Keratan sulphate in the tumour environment

Keratan sulphate (KS) is a bioactive glycosaminoglycan (GAG) of some complexity composed of the repeat disaccharide D-galactose β1→4 glycosidically linked to N-acetyl glucosamine. During the biosynthesis of KS, a family of glycosyltransferase and sulphotransferase enzymes act sequentially and in a coordinated fashion to add D-galactose (D-Gal) then N-acetyl glucosamine (GlcNAc) to a GlcNAc acceptor residue at the reducing terminus of a nascent KS chain to effect chain elongation. D-Gal and GlcNAc can both undergo sulphation at C6 but this occurs more frequently on GlcNAc than D-Gal. Sulphation along the developing KS chain is not uniform and contains regions of variable length where no sulphation occurs, regions which are monosulphated mainly on GlcNAc and further regions of high sulphation where both of the repeat disaccharides are sulphated. Each of these respective regions in the KS chain can be of variable length leading to KS complexity in terms of chain length and charge localization along the KS chain. Like other GAGs, it is these variably sulphated regions in KS which define its interactive properties with ligands such as growth factors, morphogens and cytokines and which determine the functional properties of tissues containing KS. Further adding to KS complexity is the identification of three different linkage structures in KS to asparagine (N-linked) or to threonine or serine residues (O-linked) in proteoglycan core proteins which has allowed the categorization of KS into three types, namely KS-I (corneal KS, N-linked), KS-II (skeletal KS, O-linked) or KS-III (brain KS, O-linked). KS-I to -III are also subject to variable addition of L-fucose and sialic acid groups. Furthermore, the GlcNAc residues of some members of the mucin-like glycoprotein family can also act as acceptor molecules for the addition of D-Gal and GlcNAc residues which can also be sulphated leading to small low sulphation glycoforms of KS. These differ from the more heavily sulphated KS chains found on proteoglycans. Like other GAGs, KS has evolved molecular recognition and information transfer properties over hundreds of millions of years of vertebrate and invertebrate evolution which equips them with cell mediatory properties in normal cellular processes and in aberrant pathological situations such as in tumourogenesis. Two KS-proteoglycans in particular, podocalyxin and lumican, are cell membrane, intracellular or stromal tissue–associated components with roles in the promotion or regulation of tumour development, mucin-like KS glycoproteins may also contribute to tumourogenesis. A greater understanding of the biology of KS may allow better methodology to be developed to more effectively combat tumourogenic processes

Abscisic acid-induced elevation of guard cell cytosolic Ca2+ precedes stomatal closure.

STOMATTA allow the diffusion of CO2 into the leaf for photosynthesis and the diffusion of H2O out of the leaf during transpiration1,2. This gaseous exchange is regulated by pairs of guard cells that surround each stomatal pore. During water stress the loss of water through transpiration is reduced in response to abscisic acid3, a naturally occurring plant growth regulator which is also present in certain mammals4, algae5 and fungi6, by the promotion of stomatal closure and inhibition of opening7. This involves alterations to guard cell turgor, causing the cells to shrink and thereby reducing the size of the stomatal pore. These changes are driven by cation and anion effluxes8. It has been proposed that an abscisic acid-dependent increase in the concentration of guard cell cytosolic free calcium triggers the intracellular machinery responsible for stomatal closure9(for a review, see ref. 10), but attempts to test this hypothesis by measuring [45Ca] fluxes have produced equivocal results11. Using the fluorescent calcium indicator fura-2, we report that abscisic acid induces a rapid increase in guard cell cytosolic free Ca2+ in Commelina communisL., and that this increase precedes stomatal closure. These results strongly support the suggestion that Ca2+ is an intracellular second messenger in this response