Misincorporation of Galactose by Chondroitin Synthase of Escherichia coli K4: From Traces to Synthesis of Chondbiuronan, a Novel Chondroitin-Like Polysaccharide.

Chondroitin synthase KfoC is a bifunctional enzyme which polymerizes the capsular chondroitin backbone of Escherichia coli K4, composed of repeated ß3N-acetylgalactosamine (GalNAc)-ß4-glucuronic acid (GlcA) units. Sugar donors UDP-GalNAc and UDP-GlcA are the natural precursors of bacterial chondroitin synthesis. We have expressed KfoC in a recombinant strain of Escherichia coli deprived of 4-epimerase activity, thus incapable of supplying UDP-GalNAc in the bacterial cytoplasm. The strain was also co-expressing mammal galactose ß-glucuronyltransferase, providing glucuronyl-lactose from exogenously added lactose, serving as a primer of polymerization. We show by the mean of NMR analyses that in those conditions, KfoC incorporates galactose, forming a chondroitin-like polymer composed of the repeated ß3-galactose (Gal)-ß4-glucuronic acid units. We also show that when UDP-GlcNAc 4-epimerase KfoA, encoded by the K4-operon, was co-expressed and produced UDP-GalNAc, a small proportion of galactose was still incorporated into the growing chain of chondroitin.

Design of biomimetic cell-interactive substrates using hyaluronic acid hydrogels with tunable mechanical properties.

Hyaluronic acid (HA) is a natural polysaccharide abundant in biological tissues with excellent potential for constructing synthetic extracellular matrix analogues. In this work, we established a simple and dependable approach to prepare hyaluronic acid-based hydrogels with controlled stiffness and cell recognition properties for use as cell-interactive substrates. This approach relied on a new procedure for the synthesis of methacrylate-modified HA macromers (HA-MA) and, on photorheometry allowing real time monitoring of gelation during photopolymerization. We showed in this way the ability to obtain gels that encompass the range of physiologically relevant elastic moduli while still maintaining the recognition properties of HA by specific cell surface receptors. These hydrogels were prepared from HA macromers having a degree of methacrylation <0.5, which allows to minimize compromising effects on the binding affinity of HA to its cell receptors due to high substitution on the one hand, and to achieve nearly 100% conversion of the methacrylate groups on the other. When the HA hydrogels were immobilized on glass substrates, it was observed that the attachment and the spreading of a variety of mammalian cells rely on CD44 and its coreceptor RHAMM. The attachment and spreading were also shown to be modulated by the elastic properties of the HA matrix. All together, these results highlight the biological potential of these HA hydrogel systems and the needs of controlling their chemical and physical properties for applications in cell culture and tissue engineering.

Alkylamino hydrazide derivatives of hyaluronic acid: synthesis, characterization in semidilute aqueous solutions, and assembly into thin multilayer films.

A series of biodegradable alkylamino hydrazide hyaluronic acid (HA) derivatives were prepared and used to design new biocompatible films able to release hydrophobic drugs in a controlled manner. The first step of this work thus consisted in optimizing the synthetic conditions of hydrazide HA derivatives bearing pendant hexyl, octyl, decyl, and citronellyl chains with a degree of substitution of 0.05 or 0.10. The behavior in aqueous solution of these water-soluble modified HA samples was then examined in the semidilute regime. The decylamino hydrazide derivatives of HA exhibited remarkable associating properties, giving rise to transparent gels. These gels were found to be more resistant to degradation by hyaluronidase compared to solutions of nonmodified HA at the same concentration. The other derivatives of which the lengths of grafted alkyl chains range from 6 to 8 carbon atoms lead to more or less viscous solutions. Different viscometric features for these derivatives could be observed as a function of the molecular weight of HA. As derivatives prepared from a HA sample of 600,000 g/mol (HA-600) exhibited a much higher tendency to self-aggregate than their counterparts prepared from a HA sample of 200,000 g/mol (HA-200), the latter derivatives were selected for the build up of multilayer films. The topography and z-section of (PLL/HA derivatives) films assembled layer-by-layer were observed by atomic force microscopy (AFM) in liquid and confocal laser scanning microscopy (CLSM) using PLL(FITC) as ending layer. Moreover, the ability of the films made of the different HA derivatives to incorporate the hydrophobic dye nile red (NR) was investigated. Films containing decylamino hydrazide HA derivatives were the most efficient for incorporating and retaining nile red, which confirms the formation of stable hydrophobic nanodomains in the films.