Regulation of neurite outgrowth by integrin activation

The role of integrins in brain health and neurodegenerative diseases

Integrins are heterodimeric membrane proteins expressed on the surface of most cells. They mediate adhesion and signaling processes relevant for a wealth of physiological processes, including nervous system development and function. Interestingly, integrins are also recognized therapeutic targets for inflammatory diseases, such as multiple sclerosis. Here, we discuss the role of integrins in brain development and function, as well as in neurodegenerative diseases affecting the brain (Alzheimer's disease, multiple sclerosis, stroke). Furthermore, we discuss therapeutic targeting of these adhesion receptors in inflammatory diseases of the brain.

Improved Neural Inductivity of Size-Controlled 3D Human Embryonic Stem Cells Using Magnetic Nanoparticles

Background: To improve the efficiency of neural development from human embryonic stem cells, human embryoid body (hEB) generation is vital through 3-dimensional formation. However, conventional approaches still have limitations: long-term cultivation and laborious steps for lineage determination. Methods: In this study, we controlled the size of hEBs for ectodermal lineage specification using cell-penetrating magnetic nanoparticles (MNPs), which resulted in reduced time required for initial neural induction. The magnetized cells were applied to concentrated magnetic force for magnet-derived multicellular organization. The uniformly sized hEBs were differentiated in neural induction medium (NIM) and suspended condition. This neurally induced MNP-hEBs were compared with other groups. Results: As a result, the uniformly sized MNP-hEBs in NIM showed significantly improved neural inductivity through morphological analysis and expression of neural markers. Signaling pathways of the accelerated neural induction were detected via expression of representative proteins; Wnt signaling, dopaminergic neuronal pathway, intercellular communications, and mechanotransduction. Consequently, we could shorten the time necessary for early neurogenesis, thereby enhancing the neural induction efficiency. Conclusion: Overall, this study suggests not only the importance of size regulation of hEBs at initial differentiation stage but also the efficacy of MNP-based neural induction method and stimulations for enhanced neural tissue regeneration.

Extracellular molecular signals shaping dendrite architecture during brain development

Proper growth and branching of dendrites are crucial for adequate central nervous system (CNS) functioning. The neuronal dendritic geometry determines the mode and quality of information processing. Any defects in dendrite development will disrupt neuronal circuit formation, affecting brain function. Besides cell-intrinsic programmes, extrinsic factors regulate various aspects of dendritic development. Among these extrinsic factors are extracellular molecular signals which can shape the dendrite architecture during early development. This review will focus on extrinsic factors regulating dendritic growth during early neuronal development, including neurotransmitters, neurotrophins, extracellular matrix proteins, contact-mediated ligands, and secreted and diffusible cues. How these extracellular molecular signals contribute to dendritic growth has been investigated in developing nervous systems using different species, different areas within the CNS, and different neuronal types. The response of the dendritic tree to these extracellular molecular signals can result in growth-promoting or growth-limiting effects, and it depends on the receptor subtype, receptor quantity, receptor efficiency, the animal model used, the developmental time windows, and finally, the targeted signal cascade. This article reviews our current understanding of the role of various extracellular signals in the establishment of the architecture of the dendrites.

Implantable Biomaterials for Peripheral Nerve Regeneration–Technology Trends and Translational Tribulations

The use of autografted nerve in surgical repair of peripheral nerve injuries (PNI) is severely limited due to donor site morbidity and restricted tissue availability. As an alternative, synthetic nerve guidance channels (NGCs) are available on the market for surgical nerve repair, but they fail to promote nerve regeneration across larger critical gap nerve injuries. Therefore, such injuries remain unaddressed, result in poor healing outcomes and are a limiting factor in limb reconstruction and transplantation. On the other hand, a myriad of advanced biomaterial strategies to address critical nerve injuries are proposed in preclinical literature but only few of those have found their way into clinical practice. The design of synthetic nerve grafts should follow rational criteria and make use of a combination of bioinstructive cues to actively promote nerve regeneration. To identify the most promising NGC designs for translation into applicable products, thorough mode of action studies, standardized readouts and validation in large animals are needed. We identify design criteria for NGC fabrication according to the current state of research, give a broad overview of bioactive and functionalized biomaterials and highlight emerging composite implant strategies using therapeutic cells, soluble factors, structural features and intrinsically conductive substrates. Finally, we discuss translational progress in bioartificial conduits for nerve repair from the surgeon’s perspective and give an outlook toward future challenges in the field.

Bruch's-Mimetic Nanofibrous Membranes Functionalized with the Integrin-Binding Peptides as a Promising Approach for Human Retinal Pigment Epithelium Cell Transplantation

Background: This study aimed to develop an ultrathin nanofibrous membrane able to, firstly, mimic the natural fibrous architecture of human Bruch’s membrane (BM) and, secondly, promote survival of retinal pigment epithelial (RPE) cells after surface functionalization of fibrous membranes. Methods: Integrin-binding peptides (IBPs) that specifically interact with appropriate adhesion receptors on RPEs were immobilized on Bruch’s-mimetic membranes to promote coverage of RPEs. Surface morphologies, Fourier-transform infrared spectroscopy spectra, contact angle analysis, Alamar Blue assay, live/dead assay, immunofluorescence staining, and scanning electron microscopy were used to evaluate the outcome. Results: Results showed that coated membranes maintained the original morphology of nanofibers. After coating with IBPs, the water contact angle of the membrane surfaces varied from 92.38 ± 0.67 degrees to 20.16 ± 0.81 degrees. RPE cells seeded on IBP-coated membranes showed the highest viability at all time points (Day 1, p < 0.05; Day 3, p < 0.01; Days 7 and 14, p < 0.001). The proliferation rate of RPE cells on uncoated poly(ε-caprolactone) (PCL) membranes was significantly lower than that of IBP-coated membranes (p < 0.001). SEM images showed a well-organized hexa/polygonal monolayer of RPE cells on IBP-coated membranes. RPE cells proliferated rapidly, contacted, and became confluent. RPE cells formed a tight adhesion with nanofibers under high-magnification SEM. Our findings confirmed that the IBP-coated PCL membrane improved the attachment, proliferation, and viability of RPE cells. In addition, in this study, we used serum-free culture for RPE cells and short IBPs without immunogenicity to prevent graft rejection and immunogenicity during transplantation. Conclusions: These results indicated that the biomimic BM-IBP-RPE nanofibrous graft might be a new, practicable approach to increase the success rate of RPE cell transplantation.

R-Ras1 and R-Ras2 Expression in Anatomical Regions and Cell Types of the Central Nervous System

Since the optic nerve is one of the most myelinated tracts in the central nervous system (CNS), many myelin diseases affect the visual system. In this sense, our laboratory has recently reported that the GTPases R-Ras1 and R-Ras2 are essential for oligodendrocyte survival and maturation. Hypomyelination produced by the absence of one or both proteins triggers axonal degeneration and loss of visual and motor function. However, little is known about R-Ras specificity and other possible roles that they could play in the CNS. In this work, we describe how a lack of R-Ras1 and/or R-Ras2 could not be compensated by increased expression of the closely related R-Ras3 or classical Ras. We further studied R-Ras1 and R-Ras2 expression within different CNS anatomical regions, finding that both were more abundant in less-myelinated regions, suggesting their expression in non-oligodendroglial cells. Finally, using confocal immunostaining colocalization, we report for the first time that R-Ras2 is specifically expressed in neurons. Neither microglia nor astrocytes expressed R-Ras1 or R-Ras2. These results open a new avenue for the study of neuronal R-Ras2’s contribution to the process of myelination.

R-Ras Deficiency in Pericytes Causes Frequent Microphthalmia and Perturbs Retinal Vascular Development

PURPOSE

The retinal vasculature is heavily invested by pericytes. Small GTPase R-Ras is highly expressed in endothelial cells and pericytes, suggesting importance of this Ras homolog for the regulation of the blood vessel wall. We investigated the specific contribution of pericyte-expressed R-Ras to the development of the retinal vasculature.

