced alpha11 transcription in a cell-type specific manner. Using the human fibrosarcoma cell line HT1080 and human skin fibroblasts, we now report that the majority of the activity in the proximal promoter resides in a region spanning nt +25 to nt -176. Mutation and deletion analyses using luciferase reporter assays showed that tandem low affinity Sp1/Sp3 binding sites, together with an Ets-1-like binding site, were needed for the proximal promoter activity in mesenchymal cells. EMSAs and supershift assays showed that Sp1 and Sp3 both bind to the Sp1/Sp3 binding sites, whereas occupation of the Ets-1 binding site appears to be Sp3-dependent. Chromatin immunoprecipitation assays verified that Sp1, Sp3 and Ets-1 can bind the promoter in vivo. In heterologous Drosophila SL2 cells, Sp1, Sp3 and Ets-1 all transactivated the alpha11 promoter, with Sp1 being the most efficient activator. The lack of any synergistic effect of Sp1/Sp3 and Ets-1 in SL2 cells indicates that an Ets family member other than Ets-1 might be involved in regulating alpha11 transcription in mesenchymal cells. The central role of Sp1 in regulating alpha11 RNA transcription was further verified by the ability of the Sp1 inhibitor mithramycin A to efficiently attenuate alpha11 RNA and protein levels in primary fibroblasts. The proximal promoter itself was able to confer cell-type specific transcription on HT1080 cells and embryonic fibroblasts but not on U2OS and JAR cells. We speculate that the "mesenchymal signature" of alpha11 integrin gene expression is controlled by the activity of Sp1/Sp3, fibroblast-specific combinations of Ets family members and yet unidentified enhancer-binding transcription factors.

Active sites in the carboxyl-terminal region of the laminin alpha chain in Drosophila neuronal cell spreading

An established Drosophila neuronal cell line (BG2-c6) proved to be useful to analyze laminin-mediated cell spreading and signal transduction [Takagi et al. (2000) Biochem Biophys Res Commun 270:482-487]. Here, we report, in addition to the whole molecule, the truncated alpha chain of Drosophila laminin (containing the entire carboxyl-terminal globular domain) and two dodecapeptides corresponding to the cell-binding sites identified in the alpha1 chain of mouse laminin were also active to stimulate BG2-c6 cell spreading. Our previous study [Takagi et al. (1996) J Biol Chem 271:18074-18081] revealed that these recombinant protein and synthetic peptides promoted neurite outgrowth in the primary cell culture system prepared from Drosophila embryo. Therefore, the similar effects by these proteins and peptides suggest the presence of a common mechanism of laminin and neuronal cell interaction working in both primary and established cells. One of the two active peptides contains the sequence SIKVGV. Its murine counterpart carries the sequence SIKVAV by which the interaction of laminin and cells is mediated. Furthermore, laminin-dependent BG2-c6 cell spreading was inhibited by heparin. This observation suggests that cell surface glycoproteins participate in the interaction of laminin and BG2-c6 cells.

Neural cell response to multiple novel sites on laminin-1

The basement membrane protein laminin-1 is a potent stimulator of neurite outgrowth for a variety of neuronal cell types. Previous studies have identified neurite outgrowth activity in several distinct regions of the laminin-1 molecule. In this study, 545 overlapping 12- to 14-mer synthetic peptides, corresponding to most of the amino acid sequence of the alpha1, beta1, and gamma1 chains of laminin-1, were screened for cell attachment and neurite outgrowth activity using primary cultures of mouse cerebellar granule neurons and two neuronal cell lines. We identified 48 peptides derived from novel regions of the laminin-1 molecule that were positive for neural cell adhesion activity. Only the cerebellar cells were found to have true neurite outgrowth activity with certain of the peptides, whereas some peptides induced short spike-like process with the cell lines. Although 23 of these peptides were active on all 3 cell types screened, 25 others showed cell-type specificity in their activity. These studies show that (1) there are multiple and distinct sites on laminin-1 for cell adhesion and neurite-like outgrowth and (2) that there are neural cell-type-specific active domains. The multiple active sites found explains, in part, the potent activity of laminin-1 on neurite outgrowth.

