Osteocyte-derived HB-GAM (pleiotrophin) is associated with bone formation and mechanical loading

HB-GAM (also known as pleiotrophin) is a cell matrix-associated protein that is highly expressed in bone. It affects osteoblast function, and might therefore play a role in bone development and remodeling. We aimed to investigate the role of HB-GAM in bone in vivo and in vitro. The bones of HB-GAM deficient mice with an inbred mouse background were studied by histological, histomorphometrical, radiological, biomechanical and mu-CT analyses and the effect of immobilization was evaluated. HB-GAM localization in vivo was studied. MLO-Y4 osteocytes were subjected to fluid shear stress in vitro, and gene and protein expression were studied by subtractive hybridization, quantitative PCR and Western blot. Human osteoclasts were cultured in the presence of rhHB-GAM and their formation and resorption activities were assayed. In agreement with previous reports, the skeletal structure of the HB-GAM knockout mice developed normally. However, a growth retardation of the weight-bearing bones was observed by 2 months of age, suggesting a link to physical activity. Adult HB-GAM deficient mice were characterized by low bone formation and osteopenia, as well as resistance to immobilization-dependent bone remodeling. HB-GAM was localized around osteocytes and their processes in vivo and furthermore, osteocytic HB-GAM expression was upregulated by mechanical loading in vitro. HB-GAM did not affect on human osteoclast formation or resorption in vitro. Taken together, our results suggest that HB-GAM is an osteocyte-derived factor that could participate in mediating the osteogenic effects of mechanical loading on bone.

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.

Involvement of Pleiotrophin in CNTF-mediated differentiation of the late retinal progenitor cells

Ciliary neurotrophic factor (CNTF) participates in retinal development by inhibiting rod differentiation and promoting bipolar and Müller cell differentiation. In order to identify genes which are regulated by CNTF in the developing retina, we carried out a subtractive hybridization study. By this approach, we identified the Pleiotrophin (Ptn) as an upregulated gene in postnatal day 0 (P0) retinal explants upon addition of CNTF. Correlation of overall expression patterns between different retinal cell markers and Ptn in situ hybridization suggest that Ptn transcripts are initially expressed in progenitor cells then in postmitotic precursors of the INL expressing the Chx10 gene, and later in some differentiated retinal Müller glial (RMG) cells and rod-bipolar cells. Overexpression of Ptn by in vitro electroporation of P0 rat retinal explants partially blocks rod differentiation and promotes bipolar cell production, similar to effects of exogenous CNTF and leukemia inhibitory factor (LIF). Furthermore, in P0 retinal explants from mice lacking Ptn, the inhibitory effect of CNTF and LIF on rod differentiation is partially reduced and the cytokine-induced bipolar cell differentiation is largely prevented. Together, these results demonstrate that influence of CNTF family of cytokines on the differentiation of late retinal progenitor cell population is partially mediated by the release of Ptn.

Glucocorticoid hormones decrease proliferation of embryonic neural stem cells through ubiquitin-mediated degradation of cyclin D1

Corticosteroids can influence brain function, and glucocorticoid hormone receptors (GRs) are present in brain tissue. We observed that GR and also mineralocorticoid receptor (MR) are expressed by embryonic rat neural stem cells (NSCs). NSCs in developing ventricular epithelium were positive for GR. Stimulation of cultured NSCs with the specific receptor ligands dexamethasone and corticosterone reduced cell proliferation, shown by 5'-bromo-2-deoxy-uridine labeling. The effect of the hormones was dose dependent and inhibited by the GR blocker mifepristone but not by spironolactone, blocking MR. Dexamethasone inhibited the cell cycle by decreasing the levels of cyclin D1 in NSCs. The hormone-induced decline was inhibited by MG132 (benzyloxycarbonyl-leucyl-leucyl-leucinal), showing an involvement of the ubiquitin proteasome system, In keeping with this, dexamethasone increased the ubiquitination of cyclin D1. In embryonic brain, dexamethasone inhibited cell proliferation of NSCs. This demonstrates that embryonic NSCs are critically influenced by glucocorticoids, which can have long-term effects in the brain.