METHODS

The effect of R-Ras deficiency in pericytes was analyzed in pericyte-targeted conditional Rras knockout mice at birth and during the capillary plexus formation in the neonatal retina.

RESULTS

The offspring of these mice frequently exhibited unilateral microphthalmia. Analyses of the developing retinal vasculature in the eyes without microphthalmia revealed excessive endothelial cell proliferation, sprouting, and branching of the capillary plexus in these animals. These vessels were structurally defective with diminished pericyte coverage and basement membrane formation. Furthermore, these vessels showed reduced VE-cadherin staining and significantly elevated plasma leakage indicating the breakdown of the blood-retinal barrier. This defect was associated with considerable macrophage infiltration in the retina.

CONCLUSIONS

The normal retinal vascular development is dependent on R-Ras expression in pericytes, and the absence of it leads to unattenuated angiogenesis and significantly weakens the blood-retinal barrier. Our findings underscore the importance of R-Ras for pericyte function during the normal eye development.

Osteopontin and Integrin Mediated Modulation of Post-Synapses in HIV Envelope Glycoprotein Exposed Hippocampal Neurons

The advent of Human Immunodeficiency Virus (HIV) antiretrovirals have reduced the severity of HIV related neurological comorbidities but they nevertheless remain prevalent. Synaptic degeneration due to the action of several viral factors released from infected brain myeloid and glia cells and inflammatory cytokines has been attributed to the manifestation of a range of cognitive and behavioral deficits. The contributions of specific pro-inflammatory factors and their interplay with viral factors in the setting of treatment and persistence are incompletely understood. Exposure of neurons to chemokine receptor-4(CXCR4)-tropic HIV-1 envelope glycoprotein (Env) can lead to post-synaptic degradation of dendritic spines. The contribution of members of the extracellular matrix (ECM) and specifically, of perineuronal nets (PNN) toward synaptic degeneration, is not fully known, even though these structures are found to be disrupted in post-mortem HIV-infected brains. Osteopontin (Opn, gene name SPP1), a cytokine-like protein, is found in abundance in the HIV-infected brain. In this study, we investigated the role of Opn and its ECM integrin receptors, β1- and β3 integrin in modifying neuronal synaptic sculpting. We found that in hippocampal neurons incubated with HIV-1 Env protein and recombinant Opn, post-synaptic-95 (PSD-95) puncta were significantly increased and distributed to dendritic spines when compared to Env-only treated neurons. This effect was mediated through β3 integrin, as silencing of this receptor abrogated the increase in post-synaptic spines. Silencing of β1 integrin, however, did not block the increase of post-synaptic spines in hippocampal cultures treated with Opn. However, a decrease in the PNN to βIII-tubulin ratio was found, indicating an increased capacity to support spine growth. From these results, we conclude that one of the mechanisms by which Opn counters the damaging impact of the HIV Env protein on hippocampal post-synaptic plasticity is through complex interactions between Opn and components of the ECM which activate downstream protective signaling pathways that help maintain the potential for effective post-synaptic plasticity.

Osteopontin counters human immunodeficiency virus type 1-induced impairment of neurite growth through mammalian target of rapamycin and beta-integrin signaling pathways

Despite the fact that human immunodeficiency virus type 1 (HIV-1) does not enter or replicate in neurons, its infection of a subset of resident brain glia cells (microglia and astrocytes) induces via disparate mechanisms, dysregulation of glutamate metabolism, neurotoxicity, and inflammation. Antiretroviral therapies suppress viral load, but cellular activation and release of proinflammatory factors, some of which is likely related to viral reservoirs, continue to promote a microenvironment that is injurious to neurons. However, the molecular mechanisms remain to be identified. Osteopontin (OPN) is a proinflammatory cytokine-like, extracellular matrix protein that is elevated within the brain and CSF in several neurodegenerative disorders, including HIV-associated cognitive disorder. However, the impact of elevated OPN on neuronal integrity and function in HIV-infected individuals who exhibit cognitive dysfunction remains unknown. In this study, using a neuronal cell line and primary cultures of cortical rat neurons, we identify the mammalian target of rapamycin pathway involvement in a signaling interaction between OPN-β1-integrins and the HIV-1 envelope glycoprotein, which stimulates neurite growth. These findings link for the first time HIV X4-envelope receptor engagement and osteopontin-mediated signaling through β1-integrin receptors to the mTOR pathway and alterations in the cytoskeleton of cortical neurons.

How the extracellular matrix shapes neural development

During development, both cells and tissues must acquire the correct shape to allow their proper function. This is especially relevant in the nervous system, where the shape of individual cell processes, such as the axons and dendrites, and the shape of entire tissues, such as the folding of the neocortex, are highly specialized. While many aspects of neural development have been uncovered, there are still several open questions concerning the mechanisms governing cell and tissue shape. In this review, we discuss the role of the extracellular matrix (ECM) in these processes. In particular, we consider how the ECM regulates cell shape, proliferation, differentiation and migration, and more recent work highlighting a key role of ECM in the morphogenesis of neural tissues.

Integrins promote axonal regeneration after injury of the nervous system

Integrins are cell surface receptors that form the link between extracellular matrix molecules of the cell environment and internal cell signalling and the cytoskeleton. They are involved in several processes, e.g. adhesion and migration during development and repair. This review focuses on the role of integrins in axonal regeneration. Integrins participate in spontaneous axonal regeneration in the peripheral nervous system through binding to various ligands that either inhibit or enhance their activation and signalling. Integrin biology is more complex in the central nervous system. Integrins receptors are transported into growing axons during development, but selective polarised transport of integrins limits the regenerative response in adult neurons. Manipulation of integrins and related molecules to control their activation state and localisation within axons is a promising route towards stimulating effective regeneration in the central nervous system.

Dendrite regeneration of adult Drosophila sensory neurons diminishes with aging and is inhibited by epidermal-derived matrix metalloproteinase 2

DeVault et al. show that the capacity for regeneration was present in adult neurons but diminished as the animal aged. The regenerated dendrites showed preferential alignment with the extracellular matrix, and inhibition of matrix metalloproteinase 2 led to increased dendrite regeneration.

Dendrites possess distinct structural and functional properties that enable neurons to receive information from the environment as well as other neurons. Despite their key role in neuronal function, current understanding of the ability of neurons to regenerate dendrites is lacking. This study characterizes the structural and functional capacity for dendrite regeneration in vivo in adult animals and examines the effect of neuronal maturation on dendrite regeneration. We focused on the class IV dendritic arborization (c4da) neuron of the Drosophila sensory system, which has a dendritic arbor that undergoes dramatic remodeling during the first 3 d of adult life and then maintains a relatively stable morphology thereafter. Using a laser severing paradigm, we monitored regeneration after acute and spatially restricted injury. We found that the capacity for regeneration was present in adult neurons but diminished as the animal aged. Regenerated dendrites recovered receptive function. Furthermore, we found that the regenerated dendrites show preferential alignment with the extracellular matrix (ECM). Finally, inhibition of ECM degradation by inhibition of matrix metalloproteinase 2 (Mmp2) to preserve the extracellular environment characteristics of young adults led to increased dendrite regeneration. These results demonstrate that dendrites retain regenerative potential throughout adulthood and that regenerative capacity decreases with aging.

The effect of laminin-1 on enteric neural crest-derived cell migration in the Hirschsprung's disease mouse model

BACKGROUND/AIM

Laminin-1 regulates neurite outgrowth in various neuronal cells. We have previously demonstrated that laminin-1 promotes enteric neural crest-derived cell (ENCC) migration by using Sox10-VENUS transgenic mice, in which ENCCs are labeled with a green fluorescent protein, Venus. Mice lacking the endothelin-B receptor gene, Ednrb ^-/- mice, are widely used as a model for Hirschsprung's disease (HD). The aim of this study was to investigate the effects of laminin-1on ENCC migration in Sox10-VENUS⁺/Ednrb ^-/- mice, a newly created HD mice model.

