Mary Vinson

Inhibitor of neurite outgrowth in humans.

Axons are generally believed to be incapable of regeneration in the adult central nervous system. Inhibition results from physical barriers imposed by glial scars, a lack of neurotrophic factors, and growth-inhibitory molecules associated with myelin, the insulating axon sheath. These molecules include proteoglycans, myelin-associated glycoprotein and, in bovine brain, two proteins called Nogo. We have used this bovine sequence to identify the human Nogo gene and have isolated complementary DNA clones encoding three different Nogo isoforms that are potent inhibitors of neurite outgrowth and which may help block the regeneration of the central nervous system in adults.

Neurobiology: Inhibitor of neurite outgrowth in humans

Axons are generally believed to be incapable of regeneration in the adult central nervous system. Inhibition results from physical barriers imposed by glial scars, a lack of neurotrophic factors, and growth-inhibitory molecules associated with myelin, the insulating axon sheath. These molecules include proteoglycans, myelin-associated glycoprotein and, in bovine brain, two proteins called Nogo. We have used this bovine sequence to identify the human Nogo gene and have isolated complementary DNA clones encoding three different Nogo isoforms that are potent inhibitors of neurite outgrowth and which may help block the regeneration of the central nervous system in adults.

Myelin-associated Glycoprotein Interacts with Ganglioside GT1b A MECHANISM FOR NEURITE OUTGROWTH INHIBITION

Myelin-associated glycoprotein (MAG) is expressed on myelinating glia and inhibits neurite outgrowth from post-natal neurons. MAG has a sialic acid binding site in its N-terminal domain and binds to specific sialylated glycans and gangliosides present on the surface of neurons, but the significance of these interactions in the effect of MAG on neurite outgrowth is unclear. Here we present evidence to suggest that recognition of sialylated glycans is essential for inhibition of neurite outgrowth by MAG. Arginine 118 on MAG is known to make a key contact with sialic acid. We show that mutation of this residue reduces the potency of MAG inhibitory activity but that residual activity is also a result of carbohydrate recognition. We then go on to investigate gangliosides GT1b and GD1a as candidate MAG receptors. We show that MAG specifically binds both gangliosides and that both are expressed on the surface of MAG-responsive neurons. Furthermore, antibody cross-linking of cell surface GT1b, but not GD1a, mimics the effect of MAG, in that neurite outgrowth is inhibited through activation of Rho kinase. These data strongly suggest that interaction with GT1b on the neuronal cell surface is a potential mechanism for inhibition of neurite outgrowth by MAG.

Lipid rafts mediate the interaction between myelin-associated glycoprotein (MAG) on myelin and MAG-receptors on neurons.

The interaction between myelin-associated glycoprotein (MAG), expressed at the periaxonal membrane of myelin, and receptors on neurons initiates a bidirectional signalling system that results in inhibition of neurite outgrowth and maintenance of myelin integrity. We show that this involves a lipid-raft to lipid-raft interaction on opposing cell membranes. MAG is exclusively located in low buoyancy Lubrol WX-insoluble membrane fractions isolated from whole brain, primary oligodendrocytes, or MAG-expressing CHO cells. Localisation within these domains is dependent on cellular cholesterol and occurs following terminal glycosylation in the trans-Golgi network, characteristics of association with lipid rafts. Furthermore, a recombinant form of MAG interacts specifically with lipid-raft fractions from whole brain and cultured cerebellar granule cells, containing functional MAG receptors GT1b and Nogo-66 receptor and molecules required for transduction of signal from MAG into neurons. The localisation of both MAG and MAG receptors within lipid rafts on the surface of opposing cells may create discrete areas of high avidity multivalent interaction, known to be critical for signalling into both cell types. Localisation within lipid rafts may provide a molecular environment that facilitates the interaction between MAG and multiple receptors and also between MAG ligands and molecules involved in signal transduction.

Identification of Neuroprotective Properties of Anti-MAG Antibody: A Novel Approach for the Treatment of Stroke?

