The Probable, Possible, and Novel Functions of ERp29.

The luminal endoplasmic reticulum (ER) protein of 29 kDa (ERp29) is a ubiquitously expressed cellular agent with multiple critical roles. ERp29 regulates the biosynthesis and trafficking of several transmembrane and secretory proteins, including the cystic fibrosis transmembrane conductance regulator (CFTR), the epithelial sodium channel (ENaC), thyroglobulin, connexin 43 hemichannels, and proinsulin. ERp29 is hypothesized to promote ER to cis-Golgi cargo protein transport via COP II machinery through its interactions with the KDEL receptor; this interaction may facilitate the loading of ERp29 clients into COP II vesicles. ERp29 also plays a role in ER stress (ERS) and the unfolded protein response (UPR) and is implicated in oncogenesis. Here, we review the vast array of ERp29's clients, its role as an ER to Golgi escort protein, and further suggest ERp29 as a potential target for therapies related to diseases of protein misfolding and mistrafficking.

Correction: ERp29 as a regulator of Insulin biosynthesis.

[This corrects the article DOI: 10.1371/journal.pone.0233502.].

ERp29 as a regulator of Insulin biosynthesis.

The environment within the Endoplasmic Reticulum (ER) influences Insulin biogenesis. In particular, ER stress may contribute to the development of Type 2 Diabetes (T2D) and Cystic Fibrosis Related Diabetes (CFRD), where evidence of impaired Insulin processing, including elevated secreted Proinsulin/Insulin ratios, are observed. Our group has established the role of a novel ER chaperone ERp29 (ER protein of 29 kDa) in the biogenesis of the Epithelial Sodium Channel, ENaC. The biogenesis of Insulin and ENaC share may key features, including their potential association with COP II machinery, their cleavage into a more active form in the Golgi or later compartments, and their ability to bypass such cleavage and remain in a less active form. Given these similarities we hypothesized that ERp29 is a critical factor in promoting the efficient conversion of Proinsulin to Insulin. Here, we confirmed that Proinsulin associates with the COP II vesicle cargo recognition component, Sec24D. When Sec24D expression was decreased, we observed a corresponding decrease in whole cell Proinsulin levels. In addition, we found that Sec24D associates with ERp29 in co-precipitation experiments and that ERp29 associates with Proinsulin in co-precipitation experiments. When ERp29 was overexpressed, a corresponding increase in whole cell Proinsulin levels was observed, while depletion of ERp29 decreased whole cell Proinsulin levels. Together, these data suggest a potential role for ERp29 in regulating Insulin biosynthesis, perhaps in promoting the exit of Proinsulin from the ER via Sec24D/COPII vesicles.

The Regenerating Adult Zebrafish Retina Recapitulates Developmental Fate Specification Programs.

Adult zebrafish possess the remarkable capacity to regenerate neurons. In the damaged zebrafish retina, Müller glia reprogram and divide to produce neuronal progenitor cells (NPCs) that proliferate and differentiate into both lost neuronal cell types and those unaffected by the damage stimulus, which suggests that developmental specification/differentiation programs might be recapitulated during regeneration. Quantitative real-time polymerase chain reaction revealed that developmental competence factors are expressed following photoreceptor damage induced by intense light or in a genetic rod photoreceptor cell ablation model. In both light- and N-Methyl-D-aspartic acid (NMDA)-damaged adult zebrafish retinas, NPCs, but not proliferating Müller glia, expressed fluorescent reporters controlled by promoters of ganglion (atoh7), amacrine (ptf1a), bipolar (vsx1), or red cone photoreceptor cell competence factors (thrb) in a temporal expression sequence. In both damage paradigms, atoh7:GFP was expressed first, followed by ptf1a:EGFP and lastly, vsx1:GFP, whereas thrb:Tomato was observed in NPCs at the same time as ptf1a:GFP following light damage but shifted alongside vsx1:GFP in the NMDA-damaged retina. Moreover, HuC/D, indicative of ganglion and amacrine cell differentiation, colocalized with atoh7:GFP prior to ptf1a:GFP expression in the ganglion cell layer, which was followed by Zpr-1 expression (red/green cone photoreceptors) in thrb:Tomato-positive cells in the outer nuclear layer in both damage paradigms, mimicking the developmental differentiation sequence. However, comparing NMDA- to light-damaged retinas, the fraction of PCNA-positive cells expressing atoh7:GFP increased, that of thrb:Tomato and vsx1:GFP decreased, and that of ptf1a:GFP remained similar. To summarize, developmental cell specification programs were recapitulated during retinal regeneration, which adapted to account for the cell type lost.

The KDEL receptor has a role in the biogenesis and trafficking of the epithelial sodium channel (ENaC).

Endoplasmic reticulum protein of 29 kDa (ERp29) is a thioredoxin-homologous endoplasmic reticulum (ER) protein that regulates the biogenesis of cystic fibrosis transmembrane conductance regulator (CFTR) and the epithelial sodium channel (ENaC). ERp29 may promote ENaC cleavage and increased open probability by directing ENaC to the Golgi via coat complex II (COP II) during biogenesis. We hypothesized that ERp29's C-terminal KEEL ER retention motif, a KDEL variant that is associated with less robust ER retention, strongly influences its regulation of ENaC biogenesis. As predicted by our previous work, depletion of Sec24D, the cargo recognition component of COP II that we previously demonstrated to interact with ENaC, decreases ENaC functional expression without altering ß-ENaC expression at the apical surface. We then tested the influence of KDEL ERp29, which should be more readily retrieved from the proximal Golgi by the KDEL receptor (KDEL-R), and a KEEL-deleted mutant (ΔKEEL ERp29), which should not interact with the KDEL-R. ENaC functional expression was decreased by ΔKEEL ERp29 overexpression, whereas KDEL ERp29 overexpression did not significantly alter ENaC functional expression. Again, ß-ENaC expression at the apical surface was unaltered by either of these manipulations. Finally, we tested whether the KDEL-R itself has a role in ENaC forward trafficking and found that KDEL-R depletion decreases ENaC functional expression, again without altering ß-ENaC expression at the apical surface. These results support the hypothesis that the KDEL-R plays a role in the biogenesis of ENaC and in its exit from the ER through its association with COP II. The cleavage of the extracellular loops of the epithelial sodium channel (ENaC) α and γ subunits increases the channel's open probability and function. During ENaC biogenesis, such cleavage is regulated by the novel 29-kDa chaperone of the ER, ERp29. Our data here are consistent with the hypothesis that ERp29 must interact with the KDEL receptor to exert its regulation of ENaC biogenesis. The classically described role of the KDEL receptor is to retrieve ER-retained species from the proximal Golgi and return them to the ER via coat complex I machinery. In contrast, our data suggest a novel and important role for the KDEL receptor in the biogenesis and forward trafficking of ENaC.