Calreticulin enhances the secretory trafficking of a misfolded α-1-antitrypsin

Endogenous retrovirus-like proteins recruit UBQLN2 to stress granules and alter their functional properties

The human genome is replete with sequences derived from foreign elements including endogenous retrovirus-like proteins of unknown function. Here we show that UBQLN2, a ubiquitin-proteasome shuttle factor implicated in neurodegenerative diseases, is regulated by the linked actions of two retrovirus-like proteins, RTL8 and PEG10. RTL8 confers on UBQLN2 the ability to complex with and regulate PEG10. PEG10, a core component of stress granules, drives the recruitment of UBQLN2 to stress granules under various stress conditions, but can only do so when RTL8 is present. Changes in PEG10 levels further remodel the kinetics of stress granule disassembly and overall composition by incorporating select extracellular vesicle proteins. Within stress granules, PEG10 forms virus-like particles, underscoring the structural heterogeneity of this class of biomolecular condensates. Together, these results reveal an unexpected link between pathways of cellular proteostasis and endogenous retrovirus-like proteins.

Calreticulin from Apostichopus japonicus relieves endoplasmic reticulum stress induced by Vibrio splendidus through autophagy

When organisms are exposed to external stimuli, misfolded proteins accumulate continuously, resulting in endoplasmic reticulum (ER) stress. Autophagy is of great significance for eliminating aggregated proteins and maintaining cellular homeostasis. However, the molecular mechanism of activating autophagy in response to ER stress in sea cucumber is remain unclear. In the current study, we demonstrated that the pathogen Vibrio splendidus can cause ER stress in Apostichopus japonicus coelomocytes and identified a Ca2+ binding partner calreticulin (designated as AjCRT), which increased with the occurrence of ER stress. The nucleotide sequence analysis showed that the open reading frame of AjCRT was 1242 bp and encoded a 413-amino-acid residue polyprotein with calreticulin domains. The spatial expression analysis revealed that AjCRT was ubiquitously expressed in all examined tissues with large magnitude in the coelomocytes and was minimally expressed in muscle. Furthermore, silencing AjCRT in vivo could significantly exacerbate ER stress induced by V. splendidus and resulted in the significant reduction of coelomocyte autophagy. These findings indicate a calreticulin-based mechanism that positively regulates autophagy in response to ER stress induced by pathogen infection. The results will provide a basis for understanding the way of host alleviating ER stress through autophagy, and pharmacological approaches may have potential for managing ER stress induced by pathogen and related cellular disorders.

Manipulation of Proteostasis Networks in Transgenic ZAAT Zebrafish via CRISPR-Cas9 Gene Editing

The CRISPR-Cas9 genome editing system is used to induce mutations in genes of interest resulting in the loss of functional protein. A transgenic zebrafish α1-antitrypsin deficiency (AATD) model displays an unusual phenotype, in that it lacks the hepatic accumulation of the misfolding Z α1-antitrypsin (ZAAT) evident in human and mouse models. Here we describe the application of the CRISPR-Cas9 system to generate mutant zebrafish with defects in key proteostasis networks likely to be involved in the hepatic processing of ZAAT in this model. We describe the targeting of the atf6a and man1b1 genes as examples.

Mechanism of mutant calreticulin-mediated activation of the thrombopoietin receptor in cancers

Myeloproliferative neoplasms (MPNs) are frequently driven by mutations within the C-terminal domain (C-domain) of calreticulin (CRT). CRT_Del52 and CRT_Ins5 are recurrent mutations. Oncogenic transformation requires both mutated CRT and the thrombopoietin receptor (Mpl), but the molecular mechanism of CRT-mediated constitutive activation of Mpl is unknown. We show that the acquired C-domain of CRT_Del52 mediates both Mpl binding and disulfide-linked CRT_Del52 dimerization. Cysteine mutations within the novel C-domain (C400A and C404A) and the conserved N-terminal domain (N-domain; C163A) of CRT_Del52 are required to reduce disulfide-mediated dimers and multimers of CRT_Del52. Based on these data and published structures of CRT oligomers, we identify an N-domain dimerization interface relevant to both WT CRT and CRT_Del52. Elimination of disulfide bonds and ionic interactions at both N-domain and C-domain dimerization interfaces is required to abrogate the ability of CRT_Del52 to mediate cell proliferation via Mpl. Thus, MPNs exploit a natural dimerization interface of CRT combined with C-domain gain of function to achieve cell transformation.