The inhibitory activity of myelin-associated glycoprotein (MAG) on neurons is thought to contribute to the lack of regenerative capacity of the CNS after injury. The interaction of MAG and its neuronal receptors mediates bidirectional signaling between neurons and oligodendrocytes. The novel finding that an anti-MAG monoclonal antibody not only possesses the ability to neutralise the inhibitory effect of MAG on neurons but also directly protects oligodendrocytes from glutamate-mediated oxidative stress-induced cell death is reported here. Furthermore, administration of anti-MAG antibody (centrally and systemically) starting 1 hour after middle cerebral artery occlusion in the rat significantly reduced lesion volume at 7 days. This neuroprotection was associated with a robust improvement in motor function compared with animals receiving control IgG1. Together, these data highlight the potential for the use of anti-MAG antibodies as therapeutic agents for the treatment of stroke.

Tissue sparing and functional recovery following experimental traumatic brain injury is provided by treatment with an anti‐myelin‐associated glycoprotein antibody

Axonal injury is a hallmark of traumatic brain injury (TBI) and is associated with a poor clinical outcome. Following central nervous system injury, axons regenerate poorly, in part due to the presence of molecules associated with myelin that inhibit axonal outgrowth, including myelin‐associated glycoprotein (MAG). The involvement of MAG in neurobehavioral deficits and tissue loss following experimental TBI remains unexplored and was evaluated in the current study using an MAG‐specific monoclonal antibody (mAb). Anesthetized rats (n = 102) were subjected to either lateral fluid percussion brain injury (n = 59) or sham injury (n = 43). In surviving animals, beginning at 1 h post‐injury, 8.64 µg anti‐MAG mAb (n = 33 injured, n = 21 sham) or control IgG (n = 26 injured, n = 22 sham) was infused intracerebroventricularly for 72 h. One group of these rats (n = 14 sham, n = 11 injured) was killed at 72 h post‐injury for verification of drug diffusion and MAG immunohistochemistry. All other animals were evaluated up to 8 weeks post‐injury using tests for neurologic motor, sensory and cognitive function. Hemispheric tissue loss was also evaluated at 8 weeks post‐injury. At 72 h post‐injury, increased immunoreactivity for MAG was seen in the ipsilateral cortex, thalamus and hippocampus of brain‐injured animals, and anti‐MAG mAb was detectable in the hippocampus, fimbria and ventricles. Brain‐injured animals receiving anti‐MAG mAb showed significantly improved recovery of sensorimotor function at 6 and 8 weeks (P < 0.01) post‐injury when compared with brain‐injured IgG‐treated animals. Additionally, at 8 weeks post‐injury, the anti‐MAG mAb‐treated brain‐injured animals demonstrated significantly improved cognitive function and reduced hemispheric tissue loss (P < 0.05) when compared with their brain‐injured controls. These results indicate that MAG may contribute to the pathophysiology of experimental TBI and treatment strategies that target MAG may be suitable for further evaluation.

P1-033: The identification of distinct pools of Aβ in the brains of TASTPM mice

compared to controls. BDNF mRNA levels were also significantly reduced in 15-18 month old APP and APP/PS-1 mice, but not in mice carrying only the PS-1 mutation. BDNF levels were unchanged in 18-21 month old Ts65Dn mice compared to controls, indicating that over-expression of murine APP without mutation is insufficient to down-regulate BDNF, and that defective retrograde transport in these mice, which is known to result in accumulation of NGF in cholinergic target tissues, does not influence cortical BDNF mRNA levels. In summary, mutation(s) in APP alone are sufficient to down-regulate BDNF mRNA in cortical tissue of transgenic mice.

Inhibitor of neurite outgrowth in humans: Neurobiology

Axons are generally believed to be incapable of regeneration in the adult central nervous system. Inhibition results from physical barriers imposed by glial scars, a lack of neurotrophic factors, and growth-inhibitory molecules associated with myelin, the insulating axon sheath. These molecules include proteoglycans, myelin-associated glycoprotein and, in bovine brain, two proteins called Nogo. We have used this bovine sequence to identify the human Nogo gene and have isolated complementary DNA clones encoding three different Nogo isoforms that are potent inhibitors of neurite outgrowth and which may help block the regeneration of the central nervous system in adults.