Heikki Rauvala

An 18-kd heparin-binding protein of developing brain that is distinct from fibroblast growth factors.

An 18‐kd heparin‐binding protein (p18) was isolated from perinatal rat brain. Although the protein closely resembles the fibroblast growth factors in its strong binding to heparin and in its apparent molecular mass, it has a distinct structure. This was concluded from the amino‐terminal sequence analysis that identified a unique structure containing a cluster of lysine residues. Antipeptide antibodies were raised in rabbits according to the sequence analysis and affinity purified using a synthetic peptide. The antibodies were shown to bind specifically to p18, which was immunochemically distinct from the basic fibroblast growth factor. The antipeptide antibodies detected p18 in brain but not in liver, kidney, heart or skeletal muscle. The content of the protein was shown to undergo a remarkable developmental change corresponding to the time period of rapid sprouting of axons and dendrites in brain. The content of p18 was rapidly increased at the time of birth until the postnatal age of approximately 1 week, after which it was decreased to values less than 10% in young adults as compared to the content found in perinatal rats. p18 also enhanced neurite outgrowth in brain neurons in vitro. The protein was stained in neurons in cells dispersed from perinatal brain. The properties of p18 suggest that it has a role in the growth and maturation of brain.

Transgenic expression of syndecan-1 uncovers a physiological control of feeding behavior by syndecan-3.

Transgenic expression in the hypothalamus of syndecan-1, a cell surface heparan sulfate proteoglycan (HSPG) and modulator of ligand-receptor encounters, produces mice with hyperphagia and maturity-onset obesity resembling mice with reduced action of alpha melanocyte stimulating hormone (alphaMSH). Via their HS chains, syndecans potentiate the action of agouti-related protein and agouti signaling protein, endogenous inhibitors of alphaMSH. In wild-type mice, syndecan-3, the predominantly neural syndecan, is expressed in hypothalamic regions that control energy balance. Food deprivation increases hypothalamic syndecan-3 levels several-fold. Syndecan-3 null mice, otherwise apparently normal, respond to food deprivation with markedly reduced reflex hyperphagia. We propose that oscillation of hypothalamic syndecan-3 levels physiologically modulates feeding behavior.

AMIGO, a transmembrane protein implicated in axon tract development, defines a novel protein family with leucine-rich repeats

Ordered differential display identified a novel sequence induced in neurons by the neurite-promoting protein amphoterin. We named this gene amphoterin-induced gene and ORF (AMIGO), and also cloned two other novel genes homologous to AMIGO (AMIGO2 and AMIGO3). Together, these three AMIGOs form a novel family of genes coding for type I transmembrane proteins which contain a signal sequence for secretion and a transmembrane domain. The deduced extracellular parts of the AMIGOs contain six leucine-rich repeats (LRRs) flanked by cysteine-rich LRR NH2- and COOH-terminal domains and by one immunoglobulin domain close to the transmembrane region. A substrate-bound form of the recombinant AMIGO ectodomain promoted prominent neurite extension in hippocampal neurons, and in solution, the same AMIGO ectodomain inhibited fasciculation of neurites. A homophilic and heterophilic binding mechanism is shown between the members of the AMIGO family. Our results suggest that the members of the AMIGO protein family are novel cell adhesion molecules among which AMIGO is specifically expressed on fiber tracts of neuronal tissues and participates in their formation.

Heparan sulfate proteoglycan syndecan-3 is a novel receptor for GDNF, neurturin, and artemin.

Syndecan-3 may act alone or as a coreceptor with RET to promote cell spreading, neurite outgrowth, and migration of cortical neurons by GNDF, NRTN, and ARTN.

Neuronal regulation of immune responses in the central nervous system

The central nervous system (CNS) has traditionally been considered to be immunologically privileged, but over the years there has been a re-evaluation of this dogma. To date, studies have tended to focus on the immune functions of glial cells, whereas the roles of neurons have been regarded as passive and their immune-regulatory properties have been less examined. However, recent findings indicate that CNS neurons actively participate in immune regulation by controlling their glial cell counterparts and infiltrated T cells. Here, we describe the immune-regulatory roles of CNS neurons by both contact-dependent and contact-independent mechanisms. In addition, we specifically deal with the immune functions of neuronal cell adhesion molecules, many of which are key modulators of neuronal synaptic formation and plasticity.

HB-GAM (HEPARIN-BINDING GROWTH-ASSOCIATED MOLECULE) AND HEPARIN-TYPE GLYCANS IN THE DEVELOPMENT AND PLASTICITY OF NEURON-TARGET CONTACTS

HB-GAM is a secretory, extracellular matrix-associated protein that was isolated by screening for factors that enhance neurite outgrowth in rat brain neurons. The HB-GAM sequence clearly (about 50%) is homologous to that of MK (midkine) sequence, a protein discovered through screening for factors that mediate retinoic acid-induced cell differentiation. These lysine- and cysteine-rich sequences define a novel family of differentiation/growth factors, which are conserved in their structures from mammals to amphibians. HB-GAM is expressed strongly along axon pathways and target regions of axons during and prior to the stage of axonal growth in tissues. These findings, together with in vitro interactions with neurons, suggest that HB-GAM is a cell matrix-associated cue for growth cone migration. N-syndecan (syndecan-3) functions as a receptor/coreceptor in HB-GAM-induced neurite outgrowth in perinatal rat brain neurons. In addition to enhancing neurite growth in a developmentally regulated manner in early neurons, HB-GAM is accumulated at the growth cone-target interphase accompanying the onset of synaptogenesis, as evidenced by its presence at the neuromuscular junction of Xenopus and rat. In vitro studies suggest that HB-GAM functions as a local, synaptic matrix-associated factor that enhances both presynaptic and postsynaptic differentiation during development. In addition, a role in adult plasticity is suggested by studies on injury-induced and activity-dependent plasticity in rat hippocampus.

