Department of Biology, Georgia State University, Atlanta, GA 30303, USA.
Imaging Core Facility, Georgia State University, Atlanta, GA 30303, USA.
Int J Mol Sci. 2024 Nov 1;25(21):11772. doi: 10.3390/ijms252111772.
Glycogen, a branched polysaccharide organized into glycogen granules (GGs), is delivered from the cytoplasm to the lysosomes of hepatocytes by STBD1-driven selective autophagy (glycophagy). Recently, we developed yeast as a simple model of GG autophagy and found that it proceeds non-selectively under nitrogen starvation conditions. However, another group, using as a model, found that glycogen is a non-preferred cargo of nitrogen starvation-induced bulk autophagy. To clarify cargo characteristics of GGs, we used the same glycogen synthase-based reporter (Gsy1-GFP) of GG autophagy in as was used in . The Gsy1-GFP marked the GGs and reported on their autophagic degradation during nitrogen starvation, as expected. However, unlike in , glycogen synthase-marked GGs were delivered to the vacuole and degraded there with the same efficiency as a cytosolic glycogen synthase in glycogen-deficient cells, suggesting that glycogen is a neutral cargo of bulk autophagy in . We verified our findings with a new set of reporters based on the glycogen-binding CBM20 domain of human STBD1. The GFP-CBM20 and mCherry-CBM20 fusion proteins tagged GGs, reported about the autophagy of GGs, and confirmed that GGs in are neither preferred nor non-preferred substrates of bulk autophagy. They are its neutral substrates.
糖原是一种支链多糖，组织成糖原颗粒（GGs），通过 STBD1 驱动的选择性自噬（糖噬作用）从细胞质输送到肝细胞的溶酶体中。最近，我们开发了酵母作为 GG 自噬的简单模型，发现它在氮饥饿条件下非选择性进行。然而，另一组使用作为模型，发现糖原是氮饥饿诱导的批量自噬的非首选货物。为了阐明酵母 GG 的货物特征，我们使用了与酵母中 GG 自噬相同的基于糖原合酶的报告基因（Gsy1-GFP）。Gsy1-GFP 标记 GG，并报告在氮饥饿期间其自噬降解，正如预期的那样。然而，与酵母不同的是，在酵母中，标记的糖原合酶标记的 GG 被输送到液泡中，并以与糖原缺乏细胞中细胞质糖原合酶相同的效率降解，表明糖原是酵母中批量自噬的中性货物。我们使用基于人类 STBD1 的糖原结合 CBM20 结构域的新报告基因集验证了我们的发现。GFP-CBM20 和 mCherry-CBM20 融合蛋白标记 GG，报告 GG 的自噬，并证实酵母中的 GG 既不是批量自噬的首选底物，也不是非首选底物。它们是其中性底物。
