Heparan sulfate proteoglycans regulate autophagy in <i>Drosophila</i>

Heparan sulfate-modified proteoglycans (HSPGs) are important regulators of signaling and molecular recognition at the cell surface and in the extracellular space. Disruption of HSPG core proteins, HS-synthesis, or HS-degradation can have profound effects on growth, patterning, and cell survival. The <i>Drosophila</i> neuromuscular junction provides a tractable model for understanding the activities of HSPGs at a synapse that displays developmental and activity-dependent plasticity. Muscle cell-specific knockdown of HS biosynthesis disrupted the organization of a specialized postsynaptic membrane, the subsynaptic reticulum (SSR), and affected the number and morphology of mitochondria. We provide evidence that these changes result from a dysregulation of macroautophagy (hereafter referred to as autophagy). Cellular and molecular markers of autophagy are all consistent with an increase in the levels of autophagy in the absence of normal HS-chain biosynthesis and modification. HS production is also required for normal levels of autophagy in the fat body, the central energy storage and nutritional sensing organ in <i>Drosophila</i>. Genetic mosaic analysis indicates that HS-dependent regulation of autophagy occurs non-cell autonomously, consistent with HSPGs influencing this cellular process via signaling in the extracellular space. These findings demonstrate that HS biosynthesis has important regulatory effects on autophagy and that autophagy is critical for normal assembly of postsynaptic membrane specializations.

硫酸乙酰肝素修饰的蛋白聚糖（Heparan sulfate-modified proteoglycans, HSPGs）是细胞表面及细胞外间隙中重要的信号调控与分子识别调节因子。HSPG核心蛋白、HS合成或HS降解过程的紊乱，会对机体生长、模式形成以及细胞存活产生显著影响。果蝇（Drosophila）的神经肌肉接头为研究突触处HSPGs的活性提供了一个易于操作的模型——该突触兼具发育依赖性与活动依赖性可塑性。对肌肉细胞进行HS生物合成的特异性敲低，会破坏特化突触后膜——突触下网状结构（subsynaptic reticulum, SSR）的组织形态，并影响线粒体的数量与形态。我们的研究证实，上述变化源于巨自噬（macroautophagy，下称自噬）的失调。自噬的细胞与分子标志物均表明，在正常HS链合成与修饰缺失的情况下，自噬水平显著升高。在果蝇的脂肪体——这一果蝇中枢性的储能与营养感应器官——中，HS的合成同样是维持正常自噬水平所必需的。遗传嵌合分析显示，HS对自噬的调控以非细胞自主性的方式进行，这与HSPGs通过细胞外间隙的信号通路影响这一细胞过程的假说相符。本研究结果表明，HS生物合成对自噬具有重要的调控作用，而自噬对于突触后膜特化结构的正常组装至关重要。