Study finds therapeutic target for removal of misfolded, unwanted proteins
When the UPS mechanism fails, cells engage in a compensatory protein clearance process known as "aggrephagy," in which protein aggregates are destroyed in a controlled manner by the cell.
Representative Image (Photo: ANI)
WASHINGTON DC: Biological cells have "housekeeping" systems built in to take care of damaged cellular structures. This includes the ubiquitin-proteasome system (UPS), which selectively tags and clears undesirable proteins with the ubiquitin molecule.
When the UPS mechanism fails, cells engage in a compensatory protein clearance process known as "aggrephagy," in which protein aggregates are destroyed in a controlled manner by the cell. However, the mechanism underlying aggrephagy has remained elusive.
The study was published on in Volume 13, Issue 1 of Scientific Reports. The discovery revealed the fundamental process. "Our findings show that the transcription factor NRF1 (NFE2L1) activates aggrephagy in response to proteasome dysfunction," says Atsushi Hatanaka, a doctoral student at Doshisha University in Japan and the study's first author.
Sota Nakada and Akira Kobayashi from Doshisha University's Laboratory for Genetic Code and Graduate School of Life and Medical Sciences were also members of the research team. NRF1, a protein involved in the transcription of DNA to RNA, plays a key role in the balance and regulation of proteins.
It upregulates proteasome genes when the proteasome is damaged. As a part of the study, the team first used small interfering RNA (siRNA) to reduce the activity of the NRF1 synthesis gene in a cellular model.
They then used the inhibitor MG132 to block proteasome-mediated protein recycling. These treatments led to the accumulation of undesired proteins in the cellular model, indicating that the absence of NRF1 effectively inhibited aggrephagy activation, which is otherwise typically prevalent in a cell.
The researchers then examined the effect of proteasome inhibition on genes that were direct targets of NRF1. During the course of experiments, the team noticed that in response to proteasomal failure, NRF1 caused an increase in the levels of the autophagy-related genes p62 and GABARAPL1.
The elevated levels of the corresponding proteins p62 and GABARAPL1 resulted in the removal of proteins that were tagged for removal by the housekeeping protein ubiquitin. In other words, NRF1 was found to trigger the process of aggrephagy via p62 and GABARAPL1 as a direct physiological response to proteasomal failure.
Next, the team also discovered that the presence of NRF1 was necessary for the formation of p62-positive puncta--tiny round cellular structures loaded with large amounts of the protein p62. Moreover, it became evident that the colocalization (physical proximity) of the proteins p62, ULK1, and TBK1 was necessary for the activation of p62.
This activation is considered to be the direct result of phosphorylation--the addition of phosphate groups to proteins. A series of experiments revealed that the phosphorylation of p62 was facilitated by NRF1. The phosphorylated p62 then contributed to the process of aggrephagy.
As mentioned before, a similar increase in aggrephagy was also observed after the NRF1-mediated increase in the levels of the protein GABARAPL1.
Explaining the novelty of the research, Hatanaka said, "Although NRF1 has been previously shown to upregulate proteasome genes when the proteasome is dysfunctional, our genome-wide transcriptome analyses showed that NRF1 directly upregulates autophagy-related genes p62 and GABARAPL1."