UBQLN2 restrains the domesticated retrotransposon PEG10 to maintain neuronal health in ALS

Abstract

Amyotrophic Lateral Sclerosis (ALS) is a fatal, neurodegenerative disease characterized by progressive motor neuron dysfunction and loss. A portion of ALS is caused by mutation of the proteasome shuttle factor Ubiquilin 2 (UBQLN2), but the molecular pathway leading from UBQLN2 dysfunction to disease remains unclear. Here, we demonstrate a function of UBQLN2 in regulating activity of the domesticated gag-pol retrotransposon 'paternally expressed gene 10' (PEG10) in human cells and tissues. In cells, the PEG10 gag-pol protein cleaves itself in a mechanism reminiscent of retrotransposon self-processing to generate a liberated 'nucleocapsid' fragment, which uniquely localizes to the nucleus and changes expression of genes involved in axon remodeling. In spinal cord tissue from ALS patients, PEG10 gag-pol is elevated compared to healthy controls. These findings implicate the retrotransposon-like activity of PEG10 as a contributing mechanism in ALS through regulation of gene expression, and restraint of PEG10 as a primary function of UBQLN2.

The gag-like geneRTL8antagonizes PEG10-mediated virus like particles

“…During embryonic development, there is conflict between paternally expressed genes, which promote growth of the offspring even at the expense of the mother, and maternally expressed genes, which limit the activity of paternal genes to maintain maternal health 42,43 . PEG10 is a paternally expressed gene which is essential for placental development, but is linked to cancer and neurological disease [15][16][17]19,22 . UBQLN2 and RTL8 are located on the X-chromosome and in mice, placental expression of genes on the Xchromosome occurs exclusively from the maternal allele 44 .…”

“…1c,e). Elevated PEG10 protein expression has been implicated in the neurodegenerative disease Amyotrophic Lateral Sclerosis (ALS) and the neurodevelopmental disease Angelman syndrome 18,19,22 . In both conditions, primary cultured neurons are commonly used as a model of cellular dysfunction.…”

“…Elevation of the ratio of gag-pol:gag protein levels in HepG2 and hTR-8 cells, paired with the enhanced VLP abundance in these cell lines, suggested that a component of the pol open reading frame may increase the efficiency of PEG10-derived VLP formation and/or release. Recent studies have shown that the proteasome shuttle factor ubiquilin 2 (UBQLN2) specifically regulates the abundance of PEG10 gagpol, but not gag, by targeting the protein for proteasomal degradation 18,22 . To test whether UBQLN2 similarly restricts VLP abundance, we stably introduced shRNAs targeting UBQLN2 to HepG2 and hTR-8 cells (Fig.…”

“…Virus-like particles were harvested from cultured media as previously described 22 . Briefly, T75 flasks were plated at 70% confluency (for endogenous VLP production), or 6-well plates (for overexpression experiments) were transfected as described above.…”

“…The domesticated retrotransposon Paternally Expressed Gene 10 (PEG10) embodies a balance between adaptation and pathology: PEG10 is necessary for placentation in mice 13,14 , but also drives cancer and neurological disease in humans [15][16][17][18][19] . The mechanisms of PEG10-mediated adaptive and pathological roles remain poorly understood, but may involve the protein's rare ability to form virus-like particles (VLPs) that are exported from the cell [20][21][22] . For the domesticated gene Arc, formation and release of a VLP is essential to its adaptive function in the organism 23,24 .…”