Complex cooperative functions of heparan sulfate proteoglycans shape nervous system development in Caenorhabditis elegans.
Bottom Line: Specifically, lon-2/glypican and unc-52/perlecan act in parallel genetic pathways and display synergistic interactions with sdn-1/syndecan to mediate kal-1 function.Because all of these heparan sulfate core proteins have been shown to act in different tissues, these studies indicate that KAL-1/anosmin-1 requires heparan sulfate with distinct modification patterns of different cellular origin for function.Our results support a model in which a three-dimensional scaffold of heparan sulfate mediates KAL-1/anosmin-1 and intercellular communication through complex and cooperative interactions.
Affiliation: Department of Genetics, Albert Einstein College of Medicine, Bronx, New York, 10461.Show MeSH
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Mentions: The repeated identification of mutants in heparan sulfate modification enzymes in the screens as suppressors of the kal-1-dependent branching phenotype prompted the question, which HSPG core protein(s) carry the responsible HS chains? The C. elegans genome encodes the canonical HSPGs sdn-1/syndecan, gpn-1/glypican, lon-2/glypican, and unc-52/perlecan (Bülow and Hobert 2006). We thus tested mutants in sdn-1/syndecan, lon-2/glypican, gpn-1/glypican, and in a splice variant–specific allele of unc-52/perlecan (Rogalski et al. 1993) (Figure 3A). All four HSPG core protein mutants displayed significant suppression of kal-1-dependent branching, but surprisingly nowhere similar to mutations in HS modifying enzymes. This suggested that either more than one HSPG or an unidentified HSPG is required for kal-1-dependent branching. Alternatively, the function of other splice variants of unc-52/perlecan could be sufficient to mediate most of the function required for kal-1-dependent branching. To discriminate between these possibilities, we constructed double mutants and found that only the unc-52(e998); lon-2(e678) double mutant displayed an enhanced level of suppression (51%; N = 100) that was different from the single mutants or the other double mutants (Figure 3A). This indicated that lon-2/glypican and unc-52/perlecan act in parallel genetic pathways to mediate kal-1-dependent branching in AIY. The unc-52(e998); lon-2(e678) sdn-1(zh20) triple mutant further suppressed branching to 15% (n = 100), indicating the presence of an additional parallel genetic pathway. This was surprising because the lon-2; sdn-1 double mutant did not display significantly enhanced suppression compared with either of the single mutants. However, in the absence of unc-52/perlecan, lack of both sdn-1/syndecan and lon-2/glypican at the same time appeared to result in synergistic rather than additive effects. Thus, it appears as if unc-52/perlecan can substitute for both sdn-1/syndecan and lon-2/glypican. Interestingly, we observed similar genetic synergy between HSPG core proteins in a loss of function setting. Specifically, we found that midline guidance of PVQ neurites requires sdn-1/syndecan and lon-2/glypican redundantly (Figure S4). Taken together, these findings suggest that HSPGs, at least sdn-1/syndecan, lon-2/glypican and unc-52/perlecan, act redundantly to mediate kal-1-dependent branching and likely neural development in other cellular contexts.
Affiliation: Department of Genetics, Albert Einstein College of Medicine, Bronx, New York, 10461.