Glycosylation in Cellular Mechanisms of Health and Disease

Simple and complex carbohydrates (glycans) have long been known to play major metabolic, structural and physical roles in biological systems. Targeted microbial binding to host glycans has also been studied for decades. But such biological roles can only explain some of the remarkable complexity and organismal diversity of glycans in nature. Reviewing the subject about two decades ago, one could find very few clear-cut instances of glycan-recognition-specific biological roles of glycans that were of intrinsic value to the organism expressing them. In striking contrast there is now a profusion of examples, such that this updated review cannot be comprehensive. Instead, a historical overview is presented, broad principles outlined and a few examples cited, representing diverse types of roles, mediated by various glycan classes, in different evolutionary lineages. What remains unchanged is the fact that while all theories regarding biological roles of glycans are supported by compelling evidence, exceptions to each can be found. In retrospect, this is not surprising. Complex and diverse glycans appear to be ubiquitous to all cells in nature, and essential to all life forms. Thus, >3 billion years of evolution consistently generated organisms that use these molecules for many key biological roles, even while sometimes coopting them for minor functions. In this respect, glycans are no different from other major macromolecular building blocks of life (nucleic acids, proteins and lipids), simply more rapidly evolving and complex. It is time for the diverse functional roles of glycans to be fully incorporated into the mainstream of biological sciences.

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Biological Roles of Glycans

In the CST, there was a first fixation period of 1 s, followed by a brief presentation (100 ms) of a cue at 208 to the left or to the right The monkey had to maintain his gaze at the fixation spot during a delay period of 1 s The fixation spot then disappeared and the target appeared at the same position as the cue In half of the trials, another spot appeared at the alternative position (distracter) The monkey had to make a saccade to the target position within 500 ms, and was rewarded with a drop of water for a correct saccade After recording neurons in the CST (experiment 2), we re-applied the BST (experiment 1) for at least 80 trials to confirm the reproducibility of neuronal activity Data analysis Pre-target neurons were defined as neurons that showed a statistically reliable increase in the spike count 215 s to 0 s before target onset (‘pre-target window’) as compared with the spike count 2 3s to215 s before target onset All pair-wise comparisons were evaluated by two-tailed t-tests, P , 001 We used the Bonferroni procedure to correct for family-wise error with multiple t-tests To quantify the separation of population distributions from contralateral versus ipsilateral conditions, we calculated the area under the ROC in a sliding window of 200 ms To test the adaptation of saccade latency and pre-target activity to a reversal of position-reward contingency, we compared the second trial after a reversal against the third trial after a reversal (test 1) We also compared the second trial after a reversal against the pooled data from the sixth to twentieth trial (test 2) Both tests consisted of paired twotailed t-tests on the mean data from individual neurons Adaptation was judged complete if there was no significant difference between the measures Tests 1 and 2 produced similar results in all cases For comparison between the two tasks (BST versus CST), we considered the neuronal activity from 2500 to 0 ms before target onset in both tasks, in the computation of absolute firing rates as well as ROC areas

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Post-translational disruption of dystroglycan-ligand interactions in congenital muscular dystrophies

Increasing recognition of malformations of cortical development and continuing improvements in imaging techniques, molecular biologic techniques, and knowledge of mechanisms of brain development have resulted in continual improvement of the understanding of these disorders. The authors propose a revised classification based on the stage of development (cell proliferation, neuronal migration, cortical organization) at which cortical development was first affected. The categories are based on known developmental steps, known pathologic features, known genetics (when possible), and, when necessary, neuroimaging features. In those cases in which the precise developmental and genetic features are uncertain, classification is based on known relationships among the genetics, pathologic features, and neuroimaging features. The major change since the prior classification has been a shift to using genotype, rather than phenotype, as the basis for classifying disorders wherever the genotype-phenotype relationship is adequately understood. Other substantial changes include more detailed classification of congenital microcephalies, particularly those in which the genes have been mapped or identified, and revised classification of congenital muscular dystrophies and polymicrogyrias. Information on genetic testing is also included. This classification allows a better conceptual understanding of the disorders, and the use of neuroimaging characteristics allows it to be applied to all patients without necessitating brain biopsy, as in pathology-based classifications.

A developmental and genetic classification for malformations of cortical development

Researchers have long predicted that complex carbohydrates on cell surfaces would play important roles in developmental processes because of the observation that specific carbohydrate structures appear in specific spatial and temporal patterns throughout development. The astounding number and complexity of carbohydrate structures on cell surfaces added support to the concept that glycoconjugates would function in cellular communication during development. Although the structural complexity inherent in glycoconjugates has slowed advances in our understanding of their functions, the complete sequencing of the genomes of organisms classically used in developmental studies (e.g., mice, Drosophila melanogaster, and Caenorhabditis elegans) has led to demonstration of essential functions for a number of glycoconjugates in developmental processes. Here we present a review of recent studies analyzing function of a variety of glycoconjugates (O-fucose, O-mannose, N-glycans, mucin-type O-glycans, proteoglycans, glycosphingolipids), focusing on lessons learned from human disease and genetic studies in mice, D. melanogaster, and C. elegans.

Role of Glycosylation in Development

Muscular dystrophy and neuronal migration disorder caused by mutations in a glycosyltransferase, POMGnT1.

Recent studies have shown that O-mannosyl glycans are present in several mammalian glycoproteins. Although knowledge on the functional roles of these glycans is accumulating, their biosynthetic pathways are poorly understood. Here we report the identification and initial characterization of a novel enzyme capable of forming GlcNAc beta 1-2Man linkage, namely UDP-N-acetylglucosamine: O-linked mannose beta-1,2-N-acetylglucosaminyltransferase in the microsome fraction of newborn rat brains. The enzyme transfers GlcNAc to beta-linked mannose residues, and the formed linkage was confirmed to be beta 1-2 on the basis of diplococcal beta-N-acetylhexosaminidase susceptibility and by high-pH anion-exchange chromatography. Its activity is linearly dependent on time, protein concentration, and substrate concentration and is enhanced in the presence of manganese ion. Its activity is not due to UDP-N-acetylglucosamine: alpha-3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I (GnT-I) or UDP-N-acetylglucosamine: alpha-6-D-mannoside beta-1,2-D-acetylglucosaminyltransferase II (GnT-II), which acts on the early steps of N-glycan biosynthesis, because GnT-I or GnT-II expressed in yeast cells did not show any GlcNAc transfer activity against a synthetic mannosyl peptide. Taken together, the results suggest that the GlcNAc transferase activity described here is relevant to the O-mannosyl glycan pathway in mammals.

Seiichiro Takahashi

Papers

Muscular dystrophy and neuronal migration disorder caused by mutations in a glycosyltransferase, POMGnT1.

A new β-1,2-N-acetylglucosaminyltransferase that may play a role in the biosynthesis of mammalian O-mannosyl glycans