Heparan sulphate proteoglycans in glia and in the normal and injured CNS: expression of sulphotransferases and changes in sulphation

Endothelial heparan sulfate 6-O-sulfation levels regulate angiogenic responses of endothelial cells to fibroblast growth factor 2 and vascular endothelial growth factor

Fibroblast growth factor 2 (FGF2) and vascular endothelial growth factor 165 (VEGF(165)) are potent pro-angiogenic growth factors that play a pivotal role in tumor angiogenesis. The activity of these growth factors is regulated by heparan sulfate (HS), which is essential for the formation of FGF2/FGF receptor (FGFR) and VEGF(165)/VEGF receptor signaling complexes. However, the structural characteristics of HS that determine activation or inhibition of such complexes are only partially defined. Here we show that ovarian tumor endothelium displays high levels of HS sequences that harbor glucosamine 6-O-sulfates when compared with normal ovarian vasculature where these sequences are also detected in perivascular area. Reduced HS 6-O-sulfotransferase 1 (HS6ST-1) or 6-O-sulfotransferase 2 (HS6ST-2) expression in endothelial cells impacts upon the prevalence of HS 6-O-sulfate moieties in HS sequences, which consist of repeating short, highly sulfated S domains interspersed by transitional N-acetylated/N-sulfated domains. 1-40% reduction in 6-O-sulfates significantly compromises FGF2- and VEGF(165)-induced endothelial cell sprouting and tube formation in vitro and FGF2-dependent angiogenesis in vivo. Moreover, HS on wild-type neighboring endothelial or smooth muscle cells fails to restore endothelial cell sprouting and tube formation. The affinity of FGF2 for HS with reduced 6-O-sulfation is preserved, although FGFR1 activation is inhibited correlating with reduced receptor internalization. These data show that 6-O-sulfate moieties in endothelial HS are of major importance in regulating FGF2- and VEGF(165)-dependent endothelial cell functions in vitro and in vivo and highlight HS6ST-1 and HS6ST-2 as potential targets of novel antiangiogenic agents.

Matrix metalloproteinase dependent cleavage of cell adhesion molecules in the pathogenesis of CNS dysfunction with HIV and methamphetamine

Physiologically appropriate levels of matrix metalloproteinases (MMPs) are likely important to varied aspects of CNS function. In particular, these enzymes may contribute to neuronal activity dependent synaptic plasticity and to cell mobility in processes including stem cell migration and immune surveillance. Levels of MMPs may, however, be substantially increased in the setting of HIV infection with methamphetamine abuse. Elevated MMP levels might in turn influence integrity of the blood brain barrier, as has been demonstrated in published work. Herein we suggest that elevated levels of MMPs can also contribute to microglial activation as well as neuronal and synaptic injury through a mechanism that involves cleavage of specific cell and synaptic adhesion molecules.

Normal sulfation levels regulate spinal cord neural precursor cell proliferation and differentiation

Background

Sulfated glycosaminoglycan chains are known for their regulatory functions during neural development and regeneration. However, it is still unknown whether the sulfate residues alone influence, for example, neural precursor cell behavior or whether they act in concert with the sugar backbone. Here, we provide evidence that the unique 473HD-epitope, a representative chondroitin sulfate, is expressed by spinal cord neural precursor cells in vivo and in vitro, suggesting a potential function of sulfated glycosaminoglycans for spinal cord development.

Results

Thus, we applied the widely used sulfation inhibitor sodium chlorate to analyze the importance of normal sulfation levels for spinal cord neural precursor cell biology in vitro. Addition of sodium chlorate to spinal cord neural precursor cell cultures affected cell cycle progression accompanied by changed extracellular signal-regulated kinase 1 or 2 activation levels. This resulted in a higher percentage of neurons already under proliferative conditions. In contrast, the relative number of glial cells was largely unaffected. Strikingly, both morphological and electrophysiological characterization of neural precursor cell-derived neurons demonstrated an attenuated neuronal maturation in the presence of sodium chlorate, including a disturbed neuronal polarization.

Conclusions

In summary, our data suggest that sulfation is an important regulator of both neural precursor cell proliferation and maturation of the neural precursor cell progeny in the developing mouse spinal cord.