METHODS

Fetal guts were dissected on embryonic day 12.5 (E12.5). Specimens were incubated either with, or without laminin-1 for 24 h and images were taken under a stereoscopic microscope. The length from the stomach to the wavefront of ENCC migration (L-E) and the total length of the gut (L-G) were measured. Changes in the ratio of L-E to L-G (L-E/L-G) after 24 h were calculated.

RESULTS

On E12.5, the wavefront of ENCC migration in the HD gut samples was located in the midgut, whereas the wavefront of ENCC in Sox10-VENUS⁺/Ednrb ^+/+ (WT) samples had reached the hindgut. After 24 h, L-E/L-G had increased by 1.49%, from 34.97 to 36.46%, in HD gut and had increased by 1.07%, from 48.08 to 49.15%, in HD with laminin-1, suggesting there was no positive effect of laminin-1 administration on ENCC migration in HD.

CONCLUSIONS

Our results suggest that laminin-1 does not have a positive effect on ENCC migration in HD mice on E12.5, in contrast to the phenomenon seen in normal mice gut specimens, where laminin-1 promotes ENCC migration during the same period. This suggests that there is an impairment in the interaction between ENCC and extracellular environmental factors, which are required for normal development of the enteric nervous system, resulting in an aganglionic colon in HD.

Post-endocytic sorting of Plexin-D1 controls signal transduction and development of axonal and vascular circuits

Local endocytic events involving receptors for axon guidance cues play a central role in controlling growth cone behaviour. Yet, little is known about the fate of internalized receptors, and whether the sorting events directing them to distinct endosomal pathways control guidance decisions. Here, we show that the receptor Plexin-D1 contains a sorting motif that interacts with the adaptor protein GIPC1 to facilitate transport to recycling endosomes. This sorting process promotes colocalization of Plexin-D1 with vesicular pools of active R-ras, leading to its inactivation. In the absence of interaction with GIPC1, missorting of Plexin-D1 results in loss of signalling activity. Consequently, Gipc1 mutant mice show specific defects in axonal projections, as well as vascular structures, that rely on Plexin-D1 signalling for their development. Thus, intracellular sorting steps that occur after receptor internalization by endocytosis provide a critical level of control of cellular responses to guidance signals.

Molecular mechanisms controlling axonal growth cone behaviour are only partially understood. Here the authors reveal a role of an adaptor protein GIPC1 in Plexin-D1 receptor recycling, and show that this process is required for axon track formation and vascular patterning in mice.

Epac1 interacts with importin β1 and controls neurite outgrowth independently of cAMP and Rap1

Exchange protein directly activated by cAMP-1 (Epac1) is a cAMP sensor that regulates multiple cellular functions including cellular migration, proliferation and differentiation. Classically, Epac1 is thought to exert its effects through binding of cAMP leading to a conformational change in Epac1 and its accumulation at the plasma membrane (PM) where it activates Rap1. In search for regulators of Epac1 activity, we show here that importin β1 (impβ1) is an Epac1 binding partner that prevents PM accumulation of Epac1. We demonstrate that in the absence of impβ1, endogenous as well as overexpressed Epac1 accumulate at the PM. Moreover, agonist-induced PM translocation of Epac1 leads to dissociation of Epac1 from impβ1. Localization of Epac1 at the PM in the absence of impβ1, requires residue R82 in its DEP domain. Notably, the PM accumulation of Epac1 in the absence of impβ1 does not require binding of cAMP to Epac1 and does not result in Rap1 activation. Functionally, PM accumulation of Epac1, an Epac1 mutant deficient in cAMP binding, or an Epac1 mutant tethered to the PM, is sufficient to inhibit neurite outgrowth. In conclusion, we uncover a cAMP-independent function of Epac1 at the PM and demonstrate that impβ1 controls subcellular localization of Epac1.

Structure, functional regulation and signaling properties of Rap2B

The Ras family small guanosine 5'-triphosphate (GTP)-binding protein Rap2B is is a member of the Ras oncogene family and a novel target of p53 that regulates the p53-mediated pro-survival function of cells. The Rap2B protein shares ~90% homology with Rap2A, and its sequence is 70% identical to other members of the Rap family such as RaplA and RaplB. As a result, Rap2B has been theorized to have similar signaling effectors to the GTPase-binding protein Rap, which mediates various biological functions, including the regulation of sterile 20/mitogen-activated proteins. Since its identification in the early 1990s, Rap2B has elicited a considerable interest. Numerous studies indicate that Rap2B exerts specific biological functions, including binding and stimulating phospholipase C-ε and interferon-γ. In addition, downregulation of Rap2B affects the growth of melanoma cells. The present review summarizes the possible effectors and biological functions of Rap2B. Increasing evidence clearly supports the association between Rap2B function and tumor development. Therefore, it is conceivable that anticancer drugs targeting Rap2B may be generated as novel therapies against cancer.

Recruitment and activation of SLK at the leading edge of migrating cells requires Src family kinase activity and the LIM-only protein 4

The Ste20-like kinase SLK plays a pivotal role in cell migration and focal adhesion turnover and is regulated by the LIM domain-binding proteins Ldb1 and Ldb2. These adapter proteins have been demonstrated to interact with LMO4 in the organization of transcriptional complexes. Therefore, we have assessed the ability of LMO4 to also interact and regulate SLK activity. Our data show that LMO4 can directly bind to SLK and activate its kinase activity in vitro and in vivo. LMO4 can be co-precipitated with SLK following the induction of cell migration by scratch wounding and Cre-mediated deletion of LMO4 in conditional LMO4(fl/fl) fibroblasts inhibits cell migration and SLK activation. Deletion of LMO4 impairs Ldb1 and SLK recruitment to the leading edge of migrating cells. Supporting this, Src/Yes/Fyn-deficient cells (SYF) expressing very low levels of LMO4 do not recruit SLK to the leading edge. Re-expression of wildtype Myc-LMO4 in SYF cells, but not a mutant version, restores SLK localization and kinase activity. Overall, our data suggest that activation of SLK by haptotactic signals requires its recruitment to the leading edge by LMO4 in a Src-dependent manner. Furthermore, this establishes a novel cytosolic role for the transcriptional co-activator LMO4.

Full length talin stimulates integrin activation and axon regeneration

Integrin function is regulated by activation involving conformational changes that modulate ligand-binding affinity and downstream signaling. Activation is regulated through inside-out signaling which is controlled by many signaling pathways via a final common pathway through kindlin and talin, which bind to the intracellular tail of beta integrins. Previous studies have shown that the axon growth inhibitory molecules NogoA and chondroitin sulfate proteoglycans (CSPGs) inactivate integrins. Overexpressing kindlin-1 in dorsal root ganglion (DRG) neurons activates integrins, enabling their axons to overcome inhibitory molecules in the environment, and promoting regeneration in vivo following dorsal root crush. Other studies have indicated that expression of the talin head alone or with kindlin can enhance integrin activation. Here, using adult rat DRG neurons, we investigate the effects of overexpressing various forms of talin on axon growth and integrin signaling. We found that overexpression of the talin head activated axonal integrins but inhibited downstream signaling via FAK, and did not promote axon growth. Similarly, co-expression of the talin head and kindlin-1 prevented the growth-promoting effect of kindlin-1, suggesting that the talin head acts as a form of dominant negative for integrin function. Using full-length talin constructs in PC12 cells we observed that neurite growth was enhanced by the expression of wild-type talin and more so by two ‘activated’ forms of talin produced by point mutation (on laminin and aggrecan–laminin substrates). Nevertheless, co-expression of full-length talin with kindlin did not promote neurite growth more than either molecule alone. In vivo, we find that talin is present in PNS axons (sciatic nerve), and also in CNS axons of the corticospinal tract.

