Scientists have discovered the main pathway to repair lysosomes in cells

Lysosomes are digestive enzyme sacs that organize cells
by breaking down waste products.
But they can also cause trouble: When their outer surface is damaged, their destructive proteins begin to spill into the cytoplasm, harming cells
.
In fact, the frequency of this leakage increases as a person ages and may play a role in aging-related diseases such as neurodegenerative diseases
.
Now, a study published Sept.
7 in the journal Nature reveals a previously unknown route that cells use to repair leaking lysosomes, which could have implications
for treating these diseases.

Marja Jäättelä, Professor of Cell Death and Metabolism at the Danish Cancer Society's Research Centre, said it was a "very complete and well-designed" study and the first to link lipid transport to non-metabolic biological processes
.
Marja Jäättelä was not involved in the study
.

Research has established a way
for cells to repair leaking lysosomes.
Previously, a group of proteins known as the ESCRT mechanism were found to patch holes
in the organelle membrane.
Jay Tan, a cell biologist at the University of Pittsburgh and co-author of the study, believes "something is missing
.
" Even if the ESCRT complex is inhibited, up to 90% of lysosomal damage is repaired, suggesting a different pathway for most of the repair
.

To pinpoint individual components of the missing pathway, Tan uses a lentiviral to express in human cells an enzyme called Turbo-ID, which can affix biotin tags
to any protein within a 10-nanometer radius.
Using an enzyme localized to lysosomes and a chemical known to penetrate lysosomal membranes, he was able to biotinylize all proteins around damaged organelles
.

Tan isolates the biotin-labeled protein and identifies
it by mass spectrometry.
There are components of the ESCRT complex in the purified molecule, but there are also proteins known to interact with phosphoinositol, a family of lipids known to regulate key cellular processes, including proliferation and migration
。 Further experiments have shown that when the lysomal membrane is damaged, an enzyme called phosphatidylinositol-4-kinase type 2a (PI4K2A) is absorbed into the surface of the organelles, which may be a reaction to the calcium ions leaking out of the lysosome, PI4K2A produces a lipid called phosphatidylinositol-4-phosphate (PI4P), which acts as a red flag and absorbs some proteins called ORPs (hydroxysterol-binding protein-related proteins), These proteins connect the endoplasmic reticulum to lysosomes
.

These ORPs then exchange PI4P with lipids from the endoplasmic reticulum, including phosphatidylserine, which recruits the lipid transporter ATG2 – the final component of the
pathway.
Study co-author Toren Finkel, a professor of medicine at the University of Pittsburgh, said
.
For lipids, "ATG2 is like a fire hose," pumping molecules into the membrane to plug the holes
.

Subsequently, the researchers mutated various subunits of ATG2 to confirm that lipid transporters are an important part of
the pathway.
"We set up a bunch of molecular toll booths by changing amino acids instead of smooth tunnels, which makes lipids harder to flow
," Finkel said.
These changes do hinder lipid transport and lysosomal repair
in cell culture.
In praise of the University of Pittsburgh, the two scientists named this pathway phospholipositol-initiated membrane ties and lipid transport, or PITT
for short.

Tan believes that these two mechanisms can repair different types of damage, with ESCRT complexes repairing small pores and PITT pathways repairing larger pores
.

The new pathway probably does most of the cell's work
.
The researchers found that cells typically take about an hour to repair damaged lysosomes, but in cells deficient in PI42KA, this healing takes up to 11 hours
.
Antony Galione, a pharmacologist at the University of Oxford in the United Kingdom who was not involved in the study, said: "This looks like a very important pathway, probably more important
than the ESCRT pathway.
"

Tan said the discovery could point to drug targets for neurodegenerative diseases, such as Alzheimer's, because protein aggregates such as tau proteins escape in lysosomal membranes, preventing them from being destroyed and promoting their spread
between neurons.
In fact, the researchers found that deletions of PI4K2A (the promoter of the PITT pathway) increased the spread
of tau protein in cell culture.

Caroline Mauvezin, a molecular biologist at the University of Barcelona in Spain who was not involved in the study, said: "This is definitely an important process
for further studying the neurodegenerative disease model.
" But this pathway may function differently in other cells, including neurons, so more research
is needed.

The researchers plan to start with reusable drugs currently available and screen for the drug's ability to
activate that pathway.
An interesting clue, Tan said, is ginseng, a plant used in traditional Chinese medicine whose ingredients appear to activate PI42KA
.

A phosphoinositide signalling pathway mediates rapid lysosomal repair