Laminin-like proteins are differentially regulated during cerebellar development and stimulate granule cell neurite outgrowth in vitro

The Interaction of LAMA2 and Duration of Illness Affects the Thickness of the Right Transverse Temporal Gyrus in Major Depressive Disorder

Background

Depression is a heritable brain disorder. Laminin genes were recently identified to affect the brain's overall thickness through neurogenesis, differentiation, and migration in depression. This study aims to explore the effects of the LAMA2's single nucleotide polymorphisms (SNP), a subunit gene of laminin, on the detected brain regions of patients with major depressive disorder (MDD).

Methods

The study included 89 patients with MDD and 60 healthy controls with T1-weighted structural magnetic resonance imaging and blood samples for genotyping. The interactions between LAMA2 gene SNPs and diagnosis as well as duration of illness (DOI) were explored on brain measures controlled for age, gender, and site.

Results

The right transverse temporal gyrus and right parahippocampal gyrus showed reduced thickness in MDD. Almost all seven LAMA2 SNPs showed significant interactions with diagnosis on both gyrus (corrected p < 0.05 or trending). In MDD, rs6569604, rs2229848, rs2229849, rs2229850, and rs2784895 interacted with DOI on the right transverse temporal gyrus (corrected p < 0.05), but not the right parahippocampal gyrus.

Conclusion

The thickness of the right transverse temporal gyrus in patients with MDD may be affected by LAMA2 gene and DOI.

Lactate promotes neuronal differentiation of SH-SY5Y cells by lactate-responsive gene sets through NDRG3-dependent and -independent manners

Lactate serves as the major glucose alternative to an energy substrate in the brain. Lactate level is increased in the fetal brain from the middle stage of gestation, indicating the involvement of lactate in brain development and neuronal differentiation. Recent reports show that lactate functions as a signaling molecule to regulate gene expression and protein stability. However, the roles of lactate signaling in neuronal cells remain unknown. Here, we showed that lactate promotes the all stages of neuronal differentiation of SH-SY5Y and Neuro2A, human and mouse neuroblastoma cell lines, characterized by increased neuronal marker expression and the rates of neurites extension. Transcriptomics revealed many lactate-responsive genes sets such as SPARCL1 in SH-SY5Y, Neuro2A, and primary embryonic mouse neuronal cells. The effects of lactate on neuronal function were mainly mediated through monocarboxylate transporters 1 (MCT1). We found that NDRG family member 3 (NDRG3), a lactate-binding protein, was highly expressed and stabilized by lactate treatment during neuronal differentiation. Combinative RNA-seq of SH-SY5Y with lactate treatment and NDRG3 knockdown shows that the promotive effects of lactate on neural differentiation are regulated through NDRG3-dependent and independent manners. Moreover, we identified TEA domain family member 1 (TEAD1) and ETS-related transcription factor 4 (ELF4) are the specific transcription factors that are regulated by both lactate and NDRG3 in neuronal differentiation. TEAD1 and ELF4 differently affect the expression of neuronal marker genes in SH-SY5Y cells. These results highlight the biological roles of extracellular and intracellular lactate as a critical signaling molecule that modifies neuronal differentiation.

Brain Dysfunction in LAMA2-Related Congenital Muscular Dystrophy: Lessons From Human Case Reports and Mouse Models

Laminin α2 gene (LAMA2)-related Congenital Muscular Dystrophy (CMD) was distinguished by a defining central nervous system (CNS) abnormality—aberrant white matter signals by MRI—when first described in the 1990s. In the past 25 years, researchers and clinicians have expanded our knowledge of brain involvement in LAMA2-related CMD, also known as Congenital Muscular Dystrophy Type 1A (MDC1A). Neurological changes in MDC1A can be structural, including lissencephaly and agyria, as well as functional, including epilepsy and intellectual disability. Mouse models of MDC1A include both spontaneous and targeted LAMA2 mutations and range from a partial loss of LAMA2 function (e.g., dy^2J/dy^2J), to a complete loss of LAMA2 expression (dy^3K/dy^3K). Diverse cellular and molecular changes have been reported in the brains of MDC1A mouse models, including blood-brain barrier dysfunction, altered neuro- and gliogenesis, changes in synaptic plasticity, and decreased myelination, providing mechanistic insight into potential neurological dysfunction in MDC1A. In this review article, we discuss selected studies that illustrate the potential scope and complexity of disturbances in brain development in MDC1A, and as well as highlight mechanistic insights that are emerging from mouse models.

Cerebellar Cells Self-Assemble into Functional Organoids on Synthetic, Chemically Crosslinked ECM-Mimicking Peptide Hydrogels

Hydrogel-supported neural cell cultures are more in vivo-relevant compared to monolayers formed on glass or plastic substrates. However, there is a lack of synthetic microenvironment available for obtaining standardized and easily reproducible cultures characterized by tissue-mimicking cell composition, cell-cell interactions, and functional networks. Synthetic peptides representing the biological properties of the extracellular matrix (ECM) proteins have been reported to promote the adhesion-driven differentiation and functional maturation of neural cells. Thus, such peptides can serve as building blocks for engineering a standardized, all-synthetic environment. In this study, we have compared the effect of two chemically crosslinked hydrogel compositions on primary cerebellar cells: collagen-like peptide (CLP), and CLP with an integrin-binding motif arginine-glycine-aspartate (CLP-RGD), both conjugated to polyethylene glycol molecular templates (PEG-CLP and PEG-CLP-RGD, respectively) and fabricated as self-supporting membranes. Both compositions promoted a spontaneous organization of primary cerebellar cells into tissue-like clusters with fast-rising Ca2+ signals in soma, reflecting action potential generation. Notably, neurons on PEG-CLP-RGD had more neurites and better synaptic efficiency compared to PEG-CLP. For comparison, poly-L-lysine-coated glass and plastic surfaces did not induce formation of such spontaneously active networks. Additionally, contrary to the hydrogel membranes, glass substrates functionalized with PEG-CLP and PEG-CLP-RGD did not sufficiently support cell attachment and, subsequently, did not promote functional cluster formation. These results indicate that not only chemical composition but also the hydrogel structure and viscoelasticity are essential for bioactive signaling. The synthetic strategy based on ECM-mimicking, multifunctional blocks in registry with chemical crosslinking for obtaining tissue-like mechanical properties is promising for the development of fast and well standardized functional in vitro neural models and new regenerative therapies.