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Full length talin can activate integrins and stimulate axon regeneration on inhibitory CSPGs.

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The talin head domain can activate integrins, but acts as a dominant negative for talin function.

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Talin and kindlins do not have additive effects on axon growth.

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Talin is transported into the axons of the sciatic nerve and the corticospinal tract.

OSBP-related protein 3 (ORP3) coupling with VAMP-associated protein A regulates R-Ras activity

ORP3 is an R-Ras interacting oxysterol-binding protein homolog that regulates cell adhesion and is overexpressed in several cancers. We investigated here a novel function of ORP3 dependent on its targeting to both the endoplasmic reticulum (ER) and the plasma membrane (PM). Using biochemical and cell imaging techniques we demonstrate the mechanistic requirements for the subcellular targeting and function of ORP3 in control of R-Ras activity. We show that hyperphosphorylated ORP3 (ORP3-P) selectively interacts with the ER membrane protein VAPA, and ORP3-VAPA complexes are targeted to PM sites via the ORP3 pleckstrin homology (PH) domain. A novel FFAT (two phenylalanines in an acidic tract)-like motif was identified in ORP3; only disruption of both the FFAT-like and canonical FFAT motif abolished the phorbol-12-myristate-13-acetate (PMA) stimulated interaction of ORP3-P with VAPA. Co-expression of ORP3 and VAPA induced R-Ras activation, dependent on the interactions of ORP3 with VAPA and the PM. Consistently, downstream AktS473 phosphorylation and β1-integrin activity were enhanced by ORP3-VAPA. To conclude, phosphorylation of ORP3 controls its association with VAPA. Furthermore, we present evidence that ORP3-VAPA complexes stimulate R-Ras signaling.

R-Ras contributes to LTP and contextual discrimination

The ability to discriminate between closely related contexts is a specific form of hippocampal-dependent learning that may be impaired in certain neurodegenerative disorders such as Alzheimer's and Down Syndrome. However, signaling pathways regulating this form of learning are poorly understood. Previous studies have shown that the calcium-dependent exchange factor Ras-GRF1, an activator of Rac, Ras and R-Ras GTPases, is important for this form of learning and memory. Moreover, the ability to discriminate contexts was linked to the ability of Ras-GRF1 to promote high-frequency stimulation long-term potentiation (HFS-LTP) via the activation of p38 Map kinase. Here, we show that R-Ras is involved in this form of learning by using virally-delivered miRNAs targeting R-Ras into the CA1 region of the dorsal hippocampus and observing impaired contextual discrimination. Like the loss of GRF1, knockdown of R-Ras in the CA1 also impairs the induction of HFS-LTP and p38 Map kinase. Nevertheless, experiments indicate that this involvement of R-Ras in HFS-LTP that is required for contextual discrimination is independent of Ras-GRF1. Thus, R-Ras is a novel regulator of a form of hippocampal-dependent LTP as well as learning and memory that is affected in certain forms of neurodegenerative diseases.

Polylaminin recognition by retinal cells

Polylaminin (polyLM) is a flat biomimetic polymer of laminin capable of promoting axonal growth both in vitro and in vivo. It is assembled in a cell-free system when laminin 111 is incubated in acidic pH, whereas incubation in neutral buffer leads to the formation of bulky and irregular polymers (LM). In the present work, we compared the behaviors of cells isolated from the P1 rat retina on polyLM and LM. PolyLM induced cellular spreading and the outgrowth of neurites in contact with the substrate, whereas LM led to the formation of large clusters of cells, with neurites growing only inward. After 24 hr in culture, the number of cells on polyLM increased threefold, and this increase was inhibited by 60% in the presence of the PKA inhibitor H89 and by 41% in the presence of the PKC inhibitor chelerythrine chloride, whereas both inhibitors abolished neuritogenesis. Neither the cell number nor the outgrowth of neurites was affected by the ERK1/2 inhibitor PD98059 on polyLM. On the other hand, PD98059 was able to reduce the cell number on LM, whereas the other inhibitors were not. Immunostaining of P1 retina with an antilaminin antibody revealed that the protein was expressed not only at its inner surface but also within the neuroblast layer in close contact with individual cells. Our results indicate that, when provided in its active polymerized form, laminin can influence both neuritogenesis and proliferation of retinal cells.

Proregenerative properties of ECM molecules

After traumatic injuries to the nervous system, regrowing axons encounter a complex microenvironment where mechanisms that promote regeneration compete with inhibitory processes. Sprouting and axonal regrowth are key components of functional recovery but are often counteracted by inhibitory molecules. This review covers extracellular matrix molecules that support neuron axonal outgrowth.

Integrins and cAMP mediate netrin-induced growth cone collapse

Growth cones integrate a remarkably complex concert of chemical cues to guide axons to their appropriate destinations. Recent work suggests that integrins contribute to axon guidance by interacting with a wide range of extracellular molecules including axon guidance molecules, by mechanisms that are not fully understood. Here, we describe an interaction between integrins and netrin-1 in growth cones that contributes to growth cone collapse. Our data show that netrin-1 causes growth cone collapse in a substratum-specific manner and is integrin-dependent. Netrin-1 causes collapse of cultured chick dorsal root ganglion (DRG) growth cones extending on high levels of laminin-1 (LN) but not growth cones extending on low levels of LN or on fibronectin. Blocking integrin function significantly decreases netrin-induced growth cone collapse on high LN. Netrin-1 and integrins interact on growth cones; netrin-1 causes integrin activation, a conformational shift to a high ligand-affinity state. Netrin-1 directly binds to integrin α3 and α6 peptides, further suggesting a netrin-integrin interaction. Interestingly, our data reveal that netrin-1 increases growth cone levels of cAMP in a substratum-specific manner and that netrin-induced growth cone collapse requires increased cAMP in combination with integrin activation. Manipulations that either decrease cAMP levels or integrin activation block netrin-induced collapse. These results imply a common mechanism for growth cone collapse and novel interactions between integrins, netrin-1 and cAMP that contribute to growth cone guidance.

Contributions of VLDLR and LRP8 in the establishment of retinogeniculate projections

BACKGROUND

Retinal ganglion cells (RGCs), the output neurons of the retina, project to over 20 distinct brain nuclei, including the lateral geniculate nucleus (LGN), a thalamic region comprised of three functionally distinct subnuclei: the ventral LGN (vLGN), the dorsal LGN (dLGN) and the intergeniculate leaflet (IGL). We previously identified reelin, an extracellular glycoprotein, as a critical factor that directs class-specific targeting of these subnuclei. Reelin is known to bind to two receptors: very-low-density lipoprotein receptor (VLDLR) and low-density lipoprotein receptor-related protein 8 (LRP8), also known as apolipoprotein E receptor 2 (ApoER2). Here we examined the roles of these canonical reelin receptors in retinogeniculate targeting.

RESULTS

To assess the roles of VLDLR and LRP8 in retinogeniculate targeting, we used intraocular injections of fluorescently conjugated cholera toxin B subunit (CTB) to label all RGC axons in vivo. Retinogeniculate projections in mutant mice lacking either VLDLR or LRP8 appeared similar to controls; however, deletion of both receptors resulted in dramatic defects in the pattern of retinal innervation in LGN. Surprisingly, defects in vldlr(-/-);lrp8(-/-) double mutant mice were remarkably different than those observed in mice lacking reelin. First, we failed to observe retinal axons exiting the medial border of the vLGN and IGL to invade distant regions of non-retino-recipient thalamus. Second, an ectopic region of binocular innervation emerged in the dorsomedial pole of vldlr(-/-);lrp8(-/-) mutant dLGN. Analysis of retinal projection development, retinal terminal sizes and LGN cytoarchitecture in vldlr(-/-);lrp8(-/-) mutants, all suggest that a subset of retinal axons destined for the IGL are misrouted to the dorsomedial pole of dLGN in the absence of VLDLR and LRP8. Such mistargeting is likely the result of abnormal migration of IGL neurons into the dorsomedial pole of dLGN in vldlr(-/-);lrp8(-/-) mutants.

