Peptide: N-glycanase (NGLY1) is an evolutionarily conserved enzyme to remove nitrogen-bound glycans (N-glycans) from glycoproteins and is involved in the proteostasis of N-glycoproteins in the cytosol. In 2012, a rare genetic condition called NGLY1 deficiency was discovered by exome analysis.
Symptoms in patients with NGLY1 deficiency include overall developmental delay, hypotonia, hypo / alacrima, movement disturbances, scoliosis, abnormal liver and brain functions, and peripheral neuropathy. Unfortunately, no therapeutic treatment is currently available for this devastating disease, as the exact mechanism of damage is not yet understood.
So far, Dr. Suzuki’s team at RIKEN and T-CiRA have developed various animal models for NGLY1 deficiency and reported that deletion of NGLY1 in B6 mice is embryonically fatal.
In the cytosol, there are other proteins involved in the processing of N-glycoproteins. One of these groups of proteins is made up of the glycoprotein-specific ubiquitin ligase subunits, FBS (sugar chain-recognizing F-box proteins), which ubiquitinates unfolded glycoproteins in the cytosol.
The ubiquitination of proteins serves as signals for proteolysis by the proteasome, a proteolytic machinery essential for cellular proteostasis and health. The researchers found that deletion of the gene encoding FBS2, one of the FBS proteins, restored lethality in NGLY1-KO B6 mice and that FBS2; NGLY1 double-KO mice developed without apparent abnormalities. This finding reveals that FBS2 activity could induce lethality in NGLY1-KO mouse embryos.
Adverse effects of FBS2 in the absence of NGLY1 were also observed in cultured cells. The researchers noticed that the overexpression of FBS2 in the NGLY1-KO cell lines suppressed cell growth and ultimately induced cell death.
FBS2 ubiquitinated glycoproteins which failed to remove N-glycans in the absence of NGLY1, and ubiquitinated glycoproteins were not degraded and accumulated in cells. Ubiquitinated proteins are normally degraded by the proteasome, but ubiquitinated glycoproteins induce proteasome inhibition. Prolonged proteasome inhibition leads to cell death.
In order to maintain the function of the proteasome, the cells induce the synthesis of new proteasomes by the transcriptional factor NRF1 during proteasome dysfunction. Interestingly, NRF1 is a glycoprotein that resides in the endoplasmic reticulum and degraded by the proteasome under normal conditions. When proteasome activity is suppressed, NRF1 is stabilized and translocated into the nucleus where it activates transcription of the proteasome.
However, in the absence of NGLY1, FBS2 ubiquitinated NRF1 and suppressed its proteasome transcriptional activity. Thus, FBS2 inhibited the activity of the proteasome and also damaged the recovery function of the proteasome upon loss of NGLY1. The presence of ubiquitinated NRF1 was also observed in NGLY1-KO mouse embryos.