Laminin-1 induces endocytosis of 67KDa laminin receptor and protects Neuroscreen-1 cells against death induced by serum withdrawal

Although the function of laminin in the basement membrane is known, the function of soluble "neuronal" laminin is unknown. Since laminin is neuroprotective, we determined whether the soluble laminin-1 induces signaling for neuroprotection via its 67KDa laminin-1 receptor (67LR). Treatment of Neuroscreen-1 (NS-1) cells with laminin-1 or YIGSR peptide, which corresponds to a sequence in laminin-1 β1 chain that binds to 67LR, induced a decrease in the cell-surface expression of 67LR and caused its internalization. Furthermore, intracellular cAMP-elevating agents, dibutyryl-cAMP, forskolin, and rolipram, also induced this internalization. Both soluble laminin-1 and YIGSR induced a sustained elevation of intracellular cAMP under defined conditions, suggesting a causal role of cAMP in the endocytosis of 67LR. This endocytosis was not observed in cells deficient in protein kinase A (PKA) nor in cells treated with either SQ 22536, an inhibitor for adenylyl cyclase, or ESI-09, an inhibitor for the exchange protein directly activated by cAMP (Epac). In addition, when internalization occurred in NS-1 cells, 67LR and adenylyl cyclase were localized in early endosomes. Under conditions in which endocytosis had occurred, both laminin-1 and YIGSR protected NS-1 cells from cell death induced by serum withdrawal. However, under conditions in which endocytosis did not occur, neither laminin-1 nor YIGSR protected these cells. Conceivably, the binding of laminin-1 to 67LR causes initial signaling through PKA and Epac, which causes the internalization of 67LR, along with signaling enzymes, such as adenylyl cyclase, into early endosomes. This causes sustained signaling for protection against cell death induced by serum withdrawal.

Phage-Enabled Nanomedicine: From Probes to Therapeutics in Precision Medicine

Both lytic and temperate bacteriophages (phages) can be applied in nanomedicine, in particular, as nanoprobes for precise disease diagnosis and nanotherapeutics for targeted disease treatment. Since phages are bacteria-specific viruses, they do not naturally infect eukaryotic cells and are not toxic to them. They can be genetically engineered to target nanoparticles, cells, tissues, and organs, and can also be modified with functional abiotic nanomaterials for disease diagnosis and treatment. This Review will summarize the current use of phage structures in many aspects of precision nanomedicine, including ultrasensitive biomarker detection, enhanced bioimaging for disease diagnosis, targeted drug and gene delivery, directed stem cell differentiation, accelerated tissue formation, effective vaccination, and nanotherapeutics for targeted disease treatment. We will also propose future directions in the area of phage-based nanomedicines, and discuss the state of phage-based clinical trials.

Synergistic regulation of cerebellar Purkinje neuron development by laminin epitopes and collagen on an artificial hybrid matrix construct

The extracellular matrix (ECM) creates a dynamic environment around the cells in the developing central nervous system, providing them with the necessary biochemical and biophysical signals. Although the functions of many ECM molecules in neuronal development have been individually studied in detail, the combinatorial effects of multiple ECM components are not well characterized. Here we demonstrate that the expression of collagen and laminin-1 (lam-1) are spatially and temporally correlated during embryonic and post-natal development of the cerebellum. These changes in ECM distribution correspond to specific stages of Purkinje neuron (PC) migration, somatic monolayer formation and polarization. To clarify the respective roles of these ECM molecules on PC development, we cultured cerebellar neurons on a hybrid matrix comprised of collagen and a synthetic peptide amphiphile nanofiber bearing a potent lam-1 derived bioactive IKVAV peptide epitope. By systematically varying the concentration and ratio of collagen and the laminin epitope in the matrix, we could demonstrate a synergistic relationship between these two ECM components in controlling multiple aspects of PC maturation. An optimal ratio of collagen and IKVAV in the matrix was found to promote maximal PC survival and dendrite growth, while dendrite penetration into the matrix was enhanced by a high IKVAV to collagen ratio. In addition, the laminin epitope was found to guide PC axon development. By combining our observations in vivo and in vitro, we propose a model of PC development where the synergistic effects of collagen and lam-1 play a key role in migration, polarization and morphological maturation of PCs.

Optimising parameters for the differentiation of SH-SY5Y cells to study cell adhesion and cell migration

BACKGROUND

Cell migration is a fundamental biological process and has an important role in the developing brain by regulating a highly specific pattern of connections between nerve cells. Cell migration is required for axonal guidance and neurite outgrowth and involves a series of highly co-ordinated and overlapping signalling pathways. The non-receptor tyrosine kinase, Focal Adhesion Kinase (FAK) has an essential role in development and is the most highly expressed kinase in the developing CNS. FAK activity is essential for neuronal cell adhesion and migration.

RESULTS

The objective of this study was to optimise a protocol for the differentiation of the neuroblastoma cell line, SH-SY5Y. We determined the optimal extracellular matrix proteins and growth factor combinations required for the optimal differentiation of SH-SY5Y cells into neuronal-like cells and determined those conditions that induce the expression of FAK. It was confirmed that the cells were morphologically and biochemically differentiated when compared to undifferentiated cells. This is in direct contrast to commonly used differentiation methods that induce morphological differentiation but not biochemical differentiation.