Intracerebral chondroitinase ABC and heparan sulfate proteoglycan glypican improve outcome from chronic stroke in rats

Physical and chemical constraints imposed by the periinfarct glial scar may contribute to the limited clinical improvement often observed after ischemic brain injury. To investigate the role of some of these mediators in outcome from cerebral ischemia, we treated rats with the growth-inhibitory chondroitin sulfate proteoglycan neurocan, the growth-stimulating heparan sulfate proteoglycan glypican, or the chondroitin sulfate proteoglycan-degrading enzyme chondroitinase ABC. Neurocan, glypican, or chondroitinase ABC was infused directly into the infarct cavity for 7 d, beginning 7 d after middle cerebral artery occlusion. Glypican and chondroitinase ABC reduced glial fibrillary acidic protein immunoreactivity and increased microtubule-associated protein-2 immunoreactivity in the periinfarct region, and glypican- and chondroitinase ABC-treated rats showed behavioral improvement compared with neurocan- or saline-treated rats. Glypican and chondroitinase ABC also increased neurite extension in cortical neuron cultures. Glypican increased fibroblast growth factor-2 expression and chondroitinase ABC increased brain-derived neurotrophic factor expression in these cultures, whereas no such effects were seen following neurocan treatment. Thus, treatment with glypican or enzymatic disruption of neurocan with chondroitinase ABC improves gross anatomical, histological, and functional outcome in the chronic phase of experimental stroke in rats. Changes in growth factor expression and neuritogenesis may help to mediate these effects.

Attenuation of astrocyte activation by TAT-mediated delivery of a peptide JNK inhibitor

Astrocyte activation contributes to the brain's response to disease and injury. Activated astrocytes generate harmful radicals that exacerbate brain damage including nitric oxide, peroxides and superoxides. Furthermore, reactive astrocytes hinder regeneration of damaged neural circuits by secreting neuro-developmental inhibitors and glycosaminoglycans (GAGs), which physically block growth cone extension. Therefore, targeted therapeutic strategies to limit astrocyte activation may enhance recovery from many neurodegenerative states. Previously, we demonstrated that the HIV-1 TAT cell-penetrating peptide, a short non-toxic peptide from the full-length TAT protein, delivered a protein cargo to astrocytes in a process dependent on cell-surface GAG. Since activated astrocytes produce GAG, in this study we tested whether TAT could transduce activated astrocytes, deliver a biologically active cargo, and produce a physiological effect. Astrocyte activation was induced by IL-1β, lipopolysaccharide (LPS), or mechanical stretch injury, and quantified by increased GAG and nitrite content. TAT-mediated delivery of a mock therapeutic protein, GFP, increased significantly after activation. Nitrite production, GAG expression, and GFP-TAT transduction were significantly attenuated by inhibitors of JNK, p38, or ERK. TAT fused to a peptide JNK inhibitor delivered the peptide inhibitor to activated astrocytes and significantly reduced activation. Our study is the first to report significant and direct modulation of astrocyte activation with a peptide JNK inhibitor. Our promising in vitro results warrant in vivo follow-up, as TAT-mediated protein delivery may have broad therapeutic potential for preventing astrocyte activation with the possibility of limiting off-target, negative side effects.

A role for heparan sulfate proteoglycans in Plasmodium falciparum sporozoite invasion of anopheline mosquito salivary glands

HS (heparan sulfate) has been shown to be an important mediator of Plasmodium sporozoite homing and invasion of the liver, but the role of this glycosaminoglycan in mosquito vector host-sporozoite interactions is unknown. We have biochemically characterized the function of AgOXT1 (Anopheles gambiae peptide-O-xylosyltransferase 1) and confirmed that AgOXT1 can modify peptides representing model HS and chondroitin sulfate proteoglycans in vitro. Moreover, we also demonstrated that the mosquito salivary gland basal lamina proteoglycans are modified by HS. We used RNA interference-mediated knockdown of HS biosynthesis in A. gambiae salivary glands to determine whether Plasmodium falciparum sporozoites that are released from mosquito midgut oocysts use salivary gland HS as a receptor for tissue invasion. Our results suggest that salivary gland basal lamina HS glycosaminoglycans only partially mediate midgut sporozoite invasion of this tissue, and that in the absence of HS, the presence of other surface co-receptors is sufficient to facilitate parasite entry.

