Functional Requirements for Heparan Sulfate Biosynthesis in Morphogenesis and Nervous System Development in C. elegans

Abstract

The regulation of cell migration is essential to animal development and physiology. Heparan sulfate proteoglycans shape the interactions of morphogens and guidance cues with their respective receptors to elicit appropriate cellular responses. Heparan sulfate proteoglycans consist of a protein core with attached heparan sulfate glycosaminoglycan chains, which are synthesized by glycosyltransferases of the exostosin (EXT) family. Abnormal HS chain synthesis results in pleiotropic consequences, including abnormal development and tumor formation. In humans, mutations in either of the exostosin genes EXT1 and EXT2 lead to osteosarcomas or multiple exostoses. Complete loss of any of the exostosin glycosyltransferases in mouse, fish, flies and worms leads to drastic morphogenetic defects and embryonic lethality. Here we identify and study previously unavailable viable hypomorphic mutations in the two C. elegans exostosin glycosyltransferases genes, rib-1 and rib-2. These partial loss-of-function mutations lead to a severe reduction of HS levels and result in profound but specific developmental defects, including abnormal cell and axonal migrations. We find that the expression pattern of the HS copolymerase is dynamic during embryonic and larval morphogenesis, and is sustained throughout life in specific cell types, consistent with HSPGs playing both developmental and post-developmental roles. Cell-type specific expression of the HS copolymerase shows that HS elongation is required in both the migrating neuron and neighboring cells to coordinate migration guidance. Our findings provide insights into general principles underlying HSPG function in development.

Diverse roles for glycosaminoglycans in neural patterning

“…For instance, dackel (EXT2) is required for axon pathfinding of sensory axons toward the skin (Wang et al, ), while boxer (EXTL3) is needed for proper longitudinal axon guidance of dopaminergic neurons in zebrafish (Kastenhuber et al, ). The general requirement of HS for neuronal patterning appears evolutionarily conserved based on studies of hypomorphic (and hence viable) alleles of the C. elegans homologs of the exostosis genes rib‐1 /EXT1 and rib‐2 /EXT2 (Blanchette et al, ). Systematic analyses of the neuroanatomy in these mutants suggest that in C. elegans HS is required for the majority of neuronal migrations and axonal guidance events tested (Blanchette et al, ).…”

The NCLX-type Na+/Ca2+ Exchanger NCX-9 Is Required for Patterning of Neural Circuits in Caenorhabditis elegans

“…Several components of the HS side chain biosynthesis machinery are expressed and functional in the seam cells, in which NCX-9 functions. The rib-1 gene is required for elongation of HS side chains and, similar to NCX-9, is expressed in the seam cells (29). Furthermore, rib-1 regulates the left/right asymmetric patterning of DD/VD neu-rons (29).…”

“…The rib-1 gene is required for elongation of HS side chains and, similar to NCX-9, is expressed in the seam cells (29). Furthermore, rib-1 regulates the left/right asymmetric patterning of DD/VD neu-rons (29). Two genes that are critical for the sulfation of HS side chains, pst-1 and pps-1, are also expressed and functional in the seam cells (30,31).…”

Galectins in Invertebrates with a focus on <i>Caenorhabditis elegans</i>

“…Although the interaction of LEC-1 and glycosaminoglycan has not been reported so far, its existence might be of interest, because glycosaminoglycan takes important roles in oxidative stress and development in C. elegans (33,34). Therefore, binding of a galectin to chemically synthesized Galβ1-4Fuc disaccharide does not necessarily imply that its endogenous ligand is a Galβ1-4Fuc epitope of natural Nglycan.…”