CONCLUSIONS

In contrast to our expectations, the development of both the LGN and retinogeniculate projections appeared dramatically different in mutants lacking either reelin or both canonical reelin receptors. These results suggest that there are reelin-independent functions of VLDLR and LRP8 in LGN development, and VLDLR- and LRP8-independent functions of reelin in class-specific axonal targeting.

A genome-scale RNA-interference screen identifies RRAS signaling as a pathologic feature of Huntington's disease

A genome-scale RNAi screen was performed in a mammalian cell-based assay to identify modifiers of mutant huntingtin toxicity. Ontology analysis of suppressor data identified processes previously implicated in Huntington's disease, including proteolysis, glutamate excitotoxicity, and mitochondrial dysfunction. In addition to established mechanisms, the screen identified multiple components of the RRAS signaling pathway as loss-of-function suppressors of mutant huntingtin toxicity in human and mouse cell models. Loss-of-function in orthologous RRAS pathway members also suppressed motor dysfunction in a Drosophila model of Huntington's disease. Abnormal activation of RRAS and a down-stream effector, RAF1, was observed in cellular models and a mouse model of Huntington's disease. We also observe co-localization of RRAS and mutant huntingtin in cells and in mouse striatum, suggesting that activation of R-Ras may occur through protein interaction. These data indicate that mutant huntingtin exerts a pathogenic effect on this pathway that can be corrected at multiple intervention points including RRAS, FNTA/B, PIN1, and PLK1. Consistent with these results, chemical inhibition of farnesyltransferase can also suppress mutant huntingtin toxicity. These data suggest that pharmacological inhibition of RRAS signaling may confer therapeutic benefit in Huntington's disease.

β1 integrin-focal adhesion kinase (FAK) signaling modulates retinal ganglion cell (RGC) survival

Extracellular matrix (ECM) integrity in the central nervous system (CNS) is essential for neuronal homeostasis. Signals from the ECM are transmitted to neurons through integrins, a family of cell surface receptors that mediate cell attachment to ECM. We have previously established a causal link between the activation of the matrix metalloproteinase-9 (MMP-9), degradation of laminin in the ECM of retinal ganglion cells (RGCs), and RGC death in a mouse model of retinal ischemia-reperfusion injury (RIRI). Here we investigated the role of laminin-integrin signaling in RGC survival in vitro, and after ischemia in vivo. In purified primary rat RGCs, stimulation of the β1 integrin receptor with laminin, or agonist antibodies enhanced RGC survival in correlation with activation of β1 integrin’s major downstream regulator, focal adhesion kinase (FAK). Furthermore, β1 integrin binding and FAK activation were required for RGCs’ survival response to laminin. Finally, in vivo after RIRI, we observed an up-regulation of MMP-9, proteolytic degradation of laminin, decreased RGC expression of β1 integrin, FAK and Akt dephosphorylation, and reduced expression of the pro-survival molecule bcl-xL in the period preceding RGC apoptosis. RGC death was prevented, in the context of laminin degradation, by maintaining β1 integrin activation with agonist antibodies. Thus, disruption of homeostatic RGC-laminin interaction and signaling leads to cell death after retinal ischemia, and maintaining integrin activation may be a therapeutic approach to neuroprotection.

Integrin manipulation to improve regeneration

After central nervous system (CNS) insults, such as spinal cord injury or traumatic brain injury, neurons encounter a complex microenvironment where mechanisms that promote regeneration compete with inhibitory processes. Sprouting and axonal re-growth are key components of functional recovery, but are often counteracted by inhibitory molecules. Several strategies are being pursued whereby these inhibitory molecules are either being neutralized with blocking antibodies, with enzymatic degradation or downstream signaling events are being interfered with. Two recent studies ( 1) (,) ( 2) show that activating integrin signaling in dorsal root ganglion (DRG) neurons renders them able to overcome inhibitory signals, and could possibly lead to new strategies to improve neuronal regeneration.

R-Ras controls axon branching through afadin in cortical neurons

Regulation of axon growth, guidance, and branching is essential for constructing a correct neuronal network. R-Ras, a Ras-family small GTPase, has essential roles in axon formation and guidance. During axon formation, R-Ras activates a series of phosphatidylinositol 3-kinase signaling, inducing activation of a microtubule-assembly promoter-collapsin response mediator protein-2. However, signaling molecules linking R-Ras to actin cytoskeleton-regulating axonal morphology remain obscure. Here we identify afadin, an actin-binding protein harboring Ras association (RA) domains, as an effector of R-Ras inducing axon branching through F-actin reorganization. We observe endogenous interaction of afadin with R-Ras in cortical neurons during the stage of axonal development. Ectopic expression of afadin increases axon branch number, and the RA domains and the carboxyl-terminal F-actin binding domain are required for this action. RNA interference knockdown experiments reveal that knockdown of endogenous afadin suppressed both basal and R-Ras-mediated axon branching in cultured cortical neurons. Subcellular localization analysis shows that active R-Ras-induced translocation of afadin and its RA domains is responsible for afadin localizing to the membrane and inducing neurite development in Neuro2a cells. Overall, our findings demonstrate a novel signaling pathway downstream of R-Ras that controls axon branching.

14-3-3ε couples protein kinase A to semaphorin signaling and silences plexin RasGAP-mediated axonal repulsion

The biochemical means through which multiple signaling pathways are integrated in navigating axons is poorly understood. Semaphorins are among the largest families of axon guidance cues and utilize Plexin (Plex) receptors to exert repulsive effects on axon extension. However, Semaphorin repulsion can be silenced by other distinct cues and signaling cascades, raising questions of the logic underlying these events. We now uncover a simple biochemical switch that controls Semaphorin/Plexin repulsive guidance. Plexins are Ras/Rap family GTPase activating proteins (GAPs) and we find that the PlexA GAP domain is phosphorylated by the cAMP-dependent protein kinase (PKA). This PlexA phosphorylation generates a specific binding site for 14-3-3ε, a phospho-binding protein that we find to be necessary for axon guidance. These PKA-mediated Plexin-14-3-3ε interactions prevent PlexA from interacting with its Ras family GTPase substrate and antagonize Semaphorin repulsion. Our results indicate that these interactions switch repulsion to adhesion and identify a point of convergence for multiple guidance molecules.

Neural regenerative strategies incorporating biomolecular axon guidance signals

There are currently no acceptable cures for central nervous system injuries, and damage induced large gaps in the peripheral nervous system have been challenging to bridge to restore neural functionality. Innervation by neurons is made possible by the growth cone. This dynamic structure is unique to neurons, and can directly sense physical and chemical activity in its environment, utilizing these cues to propel axons to precisely reach their targets. Guidance can occur through chemoattractive factors such as neurotrophins and netrins, chemorepulsive agents like semaphorins and slits, or contact-mediated molecules such as ephrins and those located in the extracellular matrix. The understanding of biomolecular activity during nervous system development and injury has generated new techniques and tactics for improving and restoring function to the nervous system after injury. This review will focus on the major neuronal guidance molecules and their utility in current tissue engineering and neural regenerative strategies.

Bidirectional remodeling of β1-integrin adhesions during chemotropic regulation of nerve growth

Background

Chemotropic factors in the extracellular microenvironment guide nerve growth by acting on the growth cone located at the tip of extending axons. Growth cone extension requires the coordination of cytoskeleton-dependent membrane protrusion and dynamic adhesion to the extracellular matrix, yet how chemotropic factors regulate these events remains an outstanding question. We demonstrated previously that the inhibitory factor myelin-associated glycoprotein (MAG) triggers endocytic removal of the adhesion receptor β1-integrin from the growth cone surface membrane to negatively remodel substrate adhesions during chemorepulsion. Here, we tested how a neurotrophin might affect integrin adhesions.