Suspension matrices for improved Schwann-cell survival after implantation into the injured rat spinal cord

Trauma to the spinal cord produces endogenously irreversible tissue and functional loss, requiring the application of therapeutic approaches to achieve meaningful restoration. Cellular strategies, in particular Schwann-cell implantation, have shown promise in overcoming many of the obstacles facing successful repair of the injured spinal cord. Here, we show that the implantation of Schwann cells as cell suspensions with in-situ gelling laminin:collagen matrices after spinal-cord contusion significantly enhances long-term cell survival but not proliferation, as well as improves graft vascularization and the degree of axonal in-growth over the standard implantation vehicle, minimal media. The use of a matrix to suspend cells prior to implantation should be an important consideration for achieving improved survival and effectiveness of cellular therapies for future clinical application.

Quantitative and qualitative alterations of heparan sulfate in fibrogenic liver diseases and hepatocellular cancer

Heparan sulfate (HS), due to its ability to interact with a multitude of HS-binding factors, is involved in a variety of physiological and pathological processes. Remarkably diverse fine structure of HS, shaped by non-exhaustive enzymatic modifications, influences the interaction of HS with its partners. Here we characterized the HS profile of normal human and rat liver, as well as alterations of HS related to liver fibrogenesis and carcinogenesis, by using sulfation-specific antibodies. The HS immunopattern was compared with the immunolocalization of selected HS proteoglycans. HS samples from normal liver and hepatocellular carcinoma (HCC) were subjected to disaccharide analysis. Expression changes of nine HS-modifying enzymes in human fibrogenic diseases and HCC were measured by quantitative RT-PCR. Increased abundance and altered immunolocalization of HS was paralleled by elevated mRNA levels of HS-modifying enzymes in the diseased liver. The strong immunoreactivity of the normal liver for 3-O-sulfated epitope further increased with disease, along with upregulation of 3-OST-1. Modest 6-O-undersulfation of HCC HS is probably explained by Sulf overexpression. Our results may prompt further investigation of the role of highly 3-O-sulfated and partially 6-O-desulfated HS in pathological processes such as hepatitis virus entry and aberrant growth factor signaling in fibrogenic liver diseases and HCC.

Common astrocytic programs during brain development, injury and cancer

In addition to radial glial cells of neurohistogenesis, immature astrocytes with stem-cell-like properties cordon off emerging functional patterns in the developing brain. Astrocytes also can be stem cells during adult neurogenesis, and a proposed potency of injury-associated reactive astrocytes has recently been substantiated. Astrocytic cells might additionally be involved in cancer stem cell-associated gliomagenesis. Thus, there are distinguishing roles for stem-cell-like astrocytes during brain development, in neurogenic niches in the adult, during attempted reactive neurogenesis after brain injury or disease and during brain tumorigenesis.

The Rheb-mTOR pathway is upregulated in reactive astrocytes of the injured spinal cord

Astrocytes in the CNS respond to tissue damage by becoming reactive. They migrate, undergo hypertrophy, and form a glial scar that inhibits axon regeneration. Therefore, limiting astrocytic responses represents a potential therapeutic strategy to improve functional recovery. It was recently shown that the epidermal growth factor (EGF) receptor is upregulated in astrocytes after injury and promotes their transformation into reactive astrocytes. Furthermore, EGF receptor inhibitors were shown to enhance axon regeneration in the injured optic nerve and promote recovery after spinal cord injury. However, the signaling pathways involved were not elucidated. Here we show that in cultures of adult spinal cord astrocytes EGF activates the mTOR pathway, a key regulator of astrocyte physiology. This occurs through Akt-mediated phosphorylation of the GTPase-activating protein Tuberin, which inhibits Tuberin's ability to inactivate the small GTPase Rheb. Indeed, we found that Rheb is required for EGF-dependent mTOR activation in spinal cord astrocytes, whereas the Ras-MAP kinase pathway does not appear to be involved. Moreover, astrocyte growth and EGF-dependent chemoattraction were inhibited by the mTOR-selective drug rapamycin. We also detected elevated levels of activated EGF receptor and mTOR signaling in reactive astrocytes in vivo in an ischemic model of spinal cord injury. Furthermore, increased Rheb expression likely contributes to mTOR activation in the injured spinal cord. Interestingly, injured rats treated with rapamycin showed reduced signs of reactive gliosis, suggesting that rapamycin could be used to harness astrocytic responses in the damaged nervous system to promote an environment more permissive to axon regeneration.