Cell binding sequences in mouse laminin alpha1 chain

Laminin-1, a multifunctional glycoprotein of the basement membrane, consists of three different subunits, alpha1, beta1, and gamma1 chains. Previously, we used synthetic peptides to screen for biologically active sequences in the laminin alpha1 chain C-terminal globular domain (G domain) and identified several cell binding sequences (Nomizu, M., Kim, W. H., Yamamura, K., Utani, A., Song, S. Y., Otaka, A., Roller, P. P., Kleinman, H. K., and Yamada, Y. (1995) J. Biol. Chem. 270, 20583-20590). Here, we identify new cell binding sequences on the remainder of the laminin alpha1 chain by systematic peptide screening, using 208 overlapping synthetic peptides encompassing the central and N-terminal portions of the alpha1 chain. HT-1080 cell attachment activity to the peptides was evaluated using peptide-coated plastic substrates and peptide-conjugated Sepharose beads. Twenty five peptides showed cell attachment activities on either the peptide-coated plastic substrates and/or the peptide-conjugated Sepharose beads. A-13 (RQVFQVAYIIIKA) showed strongest cell attachment activity in both the assays. Cell attachment to 14 of the peptides was inhibited by heparin. EDTA and integrin antibodies inhibited cell adhesion to two of the peptides, A-13 and A-25, suggesting that these sites likely bind to integrins. These peptides inhibited cell attachment to laminin-1 but not to collagen I, suggesting these active sites are available on the intact molecule. Most of active sequences were localized on globular domains suggesting that these structures play a critical role in binding to cell-surface receptors.

Modulation of synapsin I gene expression in rat cortical neurons by extracellular matrix

1. Neuronal differentiation depends on crosstalk between genetic program and environmental cues. In this study we tried to dissect this complex interplay by culturing neurons from fetal rat brain cortices in a chemically defined, neuron-specific, medium and on different substrata, either artificial (poly-D-lysine) or natural. 2. Among the extracellular matrix compounds used in this study, two (collagen I and fibronectin) allowed only a weak attachment of cortical neurons to the substratum, while the others (collagen IV, laminin, and basal lamina from Engelbreth-Holm-Swarm sarcoma) allowed both firm attachment and moderate to extensive neurite outgrowth from neuronal cell bodies. 3. By using synapsin I gene expression as a parameter of neuronal differentiation, we found that neurite outgrowth and neuronal differentiation are not linearly linked. Synapsin I gene expression, in fact, was maximal in neurons cultured on laminin, while the fastest neuritic outgrowth was recorded in cultures on poly-D-lysine. 4. The data presented in this paper are consistent with the hypothesis that the extracellular matrix plays an active role in modulating the differentiative program of neurons.

Splice variants of the Drosophila PS2 integrins differentially interact with RGD-containing fragments of the extracellular proteins tiggrin, ten-m, and D-laminin 2

Two new potential ligands of the Drosophila PS2 integrins have been characterized by functional interaction in cell culture. These potential ligands are a new Drosophila laminin alpha2 chain encoded by the wing blister locus and Ten-m, an extracellular protein known to be involved in embryonic pattern formation. As with previously identified PS2 ligands, both contain RGD sequences, and RGD-containing fragments of these two proteins (DLAM-RGD and TENM-RGD) can support PS2 integrin-mediated cell spreading. In all cases, this spreading is inhibited specifically by short RGD-containing peptides. As previously found for the PS2 ligand tiggrin (and the tiggrin fragment TIG-RGD), TENM-RGD induces maximal spreading of cells expressing integrin containing the alphaPS2C splice variant. This is in contrast to DLAM-RGD, which is the first Drosophila polypeptide shown to interact preferentially with cells expressing the alphaPS2 m8 splice variant. The betaPS integrin subunit also varies in the presumed ligand binding region as a result of alternative splicing. For TIG-RGD and TENM-RGD, the beta splice variant has little effect, but for DLAM-RGD, maximal cell spreading is supported only by the betaPS4A form of the protein. Thus, the diversity in PS2 integrins due to splicing variations, in combination with diversity of matrix ligands, can greatly enhance the functional complexity of PS2-ligand interactions in the developing animal. The data also suggest that the splice variants may alter regions of the subunits that are directly involved in ligand interactions, and this is discussed with respect to models of integrin structure.

