Homeostatic regulation of NCAM polysialylation is critical for correct synaptic targeting

Abstract

During development, axonal projections have a remarkable ability to innervate correct dendritic subcompartments of their target neurons and to form regular neuronal circuits. Altered axonal targeting with formation of synapses on inappropriate neurons may result in neurodevelopmental sequelae, leading to psychiatric disorders. Here we show that altering the expression level of the polysialic acid moiety, which is a developmentally regulated, posttranslational modification of the neural cell adhesion molecule NCAM, critically affects correct circuit formation. Using a chemically modified sialic acid precursor (N-propyl-D: -mannosamine), we inhibited the polysialyltransferase ST8SiaII, the principal enzyme involved in polysialylation during development, at selected developmental time-points. This treatment altered NCAM polysialylation while NCAM expression was not affected. Altered polysialylation resulted in an aberrant mossy fiber projection that formed glutamatergic terminals on pyramidal neurons of the CA1 region in organotypic slice cultures and in vivo. Electrophysiological recordings revealed that the ectopic terminals on CA1 pyramids were functional and displayed characteristics of mossy fiber synapses. Moreover, ultrastructural examination indicated a "mossy fiber synapse"-like morphology. We thus conclude that homeostatic regulation of the amount of synthesized polysialic acid at specific developmental stages is essential for correct synaptic targeting and circuit formation during hippocampal development.

Tissue-based metabolic labeling of polysialic acids in living primary hippocampal neurons

“…We envisioned that the primary neurons would tolerate external perturbations better when they were interconnected to each other and supplemented by tissue scaffolds and nutrients. Our hypothesis was supported indirectly by a previous report, in which ManNProp, a potent inhibitor of a polysialyltransferase (ST8SiaII) (41,46), was metabolized and expressed on cell membranes in hippocampal slice culture and in vivo, although at an extremely low efficiency, probably by another polysialyltransferase (ST8SiaIV) (7,47). The use of tissue culture was also hinted as a means of increasing the cellular tolerance to unnatural sialic acids and their precursors, although no data were presented (43).…”

“…The glycans modulate a multitude of cellular functions, such as adhesion, differentiation, cell-cell interactions, and recognition of external molecules, by mediating intracellular signaling events and also, intercellular communications (2,3). The regulatory functions of glycans are also considered to be significant in neurons, and previous biological work has suggested the intimate involvement of glycosylation in neurite development, synaptic plasticity, and neural regeneration (4)(5)(6)(7). The functional modulation of neural celladhesion molecule (NCAM) with polysialic acid (PSA), in particular, has been a primary target in the investigation of glycan roles in neurons.…”

Biomimetic neural scaffolds: a crucial step towards optimal peripheral nerve regeneration

“…158–160 These lines of evidence indicate the feasibility of glycoengineering precursors, and highlight the potential for future development and optimization of neural scaffolds towards clinical translation.…”

Metabolic Glycoengineering with N‐Acyl Side Chain Modified Mannosamines

“…[365,366] MGE can be used as at emplate for in vivo experiments,a nd has advantages over other approaches,f or example,k nock-out experiments of polysialyltranserases in mice.R ecently,i tw as shown that inhibition of NCAM polysialylation by ManNProp induces aberrant border formation and fiber tract outgrowth in the hippocampus of newborn mice (Scheme 14). [367] Presumably, homeostatic regulation of the quantity of polySia at restricted stages of development is essential for correct synaptic targeting and circuit formation during hippocampal development. MGE by ManNProp is au seful method to interfere with polysialylation and signal transduction in vivo.…”