CONCLUSIONS

We conclude that we have optimised a protocol for the differentiation of SH-SY5Y cells that results in a cell population that is both morphologically and biochemically distinct from undifferentiated SH-SY5Y cells and has a distinct adhesion and spreading pattern and display extensive neurite outgrowth. This protocol will provide a neuronal model system for studying FAK activity during cell adhesion and migration events.

Signaling mechanisms regulating myelination in the central nervous system

The precise and coordinated production of myelin is essential for proper development and function of the nervous system. Diseases that disrupt myelin, including multiple sclerosis, cause significant functional disability. Current treatment aims to reduce the inflammatory component of the disease, thereby preventing damage resulting from demyelination. However, therapies are not yet available to improve natural repair processes after damage has already occurred. A thorough understanding of the signaling mechanisms that regulate myelin generation will improve our ability to enhance repair. in this review, we summarize the positive and negative regulators of myelination, focusing primarily on central nervous system myelination. Axon-derived signals, extracellular signals from both diffusible factors and the extracellular matrix, and intracellular signaling pathways within myelinating oligodendrocytes are discussed. Much is known about the positive regulators that drive myelination, while less is known about the negative regulators that shift active myelination to myelin maintenance at the appropriate time. Therefore, we also provide new data on potential negative regulators of CNS myelination.

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

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

A hybrid nanofiber matrix to control the survival and maturation of brain neurons

Scaffold design plays a crucial role in developing graft-based regenerative strategies, especially when intended to be used in a highly ordered nerve tissue. Here we describe a hybrid matrix approach, which combines the structural properties of collagen (type I) with the epitope-presenting ability of peptide amphiphile (PA) nanofibers. Self-assembly of PA and collagen molecules results in a nanofibrous scaffold with homogeneous fiber diameter of 20-30 nm, where the number of laminin epitopes IKVAV and YIGSR can be varied by changing the PA concentrations over a broad range of 0.125-2 mg/ml. Granule cells (GC) and Purkinje cells (PC), two major neuronal subtypes of cerebellar cortex, demonstrate distinct response to this change of epitope concentration. On IKVAV hybrid constructs, GC density increases three-fold compared with the control collagen substrate at a PA concentration of ≥0.25 mg/ml, while PC density reaches a maximum (five-fold vs. control) at 0.25 mg/ml of PA and rapidly decreases at higher PA concentrations. In addition, adjustment of the epitope number allowed us to achieve fine control over PC dendrite and axon growth. Due to the ability to modulate neuron survival and maturation by easy manipulation of epitope density, our design offers a versatile test bed to study the extracellular matrix (ECM) contribution in neuron development and the design of optimal neuronal scaffold biomaterials.

Integrin signaling in oligodendrocytes and its importance in CNS myelination

Multiple sclerosis is characterized by repeated demyelinating attacks of the central nervous system (CNS) white matter tracts. To tailor novel therapeutics to halt or reverse disease process, we require a better understanding of oligodendrocyte biology and of the molecular mechanisms that initiate myelination. Cell extrinsic mechanisms regulate CNS myelination through the interaction of extracellular matrix proteins and their transmembrane receptors. The engagement of one such receptor family, the integrins, initiates intracellular signaling cascades that lead to changes in cell phenotype. Oligodendrocytes express a diverse array of integrins, and the expression of these receptors is developmentally regulated. Integrin-mediated signaling is crucial to the proliferation, survival, and maturation of oligodendrocytes through the activation of downstream signaling pathways involved in cytoskeletal remodeling. Here, we review the current understanding of this important signaling axis and its role in oligodendrocyte biology and ultimately in the myelination of axons within the CNS.

Plasminogen enhances neuritogenesis on laminin-1

Proteins of the plasminogen activation system are broadly expressed throughout the nervous system, and key roles for these proteins in neuronal function have been demonstrated. Recent reports have established that plasminogen is synthesized in neuroendocrine tissues, making this protein and the proteolytic activity of the product of its activation, plasmin, available at sites separated anatomically from circulating, hepatocyte-derived plasminogen. Results with plasminogen-deficient humans and mice suggest a role for plasminogen in neuritogenesis. To elucidate the role of the plasminogen activation system in these processes, the function of plasminogen during neuritogenesis and neurite outgrowth was studied. It is shown here that plasminogen participates in neuritogenesis, as plasmin inhibitors reduced both neurite outgrowth and neurite length in PC-12 cells. The addition of exogenous plasminogen enhanced neurite outgrowth and neurite length in both PC-12 cells and primary cortical neurons. The proteolytic activity of plasmin was required, since mutation of the catalytic serine residue completely abolished the stimulatory activity. Furthermore, mutation of the lysine binding site within kringle 5 of the plasminogen molecule also reduced the neuritogenic activity of plasminogen. Additionally, we demonstrate that plasminogen specifically bound to laminin-1, the interaction resulted in increased plasminogen activation by tissue-type plasminogen activator, and was dependent on a functional lysine binding site within plasminogen kringle 5. Moreover, during NGF-induced neuritogenesis, laminin-1 was degraded, and this cleavage was catalyzed by plasmin. This study provides the first direct evidence that plasminogen participates in neurite outgrowth and also suggests that laminin-1 degradation by plasmin contributes to the process of neuritogenesis.

Isolation and culture of post-natal mouse cerebellar granule neuron progenitor cells and neurons

The cerebellar cortex is a well described structure that provides unique opportunities for studying neuronal properties and development^1,2. Of the cerebellar neuronal types (granule cells, Purkinje cells and inhibitory interneurons), granule neurons are by far the most numerous and are the most abundant type of neurons in the mammalian brain. In rodents, cerebellar granule neurons are generated during the first two post-natal weeks from progenitor cells in the outermost layer of the cerebellar cortex, the external granule layer (EGL). The protocol presented here describes techniques to enrich and culture granule neurons and their progenitor cells from post-natal mouse cerebellum. We will describe procedures to obtain cultures of increasing purity^3,4, which can be used to study the differentiation of proliferating progenitor cells into granule neurons^5,6. Once the progenitor cells differentiate, the cultures also provide a homogenous population of granule neurons for experimental manipulation and characterization of phenomena such as synaptogenesis, glutamate receptor function⁷, interaction with other purified cerebellar cells^8,9 or cell death⁷.