Results

We report that brain-derived neurotropic factor (BDNF) positively regulates the formation of substrate adhesions in axonal growth cones during stimulated outgrowth and prevents removal of β1-integrin adhesions by MAG. Treatment of Xenopus spinal neurons with BDNF rapidly triggered β1-integrin clustering and induced the dynamic formation of nascent vinculin-containing adhesion complexes in the growth cone periphery. Both the formation of nascent β1-integrin adhesions and the stimulation of axon extension by BDNF required cytoplasmic calcium ion signaling and integrin activation at the cell surface. Exposure to MAG decreased the number of β1-integrin adhesions in the growth cone during inhibition of axon extension. In contrast, the BDNF-induced adhesions were resistant to negative remodeling by MAG, correlating with the ability of BDNF pretreatment to counteract MAG-inhibition of axon extension. Pre-exposure to MAG prevented the BDNF-induced formation of β1-integrin adhesions and blocked the stimulation of axon extension by BDNF.

Conclusions

Altogether, these findings demonstrate the neurotrophin-dependent formation of integrin-based adhesions in the growth cone and reveal how a positive regulator of substrate adhesions can block the negative remodeling and growth inhibitory effects of MAG. Such bidirectional remodeling may allow the growth cone to rapidly adjust adhesiveness to the extracellular matrix as a general mechanism for governing axon extension. Techniques for manipulating integrin internalization and activation state may be important for overcoming local inhibitory factors after traumatic injury or neurodegenerative disease to enhance regenerative nerve growth.

Endothelium-derived Netrin-4 supports pancreatic epithelial cell adhesion and differentiation through integrins α2β1 and α3β1

BACKGROUND

Netrins have been extensively studied in the developing central nervous system as pathfinding guidance cues, and more recently in non-neural tissues where they mediate cell adhesion, migration and differentiation. Netrin-4, a distant relative of Netrins 1-3, has been proposed to affect cell fate determination in developing epithelia, though receptors mediating these functions have yet to be identified.

METHODOLOGY/PRINCIPAL FINDINGS

Using human embryonic pancreatic cells as a model of developing epithelium, here we report that Netrin-4 is abundantly expressed in vascular endothelial cells and pancreatic ductal cells, and supports epithelial cell adhesion through integrins α2β1 and α3β1. Interestingly, we find that Netrin-4 recognition by embryonic pancreatic cells through integrins α2β1 and α3β1 promotes insulin and glucagon gene expression. In addition, full genome microarray analysis revealed that fetal pancreatic cell adhesion to Netrin-4 causes a prominent down-regulation of cyclins and up-regulation of negative regulators of the cell cycle. Consistent with these results, a number of other genes whose activities have been linked to developmental decisions and/or cellular differentiation are up-regulated.

CONCLUSIONS/SIGNIFICANCE

Given the recognized function of blood vessels in epithelial tissue morphogenesis, our results provide a mechanism by which endothelial-derived Netrin-4 may function as a pro-differentiation cue for adjacent developing pancreatic cell populations expressing adhesion receptors α2β1 and α3β1 integrins.

Student award winner in the undergraduate's degree category for the Society for Biomaterials 35th Annual Meeting, Orlando, Florida, April 13-16, 2011. Neurite growth in PEG gels: effect of mechanical stiffness and laminin concentration

Within a 3D environment, the chemical and mechanical properties of a scaffold can significantly influence nerve behavior. How these properties influence with nerve cells is important for optimizing neurite extension within a scaffold. The purpose of this study was to investigate the effect of low concentration poly(ethylene glycol) (PEG) with added laminin on 3D growth of dissociated dorsal root ganglia. Because of its high affinity for neurite adhesion and ability to promote extension, laminin was conjugated to the PEG chain, as well as mixed in the gel, at various concentrations to provide chemical cues. Gel stiffness, as determined by G*, significantly decreased with decreasing PEG concentration and with increasing laminin conjugate. Extension within the gels increased as the concentration of laminin increased with no difference between how laminin was presented (mixed or conjugated) to the cells. For example, in 3% PEG, extension increased from 92.29 ± 5.27 μm to 146.35 ± 13.12 μm as laminin conjugate concentration increased from plain to 100 μg/ml. Results indicated that the chemical properties of the scaffold influenced neurite growth more than the mechanical properties as laminin concentration had a greater impact on growth than the stiffness of the gel over the range studied. Neurite length as a function of scaffold stiffness and adhesion properties was also characterized and demonstrated a positive linear relationship between rate of neurite extension and laminin concentration. This study further demonstrates the importance of characterizing interactions between cell behavior and the chemical and mechanical environment.

Characterization of poly(ethylene glycol) gels with added collagen for neural tissue engineering

Over the past decade, it has been increasingly recognized that both chemical and mechanical properties of scaffolds influence neural cell behavior, ranging from growth to differentiation to migration. However, mechanical properties are difficult to control for in the design of scaffolds for nerve regeneration, as properties change over time for most biologically derived scaffolds. The focus of this project was to examine how the mechanical properties of a nondegradable scaffold, poly(ethylene glycol) (PEG) gels, influenced nerve cell behavior. Low concentration PEG gels, of 3, 4, or 5% PEG, with added collagen to alter chemical properties were examined for both their mechanical properties and their ability to support nerve expression and extension. Stiffness (G*) significantly increased with increased PEG concentration. The addition of chemically conjugated collagen significantly decreased the stiffness compared to plain gels. This phenomenon was confirmed to be an effect of the conjugate, and not the protein itself, as G* of gels containing conjugate, but no protein, was not significantly different than G* of gels with conjugated protein. PC12 cell neurite expression increased with decreasing PEG and increasing collagen concentration. At its best, the expression approached the value on collagen-coated tissue culture plastic, which is a substantial improvement over previous studies on PEG. Neurite extension of dorsal root ganglia was also improved on these same gels over gels with either higher PEG concentration or lower collagen amount. Overall, these results suggest that exploration of lower stiffness materials is necessary to improve neurite growth and extension in three-dimensional synthetic scaffolds.

Schwann cell migration is integrin-dependent and inhibited by astrocyte-produced aggrecan

Schwann cells transplantation has considerable promise in spinal cord trauma to bridge the site of injury and for remyelination in demyelinating conditions. They support axonal regeneration and sprouting by secreting growth factors and providing a permissive surface and matrix molecules while shielding axons from the inhibitory environment of the central nervous system. However, following transplantation Schwann cells show limited migratory ability and they are unable to intermingle with the host astrocytes. This in turn leads to formation of a sharp boundary and an abrupt transition between the Schwann cell graft and the host tissue astrocytes, therefore preventing regenerating axons from exiting the graft. The objective of this study was to identify inhibitory elements on astrocytes involved in restricting Schwann cell migration. Using in vitro assays of cell migration, we show that aggrecan produced by astrocytes is involved in the inhibition of Schwann cell motility on astrocytic monolayers. Knockdown of this proteoglycan in astrocytes using RNAi or digestion of glycosaminglycan chains on aggrecan improves Schwann cell migration. We further show aggrecan mediates its effect by disruption of integrin function in Schwann cells, and that the inhibitory effects of aggrecan can overcome by activation of Schwann cell integrins.

