K.H. Smalla, W. Tischmeyer, K. Pohlmann

Glycoproteins are major constituents of synaptic membrane structures. Termination of carbohydrate chains with residues like sialic acid and fucose have been shown to have an impact on synaptic plasticity . In particular, protein fucosylation seems to be crucial in long-term memory formation and for maintenance of hippocampal long-term potentiation (LTP). We have shown that inhibitors of protein glycosylation like tunicamycin, swainsonine or brefeldin A reduce the incorporation of H [3] -fucose and block the maintenance of LTP in vitro (Matthies et al 1999) whereas application of fucose significantly enhances LTP maintenance in vivo and in vitro (Krug et al 94; Matthies et al. 97; Matthies et al 2000) Using affinitychromatography based on binding of fucose-containing carbohydrate moieties to either fucose-specific antibodies or fucose-specific lectins we were able to identify several synaptic fucosylated proteins among them NCAM 180, NCAM140, N-Cadherin and Neuroplastin (Smalla et al 98). Currently we focus on further identification of fucosylprotein-complex components.

Fig. 1: Lectinhistochemical staining of rat hippocampus with close-up to CA1 (upper right), CA3 (bottom left) and dentate gyrus (bottom right) (by courtesy of Dr. K. Richter)

In a detailed study we investigated the role of neuroplastins in synaptic plasticity which we have identified to be potential target proteins for activity-dependent fucosylation. Neuroplastin-65 and -55 (previously known as gp65 and gp55) are glycoproteins of the Ig superfamily that are enriched in rat forebrain synaptic membrane preparations. Whereas the two-Ig domain isoform neuroplastin-55 is expressed in many tissues, the three-Ig domain isoform neuroplastin-65 is brain-specific and enriched in postsynaptic density (PSD) protein preparations. Immunocytochemical studies with neuroplastin-65-specific antibodies differentially stain distinct synaptic neuropil regions of the rat hippocampus. Kainate-induced seizures cause a significant enhancement of neuroplastin-65 association with PSDs. Similarly, long-term potentiation (LTP) of CA1 synapses in hippocampal slices enhanced the association of neuroplastin-65 with a detergent-insoluble PSD-enriched protein fraction. Several antibodies against the neuroplastins, including one specific for neuroplastin-65, inhibited the maintenance of LTP. A similar effect was observed when recombinant fusion protein containing the three extracellular Ig domains of neuroplastin-65 was applied to hippocampal slices before LTP induction. Microsphere binding experiments using neuroplastin-Fc chimeric proteins show that constructs containing Ig1–3 or Ig1 domains, but not Ig2–3 domains mediate homophilic adhesion. These data suggest that neuroplastin plays an essential role in implementing long-term changes in synaptic activity, possibly by means of a homophilic adhesion mechanism (Smalla et al 2000).

Fig. 2: Modelling of the homophilic interaction of the extracellular parts of neuroplastin-65 (by courtesy of C. Reissner)

Relevant Publications:

Krug, M., Wagner, M., Staak, S. Smalla, K.H. (1994) Fucose and fucose-containing sugar epitopes enhance hippocampal long-term potentiation in the freely moving rat. Brain Res. 643, 130-135.

Matthies, H. Jr., Staak, S., Smalla, K.H. , Krug, M. (1997) Enhancement if hippocampal long-term potentiation in vitro by fucosyl-carbohydrates. In: Neurochemistry: Cellular, Molecular and Clinical Aspects; Eds A. Teelken and J. Korf, Plenum Press 1997, New York and London, 905-908

Smalla, K.H. , Angenstein, F., Richter, K., Gundelfinger, E.D. and Staak, S. (1998) Identification of fucose-alpha[1-2]-galactose epitope-containing glycoproteins from rat hippocampus. NeuroReport 9 , 813-817

Matthies,H. Jr., Kretlow, J,,Matthies, H., Smalla, K.H. , Staak, S., Krug, M. (1999) Glycosylation of proteins during a critical time window is necessary for the maintenance of long-term potentiation in the hippocampal CA1 region. Neuroscience 91 , 175-183

Matthies, H., Schröder, H., Smalla, K.H. and Krug, M. (2000) Enhancement of glutamate release by L-fucose changes effects of glutamate receptor antagonists on long-term potentiation in the rat hippocampus. Learning & Memory 7 , 227-234

Smalla, K.H. , Matthies, H., Langnaese, K., Boeckers, T.M. Wyneken, U., Staak, S., Krug, M., Beesley, P. W. and Gundelfinger, E.D. (2000) The synaptic glycoprotein neuroplastin is involved in long-term potentiation at hippocampal CA1 synapses. Proc. Natl. Acad. Sci. USA 97 , 4327-4332

Collaborations:

• Philip W. Beesley, Royal Holloway University, London, U.K.
• Ursula Wyneken, Universida de los Andes, Santiago , Chile
• Manfred Krug, Medical Faculty, Otto von Guericke University Magdeburg , Germany
• Henry Matthies, Medical Faculty, Otto von Guericke University Magdeburg , Germany
• Eckart D. Gundelfinger, Dep. Neurochemistry
• Karin Richter, Medical Faculty, Otto von Guericke University Magdeburg , Germany