Lysosomal proteome and lipidome analyses of intestinal cells reveal the crucial role of bis(monoacylglycero)phosphate for autophagosome-lysosome fusion

Analyzing intestinal cells, researchers found that a compound called bis(monoacylglycero)phosphate (BMP) plays a critical role in the fusion of autophagosomes and lysosomes. This process is essential for cellular waste disposal and recycling. To investigate this, scientists studied a mouse cell line with a genetic mutation in Gnptab, a gene responsible for producing enzymes that form mannose 6-phosphate signals. These signals are vital for tagging proteins for transport to lysosomes.

When Gnptab was absent, the study observed a loss of multiple lysosomal enzymes and an accumulation of certain lipids, such as sphingomyelins, ceramides, and cholesterol. Importantly, lysosomes in these deficient cells had lower levels of BMP compared to normal cells. By examining the lipid content of lysosomes, the team discovered two groups of lipid-modifying enzymes that interact with BMP.

One group was linked to neutral ceramides, which are involved in early stages of autophagosome formation, while the other group was associated with autophagosome-lysosome fusion, proteins that require negatively charged BMP.

These findings suggest that the imbalance between high cholesterol levels and low BMP levels in lysosomes may directly contribute to impaired autophagic flux, the process by which cells break down and recycle waste. The researchers propose that targeting three lysosomal proteins could potentially enhance autophagic flux in cells with dysfunctional lysosomes. To validate their hypothesis, they established an intestinal organoid model, which provides a novel experimental tool to study this phenomenon.