Integrin activation or alpha 9 expression allows retinal pigmented epithelial cell adhesion on Bruch's membrane in wet age-related macular degeneration

Retinal pigment epithelial cell malfunction is a causative feature of age-related macular degeneration, and transplantation of new retinal pigment epithelial cells is an attractive strategy to prevent further progression and visual loss. However, transplants have shown limited efficacy, mainly because transplanted cells fail to adhere and migrate onto pathological Bruch's membrane. Adhesion to Bruch's membrane is integrin-mediated. Ageing of Bruch's membrane leads to a decline in integrin ligands and, added to this, wet age-related macular degeneration leads to upregulation of anti-adhesive molecules such as tenascin-C. We have therefore investigated whether manipulation of integrin function in retinal pigment epithelial cells can restore their adhesion and migration on wet age-related macular degeneration-damaged Bruch's membrane. Using spontaneously immortalized human retinal pigment epithelial cells (adult retinal pigment epithelium-19), we show that adhesion and migration on the Bruch's membrane components is integrin-dependent and enhanced by integrin-activating agents manganese and TS2/16. These allowed cells to adhere and migrate on low concentrations of ligand, as would be found in aged Bruch's membrane. We next developed a method for stripping cells from Bruch's membrane so that adhesion and migration assays can be performed on its surface. Integrin activation had a moderate effect on enhancing retinal pigmented epithelial cell adhesion and migration on normal human and rat Bruch's membrane. However, on Bruch's membrane prepared from human wet age-related macular degeneration-affected eyes, adhesion was lower and integrin activation had a much greater effect. A candidate molecule for preventing retinal pigmented epithelial interaction with age-related macular degeneration-affected Bruch's membrane is tenascin-C which we confirm is present at high levels in wet age-related macular degeneration membrane. We show that tenascin-C is anti-adhesive for retinal pigmented epithelial cells, but after integrin activation, they can adhere and migrate on it using alphaVbeta3 integrin. Alternatively, we find that transduction of retinal pigmented epithelial cells with alpha9 integrin, a tenascin-C-binding integrin, led to a large increase in alpha9beta1-mediated adhesion and migration on tenascin-C. Both expression of alpha9 integrin and integrin activation greatly enhanced the ability of retinal pigment epithelial cells to adhere to tenascin-rich wet age-related macular degeneration-affected Bruch's membranes. Our results suggest that manipulation of retinal pigment epithelial cell integrins through integrin activating strategies, or expression of new integrins such as alpha9, could be effective in improving the efficacy of retinal pigment epithelial cell transplantation in wet age-related macular degeneration-affected eyes.

Laminin-1 induces neurite outgrowth in human mesenchymal stem cells in serum/differentiation factors-free conditions through activation of FAK-MEK/ERK signaling pathways

Mesenchymal stem cells (MSCs) can be differentiated into cell types derived from all three germ layers by manipulating culture conditions in vitro. A multitude of growth and differentiation factors have been employed for driving MSCs towards a neuronal phenotype. In the present study, we investigated the potential of extracellular matrix (ECM) proteins-fibronectin, collagen-1, collagen-IV, laminin-1, and laminin-10/11, to induce a neuronal phenotype in bone marrow derived human MSCs in the absence of growth factors/differentiating agents. All of the ECM proteins tested were found to support adhesion of MSCs to different extents. However, direct interaction only with laminin-1 triggered sprouting of neurite-like processes. Cells plated on laminin-1 exhibited neurite out growth as early as 3h, and by 24h, the cells developed elaborate neurites with contracted cell bodies and neuronal-like morphology. Function-blocking antibodies directed against alpha6 and beta1 integrin subunits inhibited neurite formation on laminin-1 which confirmed the involvement of integrin alpha6beta1 in neurite outgrowth. Mechanistic studies revealed that cell adhesion to laminin-1 activated focal adhesion kinase (FAK), and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (MEK/ERK) signaling pathways. Abrogation of FAK phosphorylation by herbimycin-A inhibited neurite formation and also decreased activities of MEK and ERK. Pharmacological inhibitors of MEK (U0126) and ERK (PD98059) also blocked neurite outgrowth in cells plated on laminin-1. Our study demonstrates the involvement of integrin alpha6beta1 and FAK-MEK/ERK signaling pathways in laminin-1-induced neurite outgrowth in MSCs in the absence of serum and differentiation factors.

Overexpression of the partially activated alpha(IIb)beta3D723H integrin salt bridge mutant downregulates RhoA activity and induces microtubule-dependent proplatelet-like extensions in Chinese hamster ovary cells

BACKGROUND

We have recently reported a novel mutation in the beta3 subunit of the platelet fibrinogen receptor (alpha(IIb)beta3D723H) identified in a patient with dominantly inherited macrothrombocytopenia, and we have shown that this mutation promotes a new phenotype in Chinese hamster ovary (CHO) cells, characterized by fibrinogen-dependent, microtubule-driven proplatelet-like cell extensions.

RESULTS

Here we demonstrate that the partially activated alpha(IIb)beta3D723H or alpha(IIb)beta3D723A salt bridge mutants, but not fully activated alpha(IIb)beta3 mutants, cause this phenotype. Time-lapse videomicroscopy clearly differentiated these stable microtubule-driven and nocodazole-sensitive extensions from common dynamic actin-driven pseudopodia. In addition, overexpression of a mitochondrial marker confirmed their functional role in organelle transport. Comparative immunofluorescence analysis of the subcellular localization of alpha(IIb)beta3, the focal adhesion proteins talin or vinculin and actin revealed a similar membrane labeling of CHO cell extensions and CD34+-derived megakaryocyte proplatelets. Mutant alpha(IIb)beta3D723H signaling was independent of Src, protein kinase C or phosphoinositide 3-kinase, but correlated with decreased RhoA activity as compared with wild-type alpha(IIb)beta3 signaling, reminiscent of integrin signaling during neurite outgrowth. Accordingly, overexpression of constitutively active RhoA in CHO alpha(IIb)beta3D723H cells prevented protrusion formation on fibrinogen. Most interestingly, RhoA/ROCK inhibition was necessary, but not sufficient, and integrin activity was additionally required to induce CHO cell extension formation.

CONCLUSIONS

CHO alpha(IIb)beta3D723H cell protrusions and megakaryocyte proplatelets, like neuronal cell neurites, result from a common integrin-dependent signaling pathway, promoting strongly decreased RhoA activity and leading to microtubule-driven formation of cytoplasmic extensions.

Applied electric field enhances DRG neurite growth: influence of stimulation media, surface coating and growth supplements

Electrical therapies have been found to aid repair of nerve injuries and have been shown to increase and direct neurite outgrowth during stimulation. This enhanced neural growth existed even after the electric field (EF) or stimulation was removed, but the factors that may influence the enhanced growth, such as stimulation media or surface coating, have not been fully investigated. This study characterized neurite outgrowth and branching under various conditions: EF magnitude and application time, ECM surface coating, medium during EF application and growth supplements. A uniform, low-magnitude EF (24 or 44 V m(-1)) was applied to dissociated chick embryo dorsal root ganglia seeded on collagen or laminin-coated surfaces. During the growth period, cells were either exposed to NGF or N2, and during stimulation cells were exposed to either unsupplemented media (Ca(2+)) or PBS (no Ca(2+)). Parallel controls for each experiment included cells exposed to the chamber with no stimulation and cells remaining outside the chamber. After brief electrical stimulation (10 min), neurite length significantly increased 24 h after application for all conditions studied. Of particular interest, increased stimulation time (10-100 min) further enhanced neurite length on laminin but not on collagen surfaces. Neurite branching was not affected by stimulation on any surface, and no preferential growth of neurites was noted after stimulation. Overall, the results of this report suggest that short-duration electric stimulation is sufficient to enhance neurite length under a variety of conditions. While further data are needed to fully elucidate a mechanism for this increased growth, these data suggest that one focus of those investigations should be the interaction between the growth cone and the substrata.