Building epithelial tissues from skin stem cells

The skin epidermis and its appendages provide a protective barrier that guards against loss of fluids, physical trauma, and invasion by harmful microbes. To perform these functions while confronting the harsh environs of the outside world, our body surface undergoes constant rejuvenation through homeostasis. In addition, it must be primed to repair wounds in response to injury. The adult skin maintains epidermal homeostasis, hair regeneration, and wound repair through the use of its stem cells. What are the properties of skin stem cells, when do they become established during embryogenesis, and how are they able to build tissues with such remarkably distinct architectures? How do stem cells maintain tissue homeostasis and repair wounds and how do they regulate the delicate balance between proliferation and differentiation? What is the relationship between skin cancer and mutations that perturbs the regulation of stem cells? In the past 5 years, the field of skin stem cells has bloomed as we and others have been able to purify and dissect the molecular properties of these tiny reservoirs of goliath potential. We report here progress on these fronts, with emphasis on our laboratory's contributions to the fascinating world of skin stem cells.

Identification of dystroglycan as a second laminin receptor in oligodendrocytes, with a role in myelination

Developmental abnormalities of myelination are observed in the brains of laminin-deficient humans and mice. The mechanisms by which these defects occur remain unknown. It has been proposed that, given their central role in mediating extracellular matrix (ECM) interactions, integrin receptors are likely to be involved. However, it is a non-integrin ECM receptor, dystroglycan, that provides the key linkage between the dystrophin-glycoprotein complex (DGC) and laminin in skeletal muscle basal lamina, such that disruption of this bridge results in muscular dystrophy. In addition, the loss of dystroglycan from Schwann cells causes myelin instability and disorganization of the nodes of Ranvier. To date, it is unknown whether dystroglycan plays a role during central nervous system (CNS) myelination. Here, we report that the myelinating glia of the CNS, oligodendrocytes, express and use dystroglycan receptors to regulate myelin formation. In the absence of normal dystroglycan expression, primary oligodendrocytes showed substantial deficits in their ability to differentiate and to produce normal levels of myelin-specific proteins. After blocking the function of dystroglycan receptors, oligodendrocytes failed both to produce complex myelin membrane sheets and to initiate myelinating segments when co-cultured with dorsal root ganglion neurons. By contrast, enhanced oligodendrocyte survival in response to the ECM, in conjunction with growth factors, was dependent on interactions with beta-1 integrins and did not require dystroglycan. Together, these results indicate that laminins are likely to regulate CNS myelination by interacting with both integrin receptors and dystroglycan receptors, and that oligodendrocyte dystroglycan receptors may have a specific role in regulating terminal stages of myelination, such as myelin membrane production, growth, or stability.

Integrin-linked kinase regulates Bergmann glial differentiation during cerebellar development

We demonstrate here that integrin-linked kinase (ILK), a serine/threonine kinase that binds to the beta1 integrin cytoplasmic domain, regulates cerebellar development. Mice with a CNS-restricted knock-out of the Ilk gene show perturbations in the laminar structure of the cerebellar cortex that are associated with defects in Bergmann glial fibers and the formation of meningeal basement membranes. Similar defects have been observed in mice lacking beta1 integrins in the CNS. ILK and beta1 integrins are coexpressed in Bergmann glial cells, and studies with primary cells in culture demonstrate that ILK and CDC42 are required for beta1-integrin-dependent glial process outgrowth. Consistent with these findings, the amount of GTP-bound CDC42 is impaired in the cerebellum of Ilk-deficient mice. We conclude that beta1 integrin, ILK and CDC42 are components of the signaling machinery that regulates glial process outgrowth in the cerebellum. We also show that granule cell precursor proliferation is affected in ILK-deficient mice, but our findings provide strong evidence that proliferative defects are a secondary consequence of ILK function in glia.

Beta1-integrin signaling mediates premyelinating oligodendrocyte survival but is not required for CNS myelination and remyelination

Previous reports, including transplantation experiments using dominant-negative inhibition of beta1-integrin signaling in oligodendrocyte progenitor cells, suggested that beta1-integrin signaling is required for myelination. Here, we test this hypothesis using conditional ablation of the beta1-integrin gene in oligodendroglial cells during the development of the CNS. This approach allowed us to study oligodendroglial beta1-integrin signaling in the physiological environment of the CNS, circumventing the potential drawbacks of a dominant-negative approach. We found that beta1-integrin signaling has a much more limited role than previously expected. Although it was involved in stage-specific oligodendrocyte cell survival, beta1-integrin signaling was not required for axon ensheathment and myelination per se. We also found that, in the spinal cord, remyelination occurred normally in the absence of beta1-integrin. We conclude that, although beta1-integrin may still contribute to other aspects of oligodendrocyte biology, it is not essential for myelination and remyelination in the CNS.

Laminin activates CaMK-II to stabilize nascent embryonic axons

In neurons, the interaction of laminin with its receptor, beta1 integrin, is accompanied by an increase in cytosolic Ca2+. Neuronal behavior is influenced by CaMK-II, the type II Ca2+/calmodulin-dependent protein kinase, which is enriched in axons of mouse embryonic neurons. In this study, we sought to determine whether CaMK-II is activated by laminin, and if so, how CaMK-II influences axonal growth and stability. Axons grew up to 200 microm within 1 day of plating P19 embryoid bodies on laminin-1 (EHS laminin). Activated CaMK-II was found enriched along the axon and in the growth cone as detected using a phospho-Thr(287) specific CaMK-II antibody. beta1 integrin was found in a similar pattern along the axon and in the growth cone. Direct inhibition of CaMK-II in 1-day-old neurons immediately froze growth cone dynamics, disorganized F-actin and ultimately led to axon retraction. Collapsed axonal remnants exhibited diminished phospho-CaMK-II levels. Treatment of 1-day neurons with a beta1 integrin-blocking antibody (CD29) also reduced axon length and phospho-CaMK-II levels and, like CaMK-II inhibitors, decreased CaMK-II activation. Among several CaMK-II variants detected in these cultures, the 52-kDa delta variant preferentially associated with actin and beta 3 tubulin as determined by reciprocal immunoprecipitation. Our findings indicate that persistent activation of delta CaMK-II by laminin stabilizes nascent embryonic axons through its influence on the actin cytoskeleton.

