Polysialic acid immobilized on silanized glass surfaces: a test case for its use as a biomaterial for nerve regeneration

Polysialic acid production using Escherichia coli K1 in a disposable bag reactor

Polysialic acid (polySia), consisting of α-(2,8)-linked N-acetylneuraminic acid monomers plays a crucial role in many biological processes. This study presents a novel process for the production of endogenous polySia using Escherichia coli K1 in a disposable bag reactor with wave-induced mixing. Disposable bag reactors provide easy and fast production in terms of regulatory requirements as GMP, flexibility, and can easily be adjusted to larger production capacities not only by scale up but also by parallelization. Due to the poor oxygen transfer rate compared to a stirred tank reactor, pure oxygen was added during the cultivation to avoid oxygen limitation. During the exponential growth phase the growth rate was 0.61 h^-1. Investigation of stress-related product release from the cell surface showed no significant differences between the disposable bag reactor with wave-induced mixing and the stirred tank reactor. After batch cultivation a cell dry weight of 6.8 g L^-1 and a polySia concentration of 245 mg L^-1 were reached. The total protein concentration in the supernatant was 132 mg L^-1. After efficient and time-saving downstream processing characterization of the final product showed a protein content of below 0.04 mg_protein/g_polySia and a maximal chain length of ∼90 degree of polymerization.

Effects of polysialic acid on sensory innervation of the cornea

Sensory trigeminal growth cones innervate the cornea in a coordinated fashion during embryonic development. Polysialic acid (polySia) is known for its important roles during nerve development and regeneration. The purpose of this work is to determine whether polySia, present in developing eyefronts and on the surface of sensory nerves, may provide guidance cues to nerves during corneal innervation. Expression and localization of polySia in embryonic day (E)5-14 chick eyefronts and E9 trigeminal ganglia were identified using Western blotting and immunostaining. Effects of polySia removal on trigeminal nerve growth behavior were determined in vivo, using exogenous endoneuraminidase (endoN) treatments to remove polySia substrates during chick cornea development, and in vitro, using neuronal explant cultures. PolySia substrates, made by the physical adsorption of colominic acid to a surface coated with poly-d-lysine (PDL), were used as a model to investigate functions of the polySia expressed in axonal environments. PolySia was localized within developing eyefronts and on trigeminal sensory nerves. Distributions of PolySia in corneas and pericorneal regions are developmentally regulated. PolySia removal caused defasciculation of the limbal nerve trunk in vivo from E7 to E10. Removal of polySia on trigeminal neurites inhibited neurite outgrowth and caused axon defasciculation, but did not affect Neural Cell Adhesion Molecule (NCAM) expression or Schwann cell migration in vitro. PolySia substrates in vitro inhibited outgrowth of trigeminal neurites and promoted their fasciculation. In conclusion, polySia is localized on corneal nerves and in their targeting environment during early developing stages of chick embryos. PolySias promote fasciculation of trigeminal axons in vivo and in vitro, whereas, in contrast, their removal promotes defasciculation.

Comparison of morphology and biocompatibility of acellular nerve scaffolds processed by different chemical methods

To investigate the morphological differences among acellular rat nerve scaffolds processed by different chemical methods and compare the biocompatibility between rat nerve grafts processed by different chemical methods and rat adipose-derived stem cells in vitro. Acellular rat sciatic nerve scaffolds processed by two different chemical methods (the Sondell method and the optimized method) and normal rat sciatic nerves were used as control. The structure and components of nerve scaffold were observed under microscopy, the degrees of decellularization and demyelination of nerve scaffold and integrity of nerve fiber tubes were assessed. The rat adipose-derived stem cells growth and adherence on scaffold were studied by scanning electron microscopy, the activity and adhesive ratio of rat adipose-derived stem cells in the nerve scaffold were compared. The basal lamina tubes and the extracellular matrix in the epineurium and perineurium in the nerve graft of optimized method were better preserved than the nerve graft of the Sondell method. After co-cultured with scaffolds, the difference of cell activity between three groups (two cell-scaffold combinations and control group) at the same observation time were not statistically significant (P > 0.05),the adhesive ratio of rat adipose-derived stem cells in the scaffold of the optimized method was better than that of the Sondell method. The scaffold of the optimized method is more effective than the scaffold of the Sondell method for peripheral nerve tissue engineering.

