​Lysosomes are subcellular organelles that degrade macromolecules and pathogens, and are important for nutrient recycling and signalling. The intraluminal pH of lysosomes is low, and their content of acid hydrolyses and Ca²⁺ high. Hence, rupture of lysosomes poses a potential hazard to cells. Given the detrimental consequences of impaired lysosomal integrity and the high frequency of lysosomal membrane permeabilization in diseases and normal aging, cells have evolved essential mechanisms to rapidly repair damaged lysosomes. When severely damaged, lysosomes are engulfed by autophagic membranes in the process known as lysophagy, which is initiated by the recognition of luminal glycoprotein domains by cytosolic lectins such as Galectin-3.

We have shown that various lysosome membrane injuring conditions trigger recruitment of the endosomal sorting complex required for transport (ESCRT). This recruitment precedes Galectin-3 and the lysophagy machinery. Interference with ESCRT recruitment impairs lysosome repair and causes otherwise reversible lysosome damage to become lethal (Figure 1).

Figure 1. ESCRT proteins, originally identified as regulators of protein sorting, promote the repair of lysosomal membrane and bacteria-containing vacuoles to protect cells against cell death and provide replicative advantage to pathogens, respectively.

Recently, we described an additional mechanism for lysosome repair that is activated independently of ESCRT recruitment. This process involves rapid formation of contact sites between damaged lysosomes with the endoplasmic reticulum (ER) (Figure 2). Briefly, the ER contacts are activated on damaged lysosomes in parallel to ESCRTs to provide lipids for membrane repair, and that PtdIns4P generation and removal are central in this response.

Figure 2. ESCRT-independent repair mechanism that involves cholesterol transfer mediated by contact sites between lysosomes and the ER. Here, the ER proteins VAPA/B form contacts between ER and lysosomes and recruit the cholesterol-binding protein ORP1L. PI4K2A-dependent accumulation of PtdIns4P on the damaged lysosomes is essential for ORP1L recruitment and lysosome repair. Depletion of the cholesterol–PtdIns4P transporter OSBP causes PtdIns4P hyperaccumulation on damaged lysosomes and impairs cell viability.

We are interested to understand the mechanism of lysosome repair and how ruptured membranes are sealed. We are also investigating additional mechanisms for lysosome repair, which protect against lysosomal damage-induced cell death. 

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