A Dictyostelium model for BPAN disease reveals a functional relationship between the WDR45/WIPI4 homolog Wdr45l and Vmp1 in the regulation of autophagy-associated PtdIns3P and ER stress

PROPPINs are conserved PtdIns3P-binding proteins required for autophagosome biogenesis that fold into a characteristic group of seven-bladed beta-propellers. Mutations in WDR45/WIPI4, a human member of this family, lead to BPAN, a rare form of neurodegeneration. We have generated mutants for the two PROPPIN proteins present in the model system <i>Dictyostelium discoideum</i> (Atg18 and Wdr45l) and characterized their function. Lack of Wdr45l greatly impairs autophagy, while Atg18 only causes subtle defects in the maturation of autolysosomes. The strong phenotype of the Wdr45l mutant is strikingly similar to that observed in <i>Dictyostelium</i> cells lacking Vmp1, an ER protein required for omegasome formation. Common phenotypes include impaired growth in axenic medium, lack of aggregation, and local enrichment of PtdIns3P as determined by the use of lipid reporters. In addition, Vmp1 and Wdr45l mutants show a chronically active response to ER stress. For both mutants, this altered PtdIns3P localization can be prevented by the additional mutation of the upstream regulator Atg1, which also leads to recovery of axenic growth and reduction of ER stress. We propose that, in addition to an autophagy defect, local autophagy-associated PtdIns3P accumulation might contribute to the pathogenesis of BPAN by disrupting ER homeostasis. The introduction of BPAN-associated mutations in <i>Dictyostelium</i> Wdr45l reveals the impact of pathogenic residues on the function and localization of the protein.

PROPPIN家族蛋白（PROPPINs）是一类保守的磷脂酰肌醇3-磷酸（PtdIns3P）结合蛋白，参与自噬体（autophagosome）的生物发生，其折叠形成特征性的七叶β-螺旋桨结构（seven-bladed beta-propellers）。该家族的人类成员WDR45/WIPI4发生突变可引发BPAN（β-螺旋桨蛋白相关神经退行性疾病），一种罕见的神经退行性病症。我们在模式生物盘基网柄菌（Dictyostelium discoideum）中构建了该家族的两种PROPPIN蛋白突变体（Atg18与Wdr45l），并对其功能开展了系统性表征。Wdr45l的缺失会严重损伤自噬进程，而Atg18仅会导致自噬溶酶体（autolysosomes）的成熟过程出现轻微缺陷。Wdr45l突变体的强烈表型与缺失Vmp1的盘基网柄菌细胞表型高度相似——Vmp1是一种参与ω小体（omegasome）形成的内质网蛋白（ER protein）。二者共有的表型包括：无菌培养基中生长受阻、无法发生细胞聚集，以及通过脂质探针（lipid reporters）检测到的磷脂酰肌醇3-磷酸（PtdIns3P）局部富集。此外，Vmp1与Wdr45l突变体均表现出内质网应激（ER stress）的持续性激活反应。对于这两种突变体，额外突变上游调控因子Atg1即可恢复其磷脂酰肌醇3-磷酸（PtdIns3P）的正常定位，同时还能使其恢复无菌培养基中的生长能力，并缓解内质网应激。我们提出，除自噬缺陷外，与自噬相关的局部磷脂酰肌醇3-磷酸（PtdIns3P）积累可能通过破坏内质网稳态，参与BPAN的发病机制。在盘基网柄菌Wdr45l中引入BPAN相关突变，可揭示致病氨基酸残基对该蛋白功能与定位的影响。