The two thrombospondin type I repeat domains of the heparin-binding growth-associated molecule bind to heparin/heparan sulfate and regulate neurite extension and plasticity in hippocampal neurons

HB-GAM (heparin-binding growth-associated molecule, also designated as pleiotrophin) and midkine form a two-member family of extracellular matrix proteins that bind tightly to sulfated carbohydrate structures such as heparan sulfate. These proteins are used by developing neurons as extracellular cues in axonal growth and guidance. HB-GAM was recently reported to enhance differentiation of neural stem cells. Based on the solution structure of HB-GAM, we have recently shown that HB-GAM consists of two beta-sheet domains flanked by flexible lysine-rich N- and C-terminal tails with no apparent structure. These domains are homologous to thrombospondin type I repeats present in numerous extracellular proteins that interact with the cell surface. Our findings showed that the two beta-sheet domains fold independently. We showed that the domains (but not the lysine-rich tails) in HB-GAM are required and sufficient for interaction with hippocampal neurons. The individual domains bind heparan sulfate weakly and fail to produce significant biological effects in neurite outgrowth and long term potentiation assays. The amino acids in the linker region joining the two domains may be replaced with glycines with no effect on protein function. These results suggest a co-operative action of the two beta-sheet domains in the biologically relevant interaction with neuron surface heparan sulfate.

HB-GAM inhibits proliferation and enhances differentiation of neural stem cells

Proliferation of neural stem cells in the embryonic cerebral cortex is regulated by many growth factors and their receptors. Among the key molecules stimulating stem cell proliferation are FGF-2 and the FGF receptor-1. This ligand-receptor system is highly dependent on the surrounding heparan sulfates. We have found that heparin-binding growth-associated molecule (HB-GAM, also designated as pleiotrophin) regulates neural stem cell proliferation in vivo and in vitro. Deficiency of HB-GAM results in a pronounced, up to 50% increase in neuronal density in the adult mouse cerebral cortex. This phenotype arises during cortical neurogenesis, when HB-GAM knockout embryos display an enhanced proliferation rate as compared to wild-type embryos. Further, our in vitro studies show that exogenously added HB-GAM inhibits formation and growth of FGF-2, but not EGF, stimulated neurospheres, restricts the number of nestin-positive neural stem cells, and inhibits FGF receptor phosphorylation. We propose that HB-GAM functions as an endogenous inhibitor of FGF-2 in stem cell proliferation in the developing cortex.

Syndecan-3-deficient mice exhibit enhanced LTP and impaired hippocampus-dependent memory

Syndecan-3 (N-syndecan) is a transmembrane heparan sulfate proteoglycan expressed predominantly in the nervous system in a developmentally regulated manner. Syndecan-3 has been suggested to play a role in the development and plasticity of neuronal connections by linking extracellular signals to the regulation of the cytoskeleton. To study its physiological functions, we produced mice deficient in syndecan-3 by gene targeting. The mutant animals are healthy, are fertile, and have no apparent defects in the structure of the brain. We focused on characterizing the functions of the hippocampus, a brain area where expression of syndecan-3 is prominent in adults. Mice lacking syndecan-3 exhibited an enhanced level of long-term potentiation (LTP) in area CA1, while basal synaptic transmission and short-term plasticity were similar to those in wild-type animals. Further, the mutant mice were not responsive to the syndecan-3 ligand heparin-binding growth-associated molecule, which inhibits LTP in area CA1 in wild-type animals. Behavioral testing of the syndecan-3-deficient mice revealed impaired performance in tasks assessing hippocampal functioning. We suggest that syndecan-3 acts as an important modulator of synaptic plasticity that influences hippocampus-dependent memory.

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.

Enhanced hippocampal long-term potentiation in mice lacking heparin-binding growth-associated molecule

Heparin-binding growth-associated molecule (HB-GAM) (pleiotrophin) is a highly conserved extracellular matrix-associated protein implicated in a diverse range of developmental processes, including the formation and plasticity of neuronal connections. Using gene targeting, we have in the present study created HB-GAM-deficient mice that are viable and fertile and show no gross anatomical abnormalities. The hippocampal structure as well as basal excitatory synaptic transmission in the area CA1 appear normal in the mice lacking HB-GAM. However, hippocampal slices from HB-GAM-deficient mice display a lowered threshold for induction of long-term potentiation (LTP), which reverts back to the wild-type level by application of HB-GAM. HB-GAM expression in hippocampus is activity-dependent and upregulated in several neuropathological conditions. Thus, we suggest that HB-GAM acts as an inducible signal to inhibit LTP in hippocampus.