Dynamic regulation of integrin activation by intracellular and extracellular signals controls oligodendrocyte morphology

BACKGROUND

Myelination requires precise control of oligodendrocyte morphology and myelin generation at each of the axons contacted by an individual cell. This control must involve the integration of extracellular cues, such as those on the axon surface, with intrinsic developmental programmes. We asked whether integrins represent one class of oligodendrocyte cell-surface receptors able to provide this integration.

RESULTS

Integrins signal via a process of activation, a conformational change that can be induced either by "outside-in" signals comprising physiological extracellular matrix ligands (mimicked by the pharmacological use of the divalent cation manganese) or "inside-out" signalling molecules such as R-Ras. Increasing levels of outside-in signalling via the laminin receptor alpha6beta1 integrin were found to promote oligodendrocyte processing and myelin sheet formation in culture. Similar results were obtained when inside-out signalling was increased by the expression of a constitutively-active R-Ras. Inhibiting inside-out signalling by using dominant-negative R-Ras reduces processes and myelin sheets; importantly, this can be partially rescued by the co-stimulation of outside-in signalling using manganese.

CONCLUSION

The balance of the equilibrium between active and inactive integrins regulates oligodendrocyte morphology, which is itself regulated by extrinsic and intrinsic cues so providing a mechanism of signal integration. As laminins capable of providing outside-in signals are present on axons at the time of myelination, a mechanism exists by which morphology and myelin generation might be regulated independently in each oligodendrocyte process.

Human diseases reveal novel roles for neural laminins

Extracellular matrix molecules such as laminins have a central role in regulating cell behaviour. However, our understanding of their functions in the mammalian nervous system is incomplete. It is important to establish these functions, both for an understanding of normal development and to devise strategies to enhance repair. Here, we review how insights gained from human diseases caused by genetic mutations in laminins or their receptors have revealed significant and sometimes unexpected roles for laminins in neural stem cells, migrating neurons and myelinating glia, in both the PNS and CNS.

Integrin-growth factor interactions as regulators of oligodendroglial development and function

Central nervous system (CNS) development requires mechanisms for the regulation of cell number. Although growth factors are essential determinants of the proliferation and apoptosis that determine final numbers, the long-range nature of signals from diffusible growth factors makes them insufficient for the provision of the precise and localized signals required. Integration of integrin and growth factor receptor signaling in controlling cell behavior has been an important theme of research over the past several years. The focus of this review is on the mechanisms by which integrin-growth factor interactions regulate the development of oligodendrocytes and provide a mechanism for controlling, both in space and in time, oligodendrocyte numbers in the developing CNS.

Integrins direct Src family kinases to regulate distinct phases of oligodendrocyte development

Specific integrins expressed on oligodendrocytes, the myelin-forming cells of the central nervous system, promote either differentiation and survival or proliferation by amplification of growth factor signaling. Here, we report that the Src family kinases (SFKs) Fyn and Lyn regulate each of these distinct integrin-driven behaviors. Fyn associates with alpha6beta1 and is required to amplify platelet-derived growth factor survival signaling, to promote myelin membrane formation, and to switch neuregulin signaling from a phosphatidylinositol 3-kinase to a mitogen-activated protein kinase pathway (thereby changing the response from proliferation to differentiation). However, earlier in the lineage Lyn, not Fyn, is required to drive alphaVbeta3-dependent progenitor proliferation. The two SFKs respond to integrin ligation by different mechanisms: Lyn, by increased autophosphorylation of a catalytic tyrosine; and Fyn, by reduced Csk phosphorylation of the inhibitory COOH-terminal tyrosine. These findings illustrate how different SFKs can act as effectors for specific cell responses during development within a single cell lineage, and, furthermore, provide a molecular mechanism to explain similar region-specific hypomyelination in laminin- and Fyn-deficient mice.

Integrins: versatile integrators of extracellular signals

Extracellular matrix (ECM) molecules and growth factors have a crucial role in the signalling that controls cell behaviour during development. Integrins, which are cell-surface receptors for ECM molecules, and growth factor receptors cooperate with each other to regulate this signalling by several mechanisms. In particular, direct interactions between the integrin and growth factor receptors themselves, which often occur within a single macromolecular complex, amplify signalling by mechanisms that include posttranslational modifications and integrin shape changes that are related to activation. As a result, growth factor concentrations in the physiological range, which are too low to initiate signalling alone, do so in the presence of the ECM, enabling integrins to control the time and space of growth factor signalling.

Differential regulation of laminin b1 transgene expression in the neonatal and adult mouse brain

Laminins are the major glycoproteins present in basement membrane, a type of extracellular matrix. We showed that the LAMB1 gene, which encodes the laminin beta1 subunit, is transcriptionally activated by retinoic acid in embryonic stem cells. However, little information is available concerning LAMB1 developmental regulation and spatial expression in the adult mouse brain. In this study we used transgenic mice expressing different lengths of LAMB1 promoter driving beta-galactosidase to investigate developmental and adult transcriptional regulation in the regions of the brain in which the laminin beta1 protein is expressed. CNS expression was not observed in transgenic mice carrying a 1.4LAMB1betagal construct. Mice carrying a 2.5LAMB1betagal construct expressed the LAMB1 transgene, as assayed by X-gal staining, only in the molecular layer of the neonatal cerebellum. In contrast, a 3.9LAMB1betagal transgene showed broad regional expression in the adult mouse brain, including the hippocampus, entorhinal cortex, colliculi, striatum, and substantia nigra. Similar expression patterns were observed for the endogenous laminin beta1 protein and for the 3.9LAMB1betagal transgene, analyzed with an antibody against the beta-galactosidase protein. The 3.9LAMB1betagal transgene expression in the hippocampal tri-synaptic circuit suggests a role for the LAMB1 gene in learning and memory.