Physiological and pathophysiological outcomes of the interactions of HMGB1 with cell surface receptors

Extracellularly occurring HMGB1, either released during cell injury or actively secreted from cells, has profound effects on behaviour of a wide variety of cell types. Extracellular HMGB1 regulates migratory responses of many cell types, including neuron and growth cone migration, invasive migration of tumour cells, and migration of endothelial and immune cells. RAGE (Receptor for Advanced Glycation End Products) plays a key role as a cell surface receptor in most, if not all HMGB1-dependent migration mechanisms. HMGB1 binds to the distal immunoglobulin-like domain of RAGE, activating a signalling pathway that ends up in modulation of the cytoskeleton for regulation of cell motility. In addition to RAGE, proteoglycans and sulfated carbohydrate epitopes of glycolipids and glycoproteins may play a role as cell surface binding sites of HMGB1, affecting migratory behaviour of cells. In addition to physiological and pathophysiological cell migration control, HMGB1 has been widely studied as a molecule linking tissue injury to inflammatory mechanisms. HMGB1 by itself has little if any proinflammatory activity but it appears to activate innate immunity mechanisms as a complex with DNA, lipids and/or proinflammatory cytokines. The inflammation-inducing activity of HMGB1/DNA complexes may depend on both RAGE and Toll-like receptors of the immune cell surface. In addition to the receptors activating innate immunity, receptors downregulating inflammation upon HMGB1 release have been recently found, and include thrombomodulin and the CD-24/Siglec pathway.

N-syndecan deficiency impairs neural migration in brain

N-syndecan (syndecan-3) is a transmembrane proteoglycan that is abundantly expressed in the major axonal pathways and in the migratory routes of the developing brain. When ligated by heparin-binding (HB) growth-associated molecule (GAM; pleiotrophin), N-syndecan mediates cortactin–Src kinase-dependent neurite outgrowth. However, the functional role of N-syndecan in brain development remains unexplored. In this study, we show that N-syndecan deficiency perturbs the laminar structure of the cerebral cortex as a result of impaired radial migration. In addition, neural migration in the rostral migratory stream is impaired in the N-syndecan–null mice. We suggest that the migration defect depends on impaired HB-GAM–induced Src kinase activation and haptotactic migration. Furthermore, we show that N-syndecan interacts with EGF receptor (EGFR) at the plasma membrane and is required in EGFR-induced neuronal migration.

Role of heparin-binding growth-associated molecule (HB-GAM) in hippocampal LTP and spatial learning revealed by studies on overexpressing and knockout mice

Heparin-binding growth-associated molecule (HB-GAM) is an extracellular matrix-associated protein with neurite outgrowth-promoting activity and which is suggested to be implicated in hippocampal synaptic plasticity. To study the functions of HB-GAM in adult brain we have produced HB-GAM overexpressing mice and compared phenotypic changes in the transgenic mice to those in the HB-GAM null mice. Both mutants were viable and displayed no gross morphological abnormalities. The basal synaptic transmission was normal in the area CA1 of hippocampal slices from the genetically modified mice. However, long-term potentiation (LTP) was attenuated in the mice overexpressing HB-GAM, whereas enhanced LTP was detected in the HB-GAM-deficient mice. Changes in LTP seen in vitro were paralleled by behavioral alterations in vivo. The animals overexpressing HB-GAM displayed faster learning in water maze and decreased anxiety in elevated plus-maze, while the HB-GAM knockouts demonstrated an opposite behavioral phenotype. These results show that HB-GAM suppresses LTP in hippocampus and plays a role in regulation of learning-related behavior.

RAGE-mediated cell signaling.

RAGE (receptor for advanced glycation end products) is a multi-ligand receptor that belongs to the immunoglobulin superfamily of transmembrane proteins. RAGE binds AGEs (advanced glycation end products), HMGB1 (high-mobility group box-1; also designated as amphoterin), members of the S100 protein family, glycosaminoglycans and amyloid β peptides. Recent studies using tools of structural biology have started to unravel common molecular patterns in the diverse set of ligands recognized by RAGE. The distal Ig domain (V1 domain) of RAGE has a positively charged patch, the geometry of which fits to anionic surfaces displayed at least in a proportion of RAGE ligands. Association of RAGE to itself, to HSPGs (heparan sulfate proteoglycans), and to Toll-like receptors in the cell membrane plays a key role in cell signaling initiated by RAGE ligation. Ligation of RAGE activates cell signaling pathways that regulate migration of several cell types. Furthermore, RAGE ligation has profound effects on the transcriptional profile of cells. RAGE signaling has been mainly studied as a pathogenetic factor of several diseases, where acute or chronic inflammation plays a role. Recent studies have suggested a physiological role for RAGE in normal lung function and in neuronal signaling.