Roles of Glycolipids in the Development and Maintenance of Nervous Tissues

Investigation of a chondroitin sulfate-based bioactive coating for neural interface applications

Invasive neural implants allow for high-resolution bidirectional communication with the nervous tissue and have demonstrated the ability to record neural activity, stimulate neurons, and sense neurochemical species with high spatial selectivity and resolution. However, upon implantation, they are exposed to a foreign body response which can disrupt the seamless integration of the device with the native tissue and lead to deterioration in device functionality for chronic implantation. Modifying the device surface by incorporating bioactive coatings has been a promising approach to camouflage the device and improve integration while maintaining device performance. In this work, we explored the novel application of a chondroitin sulfate (CS) based hydrophilic coating, with anti-fouling and neurite-growth promoting properties for neural recording electrodes. CS-coated samples exhibited significantly reduced protein-fouling in vitro which was maintained for up to 4-weeks. Cell culture studies revealed a significant increase in neurite attachment and outgrowth and a significant decrease in microglia attachment and activation for the CS group as compared to the control. After 1-week of in vivo implantation in the mouse cortex, the coated probes demonstrated significantly lower biofouling as compared to uncoated controls. Like the in vitro results, increased neuronal population (neuronal nuclei and neurofilament) and decreased microglial activation were observed. To assess the coating's effect on the recording performance of silicon microelectrodes, we implanted coated and uncoated electrodes in the mouse striatum for 1 week and performed impedance and recording measurements. We observed significantly lower impedance in the coated group, likely due to the increased wettability of the coated surface. The peak-to-peak amplitude and the noise floor levels were both lower in the CS group compared to the controls, which led to a comparable signal-to-noise ratio between the two groups. The overall single unit yield (% channels recording a single unit) was 74% for the CS and 67% for the control group on day 1. Taken together, this study demonstrates the effectiveness of the polysaccharide-based coating in reducing biofouling and improving biocompatibility for neural electrode devices.

CHARMM-GUI Membrane Builder for Complex Biological Membrane Simulations with Glycolipids and Lipoglycans

Glycolipids (such as glycoglycerolipids, glycosphingolipids, and glycosylphosphatidylinositol) and lipoglycans (such as lipopolysaccharides (LPS), lipooligosaccharides (LOS), mycobacterial lipoarabinomannan, and mycoplasma lipoglycans) are typically found on the surface of cell membranes and play crucial roles in various cellular functions. Characterizing their structure and dynamics at the molecular level is essential to understand their biological roles, but systematic generation of glycolipid and lipoglycan structures is challenging because of great variations in lipid structures and glycan sequences (i.e., carbohydrate types and their linkages). To facilitate the generation of all-atom glycolipid/LPS/LOS structures, we have developed Glycolipid Modeler and LPS Modeler in CHARMM-GUI ( http://www.charmm-gui.org ), a web-based interface that simplifies building of complex biological simulation systems. In addition, we have incorporated these modules into Membrane Builder so that users can readily build a complex symmetric or asymmetric biological membrane system with various glycolipids and LPS/LOS. These tools are expected to be useful in innovative and novel glycolipid/LPS/LOS modeling and simulation research by easing tedious and intricate steps in modeling complex biological systems and shall provide insight into structures, dynamics, and underlying mechanisms of complex glycolipid-/LPS-/LOS-containing biological membrane systems.

The distribution of the proteoglycan FORSE-1 in the developing mouse central nervous system

Glycosylation is a major post-translational modification in which a carbohydrate known as a glycan is enzymatically attached to target proteins which regulate protein folding and stability. Glycans are strongly expressed in the developing nervous system where they play multiple roles during development. The importance of these glycan epitopes in neural development is highlighted by a group of conditions known as congenital disorders of glycosylation which lead to psychomotor difficulties, mental retardation, lissencephaly, microencephaly and epilepsy. One of these glycan epitopes, known as Lewis X, is recognised by the FORSE-1 antibody and is regionally expressed in the developing nervous system. In this study, we report the regional and temporal expression patterns of FORSE-1 immunolabelling during the periods of neurogenesis, gliogenesis and axonogenesis in developing mouse nervous system. We demonstrate the localisation of FORSE-1 on subsets of neuroepithelial cells and radial glial cells, and in compartments corresponding to axon tract formation. These spatial, temporal and regional expression patterns are suggestive of roles in the determination of different cell lineages and in the patterning of white matter during development, and help provide insights into the neuroanatomical regions affected by congenital disorders of glycosylation.

Reduced motor and sensory functions and emotional response in GM3-only mice: emergence from early stage of life and exacerbation with aging

Sialic acid-containing glycosphingolipids (gangliosides) have been believed to play a role in the regulation and protection of nervous tissues. To clarify their function in the nervous system in vivo, double knockout (DKO) mice of GM2/GD2 synthase and GD3 synthase genes were generated and abnormal behaviors were analyzed. Mutant mice exhibited reduced weight and a round shape of the whole brain that progressively emerged with aging, and displayed motor dysfunction in the footprint, traction, open-field, and 24h locomotion activity tests. Sensory functions were also reduced in the von Frey and hot plate tests and greatly reduced in the acoustic startle response test. For emotional behavior, fear response was clearly decreased. Numerous neuronal dysfunctions were found even in younger mutant mice examined at 10-23 weeks after birth, which were exacerbated with aging. These results suggest that a lack of gangliosides other than GM3 induces severe neuronal degeneration in the early stage of life, and that the expression of complex gangliosides is essential to maintain the integrity of the nervous system throughout life.

Glycosphingolipids with extended sugar chain have specialized functions in development and behavior of Drosophila

Glycosphingolipids (GSL) are glycosylated polar lipids in cell membranes essential for development of vertebrates as well as Drosophila. Mutants that impair enzymes involved in biosynthesis of GSL sugar chains provide a means to assess the functions of the sugar chains in vivo. The Drosophila glycosyltransferases Egghead and Brainiac are responsible for the 2nd and 3rd steps of GSL sugar chain elongation. Mutants lacking these enzymes are lethal and the nature of the defects that occur has suggested that GSL might impact on signaling by the Notch and EGFR pathways. Here we report on characterization of enzymes involved in the 4th and 5th steps of GSL sugar chain elongation in vitro and explore the biological consequences of removing the enzymes involved in step 4 in vivo. Two beta4-N-Acetylgalactosyltransferase enzymes can carry out step 4 (beta4GalNAcTA and beta4GalNAcTB), and while they may have overlapping activity, the mutants produce distinct phenotypes. The beta4GalNAcTA mutant displays behavioral defects, which are also observed in viable brainiac mutants, suggesting that proper locomotion and coordination primarily depend on GSL elongation. beta4GalNAcTB mutant animal shows ventralization of ovarian follicle cells, which is caused by defective EGFR signaling between the oocyte and the dorsal follicle cells to specify dorsal fate. GSL sequentially elongated by Egh, Brn and beta4GalNAcTB in the oocyte contribute to this signaling pathway. Despite the similar enzymatic activity, we provide evidence that the two enzymes are not functionally redundant in vivo, but direct distinct developmental functions of GSL.