Use of Skeletal Muscle Tissue in Peripheral Nerve Repair: Review of the Literature

The management of peripheral nerve injury continues to be a major clinical challenge. The most widely used technique for bridging defects in peripheral nerves is the use of autologous nerve grafts. This technique, however, necessitates a donor nerve and corresponding deficit. Many alternative techniques have thus been developed. The use of skeletal muscle tissue as graft material for nerve repair is one example. The rationale regarding the use of the skeletal muscle tissue technique is the availability of a longitudinally oriented basal lamina and extracellular matrix components that direct and enhance regenerating nerve fibers. These factors provide superiority over other bridging methods as vein grafts or (non)degradable nerve conduits. The main disadvantages of this technique are the risk that nerve fibers can grow out of the muscle tissue during nerve regeneration, and that a donor site is necessary to harvest the muscle tissue. Despite publications on nerve conduits as an alternative for peripheral nerve repair, autologous nerve grafting is still the standard care for treatment of a nerve gap in the clinical situation; however, the use of the skeletal muscle tissue technique can be added to the surgeon's arsenal of peripheral nerve repair tools, especially for bridging short nerve defects or when traditional nerve autografts cannot be employed. This technique has been investigated both experimentally and clinically and, in this article, an overview of the literature on skeletal muscle grafts for bridging peripheral nerve defects is presented.

Integrin-linked kinase is required for laminin-2-induced oligodendrocyte cell spreading and CNS myelination

Early steps in myelination in the central nervous system (CNS) include a specialized and extreme form of cell spreading in which oligodendrocytes extend large lamellae that spiral around axons to form myelin. Recent studies have demonstrated that laminin-2 (LN-2; alpha2beta1gamma1) stimulates oligodendrocytes to extend elaborate membrane sheets in vitro (cell spreading), mediated by integrin alpha6beta1. Although a congenital LN-2 deficiency in humans is associated with CNS white matter changes, LN-2-deficient (dy/dy) mice have shown abnormalities primarily within the peripheral nervous system. Here, we demonstrate a critical role for LN-2 in CNS myelination by showing that dy/dy mice have quantitative and morphologic defects in CNS myelin. We have defined the molecular pathway through which LN-2 signals oligodendrocyte cell spreading by demonstrating requirements for phosphoinositide 3-kinase activity and integrin-linked kinase (ILK). Interaction of oligodendrocytes with LN-2 stimulates ILK activity. A dominant negative ILK inhibits LN-2-induced myelinlike membrane formation. A critical component of the myelination signaling cascade includes LN-2 and integrin signals through ILK.

The control of cell number during central nervous system development in flies and mice

Growth is confined within a size that is normal for each species, revealing that somehow an organism 'knows' when this size has been reached. Within a species, growth is also variable, but despite this, proportion and structure are maintained. Perhaps, the key element in the control of size is the control of cell number. Here we review current knowledge on the mechanisms controlling cell number in the nervous system of vertebrates and flies. During growth, clonal expansion is confined, the number of progeny cells is balanced through the control of cell survival and cell proliferation and excess cells are eliminated by apoptosis. Simultaneously, organ architecture emerges and as neurons become active they also influence growth. The interactive control of cell number provides developmental plasticity to nervous system development. Many findings are common between flies and mice, other aspects have been studied more in one organism than the other and there are also aspects that are unique to either organism. Although cell number control has long been studied in the nervous system, analogous mechanisms are likely to operate during the growth of other organs and organisms.

Expression of laminin chains by central neurons: analysis with gene and protein trapping techniques

Laminins exert numerous effects on neurons in vitro, but expression of laminin subunit genes by neurons in vivo remains controversial. To reexamine this issue, we generated mice from ES cells in which the laminin alpha1, alpha5, beta1, and gamma1 genes had been "trapped" by insertion of a histochemically detectable selectable marker, betageo (beta-galactosidase fused to neomycin phosphotransferase). The presence of laminin-betageo fusion proteins was assayed histochemically and immunochemically, revealing expression of laminin beta1 and gamma1 genes, but not alpha chain genes, by defined subsets of neurons in brain and retina. We also used the gene traps in a novel way to assay expression of endogenous laminin subunits, which were barely detectable by ordinary immunohistochemical methods. The trapping vector included a transmembrane domain that anchors proteins otherwise destined for secretion. Laminin alpha/beta/gamma heterotrimers are assembled intracellularly, and we show that the trapped laminin gamma1 fusion protein "co-trapped" endogenous beta1 intracellularly. The laminin gamma1 fusion was also able to co-trap transgene-derived alpha chains, but we detected no co-trapped endogenous alpha chains. The co-trapping method may be generally useful for identifying proteins or isolating protein complexes associated with trapped gene products.

CNS integrins switch growth factor signalling to promote target-dependent survival

Depending on the stage of development, a growth factor can mediate cell proliferation, survival or differentiation. The interaction of cell-surface integrins with extracellular matrix ligands can regulate growth factor responses and thus may influence the effect mediated by the growth factor. Here we show, by using mice lacking the alpha(6) integrin receptor for laminins, that myelin-forming oligodendrocytes activate an integrin-regulated switch in survival signalling when they contact axonal laminins. This switch alters survival signalling mediated by neuregulin from dependence on the phosphatidylinositol-3-OH kinase (PI(3)K) pathway to dependence on the mitogen-activated kinase pathway. The consequent enhanced survival provides a mechanism for target-dependent selection during development of the central nervous system. This integrin-regulated switch reverses the capacity of neuregulin to inhibit the differentiation of precursors, thereby explaining how neuregulin subsequently promotes differentiation and survival in myelinating oligodendrocytes. Our results provide a general mechanism by which growth factors can exert apparently contradictory effects at different stages of development in individual cell lineages.

