Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease characterized by the progressive deterioration of motor neurons in the brain and spinal cord. This leads to disrupted communication between the brain and muscles, resulting in muscle wasting and loss of voluntary muscle control.1 Some ALS patients also experience cognitive decline and behavioral changes.
Researchers have linked familial ALS, an inherited form of the disorder, to mutations in several genes, one of which codes for the protein Ubiquilin 2 (UBQLN2).2 UBQLN2 shuttles damaged or unneeded proteins to the proteasome complex—cellular machinery that breaks down and recycles the undesirable cargo. “When UBQLN2 is mutated, which is what happens in some cases of familial ALS, it can’t perform that activity, and the proteins that it shuttles start to build up,” explained Alexandra Whiteley, a biochemist at the University of Colorado Boulder, in an email.
In a study recently published in the journal eLife, a research team led by Whiteley and her former research technician Holly Black further explored the mechanism connecting UBQLN2 with ALS.3 Previously, Whiteley and her colleagues found that several proteins accumulated in the brain and spinal cord of mice that had dysfunctional UBQLN2.3 One of these was Paternally Expressed Gene 10 (PEG10). This protein, an example of a “domesticated retrotransposon,” is derived from a virus-like genetic element that long ago integrated into the human genome and evolved to serve necessary roles.
While PEG10 is important for placental development,4 an accumulation of the protein can lead to suboptimal outcomes. Scientists have found that increased levels of PEG10 during mouse brain development might alter neuronal development and play a role in neurological disorders such as Angelman’s syndrome.5
Whiteley and her team showed that dysfunctional UBQLN2 unleashes the dormant virus-like activity of PEG10, leading to gene expression changes that could potentially play a role in ALS progression. PEG10 uses ribosome frameshifting, a rare mechanism in humans but common in viruses, to produce two proteins, gag and gag-pol, from one mRNA.6 The researchers first investigated human embryonic stem cells that lacked the UBQLN2 gene. They observed that only the gag-pol variant of PEG10 built up, while the gag variant remained unaffected, suggesting that UBQLN2 exclusively controls the regulation of the gag-pol form.
Gag-pol contains a retroviral protease domain, which it uses to cut itself and the gag form into pieces. Using a combination of protein biochemistry and cell biology techniques, the researchers found that one part of the cleaved protein, a nucleocapsid fragment that has been reported to bind nucleic acids,7 travels to the nucleus from the cytoplasm.
To test whether the nucleocapsid fragment could induce gene expression changes, the team transfected cells with either PEG10 gag-pol, gag, or the nucleocapsid fragment alone, and analyzed changes using RNA-sequencing. They found that only the gag-pol and nucleocapsid protein fragments caused alterations in neuronal gene expression, including in genes involved in axon remodeling.
“As opposed to the previous consensus that loss of UBQLN2 function was causing generalized protein aggregation leading to cellular dysfunction, our data suggests that the loss of UBQLN2 leads to this virus-like protein going a little haywire and changing cell behavior in more precise ways,” said Whiteley.
The team also found elevated levels of PEG10 gag-pol in postmortem spinal cord tissue from sporadic and familial ALS patients in comparison with control samples from people who died from causes other than neurodegenerative disease. The detection of gag-pol buildup in sporadic ALS suggests that PEG10 might be involved in more than just the UBQLN2-mediated form of the disorder.
“The role of retroviruses in sporadic ALS is particularly interesting since there is no specific underlying cause for sporadic ALS,” said Carlos Castañeda, a biochemist at Syracuse University who was not involved in this study. He added that it remains to be seen what stimulates PEG10 dysregulation in this case. “Could cell stress and/or aging activate these retroviral proteins in cells and dysregulate their control mechanisms, such as UBQLN2’s role with PEG10?”
To understand the exact contribution of PEG10 to ALS, the research team is currently investigating the effects of reducing its mRNA expression in a UBQLN2-knockout mouse model.
- Masrori P, et al. Amyotrophic lateral sclerosis: a clinical review. Eur J Neurol. 2020;27(10):1918-1929
- Deng HX, et al. Mutations in UBQLN2 cause dominant X-linked juvenile and adult-onset ALS and ALS/dementia. Nature. 2011;477(7363):211-215
- Black HH, et al. UBQLN2 restrains the domesticated retrotransposon PEG10 to maintain neuronal health in ALS. Elife. 2023;12:e79452
- Ono R, et al. Deletion of Peg10, an imprinted gene acquired from a retrotransposon, causes early embryonic lethality. Nat Genet. 2006;38(1):101-106
- Pandya NJ, et al. Secreted retrovirus-like GAG-domain-containing protein PEG10 is regulated by UBE3A and is involved in Angelman syndrome pathophysiology. Cell Rep Med. 2021;2(8):100360
- Clark MB, et al. Mammalian gene PEG10 expresses two reading frames by high efficiency -1 frameshifting in embryonic-associated tissues. J Biol Chem. 2007;282(52):37359-37369
- Segel M, et al. Mammalian retrovirus-like protein PEG10 packages its own mRNA and can be pseudotyped for mRNA delivery. Science. 2021;373(6557):882-889