Cell Type-Specific Requirements for Heparan Sulfate Biosynthesis at the Drosophila Neuromuscular Junction: Effects on Synapse Function, Membrane Trafficking, and Mitochondrial Localization

Abstract

Heparan sulfate proteoglycans (HSPGs) are concentrated at neuromuscular synapses in many species, including Drosophila. We have established the physiological and patterning functions of HSPGs at the Drosophila neuromuscular junction by using mutations that block heparan sulfate synthesis or sulfation to compromise HSPG function. The mutant animals showed defects in synaptic physiology and morphology suggesting that HSPGs function both presynaptically and postsynaptically; these defects could be rescued by appropriate transgene expression. Of particular interest were selective disruptions of mitochondrial localization, abnormal distributions of Golgi and endoplasmic reticulum markers in the muscle, and a markedly increased level of stimulus-dependent endocytosis in the motoneuron. Our data support the emerging view that HSPG functions are not limited to the cell surface and matrix environments, but also affect a diverse set of cellular processes including membrane trafficking and organelle distributions.

Extracellular matrix and its receptors indrosophilaneural development

“…Finally, heparan sulphate is enriched at Drosophila NMJs. The HSPGs Dlp and Sdc presumably contribute prominently to this local synaptic heparan sulphate enrichment (in agreement with their functional requirement at the NMJ; see previous section), and it can be abolished when deleting the heparan sulfate synthesizing enzymes tout-velu /Ext1 ( ttv ) and brother of ttv /Extl3 ( botv ) (Lin, 2004; Johnson et al, 2006; Ren et al, 2009). Loss of heparan sulphate in ttv/botv mutants, or of specific sulfate modifications in N-deacetylase/N-sulfotransferane mutants (encoded by sfl ), causes differential structural and functionional defects at NMJs, including elevated synaptic vesicle endocytosis (Ren et al, 2009).…”

“…At vertebrate NMJs, further prominent ECM constituents include the secreted HSPGs agrin and perlecan, which are important regulators of synapse formation, including acetylcholine receptor (AChR) clustering and acetylcholine esterase localization (Singhal and Martin, 2011). Specific probes similarly indicate localization of a heparan sulfate matrix at the Drosophila NMJ (Ren et al, 2009). Drosophila does not possess a defined agrin homologue, but the HSPGs Dlp and Sdc are both localized to the Drosophila NMJ (Fig.…”

Two matrix metalloproteinase classes reciprocally regulate synaptogenesis

“…Indeed, studies from our lab and others show that Wg signaling is sensitive to perturbations in HSPG chain biosynthesis and HS modifying enzymes, which modulate both NMJ structure and function Menon et al, 2013;Reichsman et al, 1996;Ren et al, 2009). It is easy to envision how tissue-and development-stage-specific HS modifications could coordinate HSPG/Mmp-dependent functions, thereby differentially regulating diverse signaling events, which enable context-specific responses instructed by the extracellular environment.…”

Diverse roles for glycosaminoglycans in neural patterning

“…The defects in tout velu mutants could be transgenically rescued by expression from muscle and partially rescued from neurons. However, many of these defects were not rescued when tout velu was expressed ubiquitously (Ren et al, ). These results suggest that the expression of HS is tightly regulated in different tissues to modulate synaptic function.…”