Identification of cell binding sites in the laminin alpha5-chain G domain

The laminins consist of at least 11 polypeptides (5 alpha-chains, 3 beta-chains, and 3 gamma-chains) specific to basement membranes. Here we investigate the biological activity associated with the G domain of the newly identified laminin alpha5-chain using 113 overlapping synthetic peptides (positions 2679-3635). Using HT-1080 cells, 21 peptides showed attachment activity either on peptide-coated tissue culture plates or to peptide-conjugated Sepharose beads. Heparin inhibited cell attachment to 16 peptides, while ethylenediaminetetraacetic acid exhibited no inhibitory activity. Peptides A5G-27, A5G-65, and A5G-71 showed the strongest cell attachment, with the minimum active core sequences of the peptides being GIIFFL, HQNMGSVNVSV, and YLQFVG, respectively. Furthermore, these 16 peptides were tested for their ability to stimulate neurite outgrowth in the PC12 cells. A5G-3, A5G-33, A5G-71, A5G-73, A5G-81, and A5G-101 were the only peptides of the 16 that demonstrated the ability to promote neurite outgrowth. These results demonstrate that synthetic peptides with alpha5-chain G domain primary amino acid sequences possess some of the same biological activities attributable to the whole laminin and the alpha5-chain G domain. Therefore, these peptides may be useful in the investigation of laminin-receptor interactions and possibly mechanisms of laminin signal transduction.

Modulation of glial glutamate transport through cell interactions with the extracellular matrix

Glial glutamate transport plays a pivotal role in maintaining glutamate homeostasis in the central nervous system. Expression of glutamate transporters is highly regulated during brain development, and a number of pathological conditions are associated with deficits in expression and/or function of glutamate transports. While several soluble factors have been shown to regulate the expression of glutamate transporter, the contribution of cell-cell interaction and cell-environmental interaction in the regulation of glutamate transport is unknown. Extracellular matrix (ECM) molecules are essential components in cell-cell and cell-environmental interactions, and the ECM has been shown to play critical role in normal development and during brain pathogenesis. We, therefore, investigated the possibility that ECM molecules may regulate astrocytic glutamate transport. Therefore, we cultured rat cortical astrocytes with different ECMs and determined expression levels of the two astrocytic glutamate transporters GLT-1 and GLAST by Western Blot and determined transporter activity through measurements of 3H-D-aspartate uptake. Astrocytes grown on poly-ornithine or poly-D/L-lysine showed approximately two-fold higher GLT-1 expression than sister cells grown on plastic dishes without ECM. Naturally occurring ECM's, including laminin and collagen, showed a dose-dependent regulation of GLT-1 protein expression. These effects were specific for GLT-1 as GLAST expression was unaffected by different ECMs. Surprisingly, however, none of the examined ECMs altered the apparent glutamate uptake activity. In probing blots side-by-side for expression of Na(+)/K(+)-ATPase, we found that ECMs affected expression of Na(+)/K(+)-ATPase and GLT-1 in a reciprocal fashion. Poly-ornithine, for example, enhanced GLT-1 expression, but reduced expression of Na(+)/K(+)-ATPase. Na(+) transport may, thus, be a limiting factor for glutamate uptake.

Survival and differentiation of neurons cultured from the mouse cochlear nucleus on extracellular matrix components

This study was designed to determine the effects of several extracellular matrix (ECM) proteins on the survival and differentiation of cochlear nucleus (CN) neurons grown in primary cell culture. CN neurons were successfully grown on laminin, collagen and fibronectin, but neuronal survival was significantly decreased on all three substrates relative to poly-D-lysine controls. In addition, analysis of dendrite organization patterns indicated that CN neurons branched more extensively when grown on poly-D-lysine than on any of the ECM components tested. Although dendrite growth has been shown to depend strongly on the presence of ECM components in other neuronal systems, the data from these experiments do not support the hypothesis that laminin, collagen or fibronectin is necessary for the survival and differentiation of CN neurons.

The laminins in the murine inner ear: developmental transitions and expression in cochlear basement membranes

The laminins are a family of heterotrimeric extracellular matrix molecules that form suprastructural networks in basement membranes and elsewhere. They interact with integrin receptors, playing key roles in modulating programs of cytodifferentiation and maintaining tissue homeostasis in animals. Earlier studies have demonstrated an extensive laminin network in both the developing and adult cochlea, primarily associated with the basement membranes. These studies, however, did not address the laminin chain composition of these networks. In this study, we used antibodies specific for the known laminin chains to examine the composition of laminins in both the developing and adult murine cochlea. The results illustrate a complex and dynamic postnatal developmental regulation pattern for most of these chains, and suggest that an unusually large number of laminin heterotrimers are present in both the developing and adult cochlea. The laminin composition at postnatal day 2 is relatively simple. By postnatal day 7, however, activation of several laminin chains results in a very complex laminin composition. In the basement membrane underlying the region of the basilar membrane under the developing organ of Corti, eight of the 11 known basement membrane laminins are possible by co-localization inference. Dynamic changes in expression continue through day 14, but simplify by adulthood. Thus, the most dynamic period for laminin expression in the mouse cochlea coincides with terminal cytodifferentiation of the cochlear epithelial structures. Considering the well established role of laminins in regulating both embryonic and organ development in other systems, these data suggest a closer look at the role of the laminins in cochlear development and function may be warranted.