Plexin-B1 is a GTPase activating protein for M-Ras, remodelling dendrite morphology

Plexins are receptors for axonal guidance molecules known as semaphorins. We recently reported that the semaphorin 4D (Sema4D) receptor, Plexin-B1, induces axonal growth cone collapse by functioning as an R-Ras GTPase activating protein (GAP). Here, we report that Plexin-B1 shows GAP activity for M-Ras, another member of the Ras family of GTPases. In cortical neurons, the expression of M-Ras was upregulated during dendritic development. Knockdown of endogenous M-Ras-but not R-Ras-reduced dendritic outgrowth and branching, whereas overexpression of constitutively active M-Ras, M-Ras(Q71L), enhanced dendritic outgrowth and branching. Sema4D suppressed M-Ras activity and reduced dendritic outgrowth and branching, but this reduction was blocked by M-Ras(Q71L). M-Ras(Q71L) stimulated extracellular signal-regulated kinase (ERK) activation, inducing dendrite growth, whereas Sema4D suppressed ERK activity and down-regulation of ERK was required for a Sema4D-induced reduction of dendrite growth. Thus, we conclude that Plexin-B1 is a dual functional GAP for R-Ras and M-Ras, remodelling axon and dendrite morphology, respectively.

Key roles of Ephs and ephrins in retinotectal topographic map formation

Cellular and molecular mechanisms involved in the development of topographic ordered connections in the central nervous system (CNS) constitute a key issue in neurobiology because neural connectivities are the base of the CNS normal function. We discuss the roles of the Eph/ephrin system in the establishment of retinotopic projections onto the tectum/colliculus, the most detailed studied model of topographic mapping. The expression patterns of Ephs and ephrins in opposing gradients both in the retina and the tectum/colliculus, label the local addresses on the target and give specific sensitivities to growth cones according to their topographic origin in the retina. We postulate that the highest levels of these gradients could signal both the entry as well as the limiting boundaries of the target. Since Ephs and ephrins are membrane-bound molecules, they may function as both receptors and ligands producing repulsive or attractant responses according to their microenvironment and play central roles in a variety of developmental events such as axon guidance, synapse formation and remodeling. Due to different experimental approaches and the inherent species-specific differences, some results appear contradictory and should be reanalyzed. Nevertheless, these studies about the roles of the Eph/ephrin system in retinotectal/collicular mapping support general principles in order to understand CNS development and could be useful to design regeneration therapies.

Binding of laminin-1 to monosialoganglioside GM1 in lipid rafts is crucial for neurite outgrowth

Laminin-1, an extracellular matrix molecule, promotes neurite outgrowth through the interaction of integrin and actin. Monosialoganglioside GM1 in the lipid rafts associates with and activates the NGF receptor TrkA, and enhances neurite outgrowth. However, the role of GM1 in laminin-1-induced neurite outgrowth was still unclear. Here, we describe that laminin-1 binds to GM1 through a carbohydrate moiety and a specific conformation of GM1, induces focal formation of large clusters of GM1, and enhances the relocation of TrkA in the membrane of dorsal root ganglion (DRG) and PC12 cells. We found that laminin-1-mediated clustering of GM1 causes the translocation and enrichment of beta1 integrin in lipid rafts--where TrkA colocalizes with beta1 integrin--and the activation of Lyn, Akt and MAPK to promote the outgrowth of neurites. Our results suggest that the binding of laminin-1 to GM1 facilitates the formation of a focal microdomain in the membrane, and enhances signal transduction that promotes neurite outgrowth by linking NGF-TrkA signaling with the laminin-integrin signaling pathways.

Improving RPE adhesion to Bruch's membrane

Age-related macular degeneration is the leading cause of blindness in the developing world. Retinal pigmented epithelium (RPE) transplantation in subretinal space, has been assessed in various animal models of age-related macular degeneration and in humans as a potential technique to preserve the visual function. However, the RPE cell survival posttransplantation is limited because of lack of attachment of the transplanted cells to the pathological Bruch's membrane and also partly because of iatrogenic removal of adhesive elements in the membrane during the removal of choroidal new vessels before transplantation procedure. Although pathological Bruch's membrane is well studied, there is still much debate as to why and how changes in the structure and components of this membrane leads to loss of RPE cells and disruption of their function and subsequent death of photoreceptors leading to visual loss. Integrins on RPE cells have been characterized and shown to be important for attachment of cells to Bruch's membrane. Considering the essential role of integrins in functions such as cell migration and adhesion, it is plausible that lack of attachment of RPE cells posttransplantation can be overcome by improving integrin function. Here, we have focused on some of the recent findings on the use of integrins and modulation of their function to improve the adhesion of RPE cells to normal and pathological Bruch's membrane. This work also aims at elucidating a potential mechanism by which accumulating inhibitory molecules in the Bruch's membrane in the pathological state, interferes with integrin function.

M-Ras evolved independently of R-Ras and its neural function is conserved between mammalian and ascidian, which lacks classical Ras

The Ras family small GTPases play a variety of essential roles in eukaryotes. Among them, classical Ras (H-Ras, K-Ras, and N-Ras) and its orthologues are conserved from yeast to human. In ascidians, which phylogenetically exist between invertebrates and vertebrates, the fibroblast growth factor (FGF)-Ras-MAP kinase signaling is required for the induction of neural system, notochord, and mesenchyme. Analyses of DNA databases revealed that no gene encoding classical Ras is present in the ascidians, Ciona intestinalis and Halocynthia roretzi, despite the presence of classical Ras-orthologous genes in nematode, fly, amphioxus, and fish. By contrast, both the ascidians contain single genes orthologous to Mras, Rras, Ral, Rap1, and Rap2. A single Mras orthologue exists from nematode to mammalian. Thus, Mras evolved in metazoans independently of other Ras family genes such as Rras. Whole-mount in situ hybridization showed that C. intestinalis Mras orthologue (Ci-Mras) was expressed in the neural complex of the ascidian juveniles after metamorphosis. Knockdown of Ci-Mras with morpholino antisense oligonucleotides in the embryos and larvae resulted in undeveloped tails and neuronal pigment cells, abrogation of the notochord marker brachyury expression, and perturbation of the neural marker Otx expression, as has been shown in the experiments of the FGF-Ras-MAP kinase signaling inhibition. Mammalian Ras and M-Ras mediate nerve growth factor-induced neuronal differentiation in rat PC12 cells by activating the ERK/MAP kinase pathway transiently and sustainedly, respectively. Activated Ci-M-Ras bound to target proteins of mammalian M-Ras and Ras. Exogenous expression of an activated Ci-M-Ras in PC12 cells caused ERK activation and induced neuritogenesis via the ERK pathway as do mammalian M-Ras and Ras. These results suggest that the ascidian M-Ras orthologue compensates for lacked classical Ras and plays essential roles in neurogenesis in the ascidian.

The influence of sub-micron inhibitory clusters on growth cone substratum attachments and CD44 expression

Proteoglycan expression patterns in the central nervous system guide neuronal pathfinding during development, but also disrupt regeneration after injuries. To deepen our understanding of the molecular level effects of proteoglycan spatial arrangements on neuronal pathfinding, we designed micropatterning stamps for the precise placement of near single molecule chondroitin sulfate proteoglycan (CSPG) clusters into regularly spaced arrays. Actin ultrastructural analysis in dorsal root ganglion neurons grown on laminin-coated substrata patterned with aggrecan cluster arrays revealed filopodial and lamellapodial edge contact avoidance of individual clusters, while growth cone lamellapodia and central domains were able to span multiple clusters over a range of cluster densities. Total internal reflection fluorescence microscopy interrogation of growth cone substratum morphology further revealed persistence of integrin mediated substratum adhesion and local out-of-plane membrane bending over clusters on the height scale of aggrecan glycosaminoglycan side chains. Direct imaging of cell adhesion molecule CD44 expression in growth cones revealed an aggrecan dose dependent upregulation in CD44 molecules. Evidence of CD44 clustering coinciding with underlying aggrecan molecules implies CSPG-CD44 interactions. The results reveal the limited local repulsive effect of CSPGs on neuronal structures and provide evidence that CD44 upregulation in neurons is affected by local CSPG expression.