Autophagy dysfunction and ubiquitin-positive protein aggregates in Dictyostelium cells lacking Vmp1

Ubiquitin-positive protein aggregates are a hallmark of many degenerative diseases. Their presence can be induced by dysfunction in protein degradation pathways such as proteasome and autophagy. We now report several lines of evidence suggesting a defect in autophagy in Dictyostelium cells lacking Vmp1 ( vacuole membrane protein 1), an endoplasmic reticulum (ER)-resident protein involved in pathological processes such as cancer and pancreatitis. vmp1-null cells are unable to survive starvation or undergo autophagic cell death under the appropriate inductive signals. Moreover, confocal studies using the autophagy marker Atg8 and previous transmission electron microscopy analysis showed defects in autophagosome formation. Although Vmp1 is localized in the ER, we found colocalization with Atg8 suggesting a contribution of both Vmp1 and ER in autophagosome biogenesis or maturation. Interestingly, vmp1-mutant cells showed accumulation of huge ubiquitin-positive protein aggregates containing the autophagy marker GFP-Atg8 and the putative Dictyostelium p62 homologue as described in many degenerative human diseases. The analysis of other Dictyostelium autophagic mutants (atg1(-), atg5(-), atg6(-), atg7(-) and atg8(-)) showed a correlation in the severity of their corresponding phenotypes and the presence of ubiquitin-positive protein aggregates suggesting that the deleterious effects associated with development of these aggregates might contribute to the complex phenotypes observed in autophagy deficient mutants. Our results suggest that Vmp1 is required for the clearance of these ubiquitinated protein aggregates through autophagy and highlight a potential role for Vmp1 in protein-aggregation diseases.