Expression of dominant-negative and chimeric subunits reveals an essential role for beta1 integrin during myelination

Myelination represents a remarkable example of cell specialization and cell-cell interaction in development. During this process, axons are wrapped by concentric layers of cell membrane derived either from central nervous system (CNS) oligodendrocytes or peripheral nervous system Schwann cells. In the CNS, oligodendrocytes elaborate a membranous extension with an area of more than 1000 times that of the cell body. The mechanisms regulating this change in cell shape remain poorly understood. Signaling mechanisms regulated by cell surface adhesion receptors of the integrin family represent likely candidates. Integrins link the extracellular environment of the cell with both intracellular signaling molecules and the cytoskeleton and have been shown to regulate the activity of GTPases implicated in the control of cell shape. Our previous work has established that oligodendrocytes and their precursors express a limited repertoire of integrins. One of these, the alpha6beta1 laminin receptor, can interact with laminin-2 substrates to enhance oligodendrocyte myelin membrane formation in cell culture. However, these experiments do not address the important question of integrin function during myelination in vivo, nor do they define the respective roles of the alpha and beta subunits in the signaling pathways involved. Here, we use a dominant-negative approach to provide, for the first time, evidence that beta1 integrin function is required for myelination in vivo and use chimeric integrins to dissect apart the roles of the extracellular and cytoplasmic domains of the alpha6 subunit in the signaling pathways of myelination.

Vitronectin regulates Sonic hedgehog activity during cerebellum development through CREB phosphorylation

During development of the cerebellum, Sonic hedgehog (SHH) is expressed in migrating and settled Purkinje neurons and is directly responsible for proliferation of granule cell precursors in the external germinal layer. We have previously demonstrated that SHH interacts with vitronectin in the differentiation of spinal motor neurons. Here, we analysed whether similar interactions between SHH and extracellular matrix glycoproteins regulate subsequent steps of granule cell development. Laminins and their integrin receptor subunit alpha6 accumulate in the outer most external germinal layer where proliferation of granule cell precursors is maximal. Consistent with this expression pattern, laminin significantly increases SHH-induced proliferation in primary cultures of cerebellar granule cells. Vitronectin and its integrin receptor subunits alpha(v) are expressed in the inner part of the external germinal layer where granule cell precursors exit the cell cycle and commence differentiation. In cultures, vitronectin is able to overcome SHH-induced proliferation, thus allowing granule cell differentiation. Our studies indicate that the pathway in granule cell precursors responsible for the conversion of a proliferative SHH-mediated response to a differentiation signal depends on CREB. Vitronectin stimulates phosphorylation of cyclic-AMP responsive element-binding protein (CREB), and over-expression of CREB is sufficient to induce granule cell differentiation in the presence of SHH. Taken together, these data suggest that granule neuron differentiation is regulated by the vitronectin-induced phosphorylation of CREB, a critical event that terminates SHH-mediated proliferation and permits the differentiation program to proceed in these 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.

Integrins mediate a neuronal survival signal for oligodendrocytes

Target-dependent survival of newly differentiated cells is an important part of neural development. In the case of myelin-forming oligodendrocytes, it matches the number of oligodendrocytes to the available axons [1]. In addition to growth factors, an axonal signal regulates this survival: when axons are transected, oligodendrocytes die and, conversely, when the number of axons is increased by genetic manipulation, oligodendrocyte numbers increase [2] [3]. Newly formed oligodendrocytes that fail to contact axons undergo apoptosis, and co-culture experiments that model axon-glial interactions in vitro reveal a neuronal survival effect not present in neuron-conditioned medium [4] [5], suggesting that the signal is non-diffusible and present on the surface of axons. The nature of these neuronal signals is unknown, as are the mechanisms by which they interact with growth-factor-mediated survival signals. As integrins can regulate survival in other cell types [6] [7] [8], we determined whether integrins are involved in the neuronal survival effect. We found that the laminin receptor alpha6beta1 integrin, which is expressed on oligodendrocytes, enhances the sensitivity of oligodendrocytes to the survival effect of growth factors. On the basis of this interaction between integrin and growth-factor-mediated signalling, we propose a simple model by which signals from axons and other cell types might interact to regulate oligodendrocyte cell numbers.

From neural stem cells to myelinating oligodendrocytes

The potential to generate oligodendrocytes progenitors (OP) from neural stem cells (NSCs) exists throughout the developing CNS. Yet, in the embryonic spinal cord, the oligodendrocyte phenotype is induced by sonic hedgehog in a restricted anterior region. In addition, neuregulins are emerging as potent regulators of early and late OP development. The ability to isolate and grow NSCs as well as glial-restricted progenitors has revealed that FGF2 and thyroid hormone favor an oligodendrocyte fate. Analysis of genetically modified mice showed that PDGF controls the migration and production of oligodendrocytes in vivo. Interplay between mitogens, thyroid hormone, and neurotransmitters may maintain the undifferentiated stage or result in OP growth arrest. Notch signaling by axons inhibits oligodendrocyte differentiation until neuronal signals--linked to electrical activity-trigger initiation of myelination. To repair myelin in adult CNS, multipotential neural precursors, rather than slowly cycling OP, appear the cells of choice to rapidly generate myelin-forming cells.

Laminin-2/integrin interactions enhance myelin membrane formation by oligodendrocytes

To examine the role of extracellular matrix (ECM)/integrin interactions in myelination we have analyzed oligodendrocyte differentiation and myelin membrane formation in oligodendrocytes grown in cell culture. We have found that the ECM substrates fibronectin, vitronectin, and laminin-2 (merosin) have no effect on differentiation, as measured by the appearance of myelin basic protein-expressing cells, but that laminin-2 substrates dramatically enhance myelin membrane formation. Blocking antibody and immunolocalization studies suggest that this effect is mediated via 1 integrins. The v integrins expressed on oligodendrocytes, in contrast, are less effective at promoting membrane formation. These results show that the interaction between laminin-2 expressed in white matter tracts and oligodendrocyte laminin-binding integrins may be an important part of the signalling mechanisms that stimulate oligodendrocytes to elaborate the extensive myelin membrane required to wrap the axon and form the myelin sheath. The results also provide a logical explanation for the abnormalities of myelination observed in humans with merosin-deficient congenital muscular dystrophy.

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.