Laminin-dependent integrin clustering with tyrosine-phosphorylated molecules in a Drosophila neuronal cell line

To gain more insight into the molecular and cellular aspects of basement membranes during Drosophila morphogenesis, especially in neural development, we carried out cell biological screening to establish a cell culture system in which Drosophila cell-matrix interaction could be reconstituted. The screening showed that a Drosophila neuronal cell line, BG2-c6, established from the third-instar larval central nervous system, had a strong adhesion activity when purified Drosophila laminin was used as a substrate. Outgrowth of axon-like structures was stimulated on laminin. Histochemical analysis revealed clusters of integrin together with phosphotyrosine and alpha-actinin. These data indicate that the Drosophila integrin cascade triggered by the interaction between BG2-c6 and laminin was initiated at the integrin cluster with tyrosine-phosphorylated proteins, similar to the observations in vertebrate cells.

Identification of cell binding sequences in mouse laminin gamma1 chain by systematic peptide screening

Laminin-1, a major component of basement membranes, consists of three different chains designated alpha1, beta1, and gamma1 and has diverse biological functions. We have identified cell binding sites on the mouse laminin gamma1 chain, using systematic screening of 165 overlapping synthetic peptides covering the entire chain. We identified 12 cell binding sequences using HT-1080 human fibrosarcoma and B16-F10 mouse melanoma cells in two independent assays employing peptide-conjugated Sepharose beads and peptide-coated dishes. Four peptides (C-16, C-28, C-64, and C-68) located on the globular domains of the gamma1 chain were the most active and showed dose-dependent cell attachment. Cell attachment to C-68 was inhibited by EDTA and by anti-alpha2beta1 integrin antibodies. Cell attachment to C-16 and C-64 was partially inhibited by EDTA but was not inhibited by anti-integrin antibodies. EDTA and anti-integrin antibodies did not affect cell attachment to C-28. The four peptides were tested in adhesion and differentiation assays with endothelial, neuronal, and human salivary gland cells. C-16 was the most active for all of the cells, whereas the other three peptides showed cell type specificity in their activities. The active core sequences of C-16, C-28, C-64, and C-68 are YVRL, IRVTLN, TTVKYIFR, and SIKIRGTY, respectively. These sequences are highly conserved among the different species and in the laminin gamma2 chain. These results suggest that the specific sequences on the laminin gamma1 chain are biologically active and interact with distinct cell surface receptors.

The alpha1 subunit of laminin-1 promotes the development of neurons by interacting with LBP110 expressed by neural crest-derived cells immunoselected from the fetal mouse gut

A plasmalemmal protein, LBP110, which binds to the alpha1 chain of laminin-1, is acquired by the neural crest-derived precursors of enteric neurons after they colonize the gut. We tested the hypothesis that laminin-1 interacts with LBP110 to promote enteric neuronal development. The effects of laminin-1 on neuronal development were studied in cultures of cells immunoselected from fetal mouse gut (E14-15) with antibodies to LBP110 or p75NTR, a marker for enteric crest-derived cells. No matter which antibody was used, the development of cells expressing neuronal markers was increased three- to fourfold by culturing the cells on a laminin-1-containing substrate. To determine whether this effect of laminin-1 is due to the selective adherence of a neurocompetent subset of precursors, immunoselected cells were permitted to preadhere to poly-D-lysine. Addition of soluble laminin-1 24 h later promoted neuronal but not glial development. The laminin-1-induced increment in neuronal development was abolished both by a peptide containing the sequence of the LBP110-binding domain, IKVAV, and by antibodies to laminin alpha1 that recognize the IKVAV domain. Neither reagent affected the total number of cells. In contrast, the response to laminin-1 was not affected by control peptides, preimmune sera, or antibodies to laminin beta1. Laminin-1 transiently induced the expression of nuclear Fos immunoreactivity; this action was blocked specifically by the IKVAV peptide. These data are consistent with the hypothesis that LBP110 interacts with the IKVAV domain of laminin alpha1 to promote the differentiation of neurons from enteric crest-derived precursors.