UBQLN2 Helps a Proteasome Tackle Aggregated Proteins

August 16, 2016 - als

Ubiquilin-2 (UBQLN2) partners with a protein HSP70 to convey many-sided proteins to a proteasome for degradation, according to a new investigate published Aug 11 in Cell. The formula brand a new mechanism, eccentric of autophagy, by that a dungeon can discharge protein aggregates, and serve implicate a forsake in protein clearway in ALS pathogenesis. They also advise that mutations in UBQLN2 means ALS by a detriment of function, nonetheless other mechanisms might also be during play, experts say.

“Our work shows that UBQLN2 acts as a executive heart for a disaggregation and a successive plunge of many-sided proteins,” remarkable principal questioner Thimo Kurz of a University of Glasgow, United Kingdom to a ALS Research Forum. “It unequivocally extends a apparatus pack of a proteasome.”

UBQLN2 translocates to a iota underneath conditions of feverishness stress. [Image: Hjerpe, Bett et al., Cell, 2016, underneath CCBY 4.0].

UBQLN2 translocates to a iota underneath conditions of feverishness stress. [Image credit: Hjerpe, Bett et al., Cell, 2016, underneath CC BY 4.0].

Proteins tagged with ubiquitin bondage are targeted to a proteasome for plunge in one of dual ways, Kurz explained: possibly by approach communication with proteasome-bound receptors, or around “shuttles” that couple a accumulation of poly-ubiquitylated proteins to a proteasome (Glickman and Ciechanover, 2002; Verma et al., 2004). A leavening convey protein, called Dsk2, is homologous to 4 vertebrate proteins, including UBQLN2, a protein compared with ALS and frontemporal insanity (see Aug 2011 news). UBQLN2 has a executive domain with homology to a feverishness startle contracting protein called STI1, that interacts with a protein identical to a famous chaperone protein, feverishness startle protein 70 (HSP70) (Kaye et al., 2000; March 2016 news). The probable communication between a UBQLN2 and a protein folding machine led Kurz, co-first authors Roland Hjerpe and John Bett, and colleagues to try a duty of UBQLN2 and a intensity purpose in controlling protein misfolding and degradation.

Working in dungeon culture, they found that a properties of UBQLN2 altered after feverishness shock: it became activated and it interacted increasingly with HSP70, as good as with a proteasome and poly-ubiquitylated proteins slated for clearance.  Depletion of UBQLN2 regulating tiny interfering RNAs (siRNAs) caused cells to be hypersensitive to feverishness shock, and also marred clearway of ubiquitinated protein aggregates.  This response occurred underneath highlight conditions other than feverishness startle stress, given plunge of unfolded proteins that amassed in response to puromycin diagnosis was also marred by UBQLN2 depletion.

An intriguing aspect of UBQLN2 duty is a purpose in compelling clearway of protein aggregates in a nucleus, where autophagy pathways are not active. In cells expressing GFPu-NLS, a indication unfolded chief protein, UBQLN2 was required to foster clearway of chief GFPu-NLS aggregates that shaped following feverishness shock.

What promotes contracting of UBQLN2 to HSP70? The authors explored this in an in vitro model, and found that contracting of these dual proteins was prompted in a participation of HSP70 substrates, such as a denatured protein. These denatured proteins also strongly prompted recruitment of a ubiquilin-HSP70 formidable to a proteasome, suggesting that a proteins slated for plunge trigger public of a plunge complex.

“These information strongly support a indication whereby contracting of clients to HSP70 triggers communication with UBQLN2,” a authors wrote, “which afterwards bridges contracting to a proteasome to intercede degradation.”

The group subsequent explored how UBQLN2 mutations impact a duty in vivo by building constitutive knock-in mice that demonstrate mutant UBQLN2. Using rodent rudimentary fibroblasts from a masculine knock-in mice, they found that mutant UBQLN2 contracting to HSP70, ubiquitylated substrates, and a proteasome was reduced. Although mutant UBQLN2 still translocated to a nucleus, it did not effectively partisan protein aggregates; influenced cells had increasing ionization to both feverishness startle and puromycin treatment. “Together, these information advise that a disease-linked forms of UBQLN2 are loss-of-function mutations,” they concluded.

Mice carrying UBQLN2 mutations were cognitively impaired, though did not vaunt a clever engine phenotype. UBQLN2 and p62 were found in inclusions in hippocampus, mind stem, and cortex. The investigators subsequent incited to examined UBLQLN2 in a Huntington’s illness (HD) rodent model, formed on commentary that UBQLN2 binds huntingtin (HTT) aggregates in pathological samples from HD patients (Rutherford et al., 2013). Wild-type UBQLN2 firm to stretched HTT protein in a R6/2 transgenic rodent indication of HD. When these mice were crossed with UBQLN2 knock-in mutant mice, double mutants had aloft levels of many-sided HTT and reduce levels of soluble HTT, suggesting that UBQLN2 promotes total clearway in vivo.

“The proteasome has been suspicion to usually reduce soluble proteins,” commented Kurz to a ALS Research Forum. “We uncover for a initial time that it can also reduce aggregates, though in sequence to do so, it requires UBQLN2.” Based on his results, he suggested a indication for UBQLN2 function, in that proteotoxic highlight triggers a contracting of HSP70 to a substrates, exposing a contracting site for ubiquilin-2 on HSP70. That communication promotes a contracting of ubiquilin-2 to a proteasome. HSP70 afterwards unfolds a many-sided protein, permitting it to be fed into a proteasome for degradation. “The proteasome can usually accommodate a singular polypeptide in a chamber,” Kurz forked out. “An total doesn’t fit in, so we need something to get these proteins untangled first.”

Ian Blair of Macquarie University, Australia, who was not concerned in a study, remarkable that for UBQLN2 mutations, “this investigate has teased out where a intensity forsake in a smoothness complement is” for misfolded proteins targeted to a proteasome. “This might give us improved opportunities to aim that forsake therapeutically.” While UBQLN2 mutations are a singular means of ALS, a import of a protein peculiarity control complement means that proteostasis is “likely to play a purpose in a most wider operation of ALS,” he said. One of a engaging facets of a UBQLN2 story, he noted, is that it is not a convey for all proteins—its substrates are singular in ways that are not nonetheless clear. “The subsequent step would be to find out what those substrates are, as that might explain because mutations privately impact a spinal or cortical neurons in ALS and FTD”. In addition, a temperament of those substrates might indicate to new healing targets.

“This paper provides some really critical insights as to how UBQLN2 might be behaving in certain circumstances,” pronounced Teepu Siddique of Northwestern University in Chicago, IL, who was not concerned in a study. “If this was a elementary detriment of duty of UBQLN2, there would be pathology or impairment,” Siddique said. “This is a really engaging and minute paper that shows some of a properties of UBQLN2”, however, he noted, a end that UBQLN2 mutations simply means a detriment of duty might not be a whole story, formed on work display that a knock-out rodent has no phenotype (Wu et al., 2015).

Primary Reference:
Hjerpe R, Bett JS, Keuss MJ, Solovyova A, McWilliams TG, Johnson C, Sahu I, Varghese J, Wood N, Wightman M, Osborne G, Bates GP, Glickman MH, Trost M, Knebel A, Marchesi F, Kurz T. UBQLN2 Mediates Autophagy-Independent Protein Aggregate Clearance by a Proteasome. Cell. 2016 Aug 11;166(4):935-49. Epub 2016 Jul 28. [Pubmed].

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