How cells decide the way they want to recycle their content

May 14, 2020

Researchers from Tokyo Medical and Dental University (TMDU) identify a novel phosphorylation site of the protein Ulk1 to be an essential regulating factor for alternative autophagy, but not canonical autophagy.

Tokyo, Japan - Autophagy is a housekeeping process through which cells remove dysfunctional contents to balance energy sources during times of stress. Now, researchers from Tokyo Medical and Dental University (TMDU) identified a novel molecular mechanism by which a type of autophagy, called alternative autophagy, is activated. In a new study published in Nature Communications, they showed how a specific phosphorylation site of the protein Unc51-like kinase 1 (Ulk1) is essential for the cell to go down the alternative autophagy path.

As living structures, cells ensure homeostasis by carrying out specific processes by which they build and degrade their contents. Particularly at times of stress, for example during exposure to toxins, autophagy helps to ensure an orderly turnover process by which cells can recycle their contents to survive. Interestingly, the process of autophagy can take place by several distinct molecular mechanisms, two of which are canonical and alternative autophagy. While the protein Ulk1 is known to initiate both types of autophagy, the mechanism by which Ulk1 differentially regulates them has remained unclear.

"Autophagy is a very elaborate process by which cells recycle their contents," says the corresponding author of the study Shigeomi Shimizu. "The goal of our study was to understand how Ulk1 that has control over two types of autophagy, differentially regulates them."

To achieve their goal, the researchers used mouse embryonic fibroblasts (MEFs) deficient in the protein Atg5 to turn off canonical autophagy. By exposing them to etoposide, a DNA-damaging reagent, they then induced alternative autophagy. Using mass spectrometry, the researchers found that Ulk1 carried an additional phosphoryl group at its amino acid serine in position 746 (Ser746; p-Ulk1746), also called phosphorylation, when exposed to etoposide but not when left untreated. By developing a new antibody against p-Ulk1746, the researchers then showed that the protein localized to the Golgi complex within the cells. The Golgi complex is an organelle participating in many cellular processes, including alternative autophagy.

"While these were already exciting findings, our goal was to understand whether the specific phosphorylation of Ulk1 at the serine 746 site is required for alternative autophagy and which kinase is responsible for this phosphorylation step," says lead and the corresponding author of the study Satoru Torii.

To analyze the causal relationship between Ulk1 Ser746 phosphorylation and alternative autophagy, the researchers used a fluorescent tandem protein consisting of red fluorescent protein (RFP) and green fluorescent protein (GFP). Because GFP does not fluoresce within acidic environments, the tandem protein made autolysosomes, cellular compartments that are created during autophagy, become red. While the red fluorescence appeared after etoposide treatment, it was not generated in cells producing Ulk1 nonphosphorylated mutant, indicating that p-Ulk1746 is required for alternative autophagy. Next, the researchers demonstrated that receptor-interacting protein kinase 3 (RIPK3), a protein that phosphorylates other proteins involved in necroptosis, is responsible for the generation of p-Ulk1746 by showing that p-Ulk1746 and alternative autophagy occurred in normal cells but not in cells deficient in RIPK3. Intriguingly in MEFs that expressed Atg5, canonical autophagy was not affected by RIPK3-deficiency, indicating that p-Ulk1746 is not involved in canonical autophagy.

"These are striking results that shed new light on how cells regulate the complex process of autophagy," says Shimizu. "We hope that our findings will be helpful in understanding the role of alternative autophagy in normal biology and disease."
-end-
The article, "Identification of a phosphorylation site on Ulk1 required for genotoxic stress-induced alternative autophagy," was published in Nature Communications at DOI: 10.1038/s41467-020-15577-2

Tokyo Medical and Dental University

Related Autophagy Articles from Brightsurf:

Surprising insights into the role of autophagy in neuron
Autophagy protects our neurons in the brain, but for entirely different reasons than previously assumed, as researchers from the Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP) and Charité in Berlin have shown.

Revealing the identity of the last unknown protein of autophagy
Japanese scientists discovered that Atg9, one of the proteins that function to mediate autophagy, has phospholipid-translocation activity (the lipid scramblase activity) between the two layers of the lipid bilayer?and elucidated that the protein's activity brings about autophagosome membrane expansion.

Lipids, lysosomes, and autophagy: The keys to preventing kidney injury
Lysosomes are cellular waste disposal organelles containing potent enzymes that cause cellular damage if they leak out of ruptured lysosomes.

How zika virus degrades essential protein for neurological development via autophagy
Researchers at the University of Maryland (UMD) shed new light on how Zika virus hijacks our own cellular machinery to break down an essential protein for neurological development, getting it to ''eat itself''.

Autophagy: the beginning of the end
Autophagy, from the Greek for 'self-eating', is an essential process that isolates and recycles cellular components under conditions of stress or when resources are limited.

Cellular cleanup! Atg40 folds the endoplasmic reticulum to facilitate its autophagy
Scientists at Tokyo Institute of Technology (Tokyo Tech) and Institute of Microbial Chemistry investigated 'ER-phagy,' the degradation mechanism of the endoplasmic reticulum (ER), an important organelle with multiple biologically necessary functions like the synthesis of proteins and lipids.

How cells decide the way they want to recycle their content
Researchers from Tokyo Medical and Dental University (TMDU) identified a new phosphorylation site of Ulk1 as a novel regulating mechanism of alternative autophagy.

Autophagy: Scientists discover novel role for self-recycling process in the brain
Proteins classically associated with autophagy regulate the speed of intracellular transport.

Insights into the diagnosis and treatment brain cancer in children
In a recent study published in Autophagy, researchers at Kanazawa University show how abnormalities in a gene called TPR can lead to pediatric brain cancer.

Autophagy degrades liquid droplets, but not aggregates, of proteins
Autophagy is a mechanism through which cellular protein is degraded.

Read More: Autophagy News and Autophagy Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.