Functions of the L2/HNK-1 carbohydrate in the nervous system

This chapter discusses the functions of L2/HNK-1 (L2) carbohydrate in the nervous system. The L2 carbohydrate is carried by several neural recognition molecules. Its receptors are present on cell surface and extracellular matrix recognition molecules. Thus, the L2 carbohydrate is an important ligand in cell interactions in the nervous system and in the immune system, possibly triggering interactions between the cells of the neural and lymphoid lineages. The finely tuned regulation of L2 expression by the subpopulations of myelinating Schwann cells in the adult peripheral nervous system of mice raises the possibility that it may favor target preferential reinnervation by motor axons. The fact that recognition molecules can be both receptors and donors of functionally important carbohydrates points to an intricate network of multiple combinations in cell-to-cell and cell-to-matrix interactions in the nervous system, particularly because carbohydrate structures are subject to finely tuned-developmentally and cell type-specifically regulated synthetic mechanisms. The L2 carbohydrate is an eminent example of the ingenuity of nature to use carbohydrate structures in functional contexts.