Comparative evaluation of chitosan, cellulose acetate, and polyethersulfone nanofiber scaffolds for neural differentiation

Based on accumulating evidence that the 3D topography and the chemical features of a growth surface influence neuronal differentiation, we combined these two features by evaluating the cytotoxicity, proliferation, and differentiation of the rat PC12 line and human neural stem cells (hNSCs) on chitosan (CS), cellulose acetate (CA), and polyethersulfone (PES)-derived electrospun nanofibers that had similar diameters, centered in the 200-500 nm range. None of the nanofibrous materials were cytotoxic compared to 2D (e.g., flat surface) controls; however, proliferation generally was inhibited on the nanofibrous scaffolds although to a lesser extent on the polysaccharide-derived materials compared to PES. In an exception to the trend toward slower growth on the 3D substrates, hNSCs differentiated on the CS nanofibers proliferated faster than the 2D controls and both cell types showed enhanced indication of neuronal differentiation on the CS scaffolds. Together, these results demonstrate beneficial attributes of CS for neural tissue engineering when this polysaccharide is used in the context of the defined 3D topography found in electrospun nanofibers.

Glycomic strategy for efficient linkage analysis of di-, oligo- and polysialic acids

Sialic acid polymers of glycoproteins and glycolipids are characterized by a high diversity in nature and are involved in distinct biological processes depending inter alia on the glycosidic linkages between the present sialic acid residues. Though suitable protocols are available for chain length and sialic acid determination, sensitive methods for linkage analysis of di-, oligo-, and polysialic acids (di/oligo/polySia) are still pending. In this study, we have established a highly sensitive glycomic strategy for this purpose which is based on permethylation of di/oligo/polySia after tagging their reducing ends with the fluorescent dye 1,2-diamino-4,5-methylenedioxybenzene (DMB). Using DMB-labeled sialic acid di/oligo/polymers glycosidic linkages could be efficiently determined and, optionally, the established working procedure can be combined with HPLC for in depth characterization of distinct di/oligo/polySia chains. Moreover, the outlined approach can be directly applied to mammalian tissue samples and linkage analysis of sialic acid polymers present in biopsy samples of neuroblastoma tissue demonstrating the usefulness of the outlined work flow to screen, for example, cancer tissue for the presence of distinct variants of di/oligo/polySia as potentially novel biomarkers. Hence, the described strategy offers a highly sensitive and efficient strategy for identification of glycosidic linkages in sialic acid di/oligo/polymers of glycoproteins and glycolipids.

Endosialidases: Versatile Tools for the Study of Polysialic Acid

Polysialic acid is an α2,8-linked N-acetylneuraminic acid polymer found on the surface of both bacterial and eukaryotic cells. Endosialidases are bacteriophage-borne glycosyl hydrolases that specifically cleave polysialic acid. The crystal structure of an endosialidase reveals a trimeric mushroom-shaped molecule which, in addition to the active site, harbors two additional polysialic acid binding sites. Folding of the protein crucially depends on an intramolecular C-terminal chaperone domain that is proteolytically released in an intramolecular reaction. Based on structural data and previous considerations, an updated catalytic mechanism is discussed. Endosialidases degrade polysialic acid in a processive mode of action, and a model for its mechanism is suggested. The review summarizes the structural and biochemical elucidations of the last decade and the importance of endosialidases in biochemical and medical applications. Active endosialidases are important tools in studies on the biological roles of polysialic acid, such as the pathogenesis of septicemia and meningitis by polysialic acid-encapsulated bacteria, or its role as a modulator of the adhesion and interactions of neural and other cells. Endosialidase mutants that have lost their polysialic acid cleaving activity while retaining their polysialic acid binding capability have been fused to green fluorescent protein to provide an efficient tool for the specific detection of polysialic acid.

Neural cell type-specific responses to glycomimetic functionalized collagen

Despite their noted functional role, glycans have had limited therapeutic use due to difficulties in synthesis and quick degradation in vivo. The recent discovery of glycomimetics has provided new opportunities for their application. In this study, we have functionalized type I collagen with peptide mimics of two glycans: (1) polysialic acid (PSA) and (2) an epitope first discovered on human natural killer cells (HNK-1). These glycans and their glycomimetic counterparts have been shown to be important regulators of repair following injury through their unique and phenotypically specific effects on neural behavior. We show that these molecules retain their bioactivity following functionalization to the collagen backbone. Grafted HNK-1 encouraged motor neuron outgrowth, while grafted PSA encouraged sensory and motor neuron outgrowth and enhanced Schwann cell proliferation and process extension. These data support the potential of glycomimetic-functionalized collagen as a biomaterial strategy to increase the efficiency of synaptic reconnection following nervous system injury.